The Cambridge natural history, Vol. 02 (of 10)

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Title: The Cambridge natural history, Vol. 02 (of 10)
Author: Gamble, Frederick William, Harmer, S. F. (Sidney Frederic), Hartog, Marcus, Sheldon, Lilian, Shipley, A. E. (Arthur Everett), Sir
Language: English
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* * * * *

THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, M.A., Fellow of King's College, Cambridge; Superintendent of
the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., Fellow of Christ's College, Cambridge; University
Lecturer on the Morphology of Invertebrates

VOLUME II

[Illustration]

FLATWORMS AND MESOZOA

By F. W. GAMBLE, M.Sc. (Vict.), Owens College

NEMERTINES

By Miss L. SHELDON, Newnham College, Cambridge

THREAD-WORMS AND SAGITTA

By A. E. SHIPLEY, M.A., Fellow of Christ's College, Cambridge

ROTIFERS

By MARCUS HARTOG, M.A., Trinity College, Cambridge (D.Sc. Lond.),
Professor of Natural History in the Queen's College, Cork

POLYCHAET WORMS

By W. BLAXLAND BENHAM, D.Sc. (Lond.), Hon. M.A. (Oxon.), Aldrichian
Demonstrator of Comparative Anatomy in the University of Oxford

EARTHWORMS AND LEECHES

By F. E. BEDDARD, M.A. (Oxon.), F.R.S., Prosector to the Zoological
Society, London

GEPHYREA AND PHORONIS

By A. E. SHIPLEY, M.A., Fellow of Christ's College, Cambridge

POLYZOA

By S. F. HARMER, M.A., Fellow of King's College, Cambridge

London
MACMILLAN AND CO., LIMITED
NEW YORK: THE MACMILLAN COMPANY
1901

_All rights reserved_

'Nous allons faire des vers ensemble'
ANDRÉ DE CHÉNIER

_First Edition 1896. Reprinted 1901_

CONTENTS

PAGE

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK ix

PLATYHELMINTHES AND MESOZOA

CHAPTER I

TURBELLARIA

INTRODUCTION—DESCRIPTION OF THE POLYCLAD _LEPTOPLANA TREMELLARIS_
—APPEARANCE—HABITS—STRUCTURE: POLYCLADIDA—CLASSIFICATION—HABITS
—ANATOMY—DEVELOPMENT: TRICLADIDA—OCCURRENCE—STRUCTURE
—CLASSIFICATION: RHABDOCOELIDA—OCCURRENCE—HABITS—REPRODUCTION
—CLASSIFICATION 3

CHAPTER II

TREMATODA

CHARACTERS OF TREMATODES—HABITS AND STRUCTURE OF TREMATODA
ECTOPARASITICA (MONOGENEA)—LIFE-HISTORIES OF _POLYSTOMUM
INTEGERRIMUM_, _DIPLOZOON PARADOXUM_, AND _GYRODACTYLUS ELEGANS_
—TREMATODA ENDOPARASITICA (DIGENEA)—OCCURRENCE AND HABITS OF DIGENEA
—LIFE-HISTORY OF _DISTOMUM MACROSTOMUM_—_DISTOMUM HEPATICUM_ AND ITS
EFFECTS—_BILHARZIA HAEMATOBIA_—BISEXUAL TREMATODES—TABLE OF HOSTS
—CLASSIFICATION 51

CHAPTER III

CESTODA

INTRODUCTION—NATURE OF CESTODES—OCCURRENCE OF CESTODES—THE TAPE-WORMS
OF MAN AND DOMESTIC ANIMALS—TABLE OF LIFE-HISTORIES OF PRINCIPAL
CESTODES OF MAN AND DOMESTIC ANIMALS—STRUCTURE AND DEVELOPMENT OF
CESTODES—TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF
MAN AND DOMESTIC ANIMALS—CLASSIFICATION 74

CHAPTER IV

MESOZOA

DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE—OCCURRENCE
—STRUCTURE: _TRICHOPLAX: SALINELLA_ 92

NEMERTINEA

CHAPTER V

NEMERTINEA

INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT
—HABITS—REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND,
FRESH-WATER, AND PARASITIC FORMS—AFFINITIES 99

NEMATHELMINTHES AND CHAETOGNATHA

CHAPTER VI

NEMATHELMINTHES

INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE
—STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE
—ENOPLIDAE—PARASITISM: NEMATOMORPHA—ANATOMY—CLASSIFICATION
—LIFE-HISTORY: ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION 123

CHAPTER VII

CHAETOGNATHA

STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION—TABLE OF
IDENTIFICATION [see also p. 534] 186

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

CHAPTER VIII

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENTS—ANATOMY—REPRODUCTION
—EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES: GASTROTRICHA:
KINORHYNCHA 197

ARCHIANNELIDA, POLYCHAETA, AND MYZOSTOMARIA

CHAPTER IX

THE CHAETOPODOUS WORMS—THE ARCHIANNELIDA—ANATOMY OF _NEREIS_, AS
TYPICAL OF THE POLYCHAETA 241

CHAPTER X

CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE—GILLS
—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS
—BUDDING—FISSION—BRANCHING—REGENERATION 257

CHAPTER XI

NATURAL HISTORY OF POLYCHAETES—GENERAL HABITS—CHARACTER OF TUBE AND
ITS FORMATION—COLOURING—PROTECTIVE AND MIMETIC DEVICES
—PHOSPHORESCENCE—FOOD—USES—ASSOCIATED WORMS—WORMS AS HOSTS
—DISTRIBUTION—FOSSIL REMAINS 284

CHAPTER XII

CHARACTERS OF THE SUB-ORDERS OF POLYCHAETES—CHARACTERS OF THE
FAMILIES—DESCRIPTION OF BRITISH GENERA AND SPECIES: THE
MYZOSTOMARIA 303

OLIGOCHAETA (EARTHWORMS, ETC.), AND HIRUDINEA (LEECHES)

CHAPTER XIII

OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES)

INTRODUCTION—ANATOMY—REPRODUCTION—BIONOMICS—DISTRIBUTION
—CLASSIFICATION—MICRODRILI AND MEGADRILI 347

CHAPTER XIV

HIRUDINEA (LEECHES)

INTRODUCTION—ANATOMY—REPRODUCTION—CLASSIFICATION—RHYNCHOBDELLAE
AND GNATHOBDELLAE 392

GEPHYREA AND PHORONIS

CHAPTER XV

GEPHYREA

INTRODUCTION—ANATOMY—DEVELOPMENT—SIPUNCULOIDEA—PRIAPULOIDEA
—ECHIUROIDEA—EPITHETOSOMATOIDEA—AFFINITIES OF THE GROUP 411

CHAPTER XVI

PHORONIS

HISTORY—HABITS—STRUCTURE—REPRODUCTION—LARVA—METAMORPHOSIS—LIST OF
SPECIES AND LOCALITIES—SYSTEMATIC POSITION 450

POLYZOA

CHAPTER XVII

POLYZOA

INTRODUCTION—GENERAL CHARACTERS AND TERMINOLOGY—BROWN BODIES—HISTORY
—OUTLINES OF CLASSIFICATION—MARINE POLYZOA—OCCURRENCE—FORMS OF
COLONY AND OF ZOOECIA—OVICELLS—AVICULARIA—VIBRACULA—ENTOPROCTA 465

CHAPTER XVIII

POLYZOA—_continued_

FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OF
_CRISTATELLA_—DIVISION OF COLONY—MOVEMENTS OF COLONY—RETRACTION AND
PROTRUSION OF POLYPIDES IN POLYZOA—STATOBLASTS—TABLE FOR
DETERMINATION OF GENERA OF FRESH-WATER POLYZOA—REPRODUCTIVE PROCESSES
OF POLYZOA—DEVELOPMENT—AFFINITIES—METAMORPHOSIS—BUDDING 492

CHAPTER XIX

POLYZOA—_continued_

CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE
EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE
DETERMINATION OF THE GENERA OF BRITISH MARINE POLYZOA 515

ADDENDUM TO CHAETOGNATHA 534

INDEX 535

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

PLATYHELMINTHES (p. 3)

TREMATODA (pp. 3, 51)
|
+—— MONOGENEA (pp. 5, 52) = Heterocotylea + Aspidocotylea (p. 73)
| |
| +—— Temnocephalidae (pp. 53, 73).
| +—— Tristomatidae (pp. 53, 73).
| +—— Polystomatidae (pp. 53, 73).
| +—— Gyrodactylidae (pp. 53, 61, 73).
| +—— Aspidobothridae (p. 73).
|
+—— DIGENEA (pp. 5, 52, 62) = Malacocotylea (p. 73)
|
+—— Holostomatidae (p. 73).
+—— Amphistomatidae (p. 73).
+—— Distomatidae (p. 73).
+—— Gasterostomatidae (p. 73).
+—— Didymozoontidae (p. 73).
+—— Monostomatidae (p. 73).

CESTODA (pp. 3, 74) ——————+
|
+—— Cestodariidae = Monozoa (p. 91).
+—— Bothriocephalidae (p. 91).
+—— Tetrarhynchidae (p. 91).
+—— Tetraphyllidae (p. 91).
+—— Taeniidae (p. 91).

MESOZOA

MESOZOA (pp. 3, 92) ——————+
|
+—— Dicyemidae (p. 93).
+—— Orthonectida (p. 94).

NEMERTINEA (p. 99)

HOPLONEMERTEA (p. 110) = Metanemertini (p. 112).
SCHIZONEMERTEA (p. 111) = Heteronemertini (_ex parte_) (p. 113).
PALAEONEMERTEA (p. 111) = Protonemertini (p. 112).
+ Mesonemertini (p. 112).
+ Heteronemertini (_ex parte_) (p. 113).

NEMATHELMINTHES (p. 123)

NEMATODA (pp. 123, 124) ——+
|
+—— Ascaridae (p. 138).
+—— Strongylidae (p. 142).
+—— Trichotrachelidae (p. 144).
+—— Filariidae (p. 147).
+—— Mermithidae (p. 150).
+—— Anguillulidae (p. 154).
+—— Enoplidae (p. 157).
+—— Chaetosomatidae (p. 158).
+—— Desmoscolecidae (p. 159).

NEMATOMORPHA (pp. 123, 164)
|
+—— Gordiidae (p. 164).

ACANTHOCEPHALA (pp. 123, 174)
|
+—— Echinorhynchidae (p. 182)
+—— Gigantorhynchidae (p. 183).
+—— Neorhynchidae (p. 184).
+—— Arhynchidae (p. 185).

CHAETOGNATHA (p. 186)

ROTIFERA (p. 197)

FLOSCULARIACEAE (p. 220) —+
|
+—— Flosculariidae (p. 221).
+—— Apsilidae (p. 221).

MELICERTACEAE (p. 221) ———+
|
+—— Melicertidae (p. 221).
+—— Trochosphaeridae (p. 221).

BDELLOIDA (p. 222) ———————+
|
+—— Philodinidae (p. 222).

ASPLANCHNACEAE (p. 222) ——+
|
+—— Asplanchnidae (p. 223).

SCIRTOPODA (p. 223) ——————+
|
+—— Pedalionidae (p. 223).

PLOIMA (p. 223)
|
+—— ILLORICATA (p. 223)
| |
| +—— Microcodonidae (p. 224).
| +—— Rhinopidae (p. 224).
| +—— Hydatinidae (p. 224).
| +—— Synchaetidae (p. 224).
| +—— Notommatidae (p. 224).
| +—— Drilophagidae (p. 224).
| +—— Triarthridae (p. 224).
|
+—— LORICATA (p. 224)
|
+—— Rattulidae (p. 225).
+—— Dinocharididae (p. 225).
+—— Salpinidae (p. 225).
+—— Euchlanididae (p. 225).
+—— Cathypnidae (p. 225).
+—— Coluridae (p. 225).
+—— Pterodinidae (p. 225).
+—— Brachionidae (p. 225).
+—— Anuraeidae (p. 225).

SEISONACEAE (p. 225) —————+
|
+—— Seisonidae (p. 226).

GASTROTRICHA

GASTROTRICHA (p. 231).
|
+—— EUICHTHYDINA (p. 235)
+—— APODINA (p. 235)

KINORHYNCHA (p. 236)

CHAETOPODA (p. 241)

ARCHIANNELIDA (p. 241)

MYZOSTOMARIA (pp. 241, 341)

HIRUDINEA (p. 392)

RHYNCHOBDELLAE (p. 405) ——+
|
+—— Ichthyobdellidae (p. 406).
+—— Glossiphoniidae (p. 406).

GNATHOBDELLAE (p. 407) ———+
|
+—— Gnathobdellidae (p. 407).
+—— Herpobdellidae (p. 407).

GEPHYREA (p. 411)

SIPUNCULOIDEA (pp. 412, 420).
PRIAPULOIDEA (pp. 412, 430).
ECHIUROIDEA (pp. 412, 434).
EPITHETOSOMATOIDEA (pp. 412, 444).

_PHORONIS_ (p. 450)

POLYZOA (p. 465)

ENTOPROCTA (pp. 475, 487)

PLATYHELMINTHES AND MESOZOA

F. W. GAMBLE, M.Sc. (VICT.)

Demonstrator and Assistant-Lecturer in Zoology in the Owens College,
Manchester.

{3}CHAPTER I

TURBELLARIA

INTRODUCTION: DESCRIPTION OF THE POLYCLAD _LEPTOPLANA TREMELLARIS_—
APPEARANCE—HABITS—STRUCTURE: POLYCLADIDA—CLASSIFICATION—HABITS—
ANATOMY—DEVELOPMENT: TRICLADIDA—OCCURRENCE—STRUCTURE—CLASSIFICATION:
RHABDOCOELIDA—OCCURRENCE—HABITS—REPRODUCTION—CLASSIFICATION.

The Platyhelminthes, or Flat Worms, form a natural assemblage of animals,
the members of which, however widely they may differ in appearance, habits,
or life-history, exhibit a fundamental similarity of organisation which
justifies their separation from other classes of worms, and their union
into a distinct phylum. Excluding the leeches (Hirudinea), and the long
sea-worms (Nemertinea)—which, though formerly included, are now treated
independently—the Platyhelminthes may be divided into three branches: (1)
Turbellaria (including the Planarians), (2) Trematoda (including the
liver-flukes), and (3) Cestoda (tape-worms). The Mesozoa will be treated as
an appendix to the Platyhelminthes.

The Turbellaria were so called by Ehrenberg[1] (1831) on account of the
cilia or vibratile processes with which these aquatic animals are covered,
causing by their incessant action, tiny currents ("turbellae,"
disturbances) in the surrounding water. The ciliary covering distinguishes
this free-living group from the parasitic Trematodes and Cestodes, some of
which possess such an investment, but only during their early free
{4}larval stage, for the short period when they have left the parental host
and are seeking another (Figs. 26, 27, 42).

Some Turbellaria (Rhabdocoelida) resemble Infusoria in their minute size,
shape, and movements. Nevertheless they possess an organisation of
considerable complexity. The fresh-water Planarians (Fig. 14), abounding in
ponds and streams, vary from a quarter to half an inch in length, and are
elongated and flattened. Their body is soft, and progresses by a
characteristic, even, gliding motion like a snail. The marine Planarians or
Polyclads (Fig. 8) are usually broad and leaf-like, sometimes attaining a
length of six inches, and swim or creep in a most graceful way. Land
Planarians occur in this country (Fig. 15), but far more abundantly in
tropical and sub-tropical districts, in moist places, venturing abroad at
night in pursuit of prey. They are elongated and cylindrical, in some cases
measuring, when fully extended, a foot or more in length, and are often
ornamented with brilliantly coloured, longitudinal bands.

Turbellaria are carnivorous, overpowering their prey by peculiar cutaneous
offensive weapons, and then sucking out the contents of the victim by the
"pharynx." Land Planarians feed on earthworms, molluscs, and wood-lice;
fresh-water Planarians on Oligochaet worms, water-snails, and
water-beetles; marine forms devour Polychaet worms and molluscs. Some
Turbellaria seem to prefer freshly-killed or weakly examples of animals too
large to be overpowered when fully active. Certain Rhabdocoelida are
messmates of Molluscs and Echinoderms, and a few others are truly
parasitic—a mode of life adopted by all Trematodes save _Temnocephala_.

The Trematodes[2] may be divided into those living on the outer surface of
various aquatic animals, usually fish (Ectoparasites); and those which
penetrate more or less deeply into the alimentary canal or the associated
organs of the host (Endoparasites). They are oval, flattened
Platyhelminthes ranging from a microscopic size to a length of three feet
(_Nematobothrium_, Fig. 22), and are provided with organs of adhesion by
which they cling to the outer surface, or to the interior, of the animals
they inhabit. Trematodes occur parasitically in all groups of Vertebrates,
but, with the exception of the liver-flukes of the sheep (_Distomum
hepaticum_ and _D. magnum_), and of _Bilharzia haematobia_ found in man (in
the blood-vessels of the urinary bladder) over the greater part {5}of
Africa, their attacks are not usually of a serious nature. Ectoparasitic
Trematodes are _Monogenetic_; that is, their larvae grow up directly into
mature forms. The Endoparasitic species, however, are usually _Digenetic_.
Their larvae enter an Invertebrate and produce a new generation of
different larvae, and these another. The last are immature flukes. They
enter a second host, which is swallowed by the final Vertebrate host in
which they become mature.

The Cestodes or Tape-worms have undergone more profound modifications both
in structure and in mode of development. They are all endoparasitic, and,
with one exception (_Archigetes_), attain maturity solely within the
alimentary canal of Vertebrates. In length they range from a few
millimetres to several metres, but this great size is attained from the
need for the rapid production and accumulation of enormous numbers of eggs.
The "head" or "scolex" is attached to the mucous membrane of the host by
suckers or hooks, but there is no mouth nor any certain trace of a
digestive tract at any stage of the life-history of Cestodes. For
nourishment they absorb, through the skin, the previously-digested food (of
the host) that bathes them. In a few Cestodes the body is simple and not
divided into "proglottides" or generative segments, but in most cases it is
jointed in such a way that the last segment is the oldest, and each
contains a set of reproductive organs. The life-histories of Cestodes are
most remarkable. The proglottides containing the eggs pass out of the final
host along with the faeces and enter the intermediate host with the food.
The larvae hatch, and boring their way into the blood-vessels, are carried
by the circulation to various internal organs. Here they usually become
"bladder-worms," and develop the "head" of the future sexual form. Then,
if, as is usually the case, the intermediate host is preyed upon by the
final host, the larval Cestodes enter the alimentary canal of the latter.
The head of the larva alone survives digestion, and from it the mature worm
is formed.

Of these three branches of the phylum Platyhelminthes, the Turbellaria
possess features of special interest and importance. Not only do they
furnish the explanation of the structure of the two parasitic groups (which
have probably arisen from Turbellarian-like ancestors), but they occupy the
lowest position in the whole group of worms. There are reasons for thinking
that this is the simplest group of bilateral animals which adopt the habit
of {6}creeping. The Turbellaria are most closely allied to that great
extinct group from which they, the Nemertinea, Rotifera, and even the
Annelids, offer increasingly convincing evidence of having been derived.
Many questions relating to the affinities of, or the origin of organs in,
the Annelids, resolve themselves into similar questions about the
Turbellaria. For these reasons, this group is here dealt with at greater
length than the others, the interest of which is of a more special nature.

The history of our knowledge of the Cestodes dates back to ancient times,
as the presence and effects of tape-worms early attracted the attention of
physicians. Trematodes are first distinctly referred to in the sixteenth
century, while Turbellaria first figure in Trembley's memoir on Hydra
(1744).[3] The whole subject of the increase in our knowledge of parasitic
Platyhelminthes is dealt with in the standard work, _The Parasites of Man_,
by Leuckart,[4] and a complete list of references in zoological literature
to Cestodes and Trematodes is to be found in Bronn's _Thierreich_.[5] O. F.
Müller[6] and Ehrenberg founded our knowledge of the Turbellaria, but for a
long time the group remained in a most neglected condition. In this country
Montagu, G. Johnston, and in Ireland, William Thompson, discovered several
marine species, one of which, _Planocera folium_ (from Berwick), has not
again been met with on British shores. Dalyell[7] conducted classical
researches on the habits of Planarians, and Faraday[8] made interesting
experiments on their power of regenerating lost parts. The credit of
assigning the correct interpretation to most of the various organs of
fresh-water Planarians belongs to von Baer[9] and Dugès,[10] while
Mertens[11] effected a similar service for the marine forms, or Polyclads.
The minute Rhabdocoels were first successfully investigated and classified
by Oscar Schmidt.[12] The great work on this group is, however, the
{7}monograph by von Graff.[13] A similarly comprehensive and indispensable
treatise by Lang, on the Polycladida,[14] contains references to all
previous publications on the group, among which the papers by Quatrefages,
Johannes Müller, Keferstein, Minot, and Hallez stand out conspicuously.
Moseley's work[15] on the Land Planarians of Ceylon is undoubtedly the most
revolutionary paper referring to this group, and the best contribution
towards elucidating the structure of the Tricladida at a time when the
subject was very obscure. A monograph on Land Planarians is being prepared
by von Graff.

The Turbellaria are divided into: (1) _Polycladida_, marine forms with
multiple intestinal branches; (2) _Tricladida_, marine, fresh-water, and
terrestrial Planarians with three main intestinal branches; (3) the
_Rhabdocoelida_, as varied in habit as the Triclads, but possessing a
straight and simple or slightly lobed, intestine. A detailed description of
an example of the Polyclads, and then a comparative account of each
division, will now be given.

[Illustration: FIG. 1.—_Leptoplana tremellaris_ O. F. M. Seen from the
dorsal surface. The alimentary canal runs down the middle line and sends
branches to the margin of the body. × 6.]

TURBELLARIA. I. POLYCLADIDA.

_Description of Leptoplana tremellaris._

APPEARANCE AND HABITS.—An account of the Polyclad Turbellaria may be fitly
prefaced by a description of a very common representative, _Leptoplana
tremellaris_, so called on account of the thin, flat body which executes
when disturbed, quivering or tremulous swimming movements.

{8}Like all Polyclads, _Leptoplana_ is marine. It is probably found on all
European shores, northwards to Greenland and southwards to the Red Sea,
while vertically it ranges from the littoral zone down to fifty fathoms.
There is, however, an apparently well-marked difference between the
littoral specimens, which vary from three-quarters to one inch in length,
are brownish in colour and firm in consistency, and the more delicate
examples half an inch long, white with a brown tinge, which occur in deeper
water.

[Illustration: FIG. 2.—_Leptoplana tremellaris._ Three-quarters view from
the ventral surface. The pharynx (_ph_) is widely protruded through the
month (_mo_) as in the act of attacking prey. _br_, Brain with nerves,
close to which are the four groups of eyes; _mg_, stomach; _mgc_, "marginal
groove"; _pe_, penis; _sc_, sucker; _ut_, uterus; _vd_, vasa deferentia; ♀,
female genital aperture surrounded by the shell-gland; ♂, male aperture.
(Semi-diagrammatic, and × 6.)]

At low water _Leptoplana_ may be found buried in mud or on the under
surface of stones, in pools where darkness and dampness may be ensured till
the return of the tide. It is, however, by no means easy to detect and
remove it from the encrusting Polyzoa, Ascidians, or Sponges with which it
is usually associated. The flat, soft, unsegmented body is so closely
appressed to the substratum that its presence is usually only betrayed by
its movement, an even gliding motion of the mobile body, which suggested
the apt name "la pellicule animée" to Dicquemare. The creeping surface is
called ventral, the upper one dorsal, and as the broader end of the body
always goes first, it is anterior as opposed to the more pointed posterior
extremity. With a lens the characters shown in Figs. 1 and 2 may be
observed. The eyes are seen as black dots near the anterior end, and are
placed at the sides of a clear oval space, the brain. Along the transparent
margin of the body, the ends of the intestinal branches may be seen. These
ramify from a lobed stomach or main-gut, and should the specimen be mature,
the "uterus" loaded {9}with eggs forms a dark margin round the latter
(Figs. 1 and 2, _ut_). The ventral surface is whitish, and through it the
"pharynx," a frilled protrusible structure, may be dimly observed. The
"mouth,"[16] through which the pharynx at the time of feeding is thrust out
(Fig. 2, _mo_), is almost in the centre of the ventral surface. Behind
this, a white, V-shaped mark (_vd_) indicates the ducts of the male
reproductive organs, and still further back is the irregular opaque mark of
the "shell-gland," by which the egg-shells are formed (Fig. 2, ♀).

[Illustration: FIG. 3.—_Leptoplana tremellaris_ in the act of swimming. A,
Seen from the right side during the downward stroke (the resemblance to a
skate is striking); B, from above, showing the upward stroke and
longitudinal undulations of the swimming lobes; C, side view during the
upward stroke; D, transverse sections of the body during the strokes. × 5.]

{10}_Leptoplana_ employs two kinds of movement, creeping and swimming.
Creeping is a uniform gliding movement, caused by the cilia of the ventral
surface, aided perhaps by the longitudinal muscular layers of this surface,
and is effected on the under side of the "surface-film" of water almost as
well as on a solid substratum. Swimming is a more rapid and elegant
movement, employed when alarmed or in pursuit of prey. The expanded
fore-parts of the body act as lobes, which are flapped rapidly up over the
body and then down beneath it, undulations running rapidly down them from
before backwards. The action in fact is somewhat similar to that by which a
skate swims, a resemblance pointed out long ago by Dugès[17] (Fig. 3).

We have few direct observations on the nature of the food of _Leptoplana_,
or the exact mode by which it is obtained. Dalyell,[18] who observed this
species very carefully, noticed that it was nocturnal and fed upon a
_Nereis_, becoming greatly distended and of a green colour after the meal,
but pale after a long fast. Keferstein[19] noticed a specimen in the act of
devouring a _Lumbriconereis_ longer than itself, and also found the radulae
of _Chiton_ and Taenioglossate Molluscs in the intestine. That such an
apparently weak and defenceless animal does overpower large and healthy
Annelids and Mollusca, has not hitherto been definitely proved. Weak or
diseased examples may be chiefly selected. The flexible _Leptoplana_
adheres firmly to its prey, and the rapid action of the salivary glands of
its mobile pharynx quickly softens and disintegrates the internal parts of
the victim. The food passes into the stomach (Fig. 2, _mg_), and is there
digested. It is then transferred to the lateral branches of the intestine,
and, after all the nutritious matters have been absorbed, the faeces are
ejected with a sudden contraction of the whole body through the pharynx
into the water.

_Leptoplana_ probably does not live more than a year. In the spring or
summer, batches of eggs are laid and fixed to algae or stones by one
individual, after having been fertilised by another. Young _Leptoplana_
hatch out in two to three weeks, and lead a {11}pelagic existence till they
are three or four millimetres in length. In late summer, numbers of such
immature examples may be found among sea-weeds and _Corallina_ in tide
pools. In the succeeding spring they develop first the male and then the
female reproductive organs.

[Illustration: FIG. 4.—Portion of a transverse section of _Leptoplana
tremellaris_ in the hinder part of the body. × 100. _bm_, Basement
(skeletal) membrane; _cil_, cilia; _d.m_, diagonal muscles; _d.v.m_,
dorso-ventral muscles; _ep_, epidermis; _f.p_, food particles; _l.g_,
lateral intestinal branches cut across; _l.m ext_, external, and _l.m int_,
internal longitudinal muscle layers; _m.c_, glandular (mucous) cells; _md_,
their ducts; _N_, longitudinal nerve; _Nu_, nuclei of the intestinal
epithelium; _ov_, ovary; _ovd_, oviduct; _par_, cells of the parenchyma;
_r.d_, vasa deferentia, with spermatozoa; _rm_, circular musculature; _rh_,
rhabdites; _sh_, cells of the shell-gland; _te_, testes; _ve_, vasa
efferentia; _y.c_, "yellow cells." (After Lang.)]

ANATOMY OF LEPTOPLANA TREMELLARIS.—_Leptoplana_ may be divided into
corresponding halves only by a median vertical longitudinal plane. The body
and all the systems of organs are strictly bilaterally symmetrical.
Excepting the cavities of the organs themselves, the body is solid. A
connective "parenchyma" (Fig. 4, _par_) knits the various internal organs
together, while it allows free play of one part on another. These organs
are enclosed in a muscular body-wall, clothed externally by the ciliated
epidermis, which is separated from the underlying musculature by a strong
membrane (Fig. 4, _bm_), the only skeletal element in the body.

{12}BODY-WALL.—The epidermis (Fig. 4, _ep_) is composed of a single layer
of ciliated cells, containing small, highly refractive, pointed rods or
"rhabdites" (_rh_), and gives rise to deeply-placed mucous cells (_m.c_),
which are glandular and pour out on the surface of the body a fluid in
which the cilia vibrate. The tenacious hold on a stone which _Leptoplana_
exerts if suddenly disturbed, or when grasping its prey, is probably due to
the increased glutinous secretion of these glands, aided perhaps by
rhabdites, which on such occasions are shot out in great numbers. The
basement membrane is an elastic skeletal membrane composed of stellate
cells embedded in a firm matrix. It serves chiefly for the origin and
insertion of the dorso-ventral muscles (_d.v.m_). Under the basement
membrane lies a very thin layer of transverse muscular fibres (Fig. 4,
_rm_), which are, however, apparently absent on the ventral surface. Then
follows a stout layer of longitudinal fibres (_l.m ext_), and beneath this
a diagonal layer (_d.m_), the fibres of which intersect along the median
line in such a way that the inner fibres of one side become the outer
diagonal fibres of the other. Lastly, within this again, on the ventral
surface, is a second stout longitudinal layer (_l.m int_). The sucker
(_sc_, Figs. 2 and 5) is a modification of the body-wall at that point. In
addition to the dorso-ventral muscles, there exists a complex visceral
musculature regulating the movements of the pharynx, intestine, and
copulatory organs.

PARENCHYMA.—The spaces between the main organs of the body are filled by a
tissue containing various kinds of cells, salivary glands, shell-glands,
and prostate glands. Besides these, however, we find a vacuolated,
nucleated, thick-walled network, and to this the word parenchyma is
properly applied. Besides its connective function, the parenchyma confers
that elasticity on the body which _Leptoplana_ possesses in such a high
degree. Pigment cells are found in the parenchyma in many Polyclads.

DIGESTIVE SYSTEM.—The general arrangement of this system may be seen in
Figs. 2, 5, and 7; and may be compared, especially when the pharynx is
protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth"
(there is no anus) is placed almost in the centre of the ventral surface.
It leads (Fig. 7, B, _phs_) into a chamber (the peripharyngeal space)
divided into an upper and a lower division by the insertion of a muscular
collar-fold (the pharynx, _ph_), which may be protruded, its free lips
{13}advancing, through the mouth (Fig. 2), and is then capable of enclosing
by its mobile frilled margin, prey as large as _Leptoplana_ itself. The
upper division of the chamber communicates by a hole in its roof[20] (the
true mouth, Figs. 5 and 7, _g.m_) with the cavity of the main-gut or
stomach (_m.g_), which runs almost the length of the body in the middle
line, forwards over the brain (Fig. 5, _up_). Seven pairs of lateral
gut-branches convey the digested food to the various organs, not directly
however, but only after the food mixed with sea-water has been repeatedly
driven by peristalsis first towards the blind end of the gut-branches and
then back towards the stomach. Respiration is probably largely effected by
this means. The epithelium of the intestine (Fig. 4, _l.g_) of a starving
specimen is composed of separate flagellated cells frequently containing
"yellow cells."[21] After a meal, however, the cell outlines are invisible.
Gregarines, encysted Cercariae, and Orthonectida[22] occur parasitically in
the gut-branches.

An EXCRETORY SYSTEM of "flame-cells" and fine vessels has hitherto been
seen only by Schultze[23] in this species, which will not, however, resist
intact the compression necessary to enable the details to be determined.
They are probably similar to those of _Thysanozoon_ described on p. 25.

NERVOUS SYSTEM.—The brain, which is enclosed in a tough capsule (Fig. 5,
_br_), is placed in front of the pharynx, but some distance behind the
anterior margin of the body. It is of an oval shape, subdivided
superficially into right and left halves by a shallow depression, and is
provided in front with a pair of granular-looking appendages, composed of
ganglion-cells from which numerous sensory nerves arise, supplying the eyes
and anterior region. Posteriorly the brain gives rise to a chiefly motor,
nervous sheath (Fig. 5, _nn_), which invests the body just within the
musculature. This sheath is thickened along two ventral lines (Fig. 5,
_ln_) and two lateral lines (_n.s_), but is very slightly developed on the
dorsal surface. Ganglion-cells occur on the course of the nerves, and are
particularly large at the point of origin of the great motor nerves.

{14}[Illustration: FIG. 5.—Diagrammatic view of the structure of
_Leptoplana tremellaris_ as a type of the Polycladida. The body is cut
across the middle to show the relative position of organs in transverse
section. In the posterior half the alimentary canal has been bisected and
removed from the left side, to exhibit the deeply placed nervous sheath
(_nn_) and the male reproductive organs. _br_, Brain; _dp_, "diaphragm";
_e_, cerebral group of eyes; _et_, tentacular eye-group; _gr_, marginal
groove; _gm_, true mouth; _lg_, lateral gut-branch; _ln_, longitudinal
nerve stem; _m_, external mouth; _mg_, _mg'_, main-gut, whole, and
bisected; _n_, sensory nerve supplying the eyes; _nn_, nervous network
lying on the ventral musculature; _n.s_, lateral nerve; _od_, oviduct;
_ov_, ovary; _pe_, penis (in section); _ph_, pharynx; _pr_, prostate or
"granule gland"; _sc_, sucker; _sg_, shell-gland; _te_, testes; _up_,
anterior unpaired gut-branch; _ut_, uterus; _va_, vagina (in section);
_vd_, vas deferens; _ve_, vasa efferentia; ♂, male genital pore; ♀, female
pore.]

{15}SENSE ORGANS.—_Leptoplana_ possesses eyes, stiff tactile, marginal
cilia, and possibly a sense organ in the "marginal groove." The eyes, which
are easily seen as collections of black dots lying at the sides of the
brain, may be divided into two paired groups: (1) cerebral eyes (Fig. 5,
_e_), and (2) tentacle eyes (_et_), which indicate the position of a pair
of tentacles in allied forms (Fig. 8, A, _t_ and B). Each ocellus consists
of a capsule placed at right angles to the surface of the body in the
parenchyma, below the dorsal muscles, and with its convex face outwards. It
is a single cell in which pigment granules have accumulated. The light,
however, can only reach the refractive rods, which lie within it, obliquely
at their outer ends. These rods are in connexion with the retinal cells,
and thus communicate by the optic nerve with the brain. The cerebral eyes
are really paired, and are directed some upwards, some sideways, some
downwards.

The "marginal groove" is a shallow depression of the epidermis (Fig. 5,
_gr_) lined by cilia, and containing the ducts of very numerous
gland-cells. It runs almost parallel to the anterior margin of the body, a
short distance from it, but we have no observations on its functions.

[Illustration: FIG. 6.—Diagram of an eye of _Leptoplana_ from the tentacle
group. × 600. (After Lang.)]

REPRODUCTIVE ORGANS.—_Leptoplana_ is hermaphrodite, and, as in most
hermaphrodites, the reproductive organs are complicated. The male organs
are the first to ripen, but this does not appear to prevent an overlapping
of the periods of maturity of the male and female products, so that when
the eggs are being laid, the male organs are, apparently, still in a
functional state. The principal parts are seen in Fig. 5. The very numerous
_testes_ (_te_) are placed ventrally, and are connected with fine vasa
_efferentia_ (_ve_), which form a delicate network opening at various
points into the two _vasa deferentia_ (_vd_). These tubes, especially when
distended with spermatozoa, may easily be seen (Fig. 2, _vd_) converging at
the base of the penis, and connected posteriorly by a loop that runs behind
the female genital pore (Fig. 5). The _penis_ (_pe_) is pyriform and
muscular, and is divided into two chambers, a large upper one for the
spermatozoa, and a smaller lower one for the secretion of a special
{16}"prostate" gland. The apex of the penis is eversible and not merely
protrusible, being turned inside out when evaginated. The _ovaries_ (Fig.
5, _ov_) are numerous and somewhat spherical. They are dorsally placed, but
when fully developed extend deeply wherever they can find room to do so,
and they not only furnish the ova, but elaborate food-yolk in the ova, as
there are no special yolk-glands. The slender _oviducts_ (_od_) open at
several points into the "uterus" (_ut_) (a misnomer, as no development
takes place within it), which encircles the pharynx, and opens by a single
duct into the _vagina_ (_va_). Here the ova are probably fertilised, and
one by one invested by the _shell-gland_ (_sg_) with a secretion which
hardens and forms a resistant shell. They are then laid in plate-like
masses which are attached to stones or shells. The development is a direct
one, and the young _Leptoplana_, which hatches in about three weeks, has
the outline of a spherical triangle, and possesses most of the organs of
the adult. After leading a floating life for a few weeks it probably
attains maturity in about nine months.

CLASSIFICATION, HABITS, AND STRUCTURE OF THE POLYCLADIDA.

The Polyclads were so called by Lang on account of the numerous primary
branches of their intestine. They are free-living, purely marine
Platyhelminthes, possessing multiple ovaries, distinct male and female
genital pores (Digonopora), but no yolk-glands. The eggs are small, and in
many cases give rise to a distinct larval form, known as "Müller's larva"
(Fig. 12). The Polyclads, with one exception,[24] fall into two sub-groups,
Acotylea and Cotylea:—

Character. Acotylea. Cotylea.

Sucker A sucker absent.[25] A sucker always present (Figs.
8, D, _s_; 7, A, _sc_).

Mouth In the middle, or behind the In the middle, or in front of
middle, of the ventral the middle, of the ventral
surface. surface.

Pharynx More or less intricately Rarely folded. Usually
folded. cylindrical or trumpet-
shaped.

Tentacles A pair of dorsal tentacles A pair of marginal tentacles
usually present. (except in _Anonymus_).

Development Usually direct. Larva when Müller's larva present.
present, not a typical Metamorphosis, however,
Müller's larva. extremely slight.

{17}Fig. 8 shows that, starting with a member (A, D) of each division, in
which the mouth is almost in the middle of the ventral surface, and the
brain and sense organs somewhat remote from the anterior end, we find in
the Acotylea a series leading to an elongated form (Cestoplanidae), in
which the mouth, pharynx, and genital pores are far back near the hinder
end of the body; while in the Cotylea the series leads similarly to the
elongated Prosthiostomatidae, in which, however, the pharynx and external
apertures are in the front part of the body. This view of the morphology of
the Polyclads is due to Lang, and is based on the assumption that the more
radially-constructed forms (Fig. 8, A, D) are the primitive ones.

[Illustration: FIG. 7.—Diagrammatic vertical longitudinal sections: A, Of
_Prosthiostomum_ (type of Cotylea); B, of _Leptoplana_; C, of _Cestoplana_
(types of Acotylea). (After Lang.) These figures illustrate the changes
which follow the shifting of the mouth from a central position (B) to
either end of the body. _br_, Brain; _dphm_, "diaphragm"; _gm_, true mouth;
_lg_, openings of lateral gut-branches; _m_, mouth; _mg_, main-gut or
stomach; _mgbr_, median gut-branch; _ph_, pharynx; _ph.m_, aperture in
pharyngeal fold; _phs_, peripharyngeal sheath; _sc_, sucker; ♂, male, and
♀, female, genital aperture.]

{18}[Illustration: FIG. 8.—Chief forms of Polycladida: A-C, Acotylea; D-F,
Cotylea. A, _Planocera graffii_ Lang, nat. size; B, _Stylochoplana
maculata_ Stimps, × 7; C, _Cestoplana rubrocincta_ Lang, × 4/3; D,
_Anonymus virilis_ Lang, × 3, ventral surface; E, _Thysanozoon brocchii_
Grube, nat. size; the head is thrown back and the pharynx (_ph_) is
protruded. F, _Prosthiostomum siphunculus_ Lang, × 3. _Br_, Brain; _CG_,
cerebral eye group; _DM_, true mouth; _Ey_, marginal eyes; _m_, mouth;
_MG_, main-gut or stomach; _P_, dorsal papillae; _Ph_, pharynx; _s_, sucker
(ventral); _T_, tentacles; _UP_, dorsal median gut-branch. ♂, male, and ♀,
female, genital aperture, except in D, where ♂ refers to the multiple
penes. (After Lang and Schmidt.)]

{19}CLASSIFICATION OF POLYCLADIDA.

ACOTYLEA.

Family. Genus. British Representatives.

{_Planocera_
{ (Fig. 8, A). {_Planocera folium_
{_Imogine._ { Grube. Berwick-on-
PLANOCERIDAE. {_Conoceros._ { Tweed.
With dorsal tentacles. {_Stylochus._ {_Stylochoplana maculata_
Mouth sub-central. {_Stylochoplana_ { Quatref. Among brown
{ (Fig. 8, B). { weeds in Laminarian
{_Diplonchus._ { zone.
{_Planctoplana._

{_Leptoplana tremellaris_
{_Discocelis._ { O. F. Müll.
LEPTOPLANIDAE. {_Cryptocelis._ {_L. fallax_ Quatref.
Without dorsal tenta- {_Leptoplana._ { Plymouth.
cles. Penis directed {_Trigonoporus._ {_L. droebachensis_ Oe.
backwards. {?_Polypostia_ { Plymouth Sound.
{ (see p. 27). {_L. atomata_ O. F. Müll.
{ Doubtful species.

CESTOPLANIDAE. {_Cestoplana_ (Fig. 8, C).
No tentacles. Body { In Mediterranean and
elongated. Penis { on French side of the
directed forwards. { Channel.

ENANTIIDAE. {
No sucker. No tenta- {
cles. Main-gut very {_Enantia._
short. External { Adriatic Sea.
apertures as {
in _Euryleptidae_. {

COTYLEA.

ANONYMIDAE. {
Mouth central. No {_Anonymus_ (Fig. 8, D).
tentacles. With two { Naples (two specimens).
rows of penes. {

PSEUDOCERIDAE. {_Thysanozoon_ (Fig. 8, E).
Marginal tentacles {_Pseudoceros._
folded. Mouth in {_Yungia._
anterior half. {

{_Prostheceraeus vittatus_
{ Mont. On west coast.
{_P. argus_ Quatref.
{ Guernsey.
EURYLEPTIDAE. {_Prostheceraeus._ {_Cycloporus papillosus_
Tentacles usually {_Cycloporus._ { Lang. On Ascidians in
present and pointed, {_Eurylepta._ { 2-30 fms.
or represented by two {_Oligocladus._ {_Eurylepta cornuta_
groups of eyes. Mouth {_Stylostomum._ { O. F. Müll. On sponges
close to anterior end. {_Aceros._ { and shells, 2-10 fms.
Pharynx cylindrical. { {_Oligocladus sanguinolentus_
{ Quatref.
{_O. auritus_ Clap. Doubtful.
{_Stylostomum variabile_
{ Lang.

PROSTHIOSTOMATIDAE. {
Tentacles absent. Body {
elongated. Pharynx {_Prosthiostomum_ (Fig. 8, F).
long, cylindrical. {
Penis with accessory {
muscular vesicles. {

{20}APPEARANCE AND SIZE OF POLYCLAD TURBELLARIA.—Polyclads are almost
unique amongst animals in possessing a broad and thin, delicate body that
glides like a living pellicle over stones and weeds, moulding itself on to
any inequalities of the surface over which it is travelling, yet so fragile
that a touch of the finger will rend its tissues and often cause its speedy
dissolution. The dorsal surface in a few forms is raised into fine
processes (_Planocera villosa_), or into hollow papillae (_Thysanozoon
brocchii_), and in very rare cases may be armed with spines (_Acanthozoon
armatum_,[26] _Enantia spinifera_); in others, again, nettle-cells
(nematocysts) are found (_Stylochoplana tarda_, _Anonymus virilis_). Some
Polyclads, especially the pelagic forms, are almost transparent; in others,
the colour may be an intense orange or velvety black, and is then due to
peculiar deposits in the epidermal cells. Between these two extremes the
colour is dependent upon the blending of two sources, the pigment of the
body itself and the tint of the food. Thus a starved _Leptoplana_ is almost
or quite white, a specimen fed on vascular tissue reddish. Many forms are
coloured in such a way as to make their detection exceedingly difficult,
but this is probably not merely due, as Dalyell supposed, to the substratum
furnishing them with food and thus colouring them sympathetically, but is
probably a result of natural selection.

The largest Polyclad, the bulkiest Turbellarian, is _Leptoplana gigas_ (6
inches long and 4 in breadth), taken by Schmarda, free-swimming, off the
coast of Ceylon. The largest European form is _Pseudoceros maximus_, 3½
inches in length and stoutly built. A British species, _Prostheceraeus
vittatus_, attains a length of from 2 to 3 inches. These large forms,
especially the Pseudoceridae (pre-eminently the family of big Polyclads),
are brightly coloured, and usually possess good swimming powers, since,
being broad and flat, they are certainly not well adapted for creeping
rapidly, and this is well shown by the way these Polyclads take to swimming
when in pursuit of prey at night. The size of any individual is determined,
amongst other factors, by the period at which maturity sets in, after which
probably no increase takes place. Polyclads apparently live about twelve
months, and mature specimens of the same species vary from ½ inch to 2½
inches in length (_Thysanozoon brocchii_), {21}showing that growth is,
under favourable conditions, very rapid.

HABITS OF POLYCLAD TURBELLARIA.—Polyclads are exclusively marine, and for
the most part littoral, animals. Moreover, there is no evidence of their
occurrence in those inland seas where certain marine animals (including one
or two species of otherwise characteristically marine Rhabdocoelida, p. 46)
have persisted under changed conditions. From half-tide mark down to 50
fathoms, some Polyclads probably occur on all coasts, but as to their
relative abundance in different seas we have very little accurate
information. The southern seas of Europe possess more individuals and
species than the northern, and probably the maximum development of the
group takes place on the coasts and coral islands of the tropics.[27] No
Polyclads have been taken below 60 fathoms; but their delicacy and
inconspicuousness render this negative evidence of little value. Six truly
pelagic forms, however, are known,[28] and these are interesting on account
of their wide distribution (three occurring in the Atlantic, Pacific, and
Indian oceans), and also from the distinct modifications they have
undergone in relation to their pelagic existence.

Whatever may be the interpretations of the fact, Polyclads are notoriously
difficult to detect, and this fact doubtless explains the scanty references
to them by the older naturalists who collected even in tropical seas. Lang,
who worked seven years at Naples, added to the Mediterranean fauna as many
Polyclads as were previously known for all Europe, in spite of the
assiduous labours of his predecessors, Delle Chiaje and Quatrefages. Again
Hallez, collecting at Wimereux at low-water, obtained some twenty specimens
of _Leptoplana tremellaris_ in an hour, while some other collectors working
by his side could only find two or three. Yet, even making allowance for
the difficulty of finding Polyclads, few of them appear to be abundant.

_Leptoplana tremellaris_ is frequently associated with colonies of
_Botryllus_, and if separated soon perishes, whereas the free-living
individuals are distinctly hardy (Hallez). A closely allied but possibly
distinct form lives upon the surface of the Polyzoon {22}_Schizoporella_,
on the French side of the Channel, and cannot long endure separation from
its natural habitat, to which it is adaptively coloured. A striking case of
protective mimicry is exhibited by _Cycloporus papillosus_, on the British
coasts. This species, eminently variable in colour and in the presence or
absence of dorsal papillae, is usually a quarter of an inch in length and
of a firm consistency. Fixed by its sucker to Polyclinid and other
Ascidians, _Cycloporus_ appears part and parcel of the substratum, an
interesting parallel to _Lamellaria perspicua_,[29] though we are not
justified in calling the Polyclad parasitic. Indeed, though a few cases of
association between Polyclads and large Gasteropods, Holothurians, and
Echinids are known,[30] there is only one case, that of _Planocera
inquilina_,[31] in the branchial chamber of the Gasteropod _Sycotypus
canaliculatus_, which would seem to bear the interpretation of parasitism.
The jet-black _Pseudoceros velutinus_ and the orange _Yungia aurantiaca_ of
the Mediterranean, are large conspicuous forms with no attempt at
concealment, but their taste, which is not known, may protect them. Other
habits, curiously analogous with devices employed by Nudibranch Mollusca
(compare _Thysanozoon brocchii_ with _Aeolis papillosa_), emphasise the
conclusion that the struggle for existence in the littoral zone has adapted
almost each Polyclad to its particular habitat.

As regards the vertical distribution of this group on the British coasts,
_Leptoplana tremellaris_ has an extensive range, and appears to come from
deeper to shallower water to breed.[32] In the upper part of the Laminarian
zone, _Cycloporus papillosus_, and, among brown weeds, _Stylochoplana
maculata_ are found. At and below lowest water-mark _Prostheceraeus
vittatus_, _P. argus_, and _Eurylepta cornuta_ occur. _Stylostomum
variabile_ and _Oligocladus sanguinolentus_, though occasionally found
between tide-marks, especially in the Channel Islands, are characteristic,
along with _Leptoplana droebachensis_ and _L. fallax_, of dredge material
from 10 to 20 fathoms.

LOCOMOTION.—Locomotion is generally performed by Polyclads at night when in
search of food, and two methods, creeping {23}and swimming, are usually
employed—creeping by the cilia, aided possibly, as in the case of some
Gasteropod Mollusca, by the longitudinal muscles of the ventral surface;
and swimming, by undulations of the expanded margins of the body. In the
former case the cilia work in a glandular secretion which bathes the body,
and enables them to effect their purpose equally well on different
substrata. The anterior region is generally lifted up, exploring the
surroundings by the aid of the tentacles, which are here usually present.
The rest of the body is closely appressed to the ground.

Swimming is particularly well performed by the Pseudoceridae, certain
species of _Prostheceraeus_, the large Planoceridae, some _Stylochoplana_,
_Discocelis_, and _Leptoplana_, and in the same manner as in _Leptoplana
tremellaris_ (p. 9). In _Cryptocelis_, _Leptoplana alcinoi_, and _L.
pallida_, however, the whole body executes serpentine movements like an
active leech (e.g. _Nephelis_); a cross section of the body would thus
present the same appearance during the whole movement. Many Polyclads,
notably _Anonymus_ (Lang), if irritated, spread out in all directions,
becoming exceeding thin and transparent.

[Illustration: FIG. 9.—_Discocelis lichenoides_ Mert. (after Mertens),
creeping on the inner side of a glass vessel by means of the lobes of the
extended and exceedingly mobile pharynx (_ph_). These lobes also serve to
enclose _Crustacea_ (_a_), and one lobe may then be withdrawn independently
of the rest, back into the body (_b_). The brain (_br_) and shell-gland
(_sg_) are shown by transparency.]

_Discocelis lichenoides_, _Planocera graffii_, and _Anonymus virilis_ have
peculiar modes of progression. The first, according to Mertens, will climb
up the sides of a vessel by means of the expanded lobes of the pharynx
(Fig. 9, _ph_), a habit of considerable interest, since we know that
certain Ctenophores—_Lampetia_, for instance—progress when not swimming on
the expanded lobes of their "stomach."[33] _Planocera_ and _Anonymus_
{24}creep by extending parts of the anterior margin and dragging the rest
of the body behind. In consequence, the brain and dorsal tentacles may come
to lie actually behind the middle of the body, and thus no definite
anterior end or "head" advances first. Along with this curious habit it may
be noticed (Lang) that the radial symmetry of the body is well marked; but
even without accepting this author's suggestion of the concurrent
development of a "head" with locomotion in a definite direction, the facts,
whether these two forms are primitive or not, are highly interesting.

FOOD.—Though we are probably right in calling Polyclads a carnivorous
group, the food of very few forms has been ascertained. Those which possess
a large frilled pharynx (most Acotylea) probably enclose and digest large,
and, it may be, powerful prey, as appears to be the case in _Leptoplana
tremellaris_. _Cryptocelis alba_ has been seen by Lang with the pharynx so
distended, owing to a large _Drepanophorus_ (Nemertine) which it contained,
as to resemble a yolk-sac projecting from the under surface of an embryo.
The Cotylea such as _Thysanozoon_, with a bell- or trumpet-shaped pharynx,
are fond of fixing this to the side of the aquarium, but whether they thus
obtain minute organisms is not clear. _Prosthiostomum_ shoots out its long
pharynx with great vehemence (Fig. 8, F) and snaps up small Annelids by its
aid (Lang). Those Polyclads which, as _Cycloporus_ and others, are
definitely associated with other organisms are not certainly known to feed
upon the latter, though "_Planaria velellae_" has been seen by Lesson[34]
devouring the fleshy parts of its host. The salivary glands which open on
the lips and the inner surface of the pharynx powerfully disintegrate the
flesh of the prey. Digestion takes place in the main-gut, and the
circulation of the food is accomplished by the sphinctral musculature of
the intestinal branches (conf. _Leptoplana_, p. 13).

[Illustration: FIG. 10.—Diagram of the musculature, causing peristaltic
movements of the intestinal branches of Polyclads. (After Lang.)]

A distinct vent or anus is always absent. After a meal the {25}faecal
matter collects in the main-gut, and is discharged violently by the pharynx
into the water. In a few species, however, the intestinal branches open to
the exterior (Lang). _Yungia aurantiaca_, a large and abundant Neapolitan
form, possesses such openings over the greater part of the dorsal surface;
_Cycloporus papillosus_ has marginal pores; _Oligocladus sanguinolentus_
apparently possesses an opening at the posterior end of the main-gut; and
_Thysanozoon brocchii_ frequently rends at this point, in consequence of
the accumulation of food.

RESPIRATION.—The oxygen of the atmosphere dissolved in the sea-water is, in
default of a special circulatory fluid, brought to the tissues of Polyclads
in two ways. The ciliated epidermis provides a constant change of the
surrounding water, by which the superficial organs may obtain their supply;
and the peristaltic movements of the digestive system, aided by the cilia
of the endoderm cells, ensure a rough circulation of the sea-water, which
enters along with the food, to the internal organs. The papillae of
_Thysanozoon brocchii_, containing outgrowths of the intestinal branches,
are possibly so much additional respiratory surface, although still larger
forms (other Pseudoceridae) are devoid of such outgrowths.

EXCRETION.—The excretory system of only one Polyclad (_Thysanozoon
brocchii_) is accurately known. Lang, by compressing light-coloured
specimens, found the three parts of the system known to occur in many
Platyhelminthes: (1) the larger longitudinal canals, and (2) the capillary
vessels, which commence with (3) the flame-cells in the parenchyma of the
body. The mode of distribution of these parts is not, however, ascertained.
The canals are delicate, sinuous, apparently intracellular tubes, coursing
close to the margin of the body and sending offsets which suspend the
canals to the dorsal surface, where possibly openings may occur. In
dilatations of these vessels bunches of cilia, and occasionally
flame-cells, are found. Usually, however, flame-cells occur at the
commencement or during the course of the capillaries, which are straight,
rarely branching, tubes of exceeding tenuity, and appear (Lang) to be
outgrowths of the flame-cells, just as the duct is an outgrowth of a
gland-cell. In fact there is little doubt that the stellate flame-cells are
modified parenchymatous gland-cells, containing a lumen filled with a fluid
into which a number of cilia project and vibrate synchronously. The cells
excrete {26}nitrogeneous waste substances, which are then discharged into
the capillaries, whence the cilia of the main vessels drive them presumably
to the exterior, though external openings of the excretory system are not
known. Traces of this system have been observed in young _Leptoplana_
(first by Schultze in 1854) and also in _Cestoplana_.

SENSATION.—A nervous sheath, with scattered ganglion cells, everywhere
underlies the musculature. It is exceedingly faintly marked on the dorsal
surface, but laterally and ventrally forms a dense network with polygonal
meshes. Thickenings of this sheath give rise to lateral nerves, and also to
a pair of stout longitudinal nerves from which the internal organs are
probably innervated. The brain, hardly distinct in pelagic Polyclads, in
most forms does not differ greatly from that of _Leptoplana_ (p. 13).

The sense organs of Polyclads have the form of tentacles, eyes, otocysts
(in _Leptoplana otophora_), and stiff tactile cilia. The solid dorsal
tentacles of Planoceridae contrast strongly with the folded or pointed
hollow processes of the Cotylea. The former (Fig. 8, A, _T_) are muscular
and very contractile, and are placed near the brain some distance from the
anterior end. The latter are outgrowths of the front margin of the body,
and are sometimes (_Yungia_) provided superficially with olfactory pits and
internally with eyes and intestinal coeca.

The eyes which occur in Polyclads may be divided into (_a_) a pair of
cerebral groups overlying the brain; (_b_) those embedded in the tentacles
(tentacular group); and (_c_) the marginal eyes, which in _Anonymus_ occur
all round the margin. A complex form is sometimes assumed by the cerebral
eyes of Pseudoceridae, resulting probably from incomplete fission (Fig.
11). _Leptoplana otophora_ was obtained by Schmarda on the south coast of
Ceylon. On each side of the brain is a capsule containing two otoliths.
This is the only known case of the occurrence of these organs in Polyclads.

REPRODUCTION.—Although Polyclads are able to repair the result of injuries
to a very considerable extent, they are not known to multiply asexually.
The two processes are intimately associated, but, though probably all
Turbellaria can regenerate certain lost parts, asexual reproduction only
occurs sporadically.

{27}[Illustration: FIG. 11.—Double eye from the cerebral group of
_Pseudoceros maximus_. (After Lang.)]

All known Polyclads are hermaphrodite. The male organs, scattered, like the
testes of _Leptoplana_, over the ventral surface, develop earlier than the
ovaries, though the periods of maturation overlap; hence the possibility of
self-fertilisation, though remote, is still worth consideration. The
genital apertures, through which, in the male, spermatozoa, and in the
female, ova, are emitted, are usually situated as in _Leptoplana_ (Figs. 2
and 5, ♂ and ♀). In _Trigonoporus_, a genus once found at Naples, a
secondary female aperture has been discovered leading into the female
genital canal[35]; and in _Anonymus_, _Polypostia_, and _Thysanozoon_ (Fig.
7, E, ♂) two or more male pores and penes have been found. _Anonymus_ has
several penes (Fig. 7, D, ♂) arranged radially round the body.
_Polypostia_, a remarkable form described by Bergendal,[36] belonging to
the Acotylea, possesses about twenty such structures ranged round the
female genital aperture. Lang, whose attention was attracted by these
singular facts, made the interesting discovery that _Thysanozoon_ uses its
penes as weapons of offence rather than as copulating organs, burying them
in the skin of another Polyclad (_Yungia_) that happened to cross its path,
spermatozoa being of course left in the wound. Lang further found that
_Prostheceraeus albocinctus_ and _Cryptocelis alba_ in this way implanted a
spermatophore in the skin of another individual of the same species, and he
suggested that from this point the spermatozoa wandered through the tissues
till they met with and fertilised the eggs. It is now known that a similar
process of "hypodermic impregnation" occurs sporadically in several groups
of animals.[37] {28}Nevertheless, in some Polyclads it is probable, and in
_Stylochus neapolitanus_ it is certain, that normal copulation takes place.
The sperm-masses are transferred to a coecal diverticulum of the female
genital canal, and then by a delicate mechanism, of which we know only the
effects, one spermatozoon obtains entrance into one matured ovum, which
differs from the ova of most Turbellaria in that it contains in its own
protoplasm the yolk necessary for the nutrition of the embryo. In other
words, there are no special yolk-glands. After fertilisation, the ovum in
all Polyclads is coated with a shell formed by the shell-gland, which also
secretes a substance uniting the eggs together. They are deposited on
stones and shells, either in plate-like masses or in spirals (like those of
Nudibranchs). _Cryptocelis alba_ lays masses of an annular shape, with two
ova in each shell, and buries them in sand.

DEVELOPMENT.[38]—The first stages in the embryology of Polyclads appear to
be very uniform. They result, in all Cotylea and in certain Planoceridae,
in the formation of a Müller's larva (Fig. 12) about a couple of weeks
after the eggs are laid. This larva (1-1.8 mm. long), which is modified in
the Planoceridae, is distinguished by the presence of a ciliated band,
running somewhat transversely round the body, and usually produced into a
dorsal, a ventral, and three pairs of lateral processes. When swimming the
body is placed as in Fig. 12, and twists round rapidly about its
longitudinal axis by means of the strong locomotor cilia placed in
transverse rows upon the processes. The cilia of each row vibrate
synchronously, and recall the action of the swimming plates of a
Ctenophore. It is noteworthy that whereas _Stylochus pilidium_ passes
through a modified or, according to some authors, a primitive larval stage,
its near ally, _S. neapolitanus_, develops directly. Most {29}Acotylea
indeed develop directly, and their free-swimming young differ from Müller's
larva merely in the absence of the ciliated band and in the mode of
swimming.

[Illustration: FIG. 12.—Section through Müller's larva of _Thysanozoon
brocchii_ (modified from Lang). The right half is seen from inside. × 150.
Semi-diagrammatic. _br_, Brain; _dl_, dorsal ciliated lobe; _dr_, salivary
gland-cells of pharynx; _e_, eye; _ep_, ciliated epidermis containing
rhabdites; _mg_, stomach or main-gut; _mg_{1}_, unpaired gut branch over
the brain; _mo_, "mouth" of larva; _n_, _n_{1}_, section of nerves; _oe_,
ectodermic pit forming oesophagus of larva; _par_, parenchyma filling the
space between the alimentary tract and the body wall; _ph_, pharynx lying
in the cavity of the peripharyngeal sheath, the nuclei of which are
visible; _sl_{1}_, _sl_{2}_, _sl_{3}_, lateral ciliated lobes of the right
side; _vl_, ventral ciliated lobe.]

[Illustration: FIG. 13.—Diagrammatic transverse sections of a larval
Polyclad at different stages, to illustrate the development of the pharynx.
(After Lang.) A, Larva of the eighth day still within the shell. The
main-gut (_mg_) is still solid, the epidermis is slightly invaginated, and
a pair of muscular mesodermic thickenings (_ms_) are present. B, Young
pelagic larva. The epidermic invagination has deepened and developed
laterally. C, The lateral pouches have formed the wall of the
peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or
pharyngeal fold (_ph_). (Compare Fig. 12.) Towards the end of larval life,
when the ciliated processes (_sl_, Fig. 12) have aborted, the stage D is
reached. By the opening outwards of the pharyngeal sheath (_ph.sh_) the two
apertures _gm_, or true mouth, and _m_, or external mouth, are formed,
which together correspond with the oesophageal opening of the younger
larva. (Compare the transverse section in Fig. 5.)]

Polyclads possess an undoubted mesoderm, which gives rise to the muscles,
the pharyngeal fold, and the parenchyma. The ectoderm forms the epidermis,
in the cells of which the {30}rhabdites (Fig. 12) arise, apparently as so
many condensed secretions. From the ectoderm the brain arises as two pairs
of ingrowths, which fuse together, and from these the peripheral nervous
system grows out. Three pigmented ectoderm cells give rise, by division, to
the eyes—an unpaired cell (Fig. 12, _e_) to the cerebral group of eyes, and
the other two to the marginal and tentacular groups. The copulatory organs
apparently arise to a large extent as ingrowths from the ectoderm, from
which the accessory glands (prostates, shell-glands) are also formed. The
endoderm forms the lining of the main-gut and its branches. The pharynx is
developed as in Fig. 13, which shows that the "mouth" of the young larva
(C) does not correspond exactly with that of the adult (D). The salivary
glands arise from ectoderm cells, which sink deeply into the parenchyma.
The reproductive organs (ovaries and testes) possibly arise by
proliferation from the gut-cells (Lang, v. Graff). The change from the
larva to the adult is gradual, the ciliary band being absorbed and the
creeping mode of life adopted.

TURBELLARIA. II. TRICLADIDA.

The Triclads are most conveniently divided into three groups[39]: (i.)
_Paludicola_, the Planarians of ponds and streams; (ii.) the _Maricola_,
the Triclads of the sea; and (iii.) _Terricola_ or Land Planarians. From
the Polyclads they differ in their mode of occurrence; in the elongated
form of their body and almost constant, mid-ventral position of the mouth;
in possessing a single external genital pore (Monogopora); and in the
production of a few, large, hard-shelled eggs provided with food-yolk.

OCCURRENCE OF THE PALUDICOLA.—The Planarians of our ponds and streams are
the most familiar and accessible Turbellaria. Their elongated, flattened
bodies, and gliding movements, render them conspicuous objects on the under
surface of stones and on the leaves of aquatic plants, where they live
gregariously. The variable _Polycelis nigra_ (Fig. 14, H) is very abundant
in stagnant water and slowly-moving streams, whereas its ally, _P. cornuta_
(Fig. 14, G), distinguished by a pair of tentacles, is more local.
_Planaria_ (_Dendrocoelum_) _lactea_ (A), _P. polychroa_ (I), _P. torva_,
and _P. punctata_ are not infrequently found together, but the last is at
once the largest and rarest.

{31}[Illustration: FIG. 14.—Forms of Triclads, with the distinguishing
specific characters of certain British forms. A, _Planaria lactea_ O. F.
M., × 2; B, _Planaria alpina_ Dana, × 4 (after Kennel); C, _Phagocata
gracilis_ Leidy (after Woodworth), × 6; C', the same with the pharynges
(_ph_) extruded; D, _Gunda ulvae_ Oer., × 4; E, _Planaria gonocephala_ Dug.
(after Schmidt), × 4; F, genitalia of _Gunda ulvae_ (after Wendt); G, head
of _Polycelis cornuta_ Schm.; H, head of _Polycelis nigra_ Ehr.; I, head of
_Planaria polychroa_ Schm. K to N show the distinctive characters of the
genital ducts in K, _Polycelis nigra_; L, _Planaria polychroa_; M,
_Planaria alpina_; N, _Planaria torva_ Schultze (after Iijima and
v. Kennel). _ga_, Genital atrium; _go_, common genital opening; _mgr_,
"musculo-glandular organ"; _mo_, "mouth"; _ovd_, oviduct; _pe_, penis;
_ph_, pharynx; _pyr_, pyriform organs of unknown significance; _sc_,
sucker; _sp_, spermatophore lying in (_ut_) uterus; _vd_, vesicula
seminalis. (All except C and E are found in England.)]

_Planaria alpina_ (Fig. 14, B) is characteristic of cold mountain streams,
but occurs down to {32}sea-level in England, the Isle of Man, and Ireland,
and from its abundance in spring water, probably enjoys a wide distribution
underground. In the Swiss Alps it has been found at altitudes of over 6000
feet, at lower levels in the Rhone, and also in the Lake of Geneva. This
wide distribution may perhaps be accounted for, partly, by its faculty for
asexual reproduction in summer, and also, by the production, later in the
year, of hard-shelled eggs which are laid loosely, not attached to stones
or plants.[40] But we have no really direct evidence of the means of
dispersal of this or of any of the foregoing species, although they all
have a wide distribution in Europe. Of extra-European forms the accounts
that exist are very fragmentary. The only indubitable diagnostic character
of a Triclad is the structure of its genital ducts, and this is accurately
known in only a few cases. Several species such as _Dicotylus pulvinar_
(Fig. 16, B), at present known only from Lake Baikal,[41] and others
(_Planaria mrazekii_, _P. albissima_) from Bohemia,[42] will doubtless be
found elsewhere when they are carefully looked for. _Phagocata gracilis_ is
a remarkable North American form, possessing several pharynges (Fig. 14, C
and C'), recalling the independent movement of the pharyngeal lobes of
_Discocelis lichenoides_ (Fig. 9).[43]

OCCURRENCE OF THE MARICOLA.—Little as we accurately know of the
distribution of the fresh-water Planariae, our knowledge of the occurrence
of the marine forms is still more limited. _Gunda_ (_Procerodes_) _ulvae_
(Fig. 14, D) is the commonest European form, occurring abundantly in the
upper part of the littoral zone, on the shores of the Baltic. _G.
segmentata_ from Messina has been carefully described by Lang,[44] but
these are almost the only species of Maricola which can be accurately
determined. They differ from the Paludicola in the position of the "uterus"
behind the genital pore and in the absence of a "musculo-glandular organ"
(Fig. 14, F). A special interest attaches to the Bdellouridae, a family
containing three species, all parasitic on _Limulus_ from the east coast of
America. These remarkable Triclads usually have a sucker at the hinder end
of the body, by which they attach themselves firmly to the cephalo-thoracic
appendages and to the {33}gill-plates, upon which the eggs may be found in
considerable numbers. One species, _Syncoelidium pellucidum_, possesses a
pair of problematical organs in the hinder part of the body, opening to the
exterior ventro-laterally by a couple of chitinous mouth-pieces, but having
no connexion with the genital ducts.[45]

OCCURRENCE AND DISTRIBUTION OF LAND PLANARIANS.—The terricolous Triclads or
Land Planarians are the most interesting division of the group. Some forms,
such as _Bipalium kewense_, attain large dimensions, being usually 6 to 9
inches in length, and specimens fully extended have measured 18 inches.
Their bodies are frequently banded or striped with brilliant colours.
_Geoplana coerulea_ Mos. has a blue ventral surface and is olive green or
dark Prussian blue above. _G. splendens_ Dendy, is marked dorsally by three
stripes of emerald green alternating with four dark brown longitudinal
bands. The mode of coloration, though somewhat variable, is an important
specific character. Its significance, however, is not clearly understood.
The colours may be a warning signal, as some _Geoplana_ at least are
disagreeable to the taste of man and some birds[46]; but since Land
Planarians are largely nocturnal animals, living by day under logs, banana
leaves, and in other moist and dark situations, this explanation is clearly
insufficient. Two _Geoplana_ have been noticed by Mr. Dendy which seem to
be protectively coloured. _G. triangulata_ var. _australis_ occurs
abundantly in the beech forest in the South Island of New Zealand, and its
brown back and yellow or orange ventral surface match the leaves around its
haunts. _G. gelatinosa_ again looks like a mere slimy patch on the rotten
bark where it is found. In arid districts, during the dry season, Land
Planarians burrow in the soil and form a cyst, in which they lie coiled up,
after the manner of earthworms.[47] The glutinous investment of their
delicate bodies forms a moist medium in which the cilia covering the body
(and especially the ventral surface) may constantly and evenly vibrate, and
by which they adhere firmly to their prey. In some tropical Planarians, in
addition to possessing offensive properties, the mucus is so copious in
amount and hardens with such rapidity, that {34}these Triclads may creep
over bridges of it, and may even be blown from one stem or branch of a
plant to another, hanging at the ends of their threads.[48]

[Illustration: FIG. 15.—Some Land Planarians found in Europe. A, _Bipalium
kewense_ Mos. × ⅓ (after Bergendal); B, _Rhynchodemus terrestris_ O. F. M.,
× 2; C, _Geodesmus bilineatus_ Metsch., × 2½ (after Metschnikoff). _mr_,
Region of mouth; _gp_, region of genital pore.]

In Europe there are only two or three indigenous Land Planarians, of which
_Rhynchodemus terrestris_ O. F. M. (Fig. 15, B) is the most widely
distributed, and has been found in moist situations for the most part
wherever it has been carefully looked for. It measures about ¾ inch in
length, and is dark grey above, whitish below, and bears a pair of eyes
near the anterior extremity (Fig. 15, B). _Bipalium kewense_ (Fig. 15, A),
which has been found in the forests of Upolu, Samoa, by Mr. J. J. Lister,
has been accidentally imported, from the (unknown) districts where it is
indigenous, with plants and soil to various parts of the world—England,
Germany, the Cape, and also to Sydney, where it appears to have established
itself. In these Bipalia living in hothouses, the genitalia never appear to
attain maturity, and apparently multiple fission and subsequent reparation
of the missing parts is the only mode of reproduction. _Geodesmus
bilineatus_ (Fig. 15, C), which has occurred at Giessen, Würzburg, and
Dresden, has, in all probability, been introduced with ferns from the West
or East Indies. _Microplana humicola_, described by Vejdovsky from
dunghills in Bohemia, is doubtfully indigenous.

In marked contrast with the poverty of the temperate zones in Land
Planarians, is the abundance and great variety of this group in Southern
Asia, South America, and especially in Australasia, where the rich Land
Planarian fauna has been carefully investigated by Spencer, Dendy,
Fletcher, and others, in {35}certain parts of Victoria, New South Wales,
and New Zealand.[49] About forty species of Planarians have been discovered
on the Australian continent, thirty-five of which belong to the predominant
genus _Geoplana_, distinguished by the presence of numerous eyes along the
border of the simple anterior extremity. Of the remaining five, four belong
to the genus _Rhynchodemus_, with, lastly, the introduced _Bipalium
kewense_. The distribution of any one species, however, is so limited that
only three forms are common to the two former colonies; and although some
of the twenty known New Zealand Planarians (chiefly species of _Geoplana_),
are identical with Australian species, yet only one, or possibly two,
varieties of these species are Australian also. In addition to their
prevalence in Australasia, the _Geoplanidae_ also occur in South America,
South Africa, Japan, and the East Indies. The _Bipaliidae_ are
characteristic of the Oriental region, being found in China, Borneo,
Bengal, and Ceylon. The _Rhynchodemidae_ are a cosmopolitan family,
occurring in Europe, North and South America, the Cape of Good Hope,
Ceylon, the East Indies, Australia (particularly Lord Howe Island), and
Samoa.[50]

HABITS AND STRUCTURE OF TRICLADS.—The common _Planaria_ (_Dendrocoelum_)
_lactea_, which usually progresses by ciliary action, aided, it is said, by
muscular contractions of the ventral surface, performs, if alarmed, a
series of rapid "looping" movements, by affixing a sucker (Fig. 14, A,
_sc_), placed on the under side of the head, to the substratum, and pulling
the posterior end close to this. The sucker, discovered by Leydig, is even
better developed in _P. punctata_ (Fig. 16, A), _P. mrazekii_, and _P.
cavatica_, and is an efficient adhering-organ which has probably been
developed from a similar but simpler structure found in a considerable
number of both fresh-water and marine Triclads (_P. alpina_, Fig. 16, E).
Probably the sucker of the Land Planarian _Cotyloplana_ (D) is the same
structure, but the two suckers of _Dicotylus_ (B) are at present unique.
_Planaria dioica_, found by Claparède on the coast of Normandy,[51] is
covered with minute adhesive papillae, {36}similar to those of certain
Rhabdocoelida (e.g. _Monotus_, Fig. 19, D), enabling it to cling tightly to
the _Zostera_, and so to resist the loosening action of the waves.

[Illustration: FIG. 16.—Suckers of Triclads. A, _Planaria punctata_ Pall.;
_a_, dorsal surface of head; _b_, ventral surface (freely moving) showing
the sucker; _c_, sucker contracted (after Hallez): B, ventral surface of
head of _Dicotylus pulvinar_ Gr., from Lake Baikal (after Grube): C, dorsal
surface of _Procotylea fluviatilis_ Gir. (after Girard): D, sucker of
_Cotyloplana whiteleggei_ Sp. (after Spencer): E, ventral view of head of
_Planaria alpina_ Dana (preserved specimen); _hg_, adhering groove; _m_,
thickened musculature forming the margin of the sucker; _sc_, sucker; _t_,
tentacles.]

The movements of Land Planarians are somewhat peculiar. The ventral surface
of _Bipalium_ has a median groove, into which the ducts of numerous
mucus-glands open. This is bordered by two ridges clothed with long and
powerful cilia, which perform the chief part in propelling the animal,
aided, according to Lehnert,[52] by muscular waves which pass from the
head, backwards, _i.e._ opposite in direction to those by which a snail
slides along. This observation, however, needs confirmation. The whole body
executes sinuous movements, during which the crescentic head, lifted
slightly above the ground (Fig. 15, A), is constantly altering and
regaining its normal shape, somewhat as a _Planaria lactea_ uses the lobes
of its head. Further examination shows that the margin of the head of
_Bipalium_ is not only provided with eyes, but in addition, with ciliated,
(probably) olfactory pits. Such depressions, innervated directly from the
cerebral ganglia, have been found in sixteen species of _Geoplana_, {37}and
in one or two species of _Rhynchodemus_.[53] Some Land Planarians (a
species of _Rhynchodemus_ from Ceylon, and a _Dolichoplana_ from the
Philippines) wriggle out of a box or the hand with great speed (Moseley).

The skin of Triclads is full of minute rods or rhabdites, which are shot
out in great numbers when the animal is irritated, and doubtless serve an
offensive purpose. The Terricola possess two kinds of these: (1)
needle-like rods; and (2) in _Bipalium kewense_, flagellated structures,
bent into a V-form and with a slender thread attached to one end (Shipley).
In _Geoplana coerulea_ these bent rods furnish the blue colour of the
ventral surface. The rhabdites arise in all Triclads in cells below the
basement-membrane, which they are said to traverse in order to reach the
epidermis, thus differing in origin, and also in structure, from the rods
of Polyclads.

FOOD.—Triclads are largely if not wholly carnivorous animals, feeding upon
Annelids, Crustacea, Insects, Insect-larvae, and Molluscs. The mouth is
usually mid-ventral or behind the middle of the body, but in the anomalous
_Leimacopsis terricola_ Schm. from the Andes[54] and in _Dolichoplana_ it
is near the anterior end. The pharynx (Figs. 17, 18, _ph_) is cylindrical
or bell-shaped, exceedingly dilatable and abundantly supplied with glands
and nervous tissue. It opens into the three main intestinal branches, one
of which runs in the median plane forwards, the others backwards right and
left, enclosing a space in which the genital ducts lie (Figs. 14, A, 17).
The fresh-water Planarians prey upon Oligochaeta, Hydrophilidae (aquatic
beetles), and the commoner pond-snails. _Bipalium kewense_ pursues
earthworms, seizes the upper surface of the anterior end by the glutinous
secretion of its ventral surface, and then proceeds to envelop part or the
whole of the worm within its pharynx, which is stretched as a thin skin
over the body of its struggling prey (Lehnert). The tissues of the latter
pass into the intestine of the Planarian, and distend it greatly. After
such a meal, which lasts from one to five hours, a _Bipalium_ may remain
for three months without seeking food. _Geobia subterranea_, a white
eyeless form from Brazil, pursues earthworms (_Lumbricus corethrurus_) in
their burrows, and has been seen by Fritz Müller sucking the blood out of a
young {38}worm.[55] _Geoplana typhlops_, a Tasmanian species, is also
blind, and pursues worms, as does _G. triangulata_ (Dendy). In Trinidad,
von Kennel[56] observed that land-snails (Subulinae) were the food of
certain Land Planarians, the name of which, however, he does not state. The
pharynx was employed to suck out the soft parts of the snail even from the
upper whorls of the shell.

REPRODUCTION.—In _Planaria lactea_ the numerous testes (Fig. 17, _te_) are
placed both above and below the alimentary canal throughout the greater
part of its course. The membrane of each gonad is continued into a minute
vas efferens, which unites with those of neighbouring testes. Two vasa
deferentia (_v.d_) arise thus on each side, one from the posterior, the
other from the anterior testes of the body, and open into the vesiculae
seminales (_v.s_), which may be seen in the living animal as tortuous
whitish tubes at the sides of the pharynx (Fig. 14, A). These open into the
penis (Figs. 14, A; 17, _pe_), a large pyriform organ, the apex of which,
when retracted, points forwards, projecting into the penial cavity. When
this apical portion is evaginated and turned inside out, it is of
considerable length, and is able to pass into the long slender duct of the
uterus (_ut_) of another individual. The penial sheath (_ps_) is part of
the genital atrium (_gs_), which is developed as a pit from the skin, and
invests the end of the genital ducts, the mouth of the pit forming the
common genital pore (_go_), through which both male and female genital
products are emitted.

There are two ovaries (_ov_) placed far forwards, between the third and
fourth pairs of intestinal coeca. The oviducts (_ovd_) lie just over the
lateral nerves, and have a slightly tortuous course, at each outward bend
receiving the duct (_yo_) of a yolk-gland (_yg_), so that ova and yolk are
already associated when the oviducts open by a short unpaired tube into the
genital atrium. The yolk-glands develop rapidly,[57] and when fully formed
are massive glands occupying the spaces between the intestinal branches and
the testes which are then aborting. The so-called _uterus_ (_ut_),
apparently at first a diverticulum of the genital atrium, expands behind
the pharynx into a receptacle lined by long glandular columnar cells,
which, however, are not all of the same kind. The uterine duct opens into
the atrium just above the aperture of a problematical, eversible,
"musculo-glandular organ" (_mgr_).

{39}[Illustration: FIG. 17.—Diagrammatic view of the structure of
_Planaria_ (_Dendrocoelum_) _lactea_. × 7. The body has been cut across and
a portion removed. In the posterior half the alimentary tract of the left
side is removed and the uterus, penis, and muscular organ sliced open
horizontally. The nervous system is represented by black, and the
yolk-glands by dotted lines. _br_, Brain; _ey_, eye with lens and optic
nerve; _go_, external genital aperture for both male and female products;
_gs_, genital atrium; _lg_, paired lateral intestinal branch; _ln_,
longitudinal nerve; _mg_, unpaired anterior intestine, the branches of
which are cut off close to the main stem; _mgr_, eversible
"musculo-glandular organ"; _nc_, nerve-cells in the pharynx; _nn_, lateral
nerve-twigs; _ns_, nerve-sheath; _ov_, ovary; _ovd_, oviduct; _pe_, the
eversible penis, the corrugated inner white portion of which is the apex;
_ph_, pharynx; _phs_, pharyngeal sheath; _pr_, "prostate" or granule-gland
(represented by dotted lines opening into the penis); _ps_, penial sheath;
_te_, testes; _to_, tactile lobe of the head; _ut_, "uterus" opening into
the genital atrium just above _mgr_; _vd_, vasa deferentia; _vs_, vesicula
seminalis; _yg_, yolk-glands; _yo_, openings of the yolk-ducts into the
oviducts.]

{40}Fertilisation appears to occur in the uterus, where ova, yolk, and
spermatozoa, or (in _P. torva_) spermatophores (Fig. 14, N, _sp_), are
found. The formation of the cocoon in _Planaria lactea_ is probably begun
in the "uterus," but is undoubtedly completed in the genital atrium. In _P.
polychroa_, however, the stalked cocoon is formed wholly in the "uterus."
Thus we find two types of cocoons in different species of the genus
_Planaria_ associated with two types of reproductive organs (Hallez):—

I. Planariae in which the two oviducts open separately into the posterior
part of the duct of the uterus. A musculo-glandular organ is absent. The
cocoons are spherical and stalked. Examples—_Planaria polychroa_ (Fig. 14,
L), _P. albissima_, _P. gonocephala_.

II. Planariae in which the two oviducts open by means of an unpaired duct
into the genital atrium. A musculo-glandular organ present (_Planaria
torva_ (Fig. 14, N), _P. mrazekii_, _P. lactea_, _P. cavatica_), or absent
(_P. alpina_, Fig. 14, M). The cocoons are sessile.

The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very
much from those of _Planaria_. The uterus (greatly reduced in the Land
Planarians) lies behind the genital pore, and several ova, together with
much milky yolk, are enclosed in a capsule which is formed in the genital
atrium.

ASEXUAL REPRODUCTION.[58]—It has long been known that fresh-water
Planarians have not only great powers of repairing injuries, but that they
use this faculty in order to multiply by transverse fission. _Planaria
alpina_ and _Polycelis cornuta_, in summer, separate off the posterior part
of the body, and this ultimately becomes an entire individual. _P.
albissima_, and especially _P. subtentaculata_, anticipate matters so far,
that before fission is complete, the new individual has a head nearly fully
formed. The new organs are largely regenerated in both parent and young,
{41}apparently by the division and specialisation of scattered embryonic
cells in the parenchyma. The asexual reproduction of Land Planarians is not
fully proved, though it is known that they repair injuries to the body
completely, and that _Bipalium kewense_ is often found in hothouses,
divided into fragments which regenerate all the organs of the parent, but
like the latter, do not mature their sexual organs.

[Illustration: FIG. 18.—Semi-diagrammatic view of the excretory system of
_Planaria lactea_. (Partly after Chickoff.) _can_, Capillary network on
both dorsal and ventral surfaces; _g.br_, branches of the intestine; _lg_,
lateral branches of the digestive system; _ln_, longitudinal nerve; _ph_,
pharynx, with intermuscular capillary excretory network arising from the
point marked _pht_; _tp_, principal vessels of the excretory system, the
external opening of which is not certainly known; _vs_, vesicula
seminalis.]

EXCRETION.—The excretory organs of Triclads consist of flame-cells,
canaliculi, and a pair of longitudinal canals, the external openings of
which, have not been satisfactorily ascertained. The flame-cells are
difficult to detect in _Planaria lactea_, and the latest observer,
Chickoff,[59] was unable to see them, although to him we are indebted for
figures of this system in _P. lactea_ (Fig. 18) and _P. alpina_ (_P.
montana_). In the latter, the flame-cells are distinct, and may open
directly into the two main canals or indirectly through unbranched
canaliculi. The pharynx possesses a special supply of excretory tubules
communicating with the main canals. A similar system has been described and
figured in _Gunda segmentata_ by Lang.[60]

{42}CLASSIFICATION OF TRICLADIDA.

PALUDICOLA.

Family. Genus and British Species.

PLANARIIDAE _Planaria lactea_ O. F. M., _P. punctata_ Pall.,
_P. polychroa_ Schm., _P. torva_ M. Sch.,
_P. alpina_ Dana.
_Polycelis nigra_ Ehr., _P. cornuta_ Schm.
_Anocelis._
_Oligocelis._ } Doubtful
_Procotyla._ } genera.
_Sorocelis._
_Dicotylus._

MARICOLA.

PROCERODIDAE _Procerodes_ (= _Gunda_) _ulvae_ Oersted,
_P. littoralis_ van Beneden.
(= Gundidae). _Cercyra._
_Uteriporus._

BDELLOURIDAE _Bdelloura._
_Syncoelidium._

TERRICOLA.

BIPALIIDAE _Bipalium kewense_ Moseley (introduced).

GEOPLANIDAE _Geoplana._
_Geodesmus._

RHYNCHODEMIDAE _Rhynchodemus terrestris_ O. F. M.

Belonging to { _Dolichoplana._
undetermined { _Polycladus._
Families { _Microplana._
{ _Leimacopsis._

TURBELLARIA. III. RHABDOCOELIDA.

The Rhabdocoelida include a very heterogeneous assemblage of usually minute
Turbellaria, distinguished collectively from the Polyclads and Triclads by
the form of the digestive tract. This is a simple or slightly lobed sac,
except in the Bothrioplanidae, which in this and many other points closely
resemble the Triclads. It is to the straight, rod-like nature of the
alimentary canal that the name of the group refers. The size and form of
the body, and the structure of the pharynx and genitalia, vary within wide
limits.

The Rhabdocoelida are subdivided into three tribes:—

(1) _Acoela_, in which a sub-central mouth and pharynx are present, but
lead into the parenchyma of the body, not into an intestine with proper
walls. An excretory system has not hitherto been seen. Yolk-glands are
absent. An otolith underlies the brain. The _Acoela_ are marine.

{43}(2) _Rhabdocoela_, which possess a complete alimentary tract separated
from the body-wall (except for a few suspensory strands) by a space or
body-cavity, filled with fluid. This space is sometimes (_Vortex viridis_)
lined by an endothelium of flattened parenchymatous cells. There are two
compact testes, which are enclosed (as are the ovaries and yolk-glands) in
a distinct membrane. An otolith is present in some genera and species.
Terrestrial, fresh-water, marine.

(3) _Alloeocoela_, in which the body-cavity is greatly reduced. Except in
the Bothrioplanidae, the gonads have no distinct membrane. Testes numerous;
yolk-glands present. Marine with a few exceptions.

OCCURRENCE AND HABITS OF THE RHABDOCOELIDA.—The _Acoela_ are usually
minute, active Turbellaria abounding amongst weeds throughout the lower
half of the Littoral, and the whole of the Laminarian zone, but are most
plentiful in the pools exposed during spring-tides on our coasts,
especially on the shores of Devonshire. The species of _Haplodiscus_,
however, and _Convoluta henseni_ are modified pelagic forms found in the
Atlantic Ocean.[61] _Convoluta paradoxa_ (Fig. 19, B) is the commonest
British species. It is from 1 to 9 mm. in length, and of a brown colour,
marked above by one or more transverse white bars. The brown colour is due
to a symbiotic alga, the nature of which has not been thoroughly
investigated. In an allied species, however (_C. roscoffensis_), from the
coast of Brittany, the alga, which is here green, has been carefully
examined by Professor Haberlandt,[62] and it appears from his researches
that the algae form a special assimilating tissue, enabling the _Convoluta_
to live after the fashion of a green plant. At Roscoff, these elongated
green _Convoluta_ live gregariously in the sandy tide-pools, fully exposed
to the sun's rays, and have the appearance of a mass of weed floating at
the surface of the water. Access to the atmosphere and to sunlight are
necessary in order to enable the assimilating tissue to form the
carbohydrates, upon which this form lives exclusively. Not only has the
alga itself undergone such profound changes (loss of membrane, inability to
live independently after the death of the host) as to disguise its true
nature (a tissue-cell derived from algal ancestors), but the _Convoluta_
has also undergone {44}concomitant changes, in form, in the loss of a
carnivorous habit, and in the development of marked heliotropic movements,
thus adapting itself to an holophytic or plant-like mode of nutrition.
Nevertheless the Acoela, as a group, are carnivorous, feeding upon Diatoms,
Copepoda, and small Rhabdocoela, the absence of a digestive tract indeed
being probably more apparent than real.[63]

The _Rhabdocoela_ live under varied conditions. One form, _Prorhynchus
sphyrocephalus_, has been found among plants far from water in the
neighbourhood of Leyden, by De Man.[64] With this exception the group is
purely aquatic, and though a few genera and even individuals of the same
species occur both in salt and fresh water, whole sub-families and genera
are either marine or paludicolous. Among the latter, _Mesostoma_,
_Castrada_, _Vortex_, and _Derostoma_ are common in brooks and ponds,
especially at certain times, often only for one month (May or June) in the
year. Species of _Macrostoma_, _Stenostoma_, and _Microstoma_ are also
abundant in similar places. The two latter occur in chains formed by
fission; but the sexual individuals (which are of distinct sexes, contrary
to the usual hermaphrodite condition of Flat Worms) only appear at stated
times and are not well known. A large number of genera are purely marine,
and one family, the Proboscidae (distinguished by having the anterior end
invaginated by special muscles and converted into a sensory organ), is
entirely so. The most cursory examination of littoral weeds reveals species
of _Macrorhynchus_, _Acrorhynchus_, _Promesostoma_, _Byrsophlebs_, and
_Proxenetes_, the character of which may be gathered from von Graffs great
monograph, or from Gamble's paper on the "British Marine Turbellaria."[65]
Much, however, still remains to be done before we possess an adequate idea
of the occurrence of this group on our coasts.

{45}[Illustration: FIG. 19.—Forms of Rhabdocoelida. A, _Mesostoma
tetragonum_ O. F. M. (Rhabdocoela), × 10; B, _Convoluta paradoxa_ Oe.
(Acoela), × 10; C, _Vorticeros auriculatum_ O. F. M., × 6; D, _Monotus
fuscus_ Oe. (Alloeocoela), × 4. _ap_, Adhesive papillae; _d_, intestine;
_m_, pharynx; _ot_, otolith; _rh_, rhabdites; _te_, testes; _ut_, uterus
with eggs; _yg_, yolk-glands; ♂, male, ♀, female genital pores. (A after
Braun.)]

Some Rhabdocoels are parasitic. _Fecampia erythrocephala_, which occurs in
the lacunar spaces and alimentary canal of young shore crabs (_Carcinus
maenas_), is a white cylindrical animal ¼ inch long, with a red snout.
After attaining maturity it works its way out of the crab and encysts under
stones, forming a pyriform mass in shape like a "Prince Rupert's drop."
Within this case the eggs develop, and the young probably emerge through
the open narrow end of the hard white tube, but how they reach the crab is
not known. _Graffilla muricicola_ is found in the kidney of _Murex
brandaris_ and _M. trunculus_, at Naples and Trieste; _G. tethydicola_ in
the foot of _Tethys_. _Anoplodium parasiticum_ occurs among the muscles
which attach the cloaca of _Holothuria tubulosa_ to the body-wall; and _A.
schneideri_ occurs in the sea-cucumber, _Stichopus variegatus_. These are
truly parasitic forms, constituting a special sub-family. They have no
rhabdites in the skin; the nervous system and sense-organs are only
slightly developed; and the pharynx has undergone a notable reduction in
relation to the simpler mode of obtaining nourishment. Other cases of
association between certain Rhabdocoels (closely allied to, if not
identical with, certain free-living species) and Lamellibranchs or
Sea-urchins, are, however, of another kind. Thus on the gills or in the
mantle cavity of species of _Mytilus_, _Cyprina_, _Tellina_, and upon the
test of _Clypeaster_, such forms as _Enterostoma mytili_, _Acmostoma
cyprinae_, and _Provortex tellinae_ have been found. But it is probable
that these Turbellaria here obtain merely a temporary {46}shelter and
possibly a supply of the food of the mussel or sea-urchin.

The _Alloeocoela_ afford a well-established case of association. _Monotus
fuscus_ (Fig. 19, D), an abundant, active, elongated animal, lives on our
coasts in the upper part of the littoral zone among _Patella_, _Balanus_,
and sometimes _Chiton_. When the tide is low, the _Monotus_, to obtain
moisture and darkness, creeps between the mantle-folds of these animals,
where it may readily be found. Upon the return of the tide it leaves its
retreat and creeps or swims about freely. Other Alloeocoela collect in
great numbers in tufts of red-seaweeds (Florideae). By placing such tufts
in vessels, the sea-water, especially as darkness sets in, begins to swarm
with _Cylindrostoma 4-oculatum_, species of _Enterostoma_ and
_Plagiostoma_; _P. vittatum_, with three violet bands across the white
body, being a particularly obvious form. _Vorticeros auriculatum_ (Fig. 19,
C), another abundant species, is remarkable for the long tentacles which
can be completely withdrawn, and in this condition it completely resembles
a _Plagiostoma_.

The presence of a species (_P. lemani_) of the characteristically marine
genus _Plagiostoma_, in the Lake of Geneva, and in one or two other Swiss
lakes, at depths varying from 1 to 150 fathoms, is very interesting, and is
perhaps the only well-established case of the survival of a once marine
Rhabdocoelid under changed conditions. _Plagiostoma lemani_ is by far the
biggest of the group to which it belongs, being over half an inch in
length. It is usually found in fine mud, sometimes among _Chara hispida_,
and has the general appearance of an inactive white slug. We are indebted
to Forel and Duplessis for the discovery of this species, and also of
_Otomesostoma morgiense_, a _Mesostoma_ with an otolith, dredged in 10 to
50 fathoms in the Lake of Geneva, the Lake of Zürich, and found recently
also by Zacharias in the Riesengebirge. The genus _Bothrioplana_, first
found by Braun in the water-pipes of Dorpat, has been carefully
investigated by Vejdovsky,[66] who places it in a special family,
Bothrioplanidae, among the Alloeocoela. One species has recently been found
near Manchester.

A comprehensive survey of the Rhabdocoelida shows that, with the chief
exception of the Proboscidae, the more lowly organised forms, the Acoela
and Alloeocoela, are marine, whereas the fresh-water forms are in most
cases the most highly organised {47}genera (_Mesostoma_, _Vortex_). But
_Macrorhynchus helgolandicus_, though minute (1.5-2 mm. long), has a more
complex structure[67] than any other species of the specialised marine
genus to which it belongs, and is a remarkable instance of great complexity
being associated with small size.

REPRODUCTION.—The Rhabdocoelida present the greatest diversity in the
development of the reproductive system. The Acoela and Alloeocoela have the
simplest arrangement. Scattered testes, often without a distinct membrane,
form the spermatozoa, which in most cases wander into parenchymatous
spaces, but in _Monoporus rubropunctatus_ and _Bothrioplana_, into distinct
vasa deferentia. In both groups a protrusible penis opens independently to
the exterior, and may be simply muscular or provided with a chitinous
armature. Two ovaries are present, and the oviducts, if distinct, are
continuations of the ovarian membrane. In most forms a "bursa seminalis,"
which receives the spermatozoa of another individual, is appended to the
female genital canal. In many of the Alloeocoela, however, a portion of the
ovary is sterile, and its cells, forming a yolk-gland, feed the fertile
portion, the whole structure being then spoken of as a germ-yolk-gland. In
many others (Monotidae) this sterile part has become an independent
yolk-gland, which communicates by yolk-ducts with the oviducts. The Acoela
form no egg-case, the body of the parent becoming a bag for the ova, which
elaborate their own food-yolk. The Alloeocoela lay hard-shelled eggs, which
are produced in _Bothrioplana_ and _Automolos_ by the activity and
interaction of reproductive organs, resembling closely those of certain
Triclads.[68]

The Rhabdocoela exhibit every stage in the development of a complex
reproductive system, from the simple ovaries and testes of a _Microstoma_
or _Macrostoma_, to the intricate system of ducts and glands of a
_Macrorhynchus_ (Proboscidae), in which there is still much to be made out.
The complications of the copulatory organs chiefly arise from the way in
which the spermatozoa are brought into contact with a nutritive prostatic
fluid, or are formed into spermatophores; and also from the penial
armature, {48}which is often very complex, and may consist of a curved
chitinoid hook or a coiled loop (_Promesostoma_), of hooks (Proboscidae),
or of an intricate arrangement of plates (_Proxenetes_); or the penis may
take on a complex corkscrew-like form (_Pseudorhynchus_). The (frequently
armed) female genital canal usually possesses a bursa seminalis for the
fertilisation of the eggs, but a receptaculum seminis or spermatheca may
serve for the reception, the bursa, for the lodgment of the spermatozoa of
another individual. The fertilised ovum is provided with a supply of
food-yolk and with a shell, which may be formed in a special diverticulum,
the "uterus." The development of these organs strains the resources of the
animal to the utmost, and in some Proboscidae the alimentary canal is
squeezed out and disintegrates, in order to make room for them.

A few _Mesostoma_ (_M. ehrenbergii_, _M. productum_, _M. lingua_) produce
two kinds of eggs—thin- and thick-shelled. The latter are laid throughout
the summer, and lie dormant through winter. The young which hatch in spring
out of these "winter" eggs develop rapidly, and when only 7 to 8 mm. long
(_i.e._ one-third the size of the parent) already possess functional
genital organs; the penis, however, is rudimentary, and incapable of being
used for copulation. Hence it is probable that this stunted progeny
self-fertilise their thin-shelled or "summer" eggs. After the formation of
these eggs the same parent is said (Schneider[69]) to produce thick-shelled
or winter eggs, but however that may be, the first young which hatch from
the thin-shelled ova are produced in great numbers at a time (April to May)
when food is abundant. These grow rapidly to the full size, and then having
attained maturity, cross-fertilise one another's ova, which become encased
in a thick brown shell; and it is these numerous "winter" eggs that lie
dormant throughout the autumn and winter. Many _Mesostoma_, and practically
all other Rhabdocoela, however, produce only thick-shelled eggs, and in all
cases it is probable that to these many species owe their wide
distribution, the exact range of which is, however, unknown, as is also the
means of dispersal.

{49}CLASSIFICATION OF RHABDOCOELIDA.

ACOELA.

Family. Genus and British species.

PROPORIDAE _Proporus venenosus_ O. Sch. Plymouth.
_Monoporus rubropunctatus_ O. Sch. Plymouth.
_Haplodiscus._

APHANOSTOMATIDAE _Aphanostoma diversicolor_ Oe. Common.
_A. elegans_ Jen. Plymouth.
_Convoluta saliens_ Grff. Plymouth, Millport.
_C. paradoxa_ Oe. (Fig. 19, B). Common.
_C. flavibacillum_ Jen. Plymouth, Port Erin,
Millport.
_Amphicoerus._
_Polychoerus._

RHABDOCOELA.

MACROSTOMATIDAE _Mecynostoma._
_Macrostoma hystrix_ Oe. Stagnant water.
_Omalostoma._

MICROSTOMATIDAE _Microstoma lineare_ Oe. Fresh water.
_M. groelandicum_ Lev. Plymouth, among _Ulva_.
_Stenostoma_ (_Catenula_) _lemnae_ Dug. Near Cork.
_S. leucops_ O. Sch. Common in fresh water.
_Alaurina claparedii_ Grff. Skye.

PRORHYNCHIDAE _Prorhynchus stagnalis_ M. Sch. In Devonshire rivers.
_Promesostoma marmoratum_ M. Sch. Common.
_P. ovoideum_ O. Sch., _P. agile_ Lev. Plymouth.
_P. solea_ O. Sch. Plymouth, Port Erin.
_P. lenticulatum_ O. Sch. Port Erin.

MESOSTOMATIDAE _Byrsophlebs graffii_ Jen. Plymouth, Millport.
_B. intermedia_ Grff. Millport, Port Erin.
_Proxenetes flabellifer_ Jen. Millport, Plymouth,
Port Erin.
_P. cochlear_ Grff. Millport.
_Otomesostoma._
_Mesostoma productum_ Leuck., _M. lingua_ O. Sch.,
_M. ehrenbergii_ O. Sch., _M. tetragonum_ O. F. M.
(Fig. 19, A). All at Cambridge.
_M. rostratum_ Ehr. Widely distributed.
_M. viridatum_ M. Sch. Manchester.
_M. robertsonii_ Grff., _M. flavidum_ Grff. Both
at Millport.
_Bothromesostoma personatum_ O. Sch. Preston.
_Castrada._

PROBOSCIDAE _Pseudorhynchus bifidus_ M‘Int. Millport, St. Andrews,
Port Erin.
_Acrorhynchus caledonicus_ Clap. Generally distributed.
_Macrorhynchus naegelii_ Köll., _M. croceus_ Fabr.
Plymouth, Millport.
_M. helgolandicus_ Metsch. West coast.
_Gyrator hermaphroditus_ Ehrbg. St. Andrews. Also
common in fresh water.
_Hyporhynchus armatus_ Jen. Plymouth, Port Erin.
_H. penicillatus_ O. Sch. Plymouth.

VORTICIDAE _Schultzia._ {50}
_Provortex balticus_ M. Sch. Generally distributed.
_P. affinis_ Jen., _P. rubrobacillus_ Gamb. Plymouth.
_Vortex truncatus_ Ehrbg. Abundant in fresh water.
_V. armiger_ O. Sch. Millport (fresh water). _V.
schmidtii_ Grff., _V. millportianus_ Grff.
Millport. _V. viridis_ M. Sch. Generally
distributed.
_Jensenia._
_Opistoma._
_Derostoma unipunctatum_ Oe. Edinburgh.
_Graffilla._
_Anoplodium._
_Fecampia erythrocephala_ Giard. Plymouth, Port Erin.

SOLENOPHARYNGIDAE _Solenopharynx._

ALLOEOCOELA.

PLAGIOSTOMATIDAE _Acmostoma._
_Plagiostoma dioicum_ Metsch., _P. elongatum_ Gamb.,
_P. pseudomaculatum_ Gamb., _P. sagitta_ Ulj.,
_P. caudatum_ Lev., _P. siphonophorum_ O. Sch.,
_P. ochroleucum_ Grff. All at Plymouth.
_P. sulphureum_ Grff. Port Erin. _P. vittatum_ F. and
Leuck. Millport, Plymouth, Port Erin. _P. koreni_
Jen. Plymouth, Millport. _P. girardi_ O. Sch.
Plymouth, Port Erin, Valencia.
_Vorticeros auriculatum_ O. F. M. (Fig. 19, C).
Port Erin, Plymouth. _V. luteum_ Grff. Plymouth.
_Enterostoma austriacum_ Grff. Plymouth, Port Erin.
_E. fingalianum_ Clap. Skye, Plymouth. _E. coecum_
Grff. Millport.
_Allostoma pallidum_ van Ben. Millport.
_Cylindrostoma 4-oculatum_ Leuck. Skye, Millport,
Plymouth.
_C. inerme_ Hall, _C. elongatum_ Lev. Plymouth.
_Monoophorum striatum_ Grff. Plymouth.

BOTHRIOPLANIDAE _Bothrioplana._
_Bothrioplana_ sp.? Manchester.
_Otoplana._

MONOTIDAE _Monotus lineatus_ O. F. M., _M. fuscus_ Oe.
(Fig. 19, D). Both common littoral forms.
_M. albus_ Lev. Plymouth.
_Automolos unipunctatus_ Oe. Skye, St. Andrews,
Plymouth.
_A. horridus_ Gamb., _A. ophiocephalus_ O. Sch.
Plymouth.

{51}CHAPTER II

TREMATODA

CHARACTERS OF TREMATODES—HABITS AND STRUCTURE OF TREMATODA ECTOPARASITICA
(MONOGENEA)—LIFE-HISTORIES OF _POLYSTOMUM INTEGERRIMUM_, _DIPLOZOON
PARADOXUM_, AND _GYRODACTYLUS ELEGANS_—TREMATODA ENDOPARASITICA
(DIGENEA)—OCCURRENCE AND HABITS OF DIGENEA—LIFE-HISTORY OF _DISTOMUM
MACROSTOMUM_—_DISTOMUM HEPATICUM_ AND ITS EFFECTS—_BILHARZIA
HAEMATOBIA_—BISEXUAL TREMATODES—TABLE OF HOSTS—CLASSIFICATION.

From the Turbellaria we now pass on to a consideration of the second great
subdivision of the Platyhelminthes, the Trematodes or "flukes," of which
the "liver-fluke" is the best known, since it is one of the most dangerous
parasites that infest domestic animals.

It has been pointed out that the Polyclads, Triclads, and Rhabdocoels are
carnivorous, and that in each of these groups sporadic cases of parasitism
occur. In other words, when the prey is much larger than the Turbellarian,
the latter tends to become a parasite, and we can trace the development of
the parasitic habit from the gradual association of Turbellaria with
Ascidians, Crustacea, Molluscs, and Polyzoa merely for protective purposes,
through the adoption, not only of the body of the host for shelter, but of
its flesh for food; though it is only in some Rhabdocoels (_Graffilla_,
etc.) that there exists a degeneration corresponding to the easier mode of
nutrition and simpler life. The Trematodes,[70] however, are wholly
parasitic, either on the outer surface, the gills, or internal organs of
their host, which is almost always a {52}Vertebrate. Some Trematodes lodge
in the mouth; others wander down the oesophagus into the stomach or
intestine, where they fix themselves to the mucous membrane. Again, others
work their way into the digestive glands by the ducts, and thus become
further and further removed from the external world, and more adapted to
live in the particular organs of that host in which they best flourish. The
most important result of the adoption of this internal habitat by
endoparasitic Trematodes is, however, seen in their life-history. If a
liver-fluke were to deposit its million or so of eggs in the bile-ducts of
the sheep, and these were to develop _in situ_, the host could not
withstand the increased drain upon its vital resources, and host and
parasites would perish together. Hence it is clear that the infection of a
second host by Trematodes is highly necessary, whether they be
_ectoparasitic_, in which case the infection is easily effected, when two
hosts are in contact, by the adult worms, as well as when they are apart,
by free-swimming larvae. In _endoparasitic_ Trematodes it is brought about
by the migration of the young to the outer world, their entrance into a,
usually, Invertebrate host and their asexual multiplication within it, and
the capture and deglutition of this "intermediate host" by the final
Vertebrate one. Within the latter the immature parasites find out the organ
in which their parents flourished, and here they too grow and attain
maturity. The chances of any one egg of an endoparasitic Trematode
producing eventually an adult are, therefore, far less favourable than in
the case of an ectoparasitic form. In other words, while the former must
lay a great number of small eggs, the latter need only deposit a
(comparatively) few large ones, and this fact has a corresponding influence
on the structure of the genitalia in the two cases. The Digenea, which
employ two hosts in a lifetime, have accordingly a different generative
mechanism from that of the Monogenea. The great need of the latter is a
powerful apparatus for adhering to the surface of the body of its host;
while the adaptations which the endoparasite requires are, in addition, (1)
protection against the solvent action of the glands of its host, (2) the
power of firm adhesion to a smooth internal surface, and (3) the ability
not only to produce a large quantity of spermatozoa and ova, but in the
absence of a fellow-parasite, to fertilise its own ova; and we find these
conditions abundantly satisfied.

{53}TREMATODA MONOGENEA (_ectoparasitica_).

There are four subdivisions of the Monogenea:—

I. _Temnocephalidae_, with four to twelve tentacles, and one sucker
posteriorly (Fig. 20).

II. _Tristomatidae_, with two lateral, anteriorly-placed suckers. Oral
suckers are absent, a large posterior sucker is constant, and is often
armed with hooks (Fig. 22, C).

III. _Polystomatidae_, with, usually, two oral suckers and a
posteriorly-placed adhesive disc armed with suckers and hooks (Figs. 23 and
24).

IV. _Gyrodactylidae_ (Fig. 29).

HABITS AND STRUCTURE OF ECTOPARASITIC TREMATODES.

I. _Temnocephalidae._—These interesting forms, of which a good account has
lately been written by Haswell,[71] occur on the surface (rarely in the
branchial chamber) of fresh-water crayfish and crabs in Australasia, the
Malay Archipelago, Madagascar, and Chili. Others have been found on the
carapace of a fresh-water tortoise, and in the branchial chamber of the
mollusc _Ampullaria_ from Brazil. Wood-Mason discovered others, again, in
bottles containing spirit-specimens of Indian fish. _Temnocephala_ is
rarely more than a quarter of an inch long, and looks like a minute
Cephalopod or a broad flattened _Hydra_. By the ventral sucker each species
adheres to its own particular host, the tentacles being used as an anterior
sucker for "looping" movements. The food, consisting of Entomostraca,
Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig.
20, _ph_), which can be protruded through the ventrally-placed mouth, and
is then received into a simple lobed intestine (_d_). The skin, especially
on the surface of the tentacles, is provided here and there with patches of
cilia borne by the cellular epidermis,—the only undoubted case of external
cilia occurring in an adult Trematode. Minute rhabdites formed in special
gland-cells, occur plentifully on the tentacles, and are another distinctly
Turbellarian feature. The excretory system is peculiar (Fig. 21). Fine
ducts proceed from the various organs of the body, and open to the exterior
by means of a pair of contractile sacs {54}placed on the dorsal surface.
Each sac is a single cell, and within it not one merely, but several
"flames," or bunches of rhythmically contractile cilia, are present. These
are placed on the course of excessively fine canals, which perforate the
protoplasm of this cell. The terminal branches of the excretory canals end
in branched cells, apparently devoid of "flames."

[Illustration: FIG. 20.—_Temnocephala novae-zealandiae_ Has. × 10. Ventral
view to show the digestive and reproductive systems. (After Haswell.)

FIG. 21.—The same from the dorsal surface, to show the excretory system
(double line), and the nervous system (black and shaded). (After Haswell.)

_d_, Intestine; _dln_, dorso-lateral nerve; _dn_, dorsal nerve; _ex.o_,
excretory aperture on dorsal surface; _ex.s_, terminal excretory sac; _m_,
mouth; _ov_, ovary; _ovd_, oviduct; _ph_, pharynx; _rh_, rhabdites; _rh.c_,
cells in which the rhabdites are formed; _rv_, yolk receptacle; _sc_,
sucker; _sh_, shell-gland; _te_, testes; _ut_, uterus; _vg_, vagina; _vn_,
ventral nerve; _vs_, vesicula seminalis; _yd_, yolk-duct; _yg_, yolk-gland.
♀, ♂, common genital pore.]

The reproductive system is very similar to that of certain Rhabdocoels. An
armed penis and the female genital duct open into a genital atrium, and
this by a single aperture (♀, ♂, Fig. 20) to the exterior. The fertilised
ovum and yolk are enclosed in a stalked shell formed in the uterus.

The interest and importance of the Temnocephalidae lies in the fact that
they are almost as much Turbellaria as Trematodes. {55}In habits, in the
character of the skin, the muscular, digestive, and reproductive systems,
they find their nearest allies in Rhabdocoels (Vorticidae). But in the
excretory and nervous systems, the latter composed of two dorsal, two
lateral, and two ventral trunks all connected together (Fig. 21), they are
Tristomid Trematodes. Thus they may fitly connect an account of the two
great groups.

[Illustration: FIG. 22.—A, _Nematobothrium filarina_ van Bened. Nat. size.
Two individuals (_a_ and _b_) are found together, encysted on the branchial
chamber of the Tunny. B, _Udonella caligorum_ Johns. A Tristomid, several
of which are attached to the ovary of a Copepod (_Caligus_), itself a
parasite on the gills of the Hake. × 8. C, _Epibdella hippoglossi_ O. F. M.
A Tristomid found on the body of the Halibut. Nat. size. _m_, Mouth; _ms_,
lateral suckers; _ov_, ovary; _ps_, posterior sucker; _te_, testes. (All
after P. J. van Beneden.)]

II. _Tristomatidae and_ III. _Polystomatidae_.[72]—The members of these
families are found on the body, or attached to the gills, of fresh-water
and marine fishes. The edible and inedible fish of our coasts have each
their particular ectoparasitic Trematodes; while the Minnows, Sticklebacks,
and Miller's Thumbs of streams and ponds are attacked by _Diplozoon_,
_Gyrodactylus_, and other forms. The aquatic Amphibia also harbour a
number. _Polystomum integerrimum_ is common in the bladder of Frogs, where
it leads a practically aquatic life. Other species of _Polystomum_ inhabit
the buccal and nasal cavities of certain Chelonia, but naturally no
terrestrial Vertebrates are infested externally by these {56}Trematodes.
The blood and epithelia of the host are sucked, and to this end the pharynx
has frequently a chitinous armature to aid in the abrasion or inflammation
of the tissues upon which the parasite feeds. In the case of a Sturgeon
attacked by _Nitzschia elongata_, a Tristomid, the mouth of the host
appeared to be highly inflamed by these attacks (v. Baer).

[Illustration: FIG. 23.—_Octobothrium merlangi_ Kuhn, from the gills of the
whiting, × 8. _int_, Intestine; _ms_, mouth; _sc_ suckers with chitinoid
armature; _yk_ yolk-glands. (After v. Nordmann.)]

The suckers, in the two families under consideration, vary in number and
complexity. There is always a powerful apparatus at the hinder end of the
body securing the Trematode firmly to the slimy body or gills of its host,
and, usually in the Polystomatidae, a pair of suckers at the sides of the
mouth accessory to the pumping action of the pharynx. In _Axine_, and to a
less extent in _Octobothrium_ (Fig. 23), the suckers are strengthened by a
complex hingework of chitinoid bars or hooks, which serve as insertions for
the muscles of the suckers, and thus increase their efficiency.

The mouth is invariably present just beneath the anterior end of the body.
It leads into a muscular, pumping pharynx (Fig. 24, _ph_), and this into a
bifurcated intestine which ends blindly. The two openings of the excretory
system lie on the dorsal surface (as in _Temnocephala_), and the excretory
canals branch through the substance of the body, ending usually in
"flame-cells." The nervous system is highly developed, and resembles that
of _Temnocephala_ (Fig. 21) in detail. Upon the brain one or even two pairs
of eye-spots are present in the larvae, and may persist throughout life.
Tactile setae occur in _Sphyranura_, a parasite of the North American
Amphibian _Necturus_, but a cellular epidermis is apparently rendered
impossible, perhaps from the nature of {57}the mucus in which the body is
bathed, or to the attempts of the host to free itself from these parasites;
and hence an investing membrane is present, which morphologically is either
a modified epithelium, or a cuticle formed by the glandular secretion of
the parenchyma.

[Illustration: FIG. 24.—_Polystomum integerrimum_ Fröh., from the bladder
of the Frog, and seen from the ventral surface. The alimentary canal is
black, the white dots upon it being the yolk-glands, _dvi_, Ductus
vitello-intestinalis (probably homologous with the Laurer's canal or
"vagina" of Digenea); _eh_, hooks of sucking disc; _int_, intestine; _m_,
mouth; _ov_, ovary; _pe_, penis; _ph_, pharynx; _sc_, suckers with an
embryonic hook persisting in each; _te_, testes; _ut_, uterus with eggs;
_vag_, left vagina; _vd_, vas deferens; _yd_, yolk-duct; _yg_, yolk-glands;
♂ ♀, common genital aperture. (Modified from Zeller.) × 8.]

The reproductive organs of the Polystomatidae may be understood from Figs.
24, 27, and 28. At the point of union of the oviduct (Fig. 28, _ovd_), the
vitelline ducts (_yd_), and the commencement of the uterus (_ut_), a
slender duct is given off which opens into the intestine, and is known as
the "vitello-intestinal canal" (Fig. 24, _dvi_; Fig. 28, _gic_). This duct
has apparently the same relations as the "canal of Laurer" of Digenea,[73]
except only that the latter opens to the exterior directly. In connexion
with this vitello-intestinal canal a "vagina" is present, which in
_Polystomum_ and most Monogenea is paired (Fig. 24, _vag_), in _Diplozoon_
and in one {58}or two other forms, however, unpaired. The vagina receives
the penis of another individual during copulation (Fig. 26), and does not
appear to have an homologue in the liver-fluke or other Digenea.

[Illustration: FIG. 25.—Eggs of Monogenea. A, Eggs of _Encotylabe pagelli_
v. Ben.-Hesse; B, eggs of _Udonella pollachii_ v. Ben.-Hesse (with young
forms just hatching out); C, egg of _Microcotyle labracis_ v. Ben.-Hesse.
(After van Beneden and Hesse.) × 50.]

LIFE-HISTORIES OF THE POLYSTOMATIDAE.[74]—_Polystomum integerrimum._ After
the mutual fertilisation of two individuals, the eggs are laid in the water
by the protrusion of the body of the parent through the urinary aperture of
the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day
for ten days. After about six weeks, the larva (.3 mm. long) hatches out,
and swims about freely by means of bands of large ciliated cells (Fig. 26,
A); but if it does not meet with a tadpole within twenty-four hours, it
dies. Should it, however, encounter one, the larva creeps along it in a
looping fashion until it approaches the opercular spout, or opening of the
branchial chamber, on the left side; into this it darts suddenly, fixes
itself, and throws off its cilia. Here it remains eight or ten weeks,
feeding, increasing in size, and forming the suckers from behind forwards.
{59}At the time of the tadpole's metamorphosis, the young _Polystomum_
works its way down the pharynx into the oesophagus and along the intestine,
till it reaches and enters the opening of the bladder. Three years
afterwards it becomes mature.

Sometimes, however, _Polystomum_ experiences another fate. The larvae
settling down on the external gills of a young, recently-hatched tadpole,
and obtaining a richer supply of blood than in the previous case, grow far
more rapidly, so that in five weeks they are mature, although still in the
branchial chamber of the tadpole. They do not then wander into the
alimentary canal, but usually, having discharged their eggs, die at the
time of the tadpole's metamorphosis. Still more interesting, however, is
the difference between the genitalia in these and in the normal
_Polystomum_. In contrast with the latter, these possess (1) one testis and
a rudimentary penis; and their spermatozoa differ in structure and shape
from those of the normal _Polystomum_. (2) The vaginae are absent, a fact
connected with the absence of a functional copulatory organ. (3) In
compensation for the loss of these, a duct connects the single testis and
the point of union of oviduct and yolk-ducts, and by this
self-fertilisation occurs. (4) The uterus is absent; the "ootype" or duct
into which the shell-gland opens, communicating directly with the exterior.
In (1) and (4) these aberrant _Polystomum_ resemble _P. ocellatum_, from
the Tortoise _Emys europaea_.

[Illustration: FIG. 26.—_Polystomum integerrimum._ A, Free-swimming larva,
seen from the ventral surface. × 80. B, Two mature individuals in mutual
coition attached to the bladder of a Frog. × 5. (After Zeller.) _d_,
Intestine; _ex.o_, excretory pore, dorsal in position, seen here by
transparency; _ey_, eye-spots; _gl_, frontal glands; _m_, mouth; _ph_,
pharynx; _sd_, adhering disc; _vag_, vagina.]

{60}[Illustration: FIG. 27.—A, Egg of _Diplozoon paradoxum_ v. Nord.,
consisting of a shell enclosing _ov_, the actual ovum, surrounded by _yc_,
the yolk-cells; B, larva just hatched (× 125); C, two _Diporpa_ (I and II)
about to unite; D, conjugation in progress but not yet complete. _dt_,
Dorsal papilla; _e_, eye; _g_, intestine; _m_, mouth; _sc_, ad-oral sucker;
_th_, spirally-wound thread attaching the egg to the gill of the Minnow;
_vs_, ventral sucker; (in D) I, I, one _Diporpa_, ventral view; II, II, the
other, dorsal view. (After Zeller.)]

[Illustration: FIG. 28.—Hinder part of the body of _Diplozoon paradoxum_.
The fusion of the two _Diporpa_, where they come into contact, is now
complete. They now cross each other like an X, and are twisted, so that
_Diporpa_ I, in front of the point of fusion, is seen from the dorsal
surface; behind, from the ventral surface; and the reverse is the case with
_Diporpa_ II. The compound animal is seen from the opposite surface to that
shown in Fig. 27, D. The digestive and excretory organs are omitted. (After
Zeller.) I _Ant. dorsal_, dorsal surface of _Diporpa_ I, facing the
anterior end; I _Post. ventral_, ventral surface of _Diporpa_ I, posterior
end; and similarly for II _Ant. ventral_ and II _Post. dorsal_. _d_, Piece
of the intestine showing opening of, _gic_, vitello-intestinal canal; _ov_,
ovary; _ovd_, point of union of female genital ducts; _sc_, suckers; _te_,
testis; _ut_ (in _Diporpa_ I), "ootype" or chamber into which shell-glands
open. This is continuous with the uterus (_ut_) of _Diporpa_ I; _uto_,
ventral opening of uterus; _vag_, vagina, with _vd_, vas deferens,
permanently inserted into it through the genital pore; _yd_, yolk-ducts;
_yg_, yolk-glands.]

{61}_Diplozoon paradoxum._—The life-history of _Diplozoon_ is unique. For
whereas the larvae of most animals grow up, each into a single adult, in
_Diplozoon_, of the few larvae that survive the dangers of their
free-swimming existence, only those become mature which conjugate
permanently with another individual. But although there are thus only half
as many adult _Diplozoon_ as there were conjugating larvae (or _Diporpa_,
as they were called when they were considered distinct forms), yet the
total number of eggs produced is probably as great as if each larva became
individually mature.

[Illustration: FIG. 29.—_Gyrodactylus elegans_ v. Nord., from the fins of
the Stickleback. (After v. Nordmann.) × 125. _emb_, Embryo.]

_Diplozoon paradoxum_ lays its eggs on the gills of the Minnow, which it
frequently infests in great numbers. The ovum divides rapidly at the
expense of the yolk-cells, and in a fortnight a larva (.2 mm. long) of the
shape and complexity shown in Fig. 27, B, hatches out, which, however,
succumbs if it does not meet with a Minnow in five or six hours. Should it
survive, a dorsal papilla, a median ventral sucker, and a second pair of
posterior suckers develop. Thus the _Diporpa_ stage is attained. These
_Diporpa_ may acquire a third and even a fourth pair of suckers, and
continue to live three months, but they only develop and mature their
reproductive organs, if each conjugates with another _Diporpa_ (Fig. 27, C,
D), and this only occurs in a small percentage of instances. Each grasps
the dorsal papilla of the other by its own ventral sucker, thus undergoing
a certain amount of torsion. Where the two bodies touch, complete fusion
occurs, and, as shown in Fig. 28, the united _Diporpa_ (or _Diplozoon_, as
the product is now called) decussate, each forming one limb of the X-shaped
_Diplozoon_, within which the two sets of complex genitalia develop (Fig.
28).

IV. _Gyrodactylidae._—_Gyrodactylus_ (Fig. 29), the structure of which is
in many ways peculiar, produces one large egg at a time. An embryo, in
which the large and smaller hooks of the adhesive disc can be seen (_emb_),
develops from this egg while still within the body of the parent, and may
give rise to yet another generation within itself. The details of the
process have not, however, been well ascertained.

{62}TREMATODA DIGENEA (_endoparasitica_).

OCCURRENCE AND HABITS OF DIGENEA.—Endoparasitic Trematodes have been found
in almost all the organs of Vertebrate hosts excepting in the nervous,
skeletal, and reproductive systems. The alimentary canal, however, is the
most usual habitat. From the buccal cavity to the large intestine, or even
to the cloaca, its different regions are the resorts of various Trematodes.
No Digenea have been found in the mouth, pharynx, or oesophagus of Mammals;
but in Birds, Reptiles, Amphibia, and especially in Fishes, these parts are
largely affected. It is a striking fact that Trematodes should occur in the
stomach of (chiefly) large predaceous fishes, such as the Pike, Sharks, the
Angler-fish, and others, considering the powerful digestive action of the
gastric juice of these carnivores. The peculiar nature of the defence which
must be employed by the parasites against this digestive action, becomes
still more marked when it is considered that if a Trematode normally living
in the stomach of one host be transferred to that of another, it is usually
speedily digested, as is shown (p. 65) in the case of _Distomum
macrostomum_. From these considerations the suggestion has been made that
the cutaneous secretions of these Trematodes must act, not only as a
protection against digestive or other ferments, but that the action in each
case must be a specific one (Frenzel, Braun).

[Illustration: FIG. 30.—_Distomum luteum_ v. Baer (immature), to show the
arrangement of the excretory vessels. × 50. _ex.o_, Excretory aperture by
which the terminal contractile duct opens—the finer vessels end in
flame-cells; _int_, intestine; _m_, mouth-sucker; _ph_, pharynx; _vs_,
ventral sucker. (After la Valette.)]

It is, however, in the small intestine that most Trematodes occur, as the
examination of the common Frog[75] will readily demonstrate. Both this and
the edible Frog are attacked by a dozen Distomatidae, only a few of which,
however, are common {63}to both hosts, and a number of Holostomatidae also
pass a stage of their development within these Amphibia. Some idea of the
extent to which animals, whose habits lead to infection, may be attacked by
Trematodes (to say nothing of Cestodes and Nematodes, which often occur
also) may be gathered from the fact that in dissecting a black stork,
Nathusius found several hundred _Holostomum excavatum_ and about a hundred
_Distomum ferox_ in the small intestine, twenty-two _D. hians_ in the
oesophagus, five others in the stomach, and one _D. echinatum_ in the
intestine. Snipe, Woodcock, Sandpipers, Dunlin, Gulls, Bittern, Geese, and
Wild Ducks are, to mention a few cases, greatly infested by members of this
group.

The following Trematodes have occurred in man[76]:—

_Distomum hepaticum_ Abild.
" _lanceolatum_ Mehlis.
" _conjunctum_ Cobbold.
" _spathulatum_ Leuckart (= _D. sinense_ Cobb.,
_D. japonicum_ R. Blanch.).
" _rathouisi_ Poir. (probably = _D. crassum_ Busk,
_D. buskii_ Lank.).
" _heterophyes_ v. Sieb.
" _pulmonale_ Bälz (= _D. ringeri_ Cobb., _D. westermanni_ Kerb.).
" _oculi humani_ Ammon (= _D. ophthalmobium_ Dies.).
_Monostomum lentis_ v. Nord.
_Amphistomum hominis_ Lewis and M‘Connell.
_Bilharzia haematobia_ Cobb.

LIFE-HISTORIES OF THE DIGENEA.—The classification of Trematodes according
to their life-histories, expressed in the divisions Monogenea and Digenea,
though a very useful one, breaks down entirely in the case of certain
forms. Thus the life-history of _Gyrodactylus_ is probably digenetic rather
than monogenetic. _Aspidogaster conchicola_,[77] which lives in the
pericardial cavity of the fresh-water mussel (possibly the only case of a
Trematode becoming normally mature in an Invertebrate host, since other
species of _Aspidogaster_ live in Chelonia), produces larvae which enter
another _Anodonta_ and develop directly into the sexual form. In other
words, _Aspidogaster_, though structurally a digenetic form, possesses a
life-history which is direct and simple, _i.e._ monogenetic.

The Holostomatidae, which live in birds of prey and aquatic birds, give
rise to eggs from which a minute larva escapes. The fate of this aquatic
larva is not directly known, {64}but in all probability after entering a
host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what
has long been known as a _Tetracotyle_, from the frequent presence of four
(sometimes only three) adhering organs. Fig. 31 exhibits a species which is
abundant in the lens and vitreous humour of the eye of the Perch. Its
further history is not known, but presumably the Perch is presently
devoured by the final host in which the _Diplostomum_ attains maturity.
Thus the Holostomatidae are "metastatic" (Leuckart), their (probably)
direct development requiring the presence of two hosts.[78]

The other Digenea, the life-histories of which are known, belong to the
Distomatidae and Amphistomatidae, and we may distinguish the steps by which
the complex life-history of the liver-fluke (_Distomum hepaticum_) has been
brought about, by a consideration of that of _Distomum macrostomum_.

[Illustration: FIG. 31.—_Diplostomum (Tetracotyle) volvens._ (After
v. Nordmann.) × 130. _cv_, Contractile excretory vesicle; _d_, intestine;
_e_, calcareous bodies in excretory tubules; _ex.o_, excretory aperture;
_gl_, glandular adhesive body; _ms_, oral sucker; _ph_, pharynx; _vs_,
ventral sucker.]

_Distomum macrostomum._—This form occurs in the intestine of several common
Passerine birds. It is remarkable not only for the large oral sucker, but
also on account of the position of the common genital pore at the hinder,
and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs
pass out through this pore, and are discharged with the bird's excrement.
Should a certain snail (_Succinea putris_) happen to rasp off the epidermis
of a leaf upon which the faeces have fallen, the eggs are swallowed and a
minute active larva is set free (Fig. 32, B). This penetrates through the
thin wall of the digestive tract of the snail, and passing into the
connective tissue, throws off its cilia and assumes the shape of Fig. 32,
C. This _sporocyst_, as the larva is now termed, grows rapidly in all
directions (Fig. 32, D) at the expense of the snail's tissues, until it
becomes impossible to separate parasite and host completely.

{65}[Illustration: FIG. 32.—Life-history of _Distomum macrostomum_ Rud. A,
Immature _Distomum_ (really a tailless _Cercaria_) found in the swollen
terminal parts of _Leucochloridium_ (Fig. 33, B) and enclosed in two
protective membranes, × 40; B, larva which hatches out of the egg of _D.
macrostomum_, × 125; C, the metamorphosed larva (sporocyst) fourteen days
after having entered _Succinea putris_, and pierced through its intestinal
wall; D, actively growing sporocyst. (After Heckert.) _go_, Genital
aperture; _int_, intestine; _ms_, mouth sucker; _n_, nervous system; _ov_,
ovary; _ps_, ventral sucker; _te_, testis.]

Those branches which lie superficially in the cephalic region of the snail
become greatly swollen, cylindrical, and contractile. They are banded with
green and white, ornamented with red terminal spots, and pulsate rapidly.
Hence these fertile branches of the sporocyst (which in this condition was
known as _Leucochloridium paradoxum_, Fig. 33, B) naturally attract the
attention of insectivorous birds, which peck off the tentacles of the
snail, and with it the swollen sporocyst-branch. A sphincter muscle closes
the cut end of the fertile sac when the bird's bill nips it off. The sac
contains large numbers of young _D. macrostomum_ (Fig. 32, A), produced by
the division of embryonic cells of the larva (Fig. 32, B), which are
apparently blastomeres of the egg reserved for this future use. It is a
remarkable circumstance that the old bird itself is immune from infection,
and if it swallows these young Distomes, they are digested. Should,
however, the snail's tentacle and its contents be offered as food to the
nestlings, their weaker digestive powers merely set the Distomes free from
the protective membranes (Fig. 32, A), and thus the Blackcaps, Sparrows,
and other birds infested by _D. macrostomum_ have acquired the parasite
when they were {66}nestlings by the unintentional agency of their
parents.[79] The snail regenerates its lost tentacles only for the
sporocyst to again bud off fertile branches into them.

[Illustration: FIG. 33.—A, _Succinea putris_, infested by B,
_Leucochloridium paradoxum_, or the fully-formed sporocyst of _Distomum
macrostomum_. (After Heckert.) A, Natural size; B, × 7.]

The egg of this Distome thus gives rise to a larva which enters the tissues
of one particular Mollusc. Here it becomes a branched sporocyst within
which the sexual worms are formed, apparently each from a single embryonic
blastomere ("Keimzelle"), by a process comparable with the development of a
parthenogenetic ovum, and the whole cycle has been termed _Alloiogenesis_,
_i.e._ alternation of sexual and parthenogenetic generations (Grobben).[80]
Leuckart[81] and Looss,[82] however, consider that what was once a
metamorphosis of an individual (as in the {67}Holostomatidae) has now
become, by maturation of the Cercaria in the comparatively modern
warm-blooded bird, a metamorphosis extending over two or more generations.

_Distomum (Fasciola) hepaticum._—The liver-fluke of the Sheep, which
produces the disastrous disease, liver-rot, has a distribution as wide as
that of a small water-snail, _Limnaea truncatula_, the connexion between
the two being, as Thomas[83] and Leuckart discovered, that this snail is
the intermediate host in which the earlier larval, sporocyst, and redia
stages are passed through, and a vast number of immature flukes (Cercariae)
are developed. These leave the snail and encyst upon grass, where they are
eaten by the sheep. Over the whole of Europe, Northern Asia, Abyssinia, and
North Africa, the Canaries, and the Faroes, the fluke and the snail are
known to occur, and recently the former has been found in Australia and the
Sandwich Islands, where a snail, apparently a variety of _Limnaea
truncatula_, is also found.[84] Over these vast areas, however, the disease
usually only occurs in certain marshy districts and at certain times of the
year. Meadows of a clayey soil, liable to be flooded (as in certain parts
of Oxfordshire), are the places where this _Limnaea_ occurs most
abundantly, and these are consequently the most dangerous feeding-grounds
for sheep. The wet years 1816, 1817, 1830, 1853, and 1854—memorable for the
occurrence of acute liver-rot in England, Germany, and France—showed that
the weather also plays a considerable part in extending the suitable ground
for _Limnaea_ over wide areas, which in dry years may be safe pastures. In
1830 England lost from this cause,[85] one and a half million sheep,
representing some four millions of money, while in 1879-80 three millions
died. In 1862 Ireland lost 60 per cent of the flocks, and in 1882 vast
numbers of sheep perished in Buenos Ayres from this cause. In the United
Kingdom the annual loss was formerly estimated at a million animals, but is
now probably considerably less. After infection during a wet autumn, it is
usually in the succeeding winter that the disease reaches its height.

{68}The symptoms of "rot" appear about a month after infection, more
acutely in lambs than in sheep, and again, less in oxen than in sheep. At
first, death may result from cerebral apoplexy, but if the first few weeks
are passed through, a pernicious anaemia sets in, the sheep are less lively
and fall at a slight touch, the appetite diminishes, and rumination becomes
irregular. The conjunctiva is of a whitish-yellow colour, the dry, brittle
wool falls off, and there is sometimes fever and quickened respiration. In
January, about three months after infection, the wasting, or fatal, period
sets in. Oedemas or swellings, usually visible before, become larger at the
dependent parts of the body, a large one in the submaxillary region being
especially well marked, and this is considered one of the most
characteristic symptoms ("watery poke"). Through this period few of the
infected sheep survive, but should they do so, the flukes begin to migrate,
though some remain much longer within the liver. Migration is effected
through the bile-duct into the duodenum and outwith the faeces, in which
the altered remains of the _Distomum_ are sometimes scarcely recognisable.
Under these circumstances (or owing to death of the fluke _in situ_) the
sheep recover more or less fully.

The preventive measures seem to be: (1) Destruction of the eggs and of the
manure of rotten sheep; (2) slaughter of badly fluked sheep; (3) adequate
drainage of pastures; (4) an allowance of salt and a little dry food to the
sheep; and (5) dressings of lime or salt on the ground to destroy the
embryos.[86]

_Distomum hepaticum_, contrary to most Trematodes, enjoys a wide range of
hosts. Man himself occasionally falls a victim; thus in Dalmatia, in the
Narenta Valley, the disease is endemic but slight in its effects. The
horse, deer, camel, antelopes, goat, pig, rabbit, kangaroo, beaver, and
squirrel have all been known to harbour this fluke occasionally. In the
Italian deer-parks at Mandria a large species, _D. magnum_, decimated the
herds some years ago; and this species, probably imported from Italy, is
now almost as dangerous a parasite on the western plains of the United
States as _D. hepaticum_.

_Bilharzia haematobia._[87]—This formidable parasite was discovered by
Bilharz in 1853 in the veins of the bladder of patients {69}at the Cairo
Hospital, and is remarkable from its abundance on the east coast and inland
countries of Africa from Egypt to the Cape, as well as in the districts
bordering Lake Nyassa and the Zambesi river, while westwards it occurs on
the Gold Coast. Mecca is a source of infection whence Mohammedans carry the
disease to distant places. In Egypt about 30 per cent of the native
population is affected by the serious disease known as Haematuria,
resulting from the attacks of _Bilharzia_, so that, of the many scourges
from which in Africa man suffers, this one is perhaps the most severe.

[Illustration: FIG. 34.—_Bilharzia haematobia_ Cobb. × 10. The female (♀)
lying in the gynaecophoric canal of the male (♂). _d_, Alimentary canal;
_ms_, oral sucker of male; _vs_, ventral suckers. (After Leuckart.)]

The worm is found usually in couples, which have been proved to be male and
female individuals (Fig. 34), often in considerable numbers in the veins of
the pelvic region, chiefly the veins of the bladder and of the large
intestine, and it is tolerably certain that _Bilharzia_ enter these vessels
from the portal vein. Their long slender bodies enable them to penetrate
into the finer vessels, which get partially or entirely choked up, and the
circulation accordingly impeded. But the most serious consequences are
observed in the urinary bladder. The mucous membrane is swollen and
inflamed here and there, chiefly on the dorsal surface, the capillaries
appear varicose and covered with mucus, mixed with blood-extravasations in
which _Bilharzia_-eggs are noticeable. The eggs also cause numerous swollen
knots in the submucous tissue. Should the disease not pass beyond this
stage (and such is usually the case, especially in South Africa), a
temporary haematuria ensues. The urine, which is only expelled with great
effort, accompanied by intense pain, is mixed with blood, mucous clots, and
masses of _Bilharzia_-eggs, from which some of the embryos have already
hatched out. The symptoms, however, may gradually pass away, and a more or
less complete recovery accomplished. The disease may indeed be of a far
less severe character, and may not interfere with the usual occupations of
the patient; but, on {70}the other hand, a far more extensive thickening of
the wall of the bladder sometimes occurs; hard masses of eggs, uric acid
crystals, and other deposits, may lead to the formation of stones,
degeneration of the substance of the ureter, and eventually to that of the
kidney itself. The stone, indeed, has long been known to be a prevalent
disease in Egypt, and it is now known to arise from concretions formed
round masses of _Bilharzia_ eggs. From the portal vein, again, other
_Bilharzia_ may gain access to the rectum, or the liver, and it has also
been found in the lungs, and may give rise to most serious complications,
if indeed the patient lives.

How infection occurs is a question to which at present no satisfactory
answer can be made. The attempt to introduce embryos of _Bilharzia_ into
the common fresh-water animals of Alexandria has hitherto proved fruitless
(Looss[88]), although there seems little doubt that the comparative
immunity of Europeans from the disease is in some way owing to their
drinking purer water than the natives. Possibly, as Leuckart suggests, the
embryo becomes a sporocyst in man himself, somewhat as _Taenia murina_ is
known to develop in the rat without an intermediate host.[89] The immense
numbers of the parasite in one host would then readily receive an
explanation.

A _Bilharzia_, possibly _B. haematobia_, was found by Cobbold in the portal
vein of _Cercopithecus fuliginosus_; and _B. crassa_ infests the cattle of
Egypt, Sicily, and certain parts of India, but does not produce haematuria.

Of the other Trematodes of man and domestic animals there is not room to
speak fully. _Distomum pulmonale_, which occurs in the lungs of the cat,
tiger, and dog, as well as in man, is especially common in Japan, China,
Corea, and Formosa. _D. sinense_ and _D. rathouisi_ have been also found in
inhabitants of these countries.

BISEXUAL TREMATODES.—Zoologically, _Bilharzia_ is interesting from its
bisexual condition. It is not, however, the only bisexual Trematode. In
cysts in the branchial chamber of Ray's bream, _Brama raii_, two worms are
found, which are probably the slender male and the swollen female of the
same species (_Distomum okenii_). The only doubt that can arise proceeds
from the tendency in all Trematodes for the male organs to ripen before
{71}the female organs. Until we certainly know that the swollen egg-bearing
form (♀) does not arise from a previously male form (♂), the case is open
to suspicion. Since, however, Kölliker[90] never found intermediate
hermaphrodite conditions, this _Distomum_ may be almost certainly regarded
as of distinct sexes. _Didymozoon thynni_ (_Monostomum bipartitum_), from
cysts on the gills of the Tunny (_Thynnus_), is another case. Two slender
worms flattened posteriorly, come together, and the body of one becomes
folded to receive that of the other. They fuse completely except for a
small lateral opening through which the anterior parts of both worms may
freely protrude. The enclosing individual contains a coiled uterus filled
with eggs, and is the female, whereas the smaller individual never
possesses eggs, and is probably the male.[91] _Nematobothrium_ (Fig. 22,
A), which occurs also in the Tunny, in the form of two immensely long
individuals intricately wound about each other in a cyst, is, however, not
bisexual.

[Illustration: FIG. 35.—_Distomum okenii_ Köll. Showing male and female as
they occur together in the branchial cavity of _Bramaraii_ (Ray's bream).
(From Bronn, after Kölliker.) Nat. size.]

TABLE OF DIGENETIC TREMATODES AND THEIR LIFE-HISTORIES.[92]

Host into which the Host into which the
larva enters, and in Cercariae migrate and
Species. Final host. which Cercariae are encyst; eaten by
eventually formed. final host.

_Diplodiscus_ Smaller species Insect-larvae, _Rana_,
(_Amphistomum_) _Rana_, _Bufo_, of _Planorbis_ _Bufo_, but frequently
_subclavatus_ _Triton_ and _Cyclas_ omitted
Göze

_Distomum advena_
Duj. _Sorex araneus_ Not known _Limax_
(_D. migrans_ Duj.)

_D. appendiculatum_ _Clupea alosa_ Not known _Lucullus acuspes_,
Rud. _Centropages hamatus_
(Copepoda)

_Limnaea
_D. ascidia_ Species of Bats stagnalis_ _Ephemera_, _Perla_,
v. Ben. _Planorbis _Chironomus plumosus_
corneus_

_D. atriventre_ Frogs and Toads _Physa heterostropha_
Weinl. of N. America Not known

_D. brachysomum_ The Dunlin Not known _Anthura gracilis_
Crepl. (_Tringa alpina_)

_D. caudatum_ Hedgehog _Helix hortensis_
v. Linst. (_Erinaceus europaeus_)

_D. clavigerum_ _Limnaea ovata_ Not known {72}
Rud. _Rana_ _Planorbis corneus_

_D. cygnoides_ _Pisidium_, _Limnaea_ sp.
Zed. _Rana_ _Cyclas_ (_Cercaria macrocerca_
Fil.)

_D. cylindraceum_ _Limnaea ovata_
Zed. _Rana_ _Ilybius fuliginosus_

_D. dimorphum_ _Ardea_, Different species of
Dies. _Ciconia_ Not known Fishes
(Brazil)

_D. echinatum_ _Cygnus_, Species of Species of _Limnaea_,
Zed. _Anser_, Limnaea _Paludina vivipara_
_Anas_

_D. endolobum_ _Rana_ _Limnaea _L. stagnalis_,
Duj. stagnalis_ _Gammarus pulex_,
larvae of
_Limnophilus rhombicus_

_Limnaea stagnalis_,
_L. ovata_,
_D. globiporum_ Not known _Succinea pfeifferi_,
Rud. _Perca fluviatilis_ _S. putris_,
_Physa fontinalis_,
_Planorbis marginatus_

_D. hepaticum_ Sheep, Oxen, _Limnaea truncatula_
Abild. Man, etc. Omitted

_D. hystrix_ _Lophius piscatorius_ Marine Fishes
Duj. Not known

_D. macrostomum_ Warblers, Tits, Omitted
Rud. Woodpeckers, etc. _Succinea putris_

_D. militare_ Common Snipe _Paludina vivipara_ _P. vivipara_
v. Ben.

_D. nodulosum_ _Bithynia tentaculata_ _Cyprinus_,
Zed. _Perca fluviatilis_ _Acerina cernua_

_D. ovocaudatum_ Species of _Planorbis_ Probably omitted.
Vulp. _Rana esculenta_ (_Cercaria_ known
as _C. cystophora_
Wag.)

_D. retusum_ _Rana_ _Limnaea stagnalis_ _L. stagnalis_, larvae
Duj. of Phryganeidae

_D. squamula_ Polecat Unknown _Rana temporaria_
Dies.

_D. signatum_ _Tropidonotus Unknown _Rana_
Duj. natrix_

_D. trigonocephalum_ _Paludina vivipara_
Rud. Badger, Polecat Unknown

Dogfish, Rays _Ostrea edulis_, _Belone vulgaris_
_Gasterostomum_ sp. _Cardium rusticum_,
_C. edule_

_Unio_, _Anodonta_
_G. fimbriatum_ (_Cercaria_ known _Leuciscus
v. Sieb. _Perca_, _Esox_ as _Bucephalus erythrophthalmus_
polymorphus_)

_G. gracilescens_ Unknown Species of _Gadus_
Rud. _Lophius piscatorius_ (e.g. _G. aeglefinus_),
_Molva_, _Lophius_

_Monostomum flavum_ _Planorbis corneus_
Mehl. _Anas_ Omitted

CLASSIFICATION OF TREMATODES.—We have seen (p. 63) that it is hardly
possible to carry out fully the division of Trematodes into Monogenea and
Digenea. Nevertheless, pending further investigation on the doubtful
points, this classification may still be {73}used. Monticelli[93] has
proposed the main divisions of a new classification, which has been also
adopted by Braun, and is based on the nature of the suckers. These
divisions are indicated below in brackets.

