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TEXT- BOOK

OF THE

EMBRYOLOGY OF INVERTEBRATES

In Preparation.
Parts 11. and III. of Drs. KORSCHELT and HEIDER'S
"TEXT-BOOK OF THE EMBRYOLOGY OF INVERTE-
BRATES," Translated and Edited by H. J. CAMPBELL, M.D.,
Senior Demonstrator of Biology and Demonstrator of Physiology in
the Medical School of Guy's Hospital.

Swan Sonnenschein & Co., Ld., London ;
Macmillan & Co., New York.

TEXT-BOOK

OF THE

^f/. 5'SZ

Ifrs

EMBRY0L06Y OF INVERTEBRATES

BY

Dr. E. KORSCHELT

Professor of Zoology &^ Comparative

Atiatomy in the University of

Marburg

Dr. K. HEIDER

Professor of Zoology in the
University of Berlin

Translated from the German

BY

EDWARD L. MARK Ph.D

Hersey Professor of Anatomy i?i
Harvard University

W. Mc M. WOODWORTH Ph.D

Instructor in Microscopical Aftatomy
in Harvard University

With Additions by the Authors and Translators

PART I

PORIFERA, CNIDARIA, CTENOPHORA, VERMES,
ENTEROPNEUSTA, ECHINODERMA TA

SWAN SONNENSCHEIN & CO., Limd

NEW YORK : MACMILLAN & CO

1895

TRANSLATORS' PREFACE

The value of the Lehrbuch der vergleichsnden Entwicklungs-
geschichte der wirhsllosen Thiere for students of animal
morphology is too well understood by those who are familiar
with its scope and execution to require any statement of our
aims in undertaking an English translation of it.

In presenting to zoologists the First Part of this work
we consider ourselves fortunate in having had the valuable
aid of the authors in supplementing the original text by
numerous additions, made desirable by the rapid advance of
the science since the date of first publication. Although the
scope of the work has permitted the addition of only the
most succinct statement of the results reached by embryo-
logists in the last five years, these additions must prove
to be of assistance to all students, and will, we believe, be
especially acceptable to those who are already familiar with
the original edition.

In order to spare the reader the labor of comparing
original and translation for the purpose of ascertaining
what is new, the plan has been adopted of enclosing ill
brackets [ ] all new matter, which, so far as practicable,
has been put in the form of footnotes. Each of these addi-
tions is followed by the initial of the author, or by the word
" Translators," to indicate the persons responsible for the new
matter. Owing to an oversight, the initial has been omitted
from several of the additions in the earlier chapters. It
should be stated, therefore, that, unless otherwise indicated,
the additions to Chapters I. — III. were made by Professor
Heider, those to Chapters IV. — XIV. by Professor Korschelt.
Brackets have also been freely used in the text to enclose
such words or brief explanations as the translators deemed

VI TRANSLATORS PREFACE

nseful supplements to the more literal translations of the
original. In such cases an indication of the authority has
been omitted, since no uncertainty is likely to result from
the omission.

To avoid confusion in citation and to indicate at a sflance
the additions to the Literature of the several chapters, the
references not included in the original have been put in the
form of Appendices and numbered with Roman numerals. It
has been the aim to make these additions include all the
important papers which have appeared since this Part was
first issued.

In translating Anlage we have employed the word funda-
ment— a use which one of us has suggested and defended in
the Translators' Preface to Text -hook of the Embryology of
Man and Mammals, by Dr. Oscar Hertwig, etc. (Swan
Sonnenschein & Co : London, 1892).

We are under deep obligation to onr colleagues Doctors
C. B. Davenport and Gr. H. Parker for their friendly and
self-sacrificing assistance, and we desire to thank both of
them for their aid — Dr. Davenport for having rendered us
valuable service in revising the whole of the manuscript ;
Dr. Parker for assistance in revising parts of the manuscript
and reading the whole of the proof. •

It is with reluctance that we have felt compelled by the
pressure of other duties to relinquish to others the task of
completing the translation of this admirable work. We
trust that one of the advantages of this change will be the
more rapid publication of the translation of the remaining
parts than could possibly have been hoped for from us.

THE TRANSLATORS.
Cambridge, Mass., U.S.A.

AUTHORS' PREFACE

The facts that the comparative embryology of Invertebrates
has not had a broad and comprehensive presentation since the
appearance of Balfour's Treatise on Comparative Embryology,
and that the special literature of this subject has undergone
an enormous increase since that time, have forced upon
every one who has been concerned with questions of com-
parative embryology the pressing need of a more modern
treatment of the subject. Inasmuch as we had occasion to
go over a considerable part of the literature of this subject
during the last few years — partly for the purpose of courses
of lectures to be given, partly from the requirements of
special investigations — it was natural that the idea should
have occurred to us to utilize this preliminary work, and by
arranging the material acquired and further elaborating it
to issue the whole in book form — a venture which was
undertaken, and the first results of which have assumed the
form of the present part.

Since it has beea our plan in writing the present work to
proceed from the special to the general, and since naturally
some time will elapse before the completion of the whole,
we have thought that we should secure the gx-atitude of the
reader if we published the first half of the special part at
once. The second half of this part, embracing the Arthro-
pods, Molluscs, Molluscoidea, Tunicates, and Amphioxus,
will appear shortly, while we hope to finish the general
part, and therewith the whole book, in the course of the
year 1890.

Not to begin the special part of this work too abruptly,
we have prefaced it with a shoi*t general introduction.

Our decision to limit the subject matter to the so-called

via AUTHORS PREFACE

invertebrate animals may require an explanation, and perhaps
also an excuse. We have been guided exclusivelj by prac-
tical considerations, especially the fact that the comparative
embryology of Vertebrates has very recently been compre-
hensively treated in an excellent manner, and further the
reflection that, with the limitation of our field to the In-
vertebrates, the treatment of them might be so much the
more thorousfh.

We take the liberty of expi'essing here our best thanks to
Herrn Geheimrath F. E. Schulze, who has aided us in the
most amiable manner, both by his advice and by his assist-
ance in procuring literature; and likewise to our publisher,
who has made it possible to issue the book in its present
form.

THE AUTHORS.

CONTENTS OF PART FIRST

Introduction .....
Types of cleavage and gastrulation
Formation of the mesoderm
Body cavity

Chapter I. Porifera. By K. Haider
I. Amphiblastula type .
II. Coeloblastula type
III. Parenchymula type .

Development of Spongilla

General considerations on the development

teniatic position of the Porifera
Origin of the different types of canal system
Non-sexual reproduction of sponges
Development of the gemmula
Literature ....

Chapter II. Cnidahia.

Systematic .

I. Hydrozoa .

By K. Heider

Medusae

2.

///;/

Hydroidea ....
Development of metagenetic
Budding of Hydroid polyps
Development of the Medusa
Comparison of polyp and Medusa
Budding and division in Medusee
Frustulation ....
Development of polyps with sessile gonophores
Development of hypogenetic Medusae
Developmental cycle of the Cuninas
Siphonophora .....
Systematic .....
(«) Physophoridse

Disconula larva of the Velellidse
Laws of budding in the Siphonophore
stock
(h) Calycophoridse

General considerations on the origin of
the Siphonophora . . . . .
ix

and

sys

1
3

10
11

13

14
20
22
25

28
30
82
34
36

39
39

39
39
40
43
43
4.5
48
49
50
53
58
59
59
60
65

66
68

X CONTENTS

PACK

II. Anthozoa 75

(a) Alcyonaria 75

Cleavage, and the development of the planula 76

Attachment and development of the polyp . 77

Budding 82

(6) Zoantharia 88

Cleavage and development of the planula . . 88

Development of the septa 90

Formation of the calcareous skeleton . . 98

Division and budding 100

III. Scyphomedusae 102

(a) Lucernaridse 102

(6) Chary bdeidse 103

(c) Discophora 103

Alternation of generations in the Discophora . 105

Development of the Scyphistoma . . . 105

Strobilation 113

Hypogenetic development of Pelagia . . 117

Metamorphosis of the Eph3a-a .... 119

General considerations regarding the Scypho-
medusae ........ 122

General considerations regarding the Cnidaria 125

Literature ........ 127

Chaptkr III. Ctenophora. By K. Heider .... 186

Tectonic 136

Embryonic development 138

Cleavage 139

Epiboly 142

Formation of the gastrovascular system 146

Development of the mesoderni 148

Metamorphosis .......... 151

Heterogeny 153

General considerations 154

Literature 158

Chai-tkk IV. Platvhelminthes. By E. Korschelt . . . 159

I. Turbellaria 159

Systematic survey ; different forms of development . 159

1. Polycladida 160

A. Direct development. Oviposition; cleavage 160

Formation of the germ-layers . . . 161
External and internal development of the

body 163

B. Indirect development ..... 165

Miiller's larva; transition to the adult

animal 165

Aberrant larval forms (Oligocladus, Sty-

lochus pilidium) 167

CONTENTS XI

PAGB

2. Tricladida. Oviposition ; yolk-cells ; cleavage . 169

Formation of the embryo and its organs . . 169

3. Rhabdocoelidse 173

[Acoela] 175

General considerations : relation of the Tur-
bellaria to the Ctenophora (Ctenoplana and

Coeloplana) 176

II. Trematoda 178

Composition of the egg from egg-cells and yolk-cells . 178

1. Distomidse 178

Embryonic development 179

Further course of development; Distomum

hepaticum ISO

Sporocyst ; Redia ; Cercaria .... 180
Differences in the development of various

Distomidse 185

Cercaria satifera, Bucephalus, Leucochlori-

dium, etc. 186

2. Polystomidse ........ 188

Diplozoon, Gyrodactilus, etc 189

III. Cestoda 190

Character of the eggs ; resemblance to those of Tre-

matodes ......... 190

Embryonic development of the Bothriocephalidse . 191

Embi-yonic development of the Tseniadse . . . 193

Further development ; transference of the eggs ;

cysticercus 194

Formation of the scolex ; transference of the cj'sti-

cercus ; development of the tapeworm , . . 197

General considerations : significance of the develop-
ment (Archigetes, Caryophyllseus, Amphiptyches,

etc.) 198

Taenia coenurus and T. echinococcus ; relation to the

Trematodes (Amphilina) and Turbellaria . . 200

Literature 201

Chapter V. Orthonectid.e and Dicyemid^e. By E. Korschelt 206

I. Orthonectidae 206

Systematic survey ; occurrence ; shape of the body ;

organization; eggs ....... 207

Development of the male 208

Development of the female 209

II. Dicyemidae .209

Systematic occurrence; organization .... 209

Development of the vermiform embryos . . . 210

Structure and development of the infusoriform embryos 211

Xll CONTENTS

General considerations; interpretation of the Ortho-

nectidse, etc 215

Literatui'B 215

Chapter VI. Nkmertini. By E. Korschelt .... 217

Oviposition ; different forms of development .... 217

1. Development through the Pilidium larva . . . 217

Blastula ; gastrula ; Pilidium 218

Origin of the worm in the Pilidium .... 221

2. Development after the type of Desor .... 224

3. Direct develoi^ment ........ 227

General considerations : interpretation of the various

forms of development 229

Relationships of the larvae and adult animals to other

groups 230

Literature . • 232

Chapter VII. Nemathelminthes. By E. Korschelt . . . 234

I. Nematoda 234

Embryonic development 234

Cleavage ; formation of the germ-layers .... 234

Formation of the embryo 237

The post-embryonic development ; Trichocephalus ;

Heterakis 239

Dochmius; Mermis 239

Sphserularia ; Atractonema 240

Heterodera ; Allantonema 241

Hhabditis nigrovenosa ; Khabdonema ; Cucullanus . 242

Dracunculus; Spiroptera ; Trichina .... 243

II. Gordiidae .244

Embryonic and subsequent development . . . 244
General considerations : Gordiidae, Nematoda, and

Acanthocephali 246

Literature 247

Chapter VIII. Acanthocephali. By E. Korschelt . . . 249

Cleavage ; embryonal membrane 249

Embryo and larva : their migration and further development 250

Literature 255

Chapter IX. Rotatoria. By E. Korschelt .... 256
Reproduction by means of summer, winter, and male eggs ;

cleavage 256

Formation of the germ-layei's and further development . . 257
General considerations : TrochosphiBra aequatorialis, com-
parison with the Trochophore larva of the Annelida, etc. . 259

Relationships to the Arthropoda (?) 260

Literaturti 261

Chapter X. Annelida. By E. Korschelt 262

I. Chaetopoda and Archiannelida 262

CONTENTS Xlll

PAGE

1. Development through free-swimming larvse (Poly-

chsbta and Archiannelida) 262

Oviposition ; brooding 262

Embryonic development (cleavage, blastula) . 263

Formation of the germ-layei's ; mesoderm . . 265

Trochophore [larva] ...... 266

Metamorphosis of the Trochophore larva into the

worm ......... 268

The various larval forms (atrocha, monotrocha,
telotrocha, etc.), with remarks on their meta-
morphoses 270

Suppression of the larval form through brooding 280

2. Development without free-swimming larvse (Oli-

gochseta) 281

Oviposition (cocoons) ; cleavage .... 281
Formation of the germ-layers (mesodermal

bands) 282

The embryos in the larval condition (head kid-
ney) 283

Formation of the worm 284

Primitive segments ; formation of the mesoderm 285

3. Formation of the organs 286

Ectodermal structures : epidermis ; setigerous

sacs ; nervous system and sensory organs . 287
Mesodermal structures : body cavity ; muscula-
ture ; blood-vessels 289

Head kidney and segmental organs . . . 295

Genital organs 297

Entodermal structures : intestinal canal . . 299
Non-sexual reproduction ; regeneration ; divi-
sion (Lumbriculus, Ctenodrilus) . . . 801
" Budding " of the Naididse, Autolytus, Syllidse,

Nereis 302

Stock-formation of Syllis ramosa .... 304

II. Echiuridae 304

1. Oviposition; cleavage and formation of the germ-

layers 304

2. Larval form and metamorphosis of Echiurus and

Thalassema 306

3. Larval form and metamorphosis of Bonellia . . 310

Development of the male of Bonellia ; general

considerations 312

IIL Dinophilus 313

Resemblance to polytrochal Annelid larvae . . 313

Oviposition ; development ; sexual dimorphism . 314

IV. Myzostoma 315

Embryonic development and larval form . . , 315

Resemblance to Annelid larva? ; metamorphosis . . 316

XIV CONTENTS

Y. Hirudinea 3^8

Oviposition (cocoons) 318

1. Cleavage ; formation of the germ-layers and estab-

lishment of the external form of the body

(Ehyncobdellid*) 319

Formation of the germ band 321

Development of the embryo 323

Gnathobdellidae : cleavage ; germ bands . . 325

2. LarvcB of the Gnathobdellidse (ciliation and primi-

tive kidneys) 327

3. The further development of the body ; establish-

ment of head and trunk 329

4. Formation of the organs 331

General considerations on the development and

interpretation of the Hirudinea . . . 386

VI. Branchiobdella 336

Systematic position 336

Oviposition ; cleavage ; formation of the germ-
layers, etc 337

General considerations regarding Annelida : larval

forms (Trochophore) 341

Trochosphsera, Pilidium, and other larvae : descent . 842

Head and trunk ; segmentation 346

Literature 350

Chapter XI. Sri'UNCULiD^. By E. Korschelt .... 357

1. The development of Sipunculus (blastula, formation of

the germ-layers) 357

Development of the embryo 358

The larva of Sipunculus 361

Metamorphosis into the adult animal .... 368

2. The development of Phascolosoma 364

General considerations : Sipunculidse and Echiuridse as

Gephyreans ; relationships to the Annelida . . 365

Literature 366

Chapter XII. Ch/etognatha. By K. Heider .... 367
Systematic position ; oviposition ; cleavage; formation of the

germ-layers 367

Further development 370

General considerations : relationships to the Annelida ;

absence of the larva ; coelomic sacs 871

Literature 371

Chapter XIII. Enteropneusta. By E. Korschelt . . . 378

Anatomy 378

Development without Tornaria larva: cleavage; formation of

the germ-layers; development of the embryo; larva . . 879

Development by means of the Tornaria 381

CONTENTS XV

PAGE

Further developmental processes of both types : coelomic

sacs, gills, genital organs, nervous system, etc. . . . 384

General considerations: relationships to the Echinodermata ;

segmentation ; relationships to the Chordata . . . 388

Literature 390

Chapter XIV. Echinodermata. By E. Korschelt . . . 892

1. Formation of the primary germ-layers, the mesenchyma,

mouth, and anus 392

Holothurioidea 393

Echinoidea 398

Asteroidea 401

Ophiuroidea ; Crinoidea 403

2. The origin of the enterocoele and the hydrocoele. . . 405

Asteroidea 405

Ophiuroidea 406

Asteroidea (Asterina) 407

Echinoidea ; Holothurioidea 409

Crinoidea 411

Divergent statements in regard to the formation of the
entero-hydrocoele : Ophiuroidea ; Echinoidea (two

pairs of enterocoeles, internal segmentation) . . 413

3. Development of the typical larval forms ; simple funda-

mental form 415

Crinoidea 415

Holothurioidea : Auricularia, and direct development . 417

Asteroidea : Bipinnaria, Brachiolaria .... 419

Aberrant larval forms of the Asteroidea . . . 421

Ophiuroidea : Pluteus, direct development . . . 422

Echinoidea : Pluteus, direct development . . , 424

4. Metamorphosis of the larva into the echinoderm . . 426

Holothurioidea 426

Asteroidea 432

Ophiuroidea 437

Echinoidea 438

Crinoidea 443

Regeneration and division 455

General considerations : common features of the develop-
ment 456

Radial structure of the adult forms : its derivation , . 458
Relationships to other divisions (value of the larval forms,

ambulacral system) 460

Literature 461

CORRIGENDA

p. 30, 1. 6
p. 33, 1. 5
p. 43, footnote ...

p. 49, 1. 12 fr. bot.

„ 1. 13 „

„ 1. 14 „
p. 51, 1. 2

„ 1. 14fr.bot.

„ 1. 15 „
p. 52, 1. 14 „

„ 1. 16 „
p. 53, 1. 1

„ 1. 11
p. 58, last 1.
p. 120, 1. 3
p. 205, 1. 4

Schulze, No. 12

(No. 4)

Literature on Cnidaria

No. 39

No. 37

Stromobrachium
(No. 42); (No. 21) ...

(No. 33)

(No. 32); (No. 35) ...

(No. 28)

Mereschowsky
Hydrocorallia

(No. 30)

(No. VII.)

CyanidfD

prefix " V/' to the line.

change tc Schulze, No. 26.
,, (Ganin, No. 4).
,, Literature on

Hydroidea.

No. 38.

No. 35.
,, Stomobrachium.

(No. 43); (No. 22).

(No. 32).

(No. 33); (No. 37).

(No. 26).
,, Mereschkowsky.
,, the Hydrocorallice.

(No. 29).

(No. v.).
,, CyaneidsB.

INTRODUCTION

Zoological research of the last decade has led to a sharp
separation of two chief divisions of the animal kingdom:
the Protozoa and the Metazoa. In the group Protozoa the
individual can, fi*om its structure, be referred to the funda-
mental type of a cell. These unicellular individuals exist
either separately or united in great numbers to form colonies
or corms. In the latter case, however, the different indi-
viduals remain equivalent to one another in structure and
function. In the group Metazoa, or Germ-layer animals,
on the contrary, there always results a multicellular or-
ganism (cell-community or cell-corm), in which the single
cells give up their independence for the good of the com-
munity, and accommodate themselves to a division of labour,
in consequence of which there is brought about a diversity
in the structure and function of the cells of the Metazoan
organism. While the development and differentiation of
distinct tissues with specific functions result from this poly-
morphism of the cells, the entire colony gains a higher
functional capacity and a more complete unity. In this
way there arises an individual of higher rank or second
degree, which we designate as person. These Metazoan in-
dividuals also may, through incomplete separation after
budding, remain united in colonies, and then there results
an individual of the third degree, the stock or corm. By
adaptation of the stock-forming persons to various functions,
accompanied with their polymorphous development, a higher
functional unity may be reached in this case also.

As a result of the division of labour which is effected
among the cells of the Metazoan organism, it comes about

K. H. E. 1 B

2 INTRODUCTION

that the ability of reproducing the entire organism does not
belong alike to all the cells. It is confined rather to very
special cells, ■which are known as reproductive cells (egg-cells
and sperm-cells) ; these are cells which for the most part
are developed only in definite regions of the organism {genital
organs, gonads). The development of the Metazoan begins
with the fusion of two morphologically different repi-oductive
cells derived, as a rule, from two different individuals
{fertilization). This kind of reproduction, known as sexttal
reproduction, is typical for all Metazoa. In many forms,
however, non-sexual modes of reproduction (by division or
budding) are interpolated in the life-history. If such an
interpolation is the rule, so that two morphologically differ-
ent generations, one of which multiplies by sexual and the
other by non-sexual reproduction, regularly alternate with
each other, then this condition is known as metagenesis or
alternation of generations. It may also happen, however, that
there is a regular alternation of sexual generations, in one
of which reproduction is hermaphroditic or parthenogenetic,
while in the other it is by means of separate sexes. Here
also there occurs a heteromorphous development of the two
generations. We call this condition heterogeny.

Inasmuch as the individual Protozoan has the morpho-
logical value of a single cell, the embryology of the Protozoa
belongs to the province of cell morphology. For this reason
it is usually excluded from the domain of compai'ative em-
bryology of animals in the stricter sense ; in this book, too,
it will receive no consideration. Comparative embryology
accordingly has to do with the development of the Metazoa,
and, above all, with their development from the fertilized
egg. Its chief problems consist in the investigation of the
formation of the germ-layers, the origin of organs, and the
development of the general form of the body. Its purpose
is the recognition of the laws of development, the determina-
tion of the homologies of organs, and the deduction of the
ancestral history of the Metazoa.

The Metazoa constitute a single stem of the animal king-
dom. It is very probable that all Metazoa can be referred
to a common ancestral form, and that certain correspond-

INTRODUCTION 3

ing features in the mode of development are the result of
this common descent. The earliest stages of development
in the Metazoa can readily be reduced to a uniform plan
characterized by the appearance of the blastula- and gastrula-
stages at the end of cleavage. One is justified in the as-
sumption that in these two stages there exists a repetition
of ancestral forms which are common to all the Metazoa.

In the first stages of development of the Metazoa the
existence of a chief or primary axis can be recognized, the
ends of which are distinguished as the animal pole and the
vegetative pole, because in th.e differentiation of the two
primary germ-layers, which soon follows, the layer arising
in the vicinity of the animal pole (ectoderm) presides over
the animal functions (sense perception, locomotion), while
the germ-layer at the opposite pole (entoderm) is mainly de-
voted to the functions of vegetation (e.g., nutrition). The
Metozoa accordingly at first show a monaxial, heteropolar
structure. Frequently the chief axis can be recognized in
the egg-cell of the Metazoa before the beginning of
development, since the germinative vesicle (nucleus of
the egg-cell) and a dense accumulation of protoplasm are
situated near the animal pole, whereas in the region of the
vegetative half of the egg a great accumulation of yolk
particles can be recognized. The animal pole, furthermore,
is characterized by being the place at which the expulsion
of the polar globules takes place before fertilization.

The process of the cleavage of the egg, by which, after
fertilization has taken place, the embryonic development is
initiated, is essentially an ever-px'ogressing division of the
egg, which takes place according to fixed laws, and by which
the egg is divided into a number of cells (cleavage spheres,
hlastomeres), which at first are still undifferentiated. Ac-
cording to the direction which the planes of cleavage occupy
in this process, we distinguish meridional and equatorial
furrows, the former coinciding Avitli the chief axis, the latter
being perpendicular to it. In this manner there arise
blastomeres that are at first spherical, but in later stages
more or less pyramidal in form, and which are arranged
radially about a point occupying the centre of the egg. By

4 INTRODUCTION

separation of the cells there soon arises a central cavity, the
cleavage cavity or Vmi Baers cavity {hlaf;toc<ele), which con-
tinually increases during the succeeding cell divisions, while
the blastomeres ari-ange themselves about this cavity in a
single-layer epithelium (blastoderm). The stage of develop-
ment thus reached is known as the hlastula or blastosphere.
In the one-layer hlastula an arrangement of the parts of the
egg about the chief axis is also clearly recognizable. The
cells in the vicinity of the animal pole are, as a rule, smaller
and not so rich in nutritive yolk particles ; Avhereas the cells
of the vegetative portion are larger and richer in yolk, and,
in consequence of the impeding influence offered by the
nutritive yolk, divide more slowly.^ The Avail of the single-
layer blastosphere represents the first of the primitive organs
of the Metazoan body.

In the simplest cases a gastrvla-stage is developed out of
the blastula-stage by the cell-layer of the vegetative half
becoming flattened and gradually depressed, so that there
arises an ever-deepening invagination at the vegetative pole.
In this way the cleavage cavity (primitive body cavity)
becomes gradually reduced, and often is presex'ved only as
a narrow cleft between the two layers of the body-wall
produced by the metamorphosis already desci'ibed. The
gastrula-stage has substantially the form of a sac. Ifc
encloses a cavity which has arisen by invagination, called
the arche7iteric cavity, and opens to the exterior in the region
of the vegetative pole by means of the primitive month or
prostoma. (blastopore). The wall at this stage consists of two
cell-layers : an outer, the ectoderm, which is derived from the
cells of the animal portion of the blastosphei-e, and an inner,
the entoderm, which consists of the cells of the former
vegetative half, and which has reached the inside of the
embryo by the process of invagination. In the region of
the blastopore the ectodermal and entodermal la3^ers be-
come continuous with each other. Ectoderm and entoderm

1 There are reasons for thinking that the rate of cleavage is not wholly
dependent on the proportion of nutritive yolk in the blastomere. (See
KoFoiD, C. A., " On Some Laws of Cleavage in Liniax," Proc. Amcr.
Acad. Arts and Sciences, vol. xxix., p. 180, 1894) [Translators].

INTRODUCTION O

repi'esent the two primitive organs — or, as they are called,
the two primary germ-layers — resulting from the differentia-
tion of the simple blastosphere. The gastrula-stage, which
recurs under various modifications in all the Metazoa, ap-
pears as the recapitulation of a hypothetical ancestral form
(Gastrea), which was characterized by the development of
the archentei-on. Among the Metazoa now living many of
the Cnidaria have retained essentially the structure of this
hypothetical ancestor. In the more highly developed forms
the two primary germ-layers undergo vai-ious modifications,
whereby additional organs are differentiated. A third layer,
the Tnesodervi or tniddle germ-layer, also grows in between
the two cell-layei'S. Concerning the origin of this we shall
speak farther on. Of the primary germ-layei^s, the entoderm
retains, even in the higher Metazoa, the function originally
belonging to it: that of receiving and digesting food. In
genei'al it constitutes the epithelium of the mid-gut. From
the ectoderm, on the other hand, arise usually the epidermis,
and the nervous system, and sense organs, as well as the
epithelial lining of the stomodeal and proctodeal invagi-
nations.

We have described above a method of origin of the
blastula- and gastrula-stages as it occurs in some, but by no
means all, Metazoa. It was chosen as the type because,
with due regard to the disturbing influences px'esent,
many of the aberrant modes of development can readily be
i-educed to the plan hei-e presented. Frequently cleavage,
the formation of the blastosphere, and the process of gastru-
lation are modified by the presence and definite arrangement
of large masses of food-yolk.

Certain eggs with little food-yolk approach most nearly to
the plan presented above (e.g., those of Amphioxus, Sagitta,
and the Echinoderms). In such cases cleavage results in the
production of blastoraeres which are nearly uniform in size,
so that in the completed blastosphere only a slight difference
can be detected between the size of the blastomeres of the
animal and vegetative poles. However, even here those of
the vegetative pole are, as a rule, a little more voluminous.
This kind of cleavage is called total and eqtial cleavage. It

6 INTRODUCTION

is called total because the entire mass of tlie egg is separated
bj the division into blastomeres, and it is called equal on
account of the approximately equal size of the resulting
blastomeres. The blastula-stage Avhich, with large central
segmentation cavity, is produced by this cleavage, is called
a ccelohlastitla or archihlastula, whereas the gastrula arising
from the latter by a process of enfolding is called an inva-
qination gastrula or evihoUc gastrula.

In the eggs of some Cnidaria, especially Hydroids, whose
earliest development is accomplished exactly in the manner
desci'ibed above — that is, by total and equal segmentation
and subsequent development of a coeloblastula — there exists
a method of entoderm formation (gastrulation) which differs
somewhat from the method by invagination just described,
although it can be reduced to the same. This is the forma-
tion of the entoderm by polar ingression. In this case the
entoderm does not arise by an invagination of the cells of
the vegetative pole, but the latter detach themselves from
the blastoderm and migrate into the blastoccele, which in
this way gradually becomes filled with a closely packed mass
of entoderm cells. It is only secondarily that the archenteric
cavity arises in this mass as a fissure, and that a mouth is
foi'med by dehiscence of the wall. One sees that this kind
of entoderm formation can readily be derived from that by
invagination, inasmuch as the essential difference from that
method of formation consists in the fact that the entoderm
cells give up their epithelial continuity at the beginning of
the ingrowth.

Closely allied to the above-described type of total and
equal cleavage are forms in which a more or less con-
sidei'able amount of food-yolk is deposited in the vegetative
half of the egg. On account of this accumulation the vege-
tative portion of the egg considerably exceeds the animal
portion in mass. It follows fx^om this that in the course of
cleavage, which here also is total, the cleavage cavity appears
relatively small, and occupies a very eccentric position near
the animal pole. The wall of the blastosphere, which can
still be called a cadoblastula, in this case pi-esents a con-
siderable difference in thickness at the animal and vegetative

INTRODUCTION /

poles. We call this kind of cleavage total unequal cleavagr,
and group together as JioJohlastic eggs the forms belonging
to this type and those previously mentioned. The unequal-
walled blastula that has arisen through total unequal seg-
mentation can in its further progress lead to the formation
of a gastrula by invagination, but in this case the gastrula
cavity will be relatively shallow, corresponding to the small
size of the segmentation cavity.

In some other cases, on the contrary (e g., in some
Annelids), after the conclusion of the process of total unequal
cleavage, there is formed a blastula in which the cleavage
cavity is reduced to a minimum. Accordingly there results
from segmentation a more or less solid cell-mass (sterroblas-
tula), in which we can distinguish a portion composed of large
entodermic elements rich in food-yolk from an ectodermic
portion consisting of small cells. The latter is placed upon
the former like a small cap in the region of the animal pole.
Here gastrulation by invagination cannot take place ; but the
gastrula-stage is formed by the growth and consequent
increase in size of the cap-shaped ectodermic part, whereby
its edges push themselves more and more over the entodermic
mass, so that finally the latter is entirely included within
the ectodermic sac. We designate the solid gastrula-stage
arising in this manner as a circumcrescent or epiboUc gastrula
(sterrogastrula) . By this means a gastrula cavity is not
formed primarily, but arises only secondarily as a fissure in
the entodermic cell-mass. The edges of the spreading ecto-
dermic layer must be regarded as the blastopore, which
accordingly is filled by a so-called yolk-plug.

The presence of large quantities of yolk matter in the
region of the vegetative half of the egg presents obstacles
to the progress of cleavage in that region. The accumula-
tion of large masses of yolk can go so far that this portion
of the egg does not at first take part in the segmentation ;
bat only a small portion, situated in the vicinity of the
animal pole and consisting principally of formative yolk,
is divided into blastomeres. Such, which undergo only a
partial cleavage, are known as meroblastic eggs, in contra-
distinction to holoblastic ones. There is developed in this

8 INTRODUCTION

way a disc-shaped embryonal part which is situated on the
unsegraented yolk-mass at the animal pole. We call this
type of segmentation, which represents the most extreme
case of unequal cleavage, discuidal cleavage. It occurs, for
example, in the Cephalopods.

A particular type of cleavage, which does not fit into the
above series, occurs in the class of Arthropods. Whereas
all eggs thus far considered were characterized by a more
or less considerable accumulation of yolk in the region of
the vegetative half (telolecithal eggs), the distribution of
the yolk being accordingly eccentric, the eggs of Arthro-
pods exhibit a regular distribution of the yolk masses of
such nature that their centre coincides with the middle
point of the egg (centrolecithal eggs). The first cleavage
nucleus here lies in the centre of the egg, where by divi-
sion it separates into a large number of nuclei, which are
distributed uniformly at the periphery of the egg, and thus
give rise to the formation there of a layer of small uniform
blastomeres. This cell-layer represents the blastoderm,
while the cleavage cavity of the blastula-stage produced in
this way is filled with the unsegmented yolk-mass. This
kind of cleavage is known as superficial cleavage.

The modifications of development hitherto considered ap-
pear to be dependent principally upon the amount and mode
of distribution of the yolk matter. We have still to consider
some forms which in the mode of distribution of the yolk
recall the centrolecithal eggs, but which by their peculiar
mode of entoderm formation prove to be aberrant forms. In
the first place, there should be mentioned in this connection
the kind of eidodcrm formatiu7i (by delaminatinn) occurring in
the Cnidaria (Hydroids). The typical case of this kind
exists in the development of the Geryonidse. A coeloblastula
is here formed by total and equal cleavage, and there follows
a division of the cells in such a manner that an inner portion
rich in yolk becomes separated from a superficial part with
little yolk matter. In this way there arises out of the one-
layer sphere an arrangement of the cells into two concentric
hollow spheres, of which the inner contains the elements of
the entoderm, and the outer those of the ectoderm. One

INTRODUCTION 9

sees that in this mode of foi*mation, which cannot be com-
pared to the plan of gastrulation by invagination, the gastrula
cavity arises from the cleavage cavity.

Apparently a transition between the formation of the entoderm by
delamination and by polar ingression is effected by a kind of entoderm
formation which has been observed by Metschnikoff in different Hydroids,
and which is designated as multipolar ingression (i.e., ingression from all
sides), in which single cells of the blastoderm migrate into the blastocoele
from different points of the sm-face and here form an entodermic cell-
mass. Nevertheless the process of forming entoderm by delamination
remains, in contrast with the other types of entoderm formation, some-
what isolated and unexjilained.

Closely related to delamination is a kind of entoderm formation which
was formerly held to be of frequent occurrence, but whose range of dis-
tribution has become more and more restricted by careful investigation of
the individual cases. They are the eases in which the blastomeres present
no radial arrangement about a point within and no definite relation to a
cleavage cavity. Such a stage, which is an apparently irregular solid mass
of cells, without cleavage cavity, has been designated as morula ; and it
is assumed that by a rapid division of the cells at the surface an outer
cell-layer is differentiated from the inner cell-mass, so that hei-e also the
separation of ectoderm from entoderm would be brought about by a
splitting oft' which takes place uniformly over the entire circumference.
We shall see that examples of such a mode of origin of the two primary
germ-layers are still ascribed to many Hydroids and Anthozoa, though
probably the greater part of the cases referred to this method can be
reduced to epibolic gastrulation, in which event the morula-stage, as being
a Schema founded on erroneous assumptions, would have to be omitted.

Even though the last-mentioned modes of entoderm formation, re-
stricted as they are to a few kinds of Metazoa, place many difficulties in
the way of the conception that there is uniformity in this process, it is
probable that more careful investigation may succeed in bringing them
into accord with the less aberrant types already mentioned.

We have seen that the chief axis of the gastrula-stage
unites the anterior or apical (animal) and the posterior or
prostomial poles with each other. In the lowest types of
the Metazoa — the Porifera, Cnidaria and Ctenophora — this
primitive axis becomes the permanent chief axis of the body ;
therefore these groups have been contrasted byHATsCHEK^
as Protaxonia with the rest of the Metazoa, which he terms

' Compare Hatschek's Lehrbuch der Zoologie. Jena, 1888, p. 40, as
well as p. 69 et seq.

10 INTRODUCTION

Heteraxonia or JBilateria. In the latter the blastopore
undergoes a secondary shifting, so that the later chief axis
can no longer be identified with the primitive axis.

The layered structure of the Metazoa becomes more com-
plicated by the appearance of a cell-layer introduced between
the ectoderm and entoderm, which takes a position in the
primitive body cavity, the remnant of the cleavage cavity,
and is designated as mesoderm or middle germ-layer. This
name is applied to any cell-layer introduced between ecto-
derm and entoderm and separated from both by a sharp
boundary, but it is not intended thereby to maintain the
liomology of this layer for all the Metazoa. On the contrary,
it appears that in the Protaxonia mesodermic layers were
independently acquii^ed in various ways. Even in the
Bilateria the homology of the mesoderm in all groups is
not absolutely established, although it may be assumed as
probable.

The mesoderm of the Bilateria arises as a rule out of the
primary entoderm, which in such cases is divided into two
parts : mesoderm and secondary entoderm. In regard to
the mode of origin, we can distinguish two sharj^ly separated
types : the formation out of tioo primitive mesoderm cells
and the formation by the production of diverticula of the
arclienteron} The first type is widely distributed among the
Bilateria. At an early period there become conspicuous at
the prostoma of the gastrula-stage two peculiar cells, b}"-
whose position the median plane, which passes between the
two, is determined. These cells are known as the primitive
mesoderm cells. They move into the space between the
ectoderm and entoderm (therefore into the primitive body
cavity), and by proliferation give rise to two paired cell-
bands, which are called the mesoderm bajids, and out of
which the oi-gans of the mesoderm are consti'ucted. The
formation of the mesoderm by the production of diverticula

' As a third type of mesoderm formation one might jierhaps cite the
formation of a niescnchyma (compare p. 11) in those cases in which, as
in the Nemerteans and Echinoderms, numerous wandering cells migrate
into the blastoccele at an early period. Yet this type could perhaps be
reduced to one of those mentioned above.

INTRODUCTION 1 1

of the arcbenteron, as it occurs in the Chsetognatha, Brachio-
poda, and Chordonia, consists in the development of paired
sac-like diverticula of the archenteron, which become
constricted off, and then as independent coelomic sacs give
rise to the systems of organs of the mesoderm. Different
as these two kinds of mesoderm formation appear to be,
they nevertheless can be reduced, like the processes of
gastrulation by invagination and by polar ingression already
described, to a uniform plan, if w^e assume that in the first
case the mesodermic elements at an early period abandon
(as primitive mesoderm cells) epithelial continuity with the
entoderm, whereas in the second case the mesodermic cell-
mass retains provisionally its epithelial continuity, and only
later becomes separated from the entoderm by the formation
of the diverticula.

As regai'ds the subsequent fate of the mesoderm, we can,
if we disregard the formation of the individual organs,
distinguish two types. In the one case the union of the
mesodermic elements is loosened, and these distribute them-
selves in the manner of amoeboid wandering cells in the
si^ace of the primitive body cavity, which eventually they
completely fill with a tissue consisting of stellate migratory
cells embedded in a gelatinous stroma. This tissue is
known as mesenchyma (0. and R. Hertwig). By separation
of the cells of the mesenchymatous tissue, spaces (lacuna?)
may be formed in it, which may coalesce to form larger
spaces, and so apparently represent a kind of body cavity.
To such spaces the name of pseudocmle is given.

In other cases the largest part of the mesoderm is employed
in the formation of paired sacs, the coelomic sacs, the walls
of which consist of a continuous epithelium of mesoderm
cells. The cavities contained in them represent the true
body cavity or ccelom. The paii^ed coelomic sacs entirely
surround the intestinal canal, so that the walls of the sacs
come together in the middle line above and below the intes-
tine to form the so-called mesenteries. The body cavity
divides the mesoderm into two layers. The outer layer, the
one applied to the ectoderm, is known as the somatic layer, the
inner one, applied to the entoderm, as the splanchnic layer.

12 INTRO AUCTION

There are a number of animals in which the mesoderm
pi'oduces, in addition to the specific organs that have arisen
from it (genital organs, excretory oi-gans), only mesench jma.
Such is the case in the Platyhelminthes. In the great
majority of the Bilateria, on the contrary, the formation of
raesenchyma and coelom occurs together, and there is there-
fore a sort of competition between the two methods of
mesoderm development, so that in one case (Annelids, Sagitta,
Phoronis) the formation of a coelom predominates, in the
other (Mollusca, Arthropoda) that of a pseudocoele (mesen-
chyma).

In the Bilateria there arise from mesoderm the muscula-
tui'e, the genital organs,^ the excretory organs known as
nephridia, the connective tissue, and fatty tissue.

' [The statement that the fundament of the genital organs of the
Bilateria comes from the mesoderm ought to be considerably restricted.
In recent times the observations have been increasing which appear to
supi^ort the Weismannian doctrine of the continuity of the germ cells.
Gkobbex some time ago observed the early differentiation of the sexual
cells in Moina. The same has been known for a long time to be true
of Diptera and Aphid®. Recently Heyiions [Sitz.-Ber. Gi'.-^ell. Ndt.
Freuiule, Berlin Jahrg., 18U3, p. 263) has found similar conditions in
various Orthoptera. The investigations of Fatjssek on Phalangidaj [Biol.
Centralbl., Bd. xii., p. 1, 1892J and the very recent observations of A.
Bkauee (Zeitsclir. f. loiss. Zool., Bd. Ivii. 1894) on the scorpion have
shown the early independence of the genital fundaments in these forms.
Special importance in the present question is to be attached to the
observations of Boveei on Ascaris (Sitz.-Ber. Gesell.f. Morpli. u. I'liysiol.
Miiiichen, Bd. viii., 1892), according to which the sexual cells are dis-
tinguished from the somatic cells even in the first stages of cleavage
owing to the special structure of the chromatic elements of tlieir naiclei.]

CHAPTER I.

PORIFERA.

Sponges reproduce by sexual and non-sexual means. To
the non-sexual kinds of reproduction belong — (1) sprouHnr}
or budding, which may lead to the formation of complicated
stocks or colonies ; (2) the formation of small buds which
separate from the parent body and grow up independently
into new individuals ; (3) reproduction by means of
gemmulce.

The investigations on the development of sponges from
the fertilized egg have not up to the present time yielded
a uniform plan for the embryology of this group, and they
frequently contradict one another. The following may be
mentioned as features common to the development of all
sponges. ^

(1) The sexual products
arise in the connective
tissue of the so-called
mesoderm out of cells
which at first are not to
be distinguished from the
connective tissue cells of
this layer.

(2) The eggs are not
surrounded by any cuti-
cular envelope (chorion)
or vitelline membrane'.
They lie naked in a cavity
lined, with endothelium
(Fig. 1 e) in the mesoderm of the parent body. Here the
expulsion of the polar globules, fertilization, and early de-
velopment take place.

13

Fig. 1.— Egg of Placina trilopha in the
parent body (after Magdeburg), r, polar
globules ; e, endothelial lining.

14 EMBRYOLOGY

The polar globules of the sponges have been overlooked up to the pre-
sent time. According to recent observations by Magdeburg, not yet
published, they present in Placina the appearance typical for most of the
other Metazoa (Fig. 1 /•). Also the i^rocesses of their formation may
well find place in the general plan, while, according to Fiedler's com-
munication (Zeitschr. f. wiss. ZooL, Bd. 47) on Spongilla, it would almost
seem as if there existed here a peculiar type of formation.

(3) The eggs undergo total cleavage, and develop in the
parent body into spheroidal or ovate embryos covered on the
surface with flagella.

(4) When the embryos have reached the stage of the
oval, flagellate, so-called plcmula larva, they emerge and pass
through a swarming stage, during which development makes
but little progress.

(5) After attachment to a fixed support is effected there
follows a rapid transformation into a young sponge, resem-
bling substantially the parent.

We may best arrange the types of sponge development
hitherto known in accordance with the characteristic con-
dition of the swarming stage.

I. — Type of development through a so-called
Am phi blastu la-stage.

The development of Sycandra raphanus, which has been
described by Metschnikoff (Nos. 12 and 13) and F. E.
ScHULZE (Nos. 19 and 22), serves as an example of this type.

The egg of this calcareous sponge undergoes a total and
nearly equal cleavage, but the couT'se of cleavage is somewhat
modified by the relation which the embryo acquii'es to the
wall of one of the radial tubes of the parent (Fig. 2).

The egg is a naked cell, and lies in the parenchyma close
to the wall of a radial tube. It is first divided into two
blastomeres of equal size (Fig. 2 A) by means of a furrow
which is perpendicular to the radial tube, and in relation to
the orientation of the developing embryo must be con-
.sidered as meridional. By means of another meridional
furrow perpendicular to the first one, the two cleavage
spheres separate into four blastomeres, now arranged in the
form of a cross (Fig. 2 B), which are applied to the radial

PORIFERA

15

tube with a flattened basal surface; and since they do not
come into close contact with one another at the centre, they
enclose between them a cavity (cleavage cavity) open above
and below. With the next act of cleavage each of these four
cells is divided into two equal parts by a new meridional
furrow (Fig. 1 G and D) . The embryo now consists of a circle

Pig. 2.— Cleavage stages of Sycandra raphanus (after F. E. Schulze). A, two-
cell stage ; B, four-cell stage ; C eight-cell stage ; D, the same in vertical section in
its relation to the collared epithelium of the maternal radial tube (diagram) ; E,
sixteen-cell stage ; F, the same in vertical section (diagram) ; G later stage of
cleavage with eight granular (ectodermal) cells at the lower pole; H, blastosphere
stage in side view. In the interior the cleavage cavity ; granular cells below,
otherwise an epithelium of tall columnar cells.

IG

EMBRYOLOGY

of eiglit cells, which enclose the cleavage cavity. Since the cells
are applied to the wall of the tube with their broad bases,
and taper conically in the opposite dii^ection, the embryo has
nearly the form of a cup-cake (Fig. 2 D). By means of a
subsequently appearing equatorial furrow each of these
8 cells is separated into an upper smaller [entodermal] and
a lower larger [ectodermal] segment ; and at the same time the
whole shape of the embryo changes in this 16-cell stage,
taking on the form of a biconvex lens by the bulging out of
its basal surface (Fig. 2 E and F). The cleavage cavity is
still open at both poles, although the opening of the upper

ec

c.s.

Pig. 3.— Swarming larval stage of Sycandva raphanua (after F. E. ScnuLZK, from
Balfour's Comparative Embryology). A, amphiblastula-stage; B, stage at the
beginning of the gastrula invagination; cs, cleavase cavity; ec, future ectoderm
colls ; en, future entoderm cells. [The orientation of the larva in this figure is the
reverse of that in Pig. 2 D, Pand If, the entodermal pole being there the upper
one.]— Tbanslatobs.

side is already considerably narrower than that of the lower
one. By means of new meridional and equatorial furrows
the embryo gradually passes into a multicellular stage,
which has an almost spherical shape, corresponding to which
there is an extensive cleavage cavity within. The opening
at the upper pole has disappeared by the apposition of the
cells, while the one corresponding to the former basal sur-
face is still retained (Fig. 2 G). It is surrounded by eight
[ectodermal] cells, which are soon distinguished by increasing

PORIFERA

17

size and by their granular plasma. With the closure of this
lower opening the embryo becomes a spherical blastosphere.
The granular cells now enlarge and multiply to the num-
ber of about thirty-two ; the other cells meanwhile increase in
numbers, and lengthen out into tall columnar prisms (Fig. 2
H), each of which develops a flagellum at the sui'face. The
large, richly granular cells now fold into the segmentation
cavity, and the last stage to be passed in the body of the parent,
the so-called pseudo-gastrula stage, is thus reached. It has
nothing to do with the true process of gastrulation, but re-
presents a transient condition, which was perhaps acquired
in connection Avith the mechanism of hatching.

s^=;nv''v7?5;

Fig. 4. — Attached gastriila-stagre of Sycandra raphanns (after F. E. ScHur.zn).
Ec, ectoderm; En, entoderm; m, gelatinous secretion between the two lasers
(remnant of the cleavage cavity).

When the embryo is hatched the invaginated part (ecto-
derm) returns to its former position, and an elongation in
the direction of the chief axis follows. The ovate swarm-
ing stage now reached is known as the amphiblastula
(Fig. 3 A). It consists of histologically differentiated
halves. The half of the body directed forwards in swimming
is composed of tall cokimnar flagellate cells, whereas the
large granular cells of the posterior half of the body bear no
flagella. Within is seen the considerably reduced cleavage
cavity (csy.

• [According to recent investigations of Dendy (Appendix to Literature
on Porifera, No. II.), small cells, which perhaps become mesoderm cells,
make their appearance in this cavity at an early period.]

K. H. E. C

18

EMBRYOLOGY

After the completion of the swarming stage, and shortly
before the attachment of the larva, a shortening in the
dii'ection of the chief axis takes place, which is accomplished
principally by a flattening of the flagellate, formerly bulging
cell-layer; and an invagination of this layer quickly follows
the flattening, the result being that the cleavage cavity is
entirely obliterated. In this way a cap-shaped gastrula-stage
(Fig. 3 B) is reached. The outer granular layer of cells can
henceforth be considered as the ectoderm, and a circle of

Fig. 5a. — Young, mortar-shaped Olynthus stage of Sycandra rnp/iaiuis (after
F. B. Schulze). Os, osculutn ; }-.o, lateral inhalent pores of the body-wall.

about sixteen of these cells, which are particularly prominent
and are known as marginal cells {Bandzellen), surrounds the
wide gastrula mouth or blastopore, while the invaginated
flagellate layer represents the entoderm.

The attachment of the larva now takes place by the
fixation of the edge of the gastrula mouth by means of
pseudopodia-like processes of the marginal cells to some

PORIFEKA

19

support (Fig. 4). The entire process of gastrulatiou and
attachment proceeds with uncommon rapidity.

In the gas trula- stage ectoderm and entoderm are not
closely applied to each other ; but one notices between them
a space which must be interpreted as the remains of the
segmentation cavity (Fig. 4 m), and which is filled with a
gelatinous hyaline mass. Accoi-ding to Metschnikoff, in-
dividual cells of the granular ectodermal layer migrate into
this mass, and lead to the formation of the mesenchyma, the
so-called mesoderm, between the two primary layers. The
first skeletal structures arise in these cells in the form of
small rod-like needles ; triradiate
ones are formed later, and finally
quadriradiate ones.

After the gastrula mouth has be-
come narrowed and finally closed,
the hollow body of the larva, which
l)as no external opening, elongates
in the direction of the chief axis,
and grows out into a cask-like or
cylindrical form (Fig. 5a), the
upper surface of which consists of
a thin membrane, which acquires
at its centre a circular opening,
the beginning of the exhalent
orifice (oscnlum, Os), which soon
enlarges. At the same time the
inhalent openings or pores (po)
appear as perforations in the
lateral walls. Since, moreover,
the epithelial layer of the ento-
derm acquires the character of
flagellate collar-epithelium, the characters typical of the
Porifera are completed in this ascon-like stage (Fig. Sa,
Olijnthus). The development into the Sycon takes place by
the radial tubes becoming established as simple evaginations
of the body- wall (Fig. 56) ; at first a circle of radial tubes
makes its appearance at about the middle of the body ; to
this a second is soon added, and so on.

Fig. Sh. — Older, attached
stage of Sxjcandra ra'phanus with
the fundaments of the first
radial tubes, r. Po, inhalent
pores.

20 EMDRYOLOGY

The araphiblastula-stage seems to occur in the hfe-history of many
Calcarea. It has also been found in Ascandra contorta (Bakeois), in
Ascandra Lieberkilhnii (Keller), and Leucandra aspera (Keller,
Metschmkoff). The genus Ascetta develops according to another tyi)e.

II.— Type of Development through a Swarming
Coeioblastu la-stage.

The egg of Oscarella lohularis {Halisarca lohularis) de-
velops in the trabeculfe of the tissue in the internal parts of
the parent, which are without flagellate ampullse, and under-
goes total and equal cleavage, by which there are formed at
first two, then four, eight, sixteen, etc., blastomeres of equal
size. At the sixteen-cell stage a distinct cleavage cavity can
be recognized within. By further cell proliferation there is
formed a hollow sphere (coiloblastula or archiblastula), the
wall of wliich is composed exclusively of cubical cells of equal
size arranged in a single layer (Carter, No. 3 ; Barrois,
No. 2 ; F. E. ScHULZE, No. 20).

Shortly before swarming, the elements of the body-wall
lengthen out into columnar epithelial cells, each of which
acquires a flagellum at its outer end. The swarming larva
(Fig. 6 A) possesses an approximately ovate form, and ex-
hibits a blunt yellowish pole, which is directed forwards in
swimming, and a posterior, more pointed bi'ownish-red pole.
The wall of the blastula consists of a single layer of
cylindrical flagellate cells. The internal cavity contains no
cells, and is filled with an albuminous fluid (F. E. Schulze).

By the invagination of one pole of the larva this stage
passes into a hemispherical gastrula-stage, which, like that
of Sycundra, now attaches itself by its gastrula mouth to a
support (Fig. QB). Thus there arises a shallow, cap-shaped
larva, the wall of which consists of two layers (ectoderm
and entoderm) and the inner cavity of which must be con-
sidered as the archenteron. The gradual closure of the
wide gasti'ula mouth now follows ; and, at the same time, by
a complicated process of folding, the first flagellate ampulla?
arise as diverticula) of the archenteron (Fig. GC^). The
mesodermal connective-tissue layer originates by the mi-
gration of cells into the space embraced between the

PORT F ERA

21

ectoderm and entoderm. Lastly, there occurs at the apical
pole an evagination of the body- wall like a chimney-pot, at

r^iiJJnA^ ■ —

Fig. 6. — Development of Osearella, diagrammatic (after Hbideh). A, swarming
blastnla larva ; B, attached gastrula-stage ; C, stage initiating the closure of the
gastrula mouth {Gm) and the folding of the entodermic sac ; D, young sponge ; Os,
osculum })o, inhalent pores ; Ee, ectoderm ; Ea, entoderm.

22 EMBRYOLOGY

the apex of which the osculum (0*) breaks through (Fig. 6 D)
The inhalent pores (po) are formed as perforations at places
where the ampullae lie close to the ectoderm. The system
of inhalent canals, which ai-ises later, must be referred to
invaginations of the ectoderm, that of the exhalent canals to
evaginations of the entoderm. The larva is not attached by
its entire base, but rests upon a few foot-like supports
(K. Heider, No. 8).

The development of the PlakinidaB appears to be closely related to that
of Oscarella. The swarming larvae can on the whole be included in the
type just described. The metamorphosis into the attached stage has
not been accurately investigated ; it is certain, however, that here also
the amijullse arise as diverticula of a common central cavity (F. E.
Schulze).

III.— Type of Development through a
Parenchymula Stage.

The superficial layer of the swarming larva consists of a
cylinder epithelium, composed of long flagellate cells, and
encloses an internal space filled with embryonic connective
tissue.

(a) The superficial layer presents on all sides cells of
nearly uniform condition.

Afcetta. — By total and equal cleavage there is first produced a
eceloblastula-stage, which is similar to that of Oscarella. But even
before the hatching of the embryo, which attains an ovate form in
later stages, the immigration of cells into the internal cavity takes
place ; and this occurte at the posterior pole of the larva. In this
manner the primitive body cavity becomes filled with a connective-tissue
mesenchyma, the common fundament of the mesoderm and entoderm,
in which the permanent gastral cavity subsequently appears as a fissure.
The entoderm cells arrange themselves about this in the form of a
unilaminar epithelium (0. Schmidt, Metschnikoff).

(h) The superficial layer in the region of the posterior
pole of the larva presents an altered condition of the cells. ^

' [The investigations of DiCLAfiE (No. I., Appendix to Literoture) and
those of Maas (Nos. IV. and V.) on the development of the Cornacuspongia
(Ceratosa and Silicea) are of great importance. A satisfactory founda-
tion for the interpretation of the development of sponges in general has

PORIFERA

23

Ceratosa. — The embryos of Spongelia pallescens (Fig. 7) which are
ready to swarm possess a cylindrical form with one end convexly rounded
and with a shallow depression at the other. In the region of this shallow
invagination the flagellate cells are pigmented brownish red. The inside
of the embryo is filled with a gelatinous connective tissue. The embryo
of Euspongia officinalis before swarming presents a very similar struc-

nt4

Pig. 7.— Longitudinal section tbrough a larva of Spongelia pallesceus (a'^tcr
F. B. Schulze). a, pigmented epithelial cells of the posterior pole of the body;
ms, gelatinous connective tissue inside the larva.

(The superficial covering of flagella has been omitted in the illustration.)

thereby been acquired, inasmuch as the development of the Cornacu-
spongia fits in completely with that of Sycandra. The free-swimming
larval stage corresponds to the pseudo-gastrula stage of Sycandra. The
superficial flagellate epithelial cells of the larva subsequently become
the collared entoderm cells of the adult sponge ; whereas the inner cell-
mass, which in many cases is exposed at the posterior pole of the larva,
represents the common fundament of the ectoderm and mesoderm. The
ectoderm and mesoderm of sponges must be regarded as an undivided
whole. The larvae attach themselves by the anterior pole, and at the
same time occurs that inversion of the layers which must be compared
with the process of actual gastrulation in Sycandra.]

24 EMBRYOLOGY

ture, save that the parenchyma fiHinf; the internal cavity is of a different
histological character ; it consists of a tissue comparable to hyaline
cartilage. The cleavage in this form is total and equal, although a
cceloblastula-stage is never developed (F. E. Schulze, Nos. 23 and 24).

Chalinida;. — The egg of Chalinula fertilis segments totally and un-
equally, according to C. Keller (No. 9). The first act of cleavage
results in a larger and a smaller blastomere. The next stage which has
been observed shows three small and one large blastomere. By the
succeeding division of the small cells there results a stage in which six
small cells rest like a cap upon the large undivided blastomere, which
also soon divides. The small cleavage spheres are said to represent the
fundament of the ectoderm, the large ones that of entoderm and mesoderm
together. In the further course of cleavage, the small cells, having the
form of a shell, grow around the large ones in such a manner that there
results an epibolic gastrula, the mouth of which is entirely filled by the
solid cell-mass of the i^rimary entoderm, which is plainly visible at this
point. The embryo now develops fiagella on its entire surface, acquires
a more elongated form, and emerges as a planula larva. At its i^osterior
l^ole it presents a darker-coloured area of the superficial fiagellate layer,
and this corresponds to the superficial entodermic portion. The earliest
spicules are very soon developed in the cells of the internal parenchyma.
The larva now attaches itself by its posterior pole, but very soon turns
over, so that it adheres to the support by the entire broad side of the
body. It now acquires the form of an irregular flat cake. The forma-
tion of the ampullae within is said to j^roceed in a different manner from
that described for Oscarella (p. 22) ; that is to say, individual entodermic
cells unite into compact groups, within which a cavity aj^pears later.
The fundaments of the ciliated chambers (ampullffi), which at first were
independent, then establish relations with a large central cavity arising
in the i^arenchyma, which soon breaks through to the outside at the apex
of the larva, thus producing the osculum. The larva3 of most siliceous
sponges appear to belong to a type similar to that of the swarming larva
of Chalinula, thus the larvfB of Esi^eria, Amorphina, Easpailia, and
Reniera (0. Schjiidt, Metschnikoff), further those of Isodyctia and
Desmacidon, made known by Ch. Bahkois.

Reniera. — The larva of Reniera filigrana resembles the ones previously
described. It consists of a flagellate columnar epithelium and an in-
ternal cellular parenchyma. In the course of the further development
the layer of columnar cells ruptures at the anterior and jiosterior ends,
so that the internal parenchyma is exposed. The larva attaches itself
by the anterior pole, loses the covering of flagella belonging to the super-
ficial layer, and takes on the form of a flattened cake, while in the
internal ijarenchyma, the common fundament of entoderm and meso-
derm, a cavity apjiears in the form of a fissure, about which the nearest
cells group themselves in the form of an epithelium. In this manner
the entodermic epithelium is separated from the mesoderm. The first

PORIFERA

25

ampullae and all of the canals arise as evaginations of this inner cavity.
Later the osculum breaks through, and siliceous spicules are formed in
the cells of the mesodermic layer (W. Maeshall, No. 10).

Halisarca. — By total and equal cleavage (F. E. Schulze, No. 20) there
originates a blastula, into which migrate cells that entirely fill the
cleavage cavity, and there form a connective-tissue mesenchyma. Upon
emerging the larva presents at the jDOsterior pole an area consisting of large
granular flagellate cells. After the larva has attached itself and assumed
a cake-like form, the ectoderm loses the flagella, and is changed into a
pavement epithelium. In the internal parenchyma there now arise
fundaments of the ampullar and canals which at first are separate, but
subsequently unite into a common system (Metschnikoff, No. 1-4).

A unique type of development, which perhajis most
resembles Reniera, appears to exist in Spongilla. The de-

Fig. 8. — Late stage of cleavage (beginning gastrulation) of Spongilla (Epkydatia)
fiuviatilis (after Goette). Ec, ectoderm cells; En, entoderm cells; d, central
entodermlc cavity.

velopment of this fresh- water sponge has been described by
Ganin (Nos. 4 and 5) and Goetie (No. 6) ; but in many
points the statements of these investigators do not altogether
agree. In our presentation we follow^ the detailed descrip-
tion of Goette without forming any preliminary judgment
as to the way in which the development of Spongilla is
related to that of other sponges. A tinal judgment will be
possible only when new observations have been made on
the development of sponges of the most widely diffei'ing
groups.

The egg of Spongilla (Ephydatia) fiuviatilis undergoes

26 EMBRYOLOGY

total unequal cleavage, by means of which there arises an
embryo consisting for the greater part of large blastomeres
rich in yolk (entodermic portion, Fig. 8 Eii), and presenting
only at its upper pole a cap of small blastomeres containing
little yolk (ectodermic part, Fig. 8 Ec). Since the cap of
ectoderm cells gradually grows around the entire embryo,
there is formed in this way a kind of epibolic gastrula. At
an early period there appears in the entodermic mass an
eccentrically placed irregular cavity, which can be referred
neither to a cleavage cavity nor to an ai'chenteron, and is
known as the entodermic cavity. The chief axis of the
embryo is recognized by the eccentric position of this cavity,
for it always lies close to the apical pole (the subsequent
anterior pole). The embi-yo, which at first is comparable to
a plano-convex lens, now elongates in the direction of the
chief axis, and is covered on the surface with a coat of
cilia. The swarming larva (Fig. 9) is generally ovate
in form, and in floating in the water has the broadened
anterior end of the body, in which the spacious entodermic
cavity is situated, always directed upwards. The larva
possesses a superficial, unilaminar, flagellate epithelium, all
the cells of which present the same character. Within the
posterior half of the body is found a solid entodermic
mass, which, in the course of the further development, takes
on the character of embryonic connective tissue. The cells
lying in the vicinity of the cavity are flattened, and form a
reticulated layer of amoeboid elements. A similar layer of
flattened cells is found at the sui'face of the solid entodermic
core, where it is in close contact with the ectoderm. At an
early period spicules are developed in cei-tain cells of this
core.

The larva attaches itself by the apical pole, the layer
covering the entodermic cavity being split open, and the
edges of the bx^each thus formed bending outward. In this
way the cells of the entodermic layer come in contact with
the support, to which they adhere by means of pseudopodia-
like processes. In the majority of cases the larva after this
fii-st attachment bends over in order thus to attach itself by
a broader surface. According to Goette, there ensues the

POEIFERA

27

complete loss of the ectoderm, which breaks up and is detached
from the surface in shreds, so that the entire body of the
young sponge consists of entoderm. In this now solid mass
(for the entodermic cavity has disappeared in the process
of attachment) ampullae arise as separate fundaments ; they
are produced by groups of cells (each of which has ai-isen
from a single cell) acquiring cavities within them. The
canals and cavities of the body develop in the same manner
in many separate regions, and afterwards unite with one
another and with the ampullfe. The most superficial layer
of the body acquires the character of a pavement epithelium,

^c

-- ZW

Fig. 9.— Free-swimming larva of Spongilla fluviatilis (after Goeitk). The
covering of flagella has been omitted. Ec, ectoderm; En, entoderm; d, ento-
dermic cavity.

and forms the permanent epidermis of Spongilla. All or-
gans therefore arise by histological differentiation from a
single layer, the primitive entoderm.

In Goette's account, which we have given here, several points must
appear to be still doubtful, thus, above all, the alleged complete casting off
of the ectoderm. To be sure, a rupture and partial loss of the ectoderm
has also been maintained for other sponges (Beniera, Esperia) by earlier
observers, to which attention has already been called (p. 24). But such
appearances are probably for the most part to be referred to pathological
or abnormal processes. Ganin has assumed that in Spongilla the ecto-
derm of the larva becomes the permanent epidermis of the sponge ; and

28 EMBRYOLOGY

that statement has recently been confirmed by the observations of
0. M.^\s {Zool. Anz., 1889), who has convinced himself on the same object
that the ectoderm of the larva is not cast off, but gradually passes over
into the superficial pavement epithelium of the adult sponge. Also in
regard to the origin of the ampullfe and canal system Gaxin does not
agree with Goette. According to Ganin, the entodermic cavity is to be
explained as an archenteron, and represents the earliest fundament of
the canal system, from which the ampuUffi arise as diverticula. A mode
of origin for the amiDullae similar to that communicated by Goette for
Spongilla has recently been maintained by Dexky (Quart. Juiir. ][licr.
Sci., 1888) for one of the horn sponges (Stelospongia).

The distribution of the recognized types of development among the
different groujjs of sponges, then, is represented as follows : In the
calcareous sponges (Calcarea) the ami)hiblasiula is found in most of
the cases observed up to the present time. Perhaps this larval form
is confined to the Calcarea. The coeloblastula appears in Oscarella and
in the family of the Plakinidae, whereas the parenchymula seems to be
present generally in the Geratosa and siliceous sponges. Furthermore
Halisarca and Ascetta exhibit a parenchymula stage.

As is to be seen from the preceding, a uniform plan of
development for sponges cannot at tlie present time be
formulated. "^ The statements differ too widely. In certain
cases we find a coelogastrula-stage, which attaches itself to
some support by the circumference of the large gastrula
mouth. This is of importance as a distinguishing charac-
teristic in contrast with the Cnidaria, in which the attach-
ment is always effected by the aboral pole of the two-layer
planula larva. In other cases there is formed a parenchymula,
the genesis of Avhich for many forms is as obscure as the
further development of this stage into the adult sponge.
We can only assume conjectural ly that this stage is in all
cases brought about by epibolic g;xetrulation or by the
process of migi-ation of certain cells from the entodermic
pole. The most obscui*e point in the development of
sponges is the metamoi-phosis accomplished at the moment
of the attachment of the swarming larva. Even the state-
ments regarding the pole by which the larva attaches itself
vary for the different forms. Authors likewise differ in
regard to the organogeny, above all as regards the origin
of the canal system and the flagellate ampullae. In certain

' Compare footnote, p. 22 [Tkanslatorsj •

PORIFEBA 29

cases the common fundament of the ampullas and exhalent
canal system is afl&rmed to have the form of a central
cavity (archenteron), from which the ampulla? and exhalent
canals arise by repeated foldings of the wall. Opposed to
this are the observations of other authors, according to
whom the different ampullae are formed independently, and
become united by means of the canals which appear later
on, and which only gradually unite into a common system
of canals.

In such a condition of affairs it is hardly possible to
arrive at any genei'al conclusions without in one way or
another doing violence to the individual statements. This
much, however, seems with some certainty to result from all
the observations : that we have before us in the sponges an
independent stem of the Metazoa, which is connected with
the other types only at its roots. We adhere to the view
that the sponges have a common origin with the rest of tlie
Metazoa. In the embryology of the sponges we find true
blastula- and gastrula-stages, which appear to point to
an ancestral form common to the Porifera and all other
Metazoa. Characteristics of histological differentiation (the
formation of columnar and pavement epithelium, of con-
nective tissue and cartilaginous tissue) likewise point to this
community of origin. As opposed to these characteristics, the
single fact of the occurrence of the collar-bearing flagellate
cells of the entoderm does not seem sufficient to warrant the
derivation of the Porifera as an independent group from the
Choanoflagellata and the denial of their phylogenetic
relationships to the rest of the Metazoa (Sollas, No. 15 ;

BiJTSCHLl).

That the Porifera do not stand in any close relationship
to the Cnidaria (Coelenteratain the stricter sense) (Marshall,
No. 11) appears to be beyond doubt. We lay less stress upon
the absence of nettling capsules, as being a purely histological
character, than on tectonic points. The attempts to reduce
the structure of sponges to the fundamental form of the
Polyp must lead to contradictions. Above all must be
emphasized the fact that the exhalent opening of the canal
system, the so-called osculum, is not homologous with the

30 EMBRTOLOO.Y

moath of Coelenterates, furthermore that the Porifera in
general are derived from a monaxial, heteropolar funda-
mental form in which the production of secondary axes in
definite numbers has not yet taken place, whereas the radial
type with four rays lies at the foundation of the Cnidaria
(compare F. E. Schulze, No. 12; A. Goette, No. 6 ; Heider,
No. 8). The absence of movable processes of the body
(tentacles) and the low grade of histological differentiation
serve as substantiating facts in support of this view.

The Porifera possess no true muscle fibres. The proi^erty of con-
tractility appears rather to belong to all the cells in about the same
degree, and the " contractile fibre-cells " occurring in the mesoderm of
many sponges are distinguished from true muscle fibres by the fact that
the contractile substance in them has not yet become separated as a
distinct portion of the cell. The absence of a nervous system has not
yet been proved, it is true ; but the presence of such a system does not
appear to be firmly established, for up to the present time the groups of
cells claimed by Lendenfeld as the nervous system of sponges have
remained doubtful as regards this interpretation.

In respect to the origin of the canal system of sponges,
reference must be made to those primitive forms which are
found more especially among the calcareous sponges, and
by comparison of which it is most clearly proved that the
complicated canal system of the siliceous and horny sponges
has been evolved by a continuous process of folding of the
wall of the sacular olynthus-like primitive form, whereby
the collar epithelium eventually becomes localized in
particular parts (ampullae) of the canal system. If the
diagram Fig. 10 A represents the wall of a simple ascon
perforated by pores, and if we bear in mind that the entire
inner surface is covered with collar-cells, then Fig. 10 B
shows the origin of the radial tubes of a sycon by means
of a folding of this walL At the same time the entoderm
lining the common central cavity is transformed into a pave-
ment epithelium ; and alternating with tlie evaginations of
the radial tubes, enfoldings (a) of the outer surface of the
body lined with ectodei'm have also been formed, the funda-
ments of the inhalent canal system. Fig. 10 C shows how,
by a repeated process of folding, diverticular spaces (h) of

PORIFERA

31

.,,-.- I-// 1 t/

V

/? a^

P'-''-''' "■;;"!," ^

." ■: ''^

Fig. 10.— Diagram of the development of the canal system in various sponges.
The ectoderm is indicated by a continuous, and the entoderm by a broken, line.
The collar epithelium of the entoderm is e.xpressed by perpendicular striations.
A, cross section through a part of the wall of an ascon ; B, cross section through
the wall of a sycon; O, cross section through the wall of Leucilla connexiva (after
Polejaeff) ; D, vertical section through Oscarella ; a, spaces of the inhalent
canal system; b, spaces of the exhalent canal system ; os, osculum.

32 EMBRYOLOGY

the central cavity ai-ise, in which tlie fundaments of an
exhalent canal system, lined with entoderm, ai-e to be
recognized, so that in this manner we arrive by gradual
transitions at the distribution of the canals shown in Fig.
10 D, which serves as a plan for most sponges.

This series, resulting from the comparison of different
genera, makes it probable that the ontogenetic origin of the
gastro-canal S3'stem by the formation of diverticula from a
single common central cavity, as it has been observed in
many forms, represents the original mode of development.
As to the development of the calcareous or siliceous spicules,
it appears to be certain that they are formed within skeleto-
genous mesoderm cells. The circumstance that the forms of
the spicules frequently merge into one another, and that in
many of the rod-like spicules an axial cross has been ob-
sei'ved in connection with the central canal, suggesting their
origin from triaxial spicules, gave rise to certain specula-
tions on the fundamental form of the spicules occurring in
the different groups and their derivation from the soft parts
of the body through simple mechanical conditions. Thus
F. E. ScHULZE (Abh. hgl. Acad. Berlin, 1887) found that the
fundamental form of the regular triaxial spicules of the cal-
careous sponges was determined by the regular altei'nating
position of the pores in the wall of the original ascon-like
animal, and that the peculiar initial quadriradiate form of the
Tetractinellidae (as well as that of the sponges derived from
them, the Monactinellidae and the horn sponges) were explain-
able by the closely crowded position of the spherical ampullae
and the resulting forms of the soft parts of the body, whereas
in the Hexactinellidi« the arrangement of the trabeculje of the
soft parts led to the fundamental form of the regulai- sexi-
radiate spicules.

Whereas the hard parts just mentioned develop within
cells, the horn fibi-es must be considered as cuticular secre-
tions, for, according to F. E. Schui-ze, they are deposited on
the inner surface of mesoderm cells (so-called sponglohlasts)
arranged like an epithelium.

Non-sexual Reproduction of Sponges. — In this con-
nection is to be mentioned the formation, by means of budding.

PORIFERA

38

of new individuals, which remain united to the parent
organism throughout life, thereby permitting the formation
of extensive colonies. In many cases (Sympagella nux,
Hexact.) the single individuals of the colony can readily be
recognized as such (No. 4), whereas in the majority of cases
their connection becomes so intimate that it is only the pre-
sence of the oscula which makes the recognition of the
individuals to a certain extent possible.

Fig. 11. — Lophocalyx (Polyloplms) philipiyinensis with buds (after F. E. Schulze).
a, young bud ; b, older bud constricted off from the parent and attached to it by
the siliceous spicules of the parent only.

Furthermore, a kind of reproduction occurs in many
sponges by means of buds, which separate from the parent
in a partially developed condition, and grow up into a new
sponge organism. A comparatively simple case of this kind
appears to exist in Leucosolenia (Vassbur, No. 36.) The
young bud is here a simple outfolding of the body- wall, which
is soon separated as an independent sac-like body, and after
becoming attached, grows up, and by the production of an
osculum becomes a young Leucosolenia. In a similar man-

K. H. E. D

34 EMBRYOLOGY

ner the budding in Tethja, Tetilla, Rinalda, etc. (Dfiso, No.
29; Mekejkowsky, No.31; Selenka, No.34), and alsoinLopho-
caljxi. e. Polylophus (F. E. Schulze, No. 33), appears to de-
pend npon the outgrowth and abstriction of a portion of the
parent body, in which is included a part of tlie canal system
of the latter, while the tissues exhibit an active cell-
proliferation. The separation from the parent frequently
takes place in these cases by an emigration along pi"ojecting
siliceous spicules (Fig. 11). After separation has taken place,
the bud grows up into a young animal resembling the parent
organism. To this class belong also the transportable brood-
buds of Oscarella described by F. E. Schclze (No. 32), which
are very similar in structure to the lar^a of this form (Fig. G
D), since they contain a considerable internal cavity. These
vesicular bodies, after they have become separated from the
parent, are for a time carried about by currents, and then
fall to the bottom, whei'e they grow into young sponge-
crusts.

Reproduction by budding in these forms depends upon the
fact that the superficially located parts of the sponge tissue
become separated and acquire the power of I'eproducing the
entire form of the parent organism. If we consider a simi-
lar process to take place within the tissues of the sponge,
whereby the separation of such a group of cells is accom-
plished by an encystment, then perhaps the manner is indi-
cated by which we are to consider the fii^st formation of
gemmula; to have taken place. Reproduction by means of
gemmulae occurs principally among the fresh-water sponges
(Spongilla), although the occurrence of gemmula-like
structures has also been affirmed for a few marine forms
(Topsent, No. 35). The fully developed gemmula (Fig. 12)
consists of a multicellular germ (rf), whose large cells, poly-
gonally flattened by mutual pressure, are filled with yolk
particles, and present one, frequently two or more, nuclei
(Petr, Weltner). This germ-body is sux'rounded by an
envelope often very complicated in structure, which opens to
the exterior by means of a pore (p) provided with an oper-
cular apparatus. There is always found a thick cuticular
layer (c), to which there is generally adiled externally a

PORIFERA

35

porous meshwork containing air (air-chamber layer), in
which skeletal elements (needles or ampliidiscs) are often
found embedded (6), while outside of all there may be added
still another cuticular layer (a). Furthermore the germ-
body is said to be immediately enveloped in a delicate
membrane (Carter).

The gemmulse are found in the midst of the mesodermal tissue of the
parent body. Many views have been advanced in regard to their origin.
According to Goette (No. 6), there is a kind of cell-proliferation that
aiJects a particular territory and also involves the ampullas and canals of
this region, whereas, ac-
cording to Marshall (No. ^
30), certain mesodermic
cells, filled with reserve
food- stuff, creep together
in groups to form the
gemmulfe. The earliest
fundament of the gem-
mula, which is essentially
a mass of cells having
embryonic characters,
soon exhibits a differentia-
tion of two layers (Goette,
WiEEZEJSKi). The central
mass is composed of large
cells in which yolk par-
ticles become embedded in
ever-increasing quantities.
The cells of the outer
layer, according to Goette,
become club-shaped, and
arrange themselves into a
kind of elongated ei^ithe-
lium enveloping the central mass. This layer, like the spongioblasts, at
first secretes a thick cuticula on the inside (the fundament of the inner
cuticular membrane, Fig. 12 c) ; the amphidiscs are then formed in the
cells of this layer, whereupon it moves outward in order to secrete, likewise
from its inner surface, the outer cuticular membrane, Fig. 12 a (Goette).
According to Wieezejski (No. 39), the amphidiscs are not formed in the
layer of cylindrical cells mentioned, but in the surrounding tissue, and
only later move into this epithelial layer, in which they become arranged
in a definite manner.

The formation of the gemmulse takes place principally in the fall in
parts of the sponge which generally die after gemmulation has taken

Fig. 13. — Gemmula of Spongilla (Ephydatia)
fluviatilis (after Vbjdovskt). o, outer cuticular
layer ; b, amphidisc layer ; c, inner cuticular
layer ; d, germ-body ; p, pore.

36 EMBRYOLOGY

place. In the following spring the germ-body crawls out hrough the
pore, to become attached and metamorphosed into a young sponge, by
means of processes of development which have not yet been accurately
studied.*

From what has been said it follows that in the gemmulae
we have to do with an encysted portion of the parent body,
provided with food-yolk and retrograded to an embryonic
condition, which acquires the power to regenerate the parent
organism. On this account gemmulation has also been
designated as a kind of internal budding. In support of
another current interpretation, according to which the
gemmules would represent the winter eggs of Spongilla,
evidence is as yet entirely wanting, for in this case it would
be necessary to prove that the germ- body originates by the
division of a single original cell, but all observations made
up to the present time are opposed to this.

Literature.

Earlier writings by Geant, Liebekkuhn, and Miclucho-Maclay.

1. Balfour, F. M. On the Morphology and Systematic Position of the

SpongidiB. Quart. Jour. Micr. Sci. Vol. xix. 1879.

2. Bareois, C. Memoire sur I'embryologie de quelques Eponges de la

Manche. Aim.sci.7iat.(Ser.4). Tom. iii. 1876.

3. Carter, H. J. Development of the Marine Sponges, etc. Ann. a)td

Slag. Nat. Hist. Vol. xiv. 1874.

4. Ganin, M. S. Zur Entwicklung der Spongilla fluviatilis. Zool.

Anzeigcr. Jahrg. i. , No. 9. 1878.

5. Ganin, M. S. Materialien zur Kenntniss des Baues und der Ent-

wicklung der Spongien. Warschau. 1879 (Russian).

6. GoETTE, A. Untersuchungen zur Entwicklungsgeschichte von Spon-

gilla fluviatilis. Hamburg u. Leipzig. 1886. Also Zool. Ameiger.
Jahrc). vii. ti. viii.

7. Haeckel, E. Die Kalkschwamme. Berlin. 1872.

8. Heider, K. Zur Metamorphose der Oscarella lobularis. Arbeitcn

a. d. zool. I7ist. z. Wien. Bd. vi. 1886.

9. Keli;Eh, C. Studien liber die Organisation und die Entwicklung der

Chalineen. Zeitschr.f. iciss. Zool. Bd. xxxiii. 1880.
10. Marshall, W. Die Ontogenie von lleniera filigrana. Zeitschr.f.
wiss. Zool. Bd. xxxvii. 1882.

1 [Compare Zykoff (Nos. VIII. and IX., Appendix to Literature on
Porifera).]

PORIFERA 37

11. Marshall, W. Bemerkungen iiber die Coelenteratennatur der

Spongien. Jena Zeitschr. Bd. xviii. 1885.

12. Metschnikoff, E. Zur Entwicklungsgesehichte der Kalkschwam-

me. Zeitschr. f. wiss. Zool. Bd. xxiv. 1874.

13. Metschnikoff, E. Beitrage zur Morphologie der Spongien. Zeit-

schr. f. loiss. Zool. Bd. xxvii. 1876.

14. Metschnikoff, E. Spongiologische Studien. Zeitschr. f. wiss.

Zool. Bd. xxxii. 1879.

15. Schmidt, 0. Zur Orientirung iiber die Entwicklung der Schwamme.

Zeitschr. f. wiss. Zool. Bd. xxv. Suppl. 1875.

16. Schmidt, 0. Nochmals die Gastrula der Kalkschwamme. Arch. f.

mikr. Anat. Bd. xii. 1876.

17. Schmidt, 0. Das Larvenstadium von Ascetta primordialis und A.

clathrus. Arch. mikr. Anat. Bd. xiv. 1877.

18. Schulze, F. E. Untersuchungen iiber den Bau und die Entwick-

lung der Spongien. I. Mitth. Ueber den Bau und die Entwick-
lung von Sycandra raphanus. Zeitschr. f. wiss. Zool. Bd. xxv.
Suppl. 1875.

19. Schulze, F. E, Untersuchungen, etc., II. Die Gattung Halisarca.

Zeitschr. f. iviss. Zool. Bd. xxviii. 1877.

20. Schulze, F. E. Untersuchungen, etc., IV. Die Familie der Aply-

sinidse. Zeitschr. f. wiss. Zool. Bd. xxx. 1878.

21. Schulze, F. E. Untersuchungen, etc., V. Die Metamorphose von

Sycandra raphanus. Zeitschr. f. wiss. Zool. Bd. xxxi. 1878.

22. Schulze, F. E. Untersuchungen, etc., VI. Die Gattung Spongelia.

Zeitschr. f. loiss. Zool. Bd. xxxii. 1879.

23. Schulze, F. E. Untersuchungen, etc., VII. Die Familie der Spon-

gidae. Zeitschr. f. wiss. Zool. Bd. xxxii. 1879.

24. Schulze, F. E. Untersuchungen, etc., IX. Die Placiniden. Zeit-

schr. f. loiss. Zool. Bd. xxxiv. 1880.

25. Schulze, F. E. Untersuchungen, etc., X. Corticiura candelabrum.

Zeitschr. f. vnss. Zool. Bd. xxxv. 1881.

26. Schulze, F. E. Ueber das Verwandtschaftsverhaltniss der Spongien

und Choanoflagellaten. Sitzungsb. d. preuss. Akad. d. IVissensch.
Berlin. 1885.

27. SoLLAS, W. J. Development of Halisarca lobularis. Quart. Jour.

Micr. Sci. Vol. xxiv. 1884.

28. Vosmaer, G. C. J. Porifera in : Bronn's Classen und Ordnungen

des Thier-Eeichs. Leipzig u. Heidelberg. 1887.

NoN-SEXUAL EePRODUCTION OF SpONGES.

29. Deso, B. Die Histologic und Sprossenentwicklung der Tethyen.

Arch.f. mikr. Anat. Bd. xvi. 1879 u. Bd. xvii. 1880.

30. Marshall, W. Vorl. Bemerkungen iiber die Fortpflanzungsverhalt-

nisse von Spongilla lacustris. Sitzungsb. Natiirf. Ges. Leipzig.
Jahrg. xi. 1884.

38 EMBRYOLOGY

31. Meeejkowsky, C. de. Reproduction des ifeponges par Bourgonne-

ment exterieur. Arch. d. Zool. exper. et gen. Tom. viii. 1879 — 1880.

32. ScHULZE, F. E. Ueber die Bildung freischwebender Brutknospen

bei einer Spongie, Halisarca lobularis. Zool. Anzeiger. Jahrg. ii.
1879.

33. ScHULZE, F. E. Report on the Hexactinellida collected by H.M.S.

Challenrfer, etc. '' Challenger" Reports. Vol. xxi. 1887.

34. Selenka, E. Ueber einen Kieselschwamm von achtstrahligem Bau

und iiber Entwicklung von Schwammknospen. Zeitschr. f. wiss.
Zool. Bd. xxxiii. 1880.

35. ToPSENT, C. Gemmules of Silicispongiffi. Ahstr. Jour. Eog. Micr.

Soc, London, 1888, p. 596.

36. Vassetjr, G. Reproduction asexuelle de la Leucosolenia botryoides

(Ascandra variabilis H.). Arch. d. Zool. exper. etgen. Tom. viii.
1879—1880.

37. Vejdovsky, F. Revisio faunje Bohemieae P. I. Die Siisswasser-

schwamme Bohmens. Abh. d. k. Bohni. Ges. d. Wiss. z. Prag.
Bd. xii. 1883.

38. WiERZEjsKi, A. Ueber die Entwicklung der Gemmulse, etc. (Polish).

Abh. Acad. Krakau. Bd. xii. 1884.

39. WiERZEJSKi, A. Le developpement des gemmules des Eisonges

d'eau douce d'EuroiJe. Arch. d. Biol. Slav. Tom. i. 1886.

Appendix to Literature on Porifera.

I. Delage, Yves. Embryogenie des Eponges ; Developpement post-
larvaire des eponges silicieuses et fibreuses marines et d'eau
douce. Arch. Zool. exper. et gen. Tom. x. 1892.
II. Dendy, a. On the Pseudogastrula-stage in the Development of
Calcareous Sponges. Proc. Roy. /Soc, Victoria. 1890.

III. Maas, 0. Ueber die Entwicklung des Siisswasserschwammes.

Zeitschr.f.wixs. Zool. Bd. 1. 1890.

IV. Maas, 0. Die Metamorphose von Epeira lorenzi 0. S. nebst

Beobachtungen an Anderen Schwammlarven. Mitth. Zool.

Sta. Neapel. Bd. x. 1892.

V, Ma-^s, 0. Die Embryonalentwicklung und Metamorphose der

Cornacuspongien. Zool. Jahrb., Ahth. f. Anat. Bd. vii. 1893.

VI. Weltner, W. Spongilledenstudien I. (contains bibliography of

487 numbers) and II. (on gemmules, etc.). Arch. f. Maturg.

Jahrg. 59. Bd. i., p. 209. 1893.

VII. Wilson, H. V. Notes on the Development of some Sponges.

Jour. Morph. Vol. v. 18<)1.
VIII. Zykoff, W. Die Entwicklung der Genimulie der Ephydatia
iiuviatilis. Zool. Anzeiger. Jahrg. xv. 1892.
IX. Zykoff, W. Entwicklungsgeschichte von Ephydatia miilleri aus
den Gemraula;. Biol. Centralbl. Bd. xii. 1892.

CHAPTER 11.
CNIDARIA.

Systematic : I. Hydrozoa.

1. Hydroidea

2. Siplionopliora.
II Anthozoa.

III. SCYPHOMEDUSiE.

I. HYDROZOA.

I. Hydroidea.

The sexual products of the Hydroidea are usually matured
in specially organized individuals, which are either free-
swimming, and then attain to the high degree of organization
of the hydroid medusa, or remain united throughout life with
the polyp colony, and then, as sessile medusoid gonopJiores
(Sporosacs), exhibit that organization only in a degenerated
condition. In Hydra, on the contrary, the sexual products
develop in the ectoderm of the body-wall of the polyp.

The eggs of the hydroid medusEe are generally extruded,
by the dehiscence of the wall of the gonad, into the sea-
water, where they are fertilized and undergo development.
But in those forms which possess sessile gonophores the
first stages of development take place within the gonophore,
and the embryo does not become free until it attains the
stage of a planula or actinula.

In the following account we separate as metagenetic forms
those which produce free-swimming medusae (forms with
alternation of generations) from those whose sexual indi-
viduals remain sessile, as medusoid buds (forms with masked
alternation of generations, Hatschek). A third group em-
braces those Hydroids in which there is developed out of the

40

EMBRYOLOGY

ogg not a polyp, but a free-swimming' larva, which passes
into the form of a medusa by a simple metamorphosis {hypo-
ynietic fcrrms with suppressed alternation of generations).

Metagenetic Medusae. — We begin with the description
of the better-known cases of development of the eggs of
hydroid medusa^, and follow principally the accounts of
Claus (No. 3) and Metschnikoff (No. 12). The spheroidal
eggs of the craspedote medusas are for the most part colour-
less, transparent, and destitute of a membrane. There may
be distinguished in them a layer of ectoplasm, consisting of
a viscid formative yolk, and an endoplasm filled with coarse

Fig. 13. — Develo|)ment of the epg of JJatJifcea /nsctculafa (after Metschnikoff).
A, an egg immediately after deposition ; r, polar corpuscles ; B, stage of first divi-
sion ; C, eight-cell stage ; D, blastula-stage in optical section ; E, planula-stage with
formation of entoderm, ew.

granules of nutritive yolk (Fig. 13 A). After fertilization
they undergo a total, and in most cases a nearly equal, cleavage.
By the formation of the first two meridional and mutually
perpendicular furrows (extending from the animal to the
vegetative pole, Fig. 13 i)), there arise four blastonieres
placed in the form of a cross, and by a succeeding equatorial
furrow there is produced an eight-cell stage (Fig. 13 C),
while two additional meridional furrows lead to the formation
of a sixteen-cell stage. Only in certain cases (^quoi'ea) i.s
the cleavage more unequal, the blastomei-es of the animal

CNIDARIA

41

zone being less voluminous than those of the vegetative. In
early stages the blastomeres move away from the centre, so
that there is foi'med a gradually enlarging cleavage cavity
within. By additional but less regular cleavages the blasto-
meres inci-ease in numbers, and arrange themselves into a
single layer of epithelial cells surrounding the cleavage
cavity, thus attaining the typical hlastula- stage (Fig. 13 jD).
This cell-vesicle now elongates, so that it becomes ovoid, or
sausage-shaped ; and its surface becomes covered with flagella

Fig. 11.— Formation of the entoderm by polar ingression in the planul® of
Aequora (after Claus, from Hatschek's Lehrhuch).

(Fig. 13 -£/), by the motions of which it swims about with
the broader end of the body directed forward. The forma-
tion of the entoderm now takes place by polar ingression, at
first a few, and then numerous, cells migrating from the
posterior end of the body into the cleavage cavity, so that,
advancing from behind forward, they gradually fill it up
(Fig. 13 -£/, Fig. 14). In this way there arises a larva which
is eminently characteristic for the Hydi'oids, and was named
by Dalyell the planula (Fig. 15 A); it has also been called
a parenchymula, on account of the embryonic cell-mass filling
its interior (Metschnikoff). During further development
there are formed in the ectoderm nettle-capsules, which seem
to be especially concentrated about the posterior pole, while
within the mass of entoderm cells there arises a fissure, the
first trace of the gastral cavity, around which the entodermal
cells assume an epithelial arrangement. Preparation is now

42

EMBRYOLOGY

made for the process of attachment.^ The larvse sink to the
bottom, their motions become slower, and finally they attach
themselves by means of the disc-like enlargement of the
anterior end of the body (Fig. 15 G). They then lose their
cilia ; and the surface becomes covered with a thin cuticular
secretion, the perisarc (Fig. 15 D). The disc-shaped region
of attachment often acquires a lobed appearance, due to the
formation of notches (Fig. 15 E). The disc constitutes the
first fundament of the hydrorhiza, while the posterior end of

Fig. 15. — Attachment and growth of the hxrval planula of Endendrium (after
Allman). a, planula ; B and C, stages of attachment by means of the disc-like
enlargement of the anterior end ; D, beginning of the formation of the hjdranth
and perisarc, p ; E, formation of the tentacles ; F, expansion of the hydranth.

the larva, now directed upward, grows up into the first
hydranth of the young polyp colony. The fundaments of the
tentacles (Fig. 15 E) arise as lateral diverticula, and the
mouth-opening is formed at the apex by a breaking through

' The development of the eggs of medusm and of hydroid polyps with
gonophorcs is from the planula-stage onward alike, so that Eudendrium
may serve in this place as an example.

CNIDARIA 43

of the body-wall. Finally, the perisarc is ruptured at this
place, and the polyp becomes fully expanded (Fig. 15 F).

The origin of the polyp stock does not always take place exactly accord-
ing to the plan here given. In certain cases the larva becomes attached
by its side, and is almost wholly enii^loyed in the formation of the hydro-
rhiza, while the first hydranth grows out of it by a kind of budding
(Mitroeoma, Metschnikoff).

The hydroid polyp thus produced propagates principally
by means of lateral budding. There is formed at first a
hernia-like protrusion of the body-wall, the cavity of which
communicates with the gastral cavity of the parent animal,
and the wall of which consists of the same layers as that of
the parent (ectoderm, entoderm, and the sustentative lamella
between them).^ The bud is converted into a hydranth by
the progressive abstriction of this protrusion from the parent
animal, by the production of a crown of tentacles, and by the
acquisition of a mouth-opening through dehiscence at the
anterior end. Only rarely do the hydranths thus produced
detach themselves from the parent, and become independent
(Hydra) ; in most cases extensive polyp colonies are formed
by continued budding. The laws of budding, by which the
extraordinarily manifold shape as well as habit of the polyp
colonies is determined, have recently been subjected to a
critical study by Weismann (No. 49) and by H. Driesch
(Inaiig.-Diss., Jena, 1889).

The individuals produced by budding do not always have the same
form as that of the first hydranth. A more or less pronounced poly-
morphism often arises among the individuals of a colony. There are
produced defensive polyps (spiral zooids), abundantly supplied with
nettle-capsules, but destitute of tentacles and oral opening, hard-shelled
spiny protective polyps, nematophores, etc. The so-called blastostyle,
which occurs in many Hydroids, is also to be interjjreted as a meta-
morphosed, non-tentacular polyp, upon the lateral walls of which the
gonophores are produced by budding.

The foi'mation of the individuals destined to become
sexually mature (medusas, sessile medusoid buds) is the

' [Eecently Alb. Lang (No. IX., Appendix to Literature on Cnidaria)
has maintained that the whole of the bud in Hydroids is derived from
the ectoderm of the parent.]

44 EMBRYOLOGY

result of a lateral budding, which during its earliest stages
pursues a course quite similar to that described above. Here
too there is formed at first a small spheroidal bilaminar
bud (Fig. 16 A), between the two cell-layers (ectoderm and
entoderm) of which there can be recognized a hyaline sus-
tentative membrane. The next change, which takes place
at the same time with the progressive abstriction of the
bud, is the formation of an ectodermal thickening at the
free distal pole, which is developed (Fig. 16 B) into a knob,
the so-called nucleus of the bud (nucleus of the bell). By the
growth of the latter into the interior of the bud, the ento-
dermal sac is invaginated, so that it now assumes the form
of a cup (Fig. 16 B). While a fissure (the beginning of the
cavity of the bell) makes its appearance (Fig. 16 D) in the
nucleus of the bud, the two facing layers of the cup-shaped
entoderm sac come into close contact, and fuse along
four meridians (Fig. 16 E), so that of the cavity of the
entoderm sac only four perradial regions (i.e. places cor-
responding with the four chief radii) remain open ; they are
the fundaments of the four radial canals. It is to be
observed that these four radial canals are connected with one
another through the remnant of the obliterated entoderm sac
(Fig. 16 E, i), in other words through the originally bila-
minar so-called vascular lamella (cathammal plate) (L. Agassiz,
No. 2 ; Claus, No. 62). While with the further enlargement
of the bud the bell-cavity inci'eases in size and breaks through
to the outside, and while the manubrium grows out at the
bottom of it, the sustentative lamella of the wall of the
bell is converted into the gelatinous substance of the
umbrella. The radial vessels have become relatively nar-
rower and farther separated from one another. The ring-
canal at the margin of the bell appears to arise by a
secondary separation of the two layers of the vascular
lamella. With these metamorphoses and with the breaking
through of the mouth-opening and the formation of the
velum,^ the medusa is essentially completed and ready to be
detached (Fig. 16 i^).

' The velum does not arise by the formation of a fold, but directly
from that ectodermal membrane which in early stages (Fig. 10 D) sepa-

CNIDARIA

45

" Alternation of generations " in Hydrolds depends upon
the regular alternation of non-sexual generations (hydroid
polyps), increasing by lateral budding, with a sexually
developed generation (hydromedusa or sessile gonophore).

The account of the development thus far affords some

■d

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ri q

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suggestions of the manner in which the hydroid polyps and
the hydromedusfe may be referred back to one and the

rates the cavity of the bell from the outside world and in which a two-
layer arrangement of the ectodermal cells can be recognized at an early
epoch (Weismann, No. 49, p. 260).

46

EMBRYOLOGY

same initial form. For if we assume that the alternation
of generations in Hydroids has arisen as a result of division
of labour, whereby the capabilities of sexual and non-sexual
reproduction have been distributed to different individuals
(Leuckart, No. 11), we must regard the diffei'ent shapes of
these individuals as having been evolved from the same
fundamental form (Allman, No. 15 ; Claus, No. 62 ; 0. and R.
Hertwig, No. 8), the sessile individuals, which are repro-
duced exclusively by
budding, having under-
gone development more
in the vegetative direc-
tion, while the free-
swimming medusJB,
which become sexually
mature, have allowed
the systems of organs
pertaining to the animal
functions to attain to
complete development.
Various circumstances *
indicate that in the ses-
sile form of the hydroid
polyp we have to do
with the primitive con-
dition, so that we may
characterize the hydro-
medusa as a metamor-
phosed hydroid polyp
which has acquired the
power of independent
locomotion. Then the
mouth of the medusa
would be homologous to

..^.:^ 5^

FiO. \7.—A, diagram of a hj-rtroiJ polyp;
B, of a craspedote medusa (after O. und R.
Hertwig, from Lang's Lehrhuch). o, mouth ;
g, gastral cavity ; t, tentacle; si, snstentative
lamella; gt, gelatinous mass between ecto-
derm nnd entoderm; rfc, radial canal; yl,
vascular lamella or caibammal plate; v,
velum ; rifc, ring-canal.

1 As such is to be considered the fact that the sequence in budding
is from hydroid polyp to medusa, and never the reverse, further the total
absence of the production of orf,'ans, especially sensory organs, on the
exumbrellar side of the medusa-bell, which points to an antecedent
sessile condition ; this is of importance in comparison with the condition
in Ctenophores.

CNIDARIA 47

that of the polyp (Fig-. 17 o), and the manubrium of the
former to the oral cone (peristome, hypostome) of the latter.
The cavity of the medusa-bell would be represented by a
concavity of the peripheral part of the peristome, which
exists in many hydroid polyps, while the polyp's crown
of tentacles would be equivalent to the marginal tentacles
of the medusa (Fig. 17 t). According to this interpretation,
the aboral part of the polyp, broadened and flattened, would
be metamorphosed into the exumbrella of the medusa,
while the gastral cavity of the latter is differentiated into
a central stomach cavity and a peripheral intestine (^Kranz-
darm), consisting of radial canals and circular canal, together
with the vascular lamella lying between them (Fig. 17).
The velum, produced by a fold of the ectoderm, would be a
new structure, not present in the polyp. To the diffei-entia-
tions which characterize the medusa belong the greater
development of the musculature and the nervous system
(double nerve-ring of the margin of the bell) and the
evolution of sensory organs.

In many Hydroids, pari passu with an acceleration of sexual maturity
accomplished by a dislocation of the germarium (Weisjiann, No. 50), the
sexual persons have lost the power of free locomotion, and have been
metamorphosed into sessile medusoid buds (gonophores). They must
be regarded as reduced medusffi (v. Koch, No. 34), in which the marginal
tentacles, the sensory bodies, and the velum — and often the opening of
the bell also— have disappeared, while the peripheral intestine has under-
gone considerable reduction (Fig. 18). According to the degree of de-
generation, there may be distinguished with Weismann (No. 49) the
following five stages: — (1) medmoids with canals in the bell, but with-
out marginal tentacles, usually also destitute of velum and sensory
bodies, manubrium without mouth, usually becoming detached at
maturity (Pennaria) ; (2) sessile medusoids, bell usually without canals
or with incomplete ones, but with bell-cavity and bell-mouth (Tubu-
laria) ; (3) sessile gonophores, the wall of whose bell — still retaining the
entodermic lamella and two layers of ectoderm, but without canals or
bell-mouth — immediately encloses the manubrium (Clava, Hydractinia) ;
(4) sessile gonophores, the medusa-layers of whose wall are incomplete
(female Campanularia) ; (5) sporophores, i.e. sessile gonophores without
any trace of medusoid structure (Cordylophora).

It is still questionable whether the sexual organs of Hydra are related
to the last of these groups, according to which Hydra would be an
extremely modified form, or whether we are not perhaps to regard

48

EMBRYOLOGY

Hj'dra as a polyp that has reached sexual maturity, and therefore as a
very primitive form of hydroid.

In general the sexual organs, both in the medusas and in the sessile
gonophores, lie in the ectodermal wall of the manubrium (Fig. 18) or
(Leptomedusae. Haeckel) on the inner wall of the bell in the course of
the radial canals.

The investigations of Weismann have yielded new results concerning
the early stages in the formation of the sexual products. In the
original condition the sexual cells were developed and reached maturity
in the ectoderm of the manubrium of the medusa. In other cases (in
forms with sessile gonophores) the development of the sexual cells took
place even before the gonophore itself was fully formed ; there ensued
therefore a secondary displacement (phyletic) of the germarium, first

Fig. 18. — Diagrammatic section through two sexual hydroid individuals. A,

young medusa, still attached ; B, sessile gonophore ; ov, gonads Covarium) ; in,
manubrium ; r, radial vessel ; f, tentacle ; v, velum ; g, vascular lamella.

into the ectoderm of the bud, then into the entoderm of the same, and
finally into the entoderm of the stem and the branches before the de-
velopment of the bud. From this location the sexual cells are compelled
to migrate (ontogenetically) to the seat of their maturation. It is to be
seen that this displacement of the germarium was established in the
interest of the greatest possible acceleration of sexual maturity. In this
case the displacement (phyletic) of the germarium is in centripetal
direction. In the Leptomedusre (Haeckel), on the contrary, the seat of
maturation is displaced in centrifugal direction, for Haetlaub has been
able to show in Obelia that the sexual cells arise in the manubrium, and
reach the radial canals only secondarily.

Reproduction by budding occurs not only in hydroid polyps, but also

CNIDARIA

49

in hydroineclusae. In the case of the latter the buds may be developed
on the manubrium (Sarsia siphonophora, Lizzia), at the base of the
tentacles (Sarsia prolifera, Codonium codonophorum), on the ring-canal,
or at other places. Concerning budding in the Cuninas consult p. 58.
A remarkable kind of reproduction by budding has been observed by
Brooks (No. 18) in a Eucopid, Epenthesis McCradyi. In this case
numerous blastostyles enclosed in a chitinous gonangium sprout out'
from the surface of the four gonads belonging to the radial canals; by
further budding small medusa are produced on the blastostyles. Ac-
cordingly, if the interpretation of these blastostyles as metamorphosed
hydranths should be definitely established, we should have in this case
an exception to the rule that by the budding of a medusa there can
never be produced anything but a medusa.

From the form of non-sexual reproduction— lateral budding— thus far
treated of, must be distinguished the reproduction of a hydranth from the
free end of the stalk, such as has been observed after injuries, after the
spontaneous detachment of the hydranths, as in Tubularia (Dalyell,
No. 4 ; Allman, No. 15), and after the death of the polyps in consequence
of their being overgrown by algaj, as in the case of Campanularians
(v. Lendenfeld, No. 38).

In addition, reproduction by division has been observed in Hydroids
in certain cases, thus by Metschnikoff (No. 12) in the blastula-stage
of Oceania armata, by Ussow (No. 48) in the buds and in the mature
animals of Polypodium hydriforme, which is parasitic in its early stages
in the eggs of the sturgeon, and also as the only method of reproduction
hitherto observed in the North American fresh-water Microhydra Ryderi
and in Protohydra (Geeeff). Furthermore the occurrence of spontaneous
division in Hydra has been maintained by the earlier observers.

Reproduction by means of division has also been observed in the young
of certain medusae ; it is introduced by the budding of a new stomach ;
this is followed by a grouping of the radial vessels around the two
existing manubria as centres and a fission of the disc, which begins at
the margin. Such is the case with Stromobrachium mirabile (Kolliker,
No. 37), Phialidium variabile (Davidoff, No. 23), and Gastroblasta
Raffaeli (Lang, No. 39). The newly formed radial canals grow out from
the marginal canal as centripetal vessels. Owing to the fact that in later
stages the budding of the gastral sacs continues and the division of the
individuals does not keep pace with it, in fact ceases altogether, colonies
are produced (Gastroblasta Raffaeli and timida — Keller).

Another method of non-sexual reproduction, which has been called
frustulation (Allman, No. 15), may best be defined as an early
abstriction of an only slightly developed lateral bud. In the case of
Schizocladium ramosum, a Campanularian, there are on the polyp
colony lateral branches which bear no hydranths. From the ends of
these are constricted off small portions, which, except for the absence of
cilia, resemble a planula ; for they attach themselves, become surrounded
K. H. E. E

r»0 EMBRYOLOGY

with a perisarc, and grow out as the hydrorhiza of a new colony, the
first hydranth arising from them by a process of budding. In the
remarkable Corymorpha, which does not produce colonies, but remains
solitary, a very similar abstriction of frustules takes place from the
lower part of the polyp. Perhaps we should recognize in this process
the last trace of stock-formation.

Hydroid Polyps with Sessile Gonophores.— The

course of tlie development of embryos which are formed in
sporosacs is, according' to Allman (No. 15), F. E. Schdlze
(No. 46), Hamann (No. 27), and Metschnikoff (No. 12), some-
what different from that just described, especially in the
formation of the entoderm, and is more closely related to the
development of the hypogenetic medusa (see p. 53). It is
maintained that in the case of sessile gonophores there arises,
by a total and usually equal cleavage, at first a spheroidal
solid embryo destitute of a cleavage cavity (a so-called
morula stage), the superficial cells of which by more rapid
division become separated off as a distinct layer (ectoderm)
from the internal cell-mass (entodenn). As is evident, this
process is closely related to the formation of entoderm by
delamination, which is to be described further on. Tlie
bilaminar embryo thus formed elongates and acquires a coat
of flagella and, by the dissociation of the entoderm cells,
the beginnings of a gastral cavity. In most instances it
becomes free as a planula.

A distinctly unequal cleavage and subsequent formation of a gastrula-
stage by epiboly has been described by Ciamiciak (No. 22) for Tubularia. '

^ [The development of Tubularia has recently been thoroughly in-
vestigated by A. Bdauer (No. II., Appendix to Literature on Hydroidea).
There are two types of cleavage. In the one case it is approximately
regular. There are, however, diiJerences in the size of the blastomeres ;
but no regular distribution of these is recognizable. At length a coelo-
blastula arises. The entoderm is produced by division of the blastoderm
cells according to the multipolar type, so that finally the cleavage cavity
is filled with entodermal elements given off from the inside of the blastula.
This stage looks like a morula, but it is already a bilaminar germ.
(Compare also Gekd, No. III., Appendix to Literature on Hydroidea.)

The second method of cleavage exhibits at first only a multiplication
of the nuclei ; then the cleavage, beginning at the animal pole, progresses
toward the opposite side.]

CNIDARIA 51

But his investigations have been refuted by Hamann (No. 27), Metschni-
KOFF (No. 42), and Conn (No. 21), according to whom the development of
the egg of Tubularia takes place in accordance with the type described
above. On the other hand, it appears as though Tichomikoff (No. 47)
had expressed himself in favour of Ciamician's observations. The bi-
laminar embryo has at first the form of a cake, but soon becomes spindle-
shaped, owing to the budding forth of two tentacles at opposite points.
Then follows the formation of the gastral cavity and of new tentacles in
the equatorial plane. The latter are at first curved toward the aboral
side. The embryo now generally undergoes an elongation in the direction
of the chief axis ; and while at its oral pole the beginnings of the oral
tentacles appear and the mouth-opening breaks through, and while the
main tentacles curve orally, the posterior end becomes narrower and to a
certain extent constricted off from the main body by a circular furrow.
With this the so-called actinula-stage (Fig. 19) is reached, and the small
polyp quits the mother (gonophore) for the purpose of attaching itself and
growing up into a new colony. The agreement with the development of
the larva of the iEginidfe to be described further on (p. 57) is note-
worthy.

The egg of Hydra ' develops in an ovarium which belongs to the
ectodermic layer of the body-wall of the polyp, and which has arisen by
an increase of the cells of the so-called interstitial tissue. Of the cells
composing the ovarium only one (in rare cases two) is developed into a
mature egg, whereas the remaining ones serve as food for it, and are in-
corporated into the egg by means of its pseudopodia. The mature egg,
which contains numerous yolk elements called pseudo cells, escapes by
the rupture of the enclosing ectodermic layer of the parent, to which it,
however, remains attached for a long time. The part of the egg which
is directed away from the body of the mother marks the animal pole,
that which adheres to it the vegetative pole, of the egg. Then follow the
detachment of the polar globules and fertilization. The development of
the egg has been studied by Kleinenberg (No. 32), Korotneff (No. 3.5 \
andKERscHNEE (No. 33). According to Kerschnee, a solid morula is not
formed ; but there is produced by total and tolerably equal cleavage a
blastula, from the lower (vegetative) pole of which there is a migration
into the cleavage cavity of cells which go to form the entoderm. In this
case, then, the entoderm arises by polar ingression ; and since in Halecium

1 [In regard to the development of Hydra the reader is referred to the
important recent investigations of A. Brauer (No. I., Appendix to Litera-
ture on Hydroidea). A cceloblastula with a large central cavity is pro-
duced by total and equal cleavage. The formation of the entoderm is
multipolar, and results from the inward migration or the division of the
blastoderm cells. The ectoderm secretes an outer and an inner germinal
membrane ; it is itself preserved, however, and persists as the permanent
ectoderm. The layer of interstitial cells arises from the ectoderm. The
future oral pole is identical with that of the polar bodies.]

52

EMBRYOLOGY

Fig. 19.— Actinula of riibidana (after
CiAMiciAN). m, incipient oral tentacles.

tenellum Hasunn (No. 27) and in Campanularia caliculata (?) Metschxi-
KOFF (No. 12) have observed the formation, of the entoderm by immigra-
tion, this type appears to be more wide-spread among the hydroid polyps
than has been assumed hitherto.

After the cleavage cavity is completely filled with entoderm cells, a

double egg-membrane is secreted,
an inner germ envelope and an
outer, harder chitinous shell.
Whereas, according to Kleinen-
BERG and KoROTNEFF, the ectoder-
mic layer is wholly consumed in
the formation of the latter, Ker-
scHNER was able to show that the
ectoderm persists. The egg now
detaches itself from the body of the
mother and sinks to the bottom,
■the mass of entodermic cells, by
the formation of numerous con-
necting cords of protoplasm and interstices between them, then assumes an
appearance similar to that of connective tissue (Kerschner) ; and then
the gastral cavity makes its appearance in this mass. Finally, the outer
(chitinous) shell of the germ goes to pieces ; and the embryo, still enclosed
in the inner envelope, emerges from it. The tentacles now arise as
evaginations of the wall, and the mouth-opening is formed by a breaking
through of the wall at a place which corresponds to the vegetative pole
(Kerschner), so that after the dissolution of the inner membrane the
young Hydra becomes free in a form that resembles an actinula.

Statements concerning the laws which govern the appearance of the
tentacles in Hydra have thus far disagreed. Kleinenberc. maintains
that all the tentacles appear at the same time, whereas Korotneff asserts
that they arise in pairs, as Mebeschowsky has affirmed for buds. While
Jung (No. 31) was unable to recognize in the latter any definite law,
Haacke (No. 28) believed that he had observed that the Hydras, apart
from the green form, were divisible into two species, which he dis-
tinguished as H. Tremblyi and H. Eoeselii. On the buds of the former
all (six) tentacles arise simultaneously ; the appearance of the tentacles
on the buds of H. Koeselii, on the contrary, discloses a definite orienta-
tion in relation to the maternal organism, inasmuch as (the insertion of
the bud being perpendicular) the two tentacles first to ajipear lie in a plane
dividing the maternal organism transversely, while the third sprouts out
in a plane perpendicular to the first and toward the oral side of the
mother, the fourth opposite the latter, etc. Such examples prove
that in stock-forming radiate animals the bilateral symmetry of the bud
is caused by its relation to the parent organism. We must therefore
attribute the bilateral structure of many Coelenterates (Anthozoa, young
Scypho-polypi) to stock-formation.

CNIDARIA 53

Concerning the development of Hijdrocorallia there are as yet only
scattered observations. Mosexey (No. 44) found in the Stylasteridai
well-developed planulas within the gonophores. The larv£e of Millepora
also appear to become free at this stage. In this case the very small
eggs, with scanty yolk, pass through the first stages of development in the
entoderm of the coenosarc, where they are often attached by a stalk-like
pseudopodium to the supi^orting lamella. Subsequently they migrate
into the entoderm of the basal plate of the gastrozooid, where they
develop into 23lanulas. It is remarkable that the early development is
here accomj^anied by a considerable increase in the number of the em-
bryonic nuclei, but without distinct cleavage (Hickson, No. 30, and Nos.
VI. and VII., Appendix to Literature on Hydroidea).

Hypogenetic Medusae. — In the groups of the Traclw-
mednsm and Narcomedusai the alternation of generations, con-
sisting in the regular recurrence of polyps and medusae, is
wanting, since in these instances the polyp-generation
appears to be suppressed. The young medusa ai-e developed
from the eg^ directly, but in many cases still have to pass
through a metamorphosis. In the Cuninas, hovs^ever, there is
a secondary introduction of alternation of generations, the
larva developed from the egg giving rise by a process of bud-
ding to medusae (parasitic bud-spikes [Knospendhren'] of the
Cuninas).

The development of the egg of the Geryonidse has been
studied in several species by Metschnikoff (Nos. 42 and 12),
FoL (No. 25), and Brooks (No. 17). The Geryonid egg,
which is expelled from the mother's mouth, is surrounded by
a mucilaginous envelope, and shows a distinct separation into
a granular ectoplasm and a foam-like, clearer endoplasra. By
total and equal cleavage there are produced two, four, eight,
etc., blastomeres, in which a superficial ectoplasmic and an
inner endoplasmic portion can be recognized (Fig. 20 J.). In
the sixteen-cell stage there is usually to be seen a cleavage
cavity (Fig. 20 A, h) produced by separation of the blasto-
meres. If this represents the blastula-stage, the following
stages inaugurate the formation of the entoderm, which,
according to the concurrent testimony of the investigators
mentioned above, takes place by means of a so-called delamina-
tion process.

