THE WORKS

OF

FRANCIS MAITLAND BALFOUR,

VOL. II.

<£iuttom

Cambrfofle :

PRINTED BY C. J. CLAY, M.A. AND SON,
AT THE UNIVERSITY PRESS.

<0&ftton.

THE WORKS

OF

FRANCIS MAITLAND BALFOUR,

M.A., LL.D., F.R.S.,

FELLOW OF TRINITY COLLEGE,

AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF
CAMBRIDGE.

EDITED BY
M. FOSTER, F.R.S.,

PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;
AND

ADAM SEDGWICK, M.A.,

FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.

VOL. II.

A TREATISE ON COMPARATIVE EMBRYOLOGY.
Vol. I. Inventebrata.

Honiron :

MACMILLAN AND CO.
1885

[The /tight of Translation is reserved.]

PREFACE.

MY aim in writing this work has been to give such an
account of the development of animal forms as may prove
useful both to students and to those engaged in embryo-
logical research. The present volume, save in the intro-
ductory chapters, is limited to a description of the develop-
ment of the Invertebrata: the second and concluding
volume will deal with the Vertebrata, and with the
special histories of the several organs.

Since the work is, I believe, with the exception of a
small but useful volume by Packard, the first attempt to
deal in a complete manner with the whole science of
Embryology in its recent aspects, and since a large
portion of the matter contained in it is not to be found in
the ordinary text books, it appeared desirable to give
unusually ample references to original sources. I have
accordingly placed at the end of each chapter, or in some
cases of each section of a chapter, a list of the more
important papers referring to the subject dealt with. The
papers in each list are numbered continuously, and are
referred to in the text by their numbers. These lists are
reprinted as an appendix at the end of each volume. It
will of course be understood that they do not profess to
form a complete bibliography of the subject.

B. II. B

PREFACE.

In order to facilitate the use of the work by students
I have employed two types. The more general parts of
the work are printed in large type; while a smaller type
is used for much of the theoretical matter, for the details
of various special modes of development, for the histories
of the less important forms, and for controversial matter
generally. The student, especially when commencing his
studies in Embryology, may advantageously confine his
attention to the matter in the larger type; it is of course
assumed that he already possesses a competent knowledge
of Comparative Anatomy.

Since the theory of evolution became accepted as an
established doctrine, the important bearings of Embryo-
l°&y on a^ morphological views have been universally
recognised; but the very vigour with which this depart-
ment of science has been pursued during the last few
years has led to the appearance of a large number of
incomplete and contradictory observations and theories;
and to arrange these into anything like an orderly and
systematic exposition has been no easy task. Many
Embryologists will indeed probably hold that any attempt
to do so at the present time is premature, and therefore
doomed to failure. I must leave it to others to decide
how far my effort has been justified. That what I have
written contains errors and shortcomings is I fear only
too certain, but I trust that those who are most capable
of detecting them will also be most charitable in excusing
them.

The work is fully illustrated, and most of the figures
have been especially engraved from original memoirs or
from my own papers or drawings by Mr Collings, who
has spared no pains to r« ndcr the woodcuts as clear and

PREFACE. vii

intelligible as possible. I trust my readers will not be
disappointed with the results. The sources from which
the woodcuts are taken have been in all cases acknow-
ledged, and in the. cases where no source is given the
illustrations are my own.

I take this opportunity of acknowledging my great
obligations to Professors Agassiz, Huxley, Gegenbaur,
Lankester, Turner, Kolliker, and Claus, to Sir John
Lubbock, Mr Moseley, and Mr P. H. Carpenter, for the
use of electrotypes of woodcuts from their works.

I am also under great obligations to numerous friends
who have helped me in various ways in the course of my
labour. Professor Kleinenberg, of Messina, has read
through the whole of the proofs, and has made numerous
valuable criticisms. My friend and former pupil, Mr
Adam Sedgwick, has been of the greatest assistance to
me in correcting the proofs. I have had the benefit of
many useful suggestions by Professor Lankester es-
pecially in the chapter on the Mollusca, and Mr P. H.
Carpenter has kindly revised the chapter on the Echino-
dermata.

I am also much indebted to Dr Michael Foster, Mr
Moseley, and Mr Dew-Smith for aid and advice.

CONTENTS OF VOLUME I.

INTRODUCTION. Pp. i — 16.
CHAPTER I. THE OVUM AND SPERMATOZOON.

General history of the Ovum, pp. 17 — 25. Special history of the Ovum in
different types, pp. -26 — 65. The Spermatozoon, pp. 65 — 67.

CHAPTER II. THE MATURATION AND IMPREGNATION OF THE OVUM.

Maturation of the Ovum, and formation of the polar bodies, pp. 68 — 79,
Impregnation of the Ovum, pp. 79—86. Summary, pp. 86.

CHAPTER III. THE SEGMENTATION OF THE OVUM.

Internal phenomena of Segmentation, pp. 88 — 92. External features of
Segmentation, pp. 92 — 122.

INTRODUCTION TO SYSTEMATIC EMBRYOLOGY. Pp. 125 — 130.

CHAPTER IV. DICYEMID^E AND ORTHONECTID/E. Pp. 131 — 137.
CHAPTER V. PORIFERA. Pp. 138 — 151.
CHAPTER VI. CCELENTERATA.

Hydrozoa, pp. 152 — 167. Actinozoa, pp. 167 — 173. Ctenophora, pp. 173
— 178. Summary, etc., pp. 178—182. Alternations of generations, pp. 182—

187.

CHAPTER VII. PLATYELMINTHES.

Turbellaria, pp. 189 — 196. Nemertea, pp. 196 — 204. Trematoda, pp. 205
— 210. Cestoda, pp. 210 — 218.

CHAPTER VIII. ROTIFERA. Pp. 221 — 224.

CONTENTS OF VOLUME I.

CHAPTER IX. MOLLUSCA.

Formation of the layers and larval characters, pp. 225 — 273. Gasteropoda
and Pteropoda, pp. 175—742. Cephalopo<ia, pp. 242—254. Polyplacophora,
pp. am — 357. Scaphopoda, pp. 257, 758. Lamellibramhiata, pp. 258 — 269
General review of Mollnscan Larva, pp. 270—273. Development of organs,
pp.

CHAPTER X. POLYZOA.

Entoprocta, pp. 291—797. Ectoprocta, pp. 297—305. Summary and
general considerations, pp. 305 — 308.

CHAPTER XI. BRACHIOPODA.

Development of the layers, pp. 31 1—313. The history of the larva, pp. 313—
317. Development of organs, p. 317. General observations on the affinities of
the Brachiopoda, pp. 317, 318.

CHAPTER XII. CHJETOPODA.

Formation of the germinal layers, pp. 319 — 325. The larval form, pp. 325 —
338. Formation of organs, pp. 338 — 342. Alternations of generations, pp.
34». 343-

CHAPTER XIII. DISCOPHORA.

Formation of layers, pp. 347—350. History of larva, pp. 351 — 354.

CHAPTER XIV. GEPHYREA.

Gephyrea nuda, pp. 355 — 361. Gephyrea tubicola, pp. 361 — 364. General
considerations, 364.

CHAPTER XV.

Chartognatha, pp. 366—369. Myzostomea, pp. 369, 370. Gastrotricha,
P- 370-

CHAPTER XVI.

Nematelminthes, pp. 370—379. Acanthocephala, pp. 379—381.

CHAPTER XVII. TRACHEATA.

I'rototrachcata, pp. 381—387. Myriapoda, pp. 387—395. Insecta, pp. 395 —
429. Embryonic mcmbrants and the formation oj ' the layers, pp. 400 — 406. Forma-
tion of the organs, pp. 406 — 417. Spec ial types of larvcc, pp. 417 — 419. Mctamor-
fikoiis and heterogamy, pp. 420—419. Arachnida, pp. 431 — 455. Formation
of tht layen and general development, pp. 431 — 446. Formation of the organs,
pp. 446—455' Formation of the layers and embryonic envelopes in the Tracheata,
PP- 456—458.

CONTENTS OF VOLUME I. xi

CHAPTER XVIII. CRUSTACEA.

History of larval forms, pp. 459 — 511. Branchiopoda, pp. 459 — 465. Mala-
costraca, pp. 465 — 487. Copepoda, pp. 487 — 492. Cirripedia, pp. 492 — 500.
Ostracoda, pp. 500 — 502. Phylogeny of the Crustacea, pp. 502 — 511. The forma-
tion of the germinal layers, pp. 511 — 521. Comparative development of organs,
pp. 521—529.

CHAPTER XIX.

Pcecilopoda, -pp. 534—538. Pycnogonida, pp. 538, 539. Pentastomida,
pp. 539 — 541. Tardigrada, p. 541. Summary of Arthropodan Development,
pp. 541— 543.

CHAPTER XX. ECHINODERMATA.

Development of the germinal layers, pp. 544 — 553. Development of the
larval appendages and metamorphosis, pp. 553 — 573. Summary and general
considerations, pp. 573 — 576.

CHAPTER XXL ENTEROPNEUSTA. Pp. 579 — 583.
INDEX. Pp. 584 — 590.

APPENDIX.

EMBRYOLOGY.

INTRODUCTION.

EMBRYOLOGY forms a large and important department of
Biology. Strictly interpreted according to the meaning of the
word, it ought to deal with the growth and structure of organisms
during their development within the egg membranes, before they
are capable of leading an independent existence. Modern in-
vestigations have however shewn that such a limitation of the
science would have a purely artificial character, and the term
Embryology is now employed to cover the anatomy and physi-
ology of the organism during the whole period included between
its first coming into being and its attainment of the adult state.

The subject-matter of the science of Embryology admits of a
twofold classification. It may be placed under a series of heads,
each dealing either with a special group of organisms, or with a
special department of the whole science. If classified in the
first of these ways the science will naturally be divided into
an Embryology of Plants, and an Embryology of Animals ; each
of which admits of further subdivision. In the second way
the subject falls under two primary heads ; viz. Physiological
Embryology and Anatomical Embryology.

The present treatise deals only with the Embryology of
Animals, and is further confined to those animals known as
Metazoa. The science is moreover treated from the morpho-
logical or anatomical, rather than from the physiological side.

B. II. I

EMBRYOLOGY.

The marvellous phenomenon of the evolution of a highly
complicated living being from a simple undifferentiated germ in
which it needs the aid of the most modern microscopical appli-
ances to detect any visible signs of life, has not unnaturally
attracted the attention of biologists from the very earliest periods.
Before the establishment of the cell theory the origin of the
organism from the germ was not known to be an occurrence
of the same nature as the growth of the fully formed individual,
and Embryological investigations were mixed up with irrelevant
speculations on the origin of life1.

The difficulties of understanding the formation of the indivi-
dual from the structureless germ led anatomists at one time to
accept the view "according to which the embryo preexisted,
"even though invisible, in the ovum, and the changes which
"took place during incubation consisted not in a formation of
" parts, but in a growth, i.e. in an expansion with concomitant
" changes of the already existing germ."

Great as is the interest attaching to the simple and isolated
life histories of individual organisms, this interest has been
increased tenfold by the generalizations of Mr Charles Darwin.

It has long been recognized that the embryos and larvae
of the higher forms of each group pass, in the course of their
development, through a series of stages in which they more
or less completely resemble the lower forms of the group2.
This remarkable phenomenon receives its explanation on Mr
Darwin's theory of descent. There are, according to this theory,
two guiding, and in a certain sense antagonistic principles which
have rendered possible the present order of the organic world.
These are known as the laws of heredity and variation. The
first of these laws asserts that the characters of an organism

1 To this general statement Wolff forms a remarkable exception, for though
without any clear knowledge of what we call cells he had very distinct notions on the
relations of growth and development.

* Von Boer who is often stated to have established the above generalization really
maintained a somewhat different view. He held (Ueber Entwickelungsgeschichte d.
Tkiere, p. 314) that the embryos of higher forms never resembled the adult stages of
lower forms but merely the embryos of such forms. Von Bacr was mistaken in thus
absolutely limiting the generalization, but his statement is much more nearly true than
a definite statement of the exact similarity of the embryos of higher forms to the
adults of lower ones.

INTRODUCTION.

at all stages of its existence are reproduced in its descendants at
corresponding stages. The second of these laws asserts that
offspring never exactly resemble their parents. By the common
action of these two principles continuous variation from a parent
type becomes a possibility, since every acquired variation has a
tendency to be inherited.

The remarkable law of development enunciated above, which
has been extended, especially by the researches of Huxley1 and
Kowalevsky, beyond the limits of the more or less artificial
groups created by naturalists, to the whole animal kingdom, is a
special case of the law of heredity. This law, interpreted in
accordance with the theory of descent, asserts that each organism
in the course of its individual ontogeny repeats the history of its
ancestral development. It may be stated in another way so as
to bring out its intimate connection with the laws of inheritance
and variation. Each organism reproduces the variations inherited
from all its ancestors at successive stages in its individual
ontogeny which correspond with those at which the variations
appeared in its ancestors. This mode of stating the law shews
that it is a necessary consequence of the law of inheritance.
The above considerations clearly bring out the fact that Com-
parative Embryology has important bearings on Phylogeny, or
the history of the race or group, which constitutes one of the
most important branches of Zoology.

Were it indeed the case that each organism contained in its
development a full record of its origin, the problems of Phylogeny
would be in a fair way towards solution. As it is, however, the
law above enunciated is, like all physical laws, the statement of
what would occur without interfering conditions. Such a state
of things is not found in nature, but development as it actually
occurs is the resultant of a series of influences of which that of
heredity is only one. As a consequence of this, the embryo-
logical record, as it is usually presented to us, is both imperfect
and misleading. It may be compared to an ancient manuscript
with many of the sheets lost, others displaced, and with spurious
passages interpolated by a later hand. The embryological

1 Huxley was the first to shew that the body of the Coelenterata was formed of
two layers, and to identify these with the two primary germinal layers of the Verte-
brata.

EMBRYOLOGY.

record is almost always abbreviated in accordance with the
tendency of nature (to be explained on the principle of survival
of the fittest) to attain her ends by the easiest means. The time
and sequence of the development of parts is often modified, and
finally, secondary structural features make their appearance
to fit the embryo or larva for special conditions of existence.
When the life history of a form is fully known, the most difficult
part of his task is still before the scientific embryologist. Like
the scholar with his manuscript, the embryologist has by a
process of careful and critical examination to determine where
the gaps are present, to detect the later insertions, and to place
in order what has been misplaced.

The aims of Comparative Embryology as restricted in the
present work are two-fold: (i) to form a basis for Phylogeny,
and (2) to form a basis for Organogeny or the origin and
evolution of organs. The justification for employing the results
of Comparative Embryology in the solution of the problems in
these two departments of science is to be found in the law above
enunciated, but the results have to be employed with the quali-
fications already hinted at ; and in both cases a knowledge of
Comparative Anatomy is a necessary prelude to their application.

In accordance with the above objects Comparative Embryo-
logy may be divided into two departments.

The scientific method employed in both of these departments
is that of comparison, and is in fact fundamentally the same as
the method of Comparative Anatomy. By this method it
becomes possible with greater or less certainty to distinguish
the secondary from the primary or ancestral embryonic characters,
to determine the relative value to be attached to the results of
isolated observations, and generally to construct a science out of
the rough mass of collected facts. It moreover enables each
observer to know to what points it is important to direct his
attention, and so prevents that simple accumulation of dis-
connected facts which is too apt to clog and hinder the advance
of the science it is intended to promote.

In the department of Phylogeny the following are the more
important points aimed at

(i) To test how far Comparative Embryology brings to
light ancestral forms common to the whole of the Metazoa.

INTRODUCTION. 5

Examples of such forms have been identified by various embryo-
logists in the ovum itself, supposed to represent the unicellular
ancestral form of the Metazoa : in the ovum at the close of
segmentation regarded as the polycellular Protozoon parent
form : in the two-layered gastrula, etc., regarded by Haeckel as
the ancestral form of all the Metazoa1.

(2) How far some special embryonic larval form is con-
stantly reproduced in the ontogeny of the members of one or
more groups of the animal kingdom ; and how far such larval
forms may be interpreted as the ancestral type for those groups.

As examples of such forms may be cited the six-limbed
Nauplius supposed by Fritz Miiller to be the ancestral form
of the Crustacea ; the trochosphere larva of Lankester, which he
considers to be common to the Mollusca, Vermes, and Echino-
dermata : the planula of the Ccelenterata, etc.

(3) How far such forms agree with living or fossil forms in
the adult state ; such an agreement being held to imply that
the living or fossil form in question is closely related to the
parent stock of the group in which the larval form occurs. It is
not easy to cite examples of a very close agreement of this kind
between the larval forms of one group and the existing or fossil
forms of another. The larvae of some of the Chaetopoda with
long] provisional setae resemble fossil Chaetopods. The Rotifers
have many points of resemblance to the trochosphere, especially
to that form of trochosphere characteristic of the Mollusca. The
Turbellarians have some features in common with the Ccelente-
rate planula. Some of the Gephyrea in the presence of a
praeoral lobe resemble certain trochosphere types. The larva
of the Tunicata has the characters of a simple type of the
Chordata.

Within the limits of a single group agreements of this
kind are fairly numerous. In the Craniata the tadpole of
the Anura has its living representative in the Pisces and perhaps
especially in the Myxinoids. The larval forms of the Insecta
approach Peripatus. The stalked larva of Comatula is re-
produced by the living Pentacrinus and Rhizocrinus etc.

1 The value of these identifications as well as of those below is discussed in its
appropriate place in the body of the work. Their citation here is not to be regarded
as necessarily implying my acceptance of them.

EMBRYOLOGY.

Numerous examples of the same phenomenon are found amongst
the Crustacea.

(4) How far organs appear in the embryo or larva which
cither atrophy or become functionless in the adult state, and
\vhich persist permanently in members of some other group or
in lower members of the same group. Cases of this kind are of
the most constant occurrence, and it is only necessary to cite
such examples as the gill-slits and Wolffian body in the embryos
of higher Craniata to illustrate the kind of instance alluded to.
The same conclusions may be drawn from them as from the
cases under the previous heading.

(5) How far organs pass in the course of their development
through a condition permanent in some lower form. Phylo-
genetic conclusions may be drawn from instances of this cha-
racter, though they have a more important bearing on Organology
than on Phylogeny.

The considerations which were used to shew that the an-
cestral history is reproduced in the ontogeny of the individual
apply with equal force to the evolution of organs. The special
questions in Organology, on which Comparative Embryology
throws light, may be classified under the following heads.

(1) The origin and homologies of what are known as the
germinal layers; or the layers into which the embryo becomes
divided immediately after the segmentation.

(2) The origin of primary tissues, epithelial, nervous, mus-
cular, connective, etc., and their relation to the germinal layers.

(3) The origin of organs. The origin of the primitive
organs is intimately connected with that of the germinal layers.
The first differentiation of the segmented ovum results in the
cells of the embryo becoming arranged as two layers, an outer
one known as the epiblast and an inner one as the hypoblast.
The outer of these forms a primitive sensory organ, and the
inner a primitive digestive organ.

(4) The gradual evolution of the more complicated organs
and systems of organs.

This part of the subject, even more than that dealing with
questions of Phylogeny, is intimately bound up with Com-
parative Anatomy; without which indeed it becomes quite
meaningless.

INTRODUCTION.

REPRODUCTION.

A study of reproduction logically precedes that of Embry-
ology. Reproduction essentially consists in the separation of a
portion of an organism which has the capacity of developing into
a form similar to that which gave it origin. The simplest
modes of reproduction are those which occur amongst the
Protozoa.

In this group, reproduction may take place in a great variety
of ways. These may be classified in three groups: (i) fission,
(2) budding or gemmation, (3) spore formation.

Reproduction in all these ways may take place either subse-
quently to and apparently in consequence of a very important
process known as conjugation, which consists in the temporary
or permanent fusion of two or more individuals, or spontane-
ously, i.e. independently of any such previous conjugation.

Reproduction by fission consists simply in the division of the
organism into two similar parts, the nucleus when present
becoming divided simultaneously with the cell body. This
mode of reproduction is the simplest conceivable, and is not
followed by a development, since the two organisms produced
are exactly similar, except in size, to the parent form. Besides
single fission, a process of multiple fission may take place, as
amongst the Flagellata, where Drysdale and Dallinger have
shewn that an individual enclosed within a structureless cyst
may divide first into two, then into four, and so on.

The process of budding differs mainly from that of simple
fission in the fact that the two organisms produced are dissimilar
in size, and also that the separation of the smaller organism
from the larger is preceded by a process of growth in the latter,
so that in the separation of the bud no essential part of the
parent form is removed. This mode of reproduction is found
amongst the Infusoria, Acineta, &c. An interesting variation in
it is the internal gemmation of many of the Acineta, where a
portion of the internal protoplasm with part of the nucleus is
separated off to form a fresh individual. This mode of gemma-
tion is connected by a series of gradations with the normal

S EMBRYOLOGY.

external gemmation. The organisms produced by gemmation
are not always similar at birth to the parent ; e.g. Acineta.

Both fission and gemmation when incomplete lead to the
formation of colonies.

The third mode of reproduction, by spore formation, does
not essentially differ from that by multiple fission. It consists
in the breaking up of the organisms into a number (usually very
considerable) of portions ; each of which eventually developes
into an organism like the parent form. All gradations between a
simultaneous division of the organism into such spores and simple
multiple fission are to be found, but this process of reproduction
may be sometimes distinguished from that by such fission by
the fact that the two processes may coexist in a single form,
e.g. the biflagellate monad of Drysdale and Dallinger. In the
majority of cases the spores produced differ at first from the
parent organism not only in size but in other points, such as the
possession of a flagellum, etc. They may even be without a
nucleus when the parent organism is nucleated, as in the Gre-
garinidae.

The encystment, which in many cases precedes reproduction
by any of the above processes, and more especially by
spores, is not an essential condition of their occurrence ; and is
probably in the first instance a protective arrangement which
has become secondarily adapted to and connected with re-
production.

As has been already stated, all the above modes of reproduc-
tion take place in some of the Protozoa without any anterior
process which can be regarded as of a sexual nature ; but very
often they are preceded by the temporary or permanent fusion
of two or more individuals, such fusion being known as con-
jugation.

In most cases reproduction by spores is the consequence of
conjugation, but in the Infusoria etc. where the fusion at conju-
gation is temporary (except Vorticella), there is probably merely
a renewed activity — a rejuvenescence — which most likely results
in active fission or budding. In the Gregarinidse reproduction
by spores usually follows conjugation, but may also take place
without it. In some Flagellata reproduction by spores follows
the conjugation of two individuals in a different stage of de-

INTRODUCTION. 9

velopment. Thus in the springing Monad, described by
Drysdale and Dallinger, a form produced by the fission of a
monad in an amoeboid condition fuses with an ordinary monad
to produce an individual, which then breaks up into spores.
Another instance of the fusion of dissimilar individuals is
afforded by Vorticella, where a free-swimming individual conju-
gates and is permanently united with a fixed one (Engelmann,
Biitschli). Conjugation often consists in the fusion of more than
two individuals. In conjugation where the fusion is permanent,
the nuclei of the conjugating forms usually unite before the
product breaks up into spores ; and where temporary fusion
occurs in the Infusoria a division of the paranuclei and often of
the nuclei takes place, followed by the ejection of parts of them,
and a reproduction of new paranuclei and nuclei from the
remainder of the original structures.

In order to understand the meaning of conjugation in con-
nection with reproduction, it is important to understand how the
two became in the first instance related. For the solution of
this question the fact that many Protozoa have the capacity of
temporarily or permanently fusing together without an imme-
diate act of reproduction is of great importance. A good example
of such fusion is supplied by Actinophrys. We must suppose in
fact that the simple coalescence of two or more individuals gives
a sufficient amount of extra vigour to their product, to compen-
sate the race for the loss in number of individuals so caused.
This extra vigour probably first exhibited itself especially by
increased activity in reproduction, till finally the two processes,
viz. that of conjugation and that of reproduction, came to be
inseparably connected together.

The reproduction of the forms above the Protozoa, which are
known as the Metazoa, takes place by two methods, viz. a sexual
and an asexual one. The sexual process, which occurs in every
known Metazoon1, consists essentially, as is shewn in the second
chapter of this work, in the fusion of two cells budded off from
the parent organism, viz. the female cell or ovum, and the male
cell or spermatozoon, and of the subsequent division of the
compound cell so produced into a number of parts which build

1 Dicyema, if it is a true Metazoon, would seem to form an exception to this rule.

10 EMBRYOLOGY.

themselves up into an organism resembling one of the parents.
The sexual process has obviously at first sight a very close
resemblance to the process of conjugation. Since it is a ques-
tion of fundamental importance to determine how sexual repro-
duction originated, it becomes necessary to examine how far
this apparent resemblance is a real one, and how far sexual
reproduction can be derived from reproduction following upon
conjugation.

In spite of the general similarity between the two processes
there is an obvious difficulty in comparing them, in that the
result of conjugation is usually the breaking up of the individual
formed by the fusion of two other individuals into a number of
new organisms, while the result of the fusion which takes place
in sexual reproduction is the formation of a single new organism.
This difference between the two processes, great as it is, is per-
haps apparent rather than real. It must be remembered that a
single individual Metazoon is equivalent to a number of Protozoa
coalesced to form a single organism in a higher state of aggre-
gation. It results from this that the segmentation of the ovum
which follows the sexual act may be compared to the breaking
up of the product of conjugation into spores, the difference
between the two processes consisting in the fact that in the one
case the spores separate each to form an independent organism,
while in the other they remain united and give rise to a single
compound organism.

If the above considerations are well founded it seems permis-
sible to accept the general view according to which sexual
reproduction is derived from conjugation. It is necessary to
suppose that, in a colony of Protozoa in the course of becoming
a Metazoon, the capacity of reproduction by spores became
localized in certain definite cells, and although the formation of
spores from these cells may have been possible without previous
conjugation, yet that conjugation gradually became established
as the rule. The differentiation of primitively similar conjugating
cells into male and female cells was probably a very early occur-
rence, since indications of an analogous differentiation, as has
already been mentioned, are found in certain existing Protozoa
(Monads, Vorticella, etc.). I have attempted to shew in the
second chapter that the breaking up of the cell into spores

INTRODUCTION. 1 1

without previous conjugation is perhaps provided against in the
extrusion of the so-called ' directive body '.

With the differentiation of special germinal cells, to take the
place of the whole individual in the act of conjugation, the pos-
sibility of each act of conjugation resulting in the production of
only a single organism became introduced. Germinal cells can
be indefinitely produced, and the reproductive capacity of a
single individual is therefore unlimited ; while if two whole
individuals conjugated and only produced one from the process,
the result would be a diminution instead of an increase in the
race1.

It must be admitted that, in the present state of our know-
ledge, the passage from reproduction by spores following con-
jugation, to true sexual reproduction, can only be traced in a
very speculative manner; and that a further advance in our
knowledge may prove that the steps which I have attempted to
sketch out are far from representing the true origin of sexual
differentiation. The peculiar conjugation and fusion of two
individuals to form Diplozoon paradoxum may be alluded to in
this connection. This fusion merely results in the attainment
of sexual maturity by the two conjugating individuals. It does
not appear to me probable that this conjugation is in any way
connected with the conjugation of the Protozoa, but the reverse
must be borne in mind as a possibility.

It is not easy to decide whether the hermaphrodite or the

1 In the vegetable kingdom there are numerous types of Thallophytes, which
throw a considerable amount of light on the relation between sexual reproduction and
conjugation. Subjoined are a few of the more striking cases. In Pandorina at the
time of sexual reproduction the cells which constitute a colony divide each into sixteen,
and the products of their division are set free. Pairs of them then conjugate and
permanently fuse. After a resting stage the protoplasm is set free from its envelope
after division into two or four parts. Each of these then divides into sixteen coherent
cells and constitutes a new Pandorina colony. In CEdogonium the fertilization is
effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere
(oospore) then undergoes segmentation like the ovum of an animal; but the segments,
instead of uniting to form a single organism, separate from each other, and each of
them gives rise to a fresh individual (swarm-spore) which grows into a perfect CEdo-
gonium. In Coleochaete the impregnation and segmentation take place nearly as in
CEdogonium, but the segments remain united together, acquire definite cell walls, and
form a single embryo. There is in fact in Coleochaete a true sexual reproduction of
the ordinary type. ( Vide S. H. Vines "On alternation of generation in the Thallo-
phytes." Journal of Botany, Nov., 1879.)

12 EMBRYOLOGY.

dioecious state is the primitive one, or in other words whether
the two conjugating cells, from which I have supposed the
sexual products to originate, were derived in the first instance
from one or from two colonies of Protozoa. On purely d priori
grounds it seems probable that they were originally formed in
one colony, and that their derivation from two colonies or
individuals was inaugurated when the spermatozoon became
motile. There can be no doubt that the dioecious state is a
very early one, and that the majority of existing cases of herma-
phroditism are secondary.

The above considerations with reference to the male and
female cells appear to indicate that they were primitively
homodynamous ; a conclusion which is on the whole borne out
by the history of their development.

Although the modes of reproduction amongst the Metazoa
have been divided into the classes sexual and asexual, there is
nevertheless one mode of asexual reproduction which ought to
be classified with the sexual rather than with the asexual
modes. I mean parthenogenesis, which consists essentially in
the development of the ovum into a fresh individual without
previous coalescence with the male element. This mode of
reproduction, which has a very limited range in the animal
kingdom, being confined to the Arthropoda and Rotifera, is
undoubtedly secondarily derived from sexual reproduction. The
conditions of its occurrence are discussed in the second chapter.

It is remarkable that in certain cases the absence of fertiliza-
tion causes the production of males (Bees, a Saw-fly, Nematus
ventricosus, etc.); more usually it results in the production of
females only, and there are very often in the Arthropoda a
series of successive generations of females all producing ova
which develope parthenogenetically into females; eventually
however, usually in direct or indirect connection with a change
of food or temperature, or other conditions, ova are formed
which give rise without fertilization both to males and females.

The true asexual modes of reproduction amongst the Metazoa
consist of fission and gemmation. Gemmation is by far the most
widely disseminated of the two. Various as are the methods in
which it takes place, it seems nevertheless that cells derived from
all the germinal layers, and very frequently from all the im-

INTRODUCTION. 13

portant organs of the adult, assist in forming the bud. Into the
details of the process, which require in many points a fuller
elucidation, it is not my purpose to enter.

Gemmation is a far commoner occurrence amongst the
simpler than amongst the more highly organised forms. It
appears to have been superadded to the sexual mode of repro-
duction quite independently in a number of different instances.

While there is no difficulty in understanding how gemmation
may have started in such simple types as the Coelenterata, the
manner in which it first originated in certain highly organised
forms, as for instance the Ascidians, is somewhat obscure, but it
seems probable that it began with the division of the developing
germ into two or more embryos, at a very early stage of growth.

Such a division of the germ is, as has been shewn by
Kleinenberg, normal in Lumbricus trapezoides1 and Haeckel
has shewn that an artificial division of the germ in the Siphono-
phora leads to the development of two individuals. It has been
pointed out by various naturalists that the production of double
monsters is often a phenomenon of the same nature. While it
is next to impossible to understand how production of a bud
could commence for the first time in the adult of a highly
organised form, it is not difficult to form a picture of the steps
by which the fission of the germ might eventually lead to the
formation of buds in the adult state.

The coexistence of sexual reproduction with normal asexual
multiplication, or with parthenogenesis, has led to a remarkable
phenomenon in the animal kingdom known as alternations of
generations2.

For the details of the various types of alternations of
generations, and their origin, the reader is referred to the body
of the work ; but a few general remarks on the nature and origin
of the process, and on its nomenclature, may conveniently be
introduced in this place. The simplest cases are those in which

1 The case of Pyrosoma, which might be cited in this connection, is probably
secondary.

2 For an excellent account of this subject, vide Allen Thompson's article Ovum in
Todd's Cyclopaedia. The metamorphosis of the Echinoderms included under this
head in Thompson's article is now known not to be a proper case of alternations of
generations.

14 EMBRYOLOGY.

an individual which produces by sexual means gives origin to
asexual individuals differently organised to itself, which produce
by budding the original sexual form, and so complete a cycle.
Instances of this kind are supplied by the Hydrozoa, Annelida
and Tunicata. In the case of the Tunicata (Doliolum) two
different asexual generations may be interpolated between the
sexual generations. In all these cases the origin of the pheno-
menon is easily understood. It appears, as is most clearly
shewn in the case of the Annelida, that the ancestors of the
species which now exhibit alternations of generations originally
reproduced themselves at the same time both sexually and by
budding, though probably the two modes of reproduction did
not take place at the same season. Gradually a differentiation
became established, by which sexual reproduction was confined
to certain individuals, which in most instances did not also
reproduce asexually. After the two modes of reproduction
became confined to separate individuals, the dissimilarity in
habits of life necessitated by their diverse functions caused a
difference in their organization ; and thus a complete alter-
nation of generations became established. The above is no
merely speculative history, since all gradations between com-
plete alternations of generations and simple budding combined
with sexual reproduction can be traced in actually existing forms.

The alternation of generations as it is found amongst the
Entoparasitic Trematodes and most Cestodes, is to be explained
in a slightly different way.

It appears that in these parasitic forms a complicated meta-
morphosis first arose from the parasite having to accommodate
itself to the different hosts it was compelled to inhabit, owing to
the liability of its primitive and subsequent hosts to be devoured1.
A capacity for asexual multiplication — obviously of immense
advantage to a parasite — appears to have been acquired in some
of the stages of this metamorphosis, and an alternation of
generations thus established.

1 The appearance of Vertebrata on the globe as the forms which most frequently
preyed on Invertebrate forms, and were themselves not so liable to be devoured, has
no doubt had a great influence on the metamorphosis of internal parasites, and has
amongst other things resulted in these parasites usually reaching their sexual state in
a vertebrate host.

INTRODUCTION. 1 5

A nearly parallel series to that exhibiting alternations of
sexual generations with generations which produce by budding
is supplied by the cases where sexual generations alternate with
parthenogenetic ones, or in some instances even with larvae
which reproduce sexually or else parthenogenetically.

The best known examples of this form of alternations of
generations are found amongst the Insecta1. A simple case
is that of the Aphides. The ova deposited by impregnated
females give rise to forms differently organised to the parents
but provided with an ovary2. The eggs from the ovary develope
parthenogenetically within the oviduct, and so long as there
is plenty of food and warmth the generations produced are
always parthenogenetic forms. The failure of warmth and
nutriment causes the production of true males and females, and
so the cycle is completed. We must suppose that the capacity
possessed by so many female insects of producing eggs capable
of developing without the influence of the male element, has
been, so to speak, taken hold of by natural selection, and has led
to the production of viviparous parthenogenetic forms, by which,
so long as food is abundant, a clear economy in reproduction is
effected. The continuance of the species during winter is secured
by the production of males and females, the females laying eggs
in autumn which are hatched in the spring.

In Chermes there is less modification of the primitive condi-
tion in that the parthenogenetic generations lay their eggs like
the impregnated females. In the gall-flies (Cynipidae), there is
frequently an alternation of generations of the same kind as in
Chermes ; there being no viviparous forms. The individuals of
the different generations differ from each other to some extent
in all these cases.

A second type of alternations of parthenogenetic and sexual
generations is exemplified by the cases of Chironomus and
Cecidomyia, where the larva which develope from the eggs of
the fertilized female produce parthenogenetically, by means of
true ova, forms which eventually after several generations (Ceci-
domyia) of larval reproduction give rise to sexual forms. The

1 For details vide Chapter on Insecta.

2 The distinction drawn by Huxley between ova and pseudova does not appear to
me a convenient one in practice.

1 6 EMBRYOLOGY.

explanation is here practically the same as in the case of Aphis,
and is paralleled in the gemmiparous series by the production of
buds in the larval forms of Trematodes, etc. A very similar
occurrence takes place in Ascaris nigrovenosa (vide chapter on
Nematoidea), except that larval forms, which carry on reproduc-
tion and then perish without developing farther, do so by a true
sexual process. Thus there is an alternation of generations of
adult and larval sexual forms. The Axolotl is an intermittent
example of the same phenomenon.

As might be anticipated from the mode in which alternations
of generations have become established, incomplete approxi-
mations to it are not uncommon. Such approximations are
especially found in the Arthropoda, where alternations of sexual
and parthenogenetic generations frequently take place, in which
the individuals of different generations are similarly organised
(Psychidae, Apus, &c.). Another approximation is afforded by
the parthenogenetic winter eggs of Leptodora amongst the
Phyllopods, which give rise to Nauplius larvae, while the young
hatched from the summer eggs do not pass through a meta-
morphosis. Numerous transitional cases are also found amongst
the forms in which there is an alternation of sexual and gemmi-
parous generations.

The whole of the cases to which allusion has been made in
this section may be conveniently classed under the term alterna-
tions of generations, but the cases of alternation of two sexual
generations, and of sexual and parthenogenetic generations,
are classified by Leuckart, Claus, etc. as cases of heterogeny,
which they oppose to the other form of alternation of genera-
tions. If special terms are to be adopted for the two kinds of
alternation of generations, it would be perhaps convenient to
classify the cases of alternations of sexual and gemmiparous
generations under the term metagenesis, and to employ the
term heterogamy for the cases of alternation of sexual and
parthenogenetic generations.

The term Nurse (German Amme), employed for the asexual
generations in metagenesis, may advantageously be dropped
altogether.

CHAPTER I.

THE OVUM AND SPERMATOZOON.

THE OVUM.

THE complete developmental history of any being constitutes
a cycle. It is therefore permissible in treating of this history to
begin at any point. As a matter of convenience the ovum ap-
pears to be the most suitable point of departure. The question
as to the germinal layer from which it is ultimately derived is
dealt with in a subsequent part of the work ; the present chapter
deals with its origin and growth.

General History of the Ovum.

Every young ovum (fig. i) has the cha-
racter of a simple cell. It is formed of a
mass of naked protoplasm (a\ containing
in its interior a nucleus (b), within which
there is a nucleolus (<;). The nucleus and
nucleolus are usually known as the ger-
minal vesicle and germinal spot.

The ovum so constituted is developed
either (i) from one cell out of an aggrega-
tion or layer of cells all of which have the
capacity of becoming ova ; or (2) from one
out of a number of cells segmented off
from a polynuclear mass of protoplasm, not divided into sepa-
rate cells. In both cases the cells which have the capacity of
becoming ova may be spoken of as germinal cells, and in the
case where the ova are ultimately developed from a poly-

B. II. 2

FIG. i. DIAGRAM OF
THE OVUM. (From Ge-
genbaur.)

a. Granular proto-
plasm, b. Nucleus (ger-
minal vesicle), c. Nu-
cleolus (germinal spot).

1 8 GENERAL HISTORY.

nuclear mass of protoplasm the latter structure may be called a
germogen.

In some cases the whole of the germinal cells eventually
become ova, but as a rule only a small proportion of them have
this fate, the remainder undergoing various changes to be spoken
of in the sequel.

Extended investigations have shewn that the distinction
between germinal cells which are independent cells from the
first, or derived from a germogen in which the nucleated proto-
plasm is not divided into cells, is an unimportant one ; and
closely allied forms may differ in this respect. It is moreover
probable that a germogen of nucleated protoplasm is less com-
mon than is often supposed : it being a matter of great difficulty
to determine the structure of the organs usually so described.
A germogen is stated to be found in most Platyelminthes,
Nematoidea, Discophora, Insecta, and Crustacea.

A more important distinction in the origin of the germinal
cells is that afforded by their position. In this respect three
groups may be distinguished, (i) The germinal cells may form
the lining of a sack or tube, having the form of a syncytium or
of an epithelium of separate cells (Platyelminthes, Mollusca, Ro-
tifera, Echinodermata, Nematoidea, Arthropoda). (2) Or they
may form a specialized part of the epithelium lining the general
body cavity (Chaetopoda, Gephyrea, Vertebrata). (3) Or they
may form a mass placed between the two elsewhere contiguous
primitive germinal layers (Ccelenterata1).

Types of transition between the first and second group are
not uncommon. Such types, properly belonging to the second
group, originate by a special membranous sack continuous with
the oviduct being formed round the primitively free patch of
germinal cells. Examples of this are afforded by the Discophora,
the Teleostei, etc. It is very probable that all the cases which
fall under the first heading may have been derived from types
which belonged to the second group.

The mode of conversion of the germinal cells into ova is
somewhat diverse. Before the change takes place the germinal

1 In nil the Metazoa the generative organs are placed between the primitive
germinal layers; and the peculiarity of their position in the Ccelenterata depends on
the absence of a body cavity and of a distinct mesoblast.

THE OVUM.

FIG. 2. OVUM OF
CARMARINA (GERYO-

NIA) HASTATA. (Copied

from Haeckel.)

gd. Body of ovum.
gv. Germinal vesicle.
gni. Germinal spot.

cells frequently multiply by division. The
change itself usually involves a considerable
enlargement of the germinal cell, and gene-
rally a change in the character of the ger-
minal vesicle, which in most young ova
(fig. 2) is very large as compared to the
body of the ovum. The most complicated
history of this kind is that of the ovum of
the Craniata. (Vide pp. 56, 57.)

The ovum in its young condition is obviously nothing but a
simple cell ; and such it remains till the period when it attains
maturity.

Nevertheless the changes which it undergoes in the course of
its growth are of a very peculiar kind, and, consisting as they do
in many instances of the absorption of other cells, have led
various biologists to hold that the ovum is a compound struc-
ture. It becomes therefore necessary to consider the processes
by which the growth and nutrition of the ovum is effected
before dealing with the structure of the ovum at all periods of
its history.

The ovum is of course nourished like
every other cell by the nutritive fluids in
which it is surrounded, and special provi-
sions are made for this, in that the ovary is
very frequently placed in contiguity with
vascular channels. But in addition to such
nutrition a further nutrition, the details of
which are given in the special part of this
chapter, is provided for in the germinal
cells which do not become ova.

In the simplest case, as in many Hy-
drozoa (fig. 3), the germinal cells which do
not become ova are assimilated by the
ovum much in the manner of an Amoeba.

In other cases the ovum becomes in-
vested by a special layer of cells, which
then constitutes what is known as a fol-
licle. The cells which form the follicle are
often germinal cells, e.g. Holothuria, Insecta (fig. 17), Vertebrata

2 — 2

FIG. 3. FEMALE

GONOPHORE OF TUBU-
LARIA MESEMBRYAN-

THEMUM. CONTAINING

ONE LARGE OVUM (ov)
AND A NUMBER OF GER-
MINAL CELLS (g.C.).

ep. Epiblast (Ecto-
derm). Ay. Hypoblast
(Entoderm). ov. Ovum.
g.c. Germinal cells.

20 GENERAL HISTORY.

(fig- !9)' In other cases they seem rather to be adjoining con-
nective-tissue or epithelioid cells, though it is sometimes difficult
to draw the line between such cells and germinal cells. Ex-
amples of follicles formed of ordinary connective-tissue cells,
are supplied by Asterias, Bonellia (fig. 16), Cephalopoda (fig.
14), etc.

A membrane enclosing the ovum without a lining of cells, as in many
Arachnida, vide p. 51, has no true analogy with a follicle and does not
deserve the same name.

The function of the follicle cells appears to be, to elaborate
nutriment for the growth of the ovum. The follicle cells are not
as a rule directly absorbed into the body of the ovum, though
in some instances, as in Sepia (vide p. 40), they are eventually
assimilated in this way.

In many cases some of the germinal cells form a follicle,
while other germinal cells form a mass within the follicle
destined eventually to be used as pabulum. Insects supply
the best known examples of this, but Piscicola, Bonellia (?) may
also be cited as examples of the same character. In the Cra-
niata (pp. 56 — 58) some of the germinal cells which advance a
certain distance on the road towards becoming ova, are even-
tually used as pabulum, before the formation of the follicle ;
while other germinal cells form at a later period the follicular
epithelium. A peculiar case is that of the Platyelminthes (fig. 9),
where a kind of follicle is constituted by the cells of a specially
differentiated part of the ovary, known as the yolk-gland. The
cells of this follicle may either remain distinct, and continue to
surround the ovum after its development has commenced, and
so be used as food by the embryo ; or they may secrete yolk
particles, which enter directly into the protoplasm of the ovum.

For further variations in the mode of nutrition the reader is
referred to the special part of this chapter. Suffice it to say
that none of the known modes of nutrition indicate that the
ovum becomes a compound body any more than the fact of an
Amoeba feeding on another Amoeba would imply that the first
Amoeba ceased thereby to be a unicellular organism.

The constitution of the ovum may be considered under three
heads :—

THE OVUM.

21

(1) The body of the ovum.

(2) The nucleus or germinal vesicle.

(3) The investing membranes.

The body of the ovum. The essential constituent of the
body of the ovum is an active living protoplasm. As a rule
there are present certain extraneous matters in addition, which
have not the vital properties of protoplasm. The most impor-
tant of these is known as food-yolk, which appears to be
generally composed of an albuminoid matter.

The body of the ovum is at first very small compared with
the germinal vesicle, but continually increases as the ovum
approaches towards maturity. It is at first comparatively free
from food-yolk ; but, except in the rare instances where it is
almost absent, food-yolk becomes deposited in the form of
granules, or highly refracting spheres, by the inherent activity
of the protoplasm during the later stages in the ripening of
the ovum. In many instances the protoplasm of the ovum
assumes a sponge-like or reticulate arrangement, a fluid yolk
substance being placed in the meshes of the reticulum. The
character of the food-yolk varies greatly. Many of its chief
modifications are described below. There is not unfrequently
present in the vitellus a peculiar body known as the yolk

FIG. 4. A. OVUM OF HYDRA IN THE AMCEBOID STATE, WITH YOLK SPHERULES

(PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After Kleinenberg. )
gv. Germinal vesicle.

B. SINGLE PSEUDOCELL OF HYDRA.

22

GENERAL HISTORY.

nucleus, which is very possibly connected with the formation of
the food-yolk. It is found in many Arachnida, Myriapoda,
Amphibia, etc.1

More important for the subsequent development than the
variation in the character of the food-yolk is its amount and
distribution. In a large number of forms it is distributed un-
sym metrically, the yolk being especially concentrated at one
pole of the ovum, the germinal vesicle, surrounded by a special
layer of protoplasm comparatively free from food-yolk, being
placed at the opposite pole. In the Arthropoda it has in most
instances a symmetrical distribution. Further details on this
subject are given in connection with the segmentation ; the
character of which is greatly influenced by the distribution of
food-yolk.

The body of the ovum is usually spherical, but during a
period in its development it not unfrequently exhibits a very
irregular amoeboid form, e.g. Hydra (fig. 4), Halisarca.

The germinal vesicle. The
germinal vesicle exhibits all the
essential characters of a nucleus.
It has a more or less spherical
shape, and is enveloped by a distinct
membrane which seems, however,
in the living state to be very often
of a viscous semi-fluid nature and
only to be hardened into a mem-
brane by the action of reagents
(Fol). The contents of the germi-
nal vesicle are for the most part
fluid, but may be more or less
granular. Their most characteris-
tic components are, however, a protoplasmic network and the
germinal spots8. The protoplasmic network stretches from the
germinal spots to the investing membrane, but is especially
concentrated round the former. (Fig. 5.) The germinal spot

1 For details on the yolk nucleus vide Balbiani, Lemons s. /. Gtntration d. Vertchrcs.
Paris, 1879. In this work the author maintains very peculiar views on the nature and
function of the yolk nucleus, which do not appear to me well founded.

a In the germinal vesicles of very young ova the reticulum is often absent.

FIG. 5. UNRIPE OVUM OK
TOXOPNEUSTES LiviDUS. (Copied
from Hertwig.)

THE OVUM. 23

forms a nearly homogeneous body, with frequently one or more
vacuoles. It often occupies an eccentric position within the
germinal vesicle, and is usually rendered very conspicuous by its
high refrangibility. In many instances it has been shewn to be
capable of amoeboid movements (Hertwig, Eimer), and is more-
over more solid and more strongly tinged by colouring reagents
than the remaining constituents of the germinal vesicle.

In many instances there is only one germinal spot, or else
one main spot and two or three accessory smaller spots. In
other cases, e.g. Osseous Fishes, Echinaster fallax, Eucope poly-
styla, there are a large number of nearly equal germinal spots
which appear to result from the division or endogenous prolifera-
tion of the original spot Sometimes the germinal spots are
placed immediately within the membrane of the germinal vesicle
(Elasmobranchii and Sagitta). In many Lamellibranchiata, in
the earth-worm, and in many Chsetopoda the components of the
germinal spot become separated into two nearly spherical
masses (fig. 12), which remain in contiguity along a small part
of their circumference, and are firmly united together. The
smaller of the two parts is more highly refractive than the
larger. Hertwig has shewn that the germinal spot is often
composed of two constituents as in the above cases, but that the
more highly refractive material is generally completely enclosed
by the less dense substance. By Fol the germinal spot is stated
to be absent in a species of Sagitta, but this must be regarded
as doubtful. In young ova the relative size of the germinal
vesicle is very considerable. It occupies in the first instance a
central position in the ovum, but at maturity is almost always
found in close proximity to the surface. Its change of position
in a large number of instances is accomplished during the
growth of the ovum in the ovary, but in other cases does not
take place till the ovum has been laid.

As the ovum attains maturity, important changes take place
in the constitution of the germinal vesicle, which are described
in the next chapter.

The egg-membranes. A certain number of ova when
ready to be fertilized are naked cells devoid of any form of
protecting covering, but as a rule the ovum is invested by some
form of membrane. Such coverings present great variety in

GENERAL HISTORY.

FIG. 6. OVUM OF Toxo-

PNEUSTES VARIEGATUS WITH
THE PSEUDOPODIA-LIKE PRO-
CESSES OF THE PROTOPLASM
PENETRATING THE ZONA RADI-

ATA (zr). (After Selenka.)

their character and origin, and may be conveniently (Ludwig,
No. 4) divided into two great groups, viz. (i) those derived from
the protoplasm of the ovum itself or from its follicle, which may
be called primary egg-membranes; and (2) those formed by
the wall of the oviduct or otherwise, such as the egg-shell of a
bird, which may be called secondary
egg-membranes.

The primary egg-membranes may
again be divided into two groups
(Ed. van Beneden, No. 1), viz., (i)
those formed by the protoplasm of
the ovum, to which the name vitel-
line membranes will be applied ;
and (2) those formed by the cells of
the follicle, to which the name
chorion will be applied.

The secondary egg-membranes
will be dealt with in connection with
the systematic account of the develop-
ment of the various groups. They
coexist as a rule with primary membranes, though in some
types (Cephalophorous Mollusca, many Platyelminthes, etc.),
they constitute the only protecting coverings of the ovum.

The vitelline membranes are either simple structureless
membranes or present numerous radial pores. Membranes
with the latter structure are very widely distributed, Echino-
dermata, Gephyrea, Vertebrata, etc. ( Vide figs. 5 and 7.) The
function of the pores appears to be a nutritive one. They either
serve for the emission of pseudopodia-like processes of the pro-
toplasm of the ovum, as has been very beautifully shewn in the
case of Toxopneustes by Selenka (fig. 6), or they admit (?) pro-
cesses of the follicular epithelial cells (Vertebrata), Their
presence is in fact probably caused by the existence of such
processes, which prevent the continuous deposition of the mem-
brane. The term zona radiata will be applied to perforated
membranes of this kind. Two vitelline membranes, one per-
forated and the other homogeneous, may coexist at the same
time, e.g. Sipunculida, Vertebrata. (Fig. 7.)

The chorion is often ornamented with various processes, etc.

THE OVUM.

FIG. 7. SECTION
THROUGH A SMALL
PART OF THE SURFACE
OF AN OVUM OF AN

It is in many cases doubtful whether a par-
ticular membrane is a chorion or a vitelline
membrane.

All the membranes which surround the
ovum may be provided with a special aper-
ture known as the micropyle. A micropyle
is by no means found in the majority of
types, and there is no homology between
the various apertures so named. Micropyles
have two functions, either (i) to assist in the
nutrition of the ovum during its develop-
ment, or (2) to permit the entrance of the
spermatozoa. The two functions may in
some cases coexist. Micropyles of the first
class are developed at the point of attach-
ment of the ovum to the wall of the ovary or to its follicle.
Good examples of this kind of micropyle are afforded by the
Lamellibranchiata (fig. 12), Holothuria, and many Annelida
(Polynoe, etc.). The micropyle of the Lamellibranchiata (p. 37)
probably serves also to admit the spermatozoa. The second
type of micropyle is found in many Insecta, Teleostei, etc.

fe. Follicular epi-
thelium, vt. Vitelline
membrane. Zn. Zona
radiata. yk. Yolk with
protoplasmic network.

GENERAL BIBLIOGRAPHY OF THE OVUM.

(1) Ed. van Ben ed en. " Recherches sur la composition et la signification de
1'ceuf," etc. Mem. cour. d. VAcad. roy. des Sciences de Belgique, Vol. xxxiv. 1870.

(2) R. Leuckart. Artikel '« Zeugung," R. Wagner's Handworterbuch d. Physio-
logie, Vol. iv. 1853.

(3) Fr. Leydig. " Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt
u. n. ihrer Bedeutung." Oken. Isis, 1848.

(4) Ludwig. " Ueber d. Eibildung im Thierreiche." Arbeiten a. d. zooL-zoot.
Instilut Wiirzburg, Vol. i. I8741.

(5) Allen Thomson. Article " Ovum" in Todd's Cyclopaedia of Anatomy and
Physiology, Vol. v. 1859.

(6) W. Waldeyer. Eierstock u. Ei. Leipzig, 1870.

1 A very complete and critical account of the literature is contained in this paper.

26

CCELENTERATA.

Special History of tJie Ovum tn different types.
CCELENTERATA.

(7) Ed. van Beneden. " De la distinction origiuelle d. testicule et fie 1'ovaire."
Bull. Accui. roy. Belgique, 3* se'rie, Vol. xxxvn. 1874.

(8) R. and O. Hertwig. Der Organismus d. Meduscn. Jena, 1878.

(9) N. Kleinenberg. Hydra. Leipzig, 1872.