A. MONOGENEA v. Ben. (HETEROCOTYLEA Mont.).
1. Fam. TEMNOCEPHALIDAE Hasw.
Gen. _Temnocephala_ Hasw.
2. Fam. TRISTOMATIDAE Tschbg.
Sub-Fam. 1. Tristomatinae Mont.
Gen. _Tristomum_, _Nitzschia_, _Epibdella_, _Trochopus_,
_Acanthocotyle_, _Phyllonella_, _Placunella_,
_Encotylabe_.
Sub-Fam. 2. Monocotylinae Tschbg.
Gen. _Pseudocotyle_, _Calicotyle_, _Monocotyle_.
Sub-Fam. 3. Udonellinae v. Ben.-Hesse.
Gen. _Udonella_, _Echinella_, _Pteronella_.
3. Fam. POLYSTOMATIDAE Tschbg.
Sub-Fam. 4. Octocotylinae v. Ben.-Hesse.
Gen. _Octobothrium_, _Pleurocotyle_, _Diplozoon_,
_Anthocotyle_, _Vallisnia_, _Phyllocotyle_,
_Hexacotyle_, _Platycotyle_, _Plectanocotyle_,
_Diclidophora_.
Sub-Fam. 5. Polystomatinae v. Ben.
Gen. _Polystomum_, _Onchocotyle_, _Erpocotyle_,
_Diplobothrium_, _Sphyranura_.
Sub-Fam. 6. Microcotylinae Tschbg.
Gen. _Microcotyle_, _Gastrocotyle_, _Axine_, _Pseudaxine_.
4. Fam. GYRODACTYLIDAE v. Ben.
Sub-Fam. 7. Gyrodactylinae Par. et Per.
Gen. _Gyrodactylus_, _Dactylogyrus_, _Tetraonchus_,
_Diplectanum_.
Sub-Fam. 8. Calceostominae Par. et Per.
Gen. _Calceostomum_, _Anoplodiscus_.
5. Fam. ASPIDOBOTHRIDAE Burm. (= Aspidocotylea Mont.).
Gen. _Aspidogaster_, _Platyaspis_, _Cotylogaster_,
_Macraspis_.

B. DIGENEA v. Ben. (MALACOCOTYLEA Mont.).
6. Fam. HOLOSTOMATIDAE Brandes (= Metastatica Leuckart).
Gen. _Diplostomum_, _Polycotyle_, _Hemistomum_,
_Holostomum_.
7. Fam. AMPHISTOMATIDAE Mont.
Gen. _Amphistomum_, _Diplodiscus_, _Gastrodiscus_,
_Homalogaster_, _Gastrothylax_, _Aspidocotyle_.
8. Fam. DISTOMATIDAE Mont.
Gen. _Distomum_ (and sub-genera), _Rhopalophorus_,
_Koellikeria_, _Bilharzia_.
9. Fam. GASTEROSTOMATIDAE Braun.
Gen. _Gasterostomum_.
10. Fam. DIDYMOZOONTIDAE Mont.
Gen. _Didymozoon_, _Nematobothrium_.
11. Fam. MONOSTOMATIDAE Mont.
Gen. _Monostomum_, _Notocotyle_, _Ogmogaster_,
_Opisthotrema_.

{74}CHAPTER III

CESTODA

INTRODUCTION—NATURE OF CESTODES—OCCURRENCE OF CESTODES—THE TAPE-WORMS OF
MAN AND DOMESTIC ANIMALS—TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL
CESTODES OF MAN AND DOMESTIC ANIMALS—STRUCTURE AND DEVELOPMENT OF
CESTODES—TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND
DOMESTIC ANIMALS—CLASSIFICATION.

The Cestodes or Tape-worms are exclusively endoparasitic Platyhelminthes
living, in the adult condition, in the alimentary canal of Vertebrates,
with the exception of _Archigetes_ (Fig. 37), which may become mature in
the body-cavity of _Tubifex_. In relation with this wholly parasitic
existence, the Cestodes exhibit certain characteristic modifications in
structure and mode of development, such as the formation, by the
segmentation of the "neck," of a (usually) long chain of "proglottides" or
joints, which form the "body" of the Cestode; and the entire absence of an
alimentary tract, both in the larva and adult. As an adaptation to the
fixed mode of life, the anterior end (head, scolex) is modified to form an
adhering organ. Various adaptive forms of larvae are known. These live in
the internal organs of one or more intermediate hosts, and are transferred
to the final host passively during a meal. Lastly, there is the curious
metamorphosis by which the adult is formed from a portion (scolex) of the
larva.[94]

{75}[Illustration: FIG. 36.—_Echinobothrium affine_ Dies., from the
intestine of _Torpedo_, × 43. _hd_, Head; _hk_, hooks; _hl_, lobes of the
head; _ov_, ovary; _pe_, penis; _ps_, penis-sheath; _te_, testes; _ut_,
uterus; _vag_, vagina; _yg_, yolk-glands. (After Pintner.[95])]

_Taenia solium_, from man (Fig. 39, B), or _Echinobothrium_ (Fig. 36), from
an Elasmobranch fish, is fixed to the mucous lining of the intestine of its
host by means of a radially-constructed apparatus of four suckers and a
circlet of hooks (Fig. 39), which are borne by the "head" or "scolex,"
being that part of the worm which is directly derived from part of the
larva, and which contains the central, commissural portion of the nervous
system. Firm adhesion to the host's intestine is necessary, in order to
avoid the loosening action of the peristaltic movements of the intestine as
the food passes along. The heads of different Cestodes exhibit a marvellous
variety of suckers and hooks, from a mere muscular depression in
_Schistocephalus_, to the compound proboscides of _Tetrarhynchus_[96] which
is found in Elasmobranchs. The jointed body, often of enormous length (up
to 20 yards in _Bothriocephalus latus_), is usually separated from the head
by a slender neck, from which the proglottides are segmented off from
behind forwards, and become more and more individualised as they recede
farther away from the neck by the intercalation of younger joints. Thus in
Fig. 36 the mature, distal proglottis has passed through all the stages
represented by the other segments.

{76}[Illustration: FIG. 37.—_Archigetes sieboldii_ (_appendiculatus_), from
the coelom of _Tubifex rivulorum_. × 40. _app_, Persistent larval
appendage; _go_, genital pore; _hk_, persistent larval hooks; _ov_, ovary;
_sc_, sucker; _te_, testes; _yg_, yolk-glands. (After Leuckart.)]

The longitudinal muscles, the nerves, and excretory vessels which supply
the proglottides are continuous throughout and with those of the head. Each
joint contains at first male genitalia comparable with those of a
Trematode; then the female organs develop, and finally self-fertilisation
follows. The Cestodes feed through their skin, probably by the aid of fine
protoplasmic processes, which penetrate the tough investing membrane and
absorb the already digested food which bathes them. When a proglottis of
_Calliobothrium_ is approaching maturity it separates from the parent, the
broken ends of muscles, nerves, and excretory vessels speedily heal, and it
is now capable of continued growth and of fairly active movement if it
remains in the intestine of the host. According to van Beneden, it may even
attain a size equal to, or exceeding, that of the whole parent or
"strobila."[97] These considerations led Leuckart, von Siebold, P. J. van
Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is
really a colony composed of two generations—the head and neck derived from
the larva, and the proglottides produced by the segmentation of the
neck.[98] This view of the colonial nature of jointed Cestodes was
generally adopted from 1851 to 1880. During the last fifteen years,
however, the varied interpretations of the facts of the ontogeny of this
group have led some authors to adopt the monozootic view (that a Cestode is
one individual), others are still of the older opinion, and Hatschek
(_Lehrbuch_, p. 349) and Lang take up intermediate positions. Lang
considers that the formation of the joints of a tape-worm from a small
fixed "scolex," is not only largely comparable with the strobilation of a
_scyphistoma_ and the consequent formation of a pile of medusae, as in the
life-history of _Aurelia_, but {77}that both processes have arisen from the
power of regenerating the necessary organs in each of the new segments. The
result in both cases is the rapid formation of a number of joints, which
gradually separate from the parent, to carry the eggs and young to new
stations. Just as some Coelenterata (_Lucernaria_) may be regarded as not
having advanced much beyond a scyphistoma stage, so there are unisegmental
Cestodes (e.g. _Archigetes_, Fig. 37) which have remained as a slightly
altered but sexual scolex, directly comparable with a Trematode, and, as
all authors are agreed, representing one generation only. Such monozootic
forms are now classed as a special family, the Cestodaria or Monozoa, of
which _Caryophylleus mutabilis_, from the intestine of various Cyprinoid
fish, is the most abundant representative, while _Amphiptyches_
(_Gyrocotyle_) _urna_, from _Chimaera monstrosa_ of the northern
hemisphere, is paralleled by _A. rugosa_, found in _Callorhynchus
antarcticus_ of the southern seas.

[Illustration: FIG. 38.—_Scolex polymorphus_ Rud. (larva of _Calliobothrium
filicolle_ Zschokke), from the muscles of _Apogon_, a Mediterranean fish;
also found in many Invertebrates (e.g. _Sepia_). A, Inverted scolex, with
calcareous bodies; B, everted older larva. _br_, Brain; _exo_, terminal
excretory aperture; _fc_, flame-cells; _for.sec_; secondary excretory
pores; _hk_, hooks of the adult Cestode; _inrag_, pit at the bottom of
which the head is developed; _msc_, anterior sucker; _nl_, lateral nerve;
_sc_, suckers; _tl_, _tp_, lateral and main excretory vessels. (After
Monticelli.)]

OCCURRENCE OF CESTODES.—The distribution of Cestodes and their larvae is
analogous to that of the digenetic Trematodes, although the absence of an
alimentary canal limits the habitat of the mature worms to certain sites,
such as the blood-vessels, the lymphatic and coelomic spaces, and the
digestive system, where their body may be bathed by a nutritive fluid.
Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes,
and particularly of Elasmobranchs, are distinguished by certain structural
and developmental features; those of birds by {78}others; those of mammals,
by a third set of characters. The young stages of the Cestodes of Sharks
and Rays occur encysted in the body-cavity, or in the pyloric appendages,
of Teleosteans, which probably swallow them along with those invertebrate
animals upon which they prey. The larvae of the Cestodes of carnivorous
mammals or piscivorous birds, live respectively in herbivores and fishes,
but how the latter are infected we know in very few instances. Cestode
larvae are known to occur in many Invertebrates, and occasionally are taken
free swimming in the sea, presumably crossing from one host to the next.
Ctenophores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs
especially Cephalopods, Earthworms, and other Annelids, are the
intermediate hosts of these larvae (see Fig. 38), the fate of which,
however, has been determined in but few cases.

OCCURRENCE OF CESTODES IN MAN.[99]—Tape-worms, either in the adult or
larval stages (bladder-worms), have, from ancient times, been known to
occur in man, and in the animals that serve him as food. Until
comparatively recent times, however, the true nature of these parasites,
and particularly of "hydatids" (cystic larvae), was unrecognised. Up to the
seventeenth century the larvae were regarded as abscesses or diseased
growths of the affected organs, and it was only at the close of that
century that their animal nature was even suggested. Even at the beginning
of the nineteenth century, three modes of origin of Cestodes—by "generatio
aequivoca" from the tissues of the body, or by the union of previously
distinct proglottides, or again by metamorphosis of free-living worms drunk
with water by cattle or birds (as Linnaeus suggested)—were still variously
held, at a time when Malpighi, Pallas, and Goeze had recognised the true
connexion between the cystic and segmented states of _Taenia crassicollis_
(the cat tape-worm), and when Goeze had seen the eggs of _Taeniae_, and
Abildgaard[100] had even conducted the first helminthological experiments
(conversion of the larval _Schistocephalus_, Fig. 40, into the adult form).

{79}[Illustration: FIG. 39.—A, _Taenia saginata_ Goeze. Nat. size. (From a
specimen in the Cambridge Museum.) The approximate lengths of the portions
omitted in the drawing are given. At * (after Leuckart) the branched uterus
and the longitudinal and transverse excretory vessels are shown. The
genital apertures are seen as a lateral opening on each of the larger
proglottides. B, Head (scolex) of _T. solium_ Rud. × 12. (After Leuckart.)]

Generally speaking, "a tape-worm" in Western Europe will prove to be
_Taenia saginata_ Goeze (the beef tape-worm, Fig. 39, A), exceedingly
prevalent also in the East, and indeed cosmopolitan, occurring wherever the
infected flesh of the ox is eaten in a raw or half-cooked state. Its
attacks are fortunately not usually severe. _Taenia solium_ Rud. (the pork
tape-worm) is found wherever the pig is kept as a domestic animal, and has
consequently a world-wide distribution. Its size (6-9 feet long) and powers
of adhesion would alone render _T. solium_ a formidable parasite. But the
danger of its presence in the body of man, or in the flesh of pigs, lies in
the fact that the larva or bladder-worm (known as _Cysticercus cellulosae_)
can live in the most varied organs. Thus if by accident a mature proglottis
be eaten, the embryos escape, bore their way into the wall of the stomach,
and entering the portal vein, may reach in time the muscles, the brain, the
eye, or even the heart itself, and attain the cystic condition. Even more
disastrous may be the result, should some ripe joints of a mature worm work
their way from the intestine back towards the stomach. Should this happen
(and though it has not been directly proved, the possibility is to be
reckoned with), the result would be the release of vast numbers of embryos
capable of inflicting fatal injury on the host. An abnormal _Cysticercus_
of this species is probably the _Taenia_ {80}(_Cysticercus_) _acanthotrias_
Weinl. (see, however, Leuckart, _loc. cit._ p. 711).

_Taenia_ (_Hymenolepis_) _nana_ v. Sieb.[101] is found in man in Egypt,
Italy, England, Servia, Argentine Republic, and the United States. Though
small (¾-1 inch long), its numbers usually excite digestive and nervous
disorders of considerable severity, more serious, indeed, than those caused
by the commoner tape-worms. _H. diminuta_ Rud. (_flavopunctata_ Weinl.),
normally found in Rodents, has been rarely recorded in man. _Taenia_
(_Dipylidium_) _caninum_ L. (= _T. cucumerina_ Bloch = _T. elliptica_
Batsch), the commonest parasite of pet cats and dogs, and _T._ (_Davainea_)
_madagascariensis_ Davaine, have occasionally been recorded from infants
and young children. But the attacks of these species are insignificant in
comparison with those of the cystic stage (_Echinococcus polymorphus_) of a
tape-worm (_T. echinococcus_ v. Sieb.) which lives when mature in the dog.

_Echinococcus_ is most frequent in Iceland, where it affects 2 to 3 per
cent of the population, and a still larger proportion of sheep; while in
Copenhagen, Northern Germany, some districts of Switzerland, and Victoria
it is not uncommon, but is frequently found during _post-mortem_
examinations when no definite symptoms of its presence had been previously
noticed. _Echinococcus_[102] varies greatly in size, form, and mode of
growth, but is distinguished in the formation not of one scolex only, as in
the _Cysticercus_, but in the production of a number of vesicles, usually
from the inner wall. Within these, large numbers of scolices may be
developed. The whole organism continues to swell by the formation of a
watery liquid within it, and if its growth be rapid the fluid tension may
cause the rupture of the enclosing connective-tissue capsule formed around
the parasite, at the expense of the host, and the protrusion of the
daughter vesicles. It is the consequent injury to the surrounding organs of
the host, at this critical stage, often only reached after the lapse of
several years, that occasions serious or even fatal results. Zoologically,
_Taenia echinococcus_ and _T. coenurus_ are interesting, since they exhibit
an {81}indubitable alternation of asexual generations in the larval state,
with a sexual adult stage.

_Bothriocephalus latus_ Brems., the broad tape-worm, which attains a length
of 20-30 feet, or even more, occurs in man endemically in the eastern
Baltic provinces, certain parts of Switzerland, generally throughout Russia
(especially near Kasan), in North America, and commonly in Japan,—that is,
in districts where the population partake largely of pike or other fish in
a raw or partially-cooked state. Elsewhere it occurs sporadically, and in
Munich, where it was unknown before 1880, its presence has been traced to
emigrants from infected districts, who settled on the shores of the
Starenberger Lake, from which Munich was supplied with fish. How the pike,
the usual but not invariable intermediate host, becomes infested (and its
musculature is frequently riddled with the larvae) we do not accurately
know, but some Invertebrate, the prey of the pike, is probably the first
host into which the free-swimming ciliated larva (Fig. 42) finds its way.
In Greenland, _B. cordatus_ is very common in the dog, and probably also in
man, though few cases have been recorded. _B. mansoni_ Cobb. (= _B.
liguloides_ Leuck.) was, till recently, known only in the larval state from
China and Japan. Iijima, however, has found older specimens in the latter
country. _B. cristatus_ Dav. is a species founded somewhat doubtfully on
two fragments found, one in a child, the other in a man, in France.

OCCURRENCE OF CESTODES IN DOMESTIC ANIMALS.[103]—Among domestic animals,
the dog is, undoubtedly, the most frequently attacked by Taeniae. Six
species of _Taenia_ (_T. serrata_, _marginata_, _coenurus_, _echinococcus_,
_krabbei_, and possibly _T. serialis_), _Dipylidium caninum_ (the commonest
form), _Mesocestoides lineatus_, and three or four species of
_Bothriocephalus_ have been found in the dog. The table of life-histories
(p. 83) shows that sheep, rabbits and other Rodents serve as the
intermediate hosts, in which the cystic stages of the species of _Taenia_
are found. Hence the prevalence of _T. serrata_ in a given locality is
connected with the abundance there of the rabbit and hare, in which the
larva (_Cysticercus pisiformis_) occurs. _Bothriocephalus cordatus_
develops from the young stage present in the fish which the Icelanders give
to their dogs. In Iceland and certain parts of {82}Australia _T.
echinococcus_ infests one-third to one-half the number of dogs examined; a
fact connected with the frequency of _Echinococcus_ in man in these
countries.

In sheep the most noteworthy and dangerous parasite is _Coenurus
cerebralis_ (or the cystic stage of the dog-taenia, _T. coenurus_), which
gives rise to the disease known as "gid" or "staggers." It is found in
various parts of the brain or spinal cord, and the symptoms differ
according to the position of the parasite. If this presses upon one
hemisphere the sheep describes circles and finally falls: if on the optic
lobes, the eyes are affected: if the pressure affects the cerebellum the
movements of the sheep are uncertain and incoordinated. Four or six weeks
after the appearance of the symptoms, death results from cerebral
paralysis, or from general debility, and the loss of sheep incurred by this
disease (happily less frequent in England than formerly) has been
calculated by Youatt at a million for France annually; at 35 per cent of
the flocks for England in bad seasons; and about 2 per cent for Germany.
Besides sheep, which are most subject to "gid" during their first year,
various ruminants—Goat, Ox, Moufflon, Chamois, Roe, Antelope, Reindeer,
Dromedary—are attacked in the same way. A similar form, _Coenurus serialis_
Baill., is common in the wild rabbit in this country, and in Australia in
the hare and squirrel. It forms large swellings in the connective tissue of
various parts of the body, but usually does not affect the health of the
host. It is not known in what carnivore _Taenia serialis_ Baill. normally
occurs. Experiments have, however, shown that it develops rapidly in dogs.

The preventive measures which are steadily diminishing the prevalence of
the Cestode parasites in man in some parts of Western Europe cannot be
dealt with here, but it may be noticed that the Jewish observance with
regard to swine is the surest preventive measure against taeniasis and
trichinosis. Careful inspection of meat and general cleanliness, are the
leading measures that in these hygienic matters secure the greatest
immunity from disease.

{83}TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND THE
DOMESTIC ANIMALS.

Cestode. Final host. Larva. Intermediate host.

_Taenia serrata_ _Cysticercus Rabbit, Hare, Mice
Goeze Dog pisiformis_ (liver and peritoneum)
Zed.

_T. marginata_ _Cyst. Monkeys, Ruminants,
Batsch Dog, Wolf tenuicollis_ ungulates (in
Rud. peritoneum)

_T. saginata_
Goeze (= _T. Man _Cyst. bovis_ Ox, Giraffe
mediocanellata Cobb. (in muscles)
_Küch.)

_Cyst.
_T. solium_ Man cellulosae_ Pig, Man, Monkeys,
Rud. Rud. (? _Cyst. Bear, Dog, Cat, Black
acanthotrias_ Rat (in various
Weinl.) organs)

_T. crassicollis_ Cat and other _Cyst. fasciolaris_
Rud. Felidae, Rud. Rat, Mouse, Bat (liver)
Stoat

_T. coenurus_. Dog, Arctic _Coenurus Brain of Sheep, Ox,
Küch Fox cerebralis_ Goat, Dromedary, Camel,
Rud. Antelope, Horse

_T. serialis_ ? Dog _Coenurus Rabbit (connective
Baill. serialis_ Baill. tissue)

_Echinococcus Man, Monkeys, many
_T. echinococcus_ Dog, Dingo, polymorphus_ Carnivores, Rodents,
v. Sieb. Jackal, Wolf Dies.(incl. Ungulates, Ruminants,
_E. multilocularis_ and Marsupials; also
found in Man) in Turkey and other
birds
_Moniezia expansa_ Sheep, Ox, Unknown
Rud. Goat, etc.

_Thysanosoma Sheep, Unknown
fimbriata_ Dies. Cervidae

_Stilesia
globipunctata_ Sheep Unknown
Riv.

_Anoplocephala
perfoliata_ Horse Unknown
Goeze

_Dipylidium Cysticercoid larva
caninum_ L. Man, Dog, (Fig. 43), Body-cavity of
(= _Taenia Cat _Cryptocystis_ _Trichodectes_ and
cucumerina_ trichodectis_ _Pulex_ of Dog
Bloch = _T. Vill.
elliptica_ Batsch)

_Hymenolepis_ _Cercocystis_ Usually absent
_murina_ Duj. Mouse, Rat Vill.[104] (develops
in parental host)

_H. nana_ Man Unknown
v. Sieb.

_H. diminuta_ Meal-moth, _Asopia_
Rud.(= _Taenia Man, Mouse, _Cercocystis_ (_Pyralis_)
flavopunctata_ Rat Vill. _farinalis_; also
Weinl.) certain Orthoptera
and Coleoptera

_Drepanidotaenia_ Duck, Goose, _Cercocystis_ The Ostracods {84}
_gracilis_ Zed. Wild Duck Vill. _Candona rostrata_ and
_Cypris compressa_,
and also
_Cyclops viridis_

_D. anatina_ Duck " " _Cypris incongruens_,
Krabbe and also Perch

_D. setigera_ Goose " " _Cyclops brevicaudatus_
Fröh.

_D. infundibuliformis_
Goeze Common Fowl " " House-fly

_Dicranotaenia Duck " " _Cypris ovum_
coronula_ Duj.

_Davainea proglottina_ " " ? _Limax cinereus_,
Dav. Fowl _L. agrestis_

_D. madagascariensis_ Unknown
Dav. Children

_D. friedbergeri_ Unknown ? Ants
v. Linst. Pheasant

_Mesocestoides
lineatus_ Dog Unknown
Goeze

Plerocercoid, Probably first enters
_Bothriocephalus Man, Dog, i.e. solid, an Invertebrate host,
latus_ Brems. ? Cat elongate larva, which is eaten by
with no bladder Pike, Perch, Trout,
etc.

[Illustration: FIG. 40.—A, Stickleback (_Gasterosteus aculeatus_) infested
by an advanced larva of _Schistocephalus solidus_ Crepl. B, The larva. All
× 1½. (From specimens in the Cambridge University Museum.)]

STRUCTURE AND DEVELOPMENT OF CESTODA.[105]—Of the unsegmented Cestodes,
_Caryophyllaeus mutabilis_, from the intestine of carp and other Cyprinoid
fishes, is the most easily accessible form. _Triaenophorus nodulosus_,
which is very useful for the study of the excretory system, occurs mature
in the pike. In the body-cavity of the Stickleback (Fig. 40) a large,
broad, yellow worm may sometimes {85}be found, the larva of
_Schistocephalus solidus_ Crepl., which occurs in the intestine of Terns,
Storks, Mergansers, and other birds. Species of _Ligula_ are found in the
same birds. The intestine of a _Lophius_ or _Cyclopterus_ ("lump-fish")
contains, usually, the early and intermediate stages of various Cestodes,
while the alimentary canal of Elasmobranchs often contain many peculiar
Tetrarhynchidae and other forms. For the study of development, the _Taenia
anatina_ from the duck may be used. The ripe proglottides are collected,
and the eggs placed with _Cypris ovum_ in an aquarium, with the probability
that some of the embryos will enter the Ostracod, and the peculiar
Cysticercoid may be bred.[106] _Cysticercus pisiformis_ and _Coenurus
serialis_, which occur commonly in rabbits, are also suitable objects for
examination.

A Cestode such as _Echinobothrium_ (Fig. 36) is divisible into head and
proglottides. Moniez has suggested that the head is really the
morphologically hinder end of the body, in which case the formation of
proglottides would closely resemble the mode of segmentation of an Annelid
larva. The close similarity, however, between the Cysticercoid larva (Fig.
43, F) and the Cercaria of a liver-fluke, seems to show that the anterior
end is the same in both cases, and since it bears the central part of the
nervous system, we may reasonably call it the "head." Moreover the hinder
end of a Platyhelminth usually possesses the chief excretory pore. Another
difficulty is the determination of dorsal and ventral surfaces. Authors are
agreed,—on the analogy of Trematodes, in which the testes are usually
dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a
Cestode are determined by the position of these organs, although the often
radially formed "head," the lateral or superficial position of the genital
apertures, and the variability of these features, render it a matter of
considerable doubt whether "dorsal" and "ventral" are more than useful
conventional terms. The suckers and hooks are borne on a muscular cap, the
"rostellum," which is only slightly developed in the _Ichthyotaeniae_. The
body is solid, and is divisible into an outer muscular coat—enveloped in a
(possibly epidermal) investing membrane—and an inner parenchymatous tissue
containing the chief part of the excretory, nervous, and reproductive
systems. One or two pairs {86}of longitudinal excretory vessels are
present, usually connected by transverse ducts and opening by a single
terminal pore. Occasionally a regularly paired arrangement of lateral or
secondary pores is present (Figs. 38 and 41, _for.sec_). Flame-cells occur
at the end of the fine tubules (Fig. 38), and the whole system is well
developed, but may undergo degenerative changes in the older proglottides.
The central nervous system varies according to the degree of
differentiation of the rostellum; and, owing to the difficulty of staining
the nerves and the contradictory statements of authors, we do not yet
possess a fully reliable account of the nervous system even of the commoner
Taeniae. Free nerve endings and other sensory terminations have been
recently stated to exist in the cuticle of Cestodes and Trematodes. If
true, this would tend to show that the parasitic mode of life of these
animals demands a complex nervous system comparable with that of the
Turbellaria.

[Illustration: FIG. 41.—Diagrammatic transverse section of _Schistocephalus
solidus_ Crepl., from the Wild-duck, illustrative of the Cestodes with
uterine aperture (_uto_). × 12. _cs_, Cirrus-sac; _for.sec_, one of the
paired lateral openings of the excretory vessels; _ln_, longitudinal nerve;
_ov_, ovary; _ovd_, oviduct; _par.m_, parenchymatous muscles; _r.sem_,
receptaculum seminis; _sh.gl_, shell-gland; _te_, testes; _ut_, uterus;
_uto_, uterine pore; _vag_, vagina; _vd_, vasa deferentia; _yd_, yolk-duct;
_yg_, yolk-glands (black); ♂, male, ♀, female genital aperture. (After
Riehm.)]

The reproductive organs, unlike the preceding systems, are discontinuous
from one proglottis to the next. The male and female organs and their
mutual connexions, especially in the unsegmented Cestodes, may be compared
in detail with those of Trematodes, but the difference between the
arrangement of the generative organs of various Cestodes is very
great.[107] The penis (Fig. 41, _cs_) is evaginated through the male pore
(Fig. 41, ♂), and inserted far into the vagina (♀, _vag_) of the same or
another segment of the tape-worm.

{87}[Illustration: FIG. 42.—A, Free-swimming, six-hooked larva of
_Bothriocephalus latus_ Brems. (the broad tape-worm of Man), still enclosed
in a ciliated (possibly cellular) double membrane or mantle. In this
condition it may continue to live in water for a week or more, but
eventually throws off its ciliated coat (as in B) and commences to creep
about vigorously by the aid of its hooks, in search of its first host,
which is at present unknown. (After Schauinsland.) × 600.]

From this fact and the anatomical relations of the vagina, it is becoming
increasingly probable that the so-called uterus of Trematodes is an organ
corresponding to the vagina of Cestodes, and not to the uterus of Cestodes.
The latter opens to the exterior in _Schistocephalus_, _Bothriocephalus_,
and some other Cestodes of fishes by a special pore (Fig. 41, _uto_).
Through this, some of the eggs (which in these genera give rise to ciliated
larvae) are enabled to escape, and need not wait for the detachment of the
proglottis, as must happen in the Taeniidae, where the uterus is closed.
This uterus, a true physiological one, is probably the homologue of the
"canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The
fertilised ovum and yolk are brought together into the "ootype," where the
shell-gland forms the egg-shell around them (Fig. 41, _sh.gl_) and the egg
is then passed into the uterus. The ovum segments to form a minute
six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be
ciliated. Thus in _Taenia serrata_ the proglottides are shed with the
faeces of the host (dog), and they protect the young from the desiccating
influence of the surroundings. If inadvertently eaten by a rabbit along
with herbs, the proglottis and larval envelope are digested, and by its six
hooks the tiny larva bores through the gastric wall into the portal vein,
and so into the liver. Here the hooks are thrown off, and the solid mass of
cells becomes vacuolated.

{88}[Illustration: FIG. 43.—Stages in the development of _Dipylidium
caninum_ L. (= _Taenia elliptica_ Batsch, _T. cucumerina_ Bloch), the
commonest of the Dog-Taeniae; compare Fig. 44. A, Six-hooked larva (now
often spoken of as an "Onchosphaera"); B, larva elongating; formation of a
central lacuna; C, larva further advanced; D, distinction between body and
tail is visible; E, invagination of the rostellum is commencing; F,
Cysticercoid larva with four suckers, invaginated rostellum, and excretory
vessels. _c_, Calcareous concretions in cells of the larva; _ex.o_,
excretory aperture; _ex.v_, excretory vessels; _inv_, invagination
commencing; _rost_, rostellum; _sc_, suckers. (After Grassi and Rovelli;
highly magnified.)]

[Illustration: FIG. 44.—Schematic longitudinal sections through the larvae
of _Dipylidium caninum_ L. All these stages are passed in the body-cavity
of the Dog-flea (_Pulex serraticeps_). (Compare Fig. 43 for further
details.) A, Six-hooked larva with developing rostellum (shaded) and
suckers (black). In this species the invagination (C, _invag_.) occurs
after the formation of these organs, and not, as in most Taeniae, before
it. B, Invagination commencing; the hooks are developing above the
rostellum, while beneath it the nervous system (dotted) is seen. C, The
invagination has now carried the suckers inwards. The tail has become
distinct, and the whole larva at this stage is known as a Cysticercoid.
_hk_, Larval hooks; _invag_, mouth of the invagination; _n_, central
nervous system; _rost_, rostellum and hooks; _sc_, suckers, of which only
two can be seen in a longitudinal section; four are really present. (After
Grassi and Rovelli.)]

{89}At one pole an invagination occurs, at the bottom of which the
rostellum, suckers, and hooks are gradually formed, but inside out as
compared with the head of the _Taenia serrata_. At this stage the larva
(_Cysticercus pisiformis_) has usually issued from the liver and attached
itself to the omentum. The invagination projects into the cavity of the
bladder, within which a watery fluid accumulates. Thus the "bladder worm"
is formed, the head of which is evaginated if the larva be introduced into
the digestive system of a dog. The bladder and neck of invagination are
digested, while the head, protected by these, remains, and forms the neck,
from which the proglottides are afterwards segmented off. In _Taenia_
(_Hymenolepis_) _murina_ the whole development may take place in the
parental host, the larva living in the villi, the adults in the cavity of
the same rat's intestine (Grassi). The different forms of Cestode larvae
depend largely upon the presence and degree of development of the caudal
vesicle or bladder, which in _Scolex polymorphus_ (Fig. 38) (the young
stage of _Calliobothrium filicolle_ Zsch.) is practically absent. If the
bladder be small, the larva is known as a Cysticercoid. For example, the
common _Dipylidium caninum_, which lives in the dog, has such a larva, the
development of which is explained and illustrated by Figs. 43 and 44. The
bladder becomes exceeding capacious in _Coenurus_ and _Echinococcus_.

TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC
ANIMALS.[108]

I. Scolex in most cases with hooks; uterus with a median and lateral
branches; yolk-glands simple, median; genital pore single; dorsal
excretory vessel narrower than the ventral, without a circular
commissural trunk; eggs without pyriform apparatus (processes of the
ovarian membrane) Gen. TAENIA L. (s. str.)

A. Genital ducts pass on the ventral side of the nerve and of the two
longitudinal excretory vessels _T. crassicollis_ Rud.

B. Genital ducts pass between the dorsal and ventral longitudinal
vessels.

_a._ Nerve present on dorsal side of genital ducts.

α. Head armed _T. solium_ Rud.

β. Head unarmed _T. saginata_ Goeze.

_b._ Nerve on ventral side of genital ducts.

DOG-TAENIAE[109] {90}

Head armed; genital pore marginal and
— Single

Many proglottides; strobila several centimetres long; small
hooks with guard.

Bifid hooks, which are
— 230µ-260µ long[110]; genital pore very distinct
_T. serrata_ Goeze.
— 136µ-157µ long; genital pore not very salient
_T. serialis_ Ball.

Entire large hooks, which are
— 180µ-220µ long; length of mature segments double their width
_T. marginata_ Batsch.
— 150µ-170µ long; length of mature segments treble their width
_T. coenurus_ Küch.

3-4 segments; a few mm. long _T. echinococcus_ v. Sieb.

— Double and bilateral _Dipylidium_ caninum L.

Head unarmed; two genital pores on ventral surface
_Mesocestoides lineatus_ Goeze.

II. Scolex without hooks; one or two transverse uteri present; one or two
genital pores and yolk-glands, the latter never median; genital ducts
pass on the dorsal side of the nerve; eggs with pyriform apparatus.

A. One transverse uterus present.

_a._ Uterus with bullate egg-sacs; pyriform apparatus without horns;
genital ducts between dorsal and ventral vessels
THYSANOSOMA Dies.

α. Head large (1.5 mm.); square lobed testes in median field;
posterior margin of segments fimbriated; genital pore double
_T. fimbriata_ Dies.

β. Head small; no fimbriae; pore rarely double
_T. giardii_ Riv.

_b._ Uterus without saccular dilatations; segments short, thick, and
slightly imbricate ANOPLOCEPHALA E. Blanch.

HORSE-TAENIAE.

α. Head very large

— No posterior lobes _A. plicata_ Zed.

— Four posterior lobes _A. perfoliata_ Goeze.

β. Head small, without posterior lobes
_A. mamillana_ Mehl.

B. Two uteri and two genital pores present; horns of pyriform apparatus
well developed; genital ducts pass on the dorsal side of the
longitudinal vessels MONIEZIA R. Bl.

_a._ Interproglottidal glands[111] arranged in linear series
(_planissima_ group)
_M. planissima_ S. and H. _M. benedeni_ Mz. _M. neumani_ Mz.

_b._ Interproglottidal glands saccular (expansa group)
_M. expansa_ Rud. _M. oblongiceps_ S. and H.
_M. trigonophora_ S. and H.

_c._ Interproglottidal glands absent (_denticulata_ group)
_M. denticulata_ Rud. _M. alba_ Perr.

C. Uterus single or double, without spore-like egg-sacs; eggs with a
single shell; genital pores irregularly alternate; strobila narrow;
testes absent from median part of the field STILESIA Raill.

_a._ A transverse uterus in middle part of median field; {91}
head 2 mm. diameter _S. centripunctata_ Riv.

_b._ Two lateral uteri in each segment; head less than 1 mm. in
diameter _S. globipunctata_ Riv.

III. Scolex almost invariably provided with hooks; genital pores on left
border of segment; eggs with three shells but no cornua. Segments
broader than long; posterior angles salient. HYMENOLEPIS Weinl.

_a._ Scolex with a single series of 24-30 hooks, each 14-18µ long
_H. nana_ v. Sieb. _H. murina_ Duj.

_b._ Scolex very small, unarmed _H. diminuta_ Rud.

IV. Scolex provided with two elongated muscular pits. Body segmented;
three genital apertures in middle of ventral surface
BOTHRIOCEPHALUS Rud.

Body 2-20 metres in length _B. latus_ Brems. _B. cristatus_ Dav.
(doubtful species). _B. cordatus_ Leuck. _B. mansoni_ Cobb.
(= _B. liguloides_ Leuck.)

CLASSIFICATION OF CESTODES.—The following classification, which, so far as
the Taeniidae are concerned, follows that employed by Railliet, Blanchard,
and most recent writers, includes only a few representative genera:—

1. Fam. CESTODARIIDAE Mont. (MONOZOA Lang).

Gen. _Caryophyllaeus_, _Archigetes_, _Gyrocotyle_, _Amphilina_.

2. Fam. BOTHRIOCEPHALIDAE.

Sub-Fam. 1. Bothriocephalinae. Gen. _Bothriocephalus_, _Schistocephalus_,
_Triaenophorus_ (= _Tricuspidaria_).

Sub-Fam. 2. Ligulinae. Gen. _Ligula_.

Sub-Fam. 3. Solenophorinae. Gen. _Solenophorus_, _Duthiersia_.

Sub-Fam. 4. Diphyllinae. Gen. _Echinobothrium_.

3. Fam. TETRARHYNCHIDAE.

Gen. _Tetrarhynchus_.

4. Fam. TETRAPHYLLIDAE.

Sub-Fam. 1. Phyllobothrinae. Gen. _Phyllobothrium_, _Echeneibothrium_,
etc.

Sub-Fam. 2. Phyllacanthinae. Gen. _Calliobothrium_, _Anthobothrium_,
etc.

5. Fam. TAENIIDAE.

Sub-Fam. 1. Cystotaeninae. Gen. _Taenia_ s. str.

Sub-Fam. 2. Anoplocephalinae. Gen. _Moniezia_, _Thysanosoma_, _Stilesia_,
_Anoplocephala_.

Sub-Fam. 3. Cystoidotaeninae. Gen. _Dipylidium_, _Hymenolepis_,
_Drepanidotaenia_, _Dicranotaenia_, _Echinocotyle_, _Davainea_.

Sub-Fam. 4. Mesocestoidinae. Gen. _Mesocestoides_, _Dithyridium_.

Sub-Fam. 5. Ichthyotaeninae. Gen. _Ichthyotaenia_, _Corallobothrium_.

{92}CHAPTER IV

MESOZOA

DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE—
OCCURRENCE—STRUCTURE: TRICHOPLAX: SALINELLA.

The Mesozoa are an obscure group, the position of which in the animal
kingdom is still doubtful. The name Mesozoa was given to the group by its
discoverer, E. van Beneden,[112] as he concluded that they were
intermediate between the Protozoa and the higher Invertebrates. Recent
authors, however, have called attention to the resemblance existing between
them and the "sporocysts" of Trematodes, and though we still are ignorant
of certain important points in their life-histories, the Mesozoa are most
conveniently (and probably rightly) considered as an appendix to the
Platyhelminthes.

[Illustration: FIG. 45.—A, B, C, Stages in the development of the vermiform
larva in _Dicyema typus_ van Ben. (After Ed. van Beneden.) _cal_,
"Calotte"; _gc_, germinal cell; _n_, nucleus of endodermal cell.]

The animals composing this group are minute and parasitic, and are composed
of a small number of cells. They may be divided into two families: the
_Dicyemidae_, which occur exclusively in the kidneys of certain Cephalopods
(cuttle-fish); and the _Orthonectidae_, which live in the brittle-star
_Amphiura squamata_, the Nemertine _Nemertes lacteus_, or the {93}Polyclad
_Leptoplana tremellaris_. In addition to the undoubted Mesozoa, certain
anomalous forms—_Trichoplax adhaerens_ and _Salinella salve_—may be
referred to this group.

[Illustration: FIG. 46.—_Dicyemennea eledones_ Wag., from the kidney of
_Eledone moschata_. A, Full-grown Rhombogen with infusoriform embryos
(_emb_); B, one of the latter developing; C, fully formed; D, calotte,
composed of the upper nine cells shown in the figure. (After Ed. van
Beneden and Whitman.) _emb_, Infusoriform embryo; _g_, part of
endoderm-cell where formation of these embryos is rapidly proceeding;
_n.ect_, nucleus of ectoderm-cell; _n.end_, nucleus of endoderm-cell; _p_,
"calotte."]

DICYEMIDAE.—If the kidney of _Eledone moschata_, a Cephalopod common on our
south-western shores, be opened, a number of fine, yellowish, hair-like
filaments may be seen attached at one end to its inner surface, floating in
the fluid contained in the renal cavity. These may be _Dicyemennea
eledones_ Wag., although another form, _Dicyema moschatum_ Whit., also
occurs in the same host. _D. eledones_ (Fig. 46) is 7 to 9 mm. long,
transparent, and is composed of one large inner cell with a simple nucleus
(Fig. 46, _n.end_), and of an outer layer of ciliated cells, nine of which
form the "calotte" or pole by which the animal is attached. Within the
former (endodermal) cell the formation of urn-shaped "infusoriform embryos"
takes place (B and C), the fate of which is not known, but they are
possibly the males. The individual which produces these larvae is called a
{94}"Rhombogen." Other individuals which produce a more elongated larva
("vermiform larva," Fig. 45) are called "Nematogens," and Whitman has
described a third kind, which produce first infusoriform, and then
vermiform, larvae (Secondary Nematogens).[113]

The occurrence of the known species of Dicyemids (a group which has not
been investigated on our coasts) is as follows:—

Species. Host.

_Dicyema typus_ van Ben. _Octopus vulgaris._
_D. clausianum_ van Ben. _O. macropus._
_D. microcephalum_ Whit. _O. de Filippi._
_D. moschatum_ Whit. _Eledone moschata._
_D. macrocephalum_ van Ben. _Sepiola rondeletii._
_D. truncatum_ Whit. _Rossia macrosoma_, _Sepia elegans_,
_S. officinalis._
_D. schultzianum_ van Ben. _S. biseralis, Octopus vulgaris._
_Dicyemennea eledones_ Wag. _Eledone moschata, E. aldrovandi._
_D. mülleri_ Clap. _E. cirrosa._
_D. gracile_ Wag. _Sepia officinalis._
_Conocyema polymorphum_ van Ben. _S. officinalis, Octopus vulgaris._

ORTHONECTIDA.[114]—Two species of Orthonectids are fairly well known,
_Rhopalura giardii_ Metschn. from _Amphiura squamata_, and _R. intoshii_
Metschn. from _Nemertes lacteus_. The latter appears to be very rare, the
former occurring in 2 to 5 per cent of the number of hosts examined. The
parasites occur in a granular "plasmodium," the nature of which is
uncertain. Metschnikoff regards it as formed by the Orthonectids, and he
considers that the cellular envelope, by which it is sometimes enclosed, is
developed from the neighbouring tissue of the host. These granular,
sometimes nucleated, plasmodial masses, which can perform active amoeboid
movements in sea-water, occur attached to the ventral part of the
body-cavity of _Amphiura_, and between the gut-branches and body-wall in
_Nemertes_. Should these hosts be infected by great numbers of the
Orthonectids, their sexual organs degenerate (as is the case with
pond-snails attacked by sporocysts[115]), and it is possible that the
remains of these organs may constitute the "plasmodia" (Braun).

{95}_Rhopalura giardii_ is of distinct sexes. Either males or females are
found in one _Amphiura_. Two kinds of females, flattened unsegmented, and
cylindrical segmented forms, are known. They consist of a ciliated
ectodermal layer enclosing an endodermal mass of eggs, between which is a
fibrillar layer usually considered to be of a muscular nature. The
cylindrical female gives rise to eggs which develop, probably exclusively,
into males. The flattened female produces eggs from which females alone
arise, though the origin of the two forms of this sex is not well
ascertained. The males contain spermatozoa which fertilise the eggs of the
cylindrical female, whereas the ova of the flat form probably develop
parthenogenetically.

[Illustration: FIG. 47.—_Rhopalura giardii_ Metschn. (from the brittle-star
_Amphiura squamata_). ♂, Full-grown male (× 800); ♀_{1}, flattened form of
female (× 510); ♀_{2}, cylindrical female (× 510). (After Julin.)]

TRICHOPLAX.[116]—This anomalous animal has only been found in aquaria,
originally in the marine aquarium at Graz by {96}Schulze. It has the
appearance of a large, flattened, ciliated _Amoeba_ (1.5-3 mm. in
diameter), but is distinguished by its structure. The upper surface is
composed of a flattened epithelium. The lower surface is made up of
cylindrical ciliated cells, which pass imperceptibly into the branched
cells, embedded in a hyaline matrix, which compose the middle layer of the
body. No distinct organs, and beyond simple fission, no mode of
reproduction, have been observed. One species, _T. adhaerens_, is known,
but has never been met with in a free state.

SALINELLA.[117]—This is another aquarium-animal, found by Frenzel in the
Argentine, in an artificial saline solution with which he filled some
aquaria. It measures .2 mm. in length, and has a somewhat flattened,
barrel-shaped appearance. A single layer of ciliated cells bounds a central
cavity opening at each end. Fission, and conjugation followed by
encystment, have been observed. One form, _S. salve_, is known from salines
taken from Cordova.

NEMERTINEA

LILIAN SHELDON

Staff Lecturer in Natural Science, Newnham College, Cambridge.

{99}CHAPTER V

NEMERTINEA

INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT—HABITS—
REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND, FRESH-WATER, AND
PARASITIC FORMS—AFFINITIES

The Nemertinea form a compact group, the affinities of which have not been
at present clearly determined. Several species were mentioned and described
in the works of various naturalists during the latter half of the
eighteenth century, though their anatomy was not understood until
considerably later. The first mention of any member of the group was made
by the Rev. W. Borlase in his _Natural History of Cornwall_, published in
1758. He gives a short description and a rough figure of _Lineus marinus_.
From that time the increase in the knowledge of the group was very gradual.
New species were from time to time described, but few of the descriptions
could boast of much completeness, and many erroneous views were held until
comparatively recent years. The group was very variously classified, but
the general arrangement in early times seems to have been to unite it with
the Planarians. Valuable contributions to the history of the development
were made in 1848 and the few subsequent years by Desor,[118]
Gegenbaur,[119] Krohn,[120] and Leuckart and Pagenstecher[121]; and more
recently by Metschnikoff[122] and Salensky.[123]

{100}Nemertines for the most part closely resemble one another in all
essential points, though they differ considerably in size, colour, and
external details. They vary in length from less than an inch to thirty
yards, this extreme size being attained by _Lineus marinus_.

[Illustration: FIG. 48.—_Lineus marinus_ Mont., from the living specimen in
the coiled condition. Plymouth. × 1. _a_, Anterior end; _b_, posterior
end.]

[Illustration: FIG. 49.—_L. marinus_, from the same specimen as Fig. 48, in
the expanded condition. _a_, Anterior end; _b_, posterior end.]

Nemertines are common on the British coasts; about forty species have been
recorded from this area. On turning over a stone on a sandy or muddy shore
in a pool left by the receding tide, there may often be seen a coiled mass,
having the appearance of a uniform slimy string twisted into a
complicated knot. If it be carefully removed, the ends can generally be
made out, one bluntly rounded and the other slightly tapering (Fig. 48,
_a_ and _b_). Occasionally there may be seen attached to the blunter end a
fine thread, which moves about freely. This thread may, by an instantaneous
movement, be drawn into the body, no trace of its existence being left
except at the tip of the head, where a small pore is visible; this is the
orifice through which it was withdrawn. Shortly afterwards the thread may
be again shot out, the process being instantaneous and often accomplished
with {101}great force. This thread (Fig. 50, _p_) is the proboscis, a very
important and characteristic organ in Nemertines.

Most Nemertines are marine; they are mostly indifferent to climate and to
the nature of the soil on which they live.

A few forms live on land (e.g. _Tetrastemma agricola_,[124] _Geonemertes
palaensis_,[125] and _G. chalicophora_[126]) or in fresh water (e.g.
_Tetrastemma aquarum dulcium_[127] and _T. lacustre_[128]) in various parts
of the globe. There are also parasitic forms; the best known of which is
_Malacobdella_.[129] A pelagic form, _Pelagonemertes_,[130] has been
described by Moseley.

[Illustration: FIG. 50.—Side view of head of _Cerebratulus_ (_Micrura_)
_tristis_ Hubr., showing the everted proboscis. Naples. × 2. Drawn from a
spirit specimen. _c.s_, Cephalic slit; _m_, mouth; _p_, proboscis.]

EXTERNAL CHARACTERS.—A typical Nemertine possesses an elongated worm-like
body (Fig. 49), which is usually thrown into numerous close coils (Fig.
48). In section it may be either round or more or less flattened, with the
lateral edges in some cases quite thin and almost fin-like. One or two
broad, flattened, and leaf-shaped forms are known, but such a condition is
exceptional, and the forms in which it occurs have probably assumed it
owing to the adoption of special modes of life.

In the ordinary forms the posterior end of the body is pointed either
bluntly or sharply. The head is somewhat broader than the rest of the body,
and often assumes a spatulate form. Eyes (Fig. 51, _e_) are usually present
either in one or several pairs, or in symmetrically-arranged groups on each
side of the head. The mouth (Fig. 58, _m_) is situated near the front end
of the body on the ventral surface, and is usually rendered conspicuous by
being surrounded by thick tumid lips. It varies in form from being
slit-like to elliptical. At the anterior end of the body a {102}small
terminal pore occurs; this is the external opening of the proboscis (Fig.
51, _p.p_).

Nemertines are often very diversely and brilliantly coloured, the hues most
commonly found being white, yellow, green, deep purple, and various shades
of red and pink. The ventral surface is usually paler in colour than the
dorsal, and the latter is often marked by longitudinal and transverse
stripes (Fig. 59) in contrasting colours.

The whole animal is enveloped in a layer of mucus, which sometimes becomes
hardened to form a tube, and this may be still further strengthened by an
admixture of particles of sand or earth.

The body is capable to a great extent of contraction and extension, a
Nemertine many inches long being apt, when irritated or alarmed, to
contract itself to the length of not more than half an inch. Hence, unless
the animal is kept and carefully watched, a very erroneous idea may be
conceived as to its size.

ANATOMY.—The body-wall consists of several layers (Fig. 52), which in a
typical highly-developed Nemertine are as follows:—

1. An external epidermic layer (_ep_), consisting of ciliated cells, among
which are placed numerous unicellular glands. These glands probably
secrete the mucus in which the Nemertine is usually enveloped; their
contents when in the body are very highly refracting. The epidermis rests
on a basement membrane (_b.m_).

2. The two or three muscular layers, arranged as either an external
circular and an internal longitudinal, or an inner and an outer circular
separated by a longitudinal layer, or, as in the figure (_c.m_ and _l.m_),
two longitudinal separated by a circular layer.

[Illustration: FIG. 51.—_Amphiporus lactifloreus_ Johnst., drawn from the
living specimen, from the dorsal surface. Plymouth. × 2. _e_, Eyes; _g_,
generative organs; _n.g_, nerve ganglion; _p.p_, proboscis pore; _p_,
proboscis.]

3. A fairly thick connective-tissue layer often found between the epidermis
and the muscles, into which latter it gradually merges (_s.t_).

{103}THE DIGESTIVE SYSTEM.—The mouth is placed on the ventral surface near
the anterior end of the body (Figs. 53, 58, _m_). It leads into a straight
oesophagus (Fig. 53, _oes_), whence passes off the intestine (_int_), which
is continued as a straight non-convoluted tube to the anus (_a_), situated
terminally at the posterior end of the body. The intestine is thrown out
throughout the greater part of its course into paired lateral pouches.

[Illustration: FIG. 52.—Diagrammatic transverse section of a Nemertine
(Schizonemertea) through the middle region of the body. _b.m_, Basement
membrane; _c.m_, circular muscle layer; _d.b_, dorsal blood-vessel; _ep_,
epidermis; _g_, generative organs; _int_, intestine; _l.b_, lateral
blood-vessel; _l.m_, longitudinal muscle layers; _n.c_, lateral nerve-cord;
_n.l_, nerve plexus; _p_, proboscis; _p.s_, proboscis sheath; _s.t_,
subcutaneous layer.]

The alimentary canal is lined throughout by a ciliated epithelium. The
oesophagus has, in addition to this layer, an outer thick coat of large
granular cells, which probably have a glandular function.

PROBOSCIS.—The most characteristic organ of the Nemertines is the proboscis
(Figs. 50, 53, 54). For many years its disposition and function were
misunderstood, and it was supposed to be a portion of the digestive system.
The proboscis, which lies dorsal to the alimentary canal, opens at the
extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When
retracted it is sometimes considerably folded, and lies in a long pouch or
sheath. To the walls of this sheath it is attached round its anterior
{104}end; and strong muscles unite its posterior extremity to the sheath a
short distance from the posterior end of the latter.

The proboscis seems to be exclusively a tactile and protective and
defensive organ, for which functions it is eminently fitted by the great
ease and rapidity with which it is everted or thrust out from the body. It
consists of two distinct regions (Fig. 54, _g.p_ and _m.p_). In the
retracted state the anterior part is a hollow tube with very thick muscular
walls made up of several layers. At the base of this part in many of the
Nemertines there is situated a sharp-pointed spine projecting forward into
the lumen, and several smaller stylets situated in a pair of vesicles close
to the base of the central spine. The position of the spines in the everted
proboscis is shown in Fig. 57. The posterior part of the proboscis is also
a tube, but instead of being muscular, its walls are glandular. This
posterior glandular part is never everted.

[Illustration: FIG. 53.—Diagrammatic drawing of a Nemertine from the dorsal
surface to show the position of some of the principal organs. _a_, Anus;
_c.s_, cephalic slit; _g_, generative organs; _int_, intestine with its
lateral diverticula; _m_, mouth; _n.c_, lateral nerve-cord; _n.g_, nerve
ganglion; _oes_, oesophagus; _p_, proboscis; _p.p_, proboscis pore; _p.s_,
proboscis sheath.]

The eversion is effected by a turning inside out of the anterior part of
the proboscis (Fig. 54). The process whereby the proboscis is retracted has
been very aptly compared to the effect which would be produced by the
inversion of the finger of a glove, accomplished by pulling a string
attached to its tip on the inside, the anterior muscular part being
comparable to the finger and the glandular part to the string. It is thus
obvious that in the everted condition the stylet will form the anterior tip
of the {105}proboscis, and will there be in a position for offence or
defence (Fig. 57, _s_).

NERVOUS SYSTEM.[131]—The brain is composed of two ganglionic masses (Fig.
53, _n.g_) lying at the anterior end of the body, one on each side of the
proboscis, and united by commissures passing round it (Fig. 55, _d.c_ and
_v.c_). Each ganglionic mass is often partially divided into a dorsal and
ventral lobe (_n.g.d_ and _n.g.v_). From the brain a pair of cords pass off
backwards along the sides of the body (_n.c_); these cords, which have no
ganglionic swellings, in some forms unite with one another above the anus.
Anteriorly nerves are given off from the brain to the eyes and front part
of the head (_a.n_). A nerve to the proboscis is given off from the
commissure which unites the two halves of the brain dorsal to the proboscis
(_d.n_).

[Illustration: FIG. 54.—Diagrammatic representation of the proboscis, (A)
in the retracted condition, (B) in the everted condition. _g.p_, Glandular
portion of the proboscis; _m_, muscle attaching the proboscis to its
sheath; _m.p_, muscular portion of the proboscis; _p.p_ in A, proboscis
pore; _p.p_ in B represents the position of the proboscis pore in the
retracted condition of the proboscis; _p.s_, proboscis sheath.]

In two out of the three groups into which the Nemertines are divided, the
lateral nerve-cords are in connexion with a network or plexus of nerves
lying between the muscular layers of the body-wall (Fig. 52, _n.l_), and in
some forms constituting a comparatively thick layer. In these two groups
there are no definite {106}nerve branches except the anterior ones to the
head. In the third group of Nemertines the lateral nerve-cords lie within
the muscular layers of the body-wall, and in this case paired nerve
branches are given off at definite intervals throughout the whole length of
the body. These branches divide up among the organs to which they pass, and
no nerve plexus is present.

The lateral cords vary in position in different cases. Sometimes they lie
laterally, at others the cords tend to approximate to one another in the
median dorsal or in the median ventral line, though in every case they
remain distinctly separated.

SENSE ORGANS.—Sense organs are usually present in the form of eyes arranged
at the sides of the head (Fig. 51, _e_), sometimes as a single pair and
sometimes in one or more groups on each side. The structure of the eyes
varies from a simple pigment spot to an organ which receives a special
nerve-supply from the brain, and possesses a refracting body answering to a
lens, and behind this a pigment layer and a layer of rods. Some forms are
devoid of all traces of eyes.

[Illustration: FIG. 55.—Diagram to show the relations of the nervous
system, circulatory system, and proboscis sheath in the anterior end of the
body in the _Hoplonemertea_, modified from M‘Intosh. _a.n_, Nerves to
anterior part of body and eyes; _d.c_, dorsal commissure; _d.n_, median
dorsal nerve; _d.v_, dorsal vascular trunk; _l.v_, lateral vascular trunk;
_n.c_, lateral nerve-cord; _n.g.d_, dorsal lobe of nerve ganglion; _n.g.v_,
ventral lobe of nerve ganglion; _p.p_, proboscis pore; _p.s_, proboscis
sheath; _v.c_, ventral commissure; _v.s_, vascular ring or collar.]

A pair of simple auditory capsules has been found in some of the
_Hoplonemertea_, where they occur as small vesicles on the brain.

The whole surface of the body appears to be remarkably sensitive. In a few
forms small tufts of tactile hairs are said to be present in the region of
the head, while in others there {107}are a few long hairs scattered
sparsely among the cilia of the epidermis.

_Frontal Organ._—In many Nemertines there is present at the anterior tip of
the head a disc-shaped group of cells bearing long hairs or bristles. On
this disc open the secreting ducts of a number of gland cells lying in the
head. It seems possible that this frontal organ may function as an organ of
taste.

_Side Organs._—In the Carinellidae there is a pair of circular epithelial
patches lying one on each side of the body in the region of the excretory
pore. The cells composing them are richly ciliated and provided with a
plentiful nerve-supply. The function of these epithelial patches is not
known, but it has been suggested that they may be auditory organs.

_Cephalic Slits and Cerebral Organs._—In most Nemertines there is a
peculiar pair of organs (Figs. 50, 53, _c.s_), situated in the head and in
close connexion with the brain. The function of these organs is not known.
Hubrecht has suggested that they may be respiratory, while Bürger[132]
conjectures that they may be organs which are used for discriminating the
condition of the surrounding medium. In an external examination of the
head, the cephalic slits may usually be seen as a pair of lateral furrows
or pits. Their form and direction vary considerably; they may take the form
of shallow circular depressions, or they may lie longitudinally and be
slit-like in shape (Fig. 50), or the slit may lie at right angles to the
long axis of the body and be beset with short transverse furrows. In some
forms these slits are merely superficial depressions, but in others they
are continued into ciliated ducts, which pass inwards and penetrate into
special lobes, consisting of glandular tissue and ganglion cells, in close
connexion with the brain. These lobes are called the cerebral organs.

In many forms the nervous system is charged with haemoglobin, which gives
to it a bright red colour.

CIRCULATORY OR BLOOD-VASCULAR SYSTEM.—The circulatory system consists of
three main longitudinal vessels, a median dorsal and a pair of lateral
ones. These are connected together posteriorly by a transverse trunk, and
also throughout the whole length of their course by branches, which are
given off at regular intervals. Anteriorly the three longitudinal vessels
{108}either all unite and form a collar (Fig. 55, _v.s_) round the
oesophagus, or they break up into a number of lacunar or open spaces in
free communication with one another.

The blood is usually colourless, but in some cases the corpuscles are
coloured red by haemoglobin.

[Illustration: FIG. 56.—Excretory system of Nemertines. A, _Drepanophorus
spectabilis_ Qtrf., part of one of the lateral vessels encircled by
branches of the excretory organ, × 585; _e_, main canal of the excretory
system: B, _D. crassus_ Qtrf., a terminal branch of the excretory system, ×
585; _f_, ciliated flame: C, _Malacobdella grossa_ O. F. Müll., entire
animal, slightly magnified, showing the excretory system (black) and the
vascular system; _e.a_, excretory aperture; _d.v_, dorsal vessel; _l.v_,
lateral vessel. (From Bürger.)]

EXCRETORY SYSTEM.—Max Schultze[133] found in _Tetrastemma obscurum_, on the
outer side of, but near to the lateral blood-vessels, a pair of canals. He
observed ciliary movements in the canals, but could not discover flame
cells. Further contributions to our knowledge of the excretory system were
made by Semper,[134] von Kennel,[135] Hubrecht,[136] and Oudemans.[137] The
latter states that the excretory system consists of a pair of canals
situated laterally near the anterior end of the body. Each canal
communicates with the exterior by one or more ducts having lateral
regularly-arranged apertures. In some cases he was unable to make out any
communication with the vascular system, but in others {109}a direct
communication, by means of open connexions with the lacunar blood spaces,
is said to occur.

Silliman[138] in _Tetrastemma aquarum dulcium_ describes the excretory
vessels as ending in numerous capillary branches, at the blind terminations
of which cilia are present. He states that there is no important difference
between the excretory systems of Rhabdocoeles and Nemertines.

Bürger,[139] as the result of recent investigations on the excretory system
in Nemertines, finds that the minute branches end in flame-cells (Fig. 56,
B) lying on and among the blood-vessels, but having no open connexion with
them.

GENERATIVE SYSTEM.—The Nemertines are for the most part dioecious, only a
few certainly hermaphrodite species having been described, e.g.
_Tetrastemma_ ("_Borlasia_") _kefersteinii_ Mar.[140]

The generative products in both cases are contained in sacs (Figs. 52, 53,
_g_) which lie in the lateral region of the body between the pouches of the
alimentary canal. The ova and spermatozoa are conveyed to the exterior by
short ducts. Most species are oviparous, though a few viviparous species
are known (e.g. _Prosorhochmus claparedii_).

CLASSIFICATION.—Nemertines were divided by M. Schultze[141] into:—

1. _Enopla_, in which the proboscis is armed with stylets.

2. _Anopla_, in which the proboscis is unarmed.

Although this classification was fairly correct as far as it went, since
many other distinctive features were correlated with the presence or
absence of armature in the proboscis, still there are several primitive
forms belonging to the _Anopla_, which possess characters such as render it
necessary to class them together in a separate group.

For this reason Hubrecht divided the Nemertinea into three
Orders—_Hoplonemertea_, _Schizonemertea_, _Palaeonemertea_; the first of
these Orders corresponding with the _Enopla_, and the other two with the
_Anopla_.

{110}ORDER I. HOPLONEMERTEA.

The proboscis is armed. The epidermis rests on a thick layer of connective
tissue plentifully supplied with glands, below which is a prominent
basement membrane. The muscular layers of the body are two in number, an
outer circular and an inner longitudinal. The nerve-trunks lie within the
muscular layers of the body and give off regularly-arranged branches. There
is no nerve plexus. Each of the cephalic slits generally opens by a pore
situated in the centre of a transverse groove, which is beset along one
side by a row of shorter grooves at right angles to it. The apparatus
consists of a ciliated duct surrounded by nerve tissue, and passing into
lobes of tissue which are connected with the brain by thick nerve-cords.
The mouth opens rather far forward in front of the brain. The intestinal
pouches are symmetrically arranged. Auditory organs are said to exist in
some forms, consisting of vesicles containing otoliths. The vascular trunks
are connected anteriorly by closed vessels and not by lacunar spaces.

[Illustration: FIG. 57.—Anterior end of the everted proboscis
(Hoplonemertea). _g.p_, Glandular portion of the proboscis; _l.s_, lateral
sacs containing stylets; _m.p_, muscular portion of the proboscis; _s_,
stylet; _s.b_, granular basal portion of stylet.]

The principal British genera and species[142] are:—

_Amphiporus bioculatus_ M‘Int., _A. dissimulans_ Riches, _A. hastatus_
M‘Int., _A. lactifloreus_ M‘Int., _A. pulcher_ Johnst.

_Drepanophorus rubrostriatus_ Hubr. (= _A. spectabilis_ Qtrf.).

_Tetrastemma ambiguum_ Riches, _T. candidum_ O. F. Müll., _T. dorsale_
Abildg., _T. flavidum_ Ehrenb., _T. immutabile_ Riches, _T.
melanocephalum_ Johnst., _T. nigrum_ Riches, _T. robertianae_ M‘Int., _T.
vermiculatum_ Qtrf.

_Prosorhochmus claparedii_ Keferstein.

_Nemertes carcinophila_ Köll., _N. gracilis_ Johnst., _N. neesii_ Oerst.

_Malacobdella grossa_ O. F. Müll.

{111}ORDER II. SCHIZONEMERTEA.

The proboscis is unarmed. The epidermis is separated from the layer of
connective tissue by a thin basement membrane, hence the glands in the
connective tissue are more deeply situated and have long ducts. The
muscular layers are three in number, an outer and an inner longitudinal
layer between which lies a layer of circular muscles. The lateral
nerve-cords lie between the outer longitudinal and the circular muscle
layers. They are connected throughout the body by a nerve plexus, the only
definite nerve branches given off being those to the brain, oesophagus, and
proboscis. The cephalic slits are a pair of deep longitudinal grooves at
the sides of the head. From each groove a canal passes inwards into a
posterior brain-lobe. The mouth opens behind the brain, and is an elongated
slit bounded by corrugated lips. Auditory organs have not been observed.
The longitudinal vascular trunks are connected anteriorly by lacunar
spaces, and not by closed vessels.

[Illustration: FIG. 58.—Head end of _Cerebratulus marginatus_ Ren., from
the ventral surface. Drawn from a spirit specimen. Naples. × 1. _c.s_,
Cephalic slit; _m_, mouth; _p.p_, proboscis pore.]

Principal British genera and species:—

_Lineus bilineatus_ Ren., _L. lacteus_ Mont., _L. marinus_ Mont. (= _L.
longissimus_ Gunnerus), _L. gesserensis_ O. F. Müll. (= _L. obscurus_
Desor and _L. sanguineus_ M‘Int.).

_Borlasia elizabethae_ M‘Int.

_Cerebratulus angulatus_ O. F. Müll., _C. fuscus_ M‘Int., _C.
pantherinus_ Hubr.

_Micrura aurantiaca_ Grube, _M. candida_ Bürger, _M. fasciolata_ Ehrenb.,
_M. purpurea_ J. Müll.

_Meckelia asulcata_ M‘Int.

ORDER III. PALAEONEMERTEA.

The proboscis is unarmed. The epidermis and connective tissue form one
layer, below which is the basement membrane. The muscular layers are three
in number, two circular separated by a longitudinal layer. The nerve-cords
lie altogether external {112}to the muscular layers, and are connected
together throughout by a plexus. No nerve branches are given off. The brain
is not divided into lobes. The cephalic slits are only represented by a
shallow depression on each side of the head, and no canals have been
observed leading from them. The intestine is straight, and the pouches are
usually absent or rudimentary. The circulatory system is largely made up of
lacunar spaces, the closed system being but little developed.

Principal British genera and species:—

_Carinella annulata_ Mont., _C. linearis_ (Mont., MS.) M‘Int., _C.
macintoshi_ Bürger (Fig. 59), _C. polymorpha_ Ren.

_Cephalothrix bioculata_ Oerst., _C. linearis_ Rathke.

_Valencinia lineformis_ M‘Int.

[Illustration: FIG. 59.—_Carinella macintoshi_ Bürger, drawn from the
living specimen, slightly contracted. Plymouth. Considerably magnified.
_a_, Anterior end; _b_, posterior end.]

A most important monograph by Bürger[143] on Nemertines has just been
published, but unfortunately it appeared too late to be adequately
considered here. He gives an elaborate account, illustrated by admirable
figures, of the present state of our knowledge of this group, and his work
will be indispensable to future students of the subject. The older systems
of classification are criticised, and the following scheme is adopted in
their place:—

ORDER I. PROTONEMERTINI (= part of the Palaeonemertea, e.g.
_Carinella_).—The brain and lateral nerve-cords lie outside the muscle
layers in the epithelium or below the basement membrane. The body-wall
consists of the following layers: epidermis, basement membrane, circular
muscles, and longitudinal muscles. The mouth lies behind the brain. The
proboscis is unarmed.

ORDER II. MESONEMERTINI (= part of the Palaeonemertea, e.g.
_Cephalothrix_).—The characters of this Order are similar to those of the
Protonemertini except that the brain and lateral nerve-cords lie in the
muscle layers.

ORDER III. METANEMERTINI (= Hoplonemertea).—The brain and lateral
nerve-cords lie in the parenchyma of the body internal to the muscle
layers. The layers of the body-wall are {113}similar to those of the
Protonemertini. The mouth lies in front of the brain. The proboscis is
armed. At the junction of the fore- and mid-gut a diverticulum is given off
which projects forwards beneath the fore-gut and ends blindly in front.

ORDER IV. HETERONEMERTINI (= Schizonemertea, and the genera _Eupolia_ and
_Valencinia_, placed provisionally by Hubrecht in the Palaeonemertea).—The
body-wall consists of the following layers: epidermis, thick cutis, and an
outer and an inner longitudinal muscle layer separated from one another by
a circular muscle layer. The brain and lateral nerve-cords lie between the
outer longitudinal and the circular muscle layers. The mouth lies behind
the brain. The proboscis is unarmed.

DEVELOPMENT OF THE NEMERTINEA.—The development of the Palaeonemertea is at
present not known: in the Schizonemertea a larval stage occurs; while in
the Hoplonemertea the egg develops directly without undergoing any
metamorphosis.

There are two forms of larva characteristic of the Schizonemertea, known
respectively as Pilidium and the Type of Desor. The Pilidium is hatched
early and leads a free-swimming existence, whereas the Type of Desor,
though in many respects resembling it, never passes through the
free-swimming phase.

[Illustration: FIG. 60.—Diagram of a Pilidium larva. (After Salensky.) _c_,
Tuft of cilia; _m_, muscle-fibres; _mo_, mouth, seen through one of the
lateral lobes; _n_, nerve-fibres; _n.r_, nerve-ring; _n.g_, nerve ganglion;
_oes_, oesophagus; _st_, stomach.]

The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike
dorsally, and prolonged downwards laterally into a pair of lobes. The whole
larva is covered with cilia, there being a specially strong band round its
ventral surface. The dorsal spike is composed of a bunch of strongly
developed cilia or of a long flagellum. The alimentary canal consists of a
sac constricted into {114}oesophageal and gastric regions (Fig. 60, _oes_
and _st_). In this condition the larva swims about freely in the water. The
helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin
of which is developed as ingrowths from it; these meet one another and
unite to form a complete covering round the alimentary canal; the larval
skin is then cast off, and by a series of gradual steps the embryo develops
into the adult.

HABITS.—Nemertines are often found under stones between high- and low-water
marks, lying on sandy or muddy bottoms. They are usually in the form of
coiled masses, and are generally in a state of quiescence. Hence it is
probable that their period of activity is during high-water, and that when
left by the receding tide they subside into a resting condition.

The large kinds, such as _Lineus marinus_, seem to be always found living
alone, but some of the smaller kinds, notably _Tetrastemma dorsale_ and
_Prosorhochmus claparedii_, have gregarious habits and live in masses, the
coils of the different individuals being inextricably mixed.

Some species, such as _Micrura purpurea_, _Amphiporus pulcher_, and
_Cerebratulus angulatus_, frequent empty bivalve shells, while Nemertines
are often found in empty limpet shells adhering to rocks in tidal pools.
Other smaller forms resort to no such definite protection, but live among
seaweeds; some of these remain naked, while others secrete for themselves
tubes of a membranous or gelatinous consistency. _Borlasia elizabethae_
lives in a burrow of clay.

Nemertines are commonly dredged from a depth of six or eight fathoms. They
may sometimes be found floating on the surface of the water, and some
possess the power of swimming rapidly, propelling themselves by a lateral
motion of the tail, the sides of which are in such cases prolonged into a
thin fin-like edge. This mode of progression is usually adopted by those
which frequent deep water. A pelagic Nemertine (_Pelagonemertes_) was
discovered by Moseley near the southern verge of the South Australian
current, being found in a trawl with deep-sea forms from a depth of 1800
fathoms. This animal was leaf-like in shape, bluntly pointed behind and
rather square in front.

The power possessed by Nemertines of secreting mucus is very great, their
course being often traceable by the tracks which they leave behind them.
Many of them glide along with great rapidity, a mode of progression which
is probably due to the {115}cilia covering the whole outer skin, and to the
extreme contractility of the muscles of the body-wall. In some locomotion
is effected by the proboscis; this is protruded and attaches itself by
means of its spines to some foreign body, after which the body is drawn up
after it. This has been specially observed in a land form, _Tetrastemma
agricola_, discovered by Willemoes-Suhm in the Bermudas. On solid bodies
the movement is a kind of crawling action, the head and mouth acting as
suckers in much the same way as in many Leeches.

Most Nemertines can be very readily kept in confinement. The chief apparent
effect of such a life is a loss of colour, the animal gradually becoming
pallid in hue. Owing also to the absence of proper food they diminish very
much in size, though even when all food is kept away an animal will
sometimes continue to live as long as eighteen months.

FOOD.—Nemertines are carnivorous in their habits and are very voracious,
devouring any prey which comes in their way, whether it be living or dead.
No animal food seems to come amiss to them, and they will devour creatures
of considerable size. When in contact with its prey, the Nemertine dilates
its mouth to a large extent, and the anterior end of the oesophagus is
thrust out and engulfs the animal. Chaetopods form a favourite food
material, the whole animal being swallowed quite regardless of the hard
chitinous bristles and spines with which it is beset. The soft parts are
gradually digested, the bristles and other indigestible portions being
extruded by the anus. The larger spines often pass out by perforating
passages through the wall of the intestine and through the body-wall. The
aperture thus formed appears speedily to heal after the foreign body has
been extruded.

The carnivorous habits of Nemertines even extend to cannibalism, and when
kept in confinement they frequently devour one another. For this reason it
is unsafe to keep large and small kinds together, as the small ones
speedily disappear, being used as food material by the large. If one be
divided into several pieces, the pieces are very rapidly demolished by
other individuals.

REGENERATION.[144]—This power is, no doubt, of great service to these
animals, since injury, or even violent local irritation, often causes
complete rupture at the point affected. It seems that the {116}chief power
of regeneration is situated in the head, as, if a very short piece be
broken off the anterior end of the body, it very rapidly reproduces itself
into a new individual. The hind end of the original body often lives for a
considerable time, but it does not in most cases appear to possess the
power of reproducing a head, and after existing for a time it dies. For a
while, however, it so far retains its vital powers that the generative
products continue to grow, and actually attain to perfection. Severe wounds
also heal very quickly and completely, and all local injuries are speedily
repaired.

Owing to the force with which it is shot out, the proboscis is often
completely severed from the body, and in such a case the animal grows a new
one in an extremely short space of time. The proboscis thus broken off
retains its power of movement and contractility for a considerable time,
and has been more than once mistaken for a worm. This great vital power is
probably due to the great development of nervous tissue, the proboscis
being usually richly supplied with nerve plexuses.

One large form, _Lineus sanguineus_, seems to possess great recuperative
powers. It shows a marked tendency to break up into pieces, when not only
the head end, but also the other portions develop into perfect animals,
each one growing a head and all the organs belonging to it. Thus in this
case an animal may multiply by a simple process of transverse fission, and
form numerous complete individuals.

BREEDING.—The breeding season only appears to cease in the extreme of
winter. Different genera and species seem to mature their generative
products at different times.

In the armed Nemertines the eggs are deposited separately, and are not
connected together except by such accidental mucus as the animal deposits
normally; but in the unarmed a special mucous secretion forms a thick
investment for the eggs.

M‘Intosh[145] has observed the process of the deposition of the male and
female products in _Nemertes gracilis_. He put into a glass vessel a male
and female of this species in which the products were apparently ripe. Soon
spermatozoa began to issue in wreath-like jets from the body of the male,
at first from the middle region of the body, and afterwards anteriorly and
posteriorly, until the animal was enveloped in a dense cloud of
{117}spermatozoa. The whole process only lasted a few minutes. When all the
spermatozoa had apparently been given out, the female was seen to protrude
her head from the sand; she then passed to the side of the vessel and
deposited a group of eggs about three inches distant from the spermatozoa.

With only a few exceptions Nemertines are oviparous. _Prosorhochmus
claparedii_, _Tetrastemma obscurum_, and _Monopora vivipara_ have been
observed to contain embryos at certain times of the year. In other forms
the eggs are laid when ripe, and development takes place subsequently to
their deposition.

GEOGRAPHICAL DISTRIBUTION.—Nemertines have been found in all seas from the
arctic to the equatorial regions. Many forms are found in the British Isles
both between tide-marks and also at greater depths around our coasts. Some
genera seem to be confined to warm climates and others to cold; while
others appear to be indifferent to climate, and to subsist equally well
under very various degrees of temperature. So far as is known, the land
forms are all indigenous to warm countries.

LAND FORMS.—Land forms, which occur on or in moist earth under stones or
decaying vegetable matter, have been discovered and described by
Semper,[146] Willemoes-Suhm,[146] and von Graff.[146]

The species found by Semper, and called by him _Geonemertes palaensis_,
lives under damp leaves and the roots of trees on Pelew Island in the North
Pacific. It is about 2 inches long, of a reddish-white colour, with narrow,
brownish-black, longitudinal stripes on its dorsal surface. It possesses
six eyes and very small cephalic slits and cerebral organs. The proboscis
is armed, and opens by the mouth instead of by a special pore.

The same peculiarity as to the opening of the proboscis is found in
_Geonemertes chalicophora_, discovered by von Graff in pots of _Corypha
australis_ in the palm-house at Frankfurt-on-Main. He found specimens on
and beneath the surface of the earth. As it was only found in pots in which
this Australian plant was growing, von Graff thought it almost certain that
it was a native of Australia. Those found below the surface of the earth
were surrounded by a transparent tube in which particles of earth were
embedded. The animal is small, only about two-fifths of an inch in length.
The colour is milk-white, with a small quantity of red pigment anteriorly:
there are four eyes, and the cephalic slits are absent.

{118}The species which was discovered by Willemoes-Suhm, and named by him
_Tetrastemma agricola_, lives under stones in damp earth in the Bermudas.
It differs from the other two in that the proboscis opens by a special
terminal aperture. It measures nearly an inch and a half in length, and,
like _G. chalicophora_, is milk-white in colour. It resembles it also in
possessing four eyes, and in the absence of cerebral organs and cephalic
slits.

FRESH-WATER FORMS.—In most cases the descriptions of fresh-water forms are
so vague and incomplete that it is difficult to determine whether or not
they are different species.

They are probably more numerous than is at present known, and are certainly
scattered widely over the face of the earth, since they have been found in
Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the
United States.

A form of which we have a full description is _Tetrastemma aquarum
dulcium_, found by Silliman[147] at Monroe, under stones in brooks in
company with Planarians. It is a small worm of a red or pink colour, about
half an inch in length, and it possesses usually three pairs of eyes. The
proboscis is armed, and opens by a separate aperture. The excretory system
consists of a vessel on each side of the body, each opening externally by a
pore, and internally dividing into numerous branches which end in ciliated
expansions. An individual of the same species was found by Beddard in one
of the tanks in the Botanical Gardens in Regent's Park, but as the tank is
one in which tropical plants are grown, it had almost certainly been
introduced among the roots of the plants, and cannot be considered as a
British species.

A fresh-water Nemertine belonging to the genus _Tetrastemma_ was, however,
found by Benham[148] on the roots of some water plants in the Cherwell at
Oxford. The specimen was of a bright orange colour and measured half an
inch in length.

Du Plessis[149] found another fresh-water form on the lower surface of
stones in shallow pools on the shores of the Lake of Geneva, and named it
_Tetrastemma lacustre_. It is a small animal, the largest specimens being
rather over an inch in length.

Another European genus was found in 1893 by F. E. Schulze in Berlin. It has
been fully described by T. H. Montgomery,[150] who has given it the name of
_Stichostemma eilhardii_.

{119}PARASITIC FORMS.—The genus _Malacobdella_ was found by von Kennel[151]
in large numbers living on _Cyprina islandica_, a Lamellibranch Mollusc, in
the harbour at Kiel; and it has also been described by Riches[152] as a
British form. It is attached to its host by means of a large round sucker
situated at the posterior end of the ventral surface, while the rest of the
body waves about freely in the mantle-cavity. It is perhaps hardly correct
to describe this animal as parasitic, since it does not appear to obtain
its nutriment at the expense of the host by preying on its juices. The
advantage of its position is, however, obvious, since a perpetual current
of water is kept up in the mantle-cavity of the Mollusc, and from the
stream the Nemertine is able to pick out and take for itself any food
material which it considers suitable. At the same time it is not subjected
to the influence of the winds and waves, as the shell of the mollusc acts
as a barrier to prevent the entrance of disturbing elements.

_Malacobdella_ is short and broad, somewhat flattened dorso-ventrally. The
anterior end is bluntly rounded. The mouth opens into a wide pharynx, which
is constricted behind and then passes into the intestine; this after a few
coils opens by the anus situated dorsally immediately above the sucker. The
proboscis opens into the pharynx.

[Illustration: FIG. 61.—_Malacobdella grossa_ O. F. Müll., a large female
specimen. Kiel. × 1. (From von Kennel.) A, From the dorsal surface; B, from
the ventral surface.]

PALAEONTOLOGY.—Nemertines are unknown in a fossil state; this is probably
owing to the softness of their bodies, which would render their
preservation extremely improbable.

AFFINITIES.—Until recently the Nemertines were regarded as a sub-order of
the Turbellaria. They were afterwards separated from the Turbellaria and
placed as a distinct class of the phylum Platyhelminthes.

Some zoologists have considered them to be so different in many respects
from the other classes of the Platyhelminthes as to justify their being
altogether separated from that phylum, and treated as a distinct group.

{120}If, however, the recent work of Bürger on the excretory system is to
be relied upon, the existence of flame cells would be a strong reason for
classing them among the Platyhelminthes.

Hubrecht[153] has instituted an interesting comparison between Nemertines
and Vertebrates. He compares the median dorsal nerve of Nemertines to the
spinal cord of Vertebrates; the lateral nerve-cords to the nerve of the
Vertebrate lateral line; and the lateral swellings which constitute the
brain in Nemertines to the lateral ganglia of the cephalic region in
Vertebrates. This view is strengthened by the existence of transverse
nerves connecting the lateral and dorsal nerves of Nemertines, since these
may be compared with the spinal nerves of Vertebrates. He suggests that
both Nemertines and Vertebrates may have arisen from a vermiform animal
possessing a nervous layer in the form of a plexus of nerve-fibres, the
nerve tissue having become concentrated along three lines to form a median
dorsal and two lateral nerve trunks; the former being specially developed
in the Vertebrata and the latter in the Nemertines. Hubrecht further
suggests that the notochord of Vertebrates may be a survival of the
proboscis sheath of Nemertines, while the proboscis of the latter may be
represented by the invagination to form the pituitary body in Vertebrates.

Certain authors[154] have suggested that indications exist of a
relationship between Nemertines and _Balanoglossus_.

The features which are supposed to indicate this are the elongated
vermiform shape showing no external signs of segmentation; the ciliated
smooth skin and the possession of unicellular mucous glands; and the
protrusible proboscis, which may be comparable to the non-retractile
proboscis of _Balanoglossus_, a comparison which is strengthened by the
fact that in some Nemertines a sheath of nerve-fibres exists in the wall of
the proboscis corresponding to the nerve plexus in the proboscis of
_Balanoglossus_. In both cases an ectodermic nerve plexus exists with local
thickenings along definite lines, although these lines are not the same in
the two cases. Both possess a straight alimentary canal, ending in a
terminal anus and thrown out into paired lateral caeca, between which are
the paired metamerically-arranged generative sacs.

NEMATHELMINTHES & CHAETOGNATHA

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge.

{123}CHAPTER VI

NEMATHELMINTHES

INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE—
STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE—
ENOPLIDAE—PARASITISM—NEMATOMORPHA—ANATOMY—CLASSIFICATION—
LIFE-HISTORY—ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION.

The Nemathelminthes include three sub-Orders of very different size and
importance. These are—

i. The Nematoda.
ii. The Nematomorpha (Gordiidae).
iii. The Acanthocephala.

Although the members of these groups differ considerably from one another,
on the whole there is a closer resemblance between them than between any
one of them and any other group of animals, and there is a certain
convenience in arranging them under one head.

The following characteristics are common to all three groups of the
Nemathelminthes: they are worm-like in form, and with few exceptions are
parasitic in the bodies of other animals, either Vertebrate or
Invertebrate. Some of them spend their whole existence within the bodies of
their hosts, but more commonly they are only parasitic during a certain
period of their life; a few, however, lead a free life in water or in damp
earth. None of the Nemathelminthes are segmented—that is, their bodies are
not divided into a number of parts which serially repeat each other, and
which resemble more or less closely the preceding and {124}succeeding
parts. They are not provided with any appendages or limbs, but sometimes
bear a few bristles or hooks, and in rarer cases suckers. The body, which
is elongated and, as a rule, thread-like and tapering at each end, is
enclosed in a thick cuticle or hardened secretion of the underlying cells.
In no Nemathelminth is there any closed vascular system, nor are special
respiratory organs developed.

In many respects the most remarkable peculiarity of these animals is that,
with the possible exception of the excretory organs of the Acanthocephala,
there is a complete absence of cilia throughout the whole group. In this
respect they resemble the Arthropoda. The universal presence of these small
flickering processes of cells from man down to the simplest unicellular
organisms makes the absence of these structures most remarkable. In many
animals they are the sole organs of locomotion, and in almost all they
perform very important functions, both in bringing food and oxygen to the
body, and in removing waste matter from it. At present there seems to be no
adequate explanation for their absence in the two large groups mentioned
above.

Nemathelminthes are, with hardly an exception, dioecious—that is to say,
their male and female reproductive organs are in different individuals.
Their young do not differ markedly from the adults, except in the absence
of sexual organs, but the immature stages are usually termed larvae, and
not infrequently either inhabit a different host from the adult, or are
free when the adults are parasitic, or _vice versâ_.

SUB-ORDER I. NEMATODA.

ANATOMY.—The Nematode worms, or thread-worms, form by far the largest and
most important division of the group Nemathelminthes. The number of species
is great, and although the conditions under which they live are of the most
varied kind, there is, as a rule, little corresponding difference in
structure, and hence the determination of the species is attended with no
small difficulty.

With few exceptions the shape of the body is filiform (Figs. 66 and 71),
the two ends being more or less pointed, and the posterior end of the male,
which is generally a smaller animal than the female, is usually slightly
recurved. The worms are, as a rule, {125}white, or of the colour of
polished ivory; they may be opaque or semi-transparent, but pigment spots
are rarely developed.

Minute Nematodes abound in moist soil, around the roots of plants, etc.,
and may easily be detected with the aid of a lens wriggling about amongst
the particles of sand and earth. Of the animal parasites perhaps the most
familiar is the "round worm" (_Ascaris lumbricoides_, Figs. 66 and 67),
which inhabits the alimentary canal of man; others are common in
domesticated animals, as _A. mystax_ in the cat and dog, and _A.
megalocephala_ in the horse and ox. They are also found living
parasitically in plants (Fig. 77), causing the formation of galls and other
pathological growths; _Anguillula_ (_Tylenchus_) _tritici_ causes in this
way considerable damage to corn, and others attack root-crops, cabbages,
etc. The "vinegar eel" (_Anguillula aceti_), which occurs so often in weak
vinegar, is another familiar example of this group.

THE SKIN.—The body of the worm is encased in a relatively thick,
transparent, smooth cuticle, which is turned in at the various apertures,
and lines the tubes connected with them for a greater or less distance. The
cuticle is in some cases raised to form spikes or hooks, and in certain
species, _e.g._ _Ascaris mystax_ and _A. transfuga_, it is produced into
two lateral fins, which are supported by a thickened triradiate rod of
specialised cuticle (Fig. 62); these fins, however, do not run far down the
body. As a rule the cuticle is quite smooth, but it may be ringed, as in
_Filaria laticaudata_ and in _F. denticulata_; and the rings may bear
backwardly-projecting teeth.

The skin of Nematodes consists of three layers—(i.) the above-mentioned
_cuticle_, which is presumably secreted by (ii.) the _sub-cuticle_ or
_epidermis_ which underlies it; the latter surrounds in its turn (iii.) the
_muscular layer_.

The nature of the sub-cuticle is one of the debateable points in the
morphology of the Nematoda. No cell outlines have been detected in it,
although nuclei are scattered through it; it is in fact a _syncytium_, or
protoplasmic mass in which cell limits cannot be distinguished. Many of the
cells forming it have broken down into fibrils, and these form a close
meshwork, which is occasionally specialised, as, for instance, round the
nerve-cords. Along the median dorsal and ventral lines, and along the
lateral lines, this tissue is heaped up in such a way as to divide the
{126}enclosed muscle-cells into four quadrants. These thickenings surround
dorsally and ventrally a specialised nerve-cord, and laterally the
excretory canals.

According to Jammes[155] this lack of differentiation in the sub-cuticular
layer is caused by the early appearance of the cuticle, which he thinks is
necessitated, at any rate in many of the parasitic forms, by the action
which the digestive juices of the host would have on the otherwise
unprotected body-wall.

[Illustration: FIG. 62.—A transverse section through the body of _Ascaris
transfuga_ Rud., in the region of the oesophagus: _a_, the muscular
oesophagus with its triradiate lumen; _b_, the cuticle; _c_, the
sub-cuticle; _d_, the muscular layer; _e_, the lateral nerves running in
the lateral line; _f_, the excretory canal; _g_, the dorsal, and _h_, the
ventral nerve; _i_, the triradiate rod in the fin.]

The nervous system, according to the same writer, is of the same nature as
this sub-cuticular tissue, only it is more differentiated, or perhaps we
should say it has retained more of the primitive cellular character of the
embryonic tissue. The fibres of the sub-cuticular tissue are closely
connected with the fibrils which compose the spongioplasm (Fig. 64, _d_) of
the muscles,[156] and form also the sheaths of the various nerves; in fact
the passage of these fibrils into the nerves is so gradual that it is
impossible to make any separation between them.

{127}THE NERVOUS SYSTEM.—The central organ of the nervous system is the
circumoesophageal ring which surrounds the pharynx, close to the anterior
end of the body, in _A. megalocephala_ 1½ to 2 mm. behind the mouth.[157]
Ganglion cells are found in the ring, but they are not numerous, and are
chiefly aggregated round the points of origin of the nerves.

Six short nerves, three on each side of the median line, run forward from
the ring, a pair of these ending in each of the three papillae which
surround the mouth.

Behind, the nerve-ring gives off six main nerve trunks, of which the dorsal
and ventral nerves are usually the largest. These run in the median dorsal
and ventral thickenings of the sub-cuticular tissue, and are connected one
with another by numerous fine lateral branches running through the
sub-cuticle.

[Illustration: FIG. 63.—Diagram of the nervous system at the two ends of
the body in _Ascaris megalocephala_ Cloq., ♂. (After Hesse.) _a_,
Circumoesophageal nerve-ring; _b_, opening of excretory ducts; _c_, dorsal
nerve; _d_, dorso-lateral nerve; _e_, ventro-lateral nerve becoming the
bursal nerve posteriorly; _f_, the ventral nerve; _g_, cloacal opening;
_h_, sub-cuticular nerves running from _c_ to _f_; _k_, spicules.]

The lateral nerves, which consist of two or four bundles, one or two lying
dorsal and one or two ventral to each excretory canal, have a double
origin. The dorsal branches arise directly from the nerve-ring, and at
their point of origin there is a considerable accumulation of ganglion
cells, from which two commissures on each side run into the ventral nerve
(Fig. 63, _f_). The ventral branches arise from the ventral nerve-cord
immediately in front of the excretory pore. At the posterior end the
lateral nerves pass into the two branches into which the ventral nerve
divides. Just before the point where the ventral {128}nerve splits it
swells out into an anal ganglion situated just in front of the anus. In the
male[158] this anal ganglion gives off two lateral nerves which pass round
the cloaca and form a ring, and in this sex the ventro-lateral nerve, which
is much strengthened by fibres from the ventral nerve, and has received,
owing to the mistaken impression that it was a special _nervus recurrens_,
the name of the "bursal nerve," gives off numerous branches to the sense
papillae which are found in this region of the body and on the tail. The
arrangement of these parts is shown in Fig. 63.

Sense organs are but poorly developed in the Nematoda, as is usual in
animals which are, as a rule, either parasitic or live underground. Eyes,
consisting of masses of dark pigment with or without a lens, occur in the
neighbourhood of the circumoesophageal nerve-ring in some free-living
forms. Leuckart described as possible auditory organs certain giant-cells
lying near the orifice of the excretory ducts. Later research has shown
these cells to have some phagocytic action on the contents of the
body-cavity. The chief sense organs are the papillae, of which in _A.
megalocephala_ there are two kinds, the lip papillae being distinguished
from the genital papillae by the fact that the nerve supplying them ends in
a fine point and pierces the cuticle in the former case, whilst in the
latter it swells out into an "end-organ," which is always covered by a
layer of cuticle, though sometimes by a very thin one.

MUSCULAR SYSTEM.—The muscular system is one of the most characteristic
features of the Nematoda, both as regards the histology of the muscle-cells
and the way in which the cells are arranged.

Each muscle-cell is of considerable size, and is of the shape of a somewhat
flattened spindle produced into a process near the middle. Each end of the
spindle cell is said to be continuous with the fibrils of the sub-cuticular
layer.[159] The muscle-cell consists of two portions, a contractile part
which lies next the sub-cuticle, and which usually, to some extent, wraps
round the second or medullary half. The latter consists of a fibrillar
spongioplasm, in the meshes of which lies a clear structureless hyaloplasm.
The nucleus always lies in the medullary half.

{129}The contractile portion consists of a number of columns, very
regularly arranged in two rows and close together, but allowing sufficient
space between adjacent columns for fibrils of the spongioplasm to
penetrate; and these become continuous with the fibrils of the sub-cuticle,
which is thus intimately connected with both nervous and muscular systems.

The medullary portion of the cell varies greatly in size; it may stretch
far into the body-cavity, which may be thereby almost occluded, or it may
be flattened out, leaving a large space around the alimentary canal. At one
point, usually about its middle, it is produced into a process, which bends
inwards towards the dorsal or ventral nerve-cord, and by means of this
process the muscle receives its nerve supply.

In most Nematodes there are numerous muscle-cells to be seen in any
transverse section, forming a layer within the sub-cuticle, and broken up
into four quadrants (Fig. 62) by the projection of the dorsal, ventral, and
lateral thickenings of the sub-cuticular tissue. In some genera, however,
such as _Oxyuris_, _Strongylus_, _Pelodera_, _Leptodera_, etc., there are
but eight muscle-cells in a row, two in each quadrant. Such genera are
classed together by Schneider,[160] and termed Meromyarii (_vide_ p. 137).

[Illustration: FIG. 64.—A, transverse section through the centre of a
muscle-cell; B, the same through a nerve fibre showing the sub-cuticular
fibres running into the sheath. (After Rohde.) _a_, Cuticle; _b_,
sub-cuticular fibres continuous with _d_; _c_, contractile columns; _d_,
network of spongioplasm; _e_, nucleus.]

In addition to the characteristic muscles of the body-wall there are
others, such as those which move the spicules in the male, which cross the
body-cavity obliquely near the anus, and such as sphincter muscles near the
latter orifice, which have not {130}the characteristic arrangement of
contractile and medullary parts described above.

THE BODY-CAVITY.—The skin of a Nematode, as described above, contains most
of the important organs of the body within its thickness. The chief
muscular system, the nervous system with its sense organs, and the
excretory organs are all embedded in or form part of the skin, which in its
turn encloses a cavity—the body-cavity—in which the other two systems of
organs which are found in Nematodes lie. These are the digestive system and
the reproductive system.

The body-cavity is continuous from one end of the animal to the other, and
is in no case divided up into compartments by the presence of septa or
mesenteries. It contains a coagulable fluid with numerous corpuscles; this
is, as a rule, colourless, but in _Syngamus trachealis_ Sieb. (Fig. 70),
which lives on blood, the haemoglobin of its host tinges it red, though the
colour is said to disappear if the parasite be isolated and starved.

The morphological nature of this body-cavity affords an interesting
problem. It is not a true _coelom_, such as exists in the earthworm, since
it is not surrounded by mesoderm, nor do the excretory organs, with the
possible exception of one or two genera, open into it, nor do the
generative cells arise from its walls. Essentially it is a space between
the mesodermic muscle-cells which line the skin and the endodermic cells of
the alimentary canal, and although in many of its functions it resembles
the coelom of other animals, its morphological character is quite
different.

There are no respiratory or circulatory organs in the Nematoda; possibly
the fluid in the body-cavity acts, to some extent, as a carrier of oxygen,
but from the inert and almost vegetative life of these animals it seems
probable that their respiratory processes are slow, and in fact Bunge[161]
has shown that _Ascaris mystax_, found in the intestine of the cat, will
live for four or five days in media quite free from oxygen, and that _A.
acus_ from the pike will live and exhibit movements in the same media for
from four to six days.

THE DIGESTIVE SYSTEM.—The mouth of the Nematoda is usually anterior and
terminal, and is surrounded by from two to six projecting lips, the most
common number being three. These {131}lips are well provided with sense
papillae. The mouth leads into an alimentary canal, which with hardly an
exception runs straight through the body to the anus without twists or
loops. The anus is usually placed ventrally and is not terminal, but in
_Trichina_ and _Trichocephalus_ it is at the end of the body, and in
_Mermis_, where the several parts of the alimentary canal are said not to
communicate, it is absent altogether. _Ichthyonema_, _Dracunculus_,
_Allantonema_, _Atractonema_, and other Filariae are also aproctous.

The alimentary canal is divisible into three parts—(i.) the oesophagus,
(ii.) the intestine, and (iii.) the rectum. The suctorial oesophagus is a
very muscular, thick-walled tube, lined with cuticle continuous with that
which covers the body, and like it cast from time to time. Its lumen is
usually much reduced, and is almost invariably triangular or triradiate in
section (Fig. 62). In many genera the hinder end of the oesophagus is
swollen into a muscular bulb, which is armed with teeth in _Heterakis_,
_Oxyuris_, _Pelodera_, _Leptodera_, etc. Other species, such as
_Tylenchus_, _Aphelenchus_, _Dorylaimus_, are armed with a spear, which in
_Onyx_,[162] a genus recently described and allied to the last named, is
borne on a special bulb. The use of the spear is to pierce the tissue upon
the juices of which the animal lives. A gland lies embedded in the thick
walls of the oesophagus, and opens into its lumen by a fine tube. This was
first described by Schneider[163] in _A. megalocephala_, and more recently
it has been found by Hamann[164] in a number of Ascaridae and Strongylidae
from the Adriatic, and also in _Lecanocephalus_.

With a few exceptions, such as _Mermis_, where it is blind, the oesophagus
opens posteriorly into the intestine. This is a somewhat flattened tube,
whose shape and position are often altered by the development of the
generative organs. Its wall consists of a single layer of columnar cells,
with large nuclei coated internally and externally by a layer of cuticle.
The inner layer of cuticle is usually perforated by very numerous minute
pores. In some species the intestine is degenerate, in _Mermis_ it is a
closed tube opening neither into the oesophagus nor into the rectum; in
_Trichina spiralis_ and in the larva of _Tylenchus tritici_ it consists of
a single row of cells perforated by a duct, but in the adult of the last
named there are many cells in a transverse section.

{132}[Illustration: FIG. 65.—A longitudinal section through the body of
_Strongylus filaria_ Rud. (From O. Augstein.[165]) A portion of the body,
on each side of the excretory pore, is seen in optical section. _a_, Mouth;
_b_, oesophagus; _c_, intestine; _d_, excretory canal; _e_, excretory pore,
and the opening of the poison glands, _i_; _f_, circumoesophageal
nerve-ring; _g_, ventral nerve; _h_, dorsal nerve; _i_, unicellular poison
glands; _k_, ovary, with the ova separate; _l_, oviduct; _m_, uterus, the
first egg in the uterus is surrounded by spermatozoa; _n_, opening of
uterus; _o_, inner end of ovary with the ova undifferentiated.]

{133}In some genera, _Leptodera_ and _Pelodera_, the lumen of the intestine
at any one level is bounded by two horseshoe-shaped cells, but by far the
commonest arrangement is a tube formed of fairly numerous columnar cells
crowded with granules and with large nuclei.

The rectum is usually short; its cuticular lining, like that of the
oesophagus, is cast at intervals. At its anterior end there is usually a
sphincter muscle, and its walls are divaricated by muscular strands which
run from it to the body-wall. The anus is a transverse slit, which in the
male Strongylidae is surrounded by a funnel-shaped membrane.

The food of Nematodes seems to be almost entirely fluid, and consists, at
any rate in the parasitic forms, of the elaborated juices of their hosts.
Little is known about the nutriment of the free-living forms.

THE EXCRETORY SYSTEM.—The excretory organs are peculiar, and, like many
other Nematode structures, do not fall readily into line with what is known
of similar organs in other animals. They consist of two canals embedded in
the lateral thickenings of the sub-cuticular tissue. The canals end blindly
behind, but near the anterior end of the body they bend inwards, and after
uniting, open by a common pore situated in the middle ventral line, a
little way behind the mouth. The lateral canals are in some cases continued
in front of the transverse branch, and they then end blindly in the head.
The walls of these canals consist of an internal, structureless, refractive
layer surrounded by a granular layer with nuclei. They contain a fluid, but
nothing is known of its composition.

An interesting divergence from the usual form of excretory organ has been
described by Hamann[166] in the genus _Lecanocephalus_. Here there is only
one canal, the right; anteriorly this bends towards the ventral surface and
opens by a small median pore close behind the nerve-ring. Posteriorly the
canal does not extend much beyond the middle of the body, where it forms a
coiled mass, and diminishing in size, opens into the body-cavity. The same
author also states that both canals in _Dochmius_ have a similar internal
opening; these observations, if confirmed,[167] show a conformity to the
ordinary structure of {134}excretory organs which was not supposed to exist
in the lateral canals of the Nematoda.

THE REPRODUCTIVE ORGANS.—With the exception of the genera _Angiostomum_,
_Pelodytes_, and of _Rhabdonema nigrovenosum_, which are physiologically
hermaphrodite and self-impregnating, the Nematodes have separate sexes. The
males are, as a rule, smaller than the females, and may usually be
distinguished by the posterior end of the body being curved towards the
ventral surface; a genital bursa, and one or more spicules are often found
in this sex. Further, the position of the genital opening differs; in the
male the vas deferens opens on the ventral surface of the rectum close to
the anus, but the oviduct in the female opens in the ventral middle line,
usually near the middle of the body, but sometimes close behind the
excretory pore, or in some Strongylidae just in front of the anus. The tail
of the male bears very numerous papillae, which are of considerable
systematic importance.

[Illustration: FIG. 66.—_Ascaris lumbricoides_ Cloq. ♂, natural size, cut
open along the dorsal middle line. _a_, Oesophagus; _b_, intestine; _c_,
testis; _d_, vas deferens; _h_, lateral excretory canals.]

With rare exceptions, _e.g._ _Filaria attenuata_, where it is double, the
male reproductive organ consists of a single tube divisible into a testis
proper, a vas deferens, a vesicula seminalis, where the spermatozoa are
stored up, and a ductus ejaculatorius. The tube stretches through the body
in a straight line in the small free-living forms, but is thrown into loops
and coils in the larger parasitic Nematodes. Within the testis the
mother-cells of the spermatozoa are attached to a rhachis or axial cord;
the mother-cells divide, and their products ultimately form spermatozoa.
The latter have a very peculiar shape; in accordance with the universal
absence of cilia in the Nematoda the spermatozoon has no flagellum, and at
first consists of a spherical nucleated cell, on one side of which a cap or
covering of some refractive substance appears. The cap elongates and
{135}becomes conical, whilst the protoplasmic portion of the spermatozoon
throws out pseudopodia and becomes amoeboid, but ultimately rounds itself
off again. The spermatozoa do not attain maturity until they reach the
uterus of the female.

The internal female reproductive organs are, with few exceptions
(_Trichina_, etc.), double, but the vagina, which is lined with cuticle
continuous with that covering the body, is always single. They are usually
much coiled, and may be divided into ovary, oviduct, and uterus. The ova
arise from a polynucleated mass of protoplasm or syncytium (Fig. 65, _o_)
at the upper end, and acquire distinctness as they approach the oviduct.
Fertilisation takes place in the uterus, but the segmentation may not begin
until some time after the eggs are laid; in _Dochmius_, however, it is well
advanced at this period, and in many genera, _e.g._ _Pseudalius_,
_Trichina_, _Dracunculus_, etc., the whole development of the larva takes
place in the body of the mother.

[Illustration: FIG. 67.—_Ascaris lumbricoides_ Cloq. ♀, natural size, cut
open along the median dorsal line to show the internal organs. _a_, The
muscular oesophagus; _b_, the intestine; _c_, the ovary; _d_, the uterus;
_e_, the vagina; _f_, the external opening; _h_, the excretory canals; _i_,
their opening.]

EMBRYOLOGY.—The eggs of many of the parasitic forms require a considerable
degree of warmth to develop. Those of _Ascaris lumbricoides_ require a
temperature of 20° C., those of _Trichocephalus_ 22.5° C., and those of
_Oxyuris vermicularis_, 40° C. The latter develop in a few hours, the eggs
of _Dochmius_ in a few days, whilst those of _A. {136}lumbricoides_ take
weeks or even months, and the young of _Trichocephalus_ seldom develop
within a year.[168] The ova only develop in a damp atmosphere, and they can
be arrested at almost any stage, and for considerable periods, by
desiccation.

Our knowledge of the processes by which the fertilised egg-cell develops
into the larva is very imperfect. As a rule the segmentation is complete
and equal; it results in the formation of a blastula, which may take the
form of a hollow sphere of cells—_A. megalocephala_—or the cavity may be
reduced, and the blastula may consist of a double-layered plate, as in
_Cucullanus_.[169] The distinction into cells which will form the three
embryonic layers, the ectoderm, mesoderm, and endoderm, is very early
evident,—in the eight-cell stage. By the growth of one side of the blastula
and the tucking in of the other the blastula becomes converted into a
gastrula, which is a two-layered stage with a cavity opening to the
exterior by a pore termed the blastopore. In Nematodes the blastopore is
elongated and slit-like; it either forms the mouth (_Cucullanus_) or closes
from behind forwards, the mouth ultimately arising at the point where the
blastopore finally closed (_Rhabdonema nigrovenosum_). The mesodermal cells
lie between the ectoderm and the endoderm; they ultimately develop into the
muscles of the body-wall, the lateral excretory canals, and the
reproductive organs; the last-named two systems arise each[170] from a
single cell. The nervous system arises from the ectoderm, which also forms
the sub-cuticle, and is turned in slightly at the mouth and anus; the
remainder of the alimentary canal develops from the endoderm.

The post-embryonic development, which is very variable, and in many cases
very extraordinary, will be dealt with under the several families.

CLASSIFICATION.—The classification of the Nematodes is a matter of very
considerable difficulty; their structure is unusually monotonous, and,
owing perhaps to their largely parasitic mode of life, they show
practically none of those external features which are so useful to the
systematist in other groups. Schneider in his Monograph divides the group
into three subdivisions—(i.) {137}the POLYMYARII, in which numerous muscle
cells are seen in a transverse section; (ii.) the MEROMYARII, in which only
eight are seen, two in each quadrant; and (iii.) the HOLOMYARII, in which
the muscles are either not divided, or only divided by longitudinal lines.
This grouping has, however, to some extent broken down, since Bütschli[171]
and others have shown that the third subdivision is founded on insufficient
observation, whilst the first two include, in different subdivisions,
Nematodes which are closely allied in all respects except as regards their
muscle cells.

The details of the life-history have been used by other writers as a basis
of classification. Linstow[172] enumerates fourteen distinct modifications
of the post-embryonic development (_vide_ p. 159), and Örley[173] has
grouped these under three headings. The animals which fall under each group
to some extent resemble one another in structure. Örley's groups are:—

(i.) NEMATOZOA.—Thread-worms with free larval life, the mature forms being
parasitic in animals. Enormous numbers of eggs are produced, and the
development is indirect. The genital organs are complicated by many
convolutions.

(ii.) RHABDITIFORMAE.—Small, as a rule microscopic, thread-worms, usually
living free, but rarely parasitic. They become sexually mature only in
decomposing organic substances, or in earth saturated with such substances.
They live gregariously and do not produce immense numbers of ova. The
metamorphosis is slight, or is complicated by sexual metamorphosis. The
oesophagus has two dilatations. The genital tubes are simple and not
coiled.

(iii.) ANGUILLULIDAE.—Small microscopic thread-worms, with a free existence
in mould or water, throughout all stages. They produce large eggs. They are
provided with a caudal sucker and bristles, sometimes with eyes and other
structures characteristic of a free life. Genital tube simple and not
coiled.

The disadvantage of such a system is, that to accurately place a specimen
in its proper class we must be acquainted with its life-history, and this
is known in but few cases.

The determination of the species to which a Nematode belongs is a matter of
considerable difficulty. Amongst the more important features for purposes
of classification are the arrangement of the {138}muscles, the character of
the tail in the male, especially when papillae are present, the number and
the size of the spicules, and the arrangement of the lips and mouth-parts
generally.

Cobb[174] has recently devised an ingenious formula in which measurements
of different parts of the body appear as percentages of the whole length of
the body. The nature of this will be understood by reference to Fig. 68.
Such a formula should, however, be used with caution, since it rests on the
assumption that the proportions of the various parts of the body are
constant in different individuals, and it is by no means certain that this
is the case.

[Illustration: FIG. 68.—Diagram to explain the descriptive formula used for
Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements,
while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements.
The formula in this case is

7 14 28 50 88
-------------
6 7 8 10 6

The unit of measurement is the one-hundredth part of the length of the
worm. The measurements are therefore percentages of the length.

The measurements are taken with the animal viewed in profile; the first is
taken at the base of the oesophagus, the second at the nerve-ring, the
third at the cardiac constriction, the fourth at the vulva in females and
at the middle in males, the fifth at the anus.]

Taking everything into consideration, it has seemed advisable in the
following systematic account of the Nematoda to abandon the larger groups,
and to deal directly with the families. Claus distinguishes seven of these,
and the diagnoses given at the head of each are mainly taken from his
_Grundzüge der Zoologie_.[175]

I. FAMILY ASCARIDAE.

Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae.
Buccal cavity distinct, seldom provided with chitinous armature. The
oesophagus often has two dilatations. {139}The tail of the male is
ventrally curved, and usually there are two horny spicules. The Ascaridae
are found in the intestines of their respective hosts.

Genera: _Ascaris_, _Heterakis_, _Oxyuris_, _Nematoxys_, _Oxysoma_, and many
others.

Von Linstow[176] enumerates over 250 species of _Ascaris_, of which it will
only be possible to mention here one or two. They are all parasitic in
Vertebrata.

_A. lumbricoides_ Linn. is one of the largest known Nematodes ♂ = 4-6 in.,
♀ = 10-14 in.; Figs. 66 and 67). It is a common parasite in man, and has
been found in the ox. It is now generally recognised as the same parasite
which inhabits the pig, and which Dujardin regarded as specifically
distinct, and named _A. suillae_. In the latter host, however, it never
attains the dimensions it does in man. It inhabits the upper and middle
parts of the small intestine, and has been known to escape into the
body-cavity and set up abscesses there, or to make its way into the
stomach, and to be voided through the mouth. It is practically cosmopolitan
in distribution, and is very common in Japan—Baely found it in twenty-one
out of twenty-three _post-mortems_—and in Tonquin and tropical Africa.
Heller[177] states that no one is free from these worms in Finland, and
they are common wherever there is a plentiful water supply, as in the
marshy districts of Holland and Sweden. In Iceland alone they seem absent.
When examined alive they give off an irritating vapour which seriously
affects some observers, causing catarrhal symptoms, which in Bastian's case
lasted six weeks. The usual number found in one host is small, one to six
or eight, but cases are on record where many hundreds occurred in one
person.

The details of the life-history of this form are not yet completely worked
out. The eggs leave the body of the host with the excreta, and formerly it
was thought they re-entered the alimentary canal in drinking-water, etc.,
and there developed into the adult without change of host. This view has
been combated by Leuckart, who failed to rear the Nematodes by direct
feeding, and it has been noticed that the youngest parasites found in the
{140}intestine are already 2 to 3 mm. long. Von Linstow has recently
suggested that the larval stages may be hatched out in the body of the
millipede _Julus guttulatus_, whose habits might easily lead it to eat the
eggs of the parasite in manured gardens, etc., and which is itself
sometimes unconsciously eaten when hidden in fruit or vegetables. This
would account for the frequent presence of the parasite in pigs, and also
for the fact that in man it is commonest in children who are apt to eat
windfalls, and in maniacs and people with perverted tastes.

_A. megalocephala_, which is found in the horse, ass, zebra, ox, etc.,
attains even greater dimensions than the foregoing. The male rarely exceeds
7 inches in length, but the female sometimes reaches 17 inches. They are
found in the small intestine of their hosts. Cobbold[178] succeeded in
rearing larvae which attained a high degree of organisation when the eggs
were placed amongst moist horse-dung, and it seems probable that the larvae
pass into the body of their hosts in drinking water; at any rate no
intermediate host has yet been found, and Davaine, who fed cows, and
Leuckart, who fed horses with the unhatched eggs, both failed to infect the
animals they experimented on. _A. mystax_, which lives in cats, dogs, and
other Carnivora, has also been found in man. It is provided with fin-like
extensions on the side of its head (cf. Fig. 62), and varies much in size
in different hosts. When first found in man it received the name of _A.
alata_. It becomes sexually mature in about three weeks.

One of the most remarkable cycles of development amongst the many curious
life-histories met with amongst Nematodes, is that presented by
_Rhabdonema_ (_Ascaris_) _nigrovenosum_. The free form of this, formerly
known as a distinct species, _Rhabditis nigrovenosa_, lives in the
excrement of frogs, and attains sexual maturity in a very short time. The
sexes pair, and the fertilised ova give rise to embryos which hatch out
within the body of the mother, and then begin to devour her internal
organs. After the destruction of the mother, the embryos escape and live in
water or slime, and sometimes burrow into water snails, but they undergo no
change until swallowed by a frog. Then they make their way into its lungs
and grow enormously, attaining a length of almost an inch. This form,
parasitic in the frog, is a protandrous hermaphrodite, which first produces
spermatozoa and afterwards {141}ova; the latter are fertilised by the
spermatozoa, and give rise to rhabditiform embryos, which escape by the
alimentary canal and form the free-living sexual stage mentioned above.
Thus in the life-history of this form we find an alternation of generation,
a sexual free-living form alternating with a hermaphrodite parasitic form.

Of the enormous number of other species of the genus, only a very few can
be mentioned. _A. transfuga_ Rud. inhabits bears; _A. leptoptera_ Rud.,
lions; _A. ferox_ H. and Ehrbg., Hyracoidea; _A. depressa_ Rud., vultures;
_A. rubicunda_ Schn., pythons; _A. sulcata_ Rud., turtles; _A. mucronata_
Schn., the cod and pike; _A. incurva_ Rud., the sword-fish.

[Illustration: FIG. 69.—A male and female _Oxyuris diesingi_ Ham. _in
copula_, × 60. _a_, Anus; _b_, oesophagus; _c_, bulb; _d_, testis; _e_,
intestine; _f_, ovary. (From Galeb.[179])]

_Oxyuris_ is Meromyarian (see p. 137), and is characterised by the long
capillary tail of the female. It includes another human parasite, _O.
vermicularis_, and it is one which it is difficult to get rid of. The
female has the characteristic tail and is about 10 mm. long. The male is
smaller. They are found in the caecum and rectum of man, and cause great
irritation and sometimes serious functional disturbance. The eggs are laid
in immense numbers but perish in water. If whilst still in the egg-shell
the larvae are swallowed on fruit or raw vegetables, etc., they are set
free in the stomach and small intestine by the action of the digestive
secretions. The distribution of this parasite is universal. Besides
numerous species that inhabit the alimentary canal of Vertebrates, such as
_O. ambigua_ Rud., found in hares and rabbits; _O. curvula_ Rud., in
{142}the caecum of horses; _O. megatyphlon_ Rud., in iguanas; several
species inhabit the rectum of insects, such as _O. blattae_, _O. diesingi_,
_O. blatticola_, found in the cockroach; _O. spirotheca_, and _O.
hydrophili_ in the water beetle _Hydrophilus_.[180]

The genus _Nematoxys_ has the most complex arrangement of muscles of any
Meromyarian, and forms a transition to the Polymyarian type. The whole body
of both sexes is covered with numerous irregularly scattered papillae. The
members of this genus have hitherto been found in snakes, Amphibia, and
eels; there are but few species.

_Oxysoma_ is another small genus with but three species, found in the
intestines of opossums, frogs, and turtles respectively.

II. FAMILY STRONGYLIDAE.

Mouth surrounded by papillae; an armature of teeth or spines often present.
The chitinous lining of the intestine projects into the interior as ridges.
No oesophageal bulb. The male orifice at the posterior end of the body is
surrounded by a bell-shaped bursa.

Genera: _Eustrongylus_, _Strongylus_, _Dochmius_, _Sclerostomum_,
_Cucullanus_, _Syngamus_, _Pseudalius_, _Ollulanus_, and others.

The genus _Eustrongylus_ includes two species, _E. gigas_ Rud. and _E.
tubifex_ Nitsch. The former attains in the female the gigantic length of
860 mm., with a breadth of 7 mm. and a weight of over 40 grs.[181] The male
is a quarter to a third as long as the female. This parasite inhabits the
kidney capsules of carnivorous animals, especially of those that eat fish,
such as dogs, seals, etc., and has occasionally been found in man, the
horse, and the deer. It frequently destroys the substance of the kidney.
The worms are red in colour. The eggs die when exposed to desiccation for a
few days, but have been kept alive for fifteen months in water; it is
believed by Schneider and Leuckart that they are eaten by fish, and that
the larvae form the _Filaria cystica_ found in the peritoneal membrane of
the fishes _Galaxias scriba_ and _Symbranchus laticaudatus_, and that they
pass into their final host, where they become sexually mature, by the
latter eating raw fish. _E. tubifex_ is found in aquatic birds, _e.g._
ducks, grebes, and divers, etc.

{143}The genus _Strongylus_ is easily recognised by its conspicuous genital
bursa, strengthened by variously arranged ridges which are of specific
value. There are numerous species, found in man and many other mammals, and
also in birds and reptiles. Some species inhabit the intestine, others form
aneurisms in the large blood-vessels, and cause considerable mortality
amongst horses; others live in the tracheae and lungs of cattle and sheep,
their presence often causing great loss to the farmer. No intermediate host
has been satisfactorily demonstrated; the larvae live in damp earth, and it
seems almost certain that they pass directly into their host with its food.

_Dochmius_ (_Ancylostomum_) _duodenalis_, called by Neumann[182] _Uncinaria
duodenalis_, is one of the most dangerous parasites that attack man. It
lives in the duodenum and jejunum, and the fertilised eggs leave the body
of its host with the excreta, and in damp earth develop into larvae in the
course of a few days. These at first eat voraciously, but after undergoing
several moults they cease to take food and pass into the resting stage. If
now they are swallowed with drinking water, they come to rest in the small
intestine of their host, and in a few weeks become sexually mature. They
cause great harm by burrowing in the intestinal walls and destroying the
capillaries. They are found by hundreds, and even thousands, in the same
host, and produce profound anaemia, which is frequently fatal to miners,
and was the cause of a great mortality amongst the workers in the St.
Gothard Tunnel some fifteen years ago. This species is very widely spread
over the face of the globe. _Dochmius trigonocephala_ Rud. and _D.
stenocephala_ produce similar diseases in dogs and cats, and _D. cernua_
Crep. is found in sheep and goats.

The genus _Cucullanus_ exists in the adult form in the intestines of
fishes, and more rarely of reptiles. _C. elegans_ Zed., which live in
fresh-water fish, _e.g._ the perch, is viviparous; after birth the young
pass into the water and make their way into the alimentary canal of the
small crustacean _Cyclops_, and thence into its body-cavity. Here they
undergo two moults, accompanied by certain changes in structure. If this
second host be swallowed by a fish the parasites are set free, and develop
generative organs. {144}_Ollulanus tricuspis_ Leuck., which in the adult
state is found in the cat, chiefly in the intestine but also in the bronchi
and other parts, gives rise to larvae which are of enormous size compared
with the parent; these leave the body, and if eaten by a mouse encyst in
its muscles, and if the mouse be devoured by a cat, they complete their
life-cycle by becoming sexually mature.

The genus _Syngamus_ infests the trachea and bronchi of birds, more rarely
of mammals. The _red-_ or _forked-worm_, _Syngamus trachealis_ Sieb., is
common in poultry and game birds, and causes the disease known as gapes,
which is especially common in young birds, and often gives rise to
extensive loss. The peculiarity of this genus is that the male is
permanently attached to the female, its genital bursa being so closely
adherent to the opening of the oviduct that two specimens cannot be
separated without tearing the tissues. The ova are not laid, but escape
from the body with fully-formed embryos in them, by the decay or rupture of
their parent's body. They hatch in damp earth or water in from one to six
weeks according to the temperature. When swallowed by a fowl they develop
into adults, which reproduce eggs in less than three weeks. No second host
is needed, but the embryos remain alive in the alimentary canal of
earthworms, and these doubtless to some extent serve to spread the disease.

[Illustration: FIG. 70.—_Syngamus trachealis_ Sieb., natural size and
magnified four diameters. The small ♂ is permanently attached to the
female. (From Warburton.[183])]

III. FAMILY TRICHOTRACHELIDAE.

This family is characterised by the anterior end of the body being produced
into a long whip-like neck. The mouth is small and devoid of papillae. The
oesophagus is very long, and it traverses a peculiar strand of cells.

Genera: _Trichocephalus_, _Trichina_, _Trichosoma_, and others.

{145}_Trichocephalus dispar_ Rud. (_hominis_ Gmel.) is common in man, and
also occurs in some species of monkey. It does not live freely within the
intestine, but buries its long whip-like anterior end in the mucous lining
of the caecum or colon. The eggs pass out of the body of the host. The
development of the embryo is slow, lasting many months; whilst still in the
egg-shell the embryos are swallowed, and give rise to the sexually-mature
parasite without the intervention of an intermediate host. They are by no
means uncommon. Davaine calculated that about 50 per cent of the
inhabitants of Paris were infested with them, but they give rise to little
disturbance, and only very occasionally cause serious harm. _T. affinis_
Rud. infests sheep; _T. crenatus_ Rud. the pig; _T. depressiusculus_ Rud.
the dog; and _T. unguiculatus_ Rud. the hare and rabbit.

[Illustration: FIG. 71.—_Trichocephalus dispar_ Rud., attached to part of
the human colon. × 2.]

The genus _Trichosoma_, with many species, is as a rule found in birds, but
it occurs also in mammals, as _T. plica_ Rud. in the bladder of the fox and
wolf, _T. felis cati_ in the bladder of the cat, _T. aerophilum_ Duj. in
the trachea of the fox and marten. The chief interest of this genus is
that, at any rate in _T. crassicauda_ Bel., which infests the rat, the
dwarf males live two, three, or four at a time within the uterus of the
female, a condition of things which recalls the similar arrangement found
in the Gephyrean _Bonellia_.

_Trichina spiralis_ is the cause of the well-known disease trichinosis,
which appears in two forms, intestinal and muscular, according to the
habitat of the parasite. The mature forms of {146}both sexes are found in
the intestine of man and many other mammals. They have been experimentally
developed in birds, though in the latter the larval forms have never been
observed. By keeping such cold-blooded animals as the salamander at a
constant temperature, Goujon and Legros succeeded in infecting them, but
the larvae perished as soon as the artificial heat was withdrawn. Muscular
trichinosis is unknown in fishes, but the sexual form develops in their
intestine.

The adult parasites of the intestine are scarcely visible to the naked eye;
the females are 3 to 4 mm. long and more numerous than the males, which
measure 1.4 to 1.6 mm. The eggs are very numerous, a single female
containing at one time 1200, and probably producing ten times as many
during her life. The embryos are hatched out within the uterus, and the
larvae leave the body of the mother through the generative pore. The minute
larvae bore through the intestinal walls of their host, and then, either
burrowing in the tissues or swept along in the stream of blood or lymph,
make their way all over the body, and come to rest most usually in the
muscles, but occasionally in other parts. When the larva reaches its
resting-place, it either pierces the sarcolemma and establishes itself
within the substance of the muscle-fibre, or it comes to rest between and
not in the fibres. Here its presence sets up the formation of a
spindle-shaped cyst which usually contains but one larva, though any number
up to seven have been found in one cyst. Within this the larva may remain
dormant for years, the walls of the cyst gradually undergoing a fatty or
calcareous degeneration. Almost any muscle may be affected; those most
usually infested being the muscles of the diaphragm, of the shoulder-blade,
and of the lumbar region; the larvae have also been found in the heart. The
ends of the muscles near their points of attachment are always the most
thoroughly infested.

[Illustration: FIG. 72.—_Trichina spiralis_ Owen, encysted in muscle. _a_,
Calcareous deposit. Highly magnified. (From Leuckart.)]

The number of the encapsuled larvae in one host is enormous. Leuckart
counted between 12,000 and 15,000 in a gramme of muscle, which would give a
total of thirty to forty million parasites in one host; other estimates
place the total even higher.

{147}When trichinised meat is eaten, unless it has been thoroughly cooked,
the cysts are dissolved and the larvae are set free. Within three or four
days they become sexually mature and their ova begin to segment. The males
after a time leave the body with the excreta and perish, whilst the larvae
of the new brood make their way into the tissues of the host.

Man usually acquires trichinosis by eating uncooked or improperly-cooked
pork, and the disease is so widely spread and of such a serious nature that
most civilised countries have adopted rigorous methods for the detection of
trichinised meat. The pigs either acquire the disease by eating uncooked
swine's flesh, which is frequently given them in the form of offal, or by
devouring rats, which are very susceptible to the disease.

IV. FAMILY FILARIIDAE.

Mouth with two lips, or without lips. Six oral papillae often present, and
sometimes a horny oral capsule. Four pre-anal pairs of papillae, and
sometimes an unpaired one as well. Two unequal spicula or a single one.

Genera: _Filaria_, _Ichthyonema_, _Hystrichis_, _Spiroptera_,
_Dispharagus_, and others.

The genus _Filaria_ is a very large one. Like _Ascaris_, it is confined to
Vertebrates, but usually lives in the tissues of the body and not in the
intestines. _F._ (_Dracunculus_) _medinensis_ Gmel., the guinea-worm, is
well known as a human parasite in hot countries; it also occurs in the
horse and dog. The female has an average length of 50 to 80 cm., but
gigantic forms with a length of 4 metres have been described. The
alimentary canal is degenerate. In adult females the body is completely
occupied by a uterus crowded with eggs and embryos, which can only escape
by the rupture of the mother's body, as the genital ducts have disappeared.
Its original home is tropical Asia and Africa, but it has been introduced
into South America with the negroes.

The female lives coiled up in the subcutaneous tissues, usually in those of
the legs. Its presence gives rise to painful tumours. When these break the
female protrudes, and may be withdrawn from the body by very carefully
rolling it round a stick or pencil. This must be done very slowly, a few
inches a day, as the rupture of the body sets free the contained embryos,
and may result in {148}the death of the host. The embryos normally bore
their way into the body of the fresh-water _Cyclops_, and are re-introduced
into their Vertebrate hosts with the drinking-water. It is usually stated
that the female alone is known, and that it is uncertain whether it is
hermaphrodite or whether both sexes are present in the _Cyclops_. Recently
Dr. Charles[184] has described a specimen found in the mesentery of a human
subject, from an orifice in the middle of whose body he was able to draw a
much smaller specimen, and he thinks this may be the long-sought-for male.

[Illustration: FIG. 73.—A, View of the heart of a dog infested with
_Filaria immitis_[185] Leidy; the right ventricle and base of the pulmonary
artery have been opened. _a_, Aorta; _b_, pulmonary artery; _c_, vena cava;
_d_, right ventricle; _e_, appendix of left auricle; _f_, appendix of right
auricle. B, A female _F. immitis_ removed from the heart to show its
length. Natural size.]

_Filaria immitis_ Leidy, the cruel worm, is common in dogs in China and the
East generally. It is not unknown in America and Europe. It occurs in such
large clusters in the right ventricle that it is difficult to see how the
circulation can proceed. The intermediate host is unknown, but from the
prevalence of the {149}disease in marshy country it is probably some
aquatic animal. The larvae are said by Manson to disappear from the
peripheral circulation of the dog during the day, but not to such a marked
extent as do the _F. sanguinis hominis_ Lew., var. _nocturna_ Man. They
were found by Galeb and Pourquier in the foetus of an infested bitch, a
fact which establishes the transmission of such parasites through the
placenta.

_Filaria sanguinis hominis nocturna._—The female of this parasite has been
described as living in the lymphatic glands of man. The embryos escape from
it into the lymph, and thus reach the blood. According to Manson the
intermediate host is the mosquito, in whose stomach the embryos undergo
their larval changes. When the mosquito dies the larvae escape into the
water, and then make their way into the alimentary canal of man, where they
are believed to pair, and whence the female makes its way to the
lymphatics. The presence of this _Filaria_ causes great functional
disturbance. One of the most remarkable features of it is that the larvae,
which are very numerous in the blood during the night, disappear during the
day, and are not to be found. Recently Manson[186] has described two new
varieties: _F. san. hom. diurna_, in which the conditions of things are
reversed, the larvae being found by day and not by night; and _F. san. hom.
perstans_, in which the larvae occur both by day and by night. The larvae
are long-lived, and were found by Manson in the blood of a negro who had
not been in Africa, where it is endemic, for six years. The same observer
is inclined to associate the presence of _F. san. hom. perstans_ with the
fatal disease known as "sleeping sickness." He also suggests that the
mature form of the variety _diurna_ is the _F. loa_, which is not uncommon
in the eyes of negroes, and that its intermediate host may be one of the
blood-sucking flies so common on the west coast of Africa.

The genus _Ichthyonema_ is confined to fishes. The male is very minute and
the female partly degenerate. It has no anus and no external opening to its
generative organs. The uterus fills up almost the whole of the body-cavity.
_I. sanguineum_ Rud. is found encapsuled in the peritoneum of many fish.

_Hystrichis_ and _Dispharagus_ are confined to birds, where they occur in
the oesophagus and stomach. _Spiroptera reticulata_ {150}Crep. occurs in
horses, twisted in a spiral round tendons and muscles, forming tumours
which require to be opened.

V. FAMILY MERMITHIDAE.

Nematodes without anus and with six mouth papillae. Two spicules in the
males and three rows of numerous papillae.

Genera: _Mermis_, _Bradynema_, _Atractonema_, _Allantonema_,
_Sphaerularia_, and others.

As a rule the Nematoda show but little trace of their parasitic mode of
life, but in this family there is considerable degeneration, and in extreme
cases the body of the female is reduced to a simple sac crowded with eggs.
They are exclusively parasitic in insects. In some respects their structure
shows a transition towards _Nectonema_ and the Gordiidae; especially is
this the case in the structure of their ventral nerve-cord.

The sexual form of _Mermis nigrescens_ Duj.[187] lives in damp earth, and
after storms and in the early morning is sometimes found in such numbers
crawling up the stalks of plants, as to give rise to the popular idea that
there has been a shower of worms. The male is unknown; the female lays her
eggs in the ground, and there they hatch out. It is not known exactly how
the larvae make their way into the grasshoppers in whose body-cavity they
live, but in an allied species, _M. albicans_ v. Sieb., the larvae have
been observed boring their way into small caterpillars through their skin,
and it seems probable that the larvae of _M. nigrescens_ burrow in a
similar way into young Orthoptera.

_Bradynema rigidum_ Leuck.[188] is found in the adult stage living freely
in the body-cavity of a small beetle _Aphodius fimetarius_, one of the
Scarabeidae, from two to three to as many as thirty being found in one
host, which does not seem much injured by their presence. The parasite is
without mouth, anus, or excretory pore. The eggs hatch out in the uterus of
the mother, and the larvae are male and female; they make their way into
the body-cavity of the host, and here they pass an unusually long time,
five months, soaking in osmotically the nutriment contained in the blood of
the insect. Eventually they burrow through the walls of {151}the intestine,
and leaving the body of their host through the anus, find their way to the
earth. Here, according to zur Strassen, the females die without playing any
part in the perpetuation of the species. The males, on the other hand,
having developed spermatozoa whilst in the larval stage (paedogenesis),
afterwards form ova, and are in fact protandrous hermaphrodites, and become
the mature parasites of the beetle, though how they enter the body of the
host is unknown.

[Illustration: FIG. 74.—_Allantonema mirabile_ Leuck. (From Leuckart.) A,
Male Rhabditis stage, sexually mature, × 100; B, the mature female
parasitic form, × 17, showing at the upper end part of the capsule richly
supplied with the tracheae of the host, a beetle; C, female Rhabditis
stage, sexually mature, × 100; D, the larva developed from the Rhabditis
form, × 102.]

The phenomenon presented by the hermaphroditism of _Bradynema_ is, as far
as we know, at present unique, as, though some other Nematodes are
hermaphrodite, in their case the hermaphrodite form alternates with a
bisexual generation. It is further interesting as showing a means by which
hermaphroditism may arise, by the suppression of the females and the
assumption of their functions by the male. In the case of _Rhabdonema
nigrovenosum_, no females appear in the alternate generation.

{152}[Illustration: FIG. 75.—_Atractonema gibbosum_ Leuck. (From Leuckart.)
1, Female with commencing prolapsus of the uterus and neighbouring parts, ×
130; 2, a further stage, the female being now sexually mature, × 15; 3, a
still older stage, with commencing degeneration of the body of the female,
× 15.]

A similar protandry exists in the parasitic forms of _Allantonema_,[189] of
which there are several species—_A. mirabile_ in _Hylobius pini_, _A.
sylvaticum_ in _Geotrupes sylvatica_, _A. diplogaster_ in _Tomicus
typographicus_; but in their case the male and female forms which leave
their host pair in the damp earth and give rise to larvae which make their
way into the body of the beetle-grubs. Here they undergo very extensive
retrogressive change. The body of the female, which becomes the shape of a
thick sausage, is encapsuled and surrounded by a curious hypertrophied
network of tracheae (Fig. 74). As is usually the case with the degenerate
parasitic forms, there are practically no organs but the ovary, and this is
{153}embedded in a fatty parenchyma which fills all the space within the
skin.

[Illustration: FIG. 76.—Four stages in the life-history of _Sphaerularia
bombi_ Dufour, ♀. (From Leuckart.) A, Beginning of the protrusion of the
uterus (_b_), × 66; B, later stage, × 66; C, later stage, × 12; D, the
protrusion is complete, × 6. In each case _a_ represents the Nematode, and
_b_ its protruded uterus.]

_Atractonema gibbosum_, which lives in the body-cavity of the larva of
_Cecidomyia pini_, has a similar life-history, but the parasitic form has a
structural peculiarity which merits attention (Fig. 75). At the time of
sexual maturity a swelling, which is caused by the prolapsus of the uterus
and vagina, appears at the posterior end of the body; this swelling
increases until it equals the rest of the body of the Nematode in size.
Even this is far surpassed by a similar protuberance in _Sphaerularia
bombi_, where the evaginated sac grows with such extreme rapidity that in a
few weeks its length increases from .25 mm. to 15 mm. and its volume
60,000-fold, the increase being due, according to Leuckart, to the increase
in size of the individual cells and not to their multiplication. The
Nematode which has produced this enormous growth gets relatively smaller
and smaller, and ultimately drops off (Fig. 76). The sexual larvae which
arise from the eggs in this sac leave the body of the bee in which this
species is parasitic by the anus, {154}and may live in damp earth, moss,
etc., for months without taking nourishment, until the autumn, when they
become sexually mature and, according to Leuckart, pair. The fertilised
female is believed to bore her way into the humble-bee whilst the latter is
seeking her underground winter quarters; this accounts for the fact that
only queen bees are infected. The parasite is widely distributed both in
Europe and North America; it is found in many species of _Bombus_, but most
frequently in _B. lapidarius_ and _B. terrestris_. The presence of the
_Sphaerularia_ affects the reproductive organs of the host, and reduces
their fertility, so that an infected queen bee never succeeds in forming a
colony.

VI. FAMILY ANGUILLULIDAE.

For the most part free living and of small size. The oesophagus has usually
a double swelling or two oesophageal bulbs. The male has two equal spicula.

Genera: _Diplogaster_, _Mononchus_, _Rhabditis_, _Tylenchus_, _Anguillula_,
and many others.

Many species of this family live in humus or decaying matter; others live
on, or are parasitic in, plants; some, such as _Anguillula aceti_, which is
found in vinegar and in paste, live in organic fluids.

The part played by the presence of these Nematodes in the soil is not
thoroughly understood; sometimes they occur in great numbers, and even when
not directly parasitic in plants, probably do them much damage. Cobb[190]
has recently described from Australia and Fiji over eighty species,
one-half of them new, which occur mostly in the earth, and many of them
among plant roots. They frequently crawl up on to plants, especially on to
seedlings. An instance of this is given as follows: "The edible part of
three bunches of nice-looking celery bought of a Chinaman in Sydney was cut
off as far up as it was tender, nearly to the first leaflets. It was washed
by hand in a tin dish in tank water, free from Nematodes. The washings gave
about 200 to 300 Nematodes, belonging to five different genera."

It is very probable that many of the free-living forms which have received
distinct specific names may ultimately turn out to be but stages in the
life-history of some of the parasitic species. {155}Von Linstow[191] has
pointed out that the free form of _A. diplogaster_, if found alone, would
be placed in the genus _Diplogaster_; similarly the bisexual form of
_Ascaris nigrovenosa_ is known as _Rhabditis nigrovenosa_.

Those Nematodes which live parasitically in plants, _e.g._ many of the
genera _Tylenchus_ and _Aphelenchus_ and _Heterodera_, as well as those
which only pierce the epidermis of the roots (the remaining species of the
above-named genera), are provided with a spine which works to and fro
through the mouth and assists the animal to bore into the tissues of the
plant. _Tylenchus devastatrix_ lives and reproduces in leaves and stems
(never in the roots, except in the case of hops[192]) of many cultivated
plants, such as rye, oats, onions, etc. "Clover sickness" is probably
caused by this Nematode. The plants become infected by the thread-worms in
the soil during the spring; their presence causes swellings and often kills
the plant, in which case the worms return to the soil or remain in the
straw.

_Tylenchus tritici_ Need. is the cause of "ear-cockles" in corn. These take
the form of brown or purple galls, which replace the grains of corn, and
which contain hundreds of minute Nematodes. In these galls they are
motionless, and are capable of surviving in dryness for at least twenty
years; but when moistened,—for instance, by the gall falling on damp
earth,—they resume their vitality and make their way to the young wheat
plants, and then, wriggling up the leaves and stems, find their way to the
ear. Here they pair, and producing a gall-like growth in the flower, lay
numerous eggs, from which arise the Nematodes of the ear-cockle.

[Illustration: FIG. 77.—A, _a_, Female _Heterodera schachtii_ Schmidt,
breaking through the epidermis of a root; the head is still embedded in the
parenchyma of the root: B, _a_, larvae boring their way into a root; _b_,
larva of the immobile kind surrounded by the old skin, living as an
ectoparasite on the outside of the root. (From Strubell.)]

_Heterodera schachtii_[193] Schmidt, is the cause of the "beet
{156}sickness," and forms galls or swellings on the roots of many plants,
in England especially on the roots of tomatoes and cucumbers. The free
larvae live in the earth and make their way into the smaller rootlets; here
the female larvae shed their skin, lose their characteristic Nematode form,
and become citron-shaped (Fig. 78, D). The male larvae undergo a change,
and after a period of rest cast their skin and, leaving the rootlet, seek
out the females. The female does not undergo this second ecdysis, but its
generative organs grow and mature in what is practically a larval stage.
The embryos develop within the body of the mother, and, escaping through
the uterus, ultimately cause her death. They then make their way into the
earth. The cycle of the development takes but four or five weeks, so that,
as in the case of _Tylenchus devastatrix_, there are several broods in a
year; _T. tritici_, on the other hand, has but one.

[Illustration: FIG. 78.—A, Male _Heterodera schachtii_ strongly magnified;
_a_, head lappets; _b_, mouth cavity; _c_, spine; _d_, muscle of spine;
_e_, gland; _f_, oesophagus; _g_, bulb; _h_, nerve-ring; _i_, excretory
pore; _j_, intestine; _k_, testis; _l_, intestine; _m_, muscles moving
spicule; _n_, spicule: B, first motile larva: C, second immovable parasitic
larva casting its skin: D, a female with one half of the body-wall taken
away to show the coiling generative organs; _a_, boring apparatus; _b_,
oesophageal bulb; _c_, excretory pore; _d_, alimentary canal; _e_, anus;
_f_, ovary: E, a male shortly before casting its larval skin.]

Vuillemin and Legrain[194] point out that while _Heterodera_ is injurious
to cultivated plants growing in damp soil, its presence is advantageous to
those that grow in deserts. It is very common in the Sahara, and attacks
many plants which are {157}immune from it elsewhere. It causes the rootlets
to swell out, and the bladder-like extensions thus formed act as reservoirs
for water.

Many other species attack plants; _Tylenchus millefolii_ Löw forms galls on
_Achillea_, _T. dipsaci_ Kühn. on the teazle. They all seem to have great
powers of resisting desiccation. The former species, when dried and placed
in a herbarium in May, gave rise to active worms when moistened the
following October; and the corn eel-worm is said to survive twenty-seven
years in a state of suspended animation. On the other hand, although these
Nematodes like moisture, they cannot withstand submersion in water for any
time. They can resist a considerable degree of cold, and a species,
_Aphelenchus nivalis_ Auriv.,[195] has been described from Spitzbergen,
where it lives in the snow amongst a small red alga, _Sphaerella nivalis_.

VII. FAMILY ENOPLIDAE.

Small, as a rule free-living, usually marine Nematodes, without a second
oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and
bristles sometimes surround the mouth.

Genera: _Enoplus_, _Dorylaimus_, _Enchelidium_, and others.

The genus _Enoplus_ is exclusively marine, living amongst Algae and
Hydroids in shallow water and moving actively about, but never coiling into
spirals. De Man[196] describes _Enoplus brevis_ Bast. as being attacked by
a plant parasite, probably a Bacterium, of a greenish colour, which
infested the muscles and gave them a peculiar colour.

Numerous other species have been described by De Man from the coast of
Holland. It is probable that some of them are the free stages of parasitic
forms; a brackish water species found in the East Indies (_Dorylaimus
palustris_) is regarded by Carter as the larva of _Filaria medinensis_.
_Oncholaimus echini_ Leyd. is parasitic in the intestine of the sea-urchin
_Echinus esculentus_. _Tricoma cincta_[197] has a strongly striated
cuticle, which gives it almost the appearance of segmentation. _Fimbria
tenuis_ has numerous hairs on the tail, and the mouth is surrounded by
bristle-bearing papillae.

{158}Here must be mentioned two families closely allied to the true
Nematodes.

(i.) CHAETOSOMATIDAE.—This family includes three genera: _Chaetosoma_,
_Rhabdogaster_, and _Tristicochaeta_. According to Metschnikoff,[198]
although they are not true Nematodes, they have a great likeness to the
group. He distinguishes them from the swimming members of the group as
"creeping Nematoda." _Chaetosoma_, of which two species are known, _C.
ophicephalum_ and _C. claparedii_, has a head distinct from the body (Fig.
79). The mouth is at the anterior end, surrounded by a double semicircle of
movable spicules; the whole body is covered by fine hairs, and on the
ventral surface, just in front of the anus, is a double row of about
fifteen cylindrical projections, by whose agency the animal creeps. The
female _C. claparedii_ is 1.5 mm. long, the male 1.14 mm. They were found
creeping about on sea-weeds in the neighbourhood of Salerno.

[Illustration: FIG. 79.—Mature female of _Chaetosoma claparedii_ Metschni.,
× 57. (From Metschnikoff.) _a_, Oesophagus; _b_, intestine; _c_, anus; _d_,
ovary; _e_, generative pore; _f_, ventral bristles.]

The genus _Tristicochaeta_[199] differs from the foregoing in having three
rows of locomotor projections instead of two.

[Illustration: FIG. 80.—_Tristicochaeta inarimense_ Panceri, in one of its
most usual positions, showing the triple row of ventral bristles, × 100.
(From Panceri.)]

_Rhabdogaster_ has no head distinct from the body, though the anterior part
of the body is swollen. A second swelling occurs, as is also the case with
_Chaetosoma_, in the region of the opening of the genital ducts. The female
in _Rh. cygnoides_ attains a length of 0.36 mm. In this genus the hairs are
confined to {159}the dorsal middle line. The locomotor projections are
hooked, and are much finer than those of _Chaetosoma_, and they are
situated farther forward than in the last-named genus. _Rhabdogaster_
occurs in the same surroundings as _Chaetosoma_. _Ch. ophicephalum_ is
recorded from the English Channel.

(ii.) DESMOSCOLECIDAE.—The members of this family are minute, and are
characterised by the presence of well-marked ridges which surround the body
and give it an appearance of segmentation. The head, which is somewhat
swollen, bears four bristles, and single pairs are borne by a certain
number of the ridges, some on the dorsal and some on the ventral surface.
These hairs can be moved independently of one another. Two red eye-spots
are described between the fourth and fifth rings. The sexes are distinct,
and the internal organs generally have a marked resemblance to those of the
true Nematoda. The _Desmoscolecidae_ move by looping their bodies after the
manner of the Geometrid caterpillars, as well as by creeping with their
bristles. The genus contains numerous species[200]: _D. minutus_ Clap.
(English Channel), _D. nematoides_ Greef, _D. adelphus_ Greef, _D.
chaetogaster_ Greef, _D. elongatus_ Panceri, and _D. lanuginosa_ Panceri.
They are exclusively marine.

[Illustration: FIG. 81.—Female _Desmoscolex elongatus_ Panceri, ventral
view, × 260. _a_, Ovary. (From Panceri.)]

_Trichoderma oxycaudatum_ Greef[201] is a minute animal, 0.3 mm. long,
which has no head or ventral spines, but whose body is ringed and covered
with long hair-like bristles. The male has two spicules, and the internal
organisation recalls that of other Nematodes; still its ringed body has
induced some authorities to place it near to _Desmoscolex_.

THE LIFE-HISTORY OF NEMATODES.

Although, considering the enormous number of species of Nematodes and the
remarkable diversity of the conditions under {160}which they live, their
bodily structure shows a very striking uniformity, the same is by no means
the case with their life-history, which exhibits an astounding variety. Von
Linstow[202] has arranged the various modifications, which occur under
fourteen heads. He includes in his list the Gordian worms, which we have
placed under a different heading. The following account has been taken from
his paper, with a few alterations:—

1. The embryos develop, with a larval stage and without any change of
medium, directly into the mature sexual forms. They live in fresh,
brackish, or salt water, in plants, in the earth or in decaying organic
matter: examples, _Dorylaimus_, _Enoplus_, _Plectus_, _Monhystera_.