By a transverse division of each of the cleavage spheres

54

EMBRYOLOGY

the ectoplasmic portion is separated from the endo-
plasmic (Fig. 20 B). The latter constitutes the ento-
dermic elements. The result of this process, which takes
place over the whole periphery, is the formation of a closed
two-layer cellular sac (Fig. 20 C), the outer layer of
which represents the ectoderm, the inner the entoderm,
while the central space, the former blastocoele, now becomes
the gastrocoele or archenteron. Soon there is a secretion of
a ti-anspareut jelly between the two layers (Fig. 20 C, g).
Since the embryo from this time forward swims about by
means of the flagellate motion of the ectodermal cells, this

Fig. 20. — Three stages in the development of GeryonidEe : A and C from Liriope
mucronata (after Metschnikopf) ; B from Geryonia fungiformis (after Pol). A, six-
teen-cell stage ; h, cleavage cavity ; B, beginning of delamination ; C, after com-
pletion of delamination ; g, gelatinous substance.

stage may be compared with the planula-stage of the other
Hydroids.

The next change consists in the increase of the gelatinous
secretion, whereby the ectoderm sac is greatly distended.
Inasmuch as this secretion does not take place unifoi'mly on
all sides, the entoderm sac becomes more and more eccentric
until it touches the ectoderm at one point, the oral pole
(Fig. 21 A). The cells of the ectoderm and entoderm at
this place become thickened, and here the mouth-opening is
subsequently formed by the breaking through of these layers.
On the thickened ectodermic plate surrounding the mouth
there is soon established a special thickening of the peripheral
parts, whereby a circular wall is produced, on the outer side
of which the four (or six) primary tentacles are developed

CNIDARIA

55

as slight elevations, in the interior of whicli are to be recog-
nized coi-ds of entoderm cells continuous with the wall of the
gastral cavity (Fig. 21 B). This tentacle-bearing stage,

Fig. 21. — Two stages in the development of Liriope mucronaia (after Metschni-
KOPr), diagrammatic. A, a larva of the fifth day ; B, a seven-day larva in optical
section ; ec, ectoderm; en, entoderm; o, mouth-opening ; v, fundament of the
velum ; t, that of the primary perradial tentacle.

which does not yet reveal the peculiarities of the medusa,
may well be regarded as a modified actinula-stage.

56

EMBETOLOGY

In the further course of development the larva, hitherto
spherical, undergoes a flattening, and at the same time the
entoderm sac becomes depressed. Then the velum (Fig. 21
B, v) is developed from the ring-like wall of the ectoderm.
By the enfolding of the area surrounding the mouth-opening
the beginning of the sub-umbrellar cavity is established
(Fig. 22), which soon increases in size. Since the flattened
gastral cavity likewise undergoes an enfolding, it now has
the form of a double-walled cup inverted over the sub-
umbrellar cavity. According to Brooks, its two walls (in
Liriope) come together and fuse with each other at four

Fig. 22. — Larva of Lirio'pe scutigcra (after Bbooes). i, interradial area of fusion
of the peripheral intestine (cathammal plate); r, radial vessel ; g, circular vessel;
t', primary peiTadial larval tentacle, migrated upward ; t^, interradial larval ten-
tacle ; t*, bud of a permanent perradial tentacle.

interradial places (Fig. 22 i), thus forming the cathammal
plates of the vascular lamella, while the regions that
remain unfused represent the four, at first very bi'oad,
radial vessels (r) and the ring-canal (g). The further
changes, through which the larva approaches the structure of
the adult, consist in the establishment of the interi'adial (t^)
and the permanent perradial (t^) tentacles (while the primary
tentacles disappear), the development of otocysts, the out-

CNIDARIA 57

growth of the gastrostyle, and a general flattening of the bell
(Brooks).

Upon these metamorphoses, above all upon the loss of the
solid larval tentacles — of which the perradial are always re-
sorbed, while in some forms the interradial are retained
alongside the later-developed, hollow, permanent (perradial)
ones — is based the metamorphosis of the Geryonidfe, accu-
rately described by Leuckart, Fr. Muller, and E. Haeckel.

In Aglaura and Ehopalonema the entoderm is not produced by de-
lamination, but like that of the hydroid polyiis, since there is formed at
first a solid so-called morula-stage destitute of cleavage cavity, the super-
ficial cells of which are converted into the ectodermic layer, while those
within represent the entoderm (Metschnikoff, No. 12).

Fig. 23. — Larva of jEgiwopsis three days old, with two tentacles (after Metschni-
koff, from Balfoue's CoTnparalive Embryology), m, mouth; t, tentacle.

The development of the Narcomedusce from the egg has
become known principally through Metschnikoff (Nos. 12
and 13). In ^ginopsis mediterranea the formation of the
entoderm is accomplished by multipolar ingression. In the
course of cleavage there is no distinct cleavage cavity pro-
duced, but from a very early period cells migrate into the
interior from any point whatever of the surface, and these
constitute the entodermic cell-mass. Since the ectoderm
becomes flagellate, there is produced an elongated, rod-like
planula, which has almost the appearance of a detached
tentacle of a hydroid, for its interior is filled with ento-
dermal cells, which are arranged in a single row at either
end, being more crowded in the middle portion only. Soon,
however, these afterwards bent ends are seen to grow out

58 EMBRYOLOGY

into the first tentacles of the larva, while the middle part
becomes the body of the medusa (Fig. 23). The gastral
cavity is produced by the dissociation of the entoderm
cells, the mouth breaking through later. There is developed
a second pair of smaller tentacles, which with the first
pair forms a cross. By the development of the sensory
bodies, the mesoglcea, the umbrellar cavity, and the velum,
the larva is gradually converted into the form of the
medusa (J. Muller, Metschnikoff).

Although the development of the .Eginidas thus consists of a simple
metamorphosis, much more complicated relations have been found in
the life-history of the Cuninas, which are produced by the parasitism of
the larva and the simultaneous tendency to early budding.' The con-
ditions in Cunoctantha octonaria are, according to McCkady and to
Brooks (No. 17), comparatively simple. In this case the ciliate larvse get
into the umbrellar cavity of one of the Tiaridse (Turritopsis), and there,
through stages similar to those described above for ^-Eginopsis, grow up
into an actinula-like creature, which attaches itself by means of its four
tentacles to the outer wall of the stomach of Turritopsis, while it intro-
duces its long proboscis through the mouth-opening into the stomach of
its host. This larval stage multiplies by budding until finally both the
original larva and the individuals thus produced acquire the form of
medusffi by a gradual metamoiiDhosis, and become young Cunoctanth®.
Similar are the cases in which free-swimming planulffi of Cuninas mi-
grate into the stomach of Geryonidai and there attach themselves, and
grow up into a spike of buds [Knospemihre]. Since in these cases, how-
ever, the buds alone possess the capability of being metamorphosed into
medusae, whereas the polypoid stolon developed out of the larva does not
undergo further development, the outcome is the establishment of an
alternation of generations. There have often been observed in the gas-
tral cavity of Cuninas themselves parasitic larv® of Cuninas, which
became metamorphosed into medusfe, but at the same time multiplied
asexually by budding at the aboral pole (Metschnikoff). Since the in-
dividuals thus produced often differ essentially in structure, especially
in the number of the antimera, from the forms in whose stomachs they
are found, it has remained doubtful whether one had to do in this case
with a brood differing from the parent in form or with descendants of
another species of Cuniua, which in the free-swimming stage migrate
into the gastral cavity of the host. Recently a Cunina larva (?) para-
sitizing the mantle-jelly of Salpa fusiformis has been described by
KoKOTNEFF (No. 30) as Gastrodes parasiticum."''

1 [Compare 0. Maas, No. X., Appendix to Literature on Hydroidea.]
* [In regard to the position of Gastrodes, compare Kokotneff, No.
VIII., and Heidek, No. VII., Appendix to Literature on Hydroidea.]

CKIDARIA

59

Metschnikoff (No. 12) has described and designated as sporogonia a
remarkable method of reproduction in Cunina proboscidea. This would
be the only case of parthenogenetically developing eggs among Ccelen-
terates. There are developed in the sexual organs of this form (in
addition to the reproductive elements) neutral amceboid sexual cells,
which soon migrate out of their places of origin and penetrate into the
entoderm of the gastral pockets and of the ring-canal, and also into the
gelatinous layer of the sub-umbrella. These amoeboid cells, which occur
in males as well as females, at first divide, and then one of the cells
closes around the other. The enclosed cell is converted into the embryo,
while the enveloping cell, as an enormously enlarged amoeboid cover-
cell, provides for the nutrition, the motion, and the attachment of the
embryo. With the further growth of the ciliate embryo it hangs free in
the gastral cavity of the parent animal, while the cover-cell alone effects
the attachment to the entoderm. Finally, the embryos become free in
the gastral space of the parent, where they are metamorphosed into
medusas, and at the same time produce buds from their aboral pole.
The medus* thus produced are already sexually mature at the moment
of their emergence from the body of the parent. They are, however,
essentially different from the parent. They have the characters of the
Solmarida3 in so far as they possess a simple gastral sac and a ring-
shaped gonad, whereas " otoporpae " are wanting. Here, therefore,
there is an alternation in the cycle of development of two differently
constructed sexual generations, one of which has arisen in a partheno-
genetic manner (or by budding). These conditions require further
investigation and confirmation.

II. SiPHONOPHORA.

Systeviatic : I. Physophoridse.

1. Physonectae (Haeckel).

2. Pneumatophoridae (Rhizophysa,

Physalia).

3. TracheophysEe (Vellela, Porpita).

II. Calycoplioridae.

The eggs of the Siphonophora are developed in sessile
gonophores, or in small, ultimately free, primitively quadri-
radiate craspedote medusae, and are fertilized in the sea-
water after their deposition. They are spherical, usually
naked (with the exception of Hippopodius gleba), and re-
semble the eggs of the Greryonidae and Ctenophora in so
far as a dense homogenous exoplasm and a vacuolated,

60 EMBRYOLOGY

frothj'^-looking' endoplasm can be distinguished in them.
Cleavage is always total and equal, and leads fi]\st to a
onorula-stacje, which shows no cleavage cavity within it. By
the development of a layer of small ciliated cells on its
outer surface, there is produced a two-layer spherical or
somewhat elongated planula-stage. Nothing more accurate
concerning the separation of the two germ-layers is known
up to the present time.

The development of the Siphonophora has been investi-
gated chiefly by Gegenbaur (No. 67), Haeckel (Nos. 68 and
70), Metschnikoff (No. 13), Fewkes (No. 66), and Chun
(Nos. 54 — 58). Considerable differences prevail among the
various groups regarding the further development (meta-
morphosis^) of the young Siphonophore stock.

Physophoridas. — A comparatively simple type is repre-
sented by the development of Halistemma (Stephanomia)
pictum. The first change noticeable in the planula is an
elongation in the direction of the subsequent chief axis
(Fig. 24 A) and the accumulation of red pigment at the
lower (oral) pole. Certain small cells, which have ap-
parently proceeded from a metamorphosis of the large
nutritive entoderm cells, then make their appearance under
the ectodermal cell-layer, and soon arrange themselves in a
second layer of cells (the permanent entoderm) under the
ectoderm. In the further coui-se of development the large
nutritive entodermal elements become more and more ab-
sorbed, so that an internal cavity, the gastrovascular cavity,
is developed (Fig. 24 B). The fundament of the first
organ to be formed is seen at the upper (aboral) pole. The
ectoderm here exhibits a thickening, which very .soon, like
the bud nucleus [Knospenkern^ of a medusa, grows inward
(Fig. 24 A, ep), and by a dehi.scence of the cells develops

' We here i-egarcl the entire Siphonophore stock as a unit (individual
of the third or higher degree, cww). Just as the metamorphosis of an
individual of the second degree (person) usually takes place by the loss
of larval organs and their replacement by permanent ones, so the meta-
morphosis of the Siphonophore stock is frequently accompanied by the
loss of larval parts, to which the value of a person must be ascribed, e.g.,
nectocalyces, hydrophyllia, etc.

CNIDARIA

61

a cavity at the centre. This is the earliest trace of the
•pneumatophore, which consequently is formed as a solid
ingrowth of the ectoderm. After this the fundament of the
first larval tentacle becomes noticeable as a latei'al evagina-
tion of both body-layei's (Fig. 24 B, t). The fundament
of a second deciduous larval tentacle soon follows. The
bilaterally symmetrical structure of the larva is indicated
by the appearance of the tentacle, since that side of the
body to which the above-mentioned organ belongs corre-
sponds to the zone from which subsequently all the newly

Fig. 24. — Two stages of development of Halistemma (Stephanomia) pictum (after
Metschnikoff, from Balfour's CompamKue Embryology). A, ciliated planula-stage ;
ep, fundament of the pneumatophore as an ectodermal ingrowth ; B, older stage
with central gastric cavity ; po, fundament of the first polypite ; f , fundament of
tentacle ; pp, pneumatocyst ; ep, its ectodermal envelope (pneumatosaccus) ; hy,
entoderm in the region of the pneumatophore.

appearing buds gi'ow forth, the so-called ventral side of the
Siphonophore stock. ^ At the same time, by a transverse
constriction at the base of the tentacle, a separation into an
upper portion of the body, which becomes the stem and
pneumatophore, and a lower portion is indicated. The first

* The designation of this as the ventral side can only be established
by comparison with other Siphonophore larvae. On the other hand,
Haeckel (No. 70, p. 315, Taf. xxii.) has pointed out that the primary
tentacle of similar larvae has a dorsal position.

62

EMBRYOLOGY

nutritive poljp, polypite, is developed from the latter by
the breaking through of a mouth-opening at the lower pole.

Thus a larval form is reached in Halistemma pictum, which recurs
frequently among the Physophoriclas, and consists of the apical pneumato-
cyst and a polypite with accompanying tentacle. In it we recognize the
Auronectid larva described byH.\ECKEL (No. 70), and referred to Stephalia
corona, which, in addition to the extensive pneumatophore, also exhibits
the fundament of the remarkable apparatus for the elimination of air
(aurophore). Moreover, it appears to occur among the Pneumatophoridffi
(Chun). Thus the youngest Physalid larvae, which have been made
known through Huxley and Haeckel (No. 70), are constructed after this
type (Fig. 2.5). Not until a later period do the air-sac, which increases
considerably in size, and the rudiment of the stem, assume a more
horizontal position, whereby the formerly apical pore of the pneumato-

FiG. 25. — Y'oungest IflrvHl stage of a Physalid {Aloyhnta GiJlscliiana) (after
Haeckel). p, pneumatophore; I'o, its apical stigma; a, rudiment of the stem;
in, polypite ; /, tentacle.

cyst conies to occupy the anterior end, the insertion of the primary
polypite, on the contrary, the posterior end. of the body, on the under
(ventral) side of which new groups of individuals (polypi tes, dactylo-
zooids, with tentacles and gonophores) now bud forth. Later the so-
called pneumatic plate is developed on the inner side of the air-sac
(modified pneumatic funnel), and also the dorsal comb (Chun, No. 58).

The development of Halistemma rubrum takes place, according to
Metschnikoff (No. 13), in a similar way to that of H. pictum, differing
from it principally in the early appearance of the buds of the nectosome
[Schicimmsaule], which are developed on the ventral side between the
fundament of the pneumatophore and the first tentacle. The first
nectocalyx bud is established very early, at the same time as the pneumato-

CNIDARIA

63

cyst, and both fundaments at first have almost the same appearance.
In the fm-ther course of development, however, the fundament of the
nectocalyx protrudes out over the surface of the larva, and is constricted
off from it like a bud, whereas the pneumatophore remains sunk in the

>:^jii0m^^ J-

B fk

PiG. 26. — Three stages of development of Agahna Sarsti (after Metschnikopf).
A, first fundament of the cap-shaped hjdrophyllium on the ciliated larva ; B, ab-
striciion of this fundament and development of the pneumatophore; C, stage with
fundaments of three hydrophillia. d, primary cap-shaped hydrophillium ; d', ir,
first and second heteromorphous hydrophillia; ec, ectoderm; en, entoderm; /, bud
of tentacle; g, mesogloea; gv, gastrovascular cavity; p, fundament of pneumato-
phore ; s, nutritive cells.

apical end. A further difference from H. pictum results from the
eccentric position of the gastrovascular cavity, which is crowded quite

64 EMBRYOLOGY

to the ventral side by a dorsal accumulation of nutritive entodermal
cells. This condition affords a transition to the larvas of Agalma, Crys-
tallodes, and Atorybia, in which, by an accumulation of still greater
masses of large nutritive cells on the dorsal side of the larval body, a
structure almost like a yolk-sac may be developed (Crystallodes).

The development of Agalma has been described by Metschnikoff
(No. 13) and Fewkes (No. 66). The ciliated planula-stage here retains
the spherical shape of the egg, but soon exhibits a thickening of the
ectoderm at one spot. At this jjlace, which corresponds to the subse-
quent ventral side of the embryo, an accumulation of small cells soon
takes place, which forms a second cell-layer under the ectoderm (Fig. 26
A, en). Both layers separate somewhat from the underlying large
nutritive cells, thereby forming the gastral cavity (gv), while the pro-
jection arising in this way is constricted off fi'om the rest of the larval
body by a circular furrow, and is to be recognized as the fundament of
the first primary hydrophyllium (Fig. 26 B, d). It develops further by
the secretion of a gelatinous mass {g), lying between the ectoderm and
entoderm, which soon increases greatly, so that the entodermic diver-
ticulum extending into the hydrojihillium becomes a comparatively small
plug-shaped organ. A short time after the establishment of the primary
cap-shaped hydrophillium, the pneumatophore is formed as an ecto-
dermic ingrowth (Fig. 26 B and C, p), which is soon surrounded by an
ectodermic duplicature, while the pneumatocyst is developed inside of it.
Next there bud forth on the ventral side two new fundaments of hydro-
phillia (Fig. 26 C, d', d^), which develop into the heteromorphous,
leaf-shaped, serrated larval hydrophillia, and meanwhile a new ventral
bud is developed into the provisional tentacle (/). By the enlargement
of the gastrovascular cavity {gv), the remains of the larval body, which
is filled with nutritive cells, are gradually metamori^hosed into the poly-
l)ite. The further development is accomplished by the loss of the
primary cap-shaped hydrophillium, which is replaced by a circle of
foliaceous, likewise provisional, hydrophillia, so that in this way a larval
stage is reached which resembles the condition which persists throughout
life in Atorybia (Claus).

The development of Physophora, which in general is like that of
Halistemma, is also characterized by the early development of a larval
hydrophillium which subsequently disappears, the earliest fundament of
which, as it seems, precedes that of the imeumatophore. In the further
progress a larva is developed in which the bilateral hydrophillium, which
is provided on one side with a fissure, envelops like a spathe the funda-
ment of the polypite, the pneumatophore, and the provisional tentacle.
The general resemblance of this larva to certain bilaterally symmetrical
medusffi (Hybocodon) has already been pointed out by Haeckel (No.
68), and later by Balfouk, and has been made use of in sup^jort of the
medusa theory (see p. 73).

According to this view, the larva of this stage would represent an in-
dividual of a medusoid organization, in which the manubrium of the
medusa would be represented by the polypite, and the umbrella of the

CNIDARIA

65

medusa by the larval hydroiAillium, whereas the tentacle would have
to be explained as the only remaining marginal tentacle of the medusa.
This primary individual of the Siphonophore stock, referable, from
Haeckel's point of view, to the fundamental form of a Hydromedusa,
would in the language of the medusa theory (see pp. 70 and 73) be
called a vieduaom, and the corresponding larval form a Siphonnla-stage.

Only portions of the development of the Vellflida (Tracheophysaj,
Chun) are as yet known. A number of young larv£E have been de-
scribed by A. Agassiz (No. 52), Haeckel (No. 70), Bedot (No. 53), and
Chun (No. 57). The youngest larval stage observed by Haeckel, perhaps
belonging to the developmental cycle of Porpita, was named Discoiivla ;
it exhibits a distinctly octoradial structure (Fig. 27). From the under-
surface of the discoid stem there hangs a central polypite (c), the cavity
of which is united by means of eight radial canals to a perii^heral ring-
canal and eight simple tentacles (f). In the apical part of the gelati-

'tf-^ >„

Tig. 27.— Two Dis -onula-stages (after Haeckel). ^.younger stage, seen from
the upper side; B, somewhat older stage with ramified tentacles, seen from the
lower surface; p, pneumatophore; g, buds of the blastostyle; c, central polypite
with mouth-opening ; t, tentacle.

nous disc is found a central lentiform pneumatoeysi (p), surrounded by
a circle of eight radial air-chambers, each of whi?h opens to the ex-
terior by means of a dorsal pore. Haeckel interprets this stage as the
ontogenetic reproduction of an octoradial ancestral form which would
have to be sought for among the Trachomedusas ; consequently all the
Siphonophores assignable to this group must be separated as an inde-
pendent sub-class (Disconanthse) from all the remaining ones, which are
descended from a bilateral ancestral form, of which the Siphonula larva
is the expression. In opposition to this hypothesis of the diphyletic de-
rivation of the Siphonophora, Chun has contended that the octoradial
Disconula-stage is probably preceded in the development of the Por-
pitidas and Vellelid* by a bilateral Siphonula-stage. Young Ratarice
(larvae of Vellelidffi), still with a simple, unchambered pneumatophore,
exhibited four bilaterally arranged tentacles, for a larger tentacle and
K. H. E. F

66

EMBRYOLOGY

three smaller ones arranged symmetrically were to be noticed. The
llatariffi are characterized by the possession of a sail which stands ver-
tically on the upper surface of the ellii^tical disc, and the base of which
originally occupies the direction of the long axis of the disc, so that in
general the Ratarise possess a biradial structure. It is only in subse-
quent stages that the amphitectal (klinoradial) fundamental form of the
Yellelidffi is brought out by the sail turning at an angle of forty-five
degrees to the above-mentioned axis, so that it stands diagonally.

We have still to add something
on the laws of growth of Sipho-
nophore stocks. In those forms
which are characterized by an
elongated stem, the differeiit indi-
viduals do not bud on the entii-e
circumference, but only along a
longitudinal line (Fig. 28). Since
the wall of the stem exhibits a
different structure along this line,
a ci'oss-section of the stem pre-
sents a bilaterally symmetrical
arrangement. That side of the
stem from which the individuals
bud is known as the ventral side
(Clau.s). The fact that the indi-
viduals of the stem appear to be
oriented in various directions re-
sults from a spiral twisting of the
stem, by which, for example, the
biserial or multiserial arrange-
ment of the nectocalyces on the
nectosome is brought about. It
was shown by Claus (No. 62)

Fig. 28. — Young Agalmopsis
(after Gegknbaub). a, stem; b,
])neumatophore ; c, only polypite
developed ; d, buds of tentacles
and dactyloznoids belonging to
the group of individuals of the
first polypite; e, hydropliillium ;
ij, buds (nectocalyces) of the nec-
tosome ; h, buds of the lower
portion of the stem.

that in the Physophoridae the
spiral twisting of the nectosome takes place in the opposite
direction to that of the lower portion of the stem.

As appears from Fig. 28, a budding point for the indi-
viduals of the nectosome is found at the upper end of the
stem. Another budding point, at the base of the necto-
some, supplies in general the buds for the rows of individuals
of the stem. Accoi-dingly those groups of individuals which

CNIDARIA

67

-^

lie at the lowermost end of the stem are the oldest. In
neai^ly all of the Caljcophoridne and some of the Physo-
phoridae (Apolemia) the individuals of the stem are
arranged in definite oroups (cormidia), which are separated
from one another by free portions of the stem (internodes) .
In many other forms, on the other hand,
the limits of the successive internodes are
indicated merely by the attachment of the
polypi tes with their tentacles (Fig. 29 A,
B, C, D), whereas the parts of the stem
lying between the polypites are occupied
by groups of individuals (consisting of hy-
drophyllia, dactylozooids, and gonophores).
(In the accompanying figui-e, for the sake
of simplicity, instead of these groups of
individuals, only their dactylozooids are
indicated.) Here the law that growth pro-
gresses unifoi*mly from above downwards
applies only to the polypites (A, B, G, B),
whereas each internode presents its own
zone of growth for the groups of indivi-
duals (a, 6, c, d) belonging to it, for which
in turn the uppermost end of the internode
must be looked upon as the budding point,
so that likewise in the series of groups of
individuals in each single internode the
lowermost (a) is the oldest. Each inter-
node of the stem is divided by these groups
of individuals into internodes of the second
order (.4a, ah, be, cd . . .); and each such
internode of the second order may, in the
further growth of the stem, become a zone
of growth for a series of new groups of in-
dividuals (a, 13, y . . .) (Chun, No. 57).
For the other groups [of Physophoridae]
the details of the laws of buddinsr are as
yet little known. In the VeUelidse the for-
mation of the individuals takes place in
concentrically arranged circles.

/

-J

—a

Fig. 29.— Diajjram
of Chun's law of
budding of the
groups of indivi-
duals on the stem
of Halistemma. In
place of the groups
of individuals only
the corresponding
dactylozooids are
shown.

68

EMBRYOLOGY

Calycophoridse. — The development of Epibulia auran-
tiaca (family of the Diphyidge), which has been very accurately
followed by Metschnikoff (No. IS), will be described as the
type. The ovate planula larva exhibits a thickening of the
ectoderm at the posterior pole and on one side (the subse-
quent ventral side). Here the fundaments of the first
nectocalyx (Fig. 30 B, nc) and of the tentacle (B'ig. 30 B, t)
are developed. The former is developed by the invagination
of a solid bud-nucleus (fCnospenherii) , in which the cavity

Fig. 30. — Three larval stages of EpihuUa ourantinca (after Mktschnikoff, from
Balfgub'b Comiiarativc Embryology ) . A, planula; B, stage six days old with funda-
ment s of nectocalyces (nc) and tentacles (() ; C, somewhat older stage with gastral
cavity ; nc, nectocalyces ; t, huulamont of tentacle ; )»o, polypite ; c, nutritive cells ;
no, fundament of the fo-ciiUed f^oinatocyst ; lnj, entoderm ; cp, eeioderm.

of the bell is soon formed ; the fundament of the tentacle
at first consists of a simple invagination of the body-wall,
in which two layers take part, the development of an
ectodermic layer (Fig. 30 Ji, hi/) along the ventral side, con-
sisting of small cells, having already taken place at this
stage. The next important change consists in the establish-
ment of the gastrovascular cavity (Fig. 80 C), which is

CNIDARIA

69

correlated with the disappearance of the nutritive cells. By
means of it the posterior part of the larval body (Fig. 30 C,
po) is characterized as the fundament of the first polypite,
whereas the upper dorsal part is retained for a considerable
time as an embryonal remnant, which gradually diminishes
and is converted into the stem (like the yolk-mass of the

Fig. 31. — Older larval stage of Eiiihulia aurantiaca (after Mkischnikoff, fro'ii
Ba-J.tovr' a Comparative Embryology), so, somatocjst ; iic, second nectocal^ x bud ;
hph, hydrophillium ; 2'", polypite ; t, tentacle.

Agalmidfe). At the same time the fundament of the necto-
calyx (Fig. 30 C, nc) has made considerable progress. Tlie
hollow core of the bud is enveloped by a layer of entoderm
(%), into which a part of the gastrovascular cavity is pro-
longed as the fundament of the vessels of the bell. Another
entodermal process (Fig. 30 G, so) becomes the so-called
somatocyst {SaftbehilUer). Between the entoderm and the

70 EMHRYOLOGY

outer ectoderm mesoglcea has been secreted. In general
the development of the nectocalyx is quite like the budding
of a Hjdromedusa described above (p. 43). On the funda-
ment of the tentacle {t) the individual nettling tubercles
can be seen developing as secondary evaginations (Fig. 'SO C).
The further changes (Fig. 31) consist in a considerable
enlargement of the first nectocalyx, which now, after the
]-eduction of the nuti-itive cells, is the most voluminous
structure of the young colony. The polypite (po) now
acquires its permanent structure by the breaking through of
the mouth at its distal end, while the teiitacle (t), in this
case persisting (not larval), attains its complete develop-
ment. Of interest is the appearance of new buds on the
rudiment of the stem, first of all that of a hydrophillium
(Fig. 31 hp]i), with the development of which is established
the first group of individuals (cormidium) of the subse-
quently elongated stem — consisting of a polypite, a dactylo-
zooid, and hydrophillium — which is afterwai-ds developed into
the Eudoxia. At the same time we see two smaller buds
arising, one of which must be considered as the second
nectocalyx (Fig. 31 7/c), whereas from the other the elements
of the second group of individuals of the stem bud forth.

In the stage Fig. 30 li, which in Fig. 30 C and Fig. 31 undergoes its
further development, is shown a larval stage exceedingly characteristic
of the Calycophoridffi, which has been designated by Haeckel as the
Ciilyconula, and which rej^resents essentially the SipUoinila-stage of the
CalycophurnUc. Haeckel (No. 70) regards this stage as an individual of
the second degree (iDerson), and recognizes in its component parts the
constituent organs of an Anthomedusa, which here present a remark-
able dislocation. For if the nectocalyx corresponds to the umbrella and
the polypite to the manubrium of the medusa, then it is evident that the
l^olypite is here attached to the ex-umbrellar side of the medusa-bell.
Haeckel explains this dislocation by the assumption of a ventral fissure
in the umbrella of the ancestral forms, through which a gradual emigra-
tion of the manubrium was possible. Furthermore the only marginal
tentacle of the medusa present has moved from the margin of the
nectocalyx to the base of the jtolypitc.

The assumption that the Siphonula thus characterized actually corre-
sponds to an ancestral form acquires an apparent support from the
circumstance that the same type of form is found again in the groups of
individuals of the stem (cormidi(i). For the individuals of the stem in

CNIDARIA 71

the Calycophoridae are united into groups and separated by intervals of
the stem {internodeg). They bud in such a way that the grouj) of indi-
viduals (cormidium) occurring at the lowermost end of the stem is the
oldest. In many cases (Polyphyid£B, Desmophyidfe, Praya, Galeolaria,
etc.) the groups of individuals, even if they produce sexual products,
remain united with the entire corm. In most of the Diphyidaj, on the
contrary, the oldest cormidia separate from the parent stock before they
arrive at sexual maturity, and either as Eudoxiaj or Ersffife lead an
independent life. In this way a kind of alternation of generations is
brought about, since the parent stock does not itself produce sexual
l^roducts, but separates into secondary stocks, which do not reach sexual
maturity until later on. Such a detached Eudoxia group (so the
cormidia have usually been called) consists of a polypite with tentacles,
a hydrophillium, and a gonophore which develops the sexual products in
its manubrium, and at the same time by the rhythmical contractions of
its swimmmg sack produces the locomotion of the detached Eudoxia.
Haeckel explains the hydrophillium, the polypite, and the tentacles as
the constituent jDarts of a sterile person, in which the bilaterally
symmetrical hydrophillium would represent the umbrella of the medusa.
The Eudoxia cormidium would accordingly in the simplest case be
composed of two persons : a sterile one and a fertile one (the gonophore
or sexual bell). It is to be observed that the two persons named would
represent two essentially heteromorphous medusas of the same corm.
Whereas the sterile person exhibits a bilaterally symmetrical structure
and the above-mentioned dislocation of the parts, nothing of this kind
can be recognized in the fertile person. The structure here is that of an
ordinary quadriradial Anthomedusa, and the manubrium has retained its
usual place.

Leuckart and Gegenbaur have shown that in various Eudoxije the
gonophore, after the discharge of its sexual products, is replaced by the
outgrowth of a new gonophore, and Chun showed it to be probable that
in all Eudoxi* a quite regular replacement of the gonophores takes
place, so that each Eudoxia has quite a number of gonophores developed
one after another. Now let us imagine that the first one of these
gonophores remains sterile, functioning merely as an organ of locomo-
tion ; that would lead to the form of the Ersseas (in Haeckel's sense).
As Ersasffi are designated the cormidia which bud on the stem of
Lilyopsis and Diphyopsis, and which, in addition to the parts described
for the Eudoxias, possess a so-called special swimming bell, so that
these cormidia, according to Haeckel's interpretation, embrace at least
three persons, two sterile and one fertile.

The different parts of the nectosome are also subject to a quite similar
replacement by supervening buds. Even in the Diphyidas the two necto-
calyces are not retained throughout life. Leuckart had already observed
the presence of two to three bud-like supplementary bells in Epibulia, and
Chun showed that the nectocalyces of the Diphyidte are subject to a con-

72 EMBRYOLOGY

stant replacement by reserve nectocalyces of the same form. This
replacement also plays a considerable role, as we shall see directly,
in the metamorphosis of the Calycophoridie.

The metamorphosis of the Caljcophoridte has been m.ade
known chietlj through the investigations of Chun (No. 54).
These refer principally to the development of the Mono-
phyidse, i.e. those forms which are characterized by the
possession of a single nectocalyx on the nectosome. In a
small Monopliyid called by Chun Muggiaea Kochii, and
characterized by its tall pentagonal nectocalyx, Chun was
able to prove that the larvoe arising from the eggs at first
possess a quite differently shaped, cap-like nectocalyx. By
the casting off of this primary provisional nectocalyx and
its replacement by the permanent heteroraorphous one,
these larvae, designated as Monophyes primoi'dialis, pass into
the Muggiaja form, from the stem of which the groups of
individuals at sexual maturity detach themselves as Eudoxia
Eschscholtzii.

Since Chun has recently been able to prove the presence
of these primary, heteromorphovis, deciduous nectocalyces
in the case of the Polyphyidae, it may be considered probable
that such nectocalyces also belong to the larval stages of all
CalycophoridfB. According to Chun's theory, which Haeckel
has adopted, the fundament of the pneumatophore in the
PhysophoridjB would be homologous to the deciduous, pri-
mary nectocalyx of the Calycophorida?.

General Considerations. — As regards the derivation of
the Siphonophora, there are at present two views, as yet
directly opposed to each other, undei'lying both of which is
the conception that the Siphonophore is a polymorphous
animal stock that has arisen by budding. But while some
authors (Leuckart, Claus, Chun) assume the starting-point
of this stock to be a floating hydroid-polyp stocklet, which
already had the power of producing medusae {hydroid theory),
others (Balfouk, Haeckel) derive the Siphonophore from a
medusa, which, by the budding of its manubrium (like Sai'sia
or Hybocodon), was able to produce new medusas {medusa
theory). Tlie former authors accordingly have two funda-
mental forms from which they are able to derive the mani-

CNIDARIA 73

fold parts of the Siphonophore body. They can consider
certain parts (polypites, dactylozooids, etc.) as metamorphosed
polypoid individuals, other parts (nectocalyces, hydrophillia,
gonophores) as metamorphosed medusoid individuals, which
remain united vpith the colony. The adherents of the medusa
theory, on the other hand, have at their disposal only the
hydroid medusa as a fundamental form for the derivation of
all the numerous polymorphous parts of the Siphonophore
organism, for only new medusae can ever be produced from
a medusa by budding. Since by this explanation the poly-
pites are homologized with the manubria, and the tentacles
with the marginal tentacles of a medusa, the adherents of
this theory find it necessary to assume an ancestral form in
which the medusa exhibited a bilaterally symmetrical
structure, while a single tentacle was advanced to the base
of the manubrium, and both these parts had emerged upon
the ex-umbrellar side of the medusa-bell through a fissure in
the umbrella — conditions which, as a matter of fact, do not
exist in any Hydromedusa. As a further consequence the
partisans of the medusa theory must assume the possibility
of a considerable dislocation of these diiferent primary
organs and an extensive capacity of the individuals to
multiply difPerent organs. With all these assumptions,
there arise certain difficulties which are not encountered in
the hydroid theory.^

Even if the ancestral form of the Siphonophora assumed
by the medusa theory, and described above (which is re-
capitulated in ontogeny by the Siphonula-stage and by the
sterile person of the Eudoxige), were to be derived from
bilaterally symmeti-icalAnthomedusse with only one marginal
tentacle (for example, from the Hybocodon belonging to
Corymorpha), it would still be difficult to point out in any
way the causes for the appearance of the fissure in the
umbrella and the described dislocation of the organs. The
difficulty is increased by the circumstance that these cha-
racters are lacking in the sexual individuals of the Siphono-

^ It should be stated that recently Hatschek (Lehrbuch der Zoologie)
has introduced modifications into Haeckel's medusa theory, by means of
which a part of these ditKculties seem to be set aside.

74 JIMBRYOLOGT

phora, so tliat, iu pursuance of the medusa theory, we are
required to distinguish iu the Siphonophora two highly
heteromorphous generations, the first, produced from the
egcr, consti'ucted upon the Siphonula type, and reproducing
by budding only, the second a generation of fertile indi-
viduals not bilaterally symmetrical, and without dislocation
of the primary organ. Still sharper, perhaps, is the contrast
between the Disconula of the Vellelidse, which is i-eferred
by Haeckel to certain Trachomedusae, and the structure of
the Chrysomitras.

On the other hand, for the hydroid theory there is the
difficulty of explaining how a firmly attached hydroid stocklet
could detach itself and become metamorphosed into a free-
moving, pelagic organism. If, however, "we assume that a
hydroid stocklet attached itself by means of a broadened
basal plate to the surface of the water, instead of to a fixed
body, as may occasionally be observed in the Scyphistomas,
and acquired under favourable circumstances the power to live
on in this condition, then with this conception the transition
from the attached to the free mode of life is brought about
by a floating at the surface of the water, a form of locomotion
which has been retained in Physalia and Vellela. Nay, we
need only to conceive that the flattened basal part of the
stem, which attached itself to the under-surface of the watei-,
curved inwards like a canoe, and finally became, with its peri-
sarc-covered face, completely invaginated,^ in order thus to
make the phylogenetic origin of the pneumatophore conceiv-
able, and to support this conception by the consideration
that such a course of development must have been constantly
accompanied by certain advantages to the entire colony.
Not until after the development of this hydrostatic apparatus
would a separation from the surface of the water and a
descent into greater depths become possible. The pneuma-
tophore would accordingly be the first, most primitive oi'gan
by the development of which the characteristic peculiarities

' It has actually been observed in the planula of various Cnidaria that
the future jjoint of attaclunent, which lias undergone glandular alteration,
is more or less invaginated, as in the Scyphomedusie and in Eutima
(Brooks).

CNIDAKIA 75

of the Siplionopliore organism were established. We might
perhaps be led by such considerations to recognize in those
forms with a persistent apical stigma (Rhizophysas, Physa-
lias) the most pi-imitive of the now existing Siphonophores.

In this proposed hypothesis of the derivation of the pneumatophore we
are opposed to the conception, shared in by most investigators (comp.
p. 72), tliat it is a modified medusa-bell. The latter view is founded
partly on the structure of the fully developed pneumatophore and partly
on its development. Even though the spaces of the gastrovascular
system in the vicinity of the pneumatophore, divided as they are by
septa, challenge a comparison with the radial canals of a medusa, and
even though the bud-like fundament of the pneumatophore is uncommonly
like a medusa bud, as has been stated by Metschnikoff (pjD. 62, 63), these
resemblances do not appear to us to present proofs of a compulsory
nature, the more so since the transition from a medusa into a hydrostatic
organ involves a change of function that is somewhat difficult to compre-
hend. According to our way of looking at it, on the contrary, the aj^ical
position of the pneumatophore, sunk into the uppermost end of the stem,
and its early appearance in the ontogeny of many forms, are most easily
explained.

According to our notion, the pneumatophore would be the most jn'imi-
tive locomotor organ of the Siphonophora, to which a neotosome would
be added only secondarily. Accordingly the Physophoridse would repre-
sent the more primitive forms, and the Calycophoridse derived forms,
with divergent development caused by the loss of the pneumatophore
and the in part higher differentiation of the nectocalyces. Among the
Physonectas (Haeckel) the Apolemidae, the nectosome of which is still
provided with heteromorphous individuals, would perhaps represent the
most primitive branch. Ojiposed to this theory, however, is the fact that
histologically the Calycophoridffi exhibit the simplest conditions (Korot-
neff) ; but these might have been simplified secondarily.

When with the above statements we adopt the hydroid
theory founded by Leuckart, it is hereby to be understood
that, according to our point of view, the existing facts are
most easily explained by this theory. Nevertheless we can
as yet ascribe even to it only a certain degree of proba-
bility.

II. ANTHOZOA.

Alcyonaria. — The sexual products of the Anthozoa, which
arise from the entoderm (Hertwig, No. 9), undergo the
process of ripening in sexual organs which belong to the

76 EMBRYOLOGY

mesenterial septa. It is here also that the eggs in most
cases are fertilized, and frequently undergo the first stages
of development, viz., cleavage and the formation of a
spheroidal embryo consisting of two gerra-layers. The
embryo is afterwards set free in the gastral cavity of the
parent, from which it is ejected ihi-ough the mouth-opening,
usually in the stage of a ciliated planula. While thus many
Alcyonaria are viviparous, cases have also been observed in
which the eggs, either unfertilized or immediately after
fertilization has taken place, are extruded through the mouth-
opening of the parent, either singly or united into Inrge
masses by means of a slimy substance (Alcyonium, Renilla,
Clavularia crassa).

The early development of the Alcyonaria has become
known chiefly through Lacaze-Duthiers (No. 88, Corallium),
KowALEVSKY (No. 10, Alcyonium, Gorgonia), v. Koch (No.
86, Gorgonia), E. B. Wilson (No. 98, Renilla), and Kowa-
LEVSKY ET Makion (No. 87, Clavularia, Sympodium).

The ripe egg of the Alcyonaria is usually rather rich in
granules of food-yolk, which, mixed with oil drops, is accu-
mulated especially in the inner parts, so that in certain cases
there is a sharp separation of a finely granular ectoplasm
from an endoplasm rich in food-yolk. Cleavage has been quite
variously described for the forms so far observed ; in fact, in
Renilla it exhibits remarkable individual variations. In
general it follows the total and equal type, and finally leads
to the development of a solid so-called tnonda-stage, consist-
ing of cells more or less unifoi-m in size and exhibiting even at
an early stage a difference between the mox^e finely granular
cells of the superficial layer and the coarsely granular
ones of the inner mass. An interesting modification of the
cleavage process is met with frequently in Renilla, and con-
stantly in Claviilaria crassa. Here a multiplication of the
cleavage nuclei first takes place, corresponding to which
there is only an indentation of the surface, not a real cleav-
age of the egg. This does not take place until thei'e are
sixteen cleavage nuclei, when it results in the formation of
the same number of separate bla.stomeres. We see that we
here have to do with a variation which forms a transition

CNIDABTA 77

to the type of superficial cleavage, which is wide-spread
amoBg the Arthropoda. '

In general the cleavage stages of the Alcyonaria are characterized by
the absence of the cleavage cavity. Monoxenia forms an exception.
Here, according to Haeckel, (No. 78), there are produced in the course of a
very regular cleavage a typical coeloblastula-stage and a gastrula in-
vaginata.

In the morula a difference can early be recognized (Fig.
32 A) between a superficial cell-layer (ectoderm) and an
inner cell-mass (entoderm). This difference becomes more
marked in later stages (Fig. 32 B, C). The ectoderm cells
by progressive division are metamorphosed into prismatic
elements, which constitute a columnar epithelium (Fig. 32
C). Those of the inner cells lying next to the ectoderm also
arrange themselves (Fig. 32 C, en) into an epithelial layer
(the permanent entoderm), whereas the elements lying at
the centre undergo a process of degeneration. The cell
boundaries hei-e become indistinct ; vacuolar spaces make
their appearance, and soon coalesce into a common internal
cavity (the beginning of the gastral cavity, li) ; finally, this
entire cell-mass is metamorphosed by fatty degeneration into
a kind of detritus {d), which is gradually resorbed. At the
same time a fine, structureless, hyaline membrane (the sus-
tentative lamella) is secreted between the ectoderm and
the permanent entoderm.

While these internal changes are taking place, the body
elongates and gradually assumes an ovoid or, with increasing
length, a vermiform shape, and its surface becomes covered
with close-set cilia; thus the swarming planula-stage is
developed (Fig. 32 D). The planula exhibits a somewhat
broadened (aboral) end, which is directed forwards during
motion, and a posterior (oral), more pointed pole. At the
expiration of the swarming stage the larva attaches itself
by means of its broadened anterior end to some convenient
support. By a gradual shortening in the direction of the
longitudinal axis the larva passes from the elongated into
a low placenfciform shape (Fig. 33).

At about the same time with the attachment, the in-

78

EMBRYOLOGY

vagination of the cesaphageal tnhe (si) takes place, and also the
formation of the eight mesenterial septa. The ojsophagus
arises in general as an ectodermal invagination (Fig. 33),
the bottom of which in later stages breaks through toward
the gastral cavity, thereby establishing the inner opening
of the cesophagus. The formation of the mesenterial septa
is referable to a folding of the entoderm, in which the
sustentative lamella also takes part. It seems that in the

Fig. 32.— Stape in the development of S'jmiiodunn cornllnides (after Kowali.vskt
i:t Marion). A and B, cleavage stages; C, embryo vfith permanent entoderm
(e)i)and inner mass of detritus (d), in which beginnings of the gastral cavity (h)
can be recognized; D, ciliated planula; ec, ectoderm ; en, entoderm.

Alcyonaria all the eight mesenterial septa always make their
appearance at the same time. As regards the origin of
the musculature of the septa, especially the longitudinal
muscles, authors agree that they arise from epithelio-mus-
culai' cells (myoblasts) of the entoderm lamella.

CNIDARIA 79

In Eenilla the oesophagus is developed in the form of a solid ectodermal
ingrowth, in which a fissure makes its appearance and opens to the
exterior, whereas the development of the inner opening of the oesophagus
does not take place until later.

sl

Fig. 33. — Attached stage of Sympodium coraUoides (after Kowalbtskt bt
MiBiotr). -Be, ectoderm ; en, entoderm; sl, oesophagus.

By the formation of the mesenterial septa the gastral
space is separated into a central stomach cavity and eight
pei'ipheral gastral pouches. At the upper ends of the latter
hollow, bud-like elevations now arise, in which we recognize
the earliest fundaments of the eight (subseqaently pinnate)
tentacles, which accoi'dingly owe their origin to simple
evaginations of the body-vrall.

The development of the septa, the formation of the oeso-
phagus, and even the establishment of the tentacles may
take place before attachment. In general, however, the
attachment of the swarming larva precedes the formation
of these organs. By means of the developmental processes
mentioned the typical structure of the polyp is established.
During these metamorphoses important changes in the struc-
ture of the ectoderm take place. By multiplication of the cells
this layer becomes changed into a multi-layered epithelium.
The secretion of a hyaline gelatinous substance [mesogloea]
now takes place between the cells of the deeper layers,
which thus lose their connection with one another and
assume more and more spindle or stellate shapes (Fig. 34).
By these processes two different layers arise from the
primary ectoderm : a superficial one, which from now on
preserves the name of ectoderm, and the cells of which have
retained the enithelial continuity, and a lower layer, which
assumes more and more the character of a gelatinous
connective tissue, and which will be called henceforth
mesoderm. This layer accordingly is a product of the

80

EMBRYOLOGY

ectoderm (Kowalevsky et Marion, No. 87). In it are
secreted the first calcareous spicules (sclerites) (v. Koch, Nos.
82 and 84, Kowalevsky). These arise as small, highly
refractive bodies (sp) within the mesodermal elements,
which resemble migi'atory cells, where they soon grow into
small needles having latei-al outgrowths. The ectodermal
axial skeleton of the Goi-gonidse arises later than these
mesodermal parts of the skeleton. It must be regarded as
a cuticular secretion of the ectoderm of the basal foot-plate
(v. Koch), and at its first appeai'ance consists of a thin
yellowish pellicle, which may be compared to the sheath
of Cornularia and Clavularia. There is soon noticeable on
this basal plate a small prominence, which grows up into
a process composed of concentrically arranged corneous
lamelltB, and extends up between the mesenterial septa of the

Fig. 34. — Section through the hody-wall of a young attached stage of Sym-
3)ocii«m coro (!o ides (after Kowalevskt et Maeion). cc, ectoderm ; eii, entoderm;
g, mesogloca ; ."ip, earliest fundament of the calcareous spicules in cells of the
developing mesoderm.

primary polyp. Thus the ectoderm of the foot-plate must
be correspondingly invaginated, and thus it comes about
that the axial process of the ectoskeleton contained within
the polyp is covered by a continuous ectodermal lamella
(the axial epithelium), from which the further development
of this part of the skeleton takes place. In the further
course of development, during the progressive growth in
length, the young polyp and the axial skeleton do not take
the same direction ; the latter thereby acquires greater
independence, and represents the earliest fundament of the
whole axial skeleton which lies at the foiindation of the
entire colony subsequently produced by budding (Fig. 36 B)
(v. Koch, No. 86 i).
'On the other hand, Studer {Arch. f. Natiirg. Jahrg., 1887) has

CNIDARIA

81

To explain the phylogenetic development of this axial skeleton of the
Gorgonidffi, v. Koch (No. 85) has for comparison made use of the
interesting discoveries on Gerardia (Antipatharia, Hexacorallia). These
colonies of Gerardia form flat membranoid coverings over foreign
bodies, and for this purpose commonly select the axial skeleton of dead
Gorgonidse as a support. A lamella of horn, which coats the support,
is now secreted by the ectoderm of the lower surface of these colonies.
The lamella surrounds the axis of the Gorgonia within it like a sheath.
When at length the colony of the Gerardia by growth acquires an extent
which stretches beyond the limits of the original support, then out-
growths covered with young polyps are produced, into which extend
horny skeletal processes, produced by the common basal lamella, but no
longer enclosing within them any foreign body. It is seen that here is
produced the first trace of an independent free axial skeleton, while
the basal plate of the skeleton, which in the higher forms is much
reduced and attached to a foreign supj)ort, arises from the basal lamella.

Fig. 35.— Two transverse sections through a polyp of the Alcyonarian type
(diagram after v. Koch, from Lang's Lehrhiich) ; that at the left is at the level
of the oesophagus, that at the right at tlie level of the gastral cavity, ah, plane
of symmetry. The ventral side is directed upwards.

The polyps of the Alcyonaria present a typical bilaterally
symmetrical structure, -vvhicli is evident in the first place
from the position of the longitudinal muscles in the mesen-
terial septa. Here the plane of symmetry (Fig. 35 ab)
passes through two unpaired chambers (gastral pouches),
which are distinguished from each other by the fact that
the two septa which bound the ventral chamber exhibit the
muscle ridges on the sides which are turned toward each
other, whereas this condition is reversed in the dorsal cham-
ber. On the remaining septa, in fact on all the septa, the
longitudinal muscle ridges are so arranged that they face

recently defended the interpretation of the axis of the Gorgonidse as a
mesodermal growth.

K. H. E. - G

82 EMBRYOLOGY

toward the ventral side of the polyp, whereas the surfaces of
the septa which are without longitudinal muscle bands face
toward the dorsal side. The bilateral symmetry can also be
recog-nized by the presence of a ventral ciliated groove
running along the laterally compressed oesophagus (siphono-
glyphe, Hickson), and above all by the condition of the
mesenterial filaments. Of these the pair belonging to the
dorsal septa differs from the others in structure, function,
and development. The filaments of the dorsal pair of
septa exhibit an epithelial band consisting of tall flagellate
cells, and produce a powerful upward ciliary current,
whereas the filaments of the other six septa are charac-
terized by their richness in gland cells, and they play an
important role in digestion. E. B. Wilson (No. 97) was
able to show that the latter take their origin as simple out-
growths of the entodermal epithelium of the septa, whereas
the dorsal filaments belong to the ectoderm, and are con-
tinued on to the margins of the septa as direct outgrowths
of the oesophageal epithelium.

An observation by Wilson is of general interest : that the develop-
ment of these dorsal filaments is retarded in the larvae produced from
the egg, whereas in the bud they actually outstrip the other filaments in
development. Wilson explains this by the conditions of nutrition in
the bud, which requires a powerful upward stream of nutritive fluid for
its development.

Of the various kinds of non-sexual reproduction in the
Alcyonaria, huddimj is the most prevalent; by means of it
extensive colonies (stocks, conns) are developed, owing to
the fact that the newly arising individuals remain united
with the parent. In the simplest case a latei'al "runner"
arises from the j)arent animal and grows out at its end into
a daughter individual. The portion remaining between the
two as a connective is called a stolon (Fig. 36 A). These
stolons, issuing from the base of the polyp, may form a net-
work (Cornularia), or fuse into a basal plate (Rhizoxenia).
We have seen above (j). 81) how, owing to the formation of
a basal skeletal plate upon which an axial skeleton arises,
the dendritic stocks of the Giorgonida3 can be derived from
such flatly extend.ed colonies (Fig. 36 B). In other cases

CNIDAKIA

83

the stolons do not belong exclusively to the basal part of
the polyps, bnt arise at various levels. In this way the
peculiar colony of Tubipora (Fig. 36 C) arises by the de-
velopment of stolonic plates in higher positions, from which
new buds grow out. In other forms, by the intimate fusion
and irregular branching of the stolons, there is developed an
intermediate tissue (coenenchyma) traversed by numerous
nutritive canals (Fig. 36 D), which unites the diiferent indi-
viduals. In this way the antler-like colonies of Alcyonium
are developed, and by the formation of a mesodermal axial

Fig. 36.— Diagrams of budding and stock-formation in the Alcyonaria (-ifter
V. Koch, from Lang's Lehrhuch). A, formation of the basal stolon ; B, type of
the Gorgonidffi ; C, type of Tubipora ; D, type of Alcyonarium ; s, oesophagus;
se, septa ; in/, mesenterial ridges ; dh, gastral cavity ; sfc, axial skeleton growing
upwards by means of successive layers.

skeleton the more slender forms, such as Corallium, Scleor-
gorgia, Melithsea, etc. (v. Koch).

The development of colonies by budding is of special
interest in those forms in which, owing to the regular orien-
tation of the daughter individuals to the parent polyp, there
is established a regular bilaterally symmetrical structure of
the entire colony (Pennatula, Renilla). In these forms a
well-marked polymorphism of the individuals is exhibited,

84

EMBRYOLOGY

inasmuch as polyps which bear tentacles and become
sexually mature [autozooids] can be distinguished from
sterile individuals lacking tentacles and having only two
septa, the so-called zoiiids [siphonozodids], which provide for
the inflowing of the water (Wilson).

The development of Henilla has been investigated by
E. B. Wilson (No. 08). Attachment is here suppressed, and
by the invagination of the oesophagus and the development
of the septa and tentacles there is produced from the planula-

FiG. 37.— Two stages of development of ReniUa (after E. B. Wilson). A, younff
polyp with two polyp buds (;<') and the terminal zooid (z^ ; B, central portion of a
somewhat older stage ; p', jj^, p*, p*, polyp buds; 2, terminal zooid; mz, marginal
zoiiid ; dz, dorsal zoiiid.

larva a free-moving polypoid form (Fig. 37 A), which, in
view of the development of the colony, can be called the axial
individual. The upper portion of this individual persists as
the terminal polyp, whereas the stem of the entire colony
(rachis) and its lower free part, the stalk {peduncle) , arise
from its middle and lower portions. We may also retain for
Renilla these terms, which are borrowed from the Pennatu-
lidae, because a striking similarity between these two forms
is established in their embryology. The eight septa of the

CNIDARIA

85

axial individual are developed in t.lie anterior part of the
polyp, and grow from in front backwards ; nevertheless they
are restricted, even in late stages, to the anterior parts of
the individual, whereas in most Alcyonaria the septa extend
as far as the posterior end of the body. On the other hand,
another wall is developed in Renilla by a transverse infold-

-UtZ

Fig. 39.— Older stage of development of the colony of Renilla (after E. B. Wilson).
)), terminal polyp ; z, terminal zooid ; mz, marginal zooid; dz, dorsal zooid.

ing of the entoderm from the posterior end of the body, the
so-called peduncular septum, by means of which the gastral
cavity is divided into a ventral and a dorsal half. The
peduncular septum grows from behind forwards ; and since it
grows more actively at its lateral parts, its anterior margin
assumes a curved form. Between the two entoderraal layers
of the peduncular septum is found a cell-mass which subse-
quently degenerates, and which is apparently homologous to

86

EMBRYOLOGY

the skeletogenous layer of the Pennatulidge, but which is
said by Wilson to arise from the entoderm.

At an early period the budding of the daughter individuals
begins ; these are formed strictly in pairs on the dorsal side
of the axial individual (Fig. 87 A, p^). The second pair of
polyp buds arises immediately behind the two first ones,
the third pair in front of and somewliat ventrad from the

Fig. 39.— Young colony of Pennatula phosphorcaCufter Jungkkskn). A, youngest
stage, seen from the right ; B, older stage, from the ventral side ; C, the same,
Irom the dorsal side; )), terminal polyp; 2, terminal zoiiid ; 3)', v\ polyps of the
first pinnate leaflet ; p2_ j,a, polyps of the second pinnate leaflet, etc.

first pair, the fourth pair in the angles between the third
pair and the axial polyp (Fig. 37 B, p\ p^, p^, p'^). The buds
arise separately at first, but subsequently their basal pai-ts
fuse. The individuals that have arisen in this way very
soon assume a radial position; and since the buds that appear
later ai-e formed in alternating positions and ventrad to
those first formed, and since in the further course of de-
velopment they grow so actively that they project beyond

CNIDARIA 87

these at the periphery, it follows that the oldest individuals
are more and more crowded toward the dorsal side (Fig. 3S).
The terminal polyp also shares this fate. In this way is
developed a discoid colony, the marginal individuals of which
are the youngest.

The zooiils are formed at the same time as the sexual
polyps. Even immediately after the appearance of the first
pair of polyp buds, a large terminal zooid (Fig. 37 z) can be
recognized ; this functions as an excurrent opening, and is
soon followed by the so-caWed viarginal zooids (mz), arranged
in two lateral dorsal rows, while dorsal zooids (dz) make their
appearance on the dorsal side of each of the individual
polyps.

As far as the development of Pennatula is at present
known, it is strikingly similar to that of Renilla. Lacaze-
DuTHiEBS (No. 90) has made some statements on the earliest
stages of Pteroides (Pennatula) griseum ; the later stages,
relating to budding, have been described by Jungersen (No.
81). Here also we find lying at the foundation of the colony
an axial individual which is retained for a considerable time
as the terminal polyp, and on the sides of which bud forth
the daughter individuals, which appear in pairs, but alter-
nating with one another. At the bases of these lateral
polyps, that are the first to appear, and in positions corre-
sponding to the ventral side of the axial individual, new buds
continue to arise, thereby introducing the development of
the pinnate leaflets, of which accordingly the dorsal indi-
vidual exhibiting the greatest length is the oldest. On the
dorsal side of the axis we find an unpaired terminal zooid
and other zo5ids which are arranged in two rows. The
lateral zooids, which belong to the ventral surface, are not
developed until later. In the young stages the terminal
zooid probably functions as the only excurrent opening. In
the older stages, on the other hand, there is found at the
upper end of the axis a group of apical zooids, among which
are probably to be found the terminal zooid and the degene-
rated terminal polyp, as well as the adjoining polyps, these
having assumed the function of the terminal zooids.

In the peduncular septum, which here also divides the gas-

88 EMBRYOLOGY

trie space of the axis into a dorsal and ventral canal, there is
found a calcareous axis (ectodermal according to v. Koch's
conjecture), surrounded by an axial epithelium, and two
lateral canals lying at the sides of the former, which, as
nutritive or sap canals, belong to the gastrovascular system.

From the embryology it appears that the older authors
have employed the expressions " ventral " and " dorsal " for
the Pennatula colony in the opposite sense to that which
is admissible according to the orientation of the axial polyp
(Jungersen).

Zoantharia. — In the majority of cases fertilization and
cleavage take place inside the mesenterial septa, and the
further development, as far as the complete formation of
the planula, in the gastral cavity of the parent. In this
stage the lai^vse are cast out through the moath-opening.
On the other hand, Cerianthus membranaceus and Actinia
parasitica (Adamsia Rondeletii), according to Kowalevsky,
eject the spawn in an unsegmented condition.

Considerable uncertainty still prevails regarding the
earliest developmental processes, the knowledge of which
we owe chiefly to Kowalevsky (No. 10), Jourdan (No. 80),
and H. V. Wilson (No. 99). In many cases cleavage and
the differentiation of the entoderm seem to take place in
connection with the formation of a solid morula, therefore
in a manner similar to that which has been described for
the Alcyonaria. At least there is in support of this Kowa-
levsky's observation on Actinia parasitica (Adamsia Ronde-
letii), which is described in the following manner: "Cleav-
age is regular, but as the result of it there arises not a
blastodermic vesicle, but only an aggregation of cells, which
becomes covered with cilia, and swims about as a larva;
subsequently a small depression is formed at one spot. The
opacity of the eggs made a further pursuit of the develop-
ment impossible." The author is convinced that the ento-
derm in this case is not formed by invagination, but by a
splitting off from the blastoderm, as in the Corallia. In
sections through ciliated larvae of Astroea Kowalevsky found
the two layers, ectoderm and entoderm, composed of cylin-
drical cells, and an inner contained mass, which had obvi-

CNIDARIA

89

ously arisen from cells, bat which now showed that it was
composed of nuclei and fat spherules only. A similar struc-
ture of the planula is also described for Actinia aurantiaca
and Balanophyllia regia ; Jourdan's observations show, how-
ever, that from the presence of an inner mass filling up the
planula we ai-e not at all justified in inferring the origin of
the mass from a solid morula. Balfour refers to observa-
tions of Kleinenberg according to which the cleavage of the
Zoantharia is frequently unequal ; this would allow one to
infer the formation of an epibolic gastrula. Accordingly
the formation of the entoderm by delamination from a solid
morula in this case still appears doubtful.

In another series of cases the development of a unilaminar
ciliated blastodermic vesicle has been observed, from which
the gastrula-stage is produced by invagination; thus in an
edible Actinian from Faro (Messina), closely related to
Actinia mesembryanthemum, observed by Kowalevskv.
Here the blastopore does not close completely, but is directly
converted into the inner opening of the oesophagus, while
the oesophagus, lined with ectoderm, is developed by the en-
folding of the margins of the mouth-opening. In Cerianthus
also the formation of a coeloblastula and an invaginate gas-
trula following total unequal cleavage was observed by
KowALEVSKY. Probably Caryophyllia also belongs here.

In Actinia equina, according to Jourdan, there is formed a typical in-
vaginate gastrula, whose gastral cavity is at first completely empty, and
whose entodermal cells contain but little food-yolk. Nevertheless the
stomach of the planula larva is filled with coarse yolk granules It still
remains uncertain whether these are produced by secretion or by the
partial disintegration of the cells of the entoderm.

According to the observations of H. V. Wilson on Manicina areolata,
first a cceloblastula is formed by total cleavage. Then, by the transverse
division of the tall cells of the blastosiihere — consequently by delamina-
tion—coarsely granular cells are repeatedly constricted off, and finally
fill completely the cleavage cavity. While the ectoderm becomes some-
what more shai-ply marked off from the inner cell-mass, the oesophageal
invagination arises. The larva now becomes covered with cilia and
swims about. The permanent entoderm arises, as in the Alcyonaria,
from the inner cell-mass, the cells lying next to the ectoderm arranging
themselves into an epithelium, while the central mass is finally resorbed.

At any rate, through these various processes of develop-

90 EMBRYOLOGY

ment there always arises the same larval form, with identical
structure : a bilarainar, thickly ciliated, oval, pyriform or
more elongated vermiform lAanula, which posses-^es an
ectoderm composed of prismatic or columnar cells, an
ontodermic epithelium consisting of large cubical elements,
and a homogeneous membrane (sustentative lamella), which
is secreted between the two layers at an early period. The
internal cavity of this larva (gastral cavity) is in most cases
still tilled with masses of food-yolk. In this swarming stage
there can be recognized a broader, anterior, aboral end of the
body, which subsequently serves for attachment, and is fre-
quently characterized by a long tuft of cilia and a narrower
posterior end ; here the oesophagus is formed by invagina-
tion, and at its deepest part a communication with the
gastral cavity is produced by resorption of the cells. The
further development takes place principally by the formation
of the mesenterial septa, the filaments, the tentacles, and,
finally, in the Corallia (Madreporaria), the calcareous
skeleton.

As regards the sequence in the development of the septa,
the views expressed by MrLNE-EowARDS et Hatme, based
chiefly upon the condition of the tentacles and calcareous
septa of the adult animal, were formerly generally accepted.
According to them, first six primary septa are simultaneously
developed, then six of the second order in the interspaces
between these, then twelve septa of the third order, twenty-
four septa of the fourth order, and so on, the septa of each
newly appearing cycle being interpolated, as was maintained,
between those already present. On the other hand, we owe
to the investigations of Lacaze-Duthiers (No. 89) the know-
ledge that this regular arrangement, which is based on the
number 6, is a secondary one, and that the septa of a cycle
are formed at different times, becoming equalized only sub-
sequently. Most important of all in the earliest stages is a
well-marked bilaterally symmetrical condition, and the
stages with four and with eight septa are to a certain extent
well marked, whereas the intermediate stage, with six
primary septa, is a very transitory one. As regards details,
the statements of Lacaze-Dutiiiers on the sequence in the

CNIDARIA 91

development of tlie pairs of septa first to appear must be
modified in accordance with the conjectures of O. UXD R.
Heriwig (No. 9), which have been confirmed by the observa-
tions of H. V. Wilson (No. 99) and others. The sequence in
the development of the different pairs of primary septa is
consequently as follows. At first a pair of septa arises which
is placed nearly at right angles to the elongated oral fissure
which marks the plane of symmetry (Fig. 40 i). This pair of
septa is formed as a longitudinal fold of the entoderm, inside
of which there exteiids a process of the gelatinous sustenta-
tive lamella. By the development of this first pair of septa,
which lies nearer to one oral angle than to the other, the
peripheral part of the gastral cavity is separated into two
gastral pouches, one of which is smaller than the other. By
means of the second pair of septa (B"'ig. 40 2) the larger of the
two pouches is separated into three parts. The third pair of
septa is developed in the smaller of the two primary gastral
pouches, and divides this in like manner into three parts,
whereas the fourth pair of septa is developed in the unpaired
pouch which is enclosed by the septa No. 2 (Fig. 40 3 and 4).
This stage with four pairs of septa marks a kind of resting
phase in the development. Up to this time the septa were
always established in pairs, and in such a way that each new
pair was developed in one and the same gastral chamber.
For the pairs which now follow, Nos. 5 and 6, the statements
of H. V. Wilson (No. 99) and A. C. Haddon (No. 77) agree
with those of Lacaze-Duthiers to the effect that they take
their origin in the two pairs of chambers which lie next to the
pair of septa first formed. Accordingly the septa of these
two pairs would make their appearance independently in four
different gastral pouches (Fig. 40 B). On the other hand, the
brothers Hertwig (No. 9) have observed in Adamsia diaphana
another mode of development of these two pairs of septa,
both of which here arise in the chambers lying between septa
1 and 2 (Fig. 41). Accordingly even in the Hexactinise alone
different conditions seem to prevail regarding the ai'range-
ment of the longitudinal muscles on the first eight septa
and the development of the fifth and sixth pairs of septa. ^
* [The recent investigations of Boveri (No. III., Appendix to Literature

92

EMBRYOLOGY

The twelve primary septa now arrange themselves in six
pairs, each of which encloses an intraseptal chamber (Fig. 42).
Two pairs of septa, called directive septa, lying opposite to
each other and corresponding in position to the angles of the
mouth (Fig. 42 3 and 4), bear the longitudinal muscles on the
sides which are turned away from each other, all other pairs
of septa on the sides which face each other. The gastral
pouch lying between any two intraseptal chambers is called
an interseptal chamber. New septa are never developed in
the intraseptal chambers. They always appear in pairs, and
from now on in the interseptal chambers and in cycles based
on the number 6.

1

'■ s
"en

Fig. 40. — Diagram of the growth of the septa in Hexactinians. A, stage of
Manicina arcohita with eight primary septa in cross-section (after H. V. Wilson) ; 1,
oldest pair of septa, which is in connection with the oesophagus; cc, ectoderm ;
en, entoderm ; s, sustentative lamella ; /, mesenterial filaments ; r/, part of the
ectoderm of the oesophageal tube tha'. is bent outwards and backwards at the free
end of the tube ; B, stage of Aulactinia stelloides with twelve primary septa (after
McMubrich).

on Anthozoa) are especially important in this connection. Boveri confirms
the existence of both the above-mentioned types of sejDtal growth in the
Hexactinia, of which the one was made known by Lacaze-Duthieks, the
other by Hertwig. In agreement with Haphon, McMurrich, and Dixon,
Boveri places special importance on the presence of an Edwardsia stage in
the ontogeny of the Hexactinia, and is inclined to regard the Edwardsia
type as the pliylogenetic starting-point of all the groups of Actinia, an
opinion against which doubts have recently been raised, so far as re-
gards the Cerianthere and Zoantheie, by E. van Beneden (Nos. I. and II.,
Appendix) and Cablgben (No. IV., Appendix). — H.]

CNIDARIA

93

The condition described in regard to the growth of the septa applies to
the Hexactinia and probably to all Hexacoralla. On the other hand, there
are a number of groups among the Actiniaria in which other laws of
septal growth prevail, which furnish
characters of systematic importance
(R. Heetwig). In the Paractinice
(Sicyonis, Polyopsis) there are found
two pairs of directive sejita, as in the
type described above, and the rest of
the septa also make their appearance
in pairs. On the other hand, the num-
ber of the septa is not fixed by the
numeral 6. The Edwardsidce (Fig. 43
A), like the Hexactiniae, exhibit two
(esophageal grooves [siphonoglyphes]
and two pairs of directive septa ; never-
theless the arrangement of the longi-
tudinal muscles on the septa indicates
a bilaterally symmetrical structure,
as opposed to the biradial condition
of the adult Hexactiniae. Of the

eight septa present, of which only the directive pair exhibits a paired
grouping, six bear their longitudinal muscle bands on the side directed

^ if

J 3

Fig. 41. — Transverse section of a
young Adamsia diaphana (after O.
UND R. Hebtwig), diagrammatic.
The pairs of septa 5 and 6 are iu
process of development.

Fig. 42.— Diagram of the further growth of the septa in the Hexactiniae. Of the
numbers at the left 1 to 5 refer to the type of development of Adamsia (comp. Fig. 41),
the numbers (f. ) and (FI.) to the type of development of ^u/actinia (comp. Fig. 40 B).
At the right, I. to IV. indicate the pairs of septa of the first to the fourth cycle ; r,
r, oesophageal grooves [siphonoglyphes].

94

EMBRYOLOGY

toward the ventral surface of the animal, whereas the ventral pair of
directives exhibits the longitudinal muscles on the opposite side. It
is worthy of consideration that, according to the coinciding observa-
tions of A. C. Haddon (No. 77) on Halcampa and Peachia and J. P.
McMuRRicH (No. 91) on Aulactinia, the position of the muscles on the first
four i^airs of septa agi'ees with the arrangement in the Edwardsidae
(comp. Fig. 40 B), sothataccordingly in the ontogeny of some HexactiniiE
an actual Edwardsia stage is passed through. A bilaterally symmetrical
type is also developed in the groups which now follow. In the MoiKiiilcte
(Fig. 43 B) the dorsal pair of directive septa is lacking, whereas in the
paired arrangement of their septa they aj^proach the Hexactiniffi. The
Zoanthfice (Fig. 43 C) also exhibit a paired arrangement of the septa, but
each pair consists of two unequal septa : a small microseptum, not reach-

Fig. 43.— Diagram of the position of the septa— ^, in the Edwardsidae; B, in the
Monaulese ; C, in the Zoanthese ; D, in the Cerlantheoe.

ing to the oesophagus, and a larger niacroseptum, extending to the
oesophagus. The two pairs of directive septa constitute the only excep-
tion to this, the dorsal pair exhibiting only microsepta, and the ventral
only macrosepta. The remaining mixed pairs of septa are so arranged
that they fall into a dorsal and a ventral group. In the dorsal group,
which always consists of only four pairs, each pair turns its niacroseptum
toward the dorsal pair of directive septa. The number of pairs of the
ventral group is usually considerably greater, and is increased by the
api)earance of new pairs next to the pair of ventral directive septa (at .t:
in the two adjoining interseptal chambers). Here, therefore, only two
interseptal chambers function as formative seats of new pairs of septa.

CNIDARIA 95

Each pair of these ventral groups turns its macroseptum toward the
ventral jjair of directive septa. Finally, in the Cerianthfo: (Fig. 43 D)
only one oesophageal groove [siphonoglyphe] is found. Here the numer-
ous septa are not arranged in pairs ; two particularly small septa attached
to the base of the oesophageal groove (A. von Heider) may be called direc-
tive septa. The septa lying at either side of them are the largest, and
from here the septa continually decrease in size toward the dorsal side, so
that it is probable that the zone of growth of new septa is situated at this
place (Hertwig). That the number of groups is possibly not concluded
with the types described, is proved by Gonactiiiia, which represents a
peculiar type allied to the Zoanthese (Blochmann und Hilger, No. 74).

With respect to the development of the mesenterial fila-
ments, H. V. Wilson (No. 99) has proved, at least as far as
concerns the filaments of the tvpelve primary septa, that they
take their oi-igin as outgrowths from the ectodermal epithe-
lium of the oesophagus. Even earlier A. von Heider, on the
basis of histological agreement, had argued for the ectoder-
mal natui'e of the filaments in Cerianthus, and E. B. Wilson
had conjectured that at least the lateral ciliate bands (Flim-
merstreifen) belong to the ectoderm. A. Andres also believed
that he had convinced himself that the filaments of the six
principal septa take their origin by means of outgrowths
from the ectoderm of the oesophagus. According to the
observations of H. V. Wilson on Manicina areolata, it is to
a certain extent probable that not only the lateral ciliate
bands, but also the nettle- and gland-cell bands {Nessel-
driUenstreifen^i arise from the ectoderm.

With respect to the more detailed processes of develo^jment, the mesen-
terial filaments of the first pair of septa differ from those appearing later.
The establishment of the first pair of sei)ta and the filaments belonging
to it takes place in Manicina areolata at a time in which the space
between the oesophagus and the body-wall is still filled throughout by a
solid mass of entodermal cells. This cell-mass encircling the oesophagus
is divided into two parts, corresponding to the two primary gastral
pouches, which are subsequently hollowed out. This division is eiifected
by the formation between the oesophagus and the body-wall of two par-
titions of the sustentative lamella, which constitute the foundation of the
first pair of septa. It takes place in this way : the oesojihagus ai^proaches
the body-wall until it comes in contact with it, then its sustentative
lamella fuses with that of the body-wall ; when subsequently the oeso-
phagus again separates from the body-wall, a bridge of the sustentative
lamella is preserved between the two. While the fundament of the first

96 KMRRYOLOOY

pair of septa is formed in this way, the development of the filaments
takes place by simple downgrowth of the ectoderm of the oesophagus, in
the direction of the two primary septa. The two gastral pouches first to
appear are now completely hollowed out. The new pairs of septa next
arise as foldings of the entodermal lamella of the body-wall, and their
upper ends seem to be at some distance from the ectoderm of the oeso-
phagus, so that no direct outgrowth of the latter can lead to the formation
of the filaments. In order to establish the connection between the ecto-
derm of the oesophagus and the newly formed septa, the former must bend
around at the inner opening of the oesophagus, and grow upward on the
outer surface of the oesophagus, until it reaches the uppermost part of the
newly formed septa, on to which it now advances to form the filament.
This bent-over part of the ectoderm is seen in Fig. 40 A, rf. H. V.
Wilson conjectures that the mesentei'ial filaments of all subsequently
appearing septa are formed after this type.

In general the development of the mesenterial filaments takes place in
the same sequence as that of the sejjta, so that the oldest pair of septa
bears the most developed filaments.

The tentacles arise as simple evaginations of the body-wall
over the different gastral poaches. The sequence of their
origin has been described by Lacaze-Duthiers (No. 89),
especially for Actinia mesembryanthemum. For the early
stages it is closely connected with the sequence of the appear-
ance of the different mesenteries and the formation of the
chambers dependent on it. In this connection ought speci-
ally to be mentioned the fact that the tentacle which arises
over the larger of the two first-formed gastral pouches con-
siderably outstrips the others in development, so that for a
long time the bilateral symmetry of the larva is marked
externally by the presence of this one large tentacle (Fig.
UA).

Haacke (No. 76) has called attention to the fact that in attached stock-
building forms, as in the blossoms of many Phanerogams, the bilaterally
symmetrical fundamental form may be expressed by the position of the
buds in relation to the parent animal, i.e., to the axis of the entire
colony, since the parts of the bud near to the axis undergo a different
development from those remote from it. Moseley had already shown
that in Saccophyton and Heliopora the polyps always have their dorsal
sides turned towards the axis. We may conclude from such observations
that the bilaterally symmetrical structure of the Anthozoa is caused by
the formation of stocks. The solitary forms (Actinians) would then have
to be derived from those forming colonies. Finally, we may assume that

CNIDARIA

97

the bilaterally symmetrical type, which at first is developed only in connec-
tion with budding, became so firmly established that it also found expres-
sion in the first stages of development from the egg (comp. above, p. 52).
After the formation of the first twelve tentacles, a rearrangement,
according to the number 6, takes place, so that there are two cycles of
six tentacles each. The larger ones, those of the first cycle, correspond
to the six primary intraseptal chambers, whereas the smaller ones, those
of the second cycle, alternate with them. Six large tentacles of the first
cycle thus alternate regularly with six smaller ones of the second cycle.
The appearance of new tentacles does not take place by the interpola-
tion of one in each of the twelve intervals between the elements of the
first and second cycles, but by the api^earance of six pairs, which occupy
only one half of these intervals, as is represented in Fig. 44 B. We

B

Fig. 44. — Two larva; of Actinia mesemhryanthemum ("aftei" Lacaze-Duthikbs, from
Balfour's Comparative Embryology) . 4, bilateral ciliated stage, with one large and
several small tentacle buds ; m, mouth ; B, view of an older stage fjom above. There
are twenty-four tentacles around the mouth. The sequence in the origin of the
twelve primary tentacles is a', a,b, c, d, f, e.

here see that three tentacles lie in the intervals between every two ten-
tacles of the first circle, one belonging to the second cycle and two being
new ; but these are arranged in such a way that the middle one of the
three everywhere belongs to the cycle of the youngest generation. This
one now increases greatly in size, and outstrij^s the individuals of the
former second cycle, which in this way lose their rank, and are classed
in the third cycle. In later stages, cycles which differ in size (six ten-
tacles of the first, six tentacles of the second, and twelve tentacles of the
third cycle) actually alternate regularly with one another in position. It
must be observed, however, that the present third cycle does not contain
uniform elements, but six tentacles of the youngest stage of development
and six which previously belonged to the second cycle. A rearrangement
therefore has taken place. In the same way the number of the tentacles
increases from twenty-four to forty-eight and to ninety-six by the appear-
K. H. E. H

98 EMBRYOLOGY

ance of new pairs of tentacles, half of the intervals being left empty.
Thus by rearrangement a fom-th cycle of twenty-four and a fifth one of
forty-eight tentacles are developed ; but these, like the third cycle before,
consist of elements of heterogeneous origin.

Ordinarily the attachment of the hitherto free-swimming ciliated larva
takes place in the stage in which the number of tentacles is increased
from twelve to twenty-four.

It is to be expected that in those forms which exhibit a special law of
septal growth the sequence of the appearance of the tentacles is corre-
spondingly modified. In a larva called Arachnactis by Sabs and A. Agassiz
(No. 72), conditions of organization are found which, as has recently been
shown by C. Vogt (No. 96), connect it with the Cerianthece.^ The develop-
ment of the tentacles also recalls the development of the Cerianthus larva,
made known by Haime. In Arachnactis the tentacles do not grow out
in cycles between those already present, but there is a dorsal budding
zone (as in the case of the septa ; comp. p. 95), where the youngest
tentacles are formed in pairs. The tentacles of the inner circle also are
formed in the same manner. It follows from this that the tentacles of
the ventral side must be the largest and oldest. The unpaired, perpetu-
ally dwarfed tentacle of the directive chamber, which is found between
the longest paired tentacles, foi-ms an exception.

The development of tlie calcareous skeleton of the Madre-
poraria has been studied by Lacaze-Duthiers (No. 88) and
V. Koch (Nos. 83 and 85) in Astroides calycularis. It takes
place at the stage in which the first twelve tentacles of the
larva have been developed, and in which attachment usually
occurs.

The calcareous skeleton is formed as a secretion on the
outer side of the ectoderm of the body-wall (Fig. 45). At
first a delicate circular basal plate arises as a secretion from
the ectodermal cells of the pedal disc. This basal plate, by
means of which the larva attaches itself to some suitable

^ [In regard to the development of Arachnactis, the adult form of which
has been found by Hertwio and Boveki, consult the recent statements of
E. van Beneden (No. II., Appendix to Literature on Anthozoa) and
BovEHi (No. III., Appendix to lAteratnre).

A long time ago a very remarkable Actinia larva was described by
Semi'EU, and recently by E. van Beneden more in detail. This larva is
characterized by the presence of a highly iridescent ciliate ridge running
lengthwise of the body. Van Beneden is inclined to refer it to the group
of the Zoantheie. Comp. Semper, "Ueber einige tropische Larvenfor-
men," Zcitschr. wis-i. Zool., Bd. xvii., 18()7, and Van Beneden (No. I.,
Appendix to Lilcralure on Anthozoa). — H.j

CNIDARIA 99

support, consists of roundish crystalline bodies, which sub-
sequently fuse with one another. The earliest fundaments
of the calcareous septa [sclerosepta] soon make their appear-
ance. It was shown by Milne-Edwards et Haime, and
afterwards by Lacaze-Ddthiers, that the calcareous septa
correspond in position each to a gastral pouch, and therefore
that they occur between every two mesenterial septa.. The
earliest fundaments of the twelve primary sclerosepta are
called radial ridges (Sternleisten), and at first are V" or
Y-shaped (Fig. 46). The fundament of the theca (Matier-
blatt) arises by the peripheral ends of the radial ridges soon
becoming fused with one another. All of these are struc-
tures which are secreted by the ectoderm of the pedal disc,

Fig. 45. — Development of the calcareous skeleton of Ast)oides ealycalaris (afler
V. Koch), diagrammatic. The section is made perpendicular to the pedal dit-c
in the direction of a secant. At the bottom the fundament of the basal plate; to
the left the epitheca ; to the right two radial ridges [sclerosepta] growing upwards
from below, alternating with two mesenterial septa [sarcosepta].

and naturally the more these skeletal parts rise upwards the
more the ectodermal layer of the pedal disc must undergo a
kind of invagination. It follows from this that in later stages
also those parts of the skeleton which apparently lie inside
the body of the polyp are covered by an epithelial lamella
belonging to the ectoderm of the pedal disc (calycohlast laijer,
V. HbiiDBR). But the lateral walls of the body in its lower
portions also deposit externally a calcareous layer, which
constitutes the fundament of the so-called epitheca (Fig. 45).
The so-called columella is formed by the fusion of the radial
ridges [sclerosepta] with one another at their inner, central
ends. Six of the twelve radial ridges soon become more
prominent, so that there is established an arrangement in
two cycles. Subsequently other cycles make their appeai--

100

EMBRYOLOGY

ance by the interpolation of new small septa in regular order

between the existing' ones.

Non-sexual reproduction in the form of fission and huddlng

is found widely distributed in the Zoantharia ; by this means

extensive colonies are developed
in the skeleton-forming Corals
(Sclerodermata), whereas in
the group of non-skeletal Ac-
tiniaria (Malacodermata) the
individuals produced by fission
or budding usually separate
entirely, so that, with few ex-
ceptions (Zoanthea?), the forms
in this case remain solitary.

Fig. 46.— Basal plate of a larva of
Afiroidex cfitycularis, soon after at-
tachment, with twelve radial riflges
(^after Lacaze-Ddtbiebs, from BAt,-
fouk's Comparative Emhvijology).

Budding in the Actiniaria has been
observed more rarely — Epiactis
(Verhill, ?), Gonactinia (Blochjiann
UNI) Hilger), Zoanthus.. More fre-
quently reproduction takes place by
fission. This may divide the parent
animal into two nearly equal parts: either as Jongitudinnl Jission, which
begins at the oral disc and progresses toward the base, or takes the oppo-
site direction, or as transverse division, a kind of reproduction which has
been described in detail for Zonactinia prolifera by M. Sars and by

Fig. 47.— Two stages of transverse fission of Gonactinia prvUfera, Sabs (after
Blochmann vhd Hilgbk).

Bloch.mann UNI) Hilger (No. 74), and which in its outcome presents strik-
ing resemblances to the divisions in Flabellum and Fungia described by
Semper, and to the process of strobilization in the Scyphozoa. In Go-
nactinia it is always young animals that undergo transverse division.

CNIDARIA

101

Somewhat below the middle of the parent animal is formed a circle of
bud-like projections, out of which is developed the circle of tentacles of
the lower individual. While the upper part is being constricted off, the
oral disc and the oesophagus of the lower off-
spring of the division are developed. Finally,
the upper jiart detaches itself. It aj^pears that
both parts have the power to divide again.

Another remarkable, more widely distributed
kind of division, which had already been ob-
served by DicQUEiiAEE and by Dal yell (No. 4),
has recently been studied in detail by A. Andres
(No. 73), and has been called luceraiion (Fig.
48). This consists in the abstriction of frag-
ments of a basal expansion. At the margin of
the base of an Actinian a small part is character-
ized by the opacity of its entoderm and by its
firm adherence to the support, the latter being
caused by a secretion of the ectoderm. By the
contraction of the parent animal, the modified
marginal part is torn away from it. This can
now be metamorphosed either directly into a
small Actinian, or after further separation into
smaller fragments.

Both kinds of non-sexual i-eproduction, fission
and budding, are widely distributed among the
Corallia. They here lead to the formation ot
extensive stocks of various shapes. In many
cases (Oculinacea and Astrteacea) in which it
was formerly believed that lateral budding
occurred, Studer (Nos. 94 and 95) was able to
show, upon closer investigation, that there exists
a rei^roduction by fission, one of the resultants
of division coming with further growth to
occupy a position on the lateral wall of the
other part. A similar kind of reproduction has
been observed among the Fungiaceaj in Her-
petolitha limax.

Genuine basal budding is found, for example,
in Turbinaria, where the base of the colony
exists as a common plate of ccenenchyma, at
the margin of which new individuals bud ; like-
wise in Galaxea.

The form of longitudinal fission occurring in the Corallia, which
usually begins with a constriction of the oral disc, may remain more or
less incomplete, so that the individuals remain united with one another
in series. This arrangement can be recognized even in the skeleton, since

Fig. 48.— Reproduction
in Aiptasia lacerata by
means of abstriction of
a basal part (after A.
AifDRBsJ. .4 to C, advanc-
ing abstriction ; D, E,
metamorphosis of the
fragment into a small
Actinian.

102 EMBRYOLOGY

a whole series of individuals remains enclosed by a common theca,
whereas the septa are placed peipendicular to the direction of the tortuous
valleys extending between the theca? (Meandrina).

In the stone corals also, budding and fission may lead to the formation
of individuals which separate from the parent and live independently. In
Blastotrochus there are lateral buds that separate, whereas in Flabellum
a kind of transverse division occurs. The young stages of the Fungidiu
form small coral stocks from which the solitary forms, which become
sexually mature, are abstricted by transverse division. Since one and
the same branch may undergo this process of transverse division several
times, the resemblance to the strobilization of the Scyphozoa is \evy
striking. Here also there is a true alternation of generations (Semper,
No. 93).

III. SCYPHOMEDUS/E.

Of the forms belonging here the Lucernandce and Charyb-
deidce are contrasted Avith the Discophora proper. While the
embryology of the latter has been repeatedly investigated,
we have as yet only a fragmentary knowledge of the two
groups first named.

Lucernaridse. — For. and Kokotneff have given accounts of the larvte
of the Lucernarians. The development from the egg has been more
thoroughly investigated by Kowalevsky (No. 108), whose results have
recently been contu-med by II. S. Bergh (No. 101). After the egg and
sperm have been discharged into the water fertilization takes place, at
the completion of which the egg retracts somewhat from the vitelline
membrane. Two polar globules are foi-med, and then the first cleavage
furrow arises. By means of total and equal cleavage a multicellular stage
is formed, which presents no cleavage cavity. The pointed ends of the pris-
matic cells meet at the centre. An accumulation of entoderm cells now
takes place inside this so-called morula ; this is accomplished by a contri-
bution of elements from a definite region of the egg, so that the production
of the entoderm here seems to approach the type of polar ingression.
KovvAbEvsKY believes that it is chiefly a transverse division of the pris-
matic cells in this region that leads to the contribution of entodermal
elements ; however, simple ingression is not wholly excluded. The
bilaminar stage resulting from this is at first completely spherical (Fig.
41J A), but soon elongates in the direction of the future chief axis (Fig.
4'J Jl). The entoderm cells meantime become vacuolated, and arrange
themselves more and more in a single row, so that there results from this
a rod-like planula, which, like that mentioned for iFiginopsis (p. 57),
resembles a detached hydroid tentacle (Fig. 4'J C). This planula of tiie
Lucernaridie is not ciliated, but creeps slowly about with worm-like
movements. The first nettling cells are developed a its i^osterior end.
Preparatory to assuming the polypoid form, it eventually attaches itself

CNIDARIA

103

by means of its anterior end. The further development could not be
followed. E. S. Bergh, however, mentions a young stage in which the
tentacles were not yet united into groups, but were distributed along the
margin of the bell, while the arms were not yet developed. Eight ten-
tacles lying in definite radii could be recognized as fundaments of
marginal i^apillae.

Charybdeidae. — W. Haacke (No. 106) has given a description of some
young forms of the Australian Charybdaea Eastonii, which already con-
siderably resembled the adult animal. These accounts are thus far the
only ones on the embryology of this genus. As contrasted with the
cubical form of the adult animal, the young Acalephs showed an
apjjroach to a i^yramidal shape, and the apex of the umbrella was more
strongly arched than in the adult. The youngest stage that was observed
exhibited a canal somewhat excentrically situated, and extending from

KiG. 49.— Three stages in the develop-
ment of Lucernaria (after R. S. Bebgh).

Fig. 50. — Non-sexual reproduction
of the Scyphistoma (after M. Sa.es)
— A, by the formation of stolons; B,
by lateral budding.

the central stomach to the dome of the umbrella, where it ended blindly.
Haacke regards it as the remains of a communication with a Scyphis-
toma nurse, and therefore maintains the probability of an alternation of
generations in the Charybdeidae.

From the egg of most Discophora (Discomedusde) a fixed
polypoid creature is first developed, which is attached by
one pole, and has the mouth at the opposite end, at some
distance from which a circle of tentacles is developed (Fig.
51, 3, 4,). The Lucernaridee are essentially a more highly
developed form of these scyphopoli/ps, which become sexually
mature. In all other Scyphomedusas the polypoid form

10-i

EMBRYOLOGY

(ScypMstoma) appears to lack the power of generating sexual
products, exhibiting only non-sexual reproduction, which
occurs in two modifications : (1) as budding (lateral budding
and formation of root-runners or stolons) (Fig. 50), by means
of which a scyphopolyp is always produced again — this
either separates from the parent and attaches itself independ-
ently, or may remain united with the parent, thus tem-
porarily producing small colonies (scyphopolyp stocks) — (2)

""^P^ '

7 8 9 12

Fig. 51.— Cycle of development of .4i(i-elirt auriia (from Katsckkk's Lehrhuch).
1, planula ; 2, attached larva; 3, j'oung Scyphistoma with four tentacular buds; 4,
Scyphi-toma with stdlonic growth ; 5, beginning of the strobilization, indicated by
a circular furrow; 6, 8, 9, 10, various strobila; polydisra^; 7, Scyphistoma from
above ; 11, Ephyra from the side ; 12, Ephyra from below.

as strobilization, in reality a transverse division with subse-
quent regeneration. By means of transverse constrictions
the scyphopolyp (Fig. 51, e) separates into superposed dis-
coid parts (strohila stage, Fig. 51, 5 — 10), each one of which,
by the production of marginal lobes and corresponding
internal metamorphoses, is changed into a young medusa,
which at first shows the characiteristic form of the E})hyra
stage (Fig. 51, n, 12), and is not converted into the permanent

CNIDARIA 105

form of tlie sexually mature medusa until after a metamor-
phosis, which in most cases is rather complicated.

In most of the Discophora hitherto studied development
takes place in the form of an alternation of generations
already described. This is wanting in the Lucernaridse
only, they representing a sexually mature scyphopolyp
stage, from the eggs of which individuals of the same form
arise. On the other hand, among the free-swiming acraspe-
dote medusae cases (Pelagia) of direct development are
known, in which a larva developed from the egg of the
medusa changes directly into the Ephyra stage. This is
looked upon as a case of coenogenetically abbreviated develop-
ment, since the formation of a non-sexually reproducing
nurse-form (Scyphistoma) is suppressed.

Development of the Scyphistoma. — The develop-
ment of Aurelia (A. aurita and A. flavidula) is that of which
we have the most complete knowledge ; it has been made
known through numerous investigations — those of M. Saks
(No. 112), V. SiEBOLD (No. 114), L. Agassiz (No. 2), Glaus
(Nos. 102 and 103), Haeckel (No. 107), and Goette (No.
105). In the following we adhere chiefly to the description
of Goette, by whose inve.stigations a number of new points
of view have been gained.

The eggs of Aurelia aurita pass from the ovary into the
gastral cavity of the parent, and from there through the
mouth into the folds of the oral arms, where, enveloped by a
slimy secretion fx-om the entoderm, they undergo embryonic
development as far as the stage of the swarming planula.
They are enveloped by a delicate vitelline membrane, which
is lost in the later stages of cleavage.

By total and equal cleavage (Glaus) the egg divides into
a number of equal-sized blastomeres, which arrange them-
selves in a single layer about a comparatively small cleavage
cavity (coeloblastula). While, according to Glaus (in har-
mony with the statements of Kowalevsky), the gastrula-stage
is reached by means of a process of invagination,^ in which

^ [The observations of Claus have been fully corroborated by the
recent investigations of Fbank Smith (No. VII., Appendix to Literature on
Scyphomeduste) on Aurelia jiaiidala. In this species the entoderm is pro-

106

KMRRYOLOGY

the lumen of the arehenteron can be recognized only as a
linear fissure in the pluf^-like ingrowth, another method of
formation of the lower germ-layer, that may be called polar
ing-ression, has been maintained by Goette. According to
GoETTE, the cells of the blastula have not the same form in
the entire circumference, but are somewhat shorter and
broader in one hemisphere. From this region there is a
migration of individual cells into the blastoccele, until finally
this cavity is completely filled with a solid cell-mass
(entoderm). The arehenteron arises in this in the form of a
fissure, which soon breaks through to the exterior at the
region from which the immigration of entoderm cells took
place, thereby forming the primitive mouth (prostoma).

Even during this process the embryo, originally spherical,
elongates, so that the longitudinal axis passes through the
primitive mouth and the apical pole lying opposite to it.
But the primitive mouth very soon closes completely. At
the same time the larva becomes narrowed at this end, so
that it is pyriforni. The swarming out of the ciliated
embryo (planula. Fig. 52 A) now takes place ; the broader
apical pole is directed forwards in swimming, whereas the
narrower pole, at which the closure of the primitive mouth
took place, comes to lie behind. Nettling capsules very soon
make their appearance on the swarming larva ; these arise
in great numbers at the posterior pole, whereas they are
almost wanting at the anterior end.

Even during the swarming stage a shallow depression is
developed at the anterior (apical) pole of the larva, and at
this point the epithelium acquires a glandular nature. The
larva now attaches itself by the apical pole to some support,

duced by the formation of a distinct invagination gastrula. A migration
of cells into the blastoccele, as described by Goette, was also occasionally
observed in A. Havidula. However, these cells appear to disintegrate
without taking any part in the formation of the entoderm.

On the other hand, the entoderm is formed in Cyanea arctica, according
to McMuiiKicH (Appendix to Litciature on Scyphomedusie, No. V., p. 814,
and No. VI., p. 'JO), by an inward migration of certain cells of the blasto-
sphere, and in Cyanea capillata, according to Hamann (No. IV., Appendix
to Literature), by the ingrowth from one pole of the embryo of a solid rod,
which subsequently becomes hollowed out to form the gastral cavity. — H.J

CNIDARIA 107

and thus the former anterior end becomes the foot of the
scyphopoljp ; this soon contracts a little, whereas the
posterior end widens, so that in this way the body acquires
the goblet shape characteristic of polyps (B'ig. 52 B).
During the attachment a cement, which soon hardens into a
plate with upturned margins (Figs. 54 and 55 k), is secreted
from the foot. The secretion of a raesogloea begins early
between the two layers of the larva (Fig. 52 B, g).

The next change is the formation of the permanent month,
which arises by a process of invagination. The ectodermal
layer of the prostomal pole invaginates into a gradually
deepening ectodermal pocket (Fig. 52 B, s), at the bottom of
which a perforation, leading into the gastral cavity, soon
arises. In this way an oesophagus, lined with ectoderm, is
produced (Fig. 52 C) ; the outer opening is known as the
mouth, the inner, communicating with the gastral cavity, as
the inner opening of the oesophagus (Fig. 52 G, sp). By means
of this pi'ocess of invagination the entodermal sac becomes
crowded downwards, but not throughout its entire extent.
Since the larva is compressed laterally, two glove-like ento-
dermal processes, corresponding in position to the longer of
the secondary axes, are preserved. These project upward,
and are the first two gastral pouches (Fig. 52 G m and D vi).
Very soon, however, in a plane at right angles to this, a second
pair of gastral pouches grows upward as diverticulse of the
central stomach (Fig. 52 E), so that now the radiate type
with four rays is reached. We now have an oesophagus
invaginated from the ectoderm, in the circumference of
which, at the four radii, lie gastral pouches in connection
with the gastral cavity.^ At these places, where two neigh-
bouring gastral pouches come in contact, a partition, or

1 [Our knowledge of the first processes of development m the Scyphis-
toma stage has been materially increased by recent investigations, which
have advanced information in several directions. Nevertheless it is not
possible as yet to pronounce final decision concerning these develop-
mental processes. The observations of Goette have been only partially
confirmed by Glaus (Nos. I. and II., Appendix to Literature on Scypho-
medusffi). As the result of his most recent observations, Glaus (No. II.)
denies totally the presence of an ectodermal pharynx. Of special import-
ance are the statements of Glaus (No. II.) concerning the formation of

108

EMBRYOLOGY

septum (Fig. 52 E, st), is produced by their contiguous lateral
walls. These four septa He in the interradii, whereas the four

Fig. 52.— Diagrammatic sections through various successive stages of ^invlia
(after Goette). .4, planula; ec, ectoderm ; en, entoderm ; B, attached larva with
forming ossopliageal invagination, .s ; g, mesogloea ; C, completed rupture of the
(esophageal invagination ; sji, inner opening of the oesophagus ; m, gastral pouches ;
D, transverse section through the stage represented in C at the level of the
oesophagus; E, transverse section through an older stage at the level of the
oesophagus ; F, transverse section through the same stage at a piirt nearer to the
stalk; s, oesophagus ; st, septa; t, tseniote.

primary yastral pouches lie in the four chief radii (perradii).

the proboscis in the developing Ephyra of the strobila stage, a point which
had not hitherto received any careful attention.

Eecent observations by Goette (No. III.) on Cotylorhiza tuberculata
and Pehujid noctiliico have led to the astonishing result, that of the four
primary gastral pouches, — although the first pair is of entodernial origin,
being produced from diverticula of the archenteron, — the second pair is
ectodermal in origin, since it arises by evagination from the ectodermal
pharynx.— H.] Consult also Ida H. H\i>e, No. VII. i^.— Tkanslatohs.

CNIDARIA

109

The structures occurring- between these [eight] radii are
designated as adradial.

The lower free margins of the four septa soon become
continuous with the wall of the central stomach in the form
of four longitudinal folds, which ultimately extend through
the entire length of the scyphopolyp, even into the foot.
These folds are known as the longitudinal folds, or tceniolce
(Fig, 52 F, t), and the sinuses of the central stomach limited
by them as gastral furroivs.

In the further metamorphosis of the larva the form changes,
approaching more and more the shape of a goblet (Fig. 53).

Fig. 53. — Diagrammatic lontritudinal seetion through a Scyphistoma (based on
Goktte). ^, perradial longitudinal section ; B, inlerradial longitudinal section;
pb, proboscis; f, tentacle ; tr, septal funnel ; m, gastral pouches; g, mesogloea; s,
septum. The entoderm is represented as a dark layer.

The lower narrow portion is called the stalh or peduncle
(Fig. 55 st), the prolongation of the central stomach extend-
ing into it the peduncular canal. The u^jper part of the
body becomes flattened, and thus forms the oral disc or peri-
stome, in the middle of which rises the cone-shaped j)?'o&oscis
(Fig. 53 ph) with its central four-sided mouth-opening.
The four corners of the mouth are placed perradially (Figs.
54 0 and 55).

The first four tentacles now arise over the four gastral
pouches, and, in keeping with the successive appearance of
the pouches, those over the first pair of pouches arise first,
and then, those over the second pair. A cylindrical ento-

110 EMBRrOLOGY

dermal cord grows from the apex of each gastral pouch
diagonally upwards and outwards, pushing before it the
ectoderm of the outer margin of the peristome. The ento-
derm cells in the tentacular buds soon arrange themselves
in a single row (Fig. 53 t).

Other important fundaments of organs are represented by
the septal funnels which are now established. Four funnel-
like invaginations arise from the ectoderm of the peristome in
the interradii ; these sink into the septa, and extend down-
wards as solid cords of cells, which are continued along the
tteniolge and even beyond these into the stalk (Fig. 53 B ;
Fig. 54 A and C, tr). In this solid portion, the cells appear
to be fused with one another, and on their surface longi-
tudinal muscle fibrillse ai'e differentiated, so that the four
[septal] longitudinal muscles extending in the taeniolee arise
in this way (Fig. 54 A, B, sm).

The young Scyphistoma thus produced is characterized
therefore as a goblet-shaped polyp, with four longitudinal
folds (tseniolas) of the entodermal sac extending upwards
as four septa, which are stretched between the body-wall
and the invaginated ectodermal oesophagus. The stomach is
accoi-dingly divided into a central cavity and four gastral
pouches (periphei'al intestine [Kranzdarml), which lie be-
tween the septa and are directly continuous with the gas-
tral furrows. Four perradial tentacles are attached to the
marerin of the peristome, while four interradial septal fun-
nels extend fi'om the peristome into the septa and taeniolae
(Fig. 53).

The metamorphosis into older Scyphistomoe (Figs. 54 and
55) takes place by an increase in the number of the ten-
tacles and other changes, which efface more and more the
original characters, and lead by a gradual transition to the
structural plan of the Kphyra.

The budding of the tentacles iiresents many irregularities. Heretofore
it has been believed that normally after the formation of the first four
tentacles, radial in iDOsition, the development of four interradial ones (i.e.,
placed over the septa) took place, and then, after all these eight tentacles
had reached the same length, ensued the development of eight others,
lying between them (therefore adradial), and so on. According to Goetxe,

CNIDARIA

111

however, the four tentacles that immediately succeed the four j^rimary
ones are not placed over the septa, but bud out from the corners of the
gastral pouches of the second pair, which lie next to the septa, and only
gradually move into positions over the septa. In this way, their axial,
entodermal cords acquire connections with the gastral pouches of the
second pair. Since the gastral pouches of the first pair soon follow with
the formation of four new tentacles, the equivalence of the first four

Fig. 54. — Diagrammatic representation of the structure of an older Scyphistoma
(based upon Goette, from Hatschkk's Lehrbuch). A, longitudinal section : at
the left, perradial; at the right, interradial ; AB, chief axis; o, mouth ; s, inner
opening of the oesophagus ; gt, gastral pouches ; gr, gastral furrow ; so, septal
ostium; tr, septal funnel; sm, septal muscle (the dotted line does not quite reach
to it) ; B, transverse section through the lower part of the body ; gr, gastral fur-
row; s, septum ; sm, septal muscle; C, view of the oral side (references as in A).

primary gastral pouches is established for the first time in the stage with
twelve arms. Goette, therefore, maintains that the numerical series, 4,
12, 20, 28, etc., is the primitive one for the budding of the tentacles,
whereas the actually observed series, 4, 8, 16, 24, 32, etc., corresponds to
a coenogenetically modified condition. It should be mentioned that the
formation of each new tentacle takes place by an outfolding of the cor-
responding part of the gastral pouch, so that in reality a small secondary
gastral pouch is produced with every tentacle.

112

EMBRYOLOGY

The farther metamorphosis of the developing^ Scyphistoma
consists in a widening of the central stomach, whereby the
oesophagus gradually moves into the proboscis (Fig. 54 A),
and the gastral pouches tend to become obliterated. At the
same time the entrances to the four funnels, which are widely
open toward the peristome, produce a circular, groove-like
depression, involving the entire circumference of the origin-
ally flat peristome, which thereby approaches the bell shape
of the sub-umbrella of the medusa ; the proboscis, which has
become more elevated, corresponds to the oral tube [manu-
brium], while the gastral pouches, separated by the septa,

represent the peripheral in-
testine {Kranzdarm) of the
medusa. The Scyphistoma,
by gradual metaviorphoses, has
approached in the most essential
featicres the structure of the
medusa (comp. Figs. 54 A and
57).

Ill most of the other Discophora,
the development of the Scyphistoma
seems to take place in quite the
same way, especially in Cotiilnrldza
borbonira (Kowalevsky, Goette)
and Cyanea cajMata (Sars, Van
Beneden, Agassiz), where the eggs
likewise undergo the first stages of
development attached to the oral arms, and enveloped in a slimy jelly.
On the other hand, the early develoimient in Chryxaora, a form which
is also striking on account of its hermaphroditism, presents notable de-
viations (Glaus, No. 102 and No. 3). Here fertilization and the entire
embryonic development take place within the ovary, so that the larvaj
are not born until they reach the planula stage. The very small mem-
braneless eggs are surrounded in the ovary by a pedunculated follicle,
which owes its origin to the cells of the germinal epithelium. Fer-
tilization and cleavage are transferred to an early stage in the de-
velopment, so that at the same time with the embryonic development
there is a considerable growtli of the embryo as the result of a con-
tinual supply of food material on the part of the parent. This food
supply is provided by the follicular cells. In these particulars the de-
velopment of the egg and embryo of Chrysaora recalls that of the vivi-
parous Aphidic and the I'olyphemidffi among the Cladocera. In other

Fig. 55. — Scyphistoma of Aurelia
aurlta. ph, proboscis ; tv, entrance into
the septal infumlibuliim ; t, tseniola;;
st, stalk; k, adhesive mass.

CNIDARIA 113

respects the ijhenomena of the embryonic development are essentially
the same as those we have described for Aurelia. By means of total and
equal cleavage, a cceloblastula arises, out of which by infolding an
invaginate gastrula develops, whose prostoma remains open for a con-
siderable time, but finally closes completely. From observations by
BuscH, it appears as if reproduction of the embryo, by means of longi-
tudinal division, frequently took place at this stage. This recalls the
occurrence of fission in the blastula of Oceania armata, according to
Metschnikoff (p. 49). In the stage of the ciliated i^lanula the larva?
of Chrysaora pass from the ovary into the gastral cavity of the joarent,
and thence to the outside world through its mouth. A glandular modifi-
cation of the ectoderm of the anterior pole of the larva, by means of which
the attachment subsequently takes place, can be recognized, whereas the
posterior (oral) pole is characterized by the aj^pearance of nettle capsules
(Glaus).

The opaque whitish or yellowish eggs of Xauaitlioe are laid singly, and
are characterized by a gelatinous envelope, provided with nettle capsules
(0. Hertwig). Cleavage is here total, and in the first stages unequal,
though finally a cceloblastula with walls of nearly uniform conditioii is
produced by the gradual obliteration of the great differences in size be-
tween the blastomeres. The blastula changes into an oval, ciliated swarm-
ing larva, the cells of which are thickened at the posterior jjole, where
the gastrula invagination takes place. After invagination the blastopore
becomes completely closed. Metschnikoff (No. 12), to whom we owe
the knowledge of these processes, was able to observe the attachment of
the planula, which is accompanied by the development of a discoid
basal expansion, and its metamorphosis into a small scyphoiDolyp pro-
vided with four tentacles and covered with a thin layer of periderm, so
that metagenesis has been proved for this form also. Metschnikoff
believed that he was justified in assuming that the Spongicola fistularis
of F. E. ScHOLZE (Stephanoscyphus mirabilis, Allman), which is para-
sitic in sponges, and in which Kowalevsky seems to have observed a kind
of strobilization, is the Scyphistoma form of Nausithoe.

Strobilization. — The simplest form of reproduction of
young meclusfe is represented by the nionodisc strobila (Fig.
59 A), occasionally observed even in Aurelia. In this case
only one young medusa {Ephyra) separates from the Scyphis-
toma. While the adoral tentacle-bearing portion of the scy pho-
polyp is by gradual changes converted into the form of the
Ephyra, it becomes separated by means of a circulai-, trans-
verse furrow from the basal portion of the body, and finally
detaches itself completely. The basal remnant can, by re-
generation of the oral portion, grow^ again into a complete

K. H. E. I

114 EMBRYOLOGY

scjphopoljp, and subsequently go througli the process of
strobilization again, and so on.

In most cases, however, new transverse furrows make
their appearance on the basal part before the detachment of
the first Ephjra, so that on the elongated cup of the scypho-
polyp a whole set of EphyriB (ten to thirty) are developed at
approximately the same time ; but of these any one that is
nearer to the base of the polyp is younger than those distal
to it (polydisc strobiln) (Figs, 56 and 51 c — lo). In this case
also the basal portion finally reproduces both a circle of
tentacles and the oral part of a scyphopolyp, and is thus
enabled to continue its existence as a scyphopolyp when
the production of Ephyr^ ceases. A polydisc strobila can
be derived from a monodisc. In the former new transverse
divisions follow one another so rapidly that a large number
of Ephyrge are in process of development at the same time.

The oral portion of a scyphopolyp, in metamorphosing
into an Ephyra, must undergo certain changes, part of
which make their appearance before the first indication of an
abstriction is produced by the circular furrow. The most
important internal change is introduced by the disappearance
of the septa and the peripheral communication between the
four gastral pouches which is thus brought about. Since
the entodermal columns of the four septal tentacles are con-
tinuous with the walls of both the gastral pouches adjoining
the septum (p. Ill), there is produced at this place a con-
nection between the neighbouring gasti-al pouches. At this
point a small perforation now arises in tiie septum (Fig. 54
so), but this very soon widens to such an extent that only
the thickened inner margin of the septum, which is tra-
versed by the septal infundibulum, is preserved (Fig. b7
so). By the formation of these septal ostia, the four gastral
pouches coalesce into a common peripheral gastrjil chamber
{feriplieral intestine) [^Ivranzdarvi]. The four septal infundi-
bula, clothed by an entodermal covering (remains of the
septa), now traverse the gastral space in the form of four
columns {columella'), which are not attached to the wall of
the central stomach except at its bottom.

A further change is brought about by the disappearance

CNIDARIA

115

of the Scyphistoraa tentacles and the growing out of the
margin of the peristome into a lobed crown consisting of
eight (four perradial and four interradial) marginal lobes
(Fig. 56). Since the marginal lobes are not formed by the
outer body- wall alone, but contain a corresponding diverti-
culum of the entodermal sac, the peripheral gasti^al chamber
in this way acquires eight blind sacs : the lobe-poncJies (Fig.
58 Z). The marginal or primary lobes [^Siammlappenl soon
develop three processes at their ends, of which the middle
one buds forth from the sub-umbrellar part of the
lobe at some distance from the margin,
and becomes the sensory body [_Sinneskolbe^
(sk), while the two lateral processes bud
forth from the margin and become the alar
lobes. Inside of these are found the alar
pouches (Fig. 58 /) as prolongations of the
lobe-pouches ; in the sensory body also
there is found a prolongation of the gastral
entodermal layer, which is destined to pro-
duce the otolith crystals.

The peripheral gastral chamber up to
this time was simple and undivided, and at
its periphery ran out into the eight lobe-
pouches. Since the disc of the developing
Ephyra is always flat, the upper and lower
(ex-umbrellar and sub-umbrellar) walls of
the peripheral gastral chamber are very
close to each other, and these two walls
nowgrowtogether from the margin inwards at sixteen regions
of the circumference [the shaded areas in Fig. 58], and thus
form sixteen radial fusions or concrescence-bands, which aie
situated sub-radially (i.e. between the per-, inter-, and adradii).
In this way the marginal part of the peripheral intestine is
separated into sixteen marginal pouches (radial peripheral
pouches. Fig. 58 m), which are separated from one another
by the concrescence-bands (cathammata). Eight of these
marginal pouches are situated in the per- and interradii, and
are continuous with the lobe-pouches, while eight others
are adradial and interpolated between the former. In later

Fig. 56. — fcjtrobila
polydiscaof AureUa
aurita. On the up-
permost Ephyra the
Scyphistoma ten.
tacles in process of
degenerating.

116

EMBRYOLOGY

stages the marginal pouches become narrower and raoA-e
ajDart ; the sixteen regions of fusion [cathammata] thus
spread out into a bilaminar plate, which connects all of the
marginal pouches with one another : the tnedusoid, vascular,
or cathammal plate.

The detachment, of the Ephyra now takes place, and from
this time on it moves about freely by rhythmical contractions
of its discoid body, the former point of attachment being
directed upwards, and the manubrium downwards (Fig. 51,
ii). The columellie, which are frequently the means of the
final connection with the nurse form, now degenerate. It

Fig. 57. — Tutenadial longituditia section through an Ephyra monodisca, vrith
the Scyphistoma tentacle.s still retained (dingram moditied from Goette). ph,
proboscis; f)-, septal funnel ; g/, gastral filament ; so, septal ostium ; i/, constrict-
ing annular groove.

is probable that the last metamorphosed remnant of the
septal infundibula can be recognized in the fonr sub-genital
cavities (p. 122) lying on the sub-umbi-ellar side beloAV the
gonads of the medusa. With the degeneration of the
columella' the boundary between the central stomach and the
perij)liural intestine entirely disappears, and the boundary
at which the ectoderm of the oesophagus is continuous with
the entodermal lining of the peripheral intestine is indicated
only by means of f(nir tentaculoid gastral filaments (Figs.
57, 58 r//), which have budded forth at the bases [oral ends]
of the columellae.

CNIDARIA

117

The Epliyra (Fig. 51, ii and 12, and Fig. 58), accordingly,
possesses a flat, discoid body, from the under-side of which
the manubrium hangs down. The margin is prolonged into
bifid marginal lobes, each, one of which bears a sensory body
between its alar lobes. Four of these are perradial, and
correspond to the radii of the oral cross, whereas the four
interradial ones fall in the radii of the gastral filaments. The

Fig. 58. — Diagrammatic figure of an embryo of an Ephyra. o, cruciform mouth-
opening ; gf, gastral filaments ; I, lobe-pouches ; /, al^r pouches ; c, cathammata,
or regions of fusion of the peripheral intestine ; sic, sensoiy bodies.

broad, flat gastral space is prolonged into sixteen peripheral
marginal pouches, which are connected by means of the
vascular plate. Of these pouches the eight perradial and
interradial ones are directly continuous with the lobe-ponches
and alar pouches. The ectoderm on the oral side of the disc
(sub-umbrella) forms a broad, band-like circular muscle,
while paired longitudinal muscle-bands stretch, along the
marginal lobes and into the alar lobes.