Amongst the Ccelenterata the ova are developed in imper-
fectly specialized organs, which are situated in various parts of
the body, for the most part in the space between the epiblast
and the hypoblast.

In Hydra the locality where the ova are developed only
becomes specialized at the time when an ovum is about to be
formed. At one or more points the interstitial cells of the
epiblast increase in number and form a protuberance of germinal
cells, which may be called the ovary. In this ovary a single
ovum is formed by the special growth of one cell. (Kleinenberg,
No. 9.) In the free and attached gonophores of Hydrozoa, the
ova appear either around the walls of
the stomach, or the radial canals, or
around other parts of the gastro-vas-
cular canals.

Their close relations to the gastro-
vascular canals are probably determin-
ed by the greater nutritive facilities
thereby afforded. (Hertwig, No. 8.)

In the permanent Medusa-forms
the ova have similar relations to the
gastro- vascular system. Amongst
the Actinozoa the ova are usually
developed between the epiblast and
the hypoblast in the walls of the
gastric mesenteries. Amongst the
Ctenophora the ova are situated in
close relation with the peripheral
canals of the gastro-vascular system,
which run along the bases of the
ciliated bands. There are many ex-

1'ic. H. RIPE OVUM OF

EriBULIA AURANTIACA. THE

GERMINAL VESICLE HAS BE-
COME INVISIBLE WITHOUT RE-
AGENTS.

Copied from Metschnikoff,
" Entwicklung dcr Siphonopho-
ren." Ztitschrift f. tciss. Zool.,
Vol. xxiv. 1874.

p.d. Peripheral layer of den-
ser protoplasm, p.m. Central
area consisting of a protoplasmic
mesh work.

THE OVUM. 27

amples amongst the Ccelenterata of ova which retain in their
mature state the very simple constitution which has been de-
scribed as characteristic of all young ova ; and which are, when
laid, absolutely without any trace of a vitelline membrane or
chorion. In many other cases both amongst the Medusae, the
Siphonophora, and the Ctenophora, the ripe egg exhibits a dis-
tinction into two parts. The outer part is composed of a dense
protoplasm, while the interior is composed of a network or more
properly a spongework of protoplasm enclosing in its meshes
a more fluid substance. (Fig. 8.)

In some cases the ovum while still retaining the constitution last
described becomes invested by a very delicate membrane. Such is the
constitution of the ripe ovum of Hippopodius gleba amongst the Siphono-
phora1 and of the eggs of Geryonia amongst the permanent Medusae2.
The ripe eggs of the Ctenophora usually present a similar structure3.
After being laid they are found to be invested by a delicate membrane
separated by a space filled with fluid from the body of the ovum. The
latter is composed of two layers, an outer one of finely granular protoplasm
and an inner layer consisting of a protoplasmic spongework containing in
its meshes irregular spheres. These latter are stated by Agassiz to be of a
fatty nature, and it is probable that in most cases where a protoplasmic net-
work is present, this alone constitutes the active protoplasm ; and that the
substance which fills up its meshes is to be looked on as a form of food -yolk
or deutoplasm, though it appears sometimes to have the power of assimila-
ting the firmer yolk particles.

The membrane which invests the ovum of many of the
Ccelenterata is probably a vitelline membrane.

The ova of the Hydrozoa take their origin, in most groups
at any rate4, from the deeper layer of the epiblast (interstitial
layer of Kleinenberg). The interstitial cells in the ovarian
region form primary germinal cells, and by an excess of
nutrition certain of them outstrip their fellows and become
young ova. Such ova differ from the full-grown ova already

1 Metschnikoff. Zeitschrift /. wiss. Zoologie, Vol. xxiv. 1874.

2 Herman Fol. Jenaische Zeitschrift, Vol. vil.

3 Kowalevsky. ' ' Entwicklungsgeschichte d. Rippenquallen." Memoire de
FAcad. Petersburg, 1866. And Alex. Agassiz. "Embryology of the Cteno-
phorEe." Amer. Acad. of Science and Arts, Vol. x. No. in.

4 The view of van Beneden, according to which the ova have an endodermal
(hypoblastic) origin, has been shewn to be at any rate confined to certain groups.
The whole question of the origin of the generative products from the germinal layers
in the Ccelenterata is still involved in great obscurity.

28 CCELENTERATA.

described, mainly in the fact that they have a proportionately
smaller amount of protoplasm round the germinal vesicle.
They grow to a considerable extent at the expense of germinal
cells which do not become converted into ova.

The ova of many Ccelenterata undergo changes of a more
complicated kind before attaining their full development. Of
these ova that of Hydra may be taken as the type. The ovary
of Hydra (Kleinenberg, No. 9) is constituted of angular flattish
germinal cells of which no single one can be at first dis-
tinguished from the remainder. As growth proceeds one of the
cells occupying a central position becomes distinguished from
the remaining cells by its greater size, and wedge-like shape.
It constitutes the single ovum of the ovary. After it has become
prominent it grows rapidly in size, and throws out irregular
processes. The germinal vesicle, which for a considerable time
remains unaltered, also at length begins to grow ; and the
sharply defined germinal spot which it contains after reaching a
certain size completely vanishes. After the atrophy of the
germinal spot, there appears in the middle of the ovum a
number of roundish yolk granules.

The shape of the ovum becomes more irregular, and chloro-
phyll granules, in addition to the yolk granules, make their
appearance in it. A fresh germinal spot of circular form also
arises in the germinal vesicle. Protoplasmic processes are next
thrown out in all directions, giving to the ovum a marvellous
amceboid character. (Fig. 4.) The amoeboid form of the ovum
serves no doubt to give it a larger surface for nutrition. Coin-
cidently with the assumption of an amceboid form there appear
in the ovum a great number of peculiar bodies. They are
vesicles with a thick wall bearing a conical projection into the
interior which is filled with fluid. (Fig. 46.) These bodies
are formed directly from the protoplasm of the ovum, and are
to be compared both morphologically and physiologically with
the yolk-spherules of such an ovum as that of the Bird. They
are called pseudocells by Kleinenberg, and are found with
slightly varying characters in many ova of the Hydrozoa.

They first appear as small highly refracting granules ; in these a cavity
is formed which is at first central but is eventually pushed to one side by the
formation of a conical projection from the wall of the vesicle.

THE OVUM. 29

After the growth of the ovum is completed the amoeboid
processes gradually withdraw themselves, and the ovum assumes
a spherical form ; still however continuing to be invested by the
remaining cells of the ovary. It is important to notice that the
egg of Hydra retains throughout its whole development the
characters of a single cell, and that the pseudocells and other
structures which make their appearance in it are not derived
from without, and supply not the slightest ground for regarding
the ovum as a structure compounded of more than one cell.

The development of the ova of the Tubularidae, which has
been supposed by many investigators to present very special
peculiarities, takes place on essentially the same type as that of
Hydra, but the germinal vesicle remains permanently very
small and difficult to observe. The mode of nutrition of the
ovum may be very instructively studied in this type. The
process is one of actual feeding, much as an Amoeba might feed
on other organisms. Adjoining one of the large ova of the
ovary there may be seen a number of small germinal cells.
(Fig. 3.) The boundary between these cells and the ovum is
indistinct. Just beyond the edge of the ovum the small cells
have begun to undergo retrogressive changes ; while at a little
distance from the ovum they are quite normal (g.c.y.

PLATYELMINTHES.

(10) P. Halle z. Contributions a VHistoire naturelle des Turbellaries. Lille,
1879.

(11) S. Max Schultze. Beitrdge z. Naturgeschichte d. Turbellarien. Greifs-
wald, 1851.

(12) C. Th. von Siebold. " Helminthologische Beitrage." Mullet's Archiv,
1836.

(13) C. Th. von Siebold. Lehrbuch d. vergleich. Anat. d. wirbellosen Thiere.
Berlin, 1848.

(14) E. Zeller. " Weitere Beitrage z. Kenntniss d. Polystomen." Zeit.f. wiss.
Zool., Bd. xxvu. 1876.

[Vide also Ed. van Beneden] (No. i).

This group, under which I include the Trematodes, Cestodes,

1 The above description of the ova of the Tubularidse is founded on sections of
the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me how-
ever that the absence of a distinct boundary between the germinal cells and the ovum
is not usual.

30 PLATYELMINTHES.

Turbellarians and Nemertines, has played an important part in
all controversies relating to the nature and composition of the
ovum. The peculiarity in the development of the ovum in
most members of this group consists in the fact that two organs
assist in forming what is usually spoken of as the ovum. One
of these is known as the ovary proper, and the other as the
vitellarium or yolk-gland. In the sequel the term ovum will be
restricted to the product of the first of these organs. In Trema-
todes the ovary forms an unpaired organ directly continuous
with an oviduct into which there open the ducts from paired
yolk-glands.

The ovary has a sack-like form and contains in some
instances a central lumen (Polystomum integerrimum). At the
blind end of the organ is placed the germinal tissue. This part
is, according to the accounts of the majority of investigators,
formed of a polynuclear mass of protoplasm not divided into
distinct cells. Whether it is really formed of undivided proto-
plasm or not, it is quite certain that a little lower down in the
organ distinct cells are found, which have been segmented off
from the above mass, and are formed of a large nucleus and
nucleolus, surrounded by a delicate layer of protoplasm. These
cells are the young ova. They usually assume a more or less
angular form from mutual pressure, and, in the cases where the
ovary has a lumen, constitute a kind of epithelial lining for the
ovarian tube. They become successively larger in passing
down the ovary, and, though in most cases naked, are in some
instances (Polystomum integerrimum) invested by a delicate
vitelline membrane. Eventually the ova pass into the oviduct
and become free; and at the same time assume a spherical
form.

In the oviduct the ovum receives somewhat remarkable
investing structures, derived from the organ before spoken of as
the yolk-gland. The yolk-gland consists of a number of small
vesicles, each provided with a special duct, connected with the
main duct of the gland. Each vesicle is lined by an epithelium
of cells provided with doubly contoured membranes, and con-
taining nuclei.

As the yolk-cells grow older refracting spherules become
deposited in their protoplasm, which either completely hide the

THE OVUM.

nucleus, or render it very difficult to see. In the majority of
cases the entire cells forming the lining of the vesicles constitute
the secretion of the yolk-gland. They invest the ovum, and
around them is formed a shell or membrane. In some cases
(e.g. Polystomum integerrimum) the yolk-cells retain their
cellular character and vitality till the embryo is far developed.
In other cases they lose their membrane and nucleus shortly
after the formation of the egg-shell, and break up into a fluid,
holding in suspension a number of yolk-granules. A partial
disorganisation of the yolk-cells can also take place before they
surround the ovum ; while in some species of Distomum they
completely break up before leaving the yolk-gland.

There is thus a complete series of gra-
dations between the investment of the
ovum by a number of distinct cells, and
its investment by a layer of fluid con-
taining yolk-spherules in suspension. In
neither the one case nor the other do the
investing structures take any share in the
direct formation of the embryo from the
ovum. Physiologically speaking they play
the same part as the white in the fowl's
egg-

The egg-shell, which is usually formed by a
secretion of a special shell-gland opening into the
oviduct, exhibits one or two peculiarities in the
different species of Trematodes. In Amphisto-
mum subclavatum it presents at one extremity a
thickened area, which is pierced by a narrow mi-
cropyle. In other cases one extremity of the egg-
shell is produced into a long process, and some-
times even both extremities are armed in this way.
Opercula and other types of armature are also
found in different forms.

The mode of development of the ovum in
Cestodes is very nearly the same as in Trema-
todes.

The ovum becomes enveloped in the usual secretion of the yolk-gland ;
and an egg-shell is always formed by the secretion of a special shell-gland.

Amongst the Turbellarians and Nemertines, there are greater
variations in the arrangement of the female generative glands,

FIG. 9. GENERATIVE
SYSTEM OF VORTEX VIRI-
Dis. (From Gegenbaur,
after Max Schultze.)

t. Testis. v.d. Vasa
differentia, v.s. Seminal
vesicle, p. Penis, u. Ute-
rus, o. Ovary, v. Vagina.
g.v. Yolk-glands, r.s. Re-
ceptaculum seminis.

32 PLATYELMINTHES.

than in the preceding types. In most of the Rhabdocoela and
fresh-water Dendrocoela these organs resemble in their funda-
mental characters those of the Trematodes and Cestodes. There
are present a paired or single ovary and a paired yolk-gland.
The general arrangement of the organs is shewn in fig. 9.

The blind end of the ovaries is usually (Ed. van Beneden, etc.)
stated to be formed of a polynuclear protoplasmic basis, but
Hallez (No. 10) has recently insisted that, even at the extreme
end of the ovary, the germinal cells are quite distinct, and not
confounded together.

With one or two exceptions the yolk-cells secreted by the
vitellarium retain their vitality till they are swallowed by the
embryo, after the development of its mouth. The few not so
swallowed become disintegrated. They are granular nucleated
cells, and, as was first shewn by von Siebold, are remarkable for
exhibiting spontaneous amoeboid movements.

Very important light on the nature of the vitellarium is
afforded by the structure of the generative organs in Prorhyncus
and Macrostomum.

In Prorhyncus there is no separate vitellarium, but the lower
part of the ovarian tube functionally and morphologically
replaces it. The ovum becomes surrounded by yolk-cells,
which according to Hallez (No. 10) retain their vitality for a
long time. According to Ed. van Beneden yolk-spherules are
formed in the protoplasm of the ovum itself, in addition to and
independently of the surrounding yolk-cells. In Convoluta
paradoxa a special vitellarium is stated to be absent ; though a
deposit of yolk is formed round the ovum (Claparede).

In Macrostomum again the yolk-glands are at most repre-
sented by a lower specialized part of the ovarian tube. The ova in
passing down become filled with yolk-spherules. According to
Ed. van Beneden these spherules are formed in the protoplasm
of the ovum itself; but this is explicitly denied by Hallez, who
finds that they are formed from the lining cells of the ovarian
tube, which, instead of retaining their vitality as in Prorhyncus,
break up and form a granular mass which is absorbed by the
protoplasm of the ovum.

In Prostomum caledonicum (Ed. van Beneden) the generative
organs are formed on the same plan as in other Rhabdocoela, but

THE OVUM. 33

the cells which form the yolk-gland give rise to yolk particles
which enter the ovum, instead of to a layer of yolk-cells sur-
rounding the ovum.

Amongst the marine dendrocoelous Turbellarians the ova are formed in
separate sacks widely distributed in the parenchyma of the body between
the alimentary diverticula. In these the ova undergo their complete develop-
ment, without the intervention of yolk-glands.

The ovaries of the Nemertines more nearly resemble those of the
marine Dendrocoela than those of the Rhabdoccela. They consist of a series
of sacks situated on the two sides of the body between the prolongations
of the digestive canal. The eggs are developed in these sacks in a perfectly
normal manner, and in many cases become filled with yolk-spherules which
arise as differentiations of the protoplasm of the ovum. The protecting
membranes of the ova have not been accurately studied. In some cases1
two membranes are present, an internal and an external. The former,
immediately investing the vitellus, is very delicate : the external one is
thicker and hyaline.

The constitution of the female generative organs of the
Trematodes was first clearly ascertained by von Siebold (No. 12).
He originally, though not very confidently, propounded the
view that the germinal vesicles alone were formed in the ovary
and that the protoplasm of the ovum was supplied by the
yolk-gland. This view has long been abandoned, and von
Siebold (No. 13) himself was the first to recognize that true ova
with a protoplasmic body containing a germinal vesicle and
germinal spot were formed in the ovary. The Trematodes have
however not ceased to play an important part in forming the
current views upon the development of ova, and have quite
recently served Ed. van Beneden as his type in exposing his
general view upon this subject.

His view consists fundamentally in regarding the secretion of the
yolk-glands, which in most cases merely invests the ovum, as homologous
with the yolk-spherules which fill the protoplasm of many eggs ; and he
considers the part of the ovary where in most forms the ova receive their
supply of yolk particles, as equivalent to the vitellarium of the Platy-
elminthes. He further appears to regard the primitive state as that
exemplified in Trematodes, Cestodes, etc., and holds that the ovarian
types characteristic of other forms are secondarily derived from this, by
the coalescence of the primitively distinct vitellarium with the ovary proper.

1 Amphiporus lactiflorius and Nemertes gracilis. Mclntosh. Monograph on
British Nemertines. Ray Society.

B. II. 3

34 ECHINODERMATA.

This appears to me a case of putting the cart before the horse. To my
mind the vitellarium is to be regarded, as has already been suggested by
Gegenbaur, Hallez, etc. as a special differentiation of the primitively simple
ovarian tube, and the instances of Macrostomum and Prorhyncus just cited
appear to me to indicate some of the steps in this differentiation. In
Macrostomum the cells of the lower part of the oviduct simply supply a
kind of nutriment to the ovum in the form of granular yolk particles,
while in Prorhyncus the yolk-cells of the lower part of the ovarian
tube form a complete investment of independent cells for the ovum. If
this lower part of the ovarian tube were to grow out as a special
diverticulum we should have produced a normal vitellarium. But even
with the above modification the theory of van Beneden appears to me not
completely satisfactory. The view that the yolk-spherules are of the same
nature as the yolk-cells is mainly supported by the case of Prostomum
caledonicum, where the vitellarium produces the yolk particles which fill
the ovum. The cases of Prorhyncus and Macrostomum give a different
complexion to that of Prostomum caledonicum. From the first of these
especially it appears that, even when normal yolk-cells surround the ovum,
yolk particles can be deposited independently in the protoplasm of the
ovum.

The most probable view of the nature of the vitellarium is
that of Gegenbaur, Hallez, etc., according to which it is to be
regarded as a specially modified part of the ovarian tube. On
this view the nature and function of the yolk-cells admit of a
fairly simple explanation. They are to be regarded as primary
germinal cells like those in the ovaries of Hydra, Tubularia, etc.,
which do not become converted into ova. Like these cells they
may in some instances, Macrostomum, Prostomum, etc., serve
directly in the nutrition of the ovum. In other cases they retain
their independence and serve for the late nutrition of the embryo.
In both instances they retain the faculty, normally possessed by
ova, of forming yolk particles in their protoplasm.

ECHINODERMATA.

(15) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." A't<;/<r-
liindisch. Archivf. Zoologie, Vol. I. 1871.

(16) C. K. Hoffmann. " Zur Anatomic d. Asteriden." Niederldndisch. Archiv
/. Zoologit, Vol. u. 1873.

(17) II. Ludwig. "Beitrage zur Anat. d. Crinoiden." Zeit. /. wiss. Zool.,
Vol. xxvin. 1877.

(18) Job. Miiller. " Ueber d. Canal in d. Eiern d. Holothurien." Muller's
Archiv, 1854.

THE OVUM. 35

(19) C. Semper. Holothurien. Leipzig, 1868.

(20) E. Selenka. Befruchtung d. Eies v. Toxopneustes variegatus, 1878.

[ Vide also Ludwig (No. 4), etc.]

The eggs of the Echinodermata present in their development
certain points of interest.

The ovaries themselves are usually surrounded by a special
vascular dilatation. In the Asteroidea, the Echinoidea, and the
Holothuroidea the organs have the form of sacks ; specially
surrounded in the two former groups, and probably the latter,
by a vascular sinus formed as a dilatation of one of the
generative vessels. In the Crinoids they have the form of a
hollow rachis completely surrounded by a blood-vessel. (Fig.
n, &) The proximity of the ovaries (generative organs) to the
vascular system in these forms has clearly the same physiological
significance as the proximity of the ovaries (generative organs)
to the radial vessels in the Ccelenterata.

In the Asteroidea, the Echinoidea and the Holothuroidea the
ovaries have the form of sacks lined by an epithelium of germinal
cells, and the ova are formed by the enlargement of these cells,
which, when they have reached a certain size, become detached
from the walls, and fall into the cavity of the ovarian sack. In
Toxopneustes (Selenka) and very probably in other forms only
a few of the epithelial cells undergo conversion into ova : the
remainder undergo repeated division, and, as in so many other
" cases, are eventually employed in the nutrition of the true ova.
In the nearly ripe ova of Asterias Fol has described a flattened
follicular epithelium the origin of which is unknown.

In Holothuria (Semper) a further differentiation of the
germinal cells, not destined to become ova, takes place. They
surround the enlarged cell which forms the true ovum, for which
they constitute a kind of follicular capsule. This capsule is
attached by a stalk to the walls of the ovary, and the ovum lies
freely in it except for an area nearly opposite its (the capsule's)
point of attachment, where the ovum adheres to the wall of the cap-
sule. Subsequently the follicle cells which form the capsule fuse
together, and form a definite membrane in which only the nuclei
remain distinct. Within the membranous capsule there is formed
for the ovum an albuminous zona radiata. At the point where
the ovum is attached to its capsule this membrane cannot be

3—2

ECHINODERMATA.

FIG. 10. OVUM OF Toxo-

PNEUSTES VARIEGATUS WITH
THE PSEUDOPODIA-LIKE PRO-
JECTIONS OF THE PROTOPLASM
PENETRATING THE ZONA RADI-

developed, and therefore remains in-
complete. The perforation so formed,
becomes the micropyle of the Holo-
thurian egg, which was first discover-
ed by Joh. Miiller. The albuminous
membrane just described for Holo-
thurians is also found in Asteroids
(fig- 5) an<3 Echinoids. In these
groups there is no proper micropyle,
though in Ophiothrix a nutritive pas-
sage perforates the membrane at the
attachment of the ovum before the

11,1 i /* riM^A 1 K.£\ J. II^VJ 1111, £*^i>i/V

period when the ovum becomes free ATA (zr). (After Selenka.)
(Ludwig). The formation of the zona

radiata has been studied by Selenka. It is secreted by the
protoplasm of the ovum, and has a gelatinous consistency, and
after it is formed the peripheral layer of the protoplasm of
the ovum sends out through it pseudopodia-like processes to
absorb nutriment from without. These
processes are at first large and irregular,
but soon become finer and finer (fig. 10),
and acquire a regular radiating arrange-
ment. They are withdrawn when the ovum
is ripe, but they nevertheless give rise to
the finely radiated appearance of the mem-
brane, the radii being in reality delicate
pores.

In the Crinoids the generative rachis
consists of a tube, the epithelium of which
is formed of the primary germinal cells.
(Fig. n.) While some of these cells en-
large and become ova, the remainder supply
the elements for a follicular epithelium,
which is established round the ova, exactly MATURE COMATULA.

TT , , . (From Gegenbaur, after

as m Holotnunans. Ludwig.)

p. Tentacle, g. Lumen of genital rachis. w. Water-vascular vessel, n. Nerve
cord. b. Blood-vessel on nerve cord and round genital rachis. eg. Genital canal.
cd. Dorsal section of the body cavity, cv. Ventral section of body cavity.

FIG. n. TRANSVERSE

THE OVUM.

MOLLUSCA.
Lamellibranchiata.

(21) H. Lacaze-Duthiers. "Organes genitaux des Acephales Lamelli-
branches." Ann. Sci. Nat., 4me serie, Vol. II. 1854.

(22) W. Flemming. " Ueb. d. er. Entwick. am Ei d. Teichmuschel. " Archiv
f. mikr. Anat., Vol. x. 1874.

(23) W. Flemming. " Studien lib. d. Entwick. d. Najaden." Sitz. d. k.
Akad. Wiss. Wien, Vol. LXXI. 1875.

(24) Th. von Hessling. "Einige Bemerkungen, etc." Zeit. f. wiss. Zool.,
Bd. v. 1854.

(25) H. von Jhering. " Zur Kenntniss d. Eibildung bei d. Muscheln." Zeit.
f. wiss. ZooL> Vol. xxix. 1877.

(26) Keber. De Introitu Spermatozoorum in ovuta, etc. Konigsberg, 1853.

(27) Fr. Leydig. " Kleinere Mittheilung etc." Mullet's Archiv, 1854.

Gasteropoda.

(28) C. Semper. " Beitrage z. Anat. u. Physiol. d. Pulmonaten." Zeit. f.
•wiss. Zool., Vol. vni. 1857.

(29) H. Eisig. "Beitrage z. Anat. u. Entwick. d. Pulmonaten." Zeit. f. wiss.
Zool., Vol. xix. 1869.

(30) Fr. Leydig. " Ueb. Paludina vivipara." Zeit.f. wiss. Zool., Vol. II. 1850.

Cephalopoda.

(31) Al. Kolliker. Entwicklungsgeschichte d, Cephalopoden. Zurich, 1844.

(32) E. R. Lankester. "On the developmental History of the Mollusca."
Phil. Trans., 1875.

L amellibranchiata.

The ova of the Lamellibranchiata present several points of
interest. They are developed in pouches of the ovary which are
lined by a flattened germinal epithelium, or sometimes (?) a
syncytium. Some of the cells of this epithelium enlarge and
become ova, but remain attached to the walls of their pouches
by protoplasmic stalks. Round the ovum there appears in some
forms (Anodon, Unio) a delicate vitelline membrane, which is
incomplete at the protoplasmic stalk, and is therefore perforated
by an aperture which forms the micropyle. (Fig. 12.) As the

38 MOLLUSCA.

ovum becomes ripe a large space filled with albuminous fluid
becomes established between the ovum and its membrane, but
the ovum remains attached to the membrane at the micropyle.
In Scrobicularia (von Jhering, No. 25) the membrane round the
ovum appears from the first as an albuminous layer, the outer-
most stratum of which becomes subsequently hardened as the
vitelline membrane. In this form also the protoplasmic stalk
becomes, in pouches largely filled with ova, extremely long.
The ova become eventually detached by the stalk rupturing,
and the portion of it which remains attached to the vitelline
membrane falling off. The function of the stalk and of the
micropyle during the development of the ovum is undoubtedly a
nutritive one.

In Anodon and Unio yolk granules
similar to those deposited in the proto-
plasm of the ovum are also found in the
epithelial cells of the ovarian pouches
(Flemming, 22), and there can be but
little doubt that they are directly trans-
ported from these cells into the ovum.
These cells would seem therefore to play
much the same part as the yolk-glands

of some Turbellarians (Prostomum cale-

v T ,-, , . , ,, , FIG. 12. MEDIUM-SIZED

domcum). In Scrobicularia yolk granules OVUM OF ANODONTA COM-

are not found in the epithelium of the PLAN ATA. (After Flemming.)

•. • j • ..u j-1 4. j w/. micropyle. gs. ger-

pouches, but are contained in the dilated minal spot.

disc by which the ovum is attached to

the wall of its pouch, as well as in the ovum itself.

On the ovum becoming detached the micropyle still remains
as an aperture, which probably has the function of admitting the
spermatozoa.

The shape and form of the micropyle vary greatly. In Anodon and
Unio it is a projecting trumpet-shaped structure, which after fertilization
becomes shortened and reduced to a mere aperture which is finally
stopped up. (Fig. 12.)

In other forms it is simply a perforation in the vitelline membrane
which is sometimes very large. In a species of Area, which I had an
opportunity of observing at Valparaizo, it was equal to nearly the circum-
ference of the ovum.

THE OVUM. 39

The eggs of the Lamellibranchiata are not only remarkable
in the possession of a micropyle, but in certain peculiarities of the
yolk and of the germinal vesicle.

In the fresh-water mussels there is usally found in young and
medium-sized ova a peculiar lens-shaped body — Keber's cor-
puscle— which is placed immediately internal to the micropyle.
It is probably in some way connected with the nutrition of the
ovum, though the fact that it is not always present shews that it
cannot be of great importance.

A dark body found by von Jhering in the neighbourhood of
the germinal vesicle in the ripe ovum of Scrobicularia is probably
of a similar nature to Keber's corpuscle. Both bodies may be
placed in the same category as the so-called yolk nucleus of the
spider's and frog's ova.

In all except the youngest ova of Anodon and Unio the
germinal spot is composed of two nearly complete spheres united
together for a small part of their circumference. (Fig. 12, gs.}
The smaller of these has a higher refractive index than the
larger, and often contains a vacuole : the two parts together
appear to be the separated components (though not by simple
division) of the primitive nucleolus. A nucleolus of this charac-
ter is not universal amongst Lamellibranchiata, but a similar
separation of the constituents of the germinal spot has been
found by Flemming in Tichogonia, in which however the more
highly refracting body envelopes part of the less highly re-
fracting body in a cap-like fashion.

Gasteropoda.

The ova of the Gasteropoda are developed, like those of the
Lamellibranchiata, from the epithelial cells of the ovarian acini
or pouches. In the hermaphrodite forms both ova and sperma-
tozoa are produced in the same pouches (fig. 13), some of the
epithelial cells becoming ova and others spermatozoa. The ova
are usually formed in the wall of the pouch, and the sperma-
tozoa internally (Pulmonata) (fig. 13 A), or a further differenti-
ation of parts may take place (fig. 13 B). The ova of Gastero-
pods are exceptional in the fact that a vitelline membrane is

MOLLUSCA.

rarely or never developed around them. The ovum in its pas-
sage to the exterior becomes enclosed in a secretion of the
albuminous gland, which hardens externally to form a special
membrane.

FIG. 13. FOLLICLES OF THE HERMAPHRODITE GLANDS OF GASTEROPODA.
(From Gegenbaur.)

A. Of Helix hortensis. The ova (aa) are developed on the wall of the follicle,
and the seminal masses (b) internally.

B. Of Aeolidia. The seminal portion of a follicle is beset peripherally by ovarian
saccules (a), c. Common afferent duct.

Cephalopoda.

Lankester (No. 32) has brought out some very interesting
points with reference to the nutrition of the eggs of Sepia during
their growth. The eggs develope in connective-tissue pouches
which early give rise to a double pedunculated capsule of
connective tissue. The cells of the inner layer of this capsule
soon assume an epithelial character, and become a definite
follicular epithelium, while between the two layers there pene-
trates a network of vascular channels. The follicular epithelium
becomes after the establishment of these vascular channels
folded in a most remarkable manner. The folds, which are
shewn in section in fig. 14, ic, project into and nearly com-
pletely fill up the body of the ovum. An enormous increase is
thus effected in the nutritive surface exposed by the epithelium.
Each fold is thoroughly supplied with blood-vessels. The
plications of the follicular epithelium give rise to a basket-work
tracery on the surface of the ovum. During the stage when the
follicular epithelium has the above structure, its cells have a

Offcition

THE WORKS

OF

FRANCIS MAITLAND BALFOUR

VOL. II.

A TREATISE ON COMPARATIVE EMBRYOLOGY.
Vol. I. Invertebrata.

Hmrtron :

MACMILLAN AND CO.
1885

THE OVUM.

character similar to that
of the goblet-cells of a
mucous membrane, and
pour out their metamor-
phosed protoplasm into
the body of the ovum.

After the above mode
of nutrition has gone on
for a certain time a
change takes place, and
the ridges gradually dis-
appear. This is caused
by the epithelial cells
passing off from the
ridges into the proto-
plasm of the ovum ; and
becoming assimilated,
after retaining their in-
dividuality for a longer
or shorter period. When
the absorption of the

ridges is completed the surface of the ovum assumes a perfectly
regular outline. The capsule of the ovum then bursts at the
opposite pole to the peduncle, and the ovum falls into the
oviduct.

The ova of the Cephalopoda, like those of the Gasteropoda,
are quite naked, being without a vitelline membrane or chorion.
The egg-capsule which is formed for them in their passage down
the oviduct is perforated in Sepia by a micropylar aperture.

or:

FIG. 14. TRANSVERSE SECTION THROUGH AN
OVARIAN EGG OF SEPIA. (Copied from Lankester.)

o.c. outer capsular membrane, i.e. inner cap-
sular membrane with follicular epithelium, b.v.
blood-vessels in section between the outer and
inner capsular membranes, c. vitellus.

The section shews the folds of the inner
capsule with their epithelium, which penetrate
into the substance of the ovum for the purpose
of supplying it with nourishment.

CH^ETOPODA.

(33) Ed. Claparede. " Les Annelides Chsetopodes d. Golfe de Naples."
Mem. d. I. Societ. phys. et d'hist. nat. de Geneve 1868 — 9 and 1870.

(34) E. Ehlers. Die Borstenivurmer nach system, und anat. Untersuchungen.
Leipzig, 1864 — 68.

(35) E. Selenka. "Das Gefass-System d. Aphrodite aculeata." Nieder-
landisches Archiv f. Zoo!., Vol. II. 1873.

The ova of the Chaetopoda are in most cases developed from
the special tracts of the epithelial cells lining parts of the body

42 DISCOPHORA.

cavity, which constitute a germinal epithelium (fig. 15). Very
frequently (Aphrodite, Arenicola), as is so common in other
types, these tracts of germinal cells surround the blood-vessels.

FIG. 15. A PARAPODIUM OF TOMOPTERIS. (From Gegenbaur.)
o. Collection of germinal epithelial cells lining the body cavity.

In some cases the germinal epithelium thickens to form a
compact organ, for which the outermost cells may form a
more or less definite membranous covering (Oligochaeta, etc.).
The ova are formed by the enlargement, accompanied by other
changes, of these germinal cells. During their early development
the ova are frequently surrounded by a special capsule, which is
often stalked, and provided at its attachment with a large micro-
pylar aperture. In Aphrodite and Polynoe this arrangement,
which is clearly connected with the nutrition of the ovum, is very
easily seen. The ovum is dehisced into the body cavity by the
bursting of its capsule or the rupture of the stalk. The capsule
is always eventually thrown off; but a vitelline membrane is
frequently developed after the detachment of the ovum into the
body cavity. The vitelline membrane of Spio and other Poly-
chaeta is provided with an equatorial ring of ampulliform
vesicles.

DISCOPHORA.

(36) H. Dorner. " Ueber d. Gattung Branchiobdella." Zcit. f. tviss. ZooL,
Vol. xv. 1865.

(37) R. Leuckart. Die menschlicfun Parasiten.

(38) Fr. Leydig. "Zur Anatomic v. Piscicola geometrica, etc." Zcit.f.wiss.
Zool., Vol. i. 1849.

(39) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micr. Sci.,
Vol. xvin. 1878.

The ovary of the Discophora is formed of a mass of cells en-
veloped in a membranous sack. In Branchiobdella there is

THE OVUM. 43

placed in the central axis of these cells a column of nucleated
protoplasm from which the cells themselves are budded off. The
development of the ovum takes place by the enlargement, etc. of
one of the peripheral cells, which eventually bursts the wall of
the sack and is freely dehisced into the body cavity.

In most other Leeches (except Piscicola and its allies) there
is found a more specialized arrangement of the same nature as
in Branchiobdella. There are one or more coiled egg-strings
which lie freely in a delicate sack continuous with the oviduct.
Each egg-string is formed of a central rachis and of a peripheral
layer of cells1. The ova are formed by the enlargement of the
peripheral cells accompanied by a deposition of food-yolk.
Food-yolk appears to be formed in the rachis even more ener-
getically than in the protoplasm of the ova. When ripe the ova
fall into the ovarian sack.

In Piscicola the development of the ovum is somewhat pecu-
liar but resembles in certain respects that of Bonellia (p. 45).
The ova are developed from the primitive germinal cells which
fill up the ovarian sack. The nuclei in these cells increase in
number, and a nucleated peripheral layer of each cell becomes
separated from the central part, which also contains nuclei.
This latter part next divides into numerous cells, of which one
eventually forms the ovum, and the remainder constitute a mass
of cells adjoining it as in Bonellia (fig. 16). This mass of cells
eventually disappears, and is probably employed in the nutrition
of the ovum.

The ovaries of the Leech appear to belong to the tubular
type in that the ova are not formed from part of the epithelium
lining the body cavity; but if, as seems probable, the true
affinities of the Leeches are with the Chaetopoda, the investment
of the ovaries must be of a secondary nature. It should be
noted that the ova are not, as in the ordinary tubular ovary,
developed from the epithelium lining the ovarian tube.

1 The rachis is stated by Whitman (No. 39), and other observers to be formed of
nucleated protoplasm, but further investigations on this point are still required.

44 GEPHYREA.

GEPHYREA.

(40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861.

(41) C. Semper. Holothurien, 1868, p. 145.

(42) J. W. Spengel. " Beitrage z. Kenntniss d. Gephyreen." Beitrage a. d.
zool. Station «. Neapel, Vol. i. 1879.

(43) J. W. Spengel. "Anatomische Mittheilungen lib. Gephyreen." Tagebl.
d. Naturf. Vers. MUnchen, 1877.

In the Gephyrea, as in the Chsetopoda, the ova are developed
from the lining cells of the peritoneum and frequently from the
cells surrounding parts of the vascular system (Bonellia, Thalas-
sema). In many cases (Sipunculus, Phascolosoma, Echiurus)
the main growth of the ovum takes place after it has been
dehisced into the body cavity.

In Sipunculus the ova in the body cavity are surrounded by-
a follicle which is thrown off before they become ripe.

Brandt denies the existence of this follicle or rather its cellular nature
Spengel's (43) observations are conclusive in favour of the correctness of
the original interpretation of Keferstein and Ehlers. The follicles would
seem to be formed after the ova have become free. In Phascolosoma there
is no follicle (Semper, Spengel).

In both Phascolosoma and Sipunculus a vitelline membrane
with radial pores — zona radiata — is formed, and in Phascolosoma
the external part of this is separated off as a structureless
vitelline membrane. The formation of both these membranes
from the protoplasm of the ovum is rendered certain in the
latter case by the absence of a follicular epithelium.

Some interesting observations on the growth and origin of
the ovum in Bonellia have been made by Spengel.

The ova originate from certain cells (germinal cells) in the
peritoneal investment of the ventral vessel, overlying the nervous
cord. These cells, which are well marked off from the surround-
ing flattened peritoneal elements, increase in number by division,
and form small masses surrounded by a follicle of peritoneal
cells, and attached by a stalk to the peritoneum. The central
cell of each mass grows larger than the rest, which arrange
themselves in a columnar fashion round it ; it is not, however,
destined to become the ovum. On the contrary certain of the
other cells adjoining the stalk grow larger, and finally one of
these becomes distinguished as the ovum by its greater size and

THE OVUM. 45

the character of its nucleus. The remainder of the larger cells

become of the same size as their neighbours. The ovum now

becomes more or less separate from the mass of germinal cells,

rapidly grows in size, and soon forms the most considerable

constituent of the follicle (fig. 16, ov). The

remaining germinal cells are quite passive,

and though, with the exception of the

central cell, they do not appear to atrophy,

they soon constitute a relatively small

prominence on the surface of the ovum.

By the rupture of the stalk the whole

follicle becomes eventually detached, and

the further development of the ovum takes

place in the body cavity. A vitelline ^ONEL'LIA^A MEDIUM

membrane is formed, and eventually the STAGE OF DEVELOPMENT.

y (After Spengel.)

ovum is taken into the oviduct (segmental ^ ovum ftt flattened

organ). At this time or slightly before, follicular epithelium,
the follicle cells together with the germinal mass, which through-
out exhibits no signs of atrophy, become thrown off, and the
ovum is left invested in its vitelline membrane.

NEMATODA.

(44) Ed. Claparede. De la formation et de la fecondation des ceufs chez les Vers
Nematodes. Geneve, 1859.

(45) R. Leuckart. Die menschlichen Parasiten.

(46) H. Munk. " Ueb. Ei- u. Samenbildung u. Befruchtung b. d. Nematoden."
Zeit.f. wiss. ZooL, Vol. ix. 1858.

(47) H. Nelson. "On the reproduction of Ascaris mystax, etc." Phil. Trans.
1852.

(48) A.Schneider. Monographic d. Nematoden. Berlin, 1866.

The female organs consist as a rule of two csecal tubes which
unite before opening to the exterior. Each of these is divided
into a vagina, uterus, oviduct, and ovary. The ovary constitutes
the blind end of the tube, and is formed of a common protoplas-
mic column, holding a number of nuclei in suspension. The
protoplasm becomes cleft around the nuclei in the uppermost
part of the tube ; the circumscription of the ova proceeds, how-
ever, very gradually, and since it commences at the periphery
of the column the ova remain attached by stalks to a central
axis with one end free. In this way there is formed a rod-like

46 INSECTA.

structure known as the rachis, which consists of a central axis
with a series of half circumscribed ova radiately arranged round
it. In the lowest part of the ovary the ova become completely
isolated and form separate cells.

The protoplasm of the ova, which is clear in the terminal
division of the ovary, becomes in most forms filled lower down
with yolk-spherules secreted in the body of the ova. These
commence to appear at the uppermost extremity of the rachis.

In some instances, e.g. Cucullanus elegans, yolk-spherules are not
formed. In the Oxyuridae the ova are directly segmented off from the
terminal syncytium of protoplasm without the intervention of a rachis ;
and are therefore formed in the same way as amongst Trematodes, etc.

The origin of the membrane around the ova of the Nematoda has been
much disputed.

At the time when the ovum is detached from the rachis no membrane
is present, but it nevertheless appears from Schneider's observations that the
region at which it is detached is softer than other parts, so that a kind of
micropyle is here formed which disappears after impregnation. A delicate
vitelline membrane then appears, around which there is subsequently
established an egg-shell, which is usually stated to be formed as a secretion
of the walls of the uterus ; but Schneider and Leuckart have given strong
grounds for believing that it is really a further differentiation of the vitel-
line membrane due to the activity of the protoplasm of the ovum. The
originally single membrane becomes as it thickens split into two layers.
The outer of these forms the true egg-shell, and the fertilization of the
ovum appears to be a necessary prelude to its production. Round the egg-
shell the walls of the uterus often secrete a special albuminous covering.

The egg-shell exhibits in many cases peculiar sculpturings as well
as terminal prolongations.

INSECTA.

(49) A.Brandt. Ueber das Ei u. seine Bildungsstdtte. Leipzig, 1878.

(50) T. H. Huxley. " On the agamic reproduction and morphology of Aphis."
Linnean Trans., Vol. XXII. 1858. Vide also Manual of Invertebrated Animals, 1877.

(51) R. Leuckart. "Ueber die Micropyle u. den feinern Bau d. Schalcnliaut
bei den Insecteneiern." Mutter's Archiv, 1855.

(52) Fr. Ley dig. Der Eierstock u. die Samentasche d. Insecten. Dresden, 1866.

(53) Lubbock. •• The ova and pseudova of Insects." Phil. Trans. 1859.

(54) Stein. Die weiblichen Geschlechtsorgane d. Kdfer. Berlin, 1847.

[Conf. also Glaus, Landois, Weismann, Ludwig (No. 4).]

The ovum of Insects has formed the subject of numerous
investigations, and has played an important part in the con-
troversies on the nature of the ovum.

THE OVUM.

47

The ovaries are paired organs, rarely directly connected,
each consisting of more or fewer ovarian tubes which open into
a common oviduct. The oviducts unite into a vagina, usually
provided with a spermatheca and accessory glands, which need
not be further alluded to. Each ovary is invested by a peri-
toneal covering, which assumes various characters, and either
forms a loose network covering the whole or a special tunic
round each egg-tube. It is continuous with the general peri-
toneal investment. Each ovarian tube (fig. 17) consists of three
sections: (i) a terminal thread,
(2) the terminal chamber or ger-
mogen, (3) the egg-tube proper.

The whole egg-tube is invested
in a structureless tunica propria.

The terminal threads are fine prolon-
gations of the ends of the egg-tubes usually
continued close up to the heart. At their
extremities they frequently anastomose,
or even unite into a common thread. In
some cases they are absent. They form
either direct continuations of the ger-
mogen and have the same histological
structure, or in other cases are simply
prolongations of the tunica propria, and
serve as ligaments.

The germogen usually consists
of two parts : an upper, filled with
nuclei imbedded in protoplasm,
and a lower, in which distinct cells
have become differentiated.

The lower part of the egg-tubes
is filled with ova which advance in
development towards the oviduct,
and lie in chambers more or less
distinctly constricted from each
other. In these chambers there
are in most forms in addition to
the true ova a certain number of
nutritive cells. The true egg-tubes
are moreover lined by an epithe-

FIG. 17. A. OVARIAN TUBE OF THE
FLEA, PULEX IRRITANS. (From
Gegenbaur, after Lubbock.)

o. ovum. g. germinal vesicle.
B. OVARIAN TUBE OF A BEETLE,
CARABUS VIOLACEUS. (After Lub-
bock.)

o. ovarian segment, formed of an
ovum a, and a mass of yolk-cells, b.

48 INSECTA.

Hal layer which passes in and forms more or less complete
septa between the successive chambers. The points which
have been especially controverted are (i) the relation of the
ovum to the germogen, and (2) the relation of the nutritive
or yolk-cells to the ovum. To the controversies on these points
it will only be possible to give a passing allusion.

As has been already hinted there are two distinct types
of ovaries, viz. those without the so-called nutritive or yolk-
cells and those with them1.

The formation of the ovum is most simple in the type
without yolk-cells, which will for that reason be first considered
(fig. 17 A).

The germogen is constituted of a number of nuclei imbedded
in a scanty cementing protoplasm. In the lower part of the
germogen the nuclei are larger, and become separated off from
the nucleated protoplasm above, as distinct cells with a thin
layer of protoplasm round the germinal vesicle. These cells
are the ova. As they pass down the egg-tube their protoplasm
increases in bulk, and they become isolated by ingrowths of the
epithelial cells the origin of which is still uncertain, which form
round each ovum a special follicle, so that the egg-tube is filled
by a single row of ova each in an epithelial follicle (fig. 17 A).
The larger the ova the more columnar is the epithelium of the
follicle. As the oviductal extremity of the egg-tube is ap-
proached the ova increase in size, and their protoplasm is more
and more filled with yolk particles.

In the lower part of the egg-tube the epithelium gives rise to
a chorion.

The epithelium around each ovum has been spoken of as forming a
follicle, and it is implied that the epithelium round each ovum travels down
the egg-tube with the ovum. It is however by no means clear from the
observations of the majority of writers that this is the case, and in fact the
epithelium is generally spoken of as if it were simply the epithelium of the
egg-tube. In favour of the view here adopted the following considerations
may be urged.

Firstly, there is considerable evidence that the superficial layer of
the germogen gives rise to the epithelial cells, simultaneously with the
formation of the ova from the deeper layers.

1 For a list of the genera with and without nutritive cells, vide Brandt, pp. 47
and 48.

THE OVUM. 49

Secondly, the fact that the epithelium grows in between the separate
ova appears to render it almost certain that this part of the epithelium
must travel down the egg-tubes with the ova.

Thirdly, the epithelium no doubt gives rise to the chorion, and considering
the peculiar structure of the chorion, this seems possible only on the view
that the epithelium travels down the egg-tube with the ova.

Fourthly, when, or even before, the egg is laid the epithelium under-
goes atrophy, and the remains of it have been compared to the corpora
lutea.

If the view about the epithelium here adopted is correct, the epithelium
without doubt corresponds to the follicular epithelium of other ova, and has
the same origin as the ova themselves.

The ovaries with yolk-cells differ in appearance from those
without, mainly in each ovarian chamber of an egg-tube con-
taining two elements, usually more or less distinctly separated.
These two elements are (i) at the lower end of the chamber, the
ovum, and (2) at the upper, large cells which gradually disappear
as the ovum grows larger (fig. 17 B).

The uppermost part of the egg-tube is formed, as in the pre-
vious type, by a mass of nucleated protoplasm, but the germinal
cells formed from it do not all become ova. The germinal cells
leave the germogen in batches, and in each batch one of the cells
may usually be distinguished from the very first as the ovum ;
the remainder forming the nutritive cells. In the uppermost
part of the egg-tube the whole mass of each batch is very small,
and the successive batches are very imperfectly constricted from
each other. Gradually however both the nutritive cells and the
ovum grow in size, and then as a rule, the Diptera forming a
marked exception, the chamber containing a batch becomes con-
stricted into an upper section with the nutritive cells and a lower
one with the ovum. The ovum in passing down the tube be-
comes gradually invested by a layer of epithelial cells, which in
many cases pass in and partially separate the ovum from the
nutritive cells. The epithelium appears not unfrequently to be
continued as a flat layer between the nutritive cells and the wall
of the egg-tube.

As was first shewn by Huxley and Lubbock, the protoplasm of the
ovum is often continued up as a solid cord, which terminates freely between
the nutritive cells, and serves to bring to the ovum the material elaborated
by them. It is present in its most primitive form in the somewhat

B. II. 4

50 ARANEINA.

aberrant ovary of Coccus. In this ovary the terminal chamber is filled
with cells which are united to a central rachis, as in Nematodes, and the
prolongation from the ovum is continuous with this rachis. This cord
is known as the yolk-duct (Dottergang) by German writers. Although it
is, not generally present in a distinct form, there is always a passage
connecting the ovum and yolk-cells, even when the follicular epithelium
grows in and nearly separates them.

The number of nutritive cells varies from two (one ?) to
several dozen. After they have reached a maximum they gradu-
ally atrophy, and are finally absorbed without apparently fus-
ing directly with the ovum. The two types of insect ovaries
appear fundamentally to differ in this. In the one type all the
germinal cells develop into ova ; in the other the quantity is, so
to speak, sacrificed to the quality, and the majority of germinal
cells are modified so as to subserve the nutrition of the few. It
is still undecided whether the yolk-cells absolutely elaborate
yolk particles, or are merely conveyers of nutriment to the ovum.

The egg-membranes of Insects present many points of in-
terest, which are however for the most part beyond the scope of
this work. There is always a chorion formed as a cuticular
deposit of the follicle cells, which is frequently sculptured, finely
perforated, etc., and is in many instances provided with a micro-
pyle, developed, according to Leydig, at the upper end of the
ovum.

Its development at this point appears to be due to the fact
that the follicle is here incomplete ; so that the cuticular mem-
brane deposited by it is also incomplete.

A true vitelline membrane can in many instances be demon-
strated (Donacia, etc.).

ARANEINA.

(65) Victor Carus. " Ueb. d. Entwick. d. Spinneneies.1' Zcit.f. wiss. Zool.,
Vol. ii. 1850.

(56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc."
Mailer's Archiv. 1849.

[Conf. Leydig, Balbiani, Ludwig (No. 4), etc.]

The ova of many Araneina are remarkable for the presence
in the ovum of the so-called yolk-nucleus. The ova develop
from the epithelial cells lining the ovarian sack. Certain of these
cells grow large and project outwards, invested by the structure-

THE OVUM. 51

less membrane of the ovarian wall. The stalks of projections
so formed are turned towards the lumen of the ovary, and are
plugged with the epithelial cells which line the ovarian sack.
When ripe, the ova pass from their sacks into the cavity of the
ovary. The yolk-nucleus, which appears very early, is a solid
body present in the protoplasm of the ovum. It is not found in
all genera of Araneina. At its full development it exhibits in
the fresh condition a granular structure, but very soon shews an
irregularly concentric stratification which becomes more marked
on the addition of reagents. According to Balbiani this strati-
fication is confined to the superficial layers, while internally there
is a body with all the characters of a cell. The yolk-nucleus is
still found in the nearly ripe ovum, though it always disappears
before development commences. It is probably connected with
the nutrition of the ovum, though nothing is certainly known
about its function.

CRUSTACEA.

(57) Aug. Weismann. " Ueb. d. Bildung von Wintereiern bei Leptodora
hyalina." Zeit.f. wiss. Zool., Vol. xxvii. 1876.

[For general literature vide Ludwig No. 4 and Ed. van Beneden, No. 1.]

Amongst the many interesting observations on the Crustacean
ova I will only allude to those of Weismann on the ova of Lepto-
dora, a well-known Cladoceran form.

The phenomena of the development of the ova in this form
present a close analogy with those in Insects.

The ovary js formed of (i) a germogen containing at its upper
end nucleated protoplasm and lower down germinal cells in
groups of four ; (2) of a portion formed of successive chambers
in each of which there is a row of four germinal cells. Of the
four cells only the third develops into an ovum ; the remainder
are used as pabulum. This is the mode of development in the
summer. In the winter the sacrifice of a larger number of germi-
nal cells is required for the development of the ova; and an
ovum is produced only in the alternate chambers. In the
chambers where an ovum will not be formed an epithelial invest-
ment becomes first established round the four germinal cells.
The four cells then coalesce, and form a spherical ball of proto-
plasm from which portions are budded off and absorbed by the

4—2

52 CHORDATA.

investing epithelial cells, which at the same time lose their nuclei.
When the whole of the central ball is thus absorbed by the
epithelial cells, the latter become used by the winter ovum as
food. The winter ovum at its full development is formed of a
central mass of food-yolk and superficial layer of protoplasm.

CHORDATA.
Urochorda. (Tunicata.)

(58) A.-Kowalevsky. "Weitere Studien ii. d. Entwicklung d. Ascidien."
Archivf. micr. Anat., Vol. VII. 1871.

(59) A. Kowalevsky. " Ueber Entwicklungsgeschichte d. Pyrosoma." Arch,
f. micr. Anat., Vol. xr. 1875.

(60) Kupffer. " Stammverwandtschaft zwischen Ascidien u. Wirbelthieren."
Arch.f. micr. Anat., Vol. vi. 1870.

(61) Giard. " Etudes critiques des travaux, etc." Archives Zool. experiment. ,
Vol. i. 1872.

(62) C. Semper. "Ueber die Entstehung, etc." Arbeiten a. d. zool.-zoot.
Institut Wurzburg, Bd. II. 1875.

Cephalochorda.

(63) P. Langerhans. "Z. Anatomic d. Amphioxus lanceolatus," pp. 330 — 3.
Archivf. mikr. Anat., Vol. xii. 1876.

' Craniata.

(64) F. M. Balfour. "On the structure and development of the Vertebrate
Ovary." Quart. J. of Micr. Science, Vol. xvm. 1878.

(65) Th. Eimer. " Untersuchungen U. d. Eier d. Reptilien." Archivf. mikr.
Anat., Vol. vin. 1872.

(66) PflUger. Die Eierstocke d. Sdugethiere u. d. Menschen. Leipzig, 1863.

(67) J. Foul is. "On the development of the ova and structure of the ovary in
Man and other Mammalia." Quart.}, of Micr. Science, Vol. xvi. 1876.

(68) J. Foul is. "The development of the ova, etc." Journal of Anat. and
Phys., Vol. xiii. 1878—9.