2. The larvae live in the earth, the sexual forms in plants: examples,
_Tylenchus tritici_ and _T. devastatrix_, _Heterodera schachtii_ (Figs. 77
and 78).

3. The larvae live in animals, after whose death and decay they are set
free and develop into the sexual animals in the earth: example, _Rhabditis
pellio_.

4. The bisexual forms live in the earth, and the fertilised females bore
into animals (insects), and here produce embryos: example, _Sphaerularia
bombi_ (Fig. 76).

5. The bisexual forms live in the earth; the females do not develop, but
the males make their way into Insects (Beetles), and becoming
hermaphrodite, develop ova which give rise to the bisexual form: example,
_Bradynema rigidum_.

6. The larvae live in the earth, the sexual form in Vertebrates: examples,
_Dochmius_, _Strongylus_.

7. The Nematode lives as a hermaphrodite in animals, the offspring of this,
by an alternation of generations, become sexual in the earth: example,
_Rhabdonema_ in Frog.

8. A bisexual free form gives origin to a bisexual parasitic form living in
an animal: example, _Leptodera appendiculata_ in Snails.

9. The eggs develop in the earth, and give rise to embryos which are
transferred whilst still in the egg-cell to the body of an animal. The
embryos hatch out and form bisexual parasites: examples, _Oxyuris_,
_Trichocephalus_.

10. The larvae live in insects, the sexual worms in water or in the earth:
example, _Mermis_.

11. The larva lives encapsuled and is passively transferred to {161}a
second animal: examples, _Ollulanus_, from Mouse to Cat; _Cucullanus
elegans_, from _Cyclops_ to Perch; _Spiroptera obtusa_, from Meal-worm to
Mouse.

12. The sexual form lives for a short time in the intestine of a
Vertebrate, and produces larvae which bore through the intestinal wall and
become encapsuled in the tissues: example, _Trichina spiralis_.

13. The sexual animal lives in the trachea of birds; the ova containing
embryos are coughed up and are taken into other birds with food. They quit
the egg-shell and wander into the air-sacs, and finally into the trachea:
example, _Syngamus_.

14. There are two larval forms; the first lives in water, the second in the
lungs of Amphibia, whence they wander into the intestine and become
sexually mature: example, _Nematoxys longicauda_ in _Triton alpestris_.

PARASITISM.

1. EFFECT OF PARASITISM ON THE PARASITE.—The usual effect of parasitism on
the parasitic organism is that the various organs necessary for a free life
tend to degenerate, whilst there is a multiplication and development of
organs of adhesion, by means of which the parasite maintains its hold on
its host. There is further an immense increase in the powers of
reproduction, which may take the form of an increase in the number of
fertilised eggs produced, or the parasite may at some time of its life
reproduce asexually, by budding, or fission, or parthenogetically.

Of the various classes of animals which are more or less parasitic, the
Nematodes show less difference between the free-living and parasitic
members of the group than obtains in any other class. With few exceptions,
such as _Sphaerularia_, _Allantonema_, and one or two others, the parasitic
forms have undergone but little degeneration. It is true that they have no
eyes such as the free forms often possess, but in other respects, such as
in the nervous, muscular, and digestive systems, they do not show any
marked retrogression; further, the mouth-armature is developed in many free
forms, and is not confined to the parasites.

The group has developed no methods of asexual reproduction by budding or
fission, such as are found in Platyhelminthes; and the cases of an
alternation of generations in which a sexual form alternates with a
parthenogenetic form, are rare, _e.g._ {162}_Rhabdonema nigrovenosum_; and
it seems possible that even when parthenogenesis has been described,
further observation may show that the parthenogenetic stage is really a
protandrous hermaphrodite, in which case the alternation of generations in
Nematodes, _i.e._ the hermaphrodite alternating with the dioecious form, is
a case of _heterogamy_ or the alternation of two sexual generations.

On the other hand, parasitic Nematodes produce enormous numbers of eggs.
Van Beneden states that 60,000,000 have been computed in a single Nematode,
and this multiplication of ova is absolutely necessary, for the chance of
the embryo reaching the right host, in which alone it can develop, is
always a small one.

It is a common thing to find that parasites are either hermaphrodite or
that the male is degenerate, as is the case with many of the parasitic
Crustacea, but with one or two exceptions the Nematoda are bisexual, and
although, as a rule, the males are smaller than the females, they show no
other trace of degeneracy.

In spite of the fact that the class as a whole shows but few special
modifications consequent on a parasitic mode of life, it is clear that the
Nematoda are peculiarly adapted for such a mode of life. Their elongated
thread-like bodies afford little resistance to the passage of the food,
which, as it passes through the intestine of the host, might tend to carry
the parasites out of the body. At the same time their shape enables them to
pierce and wriggle through the various tissues without making any very
serious lesions such as might prove fatal to their host. Their
extraordinary power of resisting desiccation both in the egg and in the
adult state vastly increases their chances of ultimately hitting on the
right host. They are capable of living in a state of suspended animation
for months, and even years when dried (_vide_ p. 136), and of resuming
their activity on being moistened.

The great faculty this group shows for living parasitically is evinced by
the extraordinary variety of life-history presented by the different
species. There is scarcely a stage which may not be parasitic; the eggs,
the larvae, the adults are all in some cases free, in others parasitic, and
in many cases first the one and then the other.

2. OCCURRENCE AND EFFECT OF THE PARASITE ON THE HOST.—Von Linstow states
that the only law that can be derived inductively from the study of the
life-history of Nematodes is that those that live in animals never pass
through all their stages of development in the same organ; consequently, in
considering the distribution of {163}the parasites within the body of their
host we have a double habitat to consider. Many forms, such as _Trichina
spiralis_, wander from the intestine to the muscles; others, such as
_Filaria medinensis_, from the alimentary canal to the lymphatics or blood
vessels or subcutaneous tissues. Others pass from the body-cavity to the
intestine, as the Mermithidae, which infest Insects, or from the stem and
leaves of a plant to its flower, as in the case of _Tylenchus tritici_.

With regard to their occurrence in the different classes of the animal
kingdom, they have been most frequently observed in Vertebrates and in
Insects. They are comparatively rare in the other large divisions. Many
genera are confined to certain hosts: thus _Ascaris_, _Filaria_,
_Trichosoma_ occur only in Vertebrates; _Spiroptera_ (with one exception)
in Mammals and Birds; _Cucullanus_ in Fishes and Amphibia; _Strongylus_ and
_Physaloptera_ in Mammals, Birds, and Reptiles; _Dochmius_, _Pseudalius_,
_Trichocephalus_ in Mammals; _Dispharagus_, _Hystrichis_, _Syngamus_ in
Birds; _Nematoxys_, _Hedruris_ in Amphibia and Reptiles; _Ichthyonema_ in
Fishes; and _Isacis_ and _Mermis_ in Insects.

Twenty-two species have been described as parasitic in man, of which
perhaps the most dangerous are _Filaria medinensis_, the three varieties of
_F. sanguinis hominis_; _Dochmius_ (_Ancylostomum_) _duodenalis_, and
_Trichina spiralis_. The Ascaridae, as _Ascaris lumbricoides_ and _Oxyuris
vermicularis_, though painful, seldom cause death.

The enormous number of parasites harboured by one host is shown by the fact
mentioned in Leuckart's _Parasites of Man_, that Nathusius[203] took from a
single black stork 24 specimens of _Filaria labiata_ from the lungs, 16
_Syngamus trachealis_ from the trachea, more than 100 _Spiroptera alata_
from the coats of the stomach, besides several hundred Trematodes belonging
to several different species (see p. 63). Even this has been surpassed in
the case of a young horse, in whose body Krause found 500 _Ascaris
megalocephala_, 190 _Oxyuris curvula_, several millions of _Strongylus
tetracanthus_, 214 _Sclerostomum armatum_, 287 _Filaria papillosa_, 69
_Taenia perfoliata_, and 6 _Cysticercus_ forms.

It is impossible here to enter into a full description of the destruction
caused to domesticated animals and crops by the presence of these
parasites; full details will be found in books dealing {164}especially with
this question, such as Neumann's _Parasites and Parasitic Diseases of
Domesticated Animals_. A couple of cases will show how important this
matter is to the farmer. Crisp estimates that _Syngamus trachealis_ causes
the death of half a million pullets in England every year, and Mégnin
states that in a single pheasantry 1200 victims died daily; again, the loss
of one-third the crop of beetroot is by no means uncommon when it is
infested with _Heterodera schachtii_. These show the practical importance
of what at first sight seem quite insignificant animals, and the necessity
for the minutest observation, for only when we are fully acquainted with
all the details of the life-history of a parasite are we in a position to
successfully combat it.

SUB-ORDER II. NEMATOMORPHA.

Until the last few years it has been customary to regard the Gordiidae as a
family of Nematodes. Although in external appearance and life-history they
closely resemble the members of this group, yet recent research has shown
so many important morphological differences between them and the Nematoda,
that most zoologists are now agreed in placing them in a different
sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[204]

[Illustration: FIG. 82.—A water plant around which a female _Gordius_ is
twining and laying eggs. _a_, _a_, Clump and string of eggs. (From von
Linstow.[205])]

The Gordiidae comprise but two genera, _Gordius_ and _Nectonema_. The
latter has but one species, _N. agile_ Verr., and is marine; the former, on
the other hand, is exclusively fresh-water, and contains a very large
number of species. Gordian worms are frequently to be found in ditches,
ponds, or large puddles, moving with an undulating motion through the
water, or twining and writhing round water-plants; they are scarcer in
running water. In shape they are like a piece of thin whip-cord, slightly
tapering {165}at each end; the male, however, is easily distinguished from
the female by its forked tail (Fig. 89). Not unfrequently a considerable
number are found inextricably tangled together into a knot, and the name of
the genus refers to this fact. Where numbers have suddenly appeared in
water hitherto free from them, legends have sprung up which attribute their
presence to a rain of worms; in reality they have come out of the bodies of
Insects in which they are parasitic for the greater part of their life.

The genus _Gordius_ passes through three distinct stages, of which the
first two are larval and parasitic; the third is sexually mature and lives
in water. The second larval stage closely resembles the adult, but the
reproductive organs are not developed. The following account of the
structure of this larval form and of the adult is in the main taken from
von Linstow.[206]

The whole body is covered with a well-developed two-layered cuticle, which
in the adult is marked out into areas, and bears numerous minute sensory
bristles, which are especially developed in the neighbourhood of the cloaca
of the male. Beneath this is a hypodermis which differs markedly from the
sub-cuticle of Nematodes, inasmuch as it consists of a single layer of
polygonal nucleated cells. Within this lies a single layer of longitudinal
muscle-cells, which differ from the corresponding layer of Nematodes in
having that part of their medulla which is not surrounded by the
contractile portion directed outwards towards the hypodermis, and not
inwards towards the body-cavity.

[Illustration: FIG. 83.—Transverse section through a young male _Gordius
tolosanus_ Duj. (From von Linstow.) Highly magnified. _a_, Cuticle; _b_,
hypodermis; _c_, muscular layer; _d_, parenchyma; _e_, alimentary canal;
_f_, nervous system; _g_, cells of the testis.]

The body is in the younger stages practically solid, the interior being
filled with clearly defined polygonal cells which are arranged in definite
rows; in later life certain splits arise in this tissue which {166}subserve
various functions; between these splits strands of tissue are left which
form mesenteries, and some of the cells remain lining the muscular layer
(Fig. 86). These cells have been described by Vejdovsky as a definite
somatic, peritoneal epithelium, but this was not found by von Linstow.
Besides forming the mesenteries, and acting as packing between the various
organs of the body, these cells also form the ova and the spermatozoa.

The splits which have appeared when the animal has reached the second
larval stage, are two dorsal and a ventral; the latter contains the
alimentary canal, and may be termed the body-cavity, the former will
develop the generative organs. The mouth is occluded in the older larvae,
and in the adults there is a distinct but solid oesophagus which passes
into a tubular intestine. The intestine consists of a single layer of cells
surrounding a lumen; it runs straight to the hinder end of the body, where
it opens in both sexes with the ducts of the reproductive organs.

The nervous system consists of a well-defined circumoesophageal ring with
two dorsal swellings, and, arising from this, a median ventral cord which
runs the whole length of the body. The cord consists of three longitudinal
strands with ganglionic cells below them; the latter, though they lie
within the muscle layer, maintain a connexion with the hypodermis. Behind,
the nerve-cord splits in the male, one half passing into each caudal fork.
In the adult a pair of black eyes can be detected on the head; the only
other sense organs are the tactile bristles mentioned above. Excretory
organs are unknown.

[Illustration: FIG. 84.—Section through a young female _Gordius tolosanus_.
(From von Linstow.) _a_, Cuticle; _b_, hypodermis; _c_, muscular layer;
_d_, parenchyma; _e_, alimentary canal; _f_, nervous system; _g_, egg-sac;
_h_, ovary.]

The generative organs only attain maturity in the adult, which is, in fact,
exclusively devoted to reproduction. No trace of testes {167}is found in
the larva, though the two dorsal splits from the walls of which the
spermatozoa will arise are present. They are lined by a definite epithelium
(Fig. 83), and this serves at once to distinguish them from the
body-cavity. Posteriorly the splits narrow and become the two vasa
deferentia which open one on each side into the cloaca. The cells lining
the lumen give rise to secondary cells, and these become spermatozoa, the
process extending from behind forwards. The external organs—bursa,
etc.—described by Vejdovsky were not found by von Linstow.

[Illustration: FIG. 85.—Section through a mature female _Gordius
tolosanus_. (From von Linstow.) Lettering as in Fig. 84; _g_, egg-sac; _h_,
ovary.]

[Illustration: FIG. 86.—Section through a female _Gordius tolosanus_ when
the deposition of ova is almost complete. _a_, _b_, _c_, _d_, _e_, and _f_,
as in Fig. 84; _g_, egg-sac; _h_, ovary almost empty; _i_, dorsal canal
containing eggs; _j_, receptaculum seminis.]

In the female larva two similar splits are present; these form the
egg-sacs. Posteriorly they end in two short oviducts which open into a
uterus, in which fertilisation takes place, and in which the secretion
arises which cements the eggs together. In the adult the ovaries and a
receptaculum seminis are found, in addition to the organs present in the
larva. The ovaries are formed from modifications of the packing tissue;
they begin close behind the head, and soon attain such dimensions as to
compress the egg-sacs and body-cavity to small slits. After a time the wall
between the ovary and the egg-sacs becomes absorbed, and the eggs grow into
the latter. In the old females, where the egg sacs are empty, there is a
considerable space round the exhausted ovary, into which eggs continue to
fall off; there {168}is also a median dorsal canal which contains a few
eggs. By this time the wall between the ovary and the egg-sac has again
appeared.

One of the most interesting points about the female is that, according to
Vejdovsky, the ovary is segmented, the cells which form the ova being
heaped up in segmentally-arranged masses. This observation, if correct, is
almost the only instance of segmentation recorded in the group
Nemathelminthes.

[Illustration: FIG. 87.—_Nectonema agile_ Verrill. A, The adult. Magnified.
(After Fewkes.) B, Longitudinal section through the head. × about 20. (From
Bürger.) _a_, Mouth; _b_, circumoesophageal commissure (dorsal); _c_, cell
of salivary gland; _d_, septum cutting off head from rest of body; _e_,
testis; _f_, ventral cord; _g_, oesophageal cells; _h_, lumen of
oesophagus; _i_, cerebral ganglion (ventral).]

The only other genus which is associated with _Gordius_ in the group
Nematomorpha is _Nectonema_, of which there is as yet but one species
known, _Nectonema agile_ Verr.[207] Our knowledge of the anatomy of this
worm is due mainly to Bürger[208] and Ward.[209] _Nectonema_ is a marine
worm found swimming near the surface of the sea with rapid undulatory
motion. The males are from 50 to 200 mm. long, the females from 30 to 60
mm. The body is faintly ringed, and bears two rows of fine bristles on each
side. Owing to a curious torsion of the body through a right angle, the
lateral bristles of the anterior third seem to be placed in the ventral and
dorsal middle line. They are very easily broken off. The body is divided
into a small anterior and a large posterior {169}chamber by a transverse
septum placed a little way behind the head. The anterior chamber contains
the brain and is lined by a definite epithelium, the posterior is not. The
layers of the skin correspond with those of Nematodes or of _Gordius_, but
the hypodermal cells show no cell outlines; still they are not so modified
as in the former group. The hypodermis is thickened in the median dorsal
and ventral line, and the single nerve-cord lies in the latter.

The alimentary canal is degenerate, as in _Gordius_. A mouth exists, but it
is minute, and opens into a very fine tube lined with chitin, which pierces
through the substance of a single elongated cell. This minute oesophagus,
with its coextensive cell, reaches back to the transverse partition, but
behind this a few other cells become associated with it, and ultimately the
lumen of the alimentary canal is surrounded by four cells; but the number
diminishes behind, and soon only two cells surround the tube at any one
level, and the intestine dwindles away some little distance in front of the
tail. There is no sign of an anus. A circumoesophageal nerve-ring exists,
of which the ventral part is by far the larger (Fig. 87); it gives off a
ventral nerve-cord, which swells posteriorly in the male into a large anal
ganglion, far bigger than the brain, and larger in the male than in the
female.

The testes consist of a dorsally placed sac, continuous behind with a vas
deferens; this opens at the posterior end, which is pointed and slightly
curved ventrally. The ovary is unknown; but females have been found with
their body-cavity crammed with ova; these escape, like the spermatozoa,
from a genital pore at the posterior end of the body.

CLASSIFICATION.—The separation of the Nematomorpha from the Nematoda
depends mainly on the character of the nervous system, the absence of the
lateral lines and of the dorsal line, the character of the contents of the
body-cavity, and the character of the reproductive organs. In Gordiidae the
latter are always placed dorsal to the intestine, and ovaries and testes
open alike at the hinder end of the body. The importance of the differences
in the organs just enumerated has been considered sufficient to justify the
removal of the Gordiidae from the Nematoda, and the establishment of the
special sub-Order Nematomorpha for their reception; and although
_Nectonema_ has a dorsal line, and is in some other respects intermediate
between the two groups, there can be little doubt that it is more closely
allied to _Gordius_ than to any member {170}of the Nematoda, and it must
therefore be placed with it in the Nematomorpha.

On the other hand, it ought to be mentioned that Camerano[210] found that
the chief details of the fertilisation and development of the egg in
_Gordius_ closely conform with what is known of the same processes in
Nematodes, and he is of opinion that these resemblances are sufficiently
important to justify the retention of the group among the Nematoda.

LIFE-HISTORY.—The life-history of _Gordius_ comprises four stages—the early
development of the egg, the first larval form, the second larval form, and
the sexually mature form. Both larval forms are parasitic, and during their
life they are actively engaged in feeding; the free form, on the other
hand, takes in no nourishment, and is exclusively engaged in reproduction.

[Illustration: FIG. 88.—Abdomen of _Pterostichus niger_ with the terga
removed to expose the _Gordius_ larva within. Slightly magnified. (From von
Linstow.)]

Von Linstow[211] gives the following account of the life-history of _G.
tolosanus_, a form which has been more fully worked out than any other. In
the month of April numerous specimens of the beetle _Pterostichus niger_
were found floating on the surface of the ditches and small ponds in the
fields surrounding Göttingen. Some were found dead or dying; others
appeared quite healthy, and these were swimming actively, endeavouring to
reach land. Within the abdomen of these beetles, in about 20 per cent of
those collected, the second larval form of the _G. tolosanus_ was found.
The longest larvae were 122 mm. in length, and very soft, partly snow-white
and partly brown in colour; traces of the boring apparatus of the first
larval form were still to be seen, but in other respects the larva only
differed from the free form in the immaturity of its sexual organs. Besides
the parasite hardly anything was to be found in the abdomen of the beetle,
the larva having eaten up all trace of the fat body and the generative
organs of its host. The larvae bored their way out of the body of the
beetle and became adult animals.

It is rather difficult to say what brings these essentially
{171}terrestrial beetles to the water, but von Linstow suggests that, as
they live partly on snails, and at this time of year there are not many
land-snails about, they may be in search of water-snails such as _Limnaea_.
They may also be sometimes blown into the water by wind storms, but,
whatever the cause is, their presence in water is essential for the
continuance of the life of their parasites.

Once free in the water the _Gordius_ is soon sexually mature; the
fertilisation takes place in April, and then the female may be seen
twisting and writhing round the stems of water-plants and laying the long
bead-like strands of eggs (Fig. 82). The first deposition observed by von
Linstow took place on 14th April, the last on 2nd August, and the period of
egg-laying for each female extended over four weeks. At first the eggs are
snow-white, but within twenty-four hours they turn brown in colour.

The development of the first larva within the egg takes about a month. When
it emerges from the egg-shell it is minute, .065 mm. long, ringed
anteriorly, and provided with a protrusible and retractile boring apparatus
consisting of three chitinous rods; round the base of this piercing
proboscis is a double crown of papillae, each bearing a spine (Fig. 90).

[Illustration: FIG. 89.—The tail ends of a female _Gordius_ (_a_) and a
male (_b_) _in copula_. × 1.5. (From G. Meissner.[212])]

This first larval form breaks through the egg-shell and sinks to the bottom
of the water, where it moves about sluggishly and awaits the arrival of the
right host in which to take up its abode. This host is the larva of the
Alder-fly, _Sialis lutaria_ Lin. (_vide_ vol. v. p. 444), and into this it
bores and comes to rest in the muscles or the fat body. It does not form
distinct capsules. It remains in this larva during the following winter,
and in the spring passes over into the imago _Sialis_. The complete insect
frequents the small plants growing along the water's edge, and falls an
easy prey to the predaceous beetle _Pt. niger_. The larva is eaten, and
undergoing a change becomes the second larval form mentioned above. It
remains in the body of the beetle during the second winter, and finally
returns to the water {172}as the adult some eighteen or twenty months after
it has been hatched from the egg.

[Illustration: FIG. 90.—Embryo or first larval form of _Gordius tolosanus_
taken from the egg. Highly magnified. _a_ and _b_, The bristle-bearing
papillae on the head; _c_, the boring apparatus. (From von Linstow.)]

From the above account of the life-history of _Gordius_ it will be seen
that the chances of an egg reaching maturity are comparatively small, and
to compensate for this a very large number of eggs are laid. In addition to
the risk of the larvae not finding the right host at the right time, and of
the first host not being eaten by the second, and the second not being
drowned, there is the danger that the ditches and ponds in which the adults
live may dry up, and, in fact, great numbers of worms perish by this taking
place.

The sex of the adults may be told from their colour, the males being of a
blackish brown, the females of a light clay brown; the former average 120
mm. in length, the latter 170 mm. The males are also more numerous, the
proportion being seven to three. Camerano[213] has drawn attention to the
fact that there is a certain polymorphism in size, form, and colour which
is especially common amongst the males; dwarf forms with mature
reproductive organs exist, and he is of opinion that these differences
depend both on the size of the second host and on the duration of the
parasitic life.

In addition to the larva of _Sialis lutaria_, the first larval stage has
also been found in the larva of _Ephemera_, _Tanypus_, _Corethra_, and
_Chironomus_; the second in _Carabus hortensis_ Fabr., _Procerus_
(_Carabus_) _coriaceus_ Linn., _Calathus fuscipes_ Goeze, _Molops elatus_
Fabr., several species of _Pterostichus_, and a number of other beetles. It
is probable that its normal hosts are _S. lutaria_ and _Pt. niger_, but it
is clear that it often comes to rest in other insects. The view that the
Gordiidae have no special hosts, but may either pass the whole of their
life-history within one and the same animal, or, on the other hand, may
inhabit animals belonging to very different groups, is held by Villot, who
has paid great attention to the subject. He finds the first larval form
encysted in the walls of the alimentary canal in fishes, such {173}as
_Leuciscus phoxinus_, the minnow, _Cobitis barbatula_, the loach, and
_Petromyzon planeri_, the lamprey; in the larvae of Diptera, _Ephemera_,
and beetles, in _Planorbis_ (a water snail), in _Enchytraeus_ (an
Oligochaet); the second larval form in all kinds of insects, spiders,
Crustacea, fish, frogs, birds (_Otis_), and in man, and these various
habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes
spéciaux." On the other hand, as von Linstow points out, it is contrary to
our knowledge of parasites that a single species should develop equally
well in the body of warm and cold-blooded Vertebrates and of Insects, and
the explanation of the presence of the larvae in these various forms may
either be that they belong to different species of _Gordius_ or, more
probably, that they are accidentally present, having passed into their
hosts with drinking water.

[Illustration: FIG. 91.—Tarsal joint of an Ephemerid larva into which two
_Gordius_ larvae (_a_, _a_) have penetrated. Magnified. (From G.
Meissner.)]

The number of species of _Gordius_ is large; over 100 are enumerated in the
_Compendium der Helminthologie_,[214] the great majority of which inhabit
insects.

The life-history of _Nectonema_ is practically unknown; the adults have
been found swimming near the surface of the sea at two places only:
Newport, R.I., and Wood's Holl, Mass., on the south coast of New England.
It has been fished close to the shore, from the end of June to the
beginning of October, when the tide is going out at evening and there is no
moon. This seems to indicate that it avoids the light. When first caught
the worms move actively about, coiling themselves into figures of eight and
then uncoiling; at the same time there is a rhythmical movement caused by
waves of muscular contraction passing down each side of the body
alternately; by this kind of motion they make rapid and definite progress
through the water.

It seems probable that the adult _Nectonema_ is preceded by one or more
larval stages, and what appears to be a young form has {174}been obtained
from the thoracic cavity of a prawn, _Palaemonetes_,[215] which has thus
some claim to be regarded as the host of this species, but nothing is known
about its early life-history.

SUB-ORDER III. ACANTHOCEPHALA.

The Acanthocephala, which form the third class of the Nemathelminthes,
consists of but few genera; there are, however, numerous species of very
different size, varying from 10 to 65 cm. long in the female
_Gigantorhynchus_ (_Echinorhynchus_) _gigas_, to quite minute forms a few
millimetres in length. The adult stage occurs in the alimentary canal of
Vertebrates, as a rule in those which live in, or frequent water; the
larvae are found in the bodies of certain Invertebrates, very frequently
small Crustacea.

[Illustration: FIG. 92.—Two specimens of _Echinorhynchus proteus_
Westrumb., with their anterior ends embedded in the wall of the intestine
of a Pike. Magnified with a lens. (From Hamann.)]

ANATOMY.—The body of the mature forms can usually be divided into three
sections—the proboscis, the neck, and the trunk, but the middle region is
not always discernible. The proboscis is armed with rings of hooks (Fig.
93) arranged in longitudinal rows; they are usually of two kinds, but in
_E. proteus_ of three. They have a certain specific value, but not much
stress can be laid on the number of rings, _e.g._ in _E. angustatus_ the
number varies from eight to twenty-four. The recurved hooks serve to fasten
the parasite very firmly to the tissues of the host. The proboscis is
hollow and retractile; it can be withdrawn into the body by means of
muscles attached internally to its tip. It does not, however, pass straight
into the body-cavity, but is retracted into a special cavity—the proboscis
sheath—with a double muscular wall. The proboscis sheath may perhaps be
looked upon as a septum, such as is found in some of the Nematomorpha,
dividing the body-cavity into two parts. It is inserted into the body-wall
at the junction of the neck and trunk or of the proboscis and trunk. In
addition to the muscles which withdraw the proboscis into its sheath, there
are two retractors running from the {175}outside of the sheath to the
body-wall; these serve to retract the whole sheath and its contents into
the body-cavity of the trunk.

The structure of the skin is essentially like that of Nematodes, but the
details are much more complicated. The whole body is covered by a thin
cuticle secreted by the epidermis, which, as in the other groups, breaks
down and forms a syncytium called the sub-cuticle. The minute fibrils which
penetrate this layer are much more definitely arranged than in Nematodes;
the largest of them run from without inwards, others run concentrically
round the body. Large oval or spherical nuclei are scattered in the
sub-cuticle, which is further honeycombed by a number of lacunae or spaces
which are described below.

[Illustration: FIG. 93.—A, Five specimens of _Echinorhynchus acus_ Rud.
attached to a piece of intestinal wall, × 4; B, the proboscis of one still
more highly magnified.]

Within the sub-cuticular layer is found a sheath of circularly-arranged
muscle-fibres, and within this again a sheath of longitudinal muscles which
do not extend into the proboscis; this inner layer lines the body-cavity,
there being no epithelium within it. In their minute structure the
muscle-cells resemble those of Nematodes.

The canals in the sub-cuticle form a very curious system of anastomosing
spaces, in which a clear fluid containing fat globules circulates. The
extent to which the system is developed varies in different species, but in
all there is a pair of longitudinal canals which are situated laterally,
and which give off the subsidiary channels in their course. The above
description applies to the lacunar spaces in the skin of the trunk; those
of the proboscis are quite distinct, and there is no communication between
the two sets of spaces; in fact, the sub-cuticle in which the lacunae are
formed is not continuous across the line of junction of the proboscis and
the neck, or, when the latter is absent, of the proboscis and the trunk,
but it is interrupted by the ingrowth of a thin ring of cuticle which
reaches down to the muscular layers (Fig. 94).

{176}[Illustration: FIG. 94.—A longitudinal section through the anterior
end of _Echinorhynchus haeruca_ Rud. (From Hamann.) _a_, The proboscis not
fully expanded; _b_, proboscis-sheath; _c_, retractor muscles of the
proboscis; _d_, cerebral ganglion; _e_, retinaculum enclosing a nerve; _f_,
one of the retractors of the sheath; _g_, a lemniscus; _h_, one of the
spaces in the sub-cuticular tissue; _i_, longitudinal muscular layer; _j_,
circular muscular layer; _k_, line of division between the sub-cuticular
tissue of the trunk and that of the proboscis with the lemnisci.]

All the spaces in the skin of the proboscis open ultimately into a circular
canal situated round its base; on each side the canal opens into a sac-like
structure which extends through the body-cavity towards the posterior end
of the animal. These two lateral diverticula are termed the _lemnisci_.
They have always attracted considerable attention from the workers at the
group, and numerous functions have from time to time been attributed to
them. They are more or less hollow, and their walls consist of
sub-cuticular tissue surrounded with a scanty muscular coat; they contain
the same fluid as the lacunae of the skin of the proboscis, with which they
are placed in communication by means of the circular canal; and it seems
most probable that, as Hamann[216] suggests, they act as reservoirs into
which the lacunar fluid retires when the proboscis is retracted, {177}and
which, by means of the contractions of their muscular coat, force the fluid
into the lacunae when the proboscis is everted, and thus aid in its
protrusion.

The parasitic habits of _Echinorhynchus_ have had a deeper influence on the
structure of the body than is the case with the Nematoda. All traces of an
alimentary canal have disappeared, and the animals live entirely by the
imbibition through the skin of the already elaborated fluids of their
hosts. The power of absorbing fluids is shown by the fact that they swell
up and become tense when placed in fresh water.

Until recently no definite excretory organs had been recognised, and the
function of excreting the nitrogenous matter was by some assigned to the
lemnisci. In 1893 Kaiser[217] described in _G. gigas_ two organs which he
called nephridia, placed dorsally to the ducts of the male and female
reproductive organs. Each nephridium, which somewhat resembles a
cauliflower, consists of a stalk or duct, opening at one end into the
reproductive ducts, and at the other branching and breaking up into a
number of secondary and tertiary twigs. The end of each twig is closed by a
membrane pierced with a number of most minute pores, by means of which it
communicates with the body-cavity; on the inner side the membrane bears a
number of long cilia, which keep up an active flickering. The presence of
these cilia is interesting, as elsewhere they are unknown throughout the
Nemathelminthes.

[Illustration: FIG. 95.—A, A longitudinal section through the terminal
twigs of the nephridium of _Gigantorhynchus gigas_. (From J. E. Kaiser.)
Highly magnified. _a_, Nucleus. B, A terminal twig more highly magnified;
_b_, the porous membrane.]

The nervous system consists of a central ganglion situated in the proboscis
sheath; it is oval and flattened in shape. The ganglion gives off nerves to
the proboscis, and two main trunks which pierce the proboscis-sheath and
run backward surrounded by a cluster of muscle-fibres, the whole being
termed the {178}_retinaculum_; in the male they are in connexion with a
special genital ganglion which lies near the ductus ejaculatorius.

With the exception of certain sensory papillae in the neighbourhood of the
male genital orifice, and of three similar papillae mentioned by Kaiser on
the proboscis, the Acanthocephala are devoid of sense organs.

The Acanthocephala are dioecious; their generative organs are developed in
connexion with the _ligament_, a cord-like structure which arises between
the inner and outer layer of the hinder end of the proboscis sheath and
traverses the body-cavity, ending posteriorly in connexion with the genital
ducts. The testes lie in this ligament; they are paired oval bodies which
open each into a vas deferens. The vasa deferentia each bear three lateral
diverticula, the vesiculae seminales; and three pairs of cement glands pour
their secretion into a duct which opens into the vasa deferentia; the
latter unite and open by a penis which is withdrawn into a genital bursa,
but is capable of being extruded.

[Illustration: FIG. 96.—An optical section through a male _Neorhynchus
clavaeceps_ Zed. (From Hamann.) _a_, Proboscis; _b_, proboscis sheath; _c_,
retractor of the proboscis; _d_, cerebral ganglion; _f_, _f_, retractors of
the proboscis sheath; _g_, _g_, lemnisci, each with two giant nuclei; _h_,
space in sub-cuticular layer of the skin; _l_, ligament; _m_, _m_, testes;
_o_, glands on vas deferens; _p_, giant nucleus in skin; _q_, opening of
vas deferens.]

The two ovaries are formed in the ligament of the female in a corresponding
position to that occupied by the testes in the male, but at an early stage
they break down into packets of cells, of which those of the peripheral
layer develop into ova at the cost of the central cells, which serve them
as a food supply. As these masses grow and increase in number they rupture
the walls of the ligament, and escape into the body-cavity, in which they
float. The ova are {179}fertilised whilst floating in the fluid of the
body-cavity. The eggs segment and the embryo is formed whilst still in the
body of the mother.

The embryos escape by means of a complicated apparatus the details of which
vary in the different species, but which, like many of the organs in these
animals, consists of very few cells with very large nuclei. This apparatus
consists of three parts: the bell, the uterus, and the oviduct. The bell is
a large funnel-shaped structure, which opens into the body-cavity, and is
connected with the end of the ligament; near its lower end, where it is
continuous with the uterus, is a second smaller opening situated dorsally.
By the contraction and expansion of its lips the oval embryos are swallowed
and pass on through the uterus to the oviduct, which opens at the posterior
end of the body. If the bell takes in any of the less mature eggs which are
spherical in shape, they are passed back into the body-cavity through the
above-mentioned dorsal opening, and the same orifice permits the passage of
the spermatozoa even when the bell is full of embryos.

[Illustration: FIG. 97.—An egg of _Echinorhynchus acus_ Rud. surrounded by
three egg-shells. Highly magnified. The egg has segmented, and the cells
are differentiated into _a_, the entoblast, and _b_, the ectoblast; _c_,
spines. (From Hamann.)]

EMBRYOLOGY.—After fertilisation the egg surrounds itself with several
egg-shells, three of which are usually distinguished; the embryo is already
far advanced in its development by the time it leaves the body of the
mother and passes out into the alimentary canal of the Vertebrate host. It
leaves the body of this second host with the faeces, and is eaten by the
first or larval host, usually a small Crustacean or water-insect, but in
some cases a fish, within whose alimentary canal it casts its membranes and
{180}becomes actively mobile. By means of a ring of hooks developed round
the anterior end it bores its way through the wall of the alimentary canal,
and after some time—three weeks in _E. proteus_—comes to rest in the
body-cavity of its host. By this time most of the organs of the adult, with
the exception of the reproductive glands, are already well established; the
latter only attain maturity when the first host is eaten by the second, and
the larvae find themselves in the intestine of a Vertebrate.

[Illustration: FIG. 98.—A, A larval _Echinorhynchus proteus_ Westrumb.
further developed than in Fig. 97. Highly magnified. The entoblast has
developed inside it the proboscis _a_; _b_, _b_, the giant nuclei of the
ectoblast. B, The entoblast at a more advanced stage, the ectoblast is not
shown. The outermost layer of cells will form the muscles of the body-wall;
the body-cavity has appeared; _a_, proboscis; _b_, cerebral ganglion; _c_,
body-cavity; _d_, _d_, the testes beginning to appear in the ligament; _e_,
cells which will form the generative ducts.]

Some of the details of the development are very remarkable, and a short
account of them may be given. The segmentation of the egg is unequal; it
results in the formation of a central biscuit-shaped mass of small cells
and a peripheral mass of larger cells; the former is called by Hamann[218]
the entoblast, the latter the ectoblast. From the entoblast arise all the
organs of the body but the sub-cuticle and the associated lemnisci, which
are formed from the ectoblast. The latter has a remarkable history; the
cells begin to break down and lose their outlines, whilst their nuclei fuse
together and form a small number of giant nuclei, which lie scattered
throughout the syncytium thus formed. The syncytium surrounds the entoblast
on all sides; by this time the anteriorly-placed hooks have appeared; in
_E. proteus_ there {181}are ten of these, but the number is not the same in
all species. The syncytium is in a fluid state, with a few gigantic nuclei
floating in it; these now lose their spherical shape, and throwing out
processes become amoeboid; in this way they bud off small portions of their
substance, and from these the oval nuclei of the sub-cuticle and the
lemnisci arise. The rest of the syncytium hardens into the fibrillar matrix
of the sub-cuticle, leaving, however, scattered spaces which form the
sub-cuticular sinuses of the adult. An interesting feature of _N.
clavaeceps_ and _Arhynchus hemignathi_ is that the skin of the adult
retains the larval features, and it and the lemnisci consist of a syncytium
with a very few giant nuclei scattered through it. Hamann counted only
eight in the skin and two in each lemniscus in the example figured on p.
178.

[Illustration: FIG. 99.—A, The larva of _Echinorhynchus proteus_ from the
body-cavity of _Phoxinus laevis_, with the proboscis retracted and the
whole still enclosed in a capsule. B, A section through the same; _a_, the
invaginated proboscis; _b_, proboscis sheath; _c_, beginning of the neck;
_d_, lemniscus. Highly magnified. (Both from Hamann.)]

The whole of the rest of the body is formed by the entoblast. Within the
latter a circular split arises which separates a single layer of outermost
cells from an axial strand of many cells (Fig. 98, B). The split is the
future body-cavity; the axial strand forms the proboscis, its sheath, the
cerebral ganglion, muscles, etc., and the ligament with the contained
generative organs; the outermost layer of cells forms the muscular lining
to the skin. It is interesting to note that these cells destined to become
muscle-fibres are at first arranged as a single layer of cubical epithelial
cells lining the body-cavity; most of them become circular muscle-fibres,
but a few are pushed inwards so as to lie next the body-cavity, and these
become the longitudinal fibres.

CLASSIFICATION.—Until recently the Acanthocephala were supposed to include
but one genus, _Echinorhynchus_, with several hundred species, but
Hamann[219] has pointed out that these species {182}present differences
which enabled him to divide the group into three families, each with a
corresponding genus. To these I have ventured to add a fourth family, to
include a remarkable species, _Arhynchus hemignathi_, described below. The
characters of the first three families in the account given below are taken
from Hamann's paper.

[Illustration: FIG. 100.—Fully formed larva of _Echinorhynchus proteus_
from the body-cavity of _Phoxinus laevis_. (From Hamann.) Highly magnified.
_a_, Proboscis; _b_, bulla; _c_, neck; _d_, trunk; _e_, _e_, lemnisci.]

FAMILY I. ECHINORHYNCHIDAE.—The body is elongated and smooth. The
proboscis-sheath has a double wall, and the proboscis is invaginated into
it. The central nerve-ganglion lies in the middle line, as a rule on the
posterior blind end of the proboscis-sheath. The papillae which bear the
hooks are only covered with a chitinous cap at their apex, and the hooks
have a process below. This family is by far the largest; a few species only
can be mentioned. _Echinorhynchus proteus_ lives in its mature form in
fishes; the young forms, up to a centimetre in length, are found living
freely in the intestine of numerous fresh-water fishes. Those found in
_Gobio fluviatilis_, the gudgeon; _Leuciscus virgo_; _Lota vulgaris_, the
burbot or eel-pout; young trout; _Thymallus vulgaris_, the grayling, seldom
surpass this size, but those found in _Acerina cernua_, the pope fish; in
_Abramis bipunctatus_; in _Esox lucius_,the pike, and in older trout,
attain or surpass double the length. As the parasites grow older they bury
their proboscis and neck in the wall of the intestine, the inner surface of
which is studded with the orange-coloured bodies of the parasites. The
proboscis is so deeply sunk in the wall of the alimentary canal as to form
a papilla on its outer surface (Fig. 92). The larvae of _E. proteus_ are
found in the body-cavity of _Gammarus pulex_, one of the Amphipod
Crustacea, and also in the same position in numerous fresh-water fishes;
they must have passed into this first host by the {183}mouth and alimentary
canal. If the liver of an infested minnow, _Leuciscus phoxinus_, be
examined, it will be found to contain on its surface numerous spherical or
egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these
contain the larval forms of the parasite. They develop into the adult form
when the first host is eaten by a carnivorous fish, but a complication may
take place when the larval form is found in _Gammarus_, as the latter, the
first host, may be eaten by a fish (intermediate host) in which the larva
does not become mature, and only develops sexual organs when eaten by a
carnivorous fish (second host). The larval form is also found in
_Nemachilus barbatulus_, _Gobio fluviatilis_, and the sticklebacks
_Gasterosteus aculeatus_ and _G. pungitius_.

_E. clavula_ Duj. is found in _Salmo fario_, _Abramis brama_, _Cyprinus
carpio_, _Gobius niger_, _Lepadogaster gouanii_, etc.; _E. linstowi_ Ham.
in _Leuciscus idus_, _Abramis ballerus_, _Abramis bipunctatus_, and
_Acipenser huso_; _E. lutzii_ Ham. was found by Dr. Lutz in Brazil in the
intestine of _Bufo agua_; _E. angustatus_ Rud. occurs in such numbers in
the perch, _Perca fluviatilis_, as to almost occlude the lumen of the
intestine, and one out of every three or four fish in certain districts is
infested by it. It is also found in the pike, _Esox lucius_, and the
barbel, _Barbus vulgaris_. The first or larval host of this species is the
Isopod _Asellus aquaticus_. _E. moniliformis_ Brews. is stated to attain
maturity in the human intestine. Except for the fact that _G. gigas_ has
once been observed in the same place, this is the only human parasite
amongst the Acanthocephala. Its normal second hosts are _Mus decumanus_ and
_Myoxus quercinus_, and its first or larval host, the larvae of the beetle
_Blaps mucronata_. _E. porrigens_ Rud. is found in considerable numbers in
the small intestine of a fin-whale (_Balaenoptera sibbaldii_), and _E.
strumosus_ Rud., in the small intestine of a seal (_Phoca vitulina_), and
in the body-cavity of the angler fish (_Lophius piscatorius_). _E. acus_ is
common in the whiting, _Gadus merlangus_.

FAMILY II. GIGANTORHYNCHIDAE.—Large forms with ringed, flattened, and
_Taenia_-like bodies. The hook-papillae are covered all over with
transparent chitinous sheaths with two root-like processes. The
proboscis-sheath is muscular and without a lumen. The central nervous
system is excentrically placed below the middle of the so-called sheath.
The lemnisci are long twisted tubes with a central canal.

{184}Hamann places three species in this family: _Gigantorhynchus
echinodiscus_, _G. spira_, and _G. taenioides_; but as he points out that
_E. gigas_ resembles these in its more important structural features, it
seems advisable to include it here under the name _G. gigas_. The members
of the first family often present a transversely ringed appearance after
death, but the Gigantorhynchidae are ringed when alive, and the circular
canals in the skin show a certain regularity, being arranged one between
each two rings. There is no lumen in the proboscis-sheath, which is not
attached to the boundary between the proboscis and the trunk, but to the
inner surface of the proboscis, and the whole can be retracted within the
anterior portion of the body, which is invaginable. There are always eight
cement-glands, and other differences exist in the musculature, hooks, and
position of the nervous system.

_G. gigas_ occurs in the adult state in the small intestine of swine; in
Europe its first or larval host is believed to be the grubs of _Melolontha
vulgaris_ and _Cetonia aurata_, but these beetles are absent from America,
though the parasite infests American hogs. Stiles[220] has recently made
some experiments which tend to show that in the United States the source of
infection is some species of the beetle _Lachnosterna_, and he has
succeeded in infecting the grub of _L. arcuata_ by feeding it on the eggs
of the parasite; from one larva he took 300 parasites six weeks after
feeding it. _L. arcuata_ is, like _M. vulgaris_, phytophagous, but the
grubs of both the beetles are fond of frequenting manure heaps and patches
of dung, and thus are much exposed to the dangers of infection.

_G. echinodiscus_ inhabits the intestine of ant-eaters, having been found
in _Myrmecophaga jubata_ and _Cycloturus didactylus_. _G. spira_ lives in
the king vulture _Sarcorhampus papa_, and _G. taenioides_ in _Dicholophus
cristatus_, a species of Cariama.

FAMILY III. NEORHYNCHIDAE.—Sexual maturity is reached in the larval stage.
The proboscis-sheath has a single wall. A few giant nuclei only are found
in the sub-cuticle and in the lemnisci. The circular muscle layer is very
simply developed. The longitudinal muscle-cells are only present in certain
places.

This family includes two species, _Neorhynchus clavaeceps_ and _N. agilis_,
which afford interesting examples of paedogenesis. The sub-cuticle and the
lemnisci are dominated by a few giant {185}nuclei, which remain in the
embryonic state and do not break up into numerous nuclei as in other forms.
The musculature is but little developed and the longitudinal sheath hardly
exists. The proboscis-sheath consists of a simple muscular layer, and the
short proboscis has few hooks and presents an embryonic appearance.

The sexually-mature form lives in the carp, _Cyprinus carpio_; the larval
form is found, according to Villot,[221] encysted in the fat bodies of the
larva of _Sialis lutaria_, one of the Neuroptera, and in the alimentary
canal of the leech _Nephelis octocula_, and successful experiments have
been made in infecting some species of the water snail _Limnaea_. _N.
agilis_ occurs in _Mugil auratus_ and _M. cephalus_.

FAMILY IV. ARHYNCHIDAE.—Short forms with the body divided into three
well-marked regions—head, collar, and trunk. The head is pitted, the collar
smooth, and the trunk wrinkled, not annulated, in spirit specimens. There
is no eversible introvert, and no introvert sheath and no hooks. The
sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are
long and coiled.[222]

This family resembles the Gigantorhynchidae in the length and curvature of
its lemnisci, and the Neorhynchidae in the persistence of the embryonic
condition of the nuclei in the sub-cuticle and the lemnisci; but in the
shape of the body, its division into three well-marked regions, the absence
of eversible proboscis, proboscis sheath, and hooks it stands alone, though
it is nearer to the Neorhynchidae than to either of the other families.

The single species _Arhynchus hemignathi_ was found attached to the skin
around the anus of a Sandwich Island bird, _Hemignathus proceros_. The bird
is a member of a family Drepanididae, which is entirely confined to the
Sandwich Island group. Professor Newton tells me that it is probable that
the "food of _Hemignathus_ consists entirely of insects which it finds in
or under the bark of trees," hence it is probable that the second host of
this parasite, if such exists, must be looked for amongst the Insecta.

{186}CHAPTER VII

CHAETOGNATHA

STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION

At certain seasons and at certain times of the day the naturalist who is
investigating the fauna of the surface of the sea is apt to find his
tow-net crammed with innumerable transparent spindle-shaped animals, which
by their number and the way in which they become entangled with rarer
objects, often render useless the result of his labours. These animals
belong to the class Chaetognatha, which includes three genera, _Sagitta_,
_Spadella_, and _Krohnia_. Amongst them are divided about twenty species,
some of which, however, are of doubtful value.

ANATOMY.—The body of these animals is as transparent as crystal; it is
elongated, and bears a resemblance to certain torpedos, except that the
head forms a somewhat blunt termination to the spindle-shaped body. The
tail bears a caudal fin, and _Spadella_ and _Krohnia_ have a single pair,
and _Sagitta_ two pairs, of lateral fins; all of which are flattened
horizontally.

The body is externally divisible into three regions—head, trunk, and
tail—and these correspond with the arrangement of the internal organs.

[Illustration: FIG. 101.—_Sagitta bipunctata._ _a_, Vesicula seminalis. ×
4. (After Hertwig.)]

The head is surrounded by a fold of skin, forming a hood, {187}which is
most prominent at the sides (Fig. 102, _g_); within the hood the head bears
from two to four rows of short spines, and outside these a right and left
row of sickle-shaped hooks, the free ends of which in a state of rest
converge round the mouth, but when disturbed these hooks can be widely
divaricated.

The cavity of the body, or coelom, is divided into three distinct chambers
by the presence of two thin transverse walls or septa, one situated between
the head and the trunk, the other between the trunk and the tail (Figs.
104, 105). In the head, this cavity is much reduced by the presence of
special muscles which move the spines, hooks, etc.; and in the small
species, such as _Spadella cephaloptera_, the other two cavities are almost
entirely occupied by the digestive and reproductive organs[223]; but in the
large species, e.g. _Sagitta hexaptera_, a considerable space is left
between the internal organs and the skin, and this is occupied by a
coelomic fluid. If the skin of one of these larger species be punctured the
fluid escapes and the animal shrivels up. A longitudinal partition or
mesentery, with numerous pores in it, runs through these spaces, dividing
the body-cavity into a right and left half; in the region of the trunk this
mesentery supports the alimentary canal.

In addition to certain muscles in the head, which move the hooks, etc.,
there is a muscular lining to the body-wall. This is divided into two
dorsal and two ventral bands, much in the same way as in Nematodes. The
muscle fibres are striated.

The mouth, situated either terminally—_Spadella marioni_[224]—or below the
head, leads into a pharynx; this passes into an intestine lined by a single
layer of ciliated cells with a few glandular ones intermingled. The
intestine runs straight through the body without loop or coil, and opens by
an anus situated at the junction of the trunk and the tail. In most cases
the anus is ventral or on the lower surface, but Gourret asserts that in
_Spadella marioni_ it is on the upper surface.

There are no special respiratory, excretory, or circulatory organs, unless
a glandular structure described by Gourret in the head of _Spadella
marioni_ be a real kidney.

The nervous system consists of a supra-oesophageal ganglion {188}or brain
situated in the head, and of a ventral ganglion lying in the trunk; both
these nerve centres are embedded in the epidermis, and are connected with
one another by means of two stout peri-oesophageal nerves (Figs. 102, 104).
The brain also gives off a pair of nerves to the eyes, another pair to the
olfactory organ, and a pair which ultimately meet one another and so form a
ring; on this are certain ganglia giving off nerves which supply the
muscles of the head. Both the chief ganglia give off numerous nerves, which
divide and split up into a network of fibres which permeate the whole skin.

The sense organs are comparatively simple. A pair of very small eyes lie in
the skin of the head; they are of complex structure, and to some extent
remind one of the simple eyes of certain Crustacea. Behind the eyes and
also on the upper surface of the animal is an unpaired organ which is
usually described as olfactory in function (Figs. 103, 105). This is a
ring-shaped modification of the epidermis drawn out into different shapes
in the various species. The modified epidermal cells bear long cilia. The
remaining sensory organs found in the group consist of clumps of modified
cells scattered in round groups over the surface of the body and of the
fins. The central cells of each group bear long tactile hairs, and are
surrounded by supporting cells.

[Illustration: FIG. 102.—Head of _Sagitta bipunctata_. A, Dorsal view; B,
ventral view. × about 33. (From Hertwig.) A, _a_, spines; _b_, nerves to
lateral cephalic ganglia; _c_, hooks; _d_, cephalic ganglion; _e_,
olfactory nerve; _f_, optic nerve; _g_, hood; _h_, commissure to ventral
ganglion; _j_, olfactory organ: B, _a_, _c_, and _g_ as in A; _k_, mouth.]

The Chaetognatha are hermaphrodite, and carry the female organs in the
trunk, the male in the tail. In a mature specimen the two ovaries occupy
almost all the space in the trunk between the alimentary canal and the
skin, and each is supported by a narrow lateral mesentery. The ovary is
traversed by a oviduct which often contains spermatozoa; it is not clear
how the eggs make their way into the oviduct, which seems to have {189}no
internal opening and to act largely as a receptaculum seminis. The oviducts
open externally on the upper side at the base of the lateral fin, close to
the junction of the tail and the trunk.

The cavity of the tail is divided into two lateral chambers by the
extension backward of the median vertical mesentery. In each of these a
testis and a vas deferens are found. The testes are solid ridges formed by
the growth of the lining cells of this part of the body-cavity; the cells
mature into spermatozoa, which break off and float freely in the coelomic
fluid. At the breeding season the whole tail may be crowded with masses of
spermatozoa, which are kept in a more or less regular circulation by the
ciliated cells lining the body-wall. The vas deferens opens internally into
the space where the spermatozoa lie, and at the other end into a vesicula
seminis, which opens to the exterior. The position of the latter structure
varies, and is of some systematic value.

The eggs are laid in the water and as a rule float at the surface of the
sea. _Spadella cephaloptera_ is, however, an exception to this rule, as it
attaches its eggs by means of a gelatinous stalk to sea-weeds. The
segmentation of the ovum is regular, and gives rise to a two-layered stage
or _gastrula_, which opens by a pore, the _blastopore_. This does not,
however, become the mouth, but closes up and the mouth arises at the
opposite pole. Perhaps the most interesting feature of the development of
_Sagitta_ is that the cells destined to form the reproductive organs
separate from the other cells of the embryo at a very early date, whilst it
is still in the gastrula stage. There is no larval form, but the young
hatch out from the egg in a state resembling the adult in all respects but
that of size.

[Illustration: FIG. 103.—_Spadella cephaloptera._ Dorsal view. x 30. (From
Hertwig.) _a_, Cephalic ganglion; _b_, commissure to ventral ganglion; _c_,
olfactory organ; _d_, alimentary canal; _e_, ovary; _f_, oviduct; _g_,
testis; _h_, vesicula seminalis.]

HABITS.—The Chaetognatha are essentially pelagic, and {190}resemble many
other creatures that dwell at the surface of the ocean in being almost
completely transparent. Most species have been taken far out at sea, but
some are perhaps rather more numerous near the coast, and one species,
_Spadella cephaloptera_, is littoral. They swim by means of muscular
movements of the whole body; the fins have no movement of their own, and
seem to serve as balancers, and not as locomotory organs. Although usually
found at the surface of the water, many species have been taken at
considerable depths. Chun[225] states that they are found in countless
numbers at depths of from 100 metres to 1300 metres. The commonest species
at these depths are _Sagitta hexaptera_ and _Sagitta serratodentata_.
_Sagitta bipunctata_, according to the same authority, confines itself to
the surface. Whether the change of depth is diurnal, or whether it has any
relation to sexual maturity, or to any other cause, has not been
satisfactorily determined.

The food of the Chaetognatha consists of floating diatoms, Infusoria, small
larvae, and such Copepods as _Calanus finmarchicus_, and small Amphipods as
_Phoxus plumosus_.[226] At times they also devour small larval or
post-larval fishes, and owing to their incredible numbers, they doubtless
do considerable damage to sea fisheries. It is also recorded that they eat
one another, and specimens have been taken which have ingested the whole
body of another _Sagitta_ except the head, which hangs out of the mouth of
the eater, and gives it the appearance of a double-headed monster.[227] It
has been said that they attack hydroid polypes, but here at any rate they
do not have it all their own way. Masterman[228] has figured the apical
group of five polypes of _Obelia_, three of which are engaged in ingesting
as many young _Sagitta_.

They exist in incredible numbers; Grassi describes the surface of the sea
at Messina on certain days as being literally covered with them, and they
must form the food supply of numerous animals which prey upon the pelagic
fauna. The immense number of individuals is probably accounted for to some
extent by the fact that they lay eggs all the year round, and pass
{191}through a very short and rapid development. They are not known to be
phosphorescent.

CLASSIFICATION.—The features of the Chaetognatha which have most systematic
value are the size of the adult, the relations of the length to the
breadth, and of the three divisions to one another; the size, number, and
position of the lateral fins, and of the hooks and spines on the head; the
thickness of the epidermis, and the structure of the olfactory organ; and,
finally, the form of the reproductive organs.

Strodtmann,[229] who gives the latest and most complete account of the
species of Chaetognatha, arranges them under three genera, which he
characterises as follows:—

(i.) _Sagitta_ Slabber.—Two pairs of lateral fins, two rows of spines on
the head. The lateral thickening of the epidermis absent or insignificant.

Under this genus are included nine definite species and five others—_S.
gracilis_ Verrill, _S. elegans_ Verrill, _S. darwini_ Grassi, _S. diptera_
d'Orbigny, and _S. triptera_ d'Orbigny—whose position, owing to the
inadequacy of their description, is of doubtful validity.

[Illustration: FIG. 104.—_Sagitta hexaptera._ Ventral view. × 4. (From
Hertwig.) _a_, Mouth; _b_, hooks; _c_, anterior septum; _d_, alimentary
canal; _e_, commissure from the brain to the ventral ganglion; _f_, ventral
ganglion; _g_, ovary; _h_, oviduct; _i_, posterior septum; _j_, testis;
_k_, vesicula seminalis.]

The distribution of the other species may be mentioned. _S. hexaptera_ is
the largest Chaetognath known, and reaches in the adult stage a length of 7
cm. It is very widely distributed, being found in practically all the
temperate and warm seas, usually at the surface of the water, though at
times it is found at a depth of one metre, or even deeper. _S. lyra_,
Mediterranean, very rare. _S. tricuspidata_, widely distributed. _S.
magna_, Mediterranean and Madeiran, living at the surface. _S. bipunctata_,
the most frequently described form, smaller than the preceding species, 1-2
{192}cm. in length, widely distributed, and as a rule living near the coast
line. _S. serratodentata_, Mediterranean. _S. enflata_, on the surface of
the sea, Mediterranean and Madeiran. _S. minima_, a very small species, 1
cm. in length, Mediterranean. _S. falcidens_, Atlantic, off the coast of
New Jersey.

(ii.) _Krohnia_ Langerhans.—A single lateral fin extending on to both trunk
and tail segment, no lateral epidermal extensions behind the head, only one
row of spines on the head. Trunk longer than the tail.

_Krohnia_ has but two species: _K. hamata_ Möbius, with a length of 3-4
cm., found in the North Atlantic and at considerable depths, 200 to 300
fathoms; and _K. subtilis_ Grassi, 1.5 cm. long, with an extraordinary
slender body and a relatively large head, found at Messina, but very rare;
as a rule only one specimen has been found at a time.

(iii.) _Spadella_ Langerhans.—A single pair of lateral fins; these are
situated on the tail segment. Behind the head a thickening of the epidermis
extends down each side of the body to the fin, or even farther. Two rows of
spines on the head. Small animals, not longer than 1 cm.

[Illustration: FIG. 105.—_Spadella draco._ Dorsal view. × 12. (From
Hertwig.) _a_, Cephalic ganglion; _b_, commissure between the cephalic
ganglion and the ventral; _c_, eye; _d_, olfactory organ; _e_, alimentary
canal; _f_, ovary; _g_, oviduct (the line goes a little beyond the duct);
_h_, testis; _j_, vesicula seminalis.]

_S. cephaloptera_ Busch is the smallest species of Chaetognatha, attaining
at most a length of .5 cm. The body is not so transparent as in other
species, and is of a yellowish colour. It has been found from the Orkney
Islands to the Mediterranean. Strodtmann is of the opinion that the three
species _S. mariana_ Lewes, _S. batziana_ Giard, and _S. gallica_
Pagenstecher differ from the above-named only in size, or that their
description is too indefinite to permit of accurate {193}characterisation.
He recognises three other distinct species: _S. pontica_ Uljanin, from the
Black Sea; _S. marioni_ Gourret, from the Gulf of Lyons; and _S. draco_
Krohn, Mediterranean and Madeiran, and from the Canaries.

Much confusion has been introduced into the classification of the
Chaetognatha by Grassi,[230] who calls some—but not all—of what other
writers term _Sagitta_, _Spadella_, and _vice versâ_. The following table
was compiled by Strodtmann,[231] but I have incorporated in it two species
recently described from Amboyna by Béraneck,[232] and called by him
_Sagitta bedoti_ and _Spadella vougai_ respectively:—

CHAETOGNATHA

I. Two pairs of lateral fins; two rows of spines on the head; slender
forms.

(i.) Number of spines in posterior row greater than in anterior.

_a._ Border of hooks smooth, their point not curved.

α. No interval between the two fins on each side. 3.5 cm. long; 4-7
anterior spines, 8-11 posterior spines; olfactory organ lying
entirely on the trunk. The anterior nerves of the ventral ganglion
lie close to one another as far as the head.—SAGITTA LYRA.

β. A distinct interval between the two fins on each side.

_aa._ Adult animals large; hooks 6-7; anterior spines 3-4;
posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral
areas relatively larger.—SAGITTA HEXAPTERA.

_bb._ Greatest length 1-2 cm.

αα. Thickening of the epidermis behind the head; prominently
projecting vesiculae seminales; olfactory organ very long;
hooks 8-10; anterior spines 4-6; posterior spines
10-15.—SAGITTA BIPUNCTATA.

ββ. No epidermal thickening; two caeca on the anterior end of
intestine; length 1 cm.; hooks 6-9; anterior spines 3-4;
posterior spines 7-8; point of the hooks somewhat bent
round.—SAGITTA MINIMA.

γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent;
anterior spines 3-4; posterior spines 7-8; length 2 cm.; small
head; trunk proportionately thick.—SAGITTA ENFLATA.

δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines
6-7; posterior spines 18.—SAGITTA FALCIDENS.

{194}εε. Hooks 7 on each side; length 1.3 cm.; anterior spines
8-10, posterior spines 18-22; no olfactory organ.—SAGITTA
BEDOTI.

_b._ Edge of hooks toothed and their point bent round; hooks 6-8;
anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender;
conspicuously projecting vesiculae seminales.—SAGITTA SERRATODENTATA.

(ii.) Number of the spines in posterior row smaller than in anterior.

_a._ Anterior spines 3; posterior spine 1; hooks 8; length 3.5
cm.—SAGITTA TRICUSPIDATA.

_b._ Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1
cm.; tail ⅕ of the total length.—SAGITTA MAGNA.

II. One pair of lateral fins lying on the trunk and tail; one row of
spines; body slender; epidermis not thickened.

(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young;
20-25 spines in a row; ovary reddish; length 3-4 cm.—KROHNIA HAMATA.

(ii.) Hooks 8, broad at their base but very sharply pointed; spines in
a curved row, about 18, with a constriction below like the neck of a
bottle; body thin; length 1-1.5 cm.—KROHNIA SUBTILIS.

III. One pair of lateral fins, these lie on the tail; body relatively
very broad in consequence of the thickening of the epidermis lying behind
the head; two rows of spines; greatest length 1 cm.; tail and trunk
usually the same length.

(i.) A great extension of the epidermis behind the head, consisting of
very large cells; amongst these, at the level of the ventral ganglion,
lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10;
anterior spines 6-8; posterior spines 12-18.—SPADELLA DRACO.

(ii.) Lateral extension of the epidermis not so conspicuous, and the
cells composing it smaller. Tactile organs in little depressions.
Transverse as well as longitudinal muscles in the trunk. Adhesive cells
on the ventral surface of the body. No interval between the lateral
fins and the tail fin. Two papillae on the head-hood elongated into
club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines
3-4; posterior spines 3-4.—SPADELLA CEPHALOPTERA.

(iii.) Similar to the last-mentioned species, but the tail segment is
larger than the trunk; in the above it is of the same size. No adhesive
cells. The fins are covered with papillae, and with a number of
serrated spines pointed at both ends.—SPADELLA PONTICA.

(iv.) Tactile organs and adhesive cells are unmodified epidermal cells.
Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis
colourless. Lateral fins without rays. A pair of ganglia at the
postero-lateral angle of the brain.—SPADELLA MARIONI.

(v.) Tactile organs well developed on the head, trunk, and fins; tail
segment a little shorter than the trunk. Body short, length 3-4 mm.
Hooks 9; anterior spines 4-5, posterior spines 6-7.—SPADELLA VOUGAI.

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

{197}CHAPTER VIII

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENT—ANATOMY—REPRODUCTION—
EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES—GASTROTRICHA—
KINORHYNCHA

The Rotifera are microscopic animals, the largest not exceeding one-eighth
of an inch in length. According to Hudson and Gosse,[233] they are first
recorded in an observation of the Rev. John Harris, in 1696, of "an Animal
like a large Maggot which could contract itself into a Spherical Figure,
and then stretch itself out again; the end of its Tail appeared with a
_Forceps_ like that of an _Ear-wig_."[234] This was certainly a Bdelloid
Rotifer.

In 1703 Leeuwenhoek[235] gave a fuller description of a tubicolous form,
probably _Limnias_, and noted the peculiar appearance of the ciliary wreath
as "two wheels thickset with teeth as the wheel of a watch." He also noted
a little later[236] the way in which _Melicerta_ (see p. 206) builds its
tube, and was the first to observe the revivification of certain species
after drying.[237] Joblot, a French professor of mathematics, in 1718
figured and described a large number of new genera and species with more or
less fantastic details. Baker's figures[238] are a considerable advance on
Joblot's, and his descriptions of habits are still fresh and accurate.
Eichhorn found a number of new and interesting forms; and O. F. Müller,
influenced by the new discipline of Linnaeus, not {198}only figured many
species, but gave good short diagnoses of their characters. Ehrenberg in
1838 brought out his magnificent _Infusionsthierchen_, which contains
descriptions and figures of what are now divided into Protophyta, Protozoa,
Rotifera, and Gastrotricha. Dujardin's monograph on the "Infusoires," in
the _Suites à Buffon_,[239] was in several respects an advance on
Ehrenberg, whose power of observation was so great as to render his
mistakes the more inexplicable. But Ehrenberg ever adhered to his errors as
firmly as to his facts.

The occurrence of Rotifers among microscopic plants induced the botanists
Cohn and Williamson[240] to work at their structure; the group has been
studied by men engrossed in other professional cares, such as Gosse,
Bedwell, Moxon, Rousselet, and Maupas. Huxley,[241] Leydig,[242] and
Cohn[243] studied Rotifers in the '50's and early '60's with a precision
the more remarkable when we remember the imperfect methods then available.
This period was closed by the valuable monograph published in Arlidge's
(4th) edition of Pritchard's _Infusoria_,[244] under the supervision of W.
C. Williamson. Leidy began the study of the American Rotifers.
Eckstein[245] gave a careful and interesting account of the species about
Giessen in a richly illustrated paper. In recent times the modern methods
of histological and embryological research have been applied by
Vallentin,[246] Plate,[247] Tessin,[248] and Zelinka,[249] the three
_Studien ueber Rotatorien_ of the last author being indispensable to every
student, and containing a full bibliography.

Hudson and Gosse's Monograph (1886-89) contains a history of the class to
which, as to the whole book, we are deeply indebted; and a full systematic
account of all published species.[250] C. Rousselet has introduced a
method[251] of preparation of Rotifers in microscopic slides which enables
workers to preserve the types they figure and describe for future
identification and comparison. Gunson Thorpe has collected and studied
Rotifera in China and {199}Australia. It would be unfair not to record here
the invaluable services of the late Thomas Bolton, and his son of the same
name, both of Birmingham, and of J. Hood of Dundee, who have found and
widely distributed living specimens of new, rare, and interesting species.

[Illustration: FIG. 106.—_Hydatina senta_, ventral view. (After Plate.)
_al_, Lateral antenna; _bl_, bladder; _ci_, cingulum; _e_, _e_, eggs in
uterus; _fg_, foot gland; _g_, gizzard; _gg_, gastric gland; _gm_,
germarium or ovary; _gr_, ciliated lobes of "groove"; _i_, intestine; _k_,
_k_, kidneys; _m_, mouth; _ns_, nephrostome; _oe_, oesophagus; _rc_, renal
commissure, transverse tube uniting kidneys above mouth; _s_, stomach
overlaid by reproductive organs; _tr_, trochus; _u_, uterus; _vm_,
vitellarium or yolk-gland.]

DEFINITION OF THE CLASS.—We may define Rotifera as a class of minute
bilaterally symmetrical animals, with a chitinous integument, a soft
terminal "disc" fringed by a complex ciliary "wreath," an anterior or
subventral mouth, and a dorsal cloacal aperture, beyond which the body is
usually prolonged into the "foot" or process bearing cement glands, and
serving for attachment, temporary or permanent. The body-cavity has no
epithelial lining, and is traversed by nerves and muscles. The alimentary
canal possesses a chitinous gizzard or _mastax_ of peculiar arrangement,
and it usually opens into a cloaca. The nervous centre consists of a
ganglion on the dorsal side of the pharynx, to which a second one on the
ventral side is sometimes connected to form a complete ring; eyes and
bristle-bearing feelers are usually present as sense-organs. A paired
system of renal tubes serves for excretion, opening through a median
contractile bladder into the ventral side of the cloaca. The sexes are
distinct; but the males (Fig. 107), which mostly lack digestive organs,
occur {200}rarely, and the females are usually viviparous, or carry about
the eggs till they are hatched; while, owing to the rarity of the males,
parthenogenesis is habitual. Fission and budding are alike unknown. The
fertilised eggs are of the kind termed "winter" or "resting" eggs, and
resist conditions adverse to life.

The Rotifera are of cosmopolitan distribution; most of the species inhabit
fresh water, whilst some are brackish, and a few are marine; 84 genera and
about 700 species have been described.

[Illustration: FIG. 107.—Male Rotifers. (After Hudson.[252]) 1,
_Floscularia campanulata_; 2, _Lacinularia socialis_; 3, _Notops
brachionus_; 4, _Synchaeta tremula_; 5, _Asplanchna ebbesbornii_; 6,
_Brachionus urceolaris_; 7, _Salpina mucronata_; 8, _Pedalion mirum_.]

EXTERNAL FEATURES.[253]—The body is divided into three regions: (1) the
_head_, ending in the _disc_, which bears the ciliary _wreath_; (2) the
_trunk_, containing the viscera; (3) the _foot_, which only contains
muscles, nerves, and cement-glands. The general form of the BODY varies
greatly: it is spherical in _Trochosphaera_, ovoid in Asplanchnidae,
conical in Scirtopoda, Triarthridae, and _Synchaeta_; moderately elongated
in the majority of the Ploima, among which some forms are very flat, like
_Pterodina_, _Metopidia_, and _Brachionus_; shortly elongated and
cylindrical in _Hydatina_ (Fig. 106), Notommatidae, and many others. In
_Taphrocampa_ it is cylindrical and segmented, while the segments are
{201}telescopic in the Bdelloida, both ends being retractile into the
middle segment. In most attached, tube-dwelling forms the body is ovate,
tapering behind into the elongated stalk-like foot.

The FOOT at the hinder end of the body is usually more or less jointed; in
_Pterodina_ and _Brachionus_ it is long, transversely wrinkled, and
retractile. Usually it terminates in a couple of acute, mobile toes,
perforated at the tips by the ducts of the pedal glands (Fig. 106, _fg_),
whose viscid secretion serves to anchor the animal. In _Rotifer_ there are
three of these toes, which are retractile, and in addition there are in
this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on
the more proximal joints of the foot. In _Callidina_ the spurs are often
perforated, and the toes are replaced by numerous openings on the last
joint of the foot (Fig. 109, A); while in _Discopus_ the end of the foot
expands into a large disc, with numerous pores for the exudation of the
pedal cement, and there are no spurs. In _Pedalion mirum_ the foot is
represented by two tubular processes ciliated at the apex and at the outer
side near the base (Fig. 117, _f_). These are inconstant in size and form,
that of one side being sometimes reduced or absent, while both are absent
in the closely allied species _P. fennicum_.

In Melicertidae and Flosculariidae the long foot ends in an expanded disc,
which is cupped and ciliated in the larva (Fig. 112, B) and in the
larva-like male (Fig. 107); but in two species it is prolonged into a long
flexible thread which is not contractile. The foot is also elongated in the
Bdelloid genus _Actinurus_ and the Ploimal genus _Scaridium_. It forms a
mere ventral disc in _Apsilus_ (and _Atrochus?_), and is absent in
Asplanchnidae (except _Asplanchnopus_), Triarthridae, and Anuraeidae, and
in the genera _Trochosphaera_ (Melicertaceae) and _Pompholyx_
(Pterodinidae).

The fringed spines of Triarthridae are jointed appendages moved by powerful
muscles; in _Triarthra_ one is median and ventral, the others being
attached to the shoulders. In _Polyarthra_, there are twelve flattened and
serrated spines, a bunch of three being attached to the dorsal and ventral
faces of either shoulder. An easy transition leads to the hollow appendages
of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered
with finer hairs (Fig. 117). These processes are in _Pedalion_ six in
number, two median (respectively dorsal and ventral), two antero-lateral,
and two postero-lateral. As they contain proper muscles, {202}and the
postero-lateral pair contain part of the nephridia and bear the lateral
antennae, they are true outgrowths of the body, and are not homologous with
the spines of Triarthridae.

[Illustration: FIG. 108.—Diagrammatic views of disc of Rotifers. Cingulum
represented by a black line, groove shaded; trochus dotted; the black spot
represents the mouth. 1, Simple disc of _Microcodon_; 2, Bdelloid disc of
_Rotifer_ or _Callidina_, the star represents the ciliated proboscis; 3,
disc of _Hydatina_, groove represented by lobes bearing ciliated styles; 4,
disc of _Melicerta_, the star represents the ciliated ventral cup with
openings into it from the groove; 5, disc of _Conochilus_; 6, disc of
_Stephanoceros_, cingulum (?) of setose lobes, trochus horseshoe-shaped,
mouth central.]

The front of the body constitutes the HEAD, which is scarcely distinct,
though usually separated by a slight neck-like constriction. The DISC,
which terminates the head, varies greatly in shape and in the arrangement
of its parts. Imagine a circular funnel, finely ciliated within, and with
the mouth at the bottom, the prominent rim bearing two zones of cilia, the
inner or anterior being the coarser, and termed the "trochus" or _hoop_;
the outer finer, and termed the "cingulum" or _girdle_, while a very finely
ciliated groove lies between the two zones. Either or both of these zones
may be interrupted on the dorsal or ventral median line, or both; and the
funnel-shaped mouth may be shifted—usually ventrally, so that it forms only
a dilatation of the ciliated groove. Again, the wreath as a whole may be
festooned or lobed; or the lobing may be confined to the area between the
cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very
frequently on these lobes adjacent cilia are fused together during life,
producing "vibratile styles," whose true nature is only revealed after
death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly
conforms to the primitive type; but the ciliated groove is absent, and the
"trochus" is in two separate half-elliptical bands. In the Flosculariaceae
(Fig. 108, 6) the mouth is also central, the disc is funnel-shaped,
{203}and the trochus is a horseshoe-shaped ridge, with its ends dorsal and
raised into prominent knobs. The margin of the funnel is in Flosculariidae
(Fig. 115) usually lobed, and furnished either with exceptionally strong
cilia, or else with very long bristles which are usually passive. However,
by the retraction of the lobes that bear them they are clasped together
like casting-nets to enclose prey brought into the funnel by the action of
the trochal cilia. An external ring of cilia in _Floscularia mutabilis_ and
_F. pelagica_ serves for swimming. In Apsilidae the margin of the disc
bears neither cilia nor bristles, but is either simple and ring-like, or is
produced into tentacles (Fig. 112, C). The oral funnel is probably
represented in Flosculariaceae by the continuation of the small central
mouth into a ciliated tube (Fig. 115, C, _tf_), open below, and hanging
freely down into the crop.

In all other cases the mouth is displaced, and lies in the groove and on
its ventral side (except in _Conochilus_, where it is dorsal, Fig. 108, 5).
In the Bdelloida the disc is prolonged into two great lobes like
kettle-drums, round the posterior, external, and ventral edges of which run
the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are
interrupted behind in the median line; ventrally the groove widens into the
oral funnel, the cingulum is continued into a sort of spout-like lower lip
(Fig. 109, C, D, _l_), and the trochus is absent. The body is prolonged
dorsally above the lobes into a two-jointed _proboscis_, ending in a
ciliated cup overhung by two dorsal flaps: this we regard as a detached
portion of the wreath.

This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in
the Ploimal genera _Triarthra_, _Pterodina_, and _Pompholyx_. A simpler
wreath of essentially the same type occurs in Asplanchnaceae and
Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is
thin at the rim, where it bears the triple ciliated zone, interrupted on
the dorsal median line and depressed ventrally into the oral funnel. In the
Melicertidae, moreover, the disc is widened into a great plate-like
extension, often beautifully lobed; and in many of the species a ciliated
cup lies ventral to the lips, and is connected with the groove by a short
ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler
wreath of Asplanchnidae is complicated by stronger lobes on either side
bearing vibratile styles.

The most complex discs are found in Ploima, especially in
{204}_Brachionus_, _Hydatina_, and _Synchaeta_, since the groove is
replaced by a zone of lappets, as above mentioned. In _Proales_ the whole
face of the disc is strongly ciliated. The wreath is reduced in the
parasitic genera _Drilophagus_, _Albertia_, _Balatro_, and the Seisonaceae;
in _Adineta_ and _Taphrocampa_ it is only represented by a general but
scanty ciliation of the disc.

[Illustration: FIG. 109.—_Callidina symbiotica._ (After Zelinka.) A,
Ventral view, with the disc half expanded, proboscis extended; B, lateral
view, proboscis extended; C, ventral view of anterior segments with
expanded disc; D, lateral view of same (proboscis retracted). _a_, Antenna;
_bl_, bladder (enlargement of rectum); _c_, ciliated cup of the proboscis;
_ci_, cingulum; _cl_, cloaca; _cp_, group of pores, the openings of cement
glands; _di_, disc; _g_, gizzard; _gm_, germarium (that of the opposite
side seen at a higher level); _gr_, ciliated groove; _k_, kidney; _l_, lip;
_m_, mouth; _pr_, proboscis; _sp_, spurs of foot; _tr_, large cilia of
trochus, showing vertical movements; _vm_, yolk-gland. The body muscles are
represented by shaded bands.]

The head is very frequently retractile, as a whole, by strong muscles. In
Bdelloida the disc proper is retracted when the animal crawls, while the
proboscis is exserted (Fig. 109). Ciliated patches occurring outside the
region of the disc point to {205}a primitive condition when the whole
surface of the body was ciliated, as does the partial ciliation of the foot
in certain groups. _Synchaeta_ and many Notommatidae possess a pair of
lateral, hollow, ciliated pits on the body, which can be everted to serve
as additional swimming organs; these are termed "auricles."

The _cuticle_ varies much in texture. It may be smooth and flexible, dotted
or shagreened, or in the Loricata firm and of definite shape, constituting
a _lorica_, which may be more or less distinctly divided up into areas or
separated into distinct pieces. In this case it resists decomposition, and
several species are only known by this "skeleton." In _Ploesoma_ it is much
thickened and looks like a honeycomb. A regular alternation of harder and
softer zones effects the annulation of the body in certain genera.

The _hypoderm_ or protoplasmic layer of the skin has no cellular
boundaries, though it contains large and distinct nuclei; it is usually
somewhat granular. It forms the wall of the body-cavity, which contains a
transparent liquid without corpuscles.

The principal _external glands_ are the pedal or cement-glands, which
secrete a viscid substance that sets in water and serves to anchor the
animal. They are formed from an ingrowth of the hypoderm, are usually
paired, and open by fine ducts on or near the apex of the toes, when these
processes of the foot are present (Fig. 106, _fg_). These glands are mostly
absent when there is no foot, as in most Asplanchnidae and in Anuraeidae,
but in _Asplanchna herrickii_ a small gland on the ventral side of the
cloacal aperture appears to represent the last rudiment of the foot.

In addition to these, the ciliated ventral cup below the disc of many
Melicertidae secretes a viscid substance (Fig. 116, _p_); and possibly the
whole surface of the body is secretory in those species of this group, and
of the Flosculariidae, whose tube (Fig. 115, A) is uniform and not made of
pellets. In several other species belonging to Bdelloida and
Ploima-Illoricata a viscid secretion of the surface of the body renders it
"sordid" with adherent particles of dirt.

When the secretion takes the form of a tube, the body can be wholly
withdrawn into it by the contraction of the foot. In _Floscularia_,
_Stephanoceros_, and _Conochilus_ the tube is hyaline and thin-walled; in
_Oecistes_ and _Cephalosiphon_ it is more or less floccose; and in
_Limnias_ it is thin, firm, and annulated. In _Melicerta_ and some species
of _Oecistes_ the tube thus secreted by {206}the body is only formed in a
very young state. In _M. janus_ and _M. pilula_ it is increased by the
successive deposition of ovoid faecal pellets on to the rim. In _M.
ringens_ (Fig. 116) and _M. conifera_ pellets are formed of the excess of
the food particles brought to the disc by the ciliary current; they are
carried through the gutters on either side of the projecting ventral lip or
"chin" into the ciliated glandular cup on that side of the head. Here, as
they revolve, they are cemented together into a pellet which is spheroidal
in the former species, cylindro-conoidal with a basal hollow like a
rifle-bullet in the latter. After a pellet is completed the animal stoops
down and deposits it on the edge of the tube. This may easily be verified
by furnishing a young _Melicerta_ with water containing solid particles of
carmine. _M. tubicolaria_ forms a thick tube which is laminated, the
laminae being directed upwards and outwards, and having diatom shells,
etc., between the layers. In this case we have observed that the faeces are
pellucid, and sometimes are so ejected as to lie in a sheet against the
funnel-shaped mouth of the tube, and we are inclined to believe that the
tube itself is formed altogether in this way. A similar process probably
occurs in _Oecistes crystallinus_ and _Oe. umbella_.

The _muscles_ are simple elongated fibres, usually having near the middle a
mass of granular protoplasm containing a nucleus; they may be smooth or
striated. The principal muscles of the body are conspicuously striated in
many active free-swimming forms (_Pedalion_, _Synchaeta_, _Pterodina_,
_Triarthra_).

The muscles of the body-wall are transverse and longitudinal. They are best
seen in Bdelloida. The principal muscles of the body-cavity are
longitudinal; the most conspicuous and constant are the retractors of the
disc and of the foot, protraction of these organs being usually
accomplished by the contraction of the transverse muscles. Special muscles
effect the vigorous springing of the Triarthridae and Scirtopoda; in the
former group the muscles only raise the spines, and their elastic recoil is
the actual mechanism of progression; but in the latter (Fig. 117) special
flexor muscles of the limbs are the effective agents of the leaping
movements.

MOVEMENTS.—The Rotifera vary very greatly in their movements. The cilia of
the disc, and especially of the trochus, are the principal organs of
prehension of food, and also of swimming when {207}the animal is not fixed
by its foot. In some cases, as in Bdelloida, the cilia lash downwards
successively in the longitudinal plane of the body (Fig. 109, C, D); this
motion during fixation produces a hollow vortex ring, like the rings of a
skilled cigarette-smoker, but when the animal is free it determines a
simple forward progression through the water. In other cases the animal
rotates on its long axis, or may even turn somersaults (_Synchaeta_). The
appearance of the spokes of a wheel is a pure illusion due to the greater
visibility of the cilia in their slow recovery than in their instantaneous
down-lash. The finer cilia of the groove and cingulum play a very minor
part in the act of swimming, and in the production of the great vortices at
the edge of the disc when the animal is fixed; they serve to direct the
particles brought by the vortices to the edge of the disc onwards towards
the mouth. It is easy to see that the stream must be in opposite directions
on opposite sides of the groove; its prolongation across the dorsal median
line would be useless, which explains the existence of the dorsal median
gap. At the ventral side we usually find a prominent ciliated lip, whose
cilia work outwards, and carry off the excess of food particles as by an
overflow spout. In many cases among the Notommatidae, Coluridae, etc., the
disc serves as much for creeping over organic débris as for swimming.

We have already noticed the springing bristles and limbs of the
Triarthridae and Scirtopoda respectively; the great foot of _Scaridium_ is
also used for leaping. The Bdelloida have the power of retracting their
disc and progressing in loops like a leech or looper (Geometrid)
caterpillar.

Baker, in a letter addressed to Martin Folkes, Esq., President of the Royal
Society, dated London, 16th January 1744-5,[254] gives the following lively
account of the aspect and movements of _Philodina roseola_ belonging to
this group, with figures, some of which we reproduce from the original
copper-plate engraving:—"I call it a _Water Animal_, because its Appearance
as a living Creature is only in that Element. I give it also for
Distinction Sake the Name of _Wheeler_, _Wheel Insect_ or _Animal_; from
its being furnished with a Pair of Instruments, which in Figure and Motion
appear much to resemble Wheels. It can, however, continue many Months out
of Water, and dry as Dust; in which Condition its Shape is globular, its
Bigness exceeds not a Grain of {208}Sand, and no Signs of Life appear.
Notwithstanding, being put into Water, in the Space of Half an Hour a
languid Motion begins, the Globule turns itself about, lengthens by slow
Degrees, becomes in the Form of a _lively Maggot_, and most commonly in a
few Minutes afterwards puts out its Wheels, and swims vigorously through
the Water in Search of Food; or else, fixing by its Tail, works them in
such a Manner as to bring its Food to it. But sometimes it will remain a
long While in the Maggot Form and not shew its Wheels at all....

[Illustration: FIG. 110.—_Philodina roseola._ (After Baker.) A, B,
Crawling, with extended proboscis, and showing antenna; C, D, E, attached,
with "wheels" extended for catching food; F, attached, with anterior end
retracted.]

"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves
behind, has any Thing of a red Colour, one may be almost certain of finding
them therein,[255] and, if in Summer, when all the Water is dried away, and
nothing but Dust remains, that Dust appears red, or of a dark brown, one
shall seldom fail, {209}on putting it into Water, to discover Multitudes of
minute reddish Globules, which are indeed the Animals, and will soon change
their Appearance, in the Manner just now mentioned....

"A Couple of circular Bodies, armed with small Teeth like those of the
Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and
extending sideways somewhat wider than the Diameter thereof. They have very
much the Similitude of Wheels, and seem to turn round with a considerable
Degree of Velocity, by which Means a pretty rapid Current of Water is
brought from a great Distance to the very Mouth of the Creature, who is
thereby supplied with many little Animalcules and various Particles of
Matter that the Waters are furnished with.

"As these _Wheels_ (for so from their Appearance I shall beg Leave to call
them) are every where excessively transparent, except about their circular
Rim or Edge on which the Cogs or Teeth appear, it is very difficult to
determine by what Contrivance they are turned about, or what their real
Figure is, though they seem exactly to resemble Wheels moving round upon an
Axis....

"As the Animal is capable of thrusting these Parts out, or drawing them in,
somewhat in the Way that Snails do their Horns, the Figure of them is
different in their several Degrees of Extension and Contraction, or
according to their Position to the Eye of the Observer, whereby they not
only appear in all the various Forms before represented, but seem at
certain Times as if the circular Rim of the Wheel or Funnel were of some
Thickness, and had two Rows of Cogs or Teeth, one above and the other below
that Rim."

DIGESTIVE ORGANS.—The _pharynx_ is usually a narrow ciliated tube, which
varies in length from genus to genus, but in no other important point, save
in Flosculariidae, where it assumes the form of a crop, into which the
mouth hangs freely down as a narrow ciliated tube. At its lower end is an
enlargement, the _mastax_ or gizzard.[256] This is a strong muscular sac
containing the _trophi_ or hard chitinous chewing organs, with an
{210}antero-ventral inlet from the pharynx, and a postero-dorsal outlet
through which the food passes into the stomach either directly or through a
slender gullet (Fig. 106, _oe_). In the ventral wall of the gizzard of most
Ploima is a median piece, the _fulcrum_, from which run forwards and
upwards two pieces, the _rami_, which are hinged on the fulcrum. The
Y-shaped structure formed of these three pieces is called the _incus_
(anvil). At either side of the gizzard and at a higher level is a paired
piece, the _malleus_, so called from its resemblance to a hammer, of which
the _manubrium_ (handle) looks backwards, and is embedded in the side walls
of the mastax, while the toothed claw or _uncus_ looks forwards and
inwards, and is hinged at its inner side with the tip of the ramus. As the
unci and rami are usually strongly toothed, this gizzard forms a very
efficient apparatus for chewing. In some cases, when the pharynx is short
and dilatable, the points of the unci and rami may be protruded for biting,
for clinging to the host (in the parasitic genera _Albertia_ and
_Drilophagus_), or for the prehension of food (Rattulidae, etc.).

[Illustration: FIG. 111.—Diagram of trophi. (After Hudson.) A, Malleate; B,
submalleate; C, virgate; D, forcipate; E, malleoramate (_Melicerta_); F,
incudate (_Asplanchna_); G, uncinate (_Stephanoceros_); H, ramate
(_Rotifer_). _f_, Fulcrum; _i_, incus; _ma_, manubrium (malleus in G); _r_,
ramus; _un_, uncus.]

The type we have just described is termed the "malleate" type (Fig. 111,
A). If all the trophi are slender and scarcely toothed, we have the
"virgate" type (C), which is frequently {211}asymmetrical. In the
"submalleate" type (B) the mallei only are slender; in the "forcipate" type
(D) both the unci and rami are slender and sharply pointed.[257] In the
"malleoramate" type (E) the manubrium is a curious looped structure, while
the uncus is formed of a number of parallel slender elongated teeth; this
characterises the family Melicertidae, and the genera _Triarthra_,
_Pterodina_, and _Pedalion_. In the "uncinate" type (G) the mallei are
simply incurved hooks with a few teeth at the free end, the rami are simple
or absent, and there is no fulcrum; this type occurs in Flosculariaceae
only. In Asplanchnidae the rami are large and hooked, constituting the
"incudate" mastax (F); but here reduced mallei are often present, and in
_Asplanchnopus_ they are almost as well developed as in Melicertidae,
affording a transition to the malleoramate type. In this group too the
mastax has a very peculiar form; it is divided into two chambers, dorsal
and ventral. The dorsal chamber forms a great purse-like sac or crop, with
a framework of four longitudinal bars: into this the gullet and pharynx
open. The ventral pouch is much smaller, and in its base the large rami are
inserted, so that they can be protruded into the crop. This ventral sac
with the rami may even be everted through the crop and the mouth, to
swallow the small Rotifers and Entomostraca which form the food of this
group, or to eject the undigested remains of the food. Two lateral sacs
open at the junction of the ventral pouch and the crop, but whether they
play a part in the deglutition of food or in the disgorging of faeces is
uncertain. The fact that the whole of this apparatus is lined by a
non-ciliated chitinous cuticle justifies our view that it is simply an
enlargement and specialisation of the mastax.

The trophi in Bdelloids also are only represented by the rami, which have
the form of segments of a sphere, excavated on the curved sides for the
attachment of muscles, and transversely ridged on the two flat sides; the
gizzard is here called "ramate" (H).

It will be seen that the characters of the gizzard are very useful for
classification, only breaking down indeed in the {212}Ploima; for though
the majority of these present one or other of the four varieties of the
malleate type, _Triarthra_ and _Pterodina_ (but not the other genera of
their respective families) have the gizzard malleoramate.

The _oesophagus_ is, when present, a contractile ciliated tube in which the
food makes no sojourn on its way to the stomach.

The _stomach_ may be nearly spherical, ovoid, or elongated and cylindrical.
Its walls are formed of large cells, often granular and sometimes brownish,
whence a hepatic function has been assigned to them. Its apertures are both
surrounded by constricting muscular fibres. The _intestine_ may be simple
or divided by a similar constriction into _intestine_ proper and _rectum_.
The whole of the alimentary tract, with the exception of the mastax, is
richly ciliated within. The rectum opens into the slender non-ciliated
cloaca. The intestine is sharply bent upwards and towards the back in the
tubicolous forms, but is nearly straight elsewhere; in _Trochosphaera_ and
_Apsilus_ it is bent ventrally. In Asplanchnaceae and in _Paraseison_ there
is no rectum, the stomach being a blind sac.

The so-called _salivary glands_, usually two in number, open into the
pharynx or mastax; and the paired _gastric glands_ (Fig. 106, _gg_) open
into the oesophagus or stomach. While the prehension of food is usually
accomplished by the ciliary current of the disc and pharynx, we have seen
that a more active swallowing action takes place in Flosculariaceae and
Asplanchnidae, which devour whole Algae, Infusoria, and even other
Rotifers, the long spines of _Triarthra_ not availing as a protection. Many
Ploima put out the tips of their trophi to nibble at débris, or, in the
case of _Diglena_ and _Distemma_, to attack Desmids, or the Infusorian
_Stentor_. But this use of the trophi is most efficient in _Ploesoma_.
Bilfinger[258] writes: "It has the courage to attack larger Rotifers; thus
I was able to observe under the microscope how it fell upon a _Rattulus_
but little smaller than itself and destroyed it. First it plunged the sharp
prongs of its mastax deep into the tender frontal area of its unhappy
victim; then followed a pumping action of the gizzard, and stroke by stroke
the whole contents of the victim's body passed into the brigand's stomach."
From this it is an easy transition to the ectoparasitism of _Drilophagus_,
_Balatro_, and {213}some species of _Albertia_, which cling to their host
by the exserted trophi.

RENAL ORGANS.—The kidneys consist of a pair of convoluted tubes, formed of
a succession of perforated, so-called "drainpipe" cells (Fig. 106, _k_);
they open directly or indirectly into the cloaca. Their walls are thin in
the straight parts, but thick and glandular in the coils which occur at
intervals. These tubes bear little tag-like appendages, hanging freely into
the body-cavity, often widening towards the free end, and flattened or
circular in section (Fig. 106, _ns_). They show during life a peculiar
flickering motion in their interior, like the equivalent "flame-cells" of
many Platyhelminthes (see p. 25), and are in function the representatives
of the multicellular renal funnels of Annelids. On one side, especially on
the edge of the flattened tags, the appearance is as of a tapering
whip-like lash, attached by its base to the free end of the tag and waving
in its cavity; but the side view of the flattened tags shows an appearance
of successive transverse or oblique waves. In many if not all cases the
free end of the tag is closed by a vacuolated plug of protoplasm, which
sometimes at least bears two flagella waving freely in the body-cavity. The
probable explanation of the two distinct wave appearances within the tag is
that the protoplasmic plug bears on its inner face a row or tuft of long
cilia hanging down into the cavity of the tag. The tags probably keep up a
current of liquid through the kidneys, while the contents of the
body-cavity are constantly replenished by osmosis.

The two renal tubes may end blindly below the disc, or else join by a short
transverse dorsal communication in front of the brain, as in
_Stephanoceros_, _Atrochus_ (Fig. 112, C), and _Apsilus_ among
Flosculariaceae, _Lacinularia_ among Melicertidae, and _Hydatina_ among the
Illoricate Ploima (Fig. 106, _rc_). In some species of _Asplanchna_, if not
all, a recurrent branch occurs opening at either end into the main tube of
its own side.

The kidneys unite to discharge into the cloaca near its orifice, and on its
distal (primitively ventral) side in many Melicertidae. In Bdelloida the
common duct formed by their fusion opens into the ventral side of a dilated
bladder-like section of the cloaca (Fig. 109, A, _bl_), which contracts
rhythmically to discharge the liquid; while in the majority of the class
they open singly or by a common duct into a separate contractile vesicle or
_bladder_, which also discharges at regular intervals into the cloaca on
its ventral or distal side (Figs. 106, _bl_ and 112).

{214}[Illustration: FIG. 112.—Apsilidae: A, _Apsilus lentiformis_, ♀,
dorsal view (after Metschnikoff); the square brain is seen with nerves to
the lateral antennae; B, larva of _A. lentiformis_ (?), showing the paired
eyes and ciliated cupped foot; C, adult of _Atrochus appendiculatus_, ♂
(after Wierzejski). _al_, Lateral antennae; _am_, median antenna (just in
front is seen the renal commissure); _an_, anus; _br_, brain, below which
the paired eyes are seen; _c_, cloaca; _em_, embryo; _em'_, _em'_, _em'''_,
three successive stages of embryos in the uterus of C; _k_, kidney. The
coarser muscles are striated.]

This bladder may reach when expanded one-third the diameter of the whole
animal, and contract as often as three times per minute; so that in a
period of nine minutes a bulk of water equal to that of the animal must
have diffused through the body-wall, to be removed by the kidneys. It is
obvious that while the function of the kidneys is primitively excretory,
the passage of the water through the body must bring in the oxygen
dissolved in the external {215}medium, and carry off the carbonic acid
formed in the tissues, and so fulfil the act of respiration. This mechanism
is physiologically comparable with that of the contractile vacuole of
fresh-water Protozoa. In a few genera (_Conochilus_, _Lacinularia_,
_Pterodina_) the kidneys open separately after a slight dilatation into the
cloaca.

NERVOUS SYSTEM.—The nervous centre of the Rotifera is the _brain_ (Fig.
112, C, _br_), a ganglion lying dorsal to the pharynx; and when this is
short it may be immediately below the surface of the disc (_Microcodon_).
In Bdelloida a second ganglion is present below the pharynx, and is
connected with the former by lateral cords which contain ganglion cells.
From the brain, nerves are given off to the disc, to the muscles, and to
the integument of the body, as well as to the sense organs. The largest
nerves are two given off from the sides of the brain, each of which divides
into a lateral and a ventral trunk, which run nearly the whole length of
the animal.

The brain of several Notommatidae has a curious appendage, white by
reflected light and very opaque; it is a sac full of chalky mineral matter,
which dissolves readily in dilute acids.

SENSE ORGANS.—The most widely diffused sense organs are the antennae or
feelers, which may serve for touch or smell, or possibly both. Each antenna
is a conical or tubular outgrowth of the skin; from its apex projects a
fine pencil of sense hairs borne on a protoplasmic cushion, which receives
a nerve. Often the antenna is elongated, and may then contain a muscle by
which it is retractile (lateral antennae of _Melicerta_); sometimes it is
reduced to a slight prominence bearing the setae (dorsal antenna of this
genus). There are usually three antennae—a _median dorsal_ (Figs. 109, B,
_a_, and 112, C, _am_) and two _lateral_ (Figs. 106, 112, C, and 115, A,
_al_), often approximated towards the ventral surface, and sometimes all
but fused on the middle line, or completely united (_Conochilus
dossuarius_, _Copeus caudatus_).[259]

Most Rotifers possess an organ of sight. This in its simplest form is a
refractive globule seated in a red pigmented cup through which the nerve
passes; in other cases it lies directly on the brain. Very frequently the
eye is paired (Figs. 112, B, and 115, A); and these paired eyes may lie on
the brain, and then {216}are so close together that the pigment-cups have
the shape of an _x_, or else they are seated in the dorsal region of the
head behind the disc. In some cases they lie just under the ciliary wreath,
or even within the region of the disc, and pass towards its ventral side in
_Pedalion_ (Fig. 117, A, _e_). In _Rotifer_ they lie just under the dorsal
side of the proboscis just below its apex. The median and two lateral eyes
often exist together, as in _Eosphora_; and sometimes additional paired
eyes exist. In _Furcularia longiseta_, var. _grandis_ a pair of pigment
spots (eyes?) occurs at the hinder end of the body just in front of the
foot.

The active Ploima show a spontaneity of movement and marked power of
avoiding obstacles, etc. This is still more marked in the very active
_Pedalion_, which, as Rousselet notes, clearly avoids capture by the
dropping tube, aided by its sense of sight, as he suggests, or by the
tactile or olfactory powers of the antennae. They must rank as psychically
high in the scale of creatures of simple organisation.

REPRODUCTIVE ORGANS AND REPRODUCTION.—The most conspicuous organ in the
female is the large _yolk-gland_ or _vitellarium_ (Figs. 106 and 109, A,
_vm_), which was regarded as the ovary by all the older observers. It
consists usually of eight cells, with conspicuous nuclei, lying on the
ventral side of the stomach, and frequently displaced to one side; but in
most Asplanchnidae it forms a broad transverse band of numerous cells. In
_Pterodina_ it is horseshoe-shaped, while in Seisonaceae and Bdelloida it
is paired, either gland containing four or eight cells. The true _ovary_ or
_germarium_ (Fig. 106, _gm_) lies more or less hidden between the
yolk-gland and the stomach; it is composed of numerous minute rounded
cells, of which the hindmost for the time being enlarges by nutrition from
the yolk-gland, and finally receives a membranous shell. This true ovary is
somewhat lateral in most Rotifers, but is median in Asplanchnidae, and
paired in _Pterodina_, Bdelloida, and Seisonaceae. A membranous covering is
common to the ovary and yolk-gland (paired when these are paired); it is
continued into a thin-walled tube or oviduct, which opens into the cloaca
on its ventral side beyond the bladder or common renal duct. In the
viviparous species the mature ovum (Fig. 112, _em_) usually lies in the
oviduct, dilating it into a sort of "uterus" until the birth of the young.
The ordinary eggs or "summer eggs" are formed without any
{217}fertilisation, and develop immediately; they are often hatched within
the tube of the tubicolous species.

Under certain conditions the unfertilised females produce exclusively
smaller eggs, which develop into males. Maupas[260] has demonstrated that a
rise in temperature to a minimum of 26° C. (79° F.) is the efficient
factor. But as Bergendal points out,[261] the critical temperature probably
varies with the antecedent conditions of the race, since males occur in
Greenland at a very much lower temperature; and it would seem probable that
a temperature approaching that at which the pools habitually dry up is what
is necessary for the production of males, as a provision for those
fertilised eggs, which, having a hard shell often adorned with prickly
prominences, and usually remaining for some time before development, are
capable of withstanding drought; such eggs are termed "winter eggs," but a
better term would be "resting eggs" (German, "Dauereier").[262]

[Illustration: FIG. 113.—_Diglena catellina._ (After Weber.) A, Male; B,
the pair _in copula_; C, female, _p_, Penis; _te_, testis.]

The male organs consist of a testis (Fig. 113, A, _te_) with accessory
glands, a large seminal vesicle, and a protrusible or projecting penis
(_p_). In _Notommata_ and _Diglena_ true intromission at the cloaca (B) has
been seen by many observers; but it {218}appears equally certain that in
many cases the male bores into the body-wall of the female at any point,
and deposits the spermatozoa in the body-cavity, so that they must pass
through the wall of the oviduct to effect fertilisation. Maupas finds that
the process of fertilisation is ineffective except upon such newly-hatched
females as would otherwise be the parents of small male eggs; that
fertilisation is inoperative even for these at a later age when their eggs
have begun to mature; and that it is wholly useless for those that lay
ordinary summer eggs. The parent of male or winter eggs would thus be
comparable to the queen bee, which if not fertilised produces drones. These
sexual relations find a close parallel in the Ostracod and Phyllopod
Crustacea, as well as in many plant-lice (Homoptera).

DEVELOPMENT.—This has only been fully studied in the summer egg; in
_Brachionus_ by Salensky,[263] in _Melicerta_ by Joliet[264]; in _Eosphora
digitata_ and several other species by Tessin[265]; in _Callidina_ and
_Melicerta_ by Zelinka,[266] the last two observers having utilised modern
methods of research.[267] We shall base our account on Zelinka's
observations. As in the case of most "parthenogenetic" eggs, the ovarian
egg begins by a very uneven division to form two cells: the minute "first
polar body" which undergoes no further development; and the definitive egg,
which by its repeated divisions gives rise to the tissues and organs.

Segmentation is very unequal, and recalls that of Molluscs in several
respects. The first division gives rise to a smaller and a larger cell.
Both of these divide again, the latter unequally, so that now there are
three smaller cells and one large one; and after repeated divisions of the
small cells and unequal divisions of the larger one, a stage is reached
where there are a number of small cells and one large one, which sinks in
and is overgrown by the small ones. Just prior to this the large cell
undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114,
_hyp_), and give rise to the gullet, stomach, and intestine, with their
appendages, and the generative organs; while the smaller cells constitute
the "epiblast" (_ep_), which gives rise to the body-wall and muscles, to
the cement glands, nervous system, pharynx and mastax, and probably to the
kidneys.

{219}[Illustration: FIG. 114.—Development of _Callidina_. (After Zelinka.)
A, Early stage showing involution of granular cells (_g_), to form the
mastax or gizzard. B, Involution complete. C, Second involution of epiblast
cells to form pharynx. D, The embryo bent on itself at ventral fold (_vf_).
E, Showing ingrowth of epiblast to form brain (_br_): _an_, involution of
epiblast to form cloaca; _br_, brain; _ep_, epiblast; _fg_, involution to
form cement glands of foot; _g_, granular cells; _gi_, gizzard; _hyp_,
hypoblast; _m_, mouth; _o_, ovary; _sp_, salivary glands; _vf_, limiting
body from foot.]

Owing to the elongation of the body within the narrow space of the egg the
hinder part is bent up on the ventral surface (D, E); and this part,
narrower than the rest, forms the foot, the centre of which is at first
occupied by a column of hypoblast. The cloaca is now formed by a dorsal
ingrowth of epiblast (the "proctodaeum") at the junction of the foot and
the body (_an_). The hypoblast in the body anterior to the cloacal ingrowth
forms the digestive apparatus; the part immediately behind forms the
reproductive organs (_o_); and the hindmost part apparently disappears. An
ingrowth of epiblast at the extreme tip of the foot gives rise to the
cement glands (_fg_). The muscles arise from the epiblast cells. The disc
arises from the modification of epiblast cells lateral to and behind the
mouth, enclosing a so-called "polar area"; it is completed by the
transformation of cells on the ventral side of the mouth. The brain (_br_)
is formed by the multiplication of epiblast cells; and in Bdelloida a
ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The
ciliated cup in _Melicerta_ is formed as a ventral hollow, only later on
united with the ciliated furrow of the wreath by the lateral grooves.[268]
In _Melicerta_ the two eyes are formed in the polar area. The young as
hatched {220}differs from the adult in the greater simplicity of its
ciliary wreath; and in the tubicolous forms the cupped end of the
foot-gland is ciliated, and two eyes are present on the polar area, which
later sink in, and often disappear more or less completely. It is stated
that the young hatched from winter eggs do not pass through this larval
state.

[Illustration: FIG. 115.—_Stephanoceros eichhornii._ (After Cubitt.) A,
Dorsal view of the upper part in its tube: _al_, lateral antenna; _e_, eye;
_em_, developing embryo in uterus; _g_, gizzard; _s.g_, median salivary
gland. B, Extremity of foot. C, Lateral view of base of disc: am, median
antenna; _of_, oral funnel; _s.g_, median salivary gland; _tf_, in the
crop, indicates the ciliated tube prolonging the funnel; _tr_,
horseshoe-shaped trochus.]

CLASSIFICATION.[269]

ORDER I. FLOSCULARIACEAE.—Females mostly tubicolous, attached by a long
contractile foot. Disc produced into a wide funnel-shaped contractile cup,
produced into lobes with long setae (_Floscularia_) or coarse cilia
(_Stephanoceros_), or entire (Apsilidae); an outer row of fine cilia rarely
present; trochus a horseshoe, open behind. Oral funnel a slender tube
hanging freely into a large pharyngeal crop; trophi uncinate projecting
freely into the crop. Kidneys often united by an anterior cross-piece.
Body-wall often containing a definite system of canals, filled with
refractive granules, and serving by their contraction to {221}dilate the
disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup,
and a simple wreath, with two eyes on the disc.

Fam. 1. Flosculariidae: _Floscularia_ Oken, _Stephanoceros_ E. (Fig.
115).

Fam. 2. Apsilidae: _Apsilus_ Metschnikoff (Fig. 112, A), _Acyclus_ Leidy,
_Atrochus_ Wierzejski (Fig. 112, C).

The family Flosculariidae contains some most exquisite forms;
_Stephanoceros_, the "Crown Animalcule," being probably the most lovely of
the Class, and many of the Floscules coming not far behind. The Apsilidae
are mostly mud-dwellers.

ORDER II. MELICERTACEAE.—Females (except in _Trochosphaera_) attached or
tubicolous; tube variable. Disc with a dorsal gap (except _Conochilus_)
often two-lobed or corolla-like; a ventral lip often separating off a
ventral ciliated cup continuous by a pair of gutters with the ciliated
groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in
a distinct mastax. Intestine much curved dorsally, cloaca long eversible
(except _Trochosphaera_). Males and larvae as in Order I.

[Illustration: FIG. 116.—_Melicerta ringens._ (After Joliet). A, Side view;
B, dorsal view. _al_, Lateral antennae; _ci_, cingulum seen by
transparency; _g_, gizzard; _p_, pellet in ciliated cup, about to be
deposited on edge of tube; _tr_, trochus.]

Fam. 3. Melicertidae: _Melicerta_ E. (Fig. 116), _Limnias_ Schrank,
_Cephalosiphon_ E., _Oecistes_ E., _Lacinularia_ E., _Megalotrocha_ E.,
_Conochilus_ E., _Octotrocha_ Thorpe.

Fam. 4. Trochosphaeridae: _Trochosphaera_ Semper (Fig. 118, D).

The Melicertidae embrace a large number of tubicolous forms, many of which
are social. This habit is especially noticeable in _Lacinularia socialis_,
which forms a gelatinous incrustation easily seen by the naked eye; and in
_Conochilus volvox_, which forms free-swimming globular aggregates, the
young attaching themselves when hatched to the centre of the ball, and the
ball splitting up into two as soon as undue pressure is exerted at the
periphery by overcrowding. In this genus the eyes are very conspicuous in
the adult, as they are in the similar free-swimming aggregates of
_Lacinularia racemovata_.

_Trochosphaera_ (Fig. 118, D) is remarkable for its peculiar {222}spherical
shape, the absence of a foot, the limitation of the viscera to the lower
hemisphere, and the dorsal position of the ovary. But a reference to the
figure will show that the outgrowth of a foot in the quadrant between the
mouth and anus and the flattening of the upper hemisphere would bring its
organs on the whole into close correspondence with those of the rest of the
Order. It is recorded from South China, the Philippines, and North-East
Australia, and has only been seen by Semper, the founder of the genus, and
by Thorpe, who saw the male of the first species, and described a
second.[270]

ORDER III. BDELLOIDA.—Females creeping like a leech, as well as swimming
(males unknown), susceptible of desiccation and revival ("anabiotic"). Body
telescopic at both ends. Disc (except in _Adineta_) chiefly composed of two
dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or
trunk-like prolongation of the body ends in a ciliated, sensory, and
adhesive cup used in crawling, and overhung by a pair of membranous flaps.
Trophi ramate; brain with a ventral ganglion, forming a complete ring.
Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation
of the rectum. Foot often possessing blind spurs, as well as two or three
retractile perforated toes, or forming a terminal disc perforated by
numerous pores of the cement glands, rarely ciliated.[271]

Fam. 5. Philodinidae: _Philodina_ E. (Fig. 110), _Rotifer_ Schrank,
_Actinurus_ E., _Callidina_ E. (Fig. 109), _Adineta_ H.

This group is remarkable for the great resisting powers of its members to
drought and to heat and cold when dried, a fact which may explain the
absence of males, though Janson records the occurrence of winter eggs in
four species of _Callidina_ and in _Adineta vaga_. The body is often
strongly pigmented; red in _Philodina roseola_, _Callidina scarlatina_, and
_C. russeola_, yellow in _P. citrina_, _Rotifer citrinus_, and _Discopus
synaptae_. Most of the species are dust- or moss-dwellers; some, such as
_Rotifer vulgaris_, are equally common in organic débris in infusions,
pools, and ditches. _Discopus_ adheres to the skin of the Holothurian
_Synapta_.

ORDER IV. ASPLANCHNACEAE.—Females ovoid, footless except in
_Asplanchnopus_. Disc often bearing a pair of antennae; circular, often
prolonged at the margin into two rounded lobes, interrupted {223}dorsally,
depressed at the ventral side into a deep ventral funnel. Trophi incudate
(virgate in _Ascomorpha_), mastax enlarged dorsally into a wide crop;
stomach large, blind. Kidneys large, with a "recurrent duct" and numerous
tags; bladder large. Brain large, with a median eye, and frequently paired
smaller eyes at the base of the marginal processes of the disc; anterior
antennae paired, relatively far back on dorsal surface. Males (Fig. 107, 5)
relatively large, frequently found.

Fam. 6. Asplanchnidae: _Asplanchna_ G., _Asplanchnopus_ De Guerne, (?)
_Ascomorpha_, Perty, (?) _Dinops_ Western.

ORDER V. SCIRTOPODA.—Females of conical shape, with the body prolonged into
hollow limb-like expansions (see p. 201) moved by strong muscles, and
ending in branched setose fins like the limbs of Crustacea. Disc as in
Bdelloids, but not retractile. Foot represented by two subventral toes,
ciliated, inconstant or absent. Trophi malleoramate. Eyes two,
latero-ventral, on the disc. Male (Fig. 107, 8) conical, with simple setae.

Fam. 7. Pedalionidae: _Pedalion_ H. (Fig. 117),[272] _Hexarthra_
Schmarda.[273]

ORDER VI. PLOIMA.—Free-swimming forms, more rarely parasites, often
adherent by their trophi to a host. Disc variable, often bearing within the
cingulum a number of lobes fringed with coarse compound cilia. Foot rarely
absent, marked off by a sharp constriction. Mastax variable, rarely
malleoramate, never incudate or uncinate. Intestine not blind. Males
small.[274]

SUB-ORDER A. ILLORICATA.—Ploima with a soft flexible integument; disc
variable; ciliated auricles sometimes present (Synchaetidae, Notommatidae);
foot rarely absent; trophi usually malleate.

{224}Fam. 8. Microcodonidae: _Microcodon_ E., _Microcodides_ Bergendal.

Fam. 9. Rhinopidae: _Rhinops_ H.

Fam. 10. Hydatinidae: _Hydatina_ E. (Fig. 106), _Notops_ H., _Hudsonella_
Zach., _Cyrtonia_ Rouss.

Fam. 11. Synchaetidae: _Synchaeta_ E.

Fam. 12. Notommatidae: _Notommata_ E., _Pleurotrocha_ E., _Copeus_ G.,
_Proales_ G., _Furcularia_ G., _Eosphora_ G., _Triophthalmus_ E.,
_Diglena_ E. (Fig. 113), _Distemma_ E., _Triphylus_ E., _Taphrocampa_ G.,
_Albertia_ Duj., _Balatro_ Clap.

Fam. 13. Drilophagidae: _Drilophagus_ Vejdovsky.

Fam. 14. Triarthridae: _Triarthra_ E., _Polyarthra_ E., _Pteroessa_ G.,
_Pedetes_ G.

[Illustration: FIG. 117.—_Pedalion mirum_, female. (After Hudson.) A,
Ventral view; B, side view. _a_, Median antenna; _al_, antero-lateral limb;
_an_, anus; _ci_, cingulum; _dl_, dorso-median limb; _e_, eye; _f_,
ciliated pedal processes; _l_, lip; _m_, mouth; _pl_, postero-lateral limb;
_tr_, trochus; _vl_, ventro-median limb.]

To this group belongs the eyeless _Hydatina_, a classical object of study,
common in greenish pools, whose male was the first male Rotifer to be
figured by Ehrenberg (1838), though he did not recognise its nature, and
gave it the name of _Enteroploea hydatina_. _Rhinops_ has the back of the
corona curiously prolonged forwards into a sort of proboscis bearing two
eyes. Some species of _Notommata_ and _Proales_ are distinctly annulated;
in _Taphrocampa_ the segmentation is so marked as to give the appearance of
mesenteric septa extending inwards from the body-wall to the intestine.
_Microcodon_ has a wreath which is very peculiar in its extreme simplicity,
with the mouth nearly central, and the eye lying just dorsal to the mouth.
The Triarthridae, which resemble the Scirtopoda in having strong leaping
spines fringed by fine bristles, should perhaps be placed in the next
sub-Order.

SUB-ORDER B. LORICATA.—Ploima with a firm elastic cuticle {225}of definite
form, persistent after death, continuous, or divided by thinner strips into
plates or shields, which again may be areolated. The cuticle may also be
shagreened or embossed in various ways.

Fam. 15. Rattulidae: _Rattulus_ E., _Mastigocerca_ E., _Coelopus_ G.,
_Diurella_ (?) Eyfurth.

Fam. 16. Dinocharididae: _Dinocharis_ E., _Scaridium_ E., _Stephanops_ E.

Fam. 17. Salpinidae: _Salpina_ E., _Diaschiza_ G., _Ploesoma_ Herrick,
_Diplax_ G., _Diplois_ G.

Fam. 18. Euchlanididae: _Euchlanis_ E., _Dapidia_ G., _Apodoides_ Joseph.

Fam. 19. Cathypnidae: _Cathypna_ G., _Distyla_ Eckstein, _Monostyla_ E.

Fam. 20. Coluridae: _Colurus_ E., _Metopidia_ E., _Monura_ E., _Mytilia_
G., _Cochleare_ G., _Dispinthera_ G.

Fam. 21. Pterodinidae: _Pterodina_ E., _Pompholyx_ G.

Fam. 22. Brachionidae: _Brachionus_ E., _Noteus_ E., _Schizocerca_ Daday.

Fam. 23. Anuraeidae: _Anuraea_ E., _Notholca_ G., _Eretmia_ G.

The group includes a number of very minute forms, besides others
conspicuous both for size and beauty. A soft dorsal flap above the head
occurs in _Stephanops_; also in Coluridae, a large family of minute
species, where the flap is movable, and looks in profile like a hook
overhanging the forehead. The genus _Pterodina_, like _Pedalion_ and
_Triarthra_, combines a Bdelloid disc with malleoramate trophi, while its
exsertile wrinkled foot ends in a ciliated cup like that of a larval
tubicolous species.

_Brachionus_, a large, often flat, transparent form, with a long wrinkled
foot, is a very common genus, known to the earlier observers, and
repeatedly figured by them. _Pompholyx_ has a sack-like lorica, no foot,
and carries its immense egg suspended by an elastic thread from the cloaca.
The Anuraeidae lack the foot, and often have great spines or bristles
projecting from the lorica, which no doubt facilitate floating. They are
abundant in the "plankton" or floating fauna of large lakes far from the
shore. Many marine species belong to this family.

ORDER VII. SEISONACEAE.—Marine Rotifers parasitic on the Crustacean
_Nebalia_; males resembling the females. Body elongated, with a slender
retractile neck, a much reduced disc, an elongated foot with a terminal
perforated disc as in _Callidina_. Trophi virgate exsertile. Genito-urinary
cloaca opening at the base of the neck in the male, at the hinder end of
the body in the female. Intestine complete (_Seison_) or blind
(_Paraseison_).[275]

{226}Fam. 24. Seisonidae: _Seison_ Grube; _Paraseison_ Plate;
_Saccobdella_ Van Beneden and Hesse.

HABITS.—The habitat of Rotifers is well known to the student of pond life.
Every dip from a greenish pool will give us a supply, if there be not an
excessive contamination by manure; and such pools give us some of the
largest and most beautiful forms, such as _Hydatina_ and _Brachionus_,
swimming about among the fibrous Algae and feeding on the organic débris
among them. Almost any organic infusions freely exposed to the open air
will yield Ploima shortly after the active putrefaction is completed. The
finer water-weeds yield most of the beautiful tubicolous forms. A whole
group of species and genera are quasi-pelagic in fresh and salt water,
constituting a large proportion of the "plankton" or floating life near the
surface; and some of these are found in deep water or in the depths of the
lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A
number of Loricates, such as _Notholca_ and _Eretmia_, are armed with long
spines, which doubtless render floating easier.

Among tubicolous forms _Conochilus volvox_ and _Lacinularia racemovata_
have this pelagic habit, forming floating globular or ovoid colonies, and
two species of _Floscularia_ also float freely in their tubes.

The following forms occur in salt or brackish water,[276] those marked with
an asterisk (*) also occurring in fresh water:—

_Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.*
Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,*
S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides.
Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens.
Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F.
neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha
leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca
longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus
calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza
fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus
amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C.
rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina
clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A.
aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N.
thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison
grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus,
P. ciliatus. Discobdella nebaliae._

Thus about seventy species are recorded as marine. _Synchaeta baltica_ is
truly pelagic, and contributes to the phosphorescence of the ocean.

{227}Other forms again are parasitic. _Proales werneckii_ is found in
_Vaucheria_, a coarse, dark green, thread-like Alga found in fresh water;
and the closely allied _P. parasita_ is not uncommon in the beautiful
floating green spheres of _Volvox_.[277] _Albertia_, _Drilophagus_, and
_Balatro_ are parasitic on or in fresh-water Oligochaetes; the curious
Seisonaceae are parasitic on _Nebalia_, a small Crustacean easily obtained
in masses of whelk's eggs; the aberrant Bdelloid _Discopus_ attaches itself
to the surface of the Holothurian _Synapta_. Similarly among this last
Order _Callidina parasitica_ attaches itself to the limbs of the
fresh-water Crustacea _Gammarus_ and _Asellus_. These are rather commensals
than true parasites. The species of _Brachionus_ often attach themselves
temporarily to the common water-flea _Daphnia_.

Besides a few Ploima, the vast majority of the Bdelloids live in or among
mosses and their roots. Many _Callidina_ inhabit cup-like hollows in the
leaves of the scale mosses (Jungermanniaceae), especially of the genus
_Frullania_. Almost all the members of this Order are susceptible of
desiccation and revival; certain species, such as _Rotifer vulgaris_,
_Philodina roseola_, _Adineta vaga_, etc., can be readily obtained by
moistening gutter dust. The mechanism of the process is as follows: when
desiccation is gradual the animals close up their telescopic bodies and
excrete gelatinous plugs at either end, which effectually seal them against
further drying; if, however, they be dried on a slide without any débris,
the process is too rapid for them to protect themselves, and they therefore
die. This was dimly seen by others, and clearly demonstrated by H.
Davis,[278] who records the following experiment:—The Rev. E. J. Holloway,
having found _Philodina roseola_ in gutters, placed strips of paper there
in the rainy season, and succeeded in obtaining clean gatherings, taking
dry groups of a hundred together, having a varnish-like covering all over;
and being glued to one another, mostly in one plane, and to the paper,
forming a pavement. In the dry condition they resist extremes of
temperature; thus Zelinka found {228}_Callidina_ revive after an exposure
of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C.
(158° F.). They will also resist deprivation of air in a vacuum of an
ordinary air-pump, but not the all but perfect exhaustion of the Sprengel
pump.

A very curious fact in relation to this Class is that often when a new form
is once described from a single locality, fresh and widely distant stations
for it rapidly become known.[279] Thus _Pedalion mirum_, first found at
Clifton in 1872 by Hudson, was a few years after captured in a small pool
above tide-marks on a rocky islet in Torres Straits. Since then it has been
recorded from many different European stations, and a second closely allied
species has been found in Finland. So a species of Ehrenberg's[280] was not
seen again till within the last decade or so; but since then it has been
independently found and described by six observers, who have given it as
many distinct generic names. In the case of _Pedalion_ it may well be that,
as Hudson suggests, the species is of southern origin and has followed the
flag, the winter egg being conveyed in dust by ships or travellers.

The above account of the habits gives the key to the collection of the
various forms. The weed-loving species are collected with the weeds, and
will keep with these in vessels if screened from direct sunlight and
protected against dust. The free-swimming forms may be collected by
sweeping with a net of fine gauze, with a bottle fixed in the bottom.

Except for their power of resisting desiccation, Rotifera are not very
long-lived, and the males are especially short-lived; the most exact
observations are those of Maupas on _Hydatina_. He found that the greatest
age of the unfertilised female was thirteen days, during which it could
produce some fifty eggs; the fertilised female lives for seven or eight
days, producing about sixteen eggs; while the male dies in two or three
days.

The preservation of Rotifers has been recently reduced to a fine art by
Rousselet, who uses a solution consisting of cocaine hydrochlorate, 1
gramme; water, 50 cc.; and methylated spirit, 12 cc. This will keep without
deterioration. When in use it {229}must be diluted in the proportion of two
volumes to three of water. This solution is added cautiously to the capsule
in which the Rotifers lie, and they are watched till their ciliary motions
slacken; when this happens a drop or two of osmic acid solution (½ to 1 per
cent) is added; the Rotifers are then sucked up by a capillary pipette, and
transferred to fresh water; and then into a solution of "Formaline" diluted
to contain 2½ per cent of formic aldehyde. In this solution they are
transferred to shallow cells, ground out of the centre of an ordinary glass
slide, covered with thin glass, and sealed.[281] Other methods of preparing
Rotifers for minute study will be found in the papers of Plate, Tessin, and
Zelinka.

THE ZOOLOGICAL AFFINITIES of the Rotifers have long been a subject of keen
interest. As early as 1851 Huxley[282] suggested that they represent a
primitive form, preserved, with modifications, in the larva of Molluscs,
Annelids and other worms, and Echinoderms. Similar views were later
maintained by Lankester,[283] who termed the larva of Polychaets, etc., a
"trochosphere," for which "trochophore" has been substituted in order to
avoid confusion with the Rotifer _Trochosphaera_; Balfour,[284]
Hatschek,[285] Kleinenberg,[286] and others have developed these views.
Serious difficulties, however, arise in the detailed comparison of Rotifers
with this type; and the special students of this Class have found it
practically impossible to agree in the identification of the various parts,
a difficulty especially felt in the case of the Rotiferan genus
_Trochosphaera_, though this is just the one which presents the closest
superficial resemblance to the Trochophore larva. I have been induced to
take a view of the structure of Rotifers that brings it into close
relationship with the lower Platyhelminthes, and with the more primitive
larva of the Nemertines termed _Pilidium_ (Fig. 60, p. 113). This is
hemispherical, ciliated all over, with the mouth in a ventral funnel lined
by fine cilia; while the edge is fringed with two rows of strong cilia,
separated by a finely ciliated groove, like those of the ciliary wreath of
a Rotifer.

{230}[Illustration: FIG. 118.—Diagram explaining the possible relations of
Rotifers. A, _Pilidium_; B, hypothetical Rotifer modified from
_Asplanchnopus_; C, a Ploimal Rotifer; D, _Trochosphaera aequatorialis_
(modified from Semper, the extension of the ovary into the posterior
ventral quadrant being omitted); E, Mollusc larva (Veliger); F, Trochophore
larva of Annelid. _a_, Anus; _ap_, apical organ; _at_, median antenna (near
which, in B, is a black spot, the brain); _bl_, bladder (receiving the
ramified kidney in B, C, and D); _br_, brain; _f_, foot; _fg_,
cement-glands, replacing apical organ; _g_, ovary; _k_, kidney; _m_, mouth;
_n_, supra-oesophageal ganglion; _nap_, nerve of apical organ; _nr_,
nerve-ring in section; _pot_, praeoral portion of trochus; _s.g_,
shell-gland; _s.n_, sub-oesophageal ganglion; _t_, trochus or ciliary
wreath; _tt_, posterior ciliated ring; _v_, velum, or expanded praeoral
part of trochus.]

The sides are produced on either side into lappets, which we do not take
into account. A cup-shaped depression at the apical pole is lined by
sense-cells, bearing long cilia which are probably sensory. A ring of
nerve-cells passes within the ciliated rim of the hemisphere, and the
stomach is a blind sac. If we compare this organism with a Rotifer, we find
that the wreath corresponds in both, the funnel of the disc in such forms
as Flosculariidae and _Microcodon_ leading to the mouth of _Pilidium_,
while the gut is blind in Asplanchnidae and in some of the highly developed
Seisonidae. The circular nerve-ring of _Pilidium_ is in many Rotifers only
represented by its anterior part, the brain; though in Bdelloids a
sub-oesophageal ganglion completes the ring. This leaves a difficulty with
regard to the apical sense organ; but it is easy to understand that an
organ of sensation should become an organ of fixation. In this case the
foot with its glands would correspond to the sense organ of the Trochophore
larva; and it retains its primitive ciliated character in the larvae and
males of many Rotifera, and the adult female of _Pterodina_ and _Callidina
tetraodon_. Embryology tells {231}us that the anus of Rotifers cannot be
homologous with that of Annelids, etc., for it is formed outside the area
of the blastopore: it is an independent formation, probably due to the
coalescence of the originally blind intestine at its extremity with the
earlier genito-urinary cloaca. On this view we must change the orientation
of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for
"anterior and posterior" we must substitute _oral_ (or _basal_) and
_apical_; for "dorsal" and "ventral" we must use _anterior_ and
_posterior_; while "right" and "left" are unchanged. And this correctly
expresses the actual space-relations in those Ploima like _Rattulus_ that
swim with their disc in contact with the organic débris on which they feed,
with the foot turned outwards and backwards. As these views are now
published for the first time, I have thought it wiser to keep to the
accepted relations in the general description, a course which has the
advantage of avoiding difficulties in the study of the literature of the
Class.

The supposed resemblance of _Pedalion_ to the Crustacea is probably the
result of convergence, not of consanguinity. The Polyzoa are a group of
freely-budding organisms whose structure otherwise recalls in many respects
that of the attached Rotifers; but a close investigation reveals so many
differences in structure, orientation, and development, that we cannot
regard the two groups as at all closely allied.

Thus the Rotifers may be regarded as a group apart, but probably
representing an early offshoot from a free-swimming Platyhelminth, probably
a Rhabdocoele; the modifications being the loss of the general ciliation of
the surface, the arching of the back into an elongated vault, the
conversion of the inner half of the pharynx into a gizzard, the change of
position of the genital and urinary apertures to the antero-dorsal surface,
and the opening of the intestine into the genito-urinary cloaca.

GASTROTRICHA.

This small and very homogeneous group consists of minute fresh-water
organisms, closely resembling many Ciliate Infusoria in their movements,
habit and habitat. They were first described in detail by Ehrenberg, and
placed by him and Dujardin in the neighbourhood of Rotifers. In recent
years A. C. Stokes[287] {232}in America and C. Zelinka[288] in Germany have
contributed, the former a careful description of a number of new species
and their habits, the latter a complete monograph of everything that is
known of the Order.

The Gastrotricha dwell among filamentous Algae and organic débris, and are
of frequent occurrence with Protozoa and Rotifera of similar habit. The
largest known measures only 400 µ (1/60 in.) in length, and the smallest
run as low as 74 µ (1/300 in.).

[Illustration: FIG. 119.—Gastrotricha. (From Zelinka.) A, _Chaetonotus
bogdanovii_, side view (after Schimkewitsch); B, _Gossea antenniger_ (after
Gosse); C, _Dasydetes goniathrix_ (after Gosse); D, _Dasydetes saltitans_
(after Stokes); E, _D. longisetosum_ (after Metschnikoff); F, _Chaetonotus
spinulosus_ (after Stokes); G, _Chaetonotus schultzei_ (after Gosse and
Bütschli). (Magnified.) B-F, × about 390; G, × about 125.]

We shall follow Zelinka in his description of the common species
_Chaetonotus larus_ as a type. The body is nearly circular in section,
flattened a little on the ventral side. The apertures are the terminal
mouth; the anus, nearly terminal and slightly dorsal; the two kidney
openings, ventral, nearly half-way down the trunk; besides the pore of a
cement-gland on either terminal process. The short ventral and post-anal
portion of the trunk with its processes therefore corresponds to the foot
of a Rotifer. The integument of the body is a thin nucleated hypoderm, not
{233}distinctly divided into cells, covered by a chitinised cuticle; it
bears cilia, sensory hairs, and peculiar scale-like processes, sometimes
produced into long bristles.

The cilia are chiefly arranged in two ventral bands, each extending nearly
the whole length of the body, and composed of a series of transverse rows
of single cilia; along these bands the hypoderm is thickened and more
richly nucleated. The sides of the head also bear numerous long cilia.

The scales are hollow processes of the cuticle overlapping from before
backwards. A ventral row lies between the ciliary bands; two series of
alternating dorsal rows lie on the back and sides of the animal, and in the
hirsute species it is these that are produced backward into bristles. A
single large scale, the "frontal shield," protects the head above and
behind, but does not extend down to the ventral surface. On either side of
the head is a pair of flattened oval areas, the "lateral fields." From
between these on either side springs a tuft of motile sensory hairs. Two
pairs of similar tufts arise dorsally on the front margin of the frontal
shield, and a fourth pair spring from the ventral surface a little behind
the mouth. These hairs are distinguished from ordinary cilia by their
length, and their insertion on large nucleated cells receiving nerves; two
pairs of similar hairs lie farther back on the dorsal surface, one in the
front of the neck, one near the base of the pedal processes.

The MUSCLES lie some in the body-wall, and some traverse the body-cavity;
only six pairs occur, simple, unstriated, and longitudinal. There are
neither transverse nor circular muscles.

The ALIMENTARY CANAL is very simple and nearly straight from mouth to anus;
it may be divided into pharynx, gullet, stomach, and rectum. The mouth is
circular, and looks forwards and a little downwards. From the mouth opens
the pharynx, a short chitinous tube, capable of eversion by being pushed
forwards by the gullet; it bears half-way down a circlet of curved hooks,
which open out when it is everted; within these are tooth-like thickenings.

The oesophagus or gullet is thick and muscular, extending through the whole
of the neck of the animal; its cavity, as well as the opening from the
pharynx, is triradiate like a leech-bite, but can be dilated by the action
of the muscular walls, inserted into a firm external cuticle; the internal
wall is also cuticulised, {234}not ciliated as in Rotifers. The hinder end
of the gullet is produced into a short, wide, membranous funnel projecting
freely into the midgut or stomach. The latter is elongated and oval,
composed of four rows of hexagonal cells, with large nuclei. This is
separated by a distinct constriction or sphincter from the short
pear-shaped rectum, which opens by a minute anus on the back just in front
of the pedal processes.

The food is chiefly organic débris; but Gastrotricha have been seen to
attack large Infusoria by nibbling, and to swallow the protoplasm as it
exudes from the wound in their prey.

The NERVOUS SYSTEM is chiefly composed of the large brain, a ganglion lying
like a saddle above and on the sides of the gullet, and in direct
continuity with the nerve-cells of the cephalic sense-hairs. A pair of
dorsal nerve-trunks extend along the whole length of the gullet. The
sense-hairs described with the general integument may be organs of external
taste ("smell") or of touch. Eyes have been described in several species;
and though Zelinka has failed to verify this, I have myself seen a pair of
minute red eyes in the back of the head of an animal (probably a
_Chaetonotus_), whose hasty escape into a mass of débris prevented my
determining its species.

The KIDNEYS are paired tubes lying at the sides of the front of the
stomach, and sending a simple loop into the neck. Each tube is much
convoluted, and ends at the one extremity in a long "flame-cell," like that
of a Rotifer much drawn out, and at the other by a minute pore on the outer
side of the ventral row of scales.

REPRODUCTIVE ORGANS.—Only the female is with certainty known to occur; and
the eggs, though recalling in their thick ornamented shell the fertilised
winter eggs of Rotifers, are probably unfertilised and parthenogenetic like
the summer eggs. The ovaries are two minute patches of cells lying at the
junction of the stomach and rectum. The eggs, as they mature and enlarge,
press against the side and back of the stomach, where they attain a length
of one-third to one-half that of the mother. The extrusion of the egg has
not been observed; but it is laid in the angles of weeds, the moulted
shells of Entomostraca, etc., where its development may be studied. The
sculpture of the shell serves to anchor it if laid among weeds. When
hatched the head, trunk, and pedal processes are of the full adult size,
all subsequent growth being limited to the neck.

{235}The function of testis has been ascribed by Ludwig to a minute
granular organ between the ovaries above the rectum; if this view be
correct the Gastrotricha are hermaphrodite.

The movements of the Gastrotricha are very elegant, recalling those of the
long-necked Ciliate Infusoria, like _Amphileptus, Lacrymaria_, etc., with
the characteristic exception that they always swim forwards; the grace of
their movements being due to the bending of the head and neck on the body.
Those which are provided with long motile bristles like _Dasydetes_,
alternate their gliding with leaps, like the springing Rotifers.

The Gastrotricha are divided into two sub-Orders—EUICHTHYDINA, with two
pedal appendages, containing the genera _Ichthydium_ Ehr., _Lepidoderma_
Zel., _Chaetonotus_ Ehr., and _Chaetura_ Metsch.; and the APODINA, with no
pedal appendages, comprising _Dasydetes_ G. and _Gossea_ Zel.

Their geographical distribution, like that of most microscopic fresh-water
organisms, is cosmopolitan. Few observers have enumerated the members of
this group; of their extra-temperate occurrence we have only the single
observations of Ehrenberg, Schmarda, and Voeltzkow for Nubia, Ceylon, and
Madagascar respectively.

Of the thirty-two species described, twelve are recorded by A. C. Stokes
from Maine and New Jersey only, besides five others that occur also in
Europe. In Europe nineteen species are recorded, one of which, _Ichthydium
podura_, has also been found in Nubia and Ceylon. One species, _Chaetonotus
tabulatus_ Schmarda, has been recorded by its author from Colombia (in
South America). As of the nineteen European species only seven have been
recorded as British, we may expect to find that careful study will well
repay the student in these islands.

The AFFINITIES of this group are probably with the Turbellarians and the
Nematodes; they differ from the former in the highly developed alimentary
canal, and from the latter in the possession of the ciliated ventral bands
and wreath. The general chitinisation of the skin, the primitive
body-cavity, the character of the alimentary canal, the ventral opening of
the renal canals far in front of the anus are characters shared by the
Nematodes, many of which possess bristles like this group. But their
affinity must be rather to some hypothetical ancestral group than to any
living Nematodes, which are destitute {236}of cilia. To the Rotifers the
affinity, dwelt on by Zelinka, is less close.

KINORHYNCHA.

This Class and Order comprises but one genus, _Echinoderes_ (Fig. 120),
founded in 1851 by Dujardin.[289] Reinhard's monograph[290] is the
generally accepted authority on this subject, and contains a full
bibliography, with diagnoses of the individual species, eighteen in number.

[Illustration: FIG. 120.—_Echinoderes dujardinii_ (?), drawn from a
preserved specimen taken at Worthing. × about 210. _b_, Bristle; _c.s_,
caudal spine; _ph_, pharynx; _s_ and _s'_, the spines on the two segments
of the proboscis; _s.g_, salivary glands; _st_, stomach.]

The animals of this group are found in shallow seas with muddy bottom,
below low-water mark, and feed on organic débris. They have been taken in
the Black Sea, Mediterranean, British Channel, and North Sea, and off the
Canary Islands (Lanzarote, Porto Pi, Palma di Mallorca). Their size varies
from 0.86 mm. × 0.22 mm. in _Echinoderes spinosus_, to 0.14 mm. × 0.03 mm.
in _E. kowalevskii_.[291]

The BODY is protected by a strong chitinous cuticle distinctly annulated,
forming eleven rings, besides a retractile proboscis obscurely divided into
two segments at the apex of which the mouth opens. The anus opens on the
extreme end of the last segment, which is frequently retracted; the genital
pores open right and left of the anus; and the renal pores lie on either
side of the back of the ninth segment. The first ring may be undivided, or
else distinctly divided into four plates, one dorsal, {237}two
latero-ventral, and one ventral. In the remaining segments each ring has
only three plates, one dorsal and two ventral, the two latter being
sometimes more or less fused in the last or ventral segment. These plates
all overlap from before backwards.

As the name _Echinoderes_ implies (Thorn-skin), the cuticle is produced
into points, bristles, or spines. The last segment frequently bears a large
pair of these, which have been compared, on the flimsiest grounds, with the
furcal processes of Crustacea and the perforated toes of Rotifers and
Gastrotricha.

The proboscis when extruded has the form of a truncated cone, obscurely
divided into two segments, a ring of strong spines marking the boundary
between them, and a second double ring of spines surrounding the apex. The
eversion is of the type termed by Lankester pleurembolic or acrecbolic, the
sides being first withdrawn, the apex first extruded.

As in so many Invertebrata, the epidermis is not separated by boundaries
into distinct cells. This layer sends out processes each of which lies in a
hollow in the thick cuticle, and perforates it to end in a fine bristle.
Minute orange pigment-granules occur at irregular intervals in this
hypoderm.

The MUSCLES of _Echinoderes_ are simple striated bands. Numerous bands lie
within and attached to the body-wall, extending its whole length; paired
dorsi-ventral muscles separate the intestine from the reproductive organ on
either side, and a complex system effect the movements of the proboscis.

ALIMENTARY CANAL.—The pore at the tip of the proboscis leads into a short
thin-walled tube, which is rarely evaginated; into the base of this tube
projects the short bluntly conical apex of the large ovoid muscular pharynx
(or gullet?); this is lined by an epithelial layer of nucleated protoplasm,
which secretes a strong cuticle. The stomach is a wide tube, somewhat
dilated in each segment between the paired dorsi-ventral muscles, and
tapering behind to end in the terminal anus. Four minute glands open at the
junction of the pharynx and stomach.

KIDNEYS.—These are a pair of blind pear-shaped sacs, ciliated within (the
only case of ciliation in _Echinoderes_), lying in the eighth segment, and
opening by the taper ends right and left on the back of the ninth segment.

NERVOUS SYSTEM.—All that has been clearly defined of this is a small brain
or ganglion lying dorsally at the junction of the {238}pharynx and stomach.
From two to eight eye-spots have been described by earlier writers, but
Reinhard was unable to find them in the (distinct) species which he
principally worked at, though he noted their existence in the solitary
specimen of the original species, _E. dujardini_, which he obtained.

REPRODUCTIVE ORGANS.—The sexes are distinct. The reproductive glands form a
pair of tubular sacs, opening ventrally on either side of the anus, and
extending forwards beside the gut as far forwards as the fifth to the
second segment in the male, but only to the fourth at furthest in the
female. The ova are large nucleated cells embedded in the protoplasmic
lining of the ovarian sac, and acquiring a distinct shell as they approach
its opening. Three-quarters of the testis sac is occupied with granular
protoplasm containing a quantity of small nuclei; the lower part alone
contains mature spermatozoa. Adjoining each external opening in the male
are a pair of short hollowed spines, which may perhaps serve as organs of
copulation; but nothing is really known of this process or of the
development of the egg. It is almost certain, from the absence of
developing eggs within _Echinoderes_, that the genus is not viviparous.

From the foregoing description it is obvious that _Echinoderes_ approaches
the Nematoda very closely: the two main points of difference are its
ciliated kidneys and its bilaterally paired sexual organs. Possibly the
study of such forms as _Desmoscolex_ (Fig. 81, p. 159) may reveal closer
affinities.

----

[Zelinka (_Verh. D. Zool. Ges._, 1894 and 1898), has given a preliminary
account of a new research on this group. The principal addition is the
discovery of a ventral nerve-cord, with a ganglionic dilatation in each
segment, lying in the ectoderm of the body-wall, as indeed do the brain
and nerve-collar. He divides the genus into two Orders according as the
orifice of the retracted fore-part of the body is slit-like or circular.
The former (Homalorhagae) retract the first two segments with the
proboscis; they are mud-dwellers, sluggish, eyeless: the latter group
(Cyclorhagae) only retract the first segment with the proboscis; they
crawl among algae, and mostly have paired pigmented eye-spots, each with
a lens, imbedded in the brain.—M. H., _Jan._ 1901.]

ARCHIANNELIDA, POLYCHAETA, AND MYZOSTOMARIA

W. BLAXLAND BENHAM, D.Sc. (Lond.), Hon. M.A. (Oxon.)

{241}CHAPTER IX

THE CHAETOPODOUS WORMS—THE ARCHIANNELIDA—ANATOMY OF _NEREIS_, AS TYPICAL OF
THE POLYCHAETA

Those animals which possess lateral bundles of bristles (technically termed
"chaetae") for use in locomotion constitute the group of "Bristle-worms,"
or CHAETOPODA. The body of these animals is made up of a preoral lobe or
prostomium, and a number of more or less distinct segments following one
another in a line, and repeating one another in their internal and external
structure. The Chaetopoda embrace the following smaller groups or
Orders:—I. Archiannelida, II. Polychaeta, III. Myzostomaria, IV.
Oligochaeta. The Archiannelida, although without the characteristic
chaetae, are yet anatomically so similar to the true Chaetopoda that they
must be included in the group, just as certain fishes are classed as
"Vertebrata," although they do not possess vertebrae. The old term Annelida
is sometimes used to include the above-mentioned groups, together with the
Gephyrea[292] and the Hirudinea or leeches.

ORDER I. ARCHIANNELIDA.

The Archiannelida are very simple worms, but simplicity may be, and very
frequently is, the result of degeneration; and it is not always possible to
determine whether a simple animal is primitively, _i.e._ ancestrally
simple, or whether it is secondarily simplified. Hence the term
_Haplodrili_ has been employed by Professor Lankester as the name of the
group; a term which does not prejudge the question as to whether or not the
worms are {242}"primitive." It is quite possible, and even probable, that
_Dinophilus_ is ancestrally simple; whilst many features in _Polygordius_
appear to be the result of simplification. For this reason it would be well
to separate _Dinophilus_ from the other two genera, on account of its much
less elaborate and more generalised structure,—so generalised, in fact,
that the worm is by some authorities placed amongst the Planarians; for the
present, however, the group Archiannelida may be regarded as containing
three genera: _Dinophilus_, _Protodrilus_, and _Polygordius_.[293]

[Illustration: FIG. 121.—_Dinophilus taeniatus._ (From Harmer.) The left
figure represents the dorsal surface of a young individual, × 76; the mouth
and alimentary tract are seen by transparency: _p_, prostomium, with two
bands of cilia and a pair of eyes; _a_, anus; _t_, tail; 1 to 5, the
segments with ciliated bands. The right figure shows the anatomy of the
male, × 38: _b_, rectum; _c_, body-cavity; _d_, vas deferens; _m_, muscular
organ (pharynx); _n'_, the first nephridium; _oe_, entrance to oesophagus;
_p_, penis; _st_, intestine (stomach); _s.x_, seminal vesicle (5th
nephridium).]

_Dinophilus_ is represented on our coasts by at least two species: _D.
gigas_ Weldon[294] and _D. taeniatus_ Harmer.[295] The latter is about
one-twelfth of an inch in length, bright orange in colour, and more or less
abundant, at springtime, in the rock pools around Plymouth, where it may be
found amongst green algae, or on the mud at the bottom of the pools.

{243}The animal consists of a broad prostomium, with a pair of eyes; and of
a body, distinctly constricted in immature specimens into five or six
segments, followed by a short conical tail. There are neither chaetae nor
tentacles; locomotion is chiefly effected by means of the bands of cilia
which encircle the body in a regular fashion, two bands round the head, and
two round each segment in _D. taeniatus_; in some species there is only a
single band on each segment. The whole of the ventral surface is covered
with cilia, by the aid of which the animal probably "creeps" along the
weeds.