Hypogenelic Development of the Larvae of Pelagia.
— ScHNEiDEJi(]S'o. 113) and Haeckel (No. 107) have already

118

EMnRTOLOGT

observed that the scjphopolyps of Aurelia aurita, when
they are placed in unfavoui-able conditions (for example, in
aquaria), show little inclination to form poljdiscous strobila^,
but frequently develop only monodiscous strobilae (Fig. 59
A). In fact, Haeckel observed in certain cases that the
transverse division of the scyphopolyp metamorphosing

into an Ephyra is altogether sup-
pressed, so that the entire body of
the larva is converted into the adult
animal. This is Haeckel's so-called
Ephyra pednncnlata (Fig. 59 B), which
was observed in the attached as well
as in the free-swimming condition.
Here therefore the alternation of
generations is omitted, and a simple
metamorphosis (hypogenesis) has
taken its place.

The latter condition is the normal
and only one in Pelagia noctilnca, the
development of which has been made
known through Kkohx (No. 109),
KowALEVSKY, and Metschnikoff (No.
12). In this case there is first formed
a blastula which has a large cleavage
cavity, and soon becomes covered on
the sui'face Avith Uagella. At the
same time an invagination from the
posterior pole is formed, which leads
to the development of a gastral cavity
which does not by any means completely fill the space of the
primitive cleavage cavity (Fig. 60 A). The blastopore does
not close, but becomes the mouth of the larva. A shallow
depression is very soon noticeable at the posterior end of the
free-swimming larva, in the middle of which the part sur-
rounding the mouth projects in the form of a cone (Fig.
60 B). This projection becomes the oral cone of the Ephyra
(Fig. 60 C, m), and the circular depression surrounding it
the umbrellar cavity, while on the peripheral margin a divi-
sion into eight marginal lobes is soon noticeable, into which

Fig. 59. — A, Strobila mo-
noilisca of Cyamea capillata
(after P. J. van Bknbdbn) ;
f, lobes of the Ephyra; t,
newly formed circle of
Scypliistoma tentacles on
the basal portion.

B, Ephyra pedunculata of
Aurelia aurita (after IIaec-
kkl).

CNIDARIA

119

the gastral cavity is continued in the form of lobe-pouches
(Fig. 60 (J). After the Ephyra shape has thus found ex-
pression in the region of the oral pole, the larva shortens
in the direction of the chief axis, and gradually assumes the
flat, discoid form of the Ephyra. Meantime the larva loses
the covering of flagella, and from now on moves like a medusa
by the regular contractions of the margin of the disc. In
Pelagia accordingly the larva coming from the eo;g passes
directly into the Ephyra, although Goette has pointed out
that, owing to its structure, we must regard the first stages
of this metamorphosis as free-swimming Scyphistoma stages.

Fig. 60. — Three stages of development of the free-swimming larva of Pelagia
noctiluca (after Keohn). r, marginal lobes; s, sensory bodies; m, mouth-
opening.

Metamorphosis of the Ephyra. — The metamorphosis
of the Ephyra is accompanied by a constant increase in the
size of the body. The sensory bodies of the Ephyra become
the eight marginal bodies [rhopalia] of the medusa. Since
the adjacent alar lobes, from which the ocellar lohes arise, do
not continue to grow with the same rapidity as the rest of
the body, new structures, corresponding in position to the
adradial regions, are developed in the mai'gin (adradial or
intermediate marginal lobes).

The simplest conditions directly referable to the Ephyra are found
in the Ephyropsidffi (Nausithoe), in which the sixteen alar lobes of the
Ephyra are retained comparatively well developed, while eight adradial
(intermediate) tentacles alternate with these. The pocket-like marginal
pouches separated by narrow concrescence-bands (Glaus) and the absence
of arm-like prolongations of the angles of the mouth are so many
characters derived directly from the Ephyra. In the families of the

120

EMBRYOLOGY

Pelagidffi and Cyanidse also the original character of the gastrovascular
system is jDreserved, the sixteen marginal (radial) pouches being retained
as broad spaces separated by only narrow concrescence-bands, and not
communicating by any circular sinus. More complicated conditions are
found in the Aureliidas, the metamorphosis of which froin the Ephyra
has been accurately described by Claus (No. 102 and jS'o. 3) for Aurelia
and Discomedusa (Umbrosa). In Aurelia the enlargement of the disc
is accompanied by the development of eight intermediate (adradial)
marginal lobes, on the ex-umbrellar surface of which numerous short
tentacles, arranged in a single series, are developed (Fig. Gl i). While

Fig. 51.— Development of the margin of the disc anl the canal system of
Aurelia ciurita (after Claus). A, quadrant of an Bphyra disc 3 mm. in breadth ;
B, quadrant of a young Aurelia with a disc 9 mm. in di.ameter; i, intermediate
(adradial) marginal lobes; o, ocellar lobes; s/c, sensory bodies; t, tentacles (some-
what removed to the ex-umbrellar side).

the disc thus gradually enlarges, the sixteen marginal pouches grow out
into elongated, narrow radial vessels, between which the concrescence-
bands extend as broad areas of the vascular plate. By the separation in
places of the two lamellae of this plate, secondary canals are developed,
by means of which there is formed first a zigzag, and subsequently a
peripheral circular communication between the different radial vessels
{rin;/ siniis\, besides numerous lateral branches of the radial vessels (Fig.
(51). The four oral arms, beset with papillre, arise as outgrowths from
the four angles of the mouth. That which especially interests us in the
metamori^hosis of the Ephyra of Rhizostoma, made known by Claus
(Nos. 3 and 103), is the metamorphosis of the oral stalk (manubrium).
The broadened ends of the four oral arms grow out into bifurcate lobes,
thus producing the fundaments of the eight oral arms, while by a
similar i)rocess of growth at the ends of these the fundaments of the

CI^IDARIA

121

dorsal tufts [Dorsalcrisj^en] of the lower part of the arm arise. The
fundaments of the shoulder tufts [Schitlterkraiiiieii] , or scapulettes, arise
in pairs as papillary elevations in the eight radii, and only subsequently
assume an adradial position (Glaus). The lateral margins of the arms,
which are bent under, now grow together, so that there arise from the
brachial furrows closed canals, which open to the outside world by
means of the so-called funnels, rhizostomes or oscula suctoria (originally
lateral folds of the margins of the arms). As the last trace of the
mouth, closed by concrescence, we find the central cruciform oral raphe.

Fig. 62. — Diagram of an interradial longitudinal section through a Scyphome-
dusa (based on a figure by Glaus), ij, radial vessel; Kfr, marginal bodies [rho-
palia]; ol, ocellar lobes; Gs, genital sinus; G, genital band; Gf, gastral fila-
ments; GiTi, gastro-genital membrane ; S, sub-genital sinus.

An account o.f the metamorphosis of one of the Versuridse (Stylorhiza
punctata) has been given by v. Lendenfeld (No. 110).

We have still to mention certain organs wliich are de-
veloped at the places originally marked by the four
degenerating columellfe, i.e. in the interradii. These are,
first of all, the gastral filaments and the genital hand. In the
youngest Ephyrse only one of the gastral filaments, which
originally budded forth as tentacle-like grovpths at the
base of the columellse (Fig. 57 gf), is found in each inter-
radius. However, their number is soon increased (Fig. 58
gf), and finally numerous filaments occur, usually arranged
in a curved line corresponding to the inner side of the

122 EMBRYOLOGY

genital hand (Fig. 62 G), which is now developing as a fokl
of the gastral wall. The sexual products arise from elements
of the wall of this fold, are ripened (Fig. 62) between its
two lamellge, and by the dehiscence of the wall pass into the
gastral cavity, whence they reach the outside world through
the mouth. The space undei'lying this fold and communi-
cating with the gasti'al cavity is called the genital sinus
(Fig. 62 Gs). The genital band, which is usually horse-
shoe-shaped, is often interrupted at the interradius, so that
we then find in the four interradii eight paired gonads,
which are often more or less adradially placed, a condition
which probably must be looked upon as the primitive one.
With the progressive increase in the thickness of the
raesoglcea, which grows, principally at the four corners of
the mouth, into massive pillars, there is in the interjmdial
region a more and more marked development of an invagina-
tion of the outer surface of the body, which is called the
suh-genital cavity (Fig. 62 S), and in its earliest beginnings
is perhaps to be referred to the cavity of the septal infundi-
bula. While, accordingly^ the body-wall of the medusa is
thickened by an increase of the mesoglcea all around this
place, it here remains as a very thin g astro-genital viemhrane
(Fig. 62 Gm), which in many forms (e.g. Pelagia) shows a
tendency to protrude outwards like a hernial sac, so that
in this way a genital sac {g astro- genital pouch) projecting into
the sub-genital sinus is developed.

While one might conclude from the structure of the adult genital band
that it was developed by a simple folding of the sub-umbrellar wall of
the stomach, the investigations of v. Lendenfeld and Hamann show
that the earliest fundament of the genital band is merely a thickening
of this wall, and that an elevation of this thickening in the form of a
fold does not take jjlace until later, when an invagination pushes forward
more and more from the distal side, thus producing the genital sinus.

General Considerations.— The fact that in the eggs
of most Scyphomedusoe a Scyphistoma stage is first de-
veloped, and that this stage is also indicated in the modified
development of Pelagia (GtOktti;), shows that we must
imagine the ancestral form of the Scyphozoa as an attached
Anthozoa-like polyp, which originally possessed, in addition

CNIDARIA 123

to the power of sexual reproduction, that of non-sexual
repi-oductiou bj budding and division. In reproduction by
means of transverse division, the basal peduncular entl of
the divided polyp must have reproduced a new oral part,
whereas the detached oral portion had to move away from
the place of its origin and seek a new place of attachment.
Before it could attach itself, however, it must by growing
have reproduced the apical part of the goblet- shaped body
and the stalk, so that in this way there arose two individuals
of the same form as the parent. In this migration of one of
the offspring of the division was furnished the motive for
its metamorphosis in the direction of an increased power of
locomotion, whereby a difference between the form of the
attached polyp and that of the free-swimming medusa was
initiated. Prom what has been said it is not to be wondered
at, that the two forms are connected by gradual transitions ;
nevertheless we shall have to adhere theoretically to the
differences of these two morphological conditions. The
medusa is therefore a morphological phase of the Scypho-
zoa which has proceeded from the scyphopolyp; but, owing
to the assumption of free locomotion, it is more highly deve-
loped, the presence of sensory bodies and marginal lobes
and the more highly organized musculatui-e of the sub-
umbrella, with concomitant increase of the elastic mesogla-a
of the umbrella, being characteristic of it.

In the Calycozoa the scyphopolyp reaches its highest
phase of development, whereas the Peromedusae are to be
looked upon as the most primitive medusa forms. The lat-
ter still reproduce the elongate bell-shaped form of tlie
umbrella, with its apical, stalk-like process, derived fr-omthe
attached polypoid ancestral forms, and in the possession of
large septal funnels resemble the scyphopolyps, whereas
the development of the margin of the umbrella marks them
as medusae. In contrast to them, the Ephyropsidge and the
corresponding larval Ephyra form appear as a further stage
in the developmental series, in which the apical, elongate
bell-shaped part of the umbrella and the peduncular rudi-
ment have been lost, and in which the septal funnels have
degenerated. We must explain the four interradial points

12-i EMBRYOLOGY

of adhesion (septal nodes, Haeckel), Avhich are present in
the Ephjropsida3 on the external side of the row of gastral
filaments, as the remains of the coluraellfe corresponding to
these funnels. The Semteostomfe and the Rhizostoma? are
derived by further metamorphosis from the Ephyra form.

If we imagine that in the above-supposed attached an-
cestral form a division of labour made its appearance of
such a natui'e that the power of non-sexual reproduction
was retained by the attached scyphopolyp form, while the
generation of the sexual products was confined to the free-
swimming (medusa) forms, resulting from the transverse
division, the origin of the kind of alternation of generations
characteristic of the Scypliomedusfe would in this way be
explained.

Whereas the tendency formerly was to unite the Hydrozoa
and iScyphomedusee into a common group, in moi-e recent
times our conception has led to a complete separation of
these two divisions. O. und R. Hertwig's (No. 9) doctrine
of the diphyletic origin of the medusa form and their dis-
tinguishing between Ectocarps and Entocarps first prepared
the way for this separation. Although various persons,
especially Claits (No. 102), had previously placed importance
upon the presence or absence of the t^nioloe, which are
also possessed by the polyps, as characteristic differences,
nevertheless the sharp separation between the scyphopolyps
and the hydropolyps was first established by Goette (No.
105). On the other hand, the observations of Goette,
especially the discovery of the ectodermal nature of the
oesophagus in the Scyphistoma?, have led to approximating
this group to the Anthozoa, so that recently various authors
(Lang, Hatschek), in accord with Goette, have united the
two groups as Scyphozoa. It must be mentioned, however,
that the scyphopolyps are separated from the Anthozoa by
the possession of septal infundibula, and by the ectodermal
origin of the longitudinal muscles, to which are to be
added as distinctive characters differences in the origin of
the first four gastral pouches and many differences in
general histological character — greater development of the
mesodermal tissue in the Anthozoa. Even though we

CNIDARIA 125

assume, then, that Scypliomedusae and Anthozoa are de-
scended from a common polypoid ancestral form which was
already characterized by the possession of an ectodermal
oesophagus, still the direct union of the two groups does not
seem to be as yet sufficiently established.

General Considerations on the Cnidaria. — The Cni-
daria constitute a very homogeneous, well-defined branch of
the animal kingdom. We assume that the fundamental and
ancestral form from which they are derived was a polyp
similar to Hydra, the chief axis of which was the same as
in the preceding free-swimming ancestral form. A free
oral pole and a pole of attachment can be distinguished.
The latter corresponds to the anterior pole of the free-
swimming ancestral form. The radial type in the structure
of the Cnidaria has arisen in connection with the attached
mode of life, whereas in many Cnidaria, as the result of
stock-formation, a bilaterally symmetrical type is second-
arily developed. It appears that even the ancestral form of
the Cnidaria had developed the quadriradial structure, so
that those forms in which, on account of the arrangement
of the tentacles, no definite secondary axes can be recognized,
like the Gorynidoe and Glavidce, would represent a secondary
modification. The growth of the Cnidaria frequently takes
place by the typical intercalation of new radii between those
already present (Hatschek).

The Hydrozoa are derived directly from this Hydra-like
ancestral form (Archihydra), whereas the common ancestral
form of the Anthozoa and Acraspeda is developed from it
by the formation of an ectodermal oesophagus and radial
septa. The presence of the longitudinal muscles in these
septa indicates that they were developed in connection with
the attached mode of life. In the ontogenetic series, it is
true, the septa often make their appearance before the
attachment and before the development of the tentacles,
from which Goette concluded that there was an ancestral
form, called a Scyphula, common to the Anthozoa, Acra-
speda, and Ctenophora, which was characterized by a free-
swimming mode of locomotion and by the possession of an
oesophagus and radial septa. It is possible, however, that

126 EMBRYOLOGY'

ontogeny does not represent the primitive condition in this
regard.

The attached polyp form recurs in the ontogeny of most
Cnidaria. In the Anthozoa and Lucernaridte it constitutes
the adult animal ; in the Hydrozoa it is co-ordinate with
the Hydromedusa ; whereas in the Acraspeda, in comparison
with the highly developed medusa form, it must instead be
considered as the young stage. Many medusa? (Tracheo-
medusre, Pelagia) develop directly from the free-swimming
larvffi into the medusa (comp. pp. 53 and 118). But here
also certain conditions of development can be interpi^eted
as modified polypoid stages.

The development of free-swimming sexual forms
(medusae) did not take place until after the separation
into hydropolyps and scyphopolyps, and therefore occurred
in the two groups independently. The differences in or-
ganization between the hydroid- and scyphopolyps are
explained by the different structure of their polyp forms
and by their independent development. The hydroid-
medusa is developed as a lateral bud, whereas the strobila-
tion of the Scyphomedusae is to be explained as a process of
transverse division. The medusa must be explained as a
polyp Avhich acquired powers of free movement, and as a
result of this underwent certain changes in form. The first
cause for the evolution of such locomotion we have re-
co"-nized in the miofration which in non-sexual reproduction
(division, budding) the detached portion must undertake
before attaching itself.

An opposite explanation, which is based chiefly on the occurrence of
hypogenetic forms, and which sees in these the more iirimitive conditions,
starts from a free-swimming medusoid ancestral form, the hxrvfe of
whicli, also at first leading a pelagic life, had secondarily acquired the
attached mode of life and reproduction by budding or division. The
polypoid forms would then have to be considered as coenogenetically
interpolated larval conditions (C. Vogt, No. 115 ; Bhdoks, No. 17). How-
ever, the entire structure of the medusas points to a primitive attached
ancestral form too clearly for us to grant this interpretation.

In the search after those hypothetical fi-oe-swimming
ancestral forms which preceded the attached Hydra-like
form, we must first think of such creatures as are repi-e-

CNIDARIA 127

sented in the ontogeny of Pelagia, for example, by the stage
of the gastrnla invaginata, i.e., a ciliated, ovoid, free-
swimming form, in which an archenteron opening to the
outside world by means of the prostoma was developed by
an invagination at the posterior end.

It can easily be explained how an ovoid blastula-like heteropolar an-
cestral form happened to develop the earliest beginnings of the archen-
teric invagination at the posterior pole of its body. In the case of
monaxial, heteropolar blastular larvfe which are allowed to swim through
water containing particles of carmine, it can be seen that these particles
are repulsed at the anterior and lateral parts of the body by the move-
ments of the larvae, whereas they are crowded together at the posterior
pole. Here accordingly was a favourable place for the reception of
particles of food, and by a flattening or shallow invagination of the
posterior pole these favourable conditions were increased. The archen-
teron therefore in its earliest beginnings was a pit in which to catch
particles of food.

If we incline to the view that the hypothetical ancestral
form of the Cnidaria was similar to the gastrula invaginata,
then in most Cnidaria we must assume a secondary change
in the ontogeny, for the typical lariml form of the Cnidaria
is the planula, a form in which we can recognize a ciliated
ectoderm and a more or less compact entodei'mal mass within.
The taking of food is here suppressed. This form serves
exclusively for locomotion and the consequent dissemination
of the species over a larger territory. In attached forms
such larval conditions are of great importance for the pre-
servation and distribution of the species. In the interest
of this function, the archenteric cavity appears to have
degenerated in the planula.

It is probable that the transition from the free-swimming
gastrula-like ancestral form to the attached polypoid foi-m
was brought about by means of an interpolated cj-eeping
stage, which would be recalled by the creeping planula of
many existing forms (e.gr Lucernaria).

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45. MuLLER, JoH. Ueber eine eigenthiimliche Meduse des Mittelmeeres

und ihren Jugendzustand. Arch. Anat. u. PJtys. 1851.

46. ScHULZE, F. E. Ueber den Bau und die Entwicklung von Cordylo-

phora lacusti'is. Leipzig. 1871.

47. TiCHOMiROFF, A. On the Embryology of Hydroids (Russian).

Mem. Boy. Soc. Friends of Nat. Set., Anthropol., and Ethnogr.
MoscntD. 1887.

48. Ussow, M. Eine neue Form von Siisswasser-Ccelenteraten. Morph.

Jahrb. Bd. xii. 1887.

49. Weismann, a. Die Entstehung der Sexualzellen bei den Hydro-

medusen, zugleich als Beitrag zur Kenntniss des Baues u. der
Lebenserscheinungen dieser Gruppe. Jena. 1883.

50. Weisjiann, a. Die Entstehung der Sexualzellen bei den Hydro-

medusen. Biol. Centralbl. Bd. iv. 1884.

51. Wilson, H. V. The Structure of Cunoctantha octonaria in the

Adult and Larval Stages. Stud. Biol. Lab. Johns Hopkins
Univ. Vol. iv. 1886.

Appendix to Literature on Hydroidea.

I. Brauer, a. Ueber die Entwicklung von Hydra. Zeitschr. tciss.

Zool. Bd. Iii. 1891.
II. Braueu, a. Ueber die Entstehung der Geschlechtsproducte und
die Entwicklung von Tubularia raesembryanthemum AUni.
Zeitschr. wiss. Zool. Bd. Iii. 1891.

III. Gerd, W. Zur Frage iiber die Keimbliitterbildung bei den Hydro-

medusen. Zool. Anzeiger. Jahrg. xv. 1892.

IV. Haeckeu, V. Die Furchung des Eies von Aeijuorea. Arch. mikr.

Anat. Bd. xl. 1892.
V. Heider, K. Ueber Gastrodes, eine parasitische Ctenophore. Sit-
zungsber. Gescllsch. ^uturf. Freunde Berlin. 1893.

CNIDARIA 131

VI. HiCKSON, S. J. On the Maturation of the Ovum and the Early
Stages in the Development of Allopora. Quart. Jour. Micr. Sci.,
n. ser. Vol. xxx. 1890.
VII. HicKSON, S. J. The Medusfe of Millepora Murrayi and the Gono-
phores of Allopora and Distichopora. Quart. Jour. Micr. Sci.,
n. ser. Vol. xxxii. 1891.
Vll.a. HicKSON, S. J. The Early Stages in the Development of Disticho-
pora violacea, etc. Quart. Jour. Micr. Sci., n. ser. Vol. xxxv.,
p. 129. 1893.
VII b. Hyde, Id.\ H. Entwicklungsgeschichte einiger Scyphomedusen.

Zeitschr. wiss. Zool. Bd. Iviii., p. 531. 1894.
VIII. KoROTNEFF, A. Zoologische Paradoxen. Zeitschr. tciss. Zool.
Bd. H. 1891.
IX. Lang, Alb. Ueber die Knospung bei Hydra. Zeitschr. wiss. Zool.

Bd. liv. 1892.
X. Maas, 0. Ueber Bau und Entwicklung der Cuninenknospen.

Zool. Jahrb., Abth.f. Anat. Bd. v. 1892.
XL ScHiMKEWiTSCH, W. Sur la segmentation et la formation de
I'entoderme des Hydromeduses. Revue Sci. St. Petershourg.
Annee i. 1890.

SiPHONOPHORA.

52. Agassiz, a. Exploration of the Surface Fauna of the Gulf Stream,

etc. : The Porpitids and Vellelidfe. Mnn. Mus. Comp. Zool.
Harvard Coll., Cambiidge. Vol. viii. 1883.

53. Bedot, M. Notice sur le d^veloppement des Velelles. Arch.

Sci. Phys. et Nat. Geneve. Ser. 3. Tom. xiii. 1885.

54. Chun, C. Ueber die cyclische Entwicklung und die Verwandt-

schaftsverhaltnisse der Siphonophoren. Sitzungsber. preuss.
Acad. Wiss. Berlin. 1882.

55. Chun, C. Ueber die cyclische Entwicklung der Siphonophoren.

II. SitziDigsber. preuss. Acad. Wiss. Berlin. 1885.

56. Chux, C. Ueber den Bau und die Entwicklung der Siphono-

phoren. Siiznnpsber. pi-euss. Acad. Wiss. Berlin. 1886.

57. Chun, C. Bericht iiber eine nach den Canarischen Inseln im

Winter 1887 — 1888 ausgefiihrte Eeise. Sitzungaber. preuss. Acad.
Wiss. Berlin. 1888.

58. Chun, C. Zur Morphologic der Siphonophoren. Zool. Anzeiger.

Jahrg. x. 1887.

59. Claus, C. Neue Beobachtungen iiber die Structur und Entwicklung

der Siphonophoren. Zeitschr. tviss. Zool. Bd. xii. 1863.

60. Claus, C. Ueber die Abstammung der Diplophysen und iiber eine

neue Gruppe der Dij^hyiden. Gottinger Nachrichten. 1873.

61. Claus, C. Die Gattung Monophyes und ihr Abkommling Diplo-

physa. Schriften Zool. Inhalts. Wien. 1874.

132 EMBRYOLOGY

62. Claus, C. Ueber Halistemma tergestinum n. sp. nebst Bemer-

kungen iiber den feineren Bau der Physophoriden. Arheiten
Zool. Inst. IVien. Bd. i. 1878.

63. Claus, C. Ueber das Verhaltniss von Monophyes zu den Diphyiden,

sowie iiber den phylogenet. Entwicklungsgang der Siphono-
phoren. Arbeiten Zool. Inst. Wien. Bd. v. 1883.

64. Claus, C. Ueber das Verhaltniss von Monophyes zu den Diphyiden,

etc. Zool. Anzeiger. Jahrg. viii. 1885.
Go. Claus, C. Zur Beurtheilung des Organismus der Siphonophoren
und deren phylogenetischer Ableitung. Arheiten Zool. Inat.
Wien. Bd. viii. 1889.

66. Fewkes, J. W. On the Development of Agalma. Bull. Mus.

Comp. Zvl'il. Harvard Coll., Cainhridfje. Vol. xi. 1885.

67. Gegenbaur, C. Beitrage zur niiheren Kermtniss der Schwimmpoly-

pen (Siphonophoren). Zeit>:chr. uiss. Zool. Bd. v. 1854.

68. Haeckel, E. Zur Entwicklungsgeschichte der Siphonophoren.

Utrecht. 1869.

69. Haeckel, E. System der Siphonophoren auf phylogenetischer

Grundlage. Jena. Zeitschr. Bd. xxii. 1888.

70. Haeckel, E. Eeport on the Siphonophor® collected by H.M.S.

Challenger, etc. " ChalU' Rep. Vol. xxviii. 1888.

71. KoROTNEFF, A. Zur Histologic der Siphonophoren. 31itth. Zool.

Stat. Neapel. Bd. v. 1884.

Appendix to Literature on Siphonophora.

I. Chun, C. Die Canarischen Siphonophoren. Theil i., 1890 ; Theil
ii., 1892. Abhandl. d. Senckenbergischen Naturf. Gesellsch.
Bde. xvi. u. xviii.

Anthozoa.

72. ArtASSiz, A. On Arachnactis brachiolata, a Species of Floating

Actinia, etc. Jour. Bnst. Soc. Nat. Hist. Vol. vii. 1863.

73. Andkes, a. Intorno alia scissiparita delle Actinic. 3Iitth. Zool.

Stat. Neapel. Bd. iii. 1882.

74. Blochmann, F., unu Hilgek, C. Ueber Gonactinia prolifera Sars,

eine durch Quertheilung sich vermehrende Actinic. Morph.
Jahrb. Bd. xiii. 1888.

75. Fauhot. Sur I'Adamsia palliata. Coinpt. Rend. Acad. Sci. Faris.

Tom. ci. 1885.

76. Haacke, W. Zur Blastologie der Korallen. Jena. Zeitschr. Bd.

xiii. 1879.

77. Haddon, a. C. On Larval Actinits parasitic on Hydromedusae at

St. Andrews. Aim. Mag. Nat. Hist. Ser. 6. Vol. ii. 1888.

78. Haeckel, E. Arabische Korallen. Berlin. 1875.

CNIDARIA 133

79. Hektwig, R. Die Actinien der Challengerexpeclition. Jena. 1882.

Also : Report on the Actinia, etc. " Chall." Rep. Vol. vi.,
part 15. 1882.

80. JouKDAN, E. Recherclies zoologiques et histologiques sur les Zoan-

thaires clu Golfe de Marseille. Ann. Sci. Nat. Ser. G. Tom. x.
1879—1880.

81. JoNGERSEN, H. F. E. Uebcr Bau und Entwicklung der Kolonie von

Pennatula phosphorea L. Zntschr. wiss. Znol. Bd. xlvii. 1888.

82. Koch, G. v. Das Skelet der Alcyonarien. Morph. Jahrb. Bd. iv.

1878.

83. Koch, G. v. Ueber die Entwicklung des Kalkskelets von Astroides

calycularis, etc. Morph. Jalub. Bd. viii. 1882.

84. Koch, G. v. Die morphologische Bedeutung des Korallenskelets.

Biol. Centralbl. Bd. ii. 1882.

85. Koch, G. v. Entwicklung des Kalkskelets von Astroides calycularis.

Mitth. Zool. Stat. Neapel. Bd. iii. 1882.

86. KocH, G. v. Die Gorgoniden des Golfes von Neapel. In Fauna

und Flora des Golfes von Neapel. Monogr. xv. 1887.

87. KowALEVSKY, A., ET Maeion, A. F. Documents pour I'histoire em-

bryogenique des Alcyonaires. Ann. Mus. Hist. Nat. Marseille.
Vol. i. 1883. See also Zool. Anz. , No. 38. 1879.

88. Lacaze-Duthiees, H. de. Histoire naturelle du Corail. Paris.

1864.

89. Lacaze-Duthiees, H. de. Developpement des Coralliaires. Arch.

Zool. exper. et gen. Tom. i., 1872 ; tom. ii., 1873.

90. Lacaze-Duthiees, H. de. Sur le developpement des Pennatules

(Pennatula grisea), etc. Compt. Rend. Acad. Sci. Paris. Tom.
civ. 1887.

91. McMdkrich, J. P. On the Occurrence of an Edwardsia Stage in

the Free-swimming Embryos of a Hexactinian. Johns Hopkins
Univ. Circul. Baltimore. Vol. viii. 1889.

92. Sempee, C. Ueber einige tropische Larvenformen. Zeitschr. iciss.

Zool. Bd. xvii. 1867.

93. Sempee, C. Ueber Generationswechsel bei Steincorallen, etc.

Zeitschr. loiss. Zool. Bd. xxii. 1872.

94. Studer, T. Knospung und Theilung der Madreporaria. Mitth.

Berner Nat. Gesellsch. 1880.

95. Studer, T. Ueber scheinbare Knospen von Herpetolitha limax.

Sitzungsher. Gesellsch. Naturf. Freunde. Berlin. 1880.

96. VoGT, C. Les Genres Arachnactis et Cerianthus. Arch. Biol.

Tom. viii. 1888.

97. Wilson, E. B. The Mesenterial Filaments of the Aleyonaria.

Mitth. Zool. Stat. Neapel. Bd. v. 1884.

98. Wilson, E. B. The Development of Renilla. Phil. Trans. Roy.

Soc. London. Vol. clxxiv. 1884.

99. Wilson, H. V. Development of Manicina areolata. Jour. Morph.

Vol. ii. 1889.

134 EMBRYOLOGY

Appendix to Literature on Anthozoa.

I. Beneden, E. VAN. Une larve voisine de la lai-ve de Semper. Arch.

Biol. Tom. X. 1890.
II. Benedex, E. van. Becherches sur le developpement des Arach-
nactis. Arch. Biol. Tom. xi. 1891.

III. BovEEi, T. Ueber Entwicklung und Verwandtschaftsbeziehungen

der Actinien. Zeitttchr. tdss. Zool. Bd. xlix. 1890.

IV. Caulgren, O. Zur Kenntniss der Septenmusculatur bei Ceriantheen

und der Schlundrinne bei Anthozoen. Ki/l. Vetenskaps-Akade-
miens h'orhandlingar. Stockholm. 1893.
V. Faueot, L. Sur le developpement du Cerianthus membranaceus.
Bull. Sue. Zool. France. Tom. xvii. 1893.
VI. McMuKRicH, J. P. Contributions on the Morphology of Actinozoa.
II. On the Development of the Hexactiniaj. Jour. Morph. Vol.
iv. 1891.

- SCYPHOMEDUS^.

100. Beneden, p. J. VAN. Recherches sur la faune littorale de Belgique.

Mem. Acad. Roy. Bruxelles. Tom. xxxvi. 1856.

101. Bergh, R. S. Bemaerkninger om Udviklingen af Lucernaria.

Vidensk. Meddel.fra den naturh. Foren i KJobenhdrn. 1888.

102. Claus, C. Studien iiber Polypen und Quallen der Adria. Deukachr.

Acad. Wiss. Wien. Bd. xxxviii. 1877.

103. CiAUS, C. Die Ephyren von Cotylorhiza und Rliizostoma und ihre

Entwicklung. Arheiten Zool. Inst. Wien. Bd. v. 1884.

104. Ci.Aus, C. Ueber die Classification der Medusen mit Riicksicht auf

die Stellung der sog. Peromedusen, etc. Arheiten Zool. Inst. M'ien.

Bd. vii. 1888.

105. GoETTE, A. Abhandl. zur Entwickl.-Gescli. d. Thiere. IV. Entwick-

lungsgeschichte der Aurelia aurita und Cotylorhiza tuberculata.

Uamburn u. Leipziji. 1887.
10(3. Ha.\ckk,W. Die Scyphomedusen des St. Vincent Golfes. Jeua.

Zeitschr. Bd. xx. 1887.
106a. Haacke, W. Ueber die Ontogenie der Cubomedusen. Zool.

Anzeiger. Bd. ix., p. 554. 1886.

107. Haeckel, E. Metagenesis und Hypogenesis von Aurelia aurita.

Jena. lS8l.

108. Kowalevsky, a. Zur Entwicklungsgeschichte der Lucernaria.

Zool. Anzeitjer. Jahrg.sn. 1884.

109. KuoiiN, A. Ueber die friihesten Entwicklungsstufen der Pelagia

noctiluca. Mi'dl. Arch. Anat. n. Fhijs. 1.S55.

110. Lendenfeld, R. v. Zur Metamorphose der Rhizostomen. Zool.

Anzeiger. Jahrg. vii. 1884.

CNIDARIA. 135

111. NoscHix, N. Ueber einen Generationswechsel bei Geryonia pro-

boscidalis u. die Larve von Rhizostoma Aldrovandi. Bull. Arad.
Imp. St. Petershourci. Tom. viii. 18(55. Also Melang. Bioloi/.
Tom. v., p. 28. 1866.

112. Saes, M. Ueber die Entwicklung der Medusa aurita mid Cyanea

capillata. Arch. f. Natttrt). Bd. vii. 1841.

113. ScHNEiDEE, A. Zur Entwicklungsgeschichte der Aurelia am'ita.

Arch. mikr. Anat. Bd. vi. 1870.

114. SiEBOLD, C. T. V. Beitriige zur Naturgeschichte der wirbellosen

Thiere. Neueste Schriften der naturf. Gesellschaft in Danzig.
Bd. iii. 1839.

115. VoctT, C. Sur un nouveau genre de medusaire sessile, Lipkea

Euspoliana C. V. Mem. Inst. Nat. Genevois. Tom. xvii.
Geneve. 1887.

Appendix to Literature on Scyphomedusre. '

I. Claus, C. Ueber die Entwicklung des Seyphistoma von Cotylorhiza,

Aurelia, und Chrysaora. Arheiten Zool. Inst. Wien. Tom. ix.

1890.
II. Claus, C. Ueber die Entwicklung des Seyphistoma von Cotylorhiza,

Aurelia, und Chrysaora, etc. Zweiter Theil. Arbeiten Zool. Inst.

Wien. Tom. x. 1892.
III. GoETTE, A. Vergleichende Entwicklungsgeschichte von Pelagia

noctiluca, Per. Zeitachr. wiss. Zool. Bd. Iv. 1893.
IV. Hamann, O. Ueber die Entstehung der Keimblatter. Ein Er-

kliirungsversuch. Internat. Monatsschrift Anat. u. Phijdol.

Bd. vii. 1890.
V. McMuERicH, J. p. Contributions on the Morphology of the Actino-

zoa. II. On the Development of the Hexactinise. Jour. Morpli.

Vol. iv. 1891.
VI. McMuEEicH, J. p. The Gastraa Theory and its Successors. Biol.

Lectures, Marine Biol. Lab. of Woods Holl. Boston. 1891.
VII. Sjiith, F. The Gastrulation of Aurelia flavidula, Per. et Les.

Ball. Mux. Comp. Zool., Harvard Coll. Cambridge. Vol. xxii.

No. 2. 1891.

CHAPTER III.
CTENOPHOEA.

Tectonic— The body of the Ctenopliore exhibits a chief
axis, the poles of which are marked, one by the position of
the mouth, the other by the sensory organ located at the
apex. Perpendicular to this chief axis two mutually per-
pendicular secondary axes can be distinguished ; they are of
unequal length, and are further distinguislied from each
other by the dissimilar organs occurring in their course.
The plane determined by one of these two secondary axes
and the chief axis we designate with Claus (No. 4) as the
lateral or transverse plane (Fig. 63 aa), for the tentacles are
situated in it, and thus a comparison with the lateral parts
of the body of the Bilateria is permitted. This plane is also
called by Chun (No. 3) the infundibular plane, since the
part of the gastro-canal system known as the infundibulam
attains its greatest dimensions in this direction. In accord-
ance with the comparison with the Bilateria above men-
tioned, the plane corresponding to the other secondary axis
is called the sagittal plane (Fig. 63 hb), or, according to
Chun, on account of the extensions of the stomach occurring
in this direction, the gastral plane. The body of the Cteno-
phore is divided by these planes into four quadrants, all of
which, however, are not congruent with one another, as is
the case in quadriradial animals, but only the diagonally
opposite ones, each quadrant being like a reflected image of
the neighbouring ones. Since in radiate animals each radial
part (antimere) is divided into two symmetrical halves by
the plane of its radius, it follows that in the Ctenophora
each quadrant corresponds to only the half of such a radial
part, and that it becomes an entire antimere only after the

136

CTENOPHORA

137

addition of a second adjacent quadrant. Accordingly the
Ctenophora are radiate animals ivith two rays (Fr. MiJLLER,
Clacjs). In this case it is impossible, though of no conse-
quence, to determine whether we ought to designate the
radii of the sagittal plane as perradii and those in the trans-
verse plane as interradii, or vice versa. Bj the unequal de-
velopment of organs Ijing in the plane of a secondary axis
the biradial structure may be converted into the bilaterally
symmetrical (for example, in the larval form known as Thoe
paradoxa by the development of a single tentacle).

f /

\

m ;

'9 \\ ^ O

Fig. 63. — Mertensia stage of Eucharis tnulticornis seen from the sensory pole
(after Chun), diagrammatic, aa, transverse axis ; bb, sagittal axis ; m, stomach ;
po, excretory pores ; p, polar plates of the apical sensory organ ; t, tentacular ap-
paratus ; mg, gastral vessels and r, meridional vessels in cross-section.

The bilateral symmetry of two neighbouring quadrants of the Cteno-
phore suffers a certain derangement by the position of the two excretory
pores. For the infundibulum communicates with the exterior by means
of two openings situated in the vicinity of the apical pole (Fio. 63 po),
which lie in two diagonally opposite quadrants. This derangement is,
of course, to be explained as the result of the suppression of two pores,
for probably there was originally one pore present in each quadrant,
consequently four in all, a condition which, according to E. Heetwig
(No. 12, p. 318), is retained in Callianira bialata. There is no essential
change in the biradiate structural plan of the Ctenophora owing to this
asymmetrical development of the excretory pores, just as in the Bilateria,
for instance, an organ is frequently seen to develop asymmetrically with-
out the bilateral type being thereby destroyed (Claus).

138

EMBRYOLOGY

If we regard one of the cross-axes as the perradius, and the other as
the interradius, we must, in accordance with the terminology employed
above (pp. 108, 115 ) for the medusas, designate as adnidii those falling
between perradii and interradii, by means of which each quadrant is
halved, whereas the eight radii lying between the adradii and the cross-
axes should be interpolated as .subrudii. The latter would nearly corre-
spond in position to the eight ribs, and, following the suggestion of Claus
(No. 4), we shall designate those lying next to the sagittal plane as sub-
sagittal, those lying nearer to the transverse plane as subtransverse.

Embryonic Development. — The embryonic develop-
meat of the Ctenophora has been described principally by
Allman (iS"o. 2), KowALEvsKY (No. 14), FoL (No. 7), A.
Agassiz (No. 1), Chun (No. 3), and Metschnikoff (No. 16).
It takes place in the different species in a nearly similar
manner.

The Ctenophora are hermaplu'oditic. The generation of
sexual products takes place either intermittently throughout
the entire year, as at Naples, or it is confined to the summer
months, as in Northern seas (Trieste, North American coast).
The eggs in most cases are deposited singly and fertilized
in the sea- water ; however, the laying of eggs in strings of
about ten each has been maintained for some forms (Pleuro-
brachia Flem. according to Kowalevsky, Bolina according to
A. Agassiz).

The eggs of the Ctenophora (Fig. 64) are enveloped by a

delicate structureless pellicle
(vitelline membrane), which
(Fig. 64 d) is rather widely
separated from the surface of
the egg. The space thus re-
sulting is filled with a trans-
parent jelly, in which the egg
proper is so embedded that
it always lies at about the
middle. The structure of the
latter recalls the eggs of the
Siphonophora, Geryonidae, etc.
We can distinguish a super-
ficial layer consisting of formative yolk (ectoplasm, ek) and

Fio. 64.— Egg of Lampetia pancerina
(after Chun), ek, ectoplasm ; en, endo-
plasm ; d, vitellino membrane.

CTENOPHOEA 139

an endoplasm (ew) filling the inside. The latter is foamy,
owing to the existence of a large number of spherical
vacuoles, between which only a scant reticulation and mesh-
work of protoplasm (formative yolk) is left. It is very
probable that these spherical vacuoles are all filled with a
homogeneous, slightly refractive mass, which has sub-
stantially the characters of food-yolk. Therefore in what
follows we shall occasionally designate the entire endoplasm
merely as food-yolk mass. The germinative vesicle is found
in the superficial cortex of ectoplasm (Fig. 64).

Even though the cleavage in the Cteuophora, as we shall
see, exhibits its peculiarities, yet we can on the whole desig-
nate it as total, unequal cleavage, which leads to the forma-
tion of an epibolic or circumcrescence gastrula. However,
this latter type of gastrulation is not preserved in its
purity, for eventually an invagination process participates
in the sinking of the entoderm into the embryo.

The first furrows that make their appearance are to be
designated as meridional, inasmuch as they cut through
from the animal to the vegetative pole in the direction of
the subsequent longitudinal axis. By means of the first of
these furrows the egg is divided into two equal parts (Fig.
60 A) ; by means of the second furrow, likewise extending
in a meridional direction and perpendicular to the first,
there are formed four cleavage spheres, arranged crosswise
(Fig. 65 B, F) ; these are oriented in such a manner as re-
gards the embryo resulting from them that each cleavage
sphere corresponds to one quadrant of the embryo (Fol, No.
7). The third act of cleavage leads to the appearance of
additional meridional furrows, which, as the dotted lines in
Fig. 65 F indicate, make an angle of 45 degrees with those
already present. If this cleavage were to take place regu-
larly in the way indicated, eight large cleavage spheres of
equal size lying in one plane would result. On the con-
trary, the eight-cell stage presents a variation from this
regularity which recurs in all Ctenophora, and is import-
ant for the subsequent formation of the embryo. For the
furrows that now make their appearance are shifted in
such a manner that, as is indicated by the dotted lines in

140

EMBRYOLOGY

Fig. 65 G, each cleavage sphere is divided into a larger and
a smaller part. The most striking thing in this is that by
the regular paired arrangement of the four smaller cleavage
spheres a difference between the cross-axes (secondary axes)
of the embryo can already be recognized, so that even as
early as this stage the biradiate structure is clearly ex-
pressed ; and, according to FoL (No. 7), the longer of the
two diameters cori-esponds to the transverse, and the shorter
to the sagittal, axis. The transverse plane (infundibular or
tentacular plane) therefore separates the embryo into two

Fis. 65. — Diagrammatic representation of the cleavage of the Ctenophora, based
on the figures of A. Agassiz. A, stage of division into two cells; B, four-cell
stage from the side ; C, eight-cell stage seen from above; D, the same in transverse
section ; E, two-cell stage from above ; F, four-coll stage from above ; G, plan of
the next succeeding division ; If, transition to sixteen-cell stage ; I, the same from
above ; K, L, succeeding stages with multiplication of the micromeres ; M, such a
stage in cross-section.

rows of four cells each, as is represented in Fig, 65 C.
Another peculiarity of this stage consists in the fact that
its eight cells no longer lie in one plane ; the smaller
lateral cells move to a higher level, whereby, as can be
seen in Fig. 65 G and D, the entire fundament becomes
somewhat basket-shaped. In this way a difference be-
tween the two poles of the chief axis is also indicated
even now, and the concavity of the basket-like fundament

CTENOPHORA

141

corresponds, according to Metschnikoff (No. 16), to the so-
called upper or future sensory pole. Furthermore a histo-
logical difference between the smaller and larger blastomeres
of this stage is said to be noticeable, inasmuch as a larger
amount of ectoplasm is involved in the formation of the
smaller cleavage spheres.

We have designated the furrows appearing up to this time
as meridional, because they had the same direction as the
chief axis. The next one to appear is, on the contrary, per-
pendicular to the chief axis (Fig. 65 if), and must therefore
be called an equatorial furrow. The formative yolk collects
in the upper part of the eight cleavage spheres, and is con-
stricted off in the form of small cells (Fig. 65 H), so that in
this way we obtain a stage composed of eight macromeres,
consisting almost exclusively of food-yolk, and eight micro-
meres (Fig. 65 I). Since in many instances the cells of the
embryo in this stage separate from one another, leaving a
space at the centre, the fundament becomes annular, a ring
of eight micromeres resting upon a larger one of eight macro-
meres. The cavity formed at the centre, whicli is open
above and below, as in the eight-cell and sixteen-cell stages
of Sycandra raphanus, we must designate as a cleavage
cavity (blastocoele).

Subsequently a rapid multiplication of the micromeres
takes place, on the one hand by division of those already

Fig. 66.— Three cleavage stages of a Ctenophore egg (diagrammatic), mi, micro-
meres; ma, macromeres (from Lang's Xe?iibucJi).

present (Fig. 65 K) and on the other hand by the abstric-
tion of new micromeres from the underlying macromeres
(Fig. 66 B and G). In this way the annular cell-mass of
micromeres continues to spread out, and finally rests like a

142 EMBRYOLOGY

cap upon the mass of macroineres, which it covers over (Fig.
65 M, Fig. 66 B and C).

We may from now on consider this cap-like fundament of
micromeres, in accordance with its destiny, as ectoderm. It
spreads out more and more, especially by the progressive
growth of its marginal parts, so that it soon envelops not
only the upper portions, but also the lateral parts, of the
mass of macromeres (Fig. 66 G and 67 A).

In this way the latter moves more and moi'e into the in-
terior of the embryo, so that we here see a two-layer em-
bryonic form (gastrula) produced by means of circumcres-
cence or epiboly. Frequently the forward growth of the
margin of the ectoderm does not proceed uniformly at all
points, but a more active marginal growth is shown at points
corresponding to the four radii. Finally the macromeres
are seen to be covered by ectoderm on all sides except the
lower surface, where the ectoderm still presents a large
circular gap (Fig. 67 A), which we may designate as the
gastrula-mouth or blastopore. Up to this time the domi-
nant activity of the embryo consisted in the production of
ectodermal elements. The macromeres were involved in
this only in so far as they constantly budded off new ecto-
dermal elements from their upper surfaces. When the stage
last mentioned has been reached, this kind of increase of the
ectoderm ceases, and the macromeres from now on become
active in another direction. It is noteworthy, in contrast
with the considerable multiplication of the ectoderm cells,
that up to this time the eight macromeres have undergone
no increase in numbers. But now they begin to divide, so
that stages with twelve and then with si.\teen macromeres
can be observed. Afterwards the division of the macromeres
becomes irregular. Meanwhile the basket-like arrangement
of the macromeres has vanished, and they now form a moi-e
placentiform mass (Fig. 67 A).

We have designated the micromeres as ectoderm ; we
could not, however, employ the term " entoderm " for the
macromeres, because, on the one hand, they still contained
parts which were to be constricted off by budding and added
to the ectoderm, and because, on the other hand, as we know

CTENOPHORA

143

through Metschnikoff (No. 16), they are also destined to
supply the elements of the mesoderm. For in the present
stage (Fig. 67 A) there is effected a new abstriction from
the macromei-es of small elements (me), which we may
designate as mesoderm cells. At first these form a cell-plate,
which lies at the lower, free surface of the macromeres,
when the latter are not yet covered by ectoderm. But in
the stages which now follow (Fig. 67 B and C) certain im-
portant changes are accomplished by means of which this
fundament soon reaches the inside of the embryo. In this
connection we must first glance at the upper pole of the
embryo (Fig. 67 A). Here the embryo still exhibits a small
opening, which in earlier stages (Fig. 66 B and G) was
larger and is to be referred to the inner circumfei-ence of

Fig. 67. — Three embryos of CalUanira hialata in transverse section, diagram-
matic (after Metschnikopp, from Lang's Le?irbMc7i). ec, ectoderm; eii, entoderm;
me, mesoderm J d, intestinal cavity ; st, oesophagus (fundament of stomach).

the ring of micromeres (Fig. 65 I, K, L). This opening in
the stage of Fig. 67 A is still in connection with a cavity
existing between the macromeres, which we recognize as
the remains of the cleavage cavity. Both the cleavage
cavity and the iipper opening in the ectoderm, which has
been erroneously assumed by many authors to be the blasto-
pore, now disappear by the neighbouring cells closing tightly
together. At the same time an invagination of the lower
surface of the macromeres and of the adjacent mesodermal
plate (me) is eifected ; by means of this a cavity (gastrula
cavity) is formed opening downwards, the lower portion of
which is lined with entoderm cells, the upper portion with
mesodermal elements (Fig. Q7 B). In the further course of

144 EMBRYOLOGY

development this cavity enlarges (Fig. Q*7 C, d), while the
mesodermal elements move upward more and more, and
finally spread out in the form of a plate on the inner surface
of the ectoderm (Fig. 67 C, me). Meanwhile the circuni-
crescence on the part of the ectoderm has made further pro-
gress. Even now it not only covers the under-side of the
embryo, but also grows up into the interior of the gastral
cavity, so that an invagination of the ectoderm (Fig. 67 C,
st) arises, wrhich is comparable with the oesophagus of the
Anthozoa, and from which the so-called stomach of the
Ctenophora is subsequently developed.

The statements of the different authors regarding the first stages of
cleavage are essentially in agreement with one another, but in regard to
the later stages, and more especially concerning the orientation of the
embryo, they differ somewhat, the point in question being the determina-
tion of the poles of the chief axis. If in the earlier stages we call that
pole which in our figures is the upper one the micromere pole, and the
opposite one the macromere jjole, the disputed point is whether or not
subsequently the micromere pole becomes the sensory pole, and the macro-
mere pole the oral pole. We have adhered to Metschnikoff's descrip-
tion (No. 16), which agrees with Kowalevsky's later memoir {Literature on
Cnidaria in General, No. 10), because such an orientation seems probable
through comparison with the eggs of mollusca and worms having
unequal cleavage and subsequent epibolic development, and because a
homology of the sensory body of the Ctenophora with the apical plate
of these forms appears thus to be provided for.

The development of all Ctenophora in the stages thus far described
appears to take place in very much the same way. Lampetia pancerina
alone appears, according to Chun (No. 3), to possess peculiarities,
especially, among others, the existence of a sixteen-cell stage, formed
on a strictly quadriradiate plan, etc.

The embryo has now assumed a nearly spherical form
(Fig. 07 C). But the two ends of the chief axis are dis-
tinguished by shallow depressions. In viewing the embryo
from above, one recognizes that the transverse axis still
exceeds the sagittal in length. From now oa a growth in
the direction of the chief axis is especially noticeable
(Fig. 68). The embryo thereby becomes more elongated.
Since at the same time the upper end of the body increases
in size, principally by the development of the tentacular

CTENOPHORA 145

apparatus, a pear-shaped or heart- shaped form is evolved
(Fig. 70 A).

Hand in hand with these changes goes the diiferentiation
of the ectodermal structures which are characteristic of the
Ctenophora : the tentacular apparatus, the ciliary plates,
and the apical sensory organ. At an early period are to be
noticed in the upper half of the body two ectodermal thicken-
ings (Fig. 68 t) lying opposite to each other in the ti-ans-
verse plane ; such an abundant multiplication of ectoderm
cells occurs in these places, that they become many layers
deep. These two thickened areas form each the fundament
of a so-called tentacle base (Fig. 69 B, th). Within the
territory of these a ridge, known as the tentacle stalk
(Fig 69 B, ts), soon makes its appearance, and out of it
arises the fundament of the tentacle (t). At the same time
with the earliest formation of the tentacular apparatus, four
I'ows of cells situated adradially become conspicuous by
their active proliferation. These cells are covered with
cilia, which at first are short and fine; they begin to beat
slowly backwards and forwards, and soon fuse together in
such a way as to form the swimming plates (Fig. 68 r). In
this way two rows of swimming plates arise on each of the
four fundaments, so that in these first stages the eight
ciliated ribs appear grouped in pairs. Originally each rib
exhibits only a very few (usually four to six) swimming
plates, and, as a rule, their number is not increased until
after the abandonment of the egg-membranes. The swim-
ming plates, as has been shown, are to be regarded as fused
cilia ; they are higher differentiations of a continuous coat
of cilia attributable to the ancestors of the Ctenophora.
In this connection it is interesting to know that Chun
(No. 3) Avas able to demonstrate on the embryo of
Eucharis multicornis a fine ciliation covering the whole
surface. Of this ciliation there are retained throughout life
only eight fine meridional rows, which extend from the
rows of swimming plates to the upper pole of the body, and
establish the connection with the sensory body located
there. This apical sensory organ, which is perhaps to be
considered as the centre of the nervous system, is also de-

K. H. E. L

146

EMHRYOLOGY

veloped from a thickening of the ectoderm (Fig. 68). The
otoliths, which are at first small and then increase in size,
are formed in certain of these cells ; they are finally
extruded upwards, to constitute the otolithic mass sus-
tained on the four S-
shaped springs (cilia).
In many cases the first
otoliths are formed in
the epithelium in four
groups, corresponding
to the different quad-
rants of the body of
the Ctenophore (A.
Agassiz,Fol). The bell-
shaped case developed
over the sensory organ
arises, like the swim-
ming plates, from four
groups of long cilia
fused with one another
(Figs. 68, 70, 72). The
ciliated polar plates
(Fig. 63 p) are foi*med
in connection with this
sensory body as thick-

Fio. 68.— Dinsram of a Ctenophore embryo
at the time of the formation of the entoderraal
sacs, all organs in transverse section, except
the fundaments of the ciliary plates, r, which
correspond to the outer surface, of, otoliths;
t, fundament of the tentacular apparatus; ms,
mesoderm ; en, entoderm ; ec, ectoderm ; g,
mesogloea ; m, stomach ; c, central intestinal
cavity; d, diverticula of the same (fundaments
of the entodermal sacs).

ened regions of the ectoderm, which at first are rounded, but
subsequently much elongated.

We have seen that two parts can be distinguished in the
fundament of the gastrovascular system (Fig. 67 0) : a
lower, which arose as an ectodermal invagination, the
surface of which is soon covered with a coat of

inner

cilia, and out of which the so-called stomach subsequently
arises; and an upper (cZ), bounded by entodermal cells, which
represents the fundament of the infundibiilum and the
vessels. The differentiation of this upper, entodermal
portion into its individual parts can be considered as essenti-
ally a kind of formation of diverticula. As can be seen
in Fig. 67 C, the entoderm cells, as the macromeres after
giving off the ectodermal and mesodermal elements may

CTENOPHORA

147

now be called, exhibit a tendency to orient themselves
radially about the central cavity d. Designating the point
of transition of the fundament of the stomach into that of
the infundibulum as the inner opening of the oesopTiagus or
infundibular fissure, the increase in the length of the stomach
causes this fissure to move into the central cavity, a w^all
being formed by means of which a central portion of the
cavity is separated from a lateral part (Fig. 68). This
lateral portion is not retained as a single space, but divides
into four diverticula, which, in accordance with their mode

Fi&. 69.— Further formation of the gastrovascular system after Chun). A,
embryo of Beroe, in optical transverse section at the time of formation of the four
entodermal sacs ; ek, ectoderm ; en, entoderm ; g, mesogloea. B, development of
the permanent canal system in an embryo of Eucharis muUicornis. View from
below: m, stomach ; mg, fundament of the gastral vessels; x, fundament of the
meridional vessels ; r, ciliary plates ; i/, fundament of the tentacular vessels ; ih
tentacle base [Tentakelhodcn} ; ts, tentacle stalk ; t, tentacle.

of origin, communicate at their upper parts with the central
infundibular fundament, and have their blind ends directed
orally (Fig. 68 d and Fig. 69 A). Inasmuch as the greater
portion of the entodermal cells is grouped about these four
blind sacs, it is divided from now on into the four so-called

148 EMBRYOLOGY

entodermal sacs, each one of which corresponds to a quadrant
of the Ctenophore body. The distinct separation of these
four entodermal sacs is materially promoted by the simul-
taneous appearance of the inesogJuea (Fig. 69 A, g). This
transparent secreted mass accumulates between the stomach,
the entoderm, and the surface ectoderm (Fig. 68 g), and
forms septa-like processes, which extend especially between
the entodermal sacs. The rapid increase of the mesogloea,
into which cells soon migrate, occasions the considerable
increase in the size of the embryo during this stage. By
the formation of the mesogloea in the further course of de-
velopment, the fundament of the gastrovascular system is
forced farther and farther away from the outer surface of
the body. An intimate contact is retained only at the
points corresponding to the eight ribs and the fundaments
of the tentacles (Fig. 69 B) ; by a large accumulation of
entodermal cells the places are here indicated at which, by
the formation of additional diverticula, the eight rib- vessels
(meridional canals) and tentacular vessels ai-e developed.
The origin of the two gastral vessels is to be attributed to a
similar formation of diverticula (Fig. 69 B, vig).

The mode of formation of the four entodermal sacs by the deeper
penetration of the inner opening of the oeso^jhagus, which has been
described by Chun (No. 3) and represented in his Fig. 18, Taf. vii., re-
calls the quite similar inode of formation of the two i^rimary gastral
pouches of the Scyphistoma according to Goette. (Comp. ]3. 107.)

During these changes the characteristic lateral compres-
sion of the stomach has already been effected (Fig. 69 B, m).
On the contrary, the central part of the vascular system,
which is metamorphosed into the infundibulum, exhibits a
compression, more or less distinct in all Ctenophora, in the
direction of the other (sagittal) secondary axis, so that these
conditions could be utilized by Chun (No. 3) in designating
the cross-axes. The more the vascular system is developed,
the more do the entodermal cells acquire the histological
characters of the permanent walls of the vessels.

We have traced the fundament of the mesoderm until, in
the progressing invagination of the gastral cavity, it arrives

CTENOPHORA

149

at its top, finally to spread out flat on the inner surface of the
ectoderm at the apex of the embryo. The plate thus formed,
which frees itself more and more from the entoderm, at first
elongates only in the direction of the transverse plane ; later,
however, by a new mesodermal growth from the original
centre, a cruciform mesodermal fundament (Fig. 71 m) is
formed, on which we can distinguish two longer (lateral) and
two shorter (sagittal) mesodermal bands. The former are
closely applied to the funda-
ments of the tentacles (Fig. 70
A and B), and supply the meso-
dermal axes, especially the
musculature, of the tentacles,
whereas the median (sagittal)
bands become the seat of the
formation of migratory cells
(Fig. 71 g), which wander into
the mesogloea and give rise to
the cellular elements of the
gelatinous tissue by becoming
metamorphosed there into stel-
late connective-tissue cells and
branched muscle fibres.

In regard to the development of
the mesodermal structures we have
followed exclusively Metschnikoff's
(No. 16) descrij^tion. Formerly the
origin of the elements of the gelatinous
tissue was attributed by Kowalevsky
(No. 14) and Chun (No. 3) to an im-
migration of ectodermal cells (the
superficial as well as the gastral epi-
thelium). According to Chun, this
immigration does not cease with the
embryonal stage, but throughout life
adds new muscle elements to the gela-
tinous tissue. The immigration of
ectodermal elements into the mesoglcEa
during embryonic life is directly denied

oi...

m^

Fig. 70.— Two stages of the de-
velopment of Callianira hialnta(si(tor
Metschnikoff, from Lang's Lehr-
buc?i). en, entoderm; me, meso-
derm; me'.mesenchyma;*, tentacle;
sk, sensory body ; d, intestinal
cavity; st, CEsophagus (fundament
of the stomach^ ; g, mesogloea.

by Metschnikoff. Accordingly the gelatinous tissue would be essentially
a mesodermal formation; and even though in later stages ectodermal

150

EMBRYOLOGY

Fig. 71. — Embryo of Callianira
bialata viewed from above (after
Metschnikoff). r, ciliary plates ;
t, fundament of the tentacular ap-
paratus ; m, cruciform mesodermal
fundament ; g, migratory cells in
the mesoglcea.

muscle fibres were secondarily to sink into the mesoderm, nothing in the
real nature of the gelatinous tissue would be changed by this.

In order to explain the presence of
four mesodermal bands, Kleinenberg
(Literature Annelida, i., No. 26, p. 13)
IJerceives in them an indication of the
existence of four tentacles (two lateral
and two sagittal) in the ancestral forms
of the Ctenoiihora, of which those in
the sagittal plane have become de-
generated. It is interesting to know
that in the Beroidse, which are with-
out tentacles, there exists an entirely
similar mesodermal fundament, which
extends in the transverse direction at
the apical pole, and there comes to lie
under two ectodermal thickenings
(rudiments of tentacles) (Metschni-
koff). The further fate of this meso-
dermal fundament
could not be followed.

As regards the
formation of the
sexual organs,
which does not
fall within the
period of embry-
onic development,
but occurs in later
stages, R. Hertwig
has shown by his
observations on
Callianii-a that it
is probable that
they are of ecto-
dermal origin.
The sexual pro-
ducts, to be sure,
ripen directly un-
der the epithelium

— wp

Fig. 72. — Young larva of Callianira bialata (after
KowALKvsKY, from Hatschkk's Lehrhuch). t, tenta-
cles ; ot, auditory organ ; so, apical organ ; wp, the
rows of ciliary plates; en, the four entodermal sacs;
«, CESophagup.

CTENOPHORA 151

of the meridional vessels, but a cord of cells wliicli unites
the ectoderm to the sexual organs points to their ectodermal
origin. Sac-like invaginations of the superficial epithelium
have also been observed, which perhaps represent the original
fundaments of genital sacs.

Metamorphosis. — After the proof had been pi'oduced by
the observations of J. Price and JoH. Mullek that the young
forms of the Ctenophores resemble to a certain extent the
adult animals, and that consequently no alternation of
generations was interpolated in their life-history, one was
inclined to assume for them a direct development. McCkady
was the first to show the existence of a rather marked meta-
morphosis by his observation that the young Bolin^ just
hatching from the egg were formed after the type of the
CydippidsB. Since then the details of the metamorphosis
have become known through the researches of A. Agassiz,
J. W. Fewkes, and especially of C. Chun.

Since the Gydippidcc, by the absence of anastomoses of the meridional
vessels and the bUnd termination of the gastral vessels, retain through-
out life the most primitive type of distribution of the vessels, the
metamorphosis in them is simple. Nevertheless it should be mentioned
that the Pleurobrachise, which are round in cross-section, are compressed
in young stages by the shortening of the sagittal diameter, and in this
regard recall the Mertensise (Chun). If Chun's hypothesis is confirmed,
according to which the remarkable Thoe paradoxa (which is characterized
by the possession of a single extensible tentacle projecting from a tentacle-
sheath resembling a chimney-i^ot near the sensory body) belongs in the
life-history of Lampetia pancerina, then a much more elaborate meta-
mori^hosis will have to be ascribed to some of the Cydippidse.

The metamorphosis of the Lob'itcc has been described by McCrady, A.
Agassiz (No. 1, Bolina), Fol (No. 7, Euramphaea), and Fewkes (Nos. 5
and 6, Ocyrrhoe, Mnemiopsis), and especially in Chun's (No. 3) extensive
presentation of the course of development in Eucharis multicornis. The
latter form in particular exhibits a series of larval stages differing from
the adult in habit as well as in the course of the vessels. Here again
the point of departure is a Mertensia stage having the structure of the
Cydippidae (Fig. 63), with distinctly shortened sagittal and elongated
transverse diameters, which is all the more striking since in the adult
form the opposite condition in the length of the cross-axes exists. In
the Ji7-st stage with the fundaments of lobes, which now follows, a con-
siderable increase in the length of the meridional vessels is noticeable.
At the same time the subsagittal vessels become longer than the sub-

152

EMBRYOLOGY

transverse ones, and coiTespondinfflj' the subsagittal ribs exhibit a larger
number of swimming plates. In the further course of development the
meridional vessels pass into the oral lobes, and their lower ends become
bent; in this way the subtransverse vessels come to be the longer.
Whereas in the adult the lower ends of the vessels in each lobe are
united m such a way that the two subtransverse vessels communicate
with each other, and the two subsagittal vessels with each other, at this
stage the subtransverse vessel forms with the subsagittal vessel of the
same quadrant a closed system of tubes (Fig. 73).

Fig. 73.— Medusiform stage of Eucliaris multiconiis (after Chun). A, view of
the sagittal plane. B, view from above : on the right-hand side the ribs are
omitted : m, stomach ; I, oral lobes; f, rudimentary tentacular apjiaratus; sf, sub-
transverse, f.s, subsagittal, meridional vessel. At x the subsequent connection of
the vessels is indicated by dotted lines.

There now follows a staije of medusiform habit, the course of the
vessels remaining about the same (Fig. 73), in which, as in the adult,
the sagittal diameter already exceeds the transverse. In this larva,
which, like a Medusa, moves through the water by the pulsating action
of its oral lobes, a complete degeneration of the tentacular apparatus
(t) occurs; this is replaced in the succeeding Boliiia stage by a new
tentacular fundament. In this stage is reached the form of body and
distribution of vessels typical of the Lobatro, for, on the one hand,
the connection of tlie subtransverse with the subsagittal vessels is

CTENOPHORA

153

broken, whereas the vessels of each lobe having the same name come
into communication with each other at their lower ends (Fig. 73 7^, at x),
and, on the other hand, each gastral vessel, which up to this time ended
blindly, sends out two transverse processes at its oral end, which open
into the subtransverse vessels of the same side. With the development
of (1) the c»cal pouches (characteristic of Eucharis) above the base of the
tentacles (metamorphosed tentacle-sheath) and (2) the dermal papillae
the form of the adult animal is reached.

Chun was able to establish the fact that under certain conditions the
Mertensia stage attains sexual maturity, so that the existence of a re-
markable heteroyeny is established for the Ctenophora.

The metamorphosis of the Cestidae, as Chun's observations on Cestus
show, proceeds from a Mertensia stage quite similar to that of Eucharis.
Here also the sagittal diameter is at first shorter than the transversei
although subsequently it so vastly predominates in the ribbon-like body.
That which especially characterizes the Cydippidoid early stage of Cestus
is the presence of a single swimming plate on each rib, corresponding to

Fig. 74.— Two stages of development of Cestus veneris (after Chun). A resembles
the CydippidEe in form : m, stomach ; mg, gastral vessel, with its processes ; s,
eubsagittal t, subtransverse meridional vessel ; B, somewhat older stage with the
ciliary plates in their permanent position.

the uppermost of the four embryonic swimming plates, the lower ones of
which become degenerated. The further course of the metamorphosis
is tolerably simple. At first the larva is round in cross-section ; then it is
flattened in the transverse direction (Fig. 7-i /I), so that the flat ribbon-
like shape is more and more expressed. At the same time the short
meridional vessels and the gastral vessel grow downward. The latter
soon puts forth two transverse processes (Fig. 74 w//), which extend
parallel to the lower margin of the larva. Of the meridional vessels the
subsagittal (Fig. 74 s) continue to grow out and become arched, while on
their upper parts new swimming plates are formed, which at first are
placed at right angles to the meridional vessels, but later (corresponding
to the conditions of the adult) are placed with their bases in the direction
of the meridional vessel (Fig. 74 A and B). The ends of the meridional
vessels and of the processes of the gastral vessels come together in
the lower.corners of the now trapezoidal, flattened larva (Fig. 74 B) and

154 EMBRYOLOGY

there fuse ; thus the arrangement of the vessels of the adult animal is
reached.

The metamorphosis of the Beroidce described by Allsun (No. 2) and
A. Agassiz (No. 1) takes place in an unusually simple manner. The larva
is at first round in cross-section, and later is flattened transversely. Of
t'.ie meridional vessels the subsagittal first grow the more vigorously, and
reach the edge of the mouth, where they meet two processes from the
gastral vessel of the same side, which extend along the edge of the mouth,
and fuse with them. The subtransverse vessels meet these transverse
processes only later, and then the ramifications of the vessels begin to
grow forth.

General Considerations. — The Ctenophora exhibit in
their organization a whole series of features by means of
which a closer relationship with the Cnidaria — Ccelenterata
in the narrowest sense — appears to be established. To this
series belong, if we ignore the more superficial resemblances
of the gelatinous transparent body, first of all the possession
of a very similar gastrovascular system, the presence of
tentacles, the bases of which exhibit relations to the canals
of this system, the position of the ripening sexual products
in these canals, and the similar character of the eggs. In
fact, up to the present time the Ctenophora have usually
been incorporated with the Ccelenterata; Haeckel (No. 11),
indeed, with whom Chun (No. .3) concurred, conjectured
that the group of the Cladonemidfe, and particularly
Ctenaria, which belongs to this group, constituted the inter-
mediate link between the Anthomedusfe and Ctenophora.
Even though this genus presents a remarkable resemblance
to the Ctenophora in the possession of only two marginal
tentacles and corresponding cnecal pouches, in the mesoglcea
of the umbrella (tentacle-sheaths), and in the eight ex-
umbrellar nettling ridges corresponding to the ribs, still the
view that these i-esemblances were based upon true homology
has been somewhat shaken by Hartlaub (Nos. 10 and 11),
who was able to produce evidence that in the closely allied
Eleutheria the brood-cavity lying over the stomach arises as
an ectodermal invagination fi-om the cavity of the umbrella,
and therefore could not be homologized, as Haeckel main-
tained, with the infundibulum of the Ctenophora. Even
earlier than this R. Hehtwig (No. 12, p. 444) had produced

CTENOPHORA 155

important evidence which militates against the derivation
of the Ctenopliora from the comparatively highly organized
and specialized Cladonemidse.

It appears to us, however, as if it were not these difficulties
alone, but rather grounds of a more general nature, that
have been influential recently in causing several writers (R.
Hertwig, Lang, and Hatschek) to concede a more independ-
ent position to the Ctenophora. We have learned to consider
the attached polyp, a Hydra-like creature, as the ancestral
form and archetype of the Cnidaria, and believe it probable
that in this instance the radial structure has been developed
in connection with the attached mode of life, as is so
fi'equently the case. Wherever among the Cnidaria pelagic
species occur, we can easily refer them to attached forms,
from which they have descended. The medasa must there-
fore be looked upon as a modified polyp that has attained
the power of free locomotion. All these pelagic Cnidaria
have, however, as evidence that they are organisms
secondarily derived from an attached form, the following
characteristics: () the loss of the general coat of cilia and
the development of new locomotor organs depending upon
muscular action ; (2) little tendency on the pai't of the ex-
umbrellar portion of the bell to produce any organs what-
ever. This latter feature of the Cnidarian medusa is con-
nected with the original function of its apical pole as a point
of attachment and the former comparatively unexposed
and unimportant position of the ex-umbrellar side, which
corresponds to the lower surface of the cup of the polyp.

The Ctenophora do not exhibit any polypoid stage in their
ontogeny. We would not ascribe too great an importance
to the absence of this, for the ontogeny of Geryonia and
Pelagia furnishes us with an example of how quickly in an
abbreviated development just this stage is blotted out past
identification ; it is thei'efore not the circumstance that
the ontogeny of the Ctenophora contains no indication of an
attached stage, but rather the existence of certain prominent
features of organization in the Ctenophora, which makes it
seem to us probable that an attached stage has never been
interpolated in their series of ancestors. A system depend-

156

EMBRYOLOGY

ing upon ciliary motion here functions as the principal
locomotor apparatus. This primitive form of motion ac-
quires in this case an importance and a development such
as exists nowhere else in the animal kingdom, whereas in
the Cnidaria it is not so prominent. The presence of the
sensory organ at the apical pole, which is, perhaps, to be
explained as tlie central point of the nervous system, makes
it seem unlikely that in any ancestral form there existed
at this point a region of separation, a cicatrized place of
attachment. Furthermore the abundance of organs on the
outer surface of the body (which would correspond to the
ex-umbrella) argues against direct relationships between
Medusae and Ctenophora.

From what precedes we must conclude as most probable
that the Ctenophora represent an independent stem of the
animal kingdom, which is connected with the Cnidaria
(Coelenterata in the restricted sense) only at its roots, and
has in common with them only those ancestral forms which
preceded the passage and metamorphosis into the polyp
form. The Ctenophora have most probably always retained
the original pelagic mode of life, and have brought to the
highest state of development the likewise primitive form of
motion by means of cilia, without exchanging it for the
secondary kind of movement by means of muscular action.
If we were to form a picture of the hypothetical pelagic
ancestral form of the Ctenophora, it would probably corre-
spond most nearly to certain Actinian larvce, which exhibit a
tuft of cilia at the anterior end and the mouth-opening at
the posterior pole, while within, the development of the
gastral pouches has already begun by the formation of
septa. The tuft of cilia at the anterior end of the body
would furnish the starting-point for the development of the
apical sensory organ, while the development of the ribs
would have advanced hand in hand with the further develop-
ment of the gastral pouches.

If, then, we admit that the Ctenophora and Cnidaria have
a common stem only at their very beginnings, the question
arises how far the Ctenophora exhibit relationships to the
hypothetical ancestral form of the Bilateria. Apparently

CTENOPHORA 157

the assumption of such relations cannot be altogether re-
jected. The similar position of the central nervous system
at the anterior pole of the body in the Ctenophora and
many worm larvaj, the production of the mesoderm as a
sepai^ate germ-layer in the form of four mesodermal bands
arranged crosswise, and the high state of development of
the mesenchymatous tissue, appear to argue for such an
assumption. First of all, there are, as we shall see, many
features agreeing with the development of the Turbellaria.
Accordingly there do seem to exist certain relationships
between the Ctenophora and the hypothetical ancestral form
of the Bilateria. ^Nevertheless we hesitate for many
reasons to imagine the latter to be precisely a Ctenophore.
In contrast to the Turbellaria, which by the retention of a
uniform coat of cilia recall primitive conditions, the
Ctenophora represent a side branch of the phylogenetic tree,
which has developed independently along one line, but
which scarcely furnished the basis for the direct develop-
ment of higher animal forms.

In the remarkable forms Cceloplana Metschnikowii and Ctenoplana
Koicalevskii, forms directly intermediate between the Ctenophora and
Turbellaria were thought to have been recognized (Nos. 13 and 15).
However, to us they appear to present only peculiarities that can readily
be explained from the typical structure of a Ctenophore as the result of
adaptation to a creeping mode of life. The similarity to the Turbellaria
would then rest upon mere analogy. Such an explanation is admissible,
for even among the true Ctenophora some forms have the power of
adhering to firm surfaces and of creeping about by means of the broad-
ened foot-like margins of the mouth (Lampetia), so that the starting-
point is here given for development in this direction. The fact that
with the degeneration of the ribs (ciliate bands) the general ciliation
secondarily came again into prominence ought not to be very surprising,
for Chun and K. Hertwig have shown that remnants of a general ciliation
are retained in the adult condition of the Ctenophora also.

It should be mentioned that in the origin of the four entodermal sacs,
in the presence of the four mesodermal bands, in the development of the
ribs on four adradially placed ectodermal thickenings, etc., there is mani-
fested a distinct tendency to express its quadriradial structure. Probably
the biradial structure of the Ctenophora has been developed from the
quadriradial by the different development of each pair of opposite radii,
so that the biradial structure does not represent the simplest condition
of the radial type, but corresponds to a derived condition.

158 KM BRYOLOGY

Literature.

1. Agassiz, a. Embryology of the Ctenophora. 3Iem. Amer. Acad.

Arts and Sciences. Vol. x. Cambridge. 1874.

2. Allman, G. J. Contribution to our Knowledge of the Structure and

Development of the Beroida;. Proc. Roy. Soc. Edinburgh. Vol.
iv. 18(52.

3. Chun, C. Die Ctenophoren des Golfes von Neapel. Fauna und

Flora des Golfes von Neapel. I. Leipzig. 1880.

4. Glaus, C. Ueber Deiopea kaloktenota Chun, nebst Bemerkungen

liber die Architektonik der Rippenquallen. Arbeiten Zool. Inst.
Wien. Bd. vii. 1886.

5. Fewkes, J. W. Notes on AcaleiAs of the Tortugas. Bull. Mus.

Comp. Zool. Harvard Coll , Cambridge. Vol. ix. 1883.

6. Fewkes, J. W. On the Acalephs of the East Coast of New England.

Bull. Mus. Comp. Zool. Harvard Coll., Cambridge. Vol. ix.
1883.

7. FoL, H. Ein Beitrag zur Entwicklungsgeschichte einiger Rippen-

quallen. Med. Inaug. Diss. Berlin. 18G9.

8. Gegenbaur, C. Studien iiberOrganis. und System der Ctenophoren.

Arch.Nuturg. Jahrg. xxii., Jid. i. 1856.

9. Haeckel, E. Ursprung und Stammesverwandtschaft der Cteno-

phoren. Sitznngsber. Jma. Gesellsch. Med. und Nat. 1879.

10. Haktlaub, C. Bau der Eleutheria. Zool. Anzeiger. Jahrg. ix.

1886.

11. Haetlaub, C. Zur Kenntniss der Cladonemiden (II. vorl. Mitth ).

Zool. Anzeiger. Jahrg. x., p. 651. 1887.

12. Heetwig, Ri. Ueber den Bau der Ctenophoren. Jena. Zeitschr.

Bd. xiv. 1880.

13. KoROTNEFF, A. Ctenoplana Kowalevskii. Zeitschr. wiss. Zool.

Bd. xliii. 1886.
4. KowALEVSKY, A. Entwicklungsgeschichtc der Rippenquallen. Mem.
Acad. St. Petersbourg ser. 7. Tom. x. 1866.

15. KowALEVSKY, A. Coeloplana Metschnikowii. 3Iem. Roy. Soc.

Friends of Nat. Sci., Anthropol., etc. Moscow. 1882 (Russian).
See Zool. Anzeiger, 1880, Jahrg. iii., p. 140.

16. Metschnikoff, E. Vergl. embryologische Studien. IV. Ueber die

Gastrulation und Mesodermbildung der Ctenophoren. Zeitschr.
loiss. Zool. Bd. xlii. 1885.

17. Semper, C. Entwicklung der Eucharis multicornis. Zeitschr. wiss.

Zool. Bd. ix. 1858.

CHAPTER lY.
PLATHELMINTHES.
I. TURBELLARIA.

Systematic : A. Dendroc(elid.e, with, branched intestine.

(a) Polycladida, with a median chief
intestine, which gives off numerous
branches.

(b) Tricladida, without chief intes-

tine ; three intestinal branches
are directly attached to the
pharynx.
B. Rhabdoccelid^, with straight unbranched
intestine or without intestine.

(a) Rhabdoccela, with a spacious cavity

in the region of the intestine.

(b) Alloiocoela, the cavity in the in-

testinal region reduced by the
great development of the paren-
chymatous tissue.

(c) Acoela, without distinct intestine.

The Tarbellaria which inhabit the land and fi-esh water
(Tricladida and Rhabdocoela) as well as many marine forms
(Polycladida) have a direct development, whereas other Poly-
clads undergo a metamorphosis in which there is a free-
swimming ciliated larva. The development best known is
that of the Polyclads, and of these we will first consider the
forms which develop directly. Closely related to these
Polyclads are those with a metamorphosis, for in the latter
the embryonic development proceeds in much the same way
as in the former. The embryonic development of the
Triclads, on the contrary, is different, while that of the

159

160 EMBRYOLOGY

Rhabdocceles in turn resembles that of the Poljclads ; the
Rhabdocoeles are, however, like the Triclads in the produc-
tion of yolk cells.

I. POLYCLADIDA.

A. Direct Development.

The development of the Polyclads has been described,
chiefly in the works of Goette (No. 3), Hallez (No. 6),
Selenka (No. 20), and Lang (No. 13).

The eggs, united by means of a slimy secretion, are usually
laid in the form of a unilaminar plate, in which they lie
more or less regularly side by side. In the Euryleptidce they
are attached to some support by means of a stalk (Selexka,
Lang). Ordinarily each egg is surrounded by a thin shell,
which in some cases (Fseudoceridse) is provided with an
operculum. Fertilization, which sometimes takes place after
oviposition, is usually preceded by the formation of the two
polar globules. These do not sepai-ate at once from the egg.,
but remain united to it by means of yolk-substance. The
spermatozoon then passes between them in penetrating into
the egg- Such is the process in ThysanozoiJn, according to
Selenka's observations. Since only one spermatozoon is
bestowed upon each egg, the act of fertilization in this
instance seems always to be accomplished with gi-eat cer-
tainty.

Cleavage is unequal. Even the first two blastopieres are
of different sizes. Each of them divides into two, and
these four blastomeres also differ in size. Owing to their
differences in position and size, the various regions of the
body of the embryo, it is said, are already indicated. At
first the two smallest blastomeres lie crosswise over the
larger ones (Fig. 75 A). They indicate the upper, aboral
pole, a conclusion which is confirmed by the polar globules,
since these lie above them, whereas the two large blasto-
meres correspond to the lower, oral pole. Furthermore it
is shown that even thus early the anterior end of the animal
is indicated by the smaller of the two large blastomeres,
the posterior end by the larger one, and that the two
smallest blastomeres correspond to its sides.

PLATHELMIM'HES 161

After the four blastomeres have arranged themselves in
one plane, a small cell buds forth at the upper [aboral] part
of each cell. In this manner four cells arise, from which
subsequently the entire ectodei'm takes its origin (Fig. 75 B).
As soon as the four primitive ectoderm cells have come close
together, again four cells, the primitive mesoderm cells, are
budded off at the aboral pole of the large blastomeres. These
cells lie in such a position that thej are not covered by the
ectoderm cells (Fig. 75 G). The ectoderm cells then increase
to the number of eight. Four additional mesoderm cells have
meantime been constricted off from the large blastomex-es,
and the four already present have divided into eight. Ecto-
derm and mesoderm in the form of a cap overlie the four
large blastomeres, which from now on must be considered
as entodei-m (Fig. 75 D). At the lower pole of these four
primitive entoderm cells four smaller entoderm cells are
constricted off, a process which is repeated, and in the same
manner, at the upper pole (Fig. 75 E). We will state at this
point that it is the upper and lower entoderm cells which
supply the intestinal epithelium, whereas the large middle
ones constitute a kind of food-yolk and soon disintegrate
(Fig. 76 A and B). Even before the division of the primi-
tive entoderm cells has taken place, the cells of the ecto-
derm have considerably increased in number. They move
downward and begin to grow over the mesoderm cells.
Fig. 75 JB and F show these conditions in a diagi-ammatic
way. The further growth of the ectoderm now proceeds
rapidly, and the entoderm and mesoderm are soon entirely
covered by it. The formation of the epibolic gastrula is
herewith completed. The ectoderm becomes covered with a
dense coat of short cilia, and the embryo begins to rotate in
the egg-shell.

We have represented the cleavage as Lang figures it for Liscocelis
tigrina. Although differmg in details, it still agrees on the whole with
the processes as they have been described for other Polijclads {Lcpio-
plana, Eurylepta) by Hallez and Selenka. The differences relate to the
formation of the mesoderm and entoderm. As regards the former, four
mesoderm cells are constricted off only once from the large blastomeres ;
and these by division then give rise to the mesoderm. According to
K. H. E. M

162

EMBRYOLOGY

Selenka, entodprm cells are formed at the lower pole only of the large
blastomeres. According to Goette (in Stijlochus), such a differentiation
of the mesoderm as is described by other authors does not take place.
The cells, which in his figures seem to correspond to the mesoderm cells,
he considers as ectoderm. According to Goette, Stylochus, in which
the development of a mesoderm has not yet taken place, represents a

Me*-

^^S^o-^

Fig. 75.-/1 to F, cleavage stages of tVie ep^s of Polyclads (after Lang). A, dia-
gram of a stage of four blastomeres, of which the two larger ones correspond to
the anterior (d) and posterior (/i) parts of the body, the smaller ones, lying above
them, to the sides of the body; Kto D, more advanced stages of Difcocelii' t!i]i-ina,
B and C seen from above, D from the side ; F, later stage of the epibolic
gastrula of T!ii/sniiozoiiji Bvocliii, seen from the side. Ec, ectoderm; En, ento-
derm; 0, Knand ii. Eii, upper and lower entoderm ; Jtfcs, mesoderm.

more primitive condition than the rest of the Pohjdadida. This con-
dition follows as a result of the shape of the entoderm. The central
entoderm cells do not in this case become food-substance, but with the
others form the wall of the intestine (Fig. 70 C). Everything enclosed
by ectoderm Goette looks upon as entoderm. Only after a part of the

PLATHELMINTHES

163

intestine had become useless, owing to the metamorphosis of a portion of
the entoderm cells into food-substance, and another part had been com-
pelled to move into its place, could this process give rise to a distinct
mesoderm.

The further development of the embryo of Biscocelis con-
sists first of a complete overgrowth on the part of the
ectoderm and the resulting closure of the blastopore. The
elements of the ectoderm become more like an epithelium,

Mes-4

Fig. 76. — A to C (after A. Lang). A and -B, embryos of Biscocelis tiqrina, seen
from the ventral side ; C, median longitudinal section through Goette's larva of
Stylochiis filidimn. Ec, ectoderm ; En, remains of the entodermal cells in process
of disintegration ; E.r, fundament of the excretory organ (?) ; D, branches of the
intestine and (in C) intestinal epithelium ; Mcs, mesoderm ; N, fundament of the
central nervous system.

their cilia stronger and more dense. A change in the outer
form now takes place, the aboral pole being pushed forward,
the oral backward. The anterior end is distinguished by
the appearance of the first pair of eyes, which here arise as

164 EMBRYOLOGY

small pigment spots (Fig. 76 A). Under tliem the brain is
established somewhat later in the form of two club-shaped
bodies (Fig. 7G B). These bodies develop as ectodermal
thickenings, which afterwards sink deeper and by means of
a broad commissure unite into the common mass which they
constitute in the adult. The two longitudinal nerve-trunks
arise from them by means of a backward gi'owth. Two cell-
growths, which perhaps are to be explained as parts of the
water- vascular system, make their appearance as ectodermal
structures in the posterior portion of the ellipsoidal embryo
(Fig. 76 A and B, Ex).

The development of the intestine takes place b}- the
abundant multiplication of the upper and lower entodermal
cells. In Fig. 76 A the embryo appears filled with the
mass of central entoderm cells which have been metamor-
phosed into food-yolk. The small entoderm cells are dis-
tributed over the surface of these ; they penetrate between
the yolk-spheres, the substance of which they dissolve, and
are finally converted into intestinal epithelium. This takes
place in the following manner : scattered -entoderm cells
surround a mass of yolk which has become reduced in size
by disintegration, and, as they begin to absoi^b this, form a
short tube, which unites with other intestinal cavities that
have arisen in the same way (Selexea). When, finally, the
intestine, with its branches, has arisen in this manner, the
embryo acquires the general appearance of the adult worm
(Fig. 76 B). The mouth arises at the place of the pre-
existing blastopore from an invagination of the ectoderm,
which fuses with the wall of the intestine. Fig. 7Q C shows
this condition in SfyJochns. The ectoderm supplies the
epithelial lining of the pharynx and pharyngeal pocket, the
musculature of which arises from the mesodermal elements
that are found massed in large numbers in the region of the
invagination (Fig. 76 C).

According to Hallez, as well as Selenka, the mesoderm
continues its development from its earliest beginnings by
the outgrowth of tlie primitive mesoderm cells into four
mesodermal bands j)laced crosswise ; these fuse with one
another as soon as their cells become moi'e numerous, and

PLATHELMINTHES 165

then lie under tlie ectoderm like a calotte. According to
Lang's description also, there is formed from the four groups
of mesoderm cells a continuous layer, which attains to a
greater extension on the ventral side than on the doi'sal
(Fig. 76 C). Only later does the mesoderm give rise to the
musculature of the body-wall and the connective-tissue
reticulum. By the formation of vertical mesodermal septa,
which advance from the periphery towards the median plane,
the branches of the intestine increase in length at the
expense of the central yolk-mass. New septa, which
encroach on these branches from the margin of the body,
split them up into secondary branches, so that the intestine
increases the number of its ramifications.

When, finally, the greater part of the food-yolk has been
consumed, and the previously ellipsoidal embryo has under-
gone a flattening in the dorso-ventral direction, it breaks
through the egg-membrane and reaches the outside world
as a young Turbellarian.

B. Indirect Development.

The embryonic development takes place in a manner
similar to that of the forms without metamorphosis. There-
fore even in what has preceded we might, in a number of
instances, have considered forms with indirect development.
But, instead of developing into tui'bellarian-like foi-ms, the
ovate embryo of this type acquires lobe-like processes (Fig.
77). They arise first by an elongation of the ectodermal
cells at the points affected, and then by an evagination of
the ectoderm. The typical larval form of the Turbellaria,
which arises in this way, is represented by Muller's larva,
as it has been named after its discoverer (Nos. 17 and
18). This larva (Fig. 77) possesses eight processes, three of
which are situated in the region of the mouth, two others
laterally, and three dorsally. They are provided at their
margins with a border of longer cilia. If these ciliated
appendages are to be compared to the ciliated bands of other
larva?, they would have to be designated as the preoral
ciliated band, by means of which an oral area is separated

166

EMBRYOLOGY

from an aboral one. The ejes, as well as the fundament
of the nervous system lying under them, are present in the
anterior, dorsal part of the body. Behind the middle, ven-
tral appendage, the pharynx is already seen (Fig. 77).
The intestine also is already established, and appears
branched ; in short, the internal organization of the larva
corresponds nearly to that vv'ith which we became acquainted
in the recently hatched embryo of Discocelis.

The larvae circle around in the water by the aid of their
cilia, revolving upon themselves in various directions. The

older, more elongated larva?, on the
contrary, are always seen swim-
ming with the anterior part of the
body directed upwards. They ro-
tate around the long axis only.

After the larva? have swarmed
for a considerable time, they ex-
change their primitive ovate form
for a mox-e and more elongated one.
Fig. 77 (probably a larva of Thysa-
nozoun) represents a stage older
than the embryos just hatched
fi'om the egg, which are more com-
pact. The elongation of the body
is accompanied by a broadening
of the anterior and a narrowing of
the posterior end (Fig. 78 A). It can be recognized from
Fig. 78 A that, in spite of the presence of the larval appen-
dages, the form of the worm is already expressed. This is
still more the case in the stage represented by Fig. 78 B, in
which the larval appendages are rapidly degenerating.
These finally disai)pear altogether, and the form which
characterizes the adult animal is reached by the gradual
completion of the internal organization, the increase in the
number of eyes, the outgrowth of the nervous system into
the longitudinal nerve-trunks, the differentiation of the
pharyngeal appaiutus and the rest of the muscular system
from the mesodei-m, and the development of the intestine,
with its branches.

Fig. 77. — Mullek's larva seen
from the ventral side (after Joh.
MuLLER, from Balfour's Codi-
parative Emhi-ijology). Theheavy
line indicates the ciliated band.
m, mouth; u.l, the so-called
upper lip.

PLATHELMINTHES

167

Somewhat different from Mullee's larva, though still derivable from
it, is that of Oliriocladus auritus (Fig. 79), which has been carefully
studied by Hallez, Like Mullee's larva, it also possesses eight lobe-
like processes, two of which, however, the median ventral and dorsal
ones, have moved far forward. The first one, situated in front of the

B

f—n

Fig. 78. — A and B, larvaj of Tungia nurantinca (after A. Lang) about to change
into the worm, seen from the ventral surface. The eyes are indicated for the
sake of better orientation.

mouth, attains a considerable size, so that the anterior end appears
spread out like an umbrella. Behind, as in Mullee's larva, there are
two ventral, two lateral, and two dorsal appendages surrounding the
larva. Rigid cilia at the anterior and posterior ends of the body give a
characteristic appearance to the larva.

Goette's larva of Stylochus inlidium resembles Mullee's larva less.
In it (if we make use of the expressions employed for Mullee's larva)
the two lobes situated at the sides of the mouth-opening are esi^ecially
well developed (Fig. 80). The lobe lying in front of the mouth, on the
contrary, is less developed, as is also the middle dorsal one. Other
appendages are wanting. Inasmuch as the back is arched, this part
assumes a bell-shaped appearance, and the larva acquires a resemblance
to a Nemertean pilidium, which is increased by the occurrence of rigid
cilia. The apex is marked by the dorsal lobe (Fig. 80). The larva in

168

EMBRYOLOGY

this figure (Fig. 80) has a different orientation from that of the other
turbellarian larviv, in order to bring out better its resemblance to the
pilidium larva. Its discoverer, Goette, also compares it directlj' to the

pilidium. If we consider that Stylo-
cJius has a simpler course of deve-
lopment (see supra, the absence of
nutritive yolk), then it appears not
imj)ossible that the larva of Stijlochus
represents a primitive condition, a
lower larval form, which perhaps still
has relationships to the larval forms
of the Nemerteaiis. The fact that
Mullee's larva also presents a similar
form at a certain stage is an argument
in support of this view. Mvllek's
larva itself would then represent a
more highly developed form. Lang,
to be sure, believes that Stylodnis
simjoly leaves the egg at an earlier
stage, arriving at the condition of
Muller's larva only during its free
existence, whereas Goette maintains
that it is developed directly into the
adult animal by an increase in length.

The larva of Sti/lochojisis ponticus described by Metschnikoff ' also

Fig. 79. — Larva of Oligoclndus
aitritus, Lang {Earylepta nuriculnta.
Clap.), seen from the side (after
Hallez, from Balfour's Compara-
tive Embryology).

Fig. 80.— Larva of Stylorhnx luUdiuni seen from the side (after Gobtte). D, in-
testine ; En, remains of the entoderm cells ; S, pharynx.

' This work by Metschnikoff, published in a Russian periodical,—
" Studies on the Development of the Planariio," BIcmuirs of the Nco-

PLATHELMINTHES 169

seems to resemble Goette's larva. It is said also to resemble the pili-
dinm in form.

Quite different from the larval forms hitherto considered is a planarian
larva found by A. Agassiz, which he
ascribes to Planaria angidata. This
larva, in which a branched intestine is
already present, shows a distinct exter-
nal segmentation corresponding to the
lateral branches of the intestine (Fig.
81). At first the body is cylindrical ;
it is only in the course of further deve- F''^' ^l- -Larva of Planaria an-

, /,,,.,, „ ,, , -, giilata (?) (after A. Agassiz, from

lopment that it becomes flattened and balfouk's Compayative Emhryo-
takes on the form of a turbellarian. logy).
Unfortunately a confirmation of Agas-
siz's short communication has not yet appeared.

II. Tricladida.

The difference between the development of the fresh-
water Bendroccelida (Triclads) and that of the Polycladida is
to be explained by the fact that it takes place under alto-
gether different conditions. In the cocoons laid by fresh-
water Dendrocoeles, which are disproportionately large as
compared with the size of the animal, there is found, in
addition to the egg-cell, a large number of yolk-cells.
According to Metschnikoff (No. 15), the proportion of the
two kinds of cells in Planaria polychroa is such that thei*e
are four to six egg-cells to about ten thousand yolk-cells. In
Dendroccelum lacteum, on the other hand, twenty to forty
egg-cells are present in one cocoon (Iijima, No. 8; Hallez,
No. 7). The yolk-cells surround the egg-cells in a radial
arrangement, and fill the remaining space of the cocoon.
They are able to move like amoebae by sending out pseudo-
podia.

As soon as the first stages of cleavage have taken place in
the egg-shell (Figs. 82 and 83), this remarkable phenomenon
occurs : the blastomeres do not remain united, but move far

Russian Society of Naturalists, vol. v. (Odessa), 1887, — was unfortunately
inaccessible to us, as was also one by Salensky : " The Development of
Enterostomum," Proceedings of the Society of N aturalists at Kasan, 1872-
73 (see Leuckaet, Jahresber. Arch. f. Naturg., Jahrg. xl., Bd. ii., 1874).

170

EMBRYOLOGY

apart (Figs. 83 and 84). Thej lie quite isolated, as if they had
no sort of relation to one another, as is seen, for example, in the
thirteen-cell stage of Dendrocxhim (Fig. 84). One would be
inclined to think of this as an abuormalitj if the observations
of Metschnikoff, Iijima, and Hallez did not entirely agree

on this point. The subsequent de-
velopment likewise proceeds in a
manner altogether original, its pecu-
liarities being evidently a result of
the large amount of yolk-substance
which must be taken up by the
embryo.

In the further course of develop-
ment some of the yolk-cells are dis-
solved, so that the embryo now lies
in a finely gi^anular protoplasmic
mass, in which some of the nuclei of
the yolk-cells can still be recognized (Fig. 84). The division
of the blastomeres continues, and as a result of it there is
produced a spherical heap of from seventy to eighty irregu-

Fig. 82.— Cleavage stage
of two blastomeres (Be),
with the surrounding
yolk-cells (Dz), of Dendro-
cceliiiii Incteuia (after
Iijima).

UJ^°'h

Bz^L

Figs. 83 and 8i.— Cleavage stages of Deadfoccelum lactciim (after Hallez). In
one stage four blastomeres {Be), in the other thirteen, with surrounding yolk-cells
{Dz), which in the later stage are partly fused into a common mass. Their nuclei
(shaded dark) are ttill visible in this mass.

larly arranged cells. Changes are soon manifested in this,
which result in the establishment of the germ-layers. Some
of the peripheral embryonic cells move to the edge of the

PLATHELMINTHES 171

surrounding homogeneous food-mass, and by becoming flat-
tened out and uniting with one another form there a thin
membrane [ectoderm]. Later a small group of cells in the
mass of loosely associated embryonic elements becomes dis-
tinguishable by its presenting a moi-e compact arrangement.
This spherical group of cells at first lies in the middle of the
embryonic mass, but later moves to the periphery. Here it
unites with the ectoderm. It then becomes hollow, and its
cells are differentiated into various layers, thus forming the
provisional organ known as the embryonal pharynx (Fig.
85 A). Four cells, which enclose a small space, are applied
to the inner end of this pharynx. According to Hallez, these
constitute the earliest fundament of the intestine (Fig. 85 J.).
The fundaments of the pharynx and intestine would be
looked upon, then, as entodermal, but the migratory cells
which remain between ectoderm and entoderm could not be
designated as mesoderm, for later, according to Hallez,
ectodermal as well as entodermal elements arise from them.
These migratory cells contribute at first to the formation of
the musculature of the embryonal pharynx, becoming elon-
gated and spindle-like, and being applied to its outer side.

The significance of the pharynx, which now begins to
execute swallowing movements, consists merely in its trans-
porting the yolk-cells into the embryo (Fig. 85 B). As soon
as the pharynx begins to function, the intestine becomes
rapidly filled with yolk-cells, which cause a great distension
of the intestine and the entire embryo. The inconsiderable
entoderm and likewise the ectoderm become stretched to
an extraordinary degree, so that they can be recognized only
with difiiculty. In order to prevent a bursting of these thin
layers, cells derived from the migratory elements unite with
them. Metschnikoff's statement that yolk-cells which have
migi^ated in from the outside are converted into the epithe-
lium of the intestine is not corroborated by Hallez. Accord-
ing to this observer, the primitive entoderm always forms a
wall, although a very delicate one, bounding the parenchy-
matous tissue of the embryo. This entoderm, to be sure, is
said to be of a provisional nature only. It disappears sub-
sequently, and the adjoining migratoiy cells unite to form

172

EMBRYOLOGY

the intestinal Avall. Immediately before the secondary for-
mation of the intestine takes place, the embryo would be to
a certain extent in the condition of the accclous Tarbellaria,
in which the food-bodies pass directly into the body paren-
chyma. To be sure, an intestinal cavity would exist in the
embryos, but it would be limited by tlie body parenchyma.
Should these observations be confii^med, they might perhaps
throw some light on the establishment of the conditions
which exist in the Acrela.

A B

Fig. 85.— Sections through embryos of Dnidi-oca'liim lacienm (somewhat diaffram-
matie, after Hai.lkz). Ec, ectoderm ; En, entoderm ; Dz, yolk-cells ; Ph', provisional
embryonal pharynx and (in Fig. C, Ph") permanent jiharynx ; IF:, migratory
cells.

Tlie branched form of the intestine of Triclads arises in a
similar way to that of the Polyclads, i.e., by the ingrowth of
connective-tissue septa from the periphery toward the middle
line. This tissue, like the body musculature, owes its origin
to the migratory cells, from which the sexual organs likewise
arise (Iijima).

The fundament of the nervous system was found by the three authors
mentioned lying deep in the body tissues, and they could not discover
tliat it had any connection with the ectoderm. If the statement of

PLATHELMINTHES 173

Hallez proves to be true, that portions of the migratory cells are
employed even after this in the formation of the ectodermal layer,
then such a mode of formation of the nervous system could be more
readily referred to the ectodermal method of origin, which was met with
in the Polycladida. However, it is not to be denied that the earliest
appearance of the nervous system in the Trieladida points to a meso-
dermal mode of origin, such as was attributed to it by the brothers
Hertwig (Cixlomtheoiie) even at the time they wrote. Recently, too, in
the related Nemerteans, the nervous system has been derived from the
mesoderm (Hubrecht).

When the embi^onal pharynx has fulfilled its function, the
provisional mouth- opening closes, the pharynx degenerates,
and an irregular heap of cells lies in its place. In this a
cavity then arises, the cellular lining of which represents the
internal epithelium of the pharyngeal pocket, for the per-
manent pharynx is formed at the same spot. This thei'efore
arises, as it seems, from the entoderm (or mesoderm), whereas
in the Poly dads an invagination of the ectoderm gives rise to
its formation. The cylindrical form of the pharynx is due
to the fact that the surrounding cells take part in its forma-
tion. Before the pharynx attains its final shape, the union
with the lumen of the intestine takes place, and later the
mouth-opening also breaks through to the exterior.

During the developmental processes described, the embryo
has frequently changed its form. At first ovate, it becomes
spherical after the introduction of the yolk-cells ; then at the
time of the formation of the permanent pharynx it again
elongates and becomes flattened on the ventral side (Fig. 85 C).
The pointed portion corresponds to the anterior part of the
body.

III. Rhabdoccelid^.

The development of the rhabdocoelous Turbellaria is still
the least known. Various forms belonging to the genera
Prorhynchtis, Prostomum, Mesosto7nuin, Schizostomum, and
Macrostomum were studied by Hallez (No. 6) in some stages
of development, but he studied only the winter eggs. These
eggs, which are surrounded by a firm capsule, are attached
to aquatic plants by means of a mucilaginous secretion. In
many forms {Prostomum lineare and P. Steenstrupii) the cap-

174

EMBRYOLOGY

sule is drawn out into a stalk, by means of which it adheres
to fixed objects (Fig. 86), in much the same waj^ as in
the fresh-water Dendrocceles. In each capsule there is
usually found only one egg-cell, in rare cases {Prostomum
Steenstrupii) two of them. As in the fresh- water Dendro-
cceles, the egg-cells occupy only a small part of the capsule,
the remaining space being filled with yolk-cells (Fig. ^Q).

In spite of the presence of the yolk-
cells, development proceeds in a manner
similar to that of the Polycladida. Per-
haps when the development of the
Ehahdoccelidcs becomes more accurately
known intermediate conditions will be
found here, which will explain the
aberrant condition of the Tricladida.

After the expulsion of the polar
globules and fertilization, the egg di-
vides first into two, then into four,
blastomeres of equal size. Four
smaller ones are constricted off from
these (Salensky). The further cleav-
age processes could not be observed
by Hallez ; their result, however, is
an epibolic gastrula, which entirely
resembles that of the Polycladida. The
ectoderm becomes covered with cilia, and the embryo floats
in the mass of yolk-cells. It is therefore equivalent to
a larva, which, however, does not attain to a wholly free
life, just as the larvae of the Gnathohdellidoi and Oligochcetce
live within the cocoon, and are nourished by the albumen
occurring in it.

In a later stage of the embryo the entoderm is seen to be
arranged in a continuous layer. Its cavity becomes connected
with the outer world by means of the pharynx. It appears
as if it were, as in the Tricladida, of entodermal nature. The
yolk-cells are conveyed by means of it into the intestine.
Yet the pharynx of the Rhahdocoelidce, contrary to that of the
Tricladida, at once reaches its permanent form. The
primitively spherical embryo by becoming elongated and

Fig. 8G.— Stalked egg
capsule of Prostomum
Steenstrupii with two egg-
cells (Ez) and surrounding
yolk-cells (after Hallez),

PLATHELMINTHES 175

flattened assumes the form of a flatworm. In Prostomum
lineare an invagination of the ectoderm at the anterior end
of the body gives rise to the pharyngeal sheath and the
pharynx.^

1 [The development of the Khabdoccela and Aecela has recently been
studied by Peeeyaslawzewa (see Appendix to Literature on Turbellaria,
Nos. I. and II.).

According to this author, the development is the same in all the Aecela
studied by her — Convoluta paradoxa, Aphanostoma di versicolor, Aph.
pulchella, and Darwinia variabilis — so that one description, that of Aph.
diversicolor, answers for all. The formation of polar cells and fecunda-
tion occur before the eggs are laid. The first cleavage results in two
cells of equal size. Preisaratory to the second cleavage the nuclei elon-
gate, and approach the side opposite that where the polar cells have taken
refuge. With the second cleavage each of the two cells is divided into
two, one of which is four times as large as the other. The four cells
finally assume a symmetrical arrangement around the chief axis, the two
planes of cleavage becoming mutually perpendicular. The two small cells
now divide, producing four of equal size arranged in a cross at the upper
pole. Then the two large cells divide into unequal parts, a larger basal
cell and a smaller one, lying nearer the plane of the four micromeres.
This eight-cell stage is a true blastula with cleavage cavity, and is quickly
followed by the division of the two basal cells, from which result four
basal cells of equal size (ten-cell stage), arranged not in a cross, but in a
row, which determines the long secondary axis. Two of the four basal
cells, the middle ones, are so crowded at their superficial ends by their
mates that they become wedge-shaped and finally forced into the cleavage
cavity. Thus gastrulation begins as a true emboly, but it is comiDleted by
a process of epiboly. During invagination nearly all the pigment gra-
nules are accumulated in the lower ends of the four basal cells, which are
then abstricted as four small dark cells. Somewhat prior to this, how-
ever, the two cells lying a little below the plane of the four micromeres
divide, and their iDroducts are arranged symmetrically (on either side of
the plane determined by the chief axis and the long secondary axis). The
eight cells of the upper half of the egg now divide, producing sixteen.
With this (twenty-four-cell) stage gastrulation is well advanced, but it is
completed in the following stages by the overgrowth of the products of the
sixteen micromeres. "The two lateral cells, having given rise to the two
small (?) primitive-entoderm cells, represent the third layer : the meso-
derm." We understand the author to mean by " the two lateral cells " the
two cells which constitute the end of the row of four basal cells, but the
account is not satisfactory. A small but well-marked archenteron, com-
municating with the outside by means of a blastopore, exists from the stage
when the entoderm consists of only two cells, which assume a concavo-

176 EMBRYOLOGY

General Considerations.

In considering the development of the Turbellaria, the first thing to
attract attention is the radial structure of the embryonic fundament : the
four large blastomeres from which, above and below, the entoderm cells
have separated, the radially arranged ectoderm cells, but especially the
four groups of mesoderm cells. This condition points to the affiliation
of the Turbellaria with radially constructed animals, a relationship which,
in fact, has been advocated, either on anatomical or embryological grounds,
by numerous writers (Kowalevsky, Selenka, Lang, Chun, Goette). An
attempt has been made to trace the Turbellaria back to the Ctenophora.

convex shape and enclose the archenteric fissure between them. Before the
overgrowth of ectoderm is completed the two entoderm cells and the two
mesoderm cells have each divided into two. The blastopore shifts (about
ninety degrees) from the basal side to one end of the sltort secondary
axis, and becomes the i^ermanent mouth. Thus the pole of the blasto-
pore, which is also that of the polar cells ('?), becomes the caudal end of
the worm. At the opposite pole, which protrudes somewhat, a bundle
of clear cells represents the first trace of the frontal organ, while two
slight depressions, symmetrically placed and of short duration, are ac-
companied by i^ermanent thickenings of the ectoderm, which constitute
the fundament of the nervous system. A marked feature of the early
stages is the bilateral symmetry, which is noticeable from near the
beginning of cleavage up to the time the young become ciliated. " There
is no hint of a radial appearance in either blastula or gastrula."

The only representative of the Rhahdoccela studied by Pekeyaslawzewa
was Macrostoma histrix, in which the eggs are so opaque that it is diffi-
cult to trace the early stages. After the egg is laid it undergoes contrac-
tions which result in elevations and deep excavations of all parts of its
surface. Several (four to eight) polar cells are formed at different points
on the periphery ; these have the appearance of very hard and compact
fusiform bodies (!). The first cleavage furrow divides the egg into two
unequal cells ; the second furrow, perpendicular to the first, divides each
of these into two equal parts, so that there are now two large and two
small blastomeres. The subsequent cleavage stages were not followed
accurately, but "it is certain that the two small segments representing
the ectoderm arise from two large segments." Gastrulation is by epiboly.
It is uncertain whether the blastopore becomes the permanent mouth-
opening, or is closed, and a new mouth formed at the cephalic pole. In
later stages the embryo is more transparent. The mesoderm then exists,
it is maintained, in the form of two longitudinal bands, each of which is
separated into two layers by a fissure : the body cavity. The outer layer,
uniting with the ectoderm, forms the subcutaneous muscular layer; the
inner, uniting with the entoderm, forms the muscular layer of the in-
testinal wall. Other organs are formed as in Aphanostoma. — The

TUANSLATOKS.]

PLATHELMINTHES 177

According to the recent investigations of Metschnikoff (No. 16) on
Ctenophores, the embryonic development of this group offers some
resemblances to that of the Polyclads. The ectoderm cells are constricted
off from the four blastomeres originally present, and grow over them from
above. As in the Polyclads, four mesodermal groups are present here,
and likewise take their origin, although in a somewhat different way,
from the large blastomeres. The subsequent deportment of the meso-
dermal tissue is similar in the two groups in so far as it fills the entire
space between ectoderm and entoderm. Since among the lower forms
the Ctenophora are the only ones which present mesodermal tissue of this
kind, there is in that fact an argument for placing the Turbellaria in
relation with them.

The often-attempted comparison of the systems of organs in Ctenophora
and Turbellaria, especially that of the gastrovascular apparatus, is, how-
ever, less satisfactory. On the contrary, Lang's suggestion concerning the
position of the cilia and the manner of their motion in the Turbellarian
larvffi appears to us to be of some importance. The cilia are arranged in
regular transverse rows on the ciliated band, and all the cilia of a trans-
verse row move at the same time in a manner which quite recalls the
strokes of the swimming plates of the Ctenophora. If the ciha of a row
were to fuse with one another, says Lang, then the structure arising in
this way could not be distinguished from such a swimming plate. But
how far Lang's attempted homology of the eight ciliated lobes with the
ribs of the Ctenophora has claim to validity is very doubtful.

Possibly also the brain of the Turbellaria can be referred to the apical
plate of the Ctenophora. It has been determined embryologically by
Lang that the originally aboral pole of the embryo becomes shifted
toward the anterior end of the body. The brain then arises in that
region. If the displacement were not to take place, then the brain would
arise at the aboral pole, and would consequently have the same position
as the apical plate of the Ctenophora. Even the otocysts of the Turbel-
laria, which in certain forms [Moiiotidcc, Utoiitesostonia, according to v.
Gkaff) lie close to the brain, are perhaps to be looked upon as the re-
mains of the otocysts of the Ctenophora.

Nevertheless it is to be emphasized that the Turbellaria and Cteno-
phora, even if they proceeded from a common root, have become so much
altered that the comparison can be of only a general nature. We have
already mentioned (p. 157) that we do not ascribe to the intermediate
forms Co'loplana and Ctenophma (Nos. 10 and 11), proclaimed as uniting
links between the Ctenophora and Turbellaria, any such meaning.
Nevertheless such forms are, in our opinion, valuable in showing how the
transition from free -swimming, radial animals into creeping, bilateral
forms could have been accomiolished.

K. H. E. N

178 EMrsRYOLOGV

II. TREMATODA.

The egg of Trematodes is a product of the ovarium and the
vitellaria. The latter supply to each egg a number of cells,
whicli surround the egg-cell, and in the course of develop-
ment are consumed by the embryo. [The conditions are,
therefore, much the same as in the Tui-bellaria, and what we
call the egg in the Trematodes is a composite structure
similar to the cocoon of the Triclads and Rhabdocoeles, save
that there is only a single egg-cell and that the number of
yolk-cells is much smaller. — K.] The embryo leaves the egg
usually at a stage of development which is still far removed
from the organization of the parent. Before it reaches this
it still has to undergo a complicated process of development.

I. Distomidj:.

Tlie embryonic development has been the most
elaborately treated in the investigations of Schauinsland
(No. 8). In Distomum tereticolle the egg-cell lies at that pole
of the egg which is marked by the operculum of the egg-
shell (Fig. 87 A). The remaining pai't of the egg is formed
by the yolk-cells, whicli still show their cellular structure,
but gradually undergo degeneration. The egg-cell divides
into two cells, four cells, etc., until the germ extends over a
large part of the entire egg (Fig. 87 B and C). At the apex
of the embryonic mass a cell is soon distinguished from
the rest by its losing its spherical shape and covering the
uppermost cells like a kind of cap (Fig. 87 C, Kz). It soon
divides into two cells, which grow downward, becoming in
this way attenuated into a thin membrane. Still other cells
take part in the formation of this delicate enveloping mem-
brane as soon as the nutritive yolk is entirely displaced by
the cleavage cells (Fig. 87 J) and E). At this stage the
germ is a solid mass of cells surrounded by the enveloping
membrane, which is separated by a narrow space froni the
cell-mass (Fig. 87 E). Under the enveloping membrane is
developed around the entire circumference of the embryo a
layer of flat cells, which Scuauixsland looks upon as

PLATHELMINTHES

179

ectoderm, and which he believes has arisen, like the envelop-
ing membrane, as the result of an overgrowth coming from
one side and surrounding the cell-mass. This, then, would
be an epibolic gastrula (Fig. 87 E and-F).

The further changes of the embryo consist, in the first
place, in the gradual disappearance of the nuclei of the
ectoderm cells and transformation of the entire ectoderm into

F'G. 87. — .1 to ff, embryonic development of Distomum tereticolle (after Schauinf-
LiNB). D, intestine ; Dz, yolk-cells ; Ez, egg-cell ; Ec, ectoderm ; En, entoderm ; Hm,
enveloping membrane ; Kz, cap-cell.

a thin cuticula-like layer, on the surface of which bristle-
like structures make their appearance (Fig. 87 H). A
number of the cells of the entoderm have united for the
formation of the intestine, which fills about one half of the
body (Fig. 87 0). Other entodei-m cells are applied to the
ectodermal membrane, whereas the remaining cells, lying
between these and the intestine, retain the character of
embryonic cells. They are germ cells, from which the new
generation subsequently arises. Since in the present stage
the cells of the ectoderm, as well as those which form the

180

EMBRYOLOGY

intestine, have separated from those lying between them, the
latter can he considered as belonging to a third germ-layer,
the mesoderm.