(69) C. Gegenbaur. " Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit
partieller Dottertheilung." Mailer's Archtv, 1861.

(70) Alex. Got te. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(71) W. His. Untersuchungen iib. d. Ei «. d. Eienhmcklung bei Knochenfischcn.
Leipzig, 1873.

(72) A. Kolliker. Entwicklungsgeschichte d. Menschen u. hbherer Thiere.
Leipzig, 1878.

(73) J. Mii Her. " Ueber d. zahlreichen Porenkanale in d. Eikapsel d. Fische."
Mailer's Archiv, 1854.

(74) W. H. Ransom. " On the impregnation of the ovum in the Stickleback."
Pro. R. Society, Vol. vii. 1854.

(75) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeiten a.
d. zool.-zoot. Ins tit. Wurzburg, Vol. II. 1875.

[Cf. Ludwig, No. 4, Ed. van Beneden, No. 1, Waldeyer, No. 6, &c.]

THE OVUM. 53

There are some very obscure points connected with the growth
of the ovum of the Tunicata. When quite young the ovum is a
naked cell with a central nucleus containing a single large
nucleolus. Around it is a flat follicular epithelium enclosed in
a membrana propria folliculi. The follicle cells soon be-
come larger and give rise to an envelope round the egg of the
nature of a chorion. At the same time they frequently become
cubical or even columnar, and filled with numerous vacuoles.

During or after the completion of the above changes a num-
ber of bodies usually spoken of as test-cells make their appear-
ance in the superficial protoplasm of the egg, which by the time
the egg is ripe arrange themselves in many species as a definite
layer round the periphery of the ovum. These bodies have
received their name from the opinion, now known to be erroneous
(Hertwig and Semper), that they eventually migrated into the
test or mantle of the embryo which becomes developed round
the ovum. By Kowalevsky (No. 58) these bodies are regarded
as true cells, and are believed to be formed by some of the cells
of the original follicular epithelium making their way into the
vitellus of the ovum and multiplying there. By Kupffer (No. GO),
and Giard (No. 61), and Fol, they are also regarded as true cells
but are believed to originate spontaneously in the vitellus.
Finally by Semper they are believed not to be cells, but to be
amoeboid protoplasmic bodies which are pressed out from the
vitellus under the stimulus of the sea-water or otherwise.

They do not according to this author naturally appear till the ovum
is quite ripe, though they can be artificially produced at an earlier period
by the action of reagents or sea-water. When produced in the natural
course of things the vitellus undergoes a contraction. They are without
any apparent function, and play no part in the embiyonic development.
Semper's results are very peculiar, but owing to the careful study which
his paper displays they no doubt deserve attention. Further investigations
are however very desirable. Kowalevsky from his researches on Pyrosoma
(No. 59) adheres to his first opinion, though he abandons the view that
these cells are connected with the formation of the test.

In the passage of the egg through the oviduct the vacuolated
follicle cells grow out into very peculiar long processes or villi.
In Ascidia canina these processes become as long as the whole
diameter of the vitellus (Kupffer, No. 60).

54 CHORDATA.

In Amphioxus and the Craniata the ova are developed as in
the Chaetopoda, Gephyrea, etc., from specialized germinal cells
of the peritoneal epithelium.

In Amphioxus the germinal epithelium which constitutes the
essential part of the ovary is divided into a number of distinct
segments : in the Craniata no such division is observable.

In young examples of Amphioxus the generative organs are
in an indifferent condition, and the two sexes cannot be dis-
tinguished. They form isolated horse-shoe shaped masses of
cells, which occupy a position at the base of the myotomes, in
the intervals between the successive segments ; and extend from
the hinder end of the respiratory sack to the abdominal pore.
They are situated in the proper body cavity, and are surrounded
by the peritoneal membrane. Each generative mass is at first
solid, and is formed of an outer layer of more flattened cells and
an inner mass of large rounded or polygonal cells. In its interior
there appears at a somewhat later period a central cavity. After
the cavity has appeared the sexes can be distinguished by the
different behaviour of the cells.

In all the Craniata, the ovary forms a paired ridge (unless
single by abortion or fusion) attached by a mesentery to the
dorsal wall of a more or less extended region of the abdominal
cavity. This ridge is at first identical in the two sexes, and
arises at an early period of embryonic life. It is essentially
formed of a thickening of the peritoneal epithelium, and in
Osseous Fish, Ganoids (?) and Amphibia the ovary remains
during embryonic life nearly in this condition, though a small
prominence of the adjacent stroma also becomes formed. In
other Craniata the ridge, though at first in this condition, very
soon becomes much more prominent, and is formed of a central
core of stroma enclosed in the germinal epithelium (fig. 1 8).

The thickened germinal epithelium gives rise (in the case of
the female) to the ova and the follicular epithelium. Whether
the genital ridge is provided with a core of stroma or no, the
germinal epithelium is always in contact on one side with the
stroma, from which it is at first separated by a well-marked
boundary line ; but after a certain time there appear numerous
vascular ingrowths from the stroma, which penetrate through all
parts of the germinal epithelium, and break it up into a sponge-

THE OVUM. 55

like structure formed of trabeculae of germinal epithelium inter-
penetrated by vascular strands of stroma. The trabeculae of the
germinal epithelium form the egg-tubes of Pfliiger.

With reference to the distribution of the stroma in the germi-
nal epithelium, it may be said in a general way that there is a
special layer close to the surface of the ovary, which, after the
formation of fresh ova has nearly ceased, completely isolates a
superficial layer of the germinal epithelium from the deeper and
major part of it. The superficial layer is frequently (but errone-
ously) regarded as constituting the whole of the germinal epi-
thelium. The layer of stroma below the superficial epithelium
forms in the mammalian ovary the tunica albuginea. As the
follicles are formed- in the trabeculae of germinal epithelium the
stroma grows in around them, and forms for each one of them a
special tunic.

FIG. 1 8. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF

SCYLLIIIM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING IN
THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.

The adult ovaries differ in a corresponding manner to the em-
bryonic genital ridges as to the presence of a core of stroma.
The ovaries which are without such a core in the embryo, are
also without it in the adult, and are formed of a double layer of
tissue entirely derived from the germinal epithelium with its in-
growths of stroma, and composed, for the most part, of ova in
all stages of development. In the case of the other ovaries there

56 CRANIATA.

is a hilus of stroma — the zona vasculosa — internal to the egg-
bearing region.

In Mammalia, proportionately to the ovary, the zona vasculosa is at a
maximum, and in Birds and Reptiles it is relatively far less developed. In
these forms the germinal epithelium covers the whole surface of the ovary.
In Elasmobranchii the structure of the ovary is somewhat different, owing
to the presence in the ovarian ridge of a large quantity of a peculiar
lymphatic tissue, which has no homologue in the other ovaries; .and still
more to the fact that the true germinal epithelium is in most forms entirely
confined to the outer surface of the ovary, on which it forms a layer
of thickened epithelium in the embryo (fig. 17), and of ovigerous tissue in
the adult.

In the ovary of Mammalia and Reptilia and possibly other forms there
are present in the zona vasculosa during embryonic life cords of epithelial
tissue derived from the Malpighian bodies; these cords have no function
in the female, but in the male assist in forming the seminiferous tubules.

In considering the development of the ova it is again con-
venient to distinguish between Amphioxus and the Craniata.

In Amphioxus the germinal cells destined to become ova are
first distinguished by the larger size of their germinal vesicles and
by the presence of certain refracting granules in their protoplasm.
They subsequently rapidly enlarge and form protuberances on the
surface of the ovary, which are enveloped for three-quarters of
their circumference by the flattened epithelioid cells of the peri-
toneal membrane, which thus form a kind of follicle. As the
ova become ripe yolk-granules are deposited in their protoplasm,
first in the superficial layer and subsequently throughout. The
germinal vesicle also passes from the centre to the surface. A
vitelline membrane is formed when the ova are mature.

In the Craniata the ova are developed from the cells of the
germinal epithelium. In the types with larger ova (Teleostei,
Elasmobranchii, Amphibia, Reptilia, Aves), at a very early period,
sometimes (Elasmobranchii) even before the formation of the
genital ridge, certain of the cells which are destined to form ova
become distinguished by their greater size, and by the possession
of an abundant clear protoplasm and a large spherical granular
nucleus. (Fig. i8,/0.) Such special cells form primitive germi-
nal cells, and are common to both sexes.

For a considerable period after their first formation these cells
remain stationary in their development ; but their number in-

THE OVUM. 57

creases, partly, it appears, by an addition of fresh ones, and partly
by division. Owing to the latter process the germinal cells
come to form small masses or nests. The following description
of the further changes of these cells in the female refers in the
first instance to Elasmobranchii, but holds good in most respects
for other types as well.

It is convenient to distinguish two modes in which the primi-
tive germinal cells may become converted into permanent ova,
though the morphological difference between the two modes is
of no great importance.

In the first mode the protoplasm of all the cells forming a
nest unites into a single mass containing the nuclei of the pre-
viously independent ova (fig. 19, nn). The nuclei in the nest in-
crease in number, probably by division, and at the same time the
nest itself increases in size. The nuclei while increasing in

FIG. 19. SECTION THROUGH PART OF THE GERMINAL EPITHELIUM OF THE OVARY

OF SCYLLIUM AT THE TIME WHEN THE PRIMITIVE GERMINAL CELLS ARE
BECOMING CONVERTED INTO OVA.

nn. Nests formed of agglomerated germinal cells. The nuclei of these cells are
imbedded in undivided protoplasm, do. developing ova. o. ovum with follicle.
po. primitive germinal cell. dv. blood-vessels.

number also undergo important changes. A segregation of their
contents takes place, and the granular part (nuclear substance)
forms a mass close to one side of the membrane of the nucleus,
while the remainder of the nucleus is filled with a clear fluid.
The whole nucleus at the same time increases somewhat in size.
The granular mass gradually assumes a stellate form, and finally

58 CRANIATA.

becomes a beautiful reticulum, of the character so well known in
nuclei (fig. 19, do). Two or three special nucleoli are present,
and form the nodal points of the reticulum, while its meshes are
filled up with the clear fluid constituents of the nucleus. Not all
the nuclei undergo the above changes ; but some of them stop
short in their development, undergo atrophy, and appear finally
to be absorbed as pabulum by the protoplasm of the nest. Such
nuclei in a state of degeneration are shewn in fig. 19. Thus only
a few nuclei out of a nest undergo a complete development. At
first the protoplasm of the nest is clear and transparent, but as
the nuclei undergo their changes the protoplasm becomes more
granular, and a specially large quantity of granular protoplasm
is generally present around the most developed nuclei, and these
with their protoplasm gradually become constricted off from the
nest, and constitute the permanent ova (fig. 19, do). The rela-
tive number of ova which may develop from a single nest is
subject to great variation. The object of the whole occurrence
of the fusion of primitive ova and the subsequent atrophy of
some of them is to ensure the adequate nutrition of a certain
number of them.

In the second and rarer mode of development of permanent
ova from primitive germinal cells, the nuclei and protoplasm
undergo the same changes as in the first mode, but the cells either
remain isolated, and never form part of a nest, or form part of a
nest in which no fusion of protoplasm takes place, and in which
all the cells develop into permanent ova.

The isolated ova and nests are situated, during the whole of
the above changes, amongst the general undifferentiated cells of
the germinal epithelium, but as soon as a permanent ovum be-
comes formed the cells adjoining it arrange themselves around it
as a special layer, and so give rise to the epithelium of the follicle
(fig. 19, <?). The growths of stroma into the germinal epithelium
appear shortly after the formation of the earlier follicles.

Mammalia. The development of the ovary in Mammalia
differs mainly from that just described in that the formation of
primitive germinal cells from the indifferent cells of the germinal
epithelium takes place at a relatively much later period.

The stroma grows into the germinal epithelium while it is still
formed of rounded indifferent cells, and divides it into trabeculae

THE OVUM. 59

as described above. At a later period a number of the cells in
the deeper layer of the epithelium, as well as certain cells in the
superficial part, become primitive germinal cells, while the re-
mainder of the cells become smaller and are destined to form the
follicle cells.

The most conspicuous primitive germinal cells are situated
in the superficial layer of epithelium ; and the primitive germinal
cells in the deeper layers of the germinal epithelium are not
nearly so marked as in most Craniata, so that it is difficult in some
cases to be sure of their destination till their nucleus commences
to undergo its characteristic metamorphosis.

The change of the primitive ova into permanent ova takes
place in the same manner in Mammals as in Elasmobranchii,
except that the fusion of the primitive ova into polynuclear
masses is much rarer. The formation of the at first quite simple
follicles takes place while the ova are still aggregated in large
masses ; and the first follicles are formed in the innermost part
of the germinal epithelium. Soon after their formation the folli-
cles become isolated by connective-tissue growths.

Post-embryonic development of the ova.

The ova of the Vertebrata differ greatly in size and structure.
The differences in size depend upon the quantity of the food-
yolk. In the Amphioxus and Mammalia, in which the ova are
smallest, the comparatively insignificant amount of food-yolk is
distributed uniformly through the ovum. A larger quantity of
it is present in the ova of Amphibia, Marsipobranchii and Teleostei,
and it attains an immense development in the ova of Elasmo-
branchii, Reptilia, and Aves.

The food-yolk originates from a differentiation of the proto-
plasm of the egg. It arises as a number of small highly refract-
ing particles in a stratum slightly below the surface.

In the Mammalian ovum these particles spread through the protoplasm
of the egg, but do not attain any considerable development. In other
forms the case is different. In Elasmobranch Fishes the refracting particles
appear to develop into vesicles, in the interior of which there arise
solid oval or even rectangular highly refracting bodies, in the substance
of which a stratification may usually be observed, which gives them

60 CRANIATA.

an appearance not unlike that of striated muscle. In Teleostei the
yolk assumes very different characters in different cases. It is often
formed of larger or smaller vesicles containing in their interior other
bodies. Stratified plates like those of Elasmobranchii are also not un-
common. In the ripe ovum of Teleostei the food-yolk usually resolves
itself into a large vitelline sphere, which occupies the greater part of
the ovum, and is formed of a highly refracting fluid material which
coagulates on the addition of water. It contains in many instances one
or more highly refracting bodies known as oil globules, and is invested
by a granular protoplasmic layer continuous with the germinal disc, in
which a number of normal yolk-spherules are frequently present. In the
ovum of the Herring1 no distinct investing protoplasmic layer or germinal
disc is present till after impregnation, but the ovum is formed of a super-
ficial layer with minute yolk-spherules, and of a central portion with larger
yolk-spheres.

In Amphibia the yolk very often appears in the form of oval or quadri-
lateral plates. In Reptilia the yolk-spherules are vesicles, somewhat similar
to the white yolk-spheres of Aves, but as a rule without the highly refracting
spheres in their interior. The peculiar and complicated arrangement and
structure of the white and yellow yolk in Birds is fully described in the
" Elements of Embryology," and it need only be said that the yolk develops
in Birds in the same manner as in other types, and that at first all the yolk-
spherules appear in the form of white yolk. The yellow yolk-spheres are a
peculiar modification of white yolk-spheres, formed comparatively late in the
development of the egg (fig. 20).

FIG. 20. YOLK ELEMENTS FROM THE EGG OF THE FOWL.
A. Yellow yolk. B. White yolk.

In the eggs of many Amphibia a dark granular mass known as the yolk
nucleus makes its appearance ; and is supposed, without any very clear evi-
dence, to be related to the formation of the yolk.

A body in the form of a shell enclosing a dark nucleus, which
is perhaps of the same nature, has been described by Eimer in the
Reptilian egg : it eventually resolves itself into a number of angular
fragments. In Elasmobranchii a similar body is perhaps present.

The food-yolk just described is imbedded in the active proto-
plasmic portion of the body of the ovum. In the case of the

1 Kupffer, Laichen u. Entwicklung des Ostsee-Harings. Berlin, 1878.

THE OVUM. 6 1

mammalian ovum the food-yolk is fairly uniformly distributed,
but in the case of all other craniate ova the protoplasm of the
ovum is especially concentrated at one pole, which is known as
the upper or animal pole, and the food-yolk is more especially
concentrated at the opposite pole. The Herring's ovum forms
an apparent exception to this statement, in that the concentra-
tion of the protoplasm to form the germinal disc does not take
place till after impregnation. In Amphibia the animal pole is
mainly marked by the smaller size of the yolk-spherules, but in
most other forms a small portion of the ovum in the region of
the germinal vesicle is nearly free from yolk-spherules, and then
forms a more or less specialized part known as the germinal
disc. In Aves, Reptilia, and Elasmobranchii the germinal disc
shades off insensibly into the yolk ; but in Teleostei it is more
sharply marked off, and is continued more or less completely
round the periphery of the ovum. In ova with true germinal
discs it is the germinal disc alone which undergoes segmentation.
The protoplasm of vertebrate ova frequently exhibits a reticulate
or sponge-like structure (fig. 21) and the reticulum in many
cases, e.g. Elasmobranchii and Reptilia, serves to hold the yolk-
spheres together. In the Tench it has been observed by Bam-
beke to penetrate into the vitelline sphere.

In the ova of the Craniata the germinal vesicle is generally
polynucleolar. In Amphioxus and Petromyzon there is how-
ever but a single nucleolus, and in Mammalia there is usually
one special nucleolus and two or three accessory ones. The
opposite extreme is reached in many osseous fish where the
nucleoli are extremely numerous. The protoplasmic reticulum
of the embryonic germinal vesicle may in some instances be
retained till the ovum is nearly ripe, but usually assumes a very
granular form. It is at first connected with the nucleoli which
form nodal points in it, but this relation cannot always be
detected in the later stages. A membrane, which in the case of
the larger ova becomes very thick, is always present round the
germinal vesicle. It is said to be perforated in some Reptilian
ova (Eimer). As to the position of the germinal vesicle, it is at
first situated in the centre of the ovum, but always eventually
travels to the animal pole, and as the egg becomes ripe under-
goes changes which will be more especially detailed in the next

62

CHORD ATA.

chapter. In the ova with a large amount of food-yolk it assumes
an eccentric position very early.

The homologies of the primary egg-membranes of Craniata
are still involved in some obscurity. There seem to be three
membranes, which may all coexist, and of which one or more
are almost always present. These membranes are —

(1) An outermost usually homogeneous non-perforated
membrane, which is by most authors regarded as a chorion,
but is probably a vitelline membrane — by which name I shall
speak of it.

(2) A radiately striated membrane (internal to the former
when the two coexist) which can be broken up into a series of
separate columns. These give to the membrane its radiate
striation, but it is probable that between the columns there are
pores sufficiently large to admit of the passage of protoplasmic
filaments. This membrane will be spoken of as the zona radiata.
It is a differentiation of the outermost layer of the yolk.

(3) Within the zona radiata a third and delicate membrane
is occasionally found, especially when the ovum is approaching
maturity.

In Elasmobranchii the first membrane to
be formed is the vitelline membrane, which
appears in some instances before the forma-
tion of the follicle — a fact which appears to
shew that it is really formed as a differentia-
tion of the protoplasm of the egg. In most
Elasmobranchii this membrane attains a
very considerable development. A zona
radiata is generally (if not always) present
in Elasmobranchii, but arises at a later
period than the vitelline membrane (fig. 21,
Zri). The zona radiata always disappears
long before the ovum is ripe. The vitelline
membrane also gradually atrophies, though
it lasts much longer than the zona radiata.
When the egg is taken up by the oviduct all trace of both mem-
branes has vanished. In Reptilia precisely the same arrange-
ments of the membranes are found as in Elasmobranchii, except
that as a rule the zona radiata is relatively more important.

z.n

FIG. 21. SECTION
THROUGH A SMALL
PART OF THE SURFACE
OF AN OVUM OF AN IM-
MATURE FEMALE OF
SCYLLIUM CANICULA.

fe. follicular epithe-
lium, vt. vitelline
membrane. Zn. zona
radiata. yk. yolk with
protoplasmic network.

THE OVUM. 63

The vitelline membrane is thin except in the Crocodilia. The
third innermost membrane is found according to Eimer in many
Reptilia. In birds both vitelline membrane and zona radiata
are present, but the latter atrophies early, leaving the former as
the sole membrane when the egg is ripe.

In osseous .fish the vitelline membrane is usually either
absent or may perhaps in some instances, e.g. the Perch, be
imperfectly represented. In the ripe ovum of the Herring there
is a distinctly developed membrane external to the zona radiata
which is probably the vitelline membrane. The zona radiata
attains a very great development, and is generally provided with
knobs of various shapes on its outer surface. A delicate mem-
brane internal to this — my third membrane — has often been
described, but there is still some doubt about its existence. In
some cases an external less granular layer of the ovum itself has
been described as a special membrane. In the Perch a peculiar
mucous capsule, penetrated by irregular branched prolongations
of the follicle cells, is present in addition to the ordinary mem-
branes. In Petromyzon a zona radiata appears to be present,
which in the adult is divided into two layers, both of them
radiately striated according to Calberla, but according to Kupffer
and Benecke the outer one is not perforated, and would appear
therefore to be a vitelline membrane as defined above. A
delicate membrane is formed at a comparatively late period
around the ova of the Amphibia, and is stated (Waldeyer, No. 6,
and Kolessnikow) to have a delicate radial striation. It probably
corresponds with the zona radiata.

In Mammalia a radiately striated membrane — the zona
radiata — is generally described as being present, and internal to
it, in the nearly ripe egg, a delicate membrane has been shewn
by E. van Beneden to exist. Externally to the zona radiata
there may be observed a granular membrane irregular on its
outer surface on which the cells of the discus are supported.
This membrane is more or less distinctly separated from the
zona radiata ; and by tracing back its development it appears
very probable that it is the remnant of the first-formed membrane
in the very young ovum, and therefore the vitelline membrane.

A micropyle (first discovered by Ransom, No. 74) is present
in a large number of osseous fish and in Petromyzon (Calberla).

64 CHORDATA.

Doubts have been thrown on its existence in the latter form by
Kupffer and Benecke ; and at any rate it would only seem to
perforate the zona radiata. In the osseous fish in which it has
been detected, Salmonidae, Percidae (Gasterosteus), Clupeidae,
etc., it forms a minute perforation of the zona radiata at the
animal pole, just large enough to admit a single spermatozoon.
Its characters differ slightly in different cases, but there is usually
a shallow depression, in the centre of which it is situated.

The eggs of all Craniata (except Petromyzon (?)) appear to
be enclosed in a cellular envelope known as the follicle. The
cells which form this are, as has been already explained, derived
from the germinal epithelium1, and frequently arrange themselves
around the ovum before the appearance of the growths of stroma
into the epithelium. All young follicles are nearly alike, but as
they grow older they exhibit various modifications in the
different groups. They retain their simplest condition as a flat
epithelial layer in most osseous fish and Amphibia. In most
other forms the cells become at some period columnar, and are
generally arranged in two or more layers. There is formed
externally to the epithelium a delicate membrane — the mem-
brana propria folliculi — which is in its turn enclosed in a
vascular connective-tissue sheath.

In Elasmobranchii and many Reptilia (Lacertilia, Ophidid)
some of the cells become much larger than the others, and
assume a funnel-shaped form with the narrow end in contact
with the egg-membrane. These large cells, which have a
regular arrangement in the epithelium, are probably in some
way connected with the nutrition. They have only been noticed
in large-yolked ova. Many observers have described prolonga-
tions of the follicle cells through the pores of the zona radiata in
Aves, Reptilia and Teleostei.

The most remarkable modification of the follicle is that
which is found in Mammalia. At first the follicle is similar to
that of other Vertebrata, and is formed of flat cells derived from
the germinal cells adjoining the ovum. These cells next become
columnar and then one or two layers deep. Later they become

1 For the different views maintained by Foulis, Kolliker, etc. the reader is referred
to the writings of these authors. The grounds for the view here adopted will be found
in my paper (No. 64).

THE OVUM. 65

thicker on one side than on the other, and there appears in the
thickened mass a cavity, which gradually becomes more dis-
tended and is filled with an albuminous fluid. As the cavity
enlarges, the ovum with several layers of cells around it forms
a prominence projecting into it. The whole structure with its
tunic is known as the Graafian follicle. The follicle cells are
known as the membrana granulosa, and the projection, in
which the ovum lies, as the discus or cumulus proligerus.
The cells of the discus in immediate contiguity to the ovum
usually form a more or less specialized layer and are somewhat
more columnar than the adjoining cells.

THE SPERMATOZOON.

Although there is no doubt that the spermatozoon in most
instances plays as important a part as the ovum in influencing
the characters of the organism which is evolved from the
coalesced product of the ovum and spermatozoon, yet the
actual form of the spermatozoon has not, like the form of the
ovum, a secondary influence on the early phases of development.
A comparative history of the spermatozoon is therefore of less
importance for my purpose than that of the ovum ; and I shall
confine myself to a few remarks on its general structure, and
mode of growth. The primary origin of the male germinal cells,
and their relation to the sperm-forming cells, is dealt with in
the second part of the treatise.

Although the minute size of most spermatozoa places great
difficulties in the way of a satisfactory investigation of them, yet
there can be but little doubt that they always have the value of
cells. In the vast majority of instances the spermatic cell or
spermatozoon is composed of (i) a spherical or oval portion
known as the head, formed of a nucleus enveloped in an
extremely delicate layer of protoplasm, and (2) of a motile
protoplasmic flagellum known as the tail; which together with
the investing layer of the head forms the body of the cell.

As might be anticipated, the proportion, size, and relations

of the parts of the spermatozoon are subject to great variations.

The head is often extremely elongated ; and it is in many cases

rather on theoretical grounds, than as a result of actual observa-

B. II. 5

66 THE SPERMATOZOON.

tion, that a protoplasmic layer is stated to be continued round
the nucleus which forms the main constituent of the head. In
some of the elongated forms of spermatozoa, e.g. in Insecta,
there is no marked distinction, except in the character of the
protoplasm, between the head and the tail. A connecting
element is frequently interposed between the head and tail,
which appears however to be constituted of the same material as
the tail, and sometimes forms a thickening on the tail close
below the head (Amphioxus). A very remarkable modification
of the tail is found in many Amphibia, Reptilia and Mammalia.
In these types there is attached to what appears to be a normal
tail a delicate membrane, the outer edge of which is thickened
to form a kind of secondary filament In the living spermato-
zoon this filament is in a state of constant movement. The
membrane winds spirally round the tail.

In the majority of forms the tail of the living spermatozoon
exhibits sinuous cilia-like movements. In two groups the move-
ments are however of an amoeboid character. These groups are
the Nematoda and the Crustacea ; and the spermatozoa in both
of them frequently present very abnormal forms. In Nematoda
they are pear-shaped, cylindrical, spine-shaped, etc., and are
mainly formed of protoplasm with a highly refracting nucleus.
In the Crustacea the variations of form are still greater. In the
Malacostraca they are sometimes simply spherical (Squilla),
while in Astacus and a large number of Decapoda they are
composed of a nucleated body with stellate rays. In Paludina
amongst the Mollusca there are two forms of completely deve-
loped spermatozoa existing side by side in the same individual.

The spermatozoa are formed by the breaking up of the male
germinal cells, or of cells secondarily derived from them by
division. The cells which directly give rise by division to the
spermatozoa may be called spermospores and are equivalent
to the ova or oospores.

Amongst the Sponges (Halisarca, Schultze, No. 141) a
germinal cell, similar to that which in the female becomes an
ovum, repeatedly divides and eventually gives rise to a ball of
cells (a spermosphere or sperm-morula), each constituent
cell of which becomes converted into a spermatozoon, and may
be designated by the special term 'spermoblast.'

THE SPERMATOZOON. 67

In most Hydrozoa the subepithelial epiblastic cells become
converted into germinal cells (spermospores), and then break up
to form spermoblasts, each of which becomes a spermatozoon.

In most higher Metazoa the spermospores usually form the
epithelium of an ampulla or tube, though more rarely (many
Chaetopoda, Gephyrea, etc.) they may be derived from cells lining
the body-cavity, as in the case of ova. The spermatozoa are
formed either by the direct division of the spermospores into a
number of cells, spermoblasts, each of which grows into a sper-
matozoon ; or by the nucleus of the spermospore becoming
subdivided within the cell body, the latter differentiating itself
into the tails of the spermatozoa while the segments of the
nucleus give rise to the main part of the heads.

In many instances interstitial cells which do not give rise to spermatozoa,
are intermingled with the spermospores.

In a good many cases, as first pointed out by Blomfield1, the whole of
each spermospore does not become converted into spermatozoa, but part,
either with or without a segment of the original nucleus, remains passive,
and carrying as it does the off-budded spermoblasts may be called the
* sperm-blastophor.' This passive portion of protoplasm is not employed
in the regeneration of the spermoblast. This very singular phenomenon
has been observed in Elasmobranchii, the Frog, the Earthworm, Helix, etc.2,
and probably has a much wider extension. In Elasmobranchii (Semper)
the passive portions of protoplasm are nucleated, and are placed on the
outer side of the columnar spermospores which line the testicular ampullae ;
they are not distinctly differentiated till the nuclei, segmented from the
nucleus of the primitive spermospore to form the heads of the spermatozoa,
have become fairly numerous. In the Frog the passive blastophor also
occurs as a nucleated mass of protoplasm on the outer side of the spermo-
spore. In the Earthworm the blastophor forms a central non-nucleated
portion of the spermospore ; and the whole periphery of each spermospore
becomes converted into spermoblasts.

It has been already stated in the introduction that the male
and female generative products are homodynamous, but the
consideration of the development of the products in the two
sexes shews that a single spermatozoon is not equivalent to an
ovum, but rather that the whole of the spermatozoa derived from
a spermospore are together equivalent to one ovum.

1 Quart. Journ. of Micro. Science, Vol. XX. 1880.

2 Blomfield, loc. cit., p. 83, states that he has observed this fact in Lumbricus,
Tubifer, Hirudo, Helix, Arion, Paludina, Rana, Salamandra, and Mus.

5—2

CHAPTER II.

THE MATURATION AND IMPREGNATION OF THE OVUM.

Maturation of ttu ovum and formation of tJie polar bodies.

IN the preceding chapter the changes in the ovum were described
nearly up to the period when it became ripe, and ready to be
impregnated. Preparatory to the act of impregnation there
take place however a series of remarkable changes, which more
especially concern the germinal vesicle.

The attention of a large number of investigators has recently been
directed to these changes as well
as to the phenomena of impregna-
tion. The results of their investi-
gations will be described in the
present chapter ; but for an histori-
cal account of these investigations,
as well as for a determination of
the delicate questions of priority,
the reader is referred to Fol's
memoir (No. 87), and to a paper
by the author (No. 81).

The nature of the changes
which take place in the
maturation of the ovum may
perhaps be most convenient-
ly displayed by following the
history of a single ovum.
For this purpose the eggs of
Asterias glacialis, which have
recently formed the subject of a series of beautiful researches
by Fol (87), may be selected.

The ripe ovum (fig. 22), when detached from the ovary is
formed of a granular vitellus enveloped in a mucilaginous coat,

FIG. 11. RIPE OVUM OF ASTERIAS GLA-
CIALIS ENVELOPED IN A MUCILAGINOUS
ENVELOPE, AND CONTAINING AN ECCEN-
TRIC GERMINAL VESICLE AND GERMINAL

SPOT (copied from Fol).

MATURATION OF THE OVUM.

69

the zona radiata. It contains an eccentrically-situated germinal
vesicle and a germinal spot. In the former is -present the usual
protoplasmic reticulum. As soon as the ovum reaches the sea-
water the germinal vesicle commences to undergo a peculiar
metamorphosis. It exhibits frequent changes of form, the reti-

FIG. 23. Two SUCCESSIVE STAGES IN THE GRADUAL METAMORPHOSIS OF THE

GERMINAL VESICLE AND SPOT OF THE OVUM OF ASTERIAS GLACIALIS IMMEDI-
ATELY AFTER IT is LAID (copied from Fol).

culum vanishes, its membrane becomes gradually absorbed, its
outline indented and indistinct, and finally its contents become
to a certain extent confounded with the vitellus (fig. 23).

The germinal spot .at the same time loses its clearness of out-
line and gradually disappears from view.

At this stage, and between it and the stage represented in
fig. 26, the action of reagents brings to light certain appearances
the nature of which is not yet fully cleared up for Asterias, which
have been described somewhat differently by Fol for Ast. glacialis
and Hertwig for Asteracanthion.

Fol finds immediately after the stage
just described that a star is visible
between the remains of the germinal
vesicle and the surface of the egg,
which is connected with an imperfectly-
formed nuclear spindle extending to-
wards the germinal vesicle1. At the
end of the nuclear spindle may be seen
the broken up fragments of the germi-
nal spot.

At a slightly later stage, in the
place of the original germinal vesicle
there may be observed in the fresh

1 By the term 'nuclear spindle' I refer to the peculiar form of a double striated cone
assumed by the nucleus just before division, which is no doubt familiar to all my
readers. I use the term star for the peculiar stellate figure usually visible at the poles
of the nuclear spindle. For a further description of these parts the reader is referred
to Chapter IV.

FIG. 24. OVUM OF ASTERIAS GLA-
CIALIS, SHEWING THE CLEAR SPACES
IN THE PLACE OF THE GERMINAL

VESICLE. FRESH PREPARATION (copied
from Fol). .

GERMINAL VESICLE.

ovum two clear spaces (fig. 24), one ovoid and nearer the surface, and the
second more irregular in form and situated rather deeper in the vitellus. In
the upper space parallel stride may be observed. By treatment with reagents
the first clear space is found to be formed of a horizontally-placed spindle
with two terminal stars, near which irregular remains of the germinal spot
may be seen. Slightly later (fig. 25) there may be seen on the lower side of
the spindle a somewhat irregular body, which may possibly be part of the
remains of the germinal spot, though Fol holds that it is probably part of the
membrane of the germinal vesicle. The lower clear space visible in the
fresh ovum now contains a round body, fig. 25. Fol concludes that the
spindle is formed out of part of the
germinal vesicle and not from the
germinal spot, while he sees in the
round body present in the lower of
the two clear spaces the metamor-
phosed germinal spot. He will not,
however, assert that no fragment of
the germinal spot enters into the for-
mation of the spindle.

The following is Hertwig's (No.
92) account of the changes in the
germinal vesicle in Asteracanthion.
Shortly after the egg is laid the proto-
plasm on the side of the germinal
vesicle towards the surface of the egg
develops a prominence which presses

inwards the wall of the vesicle. At the same time the germinal spot
develops a large vacuole, in the interior of which is a body consisting of
nuclear substance, and formed of a firmer and more refractive material than
the remainder of the germinal spot. In the prominence first mentioned as
projecting inwards towards the germinal vesicle first one star, formed by
radial striae of protoplasm, and then a second make their appearance ; while
the germinal spot appears to have vanished, the outline of the germinal
vesicle to have become indistinct, and its contents to have mingled with the
surrounding protoplasm. Treatment with reagents demonstrates that in the
process of disappearance of the germinal spot the nuclear mass in its vacuole
forms a rod-like body, the free end of which is situated between the two stars
which occupy the prominence indenting the germinal vesicle. At a later
period granules may be seen at the end of the rod and finally the rod itself
vanishes. After these -changes by the aid of reagents there may be demon-
strated a spindle between the two stars, which Hertwig believes to grow in
size as the last remnants of the germinal spot gradually vanish, and he
maintains that the spindle is formed at the expense of the germinal spot.
The stage with this spindle corresponds with fig. 25.

Several of Hertwig's figures closely correspond with those of Fol, and
considering how conflicting is the evidence before us, it seems necessary

FlG. 25. OVUM OF ASTERIAS GLA-
CIALIS, AT THE SAME STAGE AS FIG. 24,
TREATED WITH PICRIC ACID (copied

from Fol).

MATURATION OF THE OVUM.

to leave open for Asterias the question as to what parts of the germinal
vesicle are concerned in forming the first spindle.

A clearer view of the phenomena which take place at this stage
has been obtained by Fol in the case of Heteropods (Pterotra-
chaea). In the ovum a few minutes after it has been laid the
germinal vesicle becomes very pale, and two stars make their
appearance round a clear substance near its poles. The nucleus
itself is somewhat elongated, and commences to exhibit at its
poles longitudinal striae, which gradually extend towards the
centre at the expense of the nuclear reticulum, from a metamor-
phosis of which they are directly derived. When the striae of the
two sides have nearly met, thickenings may be observed in the
recticulum between them, which give rise, where the striae of
the two sides unite, to the central thickenings of the fibres
(nuclear plate). In this way a complete nuclear spindle is
established1.

The important result of Fol's observations on Heteropods,
which tallies also with what is found in Asterias, is that a spindle
with two stars at its poles is formed from the metamorphosis of
the germinal vesicle and surrounding protoplasm (fig. 25).

Polar cells. The spindle has up to
this time been situated with its axis
parallel to the surface of the egg, but in
somewhat older specimens a vertical
spindle is found, with one end projecting
into a protoplasmic prominence which
makes its appearance on the surface of
the egg (fig. 26). Hertwig believes that
the spindle simply travels towards the
surface, and while doing so changes the
direction of its axis. Fol asserts, how-
ever, that this is not the case, but that
between the two phases of the spindle
an intermediate one is found in which a
spindle can no longer be seen in the egg, but its place is taken
by a body with a dentated outline. He has not been able to
arrive at a conclusion as to what meaning is to be attached to

Fig. 26. PORTION OF

OF THE DETACHMENT OF
THE FIRST POLAR BODY AND
THE WITHDRAWAL OF THE
REMAINING PART OF THE
SPINDLE WITHIN THE OVUM.
PICRIC ACID PREPARATION

(copied from Fol).

For the further details on the nuclear spindle vide the next Chapter.

POLAR CELLS.

FIG. 27. PORTION OF THE OVUM
OF ASTERIAS GLACIALIS, WITH THE
FIRST POLAR CELL AS IT APPEARS
WHEN LIVING (copied from Fol).

this occurrence, which does not appear to take place in Hetero-
pods.

In any case the spindle which
projects into the prominence on
the surface of the egg divides into
two parts, one in the prominence
and one in the egg (fig. 26). The
prominence itself with the enclosed
portion of the spindle becomes con-
stricted off from the egg to form a
body, well known to embryologists
as the polar body or cell (fig. 27). Since more than one polar
cell is formed, that which is the earli-
est to appear may be called the first
polar cell.

The part of the spindle which re-
mains in the egg becomes directly con-
verted into a second spindle by the
elongation of its fibres, without pass-
ing through a typical nuclear con-
dition. A second polar cell next be-
comes formed in the same manner as
the first (fig. 28), and the portion of
the spindle remaining in the egg be-
comes converted into two or three clear vesicles (fig. 29), which
soon unite to form a single nucleus (fig.
30). The new nucleus which is clearly
derived from part of the original germinal
vesicle is called the female pronucleus,
for reasons which will appear in the sequel.

The two polar cells appear to be situ-
ated between two membranes, the outer
of which is very delicate, and only dis-
tinct where it covers the polar cells, while
the inner one is thicker and becomes,
after impregnation, more distinct, and
then forms what Fol speaks of as the
vitelline membrane. It is clear, as Hert-
wig has pointed out, that the polar bodies

FIG. 28. PORTION OF THE
OVUM OF ASTERIAS GLACIALIS
IMMEDIATELY AFTER THE FOR-
MATION OF THE SECOND POLAR
CELL. PICRIC ACID PREPARA-

FIG. 29. PORTION OF

THE OVUM OF ASTERIAS
GLACIALIS AFTER THK FOR-
MATION OF THE SF.COND
POLAR CELL, SHEWING THE
PART OF THE SPINDLE RE-
MAINING IN THE OVUM BE-
COMING CONVERTED INTO
TWO CLEAR VESICLES. PlC-
RIC ACID PREPARATION

(copied from Fol).

MATURATION OF THE OVUM.

73

originate by a regular process of cell-division and have the value
of cells.

A peculiar phenomenon makes its appearance in the eggs of Clepsine
shortly after the formation of the polar cells, which has been spoken of by
Whitman (No. 100) as the formation of the polar rings. The following is his
description of the occurrence.

" Fifteen minutes after the elimination of the polar globules (i.e. cells) a
ring-like depression or constriction appears in the yolk around the oral pole,
and in this depression a transparent liquid substance (nuclear ?) is collected
forming the first polar ring.... The same phenomena repeat themselves
later at the aboral pole.... The rings concentrate to form two discs.... Before
the first cleavage both discs plunge deep into the egg."

The nature of these rings is at present quite obscure.

Considering how few ova have
been adequately investigated with
reference to the behaviour of the
germinal vesicle, any general con-
clusions which may at present be
formed are to be regarded as pro-
visional.

There is however abundant
evidence that at the time of matu-
ration of the egg the germinal
vesicle undergoes peculiar changes,
which are, in part at least, of a
retrogressive character. These
changes may begin considerably
before the egg has reached the
period of maturity, or may not take place till after it has been
laid. They consist in an appearance of irregularity and obscurity
in the outline of the germinal vesicle, the absorption of its mem-
brane, the partial absorption of its contents in the yolk, the dis-
appearance of the reticulum, and the breaking up and disappear-
ance of the germinal spot. The exact fate of the single germinal
spot, or the numerous spots where they are present, is still obscure.

The retrogressive metamorphosis of the germinal vesicle is
followed in a large number of instances by the conversion of what
remains into a striated spindle similar in character to a nucleus
previous to division^ This spindle travels to the surface of the
ovum and undergoes division to form the polar cell or cells in the

FIG. 30. OVUM OF ASTERIAS
GLACIALIS WITH THE TWO POLAR

CELLS AND THE FEMALE PRONUCLEUS
SURROUNDED BY RADIAL STRIDE, AS
SEEN IN THE LIVING EGG (copied from

Fol).

74 POLAR CELLS.

manner above described. The part which remains in the egg
forms eventually the female pronucleus.

The germinal vesicle has up to the present time only been
observed to undergo the above series of changes in a certain
number of instances, which, however, include examples from
several divisions of the Ccelenterata, the Echinodermata, and the
Mollusca, some of the Vermes [Turbellarians (Leptoplana],
Nematodes, Hirudinea, Alciope, Sagitta], Ascidians, etc. It is
very possible, not to say probable, that such changes are univer-
sal in the animal kingdom, but the present state of our knowledge
does not justify us in saying so.

In the Craniata especially our knowledge of the formation of the polar
bodies is very unsatisfactory. In Petromyzon Kupffer and Benecke have
brought forward evidence to shew that one polar body is formed prior to
the impregnation, and a second in connection with a peculiar prominence
of protoplasm after impregnation. Part of the germinal vesicle remains in
the egg as the female pronucleus. In the Sturgeon the germinal vesicle
atrophies and breaks up before impregnation, and afterwards part is found as a
granular mass on the surface of the egg, while part forms a female pronucleus.

In Amphibia the observations of Hertwig (90) and Bambeke (77) tend to
shew that after the germinal vesicle has assumed a superficial situation at
the pigmented pole of the ovum its contents become intermingled with the
yolk, and are in part extruded from the ovum as a granular mass after
impregnation. Part of them remains in the ovum and forms a female
pronucleus. Whether there is a proper division of the germinal vesicle
as in typical cases is not known.

Oellacher (95) by a series of careful observations upon the egg of the trout,
and subsequently of the bird, demonstrated that in the ovum while still in
the ovary, the germinal vesicle underwent a kind of degeneration and
eventually became ejected, in part at any rate. My own observations on
Elasmobranchs, which require enlargement and confirmation, tend to shew
that this part may be the membrane. Ed. van Beneden (78) has contributed
some important observations on the rabbit. His account is as follows. As
the ovum approaches maturity the germinal vesicle assumes an eccentric
position, and fuses with the peripheral layer of the egg to constitute the
cicatricular lens. The germinal spot next travels to the surface of the
cicatricular lens and forms the nuclear disc: at the same time the membrane
of the germinal vesicle vanishes, though it probably unites with the nuclear
disc. The plasma of the nucleus then collects into a definite mass and forms
the nucleoplasmic body. Finally the nuclear disc assumes an ellipsoidal
form and becomes the nuclear body. Nothing is now left of the original
germinal vesicle but the nuclear body and the nucleoplasmic body, both still
situated within the ovum. In the next stage no trace of the germinal

MATURATION OF THE OVUM. 75

vesicle can be detected in the ovum, but outside it, close to the point where
the modified remnants of the vesicle were previously situated, there is
present a polar body which is composed of two parts, one of which stains
deeply and resembles the nuclear body, and the other does not stain but is
similar to the nucleoplasmic body. Van Beneden concludes that the parts of
the polar body are the two ejected products of the germinal vesicle. We may
be perhaps permitted to hold that further observations on this difficult object
will demonstrate that part of the germinal vesicle remains in the ovum to
form the female pronucleus.

With reference to invertebrate forms attention may be called to the
observations of Biitschli (80). Although in Cucullanus a normal formation
of the polar bodies takes place, yet in the Nematodes generally, Biitschli has
been unable to find the spindle modification of the germinal vesicle, but
states that the germinal vesicle undergoes degeneration, its outline becom-
ing indistinct and the germinal spot vanishing. The position of the
germinal vesicle continues to be marked by a clear space, which gradually
approaches the surface of the egg. When it is in contact with the surface
a small spherical body, the remnant of the germinal vesicle, comes into view,
and eventually becomes ejected. The clear space subsequently disappears.

In addition to the types just quoted, which may very pro-
bably turn out to be normal in the mode of formation of the
polar bodies, there is a large number of types, including the
whole of the Rotifera and Arthropoda with a few doubtful
exceptions1, in which the polar cells cannot as yet be said to
have been satisfactorily observed.

The more important of the doubtful cases amongst the Rotifera and Ar-
thropoda are the following.

Flemming (83) finds that in the summer and probably parthenogenetic
eggs of Lacinularia socialis the germinal vesicle approaches the surface
and becomes invisible, and that subsequently a slight indentation in the
outline of the egg marks the point of its disappearance. In the hollow of
the indentation Flemming believes a polar cell to be situated, though he
has not definitely seen one.

Hoek2 believes that he has found a polar body in the ovum of Balanus
balanoides, but his observations are not perfectly satisfactory.

1 The best instance of what appears like a polar cell in Arthropoda is a body
recently found by Grobben (" Entwicklungsgeschichte d. Moina rectirostris." Claus'
Arbeiten, Vol. II., Wien, 1879) near the surface of the protoplasm at the animal pole
of the summer and parthenogenetic eggs of Moina rectirostris, one of the Cladocera.
The body stains deeply with carmine, but differs from normal polar cells in not being
separated from the ovum ; and its identification as a polar cell must remain doubtful
till it has been shewn to originate from the germinal vesicle.

2 "Zur -Entwicklung d. Entomostraken." Niederlandischer Archiv. f. Zoologie,
Vol. in. p. 62.

76 FUNCTION OF POLAR CELLS.

Biitschli, who has expressly searched for the polar bodies in the ova of
Rotifera, was unable to find any trace of them, though he found that as the
egg became ripe the germinal vesicle became half its original size. In the
parthenogenetic eggs of Aphis he also failed to find a trace of polar bodies,
though the germinal vesicle, after the germinal spot had broken up into
fragments, approached the surface and disappeared.

Whatever may be the eventual result of more extended
investigation, it is clear that the formation of polar cells
according to the type described above is a very constant
occurrence. Its importance is increased by the discovery by
Strasburger of the existence of an analogous process amongst
plants. Two questions about it obviously present themselves
for solution : (i) What are the conditions of its occurrence with
reference to impregnation ? (2) What meaning has it in the
development of the ovum or the embryo ?

The answer to the first of these questions is not difficult to
find. The formation of the polar bodies is independent of
impregnation, and is the final act of the normal growth of the
ovum. In a few types the polar cells are formed while the
ovum is still in the ovary, as, for instance, in some species of
Echini, Hydra, etc., but, according to our present knowledge, far
more usually after the ovum has been laid. In some instances
the budding-off of the polar cells precedes, and in other instances
follows impregnation ; but there is no evidence to shew that in
the latter cases the process is influenced by the contact with the
male element. In Asterias, as has been shewn by O. Hertwig
and Fol, the formation of the polar cells may indifferently either
precede or follow impregnation — a fact which affords a clear
demonstration of the independence of the two occurrences.

To the second of the two questions it does not unfortunately
seem possible at present to give an answer which can be re-
garded as satisfactory.

The retrogressive changes in the membrane of the germinal
vesicle which usher in the formation of the polar bodies may
very probably be viewed as a prelude to a renewed activity of
the contents of the vesicle ; and are perhaps rendered the more
necessary from the thickness of the membrane which results
from a protracted period of passive growth. This suggestion
does not, however, help us to explain the formation of polar

MATURATION OF THE OVUM.

bodies by a process identical with cell-division. The ejection of
part of the germinal vesicle in the formation of the polar cells
may probably be paralleled by the ejection of part or the whole
of the original nucleus which, if we may trust the beautiful
researches of Butschli, takes place during conjugation in In-
fusoria as a preliminary to the formation of a fresh nucleus.
This comparison is due to Butschli, and according to it the
formation of the polar bodies would have to be regarded as
assisting, in some as yet unknown way, the process of regene-
ration of the germinal vesicle. Views analogous to this are held
by Strasburger and Hertwig, who regard the formation of the
polar bodies in the light of a process of excretion or removal of
useless material. Such hypotheses do not, unfortunately, carry
us very far.

I would suggest that in the formation of the polar cells part
of the constituents of the germinal vesicle, which are requisite
for its functions as a complete and independent nucleus, is
removed, to make room for the supply of the necessary parts to
it again by the spermatic nucleus.

My view amounts to the following, viz. that after the forma-
tion of the polar cells the remainder of the germinal vesicle
within the ovum (the female pronucleus) is incapable of further
development without the addition of the nuclear part of the
male element (spermatozoon), and that if polar cells were not
formed parthenogenesis might normally occur. A strong sup-
port for this hypothesis would be afforded were it to be definitely
established that a polar body is not formed in the Arthropoda
and Rotifera ; since the normal occurrence of parthenogenesis
is confined to these two groups. It is certainly a remarkable
coincidence that they are the only two groups in which polar
bodies have not so far been satisfactorily observed.

It is perhaps possible that the part removed in the formation of the
polar cells is not absolutely essential ; and this seems at first sight to follow
from the fact of parthenogenesis being possible in instances where impreg-
nation is the normal occurrence. The genuineness of the observations
on this head is too long a subject to enter into here1, but after admitting,

1 The instances quoted by Siebold, Parthenogenesis d. Arthropoden, are not quite
satisfactory. In Hensen's case, p. 234, impregnation would have been possible if we
can suppose the spermatozoa to be capable of passing into the body-cavity through the

78 FUNCTION OF. POLAR CELLS.

as we probably must, that there are genuine cases of such parthenogenesis,
it cannot be taken for granted without more extended observation that the
occurrence of development in these rare instances may not be due to the
polar cells not having been formed as usual, and that when the polar cells
are formed the development without impregnation is impossible.

Selenka found in the case of Purpura lapillus that no polar body was
formed in the eggs which did not develop, but in the case of Neritina,
Biitschli has found that this does not hold good.

The remarkable observations of Greeff (No. 88) on the parthenogenetic
development of the eggs of Asterias rubens tell, however, very strongly
against the above hypothesis. Greeff has found that under normal
circumstances the eggs of this species of starfish will develop without
impregnation in simple sea-water. The development is quite regular and
normal, though much slower than in the case of impregnated eggs. It is
not definitely stated that polar cells are formed, but there can be no doubt
that this is implied. GreefPs account is so precise and circumstantial that
it is not easy to believe that any error can have crept in ; but neither
Hertwig nor Fol have been able to repeat his experiments, and we may be
permitted to wait for further confirmation before absolutely accepting them.

To the suggestion already made with reference to the function of the
polar cells, I will venture to add the further one, that the function of
forming polar cells has been acquired by the ovum for the express purpose of
Preventing parthenogenesis.

The explanation given by Mr Darwin of the evil effects of self-fertiliza-
tion, viz. the want of sufficient differentiation in the sexual elements1,
would apply with far greater force to cases of parthenogenesis.

In the production of fresh individuals, two circumstances are obviously
favourable to the species, (i) That the maximum number possible of fresh
individuals should be produced, (2) That the individuals should be as
vigorous as possible. Sexual differentiation (even in hermaphrodites)
is clearly very inimical to the production of the maximum number of
individuals. There can be little doubt that the ovum is potentially capable
of developing by itself into a fresh individual, and therefore, unless the
absence of sexual differentiation was very injurious to the vigour of the
progeny, parthenogenesis would most certainly be a very constant occur-
rence ; and, on the analogy of the arrangements in plants to prevent self-
fertilization, we might expect to find some contrivance both in animals and in

open end of the uninjured oviduct ; and though Oellacher's instances are more valuable,
yet sufficient care seems hardly to have been taken, especially when it is not certain
for what length of time spermatozoa may be able to live in the oviduct. For Oellacher's
precautions, vide Zeit. fur Wiss. Zool., Bd. xxii., p. 202. A better instance is that
of a sow given by Bischoff, Ann. Sci. Nat., series 3, Vol. n., 1844. The unimpreg-
nated eggs were found divided into segments, but the segments did not contain the
usual nucleus, and were perhaps nothing else than the parts of an ovum in a state of
disruption.

1 Darwin, Cross- and Self- Fertilization of Plants, p. 443.

MATURATION OF THE OVUM. 79

plants to prevent the ovum developing by itself without fertilization. If
my view about the polar cells is correct, the formation of these bodies
functions as such a contrivance.

Reproduction by budding or fission has probably arisen as a means of
increasing the number of individuals produced, so that the co-existence of
asexual with sexual reproduction is to be looked on as a kind of compromise
for the loss of the power of rapid reproduction due to the absence of
parthenogenesis. In the Arthropoda and Rotifera the place of budding has
been taken by parthenogenesis, which may be a frequent, though not always
a necessary occurrence, as in various Branchiopoda (Apus, Limnadia, etc.)
and Lepidoptera (Psyche helix:, etc.); or a regular occurrence for the pro-
duction of one sex, as in Bees, Wasps, Nematus, etc. ; or an occurrence
confined to a certain stage in the cycle of development in which all the
individuals reproduce their kind parthenogenetically, as in Aphis, Ceci-
domyia, Gall Insects (Neuroterus, etc.), Daphnia1.