The alimentary canal is straight, and divisible into the regions shown in
Fig. 121; a muscular protrusible organ, which is a ventral outgrowth of the
foregut, is employed as a "sucker." The coelom is more or less obliterated
(or ill developed). The excretory system in the genus is varied: in some
species, as in _D. gigas_, it is stated to be constructed on the Planarian
plan; in others, as in _D. taeniatus_, the organs are definite nephridia.
Of these tubes there are five pairs, the last pair in the male serving as a
seminal vesicle. Each nephridium is a ciliated tube, the internal end of
which lies in the body-cavity and appears to be blocked by a ciliated
tongue-shaped appendage. The first pair corresponds to the "larval
nephridia" of Trochosphere larvae.

The nervous system, which is in contact with the epidermis, consists of a
brain in the prostomium, and, on each side of the body, a ventral cord with
five ganglia, connected by transverse commissures in as many segments.

The sexes are separate, and are usually similar; the male of _D.
gyrociliatus_ is, however, much smaller than the female. The generative
organs occupy the greater part of the body-cavity; in the male the testes
communicate, by means of the pair of seminal vesicles, with a median
eversible apparatus. In the female the paired ovaries communicate with a
median sac which serves as a spermatheca.

The development is simple:[296] the worm itself is more like a larval
Polychaete than a full-grown worm. _Dinophilus_ is an extremely interesting
form, and it has been suggested that, while still possessing certain
Planarian characteristics, it may be looked upon as closely resembling the
ancestor from which the Chaetopoda have arisen.

{244}_Protodrilus_ and _Polygordius_ are distinctly Annelidan in character.
_Protodrilus_[297] is found in the mud of the "Pantano," an inlet of the
sea near Messina; whilst of _Polygordius_[298] one species at least occurs
on our shores, and several others in the Mediterranean and elsewhere. The
worms are cylindrical, with many segments, but these segments are only
indistinctly marked externally—by girdles of cilia in _Protodrilus_, or by
faint grooves in _Polygordius_; but there are none of the characteristic
Chaetopod bristles or chaetae. The small prostomium which overhangs the
mouth is provided with a pair of ciliated pits, and carries a pair of
tentacles, serving as sensory organs, which, in _Protodrilus_, are also
respiratory. The anus is surrounded by glandular papillae in _Polygordius_,
by means of which the animal can fix itself; these are represented in
_Protodrilus_ by a couple of processes.

The nervous system lies entirely in the epidermis. The body-cavity is
regularly segmented by transverse septa passing from the body-wall to the
intestinal wall. The foregut presents a slight eversible portion in
_Polygordius_, whilst in _Protodrilus_ it has a peculiar U-shaped muscular
diverticulum on its ventral surface, corresponding with the similar
apparatus in _Dinophilus_; it is capable of eversion, and aids the worm in
burrowing, as well as in seizing and swallowing the mud. The vascular
system is represented by a dorsal and a ventral vessel, neither of which,
however, is contractile. In _Protodrilus_ the dorsal vessel divides into
two branches in the first segment, each of which passes to the tip of the
tentacle, and returning, joins its fellow to form the ventral vessel. In
some species of _Polygordius_ there is a pair of vessels connecting the
dorsal and ventral vessels in every segment, but no vessel to the tentacle.
The blood is colourless in some species of _Polygordius_, but may be yellow
(_P. neapolitanus_), red (_P. lacteus_), or green (_P. erythrophthalmus_).
Paired nephridia, with distinct funnels, occur regularly throughout the
body.

The sexes are separate in _Polygordius_, whilst _Protodrilus_ is
hermaphrodite, bearing ova in the first seven segments and testes in the
remaining segments. The genital cells are produced from the body-wall in
every segment; their mode of discharge is unknown in the male
_Polygordius_, though probably the nephridia {245}convey the spermatozoa to
the exterior; but in the female the body-wall ruptures to allow the ova to
escape, and then the animal dies. The development of _Polygordius_ has been
made the subject of very careful study; the larva has long been known, and
is a typical "trochosphere" of rather a depressed form. This "trochosphere"
larva is of considerable importance, as it makes its appearance in sundry
groups of animals in some form or another. Here, in _Polygordius_, it has
the appearance of a couple of wide but low cones united together by their
bases, which form the equator of the larva. This equator carries a double
girdle of cilia, dividing the animal into a preoral and postoral region;
for the mouth is placed on one side of the animal between the two girdles,
while the anus lies at the apex of the postoral cone, and is surrounded by
another girdle of cilia. The alimentary canal is divisible into three
regions; it is separated from the body-wall by an extensive space, which
contains cells destined to give rise to muscles and nephridia. A nervous
system (apical plate) is present at the apex of the preoral cone. This
little larva swims freely on the surface of the sea, moving, balancing, and
feeding by means of the girdle of cilia. It soon increases in length by the
active growth of the apex of the postoral cone, which becomes cylindrical
and then segmented externally and internally. The greater part of the
original larva remains of the same shape as before, and forms the head
(prostomium and peristomium): small tentacles grow out of the preoral lobe,
and after a gradual reduction in the relative size of the "head" by the
growth of the segmented "body," the animal becomes worm-like and develops
into a _Polygordius_.[299]

ORDER II. POLYCHAETA.

ANATOMY OF NEREIS.—In order to obtain a general idea of a Polychaete worm,
it is well to study a concrete example, and for this purpose the common
_Nereis_ serves excellently. Several species (see p. 315) occur more or
less commonly on our coasts, and the general remarks will apply to one as
well as to another.

_Nereis pelagica_ Linnaeus reaches a length of 5 to 6 inches, and is about
¼ inch across. It is convex above, nearly flat below. Its {246}colour is
brown or bronze. The worm, which is to be found in shallow water, is made
up of a considerable number of rings or segments, constituting the "trunk"
or "body," terminated at each end by modified segments known as "head" and
"tail" (Fig. 122). The segments composing the trunk are all alike, except
for small proportional differences, and it will be convenient to describe a
"typical segment" before referring to the head or tail.

A TYPICAL BODY SEGMENT carries on each side a muscular lobed outgrowth,
bearing bundles of bristles or "chaetae," and filamentous sensory organs
known as "cirri." To this lateral locomotor organ Huxley gave the name
"parapodium" (Fig. 124). Each parapodium or foot consists of a basal
portion, supporting a dorsal and a ventral process, the "notopodium"
(_ntp_) and "neuropodium" (_nrp_) respectively, each of which is bilobed.
The lobes are very vascular and glandular, and probably serve as
respiratory organs or "gills."

[Illustration: FIG. 122.—_Nereis pelagica_ L., natural size. (From
Ehlers.)]

[Illustration: FIG. 123.—Chaetae of _Nereis_; enlarged. A, from
neuropodium; B, from notopodium of _N. diversicolor_; C, swimming chaeta of
Heteronereid stage of _N. dumerilii_.]

The chaetae, or bristles, of each bundle project from the mouth of a great
sac, the lips of which are particularly prominent in _Nereis_. Each chaeta
arises from a single cell situated at the bottom of the sac. The chaetae of
_Nereis_, as of many other Polychaetes, are of a kind usually termed
compound or "jointed," each being composed of a long stalk and a small
"appendix" articulated in a cup at its {247}extremity (Fig. 123). The shape
of the cup varies; it is in some cases of equal height all round, or it is
higher on one side than on the other. Further, the appendix may be short
and curved, or more elongate and spear-like; it is generally notched or
finely toothed on one side.

[Illustration: FIG. 124.—_Nereis._ Somewhat diagrammatic transverse section
through the body. On the left the chief constituents of the vascular system
are represented; on the right side the chaetae and their muscles, as well
as the distribution of the lateral nerve, etc., are shown. _ac_, aciculum;
_bv_, network of blood-vessels; _ch_, chaetae (only a few are shown) in two
bundles; _ch.l_, lips of the chaetigerous sac; _cil.org_, dorsal ciliated
organ; _cir_, circular muscular coat; _coe_, coelom; _d.cir_, dorsal
cirrus; _d.long_, dorsal bundle of longitudinal muscles; _d.ve_, dorsal
blood trunk; _ep_, epidermis; _INT_, intestine; _int.cap_, blood
capillaries in its wall; _m.ch_, muscles which move the chaetae; _N.c_,
ventral nerve cord; _ner_, lateral sensory nerve, dividing into a ventral
branch entering the ventral cirrus, and a dorsal branch (_n.cir_) for the
dorsal cirrus; _neph_, nephridium, seen through the oblique muscle through
which its funnel passes; _nrp_, neuropodium; _nrp.lig_, neuropodial lobe or
ligule; _ntp_, notopodium; _ntp.lig_, notopodial ligule; _obl_, oblique
transverse muscle (muscle of the parapodium); _pv_, peripheral
blood-vessel; _v.cir_, ventral cirrus; _v.long_, ventral bundle of
longitudinal muscles; _v.ve_, ventral blood trunk.]

In addition to these projecting locomotor chaetae, there is embedded in
each of the two chaetigerous lobes a much stouter and dark-coloured,
needle-shaped bristle known as an "aciculum," whose point only just
projects beyond the surface. This aciculum extends into the interior of the
body much farther than do the locomotor chaetae, and it is to it that the
muscles serving to move the whole bundle of chaetae are attached. The
acicula thus serve as an internal skeleton to the parapodium. The shape of
the parapodium, the relative lengths of cirri and lobes, the shape and
arrangement of the chaetae, are all employed as specific characters.

{248}The HEAD consists of a preoral portion above the mouth, the
"prostomium," and a postoral region surrounding the mouth, the
"peristomium" (Fig. 125). The prostomium varies in shape in different
species of _Nereis_; but it always carries on its dorsal surface two pairs
of eyes. From its narrower anterior end there arises a pair of short,
somewhat conical, sensory processes known as the "prostomial tentacles." A
second pair of processes springs from the under surface, and rather to the
side of the prostomium; these are known as the "palps," and in _Nereis_ are
much more conspicuous than the tentacles; each is composed of two parts, a
large basal piece and a smaller terminal joint, capable of being withdrawn
into the former. The palps are highly muscular, and though they are sensory
organs, act also as great lateral lips.

[Illustration: FIG. 125.—_Nereis diversicolor_ Müll. × 4. Head, with buccal
region everted. A, Dorsal view; B, ventral view. _a_, Prostomium; _B_,
everted buccal region; _c_, _c'_, peristomial cirri, 1, 2, 3, 4; _d_,
denticles or paragnaths; _e_, eyes; _E_, lower lip; _P_, palp in A,
entrance to pharynx in B; _J_, jaw; T, tentacle; I, peristomium; II, foot
of apparent second segment.]

The peristomium is in many species of _Nereis_ (as in _N. pelagica_)
considerably larger than the trunk segments; it carries at its anterior
edge four filiform cirri on each side, which are directed forwards and used
as feelers. They are arranged in couples; a more anterior couple of dorsal
and ventral cirri, and a more posterior couple of dorsal and ventral cirri.

The TAIL.—As the most anterior segment is perforated by the mouth, and is
modified as described above, so the last or anal segment, which carries the
anus, differs from the rest. It is more or less elongated, cylindrical, and
without parapodia or chaetae. It retains, however, its pair of ventral
cirri, which are very long.

INTERNAL ANATOMY.—In correspondence with the external {249}metamerism there
is an internal repetition of parts. For, except in the anterior segments,
where the powerful protrusible pharynx is situated, the body-cavity or
"coelom" is divided into a series of chambers, by means of muscular septa
inserted, on the one hand, into the body-wall at the level of the grooves
between the external segments, and, on the other, into the wall of the
alimentary canal. Each of these coelomic chambers contains a pair of
nephridia, a portion of the intestine, of the vascular system, and of the
nervous system, as will be seen in Fig. 124.

The epidermis, which forms the outer part of the _body-wall_, consists of a
single layer of cells, covered externally by a thin, tough cuticle. The
latter is usually stated to consist of the chemical substance known as
chitin, but since the cuticle differs from true chitin by dissolving in
caustic potash after a time, Eisig[300] has suggested that its substance is
merely a stage in the formation of chitin. The epidermis contains
gland-cells, which are especially abundant on the lobes of the parapodia.
Below the epidermis lies the circular coat of muscles by whose contraction
the worm diminishes its diameter: it is interrupted on each side at the
junction of the parapodium with the body. Deeper still lie the longitudinal
muscles, which form four great bundles, two dorsal, separated by the
insertion of a small mesentery and dorsal blood-vessel, and two ventral
bundles separated in the middle line by the nerve-cords. These longitudinal
muscles, by their contraction, bend the worm from side to side, and are
continuous from segment to segment. A very characteristic muscle, present
in all the Polychaeta, is an obliquely transverse sheet of fibres passing
from the body-wall at the side of the nerve-cord to the parapodium, where
it spreads out and serves to move the parapodium (Fig. 124). All these
muscles consist of smooth fibres, as in the earthworm.

The ALIMENTARY CANAL may be divided into the following four regions:—(1)
buccal or eversible region, (2) pharynx, carrying the great jaws, (3)
oesophagus, (4) intestine.

The first two regions constitute an "introvert" (Lankester[301]). When
fully everted the whole of the buccal region is turned inside out, and the
terminal aperture leads directly into the pharynx, which is not everted but
merely protruded. {250}Throughout the following chapters the word "buccal"
region is used for that part—if any—which is thus everted (Figs. 125, 126).

Both the buccal and pharyngeal regions are wrapped round by several coats
of muscle, to form apparently a single muscular organ (Fig. 127, _sh_),
which occupies about eight segments in a condition of complete
introversion. The septa are absent from the anterior part of the body.

[Illustration: FIG. 126.—Diagrams to illustrate the action of the
Chaetopodan "introvert." (From Lang.) A shows the apparatus at rest; the
mouth leads into the buccal cavity (_B_), with paragnaths in its wall; _P_,
the pharynx, with jaws at its anterior end; _c_, brain; _p_, protractor
muscles; _r_, retractors. B, the pharynx has been brought forward or
protruded by the eversion, or turning inside out, of the buccal region, so
that the jaws (_j_) now lie some way in front of the head, which is
represented by the brain.]

The buccal region is lined with chitin, which is specially thickened at
certain definite spots, forming small "denticles" or "paragnaths" (Fig.
125), which have a different arrangement in the various species.

The cavity of the pharynx is narrow and the walls thick and muscular; each
side wall carries a large, dark, chitinous "jaw" (Fig. 127, J), which is
hollow at the base, into which the muscles serving to move it are inserted,
whilst the apex is solid, curved, and more or less notched. These two great
jaws are used not only for tearing prey, but for seizing it; for when the
pharynx is entirely protruded the two jaws are wide apart, and when
retraction takes place they come together and grasp the prey.

Eversion of the apparatus is partly effected by protractor muscles (Fig.
126, A, _p_) and partly by the pressure of the coelomic fluid, compressed
by the muscles of the body-wall; the eversion is stopped at a certain stage
by a sheet of muscular tissue or "diaphragm" (Fig. 127, _diaph_) inserted
round the buccal region and attached to the body-wall in the second
segment. The introversion is effected partly by the contraction of this
diaphragm and partly by the action of powerful retractor muscles (Fig. 126,
_r_) inserted into the hinder end of the pharynx and passing to the
body-wall (these are removed in Fig. 127). The {251}movement of the jaws
themselves and of the wall of the apparatus is due to other muscles.

The _oesophagus_ is quite short; into it opens a pair of sacculated
diverticula or glands. Then follows the _intestine_, which extends through
the rest of the body as a thin-walled tube, slightly dilated at the
insertion of the septa.

[Illustration: FIG. 127.—_Nereis_, laid open by removal of the dorsal
body-wall. _br_, Cerebral ganglion, from which three pairs of nerves are
represented as arising; a pair to the tentacles, a pair to the palps, and a
pair (_con_) passing one on each side of the buccal region, to join the
ventral nerve-cord; _mo_, mouth, exposed by removal of the dorsal wall of
the buccal region (_Buc_); _pgn_, the paragnaths in its wall; _Ph_,
pharynx; _J_, the large "jaws" embedded in its wall; _J.mus_, the muscles
which work the jaws; _sh_, the muscular sheath; _diaph_, the "diaphragm";
_oes_, oesophagus; _gl_, its glands; _st_, stomach; _int_, intestine;
_Sept_, septum; _d.v_, dorsal blood trunk; _p.v_, perivisceral branches,
one pair in each segment; _P_, palp; _t_, tentacle; _per_, peristomium;
_per.ci_, two of the four peristomial cirri; _ppIII_, the first parapodium
which belongs to the third true, but second apparent, segment; _cir.m_,
circular muscular coat; _lg.m_, longitudinal muscular coat of the
body-wall.]

The VASCULAR SYSTEM consists of a contractile dorsal vessel and of a
non-contractile ventral vessel extending along the whole length of the
body, from each of which paired and segmentally-arranged {252}vessels pass
to the intestinal wall and to the body-wall, and here form extensive
capillary networks (Fig. 124, p. 247). This type of vascular system is
pretty generally adhered to throughout the Order, but in the
Terebelliformia, Scoleciformia, and Cryptocephala the dorsal vessel and
capillary plexus on the intestine are replaced by a continuous blood sinus,
situated in the substance of the gut-wall. This "perienteric sinus" has the
same relation to the segmental vessels as the dorsal vessel has in the
Nereidiformia, and from it a tubular dorsal vessel arises anteriorly. In
_Arenicola_ the sinus is preceded in the young stage by a network the
branches of which gradually enlarge, meet, and fuse to form the sinus.[302]
Whether it is in all cases secondary is a moot point.

This system of vessels in the majority of Chaetopoda contains a respiratory
fluid coloured red[303] by haemoglobin in solution; in it float a very few
small oval nucleated non-amoeboid corpuscles. But the place of this red
pigment is taken by a green one, named "chlorocruorin," in the
Chlorhaemidae and many Sabelliformia;[304] whilst in _Magelona_[305] the
blood is tinted madder-pink by a number of globules of "haemerythrin." The
blood (or "haemal fluid") is driven forwards in the dorsal vessel, and
passes backwards in the ventral vessel. Respiration in _Nereis_ is carried
on by the whole surface of the body, but naturally with greater activity in
the surface of the parapodia, the lobes of which, with their extensive
vascular plexus, may be termed "gills"; but it must be borne in mind that
these organs have other functions as well.

The _coelomic fluid_, which fills the general body-cavity, is colourless,
and contains amoeboid corpuscles or "leucocytes." It corresponds to the
lymph of Vertebrates, being nutritive in function, in that it conveys
absorbed material from the wall of the intestine to the organs of the body,
and at the same time removes any waste substances from these organs; these
waste substances contain nitrogen, and are ultimately removed by the
nephridia. In _Ophelia_ many of the corpuscles contain a curious
dumb-bell-shaped rod of chitin, and it has been shown[306] that this
substance {253}is a highly complex form of excretory material,—more complex
than guanin, for instance, which exists in the corpuscles of the
Capitelliformia.

In Glyceridae, Capitelliformia, and _Polycirrus haematodes_ (a Terebellid),
the vascular system is absent, and the coelomic corpuscles become coloured
by haemoglobin, and in order that the coelomic fluid may be distributed to
the organs of the body, the peritoneum is ciliated along certain definite
tracts. The fluid in these "anangian" worms thus combines originally
separate functions, and behaves like the "blood" of Vertebrates.

The EXCRETORY SYSTEM is represented by a pair of nephridia in each segment,
with the exception of a few anteriorly and a few posteriorly. The
nephridium of _Nereis_ differs from that of most other Polychaetes hitherto
examined carefully, and rather resembles that of the Oligochaetous
Enchytraeids. It consists of a compact gland-like organ, containing a much
coiled tube, ciliated for the greater part of its length, but deprived of
cilia in its last coils; this latter part—or duct—leaves the "gland" and
pierces the body-wall, opening to the exterior at the base of the
parapodium. The ciliated canal passes forwards into the next segment, where
it opens by a funnel into the coelom. The lip of the funnel is extremely
curious, for the cells constituting it are drawn out into very long,
delicate processes covered with cilia.[307]

[Illustration: FIG. 128.—Nephridium of _Nereis_. (From Goodrich.) _f_,
Funnel; _n_, neck, which passes through a septum; _t_, coiled tubule; _c_,
connective tissue; _d_, duct.]

In most Polychaetes the nephridium is a wide, sac-like tube as in
_Arenicola_[308] (Fig. 129). Its walls are covered by a dense network of
blood-vessels, and it not only acts as an excretory organ, but also as a
genital duct (see p. 273).

Excretion, in the strict sense of the word, is carried out by {254}the
cells forming the wall of the tube; they remove waste materials from the
blood distributed over the surface of the organ. But, in addition, there is
a removal from the coelom, by means of the funnel, of any dead or dying
coelomic corpuscles which in their turn have eaten up or otherwise
destroyed foreign bodies (such as Bacteria, etc.) that may have entered the
animal.

In _Nereis_ there is in each segment, in addition to the pair of nephridia,
a pair of "dorsal ciliated organs" (Goodrich) (_cil.org_ in Fig. 124). Each
appears as a wide-mouthed funnel, greatly folded, and without any permanent
outlet. But it is possible that these organs function as genital ducts, and
that the external aperture will make its appearance temporarily at the
period of maturity. This "dorsal ciliated organ" has not been met with in
allied genera—such as _Eunice_, _Nephthys_, _Polynoë_, _Glycera_—where the
nephridium is a wide tube, and serves as a genital duct.

[Illustration: FIG. 129.—Nephridium of _Arenicola_. (From Benham.) × 4.
_d_, Dorsal lip of funnel; _v_, ventral lip of funnel; _b_, blood-vessel
(all the black lines are blood-vessels); _m_, dilated bladder; _x_, part
cut away from body-wall where the nephridium is passing to the exterior.]

The NERVOUS SYSTEM, as in all Chaetopods, consists of a dorsal cerebral
ganglion or "brain" (Fig. 127, _br_), connected by circum-buccal
commissures with the anterior end of a ventral chain of ganglia. The brain
occupies the prostomium,[309] and from it nerves pass away to the
prostomial tentacles and palps. The circum-buccal commissures spring from
the outer corner of the brain, and from each arises a nerve to the first
pair of peristomial cirri. The first ventral ganglion lies in the third
segment, and represents at least two ganglion-pairs fused together, for
from it arise (1) a pair of nerves to the second pair of peristomial cirri
and (2) a {255}pair to the first parapodium. In the remainder of the body
there is a ganglion in each segment, whence nerves pass outwards to the
parapodium and muscles of the segment (Fig. 124).

In _Nereis_ the apparently single ganglion in each segment really consists
of two halves, and the apparently single cord which traverses the whole
length of the body consists of two closely apposed cords. In some worms,
such as Serpulidae, the two cords are more or less widely separated, and
the two ganglia of each segment are thus distinct, and connected by a
transverse commissure. In _Nereis_, as well as in many other Polychaeta,
the nerve-cords lie within the body-wall, but in other cases they lie in
the epidermis, as they do in Archiannelida.

The visceral nervous system, supplying the muscles of the pharynx, is
frequently highly developed. In _Nereis_ it arises on each side by two
roots, one from the brain, the second from the circum-buccal commissure.

[Illustration: FIG. 130.—Eye of _Nereis_. (After Andrews.) × 150. A,
Section through the entire eye; _c_, cuticle; _e_, epidermis; _l_, lens;
_h_, rods; _r_, retina; _n_, optic nerve: B, isolated retinal element; _c_,
cell; _p_, pigment; _h_, rod: _f_, nerve-fibre.]

The ORGANS OF SENSE in _Nereis_ are eyes, tentacles, palps, and cirri. The
four eyes, which rest upon the brain, have the structure represented in
Fig. 130. The retina consists of a single layer of cells containing
pigment; each cell is drawn out peripherally into a nerve-fibre, whilst
centrally it forms a cuticular product—the "rod" (_h_). The edges of the
retina are continuous with the surrounding epidermis, and the cup thus
formed remains widely opened to the cuticle in a few Polychaetes, _e.g._
_Autolytus_, and in the young of _Nereis_, but more usually it has the
relations represented in the figure. The lens is produced by the retinal
cells (according to Andrews[310]), and is in some cases (_Eunice_,
_Amphinome_) continuous with the cuticle. It appears to be composed in
other cases (_Lepidonotus_) of continuations of the retinal rods. The
structure of the other sense organs {256}indicates their adaptation to a
tactile function; in each case a nerve traverses the axis of the organ, and
the nerve-fibrils terminate in sensory cells. Very probably the palps have
a certain power of testing the food—a combination of the senses of taste
and smell.

The GENERATIVE SYSTEM.—In all the Polychaeta, with very few exceptions, the
sexes are separate; and the reproductive cells—ova and spermatozoa—are
produced at certain seasons of the year by the rapid proliferation and
modification of coelomic epithelial cells surrounding the blood-vessels in
the parapodium and its immediate neighbourhood. The sexual cells remain in
the coelom till they are ripe.

The egg-cells become filled with yolk globules; a vitelline membrane is
present, and an outer coat of albuminous material. It is doubtful by what
means these sexual cells are discharged in _Nereis_. There is some evidence
that the "dorsal ciliated organ" may act as a genital duct. In some other
worms the nephridia serve this purpose, whilst in others a rupture of the
body-wall allows the products to escape into the sea. According to
Wistinghausen,[311] at the time of discharge the females of _Nereis
dumerilii_ become surrounded by a kind of gelatinous tube formed from a
secretion of the parapodial glands, and into this tube the ova are
discharged, and arranged in a single layer round its wall.

The common species _Nereis diversicolor_ is viviparous. In a large number
of species of _Nereis_ the sexually-mature individuals undergo very marked
changes in various parts of their body, so that they differ very greatly
from the immature individuals.

These changes resulting in the "heteronereid" condition will be dealt with
at some length in Chap. X. p. 276. The larvae of Polychaetes and other
facts connected with reproduction are described in the same chapter.

{257}CHAPTER X

CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE—
GILLS—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS—
BUDDING—FISSION—BRANCHING—REGENERATION.

The POLYCHAETA are marine worms whose bodies are usually elongated and
cylindrical; they either lead a free life, swimming in the open sea, or
crawling along the bottom; or they pass their life in burrows or definite
tubes of various kinds.

Each segment is normally provided on each side with a single or a couple of
bundles of chaetae, by means of which locomotion is effected. These, in the
free-living forms, are carried at the ends of lateral muscular outgrowths
of the body, known as "parapodia," which are practically limbs.

The "head" of the worm generally carries eyes, and frequently more or less
elongated tactile organs, the "tentacles" dorsally and "palps" ventrally.
The foregut is frequently provided with a masticating apparatus in its
anterior region, which is capable of protrusion; but this apparatus is
absent in many burrowing and tubicolous forms. The sexes are separate, so
that there is no such complicated system of generative organs as occurs in
the Oligochaeta. The nephridia usually act as genital ducts. In the
majority of cases the egg develops into a larva, the "Trochosphere," which
leads a free life and undergoes a greater or less metamorphosis into the
adult condition.

The CLASSIFICATION OF POLYCHAETA adopted in this work is as follows:[312]—

{258}BRANCH A. PHANEROCEPHALA.

SUB-ORDER 1. _Nereidiformia_ [= _Errantia_, _auctt._ + _Ariciidae_].

Family 1. Syllidae see p. 306
" 2. Hesionidae " 308
" 3. Aphroditidae " 309
" 4. Phyllodocidae " 313
" 5. Tomopteridae " 315
" 6. Nereidae " 315
" 7. Nephthydidae " 317
" 8. Amphinomidae " 318
" 9. Eunicidae " 318
" 10. Glyceridae " 320
" 11. Sphaerodoridae " 320
" 12. Ariciidae " 321
" 13. Typhloscolecidae " 321

SUB-ORDER 2. _Spioniformia._

Family 1. Spionidae see p. 321
" 2. Polydoridae " 323
" 3. Chaetopteridae " 323
" 4. Magelonidae " 325
" 5. Ammocharidae " 325

SUB-ORDER 3. _Terebelliformia._

Family 1. Cirratulidae see p. 325
" 2. Terebellidae " 327
" 3. Ampharetidae " 330
" 4. Amphictenidae " 330

SUB-ORDER 4. _Capitelliformia._

Family. Capitellidae, see p. 331.

SUB-ORDER 5. _Scoleciformia._

Family 1. Opheliidae see p. 331
" 2. Maldanidae " 332
" 3. Arenicolidae " 333
" 4. Scalibregmidae " 334
" 5. Chlorhaemidae " 334
" 6. Sternaspidae " 335

BRANCH B. CRYPTOCEPHALA.

SUB-ORDER 1. _Sabelliformia._

Family 1. Sabellidae see p. 336
" 2. Eriographidae " 338
" 3. Amphicorinidae " 339
" 4. Serpulidae " 339

SUB-ORDER 2. _Hermelliformia._

Family. Hermellidae, see p. 341.

COMPARATIVE ANATOMY OF THE POLYCHAETA.

GENERAL SHAPE OF THE BODY.—The majority of the Polychaeta have an elongated
and very mobile body, like that of _Nereis_, consisting of an indefinite
and usually of a considerable number of segments; a few, however, have a
shorter body, with fewer segments, definite in number, for instance
_Aphrodite_ and _Polynoë_, which have thirty to forty segments; and some
Hesionids, with only some seventeen to twenty segments.

{259}In Aphroditidae and certain Amphinomidae the body is more or less oval
in shape. In _Lipobranchius_ and _Sternaspis_ it is grub-like, short, and
cylindrical, with rounded ends; in the former it is difficult to
distinguish head and tail, or dorsal and ventral surfaces.

The segments composing the trunk may be all alike, or may constitute two
more or less sharply marked regions, the _thorax_ and _abdomen_, differing
in the character of the chaetae, or in their arrangement, or in some other
way, as in the Sabelliformia and the Capitelliformia.

As peculiar cuticular structures, the curious shields of _Sternaspis_, and
of certain of the Maldanidae may be mentioned.

The posterior extremity is generally more or less narrowed, and most of the
Nereidiformia are provided with special elongated cirri, borne by the anal
segment. In the Maldanidae and others the body terminates in a funnel, at
the bottom of which is placed the anus. Only in a few cases is the anus not
terminal; in _Notopygos_ and other Amphinomidae, as well as in some species
of _Polynoë_, it is dorsal.[313] In _Sabellaria_ and _Pectinaria_ the
hinder end of the body undergoes great degeneration; in the former it is
achaetous, but cylindrical and bent forwards alongside the body (Fig. 131).
In _Pectinaria_ (Fig. 177), this region, which is called the "scapha," is
leaf-like, and serves to close the narrower end of the tube in which the
worm lives. _Arenicola marina_, and some Terebellids have no chaetae in the
hinder, narrower part of the body.

[Illustration: FIG. 131.—_Sabellaria alveolata_ L. × 3. (After Malmgren.)
_a_, Anus.]

The HEAD.—The prostomium is, in the majority of cases, rounded or conical,
though it may be square (_Nephthys_) or elongated and jointed (_Glycera_),
or even hammer-shaped (_Tomopteris_); or it may be fused with the
peristomium, and apparently absent (_Arenicola_). In the great group
Cryptocephala, the peristomium grows forwards so as to hide the prostomium
entirely.

In a few of the Nereidiformia the prostomium is compressed, {260}and in the
Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a
leaf-like process overlapping three or more segments. In many Aphroditidae
(as well as in _Polydora_) there is a peculiar "frontal" ridge passing
forwards from the prostomial tentacle, and downwards into the mouth (Figs.
132, _c_, and 133, A, _x_).

[Illustration: FIG. 132.—_Aphrodite aculeata_ L. Ventral view of anterior
region, × 6. _a_, Prostomium; _c_, frontal ridge on prostomium; _d_,
neuropodial cirrus; _l_, lower lip; _m_, mouth; _p_, palp; _s_,
intersegmental groove; _t_, tentacle; I, foot of peristomium, which has
shifted forwards so as to lie in front of the mouth; II to V, successive
feet.]

In all the Nereidiformia, as well as in Sabelliformia and Chlorhaemidae,
the prostomium bears sensory processes of two kinds, viz. dorsal tentacles
and ventral palps. The latter are invariably two in number, and are
particularly well developed in Aphroditidae, Nereidae, Syllidae, some of
the Eunicidae, and in Chlorhaemidae. Even when they are apparently absent,
as in _Nephthys_, it is possible that they are represented by certain lobes
at the sides of the mouth, for in many Syllidae they are so fused with the
prostomium as to be scarcely distinguishable. In the Chlorhaemids the
palps[314] are grooved, and in the Sabelliformia they become considerably
branched, and extend round the prostomium so as to nearly meet dorsally and
ventrally. Each palp is, in this sub-Order, represented by a greater or
smaller number of long, mobile filaments, arising from a common base; they
are grooved along the inner side, ciliated, and provided with secondary
processes. The crown of "gills," in fact, is nothing {261}more than the
greatly subdivided and enormously elongated palps, as both Pruvot[315] and
Meyer[316] have shown. In such forms as _Haplobranchus_ and _Amphicorine_
the process of subdivision (branching) has only gone a short way. In all
the Sabelliformia each filament, in addition to its sensory function, aids
in conveying food to the mouth by the action of the cilia, and has a
blood-vessel within, thus acting as a respiratory organ. The filament may
carry compound eyes (Fig. 143) either at its apex (_Branchiomma_) or at
intervals along its course (_Dasychone_).

[Illustration: FIG. 133.—A, Anterior end of _Polydora_ enlarged. _a_,
Prostomium; _x_, frontal ridge; I, peristomium; _c'_, its long cirrus; II,
III, etc., the following segments; _c_, gill; B, head of Sabellid; _P_,
palps (branchial crown); _t_, position of tentacles; _l_, processes of
upper lip membrane; I, peristomium raised into a collar; II, III, IV,
following segments.]

In the family Serpulidae one (rarely two) of the most dorsally placed gill
filaments is enlarged terminally, and acts as a stopper or "operculum,"
which closes the mouth of the tube when the animal withdraws into it.
Further, in _Spirorbis_ this operculum is grooved on one side, and serves
as a brood pouch in which the eggs undergo development (Fig. 184, p. 341).
It will be seen, therefore, that the palps may be very important organs for
the life of the worm, and they are no less interesting to the comparative
anatomist, serving as they do as an excellent illustration of the various
uses which Nature finds for one and the same organ.

In the other sub-Orders the prostomium carries neither palps nor tentacles.

{262}[Illustration: FIG. 134.—Heads of various Polychaeta (diagrammatic).
A, Polynoid; B, Syllid; C, _Nephthys_; D, _Eunice_; E, _Phyllodoce_; F,
_Trophonia_: _a_, prostomium; _c_, normal cirrus; _c^1_, peristomial cirri;
_c^2_, cirrus of second segment; _c^3_, cirrus of third segment; _el^1_
point of attachment of elytron; _p_, palp: _s_, nuchal organ (ciliated
pit); _t_, tentacle; I, peristomium; II, III, IV, segments.]

The _tentacles_ in the Nereidiformia present a wide variation in number;
probably the typical number is three, one of which is median and two
lateral—as in Polynoids, Syllidae, and some Eunicidae. Further, there is a
certain amount of evidence in the nerve supply of the median tentacle to
show that it was originally double. The presence of four tentacles, then,
as in _Nephthys_, _Phyllodoce_, and _Glycera_, may be a primitive
condition. By the disappearance of the paired lateral tentacles the worm
possesses a single median one, as in _Aphrodite_ and Amphinomids;[317]
whilst a duplication of these lateral ones leads to the condition of
_Eunice_ and _Hyalinoecia_, which have five tentacles. In the Chlorhaemidae
the number is further increased to five or more on each {263}side,[318] and
in the Terebellidae these prostomial processes become very numerous.

In the Cryptocephala there is never more than a single pair of tentacles,
and these are generally reduced to a group of sensory cells, though in
_Sabellaria_ they retain a considerable size.

In a few genera, such as _Aphrodite_, _Nephthys_, _Capitella_, the first
postoral segment is distinguished from the succeeding segments only by its
position with regard to the mouth (Fig. 132) and by its smaller size. But
in the remainder of the Polychaeta, with here and there an exception, the
peristomium is achaetous in the adult.[319]

Except in the Nereidiformia, peristomial or tentacular cirri are rare,
being represented in the Spioniformia by the very long "tentacles." In the
Nereidiformia one or more of the following segments may be added to the
peristomium, and share in the "cephalisation," which is so characteristic a
feature in this group. In Amphinomids the first three or four chaetigerous
segments are incomplete ventrally, owing to the shifting of the mouth
backwards; these segments form lateral lips, but they are not otherwise
modified. In _Phyllodoce_, however, there are four cirri on each side of
the mouth, and from the arrangement in the Alciopids we are justified in
concluding that the segment which carries the four pairs of cirri is really
made up of three segments (Fig. 134, E). Among the Hesionids there are four
such "cephalised" achaetous segments with long cirri.

[Illustration: FIG. 135.—_Sabellaria alveolata_ L. Ventral view of anterior
region, × 10. _a_, Notopodial cirrus; _b_, notopodium; _c_, neuropodium;
_ch_, peristomial chaetae; _d_, neuropodial cirrus; _m_, mouth; _P_,
multifid palp (gill filaments); _P'_, ridges after removal of gill
filaments; _s_, ventral (tubiparous) gland shield; _T_, tentacle; I, hood
formed by peristomium; II to VI, following segments.]

{264}In a few cases, such as the Chlorhaemids and _Sternaspis_, and to a
slight degree in _Arenicola_, the "head" and even the anterior part of the
worm is capable of being withdrawn into the body.

THE PARAPODIA AND CHAETAE.—The typical parts of a parapodium have been
described in the preceding chapter; here it is only necessary to refer to
the series of diagrams (Figs. 136, 137) representing the parapodia of the
more common Polychaetes, and to add a few remarks about them.

[Illustration: FIG. 136.—Parapodia. A, _Nephthys_; B, _Amphinome_; C,
_Glycera_ (the unlettered lobe above _g_ is the notopodial cirrus); D,
_Syllis_; E, _Eunice_; F, _Phyllodoce_. _a_, Notopodial cirrus; _b_,
notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _g_, special gill;
_n_, aciculum (omitted in B); _x_, cirriform lip of chaetigerous sac.]

In most Annelids the chaetae are in two bundles on each side, but there are
certain families in which the dorsal bundle, and even the notopodium
itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the
dorsal bundle may be absent only in certain regions of the body, as in the
hind-body of Terebellids. In some Amphinomidae and Aphroditidae the
notopodium is scarcely distinct as a separate lobe, being a slight tubercle
on the upper surface of the neuropodium; but the notopodial chaetae are
present, and indeed particularly well developed in many cases.

But whilst, in the Nereidiformia, the parapodia, whether {265}consisting of
two lobes or only one, are always well developed, and project to a more or
less pronounced degree from the sides of the body, it is otherwise in the
rest of the group, where the chaetigerous lobes are usually reduced to mere
tubercles or ridges, no doubt in relation to their burrowing or tubicolous
habits. In _Sternaspis_ the chaetae issue directly from the body-wall.

Amongst the Nereidiformia we find examples in which the parapodia, instead
of being more or less conical "legs," are flattened fore and aft so as to
serve as efficient "fins," as in the active swimmers, _Nereis virens_ and
_Nephthys caeca_, and in the pelagic Phyllodocids, Alciopids,
Typhloscolecids, and _Tomopteris_.

[Illustration: FIG. 137.—Parapodia. A, _Polynoë_; B, _Scoloplos_; C,
_Euphrosyne_. (Transverse section of body.) _a'_, Accessory cirrus; _y_,
doubtful branchiae; D, _Sabella_ (thoracic). _a_, Notopodial cirrus
("elytron" in A, "gill" in B); _b_, notopodium; _c_, neuropodium; _d_,
neuropodial cirrus; _n_, aciculum (accidentally omitted in C).]

Of the typical dorsal and ventral cirri, the ventral is only absent in some
Amphinomids amongst the Nereidiformia; the dorsal is absent in _Nephthys_
and degenerate in _Glycera_, whilst in a very large number of families of
the other sub-Orders neither cirrus is present. These cirri, though
originally filamentous and sensory, may, by virtue of special blood supply,
become "gills," and this occurs in several families of different
sub-Orders. Thus in _Eunice_ this gill is comb-like; in _Amphinome_ and in
_Arenicola_ (on certain segments) it is arborescent, as it is also in one
to three segments in Terebellids; whilst in Ariciidae, Spioniformia,
Cirratulidae, Opheliidae, and _Sabellaria_ it remains more or less
finger-shaped or filamentous. In the family Serpulidae the thoracic cirri,
both dorsal and ventral, {266}become flattened and extended
antero-posteriorly, and unite with one another to form the "thoracic
membrane."[320] In Phyllodocidae the cirri are foliaceous and natatory, and
they contain a great quantity of glands of a peculiar character. The
Aphroditidae are distinguished from other Annelids by the possession of
"elytra" or dorsal scales, which appear to be the dorso-ventrally flattened
cirri, retaining their sensory nature, but adding to this function several
others.[321]

The CHAETAE or bristles are mainly used in locomotion, but it is not
unreasonable to believe that some of the stronger, serrated kinds may be
used as weapons of offence and defence; certainly the Polynoids, bristling
as they do with stiff chaetae along each side, must be rather unpleasant to
their smaller enemies.

The various bristles may be placed in three chief groups, viz. (1) simple;
(2) jointed; (3) uncini (see Fig. 138).

(1) The simple chaetae may be smooth and hair-shaped, _i.e._ "capillary,"
such as are present in nearly all families: or they may be forked
(Amphinomidae), comb-shaped (_Eunice_), notched or serrated, or provided
with a series of frills at right angles to their length, as in
Aphroditidae; or fringed along one or both sides with a membranous
expansion, as in Terebellids and Sabellids. The simple chaetae may also be
short and spine-like, as in the ventral bundles of _Arenicola_; or they may
be slightly curved at the end and notched, forming what are generally
termed "crotchets," such as are common amongst Oligochaeta. These
"crotchets" may be simple, or have numerous denticulations at the end
(Maldanidae), or be provided with a membranous hood (Spioniformia,
Capitelliformia). In _Hermione_ peculiar sheathed, spear-like bristles
occur (Fig. 138, N).

(2) Jointed chaetae have already been described (p. 246); they are confined
to the sub-Order Nereidiformia, and occur only in certain families.

(3) The uncini are very short chaetae, which are simply embedded in the
skin, and do not extend beyond the body-wall into the body-cavity. An
uncinus is a sharply curved hook, which may have more or less numerous
secondary teeth on it. They are characteristic of the Sabelliformia and the
Terebelliformia.

The chaetae appear as solid, usually fibrillated structures, of a yellow or
golden tint, transparent and refringent. Chemically {267}they consist of
_chitin_, and each chaeta is the product of a single cell. The chaetae of
_Euphrosyne_ are hollow and calcareous, being peculiar in both characters.

[Illustration: FIG. 138.—Chaetae of various Polychaetes (the magnification
is not the same in all cases). A, Doubly-fringed capillary, from
Terebellid; B, hooded crotchet, from _Polydora_; C, a fork, from
_Euphrosyne_; D, jointed chaeta, from _Phyllodoce_; E, simple chaeta, with
serrated ridges or frills, from a Polynoid; F, jointed chaeta, from
_Eunice_; G, uncinus, from _Pomatoceros_ (Serpulid); H, one of the outer
series of paleae from the hood of _Sabellaria spinulosa_; I, jointed
chaeta, from a Syllid; J, multidenticulate crotchet, from a Maldanid; K,
comb-shaped chaeta, from _Eunice_; L, uncinus of a Sabellid; M, uncinus of
Terebellid (_Amphitrite Johnstoni_); _a_, edgewise; _b_, side view; _m_,
attachments of muscles into _ba_, basal plate; _x_, accessory teeth. N,
Sheathed spear of _Hermione_; _a_, the spear-shaped capillary removed from
its sheath; _b_, the same, with sheath.]

Certain modifications of the chaetae presented by various worms deserve
mention. In _Polydora_ (Fig. 133, A) and in _Chaetopterus_ (Fig. 173, p.
324) those of one segment are especially strong, but their significance is
uncertain. In _Capitella_ those {268}of the notopodium of the eighth and
ninth segments are specially modified; they are analogous to the copulatory
chaetae of Oligochaeta. In _Aphrodite_, in addition to the ordinary
locomotor chaetae, there are brilliant, iridescent bristles and peculiar
felting threads arising from the indistinct notopodium; these latter,
however, are not true "chaetae," but are separate chitinous filaments
similar to the constituent fibres of an ordinary chaeta.[322]

While the chaetae in the Nereidiformia and others are grouped in bundles,
those of many other families are in vertical, transverse rows, as in
Maldanidae and in _Arenicola_. The uncini are always embedded in such rows,
usually slightly raised from the general level of the body surface, each
being termed a "torus uncinigerus." These tori are usually limited to the
sides of the body, but in _Myxicola_ and in _Notomastus_ they encroach upon
the dorsal surface, and in _Chaetozone_, also upon the ventral, so as
nearly to encircle the body, recalling the "perichaetous" condition of some
earth-worms.

[Illustration: FIG. 139.—_Aphrodite._ Foot, × 2. _a_, Elytron; _b_,
notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _n_, aciculum; 1,
iridescent bristles; 2, stiff chaetae; 3, felt.]

GILLS.—We have already seen that several different organs, _e.g._ the palps
in Sabelliformia, the prostomial tentacles of Chlorhaemidae, and the
notopodial cirri of sundry other Polychaetes, may take on a respiratory
function. There are, however, certain "gills" developed either on the
parapodium itself or elsewhere on the body which it is difficult to
homologise. Such are the _retractile gills_ on the parapodia of the
Glyceridae (Fig. 136, C); those of _Dasybranchus_, near the abdominal
neuropodia; those of _Mastobranchus_, near the notopodia. _Nephthys_ has a
sickle-shaped gill on the under surface of the notopodium. The long gill
filaments at the posterior end of _Sternaspis_, again, are only doubtfully
interpreted as the dorsal cirri of some of the posterior segments.

{269}Since primitively the whole skin of the worm is respiratory, any part
of the skin may become more or less specialised for this function, and
chiefly, of course, on the more actively moving parapodia. The
blood-vessels constituting the essential part of the "gill" may make use of
any already existing outgrowth (such as a cirrus or a tentacle), or may
push the body-wall out on their own account.

INTERNAL ANATOMY.

Probably those organs which have the greatest effect in modifying the shape
of the body are the SEPTA, for we find in the long, free-swimming worms
that these are regularly present throughout the body, and external
"segmentation" of the body is well marked. In burrowing and tubicolous
forms the septa are frequently incompletely developed, or more or fewer may
be absent; and the body becomes less distinctly segmented externally, tends
to vary greatly in diameter during movement, or becomes plumper. With the
disappearance of the septa there is also a diminution in the number of
NEPHRIDIA, as in _Arenicola_, with only six pairs. Further, there is
frequently a dimorphism of these organs; instead of all of them serving
equally as excretory organs and as genital ducts, some of the most anterior
in the Sabelliformia and Terebelliformia become greatly enlarged, and take
on practically the whole of the former function; whilst more or fewer of
the posterior nephridia dwindle in size, and become genital ducts. The
absence of septa allows a free communication between the successive
segments, and thus a freer flow of coelomic fluid for the distension of the
anterior end of the worm during burrowing.

The ALIMENTARY SYSTEM presents certain modifications of a systematic value.
In the Nereidiformia the muscular pharynx, which is always protrusible and
is preceded by an eversible buccal region, frequently encloses thickened
cuticular plates which serve as crushing and grasping organs. The form,
number, and arrangement of these "jaws" vary in the different families.
They form valuable fossil records of extinct worms.

In the Scoleciformia and Capitelliformia the buccal region exists, but
there are no jaws. In the Sabelliformia and Terebelliformia eversion does
not take place and jaws are absent.

{270}Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and
Hesionidae; when present they are usually set in the direct course of the
food. There may be one small tooth used for stabbing, as in some Syllids
(Fig. 141, A); or a circle of such denticles (_Autolytus_, Fig. 140, D). To
these are added powerful grasping jaws in _Nereis_ (E); or the latter may
alone be present, as in _Glycera_ (F). In _Polynoë_ the four jaws are
carried by hard pieces, to which the muscles are attached (C and G). In
_Nephthys_ there is a dorsal and a ventral jaw.

[Illustration: FIG. 140.—Jaws of various Chaetopods. A, Transverse section
of the anterior end of _Eunice_; _a_, _b_, _c_, _d_, various parts of the
upper series of denticles lying in a special chamber; _g_, oesophagus; _k_,
lower jaw: B, the denticles of _Eunice_ separated; _U_, upper series; _a_,
grinder; _b_, forceps; _c_, rasping plates; _d_, grater; _L_, lower series;
_j_, tooth; _k_, base into which muscles are inserted: C, Polynoid; _U_,
upper, and _L_, lower jaws; _j_, tooth; _k_, base: D, Diagrammatic section
across pharynx of Autolytus; E, of _Nereis_; F, of _Glycera_; G, of
_Polynoë_.]

In the Eunicidae, however, the numerous denticles are carried in a special
pouch below the food tract, with which it communicates anteriorly.[323]
They are arranged in an upper and lower series. The lower series (L)
consists of a pair of flat plates (_k_) on each side partially embedded in
and acted upon by muscles, with a harder enamelled piece—the actual lower
"tooth" (_j_)—at its anterior end. The upper series (U) consists of several
pieces, varying in shape and size in the various genera of this
{271}family; but developmentally they result from modifications of two rows
of small, similar pieces.[324]

The _intestine_ is generally straight and cylindrical, and is usually
constricted by the septa, if these are present. In the Polynoids the
intervening sacculations become so long as to receive the name of "caeca,"
which, in _Aphrodite_, become enormously elongated (Fig. 142); there are
eighteen pairs of them (_c_), each being a slender tube bent upon itself,
giving off short branches and dilated distally, where it lies in the base
of the parapodium.

[Illustration: FIG. 141.—A, Alimentary canal of Syllid: B, transverse
section of pharynx of the same; _b_, buccal region; _d_, oesophageal
outgrowth; _g_, salivary glands; _i_, intestine; _j_, tooth; _p_, pharynx;
_s_, gizzard: C, alimentary canal of _Petta_ (after Wirén); _i_, intestine;
_o_, oesophagus; _r_, rectum; _s_, stomach.]

[Illustration: FIG. 142.—Alimentary canal of _Aphrodite_. × 1. (From
Gegenbaur.) _a_, Anus; _b_, pharynx; _c_, caeca; _o_, mouth.]

The intestine is looped in a few genera only, as in _Trophonia_, or coiled,
as in _Sternaspis_, _Petta_ (Amphictenid, Fig. 141, C), and _Ammotrypane_.
In the course of the tube there may be a thick-walled muscular _gizzard_,
with hard chitinous lining, as in certain Terebellids, where it appears to
replace, in function, the pharynx of the Nereidiformia; in the Syllidae the
gizzard is present in addition to the pharynx (Fig. 141, A).

Glandular appendages of the oesophagus are present in many {272}worms.
Amongst the Nereidiformia, the Syllidae and Hesionidae possess oesophageal
diverticula (Fig. 141, A, _d_), which are used, not for secreting a
digestive fluid, but as reservoirs for water and air swallowed by the
worms; and are provided with muscular walls, by which their contents can be
driven out. They appear, in fact, to be used like the swim-bladder of
fishes.[325] Many Chaetopods take in water by the anus—no doubt for
respiratory purposes—and pass it forwards along the intestine. In the
Capitelliformia a special groove conducts the water for some distance, then
the groove becomes closed to form a canal, which, after a course forwards
as a free tube below the intestine, again enters the latter, constituting a
"siphonal apparatus," similar to that of the Echiuroids and the sea
urchins.

SENSE ORGANS.—In addition to the prostomial _eyes_, which are present in
nearly all the Nereidiformia and Spioniformia, eyes may exist elsewhere on
the body: thus _Myxicola infundibulum_ and _Fabricia_ possess a pair on the
anal segment; in _M. aesthetica_ Clap. there is a pair to every segment; in
_Branchiomma_ there is a compound eye near the tip of each gill filament
(_i.e._ palp); whilst in _Dasychone_ a series occurs along each gill
filament. All these examples belong to the Cryptocephala, in which, owing
to certain peculiar modes of life, these sense organs are required in
correspondingly peculiar positions. It is usually stated that
_Polyophthalmus_ possesses, in addition to the usual prostomial eyes,
twelve pairs on as many successive segments; but the minute structure of
these organs points rather to their function as light-producing organs.

[Illustration: FIG. 143.—A gill filament, A, of _Branchiomma_, B, of
_Dasychone_. _a_, Axis; _f_, secondary filaments; _o_, compound eye; _x_,
lappets.]

The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated
pits" or "nuchal organs" at the upper side of the head, {273}between the
prostomium and peristomium, and capable of eversion (Fig. 144). They are
most characteristically developed in the Capitelliformia, where each organ
abuts upon a special lobe of the brain. The function of these "ciliated
organs," which bear a great resemblance to those of the Nemertines, is a
matter of speculation. Similar organs, in the form of simple pits or
grooves, occur in many of the Nereidiformia, Terebelliformia, and
others.[326]

Otocysts are rare. _Arenicola_ possesses a pair at the base of the
prostomium, each of which in some species retains an opening to the
exterior.[327] They probably serve as "organs of direction" rather than of
"hearing." _Aricia_ and _Polyophthalmus_ likewise have such organs on the
prostomium; whilst _Fabricia_, _Myxicola_, _Terebella_, and a few others
possess them in the peristomium, or in some other segment of the body.

[Illustration: FIG. 144.—_Ammotrypane aulogaster_ Rathke, enlarged. (From
Cuningham.) Anterior end. _a_, Prostomium; _b_, everted buccal region; _c_,
notopodial cirrus; X, ciliated organ everted; I, II, III, first three
segments.]

REPRODUCTIVE PHENOMENA.—With a few exceptions mentioned below, the
Polychaeta are unisexual. The sexual cells are developed in all cases from
the lining epithelium of the body-cavity. The exact spot at which this
occurs varies in different cases; it may be, though rarely, on the floor of
the body-cavity; it is more usually on the wall of some blood-vessel,
either the ventral vessel or on branches of it; or on the many blind
blood-vessels of _Aphrodite_. The number of such genital organs is very
great in most worms, but in those presenting two regions of the body they
are confined to the posterior segments (Sabelliformia, Terebelliformia,
Capitelliformia). The number is very limited in _Arenicola_ and other worms
presenting but few nephridia: in the former genus there being six pairs, in
_Trophonia_ only one pair.

The following genera are hermaphrodite:—_Amphiglena_, _Salmacina_,
_Protula_, _Spirorbis_, belonging to the Sabelliformia, to {274}which must
be added some Hesionidae. In this family ova and spermatozoa are developed
around the same blood-vessel. But in the former group of worms (as also in
_Ophryotrocha_) the two kinds of cells are produced in different regions of
the body. Thus in _Protula_ the anterior abdominal segments are male, the
posterior ones female, while in _Spirorbis_ the reverse arrangement holds;
and in _Syllis corruscans_ the anterior segments of the body contain eggs,
whilst the posterior region contains spermatozoa, and this region separates
and becomes a male worm.

The eggs and spermatozoa in the Polychaeta are discharged into the sea
either by rupture of the body-wall or through the nephridia; the male and
female elements unite, and the resulting fertilised eggs undergo
development, either floating separately in the water, or embedded in jelly,
or attached to the body or to the tube of the worm.

The result of the segmentation of the egg is a free-swimming larva known as
a "Trochosphere," similar to that of _Polygordius_. The larvae of different
species present various more or less marked departures from this type, for
instead of the two girdles of cilia there may be only the anterior girdle,
or there may be several complete or incomplete girdles between the two
typical ones, or there may be (Chaetopterids) only a single girdle of cilia
about the middle of the body, the two typical girdles being absent.[328]
The postoral region, after elongation, generally becomes marked out into
three segments, and these segments develop chaetae, which are usually
temporary and specially long.

The little animal is thus equipped for an independent life: the provisional
chaetae help in keeping it balanced; and in some cases (Spionidae) serve to
protect the little soft creature, for when it is touched it curls up, and
its chaetae stick out at the sides, so that it looks like a hairy
caterpillar. But the larva is quite at the mercy of the sea, for it is
carried hither and thither by currents, and in this way the species is
disseminated. The larvae of the Polychaetes, like those of other animals,
occur at certain periods of the year in large quantities at the surface of
the sea, and serve as food for various larger animals.

{275}[Illustration: FIG. 145.—A, Trochosphere of _Nephthys_. × 65. _a_,
Anus; _b_, apical plate (brain); _c_, apical tuft of cilia; _c'_, girdle of
cilia; _i_, intestine; _m_, mouth; _st_, stomach. B, Larva of _Spio_, with
three segments, eight days old. × 100. _c_, Preoral girdle of cilia; _c'_,
preanal girdle; _ch_, long provisional chaetae; _pr_, prostomium with eyes.
(From Claparède and Metschnikoff.)]

These larvae are at first very different from the adult animal, and the
necessary changes to be passed through are more or less great according to
the species. It is not our intention to describe these changes in
detail.[329] The larva increases in size, the permanent chaetae make their
appearance in regular order, and the body exhibits segmentation, the new
segments always appearing just in front of the anal segment. The internal
organs gradually develop, and the prostomial and parapodial appendages grow
out in their turn. In the Sabelliformia the multifilamentous "gills" arise
by the continued branching of an at first simple process (the palp) arising
from the latero-ventral surface of each side of the preoral lobe.[330]
These gradually encroach dorsally and ventrally till the prostomium is more
or less encircled; meanwhile the peristomium grows forwards so as to
conceal the prostomium, which no longer increases at the same rate as does
the rest of the body.

Although most worms appear to discharge their ova directly into the sea and
take no further care of them, some make provision for their offspring
either by laying the eggs in a jelly, which will serve as food for the
young larvae—_Aricia_, _Ophelia_, _Protula_, _Phyllodoce_—or by attaching
them to their body. In certain Polynoids the eggs are attached by means of
a secretion to the back, under the elytra, where they undergo development
up to a certain stage. In _Exogone_ and some other Syllids they are
attached to the ventral cirri, or in _Grubea limbata_, all over the back.
In the female _Autolytus_ (_Sacconereis_) a ventrally-placed brood sac is
formed by the hardening of a {276}secretion; the eggs develop into embryos
inside the brood sac, and then become free, with head appendages and three
pairs of parapodia. Enormous numbers of such embryos may occur; for
instance, some 300 were counted in a brood sac of _Autolytus ebiensis_. In
the case of tubicolous worms, the eggs are frequently attached to the tube,
either inside or outside. In _Spirorbis_ and _Salmacina_ the operculum
serves as a brood pouch.

Only a very few species are known to be viviparous, viz. _Syllis vivipara_
Kr., _Cirratulus chrysoderma_ Clap., _Marphysa sanguinea_ Mont., and
_Nereis diversicolor_ Müll.

In most genera there is no external difference between a mature worm filled
with generative products and an immature one, except, it may be, in the
colour; for the yolk of the eggs is frequently tinted yellow, or pink, or
bluish, while the spermatozoa in mass are white; so that the normal
colouring of the worm may be modified when filled with these elements. But
in a few instances striking anatomical peculiarities are exhibited by the
mature worm.[331] In many species of _Nereis_, for instance, those segments
containing the generative products undergo more or less extensive changes,
while the anterior ones remain unaltered. The body of the ripe _Nereis_ is
then distinguishable into an anterior non-sexual region and a posterior
sexual region; and so great are these changes in certain species that the
mature worms were for a long time believed to belong to a different genus,
and received the name _Heteronereis_. But we now know their true relations,
thanks to the work of Claparède and others. The males in the Heteronereid
phase have fewer unaltered anterior segments than the females, so that
there is a sexual dimorphism.

[Illustration: FIG. 146.—Male "Heteronereis" of _N. pelagica_ L. × 1. A,
Non-sexual region; B, sexual, modified region. (From Ehlers.)]

The changes which _Nereis_ undergoes in its transformation affect chiefly
(_a_) the shape of the parapodia, and (_b_) the form of the chaetae of
these parapodia. Other organs may also be affected, though less noticeably;
thus the eyes become enlarged, the intestine may become so compressed by
the generative {277}products as to be functionless, and the tail develops
special sensory papillae.[332]

In the parapodia an increase in size and a sharper delineation of the
various parts take place; then flattened foliaceous outgrowths (Fig. 147,
_x_, _y_) arise from certain lobes of the feet, in which, too, the blood
supply becomes greatly increased. The old chaetae are pushed out by the
development of new ones of quite a different shape; these are jointed like
the old ones, but the appendix is, in many species at least, flattened and
oar-shaped (Fig. 123, C, p. 246); and the chaetae are arranged in a
fan-like manner. Both these modifications are in evident relation to the
free-swimming habit which the Heteronereid now adopts. The new foot serves
as a swimming organ, the old one was a walking appendage.

[Illustration: FIG. 147.—Parapodium of male "Heteronereis" of _N. pelagica_
L. × 10. (From Ehlers.) _a_, Notopodial cirrus; _b_, notopodium; _c_,
neuropodium with new chaetae; _c'_, foliaceous outgrowth; _d_, neuropodial
cirrus; _x_, _y_, foliaceous outgrowths.]

Whilst some species, such as the common British _N. diversicolor_, undergo
no change, and others become modified as just described, others, again, are
polymorphic. Claparède was the first to show that _N. dumerilii_ may occur
in at least five different mature forms; these differ from one another in
size, colour, mode of life, character of the eggs, etc. The immature forms
may become ripe and lay eggs while still retaining the "Nereid"
characteristics, or these immature forms may become "Heteronereids"[333]
whilst the sexual elements are ripening. There are then three different
kinds of males and of females in this one species, some being found at the
bottom of the sea, as the large Heteronereid form, while the small
Heteronereid swims on the surface. The relations of these various forms to
one another, and the causes leading to the {278}assumption of a
Heteronereid condition in some cases and not in others, are unknown.

A somewhat similar phenomenon is exhibited by members of the family
Syllidae.[334] In this family sexual reproduction is frequently accompanied
by the asexual modes of fission and gemmation. In some genera, such as
_Eusyllis_, _Odontosyllis_, and _Exogone_, there occur changes quite
similar to those characterising "Heteronereis"—that is, the posterior
segments in which the genital organs exist become altered, so that the worm
consists of two distinct regions, and is termed a "Heterosyllis." The most
marked change is the appearance of a dorsal bundle of long capilliform
chaetae in each of the genital segments (Fig. 148, I).

But in other genera the hinder genital region of the body becomes
_separated_, on maturity, from the anterior non-sexual region. Various
stages of this "schizogamy," or fission into a sexual and a non-sexual
zooid, have been observed in different genera. In the genus _Syllis_ the
first segment of the sexual zooid, after its separation from the asexual
zooid, proceeds to bud forth a head. The character of the head is alike in
both sexes, though different species present heads of different shapes; and
as the worms were originally described as distinct genera, the names then
given are retained as descriptive terms. Thus the "Chaetosyllis" form has
only two tentacles; the "Ioda" form has three tentacles and a pair of
palps. One and the same species (e.g. _S. hyalina_) may successively pass
through these stages.

With regard to the asexual portion, there is a regeneration of the tail
segments after the sexual zooid has separated; and the number of segments
so regenerated is usually equal to those that have become sexual. After a
time these newly formed segments will produce generative organs, and take
on the characteristic natatory chaetae, and this region will in its turn
separate.

But in other genera, such as _Autolytus_, the regeneration of segments may
commence _before_ the separation of the sexual zooid; and the head of the
sexual zooid becomes budded out _before_ separation from the asexual
portion. So that the animal now consists of two worms, each with its own
head, separated by {279}a region or zone of proliferation (Fig. 148, IV).
Moreover, in some species not only is the hinder part of the body
_converted_ into a sexual zooid, but the zone of proliferation becomes very
active, and produces by gemmation a large number of segments, which become
marked out, by the appearance of heads at intervals, into a number of
zooids, in which genital organs will later make their appearance. A chain
of as many as sixteen zooids may be formed in _Autolytus_ (Fig. 148, V)—the
hindermost by _conversion_ of the hinder part of the body of the original
"stock," the intervening zooids by _gemmation_.

[Illustration: FIG. 148.—Diagrams illustrating the various stages in the
asexual formation of a chain of zooids. (Modified from Malaquin.)

I, Heteronereid or Heterosyllid stage. A, Non-sexual; A', sexual region of
the body, with modified parapodia.

II, _Syllis_. The hinder sexual region, B, is similarly modified, and will
separate from the parent zooid, A, and become an independent zooid.

III, _Autolytus_. The hinder zooid, B, develops a head by budding before
separation.

IV, _Autolytus_, etc. A zone of budding (_z_) makes its appearance in front
of the head of B, and by its growth will give rise to a series of new
segments in the middle of the body.

V, _Myrianida_, _Autolytus_, etc. From this zone of budding a very large
number of segments have been formed, which have, further, become grouped so
as to form three individuals, C, D, E; B is the hindmost zooid, which is
either formed from the hinder segments of the parent zooid or is produced
by budding, like C, D, E.]

One original "stock," or asexual zooid, thus produces several sexual
zooids, but these are only of one sex for a given stock. The males differ
in several important characters from the females; so different, indeed, are
the two sexes that before their history was {280}worked out by Agassiz[335]
they were placed in different genera. The male zooid has thus come to be
known as _Polybostrichus_ (Fig. 149, B). It has three tentacles and two
bifid palps; there are two pairs of peristomial cirri; the testes are
confined to the four anterior segments, which are without natatory chaetae.
The female is termed _Sacconereis_, owing to the possession of a great
ventral brood sac; its head possesses no separate palps; the peristomium
carries only one cirrus on each side; ova occur in every segment of the
body, and may even extend into the hinder segments of the asexual zooid
(Fig. 149, C).

[Illustration: FIG. 149.—_Myrianida fasciata._ (From Malaquin.) The bright
red markings of the living animal are here represented black. A, An asexual
individual which has produced by budding from the zone (_z_) a chain of
twenty-nine zooids, the oldest being labelled 1, the youngest 29. B, A ripe
male zooid (_Polybostrichus_), with three tentacles and a pair of forked
palps (_p_). There are five unaltered anterior segments. C, A ripe female
zooid (_Sacconereis_) with the palps fused with the prostomium; _s_, the
ventral brood pouch projecting on each side; _t_, tentacles.]

A further development of this process of gemmiparity is exhibited by
_Myrianida_. Here, there is no conversion of the hinder segments, but the
normal preanal zone of proliferation gives rise to a large number of new
segments. After a time the most anterior of these becomes a head, and thus
a new zooid {281}is marked out. The zone of proliferation immediately in
front of the new head now proceeds to form new segments, and a second zooid
results. This process goes on till a considerable number of new worms have
been formed at the tail of the original one, the oldest of these new ones
being the most posterior, the youngest next the original "stock." In each
zooid there is a zone of activity which adds to its number of segments, so
that as we pass backwards the zooids increase in size. As many as
twenty-nine such zooids may be formed in this way entirely by gemmation;
and as each zooid becomes completed, genital organs make their appearance,
and when these are ripe the zooid separates from the "colony" and leads an
independent life. Here, as in _Autolytus_, the sexes are dimorphic, the
male and female resembling those of that genus.

The process of gemmation, as seen in _Autolytus_, closely resembles that
exhibited by certain Oligochaeta (_Naididae_), where there exists a
definite alternation of generations; the production of new individuals by
gemmation occurring throughout the greater part of the year, and sexual
reproduction recurring only at certain intervals. In the Polychaeta such
alternation exists in _Myrianida_; but it is only the terminal link of a
series, which takes its starting-point in the process exhibited by the
majority of Annelids, where no sexual character marks maturity. The next
stage is presented by "epigamous" forms like Heteronereis and Heterosyllis;
then "schizogamy" makes its appearance in certain Syllidae, resulting in
the formation of two morphologically and physiologically distinct
individuals which lead independent lives. The appearance of a head and of a
zone of proliferation leading to the formation of a chain of sexual zooids
is accompanied by a delay in the appearance of the genital organs, for in
_Autolytus_ these arise _during_ the formation of the new individuals, as
part of the general process of new formation; whilst in _Myrianida_ the
delay is prolonged, and the generative elements do not make their
appearance till _after_ the new individuals have reached some size.

More simple cases of the separation of the body into two parts, sexual and
asexual, occur also in some of the Serpulidae. Thus in _Filigrana_ and
_Salmacina_ the generative elements make their appearance in the hinder
segments, as they do throughout the Sabelliformia; and this hinder part of
the body separates {282}from the anterior region after the formation of a
new head between the two regions.[336]

Another modification of the process of budding and fission is exhibited by
_Syllis ramosa_, one of the most interesting forms of animal life which was
obtained by the "Challenger." This worm consists of a main stem, whence
arise a number of lateral branches, which may also branch so as to give
rise to an arborescent colony (Fig. 150). The branches of the first and
second and higher orders arise by budding from the sides of the original
form or branches of lower order; and some of these branches develop
generative products, and bud forth a head near the point of attachment.
These sexual branches, no doubt, separate from the colony and distribute
the ova. The worm lives in a Hexactinellid sponge, _Crateromorpha meyeri_,
living in depths of 95 to 140 fathoms in the Eastern seas.[337]

[Illustration: FIG. 150.—Portion of _Syllis ramosa_. (Reduced from
M‘Intosh.)]

REGENERATION OF LOST PARTS.—The process of budding and fission of the worm
into two parts is merely an extension of that resulting in the formation of
new segments when the worm is injured. In most of the Nereidiform
Polychaetes the number of segments forming the body continues to increase
throughout life by the formation of new segments between the anal segment
and the one in front of it; that is to say, there is normally a process of
budding taking place at this point. Now in many of the longer worms it may
be noticed that the segments of the hinder end suddenly become smaller than
the rest; these are segments newly formed to replace those lost by the
worm. But this "regeneration," though the same in principle as ordinary
growth {283}at the penultimate segment, is due to activity in a segment
(any segment) further forwards; in other words, in the less modified worms
every segment has the power of forming new tissues, just as each of the
joints of a crab's leg has the power of forming the remaining joints when
injured. It is not therefore surprising that a "zone of budding" arises in
an uninjured worm at certain seasons, viz. that of reproduction; it is a
property that each worm possesses, though generally it remains latent till
injury provides the stimulus.

Moreover, not only can new segments arise at the hinder end, but a new head
can be formed at the anterior end, as has been observed in worms belonging
to many families—in the less modified Syllidae,[338] in others of the
Nereidiformia, and even in Sabellids, where the greatly specialised gill
filaments can be reproduced. Thus Sir J. Dalyell[339] noted in _Dasychone_
that the crown of branchiae was regenerated in about a month in springtime,
while in winter the process occupied 116 days. He cut a _Dasychone_ into
three pieces; the hindermost produced a head, the anterior piece developed
an anus, and the middle portion formed both a head and tail!

These regenerated heads are of course at first smaller than the rest of the
body, but soon grow to a normal size. Naturally this extensive power of
regeneration is of extreme value to the Polychaetes, for if a fish or other
enemy bites the head off a worm, a new one can form; and it is not
difficult to see in this the origin of the reproduction by fission as a
normal process.

{284}CHAPTER XI

NATURAL HISTORY OF POLYCHAETES—GENERAL HABITS—CHARACTER OF TUBE AND ITS
FORMATION—COLOURING—PROTECTIVE AND MIMETIC DEVICES—PHOSPHORESCENCE—FOOD
—USES—ASSOCIATED WORMS—WORMS AS HOSTS—DISTRIBUTION—FOSSIL REMAINS.

All the many hundreds of species of Polychaetes are marine, with a very few
exceptions, which have been in recent years recorded from fresh (_i.e._
drinkable) water, viz. a species of _Nereis_ from a lake in Mingrelia,
another _Nereis_ and a _Lumbriconereis_ from running water in
Trinidad,[340] a Sabellid, _Manayunkia speciosa_,[341] from Philadelphia;
and another Sabellid, _Coabangia_,[342] from fresh water at Tonquin, which
lives in borings in shells of _Melania_; and it is by no means improbable
that other fresh-water Polychaetes exist in Lake Tanganyika in Africa,
where a Medusa has recently been discovered.

In brackish water of various densities many Polychaetes live; _Arenicola_
especially is regardless of the character of the medium, and _Nereis
diversicolor_ appears to withstand considerable admixture of fresh water.

The majority of the Polychaetes occur "inshore," that is, between
tide-marks and in shallow water down to 20 fathoms; but they occur at all
depths more or less abundantly, and some have been dredged from depths of
more than 3000 fathoms.

The nature of the soil composing the shore has a good deal to do with the
number of worms to be found there; thus in calcareous districts they are
fewer than in places where harder rocks, {285}such as granite, form the
shore line, for the chalk or limestone wears away more quickly, and exposes
to destruction the worms which may have sheltered in its crevices: further,
it does not give so permanent a place of attachment to seaweeds, on which
many Polychaetes feed. The calcareous rocks, too, are more likely to be
traversed by springs of fresh water, which is not to the taste of the
worms. The sand resulting from the destruction of the rocks, whether hard
or soft, is of itself unsuitable to the majority of worms, which are most
abundant where mud containing decaying vegetable matter is mixed with the
sand: this, which gives a firmer consistency to the soil, so that the
burrows retain their form better, supplies food for the burrowers.

GENERAL HABITS.—The division of the Polychaetes into the "Errantia" or
free-swimming and wandering forms, and "Sedentaria" or tubicolous and
sedentary forms, is a misleading mode of classification, for as a matter of
fact only a comparatively few forms are really free-swimming throughout
life; the majority, even if they do not form definite tubes, burrow
galleries for themselves in the soil, and these burrows are in many cases
only rarely left; this is true of both groups. Amongst the "errant"
Polychaetes nearly all the Eunicidae secrete a parchment-like tube, and
some Polynoids form mud tubes. Among the "sedentary worms" there are forms
which merely burrow; while _Myxicola_ readily leaves its gelatinous tube
and swims freely; _Pectinaria_ carries its house with it as it moves about,
and _Polycirrus_, a Terebellid, does not form any tube at all.

Owing to their sedentary habits, quite a representative collection of
genera may be made, especially at a spring tide, at any seaside place which
is provided with a sandy shore, and with rocks and seaweed. The larger
species, however, require to be dredged, and the best time is at night, for
then many forms which during the day are concealed in their burrows, will
be issuing forth to obtain food.

It may be useful to give instances of worms occurring in various situations
between tide-marks. Throughout pretty well the whole of the area left
uncovered by the tide, even up to nearly high-water mark in many parts of
the coast, the cylindrical "castings" of sand and mud, forming little
heaps, indicate the burrows of _Arenicola_, the common "lug-worm"; these
"castings" have passed through the worm's body, having been {286}swallowed
during the process of burrowing as well as for the purpose of obtaining
food, as in the case of the earthworms. Rather nearer the water may be seen
little tufts of sand-threads, about an inch high, springing from a short
piece of cylindrical, sandy tube rising up out of the sand; this is the
head end of the tube of _Terebella conchilega_ (Fig. 153).

Amongst the rocks may be found loose stones of different sizes; on lifting
them up, various kinds of worms may be brought to light, according to the
locality, the time of year, the position with respect to the sea, and so
on. _Polynoë_ is pretty sure to be present somewhere near low-tide mark;
the number of species is considerable, and their colouring very varied: but
as the worms have a habit of remaining still on the under surface of the
uplifted stone, the observer may easily overlook them.

Other worms occur below the stones, more or less buried in the sand or mud;
for instance, a small _Nereis_ may be lying in its temporary burrow
immediately underneath, and will at once withdraw from the now injured part
of the burrow; while deeper in the mud or sand, especially in rather
highly-smelling mud, little red worms are abundant, such as _Scoloplos_,
_Nerine_, _Capitella_, and others. By digging near low water one may find
_Nephthys_, _Glycera_, and others burrowing or hiding in the soil.

In rock pools, or sandy stretches amongst rocks kept moist and cool by
abundant _Fucus_, one may see under stones the red or yellow gill filaments
of _Cirratulus_ and of Terebellids protruding from their burrows and tubes,
while other worms are to be met with in clefts of the rocks, and amongst
the roots of _Laminaria_.

Still farther out, below low-water mark, where one must wade, can be seen
the beautiful branchial crowns of various Sabellids protruding from their
tubes; but care is necessary on approaching these worms, as eyes are, in
many cases, present on the branchiae and a shadow is readily perceived;
then the brightly-coloured tuft disappears, and only a piece of sandy or
muddy cylindrical tubing remains to tell where the _Sabella_ has withdrawn.
In order to obtain the worms one must dig quickly and deeply before they
have been disturbed; for the tube is of considerable length, and the
inhabitant withdraws to the bottom of it. Some of these soft-skinned worms
have the power of boring into hard rocks,[343] though by what means they do
so is {287}uncertain.[344] _Polydora ciliata_ makes a tube of mud
projecting from the mouth of U-shaped galleries in chalk, limestone,
shells, and even shale; it has no hard jaws or other structures sufficient
to account for the holes, but it is possible that the specially strong
chaetae on the sixth segment may be of some use in this work. Other
lithodomous worms are _Sabella saxicava_ and _Dodecaceria concharum_, which
is a common little borer, forming galleries in oyster-shells, etc.

The TUBES formed by these Polychaetes are very varied in constitution.[345]
In some cases a mucus, which hardens to form a firm protective envelope, is
secreted from special parts (_e.g._ the ventral gland shields of
Terebellids and Sabelliformia), or from the greater part of the general
surface of the body; in other cases the secretion serves to stick together
particles of mud or sand, or shelly fragments, so as to form a more or less
cylindrical tube (rarely branched), which is lined internally by the
hardened "mucus," having the appearance of silk.

[Illustration: FIG. 151.—_Clymene ebiensis_ in its tube (_t_) (from _Règne
Animal_). _a_, Anterior, _p_, posterior end, which is, however, injured.]

But the process of tube-making is not a simple one, for in many cases, at
least, the worms exhibit definite powers of choice. Thus some species of
_Sabella_ choose only the very finest particles of mud; _Terebella
conchilega_ chooses fragments of shell and grains of sand; _Onuphis
conchylega_ employs small stones more or less of a size; _Sabellaria_ makes
use only of sand grains. Whilst some worms, like _Terebella_, _Nicomache_,
and others, make a very irregular tube, _Pectinaria_ builds a most
remarkably neat house, open at each end, which it carries about with it,
the narrow end uppermost (Fig. 152); the grains of sand are nearly all of
the same size and only one layer in thickness, embedded in abundant
"mucus," and with the outer surface quite smooth.

Sir J. Dalyell[346] made some most interesting observations on the method
followed by sundry tube-formers in the building of {288}their tenements,
and these observations, though made nearly half a century ago, have
required very little addition or correction in modern times. In speaking of
_Sabella_, he writes as follows:—

[Illustration: FIG. 152.—The tube of _Pectinaria auricoma_. × 3. (From
M‘Intosh.) This is its natural position as carried about by the animal.]

[Illustration: FIG. 153.—The upper end of the tube of _Terebella
conchilega_. Slightly enlarged. (From M‘Intosh.)]

"Let a tall and ample crystal jar containing a _Sabella_ be emptied of its
contents and speedily replenished with sea-water; the animal, if in view,
has retreated during the short interval; the orifice of the tube is closed,
all is at rest. But soon after {289}replenishment it rises, to display its
branchial plume still more vigorously than before, and remains stationary,
as if enjoying the freshness of the renovated element, always so
grateful—the harbinger of health and strength to those whose dwelling is
there. The passing spectator would conclude that he now beholds only a
beautiful flower, completely expanded, inclining towards the light like
some of those ornaments of nature decorating our gardens. He pauses in
admiration. But if a drop of liquid mud falls amidst the element from
above, disturbing its purity, then, while the plume unfolds to its utmost
capacity, does the animal commence a slow revolution, the body also passing
around within the tube. Now are the thousands of cilia fringing the ribs
[_i.e._ the secondary filaments] of the branchiae discovered to be in
vigorous activity, and their office to be wondrous. A loose muddy mass is
soon afterwards visibly accumulating in the bottom of the funnel; meantime
the neck or first segment of the body, rising unusually high above the
orifice of the tube, exhibits two trowels beating down the thin edge as
they fold and clasp over the margin, like our fingers pressing a flattened
cake against the palm of the hand. [This refers to the lappets of the
peristomial collar.] During these operations muddy collections are seen
descending between the roots of the fans [right and left gills] towards the
trowels, while another organ, perhaps the mouth, is also occupied, it may
be, in compounding the preparation with adhesive matter. Still does the
partial or complete revolution of the plume above, and of the body within
the tube, continue; the bulk of the muddy mass diminishes, activity abates;
it is succeeded by repose, when the tube is found to have received evident
prolongation."

[Illustration: FIG. 154.—_Terebella conchilega_ Pall. Upper end of the tube
(_s_) showing the anterior end of the worm. _h_, Its head; _t_, tentacles
collecting sand grains (_y_) in their grooves; _x_, sand grains in mouth of
worm; _f_, filamentous fringe of tube. (After Watson.)]

The Terebellids use their numerous tentacles in searching {290}for
particles of sand, etc.; each tentacle is grooved along its ventral
surface, and the particle is conveyed along the furrow to the mouth. These
particles are actually taken into the mouth, and mixed with some sort of
secretion; on ejection again, each particle is placed by another tentacle
in its position at the edge of the tube, and by means of its lower lip the
Terebellid works it into place.[347]

But whereas the greater number of tubicolous worms make use of adventitious
material wherewith to strengthen the wall of their tube, the Serpulidae
secrete carbonate of lime from their tube-glands, and mould a tube of this
substance. Amongst the Eunicidae the secreted substance is of itself strong
enough to protect the animal; for in _Hyalinoecia_ and species of _Eunice_
the tube consists of a translucent, tough, parchment-like material.

Chemical analysis has been employed in a few cases to determine the
substance composing the tube. In the case of _Hyalinoecia_ (sometimes
erroneously called _Onuphis_) the material consists of a phosphoric salt
containing magnesia and a characteristic organic substance "onuphin"[348];
in _Spirographis_, a Sabellid, the name "spirographin" is given to its
special secretion, whilst in Serpulids the organic base of the calcareous
tube is "conchiolin."

[Illustration: FIG. 155.—_Eunice tibiana_ Pourt. × ½. The branching tube
(_t_) with the worm (_w_) protruding its head through one of several
openings. (From Ehlers.)]

The majority of worms are solitary, but there are a few instances of social
worms—not that there is any co-operation or distribution of labour amongst
the individuals, but they merely occur together in quantities; thus the
sandy tubes of _Sabellaria_ may form compact masses of several cubic feet,
which, left uncovered by the receding tide, look like rocks upon the shore;
as, for instance, at Paignton and Torquay. _Filigrana implexa_ and _Serpula
uncinata_ similarly intertwine their calcareous tubes to form masses.

{291}Whereas most worms live at the bottom of the sea, at various depths, a
few are to be found at the surface. Purely pelagic habits are confined to a
few families, viz. Tomopteridae, Typhloscolecidae, and the Alciopids and
others amongst the Phyllodocidae; though _Nectochaeta_, one of the
Polynoidae, and _Ophryotrocha_, one of the Eunicidae, are modified for this
mode of life.[349] Several genera become pelagic during the breeding
season. All these forms are excellent swimmers, and many of them are
transparent.

The COLOURING OF POLYCHAETES.—The majority of Polychaetes quickly lose
their colour in spirits, and become uniformly dull or light brown in
museums. There are a few, however, which retain their brilliancy, like
_Aphrodite_ and _Chloeia_, but in both cases the coloration is due to the
beautiful hair-like bristles ranged along each side of the animal; in the
former the colours of the rainbow flash from specimens which have been kept
in spirit for any length of time. The Polynoids, too, with their golden
chaetae and pigmented scales, retain to some extent their characteristic
colouring. But the colours of most Annelids are due to pigments in the
skin, together with the haemoglobin of the blood, which are soluble, or
otherwise changed, in alcohol; for instance, the bright greenish-blue tint
of the common _Phyllodoce_ of our coasts is changed to a rich chocolate
brown; but such cases are rare, most worms becoming more or less
decolorised.

The varied colouring in the Polychaetes, as in other animals, is due to a
variety of causes. The red is in many cases due to haemoglobin of the
vascular system showing through the transparent body; the green of the
tentacles of the Sabellids and Chlorhaemids is similarly due to
chlorocruorin. In other cases the contents of the intestine or the tint of
the coelomic fluid may affect the colour of the worm. In _Capitella_ the
coloured excretory products are regained in the skin; in an Eunicid living
in a yellow sponge, on which it feeds, the colouring matter is extracted
and stored in the skin; in the same kind of way green caterpillars may owe
their tint to feeding on green leaves. But many of the Polychaetes possess
distinct _pigments_ in the skin; thus in _Arenicola_ the dark pigment
{292}melanin has been recognised; in _Cirratulus_ and _Nereis_ certain
lipochromes; whilst _Eulalia viridis_ contains a pigment allied to
bonellein. These various pigments yield different absorption bands when a
solution is examined with the spectroscope; others, however, give no bands,
but are distinguished by different chemical reactions.[350] The colour of
the intestine of _Chaetopterus_ has been stated to be due to "modified
chlorophyll," but it is quite a different substance.

When seen in the living and healthy condition, however, these Polychaete
worms vie with the very butterflies in their brilliant and beautiful
colourings, and though our own worms are not lacking in beauty, many
tropical and southern forms exceed them in gayness of tint. Bright reds,
orange, yellows, greens, blues, rich violets, and sombre browns are all
displayed.[351]

The handsome _Terebella nebulosa_ of our own coasts is coloured bright red,
sprinkled with white spots. _Nicomache lumbricalis_ is pink, with red
girdles. Eunicids are frequently red or brown, and the red gills along each
side, together with a brilliant iridescence, render these worms very
beautiful. Nereids present a great range of coloration, from light green to
sundry tints of brown and red in various combinations. Amongst the
Serpulids our common _S. vermicularis_ is a very showy little worm, with
its orange body, its red gills splashed with orange, and its orange
operculum streaked with red; and a Southern form, _Placostegus coeruleus_,
occurring at the Cape of Good Hope, is provided with beautiful
lavender-blue gills. Our own Sabellids present examples of beautiful
markings on the gills, in different colours or in different shades of the
same colour. Amongst Polynoids, _P. leucohyba_, from the Antilles, has blue
elytra; _Hemilepidia erythrotaenia_, a long worm from the Cape of Good
Hope, has the anterior end of its body covered with light blue elytra,
whilst the uncovered part is orange, with a broad magenta-red band along
the dorsal surface.

The Phyllodocids are mostly very brightly coloured. The common _P.
lamelligera_ of our coast has a bluish-green body, with olive-green
parapodia; but _Lopadorhynchus erythrophyllum_, {293}from Jamaica, has a
blue body with red parapodia; whilst _Notophyllum myriacyclum_ has a brown
body with longitudinal dark-brown stripes and yellow parapodia. Both these
worms live in coral reefs, where brilliancy of colour is one of the
characteristic features of the fauna. Other worms are of various shades of
green: the dark green _Arenicola_ with red gills; the bright green _Eulalia
viridis_; the deep green _Amphinome smaragdina_, from Jamaica;
_Gnathosyllis diplodonta_, with its green and yellow body, serve as
examples.

Patterns or "markings" may be exemplified by _Lepidasthenia elegans_ (Fig.
156), and _Myrianida fasciata_, which has a bright red band on each segment
(Fig. 149, p. 280). From this brief list of examples it will be seen that
beautiful, and even brilliant, coloration is not confined to any particular
mode of life; many of the most typically tubicolous forms, like the
Terebellids and Serpulids, are as brilliantly coloured as the most
typically free-swimming genera, like the Phyllodocids. Carnivorous forms
like Amphinomids and Syllids present as wide a range of tint as the
limivorous forms like _Cirratulus_, _Sabella_, or Maldanids. Shore-lovers,
and deep-sea dwellers, and surface-swimmers, all exhibit equally bright or
equally sombre tints; it is therefore difficult and rash to dogmatise on
the "use" of these colourings to these animals, or to point to this worm as
being protectively, to the other as being warningly, coloured; for we are
too ignorant as to the habits of the worms.

[Illustration: FIG. 156.—_Lepidasthenia elegans_ Gr., × 2, to illustrate
colour-markings: the dark bands in the anterior part of the body occupy two
elytriferous, and the intermediate segments. In the hinder region, where
the elytra are in every third segment, this one is dark. _el._12, The
twelfth elytron.]

PROTECTIVE AND MIMETIC DEVICES.—From the point of view of "protection" in
the evolutionist's sense of the word, we can {294}say but little.
Protective resemblance there is undoubtedly amongst the Polynoids, for the
scales of these forms resemble more or less closely the stones or sand
amongst which they live; in the same species there is great variety in
coloration. This protective habit is carried still further in the case of
_Psammolyce_ by the attachment of sand grains to little cups on the elytra,
so that the back of the animal is concealed. Certain commensals, such as
_Polynoë arenicolae_, _P. pentactes_, are coloured so as to resemble their
associates. In a few cases it is possible that the gills of Sabelliformia
are protectively coloured; for in _Sabella pavonia_ they vary from a light
yellowish tint to a deep violet-brown, and the dark markings on them are
therefore more or less distinct. Spread out as the gills are in life, they
are in many cases difficult to recognise; it is rather their movement as
they are withdrawn that attracts one's attention to them, as the tubes of
these worms frequently serve for the attachment of brownish seaweeds, to
which the gills bear resemblance. But, as a matter of fact, little work has
been done in this direction, and speculation on the matter without evidence
is worthless. Many pelagic forms, being transparent, such as _Tomopteris_
and Alciopids, are no doubt protected by their lack of colour; yet these
forms present brightly-coloured spots,—the light-producing organs in the
parapodia of the former, and the large dark eyes of the latter.

Semper[352] mentions a case of possible mimicry in a species of _Myxicola_
which lives in the clefts of a coral, _Cladocora_. The branchial funnel,
when expanded, resembles very closely the expanded coral in size, colour,
etc.; but he points out that the species occurs in other situations, where
its colouring is not protective. Probably the "mimicry" is in other
instances merely accidental.

No doubt many Polychaetes may be "warningly coloured," but experimental
evidence is incomplete. _Polycirrus aurantiacus_ is bright red, with orange
tentacles; these worms were rejected by certain fish.[353] The animal has
given up living in tubes as all its allies do, and it is the tentacles
which appear to be distasteful to its enemies, for when irritated it coils
itself up and wraps {295}itself round with its tentacles. Moreover, when
the tentacles were cut off the fish did not reject the body of the worm.
The tentacles are thus coloured in such a way that fish recognise them, and
associate with the colour some distasteful property.

[Illustration: FIG. 157.—_Chaetopterus variopedatus_ Ren. × ½. On the left
the entire animal, with the three regions A, B, C. _c_, Peristomial cirrus;
_d_, "sucker"; _e_, the great "wings"; _f_, "fan"; _m_, mouth. On the right
the animal is represented in the dark, under stimulation, so as to exhibit
the phosphorescent portions of the body. (From Panceri.)]

PHOSPHORESCENCE.—Many worms of very different habits have the power of
emitting a light from some parts of the body, and they are then said to be
"phosphorescent."[354] Probably _Chaetopterus_ is most eminently
photogenic; the base of the great "wings," the "fans," and other parts
emit, on stimulation, an azure blue to greenish light, so bright that one
may read one's watch by it. Several species of _Polynoë_ exhibit a similar
phenomenon, each elytron, with the exception of the area of attachment,
being brilliantly illuminated. In these species the phosphorescent elytra
are frequently thrown off by the animal, so that possibly they deceive
enemies. _Polycirrus aurantiacus_ produces a beautiful violet
phosphorescence; usually its many tentacles alone show the light, but under
strong stimulation the entire body takes {296}part in the display, and no
doubt the phosphorescence has, like the colour, a "warning" purpose.

The production of the light in these various forms is apparently due to two
different processes. In some cases, _e.g._ _Chaetopterus_, Syllids,
Terebellids, it appears to be due to the oxidation of certain cell contents
which are discharged more or less freely on irritation of the nerves;
whilst in Polynoids the phenomenon is due to some purely nervous process,
for the elytra have no glands, but are provided with ganglia and a nervous
network.

In other worms, however, there are definite light-producing organs. In
_Tomopteris_ there is on each parapodium, above and below, a
brightly-coloured spherical organ, which for a long time was regarded as an
eye, but from its structure appears to be a "photogen" (Fig. 167, p. 315).
The same is very likely the true explanation of the segmental "eyes" of
_Polyophthalmus_, for their structure recalls that of the light-organs of
deep-sea fishes.

As many of the phosphorescent Polynoids are commensals, while
_Chaetopterus_ inhabits tubes, and close allies of other phosphorescent
worms have no power of emitting light, it is impossible to apply the same
explanation of its purpose to all cases alike; in some it may be
"accidental," though in others it may be of definite use in warning enemies
or in attracting prey.

The FOOD OF WORMS.—The Nereidiformia are mostly carnivorous, and feed on
small Crustacea, Mollusca, sponges, and other animals; and Polynoids are
even said to eat one another. Many worms do not disdain various seaweeds,
whilst the Spioniformia and Scoleciformia, which burrow in mud and sand,
and are without biting organs, swallow the mud and digest what animal or
vegetable débris it may contain. The Terebellids and Cryptocephala depend
on minute organisms which may be driven into the mouth by the action of the
cilia of the gills or tentacles.

In the case of deep-sea forms, it is an interesting fact that the
intestines are not unfrequently crammed with Radiolaria and Foraminifera in
a fairly fresh, uninjured condition, indicating that these Rhizopods do not
merely _sink_ to the bottom, but must actually _live_ there.[355]

The ECONOMIC PURPOSES to which Polychaetes are put are few; they are used
either as bait for fishes or as food for man.

{297}One of the commonest baits used for certain fish, as all who have done
any sea-fishing off the piers of our coasts know, is the common lug-worm
(_Arenicola marina_), whilst _Nephthys caeca_ and _Nereis fucata_ are also
used in some places; and for whiting _Nereis cultrifera_ and _N.
diversicolor_. _Marphysa sanguinea_, known to the fishermen in some parts
as "varme," is less frequently used.

A peculiar worm—_Palolo viridis_—is used as food by the natives of Samoa
and Fiji. The worm is similar to our Eunicid _Lysidice ninetta_, and lives
in fissures among corals on the reefs, at a depth of about two fathoms. At
certain days in October and November they leave the reefs and swim to the
shores of the above islands, probably to spawn; and this occurs on two days
in each of the above months—the day on which the moon is in her last
quarter, and the day before. The natives, who call the worm "Mbalolo," give
the name "Mbalolo lailai" (little) to October, and "Mbalolo levu" (large)
to November, thereby indicating the relative abundance of the worms in
these two months. The natives eat them either alive or baked, tied up in
leaves; and they are esteemed so great a delicacy that presents of them are
sent by the chiefs who live on shore to those living inland. A dark
green-blue Phyllodocid, which is called "A'oon," occurs in abundance off
Mota Island, amongst the New Hebrides, has similar habits, and is also
eaten.[356]

ASSOCIATED WORMS.—A considerable number of worms live in association with
other animals, either as commensals or as parasites, and it is not in every
case possible to decide in what relation the two animals stand.
_Labrorostratus parasiticus_, a Eunicid, is parasitic in the body-cavity of
_Odontosyllis ctenostomatus_ (Fig. 158); such an association between two
members of the same group of animals is peculiar; but still more
exceptional is the occurrence of _Haematocleptes terebellides_, as a
parasite in _Marphysa sanguinea_, for both parasite and host are members of
the same family, the Eunicidae. Another Eunicid, _Oligognathus bonelliae_,
occurs in the body-cavity of the Gephyrean _Bonellia_.

The Polynoid _Acholoe astericola_ and the Hesionid _Ophiodromus flexuosus_
occur as ectoparasites (or perhaps commensals) in the ambulacral grooves of
the starfish _Astropecten aurantiacus_. An Amphinomid is stated to live in
the branchial chamber of {298}the barnacle, _Lepas anatifera_. _Alciopina
parasitica_ lives, during the early stages of its life-history, within
_Cydippe_, and it is possible that most of the Alciopids thus make use of
Ctenophores as their nurseries.

A considerable number of the Polynoids are ectoparasitic: _P. castanea_
lodges in the peri-oral region of _Spatangus purpureus_, and in the
ambulacral grooves of _Astropecten_; _P._ (_Halosydna_) _bairdi_ lives
between the mantle and foot of the mollusc _Fissurella cratitia_; _P.
pentactes_ is found on the body of the Holothurian _Cucumaria pentactes_,
and appears to be protectively coloured. _P._ (_Antinoë_) _parasitica_
lives under the elytra of another Polynoid, and _P. acanellae_ on the coral
_Acanella normani_.[357]

[Illustration: FIG. 158.—_Odontosyllis ctenostomatus_, with (_L_)
_Labrorostratus parasiticus_ in its body-cavity. The parapodia and cirri
are omitted from the greater part of the body. (After St. Joseph.) × 4.]

As commensals there may be mentioned _Nereis fucata_, which lives in the
upper coil of whelk-shells which are inhabited by a hermit crab. The same
shell usually bears a particular sea-anemone, so that there are three
animals living together in or upon the cast-off house of a fourth.
_Siphonostoma_ is found in the "nests" made by the mollusc _Lima_. A
_Eunice_ is constantly associated with the coral _Lophohelia prolifera_,
amongst the branches of which the worm twines its tube; whilst another
Polychaete inhabits a tube formed by the interweaving of the fine branches
of the coral _Antipathes filix_,[358] found in the West Indian seas. A
species of _Polydora_ forms its tube in _Heliopora_. The Polynoids present
many instances of commensalism, a few of which may be here mentioned. _P.
johnstoni_ Marenz. is only found in the tubes of _Terebella nebulosa_;
other species occur in the tubes of other Terebellids. _P. marphysae_ lives
in tubes of the Eunicid _Marphysa sanguinea_. Two species live in the tubes
of _Chaetopterus_. _P. extenuata_ has been found in tubes of _Serpula
vermicularis_, while _P. arenicolae_ occurs on {299}the body of the common
lug-worm, with the colouring of which it closely harmonises.

WORMS AS HOSTS.—The Polychaeta serve not only as food for fishes,
Crustacea, and other predatory animals of larger size, but are also liable
to be the hosts of parasites[359] such as Gregarines, and even, as we have
seen, of other members of their own group. Sundry ectoparasitic Copepoda
have been found attached to worms between the parapodia or to the sides of
the feet, and an unnamed Copepod occurs attached, sometimes in considerable
numbers, to the sides of _Nereis cultrifera_. The Polychaeta also act as
protectors to other animals, for on the under surface of elytra of sundry
Polynoids may very frequently be found specimens of _Loxosoma_, which may
also be attached to gills of Eunicids; whilst below those of _Aphrodite
echidna_ and _Hermadion pellucidum_, _Pedicellina belgica_ occurs. Under
the felt of _A. aculeata_ the Sabellid _Branchiomma vigilans_ forms its
tube, and Vorticellids may be found on chaetae, gills, or other parts of
the body of sundry worms.

DISTRIBUTION.—Very little can be said in a brief way of the _geographical
distribution_ of these worms, for many of the genera are cosmopolitan,
although only a few species occur in all the great oceans, _e.g._ _Polynoë
imbricata_, _Hyalinoecia tubicola_, _Nerine_ (_Scolecolepis_) _cirrata_,
and _Terebellides stroemi_.

As for species, it can be said generally that the different oceanic areas
and even different coasts present different species, but we know
practically nothing of variation amongst Polychaeta, and many so called
species may be mere local varieties, for frequently the descriptions of
"new species" are scarcely intelligible. At any rate we know that certain
species occur at widely separated localities, for two or three species of
Polynoids occur in Japan, and again at Dinard on the French coast. A
considerable number of species are common to both sides of the North
Atlantic ocean, having been obtained off Norway and in the Gulf of the St.
Lawrence. A few of these which are common on our coasts may be
enumerated:—_Nereis pelagica_, _Nicomache lumbricalis_, _Glycera capitata_,
_Thelepus cincinnatus_, _Scoloplos armiger_, _Sabella pavonia_, _Ophelia
limacina_, _Aphrodite aculeata_, _Trophonia plumosa_, _Polynoë squamata_,
_Capitella capitata_, _Sthenelais limicola_.

{300}As for _bathymetrical_ distribution,[360] many genera occur at all
depths, though Polychaetes appear to be most abundant, as far as we know at
present, in "shallow water"—that is, down to twenty fathoms or so; but this
may be due to the greater facility of collection on shore and in these
slight depths, for the "Challenger" obtained considerable numbers of new
species at greater depths.

The "deep-sea" forms are chiefly tubicolous, and since these tubes are
fixed and partially embedded in the bottom, probably comparatively few are
brought up. Some genera occur at very great depths; thus the Terebellid
_Leaena abyssorum_ and the Serpulid _Placostegus benthalianus_ were brought
up from 3125 fathoms—the greatest depth from which Polychaetes were
obtained by H.M.S. "Challenger"; and it is interesting to note that species
of each of these two genera occur in shallow water, the Serpulid being
represented in our own coast fauna by _P. tricuspidata_.

Amongst our own fauna, a few examples may be given of the "replacement of
species."[361] The littoral _Sthenelais boa_ is represented by _S.
limicola_ in deeper water; _Sabellaria alveolata_ by _S. spinulosa_;
_Polynoë imbricata_ by several deep-water species. Similarly with genera:
the littoral _Pomatoceros_ is replaced by _Serpula_ in deeper water; and
the Hesionid _Psamathe_ by _Castalia_.

The limitation of species to certain regions, or to certain depths of an
ocean, may appear at first sight peculiar, in view of the unrestricted
communication between all its parts; but there are as efficient "barriers"
there as on land, for generally a particular worm can live only in a
certain temperature and at a certain pressure, and is dependent for its
food on particular organisms, which in their turn depend on the depth and
its accompaniments. It is, in fact, so much the more peculiar that certain
species are more or less cosmopolitan, or occur at widely distant points.
It is less peculiar, of course, to find different species of the same genus
at different depths or in different areas, for any slight variation in a
species advantageous to new conditions would readily be fixed, and give
rise to a new species.

The distribution of the Polychaeta depends probably on the pelagic larvae,
which are carried by currents from one part of an ocean to another. There
can be little doubt that many {301}Polychaetes are very "plastic," and can
adapt themselves to changed conditions of life with considerable ease; for
_Nereis diversicolor_, _Arenicola marina_, and others live equally well in
water of very different densities, and with a different food supply. The
great variety in the "habitats," and presumably therefore in their food
supply, etc., exhibited by many Polychaetes, as well as the great variation
observable in some species of Polynoina, and the close affinity of the
species and genera of this sub-family, lead us to the same conclusion.

EXTINCT POLYCHAETES.—The most numerous fossil records of the Polychaetes
are calcareous tubes of various shapes and sizes; they are irregularly or
spirally curved, and are very usually attached at one end, or by one
surface, to stones or to fossils. These tubes belong to the Serpulidae, and
are referred to the genera _Serpula_, _Spirorbis_, _Ditrupa_, and
others.[362]

_Spirorbis_ is the oldest unequivocal representative of the Polychaetes, as
its tubes are found more or less abundantly in the Silurian and other
Palaeozoic strata. In Palaeozoic times _Serpula_ was rare, as it was too in
the Trias and Lias, but in the Jurassic strata it becomes abundant. In the
chalk, _S. socialis_ may occur in masses like _S. uncinata_ of the present
day, forming "Serpulite chalk." In the older tertiaries the genus is
represented by _Spirulaea_.

_Terebella lapilloides_ occurs in the Lias as a cylindrical, more or less
curved tube of sand-grains.

[Illustration: FIG. 159.—_Eunicites avitus_ Ehl. A fossil worm from the
lithographic slate of Solenhofen: the jaws are seen in front, and the
acicula along each side. (From Ehlers.) Natural size.]

Amongst the Nereidiformia the remains are fewer, but the {302}acicula and
the hard jaws are preserved in certain rocks, and can be referred to
existing families. _Eunicites avitus_[363] is represented by a double
series of acicula, indicating the parapodia of the two sides; and by
remains of both upper and lower jaws (Fig. 159). Four different species of
the worm have been described from the lithographic slate of Bavaria, of
Jurassic age; and several upper jaws of other Eunicids have been discovered
in the Palaeozoic beds of Canada and Scotland, and have received the names
_Lumbriconereites_, _Oenonites_, and _Arabellites_, in reference to their
nearest allies amongst living genera.

There are, however, numerous remains, in the forms of tracks or casts, in
the earlier rocks, which have been referred to the Polychaeta. The names
_Crossopodia_, _Myrianites_, _Nereites_, _Phyllodocites_, have been given
to some of these traces, though they are open to numerous other
interpretations. Some of the "tracks" are similar to those made by living
Crustacea in walking over wet sand; others appear to be the casts of some
animals. Tubular burrows in rocks or fossils, some straight, others
U-shaped, have received such names as _Arenicolites_, _Scolithus_,
_Histioderma_; whilst under the name _Lumbricaria_ certain cylindrical,
coiled structures, resembling worm "castings," are met with in this same
lithographic stone of Solenhofen. Many of the tubes referred to Polychaetes
by the earlier palaeontologists have been transferred to other groups; thus
_Cornulites_ is now believed to be a Pteropod shell.

This very meagre geological record is quite insufficient to form any basis
for a phylogeny of the group. And this poor supply of remains is not
surprising, when we consider the soft nature of the tissues, the absence,
in the majority of families, of skeleton and of other parts which could
have been fossilised; yet we might have expected a greater abundance of
fossilised jaws than is represented at present. But it must be borne in
mind that the conditions of life of these soft-bodied animals are not
conducive to their leaving abundant fossilised remains.

{303}CHAPTER XII

CHARACTERS OF THE SUB-ORDERS OF POLYCHAETES—CHARACTERS OF THE
FAMILIES—DESCRIPTION OF BRITISH GENERA AND SPECIES—THE MYZOSTOMARIA.

SYSTEMATIC.—The Order Polychaeta may be divided into two branches, in one
of which, the PHANEROCEPHALA, the prostomium retains its ancestral
condition as a lobe overhanging the mouth, and frequently carries, in
addition to paired eyes, certain sensory processes of a simple structure,
the tentacles and palps; the body-segments are more or less alike, and
(except in some Spioniformia, some of the Terebelliformia, and the
Capitelliformia) do not present two sharply marked regions, owing to the
differential arrangement or character of the chaetae. In the second branch,
the CRYPTOCEPHALA, the peristomium grows forwards during development, so as
to compress or even hide the prostomium, which thus becomes a very
insignificant organ. The tentacles are reduced, but the palps become
greatly developed and take on sundry new functions. The body in this group,
by the character and arrangement of the chaetae, is distinguishable into a
thorax and abdomen, presenting certain internal differences.

These two branches may be supposed to have arisen from a common ancestor
having a general resemblance to a nereidiform worm, such as _Syllis_,
possessing palps and tentacles on the prostomium, definite parapodia and
cirri on the body, and internally, a well-marked and regular repetition of
organs.

The branch PHANEROCEPHALA contains the following five sub-Orders, though it
is possible that the Capitelliformia deserves a more important position in
the system:—

SUB-ORDER 1.—The _Nereidiformia_ have well-developed tentacles and palps;
the peristomium almost invariably possesses {304}special cirri; the
parapodia are well-marked locomotor organs, supported by acicula, and carry
dorsal and ventral cirri. The chaetae are usually jointed, though unjointed
ones may coexist with these; uncini are never present. An eversible buccal
region leads into a muscular pharynx, which in the majority is armed with
chitinous jaws; the septa and nephridia are regularly repeated throughout
the body. The worms lead a predaceous life, and are mostly carnivorous; a
few form tubes.

SUB-ORDER 2.—The _Spioniformia_ possess neither tentacles nor palps; the
peristomium usually carries a pair of long tentacular cirri, and extends
forwards at the sides of the prostomium. The parapodia project only to a
slight degree; the dorsal cirri may attain a considerable size, and act as
gills throughout the greater part of the body. The chaetae are unjointed;
uncini are only present in the aberrant _Chaetopterus_.[364] The body may
present two regions more or less distinctly marked externally, but without
corresponding internal differences. The buccal region may be eversible, but
there are no jaws. Septa and nephridia are regularly developed. The worms
are burrowers, or tubicolous.

SUB-ORDER 3. _Terebelliformia._—The prostomium is a more or less prominent
lobe (upper lip) with or without tentacles but without palps. The
peristomium may carry cirri or "tentacular filaments."[365] The parapodia
are feebly developed; there are no ventral cirri; the dorsal cirri may
exist and function as gills on more or fewer of the anterior segments. The
chaetae are unjointed, and uncini are usually present. The buccal region is
not eversible; there are no jaws. The septa are usually incomplete, with
the exception of one strongly-developed "diaphragm" anteriorly; the
nephridia are dimorphic, those of the anterior (prediaphragmatic) segments
are of large size and are excretory; the posterior series are mere funnels,
and act as genital ducts. These worms are burrowers or tube-formers, and in
the majority the tube-forming glands are grouped on the ventral surface of
the anterior segments to form "gland-shields."

{305}SUB-ORDER 4.—The _Capitelliformia_ have no prostomial processes, but
possess a pair of large retractile "ciliated organs." The parapodia do not
project; the chaetae are unjointed, and are hair-like in the anterior
segments and hooded "crotchets" posteriorly; this external division of the
body does not correspond with definite internal differences. There are no
cirri, though special "gills," often retractile, are frequently present.
The buccal region is eversible; there is no armed pharynx. An "accessory
gut" or "siphon" exists. The nephridia are small, and sometimes more than
one pair in a segment; special genital funnels exist in more or fewer of
the anterior segments of the hind body. There is no system of
blood-vessels; the coelomic corpuscles are red. The worms are burrowers.

SUB-ORDER 5.—The _Scoleciformia_ possess a prostomium, which rarely
(Chlorhaemidae) carries any sensory processes; the peristomium is without
cirri (except, perhaps, in the Chlorhaemidae). The parapodia are ill
developed, and may be absent; only rarely are dorsal cirri present, acting
as gills; ventral cirri are absent. The chaetae are unjointed; true uncini
are not present. The buccal region is eversible, but there is no armed
pharynx. The septa are not regularly developed, as more or fewer are
absent, and the nephridia are considerably reduced in number, it may be to
a single pair (Sternaspidae and some Chlorhaemidae), but they are all
alike.[366] The worms are mostly burrowers.

The branch CRYPTOCEPHALA contains two sub-Orders:—

SUB-ORDER 1. _Sabelliformia._—The prostomium is entirely hidden by the
forward extension of the peristomium; the tentacles are very small, being
frequently represented merely by small knobs of sense-cells; the palps, on
the other hand, are greatly developed, branched, and contain blood-vessels,
acting as respiratory as well as sensory organs. The peristomium never
carries cirri or chaetae, and it is usually raised up into a projecting
collar, used in fashioning the lip of the animal's tube. The parapodia are
but feebly developed; cirri are absent, except in the Serpulidae, where the
dorsal and ventral cirri become united to form the "thoracic membrane"
(Meyer). The chaetae are of two kinds—unjointed, hair-like, fringed
bristles and "uncini." {306}By their arrangement the body is divided into a
thorax of nine segments and an abdomen; in the former the capillary chaetae
are dorsal, and in the latter ventral. The buccal region is not eversible;
there is no pharynx. The septa are regularly developed in the abdomen, but
are absent in the thorax; the nephridia are dimorphic; there are two large
ones in the thorax opening by a median dorsal pore just above the brain;
those of the abdomen are small funnels, and act as genital ducts. The worms
are tubicolous; "gland-shields" are present on the thoracic segments.

SUB-ORDER 2. _Hermelliformia._—The peristomium (Fig. 135) is enormously
developed, and forms a bilobed hood capable of closing over the mouth; the
truncated free end of each lobe carries three semicircles of peculiar
chaetae, which act as an efficient protection when the worm is withdrawn
into its tube. The prostomium is very small, but retains a pair of
well-developed tentacles; the palps, which are subdivided as in the
Sabelliformia, have become fused with the ventral edges of the peristomium,
and appear as a series of ridges on each side, carrying numerous filaments.
The thorax consists of five segments, the notopodia of three of which are
well developed and bear strong chaetae; dorsal cirri are present along the
greater part of the body, and act as gills. The arrangement of the chaetae
and of the internal organs is as in the Sabelliformia. The worms form tubes
of sand.

BRANCH A. PHANEROCEPHALA.

SUB-ORDER 1. NEREIDIFORMIA.[367]

FAM. 1. _Syllidae._—These are small worms, the majority being less than an
inch long, so that they are not easily observed. {307}The body consists of
a fair number of segments.[368] In many genera a dorsal bundle of
unjointed, natatory chaetae makes its appearance at maturity. The palps,
which are grooved, are in some cases so united with one another and with
the prostomium as to be scarcely recognisable. (For head see p. 262, and
for feet see p. 264.) The pharynx is armed with one or more teeth. There is
a special gizzard, following the pharynx, and provided with thick, muscular
walls of peculiar structure. Following the gizzard, the oesophagus receives
in many genera a pair of T-shaped diverticula, that are used for storing
water, which is swallowed with food. These diverticula are absent in
_Autolytus_ and other free-swimming forms. The reproduction of the members
of this family is interesting, and has already been described (p. 278).

[Illustration: FIG. 160.—_Syllis armillaris_ Müll. × 2. (From Johnston.)
The head is towards the right.]

_Syllis._—The tentacles and cirri are moniliform; the palps large; there is
a single dorsal tooth, which is provided with a poison gland, the duct of
which opens near its apex; it is used rather for stabbing its prey than for
grasping and tearing. _S. krohnii_ Ehlers, is abundant under stones, and
forms tubes of sand; it is nearly an inch long, and consists of some
eighty-five to ninety-five segments marked with yellow bands. It may
readily be identified by longer dorsal cirri, terminally dilated,
alternating with shorter ones. _S. cornuta_ Rathke, has a translucent green
body, about half an inch long; no alternation {308}of cirri. Mediterranean,
Atlantic, on the Norwegian coast, off Spitzbergen, and on the Madeira
coast. _S. armillaris_ Müll. is very common at low water; it is pale
yellowish-brown, with a couple of dusky marks on each segment; and measures
2 inches. The dorsal cirri are quite short, consisting of only eight to ten
joints. In _Pionosyllis_ the tentacles and cirri are not moniliform; a
single dorsal tooth. _P. malmgreni_ M‘I. under stones. _Sphaerosyllis_.—The
dorsal cirri are swollen at the base, and are not moniliform; the long
palps are fused along nearly their whole extent. _S. hystrix_ Clap. is only
about one-eighth of an inch in length. _Exogone_ Oerst. _Grubea_ Qfg.

_Autolytus._—The small palps are entirely fused with the prostomium; the
pharynx, which is bent upon itself, is armed with a circle of denticles.
Dorsal cirri somewhat foliaceous. There are no ventral cirri. The male and
female differ from one another and from the asexual "stock" (see p. 279).
_A. pictus_ Ehl. is abundant under stones. It measures about two-thirds of
an inch in length, is darkly coloured with a median lighter band; the
anterior dorsal cirri are long. _A. prolifer_ Müll. is common.

_Myrianida fasciata_ Milne Edwards, with its foliaceous cirri, occurs off
our coasts (see Fig. 149, p. 280). Atlantic, Mediterranean.

FAM. 2. _Hesionidae._—The body is relatively short, with only a few
segments (sixteen to fifty, according to the genus); in the larger forms it
is cylindrical. The parapodium is usually uniramous; the dorsal cirri are
long and multiarticulate; the chaetae are jointed. The prostomium carries,
in addition to four eyes, two or three tentacles, and generally a pair of
jointed palps. The peristomium and two or more of the following segments
are achaetous, and carry long "peristomial" cirri. The pharynx is very long
but unarmed.

_Psamathe_ Johnston, has many segments; head with two tentacles and a pair
of three-jointed palps. _P. fusca_ Jnstn. occurs amongst coralline Algae,
to which it bears some resemblance, which is heightened by the moniliform
cirri. It is a small worm, less than an inch in length. Mediterranean.
_Castalia punctata_ Müll. is dirty green or brownish, with a narrow
purplish band on each side. It occurs in deeper water than the preceding.
In _Ophiodromus_ the head has three tentacles; the palps are two-jointed;
there are six pairs of peristomial cirri; the parapodia {309}are biramous.
_O. vittatus_ Sars is dredged in numbers off the Scotch coast, and is found
also at low tides. It measures 2 inches in length. A closely allied species
lives in the ambulacral grooves of the starfish _Astropecten_.

FAM. 3. _Aphroditidae._[369]—The most characteristic feature of this
family, and one by which its members are absolutely distinguished from all
other Chaetopods, is the possession of scales or "elytra" on the back.
These flattened dorsal cirri are of a somewhat horny texture, and are
carried, generally, on alternate segments of the body; filamentous cirri
occurring on the other segments. In the sub-families _Hermionina_ and
_Polynoina_ the elytriferous segments are 2, 4, 5, 7, 9, etc., up to 23;
then every third segment. The worms are usually short, with some
thirty-five to forty-five segments, though _Sthenelais_ and a few others
have many more. (For head see p. 262, and for parapodium see pp. 265, 268.)
The pharynx is very thick walled, and furnished with two pairs of jaws,
which are, however, not hardened in the sub-family _Hermionina_. The
intestine is provided with a number of paired longer or shorter caeca (Fig.
142). A considerable number of this family are commensal or parasitic (see
p. 297). The family is well represented on our own coasts, so that only a
few of the more readily distinguishable species can be here described.

[Illustration: FIG. 161.—_Polynoë squamata_ L. Nat. size. _c_, Notopodial
cirrus; _e_, elytron; _f_, parapodium; _p_, palp; _t_, tentacle. (From
Johnston.)]

Sub-Fam. 1. _Polynoina._—Body flattened, with nearly parallel sides,
usually short, more rarely worm-like; three tentacles; peristomium with
long dorsal and ventral cirri; the ventral cirri of the next segment are
also elongated. Jaws are present. Elytra, usually twelve to eighteen pairs,
the surface of which is more or less papillose, and may be "fringed" along
the outer border, with long processes. The colouring of the elytra is
characteristic in most cases, though liable to considerable variation in
some species. The chaetae are generally strong, and of bright golden
colour: they are all unjointed. The _Polynoina_ are generally but feeble
swimmers, {310}and are mostly found under stones at low tide. Some species
have a very wide geographical range.

_Polynoë._[370]—The body is short; none or only a few segments at the end
of the body are uncovered by the elytra, except in the long body of _P.
johnstoni_.

_A. With twelve pairs of elytra._—In _P. squamata_ Linn. the elytra
entirely cover the body and conceal the head, each elytron overlapping the
next posterior one, and those of the two sides overlapping. General colour
sandy-brown, speckled, lighter or darker. The fringed elytra are very
firmly fixed to the body. The notopodial chaetae scarcely project from
below the elytra. The worm is common between tide-marks and in the
coralline region, is about one to one and a half inches in length, and
about one-third of an inch in width. Atlantic. _P. clava_ Montagu, may
attain a larger size, though it is generally smaller. The elytra are dark,
usually grey, mottled with white or light grey, unfringed, and do not
overlap to so great an extent as in _P. squamata_, so that the middle of
the back and the hinder part of the body is more or less exposed. This is
never the case in the preceding species, but even here it is subject to
variation in extent, depending on the amount of food contained by the worm
or on the ripeness of the genital products. It occurs in the
Mediterranean.[371]

[Illustration: FIG. 162.—Elytra, A, of _Polynoë squamata_ L.; B, of _P.
clava_ Mont. × 10. _a_, Area of attachment; _e_, external margin; _f_,
fringe (the letter is at the posterior side of the elytron); _i_, internal
margin. (From Bourne.)]

_B. With fifteen pairs of elytra._—_P. imbricata_ L. is probably the
commonest species of the genus, occurring nearly everywhere under stones at
low tide. It is about an inch in length; the elytra are deciduous, and are
very variously coloured and marked; sometimes uniformly grey or even black,
sometimes {311}mottled with brown: in other specimens each elytron has its
outer half pale or white, while its inner half is darker, usually some tint
of brown or olive green, so that the worm appears to have a dark band along
the middle of its back. Other patterns occur. The body is entirely covered
by the elytra. The chaetae project considerably, and are nearly as long as
half the width of the body; those of the notopodium are brown and are
directed upwards, being nearly as long as the golden neuropodial chaetae.
This species has a very wide range, occurring on both sides of the
Atlantic, even on the shores of Nova Zembla, and reappearing again at
Japan. _P. semisculptus_ Leach is rather larger than the foregoing. The
elytra are very readily detached: they are light in colour, without a
fringe, but with large papillae near the margin. The notopodial chaetae are
thicker than those of the neuropodium. Several other species are also
common, but _P. johnstoni_ v. Marenz.[372] differs from the rest in having
an elongated body of some seventy segments, so that the posterior half is
uncovered by the elytra, which are small, greenish-grey, speckled, and have
no fringe. It is common and widely distributed, but appears to be only
found in the tubes of _Terebella nebulosa_.

[Illustration: FIG. 163.—Elytron of _Polynoë imbricata_. _a_, Area of
attachment to body; _e_, outer border; _i_, inner border.]

_C. With eighteen pairs of elytra._—_P. gelatinosa_ Sars, may attain a
length of 2 inches. The elytra are very faintly coloured, transparent and
soft, attached by rather long peduncles. In spirit they become swollen and
folded, giving the worm a very untidy appearance. The prostomium is partly
overlapped by a peculiar collar-like fold of the peristomium.

_D. With numerous pairs of elytra._—_Lepidasthenia_ has a very long body,
consisting of more than eighty segments. The elytra are quite small, and
occur throughout the body on the usual segments. There are no notopodial
chaetae. _L. elegans_ Gr. is a very elegantly marked worm, which, however,
has not been recorded from the British area; it occurs in the Mediterranean
(see Fig. 156, p. 293).

Sub-Fam. 2. _Hermionina._—The body is short, oval and {312}depressed; the
particularly strong notopodial chaetae are directed upwards and backwards
so as to protect the elytra. The neuropodial chaetae are also strong. The
prostomium carries a single tentacle and two long palps; the prostomial
ridge may be well developed. The peristomium is chaetigerous, with long
cirri. The jaws are represented merely by thickened prominences.

[Illustration: FIG. 164.—_Aphrodite aculeata_ L. (from _Règne Animal_).
Nat. size. _c_, Neuropodial chaetae; _p_, palps; 1, iridescent bristles; 2,
stiff chaetae; 3, felting bristles of notopodium.]

_Aphrodite._—The fifteen pairs of elytra, arranged as in _Polynoë_, are
concealed by a "felting" of hair-like chitinous threads arising from the
notopodium (Fig. 139, p. 268). _A. aculeata_ L.—The "sea-mouse" is one of
the most beautiful of the Polychaetes. The small tentacle is very readily
detached; the palps are very long; the parapodia of the peristomium are
directed forwards so as to form lateral lips; and its cirri are not
especially modified (see Fig. 132, p. 260). The body, which measures 3 to 6
inches, consists of thirty-five to forty segments, and is broadest in the
middle, the last dozen segments being very small; the body terminates in a
point. Some of the notopodial chaetae are brilliantly iridescent, and give
the worm its characteristic coloration. It is fairly common in the
coralline regions, and is frequently thrown ashore after storms. Atlantic
and Mediterranean.

In _Hermione_ the "felt" is absent, so that the elytra are exposed. _H.
hystrix_ Sav. occurs in ten to thirty fathoms of water all over the British
area and Mediterranean. It resembles in its general appearance a fat
Polynoid, with strong chaetae. _Laetmonice filicornis_ Kinb. also occurs on
our north-west coasts, and _L. producta_ Gr. has been dredged in 500
fathoms off the {313}west coast of Ireland; it has been recorded also from
Kerguelen and from Japan, so that it has a very wide distribution.[373]

Sub-Fam. 3. _Acoetina._[374]—The long, vermiform body has some thirty-nine
to ninety-three pairs of elytra, placed on every alternate segment
throughout. It is represented in the British area by _Panthalis_ from 75
fms., which forms a tube of black mud.

Sub-Fam. 4. _Sigalionina._—This sub-family includes forms with a long,
vermiform body; anteriorly the elytra are on alternate segments, up to the
twenty-sixth, and posteriorly on every succeeding segment; "gills" here
coexist with elytra; cirri are absent. The prostomium in _Sthenelais_ Kinb.
has a median tentacle, which is absent in _Sigalion_ Aud. and Edw. _Sth.
boa_ Jnstn. is common off our coasts near low-water mark, where it burrows
in the loose sand with rapidity. It is an elegant worm, and may attain a
length of 8 inches, though it is generally smaller; it is narrow, flat, and
only slightly tapering at each end; the elytra, which may be more than a
hundred pairs, are greyish or slightly brownish, some being lighter than
others; the margin is fringed with simple processes (which in _Sigalion_
are pinnate). Atlantic and Mediterranean. In _Psammolyce_ the elytra are
covered with sand grains. British and Mediterranean.

FAM. 4. _Phyllodocidae._—The members of this family make use of the
foliaceous cirri (Fig. 136, F) in their very active movements. The rounded
prostomium bears four or five tentacles; there are four long peristomial
cirri on each side (see Fig. 134, E).

Sub-Fam. 1. _Phyllodocina._—The body is elongated, with numerous segments;
the eyes are small; the chaetae are jointed; the dorsal and ventral cirri
are foliaceous; the pharynx is covered with papillae externally, but
contains no "jaws."

_Phyllodoce_ has a more or less depressed body; four prostomial tentacles;
four pairs of peristomial cirri. _P. lamelligera_ Jnstn.[375] (the
"paddle-worm") may reach a length of 24 inches, but is usually 8 to 12
inches long and ½ inch across. The general colour is bright bluish-green or
yellowish-green, with metallic iridescence; the parapodia olive-green or
brown, the sensory {314}processes yellow. It lurks, during day, under
stones and shells, etc., in the Laminarian zone. The green egg masses, so
frequently referred to as belonging to _Arenicola_, are laid by
Phyllodocids.[376]

[Illustration: FIG. 165.—_Phyllodoce paretti_ Blv. × ½. (From _Règne
Animal_.)]

In _Eulalia_ an additional (fifth) tentacle arises from the middle of the
back of the prostomium. _E. viridis_ Müll. is a dark green worm smaller
than the preceding; common between tide-marks, hiding in cavities and
tunnels in limestone rocks, which have been bored by the mollusc
_Saxicava_; it is rare where such rocks are absent. It might have been
thought that its vivid colour would harmonise with its surroundings, but it
is most abundant in regions where _Fucus_ abounds and _Ulva_ is absent. It
is evident then that the colour is not protective; it may perhaps be of
warning significance, for the mucus secreted in quantities by glands on the
cirri of the Phyllodocids is probably objectionable to their enemies.
_Phalacrophorus_ Grf. and _Pontodora_ Grf. may be mentioned as pelagic
genera.

Sub-Fam. 2. _Lopadorhynchina._—This includes small forms, _Lopadorhynchus_
Gr., _Pelagobia_ Grf., and other pelagic genera.

[Illustration: FIG. 166.—_Nauphanta celox_ R. Grf. × 4. (From Greef.) _e_,
The large eye.]

Sub-Fam. 3. _Alciopina._—These are surface forms, and, like most pelagic
animals, are colourless and transparent; the eyes, however, are very large,
and, with certain brown spots in each segment,[377] are the only coloured
parts in the body; in structure the eyes are much more complicated than
{315}those of other Polychaetes. The prostomium has five tentacles; there
are long peristomial cirri, and in general their anatomy agrees most
closely with that of Phyllodocids. _Alciope_, _Asterope_, _Vanadis_,
_Nauphanta_ are genera of the family;[378] none have been recorded from the
British area.

FAM. 5. _Tomopteridae._[379]—This includes but one genus, _Tomopteris_,
which is pelagic. The transparent, colourless body consists of only a few
(eighteen to twenty) segments; the parapodia are as long as the body is
wide, and carry no chaetae; each is bilobed, and fringed with a membrane;
each of these lobes contains a yellow rosette-shaped photogenic organ. The
only chaetae present in the worm are on the "head." The prostomium is
hammer-shaped, and appears to carry a pair of short filaments ventrally
(Fig. 167, _x_) each with a single chaeta within it; and a longer filament
laterally (_y_), supported by a long, very delicate chaeta. The mouth is
behind these, and they probably are the first pair of parapodia which have
shifted forwards. _T. onisciformis_ Eschscholtz is not unfrequently
obtained off our shores in the tow-net.

[Illustration: FIG. 167.—_Tomopteris rolasi_ Grf. × 10. From Guinea Isles.
_pr_, Hammer-shaped prostomium; _x_, first chaetigerous process; _y_,
second chaetigerous process; _c_, rosette (photogenic) organ on first two
parapodia; _b_, similar organ in the lobes of the following parapodia; _d_,
pigment spots; _f_, parapodium. (From Greef.)]

FAM. 6. _Nereidae (Lycoridae)._—This family contains a very large number of
species, differing from one another in small and not readily recognisable
characters, such as the relative lengths {316}of the various processes of
the head, of the lobes of the feet, the arrangement of the "paragnaths"
(see Fig. 125, _d_) and so forth. The general features of the family have
been already described. The genus _Nereis_ is represented by six fairly
common species on our coast, which are almost world-wide in distribution.

_N. diversicolor_ Müll. is about 3 to 4 inches in length, of a general
fleshy-red colour, though tending in some cases to yellowish-brown or even
greenish. It may be distinguished by two diverging brown bands, which start
on the peristomium and pass backward one along each side of the body for
several segments. The prostomium is broader than it is long. The worm
burrows in mud or sand, all round our coast between tide-marks. It has a
very wide distribution, being met with on this side of the Atlantic, and
off the coast of Greenland, and off Japan. It is even found in brackish
water at Bembridge, Isle of Wight.

_N. cultrifera_ Gr. is green or greenish-grey, with a series of small
rectangular light spots along the mid-dorsal surface, and oblique light
lines at the sides of each segment. Posteriorly the greenish pigment
becomes less and less till the hinder segments are flesh-coloured. The
prostomium is as long as it is broad. This species attains a length of 6
inches. Southern coasts: locally known as "Red Cat."

_N. dumerilii_ Aud. and Edw. is rather smaller and narrower than the two
preceding species; it is reddish-violet in colour, marked with darker
transverse lines in each segment. It is readily recognised by the two dark
brown spots on the upper surface of the base of the notopodium in most of
the segments, and by the great length of the peristomial cirri, the longest
of which reaches the fifteenth segment. It is sometimes found enclosed in a
cocoon-like tube of hardened grey mucus, more or less covered with foreign
particles, such as sand grains. Atlantic, Mediterranean, Japan.

[Illustration: FIG. 168.—_Nereis cultrifera_ Gr. × 6. Head with buccal
region everted, to show the arrangement of the jaws. (From Ehlers.) Cf. _N.
diversicolor_, Fig. 125, p. 248.]

_N. pelagica_ L. is red-brown or bronze in colour, and is generally larger
than the other species, from which it is distinguished by being widest
about the middle of the body (see Fig. 122, p. 246); whilst in the
preceding species the greatest {317}breadth occurs at the segments
immediately following the head. Further, the palps are long, the
peristomium is twice as long as the next segment, and the back of the worm
is strongly arched. At all depths on rocky and stony ground. Northern
coasts.

_N._ (_Nereilepas_) _fucata_ Sav. lives in the topmost whorls of empty
whelk shells and in those occupied by hermit crabs. The ground colour is
tile-red, with two milk-white bands along the dorsal surface. The dorsal
lobe of the foot is slightly foliaceous, glandular, and vascular.

_N._ (_Alitta_) _virens_ Sars. is a giant amongst Polychaetes, reaching a
length of 18 inches. Its name suggests its colour; it is very plentiful at
certain times at St. Andrews, and between tide-marks along the shore of the
Mersey estuary, as well as elsewhere. It forms a burrow in the clay, etc.,
of the shore, and lines it with mucus, which is abundantly secreted by the
great foliaceous lobes of the parapodia. These great leaf-like lobes of the
foot recall the modification which the foot of many species of _Nereis_
undergoes in transformation into _Heteronereis_: they are so greatly
developed that, at first sight, the worm might be mistaken for a large
_Phyllodoce_. The worm is known as the "Creeper," and is much esteemed as
bait on some parts of our coast.

[Illustration: FIG. 169.—Parapodium of _N. virens_ Sars. ×4. _a_,
Notopodial cirrus; _b_, notopodium; _c_, neuropodium; _d_, neuropodial
cirrus; _l_, foliaceous appendage. (From Ehlers.)]

FAM. 7. _Nephthydidae._—The elongated body is quadrangular in section, the
dorsal and ventral surfaces being almost flat. (For head see p. 262, and
for parapodium, p. 264.) The two lobes of the parapodium are widely
separated, and each is fringed with a membrane, while a sickle-shaped
"gill" hangs down from the under surface of the notopodium. The pharynx is
enormous. Of the genus _Nephthys_ two species, called the "Lurg" or "White
Cat" by fishermen, occur on our coasts. Their active movements and
beautiful mother-of-pearl tint are characteristic. _N. hombergii_ Aud. and
Edw. occurs on the shore, and down to 20 fathoms; it is 3 or 4 inches long,
and may be found burrowing in the sand; the chaetae exceed in length those
of _N. caeca_ {318}Fabricius, which occurs less frequently and in deeper
water, and is larger than the preceding. Both are Atlantic forms.

FAM. 8. _Amphinomidae._—The body in this family is either vermiform, as in
_Eurythoe_, or oval and flattened, as in _Euphrosyne_ and _Spinther_. The
head carries a peculiar sense organ, the "caruncle," consisting of a smooth
axis with the sides folded so as to look like a number of lamellae. The
parapodia carry gills. Most of the Amphinomids are tropical and Southern
forms.

_Eurythoe borealis_ Oerst. measuring 6 inches, occurs all round the British
area, from the Shetlands, where it occurs in deep water, to the Channel
Isles, where it lives on shore, under stones, etc. (For parapodium of
_Amphinome_ see p. 264.)

_Euphrosyne._—The body is short, oval, and flattened. The parapodia are not
distinct processes, but the chaetae extend from each side of each segment
nearly to the middle dorsal line, and are absent ventrally (Fig. 137, C, p.
265). The dorsal and ventral cirri are more or less filiform, and there is
an intermediate similar process on the back (? = lip of chaetigerous sac).
Amongst the chaetae are a number of curious branched processes—usually
called "gills."[380] The presence of these and of the chaetae give the
upper surface of the body a fluffy appearance. _E. foliosa_ Aud. and Edw.
is fairly common under stones on our southern shores. It is about an inch
in length and is of a cinnamon-red colour.

FAM. 9. _Eunicidae._—The elongated body is provided with parapodial gills
in more or fewer segments (except in _Lumbriconereis_). The "gills" may be
cirriform (_Hyalinoecia_), pectinate (_Eunice_, _Onuphis_), or more complex
(_Diopatra_). The notopodium is represented by a lobe (usually called
"cirrus") into which an aciculum projects; in some cases it even contains a
few chaetae; most of the neuropodial chaetae are jointed (Fig. 138, F). The
prostomial tentacles vary in number; they may be three or five, or five and
two short "frontal palps," or they may be absent. Peristomial cirri are
absent, though in _Eunice_, _Diopatra_, and _Onuphis_ "nuchal cirri" are
present on the dorsal surface of the second segment (Fig. 134, D). One of
the most characteristic features in the anatomy of the Eunicids is the
peculiar jaw apparatus (see p. 270). The majority of the genera form
permanent tubes of parchment-like consistency, {319}which may be further
strengthened by the addition of grains of sand, small pebbles, etc.; the
tubes may be branched.

_Eunice_ has five tentacles, two great palps, and a pair of nuchal cirri;
the gills are pectinate, and there are four anal cirri. _E. harassii_ Aud.
and Edw. is about 8 inches long. It is reddish-brown, with white spots down
the back, one to each segment, and others at the sides. The gills begin at
the sixth segment, and when fully developed have eleven branches. The
dorsal cirrus is not longer than the gill. _E. philocorallia_ Buch.[381]
forms its tube amongst the branches of _Lophohelia prolifera_, in 200
fathoms, off the west coast of Ireland.

_Marphysa_ resembles _Eunice_, but has no nuchal cirri. _M. sanguinea_
Mont. is a fine bronze colour, with bright red gills, which commence on the
twentieth segment, and have only four or five branches. The worm, which
measures 12 to 18 inches, and is as thick as one's finger, hides in clefts
in rocks and under stones below low water. Mediterranean. It is known as
"Rockworm" in the Channel Islands.

_Hyalinoecia_ Mgrn., in addition to the five prostomial tentacles and
palps, possesses a pair of small "frontal palps" arising from the anterior
border of the prostomium; there are no nuchal cirri, and the gills are
simple filiform processes. _H. tubicola_ Müll., about 3 inches long, is
yellowish-brown, and forms a transparent, parchment-like tube. Atlantic and
Mediterranean.

[Illustration: FIG. 170.—_Ophryotrocha puerilis_ Clap., Metsch. × 25. _ci_,
Bands of cilia; _cp_, ciliated pit (nuchal organ); _J_, jaws. (From
Korschelt.)]

_Onuphis_ Oerst. has a head like the preceding, from which it differs in
having pectinated gills and two nuchal cirri like _Eunice_. In making its
tube it employs small pebbles, bits of shell, and even echinid spines,
which it glues together with mucus, so that it bears a general resemblance
{320}to its surroundings. _O. conchylega_ Sars, has a flattened,
scabbard-like tube, which can be carried about by its owner. Atlantic.

_Lumbriconereis_ has a more or less conical prostomium, without any
tentacles, but with large palps: segments without gills. _L. fragilis_
Müll. is reddish or brownish, with a beautiful iridescence; it is
cylindrical, very narrow, and some 5 or 6 inches long; _L. tricolor_ Jnstn.
is much larger.

_Ophryotrocha_ (Fig. 170) is a small form often occurring in aquaria; it is
chiefly remarkable for the possession of segmentally-arranged girdles of
cilia—a permanent larval feature. _Lysidice ninetta_ Aud. and Edw. belongs
here.

FAM. 10. _Glyceridae._—Elongated worms with numerous segments. The
prostomium, though narrow, is long, conical, annulated, and carries at its
apex four very small tentacles; at its base a pair of palps. Special
retractile gills are present. The armed pharynx is very long, and when
protruded appears wider than the animal. The members of this family are
without any system of blood-vessels, but the coelomic corpuscles are
coloured red. _Glycera_ has four jaws, the parapodia are all alike (Fig.
136, C). _G. capitata_ Oerst. is 2 or 3 inches in length, is yellowish in
colour, with a dark-red median line. It may be found burrowing in sand. The
setigerous lobes of each foot are coalesced to form one large lobe with
pointed apex. The dorsal cirrus is a small wart above the base of the foot.
Atlantic and Mediterranean. A second species, which is much larger and
flesh-coloured, also occurs.

[Illustration: FIG. 171.—_Glycera meckelii_ Aud. and Edw. with pharynx
everted, × 1. (_Règne Animal._)]

_Goniada_ is distinguished from the preceding by the fact that the
parapodia suddenly change in size and character at about one-third the
length of the body. The pharynx has numerous paragnaths. _G. maculata_
Oerst. occurs off our coasts.

FAM. 11. _Sphaerodoridae._—The dorsal and ventral cirri of each segment are
spherical. The chaetae are usually jointed, and there is an aciculum to
each {321}parapodium. _Ephesia_ Rthke. (_E. gracilis_ R. = _Sphaerodorum
peripatus_ Jnstn.) is exceptional in having unjointed chaetae. North Sea,
Arctic Ocean, and the Channel. The family, which is much modified, is
allied in some respects to the Syllidae.

FAM. 12. _Ariciidae._—These worms burrow in sand between tide-marks. The
body consists of many short segments, and is nearly cylindrical. The
prostomium is more or less pointed; the chaetae are all capillary; in the
first few segments they project laterally but soon come to lie dorsally,
and are carried by slight conical papillae (supported by acicula), which
are longer in the middle of the body. Most of the segments carry filiform
"gills," representing the dorsal cirri (Fig. 137, B).

_Scoloplos armiger_ Müll. is extremely common on our coast. It is about an
inch long, yellowish, with red gills, commencing about the twelfth segment.
Each of the lobes of the parapodium possesses an aciculum, and the chaetae
are bent in a peculiar way. The everted buccal region has the form of a
six- or eight-rayed star. The spawn of this species may be found on the
shore in spring as brown, pear-shaped, jelly-like masses, each with a long
stalk, by which the mass is fixed to the sand. In the jelly are the eggs,
which may be watched passing through the earlier stages of development.
Atlantic on both shores, even off Spitzbergen, and Nova Zembla. Another
representative is _Theodisca mamillata_ Clap., which occurs amongst the
roots of _Laminaria_.

FAM. 13. _Typhloscolecidae._[382]—Pelagic, greatly modified forms,
apparently related to the Phyllodocidae, but with very uncertain
affinities. The prostomium is pointed and carries a pair of foliaceous
tentacles; each of the first two segments bears a pair of foliaceous cirri;
the remaining segments possess a dorsal and a ventral pair of foliaceous
cirri, with a small bunch of chaetae and a single aciculum. All the cirri
have peculiar rod-cells. _Typhloscolex_ Busch, _Sagitella_ Wagner, and
_Travisiopsis_ Uljanin: all small worms. North Sea Atlantic.

SUB-ORDER 2. SPIONIFORMIA.

FAM. 1. _Spionidae._—Mostly small worms, with small ridge-like prostomium
carrying a pair of eyes, but no tentacles or palps. The peristomium, which
extends forwards on each side {322}of the prostomium, bears a pair of very
long cirri (usually termed "tentacles") normally directed backwards, very
mobile, and more or less coiled. They are readily thrown off by the animal.
The notopodial cirri are long, finger-shaped, and curved over the back;
they are vascular and ciliated, and function as "gills." The neuropodia
project laterally. Both are usually provided with a "podal membrane" along
their outer margin. There are no ventral cirri; the dorsal chaetae are
fringed capillaries; the ventral are "crotchets." The buccal region is
eversible. The worms burrow in mud and sand.

_Spio seticornis_ Fabr. is a small worm less than an inch in length,
colourless except for the red blood in its vessels. It builds long and
flexible tubes of sand in the clefts of rocks and under stones in the upper
part of the littoral zone. The prostomium is notched at the anterior
margin. The gills commence on the twelfth segment, and do not extend to the
end of the body. A membrane-like cirrus exists also on the second
chaetigerous segment. The podal membrane is adnate to the gill throughout
its extent. Four short anal cirri occur. Greenland and Scandinavia.

[Illustration: FIG. 172.—_Nerine vulgaris_ Jnstn., enlarged. (From
Cunningham.) _a_, Prostomium; _c^1_, cirrus of peristomium; _c^2_, "gill";
_l_, lobes; _m_, podal membrane; I, peristomium; II, III, IV, following
segments.]

_Nerine_ is represented by two species, sometimes called "Ragworms." The
genus is very similar to _Spio_, but the worms are of larger size. The
prostomium is compressed by the forward growth of the peristomium, and
appears as a ridge on the latter segment, extending downwards in front
towards the mouth. The "gills" commence on the second segment, and are
continued in every segment except the hindmost. _Nerine_ (_Scolecolepis_)
_vulgaris_ Jnstn. is readily distinguished from other species by its
somewhat T-shaped prostomium. It is an extremely common worm under stones
and amongst seaweed at low water. It is some 3 or 4 inches in length and
more slender than the following species. Its colour is yellowish-brown, and
the red gills directed upwards and backwards give the appearance of oblique
red lines. The podal membrane does not reach the tip of the gill. North
Atlantic. It is {323}said to ascend rivers and live in brackish water. _N.
coniocephala_ Jnstn. is much the same colour, but reaches a length of 8
inches, and a diameter of ¼ inch. The prostomium is conical. The podal
membrane reaches to the tip of the gill in the anterior segments. The worm
burrows rather more deeply and nearer low-water mark than the preceding
species.

FAM. 2. _Polydoridae._—_Polydora_ Bosc (= _Leucodore_ Jnstn.) is readily
distinguished from the other Spionids, and, indeed, from any other
Polychaet (except _Chaetopterus_), by possessing specially strong chaetae
in the enlarged fifth chaetigerous segment. The anterior segments differ
from the rest in the absence of gills and in the character of the chaetae
(Fig. 133, A, p. 261).

_P. ciliata_ Jnstn. inhabits soft mud tubes near low water; it also makes
U-shaped galleries in stones and shells, and the tube projects from each
mouth. The worm is about ½ inch long, consists of some forty segments, and
is yellowish or flesh-coloured. The prostomium resembles that of _Spio_;
the peristomium is raised into a slight collar at each side. The anus is
surrounded by an incomplete funnel. The species has almost a world-wide
distribution, having been recorded from Iceland, Australia, the Philippine
Islands, as well as from the European seas. _P. coeca_ Oerst. often lives
commensally with a sponge, having a protective odour.

FAM. 3. _Chaetopteridae._—The family is represented on our coasts by
_Chaetopterus variopedatus_ Ren.,[383] which is found at the Channel
Islands, the Scilly Isles, the Isle of Man, and the west Scottish coast,
and probably at various other places, at low water and down to a depth of
some 15 fathoms. It occurs in all European seas. The animal builds a long
tube, the basis of which is a tough, parchment-like substance; this is
coated externally with sand, small pebbles, and other débris: it is of
considerable length and about ¾ inch in diameter, is U-shaped and open at
both ends, the greater part of it being embedded in sand or in crevices of
rocks. The animal, whose body-wall is thin and delicate, never leaves its
tube. The body has a bizarre appearance; three regions are readily
{324}distinguishable, which may be denoted by the letters A, B, and C. The
most anterior region, A, is flattened, and carries nine pairs of conical
lobes with delicate chaetae, though the fourth lobe possesses special
stouter chaetae (as in _Polydora_). The anterior end of the body terminates
in a wide funnel, the boundary of which is formed chiefly by the
peristomium; on its dorsal surface is a pair of tentacle-like processes
(peristomial cirri); the region between which represents the prostomium.

[Illustration: FIG. 173.—_Chaetopterus._ (From Panceri.) Natural size of a
young specimen. A is the anterior region of the body; B, the middle region;
C, the hinder region. _c_, Peristomial cirri; _d_, "sucker"; _e_, the great
"wings"; _f_, the first of the three "fans"; _m_, mouth.]