When the embryo has reached the stacje described, it
breaks through the enveloping membrane, which has become
a delicate transparent pellicle, the operculum of the egg-shell
opens, and the embryo reaches the outside world (Fig. 87 F
and G). Here it creeps about actively, and for this purpose
it makes especial use of the proboscis. The anterior part of
the intestine has become metamorphosed into an organ of
this kind, for it can be everted and retracted. In the embryo
represented in Fig. 87 H, the proboscis, together with the
anterior end of the body, is retracted. A kind of funnel

arises in this way which is
surrounded by the cliitinous
bristles.

The embryos of other Distoviiihe
develop on the cells of the ectoderm
cilia, by means of which they swim
in the water (Figs. 88 and 89 A). The
development of an enveloping mem-
brane was observed by ScH.\riNsi-ANi>
in various genera of Diatniniche. Two
Distoinidce (D. cyliitilraceuin and D.
vientula/vvi) on leaving the egg-shell
appear to cast off the ciliated ecto-
dermal layer in addition to the en-
veloping membrane.

We shall meet with similar pro-
cesses in considering the development
of tapeworms. The significance of
this will be considered there more at
length.

Fig. 88. — EmViryo of DM.nmum
(jlohlpnytna, j)rc8se(l out of the egK
(afier ScHAumsLAXij). The ectoder-
mal cells (Ec) are partly detached ;
Hin, enveloping membrane.

The Further Course of
Development. — The distomid
larva, in order to develop further, must seek another host.
The processes which are enacted during its growth we de-
scribe first for Distommnluqiatictim, whose course of develoj)-
ment has been made known chiefly through the prolonged
investigations of Lkuckart, as well as those of Tiiuii.AS.

PLATHELMINTHES 181

The eggs of Disfommn hepaticiim are found in great numbers
in the gall-bladder of the host inhabited by the worm. From
here they pass into the intestinal canal, to be voided together
with the foeces. Their development begins only after they
are outside the host. If by chance the egg reaches the
water, then the favourable conditions for development are
present. In from three to six weeks afterwards the embryo
abandons the egg-shell (variations in the time of develop-
ment are caused by higher or lower temperatures). By
means of the cilia which thickly cover it, the embryo, or
better the larva, easily moves about in the water. It pos-
sesses an x-shaped eye-spot (Fig. 89 A). Under this lies a
ganglion. The intestine is only slightly developed. Two
ciliated funnels already represent the beginning of the ex-
cretory system. The remaining part of the body is filled
with the germ cells, the origin of which has already been
considered in treating of the embryonic development, and
the significance of which consists in the production of the
subsequent developmental stages of the Distom7cni.

In this condition the larva may swim about for as many as
eight hours ; then it perishes, unless it meets with a snail,
into whose respiratory cavity it bores. In this process
its cephalic protuberance (also interpreted as tactile organ)
is said by Thomas to render good service. Limnceus minutus
{S. tnmcatidus) is now to be regarded with certainty as the
intermediate host of Distomum hepaticicin, as the investi-
gations of Leuckakt, confirmed by Thomas, have shown.
Having arrived in the respiratory cavity or other organ
of the snail, the larva casts off its coat of cilia and secretes
about itself a cuticula-like envelope. It now grows and
becomes a sac-like body, which is called a sporoci/st (Fig. 89
B). In it the germ cells become enlarged, and by
repeatedly dividing produce the cell-masses which give rise
to a new generation. The sporocyst besides has the power
of reproducing itself by transverse division. To this end it
constricts itself in the middle of the body, and produces two
new sporocysts.

The generation produced in the sporocyst consists in turn
of creatures which are sac-like, but which are more highly

182

EMBRYOLOGY

0^v;5S/T*i,^-<.*^vy.

Fio. 80. — A to G, course of development of Distomvw licqynU'cinii (after Lkuckart) :
^, larva with e.ye-spot (^), with gaiij,'lioii lying under it and germ cells (h'2); B,
young sporocyt't, with masses of germ cells inside of it, from the respiratoiy cavity
of the snail ; C, older sporocyst, with young Rediro ; D, Redia, with Redire and
perm balls inside of it, from the liver of the sj)ail ; E, Redia, with Cercaria^
and germ balls, from the liver (jf the snail; F, free-swimming Cercaria; G,
young liver-fluke from the bile-duft of the sheep, with branching of the in-
testine already liegun. A, eye-spot ; D, intestine ; Ih-, glandular mass on either
side of the body of the Cercaria; Ex-, excretory system; G, birth aperture of
the Redia; Ar, germ cells ; N, nervous system.

PLATE ELMINTHES

183

organized than the sporocyst, since they are provided with
a mouth and an intestinal canal, and since the different
portions of the body, as well as its organs, appear to be
better differentiated (Fig. 89 C). The mouth is, in fact,
surrounded by a kind of sucker, which enables the animal to
attach itself to the organs of the host. Moreover, the
pharynx executes swallowing movements, and the intestine
appears at some times more, at other times less, filled ;
hence it is functional. This new generation has been called
by the name of Bedice (Fig. 89 D and E).

As regards the origin of the Rediae, there are two opposing views ; some
observers (Leuckart, Schwaez) trace them directly to the germ cellf,
while others (Wagener, Biehkinger) maintain their origin from cells of
the body-wall. Although Schwarz argues strongly for the one and
BiEHEiNGER for the other mode of origin, yet this difference does not
seem to us to be important, for we have already seen that the parietal
cells and the germ cells are embryologically of the same origin. In a
portion of the cells of the body-wall even, a differentiation into separate
histological elements appears not to have taken place, and for this
reason they may continue to develop in the same way as the real germ
cells. In harmony with this view is the statement of Thomas, who de-
rives the Eediffi from both the germ cells and the cells of the body-wall ;
if the supply of the former were exhausted, then the latter might take
their place.

In regard to the way in which the Redue (and later the Cercaria)
arise, Schwarz explains this process as corresponding to the cleavage of
the egg. The single germ cell divides and produces a mo/«Za-like heap
of cells, from which the Redia (or the Cercaria) arises. The germ cell
therefore corresponds to the egg, and this would thus be a case of par-
thenogenetic development (Leuckart). The entire process of development
is therefore to be considered, not as an alternation of generations s. str.
{metagenesis), but as heteniijoiuj, as already suggested by Grobben
(Literature on Cestoda, No. 4).

When the Redise have attained the proper stage of
development, they abandon the sporocyst by rupturing its
walls. They migrate from the respiratory cavity into the
other organs of the snail, especially the liver. Here they
increase in size, and there can soon be recognized in them
in turn spherical masses of germ cells, from which arise
again, if the season be cold, — that is to say, in winter — RecUce
of nearly the same form as before (Fig. 89 E). If, on the

184 EMBRYOLOGY

contrary, this sta,<?e of development happens in the wai'm
season, then creatures of another shape are developed from
the o-erm cells : the tailed Cercaria- i¥\^. 89 E [contained
individuals] and F). In other cases the Cercaria3 arise only
ill the Redia? of the second generation.

The mode of origin of tlie Cc>cariip has been thoroughly studied by
ScHWARz (No. i)). As has ah-eady been mentioned, this investigator finds in
their origin a great resemblance to the development of the embryo. The
woruZrt -like heaji of cells which arose from the germ cell is further
developed in such a manner that there are dilferentiated a peripheral
cell-layer, a central compact mass of cells, and a layer between the two.
The first supi^lies the dermal layer, which is to be considered as a meta-
morphosed epithelium ; from the central mass arise the genital organs,
whereas the intermediate parts of the embryonal tissue (the " meristem "
of ScHWAEz) give rise to the other organs. Anterior to the central cell-
mass a number of cells are arranged in a regular manner. This is the
fundament of the intestine, which later becomes hollowed out and con-
tinuous with the two branches of the intestine, which have arisen in the
same way. The central part of the excretory apparatus is also formed
by means of such a regular arrangement of cells in the posterior part of
the body. The dermo-muscular layer and the fundament of the nervous
system arise nearer the periphery. The remaining part of the " meri-
stem " becomes the parenchymatous tissue of the body.

The Cercarla already exhibits to a certain extent the
organization of the adult Dutownm^ e.g., in the presence of
an anterior sucker and one situated on the ventral side
(Fig. 89 F). In the centre of tlie former lies the mouth,
which leads into tlie muscular pharynx, and thence into the
forked intestine. The oesophageal ganglion, with the two
lateral stems, and also the bipartite excretory system are
present. But a long muscular tail is attached to the
posterior portion of the body. In this condition the Cercaria
leaves the Redia through the birth aperture, which lies at
the anterior end (Fig. 89 E, G), and seeks an escape by work-
ing its way through the tissues of the host by means of its
suckers and tail. Its free life in the water lasts for only a
shoi-t time. It soon attaclies itself to plants which arc
found at the water's edge. It casts off the tail, and secretes
about itself a cyst. A large number of glands which lie on
either side in the body of the Cercaria, and which give a

rLATHELMINTHES

185

V^r"

f^'t

characteristic appearance to the animal, serve for this pur-
pose (Fig. 89 F). These glands appear on the free Cercaria
as white opaque masses ; but when their contents have
passed out during the encystment, the body of the young
worm becomes entirely transparent (Fig. 89 G). If the cyst,
together with the plant to which it is attached, is swallowed
by a sheep, the envelope is dissolved in its stomach ; the
young worm becomes free, and finally reaches the liver,
where, in the course of about six weeks, it develops into the
sexually mature Distnvi7im hepaticum.

The different DistomidsB present great
differences as to the course of their de-
velopmental processes. The eggs from
which embryos are to emerge do not
always become free, but may be taken
up directly by the intermediate host, and
hatched out only when they have reached
its intestine (DistomuTn ovocaudatmn, ac-
cording to Leuckart). It is not neces-
sary that a sporocyst should be first
developed out of the embryo, and a Redia
out of it, as in Distomuni hepaticuin, but
the sporocyst may become metamor-
phosed directly into a Redia. Sporocyst
and Redia in most cases beget directly
Cercarise. The sporocyst in Bistomum
macrostomum and Gasterostomuvi fimbri-
atum is very aberrant in shape. In this
species it develops tubular processes,
which serve for the reception of the
Cercarige. The sporocyst of Distomum
macrostomum, known as Leucochloridmin,
which inhabits the liver and other organs
of Siiccinea amphibia, attains an extra-
ordinarily large size, for it sends out
processes into the antennae of the snail,
where, on account of their external re-
semblance to insect larvae, they are seen Fig. 90.— CercoWa ra-

, , - . - loti, Monticelli (after

and eaten by birds (Zjeller, Heckert). vh.lot).

186

EMBRYOLOGY

The CercarifB produced in gei-m tubes present a variety
of forms. This applies chiefly to the caudal appendage,
as can be recognized in the peculiarly formed Cercaria
represented in Figs. 90 and 91. One of these, Cercaria
setifera Villot,^ a marine foi'm, which arises from a sporocyst
inhabiting Scrohicularia tenuis, possesses an extraordinarily
large tail, beset with bristles. The other (Fig. 91) has
two tails, which are directed forwards, howerer, in swim-
ming. This is the Cercaria of Gasterostomtim Jimhriatum,
and is known under the name of Bucephalus polymorplius.

Fig. 91.— Cercaria of Ga«terosfO(num ^mbriafiuii (after Ziegi.er).

Under certain conditions the tail is entirely wanting in
the Cercaria stage. This is the case when the Cercarise
are not compelled to undei-take a migration, but remain in
their host until, along with it, they are consumed by another
animal, the final host. Since they do not pass through a
free stage, they do not require any special organs of loco-

> The Cercaria setifera of Viluit is called Cfrairla Villoti by Monti-
CELLi, for the term setifera occurs in another species (Monticklli,
" Sulla Cercaria setifera Miiller," BoUetino di Naturalt>ti in NapoH,
vol. ii., 1888).

PLATHELMINTHES

187

motion. The tailless Cercarise of Dlstomnm maerostomum
(produced in LeucocMoridium paradoxum), together with
parts of the germ tubes, arrive in the intestine of the final
host [birds], in the cloaca of which they become sexually
mature (Zeller). As a rule the Cercaria passes, by an
active mig-ration, from its first intermediate host into a
second, which naturally is also an aquatic animal, either
another snail or a worm, crustacean, insect, mollusc, fish, or
amphibian. In this second intermediate host it casts off the
tail and becomes encysted. The young worm awakens to
new life only after its host has been taken as food and
digested by some other, usually higher, animal. In this
way the cyst is dissolved, and the young
Distomum now reaches the stage of the
sexually mature animal. But we have
seen that in Distomum hepaticum the
second intermediate host may be omitted,
and that the Cercaria, after becoming
encysted in the free condition, passes
directly into the final host. The state-
ment, often made, that tailed Cercarise
could migrate directly into the final host
(for example, the Cercaria macrocerca of
Distomum cygnoides into the urinary
bladder of the fi'og), has not been sub-
stantiated. On the contrary, these Cer-
carise appear to he obliged to pass
thixDugh the encysted stage.

Fig. 92.— Embryo of
UoiiostoiauDi mutabile,
shortly after hatching
(after v. Siebold). R,
Redia.

A most remarkable condition is presented by
the embryos of Monostomum viuiabile and 31.
flavum, two Distomidce, which are found in the

thoracic and orbital cavities of various aquatic birds. The embryos
abandon the egg-membi-ane when still in the uterus of the parent. These
Distomids are therefore viviparous. In each embryo a Eedia-like
creature is ah-eady present (Fig. 92). In this case, therefore, the
embryo produces the new generation even before it has time to find an
intermediate host, within which to develop into a sporocyst. There is
scarcely a doubt but that the bud is formed from the germ cells of the
embryo.

188

EMBRYOLOGY

II. Polystomidj:.

The eggs in the Polystomidfe also are composed of the
egg-cell proper and yolk-cells (Fig. 93). Their egg-mem-
brane is provided with an operculum, and occasionally with
a long_ filiform, and twisted process, Avhich serves for the
attachment of the eggs (Diplozol'm). The course of develop-
ment is simpler than in the Distomidae, for the embryo
while still in the egg-membrane attains nearly the form of

/©'

i

Fig. 93.— Egg of Microcotyle Morniyri.
Within its operculiited shell lies an egg-
cell surrounded with yolk-cells (after
LoRKNz, from Hatschek's Lehrhvch).

Fig. 91. — Embryo of Polystomum
integerrimum, shortly after hatch-
ing (after Zki.leu).

the parent (GyrodactyluK), or at least passes through only a
single metamorphosis, not an alternation of generations
(heterogony).

The early development has been but little studied. We
are best acquainted with it (Zeller, Nos. 16 and 17) in the
case of Polystomumintegerrirnum, which inhabits the urinary
bladder of the frog. The eggs of this species are voided into
the water, where cleavage soon begins. The result of this

PLATHELMINTHES ■ 189

is a spherical mass of cells, which subsequently becomes
elongated, and thereby exhibits, even at this stage, the form
of the embryo. The fundaments of the eyes, the sixteen
hooks of the clasping disc [retinaculum], the cavity of the
intestine, and the pharynx soon make their appearance (Fig.
94). The newly hatched embryo possesses in addition five
rows of cilia, of which the three anterior belong to the
ventral surface, the two posterior to the dorsal surface.
Furthermore there is a fringe of cilia in front on the head
(Fig, 94). The embryo, leaving the egg at this stage, now
seeks the tadpole of the frog, to the gills of which it attaches
itself by means of the hooks and suckers. Here the ciliated
cells, which are no longer of any use to the animal, degene-
rate, and the Polystomum larva approaches more and more
the form of the parent. In extraordinary cases it can
attain this condition even in the branchial cavity, but as a
rule this is not the case ; on the contrary, the ^'ouns'
Polystomum, upon the degeneration of the gills of the tad-
pole, penetrates into its mouth-cavity, migrates through the
entire length of its intestine, and finally passes from the
cloaca into the urinary bladder, where it attains sexual
maturity.

Biplozoon paradoxum, which is remarkable on account of
its subsequent habits, also leaves the egg as a ciliated larva
(Zeller, No. 18). The larva, known under the name of
Biporpa, bears suckers and hooks, by the aid of which it
attaches itself to the gills of fresh- water fishes {Fhoxinus
l(Bvis, for example). It can remain here for weeks and
months, gradually approaching the organization of the adult.
But before it arrives at this condition it is necessary for one
individual to unite with a second, and, in fact, for the rest of
their existence. This takes place by the larva seizing with
its ventral sucker a knob-like outgrowth situated on the
back of the other animal. Then the second individual turns
and twists its body, so that it too may grasp the dorsal
prominence of its mate with its ventral sucker. In this
position the two animals grow together firmly, and in this
condition reach sexual maturity.

The course of development in Gyrodactylus elegaus, one of

100 EMBRYOLOGY

the Pnlystomidip., also living on the gills of fishes, is very re-
markable. Its reproduction appi-oaches that of Monodomum,
ali-eady described, for in this species also the embryo while
still in the body of the parent contains another embryo;
indeed, the latter already exhibits Avithin itself traces of a
ne%v individual, so that four generations are included one
within the other (Wageneb, Metschnikoff). Accordingly
here, as in Monostomum, the germ cells produce the new
generation very early ; but otherwise this developmental
process is not very different from tliat of the other Trema-
toda. In order to understand the cause of this accelerated
production, one would have to know more accurately the
processes themselves, as well as the habits of the animal.

III. CESTODA.

The eggs of the Cestodes exhibit a close resemblance to
those of the Trematodes. Like these, they are composed of
the egg-cell proper and a number of yolk-cells ; where the
latter are wanting, an accessory yolk-mass corresponding to
them appears to be present. The eggs are surrounded by a
thin egg-membrane, which occasionally possesses a movable
lid. The development of the eggs takes place for the most
part in the uterus of the parent, but in many forms it occurs
only after the eggs are laid. In the latter case the mem-
brane is thicker.

The investigations of E. van Beneden and Villot on the
TceniadcB, and especially those of Schauinsland on the
Bothriocephalidce, have shown that the embryonic develop-
ment of the Cestoda takes place in a manner quite similar
to that of the Trematoda.

According to Schauinsland, the development of the Bothrio-
cephalidce is accomplished in two different ways, depending
upon whether the embryos are developed before- or after ovi-
position. The undeveloped eggs which are deposited in the
water are thick-shelled, opercnlated, and provided with a
large number of yolk-cells. From them emerge larvju which
bear a thick coat of cilia. The eggs of the second kind are
thin-shelled, without an operculum, and provided with only

PLATHELMINTHES

191

a relatively small amount of yolk material. The embryos
contained in them are naked.

The embryonic development of the Bothriocepha-
lidae approaches closely that of the Distomida3. Cleavage
takes place in much the same way as there. At an early period
two cells are differentiated at the two poles of the elongated
germ, upon which they rest like a cap. They then grow
around it, and constitute the enveloping membrane (Hiill-
membran). Afterwards another cell is separated off from
the spherical cell-mass sui-rounded by the enveloping mem-
brane, and this at first also covers the germ like a cap, and
then grows around it. Later this
external layer consists of several
cells. It is in this way that the
ectoderm is formed. The embryo
now consists of a single layer of
ectoderm and a solid entodermal
mass (Fig. 95). Sis chitinous

hooks make their appearance in
the latter. With this the forma-
tion of the embryo is completed.

It is said to be composed of the

inner (entodermal) mass only.

The ectoderm separates from it,

so that a space arises between

the two. The embryo is now

surrounded by two envelopes in

addition to the egg-membrane,

the ectodermal mantle, and the

enveloping membrane. In this

respect, too, the conditions described for the Distoraida? are

repeated, and a comparison of Fig. 95 with Fig. 88 (on p.

180) shows without farther comment the close agreement of

the two groups at this stage of development.

Whereas the embryo quitting the egg leaves the enveloping

membrane behind in the egg-shell, it takes the ectodermal

mantle with it (Fig. 95). The latter either serves actively

in locomotion when it possesses cilia, or it swells up so much

in the water that it serves the larva both as a protective

Fig. 95. — Embryn of BotKrioce-
phalus latus pressed out of the egg.
Ec, ect yderm ; Hin, enveloping
membrane (after ScHAUiNSLiNo).

192 EMBRYOLOGY

envelope and as a means of making it of nearly the same
weight as water, thereby enabling it to float. Where cilia
are present, they are at first short, and only gradually in-
crease in length. In Bothriocephalus latus the exceedingly
delicate cilia attain a very great length. After the larva
has floated about in the water for a time, under certain con-
ditions for several days, it divests itself of the mantle,
Avhether ciliated or not. In many cases (as sometimes even
in Bothriocephalus latus) it may at the very beginning strip
olf the mantle with the enveloping membrane. Even in this
naked condition the larva may live free for a time, but finally
perishes, if it finds no suitable host.

ScHAuiNSLAND explains the circumcrescence of the germ by the cap-
shaped cells, which occurs twice in nearly the same way, as an epiboly.
Accordingly he is compelled to assume a complete loss of the ectoderm in
the casting off of the superficial layer. The embryo is developed out of
the entoderm alone. He finds a support to this view in the fact that up
to the present time no actual body epithelium has been found either in
the Cestoda or in the Trematoda. This fact is in his opinion an argu-
ment that ectodermal structures are not present in these cases, a view
that Leuckart (No. 8) also maintains. In any event the origin of the
cuticula-like dermal layer merits a thorough investigation. If, as is to
be conjectured, it arises by the metamorphosis of a superficial cell-layer
(E. ZiEGLER, ScHWARZE, ct alii), then it would correspond to the body epi-
thelium. The question whether in the casting off of the outer layer the
entire ectoderm is removed, or whether certain of its cells still remain
behind, must be difficult to determine on account of the small size of the
egg.'

' [As is well known, a distinct epithelium could not be found on the
external surface in Cestodes and Trematodes. It was natural to connect
this fact, the absence of the body epithelium, with the casting off of the
external cell-layers in the embryo, and thus to assume that the entire
ectoderm was lost. A body epithelium, therefore, could not be present.
This question has often been considered, and even recently has been re-
sumed. While some investigators assume that the cuticula which covers
the body is secreted by the subcuticular layer, and that the latter is a
l^art of the body parenchyma (Bi!anl>es, Loos), others maintain that it is
a metamorphosed epithelium, and believe they see, more or less distinctly,
cell nuclei retained in it (Bhadn, Monticelli). The most of these obser-
vations refer to the Trematodes, although investigations in this direction
have also been made on Cestodes (Zooraff, Grassi ; see Ajjpendix to
Literature on Cestoda). Zookaff in particular finds that in \arious Ces-

PLATHELMINTHES 193

The formation of the larval membranes in the Trematodes and Ces-
todes recalls in a striking manner the Amnion and Pilidium in the Nemer-
teans. Since, however, similar processes do not occur in the Turbdiaria,
— to which relationships are shown by the Trematodes and Cestodes on
one side, and by the Nemerteans on the other, — and since the Turbellaria
are to be considered as the more primitive forms, we have here to do with
only analogous phenomena.

The embryonic development of the Taeniadae differs
to some extent from that of the Bothriocephalidce, but leads
finally to a similar result (Leuckart, No. 8 ; Moniez, No. 9 ;
E. VAN Beneden, No. 2). A difference is caused from the
very beginning by the yolk-material bestowed upon the egg
being less abundant, or not in the form of distinct cells. In
Tcenia serrata the egg- cell lies embedded in this yolk-material.
In other cases the yolk appears to enter into still more inti-
mate relations with the egg-cell ; however, it appeal's from
the somewhat various statements of the authors concerninsr
the different forms that even in these cases the nutritive
material becomes separated as early as the first divisions of
the egg. There are one or several rather voluminous, gene-
rally granular cells, which are thus at first constricted oft' and
then consumed, while the other cellular matter multiplies
further. In Tcenia cucnmerina, it is true, the entire egg is
said to be transformed directly by means of a rather regular
cleavage into the embryonic cell-mass (Moniez). In the
further development of the Tjeniada? we can find again the
characters which w^e observed in the BothriocephalidEe, al-
though the details of the process are somewhat different.
In the TeeniadaB also certain cells detach themselves at an
early period, and grow around the germ as its enveloping
membrane. In the Tseniadag known as the Bladder-tape-
worms, the second membrane may present an appearance
somewhat different from that with which we have thus far
acquainted ourselves. It becomes cuticularized, assumes a
radially striated appearance, and thus finally forms a firm
membrane about the embryo, which even in this stage is

todesthe subcuticular matrix is independent of the connective-tissue body
parenchyma, and explains how in the embryo an ectodermal cell-layer
still remains behind after the casting off of the ciliated mantle.— K.]
K. H. E. 0

194 EMBRYOLOGY

equipped with three pairs of hooks. Farthermore, according
to VAN Ben EDEN, a cortical layer can early be distinguished
from the differently constituted internal cell-mass ; and
SCHAUINSLAND also speaks of smaller peripheral cells and
larger central ones. It is natural to regard this as a differ-
entiation into the two germ-layers, though Schauinsland be-
lieves that such is not the case. According to him, the entire
ectoderm, with the two membranes, is excluded from f ui^ther
participation in the formation of the embryo, which consists
exclusively of a homogeneous cell-mass : the entoderm. This
point, and especially the origin of the layers of the embryo,
appears to us in urgent need of renewed investigation.

With Schauinsland, we regard the homology of the embryonal mem-
branes of the Bothriocephnlidce, Ttcniadce, and Distomidcc as unquestion-
able. The different development of the second membrane — in the one
case into a ciliated layer, in the other into a chitinous layer — is determined
by the mode of life of the particular worms. Some of them inhabit
animals which continually come in contact with water. In these the
deposited eggs develop very quickly and require no special protection.
The others inhabit land animals. Their eggs reach the outside world
while still within the proglottis, and the more the already developed
embryos are protected against desiccation, the better their prospects for
existence. Hence the development of the chitinous membrane. In such
Tccniada, on the contrary, as inhabit aquatic animals, the chitinized
embryonal membrane may be absent, and in place of it there may
appear a thin membrane, similar to the non-ciliated ectodermal mantle
of many Bothriocephalidce (Schauinsland, No. 12).

The further development of the six-hook embryo (Fig.
96 A) takes place only after it has migrated into an
intermediate host. Either this may take place dii-ectly, —
when the embryo, as in the Bothriocephalidce, is a free-
swimming larva, and so at once migrates into an aquatic
animal, — or the embryos, still enclosed in the egg-membrane,
may enter by passive means into the intermediate host.
Generally this happens by the segment of the tapeworm,
which crawls about on plants, being swallowed with the
food. The proglottis is digested in the stomach, the eggs
thereby become free, their membrane ruptures, and the
embryos now find themselves within the intestinal canal.
They do not remain there long, but penetrate into the in-

PLATHELMINTHES 195

testinal wall by means of the boring movements of their
hooklets. In this way apparently they arrive in the blood-
vessels, and are probably carried along by the blood current,
finally to take up their permanent abode in various organs,
very frequently in the liver, sometimes in the brain, in the
musculature, etc. There a vigorous growth soon begins ;
this is connected with a simultaneous activity of the sur-
rounding tissues, which foi^m a membrane about the intruded
foreign body. The latter now casts off its hooks, and on
its surface there appears a rather thick cuticula, under-
neath which circular and longitudinal muscle fibres are
differentiated. Beneath these there follows a cortical
layer resembling connective tissue, which differs from the
central parenchymatous tissue (Fig. 96 B). The latter soon
exhibits spaces, in which an aqueous fluid makes its appear-
ance. By the coalescing of these spaces with one another, a
large cavity filled with fluid finally arises within the body.
Herewith the development of the tapeworm has reached the
stage which is known as the Gysticercus, bladder-worm, or
hydatid. It has been compared to the sporocyst of the
Trematoda, although it presents no particular resemblance
to it either in structure or in regard to its further develop-
ment.^

The excretory system has the same organization in the
bladder-worm as in the tapeworm. It is composed of capil-
laries which arise in ciliated funnels in the tissues, and
discharge into larger stems. The latter unite into the chief
trunks, which may fuse to form a short sac at the posterior
end and there open to the exterior (G. Wagener, Leuckart).^

^ [In many cases the formation of a cavity in the Gysticercus is greatly
reduced or becomes entirely suppressed. There are found in the lungs
of crows and in the body-cavity of Lacerta vivipara, for example,
Cysticerci of this kind (Pietocystis variabilis and P. dytldridium Diesing),
the body of which is filled with a continuous connective tissue (Leuckart).
Such Gysticercus stages of Cestodes have been designated by the name
Plerocerci and Plerocercoids (M. Braun), — by the latter when the scolex
is only sHghtly marked off from the bladder. Such, to a certain extent
aberrant, bladder-worms are found in the Tajniadae, as well as in the
Bothriocephalidae and other Gestodes. — K.]

'■^ [The Gysticerci with long caudal appendages, which occur in in-

196

EMBRYOLOGY

The Cysticercus may remain for a longer or a shorter time
in the condition described, but may increase meantime in

Fig. 96.—^ to JT, development of tlie tnpeworm from embryo to scolex (after
Leuckakt). yl, six-hook embryo ; B, Cysticercus of Tceii in sngi'imfn ; C to J-J, cephalic
process of the hydatid {Cysticercus pisiformis) of T. sagivatn : C, before the funda-
ments of the suckers and hooks have iLade their nppearance, D, with fundaments
of the hooks and suckers, E, in the partially evaginated condition : F, fully evagi-
nated cephalic process with attached vesicle of T. aolium ; G, scolex of T. senvila
with the remains of the vesicle, which has fallen away ; H, young tapeworm (T.
serratii), which has only just left the scolex stage, and in which there are therefore
only a few segments.

vertebratecl animals, especially in Crustacea [e.p. Gammarus and Cyclops),
are very noteworthy ; their relationships, however, are not yet suHiciently
understood. The caudal appendage, which sometimes attains a very
considerable length, carries about with it the remains of the embryonal
envelopes. This stage in the development of the Cestodes thereby
acquires to a certain extent the appearance of a Cercaria. Such tail-
bearing Cysticerci, which belong especially to the genus Tamia, have
been repeatedly discovered in recent years, and carefully studied by
Hamann, MitAZEK, and Grassi e Rovelli (see Appendix to Literature on
Cestoda).— K.]

PLATHELMINTHES 197

volume. The Cysticercus of EcMnococcus, which remains for
several months in this stage, attains during this period about
the size of a walnut, but, as is well known, it may become
mnch larger ; that of Tcenia coenurus grows in five weeks to
the size of a pea. Most of the Cysticerci reach in the course
of three weeks or so the diameter of about 1 mm. Then a
rapid cell-growth is noticeable at the anterior pole. This
grows inward in the form of a knob (Fig. 96 B and G).
Corresponding to the cell-proliferation, there is a pit-like
depression on the surface of the vesicle, which increases in
depth with the growth of the knob. The entire growth
represents the fundament of the head of the tapeworm
(scolex), which therefore arises as an invagination of the wall
of the vesicle (.Fig. 96 ii?to F). It appears that the want of
space, to which the tapeworm is subjected as the result of
its mode of life, has the effect of preventing the scolex from
arising as an [external] appendage of the body, as would
seem most natural, and causes it to be formed as an invagina-
tion of the vesicle, which is only subsequently evaginated.

The suckers arise as pit-like depressions of the lateral
walls of the cavity of the cephalic knob, and the hooks of
the head of the tapeworm are developed at the bottom of
this cavity (Fig. 96 D). The head is now completely formed
in negative. Beginning with the deepest part, the future
rostellum (Fig. 96 E), the head is completely reversed by
evagination (Fig. 96 F), and thus attains its permanent
form. It then appears as an evagination of the vesicle,
which is attached to its posterior end (Fig. 96 F).

Before the later developmental processes and the meta-
morphosis into the tapeworm can be completed, it is neces-
sary for the Cysticercus to enter into another animal. This
takes place by its host being eaten in part or in whole by
the final host of the tapeworm. In the stomach of the final
host the scolex loses the caudal vesicle by its being digested.
In Fig. 96 (? a small remnant of the bladder is still seen
attached to the scolex, which has just become free. The
scolex usually passes farther back in the intestine, sinks its
hooks and suckers into the mucous membrane, and upon the
appearance of segmentation becomes an adult tapeworm

198 EMBRYOLOGY

(Fig'. 96 H). Ordinarily only the neck portion of the
scolex, which is immediately attached to the head, is said
to be included in the adult worm, whereas all the rest dis-
integrates. Lkuckakt observed such young stages of Tcenia
solium, which moved about freely in the intestine of their
host by extending their suckers like arms and again retract-
ing them. They were no longer so much elongated as is
the case after their evaginatiou from the Cysticercus (com-
pare Fig. 96 F), but had only a short stump-like appendage.
The formation of the segments takes place in such a way
that the terminal segment is the oldest, and the youngest
ones are always interpolated in the vicinity of the head.
Growth and formation of segments take place so rapidly
that the tapeworm soon attains a great length, and the
posterior segments become detached from the others. With
the faeces of the host they reach the outside world, where
they are encountered ci^eeping about slowly.

In the younger proglottides nothing can as yet be recognized of the
genital apparatus. This arises out of the parenchymatous tissue in the
central part of the proglottis, which to a certain extent still remains in
an embryonic condition, as the result of a more compact massing of the
cells. This cell-mass, which is at first spherical, later elongates and is
diiJerentiated in such a manner that three cords of cells occupying the
longitudinal axis of the worm can be distinguished. F. Schjiidt, who
studied these conditions in Bothriocephalus Intun, found that these three
cell-cords produced the sexual ducts, which, therefore, begin to develop
earlier than the germ glands. In consequence of a luxuriant cell-proli-
feration, these cords increase in length, the ventral one, which is earliest
differentiated, becoming the vagina, the dorsal becoming the vas deferens,
and the extensive cell-mass lying between them becoming the uterus.
In proglottides of Bothriocephalus which lie about 50 cm. behind the
head, the sexual ducts have become connected with the surface of the
body, and the sexual openings can be recognized. About 10 cm. behind
the head the genital fundaments appear simply as a dark longitudinal
streak in the middle line of the segments. The germ glands and vitel-
laria likewise arise from the parenchymatous tissue, but independently
of the ducts, with which they become connected by means of cords of
parenchymatous cells, which afterwards become hollow.

General Considerations. — The course of development in the
Cestodes has met with various interpretations. The older conception,
established by Stkensthup, looks upon it as a true alternation of
generations. According to this theory, inasmuch as the scolex buds

PLATHELMINTHES 19&

out from the Cysticercus by non- sexual methods, and then itself sepa-
rates by division into proglottides, each sexual generation, the product
of which is the embryo (Cysticercus), is followed by two non-sexual
generations. On the other hand, in view of the circumstance that in
all probability the continuity of the individual persists, the course of
development in the tapeworm has more recently been explained as a
metamorphosis (Geobben, No. 4 ; Glaus, No. 3). Certain very simply
organized tapeworms, such as Archigetes, and a tapeworm living in the
body-cavity of Cyclops (Leuckakt, No. 7 ; A. Geubee, No. 5), are evidence
in favour of this view. These Cestodes appear to reach the permanent
condition without first passing through the typical Cysticercus stage.
The one last mentioned is converted directly into the sexually mature
animal ; the other is metamorphosed into the sexual animal, simply by
its body becoming separated into an anterior and posterior portion,
whereby the worm acquires a Cercaria-like appearance (Leuckaet, No.
7). If one considers the posterior portion of the body as equivalent to
the bladder of the Cysticercus, this tapeworm arrives at sexual maturity
even in the Cysticercus stage.

Like Archigetes, the unsegmented Caryophyllccus, which is provided
with a single set of sexual apparatus, represents throughout life a stage
which is equivalent to the scolex of other tapeworms, together with a
single accompanying segment. Therefore the development of the
embryo into the scolex would correspond to a metamoi-phosis, in which,
however, it is to be noted that with the bladder are cast off parts of the
body which originally represented the body of the entire individual. But
a similar state of affairs exists in the origin of the Nemertean from the
Pilidium, and the starfish from the Bipinnaria, without our calling these
l^rocesses alternation of generations.

As regards the second i)rocess of non-sexual reproduction — namely, the
division into proglottides — those cases are particularly noteworthy in
which, as in some Acantliobothridcc and EcheneibotJiridce, the proglottides
after detachment are able to live for a long time, and increase to
several times their former volume. They give the impression that one
has to do with independently living individuals resembling somewhat
Distoniurn. However, one must consider even here the earliest origin
of the Cestodes, and go back to forms which, like Caryophylhens and
Ampliiptyches, exhibit only one set of genital apparatus. They might be
traced back through transitional forms like Amphilina (comp. infra,
p. 201) to forms resembling Trematodes. In the beginning only one set
of sexual apparatus was i^resent, later numerous sets made their appear-
ance, and this condition led, by reason of its advantage, to the detach-
ment of individual segments of the body. The Ligulidce may perhaps
give us some foothold in this connection. Even if the conditions which
we find in them are to be considered as regressive, they may, neverthe-
less, be looked upon as reversions to an earlier condition. In the
Ligulidce the genital organs are repeated without the appearance of an

200 EMBRYOLOGY

outward segmentation of the body. The entire animal, therefore,
corresponds to an individual with a segmented arrangement of the
organs, and not to an animal stock. The genital organs themselves
agree with those of the externally segmented Cestodes, and it appears,
therefore, as if we had before us in this case a condition which
corresponds to a more i^rimitive stage of the Cestodes.

Although, from what has just been said, the course of development
of tapeworms would have to be considered as a metamorphosis, it is
nevertheless certain that in some foi-ms it rej^resents a real alternation of
generations. This is true of those forms in which more than one scolex
arises in the Cysticercus. The Cysticercus of Tmnia ccennrus produces
within itself a large number of tapeworm heads (about 500), and in the
bladder-w^orm of Tcenia echinococcus even daughter-vesicles are formed,
which in turn give rise to tapeworm heads. Here, where the embryo
produces many individuals, each one of which acquires the organization
of the tapeworm, there is unquestionably an alternation of generations.
The heads arise by means of budding in the Cysticercus ; they grow up
into segmented worms, and produce the sexual elements. In this case,
therefore, a sexual generation alternates with a non-sexual. The con-
ditions are still more complicated when there is interpolated a gene-
ration of daughter-vesicles, which bud from the parent vesicle and in
turn alone give rise to the heads.

In conclusion we refer once more to the relation between Cestodes and
Trematodes. In addition to other anatomical characters, it is especially
the structure of the genital apparatus which brings the two groups very close
to each other. In both, the yolk glands, in addition to the germ glands,
contribute to the production of the eggs, which are therefore composed of
two kinds of cells. The development, too, proceeds in a homologous
manner, and shows, above all, a great similarity in the formation of the
embryonal membranes. In considering the further stages of the de-
velopmental cycle, we are led by such forms as ArcJdgetex (see supra,
p. 199), which must be considered as a sexually mature cysticercoid
larva, to the comparison of the Cysticercus stage of the Cestodes with the
Cercaria of the Trematodes, in which the caudal appendage of the
Cercaria is to be considered as the equivalent of the vesicular posterior
end of the Cysticercus (Glaus). In such an interpretation we must con-
sider the sporocysts and Rediffi as secondarily interpolated links of the
developmental cycle. They are essentially larval organisms, reproduc-
ing parthenogenetically, in which the organization and the form of the
Cercaria have secondarily undergone an alteration and partial degenera-
tion. In most of the Cestodes therefore the development from the egg to
the complete tapeworm must be considered as a simple metamorphosis ;
an alternation of generations being recognizable only in the Echinococcus
bladders, where the young forms (Cysticercus stage) possess the power
of reproduction by means of budding. The development of the Trema-
todes, on the contrary, ai:)pears under the form of heterogeny, in which

PLATHELMINTHES 201

several parthenogenetically reproducing generations of larval forms have
been interpolated into the life-cycle. The close relationship of the
Trematodes and Cestodes is supported not only by their anatomical and
embryological agreement, but also by the existence of a iorm—Amjihilina
foliacea— which in its external shape more nearly resembles the Trema-
odes and was formerly reckoned among them (under the name of Mono-
stomum foliaceum Eud.), but which, owing to the absence of an intes-
tinal canal and on account of the structure of the genital organs, must
be placed among the Cestodes (G. Wagener, No. 15). Its body is of a
leaf-like form, and there is only a single sexual apparatus present. The
embryonic development takes place as in the Trematodes and Cestodes
(Salensky, No. 11). The egg is composed of an egg-cell and yolk-cells.
An embryonal membrane is formed, which the embryo breaks through.
This is armed with ten hooks, similar to those of the tapeworm
embryos.

As regards the dei-ivation of the Trematodes, they are to
be referred to free-living, Turbellarian-like Plathelminthes,
which adapted themselves to a parasitic life.

Literature.

I. TURBELLARIA.

1. Agassiz, a. On the Young Stages of a Few Annelids. Ann. Lyceum

Nat. Hist. New York. Vol. iii. 1867.

2. Chun, C. Die Verwandtschaftsbeziehungen zwischen Wiirmern

und Colenteraten. Biol. Centralbl. Bd. ii. 1882—1883.

3. GoETTE, A. Untersuchungen zur Entwicklungsgeschichte der

Wilrmer. Leipzig. 1882 and 1884.

4. GoETTE, A. Zur Entwicklungsgeschichte der marinen Dendrocolen.

Zool. Anzeiger. Jahrg. v. 1882.

5. Graff, L. von. Monographic der Turbellarien. I. Ehabdoccelida.

Leipzig. 1882.

6. Hallez, p. Contributions a I'histoire naturelle des Turbellaries.

Travaux Inst. Zool. Lille. Fasc. ii. 1879.

7. Hallez, P. Embryogenie des dendrocceles d'eau douce. Paris.

1887.

8. IiJiJiA, I. Untersuchungen iiber den Bau und die Entwicklungs-

geschichte der Siisswasserdendrocolen (Tricladen). Zeitschr.
tviss. Zool. Bd. xl. 1884.

9. Keferstein, W. Beitrage zur Anatomie und Entwicklungsge-

schichte einiger Seeplanarien von St. Malo. Abhl. Gesellsch. Wiss.
Gottingen. Bd. xiv. 1868.
10. KoROTNEFF, A. Ctcnoplana Kowalevskii. Zeitschr. wiss. Zool.
Bd. xliii. 1886.

202

EMBRYOLOGY

11. KowALEvsKY, A. Uebcr Coeloplana Metschnikowii. Zool. Anzeiger.

Jahrg. iii. 1880.

12. Lang, A. Der Bau von Gunda segmentata und die Verwandtschaft

del- Plathelminthen mit Colenteraten und Hirudineen. Mitth.
Zool. Stat. Neapel. Bd. iii. 1882.

13. Lang, A. Die Polycladen (Seeplanarien) des Golfes von Neapel und

der angrenzenden Meeresabschnitte. Fauna n. Flora Neapel.
Monogr. xi. 1884.

14. Metschnikoff, E. Untersuchungen iiber die Entwicklung der

Planarien. 3Iem. Neo-Russian Soc. Naturalists. Bd. v.
Odessa. 1877.

15. Metschnikoff, E. Die Embryologie von Planaria polychroa.

Zeitschr. wiss. Zool. Bd. xxxviii. 1883.

16. Metschnikoff, E. Vergleichend embryologische Studien. Ueber

die Gastrulation und Mesoderinbiidung der Ctenophoren. Zeitschr.
wiss. Zool. Bd. xlii. 1885.

17. MuLLEB, JoH. Ueber eine eigenthiimliche Wurmlarve aus der

Classe der Turbellarien und aus der Familie der Planarien. Arch.
Anat. u. Phijs. Jahrg. 1850.

18. MuLLEE, JoH. Ueber verschiedene Formen von Seethieren. Arch.

Anat. u. Phys. Jahrg. 1854.

19. Salensky, W. Die Entwicklung von Enterostomum. Proceed.

Soc. Naturalists Kasan. 1872—1873.
Selenka, E. Zoologische Studien. II. Zur Entwicklungsgeschichte
der Seeplanarien. Leipzig. 1881.

20

Appendix to Literature on Turbellaria.

I. Pereyaslawzewa, Sophie. Sur le developpenient des Turbellaries.
Zool. Anzeiger. Jahrg. viii. 1885.
II. Pereyaslawzewa, Sophie. Monographic des Turbellaries de la mer
noire. Mem. Neo-Russian Soc. Naturalists. Vol. xvii. Odessa.
1893 (Development, pp. 164—204).

II. Trematoda.

1. Biehringer, J. Beitriige zur Anatomic und Entwicklungsgeschichte

der Trematoden. Arbeiten Zool.-zoot. Institut WUrzhurg. Bd.
vii. 1885.

2. Heckert, G. Leucochloridium paradoxum, Monograi^hischeDarstel-

lung der Entwicklungs- und Lebensgeschichte des Distomum
macrostomum. Bihliotheca Zoologica. Heft iv. 1889.

3. Leuckaht, R. Zur Entwicklungsgeschichte des Leberegels. Arch.

Naturg. Jahrg. xlviii., Bd. i, 1882.

4. Leuckart, R, Zur Entwicklungsgeschichte des Leberegels. Zool.

Anzeiger. Jahrg. v., 1882 ; also Zool. Wandtafeln, Tafel xxxiii.
with Text.

PLATHELMINTHES 203

5. Leuckart, E. Die Parasiten des Menschen. II. Auflage. Leipzig

M. Heidelberg. 1879—.

6. LiNSTOw, 0. V. Helminthologische Studien. Arch. Naturg. Jahrg.

xlviii., Bd. i. 1882.

7. LoRENZ, L. Ueber die Organisation der Gattung Axine und Micro-

cotyle. Arbeiten Zool. Inst. Wien. Bd. i. 1878.

8. ScHAUiNSLAND, H. Bsitrage zur Kenntniss der Embryonalentwick-

lung der Trematoden. Jena. Zeitschr. Bd. xvi. 1883.

9. ScHWARZE, W. Die postembryonale Entwicklung der Trematoden.

Zeitschr. iviss. Zool. Bd. xliii. 1886.

10. SiEBOLD, Th. v. Helminthologische Beitriige. Arch. Naturg.

Jahrg. i., Bd. i. 1835.

11. Thoius, a. p. The Life-history of the Liver-fluke (Distomum

hepaticum). Quart. Jour. Micr. Sci. Vol. xxiii. 1883.

12. ViLLOT, M. A. Organisation et developpement de quelques esp^ces

de Trematodes endoparasites marins. Ann. sci. nat. (ser. 0,
Zool.). Tom. viii.

13. Wagener, E. G. Beitrage zm- Entwicklungsgeschichte der Einge-

weidewiirmer. Haarlem. 1857.

14. Wagener, E. G. Helminthologische Bemerkungen. Zeitschr. unss.

Zool. Bd. ix. 1858.

15. Wagener, E. G. Ueber Gyrodactylus elegans, v. Nordm. Arch.

Anat. u. Phys. Jahrg. 1860.

16. Zeller, E. Untersuchungen iiber die Entwicklung und den Bau

des Polystomum integerrimum. Zeitschr. wiss. Zool. Bd. xxii.
1872.

17. Zellee, E. Weitere Beitrage zur Kenntniss der Polystomeen.

Zeitschr. wiss. Zool. Bd. xxvii. 1876.

18. Zellee, E. Untersuchungen iiber die Entwicklung des Diplozoon

paradoxum. Zeitschr. iviss. Zool. Bd. xxii. 1872.

19. Zeller, E. Ueber Leucoehloridium paradoxum und die weitere

Entwicklung seiner Distomeenbrut. Zeitschr. iviss. Zool. Bd.
xxiv. 1874.

20. ZiEGLER, E. Bucephalus u. Gasterostomum. Zeitschr. wiss. Zool.

Bd. xxxix. 1883.

Appendix to Literature on Trematoda.
I. Beandes, G. Zum feineren Bau der Trematoden. Zeitschr. iviss.
Zool. Bd. liii. 1892.
II. Braun, M. Ueber einige wenig bekannte resp. neue Trematoden.
Verh. Deutsch. Zool. Gesell. Vers. ii. 1892.

III. Loos, A. Die Distomen unserer Fische u. Frosche. Neue Unter-

suchungen iiber Bau u. Entwickelung des Distomenkorpers.
Bibl. Zool. Heft xvi. 1894.

IV. MoNTicELLi, F. S. Studii sui Trematode endoparasiti. Zool.

Jahrb. Suppl. iii. 1893.

204 EMBRYOLOGY

III. Cestoda.

1. Beneden, E. van. Eecherches sur le developpement embryonnaire

de quelques Tenias. Arch. Biol. Tom. ii. 1881.

2. Beneden, P. J. van. Les vers Cestoides considert-s sous le rapport

physiologique, etc. Bull. Acad. Sci. Bnixelles. Tom. xvii.
1850.

3. Claus, C. Zur morphologischen und phylogenetischen Beurtheilung

des Bandwurmkorpers. Wiener klin. Wochcnschr. Nr. 36 u. 37.
1889.

4. Gkobben, C. Doliolum und sein Generationswechsel nebst Bemer-

kungen iiber den Generationswechsel der Acalephen, Cestoden, und
Trematoden. Arbciten Zool. Inst. Wien. Bd. iv. 1882.

5. Grubeb, a. Ein neuer Cestodenwirth. Zool. Anzeiger. Jahrg. i.

1879.

6. Leuckart, E. Die Blasenwiirmer und ihre Entwicklung. Zugleich

ein Beitrag zur Kenntniss der Cysticercusleber. Giesseii. 1856.

7. Leuckart, R. Archigetes Sieboldi, eine geschlechtsreife Cesto-

denamme. Mit Bemerkungen iiber die Entwicklungsgeschichte
der Bandwiirmer. Zcitschr. wiss. Zool. Bd. xxx. Suppl.
1878.

8. Leuckart, R. Die Parasiten des Menschen. II. Aufiage. 1879 — .

9. JMoNiEz, R. Memoires sur les Cestodes. I.ere partie. Travaux Inst.

Zool. Lille. Tom. iii. 1881.

10. MoNiEZ, R. Essai monographique sur les Cysticerques. Travaux

Inst. Zool. Lille. 1880.

11. Salensky, W. Ueber den Bau und die Entwicklungsgeschichte der

Amphilina foliacea. Zeitschr. wiss. Zool. Bd. xxiv. 1874.

12. Schauinsland, H. Die embryonale Entwicklung der Bothrioce-

phalen. Jena. Zeitschr. Bd. xix. 1886.

13. Schmidt, F. Beitrage zur Kenntniss der Entwicklung der Gesch-

lechtsorgane einiger Cestoden. Zeitschr. wiss. Zool. Bd. xlvi.
1888.

14. Wagener, G. Die Entwicklung der Cestoden, nach eigenen Unter-

suchungen. Bredait. 1854.

15. Wagener, G. Enthelniinthica Nr. V. Ueber Amphilina foliacea

mihi, etc. Arch. Natiirg. Jahrg. xxiv., Bd. i. 1858.

Appendix to Literature on Cestoda.

I. Grassi, B., e Rovelli, G. Ricerche embryologiche sui Cestodi.

Catania. 1892.
IL Hamann, 0. In Gammarus pulex lebende Cysticercoiden mit

Schwanzanhiingen. Jena. Zeitschr. Bd. xxiv. 1889.
III. Hamann, 0. Neue Cysticercoiden mit Schwanzanhiingen. Jena.

Zeitschr. Bd. xxvi. 1891.

I'LATHELMINTHES 205

IV. Meazek, a. Eecherches sur le developpement de quelques Tenias
des oiseaux. Sitzungsb. b'dhin. Gesell. Wiss. Frag. Jahrg. 1891
(Bohemian with French abstract).

ZoGRAF, N. F. Zur Frage liber die Existence ectodermatischen
Hiillen bei erwachsenen Cestoden. Biol. Centmlhl. Bd. x. 1890,
ZoGKAF, N. F. Les Cestodes oli'rent-ils des tissus d'origine ecto-
dermique? Arcli. Zool. e.xj). gen. {ser. 2). Tom. x. 1892.

CHAPTER Y.
OETHONECTID.E AND DICYEMID^.

The DicyemidjB were discovered as early as 1839 by Krohn,
the Orthonectidse in the sixties by Kefkkstein and McIntosh.
They were more than once after that the object of investiga-
tion (v. KoLLiKER, G. Wagener), but a more thorough
knowledge of their structure and development was not ac-
quired until recent times. Our knowledge of the latter
division of these most simply constructed, parasitic creatures
is due principally to the exertions of A. Giard, Metschnikoff,
and JuLiN, whereas the Dicyemidte have been thoroughly
studied by E. van Beneuen and Whitman.

I. ORTHONECTID/E.

Systematic : There are only two species known : —

(1) Bhopalura Giardii Metschn. (Bh. opMocomce Giard., Tato-
shia gigas Giard.), from Amphiura squamata ;

(2) Bhopalura lutoshii Metschn. (probably synonymous with
Intoshia Linei and LeptoplauK Giard.), from Nemertes lacteus.

The OrthonectidfB, which live parasitically in Ihirhellarians,
Nemerteans, and Ophiurans, exhibit a striking sexual dimor-
phism. Male and female differ both in form and size (Fig.
97 A and B). The organization is very simple. The females
are composed of only a peripheral cell-layer and a central
cell-mass (Fig. 97/4). They are spindle-shaped and covered
on the surface with vibratile cilia. However, two forms
are distinguishable : those with a cylindrical body (formes
cylindriques of Julin) and those with a flattened body
(formes aplaties). Both forms exhibit a kind of external
segmentation. They probably migrate out of the body of
the Ophiuran (Amphiura squamata) which they inhabit, in

206

ORTHONECTIDJ; AND DICYEMIDJ]

207

order to seek a new host. In tlie body cavity of the latter,
again an Amphiura, their life-history is continued, but in a
different manner in the two forms. The flattened females
are said by Julin to break up into a number of fragments,
each one of which is composed of several central and peri-
pheral cells. These ciliated offspring develop into the
"plasmodial sacs" of Metschnikoff (No. 6). These are
sac-like structures, which consist of a granulated mass, and
exist in large numbers within the body cavity of Amphiura
and Nemertes. The centi^al cells contained in them are to be
considered as eggs, and
(in consequence of a
kind of parthenogenetic
reproduction) supply
both forms of females.
The mjlindrical females
while still in their new
host expel their central
cells — i.e., the eggs — and
these develop into in-
dividuals which differ
considerably in shape
from the females already
described. They are the
males of Bhopalura Giar-
dii, which, according to
Julin, are brought forth
by the cylindrical
females only. Whereas
the body of the female
is segmented externally
into nine rings, there are
only six rings in the male (Fig. 97 B). The second ring,
as in the females, is without cilia. The five rows of cells
which constitute it contain peculiar highly refractive bodies.
Within the animal there is differentiated an oval, sac-like
organ of a granular appearance. From it fine cords, which
are interpreted as muscle fibres, extend in the body forward
and backward. The organ itself corresponds to the testis ;

Fig. 97. — A, cylindrical female; B, male
of Blwpalura Giardii (after Julin); H, testis ;
M, muscle fibres.

208

EMBRYOLOGf

it is found to be full of spermatozoa. The latter present
the typical appeai-ance of flagellate seminal filaments
(Metschnikoff).

It has not yet been observed in what manner fertilizatioii
takes place. Julin saw that the supei-ficial cells of the male
detached themselves, and that in this way the spermatozoa
became free. Since males and females swim about fi'ce in
the water, it is possible that the seminal filaments penetrate
into the female, and that consequently fertilization is inter-
nal. The eggs destined to produce females develop inside
the " plasmodial sacs," those producing males, free in the
body cavity of the AmpMura. The statements of authors
diifer greatly regarding the embryonic development.

Development of the Male. — According to Julin, there
arises as the result of the unequal cleavage an epibolic gas-

trula (comp. Fig. 98 A
and B), the inner layer
of which is at first re-
presented by only one
large cell. Later, cells
are separated from this
above and below (Fig.
98(7). While the large
central cell, by sub-
sequently dividing
many times, becomes
the fundament of the
testis, the muscle fibres
arise from the cells
that were previously separated ofp from it, and which at
first rest upon it in the form of a cap (Fig. 98 i> and E).
The larva assumes the type of the adult animal as the result
of the appearance of the characteristic division of the sur-
face of the body into rings, the loss of cilia on the second
ring, and the formation in it of the highly refractive
bodies.

According to Metschnikoff, an epiboly does not take place, but there
arises a solid heap of cells of rather uniform size, from which the outer
layer and the genital fundament are subsequently differentiated. On the

Fig. 98. — A to E, stages in the development of
the male of Rliopnlitra Giardii (after Julin) ; U,
testis.

ORTHONECTID^ AND DICYEMIDJl 209

other hand, Giard in his first communication described the formation of
an epibolic gastrula.

Development of the Female. — The first stages of
cleavage are not known. According to Julin, there is pro-
duced here also an epibolic gastrula, the entoderm of which
consists even, at an early stage of a large number of cells.
A peripheral layer is said to be differentiated from it into a
layer of cylindrical cells, which, situated under the ectoderm,
surrounds the central mass of polj'hedral cells. When the
embryo has elongated and acquired its coat of cilia, it pre-
sents a great resemblance to the embrj'os of the Distomidoi
and Bothrtocephalidce. The outermost of its three cell-layers
would then correspond to the enveloping membrane (Hi'dl-
memhran). Out of the second cell-layer, which later be-
comes flattened, there is said by Julin to arise a system of
extremely delicate muscle fibres, which are found under the
ectoderm in the adult female.

According to Giard and Metschnikoff, during the development of the
female a regular blastula makes its appearance, out of which the two
germ-layers are formed possibly as the result of delamination.

The above presentation of the life-history and development of the
Orthonectidffi does not rest wholly upon reliable observations, but many
gaps in it have been filled by the siDeculations of the authors. We have
adhered chiefly to the account of Julin, for his work is the most complete
and is an advance upon that of Giard and Metschnikoff.

II. DICYEMID>E.

Systematic : van Beneden distinguishes four genera :
Dicyema, Dlcyemella, Dicyemina, and Dicyemopsis, which are
distributed among four genera of Cephalopods : Octopus,
Eledone, Sepia, and Sepiola. They are found in the append-
ages of the branchial veins. Whitman admits only two
genera : Dicyema (with eight cells in the head region) and
Dicyemennea (with nine cells in the head region).

The body of the Dicyemidre is elongated. It consists of
an outer layer of ciliated cells and a single large axial cell,
the latter surrounded by the former (Fig. 99 D). At the
anterior end the outer cell-layer is differentiated into a kind
of cap [polar calotte]. Elsewhere the outer cells are nearly
alike.

K. H. E. p

210 EMBRYOLOGY

A certain difference in individuals is manifested in the
manner of their reproduction. The latter consists in the
production of embryos in the axial cell. But these are of
different shapes ; vermiform and infusoriform (rhomboid)
embryos can be distinguished (Figs. 99 and 100). They
arise in different individuals, which, according to van Bene-
DEN, are recognizable even by their outward form. The
nematogenous individuals are longer and more slender, the
rhombogenous shorter and more compressed.

According to Whitman, in addition to the forms that bring forth only
vermiform embryos, and which he designates as primary Neviatoijeini,
there also occur forms in which at first infusoriform and later vermiform
embryos are produced {secondary Nematogens).

Development of the Vermiform Embryos. — There
can hardly be any doubt that the cells which constitute the
earliest fundament of the reproductive elements, and which
correspond to the genital cells of the other Metazoa, take
their origin by the division of the axial cell of the parent.
The products of this process of division are, however, not
equivalent ; moreover, the newly formed cells remain in the
axial cell (Fig. 99), whereby the appearance of an endoge-
nous cell-proliferation is produced. The production of the
germ cells begins very early, for even in embryos there is to
be seen inside the axial cell and behind its nucleus a new
cell undergoing differentiation, the first germ cell (Fig. 99 A),
and a second one soon arises in its anterior part (Fig. 99 B
and 0). Their nuclei have very probably arisen by division
from the nucleus of the axial cell. Subsequently the latter
takes absolutely no part in the foi-mation of new nuclei. It
appears to preside over the other cell functions only. The
two germ cells, on the contrary, begin to increase by division,
and soon furnish a large numberof genital cells, from which
the embryos subsequently arise.

Tlie development of the germ cells, which are eventually
present in lai'ge numbers within the axial cell of the parent,
takes place I'u situ after the manner of cleavage. An epibolic
gastrula is formed here, as in the Orthonectidce, except that its
inner large cell remains undivided. It becomes the axial cell.

ORTHONECTID^ AND DICVEMID^

211

Bj increasing in length the embryo becomes vermiform,
whence its name (Fig. 99 B and C). These embryos are not
essentially different from the
adult animal, whose shape is soon
fully assumed by the accomplish-
ment of the slight differentiations
in the outer layer of the body and
in the head region, and by the
elongation becoming more pro-
nounced (Fig. 99 G and D) . Then
the formation of new germs in
the axial cell begins very early,
in fact while the embryo still
remains within the parent. The
processes described apply there-
fore to embryos which are still
found within the parent (Fig. 99
Ato D). When they have arrived
at maturity, they break through
the outer layer of the parent, but
remain in the venous appendages
of the Cephalopods, where they
still grow considerably and pro-
duce other embryos.

Structure and Develop-
ment of the Infusoriform
Embryos. — The infusoriform
embryos differ widely from the
vermiform in shape. Of a shorter,
more compressed form, they also present numerous internal
differentiations (Fig. 100 DtoF). In swimming, the broader
end of the embryo is directed forwards. Whereas the an-
terior end is naked, the rest of the body is ciliated (Fig. 100
C and D). The entire embryo is constructed on the bilateral
plan, for two lateral parts as well as a dorsal and ventral
side can be distinguished. Anteriorly and more dorsal ly lie
two highly refractive bodies (Fig. 100 D, r), somewhat behind
them, and lying more ventrally, the organ called by van
Beneden the "urn." This peculiar organ, the function of

Fig. 99. — A to D, stages in the
development of the vejmiforni
embryos of Dicyema ; A, of Di-
cijemennea e^edones (after Whii-
man); B to D, of Dicyema typu^
(after E. van Beneden). Ax, axial
cell ; A', nucleus of the axial cell ;
Ks, germ cells.

212

EMBRYOLOGY

which is not clear, is composed of a shell-like envelope, a
granulated body contained in it, and a lid. The shell lies
with its cavity toward the ventral side TFig. 100 F). It
consists of two parts and, owing to small comma-shaped
bodies embedded in its free edge, acquires a striated appear-
ance (Fig. 100 D, E). Its contents consist of four large
cells of nearly equal size, which lie close together, and are
granular. Finally, the lid, which corresponds to the ventral
part of the urn, consists in turn of four cells, which unite, at
the point where they all abut on one another, to form the
knob of the lid (Fig. 100 B to (?, I). Within the urn van
Beneden sometimes observed a ciliation, which he ascribed
to the granulated cells.

Pig. 100.— j4toG, infusoiiformembryosandtheir development— J toD.of Dici/o«ia
typiis ; E to G, of DicyemeUa Wagnerii (after van Benbdkn, from Balfoue's Compara-
tive Embryology). A lo C, stages of development ; I), embryo seen from the vetitriil
side; E, from the right side; F, from the front ; G, the " urn" isolated; gr, granu-
lated cells contained in the urn ; I, its lid ; u, the shell, which forms the floor of
the urn; r, highly refractive bodies at the anterior end of the embryo.

The origin of the infusoriform embryo, although at first
sight quite different from that of the vermiform embryo,
can perhaps be referred to this. It takes place in the axial
cell of the rhombogenous individuals, though not directl}',
being introduced by a preparatory process (Whitman).

Near the nucleus of the axial cell there arise two new
cells, the nuclei of which in all probability originate from the
nucleus of the axial coll. These two cells soon multiply, but
not so rapidly as in the formation of the vermiform embryos.
They never exceed eight in number, and often only a few
are present. Before these cells develop further they undei-go

ORTHONECTID^ AND DICYEMID^

213

a pi'ocess which Whitman compares to the formation of
the polar globules in the eggs of the Metazoa. As the result
of a process of division a considerable portion of the nucleus
is said to be cast out of them, which, as the " paranucleus,"
can be recognized for a long time in the axial cell (Fig. lUl
B). Then ensue a cleavage of the cells and, as its result,
the formation of cell-masses which have quite the appear-
ance of an epibolic gastrula with a central cell. Such stages
had already been observed by van Beneden (Fig. 101 A).
They are entirely like those which occur in the development
of the vermiform embryos. Whitman compares them directly
to these, and looks upon them as
special individuals, which appear
early in the course of reproduction.
For in their central cells there are
soon formed new cells (Fig. 101 A and
B), which subsequently give rise to
the infusoriform embryos. On this
account Whitman calls this gastrula
stage an Infusorigen. Compared to
the "nematogenic developmental series,
the gastrula stages would correspond
to the vermiform embryos, which, as
we saw, also produce embryos at a
vei'y early period.

From the central cell (cpllide ger-
migene of van Beneden) of the gas-
trula stage, which increases in size,
arise several generations of germ
cells, which surround it in the form
of a rosette.^ The larger of these
cells become infusoriform embi'yos ;
the smaller ones are said subsequently
to divide repeatedly, and vermiform
embryos are said to arise from them
when, after the formation of the in-

' The central cell itself is to be looked
upon as the homologue of the axial cell of
the vermiform embryos.

Fig. 101. — vl, "Infuso-
rigen embryo " (after van
Benedbn) ; B, tbe same
lying in the axial cell (^a.)
of the rhombogen indi-
vidual (after Whitman).
A, ot Dicyema typus; B, of
Bicyeniennea eledones. C,
the central cell of the " In-
fusorigen embryo," whicli
has already produced new
germ cells ; K, nucleus of
the central cell ; Ke, nuclei
of the outer layer of the
rhombogen individual ; Pn,
paranucleus. On the right
side of Fig. 101 B the re-
ferences K and Ke are
transposed.

214 EMBRYOLOGY

fusoriform embrjos, the rhombogen individuals have entered
upon the second phase of their development (secondary Nema-
togens according to Whitman).

The formation of the infusoriform embrjos from the germ
cell also begins with a process of cleavage, the result of
which is an epibolic gastrula (E. van Beneden). However,
in this case several cells make their appearance in the centre,
at first four large ones (Fig. 100 A, u). Two of these be-
come the shell and two the lid of the urn ; whereas four
smaller cells, which arise later, supply the four granular
cells contained in the urn (Fig. 100 B and C, gr). In the
meantime the two highly refractive bodies have made their
appearance in the outer layer of the embryo (Fig. 100 A, 1),
;•), and its posterior portion has become covered with cilia.
Whereas at first the embryonal cells which become the urn lie
side by side, they subsequently alter their position so that
the granular cells become enclosed above and below by the
lid and shell of the urn.

Nothing definite is yet known about the significance of
the infusorifoi-m embryos. From the fact that they can be
kept alive in sea- water for days (E. van Beneden), it was
thought that these forms were probably for the purpose of
transferring the species from one cephalopod individual to
another, where they would develop into a form which, like
the vermiform embryos, produces new germs. Besides this
vievv, there is a second one, which compares the infusoriform
embryos to the male of the Orthonectidae. Van Beneden is
inclined to see in the granular and vibratory contents of the
urn the homologue of the testis of the Orthonectidae.
Whitman several times observed the penetration of infusori-
form embryos into nematogen individuals, which is perhaps
to be compared to a process of fertilization.

Related to the Dicyemidse are the Heterocyemidce {Conocyema and
Microcycma), observed by van Beneden (No. 2), which also inhabit the
appendages of the veins of Octopus and Sepia. Their shape differs from
that of the DicyemidiB inasmuch as they do not nearly attain the length
that these do, and wart-like structures are present at the anterior end,
which can be extended and withdrawn. Nematogen and rhombogen
individuals are also distinguished here. Although the vermiform embryos

ORTHONECTID^ AND DICYEMID^:

215

differ somewhat from those of the Dicyemidae, yet on the whole they
develop like them. The infusoriform embryos of Conocyevm resemble
those of the Dicyemidae.

General Considerations.
There are so many common features in the structure and
development of the Orthonectidfe and Dicyemidaj that we
cannot doubt the relationship of the two groups. Their
relations to the other divisions of the animal kingdom, on
the contrary, are more difficult to understand. In view of
the fact that they are said to be composed of only two germ-
layers, an attempt vras made to create out of them a new
division of the animal kingdom, that of the Mesozoa, which
would be interpolated between the Protozoa and the Metazoa
(E. VAN Beneden, Julin). Since it is only parasitic forms
with which we have to do, such an explanation seems to us
venturesome at least, and we consider it more probable that
these simply constructed animals are Platyhelminthes which
have become degenerated through parasitism (Leuckart,
Metschnikoff, Whitman).

The resemblance of the female of the Orthonectida? to the embryos of
the Distomidie has already been pointed out. The theory that such
embryos have reached sexual maturity has nothing improbable about it,
for such cases are also known elsewhere in the animal kingdom. Thus
Dinophilus is evidently to be regarded as an annelid larva which has
become sexually mature, and it is appropriate for comparison here,
inasmuch as its males have degenerated to nearly the condition of the
Orthonectid* and Dicyemidaj (eomp. infra, p. 313). The intestine and
other features of a higher organization having been lost, they present
within the body only a large testicular sac, similar to the males of the
Orthonectidae, which, to be sure, remain at a somewhat lower stage.

If we regard the Orthonectidae and Dicyemidae as degenerated forms,
then the Orthonectida, with their central cell-mass, would represent the
higher grade, whereas the Dicyemidae, in which only one central cell
is present, are more degenerate. However, in these also the inner por-
tion becomes multicellular as soon as the formation of the germ cells by
the division of the axial cell begins.

Literature.

ORTHONECTIDai.
1. Braun, M. Die Orthonectiden. Centralbl. Baht. u. Parasitenkunde.

Bd. ii. 1887.

216 EMBRYOLOGY

2. GiARD, A. Les Orthonectida, classe nouvelle du phylum des vermeF.

Jour. Anat. et Physiol., Norm, et Path. Tom. xv. 1879.

3. JuLiN, C. Contribution a I'histoire des Mesozoaires : recherches

sur I'organisation et le developpement embryonnaire des Ortho-
nectides. Arch, de Biol. Tom. iii. 1882.

4. Keferstein. Beitrage zur Anatomic und Entwicklungsgeschiclite

einiger Seeplanaiien von St. Malo. Abh. Gesell. Wiss. Gottiiigen.
Bd. xiv. 1868.

5. McIntosh, W. C. a Monograph of the British Annelids. Part I.

The Nemerteans. London (/?«// Society). 1874.

6. Metschnikoff, E. Untersuchungen iiber Orthonectiden. Ztitschr.

wiss. Zool. Bd. XXXV. 1881.

7. Spengel, J. W. Die Orthonectiden. FAol. Centralhl. Bd. i. 1881—

1882.

DlCYEMIDJi:.

1. Beneden, E. VA>f. Recherches sur les Dicyemides survivants actuels

d'un embranchement des Mesozoaires. Bruxelles. 1876.

2. Beneden, E. van. Contribution a I'histoire des Dicyemides. Arch.

de Biol. Tom. iii. 1882.

3. Braun, M. Ueber Dicyemiden. Zusammenfassender Bericht.

Centralhl. Bakt. ?*. Panisitenkunde. Bd. ii. 1887.

4. KoLLiKER, A. V. Ueber Dicyema paradoxum, den Schmarotzer der

Venenanhange der Cei^halopoden. 2tes Beiicht der Zool. Anstult
in Wiirzhury. 1849.

5. Krohn, a. Ueber das Vorkommen von Entozoen in den Venenan-

hangen der Cephalopoden. Froriep's " Neue Notizen.^' Bd. xi.

6. Leuckart, R. Die Parasiten des Menschen. 2te Auflage. 1879 — .

7. Wagener, G. Ueber Dicyema Kollikeri. Arch. Anat. u. Phys.

Jahrg. 1857.

8. Whitjl\n, C. O. a Contribution to the Embryology, Life-history, and

Classification of the Dicyemids. Mitth. Zoul. Stat. Neapel. Bd.
iv. 1883.

CHAPTER VI.

NEMEKTINI.

The Nemerteans lay their egg's enclosed in gelatinous enve-
lopes, either singly or balled into large masses of spawn. It
appears that fertilization may take place either outside or
inside the body of the female. In the latter case the sperma-
tozoa penetrate into the female genital organs (egg sacs)
through their efferent ducts. In many forms (Monopora
vivipard) the eggs are there developed up to the matarity of
the embiyo. The development takes place either directly oi'
by means of a metamorphosis. The latter is of various kinds,
according as a free-swimming larva differing very much from
the ultimate shape of the animal is produced, or merely a
larval form which does not differ essentially from the young
animal, but which nevertheless produces the latter within
it. In the first case, in view of the shape of the larva, one
speaks of a, Pilidimn larva, in the latter case, of development
after the type of Desor, thus named from its discoverer.

I. — Development through the Pilidium Larva.

As the result of the equal cleavage a regular hlastula arises
from the egg of L^ne^^s lacteus. It loses its regular shape,
owing to a considerable increase in the size of the cells of
the lower half and to the occurrence at the same time of a
flattening on the under-side of the blastula (Fig. 102 A).
The outer and inner germ-layers can be distinguished on the
blastula as early as this, for the cells of the ectoderm are
smaller than those of the entoderm. The first trace of the
middle germ-layer is likewise already present. In the cleav-
age cavity and in contact with the entoderm are found a
number of cells (Fig. 102 A) which to all appearances take
their origin from the entoderm (Metschnikoff, No. 20), and

217

218

EMBRYOLOGY

subsequently prove to be mesenchymatous mioTatory cells
(Fig. 102 B and C), like those which arise in the develop-
ment of the type of Desor.

After the blastula becomes covered with cilia, has assumed
its characteristic shape, and has acquired a large flagellum
at its apex (Fig. 102), it may break through the egg-mem-
brane to swarm about at large. More often, however, the
larva reaches the outside world only after invagination has
taken place, i.e. as a ga.strula. This is accomplished by the
symmetrical invagination of the already-formed entoderm

Fig. 102.—^ to C, blastula, gastrula, and pilidium of Lineus lacteus (after Metsch-
nikoff); C, combination of two of Mf,t8chnikofk's figures; s, ectodermiil in-
vaninations, which subsequently grow around the intestine as the prostomial and
metastomial discs.

(Fig. 102 i?). The blastopore is circular, and the entire larva
presents a radial form. This is soon changed, however, for
the blastopore elongates somewhat and becomes oval, while
the archenteron bends to one side, and its blind end grows
more and more toward one wall (Fig. 102 J5). In this way
the form of the larva becomes bilaterally symmetrical. The
larva assumes its permanent shape— i.e., the one which its
discoverer, Joh. Muller, designated as pilidium — by the

NEMERTINI 219

downgrowth of a lobe on either side of the blastopore (Figs.
102 G and 103). It now consists therefore of an upper bell-
shaped part, which we call the umbrella, and the two pen-
dent lateral lobes. Between the two latter, on the under-
side of the umbrella, lies the wide mouth-opening (Figs. 102 C
and 103). It leads into the oesophagus, which corresponds
to an ectodermal invagination, whereas the real entoderm is
represented by the intestinal sac back of it (Fig. 102 C).
The intestine of the larva, the cells of which are provided
with cilia, remains closed.

Like Turbellarian larvae, the pilidium is encircled by a
continuous band of cilia, which fringes the periphery of the
umbrella and the margins of the lateral lobes. The ciliation
of the band is distinguished from that of the rest of the
body by its longer cilia (Figs. 102 G and 103). The par-
ticularly stout flagellum already mentioned takes its origin
in a depression at the apex, corresponding to which there is
a thickening of the ectoderm. The latter has been compared
to the apical plate of the Trochophore larvse of the Annelida
(comp. infra, p. 266).

As in the annelid Trochophore, two muscle strands, which also seem to
contain nerve fibres, issue from the apical plate (Salensky, No. 25). The
presence of these cords would not constitute, however, the only resem-
blance to the annelid larva, but, according to Salensky, the ciliated band
is also accompanied by a nerve cord, which would correspond to the
ring-nerve in the ciliated zone of the Trochophore. This nerve cord, which
is composed of nerve fibres and ganglionic cells, is said indeed to present
a more varied histological differentiation than the ring-nerve of the an-
nelid larva. At the point where the nerve cord passes from the lateral
lobes to the umbrella, it forms ganglionic swellings, which Salensky
interprets as the central organ of the nervous system.

The inside of the larva, between ectoderm and entoderm,
is filled with a gelatinous mass, in which the variously
shaped mesenchymatous cells are found embedded (Fig. 102).
These become at first the muscle-bands which traverse the
larva at regular intervals ; subsequently they become in part
the mesodermal elements (connective tissue, musculature,
etc.) of the adult animal (Butschli, No. 2).

220 EMBRYOLOGY

The pilidia of different Nemerteans differ from one another in shape
as the typical form described above is viore or less disthictly developed.
In place of the flagellum, Pilidium gijrans bears a tuft of cilia at the
apex (Fig. 103). In Pilidium auriculatum (Leuckakt und Pagenstecher)
the two lateral lobes are only very slightly developed, and the Pilidium
hrachiatum described by E. B. Wilson, which resembles P. auriculatum,
possesses, in addition to the two slightly developed lateral lobes, three

Fig. 103.— Pi'Iidiiim gyrang, with completely formed worm inside (combined from
two of BuTSCHLi's figures). Am, amnion ; D, intestine of the pilidium already sur-
rounded by the worm; Ec, ectoderm of the worm; M, mouth of the pilidium; N,
fundament of the nervous system ; R, proboscis ; So, lateral organs.

additional ones, which have arisen by indentations of the edge of the
umbrella.

The Pilidium recurvatum found by Fewkes (No. 5) at Newport exhibits
a very aberrant form, which, by the absence of the lateral lobes, by the
lateral curvature of the upper part, and by the presence of a row of cilia
at the posterior end, ac(iuires a striking resemblance to the Toniaria larva

NEMERTINI 221

of Balanoglossux. Moreover, the metamorphosis of this larva is said by
Fewkes to be accom2)lished in a manner different from that of other
PiHdia. Whereas usually the larva remains intact even after the
maturity of th worm, and in this condition is abandoned by it, in the
present case the collapsed Pilidium, after the withdrawal of the Ne-
mertean, is said to hang to its posterior end, where it is gradually
resorbed, in the same way as the Pluteus larva is drawn into the body
of the young sea-urchin.

After Gegenbaur had expressed the view that possibly a new animal
was developed within the Pilidium, this idea was more precisely defined
by Keohn, who maintained that regularly a young Nemertean arises from
the pilidium. Leuckabt und Pagenstecher were able to raise this view
to a certainty, for they (No. 17) followed the development of the Nemer-
tean inside the pilidium. The accompanying processes were then fully
elucidated by Metschnikoff (No. 19) and Btjtschli (No. 2).

The formation of the !N'emerteaii in the pilidium is initi-
ated by the appearance of four pit-like depre.ssions of the
ectoderm in the region of the mouth. Externally these pre-
sent the appearance of round suckers, for which at one time
they were mistaken by JoH. Muller. As the depressions
become deeper they become sac-like in shape (Fig. 102 C),
and the wall directed toward the intestine of the larva is
much thicker than the outer one. The further changes of
the invaginations consist in their being constricted off from
the ectoderm, becoming considerably expanded and growing
around the intestine of the larva (Fig. 104 A and B). They
have now assumed more of a discoid shape. At the points
where they come together the discs fuse, and their thicker
wall, the one directed inwards, constitutes the superficial
layer of the body of the Nemertean, whereas their thin outer
layer forms around the body an envelope, which is known as
the am^iion (Fig. 103 Am). This separates from its con-
nection with the body of the worm, which it suri^ounds as a
delicate membrane. The anterior pair of discs becomes the
head of the Nemertean (as far back as the lateral grooves),
whereas the posterior pair gives rise to the ectoderm of the
rest of the body (Fig. 104 A and B). Consequently the an-
terior discs, which, moreover, are the first to fuse, are known
as the head- [prostomial'] discs, the posterior as the trunk-
[metastomiaV] discs. The union of the anterior with the

222

EMBRYOLOGY

posterior pair does not take plaoe until the components of
each pair have completely united with each other. At the
point where the two prostomial discs first come together an
invagination is formed, the fundament of the proboscis,
which soon grows backward a long distance (Fig. 104 A and

Fig. 104. — Diagrams to show the formation of the Nemertean (after Sai.ensky).
A, evaginatioiis of the oesophagus (considered by Hubkecht to be the fundaments
of the nephridia) ; D, intestine ; M, mouth ; N, fundament of the nervous system ;
R, proboscis; Rs, sheath of the proboscis; S,, prostomial [head-] discs; S„, meta-
stomial [trunlc-] discs ; So, lateral organs.

The position of the young worm in the pilidium is illus-
trated by Fig. 103. The larval intestine is entirely included
within the worm. Meanwhile the oesophagus continues to
pass through the body-wall of the worm, still terminating in
a wide opening, until at a later stage it fuses with the ecto-
derm of the worm and is displaced to a position relatively
further forward.

The lateral organs are said by Salensky and Hdbrecht to arise in the
«ame way as the somatic discs. They originate as two invaginations of
the wall of the i)iHdium, one on either side of the u'sophagus (Fig. 104
A, So), then grow out toward the somatic discs, and finally separate from

NEMERTINI 223

their connection with the primary ectoderm of the pilidium, in .order to
fuse with that of the somatic discs (Fig. 104 B). Thus they are said to
be formed directly as jjarts of the pilidium.

The nervous system of the young worm makes its appearance in the
form of two ectodermal thickenings (Fig. 104 N), which arise in the
region of the anterior pair of discs on either side of the invagination of
the proboscis. At this place the ectoderm cells are differentiated into
several layers, of which the more superficial are said to become the body
epithelium and the ganglionic cells, the deeper, the Punktmhstanz.
The anterior thickened parts of the fundaments correspond to the brain,
and their backward prolongations to the lateral nerve-trunks (Fig. 104 A
and B). According to this, the fundament of the central nervous system
would have nothing to do with the apical plate of the larva.

Even before the discs had separated from the ectoderm, mesenchyma
cells were applied to their inner (deeper) layer ; and since such cells were
also found in the region of the larval intestine, a considerable number
of them came to be enclosed within the worm (Butschli, Salensky).
Like the separate fundaments of the cephalic and somatic parts, the
fundament of the mesoderm is double. In the first place, a mass of
mesenchyma cells is formed on each of the two prostomial discs, and a
similar one at the apex of the invagination of the proboscis. It could
not be determined whether the latter originated from the former. Then
each disc has its own mesenchyma layer, which likewise has arisen by
an accumulation of mesenchyma cells. The anterior and posterior parts
of the body are established, therefore, quite independently. The mesen-
chyma of the trunk is said by Salensky to split into two layers, one of
which is applied to the intestine as the splanchnic layer, the other to the
body-wall as the somatic layer. A kind of ccelom thus arises, which, to
be sure, subsequently becomes reduced and breaks up into small cavities,
owing to the cells of both layers sending out processes which unite with
one another. In the head that part of the mesoderm which is ajaplied to
the prostomial discs becomes the musculature, whereas the layer in con-
tact with the proboscis splits into two cell-layers, one of which is applied
to the proboscis, while the other forms the sheath of the proboscis.
Accordingly the cavity of the proboscis-sheath would be a portion of the
ccBlom (Salensky). The proboscis and its sheath attain their subsequent
great length by growing backwards (Fig. 104 B).

[The results of a recent investigation by Bijrger (Appendix to Literature
on Nemertini) differ in several particulars from the account given above.
The formation, and especially the differentiation, of the head- and trunk-
discs, the formation of the head itself, and the development of the nerv-
ous system are there described quite differently. The musculature of
the dermo-muscular sac appears to be of double origin, inasmuch as the
outer layer of it arises from the ectoderm, but the remaining portion
from the mesoderm.

Similar statements are also made regarding the Annelids. — K.]

224

EMBRYOLOGY

When the development of the worm has progressed as
far as this, it breaks through the amnion and pilidium and
swims about free in the water by means of its covering of
cilia. At this stage it lacks the anas, which arises only
later. In some cases eje-spots are present ; in others thej
are absent.

II.— Development after the Type of Desor.

For the more intimate knowledge of the mode of develop-
ment in the type known as that of Desor, we are chiefly
indebted to J. Barrois (No. 1). Recently Hubrecht (Nos. 9
to 11) has reinvestigated the subject.

Here also, as in the development of the pilidium, an
invagination gastrula arises, which at first is radial, subse-

A

Pig. 105.—^ to C, formation
of the somatic plates by in-
vagination in Lineus ohsciiriis
(after J. Bakbois).

Pig. 106.— Section of an em-
bryo of Lineus ohscurus (after
Hubrecht). D, intestine; if,
mouth, which, however, like
the oesophagus, is closed by
cells : Mes, mesenchyma cells ;
S, discs which subseijuently
form the ectoderm of the worm.

quently bilaterally symmetrical. According to Huhkecht,
cells (the mesenchyma cells) are said to migrate into the
blastocoele from both ectoderm and entoderm (Fig. 106).
On the ventral surface of the ectoderm Bauuois found a pair
of invaginations in front of the mouth and another behind
it. He saw that these invaginations were closed by the
growth of the ectoderm over them, and that finally their
floor became separated from the rest of the ectoderm (Fig.

NEMERTINI

225

105 Ato C). In this way there arose under the ectoderm four
cell-plates, corresponding to the prostomial and metastomial
discs of the pilidium, but distinguished from them by the
fact that they are not composed of two layers, but of only
one cell-layer (Fig. 106). When subsequently they grow
around the embryonic intestine, there is thus formed only
one cell-layer, the body- wall. The amnion is wanting (Fig.
107 B). The body, of course, is still surrounded by the
larval skin, the original ectoderm. At the place where the

Fig. 107. — A, gastrula stag-e of Linevs ohscurus, seen from the side ; B and C,
older embryos of Lineus seen from the ventral surface (after Ba.beoi3, from Bal-
four's Comparative Embryology) ; ae, archenteron ; cs, lateral organs ; Is, larval
skin ; m, mouth ; me and ms, mesenchyma ; pr.d, prostomial disc ; po.i, meta-
stomial disc; pr. proboscis; st, stomach.

prostomial discs come together, the proboscis arises as a
solid ingrowth of the ectoderm, which subsequently becomes
hollow (Fig. 107 C).

HuBRECHT describes a fifth plate, derived from secondary ectoderm, in
addition to the four plates found by Barrois (Fig. 106 S). It is said to
be formed on the dorsal side of the embryo, but in a different manner

K. H. E. Q

226 EMBRYOLOGY

from the four ventral plates, namely by delamination. Hubrecht derives
the epithelium of the young Nemertean from the fusion of these five
plates.

Hubrecht likewise differs from Barrois and Salensky in his descrip-
tion of the mode of origin of the proboscis. The latter authors derive it
from the secondary ectoderm, but according to Hubrecht it arises from
the primary ectoderm and subsequently separates from this and fuses
with the secondary ectoderm. When one considers that the lateral
organs, according to the concordant statements of Hubrecht and Salen-
sky, take their origin from the primary ectoderm, then such a mode of
origin of the proboscis might perhaps be intelligible. Small argument,
it is true, is to be got from the fact that in the pilidium the proboscis
arises from the secondary ectoderm. Pilidium evidently represents the
more primitive state, and on this account one must also expect in it the
more primitive condition as regards the mode in which the organs origi-
nate. As to the lateral plates, which are to be considered as sensory
organs, it is easier to suppose that they were already present in the larva,
whereas this is scarcely probable for the proboscis.

The statement of Barrois that the mesenchyma cells are detached from
the somatic discs seems to need revision. We have already become ac-
quainted with the origin of these cells in the pilidium. In the cephalic part
of the embryo they are applied in part to the proboscis (as its musculature),
and in part they are arranged in the vicinity of it to form the proboscis-
sheath. In this particular also Hubrecht differs from Salensky, for he
considers the pocket of the proboscis to be the remains of the blastoccele,
whereas Salensky maintains that it arises (as a kind of coelom) by means
of a splitting of a mesenchyma-layer. Hubrecht also looks upon the blood
lacunae and the cavities of the vessels, the walls of which as well as the
body musculature are of a mesenchymatous nature, as remnants of the
blastoccele. He likewise derives the fundament of the nervous system
from the mesenchyma, a conclusion which is wholly discredited by
Salensky, since in the development of Pilidium this observer recognized
the nervous system to be ectodermal in nature; this, moreover, agrees
with the ordinary mode of origin of this system of organs. On the other
hand, Hubrecht is inclined to derive the genital organs, which make
their appearance at an early period, from the ectoderm.

While the proboscis and its sheath have grown considerably
farther backv^ards, striking changes have taken place in the
intestine. It consists, as in the pilidium, of a posterior
wider part and an anterior narrower portion, although in
this type even the latter is said to be of entodermal nature.
The anterior part becomes solid as the result of cell-growth
(Fig. 106), but subsequently is hollowed again, and its lumen

NEMERTINI 227

then communicates both with the lumen of the intestine and
with the outer world. Therefore the permanent mouth still
lies at the place of the blastopore. At about the time of the
closure of the blastopore the ciliated embryo breaks through
both the embryonal envelope, which is likewise ciliated, and
the egg-membrane, to continue its development in the free
condition.

According to Hubkecht, the paired fundaments of two nepbridia (?),
which only later would come into connection with the outer world, arise
as vesicular structures from the oesophagus, and consequently from the
entoderm (for the oesophagus is said to be of entodermal nature). In the
development from the pilidium these structures are found on the anterior
intestine (Fig. 104 A), which here is of ectodermal nature.^

III. — Direct Development.

A transition from the indirect to the direct development
is afforded by the Nemertean studied by Dieck: Cephalothrix
galathece. Here a ciliated blastula arises as the result of the
tolerably regular cleavage. Dieck is inclined to look upon
a wide cup-shaped depression, which makes its appearance on
the blastula, as evidence of relationship with the pilidium
form, for an extension of the edges of the depression would
result in the lateral lobes of the pilidium. But a process
which is accomplished later recalls far more the indirect
mode of development of other Neraerteans than this outward
shape of the embryo does. After the embryo has elongated
and has assumed a rather worm-like shape, the layer of ciliated
cells covering it begins to be cast off, and under it a neio coat of
cilia is imviediately developed. Apparently hei'e also, as in
the type of Desor and in the pilidium, the new covering of
the worm is formed under the larval skin ; a great simplifi-
cation in the mode of development has, however, taken
place. Special plates, which enlarge and unite to form the
new body-covering, are no longer formed as the result of
invaginations of the larval skin, but the body-covering is

* [These organs have also been recently recognized by Burger (Ap-
pendix to Literature on Nemertini) to be nephridia, and their origin is
likewise referred by this author to the ectodermal anterior intestine. — K.]

228

EMBRTOLOGT

split off directly from the larval skin. This process takes
place on the free-swimming larva, for long before this the

embryo had broken through the egg-
membrane. Even at the time of its be-
coming free, it exhibits at its anterior
and posterior ends stout cilia (Fig. 108),
which are likewise a reminiscence of the
pilidium stage.

Stout cilia, or tufts of cilia, arise at
the ends of the body of the embryo even
in those forms in which the development
has become quite direct (^Amphiporus,
Tetrastemma, Malacohdella) , and in >vhich
no other points in harmony with indirect
development are still found, — apparently
an indication that development by
means of a ciliated free-swimming larva
Fip. 108.— Embryo of is the more primitive mode, direct de-
Cevhaiothrix gaiathem yelopment, on the Contrary, the derived

just hatched (after ^ ' •' '

DiECKj. method.

Cleavage and the production of the germ-layers in the forms developing
directly do not appear always to take place in the manner which we have
hitherto considered. in Monopora vivipara, it is true, after an irregular
cleavage there arises from the hlastula an invagination gastrula (SaijEN-
s3;y) ; other forms, on the contrary {Amphiporus lactifloreus, Polia car-
cinophila, Tetrastemma varicolor), are said to possess a delamination
gastrula (Babrois, Hoffmann). The sheet of long prismatic cells which
forms the blastula splits into an outer and an inner layer. The former
corresponds to the ectoderm, whereas the latter again separates into a
double layer, the outer one the mesoderm, the inner one the entoderm.
In Tetrastemvia the differentiation of these cell-layers takes place in a
solid mass of cells, a part pf which remains at the centre and is employed
only as food material. In Malacohdella also the germ is said to consist
of a solid cell-mass, from which the ectoderm becomes detached. Indi-
vidual cells migrate from the inner cell-mass into the cavity thus pro-
duced, and constitute the middle germ-layer. The remaining cells
correspond to the entoderm, and finally arrange themselves into an
intestinal epithelium, which fuses with the ectoderm to form the mouth
and anus. The embryonic development is now completed. The ciliated
embryo reaches the outside world to develop directly into the Nemertean
(Hoffmann).

NEMERTINI 229

The origin of the different organs in the Nemerteans with direct deve-
lopment has recently been studied by Salensky (No. 24) on Monopora. It
corresponds essentially with what we have learned as occurring in the
indirect process of development. At the anterior end the central nervous
system arises in the form of two ectodermal thickenings, which soon
become detached from their connection with the ectoderm. The funda-
ments of the two brain ganglia grow backwards as the two lateral nerves.

The proboscis and the oesophagus arise in the vicinity of the ganglionic
fundaments, both of them as bud-like thickenings of the ectoderm and
both of very similar appearance. The proboscis in this form lies, even
in the adult animal, very close to the oesophagus, whereby the relation of
the fundaments of the two organs is explained. The proboscis, which
lies above the oesophagus, opens with it into a common atrium. In spite
of this, however, relations [genetic] between oesophagus and proboscis
should hardly be sought for here, as Hoffmann and Balfoue supposed ;
but the condition in Monopora is much more likely of a secondary nature.
The proboscis, at first located at the anterior end of the body, was only
subsequently [phylogenetically] united with the oesophagus by moving
backwards. Moreover, the union is a very loose one, for the proboscis
and oesophagus do not actually unite, but rather open independently of
each other into the common atrium.

In the further course of development the fundament of the oesophagus
becomes hollow and unites with the intestine. The latter, after the
closure of the blastopore, consists of a closed sac. Entodermal cells
migrate into its lumen, thereby entirely filling it. Later they become
arranged into an epithelium, and then the oesophagus also unites with
the wall of the intestine. Afterwards the anus is formed.

The proboscis in this case also is composed of ectoderm and mesoderm,
the latter giving rise to the envelope of the ectodermal invagination and
to the proboscis-sheath. Salensky argues for a formation of the body-
cavity by means of a splitting of the middle germ-layer in Monopora also.
Even before the appearance of the body-cavity, two lateral bloodvessels
and one dorsal are said to be formed, corresponding to the conditions of
the adult animal. To all appearances they owe their origin to the inner
part of the mesoderm, for they are located in the vicinity of the intestine.

We find no statements in Salensky regarding the formation of the
genital system.

General Considerations.

In conclusion, we must once more point out how closely
the different modes of development of the Nemerteans ap-
proach one another. In the pilidium the worm arises by
the formation of four vesicular ectodermal invaginations,
which assume a discoid shape, and, growing around the

230 EMBRYOLOGY

larva, unite to form the epidermis of the worm. Since
the discs, owing to their mode of origin, are bilaminar, the
body-covering of the worm, formed by the inner layer, is
enclosed by an envelope (amnion), the outer layer of the
discs. The larva itself goes to pieces. In the type of Desor,
the discs, which likewise arise from the ectoderm, are from
the beginning unilaminar only; the amnion, therefore, is
absent, whereas in other respects the developmental pro-
cesses are quite similar. Finally, the discs are no longer
formed at all. However, as a suggestion of the former
mode of development, the outer ectodermal layer separates
from the embryo and is cast off (Cephalothrix). Moreover,
the embryo bears rigid cilia, as in Pilidium. This is also
the case in those embryos which are metamorphosed directly
into the worm without having their ectodermal covering
undergo any important changes.

Accordingly the pilidium appears as the older type of
development, from which the others are derived by becom-
ing at the same time simpliBed. But even the development
of the pilidium cannot be the original form. The origin of
the worm within the larva is a secondary process, which has
probably arisen through adaptation to the conditions of life.
Originally the larva was certainly metamorphosed directly
into the worm, as is still the case in the Turhellaria and
Annelida, for example. If the statements of Fewkes (No. 5)
should be verified, those forms in which the pilidium is said
to be resorbed by the body of the worm could best explain
the original mode of development (comp. p. 220).

The shape of the Nemertean larva, irrespective of the Tornaria-Wke
form (Fewkes), points to relationships of two kinds. One of these con-
cerns the Turhellaria. The resemblance is clear without further
comment, if one considers the Stylochus-larva of Goette (Fig. 80, p.
168). This larva exhibits the two typical lateral lobes of the jnlidmin.
We have shown in treating of the Turbdlaria how it can be referred to
Mullek's larva. In the comparison of larval forms caution is of
course necessary, and especially in the case under consideration, where
the first stages of development in the two groups differ very much from
each other. Thus one might be inclined to maintain the similarity in
the outward shape to be accidental, if the adult animals did not also
possess many similar characters in their organiiiation.

NEMERTINI 231

The pilidium resembles the Trochophore of the Amielida (comp. p.
266), as well as the Turbellarian larvae. In common with this, it pos-
sesses the apical plate, the cords radiating from it, and the ring-nerve
which extends under the ciliary apparatus. The apical plate, to be sure,
does not give rise here, as in the Annelida, to the oesophageal ganglion,
for it is lost with the pilidium. For this reason, it does not seem allow-
able to homologize the brain of the Nemerteans directly with that of the
Annelida. Apart from this, the lateral nerves of the Nemerteans arise by
the growing out of the cerebral ganglion, which is already separated
from its connection with the ectoderm, whereas the ventral ganglionic
chain of the Annelida appears to take its origin by means of progressive
differentiation of the ectoderm.

The nervous system of the Nemerteans is most closely allied to that of
the Platyhelminthes, and particularly to that of the Turbellaria, with
which the Nemerteans also present other common features, such as the
uniform ciliation of the entire surface of the body, the body parenchyma,
and the lateral organs. But the shape of the proboscis appears to us
to be of particular value for the comparison of the two groups. The
proboscis, situated at the anterior end of the body, apparently having
arisen by the metamorphosis of the latter into a tactile organ and being
withdrawn into the inside of the body, presents in the two groups a
position and structure too much alike not to challenge comparison.

Other conditions separate the Nemerteans from the Turbellaria. The
intestine possesses an anal opening, which is wanting in all Platyhel-
minthes. The presence of a differentiated blood-vascular system indi-
cates a higher organization of the Nemerteans. The genital organs are
constructed quite differently from those of the Platyhelminthes, whereas
those of the Turbellaria, Trematoda, and Cestoda show great agreement
in structure. In the position of the sexual organs a segmental arrange-
ment can be recognized. Whether the indications of segmentation
furnished by the presence of the septa which constrict the intestine and
the numerous openings of the water system have any higher meaning
cannot be said in the present state of our knowledge. What we have
hitherto learned in regard to the excretory system (v. Kennel and Oude-
MANs) entitles us neither to recognize therein a higher degree of organi-
zation nor to place the Nemerteans nearer to the Platyhelminthes, though
the presence of two longitudinal vessels might point to the latter. On
account of their numerous relations to the Turbellaria (although their
organization is much higher than that of the latter), it does not seem
justifiable to separate the Nemerteans from the Platyhelminthes altogether
and to place them, as has already been done, nearer the segmented
worms. The Nemerteans would have to be separated much more
sharply from the Turbellaria, if the statements regarding the segmenta-
tion of the body and the occurrence of a true body cavity were verified.

Finally, we cannot leave unmentioned in this place a theory which
places the Nemerteans in relation with the Vertebrata. Hubrecht, the

232 EMBRYOLOGY

expounder of this view, compares to the central ner\'Ous system of the
Vertebrata the dorsal nerve cord found by him. The cerebral ganglia of
the Nemerteans are held to con-espond to the series of ganglia of the
cranial nerves of the Vertebrata, and the lateral nerves to the nervi
laterales which occur so constantly in the latter group. In the chorda
HuBRECHT sees the metamorphosed sheath of the proboscis, while the
remnant of the proboscis itself would be recognized in the hypophysis.
' For this view Hubrecht finds support in the fact that in certain Nemer-
teans the proboscis opens out in the vicinity of the oesophagus, and that
in Tetrastemma it is said even to arise from the wall of the oesophagus
(Hoffmann, No. 7). For the present these explanations have only the
value of a mere hypothesis.

Literature.

1. Barrois, J. Memoire sur I'embryogenie des Nemertes. Ann. Sci.
Nat. (s6r. vi.) Zoologie Tom. vi. 1877.

2. BuTSCHLi, 0. Einige Bemerkungen zur Metamorphose des
Pilidium. Arch. Natui-gesch. Jahrg. xxxix., Bd. i. 1873.

3. Desor, E. Embryologie von Nemertes. Arch. Anat. u. Phys.
Jahrg. 1848.

4. DiECK, G. Beitriige zur Entwicklungsgeschichte der Nemertinen.
Jena. Zeitschr. Bd. viii. 1874.

5. Fewkes, J. W. On the Development of Certain Worm Larvse.
Bull. Mus. Comp. Zobl. Harvard College, Cambridge, Mass. Vol. xi.
1883—1885.

6. Gegenbaur,"C. Bemerkungen liber Pilidium, Actinotrocha, und
Appendicularia. Zeitschr. wiss. Zool. Bd. v. 1854.

7. Hoffmann, C. K. Beitriige zur Kenntniss der Nemertinen : I. Zur
Entwicklungsgeschichte von Tetrastemma varicolor Oerst. Niederl.
Arch. Zool. Bd. iii. 1876—1877.

8. Hoffmann, C. K. Zur Anatomic u. Ontogenie von Malacobdella.
Niederl. Arch. Zool. Bd. iv. 1877—1878.

9. Hubrecht, A. A. W. Proeve eener ontwickelungsgeschiedenis van
Lineus obscurus Barrois : Prys verhandeling met goud bekroond en
uitgegeven door het provinciaal Utrechtsch genootschap van Kunsten en
Wetenschapen. Utrecht. 1885. (Only the two following communica-
tions of this author, which treat of the same subject, were accessible to
us.)

10. Hubrecht, A. A. W. Contributions to the Embryology of the
Nemertea. Quart. Jour. Micr. Sri. Vol. xxvi. 1886.

11. HuBRipcHT, A. A. W. Zur Embryologie der Nemertinen. Zool.
Anzeiger. Jahrg. viii. 1885.

12. Hubrecht, A. A. W. On the Ancestral Form of the Chordata.
Quart. Jour. Micr. Sci. Vol. xxiii. 1883.

NEMERTINI

233

13. HuBRECHT, A. A. W. Eelations of the Nemertea to the Verte-
brata. Quart. Jour. Micr. Sci. Vol. xxvii. 1887.

14. HuBRECHT, A. A. W. Eeport on the Nemertea collected by
H.M.S. Challenger. " Challenger " Reports. Vol. xix. 1887.

15. Kennel, J. von. Beitrage zur Kenntniss der Nemertinen. Arb.
Wiirzhurger Zool. Imtituts. Bd. iv. 1877—1878.

16. Krohn, a. Ueber Pilidium und Actinotrocha. Arch. Artat. u.
Phijsiol. Jahrg. 1858.

17. Leuckart, K., und Pagenstecher, A. Untersuchungen liber
niedere Seethiere. Arch. Anat. u. Physiol. Jahrg. 1858.

18. McIntosh, W. C. a Monograph of the British Annelids.
Part I. The Nemerteans. London {Ray Society). 1874.

19. Metschnikoff, E. Studien iiber die Entwicklung der Echinoder-
men und Nemertinen. Mem. Acad. St. Petersbourg (sev.7). Tom. xiv. 1869.

20. Metschnikoff, E. Vergleichend-embryologische Studien. Ueber
die Gastrula einiger Metazoen. Zeitschr. toiss. Zool. Bd. xxxvii. 1882.

21. MiJLLER, JoH. Fortsetzung des Berichts iiber einige Thierformen
der Nordsee. Arch. Anat. u. Physiol. Jahrg. 1847.

22. MiJLLER, JoH. Ueber verschiedene Formen von Seethieren. Arch.
Anat. u. Physiol. Jahrg. 1854.

23. OuDEJLANS, A. C. The Circulatory and Nephridial Apparatus of
the Nemertea. Quart. Jour. Micr. Sci. Vol. xxv. 1885.

24. Salensky, W. Eecherches sur le developpement de Monopora
(Borlasia) vivipara Uljan. Arch, de Biol. Tom. v. 1884.

25. Salensky, W. Bau und Metamorphose des Pilidium. Zeitschr.
wiss. Zool. Bd. xliii. 1886.

26. Wilson, E. B. On a New Form of Pilidium. Stud. Biol. Lab.
Juhm Hopkins TJyiiv., Baltimore. Vol. ii. 1882.

Appendix to Literature on Nemertini.

BoRGER, 0. Studien zu einer Eevision der Entwicklungsgeschiehte
der Nemertinen. Ber, Naturf. Gesell. Freiburg i. Br. Bd. viii. 1894.

CHAPTER VII.
NEMATHELMINTHES.

Systematic : I. Nematoda s. str.

II. GORDIIDJ;.

I. NEMATODA S. STR.

Embryonic Development.

The eggs of the Nematoda, which are usually oval, but
occasionally spherical, are laid at very different times.
Sometimes they are deposited very early, even before
cleavage begins, and then are surrounded by a thick shell
(Ascaris luvibricoides, Trichocephalus di^par), whereas thin-
shelled eggs begin their development when still in the
parent, and may even continue to develop here to a quite
advanced stage. Still other Nematodes, as, for example,
Trichina spiralis and some species of Ascaris, are viviparous.
The embryonic development of a number of forms is known,
though, it is to be regretted, not perfectly. As far as ascer-
tained, the cleavage appears in general to be fairly alike in
all cases. It is total and approximately equal, and leads to
the formation of a hlastula, which, to be sure, may be some-
what variously shaped. It may have the form of a mere
cluster of cells, designated by Goette as a sterroblastula
(Ehabditis nigrovenosa), or it may be a true vesicle, with,
however, only a very small cavity (Ascaris megalocephala),
or, finally, it may appear in the form of a bilaminar plate of
cells (Gucullanus elegans).

At a very early period the fundaments of the germ-layers and the
differentiation of the various regions of the body can be recognized on
the segmenting egg (Goette, Hallez). As early as the first cleavage the

231

NEMATHELMINTHES

235

egg is divided into an ectodermal and an ento-mesodermal half. In
Rliabditis nigrovenosa, according to Goette, the ventral and dorsal sides
and the anterior and posterior ends of the embryo can be recognized even
at this time. The ento-mesoderm divides first into two blastoraeres.
The ectodermal blastomere sends out a process dorsally over both of
these (Fig. 109 A), and a newly formed ectodermal sphere is then situated
at this point. In the further division
of the ectoderm and ento-mesoderm
the elements of the former push them-
selves more and more over those of the
latter, and thus as a whole come to lie
more dorsally (Fig. 109 B). In subse-
quent stages two cells lying close together
at the former ectodermal pole of the egg
indicate the tail-end of the embryo (Fig.
110 A, B), while the head-end lies oppo-
site.

Whereas Goette makes the separation
of the mesoderm from the entoderm take
place later, it occurs, according to Hallez
(in Ascaris and Rhahditis aceti), even in
the eight-cell stage, in which two meso-
derm cells are constricted off from two
entoderm cells. In the twenty-four-cell
stage, the hlastula, with a small cleavage
cavity, is formed, the dorsal part of which
is composed of the ectodermal cells, the
ventral of the entodermal and meso-
dermal cells.

eM/c.

rtu^.

Fig. 109.— .4 to D, cleavage
stages and formation of the
germ-layers in Khabditis nigro-
venosa Cafter Gobtte). ect, ec-
toderm ; ent, entoderm ; mes,
mesoderm.

Gastrulation takes place in vari-
ous ways, according to the form of
the hlastula. In Ascaris megalo-
cephala an invagination gastrula is
formed, the archenteron of which
is very shallow, owing to the
shape of the thick-walled hlastula
(Hallez). The process of gastrulation in Gucullanus elegans
takes place in a peculiar manner, as was demonstrated by
BtJTSCHLi. In this form the hlastula stage consists, as has
been mentioned, of a bilaminar cell-plate. This shape is
soon lost, however, for the cells of one layer multiply more
rapidly than those of the other, and therefore a bending
toward the latter ensues. Finally a kind of tube is formed,

236

EMBRYOLOGY

which presents an elongated fissure on one side. This con-
stitutes the blastopore of this peculiar gastrula. Also in the
forms observed by Hallez and in EJiabditis nigrovenosa the
blastopore exists in the form of a long slit (Fig. 110 B, II).
In the last-mentioned Nematode the gastrula arises by
the more active proliferation of the ectoderm cells, which
produce an epibolic overgrowth, embracing the ento-meso-
derm (Fig. 109 B, D), whereby a long slit, the blastopore,
persists on the ventral side (Fig. 110 B). Subsequently this
closes gradually from behind forwards. A transition between

-mAO

7ru«.

Fig. 110. — A to E, different stages of development of RTiabdif is nigrovenos^fi (after
Goettk). hi, blastopore ; d, intestinal canal ; ent, entoderm ; g, fundament of the
genitalia; m, mouth; iii«s, mesoderm; n, fundament of the nervous system.

gastrulation by invagination and by epiboly exists, according
to Hallez, in Oxijsoma.

As regards certain facts of the subsequent embryonic
development which are not yet wholly clear, we have to
abide chiefly by the statements of Goette for Rhabditis
nigrovenosa. According to him, when the circumcrescence is
already far advanced, the formation of the mesoderm takes
place by the squeezing out of two cells from their connection
with the ento-mesoderm at the posterior end of the embryo
(Fig. 109 D). A comparison of these two cells with the

NEMATHELMINTHES 237

primitive mesoderm cells [teloblasts] of the Annelida is
suggested, especially since in multiplying they extend toward
the anterior end, and then constitute two rows of cells lying
by the side of the entoderm, resembling the mesodermal
bands of the Annelida (Fig. 110 A and (7). Their subse-
quent development, however, is not the same as in that
group, for single cells afterwards separate from them and
take up various positions between the intestine and the
body-wall, without giving rise to a ccelom homologous to
that of the Annelida (comp. p. 268).

The embryo, which up to this time has possessed an oval
form, changes in shape, for it becomes curved toward the
ventral side (Fig. 110 D) and more elongated. The shape
of the entoderm should be considered in connection with
this. At first it forms two layers of cells, between which
only a narrow lumen exists (Fig. 110 J. and (7). The latter
soon disappears in the posterior part of the embryo, and the
cells now arrange themselves one after the other in a row
(Fig. 110 D). The lumen is retained only in the anterior
pox'tion; there is formed here a depression of the ectoderm,
the fundament of the fore-gut, which unites with the en-
toderm (Fig. 110 D and J5/). The mouth lies in the same
place where the last trace of the slit-like blastopore, which
closed from behind forwards, was visible. Later a lumen is
again formed in the remaining part of the intestine by the
splitting of the entoderm (Fig. 110 E). The entoderm cells
at the posterior end, according to the statements of Gobttk
and Hallez, fuse with the ectoderm to form the anus, with-
out any depression of the ectoderm being noticeable, whereas
Strubell (No. 10) maintains the existence of such a depres-
sion. The central nervous system arises by a thickening of
the ectoderm in the region of the mouth (Fig. 110 (7 and D,
n) ; the dorsal and ventral parts of the oesophageal ring are
said to sever their connection with the ectoderm earlier than
do its lateral parts (Ganin). The ventral longitudinal nerve
appears to arise from a paired fundament, a condition which
has led to a comparison with the ventral longitudinal nerve
cords of the Platyhelminthes. In pursuing this idea, there
has also been an inslination to refer the dorsal longitudinal

238 EMBRYOLOGY

cord to the dorsal nerves of the Platyhelminthes, and to
compare the lateral nerves of the latter to the two nerves of
the lateral lines in Nematoda. It must be noted, however,
that the facts actually established ofFer no certainty that
this comparison is justified. More uncertain still are the
observations on the further changes of the mesoderm. The
mesoderm cells multiply greatly, separate from the two cell-
rows, and migrate in various directions. They also penetrate
between the fundaments of the nervous system and the skin,
separating these from each other (Fig. 110 E, mes). Finally,
the mesoderm forms a rather even layer between the in-
testine and the epidermis, so that the originally bilateral
arrangement thus disappears. It would be important to
know more accurately about the formation of the body cavity
in the Nematoda.

The origin of the sexual organs, which in the early stages
is the same for both sexes, is better known. In each of the
mesodermal bands, which at first consists of only a few cells,
one of these cells is distinguished by its remarkable size
(Fig. 110 D and E, g). It constitutes the fundament of the
genital organs. In Bhahditis a cord of cells is developed
from it by division, the individual elements of which divide
further, and finally become the sexual products (Goette).
In other Nematoda the original cell multiplies, it is true, but
the protoplasmic bodies of the newly formed cells do not
separate from one another ; on the contrary, a syncytium with
many nuclei is formed. The sexual fundament, which is at
first saccular, grows and differentiates into germ glands and
ducts. While in the former the protoplasmic mass with the
nuclei persists as the germarium, in the latter a peripheral
epithelium is formed (Ant. Schneider).

The shape of the ripe embryo resembles on the whole that
of the Nematode, although it still has to undergo, especially
in the parasitic worms, many changes before it attains the
adult form. Several moultings are often necessary for this.
In some cases the embryo possesses provisional organs, which
appear to be adaptations to its mode of development. Thus
in Spiroptera obtusa and Cucullanus elegans a borino--tooth is
found at the edge of the mouth, and the posterior end of

NEMATHELMINTHES 239

the larva of the last-named worm is prolonged into the form
of an awl, whereas the adult worm possesses a conspicuously
blunt posterior portion.

Post-embryonic Development.

The post-embryonic development in the parasitic Nematoda
is very diverse. It appears to be simplest in those cases
where the eggs of the Nematode reach the outside world
from the place where the parasite lives — for example, from
the intestine of the host with its faeces — and then are taken
up by another animal of the same species along with its
food. The eggs may be more or less developed at the differ-
ent stages of this migration, but in any event their envelopes
are first destroyed in the intestinal canal of the new host,
and. the embryo here finds at once the conditions of life suit-
able to it. Leuckart has observed such a direct conveyance
of the eggs into the intestine of the host in Trichocephahis
affinis and Heterakis vesicularis.

The conditions are somewhat less simple when the eggs
are enveloped by only a thin shell, from which the embryos
hatch as larv». These then live and. take food in damp
earth or water, like those Nematodes which always lead a
free existence. In general they resemble the members of
the genus Bhabditis so closely that they are not distinguish-
able from them (Ledckart). During its free existence the
worm attains a certain size and development. Only when
it arrives in its host do the organs needed for a free exist-
ence degenerate ; it now adapts itself to the parasitic mode
of life. Such is the case, for example, in Dochmius tri-
gonocephalus and D. duodenalis. The Ehabditis-Yike larvag of
these worms undergo several moultings during their free
existence, are then swallowed by the dog along with its
drinking-water or by man, and, as the result of a gradual
metamorphosis, acquire the sharp mouth armature which is
peculiar to them in the adult condition. The process of
development is somewhat different in the Mermithidce, which
are found as sexually immature forms in the larvae of
insects. After prolonged periods of residence, they abandon
this place of habitation by breaking through the body-wall.

