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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^ iiJ J nA ^ ■ —

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 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.

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 migr