The second region, B, is very curiously modified; it is formed of five
segments. The most anterior is produced on either side into a great
wing-like process, which in life is directed forwards above the region A.
Each is grooved on its inner side, the ciliated grooves being continuous
with a median groove running forwards along the back of A; this apparatus
serves to bring food to the great funnel-like mouth. The next segment
(twelfth) carries a dorsal and ventral "sucker," representing the
parapodia. Each of the segments 13, 14, 15 carries a membranous fold
encircling the body. By the constant movement of these "fans," which have
nearly the same diameter as the tube, a current of water is constantly
washed over the animal. The fans represent the notopodia; the neuropodia
are bilobed rounded knobs. The region C consists in the adult of about
thirty segments, all alike, and less modified than the preceding. The
animal is the most truly tubicolous of the Polychaetes, and is much
modified on this account. No locomotor chaetae are present, though the
great wings and notopodial processes of region C contains chitinous
bristles, which, however, do not project;[384] the anterior region with its
stiff chaetae, and the neuropodial uncinal plates of the rest of the body
serve in its movements up and down the tube, while the "suckers" fix the
worm temporarily to the wall of its house.

{325}_Chaetopterus_ is highly phosphorescent (see p. 295). It is further
interesting on account of the green colouring-matter, which is extracted by
alcohol. Two commensal Polynoids occur in the tube, viz. _Polynoë glabra_
and _P. cirrosa_. The larva is "mesotrochal" (with a ciliated ring round
its middle), that region of the body lying in front of the cilia giving
rise to the region A, whilst the rest of the body gives rise to regions B
and C.

FAM. 4. _Magelonidae._—This family includes only the very peculiar worm,
_Magelona papillicornis_ Fr. Müll., which lives buried in sand, between
tide-marks, in various parts of our coast and that of the United States.
Its chief features are the large, flat, spoon-shaped prostomium; the long
peristomial cirri, slightly expanded terminally, carrying papillae along
one side; the enormous, eversible buccal region, which is an important
respiratory organ. The blood is of a madder-pink colour, and the
blood-vessels in the thorax are greatly dilated. The body of the worm is
divisible into two well-marked regions, owing to differences in the
chaetae.[385]

FAM. 5. _Ammocharidae._—This family contains only one species, _Owenia
filiformis_ D. Ch. Some of the anterior segments are longer than the hinder
ones, though the arrangement of chaetae is alike throughout. The mouth is
wide, like that of _Chaetopterus_, and is surrounded, except ventrally, by
a membrane, so deeply notched as to give rise to flattened filaments
containing blood-vessels. These "gills" appear to belong to the
peristomium. The small worm in its sandy tube is plentiful on our coasts in
about 20 fathoms. Off Greenland and the Mediterranean.

SUB-ORDER 3. TEREBELLIFORMIA.

FAM. 1. _Cirratulidae._—These worms have a cylindrical body, more or less
attenuated at each end; the segments are distinct, and similar throughout,
with capillary chaetae on each side in two bundles, carried by small
papillae. The prostomium is conical, the peristomium usually without cirri.
On more or fewer segments the dorsal cirri are long and filamentous, and
function as gills. There is a single pair of anterior nephridia: the septa
and genital ducts are repeated throughout the hinder part of the body. The
worms usually live in burrows.

{326}_Cirratulus._—The prostomium is long, sometimes annulated. In addition
to the segmental filamentous "gills" there is a transverse row of long
"tentacular filaments" across one of the anterior segments, and it has been
suggested that they are really prostomial tentacles which have shifted
backwards. They and the gills twist about in a very active fashion during
life, and look like small independent worms, especially when they are
detached. _C. cirratus_ Müll, is a brown or dirty yellowish worm about 4 to
6 inches in length, usually to be found under stones, partially embedded in
the mud or sand. The prostomium carries a pair of linear groups of
"eye-spots"; the first chaetigerous segment carries a transverse row of
tentacular filaments, and the red gill-filaments commence on the same
segment. Common. _C. tentaculatus_ Mont. is a larger worm, dark red in
colour, and is distinguished from the preceding by the absence of eyes and
by the fact that the tentacular filaments are on the seventh chaetigerous
segment, while the gills commence more anteriorly. Atlantic and the
Mediterranean.

[Illustration: FIG. 174.—_Cirratulus tentaculatus_ Mont. (½ nat. size).
(From _Règne Animal_.)]

_Chaetozone setosa_ Mgrn. occurs in the North Atlantic. _Dodecaceria
concharum_ Oerst. is about an inch in length, olive-green or brownish in
colour, and is not uncommon amongst roots of _Laminaria_. It is stated to
live also in tortuous tubes bored in shells and stones, but whether it
makes these tubes is uncertain. The worm has two thick tentacular
filaments, and the thinner gills are only on four segments.
_Hekaterobranchus shrubsolii_ Buch.[386] is a small worm some ½ inch long,
found at Sheerness, where it occurs at low tide in soft mud; here it forms
a loosely coherent tube, though it also moves freely in the mud. Its chief
features are (1) a pair of long, ciliated {327}"cephalic tentacles,"
probably peristomial, and similar to the "tentacular filaments" of
_Dodecaceria_; (2) a pair of filamentous gills (dorsal cirri) on the first
chaetigerous segment; (3) a pair of large green nephridia in the anterior
segments. The describer placed it amongst the Spionids, but the above and
other features point to Cirratulid affinities.

FAM. 2. _Terebellidae._—The body is cylindrical, and generally larger in
front than behind. The prostomium is generally flattened, and forms a
mobile upper lip, which always carries a transverse series of many
tentacles; it may bear "eye-spots," but never palps; the lower lip is
formed by the peristomium. There are one to three pairs of gills, which are
usually more or less branched, on as many segments.[387]

The chaetigerous lobes are small; the dorsal ones contain capillary
chaetae, which are frequently confined to the anterior segments, whilst the
ventral chaetae are uncini. The ventral surface of the anterior segments is
thickened by glands which secrete the mucus employed in tube-building; the
number of these "shields" and of the dorsal bundles of chaetae have to be
noted in identifying the worms. There are one to three pairs of large
anterior nephridia. A very strong "diaphragm"—usually more or less
pouched—cuts off this anterior region of the body-cavity from the rest, and
is the only complete septum in the body; from three to twelve pairs of
small generative ducts occur behind it. The family is tubicolous, foreign
materials being generally used in the formation of the tube.

[Illustration: FIG. 175.—_Amphitrite johnstoni_ (½ nat. size). _g_, Gills;
_t_, prostomial tentacles. (From Cunningham and Ramage.)]

There are six genera which are fairly common round our coast, and their
identification may be facilitated by means of the following table[388]:—

{328}

{ equal in _Amphitrite_.
{ 3 pairs, { size. 24 notopodia.
A. Capillary chaetae { which are {
confined to the { { unequal in _Terebella_.
anterior part of Gills { { size. 17 notopodia.
body; commencing ramose. {
on the fourth { { Gills equal.
segment. { { Eye-spots. _Nicolea._
{ 2 pairs. {
{ 17 notopodia. { Gills unequal
{ and of _Pista._
{ peculiar shape.

B. Capillary chaetae { arborescent; 3 pairs. _Leprea._
throughout the {
body; commencing Gills { filiform; in transverse
on the third { series in two _Thelepus._
segment. { segments.

_Amphitrite johnstoni_ Mgrn. (Fig. 175) is brown in colour, about 4 to 6
inches in length, and nearly ½ inch in breadth anteriorly. Each of the
gills consists of a curved stem; from the convex side of which arise a
number of branches, themselves dichotomously divided, the final branches
being long (Fig. 176, A). There are twelve ventral "gland shields." The
worm is fairly common between tide-marks, below stones in muddy places: the
end of its tube of mud projects above the surface. Atlantic.

_Terebella_ (_Polymnia_) _nebulosa_ Mont. is distinguished by its bright
red colour, spotted with white; it is 6 or 7 inches in length, and ½ inch
across. Large specimens of this beautiful worm may be obtained at Weymouth
and elsewhere on the south coast, where it lives in about 14 fathoms. Each
gill appears much more arborescent than in the preceding (Fig. 176, B); it
consists of a main stem, from which comparatively few branches arise; these
subdivide frequently, and the terminal branchlets are quite short. The
"gland shields" are fourteen to sixteen in number. The tube is of mud.
North Sea and Mediterranean. _T._ (_Leprea_) _lapidaria_ L. is 1 inch in
length, orange-red in colour; and has 12 ventral shields. The tube, of fine
mud, lies horizontally on the under surface of stones.

_T._ (_Lanice_) _conchilega_ Pall. (the "sand mason") forms a very
characteristic tube of sandy particles, small pebbles, and pieces of shell.
It is buried in the sand, but a short portion protrudes, and bears, set
round its edge, a fringe of branching sandy threads (Fig. 153) commonly
seen on sandy shores between tide-marks. The worm may be distinguished from
the preceding species by the fact that the series of fourteen to
{329}seventeen gland shields are red, and continuous from segment to
segment. The gill is shown in Fig. 176, C. North Sea, Atlantic, and
Mediterranean.

[Illustration: FIG. 176.—Gills of various Terebellids. × 4. A, _Amphitrite
johnstoni_ Mgrn.; B, _Terebella nebulosa_ Mont.; C, _T. conchilega_ Pall.;
D, _Nicolea_ Mgrn. (the finer branches are not indicated); E, _Pista_
Mgrn.; F, _Terebellides_ Sars (after Malmgren). _g_, Gill; _m_, mouth; _t_,
tentacles.]

_Nicolea venustula_ Mont. has only two pairs of equal, arborescent gills
(Fig. 176, D); the tentacles are comparatively few. The animal, which is
about an inch in length, is cinnamon-yellow with white spots, and has
seventeen gland shields. 20 fathoms, North Sea and Mediterranean. _Pista
cristata_ Müll. is readily recognised by the shape of the gills (Fig. 176,
E), of which there are only two pairs. Each consists of a long peduncle,
bearing a number of dichotomously dividing, rather spirally-arranged
branches, the whole having the appearance of a "bottle-brush." The worm is
2 to 4 inches long, of greyish-red to yellow colour. Atlantic east and west
(even at the mouth of the Congo), and Mediterranean. _Thelepus cincinnatus_
Fabr. is about the same length, pale red in colour, marked on its back with
clear areas, giving the appearance of lacework. The gills are represented
by numerous unbranched filaments arising separately in two transverse rows.
The tube, which is adherent to shells, etc., along its whole length, is of
thin, transparent, and flexible material like mica, covered with foreign
bodies, and even with Polyzoa and Hydrozoa. 30 fathoms, Atlantic and
Mediterranean.

{330}_Polycirrus aurantiacus_ Gr. is sometimes placed in a special
sub-family, as it has no gills. The numerous tentacles are very long, and
arise from a great hood over the mouth; the capillary chaetae commence in
the first segment and extend for about half the length of the body; the
uncini commence in the ninth segment. The ventral "shields" are paired. The
animal is highly coloured; its phosphorescence and its distastefulness have
already been mentioned on p. 294. In _Terebellides stroemi_ Sars, four
comb-like gills arise from a single common thick peduncle on the back of
the second segment (Fig. 176, F). The ventral surface of the body bends
upwards anteriorly so as to bring the mouth to the dorsal surface. 13 to 16
fathoms, muddy bottoms, North Sea and Mediterranean.

FAM. 3. _Ampharetidae._—This family differs from the Terebellids chiefly in
the shape of the head and in the presence of a bundle of strong chaetae (or
paleae) on each side of the head in front of the gills, of which there are
four on each side. Each gill is a simple filiform process, considerably
longer than the tentacles, which are very few in number. _Amphicteis
gunneri_ Sars, _Ampharete gracilis_ Mgrn., and _Melinna cristata_ Sars,
occur on our coasts.

[Illustration: FIG. 177.—_Pectinaria belgica_ Pall. Slightly enlarged. _a_,
Neuropodial chaetae; _b_, notopodium; _ch_, paleae; _g_, gills; _sc_,
scapha; _t_, prostomial tentacles; _I_, peristomium. (From Malmgren.)]

FAM. 4. _Amphictenidae._—This contains the interesting genus _Pectinaria_,
in which the head is protected by great golden chaetae on the second
segment; they are flattened, curved, and pointed, and are arranged in a
single transverse row on each side, serving as an operculum to the tube.
The posterior end of the worm has undergone great degeneration, and is
represented by a small leaf-like "scapha" which serves to close the tube
posteriorly. The worm is 1½ inches in length and consists of only twenty
segments, of which seventeen are chaetigerous. The tube is nearly
cylindrical, but wider anteriorly than posteriorly (Fig. 152, p. 288); the
sand grains are uniform in size, and are embedded in the secreted mucus in
a very regular way, the surface being smooth both {331}inside and out.
These tubes can be carried about by the worm, but may be found projecting
from the sand at very low tides. _P. belgica_ Pall. forms a straight tube,
whilst in _P. auricoma_ Müll. the tube is larger and slightly curved. The
former species appears to be confined to the North Sea; the latter occurs
in deeper water, and is also present in the Mediterranean.

SUB-ORDER 4. CAPITELLIFORMIA.

FAM. _Capitellidae._[389]—_Capitella capitata_ v. Ben. occurs pretty
frequently in the sand under stones near low tide-mark. It is a red worm,
about 1½ to 2 inches long, greatly resembling a Tubificid Oligochaete. It
may readily be distinguished from other Polychaeta by the strong genital
chaetae in the male, which replace the notopodial chaetae of segments 8 and
9; those in the former segment are pointed backwards, and in the latter
forwards. There is but a single pair of generative ducts in either sex in
the eighth segment. North Sea, Mediterranean.

_Notomastus latericeus_ Sars is a longer worm, living in shallow water, off
our coast and in the Mediterranean. The anterior twelve segments are wider
than the rest of the body. The notopodial chaetae of the anterior segments
of the hind body form a ring. _Dasybranchus caducus_ Gr., which occurs in
the Mediterranean, but not on our coast, reaches a length of 2 or 3 feet.
It has gills on the hinder segments above the neuropodia.

_Mastobranchus_ Eis. is found in the Mediterranean.

SUB-ORDER 5. SCOLECIFORMIA.

FAM. 1. _Opheliidae._—Comparatively short, rather ugly worms of a pearly
colour, no prostomial processes: parapodia obscure. The family is
represented in British waters by four species, occurring in shallow water.

_Ammotrypane aulogaster_ Rathke, is about 2 inches long; the nearly
cylindrical body has a ridge running along each side below the chaetae. The
conical prostomium is tipped with a small knob, and carries at each side a
ciliated pit (Fig. 144, p. 273). Every segment, except the first
chaetigerous, is provided with a filamentous gill (dorsal cirrus). The
segmentation {332}is very obscurely marked, for internally there are only
three complete septa, placed far forwards. The intestine is bent upon
itself. In _Ophelia limacina_ Rthk. the gills commence in the eighth
segment, and the longitudinal ridge does not extend in front of this
segment. The worm is about 1½ inches long, and occurs between tide-marks.
_Travisia forbesi_ Jnstn., North Sea. _Polyophthalmus pictus_[390] Duj. is
very abundant at some parts of the coast. There are two bundles of chaetae
on each side of every segment; each bundle contains three chaetae, of which
only one projects to any distance. Paired eye-like spots exist on the sides
of twelve segments. The worm is about an inch in length.

FAM. 2. _Maldanidae_ (= _Clymenidae_).—Represented on our coasts by four
fairly common species. They form sandy tubes, which are embedded in the
sand with a short portion projecting. In some places they are so abundant
that at low water the sand has quite a rough appearance. The prostomium is
frequently truncated and depressed, and is always fused with the
peristomium. A horny plate may be developed on the upper surface of the
head, and the skin at the side of the prostomium is frequently raised into
a more or less prominent fold. The hinder end of the body carries a funnel
surrounding the anus. There are no gills or sensory processes on the body.
Some of the segments towards the middle of the body may be longer than the
rest. Peculiar serrated hooks of characteristic shape constitute the
neuropodial "torus." The buccal region is eversible.

_Nicomache lumbricalis_ Fabr. is a rosy-pink worm with white spots
anteriorly; the chaetigerous ridges are red. The worm consists of
twenty-six segments, and measures 2 or 3 inches. It is very narrow and
readily breaks in pieces. The prostomium is laterally compressed; the anal
funnel is fringed with a number of short equal processes. Under stones in
the Laminarian zone.

[Illustration: FIG. 178.—Anal funnel of _Nicomache lumbricalis_. (From
Malmgren.)]

_Axiothea catenata_ Mgrn., which may reach a length of 3 or 4 inches,
resembles the above in general colour, though of a deeper tint. There are
only eighteen chaetigerous segments. The head has a membranous fold of skin
on each side, and the anal funnel is produced into longer and shorter
processes. Both {333}these species are also found on the west side of the
Atlantic. _Clymene lumbricoides_ Qfg. is about 8 inches long; pink, with a
light ring round each segment; the seventh segment is larger and
reddish-brown. The prostomium is laterally compressed. Anus on a cone,
which rises from the bottom of a funnel, the margin of which is entire.
Atlantic.

[Illustration: FIG. 179.—_Axiothea catenata._ × ½. _a_, Anterior end; _b_,
notopodial and _c_, neuropodial chaetae; _p_, perianal funnel. (From
Malmgren.)]

[Illustration: FIG. 180.—_Arenicola marina._ × 1. Dorsal view. The anterior
end is seen partly from the side. _III_, The first chaetigerous segment;
_IX_, the ninth chaetigerous, and first branchial segment; _XXI_, the last
branchial segment; _b_, notopodial and _c_, neuropodial chaetae; _g'_, _g_,
the first and last gills; _t_, the non-chaetigerous tail.]

FAM. 3. _Arenicolidae._—Here belongs the common "lug-worm" _Arenicola
marina_ L., which occurs all round our coasts between tide-marks, and is so
generally used as bait in fishing. The worm, which measures 5 to 8 inches,
is of a dark tint, usually brownish-green. It burrows to a depth of some 18
inches or 2 feet, and throws up a considerable quantity of "castings" or
"sand-ropes," which are noticeable on every shore consisting of mud or
muddy sand. The body of the worm is cylindrical, thicker anteriorly; the
segments are indistinct, owing to the secondary groovings and furrows on
the skin. The prostomium is in the adult fused with the peristomium; this
and the second segment are achaetous. Then follow twenty chaetigerous
segments with dorsal bundles of capillary chaetae and ventral rows of short
crotchets. The hinder region of the body is achaetous and narrower than the
rest, forming a "tail." There are twelve (sometimes thirteen) pairs of
arborescent red gills on segments 9 to 20 (21). Internally there are
{334}only four complete septa, and six pairs of nephridia, which are of
large size: the fore-gut is eversible. Atlantic and Mediterranean. A second
and smaller species, _A. ecaudata_ Jnstn., occurs on our southern coasts;
it is readily distinguished by the absence of a "tail," the chaetae and
gills being continued to the end of the body.

FAM. 4. _Scalibregmidae._—Prostomium ill-marked, fringed with small
processes. Parapodia represented by slight papillae; two bundles of
chaetae; usually cirriform lobes above and below them. _Lipobranchius
jeffreysii_ M‘I. has a grub-like body pointed at each end; forms tubes of
mud. Firth of Clyde and elsewhere in North Sea. _Sclerocheilus_ Gr. in old
oyster shells. Channel, Mediterranean. _Eumenia crassa_ Oerst. has gills on
first six segments. North Sea. _Scalibregma inflata_ Rthke. has arborescent
gills on segments 4 to 7. The anterior part of the body is dilated. North
Sea.

FAM. 5. _Chlorhaemidae._—The family derives its name from the green colour
of the blood, due to chlorocruorin. The representatives are comparatively
short worms, with capillary chaetae on all the segments, the limits of
which are not evident. The prostomium carries a pair of long grooved
yellowish processes, which are perhaps palps, and several green tentacles,
acting as "gills," arranged in a transverse series above the mouth (Fig.
134, F, p. 262). The peristomium is achaetous; the whole "head" can be
withdrawn into the body. The chaetae of the anterior segments are
especially long, and directed forwards so as to form a "cage" for the head.
The body-wall is covered with longer or shorter papillae. Internally, the
chief points of interest are the presence of only two septa (_Trophonia_)
or only one septum (_Siphonostoma_), situated somewhere in front of the
middle of the body, and forming a great backwardly-directed pouch, which
contains a part of the looped intestine, and the nephridia, of which there
are only two or four.

_Trophonia plumosa_ Müll. is about 2 to 4 inches long, yellowish-brown in
colour, with a rough skin; the head is usually retracted. It lives in the
mud amongst Laminarian roots down to 50 fathoms. North Atlantic.
_Siphonostoma_ (_Flabelligera_) _diplochaitos_ Otto, has a transparent
body-wall, so that the coloured viscera are visible. The skin carries long
papillae, which traverse a thick jelly-like envelope secreted by it, in
{335}which numerous diatoms live (symbiotically?); the surface is covered
by particles of mud, etc. This species, which may be found under stones at
low tide, occurs also in the Mediterranean.

[Illustration: FIG. 181.—_Sternaspis scutata_ Ranz. × 2. (From Vejdovsky.)
The left figure shows the ventral surface; the right represents the
internal organs as seen when the body-wall is pinned aside, having been
slit up along its dorsal surface. _a_, Anus; _c_, gills; _ch_, anterior
strong chaetae; _ch^1_, bundles of chaetae along the lateral margin of the
shield; _ch^2_, the posterior marginal chaetae; _d_, oviduct; _g_, the
external tube carrying genital pore; _i_, coiled intestine; _n_, nephridium
("brown tube"); _o_, ovary, amongst the coils of the alimentary canal; _p_,
pharynx; _pr_, prostomium, with mouth just behind it; _s_, shield (on left
figure); _s_, stomach (on right); _sh_, outline of shield seen through the
ventral body-wall (in right figure); _x_, chaetae embedded in the
body-wall, with nerves passing by them.]

FAM. 6. _Sternaspidae._—The single genus, _Sternaspis_, has not been
recorded on our coasts, but is of so peculiar a structure as to deserve a
description.[391] _S. scutata_ Ranzani, occurring in the Mediterranean, is
rather less than an inch in length, and derives its name from the
possession of a pair of peculiar "horny" plates or shields on the ventral
surface posteriorly. Around their margins are set about thirty bundles of
long capillary chaetae. There are three half rings of stronger chaetae on
each side near the anterior end of the body. The mouth is overhung by a
very {336}small rounded knob (prostomium), which in _S. spinosa_ Sluiter,
is prolonged outwards on each side to form a grooved palp-like organ,
recalling that of _Bonellia_. The anus is placed posteriorly; and in front
of it, on the dorsal surface, are two bundles of many long thread-like
gills. On the ventro-lateral surface, in front of the middle of the body,
is a pair of finger-shaped processes containing the genital ducts. The
anterior segments of the body can be withdrawn into the body, as in the
_Chlorhaemidae_. Further examination leads to the conclusion that the body
of _Sternaspis_ consists of about thirty segments, most of them provided
with paired bundles of capillary (neuropodial?) chaetae, distributed as
follows:—Each of the segments 2, 3, 4 has a half ring of strong chaetae on
each side; segments 5 to 7 are without chaetae; segments 8 to 14 have
chaetae embedded in the body-wall, but not projecting. The shields cover
the remaining segments; and along the outer edge of each are some ten
bundles of chaetae, and along the hinder edge some five or six bundles,
representing as many segments. Thus the worm consists of about thirty
segments whose outlines are nearly obliterated (as in the _Chlorhaemidae_),
and whose chaetae, except those which are specially developed, are
disappearing: while posteriorly a great shortening of the body brings the
bundles close together. A continuation of this process, involving a further
disappearance of chaetae, leads readily to the condition met with in
_Echiurus_, one of the chaetigerous Gephyrea. Internally, further evidence
of the relation between _Sternaspis_ and the Chlorhaemids with the Gephyrea
is afforded by the absence of septa, by the coiled alimentary canal, and by
the presence of a single pair of nephridia, which in the latter group act
both as excretory organs and as genital ducts.

BRANCH B. CRYPTOCEPHALA.

SUB-ORDER 1. SABELLIFORMIA.[392]

FAM. 1. _Sabellidae._—The branchial crown consists of a usually
considerable number of filaments arising from a semicircular base. The
peristomium may be reflexed to form a collar, {337}which is frequently
notched, so that a lateral and a ventral lobe on each side may be
distinguished (Fig. 133, B, p. 261). The thorax consists of nine segments,
and is provided with ventral "gland shields," which are continued along the
abdomen, where they are subdivided into two by a ciliated "faecal groove,"
which sometimes bends to one side on reaching the thorax, and may extend
forwards along the dorsal surface to the head: this groove serves to carry
the faeces out of the tube. The gill filaments are always provided with
secondary processes, and may be provided with compound eyes.[393] The tubes
of the Sabellidae are always of fine mud or of sand.

_Sabella pavonia_ Sav. is about 10 to 12 inches long and about ¼ inch
across; the tube of fine mud is considerably longer and embedded deeply in
the mud, with its free end projecting to some 2 or more inches, where it
serves for the attachment of seaweed, Polyzoa, Hydrozoa, etc. The colour of
the animal is orange-brown; the gills, which are about 1½ inches long, are
green (due to contained blood) marked with more or less extensive brown or
purple-brown spots, which may even hide the green tint. There is a pair of
dark filaments arising between the dorsalmost gill filaments, which have
been erroneously regarded as "prostomial tentacles"; they are, in fact,
prolongations of a peculiar membrane or lip round the base of the gills,
which bounds a groove leading to the mouth. These lip-processes (Fig. 133,
B, _l_) occur in other Sabellids. Atlantic, North Sea, and Mediterranean.

[Illustration: FIG. 182.—A, A gill filament of _Branchiomma_; B, of
_Dasychone_. _a_, Axis; _f_, secondary filaments; _o_, eye; _x_, dorsal
appendices.]

_Branchiomma vesiculosum_ Mont. forms a sandy tube near low tide-mark. The
animal, measuring 6-7 inches, is rich brown, darker anteriorly, abundantly
speckled with white; the ventral surface is pink; the gills are green or
{338}olive-brown, marked with white bands in a fairly regular fashion. Each
gill filament has, just below its tip, a compound eye, consisting of
several lenses and retinae. North Sea and Mediterranean.

_Dasychone bombyx_ Dalyl. is a short, comparatively stout worm usually 1 to
1½ inches long; reddish-brown in colour, with a darker spot on each side of
every segment. The gills are lighter with greenish marks. This worm may
readily be recognised, for each of the gill filaments carries some six to
ten pairs of dark compound eyes at intervals along its length, and near to
each pair there arise two short processes from the outer side of the
filament, which are known as "dorsal appendices." The worm forms a tube of
mud, more or less mixed with sand. It occurs at low water and to some depth
round the coasts of the Atlantic, North Sea, and Mediterranean.

_Chone infundibuliformis_ Kröyer may be recognised by the absence of lobes
on the collar, the presence of a membrane connecting the gill filaments,
and the passage of the faecal groove along the dorsal surface of the
thorax. The worm is 6 inches long, with purple gills, spotted with
yellowish-white. The tube is formed of yellowish membrane covered with
sand, and is fixed to stones and other objects. _Potamilla reniformis_
Müll. is about 3 inches long, with about twelve brown gill filaments, some
of which have eyes near the base. The tube is transparent and horny, with
sometimes a slight covering of sand. Found in old oyster shells. North Sea,
Atlantic, Mediterranean.

The genus _Spirographis_ contains one of the largest European Sabellids,
_S. spallanzanii_ Viv., which occurs off the Channel Islands and in the
Mediterranean. The two gill plumes are unequal; the large rone forms an
upright, spirally-coiled column.

FAM. 2. _Eriographidae._—_Myxicola infundibulum_ Mont. has its gill
filaments connected by a membrane reaching nearly to their tips. Each gill
plume forms a semicircle; there are no eyes; the peristomium does not form
a collar; no gland shields. The worm requires neither of these structures,
since it is practically a free-swimmer, envelopes itself in mucus, and
moves tail first. The faecal groove is not well marked, though continued
dorsally. In the abdomen the _tori uncinigeri_ extend dorsally and
ventrally {339}(beyond the neuropodial chaetae) and nearly encircle the
body. The animal is 4 or 5 inches long, dull green, with purplish gills.
Between tide-marks. North Sea and Mediterranean.

_Amphiglena mediterranea_ Leyd. is only about ¼ inch long, hermaphrodite,
and has eyes on the peristomium and on the anal segment. It is a very
elegant little worm, and as a living object under the microscope, with the
cilia on the gills, is very beautiful. The gills consist of six filaments
on each side, provided with the usual double row of ciliated processes.

FAM. 3. _Amphicorinidae._—Small hermaphrodite Sabellids in which each gill
tuft contains only a few branching filaments. The simplest form is
_Haplobranchus aestuarinus_ Bourne,[394] which occurs in the rather foul
mud at low tide in the estuaries of the Thames, the Liffey, and other
rivers. The animal is about ¼ inch long, with four finger-shaped processes
on each side, and a pair of larger, vascular processes on the ventral
surface. These five branches are gills (palps), although, owing to the
small size of the worm and simple vascular system, the four lateral
filaments have no blood-vessels. The animal consists of only eleven
chaetigerous segments, and lives in a tube made of mud particles.[395]
_Fabricia sabella_ Ehrenb. (_Amphicora fabricia_ Müll.) has three gills on
each side, each with a number of secondary branches of different sizes, but
so arranged as all to reach the same level. It has eyes in its tail and
swims backwards.

FAM. 4. _Serpulidae._—The thorax is provided with an undulated membrane on
each side, chiefly employed in smoothing the inside of the tube; it
represents the dorsal and ventral cirri of these segments. The gland
shields are confined to the thoracic segments. In many genera the
dorsalmost gill filament on one or both sides is terminally dilated and
serves as an operculum. The tube is calcareous, and attached to rocks,
shells, etc., for a greater or smaller part of its extent.

_Serpula vermicularis_ L. forms a pinkish tapering tube about 3 inches
long; the narrower fixed end is coiled. It is marked at irregular intervals
with encircling ridges, indicating cessation {340}in formation, and has a
circular aperture. The worm itself is about 1½ inches long. The horny
operculum is conical, with its base upwards, fringed with short processes.
20 fathoms in the North Sea and Mediterranean; also from 275 fathoms off
the west coast of Ireland.

_Pomatoceros triqueter_ L.—The white shell is adherent, with a distinct
keel along its upper surface; the aperture is overhung by a spine. The
tubes are abundant everywhere, attached to rocks, stones, shells, etc.,
between tide-marks and down to 18 fathoms. The animal is very handsome, the
thorax being deep blue, the abdomen red in the female, whitish in the ripe
male. The branchiae are barred and spotted with blue, orange, and white;
the operculum is calcareous, and furnished with a couple of horn-like
processes.

[Illustration: FIG. 183.—A group of tubes of _Serpula vermicularis_ L.,
from the mouths of two of which the animals are protruding, that on the
right being seen from above. _br_, The gill plume; _m_, thoracic membrane;
_op_, operculum; _op^1_, corresponding gill filament of the opposite side;
_t_, tube. (From Cuvier's _Règne Animal_.) Nat. size.]

_Filigrana implexa_ Berkeley is a small worm, but the slender white tubes
intertwine and adhere together in masses 3 or 4 inches high, occurring at
low tide and down to 18 fathoms in the North Sea and Mediterranean. The
animal has only eight gill filaments on each side, one of which on each
side is slightly expanded to serve as an operculum. The worm multiplies by
transverse division.

_Spirorbis borealis_ Daudin is a still smaller worm, the tube of which is
coiled in a flat spiral about 1/16 to ⅛ inch across; it is {341}common,
adhering to _Fucus_, shells, and other objects. It is represented by
fossils in the Palaeozoic rocks. Cosmopolitan.

_Protula_ (_Psygmobranchus_) _tubularia_ Mont. is a Serpulid without an
operculum; it forms a straight or slightly and irregularly curved tube.
Atlantic and Mediterranean. _Salmacina dysteri_ Huxley has no operculum; it
is a small worm incrusting seaweeds, or forming masses like _Filigrana_.

SUB-ORDER 2. HERMELLIFORMIA.

The single family _Hermellidae_ is represented by two species—_Sabellaria
alveolata_ L., which is littoral, and _S. spinulosa_ Leuck., occurring in
10 to 30 fathoms.

_S. alveolata_[396] is about an inch long; the thorax is purple, the
abdomen yellow to red. The narrow caudal region is bent sharply forwards,
so that the anus, situated at its tip, comes to lie at the orifice of the
tube, which is irregular and sandy. Great numbers of the animals live
together, so that the masses of their tubes may be 2 or 3 feet thick and
several feet long. They are well seen on the shore, at Paignton, near
Torquay, and on Hilbre Island, off the Cheshire coast. North Sea, Atlantic,
Mediterranean.

[Illustration: FIG. 184.—_Spirorbis._ A, the shell, enlarged. B, the
animal, × 50. _c_, Peristomial collar; _e_, eggs in the brood pouch at the
end of the operculum; _g_, gills; _m_, thoracic membrane (characteristic of
Serpulidae); _n_, the single pair of thoracic nephridia opening by a median
dorsal pore beneath the collar (common to all Sabelliformia); _o_, ova in
the anterior abdominal segments; _op_, operculum; _s_, spermatozoa in the
hinder abdominal segments; _st_, stomach. (From Claparède.)]

ORDER III. MYZOSTOMARIA.[397]

These animals are parasitic on Crinoids or Asteroids.[398] The {342}single
family, Myzostomatidae, contains but two genera, _Myzostoma_ F. S. Lkt. and
_Stelechopus_ v. Gr.

Some of them move, more or less actively, on the surface of their hosts,
others live in a sessile condition between the joints of the arms or
pinnules, causing a greater or less malformation thereof, sometimes leading
to the formation of a more or less globular cyst like a plant-gall, due to
overgrowths of the joint, as in _M. deformator_ v. Gr. and _M. cysticola_
v. Gr.: while _M. pulvinar_ v. Gr. is endoparasitic in the intestine. Two
species occur on our common _Antedon_ (_Comatula_) _rosacea_; one, _M.
cirriferum_ Lkt., creeps about the oral surface, especially along the food
grooves of the disc and arms; the other, _M. glabrum_ Lkt., lives close to
the mouth of the Crinoid, so that its pharynx can be inserted into the
oesophagus of the host; this species rarely moves from this position, and
carries a young one on its back.

[Illustration: FIG. 185.—_Myzostoma glabrum_ Lkt., on the disc of _Antedon
rosacea_. The hermaphrodite individual (⚥) lies over the mouth of the
_Antedon_, and carries on its back at the anterior end a young one (♂) with
only male organs fully developed. _ap_, The anal papilla of _Antedon_. ×
4.]

The Myzostomaria are circular or oval, more or less markedly convex
dorsally, flat ventrally; _Stelechopus_, however, which lives on
_Hyocrinus_, is elongated. The margin of the body is provided with ten or
more pairs of cirri, short (_M. glabrum_) or long (_M. cirriferum_), and
the general appearance of the animal is greatly changed in some species by
the great elongation of the hinder cirri, into which the viscera may extend
(_M. filicauda_ v. Gr.). On the ventral surface are five pairs of small
conical "parapodia," arranged, like the internal organs, in a radiate
manner. Each parapodium carries a couple of chaetae; one a hook, the other
serving as a "guide" for this hook. The four "suckers" on each side are
either glandular or sensory organs; and Wheeler considers them homologous
with the lateral organs of Capitellids; they are usually little developed
in those species which live inside cysts.

{343}[Illustration: FIG. 186.—_Myzostoma cirriferum._ (After Lang and
v. Graff.) The organs are supposed to be seen by transparency. On the right
side the more dorsal organs are shown, and on the left, those lying more
ventrally. _a_, Anus; _c_, ten pairs of marginal cirri; _f^1_ to _f^5_, the
five parapodia of the left side, each with two chaetae; _i_, the branches
of the intestine on the right side; _m_, mouth; _o_, the opening of the
oviduct (_od_) into the rectum; _ov_, the uterus or coelom, filled with
eggs, occupying the spaces between the lobes of the intestine; _p_, the
pharynx (acrecbolic introvert) lying in the pharyngeal sac; _r_, rectum;
_s_, the four "suckers" of the left side; these, like the parapodia, really
lie on the ventral surface; _st_, stomach; _t_, the branching testis; ♂,
the pore of the sperm-duct.]

The mouth and anus are usually ventral; but in _M. glabrum_ the anus is
dorsal, and in a few species both apertures are carried on to the back by
the great development of the ventral surface. The alimentary canal is
provided with a protrusible pharynx; the intestine is branched; amongst its
branches is the coelom, packed with eggs, and functioning as a uterus
(usually called "ovary"). The true ovary is a small mass of cells on each
side, a proliferation of the coelomic epithelium covering the intestinal
wall. A median continuation of the {344}uterus passes backwards above the
rectum, and opens either into it or by an independent pore dorsal to the
anus. The "lateral oviducts" of Nansen are nephridia with ciliated funnels
opening into the coelom (uterus), and with pores leading into the cloaca on
its ventral surface; or, in _M. belli_ Wheeler, opening to the exterior.
The two testes are branched, and each sperm-duct opens laterally on a
papilla, just outside the third parapodium of each side. Wheeler[399] has
recently shown that in the young _Myzostoma_ the spermatozoa ripen before
the ova, so that it is functionally a male; before the spermatozoa are all
discharged the ova mature, and the animal is for a time hermaphrodite;
later on, however, when all the spermatozoa are used up, the worm is a
female. Beard's "dwarf males" are therefore merely the young of
hermaphrodite forms. In cysticolous species each cyst usually contains a
large female individual and a small male. In these cases the young one
(male) discharges all its spermatozoa before the ova ripen, so that a
period of immaturity intervenes and a true hermaphrodite condition is
omitted; the animal is at first male, and later female. The Myzostomaria
are thus "protandric hermaphrodites."

The affinity of these animals has been much discussed; they superficially
resemble the Tardigrada in many anatomical features, and differ greatly
from Chaetopoda, but as they possess the characteristic chaetae or
parapodia, and pass through a larval stage[400] similar to that of the
Polychaetes, there is no doubt that they are closely allied to the group,
and indeed may be regarded as degenerate Chaetopods. It has been suggested
that they form a passage group between them and the Tardigrada; and von
Graff forms a group Stelechopoda, to include the Myzostomaria, the
Tardigrada, and the Linguatulida.

OLIGOCHAETA (EARTHWORMS, ETC.) AND HIRUDINEA (LEECHES)

F. E. BEDDARD, M.A. (Oxon.), F.R.S.

Prosector to the Zoological Society

{347}CHAPTER XIII

OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES)

INTRODUCTION—ANATOMY—REPRODUCTION—BIONOMICS—DISTRIBUTION—CLASSIFICATION—
MICRODRILI AND MEGADRILI

The Oligochaeta form a well-marked branch of that exceedingly large
assortment of animals vaguely spoken of as worms, and embracing a number of
types many of which have no near relationship to each other. From this
great and unnatural group, which has survived as "Vermes" even in some
quite modern text-books, we can separate off those forms which show a plain
segmentation or division of the body internally as well as externally into
a series of more or less similar rings, as a Class Chaetopoda. This Class,
consisting of the Orders mentioned on p. 241, includes the worms which form
the subject of the present chapter—the Oligochaeta, as they were originally
called by Grube, on account of the fewness of their chaetae as compared
with the number possessed by the majority of the Polychaeta.

Our knowledge of this group, as of so many others, dates from Aristotle,
who called the earthworms the "intestines of the earth." But it is only
very recently that the numerous and remarkable genera of exotic earthworms
have been anatomically investigated; indeed the common British species was
not really well known before the publication of the memoirs of
Lankester[401] and Claparède[402] in 1864 and 1868, in spite of the
elaborate quarto devoted to it by Morren,[403] the botanist, in 1826.
{348}Some of the aquatic species afforded material to Bonnet and
Spallanzani for their experiments upon the powers of regeneration of the
animals when cut into fragments, while the work of O. F. Müller[404] upon
various Naids is a monument of careful anatomical description. Our
knowledge of the aquatic Oligochaeta does not appear to have advanced so
rapidly as has that of the earthworms.

EXTERNAL CHARACTERS.—The most salient external characteristic of this group
of worms, which vary from 1 mm. to 2 metres in length, is of course the
segmentation. The entire body is divided into a number of rings, which are
for the most part similar to each other; a fragment of an earthworm's body
could not be accurately replaced unless it had been cut from the anterior
region. There is precisely the same regular segmentation in the aquatic
representatives of the Order. At the anterior end of the body in the common
earthworm (and in nearly all Oligochaeta) is a small unpaired lobe, which
overhangs the mouth, and is usually termed the prostomium; the mouth itself
is surrounded by the first segment of the body, which never bears any
chaetae in any Oligochaete. The prostomium is occasionally greatly
developed, and in such cases doubtless forms a tactile organ of importance.
This is especially the case with the South American genus _Rhinodrilus_,
where the lengthy prostomium can be retracted at will. The aquatic _Nais
lacustris_ (= _Stylaria proboscidea_) has also an exceedingly long
prostomium, which cannot, however, be retracted, though it is contractile.
At a certain distance from the anterior end of the body, fixed for the
species, but varying greatly from genus to genus and from species to
species, is the clitellum. This region of the body (popularly believed to
mark the spot where a worm divided by the gardener's spade has come
together again) is associated with the reproductive function, and serves to
secrete the cocoon in which the creature's eggs are deposited. It has in
the earthworm a thick glandular appearance. A more minute examination of
the worm's body will show the orifices of the reproductive ducts and of the
excretory organs which will be found described below. In addition to these,
all British earthworms and a large percentage of the tropical forms have a
row of pores along the back, which are between the successive segments in
the median line. These "dorsal pores" {349}open directly into the
body-cavity, and are mere perforations of the body-wall, not tubes lined by
a special layer of cells. Professor Spencer of Melbourne[405] has observed
a giant earthworm (_Megascolides australis_) of Gippsland which, when held
in the hand, spurts out, to a height of several inches, the fluid of the
body-cavity through its dorsal pores. The burrows, he remarks, are coated
over with the same fluid, which is regarded by him as a lubricant. This,
however, considering that the glandular cells of the epidermis can secrete
a mucous fluid, seems to be an expensive use to which to put the important
fluids of the interior of the body. It is more probable that the dorsal
pores are a means of getting rid of waste products. Lim Boon Keng[406]
suggests that the coelomic fluid possesses a bactericidal function. The
dorsal pores are missing in many earthworms, and without exception in those
Oligochaeta which live in water; but these latter worms have a pore upon
the head, which appears to be wanting in the earthworms. Dr. Michaelsen has
thought that the head-pore serves to relieve the brain from undue watery
pressure—to act, in fact, as a kind of safety-valve for the liberation of
superfluous fluid.

In some foreign worms the pores of the reproductive ducts are conspicuous
external features (Fig. 197); even in our British species the turgescent
male apertures upon the fifteenth segment are sometimes quite obvious.

STRUCTURE OF THE BODY-WALL.—The body-wall consists in all Oligochaeta of
three recognisable sheets of tissue. Outside is the epidermis, which always
consists of a single layer of cells, except in the clitellar region of
earthworms. It is a point of difference between the aquatic genera and the
terrestrial forms that in the former the clitellum is only one cell thick,
while in the higher Oligochaeta it is made up of more than one layer of
cells. The epidermis is ciliated only in the genus _Aeolosoma_, and there
only on the prostomium. It secretes a thin layer of chitin, which is
defective opposite to the glandular cells, and becomes therefore perforated
by numerous pores. The structure of the epidermis of _Lumbricus_ has been
studied by Cerfontaine, whose recent account[407] of the same is the
fullest and most accurate that exists.

Underneath the epidermis comes a layer of circular muscle-fibres, {350}and
underneath this again a layer of longitudinal muscles. In both layers the
fibres have a softer core, outside which lies the radially striated
muscular substance. The fibres are embedded in a granular matrix. It used
to be considered at one time that such medullated fibres were distinctive
of leeches as opposed to Oligochaeta. Their existence has been really known
in the Oligochaeta since the researches of Ratzel; but Cerfontaine has
fully described them, and emphasised the fact that the fibres of both
circular and longitudinal coats are alike in this respect.

[Illustration: FIG. 187.—Chaetae of Oligochaeta. × 10. (After Michaelsen,
Stolc, and Vejdovsky). 1, 2, Penial chaetae of _Acanthodrilus georgianus_;
3, _Spirosperma_; 4, _Ilyodrilus_; 5, _Lophochaeta_; 6, _Tubifex_; 7, 8,
_Nais_; 9, _Bohemilla_. Figs. 3-9 are ordinary chaetae.]

CHAETAE.—The passive organs of locomotion in these animals are the chaetae,
which are absent in only one family, Discodrilidae, and in one other genus,
_Anachaeta_. In this latter worm the chaetae are represented by large
glandular cells, which seem to correspond to the cells from which the
chaetae arise in other forms. They are in this case, as in the others,
cells of the epidermis. The chaetae of the Oligochaeta are not quite so
variable in form as in the marine Polychaeta (see Fig. 138, p. 267). Figs.
187 and 188 illustrate some of the principal shapes which these bristles
assume. The most prevalent form is an elongated S, which has been aptly
compared to the mathematical sign ∫. This kind of chaeta is found in all
earthworms, and in not a few aquatic genera such as the Lumbriculidae. In
some of the latter and in the Tubificidae and Naids there is the same form
of chaeta, which is cleft at the free end, and possibly enables the worm to
grasp the leaves of aquatic plants, and otherwise facilitates progression
in a laxer medium than the stiff soil frequented by the earthworms. Even
earthworms, at any rate the genus _Pontoscolex_, have chaetae of this kind;
some of the aquatic Oligochaeta have elongated and hair-like bristles, such
as that of _Tubifex_.

{351}[Illustration: FIG. 188.—Chaetae. × 10. 1, _Onychochaeta_; 2,
_Pontoscolex_; 3, _Trichochaeta_; 3_b_, the same, more highly magnified.]

In the Tubificid _Lophochaeta_ (Fig. 187, 5) the chaetae are ornamented on
both sides with delicate processes, which give them the appearance of
Crustacean hairs. Among earthworms the simple S-like form is sometimes
complicated by the development of sinuous ridges upon the distal end. No
doubt these bristles enable their possessor to get a firmer grip of
adjacent objects; they are very commonly found, in the family
Geoscolicidae, upon the segments of the clitellum, and permit of a firmer
union during sexual congress. In no Oligochaeta are the chaetae borne upon
parapodia, as is the case with the Polychaeta; but in many of the aquatic
forms there are a considerable number to each bundle. In earthworms the
number of chaetae varies greatly. The common earthworms of this country,
belonging to the genera _Lumbricus_, _Allolobophora_, and _Allurus_, have
only eight chaetae upon each segment of the body, and these are then, as a
rule, arranged in pairs or rather couples, two of each on each side of the
body. The genus _Perichaeta_ and some of its allies have a much larger
number of chaetae to each segment, disposed in a continuous row round the
middle of the segment. The intermediate condition is to be seen in the
genus _Deinodrilus_, where there are twelve in each segment, and in certain
members of the genus _Megascolex_, where there are eight in each segment in
the anterior region of the body, the number increasing in the posterior
segments. The four bundles of chaetae in the Naids and Tubificids have been
likened to the {352}notopodia and neuropodia of the Polychaetes; but it
does not seem certain that this comparison is justifiable. It was at one
time thought that the continuous circle of chaetae of the Perichaetidae was
the primitive condition; but Professor Bourne has lately found that in
_Perichaeta_ the young embryos have not got this continuous circle; it is
only acquired later.

BRANCHIAE.—The Oligochaeta were called by Cuvier the "Annélides abranches
sétigères." But the epithet "abranches" is now known to be inaccurate. In
fact it really was so when Cuvier wrote; for naturalists were at that time
well acquainted, chiefly through the elaborate work of O. F. Müller, with
the little fresh-water Naid _Dero_, the posterior extremity of which is
provided with a varying number of branchial processes. These are furnished
with looped blood-vessels and are covered externally by cilia, so that the
water containing oxygen is constantly renovated. The second instance of a
gilled Oligochaete was discovered in the very same family. Professor
Bourne[408] of Madras found in "tanks" a Naid which he named
_Chaetobranchus_, in which the head segments, to the number of fifty or so,
are provided with long ciliated processes, which as a rule enclose the
dorsal chaetae of their segments, and in addition a capillary loop.
Curiously enough, this very same worm made its appearance in the _Victoria
regia_ tank at the Botanical Gardens in the Regent's Park, whither it had
in all probability been accidentally imported. Two members of the family
Tubificidae were the next examples of gilled Oligochaeta made known to
science; one of these, _Branchiura sowerbyi_,[409] appeared also in the
Botanical Gardens, so that its native home is unknown. It differs from
_Chaetobranchus_ in that the gills are at the posterior end of the body,
and are contractile; during the life of the worm they are in continual
motion. A species of the South American genus _Hesperodrilus_,[410] _H.
branchiatus_, is also gilled, and, so far as can be made out from a
spirit-preserved specimen, the gills are precisely of the same pattern and
contractility as those of its ally _Branchiura_. Possibly _Branchiura_
ought to be included in the same genus with _Hesperodrilus_. A worm which
was originally described by Grube as _Alma nilotica_, should really have
been placed before the three last-mentioned {353}instances; but as this
worm was only known from a fragment, and as the description was not by any
means full, it was not thoroughly believed in; it was surmised that it
might be a member of some marine genus, perhaps of the Capitellidae. Oddly
enough, the same worm was independently described by a different name,
_Digitibranchus niloticus_, a few years later by Levinsen. Quite recently
Michaelsen has found by a reference to the original types that this worm is
really gilled, and that it is specifically identical with a worm which had
been given a totally different name, viz. _Siphonogaster_. The fact that
the gills of the latter had been overlooked was readily explained by the
circumstance that they are retractile, and not merely contractile. But all
the species of the genus _Siphonogaster_, or _Alma_, as it ought really,
following the rules of priority, to be called, have not got gills, as is
the case too with the genus _Hesperodrilus_. The gills of _Alma_ are
branched, and there is therefore no longer any justification whatever for
defining the Oligochaeta as a group of Annelids without gills. The simple
gill-like processes of _Chaetobranchus_ might have been held to be not
accurately comparable to the more complex structures which we find in the
marine worms.

[Illustration: FIG. 189.—Transverse section through _Branchiura sowerbyi_.
× 20. _d.br_, Dorsal branchia; _i_, intestine; _n_, nerve-cord; _v.br_,
ventral branchia.]

NERVOUS SYSTEM.—The central nervous system of the Oligochaeta is very
uniform in its structure in the entire group. The only family which is at
all anomalous is that of the Aphaneura. In _Aeolosoma_ there appears to be
only a pair of cerebral ganglia, which retain the primitive position of
these organs in being still in direct connexion with the epidermis. In all
other Oligochaeta there are a pair of cerebral ganglia, {354}connected by a
circumoesophageal commissure with a ventral ganglionated cord. From the
cerebral ganglia arises a system of nerve-fibres and nerve-cells, which
represents the stomatogastric nerves of other Invertebrates.

SENSES AND SENSE-ORGANS.—The only organs that can be regarded with anything
like probability as sense-organs are the pigmented eyes of certain Naids
and the tactile cells of many worms. The latter are usually elongated cells
provided at their free extremity with a stiff process; they occur
associated in groups, and often these bundles of cells have a segmental
arrangement. The head end of many of the lower Oligochaeta, for instance
the genus _Aeolosoma_, has delicate processes projecting here and there;
these appear to be also of a tactile nature, and are of course connected
with cells of the epidermis. The eyes of certain Naids are little more than
lenticular bodies embedded in a mass of pigment. In the genus _Eudrilus_
and in many Eudrilidae are peculiar integumental bodies, which were
independently discovered by Dr. Horst[411] and myself, and compared by us
to the Pacinian bodies of Mammals. Whether these structures are connected
with nerves or not is doubtful. In spite of the poor development and the
simplicity of their sense organs, the higher Oligochaeta at any rate can
feel, and can distinguish light from darkness. Darwin[412] came to the
conclusion that "light affects worms by its intensity and its duration."
And furthermore, it is only the anterior end of the body which is thus
affected. Of the sense of hearing these animals appear to be utterly
devoid. Some kept by Darwin "took not the least notice of the shrill notes
from a metal whistle, which was repeatedly sounded near them; nor did they
of the deepest and loudest tones of a bassoon." But it is always necessary
to discriminate between sound and vibrations passing through any solid
body, which would appeal rather to a sense of touch. Here worms are most
sensitive. It is quite easy, by digging with some vigour, to arouse the
worms in the neighbourhood, who will crawl to the surface and away from the
scene of action; a proceeding on their part which is sometimes put down to
a desire to escape from their enemy the mole.

Smell appears to be another sense which is somewhat deficient. {355}But
worms are epicures, and exhibit a decided taste and preference for certain
articles of diet. Like their fellow tiller of the soil, the agricultural
labourer, worms have a keen relish for onions, which, however, they must
recognise by the smell. They prefer green cabbage to red, celery to both,
and raw meat appears to be the greatest delicacy that can be offered to
them. It is only substances they are not likely to meet with, such as
perfumes, tobacco, and paraffin, that produce no impression upon the worm's
sense of smell.

COELOM AND VASCULAR SYSTEM.—When an earthworm is dissected the various
organs are seen to lie in a fairly spacious cavity, which is interrupted
and divided into a series of chambers by the mesenteries or septa which
stretch across from wall to wall of the body, and correspond roughly in
their position to the grooves which separate the body externally. This
cavity, common to all the higher animals, is known as the coelom; it is
lined by cells, which cover the intestines as well as the inside of the
body-wall; and upon the intestine assume the form so characteristic of the
group, namely, that of large yellow cells loaded with secreted matters, and
called "chloragogen-cells" by Claparède. The coelom communicates with the
exterior by means of the dorsal pores, the nephridia, and the ducts of the
reproductive organs. As in all animals which possess a coelom, the
reproductive tissues, ova and sperm, are developed on its walls.

[Illustration: FIG. 190.—_Sparganophilus tamesis_; general anatomy, × 3.
(After Benham.) I-XVIII, segments. 1, 4, 6, Perivisceral vessels (6 is one
of the hearts); 2, 3, 7, dorsal vessel; 5, spermatheca; 8, sperm sacs; 9,
intestino-tegumentary vessels; 10, ovary; 11, 12, integumentary vessels.]

The vascular system of the Oligochaeta forms a system of perfectly closed
vessels, which ramify into fine capillary networks in the body-wall, in the
coats of the alimentary canal, and upon {356}the other organs of the body.
The main trunks are a dorsal and a ventral longitudinal, which communicate
directly in the anterior end of the body by large transverse contractile
trunks, the so-called hearts (see Fig. 190, 6). The dorsal vessel is also
contractile, but not the ventral, or, when it occurs, the subnervian. The
vascular system has many degrees of complexity in different families; it is
simpler in the smaller aquatic forms. The blood is usually red, and the
pigment which is suspended in the plasma is haemoglobin. The blood is
corpusculated.

EXCRETORY ORGANS.—There appears to be a great deal more variation in the
structure of the excretory system than there is in many other groups. For a
long time only _Lumbricus_ and a few of the aquatic genera were known as
regards their excretory systems. In these there is a pair of excretory
organs or nephridia in nearly all the segments. These are much coiled
tubes, in which it is always possible to recognise three divisions. The
nephridium commences with an orifice of a funnel-like character, fringed
with long cilia, and opening into the body-cavity; from this springs a
tube, which immediately perforates the septum lying between the segment
which contains the funnel and the following one; this tube has the
peculiarity first pointed out by Claparède of being excavated in the
substance of cells; the glandular part of the nephridium is a row of cells
which are bored through by a continuous canal, the walls of which are here
and there furnished with cilia. It often happens that the main canal gives
off minute lateral ramifications, which may even form a kind of network
round the principal canal. The terminal section of the nephridium is a
muscular sac which opens on to the exterior by a pore, and from which the
products of excretion are from time to time evacuated by contractions of
its walls. This is a brief statement of the main facts in the structure of
those Oligochaeta in which there is a single pair of nephridia to each
segment of the body; small differences of more or less importance occur. In
_Chaetogaster_, for example, there is no trace of a funnel; in some genera
the terminal sac is much reduced or unusually extended, being even
sometimes provided with a caecum of moderate dimensions. In _Acanthodrilus
novae-zelandiae_ and a few other species the point of opening of the
nephridia varies from segment to segment, though it always bears some
relation to the chaetae. In these {357}species the nephridia which open
more dorsally are a little different in structure from those which open
more ventrally. One set have a caecum, and the other have not.

The nephridia of the terrestrial forms are enveloped by a richly developed
network of blood capillaries, which is absent in the smaller aquatic
genera.

A very remarkable genus, _Brachydrilus_, has lately been described by Dr.
Benham,[413] in which each segment has two pairs of nephridia instead of a
single pair. More recently, certain Australian forms, which I propose to
unite on this account into a genus _Trinephrus_, have been discovered which
have no less than three distinct and separate pairs in each segment.[414]

[Illustration: FIG. 191.—Section through body-wall of _Megascolides
australis_, highly magnified. (After Spencer.) 1, 4, 5, 6, Coils of
nephridia; 2, funnel; 3, septum; 7, external apertures.]

In many Megascolicidae there is a nephridial system of a different
character. In _Perichaeta_ when dissected the nephridia appear, on account
of their minute size, to be altogether absent. There is, however, in most
Perichaetidae, in many Acanthodrilidae, and in many Cryptodrilidae a mass
of minute tubules which cover the inside of the body-wall, and open on to
the exterior by innumerable openings; there may be in a single segment one
hundred or more of these external orifices, which are scattered about
irregularly. It is at present uncertain whether these minute tubes are
connected among {358}themselves, thus forming a network passing through the
septum and from segment to segment, or whether each tube is isolated from
its fellows, and forms a distinct nephridium, of which there are many in
each segment and entirely separate. This is, however, certain, that the
complex nephridial systems of at any rate _Octochaetus_ and _Megascolides_
are derived from the multiplication of a single pair of tubes which are
alone present in the embryo. In _Perichaeta_ the minute nephridia are
furnished with coelomic funnels; in _Octochaetus_ they are not, except in
the case of certain nephridia which open into the terminal section of the
intestine.

Both at the anterior and at the posterior end the nephridia occasionally
open into the alimentary canal. In various genera the first pair of
nephridia are larger than the others, and open into the buccal cavity; it
seems likely that they serve as salivary glands. A somewhat similar
condition of things exists in _Peripatus_ (vol. v, p. 17). In _Octochaetus
multiporus_, for example, there is a large tuft of nephridial tubes in the
anterior region of the body, which opens by a long muscular duct into the
buccal cavity. In the same species a good many of the nephridial tubes open
into the posterior section of the intestine, reminding one of the anal
vesicles of the Gephyrea (p. 436) and of the Malpighian tubes of the
Arthropods.

In many Eudrilidae the ducts of the paired nephridia form a network in the
body-wall, which opens on to the exterior by many pores.

ALIMENTARY CANAL.—The digestive tube is perfectly straight in nearly all
Oligochaeta. Only in _Plagiochaeta_ and a species of _Digaster_ is it
twisted in the intestinal region in a corkscrew-like fashion. The mouth is
under the buccal lobe (where, as in the majority of cases, this is
present); the anus is mostly terminal, or rarely, _e.g._ _Criodrilus_, a
little in advance of the end of the body on the ventral side. In the
simpler forms three regions can be distinguished, which are themselves
simple in structure. The mouth leads into a buccal cavity, which in its
turn opens into the pharynx; the latter is muscular, with thick walls. The
narrower oesophagus opens into the wider intestine, which opens
posteriorly, as already stated. In the earthworms there is as a rule some
complication. The oesophagus bears certain glandular appendages, the
calciferous glands; and a part of {359}it is modified into a gizzard. The
gizzard is merely a portion of the oesophagus with very much thickened
muscular walls and with a stout lining of chitin. It is not universally
present among earthworms, and when present varies much in position. The
rule is that one gizzard only is present. In _Digaster_, as is implied by
the name, and in some other forms there are two in successive segments; in
_Trigaster_, as the name also indicates, there are three gizzards; in
_Moniligaster_ and the Eudrilids _Hyperiodrilus_ and _Heliodrilus_ there
are four to six; and a few other forms also have a considerable number of
gizzards. The calciferous glands are diverticula of the oesophagus with
folded and sometimes ciliated walls; their epithelium secretes calcareous
particles, which are frequently of crystalline form. Darwin supposed that
this secretion was provided in order to negative the humus-acids of the
soil which is the food of earthworms. These organs are usually paired, but
in the Eudrilidae there are unpaired as well as paired glands; the unpaired
calciferous glands lie ventrally. These glands are totally wanting among
the aquatic families, with the sole exception of the Enchytraeidae. In a
few of these there are either paired or single glands of a very similar
nature; Dr. Michaelsen has suggested that the function of these is rather
absorptive than secretory. From the median unpaired gland of _Buchholzia_
arises the dorsal vessel, which at first forms a sinus round the glandular
epithelium; the epithelium, like that of the nephridia, is perforated by
the ducts. In certain Oligochaeta there are some curious modifications of
the calciferous glands. In _Stuhlmannia_ and a few other Eudrilidae the
oesophagus is beset with a larger number of paired structures than in any
other genera of the family, where the calciferous glands are more limited
in number. These glands consist of a short tube lined with epithelium
opening into the oesophagus. Round this is a mass of cellular tissue, but
the outlines of the constituent cells are lost; the whole is permeated with
abundant blood-vessels. This layer seems to be peritoneal, and the entire
gland seems to have lost its function as a secretory organ, and to have
taken on some function in connexion with the vascular system. An analogous
modification is to be found among the Enchytraeidae. In certain forms there
is a structure known as the cardiac body; this is a chord of cells lying in
the dorsal blood-vessel at the point where it springs from the intestine.
It is tempting to regard this cellular rod as being {360}the altered dorsal
glandular pouch already spoken of, which is surrounded by a blood sinus.

REPRODUCTIVE ORGANS.—All the Oligochaeta are hermaphrodite animals. But, as
is the case with other hermaphrodites, the male and female organs are in
many cases mature at different times, thus leading to a practical
unisexuality. Many of the aquatic forms appear to have fixed times for
breeding, which may be in the winter or in the summer; but the earthworms
are as a rule sexually mature the whole year round. Various accessory
organs are developed in the majority of cases. In all, the reproductive
glands lie in successive segments and are attached to the septa, from the
peritoneal covering of which they originate. Their actual position differs
greatly in different genera; the position is constant only in the
earthworms, where the testes are in the tenth and eleventh segments and the
ovaries in the thirteenth, in exactly corresponding situations. A few
earthworms have only one pair of testes. The only exception, among
terrestrial forms, to the position of the generative organs is in the
family Moniligastridae, which show so many other affinities to the lower
forms of Oligochaeta. In this family the ovaries have moved one or two
segments forward. Among the fresh-water families the position of the testes
and of the ovaries is not so uniform. They are generally more anterior than
in the terrestrial genera, particularly the ovaries.

One of the chief differences between the Oligochaeta and the Polychaeta is
that the reproductive organs of the former have special ducts to convey
their products to the exterior. In _Aeolosoma_, the only exception to this
rule, Dr. Stolc[415] has shown some reasons for believing that certain
nephridia, but slightly altered in form, serve as the conduits of the
spermatozoa, whilst the ova are extruded through a pore upon the ventral
surface of the body. In the Enchytraeidae the same pore for the extrusion
of the ova appears to exist; but a nearer examination shows that it is
really not a mere perforation of the integument, like the dorsal pores, for
example, but that its internal orifice is fringed with cells which seem to
represent a rudimentary oviduct; perhaps _Aeolosoma_ typifies a last stage
in the reduction. Even so high in the scale as in the genus _Nemertodrilus_
(Eudrilidae), there is an oviduct which can only be compared with that of
the {361}Enchytraeidae. Elsewhere the oviducts are a pair of tubes with a
wide, funnel-shaped, and ciliated mouth, which leads to the exterior by way
of a ciliated tube of varying length.

The sperm-ducts are of an essentially similar structure; but they are
commonly much longer, passing through a variable number of segments on
their way to the exterior. In most earthworms there are, moreover, two of
them on each side instead of only a single pair, as is the case with the
oviducts. Among the Tubificidae, Naids, and other aquatic families there
are only two sperm-ducts, one on each side of the body. But this is not a
character of the aquatic families, for the Lumbriculidae have generally two
pairs, as in the earthworms. It is, however, a rule with hardly an
exception, that among the aquatic Oligochaets the sperm-ducts open, as do
the oviducts in all Oligochaets, upon the segments following that which
bears internally the ciliated funnel. It is only in the Moniligastridae
among earthworms that the sperm-duct only traverses two segments in its
course. But where it is short as regards the actual distance traversed
between the two extremities, the tube itself is commonly long and coiled.

Sometimes, as in our common earthworms, the sperm-duct opens directly on to
the exterior of the body, the lips of the external orifice being swollen by
the development of cutaneous gland-cells. In the majority of cases the
sperm-duct or ducts open near or into a glandular structure which in
earthworms has been called "prostate"; in the aquatic forms, on the other
hand, "atrium." As these terms are objectionable from the different way in
which they have been used for structures of Vertebrates, I have suggested
for both the term "spermiducal glands," indicating the identity of the
structure in all Oligochaeta. The number of pairs of these glands varies,
as does also their shape and size. The typical form is perhaps illustrated
in the lower Oligochaeta, where there is but a single pair into which the
sperm-duct or ducts of the same side open. The Naids, Tubificidae,
Lumbriculidae, and Moniligastridae have a simple gland of this description
on each side of the body. These glands may consist of a tuft of pear-shaped
glandular cells attached to the organ at one side, as in most Tubificidae,
or of a complete investment of gland-cells, as in _Branchiura_. Among
earthworms it is only the Moniligastridae and the Eudrilidae in which the
sperm-duct opens directly into {362}the end of the spermiducal gland; in
the Perichaetidae the gland is differentiated into two sections; there is a
muscular duct leading to the exterior, and a lobate glandular part, which
is formed by a complicated branching of a single sac such as exists in the
Tubificidae; in the Acanthodrilidae and in many Cryptodrilidae the
spermiducal glands are of a tubular form and are not branched, though there
is the same differentiation into a duct and a secreting portion. There are
in the Acanthodrilidae two pairs of these, and as many as three pairs in
_Dichogaster_; in the latter case in three successive segments. In the
Acanthodrilidae the glands are upon the seventeenth and nineteenth
segments. In most Cryptodrilidae the sperm-ducts do not open into the duct
of the spermiducal gland, but on to the body-wall near to its orifice, the
distance varying in different genera. In the Acanthodrilidae the male pore
is on the eighteenth segment, removed therefore by the distance of a
segment from the aperture of either of the glands. It may be that a large
series of structures which exist in _Microchaeta benhami_[416] and in other
Geoscolecids, and which have been termed copulatory glands, are the
equivalents of the spermiducal glands.

[Illustration: FIG. 192.—Diagrammatic longitudinal section of _Lumbricus_,
showing the generative segments. × 3. (After Hesse.) _sp_, Spermathecal
pore; _t_, testis; _s.s_, seminal sac; _sp.s_, sperm-sac; _o_, ovary;
_e.s_, egg-sac; ♀, female pore; ♂, male pore.]

In many earthworms there are, at the external opening of the male ducts,
bundles of specially modified chaetae, which have been called, from their
supposed function, penial chaetae; they are usually ornamented at the free
end with spinelets or ridges, and frequently offer valuable specific
characters. In the Lumbricidae and the Geoscolicidae there are modified
chaetae upon the {363}clitellum; in a few forms, such as, for example,
_Acanthodrilus schmardae_, the spermathecae have bundles of similar chaetae
in their neighbourhood, often associated with glands not unlike the
spermiducal glands.

In most, perhaps in all Oligochaeta the sperm is not matured in the testes,
or even in the body-cavity; it is received into special sacs which are
called sperm-sacs, and there ripens. These sacs, the vesiculae seminales,
have been shown to be outgrowths of the septa; their cavity is thus a
portion of the body-cavity shut off more or less completely from the
general body-cavity.

The reproductive organs of the Eudrilidae, and particularly the female
organs, are so divergent in many particulars from those of other
Oligochaeta that it is convenient to treat them separately. The testes are
normal, save that they are often adherent to the posterior wall of their
segment, as, however, is the case with some other earthworms. In many
Eudrilidae, for instance in the genus _Hyperiodrilus_, the funnels of the
sperm-ducts are dependent from the anterior wall of the segment which
contains them; the narrow tube which follows projects into the segment in
front, and is there immediately dilated into a wide chamber, which again
narrows, and bending round, re-traverses the same septum; the two ducts of
each side (if there are two, which is not invariably the case) remain
separate and open separately into the glandular part of the spermiducal
gland. There is occasionally only a single median gland; and as a general
rule the two glands open by a median unpaired orifice. Penial chaetae may
or may not be present.

[Illustration: FIG. 193.—Female reproductive organs of _Hyperiodrilus_.
XII-XV, Segments of the body; 1, spermathecal sac; 2, egg-sac; 3,
spermatheca; 4, ovary.]

The structure of the female organs differs considerably in detail in the
different genera. But _Hyperiodrilus_ may be taken as an instance of a
genus in which these organs are as complicated as they are anywhere. The
ovaries (Fig. 193, 4) are {364}perfectly normal in structure and in
position. So also are the oviducts; but both are enclosed in sacs which
communicate in rather an elaborate fashion. Each ovisac is somewhat rounded
in form, and the two communicate by a narrow tube; from the ovisac also
arises another narrow tube, which soon dilates into a chamber lying in the
thirteenth segment; this contains the mouth of the oviduct and is
continuous with the egg-sac; the latter is quite normal in position. Beyond
the egg-sacs the two tubes unite round the intestine and open into a large
median sac, which contains sperm and may be called the spermathecal sac
(1). There is, however, a true spermatheca, single and median. This opens
on to the exterior in the middle of the thirteenth segment, but lies
chiefly in the right-hand sac behind the ovarian portion of the same. I
never found this spermatheca to contain sperm. Dr. Rosa inferred on
anatomical grounds, and I have been able to prove developmentally (in
_Libyodrilus_), that these sacs which involve the ovaries and oviducts, and
which also contain sperm, are derivatives of the septa; that in fact the
spaces which they enclose are coelomic. In some Eudrilids these sacs are
the only "spermathecae"; in others, as in _Hyperiodrilus_, there are in
addition blind pouches lying within them which must be regarded as true
spermathecae; these are smaller in some than in others. In fact there are
various transitions in the entire replacement of true spermathecae
apparently homologous with those of other earthworms by pouches which are
derived from the septa, and which are therefore of an entirely different
morphological significance; here is an excellent case of the substitution
of organs, analogous to the replacement of the primitive notochord of the
Vertebrate by the vertebral column.

So far as is known, all the Oligochaeta deposit their eggs in special
chitinous cases, the cocoons. They share this peculiarity with the
Hirudinea. The cocoons have long been known, but were originally mistaken
for the eggs themselves. The cocoons contain several eggs and a variable
quantity of albumen for the nutrition of the growing embryos. In the
majority of earthworms they are more or less oval with projections at the
two ends, and are of a brownish colour. In others the tint is rather to be
described as green. The genera _Criodrilus_ and _Sparganophilus_ have a
cocoon which is greatly elongated. These structures seem to be undoubtedly
formed by the clitellum, the earlier {365}opinion of D'Udekem being that
they were the product of certain glands developed in _Lumbricus_ at the
breeding season, which he thence called the capsulogenous glands. It is
more probable that these glands, which have been up to the present but
little investigated, are the seat of the formation of the albumen which is
found within the cocoons. The cocoons are deposited at varying depths in
the ground, or on the surface. Among the aquatic genera they are often
attached to aquatic plants. The process of formation has been carefully
watched by Vejdovsky[417] in the genus _Rhynchelmis_. The worm throws off
the cocoon over its head, crawling backwards to free itself therefrom. The
eggs, spermatozoa and albumen, reach the interior of the cocoon as it
passes over the orifices of the respective ducts. Out of the numerous eggs
which a single cocoon originally contains, only a few, sometimes only one,
reaches to maturity. Among the Enchytraeidae, however, quite a large number
of young emerge from a single cocoon. The development of all the
Oligochaeta is direct, there being no free larval stage. It seems to be the
rule for a process of fission to take place in the embryos of
_Allolobophora trapezoides_[418] at least, according to the observations of
Vejdovsky, in warm weather. In cold weather he found in each cocoon as a
rule single embryos, and only 10 per cent of double embryos.

[Illustration: FIG. 194.—Cocoons of Lumbricidae. (After Vejdovsky.) A,
_Lumbricus rubellus_, nat. size and × 3; B, _Allurus_, nat. size and × 6;
C, _Allolobophora foetida_, nat. size and × 3.]

HABITAT.—Earthworms are found in almost every part of the world where they
have been looked for. They occur far to the north, in Siberia and Nova
Zembla,[419] while South Georgia and Kerguelen mark their southern limits.
From arid tracts of country they are naturally absent, and also, which is
more {366}curious, from certain districts of North America. In the tropics
these animals seem to be on the whole less abundant than in more temperate
climates. But this deficiency of individuals is counterbalanced by the
greater variety of generic and specific types. From tropical Africa, little
explored as it has been from this point of view, no less than thirty
genera, including about ninety species, have been recorded; whereas in
Great Britain only four genera and seventeen species occur, and in all
probability but few remain to be discovered. The vertical range of these
Annelids is also considerable. Several species have been met with in Europe
and elsewhere at an altitude of 10,000 feet.

For the bulk of the species the term earthworm is an accurate description
of their habitat. But there are not a few which occasionally or habitually
prefer other localities. The genus _Allurus_ is equally at home in soil or
in water; I have taken it in the fast-flowing river Plym in Devonshire. The
genus _Acanthodrilus_ includes a few species which have at present only
been met with in water; _A. schmardae_ comes from fresh water in
Queensland, _A. stagnalis_ from ponds in South America; _A. dalei_ is like
_Allurus_ in that it is to be found both on land and in streams and ponds.
The Enchytraeidae are just as amphibious; _Criodrilus_ and _Sparganophilus_
appear to be purely aquatic. A more curious locality for a creature that is
so characteristically terrestrial is the margin of the sea. For a long time
a species belonging to a peculiar genus _Pontodrilus_ has been known from
the shores of the Mediterranean in the neighbourhood of Nice. It lives
there among seaweed above high-water mark, but it must at least
occasionally be splashed by the waves. Another species of the same genus
occurs on the coast of Brazil and some of the West Indian islands;
_Pontoscolex corethrurus_ and _Diachaeta littoralis_ were described by
Schmarda[420] from the shores of Jamaica. The former species is one of the
most widely distributed of earthworms, and, except in this particular part
of the world, has been always taken on the land far from the sea. There are
also partly marine forms among the Tubificidae; _Clitellio arenarius_ is
common on our coasts.

While there are several kinds of earthworms that are thus met with in fresh
water, others will live for some time submerged {367}in water. Perrier
found by experiment that various species could undergo with impunity a
prolonged immersion in water, and I confirmed his experiments myself with a
common species of _Allolobophora_. A correspondent of "Nature" stated that
a certain number of species (not particularised) of earthworms in Ceylon
could suffer with impunity the effects of sea-water. The importance of this
fact will be again dealt with in considering the geographical distribution
of the group.

Among the aquatic genera of Oligochaeta we do not as a rule meet with
amphibious species. The Enchytraeidae however, as already mentioned, are an
exception; so too appears to be the genus _Phreoryctes_, which in its
structure is to some extent intermediate between the earthworms and the
aquatic families.

TERRESTRIAL AND AQUATIC FORMS.—There are many obvious structural
peculiarities which would prevent the normally aquatic worms from being
thoroughly at home on dry land. The gills of _Branchiura_ and the other
gilled species would be injured, in all probability, by friction with the
earth; the delicate and long chaetae of Naids and _Tubifex_ are also most
unsuited for progression through dry soil; and it is to be noted that those
Oligochaeta, which, belonging to aquatic groups, are yet found away from
water, have chaetae of the simple sigmoid pattern which characterises the
earthworms.

There are other peculiarities found only in the aquatic species which have
not so obvious a relation to their habitat. In no genus that is mainly
aquatic in habit are the ova small and nearly unprovided with yolk as in
_Lumbricus_; the ova of aquatic forms are invariably large and filled with
abundant yolk.

The more delicate organisation of the aquatic Oligochaeta is not so hard to
understand. The comparatively unresisting nature of the medium in which
they live, water or fine mud, does not necessitate so strong a development
of the layers of the body-wall as is essential to the earth-living forms,
which have also thick septa in the anterior region, to protect the organs
of reproduction as the strong muscular contractions of the body force the
worm's way through the dense soil. With the weak structure of the
integument are perhaps also correlated the simplicity of other organs of
the body in the aquatic Oligochaeta. With thin body-walls, through which
gases can diffuse with great ease, there would seem to be less need for the
development of a system of {368}integumental blood capillaries. These are
indeed for the most part absent in the aquatic forms, being only faintly
developed in a few, an example possibly of degeneration.

EARTHWORMS AND THE SOIL.—Darwin has explained the enormous effects which
these soft-bodied and small creatures have had upon the superficial
structure of the earth. Their castings, brought up to the surface, are
blown about by the wind when dry, and are thus spread over the ground in a
fine layer. It has been calculated that in the space of an acre .2 of an
inch in thickness of earth is annually brought to the surface. It is clear
therefore that in a long period of years there would be a very large effect
produced. On the sides of a hill this matter brought up from below would
tend to roll down the slopes when dry, and would increase the débris
carried away to the sea by streams and rivers, so that continents formerly
deposited under the sea may owe no small proportion of their size to the
continued work of earthworms in past ages.

Darwin has also pointed out the benefits to the agriculturist which accrue
from the industry of these Annelids. The soil is thoroughly mixed and
submitted to the action of the atmosphere. The secretions of the worms
themselves cannot but have a good effect upon its fertility, while the
burrows open up the deeper-lying layers to the rain. Mr. Alvan
Millson,[421] in detailing the labours of the remarkable Yoruba worm
(_Siphonogaster millsoni_ Beddard), hints that they may serve as a check
upon the fatal malaria of the west coast of Africa. By their incessant
burrowings and ejecting of the undigested remains of their food many
poisonous germs may be brought up from below, where they flourish in the
absence of sunlight and oxygen, and submitted to the purifying influence of
sun and air.

PHOSPHORESCENCE.—Phosphorescence has been observed in several species of
Oligochaeta. The most noteworthy instance of recent times is the discovery
by Giard of the small worm which he called _Photodrilus phosphoreus_ at
Wimereux. During damp weather it was sufficient to disturb the gravel upon
the walks of a certain garden to excite the luminosity of these Annelids.
In all probability this species is identical with one whose luminosity had
been noticed some years before (in 1837) by Dugès, and named by him
_Lumbricus phosphoreus_. According {369}to Giard, the light is produced by
a series of glands in the anterior region of the body debouching upon the
exterior. This same worm has since been found in other localities, where it
has been shown to be phosphorescent, by Moniez[422] and by Matzdorf[423].
It is remarkable that in some other cases the luminosity, though it exists,
is very rarely seen. The exceedingly common Brandling (_Allolobophora
foetida_) of dunghills has been observed on occasions to emit a
phosphorescent light. This observation is due to Professor Vejdovsky,[424]
and was made "upon a warm July night of 1881." He thinks that the seat of
the light is in the secretion of the glandular cells of the epidermis, for
when this and other worms are handled the phosphorescence clings to the
fingers, as of course does the mucous secretion voided by the glands.

Phosphorescence has been observed also in some other families of
Oligochaetes. The late Professor Allen Harker noticed a small worm in
marshy ground in Northumberland which emitted a distinct light, and which
was subsequently identified as a member of the family Enchytraeidae.

GEOGRAPHICAL DISTRIBUTION.[425]—In the succeeding pages some of the details
of the geographical range of the Oligochaeta will be found. The present
section deals with a few generalities, which appear to result from an
examination of the facts.

As to the aquatic genera but little is known at present with regard to
their range; they have not been widely collected in extra-European
countries. What little is known points to the conclusion that while many
parts of the world have their peculiar genera (such as _Hesperodrilus_ in
South America, _Phreodrilus_ and _Pelodrilus_ in New Zealand), some of the
common European species are widely distributed. I have, for example,
received _Henlea ventriculosa_ from Kirghiz Tartary, and from New Zealand;
and a New Zealand _Tubifex_ appeared to me to be indistinguishable from the
common _T. rivulorum_ of our rivers and ponds. It is possible that these
and similar instances may, at least in some cases, be due to accidental
importation at the hands of man, a matter into which we shall enter later.
But the aquatic genera have, many of them, facilities for extending their
range in a natural fashion, which are greater than those possessed by
earthworms. {370}It has been pointed out that the chaetae of the aquatic
Oligochaeta are generally hooked at the extremity and bifid, which would
give them a greater chance of holding on to the feet or feathers of aquatic
birds; I am not myself disposed to lay much stress on the possibilities of
migration by these means, since the tender bodies of the small worms would
be liable to be soon dried up by wind while in the act of migration. More
likely in every way is a migration when enclosed in the cocoon. The cocoons
being small, and often deposited at the edges of ponds frequented by
aquatic birds, there would be many chances of their being carried away with
tolerable frequency; moreover, as Dr. Michaelsen has pointed out, the
cocoons of some species, particularly among the Enchytraeidae, contain a
large number of embryos; so that when such a cocoon reached a foreign shore
there would be a better chance of the species establishing itself there. I
have referred elsewhere[426] to the singular habit of forming a temporary
cyst which characterises one species of the genus _Aeolosoma_; this would
perhaps tend to facilitate its transference in the way indicated from one
spot to another.

Earthworms, on the other hand, have not such easy means of travelling from
country to country; the assistance which the cocoons in all probability
give to the smaller aquatic Oligochaeta cannot be held to be of much
importance in facilitating the migrations of the earthworms. In the first
place, the animals themselves are of greater bulk, and their cocoons are
naturally larger, and thus less easy of transportation. Secondly, they are
deposited as a rule upon dry land, where the chances of their sticking to
the feet of birds would be less; and thirdly, they are often deposited deep
in the ground, which is a further bar to their being taken up. Another
possible method by which earthworms could cross the sea is by the help of
floating tree-trunks; it is, however, the case with many species that they
are fatally injured by the contact of salt water. There are, it is true, a
few species, such as _Pontodrilus_ of the Mediterranean coast, which
habitually live within reach of the waves; but with the majority any such
passage across the sea seems to be impossible.[427] On the other hand,
rivers and lakes are not a barrier to the dispersal of the group. There are
a few species, such as _Allurus tetragonurus_, {371}which live
indifferently on land and in fresh water; and even some habitually
terrestrial species can be kept in water for many weeks with impunity. A
desert, on the other hand, is a complete barrier; the animals are
absolutely dependent upon moisture, and though in dry weather the worms of
tropical countries bury themselves deep in the soil, and even make
temporary cysts by the aid of their mucous secretions, this would be of no
avail except in countries where there were at least occasional spells of
wet weather.

The range of the existing genera and species is quite in keeping with the
suggestions and facts already put forward. But in considering them we must
first of all eliminate the direct influence of man. Every one who studies
this group of animals knows perfectly well that importations of plants
frequently contain accidentally-included earthworms; and there are other
ways in which the transference of species from one country to another could
be effected by man. There are various considerations which enable us to
form a fair opinion as to the probability of a given species being really
indigenous or imported. Oceanic islands afford one test. There are species
of earthworms known from a good many, but with a few exceptions they are
the same species as those which occur on the nearest mainland; in those
cases where it is supposed that the animal inhabitants have reached an
oceanic island by natural means of transit, it is a rule that the species
are different, and even the genera are frequently different. That the bulk
of them are the same seems to argue either frequent natural communication
with the mainland or a great stability on the part of the species
themselves. It is more probable that the identity is in this case to be
ascribed to accidental transference.

Another argument comes from the distribution of the family Lumbricidae.
This family forms the bulk of the earthworms of the European and North
American continents. But they are also found all over the world. With one
or two exceptions, such as _Allolobophora moebii_, from Madeira, the
extra-north-temperate species are identical with those found within that
region. Now, if the migration had been by natural means there would surely
in the lapse of time been some differentiation of species. Furthermore, Dr.
Michaelsen has pointed out that in South America the presumably European
forms (_i.e._ _Lumbricus_ and _Allolobophora_) are found upon the coast and
in cultivated ground; it is inland that {372}the presumably indigenous
species are met with. This again looks very like accidental transference.

A mapping of the world in regions indicative of the distribution of
earthworms produces a result which is slightly different from the accepted
division. North America, Europe, and Northern Asia so far as is known agree
in having as their distinctive earthworms the family Lumbricidae, which is
very nearly the only one represented in these parts of the world. The
majority of the species are common to the two continents; there cannot, in
fact, be a separation of Nearctic and Palaearctic; we must accept the
Holarctic region of Professor Newton. The Ethiopian region, on the other
hand, is quite as it is in other groups, being bounded to the north by the
desert of Sahara. The Neotropical region is quite distinct, and includes
Central as well as South America, and the West Indian islands, even the
Bermudas. It is, however, a question whether the more southern portions of
the continent should not be cut off from the rest and joined with New
Zealand, to form an Antarctic region. In these two countries, and also in
Kerguelen and Marion Islands, the prevailing genera are _Acanthodrilus_ and
_Microscolex_. In America _Acanthodrilus_ is found nowhere but in the more
southern regions of the southern continent, as well as in the Falklands and
South Georgia. New Zealand is characterised by other genera of
Acanthodrilids besides _Acanthodrilus_ itself; but the bulk of the species
belong to the latter genus. _Acanthodrilus_ also occurs (three species
only) in Queensland and at the Cape of Good Hope. _Microscolex_ is rather
more widely dispersed, being found in other parts of America and in Europe,
the island of Madeira (? accidentally imported); but it is undoubtedly
chiefly concentrated in South America and in New Zealand. Apart from New
Zealand, which, as already said, can only be doubtfully referred to the
Australian region, the latter appears to form one with the Oriental region
(to which, on account of its Perichaetidae, Japan should be added) of other
writers. There is, so far as earthworms are concerned, no "Wallace's line"
at all. The characteristic genera _Perichaeta_ and _Megascolex_ range from
one extremity of the Indo-Australian region to the other. It is true that
_Cryptodrilus_ and _Megascolides_ are limited to Australia itself (with the
apparent exception of a species or two in America, for I can hardly
separate _Argilophilus_ of Eisen from _Megascolides_); {373}but they are
not at all well-defined genera, and indeed the generic distinctions of the
whole family Cryptodrilidae are not in a satisfactory condition.

CLASSIFICATION.—The Oligochaeta do not shade into the Polychaeta so
imperceptibly as might be inferred from the current schemes of
classification. Apart from minor points, which are not universally
characteristic of the two groups, though never found except in one or the
other, the Oligochaeta are to be defined by the complicated reproductive
system; although in a few undoubted Polychaets there is a faint approach to
this in the specialisation of some of the nephridia as sperm-receptacles
and even as sperm-ducts. But nowhere among the Polychaeta are there the
diversified sperm-ducts and oviducts, spermathecae and sperm-sacs, that are
universal among the Oligochaeta. Moreover, no Polychaet has a clitellum,
which is so distinctive of the Oligochaeta, and of their near allies the
Leeches. Dr. Eisig has compared the glandular modification of the
integument at the mouths of the sperm-ducts in the Capitellidae to the
beginnings of a clitellum. This may be the case, but it is, in my opinion,
more comparable to the similar glandular spots at the male pores in
earthworms. The reproductive glands in the Oligochaeta (save for a few
apparently abnormal cases) are restricted to at most two pairs of each,
which occur in the same individual; the Polychaeta being dioecious. There
is, in short, no form known which cannot be definitely referred to either
the Polychaeta or the Oligochaeta, excepting perhaps _Ctenodrilus_, the
anatomy of whose reproductive organs is at present unknown.

It is a difficult task to classify the different families of the
Oligochaeta; and to enter into the historical aspect of the matter would
take too much space. I am myself disposed to divide them first of all into
two main groups, for which I use Dr. Benham's[428] names of Microdrili and
Megadrili.

The MICRODRILI are, as a rule, small and aquatic in habit; they have short
sperm-ducts which open on to the exterior in the segment which immediately
follows that which contains the internal aperture; the clitellum is only
one cell thick; the egg-sacs are large; the epoch of sexual maturity is at
a fixed period. This group, to my thinking, includes the Moniligastridae;
although Professor Bourne has denied my statement with regard {374}to the
clitellum, and in this case it is not so easy to decide their systematic
position.

The MEGADRILI are characterised by the precisely opposite characters. The
sperm-ducts are longer; the clitellum is composed of many layers of cells;
the egg-sacs are rudimentary; sexual maturity appears to be more or less
continuous.

There is, however, a substantial agreement about the families which I here
adopt, which may be fairly taken to express our present knowledge of the
Order. For fuller details the reader is referred to my Monograph of the
Order Oligochaeta.[429]

[Illustration: FIG. 195.—_Aeolosoma hemprichii_ dividing transversely, ×
30. (After Lankester.)]

I. MICRODRILI.

FAM. 1. _Aphaneura._[430]—This name was originally given to the present
family by Vejdovsky; the family contains a single genus, _Aeolosoma_, of
which there are some seven species. The name is taken from, perhaps, the
most important though not the most salient characteristic of the worms. The
central nervous system appears in all of them to be reduced to the cerebral
ganglia, which, moreover, retain the embryonic connexion with the
epidermis. The worms of the genus are fairly common in fresh waters of this
country, and they have been also met with in North and South America, and
in Egypt, India, America, and tropical Africa. They are all small,
generally minute (1 to 2 mm. long), and have a transparent body variously
ornamented by brightly-coloured oil globules secreted by the
{375}epidermis. These are reddish brown in _A. quaternarium_, bright green
in _A. variegatum_ and _A. headleyi_, in the latter even with a tinge of
blue. In the largest species of the genus, _A. tenebrarum_ they are olive
green. In _A. niveum_ the spots are colourless, and _A. variegatum_ has
colourless droplets mixed with the bright green ones. Fig. 195 shows very
well the general appearance of the species of this genus. The body has less
fixed outlines than in most worms, and the movement of the creatures is not
unsuggestive of a Planarian. As the under side of the prostomium is
ciliated, and as the movements of these cilia conduce towards the general
movement of the body, the resemblance is intelligible. One species of
_Aeolosoma_, at any rate, has a curious habit which is unique in the Order.
At certain times, for some reason at present unknown, the worm secretes a
chitinous capsule, inside which it moves about with considerable freedom;
these capsules when first observed were mistaken for the cocoons of the
worms; they are really homologous with the viscid secretion which the
common earthworm throws off when in too dry soil, and with which it lines
the chamber excavated in the earth in which it is lying. The worms of this
genus multiply by fission; sexual reproduction has been but rarely
observed.

FAM. 2. _Enchytraeidae._[431]—This family consists at present of rather
over fifty well-characterised species, which are distributed into eleven
genera. It is common in this country and in Europe generally; it has been
met with in Spitzbergen and the extreme north; it occurs in the American
continent from the north to the extreme south; it is also an inhabitant of
New Zealand. The worms of this family are nearly always of small size,
sometimes minute; they never exceed an inch or so in length, and that is a
rare occurrence. They are equally at home in water and in soil, some
species being common to the two media; a few are marine or littoral in
habit, while others are parasitic in vegetable tissues. Like most
earthworms, and unlike the majority of aquatic worms, the chaetae are
without a bifid termination; the body-wall, too, is comparatively thick.
The perivisceral fluid is often (as in certain Naids) loaded with
elliptical or rounded corpuscles. Resemblances to earthworms rather than to
the aquatic families of Oligochaeta are suggested by the long distance
which separates the {376}spermathecae from the male pores (segments 5 and
12), and by the paired or unpaired glands that have been already compared
to the calciferous glands so universally present among earthworms. On the
other hand, the male ducts are confined, as in the lower Oligochaeta, to
two segments, upon one of which the internal, upon the other the external
orifice is situated, and the oviduct is reduced to a simple pore, as in
Naids; but this may be merely a matter of convergence by degeneration.
Perhaps the most remarkable genus in the family is _Anachaeta_, which has
no chaetae, but in their place a large cell projecting into the
body-cavity, which appears to represent the formative cell of the chaeta.
The integument of this genus contains true chlorophyll, according to
Vejdovsky.

A singular character, found, however, also in _Rhynchelmis_ and _Sutroa_
among the Lumbriculidae, is the opening of the spermathecae into the
alimentary canal. This was originally discovered by Dr. Michaelsen, but has
been abundantly confirmed.

_Stercutus_ is a singular genus which was originally found in manure, and
has the peculiarity that the alimentary canal is often aborted; this
degeneration seems to bear some relation to the food and conditions of
life.

FAM. 3. _Discodrilidae._[432]—This family consists of small parasitic forms
which were at one time assigned to the Hirudinea; there seems, however, to
be no doubt that they are rightly included in the present Order.
_Branchiobdella_ is found upon the gills of the Crayfish, _Astacus
fluviatilis_; the American _Bdellodrilus_ upon _Cambarus_. The chief reason
for the former inclusion of these worms among the leeches was due to the
absence of chaetae and to the presence of chitinous jaws and of suckers;
apart from these structures there is nothing whatever leech-like about the
worms. _Bdellodrilus_ has two pairs of testes in segments 5 and 6; there
are two pairs of sperm-ducts, all opening, however, by a common "atrium" on
the sixth segment; on the fifth open a pair of spermathecae, likewise by a
common pore. The ovaries are in segment 7, and the ova escape by a pair of
pores apparently like the single pore of the Enchytraeidae. The entire worm
consists of only eleven segments.

FAM. 4. _Phreoryctidae._[433]—This family contains only two {377}genera,
_Phreoryctes_ and _Pelodrilus_. The former is widely spread, occurring in
Europe, North America, and New Zealand. _Pelodrilus_ is limited to New
Zealand. Most species of _Phreoryctes_ are distinguished by their
extraordinary length and thinness, and there is frequently a tendency to
the disappearance of the chaetae. The most important anatomical fact about
_Phreoryctes_ (at any rate _P. smithii_) is that there are two pairs of
ovaries as well as two pairs of testes, and that the ducts of all are
simple and very much alike. This seems to argue the low position of the
family in the series.

FAM. 5. _Naidomorpha._[434]—This family contains eight or nine genera,
perhaps more; they are all of them aquatic and of small size, and they
multiply by fission as well as sexually. The most noticeable peculiarity of
the family is the "cephalisation" which occurs in the head segments. In
some genera, in _Pristina_ for example, there is no such cephalisation to
be observed; but in others the dorsal bundles of chaetae commence a few
segments farther back than the ventral, the segment where they commence
being different and characteristic in the various genera. Thus in _Dero_
the first four segments are without dorsal chaetae, and in _Nais_ the first
five are in this condition. There is thus a kind of "head" formed, whence
the expression "cephalisation." _Dero_, _Nais_, and _Pristina_ are commonly
to be met with in ponds, lakes, etc., in this country. _Bohemilla_ is
rarer, and is to be distinguished by the remarkable serrated chaetae of the
dorsal bundles. Of _Dero_ and _Nais_ there are a considerable number of
different species; indeed it is usual perhaps to regard as distributable
among three genera, _Nais_, _Stylaria_, and _Slavina_, the species which I
am disposed to place in one genus, _Nais_. _Stylaria_ is defined on this
view by its extremely long prostomium, which has given rise to both its
popular and technical names. "Die gezungelte Naide" was the term applied by
one of its earliest investigators, and the name _Stylaria proboscidea_
signifies the same peculiarity. But as the same inordinately long
"proboscis" occurs in the South American _Pristina proboscidea_, belonging
to a genus of which the other member does not possess so well developed a
prostomium, it seems too variable a character upon which to differentiate a
genus. _Chaetogaster_ and {378}_Amphichaeta_ have been placed by some
systematists in a separate family. The first named contains four species
which are fairly common. It is one of those worms in which the chaetae are
not exactly related to the segmentation of other organs, which moreover
sometimes show an independence in their segmentation; thus there are more
nerve ganglia in the anterior segments of the body than there are septa.

FAM. 6. _Tubificidae._—The worms belonging to this family are of small
size, and are all inhabitants of fresh or salt water, or the margins of
pools and the sea. They differ from the last family in that asexual
reproduction never occurs, and that the reproductive organs are situated
rather farther back in the body. The male pores are upon segment 11, and
the oviduct-pores upon the following segment. This family differs from the
Lumbriculidae in the fact that there are only a single pair of sperm-ducts.

The earliest known Tubificid was the common _Tubifex rivulorum_, so widely
dispersed in this country and elsewhere; but with it was at first
confounded the somewhat similar genus _Limnodrilus_, which only differs in
that the chaetae are all of the cleft variety, and never capilliform, as in
_Tubifex_. The genera are mainly distinguished by the characters of the
chaetae and of the male ducts. At the base of the series perhaps lies
_Ilyodrilus_, which has many points in common with the Naids. The form of
the terminal chamber into which the sperm-duct opens has the same
simplicity as in that group, and the intestine is surrounded with a network
of blood-vessels as in the Naids, a structure which is otherwise wanting in
the Tubificidae. The development of the ova also is upon a plan which is
met with in the Naids. The atrium (see p. 361) becomes more complicated in
other Tubificidae. The extremity also is as a rule modified into a
retractile penis. The discrete "prostate," of which we have already spoken,
marks out a considerable number of genera, such as _Tubifex_,
_Limnodrilus_, _Spirosperma_, _Hemitubifex_. In the marine _Clitellio_
there is no such structure at all, and it is also wanting in the South
American _Hesperodrilus_. In _Branchiura_ there is a complete prostatic
investment of the atrium, and in _Telmatodrilus_ a large number of separate
aggregations forming as many distinct prostates. _Vermiculus_, a genus
consisting of but one species, found by Mr. Goodrich on the sea-shore in
the neighbourhood of Plymouth, is remarkable for the unpaired character of
the {379}generative organs, a peculiarity which is shared by Stolc's genus
_Bothrioneuron_. The gills upon the posterior segments of _Branchiura
sowerbyi_ and _Hesperodrilus branchiatus_ have been already noticed above
(p. 352). A very aberrant genus, perhaps not rightly referable to this
family, is _Phreodrilus_,[435] from New Zealand, first collected in water
from a subterranean spring. It differs from all other Tubificids except
_Hesperodrilus_ in that the spermathecae lie behind the male pores, a state
of affairs which is met with in the Lumbriculidae. Another singularity of
structure concerns the sperm-duct, which is wrapped in a thin-walled sac,
which has every appearance of being simply the outer muscular wall of the
duct. Within this are the complicated coils of the duct, and also a
quantity of free spermatozoa, whose mode of ingress is difficult to
understand. Many of the Tubificidae live in tubes fabricated by themselves,
whence the tail end protrudes. The integument in more than one species is
vascular. This integumental blood system, universal among the earthworms,
appears to be restricted to the present group among the Limicolae of
Claparède.

FAM. 7. _Lumbriculidae._[436]—This family is not a large one, and is nearly
limited in range to Europe and North America; indeed, if we omit the
doubtful _Alluroides_, entirely to the Palaearctic region. There are only
fourteen species, which are referred to eight genera. A number of dubious
forms, as is the case with other families, may possibly ultimately swell
this list. The type genus of the family, viz. _Lumbriculus_, upon which
Bonnet made his experiments in section and subsequent regeneration, has
only within the last year been thoroughly explored anatomically. But all
the other genera are well known. The Lumbriculidae are of small or moderate
size, and all of them aquatic in habitat. There are three characters which
are nearly or quite universal in the genera of the family. In all of them
the chaetae are only eight to each segment, arranged in couples, and are
either cleft at the extremity or simple. As a rule which has but two
exceptions, the genera _Alluroides_ and _Lumbriculus_, there are two pairs
of sperm-ducts, which, however, communicate with the exterior through a
single terminal chamber on each side of the body.

The dorsal blood-vessel has in the Lumbriculidae a series of
{380}contractile and blind appendages, which were at first mistaken for
caeca of the intestine itself. There are two genera of this family in North
America, which are not very different anatomically from their European
representatives. The genera described by Eisen are _Sutroa_[437] and
_Eclipidrilus_.[438] The latter lives in cold torrents at a great height in
the mountains of the Sierra Nevada of California.

FAM. 8. _Moniligastridae._—This family, terrestrial in habit, is probably
Oriental in range; but I have described a single species from the Bahamas
which may possibly be referable to the category of accidentally introduced
specimens. Our knowledge of this family is conveniently summed up in
Professor Bourne's paper[439] upon the genus _Moniligaster_. There are some
eighteen species, which range in size from an inch or so in length (_M.
bahamensis_) to about two feet; this last measurement is that of the huge
_M. grandis_, of which, together with many others, Professor Bourne gives
coloured drawings. There is a second genus, _Desmogaster_, which is mainly
characterised by the doubling of the reproductive organs. This was
described by Rosa from Burmah. The family is noteworthy on account of the
fact that every species belonging to it has at least four distinct
gizzards, sometimes more; but as this multiplication of the gizzards has
been also found in _Heliodrilus_ among the Eudrilidae, and indeed
elsewhere, it is insufficient to define the family. More characteristic is
the fact that the sperm-ducts open on to the next segment to, or even the
same segment as, that which contains their funnels; consequently the
apertures of the oviducts are behind instead of in front of them. These
pores are also situated in a very anterior position, the male pores being
upon the tenth segment or between the tenth and eleventh, and the oviducal
pores upon the following one. In these features the family presents
resemblance to the aquatic Oligochaeta, from which, however, its
stoutly-built gizzards, and vascular nephridia differentiate it.

II. MEGADRILI.

FAM. 9. _Perichaetidae._[440]—The Perichaetidae comprise a larger
{381}number of species than any other family of earthworms; but it is a
matter of considerable difficulty to divide the family satisfactorily into
genera. The family as a whole may be defined as having numerous chaetae in
most of the segments of the body.

There is no other definition which will distinguish this family from the
next two families, and even this definition is not absolutely distinctive.
There are Acanthodrilids which have a large number of chaetae in each
segment. The only difference is that in this case—in the genus
_Plagiochaeta_—the chaetae are implanted in twos; this is not the case in
the Perichaetidae. In all Perichaetidae that are known the sperm-ducts open
in common with the ducts of the spermiducal glands; they generally open
into them at some distance from the common external pore. In _Megascolex_,
_Perichaeta_, and _Pleionogaster_ the nephridia are of the diffuse type so
widely spread among these worms, and the spermiducal glands are lobate.
_Megascolex_ differs from the others in the fact that in addition to the
small scattered nephridia there are a pair of large nephridia in each
segment, and the chaetae do not form absolutely continuous circles, but are
interrupted above and below. _Pleionogaster_ has more than one gizzard but
otherwise agrees with _Perichaeta_; it is confined to the East.
_Perichaeta_ is tropical and occurs—no doubt introduced—in Europe and
America. _Megascolex_ is Old World only, and, like _Perichaeta_, Australian
as well as Oriental. But whereas _Perichaeta_ is rare in the Australian
region, _Megascolex_ is common there. _Perionyx_ and _Diporochaeta_ are the
other genera which it is possible to recognise. Both of them have paired
nephridia, and neither of them have intestinal caeca, a peculiarity which
they both share with _Megascolex_ and _Pleionogaster_. _Perionyx_
principally differs from _Diporochaeta_ in that the spermiducal glands are
lobate, whereas in the latter they are as in the Acanthodrilidae.
_Perionyx_ is Oriental; _Diporochaeta_ occurs in Australia and New Zealand.

[Illustration: FIG. 196.—_Perichaeta everetti_ F. E. B. × 1. _sp_,
Spermathecal pores; _cl_, clitellum; ♀, female pore; ♂, male pore.]

A very distinctive feature of _Perichaeta_—perhaps only of the {382}genus
_sensu stricto_—is its exceeding activity. The first specimens ever noticed
in this country, or at least of whose existence printed notice was taken,
were exhibited by the late Dr. Baird of the British Museum, at a meeting of
the Zoological Society. He remarked in that communication upon the agile
fashion in which these tropical Annelids will spring off a table when
touched or in any way interfered with. Numerous other observers have seen
the same manifestations, and the name of "eel-worm" has been given to these
_Perichaeta_ by gardeners. It is worth putting on record here that in a
species of _Acanthodrilus_ (_A. capensis_) the same irritable behaviour is
visible. When a _Perichaeta_ moves it helps itself greatly by extending, or
rather protruding, the buccal cavity, which serves as a sucker, and grips
the ground in front until the rest of the body is brought forward. It is
possibly on account of this extra facility for movement that the genus can
climb trees with such ease. A species of _Perichaeta_ has been recorded by
Mr. Willey upon an epiphyte of a palm, and Dr. Benham has found that it is
a new species, to which the name of _Perichaeta willeyi_ has been given.
The Lumbricid genus (if it be admitted as a genus), _Dendrobaena_, was so
named on account of a similar habit of climbing trees. Very singular in its
habit is the not inaptly-named _Perichaeta musica_ of Java. It is a monster
of its kind, several feet in length, and during the night makes "a sharp
interrupted sound," apparently by the friction of the chaetae against
stones. The species figured (p. 381) is, as are a few others, remarkable
for the presence of twelve or seventeen spermathecae in segments 6 and 7.

FAM. 10. _Cryptodrilidae._[441]—This family is one of the largest of the
Oligochaeta; there are rather over 120 different species, which can be
arranged in at least sixteen genera. They are found in most parts of the
world, but abound principally in the tropics. Australia may be considered
to be the headquarters of the family, which form its principal
earthworm-inhabitants. Peculiar to this continent, or at least mainly
confined to it, are the genera _Megascolides_, _Cryptodrilus_,
_Fletcherodrilus_, _Trinephrus_, and _Digaster_. _Microscolex_, though
occurring in many parts of the world, is characteristic of the more
southern regions of South America and of New Zealand. Tropical Africa has
the genera _Nannodrilus_ and _Millsonia_ limited to itself, and has besides
nearly {383}all the species of the genus _Gordiodrilus_. This family is one
which it is exceedingly difficult to define and to split up into different
genera. It shades almost imperceptibly into the Perichaetidae on the one
hand, and is very hard to differentiate from the Acanthodrilidae on the
other. A Cryptodrilid, like any member of the genus _Cryptodrilus_, with
complete circles of chaetae would be a Perichaetid; and as there are
species of _Perichaeta_ in which the anterior segments have only a few
chaetae in each segment, it is perhaps wrong to separate the two families
at all. Apart from the chaetae, there is no peculiarity in the organisation
of the family Perichaetidae that is not also met with in the
Cryptodrilidae. Even the highly characteristic intestinal caeca so
distinctive of the genus _Perichaeta_ itself, as contrasted with
_Megascolex_ and the other genera, occur, though more numerously, in the
African _Millsonia_, where there are forty or fifty pairs of them. A fairly
common feature in the family is the presence of two, or even three, pairs
of gizzards, a character which is also met with in the genus _Benhamia_
among the Acanthodrilidae, and occurs also in some other families. The
names _Digaster_, _Didymogaster_, _Perissogaster_, and _Dichogaster_ have
been founded upon this character. The excretory organs may be paired (in
_Trinephrus_ there are three pairs to each segment) or of the diffuse kind.
The male pores are usually upon the eighteenth segment, but not
unfrequently upon the seventeenth, and are often armed with long and
ornamented chaetae. Spermiducal glands are invariably present, and may be
lobate or tubular. There are two groups of small-sized genera, which in
their simplicity of organisation stand at the base of the series; but it is
very possible that the simplification is rather due to degeneration than to
primitive position. One of these groups includes the semi-marine genus
_Pontodrilus_ (with which I include the phosphorescent _Photodrilus_) and
_Microscolex_. In these forms the gizzard has disappeared, or is
represented by a rudimentary structure, and the male pores are upon the
seventeenth segment. In the other group are the genera _Ocnerodrilus_,[442]
_Gordiodrilus_,[443] and _Nannodrilus_, which are of even smaller size, and
have in the same way the male pores upon the seventeenth segment. The
species of this group are often aquatic, and there is not only no gizzard
{384}(in most of the species), but the calciferous glands have been reduced
to a single pair, which lie in the ninth segment. The latter character is
also found in the Acanthodrilid _Kerria_, which has been associated with
the above named. _Gordiodrilus_ has the peculiarity that there are, as in
Acanthodrilids, two pairs of tubular spermiducal glands.

FAM. 11. _Acanthodrilidae._[444]—This family is only with difficulty to be
distinguished from the last. The following definition applies to all the
members of the family with one exception, and does not apply to any
Cryptodrilid with, so far as is known, one exception only. There are two
pairs of spermiducal glands, opening upon the segments in front of and
behind that which bears the apertures of the sperm-ducts.

The one exception to this definition is the species _Acanthodrilus
monocystis_, which I formerly placed in a distinct genus, _Neodrilus_.
_Microscolex modestus_ is the exception among the Cryptodrilidae; in that
worm the male pores are upon the segment which follows that upon which the
spermiducal glands open. The Acanthodrilidae show a considerable range of
structural variation. This enables them to be separated into several
well-marked genera. The type genus _Acanthodrilus_ has a pair of nephridia
in each segment. It contains thirty-five species, which are all from the
southern hemisphere. These species show but little variation among
themselves. _Benhamia_ is a genus that differs from _Acanthodrilus_ in the
fact that the nephridia are of the complex type, so often met with in
earthworms with many external pores. The segment that bears the male pores
is entirely without any traces of the ventral chaetae. Here again there are
a large number of species which are nearly confined to the continent of
Africa. Dr. Michaelsen is indeed of opinion that the few species found in
the East Indies and America are accidental importations. I have proposed to
separate some of the New Zealand Acanthodrilids into a distinct genus,
_Octochaetus_, which is somewhat intermediate between _Acanthodrilus_ and
_Benhamia_. They have multiple nephridia, but only a single gizzard.
_Plagiochaeta_ of Benham, from New Zealand, is in any case clearly a
distinct form. It is mainly to be distinguished by the numerous chaetae in
each segment. _Trigaster_ Benham, is West Indian. _Deinodrilus_ (New
{385}Zealand) has twelve chaetae in each segment. _Diplocardia_, from North
America, has the male pores on segments 18, 19, 20.

FAM. 12. _Eudrilidae._[445]—This is perhaps the most remarkable family of
terrestrial Oligochaeta. Its distribution is no less curious than its
structure. Up to the present it is not known outside tropical Africa, with
the exception of the genus _Eudrilus_ itself, which is almost world-wide in
range. As, however, but one species of _Eudrilus_ is found out of Africa,
and as that species is so common in gatherings from various tropical
countries, it seems to be an instance of a species with large capacities
for accidental transference from country to country. The type genus,
_Eudrilus_, has been known since 1871, when it was originally described by
M. Perrier.[446] Since that date nineteen other genera have been described
from Africa by Dr. Michaelsen, Dr. Rosa, and myself. The most salient
external character of the group, not universal but general, is the unpaired
male and female orifices. The orifices are commonly very conspicuous (see
Fig. 197).

[Illustration: FIG. 197.—_Libyodrilus violaceus_ F. E. B. × 2. _sp_,
Spermathecal pore; _cl_, clitellum; ♂, male pore.]

The peculiarities of internal structure mainly concern the reproductive
organs, the differences in which from genus to {386}genus are often very
great. We have already referred to the remarkable branching of the
nephridial duct in the body-wall, and to the much modified calciferous
glands of _Stuhlmannia_ and some other genera. These structural variations
perhaps permit the family to be divided into two sub-families. In one there
are calciferous glands of the normal type, though peculiar in that one or
more are median and ventral in position, and are unpaired; there is no
branching of the nephridium in the body-wall; there are always, so far as
is known, the Pacinian-corpuscle-like bodies in the integument. In the
other sub-family the calciferous glands, if present (they are absent, for
instance, in _Libyodrilus_), have undergone much modification in structure;
the nephridia, where they have been investigated, have been found to branch
copiously in the body-wall; the peculiar integumental bodies hardly ever
occur.

FAM. 13. _Geoscolicidae._[447]—This family is essentially tropical, being
found in South America and the West Indies, in tropical Africa, in India,
and in some of the islands of the Malay Archipelago. But it also occurs
(_Sparganophilus_ and _Criodrilus_) in Europe and in America. A good many
of the genera are aquatic. This is the case with the two already mentioned;
the genera _Glyphidrilus_ and _Annadrilus_ of the Malay Archipelago can
live in water. The family is easily definable if we take the more typical
forms; but at one end of the series it fades into the next family, that of
the Lumbricidae. _Criodrilus_ is one of the genera which is difficult to
place. As is the case with many Geoscolicidae, _Criodrilus_ has ornamented
chaetae not only upon the clitellum, but upon the other segments of the
body. This character was until recently unknown among the Lumbricidae; it
has been lately found in _Allolobophora moebii_ and _A. lonnbergi_. The
absence of spermathecae characterises _Criodrilus_ as well as other
Geoscolicidae; but here again the character is not by any means
distinctive, for in _Allolobophora constricta_ there is the same absence of
these organs. In _Criodrilus_ the male pores are upon segment 15, as in the
Lumbricidae, but a species of _Kynotus_, which is certainly a Geoscolecid,
has these pores upon precisely the same segment. The only point in which
_Criodrilus_ is definitely a Geoscolecid, or rather not a Lumbricid, is in
the forward position of the clitellum, which begins upon the fifteenth
segment, far earlier than it does {387}in any undoubted Lumbricid. The
peculiar elongated cocoon, which much resembles that of _Sparganophilus_,
is another character which favours its Geoscolecine affinities. Dr.
Michaelsen has proposed to unite _Criodrilus_ and _Alma_ into a family
intermediate between the Geoscolicidae and the Lumbricidae.

[Illustration: FIG. 198.—_Alma millsoni_ F. E. B. × 1.]

Perhaps the most remarkable genus in the whole family is _Alma_. One
species lives in the Nile mud; another is the "Yoruba worm" of West Africa,
whose habits have been described by Mr. Millson. The most marked character
of this genus, apart from the branchiae (see p. 352) which apparently may
be present or absent according to the species, is in the two enormous
processes of the body-wall, which are illustrated in Fig. 198. These
contain the sperm-ducts, which, however, open some way in front of the free
end; they are provided on the ventral surface with a series of sucker-like
structures and with peculiar chaetae. Another interesting genus is
_Pontoscolex_, which was originally described from the sea-shore of Jamaica
by Schmarda; there are only two species which are certainly characterised,
though a variety from the Hawaian Islands may be a "good" species. It
possesses the remarkable peculiarity that the chaetae at the end of the
body are disposed in a perfectly irregular fashion, which earned for it the
name of brush-tail at the hands of its discoverer, Fritz {388}Müller. This
worm, which is universal, or nearly so, in its range, doubtless having been
transferred accidentally from country to country, invariably shows a light
spot not far from the tail; when this is examined with the microscope it is
seen that the chaetae are here absent or very small, and that the muscular
structure of the body-wall is slightly different; it was thought that this
spot was a zone of growth where fresh segments could be added after the
fashion of some of the aquatic Oligochaeta, to which, it may be remarked,
the present genus shows a curious point of likeness in the bifid character
of the chaetae. It seems, however, that there are really no grounds for the
supposition, and it is possible that we have here a "weak" spot, such as
that in the foot of certain land snails, which readily gives way when the
worm is picked up by a bird, and allows the "better half" of the creature
to escape. The Bermudian genus _Onychochaeta_ offers a very strange
peculiarity in that the chaetae on the hinder segments of the body are
enormously larger than those in front, and end in strong hooks; it seems
likely that their function is to maintain a tight hold of the ground while
the worm is leaning out of its burrow, as every one has seen the common
earthworms of this country do. _Onychochaeta_ has the same irregular
arrangement of the chaetae upon the greater part of the body, as has
_Pontoscolex_. This family, like so many others, has its giants and its
dwarfs. At one extreme is the great _Antaeus_ of South America, several
feet in length; at the other the inch-long _Ilyogenia_ of Africa. The
American _Urobenus_ has a pair of intestinal caeca like those of
_Perichaeta_, and placed in the same segment.

FAM. 14.9 _Lumbricidae._[448]—This family is to be distinguished by the
following assemblage of characters.

The male pores are usually upon segment 15, and never behind that segment;
the clitellum commences some way behind the male pores. The gizzard, which
is invariably single, is equally invariably at the end of the oesophagus.
There are three pairs of calciferous glands. The nephridia are always
paired. The spermathecae never have a diverticulum.

This family only contains three well-known genera, viz. {389}_Lumbricus_,
_Allolobophora_, and _Allurus_. The American _Bimastos_ may be distinct.
_Tetragonurus_, not allowed by some, is at present unknown except as
regards external characters; it differs from the other Lumbricidae in the
fact that the male pores are upon the twelfth segment. In _Allurus_ they
are upon segment 13, and in the remaining genera upon the fifteenth.
_Lumbricus_ is to be distinguished from _Allolobophora_ by its prostomium,
which is continued by grooves on to the buccal segment, so as to cut the
latter in half. It has also median sperm reservoirs, as well as the paired
sperm sacs which are alone present in _Allolobophora_.

[Illustration: FIG. 199.—_Allolobophora chlorotica_ Savigny. × 4. The
clitellar segments are marked in Roman numerals. _t.p_, Tubercula
pubertatis; ♂, male pore.]

[Illustration: FIG. 200.—_Allolobophora putris_ Vejd. × 5. Lettering as in
Fig. 199. The black dots represent the chaetae in both figures.]

This is the only family of earthworms which, so far as is known, can brave
the ice and snow, and what is still more {390}difficult to understand, the
perpetually frozen undersoil of the Arctic regions. Eisen has described a
number of species from Spitzbergen, and Colonel Feilden recently sent me an
example of _Allolobophora octoedra_ from Kolguiev, where Mr. Trevor-Battye
also saw another specimen. The family is characteristic of the Nearctic and
Palaearctic regions, and though found beyond them, is probably elsewhere an
accidental importation (see p. 371). There are at least fifteen species of
this family found in England and Ireland, and probably more will be
identified.

There does not exist at present any comprehensive account of the British
species of earthworms, though all of them are included in Dr. Rosa's recent
revision of the family. Most of the British forms belong to the genus
_Allolobophora_, which may be divided into two series according to whether
the chaetae are quite close together or further apart. The extent of the
clitellum and the position of those swollen eminences which appear earlier
than the clitellum, and are known as tubercula pubertatis, offer further
characters. In the following tables, extracted from those of Rosa, the
known British species of this genus are grouped according to these three
characters. With the help of these tables and Figs. 199 and 200, any of the
species ought to be easily identified.

WITH CHAETAE DISTANT.[449]

---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
|25|26|27|28|29|30|31|32|33|34|35|36|
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | |
_A. putris_ |..|——|——|——|——|——|——|..| | | | |
| | | |..|——|——| | | | | | |
| | | | | | | | | | | | |
_A. constricta_ | |——|——|——|——|——|——| | | | | |
| | | | | | | | | | | | |
_A. veneta_ | |..|——|——|——|——|——|——|——| | | |
| | | | | |——|——| | | | | |
| | | | | | | | | | | | |
_A. octoedra_ | | | | |——|——| |——|——|..| | |
| | | | | | |——|——|——| | | |
| | | | | | | | | | | | |
_A. cyanea_ (subsp. | | | | | |——|——|——|——|——|——| |
_profuga_) | | | | | | |——|——|——|——| | |
| | | | | | | | | | | | |
_A. rubida_ | | | | | |——|——|——|——|——|——| |
| | | | | |——|——|——|——|——|——| |
| | | | | | | | | | | | |
_A. mammalis_ | | | | | | |——|——|——|——|——|——|
| | | | | | | | |——|——| | |
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+

{391}WITH CHAETAE PAIRED.

-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | |
_A. rosea_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | |——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. foetida_ |..|..|——|——|——|——|——|——|——| | | | | | | |
| | | | |——|——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. eiseni_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. caliginosa_ | | | |..|——|——|——|——|——|——|——|..| | | | |
| | | | | | | |——|..|——| | | | | | |
| | | | | | | | | | | | | | | | |
_A. terrestris_ | | | |..|——|——|——|——|——|——|——|——| | | | |
| | | | | | | | |——|——|——| | | | | |
| | | | | | | | | | | | | | | | |
_A. chlorotica_ | | | | |..|——|——|——|——|——|——|——|——|——| | |
| | | | | | | |——| |——| |——| | | | |
| | | | | | | | | | | | | | | | |
_A. georgii_ | | | | |..|——|——|——|——|——|——|——| | | | |
| | | | | | | |——| |——| | | | | | |
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

There are of course other points by which the different species can be
distinguished. Colour in a few cases enables a species to be named at once
without any further aid. One of the most striking of these cases is the
Brandling, so common upon dunghills, and so dear to some anglers. This worm
is ringed with brownish purple upon a yellowish ground. The greenish _A.
chlorotica_ is often found under stones, and curls itself round into nearly
a complete circle when disturbed. _A. cyanea_, of a bluish grey colour, is
one of the earthworms very commonly met with in the early morning in London
and the neighbourhood. More generally, however, the colour is of a paler or
darker red, verging towards and attaining brown, or even blackish brown;
and is so variable that nothing in the way of identification can be
attempted from the colour alone, even with the most elaborate description.

_Lumbricus_, as already mentioned, is distinguished from all
_Allolobophora_ except _A. eiseni_, by the complete dovetailing of the
prostomium into the first segment. There are five species in this country
which may be thus distinguished by the position of the tubercula
pubertatis. The most familiar species is the common _L. rubellus_.

Tubercula pubertatis are on
_L. rubellus_ Hoff. 28, 29, 30, 31
_L. castaneus_ Sav. 29, 30, 31, 32
_L. herculeus_ Sav. 33, 34, 35, 36
_L. papillosus_ Friend 34, 35, 36, 37
_L. festivus_ Sav. 35, 36, 37, 38.

{392}CHAPTER XIV

HIRUDINEA (LEECHES)

INTRODUCTION—ANATOMY—REPRODUCTION—CLASSIFICATION—RHYNCHOBDELLAE AND
GNATHOBDELLAE

"The external appearance of the Hirudinea," remarks Professor
Vaillant,[450] "permits us, save for rare exceptions, to recognise at once
the animals which belong to that group." The leeches are distinguished as a
rule by the possession of two suckers, one at each end of the body; their
general shape usually differs from that of other Annelids by its oval
contour and its dorso-ventral flattening. _Cyclicobdella lumbricoides_ of
Grube, which Blanchard has stated to be the same species as _Nephelis
tergestina_, has, however, almost the form of an earthworm by reason of its
cylindrical shape and the inconspicuousness of the suckers, while
_Lumbricobdella_ also resembles an earthworm and has no posterior sucker at
all.[451] The Oligochaet family Discodrilidae (see p. 376) agree with the
leeches in their parasitism, in their general shape, in the presence of two
suckers, and, furthermore, in the existence of jaws, which are found in no
other Oligochaet, but occur in a large number of the Hirudinea. These
facts, indeed, though not perhaps important by themselves, are indications
of the really close resemblance of the Hirudinea to the Oligochaeta, a
group which they approach not merely in such habits as the formation of a
cocoon in which the eggs are enclosed, but also in many important points of
internal and external structure. Indeed, the fundamental differences
between the two groups are not numerous, and are not of such importance as
has been given them by some writers.

Leeches are to be found in most parts of the world, in {393}situations
which are sufficiently damp for their comfort. But we do not at present
possess enough knowledge to state much as to the facts of their
distribution. The structure of leeches is not so well known as is that of
the earthworms; for they have not been to so great an extent collected in
extra-European countries. It would even be desirable to ascertain precisely
the species which inhabit these islands, the most recent enumeration (1865)
being that contained in the British Museum Catalogue of non-parasitical
worms by the late Dr. George Johnston. For Italy this has been lately done
by Dr. Blanchard, and a good many of the species are common to the two
countries. Johnston enumerates altogether (after subtracting what are
probably synonyms) twenty-one species, distributed among the genera
_Branchellion_, _Pontobdella_, _Piscicola_, _Nephelis_, _Trocheta_,
_Haemopis_, _Hirudo_, and _Glossiphonia_ (= _Clepsine_), which number will
be possibly still further reduced. The first two genera are marine, the
remainder being fresh water or terrestrial; _Trocheta_ has been probably
introduced.

[Illustration: FIG. 201.—Anterior end of _Macrobdella sestertia_, to show
eyes and sense bodies. (After Whitman.)]

The use of _Hirudo medicinalis_ is well known to many of us from personal
experience. So extensively was this leech formerly made use of that it is
now far from being a common species either in this country or in France.
Those who desire full information as to Hirudiniculture should consult the
work of Dr. Ebrard, published in 1857.[452] The former extensive use of the
leech has led to the transfer of its name to the doctor who employs it, the
authors of the sixteenth century constantly terming a physician a leech; it
has been suggested, however, that the term was applied rather by way of
analogy. The useful blood-sucking habits of the medicinal leech have been
wrongly attributed to the innocent horseleech (_Aulastomum_)—innocent, that
is to say, of the blood of Vertebrates, for it has been described as "a
cruel and greedy worm," engulfing earthworms and even smaller specimens of
its own species. {394}"Horsleches," said an old writer, "are wholesome to
drawe foorthe foule blood, if thei are put into a hollowe rede, and one of
their endes cutte of, whereby the blood maie run forthe." But it is clearly
not easy for a creature destitute of jaws and teeth to bite, and the
similarity of general aspect has doubtless led to a confusion with the
savagely biting medicinal leech.

[Illustration: FIG. 202.—Sense body of _Macrobdella sestertia_. (After
Whitman.) _ep_, Epidermis; _s_, clear cells. Highly magnified.]

[Illustration: FIG. 203.—Section through eye of _Haemadipsa japonica_.
(After Whitman.) _ep_, Epidermis; _n_, nerve. Highly magnified.]

The Hirudinea are all distinctly segmented animals, but the segmentation
differs from that of the Oligochaeta in two points. In the first place the
number of segments is much smaller in a leech than in an Oligochaete,
although the difference does not appear great at first sight.

A leech's body may seem to be composed of seventy, eighty, or one hundred
segments, a number quite as great as is found, for example, in the genus
_Perichaeta_ among the earthworms; but the apparent number of segments in
the leech is produced by a very marked annulation of the real segments; and
this is indeed the second point of difference referred to above. But there
are earthworms which show frequently a secondary annulation,—secondary
because it appears late and does not affect other organs {395}of the body.
A segment of an earthworm may indeed have five or six distinct annulations,
but it will be bounded internally by two septa, and will bear only one set
of chaetae externally. In the leech external clues to the definition of a
segment were until recently wanting. They appear now to have been found in
the sensory organs of the skin (Figs. 201 and 202), which are, according to
Whitman,[453] disposed in a perfectly metameric fashion. Judged by this,
and also by the nephridia and nerve-ganglia, the number of segments in a
leech does not appear to exceed twenty-six, independently of the sucker,
which may represent a few fused segments, seven (in the medicinal leech)
according to Leuckart.

The eyes, which are so useful in the systematic arrangement of the group,
appear to have been evolved from these sensory organs by a further
exaggeration of their peculiarities. Figs. 202 and 203 show this point
convincingly. The segmental sense organ is shown in Fig. 202; to the
outside of certain sense cells, below which are a mass of ganglion cells,
are certain peculiar transparent cells very similar to the clear cells
found in the interior of the eye (Fig. 203). The segmental disposal of the
sensory bodies and of the eyes is shown in Fig. 201.

[Illustration: FIG. 204.—_Branchellion torpedinis._ (From the "Règne
Animal.") ×1.]

Some Hirudinea are furnished with external branchiae; this is the case with
_Branchellion_, in which genus the branchiae (Fig. 204) have an arborescent
form; in _Cystibranchus_ there are a series of paired simple vesicles which
take the place of these more complicated respiratory organs of
_Branchellion_. The Hirudinea do not, save for one exception
(_Acanthobdella_), possess chaetae; but it must be borne in mind that the
Discodrilidae and the genus _Anachaeta_ among the Oligochaeta are in the
same condition. In _Acanthobdella_[454] there are two pairs of chaetae upon
each side of the anterior five segments of the body. According to the
figure which Grube, the {396}original describer of the genus, gives of
these chaetae, the part implanted in the body is straight, while the part
extending freely beyond the body is sharply hooked.

The body of the leeches is never ciliated externally; there is, as in the
higher Oligochaeta, a cuticle secreted by the underlying epidermis. The
Hirudinea have, like the Oligochaeta, a clitellum which, as in some of the
lower members of that group, is limited to a very few segments in the
immediate neighbourhood of the generative openings. It occupies in _Hirudo_
segments 9, 10, and 11. The epidermis gives rise to many unicellular glands
which either remain _in situ_ or get moved to a deeper position. In this
the leeches exactly resemble the earthworms. There is generally a great
deal of connective tissue in the body-wall. The muscles consist of
circular, longitudinal, and radial series. The individual fibres have the
same structure as those of the Oligochaeta, consisting of a soft and
undifferentiated core, round which is a radially-striated sheath of
contractile substance.

ALIMENTARY CANAL.—The leeches are divided into the Rhynchobdellae, which
have a proboscis but no jaws, and the Gnathobdellae, which possess a series
of jaws but have no proboscis. But the division is not a hard and fast one,
for we have Whitman's genus _Leptostoma_, which should belong to the jawed
division, but which has quite rudimentary jaws without the sharp denticles
so characteristic of _Hirudo_. The pharynx is furnished with salivary
glands. The oesophagus is followed by the proventriculus, which has a
varying number of pairs of caeca; then comes the intestine and the rectum.
The anus is, as a general rule, placed dorsally to the sucker, but there
are a few rare exceptions where the anus is within the sucker. The caeca
are totally absent in _Trocheta_.

VASCULAR SYSTEM.—As will be seen from Fig. 205, the vascular system of the
Hirudinea is constructed on a plan which closely resembles that of the
Oligochaeta. The diagram represents _Glossiphonia_, one of the
Rhynchobdellae, the group which comes nearer to the Oligochaeta in many
particulars than the Gnathobdellae. We can recognise a dorsal and a ventral
vessel, which are united in the anterior part of the body by three
perioesophageal rings, the elongation of which, particularly of the last
pair (_v_), from before backwards is very marked. In the region of the
sucker the dorsal and ventral vessels are united by fourteen shorter loops,
the number of which has an interesting {397}relation to the number of
segments out of which this portion of the body is possibly formed. It will
be observed also that the dorsal vessel is double in this region, a
condition which obtains along its whole length in _Branchellion_—a
repetition of what has been described in more than one species of
Oligochaete. In the region of the last pair of digestive caeca the dorsal
vessel has appended to it copious sinuses which embrace the intestine and
supply its walls with blood. In _Hirudo_ there are only a pair of lateral
vessels, and neither dorsal nor ventral vessels; in this leech and in the
Gnathobdellae generally there are intra-epidermic capillaries, a fact first
discovered by Professor Lankester, and now known to occur also in the
Oligochaeta.

The development of the blood-vessels shows that they have no relation
whatever to the coelom, in spite of their subsequent connexion with it. The
two longitudinal stems of _Hirudo_ arise as cavities in the somatic layer
of the mesoblast after the formation of the coelom. In _Nephelis_, but not
in _Hirudo_, Dr. Bürger thinks that there is some reason for regarding the
vascular system as the remains of the primitive segmentation-cavity of the
embryo, an opinion which is held in respect of the vascular system of many
other animals.

[Illustration: FIG. 205.—_Glossiphonia marginata_, vascular and alimentary
system. ×4. (After Oka.) _a_, Dorsal vessel; _g_, intestinal caecum; _v_,
one of the hearts.]

BODY-CAVITY.—One of the most marked differences between the leeches and the
Oligochaeta is in the body-cavity. In the latter there are a series of
cavities corresponding to the segments, which are bounded in front and
behind by the intersegmental septa, and in which all the viscera lie. In
leeches such an arrangement is not recognisable save in _Acanthobdella_,
where Kowalevsky[455] has quite recently described a typical coelom divided
by septa into twenty segments. In transverse sections the body of other
leeches appears at first sight to be solid, owing to the growth of the
muscles and connective tissue. A more careful study, however, has revealed
the {398}fact that there are considerable remains of the body-cavity or
coelom which form a complicated system of spaces and channels. What has
happened, in fact, in the leech is that the coelom has become gradually and
partially obliterated by proliferation of the cells in the interior of the
body, a process of obliteration which has already commenced in the
Oligochaeta. In many of the latter, some of the principal blood-vessels
have become surrounded by a space cut off from the general body-cavity,
while in the majority a special cavity surrounds the testes and the funnels
of the sperm-ducts. This process of the formation of separate cavities for
the inclusion of the several viscera culminates in the leeches with the
marked obliteration of the greater part of the coelom. This has become so
much reduced to the condition of narrow tubes that there has been a
tendency to confuse it with the vascular system, more especially perhaps in
those forms in which the blood is tinged with haemoglobin, and in which
there is a connexion between the two systems of spaces. This confusion has
been further increased by the plan of injecting the vascular system, a
method of investigation which must be employed with great care in
delicately-organised creatures whose tissues can be easily ruptured, and so
lead to a flow of the injecting fluid into places and in directions
impossible during life.

[Illustration: FIG. 206.—Coelomic canals of _Glossiphonia complanata_. ×
10. (After Oka.) _a_, Dorsal canal containing dorsal blood-vessel; _b_,
ventral canal containing ventral blood-vessel; _l_, lateral canal; _n_,
nerve-cord.]

In transverse sections of leeches it may be seen in successful preparations
that the various organs of the body are enclosed in spaces. The funnels of
the nephridia open into lacunae which could hardly in any case be regarded
as blood spaces, while the blood-vessels themselves with their muscular
walls cannot be confounded, at least in the case of the larger trunks, with
the spaces not having muscular walls which surround them. Furthermore, it
will be pointed out immediately that the reproductive organs are produced
on the walls of spaces which are the commencement in the embryo of the
reduced coelom of the adult worm. These spaces therefore conform in every
particular to the general {399}conditions which have been laid down about
the characters of a true coelom. As to the complexity of this system,
attention may be directed, to the accompanying diagram (Fig. 206) of the
coelom of a segment of _Glossiphonia_, which has been lately worked out in
detail.[456] It will be observed that there are four main longitudinal
sinuses which are connected by a complicated system of transverse tubes and
spaces. In the anterior part of the body, before the point where the
intestinal caeca arise, the dorsal and ventral lacunae fuse to form one
larger so-called median lacuna. The cavity of this is interrupted, in
correspondence with the segmentation of the body, by septa exactly
comparable to those of Oligochaeta; but the septa in _Glossiphonia_ are not
present at every segment. So far our account of the coelom is chiefly
derived from the genus _Glossiphonia_. In _Hirudo_, which is an example of
the Gnathobdellae, the coelom is still further reduced; the lateral sinuses
in them appear to be absent. But on the other hand there is formed a series
of cavities in a form of connective tissue which has been termed botryoidal
tissue. The cells of this tissue become hollowed out, and form channels
which are in communication on the one hand with the remains of the coelom
and on the other with the vascular system. This system has certain
analogies with the lymphatic vessels of Vertebrates, and, like them, is an
intermediary between the body-cavity and the blood. Originally, however,
these botryoidal vessels have nothing whatever to do with either the
vascular or the coelomic system; their connexion with both is a purely
secondary affair, and only appears, comparatively speaking, late in life.

The development of the spaces here spoken of collectively as coelom
confirms this interpretation of their nature. In the embryos of _Hirudo_,
_Aulastomum_, and _Nephelis_ there is a ventral space,[457] which includes
the nerve-cord. Into this open a series of paired and segmentally-disposed
lateral cavities, a pair to each segment. The ventral cavity itself is
formed by fusion of the median parts of the lateral cavities. There is here
clear evidence of a coelom, developed on a plan fundamentally identical
with that of the Oligochaeta in that it is formed as a paired series of
chambers corresponding to the segmentation of the body.

NEPHRIDIA.—The "segmental organs" or nephridia are seen in {400}their
simplest form in such a type as _Glossiphonia_—the Rhynchobdellae, to which
this genus belongs, being indeed in most particulars less specialised than
the Gnathobdellae. Here we have a distinct funnel opening freely into the
median or ventral coelomic space, which is immediately followed by a
rounded swelling termed by Oka[458] the capsule; this is filled with cells,
in the interstices of which the ductules are situated and meander. There is
in this capsule a very strong likeness to the glandular brownish swelling
which immediately follows the funnel in the nephridia of certain of the
aquatic Oligochaeta, for example the Naids, where, as Vejdovsky has shown,
there is a similar "rete mirabile" of the nephridial duct. After the
capsule is a single row of cells which are disposed in a complicated coil.
These cells are perforated by the duct, which is thus, as in the
Oligochaeta, intracellular. In the first set of cells the duct is single,
and gives off numerous branchlets into the interior of each cell, a
condition which has also been observed in many Oligochaeta. Afterwards the
cells are perforated by two, or even three, main ducts, for the duct
returns upon itself and traverses the row of cells more than once; there
are also branchlets developed from one or other of the main ducts. The
terminal part of the nephridium is a short invagination from the exterior,
which is lined by cells. There is clearly a close resemblance here with the
nephridium of an Oligochaete. The nephridium, however, except for the
funnel and the narrow tube immediately following it, does not appear to be
ciliated.

[Illustration: FIG. 207.—Nephridium of _Hirudo medicinalis_. × 10. (After
Bourne.) _f_, Funnel; _v_, distal vesicle.]

There is, however, some difference of opinion as to the portions of the
nephridium where there are two ducts in a single cell. Bourne[459] thinks
that where there are two ducts there are two {401}cells, one lying inside
the other, and that there is sometimes also a telescoping of cell within
cell where the duct is single. In _Hirudo_ the same writer has described
the nephridial funnel, which has lost the simple character of that of
_Glossiphonia_. The funnel is represented by a cabbage-head-like mass (Fig.
207, _f_) of ciliated cells with no single definite outlet to the exterior
as in _Glossiphonia_. It appears to be an organ which has lost its proper
function—a degeneration of the funnel being, as a matter of fact, not
unknown in the Oligochaeta, where it may be carried to absolute extinction
(_Chaetogaster_). In _Branchellion_ and _Pontobdella_ the simple metameric
arrangement of the nephridia is to some extent lost, owing to the formation
of a network continuous from segment to segment. It will be borne in mind
that the Oligochaeta are the only other Chaetopods in which such a
nephridial network has been stated to exist.

MALE REPRODUCTIVE ORGANS.—In _Hirudo medicinalis_ there are nine,
occasionally ten, pairs of testes, which are round white bodies arranged
segmentally, _i.e._ a pair to each segment. From each arises a slender,
somewhat sinuous tube, which enters the common collecting tube of its own
side; each of these is much contorted at the upper end, the coiled portion
being termed the epididymis. From this they enter a muscular penis which
can be protruded. This is the arrangement met with in all leeches, save for
the fact that the penis is absent in some; in _Glossiphonia_ (see Fig. 208)
this is the case. The number of pairs of testes also varies; and in
_Nephelis_ they are no longer arranged metamerically.

[Illustration: FIG. 208.—Nervous system and reproductive organs of
_Glossiphonia plana_. × 2. (After Whitman.) _gl_, Prostate glands; _n_,
nerve-cord; _o_, ovary; _t_, testes.]

The testes arise as local proliferations of the epithelium of the lateral
coelomic cavities, but from the somatic wall, not from the splanchnic, as
in the case of the ovaries to be described later. A portion of the tissue
which is to form the testis grows out laterally into a thin cord, which is
to become the vas efferens of that {402}testis. Later both testis and duct
become hollowed out with a common cavity. The main portion of the vas
deferens of each side, as well as the terminal copulatory apparatus, is an
ingrowth from the epidermis which meets the downgrowths from the testes.

That there are considerable differences between the reproductive organs of
the leeches and those of Oligochaeta will be apparent from the above
description. There are, however, to begin with, certain obvious
similarities. In the first place, the origin of the reproductive glands is
identical; in both groups also the efferent ducts consist of two
portions—an invagination from the outside, and a formation of the proximal
part of the ducts near to the glands. In _Moniligaster_,
where—exceptionally—the testes develop on the posterior wall of their
segment in close contact with the funnels of the sperm-ducts, there is no
very hard and fast line to be observed between the tissues of the two. The
hollowing out of the testis in the leech, and the continuity of the cavity
thus formed with the duct, is a specialty of the leeches not found among
the Oligochaeta.

Like many Oligochaeta, the leeches may form spermatophores in which the
sperm is packed for its conveyance from one individual to another.
_Glossiphonia_ (_Clepsine_) _plana_, where the structure in question has
been elaborately described by Whitman,[460] may be selected as an example.
The spermatophore (Fig. 209) is about 8 mm. long, and is clearly formed of
two halves, each of which is formed separately in one ductus ejaculatorius,
the soldering together being effected in the common part of the male ducts,
where also a basal portion with a single lumen is added. The spermatophore
has a double wall. It is deposited not in the neighbourhood of the
generative pores, but upon the back; and Whitman has discovered the
extraordinary fact that the spermatozoa find their way through the
body-wall of the leech into the interior of its body, where fertilisation
presumably occurs.

[Illustration: FIG. 209.—Spermatophore of _Glossiphonia plana_. × 22.
(After Whitman.)]

{403}FEMALE REPRODUCTIVE ORGANS.—The ovaries of the Hirudinea appear to
differ from those of the Oligochaeta in that the ovaries are continuous
with their ducts. In _Hirudo_, however, the real ovary of each side
consists of masses of germinal tissue lying freely within a sac which
communicates with a duct; the two ducts unite to form a much convoluted
tube which opens into a thick-walled vagina, itself opening again on to the
exterior by a median unpaired opening on the seventh segment. The muscular
vagina is not always present.

The median unpaired female aperture offers now no particular difficulty,
since in many earthworms, e.g. _Perichaeta_, this orifice is in the same
condition; nor does the fusion of the oviducts and the so-called ovaries;
for in _Eudrilus_, for example, and in many Eudrilidae, the ovary is
contained in a sac into which the oviduct also opens. It will be noticed
too that the existence of short oviducts as compared with the long
sperm-ducts is a further point of likeness to at any rate the higher
Oligochaeta. But a further comparison needs first to be based upon a
consideration of the development of the different sections of the apparatus
in the leech. The independence of the ovaries and their ducts has been
proved by several observers; quite recently Bürger has dealt with the
matter in _Nephelis_, _Hirudo_, and _Aulastomum gulo_.[461] He has found
that the ovaries arise from the splanchnic wall of the lateral coelomic
cavities; they are therefore proliferations of the coelomic epithelium, as
in Oligochaeta and all Coelomates so far as is known. The peripheral layer
of the mass of indifferent cells which constitutes the ovary becomes
somewhat modified; its cells are flattened, and it at length separates
itself and forms a capsule surrounding the other cells, which are in fact,
or become, the ovary. This capsule meets and fuses with the ducts, which
are invaginations from the exterior of the body.

There are clearly differences between the ovary of a leech and that of a
typical Oligochaete like _Lumbricus_. The only point of agreement, in fact,
is the origin of the reproductive gland itself from the walls of the
body-cavity. In _Lumbricus_ and allied forms, whatever may be held with
regard to their homologies, the oviducts as a matter of fact appear first
as funnels, which afterwards bore their way to the exterior. They are
{404}purely mesoblastic structures, not—except perhaps the very
extremity—derived from an ingrowth from the epidermis. We have, however,
other Oligochaeta in which there are closer resemblances to what has just
been described in the Hirudinea. In more than one point the aberrant family
of the Eudrilidae come nearer to the Hirudinea than any other Oligochaeta,
in spite of _Branchiobdella_ with its jaws and sucker. Now in _Eudrilus_
the ovary is enclosed in a capsule which becomes continuous with the duct
of the great sperm-holding pouch, itself an invagination from the exterior;
there is no reason in this Annelid why the ova should not reach the
exterior by this system of ducts, although there is no actual experimental
proof that they do so. In any case there is also an oviduct corresponding
to that of _Lumbricus_, which opens into the opposite side of the duct of
the spermathecal pouch on the one hand, and into the receptaculum ovorum on
the other; it has no direct connexion whatever with the sac containing the
ovary. If we were to cut off the receptaculum and the oviduct and reduce
the spermathecal sac to its duct, the result would be much the same as we
find it in the leech.

COCOON.—The practice of forming a cocoon for the shelter of the eggs and of
the developing young is shared with the Oligochaeta; but not all leeches
deposit their eggs in this manner. _Glossiphonia_, for instance, carries
its eggs upon the ventral face of the body, where the young remain for some
time after they are hatched attached by the posterior sucker to their
parent's body, and from which situation of safety they make short
excursions. Other leeches deposit their eggs singly, but agglutinated
together upon stones, etc. In the medicinal leech the cocoon is ovoid in
shape, and from the end, which is closed by a temporary plug, the young
when ready escape. This cocoon is deposited, as is that of an earthworm, in
soil near to the borders of a marsh or pond, so that the young, while
enjoying the requisite degree of moisture, may not be injured by a too wet
environment. On the other hand _Pontobdella_ and some other leeches lay
their cocoons attached to bodies actually submerged in the water. The
cocoon is secreted, as in Oligochaeta, by the clitellum, and as in them, is
drawn off over the head, the ova and sperm probably flowing into it during
the process. The elasticity of the slightly hardened mucus causes the two
ends of the cocoon to {405}close up when it is free from the body; it is
then whitish and soft, and the leech fixes it, and appears to polish the
surface with its buccal sucker. In a few hours the cocoon becomes amber
brown, a colour which characterises the cocoons of a great many earthworms
and other Oligochaeta. The medicinal leech forms its cocoon in the same way
as does _Nephelis_, to which the above description refers; but when the
cocoon is formed the leech covers it with another layer of mucus, which
Vaillant, from whose work the foregoing notes are extracted,[462] thinks
may be produced from the so-called salivary glands.

CLASSIFICATION.—The number of different kinds of leeches is at present
uncertain. Seeing that no less than sixty-four varieties of the common
_Hirudo medicinalis_—colour varieties, it is true—are said to exist, it is
not wonderful that the labours of some systematists have been severe, and
have provoked much criticism and alteration on the part of others.[463] As
to genera, Vaillant, in his recent continuation of de Quatrefages'
"Annelés" in the _Suites à Buffon_, which includes the literature up to the
year 1886, allows thirty-seven, some (three) of which, however, are
admitted to be _incertae sedis_. Blanchard, who has paid a great deal of
attention to the group, reduces these by six, which he considers to be
synonyms; but on the other hand he has added or rescued from oblivion six
or seven others, and Whitman has instituted several Japanese and Australian
genera. Most of these generic types are, however, only imperfectly known,
and from external characters only. It is quite problematical how many valid
genera should be retained; in the meantime those that are fairly well known
are divided by Blanchard[464] in the following way:—

SUB-ORDER 1. RHYNCHOBDELLAE.—Hirudinea with an exsertile proboscis, without
jaws, and with colourless blood.

{406}FAM. 1. _Ichthyobdellidae._—Body formed of two regions, a narrower
anterior portion and a wider "abdomen," both anterior and posterior suckers
distinct from the body.