240 EMBRYOLOGY

and then remain in the damp earth. Here they moult and
metamorphose into the sexually mature animals. These
copulate, deposit their eggs in the earth, and the young
worms developing from them then migrate into insect larvae
again, that of Mermis albicans, for example, into young
caterpillars.

The mode of development just described for many
Nematodes, in which the worms pass through a Rhabditis
stage, may well be regarded as most nearly resembling that
form in which parasitism in the Nematoda originated ; that
is to say, a more or less fully developed worm resorted to the
body of Einother animal, or at first only became attached
to it in order to gain nourishment from its juices. The
parasitism only gradually became permanent ; it is precisely
the Nematodes that offer all transitions from a pai'tial to a
complete parasitic life, which eventually leads to a total
transformation of the form of the body. Such a metamor-
phosis of the most extreme kind is realized in Sphserularia
bombi, which was first investigated by Ant. Schneider and
more recently in detail by Leuckart (No. 7). This worm in
the adult condition consists of a thick warty sac, which
lies in the body cavity of female humble-bees. To it is
attached a diminutive worm, which can be recognized as a
Nematode only upon careful examination. The entire sac
owes its origiij to the fact that the vagina of the worm
became everted, and, growing to an enormous size, included
the other sexual organs in it. The entire animal now con-
sists, with the exception of the small attached worm, simply
of a sac filled with sexual products. In it the eggs develop.
The young worms reach the body cavity of the humble-bee,
and from there the outside world, where they attain to
sexual maturity. They copulate in the free condition, and
probably the fertilized females migrate into the humble-bees
when these seek their winter quarters in the ground. Then
the remarkable transformation of the female begins.

A transition to SpJwerularia is represented by Atractonema
gibbosum, discovered by Leuckart, in which an eversiou of
the vagina likewise takes place, although it attains no
greater size than about that of the worm itself. It is

NEMATHELMINTHES

241

appended to tlie worm as a liernia-like body. The intestine
of the latter degenerates, so that here also nutrition must
take place by endosmosis. The course of development of
Atractonema resembles that of Sphssrularia. The eggs
arrive in the body cavity of the host, and the young worms
from here repair to the outside world, where they develop
into sexually mature animals and copulate. The fertilized
females penetrate into the larvoe of a gall-fly, Cecidomya
jpini, where they undergo their further development.

To the developmental processes of the forms last con-
sidered can be added that of the Beet Nematode, Heterodera
Schachtii. Swellings containing a spherical worm filled with
eggs, which can be recognized as a Nematode by its develop-
ment, are often found on the lateral roots of the sugar-beet.
The eggs of the worm develop within it, and pass into a
slimy brood-sac, secreted by its genital ducts and attached
to its posterior end. From hei'e the larva passes to the out-
side world, and undergoes a development which dilfers
somewhat according as a male or a female arises from it.
The female, which is provided with a stiletto-like structure
on the pharynx, bores into a beet-root,'moults here, and sucks
up such a large amount of nourishment that it becomes
swollen into a plump body, and thereby causes the epidermis
of the root to burst. In this way the hind end of the
female is exposed, and it is probable that copulation does not
take place until this time (Strubell).

Probably the most profound metamorphosis undergone by
any Nematode is exhibited by AUantonenia tnirab'de, likewise
discovered by Leuckart (No. 7), a worm of a sausage-like,
stubby shape, which lives in the body cavity of one of the
CurculionidEe {Eylobius pini). Except for the form of the
sexual apparatus and its products, resemblance to a Nematode
could be recognized neither from the external nor internal
conditions of this intestineless structure. This worm is said
to be hermaphroditic, and it is maintained that self-ferti^
lization takes place. The eggs are developed in the uterus
into young Nematodes, which are set free in the body cavity,
and subsequently reach the outside world through the intes-
tine. For a considerable time the larvae lead a free life, for

K. H. E. R

242 EMBRYOLOGY

w'hicli their organization fits them. Thej develop into males
and females, which copulate and lay fertilized eggs. These
develop in the free condition, and a generation of Rhahditis-
like Nematodes arises from them. The latter most likely
migrate into the larvee of the weevil, and here change into
the Allantonema form described above. Here therefore the
process of development is farther complicated by embi^acing
two differently formed generations, of which one is free
throughout life ; the other, however, leads in part a parasitic
life. This condition, long known as heterogeny, corresponds
to the mode of development of llhahditis nigrovenosa, only
that in the latter case no such fundamental metamorphosis
of the parasitic generation takes place. The hermaphroditic
form, ordinarily known as Ascaris nigrovenosa, inhabits the
lung of the frog. It produces eggs, the development of
which we have described above. The eggs are developed in
the parent, from which the embryos emerge in the lung of
the frog. From there the embryos pass into the intestine
and then out with the faeces, and then develop into males
and females, the true lihabditis form. After copulation,
there are developed within the female a small number
of young, which abandon its body after they have been
nourished by its contents. These young worms likewise
exhibit the lihabditis form, and do not lose it until they
have migrated into the lung of the fi'og, where they ai-e
metamorphosed into the hermaphroditic form. The course
of development in Rhabdonema strongyloides is, according to
the discovery of Leuckart, also similar ; the hermaphroditic
form, hitherto known as Anguilhda intestinalis, inhabits the
intestine of man, whereas the dioecious ]ihabditis form (lihab-
ditis stercoralis) is found in a free condition.

Those forms also which, in order to reach their complete
development, must live parasitically in two different hosts,
show a very high degree of adaptation to a parasitic mode of
life. This applies, for example, to Cncullanus elcga^is, which
is found in the intestine of the perch. The young of this
vivipai'uus Nematode pass from the intestine of the host into
the water, where they may live free for several weeks, until
they meet with some suitable host. This is not the perch,

NEMATHELMIXTHES 243

as one might imagine ; but the worms migrate iato a
Cyclops by first penetrating into the intestine thi'ough the
mouth and then into the body cavity of the Crustacean.
Here they undergo several changes in form, but attain their
permanent shape only after the Cyclops which harbors them
is swallowed by a perch, and they are set free in its
intestine, where they soon become sexually mature, and in
turn bring forth young, which undergo the same course of
development. JDracunculus rnedinensis, a Nematode parasitic
in the human body, appears to have a quite similar mode of
development. Bracuncidus inhabits the subdermal connect-
ive tissue, and by its pressure against the skin causes a
tumor and finally an abscess, through which it is able to
pass out. In this way also the embryos, which are present
in the woimi in countless numbers, may reach the outside
world. During the bathing of persons afflicted with the
disease they get into the water, and, like the larva3 of
Cucullanus, migrate into CyclopidaB ; however, they penetrate
directly through the body-covering to the interior of the
host. These infected Cyclopidse ai'e probably swallowed
along with the drinking-water by the inhabitants of those
regions where the parasite abounds.

Spiroptera obtusa has a development very similar to that
of the two forms last considered, only it is still more
adapted to parasitic life, for the eggs of this worm do not
develop into a free organism, but are directly received by an
intermediate host. Spiroptera obtusa inhabits the intestine
of the mouse. The eggs, in which the embryos are already
developed, reach the outside world with the f feces. They
are swallowed by the larvse of the mealworm, Tenehrio, which
feed on the dung balls. The embryos hatch out in their
stomach, break through the wall of the intestine, and become
encysted in the fat-bodies of the mealworm. When a
mouse devours a mealworm, it becomes infected with
the Spiroptera, which wakes up to a new life in the intestine
of its host, becomes sexually mature and reproductive.

The course of development in Trichina spiralis is one of
those which are most completely adapted to parasitic life, for
this Nematode accomplishes its entire life-history within the

244 EMBRYOLOGY

bodies of two hosts. Its development differs from that of
other Nematodes in that the young born of the sexually
mature females in the intestine of the host do not reach the
outside world, but break through the walls of the intestine
and migrate into the muscles of various parts of the body of
the host, to become, after sufficient growth, encysted. In
order to wake the muscle Trichina into new life and bring
about its sexual maturity, it is necessary that the infected
muscle be consumed by some other animal, in whose intes-
tine the Trichinge then attain their complete development
and power of reproduction (Leuckart).

II. GORDIID/E.

The accounts of the development of the Gordiidae are still
superficial. The eggs are not deposited singly, but are united
into large balls or strings ; for dui'ing oviposition a tenacious
mass is poured over the eggs, which are already surrounded
by a shell. The mass hardens in the water. Since the egg-
strings are heavier than water, they sink to the bottom and
remain there until their embryonic development is completed.
This does not begin until after the laying of the eggs, and
requires quite a long time, according to Meissner about a
month or more. As regai'ds the first stages of development,
the statements of the authors (Villot, No. 16, and Camerano,
No. 11) do not agree. According to Camerano, the cleavage
is unequal, and leads to the formation of a bilaniinar cell-
plate, which, by the bending in of the edges, is metamor-
phosed into a gastrula with a long slit-like blastopoi-e, in a
manner similar to that described above (p. 235) for Cucul-
lanus. The gastrula closes, exactly as in Cucullanus, from
behind forwards. The observations of Camekaxo on Gordius
Villoti extend up to this stage, and it appears as if the
figures given by Villot for Gordms aqtiaticus could be ap-
plied to Camerano's observations. Villot describes the
cleavage as regulai*. A solid heap of cells arises which,
after fui-ther multiplication of the cells, splits into a central
cell-mass and a peripheral layer (Villot). The hitherto
spherical embryo elongates somewhat, and a deep depi'cssion

NEMATHELMTNTKRS 245

now arises at one end. In this tlie head of the embryo is
formed in such a way that it can subsequently be everted.
The head is composed of a thicker basal portion and a slender
proboscis. The former bears three circles of six hooks each,
the latter three long stylets, so that the embryo appears well
armed. 'At the time of hatching, the head, with its armature,
is everted (Fig. Ill J5), but can at any time be retracted as
before (Fig. Ill ^). In the mean-
time the intestine is formed, and
leads from the mouth, at the tip of
the proboscis, to the anus, situated
somewhat in front of the posterior
end. The efferent duct of a remark-
ably extensive gland opens into the
oesophagus, at the base of the pro-

-, . Fig. 111.— J and B, two

boscis. Externally the embryo pre- larva? of Gorduts subbi/ui-cus,
sents an annulated appearance (Fig. ^^^^ retracted and with

, , 1 X rm 1 p, 1 i 1 • everted proboscis (after

ill;. Ihe embryo alter hatching meissnbe).
lives as a larva for a long time free

in the water, and then, with the aid of its sharp armature,
penetrates through the skin into the interior of Chironomus
larvas, as was observed by Villot. This observer regards
as simply an exceptional case Meissner's statement (No. 13)
that the larvae of Ephemeridss are also infected with Gordius
larvae. The parasite becomes sui'rounded by an envelope
formed from the tissue of the Chironomus larva, and re-
mains there until the larva happens to be swallowed by a
fish (Villot, No. 16). Becoming free in its intestine, the
Gordius larva perforates the intestinal wall and again be-
comes encysted. It remains here for a long time without
undergoing essential change. Finally (at the beginning of
spring) it returns to the intestine, which it leaves with the
faeces, and then gradually assumes the form of the adult
worm, during which the cephalic armature is lost, the an-
nulation of the surface of the body is smoothed out, and the
sexual organs are developed. At the same time its intestine
suffers a partial degeneration, and the mouth becomes closed.
However, it appears to be by no means certain whether
the mode of development described is the one realized, or

246 EMBRYOLOGY

whether the life even in a single host may not prepare the
Gordms for further development (Villot, No. 17).

In addition to fishes, the Gordius larvae may also get into
fi'ogs, insects, spiders, and Crustacea, although, according
to Villot, fishes are their most common hosts. As is well
known, the GordiidiB are often found also in terrestrial in-
sects, for e.\ample beetles and grasshoppers ; but nothing is
accurately known concerning the conditions of infection in
these animals. In insects of prey it can be explained as the
result of swallowing infected insect larvae. The extraordi-
nary size and development of the GordiidoB in such terres-
trial animals is explicable by the fact that they so long
lacked the opportunity of reaching the water, the place of
their final development.

General Considerations.

The systematic ]30sition of Gordius must be briefly considered here.
Vejdovsky has recently reverted to the idea, prevalent in former times,
that the Gordiidse present much closer relationships to the Annelida than
to the Nematoda, and perhajjs are even to be looked upon as degenerate
Annelids (Nos. 14 and 15). The segment-like arrangement of the ova-
ries, and more esisecially the structural conditions of the body cavity, give
rise to this conception. The latter is said, according to Vejdovsky, to be
bounded, on its somatic wall at least, by a well-marked epithelium, and
the intestine, as well as the genital organs, is said to be united to the
body-wall by means of mesenteries. Villot denies the existence of the
mesenteries, and refers the epithelium seen by Vejdovsky to a kind of
mesenchymatous tissue, which in young stages of development fills up a
large part of the space between the intestine and the body -wall. There-
fore the body cavity of the Gordiidffi, like that of the Nematoda, is bounded
on one side by the musculature of the body-wall, which arises from
that [mesenchymatic] tissue, and on the other by the entodermal wall of
the intestine itself. On the latter Vejdovsky even could recognize no
lining epithelium, a condition which he explained, however, by the great
reduction of the intestine. However, v. Linstow has also denied recently
the presence of an epithelium lining the body-wall (No. 12), and Came-
KANo, in consequence of the early stages of development of the Gordiidce
observed by him, argues for their relationships to the Neiuathelininthes
(No. 11). Nevertheless the Gordiidie are distinguished from the Nematoda
by the peculiar condition of their genital organs and their deviations from
them in other systems of organs, especially the nerve-ring, which is pro-
longed into the ventral cord, and they can be co-ordinated with them as
a separate division. What was said in considering the Nematoda must

NEMATHELMINTHES ' . 247

be repeated here, namely, that for the determination of the position of
this division a better knowledge of the mode of origin and metamorphosis
of their middle germ-layer is most desirable.

Embryology gives no satisfactory basis for the determination of the
position of the Nemathelminthes in the system, and it is hardly possible,
in the present state of our knowledge, to decide this. It cannot be deter-
mined whether on the one side they have relationships to the Platyhel-
minthes or to the Nemertini, and whether on the other to the Annelida.
It appears to follow from the structure of the adult animal that there
exist resemblances to the organization of the Echinoderes and Gastro-
tricha. But the latter are unquestionably related to the Eotatoria, so-
that in this way relations of the Nemathelminthes to the Trochophore
would be brought about (comp. p. 259).

More obscure still than the position of the Nemathelminthes is that of
the Acanthocephali. We have not, as is customary, considered these
along with the Nemathelminthes ; it is for the reason that their associa-
tion with this group rests upon grounds of purely external nature, and is
required neither by their organization nor by the manner of their develop-
ment. Even the musculature, which has ordinarily been made use of
for the comparison of the two groups, does not appear to be the same in
the Nemathelminthes and Acanthocephali either in its arrangement or
structure (Safftigen, Koehler [Literature on Acanthocephali, Nos. 6
and 3] ).

Literature.

1. BuTSCHLi, 0. Zur Entwicklungsgeschichte des Cucullanus ele-

gans. Zeitschr. wiss. Zool. Bd. xxvi. 1876.

2. Ganin, M. Ueber die Embryonalentwicklung von Pelodera teres.

Zeitschr. iviss. Zool. Bd. xxviii. 1878 (Abstract).

3. GoETTE, A. Untersuchungen zur Entwicklungsgeschichte der

Wiirmer. II. Rhabditis nigrovenosa. Leipzig. 1882.

4. Hallez, p. Recherches sur I'embryogenie de quelques Nematodes.

Pans. 1885.

5. Leuckart, E. Die Parasiten des Menschen. Erste Auflage. Leip-

zig. 1876.

6. Leuckakt, R. Ueber Trichina spiralis. Leipzig. 1866.

7. Leuckart, R. Neue Beitrage zur Kenntniss des Baues und der

Lebensgeschichte der Nematoden. Abh.Kgl. sachs. Akad. Wiss.
Bd. xiii. 1887.

8. Schneider, Ant. Monographic der Nematoden. Berlin. 1866.

9. SiEBOLD, T. v. Beitrage zur Naturgeschichte der Mermithen.

Zeitschr. wiss. Zool. Bd. v. 1854.
10. Strubell, a. Untersuchungen iiber den Bau und die Entwicklung
des Riibennematoden, Heterodera'Schachtii. Bibliothcca Zoo-
logica (Leuckart u. Chun). Bd. i., H. ii. 1888.

248

EMBRYOLOGY

11. Camerano, L. I primi momenti dell' evoluzione dei Gordii. Mem.

Real. Acrad. Sci. Torino. Ser. ii. Tom. xl. 1889.

12. LiNSTOw, 0. V. Ueber die Entwicklungsgeschichte imd die Ana-

tomic von Gordius tolosanus. Arch. mikr. Anat. Bd. xxxiv.
1889.

13. Meissner, G. Beitrage zur Anatomie und Physiologie der Gordia-

ceen. Zeitschr. wiss. Zool. Bd. vii. 1856.

14. Vejdovsky, F. Zur Morphologie der Gordiiden. Zeitschr. %viss.

Zool. Bd. xliii. 188C.

15. Vejdovsky, F. Studien iiber Gordiiden. Zeitschr. wiss. Zool.

Bd. xlvi. 1888.
IG. ViLLOT, A. Monographie des Dragonneaux. Arch. Zool. exp. et
gen. Tom. iii. 1874.

17. YiLLOT, A. Nouvelles recherches sur rorganisation et le developpe-

ment des Gordiens. Arm. Sci. Nat. Ser. 6. Tom. xi. 1881.

18. ViLLOT, A. Sur I'anatomie des Gordiens. Ann. Sci. Nat. Ser. 7.

Tom. ii. 1887.

19. ViLLOT, A. Sur le developpement et la determination specifique des

Gordiens vivants a I'etat libre. Zool.Anzeiger. Jahrg.x. 1887.

Appendix to Literature on Nematlielminthes.

I. BovEEi, T. Ueber die Entstehung des Gegensatzes zwischen den
Geschlechtszellen u. den somatischen Zellen beiAscaris megalo-
cephala, nebst Bemerkungen zur Entwicklungsgeschichte der
Nematoden. Sitznngsber. Gesell. Mor-ph. n. Phys. Miinchen.
Bd. viii. 1892.
II. Camerano, L. I primi momenti dell' evoluzione dei Gordii. Boll.
Mm. Zool. Torino. Tom. iv. 1889.

III. Cobb, N. A. Beitrage zur Anatomie und Ontogenie der Nematoden.

Jena. Zeitschr. Bd. xxiii. 1889.

IV. Hamann, O. Zur Entstehung der Excretionsorgane der Seitenlinien

und der Leibeshohle bei den Nematoden. Centralblatt Bakt.
u. Parasitenknnde. Bd. xi. 1892.
V. LiNSTOW, O. V. Ueber die Entwicklungsgeschichte von Gordius
tolosanus Duj. Centralblatt Bakt. u. Farasitenkunde. Bd. ix.
1891.
VI. Vejdovsky, F. Studien zur Organogenie der Gordiiden. Zeitschr.
tvisH. Zool. Bd. Ivii. 1894.

VII. ViLLOT, A. L'evolution des Gordiens. Ann. Sci. Nat. Ser. 7. Tom.

xi. 1891.

VIII. Wandollkk, B. Zur Embryonalentwicklung des Strongylus para-

doxus. Arch. Naturg. Jahrg. Iviii., Bd. i. 1891.
IX. ZnR Strasskn, 0. Bradynema rigidum. Zeitschr. iviss. Zool.
Bd. liv. 1892.

CHAPTER Vlir.

ACANTHOCEPHALI.

The eggs of the Acanthocephali are detaclied from the
ovarium as membraneless, usually spindle-shaped cells, and
then come to lie in the interior of the body of the female.
Here they are fertilized, after which each egg is surrounded
by a delicate transparent membrane, and then begins to
cleave. When this (in EcMnorhynchus gigas) has advanced
as far as the formation of twelve blastomeres, a second mem-
brane is formed under the first, which has separated some
distance from the Qgg, and to which are added in the course
of the development two more protective envelopes, so that
finally four of them are present. This applies to Echino-
rhynchus gigas (Fig. 118 A). Ordi-
narily three such embryonal mem-
branes are formed, the middle one of
which acquires a considerable thick-
ness and firmness by the deposition
of concretions of a brownish colour.
These structures are particularly
noteworthy, for the reason that they
first make their appearance during
cleavage, and therefore are not to be
looked upon as egg-membranes, but
as a kind of embryonal membrane ;
still they do not appear to have any cellular structure.
They recall the embryonal membranes occurring in the
Tgeniada?, which may also acquire a considerable firmness.

During the formation of the embryonal membranes the
cleavage has continued.^ It is unequal, and, according to

* In this connection we follow, in addition to the older observations of
Greeff and especially Leuckart, the newer investigations of Kaiser on

249

A-'^<

Fig. 112. — 4 to D, four
cleavage stages of EcMnor-
hynchus protens (after Leuc-
kakt) ; eh, first embryonal
membrane.

250

EMBRYOLOGY

Leuckart, takes place in EchinorUynchus proteus and anyus-
tatus in such a manner that the spindle-shaped eg^ is divided
at right angles to its long axis into a number of cells which
are not quite equal in size (Fig. 112 A, B). After five
blastomeres have been formed in this way, they are divided
in the direction of the long axis, and a rather irregular ar-
rangement of the cleavage spheres ensues (Fig. 112 G,
B). As the result of cleavage an epibolic gastrula is pro-
duced (Kaiser), the outer layer of which is formed of a
large number of polyhedral cells, whereas the inner layer
consists of much larger cells and encloses a remnant of yolk
in the centre. Even at this stage the embryo acquires its
armature. In the middle of every group of four contiguous
ectoderm cells is formed, as the product of their secretion, a

recurved booklet which protrudes
into the space bounded by the em-
bryo and the innermost protective
envelope. The entire surface of an
embryo of EcJnnorhynchiis gigas is
beset with small spines, and in ad-
dition five larger hooks are found at
the anterior end (Fig. 113 A). The
anterior end of the body, on which
they are located, can be i-etracted,
forming a funnel-shaped depression.
In Echiiiorhynchiis angustatus it is
truncated, and five to six hooks are
always found on tlie disc thus
formed (Fig. 113 B). As in Echino-
rhyncluis gigas, it also can be drawn
in.

After the central yolk is entirely consumed, there begins
a process called by Kaiser histolysis. This consists in the
following changes: the boundaries between the cells dis-
appear, the bodies of the cells flow together, and the cell
nuclei move to the middle of the embryonic body, where

Echinorhynchus gigas, which, however, have as yet been pubhshed only
in a preHminary way and without illustrations, but which nevertheless
afford an insight into the development of these forms.

Fig. 113. — A, embryo of
Echinorhynchus gigas in the
embryonal membranes (eh) ;
B, larva of Echinorhynchus
angustatus with the disc (s)
at the anterior end bearing
the armature (after Lkuc-
kakt); efc, "embryonic nu-
cleus."

ACANTHOCEPHALI 251

they accumulate to form the structure called by Leuckart
the embryonic niTcleus or core. Moreover, layers of two
kinds can still be distinguished in the syncytium : an outer
tough one and a less firm inner one, which encloses the em-
bryonic nucleus. Leuckart had already shown that later the
greater part of the worm arises from this centi-al portion
of the embryo. Furthermore, he compared it to a rudi-
mentary intestine, and showed how the solid body which he
found lying between the cephalic disc and the embryonic
nucleus (Fig. 113 B) could be interpreted as a rudimentary
pharynx. This conception seemed satisfactory in view of
the relationship of the intestineless Acanthocephali to other
worms (Nematoda).

In the condition above described the embryos, enclosed in
their firm envelopes, pass out by means of the complicated
mechanism of the sexual conductive apparatus. They now
find themselves in the intestinal canal of the host — a fish
in the case of Echinorhynchus angustatus and proteus, the
hog in the case of Echinorhynchus giyas — and then reach
the outside world with the faeces of the animal. The em-
bryos of the latter species are swallowed by the larv» of
Cetonia cmrata along with their food, whereas those of the
two worms first mentioned are swallowed in the same way
by Asellus aquaticiis and Gammarus piilex. The embryonic
envelopes soften in the stomach of the new host, and the em-
bryo becomes free ; it immediately penetrates into the intes-
tinal wall, and comes to rest either here (Echinorhynchus gigas
and angustatus), or in the body cavity of the host. The
larva of Echinorhynchus angustatus also reaches the body
cavity later, but in a more passive manner as the result of
its active growth and the rupturing of the intestinal mus-
culature. Here (in Gammarus pidex) are also found the
young stages of Echinorhynchus polymorphus, which as an
adult worm inhabits the intestine of ducks and other aquatic
birds (Greeff).

The further development of the larva is connected with
a metamorphosis of the external shape of the body, due to
the formative processes which take place within. In Echi-
norhynchiis gigas the middle part of the body swells greatly

252

EMBRYOLOGY

as soon as the larva has located itself in the intestinal mus-
culature of the intermediate host. The developmental pro-
cesses proceed from the central part, the so-called embryonic
nucleus, for it is this which contains the formative material.
According to the observations of Leuckart, it is differenti-
ated into four groups of cells lying one behind the other
(Fig. 114 A). The hindermost of these four gi'oups soon
acquires a greater volume, sending out a peripheral layer,
which spreads out in front and on the sides, and encloses
the other groups, with the exception of the most anterior
one (Fig. 114 A). The Echinorhynchus is formed for the

most part out of these cell
groups. The most anterior is
said to become the proboscis,
the second the ganglion, the
third, which soon divides into
two, the sexual glands, and,
finally, the fourth the sexual
ducts. The cell-layer which
surrounds the groups subse-
quently splits into two layers,
which were treated by Leuc-
KART as answering to the soma-
tic and splanchnic layers of
the mesoderm. In the absence
of the intestine, the splanchnic
layer would be represented by
the so-called ligament and the
proboscis-sheath only, both of
Avhich structures arise from it.
The somatic layer, after it has
separated farther from the
splanchnic, and has left the
body cavity between them,
•would form the musculature
of the body, whereas the epidermis arises directly from the
larval skin. When the internal formative processes have pro-
gressed sufficiently to allow of it, the cuticula of the larva
is alone cast off. A new cuticula then arises. During these

Fig. 114.— a, B, two larvffi of Echi
iwrliynchvf jiroteus, in which the
" embryonic nucleus " is already un-
dergoinor its metamorphosis (after
Lbuckabt). r, proboscis; rf, pro-
boscia-sheath; 11, ganglion; g, funda-
ment of the genital glands ; I, ducts
of the genital system; in, the cell-
layers which are destined for the
formation of the musculature.

ACANTHOCEPHALI 253

processes the different parts of the " embryonic nucleus "
have also enlarged considerably, and thus have once more
approached somewhat nearer to the larva in size (Fig. 114
B). At the same time the differentiation and development
of the different organs begin.

From the preceding description, which contains the chief
features of Leuckart's discoveries, it is seen that the
largest part of the worm arises from that frequently men-
tioned central mass into which the nuclei are said to retreat
at the beginning of development. The more recent state-
ments of Kaiser also agree in general with this conclusion.
Since it appears that, owing to improved methods, certain
processes have been more elaborately worked out by him,
and since these are of a most peculiar nature, his obser-
vations will be considered here more at length, although, it
is difficult to obtain a clear idea of tbe complicated processes
from his brief communication, unaccompanied as it is by
illustrations.^

After the larva of Echinorhynchus gigas has attached itself to the
wall of the intestine, and the middle part of its body has become greatly
swollen, groups of cells are said to detach themselves one after the other,
to become sm-rounded with cytoplasm, and thus to form the cells which
produce the permanent hooks of the proboscis. The groups of cells
move forward and finally unite to form the proboscis, which at length
can be everted. At about the same time the permanent body-covering
of the worm is formed by the detachment of nuclei from the entire
periphery of the "embryonic nucleus" and their migration into the
outer layer of the body plasma (Fig. 114). Accompanying an active
division of the nuclei, there is soon formed a very regular body-epithe-
lium. Here also the cuticula of the larva appears to be cast off, just as
its provisional hooks are. The epithelium secretes a new cuticula, and
beneath it a colourless, tenacious product, the fibrous tissue of the so-
called subcuticula. The muscular elements which are found in the sub-
cuticula are said to be formed at the same time from the cells of the

^ [A paper by Hamann (Appendix to Literature on Acanthocephali) and
especially a voluminous work by Kaiser have furnished us with a new
exposition of the development of Echinorhynchus. These investigations
elucidate to a great extent the remarkable developmental processes, which
are here only briefly touched upon. We refer to these two works them-
selves, since it is not possible to give in this place the results of these
extensive studies. — K.]

254 EMBRYOLOGY

body-epithelium. They arise as "primitive musclc-fibros " in the
epitheUal cells, and pass from these into the tibrous tissue of the sub-
cuticula. When this process is completed, the body-epithelium de-
generates completely and disappears. The formation of the lemnisci
agrees with that of the skin. The nuclei, which have separated from
the central mass and moved to the anterior end of the body, here form
a circular swelling which at two diametrically opposite points is drawn
out into slender processes, the fundaments of the lemnisci. In them
the formation of the fibrous tissue takes place just as in the skin. Near
the anterior end of the body, and immediately behind the rod-like pro-
boscis, thei'e also lies an extensive mass of nuclei, the fundament of the
central nervous system, from which the peripheral nerves soon grow out
to the different organs.

The organs the development of which has thus far been described
are said to be of ectodermal origin ; this, indeed, is very probable,
although sufficient grounds for this conclusion cannot as yet be recog-
nized in Kaiser's description. The real body-musculature, the sexual
glands, and the ducts of the genital apparatus ai'ise, according to Kaiser,
from the entoderm. Leuckart had si^oken of a mesoderm, which splits
into an outer and an inner layer, but as yet Kaiser has not given
attention to this statement. Again, it is layers of nuclei which separate
from the central mass to give rise to new structures. Three such layers
of nuclei can be recognized, owing to their somewhat different shape.
The two outer ones soon migrate to the body-wall, and here, after various
metamorphoses, supply the circular musculature and the longitudinal
musculature of the body.

Behind the iwoboscis, in the neighborhood of the ganglion, are found
nuclei, arranged in definite order, concerning whose origin more accurate
knowledge would be important, for out of them arise the proboscis-sheath
and the retractors as well as other muscles of the proboscis, therefore
structures which would be ascribed to the inner layer of the mesoderm
did such exist.

The formation of the genital organs takes jjlace in quite a peculiar
manner. Behind the proboscis-sheath a prismatic protoplasmic mass
makes its appearance, from the edges of which arise four thin plates,
which divide the cavity of the body into four sectors. By this descrip-
tion one is mvoluntarily reminded of the mesenteries which unite the
fundaments of the genitalia with the body-wall, and at the same time
recalls the conditions which, according to Vejdovsky, exist in the
Gordiidae. In the female the plates unite in the dorsal and ventral
sectors to form the ligament ; in the male the jilates of one sector
degenerate. The germ glands themselves arise from the axial mass of
plasma. The resemblance of the thin plates to mesenteries, referred to
above, is increased, as far as can be judged from the brief statements of
Kaiser, by the two lateral sectors being filled with a cellular mass;
subsequently, however, this degenerates and thus gives rise to the body

ACANTHOCEPHALI 255

cavity. If then, provided we rightly understand Kaiser's statements, a
union of two plates were to take place dorsally and ventrally, the re-
semblance to mesenteries would indeed be strong. The i^lates could
certainly arise only from the above-mentioned third or inner layer, which
separated from the central mass at the time of the formation of the body-
musculature. The two outer layers would then be applied to the body-
wall, whereas the inner layer would perhaps assume the formation of
the internal organs, the proboscis-sheath, and the ligament, in some
such manner as was described by Leuckart. This is the way at least
in which we should interpret the statements of Kaiser in the absence of
his more detailed descriptions.

In regard to the origin of the genital organs, especially the extensive
conducting apparatus, we refer to Kaiser's communication, or, better still,
to the awaited complete work, for it cannot be determined from the
former what is the real origin of those elements which constitute the
genital apparatus.

The Echinorhiinchus, which even in the body of the inter-
mediate host attains in general the form of the adult worm,
becomes capable of reproduction only when the animal
harboring it is consumed by another which is adapted
to serving it as permanent host, thus, for example, the
Gamviarus by a fresh-water fish or a duck, if the species be
Echinorliynchus polyviorphus.

Literature.

1. Greeff, K. Untersuchungen liber den Bau und die Naturgeschichte

von Echinorhynchus miliarius (E. polymorphus). Arch. Natuig.
Jahrg. xxx., Bd. i. 1864.

2. Kaiser, J. Ueber die Entwicklung des Echinorhynchus gigas. Zool.

Aiizeiger. Jahrg. x. 1887.

3. KuHLER, R. Documents pour servir a I'histoire des Echinorhynques.

Journ. Anat. et Physiol. Tom. xxiii. 1887.

4. Leuckart, R. Die Parasiten des Menschen, etc. Bd. ii. Leipzig.

1876.
.5. Megnin, p. Recherches sur I'organisation et le developpement des

Echinorhynques. Bull. Soc. Zool. de France. Tom. vii. 1882.
6. Safftigex, a. Zur Organisation der Echinorhynchen. Morph.

Jahrb. Bd. x. 1885.

Appendix to Literature on Acanthocephali.

I. Hamann, O. Die Nemathelminthen. Beitrage zur Kenntniss ihre
Entwicklung, ihres Baues, und ihrer Lebensgeschichte. Jena.
Zdtschr. Bd. xxv. 1891.
II. Kaiser, F. Die Acanthocephalen und ihre Entwicklung. Bihliotheca
Zuologica. Heft 7. 1893.

CHAPTER IX.

KOTATOKIA.

The Rotatoria are peculiar in regard to their reproduction.
Three different kinds of eggs occur among them : in the
first place, thin-shelled summer eggs, which develop par-
thenogeneticallj into females ; then eggs similar to these,
but of only half the size, from which arise the simply
organized males ; and finally thick-shelled winter eggs or
resting eggs, which, as it appears, require to be fertilized.
The eggs are either deposited free in the water or cemented
to the body of the female. The development of the thin-
shelled eggs takes place in many forms even in the body
of the mother ; that of the resting eggs occurs only a loner
time after laying.

The expulsion of the polar globules precedes cleavage.
The parthenogenetically developing eggs, according to
Weismann und Ischikawa, produce only one polar globule.
Little is yet known concerning the development of the
Rotatoria. The chief descriptions are from Salensky, Joliet,
and Tessin ; they present, however, many gaps. In our
presentation of the subject we follow principally Tesrin's
work, which is occupied chiefly with the development of
iJosphora digitata.

Cleavage is fi'om the beginning unequal (Tessin, Joliet).
In the stage of four blastomeres one large and three small
cells can be distinguished (Fig. 115 A). At the time when
the latter divide into six, the abstriction of a new portion
from the large blastomere takes place, and when those cells
which subsequently supply the mesoderm are differentiated
from the cells at first produced, a division of the large
blastomere is still in progress (Fig. 115 J5). That part of
it which is now left as a rather extensive remnant represents

256

ROTATORIA

267

does not, it is
appear to be

the fundament of the entoderm, for it is subsequently over-
grown by the other cells. The small blastomeres, which now
divide repeatedly, are, however, to be considered as ectoderm
and mesoderm.

Particularly striking is the statement that the mesoderm (in the form
of three dark, granular cells) arises by division of the small blastomeres
that were first to appear, and that it still remains united to the ectoderm,
whereas even after its differentiation ectodermal elements continue to be
separated off from the large blastomeres. According to 0. Zachiarias,
however, the mesoderm is supplied directly by the large blastomercy
which on the whole corresponds more to the ordinary mode of formation
of the mesoderm, but
true,
well
established in the case
under consideration.
The conditions of for-
mation of the meso-
derm hitherto known
do not allow a com-
parison with the An-
nelida, as one would
perhaps expect from
the relationships of
the Rotatoria to these
forms.

The three meso-
derm cells lie at the
subsequently dorsal
side of the embryo
(Fig. 115 C). With
the progressive
division of the
ectoderm cells and
the commencing
circumcrescence
of the large blasto-
mere by these, the mesoderm cells are pushed farther forward
(Fig. 115 D). Meanwhile their number has doubled. Even
before the formation of the epibolic gastrula is completed
the enclosed entoderm cell has divided. As the result of

K. H. E. S

Fig. 115.—^ to F, stages of development of Eosphora
digi'tafa (after Tessin). A to C, cleavage stages; D,
epibolic gastrula. The large blastomeres are already
entirely oTergrown ; tbe mesoderm cells lie at the
blastopore. E, the mesoderm cells have moved in»
ward ; an invagination of the ectoderm follows it ;
the entoderm cells have multiplied. F, embryo, on
which the head-, tail-, and lateral lobes can be
recognized, iil, blastopore; Ec, ectoderm ;£», ento-
derm ; Mes, mesoderm.

•258 EMBRYOLOGY

the forward growth of the ectoderm, the mesoderm cells
are now forced inward, and the invagination of ectoderm
cells, which succeeds it (Fig. 115 D, E), subsequently pro-
duces the trochal apparatus and the oesophagus.

The outer form of the embryo is now changed in such a
way that an anterior, posterior, and two lateral protuberances
can be distinguished (Salensky, Tessin). On the surface
which bears the blastopore, these regions of the body are
seen to be separated from one another by shallow grooves
(Fig. 115 F). The posterior elevation bends forward, and
growing further in the same direction, forms the foot (or
caudal appendage) of the Rotifer.

TpssiN seeks to refer the anterior and lateral elevations (cephalic and
lateral lobes) to the lobular processes of the Turbellaria, especially to
those of the larva of Stylochus. Inasmuch as the Eotatoria do not pass
through any real laryal stage, the lobular processes would have become
rudimentary. In the further course of development the cephalic and
lateral processes are again smoothed out, and can no longer be recog-
nized as special structures.

Concerning the origin of the inner organs even Tessin can give little
definite information. We have already mentioned that he derives the
trochal organ and the most anterior part of the intestinal canal from an
ectodermal invagination. On the other hand, he combats the discoveries
of Salensky, for he derives the masticating stomach (pharynx), which
is provided with jaws, fi'om the entoderm ; Salensky maintains that this
part is of ectodermal origin. According to Tessin, by far the largest
part of the intestine (together with the appended glands) arises from the
entoderm, for the latter extends far backwards ; it is said even to send a
process into the caudal appendage. The hind-gut arises by means of an
invagination of the ectoderm (Salensky, Joliet).

The further fate of the mesodermal fundament remained obscure to
Tessin. The statements regarding the origin of the nervous system and
the genital organs are of too doubtful a nature for us to consider them.
Nothing is as yet known concerning the formation of the excretory
organs.

The development of the male of BracMonus urceolaris,
which, as is known, is very simply constructed, takes place,
according to Salensky, in the same way as that of the
female. The degenerative process^ which characterize tlie

ROTATORIA

259

incomplete form of the male begin only after the trochal
organ and the foot have been formed.^

General Considerations.

The development of the Rotatoria gives us as yet no
information as to their doubtful position in the system.
Such forms as that of Trochosphcera cequatoriaUs (Fig.
116), found by Semper in the Philippines, point with
almost imperative force to relationships with the Tro-

FiG. 116. — Trochosphcera wquatorialis (after Sempeb). Ce, cloaca; Dr, appendi-
cular glands of the fore-gut; Ex, duct of the excretory organs; G, brain; Ge,
female sexual organs with duct ; M, mouth ; Mu, musculature ; N, nerve that
emerges from the brain ; S, oesophagus ; Si, sense organ ; TT^, preoral, W,,, post-oral,
ciliated band.

chophore larva of the Annelida (comp. p. 266). Like the
latter, Trocliosphcera possesses a complete preoral circle of
cilia and an indication of a post-oral one. Both of these
are also to be recognized in the trochal org-an of other
Rotatoria, the form of which is different from that of the
Trochophore. The course of the intestine is similar to that
of the annelid Trochophore. The structure of the excretory
system also argues for a relationship with the Trochophore-
like forms. The excretory canals of the Rotatoria begin
with blind ciliated funnels in the body cavity, and the same

1 [Our knowledge of the development of the Rotatoria has recently
been much enlarged by the thorough investigation of Zelinka, to which
the reader is referred (see Appendix to Literature on Eotatoria). — K.]

260 EMBRYOLOGY

is said to be the case in the Trochnphore. In the Rotatoria
there are two trunks to the excretory system, which in the
Gastrotricha, which are related to the Botatoria, open to the
exterior by means of two ventral openings (Zelinka, No.
12), so that this organ thus acquires still greater resem-
blance to the so-called head kidney, the excretory system of
the Trochfiphore, for the two head kidneys also open directly
and separately from each other to the exterior (comp. p. 266)
The agreement of the Rotatoria with the Trochophore was
especially advocated by Hatschek (No. 1), Avith whom
recent investigators of the Rotatoria, such as Plate and
Zelinka, in the main agi^ee (Nos. 3, 4, 11, 12).

Tessin contends against the relationship of the Eotatoria to the An-
nelida or their ancestral form, which we have briefly indicated above,
because, owing to the origin of the trochal organ from the stomodeal
invagination and the position of the brain outside of the area included
within the trochal organ, a comparison of the trochal organ with the
ciliated rings of the Trochophore larva does not seem to him admissible.
Tessin seeks rather relationships to the Turbellaria, being influenced by
the lobular structures of the embryo. His comparison of the caudal
appendage of the Eotatoria with the post-abdomen of the Crustacea,
which he supports with the fact that a process of the entoderm is said to
extend into the tail, seems weak. This perhaps indicates that the
Eotatoria have a tendency to increase in length. We would here recall
the growing out of the Trochophore into the worm. Eelationships of
the Eotatoria to the Arthropoda have also been found in forms such as
Hexarihra pohjptera, to which attention has recently been called by Plate
(No. 4). This remarkable Eotifer, discovered by Schmarda, possesses on
the ventral side three pairs of movable setose appendages, which are like
extremities, and give to it almost the appearance of a Nan/dius. In view
of the close relationships of the Eotatoria to the Trochophore, one will
certainly not think of a descent of the Eotatoria from the Arthropoda,
especially from the Crustacea ; it is, however, interesting to see how
Trocltopliore-like beings can vary in the direction of the Arthropod type,
even if it be only in their outward shape.

Still less justifiable than a comparison of the caudal appendage (foot)
of the Eotatoria with the abdomen of the Crustacea is such a comparison
with the foot of the Mollusca, which has been attempted by various
writers, who have based their conclusions principally upon the position
of both organs between the mouth and anus, which is particularly well
expressed in embryos and larvae.

ROTATORIA 261

Literature.

1. Hatschek, B. Studien zur Entwicklungsgeschichte der Anneliden.

Arheiten Zool. Inst. Wien. Bd. i. 1878.

2. JoLiET, L. MonograiDhie des Melicertes. Arch. Zool. exper. et g(n.

Ser. ii., torn. i. 1883.

3. Plate, L. Beitrage zur Naturgeschichte der Eotatorien. Jena.

Zeitschr. Bd. xix. 1886.

4. Plate, L. Ueber die Rotatorienfauna des bottnischen Meerbusens.

Zeitschr. wiss. Zool. Bd. xlix. 1889.

5. Salensky, W. Beitrage zur Entwicklungsgeschichte von Brachionus

urceolaris. Zeitschr. 2ciss. Zool. Bd. xxii. 1872.

6. ScHMARDA, L. Zur Naturgeschichte iEgyptens. Denkschr. Akad.

Wiss. IVien. math.-jiaturw. Klasse. Bd. vii. 1854.

7. Semper, C. Zoologische Aphorismen : Trochosphsera sequatorialis,

das Kugelraderthier der Philippinen. Zeitschr. wiss. Zool. Bd.
xxii. 1872.

8. Tessin, G. Ueber Eibildung und Entwicklung der Kotatorien.

Zeitschr. wiss. Zool. Bd. xliv. 1886.

9. Weisiunn und Ischikawa. Ueber die Bildung der Eichtungskorper

bei thierischen Eiern. Berichte naturf. Gesell. Freiburg i. Br.
Bd. iii. 1887.

10. Zacharias, 0. Ueber Fortpflanzung von Eotifer vulgaris. Zeitschr.

tviss. Zool. Bd. xli. 1885.

11. Zelinka, C. Studien iiber Eaderthiere II. Zeitschr. wiss, Zool.

Bd. xlvii. 1888.

12. Zelinka, C. Die Gastrotrichen. Zeitschr. wiss. Zool. Bd. xlix.

1889.

Appendix to Literature on Rotatoria.

Zelinka, C. Studien iiber Eaderthiere III. Zur Entwicklungsgeschichte,
der Eaderthiere nebst Bemerkungen iiber ihre Anatomic u. Biologie.
Zeitschr. wiss. Zool. Bd. liii. 1891.

CHAPTER X.

ANNELIDA.

I.-CH/ETOPODA AND ARCHIANNELIDA.

The two chief divisions of the Chsetopoda are unlike as re-
gards development, this being in the Polycheeta generally
indirect, and involving a free-swimming larval stage, where-
as in the Oligochssta it is considerably abbreviated, and
free-swimming larvae are absent. The Archianuelid i are like
the PolychaBta.

1. Development through Free-swimming Larvae

(Polychasta and Archiannelida) .

In general the Polycheeta develop from free-swimming
larvae which are provided with ciliated bands. Only a few
forms bring forth living young. Such is the case in Eunice
sanguinea, Syllis vivipara, and in a Cirratuliis in which the
eggs develop either in the body cavity or in the cavity of a
segmental organ which has become a uterus. A kind of
brooding also occurs in many forms, as, for example, in Auto-
lytiis cornutiis, an extensive sac, in which the eggs are
developed, being formed on the ventral surface by the dis-
tension of the skin of the body. In Polynoe cirrata, the eggs,
which ar-e stuck together in a single mass, are carried on the
dorsal surface under the dorsal scales. Similar to this is
Grubea linibata, in the females of which at the time of sexual
maturity the entire back is thickly covered with eggs, while
Exogone gemrnifera and Sphserosyllis pyrifera carry their eggs
on the ventral side, namely on the ventral cirri (Viguier,
No. 46). In Spirorbis Fagenstecheri the tentacle which bears
the operculum of the tube is enlarged, and thus serves as a
brood-chamber ; in Spirorbis spirillum, on the contraiy, the

ANNELIDA 263

I

eggs are deposited inside the tube, and are found here be-
tween it and the body-wall. Similar to this is the brooding
in the genus Capitella, in which one finds the eggs firmly
attached like a mosaic, on the inner surface of the tube.
Other tubiculous worms attach their eggs to their habitations
on the outside ; thus in Sabella lucuUaria the eggs, which are
enveloped in a slimy mass, form a thick ring around the tube
of the parent. Many PoJychseta deposit their spawn in the
form of large gelatinous packets or clumps (Aricia, Psygmo-
hranchus) ; others discharge the eggs into the sea-water
without any other protection than the egg-membrane
(Ertpomatus, Pomatoceros et al., likewise Polygordius). In
such forms artificial fertilization can be employed with
success.

Cleavage is unequal, but in some forms may approach very
near to the equal type (Pomatoceros according to v. Deasche).
In the latter case it produces a aeloblastula, the entodermic
part of which can be distinguished by the greater thickness
{Sahellaria, Aricia), or an epibolic gastruJa is formed (Nereis,
Psygmobranchtis) . The Polychgeta, which have been studied
with this object in view by Hatschek, Goette, v. Drasche,
Salensky, and other investigators, offer all transitions between
the different types of cleavage, and correspondingly the form
of the gastrula also varies from a typical invagination to an
epibolic gastrula. In Terebella Meckelii, for instance, we find
a blastula with the wall thickened on one side, the cavity of
which soon becomes filled by the intruding macromeres, so
that we now have before us a so-called sterrogastrula (Salen-
sky). [Wilson (Appendix to Literature on Annelida, No.
XXVII.) has recently given a very detailed account of the
early stages of development in Nereis, especially of the cleav-
age, in which the fate of the individual cells is established
with great precision. — K.]

As an example of the embryonic development of a Poly-
chaete, we select that of Eupomatus (according to Hatschek).
The spherical egg is divided by means of the first two meri-
dional planes and succeeding equatorial plane of division
into eight blastomeres of almost equal size. Soon, however,
the divisions at the animal pole take place more rapidly than

264

EMBRYOLOGY

afc the vegetative pole, and thus the blastomeres at the latter
remain more voluminous. In the resulting blastula, the cells
from which the three germ-layers arise are already differen-
tiated (Fig. 117 A). The upper hemisphere, composed of
.smaller cells, gives rise to the ectoderm, and the greater
part of the lower to the entoderm; however, two cells here
are distinguished at an early period from the others by
assuming a more spherical shape: they produce the mesoderm,
and are called by Hatschek the primitive mesoderm cells
[mesodermal teloblasts (Fig. 117)]. The region where they
lie corresponds to the anal end of the larva. Even as early
as this stage a delicate equatorial cii'cle of cilia makes its
appearance, the future preoral ciliated band of the larva.

tics -At-

Fie. 117. — A, B, blastula and gastrula stages of Evpomatus vncinalus (after
H^tschbk); eh, egg-membrane; met, one of the two mesoderm cells.

Soon afterwards the tuft of cilia at the apex of the larva
makes its appearance (Fig. 117 B). The cilia perforate
the egg-membrane, which therefore most probably consists
of a soft mass.

The subsequent behaviour of the egg-membrane is of a peculiar nature,
for, according to the concurrent statements of various authors, it is pro
visionally retained, increases in extent with the growth of the larva, and
is thus formed into a cuticula-like enveloj^e (Hatschek, No. 20), which,
however, is replaced later by the permanent cuticula from the ectoderm.
Thus here the embryo is converted directly into the larva.

The entodermic part of the blastula now invaginates. At

ANNELIDA

265

the same time the two primitive mesoderm cells have moved
into the inside, having- detached themselves from their con-
nection with the other cells. In the lateral aspects seen in
Fig. 117 A and B, only one of the two cells can be recognized.
It should be mentioned here that later they divide (Fig.
118). The two primitive cells still continue to be dis-
tinguished from the newly formed ones by their greater size.
Hatschek calls them the two pole cells of the mesoderm.
They lie at the ends of the two mesodermal bands formed
by cell-proliferation. In the farther development of the

Pio. 118. — A and B, trochophores of Euponiatus in younger and older stages
of development (after Hatschek). a, anal opening ; ab, anal vesicle ; fcn, head
kidney; in, mouth-opening; mes, mesodermal bands; mit, muscles; oc, eye-spot;
ot, auditory vesicle ; sp, apical plate.

larva the intestine bends toward the anal side, in order
to fuse in later stages of development with the originally
slight depression of the ectoderm, which produces the hind-
gut and anus (Fig. 118). Some time before the completion
of this process, the blastopore had become considerably
narrowed. It assumed the form of a fissure which closed
(from behind forwards) and left only a small opening re-
maining in front. At the place of this last trace of the

266

EMBRYOLOGY

blastopore, the ectoderm becomes invaginated, and forms the
oesophagus. This is followed bj an enlargement, the sto-
mach of the larva, and this in turn by the small-gut and the
hind-gut (Fig. 118 B).

The formation of the intestine takes place less simply in the cases
where the gastrulation is epibolic and the entoderm at first consists of a
compact mass of cells. The intestinal wall is only gradually formed,
detaches itself from the central yolk-mass, and finally unites at the fore-
and hind-guts with the ectoderm (comp. the figures 128 A and B, p. 280,
of Psygmobranchvs).

Trochophore. — Even during gastrulation the embryos
rose, with the aid of their vibi-atile apparatus, from the
bottom, and betook themselves to the surface of the water.
Together with the internal changes described, alterations
have also taken place on the outer body, the upper portion
of which has become bell-shaped, while the under-portion

tapers more conically (Figs.
118 5 and 119). The band of
cilia which lies in front of the
mouth extends around the
longest periphery. Thereby is
reached the larval stage, de-
signated by Ray Lankester as
the Trochosphere, but now with
Hatschek more generally known
as the Trochophore. In addition
to the organs already mentioned
— ciliated band, intestinal tract,
and mesodermal bands — still
others can be seen in the Tro-
chophore. An ectodermal thickening at the upper pole,
which bears the tuft of cilia, if such be present, is de-
signated as the apical plate ; it represents the fundament
of the super-oesophageal ganglion (Figs. 118 A and 119).
The cells of the preoral ciliated band also form a similar
thickening. These may consist of several successive circles
of cells, and between or underneath them is placed a ring
of fine nerve fibres, which is connected with ganglionic cells,
and is considered by its discoverer, Kleinemberg, as the

an

Fig. 119.— Larva of Pofygordius
(after Hatschek, from Balfouk's
Qo'm'paralive 'Enxhr'\jo\og\f) . an, anal
0[)euin<j ; m, mouth-opening ; me. p,
mesodermal band ; njifi, head kid-
ney ; ol, stomach ; sij, apical plate.

ANNELIDA 267

central nervous system of the larva. At the base of the
ciliated cells there also lies, according to Kleinenberg, a
ring of muscle cells, vphich, like the ciliated band itself, is
made use of by the larva in locomotion. In addition, various
other muscle strands traverse the inside of the body ; thus
some extend from the apical plate to the stomach, others are
found in the lower part of the body, and one surrounds the
intestine at the point where the stomach and oesophagus
unite (Fig. 118 B). These muscle cells have become de-
tached from the mesodermal bands (Hatschek). From the
latter also arises the so-called head kidney, the paired excre-
tory organ ; it is formed from a few cells situated near the
pole cells, which increase greatly in length and become
hollow. The head kidney then extends from the pole cells,
that is, from the vicinity of the anus, as far as the oesophagus
(Fig. 118 B, Jen). It consists of a ciliated canal, which
may branch (as, e.g., in Folygordiiis), and of one or more
funnel-shaped terminations (Figs. 119 nph and 120 B, kn).
These do not open freely into the blastocoele, but are said to
end blindly (Fraipont), and in this regard therefore re-
semble more the excretory system of the Platyhelminthes and
Rotatoria. The spot v/here each of the two head kidneys
opens to the exterior can be seen from the figures 120 A
and B.

Of the ectodermal structures of the larva there should
still be mentioned as important, in the first place, the eye-
spot, consisting of an accumulation of pigment, which in the
larva of Eupomatus is located in a cell of the apical region,
but asymetrically on the right side (Fig. 118 B, oc). The
two ectodermal vesicles which arise symmetrically on the
posterior portion of the body, each from one ectoderm cell,
also constitute sensory organs (Hatschek). They are pro-
vided with fine stiff hairs, which project into their lumina ;
highly refractive concrements are also found inside of them.
Thus they are to be recognized as otolith vesicles (Fig. 118
B, ot). The large sac which in Fig. 11 8 B is seen lying at
the posterior end of the larva arises by the enlargement of an
ectodermal cell. This anal vesicle appears to have no im-
portant significance. In Eupomatus is developed another

268 EMBRYOLOGY

(perianal) circle of cilia, which is situated on the posterior
portion of the body (Fig. 118 B) ; this is lacking in many
other Annelids. Furthermore there is added on the ventral
side a ciliated area extending from the mouth backward,
the adoral ciliated zone (Figs. 118 B and 128 A, p. 280).

The metamorphosis of the Trochophore larva into
the adult worm will be followed in Polygorditis, a form in
which it takes place in a particularly clear manner. The
Polygordius larva was first discovei-ed by Lov^N, and held
to be that of a cha3tiferous worm. Ant. Schneider showed
that Lov^n's larva belonged to Puhjgordius. It has the form
of a typical Trochophore (Fig. 119). The ciliate apparatus,
which encircles the larva at its greatest diameter, is com-
posed of two rings, one in front and one behind the mouth.
The preoral ring consists of a double, the post-oral of a single,
row of cilia. A third ring, the perianal ciliated band, makes
its appearance at the posterior end of the larva (Fig. 120 B),
bat it is not formed until the later stages of development.
The development of the Trochophore into the perfect worm,
which has been thoroughly studied by Hatschek, consists first
of all in a growing out of the posterior section of its body
and a gradual reduction of the anterior part.

At first a segmentation of the larva is noticeable (Fig. 120
A), which depends upon a marked change in the two meso-
dermal bands. These, which at first consisted of only a few
cells, have become by active cell-proliferation much more
voluminous. Each of them is separated into two cell-layers
(Fig. 133 A to C, p. 290), and spreads out toward the ventral
and dorsal lines. Then a segmentation makes its appear-
ance in them, proceeding from in front backwards (Fig. 120
A), and at the same time the two layers of the bands sepa-
rate from each other by the formation of a cavity in each
segment. In this way the primitive segments arise, the
outer and inner walls of which become in each segment of
the worm the somatic and splanchnic layers of the mesoderm,
and the walls, abutting on one another, form the segmental
boundaries (dissepiments) of the body of the worm.^ Since

' A more detailed description of these conditions will be found in the
discussion of the formation of the body cavity (comp. p. 289).

ANNELIDA

269

to each segment of the body a pair of pt^imitive seg-ments be-
longs, these meet in the middle line of the ventral and dorsal
surfaces, and form there a ventral and dorsal mesentery
(Fig. 133 G, p. 290). In the figures 120 A and B (lateral
views of the larva) the primitive segments can already be
recoo-nized in the form of an internal segmentation of the
larva. The most anterior primitive segments are the oldest,
the posterior ones younger. The body is seen to be already
considerably grown out backwards, although the head por-
tion has not yet diminished in circumference. Two ciliated
tentacles, which are
still very small, now
make their appear-
ance on the apical
plate (Fig. 120 B).
The originally sac-
like mid -gut has
grown in length
with the body, and
is now cylindrical in
shape. Very near
the posterior end of
the body, a short
distance in front of
the anus, is the pos-
terior ciliated band
(Fig. 120 B).

While the pos-
terior part of the
body of the larva is
gradually passing
from the earlier conical into the cylindrical form, the head
portion attains its greatest volume, but thereafter soon
diminishes. The metamorphosis of the voluminous cephalic
vesicle into the slender cephalic process of the worm is
effected by the thickening and conical outgrowth of the
apical plate (Fig. 121 A), and by the contraction of the wall
of the head generally. The previously flat cells become
considerably thickened, whereby the circumference of the

Fig. 120.—^ and B, larvae of Polygordius (aftet
Hatschek). a, anus ; m, mouth-opening ; fcii, head
kidney ; m'^s, mesodermal bands ; sp, apieal plate.

270

EMBRYOLOGY

entire head is diminished, until it is not much larger than
the trunk. The apical plate has grown out forward in the
form of a cone. The eyes are more conspicuous than in
the larva. In the trunk the primitive segments have in-
creased in number, and made the
segmentation of the body still more
distinct, since they have enlarged
more and have applied themselves
more closely to the intestinal and
body-walls. At the posterior part
of the trunk they are less clearly
expressed. These changes are
much more evident in the last
stage of development (Fig. 121 B),
which we introduce for comparison.
There the segmental constrictions
of the intestine cause the metame-
rism to be still more distinct. The
cephalic vesicle and the vibratile
apparatus have already entirely
disappeared in this stage ; and we
have now before us in its chief
features the adult worm, although
it has not yet reached its complete
development. The worm gives up
the larval mode of life, that of
floating upright in the water, and
adapts itself to locomotion by
creeping. The papilla), which
make their appearance in front of
the posterior band of cilia, which
has now disappeared (Fig. 121),
serve the worm for the purpose of
attachment.

The Different Larval Forms.
— Polygordius was selected as an
example because it shows in a par-
ticularly instructive manner the transition of the larva into
the worm. It does not show, however, the ordinary condition

Fig. 121.—^ and li, larvw of
Polygordius (after Hatschkk).
a, anal opening ; m, mouth-
opening ; kn, head kidney.

ANNELIDA 271

of the Trochophore larva, for the anterior bell- shaped part in
the majority of cases is not retained unaltered for so long
a time. Generally also it does not surpass the trunk so
considerably in size, and it soon comes to be even smaller
than the trunk. Since in many forms the typical shape of
the Trochophore is not so strongly expressed, and, on the
other hand, the segmentation of the body of the worm makes
its appearance at an early stage, many deviations from the
shape of the Trochophore are realized. The larvae of
Annelids are very variously shaped, for some of them, owing
to the early appearance of the segmentation, are found in
phylogenetically younger stages than the Trochophore, and
others, although they stand at the same level with it.
may be modified by the occurrence of various kinds of
locomotor organs and by other external changes in form.
The principal difference in the larvoe consists in the presence
or absence of segmentation of the entire larva, not includ-
ing that of the trunk part, which is acquired only during
the metamorphosis. To be sure, this differeuce should not
be overrated, for the segmented forms likewise must pass
ontogenetically through an unsegmented stage. The Annelid
larvae have usually been distingaished according to the dis-
tribution of their cilia : as Atrochse when a ciliated band is
lacking ; Monotrochse with a preoral band of cilia, to which,
as in the Teloti'ochge, there may be added a post-oral band
lying directly behind the mouth ; Telotrochse with an anterior
and posterior (perianal) band of cilia; Mesotrochas, in which
the ciliated band is situated in the middle of the body ; and,
finally, PoJytrochse, which possess a greater or smaller num-
ber of ciliated bands, and as a result of this exhibit at an
early stage a segmentation of the body. The ciliated bands
of the Polytrochse may form either closed rings, or only half-
rings. In the latter case, according to their position on the
dorsal or ventral surface, Nototrochse and Gastrotroohse are
in turn distinguished. They are called Amphitrochse when
ventral and dorsal half-rings alternate with one another.
This classification has been made use of by different investi-
gators for distinguishing the larvae. However, Clapak^de
and Metschnikoff themselves, to whom we owe the most

272

EMBRYOLOGY

thoroagli knowledge of the Annelid larvfe, point out that
the characters cited have no great morphological value, for
larvfB occur in the same family, and even in the same genus,
which belong to more than one of these types. The differ-
ences in shape are probably due to differences in the mode
of life. Variations in regard to the development of the
locomotor apparatus — i.e., in the distribution and stoutness
of the ciliation — would easily follow, if the larvae of closely
related forms adopted different modes of life, as is actually
the case. Terebella larvoe (Terebella conchilega) are known
which must be placed among the Nototrochx, while others
belonging to this genus entirely lack the ciliated rings {Tere-
bella MeclceUi). The former are good swimmers, and lead, a
pelagic existence ; the latter, on the contrary, never move far
away from the masses of eggs from which they hatch, and
may sometimes develop into young worms, even in the jelly
surrounding the eggs.

The presence or absence of the preoral band of ciHa may well be im-
portant in the interpretation of Annelid larvte, for (according to Klei-
nenueeg) this alone possesses a ring-nerve, which is said to be lacking in
all other bands of cilia that make their appearance, with the exception of
the so-called post-oral band, which stands in close relation to the preoral.
Even where posterior ciliated bands appear without the existence of an
anterior one, as in the Mesotrochcc, the ring-nerve is said to be absent.

But these conditions are as yet
too little studied to allow one
to base on them a distinction
between the larva;.

Out of the multitude of
variously formed Aimelid
larvae, only a few of the
particularly characteristic
forms can be chosen. We
shall first consider the
unsegmented larvae.

The simplest larvae of
the Annelids are un-
doubtedly those whose
body is covered with a

Fig. 122. — A and B, so-called atroclial
Aiiiiolid InrvjB — A, of Lumbviconerci.t (?)
(after Ci.ai-aredb cnd Mktschnikoff); B,
of SIcrtKisjiis nc/utala (after Vkjdovbky).
eu, cuticula ; d, intestine; ent, entoderm.

ANNELIDA 273

uniform coat of cilia, and which at the most possess at the
anterior end of the body a tuft of cilia, which serves for
steering {Atrochce, Fig. 122 A, B).

CLAPARfeDE UND Metschnikoff describe atrochal larvae of Ltimbrico-
nereis (?), and Vejdovsky those of Sternaspis.^ Both larvse are at first
spherical, but later become elongated (Fig. 122 B). The former possess
eye-spots ; the latter do not. A differentiation in the ciliation ajopears
even in these larvae, for in Lumhriconereis narrow zones, one toward the
anterior and one toward the posterior end, remain free from cilia, and in
Sternaspis the entire posterior end is without cilia (Fig. 122 A, B).
Inside one recognizes in the first form the sac-like fundament of the
digestive tract, in the latter, on the contrary, only a compact mass formed
of large entoderm cells.

The further development of Lumbriconereis is marked by the ap-
pearance of setffi in pairs at the posterior end of the body, thus giving
expression to the segmentation. At the same time the degeneration of
the cilia begins. In Sternaspis the entire ciliation of the body dis-
appears, and the larva continues to live in this naked condition for
some time, the segmentation of the body being as yet unrecognizable
(Vejdovsky, Eietsch). Its further development was not followed in
detail.

It is difficult to say whether in the evenly ciliated larvae we have to do
with phylogenetically older stages than those represented by the Trocho-
phore. The incomplete develojiment of the intestinal canal, especially
in the larva of Sternaspis, and also the subsequent development of this
worm, make it appear as more probably a derived form. Although in
Lumhriconereis the cilia in later stages are arranged into an anterior
and posterior region, this distribution is altogether too indistinct to be
referred to the anterior and posterior ciliated bands of the Trochophore.

Although we are not justified in looking upon these
atrochal larvae as primitive forms, still it appears to follow
from the development of another Annelid that the larvse
which are provided with ciliated bands represent a stage
succeeding the atrochal forms. In Terehella Meckelii, which
was studied by Milne-Edwards, Clapari&de und Metschni-
koff, and later by Salenskt, there arises from a larva, which

* Sternaspis has been classed with the Echiuridae ; nevertheless in this
form, which is provided with set£e, such a distinct segmentation is ex-
pressed, both externally on the body and internally, in the matter of the
arrangement of the muscles and blood-vessels (Eietsch), that this group
of Annelids — very aberrant, it is true — must still be placed among the
Chaetopoda.

K. H. E. T

274 EMBRYOLOGY'

at first is rather evenly ciliated, one having a preoral and
a perianal band of cilia, v?hich is substantially in the
TrocJwphore stage.

The young larvae of Terehella Meckelii are at first spherical, then
elongate a little, and become covered with an even coat of cilia, which
leaves bare only the small part of the anterior end of the body lying in
front of the eyes. Later they become pyriform, and the cilia now cover
only the voluminous anterior part of the body, whereas the posterior
region is destitute of them. It is only in a later stage of development
that a perianal row of cilia makes its appearance. In this stage it re-
sembles the previously described larva of Lumbriconereis. The ciliation
is gradually confined to a preoral band, a i^erianal band, and a ventral
ciliated groove (Salensky). To be sure, the outer form of the larva is in
this case, on account of the small size of its bell, not that of a typical
Trochophore ; nevertheless nothing prevents us from comparing it to one
that has already begun its metamorphosis into the worm. In front of the
mouth lies the preoral band of cilia ; the intestine has the usual shape ;
at the posterior end, in the vicinity of the anus, is found the perianal band
of cilia. About midway between the anterior and posterior ciliated bands
appear indications of the two first segments, behind which others soon
follow. They become noticeable externally by the development of pro-
tuberances, which are studded with seta?. The worm grows in length ;
evaginations at its anterior end form the tentacles ; it secretes the tube
and attaches itself.

The larvas of the Chcetopteridiv, known as Mesotrochce, also
arise from uniformly ciliated embryos. In Chcetopterus

pergamentacetcs, which at first is even-
ly ciliated, there is formed a tuft of
cilia at the anterior end of the larva,
and gradually a ring also of cilia, en-
circling the body at about the middle
(Pig. 123). The inside of the larva is
pretty well filled by the large sac-like
intestinal canal. The larva of Telep-

Fia.i23.-so.caiio(imc. ««^"* coslarum is similar, only that it
sotrochai larva of c;i(B(op- lacks the anterior tuft of cilia. On

Wilson), m, mouth. _ _ , ., .

chcetopterus socialis also exhibits stouter
cilia at the anterior end; it possesses two ciliated bands
which lie close to the posterior end. A preoral band of cilia
is not pi-esent in these larvae, and the middle one cannot be

ANNELIDA 275'

directly compared to the perianal ciliated band of other An-
nelid larvae, for it is not, like that, situated at the posterior
end, but a number of segments make their appearance be-
tween it and the hind end. The anus in these larvae is
placed dorsally, for a pointed prolongation is formed pos-
teriorly on the ventral surface, a condition which also recurs
in polytrochal larvas (corap. Fig. 127).

Noteworthy is the tuft of cilia at the anterior end which we met with
in the atrochal and mesotrochal larvae, and which is also recalled by the
stout cilia found at the head end of many telotrochal and polytrochal
larvae. Such a ciliation of the apical area occurs also in Turbellarian,
Nemertean, and Molluscan larcce, and has pei'haps a higher significance
than that of a merely secondary acquisition, connected with the larval
mode of life only.

Apparently aberrant larval forms, such as those of Mitraria
(Fig. 124 A, B), are referable to the Trochophore. Mitraria
would, therefore, have to be classed with the Monotrochoe, in
which a preoral, but not a perianal, band of cilia is de-
veloped (comp. the larva of Psygmohranclius, shown in Fig.
128, p. 280). MonotrocfuE and Telotrochce cannot be separated
from each other, inasmuch as in the beginning the larvae
frequently possess only a preoral band of cilia, are therefore
monotrochal, whereas later a perianal ciliated band, which
gives them the character of Telotrochce, is developed on them.

Mitraria, the Annelid larva discovered by Joh. Mullek, and subse-
quently more thoroughly studied by Metschnikoff, can easily be recog-
nized in its young stages as a Trochophore, with a well-developed bell, but
much-reduced posterior portion (Fig. 124 A). As a result of this, the
anus and mouth are brought close together. The ciliated band lies in
front of the mouth. Later the posterior i^art of the body grows out
more, and the ciliated band, which acquires many outfoldings, there-
fore comes to lie more anteriorly (Fig. 124 B). In this figure the
beginning of the worm, which is gradually developed out of the
Mitraria, can be easily recognized. On the lower area, surrounded by
the ciKated band, two lateral jarotuberances, which bear long cilia, can
be recognized in the young larva. In the older larva they are seen
lying dorsally. The metamorphosis of the larva into the tubicolous
worm is due to the vigorous growth of the segmented posterior portion
and the degeneration of the chief part of the Mitraria, together with
its lobes and setiferous papillas. Thereupon the larva sinks to the bottom,
secretes the tube, and becomes attached.

276

EMBRYOLOGT

In Mitraria we i-ecognized a larva whicli possesses pro-
visional larval appendages in the form of long bristles,

Fig. 124.— Lateral views of Mitraria larvfe (after Metschnikoff, from Balfour's
Com-parative Emhryology). an, anus ; b and br, the lateral elevations with the pro-
visional set»; m, mouth; pr. h, preoral ciliated band; sg, apical plate.

which also occur in other Annelid larvae. Trochophore larvfB
are known "which exhibit a number of long denticulate
bristles on both sides of the body, thus, e.g., in the genera

Sabellaria, Spio, etc. Figs.
125 and 126 show larva?
more advanced in develop-
ment with richly developed,
and in part extraordinarily
long, provisional setfB. Setae
of this kind do not appear
in adult recent Chajtopoda,
but, on the other hand, are
found in fossil forms. It
has been conjectured that
they might have been inherited from unsegmented ancestors
of the existing Chastopoda. This suggestion appeared to
be supported by the fact that they are mostly found on the
anterior unsegmented part of the larva (Alkx. Agassiz).

Fig. 125. — Larva of Ntrinc (alter Alkx.
Aga-BSiz, from Balfoub'b Comparative
Embryology).

ANNELIDA

27"7

The larvge of Ophryotrocha puerilis (Fig. 127) are Poly tro-
chee— segmented larvte in the proper sense. They possess a
number of segments, each one of which
is provided with a ciliated band. In
addition, stouter tactile hairs are found
at the anterior and posterior ends of the
body. The first ciliated band belongs
to the head region of the larva. Next to
this is situated the mouth-opening, which
leads into a large pharynx provided with
a chewing apparatus. The intestine
extends straight backwards, and opens
to the exterior at the end of the last seg-
ment. The anus is situated dorsally, in-
asmuch as the last segment possesses a
pointed process on the ventral side (Fig.

127;.

The next stages of development in Ophryo-
trocha remain much like the larva described,
since the new segments formed in front of the
anal segment are also provided with bands of
cilia. Knob-like parapodia then bud out on the
segments, and in them the set^e are developed.
The number of the segments is considerably in-
creased, yet this small Annelid, which never
becomes over 2-5 mm. long, remains, as it were,
in a larval condition, since the segments retain
their ciliation throughout life. Still another
ciliated band has been developed on the head,
and two small knob-like feelers have arisen there,
which bear long cilia, just as do the two cirri
which have made their appearance near the unpaired process on the
anal segment. The two most anterior segments remain without appen-
dages (Claparede und Metschnikoff).

Fig. 126. — Annelid larva
with provisional setEe
(after Alex. Agassiz,
from Baifoue's ConiparoA
live Erabryology).

In Ophryotrocha the ciliated rings surround the entire
segment. They appear to be arranged in the same way in
Arenicola marina ; in other larvae, on the contrary, they have
the form of half-rings only, and are confined to the dorsal
or ventral surfaces (Nototrochce and Gasterotrochce) . Noto-
trochal larvse are found, for example, in Terebella conchilega,

278

EMBRYOLOGY

giisterotrochal in members of tlie genera Magelona, Nerine,
and Spio. In the two last-named genera there are found
ampMii-ochal larva? — i.e., such as possess dorsal as well as
ventral half-rings — in addition to the gasierotroclial , just as
atrochal and polytrochal larvae appear in the genus Terebella.
The polytrochal larvae sometimes appear
as a stage succeeding other larval types.
Thus those of Arenicula marina arise from
larvae which at first were monotrochal, later
became telotrochal, and finally, by the ap-
pearance of new ciliated rings between those
already present, assumed the stage of poly-
trochal larvae (Max Schulize). Also the
true polytrochal larvfe — i.e., those which
possess only the ciliated bands, but do not
yet, like many other polytrochal larvos, ex-
hibit the fundaments of the setae and other
parts of the body of the worm — appear as the
stage succeeding the Trochophore. Thus we
have just noted a stage entirely resembling
a TrochopJiore, which preceded the polytrochal
larva of an Ophryotrocha. This condition warrants the
assumption that the segmented forms are to be looked upon
as the younger, the unsegmented, on the other hand, as the
phylogenetically older.

As may be inferred from the manifold shapes of the
Annelid larvae, their metamorphosis into the worm is also
extremely varied.

This has already been briefly discussed in some forms
while considering the larval stages. The segmentation may
be expressed on the body of the larva in various ways. In
some cases the body elongates and divides into segments,
while the ciliated bands are still retained. In other forms the
setae alone first make their appearance in pairs, and indicate
the segmentation of the body, or at the same time the
parapodia are established in the form of protuberances.
Thus larvae are found which have still preserved the entire
form of the Trochophore, and yet exhibit already the two
lateral rows of setse or parapodia. At first only a few

Fig. 127.— Poly-
trochal larva of
Oi)hryoiroeha jiwe-
rilis (after Ci.apa-

BEDE UND MeTSCH-

nikoff). d, intes-
tine ; fc, jaws.

ANNELIDA 279

segments are present ; new ones are, however, continually
being interpolated behind. Since, moreover, the parapodia
acquire more and more their permanent shape, and the
larval organs, on the other hand, degenerate, the larva
approaches the shape of the adult animal.

The segmental appendages do not arise in a uniform manner in the
different divisions of the Polychaeta. In the Errantia the dorsal and
ventral parapodia arise from a common fundament, which afterwards
separates into the dorsal and ventral parts. This has been observed, for
example, in Nereis. Such a separation, however, does not take place in
the Sedentaria, but their dorsal hook-bearing segmental appendages arise
independently of the ventral parapodia (thus in Terehelhi). Accordingly
it is maintained that only the dorsal appendages of the Sedentaria cor-
respond to the common parapodial fundament in the Errantia, whereas
the ventral appendages are to be considered as new formations of a
secondary nature (Salensky).

The cirri and tentacles arise as elevations and evaginations of the
ectoderm, into which continuations of the somatic mesoderm may also
extend. Of these the unpaired median tentacle, as it occurs, for example,
in Terebella, Pileolaria, and Psygmobranclms, usually extending forward
beyond the head, is of an especially peculiar nature. It attains at first
a large size, and is provided with a considerable cavity (Terebella), but
may soon become reduced in size again {Psygmobranclms). When it is
present, there are found near it, and on either side of the head, the
lateral tentacles, the number and form of which are very variable in the
different Annelida. The tentacles may attain a pecuhar development by
putting forth bud-like evaginations, which enlarge and become the gills.
In Psygmobra7ichns the larva, by means of the trifid gills, acquires quite
a peculiar shape (Fig. 128 B). ^ The median tentacle, which was present
somewhat earlier and extended forward beyond the head, has already in
this stage degenerated. As sometimes the tentacles, so also may the
eyes, degenerate in the Sedentaria, since these sessile forms can scarcely
have further need of them. In Psygmobranclms a peculiar organ is
seen lying behind the gills (Fig. 128 B, kr), which is also developed in
other Annelida (Pileolaria, Spirurbis). This is an annular fold of the

1 The larva which is here figured has developed from a so-called
monotrochal larva (Fig. 128 A), which exhibits the form of the Trocho-
phore, provided with a preoral and post-oral ciliated band, while the
preanal band is wanting. The post-oral ciliated band is continued
into the ventral (so-called adoral) ciliated groove. The Troehophore
already possesses two eye-spots, but still lacks the intestinal canal, which
is represented by only an entodermal mass of large cells. The mouth
is already indicated.

280

EMBRYOLOGY

Clii

skin, which grows out backwards, and surrounds like a mantle the part

of the body lying behind the head.

The first two segments lying behind the head are conspicuous in many

Polychaeta {TerehcUa, Oph-
ryotrocha) by the fact that
they are destitute of seg-
mental appendages ; this
fact has caused them to be
reckoned as belonging to the
head, which is thus sup-
posed to arise from several
segments. However, the
manner of formation of
their internal organs (neural
and mesodermal), which are
begun like those of the body
segments, is an argument
against this (Salensky).
Differences of opinion exist
among the different investi-
gators concerning the origin
of the head itself, for some
of these maintain that it is
formed from the i^reoral por-
tion of the Trochophore
alone ; while, according to
others, post-oral parts of the
larva also enter into its for-
mation.

In forms which, like
Exogone gemmifera and
Grubea Umbata, brood
their eggs, the stage of
the free larva may be
altogether omitted, the
embryo breaking
through the egg en-
velope in the form of
the young worm al-
ready provided with a
number of segments,
parapodia, and cirri (Viguier). It is these conditions which
recall those in the Oligochoeta, more especially since one of

Fig. 128. — A, B, larval stages of Psygmohrnn-
chus protensus (after Salensky). X, Trochophore
with pre- and post-oral ciliated bands and adoral
ciliated groove (seen from the ventral surface);
m, region of the subsequently formed mouth,
opening ; ent, still undifferentiated entodermal
mass within; B, later stage with gills (fc); b,
fundaments of the setae ; fcr, collar ; and vd, duI,
ed, fore-, mid-, and hind-gut.

ANNELIDA 281

the forms raentioned (Exngone) is said to pass through a
stage of development which, according to the description of
ViGUiER, strongly resembles the " larvae " of the Oligochffita.

2. Development without Free-swimming Larva

(OUgochssta) .

The Oligochgeta lay their previously fertilized eggs in firm
cocoons, consisting of a chitin-like substance. The cocoons
vary greatly in shape in the different genera, and, according
to the life-habit of the worm, are found either in the earth
or attached to aquatic plants. The slender, spindle-shaped
cocoons of Criodrilus attain a considerable length (as much
as 5 cm.). In the Lumbricidce they are rounded or ovate,
and of different sizes in one and the same species, being
about as large as a pea or a bean. Correspondingly the
number of eggs which they contain is also variable. Some-
times only a very few eggs are found, while in other cases
the number may reach as many as twenty or thirty. Usually
not all of these eggs develop, but, as appears, some of them
develop at the cost of the others. Ordinarily the eggs float
in an albuminous mass. Their development is different
according as they contain a small amount of food-yolk
(Lumbricus, Criudrilus), or possess abundant yolk (Rhyn-
chelmis, Tubifex). Cleavage is always unequal, but in the
first case an invaginate gastrula is formed, while in the
second an epibolic gastrula occurs.

Cleavage and the formation of the germ-layers in the Oligochseta have
been thoroughly studied by various investigators (Kowalevsky, No. 27 ;
Hatschek, No. 18; Kleinenberg, No. 24; Vejdovsky, No. 45). In
Lumbricus a blastula is formed which is thicker on one side, and which
may be flattened so that the cleavage cavity is small ; and the gastrula,
which soon arises by invagination, is also at first rather flat (Kowalevsky,
Fig. 130 A). These characters are less marked in the case of Lumbricus
trapezoides (Fig. 129 A), in which occurs the peculiar phenomenon of the
division of the embryo in the gastrula stage, producing in this way two
embryos, which, separated from each other, develop further. Fig. 129
A represents such a stage of division of an embryo, and shows the two
embryos (which are in the same stage of development) only slightly
united.

282

EMBRYOLOGY

In case the egg is very rich in yolk, as in Rhi/in-hehnig, there arise,
according to Vejdovsky, as the result of the first divisions, four blasto-
meres, from which four much smaller blastomeres are constricted off, so
that now four micromeres and four macromeres are present. While
the micromeres increase rajjidly by division, the hindmost and largest of
the four large cleavage spheres buds off three cells of medium size : the
mesomeres. Now the macromeres also divide further ; the micromeres,
which, as well as the mesomeres, have meantime increased in number,
grow over the latter, which thereby come to lie inside. Between micro-
meres and macromeres a small cleavage cavity arises, which is soon
obliterated, when the small cells grow over the large ones further, in
this way forming an epibolic gastrula.

Ordinarily several, usually two, blastomeres are differen-
tiated before the formation of the tw^o primary germ-layers,
but apparently exhibiting relations to both of them; these
withdravF from connection with the other cells and enter the
cleavage cavity (Fig. 129 A). They constitute the fundament

Fig. 129. — A to C, sections throuuh tmlji-yos of Luiuhficiis trapezoidex (after
Ki.raNKNBKHO, from Balfocu's Vompnrntivc Emhynolnnn). A, liorizontal longi-
tiuliiml section of an embryo in the gastrula stajre, which is about to divide into
two embryos: between ectoderm and entoderm the two large pole cells of the
mesoderm (m') can be recognized, with the mesodermal liands (ms) arisinjr from
them on either side ; al, iirchcnteron ; /} and C, cross-sections of somewhat older
cmbryiis, whicli show how the nicsoderiniil bunds (m<) move toward the ventral
Bide, and how the cavity (pp) makes its appearance in tbeui.

ANNELIDA

283

of the mesodermal bands. These arise by the division of the
two cells, and by the smaller cells thus produced moving
away from them. This process can best be understood from
the figures of Lumbrictis given by Kowalevsky and Klei-
nenberCt (Figs. 129 and 130). The two large cells (pole cells)
from which the smaller cells of the mesodermal bands have
arisen by division are seen in the posterior part of the
embryo (comp. the interpretation given by Kleinenberg, p.
286). The mesodermal bands extend on both sides of the
embryo towards the mouth ; whereas they at first diverge,
later they move from the lateral position toward the ventral
surface, and now lie on either side of the median line (Fig.
129 i?, C).

mes. mes.

Fig. 130.—^, B, optical longitudinal flections of two embryos of Lumhricus
ruhcllus {Allolohophora fcetida [?], Vejdovsky) of different ages (after Kowalevskt).
bt, blastopore ; ect, ectoderm ; eiit, entoderm ; m, mouth-opening ; mes, mesodermal
bands ; p, pole cells of the mesoderm.

In the figures 130 A and B, the mesodermal bands are
seen in side view, and their first appearance (Fig. 130 J^)
can be recognized. In this case they consist fi'om the begin-
ning, not of one, but of several, cell-rows ; but even here the
pole cell of each band can be seen at the posterior end. The
bands extend further and further, and finally acquire the
considerable length which is represented in the figures 130
B and 131.

Together with the elongation of the mesodermal bands
already described, which are also, though inappropriately,
called germ bands, the formation of the embryo as a whole
has progressed fai-ther (Figs. 130 B and 131). It has en-
larged by the rapid multiplication of its cells, and now con-

284 EMBRYOLOGY

sists substantially of a bilaminar cellular vesicle, between
the two layers of which are lodged on the ventral side the
mesodermal bands. The blastopore has become the per-
manent mouth, in the neighborhood of which a lip-like
thickening of the ectoderm makes its appearance. The
cells lying around the mouth appear to be of a contractile
nature, and accordingly execute swallowing movements, in
consequence of which the intestine becomes filled with the
albumen in which the embryo floats, and which serves it as
food. As a result of this nutrition the embryo becomes
more and more distended, and increases in volume. The
embryo in this condition may be compared with the fi*ee-
swimming larvx of other Annelida, especially as it bursts
the vitelline membrane at about this stage, and now floats,
as has been mentioned, in the albumen of the cocoon. The
larva-like appearance of the embryo is increased by the fact
that in Lnmbricus trapezoides (according to Kleinenbebg)
there is found a ring of delicate cilia, surrounding the
mouth and continuing into a ventral ciliated groove, which
extends in the middle line between the mesodermal bands.
Hatschek also found an adoral ciliated zone in the embryos
of Criodrilus. Furthermore Vejdovsky proved the existence
in Rhynchehnis of a paired head kidney, which Bergh also
found in Criodrilus. It consists of a long, semicircular tube,
the blind inner termination of which lies in the vicinity of the
mouth, whereas the external opening of the ciliated canal is
situated laterally at about the middle of the body. In view
of all this, the embryos of the Oligochseta appear to be
degenerate larval forms, which float free in the albumen of
the cocoon, and here feed independently. The absence of the
anus does not enter much into the question, for we see that
in many Trochophore larvae also — for example, in Psygmo-
branchus — the anus, and even the mouth, may be wanting in
the early part of its free existence (comp. Fig. 128, p. 280).

The metamorphosis of the larva-like embryos into the
worm is accomplished principally as the result of the further
development of the mesodei-mal bands. These at first con-
tinue their growth forwards and surround the fore-gut,
which has been formed out of an invagination of the ecto-

ANNELIDA 285

derm (Figs. 130 B and 131). Also in the parts of the
embryo lying further backwards the mesodermal bands
advance from the ventral side, to which they were at first
confined, to the dorsal side, and thus separate the ectoderm
from the entoderm. The separation of the mesodermal
bands into primitive segments and the splitting of these into
somatic and splanchnic layers occurred even before this
(Pigs. 129 G and 131). The posterior part of the embryo

PiQ. 131.— Optical longitudinal section of an embryo of Lumhricus olidm (after
■Wilson, from Lang's Lehrbuch). hm, fundament of the ventral nerve cord; e,
ectoderm ; en, entoderm ; g, fundament of the supra-ceaophageal ganglion ; fc^,
head cavity ; m, mesodermal bands ; md, cavity of the mid-gut ; nb, neuroblast
cells ; 0, mouth ; pm, parietal (somatic) layer of the primitive segments ; pms, pole
cells of the mesoderm ; sh, cavity of the primitive segments ; st, stomodseum
(fundament of the fore-gut) ; iig, suboesophageal ganglion ; vm, visceral (splanchnic)
layer of the primitive segments.

is greatly distended by the albumen taken into the intestine,
and bulges out like a yolk-sac on the embryo, which in the
meanwhile has grown longer (Fig. 132 h). Also in the
posterior distended part of the embryo the primitive seg-
ments are ultimately formed and grow around the ento-
dermal sac towards the dorsal side, so that finally it is
entirely surrounded by mesoderm ; thus the most im-
portant parts in the development of the worm, as far as
regards its outer form, are completed. The anus is formed
later by an ectodermal depression at the posterior end of
the worm.

286

EMBRYOLOGY

Both the origin and the further development of the mesoderm are dis-
puted points in the development. In some cases, as, for example, in the
Lumhricus studied by Kowalevsky (and in Nereis, according to Goette),
it appears as if the first mesoderm cells had been derived from the ento-
derm cells, whereas in other cases they seem rather to have belonged to
the ectoderm. Usually their origin cannot be referred to either one or
the other of the two primary germ-layers, for they were established before
the formation of these, or on the border-line of the two. Such is the
case in various OligochiBta and also in Polychreta. In EJiynchelmis
(comp. p. 282) the so-called mesomeres are separated off from the
large blastomeres, which subsequently become the entoderm, and, to-
gether with the micromeres, overlie these ; apparently therefore they
belong to the ectoderm. It is only later that they are overgrown by the

ectoderm, and move to the inside
here to develop into the mesoder-
mal bands (Vejdovsky).

Just as the opinions of authors
are divided in regard to the deri-
vation of the mesoderm, so also
are they in regard to the manner
in which the mesodermal bands are
formed. Whereas some authors
derive them from proliferations of
the primitive mesoderm cells (Kow-
alevsky, No. 27; Hatschek, No.
18 ; Goette, No. 15), others are of
the opinion that the parts of the
ectoderm which lie over the meso-
dermal bands also supply cells for
the I'einforcement of these bands,
and that as a result ectoderm and
mesoderm are in this region con-
tinuous (Fig. 129 B). Kleinenbero
(No. 24) thus describes the condi-
tions in Lvmbricus trapezoides. Salensky (No. B7) agrees with him.
Kecently Kleinenberg (No. 26) has gone still further, for he considers
that the entire mesoderm— the existence of which as a separate layer he,
moreover, denies— has been gradually split off from the ectoderm. This
point will be referred to again in considering the organogeny.

Fig. 132.— An embryo of Lumhricus
agricola, already far advanced in deve-
lopment (after Kowalbvskt). h, pos-
terior part of the embryo, resembling
a yolk-sac ; its wall is formed of ecto-
derm and entoderm, and it gradually
becomes overgrown by the mesodermal
bands ; mes, upper limit of the left
mesodermal band ; as, oesophagus.

3. The Formation of the Organs.

So much of the formation of the individual organs as has
not been considered in the two preceding sections upon the
general form of the body will be added here. However, it

ANNELIDA 287

should be noted at the beginning that upon these matters
there prevails as yet among authors but little clearness, and
agreement to only a limited extent.

Ectodermal Structures.

The epidermis of the larva and of the adult worm arises
directly from the embryonal ectoderm, its cells multiplying
greatly, and becoming much flattened.

The setiqerous sacs arise, according to the concurrent state-
ments of KowALEVSKY, Vejdovsky, and Bergh, as club-shaped
ingrowths of the epidermis, inside of which the setse are
secreted. According to other descriptions, the setigerous sacs
originate from the mesoderm.

Nervous System and Sensory Organs. — In considering the
origin of the nervous system it seems necessary to separate
the supra-oesophageal ganglion from the ventral cord. Both
arise as thickenings of the ectoderm (Fig. 133 C), the ven-
tral chain of ganglia either as a longitudinal, unpaired
or as a paired thickening, which detaches itself from the
ectoderm, and moves to the inside, where it may be sur-
rounded by mesoderm (Kowalevsky). The further develop-
ment proceeds from in front backwards. Opinions are very
far apart regarding the origin of the supra-oesophageal
ganglion, and especially its connection with the ventral
chain of ganglia.

In Hatschek's opinion, there first arises an ectodermal thickening at
the head end of the embryo : the apical plate. From this the ectodermal
thickening progresses backwards in the form of two cords, which extend
on either side of the mouth. From the oesophageal connectives thus
formed, the thickening process continues further and further. In this way
the two lateral cords of the ventral nerve-trunk are formed, and in addition
a groove-like invagination, lying in the longitudinal median line (similar
to the medullary tube of vertebrates), takes part in the formation of the
ventral chain of ganglia. Hatschek defends the view that the entire
nervous system arises from a single fundament. In this he relies mainly
upon his embryological investigations on Crioarilus and Polygordius, and
furthermore on the comparative anatomical conditions in Protodrilus, in
which Archiannelid the oesophageal connectives are said to remain
throughout life in connection with the body epithelium as ectodermal
thickenings.

288 EMBRYOLOGY

In opposition to this theory, Kleinenbekg — with whom Goette,
Salensky, Bekgh, and Fraipont agree — esj^ouses the view that the nerv-
ous system is composed of two separate fundaments. The supra-oeso-
phageal ganglion arises as a preoral ectodermal thickening independ-
ently of the two longitudinal thickenings of the ventral side, which
represent the fundament of the ventral nerve cord. (A longitudinal
furrow corresponding to the medullary tube of the vertebrates does
not exist.) Later it puts forth lateral processes, the oesophageal con-
nectives, which unite with the already-formed ventral nerve cord.
Such is the condition in Lxnnbricus. The origin of the nervous system
in Lopadorlnjnchus, likewise studied by Kleinenberg, depends upon
much more complicated formative jirocesses. Lopadorhynclius develops
from a monotrochal larva, the posterior portion of which grows out into
the worm in the manner already described. A ciliated pit, the so-
called apical organ (Fig. 135, \>. 293), and the two ajiical tentacles
arise in the vicinity of the apical pole as provisional sense-organs.
Behind these the two pairs of permanent antennse and the olfactory pits
are formed, also as ectodermal growths. From these organs, which later
degenerate in part, the formation of the supra-cesophageal ganglion
proceeds. Ordinary ectodermal cells are metamorphosed into ganglionic
cells, which accumulate in the region of these organs, later sink in
deeper, and unite to form the supra-oesoijhageal ganglion. This finally
detaches itself from connection with the ectoderm and appears inside
the body as an independent organ.

Just as the formation of the supra-cesophageal ganglion, according to
Kleinenberg, starts from the sensory organs, so the origin of the ventral
nerve cord is also referred by this investigator in part to the influence of
the sensory organs. In the main, however, the impetus to its formation
proceeds from the locomotor organs. From the inner [deep] side of the
ventral ectoderm, the outer surface of which bears tufts of sensory hairs
(Fig. 134 C), a plate is separated off, the so-called neural jilate, in
which a right and left portion can be distinguished (Fig. 134, p. 292).
Along this plate arise segmental, paired ingrowths, the setigerous sacs
(Fig. 134 C). Dorsad and ventrad from tliese are formed as ectodermal
ingrowths the dorsal and ventral cirri. The parts of the neural plate
situated nearest to the median jjlane supply the ventral cord. They
approach more and more the middle line, and here fuse with each other.

The union of the ventral nerve cord with the supra- oesophageal gan-
glion is secondary. It is brought about by the neural plates extending
forward and sending out processes to the ring-nerve of the ciliated band.
But processes of the supra-cesophageal ganglion also pass into this, and
in this way the ot'sophageal connectives arise, whereas the ring-nerve
itself, together with the ciliated band, disappears.

Thei'efore, according to Kleinenbeuo's description, here reproduced
briefly, the brain and ventral cord appear to have a separate origin, the
impulse to which comes through sensory and locomotor organs.

ANNELIDA

289

The origin of the sensory organs lias already been touched
upon several times, as, for example, the formation of the
auditory vesicles in Eupomatus (comp. p. 267). The eyes of
the Alciopidoe are formed, according to Kleinenberg, as
invaginations of the ectoderm, which are constricted off and
unite with the brain, this union constituting the optic nerve.
The retina arises as the result of the differentiation of the
inner [deep] wall of the vesicle, whereas the outer wall
becomes very thin. Within, the lens and, by the activity of
certain gland-like cells, the vitreous body are secreted.

Mesodermal Structures.

Body Cavity ; Musculature ; Blood-vessels. — The differentia-
tion of the mesodermal bands, which results in the formation
of the segmental cavities, and thereby causes the segmenta-
tion of the body, takes place in a simple manner. The meso-
dermal bands have extended forward and in the anterior part
of the body become several rows wide and several layers
deep. Then a segmentation begins at their most anterior
end, individual parts becoming differentiated in groups, and
finally separated from one another by transverse boundaries
(Fig. 131, p. 285). These box-like, quadrangular cell-plates,
which succeed one another along the course of the meso-
dermal bands, and therefore lie side by side in two rows,
are the primitive segments, the influence of which on the
segmentation of the body we have already briefly men-
tioned in considering the development of Polygordius and
Lumhricus (pp. 268 and 284 — 286). We saw there also that
the development of the primitive segments takes place from
in front backwards. When the primitive segments are
already well formed in the anterior part of the body, the
mesodermal bands are still entirely undifferentiated in their
posterior portions, and new cell material continues to be
formed hei'C in the vicinity of the pinmitive mesoderm cells
(Fig. 131). A fissure soon makes its appearance in the
primitive segments, owing to the fact that the two or more
cell-layers out of which they are composed separate from each
other at the middle of each primitive segment (Fig. 133 B
and C). Thus the segmental cavity — that is to say, the be-

K. H. e. U

290

EMBRYOLOGY

g'innino' of the body cavity — is formed in each segment of the
body of the worm. The cavity enlarges while the walls of
the primitive segments are more and more distended and
apply themselves to the body-wall and to the wall of the
intestine as the somatic and splanchnic layers respectively
(Fig. 133 C). But of course every two of the segments abut
on each other with their anterior and posterior walls, and
thus arise the septa (dissepiments), which separate the
different segments of the body. Since each segment of the
body requires for its formation a primitive segment on the
right side and one on the left, there are formed a dorsal and a
ventral mesentery (Fig. 133 C). These mesenteries dis-

-ent

-U

Fio. 133. — A to C, transverse sections of Polygordius larvfe (after Hatschek).
A, optical croBS-BBCtion of the body of an unsepmenteil larva, immediately in
front of the anus, shovring the two primitive mesoderm cells (me.s) ; B, C, two
cross-sections of an older larva, the former from the posterior, the latter from the
anterior, part of the body; ect, ectoderm: ent, entoderm; mes, mesoderm; ii,
fundament of the nervous system; so, somatic, sp, splanchnic layer of the meso-
derm.

appear in most of the Chastopods (just as the septa also are
frequently perforated), but they persist in some of the lowest
Annelids, such as Polygordius among the Archiannelida and
Saecocirrus among the Chifitopoda.

The body musculature arises from the somatic layer of
the primitive segments, the ventral longitudinal muscles
being the first to be formed. By the arrangement of the

ANNELIDA 291

raasculafcure its sesfmenfcal oriofin can be recosfnize 1 even in
the fully developed animal. The peritoneal epithelium is
also derived from the primitive se^rments.

The splanchnic layer of the mesoderm produces so much
of the wall of the intestine as is not of entodermal or of
ectodermal origin, and the walls of the vessels also arise
from it. According to Salensky, the formation of the blood-
vascular system begins (in Psygmohranchus and Terebella)
in the form oE canals, which lie between the entoderm and
the splanchnic layer, and which are therefore really parts of
the segmentation cavity. Later these blood-filled cavities
are surrounded with a cellular wall, which comes from the
splanchnic layer. According to Kowalevsky's observations
also, mesodermal cells, which collect between the ectoderm
and splanchnic layer, form the walls of the vessels ; more-
over, the dorsal vessel (^Lumhricus and Griodrilus, according
to Vejdovsky) arises from separate paired fundaments.
These extend along the boundaries of the mesodermal
bands as they grow dorsad, and, advancing with them, finally
fuse with each other to form the dorsal vessel. This con-
dition is of especial interest from the fact that in Fleurochceta
(Megascolex) the separate fundaments of the dorsal vessel,
are retained in certain parts of the body throughout life
(Beddard).

It appears questionable whether the cephalic cavity is formed in the
same way as the segmental cavities of the body, or whether it is to be
distinguished from these. On the first assumption, the two most anterior
primitive segments would unite for its formation, andtherefore outgrowths
of the mesodermal bands must have crowded forward past the oesophagus
even into the head region. The outer wall of the head and the muscula-
ture of the oesophagus would then be formed from the somatic and
splanchnic layers respectively in the usual way. The conditions were
described in this way by Kleinenberg in his earlier work, and Vejdovsky
also derives the cephalic cavity from the two " anterior ends " of the
mesodermal bands, which he describes as being fused (RhynchelmiH).
Opposed to this is the view, supported especially by the free-living larvte,
that the cephalic cavity arises by means of a separation of the two
primary germ-layers and by an immigration of mesodermal elements
from the trunk (Hatschek). According to this explanation, the first
pair of primitive segments lies behind the head, and the mesoderm of

292

EMBRYOLOGY

the head ariges from one wall, or from the still undifferentiated meso-
dermal bands. The absence of the mesenteries in the head of Puly-
gordius supports the view that the cephalic cavity is not paired, but
unpaired, in its origin (Hatschek).

The difference between head cavity and body cavity vanishes when —
as is the case, according to Kl-einenberg, in many Annelids, for instance

'^.--

Fig. ^3i.—A to C, parts of frontal longitudinal sections of the larva of Lopad-
orhynchus, showing the splitting off of the muscle-plates (after Klkinknbebo). Is,
fundaments of setigerous sacs; cct, ectoderm ; ent, entoderm ; g, ganglionic cells
of the larval nervous system; mp, muscle-plates; np, neural plates; s, larval
sensory organs.

in Lopadorhijuckns — a regular splitting of the mesoderm into a somatic
and a splanchnic layer does not take place, but the investment of the
entoderm is effected by separate cells detached from the mesodermal

ANNELIDA

293

bands. The body cavity of the trunk in such cases therefore does not
represent the cavity between the two layers of the mesoderm, but corre-
sponds to the blastocoele (traversed by mesodermal cells), just as the
cephalic cavity does in the case mentioned above. Moreover, in Lopad-
orhynchus this also arises by an immigration of mesoderm cells into the
head portion. From this description it follows that the formation of the
body cavity does not always take place in so regular a manner as has
been described above ; in fact, according to Kleinenberg's statements,
the formation of the entire mesoderm may be effected in another manner.
It has already been mentioned (p. 286) that in Kleinenberg's opinion
the ectoderm, in addition to the pole cells, takes part in the formation of
the mesodermal bands of Lumhricus trapezoides. Cells are separated off
from the ectoderm and added to the germ bands lying under them. In

Fig. 135 — Sagittal section of a larva of Ln-paAorhynchus (after Kleinenberg);
d, intestine ; mp, muscle-plate ; np, neural plate ; as, fundament of the (permanent)
oesophagus; sg, fundament of the supra-oesophageal ganglion; so, apical organ;
st, stomodBsum (temporary fore-gut of the larva) ; u-, preoral band of cilia.

Lopadorhynchus Kleinenbebg derives the entire mesoderm from the
ectoderm. According to him, mesoderm does not exist as a separate
layer. The musculature of the Lopadorhynchus larva arises by means
of an emigration of cells from the ectoderm (Fig. 134 A to C). The so-
called muscle-plates (Fig. 135 7np) are formed by a splitting of the
thickened ventral ectoderm, first at the hind end of the larva and then
successively further forwards. The course of this cell growth, leading to
the formation of the muscle-plates, is evident from the figures 134 A to C.
The muscle-plates of the two sides are separated by a fold of the ento-
derm. The segmentation of the muscle-plates takes place after the
fundaments of the setigerous sacs have grown in from the neural plates
(comp. p. 287 and Fig. 134 C). The limits of the segments arise by the

294 EMBRYOLOGY

loosening of the texture of the tissue in successive planes at right angles
to the long axis of the body (Fig. 135). As was mentioned, individual
cells separate from the muscle-plates, in order to apply themselves (like
the splanchnic layer) to the intestine, whereas the remaining part of the
muscle-plates supplies the musculature and ei^ithelium of the body-wall.
Blood-vessels and segmental organs were not observed in Lopadorhunchus.
According to Kleinenbekg's explanation, with which, as regards the ecto-
dermal origin of the mesoderm, Salensky also concurs, the primitive
mesoderm cells occurring in other Annelids must be looked upon only as
early differentiations of ectodermal parts. But when several organs of
altogether diffei-ent kinds, such as the musculature, the blood-vascular
and excretory systems, can be referred back to such a common funda-
ment, then the theory which considers this fundament as a germ-layer
is not unwarranted, even when the fundament at times, as in Lopad-
urhynchus, makes its appearance in somewhat later stages and in a less
primitive manner, namely by the splitting off of a cell-layer from one
of the two primary germ-layers.

Wilson's view should also be mentioned here, according to which, in
addition to the two pole cells from which the mesodemial bands arise,
three other pairs of similar pole cells are present on the ventral side of
the embryos of Liiinhriciig, The three large cells mentioned, from each
of which a row of cells extends toward the anterior end of the embryo,
lie on either side of the middle line somewhat farther forward than the
pole cells of the mesoderm and somewhat more superficial, therefore
more in the region of the ectoderm. The imiermost of these three rows
is said to constitute the fundament of the nervous system, and the middle
one that of the nephridia, whei-eas the significance of the outer one re-
mained unknown to the author of this theory.*

The formation of the mesoderm and body cavity in Enchytr<eoides
takes place, according to Roule, in a very singular manner, as far as can
be judged from his biief communication. In the "morula," which re-
sults from an irregular cleavage, an outer layer, the ectoderm, is split off
from a central mass, while the latter separates by means of a similar
process into the centrally situated entoderm and the surrounding meso-
derm. The former (entoderm) by the appearance of a cavity in it becomes
the intestine, while spaces are formed in the mesoderm, which become
confluent, and thus give rise to the body cavity between the two layers

* [The conditions of the mesoderm have been further elucidated by the
recent works of Wilson (Appendix to Literature on Annelida, Nos. XXIV.
and XXV.) and BfiRon (No. VII.). The origin of the pole cells and their
relations to the organs have been followed out further. As a result more
definite relations of those parts which were previously held to be exclu-
sively mesodermal have been disclosed. A portion of the body muscula-
ture appears to be of ectodermal origin. On this point the papers of
Beroh and Wilson should be consulted. — K.]

ANNELIDA 295

of the mesoderm. The septa are said to be formed by outgrowths of
cells from the somatic layer, which unite with the splanchnic layer !

Head Kidney and Segmental Organs. — In the lai'va of
Eupomatus the head kidney arises by the outgrowth of a
cell lying in front of each pole cell of the mesodermal bands.
Some other mesodermal cells take part in their formation,
for they supply the spheroidal cells which i*est upon the
inner blind end of the head kidney and elongate into the
ligament-like threads of attachment of the organ (Fig. 118 5,
p. 265). In addition to the formation of the head kidney,
the few cells of which the mesodermal bands at first con-
sist are further employed for the formation of the larval
muscles. Only the two pole cells remain. These, by re-
peated division, supply the new mesodermal bands, which
Hatschek designates as secondary in contrast to those
primary ones which wei'e early put to use. Later they reach
the great development which has already been described.

According to Hatschek, the remaining segmental organs
originate from the head kidney, for a small ciliated canal
(in Polygordius), running in the somatic layer of the meso-
derm, branches off from each head kidney at the junction of
its two arms. The nephridia are said to be given off from
this canal, one to each segment (Fig. 136). While the head
kidney degenerates, they reach their final development.

Hatschek's description has met with little recognition, for it could not
be substantiated by subsequent investigators (Fkaipont). However, the
discoveries recently made by E. Meyer on certain Terebellidae {Lanice,
Loimia) show the observations of Hatschek in a new light. In the two
Annelids mentioned the nephridia are united by means of a [longitudinal,
blindly ending, common] duct, which extends far backward. The dis-
charge takes place through as many [successive lateral] canals as there
are nephridia present, but these canals are connected with the nephridia
only indirectly, i.e. by means of the common duct. In the Capitellidce
also, according to Eisig, connections between the different nephridia
exist in the form of ciliated canals. We do not intend to assert that
any great importance is to be ascribed to these conditions, for in the
first place their development is not known, and then the nephridial
system of the Terebellidae (and Capitellidse) is shown to be essentially
modified.

296

EMBRYOLOGY

A connection between all the segmental organs is fonnd
by Hatschek in Criodrilus also, for they are said to arise
fi'ora a cord-like thickening of the somatic layer, which
extends the whole length of the body dorsal to the ventral
longitudinal muscles. These cords are then separated
segmentally into loop-like parts, the fundaments of the
nephridia. The latter acquire lumina, and open into the
body cavity in front of the segment to which they belong
through the future funnels, and finally fuse at their posterior
ends with the ectodermal wall to form the external openings.
The funnels and nephridial ducts arise separately. But

Jl> J^ -p^

Ji

J>

Fig. 136.— Diagram of the development of the excretory system of Polygordius
(after Hatschek, from Balfoue's Comparafii'« Enxhy^oXo^))).

these statements of Hatschek also find opponents in
Vejdovsky and Bergh, according to whom the segmental
organs of the Oligochteta arise from separate fundaments
by the growth of the cells in the somatic layer and in the
partition walls of the primitive segments. Fig. 137 A
shows that at the boundary of the septum and somatic
layer there is a considerably enlarged cell {tz). It con-
tributes especially to the formation of the funnel. Behind
it other cells of the somatic layer arrange themselves into
a cord of cells (Fig. 137 J3), which constitutes the funda-
ment of the nephridial duct. In it, as also in the funnel,
a lumen makes its appearance later; the entire structure

ANNELIDA

297

becomes covered with peritoneiim (Fig. 137 C, pt), and
presses its way out towards the ectoderm, in order to fuse
with it directly, or with an invagination of it (Bergh,
T^o. 7), which forms the terminal portion of the duct, or
collective vesicle, when such is present (Vejdovsky, No. 43).

According to the observations of E. Meyer (No. 31), the thoracic
nephridia of Psygrnohranclms are composed of separate parts. The
nephridial ducts arise from large mesoderm cells, which are found in the
blastoccele of the larva ; the
funnels, on the contrary, and
the peritoneal covering of the
nephridia are supplied later by
the primitive segments. The
ends of the nephridial tubes
open to the exterior by means
of provisional pores, which
later occupy the floor of a
ciliated groove, which closes,
and represents the unpaired
ectodermal efferent duct of
these two so peculiarly con-
stituted nephridia of Psygmo-
branchiis.

Genital Organs. — The
development of the sexual
glands is very simple in
both the Polychoe.ta and
Oligochceta. They arise as
growths of the peritoneal
epithelium on the septa,
or, as frequently in the
Pohjchceta, on the invest-
ment of the blood-vessels.
The genital gland, which
in Lumbricus is distin-
guishable even during

Fig. 137.—^ to C, parts of longitudinal
sections through embrj'os of Criodrilus,
showing the development of the nephridia
(after Bebgh). ect, ectoderm; 7i, cavity of
primitive segment; pt, peritoneum (of
the nephridia); s, septa; so, somatic, sp,
splanchnic layer of the mesoderm; t,
funnel ; tz, funnel cell.

cocoon life (Bergh), sepa
rates from the peritoneum as the result of a rapid prolifera-
tion of the cells, and gradually assumes its permanent form
(Fig. 138 A to D, after E. Meyer). The genital products are

298

EMBRYOLOGY

liberated one by one (Fig. 138 B), and either undergo their
further development while floating free in the body cavity,
or, as in the case of the testicular cells of the earthworms,
reach special vesicles (vesiculge seminales), which, according
to Bergh (No. 5), arise on the septa by means of a process
of growth and invagination.

The ducts of the sexual organs are to be looked upon as
more or less modified segmental organs. They arise in the

^«

/m..

B.

g.(lr

pm..

Yv.

i4MM^i

g.ar.

Fio. 138.—^ to D, diagrammatic representation of the structure and development
of an ovary of An\y)iiiyite \uhra (after E. Mbybb). g.dr, sexual gland ; g.«, genital
epithelium; g.z, genital cells in the act of breaking away; pm, peritoneum; Y.v,
vas ventrale.

same way as the nephridia themselves, except that the
funnel is formed earlier than in the actual segmental organs
(Vljdovsky). The entirely independent origin of the efferent
sexual ducts from the nephridia and the simultaneous occui*-
rence of both organs in the same segment, as happens in the
earthworms, form no argument against the origin of the
ducts from nephridia, since in some Annelida (Capitdlidde,
according to Eisiu) several pairs of nephridia occur in the
same segment. In the earthworms especially, many things

ANNELIDA 299

seem to suggest that two pairs of nephi'idia originally be-
longed to each segment (Benham). The metamorphosed
segmental organs function in the following manner : the
funnel takes up the genital products out of the body cavity,
the nephridial ducts pass them onward, and the part dis-
tended into a terminal vesicle serves as a genital atrium.
But the terminal portion in the male apparatus of the
Oligochasta may be metamorphosed into an evertible copu-
latory organ (thus in Stylodrilus, No. 43).

The receptacula seminis have also been traced to nephridia,
of which only the ectodermal vesicular end portion is assumed
to develop ; but Bergh prefers to consider them metamor-
phosed dermal glands. They arise as tubular invaginations
of the epidermis into the interior of the body cavity, and are
surrounded by the other layers of the body- wall (Vejdovsky,
No. 43; Bekgh, No. 5).

Entodekmal Structures.

Intestinal Canal. — In the Polychaeta and Oligochseta we
saw that the intestine arises from portions of all three germ-
layers. The permanent mouth is generally found at the
place of the blastopore, a depression of the ectoderm taking-
place here, so that the fore-gut (just like the hind-gut, which
arises later) is an ectodermal structure. In those cases in
which the larva arises from an epibolic gastrula, and the
blastopore does not become the mouth, as in Rhynchelmis
and Psygmobranchus, the fundament of the intestine at first
consists of a solid entodermal mass rich in yolk (Fig. 128 A
and B, p. 280). The entodermal wall of the mid-gut, by
means of which the yolk-mass that still remains is absorbed,
arises by the disintegration of the more central cells, while
smaller cells at the periphery with less food-yolk are
separated from the rest of the mass and form an epithelium.
In this condition the intestine consists of a sac closed on
all sides. The fore- and hind-guts are formed by its union
with the ectoderm in front and behind.

The share which the two ectodermal invaginations take in the forma-
tion of the fore- and hind-guts is said to be very variable in the different

300

EMBRYOLOGY

Annelida (Salexsky). Thus the oesophagus may be formed of ectoderm
(Plleolaria, Liimbricus), but it is maintained that it may also be for the
greater part of entodermal origin (Pxi/gmobraiichus, Rhynchelmis). The
conditions in Ijopadorhy nchus are peculiar ; here the wide ciliated
stomodfEum (the larval fore-gut) is not directly converted into the
oesophagus, but constitutes a transitory structure. On the wall of the
stomodffium two cushion-like thickenings make their appearance, which
become hollow, and form two small sacs (Figs. 139 and 135 as). These
become considerably enlarged, surround the stomodffium, and finally
grow together, after the stomodaeum has closed and separated from the
ectoderm. The detached stomodjeum is now seen as a ciliated sac,
surrounded by the likewise saccular oesophagus. This finally unites
permanently with the ectoderm and entoderm (Kleinenberg).

Fig. 139(135).— Sap:ittal section of a larva of Lopadorhynchus (after Kleinenbehg).
d, intestine ; mp, muscle-plate ; nji, neural plate ; ces, fundament of the supra-ceso-
phageal ganglion; so, apical organ ; st, stomodffium (fore-gut of the larva); lo, pre-
oral ciliated band.

The musculature and peritoneal covering of the intestine
are supplied by the splanchnic layer of the mesoderm. The
chlorogogenous cells which surround the intestine are con-
sidered as excretory organs, and have the same origin and
significance as the so-called pericardial glands occurring on
the blood-vascular system, and are also outgrowths of the
same layer (Grobren). In the Lumbricida? the typhlosole
arises in the dorsal median line of the intestine as a more or
less deep groove-like infolding of its entire wall.