On my hypothesis the possibility of parthenogenesis, or at any rate its
frequency, in Arthropoda and Rotifera is possibly due to the absence of polar
cells. In the case of all animals, so far as is known to me, fertilization of
the ovum occasionally occurs2, but there are instances in the vegetable king-
dom where so-called parthenogenesis appears to be capable of recurring for
an indefinite period. One of the best instances appears to be that of
Ccelebogyne, an introduced exotic Euphorbiaceous plant which regularly
produces fertile seeds although a male flower never appears. The recent
researches of Strasburger have however shewn that in Ccelebogyne and other
parthenogenetic flowering plants, embryos are formed by the budding and
subsequent development of cells belonging to the ovule. This being the
case, it is impossible to assert of these plants that they are really partheno-
genetic, for the embryos contained in the seed of a flower which has
certainly not been fertilized, may have been formed, not by the development
of the ovum, but by budding from the surrounding tissue of the ovule.

The above view with reference to the nature of the polar bodies is not
to be regarded as forming more than an hypothesis.

Impregnation of the Ovum.

A far greater amount of certainty has been attained as to the
effects of impregnation than as to the changes of the germinal
vesicle which precede this, and there appears, moreover, to be a
greater uniformity in the series of resulting phenomena.

1 Mr J. A. Osborne has recently shewn (Nature, Sept. 4, 1879), that the eggs of a
Beetle (Gastrophysa raphani) may occasionally develop, up to a certain point at any
rate, without the male influence.

2 Dicyema, which is an apparent exception, has not yet been certainly shewn to
develop true ova. If its germs are true ova it forms an exception to the above
rule.

IMPREGNATION OF THE OVUM.

It will be convenient again to take Asterias glacialis as the
type. The part of the germinal vesicle which remains in the
egg, after the formation of the second polar cell, becomes con-
verted into a number of small vesicles (fig. 29), which aggregate

B.

FIG. 31. SMALL PORTIONS OF THE OVUM OF ASTERIAS GLACIALIS. THE SPERMA-
TOZOA ARE SHEWN ENVELOPED IN THE MUCILAGINOUS COAT. IN A. A PROMI-
NENCE IS RISING FROM THE SURFACE OF THE EGG TOWARDS THE NEAREST

SPERMATOZOON; AND IN B. THE SPERMATOZOON AND PROMINENCE HAVE MET.
(Copied from Fol.)

themselves into a single clear nucleus, which
toward the centre of the egg and around
which, as a centre, the protoplasm becomes
radiately striated (fig. 30). This nucleus is
known as the female pronucleus. By
the action of reagents a nucleolus may be
shewn in it. In Asterias glacialis the most
favourable period for fecundation is about an
hour after the formation of the female pro-
nucleus. If at this time the spermatozoa are
allowed to come in contact with the egg,
their heads soon become enveloped in the
investing mucilaginous coat. A prominence,
pointing towards the nearest spermatozoon,
now arises from the superficial layer of pro-
toplasm of the egg, and grows till it comes
in contact with the spermatozoon (fig. 31, A
and B). Under normal circumstances the
spermatozoon which meets the prominence is
the only one concerned in the fertilization,

gradually travels

1''IG. 32. 1'OKTION
OF THE OVUM OF AS-
TERIAS GLACIALIS AF-
TER THE ENTRANCE
OF A SPERMATOZOON
INTO THE OVUM, IT
SHEWS THE PROMI-
NENCE OF THE OVUM
THROUGH WHICH THE
SPERMATOZOON HAS
ENTERED. A VITEL-
LINEMEMBRANEW1TH
A CRATER-LIKE OPEN-
ING HAS BECOME DIS-
TINCTLY FORMED.

(Copied from Fol.)

IMPREGNATION OF THE OVUM.

8l

and it makes its way into the egg by passing through the promi-
nence. The tail of the spermatozoon, no longer motile, remains
visible for some time after the head has bored its way in, but its
place is soon taken by a pale conical body, which is, however,
probably in part a product of the metamorphosis of the tail
itself (fig. 32). It eventually becomes absorbed into the body of
the ovum.

At the moment of contact between the spermatozoon and
the egg the outermost layer of the protoplasm of the latter
raises itself as a distinct membrane, which separates from the
egg and prevents the entrance of other spermatozoa. At the
point where the spermatozoon entered a crater-like opening is
left in the membrane, through which the metamorphosed tail of
the spermatozoon may at first be seen projecting (fig. 32).

The head of the spermatozoon when in the egg forms a
nucleus, for which the name male
pronucleus may be conveniently
adopted. It grows in size, pro-
bably by assimilating material
from the ovum, and around it is
formed a clear space free from
yolk-spherules. Shortly after its
formation the protoplasm in its
neighbourhood assumes a radiate
arrangement (fig. 33). At what-
ever point of the egg the sperma-
tozoon may have entered, it grad-
ually travels towards the female FlG. 33> QVUM OF ASTERIAS
pronucleus. The latter, around GLACIALIS, WITH MALE AND FEMALE

PRONUCLEUS AND A RADIAL STRIA-

which the protoplasm no longer TION OF THE PROTOPLASM AROUND
has a radiate arrangement, re- THE FORMER. (Copied from FoL)
mains motionless till the rays of

the male pronucleus come in contact with it, after which its
condition of repose is exchanged for one of activity, and it
rapidly approaches the male pronucleus, apparently by means
of its inherent amoeboid contractions, and eventually fuses with
it (figs. 34—36).

As the male pronucleus approaches the female the latter,
according to Selenka, sends out protoplasmic processes which
B. n. 6

82

MALE PRONUCLEUS.

embrace the former. The actual fusion does not take place till
after the pronuclei have been in contact for some time. While
the two pronuclei are approaching one another the protoplasm
of the egg exhibits amoeboid movements.

The product of the fusion of the two pronuclei forms the first
segmentation nucleus (fig. 37), which soon, however, divides into
the two nuclei of the two first segmentation spheres.

The phenomenon which has just been described consists
essentially in the fusion of the male cell and the female cell. In
this act the protoplasm of the two cells as well as their nuclei
coalesce, since the whole spermatozoon which has been absorbed
into the ovum is a cell of which the head is the nucleus.

It is clear that the ovum after fertilization is an entirely
different body to the ovum prior to that act, and unless the use
of the same term for the two conditions of the ovum had become
very familiar, a special term, such as oosperm, for the ovum
after its fusion with the spermatozoon, would be very convenient.

FIGS. 34, 35, AND 36. THREE SUCCESSIVE STAGES IN THE COALESCENCE OF THE
MALE AND FEMALE PRONUCLEI IN ASTERIAS GLACIALIS. FROM THE LIVING
OVUM. (Copied from Fol.)

Of the earlier observations on this subject there need perhaps only be
cited one of E. van Beneden, on the
rabbit's ovum, shewing the presence of
two nuclei before the commencement of
segmentation. Butschli was the earliest
to state from observations on Rhabditis
dolichura that the first segmentation
nucleus arose from the fusion of two
nuclei, and this was subsequently shewn
with greater detail for Ascaris nigrove-
nosa, by Auerbach (76). Neither of these
authors gave at the first the correct in-
terpretation of their results. At a later
period Butschli (80) arrived at the con-
clusion that in a large number of in- FIG. 37 OVUM OF ASTERIAS
stances (Lymnaus, Nephelis, Cucullanus, GLACIALIS, AFTER THE COALESCENCE

f ' OF THE MALE AND FEMALE PRONU-

&c.), the nucleus in question was formed CLEI. (Copied from Fol.)

IMPREGNATION OF THE OVUM. 83

by the fusion of two or more nuclei, and Strasburger at first made a
similar statement for Phallusia, though he has since withdrawn it. Though
Biitschli's statements depend, as it seems, upon a false interpretation of
appearances, he nevertheless arrived at a correct view with reference to
what occurs in impregnation. Van Beneden (78) described in the rabbit
the formation of the original segmentation nucleus from two nuclei, one
peripheral and the other central, and deduced from his observations that
the 'peripheral nucleus was derived from the spermatic element. It was
reserved for Oscar Hertwig (89) to describe in Echinus lividus the en-
trance of a spermatozoon into the egg and the formation from it of the
male pronucleus.

The general fact that impregnation consists in the fusion
of the spermatozoon and ovum has now been established for
some forms in the majority of invertebrate groups (Arthropoda
and Rotifera excepted). Amongst Vertebrata also it has been
shewn by E. van Beneden that the first segmentation nucleus is
formed by the coalescence of the male and female pronucleus.
Calberla, and Kupffer and Benecke have demonstrated that a
single spermatozoon enters at first the ovum of Petromyzon.

The contact of the spermatozoon with the egg-membrane causes in Petro-
myzon active movements of the protoplasm of the ovum, and a retreat
of the protoplasm from the membrane.

In Amphibia the appearance of a peculiar pigmented streak
extending inwards from the surface of the pigmented pole of the
ovum,, and containing in a clear space at its inner extremity a
nucleus, has been demonstrated as the result of impregnation by
Bambeke (77) and Hertwig (90). There can be little doubt that
this nucleus is the male pronucleus, and that the pigmented
streak indicates its path inwards. Close to it Hertwig has
shewn that another nucleus is to be found, the female pronucleus,
and that eventually the two join together. In Amphibia the
phenomena accompanying impregnation are clearly of the same
nature as in the Invertebrata. A precisely similar series of
phenomena to those in Amphibia has been shewn by Salensky
to take place in the Sturgeon.

Although there is a general agreement between the most recent observers,
Hertwig, Fol, Selenka, Strasburger, £c., as to the main facts connected
with the entrance of one spermatozoon into the egg, the formation of the
male pronucleus, and its fusion with the female pronucleus, there still exist
differences of detail in the different descriptions, which partly, no doubt,

6—2

84 MALE PRONUCLEUS.

depend upon the difficulties of observation, but partly also upon the observa-
tions not having all been made upon the same species. Hertwig does not
enter into details with reference to the actual entrance of the spermatozoon
into the egg, but in his latest paper points out that considerable differences
may be observed in the occurrences which succeed impregnation, according
to the relative period at which this takes place. When, in Asterias, the
impregnation is effected about an hour after the egg is laid, and previously
to the formation of the polar cells, the male pronucleus appears at first to
exert but little influence on the protoplasm, but after the formation of the
second polar cell, the radial striae around it become very marked, and the
pronucleus rapidly grows in size. When it finally unites with the female
pronucleus it is equal in size to the latter. In the case when the impregna-
tion is deferred for four hours the male pronucleus never becomes so large
as the female pronucleus. With reference to the effect of the time at
which impregnation takes place, Asterias would seem to serve as a type.
Thus in Hirudinea, Mollusca, and Nematoidea impregnation normally takes
place before the formation of the polar bodies is completed, and the male
pronucleus is accordingly as large as the female. In Echinus, on the other
hand, where the polar bodies are formed in the ovary, the male pronucleus
is always small.

Selenka, who has investigated the formation of the male pronucleus in
Toxopneustes variegatus, differs in certain points from Fol. He finds that
usually, though not always, a single spermatozoon enters the egg, and that
though the entrance may be effected at any part of the surface it generally
occurs at the point marked by a small prominence where the polar cells
are formed. The spermatozoon first makes its way through the mucous
envelope of the egg, within which it swims about, and then bores with its
head into the polar prominence.

One important point has been so far only indirectly alluded
to, viz. the number of spermatozoa required to effect impregna-
tion.

The concurrent testimony of almost all observers tends to
shew that one only is required for this purpose. But the
number of cases tested is too small to admit of satisfactory
generalization.

Both Hertwig and Fol have made observations on the result
of the entrance into the egg of several spermatozoa. Fol finds
that when the impregnation has been too long delayed the
vitelline membrane is formed with comparative slowness, and
several spermatozoa are thus enabled to penetrate. Each sper-
matozoon forms a separate pronucleus with a surrounding star ;
and several male pronuclei usually fuse with the female pro-
nucleus. Each male pronucleus appears to exercise a repulsive

IMPREGNATION OF THE OVUM. 85

influence on other male pronuclei, but to be attracted by the
female pronucleus. When there are several male pronuclei the
segmentation is irregular and the resulting larva a monstrosity.
These statements of Fol and Hertwig are up to a certain point
in contradiction with the more recent results of Selenka. In
Toxopneustes variegatus Selenka finds that though impregnation
is usually effected by a single spermatozoon yet several may be
concerned in the act. The development continues, however, to
be normal up to the gastrula stage, at any rate, if three or even
four spermatozoa enter the egg almost simultaneously. Under
such circumstances each spermatozoon forms a separate pro-
nucleus and star. Selenka is of opinion (apparently rather on
a priori grounds than as a result of direct observation) that
normal development cannot occur when more than one male
pronucleus fuses with the female pronucleus ; and holds that,
where he has observed such normal development after the
entrance of more than one spermatozoon, the majority of male
pronuclei become absorbed.

It may be noticed that, while the observations of Fol and
Hertwig were admittedly made upon eggs in which the impreg-
nation was delayed till they no longer displayed their pristine
activity, Selenka's were made upon quite fresh eggs ; and it
seems not impossible that the pathological symptoms in the
embryos reared by the two former authors may have been due
to the imperfection of the egg, and not to the entrance of more
than one spermatozoon. This, of course, is merely a suggestion
which requires to be tested by fresh observations.

Kupffer and Benecke have further shewn that although only
one spermatozoon enters the ovum directly in Petromyzon yet
other spermatozoa pass through the vitelline membrane, and are
taken into a peculiar protoplasmic protuberance of the ovum
which appears after impregnation.

The act of impregnation may be described as the fusion of
the ovum and spermatozoon, and the most important feature in
this act appears to be the fusion of a male and female nucleus ;
not only does this appear in the actual fusion of the two pro-
nuclei, but it is brought into still greater prominence by the fact
that the female pronucleus is a product of the nucleus of a
primitive ovum, and the male pronucleus is the metamorphosed

86 SUMMARY.

head of the spermatozoon which, as stated above, contains part
of the nucleus of the primitive spermatic cell. The spermatic
cells originate from cells indistinguishable from the primitive
ova, so that the fusion which takes place is the fusion of morpho-
logically similar parts in the two sexes.

These conclusions tally very satisfactorily with the view
adopted in the Introduction, that impregnation amongst the
Metazoa was derived from the process of conjugation amongst
the Protozoa.

Summary.

In what may probably be regarded as a normal case the
following series of events accompanies the maturation and im-
pregnation of an ovum : —

(1) Transportation of the germinal vesicle to the surface of
the egg.

(2) Absorption of the membrane of the germinal vesicle
and metamorphosis of the germinal spot and nuclear reticulum.

(3) Assumption of a spindle character by the remains of
the germinal vesicle, these remains being probably in part
formed from the germinal spot.

(4) Entrance of one end of the spindle into a protoplasmic
prominence at the surface of the egg.

(5) Division of the spindle into two halves, one remaining
in the egg, the other in the prominence ; the prominence becom-
ing at the same time nearly constricted off from the egg as a
polar cell.

(6) Formation of a second polar cell in the same manner as
the first, part of the spindle still remaining in the egg.

(7) Conversion of the part of the spindle remaining in the
egg into a nucleus — the female pronucleus.

(8) Transportation of the female pronucleus towards the
centre of the egg.

(9) Entrance of one spermatozoon into the egg.

(10) Conversion of the head of the spermatozoon into a
nucleus — the male pronucleus.

(i i) Appearance of radial striae round the male pronucleus,
which gradually travels towards the female pronucleus.

MATURATION AND IMPREGNATION OF THE OVUM. 87

(12) Fusion of male and female pronuclei to form the first
segmentation nucleus.

(76) Auerbach. Organologische Stiidien, Heft 2. Breslau, 1874.

(77) Bambeke. " Recherchess. Embryologie des Batraciens." Bull.detAcad.
royale de Belgique, sme Ser., T. LXI., 1876.

(78) E. van Beneden. "La Maturation de 1'OZuf des Mammiferes." Bull.de
fAcad. royale de Belgique, 2me Sen, T. XL. No. 12, 1875.

(79) Idem. " Contributions a 1'Histoire de la Vesicule Germinative, &c." Bull.
de TAcad. royale de Belgique, sme Se"r., T. XLI. No. i, 1876.

(80) O. Biitschli. Eizelle, Zelltheilung, und Conjugation der Infusorien. Frank-
furt, 1876.

(81) F. M. Balfour. "On the Phenomena accompanying the Maturation and
Impregnation of the Ovum." Quart. J. of Micros. Science, Vol. xvni., 1878.

(82) Calberla. " Befruchtungsvorgang beim Ei von Petromyzon Planeri." Zeit.
f. wiss. Zool., Vol. XXX.

(83) W. Flemming. " Studien in d. Entwickelungsgeschichte der Najaden."
Sitz. d. k. Akad. Wien, B. LXXL, 1875.

(84) H. Fol. "Die erste Entwickelung des Geryonideneies." Jenaische Zeit-
schrift, Vol. vii., 1873.

(85) Idem. " Sur le Developpement des Pteropodes." Archives de Zoologie
Experimentale et Generale, Vol. IV. and V., 1875 — 6.

(86) Idem. "Sur le Commencement de 1'Henogenie." Archives des Sciences
Physiques et Naturelles. Geneve, 1877.

(87) Idem. Recherches s. /. Fecondation et I. cornmen. d. FHenogenie. Geneve,
1879.

(88) R. Greeff. " Ueb. d. Bau u. d. Entwickelung d. Echinodermen. " Sitzun.
der Gesellschaft z. Beforderung d. gesammten Naturiviss. z. Marburg, No. 5, 1876.

(89) Oscar Hertwig. "Beit. z. Kenntniss d. Bildung, &c., d. thier. Eies."
Morphologisches Jahrbuch, Vol. I., 1876.

(90) Idem. Ibid. Morphologisches Jahrbuch, Vol. in. Heft i, 1877.

(91) Idem. " Weitere Beitrage, &c." Morphologisches Jahrbuch, Vol. ill., 1877,
Heft 3.

(92) Idem. "Beit. z. Kenntniss, &c." Morphologisches Jahrbuch, Vol. IV. Heft
i and 2, 1878.

(93) N. Kleinenberg. Hydra. Leipzig, 1872.

(94) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtung am Eie d.
Neunaugen. Konigsberg, 1878.

(95) J. Oellacher. "Beitrage zur Geschichte des Keimblaschens im Wirbel-
thiereie." Archivf. micr. Anat., Bd. viil., 1872.

(96) W. Salensky. " Befruchtung u. Furchung d. Sterlets-Eies." Zoologischer
Anzeiger, No. u, 1878.

(97) E. Selenka. Befruchtung des Eies von Toxopneustesvariegatus. Leipzig,

1878.

(98) Strasburger. Ueber Zellbildung u. Zelltheilung. Jena, 1876.

(99) Idem. Ueber Befruchtung u. Zelltheilung. Jena, 1878.

(100) C. O. Whitman. "The Embryology of Clepsine." Quart. J. of Micr.
Science, Vol. xvm., 1878.

CHAPTER III.

THE SEGMENTATION OF THE OVUM.

THE immediate result of the fusion of the male and female pro-
nucleus is the segmentation or division of the ovum usually into
two, four, eight, etc. successive parts. The segmentation may
be dealt with from two points of view, viz. (i) the nature of the
vital phenomena which take place in the ovum during its
occurrence, which may be described as the internal phenomena
of segmentation. (2) The external characters of the segmenta-
tion.

Internal PJunomena of Segmentation.

Numerous descriptions have been given during the last few
years of the internal phenomena of segmentation. The most
recent contribution on this head is that of Fol (No. 87). He
appears to have been more successful than other observers in
obtaining a complete history of the changes which take place,
and it will therefore be convenient to take as type the ovum of
ToxopneusUs (Echinus] lividus, on which he made his most
complete series of observations. The changes which take place
may be divided into a series of stages. The ovum immediately
after the fusion of the male and female pronucleus contains a
central segmentation nucleus.

In the first stage a clear protoplasmic layer derived from the
plasma of the cell is formed round the nucleus, from which there
start outwards a number of radial striae, which arc rendered
conspicuous by the radial arrangement of the yolk-granules

THE SEGMENTATION OF THE OVUM. 89

between them. The nucleus during this process remains per-
fectly passive.

In the second stage the nucleus becomes less distinct and
somewhat elongated, and around it the protoplasmic layer of
the earlier stage is arranged in the form of a disc-shaped ring,
compared by Fol to Saturn's ring. The protoplasmic rays still
take their origin from the perinuclear protoplasm. This stage
has a considerable duration (20 minutes).

In the third stage the protoplasm around the nucleus
becomes transported to the two nuclear poles, at each of which
it forms a clear mass surrounded by a star-shaped figure
formed by radial striae. The nucleus is hardly visible in the
fresh condition, but when brought into view by reagents is found
to contain many highly refractive particles, and to be still
enveloped in a membrane.

In the fourth stage the nucleus when treated by reagents has
assumed the well-known spindle form. The striae of which it is
composed are continuous from one end of the spindle to the
other and are thickened at the centre. The central thickenings
constitute the so-called nuclear plate. The clear protoplasmic
masses and stars are present as before at the apices of the
nucleus, and the rays of the latter converge as if they would
meet at the centre of the clear masses, but stop short at their
periphery. There is no trace of a membrane round either the
nuclear spindle or the clear masses ; and in the centre of the
latter is a collection of granules. The striae of the polar stars
are very fine but distinct.

Between the stage with a completely formed spindle and the
previous one the intermediate steps have not been made out for
Toxopneustes ; but for Heteropods Fol has been able to demon-
strate that the striae of the spindle and their central thickenings
are formed, as in the case of the spindle derived from the
germinal vesicle, from the metamorphosis of the nuclear reticulum.
They commence to be formed at the two poles, and are then (in
Heteropods) in immediate contiguity with the striae of the stars.
The striae gradually grow towards the centre of the nucleus and
there meet.

In the fifth stage the central thickenings of the spindle
separate into two sets, which travel symmetrically outwards

QO INTERNAL PHENOMENA.

towards the clear masses, growing in size during the process.
They remain however united for a short time by delicate
filaments — named by Fol connective filaments — which very soon
disappear. The clear masses also increase in size. During this
stage the protoplasm of the ovum exhibits active amoeboid
movements preparatory to division.

In the sixth stage, which commences when the central
thickenings of the spindle have reached the clear polar masses,
the division of the ovum into two parts is effected by an
equatorial constriction at right angles to the long axis of the
nucleus. The inner vitelline membrane follows the furrow for a
certain distance, but does not divide with the ovum. All con-
nection between the two parts of the spindle becomes lost during
this stage, and the thickenings of the fibres of the spindle give
rise to a number of spherical vesicular bodies, which pass into
the clear masses and become intermingled with the granules
which are placed there. The radii of the stars now extend
round the whole circumference of each of the clear masses.

In the seventh stage the two clear masses become elongated
and travel towards the outer sides of their segments ; while the
radii connected with them become somewhat bent, as if a
certain amount of traction had been exercised on them in the
movement of the clear masses. Shortly afterwards the spherical
vesicles, each of which appears like a small nucleus and contains
a central nucleolus, begin to unite amongst themselves, and to
coalesce with the neighbouring granules. Those in each seg-
ment finally unite to form a nucleus which absorbs the substance
of the clear mass. The new nucleus is therefore partly derived
from tfie division of the old one and partly from the plasma of the
cell. The two segments formed by division are at first spherical,
but soon become flattened against each other. In each subse-
quent division of these cells the whole of the above changes are
repeated.

The phenomena which have just been described would
appear to occur in the segmentation of ova with remarkable
constancy and without any very considerable variations.

The division of the ovum constitutes a special case of cell division, and it
is important to determine to what extent the phenomena of ordinary cell
division are related to those which take place in the division of the ovum.

THE SEGMENTATION OF THE OVUM. 91

Without attempting a full discussion of the subject I will confine myself to
a few remarks suggested by the observations of Flemming, Peremeschko and
Klein. The observations of these authors shew that in the course of the
division of nuclei in the salamander, newt, etc. the nuclear reticulum under-
goes a series of peculiar changes of form, and after the membrane of the
nucleus has vanished divides into two masses. The masses form the basis
for the new nuclei, and become reconverted into an ordinary nuclear reticu-
lum after repeating, in the reverse order, the changes of form undergone
by the reticulum previous to its division.

It is clear without further explanation that the conversion of the
nuclear reticulum of the segmentation nucleus into the striae of the spindle
is a special case of the same phenomenon as that first described by Flemming
in the salamander. There are however some considerable differences. In
the first place the fibres in the salamander do not, according to Flemming,
unite in the middle line, though they appear to do so in the newt. This clearly
cannot be regarded as a fact of great importance ; nor can the existence of
the central thickenings of the striae (nuclear plate), constant as it is for the
division of the nucleus of the ovum, be considered as constituting a funda-
mental difference between the two cases. More important is the fact that
the striae in the case of the ovum do not appear, at any rate have not been
shewn, to form themselves again into a nuclear network.

With reference to the last point it is however to be borne in mind (i) that
the gradual travelling outwards of the two halves of the nuclear plate is
up to a certain point a repetition, in the reverse order, of the mode of
formation of the strise of the spindle, since the striae first appeared at the
poles and gradually grew towards the middle of the spindle : (2) that there
is still considerable doubt as to how the vesicular bodies formed out of the
nuclear plate reconstitute themselves into a nucleus.

The layer of clear protoplasm around the nucleus during its division has
its homologue in the case of the division of the nuclei of the salamander,
and the rays starting from this are also found. Klein has suggested that the
extra-nuclear rays of the stars around the poles of the nucleus are derived
from a metamorphosis of the extra-nuclear reticulum, which he believes
to be continuous with the intra-nuclear reticulum.

The delicate connective filaments usually visible between the two halves
of the nuclear plate would seem from Strasburger's latest observations
(No. 104) to be derived from the nuclear substance between the striae of the
spindle, and to become eventually reabsorbed into the newly-formed nuclei.

We are it appears to me still in complete ignorance as to the
physical causes of segmentation. The view that the nucleus is
a single centre of attraction, and that by its division the centre of
attraction becomes double and thereby causes division, appears to
be quite untenable. The description already given of the pheno-
mena of segmentation is in itself sufficient to refute this view.

92 REGULAR SEGMENTATION.

Nor is it in the least proved by the fact (shewn by Hallez) that
the plane of division of the cell always bears a definite relation
to the direction of the axis of the nucleus.

The arguments by which Kleinenberg (93) attempted to de-
monstrate that cell division was a phenomenon caused by altera-
tions in the molecular cohesion of the protoplasm of the ovum
still in my opinion hold good, but recent discoveries as to the
changes which take place in the nucleus during division probably
indicate that the molecular changes which take place in the co-
hesion of the protoplasm are closely related to, and possibly
caused by, those in the nucleus. These alterations of cohesion
are produced by a series of molecular changes, the external indi-
cations of which are to be found in the visible alterations in the
constitution of the body of the cell and of the nucleus prior to
division.

BIBLIOGRAPHY.

In addition to the papers cited in the last Chapter, vide

(101) W. F lemming. " Beitrage z. Kenntniss d. Zelle u. ihrer Lebenserschei-
nungen." Archiv f. mikr. Anat., Vol. xvi., 1878.

(102) E. Klein. "Observations on the glandular epithelium and division of
nuclei in the skin of the Newt." Quart. J. of Micr. Science, Vol. xix., 1879.

(103) Peremeschko. " Ueber d. Theilung d. thierischen Zellen." Archiv f.
mikr. Anat., Vol. xvi., 1878.

(104) E. Strasburger. " Ueber ein z. Demonstration geeignetes Zelltheilungs-
Object." Site. d. Jenaischen Gesell.f. Med. u. Naturwiss., July 18, 1879.

External Features of Segmentation.
In the simplest known type of segmentation the ovum first

,-

FIG. 38. VARIOUS STAGES IN PROCESS OF SEGMENTATION. (After Gegenbaur.)

of all divides into two, then four, eight, sixteen, thirty-two, sixty-
four, etc. cells (fig. 38). These cells so long as they are fairlylarge
are usually known as segments or spheres. At the close of such

THE SEGMENTATION OF THE OVUM.

93

a simple segmentation the ovum becomes converted into a sphere
composed of segments of a uniform size. These segments usu-
ally form a wall (fig. 39, E), one row of cells thick, round a central
cavity, which is known as the segmentation cavity or cavity
of Von Baer. Such a sphere is known as a blastosphere. The
central cavity usually appears very early in the segmentation, in
many cases when only four segments are present (fig. 39, B).

In other instances, which however are rarer than those in
which a segmentation cavity is present, there is no trace of a
central cavity, and the sphere at the close of segmentation is
quite solid. In such instances the solid sphere is known as a
morula. It is found in some Sponges, many Coelenterata, some
Nemertines, etc., and in Mammals ; in which group the segmen-
tation is not however quite regular. All intermediate conditions
between a large segmentation cavity, and a very minute central
cavity which may be surrounded by more than a single row of
cells have been described.

The segmentation cavity has occasionally, as in Sycandra, the Cteno-
phora and Amphioxus, the form of an axial perforation of the ovum open at
both extremities.

FIG. 39. THE SEGMENTATION OF AMPHIOXUS. (Copied from Kowalevsky.)
sg. segmentation cavity. A. Stage with two equal segments. B. Stage with four
equal segments. C. Stage after the four segments have become divided by an
equatorial furrow into eight equal segments. D. Stage in which a single layer of
cells encloses a central segmentation cavity. E. Somewhat older stage in optical
section.

94 REGULAR SEGMENTATION.

When the process of regular segmentation is examined some-
what more in detail it is found to follow as a rule a rather definite
rhythm. The ovum is first divided in a plane which may be
called vertical, into two equal parts (fig. 39, A). This division is
followed by a second, also in a vertical plane, but at right angles
to the first plane, and by it each of the previous segments is
halved (fig. 39, B.) In the third segmentation the plane of divi-
sion is horizontal or equatorial and divides each of the four seg-
ments into two halves, making eight segments in all (fig. 39, C).
In the fourth period the segmentation takes place in two vertical
planes each at an angle of 45° with one of the previous vertical
planes. All the segments are thus again divided into two equal
parts. In the fifth period there are two equatorial planes one on
each side of the original equatorial plane, and thirty-two spheres
are present at the close of this period. Sixty-four segments are
formed at the sixth period, but beyond the fourth and fifth periods
the original regularity is not usually preserved.

In many instances the type of segmentation just described cannot be
distinctly recognized. All that can be noticed is that at each fresh
segmentation every segment becomes divided into two equal parts. It is
not absolutely certain that there is not always some slight inequality in
the segments formed, by which, what are known as the animal and vegetative
poles of the ovum, can very early be distinguished. A regular segmen-
tation is found in species in most groups of the animal kingdom. It is
very common in Sponges and Ccelenterates. Though less common so
far as is known amongst the Vermes, it is yet found in many of
the lower types, viz. Nematoidea, Gordiacea, Trematoda, Nemertea
(apparently as a rule), Sagitta, Chcetonotus, some Gephyrea (Phoronis) ;
though not usual it occurs amongst Cha?topoda, e.g. Serpula. It is the
usual type of segmentation amongst the Echinodermata. Amongst the
Crustacea it appears (for the earlier phases of segmentation at any rate)
not infrequently amongst the lower forms, and even occurs amongst the
Amphipoda (Phronimd). It is however very rare amongst the Tracheata,
Podura affording the one example of it known to me. It is almost as rare
amongst Mollusca as amongst the Tracheata, but occurs in Chiton and is
nearly approached in some Nudibranchiata. In Vertebrata it is most nearly
approached in Amphioxus^.

Most of the eggs which have a perfectly regular segmentation
are of a very insignificant size and rarely contain much food-

1 In the Rabbit and probably other Monodelphous Mammalia the segmentation is
nearly though not quite regular.

THE SEGMENTATION OF THE OVUM. 95

yolk : in the vast majority of eggs there is present however a con-
siderable bulk of food material usually in the form of highly re-
fracting yolk-spherules. These yolk-spherules lie embedded in
the protoplasm of the ovum, but are in most instances not distri-
buted uniformly, being less closely packed and smaller at one pole
of the ovum than elsewhere. Where the yolk-spherules are few-
est the active protoplasm is necessarily most concentrated, and
we can lay down as a general law1 that the velocity of segmen-
tation in any part of the ovum is roughly speaking proportional
to the concentration of the protoplasm there ; and that the size
of the segments is inversely proportional to the concentration of
the protoplasm. Thus the segments produced from that part of
an egg where the yolk-spherules are most bulky, and where
therefore the protoplasm is least concentrated, are larger than
the remaining segments, and their formation proceeds more
slowly.

Though where much food-yolk is present it is generally dis-
tributed unequally, yet there are many cases in which it is not
possible to notice this very distinctly. In most of these cases the
segmentation is all the same unequal, and it is probable that they
form apparent rather than real exceptions to the law laid down
above. Although before segmentation the protoplasm may be
uniformly distributed, yet in many instances, e.g. Mollusca,Vermes,
etc., during or at the commencement of segmentation the proto-
plasm becomes aggregated at one pole, and one of the segments
formed consists of clear protoplasm, all the food-yolk being con-
tained in the other and larger segment.

Unequal Segmentation. The type of segmentation I now
proceed to describe has been called by Haeckel (No. 105) 'un-
equal segmentation', a term which may conveniently be
adopted. I commence by describing it as it occurs in the well-
known and typical instance of the Frog2.

The ripe ovum of the common Frog and of most other tailless
Amphibians presents the following structure. One half appears
black and the other white. The former I shall call the upper

1 Vide F. M. Balfour, " Comparison of the early stages of development in Verte-
brates." Quart. Jour, of Micr. Science, July, 1875.

2 Vide Remak, Entwicklung d. Wirbelthiere; and Gotte, Entwicklung d. Unke.

96

UNEQUAL SEGMENTATION.

pole, the latter the lower. The ovum is composed of protoplasm
containing in suspension numerous yolk-spherules. The largest

FIG. 40. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. (Copied
from Ecker.)

The numbers above the figures refer to the number of segments at the stage figured.

of these are situated at the lower pole, the smaller ones at the
upper pole, and the smallest of all in the peripheral layer of the
upper pole, in which also pigment is scattered and causes the
black colour visible from the surface.

The first formed furrow is a vertical furrow. It commences
in the upper half of the ovum, through which it extends rapidly,
and then more slowly through the lower. As soon as the first
furrow has extended through the egg, and the two halves have
become separated from each other, a second vertical furrow
appears at right angles to the
first and behaves in the same
way (fig. 40, 4).

The next furrow is equa-
torial or horizontal (fig. 40, 8).
It does not arise at the true
equator of the egg, but much
nearer to its upper pole. It
extends rapidly round the egg
and divides each of the fourpre-
vious segments into two parts,
one larger and one smaller.
Thus at the end of this stage
there are present four small
and four large segments. At
the meeting point of these a

II

FIG. 41. SECTION THROUGH FROG'S

OVUM AT THE CLOSK OK SKGMKNTATION.

sg. segmentation cavity. //. large yolk-con-
taining cells, ep. small cells at formative
pole (epiblast).

THE SEGMENTATION OF THE OVUM. 97

small cavity appears, which is the segmentation cavity, already
described for uniformly segmenting eggs. It increases in size in
subsequent stages, its roof being formed of the smaller cells and
its floor of the larger. The appearance of the equatorial furrow
is followed by a period of repose, after which two rapidly suc-
ceeding vertical furrows are formed in the upper pole, dividing
each of the four segments of which this is composed into two.
After a short period these furrows extend to the lower pole,
and when completed 16 segments are present — eight larger and
eight smaller (fig. 40, 16). A pause now ensues, after which the
eight upper segments become divided by an equatorial furrow,
and somewhat later a similar furrow divides the eight lower seg-
ments. At the end of this stage there are therefore present 16
smaller and 16 larger segments (fig. 40, 32). After 64 segments
have been formed by vertical furrows which arise symmetrically
in the two poles (fig. 40, 64), two equatorial furrows appear in the
upper pole before a fresh furrow arises in the lower ; so that there
are 128 segments in the upper half, and only 32 in the lower.
The regularity is quite lost in subsequent stages, but the upper
pole continues to undergo a more rapid segmentation than the
lower. While the segments have been increasing in number the
segmentation cavity has been rapidly growing in size ; and at the
close of segmentation the egg forms a sphere, containing an
excentric cavity, and composed of two unequal parts (fig. 41).
The upper part, which forms the roof of the segmentation cavity,
is formed of smaller cells : the lower of larger yolk-containing
cells.

The mode of segmentation of the Frog's ovum is typical for
unequally segmenting ova, and it deserves to be noticed that as
regards the first three or more furrows the segmentation occurs
with the same rhythm in the unequally segmenting ova as in those
which have an uniform segmentation. There appear two verti-
cal furrows followed by an equatorial furrow. The general laws
which were stated with reference to the velocity of segmentation
and the size of the resulting segments are well exemplified in the
case of the Frog's ovum.

The majority of the smaller segments in the segmented Frog's
ovum are destined to form into the epiblast, and the larger seg-
ments become hypoblast and mesoblast.

B. II. 7

98 UNEQUAL SEGMENTATION.

With a few exceptions (the Rabbit, Lymnaeus, etc.) the majority of the
smaller segments always become epi blast and of the larger segments hypo-
blast.

The Frog's ovum serves as a good medium type for unequally
segmenting ova. There are many cases however in which a
regular segmentation is far more closely approached, and others
in which it is less so.

One familiar instance in which a regular segmentation is
nearly approached is afforded by the Rabbit's ovum, which has
indeed usually been regarded as offering an example of a regular
segmentation.

The ovum of the Rabbit1 becomes first divided into two sub-
equal spheres. The larger and more transparent of the two may,
from its eventual fate, be called the epiblastic sphere, and the
other the hypoblastic. The two spheres are divided into four,
and then by an equatorial furrow into eight — four epiblastic and
four hypoblastic. One of the latter assumes a central position.
The four epiblastic spheres now divide before the four hypoblastic.
There is thus introduced a stage with twelve spheres. It is
followed by one with sixteen, and that by one with twenty-four.
During the stages with sixteen spheres and onwards the epiblastic
spheres gradually envelop the hypoblastic, which remain exposed
on the surface at one point only. There is no segmentation
cavity.

In Pedicellina, one of the entoproctous Polyzoa, there is a sub-
regular segmentation, where however the two primary spheres
can be distinguished much in the same way as in the case of the
Rabbit.

A very characteristic type of unequal segmentation is that
presented by the majority of Gasteropods and Pteropods and
probably also of some Lamellibranchiata. It is also found in
some Turbellarians, in Bonellia, some Annelids, etc. In many
instances it offers a good example of the type where in the course
of segmentation the protoplasm becomes aggregated at one pole
of the ovum, or of its segments, to become separated off as a clear
sphere.

The first four segments formed by two vertical furrows at

1 Van Beneden, " D^veloppement embryonnaire des Mammiftres." Bull, de
FAcad. Belgique, 1874.

THE SEGMENTATION OF THE OVUM. 99

right angles are equal, but from these there are budded off four
smaller segments, which in subsequent stages divide rapidly,
receiving however, a continual accession of segments budded off
from the larger spheres. The four larger spheres remain conspi-
cuous till near the close of the segmentation. The process of
budding, by which the smaller spheres become separated from
the larger, consists in a larger sphere throwing out a prominence,
which then becomes constricted off from it.

In the extreme forms of this unequal segmentation we find
at the end of the second cleavage two larger spheres filled with
yolk material and two smaller clear spheres ; and in the later
stages, though the large spheres continue to bud off small
spheres, only the two smaller ones undergo a regular segmenta-
tion, and eventually completely envelop the former. Such a
case as this has been described in Aplysia by Lankester1.

The types I have described serve to exemplify unequal seg-
mentation. The Rabbit's ovum stands at one end of the series,
that of Aplysia at the other ; and the Frog's ovum between the
two.

Great variations are presented by the ova with unequal seg-
mentation as to the presence of a segmentation cavity. In some
instances, e.g. the Frog, such a cavity is well developed. In
other cases it is small, e.g. most Mollusca, while not unfrequently
it is altogether absent.

Before leaving this important type of segmentation, it will be well to
enter with slightly greater detail into some of the more typical as well as
some of the special forms which it presents.

As an example of the typical Molluscan type the normal Heteropod
segmentation, accurately described by Fol2, may be selected.

The ovum divides into two and then four equal segments in the usual
vertical planes. Each segment has a protoplasmic and a vitelline pole.
The protoplasmic pole is turned towards the polar bodies. In the third
segmentation, which takes place along an equatorial plane, four small
protoplasmic cells or segments are segmented or rather budded off from the
four large segments, so that there are four small segments in one plane and
four large below these. In the fourth segmentation the four large segments
alone are active and give rise to four small and four large cells ; so that there
are formed in all eight small and four large cells. The four small cells of the

1 Phil. Trans. 1875.

2 Fol, Archives de Zoologie Experimenfale, Vol. iv. 1875.

7—2

100 UNEQUAL SEGMENTATION.

third generation next divide, forming in all twelve small cells and four large.
The small cells of the fourth generation then divide, and subsequently the
four large cells give rise to four new small ones, so that there are twenty
small cells and four large. The small cells form a cap embracing the upper
pole of the large segments. It may be noted that from the third stage
onwards the cells increase in arithmetical progression — a characteristic
feature of the typical gasteropod segmentation.

In the later stages of segmentation the large cells cease to give rise
to smaller ones in the same manner as before. One of them divides
first into two unequal parts, of which the smaller becomes pushed in to-
wards the centre of the egg. The larger cell then divides again into two,
arid the three cells so formed occupy the centre of a shallow depression.
The remaining larger cells divide in the same way, and give rise to smaller
cells which line a pit which becomes formed on one side of the ovum.
The original smaller cells continue in the meantime to divide and so form
a layer enclosing the larger, leaving exposed however the opening of the
pit lined by the latest products of the larger cells.

FIG. 47. SEGMENTATION OF ANODON PISCINALIS. (Copied rom Flemming.)
r. polar cells, v. vitelline sphere, i . Commencing division into two segments ;
one mainly formed of protoplasm, the other of yolk. 2. Stage with four segments.
3. Formation of blastosphere, and segmentation cavity. 4. Definite segmentation
of the yolk sphere.

The eggs of Anodon and Unio serve as excellent examples of the type
in which the ovum has a uniform structure before the commencement of
segmentation, but in which a separation into a protoplasmic and a nutritive
portion becomes obvious during segmentation.

In Anodon1 the egg is at first uniformly granular, but after impregnation
it throws out on one side a protuberance nearly free from granules (fig.
42, 1).

In the case of this clear protuberance and of the similar protuberances
which follow it, the protoplasm is not at first quite free from food-yolk,
but only becomes so on being separated from the yolk-containing part of the
ovum. We must therefore suppose that the production of the clear
segments is in part at least due to the yolk spherules becoming used up to
form protoplasm. Such a formation of protoplasm from yolk spherules has
been clearly shewn to occur in other types by Bobretzky and Fol.

1 Flemming, "Entwick. der Najaden," Sitz. d. Akad. Wiss. Wien, Bd. 4, 1875.

THE SEGMENTATION OF THE OVUM.

IOI

The protuberance soon becomes separated off from the larger part of
the egg as a small segment composed of clear protoplasm. From the larger
segment filled with food-yolk, a second small clear segment is next budded
off, and simultaneously (fig. 42, 2) the original small segment divides into
two. Thus there are formed four segments, one large and three small ; the
large segment as before being filled with food-yolk. The continuation
of a similar process of budding off and segmentation eventually results
in the formation of a considerable number of small and of one large
segment (fig. 42, 3). Between this large and the small segments is a seg-
mentation cavity.

Eventually the large yolk segment, which has hitherto merely budded
off a series of small segments free from yolk, itself divides into two similar
parts. This process is then repeated (fig. 42, 4) and there is at last formed
a number of yolk segments filled with yolk spheres, which occupy the
place of the original large yolk segment. Between these yolk segments
and the small segments is placed the segmentation cavity.

The segmentation of the ovum of Euaxes1 resembles that of Unio in the
budding off of clear segments from those filled with yolk, but presents
many interesting individualities.

A very peculiar modification of the ordinary Gasteropod segmentation is
that described by Bobretzky for Nassa mutabilis2.

FIG. 43. SEGMENTATION OF NASSA MUTABILIS (from Bobretzky). A. Upper half
divided into two segments. B. One of these has fused with the large lower seg-
ment. C. Four small and one large segment, one of the former fusing with the
large segment. D. Each of the four segments has given rise to a small segment.
E. Small segments have increased to thirty-six.

1 Kowalevsky, Mem. Akad. Petersburg, Series vn. 1871.

2 Archiv.f. mikr. Anat. Vol. xni. 1877.

102 UNEQUAL SEGMENTATION.

The ovum contains a large amount of food-yolk, and the protoplasm is
aggregated at the formative pole, adjoining which are placed the polar
bodies. An equatorial and a vertical furrow (fig. 43 A), the former near
the upper pole, appear simultaneously, and divide the ovum into three
segments, two small, each with a protoplasmic pole, and one large en-
tirely formed of yolk material. One of the two small segments next com-
pletely fuses with the large segment (fig. 43 B), and after the fusion is com-
plete, a triple segmentation of the large segment takes place as at the first
division, and at the same time the single small segment divides into two. In
this way four partially protoplasmic segments and one yolk segment are
formed (fig. 43 C). One of the small segments again fuses with the large
segment, so that the number of segments becomes again reduced to four,
three small and one large. The protoplasmic ends of these segments are turned
towards each other, and where they meet four very small cells become budded
off, one from each segment (fig. 43 D). Four small cells are again budded
off twice in succession, while the original small cells remain passive, so that
there come to be twelve small and four large cells. In later stages the four
first-formed small cells give rise to still smaller cells and then the next-
formed do the same. The large cells continue also to give rise to small
ones, and finally, by a continuous process of division, and fresh budding of
small cells from large cells, a cap of small cells becomes formed covering
the four large cells which have in the meantime pressed themselves together
(fig. 43 E). A segmentation cavity of not inconsiderable dimensions be-
comes established between this cap of small cells and the large cells.

Many eggs, such as those of the Myriapods1, present an irregular seg-
mentation ; but the segmentation is hardly unequal in the sense in which I
have been using the term. Such cases should perhaps be placed in the first
rather than in the present category.

The type of unequal segmentation is on the whole the most widely
distributed in the animal kingdom. There is hardly a group without ex-
amples of it.

It occurrs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst
the Ctenophora this segmentation is of the most typical kind. Four equal
segments are first formed in the two first periods. In the third period a
circumferential furrow separates four smaller from four larger segments.

This type is also widely distributed amongst the unsegmented (Gephyrea,
Turbellaria), as well as the segmented Vermes, and is typical for the
Rotifera. It appears to be very rare in Echinoderms (Echinaster Sarsif).
It is not uncommon in early stages of the segmentation of the lower
Crustacea.

For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia
it occurs in several forms (Salpa, Molgula] and amongst the Craniata it
is typical in the Cyclostomata, Amphibia, and some Ganoids, e.g. Acci-
Penser.

1 Metschnikoff, Zeitschrift f. wiss. '/.oohgie, 1X74.

THE SEGMENTATION OF THE OVUM.

103

Partial segmentation. The next type of segmentation we
have to deal with has long been recognized as partial segmenta-
tion. It is a type in which only part of the ovum, called the
germinal disc, undergoes segmentation, the remainder usually
forming an appendage of the embryo known as the yolk-sack.
Ova belonging to the two groups already dealt with are fre-
quently classed together as holoblastic ova, in opposition to ova
of the present group in which the segmentation is only partial,
and which are therefore called meroblastic. For embryological

FIG. 44. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A

FOWL'S EGG. (After Coste.)

a. edge of germinal disc. b. vertical furrow, c. small central segment, d. larger
peripheral segment.

purposes this is in many ways a very convenient classification,
but ova belonging to the present group are in reality separated
by no sharp line from those belonging to the group just
described.

The origin and nature of meroblastic ova will best be under-
stood by taking an ovum with an unequal segmentation, such as
that of the frog, and considering what would take place in
accordance with the laws already laid down, supposing the
amount of food-yolk at the vitelline pole to be enormously
increased. What would happen may be conveniently illustrated
by fig. 44, representing the segmentation of a fowl's egg. There
would first obviously appear a vertical furrow at the formative
or protoplasmic pole. (Fig. 44 A, b.} This would gradually
advance round the ovum and commence to divide it into two
halves. Before the furrow had however proceeded very far it

PARTIAL SEGMENTATION.

would come to the vitelline part of the ovum ; here, according
to the law previously enunciated, it would travel very slowly,
and if the amount of the food-yolk was practically infinite as
compared with the protoplasm, it would absolutely cease to
advance. A second vertical furrow would soon be formed,
crossing the first at right angles, and like it not advancing
beyond the edge of the germinal disc. (Fig. 44 B.)

The next furrow should be an equatorial one (as a matter of
fact in the fowl's ovum an equatorial furrow is not formed till
after two more vertical furrows have appeared). The equatorial
furrow would however, in accordance with the analogy of the
frog, not be formed at the equator, but very close to the formative
pole. It would therefore separate off as a distinct segment (fig.
44 C, c), a small central, i.e. polar, portion of each of the imper-
fect segments formed
by the previous verti-
cal furrows. By a
continuation of the
process of segmenta-
tion, with the same
alternation of vertical
and equatorial furrows
as in the frog, a cap or
disc of small segments
would obviously be
formed at the proto-
plasmic pole of the
ovum, outside which
would be a number of
deep radiating grooves
(fig- 45), formed by
the vertical furrows,
the advance of which
round the ovum has come to an end owing to the too great pro-
portion of yolk spheres at the vitelline pole.

It is clear from the above that an immense accumulation of
food -yolk at the vitelline pole necessarily causes a partial seg-
mentation. It is equally clear that the part of meroblastic ova
which does not undergo segmentation is not a new addition

FIG. 45. SURFACE VIEW OF THE GERMINAL DISC

OF FOWL'S EGG DURING A LATE STAGE OF THE SEG-
MENTATION.

c. small central segmentation spheres ; b. larger
segments outside these ; a. large, imperfectly cir-
cumscribed, marginal segments ; e. margin of ger-
minal disc.

THE SEGMENTATION OF THE OVUM. IO5

absent in other cases. It is on the contrary to be regarded
merely as a part of the ovum in which the yolk spherules have
attained to a very great bulk as compared with the protoplasm ;
sometimes even to the complete exclusion of the protoplasm.

An ordinary meroblastic ovum consists then of a small disc
at the formative pole, known as the germinal disc, composed
mainly of protoplasm in which comparatively little food-yolk is
present This graduates into the remainder of the ovum, being
separated from it by a more or less sharp line. This remainder
of the ovum, which almost always forms the major part, usually
consists of numerous yolk spherules, embedded in a very scanty
protoplasmic matrix.

In some cases, e.g. the eggs of Elasmobranchii1, the protoplasm is pre-
sent in the form of a delicate network ; in other and perhaps the majority of
cases, too little protoplasm is present to be detected, or the protoplasm may
even be completely absent. In some Osseous Fishes, e.g. Lota, the yolk
forms a homogeneous transparent albuminoid substance containing a large
globule at the pole furthest removed from the germinal disc. In this case
the germinal disc is sharply separated from the yolk. In other Osseous
Fishes the separation between the two parts is not so sharp2. In these
cases we find adjoining the germinal disc a finely granular material con-
taining a large proportion of protoplasm ; this graduates into a material with
very little protoplasm and numerous yolk spherules, which is in its turn
continuous with an homogeneous albuminoid yolk substance. In Elasmo-
branchii we find that immediately beneath the germinal disc there is present
a finely granular matter, rich in protoplasm, which is continuous with the
normal yolk.

The Elasmobranch ovum may conveniently serve as type for the Verte-
brata. The ovum is formed of a spherical vitellus without any investing
membrane. The germinal disc is recognizable on this as a small yellow spot
about i^ millimetres in diameter. In the germinal disc a furrow appears
bisecting the disc, followed by a second furrow at right angles to the first.
Thus after the formation of the second furrow the disc is divided into four
equal areas. Fresh furrows continue to rise, and eventually a circular
furrow, equivalent to the equatorial furrow of the frog's ovum, makes its
appearance, and separates off a number of smaller central segments from
peripheral larger segments. In the later stages the smaller segments at first
divide more rapidly than the larger, but eventually the latter also divide
rapidly, and the germinal disc becomes finally formed of a series of segments

1 Vide Schultze, Archiv.f. mikr. Anat. Vol. XI.; and F. M. Balfour, Monograph
on the Development of Elasmobranch Fishes.

2 Vide Klein, Quart. Joitrnal of Micr. Science, April, 1876. Bambeke, Mem.
Cour. Acad. Belgique, 1875. His, Zeit.fiir Anat. u. Entwicklung. Vol. I.

106 NUCLEI OF THE YOLK.

of a fairly uniform size. So much may be observed in surface views of the
segmenting ovum, and it may be noted that there is not much difference to
be observed between the segmentation of the germinal disc of the Fowl's
ovum and that of the Elasmobranchii. Indeed the figure of the former (fig.
44) would serve fairly well for the latter. When however we examine
the segmenting germinal discs by means of sections, there are some dif-
ferences between the two types, and several interesting features which
deserve to be noticed in the segmentation of the Elasmobranchii. In the
first stages the furrows visible on the surface are merely furrows, which
do not meet so as to isolate distinct segments ; they merely, in fact, form a
surface pattern. It is not till after the appearance of the equatorial furrow
that the segments begin to be distinctly isolated. In the subsequent stages
not only do the segments already existing in the germinal disc increase by
division, but fresh segments are continually being formed from the adjacent
yolk, and added to those already present in the germinal disc. (Fig. 46.)

i I tffl

FIG. 46. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO DURING

THE SEGMENTATION.

n. nucleus; nx. nucleus modified prior to division; nx '. modified nucleus of the
yolk ; /. furrow appearing in the yolk adjacent to the germinal disc.