These leeches are parasites of fishes and of some other animals such as
tortoises. The family contains a number of genera. _Branchellion_ (Fig.
204) has a series of leaf-like branchiae on both sides; in _Cystibranchus_
the respiratory organs are reduced to a series of round vesicles. The
latter genus occurs in Europe and North America, and is parasitic upon
marine and fresh-water fishes. _Piscicola_ is a common leech which confines
its attacks to the inhabitants of fresh water. _Pontobdella_ (Fig. 210) is
marine, and affects rays and sharks; the best known species, _P. muricata_,
is usually of a green colour.

To the family Ichthyobdellidae also belongs the large Chilian leech
_Macrobdella valdiviana_, of which there is or are also species in North
America. Philippi's figure of this leech[465] shows the distinct neck; and
as it has no jaws, it should be referred to the present family. It has got
an undue reputation for size, 2½ feet[466] having been assigned to it. As a
matter of fact Philippi's illustration depicts an Annelid of about 7 inches
in length, with a greatest diameter of about an inch. But doubtless when
extended in walking it would be longer—1½ feet, Philippi thinks. It has no
eyes, a failing which is not unusual among the leeches.

FAM. 2. _Glossiphoniidae._—Anterior sucker fused with the body, posterior
sucker distinct. No cocoon.

[Illustration: FIG. 210.—_Pontobdella muricata._ × 1. (After Bourne.)]

The members of this family all inhabit fresh water; they have the habit of
depositing the eggs separately, which are then fixed to the ventral surface
of the body, and the young when hatched are {407}still protected by the
parent, returning to its body for shelter. The type genus _Glossiphonia_ (
= _Clepsine_) is common in Europe, and has many species. The Mexican and
Amazonian _Haementeria_ contains a number of species, of which the Mexican
_H. officinalis_ is used in medicine; but according to Miguel Jimenez, its
use is apt to be attended with unpleasant symptoms. Drowsiness, a buzzing
in the ears, and the development of a painful rash are some of the effects
produced by its bite. It is disputed whether the animal's saliva or foreign
matter introduced by it into the wound are the cause of the symptoms.

_Mesobdella_ of Blanchard[467] is said to be intermediate between this
family and the next. Each segment has three annuli, as in this family, but
the leech has three jaws, as in _Hirudo_.

SUB-ORDER 2. GNATHOBDELLAE.—Hirudinea without a proboscis, generally with
jaws; the blood is red; the eggs are invariably deposited in cocoons.

FAM. 1. _Gnathobdellidae._—Pharynx with three denticulate jaws.

This family as well as the next is terrestrial or fresh-water in habit. It
contains a number of generic types, including the medicinal leech, _Hirudo
medicinalis_, and the horseleech, _Haemopis_ (_Aulastomum_) _gulo_. The
former can be distinguished from the latter by its power of contracting
itself into an oval olive-shaped form, which power is not possessed by the
horseleech; the latter has, moreover, only two caeca, while the common
leech has ten pairs of these appendages of the intestine. The genus
_Limnatis_ is called after the Greek word λιμνῆτις, which Theocritus
applied to the leech. It is found in the Nile, and caused serious
inconvenience to the army of Napoleon. His soldiers in drinking at pools
sucked up the small leeches not thicker than a horse's hair, whose presence
in the hinder part of the mouth cavity produced divers objectionable
results, such as spitting of blood and hindered respiration.

FAM. 2. _Herpobdellidae._—Pharynx without denticulate jaws, with three
unarmed chitinous plates.

A characteristic genus of this family is _Trocheta_, which is so common at
the Zoological Society's Gardens and in the Regent's Park, and which has
been met with in other places near London; it is in this country an
introduced species, but is found in many {408}parts of the continent. It is
a land-leech, and lives upon earthworms.

The genus _Haemadipsa_, which M. Blanchard places in a special sub-family,
contains a number of species which are for the most part land-leeches.
Land-leeches occur in many parts of the world, but chiefly in the
tropics—in India, Ceylon, Java, South America, etc. They lie in wait for
their prey, upon the ground as a rule; but they may ascend herbs and shrubs
to gain a better outlook when they are aware of an approaching footstep. A
vivid account of the ferocity of these tiny Annelids in Ceylon can be read
in Sir J. E. Tennent's _Natural History of Ceylon_. They have been said to
be so pugnacious and so poisonous that persons surprised in their sleep by
the pests have succumbed to their united efforts. A whole battalion of
English soldiers decamped on one occasion from a wood which was overflowing
with land-leeches. The familiar misquotation "lethalis hirudo" might well
be applied to this species. Professor Whitman has written much upon the
habits of the land-leech of Japan (_Haemadipsa japonica_), which bites so
softly that its presence cannot be detected except for the stream of blood
which trickles from the wound. While it is feeding it emits from the pores
of the nephridia a clear fluid, which, as it appears, is used to keep the
skin moist; when unduly dried the same phenomenon occurs. It is curious
that in this and other leeches the nephridia should play a part which in
the earthworm is played by the dorsal pores; in both animals the glands of
the skin are also concerned with the same duty.

The purely aquatic leeches swim by undulations, and also crawl by the help
of the two suckers, like a "Geometer" caterpillar. But when a land-leech is
dropped into the water it at once sinks to the bottom and crawls out; it
does not swim, but can survive immersion for a long period. In this it
resembles the earthworms, which can also survive a prolonged immersion, and
even in the case of some are indifferent to the medium, land or water, in
which they live; the land-leech, however, is entirely dependent upon damp
surroundings; a dry air is fatal to it. The land-leech of Japan leaves a
slimy trail behind it as it crawls, in this respect recalling the land
Planarian _Bipalium kewense_.

GEPHYREA AND PHORONIS

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge

{411}CHAPTER XV

GEPHYREA

INTRODUCTION—ANATOMY—DEVELOPMENT—SIPUNCULOIDEA—PRIAPULOIDEA—
ECHIUROIDEA—EPITHETOSOMATOIDEA—AFFINITIES OF THE GROUP.

The animals included in the above-named group were formerly associated with
the Echinodermata. Delle Chiaje[468] states that Bohadsch of Prague in 1757
was the first to give an accurate description of _Sipunculus_ under the
name of _Syrinx_, but Linnaeus, who noted that in captivity the animal
always kept its anus directed upwards, re-named it _Sipunculus_.
Lamarck[469] placed the Gephyrea near the Holothurians; and Cuvier[470]
also assigned them a position amongst the Echinoderms. He mentions
_Bonellia_, _Thalassema_, _Echiurus_, _Sternaspis_, and three species of
_Sipunculus_, one of which, _S. edulis_, "sert de nourriture aux Chinois
qui habitent Java, et qui vont la chercher dans le sable au moyen de petits
bambous préparés."

The name Gephyrea[471] was first used by Quatrefages, who regarded these
animals as bridging the gulf between the Worms and the Echinoderms. He
included in this group the genus _Sternaspis_ (_vide_ p. 335), now more
usually classed with the Chaetopoda.

The Gephyrea are exclusively marine. They are subcylindrical animals, which
can either retract the anterior end of their body—the introvert—carrying
the mouth into the {412}interior; or are provided with a long flexible but
non-retractile proboscis. The latter is easily cast off. They usually bear
spines or hooks of a hard chitinous character, secreted by the epidermis or
outermost layer of cells. The mouth is at the base of the proboscis or at
the end of the protractile part, the anus is at the other end of the body
or on the dorsal surface. The nervous system consists of a ring round the
mouth and of a ventral nerve-cord. A vascular system is present as a rule.
Nephridia are found which act as excretory organs, and in most cases also
as ducts for the generative cells. The Gephyrea are bisexual, and the male
is sometimes degenerate.

The group may be divided into four Orders:—(i.) Sipunculoidea; (ii.)
Priapuloidea; (iii.) Echiuroidea; (iv.) Epithetosomatoidea; of these the
first is by far the largest, both in number of genera and of species.

THE ANATOMY OF SIPUNCULUS NUDUS.

EXTERNAL CHARACTERS.—The body of _S. nudus_ when fully extended may attain
a length of a foot, or even a little more; in this condition it is seen to
consist of two portions, the anterior of which is, however, retracted into
the other when the animal is disturbed. The retractile portion is sometimes
termed the proboscis, but as its nature is entirely different from that of
the proboscis of the Echiuroidea, it is better to refer to it as the
_introvert_. Special retractor muscles are attached on the one hand to the
body-wall about half-way down the body, and on the other hand are fused
into a muscular sheath which surrounds the gullet, just behind the mouth.
When these muscles contract, they withdraw the introvert into the rest of
the body or _trunk_ in much the same way as the finger of a glove may be
drawn into the hand, by a thread fastened to the inside of its apex. The
introvert is protruded by the contraction of the circular muscles of the
body-wall. These exert a pressure on the fluid which fills the body-cavity,
and by this means the sides of the introvert are forced forward until
finally the head is exposed.

The introvert occupies about one-sixth or one-fifth of the total body
length. It is somewhat narrower than the trunk, and is covered by a number
of small flattened papillae, some of which lie with their free ends
directed backward, overlapping {413}one another like tiles on a roof. In
some other genera, as _Phymosoma_, the introvert bears rows of horny hooks,
which are apt to fall off as the animal grows old.

The trunk has from thirty to thirty-two longitudinal furrows, the
elevations between which correspond with a similar number of muscles lying
in the skin. This longitudinal marking is crossed at right angles by a
circular marking of similar origin, the elevations of which correspond with
the circular muscles in the skin. These two sets of markings thus divide
the skin of the trunk into a number of small square areas, very regularly
arranged (Fig. 212).

The outline of the trunk is more or less uniform, but it is capable of
considerable change according to the state of contraction of its muscles.
The circular muscles, for instance, may be contracted at one level, thus
causing a constriction at this spot. The colour of _S. nudus_ is a somewhat
glistening greyish-white.

[Illustration: FIG. 211.—Right half of the anterior end of _Sipunculus
nudus_ L., seen from the inner side and magnified. _a_, Funnel-shaped
grooved tentacular crown leading to the mouth; _b_, oesophagus; _c_,
strands breaking up the cavity of the tentacular crown into vascular
spaces; _c'_, heart; _d_, brain; _e_, ventral, and _e'_, dorsal retractor
muscles; _f_, ventral nerve-cord; _G_, vascular spaces in tentacular
crown.]

The anterior end of the fully-expanded _Sipunculus_ may be termed the head;
here the skin is produced into a frayed fringe which stands up in the shape
of a funnel round the mouth. This fringe is grooved on its internal surface
with numerous little gutters, all of them lined with cilia, which by their
constant motion keep up a current which sweeps food into the mouth.
{414}The fringe may be in the form of a simple ring round the mouth, or the
ring may be folded in at the dorsal side so as to take the form of a double
horse-shoe (Figs. 211 and 212).

BODY-WALL.—The glistening appearance of _Sipunculus_ is due to the cuticle,
a chitinoid layer which is secreted by the external layer of cells, the
_epidermis_. Beneath this lies a layer of connective tissue, which is not
always present in other Gephyrea; within this lies a layer of circular
muscles arranged in bundles, then comes a very thin sheath of oblique
muscular fibres, then a thicker layer of longitudinal muscles, and finally
a layer of peritoneal epithelial cells, which in _Sipunculus_ are for the
most part ciliated.

Scattered over the surface of the body, and opening by narrow tubes which
pierce the cuticle, are a number of glandular bodies which may be either
bi- or multi-cellular. The glandular cells are apparently enlarged and
modified epidermal cells; they are arranged in a cup-shaped manner, with
their apices directed towards the orifice. They are crowded with granules,
which are presumably poured out over the cuticle, but the exact function of
the secretion is entirely unknown. They have a well-developed nerve supply.

DIGESTIVE SYSTEM.—The mouth lies in the centre of the fringe, and is not
provided with any kind of jaw or biting armature; it leads directly into
the thin-walled alimentary canal, the first part of which is ciliated. The
alimentary canal is not marked out into definite regions, but passes as a
thin-walled semi-transparent tube to the posterior end of the body, and
then turns forward again and opens to the exterior by an anus situated
about an inch below the junction of the introvert with the trunk, on the
median dorsal line. The descending and ascending limbs of the alimentary
canal are coiled together in a spiral, which may be more or less close in
different individuals. The whole is supported by numerous fine muscular
strands, which pass from the walls of the intestine to the skin, and by a
spindle-muscle, which runs from the extreme posterior end of the trunk up
the axis of the spiral and terminates in the skin close to the anus.

No glands open into the alimentary canal at any point of its course, but
near the anus a simple diverticulum, or pocket, of unknown function arises.
The size of this outgrowth differs {415}enormously in different
individuals. The alimentary canal near the anus also bears two tuft-like
organs, which, however, do not open into the intestine, but probably have
some function in connexion with the fluid in the body-cavity.

Along the whole course of the alimentary canal there runs a ciliated
groove, into which the food does not pass, but the cilia of which probably
keep in motion a current of water whose function may be respiratory.

[Illustration: FIG. 212.—_Sipunculus nudus_ L., with introvert and head
fully extended, laid open by an incision along the right side to show the
internal organs. × 2. _a_, Mouth; _b_, ventral nerve-cord; _c_, heart; _d_,
oesophagus; _e_, intestine; _f_, position of anus; _g_, tuft-like organs;
_h_, right nephridium; _i_, retractor muscles; _j_, diverticulum on rectum.
The spindle-muscle is seen overlying the rectum.]

VASCULAR SYSTEM.—On the dorsal surface of the anterior end of the
alimentary canal lies a contractile vessel, usually termed the heart. It is
a tube about an inch long, ending blindly behind, but opening in front into
a ring-shaped space surrounding the mouth and partially enveloping the
brain. From this ring-like vessel numerous branches are given off which
pass into the fringe round the mouth, and probably the chief function of
the heart is by its contraction to force fluid into this fringe, and so to
extend it. The heart contains a corpusculated fluid. {416}A similar but
shorter tube is found on the ventral surface of the anterior end of the
alimentary canal in the species in question; it also opens into the ring
which surrounds the mouth.

RESPIRATORY SYSTEM.—There are no special respiratory organs, and it has
long been a matter of dispute where the respiration of Gephyrea is carried
on. The oxygenation of the blood probably takes place to some extent
through the walls of the oral fringe, but the blood which receives its
oxygen at this spot is limited in its distribution, and could only supply
the brain and head. It seems probable that the remaining organs are
supplied with oxygen by the fluid of the body-cavity, which bathes them on
all sides. This might obtain its oxygen from the blood in the heart, or
more probably, through the thin walls of the intestine, from the stream of
water which is maintained by the ciliated groove described above. Quite
recently a form—_S. mundanus_, var. _branchiata_—has been described[472]
with thin-walled papillae covering parts of the skin. These papillae are
full of corpuscles, and are regarded by their discoverer as branchiae.

BODY-CAVITY.—The pinkish fluid of the body-cavity contains numerous
corpuscles, the products of the reproductive organs (either ova or
spermatozoa), and some curious unicellular bodies known as "urns." The
latter are shaped like a bowl with a ciliated rim, and are formed from the
budding of certain cells on the walls of the dorsal blood-vessel.[473]
Their function is unknown, but they resemble certain multicellular bodies
found in the body-cavity of _Phascolosoma_. The generative cells found in
the body-cavity are further considered below. The true corpuscles are
either biconcave round corpuscles coloured with a chemical substance, the
haemerythrin of Krukenberg, which apparently plays the same rôle as
haemoglobin in other animals; or amoeboid corpuscles, which, though rare in
_Sipunculus_, are very numerous in _Phascolosoma_.

NERVOUS SYSTEM.—The nervous system of _Sipunculus_ consists of a brain or
cerebral ganglion, a circumoesophageal ring surrounding the gullet, and a
ventral nerve-cord. The brain is a small bi-lobed nervous mass situated on
the dorsal surface of the oesophagus, in the angle between the right and
left dorsal retractor muscles close to their point of insertion. Numerous
{417}nerves arise from it, and pass to the fringe surrounding the mouth and
to neighbouring parts. At the sides, the brain is continued into two stout
nerve-cords which encircle the oesophagus, and meeting, fuse together in
the median ventral line to form the ventral nerve-cord (Fig. 211). The
latter is of the same diameter throughout, and shows no signs of
segmentation; it is oval in section, and consists of small ganglion cells
heaped up on the ventral surface, _i.e._ next the skin, and of numerous
fibres situated dorsally. The cord gives off many nerves, which usually
arise in pairs. These pass into the skin, and forming rings, run round the
body, and give off finer nerves as they go.

The nerve-cord is supported by numerous strands of muscle which pass to it
from the skin. These are especially long in the region where the introvert
joins the trunk, and thus allow free play to the nerve-cord when the former
is being protruded or retracted.

_Sipunculus_ is not well provided with sense-organs, but in an animal which
lives buried in sand we should not expect to find these very highly
developed. On the introvert there are certain patches of epithelium bearing
long stout cilia, which have been regarded as tactile in function, and
there is a tubular infolding reaching the brain, which almost certainly has
some sensory function. Ward[474] has termed this "the cerebral organ." It
consists of a duct lined with ciliated cells, which opens to the exterior
in the middle dorsal line outside the tentacular fringe. The duct leads
down to the brain, and expands at its lower end into a saucer-shaped space,
covering that portion of the brain where its substance is continuous with
the external epithelium. In _Phymosoma_ this cavity is produced into two
finger-shaped processes, which are sunk into the brain and are lined by
cells crowded with a dense black pigment.[475] They are probably
rudimentary eyes, perhaps distinguishing only between darkness and light.
The pits appear to be absent in _Sipunculus nudus_, but Andrews states they
are found, although without pigment, in _S. gouldii_.[476]

EXCRETORY SYSTEM.—The excretory organs or "brown tubes" are typical
_nephridia_, that is to say, they consist of tubes {418}with glandular
walls which open on the one side to the exterior, and on the other by means
of a ciliated funnel-shaped opening into the body-cavity. In Gephyrea one
wall of the tube is produced into a long diverticulum or sac which hangs
down into the body-cavity, and is usually supported by muscle-fibres
running to the body-wall. The lower end of the sac is broken up into a
number of crypts or pits, lined by large glandular cells crowded with brown
pigment. The pigment-granules are secreted into the cavity of the sac, and
leave the body through the external opening; they probably consist of the
nitrogenous excreta of the animal. The upper end of the sac, into which
both the external and internal orifices open, is usually enlarged, and its
walls are very muscular. As in so many other animals, the nephridia serve
as ducts through which the reproductive cells leave the body of the parent.

REPRODUCTIVE SYSTEM.—The Gephyrea are bisexual. In _Sipunculus_ the testes
and ovaries are found in the same position in the two sexes, and are
indistinguishable without microscopic investigation. They each consist of
small ridges situated at the lower end of the ventral retractor muscles,
just where the latter take their origin from the longitudinal muscles of
the skin. At this level the cells which line the body-cavity on the inside
of the skin are heaped up, and become modified in the one case into ova or
eggs, and in the other into the mother-cells of the spermatozoa. This
method of forming the reproductive organs from modified cells lining the
body-cavity is very common in the higher animals; but it is seen in its
simplest and least modified form in the Sipunculidae.

The eggs break away from the ovary in a very undeveloped condition, but
whilst floating about in the body-cavity they increase in size and secrete
a thick membrane around them. They have a well-marked nucleus, and are oval
in outline.

The mother-cells of the spermatozoa also break away in an immature
condition, and complete their development in the nutritive fluid of the
body-cavity. They divide into a number of spermatozoa, usually eight or
sixteen, which remain in contact. They each develop a tail, which projects
outwards, and aids the cluster in swimming along. These clusters of
spermatozoa are about the same size as the ova of the female, and, like
them, make their way into the "brown tubes." The exact way in {419}which
this is accomplished is not very clear, but the cilia on the funnel-shaped
internal opening of the tube seem to have some power of selecting the
generative cells when they come within their reach, and of passing them on,
whilst they reject the much smaller corpuscles of the perivisceral fluid,
which are never found in the nephridia.[477] Once inside the internal
opening, the clusters break up and the spermatozoa escape singly into the
sea. Here they meet with and fertilise the eggs which have escaped from the
body of the female.

[Illustration: FIG. 213.—Larva of _Sipunculus nudus_ L. × 150. (After
Hatschek.) _a_, Mouth; _b_, anus; _c_, excretory organ; _d_, glandular
appendage of oesophagus; _e_, wall of stomach over which the retractor
muscle runs; _f_, invaginated sense-organ at aboral pole.]

DEVELOPMENT.—Hatschek,[478] who investigated the development of _Sipunculus
nudus_ at Pantano, an inlet of the sea near Messina, states that the
spawning takes place during the night, and ceases about July 10. The rate
of development depends upon the temperature, but the larvae usually free
themselves from the egg-membrane during the third day. When hatched the
embryos lengthen out a good deal, and take the form represented in Fig.
213. The larva swims actively by means of a ring of stout cilia, which
encircle the body just behind the mouth. Other shorter cilia are found on
the head, continuing into the lining of the mouth, and a little bunch of
them is situated at the extreme posterior end. The alimentary canal is
already formed, and is twisted, so that the anus lies dorsally, but not so
far forward as it does in the adult. A glandular structure opens into the
mouth, and another body of unknown function is connected with the
oesophagus; both these disappear during larval life. A pair of excretory
tubules, the {420}forerunners of the brown tubes, are found, and the chief
muscle tracts are already established. The nervous system is still in close
connexion with the skin, from the outer part of which it is derived; the
cerebral thickening bears two eye-spots.

The fluid of the body-cavity contains corpuscles, which are kept in active
circulation by the constant contractions of the body-wall, and by numerous
tufts of cilia which are borne on the inner surface of the skin. The dorsal
blood-vessel is one of the latest organs to arise.

The larva swims actively about for a month, during which time it increases
greatly in size; it then undergoes a somewhat sudden metamorphosis. The
ciliated ring and the structures related to the oesophagus begin to
disappear, the distinction between the head and the rest of the body is
obliterated, and the head becomes relatively small. The mouth changes its
position, and becomes terminal instead of being somewhat ventral, and the
tentacular membrane begins to appear. At the same time the larva
relinquishes its free-swimming life, and sinks to the bottom; it begins
creeping amongst the sand by protruding and retracting the anterior part of
its body, and takes on all the characters and habits of the adult.

I. ORDER SIPUNCULOIDEA.

Besides the genus _Sipunculus_, the Order Sipunculoidea includes ten other
genera. A key to these, taken for the most part from Selenka's admirable
monograph, is given on page 424.

_Phascolosoma_ contains, in comparison with _Sipunculus_, only small
species, and it is easily distinguished by the fact that the longitudinal
muscles are fused into a continuous sheath. As a rule the skin is smooth. A
few species bear hooks, which are generally scattered irregularly and not
arranged in transverse rows, as in _Phymosoma_ (Fig. 214) and most of the
other genera.

The fold which in _S. nudus_ surrounds the mouth may be in the same species
bent in so as to take the form of a double horse-shoe, the opening of which
is always dorsal, just above the brain; in this case the mouth is
crescentiform. In other genera the fold is broken up into discrete
tentacles, and these are variously arranged; in _Dendrostoma_ they are
grouped together in four or six bundles round the mouth, but the more usual
{421}arrangement is the horse-shoe-like row of tentacles which overhang the
crescentiform mouth, as in _Phymosoma_ and some species of _Aspidosiphon_.

The ventral side of each tentacle is grooved and ciliated, and the grooves
are continued into the ciliated mouth. Their dorsal surface is pigmented,
and in the hollow of the horse-shoe lies a deeply pigmented epithelium
covering the brain.

A blood-vessel courses up each tentacle, and usually two channels return
the blood to the vascular ring which surrounds the mouth. In those forms
which possess tentacles on the dorsal side of the mouth only, the ventral
part of the vascular ring lies in the lower lip, which is tumid and
swollen. The brain supplies a nerve to each tentacle.

When the introvert is retracted the tentacular ring is withdrawn and to
some extent collapsed; in this condition it would be almost touching the
rough external surface of the introvert. In some species of _Phymosoma_ the
delicate appendages of the head are guarded from the hooks on the introvert
by a thin membrane or collar,[479] which completely ensheaths the retracted
head.

[Illustration: FIG. 214.—A, _Phymosoma granulatum_ F. S. Leuck. × 2. B,
Head of the same. × 4. _a_, Pigmented pit leading to brain. The
crescentiform mouth on the lower side of the figure is overhung by the
tentacles.]

When the introvert is fully extended the dorsal blood-vessel contracts and
sends its blood forward into the vascular ring, and thence into the
tentacles or tentacular fold, which are thus erected. In several species of
_Sipunculus_, as _S. nudus_, _S. norvegicus_, _S. robustus_, _S.
tesselatus_, there is a ventral blind tube as well as a dorsal, into which
the blood is withdrawn when the head is retracted. In many other species in
various genera, such as _Phymosoma {422}weldonii_ and _Ph. asser_,
_Dendrostoma signifer_, _S. vastus_, the lumen of the dorsal vessel is
increased by numerous hollow blind processes which it bears, hanging freely
into the body-cavity. Three very small genera of Sipunculids—_Onchnesoma_,
_Petalostoma_, and _Tylosoma_—are devoid of all trace of vascular system
and of tentacles; the mouth opens in the centre of the anterior end of the
introvert. In _Onchnesoma_ the dorsal part of the lip is somewhat produced,
so that the head has somewhat the shape of a Doge's cap, and in
_Petalostoma_ there are two leaf-like processes of the body-wall which
guard the mouth.

The extent to which the intestine is coiled varies very much even in the
same species; the axis of the coil is often supported by a spindle-muscle,
but this is sometimes absent. The caecum, which opens into the rectum of
_S. nudus_, is again a very variable structure, and when it is present
varies remarkably in size.

The food of Sipunculids seems to consist almost entirely of sand, and their
only nourishment must be such small microscopic organisms or particles of
animal and vegetable débris as are to be found mixed with the sand. The
alimentary canal is, as a rule, quite full of sand, and yet in spite of the
tenuity of its walls they never seem to be ruptured. If the contents of the
digestive tube be washed out with a pipette, it will be found that it
requires considerable force to dislodge many of the sand-particles lying
next the wall. These are more or less embedded in crypts or pockets of the
wall, and as the sand passes along the intestine they probably serve as
more or less fixed hard points, against which the sharp edges of the sand
particles are worn off. Amongst the sand are usually to be found pieces of
shell, sometimes with a diameter equal to that of the alimentary canal;
these are usually rounded, but their angles may have been removed by
attrition before they entered the mouth of the Sipunculid.

In _S. tesselatus_ the sand is to some extent held together by a mucous
deposit; in those cases where there is no sand in the intestine, there is
always a coagulum of mucus, and the walls are contracted and thick; when
full of sand the walls are tensely stretched and very thin. This thinness
of the wall of the alimentary canal seems ill-adapted to a diet of sand,
nevertheless it is also met with in other great sand-eating groups of
animals, such as the Echinids and the Holothurians.

{423}The enormous amount of sand and mud which passes through the bodies of
the Sipunculids shows that they must take a considerable part in modifying
the mineral substances which form the bottom of the sea. Just as
earthworms, as shown by Darwin, play a considerable rôle in the formation
of soil, so must these animals, in conjunction with Echinids and
Holothurians, effect considerable modifications in the sand and mud which
pass through their bodies. Mr. J. Y. Buchanan[480] is "led to believe that
the principal agent in the comminution of the mineral matter found at the
bottom of both deep and shallow seas and oceans, is the ground fauna of the
sea, which depends for its subsistence on the organic matter which it can
extract from the mud." The minerals at the bottom of the sea are exposed to
a reducing process in passing through the bodies of the animals which eat
them, and subsequently to an oxidising process due to the oxygen dissolved
in the sea-water acting on the minerals extruded from the animals' bodies.

The rate at which the sand passes through the body of _Sipunculus_ is
unfortunately unknown, but that at any one moment a considerable quantity
is contained in the intestine is shown by the fact that the average weight
of five specimens of _S. nudus_ from Naples, taken at random, was 19.08
grms., whilst the average weight of sand washed out of their alimentary
canal was 10.03 grms. The sand contained in five other specimens of the
same species measured respectively 6 c.c., 7 c.c., 6.5 c.c., 7.5 c.c., and
7.5 c.c., giving an average of 6.9 c.c. for each individual.

_Onchnesoma_ and _Tylosoma_ have only one retractor muscle; _Aspidosiphon_
and _Phascolion_ have, as a rule, two; _Phymosoma_ and _Sipunculus_ have
four, and perhaps this is the more usual number.

_Phascolion_, _Tylosoma_, and _Onchnesoma_ have but one "brown tube"; in
_Phascolion_ this is the right, in _Onchnesoma_ it is sometimes the right
and sometimes the left that persists. Most other genera retain two, but
there are many exceptions; for instance, _Phascolosoma squamatum_ has but
one, and so has _Aspidosiphon tortus_, and in both cases it is that of the
left side. No Sipunculid has more than two. It has been pointed out by
Selenka that those species which have but one brown {424}tube are, as a
rule, inhabitants of tubes or shells, and do not move actively about in the
sand.

The eggs of all members of the family, with the exception of the genus
_Phymosoma_, are spherical, but those of the last-named genus are
elliptical. They are always surrounded by a thick membrane, the "zona
radiata," pierced by numerous pores.

_Aspidosiphon_ (Fig. 215) is easily recognised by the presence of two
symmetrically-arranged cuticular shields, one at each end of the trunk.
These are formed by the fusion of minute cuticular plates, such as exist in
the skin of most Sipunculids. The posterior shield is radially symmetrical,
but the anterior is somewhat like the shell of a Pecten, and symmetrical
only about one plane. The introvert is protruded from the acute angle of
the anterior shield, and when extended lies almost at right angles to the
trunk, instead of being, as is usually the case, in the same straight line
with it. In many specimens, and these seem as a rule to be the older ones,
a deposit of calcium carbonate takes place over these shields, covering
over and concealing their external markings.

_Cloeosiphon_ (_Echinosiphon_) has a calcareous ring, consisting of four or
five rows of lozenge-shaped calcareous bodies forming a close mosaic,
arranged round the base of the introvert, which when extended is in the
same straight line as the trunk. Each piece bears a brown spot, which is
said to be the pore of a gland (Fig. 217). _Golfingia_ Lankester, has a
cylindrical horny thickening at the anterior end of the trunk and another
at the posterior.

[Illustration: FIG. 215.—_Aspidosiphon truncatus_ Kef. × 2. _a_, Introvert
partially extended, but not sufficiently to show the head.]

KEY TO THE GENERA OF SIPUNCULOIDEA.[481]

I. The longitudinal muscles in the body-wall divided into 17-41 distinct
bundles. Four retractor muscles.

A. Body covered with papillae. Numerous filiform tentacles which seldom
(or never?) surround the mouth, but stand above and dorsal to it in a
horse-shoe, with the opening dorsal. No rectal {425}caecum. Hooks
usually present. Four retractors (in _Ph. Rupellii_ only two?). Heart
almost always without caeca. Eye-spots always present. Eggs oval, flat,
reddish. Almost entirely small tropical species 1. PHYMOSOMA

B. Body devoid of papillae. Tentacular membrane surrounds the mouth in
a circlet. Rectum with one or more caeca (except _S. edulis_?). Hooks
absent except in _S. australis_. Eggs spherical. The tentacular
membrane contains a vascular network. A ventral contractile vessel
usually present in addition to the heart. Mostly large forms. Found in
all seas 2. SIPUNCULUS

II. The longitudinal muscles in the body-wall form a continuous sheath,
and are not split up into bundles.

A. Two brown tubes. Numerous tentacles form a wreath round the mouth.
Alimentary canal forms a complete spiral, free behind except in _Ph.
Hanseni_. Spindle-muscle usually present. One or more ligaments
present, but only on the anterior convolutions of the intestine.
Adhesive papillae always absent. Hooks very frequently absent. Eggs
spherical. Found in all seas. 3. PHASCOLOSOMA

B. Two free brown tubes. Only four or six plumed tentacles. A complete
intestinal spiral, not attached behind. Spindle-muscle always present.
One or more ligaments present, but only on the anterior convolutions of
the intestine. Hooks are present, but sometimes fall off early in life.
Heart usually bears caeca. Found only in the tropics. 4. DENDROSTOMA

C. Only one brown tube, that of the right side, present; it is attached
to the body-wall throughout its entire length. Numerous tentacles form
a circle round the mouth. The alimentary canal forms no spiral, or an
incomplete one. No spindle-muscle, but the intestine is attached to the
body-wall throughout its length by numerous ligaments. Adhesive
papillae often present. Not more than two retractors. Spherical eggs.
Inhabits Mollusc shells or tubes. Found in all seas 5. PHASCOLION

III. At both ends of the trunk a distinct horny shield, or tube-like
cornification, or a calcareous ring at the anterior end of the trunk.
Hooks sometimes present. Longitudinal muscles continuous or split up into
bundles.

A. A shield at both ends of the trunk. Introvert excentric, arising
from the ventral side of the anterior shield. Tentacles small and few
in number, arranged in a horse-shoe above the mouth. A spindle-muscle,
which arises from the posterior end of the body, traverses the
intestinal coil. Two retractors only, these are the ventral; they are
frequently fused together from their point of origin. 6. ASPIDOSIPHON

B. A calcareous ring surrounds the anterior end of the trunk, from the
middle of which the introvert is extruded. Longitudinal muscles
continuous. Hooks bifid. Tropical. 7. CLOEOSIPHON

C. A corneous ring, from which the introvert issues, surrounds the
anterior end of the trunk, and the posterior end of the trunk is
{426}produced into a corneous spike. Six pinnate tentacles encircle the
mouth. Four retractors. Hooks present on the introvert. Longitudinal
muscles continuous. Intestine not coiled throughout in a spiral nor
fastened posteriorly. Spindle muscle present. 8. GOLFINGIA

IV. No tentacles, but two leaf-like extensions of the body-wall guard the
mouth. Four retractors. Few intestinal loops, quite free. No vascular
system. 9. PETALOSTOMA

V. No tentacles, no vascular system. One retractor, and one segmental
organ.

A. Introvert long. Body small, pear-shaped. 10. ONCHNESOMA

B. No introvert (?). Body cylindrical, thickly covered with papillae,
which are larger and more crowded at both ends of the trunk.
11. TYLOSOMA

SPECIES OF SIPUNCULOIDEA.—The genus _Phymosoma_ (Fig. 214) contains more
species than any other genus of Sipunculoidea, and they are all of fair
size. Twenty-seven species are known, of which seventeen occur in the Malay
Archipelago, thirteen being found there alone. _Phymosoma_ affects shallow
water, the deepest specimens being taken at a depth of about 50 fathoms;
this may be due to the fact that they flourish only in comparatively warm
water. With very few exceptions, they are found only in tropical seas, very
often living in tubular excavations made in soft coral rock.

The genus _Sipunculus_ contains sixteen species. They are the largest and
the most conspicuous members of the group. They have a very wide
distribution, some species, as _S. nudus_ (Fig. 212) and _S. australis_,
being almost cosmopolitan. They are most common in temperate and tropical
seas, but _S. norvegicus_ and _S. priapuloides_ are found far north, but
always at considerable depths, 100 to 200 fathoms.

The following account of the habits of _Sipunculus gouldii_ is taken from
Mr. Andrews'[482] paper on that species:—

"This _Sipunculus_ is very abundant in certain small areas of compact, fine
sand darkened by organic matter and not laid bare at ordinary low tide. In
such places, only a few square metres in extent, they pierce the sand in
all directions to a depth of more than half a metre, making burrows with
persistent lumen running from the surface downward and then laterally, but
with no regularity in direction.

"Kept in aquaria, the dependence of the animal upon the {427}nature of the
sand and its method of locomotion may be readily observed. A vigorous
individual buries itself in a few moments in the following manner: Running
out the introvert to nearly its full extent, and applying it to the surface
of the sand till some spot of less resistance is found, the animal still
further expands the introvert so that it penetrates the sand, provided this
is not too dense and firm, for then the body is merely shoved backward.
When the introvert is inserted, the contraction of the longitudinal muscles
of the body-wall brings the whole body forward somewhat, in case the
introvert is fixed in the sand. In case soft ooze was present, this
fixation did not take place, and the introvert was merely pulled out again,
but when the sand was of the right consistency the introvert was fixed by
becoming much swollen at the tip, and then constricted just posterior to
this swollen area. This bulb-like area exerts lateral pressure on the sand,
as could be seen by movements of the grains. The swelling of the anterior
end of the introvert is brought about by the body-wall contracting
elsewhere, and forcing in liquid to distend that end. Owing to the curved
form assumed by the body in the normal contracted state when first removed
from its burrow, the entrance of the introvert may often be nearly
vertical, and hence the entire body is soon raised nearly upright in the
water above the sand. If the body has thus been warped forward sufficiently
to become somewhat fixed in the sand, the introvert is rolled in and again
thrust forward from this new point of resistance, and so on till the animal
is entirely buried. This locomotion increases in speed as the creature
becomes more completely surrounded by sand, and is the only means of moving
from place to place.

"On a smooth surface, or on one not presenting the right degree of
resistance, the _Sipunculus_ does not change its position, but remains till
death finally occurs, rolling its introvert in and out and contracting its
body-wall to no purpose.

"The essential factors in the mechanism bringing about this hydrostatic
locomotion are an elongated contractile sac filled with liquid, and some
means of definitely co-ordinating the contractions of the sac.

"In natural environment the animals are found with sometimes one, sometimes
the other end nearer the surface of the sand: in the aquaria the same was
observed, but when the {428}water became stagnant and impure the anterior
end with expanded branchiae was often protruded somewhat above the surface
of the sand."

The genus _Phascolosoma_ contains at least twenty-five species, for the
most part small. _Ph. margaritaceum_, however, measures[483] 10 cm. in
length, and _Ph. flagriferum_, 13 cm. The latter is produced at the hinder
end of its trunk into a long whip-like process, which recalls the horny
spike of _Golfingia_. Most species live free, but a few inhabit the shells
of dead Gasteropods or of _Dentalium_, or the abandoned tubes of worms.
They occur in practically all seas.

[Illustration: FIG. 216.—Specimens of the Coral _Heteropsammia cochlea_,
with _Aspidosiphon heteropsammiarum_ or _A. michelini_ living in a state of
commensalism with them. (From Bouvier.)]

_Dendrostoma_ contains but five species, which are all found within the
tropics in the Pacific or in the West Atlantic. They are shallow-water
forms, and some are found between tide-marks.

_Phascolion_ is a smaller genus, containing but ten species, which may have
been derived independently from different species of _Phascolosoma_, and in
this case the genus should be broken up. The members of this genus live in
Mollusc shells, such as _Dentalium_, _Turritella_, _Buccinum_, _Chenopus_
(_Aporrhais_), _Nassa_, _Strombus_, and generally acquire the coiled shape
of their host. They are usually attached to the shell by means of certain
{429}adhesive papillae found on their posterior end. _Ph. strombi_ fills
its shell with mud, which must be kept together by some secretion of the
animal. The body lies in a tube in this mud, and the introvert projects
from the small round opening at the end of the tube, and explores the
ground in every direction. They are found in all seas, but more especially
in the colder waters.

[Illustration: FIG. 217.—_Cloeosiphon aspergillum_ Quatr. × ½. _a_,
Introvert covered with spines and partially extended, but not sufficiently
to show the head; _b_, calcareous plates surrounding the point of origin of
the introvert.]

The genus _Aspidosiphon_ includes nineteen species, which are, with few
exceptions, exclusively confined to the Indian Ocean and neighbouring seas,
including the Red Sea. The exceptions are _A. armatus_ from the Norwegian
coast, and _A. mülleri_ from the Mediterranean and Adriatic. _A. truncatus_
is also stated to occur at Panama, the Bahamas, and at Mauritius. The
remaining species almost all occur in the Malay Archipelago and
neighbouring islands, and as was the case with _Phymosoma_, this part of
the world seems to be the headquarters of the genus. _A. mülleri_ lives in
the interstices of rocks and stones, and occasionally in disused Mollusc
shells.

Two species of _Aspidosiphon_ have been described by Bouvier[484] living in
a state of commensalism with two species of Madreporarian corals,
_Stephanoceris rousseaui_ and _Heteropsammia cochlea_, which live on and
surrounding the shells of certain Molluscs at Aden (Fig. 216). Apparently
the Gephyrean takes up its abode within its house at a tender age, and
according to Bouvier, it provides for its increasing bulk by secreting a
coiled calcareous tube, the outer surface of which affords space for the
growth of the coral.

The genus _Cloeosiphon_, the _Echinosiphon_ of Sluiter, includes three
species: _C. aspergillum_ (Fig. 217), _C. molle_, and _C. javanicum_. The
first named occurs at Mauritius, the Malay Archipelago, and neighbouring
islands; the others are {430}confined to the last-named area, which thus
again forms the headquarters of a genus.

_Golfingia_, described by Lankester from a single specimen, was dredged in
St. Andrews Bay, at the depth of 10 fathoms.

_Petalostoma_ comprises but one species, _P. minutum_, which is found in
the English Channel.

_Onchnesoma_ comprises two species, _O. steenstrupii_ and _O. sarsii_, both
found off the coast of Norway at considerable depths between 200 and 300
fathoms.

_Tylosoma_ comprises one species, _T. lütkenii_, also from the Norwegian
coast. It is dredged from stony ground in 50 to 80 fathoms.

II. ORDER PRIAPULOIDEA.

ANATOMY.—This Order consists of the two genera _Priapulus_ and
_Halicryptus_. Both are cylindrical animals with the mouth at one end and
the anus at the other. The introvert is short, and is covered with rows of
chitinous spines, which are continued to some extent over the body.

The skin is folded in a series of rings, and the body is usually somewhat
swollen posteriorly. _P. caudatus_ bears a curious caudal appendage, beset
with a number of hollow lobes somewhat grape-like in appearance. This is
situated ventral to the anus; its lumen is continuous with that of the
body-cavity, but it can be separated from it by the action of a sphincter
muscle. Two such appendages exist in _P. bicaudatus_.

There cannot be said to be any head in the Priapuloidea; they have no
tentacles or tentacular fringe, no proboscis, and no distinct brain; simply
a round aperture, the mouth, which is surrounded by a groove in the skin,
at the bottom of which the circumoesophageal nerve-cord lies. The mouth
leads into a very muscular pharynx lined with stout chitinous teeth; this
passes into an intestine, which is as a rule straight, but in _P.
glandifer_ it has a single loop.

The Priapuloidea possess no vascular system and no brown tubes. Their skin
has in the main the same structure as that of the Sipunculids, with spines,
glandular bodies, and papillae with sensory hairs which resemble similar
structures on _Phymosoma varians_. Retractor muscles arise from the
longitudinal muscles {431}of the skin, and are inserted into the pharynx;
they are short and not constant in number.

The nervous system has retained throughout its primitive connexion with the
epidermis. In almost all animals the nervous system is formed from the
epiblast or outermost cellular layer of the embryo; it usually, however,
breaks away from this and sinks into the body. Thus in _Sipunculus_ it lies
within the body-cavity, and has retained its primitive connexion with the
outer layers of the skin only in the region of the brain; but in the
Priapulids the nervous system, which consists of a ring round the mouth and
of a ventral cord, lies embedded in the skin, and the nerve cells are
directly continuous with the cells of the epidermis. The nerve-ring lies at
the base of a groove in the skin, which forms a kind of gutter round the
mouth; the ventral nerve-cord is visible exteriorly as a light line which
marks the ventral surface of the animal. In no place is the ring or cord
differentiated in any way, and there cannot be said to be any brain or
special sense-organs. Numerous nerves are given off from the ring to the
pharynx and intestine, and from the cord to the body-wall.

[Illustration: FIG. 218.—_Priapulus caudatus_ Lam. Nat. size. _a_, Mouth
surrounded by spines.]

The sexes are distinct, but they differ from the other Gephyrea in the
nature of their reproductive organs. In mature specimens the ovaries or
testes are easily recognisable, lying to the right and left of the
alimentary canal. The reproductive glands are continuous with ducts, which
act as oviducts and vasa deferentia respectively. Both glands and ducts are
attached to the body-wall by a mesentery.

The excretory function is performed in the Priapuloidea by the ducts of the
generative organs. These are primarily connected with a number of branching
canals of small size which project into the body-cavity. According to
Schauinsland,[485] one or more pear-shaped cells are found at the end of
each branch, and each is {432}continued into a long cilium which hangs down
into the lumen of the canal, and by its movement produces a flickering
motion. Beyond the free end of the large cilium the canal is lined with
ciliated cells. The remarkable resemblance this form of excretory organ
presents to that of the Platyhelminthes (_vide_ p. 25) and of certain
Chaetopods is worthy of attention. In the young Priapuloidea the duct with
its branching canals is not masked by the generative organs, but as the
animals become mature, diverticula from the duct arise, and the cells
covering these become modified into ova in the female, and into spermatozoa
in the male. The presence of these follicles masks the excretory part of
the gland. The ova and spermatozoa escape through the ciliated ducts which
open to the exterior one on each side of the anus, and, contrary to what is
the case with other Gephyrea, leave the body without having ever been in
the body-cavity.

Nothing is known of the embryology of either member of this family, but
both genera appear to be sexually mature from the end of May until October.

CLASSIFICATION.—The two genera which make up the Order Priapuloidea are
characterised as follows:—

_Priapulus._—The body is continued into one or two caudal appendages, beset
with hollow papillae; these are ventral to the anus. The introvert forms ¼
to ⅓ of the total body-length; it is covered with spines in conspicuous
longitudinal rows, the rest of the body being ringed. The retractor muscles
are numerous, and are attached to the body-wall, some anteriorly and some
posteriorly.

The genus includes the following five species:—

_P. caudatus_ Lam. (Fig. 218). _Hab._ Coasts of Greenland, Norway, Great
Britain, the North Sea, and the Baltic.

_P. bicaudatus_ Dan. _Hab._ North Sea and Arctic Ocean.

_P. glandifer_ Ehlers. _Hab._ Coast of Greenland, North Sea.

_P. brevicaudatus_ Ehlers. _Hab._ North Sea and Baltic, from ten fathoms.

_P. tuberculato-spinosus_ Baird. _Hab._ Falkland Islands.

_Halicryptus._—No caudal appendages. Introvert ⅒ to 1/12 of the total body
length, with numerous spines arranged in close circles. Retractors numerous
and all attached to the body-wall anteriorly.

_H. spinulosus_ v. Sieb. (Fig. 219). _Hab._ North Sea, Arctic Ocean, and
Baltic, in from two to fifty fathoms.

{433}It will be noticed that with the exception of _P.
tuberculato-spinosus_, described by Baird from a single specimen, the whole
family is confined to northern seas.

HABITS.—The newly-captured specimens of both _P. caudatus_ and _H.
spinulosus_ are of a flesh colour, with a somewhat metallic sheen.
According to Apel, the latter lived in an aquarium for more than five
months, whilst the former died during the first month. When first
introduced into the aquarium they immediately began to busy themselves in
the mud or sand at its bottom, and very seldom showed themselves above it.
They forced their way into the sand by alternately contracting and
extending their introvert, and the _Priapulus_ arranged itself so that a
portion, often a very small one, of its caudal appendage was exposed to the
water; this fact supports the view that the appendage is respiratory in
function. When the animal buries itself deeply, the appendage does not
relinquish its position at the surface of the sand, but stretches itself
until it in some cases surpasses the length of the body. On the other hand,
_Halicryptus_ (Fig. 219), according to the same observer, lies with the
anterior end, the mouth, projecting from the surface of the sand, or else
it curves itself, so that both ends project into the water.

[Illustration: FIG. 219.—_Halicryptus spinulosus_ v. Sieb. × 6. _a_, Dark
line indicating the position of the ventral nerve-cord; _d_, mouth
surrounded by spines.]

Leckenby, who described specimens of _P. caudatus_ which were found by
fishermen searching for worms for bait in the outer harbour at Scarborough
at half tide, states that they live in sandy clay in U-shaped tubes, at a
depth of about 9 inches, the tubes opening at each end on to the surface of
the sand. The fishermen of this district call them "sea mushrooms."

_Halicryptus_ casts its cuticle in May and September; it becomes loose
first at the hinder end, and the split between it and the skin grows
forward until the animal lies free in a cuticular mantle. After some days
this is split, and the animal frees itself from it; the cast-off cuticle
includes for a short {434}distance the lining of the mouth, the anus, and
the two generative pores.

III. ORDER ECHIUROIDEA.

ANATOMY.—The most striking peculiarity of the Echiuroidea, as opposed to
the other two families of the Gephyrea, is the presence of a solid dorsal
outgrowth of a portion of the head, forming the proboscis. The nature of
this proboscis is something quite different from that of the introvert of
the Sipunculoidea; it would appear to correspond to an extension, in the
members of the last-named Order, of that part of the head which is dorsal
to the mouth and is covered by a peculiar pigment-epithelium, often in
continuity with the brain. In its outgrowth this portion of the body has
carried with it the nerve-ring and the vascular ring, which both surround
the mouth. The proboscis is found in all the genera with the exception of
the aberrant genus _Saccosoma_.

[Illustration: FIG. 220.—A, _Bonellia viridis_ Rol., ♀; B, _B. fuliginosa_.
Both nat. size. _a_, Grooved proboscis; _b_, mouth; _c_, ventral hooks;
_d_, anus.]

The body of the female _Bonellia viridis_, one of the best known species of
Echiurids, is shaped like a small sausage, and is usually about 2 inches
long. The proboscis arises from the anterior end, and is extremely
extensible. At the distal end the proboscis splits into two short arms,
which are often recurved; along the whole ventral surface runs a groove
lined with cilia, which by the approximation of its edges can be converted
into a tube. At the bottom of the proboscis the groove opens into the
mouth. {435}_Echiurus_; _Thalassema_, and the female _Hamingia_ have short
proboscides, which do not bifurcate but otherwise resemble those of the
female _Bonellia_.

[Illustration: FIG. 221.—View of a female _Bonellia viridis_ Rol., opened
along the left side, × 2. _a_, Proboscis cut short; _b_, a bristle passed
through the mouth into the pharynx; _c_, convoluted intestine; _d_, anal
tufts or vesicles; _e_, ventral nerve-cord; _f_, ovary borne on ventral
vessel running parallel with _e_; _g_, position of anus; _h_, points to
position of external opening of nephridium; _i_, nephridium. This line is
on a level with the internal funnel-shaped opening.]

The green colour of _B. viridis_ is due to a special pigment, "Bonellein,"
which at one time was thought to be identical with chlorophyll. A similar
green colour is found in _Hamingia arctica_, _Thalassema baronii_, and the
larvae of many forms.

A short distance behind the mouth, on the ventral surface, the female
_Bonellia_ and both sexes of _Thalassema_ and _Echiurus_ bear two incurved
stout chitinous hooks; these gave the name {436}Gephyrea Armata to the
above-mentioned genera. In addition to these, _Echiurus_ has a row of
chitinous bristles surrounding the posterior end of the body; the row is
single in _E. unicinctus_, double in _E. pallasii_. These bristles are
formed, like the hooks on the introvert of the Sipunculoidea, by epidermal
cells; those of _B. minor_ and of the posterior rings in _Echiurus_ are
said to arise each from a single cell, just as the bristles do in
Chaetopods.

The skin consists of very much the same layers as does that of
_Sipunculus_; the cuticle is thin, the epidermis is modified into numerous
glandular cells, papillae, and pits, from which the bristles arise. A third
layer of oblique or circular fibres is usually found inside the
longitudinal muscle-layer. The proboscis is solid, and contains much
connective-tissue and numerous muscle-fibres running in all directions; the
ventral groove is ciliated.

The alimentary canal in the Echiuroidea consists of a long thin-walled tube
with numerous convolutions; it is not coiled as in Sipunculids, but the
loops are irregularly arranged, and are supported by numerous fine muscular
strands which run from the skin. There is a ciliated groove running along
one side of the intestine, as in the Sipunculids. The anus is terminal. The
most striking peculiarity of the alimentary canal of the Echiurids is the
existence of a collateral intestine or "siphon." This is a narrow tube
which arises from the main canal not very far from the mouth, and re-enters
it again lower down. A similar structure occurs in some Echinids, and in
the Capitelliformia (pp. 272, 305). Its function is not certainly known.

Another characteristic feature of the Echiurids is the presence of "anal
vesicles," branching structures which unite into a common stem opening into
the intestine close to the anus. The free end of each of the branches
terminates in a ciliated funnel-shaped opening. The function of these
structures may be excretory, or they may control the amount of fluid in the
body-cavity.

A closed vascular system exists in Echiurids, consisting of a contractile
dorsal vessel running along the dorsal surface of the anterior end of the
alimentary canal, and continued along the axis of the proboscis. At the tip
of the proboscis it bifurcates, and each branch descends along the edge
until it reaches the base where, having encircled the oesophagus, the two
unite, and are continued as the ventral vessel which runs along the dorsal
surface of the nerve-cord, and eventually ends blindly. There is also a
vessel which {437}passes from the ventral vessel and encircles the
intestine, opening into the posterior end of the dorsal vessel. In
_Echiurus_ the same vessel encircles a stout muscle which runs from the
base of one of the ventral bristles to the other. In _Thalassema_ Lankester
states that the fluid within the vessels is colourless, and does not
contain corpuscles similar to those in the body-cavity fluid.

The "brown tubes" or nephridia vary in number in the Echiurids. In the
female _Bonellia_ there is but one; in _B. viridis_ the right, in _B.
minor_ the left usually persists. In shape, colour, contractility, and
minute structure they closely resemble those of _Sipunculus_. _Hamingia_ is
said to have a pair of brown tubes; _Echiurus_ has two pairs, except _E.
chilensis_, which has three; their internal openings are produced into long
coiled slits in some genera. _Thalassema gigas_ has one pair; _Th.
neptuni_, _Th. baronii_, _Th. formosulum_, and _Th. exilii_, two; whilst
_Th. vegrande_, _Th. moebii_, _Th. erythrogrammon_, _Th. caudex_, and _Th.
sorbillans_ have three pairs.

The nervous system consists of a ventral cord lying in the body-cavity, as
in the Sipunculoidea, but attached to the skin, and of a circumoesophageal
ring. With the growth of the proboscis this ring is drawn out, and the two
branches run along the sides of the proboscis and unite at the tip. There
is no specialisation of brain, nor are any special sense organs present,
but the ventral cord gives off paired nerves at regular intervals, which,
uniting dorsally, form rings in the skin in some and probably in all
species.

The perivisceral fluid is of a dark brown colour in _Thalassema_,
containing numerous spherical corpuscles deeply impregnated, according to
Lankester, with haemoglobin, and also containing granules of a brown
pigment. Haemoglobin is also found in certain of the muscles and in part of
the epithelial lining of the body-cavity. Lankester also describes the
presence of haemoglobin in the corpuscles of the perivisceral fluid in
_Hamingia_.

The genital glands are, like those of the Sipunculoidea, formed by a
special development of the cells lining the body-cavity. These cells are
massed together along the wall of the ventral blood-vessel. In _Echiurus_
and in _Thalassema_ the cells break off and float in the body-cavity,
developing into ova and spermatozoa. In _Bonellia_ each cell does not
become an egg, but a mass of cells breaks off, one of which increases in
size at the expense of the {438}others and forms the ovum. The mature
sexual cells leave the body through the nephridia.

[Illustration: FIG. 222.—An adult male _Bonellia viridis_ Rol. The original
was 1.5 mm. long. The nervous system is not shown. (After Selenka.). _a_,
Generative pore with spermatozoa coming out; _b_, anterior blind end of
intestine attached to the parenchymatous tissue by muscular strands; _c_,
green wandering cells containing chlorophyll; _d_, parenchymatous
connective-tissue; _e_, epidermis; _i_, intestine; _j_, vas deferens; _l_,
internal opening of vas deferens; _m_, the left anal vesicle; _n_,
spermatozoa in the body-cavity.]

_Bonellia_ and _Hamingia_ present very interesting cases of sexual
dimorphism. In both genera the female is an animal of considerable size
with the normal structure of the Echiuroidea, but the male (Fig. 222) is a
microscopic Planarian-like animal, which lives in the mouth and in the
nephridia of the female. Both in _Bonellia_[486] and in _Hamingia_ the male
is provided with a pair of hook-like ventral bristles; these are wanting in
the female _Hamingia_. The surface of the male is ciliated, and the skin
{439}contains circular and longitudinal muscle-fibres. The body-cavity is
developed, but does not reach to either end of the body. The alimentary
canal is closed, neither mouth nor anus existing; it is supported by
regularly arranged dorso-ventral muscle strands. A nerve-ring and a ventral
cord exist. There are also two rudimentary organs corresponding with the
anal vesicles of the female, and a single nephridium which acts as a duct
for the spermatozoa; the latter arise from modified cells lining the
body-cavity.

In both sexes the larvae develop to a certain stage without showing any
trace of sexual differentiation, but after this stage, the development of
the male is to a certain extent arrested; in some respects, indeed, it
undergoes retrogressive changes. At this time it is found clinging to the
proboscis of the female, thence it makes its way to the mouth, where it
undergoes its final change; and then creeping out, finds its way into the
nephridium of the female, and spends the rest of its life there in a
special recess cut off by a fold from the excretory part of this organ. In
_Hamingia_, however, Lankester, who first described the male, did not find
any in the nephridia, but found five specimens, each 1/12 inch long, within
the dilated pharynx of the female.

DEVELOPMENT.—In _Bonellia_ and _Hamingia_ it seems probable that the ova
are fertilised in the nephridium of the female; in the other genera they
are fertilised in the water after leaving the body of the mother.

In _Thalassema_ and _Echiurus_ the growth of the embryo results in the
formation of a typical Trochosphere larva, a type widely spread in the
animal kingdom, being found in the Chaetopoda (Fig. 145, A), Polyzoa (p.
510), and Mollusca. The large prae-oral lobe persists in the Echiuroidea as
the proboscis; the mouth is ventral in position, with usually a ring of
cilia encircling the body in front of and behind it; the anus is posterior
and terminal. A pair of larval excretory organs are present, and a special
nervous aggregation of cells at the apex of the prae-oral lobe is usually
indicated by the presence of a bunch of long cilia.

The trunk of the Trochosphere is unsegmented, and in certain groups of
animals it remains so, but in Chaetopods, and in _Echiurus_ and
_Thalassema_, it elongates and becomes divided {440}up into a series of
somites or segments. Of these there are fifteen in _Echiurus_, and
apparently eleven in _Th. mellita_; in this stage the Gephyrean larvae have
again so close a resemblance to the segmenting Chaetopod larvae as to be
easily mistaken for them. The segmentation is shown in the following way:
(i.) the middle layer of cells or mesoblast is typically segmented, and
forms septa, which separate each segment from its neighbours; (ii.) the
ventral nerve-cord arises as segmentally-arranged thickenings of the
epiblast, which fuse together, but retain their segmented appearance for
some time; (iii.) the skin shows the segmentation of the body both by the
arrangement of the pigment and by bands of cilia. The latter are replaced
in the adult by rows of spines, and on the fourteenth and fifteenth
segments in _Echiurus pallasii_ by the two peri-anal circles of bristles.
Each bristle, like those of Chaetopods, originates from a single cell.

The anal vesicles arise quite late in the development; when they have
acquired their openings into the body-cavity, they seem to take in water.
In _Thalassema_, as described by Conn, this is accompanied by remarkable
changes, amounting almost to a metamorphosis. The body increases in bulk
fourfold, the cilia of the prae-oral ring disappear, and the animal now
moves only by means of its proboscis; the pigment is absorbed, and all
traces of segmentation disappear. A similar intaking of water is described
by Spengel in _Bonellia_. In this genus the larva, which is coloured bright
green, and has two brown eye-spots, is not such a typical Trochosphere as
is that of _Echiurus_ and _Thalassema_.

[Illustration: FIG. 223.—_Echiurus pallasii_ Guér. × ½. _a_, Mouth at the
end of the grooved proboscis; _b_, ventral hooks; _c_, anus.]

Nothing is known of the development of _Hamingia_ or of _Saccosoma_.

SPECIES OF ECHIUROIDEA.—_Echiurus._ Proboscis not bifurcated at the end.
Two ventral hooks and a single or double peri-anal ring of bristles. The
body is to a varying extent marked {441}by rings bearing spines. Two or
three (_E. chilensis_) pairs of nephridia, their external orifice often
lengthened and spirally coiled. Both sexes alike.

Greef[487] mentions six species of _Echiurus_, viz. _E. pallasii_, _E.
forcipatus_, _E. sitchaensis_, _E. chilensis_, _E. carabaicus_, and _E.
chrysacanthophorus_; to these must be added _E. unicinctus_. It seems
probable that _E. forcipatus_ of Reinhardt is identical with _E. pallasii_,
although bigger, whilst _E. sitchaensis_, _E. carabaicus_, and _E.
chrysacanthophorus_ are inadequately described. The distribution of the
remaining three species is as follows:—

_E. pallasii_ Guérin (Fig. 223). North Sea, Atlantic, English Channel.

_E. unicinctus_ Drasche. Japan.

_E. chilensis_ Max Müller. Chili.

_Thalassema._—Proboscis rather pointed at the end, not bifurcated. No
peri-anal bristles, but two ventral hook-like bristles placed anteriorly.
One to three or four pairs of nephridia. The sexes resemble each other.

Greef mentions eight species of _Thalassema_ and Rietsch thirteen; three of
these, however, _Th. grohmanni_, _Th. lessonii_, and _Th. pelzelnii_, were
not seen by either author, and their description is taken from Diesing.
There is some reason for thinking that the two first-named species are
identical with _Th. neptuni_. Conn has established a new species for the
specimens whose embryology he worked out at Beaufort, Virginia, and Selenka
described a new species from the Challenger material.

With the exception of the three doubtful species mentioned above, the list
of species of _Thalassema_ is as follows:—

_Th. neptuni_ Gaertner (Fig. 224). English Channel (Devonshire coast),
Concarneau, Mediterranean (Gulf of Marseilles), Irish coast (Dungarvan).

_Th. gigas_ Max Müller. Trieste.

_Th. vegrande_ Lampert. Philippine Islands.

_Th. baronii_ Greef. Canary Islands (Lanzarote).

_Th. formosulum_ Lampert. Shanghai and Philippine Islands.

_Th. exilii_ Fr. Müller. Brazil (Desterro).

_Th. moebii_ Greef. Mauritius.

_Th. erythrogrammon_ Max Müller. Red Sea and East Indies (Billiton).

_Th. caudex_ Lampert. Red Sea and Indian Ocean.

_Th. sorbillans_ Lampert. Philippine Islands.

_Th. mellita_ Conn. West Atlantic (Beaufort).

_Th. faex_ Selenka. North of the Faroe Islands.

{442}_Bonellia._—Proboscis very extensible and bifurcated at the end. The
body and proboscis are coloured a bright green. Two ventral hook-like
bristles, but no peri-anal ring. A single nephridium. The above applies to
the female; the males are degenerate, and live in the nephridium or pharynx
of the female.

Three (or four?) species of this genus are known.

_B. viridis_ Rolando (Fig. 220). Mediterranean, Adriatic, North Sea
(Bergen).

_B. minor_ Marion. Mediterranean (Gulf of Marseilles, Naples).

_B. suhmii_ Selenka. Off Nova Scotia. Male not known.

_B. fuliginosa_ Rolando? (Fig. 220). Mediterranean (Naples).

_Hamingia._—Proboscis not bifurcated, about as long as body. No ventral
hook-like bristles. One or two nephridia, which open at the apex of one or
two well-marked papillae. The above applies to the female; as in the genus
_Bonellia_, the male is minute and parasitic. It has two well-marked
hook-like bristles situated behind the genital pore.

This genus was first described by Koren and Danielssen as _H. arctica_. Two
specimens were afterwards described by Horst as _H. glacialis_. Later
Lankester described two other specimens; he was the first to find the male
in the pharynx of the female. He is of the opinion that all three
descriptions apply to the same species, and for this the original name _H.
arctica_ must be retained.

_Hamingia arctica_ K. and D. Two hundred miles north of North Cape and in
the Hardanger Fjord.

_Saccosoma._—No proboscis. The body is flask-shaped. The mouth and anus are
terminal. The ovary is anterior, and there is only one nephridium. No
bristles.

Our knowledge of this remarkable Gephyrean is very incomplete, but such as
it is, it is due to the careful investigations of Koren and Danielssen, who
had only a single specimen at their disposition.

_Saccosoma vitreum_ K. and D. North of the Faroe Islands.

HABITS OF THE ECHIUROIDEA.—As a rule the members of this group conceal
their bodies in clefts and fissures of rocks and stones, keeping up
communication with the outer world by means of their proboscis.
Rietsch[488] describes a specimen of _Bonellia minor_, which he placed in
an aquarium, exploring with its {443}proboscis the nature of the bottom;
when the animal had found a convenient crevice, it fixed its proboscis in
it by means of the bifurcated end, and by its contraction drew the body up,
and entered the hole, proboscis first. It then turned round, and during
this operation doubtless the ventral hooks came into play; and then
stretching out its proboscis, it began to explore the neighbourhood. The
proboscis is evidently very sensitive, and in addition to being a locomotor
organ, it is also used for the prehension of food. If cut off near the
mouth, the animal does not long survive, but if a considerable portion is
left the scar heals, and the lost part is probably regenerated. In
captivity the animals frequently change their place of residence.

Eisig some years ago described the great extensibility of the proboscis of
_B. viridis_ when confined in the tanks of the Zoological Station at
Naples. When contracted the proboscis was but a few inches long, but at
times it was extended till it reached the length of 1½ metre, shining
through the water as a transparent green thread. The body of the _Bonellia_
was hidden under stones, but the proboscis could be seen seizing between
its two ends the bodies of certain Ascidians which covered the inside of
the tank, tearing them off the walls, and conveying them to the mouth along
its grooved ventral surface.

The food of the Echiuroidea consists of organic matter, in the main of
animal nature, but the group differs from the Sipunculoidea in not eating
sand.

Rietsch describes _Thalassema neptuni_ as being more active in its
movements and less sedentary than _B. minor_. The proboscis is still the
chief organ of locomotion, but the trunk plays a greater part in the
movements of the animal than it does in the last-named species. _Th.
neptuni_ is found in cavities of stones or in the chambers worn out by the
Mollusc _Gastrochaena_; when withdrawn from its house the body is found to
be covered by a thick layer of tenacious viscid mucus.

[Illustration: FIG. 224.—_Thalassema neptuni_ Gaert. × 2. A, The animal
lying on its ventral surface. B, Ventral view of the anterior end, showing
the grooved proboscis ending behind in the mouth, and the ventral hooks.]

{444}_Th. mellita_ was so named by Conn because it is found sheltering in
the Echinid _Mellita_. "It enters the shell at the oral opening while yet
very small, but once within its house it grows to its adult size, and is
obliged therefore to remain during the rest of its life a prisoner." Each
shell thus inhabited acquires a reddish brown horse-shoe-shaped marking,
which affords a conspicuous signal that the shell contains a _Thalassema_.

_Thalassema_ is seldom found living in sand, and _Bonellia_ never, but
_Echiurus_ is almost always found in U-shaped tubes or passages in the
sand, which it digs out for itself by the rapid contractions of its
body-wall aided by its bristles. It, like the other two genera named above,
does not long remain in the same hole, but frequently changes its home. As
a rule the _Echiurus_ sits near the mouth of its tube, which is often a
foot or even two in depth, and sends out its proboscis in every direction;
at the least sign of disturbance it withdraws into the deeper recesses. The
walls of the tube are kept from falling in by a layer of mucus, which makes
a smooth lining to the passage. The peri-anal bristles, which can be
withdrawn or protruded at will, enable the animal to fix itself at any
level in the tube.

The Echiuroidea are sometimes used by fishermen as bait. In _Echiurus
pallasii_ Greef found three parasites, all of them new species. One, a
Gregarine, he named _Conorhynchus gibbosus_; the others were
Platyhelminthes, and were named by him _Distomum echiuri_ and
_Nemertoscolex parasiticus_ respectively.

IV. ORDER EPITHETOSOMATOIDEA.

This Order includes the single Family _Epithetosomatidae_, which was
established by Koren and Danielssen to contain the remarkable Gephyrean
they described in 1881 under the name _Epithetosoma norvegicum_ (Fig. 225).

Unfortunately only two specimens were at their disposition, and these were
badly preserved, so that many details of their structure could not be made
out. The animals are of an olive-green colour, and consist of a trunk about
12 mm. long, and of a proboscis 30 mm. in length; the latter differs
essentially from the proboscis of the Echiuroidea inasmuch as it is hollow,
and seems to be a whip-like tubular extension of the skin. Its lumen opens
into the body-cavity. Ventral to the base {445}of the proboscis is the
mouth; the intestine is straight, and terminates in the anus, which is
posterior. The nervous system lies between the circular and the
longitudinal muscles of the body-wall, and contains a tube, the nature of
which is obscure. No vascular system is known. The ovary is attached to a
mesentery ventral to the anterior part of the alimentary canal, and there
is a single nephridium. No anal vesicles exist.

The most remarkable feature of the genus is a series of pore-like openings,
which are stated to lead from the outside into the body-cavity (Fig. 225,
_a_). These are arranged four on each side, at the bottom of two slit-like
depressions in the skin, which lie one on each side of the base of the
proboscis, slightly dorsal to it.

These remarkable structures are without parallel amongst the Gephyrea, and,
together with the peculiar character of the proboscis, justify the
Norwegian naturalists in adding a new family to the group.

[Illustration: FIG. 225.—A, _Epithetosoma norvegicum_ K. and D., magnified.
_a_, _a_, Right and left slits leading to the pores; _b_, mouth; _c_,
proboscis: B, the same animal opened dorsally; _a_, pores; _b_, oesophagus;
_c_, proboscis; _d_, brown tube. (After Danielssen and Koren.)]

AFFINITIES OF THE GEPHYREA.

Before considering to what other groups of animals the Gephyrea may be
allied, it is advisable to discuss the relationship of the four Orders
which compose the group.

Quatrefages, in the year 1865, divided the Gephyrea into I. _Gephyrea
Armata_, with which he included the Echiuroidea and _Sternaspis_,[489] and
II. _Gephyrea Inermia_ or Sipunculoidea. The Gephyrea Inermia, sometimes
called the Achaeta, have been extended to include the Order Priapuloidea,
and opposed to the smaller sub-group the Gephyrea Armata or Chaetifera. In
my opinion, however, these names now are no longer in accordance with our
knowledge of the structure of the {446}animals they attempt to describe,
and they should be given up. Both names had reference to the presence or
absence of the two hook-like bristles described on the ventral surface of
some of the Echiuroidea, but of the five genera of this family, two,
_Saccosoma_ and _Hamingia_ (the latter in the female or normal form), are
without these bristles, and can therefore be described neither as _Armata_
nor as _Chaetifera_. On the other hand, hook-like chitinous bristles of
somewhat the same nature, though smaller in size and varying in position,
are very common on the introvert of Sipunculoidea and on the body of the
Priapuloidea.

Again, the association of the two last-named Orders in one sub-group is, to
my mind, an error. The Priapuloidea have little in common with the
Sipunculoidea; almost the only real point of resemblance is the power of
protruding the anterior part of the alimentary canal, and withdrawing it by
the aid of retractor muscles. But in the Priapuloidea this power exists to
a very small extent, and it is a power shared by very many animals besides
the Gephyrea. The terminal anus of the former is a feature shared by the
Echiuroidea and by _Epithetosoma_, but these have little else in common
with the Priapuloidea. On the other hand, the entire absence of any head
appendages, such as the proboscis of the Echiuroidea and the tentacles or
tentacular membrane of the Sipunculoidea, the absence of a vascular system,
of nephridia or anal vesicles, taken together with the straight intestine
which occurs elsewhere only in _Epithetosoma_, the persistent connexion of
the nervous system with the epidermis, the unique character of their
excretory system and of the reproductive organs, are all features in which
the Priapuloidea differ from the more normal members of the other three
Orders. These constitute a list of peculiarities which are at least as
important, and probably even more important, than those which characterise
the Sipunculoidea and the Echiuroidea. Thus the Priapuloidea should, I
think, be regarded as a distinct Order, which occupies a very isolated
position in the group.

Until we know something about the development of _Halicryptus_ and of
_Priapulus_, it will be difficult to say whether the Order is more nearly
allied to one or the other of the two great Orders of Gephyrea, whether it
is very primitive or very specialised. The connexion of the entire nervous
system with the epidermis and the absence of a vascular system are both
{447}rather primitive features, and so is the Platyhelminthine character of
the excretory organs. With regard to the vascular system, however, it
should be pointed out that it arises very late in the larva of those
Gephyrea whose development is known, and that it does not seem to
correspond with the vascular system of other animals; it has no fine
vessels or capillaries connected with it, and apparently does not act so
much as the channel of the circulatory medium, but more as a mechanism for
the expansion of the head appendages, the tentacles in the Sipunculoidea
and the proboscis in the Echiuroidea; moreover, it is absent in some genera
of the former, such as _Onchnesoma_, _Tylosoma_, and _Petalostoma_, where
there are no tentacles.

The conclusion of the whole matter seems to be that the Priapuloidea are an
isolated Order retaining many primitive features, and having no closer
affinities to the Sipunculoidea than to the Echiuroidea.

Hatschek came to the conclusion, from his work on the development of
_Echiurus_, that the Echiuroidea are true "Annelids," and from the presence
and mode of formation of the bristles, that they are related to the
Chaetopods. In this view he is confirmed by Conn, who worked out the
development of _Thalassema_. This relationship is further confirmed by the
discovery of Sluiter's that _Sternaspis_, the genus of Chaetopods which in
other respects most nearly resembles the Gephyrea, has in one of its
species (_S. spinosa_) a well-marked bifid proboscis, which, like that of
the Echiuroidea, is thrown off at the least disturbance. Thus it seems
fairly well established that the Echiuroidea are closely connected with the
Chaetopoda, for although the only traces of segmentation they retain in the
adult are the serially-repeated nephridia of _Thalassema_ and _Echiurus
pallasii_, and the two rows of peri-anal bristles in the latter, and
possibly the circular nerves given off from the ventral cord, yet the larva
is fully segmented, and in other respects is almost typically Chaetopodan.

The relationship of the Sipunculoidea to the Echiuroidea is a more doubtful
point. Hatschek is inclined to separate them, and in this he is again
supported by Conn. Embryology unfortunately does not help us much. The
early stages and larvae of _Sipunculus nudus_ and of _Phascolosoma
elongatum_ have been investigated by Hatschek and by Selenka respectively.
In neither genus is there any trace of segmentation or of Annelid
{448}features, with the possible exception of the bristles on the larval
_Phascolosoma_. On the other hand, it must be remembered that the
development of _Sipunculus_ is remarkably abbreviated, and that such stages
may have dropped out, the larvae hardly differing more from the
Trochosphere of _Echiurus_ and _Thalassema_ than does that of _Bonellia_,
an undoubted Echiurid. Still the facts that there is never a head-kidney
present, that there is no trace of segmentation, and that at no stage is
the anus terminal, must have a certain weight.

If we leave out of account the larval history, which, although pointing to
a difference in the nature of the two families, is by no means decisive,
and consider the adult structures, we find very considerable evidences of
affinity. Taking firstly the main points of difference, we find these to be
(i.) the nature of the cephalic appendages, either a proboscis or some
modification of tentacles; (ii.) the position of the anus; (iii.) the
presence of anal vesicles; (iv.) the number of the nephridia, never more
than one pair in Sipunculids; and (v.) the difference in origin of the
chaetae. Of these most undoubtedly the first is the most important. The
Echiuroidea have retained the prae-oral lobe of the larva in the form of a
solid outgrowth of the body, which outgrowth has carried with it the
nerve-ring and vascular ring which surround the mouth. This has been lost
in the Sipunculoidea, but is, I think, represented by a modified patch of
epidermis which lies dorsal to the mouth and just above the brain. A solid
extension of the skin in this region, which involved the nervous and
vascular systems, would bring about the same relation of parts as is found
in the Echiuroidea. The tentacular membrane or tentacles of the
Sipunculoidea have such a variety of form and arrangement, whilst all
subserving the same end, that I am inclined to believe that they have
originated within the limits of the family.

The position of the anus in the Sipunculoidea is one common to very many
animals which live embedded in sand or in tubular holes; it is probably not
primitive, as in the larva of _Sipunculus_ it is near the posterior end,
and becomes more dorsal as the larva elongates.

The anal vesicles of the Echiuroidea probably have no representative in the
Sipunculoidea. In appearance and position they are very like the little
tufts which are found on the rectum of {449}_Sipunculus_, but since these
open neither into the body-cavity nor into the alimentary canal, it is
hardly fair to compare them.

The resemblances between the Orders seem to me, on the whole, to outweigh
the differences. The general structure of the skin, the coiled alimentary
canal, with its ciliated groove, supported by strands of muscles, the
vascular system which gives off no capillaries, the structure of the brown
tubes, the existence of chitinous hooks or bristles, the nervous system
with its single unsegmented ventral cord, the formation of the generative
organs, all point to a sufficiently close resemblance to justify us in
classing the two Orders together. In addition to these there are
considerable histological resemblances which cannot be discussed here, but
which have a certain weight.

To sum up, it seems probable that the Echiuroidea are derived from the
Chaetopoda, and that their nearest ally in this group is _Sternaspis_; and
that the Sipunculoidea are allied to the Echiuroidea, but have further
departed from the Annelid stock, and have lost even those traces of
affinity with the parent group which have been preserved in the development
of _Echiurus_ and _Thalassema_.

So little is known of _Epithetosoma_ that it is difficult to discuss its
affinities. The presence of the hollow proboscis and the pores leading into
the body-cavity undoubtedly justify its being placed in a separate Order,
but beyond the presence of a terminal anus, in which it resembles the
Echiuroidea, there is nothing in its structure which connects it more
nearly with one than with the other of the three larger Orders of Gephyrea.

LIST OF GEPHYREA FOUND IN THE BRITISH AREA AS DEFINED BY CANON NORMAN.

_Phascolosoma vulgare_ Blainv. English Channel and North Sea.
" _elongatum_ Kef. English Channel.
" _papillosum_ Thom. English coast.
" _eremita_ Sars North Sea.
" _procerum_ Moeb. Bass Rock.
_Phascolion strombi_ Mont. English coast (Plymouth).
_Sipunculus nudus_ L. North Sea, English Channel (Paignton,
Teignmouth).
_Golfingia macintoshii_ Lank. East coast of Scotland (St. Andrews Bay).
_Petalostoma minutum_ Kef. English Channel (Plymouth).
_Priapulus caudatus_ Lam. Scarborough, Outer Hebrides.
_Echiurus pallasii_ Guérin Coast of Scotland, English Channel.
_Thalassema neptuni_ Gaertner English Channel (Coast of Devonshire).

{450}CHAPTER XVI

PHORONIS

HISTORY—HABITS—STRUCTURE—REPRODUCTION—LARVA—METAMORPHOSIS—LIST OF SPECIES
AND LOCALITIES—SYSTEMATIC POSITION.

This interesting genus was discovered and first described by Dr. Strethill
Wright of Edinburgh, who in the year 1856 found specimens of it living on a
stone with _Caryophyllia_ sent to him from Ilfracombe. He christened the
form _Phoronis hippocrepia_,[490] the generic name being apparently taken
from an epithet applied to Io, the specific name having reference to the
beautiful horseshoe shape of its tentacular crown. Two years later a
closely allied or identical form was described by Professor P. J. van
Beneden under the name of _Crepina gracilis_.[491]

_Phoronis_ is a sedentary animal living in "colonies," but each member of
the colony is distinct, and has no organic connexion with the others, from
which it is isolated by the presence of a tube in which it lives, and into
which it can be completely withdrawn. The tube is formed from a secretion
which probably has its origin from the anterior end of the body-wall. The
secretion hardens and forms at first a transparent coating, but it soon
becomes opaque, and numerous sand particles, small pieces of shell, sponge
spicules, and other marine objects adhere to the outside of the tubes,
giving them a very characteristic appearance, and doubtless serving to
protect the inhabitants from predatory animals.

{451}What little we know about the habits of _Phoronis_ is in the main due
to the observations of Cori,[492] who studied _Ph. psammophila_ at Faro, an
inlet of the sea near Messina. The least disturbance causes the animal to
withdraw its head with lightning rapidity into the tube, from which after a
time it re-emerges very slowly, and does not expand its tentacular crown
until its body is completely extended. Cori states that not unfrequently
individuals are found either without the crown of tentacles or with the
latter in process of regeneration. These may have been bitten off by fish,
etc.; but, on the other hand, van Beneden describes in _Crepina gracilis_
(_Ph. hippocrepia_) the throwing off and regeneration of the crown of
tentacles; and Cori confirms his observation, at any rate as far as
concerns those individuals kept in captivity, and whose surroundings were
presumably somewhat unfavourable. He further observed the interesting fact
that the cast-off crown of tentacles continued to live, and suggests that
possibly it may develop a new body, in which case the phenomenon would be
an interesting case of binary fission producing two new animals.

[Illustration: FIG. 226.—A piece of a matted colony of _Ph. kowalevskii_
Cald. Slightly magnified. In most cases the tentacular head is protruding
from the tube.]

With regard to the habitation of _Ph. australis_, the largest species
known, some discrepancies have crept into the literature of the genus, and
to prevent their recurring again it may be worth while to quote the
statements of its discoverer, Mr. Haswell.[493] He says: "_Phoronis
australis_ occurs in communities of twenty to thirty, in spaces in the
substance of the wall of the tube inhabited and formed by a species of
_Cerianthus_. Each worm has a tube of its own, very delicate and
transparent, made up of several layers, the mouth opening on the outer
surface of the tube of the _Cerianthus_. The _Cerianthus_ tubes sometimes
come up empty, as we should naturally expect, the animal having dropped
out; but a sufficient number of {452}occupied tubes are found to show that,
under ordinary circumstances, a living _Cerianthus_ occupies the interior
of the tube and a community of _Phoronis_ live in its wall. This species of
_Phoronis_ is never found anywhere else, and the species of _Cerianthus_ is
very rarely found without the _Phoronis_."

_Ph. australis_ is sluggish in its movements, but other species are capable
of very active movement, and withdraw their heads in a moment at the
approach of danger. A Neapolitan species, _Ph. kowalevskii_—known to the
fishermen of that place as "Ficchetelli bianchi" or "Vermi di ceppa"—lives
chiefly on submarine posts and piles; its tubes, closely interlacing, form
a dense feltwork, upon which Ascidians and Sea-anemones often settle, and
over which Ophiurids and Polychaets creep. The tubes of this species are
rendered opaque by the excreta ejected from the body, and they do not
attach foreign substances to the outside to anything like the same degree
as _Ph. psammophila_, which live in sandy places, and are termed by the
Sicilian fishermen "Tubi di sabbia." The feltwork of _Ph. kowalevskii_
attains a thickness of 5 to 8 cm. In each case the tube is much longer than
the animal it shelters, and is so entangled with its neighbours, to which
it frequently adheres, that it is a matter of considerable difficulty to
isolate it.

[Illustration: FIG. 227.—A piece of a colony of _Ph. psammophila_ Cori.
Slightly magnified. The tubes are covered by particles of sand, small
shells, etc.]

The various species of _Phoronis_ differ a good deal in size; Cori gives
the average length as varying from 1.5 to 7.9 mm. in _Ph. hippocrepia_ and
up to 127 mm. (6 inches) in _Ph. australis_. Probably the very short
individuals of the first-named species had not attained their adult
stature. _Ph. australis_ has recently formed the subject of a memoir by Dr.
W. B. Benham,[494] from whom the following account is mainly taken.

The length of the individuals varied from three to six inches, and their
diameter, which is not very uniform, averaged one-eighth of an inch. At one
end, which, since it bears the mouth, we may call the oral end, is the very
characteristic tentacular {453}crown surrounding the mouth on all sides but
one, where there is a slight break in its continuity. The crown of
tentacles or lophophore is flattened, and the two ends drawn out, and each
is coiled into a spiral (Fig. 229); between the bases of these two spirals
three ridges can be seen, each ending in a pore; the median opening is the
anus, the two lateral are the openings of the nephridia or kidneys, which
also serve as ducts for the reproductive organs. The anus is thus
approximated to the mouth, and since the continuity of the tentacular crown
is broken at a spot just between the two, there would be nothing to
separate these orifices if it were not for the presence of the epistome, a
projection or flap of the body-wall which overhangs the mouth between it
and part of the crown of tentacles.

The extent to which the ridge bearing the tentacles is incurved at each
side varies in different species. In _Ph. kowalevskii_ and _Ph.
psammophila_ the ends are only slightly turned in, so that the crown of
tentacles is truly horse-shoe shaped; but in _Ph. australis_ they are
turned in and form three coils on each side. The number of tentacles also
varies, being very numerous in _Ph. australis_ and _Ph. buskii_—the latter
having as many as 300, whilst the other species as a rule have from 60 to
90. The bases of the tentacles are fused for a short distance with one
another, forming a thin membrane.

[Illustration: FIG. 228.—A specimen of _Ph. buskii_ M‘Int. removed from its
tube and seen from behind, × about 2. (After M‘Intosh.)]

The rest of each tentacle is free, and its inner surface, or that turned
towards the mouth, is covered with long cilia, which, by the currents they
set up, doubtless serve to bring food to the mouth. The tentacles are
hollow, and their cavity is kept open by a stiffening of the tissue, which
almost resembles an internal skeleton; the cavity communicates with the
anterior part of the general body-cavity, and up it runs a single
blood-vessel containing red blood. A single nerve is also distributed to
each tentacle.

{454}At the base of the two spirals of the tentacular crown lie two
ciliated pits, regarded by Caldwell and M‘Intosh[495] as sensory organs,
but Benham looks upon them as glandular in structure and function. Perhaps
they secrete the substance from which the tubes are formed.

The skin is covered by a delicate cuticle secreted by the underlying
epidermis; within the latter is a well-marked basement membrane, and
beneath this a layer of circular muscle fibres; these surround a layer of
longitudinally-arranged fibres, which do not form a continuous sheet but
are arranged in bundles. In both layers the fibres are unstriated. The
longitudinal fibres are covered on their inner side by a layer of flat
pavement cells, which line the general cavity of the body.

This space, the body-cavity, is divided into two parts by the presence of a
diaphragm or septum which runs across from one side of the body to the
other about the level of the ridge bearing the tentacular crown. The
anterior space is continuous with the cavities of the tentacles and of the
epistome. The partition is pierced by the blood-vessels and the oesophagus,
but the rest of the alimentary canal, including the anus, the kidneys, and
the reproductive organs, all lie in the posterior half of the body-cavity
behind the diaphragm. This portion of the body-cavity is further subdivided
by the presence of three longitudinal mesenteries supporting the alimentary
canal and running between it and the body-wall. One of these mesenteries
runs along the outside of the alimentary canal throughout its whole length,
attaching both the descending and ascending limbs of the U-shaped tube to
the body-wall. The other two are lateral mesenteries, which pass from the
body-wall to the sides of the oesophagus. These mesenteries therefore
divide the body-cavity into three spaces—one in which the rectum lies,
which may be called the rectal, and two lateral; owing to the fact that the
lateral mesenteries end before they reach the bend of the alimentary canal,
the three chambers are in free communication one with another. The
body-cavity is further traversed by irregular strands of tissue which run
from the body-wall to the various organs. It contains a corpusculated
fluid.

The alimentary canal (Fig. 230) consists of a U-shaped tube {455}which may
be divided into four regions. The mouth (_m_) leads into the oesophagus
(_oe_), which gradually enlarges into the stomach (_st_) situated just
before the bend; a constriction just at the bend separates the stomach from
the intestine (_int_), and this leads into the rectum (_r_), which
terminates in the anus (_an_). The first three divisions of the alimentary
canal are ciliated, but the rectum is not; the walls of the stomach also
contain glandular cells, but there are no special glands opening into any
part of the tract.

[Illustration: FIG. 229.—The dorsal surface of _Ph. australis_ Has.,
looking down on the head. The tentacles are cut away on the left side, and
the innermost are shortened on the right side to show the arrangement; in
reality they are of the same length throughout. _a_, Mouth; _b_, anus; _c_,
pore of left nephridium; _d_, epistome; _e_, break in the inner series of
tentacles. The drawing is to some extent diagrammatic, and is considerably
enlarged. (After Benham.)]

One of the most interesting features of _Phoronis_ is the presence of a
closed system of blood-vessels containing red blood. There are two main
blood-vessels; one, lying in the rectal chamber between the two limbs of
the U-shaped alimentary canal, has been named the afferent vessel. Just
below the diaphragm this splits into two, and each branch, after piercing
this partition, runs in a spiral course along the base of the crown of
tentacles, giving off a single blood-vessel into each tentacle. At its base
each tentacular vessel opens not only into the above-mentioned
{456}"distributing" vessel, but also into a "recipient" vessel which takes
a course parallel with the former. The two recipient vessels pierce the
diaphragm, and after running for some distance apart, fuse to form the
efferent vessel, which continues down the body on the left side of the
oesophagus. At the aboral end of the body the efferent vessel turns forward
and becomes the afferent. Both the main vessels give off numerous blood
diverticula, which are developed into plexiform sinuses on the walls of the
stomach, and in this region they are covered with the reproductive cells.
All the vessels are contractile, and Strethill Wright counted about fifteen
pulsations a minute. The blood contains numerous nucleated, disc-shaped
corpuscles differing in appearance from those of the fluid in the
body-cavity. The corpuscles contain haemoglobin, which gives the red colour
to the blood.

The two nephridia or kidneys are essentially tubes which open on the one
side into the body-cavity, and on the other to the exterior. The position
of the external pores has already been described, one being on each side of
the anus. Each pore leads into a tube which passes into that part of the
body-cavity situated below the diaphragm, where it divides, and each of the
two branches terminates in a ciliated funnel-shaped opening. The smaller of
these two funnels pierces the lateral mesentery and opens into the lateral
chamber, whilst the larger, whose opening is very much drawn out
longitudinally, opens into the rectal chamber. The whole organ is ciliated
internally.

The nervous system lies in the skin immediately below the epidermis. This
position is very primitive, and forms one of the most interesting
anatomical peculiarities of the genus. The nervous tissue is probably
diffused all over the body, but there is a special concentration or
thickening in the form of a ring which surrounds the mouth, following the
base of the tentacular spirals and giving off a nerve to each tentacle. The
ring lies at the outside of the base of the tentacles, the anus is not
included in it. Caldwell[496] has described in _Ph. kowalevskii_ an
asymmetrical nerve-cord given off from the ring and running along the left
side of the body; associated with which is a tubular structure of unknown
function. In _Ph. australis_ Benham mentions two such tubes, one on each
side of the body; their precise value is obscure.

{457}The epithelium covering the nerve-ring is slightly modified in the
neighbourhood of the kidney pore, and may have some special sensory
function; no other organs of sense are known (but see p. 454).

[Illustration: FIG. 230.—A schematic view of the interior of the body of
_Phoronis_. The middle seven-eighths of the body are omitted, _af_,
Afferent blood-vessel; _an_, anus; _ef_, efferent blood-vessel; _ep_,
epistome; _gl_, glandular pit; _int_, intestine or "second stomach"; _k_,
large funnel of the left nephridium; _ko_, opening of right nephridium, the
opening of the left is seen immediately below in section; _m_, mouth; _n_,
nerve concentration; _oe_, oesophagus; _oem_, oesophageal mesentery; _ov_,
ovary; _r_, rectum; _rm_, rectal mesentery; _rv_, right recipient
blood-vessel; _s_, septum; _st_, stomach; _t_, testis; _tm_, right lateral
mesentery. (From Benham.)]

_Phoronis_ is hermaphrodite, male and female reproductive cells being
formed in the same individual. The testes and ovaries form two white masses
lying on the left side of the stomach, one on one side and the other on the
other side of the efferent blood-vessel. The glands are traversed in all
directions by the diverticula given off from this trunk, and are thus well
supplied with blood; in fact both the ovary and the testis are formed by
the {458}multiplication and growth of the epithelial cells which cover
these diverticula. When ripe the ova and spermatozoa drop off into the
body-cavity and make their way to the exterior through the duct of the
kidney.

[Illustration: FIG. 231.—Three stages in the metamorphosis of the
Actinotrocha into _Phoronis_. A, Actinotrocha larva with the invagination
(_c_), which will form the trunk of the _Phoronis_ larva beginning to
appear. B, Stage with the invagination partly extruded. C, Stage when the
extrusion is complete and the alimentary canal has passed into it. C is
after Metschnikoff. _a_, Mouth; _b_, anus; _c_, invagination which
ultimately forms the greater part of the body of the adult.]

The ova are probably fertilised in the sea-water; they undergo the early
stages of their development whilst entangled amongst the tentacles of the
parent. The larval form to which they give rise was known long before its
connexion with the adult was demonstrated by Kowalevsky.[497] It is known
as the Actinotrocha (Fig. 231, A), and according to Caldwell has the
following structure in _Ph. kowalevskii_. The mouth is anterior, and the
anus terminal and posterior; the mouth is overhung by an immense prae-oral
lobe, which bears a special larval nerve {459}ganglion, and in some species
four eye-spots; at the base of this, but behind the mouth, is a ring of
larval tentacles. The prae-oral lobe and the tentacles are ciliated; the
margin of the lobe bears, however, specially long cilia, and there is also
a ring of long cilia around the anus.

Before the Actinotrocha stage has been reached the larva has forsaken the
shelter of its parent's tentacles, and swims actively about in the open
sea. As it grows older a finger-like involution of the skin (_c_) arises
just behind the tentacles on the ventral surface and grows into the body,
increasing greatly in length and becoming much folded. The larva now sinks
to the bottom of the sea, and after swimming round many times on its axis,
undergoes a very astonishing metamorphosis (Fig. 231, B, C). The
finger-like involution is suddenly turned inside out, and forms a large
projection on the ventral surface, into which the alimentary canal passes,
assuming a U-shape, as in the adult. This ventral process in fact forms all
the body of the adult behind the line of tentacles, and subsequently
contains, not only the alimentary canal, but the kidneys, the reproductive
organs, and a large part of the vascular system. At the same time the
prae-oral lobe breaks off, and, together with its ganglia and eye-spots,
passes into the mouth and is digested in the stomach; the larval tentacles
follow the prae-oral lobe, and are similarly digested. Their place is taken
by a ring of adult tentacles which commence to appear just behind the
larval tentacles before they fall off. The animal is now practically
adult.[498]

It is obvious that this astonishing metamorphosis is accompanied by the
rotation of the axes of the animal. The adult practically lives at right
angles to the larva. In the latter the anus marked the posterior end, and
the prae-oral lobe the anterior. The prae-oral lobe has disappeared in the
adult, but its position is marked by the mouth. The ventral surface has
enormously increased, and corresponds with the whole surface of the trunk.
To be consistent we must therefore regard the mouth of the {460}adult as
marking the anterior end of the animal, the anus the posterior. The short
line between the mouth and anus across the centre of the tentacular crown
marks the dorsal surface; and the line running all round the trunk from
anus to mouth, the ventral. In fact, in its usual position in its tube
_Phoronis_ is lying on its ventral surface, its back faces upwards, and the
anterior and posterior ends lie on one side or the other.

SPECIES AND AFFINITIES.—In his exhaustive memoir on the anatomy and
histology of _Phoronis_, Cori enumerates seven different species, and
quotes the characters of each as enumerated by eight different authors. He,
however, reserves his opinion as to the identity or distinctness of some of
these species. Benham in his account of _Ph. australis_ enumerates five
species, including amongst them _Ph. ovalis_, which, however, he regards as
probably a young form, an opinion in which Cori coincides. The latter
regards it as possibly a young form of _Ph. hippocrepia_.

Without comparing specimens of each of the alleged species, it is difficult
to come to any very satisfactory solution of the problem of how many
distinct species are at present known, but it seems probable that there are
at least six.

(i.) _Phoronis hippocrepia_ Wright.—Under this name is included the first
form, described and named by Wright in 1856; also _Ph. ovalis_, described
two years later by the same observer as a distinct form, though it now
seems probable that it is but a young form of _Ph. hippocrepia_. The
_Crepina gracilis_ of van Beneden is probably identical with this
species.

This species occurs in membranous tubes embedded in limestone, corals, or
oyster shells. Its length varies from 1.5 to 15 mm. The number of
tentacles varies from 16 to 86. It has been found off the coast of
Devonshire and in the Firth of Forth.

(ii.) _Phoronis kowalevskii_ Caldwell.—This name is given by Benham to
the species from Naples described by Caldwell, and replaces the name _Ph.
caespitosa_, which was given by Cori. This species is found in the Bay of
Naples, living in considerable colonies on submarine piles and posts. It
is not firmly attached to its substratum. The tube may be coated with
sand or other foreign particles. The length of the individuals varies
from 3 to 39 mm. The lophophore is simple, with from 50 to 100 tentacles.

(iii.) _Phoronis australis_ Haswell.—This is the giant of the genus, the
length of the individuals being from 3 to 5 (76-127 mm.) or rarely 6
inches. It lives in delicate transparent tubes, interlacing the walls of
the tube of a sea-anemone, _Cerianthus_. The arms of the lophophore coil
into two spirals. The colour is reddish or purple. Found in Port Jackson.

(iv.) _Phoronis buskii_ M‘Intosh.—This species was dredged by the
_Challenger_ {461}from a sandy bottom at a depth of 10 to 20 fathoms off
the Philippines. Its tube is covered with particles of sand, sponge
spicules, etc. Its length is 52 mm. or more (more than two inches). The
anatomy of this species closely resembles that of _Ph. australis_, and
Benham thinks that, in spite of the difference in their habitat, they may
belong to the same species.

(v.) _Phoronis architecta_ Andrews.—A species recently described by
Andrews from Beaufort, N.C. Its distinctive features are: "the formation
of isolated tubes covered by definite collections of sand grains; the
presence of special prostomial organs, possibly of use in the formation
of these tubes; the great development of the longitudinal muscles; the
presence of a ciliated groove in the digestive tract; the apparent
separation of the sexes."

(vi.) _Phoronis psammophila_ Cori.—Found in Faro, near Messina. The tube
is hyaline, and is covered by numerous grains of sand, some of
considerable size. The length of the individuals is 25 to 50 mm. There
are 60 to 90 tentacles. The colour is a fleshy red. A second species
discovered by Haswell in Port Jackson had no points of importance to
distinguish it from _Ph. psammophila_, except that no sand adheres to its
tube and the number of tentacles is slightly greater.

In addition to the various species of _Phoronis_, several distinct forms
of its larva, Actinotrocha, are known, and have been named without having
been traced into their corresponding adult form.

The position of _Phoronis_ in the animal kingdom has formed the matter of
considerable divergence of opinion amongst the naturalists who have studied
it. The earlier writers regarded _Phoronis_ as allied to the Gephyrea, and
it was for a long time classed with these animals, but placed in a separate
sub-Order, the _Gephyrea tubicola_, which was opposed to the _Gephyrea
nuda_, which comprised the true Gephyrea.

Caldwell referred _Phoronis_, the Brachiopoda, the Polyzoa, and the
Gephyrea to the same type of body structure, and Lankester subsequently
suggested the provisional name Podaxonia for this miscellaneous collection
of animals. Lankester divided his phylum Podaxonia into three classes: (i.)
the Sipunculoidea (Gephyrea), (ii.) the Brachiopoda, and (iii.) the
Polyzoa. The last-named class he divided into three sections: (_a_) the
Vermiformia, this includes the single genus _Phoronis_; (_b_) the
Pterobranchia, including the forms _Cephalodiscus_ and _Rhabdopleura_,
whose affinities with Balanoglossus were subsequently demonstrated; and
(_c_) the Eupolyzoa, including the forms treated as Polyzoa in the
following pages.

Masterman's recent researches[499] on _Phoronis_ seem to indicate {462}that
the Vermiformia, like the Pterobranchia, must in future be grouped with the
Hemichordata. He finds three well-defined coelomic spaces corresponding
with the epistome, the collar, and the trunk, and also representatives of
the collar pores, and is further inclined to believe that structures
representing the notochord exist in Actinotrocha.

Should Masterman's researches be confirmed, _Phoronis_ will be removed from
its present isolated and enigmatical position, and placed with
_Cephalodiscus_ and _Rhabdopleura_ amongst the Hemichordata, which will be
described in Vol. VII. of this work.

POLYZOA

SIDNEY F. HARMER, M.A.

Fellow of King's College, Cambridge

{465}CHAPTER XVII

POLYZOA

INTRODUCTION—GENERAL CHARACTERS AND TERMINOLOGY—BROWN BODIES—HISTORY—
OUTLINES OF CLASSIFICATION—MARINE POLYZOA—OCCURRENCE—FORMS OF COLONY AND OF
ZOOECIA—OVICELLS—AVICULARIA—VIBRACULA—ENTOPROCTA.

The following pages[500] deal with animals whose very existence is hardly
known to those who are not professed naturalists. There are but few Polyzoa
which have earned the distinction of possessing a popular name, and most of
such names as do exist cannot be found outside treatises on Natural
History. It is true that many of the members of this group have been
vaguely termed "Zoophytes"; but this term implies no more than that they
possess a superficial resemblance to certain plants, and it must be
remembered that this habit of growth is assumed by many animals which have
nothing to do with the Polyzoa. The term "Coralline" is sometimes applied
to those calcareous Polyzoa which grow into coral-like forms; and the
Tertiary deposit known as the "Coralline Crag" is so called from the large
number of fossil Polyzoa which it contains.

The Polyzoa are none the less a most attractive group. Let any one examine
a dry piece of a brown paper-like substance (Fig. 232, A), which may be
found thrown up on the beach on many parts of our coasts. Of this species
(_Flustra foliacea_), the {466}so-called "sea-mat," an old writer says:
"For curiosity and beauty, I have not, among all the plants or vegetables I
have yet observed, seen any one comparable to this seaweed."[501] Viewed
with the microscope, the frond is seen to consist of two layers, placed
back to back, of oblong chambers, each of which is the dried body-wall of a
single individual. The whole is obviously a _colony_, and to this fact the
term Polyzoa refers.

The chambers just noticed are termed "zooecia." Each is rounded at one end,
near which is the "orifice," through which the tentacles of the living
animal can be pushed out. Two short, stiff spines usually occur on each
side of the orifice; and the symmetry of this forest of spines fully
justifies the above-quoted remark.

[Illustration: FIG. 232.—_Flustra foliacea_ L., Cromer. A, Natural size, B'
indicating the portion magnified in B (× 30): _a_, avicularium with closed
mouth, to the left of which are seen two avicularia with open months; _o_,
ovicell, forming the upper part of a zooecium. Ovicells are seen on three
consecutive zooecia. The operculum, which closes the orifice of the
zooecium, is seen in different positions in the individuals figured.]

The upper part of some of the zooecia is somewhat swollen, these swellings
representing the conspicuous "ovicells" of many other genera. In the early
part of the year each ovicell protects an orange-coloured egg or embryo,
and the larvae are readily liberated if the fresh colony be placed in clean
sea-water. "At least ten thousand" were hatched out in three hours from a
colony placed in a glass by Sir John Dalyell.[502] The larva swims freely
in the water for a short time, and should it find a {467}suitable
resting-place, it fixes itself and forms the starting-point of a colony,
the number of whose individuals is continually increased by the production
of buds at the growing edge. The "avicularia" of this species will be
alluded to later (see p. 482).

_F. foliacea_ has long been known to possess in the fresh state a
remarkable odour, which is described, according to the fancy of the
observer, as a strong odour of fish, or as the smell of violets after a
shower. Others have compared it to that of the orange or verbena, or to
that of a mixture of roses and geranium.

_Flustrella hispida_, another of our commonest Polyzoa, which may be found
between tide-marks on the stalks of _Fucus_, consists of a softish brown
encrustation, about one-sixteenth of an inch thick, covered by numerous
spines. If examined undisturbed in a rock-pool, or transferred to a glass
of sea-water, the brown mass will be seen to become surrounded by a
delicate bluish halo, which is about as thick as the encrusting mass
itself, and consists of the tentacles of the numerous individuals of the
colony. The microscope shows that each individual is provided with a
circlet of some thirty or more long, delicate tentacles, which together
form a graceful funnel (as in Fig. 233). At the bottom of the funnel is the
mouth, to which Diatoms or other minute organic particles are conveyed by
the cilia which fringe the tentacles. If the tentacles be touched with a
needle, the whole funnel is retracted with great rapidity, and in this
retracted condition we see no more than the body-walls of the animals.
After an interval the tips of the tentacles are cautiously protruded; the
tentacles are _gradually_ pushed out, at first in a close bundle, but
finally separating from one another to form the funnel which we have
already noticed.

There is hardly a more surprising spectacle in the whole animal kingdom
than a living fragment of the genus _Bugula_. The colony grows in the shape
of a small tree, whose height may amount to several inches; and is
characterised, in many species, by a spiral arrangement of the branches,
which makes the genus easy to recognise at first sight (Fig. 233, A). The
stem and branches are composed of a single layer of zooecia, arranged two
or more abreast. Each zooecium bears, on its outer side, a most singular
body termed an avicularium, from its resemblance to a bird's head. Imagine
a minute eagle's head attached by a short but flexible neck to the
zooecium. Suppose further {468}that this structure moves backwards and
forwards in a deliberate but determined fashion, its lower jaw usually
widely open so as to be nearly 180° distant from its position when closed.
Suppose that the lower jaw is moved by powerful muscles which can be
distinctly seen inside the transparent head of the avicularium, and that
every now and then it closes with a snap, seizing any unfortunate worm
which may happen to be within reach with a grasp of iron. The above gives a
very faint idea of the appearance of a living _Bugula_ colony, with its
hundreds of swaying avicularia, and with its tentacular funnels protruding
from their zooecia, and withdrawing themselves capriciously from time to
time.

[Illustration: FIG. 233.—_Bugula turbinata_ Alder, Plymouth. A, A small
colony (natural size); B, portion of a branch (× 50): _a_, _a'_,
avicularia, in different positions; _ap_, "aperture" (see p. 524); _b_,
polypide-bud, attached by its stomach to _b.b_, brown body; _m_, mouth,
surrounded by the circle of tentacles; two individuals to the right show
the tentacles partially expanded; _o_, ovicell; _s_, marginal spine. The
avicularia of some of the zooecia have been omitted in B.]

GENERAL CHARACTERS.—The Polyzoa are colonies, leaf-like or tree-like in
form, and often strongly resembling seaweeds, or forming encrustations on
the surface of stones and water-plants, or taking on other shapes. The
units of the colony are complete individuals (Fig. 234). The zooecium or
body-wall encloses a body-cavity, in which lies a digestive canal, with
which are closely connected the central nervous system and the retractile,
ciliated tentacles. The structures other than the zooecium constitute the
"polypide." The mouth (_m_) leads into the ciliated pharynx (_ph_) which is
followed by the oesophagus (_oe_) which again passes into the stomach
(_s_), whose walls are coloured by a {469}characteristic yellowish pigment.
The stomach gives off the intestine (_in_), which is lined by strong cilia,
by means of which a rotatory movement is given to the faeces contained in
it. This communicates by a narrow passage with the rectum (_r_), which
opens by means of the anus (_a_).

[Illustration: FIG. 234.—_Alcyonidium albidum_ Alder, Banyuls-sur-Mer.
Diagram showing the structure of a single zooecium with its polypide
retracted: _a_, anus; _d_, diaphragm; _e_, ectocyst; _em_, ectoderm; _f_,
funiculus; _g_, ganglion; _i_, intertentacular organ; _in_, intestine; _m_,
mouth; _mm_, mesoderm of body-wall; _o_, orifice; _oe_, oesophagus; _ov_,
ovary; _p_, parietal muscles; _ph_, pharynx; _p.v_, parieto-vaginal
muscles; _r_, rectum; _r.m_, retractor muscles (contracted); _s_, stomach;
_t_, testis; _tn_, tentacles; _t.s_, tentacle-sheath or kamptoderm. (After
Prouho.[503])]

In the retracted condition the tentacles (_tn_) lie in a cavity {470}which
opens to the exterior by the orifice (_o_). The cavity is bounded by a thin
membrane termed the "tentacle-sheath" (_ts_), and it is incompletely
subdivided, near its upper end, by a diaphragm (_d_), perforated by a
circular hole through which the tentacles can be protruded. The diaphragm
bears the thin folded collar characteristic of the Ctenostomata, the group
to which the species figured belongs (see p. 477).

Fig. 238, B, shows the tentacles of _Bowerbankia_ in their fully expanded
and partially expanded condition. Comparing this with Fig. 234, it will be
clear that when protrusion is taking place, the tentacles are forced in a
bundle, tips first, through the diaphragm and next through the orifice of
the zooecium, the alimentary canal offering no resistance to this movement,
owing to the length of the oesophagus. A moment's consideration will show
that the bases of the tentacles, in passing through the orifice, will carry
with them that part of the flexible tentacle-sheath to which they are
attached; and it will further be clear that so much of the tentacle-sheath
as is thus protruded will be turned inside out. This process of
"evagination" continues until its further progress is stopped by the
retractor-muscles (_r.m_), and by the parieto-vaginal muscles (_p.v_),
which pass from the interior of the body-wall to the upper part of the
tentacle-sheath. The latter has now become the delicate layer which
connects the expanded tentacles with the zooecium; and the anus (Fig. 238,
C, _a_) opens directly to the exterior. Since the name "tentacle-sheath" is
thus descriptive of the condition of retraction only, the term
"kamptoderm"[504] has been suggested as an alternative name.

The presence of a complete digestive canal and the ciliation of the
tentacles in Polyzoa are conspicuous differences between these animals and
the Hydroids, with some of which the Polyzoa may have a marked external
similarity.

The outermost[505] layer of the body-wall is known as the "ectocyst" (Fig.
234, _e_). This may be densely calcareous, in which case the dried Polyzoon
differs little in appearance from the living animal with its tentacles
retracted; or it may be partially calcified, or it may consist entirely of
a flexible cuticle, {471}as in Fig. 234. The ectocyst is prolonged through
the orifice (_o_) as far as the diaphragm (_d_).

Forms with a calcareous ectocyst are commonly ornamented with ridges or
other patterns, which are often of great beauty. The ectocyst in these
cases is commonly interrupted at intervals by pores (Fig. 239, C), into
which processes of the "endocyst"—the living, internal part of the
body-wall—extend. These may appear as superficial pores, which apparently
open to the exterior in the dried condition, or they may perforate the
septa between adjacent individuals. This may be strikingly demonstrated by
decalcifying a branch of _Crisia_ (Fig. 237), in which the zooecia then
appear connected by numerous strands of tissue. In many marine forms the
communications between the individuals are in the form of small sieve-like
plates known as "rosette-plates."

The endocyst may consist of definite layers of ectoderm (_em_) and mesoderm
(_mm_), as in Fig. 234, but the mesoderm is commonly in the form of a loose
network, some of which is attached to the body-wall, some to the alimentary
canal, some forming connecting strands between these two layers, and other
cells floating about freely in the body-cavity. These mesodermic structures
are often spoken of as the "funicular tissue," since one or more strands of
it commonly take on the form of a definite "funiculus" (_f_). This
structure may bear the ovary (_ov_), while the testes (_t_) are found,
commonly in the same zooecium, attached to various parts of the body-wall.
The eggs and spermatozoa, when ripe, break off and float freely in the
body-cavity.

The funicular tissue was at one time described as a "colonial nervous
system." The idea expressed by this term must be considered erroneous from
the fact that no nervous co-ordination of the individuals is known to
exist, in the vast majority of cases. The actual nervous system consists of
a ganglion (_g_) placed between the mouth and anus of each polypide, and
lying in a small circular canal (not shown in Fig. 234) which immediately
surrounds the oesophagus. This canal is developed in the bud as a part of
the body-cavity, from which it becomes completely separated in marine
forms. The Polyzoa have no vascular system.

BROWN BODIES.—In the majority of cases, an extraordinary process of
regeneration takes place periodically during the life of each zooecium. The
tentacles, alimentary canal, and nervous system break down, and the
tentacles cease to be capable of being {472}protruded (Fig. 235, 1). The
degenerating organs become compacted into a rounded mass (Fig. 235, 2 and
3, _b.b_), known from its colour as the "brown body." This structure may
readily be seen in a large proportion of the zooecia of transparent
species. In active parts of the colony the body-wall next develops an
internal bud-like structure (Fig. 235, 1, _b_), which rapidly acquires the
form of a new polypide (Fig. 235, 2 and 3). This takes the place originally
occupied by the old polypide, while the latter may either remain in the
zooecium in the permanent form of a "brown body," or pass to the exterior.
In _Flustra_ the young polypide-bud becomes connected with the "brown body"
by a funiculus (Fig. 235, 1, 2). The apex of the blind pouch or "caecum" of
the young stomach is guided by this strand to the "brown body," which it
partially surrounds (3). The "brown body" then breaks up, and its fragments
pass into the cavity of the stomach, from which they reach the exterior by
means of the anus.

[Illustration: FIG. 235.—_Flustra papyrea_ Pall. Naples. × 50. Illustrating
the development of a new polypide after the formation of a "brown body." In
1, _a_, two masses formed from the alimentary canal; _b_, young
polypide-bud; _b.b_, degenerating tentacles; _c_, connective tissue: 2,
another zooecium, later stage; _b.b_, brown body; _r.m_, retractor muscles;
_s_, stomach; _t_, tentacles of new polypide; _t.s_, tentacle-sheath: 3,
the same zooecium, 191 hours later; letters as in 2. 1 and 2 are seen from
the front, 3 from the back.[506]]

There is some reason to believe[507] that these remarkable processes are
connected with the removal of waste nitrogenous matters. The Marine Polyzoa
are not known to be, in most cases, provided with definite excretory
organs, although it is {473}possible that the intertentacular organ (Fig.
234, _i_) described on p. 508 may in some cases perform excretory
functions. There can, however, be little doubt that some kind of excretion
takes place in the Polyzoa; and in considering what organs could possibly
perform this work, our attention is arrested by the alimentary canal. The
digestive organs of the young bud are perfectly colourless. As growth
proceeds, certain parts acquire a yellowish, and later a brown colour. The
degeneration of the polypide is followed by the grouping of large numbers
of the free cells of the body-cavity into a mass which closely surrounds
the incipient "brown body." Under their action, the latter becomes
considerably smaller, probably as the result of the absorption of matters
of nutritive value into other tissues. The final result is the formation of
the compact "brown body," whose colour is principally derived from the
pigment formerly present in the alimentary canal. Experiments made by
introducing into the tissues of the Polyzoa certain artificial pigments
which are known to be excreted by the _kidneys_ when injected into the
bodies of other animals, have given some reason for believing that the
appearance of the brown pigment in the wall of the digestive organs is, in
part, a normal process of excretion; although that process is not entirely
carried out by the organs in question.

Little is known with regard to the duration of life of a single polypide;
but some information bearing on this question may be obtained from a set of
observations made on _Flustra papyrea_.[508] The table gives the number of
days from the time at which the polypides were noticed to commence their
degeneration:—

Days.
2 "Brown body" partially formed, the parts of the polypide being still
easily recognisable.
5 Tentacles still just recognisable: appearance of new polypide-bud.
8 Stage shown in Fig. 235, 2.
11 Union of apex of stomach with "brown body."
16 "Brown body" half surrounded by stomach, and preparing to break up
(Fig. 235, 3).
21 "Brown body" broken up into numerous fragments, contained in the
alimentary canal of the new polypide.
35 "Brown body" almost completely absorbed.[509]

{474}These results did not hold good for all the zooecia in a single
colony. In some, the "brown body" was not completely got rid of at the end
of sixty-eight days, the conclusion of the experiment.

So striking are the facts relating to the "brown bodies" that it has been
believed[510] that what we have above described as the individual really
consists of two kinds of individuals: firstly, the "polypide" or complex of
tentacles and digestive organs; and secondly, the "zooecium," or house of
the zooid or polypide, corresponding with what has been described above as
the body-wall. The one individual, the zooecium, is on this view provided
with successive generations of the second kind of individual, the polypide;
and these latter function as the digestive organs of the two-fold organism.
This view, though fascinating at first sight, is not borne out by an
examination of all the facts of the case, especially when the Entoprocta
are taken into account.

HISTORY.—The history of the Polyzoa, as far as 1856, has been fully treated
by Allman in his great work on the Fresh-water Polyzoa;[511] but a few
words may be said on this subject.

The Polyzoa attracted comparatively little attention before the beginning
of the present century. Originally passed over as seaweeds, their real
nature was established in connexion with the discovery of the animal nature
of corals. So great a revolution could hardly be accepted without a
struggle, and even Linnaeus went no further in this direction than to place
them in a kind of half-way group of "zoophytes," whose nature was partly
animal and partly vegetable. It is hardly necessary to point out that this
view has now been abandoned by common consent; and indeed there is no more
reason for regarding an animal as showing an approach to the plants because
it grows in the external semblance of a seaweed than there would be for
supposing a bee-orchid to be allied to the animal kingdom because of the
form of its flowers.

But the claims of the Polyzoa to rank as a separate class were by no means
admitted with the discovery that they were animals. They were still
confounded with Hydroids, Alcyonarians, or Corals until their possession of
a complete alimentary canal was recognised as a feature distinguishing them
from those {475}animals. This was principally due to the observations of J.
V. Thompson[512] in Ireland, who introduced the term POLYZOA; and of C. G.
Ehrenberg[513] in Germany, who proposed the class-name BRYOZOA, or
moss-like animals.

It is impossible to avoid all mention of the controversy which has raged
with regard to these two rival terms. The controversy is for the present at
rest, the name Polyzoa being employed by the majority of English writers,
amongst whom must be mentioned Allman, Busk, Hincks, and Norman, admittedly
authorities of the first rank; while Bryozoa is employed by practically all
the Continental writers.

The priority of Thompson's name is unquestioned. While Ehrenberg, however,
definitely introduced Bryozoa _as the name of a group_, Thompson was less
precise in this respect, although he states[514] that his discovery "must
be the cause of extensive alterations and dismemberments in the class with
which they [the Polyzoa] have hitherto been associated." Thompson, in fact,
clearly understood that the Polyzoa could no longer rank with the Hydroids.
The controversy has been summarised by Hincks, in his _History of the
British Marine Polyzoa_,[515] where references to other papers on the same
subject are given.

The Polyzoa were associated by H. Milne-Edwards with the Tunicata in the
group Molluscoidea (Molluscoïdes[516]), to which the Brachiopoda were
afterwards added by Huxley.[517] A knowledge of the development of the
Tunicata has, however, shown that these animals must be withdrawn from any
association with the other two groups; while there is little real evidence
that even the Brachiopods have anything to do with the Polyzoa.

CLASSIFICATION.—The Polyzoa are divided into two sub-classes:—I, the
ENTOPROCTA; and II, the ECTOPROCTA.[518] Although the character referred to
by these terms is merely the position of the anus with relation to the
tentacles,[519] there can be no doubt that the two groups differ widely
from one another in {476}many important respects. I do not, however, accept
the view, maintained by some authors, that the Entoprocta and the
Ectoprocta are two separate classes which are not nearly related.

The base from which the whole set of tentacles springs is known as the
"lophophore."[520] In the Entoprocta (Fig. 236, 1) the lophophore is
circular; the mouth is situated near the margin of the area surrounded by
the tentacles; and the anus is found _within_ the circlet, near the end
opposite to the mouth.

In (2) and (3), representing the Ectoprocta, the anus is _outside_ the
series of tentacles. In the majority of cases, including all the marine
Ectoprocta and one or two of the fresh-water forms, the lophophore is
circular (2), the mouth occurring at the centre of the circle, and not
being provided with a lip. These forms of Ectoprocta constitute the Order
GYMNOLAEMATA,[521] the dominant group of the Polyzoa in respect of number
of genera and species. The remaining Ectoprocta belong to the exclusively
fresh-water Order PHYLACTOLAEMATA,[522] in which the mouth is protected by
an overhanging lip or "epistome"; the ground-plan of the tentacles is,
except in _Fredericella_, horse-shoe shaped (Fig. 236, 3), and the
tentacles themselves are usually much more numerous than in the other
cases.

[Illustration: FIG. 236.—Ground-plan of the lophophore in (1) Entoprocta,
(2) Gymnolaemata, (3) Phylactolaemata: _a_, anus; _ep_, epistome; _m_,
mouth. The tentacles are represented by shaded circles.]

The general characters of these divisions will be more easily understood by
referring to the figures given of living representatives of the groups. The
Entoprocta are illustrated by Figs. 243-245; the Gymnolaemata by Figs. 238,
240; and the Phylactolaemata by Figs. 247, 248.

{477}The Gymnolaemata include three Sub-Orders:—

1. CYCLOSTOMATA.[523]—Body-wall densely calcareous, the zooecia being
more or less tubular, usually with a _circular_ orifice (Fig. 237).

2. CHEILOSTOMATA.[524]—Body-wall of varying consistency. The orifice is
closed, in the retracted state of the polypide, by a chitinous _lip_ or
"operculum," which is more or less semicircular (Figs. 239, 241).

3. CTENOSTOMATA.[525]—Body-wall always soft. The cavity into which the
tentacles are retracted is closed by a frill-like membrane, the edges of
whose folds have some resemblance to the teeth of a _comb._ This
membrane, the "collar," is seen in different conditions of protrusion or
retraction in Figs. 234, 238. The stomach may, in this group, be preceded
by a muscular gizzard (Fig. 238, C, _g_).

OCCURRENCE.—By far the larger number of the Polyzoa are inhabitants of the
sea. A recently published catalogue[526] of marine Polyzoa includes nearly
1700 living species; and of these, the great majority belong to the
Gymnolaemata. This group is further known to include an enormous number of
fossil forms. Not only do we find that in living Polyzoa the members of a
single Order largely outnumber the remainder of the Polyzoa, but we may
further notice that the Cheilostomata, one of the sub-Orders of the
dominant group, are at present largely in excess of the whole of the rest
of the Polyzoa taken together.

Polyzoa may be collected with ease on almost any part of our coasts. The
fronds of the "sea-mat" (_Flustra foliacea_) are thrown up by the waves in
thousands in places where the bottom is shallow and sandy. The bases of the
larger seaweeds growing on rocks between tide-marks are nearly always
thickly covered with encrustations of _Flustrella hispida_ or of species of
_Alcyonidium_, in places where they are kept moist by being covered with a
sufficiently thick layer of other algae. Rocks which are protected from the
sun may be coated with calcareous Cheilostomes; and these are also found,
in company with branching Polyzoa of various kinds, on the bases of the
_Laminaria_ thrown up by gales or exposed at spring tides. The graceful
spirals of _Bugula turbinata_ (Fig. 233, A) may be found hanging from the
rocks at extreme low water; while colonies of _Scrupocellaria_, remarkable
for their vibracula (see p. 484), are common in many places between
tide-marks. Certain species affect the mouths of estuaries.

{478}_Membranipora membranacea_ commonly covers many square inches of the
frond of _Laminaria_ with its delicate lace-like encrustation. Nitsche[527]
has shown that this species has its calcareous matter deposited in plates,
separated by intervals of uncalcified ectocyst. The effect of this
arrangement is to make the colony flexible, and to enable it to adapt its
shape to the movements of the _Laminaria_, which is swayed to and fro by
the action of the waves. Many of the calcareous forms growing on
_Laminaria_ have no special arrangement of this kind, and they accordingly
grow in colonies whose area is so small that the greatest movements to
which the seaweed is liable are not sufficient to crack or break the
colony.

Many species show a decided, or even exclusive, preference for particular
situations; as, for instance, species of _Triticella_, which are only found
on certain Crustacea. Many encrusting forms prefer the inside of dead
shells of _Pecten_, _Cyprina_, etc., to any other habitat.
_Terebripora_[528] excavates tubular cavities in the substance of the
shells of Molluscs. _Hypophorella_[529] inhabits passages which it forms in
the walls of the tubes of the Polychaets, _Lanice_ and _Chaetopterus_.
_Lepralia foliacea_, one of the Cheilostomata, forms masses which may reach
a circumference of several feet, simulating a small coral-reef. Its
contorted plates are a regular museum of Polyzoa, so numerous are the
species which delight to find shelter in the quiet interstices of the
colony. The exquisite little colonies of _Crisia eburnea_ are commonly
found on red seaweeds, or on the branches of the Hydroid _Sertularia_.

The Polyzoa are found at all depths, certain Cheilostomes having been
recorded from 3000 fathoms. The Cyclostomes dredged by the "Challenger"
were all found in depths of 1600 fathoms or less, while the Ctenostomes are
a distinctly shallow water group, most having been found at less than 40
fathoms, and only three at so great a depth as 150 fathoms.[530]

A few forms (_Membranipora pilosa_, _Scrupocellaria reptans_, etc.) are
known to be phosphorescent;[531] but it is not known what is the purpose of
this phenomenon.

{479}EXTERNAL FORM.—The Polyzoa may be roughly divided into (1) encrusting
forms, usually calcareous, but sometimes soft; and (2) erect forms, which
are either rigid or flexible. This flexibility can coexist with a highly
calcified ectocyst, as in _Crisia_ (Fig. 237), _Cellaria_, and others in
which the branches are interrupted at intervals by chitinous joints. The
coral-like forms may assume the most exquisite shapes, pre-eminent among
which are the lovely net-like colonies of _Retepora_. Polyzoa of this type
are seldom found between tide-marks, where their brittle branches would be
liable to be snapped off by the waves. The _erect_ species which occur in
such positions are flexible, although flexible species are by no means
restricted to the zone between tide-marks.

[Illustration: FIG. 237.—_Crisia ramosa_ Harmer, Plymouth. A, End of a
branch, × 1; B, another branch, × 20, showing the chitinous joints, the
tubular zooecia characteristic of Cyclostomata, and the pear-shaped ovicell
with a funnel-shaped orifice at its upper end.]

Although the form of the colony is very different in different Polyzoa, a
pocket-lens will usually show whether a given specimen belongs to the group
or not. The surface is nearly always more or less distinctly composed of
zooecia, or at least shows their orifices. The entire colony may be built
up of these zooecia; and this is by far the commonest arrangement, both in
encrusting and in erect forms. In certain genera, however, and particularly
in some Ctenostomes (Fig. 238), and in most of the Entoprocta, the
{480}individuals grow out at intervals from a cylindrical stem or "stolon"
(_st_), which is not composed of zooecia.

The Cyclostomata may assume an erect or encrusting habit. Their zooecia are
always more or less cylindrical; the upper ends being often completely
free, although in many cases the whole zooecium is closely adnate to its
neighbours. In the breeding season the forms which belong to this group are
provided with curious "ovicells," which contain the embryos. These may
either be pear-shaped swellings on the branches (_Crisia_, Fig. 237), or
they may form inflations of the surface, between the zooecia. The mature
ovicell is provided with one or more openings, through which the larvae
escape.

[Illustration: FIG. 238.—_Bowerbankia pustulosa_ Ell. and Sol., Plymouth.
A, Fragment of a colony, natural size, showing the branching stem, bearing
tufts of zooecia: B, one of these tufts, with the growing apex of the stem
(_st_), × 27; _b_, young zooecia (buds); _c_, the "collar" characteristic
of Ctenostomata; _t_, tentacles; C, a single zooecium, with expanded
tentacles, more highly magnified; _a_, anus; _c_, collar; _g_, gizzard;
_i_, intestine; _o_, oesophagus; _s_, stomach.]

The Ctenostomata rarely have even the slightest trace of calcareous matter.
_Alcyonidium_ and its allies form soft encrustations, or may even grow into
erect masses six inches or more in height (_A. gelatinosum_). In this type
the zooecia are often so closely united that it may be difficult or
impossible to make out their limits in the living colony. Many of the
dendritic or branching {481}Ctenostomes (Fig. 238) are characterised by an
extreme delicacy of habit. The zooecia in these cases are sharply marked
off from the stem. They are either cylindrical or ovoid, being commonly
attached by a very narrow base, so that in some species they readily fall
off, and may thus be completely absent in certain parts of the colony. In
such forms as _Vesicularia spinosa_, it requires considerable experience to
recognise a stem which has lost its zooecia as being part of a Polyzoon. In
_Mimosella_ the zooecia possess a remarkable power of movement on the stem,
similar to that possessed by the leaflets of the Sensitive Plant.[532] In
certain forms (_Bowerbankia_, _Amathia_) the zooecia occur in groups
separated by intervals which are devoid of zooecia, but in other cases they
may have a more irregular arrangement. The collar to which this group owes
its name is by no means a conspicuous feature. Its position when retracted
has been shown in Fig. 234, while Fig. 238 further illustrates its
relations.

The Cheilostomata grow in a great variety of forms, and also show a wide
range of character in their zooecia. The orifice is commonly surrounded by
stiff spines (Fig. 257, p. 524), which perhaps have the function of
protecting the delicate polypides from the sudden impact of foreign bodies.
These spines may attain an enormous development, as in _Bicellaria
ciliata_, and some forms of _Electra_ (_Membranipora_) _pilosa_ (Fig. 256,
A).

The operculum is usually, though by no means always, a conspicuous feature
of the Cheilostome zooecium. It is invariably of chitinous consistency, and
is more or less semicircular in outline, the straight edge forming a hinge
on which the operculum opens. In some cases the orifice is surrounded by a
raised margin or "peristome" (Fig. 255, B, C); the operculum is then
situated at the bottom of a depression of the surface, and may be concealed
from view. In others, in which the front wall of the zooecium is membranous
(_Bugula_, Fig. 233), the operculum is merely a part of this membrane, and
so is quite inconspicuous; and in cases of this kind the membranous wall
may be protected by an arched spine, the "fornix," developed from one side
of the zooecium (Fig. 254, _f_). The ovicells are commonly a conspicuous
feature of this group, although they are believed to differ fundamentally
from those of Cyclostomata. They have the form of a helmet-like covering
overhanging the orifice (Figs. 240, 241), {482}and may be either prominent
or more or less concealed by the growth of adjacent parts of the zooecia.
The presence of ovicells of this description is perfectly distinctive of
the Cheilostomata.

AVICULARIA AND VIBRACULA.—Most singular of the external appendages of the
Cheilostomata are the extraordinary "avicularia" and "vibracula" of some
genera.[533] By the comparison of a carefully selected series of genera, it
has been established that the avicularium is a special modification of a
zooecium. One of its least modified forms is found in _Flustra foliacea_
(Fig. 232), where the avicularia (_a_) are small zooecia with a
conspicuously large operculum ("mandible"). Avicularia of a similar type
occur in _Cellaria_ (Fig. 239, A), _Schizotheca_, etc., the avicularium
occupying the place of an ordinary zooecium. These are the "vicarious"
avicularia of Mr. Busk.[534]

[Illustration: FIG. 239.—Forms of avicularia. A, _Cellaria fistulosa_ L.,
Plymouth, × 43; _a.z_, avicularian zooecium, with closed mandible; _o_,
operculum of zooecium: B, _Schizoporella unicornis_ Johnst., Scilly Is., ×
43; zooecium bearing two avicularia; _m_, opened mandible of avicularium;
_s_, sinus of orifice: C, zooecium of _Smittia landsborovii_ Johnst.,
Plymouth, × 43; the operculum is seen at the bottom of a depression
surrounded by a thin collar or "peristome," in an emargination of which is
seen an avicularian zooecium (_a.z_); _m_, mandible (opened); _p_, pores;
_t_, tooth.]

In the next stage (Figs. 239, B, 256, B) the avicularian zooecium is
further reduced; it has in most cases lost its place in the series of
individuals, and is found instead seated on some part of an ordinary
zooecium ("adventitious" avicularia). The avicularium now consists of a
much reduced zooecium, bearing the well-developed operculum or mandible.

{483}Having arrived at this point, the avicularia seem to lose all sense of
the propriety of remaining in the positions once occupied by zooecia. They
have become degraded to the rank of appendages of the zooecia, and as such
they may occur in an astonishing variety of positions. Sometimes one occurs
on each zooecium in the middle line, or asymmetrically, or even on the top
of the ovicell; in other cases the orifice is flanked by an avicularium on
each side (Fig. 239, B). Sometimes (_Cellepora_) the avicularia are of more
than one kind, some being large and some small, some having a pointed
mandible and others a mandible with a rounded spoon-like end.

In the cases so far considered, the body of the avicularium is fixed. The
highest differentiation acquired by these structures occurs in cases like
_Bugula_, where they are borne on flexible stalks, which may even exceed
the avicularia in length.[535]

[Illustration: FIG. 240.—_Bugula turbinata_, showing avicularia (_a_,
_a'_). The figure is explained on p. 468.]

In _Bugula turbinata_ (Fig. 240) each zooecium is provided with one of
these appendages, attached to the base of the outer of the two spines which
border its orifice. The avicularia of the two edges of the flattened branch
are much larger than those of the more internal zooecia. The upper jaw is
strengthened by a kind of buttress, or thickening of the ectocyst, which
passes on each side across the avicularium to the hinge-line of its
mandible. The upper part of the beak is strongly hooked, while the tip of
the mandible bears a {484}prominent spike, which fits inside the upper beak
when the jaw snaps. A great part of the head is filled with a strong
muscle, whose fibres exhibit a distinct transverse striation, and converge
into a median tendon. The latter is inserted into the middle of the
mandible. The muscle serves to close the jaws, and is the representative of
the muscles by which the operculum is closed in an ordinary zooecium. The
lower jaw is opened by means of a pair of muscles which are situated
immediately under the ectocyst of the avicularium, and pass into the
mandible close to its hinge.

[Illustration: FIG. 241.—Illustrating the transition from avicularia to
vibracula. A, _Microporella ciliata_ Pall., Scilly Is., × 62; _a_,
avicularium with short mandible (closed); _a'_, avicularium with
vibraculoid mandible (open); _m.p_, median pore; _o_, ovicell: B,
_Mastigophora dutertrei_ Aud., Shetland Is., × 47; _s_, sinus of orifice;
_v_, seta of vibraculum (or vibraculoid avicularium).]

Within the jaws, in the region which we may term the palate, is a rounded
knob, which bears a tuft of delicate sensory hairs, which doubtless enable
the avicularium to recognise the presence of any foreign body. The closure
of the mouth may, indeed, be instantaneously induced by touching it with
the point of a needle. It has been suggested that a small mass of cells
which bears these hairs may represent the rudiment of the polypide.

The "vibraculum" (Fig. 242) is regarded as an avicularium in which the
mandible has become elongated, so as to form a {485}thin, chitinous "seta,"
which from time to time moves through the water. The part of the vibraculum
which represents the zooecium commonly bears a tubular rootlet, used for
attaching the colony to the substance on which it is growing (Fig. 254, p.
517).

In _Microporella ciliata_ (Fig. 241, A) the avicularia are very variable,
and in some cases take on a "vibraculoid" character. But in the
fully-developed vibraculum (Fig. 242) there is usually no such compromise
of characters. It may, however, be noted that _Scrupocellaria scabra_ (Fig.
254), which belongs to a genus characterised by its highly differentiated
vibracula, possesses structures (_v.z_) which could hardly be distinguished
from avicularia were it not for the presence of the rootlet (_r_).

In the course of some observations which I had the opportunity of making on
_Bugula calathus_ at Naples, a fine hair offered to a small colony was
seized with such force by the avicularia that the entire colony was lifted
out of the water by the hair. The same colony had captured (1) a small
_Nereis_, which it held with several of its avicularia; (2) an Anisopod
Crustacean, 2½ mm. long; and (3) a small Amphipod, which was held by one of
its antennae. The Anisopod was held by the tip of one leg with one
avicularium, and by the penultimate joint of one of its chelae with an
avicularium of another branch. It was captured in such a way that its
chela, the "hand" of which was about half as long as the avicularium,
actually closed on to the avicularium without being able to effect its
escape. A little later the other chela was caught by another avicularium.
Curiously enough, however, an avicularium did not necessarily close even
when part of a captured animal was actually in its mouth. The avicularia
made no attempt to place themselves in an advantageous position for
catching fresh parts of the _Nereis_, which they might easily have done.
The avicularia which had captured prey remained motionless. The others
moved backwards and forwards (cf. the various positions of the avicularia
shown in Fig. 240) ten times in ¾ to 1 minute, snapping their jaws perhaps
once in that time. The two Crustacea were still retained by the avicularia
two days later. On the next day they had both disappeared; but the colony
had again caught the _Nereis_, which had previously effected its escape
with the loss of nearly all its tentacular cirri.

These observations, and others which have been recorded, do not,
unfortunately, give any information as to the purpose of the {486}movements
of the avicularia and vibracula. It is obvious that they may be defensive
in character; and it cannot be doubted that the avicularia can prevent
inquisitive worms from straying at will over the surface of the colony.
There is no evidence to show that animals are discouraged from interfering
with a _Bugula_ owing to the presence of its defensive weapons.

[Illustration: FIG. 242.—_Caberea ellisii_ Flem., Norway. × 40. Back view
of part of a branch. The large vibracular zooecia (_v.z_) occupy nearly the
whole of the surface. _s_, Seta of vibraculum; _z_, zooecia.]

It is not, indeed, certain what are the enemies against which the Polyzoa
have specially to guard. Sea-urchins and certain Molluscs are known to
browse on Polyzoa. Fresh-water Polyzoa, in which avicularia and vibracula
are absent, are attacked by the larvae of Insects, and by Triclad
Planarians. I have found the latter with their long pharynx everted and
completely buried in a _Cristatella_ colony. It is possible that some
marine Cheilostomes may be saved from attacks of this kind owing to the
existence of their armoury of avicularia and vibracula. It is also possible
that these structures are of service by removing foreign particles which
might otherwise settle on the colony, and tend to block up its orifices. It
has further been suggested that animals seized by the avicularia may be
held until they die, and that their disintegrating particles may then be
carried to the mouths of the polypides by the ciliary currents of the
tentacles; but proofs of this suggestion are {487}wanting, and it must be
admitted that the subject needs further elucidation.

The vibracula ordinarily remain stationary for some little time, every now
and then giving a sweep through the water. In the majority of cases these
structures, like the avicularia, act perfectly independently of one
another, so far as can be made out; but in _Caberea_ (Fig. 242) the
vibracula move in unison, the simultaneous action of the whole series,
after a period of quiet, being described as "positively startling."[536]

It has been stated by Busk[537] that the entire colony in _Selenaria_ and
_Lunulites_ may be moved from place to place by the large vibracula which
these forms possess.

[Illustration: FIG. 243.—_Pedicellina cernua_ Pall., Guernsey. Entire
colony. × 27. The colony has three growing ends, _a_; 1-8, individuals of
colony; 1 and 8 are quite immature; and 7 (tentacles retracted) is still
young; 2, is seen in longitudinal section; _g_, generative organ, and below
it the ganglion; _m_, mouth; _r_, rectum; _s_, stomach; between _g_ and _r_
are three embryos in the brood-pouch; the tentacles are retracted; in 5 and
6 the tentacles are expanded; in 6 two embryos are seen within the circle
of the tentacles, to the left of them is the rectum, and to the right the
mouth; 3 is in the act of losing its calyx, and has already developed the
beginning of a new polypide-bud; in 4 the primary calyx has been lost, and
the new calyx is clearly marked off from the stalk.]

ENTOPROCTA.—The Entoprocta, although a very small sub-class, deserve
special consideration, if for no other reason, from the fact that many
writers regard them as the most primitive group of Polyzoa, and
consequently as the forms which show most affinity to other classes of
animals.

Their most obvious characteristic is, as we have already seen,[538] the
position of the anus within the circle of tentacles. The individuals formed
by budding always remain more separate from one another than those of most
Ectoprocta.

{488}The commonest Entoproctous genus is _Pedicellina_, a graceful little
animal, which occurs on many parts of our coast. It may often be discovered
by looking carefully on the pink, jointed, calcareous alga, _Corallina_,
which may be found growing at the edges of deep and cool rock-pools not too
far above low-water mark. Its creeping stem or "stolon" is firmly attached
to the surface of the seaweed, and sends off vertical stems here and
there.[539] Each stem bears a "calyx," which is practically an individual
of the colony. The stolon terminates, at one or both ends, in a
growing-point (_a_), from which new individuals are budded off. The stalks
bend from time to time in a curious spasmodic manner, by which means the
calyces are moved about with an irritable and angry air. A good idea of the
way in which the tentacles are folded away when the animal is disturbed may
be obtained by putting the two wrists together, with the fingers spread out
to represent the tentacles, the retraction of which would be represented by
turning the tips of the fingers down into the space, the "vestibule,"
between the two palms. A delicate fold of skin growing from the edge of the
calyx closes over the retracted tentacles, owing to the contraction of a
sphincter muscle present in its circular edge. The body-wall is not
separated from the alimentary canal by a definite body-cavity, so that
there is no obvious distinction between the polypide and the zooecium. The
existence of the Entoprocta is in fact a strong reason for refusing to
admit that these two terms correspond with two different kinds of
individuals.

Let us now imagine the condition we should have if a large and continuous
cavity were developed between the alimentary canal and the body-wall. The
body-wall would clearly have the general relations of a zooecium, while the
alimentary canal and tentacles would obviously correspond with the
polypide. The existence of the body-cavity would make it possible for the
animal to _retract_ its tentacles instead of merely turning them in.
Regarded in this way, there is but little difficulty in comparing the
Ectoprocta with the Entoprocta.

The calyces are deciduous, _i.e._ they are lost from time to time, the end
of the stalk then producing a polypide-bud, which {489}forms the vestibule
and alimentary canal of a new calyx. Hence the phenomenon which may so
commonly be noticed in _Pedicellina_ of a "young head on old shoulders."
The loss of the calyces may have some relation to the formation of the
"brown bodies" in the Ectoprocta.

Another Entoproct, _Loxosoma_ (Fig. 245) is remarkable for being the only
Polyzoon which is not colonial. The buds, which are formed in two lateral
series, break off as soon as they are mature, and at once begin to lead an
independent existence. _Loxosoma_ is further remarkable for being almost
invariably found commensally with other animals, where it may occur in
enormous numbers. _L. phascolosomatum_, common in the Channel Islands, is
only found on the tip of the tail of _Phascolosoma_ (see p. 428), which
inhabits the mud of _Zostera_-beds. Other species are found on the external
surface of certain sponges (_Tethya_, _Euspongia_, _Cacospongia_); or on
the outside of a compound Ascidian, _Leptoclinum_, which may itself be
carried about as a detachable covering on the back of a crab (_Dromia_).
Another species is found on the ventral surface of the Polychaet
_Aphrodite_, and of its ally _Hermione_.

[Illustration: FIG. 244.—Side view of _Loxosoma annelidicola_ Van Ben. and
Hesse. × 50. (From Prouho.)]

_L. annelidicola_, an interesting species recently investigated by
Prouho,[540] was originally described in 1863 as a Trematode, under the
name of _Cyclatella_. It escaped further notice until it was again found in
the neighbourhood of Roscoff, in Brittany, on certain Polychaets belonging
to the family Maldanidae (see p. 332). The calyx has a very flattened form,
and is borne on a short stalk, which terminates in a large attaching disc,
formerly mistaken for the sucker of a Trematode. The features in which this
species differs from other members of the genus are shown by M. Prouho to
be correlated with its mode of life. The animal has the habit of lying flat
on its back, the disc at the end of its stalk being firmly attached to the
skin of the worm, and its short stalk being bent round into a curve so
{490}as to bring the calyx into a supine position, with its lophophore
directed upwards. This habit, together with its flattened form, prevents it
from being crushed between the worm and its tube. But without some further
provision its position might be merely a source of danger. For supposing
the calyx to be directed backwards in relation to the worm, a sudden
backward movement of the latter into its tube might bring the _Loxosoma_
into fatal contact with the inner surface of the tube. There would
obviously not be sufficient room to turn round in a _vertical_ plane, so as
to bring the body into a position of safety, _i.e._ into a position in
which it moves stalk first. But by a beautiful arrangement of the muscles
of its stalk this movement is effected in a horizontal plane; on touching
the _Loxosoma_ with the point of a needle it would swing round in this way
through 180° with "une rapidité qui étonne."

_Urnatella_[541] is a beautiful form with a segmented stalk, the stalks
usually arising in pairs from a common base. It has at present only been
found in fresh water in the United States.

[Illustration: FIG. 245.—Diagram of the structure of _Loxosoma_, seen from
the oesophageal side. _×_ about 70. _a_, Anus; _b_, buds; _e_, excretory
organ; _f_, foot-gland; _g_, ganglion; _gn_, generative organs; _o_,
orifice of vestibule; _oe_, oesophagus; _s_, stomach; _t_, retracted
tentacles.]

In _Pedicellina_ the plane of the lophophore is at right angles to the
stalk, which is separated from its calyx by a marked constriction. In
_Loxosoma_ the lophophore is set obliquely,[542] and there is no
constriction at the base of the calyx. In _Urnatella_ we find an
intermediate condition, the lophophore resembling that of _Loxosoma_, while
the constriction at the base of the calyx is similar to that of
_Pedicellina_. Since the latter is known to pass in its development[543]
through a stage with an oblique lophophore, it may be presumed that
_Loxosoma_ is a more archaic form than _Pedicellina_. In other respects,
the structure of the Entoprocta is very constant, whatever the genus.

{491}A pair of ciliated excretory tubes open into the vestibule. These are
similar in structure to the "head-kidneys" of the larvae of Polychaet
worms, or to the excretory organs of adult Rotifers. Flame-cells have been
described by Davenport in the stalk of _Urnatella_, but it is not known
whether they are connected with the excretory tubes of the calyx. The
animals are either hermaphrodite or have separate sexes, and the generative
organs open by ducts of their own into the vestibule. The nervous system
consists of a ganglion placed between the mouth and the anus, giving off a
set of nerves, many of which end in delicate tactile hairs placed on the
tentacles or other parts of the body.[544]

{492}CHAPTER XVIII

POLYZOA (_continued_)

FRESH-WATER POLYZOA—PHYLACTOLAEMATA—OCCURRENCE—STRUCTURE OF _CRISTATELLA_—
DIVISION OF COLONY—MOVEMENTS OF COLONY—RETRACTION AND PROTRUSION OF
POLYPIDES IN POLYZOA—STATOBLASTS—TABLE FOR DETERMINATION OF GENERA OF
FRESH-WATER POLYZOA—REPRODUCTIVE PROCESSES OF POLYZOA—DEVELOPMENT—
AFFINITIES—METAMORPHOSIS—BUDDING.

FRESH-WATER POLYZOA.—Although the Gymnolaemata are ordinarily marine
animals, fresh-water examples from this Order are not altogether wanting.
The Ctenostomata among the typically marine groups show the most tendency
to stray into fresh-water.

_Alcyonidium_ and _Bowerbankia_ (Fig. 238) flourish in estuaries, while
_Victorella_ and _Paludicella_ (Fig. 250) are only known as fresh or
brackish water forms. _Victorella_ was named after the Victoria Docks in
London, where it was first found; more recently it has also been discovered
in other parts of England and on the Continent.[545]

The systematic position of the genera _Hislopia_ and _Norodonia_,[546]
which have been described from fresh water of India and China respectively,
is at present uncertain. The undoubted Cheilostome _Membranipora_ has,
however, a British representative (_M. monostachys_), which occurs in
brackish water, in ditches on the coast of East Anglia. It is there known
to form "friable, irregularly-shaped, sponge-like masses," which grow on
water-plants.[547]

{493}The Entoprocta, as we have seen, are represented in fresh water by the
genus _Urnatella_.