ANNELIDA

301

4. Non-sexual Reproduction ; Alternation of

Generations.

The Chsetopoda possess to a high degree the power of restoring parts of
the body that have been lost ; not only the less important parts of the
body, but also the more important ones, such as the head region, includ-
ing mouth and brain, can be formed anew by them. This power of
regeneration passes into a kind of non-sexual reproduction (schizogeny),
when the body, as in Lumhriculus, separates spontaneously into several
pieces, each of which is able to regenerate itself into a perfect worm.
Approaching closely to this is the reproduction of one of the marine
Annelida, closely allied to the Oligochseta, which is found actively multi-
plying throughout the year, without at any time developing genital organs
{Ctenodrilus monostylos, according to Count Zeppelin). This worm repro-
duces in the most primitive
way : a constriction is formed
on the body immediately
behind a septum, and be-
comes deeper and deeper
until the worm falls into
two parts (Fig. 140 A). Of
the two resulting parts the
anterior is thus without an
anus, the posterior without
a head. This primitive
kind of division may go so
far that parts arise which
are destitute of both head
and anus, and at times con-
sist of only a single segment
(Fig. 140 B). Head and
terminal parts are formed
by the thickening of the
integument (hypodermis),
which sends inward plug-
like ingrowths that unite
with the intestine. In this

Fig. 140. — A, Ctenodrilus monostylos dividing
transversely (alter Count Zeppelin). B, a por-
tion of the same worm consisting of only a
single segment ; c, cirrus ; d, intestine. C, Cteno-
drilus pardalis (after v. Kennel); fcn., budding
zone, where the worm later separates into the
different parts ; d, intestine.

way the mouth and anus

arise. The new segments that are to be formed are interpolated between

the newly formed anus and the preceding segment.

Less primitive is the condition in another worm belonging to the same
genus, Ctenodrilus pardalis, which likewise was found only in a state of
non-sexual reproduction (v. Kennel). In this case thickened zones,
corresponding to the anterior and posterior ends of the worms about to
be produced (Fig. 140 C), are formed— even here in a simple way, it is
true — before the division ; that is to say, a so-called budding zone is

a

302 EMBRYOLOGY

found at this point, where the division is about to take place. In
Cteiiodrihis partbiHs each portion includes only one segment, and thus
the budding zones are seen to be repeated segmentally. While they still
remain united to one another, the cephalic lobe [prostomium] and
the brain, as well as the oral and anal invaginations, are developed on
the several portions. The degree of development in which the zones
are found increases from behind forwards (Fig. 140 C).

Some Polychffta and those Oligochseta in which non-sexual reproduc-
tion is known are like Ctenodrilus in so far as they also divide in i
condition in which no genital organs are present. In the ProUila
described by Huxley a budding zone arises between the sixteenth and
seventeenth segments ; then follows the formation of the prostomium of
a new individual in the seventeenth segment. In this case, however,
after the separation both individuals become sexually mature. The
conditions are similar in the Naidida, in which they were thoroughly
studied by Semper. These worms also reproduce by division in the
sexually immature condition only. The body of the worm may be first
separated by a budding zone into two regions ; then new budding zones
are interpolated ; that is to say, fundaments of younger animals arise in
the individuals already established. This process is kept up, not, how-
ever, serially from in front backwards, but in such a way that individuals
of quite different ages come to lie one behind another. When the chain
has reached a certain stage of development, it separates into the different
individuals, which now reach their final shape by growing considerably,
by increasing the number of their segments, and by maturing their sexual

organs.

The cases last considered were, it is true, those of animals without
sexual organs, which multiplied non-sexually ; finally, however, all in-
dividuals acquire sexual maturity, and are not distinguishable from one
another in shape. The conditions are different in those Polychreta in
which new individuals, which become sexually mature, are continually
being separated off from the hinder part of the body of an individual that
remains [sexually] sterile, a process that is to be placed alongside that of
strobilization in the Scyphomedusas. Thus in Autolytus (according to
Kkohn and Agassiz) there are formed, by the budding of the parent,
male and female animals, which lie in a chain one behind the other ; of
these the most anterior, the one lying nearest to the parent animal, is
the youngest. They separate from the chain according to their ages.
The sexually mature animals are essentially different in shape from the
budding forms, so that the two were formerly assigned to different
species. The sexual animals appear to copulate ; for in a brood-pouch
of the female the eggs develop into the worm which subsequently repro-
duces by budding. This is therefore a genuine alternatUm of (lenerations.

Similar conditions of reproduction are found in some Syllidtr, from
the budding mdividuals of which are detached sexual animals, which, by
the great development of their parapodia and selaj and by their well-

ANNELIDA

303

developed orienting apparatus, are especially well adapted to a free life.
They swarm about and secure the necessary distribution of the sexual
products, whereas the less active budding form remains at the bottom of
the sea. The extensive development of the parapodia is completed even
while the buds are still connected with the parent. This recalls the
condition of certain Nereidce, in which new setas, better adapted to
swimming, make their appearance on the posterior part of the body at

Fig. Ul.— Part of a stock of Si/lUs ramosn (somewhat diagrammatic, after
McIntosh, and from a j reparation of the Challenger material), d, intestine,
which branches throughout the entire stock. The stock is injured in some places.

the time of sexual maturity {epitokal form) ; these give to the sexually
mature animals an entirely different appearance from that of the young
form (so-called atokal form), so that here also the sexually mature and
the young forms were assigned to different species and genera. In the

304

EMBRYOLOGY

Nereida", however, the hinder part of the body, thus equipped, does not
become detached, but its better equipment serves only to facilitate the
locomotion of the sexually mature animal (Ehlers). But in any event
the conditions existing in the Nereides and Syllida are comparable with
each other.

Under the influence of special conditions of life the reproduction of
the Syllidce may assume a very peculiar form. In the sponge, Aulochone,
and other Hexactinellidse a S^jUis has been found (McIntosh), on which
new individuals arise, not only one after another, but also by lateral

budding (Sijllis ramosa, Fig. 141).
A genuine stock is formed in this in-
stance, the branches of which extend
without limit in the canal system of
the sponge, for the branches in turn
have the power of producing new
buds. These detach themselves from
the stock as male and female sexual
animals (Fig. 142) ; and since they
are provided with better swimming
apparatus, and with especially well
developed eyes, one can readily as-
sume that they abandon the sponge,
and, swarming about free in the sea,
secure a wider distribution of the
sexual products. Their descendants,
which they produce by sexual means,
then migrate back, it is to be as-
sumed, into sponges. In this in-
teresting case the alternation of
generations combined with stock-formation is particularly evident.

Fig. 142. — Anterior part of a female
individual, such as is found in the
sponge stocks inhabited by Syllis
ramosa. The animal is filled with
eggs. The large eyes can be recog-
nized on the head (after McIntosh).

II. ECHIURID/E.

Echiurus ; Thalassema ; JBonellia.

While Bonellia viridis lays its eggs in the form of a thick,
tortuous cord, consisting of a gelatinous mass, in which the
eggs lie in several rows (Spengel), the Thalassema mellita
observed by Conn discharges its eggs and spermatozoa free
in the sea, so that with this animal artificial fertilization
could be undertaken.

1. Cleavage and Formation of the Germ-layers.

The cleavage of the c^^^ was closely studied by Spengkl in
Bonellia. An animal portion of the Qg^, consisting of finely

ANNELIDA

305

granular protoplasm, can be distinguished from a yolk-laden
vegetative pole. The cleavage corresponds with this condi-
tion. At first the egg is divided into four large spheres by
cleavage along two meridional planes, but then four small
blastomeres are constricted off from these at the animal pole.
By division of the latter and the formation of new micromeres
from the macromei-es, the number of the small cleavasre
spheres rapidly increases ; they spread out over the four large
spheres, and finally envelop them completely, producing an
epibolic gastrula (Fig. 143 A). The small cells that are now
being constricted off from the raacromeres no longer reach
the surface, but remain lying under the layer of micromeres.
They form the entoderm. Inside of these the four macro-

meres are still retained for a while.

In the region of the

Fig. 143. — A and B, embryos of Bonellia (after Spkngel, from Balfour's Com-
parative Emhryology). yt, epibolic gastrula; £, formation of the mesoderm ; hi,
blastopore ; ep, ectoderm ; me, mesoderm.

blastopore thei'e appears a layer of cells (Fig. 143 B), which
surrounds the blastopore in the form of a circle. Spengel
believes this to have arisen by a migration inward of the
micromere layer. He interprets it as the fundament of the
mesoderm (Fig. 143 B, me).

Cleavage and the formation of the germ-layers in Thalas-
sema (Kowalevsky, Conn) do not take place in the same
manner as in Bonellia. In Thalassema cleavage is equal, and
its result is a blastula from which an invagination gastrula
arises. To be sure, the latter is not altogether typically ex-
pressed in Thalassema viellita, for the invagination merges

K. H. £. X

306 EMBRYOLOGY

into an ingrowth of cells (Fig. 144 A). As in the later stages
of the epibolic gastrula of Bonellia, here also the entoderm
consists of a solid mass with an outer differentiated cell-lajer
and an inner yolk-mass (Fig. 144 A).

2. Larval Form and Metamorphosis of Echiurus
and Thalassema.

The Echiuridas possess free-swimming larvfe, which exhibit
Tnore or less clearly the form of the Trochophore. The
development of the gastrula into the Trochophore takes place
in Thalassema by the appearance of a thickening of the ecto-
derm at the upper pole, the fundament of the apical plate.
This place becomes covered with a tuft of cilia in the same
way as in the larvae of the Polychteta. At an early stage
cilia make their appearance on the outer surface of the
embryo, and these are said to traverse, as in Eupomatus, the
egg-membrane, so that the latter would become the cuticula
of the larva (CoNx). Even during the gastrula stage a circle
of long cilia makes its appearance at the equatorial circum-
ference of the larva (Fig. 144 A). Below it lies the blasto-
pore. The cavity of the intestine arises as the result of the
absorption of the central yolk-mass by the rapidly multiply-
ing entoderm cells. It becomes connected with the outside
world by means of the mouth, which is formed at the place of
the blastopore. By the outgrowth of the hinder part of the
larva the mouth comes to lie more on one side (on the future
ventral surface) immediately under the band of cilia. The
latter is then differentiated into a row of cilia lying in front
of the moutli and one lying behind it (Fig. 144 B). In addi-
tion a ciliationmakes its appearance in the middle line of the
ventral surface. Tiie intestine grows so much in length that
it lies in loops. This is particularly true of the anterior part.
The terminal portion fuses at the hind end of the larva with
the ectoderm, thereby giving rise to the anus (Fig. 144 B).
Fore- and hind-guts, according to Conn, are entodermal
formations (?). Mesenchymatous muscle-cells extend be-
tween the ectoderm and entoderm of the larva, and at the
hind end lie two band-like cell complexes, the mesodermal

ANNELIDA

307

bands (Fig. 144 B, mes). Consequently the larva of Tkalas-
sema possesses the greatest similarity to the Trocliophore
of the other Annelida. The same applies to the larva
of Eclimrus (Figs. 145 and 146), the structure and meta-
morphosis of which were thoroughly studied by Hatschek,
who established the presence of a head kidney. This paired
organ consists at first of a simple canal, which opens to the
exterior on the ventral
side at the anterior end
of the mesodermal bands.
Later there is added to
this primary head kidney
a secondary branch, which
is much ramified (Fig.
145) . Altogether the larva
undergoes a number of
changes, until it arrives at
the height of its develop*
ment, and the larval organs
begin to dearenerate. This
is true of some other meso-
dermal structures as well
as of the head kidney. In
addition to the muscles
characteristic of Annelid
larvae, which extend
through the blastocoele,
there appears in Echiurus
under the ectoderm a fine
membrane, which arose by
the union of branched
mesodermal cells and is
characteristic for this
larva. The mesodermal
bands are developed in the
manner typical for the Annelida. The pole cells lie at their
posterior ends, whereas the differentiation begins at the
anterior end. It is here that they are first many-layered,
and that they separate into the primitive segments. The

-■' a

FiGi \Hi — A and B, gastrula stage and
Trochoyliore larva of Thalasseraamellita (after
Conn), a, anus; dj intestine; m, mouth;
mes, mesodermal bauds ; as, cesophagus ;
sp, apical plate.

308

EMBRYOLOGY

latter acquire cavities and enlarge in the well-known manner.
Just as in the other Annelida, so in the EcUuridce (Fig.
145), there is established an internal segmentation, corre-
sponding to which there is an outer one, in so far as a
laro-e number of segmental ciliated bands make their appear-
ance on the posterior part of the Echiurus larva. But this
segmentation is only temporary, for, like the bands of cilia, the
septa between the cavities of the segments also degenerate.
Of the fifteen primitive segments which were begun, only the
somatic and splanchnic layers remain, and, as the result of
the disappearance of the septa, the secondary body cavity of

the trunk unites with the
primitive head cavity. Like
the cavity of the trunk, the
head cavity is also tra-
versed by branched cells
(Fig. 146), and since these
are in part applied to the
ectoderm, the dermo-muscu-
lar sac, which was estab-
lished in the trunk at an
earlier period, is also deve-
loped in the head region.

The nervous system is
also established in the larva.
On either side of the ventral
ciliated groove arise thick-
enings of the ectoderm, from
which small nodular pro-
cesses grow inward, and
unite segmentally into large

Fig. 145. — TrocJiopIiorc larva of JBcliiimis
(after Hatschek). a, anus; ah, iinal
vesicle ; d, intestine ; fcii, head kidney ; m,
mouth ; mes, mesodermal bands ; n, ven-
tral chain of ganglia; sc, oesophageal con-
nective; sp, apical plate. The ciliated
bands of the posterior part of the body
are indicated by the cilia at the margins
only.

masses of cells, the ventral
ganglia (Figs. 145 and 140). In this way the lateral cords
arise, to which a middle cord is added. The latter sepa-
rates from the ectoderm of the ciliated groove. At first
the entire ventral cord is still intimately united with the
ectoderm, but the latter gradually detaches itself, and the
ventral cord thereby acquires a deeper position. The funda-
ment of the supra-oesophageal ganglion, which is small in the

ANNELIDA

309

—j'/i

fully formed animal, we have already identified as the apical
plate. From it two cords extend backwards, embrace the
mouth-opening, and unite with the ventral cord. In this
way are formed the oesophageal connectives, which are un-
usually large in the Echiuridae (Figs. 145 and 146 sc).

The anal vesicles, which open to the
exterior along with the intestine (Figs.
146 and 146), do not arise, as was sup-
posed, from the intestine, but are
formed in the somatic layer of the
mesoderm. They lie here in the ter-
minal segment of the body as two
compact cylindrical structures, which
later become hollow and unite with
the ectoderm on either side of the anus.
At the same time they grow inward.
Their middle part is distended, and the
inner end opens free into the body
cavity by means of a ciliated funnel
(Fig. 146 ah). Their entire mode of
origin proclaims the anal vesicles to
be nephridia, which only secondarily
entered into connection with the hind-
gut.

The intestine is no longer so wide in
comparison with the entire body, as is
to be seen in Fig. 146 ; on the con-
trary, it has grown more in length and
makes several turns, which subse-
quently are still more emphasized. In
the meantime the larva has also altered
externally, in that its transverse dia-
meter has decreased in proportion to its
diameter (Figs. 145 and 146). On the surface the rows of
dermal papillae become noticeable, and, just as in the
Chsetopoda, the uncinate setae which are formed in the
setigerous sacs (immediately under the ectoderm) break
through to the outside (Fig. 146). The further development
consists first of all in an active growth of the hinder portion

a

Pro. 146. — Larva of
'Echiurus (after Hats-
chek). a, anus ; ah, anal
vesicle ; b, circle of setae
at the hind end of the
body ; d, intestine ; m,
mouth; M, ventral chain
of ganglia ; sc, cesopha-
geal connective ; sp, api-
cal plate ; vh, ventral or
uncinate setae.

longitudinal

310

EMBRYOLOGY

of the body, which thus approaches the form of the adult
worm, as can be seen from the larval stage of Thalassema
shown in Fig. 147. Even in this stage the ciliated bands are
still present. With the gradual reduction of these, which

soon takes place, the larva approaches
more and more the shape of the adult
animal. The oral and preoral parts of
the larva are transformed into the pro-
stomium, whereas the hind part still
grows in length.

3. Larval Form and Metamor-
phosis of Bonellia.

The larva of Bonellia resembles the
Trochophore less than do the larvfe of
Echiunis and Thalassema, although it
also can be referred to the Trochophore.
Evidently it is much modified, as is
pi-oved by its internal organization,
which is less adapted to a free life.
We follow SpE^G^!;L in describing the
development of Bonellia.

The larva of Bonellia, which is at first
spherical, possesses, in addition to the anterior
band of cilia, a posterior one (Fig. 148^).
The anterior to all appearances corresponds to
the anterior ciliated band of the larva of
Thalassema and Ecliiurus, especially since in
the latter a band of cilia is also found in the
region of the anus. Anteriorly two eye-spots
make their appearance (Fig. 148 A and h).
The intestine is not yet differentiated as such,
but is developed later. The larva increases in
length, becomes flattened dorso-ventrally and
covered with cilia (Fig. 148 />'). It now has more the appearance of a
Turbellarian, and, as its shape indicates, it moves by creeping.

The further development of the larva atfects first its internal organi-
zation. The supra-cesophageal ganglion has become differentiated from
the ectoderm, and later the esophageal connectives and the ventral
nerve cord begin to develop. The entoderm has become a single layer

if

Fig. 14.7.— Late larvii.l
stago of Thalassema nullita
(after Conn), a, anus; ah,
anal vesicle; d, intestine ;
m, mouth ; vb, ventral er
uncinate setas.

ANNELIDA

311

of cells, which surrounds the central yolk-mass like a sac. On the front
end of this fundament of the intestine a conical appendage becomes
noticeable, the first indication of the oesophagus, which subsequently
breaks through to the exterior in the region of the anterior ciliated band
(probably behind it). Lying between ectoderm and entoderm is the
mesoderm, which has split into a somatic and a splanchnic layer in most
parts of the body, whereas in the head region it has the form of a com-
pact mass of vesicular cells. Besides the mesodermal elements which
are transformed into musculature and peritoneum, there exist still
others, which lie in the body cavity. These are transformed into
structures like blood cells, which float in the body fluid. It is only at
this time that the formation of the spacious body cavity is accomplished.
The vessels arise from the peritoneal lining of the body cavity.

er/—

Fia. 118. — Stages of developinent of Bonellia (after SpEWGaL, from Ba.lfouk's
Comparative Emhi-yology). A and B, larvse with anterior and posterior bands of
cilia. C, young Bonellia. al, intestinal canal ; an.v, anal vesicle ; w, mouth ; s,
fundament of the ventral hook ; se, excretory organs.

At the same time with the internal processes of development described,
an external change of form takes place. The ciliation of the body
disappears for the most part ; the anterior portion of the body grows
considerably in length, and its ventral side, which is still ciliated, be-
comes depressed, in this way acquiring a spoon-like form (Fig. 148 C),
and thereby realizing a stage like that in EcUurus. Later, projections
are formed on the prostomium where the eye-spots are situated. By
their further growth is brought about the bifurcation of the prostomium
which characterizes the female of Bonellia.

Of the internal changes, there is still to be considered the further
development of the intestine, whose central yolk-mass becomes absorbed.
The mouth-opening breaks through the base of the prostomium, while

312

EMBRYOLOGY

the anus is formed at the posterior end of the ventral side. The anal
vesicles are said by Spengel to arise as evaginations of the hind-gut
(Fig. 148 C). We saw that in Echiurus their origin was described
differently, and that therefore they are preferably to be considered as
nephridia (p. 309).— A pair of tubes which make their appearance behind
the mouth are considered by Spengel to be the provisional excretory
organs (Fig. 148 C). Immediately behind these the ventral setae are
formed (Fig. 148 C).

The earliest fundament of the ovary was also observed by Spengel.
It is formed, in the same way as in other Annelids, from the peritoneal
covering of the blood-vessels, in this case on the posterior part of the
ventral vessel. The duct for the sexual products is a tube, which is to
be looked upon as a nephridium, although it is not quite evident whether
it is connected, and if so by what means, with the provisional excretory
organs observed by Spengel.

The description of the development of BonelUa up to this point
applies to the female only. The development of the small male, living
in the uterus, is much simpler, since it remains in the state of the
ciliated larva. The larvae which develop into males seek the ciliated
groove on the prostomium of the female, and there attach themselves.
They lose the two ciliated bands, but retain the uniform coat of cilia.
Their internal organization corresponds on the whole to that of the
female, only certain simplifications arise ; thus, for example, the mouth
and anus are wanting. In the male also the genital products arise from
the cells of the peritoneum. Balls of spermatic cells are detached from
this and fall into the body cavity, subsequently to be taken up by the
funnels of the spermatic duct. After the males have remained for a
short time on the prostomium of the female, they migrate into the
oesophagus, in order to complete there their metamorphosis. Spengel
found as many as eighteen males in the oesophagus. Subsequently they
abandon the oesophagus and repair to the uterus, where ordinarily six,
eight, or more males are found.

General Considerations.— As regards the position of

the Echiuridi'e, we agree with Hatschkk's view (No. 51) ; he
sees in them a division of the Annelida, and brinors them
into relation with the Chsetopoda. The form and internal
organization of the larva, as well as the mode of origin of
the setae, seem fully to substantiate this view. Even though
a segmentation [metamerism] no longer exists in the adult
animal, it was nevertheless established in the larva, just
as in the Chietopoda and ArchiKunelida. The loss of the
segmentation and the reduction of the setoe, as well as the
enormous extension of the prostomium, or so-called proboscis,
make the Echiuridie appear as somewhat modified forms.

ANNELIDA

313

III. DINOPHILUS.

Although the development of Dinophilus is not yet known
in detail, we include this aberrant form in the course of our
present account, because the adult animal itself remains to
a certain extent at the stage of an Annelid larva.

In its outward shape (Fig. 149) Dinophilus presents a
striking resemblance to certain
polytrochal Annelid larv^, e.g.,
to those of Ophryotrocha and
a larval SylHs not yet de-
scribed, which we observed in
the "tow" at Trieste. This
applies not only to the ciliated
bands, the distribution of the
sensory hairs, and the ventral
tail-like appendage, but more
especially to the entire habit
of the worm. The caudal
appendage of Dinophilus, like
that of the Annelid larva in
question {Ophryotrocha sp.), is
segmented. For this reason,
as also on account of its ven-
tral position, it recalls the foot
of the Rotatoria, a comparison
which indeed does not appear
altosfether without foundation
when one considers the simi-
larity in the organization of
Trochophore larva to that of
the Rotatoria, which has al-
ready been emphasized (p. 259).
Should the statement prove to
be correct that the five pairs
of nephridia possessed by Dino-
philus (Fig. 149) end blindly
in the body cavity (E. Meyer), there would exist in this
particular also conditions such as are found in the Annelid

Fig. 149.— Female of Dinophilus
gyrociliatus (after E. Metek, from
LiNG's Lehrbuch). a, eye ; aw, anus;
ed, hind-gut ; i/i, mouth ; md, mid-gut ;
n, nephridia; o, ovary; ph, pharynx;
phd, pharyngeal glands ; ufc, ciliated
band.

'^14 EMBRYOLOGY

larva and in the Rotatoria. As regards the rest of the organi-
zation— for example, the structure of the nervous system —
Dmophihis has been compared directly to the Archiannelida.

Dinophilus lays its eggs, several united, in transparent
gelatinous capsules. In Binojphilus apatris (gyrociliatus)
there are found in the capsules, in addition to the large oval
eggs, spherical ones, which are several times smaller ; the
number of the former compared to the latter is about as two
to one. From the larger eggs arise the females, from the
smaller the males, when, as in D. apatris (gyrociliatus),
there is a great difference in the size and foi-m of the two
sexes (Korschelt).

Cleavage is unequal in both kinds of eggs, and, according
to Korschelt's statements, which are corroborated by
Harmer, leads to the formation of an epibolic gastrula.
Repiachoff, on the contrary, desci'ibes the occurrence of an
invagination gastrula for the large eggs of D. gymciliatus,
which arose from a blastula thickened on one side. The
blastopore appears to become the mouth. Two large
cells are differentiated near it, which, according to Repia-
CHOFf's observations, move into the blastoccele, and there, as
in other Annelida, produce the two mesodermal bands. The
supra-oesophageal ganglion is to be seen lying close to the
ectoderm.

However, the accounts last mentioned do not seem to be
w^ell established ; but what is known of the development of
Dinophilus harmonizes with Annelid development, and the
entire organization of the worm points to relationships with
the Annelida.

From the fact that a most striking sexual dimorphism
exists in Dinophilus, — in so far as the males are much smaller
and more simply organized than the females, lacking the
intestine, the eyes, and the segmental bands of cilia (Kor-
schelt),— relationships of this genus with i\\e Rotatoria have
also been contended for ; but these conclusions do not appear
to be justified when one reflects that, while sexual dimor-
phism makes its appearance in certain species (D. apatris,
i.e., D. gyrociliatus), in very similar species, such as D. vorti-
coides, gigas, and tceniatus (according to 0. Schmidt, Weldon,

ANNELIDA 315

and Harmer), the males, apart from the sexual characters
proper, are formed just like the females.

IV. MYZOSTOMA.

Myzostoma, the discoid parasite of the Crinoids, provided
with hook-bearing parapodia arranged in pairs, deposits its
eggs in large masses without bestowing on them any special
care. The eggs, which are enclosed in a structureless mem-
brane, are fertilized outside the parent, after the formation
of the two polar cells. The fertilized eggs sink to the
bottom. Their development was studied by Metschnikoff in
Mi/zostoma cirriferum, and afterwards somewhat more fully by
Beard in Myzostomum glabrum. The unequal cleavage leads
to the formation of an epibolic gastrula, of whose six inner
cells the two lying nearest to the blastopore are said to give
evidence by their darker appeai'ance of being mesodermal
cells. Tufts of- cilia soon afterwards make their appearance
on the ectoderm cells all ai'ound the ovate embryo, which
now breaks through the egg-shell. Its shape soon becomes
pyriforra. An ectodermal invagination, the fundament of the
mouth and fore-gut, makes its appearance in the region of
the blastopoi-e. It grows inward and unites with the
stomach, which in the meantime has been formed out of the
entoderm cells, which have increased greatly in number.
The anus arises at the posterior pointed end of the larva. It
is not easy to determine whether it also is formed by an
ectodermal invagination or merely by a fusion of ectoderm
and entoderm. In front of the anus there appears a papilla
[on the ventral side], which subsequently becomes quite large
and constitutes the end of the body, the anal opening thus
becoming displaced dorsad (Fig. 150). The subsequent
stages are chai'acterized by the fact that the cilia distributed
over the entire body become restricted to certain regions.
These are, first the anterior end of the preoral pai't of the
body, which constitutes the apical area and bears a tuft of
rigid cilia (Fig. 150), then a band of cilia lying immediately
behind the mouth and a second one in the region of the anus,
and finally a bundle of rigid cilia at the tip of the caudal
appendage (Fig. 150). At the same time with the changes in

316

EMBRYOLOGY

the ciliation, there appear on both sides of the head, behind
the post oral band of cilia, the fundaments of setge, which
soon elongate considerably, and finally reach appi'oximately
the length of the entire larva.

It is not to be denied that this Myzostoma larva possesses
a very great i-esemblance to the larvae of Annelids, even

though the absence of the pre-
oral band of cilia, which could
not be found by Beard, inter-
feres with a complete resem-
blance to the Trochophore. The
thickened apical area, the two
bands of cilia, the sensory hairs
at the anterior and posterior
ends, as well as the internal
orofanization of the larva are
quite Annelid-like. A caudal
appendage, covered in the same
way with tactile hairs and con-
stituting a prolongation of the
ventral surface, is found in the
larvge of Telepsavus and Ophrijo-
trorha. In the same way the
provisional setse of the Myzo-
stoma larva, Avhich probably
arise in ectodermal sacs, point
to the corresponding structures
of certain Annelid larvae (comp.,
for example, the drawing of
Mitraria, Fig. 124, p. 276). In
any event the similar characters in the larvae of Myzostoma
and other Annelids are very many, and the further develop-
ment also presents other common features, e.g., the formation
of the parapodia and their bristle- or hook-beainng, stump-
like processes.

After the larvae have moved about free at the bottom of
the aquarium for some seven days, they cast off the pro-
visional setae and betake themselves to an Antedon, on which
they are found crawling about like worms, for the larval

Fig. 150 — Liirva rif Myzn!'l(}}iia
glahrum (after Beako). a, anus;
h, setae; m, mouth; s, caudal ap-
pendage ; sp, apical plate.

ANNELIDA

317

ciliation meanwhile has degenerated. The further develop-
ment of the larva is quite simple. The body, which up
to this time was broad in front and narrow behind (Fig.
150), changes its shape to such an extent that it becomes
broader behind than in front. The principal changes in
the shape of the larva are brought about by the develop-
ment of parapodia, which takes place from in front back-
wards, as in the PolychiBta. Like the setge in Polychceta, the
hooks in Myzostoma are said to arise in ectodermal sacs.
A segmental differentiation of the compact mesodermal mass
lying between the integument and the intestine, a differen-
tiation which proceeds from in fi-ont backwai'ds, might be
compared with the segmentation of the mesodermal bands in
the Annelida. A large part of these mesoderm cells become
connected with the parapodia as musculature. Others are
applied to the mid- and fore-guts. The latter effect the
formation of the evertible proboscis. Up to this time the
intestinal canal has retained its simple character ; but by
the time the development of the parapodia is completed
evaginations are seen in it, and in this way its bianched
character takes its origin. As regards the formation of the
nervous system, the apical plate, which is to be looked upon
as a larval central organ, is said by Beard to degenerate ;
but since this author did not recognize the presence of a
supra- oesophageal ganglion and oesophageal ring, which
nevertheless are present, it is quite possible that the former
arises from the apical plate, and that, as in other Annelids,
a union with the ventral cord, which arises as an ectodermal
thickening, also occurs in the development. The ventral
cord, which exhibits, according to Nansen and v. Wagner,
the usual form of a chain of ganglia with transverse com-
missures, has thus a segmental arrangement.

The statements concerning the origin of the mesodermal
structures are less certain. A true body cavity is not present,
but its place is occupied by parenchymatous tissue, which is
traversed by muscle fibres, and yet the authors (Nansen,
Beard) speak of an epithelium of the body cavity, from which
the sexual products atise. It appears, then, as if the hollow
spaces which contain the sexual pi'oducts constitute remnants

318 EMBRYOLOGY

of the body cavitj. Sej^raental organs have not. been
identified, though the oviducts were held by Beakd to be
remnants of such, and Nansen believed the same of the
paired ciliated depressions of the outer surface of the body
formerly called sucking discs ; but up to the present time
su tficient grounds for this view have not been produced . The
sexual organs in Myzostoma are not always developed in the
same way. In addition to the hermaphroditic individuals,
there are living on them very much smaller male animals
(complemental males). The fact that oviducts were also
found in these (Nansen) indicates that we have to do, not
with individuals of really separate sexes (Beard), but only
with incompletely developed hermaphrodites.

The place which we assign to the genus Myzostoma appears
to be justified by the manner of its development. This
characterizes it as a branch of the Annelid stem, which, to
be sure, is T-ather aberrant, and has suffered great changes,
probably as the result of the parasitic mode of life. The
place pi'eviously ascribed to it, supplementary to the class of
Arthropods, was necessarily given up when the development
became better known. The form and internal organization
of the larva, as well as its ciliation, which is also a feature
of the adult animal, separate it shai-ply from the Arthro-
poda.

V. HIRUDINEA.

The Hirudinea, like the Oliyochieta, lay their eggs in
CQcoons, which are formed in the same way in the two
cases, namely, by a secretion from dermal glands, which
hardens. The cocoons themselves ai-e of various sizes,
according to the size of the animal. In the medicinal leech
they become more than 2 cm. in length. Their shape also
varies in different species and genera. Those of Uirudo and
Aulastoma are ellipsoidal, and exhibit outside the shell
proper a layer of spongy substance, which probably serves to
protect them against desiccation (Leuckart). They are
deposited in the earth. The flattened cocoons of Clepsine
and Nephelis are found in water, firmly glued to some fixed
object. Clepsine covers the cocoon with its body, and

ANNELIDA 319

further cares for the brood by carrying about with it, at-
tached to its ventral side, the young after they have hatched
from the cocoon. The cocoon ordinarily contains a large
number of eggs, as many as twenty in the medicinal leech.
The Gnathobdellidas and Rhynchobdellidoe are distinguish-
able by the fact that the cocoon of the former is filled with
albumen, in which the eggs are found embedded, whereas in
the Rhynchobdellidae the cocoons lack the albumen, and the
much larger eggs lie in rows and in layers alongside and
above one another in great numbers, in Clepsine, for example,
as many as 200. Correspondingly the eggs of the Gnathob-
dellidae are small, and contain little yolk ; the embryos leave
the eggs at an early stage of development, and, like the
Oligochgeta, float as larval forms in the albumen of the
cocoon, by means of which they are nourished. Only after
several weeks do they quit the cocoon. The Rhynchob-
dellidfe, on the contrary, whose large, richly yolk-laden eggs
furnish to the embryos sufficient nourishment, do not break
through the egg-membrane until a much more advanced stage
of development and soon after also abandon the cocoon.

1. Cleavage, Formation of the Germ-layers, and
Development of the Outward Form of the Body.

A. Rhtnchobdellid^.

The process of cleavage can best be followed in the Rhyn-
chohdellidas, on account of the larger size of the eggs, and
has been repeatedly studied in Clepsine. According to
"Whitman, three small blastomeres and a single larger one
are first pi'oduced by the formation of two vertical cleavage
planes, whose position indicates the subsequent orien-
tation of the body of the worm. The three smaller ones
mark the anterior end, the larger one the posterior end, of
the worm. Then four small blastomeres bud out at the
animal pole from the four large ones, whereby the familiar
stage of four macromeres and four micromeres is reached (Fig.
151 A). The further metamorphosis consi-sts in the separa-
tion of the posterior large blastomere into two of nearly the

320

EMBRYOLOGY

same size (Fig. 151 B), one of which Whitman designates as
neuronephroblast, and the other as mesoblast, in accord-
ance with their subsequent fate. The mesoblast soon divides
into two cells, which at first do not occupy bilaterally sym-
metrical positions, as would be expected of the primitive
cells of the mesoderm. One of them lies more behind, the
other more in front beneath the micromeres, the number of
which soon increases, first at the expense apparently of the

Fio. 151.— /I to C, cleavage stages of Ch-psine, diagrammatic (after Whitmvn).
I. and IL indicate the direction of the first and second planes of division in,}), c, the
macromeres which become entoblasts ; /c, the maoromere which supplies tlie germ
bands ; a', b', c, k', micromeres which arise from tlie macromeres a, b, c, and fc ;
m, mesoblast; nn, neuronephroblast; n, neuroblast; up, nephroblasts ; I, jiole cells
of the lateral cell-row of the germ bauds, ect (mifcr.), which are descendants
of the micromeres.

macromeres (Fig. 151 B). With the exception of this pro-
duction of micromeres at the animal pole, the anterior three
macromeres take no further part in the subsequent cleavage.
They contain the nutritive yolk of the ega;^ and later supply
the cell material for the formation of the mid-gut ; they are
therefore to be designated as entoblasts. At the time when
the neuronephroblast divides into eight cells symmetrically

ANNELIDA 321

placed at the posterior pole (Fig. 151 C), additional nuclei
make their appearance in the entoblasts without any corre-
sponding division of the entoblasts. In addition, however,
certain cells that are from the bes'innino' distinct are con-

o o

stricted off from the entoblasts; they lie under the layer of
micromeres, and are to be looked upon as the earliest
entodex'm cells. Later, cells that have been dilferentiated
within the entoblasts are added to them, so that a distinc-
tion between the two kinds can no longer be made.

The embryo, up to the stage to which we have followed it,
consists of a solid mass of cells formed of the three macro-
meres (entoblasts), which become partly covered over by the
disc of micromeres, which have now become very numerous
(Fig. 151 C). Under the latter, consequently between them
and the entoblasts, there already lie a number of entoderm
cells, while the nuclei that appear within the entoblasts
provide for the formation of further cell material to be
added to the entoderm. At the posterior pole appear the
two symmetrical groups of neuronephroblasts, each com-
posed of four large cells (Fig. 151 C), and below them,
sunk somewhat deeper, lie the two mesoblasts ; these, too,
are now almost symmetrically arranged, although that re-
lation cannot be recognized in a surface view, such as
Fig. 151 G.

The two groups of five cells each at the posterior end are
of great importance from the fact that the greater part of
the body of the leech arises from them. Since in their origin
they can be traced back to the hindermost of the four
original macromeres, it follows that this is the one which is
responsible for the development of by far the largest part
of the body. These two groups of cells undergo the follow-
ing change : from the anterior face of each of the ten cells
new cells are constricted off by repeated cell division, a pro-
cess which can be compared with the multiplication of the
pole cells of the mesoderm in the Chsetopoda, and also leads
to the same result. On each side, then, there arise four
adjacent rows of cells, those of the neuronephroblasts, and
one lying somewhat deeper, that of the mesoblasts. All of
them together constitute the two germ bands, which, how-

K. H. E. Y

322

EMBRYOLOGY

ever, as will be shown later, are not directly comparable to
the mesodermal bands of the Chgetopoda.

As a result of the rapid cell-proliferation, the germ bands
grow forward, and the layer of micromeres, which have in
the meanwhile increased considerably in numbers, advancing
at the same time with them, covers a greater extent. Thus
the entoblasts gradually become overgrown by the germ
bands and the descendants of the micromeres. Whereas the
two germ bands at first diverge, their ends subsequently
unite at the anterior part of the embryo (Fig. 152 A). They
now elongate to such an extent that they occupy approxi-
mately the greatest periphery of the egg (Fig. 152 B). Their
further change in position takes place in such a way that

Fig. lf)2. — Embryos of Clepsine, elucidating the development of the germ bands
(after Whitman). Between the germ bands (fcs(r) the portion already overgrown
by ectoderm is dotted ; the entoblasts (ent) are shaded by par.allel lines, m, region
of the mouth; p, pole cells of the cell-rows constituting the germ bands.

their anterior and posterior ends appear to be fixed, but they
themselves move down toward the ventral side, and thus
approach each other, so as finally to unite in the ventral
middle line (Fi^. 152 F). Fig. 152 C, which is a view at an
angle of ninety degrees with that of Fig. 152 B, shows the
beginning of this process, whereas in Fig. 152 D the fusion
of the germ bands, Avhich takes place from in front back-
wards, has progressed still farther. Fig. 152 E exhibits the
other hemisphere, and shows that here the germ bands are

ANNELIDA 323

not yet completely united. This has occurred, however, in
Fig. 152 F, which shows the embryo in profile.

Since the layer of small cells arising from the micromeres
follows the growth of the germ bands, the embryo becomes
surrounded by a superficial cell-layer, which, according to
Whitman, produces the epidermis. Furthei'more, the head
portion of the worm is said to arise from these cells, and
perhaps in the same way as the trunk is formed from the
germ bands, for the trunk alone owes its origin to these
bands (Whitman, Bergh).

During the processes described certain changes, which
give rise to the formation of the mid-gut, have also taken
place in the entoderm. Even at an earlier stage certain
cells had separated from the upper part of the entoblasts.
Others succeed these, for the nuclei move out to the surface
of the entoblasts, surround themselves with plasma, and in
the form of an epithelium — the cells of which at first are
flat, but later become cubical — are added to the cells
already present. The formation of the mid-gut begins at
the anterior end, and progresses from there backward on the
ventral side Avith unusual rapidity. Finally the completely
formed mid-gut surrounds the entoblasts, which have now
sunk to the value of mere food-yolk. At the anterior end
the pharynx, which has arisen as an ectodermal invagina-
tion, unites with the mesenteron. A shallow depression
makes its appearance at a very early period in the region of
the ectoderm cells which are first formed (micromeres) ; in
later stages this comes to lie at that point where the two
germ bands meet (Fig. 152 m). This depression indicates
the future pharyngeal invagination. The latter makes its
appearance as a solid growth of the ectoderm, which lies in
the depression. Later it becomes hollow and fuses with tho
entoderm. This (entoderm) lines a part of the proboscis,
whereas the remaining part of the proboscis and the pro*
boscis-sheath are formed of ectoderm. The anus does not
arise until later.

When the embryo has developed thus far — that is, when
the circumcrescence is completed, and its surface is entirely
closed — it abandons the egg and soon afterwards the cocoon

324 KMBRYOLOfiV

also to undergo its further development while attached to
the ventral surface of the parent. As regards the external
form of the body, a segmentation [metamerism] can be recog-
nized, which is to be referred to that of the germ bands
(Fig. 152 F). This segmentation makes its appearance in
the same way as in the Chaetopoda, namely, progressing
from in front backwards.^ Furthermore the shape changes,
in that the body, which Avas first flat on the dorsal and

strongly curved on the ventral
S. d. mr/-"' surface (Figs. 152 F and 153),

becomes straight and flat on
the ventral surface, while its
dorsal side assumes the fami-
liar arching owing to the more
active growth of that part.

Fig. 153.-Embryo of Clepsine (after -^.t this stage the body COnsists

Rathke and Whitman), d, intestine ; of thirty-three Segments, the

Icit, g-erm bands : s, pharynx : sn, , . • i , p i ■ i • j

gygjjgj.^ posterior eight or which unite

to form the posterior sucker
(Fig. 153). The anus arises dorsad of this by the fusion of
the entoderm and ectoderm.

It is a question how the formation of the germ-layers is to be ex-
plained. Whitman assigns to the ectoderm the cells that have arisen by

1 [Whitman (No. XXII., Appendix to Literature on Annelida) has
recently investigated the metamerism of the Hirudiiiea and its origin.
He endeavors to explain the segmentation of the adult animal by
means of embryological facts, and further supports his opinion by the
anatomical conditions, especially that of the nervous system. The
principal question concerns the interpretation of the head, which is
composed of the primary head segment and several trunk segments
united with it, a condition similar to that which is also assumed for
the Chtetopoda. The mouth may be placed as far back as the fourth
segment.

The segmented body is derived from an unsegmented. The origin of
the segmentation is to be sought in the reproduction by division of the
originally unsegmented worm. The individual segments therefore really
correspond to separate individuals. The increase in the number of
segments is caused by the method of life, which necessitates such an
increase. (Comi). in this connection the statements made under " General
Considerations regarding Annelida," p. 348.) — K.]

ANNELIDA 325

the division of the neuronephroblast (Fig. 151 B, C) ; in that case the
circumcrescence of the macromeres by the layer of small cells and the
germ bands would seem to produce an epibolic gastrula, an interpretation
that was in fact given by Balfour. The deep layer of the germ bands
arising from the mesoblasts would then be the mesoderm, though its
superficial layer also, by becoming overgroN\Ti with the small cells, soon
comes to lie inside. These processes recall to a certain extent those in
Rhynchelmis, in which Oligochaete the mesomeres at first lie in the region
of the ectoderm and give off to it products of their division. Perhaps
more detailed observations on this point will yield greater evidence of
agreement. At present the germ bands of the Hirudinea and the meso-
dermal bands of the Oligochceta are not to be looked upon as homologous
structures, for they are composed of different kinds of elements. How-
ever, Kleinenberg argues for a participation of the ectoderm in the
formation of the mesodermal bands, and Wilson likewise finds these
same bands of cells, which form the germ bands of the Hirudinea, even
in the Oligochaeta (comp. Kupra, p. 294).

If such stages of the embryos of CU-psine and Lumhricus as are shown
in Fig. 153 (p. 324) and Fig. 132 (p. 286) are compared, the conclusion
is natm-al that processes which led to such similar structures must have
been at the beginning of like nature, even though they are now changed
in their details.

B. GnathobdelltdtE.
A detailed investigation of the cleavage of the egg of
Nephelis has been given by Butschlt. Nevertheless, owing
to the small size of the egg, we are not as accurately informed
about the cleavage and formation of the germ-layers in the
Gnathohdellido! as about the corresponding processes in the
Rhynchobdellidce. At all events, certain differences between
the groups seem to exist.

In Nephelis there also occurs a cleavage stage of four macromeres and
four micromeres, though the latter are said not to arise from all four,
but from only three, of the macromeres, whereas the fourth, posterior
blastomere remains for a time passive. These three macromeres then
again give rise to three small cells, which are arranged, as in Clepsine,
under the micromeres first formed, and constitute the first entoderm
cells. The fourth of the four macromeres now divides into two large
blastomeres, which Whitjian interprets as corresponding to the neurone-
phroblast and mesoblast in Clepsine. According to this view, to which
Bergh also inclines, the superficial layer of the germ bands would be
derived from the former, the lower layer, on the contrary, from the
latter. The fact that the "neuronephroblast" is said to form two

326

EMBRYOLOGY

additional small cells, which are added to the four ectoderm cells already
present, does not agree with the processes in Clepsine. The " neurone-
phroblast" and the "mesoblast" each divide into two cells, which are
placed symmetrically in respect to the middle line. The edges of the
macromeres arch up more or less over the small blastomeres, so that
these at times appear to be embedded in them, a process that also takes
place in like manner in Clepsine. The fate of the different blastomeres

Fig. 151,.-^ anil B, cleavage stage and an embryo at the time of hatching of
Hephelit vu^aris (after BOtschli). cct, ectoderm; cnt, entoderm; Ifsfr, germ
bands; nm/ir, macromeres ; mifcr, micromeres; m, mouth-opening; t, pharynx.

could not be followed farther than this, though it is to be assumed
that the further differentiation is the same as in Clepsine. At all events,
two " germ bands " are also formed here (Fig. 154), which extend from
behind forwards and there (m the region of the future mouth) unite.
The metamorphosis of the entoderm is important, and in determining
the entire shape of the animal, significant. To the entoderm cells first

ANNELIDA 327

formed from the three macromeres have been added others, which like-
wise have probably been furnished from the same source. The entoderm
now lies in the form of two rows of cells upon the macromeres (Fig. 154
A), which now represent a kind of food-yolk. They are surrounded and
partly covered in by the germ bands, while the ectoderm, now increasing
more rapidly, covers over the anterior part of the embryo. A central
fissure (the fundament of the cavity of the mesenteron) soon arises be-
tween the entoderm cells, which enlarge at the exiiense of the yolk-cells
[macromeres] (Fig. 154 A). The latter are forced more toward the hind
end of the embryo, and are finally overgrown by the ectoderm, which
also spreads out backwards (Figs. 154 B, 156). In this case, there-
fore, the macromeres are not taken into the intestine, as in Clepsine,
but remain outside of it ; but in this position they too are gradually
absorbed. The mouth and pharynx finally arise at the anterior end of
the embryo in the form of an ectodermal invagination, which unites with
the intestine (Fig. 154 B).

2. The Larvae of the Gnathobdellidae.

The embryos of the Gnathobclellidge break through the
egg- membrane at a stage in which they are spherical or oval
and have attained about the condition represented in Fig.
154 B. The pharynx, still very simple in stracture, leads
into the intestine, which now begins to enlarge. The ecto-
derm has not yet quite grown over the macromeres. The
" germ bands " lie between it and the entoderm. It is seen
that the development is not so far advanced as that of
the hatching embryo of Clepsine. Whereas the latter is
converted directly into the worm, the embryo of the
Gnathobdellidfe undergoes a protracted larval existence.
Like the larvae of the Oligocheela, those of the Gnathobdel-
lidse float in the albumen of the cocoon, and take this into
the intestine by means of deglutitory movements. For this
purpose a provisional pharynx (Figs. 154 B and 156 s) is
developed, which is provided with a powerful musculature.
The larva possesses still other provisional structures which
are entirely wanting in Clepsine. In Nephelis a cephalic
process is developed, which is thickly covered with cilia (Fig.
156). This ciliation recalls that which occurs in the larvae
of the Oligochgeta, especially since, as in Lumbricus trape-
zoides, it extends on to the ventral side, where it is found in

328

EMBRYOLOGY

the median line of the entire ventral surface (RoBix). The
larvfeof the Gnatliohdellid?e also possess provisional excretory
organs which are comparable to those of the Oligochseta, even
though in number and form they are different. In Nephelis
there are two (Fig. 156 tirii and un^), in Hirudo three, and
in Aulastoma four pairs of provisional kidneys. In the last
form they are found lying on the ventral surface of the larva,
on either side of the germ bands from which, according to
Bekgh, they take their origin as cell-growths, composed at
first of one, then of several rows of cells (Fig. 155). Sub-
sequently they separate from the germ bands, and then con-
sist of structures somewhat annular in shape and composed
of two rows of cells (Fig. 156 un^). These two cell-rows
subsequently differentiate in such a way that they consist

of two adjacent canals ; one of them
becomes the stouter, chief canal, and
the other one winds several times
about it (Fig. 156 «%) . At the turn-
ing point the two are continuous
Avith each other, and therefore really
constitute only one canal. A cilia-
ticn has not been observed in the
canals. Not only is there in Ne-
phelis the ring-like canal, which is
wound about itself, but this is pro-
longed into a duct, which to a cer-
tain extent constitutes the efferent
duct of the organ, and has been
compared to such by Bergh, in the
sense that the two primitive kidneys
would correspond to the two arms
of the primitive kidney of Polygor-
dius, and that the duct would lead to the point of union of
the two. As has been stated, there are in Hirudo three and
in Aulastoma four pairs of primitive kidneys, and Leuckart
even describes in the medicinal leech their opening to the
exterior. Bkrgh, however, could not confirm this.

The primitive kidneys of the Hirudinea are said by
Bergh to have nothing to do with the permanent excretory

Fio. 155.— Origin of the
I)rimiti\'e kidiiej-sCniii to uti,)
from the germ bands (Riithj)/-
Jceim) of Aulastoma gulo (after
Bergb). 1)1, mouth; pz, pole
cella terminating the cell-rowa
of the germ bands.

ANNELIDA

329

organs, for these are not formed in the germ bands until the
primitive kidneys have already separated from them (comp.
also p. 332).

Like the primitive kidneys, other organs of the larva also
degenerate during its metamorphosis into the adult worm.
A musculature consisting of longitudinal and circular fibres,
which in the region of the mouth enlarges into a powerful
circular muscle, is found under the epidermis of the larva.
Between the muscle fibres Bergh finds spindle-like and
branched cells, which he takes to be of a nervous nature.
This entire larval skin is said by Bergh to be cast off in the
metamorphosis, and the whole body of the leech, with the
single exception of the mid-gut, arises from the so-called
trunk and head germs (Rumpf- und Kopfkeime) , of which
more will be said later. At this time the mouth closes.
The provisional pharynx of the larva is replaced by a per-
manent one. Details about these pi^ocesses will be given
in considering the formation of the organs.

3. The Further Development of the Body; For-
mation of the Head and Trunk.

A distinction between head and trunk was apparent even
in the Chsetopoda ; it was recognizable by the condition of
the mesoderm, and also probably found expression in the
m.ode in which the nervous system was formed. In the
Hirudinea this contrast is still more decided, for the funda-
ments of the nervous system of the head and trunk are
separate, and the so-called germ bands probably take no
part whatever in the formation of the head. According to
the investigations of Bergh, which to a certain degree con-
firm and extend the earlier discoveries of Leuckart and
Semper, there are two so-called head germs (Kopfkeime) in
addition to the germ bands, which we have already learned
about, and which are designated by Bergh as trunk germs
(Btivipfkeime) . These head germs, the origin of which
is still obscure, lie between the pharynx and epidermis as
two cell-masses, which become united by a connecting cord

330

EMBRYOLOGY

of cells, extending over from one to the other (Fig. 156).
From these head germs the whole head portion is said to be
formed, including even the epidermis, for the epidermis
which is now present (Fig. 156 ep) is of only a provisional
nature. In like manner the entire trunk portion (with the
exception of the mid-gut) is said to arise from the trunk
germs. The head germs and trunk germs unite in the region
of the mouth. Thus in the formation of the body a decided
difference would exist between head and trunk.

~'^,

Fio. 15(1.— Longitudinal section of a larva of Ncpkclis (after Bkroh). eni, ento-
dermal elements; ep, provisional epidermis; Ick, head germ {Kopfkeim); m,
mouth-opening ; mes, individual mesoderm cells; mii, muscle cells; pz, pole cells
of the germ band (i.e. trunk germ) ; rk, trunk germ (Rumpflceim) ; s, provisional
pharynx; titii and un,, primitive kidneys or their fundaments.

Whitman also assumes a fundamental distinction between head and
trunk portions, and is inclined to refer the origin of the former to the
four micromeres first foi-med. However, the difference in the Clepsine
observed by Whitman is not so striking as here, for in that case the
epidermis is not cast off.

If the permanent body of the medicinal leech is really formed from
four fundaments, then the comparison with the formation of the
Nemertean from the Pilidium, which was attempted by Bekgh, is a natural
one. In Pilidium also the larval skin is cast off, and the body arises
from several separate fundaments, of which the mesodermal are four in
number (two in the head and two in the trunk portion) (comp. y>. 223).
Yet these processes, as far as they are known, appear to take place in

ANNELIDA 331

the Nemertini and Hirudinea in a manner that is too inharmonious to
warrant a comparison. ^ Also the further development of the "head
germs and trunk germs," which in the Nemertini takes place by means of
ectodermal invaginations and additions of mesenchyma cells, but in the
Hirudinea as early differentiations of embryonal cells, shows little
similarity, apart from the fact that the Annelidan and Nemertean larva
themselves have only a very slight resemblance to each other.

4. Formation of the Organs.

The Body-covering. — At an early stage of embryonic de-
velopment the layer of small cells groves over the germ bands
and macromeres, and thus forms the epidermis. This epi-
dermis, beneath which muscles have already been developed,
probably from the germ bands, becomes in Clepsine the epi-
dermis of the adult worm, whereas in the Gnathobdellidae
it, together with its musculature, is said gradually to dis-,
integrate, and to be replaced by a new epithelium, which is
formed from the superficial layer of the " head germs and
trunk germs." These have united in the region of the
mouth, and thus the entire body is covered by the new epi-
dermis. At the same time the body musculature is formed
from the head germs and trunk germs. The remnants of
the larval skin are finally cast ofF.

According to the description given by Whitman for Clepsine, and by
Bergh for AulaKtoma and Nephdis, the epidermis does not appear to be
homologous in the two groups, which differ from each other to the extent
that the larval skin in one group is directly transmitted to the adult
animal, but in the other is cast off, being replaced by a layer of different
origin. However, an intermediate condition is said to exist in Clepsine,
for, according to Whitman, two cells of the germ bands take part in the
formation of the epidermis, though Whitman exphcitly denies that it
arises from these alone.

In Clepsine there is developed out of the epidermis, between it and the
ganglion underlying the pharynx, a mass composed of numerous large
gland-cells, whose secretion serves to attach to the mother the newly
hatched young until their suckers are developed (Whitman).

The Nervous System. — In the development of the Hirudinea
it is difficult to separate ectodermal and mesodermal ele-

1 It should be added that Bergh himself afterwards ceased to place
any value on this quite natural comparison.

332 EMBRVOLOGY

ments from each other. Thus the germ bands can be in-
terpreted as being formed of both kinds of elements
(Whitman). As we have seen, each germ band is composed
of four superficial rows of cells and a more deeply located
one (Fig. 151 G). The ventral chain of ganglia arises from
the innermost row of each germ band. The cells multiply,
and in this way a cord of cells consisting of several layers is
formed from the single row. A segmentation takes place in
this from in front backwards. In addition, median and
lateral parts are differentiated in the separate cell-masses,
and both cords unite in the middle line. In this way arise
the ganglia and the commissures.

Bergh, like Whitman, also derives the ventral chain of ganglia from
the germ bands, but, according to him, the permanent epidermis arises
from the same source, and consequently the nervous system takes its
origin beneath this.

Ndsbaum's (No. 75) theory of the origin of the nervous system differs
from that described. He derives the ventral chain of ganglia, as well as
the brain, from a thickening of the ectoderm — that is to say, from the
primitive epidermis — and thus adopts an intei-pretation that (more in
harmony with theoretical considerations) was also espoused by Kowa-
LEVSKY and Balfour. The statements of Nusbaum on this and other
developmental processes of the Hirudinea harmonize so little with the
statements of the other authors on this subject that any further con-
sideration of them must be omitted.

The development of the supra- oesophageal ganglion is
initiated in the head germs, underneath the layer which
supplies the epidermis, by the segregation of a compact
mass of cells, in which the Punktsubstanz can soon be
recognized (Bergh). The fundaments of the brain and
ventral chain of ganglia would then be distinct, and not
until after the concrescence of the head germs and trunk
germs would they be united by the development of the oeso-
phageal connectives.

The Nephridia. — According to Whitman, the nephridia
arise from the two middle cell-rows of each germ band,^ and,
in fact. Whitman, in opposition to Bergh (comp. supra, p.

• The fate of the fourth, outer row of cells remained unknown to
Whitman.

ANNELIDA 333

328), finds a certain resemblance between the primitive
kidneys and the permanent nephridia, in that both of them
arise from the same parts, i.e. from the middle rows of the
germ bands. However, in the Rhyncobdellidge themselves,
which Whitman studied, primitive kidneys are not pr-esent.
The origin of the nephridia from a continuous cord of cells,
which, moreover, is described by Wilson in the same way
for Lumbricus, recalls the theory advanced by Hatschek that
in Criodrilus the permanent nephridia arise from a cord of
cells in the somatic layer of the mesoderm (comp. supra, p.
296).

The development of the nephridia takes place from in
front backwards by the cord of cells becoming many-
layered and undergoing a segmental division. How the
nephridia arise from the cell-masses thus produced has not
yet been accurately determined. A pair of nephridia is
begun in each segment, though all of them do not develop,
for in the adult worm there are only sixteen pairs. ^

The Body-cavity and its Lining ; Musculature ; Blood-vessels.
— The peritoneal lining of the body cavity and the somatic
and intestinal musculature arise from the two more deeply
located cell-rows of the germ band, the pole cells of which
we have learned to designate as the mesoblasts. The two
cell-rows have changed into voluminous cords of cells by the
I'apid multiplication of their elements. These cords undergo
a segmentation from in front backwards. The primitive
segments thus produced extend out dorsally, and cavities
make their appeax^ance in them. The latter correspond to
the segmental (metameric) cavities of the Chaetopoda.
After growing completely around the intestine, they are
said to become confluent, and to form the marginal sinus,
which belongs to the lacunar portion of the blood-vascular

1 [The formation and differentiation of the rows of cells produced by
the teloblasts has been again traced by Bergh and by Apathy, as well a^
in the works of Whitjun (see Appendix to Literature on Annelida). The
subjects involved are the formation of the nervous system, the body
musculature, and the nephridia. These organs have been traced back to
definite parts of the so-called germ band, though as yet complete agree-
ment on the part of the authors has not been reached.— K.J

334 EMBRYOLOGY

system (Whitman). According to another view, however,
the two remain separate, and constitute the lateral sinuses of
the two sides. The other processes — the formation of the
septa and that of the intestinal and body musculature —
ajDpear to take place in the same way as in the Chaetopoda.
By the growth of the mesodermal elements, the body cavity
may undergo a greater or less reduction. In the Rhyn-
chobdellidse the body cavity is still well developed, and is
provided with a distinct peritoneal epithelium, whereas in
the Gnathobdellidas it is almost entirely suppressed (Bourne).
It has already been mentioned that portions of the body
cavity are metamorphosed into parts of the blood-vascular
system. It has been stated that the dorsal and ventral
trunks of the blood-vessels take their origin from the
splanchnic layer, owing to a splitting of it.*

The Genital Organs are doubtless of mesodermal origin,
though the statements which are made concerning their
formation are little to be trusted.-

The Inteslinal Canal. — In both the Rhynchobdellidae and
the Gnathobdellidae we have already become acquainted
with the origin of the raesenteron from the three entoblasts.
These give rise to a vesicle composed of large cells, which
gradually resorbs the entoblasts whether enclosed within or
lying outside it, and becomes connected with the outer
world by means of an ectodermal invagination (comp. pp.
323 and 327). The pharynx which is formed in this way
presents different conditions, according as the development
is direct or indirect. In the first case the pharnyx, pro-
duced by the collaboration of entodermal, ectodermal, and

* [Burger (Appendix to Literature on Annelida) has made an ex-
tensive investigation of the formation of the body cavity, the blood-
vascular system, and the nephridia. He traced the establishment of the
cuilom, its differentiation, and its relation to the circulatory system.
In regard to the nephridia, considerable agreement with the Oligochaita
has been found. — K.]

[However, Burgku (Appendix to Literature on Annelida) has re-
cently given a detailed account of their origin, according to which they
are referable on the whole to proliferations of the peritoneal epithelium.
Not only the sexual glands, but also the efferent ducts, arise in this
way.— K.]

ANNELIDA 335

probably mesodermal parts, is converted directly into the
oesophag-us, pharynx, and proboscis-sheath of the adult
animal. The intestinal canal attains its final shape as the
result of the ingrowth toward it of the dissepiments,
"which thus cause the cascal diverticula of the intestine. At
the same time the intestine is provided with its musculature.
In Clepsine there are six pairs of such diverticula ; the
seventh pair grows backward through five segments, and
consequently acquires constrictions similar to those of the
intestine itself. The terminal portion of the intestine
extends straight backwards and unites with the ectoderm to
form the anus.

The conditions are not so simple in the Gnathobdellidse.
Here the pharynx first formed is of a provisional nature, and
functions only in the reception of the albuminous nourish-
ment. After this office is discharged it degenerates ; the
mouth closes as the result of the concrescence of the head
germ and trunk germ (Bergh). At the same point there is
formed an invagination of the united head and trunk
germs, the fundament of the permanent pharynx, which
grows into the larval pharynx and unites with the entoderm,
while the tissue of the old pharynx is gradually absoi-bed.
The jaws arise in the pharynx as fold-like elevations covered
by a firm cuticula (Leuckart). The oral sucker is formed
as a circular elevation of the superficial layer of the body.
The development of the mid-gut takes place in a manner
similar to that already described above for Clepsine. On
the other hand, according to Bergh's observations, the hind-
gut is formed as a solid outgrowth of the tissue of the
" trunk germ," which subsequently becomes hollow, and
unites with the entoderm. Such a mode of origin agrees
with Bergh's entire theory, according to which the whole
body of the leech, with the single exception of the mid-gut,
is formed from the so-called head and trunk germs.

The degeneration and regeneration of the pharynx in the Gnathob-
dellidas recall the replacement of the larval stomodsBum by a permanent
pharynx in Lopadorhynchus as described by Kleinenberg, even though
the metamorphosis takes place there in a different way.

336 EMBRYOLOGY

General Considerations. — The development of the
Himdinea doubtless points to the fact that in dealing with
them one has to do with Annelida. Although differing in
details, the entire process of development is similar to that
of the Chaetopoda, and especially of the Oligochaeta. The
so-called germ bands of the Himdinea and the mesodermal
bands of the Chaetopoda, it is true, do not appear to be
homologous structures, but the entire manner of their
formation and their relation to the embryonic body in
general, as well as their subsequent development, indicate
that both are to be referred to like structures, and that in
the Himdinea a modification has appeared only in so far
as the more simple mesodermal bands have there acquired a
more complicated structure by the addition of ectodermal
pai'ts. In their mode of development the Hirudinea appear
to be less primitive forms than the Chaetopoda.

Just as the mode of origin of the individual organs,
especially the body cavity, the nervous system, and the
excretory system, shows the leech to be an Annelid, so, too,
does its anatomical structure. This is mentioned only for
the reason that direct relationships between the Hirudinea
and Platyhelminthes have been sought for in various direc-
tions. In this connection it is only the structure of the
genital organs and their resemblance to those of the
dendrocoelous Turbellarians that appear to be remarkable.
It would be desirable to know more than we do at present
regarding this point.

In brief it must be said that, as compared with the
Chfetopoda, the Himdinea show themselves to be in struc-
ture and development higher forms, which exhibit, many
secondary modifications.

VI. BRANCHIOBDELLA.

The systematic position of Branchiuhdella is not yet
established. There are anatomical grounds for the view
that this worm is to be assigned to the Oligochaeta, and that
it is only in consequence of its parasitic mode of life that it
has acquired certain cliaractei'S — for example, the posterior

ANNELIDA 337

sucker — which cause it to resemble the Hirudinea (Voigt,
Vejdovskt). The development exhibits in some features a
resemblance to that of the Hirudinea, but otherwise it
is so peculiar — provided we can rely on the statement of
Salensky — -that the relationship of Branchiobdella to either
branch of the Annelida cannot be inferred from it.

Branchiobdella lays its eggs, each surrounded by a firm
envelope, on the gills of the crayfish, where they are
attached bj'' means of a stalk, a prolongation of the
envelope. A cocoon proper, as in the Oligochfeta and
Hirudinea, does not exist, although the egg is surrounded,
as in these, by a special envelope ; perhaps therefore the
outer envelope is equivalent to a cocoon.

In the cleavage and the formation of the germ-layers,
conditions are exhibited which do not resemble those of the
Hirudinea or Oligochfeta, but can perhaps be referred more
readily to the latter. We begin with the stage in which one
large and three small blastomeres are formed. All four are
to be called macromeres, for soon four micromeres are
abstricted from them. By division of the micromeres and
the formation of new ones on the part of the macromeres, a
rapid increase of these small (ectoderm) cells takes place.
They soon form an ii'regalarly defined cell-plate, the sides of
which grow out and overlie the macromeres in the form of a
cap. The striking thing in this is that the micromeres are
said to correspond to the ventral side of the worm ; however,
it is also stated that for Clepsine the mouth breaks through
in the region of the first four micromeres, and it has a
similar position in Nephelis (comp. Fig. 154, p. 326). A
rather small cleavage cavity makes its appearance between
the micromeres and macromeres; it is subsequently forced
away from the macromeres by the production of new
cells. The macromeres have likewise divided and arranged
themselves as two pairs of large cells at the posterior end
(Fig. 157 A). A cord of ectoderm cells forces its way
between the two pairs. The double-row arrangement of the
macromeres persists even when they divide further (Fig.
157 B).

K. H. E. Z

338 EMBRYOLOGY

These wo rows of macromeres have been compared to the macromeres
of the Hirudinea, though, as far as can now be seen, this comparison is
not warranted, for the macromeres in Branchiobdella are said to continue,
dividing, and to give rise to the mesoderm and entoderm. But in both
the Hirudinea and the OUgochteta the separation of the two germ-layers
takes place much earlier.

The division of the macromeres advances steadily from
behind forwards. In this way two different groups of cells
ai'ise, one of which lies next to the ectoderm, and constitutes
the mesoderm, while the other, lying next to the macro-
meres, is the entoderm. What is left of the macromeres
themselves divides uninterruptedly, so that the cells arising
in this way become like the ectoderm. They cover the
posterior part of the embryo (Fig. 157 D).

Even before the macromeres separated into the different
elements in the manner described, a depression of the ecto-
derm (Fig. 157 A), which does not persist long, and perhaps
represents the fundament of the supra-oesophageal ganglion,
makes its appearance in front of them, and therefore on the
dorsal side of the embryo. This originates independently of
the ventral chain of ganglia. The latter arises, according to
Salensky, in the form of an extensive groove on the ventral
side (Fig. 157 G). The groove is veiy broad at the posterior
end of the embryo. It is bounded here by the large cells
still remaining, which, continuing to divide, contribute to
the formation of the margins of the groove. The groove
becomes narrower anteriorly, extends on to the dorsal side
of the embryo, and here bifurcates (Fig. 157 D). The part
of the ectoderm which is encircled by the two branches
probably con-csponds to the above-mentioned ectodermal
depression, and jjroduces the supra-cesophageal ganglion,
which secondarily unites with the two processes of the ven-
tral chain of ganglia by means of two ridge-like processes,
the oesophageal connectives. The ventral cord itself is said
to originate in a manner quite like that of the medullary
tube of vertebrates. The groove becomes closed by the
bending together of its upper edges (in this case, however,
from in front backwai'ds), and in this way forms a tube,
which finally separates from the overlying ectoderm, loses

ANNELIDA

339

its lumen, and lies as a cellular cord in the ventral median
line of the embryo.

On each side of the nerve cord lies a ribbon-like cord of
cells, the mesodermal band. The two mesodermal bands are
united to each other by. a median part. They have arisen
from the common ento-mesodermal mass, the origin of which
we have previously traced, by the separation of a ventral
layer, the mesoderm, from the dorsal layer, the entoderm.
A segmentation, like that
in the ventral cord, also
makes its appearance in
the mesodermal bands,
which separate into the
primitive segments. The
processes thus effected, as
well as the formation of
the body cavity and the
septa, take place in a
manner similar to that
described for the other
Annelida. The internal
segmentation is late in
finding expression on the
exterior of the body, and
is suppressed in its an-
terior and posterior parts,
where the primitive seg-
ments for the present
acquire no cavities, and
therefore remain in an
embryonic condition.
Each segment exhibits
externally a division into
a broader and a narrower portion (Fig. 157 E). The former
corresponds to a ganglion, the latter to a septum. In front
of the anterior end of the ganglionic chain lies a part of the
mesoderm, which forms the head cavity ; but regarding this,
Salensky could not determine whether it likewise arose from
the mesodermal bands.

Fig. 157.—^ to E, embryos of Branchio-
hdella in vatious stages (after Salensky).
ect, ectoderm ; gr, pit in the ectoderm on
ibe dorsal side ; ma, macromeres ; m, mouth-
opening ; n, neural groove ; s, sucker.

340 EMBRYOLOGY

At the time of the appearance of the outer segmentation
a peculiar change in the position of the embryo occurs. Up
to this time its ventral side was greatly curved, for both the
anterior and the posterior ends grew toward the dorsal side.
Later it assumes the revei'se position. This is effected by a
rotation of the embryo on its own axis. The movement
begins at the anterior and at the posterior parts of the em-
bryo, and gradually extends to the middle portion. Where-
as the ganglion chain at first lay on the convex side of the
embryo, it is now found on its concave surface. In the
course of this process the anterior and posterior parts of the
body assume their permanent shape (Fig. 157 E). The
posterior end is abruptly truncated. A depression on it,
which soon makes its appearance, repj-esents the fundament
of the sucker. The absence of segmentation at the anterior
end is noticeable ; the head, however, is distinct from the
anterior part of the body (Fig. 157 E). The mouth-opening
makes its appearance as a shallow depression of the ecto-
derm far in front, and probably at the place where the
medullary groove bifurcated. It unites with the fore-gut,
which, as well as the hind-gut, is said to arise from the en-
toderm. Tlie entoderm for along time consists of a compact
mass of cells, which increases in length with the growth of
the embryo. In the formation of the epithelium, the cells
withdraw to the periphery of the mass; and the nutritive
material, which is surrounded by them, remains at the centre
just as in the formation of the intestine in Rhynchelviis.
Tlie fore- and hind-guts are the first to be hollowed out.
The latter unites with the very short tube which forms the
anal invagination located on the dorsal side of the sucker.
The entire oesophagus, even the jaws, are said by Salensky
to be of entodermal nature ; and only the lips, with their in-
ternal lining, are formed of ectoderm. Last of all follows
the development of the mid-gut. Even in the hatching em-
bryos, which have approximately the development described
(Fig. 157 E), the mid-gut is still filled with an undigested
yolk-mass.

To enumerate once more the chief points in the develop-
ment of this unique group, which it has hitherto been im-

ANNELIDA 341

possible to unite satisfactorily either with the 01igocha3ta
or Hirudiaea, the following points, in addition to the al-
together aberrant cleavage phenomena, are remarkable: the
formation of the mesodermal bands and the very peculiar
manner in which the nervous system is formed. A germ
band in the sense of the Hirndinea is not present, but, on
the other hacd, the fundament of the nervous system differs
from that of the Chsetopoda. To be sure, the oi-igin of the
chain of ganglia from a ventral ectodermal invagination has
been repeatedly described for the Annelida, but this conclu-
sion has not gained currency. At all events, the origin of
the nervous system and mesodermal bands of Branchiobdella
merits renewed investigation.

General Considerations regarding Annelida.

The embryology of the Annelida affords us some hints
regarding the phylogenetic derivation of the Annelid stem
and its genetic relationships to other gi-oups of animals, and
also regarding the origin of metameric segmentation. These
suggestions are significant, even though they do not as yet
furnish a foundation of positive knowledge, but serve only
to support theories of greater or less probability.

If we consider the larval forms of the Annelida, we see
that their different shapes, however variously they may be
expressed, are referable to the Trochophore. The Trochu-
phore is the typical larval form, of the Annelid stem. Even in
the derived and much-modified groups, such as the Oligo-
chceta and Echiuridm, as well as in the aberrant genus Myzo-
stoma, the larval Trochophore form can be recognized more
or less distinctly. Dinophilus corresponds in its shape
and organization to a so-called polytrochal larva, which it
was possible to derive directly from the Trochophore (comp.
p. 278). The embryos of the Hirudinea exhibit the greatest
resemblance to those of the Oligochgeta. However, they are
much more modified than these, and consequently cannot be
traced directly to the Trochophore, though this may be
accomplished through the mediation of the Oligoch^ta.

Most likely the Trochophore of the Annelida embodies

342 EMBRYOLOGY

the ontogenetic recapitulation of an ancestral form which
was common to the Annelida, Mollusca, and MoUuscoidea,
and from which these animal stems branched off as inde-
pendent groups. The assumption that the Trochophore cor-
responds to an ancestral foi-rn is supported, not alone by
the circumstance that the larval forms in the groups men-
tioned can all of them be traced more or less directly to
the Trochophore : it acquires a further and important sup-
port from the fact that in the division liuHfera we see before
us forms which in their adult condition remain essentially
at the stage of organization of the unsegmented Trocho-
phore. We have already mentioned (p. 259) that not only
the Rotifer known by the generic name of Trochosphjera,
but also the rest of the Rotatoria, can readily be referred to
the plan of the Trochophore. The Rotatoria a;ccordingly
are organisms which still exhibit the closest relationships to
the Trochophore-like ancestral form whose mode of locomo-
tion and plan of organization, with some secondary changes,
they have retained.

If we take into comparison the rest of the groups of the
so-called Vermes, there is apparent, in the first place, a
striking resemblance between the Trochophore of the An-
nelida and the larval form of the Nemertini known as
Pilidinm (comp. p. 231), even though in their further de-
velopment the two groups pursue different paths. By means
of the Pilidium we are also led to bring certain larvas of the
Turhellaria into remote comparison with the Trochophore
(comp. pp. 168 and 230).

In searching for the ancestral forms from which the
Trochophore-like archetype arose one meets with great diffi-
culties. In order to arrive at an idea of this ancestral form,
the Trochophore has been compared to a Medusa. Its
pelagic mode of life, its shape, and, above all, the nerve-
ring of the ciliated band discovered by Kleinenberg, were
the things which led this author and Balfour to assume its
descent from a medusoid form. Derived in such a way, the
pi-eoral band of cilia is, from its position, referred to the
margin of the umbrella, and the aboral dome of the Trocho-
phore to the ex-umbrella, whereas the part of the larva lying

ANNELIDA 343

behind the ciliated band must be considered as the sub-ura-
brella, made to bulge downward. A more careful considera^
tion, however, offers considerable difficulties to a derivation
of this kind. Even if we disregard the fact that the Medusa
represents the most divergent and most highly developed
form of the Cnidaria type, and that forms which are highly
developed in one direction ordinarily do not become points
of departure for new developmental series, still the difficulty
of the derivation suggested is evident fi'om a comparison
of the mode of locomotion of the t\vo forms. The Medusa
moves by means of oar-like strokes of a complicated loco^
motor apparatus, depending iipon uiuscular action. On the
other hand, the Trochophore, with its trochal organ operated
by ciliary motion, represents a much more prim.itive loco-
motor apparatus, directly comparable in its mode of action to
the ciliated planula (comp. p. 154, et seq., the grounds
which have been advanced against the derivation of the
Ctenophora from Medusae). A chief difficulty in the
derivation under discussion is found in the presence of the
central nervous system at the apical region, where important
organs are never developed in the Meduste. We should then
have to look upon the nerve-ring of the Trochophore as the
chief part of the central nervous system, and the apical plate
as a subsequently acquired secondary part of it ; but in the
present state of our knowledge we are not justified in this.
We recognize that the two parts of the nervous system be-
long together, and have probably been developed in close
relation with the locomotor apparatus, as regulators of the
movements. Thus perhaps the apical plate in its earliest
origin is to be traced back to a tuft of cilia functioning as
a rudder (such as is met with at the apical pole of many
Actinian larvaa), whereas the ring-nerve, it is to be assumed,
has been formed in connection with the development of the
trochal organ, both of them as localizations of a system of
nervous internuncial fibres, distributed under the entire
ectoderm. It might be mentioned here that, in addition to
the apical plate, a nerve-ring is also met with in the Pili-
dium.

We have above adduced the difficulties which, according

344 EMBRYOLOGY

to our point of view, are opposed to a derivation of the Ti-o-
chophore from the medusoid form, and have already made
some suggestions respecting a derivation of the Trochophore
which, although based upon hypothetical grounds, nevei'the-
less appear to be better supported by the facts of compara-
tive anatomy and embryology than the former view. This
view brings the Trochophore into relation with the ancestral
forms of the Nemertini, TurbeVaria, and Ctennphora, and
regards it as having arisen tolerably directly from much
more primitive ccelenterate forms than is possible on the
assumption of derivation from Medusse. It should be ex-
pressly noted here that we necessarily abandon the realm of
positive demonstration in making these statements, which
scarcely have any higher value than that of mere conjec-
tures.

To us the facts appear to indicate that the ancestral form
aj-ose rather directly from a uniformly ciliated gastrula-
like archetype by a change in the mode of locomotion. Such
a primitive, completely and uniformly ciliated organism
exhibited an anterior apical and a posterior oral pole.
Secondary axes had not yet been developed ; the form
presented at first the monaxial heteropolar type. It is
possible, and in view of the ancestors of the Ctenophora
probable, that on this form certain differentiations made
their appearance without causing an abandonment of the
monaxial, heteropolar form, or the i-adial form that arose
from it. Among these differentiations we reckon a tuft of
cilia at the animal pole functioning as a rudder (the earliest
fundament of the apical plate), an ectodermal pharyngeal
tabe, and the formation of diverticula of the entodermal
portion of the intestine, by the regular distribution of which
around the chief axis the fii-st impetus to the formation of
definite secondary axes was pi-obably given.

It must be mentioned that many Actinian larvae present exactly the
atructure described {Scijpku a). However, this resemblance is probably
founded merely on analogy, for in the Cnidaria we assume that the
formation of radial gastral pouches took place only after attachment
and the development of an Archhydra stage, whereas the Ctenoi^hora and
Bilatcria probably never had an attached ancestral form.

ANNELIDA 34

The original mode of locomotion of the uniformly ciliated,
radial ancestor described, which had arisen from the gas-
trula stage by means of some further differentiations, was
spiral, such as we may still see in the ciliated planula3 of
many lower animals. It depended upon a combination of
a progressive movement in the direction of the longitudinal
axis with a rotation of the entire body about this axis.
The ancestral forms of the Platyhelminthes have perhaps
been directly developed from such a uniformly ciliated stock-
form by the assumption of the creeping mode of life, and
the ancestors of the Ctenophora may have been developed
by a change in the method of pelagic swarming and by the
formation of rows of ciliary plates. Whereas in the latter
case the rotation around the longitudinal axis sank into
insignificance, and the combined force of the ciliary plates
was concentrated on propulsion in the direction of the longi-
tudinal axis, in those forms which effected the transition
to the Trochophore-like ancestor a change of movement
took place. In these cases, though the body as a whole
ceased to rotate, the rotatory movement was retained in the
trochal organ in the form of a regular cii^cular wave of
contraction, whereby this organ was in position to under-
take a function in relation to the body (now progressing
in a constant position) similar to that of the ship's screw
in relation to the hull of the ship. Hand in hand with this
alteration in the mode of locomotion went a higher differentia-
tion of the ciliary locomotor apparatus, by means of which
the passage from the original uniform coat of cilia to
distinct locomotor organs was brought about. As such are
to be mentioned the tuft of cilia functioning as the rudder
and the rows of cilia, but especially the preoral ciliated
band.

It is possible that the bilaterally symmetrical distribution
of the body-masses was directly developed in connection
with the above-mentioned changes in the form of motion
by means of which the body was balanced in its forward
movement. At any rate, one of the most important factors
for the development of bilateral symmetry is to be sought
in the shifting of the mouth-opening, which now moved

346 EMBRYOLOGT

forwards from the posterior pole of the body, thus deter-
mining as ventral the side of the body on which this
shifting took place. The first cause of this forward migra-
tion of the mouth, during which the opposite edges of the
posterior parts of the blastopore successively approached
each other until the slit thus produced was at last entirely
closed, is to be sought in the significance of the trochal
organ as a food-procuring apparatus and the importance
of the approach of the mouth toward it. By such a change
in the position of the mouth, the relations of the primary
axes were disturbed, so that henceforth the chief axis of
the body can no longer be referred dii'ectly to the primary
axis ; for this reason the Bilateria are also designated as
Heteraxonia (Hatschek).

Owing to the resemblance which the Pilidium and many Turbellarian
larva; present to the Trochophore, one might also be inclined to derive
the Platyhelminthes and Nemertini directly from a Trochophore-like
ancestor. The complete ciliation of these forms would then be not
primitive, but re-acquired after the transition to the creeping mode of
life (therefore as in Coeloplana, comii. p. 157). On the other hand, it
must be pointed out that ciliated bands are developed in great variety
in pelagic larvsB, and we are certainly not in i^osition to prove the
homology of all these bands. Hence the resemblance of these larval
forms to the true Trochophore of the Annelida is perhaps to be referred
merely to analogy in develoiDment, which would have its cause in a
developmental tendency in this direction inherited from the common
ancestor.

As a result of the development of the most important
locomotor organs in the anterior half of the body, it came
about that the organs of the animal functions arose in this
region. It is this part of the body which, as head, we place
in contrast with the posterior, subsequently elongated por-
tion of the Trochophore, which is called the trunk, and gives
rise particularly to organs of vegetative functions. The
fruitfulness of the conception, in the interpretation of the
Annelid body, that head and trunk are distinct, has only
been equalled by the difficulty of determining the precise
boundaries between these two primary regions of the body.

la the first place, the question arises whether the mouth

ANNELIDA 347

is to be assigned to the head or to the trunk. In the
solution of this problem the condition of the mesoderm
plays a particularly important role. No real coelom appears
to be formed in the portion lying in front of the mouth ;
on the contrary, the first pair of primitive segments is said
to surround the pharynx. If this were so, then a distinction
would really be established between the preoral and the
oral portions, and the latter would have the greater re-
semblance to the body segments. However, this distinction
is later obliterated owing to the fact that portions of the
mesoderm from the most anterior primitive segments migrate
into the preoral region and form its musculature. Thus
interpretations differ, inasmuch as the preoral part alone
(Kleinenberg) and also that together with the oral segment
(Hatschek) have been taken to be the head portion. More-
over, induced by the peculiar phenomena in non-sexual
reproduction, some authors have gone farther than this and
considered a greater number of segments (as many as six)
as the head portion of the worm (Semper, v. Kennkl). The
first and last theories seem to us to go too far. Until the
final settlement of the question how the mouth and the
pharynx are related to the first primitive segment, we would
reckon the mouth region as belonging to the head of the
worm.

The transition from the Trochophore-like stock-form to
the real Annelid ancestors (Archiannelida) took place by
considerable growth in length, whereby the trunk portion
of the worm became larger, and the primary head portion
less and less conspicuous. At the same time a change in
the mode of life took place, the pelagic being exchanged
for the creeping habit.

The larval stages belonging to this transition are dis-
tinguished especially by the terminal growth of the body.
Near the posterior end of the body, which we can henceforth
distinguish as the anus-bearing terminal segment, is found
a zone of proliferation, from which new cell material is
continually being given off forward to the elongating trunk
portion. Since, at the same time with this growth in
length, the segmentation of the trunk is established, it

348

EMBRYOLOGY

follows that the most anterior segments of the trunk are
formed first, and therefore are the most differentiated ones
in the developing- larva, while behind follow younger and
younger ones. The growth of the Annelid body therefore
does not depend upon growth of the body in all directions,
but upon a partial (terminal) growth, since new segments
are always being supplied from a zone of proliferation lying
near the posterior end of the body (in front of the terminal
segment). This pi'oductiveness of a restricted portion of
the body strongly recalls certain kinds of non-sexual re-
production, and therefore the process has been called, even
in this case, a " budding of the segments." That, however,
is an inaccurate mode of expression. The most natural
comparisons are those with the tapeworm chain and Avith
the strobila of the Scyphomedusse. The point of comparison
m all these cases lies in the production, from a certain zone
ot proliferation, of homodynamic portions of the body, which
become to a certain extent independent. For this reason
the view has been expressed that in the segments of the
Annelid body we have before us single individuals (which
do not arrive at complete separation), and accordingly in
the entire body of the Annelid a stock or corm. It seems
scarcely favorable to this theory that the degree of in-
dependence which the individual segments present is com-
parativelj^ slight. The most important organs (nervous
system, body musculature, blood-vascular system) show
themselves to be single fundaments of the entire body, and
are also developed as such even though they also exhibit
evidences of metamerism. Even the excretory canals may
give up their segmental isolation and become united to one
another by means of longitudinal canals. The comparison
with the single fundaments of the other systems of organs
inclines us to the opinion that the development of the
nephridia from separate fundaments (Bkkgh) represents a
coenogenetically altered condition, and that the nephridial
system was originally developed by sepax'ation from a com-
mon cord (Hatschkk). By such an assumption the com-
parison of the nephridial system of the Annelida as a whole
with the excretoi-y oi'gaiis of the riatyhelniiuihus would

ANNELIDA

349

become possible, since the longitudinal stems in the two
cases could be looked upon as corresponding to each other
(whereby we even have in mind a former connection of the
permanent nephridia with the head kidney). At all events,
the anatomy and development of the Annelid body permit
the establishment of the interpretation of the entire body
as an individual. Just as in the consideration of the tape-
worm chain we were induced by the comparison with un-
segmented forms to refer the entire chain to an unsegmented
individual,^ and, on the other hand, to see in the proglottis,
not a complete individual, but only the abstricted hinder
portion of the body of the Cestode, in the same manner,
and with much more reason, we adhere to the individuality
of the Annelid body. We can accordingly recognize in
metameric segmentation only the regular repetition of certain
groups of organs in the trunk at uniform intervals.

In the question of the origin of the metameric segmenta-
tion we shall have to ascertain whether the synchronism
of the terminal growth of the body and the appearance of
metameric segmentation correspond to a palingenetic con-
dition. In other words, in the hypothetical ancestral form
were new segments successively added behind during in-
crease in length, so that forms with many segments arose
from those with few by gradual increase in the number of
segments ? The fact that the growth of the body in length
by the formation of new segments at the posterior end is
typical in all Annelids and the forms derived from them
(Arthropoda) is an argument in support of this theory. In
that case we might perhaps be inclined to the opinion, as
stated by Hatschek, that in ancestral forms enlarging by
terminal growth the diiferentiation, originally progressing
continuously, became intermittent, and thus reached the type
of the metameric animal. But another view may also be
maintained, and, as it seems to us, with quite as much
justice — a view which is based upon the assumption that
at first an unsegmented, elongated ancestral form was pro-
duced by terminal growth, whereupon the entire body be-

1 There is a considerable difference between this and the process of
strobilization.

350 EMBRYOLOGY

came separated at once into a large number of segments
bj a rearrangement of the individual organs. This assump-
tion is supported bj the consideration that with the lateral
sinuous movement of the body, and with the rigidity of the
tissues caused by increasing differentiation, the formation
of alternating regions of greater and less motility was of
considerable advantage to the individual, and rendered
possible a further elongation of the body. The first cause
for the appeai^ance of metameric segmentation would then
be sought in the manner of locomotion and in mechanical
conditions. However, this latter view is not supported in
any way by embryology.

Even though we have not been able to give a positive
decision on these difficult questions, yet it seems to us ap-
propriate, in the present state of knowledge, to indicate the
direction of future inquiry by which a possible solution of
the questions is to be sought.

Literature.

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6. Bergh, R. S. Die Entwicklungsgeschichte der Anneliden. A

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ANNELIDA 351

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16. Grobben, C. Die Pericardialdriise der chatopoden Anneliden, etc.

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18. Hatschek, B. Studien iiber Entwicklungsgeschichte der Anne-

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von Lopadorhynchus : Nebst Bemerkungen iiber die Entwicklung
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27. Kowalevsky, A. Embryologische Studien an Wiirmern und

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23. Krohn, a. Uebar die Erscheinungen bei der Fortpflanzung von

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xviii., Bd. i. 1852.
29. Loven, S. Beobachtungen iiber die Metamorphose einer Annelide.

Arch. Naturg. Jahrg. viii., Bd. i. 1842.

852 EMRRYOLOGY

30. Metschkikoff, E. Ueber die Metamorphose einiger Seethiere

(Mitraria unci Actinotrocha). Zeltxchr. wiss. Zool. Bd. xxi. 1871.

31. Meter, E. Studien iiber den Korperbau der Anneliden. 1. und

2. Theil. Mitth. Zool. Station Neapel. Bde. vii. and viii. 1887

—1889.

32. Milne-Edwards, H. Observations sur le duveloppement des

Annelides. Ann. Sci. Nat. Ser. 8 : Zoologie. Tom. iii. 1845.

33. M'Intosh, W. The Eeport on the AnneUda Polychaeta. " Chal-

lemier" Reports : Zoologij. Vol. xii. 1885.

34. QuATREFAGES, M. A. DE. Memoire sur I'embryogenie des AnncUdes.

Ann. Sci. Nat. S6r. 3 : Zoologie. Tom. x. 1848.

35. Eeetsch, M. 6tude sur le Sternaspis scutata. Ann. Sci. Nat.

Ser. 6 : Zoologie. Tom. xiii. 1882.

36. BocLE, L. Sur la formation des feuillets blastodermiques et du

coelome chez un Oligochffite limicole (Enchytrseoides Marioni).
Compt. Fend. Acad. Sci. Paris. Tom. cvi. 1888.

37. Salensky, W. Etudes sur le developpement des Annelides. Arch.

Biol. Tom. iii., 1882 ; torn, iv., 1883 ; and tom. vi., 1887.

38. Schneider, Ant. Ueber Bau und Entwicklung von Polygordius.

Arch. Anat. u. Phys. Jahrg. 1868.

39. ScHULTZE, M. Ueber die Entwicklung von Arenicola piscatorum.

Halle. 1856.

40. Semper, C. Die Verwandtschaftsbeziehungen der gegliederten

Thiere. Arbeiten zool. Inst. Wiirzhurg. Bd. iii. 1876 — 1877.

41. Semper, C. Beitriige zur Biologie der Oligochiiten. Arbeiten

z„„l. Inst. Wilrzburg. Bd. iv. 1877—1878.

42. Vejdovsky, F. Untersuchungen iiber die Anatomic, Physiologie,

und Entwicklung von Sternaspis. Denkschr. Akad. Wiss. Wien.
Math.- naturw. CI. Bd. xliii. 1882.

43. Vejdovsky, F. System und Morphologic der Oligochiiten. Frag.

1884.

44. Vejdovsky, F. Die Embryonalentwicklung von Rhynchelmis.

Sitzungsh. bdhni. Gesellsch. Wiss. 1886.

45. Vejdovsky, F. Reifung, Bcfruchtung, und die Furchungsvorgiinge

des Rhynchclmis-Eics. Entwicklungsgeschichtlichc Untersuch-
ungen. Prag. 1886.

46. ViGciEK, C. i^tudes sur les animaux inf^rieurs de la bale d' Alger.

Arch. Zool. exp. et gen. S6r. 2, tom. ii. 1884.

47. Wilson, E. B. Observations on the Early Developmental Stages

of some Polychffitous Annelids. Stud. Biol. Lab. Johns Hopkins
Univ. Baltimore. Vol. ii. 1882.

48. Wilson, E. B. The Germ-bands of Lumbricus. Jour. Morph.

Vol. i. 1887.

49. Zeppelin, Graf M. Ueber den Bau und die Theilungsvorgiinge

des Ctenodrilus monostylos. Zeitschr. wisf. Zool. Bd. xxxix.
1883.

ANNELIDA 353

ECHIURIDJ!.

50. Conn, H. W. Life-history of Thalassema. Stud. Biol. Lah.

Johm Hopkins Univ. Baltimore. Vol. iii. 1886.

51. Hatschek, B. Ueber Entwicklungsgeschichte von Echiurus, etc.

Arbeiten zool. In$t. Wien. Bd. iii. 1881.

52. KowALEVsKY, A. Mittheilungen tlber die Entwicklung von Tha-

lassema. Zeitschr. wiss. Zool. Bd. xxii. 1872.

53. EiETscH, M. Etude sur les Gephyriens armes ou Echiuriens.

Recueil zool. Suisse. Tom. iii. 1886.

54. Salensky, W. Ueber die Metamorphose des Echiurus. Morph.

Jahrb. Bd. ii. 1876.

55. Spengel, J. Beitriige zur Kenntniss der Gephyreen. Blitth. zool.

Station. Neapel. Bd. i. 1879.

DiNOPHILUS.

56. Harmer, S. F. Notes on the Anatomy of Dinophilus. Jour.

Mamie Biol. Assoc. Vol. i. 1889.

57. KoRscHELT, E. Ueber Bau und Entwicklung des Dinophilus apa-

tris. Zeitschr. wiss. Zool. Bd. xxxvii. 1882.

58. Meyer, E. Studien iiber den Korperbau der AnneHden. Mitth

zool. Station. Neapel. Bd. vii. 1887.

59. Eepiachoff. Ueber Bau und Entwicklung des Dinophilus gyro-

ciUatus. (Eussian.) Odessa. 1886.

60. Schmidt, 0. Neue Beitriige zur Naturgeschichte der Wiirmer,

etc. Jena. 1848.

61. Weldox, W. F. E. On Dinophilus gigas. Quart. Jour. M'lcr. Sci

Vol. xxvii. 1885.

Myzostoma.

62. Beard, J. On the Life-history and Development of the Genus

Myzostoma. Mitth. zool. Station. Neapel. Bd. v. 1884.

63. Grafe, L. von. Das Genus Myzostoma. Leipzig. 1877.

64. Graff, L. von. Eeport on the Myzostomida. '' Challenger'' Re-

ports : Zoology. Vol. x. 1884.

65. Metschnikoff, E. Zur Entwicklungsgeschichte von Myzostomum.

Zeitschr. wiss. Zool. Bd. xvi. 1866.

66. Nansen, F. Bidrag til Myzostomernes Anatomi og Histologi.

Berg ens Museum. 1885.

67. Wagnee, F. von. Das Nervensystem von Myzostoma. Graz.

1886.
K. H. E. A A

354 EMBRYOLOGY

HiRUDINEA.

68. Beroh, R. S. Ueber die Metamorphose von Nephelis. Zeitschr.

wits. Zool. Bd. xli. 1885.

69. Bergh, R. S. Die Metamorphose von Aulastoma gulo. Arbciten

zool. I7}st. Wiirzhurg. Bd. vii. 1885.

70. Bergh, R. S. Ueber die Deutung der allgemeinen Anlagen am Ei

der Clepsine und der Kieferegel. Zool. Anzeiger. Jahrij. ix. 1886.

71. Bourne, A. G. Contributions to the Anatomy of the Hirudinea.

Quart. Jour. Micr. Sci. Vol. xxiv. 1884.

72. BuTSCHLi, 0. Entwicklungsgeschichtliche Beitrage. Zeitschr.

wiss. Zool. Bd. xxix. 1877.

73. Hoffmann, C. K. Zur Entwicklungsgeschichte der Clepsinen.

Niederl. Arch. Zool. Bd. iv. 1877—1878.

74. Leuck-uit, R. Die menschlichen Parasiten, etc. Leipzig und

Heidelberg. First edition. 1863.

75. NusBAUM, J. Recherches sur I'organogenese des Hirudinees.

Arch. Slav. Biol. Tom. i. 1886.

76. Rathke, H. Beitrage zur Entwicklungsgeschichte der Hirudineen,

herausgegeben von R. Leuckart. Leipzig. 1862.

77. Robin, C. Memoire sur le developpement embryogenique des

Hirudinees. Paris. 1875.

78. Whitman, C. 0. The Embryology of Clepsine. Quart. Jour.

Micr. Sci. Vol. xviii. 1878.

79. Whitman, C. 0. A Contribution to the History of the Germ-layers

in Clepsine. Jour. Morph. Vol. i. 1887.

Branchiobdella.

80. Salensky, W. Etudes sur le developpement des Annelides — He.

partie : Developpement de Branchiobdella. Arch. Biol. Tom. vi.
1887, and Beitrage zur Entwicklungsgeschichte der Anneliden.
Biol. Centralblatt. Bd. ii. 1882—1883.

81. Vejdovsky, F. System und Morphologic der Oligochiiten. Prag.

1884.

82. VoiGT, W. Untersuchungen iiber die Varietiitenbildung bei

Branchiobdella varians (and other works on Branchiobdella by
the same author). Arbeiten zool. Inst. Wiirzbnrg. Bd. vii., 1885,
and Bd. viii., 1888.

Appendix to Literature on Annelida.

I. Andrews, E. A. Compound Eyes of Annelids. Jour. Morph.
Vol. V. 1891.
II. Apathy, S. Keimstreifen und Mesoblaststreifen bei Hirudineen.
Zool. Anzeiger. Jahrg. xiv. 1891.

ANNELIDA

355

III. Beddaed, F. E. Researches into the Embryology of the

Oligochaeta. 1. On Certain Points in the Development
of Acanthodrilus multiporus. Quart. Jour. Micr. Sci.
Vol. xxxiii, 1892.

IV. Benham, W. B. The Post-larval Stage of Arenicola marina.

Jour. Marine Biol. Assoc. Vol. iii. London. 1893.
V. Beeaneck, E. Etude sur I'embryogenie et sur I'histologie de
I'cEil des Alciopides. hevue Suisse Zool, Tom. i. Geneve.
1893.
VI. BiiKANECK, E. Quelques stades larvaires d'un Chetoptere.

Revue iSuisse Zool. Tom. ii. Geneve. 1894.
VII. Beegh, R. S. Neue Beitrage zur Embryologie der Anneliden :
Zur Entwicklung und Differenzirung des Keimstreifens von
Lumbricus. Zeitschr. wiss. Zool. Bd. 1. 1890.
VIII. Beegh, R. S. Die Schichtenbildung im Keimstreifen der
Hirudineen. Zeitschr. tviss. Zool. Bd. Iii. 1891.
IX. Bourne, A. Certain Points in the Development of the Earth-
worms. Quart. Jour. Micr. Sci. Vol. xxxvi. 1893.
X. Beaeji, F. Zur Entwicklungsgeschichte von Ophryotrocha
puerilis Clprd. Mecz. Zeitschr. wiss. Zool. Bd. Ivii. 1893.
XI. BiJEGER, 0. Beitrage zur Entwicklungsgeschichte der Hiru-
dineen : Zur Embryologie von Nephelis. Zool. Jahrb., Ahth.
f. Anat. u. Ontog. Bd. iv. 1891.
XII. BtJEGEE, 0. Zur Embryologie von Hirudo medicinalis und
Aulastoma gulo. Zeitschr. wiss. Zeol. Bd. Iviii. 1894.

XIII. Haecker, V. Ueber die Metamorphose der Polynoinen. Ber.

naturf. Gesell. Freiburg. Bd. ix. 1894.

XIV. KoESCHELT, E. Ueber Ophryotrocha puerilis Clprd. Mecz. und

die polytrochen Larven eines anderen Anneliden (Harpochseta
cingulata nov. gen. nov. sp.). Zeitschr. wiss. Zool. Bd. Ivii.
1893.
XV. Malaquin, a. Etude coraparee du developpement et de la
morphologic des parapodes chez les Syllidiens. Compt.
Rend. Acad. Sci. Paris. Tom. cxiii. 1891.
XVI. Racovitza, E. Zur la Micronereis variegata Clap. Compt.

Rend. Acad. Sci. Paris. Tom. cxvi. 1893.
XVII. Randolph, H. The Regeneration of the Tail in Lumbriculus.
Zool. Anzeiger. Jahrg. xiv. 1891.
XVIII. RouLE, L. Etudes sur le developpement des Annelides et
en particulier d'un oligochfete limicole marin (Enchy-
trsBoides Marioni). Ann. Sci. Nat. Ser. 7, Zool., tom. vii.
1889.
XIX. Vejdovsky, F. Entwicklungsgeschichtliche Untersuchungen :
Die Entwicklungsgeschichte von Rhynchelmis und der
Lumbriciden. Prag. 1888—1890.
XX. Vejdovsky, F. Die Organogenic der Oligochseten. Prag. 1892.

356

KMBRTOLOOY

XXI. Vejdovsky, F. Zur Entwicklungsgeschichte der Nephridial-
apparates von Megascolides australis. Arch. mikr. Anat.
Bd. xl. 1892.
XXII. Whitman, C. 0. The Metamerism of Clepsine. Festschrift
fur Lntckart. Leipzig. 1892.

XXIII. WHIT^rA^■, C. O. A Sketch of the Structure and Development

of the Eye of Clepsine. Zool. Jahrb., Abth. f. Anat. v.
Ontog. Bd. vi. 1893.

XXIV. Wilson, E. B. The Embryology of the Earthworm. Jorir.

Morph. Vol. iii. 1889.
XXV. Wilson, E. B. The Origin of the Mesoblast Bands in Anne-
lids. Jour. Morph. Vol. iv. 1890.
XXVI. Wilson, E.B. Some Problems of Annelid Moq^hology. Biol.
Lect., Mar. Biol. Lab. Woods Holl. B'^ston. 1891.

XXVII. Wilson, E. B. The Cell Lineage of Nereis. Jour. Morph.

Vol. vi. 1892.

XXVIII. WiSTiNGHATJSEN, C. V. Untersuchuugen iiber die Entwick-

lung von Nereis Dumerilii. Miith, zool. Station. Neapel.
Bd. X. 1891.

CHAPTER XI.

SIPUNCULID^.

Our knowledge of the development of the Sipunctdidx is still
very meagre. Concei'ning Siptmculus and Phascolosoma,
the embryology of which has been studied, we know that
they possess larvae which may be compared to the Trocho-
phore. The development of Sipuncidus, which has been
thoi'oughly dealt with by Hatschek, presents many peculiari-
ties, above all the formation of an embryonal membrane.
The Troc/iop/iore-like larva does not arise directly from the
embryo, but the latter is surrounded by a cellular mem-
brane, as if by an amnion.

I.— The Development of Sipunculus.

The fii'st stages in the development of Sipunculus are not
known. By pelagic fishing, Hatschek captured the embryos
in the blastula stage. In these embryos the fundaments of
the three gex'm-layers can already be recognized (Fig. 158 A).
The thickened part of the spherical blastula consists of tall
cells, the entoderm; there is prominent among these a
particularly large cell, which, in contrast to the other
(prismatic) cells, retains a more spherical shape. This is
the first mesoderm cell. It lies in the median plane between
the ectoderm and entoderm, and marks the posterior part of
the embryo (Fig. 158 A). The free space which existed
between the embryo and the egg-membrane — the latter being
traversed by radial pores — disappears during the blastula
stage, owing to the fact that the cells apply themselves to
the egg-membrane. They send out cilia through the pores
of the membrane, so that the embryo, together with the egg-

357

358 EMBRYOLOGY

membrane, now begins to rotate. The vegetative pole, which
begins to flatten, and then to invaginate, remains free from
cilia.

In the formation of the archenteron a small part of that
portion of the blastula which is still to be assigned to the
ectoderm (Fig. 158 B) is also invaginated. The boundary
between it and the entoderm is marked by the mesoderm,
which has increased to two cells (the primitive mesoderm
cells) and now moves inward. The two cells are symme-
trical in relation to the median plane. The depression of the
ectoderm already mentioned, which follows that of the
entoderm, gives the first impulse to the development of the
permanent larval skin. It sinks in deeper and deeper and
bends over forwards, forming in this way a lamella of thick
cells (Fig. 158 G and D, rp). The plate remains temporai-ily
united, by means of an amnion-like fold that is not exten-
sive, it is true, with the primitive ectoderm, which soon
appears only as the sei-osa (Fig. 158 D). Hatschek calls
the plate the trunk-plate in contrast to the head-plate,
which is also differentiated from the primitive ectoderm.
This differentiation takes place as follows : — In the region of
the animal pole (corresponding to the apical plate), which
has now also become thickened, the cell plasma retracts
from the egg-membrane in an annular furrow, and thus
gives rise to a circular space (Fig. 158 B to IJ, ha) be-
tween the permanent ectoderm and an outer layer (serosa).
The head-plate, therefore, corresponds to the apical plate.
The space between it and the serosa (the amnion is only
slightly developed here) Hatschek calls the head amniotic
cavity (ka), and that between the trunk-plate and serosa the
trunk amniotic cavity (ra). The fold which would corre-
spond to the amnion on the trunk-plate is retained for a
short time and is then included in the formation of the
trunk-plate. The trunk- and head-plates alone supply the
ectoderm of the larva. All the rest of the ectoderm of the
embryo is emploj^ed in the formation of the embiyonal
membrane (serosa). The serosa grows over the trunk-plate
and completely encloses it (Fig. 158 JJ and E, se) ; however,
this is not the case at the opposite (animal) pole. Tlie

SIPUNCULID5: 359

head-plate is not overgrown by the serosa, and consequently
a circular opening is always retained here.

In consequence of a complicated process of growth, regard-
ing the details of which the reader must be referred to
Hatschek's desci'iptions, the at first median band-like
trunk-plate spreads considerably and grows out towards the
sides and then dorsal ly, finally to unite, at the termination
of the embryonic period, with the head-plate, which has
likewise enlarged somewhat. During this circumci*escence
of the internal parts by the trunk-plate, a change in the
position of the embryo takes place. The posterior part of
the trunk-plate had even before this turned toward the
animal pole, and thereby was in a position to supply the
dorsal part of the ectoderm (comp. Fig. 158 D and U). In
the region of the blastopore, which closes, the oesophagus had
been formed from the ectoderm at an early period (Fig. 158
D, bl). This part also changes its position, for it moves
more toward the apical plate, whereas the entodermal sac is
crowded further backwards (Fig. 158 E). As a result of
this, the position of the mesoderm is necessarily altered
(Fig. 158 G, D, and E). It is moved from the posterior
pai't of the larva further forwards. Its cells have meanwhile
greatly multiplied, and two mesodermal bands have arisen
from it (Fig. 158 U, mes). The latter do not undergo a
segmentation ; on the contrary, a fissure makes its appear-
ance in them, which separates them into a splanchnic and
somatic layer. This differentiation is first noticeable in the
anterior part of the mesodermal bands, and proceeds from in
front backwards.

The complete development of the embryo is reached by
the gradual closing of the permanent ectoderm. We saw
that the primitively band-like trunk-plate curved over
toward the dorsal side, and that its end moved toward the
apical plate. Since the band-shaped trunk-plate lies in the
median line, the embryo of such a stage appears in a
median section, almost enclosed by the permanent ectoderm
(Fig. 158 E) ; however, this is not actually the case, for,
although the ventral and dorsal parts of the trunk-plate also
spread out laterally, yet they remain separated on either

360

EMBRYOLOGY

side by a broad space. The ventral and dorsal parts of the
trunk-plate now grow out more and more on the sides, and

Fig. loS.— .4 to F, stages of development of Sipunculus nudus (after Hatschkk).
J,bla8tiil;i; W.nastrula; C to K, other stages, in which the development of the head-
and trunk-platea takes place; F, embryo during hatching; a, anus; II, blasto-
pore ; dr, glandular appendage of the fore-gut; cct, ectoderm; cut, entoderm ; ka,
head amniotic cavity; m, mouth; mc», mesoderm; n, nephridium; ra, trunk
amniotic cavity; rj), trunk-plate; .s, pharynx; se, serosa; so, somatic, »p, splanch-
nic, layer of the mesoderm ; "', ciliated band. The ectoderm is finely and the ento-
derm coarsely punctate ; the mesoderm is cross-hatched.

SIPUNCULIDS 361

finally fuse in the lateral lines ; furthermore, a complete
fusion takes place with the head-plate (Fig. 158^ and F).
An ectodermal invagination at the posterior dorsal part of
the larva produces the hind-gut, and fuses with the entoderm.
The deep and voluminous fundament of the fore-gut now
does the same. Two invaginations make their appearance
on the oesophagus : an anterior, which is developed into a
gland with a ciliated efferent duct, and a postei-ior, the
fundament of the pharynx (Fig. 158 F, dr, and s). Stout
cilia make their appearance in the circumference of the
body behind the mouth-opening, and form the post-oral cili-
ated band (Fig. 158 F). The embryo is now ready to hatch.
It has up to the present retained its spherical shape; but at
the time of hatching it passes into the permanent shape of
the larva, owing to the appearance of a constriction behind
the ridge that bears the circle of cilia (Fig. 159) ; this marks
off the broad anterior part of the body from the conical pos-
terior portion. At the same time the entire body enlarges,
and its cellular walls consequently become thinner. Hatch-
ing takes place by the pointed end of the lai^va breaking
through the serosa and egg-membrane at the pole opposite
the apical plate and the emergence of the embryo at this
point (Fig. 158 F). The connection with the serosa, as far
as it still exists, breaks, and the tuft of cilia of the apical
plate is withdrawn through the pores of the egg-membrane,
to be retained by the larva. The egg-membrane itself re-
mains for a while on the larva like a helmet.

The larva of Sipunculus strongly resembles the Trocho-
phore, but differs from it in the absence of the preoral
ciliated band and the great reduction of the preoral part
of the prostomium (Fig. 159). As a result of this, the apical
plate comes to lie in the vicinity of the mouth, which is
shifted well toward the anterior end of the larva. The usual
three regions of the intestine can be recognized, though the
hind-gut opens to the exterior on the dorsal side (Fig. 159) ;
this, however, is frequently observed in Annelid larvffi. A
head kidney has not as yet been observed. The internal
organization is of a higher grade than is general in the
Trochophore, and in part already corresponds to that of

362

EMBRYOLOGY

nn.

the adult worm. This applies, for example, to the arrange-
ment of the mesoderm, which is seen clothing the walls of
the body and intestine as the somatic and splanchnic layers,
though, according to Hatschek, the somatic layer also sup-
plies the covering of the fore- and hind-guts, so that only
the covering of the enfcodermal part of the intestine (the
mid-gut) arises from the splanchnic layer. From the somatic
layer arise also the four retractors of the anterior part of
the body, which are developed even in the larva, and extend
from the head region to the anus (Fig. 159 r). In conse-
quence of this arrange,
ment, the anterior part
of the larva can be in-
vaginated into the pos-
terior part. A circular
muscle lying behind the
ciliated band (Fig. 159
rin) serves to close the
opening of the invagina-
tion in the larva, which
in this retracted condi.
tion is almost spherical.
The paired nephridia,
which in their structure
correspond to those of
the Annelida, are also
produced from the soma-
tic layer of the meso-
derm. At quite an early
stage of the embryo a
mesoderm cell was dis-
tinguishable from the
rest by its strikingly
yellow colour. Some
other cells were then
added to it. The entire
structure assumed a looped form, and a lumen was excavated
inside it (Figs. 158 F, 159 «). There are cells, likewise of
mesodermic origin, resembling blood corpuscles, which were

Fig. 159. — Larva of Si'})unculus luidus (after
Uitscekk). a, anus; tl, cells resembling
blood corpuscles; iir, glandular appendapo
of the foie-gut ; m, mouth ; n, nephridium ; r,
retractors; rm, circular muscle; s, pharynx ;
sj), apical plate.

sipunculidj: 363

detached from the peritoneal epithelium, and are found
floating free in the body cavity.

The metamorphosis of the Si'punculvs larva into the adult
animal is first indicated in the considerable growth of the
body and the reduction of the head portion. Connected
with this is the complete displacement of the mouth to the
anterior end and the further shoving forward of the anus,
the latter being brought about by the more i^apid growth of
the extreme posterior portion of the body. The ciliated
band gradually atrophies. It has nothing to do with the
development of the tentacles, which take their origin as
evaginations of the margins of the mouth. The brain arises
from the lower layers of the apical plate, which has become
several layers thick. The ventral nerve cord arises in the
ventral middle line from an ectodermal thickening, which
progresses from in front backwards. The oesophageal con-
nectives grow from its anterior end towards the brain,
therefore in a direction opposite to the growth of the ventral
cord, and contrary to the method of outgrowth in the Anne-
lida, where the apical plate grows out into the connectives.

Two additional pigment spots are added to the two wbich
had already arisen in the larva in connection with the apical
plate (Fig. 158 F). The provisional organs of the intestine —
the glands and the so-called pharynx — atrophy ; the intes-
tine itself increases in length and is thrown into several
loops (Fig. 159). On the dorsal side of the intestine there
arises from its mesodermal covering a blood-vessel ; but this
does not take place until quite late. The longitudinal and
circular muscle-layers of the dermo-muscular sac are differ-
entiated much earlier. The nephridia are said to undergo
a peculiar change, terminating internally in vesicular en-
largements, while their external openings are retained.

The condition of the nephridia recalls the statements made by Schau-
INSLAND that in the Priapulidae the nephridia are closed and, on the one
hand, function with their blind ends as excretory organs, while, on the
other hand, they are directly united with the germaria, and, in fact, accord-
ing to Schauinsland's description, even ai-ise from the latter. Thus even
in closed nephridia their function as an efferent apparatus of the genital
products would be explicable. It should be mentioned, however, that

364

EMBRYOLOGY

the nephridia of the adult Sipiinculus are described as opening towards
the body cavity, and that the sexual glands are exj^lained as growths of
the peritoneal epithelium, the products of which are set free in the body
cavity, and from there pass into the funnels of the nephridia.

II.— The Development of Phascolosoma.

According to Selenka's description, the development of
Phascolosoma elongatum is much simpler than that of Hipun-
culus. Following an unequal cleavage, there arises an
epibolic gastrula, which is said, however, to be converted into
a kind of invagination gastrula after the division and inva-
gination of the macromeres, which soon ensue. Cilia, which,
as in Siptmculus, perforate the egg-membrane, make their
appearance early. They form a tuft at the apical pole and
a post-oral ciliated band ; how^ever, a preoral band is also
present, so that in Phascolosoma the resemblance to the
Trorhophore is greater. The blastopore is said to be directly
converted into the mouth ; the anus in this case, too, lies
on the dorsal side. The formation of an embryonal mem-
brane is not described by Selenka ; on the contrary, this in-
vestigator states that the egg-membrane
becomes the cuticula of the larva, as has
already been described for some Annelids.
The embryo (the larva) then simply elon-
gates, so that here a stage quite similar
to that in Sipnuculus is reached, but in a
simpler manner.