This fact is one out of many which prove that the germinal disc is merely
part of the ovum characterized by the presence of more protoplasm than the
remainder which forms the so-called food-yolk. During the latest stages of
segmentation there appear in the yolk around the blastoderm a number of
nuclei. (Fig. 46, nx'.} These are connected with a special protoplasmic
network (already described) which penetrates through the yolk. Towards
the end of segmentation, and during the early periods of development which
succeed the segmentation, these nuclei become very numerous. (Fig. 47
A, «'.) Around many of them a protoplasmic investment is established, and
cells are thus formed which eventually enter the blastoderm.

The result of segmentation is the formation of a lens-shaped mass of
cells lying in a depression on the yolk. In this a cavity appears, the
homologue of the segmentation cavity already spoken of. It lies at first in

THE SEGMENTATION OF THE OVUM. 107

the midst of the cells of the blastoderm, but very soon its floor of cells
vanishes, and it lies between the yolk and the blastoderm. (Fig. 47 A.) Its
subsequent history is given in a future Chapter.

Segmentation proceeds in Osseous Fishes in nearly the same manner as
in Elasmobranchii. In some cases the germinal disc is small as compared
with the yolk, in other cases it is almost as large as the yolk. The only
points which deserve special notice are the following : (i) Nuclei, precisely
similar to those in the Elasmobranch yolk, appear in the protoplasmic
matter around the germinal disc ; (2) After the deposition of the ova there is
present in some forms a network of protoplasm extending from the germinal
disc through the yolk1. At impregnation this withdraws itself from the
yolk. It is to be compared to the protoplasmic network of the Elasmo-
branch ovum.

FlG. 47. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PRISTIURUS
EMBRYO AT STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE.

ep. epiblast; //.lower layer cells; m. mesoblast; hy. hypoblast; sc. segmentation
cavity ; es. embryo swelling ; ri. nuclei of yolk ; er. embryonic rim.

There are two types of meroblastic ova. In one of these
(Aves, Elasmobranchii) the germinal disc is formed in the
ovarian ovum. In the second type the germinal disc is formed
after impregnation by a concentration of the protoplasm at one
pole. This concentration is analogous to what has already been
described for Anodon and other Molluscan ova (p. 100).

The ova of some Teleostei are intermediate between the two
types.

The ovum of the wood-louse, Oniscus murarius2, may be taken as an
example of the second type of meroblastic ovum. In this egg development
commences by the appearance of a small clear mass with numerous
transparent vesicles. This mass is the protoplasm which has become

1 Vide Bambeke, loc. cit.

2 Vide Bobretzky, Zeitschrift fur wiss. Zoologie, Vol. xxiv., 1874.

108 NUCLEI OF THE YOLK.

separated from the yolk. It undergoes segmentation in a perfectly
normal fashion. Examples of other cases of this kind have been described
by Van Beneden and Bessels1 in Anchorella, and in Hessia by Van
Beneden2. It appears from their researches that the protoplasm collects
itself together, first of all in the interior of the egg, and then travels to the
surface. It arrives at the surface after having already divided into two or
more segments, which then rapidly divide in the usual manner to form the
blastoderm.

There are some grounds for thinking that the cases of partial segmen-
tation in the Arthropoda are not really quite comparable with those in
other groups, but more probably fall under the next type of segmentation
to be described. The grounds for this view are mentioned in connection
with the next type.

In most if not all meroblastic ova there appear during and
after segmentation a number of nuclei in the yolk adjoining the
blastoderm, around which cells become differentiated. (Figs. 46
and 47.) These cells join the part of the blastoderm formed by
the normal segmentation of the germinal disc. Such nuclei are
formed in all craniate meroblastic ova3. In Cephalopods they
have been found by Lankester, and in Oniscus by Bobretzky.
They have been by some authors supposed to originate from the
nuclei of the blastoderm, and by others spontaneously in the
yolk.

Some of the earliest observations on these nuclei were made by Lankes-
ter4 in the Cephalopods. He found that they appeared first in a ring-
like series round the edge of the blastoderm, and subsequently all over the
yolk in a layer a little below the surface. He observed their development
in the living ovum and found that they " commenced as minute points, gra-
dually increasing in size like other free-formed nuclei." A cell area sub-
sequently forms around them.

By E. van Beneden5 they were observed in a Teleostean ovum to appear
nearly simultaneously in considerable numbers in the granular matter
beneath the blastoderm. Van Beneden concludes from the simultaneous
appearance of these bodies that they develop autogenously. Kupffer at an
earlier period arrived at a similar conclusion. My own observations on these
nuclei in Elasmobranchii on the whole support the conclusions to be derived
from Lankester's, Kupffer's and Van Beneden's observations. As mentioned
above, the nuclei in Elasmobranchii do not appear simultaneously, but

1 Loc. cit. 2 Bulletins de FAcad. Belgique, Tom. xxix., 1870.

* Though less obvious in the ovum of the fowl than in that of some other types,
they may nevertheless be demonstrated there without very much difficulty.
4 Quart. Journ. of Micr. Science, Vol. xv. pp. 39, 40.
6 Quart. Journ. of Micr. Science, Vol. xvm. p. 41.

THE SEGMENTATION OF THE OVUM. 109

increase in number as development proceeds ; and it is possible that Van
Beneden may be mistaken on this point. No evidence came before me 01
derivation from pre-existing nuclei in the blastoderm. My observations
prove however that the nuclei increase by division. This is shewn by the
fact that I have found them with the spindle modification (fig. 46, nx'\ and
that in most cases they usually exhibit the form of a number of aggregated
vesicles1, which is a character of nuclei which have just undergone division.
It should be mentioned however that I failed to find a spindle modification
of the nuclei in the later stages. Against these observations must be set
those of Bobretzky, according to which the nuclei in Oniscus are really the
nuclei of cells which have migrated from the blastoderm. Bobretzky's obser-
vations do not however appear to be very conclusive.

It must be admitted that the general evidence at our com-
mand appears to indicate that the nuclei of the yolk in mero-
blastic ova originate spontaneously. There is however a difficulty
in accepting this conclusion in the fact that all the other nuclei
of the embryo are descendants of the first segmentation nucleus ;
and for this reason it still appears to me possible that the nuclei
of the yolk will be found to originate from the continued
division of one primitive nucleus, itself derived from the segmen-
tation nucleus.

The existence of these nuclei in the yolk and the formation
of a distinct cell body around them is a strong piece of evidence
in favour of the view above maintained, (which is not universally
accepted,) that the part of the ovum of meroblastic ova which
does not segment is of the same nature as that which does
segment, and differs only in being relatively deficient in active
protoplasm.

The following forms have meroblastic ova of the first type : the Cephalo-
poda, Pyrosoma, Elasmobranchii, Teleostei, Reptilia, Aves, Ornithodelphia (?).
The second type of meroblastic segmentation occurs in many Crustacea,
(parasitic Copepoda, Isopoda Mysis, etc.). It is also stated to be found in
Scorpio.

The ova of the majority of groups in the animal kingdom
segment according to one of the types which have just been
described. These types are not sharply separated, but form an
unbroken series, commencing with the ovum which segments
uniformly, and ending with the meroblastic ovum.

1 At the time when my observations on Elasmobranchii were carried out, this
peculiar condition of the nucleus had not been elucidated.

I IO CENTROLECITHAL SEGMENTATION.

It is convenient to distinguish the ova which segment
uniformly by some term ; and I should propose for this the
term alecithal1, as implying that they are without food-yolk,
or that what little food-yolk there is, is distributed uniformly.

The ova in which the yolk is especially concentrated at one
pole I should propose to call telolecithal. They constitute
together a group with an unequal or partial segmentation.

The telolecithal ova may be defined in the following way :
ova in which the food-yolk is not distributed uniformly, but is
concentrated at one pole of the ovum. When only a moderate
quantity of food-yolk is present the pole at which it is concen-
trated merely segments more slowly than the opposite pole ; but
when food-yolk is present in very large quantity the part of the
ovum in which it is located is incapable of segmentation, and
forms a special appendage known as the yolk-sack.

There is a third group of ova including a series of types of
segmentation nearly parallel to the telolecithal group. This
group takes its start from the alecithal ovum as do the teloleci-
thal ova, and equally with these includes a series of varieties
of segmentation running parallel to the regular and unequal
types of segmentation which directly result from the presence
of a greater or smaller quantity of food-yolk. The food-yolk is
however placed, not at one pole, but at the centre of the ovum.
This group of ova I propose to name centrolecithal. It is
especially characteristic of the Arthropoda, if not entirely con-
fined to that group.

Centrolecithal ova. As might be anticipated on the analogy
of the types of segmentation already described, the concentration
of the food-yolk at the centre of the ovum does not always take
place before segmentation, but is sometimes deferred till even
the later stages of this process.

Examples of a regular segmentation in centrolecithal ova
are afforded by Palaemon (Bobretzky) and Penaeus (Haeckel).
A type of unequal segmentation like that of the Frog occurs in
Gammarus locusta (Beneden and Bessels), where however the
formation of a central yolk mass does not appear to take place

1 For this term as well as for the terms telolecithal and centrolecithal I am indebted
Mr l.ankester.

THE SEGMENTATION OF THE OVUM.

Ill

till rather late in the segmentation. More irregular examples of
unequal segmentation are also afforded by other Crustaceans,
e.g. various members of the genus Chondr acanthus (Beneden and
Bessels) and by Myriapods. In all these cases segmentation
ends in the formation of a layer of cells enclosing a central mass
of food-yolk.

The peculiarity of the centrolecithal ova with regular or un-
equal segmentation is that (owing to the presence of the yolk in
the interior) the furrows which appear on the surface are not

FIG. 48. SEGMENTATION OF A CRUSTACEAN OVUM (PEN^EUS). (After Hseckel.)

The sections illustrate the type of segmentation in which the yolk is aggregated at
the centre of the ovum.

yk. central yolk mass.

i and 2. Surface view and section of the stage with four segments. In 2 it
is seen that the furrows visible on the surface do not penetrate to the centre of the
ovum.

3 and 4. Surface view and section of ovum near the end of segmentation. The
central yolk mass is very clearly seen in 4.

continued to the centre of the egg. The spheres which are thus
distinct on the surface are really united internally. Fig. 48,
copied from Haeckel, shews this in a diagrammatic way.

Many ova, which in the later stages of segmentation exhibit
the characteristics of true centrolecithal ova, in the early stages
actually pass through nearly the same phases as holoblastic ova.

112 CENTROLECITHAL SEGMENTATION.

Thus in Eupagurus prideauxii* (fig. 49), and probably in the
majority of Decapods, the egg is divided successively into two,
four and eight distinct segments, and it is not till after the fourth
phase of the segmentation that the spheres fuse in the centre of
the egg. Such ova belong to a type which is really intermediate

FIG. 49. TRANSVERSE SECTION THROUGH FOUR STAGES IN THE SEGMENTATION OF
EUPAGURUS PRIDEAUXII. (After P. Mayer.)

between the ordinary type of segmentation and that with a
central yolk mass. Eupagurus presents one striking peculiarity,
viz. that the nucleus divides into two, four and eight nuclei, each
surrounded by a delicate layer of protoplasm prolonged into a
reticulum, before the ovum itself commences to become seg-
mented. The ovum before segmentation is therefore in the
condition of a syncytium.

The segmentation of Asellus aquaticus2 is very similar to that of Eupagu-
rus, etc. but the ovum at the very first divides into as many segments (viz.
eight) as there are nuclei.

In Gammarus locusta the resemblance to ordinary unequal segmentation
is very striking, and it is not till a considerable number of segments have
been formed that a central yolk mass appears.

1 Mayer, Jtnaische Zeitschrift, Vol. XI.

3 Ed. van Beneden, Butt, d. fAcad. roy. Bdgique, 2me serie, Tom. Xxvm. No. 7,
1869, p. 54.

THE SEGMENTATION OF THE OVUM.

In all the above types, as segmentation proceeds, the
protoplasm becomes more and more concentrated at the surface,
and finally a superficial layer of flat blastoderm cells is com-
pletely segmented off from the yolk below (fig. 49 D).

In cases like those of Penaeus, Eupagurus, etc., the yolk in
the interior is at first nearly homogeneous, but at a later period
it generally becomes divided up partially or completely into a
number of distinct spheres, which may have nuclei and therefore
have the value of cells. In many cases nuclei have however not
been demonstrated in these yolk spheres, though probably
present ; yet, till they have been demonstrated, some doubt
must remain on the nature of these yolk spheres. It is probable
that not all the nuclei which result from the division of the first
segmentation nucleus become concerned in the formation of the
superficial blastoderm, but that some remain in the interior of
the ovum to become the nuclei of the yolk spheres.

In Myriapods (Chilognatha) a peculiar form of segmentation has been

FIG. 50. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.

(After Metschnikoff.)

In A the ovum is divided into a number of separate segments. In B a number of
small cells have appeared (bl) which form a blastoderm enveloping the large yolk
spheres. In C the blastoderm has become divided into two layers.

B. II. 8

114 CENTROLECITHAL SEGMENTATION.

observed by Metschnikoff1. The ovum commences by undergoing a per-
fectly normal, though rather irregular total segmentation. But after the
process of division has reached a certain point, scattered masses of very
small cells make their appearance on the surface of the large spheres. These
small cells have probably arisen in a manner analogous to that which
characterizes the formation of the superficial cells of the blastoderm in the
types of centrolecithal ova already described. They rapidly increase in
number and eventually form a continuous blastoderm; while the original
large segments remain in the centre as the yolk mass. In the interesting
Arachnid CJulifer segmentation takes place in nearly the same manner as in
Myriapods (fig. 50).

It is clear that it is not possible in centrolecithal ova to have
any type of segmentation exactly comparable with that of
meroblastic ova. There are however some types which fill the
place of the meroblastic ova in the present group, in as much as
they are characterised by the presence of a large bulk of food-yolk
which either does not segment, or does not do so till a very late
stage in the development. The essential character of this type of
segmentation consists in the division of the germinal vesicle in

FIG. 51. FOUR SUCCESSIVE STAGES IN THE SEGMENTATION OF THE EGG OF TETRA-
NYCHUS TELARIUS. (After ClaparMe.)

the interior, or at the surface of the ovum into two, four, etc.
nuclei (fig. 51). These nuclei are each of them surrounded by a
specially concentrated layer of protoplasm (fig. 51) which is

1 Zeitschrift fur wiss. Zoo/., Vol. xxiv. 1874.

THE SEGMENTATION OF THE OVUM. 115

continuous with a general protoplasmic reticulum passing
through the ovum [not shewn in fig. 51]. The yolk is contained
in the meshes of this reticulum in the manner already described
for other o.va.

The ovum, like that of Eupagurus before segmentation, is
now a syncytium. Eventually the nuclei, having increased by
division and become very numerous, travel, unless previously
situated there, to the surface of the ovum. They then either
simultaneously or in succession become, together with protoplasm
around them, segmented off from the yolk, and give rise to a
peripheral blastoderm enclosing a central yolk mass. In the
latter however many of the nuclei usually remain, and it also
very often undergoes a secondary segmentation into a number
of yolk spheres.

The eggs of Insects afford numerous examples of this mode
of segmentation, of which the egg of Porthesia1 may be taken as
type. After impregnation it consists of a central mass of yolk
which passes without a sharp line of demarcation into a peripheral
layer of more transparent (protoplasmic) material. In the
earliest stage observed by Bobretzky there were two bodies in
the interior of the egg, each consisting of a nucleus enclosed in a
thin protoplasmic layer with stellate prolongations. This stage
corresponds with the division into two, but though the nucleus
divides, the preponderating amount of yolk prevents the egg
from segmenting at the same time. By a continuous division
of the nuclei there becomes scattered through the interior of the
ovum a series of bodies, each formed of nucleus and a thin layer
of protoplasm with reticulate processes. After a certain stage
some of these bodies pass to the surface, simultaneously (in
Porthesia) or in some cases successively. At the surface the
protoplasm round each nucleus contracts itself into a rounded
cell body, distinctly cut off from the adjacent yolk.

The cells so formed give rise to a superficial blastoderm of a
single layer of cells. Many of the nucleated bodies remain in
the yolk, and after a certain time, which varies in different forms,
the yolk becomes segmented up into a number of rounded or
polygonal bodies, in the interior of each of which one of the

Bobretzky, Zeit.f. wiss. Z00/.,-Bd. xxxi. 1878.

8—2

CENTROLECITHAL SEGMENTATION.

above nuclei with its protoplasm is present. This process,
known as the secondary segmentation of the yolk, is really part
of the true segmentation, and the bodies to which it gives rise
are true cells.

Other examples of this type may be cited. In Aphis1 Metschnikoff
shewed that the first segmentation nucleus divides into two, each of which
takes up a position in the clearer peripheral protoplasmic layer of the egg
(fig. 52, i and 2). Following upon further division the nuclei enveloped in a
continuous layer of protoplasm arrange themselves in a regular manner, and
form a syncytium, which becomes segmented into definite cells (fig. 52, 3 and
4). The existence of a special clear superficial layer of protoplasm has been
questioned by Brandt.

FIG. 57. SEGMENTATION OF APHIS ROSAE. (Copied from Metschnikoff.)
In all the stages there is seen to be a central yolk mass surrounded by a layer of

protoplasm.

In this protoplasm two nuclei have appeared in i, four nuclei in 2. In 3 the nuclei

have arranged themselves regularly, and in 4 the protoplasm has become divided into

a number of columnar cells corresponding to the nuclei.

TV. pole of the blastoderm which has no share in forming the embryo.

In Tetranychus telarius, one of the mites, Claparede found on the surface
of the ovum a nucleus surrounded by granular protoplasm (fig. 51) ; which
is no doubt the first segmentation nucleus. By a series of divisions, all
on the surface, a layer of cells becomes formed round a central yolk mass.
The result here is the same as in Insects, but the nucleus with its granular
protoplasm is from the first superficial. In other cases, such as that of the
common fly2, a layer of protoplasm is stated to appear investing the yolk ;
and in this there arise simultaneously (?) a number of nuclei at regular inter-
vals, around each of which the protoplasm separates itself to form a distinct
cell. Closely allied is the type observed by Kowalevsky in Apis. Develop-
ment here commences by the appearance of a number of protoplasmic

1 Metschnikoff, " Embry. Stud. Insecten," Zcit. fur wiss. Zoo!., Bd. xvi. 1866.
My own observations on this form accord in the main with those of Metschnikoff.

2 Vide Weismann, Entwicklung d. Dipteren; and Auerbach, Organologische
Studien.

THE SEGMENTATION OF THE OVUM.

117

prominences, each forming a cell provided with a nucleus, the nuclei having
no doubt been formed by previous division in the interior of the ovum.
They appear at the edge of the yolk, and are separated from one another by
short intervals. Shortly after their appearance a second batch of similar
bodies appears, filling up the interspaces between the first-formed promi-
nences. In the fresh-water Gammarus fluviatilis the protoplasm is stated
first of all to collect at the centre of the ovum, where no doubt the segmenta-
tion nucleus divides. Subsequently cells appear at numerous points on the
surface, and by repeated division constitute an uniform blastoderm investing
the central yolk mass. This mode of formation of the blastoderm is closely
allied to that observed by Kowalevsky in Apis.

Between ova with a segmentation like that of Insects, and
those with a segmentation like that of Penaeus, there is more
than one intermediate form. The Eupagurus type, with the
division of the first nucleus into eight nuclei before the division

FIG. 53. THREE STAGES IN THE SEGMENTATION OF PHILODROMUS LIMBATUS.
(After Hub. Ludwig.)

of the ovum, must be regarded in this light ; but the most
instructive example of such a transitional type of segmentation
is that afforded by Spiders1.

The first phenomenon which can be observed after impreg-
nation is the conglomeration of the yolk spheres into cylindrical
columns, which finally assume a radiating form diverging from
the centre of the egg. In the centre of the radiate figure is a
protoplasmic mass, probably containing a nucleus, which sends

i Vide Ludwig, Zeit.f. wiss. Zool., 1876.

Il8 CENTROLECITHAL SEGMENTATION.

out protoplasmic filaments through the columns (fig. 53 A). After
a certain period of repose the figure becomes divided into two
rosette-like masses, which remain united for some time by a proto-
plasmic thread : this thread is finally ruptured (fig. 53 B). The
whole egg does not in this process divide into two segments, but
merely the radiate figure, which is enclosed in a finely granular
material. The two rosettes next become simultaneously divided,
giving rise to four rosettes (fig. 53 C) : and the whole process is
repeated with the same rhythm as in a regular segmentation
till there are formed thirty-two rosettes in all (fig. 54 A). The
rosettes by this time have become simple columns, which by
mutual pressure arrange themselves radiately around the centre
of the egg, which however they do not quite reach.

When only two rosettes are present the protoplasm with its
nucleus occupies a central position in each rosette, but gradually,
in the course of the subsequent subdivisions, it travels towards
the periphery, and finally occupies, when the stage with thirty-
two rosettes is reached, a peripheral position. The peripheral
protoplasm next becomes separated off as a nucleated layer

FIG. 54. SURFACE VIEW AND OPTICAL SECTION OF A LATE STAGE IN THE

SEGMENTATION OF PHILODROMUS LIMBATUS (Koch). (After Hub. Ludwig.)

bl. blastoderm ; yk. yolk spheres.

(fig- 54 B). It forms the proper blastoderm, and in it the nuclei
rapidly multiply and finally around each an hexagonal or
polygonal area of protoplasm is marked off; and a blastoderm,
formed of a single layer of flattened cells, is thus constituted.
The columns within the blastoderm now form (fig. 54 B) more
or less distinct masses, which are stated by Ludwig to be with-
out protoplasm.

THE SEGMENTATION OF THE OVUM. 119

From observations of my own I am inclined to differ from Ludwig as to
the nature of the parts within the blastoderm. My observations have been
made on Agelena labyrinthica and commence at the close of the segmenta-
tion. At this time I find a superficial layer of flattened cells, and within
these a number of large polyhedral yolk cells. In many, and I believe all,
of the yolk cells there is a nucleus surrounded by protoplasm. It is generally
placed at one side and not in the centre of a yolk cell, and the nuclei are so
often double that I have no doubt they are rapidly undergoing division. It
appears to me probable that, at the time when the superficial layer of proto-
plasm is segmented off from the yolk below, the nuclei undergo division, and
that a nucleus with surrounding protoplasm is left with each yolk column.
For further details vide Chapter on Arachnida.

Although by the close of the segmentation the protoplasm
has travelled to a superficial position, it may be noted that at
first it forms a small mass in the centre of the egg, and only
eventually assumes its peripheral situation. It is moreover clear
that in the Spider's ovum there is, so to speak, an attempt at a
complete segmentation, which however only results in an
arrangement of the constituents of the ovum in masses round
each nucleus, and not in a true division of the ovum into distinct
segments.

It seems very probable that Ludwig's observations on the segmentation
of Spiders only hold good for species with comparatively small ova.

In connection with the segmentation of the Insects' ovum and allied
types it should be mentioned that Bobretzky, to whose observations we are
largely indebted for our knowledge of this subject, holds somewhat different
views from those adopted in the text. He regards the nuclei surrounded by
protoplasm, which are produced by the division of the primitive segmenta-
tion nucleus, as so many distinct cells. These cells are supposed to move
about freely in the yolk, which acts as a kind of intercellular medium. This
view does not commend itself to me. It is opposed to my own observations
on similar nuclei in the Spiders. It does not fit in with our knowledge of the
nature of the ovum, and it cannot be reconciled with the segmentation
of such types as Spiders or even Eupagurus, with which the segmentation in
Insects is undoubtedly closely related.

The majority if not all the cases in which a central yolk mass is formed
occur in the Arthropoda, in which group centrolecithal ova are undoubtedly
in a majority. In Alcyonium palmatum the segmentation appears however
to resemble that of many insects.

One or two peculiar varieties in the segmentation of ova of this type
may be spoken of here. The first one I shall mention is detailed in the
important paper of E. Van Beneden and Bessels which I have already so
often had occasion to quote : it is characteristic of the eggs of most of the

120 SUMMARY.

species of Chondracanthus, a genus of parasitic Crustaceans. The ovum
divides in the usual way but somewhat irregularly into 2, 4, 8 segments
which meet in a central yolk mass ; but after the third division instead of
each segment dividing into two equal parts it divides at once into four, and
the division into four having started, reappears at every successive division.
Thus the number of the segments at successive periods is 2, 4, 8, 32, 128, etc.
In another peculiar case, an instance of which1 is afforded by Asellus aqua-
ticus, after each of the earlier segmentations all the segments fuse and
become indistinguishable, but at the succeeding segmentation double the
number of segments appears.

Although, as has been already stated, it does not seem possible to have a
true meroblastic segmentation in centrolecithal ova, it does nevertheless
appear probable that the apparent cases of a meroblastic segmentation in
the Arthropoda are derivatives of this type of segmentation. The manner
in which the one type might pass into the other may perhaps be explained
by the segmentation in Asellus aquaticus^. In this ovum large segments
are at first formed around a central yolk mass, in the peculiar manner men-
tioned in the previous paragraph, but at the close of the first period of seg-
mentation minute cells, which eventually form a superficial blastoderm, are
produced from the yolk cells. They do not however appear at once round
the whole periphery of the egg, but at first only on the ventral surface and
later on the dorsal surface. If the amount of food-yolk in the egg were
to increase so as to render the formation of the yolk cells impossible, and at
the same time the formation of the blastodermic cells were to take place at
the commencement, instead of towards the close of the segmentation, a mass
of protoplasm with a nucleus might first appear at the surface on the future
ventral side of the egg, then divide in the usual way for meroblastic ova, and
give rise to a layer of cells gradually extending round to the dorsal surface.
A meroblastic segmentation might perhaps be even more easily derived from
the type found in Insects. It is probable that the cases of Scorpio, Mysis,
Oniscus, the parasitic Isopoda, and some parasitic Copepoda belong to this
category ; and it may be noticed that in these cases the blastopore would be
situated on the dorsal and not on the ventral side of the ovum. The mor-
phological importance of this latter fact will appear in the sequel.

The results arrived at in the present section may be shortly
restated in the following way.

(i) A comparatively small number of ova contain very
little or no food-yolk embedded in their protoplasm; and have
what food-yolk may be present distributed uniformly. In such
ova the segmentation is regular. They may be described as
alecithal ova.

1 Ed. van Beneden, Bull. Acad. Belgique, Vol. xxvm. 1869.

THE SEGMENTATION OF THE OVUM. 121

(2) The distribution of food-yolk in the protoplasm of the
ovum exercises an important influence on the segmentation.

The rapidity with which any part of an ovum segments varies
ceteris paribus with the relative amount of protoplasm it contains;
and the size of the segments formed varies inversely to the
relative amount of protoplasm. When the proportion of pro-
toplasm in any part of an ovum becomes extremely small,
segmentation does not occur in that part.

Ova with food-yolk may be divided into two great groups
according to the eventual arrangement of the food-yolk in the
protoplasm. In one of these, the food-yolk when present is
concentrated at the vegetative pole of the ovum. In the other
group it is concentrated at the centre of the ovum. Ova belong-
ing to the former group are known as telolecithal ova, those to
the latter as centrolecithal.

In each group more than one type may be distinguished. In
the first group these types are (i) unequal segmentation, (2)
partial segmentation. The features of these three types have
been already so fully explained that I need not repeat them here.

In the second group there are three distinct types, (i) equal
segmentation, (2) unequal segmentation. These two being ex-
ternally similar to the similarly named types in the first group.
(3) Superficial segmentation. This is unlike anything which is
present in the first group, and is characterized by the appearance
of a superficial layer of cells round a central yolk mass. These
cells may either appear simultaneously or successively, and their
nuclei are derived from the segmentation within the ovum of the
first segmentation nucleus.

The types of ova in relation to the characters of the segmen-
tation may be tabulated in the following way :

Segmentation.

(1) alecithal )

v ' regular

ova j

(2) telolecithal \ (a) unequal

ova J (b) partial

, . N (a) regular (with segments united in

(3) centre- | v ' B '

, .,, , central yolk mass)

lecithal > /

ova W une(lual " » » »

(c) superficial.

122 SUMMARY.

Although the various types of segmentation which have been
described present very different aspects, they must nevertheless
be looked on as manifestations of the same inherited tendency
to division, which differ only according to the conditions under
which the tendency displays itself.

This tendency is probably to be regarded as the embryologi-
cal repetition of that phase in the evolution of the Metazoa,
which constituted the transition from the protozoon to the
metazoon condition.

From the facts narrated in this chapter the reader will have
gathered that similarity or dissimilarity of segmentation is no
safe guide to affinities. In many cases, it is true, a special type
of segmentation may characterize a whole group ; but in other
cases very closely allied animals present the greatest differences
with respect to their segmentation ; as for instance the different
species of the genus Gammarus. The character of the segmen-
tation has great influence on the early phenomena of develop-
ment, though naturally none on the adult form.

EXTERNAL FEATURES OF SEGMENTATION.

(105) E. Haeckel. "Die Gastrula u. Eifurchung." Jenaische Zeitschrift,
Vol. IX. 1877.

(106) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d.
Thierwelt u. n. ihrer Bedeutung." Oken his. 1848.

PART I.

SYSTEM A TIC EMBR YOL OG Y.

PART I.
SYSTEMATIC EMBRYOLOGY.

INTRODUCTION.

IN all the Metazoa the segmentation is followed by a series
of changes which result in the grouping of the embryonic cells
into definite layers, or membranes, known as the germinal
layers. There are always two of these layers, known as the
epiblast and hypoblast; and in the majority of instances
a third layer, known as the mesoblast, becomes interposed
between them. It is by the further differentiation of the ger-
minal layers that the organs of the adult become built up.
Owing to this it is usual, in the language of Embryology, to
speak of the organs as derived from such or such a germinal
layer.

At the close of the section of this work devoted to systematic
embryology, there is a discussion of the difficult questions which
arise as to the complete or partial homology of these layers
throughout the Metazoa, and as to the meaning to be attached
to the various processes by which they take their origin ; but a
few words as to the general fate of the layers, and the general
nature of the processes by which they are formed, will not be
out of place here.

Of the three layers the epiblast and hypoblast are to be
regarded as the primary. The epiblast is essentially the primi-
tive integument, and constitutes the protective and sensory
layer. It gives rise to the skin, cuticle, nervous system, and
organs of special sense. The hypoblast is essentially the diges-
tive and secretory layer, and gives rise to the epithelium lining
the alimentary tract and the glands connected with it.

126 INTRODUCTION.

The mesoblast is only found in a fully developed condition
in the forms more highly organized than the Coelenterata. It
gives origin to the general connective tissue, internal skeleton,
the muscular system, the lining of the body-cavity, the vascular,
and excretory systems. It probably in the first instance origi-
nated from differentiations of the two primary layers, and in all
groups with a well-developed body-cavity it is divided into two
strata. One of them forms part of the body-wall and is known
as the somatic mesoblast, the other forms
part of the wall of the viscera and is known
as the splanchnic mesoblast.

A very large number not to say the
majority of organs are derived from parts of
two of the germinal layers. Many glands
for instance have a lining of hypoblast which
is coated by a mesoblastic layer.

The processes by which the germinal
layers take their origin are largely influenced
by the character of the segmentation, which, FIG DIAGRAM
as was shewn in the last chapter, is mainly OF A GASTRULA.
dependent on the distribution of the food- ™m bl?stopore; b.

yolk. When the segmentation is regular, archenteron; c. hypo-
' blast ; d. epiblast.

and results in the formation of a blastosphere,
the epiblast and hypoblast are usually differentiated from the
uniform cells forming the wall of the blastosphere in one of the
two following ways.

(1) One-half of the blastosphere may be pushed in towards
the other half. A two-layered hemisphere is thus established
which soon elongates, while its opening narrows to a small pore
(fig- 55)- The embryonic form produced by this process is
known as a gastrula. The process by which it originates is
known as embolic invagination, or shortly invagination. Of
the two layers of which it is formed the inner one (c) is known as
the hypoblast and the outer (d} as the epiblast, while the pore
leading into its cavity lined by the hypoblast is the blastopore
(a). The cavity itself is the archenteron (b}.

(2) The cells of the blastosphere may divide themselves by
a process of concentric splitting into two layers (fig. 56, 3). The
two layers are as before the epiblast and hypoblast, and the

SYSTEMATIC EMBRYOLOGY.

127

process by which they originate is known as delamination.
The central cavity or archenteron (F) is in the case of delamina-
tion the original segmentation cavity ; and not an entirely new
cavity as in the case of invagination. By the perforation of the
closed two-walled vesicle resulting from delamination an embry-

Fig.z

FIG. 56. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION.

(From Lankester.)
Fig. i. Ovum.

Fig. 2. Stage in segmentation.

Fig. 3. Commencement of delamination after the appearance of a central cavity.
Fig. 4. Delamination completed, mouth forming at M.
In fig. i, 2 and 3 EC. is ectoplasm, and En. is entoplasm.
In fig. 4 EC. is epiblast and En. hypoblast.

onic form is produced which cannot be distinguished in structure
from the gastrula produced by invagination (fig. 56, 4). The
opening (M) in this case is not however known as the blastopore
but as the mouth.

When segmentation does not take place on the regular type
the processes above described are as a rule somewhat modified.
The yolk is usually concentrated in the cells which would, in
the case of a simple gastrula, be invaginated. As a consequence
of this, these cells become (i) distinctly marked off from the
epiblast cells during the segmentation ; and (2) very much
more bulky than the epiblast cells. The bulkiness of the

128

INTRODUCTION.

ms

7,y

hypoblast cells necessitates a
modification of the normal pro-
cess of embolic invagination,
and causes another process to
be substituted for it, viz. the
growth of the epiblast cells as
a thin layer over the hypoblast.
This process (fig. 57) is known
as epibolic invagination.
The point where the complete
enclosure of the hypoblast cells
is effected is known as the blas-
topore. All intermediate con-
ditions between epibolic and
embolic invagination have been found.

In delamination, when the segmentation is not uniform, or
when a solid morula is formed, the differentiation of the epiblast
and hypoblast is effected by the separation of the central solid
mass of cells from the peripheral cells (fig. 58 A).

FIG. 57. TRANSVERSE SECTION

THROUGH THE OVUM OF EUAXES
DURING AN EARLY STAGE OF DEVELOP-
MENT. (After Kowalevsky.)

ep. epiblast; ms. mesoblastic band;
hy. hypoblast.

FIG. 58. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM.

(After Metschnikoff.)

A. Stage after the delamination. ep. epiblastic invagination to form pneuma-
tocyst.

B. Later stage after the formation of the gastric cavity in the solid hypoblast,
po. polypite ; t. tentacle ; pp. pneumatophore ; ep. epiblastic invagination to form
pneumatocyst ; hy. hypoblast surrounding pneumatocyst.

SYSTEMATIC EMBRYOLOGY. 1 29

In the case of epibolic invagination as well as in that of the
type of delamination just spoken of, the archenteric cavity is in
most cases secondarily formed in the solid mass of hypoblast
(fig. 58 B).

In ova with a partial segmentation there is usually some
modification of the epibolic gastrula.

Many varieties are found in the animal kingdom of the types
of invagination and delamination just characterized, and in not
a few forms the layers originate in a manner which cannot
be brought into connection with either of these processes.

FIG. 59. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)

A. Stage when the four hypoblast cells are nearly enclosed.

B. Stage after the formation of the mesoblast has commenced by an infolding of
the lips of the blastopore.

ep. epiblast ; me. mesoblast ; bl. blastopore.

The mesoblast usually originates subsequently to the two
primary layers. It then springs from one or both of the other
layers, but its modes of origin are so various that it would be
useless to attempt to classify them here. In cases of invagination
it often arises at the lips of the blastopore (fig. 57 and 59), and
in other cases part of it springs as paired hollow outgrowths of
the walls of the archenteron. Such outgrowths are shewn in
fig. 60, B and C at pv. The cavity of the outgrowths forms the
body cavity, and the walls of the outgrowths the somatic and
splanchnic layers of mesoblast (fig. C. sp. and so.). The archen-
teron is in part always converted into a section of the permanent
alimentary tract; and the section of the alimentary tract so
derived is known as the mesenteron. There are however
usually two additional parts of the alimentary tract, known as
B. II. 9

130 INTRODUCTION.

FIG. 60. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
Butschli and B after Kowalevsky.) The three embryos are represented in the same
positions.

A. Represents the gastrula stage.

B. Represents a succeeding stage in which the primitive archenteron is com-
mencing to be divided into three parts, the two lateral of which are destined to form
the mesoblast.

C. Represents a later stage in which the mouth involution (»/) has become con-
tinuous with alimentary tract, and the blastopore has become closed.

m. mouth ; al. alimentary canal ; ae. archenteron ; bl. p. blastopore ; pv. perivis-
ceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs.

the stomodaeum and proctodaeum, derived from epiblastic
imaginations. They give rise respectively to the oral and anal
extremities of the alimentary tract.

BIBLIOGRAPHY.

(107) K. E. von Baer. " Ueb. Entwicklungsgeschichte d. Thiere." Konigs-
berg, 1828—1837.

(108) C. Claus. Griindzilge d. Zoologie. Marburg und Leipzig, 1879.

(109) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also Translation. Elements of Comparative Anatomy. Macmillan and Co.,
1878.

(110) E. Haeckel. Studien z, Gastraa-Theorie. Jena, 1877, and dsojenaischc
Zeitschrift, Vols. vin. and ix.

(111) E. Haeckel. Schbpfungsgeschichte. Leipzig. Vide also Translation.
The History of Creation, King and Co., London, 1876.

(112) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation. Anthro-
Pogeny (Translation). Kegan Paul and Co., London, 1878.

(113) Th. H. Huxley. The Anatomy of Invcriebratcd Animals. Churchill,
1877.

(114) E. R. Lankester. "Notes on Embryology and Classification." Quart.
J. of. Micr. Science, Vol. xvn. 1877.

(115) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
of Comparative Embryology. Holt and Co., New York, 1876.

(116) H. Rathke. Abhandlungen 2. Bildung- und Enhvicklungsgesch. d.
Menschen u. d. Thiere. Leipzig, 1833.

CHAPTER IV.

DICYEMID.E AND ORTHONECTID^.

DlCYEMHXE.

THE structure and development of these remarkable para-
sites in the renal organs of the Cephalopoda have recently been
greatly elucidated by the researches of E. van Beneden ; and
although a male element has not been discovered, yet the
embryos originate from bodies which have a close similarity to
ordinary ova.

Van Beneden has shewn that Dicyema consists in the adult
state of (i) a single layer of ciliated epiblast cells, somewhat
modified anteriorly to form a cephalic enlargement; and of (2)
one large nucleated hypoblast cell enclosed within the epiblast.
There are two kinds of embryo, both developed from germs
which originate in the hypoblast cell. The two kinds of embryo
arise in individuals of somewhat different forms. The one kind,
called by Van Beneden the vermiform embryo, arises in the
more elongated and thinner examples of Dicyema which have
been named Nematogens. These embryos pass directly into
the parent form without metamorphosis.

The second kind of embryo, called infusoriform, is very
different from the parent, and has a free existence. Its eventual
history is not known. It originates in the shorter and thicker
individuals of Dicyema; which have been called Rhombogens.

The Vermiform Embryos. The germs or cells which give
rise to the vermiform embryos originate endogenously in the
protoplasmic reticulum of the axial hypoblast cell. They appear
as small but well-defined spheres, with a minute body inj;he

9—2

132

DICYEMID^E.

centre. In these spheres a cortical layer becomes differentiated,
which gradually increases in thickness and gives rise to the body
of a cell, the nucleus and nucleolus of which are respectively
formed from the inner part of the original sphere and the minute
central body. These germs can originate in all parts of the
hypoblast cell and are frequently very numerous.

The germ when completely formed undergoes a segmentation
very similar to that of an ordinary ovum. It divides first into
two and then into four approximately equal segments. Of the
four segments one, however, remains passive for the remainder
of the development. The other three divide and arrange them-
selves so as partially to enclose in a cup-like fashion the passive
cell (fig. 6 1 A). The six cells resulting
from their division again divide, giving
rise to twelve cells, which nearly enclose
the passive cell, leaving only a small
aperture at one point. The whole pro-
cess by which the central cell becomes
enclosed is, as E. van Beneden points

out, identical with a gastrula formation

, FIG. 61. A. GASTRULA

by epibole, and the space where the STAGE OF DICYEMA TYPUS.

central cell is left uncovered is the bias-

topore. The central cell itself gives Gegenbaur, after E. van

origin to the hypoblast cell of the Beneden-)
adult, and the peripheral cells to the epiblast.

By this time the embryo has assumed an oval form, and the
blastopore is situated at the pole of the long axis of the oval
where the cephalic enlargement is eventually formed.

The subsequent development consists mainly in the closure
of the blastopore, and an increase in the number of the epiblast
cells. Before the development is completed, and while the
embryo is still in the body of the parent, two germs, destined
themselves to give rise to fresh embryos, appear in the hypoblast
cell, one on each side of the nucleus (fig. 61 B). The embryo
continues to elongate, while the anterior cells become converted
into the polar cells. Cilia appear simultaneously over the
general surface, and the embryo makes its way out of the body
of the parent, usually at the cephalic pole, and becomes itself
parasitic in the renal organ of the host in which it finds itself.

INFUSORIFORM EMBRYOS. 133

At the time of birth the embryo may contain a number of germs
and sometimes even developing embryos.

Infusoriform Embryos. The infusoriform embryos are
capable of living in sea-water and almost certainly lead a free
existence. In their most fully developed condition so far known
they have the following rather complicated structure (fig. 62 D,
E, F, G).

The body is somewhat pyriform, with a blunt extremity
which is directed forwards in swimming, and a more pointed
extremity directed backwards. The former may be spoken of as
the anterior, and the latter as the posterior extremity or tail.
At the anterior extremity are situated a pair of refractive bodies
(f) which lie above an unpaired organ which may be called the
urn.

The structure of the urn, the refractive bodies, and the tail
may be dealt with in succession.

The urn consists of three parts: (i) a wall (#), (2) a lid (/),
and (3) contents (gr). The wall of the urn is hemispherical in
form, and composed of two halves in apposition (fig. F). Its
concavity is directed forwards, and in its edge are imbedded a
number of rod-like corpuscles which appear as a ring near the
surface in a full-face view (fig. D). The lid has the form of a
low pyramid with its apex directed outwards. It is made up of
four segments (fig. D). The contents of the urn, which com-
pletely fill up its cavity, are four polynuclear cells arranged in the
form of a cross which appear with low powers as granular bodies
(fig. F). They are frequently ejected, apparently at the will of
the embryo.

The refractive bodies (r), two in number, one on each side of
the middle line, are composed of a material which is not of a
fatty nature, and which is passive to the majority of reagents.
Each is enveloped in a special capsule, and at times more than
one refractive body is present in each capsule. The tail is a
conical structure formed of ciliated granular cells.

No plausible guess has been made as to the function either of
the urn or of the refractive bodies.

The infusoriform embryos originate from germs, which have
however a different origin to the germs of the vermiform
embryos. One to five cells appear in the axial hypoblast cell, in

134

a way not clearly made out, and each of them gives rise by an
endogenous process to several generations of cells, all of which
develop into infusoriform embryos.

FIG. 62. INFUSORIFORM EMBRYO OF DICYEMA.

A. B. C. Three of the later stages in the development.

D. E. F. Three different views of the full-grown larva. D. from the front, E.
from the side, and F. from above.

G. side view of urn.

u. wall of urn ; /. lid of urn ; r. refractive bodies ; gr. granular bodies filling the
interior of the urn.

The primitive cell is called by Van Beneden a Germogen.
In its protoplasm a number of germs first appear endogenously,
but the nucleus of the germogen does not assist in their forma-
tion. They eventually become detached from the parent cell,
around which they are concentrically arranged. A second and
then a third generation of germs are formed in the same way, till
the whole of the protoplasm of the primitive cell is absorbed in
the formation of these germs, and nothing of it remains but the
nucleus. The germs so formed are arranged in about three con-
centric layers, of which the innermost is the youngest. One to
five masses of germs may be present in a single Rhombogen.
The germs undergo a division, in the course of which their nuclei
exhibit very beautifully a spindle modification. In the course of
the segmentation the embryo gradually assumes its permanent
form, and four of the cells composing it can be distinguished
from the remainder by their greater size (fig. 62 A, ;/). The two
largest of these give rise to the wall of the urn, and also give
origin to four smaller cells (fig. 62 B, gr) which eventually be-
come polynuclear and constitute the four granular cells in the
urn. The two other cells become the lid of the urn. The parts

INFUSORIFORM EMBRYOS. 135

of the urn lie at first side by side, but in the course of develop-
ment the cells which form the wall of the urn travel inwards, and
the four granular cells are carried into their concavity. At the
same time the cells which form the lid of the urn alter their
position so as to overlie the wall of the urn. The two cells
immediately above the urn give rise to the refractive bodies
(fig. 62 A, B, C, r) and the remainder of the cells of the embryo
become the tail (fig. 62 C). The embryo becomes ciliated, and
attains its nearly full development before leaving the parental
tissues. It usually passes out at the cephalic extremity.

As has already been stated, it is probable that the infusori-
form embryos leave the renal organs of their host and lead a free
existence. What becomes of them afterwards is not however
known, though there can be little doubt that they serve to carry
the species to new hosts.

Till the further development of the infusoriform embryo is
known it is not possible to arrive at a definite conclusion as to
the affinities of this strange parasite. Van Beneden is anxious
to form it, on account of its simple organization, into a group
between the Protozoa and the Metazoa. It appears however
very possible that the simplicity of its organization is the result
of a parasitic existence ; a view which receives confirmation from
the common occurrence of the process of endogenous cell-forma-
tion in the axial hypoblast cell. It has been clearly shewn by
Strasburger that endogenous cell-formation is secondarily
derived from cell-division ; so that the occurrence of this pro-
cess in Dicyema probably indicates that the hypoblast was primi-
tively multicellular. It is not improbable that the enigmatical
infusoriform embryo may develop into a sexual form, the pro-
geny of which are destined to complete the cycle of develop-
ment by becoming again parasitic in the renal organ of a
Cephalopod.

BIBLIOGRAPHY.

(117) E. van Beneden. " Recherches sur les Dicyemides." Bull. d. FAca-
dtmie roy. de Belgique, i" ser. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this
paper for a full account of the literature.

(118) A. Kolliker. Ueber Dicyema paradoxum den Schmarotzer der Venenan-
hdnge der Cephalopoden.

(119) Aug. Krohn. "Ueb. d. Vorkommen von Entozoen, etc." Froriep
Notizen, vii. 1839.

136 ORTHONECTIM:.

ORTHONECTIM;.

A number of minute parasites infesting various Nemertines, Turbella-
rians, and Ophiuroids have recently been studied by Giard and Metschnikoff,
the former of whom has placed them in a special group which he calls the
Orthonectidae. They were first discovered by W. C. Mclntosh.

In the adult state they are1 (Metschnikoff) somewhat pear-shaped bodies
formed of a kind of plasmodium of cells with irregular lobate processes.
In the interior of this body are eggs in all stages of development.
In the type observed by Metschnikoff (Intoshia gigas) the ova undergo
a regular segmentation, resulting in the formation of a blastosphere
in which an inner layer is subsequently formed by delamination. A
smaller and a larger kind of embryo are formed ; but all the embryos
in each female belong to one type. The larger become females and the
smaller males.

The female embryos are ovoid. The outer layer of cells or epiblast
becomes ciliated, and divided into nine segments, of which the second
is marked off from the remainder by the absence of cilia, and by being
provided with refractive corpuscles. The inner layer which surrounds
a central cavity, and might be supposed to be the hypoblast, becomes
according to Metschnikoff converted into ova.

The male embryos are more elongated than the female, from which they
further differ in only having six segments. The cells of the inner layer
eventually divide up into spermatozoa.

The larva} probably become free, and while in the free state impregna-
tion would appear to be effected. When the female larvae become parasitic
they undergo a metamorphosis, the stages of which have not been observed ;
but in the course of which the epiblast cells probably unite into a plasmo-
dium.

The observations of Giard are in several points irreconcilable with those
of Metschnikoff, but from the statements of the latter it appears possible
that Giard has made two genera from the males and females of one species ;
and that Giard's account of an unequal segmentation followed by an
epibolic gastrula, in one of his species, has arisen from two segmenting ova
temporarily fusing together. Giard has given a description of internal
gemmiparous reproduction, upon the accuracy of which doubts have been
thrown by Metschnikoff. The affinities of the Orthonectida: are as obscure
as those of the Dicyemida? ; though there can be but little doubt that
their organization has been much simplified in correlation with their
parasitic habits. The origin of the genital products in the axial tissue is
a feature they have in common with the Dicyemidae.

1 This at any rate holds true for the type investigated by Metschnikoff. The
full history of other forms is not yet known.

ORTHONECTID^E. 137

BIBLIOGRAPHY.

(120) Alf. Giard. " Les Orthonectida classe nouv. d. Phylum des Vers."
Journal de PAnat. et de la Physiol., Vol. XV. 1879.

(121) El. Metschnikoff. " Zur Naturgeschichte d. Orthonectidae." Zoolo-
gischer Anzeiger, No. 40 — 43, 1879.

[Ch. Julin. " Rech. sur 1'organization et le devel. d'Orthonectides." Arch.
BioL Vol. in. 1882.

E. Metschnikoff. " Untersuchungen lib. Orthonectidae." Zeit. f. Wiss. Zoo-
logic, Vol. xxxv. 1881.

For general account of Orthonectidse, vide Spengel. Biolog. Centralblatt, No. 6.]

CHAPTER V.

PORIFERA.

ALTHOUGH within the last few years greater advances have
probably been made in our knowledge of the development of the
Porifera than of any other group, yet there is much that is still
very obscure, and it is not possible to make general statements
applying to the whole group.

Calcispongiae. The form which has so far been most com-
pletely worked out is Sycandra raphanus, one of the Calcispon-
giae (Metschnikoff, Nos. 132 and 134, F. E. Schulze, Nos. 139
and 142), and I shall commence my account with the life-history
of this species.

The ovum in Sycandra as in other Spongida has the form of
a naked amceboid nucleated mass of protoplasm. From the
analogy of the other members of the group, there is no doubt
that it is fertilized by a male spermatic element, though this has
not as yet been shewn to be the case — and the changes which
accompany fertilization are quite unknown.

The segmentation and early stages of development take
place in the tissues of the parent. The segmentation is some-
what peculiar, though a modification of a regular segmentation.
The ovum divides along a vertical plane, first into two, and then
into four equal segments. But even when two segments are
formed, each of them has one end pointed and the other broader.
The pointed ends give rise to the ciliated cells of the future
larva, and the broad ends to the granular cells. Instead of the
next division taking place, as is usually the case, in a horizontal
(equatorial) plane, it is actually effected along two vertical planes

PORIFERA.

'39

intermediate in position between the two first planes of segmen-
tation. Eight equal segments are thus formed, each of which
has the form of a pyramid. All the segments are situated in a
single tier, and are so arranged as to give to the whole ovum the
form of a flat cone, the apex of which is formed by the pointed
extremities of the constituent segments (fig. 63 B). The apices
of the segments do not however quite meet, but they leave a
central space, which is an actual perforation (fig. 63 A) through
the axis of the ovum, open at both ends. The first indications
of this perforation appear when only four segments are present,

A

C

FIG. 63. SUCCESSIVE STAGES IN THE SEGMENTATION OF SYCANDRA RAPHANUS.
(Copied from F. E. Schulze.)

A. stage with eight segments still arranged in pairs, from above.

B. side view of stage with eight segments.

C. side view of stage with sixteen segments.

D. side view of stage with forty-eight segments.

E. view from above of stage with forty-eight segments.

F. side view of embryo in the blast osphere stage, eight of the granular cells which
give rise to the epiblast of the adult are present at the lower pole.

cs. segmentation cavity ; ec. granular cells which form the epiblast ; en. clear cells
which form the hypoblast.

and it is to be regarded as the homologue of the segmentation
cavity of other ova. The next plane of division is horizontal
(equatorial), and the apices of the eight cells are segmented off
as a tier of small cells. At the completion of this division (fig. 63
C), the ovum is formed of sixteen cells arranged in two superim-
posed tiers. The ovum now assumes somewhat the form of a
biconvex lens, in the axis of which the central perforation is still

140 SYCANDRA.

present. At the close of the next stage, forty-eight cells are pre-
sent arranged in four tiers (fig. 63 D and E), the two outer tiers
containing eight cells each, and the two inner sixteen. The two
inner tiers probably arise by the simultaneous appearance of two
equatorial furrows dividing the original tiers into two, and by the
subsequent simple division of the cells of the two inner of the
tiers so formed. At the -close of the stage the eight basal cells
become granular (fig. 63 F). At the same time the central part
of the segmentation cavity becomes enlarged, while its terminal
apertures become narrowed and finally, shortly after the end of
this stage, closed. The axial perforation thus acquires the

KK;. 64. LARVA OF SYCANDRA RAPHANUS AT PSEUDCMJASTRUI.A STACK, IN MM

IN THE MATERNAL TISSUES. (Copied from F. E. Schulze.)

me. mesoblast of adult ; hy. collared cells forming hypoblast of the adult ; en.
clear cells of larva which eventually become involuted to form the hypoblast ; >; .
granular cells of larva which give rise to the epiblast, which at this stage are partially
involuted.

character of a closed segmentation cavity. While the ovum
itself becomes at the same time a blastosphere.

This stage nearly completes the segmentation : in the next
one, the cells of the poles of the blastosphere increase in number,

PORIFERA.

and the cells of the greater part of the blastosphere become
columnar and ciliated, (fig. 64 en.) while the granular cells (ec.}
increase to about thirty-two in number and appear to be (parti-
ally at least) involuted into the segmentation cavity, reducing this
latter to a mere slit. This stage forms the last passed by the
embryo in the tissues of the parent. The general position of the
embryo while still in this situation may be gathered from fig. 64,
representing the embryo in situ. The embryo is always placed
close to one of the radial canals. From this situation it makes
its way through the lining cells into a canal and is thence trans-
ported to the surrounding water. By the time the larva has
become free, the semi-invaginated granular cells have increased
in bulk and become everted so as to project very much more
prominently than in the encapsuled state. To the gastrula stage,
if it deserves the name, passed through by the embryo in the
tissues of the parent, no importance can be attached.