The PHYLACTOLAEMATA are an exclusively fresh-water group, and they are
believed by Kraepelin[548] to have been derived from the Ctenostomata. Many
of their special peculiarities can, with great probability, be regarded as
adaptations to a fresh-water existence. This is particularly clear in the
all but universal habit of dying down in the winter, and in the occurrence
of the so-called statoblasts (Fig. 251), which are hard-shelled
reproductive bodies, absolutely restricted to the Phylactolaemata, and
capable of resisting the winter's cold and even a certain amount of drying
up. Phylactolaemata have indeed been recorded from the tropics; but it is
not yet sufficiently clear how they there behave in these respects. F.
Müller[549] has found these animals in Brazil, where they are said to be
more common at certain periods of the year than at others. Stuhlmann has
found them in Tropical Africa (Victoria Nyanza, etc.);[550] and
Meissner[551] has discovered the sessile statoblasts of _Plumatella_ on the
shells preserved in the Berlin Museum, of species of the Mollusc _Aetheria_
from various localities in Africa. Fresh-water representatives of a
considerable number of other groups of animals agree with the
Phylactolaemata in the possession of reproductive bodies which are
protected by hard coats. Such, for instance, are the ephippian ova of
_Daphnia_—bodies which have an extraordinary external similarity to
statoblasts—the gemmules of Spongillidae, the winter-eggs of Rhabdocoels
and Rotifers, and the cysts of Protozoa. The evolution of these bodies in
so many widely different cases may have been due to the selection of
variations calculated to minimise the dangers attendant on the drying up of
the water in summer, or on its freezing in winter.

The Phylactolaemata are by no means uncommon, although they can seldom be
found without a careful search. Their presence may often be detected by
taking advantage of the property of the free statoblasts of rising to the
top of the water, where they can be discovered by skimming the surface with
a fine hand-net.

The colonies themselves are usually found attached to water-plants, roots
of trees or stones. Most of them flourish best in {494}a zone not more than
two feet below the surface. Certain species show a preference for floating
leaves, such as those of water-lilies, where they are not liable to be
dried up by alterations in the level of the water. Some forms (e.g.
_Plumatella_, Fig. 246) are, however, able to withstand being dried for
some time. Most species prefer shady places, and accordingly settle on the
lower sides of leaves and sticks. Others (e.g. _Cristatella_, Fig. 247)
have no objection to the direct rays of the sun. Most forms prefer still
water, but one or two are found in running water.

_Fredericella_ is a common constituent of the deep-water fauna of Swiss
Lakes (down to over forty fathoms); and reaches there a size considerably
larger than the shallow-water form of the same species. _Paludicella_ is
common at thirteen fathoms. These two genera, with _Plumatella_, have been
found in absolute darkness, under a pressure of 2½-5½ atmospheres, in the
Hamburg aqueduct. The Polyzoa and other organisms growing in the
water-supply of Hamburg were accused of being concerned in the spreading of
cholera, during the recent epidemic, by choking up the water-pipes, and
creating obstructions which formed a favourable nidus for the development
of cholera-germs.

The colony may take the form of a series of delicate, branching tubes
(_Plumatella_, _Fredericella_), of more massive aggregations of parallel
tubes (as in the Alcyonelloid forms of _Plumatella_), or of gelatinous
masses of varying size (_Lophopus_, _Cristatella_).

[Illustration: FIG. 246.—A, _Plumatella_ (_Alcyonella_) _fungosa_ Pall.,
Naples (fresh water), small part of a mass, natural size; B, _Plumatella
repens_ L., R. Yare, on the leaf of a water-lily, natural size.]

_Cristatella mucedo_ (Fig. 247) is remarkable for its power of moving from
place to place; it consists of an elongated mass of greenish, gelatinous
substance, which, in its fully developed state, may reach a length of eight
inches or more, with a transverse diameter of three-eighths of an inch. It
has a flattened sole on which it crawls, while the graceful plumes of its
numerous polypides protrude as a delicate fringe from its upper side.

{495}The tentacles are about eighty to ninety in number, and they are, as
in other Phylactolaemata, united at their bases by a delicate web. The
lophophore is horse-shoe-shaped (Fig. 236, 3) throughout the group, with
the exception of _Fredericella_, in which genus it is circular.

In some Phylactolaemata the polypide has been observed to interlace its
tentacles, so that the plume becomes a kind of cage, in which the more
active Infusoria are imprisoned until their struggles have so far weakened
them that they are swept into the mouth by the action of the cilia of the
tentacles.[552]

[Illustration: FIG. 247.—_Cristatella mucedo_ Cuv. (a small colony), R.
Yare, above Norwich, × 24.]

Around the edge of the _Cristatella_ is found a zone of budding tissue,
which gives rise continuously to new individuals. Now, whereas in
Gymnolaemata the growing edge gives rise to zooecia, whose cavities become
completely cut off from that of the older ones; in Phylactolaemata the
partitions between the zooecia are never completed. The body-cavity of
_Cristatella_ is thus a continuous space, interrupted at the margin only by
vertical septa (see Fig. 247), which represent the partitions between the
zooecia of other forms.

The body-wall consists of two epithelial layers of ectoderm and mesoderm,
between which is a layer of muscular fibres. {496}Parts of the epithelium
lining the body-cavity are ciliated. Into the common body-cavity hang the
polypide-buds at the edge of the colony, and the mature polypides in the
more central regions. There are usually three rows of polypides on either
side of the middle line, in the neighbourhood of which is an area devoid of
polypides, but containing "brown bodies" and statoblasts. The polypides
nearest to the middle line pass in succession into the condition of "brown
bodies," while young buds near the margin grow up coincidently to form new
polypides.

The movement of the colony is in the direction of the long axis, although
either end may go first. Sir John Dalyell records an observation[553] on a
specimen (about one inch long) which was artificially divided into two
halves. The two halves "receded from each other as if by common consent,"
and were nearly an inch apart in twenty hours.

An observation made at Cambridge on a small colony of about 7 mm. in
greatest length gave the following results. The colony moved 13 mm. (nearly
twice its own length) in 8¼ hours: in the next 40 hours it moved 20 mm. (⅘
inch); while in the following 24 hours it moved only 6 mm. Large colonies
change their place only with reluctance.

The locomotive power possessed by _Cristatella_ is not unique among
Phylactolaemata. _Lophopus_, the first fresh-water Polyzoon of which any
description was published, was originally described by Trembley in 1744
under the name of the "Polype à pannache." Trembley observed the
spontaneous division of the colony, _followed by the gradual separation
from one another of the daughter-colonies_.[554] The power of dividing
spontaneously is also possessed by colonies of _Cristatella_ and of
_Pectinatella_.

The colonies of _Lophopus_ are surrounded by an excessively hyaline
ectocyst, and are usually triangular, as shown by Fig. 248. When division
is about to occur, the base of the triangle becomes indented, and the
indentation travels towards the apex in such a way as to bisect the
triangle. The two halves diverge from one another during the process, so
that before division is complete, they are looking, in some cases, in
opposite directions. {497}After a time the narrow connection breaks, and
two new colonies are formed.

Fig. 248 shows a colony shortly after division has taken place. The colony
had moved forwards, in a direction away from its apex, for three days in a
nearly straight line, the distances moved in each day being respectively 6,
8½, 8½ mm. These observations, for which I am indebted to Mr. Lister, show
a considerably higher speed than in those recorded by Trembley, who
observed no colony which moved more than half an inch (12.5 mm.) in eight
days.

The genus _Pectinatella_ also has some power of locomotion. This
magnificent Polyzoon occurs in masses several feet in length (as much as
six feet in _P. gelatinosa_ from Japan[555]), and four to eight inches in
thickness. The greater part of _P. magnifica_[556] consists of a thick,
opaline, and gelatinous ectocyst, the upper surface of which is covered by
hundreds of rosette-like colonies, which increase in number by division.
The masses are thus aggregations of colonies, which secrete a common basal
ectocyst. The latter decays in the autumn; and the separate rosettes, or
groups of them, may thus be set free, being found as floating masses, which
may again attach themselves to a solid object till the time of their death.
_Pectinatella_ has not yet been recorded in England, although, considering
the ease with which statoblasts are transported, it is by no means
improbable that it will eventually be recorded as a British genus. It is at
present known to inhabit America, Japan, and Hamburg.

[Illustration: FIG. 248.—_Lophopus crystallinus_ Pall., Cambridge, showing
the rate of movement. The colony and the distances moved are × 2.]

It is by no means certain what is the mechanism by which {498}movement
takes place in the above cases. The ectocyst of _Cristatella_ is confined
to the base of the colony, and there forms a thin slimy film, which
lubricates the surface over which the animal moves. It has been stated[557]
that progression is produced in the following way. The polypides are
withdrawn by means of retractor muscles, which originate from the septa and
inner surface of the sole. Thus at each retraction of any polypide, the
muscle pulls on a portion of the sole. Should the expanded polypides place
themselves in a suitable position, the movement will be in the direction of
the resultant of the forces due to the separate retractor muscles; while it
is probable that their cilia assist in the onward movement. It should be
noted that it is definitely stated that a colony in which all the polypides
are retracted can alter its position,[558] although even then the retractor
muscles might still contract to some extent.

The movement probably depends on several causes. It must probably be
conceded that the sole itself has some effect on this process. Its outer
cells are contractile, and have the power of raising themselves from the
underlying ectocyst. They may then again attach themselves, and this new
attachment does not always take place in exactly the same place as the
former one. Any movement of the muscles of the sole, or of the retractor
muscles, will thus shift the skin to a new place.[559]

PROTRUSION OF THE POLYPIDE.—While it is perfectly clear that retraction is
principally performed by the great retractor muscles acting directly on the
polypide, it is less easy to explain the converse movement. There can,
however, be little doubt that protrusion is effected by the pressure of the
fluid of the body-cavity, caused in large part by contractions of the
common body-wall.

Now since, in _Cristatella_, the body-cavity is a continuous space, any
pressure on the fluid must act uniformly on all its contents. The cause
which determines the protrusion of a polypide is thus to a large extent the
relaxation of the sphincter-muscle which surrounds its orifice, aided by
special muscles which dilate the orifice. Any polypide which is retracted
while the pressure of the fluid in the body-cavity is sufficient to keep
other polypides protruded, must therefore keep either its
{499}retractor-muscles or its sphincter in a state of contraction in order
to remain in that position. And as a matter of fact, _Cristatella_ and
_Lophopus_ differ from most other Polyzoa in the readiness with which they
expand their tentacles, after they have been induced to retract themselves
by mechanical irritation.

_Plumatella_ and other forms have a chitinous ectocyst, which, however, is
sticky when it is first formed. By virtue of this property, the branches
become attached to the leaf on which the colony is growing, and may have
their natural transparency obscured by taking up foreign bodies. The
stiffness of the ectocyst naturally involves some modification of the
process by which the polypides are protruded. In some cases, this is
effected by the separation of the endocyst from the ectocyst in the lower
parts of the tube. The muscles of the body-wall can thus press on the fluid
of the body-cavity without being restrained by the inflexible ectocyst. In
other cases, the tube of ectocyst is rendered flexible by the presence of a
thin line along one side where the chitin is deficient.

[Illustration: FIG. 249.—_Plumatella repens_ L., R. Yare, × 30. _a_, Anus;
_b_, polypide-bud; _c_, caecum of stomach; _d_, duplicature; _e_, epistome
(see p. 476); _f_, funiculus; _g_, ganglion; _m_, retractor muscle; _p_,
parieto-vaginal muscles; _ph_, pharynx; _s_, statoblasts attached to _f_.]

The upper end of the retracted tentacle-sheath is connected with the
body-wall by bands known as the parieto-vaginal muscles (Fig. 249, _p_).
These {500}serve not only to dilate the orifice when protrusion is
commencing, but also to prevent the polypide from being forced out too far.
They are arranged in such a way that a circular fold, the duplicature
(_d_), is never turned inside out, even in the state of complete protrusion
of the polypide.

The mechanism of the protrusion of the polypide in the Gymnolaemata is in
many cases obscure. The body-wall is not muscular in this group, in some
forms of which, however, short strands known as the parietal muscles (Fig.
234, _p_) pass across the body-cavity from one point to another of the
zooecium. As doubts have been thrown on the function of these muscles in
causing protrusion, it will be worth while to refer to the detailed and
convincing statements of Farre,[560] relating to this point.

Farre's observations were made on certain transparent Ctenostomes
(_Bowerbankia_ and _Farrella_). He states that the parietal muscles "were
distinctly seen to contract whenever the protrusion of the animal took
place, and to become relaxed again upon its retiring into its cell." Their
contraction may indent the outline of the ectocyst, or may cause the
separation of the endocyst from the ectocyst. The endocyst is then drawn
into longitudinal lines at the origin and insertion of these fibres. It is
further suggested that some part is played in the process by the muscular
walls of the alimentary canal, which is a good deal bent in the retracted
condition. The effort to straighten itself is believed to have some share
in forcing out the polypide. The flexible, membranous character of the
"aperture" (see p. 524) in _Membranipora_ (Fig. 256, A) is said by
Nitsche[561] to be an arrangement for the protrusion of the polypides; the
parietal muscles passing from the lateral walls of the zooecium to the
upper membranous wall, which is accordingly depressed by their contraction.

Although it is hardly possible to doubt the accuracy of Farre's
observations, which have, moreover, been confirmed by Hincks, it is by no
means certain that this is the whole explanation in all cases. Oka,[562]
for instance, states that protrusion of the polypide in Phylactolaemata can
be effected in a branch whose body-wall has been cut open. Pergens[563]
believes that the diaphragm (Fig. {501}234, _d_) acts as a pump,
introducing water from the tentacle-sheath into the body-cavity, into which
it is said by him to open, and so forcing out the polypide. It is probable
that many of the forms which have a stiff, unyielding ectocyst possess
special arrangements for introducing water in some way into the space
bounded by the ectocyst,[564] and so forcing out the polypide. Such, for
instance, may be the median pore which occurs beneath the orifice in
_Microporella_ (Fig. 241, A, _mp_), and in certain other cases.

REPRODUCTION OF PHYLACTOLAEMATA.—Sexual reproduction takes place in
_Cristatella_ from June to August. The spermatozoa are ordinarily produced
on the funiculus. The ovaries usually occur on the inner side of the common
wall of the colony, not far below the orifice of a polypide. Each ovary
matures a single egg, which develops _in situ_, the free larva leaving the
colony by the orifice of one of the degenerated polypides.

A second method of reproduction takes place by means of the statoblasts,
which are developed on the funiculus (Fig. 249). According to Verworn,[565]
each statoblast arises from a single cell of the funiculus; and on this
view, the statoblast is, as supposed by the earlier observers, a special
kind of winter-egg. According to more recent researches,[566] the funiculus
consists of a central axis, formed from the ectoderm, and of an outer
sheath of mesoderm-cells; the statoblast is developed from the two kinds of
cells of which the funiculus is composed, and is consequently comparable in
its mode of origin to an ordinary bud. Its special peculiarities are: its
origin as an internal bud, its possession of a chitinous shell, and the
fact that it is destined to leave the parent colony, and to develop, after
a period of rest, into a new colony. Germination takes place by the
formation of a polypide-bud inside the statoblast, which finally splits
along its equator into two halves. The contents emerge as a young colony
which possesses at least one fully-formed polypide.

Remarkable structures known as "hibernacula" occur in the fresh-water
Ctenostomes, _Paludicella_ and _Victorella_. These bodies are in the former
(Fig. 250, B) specially modified _external_ buds, which persist through the
winter when the rest of the colony dies down. At the close of winter the
shell splits into two {502}halves, exactly as takes place in the
statoblasts, and a young colony emerges. It is possible that the
statoblasts may have been evolved from a hibernaculum, which was at first
produced externally, but has become modified in such a way as to acquire an
internal mode of origin.[567]

The simplest known statoblast is that of _Fredericella_ (Fig. 251, A),
which differs from that of other Phylactolaemata in having no ring of
air-cells. In _Plumatella_, the statoblast (Fig. 251, B) has a broad
equatorial ring of air-cells, which enable it to float at the surface of
the water on the decay of the parent tubes. In some species, certain
statoblasts which are produced in the adherent parts of the colony remain
attached to the substratum. These "sessile statoblasts" may have no trace
of the ring of air-cells; but the fact that many sessile statoblasts have
rudiments of this structure suggests that they are a secondary modification
of the floating statoblast. In _Lophopus_ (Fig. 251, C) the ring of
air-cells is very broad, and is pointed at each end; while in _Cristatella_
(Fig. 251, D) and in _Pectinatella_ the statoblast is circular, and
possesses an armature of hooked spines. That of _Cristatella_, measures
about .75 mm. in its greatest length.

[Illustration: FIG. 250.—_Paludicella ehrenbergi_ van Beneden, × about 3.
A, Part of a colony with expanded polypides; B, remains of part of a colony
which has produced hibernacula or winter-buds (_h_); _z_, zooecium. (From
Kraepelin.)]

Kraepelin has suggested that the above order of increasing complexity of
the statoblasts corresponds with the order in which the genera to which
they respectively belong would be placed, on the assumption that the
Phylactolaemata have been derived from the Ctenostomata. Thus, in
_Fredericella_, the form of the lophophore is circular, as in the
Gymnolaemata. The number of the tentacles is comparatively small (20-24).
The arborescent form of the colony resembles that of many Ctenostomes, and
the zooecia are more or less cut off from one another by incomplete septa.

{503}In _Plumatella_, the lophophore has become horse-shoe-shaped, and the
tentacles are more numerous (38-60). In general form and in the arrangement
of the septa this genus resembles _Fredericella_, with which it may easily
be confused.

In _Cristatella_ we have the most highly modified of all the
Phylactolaemata. The individuality of the zooecium is here subordinated to
that of the colony as a whole. The branched arrangement of the zooecia is
greatly obscured. The body-cavities have become completely confluent,
although rudiments of the septa still exist. The ectocyst has been lost,
with the exception of the basal layer of the colony. The tentacles are more
numerous (80-90); and in accordance with the increase in the elaboration of
the genus, its statoblasts belong to the most complicated type known.

[Illustration: FIG. 251.—Statoblasts of Phylactolaemata. A, _Fredericella
sultana_ Blum., × 38; B, _Plumatella repens_ L., × 38; C, _Lophopus
crystallinus_ Pall., × 28; D, _Cristatella mucedo_ Cuv., × 28. (A, from
Allman; B-D, from Kraepelin.)]

The production of _floating_ statoblasts may seem a strange adaptation to
the conditions of fresh-water life, since it might be assumed, _a priori_,
that these structures would be specially liable to be frozen during the
winter. The following experiments made by Braem[568] show, however, that
the germinating power of the statoblasts is improved by a certain amount of
frost. A number of statoblasts were taken; half of these were placed in
water, which was then frozen; and these were found to germinate readily
when afterwards exposed to suitable conditions. The other half were not
subjected to the action of frost; and these could not be made to germinate,
even although the water had been cooled to a point slightly above the
freezing point. It thus appears that the buoyancy, so far from being a
risk, is a means of exposing the statoblast to the conditions which are
most favourable to its later development.

{504}Braem supposes that the beneficial action of frost is due to a
lowering of the vital energy of the statoblast. As in the case of
reproductive bodies known in many other fresh-water organisms, the
statoblast germinates only after a period of rest. Although this period is
often shortened by a lowering of the temperature, it can also be induced by
the exclusion of air, as in an experiment during which the statoblasts were
enclosed in airtight tubes. The respiratory processes were thereby
lessened, and the germinating power was materially improved.

Since the development of the statoblasts depends largely on the
temperature, the first warm weather in early spring will probably induce
the germination of those which are floating; and the young colony, leaving
the protection of the statoblast, will become susceptible to frost. But
even if the first-formed colonies are killed off by a subsequent frost,
other statoblasts which have remained in the mud during the winter are
disentangled from time to time, and germinate on reaching the surface.

DISTRIBUTION.—The protective value of the shell is also shown by the fact
that the statoblast may be kept for some months in a dry condition without
losing its power of germination. There can be little doubt that the
capability of withstanding desiccation enables the species to enlarge its
area of distribution. It is asserted that fresh-water Polyzoa decrease in
abundance in proportion to the distance from the mouth of the river in
which they are found. The current will naturally tend to bring together the
statoblasts from the Polyzoa growing in the upper waters.

Nothing is more surprising than the wide geographical distribution of the
Phylactolaemata. The European genera are all recorded from North America.
_Fredericella_, _Plumatella_, and _Lophopus_ are further recorded from
Australia; while _Plumatella_ is known to occur also in Malacca, the
Philippine Islands, India, Japan, Africa, and South America, It is even
stated that some of the Australian _species_ are identical with those found
in Europe.

Some of the fresh-water Polyzoa are extremely variable, and observers are
by no means agreed in deciding whether certain well-known forms are to be
regarded as varieties or as species. While certain genera, such as
_Cristatella_ and _Lophopus_, are comparatively constant in their form,
_Plumatella_ is excessively variable. _Plumatella_ has a number of species
greater than that of any other form, and the genus has a wider distribution
than any {505}other. This greater variation of species of the dominant
genus is in complete accordance with the general law enunciated by Darwin
that "wide-ranging, much diffused, and common species vary most."

While the ordinary forms of _Plumatella_ consist of branching colonies,
which are either completely adherent to their substratum, or grow in a more
or less erect manner, another habit which is assumed by this genus is so
different from the first that it has been considered to mark a distinct
genus, _Alcyonella_. The Alcyonelloid form (Fig. 246, A) consists of
closely packed tubes which stand more or less at right angles to their
substratum, which they may cover with a dense mass an inch thick, and with
a superficial area of several square inches. But in spite of this
difference, it is possible that _A. fungosa_ is only a variety of an
ordinary _Plumatella_ form. Whether this is so or not, a typical
_Plumatella_ may in places take on an Alcyonelloid habit; and parts of an
_Alcyonella_ may become so lax in growth as to resemble a _Plumatella_.

The British genera of fresh-water Polyzoa may be distinguished from one
another by means of the following table:—

{ Zooecia perfectly distinct from one another. Lophophore circular.
{ Statoblasts absent 2
1. { Colony formed of branching tubes composed of confluent zooecia 3
{ Colony gelatinous, not obviously formed of branching tubes.
{ Lophophore horse-shoe shaped 4

{ Colony consisting of a stolon from which new zooecia originate.
{ These may give rise to new stolons, or directly to new zooecia
{ _Victorella_
2. { Branches composed entirely of club-shaped zooecia, each of which
{ may give off two zooecia near its upper end
{ _Paludicella_ (Fig. 250)

{ Tubes hyaline or opaque, usually containing numerous oval
{ statoblasts (Fig. 251, B), most of which have a ring of
{ air-cells. Lophophore horse-shoe shaped.
{ (_a_) Tubes divergent _Plumatella_ (Fig. 246, B)
3. { (_b_) Tubes parallel with one another _Alcyonella_ form of
{ _Plumatella_ (Fig. 246, A)
{ Tubes cylindrical, usually dark brown. Statoblasts (Fig. 251, A)
{ few, without air-cells. Lophophore circular _Fredericella_

{ Colony hyaline, usually divided into three or four short lobes.
{ Ectocyst thick. Statoblasts (Fig. 251, C) pointed at each end,
{ with a broad ring of air-cells _Lophopus_ (Fig. 248)
{ Colony slug-shaped, crawling on a flattened sole. Ectocyst
4. { rudimentary. Statoblasts (Fig. 251, D) circular, with
{ marginal hooks _Cristatella_ (Fig. 247)
{ Colonies consisting of small rosettes, many of which are attached
{ to a thick basal layer of hyaline ectocyst. Statoblasts circular,
{ with marginal hooks. (Not recorded as British) _Pectinatella_

{506}REPRODUCTIVE PROCESSES OF POLYZOA IN GENERAL.

In studying the reproductive processes of Polyzoa, we have to deal with two
very distinct phenomena; firstly, with the development of eggs; and
secondly, with the formation of buds.

The process of budding usually does no more than increase the number of
individuals in a colony which already exists, and is seldom responsible for
the commencement of a new colony. In _Loxosoma_, however, the buds break
off and lead an independent existence; and in the Phylactolaemata a large
proportion of the colonies have their origin in the statoblasts. In certain
cases, again, new colonies may be formed by the detachment of parts of an
old one, as by the fission of _Cristatella_ and _Lophopus_, or by the
breaking up of a richly-branched species into several colonies by the decay
of the proximal parts.

We may then in the majority of cases look to an embryo for the foundation
of a new colony. The embryo develops into a larva, which, after a period in
which it swims freely, settles down, and is metamorphosed into the first
zooecium. This primary individual forms the starting-point of a colony, and
often differs to a considerable extent from the other zooecia which arise
from it. In Cyclostomata, for instance, the proximal end of the primary
zooecium permanently retains the disc-like shape assumed by the young larva
when it first fixed itself. The primary zooecium may be recognised with
equal ease in many Cheilostomata, and may differ from its successors by
possessing a richer development of marginal spines, or in other respects.

REPRODUCTIVE ORGANS.—Eggs and spermatozoa are commonly found in the same
colony, either in different individuals, or else in the same zooecium (see
Fig. 234, p. 469). In some cases, the zooecium first develops spermatozoa,
and later eggs. The Entoprocta have a more marked separation of the sexes
than obtains in other Polyzoa. The genus _Loxosoma_ is perhaps always
dioecious (_i.e._ with separate sexes). _Pedicellina_ is sometimes found
with ovaries and testes in the same individual, sometimes with these organs
in different individuals; and it is not clear whether a given species
always behaves alike in these respects.

The reproductive organs of the Entoprocta open by ducts of their own into
the vestibule. In the Ectoprocta they are developed in the body-cavity, and
they have no ducts.

{507}The fate of the ripe egg differs widely in different cases. In the
Entoprocta it develops in a kind of brood-pouch formed from part of the
vestibule. The fact that in _Pedicellina_ (Fig. 243) the embryos grow
largely during their development, shows that nutritive material must be
supplied to them from the parent. There is reason to believe that the
epithelium of the brood-pouch is responsible for this process. The eggs are
also known to develop at the expense of nutritive substances prepared by
the parent in the ovicells of the Cyclostomata. In other cases, as in some
species of _Alcyonidium_, the egg is large, and its copious yolk doubtless
supplies a large part of the material required for development.

In the Ectoprocta, development takes place in a variety of places. In most
Cheilostomata a single egg passes into the globular ovicell, which is
formed above the orifice of many of the zooecia. In certain
Ctenostomata,[569] Phylactolaemata,[570] and Cyclostomata,[571] the ripe
egg is taken up by a rudimentary polypide-bud, which is specially formed
for the purpose. In the Ctenostomata and in the fresh-water Polyzoa these
buds, if present, are found in ordinary zooecia which do not become
modified externally in any special way. In the Cyclostomata (_Crisia_), on
the contrary, the formation of the polypide-bud is intimately bound up with
the development of the ovicell. The number of the zooecia which produce
eggs that are capable of development is greatly restricted in this group.
The ovicell, which contains numerous embryos, is not merely a portion of a
zooecium, as in the Cheilostomata; but it is probably to be regarded as a
modification of the entire fertile zooecium or zooecia. These take on an
appearance widely differing from that of the ordinary zooecia, and in
course of time give rise to the ovicells (see Fig. 237).

In all these cases the egg develops inside the parent, and it was hardly
known, before the publication of the interesting researches of M.
Prouho,[572] that some of the Polyzoa lay eggs which develop externally. In
these cases a considerable number of eggs are produced simultaneously by a
single zooecium. {508}M. Prouho further throws light on a much contested
subject; namely, the nature of the so-called "intertentacular organ" (_i_,
Fig. 234, p. 469), described so long ago as 1837 by Farre,[573] but looked
for in vain by the majority of later observers.

The failure to find this organ, even in species which possess it, _in
certain individuals_, according to Farre's statements, is now
satisfactorily explained by M. Prouho, who shows that while it is absent in
a large number of polypides, it is normally present in those individuals
which possess an ovary, and in those only; and that its primary function is
that of an oviduct.

The intertentacular organ is an unpaired ciliated tube, which is situated
between the two tentacles which are nearest to the ganglion. In the
retracted condition of the polypide, it opens from the body-cavity into the
tentacle-sheath; and in the expanded condition, directly to the exterior.

In the remarkable case of _Alcyonidium duplex_, each zooecium normally
possesses two sexual polypides. The first of these produces a testis and
then becomes a "brown body." The second is meanwhile developed, and
produces an ovary and an intertentacular organ, a structure which was not
present in the male polypide. The eggs pass through the intertentacular
organ into the tentacle-sheath, and attach themselves to the diaphragm
(_d_, Fig. 234), where they remain during their development.

Although the intertentacular organ has been found by Prouho in female
polypides only, it would perhaps be going too far to assert that it is
confined to polypides of that sex. Hincks[574] has observed the passage of
spermatozoa in enormous numbers through the organ, although it may be noted
that there is no sufficient proof that eggs were not present as well in
these zooecia. It further appears that in some cases waste matters may be
removed from the body-cavity through the same passage.

It may be presumed that the egg is normally fertilised by a spermatozoon,
although this is at present largely a matter of inference. It is believed
by Joliet[575] that fertilisation is reciprocal, although Prouho has come
to the opposite conclusion. Joliet has, however, very justly pointed out
that the enormous number of spermatozoa developed by a single individual
would be disproportionately large, if their function were merely to
fertilise the {509}ovum in the same zooecium. According to his view, the
egg is fertilised by a spermatozoon after it has passed into the
tentacle-sheath or ovicell, or some other place where it is in free
communication with the outside water.

DEVELOPMENT AND AFFINITIES.—Few parts of the history of the Polyzoa are
more fascinating than that which deals with their development; and it is
probable that no other is capable of giving so much insight into the
affinities of the several groups to one another and to other groups of the
animal kingdom.

[Illustration: FIG. 252.—Diagrams of larvae. A, _Loxosoma_, × 208; _a_,
anus; _b_, brain, with left eye and ciliated pit; _c_, ciliated ring; _ep_,
epistome; _m_, mouth; _o_, oesophagus; _st_, stomach; _x_, aboral adhesive
organ: B, _Cyphonautes_ larva of _Membranipora_ (_Electra_) _pilosa_, ×
about 90; _a_, _m_, _o_, _st_ as in A; _c_, anterior part, and _c'_,
posterior part of the ciliated ring; _e_, epidermis; _ms_, adductor muscle
of shells; _p_, pyriform organ, of unknown function; _sh_, shell; _v_,
vestibule; the "internal sac" or sucker, by which fixation is effected, is
seen between _a_ and _ms_. (B, after Prouho.)]

The comparative study of the larvae of the Polyzoa may be said to date from
1877, when J. Barrois published an elaborate Monograph[576] on this
subject. Although some of Barrois' earlier opinions have been subsequently
modified, this work still gives the best figures of the external form of
the beautiful larvae of many genera. A detailed account of the larval forms
of Polyzoa must be omitted from want of space; and the general conclusions
only can be given.

{510}The larvae of the Entoprocta (Fig. 252, A) resemble the so-called
"Trochosphere" of Polychaeta (see p. 274). The common characters shared by
the larvae of Chaetopoda, Echiuroid Gephyrea, Mollusca, and Polyzoa, and by
adult Rotifera, may well point to the derivation of these groups from a
common ancestor. On this assumption, it is possible that the Polyzoa have
been derived from forms which existed long ages ago, which combined the
common characters of these groups, and the structure of which we can
picture to ourselves only so far as the "Trochosphere" larva can be taken
to represent it in a much simplified condition. Such a view harmonises well
with the great antiquity of the Polyzoa. Certain Ectoproct forms have a
larva, known as _Cyphonautes_ (Fig. 252, B), which closely resembles the
larval form of the Entoprocta; and it is a fact which probably has
considerable significance that this type of larva is known to occur only in
those species of _Membranipora_ (_Electra_), _Alcyonidium_, and
_Hypophorella_, which lay eggs.[577] This may perhaps be regarded as a
primitive form of development which has been lost in species in which
development takes place inside the parent. _Cyphonautes compressus_ (Fig.
252, B), one of the commonest objects taken in the surface-net off our own
coasts, is the larva of _Membranipora_ (_Electra_) _pilosa_. Whilst this
larva is provided with a well-developed alimentary canal, those of most
other Ectoprocta possess a mere rudiment of this structure, and depend for
their nutrition either on yolk present in the egg or on material supplied
by the parent. In most cases the mature larva has no recognisable trace of
a digestive system; and, although it has a free-swimming period, it does
not become truly pelagic.

The alimentary canal of the larva of _Pedicellina_ is known to persist in
the primary individual of the colony. In all other known cases, even in
that of _Cyphonautes_, the larva at fixation loses practically all its
internal organs, and becomes a mere body-wall containing a mass of
degenerated larval tissues. It is in fact a zooecium containing a "brown
body." A polypide-bud is now developed, the body-cavity appears as the
result of the shrinkage of the "brown body," and the primary individual of
the colony is thereby established.

The larvae of the Ectoprocta form a tolerably complete series, starting
from _Cyphonautes_, itself allied to the larva of the {511}Entoprocta, and
ending with the Phylactolaemata. _Alcyonidium_ (Fig. 253, B) possesses a
rudimentary alimentary canal,[578] although the most conspicuous structures
are those connected with the fixation and other phenomena of larval life.
The larvae of many of the encrusting Cheilostomes (Fig. 253, A) resemble
that of _Alcyonidium_, while those of _Bugula_, _Scrupocellaria_, etc.,
belong to a type easily derivable from that of the encrusting forms. The
branching Ctenostomes (_Bowerbankia_, etc.) have a larva which may be
regarded as derived, along slightly different lines, from that of
_Alcyonidium_. The Cyclostomata and the Phylactolaemata have the most
modified forms of larva. That of the former group may owe some of its
peculiarities to the occurrence of a remarkable process of embryonic
fission, which takes place in the ovicell, and as the result of which each
egg gives rise to a large number of larvae.[579] The Phylactolaemata have a
larva which is not unlike that of _Bowerbankia_.

[Illustration: FIG. 253.—A, Aboral view of free larva of _Lepralia
foliacea_ Ell. and Sol.; _a_, long cilia of pyriform organ; _g_, aboral
groove: B, longitudinal section of embryo of _Alcyonidium_, × 135; _c_,
ciliated ring; _g_, aboral groove; _m_, mouth; _n_, nervous system; _p_,
"pyriform organ," of unknown function; _s_, "internal sac" or "sucker," by
which fixation is effected; _st_, stomach.]

We have seen that the larva at fixation becomes a zooecium, {512}which in
the Gymnolaemata forms a polypide-bud after fixation. The peculiarities of
the Phylactolaematous larva may be explained by assuming that it becomes a
zooecium while it is still free-swimming. Thus the larva of _Plumatella_
develops one or sometimes two polypides, which actually reach maturity
before fixation takes place. That of _Cristatella_ develops from two to
twenty[580] polypides or polypide-buds at the corresponding period, and it
is in fact a young colony while still free-swimming.

Now in most colonial animals, such as Coelenterates and Ascidians, the
larva metamorphoses itself into a temporarily solitary animal, which then
gives rise to the remainder of the colony by budding. The majority of the
Gymnolaemata behave in this way; while the Phylactolaemata may not only
develop a multiplicity of polypides in their larval stage, but the
individuality of the zooecia is then just as much obscured as in the adult
state. These facts are more easily explained if we assume that
_Cristatella_ is the end-point in a series than if we suppose it to be a
starting-point.

On the view maintained by many authorities, that the Polyzoa are related,
through _Phoronis_, with the Gephyrea and the Brachiopoda, we should expect
to find in those Polyzoa which most closely resemble _Phoronis_ in their
adult state—that is to say in the Phylactolaemata—some indications of
affinity to that animal in their development. This is emphatically not the
case. The hypothesis that the Phylactolaemata are related to _Phoronis_
leads, moreover, to the improbable conclusion that the similarities between
the Entoproct-larva and _Cyphonautes_, on the one hand, and the
Trochosphere larva of Polychaeta, on the other hand, is entirely
superficial and meaningless. In spite, therefore, of the similarity between
_Phoronis_ and a single individual of the Phylactolaemata, and in spite of
the marked resemblance between its nephridia and structures which have been
described in _Cristatella_[581] and _Pectinatella_[582] the comparative
study of the development appears to indicate that the resemblances between
_Phoronis_ and the Phylactolaemata are the result of a coincidence rather
than of any close relationship.

A few points connected with the metamorphosis of the {513}Polyzoa deserve
more special notice. There is generally great difficulty in persuading
larvae to fix themselves when kept in a small quantity of water, which
becomes over-heated in the air of a laboratory. The difficulty may be
surmounted by placing colonies containing embryos, together with some clean
pieces of the seaweed on which the adults are habitually found, in a vessel
closed by a piece of fine muslin, and by leaving the vessel attached to a
buoy or in a deep tide-pool. The larvae being without an alimentary canal,
fix themselves, after a very short free life, on the seaweed.

It is probable that a great struggle for existence normally takes place at
the commencement of the metamorphosis. Any one who will examine, in June or
July, rocks covered by _Fucus_ on which _Flustrella hispida_ is growing,
will probably find numerous young fronds of _Fucus_, from half an inch to
an inch or two in length, growing under the shelter of the older fronds.
The bivalve larvae of _Flustrella_ show a marked preference for fixing on
these young fronds—perhaps in order that the duration of life of the colony
may coincide with that of the _Fucus_—and these young fronds are commonly
covered by very numerous recently-fixed larvae, and by young colonies of
various ages. Or, it is easy to observe, by placing pregnant colonies of
_Bowerbankia_ in a vessel of water, that the larvae, which are hatched out
in thousands, fix themselves in dense masses on certain parts of the wall
of the vessel. It is clear that but a small proportion of these larvae will
find room for further development.

Next with regard to the mode of fixation. Attachment always takes place by
the surface on which the mouth or its rudiment is situated, and the
permanent alimentary canal opens on the opposite surface. In _Pedicellina_,
the one case in which the larval digestive organs are known to become those
of the first adult individual, this presupposes a rotation of the
alimentary canal, in order to bring it into its new position.

It is well known that the larvae of other fixed animals may undergo a
somewhat similar change. Thus those of Ascidians and of Barnacles fix
themselves by their anterior end, and ultimately reach their adult form by
performing a kind of a somersault. The process may perhaps be explained by
supposing that some part of the anterior end or of the oral surface is
specially sensitive, and that the larva fixes itself by that portion of its
{514}body which is best fitted for ascertaining which is the proper
substance on which to fix.

BUDDING.—The formation of a new individual may take place by the outgrowth
of part of the body-wall, as in _Pedicellina_ (Fig. 243, p. 487) and in
_Bowerbankia_ (Fig. 238, p. 480). In _Pedicellina_ a young stalk is formed
by an outgrowth near one of the growing points, and the upper part of this
outgrowth becomes constricted off to form the calyx. In other cases (cf.
the growing ends of the branches in Fig. 237) a partition grows across the
body-cavity at the growing edge of the colony, and so cuts off a part
destined to become a new zooecium.

The zooecium formed in one of these ways acquires an alimentary canal by
the formation of a polypide-bud, some stages in the growth of which are
shown in Fig. 235 (p. 472). Contrary to what happens in Coelenterates and
Tunicates, in which the endoderm takes part in the budding, there is good
reason for believing that in Polyzoa the polypide-bud is developed entirely
from ectoderm and mesoderm.[583] The bud is a two-layered vesicle, attached
to the inner side of the body-wall. Its inner layer is derived from the
ectoderm, which at first projects into the body-cavity in the form of a
solid knob surrounded by mesoderm-cells. A cavity appears in the inner,
ectodermic mass, and the upper part of the vesicle so developed becomes
excessively thin, forming the tentacle-sheath, which is always developed in
the condition of retraction. The lower part becomes thicker; its inner
layer gives rise to the lining of the alimentary canal, to the nervous
system, and to the outer epithelium of the tentacles, which grow out into
the tentacle-sheath (cf. Fig. 235). The outer layer gives rise to the
mesodermic structures, such as the muscles, connective tissue, and
generative organs.

These processes are fundamentally similar, whether in the metamorphosed
larva, in a young zooecium, in an old zooecium after the formation of a
"brown body," or in the germinating statoblast of the Phylactolaemata.

{515}CHAPTER XIX

POLYZOA (_continued_)

CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE
EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE DETERMINATION OF
THE GENERA OF BRITISH MARINE POLYZOA

Our account of the Polyzoa would be manifestly incomplete without some
reference to the systematic arrangement of these animals. An outline of the
principal groups has been given on p. 475. So far, the classification is
easy, but it is otherwise when we attempt to subdivide most of the groups
any further.

Systems of classification which depend exclusively upon the external
characters of animals have been repeatedly shown to be unsatisfactory. Now
with regard to the Polyzoa, not only is it the case that the great majority
of forms are only known in their external characteristics, but current
systems of classification cannot be regarded as final, because it is not
yet certain which of the external features have most systematic value. Two
obvious points can be at once selected—namely, the character of the
zooecium and the character of the entire colony. One or two instances will
serve to show what different results are obtained by depending exclusively
on either of these characters by itself.

According to the older writers, the habit of the colony was taken as the
most important generic character; and there can indeed be no doubt that
this feature has great importance within certain limits. Any one who has
examined different species of such genera as _Flustra_, _Cellaria_,
_Bugula_, _Retepora_, etc., must feel that the form of the colony goes for
a good deal. But a consideration of other cases shows that there is great
risk in the {516}indiscriminate use of this method of arranging the
Polyzoa. The old genus _Eschara_, composed of forms with an erect
coral-like habit,[584] included species which are now placed in such
different genera as _Lepralia_, _Porella_, _Microporella_, etc. The older
works on Polyzoa include all encrusting forms of Cheilostomata, with a
completely calcareous front wall, in the genus _Lepralia_, the members of
which are now distributed in numerous widely separated genera.

As an instance of the converse arrangement—essential similarity of the
zooecia with great differences of the general habit—may be mentioned the
common _Membranipora_ (_Electra_) _pilosa_.[585] Ordinarily growing in the
form of close encrustations on seaweeds, this species may take on entirely
different habits of growth. The zooecia are now dissociated, growing in
single lines over the substratum; now forming erect tufts, composed of
single lines of zooecia or of several rows. The erect, branching habit
appears to be induced in the first instance by the character of the seaweed
on which the colony begins life. Thus colonies which encrust the thin
branches of _Corallina_ may have impressed on them something of the mode of
growth of the seaweed, so that when they extend beyond the tips of the
branches of the _Corallina_, they continue to grow in delicate branches,
which still retain more or less the same diameter as those which form their
base. An extreme variation results in the beautiful form known as _Electra
verticillata_, in which the zooecia are arranged with great regularity in
whorls, which together form erect branches.[586] But with all these
variations, the zooecia are so much alike that it is hardly possible to
regard the extreme forms as more than varieties of a single species. A
careful examination of this case would convince most observers that the
characters of the zooecium are a more trustworthy guide to classification
than those of the entire colony, a result which was first clearly stated by
Smitt, and amply confirmed by Hincks.[587]

The avicularia of the Cheilostomata afford useful help in classifying this
group; but while certain genera are always provided with avicularia, others
include some species with these organs, and other species without them.
Again, while the species {517}of some genera (e.g. _Cellepora_) possess a
great variety of forms of avicularia, the same pattern of avicularium may
characterise several widely different genera. Further, the _position_ of
the avicularium may be very different in species which are apparently
closely related. Well-developed vibracula, although constant in their
occurrence in such forms as _Scrupocellaria_ (Fig. 254) and _Caberea_ (Fig.
242), occur here and there in species of encrusting forms which are
ordinarily placed in very different families.

Now although some of these discrepancies are perhaps due to errors in
classification, whereby species which are really allied have been wrongly
placed in distinct genera, this explanation would not prove satisfactory in
all cases. Thus in _Bugula_, a genus which is specially characterised by
the high development of its avicularia, these organs are normally absent in
_B. neritina_. The fact that this species was rightly placed in the genus
has been confirmed by the discovery made by Waters[588] that avicularia
occur in specimens which are believed to be identical with that species.

[Illustration: FIG. 254.—A, Front view, and B, back view of part of a
branch of _Scrupocellaria scabra_, Van Ben., Durham Coast, × 43; _a_,
lateral avicularium; _a'_, smaller median avicularium; _ap_, membranous
aperture; _f_, fornix; _r_, rootlet; _s_, seta of vibraculum; _v.z_,
vibracular zooecium.]

1. The Cyclostomata appear to fall naturally into two main groups, (A) the
ARTICULATA, including the Crisiidae (Fig. 237), distinguished by their
erect branches, divided at intervals by chitinous joints; and (B) the
INARTICULATA, which include the remaining families, whether erect or
encrusting, agreeing in the negative character of being unjointed.

{518}2. The Cheilostomata consist of (A) the CELLULARINA, including the
flexible, erect forms, such as _Bugula_ (Fig. 233) and _Scrupocellaria_
(Fig. 254); (B) the FLUSTRINA, to which belong _Flustra_ (Fig. 232),
_Membranipora_ (Fig. 256, A, B), _Micropora_ (Fig. 256, C), and other forms
in which the front wall of the zooecium is either membranous, or depressed
and marked off by a ridge-like margin; (C) the ESCHARINA, including the
great majority of forms, in which no part of the front wall remains
membranous, the wall of the zooecium being wholly calcified.

3. The Ctenostomata comprise (A) the ALCYONELLEA or encrusting forms; and
(B) the VESICULARINA or branching forms. The zooecia in the latter
subdivision (Fig. 238) are given off from a tubular stem or stolon, which
is usually erect and branching.

We thus have the following arrangement of recent forms. The genera
mentioned are for the most part those which have already been alluded to in
the preceding account:—

Sub-class I. ENTOPROCTA.
_Loxosoma_, _Pedicellina_, _Urnatella_.

Sub-class II. ECTOPROCTA.
Order 1. Gymnolaemata.
Sub-order 1. Cyclostomata.
A. Articulata. _Crisia_.
B. Inarticulata. _Hornera_, _Idmonea_, _Tubulipora_, _Stomatopora_,
_Diastopora_, _Entalophora_, _Lichenopora_.
Sub-order 2. Cheilostomata.
A. Cellularina. _Aetea_, _Eucratea_,[589] _Catenicella_,
_Cellularia_, _Gemellaria_, _Menipea_, _Scrupocellaria_,
_Caberea_, _Notamia_ (= _Epistomia_), _Bicellaria_, _Bugula_,
_Beania_.
B. Flustrina. _Cellaria_, _Flustra_, _Membranipora_, _Electra_,
_Lunulites_, _Membraniporella_, _Cribrilina_, _Micropora_,
_Selenaria_.
C. Escharina. _Retepora_, _Microporella_, _Lepralia_, _Porella_,
_Smittia_, _Mucronella_, _Schizoporella_, _Schizotheca_,
_Mastigophora_, _Porina_, _Cellepora_.
Sub-order 3. Ctenostomata.
A. Alcyonellea. _Alcyonidium_, _Flustrella_.
B. Vesicularina. _Vesicularia_, _Amathia_, _Bowerbankia_,
_Farrella_, _Hypophorella_, _Triticella_, _Mimosella_,
_Victorella_, _Paludicella_.
Order 2. Phylactolaemata.
_Fredericella_, _Plumatella_ (including _Alcyonella_), _Lophopus_,
_Cristatella_, _Pectinatella_.

{519}Even this classification, which deals only with the larger groups,
must not be made use of without a word of warning. The division of the
Cheilostomata is a matter of great difficulty; and no scheme which has yet
been suggested can be regarded as more than tentative. The great number of
forms included in this group makes its subdivision extremely desirable from
the point of view of convenience; but a further knowledge of the anatomy
and of the development of many of the forms of doubtful systematic position
is probably necessary before any scheme which is likely to be permanent is
put forward. Those who desire to make a further study of the classification
of the Polyzoa should refer to the works of Hincks,[590] Busk,[591]
MacGillivray,[592] and Gregory.[593]

The Polyzoa do not appear to lend any valuable assistance towards settling
the disputed problems of Geographical Distribution. They are not in any
case terrestrial, while the fresh-water species do not always respect the
limits between the great zoogeographical regions. It has already been
pointed out (p. 504) that _Plumatella_, _Fredericella_, and _Lophopus_ are
believed to occur in Australia, and the first-named genus is practically
world-wide in its distribution.

Many marine forms also have a surprisingly wide distribution. Thus among
the British species which are described by Mr. Hincks as occurring from
Norway to New Zealand are _Membranipora pilosa_, _Scrupocellaria scruposa_,
_Cellaria fistulosa_, _Microporella ciliata_, and _M_. _malusii_. Even if
it should be proved that specific differences do exist between the southern
forms and our own, there can be no doubt of the wide distribution of
certain species. It was pointed out by D'Orbigny that _Bugula neritina_ has
the habit of attaching itself to the bottoms of ships, a fact which may
possibly account for the wide distribution of this species; although it
would not be safe to assume this explanation of the facts in all cases.
Other Polyzoa, on the contrary, have a more restricted range. Thus
_Catenicella_ is specially characteristic of the Australian region.

It is perhaps surprising that marine Polyzoa should in so many cases have
so wide a range. Even though it is the rule {520}for Polyzoa to have free
larvae, the period during which these larvae are free-swimming is, so far
as is known, a short one in most cases. _Cyphonautes_ is a common pelagic
form (see p. 510), and probably remains for a considerable period in the
larval condition. Other Polyzoon-larvae appear to fix themselves very soon
after their birth; and this would not appear to give much time for them to
be carried to great distances by ocean-currents. It may, however, be
suggested that it does not follow that because we know that a larva may,
under favourable conditions fix itself a few minutes after it becomes free,
we should be justified in assuming that that larva would not retain for a
long period the power of undergoing a normal metamorphosis should it be
drifted away from suitable fixing-grounds.

PALAEONTOLOGY.[594]—The number of fossil Polyzoa is enormous. D'Orbigny
devoted two hundred plates and more than a thousand octavo pages[595] to a
Monograph on the Cretaceous Polyzoa of France. Many of the fossil forms are
extraordinarily well preserved, and there is often no difficulty in
recognising the identity between certain fossil species belonging to the
more recent formations and living forms. It thus becomes necessary to
consult Palaeontological memoirs in working at recent Polyzoa.

While the great majority of fossil Polyzoa do not differ in any essential
particular from recent species, this is not altogether the case with the
Palaeozoic forms. Leaving out of account the Stromatoporoids, which have
been variously referred to the Sponges, Hydrozoa, and Foraminifera, as well
as to the Polyzoa, the Palaeozoic strata contain large numbers of peculiar
Cyclostomata, together with members of the Trepostomata, a fourth Sub-order
of Gymnolaemata, allied to the Cyclostomata. The Trepostomata are for the
most part Palaeozoic, but a few survived as late as the Jurassic
period.[596] These, with the other Polyzoa from the same formations, are
considered by Dr. Gregory in his recently published _Catalogue of the
Fossil Bryozoa in the British Museum_ (1896).

The number of Polyzoa recorded from the earlier secondary strata is small.
The majority of the known Jurassic forms {521}belong to the Cyclostomata;
and one or two Cheilostomes are recorded from the same period. Recent
papers by Walford[597] on Jurassic Polyzoa contain the description of
genera which are believed to be intermediate between the Cyclostomata and
Cheilostomata, particularly with regard to the characters of their
ovicells. Although it is not impossible there may be a connection between
the ovicells of these two groups, it has yet to be proved that the two sets
of structures are homologous.

The Cretaceous period marks the commencement of a large number of
Cheilostome genera, although the Cyclostomes still remain numerous.

In the Tertiary formations the Cyclostomes gradually become less numerous,
and although in earlier geological periods they far outnumbered the
Cheilostomes, these relations are now reversed. Certain Tertiary strata,
and particularly the Coralline Crag (Pliocene), are remarkable for the
extremely large number of Polyzoa they contain. It will be noticed that no
mention has been made of the Entoprocta, the Ctenostomata, and the
Phylactolaemata. Their absence in the fossil condition[598] need not,
however, be a matter for surprise, as none of these forms are so well
suited for being fossilised as are the calcareous Cyclostomata and
Cheilostomata. There is consequently no adequate reason for assuming that
the absence of a palaeontological record implies that these groups have
been recently evolved.

DETERMINATION OF GENERA OF MARINE POLYZOA.—The species to which a Polyzoon
belongs can only be determined, in most cases, with the assistance of the
low powers of a microscope. There are very great advantages in the use of a
binocular instrument, by means of which a microscopic preparation appears
with its parts standing up in proper relief.

In the case of the calcareous forms, the external characters may be more
readily made out in a dry preparation than in any other way. For this
purpose, the colony should be washed with fresh water, in order to remove
the salts, which otherwise crystallise out on drying and obscure the
surface. Preparations of this kind must be looked at with the aid of
reflected light. Canada-balsam or glycerine preparations are also valuable,
whether {522}stained or unstained; and are essential for the examination of
the softer forms. In the case of erect species, both surfaces of the branch
should be looked at. The opercula, avicularia, and rosette-plates afford
important systematic characters in the case of the Cheilostomata.

It must not be forgotten to take account of the condition of the zooecia at
different ages. The old zooecia often become entirely altered in form, by
the deposition of additional calcareous matter, or by the loss of certain
parts present in the younger zooecia. Thus the marginal spines may be
entirely lost in the older individuals, while in those forms which develop
a "peristome" (see Fig. 255 and p. 524), the characters of the orifice can
often be determined in the young zooecia only. It is thus essential to
examine the growing ends of the branches or the rim of the colony, as the
case may be.

[Illustration: FIG. 255.—Illustrating the nature of a secondary orifice
(Cheilostomata). A, _Mucronella coccinea_ Abildg., Scilly Is., × 40. The
ovicell (_o_) overhangs the primary orifice, which is concealed by the
great development of the peristome, produced into the mucro (_mu_); _t_,
the three teeth (denticles) within the secondary orifice; _a_, avicularium.
B, _Porella compressa_ Sowb., Norway, × 40; _p.o._ primary orifice, above
which is a concave lamina, the beginning of the ovicell. In the lower
zooecium the ovicell (_o_) is further grown. The primary orifice is still
visible, but it is partially concealed by the growth of the peristome,
which encloses a minute avicularium; _m_, mandible of avicularium. C, Older
part of the same colony; _pr_, peristome; _s.o_, secondary orifice; _o'_,
adult ovicell; _p_, pores.]

In order to make preparations with the tentacles expanded, hydrochlorate of
cocaine, chloral hydrate or spirit should be added gradually to the water.
When the animals are completely anaesthetised they may be killed by means
of a 7-10 p.c. solution of sulphate of copper (best made in distilled water
or in rain water). This method gives admirable results in the case of both
{523}fresh-water and marine Polyzoa. The use of formaline (see p. 229) may
be strongly recommended for the Vesicularina.

The only recent work dealing with all the marine BRITISH FORMS is Mr.
Hincks' invaluable _History of the British Marine Polyzoa_.[599] As the use
of this book, unaided by any artificial help, is by no means easy to the
beginner, the following key has been compiled as an index to the genera.
The Entoproct forms, _Loxosoma_ and _Pedicellina_ (see pp. 488-491), are
not included in the table.

[Illustration: FIG. 256.—Illustrating the terminology of the front surface
of the zooecium (Cheilostomata). A, _Membranipora_ (_Electra_) _pilosa_ L.,
Cromer, × 47; _ap_, the membranous "aperture;" _o_, orifice. B,
_Membranipora flemingii_ Busk, Plymouth, × 60; _ap_, the aperture, enclosed
in a calcareous "area" (_a_); _av_, avicularium; _s_, marginal spines. C,
_Micropora coriacea_ Esper, Plymouth, × 43; _a_, area (calcareous); _o_,
operculum; _ov_, ovicell.]

In order to facilitate the use of the table here given in conjunction with
Mr. Hincks' work, the nomenclature there adopted has been followed
throughout. References to other descriptions of the species may be obtained
by consulting Miss Jelly's admirable _Synonymic Catalogue of the Recent
Marine Bryozoa_.[600]

TERMINOLOGY.—A few technical terms must of necessity be employed. The
colony is _adherent_ when its zooecia are attached to the object on which
the colony is growing. The _zooecium_ is the body-wall of a single
individual; and, except in transparent species, is the only part which can
be seen from the outside in the retracted condition of the _polypide_ or
tentacles with the alimentary canal. The outermost layer of the zooecium is
known as the _ectocyst_; it may be simply membranous, or calcified, or may
be rendered opaque by foreign bodies; its surface in {524}calcareous forms
is often marked by _pores_ (Fig. 239, C, _p_), which are vacuities in the
calcareous wall, closed externally by membrane. A special _median pore_
(Fig. 241, A, _m.p_) may occur, and is in some cases at least a complete
perforation through the body-wall.

The tentacles are protruded through the _orifice_, which in Cheilostomata
is usually guarded by a movable chitinous lid, or _operculum_ (Fig. 256, A,
_o_). Should the ectocyst be thickened or raised into a ridge surrounding
the orifice, a tubular passage results, known as the _secondary orifice_
(Fig. 255), at the deeper end of which is the true orifice. The _peristome_
(Fig. 255, C, _pr_) is the raised or thickened part which gives rise to the
secondary orifice. Should the zooecium be outlined by a raised ridge, the
part so enclosed is known as the _area_ (Fig. 256, C, _a_), if calcareous.
The _aperture_ or _opesia_ (Fig. 256, A, B, _ap_) is a membranous part of
the front surface; and may consist of the whole or part of the area. The
orifice or the aperture is commonly provided with _spines_ (Fig. 256, B,
_s_).

[Illustration: FIG. 257.—A, _Cribrilina annulata_ Fabr., Norway, × 33; _c_,
calcareous bars concealing the membranous aperture: B, _Membraniporella
nitida_ Johnst., Plymouth, × 45; _a_, calcareous bars growing up round the
margin of the aperture; _b_, the same, further developed; _c_, the same,
completely formed (as in A); _av_, avicularium; _o_, immature, and _o'_,
mature, ovicell; _s_, marginal spines.]

The _avicularium_ and the _vibraculum_ are specially modified zooecia (see
p. 482), which occur in a great variety of forms, in certain Cheilostomata
only. The operculum of the ordinary zooecium is represented by the
_mandible_ (Fig. 239, B, _m_) in the avicularium, and by the _seta_ (Fig.
242, _s_) in the vibraculum. The representative of the zooecium itself is
known as the _avicularian_ (Fig. 239, A, _a.z_) or _vibracular zooecium_
(Fig. 242, _v.z_).

{525}An _ovicell_ is a swelling in which the embryo develops, in certain
Cyclostomata (Fig. 237) and Cheilostomata (Fig. 241, A, _o_). A _stolon_
(Fig. 238, B, _st_) is a stem, not formed of fused zooecia, from which new
individuals originate. An _internode_, in a jointed colony, is the part
between any two joints. The _fornix_ or _scutum_ (Fig. 254, A, _f_) is a
modified spine which in some Cheilostomata overhangs the aperture. A
_mucro_ (Fig. 255, A, _mu_) is a spike or protuberance developed just below
the orifice. A _sinus_ (Fig. 239, B, _s_) is a slight bay on the lower
margin of the orifice.

The orifice opens at the _upper_ end of the zooecium, on its _front_
surface. The _length_ of the zooecium is the distance from the upper to the
lower ends, and the _width_ the distance between its sides.

{One or more of the following characters: orifice provided with an
{ operculum; avicularia or vibracula present; a globular ovicell
{ above the orifice of certain zooecia (CHEILOSTOMATA) 7
{
{Opercula, avicularia, vibracula, and ovicells completely absent,
{ or inconspicuous. Calcareous or non-calcareous. If calcareous,
1. { the orifice is not at the end of a free cylindrical portion 3
{
{Calcareous; zooecia cylindrical, often united for the greater part
{ of their length, but usually ending in a free cylindrical
{ portion,which bears the terminal orifice. The zooecia may be
{ much obscured by calcifications surrounding their basal parts 2

{Zooecia long, tubular, with a lateral membranous region at the upper
{ end, given off quite separately from a creeping stolon _Aetea_
2. {
{Zooecia more or less united to one another, orifice without
{ chitinous operculum (CYCLOSTOMATA[601]) 63

{Zooecia without marginal spines; arising from a branching axis,
{ which is not formed of zooecia 74
{
3. {Colony adherent; _or_ erect, fleshy and slightly branched; _or_
{ erect, encrusted with earthy matter and repeatedly branched 72
{
{Characters not as above 4

{Zooecia minute, boat-shaped, united by a delicate tube. Aperture
4. { large, with marginal spines _Beania mirabilis_
{
{Colony delicate, erect; zooecia wider above than below 5

{No marginal spines 6
{
5. {Zooecia uniserial; with marginal spines. Branches arising from
{ the top of a zooecium _Brettia_

{Zooecia uniserial; branches arising just below the large aperture. An
{ ovicell may be developed above the orifice of a modified zooecium
6. { _Eucratea chelata_
{
{Zooecia somewhat pear-shaped; orifice small, semicircular
{ _Huxleya fragilis_

{526}
{Colony erect 8
{
7. {Zooecia in several layers forming confused masses 30
{
{Colony entirely adherent,[602] the zooecia usually in a single
{ layer 31

ERECT CHEILOSTOMATA.

{Branches cylindrical, calcareous, divided by chitinous joints.
{ Orifices arranged all round the branc _Cellaria_ (Fig. 239, A)
{
8. {Branches flexible, jointed or unjointed. Orifices not arranged
{ all round the branch. 9
{
{Calcareous, unjointed, rigid 21

{Branches leaf-like, flattened 10
9. {
{Branches not leaf-like 11

{Avicularia resembling birds' heads, movable _Bugula_ (Fig. 233)
{
10.{Avicularia not resembling birds' heads, unstalked; _or_ absent.
{ Colony broadly leaf-shaped, composed of a single layer or of
{ two layers of zooecia _Flustra_ (Fig. 232)

{Zooecia in pairs, at the same level 12
11.{
{Zooecia not obviously paired 13

{Branches numerous, straight. Zooecia back to back, with an oblique
{ aperture. No avicularia _Gemellaria loricata_
12.{
{Branches delicate, curved. A pair of stalked avicularia between
{ each two pairs of zooecia _Notamia_ (_= Epistomia_) _bursaria_

{conspicuous 14
13. Avicularia or vibracula {
{inconspicuous or absent 17

{Avicularia resembling birds' heads, movable. Vibracula absent 15
{
{Avicularia large, unstalked. Vibracula present or absent 16
14.{
{Avicularia inconspicuous. Setae of the vibracula large, very
{ conspicuous, on oblique vibracular zooecia, which almost cover
{ the backs of the branches _Caberea_ (Fig. 242)

{Zooecia in two series, alternate, with one or several
{ conspicuously long marginal spines _Bicellaria_
15.{
{Zooecia in two or more series. Aperture occupying most of the
{ front of the zooecium. Colony often spiral. Avicularia
{ usually large _Bugula_ (Fig. 233)

{Zooecia long, narrow below, commonly in triplets, with two lateral
{ avicularia to each triplet. Fornix present. _Menipea ternata_
{
16.{Zooecia biserial, a considerable number forming an internode
{ separated by a joint (often inconspicuous) from the next
{ internode. Lateral avicularia usually large. Vibracular zooecia
{ on the back or sides of the branches _Scrupocellaria_ (Fig. 254)

{Characters as in _Scrupocellaria_ (No. 16), but with inconspicuous
17.{ avicularia. A branched fornix _Scrupocellaria reptans_
{
{Vibracula absent 18

{A single, short, marginal spine; _or_ none 19
18.{
{Marginal spines present 20

{527}
{Characters as in No. 6 _Eucratea chelata_,
{ _Huxleya fragilis_
{
{Zooecia biserial. Aperture large, the semicircular orifice at its
{ upper end, where there is commonly a short spine
{ _Cellularia peachii_
19.{
{Zooecia in one or two series. Branches originating from the backs
{ of the zooecia, and facing in the opposite direction to the
{ parent branch. Aperture small _Scruparia clavata_

{One or more conspicuously long marginal spines. Avicularia
{ present or absent _Bicellaria_
{
20.{Zooecia uniserial (see No. 5) _Brettia_
{
{Zooecia biserial, in short internodes. An inconspicuous avicularium
{ below the aperture. Fornix present _Menipea jeffreysii_

{Colony consisting of a network of narrow branches, the zooecia
{ opening only on one of their surfaces _Retepora_
{
{Colony large, brittle, composed of contorted plates, uniting
21.{ irregularly, usually composed of two layers of zooecia.
{ Orifice large, indented laterally _Lepralia foliacea_
{
{Branches delicate, cylindrical 22
{
{Branches or lobes coarser, not necessarily cylindrical 24

{Branches composed of four rows of zooecia 23
{
22.{Zooecia in more than four regular, longitudinal rows. Peristome
{ raised, and, with the ovicell, forming a swelling on the
{ surface of the branch _Escharoides quincuncialis_

{Orifice circular. A row of pores round the margin of the zooecium.
{ A median pore resembling a small orifice below the true orifice.
23.{ Small lateral avicularia _Porina borealis_
{
{Orifice surrounded by a peristome, produced into a mucro beneath
{ the orifice. No pores _Palmicellaria elegans_

{Zooecia arranged in regular series 25
{
24.{Zooecia irregularly heaped, their long axes often perpendicular
{ to the surface of the colony. Mucro largely developed,
{ concealing the form of the orifice, and bearing an avicularium
{ _Cellepora_

{Orifice with a sinus; _or_ peristome interrupted or extended below
25.{ into a sinus-like outgrowth, which usually includes a small
{ avicularium 26
{
{Neither median sinus nor interrupted or extended peristome 28

{Orifice with a sinus _Schizoporella_ (Fig. 239, B)
26.{
{Peristome interrupted or extended below 27

{Branches of various forms. Surface of the older parts very even.
{ Secondary orifice rather long, usually wider above, enclosing a
{ small avicularium below, and appearing as a hole in the even
{ surface of the branch _Porella_ (Fig. 255, B, C)
{
27.{A prominent tooth projects into the orifice from its lower side.
{ Zooecia with thin walls _Smittia landsborovii_ (Fig. 239, C)
{
{No tooth in the orifice, at the side of which is a small avicularium.
{ Old zooecia with thick walls. Colony composed of a short stem
{ and flattened branches _Escharoides rosacea_

{A tooth projects from the lower side into the large, {528}
{ subcircular orifice, on each side of which is a small oval
28.{ avicularium (colony erect or encrusting) _Mucronella pavonella_
{
{No tooth: mucro sometimes present 29

{Branches cylindrical. Old zooecia with thick walls. Orifice in
{ young zooecia longer than broad; beneath it a median pore,
29.{ and in some cases a lateral avicularium with vibraculoid mandible
{ _Diporula verrucosa_
{A distinct mucro, which may bear an avicularium above
{ _Palmicellaria_

ENCRUSTING CHEILOSTOMATA.

{Usually growing on a small univalve shell. Orifice longer than
{ broad, indented laterally. Mucro present _Lepralia edax_
{
30.{One or two conspicuous processes, each bearing an avicularium,
{ near the orifice, which is often concealed. Avicularia in many
{ cases found on other parts of the colony _Cellepora_

{Zooecia distant; _or_ in single rows 32
31.{
{Zooecia forming continuous expansions 36

{An oval aperture, larger than the orifice 33
32.{
{No aperture 34

{A tubular process below the aperture, in some cases: zooecia very
33.{ narrow below _Eucratea chelata_, var. _repens_
{
{No tubular process below the aperture _Membranipora_

{Peristome much raised below, collar-like _Phylactella_
34.{
{Peristome not much raised below 35

{Zooecia minute, much narrowed below. Orifice small, usually with
35.{ a sinus _Hippothoa_
{
{Zooecia not narrowed below. Orifice with a sinus _Schizoporella_

{Zooecia partly separated by a thin calcareous crust. Colonies
36.{ small 37
{
{Zooecia contiguous 38

{Zooecia pear-shaped. Orifice with a sinus _Hippothoa expansa_
{
37.{Zooecia ovoid. Orifice subcircular, with a tubular peristome
{ _Lagenipora socialis_

{Orifice close to the upper end of the zooecium (unless crowned by an
{ ovicell). Front of the zooecium marked by transverse or
{ radiating furrows or lines. The very young zooecium may
38.{ possess a membranous area, which becomes roofed in by the
{ union of two lateral series of converging bars (Fig. 257) 39
{
{Characters not as above 40

{Furrows with uniserial rows of pores (often minute), which are rarely
{ irregular _Cribrilina_ (Fig. 257, A)
39.{No rows of pores. Distinct transverse lines or spaces and a
{ median longitudinal suture between the bars
{ _Membraniporella_ (Fig. 257, B)

{529}
{Zooecia arranged in regular series 41
40.{
{Zooecia irregularly[603] heaped together (cf. No. 30) _Cellepora_

{Primary orifice conspicuous; with a sinus, or with a peristome
{ extended or interrupted below, and sometimes simulating a
{ sinus 42
{
{Neither sinus[604] nor interrupted peristome 44
41.{
{Surface of the old zooecia much thickened, so that the secondary
{ orifice does not project beyond the most prominent parts of the
{ zooecium. Secondary orifice concealing the primary orifice,
{ wider above, enclosing a small avicularium below
{ _Porella_ (Fig. 255, B, C)

{Primary orifice with a sinus, but no tooth 43
{
42.{A prominent tooth projects into the orifice from its lower side.
{ Peristome interrupted or with a sinus. Surface of the old
{ zooecia not much thickened _Smittia_

{Orifice with a sinus and long spines. Peristome interrupted. Ovicell
{ with a wedge-shaped or linear longitudinal fissure. Avicularia
{ generally present, the avicularian zooecium conspicuous.
{ _Schizotheca_
{
43.{Orifice semicircular. Vibracula present, near the orifice.
{ _Mastigophora_ (Fig. 241, B)
{
{Orifice semicircular or subcircular. No vibracula; avicularia with
{ vibraculoid mandibles may occur _Schizoporella_ (Fig. 239, B)

{Zooecium with a median pore; _or_ completely tubular above 45
{
44.{Zooecium with no median pore. The orifice may be partially
{ surrounded by a collar-like development of the peristome,
{ but it is not completely tubular 49

{Orifice not tubular. A median pore _Microporella_ (Fig. 241, A)
45.{
{Orifice tubular 46

{Zooecia narrow or small 47
46.{
{Zooecia ovoid 48

{Orifice markedly tubular. Median pore conspicuous _Porina tubulosa_
{
47.{Colony very small. Zooecia irregularly arranged, with no median
{ pore _Celleporella_

{Zooecia very convex, with a granular surface; ovicells set far
{ back. Orifice wider than long _Mucronella microstoma_
{
48.{Young zooecia with stellate pores. A minute avicularium, or merely
{ a pore, on the upper and lower sides of the orifice in some
{ zooecia. _Anarthropora monodon_

{Front of zooecium with an elevated margin, enclosing an area 50
49.{
{Area not present 55

{Front wall wholly calcareous 51
50.{
{Front wall wholly or partly membranous 54

{Avicularian or vibracular zooecia replacing an ordinary {530}
{ zooecium, or at least situated between the zooecia 52
51.{
{Avicularia and vibracula absent, or if present not replacing a
{ zooecium 53

{Vibracula present. Colonies small. A pair of longitudinal slits
{ within the area _Setosella vulnerata_
52.{
{Very large avicularia present. Ovicell closed by a movable lid.
{ Orifice subcircular, with a minute lateral tooth on each side.
{ _Thalamoporella_[605] (_Steganoporella_) _smittii_

{Orifice semicircular, quite at the upper end of the zooecium; usually
{ with a knob on each side _Micropora coriacea_[606] (Fig. 256, C)
53.{
{A transverse chitinous plate lies immediately below the operculum.
{ A vibraculoid spine may occur _Megapora ringens_

{Area entirely membranous, usually bordered by spines.
{ _Membranipora_ (including _Electra_[607]) (Fig. 256, A)
54.{
{Membranous portion reduced to a small portion, which may be
{ variously lobed, enclosing the orifice.
{ _Membranipora_ (other species) (Fig. 256, B)

{Peristome present. No mucro 56
{
55.{Peristome absent; _or_ if present, with a mucro 57
{
{See also _Mucronella pavonella_, No. 28).

{Peristome collar-like, much raised below and at the sides of the
{ orifice, deficient above. No avicularia _Phylactella_
56.{
{Orifice large, longer than broad. Peristome not deficient above
{ the orifice _Lepralia_

{Wall of zooecium thin, shiny, and without pores 58
57.{
{Not agreeing with the characters given under No. 58 59

{A minute avicularium above the orifice, or where an ovicell is
{ present, situated at the summit of that structure. Zooecia not
{ quite contiguous. Mucro sometimes present.
58.{ _Chorizopora brongniartii_
{
{No avicularia. Ovicells on rudimentary zooecia, lying in a plane
{ superficial to that of the rest of the colony. Zooecia long.
{ _Schizoporella hyalina_

{A more or less distinct mucro or prominence beneath the orifice. 60
{
59.{Mucro rarely present. Orifice nearly always longer than broad, or
{ nearly circular, usually large, and slightly indented laterally.
{ _Lepralia_

{A tooth projects into the orifice from its lower side 61
60.{
{No tooth 62

{Colony glistening. Orifice much obscured by the mucro and by {531}
{ stout spines developed from the peristome. Tooth (concealed in
{ old zooecia) large, strongly curved to one side.
61.{ _Rhynchopora_ (_Rhynchozoon_[608]) _bispinosa_
{
{Tooth of the lower margin of the orifice symmetrical, sometimes
{ bifid. Avicularia may be present laterally, but are not
{ developed on the mucro _Mucronella_ (Fig. 255, A)

{Orifice at least half the width of the zooecium, bordered below by a
{ well-developed prominence or "umbo." Surface of the zooecium
62.{ strongly areolated round the margin _Umbonula_[609]
{
{Orifice considerably less than half the width of the zooecium.
{ _Schizoporella_ (Fig. 239, B)

CYCLOSTOMATA.

{Colony erect. Branches of two or one series of zooecia, divided at
{ intervals by chitinous joints. Ovicells pear-shaped.
63.{ _Crisia_ (Fig. 237)
{
{Colony erect, unjointed 69
{
{Colony in the main adherent; _or_ circular; _or_ lobed 64

{Colony more or less circular, discoidal or cup-shaped, sometimes
64.{ forming secondary colonies by marginal budding 65
{
{Colonies not circular 68

{Zooecia separated by calcified interspaces, which may contain large
{ pores, often difficult to distinguish from the orifices 66
65.{
{No large pores as above. Orifices not spiny. Zooecia nearly always
{ contiguous, except where an ovicell is developed 67

{Colony composed of one or more convex discs, bearing radial ridges,
{ each composed of many zooecia _Domopora_
{
66.{Colony encircled by a thin calcareous lamina, which gives rise to
{ new zooecia, its centre usually devoid of zooecia when adult,
{ and often bearing the orifice(s) of the ovicell. Zooecial
{ orifices often spiny. _Lichenopora_

{Zooecia with a long, tubular, free portion, in some cases curved
{ in a horizontal plane. Colony fan-shaped until a late stage.
{ _Tubulipora flabellaris_
67.{
{Tubular portion absent, or for the most part curved in a vertical
{ plane. Some of the orifices may be closed by a calcareous
{ plate. Colony circular or bluntly lobed _Diastopora_

{Zooecia in one or few series, forming a linear or branched {532}
{ colony, which is closely adherent, but may give rise to short
{ erect portions. Branches narrow, but often broadening at their
{ ends. Zooecia usually with a free upper end _Stomatopora_
{
68.{Colony broadly lobed, some of the zooecia in transverse or oblique
{ ridges composed of contiguous zooecia, arranged like a row of
{ organ-pipes _Idmonea serpens_
{
{Colony broadly lobed, or fan-shaped; zooecia in many series, which
{ are not arranged like organ-pipes _Tubulipora_

{Well branched. Orifices confined to one surface of the colony 70
69.{
{Not much branched 71

{Zooecia in transverse rows, their upper ends united in the manner
{ of a row of organ-pipes. Ovicell (when present) an inflation
70.{ of the front of the branch _Idmonea atlantica_
{
{Zooecia not in regular transverse rows. Ovicell (when present) large,
{ mostly on the back of the branch _Hornera_

{Branches cylindrical, their ends massive and raised into radial
{ ridges, which carry the orifices _Domopora stellata_
71.{
{Ends of zooecia tubular, arranged all round the branch.
{ _Entalophora_

CTENOSTOMATA.

{Colony entirely adherent, or forming thick, soft, erect lobes 73
72.{
{Colony erect, well-branched, dark and opaque, resembling seaweed.
{ Zooecia with a long tubular free portion _Anguinella palmata_

{Orifice large, with two distinct lips. A variable number of stout,
{ brown spines. Encrusting _Flustrella hispida_
{
{Orifice small, rounded, borne by a more or less distinct papilla.
73.{ Encrusting or erect. Zooecia crowded, rarely in single lines.
{ _Alcyonidium_
{
{Orifice small, rounded. Zooecia widely separated, connected by
{ narrow tubes _Arachnidium_

{Axis of colony erect, usually branched 75
74.{
{Axis creeping 79

{Zooecia in elongated clusters, which occur at intervals 76
75.{
{Zooecia not grouped; _or_ in irregular groups; _or_ in whorls 78

{Zooecia regularly biserial 77
76.{
{Zooecia long, less regularly arranged. Polypide with a gizzard.
{ _Bowerbankia_ (Fig. 238)

{Clusters of zooecia very regular, occurring immediately below a
{ bifurcation of the axis. Zooecium with a broad base, not
77.{ movable. _Amathia lendigera_
{
{Zooecia arranged like the pinnules of a leaf, with a constricted
{ base, and movable on the branch _Mimosella gracilis_

{Main stem zigzag. Branchlets delicate, many ending in sharp {533}
{ points. Zooecia small, ovoid _Vesicularia spinosa_
{
78.{Axis jointed. Zooecia small, in small clusters. Polypide without a
{ gizzard _Valkeria uva_, var. _cuscuta_
{
{Zooecia in whorls, attached to the axis by thread-like stalks,
{ much longer than themselves _Hippuraria egertoni_

{Zooecia pear-shaped, produced at the lower end into a distinct
{ stalk. Gizzard absent 80
79.{
{Zooecia not distinctly stalked, although sometimes constricted
{ at the base 81

{Stalk long. Zooecium movable on its stalk, compressed, with a
{ membranous area on one side. Twelve or more tentacles.
{ Usually found on Crustacea _Triticella_
{
80.{Stalk variable. Zooecium very transparent; orifice bilabiate. Ten
{ to sixteen tentacles _Farrella repens_
{
{Zooecium very small, much elongated and narrow. Eight tentacles.
{ _Valkeria tremula_
{(See also _Arachnidium_, No. 73).

{Zooecia short, minute, with a few short spines on each side of
81.{ its broadened base. Upper end tubular _Buskia nitens_
{
{Zooecia elongated 82

{Zooecia transparent 84
82.{
{Zooecia brown, often quite opaque 83

{Zooecia large (about 1/16 inch long), distant, constricted at the
{ base, bearing scattered bristles. Usually found on Crabs or
83.{ Hydroids. _Avenella fusca_
{
{Zooecia tall, cylindrical, not constricted at the base.
{ _Cylindroecium_

{Zooecia minute. Axis dilating at intervals into swellings, from
{ which new zooecia originate. These may give rise to new
84.{ stolons,or directly to new zooecia. No gizzard. Found in
{ brackish or fresh water _Victorella pavida_
{
{Axis not dilated, as above 85

{Zooecia small, in small groups. No gizzard _Valkeria uva_
85.{
{Zooecia long, scattered or in groups. Gizzard present.
{ _Bowerbankia_ (creeping forms)

It is highly probable that the Ctenostome genus _Hypophorella_[610] will
before long be added to the British Fauna. The animal consists of delicate
stolons, which give off small zooecia at intervals; and it is known to
excavate passages in the substance of the tubes of certain Polychaet worms
(_Chaetopterus_ and _Lanice_).

{534}ADDENDUM TO CHAETOGNATHA

Since the Chapter on the Chaetognatha was printed the following list[611]
of "The Known Chaetognaths of American Waters" has appeared:—

1. _Sagitta elegans_ Verr. This species resembles _S. bipunctata_ (_vide_
pp. 191 and 193), but differs in size, in the relative proportions of
caudal and body segments, and in the presence of diverticula from the
intestine.

2. _Sagitta flaccida_ Con. This species resembles _S. hexaptera_ (_vide_
p. 193); it is, however, smaller (length, 1.3-1.8 cm.) and has more
spines (anterior, 7-8, posterior, 10-12), and its tail segment is
relatively smaller.

3. _Sagitta tenuis_ Con. Length, 5.25 mm.; hooks, 7-8; anterior spines,
4-5; posterior spines, 7-10.

4. _Sagitta hispida_[612] Con. Length, 7-11 mm.; hooks, 8-9; anterior
spines, 4-5; posterior spines, 8-15; tail segment one-third body length;
intestine with two diverticula; sensory hairs very numerous.

5. _Sagitta hexaptera_ (_vide_ p. 193).

6. _Krohnia hamata_ (_vide_ p. 194).

7. _Spadella maxima_ Con. Length, 5.2 cm.; hooks, 6; anterior spines,
3-5; posterior spines, 5-7; epidermal thickenings round the neck.

8. _Spadella draco_ (_vide_ p. 194).

9. _Spadella schizoptera_[612] Con. An opaque, yellowish-brown species
living among algae. Length, 4 mm.; hooks, 8; anterior spines, 4-6;
posterior spines wanting. Caudal segment occupies one-half the body
length.

Professor Verrill states that the name _S. gracilis_ (_vide_ p. 191) was
due to a clerical error, the species really referred to being _S. elegans._

A. E. S.

{535}CHAPTER INDEX

Every reference is to the page: words in italics are names of genera or
species; figures in italics indicate that the reference relates to
systematic position; figures in thick type refer to an illustration; f. =
and in following page or pages; n. = note.

_Acanella_, as host, 298
_Acanthobdella_, 395
Acanthocephala, 123, 124, 174 f.;
embryology, 179;
classification, 181
_Acanthocotyle_, _73_
Acanthodrilidae, 357, 362, 381, 383, _384_
_Acanthodrilus_, 356, 363, 366, 372, 382, _384_;
chaetae, 350
_Acanthozoon_, 20
Accessory gut, of Polychaeta, 305
_Aceros_, _19_
Achaeta, 445
_Acholoe_, 297
Aciculum, 247; fossil, 302
_Acmostoma_, _50_
Acoela, 42;
occurrence and habits, 43;
reproduction, 47;
classification, 49
Acotylea, 16 f., 17, 18
_Acrorhynchus_, _49_;
occurrence, 44
Actinotrocha larva, 458
_Actinurus_, 201, _222_
_Acyclus_, _221_
Adaptation, of Trematodes, 52, 62;
of Cestodes, 74;
of Nematodes, 161
Adherent, 523
_Adineta_, 204, _222_, 227
Adventitious avicularia, 482
_Aeolosoma_, 349, 353, 354, 360, 370, 374, _375_
_Aetea_, _518_, 525
Agassiz, on Syllidae, 280
_Alaurina claparedii_, _49_
_Albertia_, 204, 210, 213, _224_, 227
_Alciope_, _315_
Alciopids (Alciopina), _314_;
head, 263;
parapodium, 265;
habit, 291;
light-organs, 294
_Alciopina parasitica_, 298
_Alcyonella_, 494, 505, _518_
Alcyonellea, _518_
_Alcyonidium_, 477, 480, 492, _518_, 532;
structure of zooecium, 469;
reproduction, 507, 508;
larva, 510, 511
Alimentary canal—see Digestive System
_Alitta_, 317
_Allantonema_, 131, _150_, 151, 161
Allman, on Polyzoa, 474, 475
Alloeocoela, 43; habits, 46;
reproduction, 47;
classification, 50
Alloiogenesis, 66 n.
_Allolobophora_, 351, 367, 369, 371, 386, 389, _390_ f.;
cocoons, 365
_Allostoma pallidum_, _50_
_Alluroides_, _379_
_Allurus_, 351, 366, 370, 389;
cocoons, 365
_Alma_, 352 f., 387
Alternation of generations, 66, 81, 281
_Amathia_, 481, _518_, 532
Ammocharidae, _258_, _325_
_Ammotrypane_, 273, _331_;
intestine, 271
_Ampharete_, _330_
Ampharetidae, _258_, _330_
Amphibia, Trematodes of, 55, 62, 71, 72;
Nematodes of, 163
_Amphichaeta_, _377_
_Amphicoerus_, _49_
_Amphicora_, 339
_Amphicorine_, gill, 261
Amphicorinidae, _258_, _339_
_Amphicteis_, _330_
Amphictenidae, _258_, _330_
_Amphiglena_, 273, _339_
_Amphileptus_, 235
_Amphilina_, _91_
_Amphinome_, eye of, 255;
_A. smaragdina_, colour, 293
Amphinomidae, _258_, _318_;
shape, 259;
caruncle, 260, 273 n.;
head, 262, 263;
parapodium, 264;
cirri, 265;
chaetae, 267, 267;
in _Lepas_, 297
_Amphiporus_, 102, 114;
British species, _110_
_Amphiptyches_, 77
Amphistomatidae, _73_ {536}
_Amphistomum_, 71, _73_;
_A. hominis_, 63
_Amphitrite_, 327, _328_;
gill, 329
_Ampullaria_, _Temnocephala_ with, 53
_Anachaeta_, 350, _376_, 395
Anal, cirri, 259;
funnel, 259, 332, 333;
vesicles, 358, 436
Anangian worms, 253
_Anarthropora_, 529
_Ancylostomum_, 143, 163
Andrews, on _Sipunculus_, 417, 426
_Angiostomum_, 134
Angler-fish, Trematodes of, 62, 72
_Anguillula aceti_, 125, _154_;
_A. tritici_, 125;
_A. diplogaster_, 155
Anguillulidae, 137, _154_
_Anguinella_, 532
_Annadrilus_, _386_
Annelida, 241
_Anocelis_, _42_
Anonymidae, _19_
_Anonymus_, 16, 18, _19_, 20;
penes, 27
Anopla, 109
_Anoplocephala_, _91_;
characters, 90:
_A. mamillana_, 90;
_A. perfoliata_, life-history, 83;
specific characters, 90:
_A. plicata_, specific characters, 90
_Anoplodiscus_, _73_
_Anoplodium_, _50_;
_A. parasiticum_, occurrence, 45
_Antaeus_, 388
_Antedon_, as host, 342
Antenna, of Rotifers, 215
_Anthobothrium_, 76 n., _91_
_Anthocotyle_, _73_
_Antinoë_, 298
_Antipathes_, as host, 298
_Anuraea_, _225_, 226
Anuraeidae, 201, 205, _225_, 226
A'oon, an edible worm, 297
Apel, on Priapuloidea, 433
Aperture, of zooecium, 468, 517, 523, 524
Aphaneura, 353, _374_
_Aphanostoma_, _49_
Aphanostomatidae, _49_
_Aphelenchus_, 131, _155_, 157
_Aphrodite_, _312_;
shape, 258;
head, 260;
peristomium, 263;
chaetae, 268;
felting, 312;
intestine, 271;
genital cells, 273;
colour, 291:
_A. aculeata_, 312;
distribution, 299;
_A. echidna_, 299
Aphroditidae, _258_, _309_;
frontal ridge, 260;
parapodium, 264;
elytra, 266, 309;
chaetae, 266
Apical plate, of Trochosphere, 245
Apodina, _235_
_Apodoides_, _225_
_Apogon_, _Scolex polymorphus_ in, 77
Apsilidae, 201, 203, 214, 220, _221_
_Apsilus_, 201, 212, 213, 214, _221_
_Arabellites_, 302
_Arachnidium_, 532
Archiannelida, _241_;
anatomy, 243 f.;
nerve cords, 255;
development, 243, 245
_Archigetes_, 5, 74, 76, _91_;
significance of, 77
Area, of zooecium, 523, 524
_Arenicola_, _333_;
perienteric sinus, 252;
nephridium, 253, 254, 269;
prostomium, 259;
body, 259;
head, 264;
gill, 265;
chaetae, 266 f.;
genital organs, 273;
otocyst, 273;
burrows, 285;
pigment, 291;
colour, 293;
_A. marina_, 333;
habits, 301;
in brackish water, 284;
as bait, 297;
eggs, 314
Arenicolidae, _258_, _333_
_Argilophilus_, 372
Arhynchidae, _185_
_Arhynchus hemignathi_, 181, _185_
_Aricia_, otocyst, 273;
eggs, 275
Ariciidae, _258_, _321_;
gill, 265
Aristotle, on Earthworms, 347
Armata, 445, 446
Arthropoda, absence of cilia in, 124
Articulata, _517_, _518_
Ascaridae, 131, _138_, 163
_Ascaris_, _139_, 163;
_A. acus_, 130;
_A. alata_, 140;
_A. depressa_, 141;
_A. ferox_, 141;
_A. incurva_, 141;
_A. leptoptera_, 141;
_A. lumbricoides_, 125, 134, 135, 139, 163;
_A. megalocephala_, 125, 127, 128, 131, 136, 140, 163;
_A. mucronata_, 141;
_A. mystax_, 125, 130, 140;
_A. nigrovenosa_, 155;
_A. rubicunda_, 141;
_A. suillae_, _139_;
_A. sulcata_, 141;
_A. transfuga_, 125, 126, 141
_Ascodictyon_, 521 n.
_Ascomorpha_, _223_
_Ascopodaria_, 488 n.
_Asellus_, Rotifers attached to, 227
Asexual reproduction, in Triclads, 40;
in Rhabdocoels, 44;
in Cestodes, 80;
in _Trichoplax_ and _Salinella_, 96;
in Polychaeta, 278 f., 279, 280, 282, 340;
in Oligochaeta, 374, 375, 377;
in Polyzoa, 496, 514
Aspidobothridae, _73_
_Aspidocotyle_, _73_
Aspidocotylea, _73_
_Aspidogaster_, 63, _73_
_Aspidosiphon_, 421, 423, 424, _425_, 428;
commensalism of, 429
_Asplanchna_, 200, 205, 210, 213, 215 n., _223_, 226
Asplanchnaceae, 203, 212, _220_, _222_
Asplanchnidae, 200, 201, 203, 205, 211, 212, 216, _223_, 226, 230
_Asplanchnopus_, 201, 211, 222, _223_, 226, 230
Ass, parasites of, 140
Association, of Rhabdocoels with Lamellibranchs and Sea-urchins, 45;
of _Monotus fuscus_ with littoral animals, 46;
significance, in Turbellaria, 51—see also Commensal and Parasitic
Asteroids, as hosts, 341 {537}
_Asterope_, _315_
_Astropecten_, as host, 297, 309
Atokous phase, 277 n.
_Atractonema_, 131, _150_, 152, 153
Atrium (genital), in _Planaria_, 38, 39;
in Oligochaeta, 361, 378
_Atrochus_, 201, 213, 214, _221_
Auditory organs, of Turbellaria, 26;
of Hoplonemertea, 106, 110;
of Nematoda, 128;
of Polychaeta, 273
_Aulastomum_, 393, 399, 403
Auricles, of Rotifers, 205
_Autolytus_, _308_;
eye, 255;
denticles, 270;
brood-sac, 275, 276;
reproduction, 278, 279;
sexual dimorphism, 281;
_A. ebiensis_, eggs, 276
_Automolos_, British species, _50_
_Avenella_, 533
Avicularian zooecium, 482, 524
Avicularium, 466, 467, 468, 482 f., 482, 516, 517, 522 f., 524;
adventitious, 482;
vicarious, 482;
vibraculoid, 484, 485;
structure, 483;
movements, 485;
function, 486
_Axine_, 56, _73_
_Axiothea_, _332_, 333

Baely, on human parasites, 139
Baird, on Oligochaeta, 382
Bait, Polychaeta as, 297
Baker, on Rotifers, 197, 207; on Polyzoa,
496 n.
_Balanoglossus_, affinities of Nemertinea with, 120
_Balatro_, 204, 212, _224_, 227
Balfour, on Trochosphere, 229
_Barentsia_, 488 n.
Barrois, on Polyzoa, 509
Basement-membrane, of _Leptoplana_, 11, 12;
of Nemertines, 102, 103, 110 f.
Bathymetrical distribution, of Polychaeta, 300
_Bdellodrilus_, _376_
Bdelloida, 201, 203 f., 211, 213, 215, 216, _222_, 227
Bdellouridae, 32, _42_
_Beania_, _518_, 525
Beddard, on _Tetrastemma aquarium dulcium_, 118;
on Oligochaeta, 347 f.;
on Leeches, 392 f.
Bedwell, on Rotifers, 198
Beneden, van, on Cestodes, 76;
on Nematodes, 162;
on _Phoronis_, 450
Benham, on a fresh-water _Tetrastemma_, 118;
on Archiannelida, 241 f.;
on Polychaeta, 245 f.;
on Myzostomaria, 341 f.;
on Oligochaeta, 357, 373, 382;
on _Phoronis_, 452 f.
_Benhamia_, _383_ f.
_Bicellaria_, 481, _518_, 526, 527
Bilfinger, on Rotifers, 212
_Bilharzia_, 4, _73_;
_B. crassa_, 70;
_B. haematobia_, 63, 68 f., 69
_Bimastos_, _389_
Bipaliidae, 35, _42_
_Bipalium_, 33, 34, _42_, 408
Birds, Trematodes of, 62, 63, 64, 72;
Cestodes of, 77 f., 84, 85;
Nematodes of, 144, 149, 163;
_Gordius_ of, 173;
Acanthocephala of, 184, 185
Bisexual, Turbellaria, 44;
Trematodes, 70 f.
Bladder, of Rotifers, 214
Bladder-worms, 5, 79 f., 89
Blanchard, on Cestoda, 91;
on Hirudinea, 392 f., 405, 408
Blastomeres, of egg of _Distomum_, 65
Blood, of Nemertinea, 108;
of _Polygordius_, 244;
of Chaetopoda, 252;
of Chlorhaemidae, 252, 334;
of _Magelona_, 252, 325;
of Sabelliformia, 252, 337
Blood-corpuscles, in Chaetopoda, 252
Body-cavity (including Coelom), of Nematoda, 130;
of _Gordius_, 166;
of Acanthocephala, 175, 178;
of Chaetognatha, 187;
of Archiannelids, 243, 244;
of Polychaeta, 249;
of _Myzostoma_, 343;
of Oligochaeta, 355;
of Leeches, 397;
of Gephyrea, 416;
of _Phoronis_, 454, 462;
of Polyzoa, 468, 488, 495
Body-wall, of Nemertinea, 102, 103;
of Nematodes, 125;
of Gordiidae, 165;
of Acanthocephala, 175;
of Chaetognatha, 187;
of Rotifers, 205;
of _Nereis_, 249;
of Oligochaeta, 349;
of Gephyrea, 414, 436;
of _Phoronis_, 454;
of Polyzoa, 470, 495, 500
Bohadsch, on Gephyrea, 411
_Bohemilla_, _377_;
chaeta, 350
Bonellein, 435, 292
_Bonellia_, 411, 434, _442_;
anatomy, 434 f., 435;
male, 438;
development, 439;
habits of, 442;
as host, 297
Bonnet, on Oligochaeta, 348, 379
Boring Worms, 286, 287
Borlase, on _Lineus marinus_, 99
_Borlasia elizabethae_, _111_, 114
Bothriocephalidae, _91_
Bothriocephalinae, _91_
_Bothriocephalus_, _91_;
_B. cordatus_, 81, 91;
_B. cristatus_, 81, 91;
_B. latus_, in man, 81, 91;
life-history, 84;
reproductive organs, 87 f.;
larva, 87;
_B. mansoni_ (= _B. liguloides_), 81, 91
_Bothriocerca_, 226
_Bothrioneuron_, _379_
_Bothrioplana_, 46, _50_
Bothrioplanidae, 42, 46, _50_
_Bothromesostoma personatum_, _49_
Bourne, on Oligochaeta, 352, 373, 377 n., 380;
on Leeches, 400
Bouvier, on commensal Gephyrea, 429
_Bowerbankia_, 470, 480, 481, 492, 500, _518_, 532, 533; {538}
larva, 511, 513;
budding, 514
Brachionidae, _225_
_Brachionus_, 200, 201, 204, 218, _225_, 226, 227
_Brachydrilus_, 357
Brackish water, Rotifers, 226;
Polychaeta, 284;
Polyzoa, 492
_Bradynema_, _150_, 151, 160
Braem, on statoblasts, 503 f.
Brain—see Nervous System
_Branchellion_, 393, 395, 397, 401, _406_
Branchial crown, 336;
regeneration of, 283
_Branchiobdella_, _376_
_Branchiomma_, _337_;
gills, 261;
eyes, 272;
_B. vigilans_ on _Aphrodite_, 299
_Branchiura_, 352, 361, 367, _378_ f.;
transverse section, 353
Braun, on Platyhelminthes, etc., 6 n., 55 n., 62, 94
_Brettia_, 525, 527
Bristles = Chaetae, _q.v._
Bristle-worms, 241
British, Polycladida, 19;
Tricladida, 42;
Rhabdocoelida, 49;
Nemertinea, 100, 110 f.;
Polychaeta, 306 f.;
Earthworms, 390;
Leeches, 393;
Gephyrea, 449;
Polyzoa, 488 n., 505, 523 f.
Brood-pouch, of _Spirorbis_, 261, 276, 341;
of _Salmacina_, 276;
of Entoprocta, 487, 507
Brood-sac, of _Autolytus_, 275;
of _Myrianida_, 280
Brown body, in Polyzoa, 468, 471 f., 472, 489, 496, 510, 514
Brown tubes (nephridia), of Sipunculoidea, 415, 417, 423, 425;
of Echiuroidea, 435, 437, 439, 441;
of Epithetosomatoidea, 445;
used as generative ducts, 418, 438;
absent in Priapuloidea, 430
Bryozoa, 475
Buccal region, in Polychaeta, 249, 250, 269;
of _Nereis diversicolor_, 248;
of _N. cultrifera_, 316
Buchanan, on marine muds, 423
_Buchholzia_, 359
Budding, in Syllidae, 279, 283 (see also Gemmation);
in Polyzoa, 467, 514 (see also Polypide-bud)
_Bugula_, 467, 468, 477, 481, 515, 517, _518_, 519, 526;
avicularia, 483, 485;
larva, 511
Bunge, on respiration in Nematoda, 130
Bürger, on Nemertinea, 109, 112;
on _Nectonema_, 168;
on Hirudinea, 397, 403
Burrows, of Polychaeta, 285, 304;
of _Cirratulus_, 286;
of _Nereis_, 286, 316, 317;
of _Arenicola_, 333;
fossil, 302;
of Earthworms, 368;
of _Sipunculus_, 426
Bursa seminalis, in Rhabdocoels, 48
Busk, on Polyzoa, 465 n., 475, 487, 519
_Buskia_, 533
Bütschli, on Nematoda, 137
_Byrsophlebs_, occurrence, 44;
British species, _49_

_Caberea_, 487, _518_, 526;
vibracula, 486, 517
Caecum, in Polyzoa, 499
_Calathus_, host of _Gordius_, 172
Calceostominae, _73_
_Calceostomum_, _73_
Caldwell, on _Phoronis_, 454, 456, 461
_Calicotyle_, _73_
_Callidina_, 201, 202, 204, 218, 219, _222_, 225, 227, 230
_Calliobothrium_, 76 n., _91_;
larva, 77
Calotte, of Dicyemids, 93
Calyx, 488
Camerano, on development of _Gordius_, 170
_Capitella_, _331_;
peristomium, 263;
special chaetae, 267, 268;
habitat, 286;
colour, 291;
_O. capitata_, distribution, 299
Capitellidae, _258_, _331_, 373
Capitelliformia, _258_, _305_;
guanin in, 253;
body, 259;
buccal region, 269;
siphon, 272;
ciliated organs, 272, 273;
genital organs, 273
_Carabus_, host of _Gordius_, 172
_Carinella_, 112;
British species, _112_
Carinellidae, side organs of, 107
Caruncle, of Amphinomidae, 260, 273 n., 318
_Caryophyllaeus_, _91_;
_C. mutabilis_, 77
_Castalia_, _308_;
distribution of, 300
Castings, of Polychaeta, 285;
fossil, 302;
of _Arenicola_, 333
_Castrada_, 44, _49_
Cat, parasites of, 80, 125, 130, 140, 143, 144, 145
_Catenicella_, _518_, 519
_Catenula_, _49_
_Cathypna_, _225_
Cathypnidae, _225_
_Cellaria_, 479, 515, _518_, 519, 526;
zooecia and avicularium, 482
_Cellepora_, _518_, 527, 528, 529;
avicularia, 483, 517
_Celleporella_, 529
_Cellularia_, _518_, 527
Cellularina, _518_
Cement-glands, of Rotifers, 205
Cephalic slits, of Nemertinea, 101, 104, 107, 111, 112
Cephalisation, in Polychaeta, 263;
in Oligochaeta, 377
_Cephalodiscus_, 461 f.
Cephalopods, parasites of, 78, 92;
list of, containing Dicyemids, 94
_Cephalosiphon_, 205, _221_
_Cephalothrix_, _112
Cercaria_, 13, 65, 67, 71 f.; {539}
_C. macrocerca_, 72;
_C. cystophora_, 72
_Cercyra_, 42
Cerebral organ, of Nemertinea, 107;
of Gephyrea, 417
_Cerebratulus_, 101, 111, 114;
British species, _111_
Cerfontaine, on Earthworms, 349, 350
Cestoda, characters of the group, 5, 74;
nature of, 76 f.;
occurrence, 77-82;
life-histories, 83;
structure and development, 84-89;
synoptic table of, 89 f.;
classification, 91
Cestodariidae (= Monozoa), _91_
_Cestoplana_, 17, 18, _19_
Cestoplanidae, _19_
Chaetae, 241;
of Polychaeta, 266, 267;
provisional, 274;
of _Nereis_, 246, 247;
of Heteronereid, 276, 277;
jointed, 246;
natatory, in sexual Syllid, 278, 307;
iridescent, 268, 291, 312;
palmate, of _Coabangia_, 339 n.;
colour, 291;
genital, of _Capitella_, 331;
of _Sternaspis_, 336;
special, of _Polydora_, 261, 267;
of _Chaetopterus_, 267, 324;
of _Myzostoma_, 342;
of Oligochaeta, 347, 350, 351, 352;
penial, 362;
of Microdrili, 375 f.;
of Megadrili, 381 f.;
of Lumbricidae, 389, 390;
of Leeches, 395, 396;
(= hooks), of Echiuroid Gephyrea, 434, 435, 438, 440 f., 446
Chaetifera, 445, 446
_Chaetobranchus_, 352
_Chaetogaster_, 356, 377, 401
Chaetognatha, 186 f., 534;
anatomy, 186;
development, 189;
habits, 189;
classification, 191;
key to, 193;
American species, 534
_Chaetonotus_, 232, _235_
Chaetopoda, 241 f.;
as food for Nemertinea, 115
Chaetopteridae, 258, _323_
_Chaetopterus_, 304 n., _323_;
anatomy, 323 f.;
special chaetae, 267;
larva, 274, 325;
pigment, 292;
phosphorescence, 295, 296;
commensals of, 298, 478, 533;
_Ch. variopedatus_, 324
_Chaetosoma_, 158
Chaetosomatidae, _158_
Chaetosyllis, form of head, 278
_Chaetozone_, _326_;
uncini, 268
_Chaetura_, _235_
Chalk, Serpulids of, 301
Charles, on male guinea-worm, 148
Cheilostomata, _477_, 506, _518_, 519, 525, 526 f.;
occurrence, 478;
external characters (see also Avicularium and Vibraculum), 481;
ovicells, 507;
reproduction, 507 f.;
larva, 511;
fossil, 521
Chiaje, Delle, on Gephyrea, 411
Chickoff, on Triclads, 41
_Chironomus_, host of _Gordius_, 172
Chitin, 249, 267;
in coelomic corpuscles, 252
_Chloeia_, colour, 291
Chlorhaemidae, _258_, 305, _334_, 336;
chlorocruorin in, 252;
head, 260, 262, 264;
palps, 260;
tentacles, 262
Chlorocruorin, 252, 334;
colour due to, 291
_Chone_, _338_
_Chorizopora_, 530
Cilia, 3;
of _Leptoplana_, 10, 11, 12, 15;
of Polyclads, 23, 25, 26;
of Müller's larva, 29;
of Land Planarians, 33;
of _Planaria lactea_, 35;
of _Temnocephala_, 53;
of Trematode-larvae, 3, 59, 60, 65;
of Cestode-larvae, 87;
absent in certain groups, 124, 396;
of Rotifers, 202 f.;
of Archiannelida, 243, 244;
of Echiuroidea, 434;
of _Phoronis_, 453;
of Polyzoa, 467, 470
Ciliated, lappets, of _Pterosyllis_, 273 n.;
pits, of _Polygordius_, 244;
pits (= nuchal organs), of Polychaeta, 272 f.;
organs, of Capitelliformia, 305
Cingulum, in Rotifers, 202
Cirratulidae, _258_, _325_;
gill, 265;
tentacular filaments, 304 n.
_Cirratulus_, _326_;
burrows, 286;
pigment, 292;
colour, 293;
viviparous, 276;
_C. tentaculatus_, 326
Cirri, of _Nereis_, 246;
of Polychaeta, 265;
of _Myzostoma_, 342;
anal, 259;
nuchal, of Eunicidae, 318;
peristomial, of _Nereis_, 248;
nerves to, 254;
of Polychaeta, 263
_Cladocora_, with _Myxicola_, 294
Claparède, on Heteronereis, 276, 277;
on Earthworms, 347, 355, 356
Claus, on Nematoda, 138;
on Seisonaceae, 225 n.
_Clepsine_—see _Glossiphonia_
_Clitellio_, 366, 378
_Cloeosiphon_, 424, _425_, 429
Clover sickness, 155
_Clymene_, _333_;
_C. ebiensis_, tube of, 287
Clymenidae—see Maldanidae
_Coabangia_, 284, _339_ n.
Cobb, on Nematoda, 131 n., 138
Cobbold, on Nematoda, 140
_Cobitis_, host of _Gordius_, 173
_Cochleare_, _225_
Cocoons, of Triclads, 40;
of Oligochaeta, 364, 365;
of Leeches, 404
Coelom—see Body-cavity
Coelomic fluid, of Polychaeta, 252;
as cause of colour, 291
_Coelopus_, _225_
Cohn, on Rotifers, 198
Collar, peristomial, of Sabellidae, 336;
of Gephyrea, 421;
of Ctenostomata, 470, 477, 480, 481
Colonial nervous system, 471
Colony, of _Myrianida_, 281; {540}
of _Syllis ramosa_, 282;
of Polyzoa, 466
Colour of Polyclads, 20;
of Land Planarians, 33;
of Nemertinea, 102;
of Polychaeta, 291, 314, 340
Coluridae, 207, _225_
_Colurus_, _225_, 226
_Comatula_, as host, 342
Commensal, Polychaeta, 297 f., 323, 325;
Gephyrea, 428, 429;
Polyzoa, 489
Conn, on development of Gephyrea, 419 n., 441, 444, 447
_Conoceros_, _19_
_Conochilus_, 202, 203, 205, 215, _221_, 226
_Conocyema_, hosts of, 94
_Convoluta_, 45;
British species, 43, _49_;
_C. henseni_, pelagic habit, 43;
_C. roscoffensis_, assimilating tissue, 43
Copepoda, on Polychaeta, 299
_Copeus_, 215, _224_
Coral reefs, Polychaeta in, 293
_Corallina_ (= Coralline Alga), 14, 488, 516
Coralline, 465
Coralline Crag, 465, 521
_Corallobothrium_, _91_
_Corethra_, host of _Gordius_, 172
Cori, on _Phoronis_, 451 f.
_Cornulites_, 302
Cotylea, 16 f., 17, 18
_Cotylogaster_, _73_
_Cotyloplana_, 35
_Crateromorpha_, as host, 282
Crayfish, _Temnocephala_ associated with, 53
Creeper, 317
_Crepina_, 450
Cretaceous, Polyzoa, 520, 521
_Cribrilina_, _518_, 524, 528
Crinoids, as hosts, 341
_Criodrilus_, 358, 366, _386_
_Crisia_, 471, 478, 479, 480, 507, _518_, 531
Crisiidae, _517_
Crisp, on Parasites, 164
_Cristatella_, 494 f., 495, 499, 501, 503-505, 512, _518_;
attacked by Planarians, 486;
movements, 494, 496, 498;
fission, 496, 506;
statoblast, 502, 503;
larva, 512
_Crossopodia_, 302
Crotchets, 266, 267, 305, 322
Crustacea, parasites of, 174, 179, 182
_Cryptocelis_, _19_, 24
Cryptocephala, _258_, _303_, _305_;
vascular system, 252;
prostomium, 259;
tentacles, 263;
eyes, 272;
food, 296
Cryptodrilidae, 357, 362, 373, _382_
_Cryptodrilus_, 372, _382_ f.
_Ctenodrilus_, 373
Ctenophores, as hosts, 298
Ctenostomata, 470, _477_, 479, 480, _518_, 532;
occurrence, 478;
in fresh water, 492;
external characters, 480;
reproduction, 507;
larva, 511;
relation to Phylactolaemata, 493, 502 f.;
fossil, 521 n.
_Cucullanus_, 136, _142_, 163;
_C. elegans_, 143, 161
_Cucumaria_, as host, 298
Cuénot, on Gephyrea, 416 n.
Cuticle, of Nemathelminthes, 125, 165, 175;
of Rotifera, etc., 205, 233, 236;
of Polyzoa, 470—see also Epidermis
Cuvier, on Oligochaeta, 352;
on Gephyrea, 411
_Cyclatella_, _489_
_Cyclicobdella_, 392
_Cycloporus_, _19_, 22, 24
_Cyclops_, parasites of, 143, 148, 161
Cyclorhagae, 238
Cyclostomata, _477_, 479, 506, _517_, _518_, 525, 531;
occurrence, 478;
external characters, 480;
ovicells, 507;
reproduction, 507, 511;
larva, 511; fossil, 520, 521
_Cydippe_, as host, 298
_Cylindroecium_, 533
_Cylindrostoma_, 46;
British species, _50_
Cyphonautes, 509, 510, 512, 520
_Cyprina_, _Malacobdella_ found on, 119
_Cyrtonia_, _224_
Cyst, of Land-Planarians, 33;
of _Myzostoma_, 342, 343, 344;
(capsules), of _Aeolosoma_, 370, 375
_Cystibranchus_, 395, _406_
Cysticercoid-larva, 83, 85, 88
_Cysticercus_-larva, 79, 80;
list of, 83;
_C. cellulosae_, 79, 80;
_C. pisiformis_, development, 81, 85, 89
Cysticolous, Myzostomaria, 344
Cystoidotaeninae, _91_
Cystotaeninae, _91_

_Dactylogyrus_, _73_
Dalyell, on habits of Turbellaria, 6, 10, 20;
on regeneration in Polychaeta, 283;
on tubes of Polychaeta, 287;
on Hirudinea, 405 n.;
on larvae of _Flustra_, 466;
on _Cristatella_, 496
Danielssen and Koren, on Gephyrea, 442, 444
_Daphnia_, Rotifers attached to, 227
_Dapidia_, _225_
Darwin, on Earthworms, 354, 359, 368
_Dasybranchus_, _331_;
gill, 268
_Dasychone_, _338_;
gills, 261;
eyes, 272;
regeneration, 283
_Dasydetes_, 232, _235_
Davaine, on Nematoda, 140, 145
_Davainea_, _91_;
_D. friedbergeri_, 84;
_D. madagascariensis_, 80, 84;
_D. proglottina_, life-history, 84
Davenport, on _Urnatella_, 491
Davis, on Rotifers, 227
Deep-sea, Polychaeta, 300;
Polyzoa, 478
_Deinodrilus_, 351, _384_
_Delagia_, 478 n.
_Dendrobaena_, _382_ {541}
_Dendrocoelum_, 30, 35, 39
_Dendrostoma_, 422, _425_, 428
Dendy, on Land Planarians, 33, 34, 38
Denticles, 248, 250, 316, 522
_Dero_, 352, _377_
_Derostoma_, 44, _50_
_Desmogaster_, _380_
Desmoscolecidae, 159
_Desmoscolex_, 159, 258
Desor, on Nemertine development, 99;
Type of, larva, 113
Development, of Polyclads, 28;
of Nemertinea, 99, 113;
of Nematoda, 135;
of _Gordius_, 171;
of Acanthocephala, 179;
of Chaetognatha, 189;
of Rotifers, 218;
of Archiannelida, 243, 245;
of Polychaeta, 274 f.;
of Oligochaeta, 365;
of Leeches, 399;
of Gephyrea, 419, 432, 439, 447;
of _Phoronis_, 458;
of Polyzoa, 506, 509—see also Life-history and Larva
_Diachaeta_, 366
Diaphragm, of _Nereis_ introvert, 250, 251;
of Terebellidae, 304, 327;
of Polyzoa, 469, 470, 500, 508
_Diaschiza_, _225_, 226
_Diastopora_, _518_, 531
_Dichogaster_, 362, _383_
_Diclidophora_, _73_
_Dicotylus_, 32, 35, 36, _42_
_Dicranotaenia_, _91_;
_D. coronula_, life-history, 84
_Dicyema_, 93;
vermiform larva of, 92;
hosts of, 94
_Dicyemennea_, 93, 93;
hosts of, 94
Dicyemidae, 92 f.
_Didymogaster_, _383_
_Didymozoon_, _73_;
_D. thynni_ (= _Monostomum bipartitum_), 71
Didymozoontidae, _73_
_Digaster_, 358, 359, _382_ f.
Digenea (Digenetic Trematodes), 5, 52, 62, _73_;
occurrence and habits, 62;
life-histories, 63 f., 71 f.
Digestive system, of _Leptoplana_, 11, 12;
of Polyclads, 24;
of Triclads, 37, 39;
of Rhabdocoelida, 42 f.;
of _Temnocephala_, 53, 54;
of _Polystomum_, 57;
of _Distomum_, 62;
absence of, in Cestodes, 74;
of Nemertinea, 103, 103, 104;
of Nematoda, 130;
of Gordiidae, 166, 169;
of Chaetognatha, 187;
of Rotifers, etc., 209, 233, 237;
of Archiannelida, 243;
of Polychaeta, 249, 269;
of Oligochaeta, 358;
of Hirudinea, 396;
of Gephyrea, 414;
of _Phoronis_, 454;
of Polyzoa, 468, 469, 487
_Digitibranchus_, 353
_Diglena_, 212, 217, _224_, 226
Digonopora, 16
Dimorphism, in _Polystomum_, 59;
sexual, of Trematodes, 70;
of Orthonectida, 95;
of _Dinophilus_, 243;
of Polychaeta, 276, 279 f.;
of Gephyrea, 438
Dinocharididae, _225_
_Dinocharis_, _225_
_Dinophilus_, 242;
_D. taeniatus_, 242;
_D. gyrociliatus_, sexual dimorphism, 243
_Dinops_, _223_
_Diopatra_, gill, 318
Diphyllinae, _91_
_Diplax_, _225_
_Diplectanum_, _73_
_Diplobothrium_, _73_
_Diplocardia_, _385_
_Diplodiscus_, _73_;
_D._ (_Amphistomum_) _subclavatum_, life-history, 71
_Diplogaster_, _154_
_Diplois_, _225_
_Diplostomum_, _73_
_Diplozoon_, 55, 60, _73_;
life-history, 60, 61;
reproductive organs, 60
_Diporochaeta_, _381_
_Diporpa_, 60, 61
_Diporula_, 528
_Dipylidium_, 81, _91_;
life-history, 83, 88, 89;
specific characters, 90
Disc, in Rotifers, 200, 202 f., 202
Discharge, of genital cells;
Nemertinea, 116;
Archiannelida, 244;
Polychaeta, 256, 274, 275
_Discobdella_, 226
_Discocelis_, _19_, 23
Discodrilidae, 350, _376_, 392, 395
_Discopus_, 201, _222_, 226, 227
_Dispharagus_, _147_, 149, 163
_Dispinthera_, _225_
_Distemma_, 212, _224_, 226
Distomatidae, _73_
_Distomum advena_ (= _D. migrans_), life-history, 71;
_D. appendiculatum_, life-history, 71;
_D. ascidia_, life-history, 71;
_D. atriventre_, life-history, 71;
_D. brachysomum_, life-history, 71;
_D. buskii_, 63;
_D. caudatum_, life-history, 71;
_D. clavigerum_, life-history, 72;
_D. conjunctum_, 63;
_D. crassum_, 63;
_D. cygnoides_, life-history, 72;
_D. cylindraceum_, 72;
_D. dimorphum_, 72;
_D. echinatum_, 63;
life-history, 72;
_D. echiuri_, 444;
_D. endolobum_, life-history, 72;
_D. excavatum_, 63;
_D. ferox_, 63;
_D. globiporum_, life-history, 72;
_D. hians_, 63;
_D. hepaticum_ (liver-fluke), 3, 63, 67 f.,72;
_D. heterophyes_, 63;
_D. hystrix_, life-history, 72;
_D. japonicum_ (= _D. spathulatum_), 63;
_D. lanceolatum_, 63;
_D. luteum_, excretory system of, 62;
_D. macrostomum_, life-history, 64, 65, 72;
_D. magnum_, 68;
_D. militare_, life-history, 72;
_D. nodulosum_, life-history, 72;
_D. oculi-humani_ (= _D. ophthalmobium_), 63;
_D. ovocaudatum_, life-history, 72;
_D. pulmonale_, 63, 70;
_D. rathouisi_, 63, 70;
_D. retusum_, life-history, 72;
_D. ringeri_ (= _D. pulmonale_), 63; {542}
_D. signatum_, life-history, 72;
_D. sinense_, 70; _D. spathulatum_, 63;
_D. squamula_, life-history, 72;
_D. trigonocephalum_, life-history, 72;
_D. westermanni_ (= _D. pulmonale_), 63
_Distyla_, _225_
_Dithyridium_, _91_
_Ditrupa_, 301
_Diurella_, _225_, 226
_Dochmius_, 133, 135, _142_, 160, 163;
_D. cernua_, 143;
_D. duodenalis_, 143, 163;
_D. stenocephala_, 143;
_D. trigonocephala_, 143
_Dodecaceria_, 287, _326_, 327
Dog, parasites of, 80 f., 90, 125, 140, 142, 143, 145
_Dolichoplana_, 37, _42_
Domestic animals, Trematodes of, 67, 68, 70, 72;
Cestodes of, 81, 89;
Nematodes of, 139 f., 163;
Acanthocephala of, 184
_Domopora_, 531, 532
Dorsal ciliated organ, 247, 254, 256
Dorsal pores, 348
_Dorylaimus_, 131, _157_, 160
_Dracunculus_, 131, 135, 147
_Drepanidotaenia_, _91_;
_D. anatina_, life-history, 84, 85;
_D. gracilis_, _D. infundibuliformis_, _D. setigera_, life-history, 84
_Drepanophorus_, excretory system, 108;
British species, _110_
Drilophagidae, _224_
_Drilophagus_, 204, 210, 212, _224_, 227
Dugès, on Planarians, 6, 10;
on Oligochaeta, 368
Dujardin, on Rotifers, 198;
on Gastrotricha, 231;
on Kinorhyncha, 236
Duplicature, 499, 500
_Duthiersia_, _91_
Dwarf males, of _Myzostoma_, 344;
of Gephyrea, 438

Ear-cockle, 155
Earthworms, 347, 365;
senses, 354;
food, 359;
effect on the soil, 368;
distribution, 369 f.;
classification, 380 f.;
British, 390—see also Oligochaeta
Ebrard, on Hirudinea, 393
_Echeneibothrium_, _91_
_Echinella_, _73_
_Echinobothrium_, 74, 75, 85, _91_
_Echinococcus_, 80, 83
_Echinocotyle_, _91_
_Echinoderes_, 236 f., 236
Echinorhynchidae, _182_
_Echinorhynchus acus_, 175, 179;
_E. angustatus_, 174, _183_;
_E. clavula_, _183_;
_E. haeruca_, 176;
_E. linstowi_, _183_;
_E. lutzii_, _183_;
_E. moniliformis_, _183_;
_E. proteus_, 174, 180, 181, 182, 182
_Echinosiphon_, 424, 429
Echiuroidea, 241 n., _412_, 434, 446;
anatomy, 434 f.;
classification, _440_;
development, 439;
parasites, 444;
habits, 442
_Echiurus_, 336, 411, 435, 440, _441_;
development, 439
Eckstein, on Rotifers, 198
_Eclipidrilus_, _380_
Economic uses, of Polychaeta, 296, 297
Ectocyst, 469, 470, 523;
of Phylactolaemata, 496 f., 503
Ectoderm, of Mesozoa, 93, 95
Ectoparasitic Trematodes, 4, 52, 53
Ectoprocta, _475_, _518_;
structure, 469;
lophophore, 476;
reproduction, 506 f.;
larva, 509, 510, 511;
compared with Entoprocta, 488
Effects, of parasites on their hosts, 56, 68, 69, 80, 94, 162
Eggs, of _Leptoplana_, 10, 16;
of Polyclads, 28;
of Triclads, 33, 40;
of Rhabdocoels, 47, 48;
of Termatodes, 52;
of ectoparasitic Trematodes, 54, 58, 60, 61;
of endoparasitic Trematodes, 63, 69;
of Cestodes, 87;
of Orthonectidae, 95;
of Nemertinea, 116;
of Nematoda, 135, 162;
of _Gordius_, 171;
of Acanthocephala, 179;
of Chaetognatha, 189;
of Rotifera, 200, 216 f.;
of Gastrotricha, 234;
of _Nereis_, 256;
of Polychaeta, 274, 275;
of Phyllodocids, 314;
of _Scoloplos_, 321;
of _Myzostoma_, 343;
of Polyzoa, 506, 507
Ehrenberg, on Turbellaria, 3, 6;
on Rotifers, 198, 220 n., 228;
on Gastrotricha, 231;
on Polyzoa (Bryozoa), 475
Eichhorn, on Rotifers, 197
Eisen, on Oligochaeta, 380, 390
Eisig, on Capitellidae, 373;
on _Bonellia_, 443
_Electra_, 481, _518_, 523, 530;
larva, 509, 510;
variation, 516
Elytra, of Polynoids, 265, 292, 294, 298, 299, 309, 310, 311;
phosphorescent, 295, 296;
as brood-pouch, 275;
of Aphroditidae, 266;
arrangement of, 309;
of Sigalionina, _313_
_Enantia_, 16 n., _19_
Enantiidae, _19_
_Enchelidium_, _157_
Enchytraeidae, 359, 360, 361, 366, 367, 370, _375_
_Enchytraeus_, host of _Gordius_, 173
_Encotylabe_, _73_;
eggs of _E. pagelli_, 58
Endocyst, 471
Endoderm, of Mesozoa, 93, 95
Endoparasitic Trematodes, 4, 52, 62
Enopla, 109
Enoplidae, _157_
_Enoplus_, 157, 160
_Entalophora_, _518_, 532
_Enteroploea_, _224
Enterostoma_, 46; {543}
British species, _50_
Entoprocta, _475_, 479, 487 f., _518_;
lophophore, 476;
reproduction, 506;
larva, 509, 510
_Eosphora_, 216, 218, _224_
_Ephemera_, host of _Gordius_, 172, 173
_Ephesia_, _321_
_Epibdella_, 55, _73_
Epidermis, of _Leptoplana_, 11, 12;
of Polyclads, 20, 25, 29;
of Trematodes, 56;
of Cestodes, 85;
of Nemertinea, 102;
of Nematoda, 125;
of Acanthocephala, 175;
of _Nereis_, 249;
of Oligochaeta, 349;
of Leeches, 396;
of Gephyrea, 414;
of _Phoronis_, 454—see also Hypodermis
Epigamous, phase, of _Nereis_, 277 n.;
worms, 281
Epistome, in _Phoronis_, 453, 455;
in Phylactolaemata, 476, 476, 499;
in larva of _Loxosoma_, 509
_Epistomia_, _518_, 526
_Epithetosoma_, 444, 445, 449
Epithetosomatoidea, _412_, 444
Epitokous, phase, of _Nereis_, 277 n.
_Eretmia_, _225_, 226
Eriographidae, _258_, _338_
_Erpocotyle_, _73_
Errantia, 258, 285
_Eschara_, 516
Escharina, _518_
_Escharoides_, 527
Euchlanididae, _225_
_Euchlanis_, _225_, 226
_Eucratea_, _518_, 525, 527, 528
Eudrilidae, 359, 360, 380, _385_, 403 f.
_Eudrilus_, 354, _385_, 403 f.
Euichthydina, _235_
_Eulalia viridis_, _314_;
pigment, 292;
colour, 293;
eggs, 314 n.
_Eumenia_, _334_
_Eunice_, 318, _319_;
nephridium, 254;
eye, 255;
head, 262;
parapodium, 264;
gill, 265;
chaetae, 266, 267;
jaws, 270;
substance of tube, 290;
commensal, 298;
_E. tibiana_, 290
Eunicidae, _258_, _318_;
palps, 260;
tentacles, 262;
jaws, 270;
tube, 285, 290;
colour, 291, 292;
parasitic, 297;
tubes containing Polynoids, 298;
Palaeozoic, 302
_Eunicites_, 301, 302
_Euphrosyne_, _318_;
parapodium, 265;
chaetae, 267
_Eupolia_, _113_
Eupolyzoa, 461
_Eurycercus_, shell of, inhabited by Rotifers, 227 n.
_Eurylepta_, _19_
Euryleptidae, _19_
_Eurythoe_, _318_
_Eustrongylus gigas_, _142_;
_E. tubifex_, _142_
_Eusyllis_, reproduction of, 278
Excretion, as cause of colour, 291
Excretory system, of _Leptoplana_, 13;
of Polyclads, 25;
of Triclads, 41;
absent in Acoela, 42;
of Temnocephala, 54;
of Digenea, 62;
of Cestodes, 86;
of Nemertinea, 108, 108, 120;
of Nematodes, 133;
of Acanthocephala, 177;
of Rotifers, 199, 213;
of Gastrotricha, 234;
of Kinorhyncha, 237;
of Polychaeta, 253;
of Polyzoa, 472 f.—see also Nephridium
_Exogone_, _308_;
attachment of eggs, 275;
reproduction, 278
Eyes, of _Leptoplana_, 8, 15;
of Polyclads, 26, 27;
development of, 30;
of larval Polystomatidae, 59, 60;
of Nemertinea, 102, 106;
of Nematodes, 128;
of _Gordius_, 166;
of Chaetognatha, 188;
of Rotifera, 215;
of Gastrotricha, 234;
of Kinorhyncha, 238;
of Polychaeta, 255, 272, 314, 337, 339;
of Oligochaeta, 354;
of Leeches, 393, 394, 395;
of Gephyrea, 417;
of Polyzoon larva, 509

_Fabricia_, _339_;
eyes, 272;
otocyst, 273
Faecal groove, of Sabellids, 337
Fans, of _Chaetopterus_, 295, 324
Faraday, on asexual reproduction of Planariae, 6, 40 n.
Farre, on Polyzoa, 500, 508
_Farrella_, 500, _518_, 533
_Fasciola_, 67
_Fecampia_, life-history, 45
Felt, of _Aphrodite_, 268, 312
Fertilisation, of Nemertinea, 117;
of Nematodes, 135;
of Gordiidae, 171;
of Acanthocephala, 179;
of Chaetognatha, 188;
of Rotifers, 217
_Filaria_, _147_, 163;
_F. attenuata_, 134;
_F. cystica_, 142;
_F. denticulata_, 125;
_F. immitis_, 148;
_F. labiata_, 163;
_F. laticaudata_, 125;
_F. loa_, 149;
_F. medinensis_, 147, 157, 163;
_F. sanguinis hominis_, 149, 163;
_F. papillosa_, 163
Filariidae, _147_
_Filigrana_, _340_;
fission in, 281;
tubes, 290
_Fimbria tenuis_, 157
Fischer, on branchiae in Gephyrea, 416 n.
Fishes, Trematodes of, 4, 53, 55, 62, 64, 71, 72;
Cestodes of, 77, 84, 85;
Nemathelminthes of, 142, 143, 149, 163, 173, 182
Fission, in _Bipalium_, 34;
in Planariae, 40;
in _Trichoplax_, 96;
in _Salinella_, 96;
in Polychaeta, 278 f., 279, 280, 282, 340;
in Oligochaeta, 374, 375, 377;
in Phylactolaemata, 496;
in embryos, 365, 511
_Fissurella_, as host, 298
_Flabelligera_, 334
Flame-cells, in _Leptoplana_, 13;
in _Thysanozoon_, 25;
in Triclads, 41;
in _Temnocephala_, 54;
in Polystomatidae and Tristomatidae, 56;
in _Distomum luteum_, 62;
in Cestodes, 86; {544}
in Nemertinea, 108, 109;
in Rotifers, 213;
in _Urnatella_, 491
_Fletcherodrilus_, _382_
_Floscularia_, 200, 203, 205, 220, _221_, 226
Flosculariaceae, 202, 211, 213, _220_
Flosculariidae, 201, 203, 205, _220_ n., _221_, 230
Flukes, 51
_Flustra_, 465, 466, 467, 472, 473, 477, 515, _518_, 526;
avicularia, 482
_Flustrella_, 467, 477, _518_, 532;
larva, 513
Flustrina, _518_
Food, of Turbellaria, 4;
of _Leptoplana_, 10;
of Polyclads, 24;
of Triclads, 37;
of Acoela, 43 f.;
of Rhabdocoela, 45;
of Trematodes, 4, 52, 62;
of _Temnocephala_, 53;
of Cestoda, 5;
of Nemertinea, 115;
of Nematodes, 131;
of Acanthocephala, 177;
of Chaetognatha, 190;
of Rotifera, 207, 212;
of Gastrotricha, 234;
of Polychaeta, 296;
of Earthworms, 359;
of Leeches, 393, 406 f.;
of Gephyrea, 422, 443;
of Polyzoa, 467
Foot, in Rotifers, 200, 201
Foraminifera, as food of Polychaeta, 296
Forceps, of _Eunice_, 270
Forcipate, 210, 211
Formula, for Nematoda, 138
Fornix, 481, 517, 525
Fossil, Polychaeta, 301;
Polyzoa, 520;
Nemertinea, absence of, 119
_Fredericella_, 476, 494, 502-505, _518_, 519;
lophophore, 495;
statoblast, 502, 503
Frenzel, on Trematoda, 62;
on _Salinella_, 96
Fresh-water, Turbellaria, compared with marine, 46;
Nemertinea, 101, 118;
Polychaeta, 284;
Polyzoa, 492
Friend, on Oligochaeta, 388 n.
Frogs, Trematodes of, 55, 58, 62, 71 f.;
Nematodes of, 140, 142, 160, 173
Frontal organ, of Nemertinea, 107
Frontal palps, of Eunicidae, 318 f.
Frontal ridge, 260
_Frullania_, inhabited by Rotifers, 227
Fulcrum, in Rotifers, 210
Funicular tissue, 471
Funiculus, 469, 471, 472, 499, 501
_Furcularia_, 216, _224_, 226

Gamble, on Platyhelminthes, 3 f.;
on Mesozoa, 92 f.
_Gammarus_, Rotifers attached to, 227
Gapes, cause of, 144
Gardiner, on development of Acoela, 44 n.
Gasterostomatidae, _73_
_Gasterostomum_, _73_;
_G. fimbriatum_, life-history, 72;
_G. gracilescens_, 72
_Gastrocotyle_, _73_
_Gastrodiscus_, _73_
_Gastrothylax_, _73_
Gastrotricha, 231 f.
Gegenbaur, on Nemertine development, 99
_Gemellaria_, _518_, 526
Gemmation, 281;
in Syllidae, 278 f., 279, 280
_Geobia_, 37
_Geodesmus_, 34, _42_
Geographical distribution, of Turbellaria, 32 f.;
of Nemertinea, 117;
of Chaetognatha, 191, 534;
of Gastrotricha, 235;
of Polychaeta, 299 f.;
of Oligochaeta, 369 f.;
of Leeches, 405;
of Gephyrea, 426 f., 432, 441 f.;
of _Phoronis_, 460;
of Polyzoa, 492 f., 504, 519
_Geonemertes_, 101;
description of _G. chalicophora_, 117
_Geoplana_, 33, 38, _42_
Geoplanidae, 35, _42_
Geoscolicidae, 351, 362, _386_
Gephyrea, 411 f.;
history, 411;
external characters, 412, 420 f., 430 f., 434 f., 444;
body-wall, 414, 436;
digestive system, 414, 422, 430, 436, 445;
vascular system, 415, 421, 436;
respiratory system, 416;
body-cavity, 416, 437, 445;
nervous system, 416, 431, 437, 445;
excretory system, 417, 423, 431, 437, 445;
reproductive system, 418, 431, 437, 445;
development, 419, 439;
food, 422, 443;
commensalism, 429;
affinities, _241_ n., 336, 445 f., 512;
British, 449
Germarium, of Rotifers, 216
Germ-yolk-gland, 47
Giard, on Oligochaeta, 368
Gid, induced by _Coenurus_, 82
Gigantorhynchidae, _183_
_Gigantorhynchus gigas_, 174, 177, _184_;
_G. echinodiscus_, _G. spira_, _G. taenioides_, _184_
Gills, of Polychaeta, 252, 265, 268 f.;
of _Arenicola_, 333;
of Chlorhaemidae, 334;
of _Cirratulus_, 326;
of _Euphrosyne_, 265, 318;
of Eunicidae, 318;
of _Nereis_, 246;
of Sabelliformia, 260, 261;
development of, 275;
of _Sabella_, 286;
colour of, 294;
of _Sabellaria_, 263;
of Serpulidae, 261;
of _Sternaspis_, 336;
of Sigalionina, 313;
of Terebellidae, 329;
of Oligochaeta, 352, 353;
of Leeches, 395;
of Gephyrea, 416
Gizzard, in Rotifers, 199, 210;
in Polychaeta, 271, 307;
in Oligochaeta, 358;
in Polyzoa, 477, 480, 532 f.
Glands, on parapodia, 249;
of Phyllodocids, 314;
oesophageal, 271, 272, 358;
tube-forming, 287, 304, 327
Gland shields, 287, 304, 306;
of Sabellids, 337;
of Terebellids, 327
Glandular papillae, of _Polygordius_, 244
_Glossiphonia_ (= _Clepsine_), 393, 396, 399, 404, _407_; {545}
anatomy, 397, 398, 401;
spermatophore, 402
Glossiphoniidae, _406_
_Glycera_, cirrus, 265;
habitat, 286;
jaws, 270;
nephridium, 254;
parapodium, 264;
prostomium, 259;
tentacles, 262;
_G. capitata_, distribution, 299;
_G. meckelii_, 320
Glyceridae, _258_, _320_;
gills, 268;
coelomic corpuscles, 253
_Glyphidrilus_, _386_
Gnathobdellae, 396 f., _407_
Gnathobdellidae, _407_
_Gnathosyllis_, colour of, 293
_Gobio_, parasites of, 182, 183
_Golfingia_, _426_, 428, 430
_Goniada_, _320_
Goodrich, on Oligochaeta, 378
Gordiidae, _123_, 164 f.
_Gordiodrilus_, _383_ f.
_Gordius_, 164 f., 164;
_G. tolosanus_, 165 f., 170
Gosse, on Rotifers, 198, 209 n., 220
_Gossea_, 232, _235_
Goujon, on _Trichina_, 146
Gourret, on Chaetognatha, 187
Graff, von, on Rhabdocoelida, 44;
on _Geonemertes_, 101, 117
_Graffilla_, _50_;
occurrence, 45
Grassi, on Cestoda, 80 n., 89;
on Chaetognatha, 190, 193
Grater, of Eunicidae, 270
Greef, on Echiuroidea, 441, 444
Gregarines, in Polychaeta, 299;
in Gephyrea, 444
Gregory, on Polyzoa, 519, 520
Grinder, of Eunicidae, 270
Grube, on Oligochaeta, 347, 352;
on Hirudinea, 395
_Grubea_, _308_;
attachment of eggs, 275
Guanin, in coelomic corpuscles, 253
Guinea-worm, 147
_Gunda_, 31, 41, _42_
Gundidae, 42
Gymnolaemata, _476_, 512, _518_;
lophophore, 476;
in fresh water, 492
_Gyrator hermaphroditus_, _49_
_Gyrocotyle_ (= _Amphiptyches_), _91_;
_G. rugosa_, 77;
_G. urna_, 77
Gyrodactylidae, _53_, 61, _73_
Gyrodactylinae, _73_
_Gyrodactylus_, 55, 61, _73_

Haberlandt, on _Convoluta_, 43
Habits, of Platyhelminthes, 3, 7, 21, 35, 43;
of Nemertinea, 114;
of Gordiidae, 170 f.;
of Chaetognatha, 189;
of Rotifers, 206, 226;
of Gastrotricha, 232, 234, 235;
of Polychaeta, 285;
of Oligochaeta, 366;
of Gephyrea, 426, 433, 442;
of _Phoronis_, 451
_Haemadipsa_, _408_;
eye, 394
Haemal fluid, 252
_Haematocleptes_, parasitic, 297
_Haementeria_, _407_
Haemerythrin, 416;
in _Magelona_, 252
Haemoglobin, in Nemertinea, 107, 108;
in Chaetopoda, 252, 253, 291, 356;
in Gephyrea, 437;
in _Phoronis_, 456
_Haemopis_, 393, _407_
_Halicryptus_, _432_, 433;
anatomy, 430
Hallez, on Turbellaria, 7, 21, 40
_Halosydna_, 298
Hamann, on Nematoda, 131, 133, 136 n.;
on Acanthocephala, 176, 180
_Hamingia_, 435 f., _442_;
males of, 438
_Haplobranchus_, _339_;
gills, 261
_Haplodiscus_, 43, _49_
Haplodrili, 241
Hares, parasites of, 141, 145
Harker, on Oligochaeta, 369
Harmer, on Polyzoa, 465 f.
Harris, on Rotifera, 197
Hartog, on Rotifera, 197 f.;
on Gastrotricha, 231 f.;
on Kinorhyncha, 236 f.
Haswell, on Temnocephalidae, 53;
on _Phoronis_, 451
Hatschek, on affinities of Polyclads, 28 n.;
on nature of Cestodes, 76;
on Trochophore, 229;
on development of Gephyrea, 419, 447
Head, of _Nereis_, 246, 248;
of Polychaeta, 259 f.;
regeneration of, 283;
of _Tomopteris_, 315;
of _Aphrodite_, 260;
of Chlorhaemidae, 334;
of _Eunice_, 262;
of _Nephthys_, 262;
of _Phyllodoce_, 262;
of _Polydora_, 261;
of Polynoid, 262;
of _Sabella_, 261;
of _Sabellaria_, 263;
of Syllid, 262;
of _Trophonia_, 262
_Hedruris_, 163
_Hekaterobranchus_, _326_
_Heliodrilus_, 359, 380
_Heliopora_, containing _Polydora_, 298
Heller, on human parasites, 139
Hemichordata, 462
_Hemilepidia_, colour of, 292
_Hemistomum_, _73_
_Hemitubifex_, _378_
_Henlea_, 369
_Hermadion_, 299
Hermaphrodite, Nemertinea, 109;
Polychaeta, 273 f.
Hermellidae, _258_, _341_
Hermelliformia, _258_, _306_, 341
_Hermione_, _312_;
chaetae, 266, 267
Hermionina, 309, _311_
Herpobdellidae, _407_
Hertwig, O., on Chaetognatha, 187 n.
Hesionidae, _258_, _308_;
segments, 258;
head, 263;
swim-bladder, 272;
genital organs, 274;
parasitic, 297
_Hesperodrilus_, 352 f., 369, _378_ f.
Hesse, on nervous system of Nematoda, 127
_Heterakis_, 131, _139_ {546}
Heterocotylea, _73_
_Heterodera_, _155_, 160, 164
Heterogamy, 66 n.
Heteronemertini, _113_
Heteronereid phase, 276 f.;
chaetae, 246, 276;
parapodia, 276, 277
Heteronereis, 276, 281—see Heteronereid
_Heteropora_, 520 n.
Heterosyllis, 278, 279, 281
_Hexacotyle_, _73_
_Hexarthra_, _223_, 226
Hibernaculum, 501, 502
Hincks, on Polyzoa, 475, 500, 508, 516, 519, 523
_Hippothoa_, 528
_Hippuraria_, 533
Hirudinea, _241_, 392 f.;
external characters, 392;
British species, 393;
eyes, 395;
branchiae, 395;
alimentary canal, 396;
vascular system, 396;
body-cavity, 397;
nephridia, 399;
reproductive organs, 401;
cocoons, 404;
classification, 405
Hirudiniculture, 393
_Hirudo_, 393, 396 f., 399, 403, 405, _407_;
nephridium, 400
_Hislopia_, 492
_Histioderma_, 302
_Histriobdella_, 242 n.
_Histriodrilus_, 242 n.
Holloway, on Rotifers, 227
Holomyarii, _137_
Holostomatidae, 63, 64, _73_
_Holostomum_, _73_;
_H. excavatum_, 63
Holothurian, as host, 298
_Homalogaster_, _73_
Hood, of _Sabellaria_, 263, 306
Hooks, of Trematodes, 53, 56, 57;
of Cestodes, 75, 85;
of Cestode-larvae, 87, 88;
taxonomic value in Cestodes, 90 f.
Hoplonemertea, _106_, 109, _110_;
auditory organs, 106, 110;
British species, 110;
characters, 110;
development, 113;
proboscis, 110
_Hornera_, _518_, 532
Horse, parasites of, 68, 83, 90, 125, 140, 142, 163
Horse-leeches, 393 f.
Horst, on Oligochaeta, 354
Hubrecht on Nemertinea, affinities, 120;
classification, 109;
excretory system, 108;
nervous system, 105
Hudson, on Rotifera, 197, 198, 215 n., 220 n., 223 n., 228
_Hudsonella_, _224_
Huxley, on Rotifers, 198, 229;
on Polychaeta, 246;
on Molluscoidea, 475
_Huxleya_, 525, 527
_Hyalinoecia_, 299, 318, _319_;
tentacles, 262;
composition of tube of, 290
_Hydatina_, 199, 200, 202, 204, 213, _224_, 226, 228
Hydatinidae, _224_
_Hymenolepis_, _91_;
generic characters, 91;
_H. diminuta_ (= _Taenia flavopunctata_), life-history, 83;
specific characters, 91;
_H. murina_ (= _Taenia murina_), 70;
life-history, 83, 89, 91;
_H. nana_, 80;
life-history, 83;
specific characters, 91
_Hyocrinus_, as host, 342
_Hyperiodrilus_, 363 f.;
reproductive organs, 363
Hypodermic impregnation, 27, 218
Hypodermis (= Epidermis) of _Gordius_, 165;
of Rotifera, etc., 205, 232
_Hypophorella_, 478, _518_, 533;
larva, 510
_Hyporhynchus_, _49_
_Hystrichis_, _147_, 149, 163

_Ichthydium_, _235_
Ichthyobdellidae, _406_
_Ichthyonema_, 131, _147_, 149, 163
_Ichthyotaenia_, _91_
Ichthyotaeninae, 85, _91_
_Idmonea_, _518_, 532
Iguana, parasites of, 142
Iijima, on yolk-glands, 38 n.
Illoricata, _223_
_Ilyodrilus_, _378_;
chaeta, 350
_Ilyogenia_, _388_
_Imogine_, _19_
Inarticulata, _517_, _518_
Incudate, 210, 211
Incus, in Rotifers, 210
Inermia, 445
Infusoriform embryos, 93
Insects, parasites of, 142, 150, 152, 153, 154, 160, 163, 179, 183, 184,
185
Internal sac, 509, 511
Internode, 525
Interproglottidal glands, 90
Intertentacular organ, 469, 473, 508
Introvert, in Polychaeta, 249, 250;
in Gephyrea, 412, 424
Investing membrane, in ectoparasitic Trematodes, 57;
in Cestodes, 85
Ioda, form of head, 278
_Isacis_, 163

Jammes, on skin of Nematoda, 126
Jaws, of _Nereis_, 248, 250, 270;
of Polychaeta, 269 f.;
of Eunicidae, 270;
of Polynoid, 270;
of _Autolytus_, 270;
fossil, 302;
of _Glycera_, 270
Jelly, Miss, on Polyzoa, 523
_Jensenia_, _50_
Jimenez, on Leeches, 407
Joblot, on Rotifers, 197
Johnston, on Hirudinea, 393
Joliet, on development of Rotifers, 218;
on Polyzoa, 508
Jungermanniaceae, inhabited by Rotifers, 227 {547}
Jurassic, Serpulids, 301;
Polyzoa, 520 f.