The larva, which, as one of the last
stages, was observed by Selenka, is elon-
gated (Fig. 160). The trunk, which is
much the more voluminous, is separated
from the small head portion by means of
a thick, collar-like ridge, which bears the
post-oral ciliated band. A large portion
of the head is occupied by the broad
preoral band of cilia, and at the anterior
end the ciliated tuft of the apical plate
projects. The head bears two eye-spots. The hooks which
constitute the armature of the adnlt animal make their ap-

FiG. 160. — Larva of
Phascolosoma elonga-
tum (after Selknka).
h, setse ; mi, mouth ;
II', preoral, ir,, i)08t-
oral, ciliated bands.

SIPUNCULID^ 365

pearance in front of the collar. Two pairs of rigid bristles
(Fig. 160) arise on the trunk, each of which belongs to an
ectodermal cell. A third pair is subsequently added to
these. Selenka is inclined to compare them to the setse of
the Annelida. The latter arise, it is true, as ectodermal
structures, though not in so simple a way as here.

General Considerations. — With the limited knowledge
that we possess of the development of the different genera
of Sipunculidse, it is difficult to pass judgment on the syste-
matic position of this group. Until quite recently the
SipuncuUdce, with the Echiuridce, were usually united into
the group of Gephyrea. The grounds which led to this
association were rather of an external nature. A comparison
of the anatomical and embryological data proves that the
two groups exhibit no special resemblances. The so-called
proboscis of the EcMuridce corresponds to the elongated
cephalic lobe of the larva; the mouth lies at its base, but
in the Sipunculidse at the tip of the proboscis. The cephalic
lobe entirely degenerates even in the larva. (Comp. Fig.
159, p. 362, and Fig. 145, p. 309.) The differences in
the structure of the nervous system, and especially the
musculature, which separate the SipuncuUdce from the
Echiuridce and also from the Annelida, are striking. It
seems very doubtful whether these differences can be main-
tained after a comprehensive knowledge of the development
of the SipuncuUdce, and if so, to what extent. The chief
point is whether or not the Sip^mcuUdce are to be derived
from segmented forms, i.e., whether they are related to
the Annelida. In the Echiuridse we saw that a segmenta-
tion was indicated in the larva, and through this and the
remaining structural conditions of the larva we acquired
an insight into their relationships to the Annelid stem. In
Sipunculus such indications are lacking. To be sure, the
mesoderm here also splits into two layers, progressing from
in front backwards, and the differentiation of the nervous
system, which, however, is aberrant in being produced from
an unpaired fundament, takes place in the same direction ;
but no transitory segmentation is indicated, a head kidney is
not present, the preoral ciliated band is lacking, and the

366 EMBRYOLOGY

head portion sinks into complete insignificance (Fig. 159,
p. 362). As regards the formation of the embryonal
membrane, this might be a later acquisition, especially
since it is said to be wanting in Phascolosoma. Moreover,
Selenka argues that the pairs of so-called setje in the
latter form might indicate a segmentation ; but this
evidence cannot be considered as conclusive.

Finally, it should be stated that the structure and develop-
ment of the Sipunculidie do not disprove relationships to the
Annelida, but that as yet no justification exists for uniting
them with these. We place them here next to the Annelida,
because definite relationships to any other branch of the
animal kingdom are not demonstrable and because in the
shape of their larva they are most nearly related to the
Annelida. A closer relationship to Phoronis and the Molliis-
coidea appears to us not yet sufficiently established.

Literature.

1. Hatschkk, B. Ueber Entwicklung von Sipunculus nudus. Arh.

Zool. Inst. Wien. Bd. v. 1884.

2. Selej^ka, E. Eifui'chung und Larvenbildung von Phascolosoma

elongatum. Zeitschr. v/ixs. Zool. Bd. xxv. 1875.

3. Selenka, E. Die Sipunculiden. Wiesbaden. 1883.

4. ScHAuiNSLAND, H. Die Excretions- und Geschlechtsorgane der

Priapuliden. Zool. Anzciger. Jahrg. ix. 1886.

CHAPTER XII.

CH^TOGNATHA.

The Chaetognatha occupy an altogether isolated position as
regards their structure and mode of development. Though
they most nearly resemble the Annelida in peculiarities of
structure, they differ from this group in important embryo-
logical features. Among the most characteristic peculiarities
of the development of the Chaetognatha are to be mentioned
the origin of the mesoderm by the formation of two archen-
teric diverticula and the early differentiation of the funda-
ment of the sexual organs. Owing to the absence of peculiar
larval forms, it is evident that the development of the
Chgetognatha is abbreviated. The developmental history of
the Chaetognatha has been made knovrn chiefly by Gegen-

BAUR, KOWALEVSKY, BtJTSCHLI, and 0. HeRTWIG.

The eggs of the Chgetognatha (Sagitta) after fertilization
has taken place are discharged into the surrounding water. ^
They are spherical, transparent, and contain numerous clear
yolk spherules. They are surrounded by a vitelline mem-
brane and an outer gelatinous mantle. Cleavage must be
considered as total and equal, and leads to the formation of
a regular blastula, which is characterized by the tall pris-
matic form of its cells, which are grouped about a compara-
tively small cleavage cavity. One half of the embryo soon
flattens and invaginates, whereby the cleavage cavity is
reduced to a fissure. In this way a very regular invagina-
tion gastrula is formed (Fig. 161 A), the blastopore of

^ [BovERi states that the eggs at the time of ovipositing in passing
through the narrow orifices assume an elongated form, but that they
recover the rounded shape in the water. When the eggs are laid the first
polar spindle is already formed, and every egg also contains a spermato*-
zoon. — K.]

367

368 EMBRYOLOGY

whicli soon narrows. At an early period two large cells,
the genital cells, become noticeable at the bottom of the
archenteric invagination, directly opposite to the blastopore.
A plane passing between these two cells would correspond
to the future plane of symmetry. In the course of further
development the genital cells withdraw from epithelial con-
tinuity in the wall of the archenteron, passing into the
archenteric cavity. Here they divide so that four
genital cells lying in the transverse axis can be distinguished
(Fig. 161 B). Of these the two middle ones represent the
fundaments of the two testes, the two outer, on the other
hand, those of the ovaries of the two sides. In the anterior
widened portion of the archenteron the formation of two
folds now takes place from in front backwards ; these push
the genital fundaments before them (Fig. 161 B), and by
their development the archenteron is divided into three
spaces lying side by side, the middle one of which represents
the cavity of the mid-gut, the two lateral ones, on the
other hand, those of the paired coelomic sacs. ^

While the embryo now increases in length, the blastopore
closes and the permanent mouth-opening breaks through,
the latter being accompanied by the development of the
fore-gut, which probably arises as an ectodermal invagina-
tion (Fig. 161 C, st). The middle one of these three
previously formed diverticula acquires in this way an open-
ing anteriorly. In the view from the dorsal side (Fig. 162
A) the blastopore and permanent mouth appear to lie
directly opposite each other, but side views show that the
blastopore is moved a little, as it seems, towards the ventral
side of the embryo, so that accordingly the permanent
longitudinal axis occupies a position oblique to the chief
axis of the gastrula.

With their further growth in length the two folds are
pushed farther and farther backwards (Fig. 161 C). In this

1 [According to Jourdain, the two evaginations of the archenteron do not
produce the ca'lom, but their cavities disappear, and at the same time
between ectoderm and entoderm there is formed in the mesoderm a
fissure, which becomes the permanent body cavity. This statement
contradicts those of the authors mentioned above.— K. ]

CHJITOGNATHA

869

way the two primitive cells of the testes are also pushed
backwards (Fig. 161 C), whereas the primitive cells of the

Fig. 161. — Three embryos of Snaitta at the stage of the formation of the germ-
layers, in frontal section (after O. Hertwig, from Lang's Lehrbuch), hi, blasto-
pore; ltd, archenteron; g, primitive cells of the sexual organs; Dm, visceral
(splanchnic), pin, parietal (somatic), layer of the mesoderm ; d, fundament of the
mid-gut; cs, coelomic sacs ; st, stomodaj am (fore-gut).

ovaries lie at the sides of the folds and in this way are
moved rather into the pair of coelomic sacs, in accordance
with their subsequent permanent position (Fig. 162). The
embryo bends more and more towards the ventral side,
during which a ventral ectodermal thickening becomes

Fig. 162. -Dorsal and lateral views of an advanced embiyo of Sagitta (after
BuTscHLi, from BKhToua'a Compurative £mbri,ologi;). in, moutli ; al, intestinal
canal; vg, fundament of the ventral ganglion; ey, ectoilerm ; c.pr, c- pi alic
part of the boiiy cavity; so, somatic, sp, splanchnic, layer of the nicsjeieriii
ge, sexual oi'gaus.

K. H. E. ' n B

370

EMBRYOLOGY

noticeable as the fundament of the ventral ganglion (Fig.
162 B, vg).

Many obscure points still exist regarding the further development.
In a species studied by Butschli two portions of the ccelomic diverti-
cula lying in the head cavities are constricted off at an early period (Fig.
162 c. pv) ; the walls of these are said to be employed chiefly in the
formation of the musculature of the head. In the species studied by 0.
Hertwig, on the other hand, the formation of such paired head cavities
could not be recognized, for here in the course of the further development
the walls of the mid-gut and the ccjelomic sacs are so closely applied to
each other that these organs soon present only a slit-like lumen, which
finally disappears entirely. A solid, laterally compressed ectodermal
cord and two, likewise solid, lateral mesodermal masses, which contain
within them the genital products, can now be distinguished. All three
of the cords grow out backwards, not only in the region of the future
trunk, but also in the tail region, so that the latter also has an ento-
dermal fundament ; it is, however, smaller than that of the trunk region.
The rudimentary tail portion of the intestinal canal is subsequently em-
l^loyed in the formation of the sagittal septum separating the two caudal
cavities from each other ; here it atrojihies without acquiring a lumen.
It has not as yet been observed either in what manner the transverse
septum between the trunk- and tail-cavities is formed, how the canal
opening is developed, or even how the efferent sexual ducts are produced.
Of interest is the great extension of the ventral ganglionic mass, which
remains united with the skin throughout life (Fig. 162 B, vg, and Fig.

103 b<i), extends in the young animal
along the ventral side and the lateral
parts of the entire trunk region, and
does not become relatively more re-
stricted until later. The transversely
stnated fibres of the four longitudinal
muscle-bands are differentiated from
] the cells of the somatic layer of the

lining of the ccelom after the type of
epithelial musculature (Fig. 1G2 A,
so). The fins arise as simple evagi-
nations of the lateral parts of the
ectoderm, whereas the cuticular
skeleton found in them probably
arises as a secretion of this ecto-
dermal cell-layer at its base. In
later stages of development the two
ccelomic sacs move in the trunk
region into close contact above and
below the intestinal canal, so that a

Fig. 1G3. — Trausversij section
tliroiigli the trunk of Sd'jitta (after
(). Hebtwig, from Lang's icJirlucft).
Ih, body cavity ; men, mesentery of
the intestine ; md, mid-gut; Im, longi-
tudinal musculature; h<j, veiitial
ganglion.

CH^TOGNATHA 871

dorsal and a ventral mesentery are formed by their contiguous walls
(Fig. 163 iiifis). The young Sagitta upon hatching from the egg exhi-
bits essentially the form of the adult animal.

General Considerations. — The problem as to the position of the
Chffitognatha in the zoological system is not brought any nearer to
solution even by embryology, and for the time being can be treated
only with the utmost reserve. The agreement which exists between
the transverse section through Sagitta and that through Polygordius
has already been pointed out by O. Hertwig. As a matter of fact,
a significant resemblance in the tectonic of the two groups is shown in
the presence of paired entodermal sacs lined with epithelium, a dorsal
and ventral intestinal mesentery, and the four longitudinal muscle-
bands arranged in pinnate lamellffi, to which in some cases the
indication of a transverse musculature is added. The chief difficulty
in arriving at a safe conclusion regarding the position of the Chcetog-
natha is our ignorance in regard to the excretory system. The sexual
organs, particularly those of the male portion of the body, exhibit an
important resemblance to the conditions in the Annelida, and if it is
l)ei-missible to refer the efferent sexual ducts to metamorphosed nephridia,
we should have to ascribe to Sagitta at least two trunk somites, and
accordingly explain the Chagtognatha as forms in which, perhaps in
connection with the manner of locomotion, a primitive segmentation of
the body has been retained in a degenerated form only.

Embryologically considered, the Chsetognatha are distmguished from
the Annelida by the absence of a Trochophore-like embryonal or larval
stage, and, above all, by the characteristic folding off of the mesoderm.
In order to harmonize this kind of mesoderm formation with the de-
velopment of mesodermal bands in the Annelida, one would have to
assume that [in the Chaetognatha] the mesodermal elements increased
considerably by proliferation as early as in the blastula and gastrula
stages, so that in this way paired mesodermal bands arose, which at first
remained lying at the surface of the walls of the archenteron, retaining
an epithelial connection with the entoderm, and only later, by the forma-
tion of diverticula, became detached. By this assumption it is compre-
hensible how, even in closely related animals, two so apparently different
kinds of mesoderm formation might be realized.

Literature.

1. BuTscHLi, 0. Zur Entwicklungsgeschichte der Sagitta. Zeitschr.
wiss. Zool. Bd. xxiii. 1873.

2. Gegenbaur, C. Ueber die Entwicklung der Sagitta. Ahh. Naturf.
Gesell. Halle, 1856. Translation : Quart. Jour. Micr. Sci. Vol. vii.,
p. 47.

3. Geassi, B. I Chfetognati. Fauna und Flora des GoJfes v. Keapel.
Monogr. v. Leipzig. 1883.

372 EMBRYOLOGY

4. Heutwig, 0. Die Chiitognathen. Jena. Zeitschr. Ed. xiv. ISSO.

0. KowALEVSKY, A. Embryologischc Studien an Wiinuern und Aith-
ropoden. Mem. Acad. St. Petershourg. Ser. 7, torn. xvi. 1871.

Appendix to Literature on Ghcetognatha.

1. BovEra, T. Zellstudien. Heft 3. Jena. Zeitschr. Bd. xxiv. 1890.
II. JouRDAiN, S. Sur rEmbryogenie des Sagitta. Compt. Rend.

Acad. Sci., Paris. Tom. cxiv. 1892.

CHAPTER XTII.

ENTEROPNEUSTA.

Under the name of Enteropneusta it is customary to place
the isolated form Balanoglosstis next to the Echinodermata,
since it scarcely presents closer relationship to any othei-
division. At the close of this chapter something further
will be said on its probable position in the system. In order
to make ourselves more easily understood concei-ning the
developmental processes, it seems necessary to discuss first
some morphological conditions.^

Anatomical. — Balanoglossus possesses anelongated vermi-
form body, on which different regions can be recognized
externally. Anteriorly the so-called acorn [balanus], less ap-
propriately called proboscis, is marked off from the rest of
the body ; upon this follows the muscular collar, and then the
branchial region, which gradually merges into the posterior
part of the body (Fig. 164). Acorn and collar areessentially
a locomotor apparatus, and therefore are largely composed of
muscle fibres, which can be distinguished as external circular
and internal longitudinal muscles. The cavities inside both
organs which are left between the longitudinal muscles and
connective-tissue cells can be filled with water from the out-
side by means of one or two dorsal pores lying at the base
of the acorn (Fig. 165 p). Similar pores also conduct water
into the cavities in the collar (Spengel). These conditions
have been compared to those of the water-vascalar system of

1 [It should be mentioned here that since the publication of our de-
scription of the development of Balanoglossus the important works of
Spengel (No. VI.) and Morgan (Nos. II., IV.) have appeared, necessitating
some modifications in the account which we have given. The most im-
portant of these will be pointed out in what follows. — K.]

373

374

EMBRYOLOGY

the Echinodermata, and it was indeed supposed that the
acorn represented a rudiment of this system, especially since
the cavity of the acorn in its earliest condition presents a
certain resemblance to the water- vascular system as it origi-
nates in the Echinodermata. It appears certain that the
acorn serves as an organ of locomotion. It was believed that
it took in water from the outside by means of the probosci.s

pore, and that there-
fore it operated in the
same way as the am-
bulaci-al feet of the
Echinoderms (Spen-
gel). However, it has
been maintained, on
the other hand, that
particles of pigment
distributed in the water
are never found inside
the acorn, and that the
proboscis pore therefore
does not serve for the
reception, but only for
the elimination, of sub-
stances from the inside
(Batesox). This ob-
servation is of parti-
cular interest, inasmuch
as the acorn contains
a o-landular structure,
which has been inter-
preted as an excretory
oriran. The locomotion
of Balanoglossus is
effected by peristaltic
movements on the part
of the acorn, which tlius pushes its way into the sand. The
collar follows it, also taking part in the same way in the pro-
gression of the animal. At the same time the sand enters
the mouth-opening, which lies at the base of the acorn,

Fig. IGl. — BalanoglossH.s Ji'oicaU't's/.ii (after
A. AoAssiz). e, acorn ; fcr, collar; J;, branchial
region; g, penital region of the body; i i>,
dorsal, vh, ventral, blood-vessel.

ENTEROPNEUSTA

375

gradually fills the entire intestine, and finally passes out
through the anus at the postei'ior end of the body as a
sausao-e-like cord. Thus the animal eats its way, as it were,
through the sand.

The intestinal canal commences immediately under the

ai

^K-

V.L

de

Fig. 165. — Sagittal section through the acorn and collar of BalanogJossus
sarniemis (made somewhat diagrammatic, after Kohler). d, intestine ; Ae, intes-
tinal epithelium; Ah, dorsal blood-vessel; Ai, diverticulum of the intestine; An,
dorsal nerve ; li, the so-called heart ; lli, the body cavity ; m, mouth ; ]), proboscis
pore ; sk, skeletal body ; vh, ventral blood-vessel ; x, the so-called proboscis gland.

acorn with the broad mouth-opening, which cannot be
closed (Fig. 165). It extends backwards tolerably straight.

876 EMBRYOLOGY

The appendicular structures which arise from the intestine,
and in part remain intimately united with it, are important
for the organization of the animal. The intestine produces
in its anterior part a dorsal evagination, which extends into
the base of the acorn (Fig. 165 di). Between the ventral
wall of this evagination and the epidermis of the acorn is
inserted the anterior part of the so-called proboscis- or acorn-
framework (Fig. 165 si-), a skeletal body, the unpaired part
of which has the position described, whereas two arms,
which project from it backwards and downwards, embrace
the fore-gut like a hoop. They lie in folds of the intestinal
wall ; they could not be introduced into the figure. The
entire skeletal bod^^ according to Kohler, Bateson, and
Morgan, is a product of the intestinal epithelium, i.e., the
above-mentioned evagination of it, [whereas Spengel (No.
VI.) interprets the acorn-skeleton as only a modification of
the bounding membrane (^Grenzviembran), and also makes
the coelomic sacs share in its formation. According to
Spengel, the histological structure of the acorn-skeleton in no
way agrees with that of the chorda doi^salis of vertebrates ;
notwithstanding, this has been repeatedly emphasized by
different observers, and has been regarded as one of the
points for comparison between Enteropneusta and Vertebrata
(comp. infra). — K.]

The gills, which are most important for the whole inter-
pretation of the animal, occur somewhat further back on the
intestine, behind the collar region of the body. They
are paired, pouch-like outfoldings of the dorsal wall of
the intestine lying on both sides of the middle line (Fig. 166
k). Each of these pockets, which are lined with cilia, sends
upwards a short duct, which opens on the dorsal surface by
means of a pore (Fig. 166 /)). Externally the rows of
branchial pockets can be recognized by the transverse arched
bands (Fig. 164 A). Behind, these transverse arches become
less extensive, which indicates that the formation of new
gills takes place even in the later stages of the animal's life.
A skeleton, formed of delicate chitinous hoops, which is
embedded in the walls of the branchial pouches, serves as a
protection for the gills. The water is taken in by the mouth,

ENTEROPNEUSTA

377

passes from the fore-gut to the gill-pockets, and from there
to the outside world by means of the dorsal pores. ^

The intestine also presents paired dorsal evaginations in
the parts which lie behind the gill region. These are the
hepatic appendages. They also influence the shape of the
body, inasmuch as they cause the skin to protrude (Fig.
164), and the musculature is only slightly developed at
these places. The hindermost portion of the intestine lacks
the appendicular structures, and extends straight to the
anus.

One mesentery extending in the dorsal and another in the
ventral line serve for the attachment of the intestine. By
means of the mesenteries the body cavity is divided into a
right and a left portion, but the two parts are confluent in
many species, owing to the
perforation of the dorsal
mesentery. The body
cavity of the collar is dis-
tinct from that of the
trunk, and also differs
from it in its mode of
origin ; moreover, it is for
the most part reduced by
being filled with connec-
tive tissue and muscula-
ture (Fig. 165). In the
tx'unk, on the contrary,
the greater part of the
body cavity is said to
persist, and its wall is
composed of the longi-
tudinal and circular mus-
culature of the somatic
and splanchnic layers (Spengel). However, according to
other statements, even the trunk cavity is said to lose the

Fig. 166. — Transverse section through
the branchial region of Balanoglossus rainu-
tux (after Spehgei). d, intestine ; db,
dorsal blood-vessel; dn, dorsal nerve;
g, genital organ; fc, gill-pockets; Ih, body-
cavity; p, pore of the gill-pockets; so,
somatic, sp, splanchnic, layer of the meso-
derm ; vh, ventral blood-vessel; vn, ven-
tral nerve.

1 [A full account of the very complicated structure of the gills is given
in Spengel's monograph (No. VI.), to which we particularly call attention
in the matter of this and other anatomical conditions, and especially in
view of the correction which it has since undergone.— K.]

378 EMBRYOLOGY

nature of a true ccelom and to be filled with connective
tissue and muscles (Kohler).

The two chief vascular trunks of Balanoglossus (Figs. 165
and 166 vh and dh) extend in the ventral and dorsal middle
lines between the wall of the intestine and that of the body.
The blood flows forward in the dorsal vessel, backward in
the ventral. They give off branches at regular intervals,
which extend to the body- wall, to the intestine, and to other
oro-ans. According to Kowalevsky, there are also two
lateral trunks which receive vessels from the intestine and
from the gills. Their presence was confirmed by Kohlkr.
It still appears doubtful whether the saccular structure lying
at the base of the acorn and at least connected with the
vessels of the body, which was maintained by Kohler and
Bateson to be the central organ of the blood-vascular system,
is to be looked upon as a heart. In Fig. 165 it (Ji) is seen
lying on the dorsal side of the intestinal diverticulum. An
organ (x) lies above it the significance of which is still less
certain. It is a closed saccular body, the epithelial lining of
which is greatly thickened anteriorly (Fig. 165 x). Owing
to its intimate relation to the blood-vascular system, due to
its position, Spengel looked upon the anterior part of this
organ as an internal gill (acorn gill), whereas Bateson and
Kohler explain it as a gland (proboscis gland), which has
an excretory function. To be sure, a difficulty occurs with
this explanation, namely, the absence of the efferent duct ;
for it is not evident how the proboscis pore conveys away the
products of this " gland," which is a closed sac. Apart from
this, and in the absence of any other excretory organ, this
interpretation is nevertheless natural.

A thick cord, which lies in the dorsal mid-line of the collai*,
is to bo looked upon as the central organ of the nervous
system (Fig. 165 cin). It is said to possess a cavity which
would be comparable to the central canal of the spinal cord
of the Vertebrata (Bateson), but this is denied by Spengel.
According to both Spengel and Kohler, the cavity is
traversed by coi-ds of cells, so that only irregular spaces
appear in it. Kohler further states that the following pecu-
liar condition exists: the internal space of the nerve cord

ENTEROPNEUSTA 379

opens to the outside at its posterior end, the cells of its walls
merging into the epithelial cells of the body-wall. Similar
communications of the inner space are also said to exist at
the anterior end (neuropore according to Batkson). A stout
nerve, which extends along the entire dorsal mid-line of the
body, is given off from the central organ. This in turn gives
off two nerves just behind the collar, which extend down-
wards (ventrad), unite in the region of the first pair of gill-
pockets, and extend backwards in the body as the ventral
median nerve (Fig. 166 vn).

The genital organs of Balanoglossus either belong to the
branchial region of the body or lie behind this. Balanoglossus
is dioecious. Male and female organs are entirely alike as
regards form and position. The genital glands lie in the
form of simple or branched tubes on both sides of the body,
and their external openings are found one behind the other,
forming two rows on the dorsal surface (Fig. 166 g). In
addition to these lateral rows of sexual oi-gans (Fig. 164 g),
two others (median) lying between the gill-pockets and the
dorsal blood-vessel may make their appearance. In many
species the part of the body succeeding the gill-pouches can
also be called the genital region, for the sexual organs are
especially well developed there. Owing to the fact that the
parts which contain the sexual glands undergo a great flatten-
ing and lateral extension, wing-like extensions of the body are
produced in certain species, e.g. B. claviger and B. minuiwi,
studied by Kowalevsky.

Development without the Tornaria Larva. — The

fertilization of the eggs takes place outside the body in the
sea-water, into which in the American species studied
by Bateson (Balanoglossus KowalevsMi) both kinds of
sexual products are said to pass by the rupturing of the
body-wall. Artificial fertilization could not be under-
taken, though Bateson found the eggs in large quantities in
the slimy sand inhabited by the adult animals. The eggs
are closely enveloped by a delicate membrane, which sepa-
rates fi'om the egg when fertilization has taken place.
Cleavage is total and tolerably equal. A blastula arises as

380

EMBRYOLOGY

the result of it, which is at first spherical, but subsequently
becomes flattened on one side. On this side an invasrination
then takes place, and the result is a typical invagination
gastrula. Soon, however, the originally wide blastopore
contracts to a short, nari-ow fissure. At the same time the
external surface of the embryo becomes covered with short
cilia, and a circle of stouter cilia makes its appearance in
the vicinity of the blastopore. Subsequently the blastopore
entirely closes. The two primary germ-layers remain
united at this point, but only by a plug of cells ; finally, they

separate entii-ely from each
other, so that the embryo
then consists of two sepa-
rate cell-vesicles lying one
within the other. At the
same time the embryo
elongates somewhat and
then assumes a shape such
as is represented in Fig.
168 J.. At about this stage
or even somewhat earlier,
the embryo breaks through
the egg-membrane and be-
comes a free larva, which
does not, however, lead a
pelagic existence, but lives
on the bottom and is found
in places where the water
is not very deep.

The internal structure of
the larva is soon changed
in such a way that an in-
ternal segmentation can be recognized. The elongated, com-
pletely closed archenteron bulges out at its anterior end,
and forms a pair of diverticula, which are directed back-
wards (Fig. 167 Cj). These are constricted off from the
archenteron in connection with each other, and lie in front
of it as closed vesicles. In the same way two pairs of
ccclomic sacs are formed further back as evaginations of

Fig. 16".— Diagram of a longittuliiial
.section through a larva of B(ilano[ilos>.ns
Kowalevski Hiifter Ba.teson). c,, anterior,
til, middle, cm, posterior, coelotnic sacs ;
d, intestine.

ENTEEOPNEUSTA

381

A

B

.'.liJSi'.C;'.

ji'4!toi<^.

fc

0.

the archenteron (Fig. 167 Cjj and Cm) ; tliese also become
detached, and are subsequently found next to the intestine
as flattened sacs. The mouth is formed at a somewhat later
stage on the ventral surface at the point where a transverse
furrow has made its appearance on the outside of the larva
(Fig. 168 A). The anus arises at the posterior end of the
larva near the place where the blastopore closed. Both
month and anus are formed by the fusion of the inner with
the outer germ-layer. During these processes the external
shape of the larva undergoes important changes. At first a
transverse furrow, which gradually deepens, and behind
which a second one
soon makes its appear-
ance, arises at about
the middle of the body
(Fig. 168 A and B).
While the first furrow,
even as early as this,
marks off the anterior
portion — namely, the
acorn — from the rest of
the body, the second
furrow, together with
the first, bounds the
future collar. Behind
this the gill region is
now also indicated by
the appearance of two
pores as evidence of
the first pair of gill-
pouches (Fig. 168 B
and (7, ^•). The part of the larva lying behind the ciliated
band gradually elongates (Fig. 168 C). Thus the principal
parts of the body of the adult animal are established in the
larva even at this stage.

4r(l

i

■'■^^ftrf'rf

,.•>'

-< tir

Fig. 168. — A to C, free-swimming larvas of
'BaXa.yiOOiXo&ius Kou-alevskii in different stages of
development (after Bateson). e, acorn ["pro-
boscis"]; fc, branchial openings ; fcv, collar.

Development by means of the Tornaria Larva. —

Not all species of Balanoglossus, however, develop from
the egg into the form of the adult animal in so simple a

382

EMBRYOLOGY

manner as that described above, for some species pass
through a larval stage, the shape of which recalls the larva?
of the Echinodermata. The larva called Tornaria was de-
scribed bj JoH. MuLLER as an Echinoderm larva. Its
shape, which, moreover, exhibits modifications in the different
species, is illustrated by Fig. 169. On the ventral side of
the bell-shaped larva lies the mouth-opening, from Avhich
the oesophagus ascends, and then bends backwards, to be-
come continuous with the capacious stomach. Upon this

Fig. \&3.—A an I B, Toriuiria and later stage of development of Balanoglossuti
(after Kowalevskt, from Balfouk's Comimrative Embryology). The broad black
lines indicate the ciliated band and the ring of cilia behind it. an, nnus; br,
gill-pocket; c, body cavity; /i', " heart " ; m, mouth ; w, the so-called water-vas-
cular vesicle.

follows the hind-gut, which opens to the exterior through
the anus at the posterior end of the larva. The surface of
the larva becomes engirdled by ciliated bunds, which, how-
ever, are distinguished from those of the Echinoderm larva?
by their different parts acquiring a gi-eater independence.
In the first place, we distinguish a preoral from a post-oral

ENTEROPNEDSTA 383

ciliated band, both uf whicli are provided with several
flexures (Fig. 169 ^-1). They almost come in contact with
each other at the anterior end of the larva. At this point
is found an ectodermal thickening, comparable to the apical
plate of the Annelid larvae, with two eye-spots lying over it.
From this region a contractile band extends backwards.
Mesenchymatous cells seem to arise between the intestine
and the body- wall. The posterior part of the larva is en-
circled by a ring of cilia which is independent of the other
ciliated band (Fig. 169 A), and in later stages of the larva
another one may also make its appearance behind this.

[The ciliated band of the anterior part of the body may undergo more
or less extensive outfoldings, by which the external form of the larva is
considerably modified. These outgrowths are sometimes large, such as
we shall find in the larvas of Echinoderms, and these influence, as has
been said, the entire form of the body. Continual outgrowths of the
ciliated band of limited extent result in the formation of tentacle-like
structures. — K.]

The condition of the larva described is only gradually
reached during its free pelagic life. At first the transverse
(posterior) rings of cilia are lacking, and the preoral and
post-oral ciliated bands have a more simple course. In the
further development of the Tornaria its anterior end elon-
gates and becomes the acorn of the Balanoglossus. Preoral
and post-oral ciliated bands then disappear, and instead of
these the entire body becomes covered with cilia (Fig.
169 B). The eye-spots are still retained for a while at the
tip of the anterior end. The middle region of the body is
encircled by the transverse ciliated band, and it can thus be
seen that the parts lying behind it have also elongated.
Two openings make their appearance externally on the
dorsal side of the anterior part of the body, the external
openings of the gill-pockets. With this nearly the same
stage is reached which we saw arising by direct means from
the larva described by Bateson. The simpler mode of de-
velopment is doubtless to be considered as the derived, and
that of the Tornaria as the more primitive, since the absence
of mouth and anus in a free-swimming larva does not
represent a primitive condition.

3^4

EMBRYOLOGY

Further Developmental Processes of Both Types.

— Thus far we have cliietiy considered the external shape of
the Tornaria. As regards the internal development, we find
confirmed the processes which, following Batesox, we have
already described. In the youngest stages of Tornnria yet
observed (Fig. 170), the archenteron, which in this case
never loses its connection with the ectoderm, develops an
unpaired evagination. This is said to be the fundament of
the so-called water-vascular vesicle, which, like the corre-
sponding organ of the Echinoderm larva, opens out by means
of a pore on the dorsal surface (Fig. 169 A). This in
particular has given rise to a comparison with the Echino-
derm larvse. In addition to this diverticulum, two pairs of
evaginations arise farther back on the intestine (Agassiz).

They are the ccelomic sacs, which soun
become detached from the intestine,
and lie close to it on either side as
two pairs of flattened, vesicles. They
soon become considerably enlarged,
and then their walls are applied to the
wall of the body and that of the in-
testine as the somatic and splanchnic
layers.^ A mesentery is developed
dorsally and ventrally, separating the
sacs of the body cavity of the two
sides ; but, according to Spengel, the
dorsal mesentery may afterwards de-
o-enerate. The hindermost pair of
ccelomic sacs supplies the greater
part of the body cavity, — namely, that of the entire trunk,
— whereas the body cavity of the collar arises from the
anterior pair, and the cavity of the acorn is developed from
the so-called water-vascular vesicle (Si'ENGEI,)- The re-
semblance of the latter to the paired structures points to
the fact that they originally hail the same significance, and
the appearance of two acorn pores in Balanoglnssus Kupferi

1 As is to be seen in Fij,'. 172 (p. 387), entirely similar conditions
appear also in Bolunoijloiisus Kowalei skii, which does not develop by
means of a Tornaria.

Fig. 170.— Kiu-ly 8taj;e of
a Toriiavia (after Goettk,
from Bai.foub'8 Co/ujiaivi-
tim Emhryologij). in, mouth ;
oil, anus; W, so-called
water-vascular vesicle.

ENTEROPNEUSTA 385

seems to indicate the paired origin of even this anterior
coelomic sac. The acorn pore arises from the dorsal pore
of the Tornaria. In the Balanoglossus which does not pass
through the Tornaria stage, the anterior coelomic sac, as
Bateson unreservedly calls it, forms a pointed process, the
end of which fuses with the ectoderm and breaks through
to the exterior.

After the coelomic sacs have separated from the archen-
teron, the remaining entoderm produces, in the form of a
forward evagination, the intestinal diverticulum, which lies
at the base of the acorn (comp. Fig. 165 di), and from
this the formation of the acorn stalk probably takes place.
Even earlier than this, the gill-pockets develop as paired
evaginations from a portion of the intestine behind the
diverticulum. They are dii'ected toward the dorsal surface
(Fig. 169 B), with which they soon unite, since they open to
the exterior by means of pores which are at first rather large
(Fig. 168 C). In several forms at first only one pair of gill-
pockets is to be observed (Figs. 168 G and 169 B) ; in the
Tornaria studied by Agassiz, on the contrary, four pairs of
them make their appearance simultaneously (Fig. 171).
The form of the gill-pockets, which is at first so simple,
is later much more complicated, for their walls become
folded, and the skeletal hoops are developed between them.
The formation of new gill-pockets continues to take place
even when the Balanoglossus has long since assumed its
permanent shape. After the posterior part of the body has
considerably increased in length, the paired evaginations of
the intestine which have been interpreted as hepatic append-
ages ai'ise behind the gill region.

In the Tornaria there is found next to the so-called water-vascular
vesicle, or even sunk into a depression in it, a spherical vesicle, which
ordinarily is called the heart of the Tornaria (Figs. 169 B and 171
ht [not h, Fig. 1G5] ). It does not merit this name, for, according to
Spengel, it is developed into the organ which Bateson and Kohler have
called the saccular posterior portion of the "proboscis gland" (comp.
Fig. 165). In Balnnoijlossus Kowalevskii the proboscis gland arises by
delamination from the tissues of the anterior coelomic sac after this has
already spread out inside the acorn. This mode of origin seems to us
K. H. E. CO

386

EMBRYOLOGY

to indicate that even the so-called heart of the Tornaria might arise from
the water-vascular vesicle or, what is the same thing, from the anterior
coelomic sac* The early appearance of the organ in the Tornaria is
favorable to the explanation (excretory) which has been given to the
fully developed organ. In other divisions of the animal kingdom also
we see the excretory system established at a very early i?eriod.

Likewise the organ which is interpreted as the real heart first appears,
according to Bateson, as a fissure in the mesodermal tissue. This fissure
makes its appearance between the fundament of the "proboscis gland"
and the intestinal diverticulum, and is only gradually surrounded by a
firm wall. It has not been determined whether or not it is from the
beginning connected with the blood-vessels of the body. The blood-
vessels probably arise from the mesoderm in
the same way as the supposed central organ.

[The observations of authors are not in
agreement respecting the origin of the coelomic
sacs, for Spengel (No. VI., Appendix to Lii^ra-
tnre on Enteropneusta) and Bourne (No. I.,
Appendix) maintain that they arise from the
hind-gut, whereas Morgan (Nos. III., IV.)
refers them to the entoderm, as was formerly
done. Morgan, moreover, assumes a different
method of origin in the different species of
Balanoglossus, for in one case (Tornaria from
the Bahamas) he even refers them to scattered
raesenchyma cells of the primary body cavity.
Also the origin of the so-called heart-sac
is not yet sufficiently clear. Morgan would
refer it likewise to an accumulation of mesen-
chyma cells, whereas Spengel adheres to his
former account of the ectodermal origin of
this organ. Moreover, it seems to be im-
possible to re«oncile the new results with the
earlier account, and for this reason we must
refer to the original articles. — K.J

The earliest fundaments of the genital
organs occur as pyriform sacs, and are found
in close connection with the ectoderm, a fact

«^

Fig. 171.— Stage of de-
velopment of BalanogJosifus
lafter Agasbiz, from Ba.l-
four's Comparative Embryo-
logy), an, anus; hr, gill-
]iocket8; lit, "heart"; m,
mouth ; W, the BO-called
water-vascular vesicle.

1 Spengel, however, affirms that the " heart " is formed as a thicken-
ing of the epidermis next the acorn pore ; but perhaps this statement
can be harmonized with the opinion expressed above by assuming that
in this case the development of the so-called heart took place later, that
is to say, when the lining of the internal cavity of the acorn by means
of the water-vascular vesicle had already been accomplished. This,
however, is only conjecture.

ENTEROPNEUSTA

387

which caused Bateson to believe them derived from it, and not from the
mesoderm, as would seem more natural, especially since at this time
the mesodermal tissue is already found closely applied to the ectoderm.
However, Bateson states that the origin of the genital organs is as yet
not certainly determined.

[Spengel found the union of the nascent gonads with the ectoderm less
intimate, and was inclined to refer their production to the mesenchyma
of the body cavity. The connection with the ectoderm is only secondary.
According to Morgan, they arise as iM'oliferations of the wall of the
coelomic sacs of the trunk, which at first have no connection with the
ectoderm. — K.]

The central part of the nervous system arises, according
to Bateson, as follows : — A part of the cells of the deepest
layer of the ectoderm in the median line of the collar is

Fig. 172. — Transverse section through the anterior part of the collar of a larva
of Balanoglossus Koxcalcvskii which is at about the stage of Fig. 168 B (p. 381)
(after Bateson). Above is seen the dorsal ciliated groove, d, intestine ; u, funda-
ment of the nerve cord ; Cn, cavity of the middle ccelomic sac, which is already
applied to the wall of the body and that of the intestine as somatic and splanch-
nic layers.

differentiated in a peculiar way, and is finally detached from
the ectoderm along the entire length of the collar (Fig.
172 n). This process, moreover, is said to be accompanied
by a superficial depression of the ectoderm, which is notice-
able as a dorsal, longitudinal ciliated groove on the recently
developed collar of the young larva (Fig. 172, after Bateson).
Spengel also speaks of the development of the nervous
system as the result of a dorsal invagination in the middle

888 EMBRYOLOGY

line of the collar. The cavities, however, which occur in
the central nervous system of the adult animal, are not to
be referred to a formation comparable with the neural tube
of the Yertebrata, but arise in the cell-layer which Avas split
off from the ectoderm, in all probability by the appearance
of fissures. However, at the end of the central cord, where
it mersres into the indifferent cells of the ectoderm and
where the latter is considerably thinner, a kind of foldinj?
process seems to take place ; at this point also the lumen of
the central cord is said to communicate with the outside
world (neuropore?). There seems to be no relation between
the dorsal groove and the blastopore ; for the groove does
not extend so far back. A direct connection with the
conditions occurring in the Chordonia is therefore not in-
dicated by this (comp. infra)} Like the chief, central parts
of the nervous system, its peripheral portions are also
differentiated from the lower cell-layers of the ectoderm,
which, according to Bateson, everywhere exhibits large
accumulations of sensory cells.

General Considerations. —The external resemblance
of the Tornaria to the Echinoderm larvce and the oc-
currence of the water-vascular vesicle, opening out by
means of a dorsal pore, have caused Balanoglossus to be
brought into relation with the Echinodermata. Corre-
spondingly, the acorn, the lining of which is supplied by
the so-called water-vascular vesicle, has been explained as
the last remnant of the water-vascular system, as the single
remaining ambulacral tentacle. The nature of the skin,
provided with calcareous structures, is, in addition to the
water-vascular system, characteristic of the Echinodermata.
The entire absence of calcareous bodies in Balanoglossus
and the different condition of the skin, together with
the other peculiarities in the entire structure of the body,

» [According to Spenoel's description, it must be assumed that the
account given by the previous observers does not at all relate to the first
fundament of the nervous system, and that in its formation, which takes
place at an early period, there is no invagination. Differentiations of
the ectoderm without any invagination give rise to the nervous system,
which only subsequently sinks in deeper. — K.]

ENTEROPNEUSTA 389

do not allow us to put any great weight on such an in-
terpretation of the acorn. A comparison of the Tornaria
with the Echinoderm larva is difficult to carry out, for the
ciliated bands so characteristic of the latter present here
quite a different distribution. Moreover, the Tornaria ap-
pears to possess a kind of apical plate, which is absent in
the Echinoderm larvas. The latter likewise exhibit no
eye-spots. The resemblance between the Tornaria and the
Echinoderm larvee is therefore of a rather superficial nature.
The possession of an apical plate and the cords radiating
from it point rather to relationships of the Tornaria with
the Trochophore.

The occurrence in Balanoglossus of paired coelomic sacs,
lying one behind the other, indicates a segmentation. In
this, it is true, a resemblance to the Echinodermata would
exist, if the statement should be confirmed that in the
latter also sevei-al pairs of ccBlomic sacs are developed
(comp. p. 414). This internal segmentation of the larva
subsequently disappears, and the segmentation which can
be recognized on the adult Balanoglossus has nothing to
do with it.

In searching through the animal kingdom after relationships for
Balanoglossus, characterized as it pre-eminently is by the possession
of gills, a comparison with the Chordata has been reached ; but there is
as yet no adequate basis for this comparison. It is Amphioxus which
has been especially in mind, and the comijarison has been based chiefly
on the gills, on the intestinal diverticulum, called by authors the
chorda, and its skeletal body, and on the formation of the nervous
system. A striking resemblance is noticeable between the anterior
ccelomic sacs of Balanoglossus and the most anterior archenteric diver-
ticula of Amphioxus, which also make their appearance very early, and
one of which greatly enlarges and opens to the exterior by means of
a ciliated canal, like the so-called water-vascular vesicle or the anterior
coelomic sac in Balanoglossus.

[To what precedes we append the following : The supposed relations
between Enteropneusta and Chordata have become, according to recent
observations at least, very doubtful. The diverticulum of the intestine
in the acorn, or rather the acorn skeleton, is, as it appears, comjjarable
with the chorda neither in regard to its origin nor structure. The needed
agreement in the formation of the nervous system seems, in fact, to be
wanting. The gills of Balanoglossus, it is true, are strikingly similar

390

EMBRYOLOGY

in structure to those of Amiihioxus ; but a detailed comparison reveals
in them organs of different phylogenetic origin (Spengel). The ccelom
of the Enteroimeusta exhibits other conditions than in the Chordata.
The probability of the relationship of Balanoglossus to the Echinodenm,
on the conti-ary, has, it may be said, been increased by recent investiga-
tions. A comparison of Tornaria with the larviB of Echinoderms, so far
as regards the external form, and particularly the structure, is perhaps
still possible. The apical plate present in Tornaria is, according to
recent observations, also found in the larvs of Echinoderms — e.g., in •
Antedon — although in a greatly reduced condition. The coelom seems to
be segmented in the Echinodemis as well as in the Enteropneusta— a
fact which is certainly of importance. However, it is precisely the
interpretation of the coelom in Balanoglossus which is not yet adequate,
since the different portions of the intestine, from which it takes its
origin, are not fully understood. Unfortunately it must be admitted
that even yet the relationships of Balanoglossus are shrouded in dark-
ness.— K.]

Literature.

1. Agassiz, a. Notes on the Embryology of Starfishes (Tornaria).

Ann. Lyceum Nat. Hist. Neio York. Vol. viii. 1867.

2. Agassiz, A. The History of Balanoglossus and Tornaria. Mem.

Amer. Acad. Arts and Sciences. Vol. ix. 1867.
y— 5. Bateson, W. Early and Later Stages in the Development of

Balanoglossus. Quart. Juur. Micr. Sci. Vols. xxiv. — xxvi.

1884—1886.
•5a. Bateson, W. The Ancestry of Chordata. Quart. Jour. Micr. Sci.

Vol. xxvi. 1886.
C. Fewkes, J. W. On the Development of Certain Worm Larvae. Bull.

Mus. Comp. Zool. Harvard Coll., Cambridge, Mass. Vol. xi.

1883 — 1885 (description of a Tornaria found at NeNvport).

7. GiAiU), A. Systematic Position of Balanoglossus. Jour. Boy. Micr.

Soc. Vol. ii. 1882.

8. GoETTE, A. Vergleichende Entwicklungsgeschichte der Comatula

Mediterranea. Arch. Mikr. Anat. Bd. xii. 1876.
a. KoEHLEK, K. Contributions a I'^tude des Enteropneustes. Inter-
nut. Monatsschr. Anat. u. Hist. Bd. iii. 1886.

10. KoKHLEii, 11. Sur la parente du Balanoglossus. Zool. Anzeiger.
Jahrg. ix., No. 230, p. 506. 1886.

lU KowALEvsKY, Alex. Auatomie des Balanoglossus delle Chiaje.
Mem. Acad. St. Petershourg. Ser. 7, torn. x. 1867.

12. Mahion, a. F. Etudes Zoologiques sur deux especes d'Entero-

pneustes, etc. Arch. Zool. e.vper. et gen. Ser. 2, tom. iv. 1886.

13. Metschnikofi-, E. Untersuchungen iiber die Metamorphose einiger

Seethiere. L Ueber Tornaria. Zcitschr. iciss. Zool. Bd.
xxii. 1870.

ENTEROPNEUSTA 391

14. Metschnikoff, E. Ueber die systematische Stellung von Balano-

glossus. Zool. Anzeiger. Jalirg. iv., Nos. 78, p. 139, and 79.
p. 153. 1881.

15. MtJLLEE, J. Ueber die Larven und die Metamorphose der Ecliino-

dermen. Ahhandl. Aknd. Wiss. Berlin, 1849, 1850.

16. ScHiMKEWiTSCH, W. Ueber Balanoglossus Mereschk. Zool. Anzei-

ger. Jahrg. xi., No. 280, p. 280. 1888.

17. Spengel, J. W. Ueber den Bau und die Entwicklung des Balano-

glossus. Amtl. Ber. der 50. Vers. Deutsch. Naturf. u. Aerzte in
Milnchen. 1877.

18. Spengel, J. W. Zur Anatomic des Balanoglossus. Mittheil. Zool.

Stat. Neapel. Bd. v. 1884.

Appendix to Literature on Enteropneusta.

I. BouENE, G. C. On a Tornaria found in British Seas. Jour.
Marine Biol. A^soc. Ser. 2, vol. i. London. 1889.
II. Morgan, T. H. The Growth and Metamorphosis of Tornaria.
Jour. Morphol. Vol. v. 1891.

III. Morgan, T. H. Balanoglossus and Tornaria of New England.

Zool. Anzeiger. Jahrg. xv. 1892.

IV. Morgan, T. H. The Development of Balanoglossus. Jour.

Morphol. Vol. ix. 1894.
V. KiTTER, W. E. On a New Balanoglossus Larva from the Coast
of California, etc. Zool. Anzeiger. Jahrg. xvii. 1894.
VI. Spengel, J. W. Die Enteropneusten. Fauna u. Flora des Golf es
von Neapel. Mem. xviii. 1893.

CHAPTER XIV,

ECHINODERMATA.i

Development in the five divisions of the Echinodermata
offers so much in common that we shall treat of these
together as far as possible. In the development of the
Echinodermata we distinguish the following four periods : —

1. The formation of the primary germ-layers and the
mesenchyma, together ^vith the establishment of mouth and
anns.

2. The origin of the enteroccele and hydrocoele.

3. The formation of the typical larval form.

4. The metamorphosis of the larva into the Echinoderm.

I. THE FORMATION OF THE PRIMARY GERM-LAYERS
AND THE MESENCHYMA, TOGETHER WITH THE
ESTABLISHMENT OF MOUTH AND ANUS.

orr

As far as is known, the cleavage of the Echinoderm e,
is always total. In the Holothurioidea (Synapta), more-
over, it is strictly equal ; whereas in the star-fishes and
sea-urchins it takes place less regularly. Within the mass
of cells which has arisen by cleavage there is found even
during this period a cavity which, in ^the further course of
cleavage, continues to enlarge, and becomes an extensive
blastocoele. The result of cleavage is always a ccelo-
blastula. The next stage of development, too, exhibits
an essential agreement in the different groups of Echino-
dernis, for in all of them it consists of an invagination
gastrula. In the details, however, certain deviations from
the common plan of development occur in the different
forms.

' The maturation and fertilization of the Echinoderm egg will be
discussed in the general part.

392

ECHINODERMATA

393

Holothurioidea. — The earliest stages of development
are the simplest in the Holothurioidea. We follow Selenka's
account (No. 54) of the development of Synapta digitata.

Cleavage is entirely regular. By means of the first
division the egg is halved. Since the newly iormed blas-
tomeres always divide into equal parts, this process being
repeated nine times consecutively, there finally arises a
stage the prismatic cells of which are approximately equal
in size, and arranged in the form of a hollow sphere. They
have already acquired cilia, although the blastula is still
enclosed by the vitelline membrane. (A similar stage in
Holothtiria is shown in Fig. 180 A.) At this stage the

Fig. 173.— Blastosphere of Sxjnapta digitata at the beginning of gastrulation, still
lying witliin the egg-membrane (after Selenka).

further division of the blastomeres is suspended for a con-
siderable time, only subsequently to proceed slowly at the
vegetative pole, and at first at this pole alone. Gastrulation
is initiated by means of this cell-proliferation realized at
the vegetative pole (Fig. 173). The result is a regular
gastrula with a small archenteron (Fig. 174). In this
stage the embryo becomes a free-swarraing larva, which
moves about by the aid of its long cilia. The gastrula very
soon undergoes a change, for the archenteron bends towards
the wall of the gastrula, and unites with the ectoderm

394

EMBRYOLOGY

(Fig. 175). This region corresponds to the dorsal surface
of tlie larva. After the fusion of ectoderm and entoderm,
the lumen of the archenteron communicates with the
outer world, thus establishing the so-called dorsal pore
(Fig. 176). Johannes Muller, who, even in his time, was
acquainted with this process, considered the dorsal pore to
be the mouth of the larva ; but that is not its fate, for the
archenteron soon separates into two portions, of which the
one connected with the dorsal pore constitutes the funda-

FiG. 174,

Figs. 174 and 175. — Gastrula stages of Synapta digitat a (after Semsnea). In Fig. 175
the mesenchyma begins to develop. Bl, blastopore.

ment of the water-vascular system and body cavity, while
the other becomes the intestine. With the multiplication
of its cells the latter acquires a knee-like bend (Fig. 177),
a,ud, while increasing in length, turns toward the ventral
side. Even before it reaches tliis, the communication be-
tween the upper and lower portions of the archenteron is
interrupted (Figs. 177 and 178). Of these two portions

ECHINODERMATA

395

only the lower interests us for the present. Its blind end
enlarofes a little, and conies into contact with the ventral
wall of the larva (Fig. 178). The corresponding part
of the wall sinks in, forming a cup-like depression (Fig.
179), the intestine fuses with it, and an opening leading
into the intestine now breaks through at this point: the
mouth-opening of the larva. The mouth-opening is there-
fore a new formation. The intestine opens to the outside
at the posterior end by means of the blastopore ; the

Fig. 176.

Fig. 177.

FiGa. 176 and 177. — Larvse of Synapta diijitata, showing the formation of the dor-
sal pore (P) and the vaso-peritoneal vesicle (after Sblknka). Bl, blastopore.

gastrula mouth consequently has become the anus of the
larva.

With these changes the larva has also undergone a certain
differentiation in external shape. Its bilateral symmetry is
already expressed. The mouth and anus mark definite
regions of the body. The former lies on the ventral surface,
the latter at the posterior end of the larva. Moreovei', as we
will mention in anticipation of the sequel, the ventral surface

396

EMBRYOLOGT

is ordinarily flattened somewhat, wliereas the dorsal side is
more convex.

Before concluding the consideration of the first develop-
mental processes of Synapta, we must refer to a process
which takes place before the change in position of the arch-
enteron already described ; this is the formation of the mesen-
chyma. At the time when the blind end of the archenteron
begins to bend toward the dorsal surface, there appear at its
apex two cells which project out beyond the other cells (Fig.
175), and are called by Selenka the two primitive cells of the

Figs. 178 and 179.— Larvfe of Synaida digiUita, showing the formation of the
intestine and vaso-peritoneal vesicle (after Selenka.). Bl, blastopore; M, mouth;
P, dorsal pore.

mesenchyma. These cells then separate from their connec-
tion with the archenteron, migi-ate into the blastoccele, and
apply themselves to the ectoderm, but not at predetermined
points. Subsequently a large number of such mesenchyma-
tous or migratory cells are found in the blastoctt'le (Figs.
17G to 179). According to Siclenka's description, they arise by
the division of the two primitive mesenchyma cells ; but
the process of mesenchyma formation in other Holothurioidea

ECHINODERMATA

397

shows that it is not two primitive mesenchyma cells that
give rise to the entire mesenchjma, but that a large num-
ber of cells separate from their connection with the others
and migrate into the blastocoele, where they subsequently
increase in numbers (Fig. 180 B). In Cuctimaria doliolum
and Holothiiria tubulo^a the formation of the mesenchyma
precedes gastrulation, or takes place at the same time with it
(Fig. 180 B). The pi-eviously somewhat thickened place of
the blastula indicates the region from which the migratory
cells detach themselves. From four to ten cells enter the
blastocoele, where they remain until they are forced farther

Fig. 18').— .4, blastula staffe still within the egg-meml)r;ine, and B, larva of
Holothiiria tubulosa at the stage when gastrulation and the formation of the mesen-
chyma begin (after Selknka, from Baifodr's Comparative Embryology), ae, archen-
teric cavity; hi, blastula; ep, ectoderm; /, egg-membrane; hy, entoderm; mr,
micropyle ; ms, mesenchymatous cells ; sc, cleavage cavity.

inwards by the first steps of invagination. The process is,
on the whole, the same as in Synapta. The formation of the
mesenchyma takes place on the same part of the larva, only
it occurs a little earlier. We shall see that in the sea-
urchins the mesenchyma takes its origin still eai-lier, even in
the blastula stage.

The migratory cells, according to the observations of some authors,
move about in the blastocoele with great facility, so that it appears as if
the space between ectoderm and entoderm were filled with fluid. This

398

EMBRYOLOGY

view is advocated, for example, by Ludwig (No. 35), whereas other inves-
tigators (Hensen, Selenka) ascribe a gelatinous consistency to the con-
tents of the cleavage cavity.

Echinoidea. — According to the recent investigations of
Selenka on Strongylocentrotus lividvs, SphferecMnus granu-
laris, and Echinus microtuberculatus, and of Fleischmann on
Echinocardium cor datum, cleavage does not take place so
regularly in the sea-urchins as in the Holothurians. During
the first four cleavage phases only do the farrows extend to
all the blastonieres ; then for a time some of the elements of
the circles of cells now present take no part in the further
cleavage, so that they soon greatly surpass in size the blasto-

FiG. 181.

Fig. 181. — Blastula stage of
StrongyJocentrotus lividus (after
Selenka). The ciliation of the
larva is omitted in this and most
of the following figures.

Fig. 182.

Fig. 182. — Blastula stage of StrongyJo-
centrotus lividns showing the migration
of mesenchyma cells (after Korschklt).
The flagella are represented too stout.

meres at the opposite pole of the egg. Since, however, the
difference in the size of the blastomeres disappears as cleav-
age progi^esses, a regular blastula, consisting of a layer of
rather tall cells of nearly equal size, results fi'ora cleavage
even in the Echinoidea (Fig. 181).

In the Echinoidea the formation of the mesenchyma regu-
larly precedes gastrulation. At the end of cleavage there
occur in the blastula a flattening of the cells at the ajiimal
pole and, on the other hand, a thickening of those at the

ECHINODERMATA

399

vegetative pole (Fig, 181). The cleavage cavity is diminished
in size as a result of the considerable elongation of the cells
at the vegetative pole, and the embryo becomes slightly oval
in shape. A long flagellum makes its appearance on each of
the cells of the blastoderm ; the embryo begins to rotate within
the vitelline membrane, and finally breaks through the latter
to swarm out as a larva. At the same time a more active
multiplication of the cells begins at the thickened part of
the blastula, as the result of which some of them soon be-
come forced into the cleavage cavity (Fig. 182), where, like
Amoebas, they creep about as migratory cells. These are
followed by others ; after multiplying rapidly in the blasto-
coele, they almost fill it.

Fig. 183.

Fig. 184.

Figs. 183 and 184. — Blastula stages showing the commencement of formation of
mesenchyma (primitive mesenchyma cells) (afcer Selbnka. and Hatschek). The
flagella are omitted here, as also in Fig. 185.

We have described the formation of the mesenchyma as it appeared to
us from personal observations (No. 26). However, other statements have
been made on this point. According to the observations of Selbnka and
Hatschek (No. 54), there arises some time after the swarming out of the
larva a funnel-like depression (Figs. 183 and 184) at the vegetative pole,
which owes its production to the shortening and thickening of two cells
lying at that pole of the blastula. These two cells were looked upon by
the observers last named as the primitive cells of the mesenchyma ; to this
interpretation Fleischmann also adheres. According to him, there are
present at the vegetative pole four such primitive mesenchyma cells,
which have been differentiated there during cleavage. The primitive
mesenchyma cells are said to correspond to the primitive cells of the

400

EMBRYOLOGY

\^ mesoderm, which constitute in the Annelida, as well as in some other
forms, the starting-point for the formation of the mesoderm (comp. pp.
264 and 282).

Two groups of cells arise by the multiplication of the primitive mesen-
chyraa cells, either by their being forced into the blastula by the pressure
of the surrounding cells (Fig. 185, after Selenka), or by new cells being
constricted off from them (Flkischmank). Out of these groups there are
said finally to arise two bilaterally symmetrical cell-bands, which corre-
spond to the mesodermal bands of the Annelida. As a result of these
processes, the larva would show at a very early stage bilateral symmetry,
which becomes intensified by the flattening which takes place from the
dorsal to the ventral side.

This conception of the origin of the mesenchyma from the two
primitive mesenchyma cells was generalized by Selenka, for he also

found the two primitive cells
in other Echinoderms (Holo-
thiirioidea and Ophiuroidea).
Thus in Synapta there are the
two cells lying at the summit
of the archenteron (Fig. 175),
and also in Ophioglypha two
cells which detach themselves
from the cells of the blastula.
This kind of mesenchyma de-
velopment is not by any means
so typical as it is said by
Selenka and Hatschek to be
in the Echinoidea, and recalls
much more the formation at
pleasure of mesenchyma cells
which, like the succeeding ones,
separate from the cell-wall of
the blastula or gastrula, as is
known to take place in the
Agteroidea and Crinoidea (comp. pp. 402 and 403). Metschnikoff has
accordingly opposed the theory of mesenchyma formation espoused by
Selenka and Hatschek, and, on the whole, we agree with his con-
clusions.

The appearances which led Hatschek and Selenka to believe in the
existence of primitive mesenchyma cells are to be explained by the fact
that the cells of the blastula at the time of dividing are shortened, and
become stouter. Thus it happens that directly after the division of such
a shortened cell two small cells, surrounded by tall, prismatic cells, come
to lie side by side, and thus such stages arise as those of Hatschek and
Selenka (Figs. 183 and 184). Such shortened cells occur in various
parts of tlie circumference of the blastula, when the development of the

Fig. 185. — Blastula stage of Strong]ilo-
centrolus lioidus, with mesenchyma cells
which have migrated into the blastocoele
(after Sklbnka)

ECHINODERMATA

401

mesenchyma at the thickened pole is ah-eacly under way (Fig. 182). It
can be observed in the living blastula that the shortened cells elongate,
and soon attain the length of the surrounding cells. The mesenchyma,
then, does not take its origin from primitive mesenchyma cells, but by
the proliferation of a large number of cells. Moreover, the migratory
cells which have entered the blastocoele do not form mesenchyma bands,
but are irregularly scattered.

Gastrulation in the Echinoidea takes place after the
detachment of the mesenchyma cells, in the ordinary manner
(Selenka, N'o. 53). The archenteron grows out in half a day
to a comparatively long sac-like tube. Between it and the
ectoderm there are frequently stretched some of the mesen-

FiG. 186.— Gastrula stage of Toxopneustes hrevispinosus (after Selen-ka). The
mesenchyma cells stretch across suspensor-like between the ectoderm and archen-
teron.

chyma cells, which probably serve as suspensors for the
archenteron (Fig. 186). The larval mouth in the Echinoidea
is formed in a direct manner, for the end of the archenteron
(after the abstriction of the entero-hydrocoele) bends toward
the ventral surface and fuses with the ectoderm, whereupon
the mouth-opening breaks through. The gastrula mouth
here too becomes the anus.

Asteroidea. — Cleavage, although unequal, differs only a
little from the equal type. The difference in the size of the

K. H. E. D D

402

EMBRYOLOGY

blastomeres becomes imperceptible even in the sixteen-cell
stage, and from that onwards. LUDWIG, in his description of
the development of Asterina gihhosa, laj's particular stress on
the fact that from the very beginning a cavity exists between
the cleavage spheres, and consequently the blastosphere is
formed at a very early period. The result of cleavage in
the star-fishes is also a hlastula, which is formed of a layer
of equal-sized cells. The gastrula arises from this by invagi-
nation.

The formation of the mesenchyraa takes place, according
to the observations of Metschnikoff on Astropecten, after

Fig. 187

FiQ. 188.

Figs. 187 and 183. — Blind end of the archenteron of gastrula stages of Astropecten
pentacanthus during tbe formation of the mesenchyma (after Meibohnikoff).

gastrulation is completed. The originally cylindrical cells of
the archenteron at its blind end become flattened out (Fig.
187) ; then they begin to put forth short pseudopodial pro-
cesses, and finally some of them detach themselves from the
rest. Others, usually four or five at a time, soon follow
these (Fig. 188). It is said that the migration of entoderm
cells may be so active that a more or less considerable open-
ing arises at the upper end of the archenteron.

It is seen that the conditions described by Metschnikoff resemble
those of which we have learned through Selenka as existing in the

ECHINODERMATA 40;i

Holothurians. There also the mesenchyma cells separate from the apex
of the archenteron. To be sure, according to Sklenka, only two cells —
namely, the primitive mesenchyma cells — arise in this way. Metschni-
KOFF declares that his search for such a stage with two cells has always
been in vain. His observations allow him to reject even in the case of the
Holothurians the idea of the origin of the mesenchyma from two i^rimi-
tive cells, and to assume instead a continuous emigration of cells from the
entoderm, especially since certain observations of Selenka seem to him
to corroborate his own view. For Selenka found larvfe in which the free
end of the archenteron was irregularly outlined or covered with stellate
cells. Selenka explains this phenomenon as being pathological, whereas
Metschnikoff considers it warrantable to look upon such larv£e as indi-
viduals in which an emigration of numerous entoderm cells is now taking
place.

The mouth-opening and the stomodteam in Asteriua arise
in the form of a hollow plug, which is invaginated at the
front end of the ventral side of the embryo, and fuses with
the entoderm at that point (Ludwig). At this stage of
development the embryo abandons the egg-membrane, and,
as an approximately pyriform larva, swims about free in the
water by means of cilia, which cover its entire external
surface.

Ophiuroidea. — Cleavage appears to take place in the
same way as in the Asteroidea (Ludwig, Selenka). The
blastula, which also exists here, exhibits a thickening at its
vegetative pole. The mesenchyma is formed within it in
the same way as in the Echiuoidea ; according to Selenka,
it originates from the two primitive cells, but according to
Metschnikoff from a continuous emigration of cells from
the thickened wall.

Not much weight should be given to the view, advocated by Fewkes,
that there is a bilaterally symmetrical arrangement of the mesenchyma
in OphiophoHs, especially since the author himself did not find the
mesenchyma bands in the sea-urchin (Eclihiarachinus parma) studied by
him (No. 13).

Crinoidea. — Nothing is known of the earliest develop-
mental processes of the Crinoidea, except the statements
concerning Atitedon. In this form, too, a blastula is deve-
loped after equal cleavage, and the gastrula is formed by
invagination. The formation of the mesenchyma takes

404

EMBRYOLOGY

place from the arclienteron, but only after j^astrulation
(Baurois, No. 6 ; Bury, No. 7), as in Sjnapta and the Aste-
roidea. The cells of the archenteron, especially those lying
at its apex, lose their regular arrangement, apparently as the
result of a rapid cell-proliferation occurring at this point, so
that the archenteron no longer presents strictly a single
layer of cells, but is composed of cells irregularly grouped.
A large number of these cells migrate into the cleavage
cavity, and form the mesenchyma (Fig. 189).

The various kinds of mesenchyma formation are not so different from
one another as they at first sight appear to be. It is always substantially
the same region of the blastula from which the mesenchyma takes its
origin, the only difference being that in the one case this has already
undergone invagination, while in the other the invagination does not
take place until later. It seems as if those methods of mesenchyma

formation were the more primitive in

-C^^^Sf^h). which the cells take their origin from the

-^^^ *^4 *' •i^^^'-' archenteron. Later the ccelomic sacs are

t^''^')^'' -'* ' ''p^'Vi^ ^^so developed from the archenteron, and

/^ J*_^", ...|kC»' ^ thus the development of the mesenchyma

W'ii^^'l-':,^^'^'^:*^^ ^^^ the mesoderm could be correlated.

~<S2^£}^^ '~^\kwi There are found various transitional stages

of this anticijjatory misplacement, i.e.,

between the origin of the mesenchyma

from the archenteron and its development

Fig. 189.— Dovelopinentof the from the thickened pole of the hhintiild,

mesencbyma in the gast.uhi „f ^s, for example, in the Holothurians. In

Antedon rosacea (after Buhy). c , xi i • .. ,i

hi blasto 30 e iiynai^ta tne mesenchyma arises from the

archenteron, whereas in Ilolothuria it
begins at the same time as gastrulation.

The metamorphoses which the archenteron of Antedon
undergoes are different from those in other Echinodermata
in so far as the blastopore does not become the anus, but
closes, after which the archenteron is constricted off from
the ectoderm and lies as a spacious sac in the interior of the
embryo. Mouth and anus arise as new formations, but not
until much later. There is certainly a connection between
these complicated formative processes and the fact that the
larva of Antedon abandons the egg-membrane much later
(the seventh day of development) than other Echinoderm
larva).

7 «••>.;

ECHIXODERMATA 405

The statements of authors differ regarding the position of the blasto-
pore ; according to Barrois, it is found at one pole of the larva, whereas
Bury and Goette locate it more toward the future ventral side (Fig. 189).

II. THE ORIGIN OF THE ENTEROCGELE AND
HYDROCCELE.

In the first part of this chapter we traced the develop-
ment of the Echinoderm larva to the point at which the
mesenchyma was formed and the larval mouth had broken
through on the ventral side. Even before this occurs, im-
portant developmental processes, which result in the absti-ic-
tion of the fundaments of the body cavity and water- vascular
system, take place in the archenteron. Both of these arise
in all Echinoderms as diverticula of the archenteron. We
apply to them the names employed by Ludwig, eiiteroccele
and hydrocuele, without, however, abandoning for the common
fundament of both the older, but very significant, name of
vaso-peritoneal vesicle (Selenka).

The formation of the body cavity and the water- vascular
system, although taking place in various ways in the differ-
ent divisions of the Echinodermata, nevertheless exhibits
close relationships between the different groups. We shall
accordingly treat them here in the order in which they
follow each other niost naturally.

Asteroidea [Agassiz (No. 1), Metschxikoff (No. 37),
Greeff (Nos. 18 and 19)]. — Before the mouth-opening of
the larva is formed, two bilaterally symmetrical outfoldings
arise at the blind end of the archenteron. These soon
become considerably enlarged by growing out toward the
posterior end of the larva (Fig. 190 A and B). Then
each of the two vesicles separates from the intestine, and
the left-hand one unites by means of a tube with the dorsal
surface of the larva. This pair of vesicles constitutes the
fundaments of the body cavity and water- vascular system.
Their further development takes place in such a way that the
left vesicle is consti'icted in its posterior portion, whereby
a part finally becomes separated off. The abstricted
anterior part constitutes the earliest fundament of the
water-vascular system [hydrocoele]. It is soon transformed

406

EMRRTOLOGY

into a five-lobed, rosette-like structure, in which the sub-
sequent shape of the water- vascular system, with its five
chief stems, is already expressed. The two chief vesicles,
which remain after the abstriction of the hydroccele, undergo
a metamorphosis similar to that which Ave shall describe
farther on for Asterina gibbosa. We only add at present
that they represent the fundament of the body cavity, the
•enteroccele.

It can be seen from the above description that in the
establishment of the vaso-peritoneal vesicles a bilateral
symmetry is expressed, which, however, is again deranged

0[f^

Fig. 190. — A and B, two starfish larvfp at the time of the (level npmcnt of the
■entero-hydrocciele (after Metschnikoff). D, intestine ; I'p, vaso-peritoneal vesicle,
the dorsal ])ore of which can be recognized in B ; p, peritoneal vesicle (right-hand
enteroccele).

by the development of only one hydroccTple. A more exact
bilateral symmetry, extending to the formation of the
hydrocoole, appears, on the other hand, to exist during these
stages in the

Ophiuroidea. — In this group also, according to the
statements of Metschnikoff (No. 87), two vaso-peritoneal
vesicles are constricted off from the intestine, but each of
them is said to divide into an enterocoelic and a hydrocadic
vesicle. Oi-dinarily, however, only the anterior left one of
the two hydrococles develops further, whereas the right

ECHINODERMATA 407

one in most cases degenerates, only occasionally giving
rise to a right-hand watei^-vascular rosette with a dorsal
pore (MiJLLER, Metschnikoff). In exceptional cases in
the Asteroidea also, there is said to be formed, in addition
to the left-hand one, a right-hand hydrocoelic vesicle, which,
quite like the other, develops into a five-rayed water- vascular
rosette, provided with a dorsal pore. In that case, a hydro-
coele and an enteroccele would therefore be present on both
sides, and the symmetry would be complete.

The development of enterocoele and hydrocoele does not
take place in all Asteroidea in the way described, for
the double character in the fundament of the entero-
hydrocoele may not be so prominent, owing to the fact
that the vesicles of the two sides are no longer constricted
off from the archenteron separately, a condition which is

Fig. 191. — A, intestine of Asteracanthion glacialis and the vaso-peritoneal vesicle
constricted off from it (after Goette). The dorsal pore is already formed ; r and I,
right and left sacs of the vaso-peritoneal vesicle; A, anus; M, region of the
mouth, which does not develop until later. B, vaso-peritoneal vesicle of ^sf«ri>ia
gihhosa, with its riglit and left sacs (r and !), on the latter the fundament of the
hydrocoele (B) (after Ludwig).

important for the reason that it is a transition to the corre-
sponding process in the Echinoidea. Goette observed in
larvae of Asteracanthion glacialis that the process usually
takes place in the manner described, but that during the
abstriction from the archenteron the vesicles may in this
same form remain united with each other (Fig. 191 A).
Now, according to Ludwig, this latter condition is the
common one in Asterina. The vaso-peritoneal vesicle
appears in the form of two lateral outfoldings at the blind
end of the archenteron (Fig. 192 A). The two outfoldings
grow toward the posterior pole of the larva (Fig. 192 JS),

408

EMBRTOLOGT

and the bipartite vesicle separates off from the archenteron
(Fig. 192 C). Meanwhile the blastopore has closed. In
their further development the two arms of the vesicle grow
around the intestine. They come in contact with each other
behind [and dorsad of] the intestine, and there form the
mesentery, which extends from the intestine to the body-
wall [in a direction oblique to the sagittal plane]. Tlie
fundament of the water- vascular system now first makes its
appearance in the vaso-peritoneal vesicle as an outfolding
of the left half of the vesicle (Fig. 191 B, H). Projecting
at first only a little beyond the wall of the vaso-peritoneal

c.

Fig. 192.—^ to C, sections through larvw of Asteriva gihhosa (after Ludwig). Bl,
blastopore; D, intestine; Vp, vaso-peritoneal vesicle; r and 1, right and left
sides.

vesicle, it soon gives rise to five lobes, thus producing the
earliest fundament of the five radial stems of the water-
vascular system. At about the same time, an invagination
of the ectoderm takes place on the dorsal side of the larva
opposite the larval mouth ; it grows inwards, and opens into
the left half of the A^esicle. This is the dorsal pore of the
larva, which therefore effects a communication of the out-
side world with enterocoele and hydrocoele, for the separji-
tion of the latter from the enteroca'le does not take place
until later.

ECHINODERMATA

409

The processes which we have followed in the development
of the enterocoele and hydrocoele of Asterina bear a close
resemblance to those which give rise to the formation of the
vaso-peritoneal system of the

Echinoidea. — As in Asterina, the blind end of the arch-
enteron is transformed into the fundament of the vaso-
peritoneal vesicle (Selenka, No. 53). Two outpocketings
are developed from it, the two being constricted off in
common from the intestine (Fig. 193 J. to C). It is not until
later that they separate into a right and left vesicle ; the
former constitutes a part of the enterocoele, but the latter
represents not only the other part of the enterocoele, but also
the hydrocoele. Accordingly by another constriction the
left vesicle is divided into two, and in this manner gives rise

Fig. 193. — A to C, longitudinal sections of the archenteron of Echinus miliaris,
showing the development of the vaso-peritoneal vesicle {after Selenka).

to the left enterocoelic sac and the hydrocoele. The same
process is said by Metschnikoff to take place in the right
vesicle, so that a hydrocoele is formed on the right side also,
in a manner similar to that described for the Asteroidea and
Ophiuroidea. The right hydrocoele is said subsequently to
degenerate. These statements are noteworthy only for the
reason that they may indicate that the water- vascular
system is traceable to an organ of paired origin.

A mode of origin of the hydro-enterocoele still more
modified than that in the forms hitherto considered is
exhibited in the

Holothurioidea, although it can be referred to the same
plan. We have already seen in Synopta (comp. Figs. 176 to
179, pp. 395, 396) that a part of the archenteron detaches

410

EMBRYOLOGY

itself from the rest, and unites with the dorsal wall of the
larva. As its further development shows, this abstricted
portion of the archenteron corresponds to the vaso-peritoneal
vesicle of the other Echiiiodermata (Fig. 179). It communi-
cates directly with the outside woi^ld by means of the dorsal
pore, just as in certain Asteroidea. It is not until this stage
that a constriction makes its appearance ia the posterior

Fig. 191..

Fig. 191. — Optical longi-
tudinal section of an Auricii-
laria larva of Holothurii tabu-
losa (after Selbnka). A,
auus ; D, intestine; JB, onteni-
ccBle ; H, hydrncojle ; M,
mouth ; P, dorsal pore ; W,
ciliated band.

Fig. 195.

Fig. 195. — Longitudinal section of a larva
of Citctii?iana (JolioIu7a, Bomewhat diagram-
matic (after Selenka). A, anus; Am, ambu-
lacral (radial) vessels; D, intestine; E, entero-
coeles; l\ feet; 31, mouth ; /', di)rsal pore,
leading through the stone canal to the water-
vascular ring, Wv ; T, tentacular vesicles ;
IK)-, water- vascular ring.

third of the vaso-peritoneal vesicle (in Holothuria tubulosa),
which divides the vesicle into the hydrocoele, connected with
the dorsal pore, and the less extensive enteroccele (Fig. 19-1

ECHINODERMATA 411

jETand E). The former is soon transformed into a five-lobed
structure, which grows around the stomod^um of the larva,
and thus proves to be the water- vascular ring of the animal
(Fig. 195). Five outgrowths from it indicate the primary
tentacles.

The enterocoele has in the meantime grown out into a sac,
which has bent around under the intestine and then divided
into a right and a left peritoneal vesicle. These are sym-
metrically placed on the intestine (Fig. 195). They enlarge
and finally obliterate the cleavage cavity. Their cavity be-
comes the permanent body cavity, and their walls the peri-
toneum. Where their walls come together the mesentery of
the intestine arises.

In the development of the entero-hydrocoele the Holothurioidea differ
from the forms previously considered, owing to the fact that the vaso-
peritoneal vesicle does not present a bilaterally symmetrical shape from
the beginning, this being expressed very late, not until the abstriction of
the enterocoele from the hydroccele has taken place. The hydrocoele
in the Holothurioidea, in contrast to the other forms, separates very early
from the enterocoele.

In the Crinoidea the conditions are quite peculiar, pro-
bably owing to the fact that the archenteron surrenders its
connection with the ectoderm at a very early period, and
lies as an isolated sac in the interior of the blastula. On
the third day this sac acquires an annular constriction
(Fig. 196 A), which later becomes considerably deeper, so
that there arise two vesicles, which are connected by only
a narrow neck. The two vesicles may be distinguished as
anterior and posterior, for, corresponding to the subsequent
development of the embryo, an anterior and a posterior part
of the body can be distinguished even now. The anterior pole
is marked by the accumulation of numerous mesenchyma
cells, and the posterior by the position of the vesicular archen-
teron. The posterior of the two vesicles now changes its
shape by elongating in the transverse direction and then ac-
quiring a slight constriction at the middle (Fig. 196 B).
Two hollow processes grow out backward from the point of
junction of the two vesicles, and bend around the constricted

412

KM BRYOLOGY

part of the posterior vesicle. Moreover, the anterior vesicle
has also altered in shape, for it divides into an extensive
saccular and a more narrow, canal-shaped portion (Fig. 196
B). The fundaments of the most important parts are thus
produced. The posterior bipartite vesicle constitutes the
fundament of the enterocoeles. It first separates from the
other parts, and then divides into a right and left coelomic
sac. Of the two processes from the neck between the anterior
and posterior vesicles, only the larger, dorsal one is said
to be retained, to constitute the fundament of the intestine,
whereas the smaller, ventral one disappears (Barrois). The
anterior vesicle, the hjdroca?le, which already exhibits the

mes-.

-ent

Fig. 19G.— j4 and B, embryos of Aniedon rosacea in optical section ; development
of the enterocoele (ent) and hydrocoele (h) (after J. Babeois). d, intestine; mes,
cells of the mesenchyma; st, stone canal.

separation into water-vascular vesicle and stone canal,
remains united with the intestine for some time (Fig. 19G
B). Later the hydrocoele also se[)arates from the intestine,
and then, as in other Echinoderm larvae, the two enterocceles
and the hydrocoele are found lying next to the intestine,
which has now considerably enlai-ged. However, the oral
and anal openings, as well as the water-vascular pore, are
still lacking.

A vaso-peritoneal vesicle can scarcely be spoken of in
Antedon, unless the condition in which the already paired
enterocceles are still connected with the hydrocoele by means

ECHINODERMATA

413

of tlie narrow neck is to be considered as such. The entero-
coeles in Antedon separate from the common fundament of
the intestine and hydrocoele at a very early period.

In the development of Antedon the statements of Goette, Barrois, and
Bury are opposed to one another. In what precedes we have followed
those of Barrois, for they agree fairly well with those of Bury. Differ-
ences between these two authors exist in so far as, according to Bury, the
two processes from the neck, connecting anterior and posterior vesicles,
unite and together constitute the intestine, which at first is circular in
form, and through which there extends for a while a solid cord, uniting
the two enterocoeles. Furthermore, according to Bury, only the larger
portion of the anterior vesicle represents the fundament of the hydro-
coele; the smaller portion, after separating from the hydrocoele, still
supplies a part of the body cavity (anterior body cavity), and later is
connected with the outer world by means of a canal, whereas a union
of this part of the body cavity with the hydrocoele and the formation of
the stone canal take place only secondarily. According to this, a part
at least of the enterocoele would arise at the same time as the hydro-
coele. According to the statements of Barrois, on the other hand, the
hydrocoele in this case, contrary to all other Echinoderms, takes its origin
independently of the enterocoele. The same is to be gathered from
Goette's description, which, however, it is difficult to harmonize with
those of Barrois and Bury.

Features appear in the development of Antedon which,
besides influencing the shape of the archenteron, also modify
its derivatives, the enterocoele and hydrocoele. In accord-
ance with the attached mode of life of the later larval
stages, the farther development also exhibits important
deviations from the development of the other Echinoderms.

Statements on the Development of the Entero-hydrocoele not
in accord with the Preceding.— The studies of N. C. Apostolides on
the development of two Ophiurans {Ophiotrix versicolor [Lusitanica Lin.,
cf. Fewkes, No. 13] and Amphiura squarnata) also contain statements
on the mode of formation of the water-vascular system, but the results
are so different from anything hitherto known of Echinoderm develop-
ment that we mention them only on account of their remarkableness.
In both forms the gastrula is said to arise not by invagination, as is
known to be the case in other Echinoderms, but rather by delamination.
Likewise the fundament of the loater-vascular system is said to be de-
veloped in a different way, namely, by an accumulation of two masses of
cells, one on either side of the archenteron. These, according to the

414 EMBRYOLOGY

description of the author, evidently must come from the mesenchyma.
Examples of both genera were previously studied by Balfour (C«?)i-
parative Embryolofiy) and Metschnikoff, and were found to be like other
Echinoderms in regard to the structures in question.

The studies of Bury (No. 8) on the early development of the entero-
hydrocoele are, on the contrary, important. If, in spite of this, we have
not given them a correspondingly prominent place in our account of these
conditions, it is owing chiefly to the fact that Bury's statements on this
subject are almost directly opposed to those of other authors, and that,
furthermore, they neither trace the fundaments of those organs back to
the earliest stages, nor devote sufficient attention to the later history
of them. For these reasons, Bury's investigations, which, after all,
include only a few stages from the middle, do not seem to us to be sufli-
ciently conclusive upon the development of this important system of
organs, deviating so fundamentally as they do from all other descriptions.

Bury assumes that all Echinoderm larvfe have not two enterocoelic
sacs, as had previously been believed, but two pairs of them, either
actually present or to be recognized from their fundaments. Thus the
larval body would exhibit an internal segmentation. These conditions
can be clearly seen in the larvae of Ophiurans and Ecliinnids, in which
the larger, anterior enterocoeles lie at the side of the oesophagus, and the
smaller, posterior ones next to the stomach. The anterior and posterior
pairs have arisen by division of the primary enterocoeles. The left
anterior enterocoelic sac opens to the exterior by means of the water
pore. The union of the latter with the enterocoelic sac does not corre-
spond to the subsequent stone canal, for the hydrocoele does not arise
until later, and then either from the anterior or posterior enterocoele,
from which it is constricted off ; it is only secondarily that it unites with
the anterior enterocoele. Originally, then, only the body cavity commu-
nicates (by means of the dorsal pore) with the outer world. The hydro-
ccjele does not unite with the body cavity, and thus with the outer world,
until later. Such conditions were also found by Ludwig in later stages
of Aster ina (comp. pp. 408 and 436), and are retained throughout life
in the Crinoidea (comp. p. 447 and Fig. 224, p. 4o3).

Bury's observations seem to coincide with those of Metschnikoff, who
also observed a division of the right enterocoelic vesicle in Ophiurans and
Echinoids. but referred it to the formation of a right hydroca-le, which
subsequently degenerates. Thus Metschnikoff argues for a primitively
paired fundament of the hydrocoele, whereas Bury, like other authors,
derives it as an unpaired structure from one of the two enterocoeles of
the left side.

In other Echinoderm larvte Bury finds the internal segmentation less
sharply expressed. In the Astcroidea an anterior and posterior entero-
cciile can still be distinguished ; they are, however, no longer sejiarate,
but coalesce with each other. The Ilolothurioidea are said to have,
in addition to the two posterior enterocoeles, a left anterior one, which,

ECHINODERMATA 415

however, from the beginning exists only as an indistinct appendage of the
hydrocoele; likewise in the Crinoidea (Antedon) only an anterior entero-
coele is present, which, united with the hydrocoele, is constricted off
from the archenteron. What Bury here considers as anterior enterocoele
and hydrocoele together, other authors look upon as hydrocoele only.
Where the conditions are of such a character as they are in the Holo-
thurioidea and Crinoidea, and to some extent in the Asteroidea, Bury
conceives a partial degeneration of the originally paired and segmentally
arranged enterocceles.

It is not to be denied that in most forms the common entero-hydrocoele,
the so-called vaso-peritoneal vesicle, communicates (by means of the
dorsal pore) with the outside world ; but whether it was the enterocoele
alone which originally possessed this union, and whether the hydrocoele
was united with it only secondarily, does not yet seem to be proved by
Bury's investigations as long as the origin and subsequent fate of his
anterior and posterior enterocoeles remain unknown.

III. THE DEVELOPMENT OF THE TYPICAL LARVAL

FORMS.

Having become acquainted with the most important pro-
cesses which take place within the body of the larva, we
turn to the consideration of its external shape. This is very
different in the separate groups of Echinoderms. Like
MtJLLER, we start with a simple fundamental form, from
which to derive the different larval forms. This funda-
mental type is an elongate, oval to pyriform larva, which is
somewhat flattened on the ventral side. This larval form
arose from the gastrula, the blastopoi^e of which is meta-
morphosed into the anus, while the archenteron bends
around toward the ventral side and here connects with
the outside world by means of the larval mouth. The larva
possesses still another opening in addition to these two,
namely, the dorsal pore of the water-vascular system. The
flao-ella with which the larva was at first uniformly covered
disappear in part, and are retained only on limited areas,
which are called ciliated bands.

Crinoidea. — The larva of Antedonis one of the most sim-
ply constructed of Echinoderm larvae. At first of fairly uni-
form, oval shape, it is subsequently slightly curved toward
the somewhat flattened ventral surface. In place of the com-

416

EMBRYOLOGY

Gr

plete covering of cilia of the first few days, it subsequently
acquires five ciliated rings, which encircle the body trans-
versely, and a tuft of long cilia at the anterior end. The
most anterior of the ciliated rings is less sharply marked
than the others (Bury), owing to which eai'lier authors
spoke of only four rings of cilia. The larva swims with the
tuft of cilia directed forwai-ds. [Under the tuft lies a
thickening of the ectoderm which was recognized already by
Nachtkieb (No. XXa, Appendix to Literature) as an apical

plate. It has been confirmed
as such by the recent thorough
investigations of Seeliger (No.
XXVI.) . Indications of an
apical plate also appear in the
younger stages of other Echi-
noderm larvae (Asteroidea and
Echinoidea). — K.] If we com-
pare the larva of Antedon with
that of the other Echinoderms,
in which the blastopore be-
comes the anus, we must look
upon the end opposite the anus
• — namely, the one provided
with the tuft of cilia — as the
anterior end of the larva, and
more especially because the so-
called larval mouth lies nearer
to this end. Recent authors
designate as the mouth a cili-
ated depression which lies on
the ventral side between the
second and third rings of cilia
(Fig. 197). This pit does not
represent an actual mouth-opening, for it is not connected
with the intestine, but the so-called vestibule is subsequently
formed at this jioint, and the mouth-opening arises at its
bottom.

A small pit is found on the ventral surface near the
anterior end of the l^irva, which later serves the larva in

Lju

\

x

\

/

Fig. 197. — Larva of /I ntf don rosacea,
with ciliated rings and tuft of cilia
(after Goette and W. Thomson) The
earliest skeletal pieces are aheady
formed as fenestrated plates. Gr, pit
which serves the larva in attaching
itself; Lm, the so-called larval mouth.

ECHINODERMATA 417

attaching itself. Furtliermoi'e, on the left side, between the
third and fourth ciliated bands, the water pore makes its
appearance as a clear spot on the yellowish-brown larva.
In addition to the systems of organs already considered,
the earliest fundaments of the skeleton can be recognized
(Fig. 197j.

Holothurioidea. — The larvae of the Holothurioidea usu-
ally exhibit a typical form, which was designated by Joh.
MtJLLER as the Auricularia. Its derivation from the funda-
mental form of the Echinoderm larva is illustrated by the
following diagram^ (Fig. 198), in which the shaded part
represents the deep depression of the body, within which the
mouth-opening (m) lies. This part is surrounded by a band
of cilia, the transverse tracts of which, those lying in front of
the mouth and in front of the anus, have been distinguished

an

Fig. 198. — A to D, development of the Auricularia from the fundamental form of
the Echinoderm larva' (diagram after Joh. Muller, from Balfour's Comimrative
Embryology). The broad black line indicates the ciliated band, the shaded area
the depressed part of the surface, an, anus; in, mouth.

from the tracts extending lengthwise on both sides, the
so-called longitudinal parts of the ciliated band (Figs. 198
A and B). The anus lies near the posterior pole of the larva.
In its further development the body of the larva, in front
as well as behind, becomes more hollowed out on both sides,
while the elevated parts of the ventral surface persist and
grow toward each other (Fig. 198 B and C). In this way
there results a larval form, on the ventral side of which an

1 The manner of orientation chosen by Joh. Muller has been retained
in this and in the following diagrammatic figures (200, 202, and 203) for
practical reasons only. It would be better if they were placed with the
mouth upwards and the anus down, as has been done, for example, in
Figs. 199, 204, 209, and 211.

K. H. E. E E

418

EMBRYOLOGY

anterior, so-called preoral, and a posterior, anal area can be
distinguished as elevated parts from the depressed portions
(Fig. 198 G and D). At the anterior and posterior ends the
two areas bend around toward the dorsal surface. Fig.
199 il shows a larva at about this stage as seen from the
side. The further development of the shape of the larva

is finally attained by
the extension of the
depressed, or hollowed-
out, region more to the
periphery, and by the
production of lobular
processes at the mar-
gins of the body, owing
to the outgrowth of cer-
tain parts (Fig. 198 D).
Calcareous deposits,
having the shape of
delicate miniature
wheels, may make their
appearance in these
ear-like appendages
(Fig. 205, p. 426). Along the periphery of the lobes runs an
uninterrupted ciliated band, which borders the two ventral
areas as well as the dorsal surface.

In each of the two depressed lateral surfaces of the Auricularia larva
lies a structure resembling a ciliated band ; but these structures bear no
relation to the ciliated band itself. Each of these two bands exhibits the
form of a blunt angle opening toward the ventral surface. The cords
consist of ciliated cells and fine longitudinal fibres lying under them.
Strands of fibres pass from them to the ciliated band. Accordingly
Metschnikokf (No. 37) and Semon (No. 55) interpret the two cords as the
central nervous system of the larva. They also occur under similar cir-
cumstances—to anticipate— in the Pluteus larvae of the Ophiuroidea.
On the other hand, corresponding structures do not occur in the larvre of
the Echinoidea and Asteroidea. According to Semon, however, fine
fibres, similar to those in the nerve cords of the Auricularia larva;, occur
in the ciliated band of these larvae, so that the nerve apparatus would be
connected with the ciliated bands in the same way as in the larvae of the
Annelida (comp. p. 20(5).

[A very large Holothurian larva (Auricularia nudibranchiata), which

Fig. 199. — A and B, larva of a Holothurioid
and an Asteroid respectively seen from the side
(from Balfour's Comparative Embryology), a,
anus ; l.c, ciliated band ; m, mouth ; pr.c, adoral
band of cilia of the Bipinnaria ; st, stomach.

ECHINODERMATA 419

attains a length of 6 mm. and is characterized by the complicated form of
its ciliated band, has been recently described by Chun (No. VII., Appendix
to Literature). The ciliated band of this larva is extraordinarily tortuous,
and exhibits arabesque-like foldings. Somewhat similar conditions were
previously mentioned in the case of a very large Tornaria. Chun's
paper, which is important in many respects, contains an account — to
which attention may be called here— of a sac-like invagination of the
hind-gut of the larva, from which, according to Chun's conjecture, the
respiratory trees of the Holothurian may arise. — K. ]

The Aaricularia larva does not occur in all the Holo-
thurioidea. Thus, for instance, the larva of Cucumaria
doUolum at the time of the formation of the mouth assumes
a cylindrical form (Selenka). The flagella disappear zone
by zone, until the larva retains only four to five bands of
cilia, a ciliated anal area, and a ciliated cephalic zone. With
this the so-called pupal stage is reached, which does not make
its appearance in the development of other Holothurians
until later (comp. p. 427). Another Holothurian, Psolinus
hrevis, develops, according to Kowalevskt (No. 28), alto-
gether without a metamorphosis. The young Holothurians
arise directly from the eggs, which are laid in the sea- water.
In Phyllopliorus urna the larvoe, which are pi-obably com-
pletely and uniformly ciliated, are said to swim about in the
body cavity of the parent. When they abandon the parent,
they already possess five tentacles and two feet. A similar
condition is found, according to Ludwig (No. 33), in the
likewise viviparous Chirodota rotifera.

Asteroidea. — The larval form of the Asteroidea, like
that of the Holothurioidea, can be derived from the funda-
mental form. If Figs. 200 B and 198 (7, from JoH. Muller's
diagrams, are compared, one sees that in the Asteroid larva
the preoral area of the ventral surfaces, together with the
part of the ciliated band surrounding it, is isolated. The de-
pression on the ventral surface is continued farther forward
here than in the Holothurian larva. In this way the con-
nection of the preoral area with the dorsal surface is inter-
rupted, and the ciliated band is separated into two parts.
Thus two ciliated bands arise, which, from their positions,
may be designated as the adoral and adanal (Figs. 200 A

420

EMBRYOLOGY

Of these the latter is mueli the longer (Fig. 200

and 199 B).
AtoD).

By the bulging and gi'owing outward of the peripheral
parts of the larva, there arise longer and shorter processes,
which are bordered by the ciliated bands (Fig. 200 C). This
larval form received from its discoverer, Sars, the name of
^^ Bipvnnaria" (asterigera), which it continued to bear even
after its relation to the starfishes was recognized.

a/i

Fig. 200.— Development of the Bipimiai-i'i and Brachiolaria from the funda-
mental form of the Echinoderm larva (diagrumafter Joh. Mullbb, from Balfoub's
Comparative Emhryoloijy). The broad black line indicates the ciliated bund, the
shaded part the depressed portion of the surface. (Comp. footnote on p. 417 in
regard to the orientation of the figures.) aii, onus; 111, mouth.

The opinion which Semon advances concerning the origin of the Bi-
jniinaria larva does not agree with the description of it which we have
just given. Semon finds in the Echinoderm larva a ciliated band surround-
ing the mouth, a loop of which occasionally extends into the oesoijhagus
(as in the Auricularia). This " adoral " ciliated band has nothing to do
with the continuous ciliated band, but exists independently of it. In the
Bipinnaria the " adoral " ciliated band of Semon is said also to supply
that part of the ciliated band which we called the adoral part, so that the
latttr is not, as we described it, to be looked upon as a detached part of
the continuous ciliated band. As long as strict proof of such a mode of
origin is not forthcoming, we are unable to accept this opinion. The
agreement of the preoral area in Auricularia and Bipinnaria is too
striking for one not to assume that its isolation took jilace by means of

ECHINODERMATA

421

a deeper and deei^er incision on the i^art of the lateral depressions. ^
(Comp. also in this connection Figs. 200 and 198, as well as Fig. 199
A and B.) Job. Muller figures Auricularians in which the two depres-
sions almost meet at the anterior end of the larva. Furthermore, the
breaking up of the ciliated band discovered by Semon and the metamor-
phosis of the component parts into the epithelium of the fore-gut men-
tioned by him make the band appear to be more probably an oral ciliary
api^aratus, serving for the capture of food (comp. p. 427).

The Brachlolaria arises from the Bipinnaria of the star-
fish as a subsequent stage by the formation of two addi-
tional processes at the base of the longer (doi'sal) appendage
(Fig. 200 D). In this way are formed the so-called
Brachiolarian arms, which ai'e diffei'ent from the others.
They are not bordered by a ciliated band, but possess wart-
like elevations at the ends, which probably serve the larva
for attachment in later stages.

In the starfishes, too, many exceptions to the typical form
of the larva are found. This is the case in Asterina gibbosa,
the development of which has been made known through
the thorough researches of H. Ludwig. The larva, which
at first is pyriform, acquires a ridge-like thickening, en-

Fis. 201. — A and B, larvae of Asterina gihhosa (after Ludwig). A, a younger
stage seen from in front; B, older stage seen, from the side; Lo, larval organ;
ni.rnoutb.

closing a depressed area, at the anterior end (Fig. 201
Lo). This thickening finally acquires a volume surpassing
that of the rest of the larval body (Fig. 201 B). The
peculiar organ consists of two lobes, and since the anterior

1 [This interpretation is confirmed by the investigations carried on
since the above statement was written. — K.]

422 EMBRYOLOGY

one of these occasionally divides, a certain resemblance
to the Braehiolarian arm is pi'oduced. For in this way
there arise two lobes, which lie symmetrically in front
of the mouth, and a third unpaired lobe, which is farther
removed from the mouth-opening. But the arms of the
Brachiolaria which lie in front of the mouth have a similar
position, and therefore Ludwig homologizes the larval
organs of Asterina with the latter. The organ is muscular,
and serves the larva for attachment. Similar, but multifid,
larval appendages have also been described by Saks, Joh.
MtJLLER, Agassiz, THOMPSON, and others, for Echinaster and
Asleracanthion (AI idler i) .

Nothing more detailed has been learned of a vermiform
Echinoderm larva described by Joh. MOlleb, which was
divided into five segments by transverse constrictions, and
to the nndei'-surface of which a five-lobed star was attached.
According to Joh. Muller's account, it develops into a star-
fish.

Ophiuroidea. — The Pluteus larvoeof the Ophiuroidea e.x-
hibit an es.sentially different shape from that of the larvae
thus far considered. But they also arise from the same
fundamental form. As in the cases previously considered,
there is a continuous ciliated band, which boi'ders the deep
depressions of the body (Fig. 202 A). The subsequent
characteristic shape of the larva depends, in the first place,
upon the fact that the anal ai-ea increases considerably in
extent, while the preoral ai'ea, on the contrary, almost
entirely disappears (Fig. 202 i>). Apart from this, the
shape of the larva is determined by the long processes into
which its peripheral portions grow out (Fig. 202 G and D).
These are bordered by the ciliated band, which is still, and
always remains, continuous. As the form of the Plutens is
reached, the anal area becomes pointed (Fig. 202 D). The
two ventral, posterior arms are especially well developed.
They are also significant for the reason that they are always
present, whereas the other arms may be more or less sup-
pressed.

The Pluteus larvae, unlike the Aaricularia and Bipinnaria,
possess a calcareous skeleton. As early as the gastrula

ECHINODERMATA

423

stage of the larva two triradiate calcareous bodies are
secreted by tlie mesenchyma cells in the vicinity of the
blastopore. These soon elongate owing to the activity of
the mesenchyma cells. They increase considerably in
volume, branch, and send out rod-like processes into the
arms as soon as the latter are developed. The calcareous
i-ods fuse at the posterior end of the larva, and appear to be
united here by a kind of ring having a transverse position
(Fig. 211, p. 438). In this way there arises an excellent
supporting apparatus for the larva and its appendages.

Fift. 202.—^ to D, evolution of the Oplduvan Pluteus from the fundamental form
of the Echinoderm larva (diagram after Joh. Mijllkk, from Balpoub's Comjiarative
Emhi-yology). The Ijroad black line indicates the ciliated band, the shaded area
the depressed part of the external surface. In regard to the orientation, what is
said on p. 417 (footnote) applies here also, an, anus ; m, mouth. The other letters
refer to the nomenclature of the appendages, which is not further considered here.

AmpJiiura squamata develops without any real meta-
morphosis. Amphiura is viviparous. The earliest develop-
mental processes are nearly the same as those that we have
already learned about. There arises an oval embryo, which
assumes a bilaterally symmetrical shape, but which does
not develop into a ciliated larva, passing, on the contrary,
directly into the five-rayed star. The young, even at the
time when they come into the world, exhibit the organization
of the parent. It is interesting, however, that, despite this,
the larval skeleton of the Pluteus is begun in the embryos.

4-24

EM BR VO LOGY

This points to the fact that even in Aviphiura or its an-
cestors a metamorphosis took place, which, however, was
abandoned owinsr to a change in the mode of life.

Echinoidea. — The larva resembles that of the Ophi-
iiroidea, and, like it, is called a Phiteus. In it also the anal
area preponderates on the ventral surface. The ciliated
band is simple. A calcareous skeleton is found inside the
body and its appendages (Figs. 204 and 212, p. 440).

The derivation of the Echinoid Pluteus from the funda-
mental form is nearly the same as in the Ophiurans, and is
explained by the diagrams of Fig. 203. The shape of the
different sea-urchin larvas is quite varied, according to the
greater or less development of the arms. The larvee of

Fig. 203. — Evolution of the Echinoid Plutcns from ttie fundamental form of the
Kchinoderm larva (ciiagram after Joh. Mum.bk, from Bai.fouk's Comparative Ein-
hryology). For further i)articulars consult the explanation of Fig. 202.

Echinus and Spatangus may be distinguished as particularly
characteristic forms. On the anal ai'ea of the former, after
the development of all eight processes, the so-called ciliated
epaulettes make their appearance (Fig. 202 B). These are
two pairs of ciliated projections of the body, which lie on
either side immediately behind the ciliated band, but isolated
from it. According to A. Agassiz, they should be interpreted
as detached parts of the ciliated band.

The larvae of Spatangus do not possess the ciliated epau-
lettes, but have throe processes on the anal area (Fig. 203

ECHINODERMATA

425

E), which are supported by calcareous rods, like the other
processes of the body. In the Plnteus of' Arbacia there are
only two processes on the anal area (Fig. 204), but they are
particularly long. Furthermore, in addition to the ordinary
Pluteus-arms, it possesses two pairs of auricular processes
(Fig. 204) which, like the arms, are surrounded by the
ciliated band (JoH. Muller, Fewkes). A pedicellaria of the
future sea-urchin can already be recognized on the anal area
of this larva.

Fig. 20i.— Pluteus larva of Arbacia pustulosa (after Joh. Muller). P, pedi-
cellaria. The skeletal rods are dark.

The skeletal parts are developed very early as products of the me-
senchyma (Selenka, No. 53 ; Ludwig, No. 34). There is first secreted
between two cells a calcareous concretion, which soon enlarges and
becomes triradiate. The skeletogenous cells then migrate along their
respective rays, gradually moving farther away, while they continue to
secrete calcareous salts. In this way finally arise the long skeletal rods,
which may be many times branched and perforated like a network
(Figs. 204 and 212, p. 440).

The typical larval form may also be omitted in the sea-
urchins. Thus A. Agassiz (No. 4) describes a viviparous

426

EMBRYOLOGY

Spatangoid, Hemiaster austrah's, the eggs of which develop
within the ovary, and then pass into a kind of brood-cavity,
which lies over the ambulacral furrow and is formed of
close-set and connivent spines. Here the young sea-urchins
undergo direct development.

IV. THE METAMORPHOSIS OF THE LARVA INTO
THE ECHINODERM.

The metamorphosis of the larva into the Echinoderm takes
place most simply in the Holothurioidea, and therefoi-e we
consider this group first.

Fig. 205. — A and B, metamorphosis nf the Auricularia larva of Synapta ^ligit^lta
into the pupal form (after Semox). a, anus; cd, proctodoeum ; cut, enteroccele ;
ki; calcareous wheels; m, oral funnel; mg, stomach; 11, nerve bands; u', water-
vascular ring with the evaginations (tentacular and radial vessels).

Holothurioidea. — The metamorphosis of the Auricularia
into the Halothurian is manifested in the external shape of
the larva by the gradual disappearance of its lobular pro-
cesses and the alteration of the ciliated band, which breaks
up into several pieces (Fig. 205 A and B). The larger
number of these pieces alter their positions by acquiring
a transverse position in place of a longitudinal one (Fig.
205 B). At the same time the protuberances of the larval
body disappear, and it assumes more of a cylindrical form,
whei-eby, according to Sicmon, its circumference is strikingly
diminished. Finally the different pieces of the ciliated

ECHINODERMATA

427

band grow together into five rings, wliicli surround the
larva like the hoops of a barrel (Fig. 206). This is the
so-called pupal stage, which is also assumed by some Holo-
thurians {e.g. Gucumaria) without passing through the
Auricularia form. This stage is remarkable owing to its
resemblance to the larva of Antedon, with which it has in
common even the number of the ciliated rings.

o

According to Sejion, we may imagine that the rearrangement and loss
of continuity in the ciliated band are the result of the migration of the
band together with the adjacent body epithelium, probably in con-
sequence of internal processes of growth.

In the metamorphosis of the ciliated band we have not
yet considex-ed the parts lying near the mouth, which do
not share in the formation of the external ciliation of the
pupal stage. Parts of the longitudinal and transverse
portions of the ciliated band approach very close to the
region of the mouth-opening (Fig. 205 A). After the
breaking up of the ciliated band, four parts can be dis-
tinguished, which closely surround the mouth, and finally
form a continuous ring about it. They gradually move
more into the infundibular depression which leads to the
mouth-opening. By a marked narrowing of the funnel
they come to lie inside the larva, and are employed to
clothe the tips of the five anteriorly directed evaginations
of the hydrocoele (therefore for the formation of tentacles)
(Fig. 205 B). The nerve bands have moved down into the
funnel even before the parts of the ciliated band have, and
are to a certain extent forced down by them (Fig. 205 A,
n), for the nerve bands occupied a position nearer to the
mouth-opening than the ciliated bands. Their free ends
then unite at the bottom of the f annel and there form the
nerve-ring of the Synapta (Semon). It was precisely those
four parts of the ciliated band that moved into the funnel
which were united to the nerve bands by means of nerve
fibres. Probably this connection is retained during the
metamoi'phosis, and as soon as the ciliated band has covered
the five tentacles the five large tentacular nerves are es-
tablished on the nerve-ring, whereas the five radial nerves

428

EMBRYOLOGY

do not bud forth from it until later (Semon). The parts of
the nervous system, Avhich at first lie superficially, are
finally overgrown by the rest of the ectoderm, and since
mesenchyma cells crowd in over them, they come to lie at a
still greater depth.

The ciliated band, which, according to Semon, encircles the mouth,
changes its position during the raetamoriihosis of the larva by coming to
lie wholly in the stomodsBum. Here its cells are said to spread them-
selves out on the wall and constitute the epithelium.

Our previous account of the internal organization of the
Holothurian larva was confined to the formation of the
intestine, the two enterocoelic sacs, which extended between

the intestine and the body-wall,
and the hydrocoele. We saw
the latter growing around the
stomoda?um of the larva as a
five-lobed structure. It already
represented the fundament of
the water- vascular ring and the
five tentacles of the Holothurian.
Five secondary evaginations of
the water-vascular ring arise
between the five primary ten-
tacles, and at first are also
directed upwards (Fig. 205 B).
Later, however, five of the ten
evaginations of the water-vas-
cular ring now present bend
over the calcareous arches and
grow out backwards (Figs. 206
and 207), so that there are now five tentacles and five radial
vessels (Semon). The question now arises whether it is the
five vessels first developed (the so-called primary tentacles)
which bend over backwards, and thus correspond to the
i-adial vessels of other Echinoderms — as seems most natural,
though this has been denied — or whether it is the five vessels
of the second group, which correspond to radial vessels. As
to the homologies of the ambulacral vessels in the different

Fig. 206. — Pupal stage of the
larva of Synapta digitata (after
Skmon). ed, proctodfeum ; ent, en-
terocoele; in, oral funnel; u-, water-
va.scular ring with evaginations
forwards (tentacles [tj) and back-
wards (radial vessels and Polian
vesicles [ji]).

ECHINODERMATA

429

divisions of Echinoderras, which apparently seem to be so
clear, the opinions of authors are nevertheless at variance
(comp. Semon, No. 55).

[Through Ludwig's (No. XV.) recent investigations of Cucumaria
planci, these conditions have been satisfactorily elucidated. The five
invaginations of the water-vascular ring first formed really produce
the radial vessels. They are, it is true, at first directed forward, but
soon bend backward, thus marking the radii. The tentacular vessels
do not arise directly from the ring-canal, but branch off from the radial
canals ; however, every radial canal does not give rise to a tentacular
vessel, the latter being distributed unsymmetrically to three radial
canals only. — K.]

The internal condi-
tions are more evident
in Fig. 207, a longi-
tudinal section of the
pupal stage of Cucn-
nnaria (after Selenka).
The tentacular evagi-
nations are seen coming
off from the water-
vascular ring forwards,
and posteriorly those of
the radial vessels. The
Polian vesicles also take
their origin as evagina-
tions of the water-vas-
cular ring. In the stasre
under consideration the
ring is still in connec-
tion with the outside
world by means of the
stone canal and the
dorsal pore. This con-
nection is afterwards
broken, since a cluster
of mesenchyma cells
subsequently applies
itself to the stone canal

Fio. 207. — Longitudinal section of a larva of
Cucumaria doKoIitm, somewhat diagrammatic
(after Selbnka). A, anus; Am, ambulacra!
(radial) vessels ; D, intestine ; E, enterocceles ;
F, feet ; M, mouth ; P, dorsal pore, leading
through the stone canal to the water-vascular
ring ; T, tentacular vessels ; Wr, water-vascular
ring.

t

430 EMBRYOLOGY

at about the middle of its course, and here deposits a semi-
lunar calcareous i-idge, which must be looked upon as cor-
responding to the madreporic plate. Where it rests upon
the stone canal, the latter is cut through by a constriction,
and one half henceforth hangs down from the ring-canal
free in the body cavity, whereas the other half is gradually
obliterated.

Externally the pupal stage now approaches more the
adult Holothurian, owing to the fact that the first two feet,
the development of which is to be traced to evaginations of
the corresponding radial vessel, make their appearance on
the posterior part of the ventral surface (Figs. 207 F and
208/). At the rarae time the tentacles also advance in their
development. We saw in Synapta that a part of the ciliated
band moved down into the oral funnel to supply the ecto-
dermal covering of the tentacular vessels, which consists
partly of sensory cells. The oral funnel then closed to an
extremely narrow fissure, and there was thus formed a
kind of vestibule (corap. the corresponding processes in the
development of the vestibule of Antedon, p. 446). The
tentacles, to which the nerve-ring is still joined, lie in the
vestibule. The nerve-ring lies at the point where the calca-
reous ring,^ — the supporting apparatus of the tentacles, con-
sisting at first of five and later of ten I'ods, — is attached to
the tentacles. When the tentacles have reached the neces-
sary development, they are extended out through the fissure,
which widens again (Fig. 208), and the young Holothurian
now moves both by means of those ciliated bands which still
remain, and by means of adhesion with the tentacles and

feet when the latter are present.
In Synapta, as is known, the
feet are not developed, even the
radial vessels degenerating.

The shape of the Holothurian
would thus be attained if the
^"^' -^ young animal did not lack the
Pin. 208.— HoiothnriiuiiarvinviUi larger number of tentacles and

ciliatc.1 bands, extemled tentacles f^^j. ^^^ jf j^^ body-COVering
(T), anri developing feet (;) (after .

joH. mcllkk). already possessed its permanent

ECHINODERMATA 431

structure. Additional tentacles and feet are developed in
the san;e way as those which we have already learned about,
namely, by evaginations of the parts of the water-vascular
system already formed. The ciliated bands are said by
Semon to disappear owing to their cells spreading themselves
out over the entire surface of the larva, and substituting
themselves for the originally flat body epithelium, which now
makes way for a thick epithelial layer. With progressing
growth the mouth and anus of the larva are shifted fi-om
the more ventral position toward the anterior and posterior
ends of the animal respectively.

Now that we have followed the Holothurian larva in its development
as far as the young animal, there remain for consideration only a few
important internal developmental pi'ocesses, which relate to the deriva-
tion of the middle germ-layer. As we were obliged in the previous
descrii^tion to take into consideration various forms, so are we under the
same necessity in the following statements, which for the greater part we
take from the works of Selenka and Semon.

Having treated of the origin of the mesenchyma cells in the first
division of the chapter, we intentionally left the subject for the time
being. By far the greater part of these cells become connective tissue.
Whereas they are usually api^lied to the inner surface of the ectoderm as
isolated cells, they are accumulated on both sides of the proctodeum in
larger groups, which give rise to the calcareous spherules and calcareous
wheels. They also occur in large numbers in the region of the stone
canal and of the water-vascular ring, forming in one place the well-
known calcareous deposits and in the other the calcareous ring surround-
ing the oesophagus. The mesenchyma cells give rise by multiplication
to a kind of cutis under the entire ectoderm (Metschnikoff, No. 37).
Underneath the ciliated baud they form, according to Semon, groove-like
sheaths, which probably serve as supports for the ciliary apparatus.
Fissures in the mesenchyma are said to give rise to the blood-vessels of
the Holothurian. Thus the vessels accompanying the intestine first
make their api^earance as lacunar spaces in the mesenchyma lying
dorsad and ventrad of the intestine. The blood cells, on the contrary,
are said to have been detached from the walls of the hydro-enterocoele
and to have taken part in the formation of these vessels. These free
cells, which are found in the body cavity as well as in the ambulacral
vessels and blood-vessels, would therefore, according to this view, not
arise from the primitive mesenchyma (Semon).

Of all the musculature only that of the stomodseum originates from
the mesenchyma. It persists, being carried over from the larva to the
young animal (Selenka). The rest of the musculature arises partly

432 EMBRYOLOGY

from the water-vascular system, the epithelium of which produces an
external layer of contractile cells. According to Selenka, the live power-
ful longitudinal muscles of the Holothurian arise in this way from the
radial ambulacral vessels. This, however, is denied by Semon. Accord-
ing to him, the circular and longitudinal muscle layers of the body-wall
and the intestine take their origin from the enterocoele. Its two sacs
have expanded considerably and applied themselves to the wall of the
body and that of the intestine as the somatic and splanchnic layers,
thus giving rise to the musculature and epithelial lining of the body
cavity. A mesentery is developed on the dorsal and another on the
ventral side of the intestine. While, however, the ventral one is broken
through, and the two enterocceles become confluent here, the dorsal one
persists, and thus throughout life indicates the origin of the body cavity
from two separate sacs, as well as the bilateral plan of the body in general.

The fundament of the nervous system has already been discussed. As
regards the development of the genital system, authors are not able to
make any very trustworthy statements.

[The organogeny of the Holothurians has received a new and thorough
treatment in Ludwig's memoir on Cucumaria planci. — K.]