The larva, after it has left the parental tissues, has an oval
form and is transversely divided into two areas (fig. 65 A). One

enl

c .§.

FlG. 65. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS.

(Copied from Schulze.)

A. Amphiblastula stage.

B. A later stage after the ciliated cells have commenced to become invaginated.
cs. segmentation cavity ; ec. granular cells which will form the epiblast ; en.

ciliated cells which become invaginated to form the hypoblast.

of these areas is formed of the elongated, clear, ciliated cells,
with a small amount of pigment near their inner ends (en.), and

1 42

SYCANDRA.

the other and larger area of the thirty-two granular cells already
mentioned (ec.). Fifteen or sixteen of these are arranged as a
special ring on the border of the clear cells. In the centre of the
embryo is a segmentation cavity (c.s.) which lies between the
granular and the clear cells, but is mainly bounded by the
vaulted inner surface of the latter. This stage is known as the
amphiblastula stage. During the later periods of the amphi-
blastula stage a cavity appears in the granular cells dividing them
into two layers. After the larva has for some time enjoyed a
free existence, a remarkable series of changes take place, which
result in the invagination of the half of it formed of the clear
cells, and form a prelude to the permanent attachment of the
larva. The entire process of invagination is completed in about
half an hour. The whole embryo first becomes flattened, but
especially the ciliated half, which gradually becomes less promi-
nent (fig. 65 B); and still later the cells composing it undergo a
true process of invagination. As a result of this invagination
the segmentation cavity is obliterated, and the larva assumes a
compressed plano-
convex form, with
a central gastrula
cavity, and a blasto-
pore in the middle
of the flattened sur-
face. The two layers
of the gastrula may
now be spoken of as
epiblast and hypo-
blast. The blasto-
pore becomes gradu-
ally narrowed by the
growth over it of the
outer row of granu-
lar cells. When it has
become very small
the attachment of

re

FIG. 66. FIXED GASTRULA STAGE OF SYCANDRA
RAPHANUS. (Copied from Schulze.)

The figure shews the amoeboid epiblast cells (ec.)
derived from the granular cells of the earlier stage, and
the columnar hypoblast cells, lining the gastrula cavity,
derived from the ciliated cells of the earlier stage. The
larva is fixed by the amoeboid cells on the side on
which the blastopore is situated.

the larva takes place by the flat surface where the blastopore
is situated. It is effected by protoplasmic processes of the
outer ring of epiblast cells, which, together with the other

PORIFERA. 143

epiblast cells, now become amoeboid. They become at the
same time clearer and permit a view of the interior of the
gastrula. Between the epiblast cells and the hypoblast cells which
line the gastrula cavity there arises a hyaline structureless layer,
which is more closely attached to the epiblast than to the hypo-
blast, and is probably derived from the former. A view of the
gastrula stage after the larva has become fixed is given in fig. 66.

There would seem according to MetschnikofT's observations
(No. 134) to be a number of mesoblast cells interposed between
the two primary layers, which he derives from the inner part of
the mass of granular cells.

After invagination the cilia of the hypoblast cells can no
longer be seen, and are probably absorbed ; and their disappear-
ance is nearly coincident with the complete obliteration of the
blastopore, an event which takes place shortly after the attach-
ment of the larva.

Not long after the closure of the blastopore, calcareous
spicules make their appearance in the larva as delicate un-
branched rods pointed at both extremities. They appear to be
formed on the mesoblast cells situated between the epiblast and
hypoblast1. The larva when once fixed rapidly grows in length
and assumes a cylindrical form (fig. 67 A). The sides of the
cylinder are beset with calcareous spicules which project beyond
the surface, and, in addition to the unbranched forms, spicules
are developed with three and four rays as well as some with a
blunt extremity and serrated edge. The extremity of the
cylinder opposite the attached surface is flattened, and, though
surrounded by a ring of four-rayed spicules, is itself free from
them. At this extremity a small perforation is formed leading
into the gastric cavity, which rapidly increases in size and forms
an exhalent osculum (vs.). A series of inhalent apertures is also
formed at the sides of the cylinder. The relative times of
appearance of the single osculum and the smaller apertures are
not constant for the different larvae. On the central gastrula
cavity of the sponge becoming placed in communication with
the external water, the hypoblast cells lining it become ciliated

1 Metschnikoff was the first to give this account of the development of the spicules
in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the
same result.

144 SYCANDRA.

afresh (fig. 67 B, rn.) and develop the peculiar collar characteris-
tic of the hypoblast cells of the Spongida (vide fig. 64, hy.).
When this stage of development is reached we have a fully-
formed sponge of the type made known by Haeckel as
Olynthus.

FIG. 67. THE YOUNG OF SYCANDRA RAPHANUS SHORTLY AFTER THE DEVELOP-
MENT OF THE SPICULA. (Copied from Schulze.)

A. View from the side.

B. View from the free extremity.

os. osculum ; cc. epiblast ; en. hypoblast composed of ciliated cells. The terminal
osculum and lateral pores are represented as oval white spaces.

When young examples of Sycandra come in contact shortly
after their attachment they appear to fuse together temporarily
or else permanently. In the latter case colonies are produced
by their fusion.

Amongst other calcareous sponges the larva of Ascandra contorta
(Haeckel No. 126, Barrois No. 122) presents the typical amphiblastula stage,
and so probably does that of Ascandra Lieberkiihnii (Keller No. 128). In
Leucandra aspera (Keller No. 128, Metschnikoflf No. 134) the larva passes
through an amphiblastula stage, but the characters of the cells of the two
halves of the larva do not differ to nearly the same extent as in Sycandra.

Although the majority of calcareous sponges appear to agree in their

PORIFERA.

145

mode of development with Sycandra, nevertheless the concordant researches
of O. Schmidt (No. 138) and Metschnikoff (No. 134) have shewn that this
is not true for the genus Ascetta (As. primordialis, dathrus and bianco).

The larvae of these forms are very differently constituted to those
of Sycandra. They have an oval form and are composed of a single
row of ciliated columnar cells : their two extremities only differ in the cells
at one extremity being longer than those at the other. Especially at the
pole where the shorter cells are situated (Schmidt) a metamorphosis of the
cells takes place. One after the other they lose their cilia, become granular,
and pass into the interior of the vesicle. Here they become differentiated
into two classes (Metschnikoff) ; one of larger and more granular cells,
and the other of smaller cells with clearer protoplasm. Cells of the former
class are mainly found at one of the poles. When the larva becomes
free the cells in the interior of the vesicle increase in number and nearly
fill up its central cavity. After a short free existence the larva becomes
fixed, and the epiblast cells lose their cilia and become flattened. At a later
period the large granular cells assume a radiate arrangement round a central
cavity and become clearly marked out as the hypoblast cells. The smaller
cells become placed between the epiblast and hypoblast and constitute the
mesoblast.

Myxospongiae. In this group Halisarca has been investi-
gated by Carter (No. 123), Barrois (No. 122), Schulze (No. 141)
and Metschnikoff (No. 134). The ova develop in the mesoblast,
and when ripe occupy special chambers lined by a layer of
epithelial cells. Schulze has found the spermatozoa of this
genus of sponge and has been able to shew that the sexes may
be distinct, though many species of Halisarca are hermaphrodite.

The segmentation is, roughly speaking, regular, and a seg-
mentation cavity is early formed, which is never, as in Calci-
spongise, open at the poles. When the larva leaves the parent it
is an oval vesicle formed of a single layer of columnar ciliated
cells. Slight differences may be observed between the two
extremities of the larvae of most species. One of these — the
hinder extremity — is directed backwards in swimming.

The further history of the larva has been investigated by
Metschnikoff. He has found that the interior of the vesicle
becomes gradually filled with mesoblast cells of a peculiar type,
called by him rosette-cells, which are probably derived from the
walls of the vesicle.

When the metamorphosis commences, the larva assumes a
flattened form, and cells of a new type, viz. normal amoeboid
B. II. 10

146 CERATOSPONGI^:.

cells, grow in amongst the rosette cells. The new cells are also
derived from the epiblast. The larvae appear to fix themselves
by the hinder extremity. The cilia gradually disappear, and
the epiblast cells flatten out and form a kind of cuticle. For
some time the larva remains in the two-layered condition, but
gradually canals (? ciliated chambers) lined by hypoblast cells
become formed. They appear as closed spaces with walls
of ciliated cells derived from the amoeboid cells, and the
different parts of the system of chambers are established inde-
pendently. In H. pontica the ciliated chambers are formed
before the attachment of the larva. The development was not
followed up to the formation of the pores placing the canal
system in communication with the exterior.

The young sponges at a somewhat later stage have been
studied by Schulze and Barrois. They are formed of an
external layer of flattened cells, not clearly ciliated as in the
adult, within which are a normal mesoblastic tissue, and several
spherical chambers lined by ciliated cells exactly like the ciliated
chambers of the full-grown sponge. Irregular invaginations of
the epiblast give to the young sponge a honeycombed structure.
The ciliated chambers in the youngest condition of the sponge
are closed ; but in slightly older examples they come into com-
munication with the passages lined by hypoblast, and so indirectly
with the external medium.

CeratospongiSB. Amongst the true Ceratospongias the embryos of
two of the Aplysinidae, and of Spongelia and Euspongia have been to some
extent worked out by Barrois and Schulze. The form worked out by Barrois
is called by him Verongia rosea. The segmentation is nearly regular, but
from the first the segments may be divided according to their constitution
into two categories. At the close of segmentation the embryo is oval and
covered by a single layer of columnar ciliated cells ; these cells may however
be divided into two categories, corresponding with those observable during
the segmentation. A certain number are coloured red and form a definite
circular mass at one pole, while the remainder, which constitute the major
part of the embryo, have a pale yellowish colour. Those at the red pole
lose their cilia in the free larva, but around the area formed by them is a
special ring of long cilia. The chief peculiarity of the embryo (made known
by Schulze) consists in the fact that the layer of cells which covers the
embryo does not, as in other sponge embryos, simply enclose a space,
but the interior of the embryo is formed of a mass of stellate cells like the
normal mesoblast of full-grown sponges.

PORIFERA. 147

This feature is also characteristic of the embryos of Spongelia and
Euspongia.

The embryo of the Gummineae (Gummina mimosa} has been in-
vestigated by Barrois (No. 122), and has been shewn closely to resemble the
typical larvae of calcareous sponges ; one-half being formed of elongated
ciliated cells and the other of rounded granular ones.

Silicispongiae. The development of marine silicious sponges is but
very imperfectly understood. The larvae of various forms — Reniera (Iso-
dyctia), Esperia (Desmacidon), Raspailia, Halichondria, Tethya — have been
described. Barrois has shewn that the egg segments regularly and that in
the earlier stages a segmentation cavity is present. In the later stages the
embryo appears to become solid. Externally there is a layer of ciliated cells,
and within a mass of granular matter in which the separate cells cannot be
made out. The granular matter projects at one pole, and forms a prominence
possibly equivalent to the granular cells of Sycandra. In some forms, e.g.
Reniera, the edge of the unciliated granular prominence may be surrounded
by a row of long cilia. In later stages the granular material may project at
both poles or even at other points. One remarkable feature in the
development of the Silicispongiae is the appearance of spicula between the
ciliated cells and the central mass, while the larva is still free.

Professor Schulze has informed me that these spicula are developed
in mesoblast cells ; while the horny fibres of the sponge are developed
as cuticular products of special mesoblast cells (spongioblasts).

The attachment and accompanying metamorphosis are so diversely
described that no satisfactory account can be given of them. The general
statements are in favour of the attachment taking place by the posterior
extremity where the granular matter projects.

Carter especially gives a very precise account, with figures, of the
attachment of the larva in this way. He also figures the appearance of an
osculum at the opposite pole1.

A very elaborate account of the development of Spongilla has been
published in Russian by Ganin, of which a German abstract has also
appeared (No. 124).

The ovum undergoes a regular segmentation and becomes a solid ova
morula. An epiblast of smaller cells is early differentiated, and in the
interior of the inner cells an archenteron becomes subsequently formed.
The inner cells next become divided into an hypoblastic layer lining the

1 Keller (No. 129) has recently given an account of the development of Halichon-
dria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by
a partial epibolic invagination, the inner mass of cells remaining exposed at one pole
and forming there a prominence, equivalent to the granular prominence in the larvae
of other Silicispongiae. The free swimming larva resembles the larva of other Sili-
cispongiae in the possession of spicula, etc., and after becoming laterally compressed
attaches itself by one of the flattened sides. A central cavity is formed in the interior
with ciliated chambers opening into it, and is subsequently placed in communication
with the exterior by the formation of an aperture which constitutes the osculum.

IO — 2

148 SILICISPONGLE.

archenteron, and a mesoblastic layer between this and the now ciliated
epiblast. At the narrow hinder end of the embryo the mesoblast becomes
thickened, and largely obliterates the archenteron. In this part of the
mesoblast silicious spicula are formed. The larva becomes attached by
its hinder extremity, and in the course of this process flattens itself out
to a disc-like form. From the nearly obliterated archenteric cavity out-
growths take place which give rise to the ciliated chambers. These
are not placed directly in communication with the exterior, but open, if I
understand Ganin rightly, into a space in the mesoblast, which subsequently
acquires an exterior communication — the primitive osculum. The subse-
quent pores and oscula are also formed as openings leading into the meso-
blastic cavity, which communicates in its turn with the ciliated chambers.

It appears that in the present unsatisfactory state of our
knowledge the larvae of the Porifera may be divided into two
groups : viz. (i) those which have the form of a blastosphere or
else of a solid morula ; (?) those which have the amphiblastula
form.

In the former type the mesoblast and hypoblast are formed
either from cells budded off from the outer cells of the blasto-
sphere or from the solid inner mass of cells ; while the outer
ciliated cells become the epiblast. This type of larva, which is
found in the majority of sponges, is very similar in its general
characters and development to many Ccelenterate planulae.

The second type of larva is very peculiar, and though in its
fully developed form it is confined to the Calcispongiae, where it
is the usual form, a larval type with the same characters is
perhaps to be found in other sponges, e.g. amongst the Gum-
minese, and amongst the Silicispongiae where one-half of the
embryo is without cilia, though in the case of the Silicispongise
the cells of the ciliated part of the embryo correspond to the
granular cells of the larva of Sycandra.

The later stages in the development of the larvae of the Pori-
fera are not similar to anything we know of in other groups.

It might perhaps be possible to regard sponges as degraded descendants
of some Actinozoon type such as Alcyonium, with branched prolongations of
the gastric cavity, but there does not appear to me to be sufficient evidence
for doing so at present. I should rather prefer to regard them as an
independent stock of the Metazoa.

In this connection the amphiblastula larva presents some points of
interest. Does this larva retain the characters of an ancestral type of the
Spongida, and if so, what does its form mean ? It is, of course, possible that

PORIFERA. 149

it has no ancestral meaning but has been secondarily acquired ; but, assuming
that this is not the case, it appears to me that the characters of the larva
may be plausibly explained by regarding it as a transitional form between
the Protozoa and Metazoa. According to this view the larva is to be
considered as a colony of Protozoa, one-half of the individuals of which
have become differentiated into nutritive forms, and the other half into
locomotor and respiratory forms. The granular amoeboid cells represent
the nutritive forms, and the ciliated cells represent the locomotor and
respiratory forms. That the passage from the Protozoa to the Metazoa
may have been effected by such a differentiation is not improbable on
a priori grounds.

While the above view seems fairly satisfactory for the free swimming
stage of the larval sponge, there arises in the subsequent development a
difficulty which appears at first sight fatal to it. This difficulty is the
invagination of the ciliated cells instead of the granular ones. If the
granular cells represent the nutritive individuals of the colony, they, and
not the ciliated cells, ought most certainly to give rise to the lining of the
gastrula cavity, according to the generally accepted views of the morphology
of the Spongida. The suggestion which I would venture to put forward in
explanation of this paradox involves a completely new view of the nature
and functions of the germinal layers of adult Spongida.

It is as follows : — When the free swimming ancestor of the Spongida
became fixed, the ciliated cells by which its movements used to be effected
must have to a great extent become functionless. At the same time the
amoeboid nutritive cells would need to expose as large a surface as possible.
In these two considerations there may, perhaps, be found a sufficient expla-
nation of the invagination of the ciliated cells, and the growth of the
amoeboid cells over them. Though respiration was, no doubt, mainly
effected by the ciliated cells, it is improbable that it was completely
localized in them, but they were enabled to continue performing this
function through the formation of an osculum and pores. The collared cells
which line the ciliated chambers, or in some cases the radial tubes,
are undoubtedly derived from the invaginated cells, and, if there is any
truth in the above suggestion, the collared cells in the adult sponge must
be mainly respiratory and not digestive in function, while the epiblastic
cells, which in most cases line the inhalent passages through its substance1,
ought to be employed to absorb nutriment. The recent researches of
Metschnikoff (No. 134) on this head shew that the nutriment is largely
carried into the mesoblast cells, which in Sycandra appear to be derived

1 That the greater part of the flat cells which line the passages of most Sponges
are really derived from epiblastic invaginations appears to me to be proved by Schulze's
and Barrois' observations on the young fixed stages of Halisarca. Schulze's (No. 140)
observations have however proved that the flat cells lining the axial gastric chamber
of Sycandra are hypoblastic in origin, and the observations of Keller (No. 129) and
Ganin (No. 124) have led to the same result for the flat epithelium lining part of the
passages of the Silicispongise.

150 SUMMARY.

from the granular cells, and also that it is taken up by the cells which line
the passages, though not by the superficial epiblast cells. Whether the
collared cells generally absorb nutriment is not clear from his statements :
but he finds that they do not do so in Silicispongice.

Professor Schulze has informed me by letter that he finds the collared
cells to be respiratory in function, while the cells derived from the granular
cells in Sycandra are nutritive. Carter1, on the contrary, from his obser-
vations on Spongilla, has fully satisfied himself that the food is absorbed
by the cells lining the ciliated chambers.

If it is eventually proved by further experiments on the nutrition of
sponges, that digestion is mainly carried on by the general cells lining the
passages and the mesoblast cells, and not for the most part by the ciliated
cells, it is clear that the epiblast, mesoblast and hypoblast of sponges will
not correspond with the similarly named layers in the Ccelenterata and other
Metazoa. The invaginated hypoblast will be the respiratory layer and
the epiblast and mesoblast the digestive and sensory layers ; the sensory
function being probably mainly localized in the epithelium on the sur-
face, and the digestive one in the epithelium lining the passages and in
the mesoblast. Such a fundamental difference in the primary function of
the germinal layers between the Spongida and the other Metazoa, would
necessarily involve the creation of a special division of the Metazoa for the
reception of the former group.

BIBLIOGRAPHY.

(122) C. Barrois. "Embryologie de quelques eponges de la Manche." An-
nales des Sc. Nat. ZooL, vi. ser., Vol. ill. 1876.

(123) Carter. " Development of the Marine Sponges." Annals and Mag. of
Nat. Hist., 4th series, Vol. xiv. 1874.

(124) Can in2. "Zur Entwicklung d. Spongilla fluviatilis." Zoologischer An-
zeiger. Vol. I. No. 9, 1878.

(125) Robert Grant. "Observations and Experiments on the Structure and
Functions of the Sponge." Edinburgh Phil, jf., Vol. xm. and xiv., 1825, 1826.

(126) E. Haeckel. Die Kalkschwamme, 1872.

(127) E. Haeckel. Studien zur Gastraa-Theorie. Jena, 1877.

(128) C. Keller. Untersuchungen uber Anatomic und EntwicklungsgeschichU
einiger Spongien. Basel, 1876.

(129) C. Keller. "Studien ub. Organisation u. Entwick. d. Chalineen."
Zeit.f. wiss. Zool., Bd. xxvin. 1879.

(130) Lieberkiihn. " Beitr. z. Entwick. d. Spongillen." Mailer's Archiv,
1856.

(131) LieberkUhn. " Neue Beitrage zur Anatomic der Spongien." Miiller's
Archiv, 1859.

1 "On the Nutritive and Reproductive Processes of Sponges." Ann. and Mag.
of Nat. Hist., Vol. iv. Ser. v. 1879.

2 There is a Russian paper by the same author, containing a full account, with
clear illustrations, of his observations.

PORIFERA. I 5 I

(132) El. Metschnikoff. " Zur Entwicklungsgeschichte der Kalkschwamme."
Zeit.f. wiss. Zool., Bd. xxiv. 1874.

(133) El. Metschnikoff. " Beitrage zur Morphologic der Spongien." Zeit.
f. wiss. Zool., Bd. xxvu. 1876.

(134) El. Metschnikoff. *' Spongeologische Studien." Zeit. f. wiss. Zool.,
Bd. xxxn. 1879.

(135) Miklucho Maklay. " Beitrage zur Kenntniss der Spongien." Jenaische
Zeitschrift, Bd. iv. 1868.

(136) O. Schmidt. " Zur Orientirung iiber die Entwicklung der Schwamme."
Zeit.f. wiss. Zool., Bd. xxv. 1875.

(137) O. Schmidt. " Nochmals die Gastrula der Kalkschwamme." Archiv
fur mikrosk. Anat., Bd. xii. 1876.

(138) O. Schmidt. "Das Larvenstadium von Ascetta primordialis und Asc.
clathrus." Archiv fur mikrosk. Anatomic, Bd. xiv. 1877.

(139) F. E. Schulze. " Ueber den Bau und die Entwicklung von Sycandra
raphanus." Zeit. f. wiss. Zool., Bd. xxv. 1875.

(140) F. E. Schulze. " Zur Entwicklungsgeschichte von Sycandra." Zeit.f.
wiss. Zool., Bd. xxvu. 1876.

(141) F. E. Schulze. " Untersuchung lib. d. Bau, etc. Die Gattung Hali-
sarca." Zeit.f. wiss. Zool., Bd. xxvni. 1877.

(142) F. E. Schulze. " Untersuchungen iib. d. Bau, etc. Die Metamorphose
von Sycandra raphanus." Zeit.f. wiss. Zool., Bd. xxxi. 1878.

(143) F. E. Schulze. "Untersuchungen ii. d. Bau, etc. Die Familie Aply-
sinidse." Zeit.f. wiss. Zool., Bd. xxx. 1878.

(144) F. E. Schulze. " Untersuchungen ii. d. Bau, etc. Die Gattung Spon-
gelia." Zeit.f. wiss. Zool., Bd. xxxn. 1878.

CHAPTER VI.

C(ELENTERATA '.

Hydroidea. The most typical mode of development of the
Hydroidea is that in which the segmentation leads directly to
the formation of a free ciliated two-layered larva, known since
Dalyell's observations as a plan u la. The planula is characteris-
tic of almost all the Hydromedusae with fixed hydrosomes
including the Hydrocoralla (Stylasteridse and Millepora), the
most important exceptions being the genus Tubularia and one
or two other genera, and the fresh-water Hydra.

In a typical Sertularian the segmentation is approximately
regular8 and ends according to the usual accounts in the forma-
tion of a solid spherical mass of cells. A process of delamina-
tion now takes place, which leads to the formation of a superficial
layer of cubical or pyramidal cells, enclosing a central solid
mass of more or less irregularly arranged cells.

The embryo, in the cases in which it is still contained within
the sporosack, now begins to exhibit slight changes of form, and

1 I. HYDROZOA.

\Hydroidca.

1. Hydromedusse. \>/rachymedu^

2. SiphODOphora. \Cafycop**rM*.

( Physophonda.

3- Acraspeda.

II. ACTINOZOA.

1. Alcyonaria. (Octocoralla.)

2. Zoanthaiia. (Hexacoralla.)

III. CTENOPHORA.

9 For a detailed description of the development of a single species the reader
referred to Allman's description of Laomedia flexuosa, No. 149, p. 85 / seq.

CtELENTERATA.

153

one extremity of it begins to elongate. It soon becomes free,
and rapidly assumes an elongated cylindrical form, while a
coating of cilia, by means of which it moves sluggishly about,
appears on its outer surface. A central cavity appears in the
interior, and the inner cells form themselves into a definite
hypoblast. The larva has now become a planula, and consists of
a closed sack with double walls. It continues for some few days
to move about, but eventually drops its cilia, and becomes
dilated at one extremity, by which it then becomes attached.
The base of attachment becomes gradually enlarged so as to
form a disc, which spreads out and is frequently divided by
fissures into radiating lobes. The free extremity becomes en-
larged to form the eventual calyx.

Over the whole exterior a delicate pellicle — the future peri-
sarc — now becomes secreted. Round the edge of the anterior
enlargement a row of tentacles makes its appearance. These, in
the embryos of the Tubularian genera, lie some little way behind
the apex of the body. After a certain time the perisarc, which
has hitherto been continuous, becomes ruptured in the region of

C

FIG. 68. THREE LARVA STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.)

A. Blastosphere stage with hypoblast spheres becoming budded off into the
central cavity.

B. Planula stage with solid hypoblast.

C. Planula stage with a gastric cavity.

ep. epiblast ; hy. hypoblast ; al. gastric cavity.

the calyx, and the tentacles become quite free. At about the
same period a mouth is formed at the oral apex.

HYDROIDEA.

The development of Eucope polystyla (fig. 68), one of the
Campanularidae, deviates according to Kowalevsky (No. 147) in
somewhat important points from the usual type. The whole
development takes place after the deposition of the ovum. The
segmentation results in the formation of a single-walled blasto-
sphere with a large central cavity (fig. 68 A). This cavity,
somewhat as in Ascetta, becomes filled up with a not clearly (?)
cellular material derived from the walls of the blastosphere,
which must be regarded as the hypoblast (fig. 68 B). The larva
elongates and becomes ciliated, and the epiblast at its two
extremities becomes thickened, and is stated by Kowalevsky
also to become divided into two layers. The alimentary cavity
appears as a slit in the middle of the hypoblast (fig. 68 C). The
cilia after a time disappear, and the larva then becomes fixed by
one extremity. It flattens itself out into a disc-like form, becomes
divided into four lobes, and covered by a cuticle (perisarc).
From the disc the stalk grows out which dilates at its free ex-
tremity into the calyx.

In both the groups (Tubularia and Hydra)
having a ciliated planula stage, its absence may
be put down to an abbreviation of the develop-
ment, and in fact a two-layered quiescent stage,
through which the embryo passes, may be
regarded as representing the planula stage.

The development of Tubularia, which has
been described in detail by Ciamician, takes
place in the gonophore1. The segmentation
is irregular and leads to the formation of an
epibolic gastrula, four large central cells con-
stituting the hypoblast2. The larva now elon-
gates, and grows out laterally into two pro-
cesses which constitute the first pair of
tentacles. At this stage it closely resembles
the larvae of some Medusas. Additional ten-
tacles are soon formed ; and a central cavity
appears in the hypoblast, the cells of which
have in the meantime become more numerous
(fig. 69). The tentacles are directed towards

which are exceptional in not

FIG. 69. LONGITUDINAL

SECTION THROUGH A LARVA OF
TUBULARIAMESEMBRYANTHE-
MUM WHILE STILL IN THE

GONOPHORE. The lower end
is the oral one.

ep. epiblast; hy. hypoblast
of tentacle ; en. enteric cavity.

1 Vide Ciamician, Zeit.f. wiss. Zool., Bd. xxxn. 1879.

3 In examining the segmentation by means of sections I have failed to detect an
epibolic gastrula or such irregularity as is described by Ciamician. Prof. Kleinenberg
informs me that he has been equally unsuccessful.

CCELENTERATA. 155

the aboral side, which is considerably more prominent than the oral one.
They contain a hypoblastic axis. The aboral end continues to grow and
the tentacles gradually assume a horizontal position. A constriction now
appears, dividing the larva into an aboral portion which will eventually form
the stalk, and an oral portion. At the apex of the latter a row of short
tentacles — the future oral tentacles — now appears. The larva has at this
stage the form known as Actinula. In this condition it becomes hatched,
and shortly afterwards it becomes fixed by the aboral end and grows into
a colony.

The development of Myriothela (Allman, No. 150) takes place on the
Tubularian type. The ovum invested by a delicate capsule becomes freed
by the rupture of the gonophore, and is then taken up by the remarkable
claspers characteristic of the genus. In the claspers it becomes fecundated
and undergoes its further development. After segmentation a gastric
cavity is formed, and provisional tentacles arise as a series of conical
involutions which subsequently become evoluted. Permanent tentacles are
formed as conical papillae on a truncated oral process. After hatching it
has a few days' free existence, and then becomes attached, and loses its
provisional tentacles.

Although Hydra itself constitutes the simplest type of Hydrozoon, its
development, which has been fully investigated by Kleinenberg (No. 161), is
in some respects a little exceptional. The segmentation is regular, but a
segmentation cavity is not formed. The peripheral layer of cells gradually
becomes converted into a chitinous membrane, which is perhaps homologous
with the perisarc of marine forms. Between the membrane and the germ a
second pellicle makes its appearance. The above changes require about
four days for their completion, but there next sets in a period of relative
quiescence which lasts for some 6 — 8 weeks. During this period the
remaining development is completed. The cells of the germ first fuse
together. In the interior of the protoplasm a clear excentric space arises,
which gradually extends itself and forms the rudiment of the gastric cavity.
The outer shell in the meantime becomes less firm, and is finally burst and
thrown off, owing to the expansion of the embryo within.

The outermost layer of the protoplasm becomes, relatively to the inner
layer, clear and transparent, and there thus arises an indication of a division
of the walls of the archenteric cavity into two zones, or layers. These layers,
which form the epiblast and hypoblast, are definitely established on the
appearance of cells with contractile tails1 in the clear outer zone, between
which the interstitial epiblast cells subsequently arise.

The embryo, still forming a closed double-walled sack, elongates itself,
and at one pole its wall becomes very thin. And at this point a rupture
takes place which gives rise to the mouth. Simultaneously with the mouth
the tentacles become formed as hollow processes, according to Mereschkowsky
two being formed first and subsequently the others in pairs. Very shortly

1 These cells are the so-called nerve-muscle cells. Their nature is discussed in
the second part of this work.

56 TRACHYMEDUS^E.

afterwards the hitherto uniform hypoblast becomes divided up into distinct
cells. The thin inner pellicle which persists after the rupture of the outer
membrane becomes in the meantime absorbed. With these changes the
embryo practically acquires the characters of the adult.

TrachymedusaB. Amongst the Trachymedusae, which as
has now been satisfactorily established develop directly without
alternations of generations, the embryology of species both of
the Geryonidae and the ^Eginidae has been studied.

In all the types so far investigated the hypoblast is formed
by delamination, and there is a more or less well-marked planula
stage.

The development of Geryonia (Carmarina) hastata has been
studied by Fol (No. 155) and MetschnikofT (No. 163)1. The
ovum, when laid, is invested by a delicate vitelline membrane
and mucous covering. Its protoplasm is formed of an outer
granular and dense layer, and a central mass of a more spongy
character. The segmentation is complete and regular, and up
to the time when thirty-two segments
have appeared each segment is composed
of both constituents of the protoplasm
of the ovum. A segmentation cavity
appears when sixteen segments are
formed, and becomes somewhat larger at
the stage with thirty-two. At this stage
the process of delamination commences.
Each of the thirty-two segments, as
shewn in the accompanying diagram

(fig. 7O), becomes divided into two unequal DELAMINATION OF THE
' , r , . r , OVUM OF GERYONIA.

parts. The smaller of these is formed (Copied from Fol.)

almost entirely of granular material ; «• segmentation cavity ;

. , . . r . . a. endoplasm ; b. ectoplasm.

the larger contains portions of both The dotted lines shew the
kinds of protoplasm. In the next seg- C?u.r55e of the next planes of

division.

mentation the thirty-two large cells only

are concerned, and in each of these the line of division passes
between the granular and the transparent protoplasm. The
sixty-four lenticular masses of granular protoplasm thus formed
constitute an outer closed epiblastic vesicle, within which the

1 In the succeeding account I have followed Fol, who differs in some nvnor points
from Metschnikoff.

CCELENTERATA.

157

thirty-two masses of transparent protoplasm form an hypoblastic
vesicle. The embryo at this stage is shewn in optical section in
fig. 71.

The epiblastic vesicle now grows rapidly, while the hypo-
blastic vesicle remains nearly passive and becomes somewhat
lens-shaped. At one point its wall comes in close contact with
the epiblast. Elsewhere a wide cavity is developed between the
two vesicles which becomes filled with gelatinous tissue. At this
period cilia appear on the surface, and the larva becomes a planula.

The succeeding changes lead rapidly to the formation of a
typical Medusa. Where the epiblast and
hypoblast are in contact the former layer
becomes thickened and forms a disc-shaped
structure. The centre of this becomes
somewhat protuberant, fuses with the hy-
poblast and then becomes perforated to
form the mouth (fig. 72 <?). The edge of
the disc forms a thickened ridge, the
rudiment of the velum (v), which is en-
tirely formed of epiblast. At its edge six
tentacles (t] arise, into which are con-
tinued solid prolongations of the wall of the now somewhat
hexagonal gastric chamber. The hypoblastic axes of the tenta-
cles soon lose their connection with the gastric wall.

Up to this time the larva _==___=_^_

has retained a more or less
spherical form, and the cavity
on the under side of the
umbrella has not yet become
developed. The latter now
becomes established by the
whole disc assuming a vault-
ed form with the concavity
directed downwards. The
lining of the cavity so formed
is derived from the epiblast
of the disc already spoken of.

The exact mode of formation of the gastrovascular canals has not been
worked out. It has however been established by the researches of the

FIG. 71. EMBRYO OF
GERYONIA AFTER DELAMI-
NATION. (After Fol.)

ep. epiblast; hy. hypo-
blast.

FIG. 72. OPTICAL SECTION THROUGH
THE ORAL POLE OF GERYONIA AFTER THE

APPEARANCE OF THE GELATINOUS TISSUE

OF THE DISC. (After Fol.)

o. mouth; v. velum; /. tentacle.
The shaded part represents the gelati-
nous tissue.

158 TRACHYMEDUS/E.

Hertwigs (No. 146) and Glaus (No. 153) that the radial and circular vessels
of this system are connected together in adult Medusae by an hypoblastic
lamella ; so that these canals would seem to be the remnants of an once-
continuous gastric cavity. This mode of formation is established in the case
of the medusiform buds ; and it would therefore seem, as pointed out by the
Hertwigs, a fair deduction that it occurs in the larva — a conclusion which is
confirmed by the primitive extension of the gastric cavity to the edge of the
disc at the time when its walls give rise to the solid axes of the tentacles. In
the course of the subsequent retirement of the gastric cavity from the edge of
the disc the gastrovascular canals probably take their origin, though Fol was
unable to follow the changes which result in their formation.

On the completion of the above changes the larva has become
a fully formed Medusa, but it undergoes a not inconsiderable
metamorphosis before the attainment of the adult state.

Two species of ^Eginidae have been studied by Metschnikoff
(163), viz. Polyxenia leucostyla (^Egineta flavescens}, and ^Egi-
nopsis mediterranea. In both of these forms the segmentation

FIG. 73. A THREE-DAYS' LARVA OF ^GINOPSIS WITH TWO TENTACLES.
(After Metschnikoff.)
m. mouth; /. tentacle.

results in the formation of an elongated two-layered ciliated
planula, without a central cavity. The two ends of this grow
out into two long processes — the rudiments of a pair of at first
aborally directed arms — which contain a solid hypoblastic axis
(fig. 73). At this stage the larva closely resembles the larva of
Tubularia. An alimentary cavity is hollowed out in the centre
of the hypoblast which soon opens by a wide oral aperture (m).
A second pair of arms becomes formed, which are at first much
shorter than the original pair; with their formation a radial
symmetry is acquired. Sense-organs become at the same time

OELENTERATA.

159

developed, and the whole embryo assumes a medusiform character.
Fresh tentacles arise, the velum and cavity of the umbrella become
established, but these changes do not involve any points of very
special interest.

Siphonophora. The development of the Siphonophora has
been the subject of careful investigation by Haeckel (158) and
Metschnikoff (163). The ova are large and usually (except
Hippopodius) without a membrane.

They are formed of a peripheral denser layer of protoplasm
and a central spongy mass. They usually undergo their entire
development in the water. In some instances they have been
successfully reared by artificial impregnation.

As an example of the Calycophoridae I shall take Epibulia
aurantiaca, a form allied to Diphyes, the development of which
has been studied by Metschnikoff1.

FIG. 74. THREE LARVAL STAGES OF EPIBULIA AURANTIACA. (After Metschnikoff.)

A. Planula stage.

B. Six-days' larva with nectocalyx (nc) and tentacle (/").

C. Somewhat older larva with gastric cavity.

ep. epiblast; hy. hypoblast; so. somatocyst; nc. nectocalyx; /. tentacle; c. large
yolk cells ; po. polypite.

1 In my description of the development of the Siphonophora I employ Huxley's
terminology.

i6o

SIPHONOPHORA.

There is a regular segmentation, unaccompanied by the
formation of a segmentation cavity. At its close the ovum
becomes a spherical ciliated embryo. This embryo soon becomes
elongated, and its cells differentiate themselves into a central
and a peripheral layer — the epiblast and the hypoblast (fig. 74 A).
At this stage the larva has the typical planula form. The epiblast
is especially thickened at a pole, which may be called the oral
pole, and towards the side of this, which will be spoken of as
the ventral side. Adjoining this thickened layer of epiblast a
special thin layer of hypoblast becomes differentiated, which in
opposition to the main mass of large nutritive cells forms the
true hypoblastic epithelium
(fig. 74 B, hy\ On this
thickening two prominences
make their appearance (fig.
74 B). The oral of these
is the rudiment of a ten-
tacle (/), and the aboral of
a nectocalyx (nc).

The former of these
elongates itself in succeed-
ing stages into a process of
both epiblast and hypoblast.
The central part of the
nectocalyx on the other hand
appears to originate from a
thickening of the epiblast
in which the cavity of the
bell becomes subsequently
hollowed out. Between
this part and the external
epiblast which gives origin
to the outermost layer of
the nectocalyx a layer of
hypoblast is interposed.
When the nectocalyx has
become to a certain extent
established a cavity — the
commencement of the

FlG. 75. AN ADVANCED LARVA OF EPI-
BULIA AURANTIACA WITH ONE LARGE NECTO-
CALYX. (After Metschnikoff.)

so. somatocyst ; nc. second imperfectly de-
veloped nectocalyx; hph. hydrophyllium; po.
polypite; /. tentacle.

CCELENTERATA.

161

primitive gastrovascular cavity of the adult — appears in the
general hypoblast between the epithelial and nutritive layers in
the immediate neighbourhood of its attachment. This cavity
becomes prolonged into the nectocalyx to form the four gastro-
vascular canals; while the hypoblast at the upper end of the
nectocalyx forms the somatocyst (fig. 74 C, so). The primitive
enteric cavity once formed rapidly extends, especially in an oral
direction (fig. 74 C), and forms a widish cavity in the oral part of
the embryo. At the pole of this part (fig. 74, pd] is eventually
formed the opening of the mouth, and the contained cavity
becomes in a special sense the gastric cavity. This region of the
embryo may be spoken of as the polypite. The nectocalyx
grows with great rapidity and soon forms by far the most
prominent part of the larva (fig. 75). The true gastric region or
polypite (fig. 75, po) continues also to grow, and a mouth becomes
formed at its extremity. The aboral end of the original body of
the embryo gradually atrophies.

FlG. 76. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM.

(After Metschnikoff.)

A. Stage after the delamination. ep. epiblastic invagination to form pneuma-
tocyst.

B. Later stage after the formation of the gastric cavity in the solid hypoblast,
po. polypite; A tentacle; pp. pneumatophore ; ep. epiblastic invagination to form
pneumatocyst ; hy. hypoblast surrounding pneumatocyst.

At the junction of the nectocalyx and polypite the ccenosarc
becomes formed, and rudiments of a second nectocalyx (nc) and
B. II. Ir

162 SIPHONOPHORA.

second polypite early become visible ; while a hydrophy Ilium is
formed as a bud which covers over the first polypite and tentacle
(JipJi). With the development of the hydrophyllium the first
segment, if the term may so be used, is complete. The second
segment of which a rudiment is already present as a second
polypite is intercalated between the first segment and the necto-
calyces.

Amongst the Physophoridae there is a considerable range of
variation in development ; though the variations concern for the
most part not very important points. The simplest type hitherto
observed is that of StepJianomia (Halistemma) pictum. The
segmentation and formation of a two-layered planula (fig. 76)
take place in the usual way. Between the solid central mass of
nutritive hypoblast cells and the epiblast an epithelial hypoblastic
layer becomes interposed which undergoes a special thickening
at the aboral pole. At this pole a solid involution of epiblast
next becomes formed, to which a layer of hypoblast becomes
applied. The structure so formed is the rudiment of the pneuma-
tocyst (<?/). In the next stage the air-cavity of the pneumatocyst
becomes established within the epiblast.

The gastrovascular cavity is formed in the midst of the
nutritive hypoblast cells, which then become rapidly absorbed
leaving the gastrovascular cavity entirely enclosed by the epi-
thelial layer of hypoblast (fig. 76 B).

By the above changes the more important organs of the larva
have become established. The one end forms the pneumatophore,
and the other, the oral part, the polypite. Between the two there
is already present the rudiment of a tentacle, and a second tenta-
cle soon becomes formed. The mouth arises as a perforation at
the oral end of the larva.

The pneumatophore contains a prolongation of the gastro-
vascular cavity, the fluid in which bathes the outer hypoblastic
wall of the pneumatocyst. It has however no communication
with the enclosed cavity of the pneumatocyst. In the later
developmental stages the size of the pneumatophore becomes
immensely reduced in comparison with the remainder of the
larva.

The development of Physophora agrees closely with that of Stephanomia
except in one somewhat important point, viz. in the development of a

CCELENTERATA. 163

provisional hydrophyllium. This arises as a prominence at the aboral pole,
containing a prolongation of the gastrovascular cavity. Between the epiblast
and hypoblast of the prominence gelatinous tissue becomes deposited, and
the hydrophyllium is thus converted into a large umbrella-like organ enclosing
the polypite. The two together have a close resemblance to an ordinary
Medusa, the polypite forming the manubrium, and the hydrophyllium the
umbrella. The hydrophyllium is eventually thrown off.

An important type of Physophorid development is exemplified in Crystal-
loides, a genus closely allied to Agalma. In this type the greater part of the
original ovum, instead of directly giving rise to the polypite, becomes a kind
of yolk-sack, from which the polypite is secondarily budded (fig. 77, yk).
Agalma sarsii is in this respect intermediate between Crystalloides and
Physophora. Both these types are remarkable for developing a series of
provisional hydrophyllia (fig. 77, h.ph.}. In both genera the first of these
develops as in Physophora, and for a long time is the only one functional.

The conclusions to be drawn from the above description may
be summed up as follows. In all the Siphonophora, so far
observed, the starting-point for further development is a typical
ciliated two-layered planula. The inner layer or hypoblast is
mainly formed of large nutritive cells. From these cells an
epithelial hypoblastic layer becomes secondarily differentiated,
the exact relations of which differ somewhat in the various
types. The nutritive cells themselves do not appear to become
directly converted into the permanent hypoblastic tissues. The
development of the adult from the planula commences by the
thickening of the epiblastic layer, usually at one pole (the future
proximal or aboral pole), and the formation at this pole of a
series of bud-like structures (in the growth of which both
embryonic layers have a share), which become converted into
the hydrophyllia, nectocalyces etc. The main oral part of the
planula becomes generally converted into the polypite, though
in some instances (Crystalloides) it remains as a yolk-sack, and
only secondarily gives rise to a polypite.

Two very different views have been taken as to the nature of
the various component parts of the Siphonophora, and the
embryological evidence has been appealed to by both sides in
confirmation of their views. By Huxley and Metschnikoff the
various parts — nectocalyces, hydrophyllia, hydrocysts, polypites,
generative gonophores etc. are regarded as simple organs, while
by Leuckart, Haeckel, Glaus etc. they are regarded as so many
different individuals forming a compound stock. The difference

II — 2

164

SIPHONOPHORA.

between these two views is not merely as to the definition of an

individual1. The question really is, are these parts originally

derived by the modification

of complete zooids like the

gonophores and trophosomes

of the fixed Hydrozoa stocks,

or are they structures derived

from the modification of the

tentacles or some other parts

of a single zooid ?

The difficulty of deciding
this point on embryological
evidence depends on the fact
that ontologically a tentacle
and a true bud arise in the
same way, viz. as papilliform
outgrowths containing pro-
longations of both the primi-
tive germinal layers. The
balance of evidence is never-
theless in my opinion in
favour of regarding the Si-
phonophora as compound stocks, and the views of Claus on this
subject (Zoologie, p. 271) appear to me the most satisfactory.

FIG. 77. LARVA OF CRYSTAI.LOJDES.
(After Haeckel.)

h.ph. hydrophyllium ; h. hydrocyst ; /.
tentacle ; pp. pneumatophore ; po. polypite ;
yk. yolk-sack.

The most primitive condition is probably that like Physophora in an
early stage with an hydrophyllium enclosing a polypite (cf. Haeckel and
Metschnikoff). In this condition the whole larva may be compared to a
single Medusa in which the primitive hydrophyllium represents the umbrella
of the Medusa, and the polypite the manubrium. The tentacle which
appears so early is probably not to be regarded as a modified zooid, but as
a true tentacle. The absence of a ring of tentacles is correlated with the
bilateral symmetry of the Siphonophora.

The primitive zooid of a Siphonophora stock is thus a Medusa. Like
Sarsia and Wilsia this Medusa, must be supposed to have been capable of
budding. The ordinary nectocalyces by their resemblance to the umbrellas
of typical Medusae are clearly such buds of the medusiform type. The same
may be said of the pneumatophore, which, as pointed out by Metschnikoff,
is identical in its development with a nectocalyx. Both are formed by a

1 From the expressions used by Huxley, Anatomy of Inverttb rated Animals,
p. 149, it appears to me possible that his opposition to Leuckart's view is mainly as to
the nature of the individual.

CCELENTERATA. 165

solid process of epiblast in which a cavity— the cavity of the nectocalyx or
pneumatocyst — is eventually hollowed out. Around this there appears a
double layer of hypoblast containing a prolongation of the gastrovascular
cavity ; and this is in its turn enclosed by a layer of epiblast which forms the
covering of the convex surface of the nectocalyx and the external epiblast of
the pneumatophore.

The generative gonophores are clearly also zooids, and the hydrophyllia
are probably a rudimentary form of umbrella. In many cases (Epibulia,
Stephanomia, Halistemma etc.) the hydrophyllium of the primitive polypite
(manubrium) is absent. In such instances it is necessary to suppose that
the umbrella of the primitive zooid of the whole colony has become aborted.
Leuckart originally took a somewhat different view from the above in that
he regarded the starting-point of the Siphonophora to be a compound fixed
Hydrozoon stock, which became detached and free-swimming.

Acraspeda1. The embryonic development of several of the
forms of the Acraspeda has been investigated by Kowalevsky
(No. 147) and Claus (No. 153). Their observations seem to
point to an invaginate gastrula being characteristic of this group.

Amongst the forms with alternations of generations and a
fixed larval form Chrysaora and Cassiopea have been most fully
investigated. The ovum of the former undergoes the first embry-
onic phases while still in the ovary. In the latter it is enclosed
amongst the oral processes. A complete and more or less
regular segmentation leads to the formation of a single-walled
blastosphere with a small segmentation cavity. The wall of the
blastosphere next becomes invaginated, giving rise to an arch-
enteron (fig. 78 A). The blastopore soon closes up, and the
archenteron is converted into a closed sack completely isolated
from the epiblast (fig. 78 B). The surface of the larva becomes
in the meantime covered with cilia. The free larval stage thus
reached is similar to the ordinary Hydrozoon planula. After
the closure of the blastopore the larva becomes elongated, and
one end becomes narrowed. By this narrowed extremity the
larva soon attaches itself, and at the opposite and broader end a
fresh involution of the epiblast appears (fig. 78 C) ; this gives
rise to the stomodseum, which is placed in communication with
the archenteron on the absorption of the septum dividing them.
The relation of the stomodaeum to the original blastopore has
not been determined.

! I use this term for the group, often known as the Discophora, which includes the
Pelagidte, Rhizostomidae, and Lucernaridse.

1 66 ACRASPEDA.

At the point of attachment there is developed a peculiar
pedal disc, and around the mouth there appears a fold of epiblast
which gives rise to an oral disc (fig. 78 D). Two tentacles first
make their appearance, but one of these is primarily much the
largest, though eventually the second overtakes it in its growth.

FlG. 78. FOUR STAGES IN THE DEVELOPMENT OF CHRYSAORA. (After Glaus.)

A. Gastrula stage.

B. Stage after closure of blastopore.

C. Fixed larva with commencing stomodaeum.

D. Fixed larva with mouth, short tentacles, etc.

ep. epiblast; hy. hypoblast; st. stomodasum; m. mouth; bl. blastopore.

A second pair of tentacles next becomes formed, giving to the
larva a 4-radial symmetry. Between these four new tentacles
subsequently sprout out, and in the intermediate planes four
ridge-like thickenings of the hypoblast, projecting into the cavity
of the stomach, make their appearance. They imperfectly divide
the stomach into four chambers, to each of which one of the
primary tentacles corresponds ; they may be regarded as homo-
logous with the mesenteries of the Actinozoa. The number of
tentacles goes on increasing somewhat irregularly up to sixteen.
All the tentacles contain a solid hypoblastic axis. Muscular
elements are developed from the epiblast.

With the above changes the so-called Hydra tuba or Scyphi-
stoma form is reached (vide fig. 85). The peculiar strobilization
of this form is dealt with in the section devoted to the meta-
morphosis.

CCELENTERATA. 167

Aurelia is stated by Kowalevsky to develop in the same way as Cassiopea;
and the one stage of Rhizostoma observed is that in which it has a (probably
invaginate) gastrula form.

In Pelagia the ovum directly gives rise to a form like the parent. The
segmentation and the invagination take place nearly as in Cassiopea, but the
archenteric cavity is relatively much smaller, and the large space between it
and the epiblast becomes filled with the gelatinous tissue which forms the
umbrella. The blastopore does not appear to close but to become directly
converted into the mouth. As in Cassiopea the larva takes a somewhat
four-sided pyramidal form. The mouth is placed at the base. The pyramid
becomes subsequently flatter, and at the four corners four tentacles grow out
which increase to eight by division. The flattening continues till the larva
reaches a form hardly to be distinguished from the Ephyra resulting from
the strobilization of the fixed Scyphistoma form of other Acraspeda.

Alcyonidae. In the Alcyonidae the segmentation appears
always to lead to the formation of a solid morula, which becomes
a planula by delamination. The true enteric cavity is formed
by an absorption of the central cells, but the axial portion of the
gastric cavity and mouth are formed by an epiblastic invagi-
nation.

The development of these types has been mainly studied by Kowalevsky
(147), and my knowledge of his results is derived from German abstracts of
the original Russian memoirs.

In Alcyonium palmatum the impregnation is external. The segmenta-
tion is very exceptional in character. It commences with the formation of a
series of irregular prominences on the surface of the ovum, which become
segmented off to form a superficial layer of epiblast cells. The inner mass
of protoplasm then divides up into polygonal cells to form the hypoblast,
which would thus seem to be formed by a kind of delamination. In Clavu-
laria crassa (No. 168) there is a complete segmentation followed by a
delamination. The larva of AL palmattim elongates and becomes ciliated,
and so assumes the characters of a typical planula. The central hypoblast
is formed of an outer granular stratum with imperfectly differentiated cells
— the true hypoblast — and an inner homogeneous mass with vacuoles.

Some of the larvae become fixed, while others coalesce together and
form a large mass, the fate of which has not been further studied. An
invagination of epiblast takes place at the free end of the fixed larva, which
gives rise to the so-called gastric cavity, i. e. the axial portion of the general
enteric cavity, which would appear to be in reality a kind of stomodseum.
Around the gastric cavity the hypoblast forms eight mesenteries, the
chambers between which are filled with the homogeneous material which
occupied the centre of the ovum in the previous stage. It is to be
presumed, though not stated, that by an absorption of the blind end
of the stomodaeal invagination the gastric chamber is placed in free

168 ZOANTHARIA.

communication with the spaces between the mesenteries1. During the next
stage the young Alcyonium also acquires eight tentacles, which arise as
hollow papillae opening into the eight mesenteric chambers. By this stage
also the matter filling up the mesenteric chambers is nearly absorbed.

Between the epiblast and hypoblast there is formed an homogeneous
membrane, which penetrates in between the two layers of hypoblast
which form the mesenteries. On the outer side of this membrane, and
therefore presumably derived from the epiblast, is a layer of connective-
tissue cells, which eventually gives rise to the abundant gelatinous tissue
(ccenenchyma) in which the skeletal elements are deposited. In Sympodimn
coralloides Kowalevsky (No. 168) has shewn still more completely the deriva-
tion of the stellate mesoblast cells from the epiblast. He finds that the
calcareous spicula develop in these cells as in the mesoblast cells of
sponges. The branched gastrovascular canals in this tissue are out-
growths of the primitive enteric cavity. A layer of circular muscles is
formed at a late period from the epiblast, but the longitudinal muscles of
the mesenteries on the inner side of the homogeneous membrane are
regarded by Kowalevsky as hypoblastic.

A ciliated planula with delaminated hypoblast is also found in Gorgonia
and Corallium rubrum. In the former genus at the time when the larva
becomes fixed, the hypoblast is formed of two strata, an outer one of
columnar cells, and an inner one of round ciliated cells lining a central
enteric cavity. The inner layer is believed by Kowalevsky to become
eventually absorbed and to be homologous with the inner granular mass of
Alcyonium.