Kaiser, on Acanthocephala, 177
Kamptoderm, 469, 470
Keferstein, on Polycladida, 7, 10
Kennel, von, on Nemertinea, 108;
on _Malacobdella_, 101 n., 119
_Kerria_, _384_
Kinorhyncha, 236 f.
Kleinenberg, on Trochophore, 229
_Koellikeria_, _73_
Kölliker, on _Distomum okenii_, 71
Koren and Danielssen, on Gephyrea, 442, 443
Kowalevsky, on Hirudinea, 397;
on _Phoronis_, 458
Kraepelin, on Polyzoa, 493, 502
Krause, on parasites, 163
Krohn, on Nemertine development, 99
_Krohnia_, _186_;
species, 192, 194;
American species, 534
Krukenberg, on haemerythrin, 416
_Kynotus_, _386_

_Labrorostratus_, 297
_Lacinularia_, 200, 213, 215, _221_, 226
_Lacrymaria_, 235
_Laetmonice_, _312_
_Lagenipora_, 528
Lamarck, on Gephyrea, 411
Land, Planarians, 4, 30, 33, 34, 36;
Nemertines, 101, 117;
Oligochaeta, 347 f.;
Leeches, 408
Lang, on Polyclads, 7, 17, 21 f., 27, 28 n.;
on nature of Cestodes, 76
_Lanice_, 328;
_Hypophorella_ in tubes of, 478, 533
Lankester, on Trochosphere, 229;
on Earthworms, 347;
on Hirudinea, 397;
on Gephyrea, 430, 437, 439;
on Podaxonia, 461
Larva, Müller's, 29;
of _Polystomum_, 59;
of _Diplozoon_, 60;
of _Gyrodactylus_, 61;
of Holostomatidae, 63, 64;
of _Distomum_ 64, 65, 67;
hosts of larvae of digenetic Trematodes, 71 f.;
of _Calliobothrium_, 77;
of Cestodes, 79 f.;
table of Cestode larvae, 83, 85, 87;
of _Schistocephalus_, 84;
of _Bothriocephalus_, 87;
of _Dipylidium_ (Cysticercoid), 83, 88 f.;
of Mesozoa, 92, 93;
of Nemertinea, 113;
Trochosphere, 229, 274, 439, 510;
of _Polygordius_, 245;
of Polychaeta, 274, 275;
of _Chaetopterus_, 325;
of _Myzostoma_, 344;
of Gephyrea, 419, 439;
of _Phoronis_, 458;
of Polyzoa, 466, 509, 511, 520
Lateral organs, of Capitellidae, 343—see also Ciliated pits
Laurer's Canal (= Laurer-Stieda canal), 57, 87
_Leaena_, 300, 327 n.
_Lecanocephalus_, 131, 133
Leckenby, on _Priapulus_, 433
Leeches, _241_, 392 f.—see also Hirudinea
Leeuwenhoek, on Rotifera, 197
Legrain, on Nematodes in deserts, 156
Legros, on _Trichina_, 146
Lehnert, on _Bipalium_, 36, 37
Leidy, on Rotifers, 198
_Leimacopsis_, 37, _42_
Lemnisci, 176
_Lepas_, as host, 298
_Lepidasthenia_, 293, _311_
_Lepidoderma_, _235_
_Lepidonotus_ (subgenus of _Polynoe_), eye, 255
_Lepralia_, 516, _518_, 528, 530, 531 n.;
_L. foliacea_, 478, 527;
larva, 511
_Leprea_, _328_
_Leptodera_, 129, 131, 133, 160
_Leptoplana_, 7, 8, 9, 11, 14, 17;
British species, _19_;
habits, 8 f.;
anatomy, 11 f.
Leptoplanidae, _19_
_Leptostoma_, 396
Lesson, on Polyclads, 24
_Leuciscus_, parasites of, 173, 182
Leuckart, on Platyhelminthes, 6, 64 f., 70, 76;
on Nemertinea, 99;
on Nematoda, 136, 139, 140, 142, 146, 163;
on Hirudinea, 395
_Leucochloridium_, 65, 66
Leucocytes, of Polychaeta, 252
_Leucodore_—see _Polydora_
Leydig, on Rotifers, 198
Lias, Serpulids in, 301
_Libyodrilus_, 364, 385, 386
_Lichenopora_, _518_, 531
Life-history, of Trematodes, 4;
of Polystomatidae (_Polystomum_, _Diplozoon_, _Gyrodactylus_), 58 f.;
of digenetic Trematodes, 63 f.;
table of, 71;
of Cestodes, 5 f., 87;
table of life-histories of Cestodes, 83;
of _Dipylidium_, 88;
of Dicyemidae, 93;
of Nematoda, 159;
of _Gordius_, 170;
of Acanthocephala, 179—see also Development
_Ligula_, _91_;
occurrence, 85
Ligulinae, _91_
Lim Boon Keng, on Earthworms, 349
_Lima_, nests of, 298
Lime, secreted by Serpulidae, 290
_Limnaea truncatula_, host of larvae of _Distomum hepaticum_, 67, 72
_Limnatis_, _407_
_Limnias_, 205, _221_;
early description of, 197
_Limnodrilus_, _378_
_Lineus_, _111_;
_L. marinus_ (= _L. longissimus_), 99, 100, _111_, 114;
Borlase on, 99;
size of, 100;
_L. gesserensis_ (= _L. obscurus_ = _L. sanguineus_), recuperative
powers of, 116
Linguatulida, affinities of, 344 {548}
Linnaeus, on Cestodes, 78;
on Gephyrea, 411;
on Polyzoa, 474
Linstow, von, on classification of Nematodes, 137;
on life-history of _Ascaris lumbricoides_, 140;
on parasitism, 162;
on _Gordius_, 166, 171, 173
Lip, lip-membrane, lip-processes, of Sabellidae, 261, 337
_Lipobranchius_, _334_;
shape of body, 259
Lipochromes, in Polychaeta, 292
Lister, on Polyzoa, 497
Lithodomous, Polychaeta, 287
Lithographic slate, fossil Polychaeta from, 301, 302
Liver, -fluke, 51;
-rot, 68
Locomotion, of Planarians, 9, 10, 36;
of Polyclads, 22 f.;
of Nemertinea, 114, 115;
of Chaetognatha, 190;
of Rotifers, etc., 206, 235;
of _Dinophilus_, 243;
of Phylactolaemata, 496
Looss, on Trematoda, 62 n., 66, 70
Lopadorhynchina, _314_
_Lopadorhynchus_, _314_;
_L. erythrophyllum_, colour of, 292
_Lophochaeta_, 351;
chaeta, 350
_Lophohelia_, with _Eunice_, 298, 319
Lophophore, in _Phoronis_, 453;
in Polyzoa, 476, 476, 495
_Lophopus_, 494, 499, 504, 505, _518_, 519;
movements, 496, 497;
fission, 496, 506;
statoblast, 502, 503
Lorica, in Rotifers, 205
Loricata, 205, _224_
_Lota_, parasites of, 182
_Loxosoma_, 489 f., 489, 490, 506, _518_;
on Polychaeta, 299, 489;
on other animals, 489;
buds, 489, 490, 506;
larva, 509
Lug-worm, castings of, 285;
as bait, 297
_Lumbricaria_, 302
Lumbricidae, 362, 371 f., _388_
_Lumbricobdella_, 392
_Lumbriconereis_, 318, _320_;
from fresh water, 284
_Lumbriconereites_ (misprinted in text), 302
Lumbriculidae, 350, 361, _379_
_Lumbriculus_, _379_;
as host of Rotifer, 211 n.
_Lumbricus_, 349, 351, 356, 367, 368, 371, 389 f., 403 f.;
generative organs, 362;
cocoon, 365
_Lunulites_, _518_;
vibracula, 487
Lurg, 317
Lycoridae, _315_
_Lysidice_, 297, _320_

MacGillivray, on Polyzoa, 519
M‘Intosh, on Nemertinea, 110 n., 115, 116;
on _Phoronis_, 454
_Macraspis_, _73_
_Macrobdella_, 393, _406_;
sense-body, 394
_Macrorhynchus_, 44, 47;
British species, _49_
_Macrostoma_, 44, 47;
British species, _49_
Macrostomatidae, _49_
_Magelona_, _325_;
haemerythrin in, 252
Magelonidae, 258, _325_
Main-gut, of _Leptoplana_, 8, 13;
of Polyclads, 17
_Malacobdella_, 101;
description of, 119;
_M. grossa_, _110_, 119;
excretory system of, 108
Malacocotylea (= Digenea), _73_
Maldanidae (= Clymenidae), _258_, _332_;
shields, 259;
anal funnel, 259;
chaetae, 266, 267, 268;
colour, 293
Male, of Rotifers, 199, 200, 217, 223 n.;
of Echiuroids, 438
Malleate, 210
Malleoramate, 210, 211
Malleus, 210
Mammals, Trematodes of, 62, 63, 67-70, 71 f.;
Cestodes of, 77-84, 89-91;
Nematodes of, 163;
Acanthocephala of, 183, 184
Man, Trematode-parasites of, 63, 68-70;
Cestodes of, 74, 78-81, 83, 89 f.;
Nematodes of, 125, 139, 140, 143, 145, 147, 163;
_Gordius_ of, 173;
_Echinorhynchus_ of, 183
Man, De, on free-living Nematodes, 157
_Manayunkia_, 284, _339_ n.
Mandible, of avicularium, 482, 524
Manson, on _Filaria_, 149
Manubrium, 210
Marginal groove, of _Leptoplana_, 15
Maricola, 30, 32, _42_
Marine, Rotifers, 226;
Oligochaeta, 366;
Leeches, 406
_Marphysa_, 276, _319_;
as bait, 297;
as host, 297
Mastax, of Rotifers, 199, 210
Masterman, on Chaetognatha, 190;
on _Phoronis_, 461
_Mastigocerca_, _225_
_Mastigophora_, _518_, 529;
vibraculum, 484
_Mastobranchus_, gill of, 268, _331_
Matzdorf, on Leeches, 369
Maupas, on Rotifers, 217
Mbalolo, 297
_Meckelia asulcata_, _111_
_Mecynostoma_, _49_
Median pore, in Cheilostomata, 484, 524
Megadrili, _373_, 374, _380_ f.
_Megalotrocha_, _221_
_Megapora_, 530
_Megascolex_, 351, 372, _381_, 383
_Megascolides_, 349, 358, 372, _382_;
body-wall and nephridia, 357
Mégnin, on parasites, 164
Meissner, on Polyzoa, 493
_Melania_, _Coabangia_ in shell of, 284
Melanin, 292
_Melicerta_, 202, 205, 206, 210, 215, 218, 219, 221, 226; {549}
early description of, 197
Melicertaceae, 203, _221_
Melicertidae, 201, 203, 205, 211, 213, _221_
_Melinna_, _330_
_Membranipora_, 481, 492, _518_, 519, 523, 528, 530;
phosphorescence, 478;
function of aperture, 500;
larva, 509, 510;
variation, 516
_Membraniporella_, _518_, 524, 528
_Menipea_, _518_, 526, 527
_Mermis_, 131, _150_, 160, 163
Mermithidae, _150_, 163
Meromyarii, 129, _137_, 142
Mertens, on Polyclads, 6, 23
_Mesobdella_, _407_
_Mesocestoides_, _91_;
_M. lineatus_, life-history, 84;
specific characters, 90
Mesonemertini, _112_
_Mesostoma_, 44, 45;
reproduction, 48;
British species, _49_
Mesostomatidae, _49_
Mesotrochal larva, 325
Mesozoa, 92 f.
Metamerism, 249
Metamorphosis, in Polycladida, 16, 28;
in Trematodes, 5, 66 f.;
in Cestodes, 5, 74, 76, 87-89;
in _Phoronis_, 459;
in Polyzoa, 512
Metanemertini, _112_
Metastatica (= Holostomatidae), _73_;
life-histories, 64
_Metopidia_, 200, _225_
Metschnikoff, on Orthonectida, 94;
on Nemertines, 99;
on Chaetosomatidae, 158
Meyer, on Polychaeta, 261
Michaelsen, on Earthworms, 349, 353, 359, 370 f., 375, 385
_Microchaeta_, 362
_Microcodides_, _224_
_Microcodon_, 202, 215, _224_, 230
Microcodonidae, 202, _220_ n., _224_
_Microcotyle_, _73_;
eggs of _M. labracis_, 58
Microcotylinae, _73_
Microdrili, 373, 374 f.
_Microplana_, 34, _42_
_Micropora_, _518_, 530;
zooecium, 523
_Microporella_, 516, _518_, 519, 529;
avicularia, 484, 485;
median pore, 501
_Microscolex_, 372, _382_, 383, 384
_Microstoma_, 44;
asexual reproduction, 44;
sexual organs, 47;
British species, _49_
Microstomatidae, _49_
_Micrura_, 101, 114;
British species, _111_
Migrations, of Trematode-larvae, 5, 52, 63 f.;
of Cestode-larvae, 5, 74, 83, 87
Millson, on Oligochaeta, 368, 387
_Millsonia_, _382_ f.
Milne-Edwards, on Polyzoa, 475
Mimicry, in Polychaeta, 293 f.
_Mimosella_, _518_, 532;
movement of zooecia, 481
Molluscoidea, 475
_Molops_, host of _Gordius_, 172
_Monhystera_, 160
Moniez, on Cestodes, 84 n., 85;
on Earthworms, 369
_Moniezia_, 83, _91_;
characters of genus, 90;
_M. alba_, _M. benedeni_, _M. denticulata_, 90;
_M. expansa_, 83, 90;
_M. neumani_, _M. oblongiceps_, _M. planissima_, _M. trigonophora_, 90
_Moniligaster_, 359, _380_, 402
Moniligastridae, 361, 373, _380_
Monkey, parasites of, 145
_Monocotyle_, _73_
Monocotylinae, _73_
Monogenea (Monogenetic Trematodes), 5, 53;
classification, 73
_Mononchus_, _154_
_Monoophorum striatum_, _50_
_Monopora vivipara_, 117
_Monoporus rubropunctatus_, _49_;
reproductive organs of, 47
Monostomatidae, _73_
_Monostomum_, 71, _73_;
life-history, 72
_Monostyla_, _225_, 226
Monotidae, _50_
_Monotus_, 36;
British species, _50_;
_M. fuscus_, 45;
habits, 46
Monozoa, _91_
Montgomery, on _Stichostemma eilhardii_, 118
_Monura_, _225_
Morren, on Earthworms, 347
Moseley, on Land Planarians, 7, 35 n., 37;
on _Pelagonemertes_, 101, 114
Movements, in Rotifers, etc., 206, 235
Moxon, on Rotifers, 198
Mucro, 522, 525
_Mucronella_, _518_, 522, 528, 529, 530, 531
Müller, F., on Triclads, 37;
on Polyzoa, 493
Müller, O. F., on Turbellaria, 6;
on Rotifers, 197;
on Oligochaeta, 348, 352
Müller's larva, 16, 28, 29
Muscles, of _Leptoplana_, 12;
of intestine in Polyclads, 24;
of Nemertinea, 102, 103;
of Nematoda, 128;
of Gordiidae, 165;
of Chaetognatha, 187;
of Rotifers, etc., 206, 233, 237;
of _Nereis_, 247, 249 f.;
of Polyzoa, 469, 470, 472, 499, 500—see also Body-wall
Musculo-glandular organ, in Triclads, 39, 40
_Myrianida_, 280, _308_;
gemmiparity, 279, 280;
markings, 293
_Myrianites_, 302
_Mytilia_, _225_, 226
_Myxicola_, _338_; {550}
tori uncinigeri, 268;
eyes, 272;
otocyst, 273;
tube, 285;
supposed mimicry, 294
_Myzostoma_, anatomy, 342 f.;
_M. cirriferum_, 343;
_M. glabrum_, 342
Myzostomaria, _241_;
structure and affinities, 342 f.
Myzostomatidae, _342_

Naidomorpha (= Naididae and Naids), 348, 352, 375 f., _377_, 378, 400;
chaetae, 350, 367;
reproductive organs, 361;
asexual reproduction, 281, 377
_Nais_, 348, _377_;
chaetae, 350
_Nannodrilus_, _382_ f.
Natatory chaetae, of Nereidae, 277;
of Syllidae, 278, 307
Nathusius, on parasites, 63, 163
_Nauphanta_, 314, _315_
_Nebalia_, Rotifers parasitic on, 225
_Nectochaeta_, 291
_Nectonema agile_, 164, 168, _169_, 173
Nemathelminthes, 123 f., 168
_Nematobothrium_, 4, 55, 71, _73_
Nematoda, _123_;
anatomy, 124 f.;
embryology, 135;
classification, 136;
life-history of, 159 f.
Nematogen, 94;
Secondary Nematogen, 94
Nematomorpha, _123_, 164 f.
_Nematoxys_, _139_, 142, 161, 163
Nematozoa, _137_
_Nemertes_, British species, _110_;
deposition of ova and spermatozoa in, 116
Nemertinea, affinities, 119;
anatomy, 102;
body-wall, 102;
breeding, 116;
cephalic slits and cerebral organs, 107, 273;
circulatory system, 106, 107;
classification, 109 f.;
colour, 102;
contractility, 102;
development, 113;
diagram, 104;
digestive system, 103;
excretory system, 108, 108;
external characters, 101;
food, 115;
fresh-water forms, 101, 118;
generative organs, 109;
geographical distribution, 117;
habitat, 100;
habits, 114;
land forms, 101, 117;
locomotion, 114, 115;
nervous system, 105, 106;
parasitic forms, 101, 119;
pelagic forms, 101, 114;
proboscis, 100, 101 f., 103, 104, 110, 115;
regeneration, 115;
sense organs, 101, 106, 107;
size, 100
_Nemertodrilus_, 360
_Nemertoscolex_, 444
_Neodrilus_, _384_
Neorhynchidae, _184_
_Neorhynchus clavaeceps_, 178, 181, _184_;
_N. agilis_, _184_
_Nephelis_, 392 f., 397, 399, 401, 403, 405
Nephridium, of Archiannelida, 243, 244;
of _Nereis_, 253;
of _Arenicola_, 253, 254;
of Polychaeta, 253;
dimorphism of, in Polychaeta, 269;
thoracic, of Sabelliformia, 306;
of Terebellidae, 327;
of Chlorhaemidae, 334;
of _Myzostoma_, 344;
of Oligochaeta, 356 f., 381 f., 400;
of Leeches, 399;
of Gephyrea—see Brown tubes;
of _Phoronis_, 456
Nephthydidae, _258_, _317_
_Nephthys_, _317_;
nephridium, 254;
prostomium, 259, 260;
head, 262;
tentacles, 262;
peristomium, 263;
parapodium, 264, 265;
cirrus, 265;
gill, 268;
jaws, 270;
habitat, 286;
as bait, 297
Nereidae, _258_, _315_;
palps, 260;
colour, 292
Nereidiformia, _258_, _303_;
vascular system, 252;
anal cirri, 259;
prostomium, 259, 260;
tentacles, 262;
peristomial cirri, 263;
parapodium, 264 f.;
cirri, 265;
chaetae, 266;
jaws, 269;
eyes, 272;
ciliated organ, 273;
regeneration in, 283;
food, 296;
fossil, 301 f.
_Nereilepas_, 317
_Nereis_, 246, 299, 301, _316_ f.;
anatomy, 245 f., 247 f.;
transverse section, 247;
parapodium, 246, 247, 265, 317;
chaetae, 246;
head, 246, 248, 316;
alimentary canal, 249 f., 251;
jaws, 248, 250, 270, 316;
nervous system, 254, 255;
eye, 255;
vascular system, 247, 251;
dorsal ciliated organ, 247, 254, 256;
nephridium, 253;
reproduction, 256;
sexually mature, 276, 276 f.;
sexual dimorphism and Heteroneid phase, 276 f.;
epitokous (= epigamous) and atokous phase, 277 n.;
from fresh water, 284;
burrow, 286;
pigment, 292;
as bait, 297;
commensalism, 298;
as host, 299;
British species, 316 f.
_Nereites_, 302
_Nerine_, 299, _322_;
habitat, 286;
_N. vulgaris_, head, 322
Nerve plexus of Nemertinea, 103, 105
Nervous system, of _Leptoplana_, 13, 14;
of Polyclads, 26, 30;
of _Planaria lactea_, 39;
reduced in parasitic Rhabdocoels, 45;
of _Temnocephala_, 54;
of Polystomatidae, 56;
of Cestodes, 75, 86;
of Nemertinea, 105, 106;
of Nematoda, 127;
of Gordiidae, 166;
of Acanthocephala, 177;
of Chaetognatha, 187;
of Rotifera, etc., 215, 234, 237;
of Archiannelida, 243, 244;
of Polychaeta, 254;
visceral, of Polychaeta, 255;
of Oligochaeta, 353, 374;
of Gephyrea, 416, 431, 437, 440, 445;
of _Phoronis_, 456;
of Polyzoa, 471
Neumann, on parasites, 164
Neuropodium, 246, 247, 264, 266, 268
Newton, on zoo-geographical regions, 372
_Nicolea_, _328_;
gill, 329
_Nicomache_, _332_;
tail, 332;
tube, 287;
_N. lumbricalis_, colour, 292;
distribution, 299
Nitsche, on Polyzoa, 475 n., 478, 500
_Nitzschia_, 56, _73_ {551}
Norman, on Polyzoa, 475
_Norodonia_, 492
_Notamia_, _518_, 526
_Noteus_, _225_
_Notholca_, _225_, 226
_Notocotyle_, _73_
_Notomastus_, _331_;
chaetae, 268
_Notommata_, 217, _224_, 226
Notommatidae, 200, 205, 207, 215, 223, _224_
_Notophyllum_, colour, 293
Notopodium, 246, 247, 264, 265, 268
_Notops_, 200, _224_
_Notopygos_, 259
Nuchal cirri of Eunicidae, 318;
Nuchal organ—see Ciliated pits

_Ocnerodrilus_, _383_
_Octobothrium_, 56, _73_
_Octochaetus_, 358, _384_
Octocotylinae, _73_
_Octotrocha_, _221_
_Odontosyllis_, reproduction of, 278;
as host, 297.
_Oecistes_, 205, 206, _221_
_Oenonites_, 302
Oesophageal glands, 271, 358
_Ogmogaster_, _73_
Oka, on Hirudinea, 399 f.;
on Polyzoa, 500
Olfactory pits, in Polyclads, 26;
in Triclads, 36
_Oligocelis_, _42_
Oligochaeta, _241_, 347 f.;
external characters, 348;
body-wall, 349;
chaetae, 350, 351;
branchiae, 352;
nervous system, 353;
sense-organs, 354;
coelom and vascular system, 355;
excretory organs, 356;
alimentary canal, 358;
reproductive organs, 360;
habitat, 365;
phosphorescence, 368;
distribution, 369;
classification, 373;
Rotifers parasitic on, 227
_Oligocladus_, _19_, 22
_Oligognathus_, 297
_Ollulanus_, _142_;
_O. tricuspis_, 144, 161
_Omalostoma_, _49_
_Onchnesoma_, 422, 423, _426_, 430, 447
_Onchocotyle_, _73_
_Oncholaimus_, _157_
Onchosphaera-larva of Cestodes, 87, 88
Onuphin, 290
_Onuphis_, 290, 318, _319_;
_O. conchylega_, tube, 287
_Onychochaeta_, _388_;
chaeta, 351
_Onyx_, 131
Ootype, in _Polystomum_, 59.
Operculum, of Serpulidae, 261, 276, 339;
of _Spirorbis_, as brood-pouch, 261, 276;
of Cheilostomata, 466, 477, 481, 482, 522, 524
Opesia, 524
_Ophelia_, 299, _332_;
eggs, 275;
coelomic corpuscles, 252
Opheliidae, _258_, _331_;
gill, 265;
ciliated pits, 272
_Ophiodromus_, _308_;
_O. flexuosus_, parasitic, 297
_Ophryotrocha_, 310, _320_;
pelagic, 291;
genital organs, 274
_Opisthotrema_, _73_
_Opistoma_, _50_
Orbigny, D', on Polyzoa, 519, 520
Orifice, of zooecium, 466, 469, 470, 524;
secondary, 522, 524
Örley, on classification of Nematodes, 137
Orthonectidae, 13, 92, 94 f.
Otocyst (and Otolith), of Turbellaria, 26;
of Nemertinea, 106, 110;
of Polychaeta, 273
_Otomesostoma_, 46, _49_
_Otoplana_, _50_
Oudemans, on Nemertinea, 108
Ovary (and Oviduct), of _Leptoplana_, 11, 14, 16;
of Polyclads, 27;
of _Planaria_, 38, 39;
of Rhabdocoelida, 47;
of _Temnocephala_, 54;
of Polystomatidae, 57;
of _Diplozoon_, 60;
of _Gyrodactylus_, 61;
of _Distomum macrostomum_, 65;
of _Calliobothrium_, 75;
of _Archigetes_, 76;
of _Schistocephalus_, 86;
of _Rhopalura_, 95;
of Myzostoma, 343—see also Reproductive organs
Ovicell, in Cheilostomata, 466, 466, 468, 481, 482, 484, 522 f., 525;
in Cyclostomata, 479, 480, 521, 525
Oviduct—see Ovary
_Owenia_, _325_
Ox, parasites of, 79, 83, 125, 139, 140, 143
_Oxysoma_, _139_, 142
_Oxyuris_, 129, 131, 135, _139_, _141_, 160;
_O. ambigua_, 141;
_O. curvula_, 141, 163;
_O. diesingi_, 141, 142;
_O. vermicularis_, 141, 163;
_O. blattae_, _O. blatticola_, _O. hydrophili_, _O. megatyphlon_,
_O. spirotheca_, 142

Paddle worm, _313_
Paedogenesis, 151
Pagenstecher, on Nemertinea, 99
_Palaemonetes_, host of _Nectonema_, 174
Palaeonemertea, _109_, 112;
characters, 111;
development, 113
Palaeozoic, Serpulidae, 301;
Eunicidae, 302;
Polyzoa, 520
Paleae, of _Sabellaria_, 267
_Palmicellaria_, 527, 528
_Palolo viridis_, as food, 297
Palps, of _Nereis_, 248, 255;
nerves to, 254;
of Polychaeta, 260 f.;
development of, in Sabelliformia, 275;
of Hermelliformia, 306;
of Syllidae, 307
_Paludicella_, 492, 494, 501, 502, 505, _518_
Paludicola, 30, _42_ {552}
_Panthalis_, _313_
Paragnaths, 248, 250, 316
Parapodium, of _Nereis_, 246, 247;
of Polychaeta, 264 f.;
of Heteronereid, 276, 277;
muscles of, 247;
glands of, 249, 314;
of _Myzostoma_, 342
_Paraseison_, 212, 225, _226_
Parasitic, Turbellaria, 51;
Polyclads, 22;
Triclads, 32;
Rhabdocoela, 45, 51;
Nemertinea, 101, 119;
Rotifers, 204, 227;
Polychaeta, 297;
Leeches, 406
Parasitism, effect on the parasite, 161, 177;
effects on the host, 162
Parenchyma, in _Leptoplana_, 11, 12;
of Müller's larva, 29;
in Triclads, 41;
in Acoela, 42;
in Cestodes, 85, 86
Parovaria, of _Phagocata_, 38 n.
Parthenogenesis, amongst Rotifers, 200
_Pectinaria_, _330_;
body, 259;
tube, 285, 287;
_P. auricoma_, tube, 288;
_P. belgica_, 330
_Pectinatella_, 496, 497, 505, 512, _518_;
statoblast, 502
_Pedalion_, 200, 201, 206, 211, 216, _223_, 224, 225, 228, 230
Pedalionidae, _223_
_Pedetes_, _224_
_Pedicellina_, 487, 488, 490, 506, 507, _518_;
larva, 510, 513;
budding, 514;
on Polychaeta, 299
Pelagic, Nemertinea, 101, 114;
Chaetognatha, 189; Rotifera, 226;
Polychaeta, 291, 294, 314;
larvae of Polychaeta, 300, of Polyzoa, 520
_Pelagobia_, _314_
_Pelagonemertes_, 101, 114
_Pelodera_, 129, 131, 133
_Pelodrilus_, 369, _377_
_Pelodytes_, 134
Penis, of _Leptoplana_, 14, 15;
of Polyclads, 27;
of _Planaria_, 38, 39;
of Rhabdocoelida, 47;
of _Temnocephala_, 54;
of _Calliobothrium_, 75;
of _Schistocephalus_ (cirrus-sac), 86
Pennant, on Hirudinea, 406 n.
Pereyaslawzewa, on Acoela, 44 n.
Pergens, on Polyzoa, 500
_Perichaeta_, 351, 357, 358, 372, 381, 381 f., 388, 394, 403
Perichaetidae, 357, 362, _380_
Perienteric, blood-sinus, 252
_Perionyx_, _381_
_Perissogaster_, _383_
Peristome, 481, 482, 522, 524
Peristomial (tentacular) cirri, of _Nereis_, 248;
of Polychaeta, 263;
nerves to, 254
Peristomium, of _Nereis_, 248;
of Polychaeta, 263;
of Sabellidae, 336
Perrier, on Oligochaeta, 367, 385
_Petalostoma_, 422, _426_, 430, 447
_Petromyzon_, host of _Gordius_, 173
_Petta_, intestine of, 271
_Phagocata_, 31, 32
_Phalacrophorus_, _314_
Phanerocephala, _258_, _303_
Pharynx, 4;
of _Leptoplana_, 8, 9, 12, 14;
of Polyclads, 17, 24;
of _Discocelis_, 23;
development of, in Polyclads, 29, 30;
of Triclads, 31, 37, 39;
of _Temnocephala_, 53, 54;
of Polystomatidae, 56;
of Digenea, 62, 64;
of _Nereis_, 249, 250, 251;
of Polychaeta, 269
_Phascolion_, 423, _425_, 428
_Phascolosoma_, 416, 420, 423, _425_, 428, 447;
as host of _Loxosoma_, 489
Philippi, on Hirudinea, 406
_Philodina_, 208, _222_, 227
Philodinidae, _222_
_Phoronis_, 450 f., 451, 452, 453, 455;
habits, 451;
anatomy, 453 f., 457;
development, 458, 458;
species, 460;
affinities, 461, 512
Phosphorescence (and light-producing organs), in Rotifers, 226;
in Polychaeta, 272, 295, 296;
in Oligochaeta, 368;
in Polyzoa, 478
_Photodrilus_, 368, _383_
Photogen (light-producing organ), of _Polyophthalmus_, 272;
of _Tomopteris_, 296, 315
_Phreodrilus_, 369, _379_
_Phreoryctes_, 367, _377_
Phreoryctidae, _376_
_Phylactella_, 528, 530
Phylactolaemata, _476_, 493 f., _518_;
lophophore, 476, 495;
occurrence, 493;
movements, 494, 496 f.;
reproduction, 501, 506, 507;
larva, 511, 512;
distribution, 493, 504;
affinities, 512
Phyllacanthinae, _91_
Phyllobothrinae, _91_
_Phyllobothrium_, 76 n., _91_
_Phyllocotyle_, _73_
_Phyllodoce_, _313_, 314;
head, 262, 263:
eggs, 275, 314 n.;
colours, 291, 292;
parapodium, 264;
chaeta, 267
Phyllodocidae, _258_, _313_;
parapodial cirri, 266;
as food, 297;
eggs, 314;
colours, 292, 293
_Phyllodocites_, 302
_Phyllonella_, _73_
_Phymosoma_, 413, 420, 421, 423 f., _425_, 426
_Physaloptera_, 163
Pig, parasites of, 68, 79, 139, 147, 184
Pigments, of Polychaeta, 291 f.;
of Gephyrea, 435
Pike, Trematode of, 62;
Cestode of, 81, 84
Pilidium larva, 113, 113, 229, 230
_Pionosyllis_, _308_
_Piscicola_, 393, _406_
_Pista_, _328_;
gill of, 329
Pits, ciliated, of Polychaeta, 272, 273
_Placostegus_, colour, 292; {553}
from deep sea, 300
_Placunella_, _73_
_Plagiochaeta_, 358, _381_, _384_
_Plagiostoma_, 46;
British species, _50_
Plagiostomatidae, _50_
_Planaria_, 30, 31 f., 39;
British species, _42_
Planarians, 3, 7;
_Dinophilus_, compared with, 242, 243
Planariidae, _42_
_Planctoplana_, _19_
Plankton, Rotifers in, 225
_Planocera_, 18, _19_, 20
Planoceridae, _19_, 23
_Planorbis_, host of _Gordius_, 173
Plants, parasites on, 154, 155, 157, 160
Plasmodium, nature of, in Orthonectids, 94
Plate, on Rotifers, 198, 225 n.
_Platyaspis_, _73_
_Platycotyle_, _73_
Platyhelminthes, 3 f.;
Nemertinea classed with, 119
_Plectanocotyle_, _73_
_Plectus_, 160
_Pleionogaster_, _381_
Plerocercoid larva, 84
Plessis, du, on _Tetrastemma lacustre_, 101 n., 118
_Pleurocotyle_, _73_
_Pleurotrocha_, _224_, 226
_Ploesoma_, 212, _225_
Ploima, 202, 203, 212, 213, 216, _220_, _223_, 226, 227
_Plumatella_, 493, 494, 499, 503-505, _518_, 519;
protrusion of polypides, 499;
statoblasts, 499, 502, 503;
larva, 512
Podal membrane, of Spionidae, 322
Podaxonia, 461
_Polyarthra_, 201, _224_, 226
Polybostrichus, 280
_Polycelis_, 30, 31, 40, _42_
Polychaeta, _241_, 245 f.;
classification, 257, 258, 303 f,;
head, 248, 259 f.;
parapodium, 246, 264 f.;
chaetae, 246, 266 f.;
coelomic fluid, 252;
nervous system, 254;
sense-organs, 255, 272;
ciliated pits, 272;
alimentary canal, with pharynx, 249, 250, 251, 269, 270, 271;
oesophageal glands, 271 f.;
nephridium, 253, 254, 269, 274;
genital duct, 254, 269;
genital cells, 256, 273 f.;
hermaphrodite, 273;
regeneration, 278, 282;
habits, 285;
carnivorous, 304;
distribution, 299;
from fresh water, 284;
from deep sea, 300;
pelagic, 291, 314;
boring, 287;
tubes, 287;
pigments, 291;
colours, 291 f.;
warning colours, 294;
protective devices and mimicry, 293;
phosphorescent, 295;
food of, 296, 299;
as bait, 296, 297;
as food for man, 297;
commensalism, 297 f.;
parasitic, 297 f.;
as hosts, 299;
extinct, 301, 302;
larva, 274, 276, 300;
provisional chaetae, 274
_Polychoerus_, _49_;
development of, 44 n.
_Polycirrus_, _330_;
habits of, 285;
_P. aurantiacus_, warning colours, 294;
phosphorescence, 295;
_P. haematodes_, coelomic corpuscles, 253
Polycladida (Polyclads), 4 f., 7;
classification, 16 f.;
development, 28 f.;
British species, 19, 20, 22
_Polycladus_, 42
_Polycotyle_, _73_
_Polydora_ (= _Leucodore_), _323_;
frontal ridge, 260;
head, 261;
special chaetae, 267;
with _Heliopora_, 298;
_P. ciliata_, borings, 287
Polydoridae, 258, _323_
_Polygordius_, 242, 244;
development, 245
_Polymnia_, 328
Polymorphism, of _Nereis_, 277
Polymyarii, _137_, 142
_Polynoe_ _310_;
segments 258;
parapodium, 265;
jaws, 270;
anus, 259;
nephridium, 254;
habits, 286;
as ectoparasites or commensals, 294, 298, 325;
distribution, 299, 300;
British species, 299, 310 f.;
_P. squamata_, 309;
elytron, 310;
_P. clava_, elytron, 310;
_P. imbricata_, elytron, 311
Polynoina (= Polynoids), _309_;
head, 262;
chaetae, 266, 267;
jaw, 270;
intestine, 271; eggs, 275;
sexual dimorphism, 276 n.;
tubes, 285;
colours, 291, 292;
protective resemblance, 294;
phosphorescence, 295, 296;
food, 296;
parasitic and commensal, 297, 325;
elytra, 275, 294, 295, 299, 309 f.
_Polyodontes_, _313_ n.
_Polyophthalmus_, _332_;
segmental eyes, 272, 296;
otocyst, 273
Polype à pannache, 496
Polypide, 468, 469, 474, 488, 523;
retraction and protrusion, 498 f.
Polypide-bud, 468, 472, 487, 496, 499, 501, 510;
connected with reproduction, 507
_Polypostia_, _19_;
penes, 27
Polystomatidae, _53_, 55, _73_
Polystomatinae, _73_
_Polystomum_, 55, 57;
life-history, 58, 59
Polyzoa, 465 f., 475;
external characters, 465 f., 479 f.;
anatomy, 468 f., 469;
brown bodies, 471 f., 472;
history, 474 f.;
classification, 475 f., 515, 517 f.;
occurrence, 477 f.;
avicularia and vibracula, 482;
enemies, 486;
Entoprocta, 487;
fresh water, 492 f.;
reproduction, 501, 506;
development, 509;
affinities, 461, 509, 510;
metamorphosis, 512;
budding, 514;
distribution, 493, 504, 519;
palaeontology, 520;
terminology, 523;
determination of British genera, 505, 521, 525
_Pomatoceros_, habitat, 300, _340_ {554}
_Pompholyx_, 201, 203, _225_
_Pontobdella_, 393, 401, 404, 406
_Pontodora_, _314_
_Pontodrilus_, 366, 370, _383_
_Pontoscolex_, 350, 366, _387_ f.;
chaeta, 351
Pore, in Polyzoa, 471, 482, 522, 524;
median, 484, 524;
dorsal, 348
_Porella_, 516, _518_, 522, 527, 529
_Porina_, _518_, 527, 529
_Potamilla_, _338_
Praeoral lobe (= Prostomium), 245, 439
Predaceous worms, 304
Priapuloidea, _412_, 446;
anatomy, 430;
classification, 432;
habits, 433
_Priapulus_, 430, 431, _432_;
anatomy, 430 f.
_Pristina_, _377_
_Proales_, 204, _224_, 226, 227
Proboscidae, _49_;
occurrence, 44
Proboscis, of Nemertinea, 100, 101 f., 103 f.;
of Hoplonemertea, 104, 110;
opening by mouth, 117, 119;
severance of, 116;
of Acanthocephala, 174 f.;
of Rotifers, 203;
of Kinorhyncha, 237;
of Echiuroidea, 434
Proboscis-pore of Nemertinea, 102, 103
Proboscis-sheath of Nemertinea, 103, 103 f.
_Procerodes_, _42_
Procerodidae, _42_
_Procerus_, host of _Gordius_, 172
_Procotylea_, 36, _42_
Proglottis, 5, 74, 75, 79, 85
_Promesostoma_, occurrence, 44;
British species, _49_
Proporidae, _49_
_Proporus venenosus_, _49_
Prorhynchidae, _49_
_Prorhynchus sphyrocephalus_, terrestrial habit, 44;
_P. stagnalis_, _49_
_Prosorhochmus claparedii_, _110_, 114, 117
Prostate-gland, of _Leptoplana_, 16;
of Polyclads, 30;
of _Planaria_, 39;
of Rhabdocoela, 47;
of Oligochaeta, 361
_Prostheceraeus_, _19_, 22;
spermatophores, 27
Prosthiostomatidae, _19_
_Prosthiostomum_, 17, 18, _19_, 24
Prostomial tentacles, 248, 262
Prostomium, 241;
of _Dinophilus_, 243;
of _Polygordius_, 244;
of Trochosphere, 245;
of _Nereis_, 248;
of Polychaeta, 259;
of Glyceridae, 320;
of Terebellidae, 327;
of Oligochaeta, 348
_Protodrilus_, 242, 244
Protonemertini, _112_
_Protula_, _341_;
genital organs, 273, 274;
eggs, 275
Prouho, on Polyzoa, 489, 507 f.
Provisional chaetae, 274
_Provortex_, British species, _50_
_Proxenetes_, 44;
British species, _49_
Pruvot, on Polychaeta, 261
_Psamathe_, 300, _308_
_Psammolyce_, _313_;
elytra, 294, 313
_Pseudalius_, 135, _142_, 163
_Pseudaxine_, _73_
Pseudoceridae, _19_, 20
_Pseudoceros_, _19_, 20
_Pseudocotyle_, 73
_Pseudorhynchus bifidus_, _49_
_Psygmobranchus_, _341_
Pterobranchia, 461
_Pterodina_, 200, 201, 203, 206, 211, 215, 216, _225_, 226, 230
Pterodinidae, 201, _225_
_Pteroessa_, _224_
_Pteronella_, _73_
_Pterostichus niger_, infested by _Gordius_, 170, 170, 172
_Pterosyllis_, ciliated lappets, 273 n.
Pyriform organ, 509, 511

Quatrefages, on Gephyrea, 411, 445

Rabbit, parasites of, 141, 145
Ragworm, 322
Railliet, on Cestodes, 91
Rami, in Rotifers, 210
Rasping plate, of Eunicidae, 270
Rattulidae, 210, _225_
_Rattulus_, 212, _225_, 226
Ratzel, on Earthworms, 350
Red Cat, 316
Regeneration of lost parts, in Polyclads, 26;
in Triclads, 40;
in Cestodes, 77;
in Nemertinea, 115;
in Polychaeta, 278, 282;
in Oligochaeta, 348, 379;
in Polyzoa, 471, 488
Repetition of parts, 249
Replacement of species, 300
Reproduction (and Reproductive organs), of _Leptoplana_, 14 f.;
of Polyclads, 26, 30;
of Triclads, 31, 38, 39;
of Rhabdocoelida, 45, 47 f.;
of _Temnocephala_, 54;
of Polystomatidae, 57 f.;
of _Diplozoon_, 60;
of Digenea, 65;
of _Calliobothrium_, 75;
of _Schistocephalus_, 86;
of Mesozoa, 93 f.;
of Nemertinea, 102, 103, 104, 109;
of Nematoda, 134;
of Nematomorpha, 166, 169;
of Acanthocephala, 178;
of Chaetognatha, 188;
of Rotifera, etc., 216, 234, 238;
of Archiannelida, 243 f.;
of Polychaeta, 253, 254, 256, 269, 273;
of _Myzostoma_, 343;
of Oligochaeta, 360;
of Leeches, 401;
of Gephyrea, 418, 431, 437;
of _Phoronis_, 457;
of Polyzoa, 471, 490, 501, 506—see also Ovary and Asexual reproduction
Reptiles, parasites of, 163
Respiration, in _Nereis_, 252;
in Chaetopoda, 272;
in Gephyrea, 416
_Retepora_, 479, 515, _518_, 527
Rhabdites (rods), of _Leptoplana_, 11, 12;
in Polyclads, 29;
in Triclads, 37; {555}
absent in parasitic Rhabdocoela, 45;
in _Temnocephala_, 53, 54
Rhabditiformae, _137_
_Rhabditis_, _154_, 160;
_R. nigrovenosa_, 140, _155_
Rhabdocoelida, 4, _7_, 36, 42 f.;
occurrence and habits, 43;
parasitic forms, 44;
reproduction, 47;
classification, _49_;
British species, 43, 44, 49
_Rhabdogaster_, 158
_Rhabdonema nigrovenosum_, 134, 136, _140_, 151, 160, 161
_Rhabdopleura_, 461 f.
_Rhinodrilus_, 348
Rhinopidae, _220_ n., _224_
_Rhinops_, _224_;
male of, 223 n.
Rhizopoda, as food for Polychaeta, 296
Rhizota, _220_ n.
Rhombogen (form of Dicyemid), 93
_Rhopalonaria_, 521 n.
_Rhopalophorus_, _73_
_Rhopalura giardii_, occurrence and structure, 94, 95;
_R. intoshii_, 94
_Rhynchelmis_, 365, _376_
Rhynchobdellae, 396 f., _405_
Rhynchodemidae, 35, _42_
_Rhynchodemus_, 34, 35, _42_
_Rhynchopora_, 531
_Rhynchozoon_, 529 n., 531
Riches, on British Nemertinea, 110;
on _Malacobdella_, 119
Rietsch, on Gephyrea, 443
Rockworm, 319
Rods—see Rhabdites
Rohde, on muscles of Nematoda, 128 f.
Rootlet, in Polyzoa, 485, 517
Rosa, on Oligochaeta, 364, 380, 385, 390
Rosette-plates, 471, 522
_Rotifer_, 201, 202, 210, 216, _222_, 226, 227
Rotifera, 197 f.;
distribution, 200;
parasitic, 204;
digestive organs, 209;
renal organs, 213;
nervous system and sense organs, 215;
reproduction and development, 216;
classification, 220;
habits 226;
preservation, 228;
affinities, 229
Rousselet, on Rotifers, 198, 216, 228

_Sabella_, 299, _337_; parapodium, 265;
habitat, 286;
tube, 287;
tube-building, 288;
colour, 293, 294;
_S. saxicava_, habits of, 287
_Sabellaria_, _341_;
body, 259;
cirri, 265.;
tube, 287, 290;
_S. alveolata_, 259, 300;
_S. spinulosa_, paleae, 267, 300
Sabellidae, _258_, _336_;
head, 261;
chaetae, 266, 267;
regeneration, 283;
from fresh water, 284;
colour, 292
Sabelliformia, _258_, _306_, 336;
chlorocruorin in, 252; body, 259;
head, 260, 261;
uncini, 266, 267;
nephridia, 269, 306;
genital organs, 273;
development of gills, 275;
gland shields, 287
_Saccobdella_, _226_
_Sacconereis_, 275, 276, 280
_Saccosoma_, 434, 440, _442_
_Sagitella_, _321_
_Sagitta_, _186_, 186, 191, 534;
anatomy, 186 f., 188;
development, 189;
habits, 190;
species, 191, 193;
American species, 534
Salensky, on development of Nemertinea, 99;
of Rotifers, 218
_Salinella_, 93, 96
Salivary glands, in Polyclads, 10, 24;
in Leeches, 396
_Salmacina_, 273, _341_;
brood-pouch, 276;
fission, 281
_Salpina_, 200, _225_
Salpinidae, _225_
Sandmason, 328
_Saxicava_, _Eulalia_ in borings of, 314
Scales, of Gastrotricha, 233
_Scalibregma_, _334_
Scalibregmidae, _258_, _334_
Scapha, 259, 330
_Scaridium_, 201, 207, _225_
_Schistocephalus_, 75, _91_;
reproductive organs, 86;
larva, 84;
life-history, 78, 85
_Schizocerca_, _225_
Schizogamy, in Syllidae, 278, 279, 281
Schizonemertea, _109_;
characters, 111;
development, 113;
transverse section, 103
_Schizoporella_, _518_, 527, 528, 529, 530, 531;
zooecium and avicularia, 482;
_Leptoplana_ on, 22
_Schizotheca_, 482, _518_, 529
Schmarda, on Oligochaeta, 366, 387
Schmidt, on Rhabdocoels, 6
Schneider, on life-history of certain _Mesostoma_, 48;
on classification of Nematoda, 129;
on oesophageal glands, 131;
on Strongylidae, 142
Schultze, on Polyclads, 13, 26;
on Nemertinea, 108, 109
_Schultzia_, _50_
Schulze, F. E., _Stichostemma_ found by, 118;
_Trichoplax_ found by, 96
Scirtopoda, 200, 201, 203, 206, 207, _223_
_Sclerocheilus_, _334_
_Sclerostomum_, 163
Scoleciformia, 258, _305_, _331_ f.;
vascular system, 252;
buccal region, 269;
food of, 296
_Scolecolepis_, 299, _322_
Scolex, 5, 74, 75 f., 89;
_S. polymorphus_, 77;
of _Taenia solium_, 79
_Scolithus_, 302
_Scoloplos_, 299, _321_;
parapodium, 265;
habitat, 286
_Scruparia_, 527
_Scrupocellaria_, 517, _518_, 519, 526;
vibracula, 477, 485, 517;
phosphorescence, 478;
larva, 511
Scutum, 525 {556}
Seals, parasites of, 142, 183
Sea-mat, 466, 477
Sea-mouse, 312
Secondary orifice, 522, 524
Sedentaria, 285
Segment, 241;
of _Nereis_, 246, 247
_Seison_, _226_
Seisonaceae, 204, 216, 220 n., _225_, 227
Seisonidae, _226_
_Selenaria_, _518_;
vibracula, 487
Selenka, on Sipunculids, 424 n., 447
Self-fertilisation, in certain _Mesostoma_, 48;
in Trematodes, 52, 58;
in Cestodes, 86
Semper, on excretory system of Nemertinea, 108;
on _Geonemertes palaensis_, 101 n., 117;
on mimicry in Polychaeta, 294
Sense-organs, of _Leptoplana_, 13;
of Polyclads, 26;
of Triclads, 36;
of Trematodes, 56, 86;
of Cestodes, 86;
of Nemertinea, 106;
of Nematoda, 128;
of _Gordius_, 166;
of Acanthocephala, 178;
of Chaetognatha, 188;
of Rotifera, etc., 215, 233, 234;
of Polychaeta, 255, 272;
of Oligochaeta, 354;
of Leeches, 395;
of Gephyrea, 417;
of _Phoronis_, 457
Septum, of Archiannelida, 244;
of _Nereis_, 249, 251;
of Polychaeta, 269;
of Chlorhaemidae, 334;
of Oligochaeta, 355;
of Gephyrea, 440
_Serpula_, 300, _339_, 340;
fossil, 301;
tubes, 290, 301;
commensal with Polynoid, 298;
colour, 292
Serpulidae, _258_, _339_;
nerve cords, 255;
gills, 261;
operculum, 261;
cirri, 265;
thoracic membrane, 266;
uncinus, 267;
fission, 281;
tube, 290;
colour, 292, 293;
from great depth, 300;
fossil, 301
Serpulite chalk, 301
Seta, of vibraculum, 484, 485, 486, 517, 524
_Setosella_, 530
Sharks, Trematodes of, 62, 72;
Cestodes of, 78
Sheep, parasites of, 67, 81, 82, 83
Sheldon, Miss, on Nemertinea, 99 f.
Shell-gland, of _Leptoplana_, 8, 9, 14, 16;
of Polyclads, 28;
of Trematodes, 59;
of Cestodes, 86
Shield, cuticular, of Polychaeta, 259;
of _Sternaspis_, 335;
glandular—see Gland shields.
Shipley, on _Bipalium_, 37;
on Nemathelminthes, 123 f.;
on Chaetognatha, 186 f., 534;
on _Gephyrea_, 411 f.;
on _Phoronis_, 450 f.
_Sialis lutaria_, host of _Gordius_, 171, 172;
host of Acanthocephala, 185
Side organs, of Carinellidae, 107
Siebold, von, on Tape-worms, 76
_Sigalion_, _313_
Silliman, on Nemertinea, 101, 109, 118
Silurian, Polychaeta, 301
Sinus, in Polyzoa, 482, 484, 525
Siphon, of Capitelliformia, 272, 305;
of Gephyrea, 436
_Siphonogaster_, 353, 368
_Siphonostoma_, _334_;
commensal, 298
Sipunculoidea, _412_, 420, 446;
species, 426
_Sipunculus_, _425_;
history, 411;
species, 426;
anatomy, 412 f., 413, 415;
development, 419, 419;
food, 422;
habits, 426
Size, of Cestodes, 5;
of Polyclads, 20;
of Land Planarians, 33;
of Cestodes, 75;
of Nemertinea, 100
_Slavina_, _377_
Sluiter, on Gephyrea, 429, 447
Smitt, on Polyzoa, 516
_Smittia_, _518_, 527, 529;
zooecium and avicularium, 482
Snakes, parasites of, 142
Solenopharyngidae, _50_
_Solenopharynx_, _50_
Solenophorinae, _91_
_Solenophorus_, _91_
_Sorocelis_, _42_
_Spadella_, _186_, 189, 192;
anatomy, 186 f.;
eggs, 189;
habits, 190;
species, 192, 194;
American species, 534
Spallanzani, on Oligochaeta, 348
_Sparganophilus_, 366, _386_;
anatomy, 355.
_Spatangus_, as host, 298
Spencer, on Land-Planarians, 34;
on earthworms, 349, 380
Spengel, on Gephyrea, 440
Spermatheca, of _Dinophilus_, 243;
of Oligochaeta, 362, 363, 364
Spermatophores, 27, 402
Spermiducal gland, 361
Sphaerodoridae, _320_
_Sphaerodorum_, _321_
_Sphaerosyllis_, _308_
_Sphaerularia_, _150_, 153, 160, 161
_Sphyranura_, _73_;
setae in, 56
Spine, of Polyzoa, 481, 523 f., 524
_Spinther_, _318_
_Spio_, _322_
Spionidae, _258_, _321_;
larva, 274, 275
Spioniformia, _258_, _304_, 321;
peristomial cirri, 263;
gill, 265;
chaetae, 266, 267;
eyes, 272;
food, 296
Spirographin, 290
_Spirographis_, _338_;
substance of tube, 290
_Spiroptera_, _147_, 163;
_S. reticulata_, 149;
_S. obtusa_, 161;
_S. alata_, 163
_Spirorbis_, _340_, 341;
operculum, 261, 341;
genital organs, 273, 274;
brood-pouch, 261, 276;
fossil, 301; shell, 341
_Spirosperma_, _378_;
chaeta, 350
_Spirulaea_, 301
Sporocysts, 92; {557}
of _Distomum macrostomum_, 64, 65;
of _D. hepaticum_, 67;
hosts of, 71
Staggers, induced by _Coenurus_, 82
Statoblast, 493, 499, 501 f., 506;
sessile, 502;
germination, 501, 503, 514;
resemblance to ephippian ova, 493
Steenstrup, on Tape-worms, 76
_Steganoporella_, 530
Stelechopoda, 344
_Stelechopus_, _342_
_Stenostoma_, 44, _49_;
asexual reproduction, 44
_Stephanoceros_, 202, 205, 210, 213, 220, _221_
_Stephanops_, _225_
_Stercutus_, _376_
Sternaspidae, _258_, _335_;
nephridia of, 305
_Sternaspis_, _335_, 411, 445;
anatomy, 335, 336;
shape, 259;
shield, 259;
head, 264;
chaetae, 265;
gills, 268;
intestine, 271;
compared with Gephyrea, 336, 447, 449
_Sthenelais_, 299, 300, 309, _313_
_Stichostemma eilhardii_, 118
_Stilesia_, _91_;
generic characters, 90;
_S. centripunctata_, 91;
_S. globipunctata_, 91
Stock, asexual, of _Autolytus_, 279;
of _Myrianida_, 281
Stolc, on Oligochaeta, 360
Stolon, 480, 488, 518, 525
Stolonata, 518 n.
_Stomatopora_, _518_, 532
Stork, parasites of, 63, 163
Strobila, 75, 76
Strobilation, 76
Strodtmann, on Chaetognatha, 191
Stromatoporoids, 520
Strongylidae, 131, _142_
_Strongylus_, 129, _142_, 143, 160, 163;
_S. filaria_, 132;
_S. tetracanthus_, 163
Stuhlmann, on Polyzoa, 493
_Stuhlmannia_, 359, _386_
_Stylaria_, 348, _377_
Stylets of Nemertine proboscis, 104, 110
_Stylochoplana_, 18, _19_, 20
_Stylochus_, _19_;
development, 28
_Stylostomum_, _19_, 22
Sub-cuticle, 125, 175
Submalleate, 210, 211
_Succinea putris_, infested by larvae of _Distomum macrostomum_, 64, 66
Sucker, of _Leptoplana_, 8, 16 n.;
of Triclads, 35, 36;
of _Temnocephala_, 53, 54;
of Monogenea, 53, 56, 57, 60;
of Digenea, 62, 64, 65, 69;
of Cestodes, 75, 79;
of _Dinophilus_, 243;
of _Chaetopterus_, 324;
of _Myzostoma_, 342;
of larva of Polyzoa, 509, 511
Summer-eggs, of _Mesostoma_, 48;
of Rotifera, 216
_Sutroa_, 376, _380_
Swim-bladder, of Syllidae, 272
Swimming, of _Leptoplana_, 9, 10;
of Polyclads, 23;
of Rotifers, etc., 206, 235
Syllidae, 258, _306_;
palps, 260;
tentacles, 262;
head, 262;
parapodium, 264;
jaw, 270, 271;
alimentary tract, 271;
swim-bladder, 272;
asexual reproduction, 278 f., 279;
regeneration, 278, 283;
colours, 293;
phosphorescence, 296;
ancestral, 303
_Syllis_, 274, _307_;
development, 278;
_S. armillaris_, 307;
_S. ramosa_, 282;
_S. vivipara_, 276
_Synapta_, bearing Rotifers, 222, 227
_Synchaeta_, 200, 204 f., _224_, 226
Synchaetidae, _223_, _224_
_Syncoelidium_, 33, _42_
Syncytium, 125
_Syngamus trachealis_, 130, _142_, 144, 161, 163, 164
_Syrinx_, 411

_Taenia_, 74, 78, 79, _91_;
life-histories of species of, 83;
table of species, 89;
_T._ (_Cysticercus_) _acanthotrias_, 80;
_T. coenurus_, 81, 82;
life-history, 83;
specific characters, 90;
_T. crassicollis_, life-history, 78, 83;
specific characters, 89;
_T. echinococcus_, 80;
life-history, 83;
specific characters, 90;
_T. krabbei_, 81;
_T. marginata_, 81;
life-history, 83;
specific characters, 90;
_T._ (_Hymenolepis_) _murina_, 70, 80 n., 89;
life-history, 80, 83;
specific characters, 91;
_T. perfoliata_, 163;
_T. saginata_ (= _T. mediocanellata_), 78, 79;
life-history, 83;
specific characters, 89;
_T. serialis_, 82;
life-history, 83;
specific characters, 90;
_T. serrata_, 81;
life-history, 83, 87, 89;
specific characters, 90;
_T. solium_, 79;
life-history, 79, 83;
specific characters, 89
Taeniasis, 82
Taeniidae, _91_
Tail, of _Arenicola_, 333;
of _Nereis_, 246, 248;
regeneration of, 283
_Tanypus_, host of _Gordius_, 172
Tape-worms, 5, 74
_Taphrocampa_, 200, 204, _224_
Tardigrada, affinities, 344
_Telmatodrilus_, _378_
_Temnocephala_, 4, 53, 54, _73_
Temnocephalidae, _53_, _73_;
habits and structure, 53 f.;
affinities, 54
Tennent, on land-leeches, 408
Tentacles, in Polyclads, 15, 26;
in Triclads, 30, 36;
in _Vorticeros_, 45, 46;
in Trematodes, 53;
(peristomial), of Spionidae, 322;
(prostomial), 255, 260, 262 f.;
of _Nereis_, 248;
of _Polycirrus_, 294, 295;
of _Polygordius_, 244;
of Terebellids, use of, 289;
nerves to, 254
Tentacle-sheath, in Polyzoa, 470 {558}
Tentacular cirri = Peristomial cirri, _q.v._
Tentacular filaments, 304;
of Cirratulids, 326, 327
_Terebella_, _328_;
otocyst, 273;
fossil, 301;
_T. conchilega_, tube, 286, 287, 288;
building of tube, 289, 290;
gill, 329;
_T. nebulosa_, colour, 292;
as host, 311;
gill, 329
Terebellidae, _258_, _327_;
shape, 259;
tentacles, 263;
gill, 265;
chaetae, 266, 267;
gizzard, 271;
tube, 286;
use of tentacles, 289;
colour, 293;
phosphorescence, 296;
food of, 296;
tube containing Polynoid, 298
_Terebellides_, 299, _330_;
gill, 329
Terebelliformia, _258_, _325_;
definition, 304;
genital organs, 273;
gland shields, 287;
nephridia, 269;
nuchal organs, 273;
uncini, 266, 267;
vascular system, 252
_Terebripora_, 478
Terricola, 30, _42_
Tertiary, Polyzoa, 521
Tessin, on Rotifers, 198, 218
Testes, of _Leptoplana_, 14, 15;
of _Planaria lactea_, 38, 39;
of Acoela and Alloeocoela, 47;
of _Temnocephala_, 54;
of _Polystomum_, 57;
of Cestodes, 75, 76, 86—see also Reproductive organs
_Tetragonurus_, _389_
_Tetraonchus_, _73_
Tetraphyllidae, _91_
Tetrarhynchidae, _91_
_Tetrarhynchus_, 75, 76 n., 85, _91_
_Tetrastemma_, British species, 110;
land forms, 101, 115, 118;
fresh-water forms, 101, 118;
excretory system, 108, 109;
habits, 114;
hermaphrodite species, 109;
viviparous species, 117
_Thalamoporella_, 530
_Thalassema_, 411, 435 f., _441_, 443;
development, 439;
habits, 443
_Thelepus_, 299, _329_
_Theodisca_, _321_
Thompson, J. V., on term Polyzoa, 475
Thoracic membrane, 266, 305
Thorax, of Polychaeta, 259, 306, 337
_Thysanosoma_, _91_;
generic characters, 90;
_T. fimbriata_, life-history, 83;
specific characters, 90;
_T. giardii_, specific characters, 90
_Thysanozoon_, 13, 18, _19_, 20
Tomopteridae, 258, 291, _315_
_Tomopteris_, _315_;
colour, 294;
light-producing organ, 296;
prostomium, 259;
_T. rolasi_, 315
Tooth, in Polyzoa, 482, 522
Tortoise, _Temnocephala_ associated with, 53
Torus uncinigerus, 268
Tracks, fossil, 302
_Travisia_, _332_
_Travisiopsis_, _321_
Trematoda, 4, 51 f.;
life-histories, 71;
classification, 73
Trembley, on Turbellaria, 6;
on Polyzoa, 496, 497
Trepostomata, 520
_Triaenophorus_ (= _Tricuspidaria_), _91_;
excretory system, 84
_Triarthra_, 201, 203, 206, 211, _224_, 225, 226
Triarthridae, 200, 201, 202, 206, 207, _224_, 226
Trias, Serpulid in, 301
_Trichina_, 131, 135, _144_, 161;
_T. spiralis_, 146, 163
Trichinosis, 82, 146, 147
_Trichocephalus_, 131, 135, 136, _144_, 160, 163;
species of, 145;
_T. dispar_, 145
_Trichochaeta_, chaeta, 351
_Trichoderma_, _159_
_Trichoplax_, 93, 95
_Trichosoma_, _144_, 163;
species of, 145
Trichotrachelidae, _144_
Tricladida, _7_, 30 f.;
habits, 35 f.;
sexual reproduction, 38;
asexual reproduction, 40;
classification, 42;
British species, 31, 32, 34, 42
_Tricoma cincta_, _157_
_Tricuspidaria_, _91_
_Trigaster_, 359, _384_
_Trigonoporus_, _19_, 27
_Trinephrus_, 357, _382_ f.
_Triophthalmus_, _224_
_Triphylus_, _224_
_Tristicochaeta_, 158
Tristomatidae, _53_, 55, _73_
Tristomatinae, _73_
_Tristomum_, _73_
_Triticella_, 478, _518_, 533
_Trocheta_, 393, _407_
Trochophore, 229
_Trochopus_, _73_
_Trochosphaera_, 200, 201, _221_, 229, 230
Trochosphaeridae, _221_
Trochosphere, of Archiannelida, 243, 245;
of Polychaeta, 274, 275, 510, 512;
of Echiuroidea, 439, 510;
of Polyzoa, 510
Trochus, in Rotifers, 202, 204
Trophi, of Rotifers, 209, 210
_Trophonia_, 299, _334_;
genital organs, 273;
head, 262;
intestine, 271
Trunk, of _Nereis_, 246;
of Polychaeta, 259;
of Gephyrea, 412 f.
Tube, of Rotifers, 205;
of Polychaeta, 287 f.; composition of, 290;
fossil, 301, 302;
of Chaetopterus, 323;
of _Clymene_, 287;
of _Dodecaceria_, 326;
of _Eunice tibiana_, 290;
of Eunicidae, 285, 290, 318;
of _Haplobranchus_, 339;
of _Hekaterobranchus_, 326;
of _Hyalinoecia_, 290, 319;
of Maldanidae, 332;
of _Myxicola_, 285, 338;
of _Nereis_, 316;
of _Nicomache_, 287;
of _Onuphis_, 287, 319;
of _Owenia_, 325;
of _Pectinaria_, 285, 288, 330;
of _Polydora_, 323; {559}
of Polynoids, 285;
of _Panthalis_, 313;
of _Sabella_, 287 f.;
of _Sabellaria_, 287;
of Sabellidae, 337;
of Serpulidae, 290, 339 f., 340;
of Terebellidae, 286, 287, 288, 289, 327 f.;
of Priapuloidea, 433;
of _Echiurus_, 444
Tube-forming glands, 304
Tube-making, of Polychaeta, 287 f.
Tubicolous Polychaeta, 285, 300, 304, 306
_Tubifex_, 351, 367, 369, _378_;
chaetae, 350
Tubificidae, 350, 361, 366, _378_
_Tubulipora_, _518_, 531, 532
Turbellaria, 3 f.
Turtles, parasites of, 142
_Tylenchus_, 131, _154_, 155, 157, 160, 163
_Tylosoma_, 422, 423, _426_, _430_, 447
Typhloscolecidae, _258_, 291, _321_;
nuchal organ, 273 n.
_Typhloscolex_, 321
_Typosyllis_, regeneration of head, 283 n.

Udekem, D', on Oligochaeta, 365
_Udonella_, _73_;
_U. caligorum_, 55;
_U. pollachii_, eggs of, 58
Udonellinae, _73_
_Umbonula_, 531
_Uncinaria_, 143
Uncinate, 210, 211
Uncini, of Polychaeta, 266, 267, 304, 305
Uncus, 210
_Urnatella_, 490, 493, _518_
_Urobenus_, _388_
_Uteriporus_, _42_
Uterus, of _Leptoplana_, 8, 14;
of _Planaria_, 38, 39;
of Triclads, 40;
of Rhabdocoela, 48;
of _Temnocephala_, 54;
of _Polystomum_, 57, 59;
of Diplozoon, 60;
of _Didymozoon_, 71;
of _Calliobothrium_, 75;
of _Taenia_, 79;
of _Schistocephalus_, 86;
in Bothriidae, 87;
of Rotifers, 216

Vagina, of _Leptoplana_, 16;
in ectoparasitic Trematodes, 57 f.;
in Cestodes, 86, 87
Vaillant, on Hirudinea, 392, 405
_Valencinia_, _113_;
_V. lineformis_, _112_
_Valkeria_, 533
Vallentin, on Rotifers, 198
_Vallisnia_, _73_
_Vanadis_, _315_
Varme, 297
Vasa deferentia, of _Leptoplana_, 14, 15;
of _Planaria_, 38, 39;
of Acoela and Alloeocoela, 47;
of _Diplozoon_, 60;
of _Schistocephalus_, 86
Vasa efferentia, of _Leptoplana_, 14, 15;
of Triclads, 38
Vascular System, of Nemertinea, 106, 107;
of Archiannelida, 244;
of _Nereis_, 251 f.;
of Polychaeta, 251 f.;
of Cryptocephala, 252;
of Scoleciformia, 252;
of Terebelliformia, 252;
absence of, in certain Polychaeta, 253;
of Oligochaeta, 355;
of Leeches, 396;
of Gephyrea, 415, 436, 447;
of _Phoronis_, 455
_Vaucheria_, Rotifers in, 227
Vejdovsky, on Rhabdocoels, 46;
on _Gordius_, 164, 166;
on Oligochaeta, 365, 369, 374, 400
Vermes, 347
_Vermiculus_, _378_
Vermiformia, 461
Verrill, on Chaetognatha, 534
Vertebrates, parasites of, 163, 174, 179, 183
Verworn, on statoblasts, 501
Vesicula seminalis, of _Planaria_, 39
_Vesicularia_, 481, _518_, 533
Vesicularina, _518_, 523
Vestibule, 488, 490;
of larva, 509
Vibracular zooecium, 485, 486, 517, 524
Vibraculum, 477, 484, 485, 517, 524;
movements, 487;
function, 486
Vicarious avicularia, 482
_Victorella_, 492, 501, 505, _518_, 533
Villot, on life-history of _Gordius_, 172
_Vinella_, 521 n.
Virgate, 210
Visceral nervous system, of _Nereis_, 255
Vitellarium = Yolk-gland, _q.v._
Vitello-intestinal canal, in Polystomatidae, 57
Viviparous, Nemertinea, 109, 117;
Rotifers, 200, 216 f.;
Polychaeta, 256, 276
_Volvox_, Rotifers in, 227
Vortex, 44;
British species, _50_;
body-cavity, 43
Vorticellids, on Polychaeta, 299
_Vorticeros_, British species, 45, 46, _50_
Vorticidae, _50_
Vuillemin (misprinted in text), on Nematodes in deserts, 156

Walford, on Polyzoa, 521
Ward, on _Nectonema_, 168;
on _Sipunculus_, 417
Warning colours, in Polychaeta, 294, 314
Waters, on Polyzoa, 517
Wheeler, on Myzostomaria, 344
Whelk, shell occupied by _Nereis_, 298
White Cat, 317
Whitman, on Dicyemidae, 94;
on Hirudinea, 395 f., 402, 405 f.
Willemoes-Suhm, von, on _Tetrastemma agricola_, 101, 115, 117, 118
Willey, on affinities of Nemertinea, 120 n.;
on Oligochaeta, 382
Wings, of _Chaetopterus_, 295, 324
Winter-eggs, of _Mesostoma_, 48;
of Rotifers, 217;
compared with statoblasts, 493
Woodworth, on yolk-glands, 38 n.
Wreath, in Rotifers, 200
Wright, on _Phoronis_, 450, 456

Yellow-cells, in _Leptoplana_, 13 {560}
Yolk-gland, in _Planaria_, 38, 39;
in Rhabdocoelida, 47;
in _Temnocephala_, 54;
in Polystomatidae, 57;
in _Calliobothrium_, 75;
in _Schistocephalus_, 86;
in Rotifers, 199, 216
Yoruba Worm, 368, _387_
Youatt, on _Coenurus_, 82
_Yungia_, _19_, 25

Zebra, parasites of, 140
Zelinka, on Rotifers, 198, 215 n., 218, 219, 227, 229;
on Gastrotricha, 232
Zooecium, 466, 469, 474, 488, 523;
of Phylactolaemata, 495;
loss of zooecia, 481, (= calyces), 488;
primary, 506;
alterations with age, 522
Zone of budding, 279, 283
Zooid, sexual and asexual, 278 f.
Zoophytes, 465, 474

END OF VOL. II

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NOTES

[1] Hemprich and Ehrenberg, _Symbolae physicae_, Berlin, fol. 1831.

[2] Τρῆμα, a hole; referring to the orifices of the suckers.

[3] _Mémoires pour servir à l'histoire d. Polypes d'eau douce_, Leyden,
1744.

[4] _Die Parasiten des Menschen_, 1879——. Engl. Transl. by W. E. Hoyle,
i. 1886.

[5] Band 4, by M. Braun. (Mesozoa and Trematoda completed; Cestoda in
progress.)

[6] _Verm. terr. et fluv. ... succincta historia_, 1773; _Zool. Danica_,
1777.

[7] _Observations on Planariae_, Edinburgh, 1813.

[8] M. Faraday, "On the Planariae," _Medical Gazette_, Feb. 1832; and in
_Edinburgh New Philosoph. Journal_, vol. xiv. 1833, pp. 183-189.

[9] _Nov. Act. Acad. Caes. Leop.-Carol._ tom. xiii. 1827.

[10] _Ann. Sci. Nat._ (Zool.) I. tom. xv. 1828.; _ibid._ tom. xxi. 1830.

[11] _Mém. Acad. St. Pétersbourg_, 5th ser. tom. ii. 1832.

[12] _Die rhabdocoelen Turbellarien des Süsswassers._ Jena 1848.

[13] _Monographie d. Turbellarien._ I. Rhabdocoelida, 1882. Die Acoela,
Leipzig, 1892.

[14] "Die Polycladen," _Fauna u. Flora d. Golfes v. Neapel_, Monogr. XI.
1884.

[15] _Phil. Trans._ 1874, p. 105.

[16] Since no food, but only the pharynx, passes through this "mouth," the
term is unfortunate. Moreover the true mouth is the aperture placing
the stomach in communication with the pharynx (Fig. 5, _gm_).

[17] _Ann. Sci. Nat._ 1 sér. tom. xv. 1828, p. 146. "La Planaire
trémellaire ... peut parcourir ... en faisant battre rapidement ses
parties latérales à la manière des larges nageoires des Raies."

[18] _Observations on Planariae._ Edinburgh, 1813, p. 12.

[19] "Zur Anat. u. Entwickl. einiger Seeplanarien v. St. Malo," _Abh. K.
Gesellschaft d. Wiss._ Göttingen, 1868.

[20] The roof of the peripharyngeal chamber is hence known as the
"diaphragm."

[21] See Brandt, _Fauna u. Flora d. Golfes v. Neapel_, Monogr. XIII. 1885,
p. 65.

[22] See p. 94.

[23] _Verhandlungen d. med. Gesellschaft zu Würzburg_, iv. 1854, p. 223.

[24] _Enantia spinifera_ Grff. _Mittheil. d. Naturwiss. Verein. f.
Steiermark_, 1889.

[25] The sucker of _Leptoplana tremellaris_ probably does not correspond
with that of the Cotylea.

[26] Collingwood, _Trans. Linn. Soc._ 2 ser. vol. i. pt. 3, 1876, p. 83.

[27] Von Stummer-Traunfels, _Zeitschr. f. wiss. Zool._ Bd. lx. 1895, p.
689.

[28] _Planocera pellucida_ Mertens, _P. simrothi_ v. Grff., _P. grubei_
Grff., _Stylochoplana sargassicola_ Mertens, _S. californica_
Woodworth, _Planctoplana challengeri_ Grff., all belonging to the
Planoceridae. See v. Graff, "Pelagische Polycladen," _Zeitschrift f.
wiss. Zoologie_, Bd. lv. 1892, p. 190.

[29] _Cambridge Natural History_, vol. iii. p. 74.

[30] Lang, "_Polycladen_," p. 629.

[31] Wheeler, _Journal of Morphology_, vol. ix. part 2, 1894, p. 195.

[32] Many Nudibranchiate Mollusca undergo this change of habitat. See
Garstang, _Journal of the Marine Biological Assoc._ n.s. i. No. 4,
1890, p. 447.

[33] Chun, "Ctenophoren," _Fauna u. Flora G. v. Neapel_, Monogr. I. 1880,
p. 180.

[34] See Lang, "_Polycladen_," p. 607.

[35] Lang, "_Polycladen_," Pl. 30, Fig. 8.

[36] _Kongl. Fysiograf. Sällskapets Handlingar_, Bd. iv. Lund, 1892-93.

[37] Whitman, _Journal of Morphology_, vol. iv. 1890, p. 361.

[38] A full account of Polyclad development is contained in Lang's
"_Polycladen_," with references to the literature of the subject.
Since the date of that work (1884) the embryology of Ctenophora has
become better known, but, though the segmentation of the egg and
early stages of development are very similar in both cases, the
elaborate investigations of E. B. Wilson (_Journ. Morphology_, vol.
vi. p. 361) show that the segmentation of Polychaet worms is again
similar. The question of the affinities of the Polycladida is also
discussed by Lang ("_Polycladen_" p. 642 _et seq._). The work of the
last decade has neither proved nor disproved his suggestion that the
Ctenophores and Polyclads have been derived from common ancestors. On
this subject the remarks made by Hatschek (_Lehrbuch d. Zoologie_, p.
319) are some of the weightiest that have appeared.

[39] Hallez, _Revue Biologique du Nord de la France_, tom. ii. 1889-90.

[40] Voigt, _Zool. Anz._ xv. p. 238.

[41] Grube, _Archiv f. Naturgeschichte_, 38 Jahrg. Bd. i. 1872, p. 273.

[42] Vejdovsky, _Zeitschr. f. wiss. Zoologie_, Bd. lx. 1895, p. 200.

[43] Woodworth, _Bulletin Mus. Comp. Zoology, Harvard_, vol. xxi. No. 1,
1891.

[44] _Mitth. Zool. Stat. Neapel_, 1882, p. 187.

[45] Wheeler, _Journal of Morphology_, vol. ix. 1894, p. 167.

[46] Dendy, _Trans. Roy. Soc. Victoria_ 1890, p. 65; Id. _Austral. Assoc.
Brisbane_, 1895, "Presid. Add. to Sect. D," p. 15.

[47] Darwin, _Ann. and Mag. Nat. Hist._ vol. xiv. 1844, p. 241.

[48] Shipley, _Proc. Camb. Phil. Soc._ vol. vii. pt. 4, 1891 (with
literature).

[49] _Trans. Roy. Soc. Victoria_ from 1889 onwards. _Trans. New Zealand
Institute_, 1894-95.

[50] Moseley, _Phil. Trans._ 1874, p. 105; Id. _Quart. Journ. Micr. Sci._
vol. xlvii. 1877, p. 273; Loman, _Bijdrag tot d. Dierkunde_, Aflev.
14, 1887, p. 71; Id. _Zool. Ergeb. ein. Reise in Nieder-Ost-Indien_,
Hft. 1, p. 131; Beddard, _Zoogeography_, 1895, p. 53.

[51] _Beobachtungen ü. Anat. u. Entwickel. an der Küste von Normandie_,
1863, p. 18.

[52] _Archiv f. Naturgeschichte_, 57 Jahrg. Bd. i. Hft. 3, 1891, p. 308.

[53] Dendy, _Proc. Roy. Soc. Victoria_, vol. iv. n.s. i. 1892.

[54] Schmarda, _Neue wirbellose Thiere_, Leipzig, 1859, I. i. p. 30.

[55] _Abhandl. d. Naturf. Gesell. zu Halle_, Bd. iv. 1857, p. 33.

[56] _Arb. Zool.-Zoot. Instit. Würzburg_, Bd. v. 1882, p. 120.

[57] Woodworth (_loc. cit._ p. 38) states that in _Phagocata_ the
yolk-glands arise by proliferation from two parovaria, placed just in
front of the ordinary ovaries. Iijima, however (_Zeitschr. f. wiss.
Zool._ Bd. xl. 1883, p. 454), regarded them as derivatives of the
parenchyma.

[58] The extensive literature on this subject is fairly completely
summarised by Voigt in _Biol. Centralblatt_, vol. xiv. Nos. 20, 21,
1894. Faraday's observations (cf. p. 6, note 8) have been generally
overlooked.

[59] _Archives d. Biologie_, tom. xii. 1892, p. 437.

[60] _Mitth. Zool. Stat. Neapel_, Bd. iii. 1882, p. 187.

[61] Böhmig, _Ergebnisse d. Plankton Expedition_, Bd. ii. H. g. 1895.

[62] von Graff, _Die Acoela_, Leipzig, 1892. Appendix.

[63] The development of the _Acoela_ has been worked out recently by
Mdlle. Pereyaslawzewa (_Zapiski Novoross. Obshch. Odessa_, 17 Bd.
1892) and Gardiner (_Journal of Morphology_, xi. No. 1, 1895, p. 155)
with conflicting results. The former finds four endoderm cells, which
give rise to a larval intestine. The _Acoela_ are for her,
_Pseudacoela_. Gardiner, on the other hand, finds no trace of an
endoderm at any stage of the development of _Polychoerus caudatus_.

[64] _Tijdschr. Nederland. Dierk. Ver._ Deel ii. 1875.

[65] Von Graff, _Monographie d. Turbellarien: I. Rhabdocoeliden_, 1882.
Gamble, _Quart. Journ. Microscop. Science_, vol. xxxiv. 1893, p. 433.

[66] _Zeitschr. f. wiss. Zoologie_, Bd. lx. 1895, p. 163.

[67] See von Graffs _Monographie_, pl. ix.; and Jensen, _Turbellaria ad
Litora Norvegiae_, Bergen, 1878, pl. iv.

[68] For the reproductive organs of Rhabdocoelida, consult von Graff,
_Monographie_, "Die Acoela"; and Böhmig, _Zeitschr. f. wiss. Zool._
Bd. li. 1891, p. 167.

[69] _Untersuchungen ü. Platyhelminthen_, Giessen, 1873, p. 101.

[70] Compare the remarks on Trematodes, pp. 4-5.

[71] Haswell, _Monograph of the Temnocephaleae_. Macleay Memorial Volume.
Mem. iii. 1893.

[72] Braun, in Bronn's _Klassen u. Ordn. d. Thierreichs_, vol. iv. p. 407,
gives a valuable summary of our knowledge of this group. For figures,
see van Beneden and Hesse, _Mémoires de l'Acad. roy. de Belgique_,
tom. xxxiv. 1864, pp. 1-169. A valuable paper (with synoptic tables)
on Japanese Monogenea, by Goto, _Journ. Coll. Sci. Japan_, vol. viii.
pt. 1, 1894, has recently appeared.

[73] See Leuckart, "_Parasiten_" Bd. ii. p. 238.

[74] Zeller, _Zeitschr. f. wiss. Zool._ xxii. 1872, pp. 1, 168; also Bd.
xxvii. 1876, p. 238; xlvi. 1888, p. 233.

[75] An excellent and beautifully illustrated account, by Looss, of the
Distomatidae of Frogs and Fishes may be found in Leuckart and Chun's
_Bibliotheca Zoologica_, Heft 16, 1894.

[76] Leuckart, _Parasiten d. Menschen_, "Trematoden," 1892-94; R.
Blanchard, _Traité d. Zool. médicale_, i. 1889; H. B. Ward, _Report
for 1894 of Nebraska State Board of Agric. Lincoln_, U.S.A. 1895, p.
225.

[77] Huxley, _Anat. of Invert. Animals_, 1877, p. 194.

[78] Braun, Bronn's _Thierreichs_, Bd. iv. p. 792; Leuckart, _Parasiten d.
Menschen_, 11 Abth. p. 158; Brandes, in _Spengels Zool. Jahrb. Syst.
Abtheil._ Bd. v. 1890, p. 849; v. Nordmann, _Mikr. Beitr._ i. Berlin,
1832.

[79] Heckert, _Bibliotheca Zoologica_ (Leuckart and Chun), Heft 4, 1889. I
am not aware that _Leucochloridium_ has been noticed in England.

[80] "Heterogamy" usually means the alternation of bisexual and unisexual
generations (e.g. _Rhabdonema nigrovenosum_), but is, unfortunately,
also used in the sense of Alloiogenesis, as defined above. See
Grobben, _Arbeit. Zool. zoot. Ints. Wien_, Bd. iv. 1881, p. 201.

[81] _Parasiten_, Bd. i. Abth. II. p. 152.

[82] _Festschrift f. Leuckart_, Leipzig, 1892, p. 167.

[83] _Quart. Journ. Micros. Sci._ vol. xxiii. 1883, p. 90.

[84] The intermediate host in the Sandwich Islands is said to be _Limnaea
peregra_. See Lutz, _Centralbl. f. Bakter._ xi. 1892, p. 783.

[85] The mortality in wet years, however, is said to be largely due to
pulmonary inflammation. This and other causes of death are not always
discriminated in the returns.

[86] See Thomas, _Quart. Journ. Micros. Science_, xxiii. 1883. Neumann,
_Parasites of Domesticated Animals_, translated by Fleming, 1892.

[87] Leuckart, _loc. cit._; Looss, _Archiv f. mikroskop. Anatomie_, Bd.
xlvi. 1895, p. 1.

[88] In Leuckart, _Die Parasiten d. Menschen_, pp. 521-528, 1894.

[89] Cf. p. 89.

[90] See Braun. Bronn's _Klassen u. Ordnungen d. Thierreichs_, vol. iv. p.
572.

[91] Braun, _loc. cit._ p. 573.

[92] Taken largely from Braun, _Ibid._ pp. 864-866, where the literature
of the subject is referred to fully.

[93] _Festschr. f. Leuckart_, 1892, p. 134.

[94] Cf. p. 5.

[95] _Arbeit. Inst. Wien_, iii. 1881, p. 163; see also _ibid._ ix. 1890,
p. 57.

[96] For figures of various scolices see van Beneden, _Mémoire sur les
vers Intestinaux_, 1861; Braun in Bronn's _Thierreich, Cestoda_ (in
progress), Bd. iv. Pl. xxxviii.-xlv.

[97] The mature proglottis of _Calliobothrium eschrichti_ is 8-9 mm. long,
whereas the strobila only measures 4-5 mm. in length. Species of
_Phylliobothrium_, _Anthobothrium_, and _Tetrarhynchus_ show a
similar but not an equal contrast between the size of the parent and
proglottis (P. J. van Beneden, "Les Vers Cestoides," _Nouv. Mém. de
l'Acad. Roy. d. Belgique_, tom. xxv. 1850).

[98] The difficult question of the nature of the Cestode body and Cestode
larvae is adequately discussed by Braun, _loc. cit._ p. 1167.

[99] Leuckart, _Die Parasiten d. Menschen_ [English trans. by W. E.
Hoyle]; Blanchard, _Traité de Zoologie médicale_, 1893.

[100] For a full account of the history of this subject see Leuckart,
_Parasiten d. Menschen_, p. 28; Braun, _loc. cit._ Bd. iv. p. 929 _et
seq._; Huxley, _Collected Essays_, vol. viii. p. 229.

[101] By Grassi this form is considered identical with _T. murina_. The
latter species is known, from this author's researches, to develop in
rats without migration into an intermediate host. Should Grassi's
synonymy prove correct, the presence of large numbers of this
tape-worm in man would readily receive its explanation.

[102] Leuckart, _loc. cit._ p. 752 _et seq._

[103] The distinctive features of these and the foregoing tape-worms are
given on pp. 89-90

[104] For description of the _Cercocystis_-larva see Villot, _Ann. Sci.
Nat. (Zool.)_ (6), xv. 1883, Art 4; and compare Leuckart's criticism
of this paper, "_Parasiten_," p. 979.

[105] Moniez, "Sur les Cysticerques," Paris, 1880; _Id._ "Sur les
Cestodes," 1881; Zschokke, "Recherches sur la structure anatomique et
histologique d. Cestodes," Genève, 1888.

[106] Schmidt, _Archiv f. Naturgeschichte_, Jahrg. lx. Bd. 1, 1894, p. 65.

[107] For example, the genitalia in _Dipylidium caninum_ are duplicated in
each proglottis. Other differences are noted in the following table
(pp. 89-90).

[108] See Stiles, _Centralbl. f. Bakt. u. Parasitenkunde_, 1893, xiii. p.
457 (conf. note, p. 90).

[109] Taken from Neumann, _Parasites of Domesticated Animals_, 1892, p.
448.

[110] µ = 1/1000 millimetre.

[111] For a description of these glands, and for further diagnostic details
and literature, see Stiles and Hassall, _U. S. Department of
Agriculture, Bureau of Animal Industry_, Bulletin 4, 1893.

[112] Ed. van Beneden, _Bull. Acad. Roy. Belgique_, 1876, p. 35.

[113] Whitman, _Mittheil. Zool. Stat. Neapel_, Bd. iv.; see also Braun, in
Bronn's _Thierreich_, Bd. iv. p. 253.

[114] Braun, _loc. cit._ p. 281 (with literature).

[115] Giard, "La Castration parasitaire," _Bull. Sci. d. France et de
Belgique_, 3 sér. i. 1888, p. 12.

[116] Schulze, _Abh. Akad. Berlin_, 1891, p. 1.

[117] _Arch. Naturg._ lviii. 1891, p. 66.

[118] _P. Boston Soc._ vol. vi. 1848.

[119] _Zeitschr. wiss. Zool._ Bd. v. 1854, p. 344.

[120] _Arch. Anat._ 1858, p. 289.

[121] _Ibid._ 1858, p. 558.

[122] _Mem. Ac. St. Petersb._ ser. vii. tom. xiv. 1869.

[123] _Zeitschr. wiss. Zool._ Bd. xliii. 1886, p. 481.

[124] R. von Willemoes-Suhm, _Ann. Nat. Hist._ ser. iv. xiii. 1874, p. 409.

[125] Semper, _Zeitschr. wiss. Zool._ Bd. xiii. 1863, p. 558.

[126] L. von Graff, _Morphol. Jahrb._ Bd. v. 1879, p. 430.

[127] W. A. Silliman, _Zeitschr. wiss. Zool._ Bd. xli. 1885, p. 48.

[128] du Plessis, _Zool. Anz._ vol. xv. 1892, p. 64.

[129] J. von Kennel, _Arb. Inst. Würzburg_, Bd. iv. 1877-78, p. 305.

[130] H. N. Moseley, _Ann. Nat. Hist._ ser. iv. vol. xv. 1875, p. 165.

[131] See Hubrecht, in _Verh. Ak. Amsterdam_, vol. xx. 1880; and in _Quart.
J. Micr. Sci._ vol. xx. 1880, p. 431.

[132] "Nemertinen," _Fauna und Flora G. von Neapel_, 22 Monogr. 1895.

[133] _Beiträge zur Naturgeschichte der Turbellarien_, Griefswald, 1851.

[134] _Arb. Inst. Würzburg_, Bd. iii. 1876, p. 115.

[135] _Ibid._ Bd. iv. 1877, p. 305.

[136] _Zool. Anz._ vol. viii. 1885, p. 51.

[137] _Quart. J. Micr. Sci._ vol. xxv. 1885, suppl. p. 1.

[138] _Zeitschr. wiss. Zool._ Bd. xli. 1885, p. 48.

[139] _Ibid._ Bd. liii. 1892, p. 322, and _Fauna und Flora G. von Neapel_,
22 Monogr. 1895.

[140] _Ann. Sci. Nat._ (5) vol. xvii. 1873.

[141] _Zeitschr. wiss. Zool._ Bd. iv. 1853, p. 178.

[142] Our knowledge of British species is mainly due to M‘Intosh (_British
Annelids_, Ray Society, 4to, 1873) and Riches (_Journ. Mar. Biol.
Ass._ vol. iii. 1893-1895, p. 1).

[143] _Fauna und Flora G. von Neapel_, 22 Monogr. 1895.

[144] See M‘Intosh, _British Annelids_, Ray Society, 4to, 1873.

[145] _Loc. cit._

[146] References to these works are given on p. 101.

[147] _Zitschr. wiss. Zool._ Bd. xli. 1885, p. 48.

[148] _Nature_, vol. xlvi. 1892, p. 611.

[149] _Zool. Anz._ vol. xv. 1892, p. 64.

[150] _Zeitschr. wiss. Zool._ Bd. lix. 1895, p. 83.

[151] _Arb. Inst. Würzburg_, Bd. iv. 1877-1878, p. 305.

[152] _Journ. Mar. Biol. Ass._ vol. iii. 1893-1895, p. 22.

[153] _Quart. J. Micr. Sci._ vol. xxiii. 1883, p. 349; _Ibid._ vol. xxvii.
1887, p. 605.

[154] Cf. Willey, _Amphioxus and the Ancestry of the Vertebrates_,
Macmillan, 1894.

[155] _Ann. Sci. nat._ 7, sér. vol. xiii. 1892, p. 321.

[156] E. Rohde, _SB. Ak. Berlin_, 1892, p. 515.

[157] R. Hesse, _Zeitschr. wiss. Zool._ Bd. liv. 1892, p. 548.

[158] E. Rohde, _Zool. Beitr._ Bd. i. 1885, p. 11.

[159] E. Rohde, _Zool. Anz._ xvii. 1894, p. 38.

[160] _Monographie der Nematoden_, 4to, Berlin, 1866.

[161] _Zeit. Physiol. Chem._ vol. xiv. 1890, p. 318.

[162] N. A. Cobb, _P. Linn. Soc. N.S. Wales_, 2nd ser. vol. vi. 1891, p.
143.

[163] _Monographie der Nematoden_, Berlin, 1866, p. 192.

[164] _Zool. Anz._ vol. xvi. 1893, p. 432.

[165] _Arch. Naturg._ 60 Jahrg. Bd. i. 1894, p. 255.

[166] _SB. Ak. Berlin_, 1891, p. 57.

[167] [Hamann subsequently withdrew these statements.]

[168] Leuckart, _The Parasites of Man_, English Trans. by W. E. Hoyle,
Edinburgh, 1886, p. 56.

[169] O. Bütschli, _Zeitschr. wiss. Zool._ Bd. xxvi. 1876, p. 103.

[170] O. Hamann, _Centrlb. Bakter._ vol. xi. 1892, p. 501.

[171] _Zeitschr. wiss. Zool._ vol. xxiii. 1873, p. 402.

[172] _Ibid._ vol. xlii. 1885, p. 708.

[173] _Ann. Nat. Hist._ 5th ser. vol. ix. 1882, p. 301.

[174] _Macleay Memorial Volume_, Sydney, 1893, p. 252; and _Proc. Linn.
Soc. N.S.W._ 2nd ser. vol. v. 1890, p. 449.

[175] 4th edition, 1880.

[176] _Compendium der Helminthologie_, Hannover, 1878, and _Nachtrag_,
1889.

[177] A. Heller, "Darmschmarotzen" in v. Liemssen's _Handb. d. sp. Path. u.
Ther._ vol. vii.

[178] Cobbold's _Parasites_, London, 1879, p. 246.

[179] _Arch. Zool. exper._ 1 sér. tom. vii. 1878, p. 283.

[180] _Arch. Zool. exper._ 1 sér. tom. vii. 1878, p. 283.

[181] Balbiani, _Anat. Physiol._ 7th year, 1870-71, p. 180.

[182] _A Treatise on Parasites and Parasitic Diseases._ English Trans. by
G. Fleming, London, 1892.

[183] _Journ. Roy. Agric. Soc._ 3rd series, vol. iv. 1893.

[184] _Sci. Mem. Medic. Officers, Army of India_, vol. vii. 1892, p. 51.

[185] Shipley, _Proc. Phil. Soc. Camb._ vol. viii. 1892-95, p. 211.

[186] "The Distribution, etc., of _Filaria sanguinis hominis_," _Trans. of
7th Inter. Congress of Hygiene_, vol. i. 1892, p. 79.

[187] v. Linstow, _Arch. mikr. Anat._ vol. xl. 1892, p. 498.

[188] zur Strassen, _Zeitschr. wiss. Zool._ vol. liv. 1892, p. 655.

[189] Rud. Leuckart, _Abh. Sachs. Ges._ vol. xiii. 1887, p. 567.

[190] _Macleay Memorial Vol._ Sydney, 1893, p. 253.

[191] _Centrbl. Bakter._ vol. viii. 1890, p. 489.

[192] J. Percival, _Nat. Sci._ vol. vi. 1895, p. 187.

[193] A. Strubell, _Bibl. Zool._ Bd. i. Heft 2, 1888, p. 1.

[194] _C. R. Ac. Sci._ cxviii. 1894, p. 549.

[195] _Bihang Svenska Ak. Handl._ viii. No. 11, 1883.

[196] _Anat. Untersuch. ü. freilebende Nordsee-Nematoden_, Leipzig, 1886.

[197] Cobb, _P. Linn. Soc. N. S. Wales_, 2nd ser. viii. 1893, p. 389.

[198] _Zeitschr. wiss. Zool._ Bd. xvii. 1867, p. 539.

[199] Panceri, _Atti Acc. Napoli_, vii. 1878, No. 10.

[200] Panceri, _Atti Acc. Napoli_, vii. 1878, No. 10.

[201] _Arch. Naturg._ 35 (i.), 1869, p. 112.

[202] _Zeitschr. wiss. Zool._ Bd. xlii. 1885, p. 708.

[203] _Arch. Naturg. Jahrg._ iii. Bd. i. 1837, p. 52; and van Beneden,
_Animal Parasites_, p. 91. International Sci. Series.

[204] F. Vejdovsky, _Zeitschr. wiss. Zool._ Bd. xliii. 1886, p. 369;
_Zeitschr. wiss. Zool._ Bd. xlix. 1888, p. 188.

[205] _Arch. mikr. Anat._ Bd. xxxvii. 1891, p. 239.

[206] _Arch. mikr. Anat._ Bd. xxxiv. 1889, p. 248.

[207] A. E. Verrill, _P. U. S. Mus._ vol. ii. 1879, p. 165.

[208] O. Bürger, _Zool. Jahrb. Anat._ Bd. iv. 1891, p. 631.

[209] H. B. Ward, _Bull. Mus. Harvard_, vol. xxiii. 1892-93, p. 135.

[210] _Mem. Acc. Torino_, 2nd ser. vol. xl. 1890, p. 1.

[211] _Centrlb. Bakter._ Bd. ix. 1891, p. 760.

[212] _Zeitschr. wiss. Zool._ Bd. vii. 1856, p. 1.

[213] _Zool. Anz._ vol. x. 1887, p. 602.

[214] Von Linstow, Hannover, 1878, and Nachtrag, 1889.

[215] H. B. Ward, _P. Amer. Ac._ new ser. vol. xix. 1892, p. 260.

[216] _Jen. Zeitschr._ Bd. xxv. 1891, p. 113.

[217] _Bibl. Zool._ Bd. ii. Heft 7. 1893.

[218] _Jen. Zeitschr._ Bd. xxv. 1891, p. 113.

[219] _Zool. Anz._ Bd. xv. 1892, p. 195.

[220] _Zool. Anz._ vol. xv. 1892, p. 52.

[221] _Zool. Anz._ vol. viii. 1885, p. 19.

[222] Shipley, _Quart. J. Micr. Sci._ vol. xxxix. 1896.

[223] O. Hertwig, _Jen. Zeitschr._ Bd. xiv. 1880, p. 196.

[224] P. Gourret, _Ann. Mus. Marseille_, tom. ii. Mem. 2, 1884, p. 103.

[225] _Bibl. Zool._ vol. i. 1888-89, p. 1.

[226] Scott, _Annals of Scottish Natural History_, 1892 and 1893.

[227] E. Béraneck, _Rev. Zool. Suisse_, vol. iii. 1895, p. 137.

[228] _Ann. Mag. Nat. Hist._ 6th ser. vol. xiii. 1894, p. 440.

[229] _Archiv Naturg._ 58 Jahrg. Bd. i. 1892, p. 333.

[230] I Chetognati, _Flora u. Fauna d. Golfes von Neapel_, Mon. v. 1883.

[231] _loc. cit._

[232] _loc. cit._

[233] _The Rotifera_, two vols, and supplt. London, 1886-89.

[234] _Phil. Trans._ vol. xix. No. 220, p. 254 (abridged ed. vol. iii.
1705, p. 651).

[235] _Ibid._ vol. xxiii. No. 283, p. 1304 (abridged ed. vol. v. p. 6).

[236] _Ibid._ vol. xxiii. No. 295, p. 1784 (abridged ed. vol. v. p. 175).

[237] _Ibid._ No. 337, vol. xxviii. 1714, p. 160.

[238] _Employment for the Microscope._ London, 1785.

[239] Paris, 1841.

[240] _Quart. Journ. Micr. Sci._ vol. i. 1853, pp. 3-8, 65-76.

[241] _Trans. Micr. Soc. London_, vol. i. (n.s.), 1853, pp. 1-19.

[242] _Verh. Ges. Würzb._ vol. iv. 1854; _Zeitschr. wiss. Zool._ vols. iii.
vi. 1851-55.

[243] _Zeitschr. wiss. Zool._ vols. vii. ix. xii. 1856-58-63.

[244] London, 1861.

[245] _Zeitschr. wiss. Zool._ vol. xxxix. 1883.

[246] _Ann. Nat. Hist._ ser. 6, vol. v. 1890, p. 1; viii. 1891, p. 34.

[247] _Jen. Zeitschr. Nat._ vol. xix. 1886; and _Zeitschr. wiss. Zool._
vols. xliii. xlix. 1886-90.

[248] _Zeitschr. wiss. Zool._ vol. xliv. 1886, p. 273.

[249] _Ibid._ vol. xliv. p. 396; xlvii. 1888, p. 353; liii. 1892, p. 1.

[250] For additions see Rousselet, _J. Roy. Micr. Soc._ 1893 and 1897.

[251] See p. 228.

[252] _Quart. Journ. Micr. Soc._ (n.s.) vol. xxiv. 1884, p. 352.

[253] The definition of the Orders and systematic position of the genera
and species referred to under this head will be found in a following
section (pp. 220 f.).

[254] Reprinted in Baker's _Employment for the Microscope_, 1785, pp. 267
f.

[255] "_Wheel Animals_, though found with most Certainty in Leaden Gutters,
etc. are often discovered in the Waters of some Ditches, and likewise
in Water that has stood a considerable Time even in the House; for I
have often met with them, in sufficient Plenty, in a Sort of slimy
Matter that is apt to be produced on the Sides of Glasses and other
Vessels, that are kept long with the _Infusions of Hay_ or other
_Vegetables_; and probably they are wafted thither by the Air, when
in the Condition of little dry Globules."

[256] Gosse's account of the "Structure, Functions, and Homologies of the
Manducatory Organs in the Class Rotifera" (in _Phil. Trans._ 1856)
remains as the most complete anatomical account we have, though his
attempt to identify these parts with the modified limbs of the
Arthropod mouth has met with no support from subsequent workers.
Gosse rendered these parts clearly visible by the use of dilute
caustic alkali.

[257] A modification of this type is seen in the parasite _Drilophagus_,
where the unci and rami are two-pronged at the end, but the trophi
are not movable on one another, but protrusible as a whole to serve
as an organ of attachment to the Oligochaete _Lumbriculus_, to which
this Rotifer attaches itself. See Vejdovsky, "Ueb. _Drilophaga
bucephalus_," etc., in _SB. Böhm. Ges. Jahrg._ 1882 (1883), p. 390.

[258] "Zur Rotatorien Württemburgs," in _Jahresb. Ver. Würt._ vol. l. 1894,
p. 57.

[259] Similarly Hudson and Zelinka both regard the dorsal antenna as formed
by the coalescence of two antennae. These retain their distinctness
in _Asplanchna_; in some Bdelloida the single antenna is supplied by
a pair of nerves.

[260] _C. R. Ac. Sci._ cxi. 1890, p. 310; cxiii. 1891, p. 388.

[261] _Acta Univ. Lund._ xxviii. 1891-92.

[262] [See, however, Calman, _Natural Science_, xiii. 1898, p. 43.]

[263] _Zeitschr. wiss. Zool._ xxii. 1872, p. 455.

[264] _Arch. Zool. Exp._ sér. 2, i. 1883, p. 131.

[265] _Zeitschr. wiss. Zool._ xliv. 1886, p. 273.

[266] _Ibid._ liii. 1892, p. 1.

[267] [See further Jennings, _Bull. Mus. Harvard_, xxx. 1896, p. 1;
Erlanger and Lauterborn, _Zool. Anz._ xx. 1897, p, 452; and Lenssen,
_Zool. Anz._ xxi. 1898, p. 617.]

[268] It does not appear to us that Zelinka is justified by his account of
the development in regarding this cup as other than a part of the
disc.

[269] The classification we have adopted is a modification of that made by
Hudson and Gosse; we have divided up their first Order Rhizota into
two, and split off from Flosculariidae the family Apsilidae; removed
the Asplanchnaceae from the admittedly heterogeneous Order Ploima,
made distinct families in the Ploima for Microcodonidae and
Rhinopidae, and created a third new Order for the Seisonaceae.
Ehrenberg, Gosse, and Hudson, being the authors of most of the
genera, are designated by their initials only.

[270] This second species has also been found in the Northern United
States.

[271] This Order has been monographed recently by Janson in _Abh. Ver.
Brem._ Bd. xii. Beilage, 1893, p. 1.

[272] See Hudson in _Month. Micr. Journ._ vol. vi. 1871, pp. 121, 215, and
_Quart. Journ. Micr. Sci._ (n.s.) xii. 1872, p. 333; Lankester,
_ibid._ p. 338; Levander in _Act. Soc. Faun. Fenn._ xi. 1894.

[273] In _Denk. Ak. Wien_, vol. vii. 1854, 2 Abth., p. 15. As has been
suggested by Deby and by Daday, it is not impossible that _Hexarthra_
is identical with _Pedalion_ (and in this case the latter name, as
newer, should be suppressed in favour of the former); but we must
suppose that Schmarda's figure of the front view is a combination,
more or less from memory or notes, of two sketches or notes taken
some time before publication; the one a side view somewhat obliquely
flattened, showing the two eyes as in Levander's Fig. 3; the other a
front view, showing the two pairs of lateral limbs in their correct
positions under pressure.

[274] The male of _Rhinops vitrea_ is exceptional in possessing a complete,
functional alimentary canal, with mastax, stomach, and intestine
(Rousselet). That of _Proales werneckii_ has a mastax, but no
intestine (Rothert).

[275] For a full account of this group see Claus in _Festschr. Z.-B. Ges.
Wien_, 1876, p. 75; and Plate in _Mt. Stat. Neapel_, vol. vii.
1886-87, p. 234; _Ann. Nat. Hist._ ser. 6, vol. ii., 1888, p. 86.

[276] [Eighteen more have since been recorded.]

[277] I have recently found a large species of this genus dwelling in the
shell of the large Cladoceran Crustacean, _Eurycercus lamellatus_. It
is remarkable for its power of completely telescoping its extremities
within the middle segments, and for its immense foot-glands, both
characters being doubtless correlated with its habitat. Rousselet
identifies it with _P. petromyzon_.

[278] _Month. Micr. Journ._ vol. ix. 1873, p. 287; _Journ. Quekett Club_,
ser. 2, vol. ii. 1884-86, p. 231.

[279] See Dr. Hudson's very suggestive presidential addresses to the Royal
Microscopical Society, published in their Journal, vols. ix.-xi.
1889-91.

[280] _Euchlanis lynceus._—This is clearly not an _Euchlanis_, and of the
six names referred to—_Ploesoma_, _Gomphogaster_, _Gastropus_,
_Gastroschiza_, _Bipalpus_, and _Dictyoderma_—the first has priority,
and the other five drop by the laws of zoological nomenclature.

[281] _Journ. Quekett Club_, ser. 2, vol. v. 1892-94, p. 205.

[282] _Trans. Micr. Soc._ (n.s.) i. 1853, p. 18 (read Dec. 31, 1851): "We
may say, therefore, that the Rotifera are organized upon the plan of
an Annelid larva.... I do not hesitate to draw the conclusion ...
that _the Rotifera are the permanent forms of Echinoderm larvae_, and
hold the same relation to the Echinoderms that the Hydriform Polypi
hold to the Medusae, or that _Appendicularia_ holds to the
Ascidians."

[283] _Quart. Journ. Micr. Sci._ (n.s.) vol. xvii. 1877, p. 399.

[284] _Ibid._ (n.s.) vol. xx. 1880, p. 381.

[285] _Arb. Z. Inst. Wien_, vols. i. iii. v. 1878-84; _Lehrbuch der
Zoologie_, part iii. 1891.

[286] _Zeitschr. wiss. Zool._ vol. xliv. 1886, p. 1.

[287] _The Microscope_ (Detroit), 1887-88.

[288] _Zeitschr. wiss. Zool._ xlix. 1890, p. 209.

[289] _Ann. Sci. Nat._ ser. 3, vol. xv. 1851, p. 158.

[290] _Zeitschr. wiss. Zool._ vol. xlv. 1887, pp. 401-467, t. xx-xxii.

[291] The breadth of the latter is estimated from Reinhard's figure.

[292] The Echiuroid Gephyrea (see p. 434) are by some authorities
considered to be a division of the Chaetopoda.

[293] Another worm, _Histriobdella_ (_Histriodrilus_) _homari_, which is
parasitic on the eggs of the lobster, and which occurs on our coast,
has been placed amongst the Archiannelida. It is a minute form, with
peculiarities in its anatomy which render its affinities uncertain.

[294] _Quart. J. Micr. Sci._ xxvii. 1887, p. 109.

[295] _J. Mar. Biol. Assoc._ vol. i. (n.s.) 1889-90, p. 119.

[296] Schimkewitsch, _Zeitschr. f. wiss. Zool._ lix. 1895, p. 46.

[297] Hatschek, _Arb. Zool. Inst. Wien_, iii. 1881, p. 79.

[298] Fraipont, "Le Genre Polygordius," _Fauna u. Flora des Golfes v.
Neapel_, Monogr. xiv. 1887.

[299] T. J. Parker, _Lessons in Elementary Biology_, London, 1891, p. 267,
gives a full account of the anatomy and development of _Polygordius_.

[300] "Die Capitelliden," _Fauna u. Flora d. Golfes v. Neapel_, Monogr.
xvi. 1887, p. 350.

[301] _Encyclopaedia Britannica_, 9th ed., Art. "Mollusca," p. 652.

[302] Benham, "The Post-Larval Stage of _Arenicola_," _J. Mar. Biol.
Assoc._ iii. (n.s.) 1893, p. 48.

[303] The blood is colourless in Syllidae and Nephthydidae.

[304] Ehlers states that some Eunicidae have green blood.

[305] Benham, _Quart. J. Micr. Sci._ xxxix. 1896, p. 1.

[306] Schaeppi, _Jena. Zeit._ xxviii. 1894, p. 217.

[307] Goodrich, _Quart. J. Micr. Sci._ xxxiv. 1893, p. 387.

[308] Benham, _Quart. J. Micr. Sci._ xxxii. 1891, p. 325. See also Bourne
(nephridium of _Polynoë_), _Tr. Linn. Soc._ (_Zool._), ii. 1883, p.
357; Meyer, for nephridium of Terebellidae, Sabellidae, and
Cirratulidae, in _Mt. Zool. Stat. Neapel_, vii. 1887, p. 592.

[309] It is worthy of note that in _Aeolosoma_ alone amongst the
Oligochaeta does the brain lie in the prostomium in the adult.

[310] Andrews, "The Eyes of Polychaetes," _J. Morph._ vii. 1892, p. 169.

[311] Wistinghausen, "Entwick. v. _N. dumerilii_," _Mt. Zool. Stat.
Neapel_, x. 1891, p. 41.

[312] This is a modification of the classification proposed by me at the
meeting of the British Association at Oxford, 1894 (see Report, p.
696). For further characteristics of these Orders and sub-Orders see
below Chap. XII. Ehlers, "Die Borstenwürmer," 1864, gives a
historical survey of the group, and enumerates the earlier
classifications.

[313] In _Coabangia_ (see p. 284) the anus is near the anterior end, on the
ventral surface.

[314] It is doubtful whether these organs are palps or only lateral lips.

[315] Pruvot traced the nerve supply to these organs, and thus established
their homology. _Arch. d. Zool. Expér._ (ser. 2) iii. 1885, p. 211.

[316] Meyer, "Stud. ub. d. Körperbau der Anneliden," _Mt. Zool. Stat.
Neapel_, vii. 1887, p. 592; viii. 1888, p. 462. In this work a great
number of important and interesting anatomical facts are recorded
with respect to the Terebelliformia and Sabelliformia, as well as
certain details as to the structure and development of the nephridia.

[317] In some of the members of this family paired lateral tentacles appear
to exist.

[318] It is possible that some of these may be peristomial.

[319] Individual cases in which chaetae are present have been recorded.

[320] Meyer, _loc. cit._

[321] Haswell, _P. Linn. Soc. N.S. Wales_, vii. 1883, p. 251.

[322] Eisig, "Die Capitelliden," _Fauna u. Flora G. v. Neapel, Monogr._
xvi. 1887, p. 331.

[323] Compare with this the muscular organ of _Dinophilus_, p. 243,
_Protodrilus_, and a similar structure which occurs in Terebellids.

[324] Korschelt, "Über Ophryotrocha puerilis," _Zeitschr. f. wiss. Zool._
lv. 1893, p. 224.

[325] Eisig, _Mt. Zool. Stat. Neapel_, ii. 1881, p. 255.

[326] They are specially large also in the Typhloscolecidae; while
Racovitza (_Ann. Mag. N. H._ (ser. 6), xv. 1895, p. 279) has recently
suggested that the caruncle of Amphinomidae belongs to the category
of nuchal organs, and compares it with the ciliated lappets of
_Pterosyllis_.

[327] Ehlers, _Zeitschr. f. wiss. Zool._ liii. 1892, p. 217.

[328] See Claparède and Metschnikoff, "Beit. zur Kennt. d. Entwick der
Chaetopoden," _Zeitschr. f. wiss. Zool._ xix. 1869, p. 163; and
Fewkes, "On the Development of certain Worm Larvae," _Bulletin Mus.
Harvard_, xi. 1883, p. 167.

[329] For an account of the anatomy and development of a Trochosphere, see
Hatschek, on _Eupomatus_, in _Arbeit. Zool. Inst. Wien_, vi. 1885.
Also Meyer, _Mt. Zool. Stat. Neapel_, viii. 1888, p. 462; and for
Polynoid larva see Häcker, _Zool. Jahrb. Abth. Anat._ viii. 1895, p.
245.

[330] See Meyer (ref. on p. 261).

[331] Many of the Polynoids are sexually dimorphic.

[332] Claparède, "Annélides Chétopodes du Golfe de Naples," Supplement,
1870; and Wistinghausen, _Mt. Zool. Stat. Neapel_, x. 1891, p. 41.

[333] Claparède used the term "epigamous" for this phase; Ehlers employed
the term "epitokous," whilst he called the "Nereid" phase "atokous,"
under the impression that the worm did not become mature in this
condition.

[334] Malaquin gives a detailed account of the asexual reproduction in
Syllidae in _Recherches sur les Syllidiens_, Lille, 1893, and in
_Revue Biol. d. Nord de la France_, iii. 1891. See also St. Joseph,
"Les annelides polychétes des côtes de Dinard," _Ann. Sci. Nat.
Zool._ (7th ser.) i. 1886, p. 134.

[335] Alex. Agassiz, _Boston J. Nat. Hist._ vii. 1863, p. 384.

[336] Huxley, _Edinb. New Philosoph. Journ._ 1855, i. p. 113.

[337] "Challenger" Reports, vol. xii. 1885, "Polychaeta," p. 198; and Oka,
_Zoolog. Centralbl._ ii. 1895, p. 591.

[338] Two new heads have been observed in _Typosyllis variegata_ by
Langerhans, and two new tails in another _Syllis_.

[339] Dalyell, _The Powers of the Creator revealed, etc._, vol. ii. 1853,
p. 225 _et seq._

[340] von Kennel, _Arb. Zool. Instit. Würzburg_, vi. 1883, p. 259.

[341] Leidy, _Proc. Acad. Nat. Hist. Philadelphia_, 1883, p. 204.

[342] Giard, _C. R. Soc. Biol._ v. 1893, p. 473.

[343] See M‘Intosh, _Ann. Mag. Nat. Hist._ (ser. 4) ii. 1868, p. 276.

[344] Lankester has suggested that a strong acid is secreted for the
purpose, see _Ann. Mag. Nat. Hist._ (ser. 4) i. 1868, p. 233.

[345] M‘Intosh, _Ann. Mag. Nat. Hist._ (ser. 6) xiii. 1894, p. 1.

[346] Dalyell, _The Powers of the Creator revealed_, ii. 1853, p. 217.

[347] Watson, _Journ. R. Mic. Soc._ 1890, p. 685; see also Dalyell, _loc.
cit._ ii. p. 195.

[348] Schmiedeberg, _Mt. Zool. Stat. Neapel_, iii. 1882, p. 373.

[349] For pelagic forms, see Camille Viguier, _Arch. de Zool. Expér._ (ser.
2) iv. 1886, p. 347; also Reibisch, _Die pelag. Phyllodociden u.
Typhloscoleciden d. Plankton Exped._ 1895.

[350] Lankester, _Journ. Anat. and Physiol._ 1868, p. 114; and 1870, p.
119; see also MacMunn, "On the Chromatology of the Blood in some
Invertebrates," _Quart. J. Micr. Sci._ xxv. 1885, p. 469.

[351] For coloured pictures of worms consult Schmarda, "Neue wirbellose
Thiere," 2nd part, 1861; Milne Edwards in Cuvier's "Règne Animal"
(Ed. Disciples de Cuvier).

[352] Semper, _Animal Life_, "Internat. Sci. Series," 1881, p. 401.

[353] The experiments were made by Mr. Garstang at the Laboratory of the
Marine Biological Association, and are recorded by Poulton in _The
Colours of Animals_, "Internat. Sci. Series," 1890, p. 201.

[354] Panceri, _Atti Acad. Sci. Napoli_, vii. 1875.

[355] M‘Intosh, H.M.S. "Challenger" Reports, "Polychaeta," vol. xii. p. ix.

[356] For an account of these worms see M‘Intosh, _loc. cit._ p. 257.

[357] For a list of parasitic Polychaetes see St. Joseph, _Ann. Sci. Nat._
(ser. 7) v. 1888, p. 141.

[358] Semper, _loc. cit._ p. 340.

[359] See "Challenger Reports," and St. Joseph, _loc. cit._

[360] "Challenger" Reports, _loc. cit._ p. xxx.

[361] See Hornell, _Fauna of Liverpool Bay_, Report III. 1892, p. 126.

[362] Zittel, _Handbuch d. Palaeontologic_ (_Palaeozoologie_), i. 1876-80,
p. 562.

[363] Ehlers, _Zeitschr. f. wiss. Zool._ xviii. 1868, p. 241.

[364] The Chaetopteridae may have to be placed elsewhere in the system, as
they are peculiarly modified, and present features recalling the
Cryptocephala, from which it is possible they have descended.

[365] Meyer (_Mt. Zool. Stat. Neapel_, vii. 1887, p. 669, note) suggests
that the tentacular filaments of Cirratulids are really prostomial,
but have shifted back on to the peristomium, or even farther.

[366] It is probable that the genital ducts of _Sternaspis_ and
Chlorhaemids are modified nephridia.

[367] The character of head and parapodium in each family will be gathered
from the figures accompanying the general description in Chap. X., so
that _detailed_ description is unnecessary. In all cases the chaetae
form valuable specific characters.

The examples of the various families are British, unless the opposite
is expressly stated; but most of them are not confined to our shores,
and the foreign localities are usually given. No attempt is made to
enumerate all the British species.

The following books may be found useful for identifying the worms:—

Claparède, _Recherches anat. sur les Annélides observées dans les
Hebrides_, 1861; _Annélides Chétopodes du golfe de Naples_, 1868,
and Suppl., 1870.

Cunningham and Ramage, "Polychaeta Sedentaria of the Firth of
Forth," _Trans. Roy. Soc. Edinburgh_, xxxiii. 1888, p. 635.
80Ehlers, _Die Borstenwürmer_, 1868.

Johnston, "British Museum Catalogue of Non-Parasitical Worms,"
1865.

M‘Intosh, "British Annelida," _Trans. Zool. Soc._ ix. 1877, p. 371;
"Invert. Marine Fauna of St. Andrews; Annelida," _Ann. Mag. Nat.
Hist._ (4) xiv. 1874, p. 144.

Malmgren, "Nordiska Hafs-Annulater," _Öfversigt af K. Vet.-Akad.
Förhandlingar_, 1865, pp. 51, 181, 355; and "Annulata Polychaeta,"
_ibid._ 1867, p. 127.

St. Joseph, "Les Annélides Polychétes des côtes de Dinard," _Ann.
Sci. Nat._ (_Zool._) (7) vol. i. 1886, p. 127; v. 1888, p. 141;
xvii. 1894, p. 1; xx. 1895, p. 185.

[368] Malaquin, _Recherches sur les Syllidiens_, 1893; for structure of the
gizzard, see also Haswell, _Quart. J. Micr. Sci._ xxvi. 1886, p. 471;
and xxx. 1889, p. 31.

[369] See M‘Intosh's Memoirs, _loc. cit._

[370] Herein are included the various genera formed by Kinberg, Malmgren,
and others.

[371] It appears to be the same as _P. grubiana_ Clap.

[372] Marenzeller has shown that Johnston's _P. scolopendrina_ is not
identical with that of Savigny, and suggests the above name for it.

[373] F. Buchanan, "Report on Polychaetes, Part I." _Sci. Proc. Roy. Dublin
Soc._ vii. (n.s.) 1893, p. 169.

[374] _Polyodontes_ Ran. deserves mention as being a large, rare form with
peculiar pedal gland; cf. Eisig (ref. on p. 268), p. 324; and
Buchanan, _Quart. J. Micr. Sc._ xxxv. 1894, p. 433.

[375] Many authorities regard this species as synonymous with Savigny's _P.
laminosa_.

[376] According to a verbal communication from Mr. J. Hornell of Jersey,
they belong to _P. maculata_ Müll., while Mr. Garstang believes them
to belong to _Eulalia viridis_.

[377] These segmentally-arranged brown spots may perhaps be photogenic.

[378] Greef, _Acta Ac. German._, xxxix. 1877.

[379] Greef, _Zeitschr. f. wiss. Zool._ xlii. 1885, p. 432.

[380] Buchanan, _Quart. J. Micr. Sci._ xxxv. 1894, p. 445.

[381] Buchanan, _Sci. Proc. R. Dublin Soc._ viii. (n.s.) 1893, p. 169.

[382] Reibisch, _Phyllodociden u. Typhloscoleciden d. Plankton Exped._
1895.

[383] The British species is usually referred to as _C. insignis_ Baird,
but Joyeux Laffuie (_Arch. Zool. Exp._ (ser. 2) viii. 1890, p. 244)
has shown that there is only one European species. It is possible
that there is a closer affinity with the Sabelliformia than is at
present supposed.

[384] Compare _Sternaspis_, p. 336.

[385] For literature, see Benham, _Quart. J. Micr. Sci._ xxxix. part 1,
1896, p. 1.

[386] F. Buchanan, _Quart. J. Micr. Sci._ xxxi. 1890, p. 175.

[387] In some genera there are no gills, _e.g._ _Leaena_.

[388] These characters are not necessarily generic.

[389] Eisig, "Die Capitelliden," _Fauna u. Flora G. v. Neapel_, Monogr.
xvi. 1887.

[390] Ed. Meyer., _Arch. mikr. Anat._ xxi. 1882, p. 769.

[391] Vejdovsky, _Denk. Akad. Wien_, xliii. 1882, part 2, p. 33; and
Rietsch, _Ann. Sci. Nat._ (_Zool._) ser. 6, xiii. 1882, art. 5.

[392] For anatomy see Meyer, _Mt. Zool. Stat. Neapel_, vii. 1887.

[393] Andrews, _Journ. Morph._ v. 1891, p. 271.

[394] A. G. Bourne, _Quart. J. Micr. Sci._ xxiii. 1883, p. 168.

[395] Closely allied is _Manayunkia_ Leidy, which occurs in fresh-water
lakes of America. Another fresh-water genus is _Coabangia_ Giard,
which perhaps deserves the creation of a special family. The anus is
ventral and anterior. The chaetae are peculiarly arranged, dorsal
uncini being present only on four segments. The first body segment
carries a ventral bundle of five great "palmate" chaetae.

[396] For the anatomy see Meyer, _Mt. Stat. Neapel_, vii. 1887; see also
above, p. 306.

[397] von Graff, "Myzostomida," "Challenger" Reports, part 27, vol. x.
1884; and "Supplement," part 61, vol. xx. 1887.

[398] Marenzeller, _Anz. Akad. Wien_, xxxii. p. 192.

[399] _Mt. Zool. Stat. Neapel_, xii. 1896, p. 227; where, too, see
literature.

[400] Beard, _Mt. Zool. St. Neap._ v. 1884, p. 544.

[401] _Quart. J. Micr. Sci._ (n.s.) vol. iv. 1864, p. 258; and v. pp. 7,
99.

[402] _Zeitschr. wiss. Zool._ xix. 1869, p. 563.

[403] _De Lumbrici terrestris Historia naturali_, Brussels, 1829.

[404] _Naturg. ein. Wurm-Arten d. süssen u. salzigen Wasser_, Copenhagen,
1771.

[405] _Trans. Roy. Soc. Victoria_, vol. i. 1888, p. 1.

[406] _Phil. Trans._ clxxxvi. 1895, A, p. 383.

[407] _Mém. cour. Ac. Belg._ lii. 1890-93.

[408] _Quart. J. Micr. Sci._ xxxi. 1890, p. 83.

[409] Beddard, _Ibid._ xxxiii. 1892, p. 325.

[410] Beddard, _Ann. Mag. Nat. Hist._ (6) xiii. 1894, p. 205.

[411] _Mém. Soc. Zool. France_, iii. 1890, p. 223.

[412] _Vegetable Mould and Earthworms_, London, 1881.

[413] _Zool. Anz._ xi. 1888, p. 72.

[414] See Fletcher, _P. Linn. Soc. N.S.W._ (2) iii. 1889, p. 1542.

[415] In _Sitzungs-Ber. Böhm. Ges._ 1889, p. 183.

[416] See Dr. Rosa in _Ann. Hofmus. Wien_, vi. 1891, p. 379.

[417] _Entwickelungsgeschichtliche Untersuchungen_, Prag, Heft i. 1888, p.
33.

[418] See Kleinenberg, _Quart. J. Micr. Sci._ xix., 1879, p. 206.

[419] Both Col. Feilden and Mr. Trevor-Battye found specimens in Kolguiev.

[420] _Neue wirbellose Thiere_, Leipzig, ii. 1861, p. 11.

[421] _Kew Bull. Misc. Information_, No. 46, 1890.

[422] _Rev. Biol. Nord France_, i. 1889, p. 197.

[423] _SB. Ges. naturf. Berlin_, 1893, p. 19.

[424] _System u. Morph. d. Oligochaeten_, Prag, 1884.

[425] See my text-book of Zoogeography (Cambridge, 1895) for fuller
treatment.

[426] _Ann. Mag. Nat. Hist._ (6) ix. 1892, p. 12.

[427] Darwin, _Vegetable Mould and Earthworms_, p. 121.

[428] "An Attempt to classify Earthworms," _Quart. J. Micr. Sci._ xxxi.
1890, p. 201.

[429] Oxford, 1895.

[430] See especially Vejdovsky, _Syst. u. Morph. Olig._ Prag, 1884.

[431] Vejdovsky, _Monographie der Enchytraeiden_, Prag, 1879. Michaelsen,
"Synopsis der Enchytraiden," _Abh. Ver. Hamburg_, xi. 1889, p. 1.

[432] J. P. Moore, "The Anatomy of _Bdellodrilus_," _J. Morphol._ x. 1895,
p. 497.

[433] Beddard, _Trans. Roy. Soc. Edin._ xxxv. 1890, p. 629, and xxxvi.
1892, p. 1.

[434] A. G. Bourne, "On the Naidiform Oligochaeta," _Quart. J. Micr. Sci._
xxxii. 1891, p. 335.

[435] F. E. Beddard, _Trans. Roy. Soc. Edin._ xxxvi. 1892, p. 273.

[436] Vejdovsky, _System u. Morph. d. Oligochaeten_, Prag, 1884.

[437] "Anatomical Notes on _Sutroa_," _Zoe._ ii. 1892, p. 321.

[438] "Pacific Coast Oligochaeta," _Mem. California Acad. Sci._ vol. ii.

[439] _Quart. J. Micr. Sci._ xxxvi. 1894, p. 307.

[440] See Spencer, _Proc. Roy. Soc. Vict._ v. 1893, and Fletcher, _P. Linn.
Soc. N.S.W._ 1886-1888, for Australian forms; Rosa, _Ann. Mus. civ.
Genova_, vi. 1886, x. 1890, and xii. 1892, for Oriental species, etc.

[441] See Fletcher and Spencer, already quoted, for Australian species.

[442] Eisen, "Anat. Studies on _Ocnerodrilus_," _Proc. Calif. Acad._ (2)
iii. 1892, p. 228.

[443] Beddard, _Ann. Mag. Nat. Hist._ (6) x. 1892, p. 74.

[444] Beddard, _P. Z. S._ 1885 and 1895, for Antarctic Acanthodrilids;
Michaelsen, in _Jahrb. Hamburg. Anst._ 1888-95, for _Benhamia_.

[445] For a general account of the Eudrilidae, see my Monograph of the
Order Oligochaeta, Oxford, 1895.

[446] _Nouv. Arch. Mus. Paris_, viii. 1872, p. 5.

[447] The scattered literature of this family is due to Benham, Michaelsen,
Perrier, Rosa, and others.

[448] Rosa, "Revisione dei Lumbricidae," _Mem. Acc. Torino_ (2), xliii.
1893, p. 399; also the Rev. H. Friend's numerous and useful papers,
and especially "A New Species of Earthworms," _Proc. Roy. Irish Ac._
(3) ii. 1891-93, p. 402; and "The Earthworms of Ireland," _Irish
Nat._ v. 1896, p. 69, etc.

[449] In the tables the figures refer to the segments of the body. Opposite
the name of each species are two sets of lines; the upper series
indicate the segments occupied by the clitellum; the lower series
those occupied by the tubercula pubertatis. The dots indicate the
occasional extension of the clitellum or of the tubercula.

[450] "Annelés," vol. iii. 1889-90, p. 477, in the _Suites à Buffon_.

[451] See v. Kennel, _Zool. Jahrb._ ii. 1887, p. 37.

[452] _Nouvelle Monographie des Sangsues médicinales._ Paris, 1857.

[453] _Quart. J. Micr. Sci._ xxvi. 1886, p. 317.

[454] See Grube, "Annulaten" of Middendorff's _Sibirische Reise, Zoology_,
1851, p. 20; and Kowalevsky, _Bull. Ac. St. Petersb._ v. June 1896.

[455] See ref. on p. 395.

[456] Asajiro Oka, _Zeitschr. wiss. Zool._ lviii. 1894, p. 79.

[457] See Bürger, quoted on p. 403.

[458] _Loc. cit._

[459] _Quart. J. Micr. Sci._ xxiv. 1884, p. 419; see also _ibid._ xxxiv.
1893, p. 545, which is mainly a criticism of Bolsius' additions to
the very considerable literature upon the Leech nephridium.

[460] "Spermatophores as a Means of Hypodermic Impregnation," _J. Morphol._
iv. 1891, p. 361.

[461] _Zeitschr. wiss. Zool._ lviii. 1894, p. 440; and _Zool. Jahrb. Anat._
iv. 1891, p. 697.

[462] "Annelés," vol. iii. 1889-90, p. 493, in the _Suites à Buffon_.

[463] Whitman quotes with regretful approval (_Proc. Americ. Acad._ xx.
1884-85, p. 76) Sir J. Dalyell's remark, "It does not appear that the
history of the leech has advanced in proportion to the number of
_literati_ who have rendered it the subject of discussion," and adds
on his own account the following severe indictment of his
predecessors: "As a considerable share of the work done in this
direction is purely systematic, it is somewhat surprising that not a
single description of any Hirudo has been given with sufficient
accuracy and completeness for a close comparison of even its more
important external characters with those of other species."

[464] "Hirudinées de l'Italie," etc., _Boll. Mus. Zool. Torino_, vol. ix.
1894, No. 192. See also Apathy, "Süsswasser-Hirudineen," _Zool.
Jahrb. Syst._ iii. 1888, p. 725.

[465] _Zeitschr. f. die gesammt. Naturwiss._ vi. 1872, p. 422.

[466] But Pennant in his _British Zoology_ has referred to a leech which is
even larger. Upon the huge Basking shark (_Selache_) the fishermen
sometimes observe a leech, which invariably drops off when the fish
is brought to the surface, "of a reddish colour and about 2 feet in
length"; this may be a _Pontobdella_.

[467] _Ann. Mag. Nat. Hist._ (6) xii. 1893, p. 75.

[468] _Memorie sulla Storia e Notomia degli Animali senza Vertebre_, 1823.

[469] _Histoire Naturelle des Animaux sans Vertèbres_, vol. iii. 1816, p.
76.

[470] _Le Règne Animal_, 2nd ed. 1830.

[471] γέφῦρα = a bridge, _Ann. Sci. Nat._ (3), vol. vii. 1847, p. 340.

[472] Fischer, _Abh. Ver. Hamburg_, Bd. xiii. 1895, p. 1.

[473] Cuénot, _Arch. Zool. exp._ (2) ix. 1891, p. 593.

[474] _Bull. Mus. Harvard_, vol. xxi. 1891, p. 143.

[475] Shipley, _Quart. J. Micr. Sci._ vol. xxxi. 1890, p. 1.

[476] _Stud. Johns Hopkins Univ._ vol. iv. 1887-90, p. 389.

[477] Conn, _Stud. Johns Hopkins Univ._ vol. iii. 1884-87, p. 351.

[478] _Arb. Instit. Wien_, Bd. v. 1884, p. 61.

[479] Shipley, _Quart. J. Micr. Sci._ vol. xxxii. 1891, p. 111.

[480] _Proc. Roy. Soc. Edin._ xviii. 1892, p. 17.

[481] Selenka, _Die Sipunculiden_. Semper's _Reisen im Archipel d.
Philippinen_, vol. iv. 1883.

[482] _Stud. Johns Hopkins Univ._ vol. iv. 1887-90, p. 389.

[483] Selenka, _Challenger Reports_, vol. xiii. 1885.

[484] _Ann. Sci. nat._ (7) vol. xx. 1895, p. 1.

[485] _Zool. Anz._ ix. 1886, p. 574.

[486] This is not true of all species.

[487] _Acta Ac. German, Halle_, xli. Part II. No. 1, 1879.

[488] _Recueil Zool. Suisse_, iii. 1886, p. 313.

[489] _Vide_ p. 335.

[490] _P. Phys. Soc. Edinb._ vol. i. 1856, p. 165; and _Edinb. New Phil.
Journ._ vol. iv. (n.s.) 1856, p. 313; _Ann. Sci. Nat._ 4th ser. vol.
xi. 1859, p. 150; and F. D. Dyster, _Tr. Linn. Soc. London_, vol.
xxii. 1859, p. 251.

[491] _Ann. Sci. Nat._ 4th ser. vol. x. 1858, p. 11.

[492] "Beiträge zur Anatomie der Phoronis," _Inaug. Dissert. Prag._ 1889,
and _Zeitschr. wiss. Zool._ vol. li. 1891, p. 480.

[493] _P. Linn. Soc. N. S. Wales_, 1st ser. vol. vii. 1883, p. 606; and 2nd
ser. vol. vii. 1893, p. 340.

[494] _Quart. J. Micr. Sci._ vol. xxx. 1890, p. 125.

[495] _Challenger Reports_, vol. xxvii. 1888; and _Proc. Roy. Soc. Edinb._
vol. xi. 1882, p. 211.

[496] _Proc. Roy. Soc. London_, vol. xxxiv. 1883, p. 371.

[497] _Zapiski Acad. St. Petersb._ vol. xi. No. 1, 1867 (Russian). Abstract
in _Arch. Naturg._ Jahrg. xxxiii. 1867, Bd. ii. p. 235.

[498] Caldwell, _loc. cit._ Foettinger, _Arch. Biol._ vol. iii. 1882, p.
679; Gegenbaur, _Zeitschr. wiss. Zool._ vol. v. 1854, p. 345; Krohn,
_Arch. Anat._ Jahrgang 1858, p. 289; Metschnikoff, _Nachricht. k.
Ges. Wiss. Göttingen_, No. 12, 1869, p. 227, and _Zeitschr. wiss.
Zool._ vol. xxi. 1871, p. 233; J. Müller, _Arch. Anat._ Jahrgang
1846, p. 101; Schneider, _Monatsber. Ak. wiss. Berlin_, 1861, p. 934,
and _Arch. Anat._ Jahrgang 1862, p. 47; Wagener, _Arch. Anat._
Jahrgang 1847, p. 202; Wilson, _Amer. Natural._ vol. xiv. 1880.

[499] _Proc. Roy. Soc. Edinb._ vol. xxi. 1896, p. 59; and _Zool. Anz._ xix.
1896, p. 266.

[500] The account given in the following pages has been deliberately
restricted, for the most part, to British species. Our own fauna
contains an assemblage of Polyzoa which is so representative that it
has seemed better to do some justice to the British forms than to
attempt to cover the whole ground in the limited number of pages
devoted to this group. Those who desire to make a wider study of the
subject should refer, for marine forms, to Busk's _Catalogue of
Marine_ _Polyzoa in the Collection of the British Museum_, Parts
I.-III. 1852, 1854, 1875; to the _Challenger_ Reports on Polyzoa,
Parts 30 (1884), 50 (1886), and 79 (1888); for references and lists
of species, to Vine's _Report on Recent Marine Polyzoa, Cheilostomata
and Cyclostomata_ (Report, 55th meeting Brit. Ass. Aberdeen, 1885,
pp. 481-680); [and to Nickles and Bassler, _Synopsis Amer. Foss.
Bryozoa incl. Bibliography_ (Bull. U.S. Geol. Survey, No. 173,
1900)]. References to the literature of the fresh-water forms will be
found below, in Chap. XVIII.

[501] Hooker, quoted by Landsborough, _Hist. Brit. Zoophytes_, 1852, p.
346.

[502] _Rare and Remarkable Animals of Scotland_, ii. 1848, p. 15.

[503] _Arch. Zool. Exp._ 2 sér. x. 1892.

[504] Kraepelin, _Abh. Ver. Hamburg_, x. 1887, No. ix. p. 19; κάμπτειν, to
bend; δέρμα, skin.

[505] Parts of the ectocyst of some calcareous forms are covered by an
external investment of cells, which give rise to secondary
thickenings, ridges, and other growths.

[506] From the _Quart. J. Micr. Sci._ xxxiii. 1892.

[507] _Ibid._ p. 123.

[508] _Quart. J. Micr. Sci._ xxxiii. 1892, p. 147. The experiment was
conducted in a laboratory, and the results may not be perfectly
normal with regard to the time occupied.

[509] See also Joliet, _Arch. Zool. Exp._ vi. 1877, p. 202, and explanation
of plate viii. for another series of observations.

[510] See especially G. J. Allman, _Monograph of the Fresh-water Polyzoa_,
Ray Society, 1856, p. 41; and H. Nitsche, _Zeitschr. wiss. Zool._
xxi. 1871, p. 479.

[511] Ray Society, 4to, 1856.

[512] _Zoological Researches and Illustrations_, v. "On Polyzoa." Cork,
1830.

[513] "Symbolae Physicae," 1831, and _Abh. Ak. Berlin_, 1832, i. p. 377,
etc.

[514] _T. cit._ p. 92.

[515] Vol. i. 1880, Introduction, p. cxxxi.

[516] _Élémens de Zoologie_, 2nd ed. _Animaux sans Vertèbres_, 1843, pp.
238, 312. Prof. A. Milne-Edwards has kindly written to me, informing
me that he believes this to have been the first occasion on which the
term was thus used.

[517] _Phil. Trans._ vol. cxliii, 1853, p. 62.

[518] Nitsche, _Zeitschr. wiss. Zool._ xx. 1870, p. 34.

[519] πρωκτός, anus; ἐντός, within; ἐκτός, without.

[520] λοφός, crest or tuft.

[521] γυμνός, naked; λαιμός, throat.

[522] φυλάσσω, I guard.

[523] κύκλος, circle; στόμα, mouth.

[524] χεῖλος, lip.

[525] κτείς, κτενός, comb.

[526] Miss E. C. Jelly, _Synonymic Cat. Recent Marine Bryozoa_, London,
1889.

[527] _Zeitschr. wiss. Zool._ xxi. 1871, p. 421.

[528] Fischer, _Nouv. Arch. Mus. Paris_, ii. 1866, p. 293.

[529] Ehlers, _Abh. Ges. Göttingen_, xxi. 1876, p. 3, and Joyeux-Laffuie,
(as _Delagia_) _Arch. Zool. Exp._ 2 sér. vi. 1888, p. 135.

[530] Busk, "Challenger" Reports, Parts 30 and 50.

[531] Hincks, _Brit. Marine Polyzoa_, Introduction, p. cxxxv.

[532] Hincks, _Brit. Mar. Polyzoa_, i. p. 558.

[533] See Hincks, _Brit. Mar. Polyzoa_, i. p. lxiv.; and Busk, _Cat. of
Marine Polyzoa in the British Museum_, part ii. 1854, p. 103.

[534] _J. Linn. Soc._ xv. 1881, p. 359.

[535] "Challenger" Report, part xxx. 1884, pl. ix.

[536] Hincks, _Brit. Mar. Polyzoa_, i. p. 58.

[537] _Brit. Mus. Cat._ part ii. 1854, p. 106; Hincks, _t. cit._ p. 181 n.

[538] p. 475.

[539] _Barentsia_ Hincks (= _Ascopodaria_ Busk) differs from _Pedicellina_
in that each stem has a muscular swelling at its base. The genus is
represented by two British species, _B. gracilis_ Sars and _B.
nodosa_ Lomas.

[540] _Arch. Zool. Exp._ 2 sér. ix. 1891, p. 91.

[541] For structure, see Davenport, _Bull. Mus. Harvard_, xxiv. 1893, p. 1.

[542] λοξός, oblique; σῶμα, body.

[543] _Quart. J. Micr. Sci._ xxvii. 1887, pl. xxi. Fig. 10.

[544] For a recent account of the Entoprocta, see Ehlers, "Zur Kenntniss d.
Pedicellineen," _Abh. Ges. Göttingen_, xxxvi. 1890, No. iii.

[An important account of the structure of marine Ectoprocta is given
by Calvet, "Contribution à l'Histoire Naturelle des Bryozoaires
Ectoproctes Marins," _Trav. Inst. Zool. Montpellier_, N.S., Mém. No.
8, 1900.]

[545] Kraepelin, K., "Die deutschen Süsswasser-Bryozoen."—_Abh. Ver.
Hamburg_, x. 1887, No. 9, p. 95.

[546] Jullien, _Bull. Soc. Zool. France_, x. 1885, p. 92.

[547] Hincks, _Brit. Marine Polyzoa_, i. p. 132.

[548] _T. cit._, p. 167.

[549] Quoted by Kraepelin, _t. cit._, p. 83.

[550] Kraepelin, _Abh. Ver. Hamburg_, xii. 1893, No. 2, p. 65.

[551] _Zool. Anz._, xvi. 1893 (1894), p. 385.

[552] Hyatt, _Proc. Essex Institute_ (U.S.A.) (reprint from vols. iv., v.
1866-1868), p. 9.

[553] _Rare and Remarkable Animals of Scotland_, ii. 1848, p. 93.

[554] Trembley, _Mém. Hist. Polypes_, 1744; iii. _Mém._, p. 217. The same
processes are described by Baker, _Employment for the Microscope_,
new ed. 1785, p. 311.

[555] Oka, _J. Coll. Japan_, iv. 1891, p. 90.

[556] Hyatt, _t. cit._ p. 99.

[557] Verworn, _Zeitschr. wiss. Zool._ xlvi. 1888, p. 119.

[558] Dalyell, _t. cit._ p. 94.

[559] Kraepelin, _Abh. Ver. Hamburg_, x. 1887, No. 9, p. 141.

[560] _Phil. Trans._ 1837, p. 396.

[561] _Zeitschr. wiss. Zool._ xxi. 1871, p. 426.

[562] _J. Coll. Japan_, iv. 1891, p. 113.

[563] _Zool. Anz._ xii. 1889, p. 508. This paper contains references to M.
Jullien's writings on the mechanism of protrusion.

[564] [See _P. Cambridge Soc._ vol. xi. Part 1, 1901.]

[565] _Zeitschr. wiss. Zool._ xlvi. 1888, p. 124.

[566] Kraepelin, _Abh. Ver. Hamburg_, xii. 1893, No. 2, p. 47; Braem,
_Bibl. Zool._ (Bd. ii.) Heft 6, 1890, pp. 66 f.

[567] Cf. Kraepelin, _Abh. Ver. Hamburg_, x. 1887, No. 9, pp. 154 f.

[568] _T. cit._ p. 83.

[569] Joliet, _Arch. Zool. Exp._ vi. 1877, p. 262.

[570] Kraepelin, _Abh. Ver. Hamburg_, xii. 1893, No. 2, p. 22.

[571] Harmer, _Quart. J. Micr. Sci._ xxxiv. 1893, p. 211.

[572] _Arch. Zool. Exp._ 2 sér. x. 1892, p. 557.

[573] _Phil. Trans._ 1837, p. 408.

[574] _Brit. Marine Polyzoa_, Introduction, pp. lxxxvi, xc.

[575] _Arch. Zool. Exp._ vi. 1877, p. 261.

[576] _Recherches sur l'Embryologie des Bryozoaires_, 4to Lille, 1877.

[577] Prouho, _loc_. _cit_.

[578] _Arch. Zool. Exp._ 2 ser. v. 1887, p. 446.

[579] _Quart. J. Micr. Sci._ xxxiv. 1893, p. 199; xxxix. part i. 1896, p.
71.

[580] Jullien, _Mém. Soc. Zool. France_, iii. 1890, p. 381.

[581] Cori, _Zeitschr. wiss. Zool._ lv. 1893, p. 626.

[582] Oka, _J. Coll. Japan_, iv. 1891, p. 109; viii. 1895, p. 339.

[583] Cf. Seeliger, _Zeitschr. wiss. Zool._ xlix. 1890, p. 168; and l.
1890, p. 560.

[584] Cf. Milne-Edwards (H.), _Ann. Sci. Nat._ 2 ser. vi. 1836, pp. 5, 321.

[585] See Norman, _Ann. Nat. Hist._ ser. 6, xiii. 1894, p. 114.

[586] See Holdsworth, _P. Zool. Soc._ pt. xxvi. 1858, p. 306.

[587] _Brit. Mar. Polyzoa_, Introduction, p. cxxii.

[588] _Ann. Nat. Hist._ ser. 5, xx. 1887, p. 91.

[589] _Aetea_, _Eucratea_, and certain other forms were separated off by
Mr. Busk as a distinct division, the Stolonata.

[590] Most of the writings of this author are referred to on pp. 277, 278
of Miss Jelly's _Synonymic Catalogue_, referred to on p. 523.

[591] _Catalogue of Marine Polyzoa in the Collection of the British
Museum_, parts i.-iii. 1852-1875; and _Challenger Reports_, Parts 30
(1884) and 50 (1886).

[592] _Trans. and Proc. R. Soc. Victoria_, xxiii. 1887, p. 187, and _Tr. R.
Soc. Victoria_, iv. 1895, p. 1.

[593] _Tr. Zool. Soc._ xiii. 1895, p. 223.

[594] Zittel, _Text Book of Palaeontology_ (Eng. Trans.), 1900, p. 257
(Bryozoa, by E. O. Ulrich).

[595] _Paléontologie Française. Terrains Crétacés_, tome v., Bryozoaires,
8vo. Paris, 1850-1851. This great work refers, however, to recent as
well as to fossil species.

[596] _Heteropora_, of which recent species exist, is placed by Dr. Gregory
in the Trepostomata.

[597] _Quart. J. Geol. Soc._ l. 1894, pp. 72, 79.

[598] See, however, Vine, _Ann. Nat. Hist._ ser. 5. xiv. 1884, pp. 87, 88,
and _P. Yorksh. Geol. Soc._ xii. 1891, p. 74, for possible Palaeozoic
Ctenostomes (_Ascodictyon_, _Rhopalonaria_, and _Vinella_).

[599] Two vols. 8vo. London (Van Voorst), 1880.

[600] 8vo. London (Dulau), 1889.

[601] One or two genera of Cheilostomata may be mistaken for Cyclostomata.
In case of doubt, 7 _et seq_. must be worked through.

[602] Certain varieties of adherent species occasionally assume an erect
form.

[603] For _Celleporella_ (colony minute: orifice tubular), see 41 _et seq._

[604] _Rhynchozoon_ (see No. 61), in which the primary orifice becomes much
obscured by the development of a large mucro, is placed in this
section.

[605] Hincks, _J. Linn. Soc._ xxi. 1889, p. 123.

[606] _Micropora complanata_, Norman, should be placed in the genus
_Lepralia_. See Hincks, _Ann. Nat. Hist._ 5 ser. xix. 1887, p. 304.

[607] See Norman, _Ann. Nat. Hist._ ser. 6, xiii. 1894, p. 113.

[608] Hincks, "Marine Polyzoa" (reprints from _Ann. Nat. Hist._ 1880-91),
Index, p. v. note. (Replacing _Rhynchopora_, preoccupied for a
Brachiopod.)

[609] A form of _Lepralia pallasiana_, in which a mucro is developed, may
be mistaken for _Umbonula_ (see characters given for _Lepralia_ under
No. 59).

[610] See _Arch. Zool. Exp._ 2 ser. vi. 1888, p. 135 (as _Delagia_), and
_ibid._ x. 1892, p. 594. [See also _J. Mar. Biol. Ass._ v., 1897-99,
p. 51.]

[611] F. S. Conant, _Johns Hopkins Univ. Circ._ vol. xv. 1896, p. 82.

[612] _Ibid._ vol. xiv. 1896, p. 77.

*** End of this LibraryBlog Digital Book "The Cambridge natural history, Vol. 02 (of 10)" ***

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