Asteroidea. — Whereas earlier investigators were inclined
to believe that the starfish arose as a bud, so to speak, on the
larva, we know to-day that even here there is a metamorphosis
of the larval body into that of the adult animal. It is true
that certain modifications make their appearance in this
connection, for the body of the Echinoderm is at first estab-
lished in only a comparatively small portion of the larval
body. It is but gradually that the larger part of the larval
body is employed in the formation of the starfish. la
certain cases, it is true, this latter process seems to be
omitted, and the Echinoderm then takes its origin from only
one part of the body of the larva. In the latter method of
development the young starfish is detached from the body of
the larva, and the latter is said to be able to live for a con-
siderable time (JoH. MiJLriER, Koren et Danielssen). Such
a condition might cause the process to be looked upon as
budding. This view, however, is disproved by the meta-
morphosis of the entire body of the larva in other starfishes,
which will be described later.

The metamorphosis of Bipinnaria or BracJdolaria into the
starfish has been studied most exhaustively by A. AoAssiz
(No. 1) and Metschnikoff (No. 37),

BCHINODERMATA

438

The earliest trace of the starfish is formed in the posterior
part of the larval body. The five-rayed f andament of the
water- vascular system lies at the left of the stomach (Fig.
209 -ff), while on the right side of it an accumulation of
mesenchyma cells takes place, in which the first skeletal
parts are already secreted (Fig. 209 K). The ventral or
ambulacral side of the starfish is indicated by the water-
vascular rosette, and the dorsal or antambulacral side bv

B-y

Fig. 209.

Fig. 210.

Figs. 209 and 210. — Bipinnaria larvae with the fiindament of the starfish in
different stages of development (after Joh. Mullke). A, anus ; D, intestine ; H,
water-vascular rosette ; K, calcareous secretions lying in the fundament of the
antambulacral surface of the starfish j M, mouth ; p/, preoral area of the larva ;
W, ciliated band.

the skeletal structures. The two sides are established inde-
pendently. Even at a very early period the five-rayed
structure of the starfish can be recogrnized in them. This
is indicated on the ambulacral surface by the fact that five
folds arise in the larval skin immediately over the radii of
the water-vascular rosette, while on the antambulacral sur-
face the calcareous rods are deposited in the form of a
pentagon. On both sides a thickening of the epidermis is
connected with the diffei'entiation of the cutis-forming

K. H. E.

F F

434 EMBRYOLOGY

mesencLyma cells at the sides of the stomach. The com-
prehension of this process is rendered more difficult by the
fact that the ambulacra! and antambulacral surfaces are not
parallel, but nearly at right angles to each other. Between
the two lies the capacious stomach. In Fig. 209, which,
however, corresponds to a somewhat earlier stage, the water-
vascular rosette (H) is seen to be partly covered by the
stomach, whereas this in turn is partly covered by the funda-
ment of the antambulacral surface. The latter develops
further in such a way that from the calcareous concretions
a number of plates are formed (comp. infra), which cover
a pentagonal area. This grows out then into five processes,
thus establishing the dorsal surface of the arms, upon which
there appear wart-like elevations, from which the spines
arise later.

At this stage the starfish already approaches the shape of
the adult animal, at least as far as regards its dorsal external
surface, and is seen attached to the larva, the posterior
end of which it has quite absorbed (Fig. 210). Its anterior
portion is still well preserved. Now, however, degeneration
also begins here. It gradually shrinks, its substance being
consumed by the phagocytic mesenchyma cells, undergoing
intracellular digestion, and being doubtless employed in the
formation of the new body (Metschnieoff, No. 40). At the
same time with these processes the size of the stomach
decreases, as a result of which the two surfaces of the star-
fish, which were separately developed, are able to approach
each other. They cover each other, and finally fuse. The
hitherto unclosed water- vascular rosette grows around the
oesophagus, and its radii elong.ate to form the ambulaoral
vessels, which in their turn give rise to the feet. In this
process the distal end of the vascular fundament becomes
the so-called tentacle, but the feet are established laterally
in pairs. The youngest feet are always found next to the
tentacle, therefore at the tip of the arm, whereas the oldest
are crowded toward its base. The eye makes its appearance
as an accumulation of red pigment at the base of the tentacle
at a very early period.

Even before this, there have been produced on the antam-

ECHINODERMATA 435

bnlacral surface secretions of calcareous salts, which at first
formed delicate rods and subsequently united into reticular
plates. Eleven such plates can soon be recognized, a central
one and (arranged about it in a circle) two rows of alter-
nating plates, the fundaments of the radial and interradial
plates. One of the former, which at first lies at the left next
to the dorsal pore, subsequently grows around it, and thus
becomes the madreporic plate (LuDWiCx). According to
LuDWiG, the ambulacral or vertebral plates of the arms
make their appearance very early as five pairs of calcareous
bodies at the base of the five hydrocoele pockets. They
therefore have even now the position which they retain
afterwards, namely, on the outer side of the future am-
bulacral vessel. The other skeletal pieces of the ai-m are
not developed until later.

The question now is. What relation does the larval intes-
tine have to the newly formed starfish ? The older state-
ments are not precise on this point ; for this reason we
adhere to the recent investigations of Ludwig on Asterina
gibbosa, a form, however, which is developed neither from a
Bipinnaria nor from a Brachiolaria (comp. supra, p. 421).
Yet in this species the two surfaces of the starfish are
established independently, and afterwards unite as described
above. From this, one may perhaps conclude that the
processes in question resemble those in the typical larva.
In Asterina the stomodaeum of the larva separates from the
vstomach and hangs down from the larval mouth as an
internal blind rudiment. For a time the intestine is without
any connection with the outside world. The permanent
mouth of the starfish is then developed by an outfolding of
the stomach, growing out toward the body-wall and finally
breaking: through to the outside world. The stomach itself
is transmitted to the starfish. It subsequently acquires
five outpocketings, which bifurcate at their tips, the funda-
ments of the five pairs of intestinal C83ca. The larval anus
is obliterated even before the union of the intestine with the
mouth takes place, and the new anus does not arise until
after the formation of the mouth-opening. It breaks
through at the margin of the central plate, between it and

436 EMBRYOLOGY

an interradial plate. According to the observation of
Agassiz, the mouth arises by a shortening of the long
oesophagus, and the anus persists.

The condition of the dorsal pore and the stone canal, as described by
LuDwiG for Asterina, is interesting. In this form, after the separation
of the enteroccele and hydrocoele, a canal is developed on the latter,
which, attached to the water-vascular rosette, opens into the enteroccele
quite near the place where the dorsal pore is connected with the entero-
ccele. This is the stone canal, which does not unite with the dorsal
pore until later. Thus there is a stage in which the stone canal does
not open directly from the water-vascular ring to the outside world, but,
on the contrary, leads into the body cavity. This, however, is in turn
connected with the outside world by means of the dorsal pore. Ludwig
compares this condition to that which he described for the Crinoids (Nos.
30, 32). In them the water jJenetrates by means of the pores in the cup
(Kekhporen) into the body cavity, to be taken from there and con-
ducted into the water-vascular ring by the stone canals, of which there
are several hanging down from the water-vascular ring into the body
cavity.

The earliest fundament of the blood vasrular si/stem arises, according
to Ludwig, at the place where the intestine grows out to form the oeso-
phagus. In the mesenchymatous layer lying between the walls of the
hydrocoele, the enteroccele, and the intestine, there is formed a fissure,
which exhibits a lining of very flat cells. This is the fundament of the
first blood-vascular ring. The structure ordinarily described as the
central i^lexus of the blood-vascular system also arises as a fissure next
to the stone canal (comp. General Considerations, p. 456).

The nervous system of Asterina is first established in the form of a
circular epithelial thickening, which surrounds the region of the future
mouth-opening. Its development is certainly similar to that of the
central nervous system of the Holothurioidea, with which we are already
familiar.

The metamorphosis of those starfish larvis which differ from the Bipin-
naria- and Brachiolaria-forms, as, for example, that of Asterina f/ibbosa
(Fig. 201, p. 421), is likewise accompanied by the transmission of the
greater part of the larval organs to the starfish (Ludwig). Only the mouth
and anus have to be formed anew, and the larval organ suffers degenera-
tion, being gradually absorbed. Here also the starfish arises from an
ambulacral and an antambulacral fundament, which at first are separate.
The development of Pteraster vtilitaris seems to resemble that of
Asterina (Kouen kt Danielssen). In this starfisii, however, a kind of
brooding occurs ; a membrane stretches out over the spines on the back
of the animal, forming a brood-chamber. Into this the eggs pass, and
there develop into the larva; and young starfishes.

ECHINODERMATA 437

Ophiuroidea. — Although the larvae of the Ophiuroiclea
and Asteroidea are so different in shape, their metamor-
phosis presents a certain resemblance. In the Pluteus larva
too the ambulacral and antambulacral surfaces are begun
separately, and only after their subsequent union give rise
to the complete star (JoH. Muller, Metschnikoff). In the
Pluteus the five-rayed water-vascular rosette, which opens
to the outside world on the dorsal side of the larva, lies
A^entrad of the oesophagus. It is on this that the first steps
in metamorphosis are manifested, for it is over its different
radii that the mesenchyma and contiguous larval skin be-
come thickened. In this way the fundament of the ambu-
lacral surface of the star is produced. Each of the five
radii of the rosette, which repi^esent the future ambulacral
vessels, produces two lateral evaginations ; thus the larvae
acquire the fundaments of the first feet, which are soon
followed by a second and a third pair, etc. While these
processes are taking place on the ventral side of the Pluteus,
the first indications of the antambulacral surface of the
Ophiurau have made their appeai"anee on its dorsal surface,
in the form of five outgrowths of the larval skin. They are
arranged in a line, so that three of them lie on the larger
and two on the smaller part of the umbrella. The five
skeletal pieces arise in them as products of the mesenchyma
cells. Although the principal parts for the production of
the star are now present, nevertheless a total rearrange-
ment must take place to accomplish its formation. This
begins by the growth of the hei'etofore semicircular water-
vascular rosette, together with its appendages, around the
oesophagus, to form the water- vascular ring. With the
closure of the ring, the two vessels which at first were
farthest apart naturally have come to lie close together, and
at the same time the form of a star has now been reached,
first on the ambulacral surface. This is, however, not the
case on the antambulacral surface. Here also the dermal out-
growths (dorsal) undergo considerable changes in position ;
but it is not until the larval appendages begin to degenerate
that the antambulacral parts cover the ambulacral, and thus
complete the star. The internal parts of the larva — the

438

EMBRYOLOGY

enterocoelic body cavity, the intestine, etc. — then form a part
of the permanent star ; the mouth is said to persist, whereas
the anus disappears. Upon the completion of these processes,
which result in the establishment of the- permanent shape
of the animal, the calcareous skeleton of the Plnteus disin-
tegrates. The rods break up into pieces; as a result of this,
the arms collapse, and the skeleton, together with the larval
body, appears finally to be resorbed by the young Ophiuran.

'"»l^^^

Fig 211.— Pluteus larva with the fun-
datuent of the Ophiuran (after Joh.
Mulleb). The rods of the larval skele-
ton are dark.

Like the arms of the starfish,
those of the Ophiuran grow at the
tips, with the exception of the
terminal pieces, which correspond
to the skeletal i^ieces first formed
on the dorsal surface. Therefore
the new pieces are interpolated
between the terminal pieces and
the adjacent ones. The skeletal
parts thus follow a law quite
similar to that of the feet, the de-
velopment of which always takes
place between the (terminal) ten-
tacle and the pair of feet next to
it. The origin of the arm-plates
is interesting ; according to LuDWia
(No. 34), they result from the
fusion of two calcareous plates ly-
ing on either side of the median
line of the arm.

Echinoidea. — According to Mktschnikoff's description
(No. 37), a difference exists between the metamorphosis of
the Echinoidea and that of other Echinoderms, inasmuch as
there is developed an invagination of the larval skin, at the
bottom of which the earliest fundament of the body of the
sea-urchin makes its appearance. Thus it happens that
the earliest fundament is not exposed, but is covered by a
fold of the larval skin as though by an amnion. Since, how-
ever, the larval skin here too becomes directly converted
into that of the sea-urchin, this difference does not appear
to us to be in any way important.

The processes of the metamorphosis of the Plnteus into

ECHINODERMATA 439

the sea-urchin are as follows : inside the Phdeus of Strongylo-
centrotus lividus, which is provided with four arms, we find
nearly the same conditions that have been described apropos
of the development of the enterocoele and hydrocoele. The
enterocoelic sacs lie to the right and left of the stomach ; the
hjdrocoele lies over the left one of these, and has the form
of a retoi't, the neck of which opens to the exterior on the
back of the larva, somewhat as in Figs. 212 and 213,
figures of a Spatangoid ; the conditions in these forms are,
however, somewhat different, as will be mentioned farther
on. Later, when the Pluteun has become six-armed, an
invagination of the outer skin is formed over the hydrocoele
(Fig. 212). This arises from a thickening of the epidermis,
which gradually sinks in and finally rests with its bottom
upon the hydroccBle.^ The thickened discoid bottom of the
dermal invagination is the earliest fundament of the lower
(oral) surface of the body of the sea-urchin (called " Echinoid
disc" by JoH. Muller). The much thinner lateral jDarts of
the invagination overlie this as an amnion-like covering
(Fig. 213). The opening of the invagination has narrowed,
but persists, whereas in the Spatangoids other conditions
subsequently make their appearance (comp. infra). The
hydrocoele now grows out into five processes, and the
Echinoid disc does the same, by developing a dermal cover-
ing over each of the five hydrocoele processes. In this way
the first five feet of the sea-urchin arise. They extend into
the cavity of the invagination, almost filling it.

During the changes described in the region of the Echinoid
disc, the first indications of the dorsal surface of the sea-
urchin also become noticeable. Two roundish dermal eleva-

1 Figures which Fewkes (No. 13) gives of the developmental stages of
Echinarachnius parma may contirm Metschxikoff's description, although
this cannot be gathered from the text of the work. Likewise it seems to
us from the figures of Colton and Gakjian (No. 11) that the metamor-
phosis of Aiitacia is like that described by Metschnikoff for Echlnoids
and Spatangoids. A cavity appears on the Platens, in which the first
formed feet become visible. The work of Colton and Gakman was un-
fortunately not accessible to us, and is known only through the descrip-
tion of Bkooks {Handbook of Invertebrate Zoology, Boston, 1882),

440

EJI BRYOLOGY

tions are developed on the umbrella of the Pluteus, one on
the dorsal surface and one on the anal area. Each of these
soon assumes a trilobed form, and they prove to be the two
first pedicellaria of the sea-urchin. Fig. 214 shows this
condition in another sea-urchin {Arhacia pustule so) } With
the progress of development the disc continually increases

Fig. 2

Fig. 212.

Figs. 212 and 213.— Parts of a S)taiangoi& Pluteus (after Metschnikoff). D,

intestine; Ei, invagination of the larval skin, which in Fig. 213 covers the

hydroccele (H). The latter opens to the exterior by means of the dorsal pore. H,
hydroccele ; P, dorsal pore ; Sfc, larval skeleton.

in circumference, and at the same time the opening of the
invagination also widens again. The contractile feet are
finally extruded through the latter, and are now seen to

* [Very thorough and accurate investigations of these features of de-
velopment have recently been carried on by Thkel (No. XXIX.) on
Echinocyamus, a form in which the larval development and the meta-
morphosis could be established almost without a gap. Unfortunately
his fine results cannot be stated here in brief form, and we are compelled
to refer to the original paper. — K.j

ECHINODERMATA

441

execute tactile movements. At this time the larval skeleton
begins to break np, and the arms of the Fluteiis degenerate
as a result of this (Fig 215). The body thereby assumes
nearly the form of a hemisphere with the disc as the base.
The circumference of the disc has increased more and more,
and correspondingly the opening of the invagination has
also become enlarged. The amnion-like envelope meantime
gradually diminishes in prominence ; at length it forms only
a circular fold, surrounding the circumference of the disc,

Fig. 214.— Pluteus larva of Arhacia pustulosa (after Joh. Mullee). The skeletal
rods are dark. P, pedicellaria.

and finally disappears. Thus the amnion seems to become
directly converted into the skin of the sea-urchin, and, in
fact, would seem to supply that part of the skin which unites
the sole-like ventral surface with the arched back. Fig.
215 represents a young sea-urchin which still possesses, in
addition to the feet, some of the Pluteus arms. Its feet are
already employed as locomotor organs. In Fig. 216 the
young sea-urchin spines are already seen making their

442

EMBRYOLOGY

appearance by the side of the pedicel lariao. They arise as
evaginations of the skin in which reticulated calcareous rods
are deposited. The first of the dorsal plates to make its
appearance is the central one, which is perforated by the
anus. Other plates are then secreted about it, in a spiral
line, i.e. in such a way that the newly arising plates crowd
away the older ones from the anal plate, since they are
interpolated between it and the older plates (Agassiz).

Fig. 215. — Young sea-urchin [Arhacia punclulatn) with parts of the Pluteus Inrva
attached (after Colton and Gabman, from Brooks's Handhoolc). a, partially de-
generated arms of the Pluteus ; /, feet ; St, spines.

The internal larval organs become a part of the sea-urchin, although
a new oesophagus is said to be developed, which is not grown around
by the water-vascular rosette, but grows through the previously formed
water-vascular ring (Uuky), — a condition, therefore, somewhat different
from that which we observed in the development of the other groups of
F.chinndeinm. The dorsal pore persists, and also its connection with the
water-vascular ring by means of the stone canal. The outgrowth of the
water-vascular rosette into the ambulacral trunks appears to take place,
according to Agassiz, in the same way as in the starfishes, for new feet
are continually interpolated between the terminal foot and the adjoining
pair.

ECHINODERMATA

443

In the metamorphosis of the Spatangoid Phiteus into the sea-urchin
the invagination is said by Metschnikoff to close. At the bottom of it
the earliest fundament of the sea-urchin then makes its appearance.
Furthermore the " amnion " is said to become detached from the larval
skin. In the protrusion of the feet the amnion, as well as part of the
larval skin, would therefore have to be broken through.

Crinoidea. — We left the larva of Antedon rosacea at a
stage in w^hich the nearly ovate form exhibited slight curv-
ing toward the ventral side. The further development is
characterized by the fact that the larva abandons its free
life and grows into an attached stalked form. It therefore
passes through a stage in which it resembles a stalked
Crinoid. This is known as the pentacrinoid stage. Traces

Fig. 216. — Young sea-urchin {Arhacia pwstiilosa) with degenerated Pluteus arms
attached (after Joh. Mullbb). /, feet; P, pedicellarise; St, spines.

of this stage are already shown in the free-swimming larva
through the fundaments of the skeleton, which make their
appearance in the mesenchymatous tissue of the larva.
They are first seen as small granules, which, however, soon
enlarge into triradial and quadriradial forms, and finally
become fenestrated plates (Fig. 217). Two rows of five
plates each can be distinguished, — the oralia and hasalia con-
stituting the calyx, — and a piece lying below these, the future
terminal plate of the stem (Figs. 217, 221, and 222, p. 450).
According to BuRY, it is this plate of the skeleton which first
makes its appearance deep in the body of the larva. Inas-

444

EMBRYOLOGY

much as new segments (the stem-joints) are interpolated
between it and the basalia, it moves farther downward. The
stem-joints take their origin at the base of the calyx. The
youngest, therefore, lie next to this, the oldest, on the other
hand, next to the terminal plate. At first they form annular
plates, but soon alter their shape and become thick segments
by the secretion of rod-like calcareous concretions on both
their surfaces.

Between the uppermost
stem-joints and the basalia
lies a larger skeletal piece,
which has been called the
centrodorsal plate (Fig. 223
cd, p. 451). It forms the
important foundation of the
basal plate of the calyx.
According to Bury, it arises
by the union of several skele-
tal pieces. For three sub-
basal plates make their ap-
pearance below the basalia ;
these at length fuse with one
another into a five-pointed
star and finally unite with
the uppermost joint of the
stem to form the centrodor-
sal plate. This condition is
impoi'tant, because certain
fossil Crinoids (Ichthyocri-
noidc'e) also possess three
sub-basal plates with the

im

Lm.

\

Fir,. 217, — Larva of AixlfAon rosacea,
with ciliated bands and tuft of cilia, as
well as fundaments of the skeletal
plates inside. Qx, pit, by means of
which the larva attaches itself; L\>x,
the so-called larval mouth.

same airangement.

The series of plates of the calyx are at first not arranged in a closed
ring, but in the form of a horseshoe, the open side of which corresponds
to the position of the " larval mouth."

Before the skeleton attains the development described,
the larva has already given up its free life. After about
twelve hours of swarming it attaches itself by means of the

ECHINODERMATA

445

preoral pit (Fig'. 217), which then quickly broadens into an
adhesive disc. At this stage of attachment the larva lies
with its entire venti^al surface on the object to which it
attaches itself. At first it still possesses its typical ciliation,
but that is soon lost. At the same time its shape changes,
the anterior end, with which the larva attaches itself and
which subsequently grows out into the stem, diminishing in
size and the opposite end becoming broader. The club-
shaped larva now rises from its support, to which only the
small end remains united. Accordingly we now designate
the club-shaped portion, which becomes the calyx, as the
upper part, the narrowed portion as the lower part, of the
larva (Fig. 218).

■v-

Fig. 218.— 4 to C, early stages of development of the attached larva of Anteion
rosacea (after J. Baebois). Development of the vestibule (F) by invagination of
the ectoderm {ect). D, intestine; Is, subambulacral, Lv, visceral, body cavity;
S, stalk of the larva ; T, tentacular vessel ; x, stone canal (?).

The most important change which takes place in the
larva after this metamorphosis of its external shape affects
its ventral surface. The wide pit which is found there, and
which is called the larval mouth, becomes obliterated during
the attachment of the larva, but a new invagination of the
ectoderm takes place at the same spot, which, is deeper than
the pre-existing one. Here also, as in the region of the
"larval mouth," the ectoderm is greatly thickened (Fig.
218 4). The invagination soon enters into relationships
with the internal organs, for its upper margin extends out

446 EMBRYOLOGY

toward the upper pole of the larva, and thus the floor of the
pit comes to lie over ag'ainst the internal organs (Fig. 218 B).
At the same time the opening of the invagination nai'rows,
and is finally entirely closed and [the invaginated layer]
detached. In this way the invaginated part of the ectoderm
comes to lie inside as a closed sac, and since it follows still
more the tendency existing from the beginning, it moves
quite to the upper end of tlie larva (Fig. 218 G). This sac
subsequently changes in such a way that its floor overlies
the evaginations of the water-vascular system (tentacular
vessels), and its roof unites with the mesenchyma and the
outer ectodermal lamella to form the roof of the vestibule
(Figs. 219 and 220) — the chamber on the floor of which the
mouth subsequently arises, and the roof of which disappears
to set free the tentacles. However, before the beginning of
these processes, which bring the larva nearer to its per-
manent shape, important changes take place in the internal
organs.

Just as the Antedon larva, with its five ciliated rings, recalls the cask-
like shape of the Holothurian larva, so, too, the development of the
vestibule and the investment of the tentacles by its floor show a certain
resemblance to the formation in the Holothurian larva of the vestibule
in which the tentacles lie (comp. it. 4*27). Here, as there, it is a de-
l^ression of the ectoderm which forms the vestibule and supplies the
external covering of the tentacular vessels. In both cases the process
takes place in the region of the mouth, which, however, exhibits a
different position in regard to the ciliated rings.

We left the internal organs at a stage of development at
which the two enterocoeles and the hydrocoele lay at tlie
side of the saccular intestine. The latter, which at first lies
ventrad of the intestine, moves, with the metamorphosis of
the larva into the pentacrinoid form, to a position over the
intestinal sac (Figs. 218 and 219), and grows out into the
shape of a horseshoe, its two arms finally uniting into a ring.
At the same time there are formed five upward evaginations,
which are covered over by the ectodermal cell-layer which
forms the floor of the vestibule (Figs. 218 C and 219). The
prolongation of the hydrocoele, which was recognizable even

ECHINODERMATA

447

at the time of its establishment, has in the meantime elon-
gated as far as the outer body- wall and fused with it (Fig.
219), forming in this way the stone canal (Barrois). As in
the other Eohinoderms, so also in the Crinoids, at least while
they are young, a communication exists between the water-
vascular system and the outside world ; this fact was estab-
lished by Perrier and confirmed by Barrois.

As is well known, a large number of stone canals hanging clown into
the body cavity occur in the adult Crinoids. Ludwig (No. 32) had
already shown that in the penta-
crinoid stage of the Antedon larva
at first only one stone canal is pre-
sent ; but he believed that this
also, arising from the water-vas-
cular ring, ended free in the body
cavity, wheiice it took up into it
the water which entered through
a pore in the body-wall. This
view corresponds nearly to that
which was defended upon embryo-
logical grounds by Bury. Ac-
cording to him, the free process
of the fundament of the hydro-
coele, which was considered by
Bareois as a stone canal, is rather
a third ccslomic sac. This en-
larges, comes into connection with
the body-wall by means of a pro-
cess (parietal canal), and thus
opens to the outside world by
means of the water-vascular pore.
It is only secondarily then that
the hydrocoele, by means of a
stone canal, is united to this part
of the body cavity. The descrip-
tion of these conditions coincides
with that given by Ludwig for
Asterina, where the stone canal

also opens into the enterocoele, and is connected with the dorsal pore
only by means of this (comp. supra, p. 436). According to Perrier,
the pore described by Ludwig, which lies in one of the oral plates, cor-
responds to the external oi^ening of the stone canal. The stone canal
is said to be easily separated from the pore in dissecting, and then hangs
from the water-vascular ring free in the body cavity.

Pig. 219.— Longitudinal section of an
Antedon larva (after figures by Peeriee).
D, intestine ; is, subambulacral, Lv,
visceral, part of the body cavity ; N,
stalk of the larva ; St, stone canal ; T,
tentacle ; V, vestibule ; Wr, water-vas-
cular ring, from which spring the ten-
tacular vessels and the tstone canal.

448 EMBRYOLOGY

In later stages of the larva still other canals are developed as evagina-
tions of the water-vascular ring and the peritoneum which covers it.
They grow out toward the body-wall and unite with it. At the time
when the larva detaches itself, five such canals are present, all of which
communicate with the outside world. Afterwards, however, the forma-
tion of the evaginations and the pores of the cup is said no longer to
take place simultaneously, so that the former may niultij^ly independently
of the latter, and vice vernd. Thus a condition would arise such as was
described by Ludwig, — cup-pores leading to the interior, and free appen-
dages of the water-vascular ring opening into the body cavity. The canal
which was first formed attains an extraordinary development, and
Perrier regards it alone as homologous to the stone canal of other
Echinoderms, those formed later being of a secondary nature.

The conditions of the body cavity of the Antedon larva are
complicated. The two coelomic sacs, which at first lie at
the right and left of the intestine, subsequently, when the
larva passes into the pentacrinoid stage, arrange themselves
above and below the intestine (Figs. 218 and 219). Perkier
designates the parts of the body cavity as the subambulacral
and visceral portions. Where the two come together there
is produced a mesentery, extending transversely across the
body (Figs. 219 and 220). Furthermore, according to Bury,
two longitudinal mesenteries are foi-raed, owing to the facts
that the two coelomic sacs are (in cross-section) nearly
horseshoe-shaped, and that the two arms of each sac grow
out toward each other. In each case they meet in a longi-
tudinal mesentery, the one belonging to the upper entero-
coele lying in the anal radius and that of the lower (visceral)
enteroccele in the preceding radius (I'eckoned according to
the course of the intestine). As the coelomic sacs enlarge
they apply themselves to the intestine and water-vascular
ring as the splanchnic layer, and to the mesenchymatous
tissue of the body-wall as the somatic layer. The aboral
body cavity, as was pointed out by Goette, sends a process
into the narrow posterior part of the larval body (Fig, 220).
According to Pkrrier, this process consists of both layers of
the mesoderm (Fig. 219), and Goette's conjecture is con-
firmed, viz. that the chambered organ, which in the adult
animal lies within the centrodorsal plate as an important
part of the blood-vascular system, arises from this posterior

ECHINODERMATA

449

process of the body cavity. We shall revert to this part of
the body cavity and its derivatives later.

An entirely clear insight into the structural conditions of the body
cavity, which are obviously difficult to follow, cannot be gained from the
authors' statements (Goette, Pekrier, Barrois,Bury), since they do not
agree. The older statement of Goette, according to which the body
cavity also takes part in the foi'mation of the vestibule, appears in
another light since the descriptions which Barrois and Bury give of

ae.

i'jr~.^.

Fig. 220.— Longitudinal section through the cup of an AnteHon larva, the vesti-
bule of which is still closed (after Goette, from Balfour's Comparative Bynhr]jO-
\ogi^. ae, depression of the vestibular epithelium to form the mouth (m); al, intestinal
canal; an, region of the anus; Ip, subambulacral body cavity; Ip', vestibule; m,
mouth ; m(, transverse mesentery ; r, roof of the vestibule ; rp, visceral part of the
body cavity and its prolongation (rp') into the stalk of the larva; f, tentacle; ur,
water-vascular ring.

this process. According to these authors, the lining of the vestibule
is not mesodermal, but ectodermal. The penetration of an enteroccelic
diverticulum into the stem, observed by Goette, Perrier, and Bury, is
denied by Barrois. According to him, these axial structures arise rather
by an accumulation of mesenchyma cells. On the other hand, according
to Barrois, a process of the subambulacral body cavity penetrates axially
toward the stem. Bury's contention that, in addition to the right and
K. H. E. G G

450

EMBRYOLOGY

left portions of the body cavity, there is developed still a third part, was
already mentioned in considering the hydrocoele (comp. p. 413).

Figs. 221 (and 222).— Pentacrinoid larva (C) and swarming larvae {A and B) of
Anlednn (after Thomson, from Bilfoub's Com'parative Embryology). The moat
anterior ciliated ring, described by Bubv, is lacking on the swarming larvw,
which are placed in the positions in which thev aubaeauently become attached
(comp. Fig. 217, p. 441).

ECHINODERMATA

451

Hitherto we have learned of the intestinal canal of the
Antedon larva only as a closed sac. The mouth and oeso-
phagus do not arise until the formation — on the floor of the
vestibule in the middle of the
water- vascular ring — of a depression,
which fuses with the intestine (Fig.
220 m). The intestine therefore
does not even yet open directly to
the outside world, but into the
vestibule. Its interior at this time
is not empty, but filled with cells
(Bury) or with a kind of nutritive
yolk (Barbois). The entodermal
mass elongates backwards (basal-
wards) to form the intestine, and
winds spirally about the axial part
of the body cavity. Its end then
moves in the transverse mesentery,
at about the height of the upper
margin of the basal plates, up to
the body- wall (Fig. 220), with
which it fuses, subsequently break-
ing through to the exterior. The
anus comes to lie in the vicinity of
the water- vascular pore. Subse-
quently it is shifted to its final po-
sition on the ventral wall of the cup.
As in the rest of the Echinoderms,
the anus seems to have no direct
relation to the blastopore.

Having considered the internal
developmental processes, we turn
again to the external shape of the
larva, which in the meanwhile has
essentially changed. These changes
are partly due to the metamorpho-
sis of the hydroccele. Each of the
five primary tentacles, which we have already seen to be
an evagination of the water-vascular ring, splits into three

Fig. 223.— Diagram of a
pentacrinoid larva of Antedon
rosacea (after Thomson, from
BALronu's Comparative Em-
bryology), cd, centrodorsal
plate ; or, oralia ; 4, radialia ;
3, basalia ; 1, terminal plate.

452 EMBRYOLOGY

parts, SO that fifteen tentacles can now be recognized.
By the addition of two new tentacle-buds to each of the
five groups, the number of tentacles soon increases to twenty-
five, arranged in five radial groups. The tentacles (Figs. 219
and 220 t) project into the vestibule, the roof of which is
stretched out between the upper margins of the oralia.
This roof is at first thick, but gradually becoming thinner
(Fig. 220 r), finally disappears entirely. The gi-adual dis-
appearance of the roof is partly a result of growth, partly
brought about by histolytic processes. According to Bl^ry,
such processes can also be recognized on the rest of the
larval body, and cause a disappearance of the histological
differentiation. Probably migratory cells make their appear-
ance in this connection as phagocytes.

After the disappearance of the roof of the vestibule, the
tentacles, on which papillae subsequently bud forth, project
free to the exterior (Fig. 221 C). The under-part of the
larva has elongated into the stalk, and it now rests with its
terminal plate on some support. The fundaments of the
arms bud forth on the upper part of the cup as five pro-
jections (Fig. 221 C). The tips soon split into two branches
corresponding to the permanent forking of the arms. One
of the radial tentacles, each of which has likewise split into
two, unites with the fundament of each of the arms. Sur-
rounded by this, it grows out with it and becomes the ambu-
lacral canal of the arm. By means of lateral budding it
gives rise to the tentacles of the arm. The tentacle which
is first formed always remains at the tip of the arm. The
new tentacles arise at its base in groups of three. The mode
of formation of the tentacles is therefore similar to that of
the ambulacral feet of other Echinoderms. The development
of the pinnules is the result of a forking of the arms, which
occurs alternately to the right and to the left (W. Carpenter,
Perrier). This explains the alternating position of the
pinnules.

Important changes have taken place in the skeleton of the
larva. Between the basal and oral plates, alternating with
the latter, five new skeletal pieces, the radialia, have made
their appearance (Fig. 223^); these become greatly enlarged,

ECHINODERMATA

453

and serve for the support of the arms (Fig. 224 Ti-rm).
Through the vigorous growth of the radial pieces, to each
one of which two other plates are added, the oral plates
are crowded on to the oral surface, where they finally
undergo resorption. In other Crinoids (e.g., Uhizoorinus),
on the contrary, the oralia are said to persist throughout
life. Another change has taken place at the base of the
cup : the centrodorsal plate has gradually overgrown the
ha-'alia and the lower radialia, so that nothing more remains
visible of the chief pieces of the original cup. The basalia
have fused into an unpaired piece, the so-called rosette. The

si ^^

';r^^{fr'^^ '

^'

>r ^-'i

r„ mu.

Fig. 224 — Vertical axial section throagh the disc and point of attachment of an
arm of Antedoa rosacea (with slight changes, after H. Ludwig). At the left the
section passes through an interradius, at the right through one of the radii, ax,
axial body cavity; bi"i_ 6)",, bi'm br^, brachialia (skeletal pieces); cl, circum-
visceral body cavity; cr, cirri; d, intestine; dc, dorsal canal of the arm; dn,
dorsal nerve ; do, dorsal organ ; e, epithelium of the ambulacral groove ; g,
genital canal; gelc. o, chambered organ; il, intervisceral body cavity; m, mouth-
opening; mit, musculature of the arm; p, pores of the cup; r^ Tj, r^j radialia
(skeletal pieces) ; rh, radial blood-vessel ; rn, radial (ventral) nerve ; rw, radial
water-vessel; st, stone canals; t, tentacle; vc, ventral canal of the arm; wr,
water-vascular ring and the stone canal (st) proceeding from it.

stem of the pentacrinoid stage now also degenerates. In
place of it there are developed from the centrodorsal plate
(according to Perrier as evaginations of the chambered
organ) at first five and later additional cirri, by means of
which the young Rhizocrinus attaches itself (Fig. 224 cr).

454

EMBRYOLOGY

Some internal developmental processes still remain for us to consider,
in which, however, we must confine ourselves to the fundaments of the
chief systems of organs.

We saw that the body cavity of the young larva consists of two separate
spaces, namely the subambulacral and the visceral body cavities (Figs.
219 Ls and Lv and 220 Ij) and rp). In the development of the arms
these two spaces are continued into them and give rise to their ventral
and dorsal canals (Perrier). The separation of the body cavity into
these primitive spaces does not, however, persist long. The mesentery
lying between the subambulacral and visceral body cavities jjartly dis-
appears, and the two thus coalesce. On the other hand, a membrane,
which marks off a central space (Fig. 224 ax) — lying approximately in
the perpendicular axis of the body — from the rest of the body, makes
its appearance as a new structure. Another such boundary is developed
in the periphery of the intestine (Fig. 224 (7) ; it is called the visceral sac.
The part of the body cavity lying external to it is called by Ludwig the
circumvisceral, the inner part, on the other hand, the intervisceral, body
cavity 1 (Fig. 224 cl and (7). Enclosed in the latter lies the axial cavity
(ax) already mentioned. After the disappearance of the primary mesen-
teries and the development of these spaces, the ventral canal of the arm
coalesces with the axial, and the dorsal cavity with the circumvisceral
cavity (Fig. 224).

The genital canal, which encloses the genital cavity, extends in the
arms between the dorsal and ventral canals (Fig. 224 g). These structures
also take their origin from the disc. According to Perkier, the genital
apparatus is established at an early period, even before the pentacrinoid
stage is reached. It then consists of a thickening of the splanchnic layer
of the visceral body cavity and occupies an axial position in the lower
part of the cup. It remains here for a while, and so changes as to
acquire a racemose appearance and, within, a cavity. After the develop-
ment of the arms has taken place, it splits at the tip and sends a branch
into each arm. This mode of origin of the genital apparatus agrees with
the recent description of Hamann (No. 21), according to whom there is
in other Echinoderms also a central part of the genital apparatus, from
which branches go off to the different radii. The additional branching
of the genital tubes to the pinnules in the Crinoids is then homologous
with the development of the genital sacs of other Echinoderms.

As regards the blood-vascular system, we have already learned that
the chambered organ lying in the centrodorsal piece takes its origin from
the outer enterocoelic lamella, which penetrates into the larval stalk
(Peuhieb). This splits into five cords, which acquire cavities and then

' The presence of separate spaces of the body cavity is not admitted
by Hamann {Histologie der Crinoiden, Jena, 1889). We have followed the
statements of Ludwig, and as regards embryological questions have
adhered to those of Perkier,

ECHINODERMATA 455

form the five chambers of the organ. By means of an invagination each
chamber gives rise to one of the five primary cirri. The dorsal organ
(Fig. 224 do) is united with the chambered organ, in which, as in the
latter, one is inclined to see the central organ of the blood-vascular
system (Ludwict).i According to Peerier, the dorsal organ is composed
of the genital fundament previously mentioned and a vascular plexus.
The latter would also take its origin from the inner enterocoelic lamella
of the visceral body cavity. A coalescence of the genital canals of the
arms in the dorsal organ was conjectured even by W. B. Carpenter and
by LuDWiG. A number of other vascular plexuses are distinguished by
Perrieb, and their development described. They are said to be directly
connected with the ambulacral system. Inasmuch as portions of the
body cavity also communicate with the so-called blood-vessels, the circu-
lation is said to be general.

The views on the development of the nervous system are not yet clear
enough to admit of a brief description. When Perrier derives parts of
the nervous system from mesenchymatous elements, he is in opposition

Fig. 225. — LincHa midtifora (after P. und F. Saeasin).

to the prevailing views. [To supplement what is stated here, reference is
made to the work of Seeligek (No. XXVI.), which gives a very thorough
account of the embryology of Comatula. — K.]

Regeneration and Division.— Starfishes possess to a high degree
the power of replacing lost arms. Single arms, which have become
detached from the disc, are again made good ; in fact, a detached arm is
able to develop an entirely new disc with the normal number of arms.
The so-called comet form of the starfish (von Martens, Haeckel) is due
to the newly formed parts, which at first are relatively small, being
appended, as it were, to the large arm. The mere replacement of lost
parts seems here to merge into a reproduction by division. Such really
takes place in those forms in which the disc constricts spontaneously

1 In the recent work of Hamann mentioned in the preceding note, the
connection of the dorsal organ with the chambered organ is denied, and
likewise the important relation of the former to the blood-vascular system
is regarded as being without evidence.

456 EMBRYOLOGY

through the middle and produces two parts, which again grow into new
individuals (Kowaxevsky, Simkoth). Each part acquires a new mouth
and the complete organization of the normal animal. A division of this
kind, such as is found, for example, in Asteracanthion temiupinus and
Ophiactis vireiu, may be called schizogony.

An interesting form of regeneration, which can scarcely be called by
that name, since it transcends the meaning of that word and appears to
approach non-sexual reproduction, is described by P. und F. Sarasin
(No. 46). They saw new arms budding forth on the stump of an arm of
Linckia multifora in such a way as to seem to form a new star, which
remained attached to the stump of the arm (Fig. 225). P. uxd F. Sarasin
look upon this process as a budding of the starfish, and it is not to be
denied that it would merit this term, if a niouth-o]3ening could be demon-
strated on the young starfish.

General Considerations. — The development of the
various divisions of Echinoderms oifers many common
features, in which a close relationship of the five divisions
can be recognized. We saw that cleavage was always total,
tolerably equal, and its result a ciliated blastula. A typical
invagination gastrula arises from it. The mesenchyma cells
detach themselves from the invaginated part (Antedon, Astro-
pecten, Synapta). The fact that the development of the
mesenchyma in other forms {Echinoidea, Opliinroided) takes
place even before gastrulation is completed does not seem
to constitute an important distinction, because in such cases
it is from the same region — namely, the entodermal part of
the blastula — that the detachment of the mesenchyma cells
takes place, and because in another case (Holuthuria) this
occurs during the beginning of gastrulation. The mesen-
chyma is doubtless to be traced to the same origin as the
mesodermal structures, which as cwlomic sacs detach them-
selves from the archenteron. This assumption gains in pro-
bability from the fact that in a later stage of development
another separation of cells takes place from the epithelium
of the entero-hydroca^le. Euterocoole and hydrocele are
the derivatives of the two ca^lomic sacs, which are constricted
off together from the apex of the archenteron. They then
separate into the two enteroca^les and the hydrocoele, which
latter enters into communication with the outside world by
means of the dorsal pore.

ECHINODERMATA 457

The blastopore, in those cases where it persists, becomes
the anus. The mouth arises bj the union of the archenteron
with the ectoderm. Certain differences occur in regard to
the formation of mouth and anus as the result of differences
in the mode of life and the consequent alteration of the
shape of the larva. The larvae are quite different in shape
in the different groups of Echinoderms, although common
features are not lacking. Apart from the internal organiza-
tion already mentioned, in regard to which thej are entirely
comparable with one another, the external characters can
also be compared, in spite of the differences in the shape of
the body and, above all, the ciliated band, which, together
with the shape of the larva, is derivable from a common
fundamental larval form. Even the cask-shaped larva of
Antedon, differing as it does from the other larval forms of
the Echinoderms, resembles in shape the so-called pupa of
the Holothurians, which, like it, possesses five ciliated bands.

The further development must in turn present differences
corresponding to the variations in shape of the larvae ; but,
on the other hand, it also exhibits certain resemblances, as
the usual similarity of structure in the different systems of
organs demands. Thus not only the fundament, but also
the further development, of the water-vascular system re-
curs in nearly the same way in all cases. The development
of the nervous system, as far as it is known, also shows
many things in common, and the same is true to a still
greater extent for the musculature. The early development
and union of the ambulacral and subambulacral surfaces of
the starfish and the Ophiuran in the Brachiolaria and
Pluteus larvae, which differ so much from each other in
shape, takes place in a strikingly similar way. In the
larvae of Crinoids and Holothurians, a certain resemblance
in regard to the development of the tentacles at the bottom
of the vestibule or oral funnel cannot be mistaken.

As regards the development of the skeleton, it is not
possible from what is at present known to discover any posi-
tively established relations between the different divisions
of Echinoderms. To be sure, reference has been made to the
position of the forming plates in relation to the internal

458 EMBRYOLOGY

organs (H. P. Carpenter, Bury), but these relations are
quite uncertain still. Even the conception of the homology
of the plates founded by Lov^N and championed by Car-
penter, especially those which in the different groups of
Echinoderms lie about the apical pole, is not to be considered
as assured.

All Echinoderms have a radial structure ; the larvae, on the
contrary, are bilaterally symmetrical as regards both their
internal and their external organization. It has been shown
by different examples how the radial structure arises onto-
genetically from the bilateral ; but the question now presents
itself, How is the shape of the Echinoderms to be explained
phylogenetically ? A reply to this involves another question,
namely, whether the different groups of Echinoderms are
derived one from the other, and in that event which of them
stands the highest and which the lowest. Recently the
Holothurians, and especially the apodal Holothurians
(Synapta) , have been looked upon as the lowest forms, and
transitions have been sought from them to the Crinoids on
the one hand and to the Echinoids on the other, because certain
Holothurian characters were found on the one hand in the
(fossil) Cystidce, on the other hand in the soft-shelled Echino-
ihuridce (P. und F. Sarasin, No. 47). Additional transitions
to the Asteroids and Ophiuroids are also demonstrable.
This theory, to be sure, traces the Echinoderms back to
simple forms, but gives no explanation of the origin of the
radial structure. Even the apodal Holothurians are still
radially constructed, and it does not seem at all impossible
that the simplicity of their structure is only a phenomenon
of degeneration. The Echinoderms, however, as we may con-
clude from their ontogeny, are to be referred to bilateral
forms as their source.

Another theory is that which traces back the different
divisions separately to a common ancestral form known as the
Pentactcea (Semon, No. 55). This ancestral form corresponds
to that stage in the ontogeny in which the larva had already
passed from the bilateral to the radial form by the closing
of the water-vascular ring, and by the putting forth of
five evaginations. Semon tinds such a Pentactula stage in

ECHINODERMATA 459

the ontogeny of all five groups, and from this argues for a
common ancestor having that form. This theory encounters
the difficulty that the five groups, if they had assumed such
an independent development, would scarcely show so great
a resemblance in their organization as they in fact do. It
seems to ns more justifiable to search for the ancestral forms
of the Echinodermata among the existing material which is
ofFered to us by palaeontology. In this, however, the other
difficulties ainse that the material is not complete, since
delicate forms have not been preserved, and that it allows
only the external shape to be recognized.

At all events, it will be the stalked forms among which
we are to search for the ancestors of the Echinodermata;
for at any rate it was the influence of the attached mode of
life which in the Echinoderms, as in other groups of animals,
called forth the radial structure. Such forms as the Gystidce
— which are in part stalked and in part without stalks, and
of which one part obviously led an attached life, while the
other part, on the contrary, led a free existence — seem best
fitted to stand as the ancestral forms of the Echinodermata
(comp. also Neumatr, No. 43). Their shape is spherical,
not yet being prolonged into arms. In many of them the
plates are irregularly arranged, and then no trace of a radial
arrangement is noticeable. On the other hand, five radial
furrows may extend out from the mouth, similar to the
ambulacral furrows on the disc of a Crinoid or Asteroid.
However, it is the relationships which exist between the
Cystidse and the other groups of Echinoderms that seem
to be especially important. The Cystidse, by means of tran-
sitional forms, are said to stand in relation to the Crinoids,
the Asteroids, and the Echinoids (Neumayr, No. 43). Since,
however, the Echinoids have been placed nearer to the Holo-
thurioids by P. und F. Sarasin, and since evident relation-
ships between the Cystidae themselves and the Holothuri-
oidea have been discovered by the same authors, this latter
group can also be compared with the rest of the Echino-
dermata. To us it seems probable that the Echinoderms
established their radial structure by an attached mode of
life long continued, and only later returned again to a free

460 EMBRYOLOGY

life, which to-day characterizes the most of them. The
shape of the larva seems to have been produced indepen-
dently of this course of development.

As regards the question what may have been the nature
of the bilateral progenitors of that radial ancestral form,
we are altogether in the dark. Ontogeny gives no answer
to this question, since, on the one hand, the larvae are much
changed, owing probably to phenomena of adaptation, and
since, on the other hand, no true relationships to other larval
forms — e.g., those of the worms — can be recognized. One
would most naturally compare the larvae of the Echinoder-
mata with those of the Turhellaria and Nemerfeans or with
the TrochopJwre, but the difference in the distribution of the
ciliation and the absence of the apical plate make this diffi-
cult. Such larv^ as that of Antedon, the Holothurian pupa,
and the vermiform larva of the Asteroidea (Joh. Ml'LLKk)
recall segmented forms; but they may quite as well repre-
sent secondarily acquired stages of development. This is
especially difficult to determine in the case of the larva of
Antedon, for it is not impossible that larvae of the typical
form of the Echinoderm larva3 make their appearance in the
development of the Crinoids, which is still little known.
The larva of Antedon even is modified, as the disappearance
of the blastopore shows. Nevertheless, the resemblance
to the Holothurian pupa is striking, the latter certainly
representing a secondary stage of development.

The Echinoderm larvae, as far as regards their internal
organization, resemble most closely such forms as the Anne-
lida, owing to the occurrence of coelomic sacs. We are
inclined to refer the formation of such a body cavity as
takes place in the Annelida to a like origin with that of the
Echinodermata, and accordingly to regard the mesoderm of
the Echinodermata and that of the Annelida as homologous
structures. Indeed, indications which point to relationships
of the Echinodermata Avith segmented forms are not lack-
ing.

An internal segmentation would find expression in the body, if tlie con-
dition described by Buky (No. 8) — the development of two pairs of

ECHINODERMATA 461

enterocoeles — should be confirmed. This would indicate an approach
to segmented forms. But in this connection one is involuntarily re-
minded of the condition of the larva of Balanoglossus, in which, according
to Bateson, an internal segmentation is expressed by the establishment
of three pairs of coelomic sacs (comp. Fig. 167, p. 380). Even in its
external shape the Tornaria of Balanoglossus seems to possess a certain
resemblance to the Echinoderm larvae. Added to this is the fact that
the so-called water-vascular vesicle of Balanoglossus may exhibit a bipar-
tite fundament. Also the water-vascular vesicle of the Echinoderms in
certain cases (especially in the Ophiuroidea, and occasionally in the
Asteroidea) is said to be begun as a paired structure (Metschnikoff).
Should this statement (hitherto held to be insufficiently authenticated)
be confirmed, then one would be able to compare the fundament of this
system of organs — so important for the Echinoderms, but so obscure as
regards its phylogenetic origin — with an embryonal excretory apparatus
(primitive kidney). This view is supported by the discoveries of P. und
F. Sarasin (No. 47), who explain the glandular structure, which opens to
the exterior at the same time as the stone canal — the so-called heart of
the sea-urchin — as an excretory organ communicating freely with the
body cavity by means of an open ciliated funnel. The assumption of
the Sarasins that the excretory system is the more primitive, and the
water-vascular system, with its locomotor function, is only an organ
derived from it, seems to be one that is required by the natural course of
events.

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Echinodermen, etc. Zool. Jahrh., Abth. Anat. Bd. iv. 1889.

27. KowALEVSKY, A. Sitzungsb. der 8. Vers. russ. Naturforscher in

Kiew. Zeitschr. wiss. Zool. Bd. xxii. 1872.

28. KowALEVSKY, A. Beitrage zur Entwicklungsgeschichte der Holo-

thurien. 3Iem. Acad. St. Petersbourg. Ser. 7, tom. xi. 1867.

29. Kroun, a. Ueber die Entwicklung einer lebendig gebiirenden

Ophiura. Arch. Anat. u. Physiol. Jahrg. 1851.

30. Lddwio, H. Beitriige zur Anatomic der Crinoiden. Zeitschr. wiss.

Zool. Bd. xxviii. 1877.

31. LuDwio, H. Beitrage zur Anatomie der Asteriden. Zeitschr. wiiS.

Zool. Bd. xxx. 1878.

ECHINODERMATA 463

32. LuDwiG, H. Ueber den primaren Steincanal der Crinoiden, nebst

vergl. anat. Bemerkungen iiber die Echinodermen iiberhaupt.
Zeitschr. wiss. Zool. Bd. xxxiv. 1880.

33. LuDWiG, H. Ueber eine lebendig gebarende Synaptide und zwei

andere neue Holothurienarten der brasilianischen Kuste. Arch,
de Biol. Tom. ii. 1881.

34. LuDwiG, H. Zur Entwicklungsgeschichte des Ophiurenskelets.

Zeitschr. loiss. Zool. Bd. xxxvi. 1881.

35. LuDWiG, H. Entwicklungsgeschichte der Asterina gibbosa. Zeit-

schr. wiss. Zool. Bd. xxxvii. 1882.

36. Maetens, E. v. Ueber ostasiatische Echinodermen. Arch, Naturg.

Jahrg. xxxii., Bd. i. 1866.

37. Metschnikoff, E. Studien iiber die Entwicklungsgeschichte der

Echinodermen u. Nemertinen. Mem. Acad. St.Petersbourg. Ser. 7,
tom. xiv. 1869.

38. Metschnikoff, E. Studien iiber die Entwicklung der Medusen und

Siphonophoren. Zeitschr. wiss. Zool. Bd. xxiv. 1874.

39. Metschnikoff, E. Embryologische Mittheilungen iiber Echinoder-

men. Zool. Anzeiger. Jahrg. vii. 1884.

40. Metschnikoff, E. Untersuchungen iiber die intracellulare Verdau-

ung bei wirbellosen Thieren. Arbeiten Zool. Inst, llien. Bd. v.
1884.

41. Metschnikoff, E. Vergleichend-embryologische Studien : Ueber

die Bildung der Wanderzellen bei Asteriden und Echiniden.
Zeitschr. wiss. Zool. Bd. xlii. 1885.

42. MxJLLEE, J. Abhandlungen iiber die Larven und Metamorphose der

Echinodermen. Abh. Kgl. Akad. Wiss. Berlin. 1848 — 1850,
1852, 1853, 1855.

43. Neumayr, M. Die Stamme des Thierreiches. Wien und Frag.

1889.

44. Peekiee, E. Memoire sur I'organisation et le developpement de

la Comatule de la Mediterranee (Antedon ros.). Paris. 1886.

45. Peouho, H. Eecherches sur le Dorocidaris papillata et quelques

autres Echinides de la Mediterranee. Arch. Zool. exper. et gen.
Ser. 2, tom. v. 1887.

46. Saeasin, p. und F. Knospenbildung bei Linckia multifora Lam.

Ergebn. naturw. Forsch. auf Ceylon in den Jahren, 1884 — 1886.
Bd. i. Wiesbaden. 1888.

47. Saeasin, P. und F. Ueber die Anatomic der Echinothuriden und

die Phylogenie der Echinodermen. Ergebn. naturw. Forsch. auf
Ceylon in den Jahren, 1884 — 1886. Bd. i.

48. Saes, M. Beskrivelser og lagttagelser over nogle mserkelige eller

nye i Havet ved den Bergenske Kyst levende Dyr, etc. Bergen.
1835.

49. Saes, M. Fauna littoralis Norvegiae. Christiania. 1846.

50. Saes, M. Ueber die Entwicklung der Seesterne. Arch. Naturg.

Jahrg. x., Bd. i. 1844.

464

EMRRYOLOGY

51. ScHTTLTZE, M. Uebei' die Entwicklung von Ophiolepis squamata.

Arch. Anat. u. Phys. Jahrg. 1851.

52. Selenka, E. Zur Entwicklung der Holothurien : Ein Beitrag zur

Keimbliittertheorie. Zeitschr. ivi>ss. Zool. Bd. xxvii. 1876.

53. Selenka, E. Keimbliitter und Organanlage der Echiniden. Zeitschr.

ivisg. ZooL Bd. xxxiii. 1880.

54. Selenka, E. Studien iiber die Entwicklungsgeschichte der Thiere.

Heft 2 : Die Keimbliitter der Echinodermen. Wiesbaden. 1883.

55. Semon, E. Die Entwicklung der Synapta digitata und die Stam-

mesgeschichte der Echinodermen. Jena. Zeitschr. Bd. xxii.
1888.

56. SiJiEOTH, H. Schizogonie der Ophiactis virens. Zeitschr. wiss,

Zool. Bd. xxviii. 1877.

57. Thomson, C. W. On the Embryology of the Echinodermata. Nat.

Hist. Review, Dublin, vol. ill., 1863, and vol. iv., 1864.

58. Thomson, C. W. On the Embryogeny of Antedon rosaceus. P}iil.

Tiuns. Roy. Soc. London. Vol. civ. 1865.

Appendix to Literature on Echinoderviata.

I. Bkide, E. W. Mac. The Development of the Genital Organs,
Pseudo-heart, etc., in Amphiura squaraata. Zool. Anzeiyer.
Jahrg. xv., p. 234 and p. 449. 1892. Also Quart, Jour. Micr.
Sci. Ser. 2, vol. xxxiv. 1892.
II. Bride, E. W. Mac. The Development of the Dorsal Organ,
Genital Eachis, etc., in Asterina gibbosa. Zool. Anzeiyer.
Jahrg. xvi. 1893.

III. Bride, E. W. Mac. Organogeny of Asterina gibbosa. Proc.

Roy. Sac. London. Vol. liv. 1894.

IV. BiJTscHLi, 0. Versuch der Ableitung des Echinoderms aus

einer bilateralen Grundform. Zeitschr. wiss. Zool. Bd.
xliii. 1892.
V. Chadwick, H. C. Notes on Cucumaria planci. Trans. Liver-

j)ool Biol. Soc. Vol. v. 1891.
VI. Chun, C. Die Bildung der Skelettheile bei den Echinoder-
men. Zool. Anzeiger. Jahrg. xv. 1892.
VII. Chun, C. Atlantis. Biologische Studien iiber pelagische
Organismen. BibUotheca Zooloyica. Heft 19. Stuttgart.
1895.
VIII. Edwards, C. L. Notes on the Embryology of Miilleria
Agassizii, a Holothurian, etc. Johns IL)pkins Univ. Circu-
lars. Vol. viii. 1889.

IX. Field, G. W. The Larva of Asterias vulgaris. Quart. Jour.

Micr. Sci. Ser. 2, vol. xxxiv. 1892.

X. Gaustano, W. On some Bipinnarire from the English Chan-

nel. Quart. Jour. Micr. Sci. Ser. 2, vol. xxxv. 1893.

ECHINODERMATA

465

XI. Jaekel, 0. Eutwurf einer Morphogenie und Phylogenie der
Crinoiden. Sitzungsb. Gesell. Naturf. Fri'unde. Berlin. 1894.
XII. KiSHiNouYE, K. Note on the Development of a Holothurian
Spicule. Zool. Avzeiger. Jahrg. xvii. 1894.

XIII. Leipoldt, F. Das angebliche Excretionsorgan der Seeigel,

untersucht an Sphaerechinus granulans und Doroeidaris
papillata. Zeitschr. iciss. Zool. Bd. Iv. 1893.

XIV. LovEN, S. Echinologica. Bihang Svenska Vet. Akad.,Handl.

Bd. xviii., Afdel. iv. 1893.
XV. LuDwiG, H. Zur Entwicklungsgeschichte der Holothurien.
Sitzungsb. Akad. IViss. Berlin. 1891.
XVI. LuDwiG, H. Ueber die Eadchen der Synaptiden. Zeitschr.

wiss. Zool. Bd. liv. 1892.
XVII. Ludwict, H. Echinodermen. Bronn^s " Classen vnd Ordnuvgen

des Tliierreichs." Bd. ii., Abth. iii. 1889—1894.
XVIII. LuDwiG, H. Notiz iiber die von Kishinouye beschriebenen
Holothurienkalkkorper. Zool. A)izeiger. Jahrg. xvii. 1894.
MacBride, E. W. See Beide, E. W., Mac.
XIX. MoRTENSEN, T. Ucber Ophiopus arcticus, etc. Zeitschr. wiss.
Zool. Bd. Ivi. 1893.
XX. MoRTENSEN, T. Zur Anatomie und Entwicklungsgeschichte
der Cucumaria glacialis. Zeitschr. wiss. Zool. Bd. Ivii.
1894.
XX.a. Nachteeeb, H. F. Preliminary Notes on the Echinoderms of
Beaufort. Johns Hopkins Univ. Circulars. Vol. iv., No.
38, pp. 67, 68. 1885.
XXI. Peeeiee, E. Memoire sur I'organisation et le developpement
de la Comatule de la Mediterran6e (suite et fim). NouvelLs
Arch. 31us. Hist. Nat. Paris. Ser. 3, torn. ii. 1891.
XXII. Eusso, A. Die Keimblatterbildung bei Amphiura squaraata.
Zool. Anzeiger. Jahrg. xiv. 1891.

XXIII. Eusso, A. Fasi di sviluppo del sistema aquifero, etc., nell'

Amphiura squamata. Anut. Anzeiger. Jahrg. vi. 1891.

XXIV. Eusso, A. Contribuzione all' embriologia degli Echinodermi el

sviluppo dell' Asterias glacialis. Boll. Soc. Nat. Napoli.
Vol. vi. 1892.
XXIV.a. Eusso, A. Embriologia dell' Amphiura squamata (Morf. del
apparecchio riproduttore). Atti Accad. Sci.fis. mat. Napoli.
Ser. 2, vol. v. 1893."
XXV. Eusso, A. Contribuzione alia genesi degli organi negli Stelleridi.
Rend. Accad, Sci. fis. mat. Napoli. Ser. 2, vol. viii. 1893.
Also Mo7iitor Zool. Ital. Ann. v. 1893.
XXVI. Seeligee, O. Studien zur Entwicklungsgeschichte der Crinoi-
den. Zool. Jahrb., Abth. Anat. Bd. vi. 1892.
XXVII. Semon, E. Die Homologien innerhalb des Echinodermen-
stammes. Morph. Jahrb. Bd. xv. 1889.
K. H. E. H H

46G EMBRYOLOGY

XXYIII. Semon, R. Zur Morphologie der bilateralen Wimperschniire
der Echinodermenlarven. Jena. Zeitschr. Bd. xxv. 1890.
XXIX. Theel, H. On the Development of Echinocyamus pusillus.
Nov. Acta Reg. Soc, Sci., Upsala. Ser. 3, vol. xv., fasc. ii.
1892,
XXX. Theel, H. Notes on the Formation and Absorption of the
Skeleton in Echinoderms. K. Svensk. Vet. Akad. Handl.
No. VIII. Stockholm. 1894.

SUBJECTS INDEX.

Acanthobothridae, 199.
Acaathocephala, 219.
Accela, 159, 172, 175.
Actinaria, 100.
Actinia aurautiaca, 89.

equina, 89.

meserabryanthemum, 89.

parasitica, 88.
Actinula, 39, 51, 52.
Adamsia Roudeletii, 88.

diapbana, 91.
Jilginidfe, 51.
JEginopsis, 57.
JSquorea, 40.
Agalma, 64.
Agalmidse, 69.
Agalmopsis (Fig. 28), 66.
Aglaura, 57.
Alciopidffi, eyes of, 289,
Alcyonaria, 75, 78, 82.
Alcyonium, 76, 83.
Allantonenia, 211.
AUoiocoela, 159.
Alophota (Fig. 25), 62.
Alternation of generations, 2.
Amnion (Pilidium), 221.
Amorpbina, 21.
Ampbiblastula, 14, 17, 20.
Amphilina, 199, 200.
Ampbiporus, 228.
Amphiptycbes, 199.
Ampbitrite (Fig. 138), 298.
Ampbitrochse, 271, 278.
Ampbiura, enterobydrocoele of,

413.
Ampbiura squamata, 404.
Anguillula, 242.
Animal pole, see Pole, animal.
Annelida, 262.

relationships of, 341.
Antedou, 404.

larva of, 415, 457.

mesencbyma of, 450.

pentacrinoid stage of, 443.

vasoperitoneal vesicle of, 412.

Antbozoa, 39, 75.
Apbanostoma diversieolor, 175.

pulchella, 175.
Aphidse, sexual cells of, 12.
Apolemia, 67.
Apolemidse, 75.
Aracbnactis, 98.
Arbacia, Pluteus of, 425.

pedicellaria of, 440.
Arebenteric cavity, 4.
Arcbiannelida, 261, 347.
Arcbiblastula, see Cceloblastula,
Archigetes, 199, 200.
Arcbibydra, 125.
Arenicola, 277, 278.
Aricia, 363.
Ascandra contorta, 20.

Lieberkiibnii, 20.
Ascaris lumbricoides, 234.

megalocepbala, 234, 235.

nigrovenosa, 236.

sexual cells of, 12.
Ascetta, 22, 28.

Asteracantbion Miilleri, enteroby-
drocoele of, 407.

larval appendages of, 422.

tenuispinus, reproduction by
division in, 456.
Asterina, 421.

dorsal pore and stone canal
of, 436.

enterocoele of, 406.

enterobydrocoele of, 414.

intestine of, 435.

metamorpbosis of, 436.

nervous system of, 436.

stomodseum of, 403.

vasoperitoneal vesicle of, 407.
Asteroidea, 419, 432.

mesencbyma of, 400.
Astrseacea, 101,
Astroides, 98.
Astropecten, mesencbyma of, 402,

456.
Atokal forms of Nereidse, 303.

467

468

SUBJECTS INDEX

Atorybia, 64.

Atrochae, 271, 273,

Attractonema, 240.

Aulactinia, 94.

Aulastoma, 318, 328.

Aulochone (Sponge), inhabited by

Syllis, 304.
Aurelia aurita, 105, 120.

flavidula, 105, 106.
Aiireliidae, 120.
Auricularia, 417, 418, 420.

metamorphosis of, 426.
Aurophore, 62.
Autolytus, 262, 302.
Axis, chief or primary, 3.

Baer, see von Baer.
Balauoglossus, 373.

claviger, 379.

fertilization in, 379.

genital organs of, 379.

Kowalevskii, 379, 385.

Kupfferi, 384.

minutus, 379.

relationships of, 388.
Balanophyllia, 89.
Beroidffl, 150, 154.
Bilateria, 10.
Bipiunaria, 420.

metamorphosis of, 433.
Bladder- worm (Taeniadffi), 193,

195.
Blastocoele, see Cleavage cavity.
Blastoderm, 4.
Blastoniere, 3.
Blastopore, 4.
Blastosphere, see Blastula.
Blastotrochus, 102.
Blastula, 4.
Boliua, 138, 151.
Boliute, 151.
Bouellia, 304, 310.
BothriocephaluB, 192, 194, 198.
Brachiohiria, 421.

metamorphosis of, 432.
Branchiobdella, 336.
Bucephalus, 186.

Calcarea, 20, 28.
Callianira, 137, 150.
Calvconula, 70.

Calycoplioridffi, 59, 67, 68, 70, 72.
Calycozoa, 123.
Caiiipaiiularia, 47, 49, 52.
Capit(dhi, 263.
Capitellidaa, 295, 298.
Caryophyllu3Us, 199.

Caryophyllia, 89.
Centrolecithal eggs, 8.
Cephalothrix, 230.
Ceratosa, 22, 23, 28.
Cercaria (Distomidss), 183, 184.
macrocerca, 187.
setifera, 186.
Villoti, 186.
CerianthesB, 95.
Cerianthus, 88, 95.
Cestidaj, 153.
Cestoda. 190.

relationships of, to Treraatoda,
200.
Cestiis, 153.
Cbsetognatha, 367.

relationships of, 371.
Chaetopoda, 262.
Chsstopteridffi, 274.
Chffitopterus, 274.
Chalinidse, 24.
Ohalinula, 24.
Charybdaea. 103.
Charybdajidae, 102, 103.
Ghirodota, larva of, 419.
Choanoflagellata, relation of to

Porifera, 29.
Chrysaora, 112.
Chrysomitra, 74.
Cirratulus, 262.
Cladonemida3, 154.
Clava, 47.
Clavidse, 125.
Clavularia, 76.
Cleavage, 8.
cavity, 4.
discoidal, 8.

spheres, see Blastomere.
superficial, 8.
total and equal, 5.
total and unequal, 7.
Clepsine, 318.
Cnidaria, 39, 124.
Codouium, 49.
Ccjeloblastula, 6.
Coelom, 11.
radouiic sacs, 11.
CcEloplana, 157, 177.
Comatula, nervous system of, 455.
Complemental males (Myzostoma),

318.
Conocyema, 214, 215.
Convoluta, 175.
Corallia, 90, 101.
Corallium, 83.
Cordylo'phora, 47.
Corm, 1.

SUBJECTS INDEX

469

Cornacuspongia, 22.
Cornularia, 80, 82.
Corymorpha, 50, 73.
Corynidffl, 125.
Cotylorhiza, 108, 112.

borbouica, 112.

tuberculata, 108.
Crinoidea, 443.

mesenchvma of, 400.
Criodrilus, 28^1, 281, 287. 291, 296.

monostylus, 301.

pardalis, 301.
Crystallodes, 64.
Ctenaria, 154.
Ctenophora, 136.
Ctenoplana, 157, 177.
Cucullauus, 234, 23-5, 238, 242.
Cucumaria doliolum, larva of, 419.

meseuchyma of, 397.

planci, 429, 432.

pupal stage of, 427.
CuninisB, 49, 53, 58.
Cunoctantba, 53.
Cyauea, 106, 112.

arctica, 106.

capillata. 112.
CyaueidsB, 120.
Cydippidffi, 151.
Cysticercus (TEeniadas), 195.

pisiformis (Fig. 96), 196.
Cystida3, 458, 459.

Darwinia, 175.

Delamiuation, gastrulation by, 8.

Dendrocoelidffi, 159, 169.

Dendrocoelum lacteum, 169, 170.

Desmacidon, 24.

Desmophyidffl, 71.

Desor's type of development (Ne-

mertini), 225.
Dicyema, 209.
Dicyemella, 209.
Dicyemennea, 209.
Dicyemidffl, 206, 209.

relationships of, 215.
Dicyemina, 209.
Dicyemopsis, 209.
Dinophilus, 215, 313.

apatris (gyrociliatus), 314.

giiias, 314.

relationships of, 314.

tseniatus, 314

vorticoides, 314.
Diphyidaj, 68, 71.
Diphyopsis, 71.
Diplozoon, 188, 189.
Diptera, sexual cells of, 12.

Discocelis, 161, 163.
Discomedusse, 103, 120.
Disconanthae, 65.
Disconula, 65.
Discophora, 102, 103.
Distomidffi, 178, 180.
Distomum cygnoides, 187.

cylindraceum, 180.

hepaticum, 180, 181, 185, 187.

raacrostomum, 185.

mentulatuin, 180.

ovocaudatum, 185,

tereticolle, 178.
Doclimius duodenalis, 239.

trigonocephalus, 239.
Dracunculus, 243.

Echinarachnius, 403, 439.
Echinaster, larval appendages of,

422.
Echinobothridse, 199.
Echinocardium, cleavage in, 398.
Echinoeoccus, 197.
Echinocyamus, 440.
Echinodermata, 392.

regeneration and division in,
456.

relationships of, 459, 460.
Echinoidea, 398, 424, 438.
Echinorhynchus angustatus, 250,
251.

gigas, 249, 250, 251, 258.

pol.ymorphus, 252, 255.

proteus, 250, 251.
Echinothuridse, 458.
Echinus, 424.

microtuberculatus, cleavage
in, 398.
Echiuridffl, 304.

compared to the Sipunculidae,
365.
Echiurus, 304, 306.
Ectocarp meduste, 124.
Ectoderm, 3, 4.
Edwardsia, 92.
Edwardsidas, 93.

Embolic gastrula, see Gastrula, em-
bolic.
Enchvtrseoides, 294.
Enteropneusta, 373.
Entocarp medusas, 124.
Entoderm, 3, 4.
Eosphora, 256.
Epenthesis, 49.
Ephydatia, 25.
Ephyra, 104, 113, 114, 119.

pedunculata, 118.

470

SUBJECTS INDEX

Ephyropsidee, 119, 123.

Epiactis, 100.

Epibolic gastrula, see Gastrula,

epibolic.
Epibulia, (58, 71.
Epitokal forms of Nereidse, 303.
Errantia, segmeutal appendages of,

279.
Ersffite, 71.
Esperia, 21, 27.
Euchaiis, 145, 151, 153.
Eudendrium, 42.
Eudoxia, 70-72.
Euletheria, 154.
Eunice, 2<52.

Eupomatus, 263, 267, 289, 295.
Eurampba, 151.
Eurylepta, 161.
Euryleptidse, 1(50.
Euspongia, 23.
Eutima, 74.
Exogone, 262, 280.

Fertilization, 2.
Flabellum, 100, 102.
Frustulation, 49.
Fuugia, 100.
Fungiacese, 101.
Fuugidse, 102.

Galaxea, 101.
Galeolaria, 71.
Gasterostomum, 184.
Gasterotrochee, 271, 277.
Gastrea, 5.
Gastroblasta, 49.

Kaffaeli, 49.

timida, 49.
Gastrodes, 58.
Gastrula, 4.

circumcrescence, see Gastrula,
epibolic.

embolic, 6.

epibolic, 7.

invagination, see Gastrula,
embolic.
Gemmulse, 13, 34, 35.
Gephyrea, 365.
Germinative vesicle, 3.
Geryonidoe, 53.
Gnatbobdellidffi, 325.
Gonactinia, 95, 100.
Gonad, 2.
Gordiida), 244.
Gordius aquaticus, 244.

relationships of, 246.

Villoti, 244.

Grubea, 262, 280.
Gyrodactylus, 188, 189.

Halcampa, 94.
Halecium, 51.
Halisarca, 20, 25, 28.
Halistemma, 60-62.

pictum, 62

rubrum, 62.
Herpetolitha, 101.
Heterakis, 239.
Heteraxonia, 10.
Heterocyemidffi, 214.
Heterodera, 241.
Heterogeny, 2.
Hexacorallia, 93.
Hexactinellidffi, 32.
Hexactiuise, 91.
Hexarthra, 260.
Hippodius, 59.
Hirudinea, 318.

relationshii^s of, 336.
Hirudo, 318, 328.
Holoblastic eggs, 7.
Holothuria, 393.

mesencbymaof, 397, 404, 456.

vasoperitoneal vesicle of, 410.
Holothurioidea, 392, 417, 426.

cleavage in, 392.

mesenchyma of, 400.
Hybocodon, 64, 72.
Hydra, 39, 42. 47,51, 52.

Koeselii, 52.

Tremblyi, 52.
Hydracorallia, 53.
Hydractinia, 47.
Hydroid medusa, 39, 40, 126.

polyp, 43, 50.

theory, 39.
Hydroidea, 39.
Hydrozoa, 39.

Ichthyocrinoidea, 444.
Infusorigen embryos of Dicye-

midse, 213.
Ingression, multipolar, 9.

polar, 6.
Intosbia gigas, 206.

Linei, 206.
Isodyctia, 24.

Kopfkeim (Hirudinea), 329.

Laceration, reproduction by, 101.
Lampetia, 144, 151.
Lanice, 295.
Leptomedusae, 48.

SUBJECTS INDEX

471

Leucandra, 20.
Leucochloridium, 185, 187.
Leucosolenia, 33.
Lilyopsis, 71.
Lineus lacteus, 217.

obscurus (Figs. 105-107), 224-5.
Liriope, 56.
Lizzia, 49.
Lobatffi, 151, 152.
Loimia, 295.
Lopadorhyncbus, 288, 292-294,

300.
Lophocalyx, 34.
Lucernaridse, 102.
Lumbricidffi, 231, 300.
Lumbriconereis, 275.
Lumbriculus, 301.
Lumbricus, 281, 283, 284, 291, 293,

294, 297, 300.

Macrostomum, 173, 176.
Madreporaria, 98.
Magelona, 278.
Malacobdella, 228.
Malacodermata, 100.
Mauicina, 89, 95.
Meandrina, 102.
Medusoid theory, 72.
Medusom, 65.
Megascolex, 291.
Melithsea, 83.

Meridional cleavage furrows, 3.
Mermis, 240.
MermitliidEe, 239.
Meroblastic eggs, 7.
Mertensise, 151.
Mesenchyma, 11.
Mesentery, 11.
Mesoderm, 5, 10.

somatic, 11.

splanchnic, 11.
Mesodermic bands, 10.
MesoglcEa, 79.
Mesotrochae 271, 272, 274.
Mesozoa, 215.
Metagenesis, 2.

Metastomial discB (Pilidium), 221.
Metazoa, 1.

Microcotyle (Fig. 93), 188.
Microcyema, 214.
Microhydra, 49.
Millepora, 53.
Mitrana, 275, 276, 316.
Mitrocoma, 43.
Mnemiopsis, 151.
Monactinellidse, 32.
Monaulese, 94.

Monophyes, 72.
Monophyidse 72.
Monopora, 217, 228, 229.
Monostomum, 190.

flavum, 187.

foliaceum, 200.

mutabile, 187.
Monotidae, 177.
Monotrochse, 271, 275.
Monoxenia, 77.
Muggisea, 72.

Miiller's larva, 165, 167, 168.
Multipolar ingression, see Ingres-

sion, multipolar.
Myzostoma cirriferum, 315.

complemeotal males of, 318.

glabrum, 315.

Naididae, non-sexual reproduction

of, 302.
Narcomedusse, 53, 57.
Nausithoe, 113, 119.
Nemathelminthes, 234.
Nematoda, 234.

relationships of, 246.
Nematogen embryos of Dicyemidse,

210.
Nemertini, 217.

relationships of, 230.
Nephelis, 318, 325, 328.
Nereidae, atokal forms of, 303.

epitokal forms of, 303.

non -sexual reproduction of,
303.
Nereis, 3, 263, 279, 286.
Nerine, 278.
Neuronephroblast (Hirudinea),

320.
Neuropore of Balanoglossus, 338.
Non-sexual reproduction, 12.
Nototrochae, 271, 277.

Obelia, 48.

Oceania, 49, 113.

Oculinacea, 101.

Ocyrrhoe, 151.

Oligochaeta, 281.

Oligocladus, 167.

Olynthus, 19.

Ophiactis, reproduction by division

in, 465.
Ophioglypha, mesenchyma of, 400.
Ophiopholis, mesenchyma of, 403.
Ophiotrix, enterohydrocoele of, 413.
OiDhiuroidea, 422, 437.
mesenchyma of, 400.

472

SUBJECTS INDEX

Ophryotroeha, 277, 278, 280, 313,

316.
OrthonectidBe, 206.

relationships of, 215.
Orthoptera, sexual cells of, 12.
Oscarella, 20, 22, 28, 3i.
Otomesostoma, 177.
Oxysoma, 236.

Paractinia3, 93.

Parenchymula, 22, 41.

Peachia, 94.

Pelagia, 105, 118, 122.

Pelagidie, 120.

Pennaiia, 47.

Pennatula, 83, 87.

Pentacta?a, 458, 459.

Peromedusffi, 123.

Person, 1.

Phalangidae, sexual cells of, 12.

Phascolosoma, 357, 364.

Phialidium, 49.

Phorouis, relationships of, to

Sipunculidte, 365.
Phyllochsetopteras, 274.
Phyllophorus, larva of, 419.
Physalia, .59, 62, 74, 75.
PhysonectsB, 59, 75.
Physophora, 64.
PhysophoridiB, 59, 60, 67.
Pietocystis dythiiidium, 195.

variabilLs. 195.
Pileolaria, 279,- 300.
Pilidium, 217.

auriciilatum, 220.

brachiatum, 220.

gyrans, 220.

recur vatum, 220.
Placina, 14.
PlakinidfB, 22, 28.
Planaria angulata, 169.

polychroa, 169.
Planula, 14.
Platyhelminthes, 159.
Plerocerci (Taeniadas), 195.
Plerocercoids (TiBuiadoe), 195.
Plcurobrachia. 138.
PleurochtKta, 291.
Pluteus, 422, 4i4.
Piieumatophorida), 59, 62.
Polar globules, 3.

ingressioa, see Ingression,
l^olar.
Pole, animal, 3.

vegetative, 3.
Polia, 228.

Polychsera, 262.
Polycladida, 160, 173. 174.
Polygordius, 263, 267, 268, 237,

290, 292, 295.
Polylophns, 34.
Polyuoe, 262.
Polyopsis, 93.
Polypbyidae, 71, 72.
Polvpodium, 49.
Polystomidffi, 188, 190.
Polvstomum, 188.
Polytrochfe, 271, 273.
Pomatoceros, 3, 263.
Porifera, 13, 28, 32.
Porpita, 59, 65.
PorpitidsB, 65.
Pray a, 71.

Primary germ-layers, 5.
Primitive mesoderm cells, 10.

mouth, see Blastopore.
Prorhynchus, 173.
Pi'ostonia, see Blastula.
Prostomial discs (Pilidium), 221,
Pi'ostomum lineai'e, 173, 175.

Steenstrupii, 173, 174.
Protaxonia, 9.
Protodrilus, 287.
Pi'otohydra, 49.
Protozoa, 11,
Protiila, 302.
PseudoceridsB, 160.
Pseudocoele, 11.
Pseudogastrula, 17.
Psolinus, 419.
Psygmobrauchus, 263, 275, 279,

284, 291, 297, 299, 300.
Pteraster, brooding iu, 436.

Raspailia, 24,
Ratariae, 65.
Rathkea (Fig. 13), 40.
Redia (Distomidie), 183.
Reniera, 24, 27.
Renilla, 76, 83, 88.
Rhabditis, 238.

aoeti, 235.

nigrovenosa, 234, 242.

stercoralis, 242.
Rliabdoccela, 159.
RhabdoccelidfB, 159, 173, 174.
Rhabdonema, 242.
Rhizocriuus, 453.
Rhizophysa, 59. 75.
Rhizostoma, 120.
Rhizostomne, 124.
Rhizoxonia, 82.
Rhopalouema, 57.

SUBJECTS INDEX

473

Ehopalura, 206, 207.

Giardii, 206.

lutoshii, 206.

ophiocoruffi, 206.
Ehynchelmis, 281, 282, 284, 286,

291, 299, 300.
Ehvnchobdellidsa, 319.
Einalda, 34.
Eotatoria, 2-56.

relatiousliips of, 2.59.
Eumpfkeim (Hirudinea), 329.

Sabella, 3, 263.

Sabellaria, 263, 276.

SaccociiTus, 290.

Sagitta, 367.

Sarsia prolifera, 72.
siphonophora, 72.

Scbizocladium, 49.

Sc.bizostomum, 17-5.

Scleiodermata, 100.

Sclerogorgia, 83.

Scolex (Tffiniadffi), 197.

Scyphistoma, 104, 105, 110.

Seypbomedusffi, 39, 102, 103, 122.

Scyijbopolyps, 103.

Scyphozoa, 124.

Scyphula, 125.

Sedeutaria, segmental appendages
of, 279.

SemEeostomae, 124.

Sexual reproduction, 2.

Silicea, 22.

Sipbouophora, 39, 59.

Siphonula, 65, 70.

Sipunculidfe, 357, 365..

relationsbips of, to Annelida,
365.

Sipunculus, 357.

larva of, compared to Trocbo-
phore, 361.

Somatic mesoderm, see Mesoderm,
somatic.

Spatangus, 424.

Spbaarechinus, cleavage in, 398.

Spbserosyllis, 262.

Spbffirularia, 240.

Spio, 276, 278.

Spiroptera, 238, 243.

Spirorbis, 279.

Pagenstecberi, 262.
spirillum, 262.

Splanchnic mesoderm, see Meso-
derm, splanchnic.

Spongelia, 23.

Spougicola, 113.

Spongilla, 25, 27, 34.

Spongioblasts, 32.

Sporocyst (Distomidse), 181.

Si'orogouia, 59.

Sporophores, 47.

Sporosacs, 39.

Sprouting (Porifera), 28.

Stelospougia, 28.

Stephalia, 62.

Stei^hanomia, 60.

Stephanoscyplius, 103.

Sternaspis, 273.

Sterroblastula, 7.

Sterrogastrula, see Gastrula, epi-

bolic.
Stock, 1.

Stomobracbium, 49.
Strobila, 104, 113, 114.
Strongylocentrotus, cleavage in,
398.

metamorphosis of, 439.
Stylasteridse, 53.
Stvlochopsis, 168.
Stylocbus, 162, 164, 167, 168.
St\lodrilus, 299.
Stylorbiza, 121.
Sycandra, 14.
Sycou. 19.
Syoyonis, 93.
Syllidffi, nonsexual reproduction

of, 302, 304.
Syllis ramosa, 304.

resemblance of larva to Dino-
philus, 313.

vivipara, 262.
Sympagella, 33.
Sympodiiim, 76, 77.
Synapta, 398.

cleavage in, 392.

mesenchyma, 396, 397, 404,
456.

metamorphosis of ciliated
band, 430.

nerve-ring of, 427.

vasoperitoneal vesicle of, 409.

Taenia coenurus, 197, 200.

cucumerina, 193.

echiuococcus, 200.

saginata (Fig. 96), 196.

serrata, 193.

solium, 198.

tailed cvsticerci of, 196.
Tasuiadee, 193.

embryonal membranes of, 194.
Telepsavus, 274, 316.

474

SUBJECTS INDEX

Telolecithal eggs, 8.
Telotrochffi, 271, 275.
Terebella, 279, 280, 291.

Meckelii, 263, 272-274.
Terehellidie, 295.
Tethya, 34.
Tetilla, 84.
Tetractinellidse, 32.
Tetrasteiuma, 228.
Thalassema, 304-306, 310.
Thoe, 151.

Thysanozoon, 160, 166.
TiaridsB. 58.
Tornaria, 381, 382.

heart of, 385, 386.
Toxopneustes (Fig. 186), 401.
Tracheophysse, 59, 65.
Trachomedusffi, 53, 126.
Trematoda, 178.

derivation of, 201.

larval membranes of, compared
to amnion of Pilidium, 19'^.

relationships of, to Cestoda,
200.
Trichina, 234, 243.
Trichocephalus aftinis, 239.

dispar, 234.
Tricladida, 169, 174.

Trochophore (Annelida), 266, 341.

compared to Medusa, 342.

(Pilidium). 219.

(Rotatoria), 259.
Trochosphsera, 259.
Trochosphere, 266.
Tubifex. 281.
Tubipora, 83.
Tubularia, 49, 50.
Turbellaria, 159.

relationships of, 176.
Turbinaria, 101.
Turritopsis, 58.

Umbrosa, 120.

Vegetative pole, see Pole, vegeta-
tive.

Velella, 59, 74.

Velellidffi, 65, 67.

Versuridffi, 121.

Von Baer's cavity, see Cleavage
cavity.

Zoantharia, 88, 100.
Zoantheffi, 94.
Zoanthus, 100.
Zonactinia, 100.

AUTHORS INDEX.

Agassiz, a.

Annelida, 276.

Arachnactis, 98.

Asteroidea, 405, 422, 436.

Autolytus, 302.

Balanoglossus (Fig. 164), 374.

Beroidse, 154.

Bipinnaria and Brachiolaria,
432.

Bolina, 138, 151.

Ctenophora, 138, 146, 151.

Cyanea, 112.

Echinoidea, 442, 443.

Hemiaster, 436.

Planaria angulata, 169.

Velellidffi, 65.
Agassiz, L.

Aurelia, 105.

Hydrozoa, 44.
Allman, G. J.

Beroidffi, 154.

Ctenophora, 138.

Hydroidea, 46, 49, 50.

Stephanoscyphus, 103.
Andres, A.

Zoantharia, 95, 101.
Andbews, E. a.

Annelida, 357.
Apathy, S.

Hirudinea, 333.
Apostolides, N. C.

Ampbiura, 413.

Ophiotrix, 413.

Balfour, F. M.

Annelida, 342.

Echinodermata, 414.

Hirudinea, 325, 332.

Nemertines, 229.

Physophora, 64.

Porifera, 36.

Siphonophora, 72.
Baerois, Ch>

Antedon, 447, 451.

Baerois, Ch.

Ascandra, 20.

Crinoidea, 405, 412, 413.

Desmacidon, 24.

Halisarca, 20.

Isodyctia, 24.

Nemertini, 224-226, 228.

Oscarella, 20.
Bate son, W.

Balanoglossus, 374-387.

Tornari^,, 385, 386.
Beard, J.

Myzostoma, 315.
Beddard, F. E.

PleurocliBeta, 291.
Bedot, M.

Velellidse, 65.
Beneden, E. van.

Arachnactis, 98.

Cerianthese, 92.

Dicyemidffi, 206-215.

Tffiuiadse, 190, 193.

Zoanthese, 92.
Beneden, P. J. van.

Cestoda, 204.

Cyanea, 112.
Benham, W. B.

Oligochseta, 299.
Beraneck, E.

Annelida, 355.
Bergh, E. S.

Annelida, 287, 288, 293.

Aulastoma, 331.

Clepsine, 322.

Criodrilus, 284.

Hirudinea, 328, 329, 335.

Lucernaria, 102.

Lumbricus, 297, 298.

Nephelis, 331.

Oligochseta, 296, 297, 299.
Biehringer, J.

Distomidee, 183._
Blochman, F., und Hilger, 6.

Gonactinia, 95, 100.

475

476

AUTHORS INDEX

BOUBNE, A. G.

Balanoglossus, 386.
Hirudiuea, ij34.

BOVERI, T.

Arachnactis, 98.

Ascaris, 20.

Hexactiiiiffi, 91.

Sagitta, 367.
Braf.m, F.

Aunelida, 355.
Brandes, G.

Trematoda, 233.
Brauer, a.

Hydra, 51.

Tubularia, 50.
Braun, M.

Dicyemidse, 216.

Orthonectidse, 216.

Pieurocerci (Trematoda), 195.

Trematoda, 192.
Bride, E. W. Mac.

Ecbinodermata, 464.
Brooks, W. K.

Cunoctautha, 58.

Epenthesis, 49.

Eiitima, 74.

Geryonidse, 53.

Liriope, 66.

Medusa, origin o'', 126.
BtJLow, G.

Lumbriculus, 350.

BtJHGER, 0.

Hirndinea, 334.

Nemertini, 223, 227.
Bury, H.

Antedon, 416, 443-448, 451,
452.

Crinoidea, 404, 405, 413.

Ecbinodermata, 414, 415, 453,
461.

Echnioidea, 442.
BuscH, W.

Cbrysaora, 113.

BUXSCHLI, O.

Cbaetognatha, 367, 370.
Caculhinus, 235.
Nemertini, 223.
Nephelis, 325.
Pilidium,2l9, 221.
Porifera, 29.

Camerano, L.

Gordius, 244, 246.
Carloken, 0.

Ceriantbere, 92.

Zoantheaj, 92.

Carpenter, P. H.

Ecbinodermata, 458.
Carpenter, W. B.

Antedon, 452, 455.
Carter, H. J.

Halisarca, 20.

Oscarella, 20.

Porifera, 35.
Chadwick, H. C.

Ecbinodermata, 464.
Chun, C.

Auricularia nudibrancbiata,
419.

Calycophoridse, 72.

Cestus, 153.

Ctenopbora, 136, 138, 148,
149, 151.

Diphyidae, 71.

Encbaris, 145.

Eudoxia, 72.

Holotburioidea, 419.

IjHmpetia, 138, 144, 151.

Monopbyes, 72.

Muggiaea, 72.

Sipbonophora, 60, 67, 72.

Turbellaria, 176.

Velellidte, 65.

ClAMICIAN, J.

Tubularia, 50.

CLAPARfcDE, E.

Annelida, 271.
Ophryotrocba, 277.
Terebella, 273.

CLAPARfcDE, E , UND MeTSCHNIKOFF,

E.

Annelida, 350.
Claus, C.

Atorybia, 64.

Aurelia, 105, 120.

Cestoda, 199, 200.

Cbrysaora, 112.

Ctenopbora, 136-138.

Diseomedusffl 120.

Hydroidea, 46.

Hydrozoa, 40, 44.

Nausitboe, 119.

PiiysopboridiB, 66.

llhizostoma, 120.

Soypliistoma, 107.

Sipbonopbora, 72.

Umbrosa, 120.
CoBB, N. A.

Nematbelmintbes, 248.
CoLTON, B. p., and Garman, H.

Arbacia, 439.
Conn. H. W.

Tlialassema, 304, 305.

AUTHORS INDEX

477

Conn, H. W.

Tubularia, 51.

Dalyell, J. G.

Hydroidea, 49.

Planula, 41.

Zoantharia, 101.
Davidoff. M. von.

Pbialidium, 49.
Delages, Yves.

Cornacuspongia, 22.
Dendy, a.

Sycandra, 17.
Deso, B.

Porifera, 34.

DiCQUEMARE, J F.

Zoantharia, lOl.
Dieck, G.

Cephalothrix, 227.
Dixon,

Hexactinise, 92.
Drasche, R. von.

Polycbffita, 263.
Driesch, H.

Hydrozoa, 43.
Dujardin, F.

Hydroidea, 129.

Edwards, C. L.

Echinodermata, 464.
Edwards, H. Milne, see Milne-
Edwards, H.
Ehlers, E.

Nereidae, 304.
EisiG, H.

Capitellidse, 295, 298.

Faurot, L.

Anthozoa, 132.
Fewkes, J. W.

Agalma, 64.

Amphiura, 413.

Annelida, 357.

Arbacia, 425.

Ctenophora, 151.

Echinaracbnius, 403, 439.

Mnemiopsis, 151.

Nemertini, 230.

Ocyrrboe, 151.

Ophiopholis, 403.

Pilidium, 220, 221.

Siphonophora, 60.

Tornnria, 390.
Fiedler, K.

^l.ongilla, 14.
Field, G. W.

Echinodermata, 4G4.

Fleischman, a.

Echinocardium, 398.

Echinoidea, 399.
FoL, H.

Ctenophora, 138-140, 146.

Euramphtea, 151.

Geryonidffi, 53.

Lucernaridfe, 102.
Fraipont, J.

Annelida, 288.

Polygordius, 267, 295.

Ganin, M. S.

Nematoda, 237.

Pelodera, 247.

Spongilla, 25, 28.
Garstang, W.

Echinodermata, 464.
Gegenbaur, V.

ChsBtognatha, 367.

Eudoxia, 71.

Pilidium, 221.

Siplionophora, 60.
Gerd, W.

Tubularia, 50.

GlARD, A.

Balanoglossus, 390.

Ortbonectidae, 209, 260.
Goette, a.

Annelida, 288.

Antedon, 448.

Ascaris niKrovenosa, 236.

Asteracanthion, 407.

Aurelia, 105.

Cotylorhiza, 108, 112.

Crinoidea, 405, 413.

Ephyra, 119.

Nematoda, 234, 237.

Nereis, 263, 286.

Polycladida, 160.

Porifera. 30, 35.

Ebabditis, 234, 235, 238.

Scypbistonia, 107.

Scyphomedusse, 124.

Scyphula, 125.

Sijongilla, 25-27.

Stylochus, 162, 168.

Tornaria (Fig. 170J, 384.

Turbellaria, 176.

GUAFF, L. VON.

Monotidfe, 177.
Myzostoma, 353.
Otomesostoma, 177.
Turbellaria, 201.
Grassi, B.

Cestoda, 192.
Choetognatha, 371.

478

AUTHORS INDEX

Grassi, B., e. Rovelli, G.

Cysticercus, 196.
Greef, R.

Echinorhyuchus, 249, 251.

Protohydra, 49.
Grobben, C.

Annelida, 300.

Cestoda, 199.

Distomidse, 183.

Moina, 12.
Grubek, VV.

Cestoda, 199.

Haacke, W.

Charybdsea, 103.
Hydra, 52.
Zoautharia, 96.
Haddon, a. C.

Halcampa, 94.
Hexactinise,- 92.
Peacliia, 94.
Zoautharia, 91.
Haeckel, E.

Asteroidea, 455.
Amelia, 105.
Caliconula, 70.
Ctenophora, 154.
Disconula, 65.
Ephyra pedunculata, 118.
Ephyropsidffi, 124.
GeryonidsB, 57.
Leptomedusse, 48.
Monoxenia, 77.
Pelagia, 118.
Physalid larvs, 62.
Physonectas, 75.
Porifera, 36.
Siphouophora, 60, 72.
Siphonula. 65.
Starfish, 455.
Stephalia, 62.
Velellidae, 65.
Haecker, V.

Hydroidea, 130.
Haime, .T.

Cerianthus, 98.
Hallez, p.

Ascaris, 2:^5.
Dendrocoelum, 169, 170.
Leptoplaua, 161.
Nematoda, 234, 236, 237.
01i{,'ocladus, 167.
Oxysoma, 236.
Poiycladida, 160, 164.
Rhabditis, 235.
Rhabdoc(julid£E, 173, 174.
Tricladida, 170, 171, 173.

Hamann, 0.

Autedon, 454, 455.
Cyanea, 106.
Cysticercus, 196.
Echinorhynchus, 253.
Halecium, 51.
Hydroidea, 50.
Scyphomediisse, 122.
Tubularia, 51.
Harmer, S. F.

Dinophilus, 314, 315.
Hartlaub, C.

Eieutheria, 154.
Obelia, 48.
Hatschek, B.

Annelida, 287.
Cnidaria, 125.
Criodrilus, 284, 296.
Ctenophora, 155.
EchiQoidea, 349, 400.
Echiurus, 307.
Eupomatus, 263.
Hydroidea, 39.
Oligochffita, 281, 286, 296.
Polychseta, 263.
Polygordius, 268, 292, 295.
Protaxonia, 9.
Rotatoria, 260.
Scyphozoa, 124.
Siphonophora, 73.
Sipunculus, 357.
Trochophore, 266.
Heckert, G.

Leucochloridium, 185.
Heider, a. von.

Calycoblasts, 99.
Cerianthus, 95.
Zoautharia, 99.
Heider, K.

Gastrodes, 58.
Oscarella, 22.
Porifera, 30.
Hensen, V.

Holothurioidea, 398.
Hertwig, 0.

Chajtognatba, 367, 370.
Nausitboe, 113.
Sagitta, 371.
Hertwig, R.

Actiuaria, 03.
Aracbuactis, 98.
Calliauira, 137, 150.
Ctenophora, 154.
Hertwig, 0. und R.
Adarasia, 91.
Alcyonaria, 75.
Coelomtheorie, 173.

AUTHORS INDEX

479

HeRTWIG, 0. UND E.

Hydroidea, 46.
Medus£e, origiu of, 124.
Mesenchyma, 11.
Zoantharia, 91.

HiCKSON, S. J.

Alcyonaria, 82.

Hydrocorallia, 53.
HiNKS, T.

Hydroidea, 129.
Hoffman, G. K.

Clepsine, 352.

Nemertini, 228, 229, 232.

HORST, E.

Annelida, 351.

Hermella, 237.
HUBRECHT, A. A. W.

Chordata, 232.

Nemertini, 173, 224, 227, 231.

Pilidium, 222.
Huxley, T. H.

Physalid, 61.

Protula, 302,
Hyde, I. H.

Hydroidea, 131.

IlJIMA, I.

Tricladida, 169, 170, 172.

Jaekel, 0.

Echinodermata, 465.

JOLIET, L.

Eotatoria, 256.

JOURDAIN, S.

Sagitta, 368.

JOURDAN, E.

Actinia, 89.
Zoantharia, 88, 89.

JULIN, C.

Dicyemidge, 215.
Oi'thonectidffi, 206, 209.
Jung, H.

Hydra, 52.

JUNGERSEN, H. F. E.

Pennatula, 87.
Pteroides, 87.

Kaiser, J.

Echinorhynchus, 249, 250,
253, 254.
Keferstein, W.

Orthonectidffi, 206.

Planaria, 201.
Keller, C.

Ascandra, 20.

Chalinula, 24.

Gastroblasta, 49.

Leucandra, 20.

Kennel, J. von.

Annelida, 347.

Ctenodrilus, 301.

Nemertini, 231.
Kerschner, L.

Hydra, 51, 52.

KiSHINOUYE, K.

Echinodermata, 465.
Kleinenberg, N.

Alciopidae, 289.

Annelida, 272, 291.

Ctenophora, 150.

Hydra, 51, 52.

Lopadorhynchus, 288, 292.

Lumbricus, 282, 284, 286, 293.

Oligochseta, 281.

Trochophore, 266.

Zoantharia, 89.
Koch, G. von.

Alcyonaria, 76, 80.

Antipatharia, 81.

Astroides, 98.

Corallium, 83.

Gerardia, 81.

Gorgonia, 76.

Hexacorallia, 81.

Hydroidea, 47.

Melithffia, 83.

Sclerogorgia, 83.
KOFOID, C. A.

Limax, cleavage in, 4.

KOHLER, R.

Acanthocephali, 255.
Balanoglossus, blood-vascular

system of, 376-378.
Tornaria, 385.

KOLLIKER, A. VON.

Ctenophora, 177.
Dicyema, 206.
Stomobrachium, 49.

KOREN ET DaNIELSSEN.

Asteroidea, 432.
Pteraster, 436.

KOROTNEFF, A.

Calycophoridae, 75.
Ctenoplaua, 201.
Gastrodes, 58.
Hydra, 51, 52.
Lucernaridse, 102.

KORSCHELl', E.

Dinophilus, 314.
Strongylocentrotus (Fig. 182),
398.
Kowalevsky, A.
Actinia, 88.
Adamsia, 88.
Alcyonaria, 80.

480

AUTHORS INDEX

K0WAI,EVSKT, A.

Annelida, 287, 288, 291.
Asteroidea, 456.
Astnea, 88.
Aurelia, 105.

Balanoglossus, 378, 379, 390.
Ceriantbus, 89.
Chsetognatha, 367.
Clavularia, 76.
Coeloplana, 201.
Cotylorhiza, 112.
Cteuopbora, 138, 144.
Gorgonia, 76.
Hirudinea, 332.
Lucernaria, 102.
Lumbiicus, 283, 286.
OliKochffita, 281, 286.
Pelagia, 118.
Pleurobrachia, 138.
Psolinus, 419.
Spongicola, 113.
Sympodium, 76.
Tbalassema, 305.
Tuibellaria, 176.
Zoautbaria, 88.

KOWALEVSKY, A., ET MaEION, A. F.

Alcyouaria, 76, 80.
Kbohn, a.

Autolytus, 302.
Dicyemidfe, 206.
Opbiuroidea, 762.
Pebagia, 118.
Pilidium, 221.

Lacaze-Duthiers, H. de.

Actinia, 96.

Alcyouaria, 76.

Astroides, 98.

Corallium, 76.

Pennatula, 87.

Pteroides, 87-

Zoautbaria, 90, 99.
Lang, A.

Cteuopbora, 155.

Cteuoplaua, 155.

Discocelis, 161-

Gastroblasta, 49.

Gmida, 21)2.

Polytdadida, 160, 164.

Scypbozoa, 121.

Turbellaria, 176, 177.
Lang, Alb.

Hydrozoa, 43.
Lankesteh, E. 11 \y, see Bay

Lankesxer, E.
LeH'oi.dt, F.

Ecbinodermata, 465.

Lendknfeld, K. von.

Campanularia, 49.

Hydroidea, 49.

Porifera, 30.

Scyphomedusse, 122.

Stylorbiza, 121.

Versuridffi, 121.
Leuckart, K.

Allautouema, 241.

Arcbigetes, 199.

Attractonema, 240.

Aulastoma, 318.

Dicyemidee, 215.

Distomum bepaticum, 180.

tcbiuorbyncbus, 250-254.

Fpibulia, 71.

Eudoxiffi, 71.

Gpryduidte, 57.

Hirudinea, 335.

Hirudo, 318, 328.

Hydroidea, 46.

Nematoda, 239.

Ortbouectidffi, 215.

Pietocystis, 195.

Eedia, 183.

Ebabdouema, 242.

Sipbouophora, 72.

Sjibferularia, 240.

Tffiuia solium, 198.

Tseniadae, 193, 195.

Trematoda, 183, 192.

Tricbina, 244.

Tricbocepbalns, 239.
Leuckart, R., und Pagenstecher,
A.

Pilidium, 220, 221.

LiNSTOW, O. VON.

DistomidiE, 203.
GordiidsB, 246.
Loos, A.

Cestoda and Trematoda, 192.

LORENZ, L.

Axine, 203.

Microcotyle (Fig. 93), 203.
LOVEN, S.

Ecbinodermata, 458.

Polygordius, 268.
LUDWUJ, ri.

Antedou, 447, 448, 454, 455.

Asterina, 402, 403, 407, 414,
421, 422, 435, 436.

Asteroidea, 439, 435.

Ciiirodota, 419.

(Incuinaria, 429, 432.

Ilolotliurioidea, 398.

Opbiuroidea, 403, 438.

Pluteus, 425.

AUTHORS . INDEX

481

Maas, 0.

Cornacuspongia, 22.

Spongilla, 28.
MacBride, E. W., see Bkide, E.

W. Mac.
McCrady, J.

Bolina, 151.

Cunoctantba, 58.
McIntosh, W. C.

Nemertini, 233.

OrthonectidsB, 206.

Syllis, 304.

McMURRICH, J. p.

Aulactinia, 91.

Cyanea, 106.

Hexactinite, 92.
Magdeburg.

Placina, 14.
Malaquin, a.

Annelida, 355.
Marion, A. F.

Balanoglossus, 390.
Marshall, W.

Porifera, 29, 35.

Eeniera, 25.
Martens, E. von.

Asteroidea, 455.
Megnin, p.

Ecbinorbynchus, 255.
Meissner, G.

Gordiidae, 244, 245.
Merejkowsky, C. de. See also
Mereschkowskt, K.

Porifera, 34.
Mereschkowsky, K.

Hydroidea, 52.
Metschnikoff, E.

iEeginopsis, 57, 58.

Agalma, 64.

Agiaura, 57.

Annelida, 271.

Ascetta, 22.

Asteroidea, 405.

Astropecten, 402.

Auricularia, 418.

Balanoglossus, 390.

Bipinnaria, 432, 435.

Brachiolaria, 432, 435.

CamiJanularia, 52.

Ctenophora, 138, 141, 143, 144,
149.

Cunina, 59.

Echinodernaata, 400, 414, 461.

Echinoidea, 409, 414, 438, 439.

Epibulia, 68.

Esperia, 34.

Geryonidse, 53.

Metschnikoff, E.

Halisarca, 25.

Halistemma, 62.

Holotburioidea, 431.

Hydroidea, 39, 40, 50.

Leucandra, 20.

Lineus, 217.

Mitraria (Fig. 124), 276.

Mitrocoma, 43.

Monostomum, 190.

Myzostoma, 315.

Narcomedusae, 57, 58.

Nausithoe, 113.

Oceania, 113.

Opbiuroidea, 406, 407, 414,
437.

Opbryotrocba, 277.

Ortbonectidas, 206, 208, 209,
215.

Parenchymula, 41.

Pelagia, 118.

Pilidium, 221.

Planaria, 169.

Rbopalonema, 57.

Siphonopbora, 60.

Spatangus, 489, 443.

Spongicola, 113.

Stylochopsis, 168.

Sycandra, 14, 19.

Terebella, 273.

Tricladida, 170, 171.

Tubularia, 51.

Turbellaria, 176.
Meyer, E.

Ampbitrite, 298.

Annelida, 295.

Dinopbilus, 313.

Psygmobrancbus, 297.
Milne-Edwards, H.

Terebella, 273
Milne-Edwards, H., et Haime, J.

Zoantbaria, 90, 99.
Moniez, R.

Taenia cucumerina, 193.

Taeniadae, 193.
Monticelli, F. S.

Cercaria Villoti, 186.

Trematoda, 192.
Morgan, T. H.

Balanoglossus, 373, 376, 386.

MORTENSEN, T.

Ecbinodermata, 465.

MOSELEY, H. N.
Heliopora, 96.
Hydrocoralliffi, 53.
Saccopbyton, 96.
Stylasteridse, 53.

K. H. E.

I I

482

AUTHORS INDEX

Mrazek, a.

Cysticercus, 196.

MtJLLEB, JOH.

Arbacia, 419, 422, 425, 432.
Asteroidea 422. 432.
Auiicularia, 417, 421.
Ctenophora, 151.
Mitraria, 275.
Narcomedusse, 58.
Ophiuroidea, 407, 437.
Pilidium, 218, 221.
Pletus, 419.
Tornaria, 382.
Turbellaria, 202.
MULLER, 0. F.

Ctenophora, 137.
Geryonidse, 57.

Nachtrieb, H.F.

Autedon, 416.
Nansen, F.

Myzostoma, 317, 318.
Neumatr, M.

CystidaB, 459.

NOSCHIN, N.

Scyphomedusae, 135.

NUSBAUM, J.

Hirudinea, 332.

OUDEMANS, A. C.

Nemertini, 231.

Pereyaslawzewa, S.

Accela, 174.

Aphanostoma, 175.

Convoluta, 175.

Darwinia, 175.

Macrostomum, 175.
Perkier, E.

Antedon, 447, 448, 452, 454,
455.

Rhizocrinus, 453.
Petr, F.

Spongilla, 34.
Plate, L.

Eotatoria, 200.
Price, J.

Ctenophora, 151.

Quatrefages, a. de.
Annelida, .352.

Racovitza, E.

Annehda, 355.
Randolph, H.

Annelida, 355.

Rathke, H.

Clepsine (Fig. 153), 324.
Ray Lankester, E.

Troehosphere, 206.
Repiachoff, W.

Diuophilus, 314.

RiETSCH, M.

Echiuridae, 353.
Sternaspis, 273.
RiTTER, W. E.

Balanoglossiis, 391.
Robin, C.

Hirudinea, 327.
ROULE, L.

Enchytraeoides, 293.
Russo, A.

Echinodermata, 465.

S^FFTIGEN, A.

Echinorhynchus, 255.
Salensky, W.

Amphilina, 200.

Annelida, 279, 288, 294, 300.

Branchiobdella, 337-340.

Echiurus, 353.

Enterostonium, 169.

Lumbricus, 286.

Mouopora, 228, 229.

Nemertini, 223, 226.

Ophryotrocha, 280.

Pilidium, 219, 222.

Polychffita, 203.

PsygDDobranchus, 291, 292.

Rhabdocoilidae, 174.
. Rotatoria, 256, 258.

Terebella, 273, 274, 289, 291.
Sarasin, p. dnd F.

Echinodermata, 459.

Echinoidea, 461.

Echinothuridffi, 458.

Liuckia, 450.
Sars, M.

Arachnactis, 98.

Asteroidea, 422.

Aurelia, 105.

Bipinnaria, 420.

Cyauea, 112.

Gouactinia, 100.
Schauinsland, H.

Bothriocephalida}, 190, 194.

Bothriocephalus, 192.

Cestoda, 194.

Distoiuidae, 180.

Distomum, 178.

Priapulidte, 363.

Trematoda, 194.

AUTHORS INDEX

483

SCHIJIKEWITSCH, W.

Balanoglossus, 391.

Hydroidea, I'Sl.
SCHMAKDA, L. E.

Hexarthra, 260.

Rotatoria, 261.
Schmidt, F.

Bothriocephalus, 198.
Schmidt, 0.

Ascetta, 22.

Dinophilus, 314.

Esperia, 24.
ScHNEiDEK, Ant.

Nematoda, 238.

Polygordius, 268.

Spbterularia, 240.
ScHULTZE, Max.

Arenicola, 278.
SCHULZE, F. E.

Halisarca, 20, 25.

Hydroidea, -50.

Lopliocalyx, 34.

Oscarella, 20.

Plakinidffi, 22.

Polygordius, 34.

Porifera, 30.

Spongelia, 24.

Spongicola, 113.

Spongioblasts, 32.

Sycandra, 24.

SCHWARZ, W.

Cercaria, 184.

Eedia, 183.

Trematoda, 192.
Seeligee, O.

Antedon, 45-5.

Asteroidea, 416.

Echinoidea, 416.
Selenka, E.

Cucumaria, 419, 429.

Discocelis, 164.

Echinoidea, 399, 401, 409.

Echinus, 398.

Eurylepta, 161, 162.

Holothurioidea, 398, 400, 403,
431.

Ophimoidea, 400, 403.

Phascolosoma, 364.

Pluteus, 425.

Polycladida, 160, 164.

Sphffirechinus, 398.

Strongylocentrotus, 398.

Synapta, 396.

Tetilla, 34.

Thysanozoon, 160.

Turbellaria, 176.

I Semon, R.

Auricnlaria, 418, 421, 426.
Bipinnaria, 420.
Holothurioidea, 427, 428, 431,

432.
Pentactea, 458, 459.
Synapta, 427.
Semper, C.

Flabellum, 100, 102.
Fungia, 100.
Hirudinea, 229.
Naidida?, 302.
TrochospbEera, 259.
Zoantharia, 102.

SlEBOLD, C. T. E. VON.

Aurelia, 105.
Mermis, 162.
Nemathelminthes, 247.
Trematoda, 132.

SiMEOTH, H.

Echinoderniata, 456.
Smith, F.

Aurelia, 105.
SOLLAS, W. J.

Porifera, 29.
Spengel, J. W.

Balanoglossus, 373, 374, 376-
378, 384, 387, 388.

Bonellia, 304, 310, 312.

Orthonectidfe, 216.

Tornaria, 385, 386.
Steenstrup, J. J. S.

Cestoda, 198.

Cnidaria, 128.
Strubell, a.

Heterodera, 241.

Nematoda, 237.
Studee, T. ,

Astraeacea, 101.

Gorgonidae, 80.

Oculinacea, 101.

Tessin, G.

Eosphora, 256.

Rotatoria, 256, 258.
Theel, H.

Echinocyamus, 440.
Thomas, A. P.

Distomum hepaticum, 180.
Thomson, C. W.

Antedon (Fig. 223), 451.

Asteroidea, 422.

TlCHOMIUOFF, A.

Tubularia, 51.

TOPSENT, E.

Porifera, 34.

484

AUTHORS INDEX

Ussow, M.

Pol jijodium , 49.

Vasseor, G.

Leucosolenia, 33.
Vejdovsky, F.

Aunelida. 287, 298.

Brancbiobdella, 337.

Criodrilus, 291.

Gordiidaj, 24G, 248.

Lumbiieus, 291.

Oligochasta, 281, 296, 297,
299.

Ehynchelmis, 282, 284, 286,
291.

Sternaspis, 273.
Veeeil, a. E.

Epiactis, 100.

VlQUIEB, C.

Exogone, 280, 281.
Grubea, 280.
SpbfBi-osyllis, 262.

ViLLOT, A.

Cercaria setifera, 186.
Goidiidse, 244, 246.
Gordius, 244. 245.
Ta?niada3, 190.

VOGT, C.

Aiachnactis, 98.
Medusffi, 126.

VOIGT, W.

Brancbiobdella, 337.
VOSMAER, G. C. J.
Porifera, 37.

Wagener, G.

Amphilina, 200.

Dicyemidae, 200.

Monostomum, 190.

Redia, 183.

Tseniadffi, 195.
Wagner, F. von.

Myzostoma, 317.
Wandollek, B.

Nematbelminthes, 278.
Weikmann, A.

Hydroidea, 47, 48.

Hydrozoa, 43, 45.

WeISMANN, a. , UND ISCHIKAWA, C.

Rotatoria, 256.
Weldon, W. F. R.

Dinopbilus, 314.
Weltner, W.

Spongilla, 34.
Whitman, CO.

Clepsiue, 319, 322, 330-332.

Dicyemida?, 20G, 209, 210, 213,
215.

Hirudinea, 324, 332, 333.

Nephelis, 325.

WiERZEJRKI, A.

Spougilla, 35.
Wilson, E. B.

Alcyonaria, 76, 82.

Lumbricus, 293.

Nereis, 263.

Pilidium, 220.

Renilla, 76, 84.
Wilson, H. V.

Hydroidea, 129.

Manicina, 89, 95.

Porifera, 38.

Zoantbaria, 88, 91.

WiSTINGHAUSEN, C. VON.

Annelida, 356.

Zacharias, O.

Rotatoria, 257.
Zelinka, C.

Gastrotricha, 260.

Rotatoria, 259.
Zeller, E.

Diplozoon, 189.

Leucocbloridiuin, 185, 187.

Polystomum, 188.
Zeppelin, M.

Ctenodrilus, 301.
Ziegleh, H. E.

Gasterostomum (Fig. 91), 186.

Trematoda, 192.

ZOGRAFF, N.

Cestoda, 192.
ZuR Strassen, 0.

Nematbelminthes, 278.
Zykoff, W.

Porifera, 38.

Butler & Tanner, The Selwood Printing Works, Fromo, and London.

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