Zoantharia. Amongst the Zoantharia several forms have
been investigated by Kowalevsky (147) and Lacaze Duthiers
(170), of which some are stated by the former author to pass
through an invaginate gastrula stage, while in other instances
the hypoblast is probably formed by delamination.

To the first group belongs an edible form of Sea Anemone
found near Messina, Cerianthus, and perhaps also Caryophyllium.
In the first of these segmentation results in the formation of a
blastosphere. A normal invagination obliterating the segmenta-
tion cavity then ensues, and the blastopore narrows to form the
mouth. The borders of the mouth bend inwards and so give
rise to the gastric cavity (stomoda^um) which as in the Alcyonidae
is lined by epiblast. Simultaneously with the formation of the
mouth there appear the two first mesenteries.

In Cerianthus the segmentation is unequal, the early stages are the
same as in the Actinia just described, but the hypoblast cells give rise

1 The German abstract is very obscure as to the formation of the muulh.

CCELENTERATA. 169

to a mass of fatty material filling up the enteric cavity, which becomes
eventually absorbed.

In the majority of the Zoantharia so far investigated, includ-
ing species of Actinia, Sagartia, Bunodes, Astroides, Astraea, the
segmentation, which is often unequal1 and not accompanied by
the formation of a segmentation cavity, results in a solid two-
layered ciliated planula. In these forms the impregnation takes
place in the ovary, and the early stages of development are
passed through in the maternal tissues.

One end of the planula becomes somewhat oval and develops
a special bunch of cilia. At the other end a shallow depression
appears, which becomes deeper and forms an involution lined by
epiblast. This involution is the stomodaeum, and becomes the
so-called gastric cavity. The true enteric cavity lined by hypo-
blast is for some time filled with yolk material. The larva
always swims with the aboral end directed forwards.

Between the two embryonic layers a homogeneous membrane
is formed, similar to that already described in the Alcyonidae.

The further development of the larvae especially concerns the formation
of mesenteries, tentacles and calcareous skeleton. With reference to this sub-
ject the observations of Lacaze Duthiers are especially valuable and striking.

In the adult it is usually possible to recognise in the tentacles a
symmetry of six. There are six primary tentacles, six secondary, twelve
tertiary, twenty-four quaternary, etc. In the hard septa of the skeleton
the same law is followed up to the third cycle, but beyond that, in the
cases where the point can be verified, there appear to be only twelve septa
in each additional cycle. The observations of Lacaze Duthiers have shewn
that this symmetry is only secondarily acquired and does not in the least
correspond with the succession of the parts in development.

His observations were conducted on three species of Zoantharia without
a skeleton, viz. Actinia mesembryanthemum, Sagartia, and Bunodes gem-
macea ; while Astroides calycularis served as the type for his investiga-
tions on the corallum. It will be convenient to commence with his
results on Actinia mesembryanthemum which served as his type.

The free cylindrical embryo, with the aboral end directed forwards in
swimming, first becomes somewhat flattened and the mouth elongated. A
bilateral symmetry is thus brought about. Two mesenteries now make
their appearance transversely to the long axis of the mouth, which divide
the enteric cavity into two unequal chambers. The mesenteries consist of
a fold of hypoblast with a prolongation of the epiblast between the two

1 I have this on the authority of Kleinenberg. The existence of an unequal
segmentation probably indicates an epibolic gastrula.

ZOANTHARIA.

limbs of the fold. The larger chamber next becomes divided by two fresh
mesenteries into three, and a similar division then takes place in the smaller
chamber. The stage with six chambers is almost immediately succeeded
by one with eight, owing to the appearance of two fresh mesenteries in the
second-formed set of chambers. At the stage with eight chambers there is a
marked period of repose. The number of chambers is increased to ten by
the division of the third-formed set of chambers, and to twelve by the
division of the fourth-formed set. It will be observed that the number of
the chambers increases in arithmetical progression by the continual addition
of two, alternately cut off from the primitive large and small chambers.
The freshly formed chambers are always formed immediately on one side of
the primitive mesenteries. The stages with six and ten are of very short
duration. The two primitive chambers are necessarily at the ends of the
long axis of the mouth. After the division of the enteric cavity into twelve
chambers, these chambers become about equal in size, and the formation of
the tentacles commences. The law regulating the appearance of the
tentacles is nearly the same as that for the mesenteries, but is not quite so
precise. One tentacle makes its appearance for each chamber. The most
remarkable feature in the appearance of the tentacles is due to the fact that
the tentacle surmounting the primitive largest chamber arises before any
of the others, and long retains its supremacy (fig. 80 A). This fact, coupled
with the inequality of the two primitive chambers, supplies some grounds
for speculating on a possible descent of the Ccelenterata from bilaterally
symmetrical forms with dis-
tinctly differentiated dorsal and
ventral surfaces. The supre-
macy of the first-formed tentacle
is not confined to the Actinozoa,
but as has already been indi-
cated, is also found in the Scy-
phistoma (p. 166) of the Acras-
peda.

After the twelve tentacles
have become established they
become secondarily divided into
two cycles of six respectively
larger and smaller tentacles,
which alternate with each other.
The two tentacles pertaining to
the two original chambers be-
long to the cycle of larger ten-
tacles. The mesenteric fila-
ments appear first of all on the
primary pair of septa. The
increase in the number of ten-
tacles and chambers from 12 to 24 has been found to take place in a very

FlG. So. Two STACKS IN Till', HKVKI.OP-

MENT OF ACTINIA MESKMHRYANTHEMUM.
(After Laea/.e Duthicrs.)

In the younger ciliated embryo A, vkuol
from the side, only one tentacle is developed.
m. mouth.

The oMer larva B is viewed from the face
when 24 tentacles have just become establislu-d.
The letters shew the true order of succession of
the tentacles; but c and/are transposed.

CCELENTERATA. 171

remarkable and unexpected way. The law is expressed by Lacaze Duthiers
as follows. "The appearance of the new chambers is not, as has been
believed, a consequence of the production of a single chamber between each
of the twelve already existing chambers, but of the birth of two new
chambers in each of the six elements (chambers) of the smaller cycle." The
result of this law is that a pair of tentacles of the third cycle is placed in
every alternate space, between a large and a small tentacle, of the two
already existing cycles, which may conveniently be called the first and
second cycles (fig. 80 B).

The twenty-four tentacles formed in the above manner are obviously at
first very irregularly arranged (fig. 80 B), but they soon acquire a regular
arrangement in three graduated cycles of 6, 6 and 12. The first cycle of the
six largest tentacles is the large cycle of the previous stage, but the two
other cycles are heterogeneous in their origin, each of them being composed
partly of the twelve tentacles last formed, and partly of the six tentacles
of the second cycle of the previous stage.

The further law of multiplication has been thus expressed by Lacaze
Duthiers: "The number of chambers and still later that of the corre-
sponding tentacles is carried from 24 — 48 and from 48 — 96 by the birth of
a pair of elements in each of the 12 or 24 chambers, above which are placed
the smallest tentacles which together constitute the fourth or fifth cycle.
Since, after the formation of each fresh cycle, the arrangement of the
tentacles again becomes symmetrical, it is obvious that all the equal sized
cycles except the first are formed of tentacles entirely heterogeneous as to
age."

The fixation of the free swimming larva takes place during the period
when the tentacles are increasing from 12 to 24.

The general formation of the chambers in Bunodes and Sagartia is
nearly the same as in Actinia.

In the two types of Actinozoa with an embolic gastrula stage the laws
as to the formation of the tentacles do not appear to be the same as those
regulating the forms observed by Lacaze Duthiers.

In Cerianthus four tentacles are formed simultaneously at the period
when only four chambers are present. In Arachnitis (Edwarsia) the suc-
cession of the tentacles is stated (A. Agassiz, 166) to resemble that in
Cerianthus. There are originally four tentacles, and at one extremity of the
long axis of the mouth are the oldest tentacles, while at the other tentacles
are constantly added in pairs. An odd tentacle is always found at the
extremity of the mouth opposite the oldest tentacles.

In the other species with an embolic gastrula eight tentacles would seem
to appear simultaneously at the period when eight chambers are present ;
though on this point Kowalevsky's description is not very clear. The
presence of such a stage would seem to indicate a close affinity to the
Alcyonidas.

Amongst the sclerodermatous Actinozoa, except Caryophyllium, the
embryo closely resembles that of the delaminate Malacodermata. The first

ZOANTHARIA.

stages occur in the ovary, and the larva is dehisced into the body cavity as
a two-layered ciliated planula.

The laws affecting the formation of the first twelve tentacles and septa
appear to be nearly the same as for the Malacodermata. The hard parts
begin as a rule to be formed when twelve tentacles have appeared, at which
period also the fixation of the larva takes place. On fixation the larva
becomes very much flattened.

The first parts of the corallum to appear are twelve of the septa, which
arise simultaneously in folds of the enteric wall in the chambers between
the mesenteries, and correspond therefore with the tentacles and not, as
might be supposed, with the mesenteries. Each septum is formed by the
coalescence of three calcareous plates which originate in separate centres of
calcification. The concrescence of the three produces a Y-shaped plate
with the single limb directed inwards and the two limbs outwards (fig. 81).
The theca does not arise till after the septa have become formed, and is at
first a somewhat membranous cup quite distinct from the septa. The
columella is formed still later by the coalescence of a series of nodules which
are formed in a central axis enclosed by the inner ends of the septa.

After the formation of the theca the
septa become divided into two cycles by
the predominant growth of six of them.
On the coalescence of the septa with the
theca the space between the two limbs of
the Y becomes filled up with calcareous
tissue. The law of the formation of the
third cycle of septa (12 — 24) has not been
worked out, so that it is not possible to
state whether it follows the peculiar prin-
ciples regulating the growth of the tentacles.

The whole of the skeletal parts occupy
a position between the epiblast and hypo-
blast, and are exactly homologous in this
respect with the skeleton of the Alcyonidae.
By Lacaze Duthiers they are however be-
lieved to originate in the hypoblast, but
from the observations of Kowalevsky there
can be little doubt that they arise in the
connective tissue between the two embry-
onic layers which is probably epiblastic in
origin.

A peculiar larva, probably belonging to the Actinozoa, has been described
by Semper1. It has an elongated form and is provided with a longitudinal
ridge of cilia. There is a mouth at one end of the body and an anus at
the opposite extremity. The mouth leads into an oesophagus, which opens

FIG. 81 . LARVA OF ASTROIDES
CALYCULARIS SHORTLY AFTER IT

HAS BECOME ATTACHED. (After

Lacaze Duthiers.)

The figure shews the develop-
ment of the Y-shaped septa in the
intervals between the mesenteries.
The position of the latter is in-
dicated by the faint shading. The
theca has become developed ex-
ternally.

Ueb. einige tropische Larven-formen." Zeit. f. iviss. ZooL, vol. xvn. 1867.

CCELENTERATA. 173

freely into a stomach with six mesenteries. In the skin are numerous thread-
cells. A mesotrochal worm-like larva, also provided with thread-cells, and
found at the same time, was conjectured by Semper to be a younger form of
this larva.

Cfcenophora. The ovum of the Ctenophora is formed of an
outer granular protoplasmic layer and an inner spongy mass with
fatty spherules. It is enveloped in a delicate vesicle, the diame-
ter of which is very much greater than that of the contained
ovum. This vesicle appears to be filled with sea-water, in which
the ovum floats.

Fertilized ova may usually be easily obtained by keeping the
captured adults in water from 12 — 24 hours. The two main
authorities on the development of these forms (Kowalevsky, No.
147 and 178 and Agassiz, No. 172) are unfortunately at variance
on one or two of the most fundamental points. It seems how-
ever that the embryonic layers are formed by a kind of epibolic
gastrula ; while the true gastric cavity, as distinct from the gas-
trovascular, is formed by an invagination, and deserves therefore
to be regarded as a form of stomodaeum.

The early stages are very closely similar in all the types so far
observed. Segmentation commences by the outer layer of the
ovum, which throughout behaves as the active layer, forming a
protuberance at one pole, which may be called the formative
pole. Close below this protuberance is placed the nucleus. In
the median line of the protuberance a furrow appears (fig. 82 A),

FlG. 82. FIVE STAGES IN THE DEVELOPMENT OF IDYIA ROSEOLA. (After Agassiz. )

The protoplasmic layer of the ovum is represented in black.

which gradually deepens till it divides the ovum into two. The
granular layer follows the furrow so that each of the fresh seg-
ments, like the original ovum is completely invested by a layer

1/4 CTENOPHORA.

of granular protoplasm. Each segment contains a nucleus. A
second similar division at right angles to the first gives rise to
four segments (fig. 82 B), and the segments so formed become
again divided into eight (fig. 82 C). In the division into eight,
which takes place in a vertical plane, the segments formed are of
unequal size, four of them being much smaller than the others.
The eight segments are arranged in the form of a slightly curved
disc round a vertical axis — the future long axis of the body ; —
and there is a cavity in this axis which, like the segmentation
cavity of Sycandra raphanus^ is open at both extremities. The
disc with its concavity on the side of the formative pole has the
shape sometimes of an ellipse (fig. 82 C) and sometimes of a
rectangle, in which the four small spheres occupy the poles of
the longer axis. A bilateral symmetry is thus even at this stage
clearly indicated.

In the next phase of segmentation the granular layer sur-
rounding each segment again forms a protuberance at the forma-
tive pole, but, instead of each segment becoming divided into
two equal parts, the protoplasmic protuberance alone is divided
off from the main segment. In this way sixteen spheres become
formed, of which eight are large and are formed mainly of the
yolk material of the inner part of the ovum, and eight are small
and entirely composed of the granular protoplasm. The eight
small spheres form a ring on the formative surface of the large
spheres (fig. 82 D).

The small spheres now increase very rapidly (fig. 82 E), partly
by division and partly by the formation of fresh cells from the
large spheres ; and spread over the large spheres, forming in this
way an epibolic gastrula. They constitute a layer of epiblast.
(Fig. 83 A.) The large cells in the meantime remain relatively
passive, though during the process they divide, in some cases
more or less irregularly, while in Eucharis they divide into six-
teen. The axial segmentation cavity would seem during the
process to become obliterated.

There is an important discrepancy between the statements of Kowalevsky
and Agassiz as to the course of the growth of the small cells. According
to Agassiz the small cells grow most rapidly at the formative pole and cover
this before they meet at the opposite pole. The reverse statement is made
by Kowalevsky. It would seem that the above discrepancy is due to an

CCELENTERATA.

175

interchange on the part of the one or the other of these authors of the two
poles of the embryo, in that according to Agassiz the formation of the mouth
takes place at the formative pole, and according to Kowalevsky at the pole
opposite to this.

Without attempting to decide between the above views, we shall speak of
the pole at which the mouth is formed as the oral pole.

The formation of the alimentary cavity commences shortly
after the complete investiture of the embryo by the epiblast
cells. At the oral pole an invagination of epiblast cells takes
place (fig. 83 B), which makes its way towards the opposite pole.
More especially from the figures given by Agassiz, and from the
explanation of his plates, it would seem that a large chamber is
formed in the hypoblast at the end of the invaginated tube, into
which this tube soon opens (fig. 83 C). The invaginated tube
would seem to give rise to the so-called stomach, while the
chamber at its aboral extremity is no doubt the infundibulum,
which as may be gathered from Kowalevsky 's statements, is lined
by a flattened epithelium. At a later period the gastrovascular
canals grow out from the infun-
dibulum as four pouches, which
are surrounded by, and grow at
the expense of, the large central
cells, which have in the mean-
time arranged themselves in
four masses, and appear to serve
as a kind of yolk. The nuclei
of these large cells according to
Kowalevsky disappear, and the
cells themselves break up into
continually smaller masses.

FlG. 83. FOUR STAGES IN THE DE-
VELOPMENT OF IDYIA ROSEOLA. (After
Agassiz.)

s.c, sense capsule; st. stomodreum.

The main difficulty in the above
description of Agassiz is the origin of
the infundibulum. In the absence of
definite statements on this head it
seems reasonable to conclude that it arises as a space hollowed out in the
central cells, and that its walls are formed of elements derived from the yolk
cells1. On this interpretation the alimentary canal of the Ctenophora would

1 Chun (No. 174) gives a short statement of his observations, which accords with
the interpretation in the te*t.

176 CTENOPHORA.

consist, as in the Acraspcdote Medusa: and Actinozoa, of two sections: (i)
A true hypoblastic section consisting of the infundibulum and the gastro-
vascular canals derived from it ; and (2) an epiblastic section— the stomo-
d.i-um — forming the stomach.

The observations of Kowalevsky on the alimentary system do not wholly
tally with those of Agassiz. He finds that the oral side of the embryo
becomes hollowed out, and that the hollow, lined by flattened cells, becomes
constricted off as the infundibulum, from which the radial canals subse-
quently grow out. To the infundibulum there leads a narrow canal lined by
a columnar epithelium which becomes the gastric cavity.

While the alimentary canal is becoming formed a series of
important changes takes place in other parts of the embryo.
The rows of locomotive paddles first appear as four longitudinal
equidistant linear thickenings of the epiblast near the aboral pole
(fig. 83 D). On the projecting surface of these ridges stiff cilia
appear which coalesce together to form the paddles. While the
embryo is still within the egg the rows of paddles are quite short
and also double. There arc in Pleurobrachia about eight or nine
pairs of paddles in each row. Each double row eventually sepa-
rates into two.

In all the forms except the Eurostomata (Beroe) two tentacles
grow out as thickenings of the epiblast (fig. 84 B, /.). They are
placed at the opposite poles of the long transverse axis of the
embryo.

A process of the contractile gelatinous tissue of the body, the
origin of which is described below, makes its way, according to
Kowalevsky, into the tentacles.

The central apparatus of the nervous system and the otoliths
are formed at the aboral pole from a thickening of the epiblast,
but the full details of their formation have not been elucidated.
It may be well to preface my account of their development with
a short statement of their adult structure.

They consist in the adult of a vesicle with a ciliated lining situated at
the bifurcation of the two anal tubes, and of certain structures connected
with this vesicle. From the floor of the vesicle is suspended a mass of
otoliths by four leaf-like bodies known as suspenders. The roof is very
• tc ami IMS the form of a four-sided pyramid. Six openings lead into
the vesicle. Through four of these, placed at the four corners, there pass
out four ciliated grooves continuous with the suspenders. These grooves,
after leaving the otolithic vesicle, bifurcate and pass to the eight rows of
paddies. At the two sides the walls of the vesicle are continuous with two

CCELENTERATA. 177

thickened ciliated plates with swollen edges, opposite the centres of which
are two lateral openings into the vesicle, completing the six openings.
Through the lateral openings the sea water is driven by the action of the
cilia of the plates.

The development of these parts is as follows — In the aboral
thickening of epiblast a cavity makes its appearance, the walls
of which constitute the rudiment of the otolithic vesicle (fig. 83 B
and C, s.c.). The roof of the cavity is extremely delicate. On
each side of it a thickening of cells becomes established, regarded
by Kowalevsky as the rudiment of the nervous ganglia. These
thickenings appear to give origin to the lateral ciliated plates.
The otoliths arise from cells at four separate points at the corners
of the ciliated plates opposite the rows of paddles (fig. 84 A, ot.).

In Pleurobrachia there is at first only one otolith at each
corner. The otoliths are gradually transported towards the
centre of the vesicle (fig. 84 B, ot) and are there attached, though
the four leaf-like suspenders do
not arise till very late. The oto-
liths go on increasing in number
throughout life.

The gelatinous tissue of the
Ctenophora appears as a homoge-
neous layer between the epiblast
and the yolk-cells, and is probably
homologous with the layer formed FlG. ^ Two STAGES IN THE

in the same situation in all other DEVELOPMENT OF PLEUROBRAcnrA

, r TII RHODODACTYLA. (After Agassiz.)

ccelenterate forms. Into the layer

* of. otolith ; /. tentacle.

a number of anastomosing cells,

mainly derived from the epiblast, though according to Chun
(No. 174) also in part from the hypoblast, make their way.
These cells would appear to be mainly, if not entirely (Chun),
of a contractile nature. It is probable that the great mass of the
gelatinous tissue of the adult is an intercellular substance derived
from these cells.

The whole of the above changes are completed while the
embryo is still enclosed in the egg capsule. During their
accomplishment the oro-anal axis, which was originally very
short, increases greatly in length (fig. 83), so that the embryo
acquires an oval form similar to that of the adult.

B. IT. 12

I78

SUMMARY.

The exact period of leaving the egg does not appear to be very constant
but the hatching never takes place till the embryo has practically acquired
all the organs of the adult.

In the majority of types the differences between the just hatched larva
and the adult are inconsiderable, and in all cases the larva has a somewhat
oval form. In the case of the Tasniatae (Cestum, etc.), the larva has the
characteristic oval form, and the subsequent changes amount almost to a
metamorphosis.

The larva of the Lobatae, such as Eucharis, Bolina, etc., can hardly be
distinguished from Pleurobrachia, and undergoes therefore considerable
changes after hatching.

Eucharis multicornis while still in the larval condition is stated by Chun
to become sexually mature.

The new genus Ctenaria recently described by Haeckel,
which is intermediate between the Ctenophora and the Medusae
clearly proves that the Ctenophora are more closely related to
the Medusae than to the Actinozoa ; but their development,
especially the presence of a stomodaeum, shews that they have
affinities (in spite of the rudimentary velum of Ctenaria) with the
Acraspedote as well as with the Craspedote Medusae ; and it
may be noted that the Acraspeda have undoubted affinities with
the Actinozoa.

Summary and general considerations.

Even in the adult condition the lower forms of Ccelenterata
do not rise in complexity much beyond a typical gastrula.
Ontogeny nevertheless brings clearly to light the existence of a
larval form — the planula — which recurs with fair constancy
amongst all the groups except the Ctenophora.

We are probably justified in assuming that the planula is a
repetition of a free ancestral form of the Ccelenterata. The pla-
nula, as it most frequently occurs, is a two-layered ciliated nearly
cylindrical organism, with at most a rudimentary digestive cavity
hollowed out in the inner layer, and as a rule no mouth. In the
outer layer are numerous thread-cells.

How many of these characters did the ancestral planula possess ? I think
it is not unreasonable to assume that the only two characters about which
there can be much doubt are the rudimentary condition of the digestive
cavity and the absence of a mouth. Paradoxical as it may seem, it appears to
me not impossible that the Ccelenterata may have had an ancestor in which a
digestive tract was physiologically replaced by a solid mass of amoeboid cells.

COELENTERATA. 179

This ancestor was perhaps common to the Turbellarians also. The constant
presence of thread-cells in the inner layer of their epiblast fits in with their
derivation from a form similar to the planula. While the solid parenchy-
matous digestive canal of Convoluta and Schizoprora and other forms
amongst the Turbellarians, though very probably secondary, may perhaps
be explained by such a view of their origin.

The planula in its primitive condition is not bilaterally symmetrical, but
frequently, as amongst the Actinozoa, it becomes flattened on two sides before
undergoing its conversion into the adult form. Perhaps the bilateral form
of planula is the starting point both for the Ccelenterata and the Turbellaria.
In this connection the peculiar unilateral development of a tentacle in
Scyphistoma and Actinia should be noted.

The planula occurs in the majority of sessile forms of Hydro-
zoa except the Tubularidae and Hydra. It is also characteristic
of the Trachy medusae and Siphonophora. Amongst the Acras-
peda it is also present, but has an exceptional mode of ontogeny
which is discussed in connection with the germinal layers.

It is characteristic both of the Octocoralla and Hexaco-
ralla, but is not found in the Ctenophora.

In the Tubularidae and in Hydra an abbreviated develop-
ment leads no doubt to the absence of a free planula stage, and
the absence of a larval form amongst the Ctenophora may, as has
already been stated, be probably explained in the same way.

The Ccelenterata of all the Metazoa are characterized by the
greatest simplicity in the arrangement of their germinal layers ;
and for this reason very considerable interest attaches to the
mode of formation of the layers amongst them. Two germinal
layers are constantly found, which correspond in a general way
to the epiblast and hypoblast. It might have been anticipated
that a certain amount of uniformity would have existed in the
mode of formation of the layers. This however is not the case.
In perhaps the majority of forms they become differentiated by
a process of delamination, but in a not inconsiderable minority
the two layers owe their origin to an invagination.

Delamination is constant (with the doubtful exception of
some Tubularidae) amongst the Hydromedusae and Siphono-
phora. It is perhaps in the main characteristic of the Actino-
zoa.

Invagination by embole takes place, so far as is known, con-
stantly amongst the Acraspeda and frequently amongst the

12 — 2

,gO SUMMARY.

Actinozoa ; and an epibolic invagination is characteristic of the
Ctenophora.

If confidence is to be placed in the recorded observations on
which this summary is founded, and there is no reason why in a
general way it should not be so placed, the conclusion is inevit-
able that of the above modes of development the one must be
primitive and the other a derivative from it, for, if this conclusion
be not accepted, the absolutely inadmissible hypothesis of a
double origin for the Coelenterata would have to be adopted.

Two questions arise from these considerations : —

(1) Which is the primitive, delamination or invagination ?

(2) How is the one of these to be derived from the other ?

There is a great deal to be said in favour of both delamin-
ation and invagination ; but it will be convenient to defer all
discussion of the question to the general chapter on the forma-
tion of the layers throughout the animal kingdom.

The hypoblast cells are often filled with yolk material, and
secondary modifications are thus produced in the development.
The most important examples of such modifications are found in
the Siphonophora and Ctenophora.

In the simplest forms amongst the Hydrozoa there is no trace
of a third layer or mesoblast. The epiblast is typically formed,
as was first shewn by Kleinenberg, of an epithelial layer and a
subepithelial interstitial layer of cells. The cells of the former
are frequently produced into muscular or nervous tails, and those
of the latter give rise to the thread cells and generative organs
and in some cases to muscles1. In many cases, amongst all the
Ccelenterate groups, and constantly amongst the Ctenophora the
epiblast is simplified and reduced to a single layer. The hypo-
blast undergoes in most cases no such differentiation but simply
forms a glandular layer lining the gastric chamber and its pro-
longations into the tentacles ; but in the Actinozoa it appears to
give rise to muscles, and strong evidence has been brought for-
ward to shew that in some groups it gives rise to the generative
organs.

Between the epiblast and hypoblast a structureless lamella
appears always to be interposed.

1 The questions relating to the generative organs of the Coelenterata are dealt with
in the second part of this work.

CCELENTERATA. l8l

In many Coelenterata further differentiations of the epiblast
are present. In many forms the layer gives rise to a hard exter-
nal skeleton. This is most widely spread amongst the Hydrozoa,
where in the majority of cases it takes the form of the horny
perisarc, and in the Hydrocoralla (Millepora and Stylasteridse)
of a hard calcareous skeleton. The skeleton in these forms,
though closely resembling the mesoblastic skeleton of the Actin-
ozoa, has been shewn by Moseley (164) to be epiblastic.

In the Actinozoa an epiblastic skeleton is exceptional, and
according to most authorities absent. Quite recently however
Koch (167) has found that the axial branched skeleton of most
of the Gorgonidae, viz. the Gorgoninae and Isidinae, is separated
from the ccenosarc by an epithelium, which he believes to be
epiblastic, and to which no doubt the axial skeleton owes its
origin. A similar epithelium surrounds the axis of the Penna-
tulidae.

In the Medusae the epiblast also gives rise to a central
nervous system, which however continues to form a constituent
part of the layer, and to the organs of special sense1.

A special differentiation of the hypoblast is found in the
solid axis of the tentacles. This axis replaces the gastric
prolongation found in many forms, and the cells composing
it differentiate themselves into a chorda-like tissue, which has
a skeletal function, and is no longer connected with nutrition.
This axis is placed by many morphologists amongst the meso-
blastic structures.

In all the higher Ccelenterata certain tissues become inter-
posed between the epiblast and hypoblast, which may be classi-
fied together as the mesoblast.

The most important of these are

(1) The various distinct muscular layers.

(2) The gelatinous tissue of the Medusae and Ctenophora.

(3) The skeletogenous tissue of the Actinozoa.

In most cases the muscular fibres are connected with epithe-
lial cells, but in certain forms amongst the Medusae and in the
majority if not all the Actinozoa they constitute a distinct layer,
sometimes separated from the epiblast by a structureless mem-

1 The differentiation of the nervous and muscular systems in the Hydrozoa is
treated of in the second part of this work.

ALThK NATIONS OF GENERATIONS.

branc, ALquorca Mitrocoma. Such layers when on the outer
of the membrane separating epiblast and hypoblast are
undoubtedly epiblastic in origin, but in some cases amongst the
Actinozoa they adjoin the hypoblast, and are very probably
derived from this layer.

The origin of the gelatinous tissue is still involved in much
obscurity.

It originates as a homogeneous layer between epiblast and
hypoblast, which in the Hydromedusae never becomes cellular
though traversed by elastic fibres.

In the Acraspeda it contains anastomosing cells in the main
apparently (Claus) derived from the hypoblast, and in the Cteno-
phora it is richly supplied with muscular stellate cells for the
most part of epiblastic origin, though some are stated by Chun
to come from the hypoblast. On the whole it seems probable,
that the gelatinous tissue may be regarded as a product of both
layers ; and there are some grounds for thinking that it is an
immense development of the membrane always interposed be-
tween the two primary layers. It must however be borne in mind
that a membrane, regarded by the Hertwigs as the equivalent of
the ordinary membrane between the epiblast and hypoblast, can
be usually demonstrated on both surfaces of the gelatinous
tissues in Medusae. The skeletogenous layer of the Actinozoa
is probably the morphological homologue of the gelatinous
tissue ; but the evidence we have is on the whole in favour of the
connective-tissue cells it contains being epiblastic in origin. It
gives rise to the skeleton of the Hexacoralla, to the spicular
skeleton of Alcyonium, the axial skeleton of Corallium, and the
skeleton of the Helioporidae and Tubiporidae.

Alternations of generations.

Alternation of generations is of common occurrence amongst
the Hydrozoa, and something analogous to it has been found to
take place in Fungia amongst the Actinozoa. It is not known
to occur in the Ctenophora.

The chief interest of its occurrence amongst the Hydro-
medusae and Siphonophora is the fact that its origin can be

CCELENTERATA. 183

traced to a division of labour in the colonial systems of zooids
so characteristic of these types.

In the Hydromedusae an interesting series of relations
between alternation of generations and the division of the zooids
into gonophores and trophosomes can be made out. In Hydra
the generative and nutritive functions are united in the same
individual. The generative swellings in these forms cannot, as
has been ably argued by Kleinenberg, be regarded as rudimen-
tary gonophores, but are to be compared to the generative bands
developed in the Medusae around parts of the gastro-vascular
system. A condition like that of Hydra, in which the ovum
directly gives rise to a form like its parent, is no doubt the
primitive one, though it is not so certain that Hydra itself is a
primitive form. The relation of Hydra to the Tubularidae and
Campanularidae may best be conceived by supposing that in
Hydra most ordinary buds did not become detached, so that a
compound Hydra became formed ; but that at certain periods
particular buds retained their primitive capacity of becoming
detached and subsequently developed generative organs, while
the ordinary buds lost their generative function.

It would obviously be advantageous for the species that the
detached buds with generative organs should be locomotive, so
as to distribute the species as widely as possible, and such buds
in connection with their free existence would naturally acquire a
higher organization than the attached trophosomes. It is easy
to see how, by a series of steps such as I have sketched out, a
division of labour might take place, and it is obvious that the
embryos produced by the highly organized gonophores would
give rise to a fixed form from which the fixed colony would be
budded. Thus an alternation of generations would be estab-
lished as a necessary sequel to such a division of labour. To
test the above explanation it is necessary to review the main
facts with reference to alternations of generations amongst the
Hydromedusae.

Hydromedusae1. In many instances amongst the Tubula-
ridae, Sertularidae and Campanularidae medusiform buds are
produced which become detached and develop sexual organs.

1 For a full account of this subject the reader is referred to the beautiful memoir
of Allman (No. 149).

IS4 ALTERNATIONS OF GENERATIONS.

Such Medusa arc divided into two great groups, the Ocellata and
Vesiculata, according to the characters of the marginal sense organs.
In the Ocellata the sense organs have the form of eyes, and in the Vesiculata
of auditory vesicles. The latter seem to be usually budded off from the
Campanularia stocks, and the generative organs extend in folded bands over
the radial canals. These bands have been regarded by Allman as composed
of rudimentary gonophores, and he called the Medusae which give rise to
them blastochemes. He regards them as representing a more complicated
type of alternation of generations with three instead of two generations in
the series. The Hertwigs have brought what appear to me conclusive
grounds for rejecting this view, and have demonstrated that the generative
organs of these types resemble those of ordinary Medusas.

In many forms the medusiform buds though fully developed
do not become detached ; whether detached or not they are
known as phanerocodonic gonophores. In other forms
again buds which begin as if they were going to form Medusae
never reach that condition but remain permanently in an unde-
veloped state. They have been called by Allman adelocodonic
gonophores.

In all the above cases two generations at the least interpose
between the successive sexual periods, viz. : —

(1) A trophosome produced directly from the ovum.

(2) A gonophore budded from this.

In a very large number of types the gonophores do not
develop directly on the hydroid stem, but arise on specially
modified zooids resembling rudimentary trophosomes which
have been named blastostyles by Allman. On the sides of
each blastostyle a series of gonophores usually becomes de-
veloped. The blastostyles either remain exposed as in all the
Gymnoblastic or Tubularian Hydroids, or as in all the Calypto-
blastic Hydroids (Sertularidse and Campanularidae) they become
invested by a special case — known as the gonangium — which
is formed of perisarc lined by epiblast. In the forms with
blastostyles three generations interpose between the successive
stages of sexual reproduction, (i) the trophosome developed
directly from the ovum, (2) the blastostyle budded from this, (3)
the gonophore budded from the blastostyle.

Such being the main facts, in order to prove that the existing condition
of polymorphism amongst the Hydromedusae is to be explained as hypo-
t helically suggested above, it is still necessary to shew that (i) the free

CCELENTERATA. 185

medusiform gonophores are really only modified trophosomes, or rather that
the trophosomes and gonophores are both modifications of some common
type, and (2) that the fixed so-called adelocodonic gonophores are retrograde
derivatives of the free medusiform gonophores. Unless these points can be
established it might be maintained that the Medusae were special zooids,
developed de novo and not by a modification of trophosome zooids. To
demonstrate these propositions at length would carry me too far into the
region of simple Comparative Anatomy, and I content myself with referring
the reader to a discussion of the Hertwigs (No. 146, p. 62) where the first
point appears to me fully established. With reference to the second point I
will only say that the structure and development of the adelocodonic gono-
phores can only be explained on the assumption that they are retrograde
forms of the phanerocodonic gonophores, and that the opposite view, that
the phanerocodonic gonophores are derived from the adelocodonic, leads to
a series of untenable positions.

The Trachymedusae, as has been shewn above, develop directly. They
are probably derived from gonophores in which the trophosome has dis-
appeared from the developmental cycle.

To sum up, three types of development are found amongst
the Hydromedusae.

(1) No alternations of generations. Permanent form, a
sexual trophosome. Ex. Hydra.

(2) Alternations of generations. Trophosome fixed, gono-
phore free or attached. Ex. Gymnoblastic and Calyptoblastic
Hydroids, and Hydrocoralla.

(3) No alternations of generations. Permanent form, a
sexual Medusa. Ex. Trachymedusse.

Siphonophora. In the Siphonophora alternations of gener-
ations take place in the same way as in the Hydromedusae, but
the starting point appears to be a Medusa. The gonophores
may remain fixed or become detached.

Acraspeda. With the exception of Pelagia and Lucernaria,
in which the development involves a simple metamorphosis, all
the Acraspeda undergo a form of alternations of generations.
The ovum, as already described, develops into a fixed form — the
Scyphistoma — which increases asexually by normal budding,
and can even form a permanent colony.

The formation of the sexual Medusa form takes place by a
kind of strobilization of the body of the fixed Scyphistoma.
A series of transverse constrictions becomes formed round
the body below the mouth, dividing it up into corresponding

186

ALTERNATIONS OF GENERATIONS.

rings, each of which eventually gives rise to a Medusa known
as an Ephyra (fig. 85). In each
of these rings is a dilation of the
stomach, and a section of each of
the four rudimentary mesenteries
described in connection with the de-
velopment of the Scyphistoma. As
the constrictions become deeper the
segments of the body between them
become disc-like, and their edges
are produced into eight lobes con-
taining prolongations of the gastric
cavity (fig. 85 C). The lower sur-
face of each disc, which forms the
future aboral surface of the Medusa,
becomes convex, in part owing to
the development of gelatinous tis-
sue. On the opposite surface a
muscular layer becomes developed.
the body of the Scyphistoma gradually grows in length and
continues to be segmented, so that a series of Ephyrae are
uninterruptedly formed, of which those near the base are the
youngest. The original terminal ring of tentacles of the
Scyphistoma gradually atrophies.

In the further development of the Ephyrae each of their eight
lobes becomes bifid at its extremity.

As the Ephyrae successively reach this condition they be-
come detached, and by a series of remarkable changes, amount-
ing almost to a metamorphosis, and accompanied by an enor-
mous growth in size, reach the adult condition.

The alternation of generations in the Acraspeda cannot be
quite so simply explained as in the Hydromedusae, though the
principle is probably the same in the two cases.

Actinozoa. Amongst the Actinozoa there occurs in Fungia a
peculiar process which is, as shewn by Semper (171), in many
ways analogous to alternations of generations1. From the larva
a nurse-stock is developed, at the end of which a cup-like coral

B

FIG. 85. THREE STAGES IN

THE ALTERNATIONS OF GENERA-
TIONS OF AURELIA AURITA. (From

Gegenbaur.)

A. Polype stage.

B. Commencing strobilization.

C. Completed strobilization.

During the above process

Vide also Moselcy. Notes by a Naturalist of the Challenger •, pp. 524 and 525.

CCELENTERATA. 1 87

resembling the adult is formed as a bud. The bud becomes
detached and then gives rise to a permanent sexual Fungia.
From the nurse-stock there is formed however a fresh bud at
the centre of the scar left on the detachment of the old one.
The fresh bud eventually becomes separated from the nurse-stock
leaving a small portion of its stem behind ; each succeeding bud
similarly leaves a small portion of its stem, so that the nurse-
stock eventually acquires a jointed appearance. In the above
process we clearly have, as in the Hydromedusae, a non-sexual
form — the nurse-stock — produced directly from the larva, giving
rise by budding to a sexual form ; all the conditions of an alter-
nation of generations are therefore fulfilled. It seems however
possible that the nurse-stock itself may eventually become sexual.

BIBLIOGRAPHY.
Coelenterata. General.

(145) Alex. Agassi z. Illustrated Catalogue of the Museum of Comparative
Anatomy at Harvard College, No. II. American Acalephse. Cambridge, U. S., 1865.

(146) O. and R. Hertwig. Der Organismus d. Medusa u. seine Stellung z.
Keimbldttertheorie. Jena, 1878.

(147) A. Kowalevsky. " Untersuchungen lib. d. Entwicklung d. Coelente-
raten." Nachrichten d. kaiser. Gesell. d. Freunde d. Naturer kenntniss d. Anthro-
pologie u. Ethnographic. Moskau, 1873. (Russian.) For abstract vide Jahresberichte
d. Anat. u. Phys. (Hoffman u. Schwalbe), 1873.

Hydrozoa.

(148) L. Agassiz. Contributions to the Natural History of the United Slates of
America. Boston, 1862. Vol. IV.

(149) G. J. Allman. A Monograph of the Gymnoblastic or Tubularian Hydroids.
Ray Society, 1871-2.

(150) G. J. Allman. "On the structure and development of Myriothela." Phil.
Trans., Vol. CLXV. p. 2.

(151) P. J. van Beneden. "Mem. sur les Campanulaires de la Cote d'Ostende
considered sous le rapport physiologique, embryogenique, et zoologique." Now.
Mem. de ? Acad. de Srux., Tom. xvn. 1844.

(152) P. J. van Beneden. " Recherches sur PEmbryog&ne des Tubulaires et
1'histoire naturelle des differents genres de cette famille qui habitent la Cote d'Ostende."
Nouv. Mem. de F Acad. de Brux., Tom. xvn. 1844.

(153) C. Claus. " Polypen u. Quallen d. Adria." Denk. d. math.-naturwiss.
Classed, k. k. Akad. d. Wiss. Wien, Vol. xxxvui. 1877.

(154) J. G. Daly ell. Rare and Remarkable Animals of Scotland. London,
1847.

(155) H. Fol. "Die erste Entwicklung d. Geryonideneies. " Jenaische Zeit-
schrift, Vol. VII. 1873.

(156) Carl Gegenbaur. Zur Lehre vom Generationswechsel und der Fort-
pflanzung bei Medusen und Polypen. WUrzburg, 1854.

1 88 BIBLIOGRAPHY.

(157) Thomas Hincks. "On the development of the Hydroid Polypes, Clava-
lella and Stauridia; with remarks on the relation between the Polype and the Medu-
soid, and between the Polype and the Medusa." Brit. Assoc. Rep., 1861.

(168) E. Haeckel. Zur Entwicklungsgeschichte d. Siphonophoren. Utrecht,
1869.

(159) Th. H.Huxley. Oceanic Hydrozoa. Ray Society, 1858.

(160) Geo. Johnston. A History of British Zoophytes. Edin. 1838. 2nd
Edition, 1847.

(161) N. Kleinenberg. Hydra, eine anatomisch-entwicklungsgeschichtliche
Untersuchung. Leipzig, 1872.

(162) El. Metschnikoff. "Ueber die Entwicklung einiger Ccelenteraten. "
Bull, tie CAcad. dt St Petersbourg, XV. 1870.

(163) El. Metschnikoff. "Studien iiber Entwicklungsgeschichte d. Medusen
u. Siphonophoren. Zeit.f. wiss. Zool., Bd. xxiv. 1874.

(164) H. N. Moseley. "On the structure of the Stylasteridae." Phil. Trans.
1878.

(165) F. E. Schulze. Ueber den Bau und die Entwicklung von Cordylophora
lacustris. Leipzig, 1871.

Actinozoa.

(166) Al. Agassiz. "Arachnitis (Edwarsia) brachiolata." Proc. Boston Nat.
Hist. Society, 1860.

(167) Koch. "Das Skelet d. Alcyonarien." Morpholog. Jahrbuch, Bd. iv.
1878.

(168) A. Kowalevsky. "Z. Entwicklung d. Alcyoniden, Sympodium coral-
loides und Clavularia crassa." Zoologischtr Anzeiger, No. 38, 1879.

(169) H. LacazeDuthiers. Histoire nat. du Corail. Paris, 1864.

(170) H. Lacaze Duthiers. " Developpement des Coralliaires. " Archives de
Zoologie experimentale et generate, Vol. i. 1872 and Vol. II. 1873.

(171) C. Semper. " Ueber Generationswechsel bei Steinkorallen etc." Zeit.
/. wiss. Zool., Bd. xxn. 1872.

Ctenophora.

(172) Alex. Agassiz. " Embryology of the Ctenophorse. " Mem.ofthe Amer.
Acad. of Arts and Sciences, Vol. x. No. in. 1874.

(173) G. J. Allman. "Contributions to our knowledge of the structure and
development of the Beroidae." Proc. Roy. Soc. Edinburgh, Vol. iv. 1862.

(174) C. Chun. Das Nervensystem u. die Musculatur d. Rippenquallcn."
Abkand. d. Senkenberg. Gesellseh., B. XI. 1879.

(175) C. Claus. "Bemerkungen u. Ctenophoren u. Medusen." Zeit. f. wiss.
Zool., xiv. 1864.

(176) H. Fol. Ein Beitrag z. A nat. u. Entwickl. einiger Rippenquallen. 1869.

(177) C. Gegenbaur. "Studien ii. Organis. u. System d. Ctenophoren."
Archivf. Natnrgesch., xxil. 1856.

(178) A. Kowalevsky. "Entwicklungsgeschichte d. Rippenquallen." Mem.
Acad. St Petersbourg, VII. serie, Tom. x. No. 4. 1866.

(179) J. Price. "Embryology of Ciliogrades." Proceed, of British Assoc.,
1846.

(180) C. Semper. "Entwicklung d. Eucharis multicornis." Zeit. f. wiss.
Zool., Vol. tx. 1858.

CHAPTER VII.

PLATYELMINTHES '.
TURBELLARIA.

ALTHOUGH there is perhaps no group in the animal kingdom
the ontogeny of which would better repay a thorough investiga-
gation than the Turbellarians, yet the difficulties to be overcome
have hitherto proved too great.

The fresh-water Rhabdocoela and Dendroccela do not under-
go any metamorphosis, and leave the ovum in a condition in
which they cannot easily be distinguished in their general appear-
ance from Infusoria. Many marine Dendroccela also develop
directly, while, as was first shewn by Joh. Miiller, other marine
Dendroccela undergo a more or less complicated metamorphosis.

Marine Dendroccela. Of the marine Dendroccela which do
not undergo a metamorphosis the form most fully worked out is
Leptoplana tremellaris — (vide Keferstein, No. 187, and Hallez,
No. 185).

The ova are surrounded by large albuminous capsules
secreted by a special gland. They are laid a great number at a

1 I. Turbellaria.

1. Dendrocoela.

2. Rhabdocoela.

II. Nemertea.

1. Anopla.

2. Enopla.

Hi. Trematoda.

1. Distomese.

2. Polystomeae.

iv. Cestoda,

TURBELLARIA.

time, and adhere together so as to form masses not unlike the
spawn of nudibranchiate Molluscs.

Within the egg-capsule the ovum floats freely and undergoes
a segmentation similar in many respects to the characteristic
molluscan type. The ovum divides into two, and then into four
parts, from each of which a small segment is then separated off.
The four small segments, which appear to give rise to the epi-
blast, increase in number by division and gradually envelop the
large segments1; so that an epibolic invagination clearly takes
place. Between the small and the large cells is a small segmen-
tation cavity, fig. 86 A and B. At the time when twelve epiblast
cells are present, each of the four large cells divides into two un-
equal parts (Hallez), fig. 86 A. In this way four large (Jiy) and
four small cells (;«) are formed. The latter are placed at the
opposite pole of the ovum to the epiblast cells, and give rise to
the mesoblast, while the four large cells remain as the hypoblast.

In the course of the enclosure of the hypoblast cells by the

FIG. 86. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN THREE
STAGES OF DEVELOPMENT. (After Hallez.)

ep. epiblast; m. mesoblast; hy. yolk cells (hypoblast); bl. blastopore.

epiblast, the mesoblast cells gradually travel towards the forma-
tive pole (fig. 86 B). In the process they become first of all
divided so as to form four linear streaks, and finally unite into a
continuous layer between the epiblast and hypoblast, which
obliterates the segmentation cavity (fig. 86 C, m).

Before the completion of the epibole a closely packed layer
of fine cilia appears, which causes a rotation of the embryo within
the egg-capsule. During the above changes a fifth hypoblast
cell is formed by the division of one of those already present ;
and at a later period four of the hypoblast cells give rise within

1 It is probable, though it has not been observed, that the growth of the layer of
small rclls is a^isted by the formation of fresh cells from the hypoblast spheres.

PLATYELMINTHES. 191

the nearly closed blastoporic area to four small cells. In con-
nection with these cells a complete hyploblastic wall becomes
subsequently established, which encloses the original large hypo-
blast cells. The latter then become resolved into a vitelline
mass.

From a comparison with other types it may be regarded as
probable that the enteric wall originates by a process of continu-
ous budding off of small cells from the large cells, which com-
mences with the formation of the four cells above mentioned.

The blastopore becomes nearly obliterated, but whether it
gives rise to the mouth, which is formed in the same place, has
not been determined. In front of the mouth a small and very
transitory rudiment of an upper lip makes its appearance. The
protrusible pharynx is stated by Hallez to arise as an hypoblastic
bud, while its sheath has an epiblastic origin. Two pairs of
eyes and the supra-cesophageal ganglia also become early
developed.

The peripheral ciliated layer of small cells becomes divided
into two strata, of which the outer remains ciliated and forms
the true epiblast : the inner probably forms the cutis. In it are
developed rod-like bodies, which seem to be homologous with
the thread cells of the Ccelenterata, so that if the views put
forward in the previous chapter as to the similarity of the turbel-
larian and ccelenterate larvae are correct, the cutis corresponds
with the deeper layer of the ccelenterate epiblast. The meso-
blast, like the epiblast, becomes divided into two strata. The
outer one is stated to form the circular and longitudinal muscles;
the inner one to give rise to a muscular reticulum, the spaces
within which constitute the parenchymatous body cavity.

The later changes are not of great importance. At a period slightly
after the formation of the mouth and ganglia two pairs of stiff hairs become
formed at the sides of the body. The embryo has by this time grown so as
to fill up its capsule, in which however it continues rapidly to rotate, and also
commences to exhibit active contractions. It next becomes hatched, and
passes from a spherical to a flattened elongated form. The ventral oral
opening is at first central, but soon, by a process of unequal growth, becomes
carried towards the posterior end of the body. The pairs of stiff hairs in the
meantime considerably increase in number. The remains of the yolk cells
now disappear, and the enteric walls become more distinct. The alimentary
canal, which is at first simple in outline like that of a rhabdoccelous Turbel-

192

TURBELLARIA.

larian, soon assumes a dendritic form. The young animal after these changes
resembles its parent, except in the possession of only two pairs of eyes and
in the absence of generative organs.

Of the types with a complete metamorphosis the free larvae
of various species of Thysanozoon have been observed by Joh.
Miiller (190) and Moseley (189),
and the complete development of
Eurylepta auriculata has been
studied by Hallez.

The stages within the egg of
this latter type agree precisely
with those already described in
Leptoplana. After the formation
of the mouth the body elongates,
remaining however cylindrical. A
fold forms on the anterior side of
the mouth, giving rise to a large
upper lip. Two posterior processes FIG. 87. LARVA OF EURYLEPTA

AURICULATA IMMEDIATELY AFTER

are next formed, and other pro- HATCHING.

cesses soon arise, constituting the SID^ j£J

whole of those found in the free

larva. The embryo next shakes off its egg membranes by a

series of vigorous contractions. When free it has the form repre-

sented in the annexed figure (fig. 87).

It is so similar to Miiller's (fig. 88) and Moseley's larvae that
all three may be dealt with together.

The body is somewhat oval, and slightly pointed behind.
At the anterior end are placed the eyes, two in the youngest
larva of Miiller, and twelve in the older larva (fig. 88), and in
the middle of the ventral surface is the mouth. It is surrounded
by a strong fold, and leads into an alimentary canal, which is at
first simple, but in the older larvae is much branched. A bilobed
ganglion connected with two nerve cords is placed anteriorly.
The superficial epithelium is ciliated, and below it is a layer of
cells (cutis) derived from the primitive epiblast, in which are
formed the usual rods (Hallez). The chief peculiarity of the
larva consists in the presence of elongated processes covered
with long cilia, and so connected together by a ciliated band
that the whole together forms, in Miiller's larva at any rate, a

VIEWED FROM THE
Hallez.)

PLATYELMINTHES.

193

lobed prceoral ciliated band (fig. 88). This band is not quite so
clear in Hallez' figures. Miiller's youngest larva was provided
with eight very long lobes; three were dorsal, viz. a median
anterior, and two lateral placed far back ; three ventral, viz. a
median in the front of the mouth forming a large upper lip, and
two processes at the sides of the mouth. The number was com-
pleted by two lateral processes of the
body. All the processes except the
dorsal median one are shewn in fig. 88.
In Hallez' larva, fig. 87, the six posterior
processes form a rather definite ring,
while one flagellum projects from the
front end of the body immediately below
the eyes, and a second flagellum behind.
In Moseley's youngest larva six, pro-
cesses only were present, though subse-
quently eight became formed as in Miiller's
larvae.

The metamorphosis consists in the
whole animal growing longer and flatter,
and in the arms becoming gradually
shorter and shorter till they finally dis-
appear altogether, and the larva acquires
the ordinary adult form.

The lobed larval form of the Turbellaria has some points of
resemblance to the Pilidium form of nemertine larva described
below, yet its resemblance to this interesting larva is less close
than would appear to be the case with certain turbellarian larval
forms recently described by Gotte and Metschnikoff, which are
in some respects intermediate in character between the larva of
Leptoplana and those just described.

The observations of Gotte (No. 184) were made on Planaria Neapolitana
and Thysanozoon Diesingi, and those of Metschnikoff (No. 188) on Stylo-
chopsis ponticus. The larvae of all these forms undergo more or less of a
metamorphosis, but the accounts of their development are not easily
reconciled1. The early stages of Planaria are like those of Leptoplana, as

1 The account of Metschnikoff s observations on Stylochopsis ponticus given in
the German abstract is too obscure to be placed in the text, but the following are the
more important points which can be gleaned from it.

The ovum becomes first divided into eight segments. By further division along
the equatorial zone, a ring of small cells is formed which becomes the epiblast. The

B. II. 13

FIG. 88. MULLER'S TUR-
BELLARIAN LARVA (PRO-
BABLY THYSANOZOON).
VIEWED FROM THE VEN-
TRAL SURFACE. (After
Miiller.)

The ciliated band is re-
presented by the black line.

m. mouth ; «./. upper
lip.

194

TURBELLARIA.

FIG. 89. PLANA-
RIAN LARVA (PRO-

BABLY PLAN ARIA

ANGULATA). (From
Agassi z.)

described by Keferstein. Four large hypoblast cells become surrounded by
small epiblast cells, which commence to be formed on the dorsal side. The
hypoblast cells divide and arrange themselves in two bilaterally-symmetrical
rows. A small blastopore is left by the small cells on the ventral surface,
which communicates with an otherwise closed and ciliated cavity which is
formed between the two rows of hypoblast cells. The blastopore would
seem to remain permanently open, and to be placed at the base of a deep
pit, lined by epiblast cells, which constitutes the stomodaeum.

The embryo now becomes dorsally convex, while the ventral surface
becomes marked with a median furrow and grows out laterally into two
lobes, and anteriorly into a ventrally-directed upper lip. The whole surface
becomes ciliated, and the cilia are especially prominent on the ventral
processes and the summit of the dorsal dome. A bunch
of strong cilia becomes formed in front of the dome,
and a less marked bunch behind. The larva is now
stated by Gotte closely to resemble a Pilidium. It soon,
however, extends itself, and the two bunches of cilia
become situated at the anterior and posterior extremities
of the body. The ventral processes become incon-
spicuous prominences of the side of the body. Gotte
believes that the larva undergoes no further metamor-
phosis.

A type of Planarian larva (figs. 89 and 90)— possibly Plan, angulata,
observed by Alex. Agassiz (No. 181), — is very
different from any other so far described, and
is remarkable for being divided into a series of
segments corresponding in number with the
diverticula of the digestive cavity. In the
youngest specimen (fig. 89) the body was nearly
cylindrical, and divided into eleven rings, cor-
responding with as many digestive diverticula.
Two eye-spots were present. In a later stage
two poles are at this time formed of large cells. At one pole four small cells appear,
which are compared by Metschnikoff to the pole cells of the Diptera (vide Chapter
on the development of the Insecta). At the opposite pole a blastopore is formed
leading into a small segmentation cavity. The epiblast also now gradually grows
over the large cells. At the blastopore pole the large cells give rise to the hypoblast
and the small cells at the opposite pole assist in forming the epiblast. The blastopore
dtappeUB, and with it the segmentation cavity, while the hypoblast, forming a solid
mass, becomes divided into two halves (Cf. Planaria Neapolitana). The embryo be-
comes ciliated and begins to rotate; and the eyes, and somewhat later (?) the nervous
ganglion make their appearance.

In the interior a wide cavity develops between the hypoblast cells, which becomes
ciliated and is placed in communication with the exterior by an invaginated stoma-
dseum which forms the pharynx.

The larva now, as in Planaria Neapolitana, takes on a Pilidium-like form. Lateral
I- >!•(.•> and an anterior lip grow out from the under surface, and become covered with
long cilia, while at the upper pole a long flagellum makes its appearance.

FIG. 90. PLANARIAN LARVA
(PROBABLY PLANARIA ANGU-
LATA). (From Agassiz.)

PLATYELMINTHES. 195

(fig. 90) the body was considerably flattened and had approached more to
the planarian form.

If Agassiz' interesting observations can be trusted we have in this larva
indications of a distinct segmentation, which are of some morphological
importance, especially when taken in connection with the traces of segmen-
tation found amongst the Nemertines.

A further type, with an incomplete metamorphosis, has been observed by
Girard (183). It is remarkable for having an uniform.segmentation, and for
presenting a quiescent stage after passing through a free larval condition
with a large upper lip.

Fresh-water Dendroccela. The development of the fresh-
water Dendrocoela has been especially investigated by Knappert
(No. 186) and Metschnikoff (No. 188).

The ova are very delicate minute naked cells, which to the
number of 4 — 6 or more become enveloped in a capsule or
cocoon together with a large mass of yolk cells derived from the
vitellarium. The yolk cells exhibit peristaltic movements and
send out amoeboid processes. Each ovum when laid becomes
surrounded by an extremely delicate membrane, which dis-
appears during the course of development. The capsules consist
of a spherical case and a stalk. The latter is first emitted from
the female opening as a thread-like body. Its free end becomes
attached, and then the remainder of the capsule is ejected.

Impregnation takes place before the formation of the capsule. The
segmentation is complete. The ovum first divides into two segments. One
of these next divides, forming three segments. There are subsequently
stages with four, eight, sixteen, and thirty-two segments.

Metschnikoff's results on the stages subsequent to the segmentation are
not in complete harmony with those of Knappert ; but no doubt represent
an advance in our knowledge, and I shall follow them here. His observa-
tions were made on Planaria polychroa.

In the earliest stage observed by him the segmentation was already far
advanced, but no membrane was present round the ovum. At a later stage
the ovum becomes more or less bell-shaped or hemispherical, and encloses
within its concavity a mass of yolk elements. It is now formed of three
concentric layers. An outer layer of flattened cells — the epiblast, a middle
layer of fused cells — the mesoblast, and an inner solid mass of yolk cells —
the hypoblast.

At the upper pole is formed the protrusible pharynx (cf. Knappert), which
is provided with a provisional musculature and a lumen. By its contractions
it takes up the yolk elements which surround the embryo, and the rapid
growth of the embryo no doubt takes place at their expense. The embryo

13—2

196 NEMERTEA.

gradually loses its hemispherical form, and assumes an elongated and
flattened shape. It acquires a coating of cilia by means of which it rotates.
On the fifth day it is hatched.

The alimentary tract long remains solid, even after it has acquired its
branched form. The pharynx becomes withdrawn as soon as the larva is
hatched. It loses its provisional muscles, and subsequently acquires a
permanent musculature. The young after hatching attach themselves to the
body of their parent, on which they feed (?).

Rhabdoccela. The development of some of the Rhabdoccela
has recently been studied by Hallez. The ova are mostly laid
in capsules, one in each capsule. Sometimes the development
commences before the capsules are laid, at other times not till
afterwards. In certain forms (Mesostomum) there are summer
eggs with thin capsules which develop within the parent, while
hard capsules, forming what are known as winter eggs, are laid
in the autumn, and the embryo hatched in the spring.

The ova of the Rhabdoccela like those of the fresh-water Dendroccela
are enveloped in yolk elements derived from the vitellarium.

The segmentation probably takes place in the same way as in Lepto-
plana. A stage with four equal cells has been observed by Hallez, and
there is subsequently an epibolic gastrula. The embryo becomes ciliated
while still within the capsule and, according to Hallez, the pharynx arises
as a bud of the hypoblast. The proboscis in Prostomum originates as an
epiblastic invagination.

NEMERTEA.

Some Nemertea develop without and some with a meta-
morphosis.

The most remarkable type of Nemertine development with a
metamorphosis is that in which the ovum develops into a
peculiar larval form known as Pi lid ium, within which the perfect
worm is subsequently evolved. Closely allied to this type is one
in which the sexual worm is developed within a larval form as in
1'ilidium, but in which the larva has no free swimming stage, and
is therefore without the characteristic appendages of the Pilidium.
This is known as the type of Desor and is confined (?) to the
genus Lineus. The Pilidium and the Desor type may be first
considered (vide Barrois, No. 192).

The type of Desor. The segmentation is regular and leads
to the formation of a blastosphere with a large segmentation

PLATYELMINTHES. 197

cavity. The blastosphere is converted by invagination into a
gastrula (fig. 91 A). The blastopore is soon carried relatively

FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois.)

A is a side view in optical section.

B and C are two later stages from the ventral (oral) surface.

ae. archenteron ; sc. segmentation cavity ; hy. hypoblast ; me. mesoblast ; ep. epi-
blast ; m. mouth ; st. stomach ; pr. d. prostomial disc ; po. d. metastomial disc ; pr.
proboscis.

forwards by the elongation backwards of the archenteron, and,
according to Barrois, actually forms the mouth. Owing to the
elongation of the archenteric cavity the embryo assumes a bila-
teral form (fig. 92 A) in which the dorsal and ventral surfaces
can be distinguished, the mouth (m.) being situated on the
ventral surface.

Immediately after the completion of the gastrula a remarkable
series of phenomena takes place. The embryo when viewed
from the ventral surface assumes a pentagonal form (fig. 91 B),
and four invaginations of the epiblast make their appearance on
the ventral surface (fig. 92 A), two in front of {pr. d.) and two
behind {po. d.) the mouth ; they result in the formation of four
thickened discs. These discs soon become separated from the
external skin, which closes in forming an unbroken layer over
them (fig. 91 C). The discs grow rapidly, and first the prosto-
mial pair and subsequently the metastomial fuse together, and
finally the whole four unite into a continuous ventral plate ;
analogous it would seem to the ventral plate of chsetopodan and

198

NEMERTEA.

arthropodan embryos. The plate so formed gradually extends
itself so as to close over the dorsal surface, and to form a
complete skin within the primitive larval skin, which at this
period is richly ciliated, though the embryo is not yet hatched

FIG. 91. THREE STAGES IN THE DEVELOPMENT OF LINEUS. (After Barrois.)

A. Side view of an embryo at a very early stage as an opaque object.

B and C. Two late stages, seen as transparent objects from the ventral surface.

at. archenteron; m. mouth; pr, d. prostomial disc; po.d. metastomial disc;
cs. lateral pit developing in B as a diverticulum from the oesophagus; pr. proboscis ;
ms. muscular layer (?); Is. larval skin about to be thrown off; me. mesoblast;
st. stomach.

(fig. 91 C). While these changes are taking place, there are
budded off from the invaginated discs a number of fatty cells,
which fill up the space between the discs and the archenteron,
and eventually form the mesoblastic reticulum. During this
stage the rudiment of the proboscis also makes its appearance as
a solid process of epiblast, which grows backwards from the
point of fusion of the two prostomial discs at the front end of
the embryo (fig. 91 C, pr.). A lumen is excavated in it at a
later period. The lateral organs or cephalic pits arise in a
somewhat unexpected fashion as a pair of diverticula from the

PLATYELMINTHES. 199

oesophagus (fig. 92 B, cs.)1, which soon fuse with the walls of the
body at the junction of the prostomial and metastomial plates
(fig. 92 C, cs.), although they remain for some time attached to
the oesophagus by a solid cord.

During these changes the original larval skin separates itself
from the subjacent layer formed by the discs (fig. 92, B and C),
and is soon thrown off completely, leaving the already ciliated
(fig. 92 C) external layer of the invaginated discs as the external
skin of the young Nemertine. During, and subsequently to, the
casting off of the embryonic skin, important changes take place
in the constitution of the various layers of the body, resulting in
the formation of the vascular system and other mesoblastic
organs, the nervous system, and the permanent alimentary tract.
These changes appear to me to stand in need of further elucida-
tion ; and the account below must be received with a certain
amount of caution.

It has been already stated that the two discs give rise to fatty cells,
which occupy the space between the walls of the body and the archenteron.
At the period of the casting off of the embryonic skin fresh changes take
place. The discs become very much thickened, and then divide into two
layers, which become the epidermis and subjacent muscular layers. The
muscular layers arise in two masses, separated by the two cephalic sacks.
The anterior mass is formed as an unpaired anterior thickening, followed by
two lateral thickenings. The posterior mass is much thinner, in correspond-
ence with the rapid elongation of the metastomial portion of the embryo.

The cells originally split off from the discs undergo considerable changes,
some of them arrange themselves around the proboscis as a definite mem-
brane, which becomes the proboscidean sheath, some also form a true
splanchnic layer of mesoblast, and the remainder, which are especially con-
centrated during early embryonic life in the anterior parts of the body, form
the general interstitial connective tissue. The cephalic ganglia are stated to
become gradually differentiated in the prostomial mesoblast, and the two
cords connected with them in the metastomial mesoblast.

At the time when the larval skin is cast off the original mouth becomes
closed, and it is not till some time afterwards that a permanent mouth is
formed in the same situation. During the early part of embryonic life the
intestine is lined with columnar cells, but, before the loss of the larval skin,
the walls of the intestine undergo a peculiar metamorphosis. Their cells
either fuse or become indistinguishable, and their protoplasm appears to
become converted into yolk-spherules, which fill up the whole space within

1 Biitschli for Pilidium regards these pits as formed by imaginations of the epiblast,
but Metschnikoff s statements are in accordance with those in the text.

2OO NEMERTEA.

the walls of the body, and are only prevented from extending forwards by
a membrane of connective tissue. This mass gradually forms itself into a
distinct canal, lined by columnar cells.

Pilidium. In the case of the true Piltdium type, the larva is
hatched very early and leads the usual existence of surface
larvae. A regular segmentation is followed by an invagination
which does not however cause the complete obliteration of the
segmentation cavity (fig. 93 A, a.e.).

The primitive alimentary tract so formed becomes divided
into cesophageal and gastric regions (fig. 93 B, oe. and .$•/.). Even
while the invagination of the archenteron is proceeding, the
larva becomes ciliated throughout, and assumes a somewhat
conical form, the apex of the cone being opposite the flat ventral
surface on which the mouth is situated (fig. 93, A and B). From

FIG. 93. Two STAGES IN THE DEVELOPMENT OF PILIDIUM. (After Metschnikoff.)
of. archenteron ; <r. oesophagus ; st. stomach ; am. amnion ; pr.d. prostomial disc ;
PO. d. metastomial disc; c.s. cephalic sack.

the apex a flagellum projects in many forms, giving the larva a
helmet-like appearance. In other forms a bunch of long cilia
takes the place of the flagellum (fig. 94), and in others again the
flagellum is not represented. After the completion of the inva-
gination a lobe grows out on each side of the mouth, and less
well developed lobes may appear anteriorly and posteriorly.
Round the edge of the ventral surface a ciliated band makes its
appearance.

PLATYELMINTHES.

201

Two pairs of imaginations of the skin, just as in the type of
Desor, now make their appearance, one pair in front of and the
other behind the mouth (fig. 93 B, pr.d. and po.d.}, and each of
them by the closure
of the opening of
invagination forms
a sack, the outer
wall of which be-
comes very thin
and the inner
wall (correspond-
ing with the whole
invagination of the
type of Desor) very
thick. The inner
walls of the four
thickenings, which
I may speak of as
discs, now fuse to-
gether, each disc
first uniting with
its fellow, and
finally the two
pairs uniting.

A ventral ger-
minal plate is thus
established, which
gradually grows
round the intestine
of the Pilidium to
form the skin of
the future Nemer-
tine. The outer
thin layer of each of the discs grows part passu with the inner
layer, and furnishes an amnion-like covering for the embryo
which is forming within the Pilidium (fig. 94, an}.

In connection with the young vermiform Nemertine there is
formed on each side an outgrowth from the oesophagus (fig. 94)
which is eventually placed in communication with the exterior

FIG. 94.

A. PILIDIUM WITH AN ADVANCED NEMERTINE
WORM.

B. RIPE EMBRYO OF THE NfiMERTEA IN THE
POSITION IT OCCUPIES IN PlLIDIUM. (Both after

Butschli.)

&. oesophagus ; st. stomach ; i. intestine ; pr. pro-
boscis ; Ip. lateral pit ; an. amnion ; n. nervous system.

202 NEMERTEA.

by a ciliated canal1. The proboscis arises as an hollow invagi-
nation at the point where the two anterior discs fuse in front.

When the young Nemertine has become pretty well formed
within the Pilidium it becomes ciliated, begins to move, and
eventually frees itself and leads an independent existence,
leaving its amnion in the Pilidium which continues to live for
some time.

The central nervous system (fig. 94) is developed either
before or after the detachment of the young Nemertine, accord-
ing to Metschnikoff as a thickening of the epiblast. The young
Nemertine is at first without an anus.

The development of the Nemertine within the Pilidium is
clearly identical with that of the Lineus embryo within the
larval skin ; the formation of an amnion in the Pilidium consti-
tuting the only important difference which can be pointed out
between the modes of origin of the young Nemertine in the two
types.

So far as is known the forms which develop in a Pilidium, or
according to the type of Desor, all belong to the division of the
Nemertines without stylets in the proboscis, known as the
Anopla.

Development without Metamorphosis. The majority of
the Nemertea, including the whole (?) of the Enopla, develop
without a metamorphosis. The observations which have been
made on this type are not very satisfactory, but appear to
indicate that the formation of the hypoblast may take place
cither by invagination or by delamination.

Invaginate types have been observed by Barrois (No. 192), Dieck (No.
196) and Hubrecht.

Barrois' fullest observations were made on Amphipoi~us lactifloreus (one
of the Enopla), and those of Dieck on Cephalothrix galathece (one of the
Anopla).

A regular segmentation is followed by a blastosphere stage with a small
segmentation cavity. In Barrois' type the inner ends of the cells of the
blastosphere are stated to fuse into a kind of syncytium. A small invagina-
tion takes place, and the cells which take part in it separate from the

1 This is the view of both Metschnikoff (No. 202) and Leuckart and Pagcnstecher
(No. 201), and is further confirmed by Barrois, but Biitschli (No. 193), though he has
not observed the earliest stages of their outgrowth, believes them to be invaginations of
the Nemertine skin.

PLATYELMINTHES. 203

epiblast, and then fuse with the syncytium within the blastosphere. Dieck
finds that in Cephalothrix the invaginated mass simply vanishes.

Barrois' statements about the fusion of the syncytium derived from the
epiblast cells with the invaginated cells must be regarded as very doubtful.
The formation of the germinal layers takes place, according to Barrois,
by the separation of the internal mass of cells into mesoblast and hypoblast.
The proboscis is formed, according to this author, from the mesoblastic
tissues. Dieck, on the other hand, with greater probability, states that the
proboscis is formed by an invagination. In Cephalothrix a further point
deserves notice, in that the whole of the primitive epiblast becomes shed.
In this fact there may perhaps be recognised the last trace of a metamor-
phosis like that in the type of Desor.

Delaminate types have been studied by Barrois (No. 192) and Hoffman
(No. 198), both of whom give circumstantial accounts of their develop-
ment.

Hoffman's account is especially deserving of attention, since his observa-
tions were, to a great extent, made by means of artificial sections. The
following account is taken from him. His observations were made on
Tetrastemma varicolor, and Tetrastemma appears to be the genus in which
this type of development has been most completely made out. After a
regular segmentation the embryo forms a solid mass of cells, the outermost
of which soon become distinguished as a separate epiblastic layer. At the
same time the larva leaves the egg, and the epiblast cells become coated by
an uniform covering of cilia. At the anterior extremity of the body is a
bunch of long cilia ; and at the hinder end two stiff bristles are formed, but
soon disappear.

The internal mass of cells is still quite uniform, but as the larva grows in
length the outermost of them arrange themselves as a columnar layer,
constituting the mesoblast. Of the cells internal to the mesoblast the outer
become columnar, and are converted into the walls of the alimentary tract,
while the inner ones undergo fatty degeneration, and form a kind of food-
yolk. In the later development the characters of the adult are gradually
acquired without metamorphosis, and the larval skin passes directly into
that of the adult. Both mouth and anus are formed nearly simultaneously
by a rupture of the enteric wall from within. The nervous system arises as
a thickening of the epiblast, which Hoffman states he has been able to see
in sections. Hoffman also states that the epithelium of the proboscis is
formed as a diverticulum of the alimentary tract, and that its sheath is
formed by a special mesoblastic growth.

Barrois is less precise than Hoffman, from whom he differs in certain
particulars. Hoffman's statements about the proboscis are important if
accurate, but require further confirmation.

Malacobdella. The early stages in development of the peculiar ecto-
parasitic Nemertine Malacobdella have been worked by Hoffman (No. 199)
by means of sections, and there appears to be a close agreement between
the development of Malacobdella and that of Tetrastemma.

The segmentation is uniform, and there is no trace of a segmentation

204 NEMERTEA.

cavity. On the third day after impregnation the outermost cells of the
embryo become flattened and ciliated, and distinguished from the remain-
ing spherical cells of the embryo as the epiblast. With the appearance
of cilia a rotation of the embryo commences. On the fourth day the
embryo becomes oval, and at one of the poles — the future anal pole — a
separation takes place between the epiblast and the inner cells, giving rise
to the body cavity. In it are a number of loose oval cells, which soon
become stellate, and form a mesoblastic reticulum connecting the body-wall
nnd central cells of the embryo, which may now be spoken of as hypoblast.
The body-cavity increases in size, leaving at last the hypoblast and epiblast
united only at one point— the oral pole— at which, on the fifth day, a crown
of long cilia appears. The solid mass of hypoblast in the interior becomes
differentiated into an outer layer of cells — the true glandular epithelium of
the alimentary tract — and an inner core, the cells of which soon undergo
fatty degeneration, and serve as food-yolk.

The later stages of development, and the formation of the proboscis,
etc., have not been worked out.

General considerations. Of the types of larvae hitherto
found amongst the Nemertea, those with a metamorphosis, viz.
the Pilidium type and that of Desor, are to be regarded as the
primitive. But even in Pilidium there are evidences of a great
abbreviation in development. Pilidium itself is probably a more
or less modified ancestral form, while the peculiar development of
the Nemertine within it is to be explained as a very much short-
ened record of a long series of changes by which the Pilidium be-
came gradually converted into a Nemertine. The formation of
the body wall of the Nemertine by four epiblastic invaginations
is a remarkable cmbryological phenomenon, for which it is not
easy to assign a satisfactory meaning ; and it is probable that it is
merely a secondary process of growth similar to the formation of
imaginal discs in the larvae of Diptera (vide Chapter on Trache-
ata), which has had its origin in the abbreviation of the develop-
ment just alluded to. The development on the type of Desor is
clearly a simplification of the Pilidium type, and its peculiarities
are to be explained by the fact that the first larval form has no
free existence. The types without metamorphosis have no doubt
a development of a still more simplified character ; they are re-
markable however in presenting us, if the existing descriptions
are to be trusted, with examples of delamination and invagination
coexisting in closely allied forms.

PLATYELMINTHES. 20$

TREMATODA.

The eggs of the Trematoda consist of a germ or true ovum
enclosed in a mass of yolk cells, which undergo disintegration
and subsequent absorption at varying periods of the develop-
ment. From the observations of E. van Beneden (No. 218)»
Zeller (No. 217), etc. it is known that the segmentation is
usually complete, but generally somewhat irregular.

Unfortunately we are still completely in the dark as to the
mode of formation of the germinal layers. The embryos of the
entoparasitic forms or Distomeae become free in a very imperfect
condition, and the ova are small ; while in the Polystomeae the
development is as a rule nearly completed before hatching, and
the ova are large. It will be convenient to treat separately the
development of the two groups.

Distomeae. The embryos of the Distomeae are hatched
either in some moist place or more usually in water. In the
majority of genera the larvae pass through a complicated meta-
morphosis, accompanied by alternations of generations. But for
some genera, e.g. Holostomum, etc., the life history has not yet
been made out. The whole life history of comparatively few
forms has been followed, but sufficient fragments are known
to justify us in making certain general statements, which no
doubt hold true for a large proportion of the Distomeae.

The larvae are usually ciliated (fig. 95 A), but sometimes
naked.

The ciliated forms are generally completely covered with cilia, but in
Distommn lanceolatum the cilia are confined to an area at the front end of
the body, in the centre of which a median spine is placed. An x shaped
pigment spot, sometimes provided with a rudimentary lens (Monostomum
mutabile\ is also generally situated on the dorsal surface.

In some intances a more or less completely developed alimentary tract is
present (Monostomum capitellum, Amphistomum subclavatum\ but usually
there can only be distinguished in the interior of the larva a transparent
mass of cells bounded by a more or less distinctly marked body wall with
ciliated excretory channels.

Ed. van Beneden has shewn that the ciliated covering is developed
while the embryo is still in the egg, and long before the yolk cells are com-
pletely absorbed. It would seem that even before hatching this ciliated
covering is to a great extent independent of the mass within. In the

206 TREMATODA.

larva of Monostomum mutabile (fig. 95 A), which offers an example of
an extreme case of the kind, there is present within the ciliated epidermis
a fully-developed independent worm.

The non-ciliated larvae are less highly organized than the ciliated forms,
and are covered by a cuticle : their anterior extremity is sometimes provided
with a circular plate armed with radiate ridges and spines.

The free-swimming or creeping embryos make their way into
or on to the body of some invertebrate (occasionally vertebrate)
form, usually a Mollusc, to undergo the first stage in their
metamorphosis. They may either do this on the gills of their
host, or very frequently they bore their way into the interior of
the body. Soon after the larvae have reached a satisfactory
position the epidermis becomes stripped off, and there emerges a
second larval form developed in the interior of the first larva,
much as a Nemertine is developed within the larva of Desor.
In the case of Monostomum mutabile the new worm is, as
stated above, fully formed within the ciliated larva at the time
of hatching.

The worm which proceeds from the above metamorphosis
has different characters corresponding with those of the larva
from which it proceeded. If the original larva had an alimen-
tary canal it has one also, and then grows into the form known
as a Redia (Fig. 95, B and C).

The Redia has anteriorly a mouth leading into a muscular
pharynx and thence into a caecal stomach. Posteriorly the body
is prolonged into a kind of blunt caudal process, at the com-
mencement of which are a pair of lateral papillae. There is a
perivisceral cavity, and the body walls are traversed by excretory
tubes.

If the original larva is without an alimentary tract, the
second form becomes what is known as a Sporocyst. The
Sporocyst is a simple elongated sack with a central body cavity ;
when derived from the metamorphosis of a ciliated embryo its
walls are provided with excretory tubes, but such tubes are
absent in Sporocysts developed from non-ciliated larvae. Some
Sporocysts send out numerous branches amongst the viscera of
their hosts.

The Rediiu and Sporocysts rapidly grow in size and some-
times increase by transverse division. In the course of their

PLATYELMINTHES.

2O7

further development one of two things may happen. They may
either (i) develop fresh Rediae or Sporocysts by a process of
internal budding (fig. 95 C) ; or else (2) there may be formed in
them, by an analogous pro-
cess, larvae with long tails
known as Cercariae (fig. 95
D.) The direct develop-
ment of Cercariae is the
usual course, though in
Distomum globiparum the
reverse is true ; but where
this does not take place the
Rediae or Sporocysts of the
second generation give rise
to Cercariae.

The Cercarias are deve-
loped from spherical masses
of cells found in the body
cavity of the Sporocyst or
Redia. The exact origin of
these masses is still some-
what obscure, but they are
stated by Wagener (No. 212)
to be derived from the body wall,
regarded as internal buds.

The spherical bodies grow rapidly in size, their posterior
extremity is prolonged into a process which forms the tail, while
the anterior part forms the trunk. When fully formed (fig. 95 E),
the trunk has very much the organization of an adult Distomum.
There is an anterior and a ventral sucker, the former of which
contains the opening of the mouth, and is often provided with a
special chitinous armature. The mouth leads into a muscular
pharynx, and this into a bilobed caecal alimentary tract. An
excretory system of the ordinary type is present, consisting of
longitudinal contractile trunks continuous anteriorly with branch-
ed ciliated canals, which, as has recently been shewn by Biitschli,
may be provided with funnel-shaped ciliated internal openings1.

FIG. 95. VARIOUS STAGES IN THE META-
MORPHOSIS OF THE DISTOME^; (from Huxley.)

A. Ciliated larva of Monostomum muta-
bile. a. larval skin. b. Redia developed
within it. B. Redia of Monostomum muta-
bile. C. Redia of Distomum pacificum, with
germs of a second brood of Rediae. D. Redia
containing Cercariae. E. Cercaria. F. Full-
grown Distomum.

They are probably to be

1 O. Biitschli, "Bemerkungen iib. d. excretorischen Gefassapparat d. Trematoden."
Zoologischer Anzeiger, 1879, No. 42.

208 TREMATODA.

The contractile trunks unite posteriorly, but instead of opening
directly to the exterior are prolonged into a vessel which
traverses the substance of the tail, and after a longer or shorter
course bifurcates into two branches which open laterally.

The tail is provided with an axial rod of hyaline connective
tissue, like the notochord of the tail of a larval Ascidian, and is
frequently provided with membranous expansions. It is used as
a swimming organ. Beneath the epidermis are layers of circular
and longitudinal muscular fibres, the latter arranged in the tail
as two bands.

The Cercariae when fully developed leave the Sporocyst or
Redia, and then their host, and become free. In most Rediae
there is a special opening, not far from the mouth, by which they
pass out. There is no such opening in Sporocysts, but the
Cercariae bore their way through the walls.

After leaving their parent the Cercariae pass into the external
medium, and for a short period have a free existence. They
soon however enter a new host, making their w; y into its body
by a process of boring, which is effected by the head (especially
when armed with chitinous processes) assisted by movements of
the tail.

The second host is usually some Invertebrate (Mollusc,
Worm, Crustacean, Insect larva, &c.), but occasionally a Fish or
Amphibian or even a vegetable. The tail is very often lost as
the Cercaria bores its way into its host, but whether it is so or
not, the Cercaria, after it has once reached a suitable post in its
new host, assumes a quiescent condition, and surrounds itself
with a many-layered capsule. The cephalic armature and tail
(if still present) are then exuviated, and the generative organs
gradually become apparent though very small. In other respects
the organization is not much altered.

Though an encysted Cercaria may remain some months
without further change, it eventually dies unless it be introduced
into its permanent vertebrate host, an act which is usually
effected by the host in which it is encysted being devoured.
It then becomes freed from its capsule as a fully formed Trema-
tode, in which the generative organs rapidly complete their
development.

In some cases the Rediae or Sporocysts do not give rise to

PLATYELMINTHES. 2OQ

tailed Cercariae, but to tailless forms. In such cases, as a rule,
the encystment takes place in the host of the Redia or Sporocyst,
but the tailless larvae sometimes pass through a free stage like
the Cercariae. In the case of Distomum cygnoides, parasitic in
the bladder of the Frog, the Cercaria passes directly into the
adult host without the intervention of an intermediate host.

The life history of a typical entoparasitic Trematode is shortly
as follows :

1 i ) It leaves the egg as a ciliated or non- ciliated free larva.

(2) This larva makes its way on to the gills or into the
body of some Mollusc or other host, throws off its epidermis and
becomes a Redia or Sporocyst.

(3) In the body cavity of the Redia or Sporocyst nume-
rous tailed larvae, known as Cercariae, are developed by a process
of internal gemmation.

(4) The Cercariae pass out of the body of their parent,
and out of their host, and become for a short time free. They
then pass into a second, usually invertebrate host, and encyst.

(5) If their second host is swallowed by the vertebrate
host of the adult of the species, the encysted forms become free,
and attain to sexual maturity.

The majority of these stages are simply parts of a complicated
metamorphosis, but in the coexistence of larval budding (giving
rise to Cercariae or fresh Rediae) with true sexual reproduction
there is in addition a true alternation of generations.

Polystomeae. The ova of the Polystomeae are usually large
and not very numerous, and they are in most cases provided
with some process for attachment. Some species of Polystomeae,
e.g. Gyrodactylus, are however viviparous. The young leave the
egg in a nearly perfect state, and at the utmost undergo a slight
metamorphosis and no alternations of generations. Some how-
ever (Polystomum, Diplozoon) are provided with temporary cilia,
but the number investigated is too small to determine whether
ciliation is the rule or the exception. The ciliated larvae have a
short free existence. The cilia are developed on special cells
which may be arranged in transverse bands in the same way as
in the larvae of many Chaetopods, but are not, in the larvae at
present known, distributed uniformly. When the free larvae
become parasitic the cells with cilia shrink up.

B. II. H

2IO ' 1'^TODA.

In Polystotninn inlc^rrimum, which lives in the urinary bladder of Rana
temporaria, the eggs when laid in the spring pass out into the water. The
segmentation is complete, and the embryo when hatched is provided with
most of the adult organs, but presents certain striking larval characters.
It has five rings of ciliated cells. Three of these are placed anteriorly, and
are especially developed on the ventral surface, the posterior one being
incomplete dorsally ; two are placed posteriorly, and are especially devel-
oped on the dorsal surface. Anteriorly there is a tuft of cilia.

The larva itself resembles somewhat an adult Gyrodactylus, and is pro-
vided (i) with a large posterior disc armed with hooks, and (2) with two
pairs of eyes which persist in the adult state. After a certain period of free
existence the larva attaches itself to the gills of a tadpole. The rings of cili-
ated cells shrink up, and some of the six pairs of suckers found in the adult
commence to be formed on the posterior disc. When the bladder of the tad-
pole is developed, the young Polystomum passes down the alimentary tract to
the cloaca, and thence to the urinary bladder, where it slowly attains to sexual
maturity. When the larva becomes attached to the gills of a very young
tadpole, its development is somewhat more rapid in consequence of better
nutrition from the more delicate gills. It then reaches its full development
in the gill cavity, and. though smaller and provided with differently
organised generative organs to the normal form, produces generative pro-
ducts and dies without being transported to the bladder (vide Zeller,
Nos. 216 and 217).

The ova of Diplozoon, a form parasitic on the gills of freshwater fish
(Phoxinus, etc.), are provided with a long spiral filament (Zeller, No. 215).
The embryo has five ciliated areas, four lateral and one posterior. The
young form is known as Diporpa. Sexual maturity is not attained till two
individuals unite permanently together. They unite by the ventral sucker
of each of them becoming attached to the dorsal papilla of the other. Sub-
sequently these parts coalesce, and the ventral suckers disappear in the
process. Gyrodactylus, parasitic, like Diplozoon, on the gills of freshwater
fishes (Gasterosteus, etc.), is remarkable for its mode of reproduction. It is
viviparous, producing a single young one at a time, and, what is still more
remarkable, the young while still within its parent produces a young one,
and this again a young one, so that three generations may be present within
the parent. It seems probable that the second and third generations are
produced asexually, the generative organs not being developed ; while the
young Gyrodactylus of the first generation springs from a fertilized ovum
(Wagener, No. 214).

CESTODA.

On anatomical grounds the affinity of the Cestoda to the
Trematoda has been insisted on by the majority of anatomists.
The existence of such intermediate forms as Amphilina tends to

PLATYELMINTHES. 211

strengthen this view ; and the striking resemblances between
the two groups in the structure of the egg and characters of
the metamorphosis appear to me to remove all doubt about the
matter.

The ripe egg is formed of a minute germ enveloped in yolk
cells, the whole being surrounded by a membrane, which is very
delicate in most forms, but in certain types has a firmer consist-
ency, and is provided with an aperture, covered by an operculum,
by which the larva escapes.

The early development, up to the formation of a six-hooked
larva, generally takes place in the uterus, but in the types with a
firmer egg-shell it takes place after the egg has been deposited
in water.

The segmentation (E. van Beneden, No. 218, Metschnikofif,
No. 228) is complete, and during its occurrence the yolk cells
surrounding the germ are gradually absorbed, so that the mass
of segmentation spheres grows in size, till at the close of segmen-
tation it fills up nearly the whole egg-shell.

As was first shewn by Kolliker for Bothriocephalus salmonis,
the embryonic cells separate themselves at the close of segmen-
tation into a superficial layer and a central mass.

The further development takes place on two types. In the
cases where the egg-shell is strong, and the egg is laid prior to
the formation of the embryo, a ciliated larva is developed (Bo-
thriocephalus latus, ditremus, Schistocephalus dimorphus, Ligula
simplicissima, etc.1).

Of these forms Bothriocephalus latus may be taken as type.

The development of the embryo requires many months for
its completion. The outer layer becomes ciliated while the
central mass has already become developed into a six-hooked
embryo. The embryo leaves its shell by the opercular aperture,
and for some time swims rapidly about by means of its long cilia.
The ciliated coating is eventually stripped off, and the six-hooked
larva emerges.

In the second type of embryo the external cellular layer does
not become ciliated. This is the most usual arrangement, and is
even found in many species of Bothriocephalus.

1 Vide for list of such forms at present known Willemoes Suhm, No. 231.

14—2

212 CESTODA.

The central mass of cells becomes developed, as in the other
type, into a six-hooked (rarely four-hooked) embryo (fig. 96 G),
but the superficial layer separates from the central, and either
disappears or becomes (Bothriocephalus proboscideus] a cuticular
layer. Between the six-hooked embryo and the outer layer of
cells one or more thick membranes become deposited (E. van
Beneden). The eggs are carried out of the alimentary canal in
the proglottis and transported to various situations on land or
in water. They usually remain within the proglottis, invested
by their thick shell, till taken up into the alimentary canal of
a suitable host, or they may be swallowed after the death and
decay of the proglottis. They are subsequently hatched after
their shell has become softened by the action of the digestive
fluids.

Before proceeding to describe their further history, the close
resemblance between the first developmental stages of Cestoda,
especially in the case of the ciliated larvae, and those of Trema-
toda, may be pointed out.

In both there is a ciliated larva, and in both there is developed

FK;. 96. DIAGRAMS OF VARIOUS STAGES IN TMK DKVKI.OPMENT OF THE
••DA. (From Huxley.)

A. Cysticercus. H. and C. Cysticerci in the everted (B) and inverted (C) con-
dition. I), (''"'minis. K. and F. I )iat;rams of Kchinococcus. It is most probable that
T;unia heads are not developed directly from the wall of the cyst as represented in
the diagram, (i. Six-hooked embryo.

within the ciliated skin a second larva, which becomes freed by
the stripping off of the ciliated skin.

The type of development has moreover many analogies with
that of the Nemcrtine larva of Desor, p. 163 (cf. Mctschnikoff),
and is probably like that an abbreviated record of a long history.

The suitable host for the six-hooked embryo to enter is

PLATYELMINTHES. 213

rarely the same as the host for the sexual form. The embryos
having become transported into the alimentary canal of such a
host, and become free, if previously invested by the egg-shell,
soon make their way, apparently by the help of their hooks,
through the wall of the alimentary tract, and are transported in
the blood or otherwise into some suitable place for them to
undergo their next transformation. This place may be the liver,
lungs, muscles, connective tissue, or even the brain (e.g. Ccenurus
cerebralis in the brain of sheep).

Here they become enclosed in a granular deposit from the
surrounding tissues, which becomes in its turn enclosed in a con-
nective-tissue coat. Within lies the solid embryo, the hooks of
which in many cases disappear or become impossible to make
out. In other forms, e.g. Cysticercus limacis, they remain visible,
and then mark the anterior pole of the worm (fig. 98, c.}. The
central part of the body next becomes transformed into a material
composed of clear non-nucleated vesicles. Accompanying these
changes the embryo grows rapidly in size ; a cuticle is deposited
by its outer layer, in which also an external layer of circular
muscular fibres and an internal layer of longitudinal fibres become
differentiated, and internal to both there is formed a layer of
granular cells.

With the rapid growth of the body a central cavity is formed,
which becomes filled with fluid, and the embryo assumes the form
of a vesicle. At the same time a system of excretory vessels,
sometimes opening by a posterior pore, becomes visible in the
wall of the vesicle.

The embryo has now reached a condition in which it is known
as a cystic- or bladder-worm, and may be compared in almost
every respect with the sporocyst of a Trematode (Huxley).

The next important change consists in the development of a
head, which becomes the head of the adult Tamia. This is
formed in an involution of the outer wall of the anterior ex-
tremity of the cystic worm. This involution forms a papilliform
projection on the inner surface of the wall of the cystic worm,
with an axial cavity opening by a pore on the outer surface.
The layer of cells forming the papilla soon becomes divided
into two laminae, of which the outer forms a kind of investing
membrane for the papilla. The papilla itself now becomes

2I4

CESTODA.

Kic. 97. CYS-

TICERCUS CELLU-
LOSE. (From Ge-
fenbaur, after von
iebold.)

a. Caudal ve-
sicle, c. Anterior
part of body, d,
head.

moulded into a Cestode head, which however is developed in
an inverted position. The suckers and hooks
(when present) of the head are developed on a
surface bounding the axial lumen of the papilla,
which is the true morphological outer surface,
while the apparent outer surface of the papilla
is that which eventually forms the interior of
the (at first) hollow head. Before the external
armature of the head has become established,
four longitudinal excretory vessels, continuous
with those in the body of the cystic worm, make
their appearance. They are united by a circular
vessel at the apex of the head. The develop-
ment is by no means completed with the simple
growth of the head, but the whole inverted papilla continues to
grow in length, and gives rise to what afterwards becomes part
of the trunk. The whole papilla eventually becomes everted,
and then the cystic worm takes the form (fig. 97) of a head and
unsegmented trunk with a vesicle — the body of the cystic worm
— attached behind. The whole larva is known as a Cysticercus.
The term scolex, which is also sometimes employed, may be
conveniently retained for the head and trunk only. The head
differs mainly from that of the adult in being hollow.

There are great variations in the relative size of the head and the
vesicle of Cysticerci. In some forms the
vesicle is very small (fig. 98), e.g. Cysticercus
limacis ; it is medium-sized in Cysticercus
cellulosce (fig. 97), and in some forms is much
larger. The embryonic hooks, when they
persist, are found at the junction of the trunk
and the vesicle (fig. 98 A, c}. Though the
majority of cystic worms only develope one
head, this is not invariably the case. There
is a cystic worm found in the brain of the
sheep known as Ccenurus cerebralis — the larva
of Tcenia caenurus, parasitic in the intestine
of the dog — which forms an exception to this
rule. There appears, to start with, a tuft of
three or four heads, and finally many hun-
dred heads are developed (fig. 96 D). They

FK;. 98. CYSTICERCUS

\\Y\\\ SMALL CAUl'Al VESICLE,

A. Head involuted. B.
Head everted.

a. Scolex. />. caudal vesicle.
c. (in A) six embryonic hooks.

are arranged in groups at one (the anterior?) pole of the cystic worm.

PLATYELMINTHES.

A still more complicated form of cystic worm is that known as Echino-
coccus, parasitic in the liver, lungs, etc. of man and various domestic Un-
gulata. In the adult state it is known as Tcenia echinococcus and infests
the intestine of the dog. The cystic worm developed from the six-hooked
embryo has usually a spherical form, and is invested in a very thick cuticle
(fig. 96 E and F, and fig. 99). It does not itself directly give rise to Taenia
heads, but after it reaches a certain size there are formed on the inner
side of its walls small protuberances, which soon grow out into vesicles
connected with the walls of the cyst by narrow stalks (figs. 96 F and 99 C).
In the interior of these vesicles a cuticle is developed. It is in these
secondary vesicles that the heads originate. According to Leuckart, they
either arise as outgrowths of the wall of the vesicle on the inner face
of which the armature is developed, which subsequently become involuted
and remain attached to the wall of the vesicle by a narrow stalk, or they
arise from the first as papilliform projections into the lumen of the vesicle,
on the outer side of which the armature is formed. Recent observers only
admit the second of these modes of development. The Echinococcus larva,
in addition to giving rise to the above head-producing vesicles, also gives
rise by budding to fresh cysts, which resemble in all respects the parent
cyst. These cysts may either be detached in the interior (fig. 96 F) of
the parent or externally. They appear to spring in most cases from the
walls of the parent cyst, but there are some discrepancies between the
various accounts of the process. In the cysts of the second generation
vesicles are produced in which new heads are formed. As the primitive
cyst grows, it naturally becomes more and more complicated, and the num-
ber of heads to which one larva may give rise becomes in this way almost
unlimited.

Cysticerci may remain a long time without further develop-
ment, and human beings have been known to be infested with
an Echinococcus cyst for over thirty years. When however the
Cysticercus with its head is fully developed, it is in a condition
to be carried into its final host. This takes place by the part of
one animal infested with cysticerci becoming eaten by the host
in question. In the alimentary canal of the final host the con-
nective-tissue capsule is digested, and then the vesicular caudal
appendage undergoes the same fate, while the head, with its
suckers and hooks, attaches itself to the walls of the intestine.
The head and rudimentary trunk, which have been up to this
time hollow, now become solid by the deposition of an axial
tissue; and the trunk very soon becomes divided into segments,
known as proglottides (fig. 99 A). These segments are not
formed in the same succession as those of Chaetopods ; the

216

CKSTODA.

youngest of them is that nearest to the head, and the oldest
that furthest removed from it. Each segment appears in fact
to be a sexual individual, and is
capable of becoming detached and
leading for some time an indepen-
dent existence. In some cases,
e.g. Cysticcrcus fasciolaris, the seg-
mentation of the trunk may take
place while the larva is still in its
intermediate host.

The stages in the evolution of
the Cestoda are shortly as follows :

1. Stage with embryonic epi-
dermis either ciliated (Bothrioce-
phalus, etc.) or still enclosed in the
egg-shell. This stage corresponds
to the ciliated larval stage of the
Trematoda.

2. Six-hooked embryonic stage
after the embryonic epidermis has
been thrown off. During this stage
the embryo is transported into the
alimentary tract of its intermediate
host, and boring its way into the
tissues, becomes encapsuled.

3. It develops during the en-
capsuled state into a cystic worm,
equivalent to the sporocyst of
Trematoda.

4. The cystic worm while still encapsuled develops a head
with suckers and hooks, becoming a Cysticercus. In some forms
(Ccenurus, Echinococcus) reproduction by budding takes place at
this stage. The head and trunk are known as the scolex.

5. The Cysticercus is transported into the second and
permanent host by the infested tissue being eaten. The bladder-
like remains of the cystic worm are then digested, and by a
process of successive budding a chain of sexual proglottides are
formed from the head, which remains asexual.

The above development " is to be regarded as a case of

FIG. 99. ECHINOCOCCUS VETE-
RINORUM. (From Huxley.)

A. Tsenia head or scolex. a.
hooks, b. suckers, c. cilia in
water vessel, d. refracting parti-
cles in body wall.

B. single hooks.

C. portion of cyst. a. cuticle.
b. membranous wall of primary
cyst. c. and e. scolex heads, d.
secondary cyst.

PLATYELMINTHES.

217

FIG. 99 A. TETRARHYNCUS. (From
Gegenbaur ; after Van Beneden.)

A. Asexual state.

B. Sexual stage with ripe proglottides.

complicated metamorphosis secondarily produced by the neces-
sities of a parasitic condition, to which an alternation of sexual
and gemmiparous generations
has been added. The alter- A-
nation of generations only
occurs at the last stage of the
development, when the so- B|
called head, without generative
organs, produces by budding
a chain of sexual forms, the
embryos of which, after pass-
ing through a complicated
metamorphosis, again become
Cestode heads.

In the case of Ccenurus and Echinococcus two or more
asexual generations are interpolated between the sexual ones.
It is not quite clear whether the production of the Taenia head
from the cystic worm may not be regarded as a case of budding.
There are some grounds for comparing the scolex to the Cercaria
of Trematodes, cf. Archigetes.

As might be anticipated from the character of the Cestode metamor-
phosis, the two hosts required for the development are usually forms so
related that the final host feeds upon the intermediate host. As familiar
examples of this may be cited the pig, the muscles of which may be
infested by Cysticercus cellulosce, which becomes the Tcenia solium of man.
Similarly a Cysticercus infesting the muscles of the ox becomes the TcBnia
mediocanellata of man. The Cysticercus piscifonnis of the rabbit becomes
the Tcenia serrata of the dog. The Coenurus cerebralis of the sheep's
brain becomes the Tcenia ccenurus of the dog. The Echinococcus of man
and the domestic herbivores becomes the Tcenia echinococcus of the dog.

Cystic worms infest not only Mammalian forms, but lower Vertebrates,
various fishes which form the food of other fishes, and Invertebrates liable
to be preyed on by vertebrate hosts. So far the Cestodes (except Archi-
getes) are only known to attain sexual maturity in the alimentary tracts of
Vertebrata.

The rule that the intermediate host is not the same as the final host does
not appear to be without exception. Redon1 has shewn by experiments
on himself that a Cysticercus (celluloses) taken from a human subject
developes into Tcenia solium in the intestines of a man. Redon took
four cysts of a Cysticercus from a human subject, and after three months
passed some proglottides, and subsequently the head of Tania solium.

1 Annal. d. Scien. Nat., 6th Series, Vol. vi. 1877.

2l8 CESTODA.

Some important variations of the typical development are known.

The so-called head or scolex may be formed without the intervention
of a cystic stage. In Archigetes (Leuckart, No. 227), which infests, in
the Cysticercus condition, the body-cavity of various invertebrate forms
(Tubifex, etc.), the six-hooked embryo becomes elongated and divided into
two sections, one forming the head, while the other, with the six embryonic
hooks, forms an appendage, homologous with the caudal vesicle of other
Cysticerci.

The embryo of Tcenia elliptica similarly gives rise to a Cysticercus
infesting the dog-louse (Trichodectes cants], without passing through a
vesicular condition ; but the caudal vesicle disappears, so that it forms
simply a scolex. These cases may, it appears to me, be probably regarded
as more primitive than the ordinary ones, where the cystic condition has
become exaggerated as an effect of a parasitic life.

In some cases the larva of a Taenia has a free existence in the scolex
condition. Such a form, the larva of Phyllobothrium, has been observed by
Claparede1. It was not ciliated, and was without a caudal vesicle; and
was no doubt actively migrating from an intermediate host to its permanent
host.

Scolex forms, without a caudal vesicle, are found in the mantle cavity
of Cephalopoda, and appear to be occupying an intermediate host in their
passage from the host of the cystic worm to that of the sexual form.

Archigetes, already mentioned, has been shewn by Leuckart (No. 227)
to become sexually mature in the Cysticercus state, and thus affords an
interesting example of paedogenesis. It is not known for certain whether
under normal circumstances it reaches the mature state in another host.

Amphilina. The early stages of this interesting form have been
investigated by Salensky (No. 229), and exhibit clear affinities to those of
the true Cestoda. An embryonic provisional skin is formed as in Cestodes ;
and pole-cells also appear. Within the provisional skin is formed an
embryo with ten hooks. After hatching the provisional skin is at once
thrown off, and the larva, which is then covered by a layer of very
fine cilia, becomes free. The further metamorphosis is not known.

BIBLIOGRAPHY.
Turbellaria.

(181) Alex. Agassiz. "On the young stages of a few Annelids" (Planaria
angulatd). Annals Lyceum Nat. Hist, of Nav York, Vol. vili. 1866.

(182) Dalyell. " Powers of the Creator. "

(183) C. Girard. " Embryonic development of Planocera elliptica." Jour, of
Acad. of Nat. Set. Philadelphia. New Series, Vol. 1 1. 1854.

(184) Alex. Gotte. "Zur Entwicklungsgeschichte d. Seeplanarien. " Zoolo-
gischtr Anzeiger, No. 4, 1878.

1 Beobachtungen iib. Anat. H. lint-wick. Wirbell. Thiere. Leipzig, 1863.

PLATYELMINTHES. 219

(185) P. Halle z. Contributions a Chistoire natitrelle des Ttirbellaries. Thesis a
la faculte des Sciences p. le grade d. Docteur es-sciences naturelles, Lille, 1879.

(186) Knappert. " Bijdragen tot de Ontwikkelings-Geschiedenis der Zoet-
water-Planarien." Provinciaal Utrechtsch Genootschap van Kunsten en Wetenschap-
pen. Utrecht, 1865.

(187) W. Keferstein. " Beitrage z. Anat. u. Entwick. ein. Seeplanarien von
St. Malo." Abh. d. konig. GeselL d. Wiss. zu Gottingen. Bd. xiv. 1868.

(188) El. Metschnikoff. " Untersuchungen lib. d. Entwicklungd. Planarien."
Notizen d. neurussischen Gesellschaft d. Naturforscher. Odessa, Bd. V. 1877. Vide
Hoffman and Schwalbe's Bericht for 1878.

(189) H. N. Moseley. "On Stylochus pelagicus and a new species of pelagic
Planarian, with notes on other pelagic species, on the larval forms of Thysanozoon,
etc." Quart. Journ. of Micr. Science. Vol. xvn. 1877.

(190) J. Miiller. " Ueber eine eigenthtimliche Wurmlarva a. d. Classe d. Tur-
bellarien, etc." Miiller's Archiv f. Anat. u. Phys. 1850.

(191) "Ueber verschiedene Formen von Seethieren." Miiller's Archiv f.

Anat. und Phys. 1854.

Nemertea.

(192) J. Barrois. " L'Embryologie des Nemertes." An. Sd. Nat. Vol. VI.
1877.

(193) O. Biitschli. Archiv f. Naturgeschichte, 1873.

(194) A. Krohn. " Ueb. Pilidium u. Actinotrocha." Miiller's Archiv, 1858.

(195) E. Des or. " Embryology of Nemertes." Proceedings of the Boston Nat.
History Society, Vol. vi. 1848.

(196) G. Dieck. " Entwicklungsgeschichte d. Nemertinen." Jenaische Zeit-
schrift, Vol. viii. 1874.

(197) C. Gegenbaur. " Bemerkungen iib. Pilidium gyrans, etc." Zeitschrift
fur wiss. Zool., Bd. v. 1854.

(198) C. K. Hoffmann. " Entwicklungsgeschichte von Tetrastemma tricolor."
Niederldndisches Archiv, Vol. ill. 1876, 1877.

(199) "Zur Anatomic und Ontogenie von Malacobdella." Niederldndisches

Archiv, Vol. IV. 1877.

(200) W. C. Mc In tosh. British Annelids. The Nemerteans. Ray Society,

1873-4-

(201) Leuckart u. Pagenstecher. " Untersuchungen iib. niedere Seethiere."
Miiller's Archiv, 1858.

(202) E. Metschnikoff. " Studien iib. die Entwicklung d. Echinodermen u.
Nemertinen." Mem. Acad. imp. Petersburg, vn. Ser. Tom. xiv. No. 8, 1869.

Trematoda.

(203) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.

(204) Parasites ; a Treatise on the Entozoa, etc. Churchill, 1879.

(205) Filippi. Mem. p. servir a Fhistoire genetiqite dts Tremalodes. Ann.
Scien. Nat. 4th Series, Vol. u. 1854, and Mem. Acad. Torino, 1855 — 1859.

(206) R. Leuckart. Die menschlichen Parasiten, Vol. I. 1863, p. 485, et seq.

(207) H. A. Pagenstecher. Trematoden u. Trematodenlarven. Heidelberg,
1857.

220 mm i' H.KAPHY.

(208) C. Th. von Siebold. Lehrbttch d. vergleich. Anat. wirhclloscr Thicrc.
Merlin, 1848.

(209) J. J. S. Steenstrup. Gcnerationswcchsel. 1842.

(210) R. v. Willemoes-Suhm. " Zur Naturgeschichte d. Polystomum inte-
gerrimum, etc." Zeit. f. wiss. Zool. Vol. xxn. 1872.

(211) - - "Helminthologische Notizen III." Zeit. /. wiss. Zool. Vol. xxm.
1873. Vide this paper for a summary of known observations and literature.

(212) G. R. Wagener. Bt'ilriige zur Entwicklnngsgeschichte d. Eingeweidewiir-
mtr. Haarlem, 1855.

(21H) G. R. Wagener. " Helminthologische Bemerkungen, etc." Zeit. f. wiss.
Zool. Vol. ix. 1850.

(214) G. R. Wagener. " Ueb. Gyrodactylus elegans." Archiv f. Anat. u.
Phys. 1860.

(215) E. Zeller. " Untersuchungen lib. d. Entwicklung d. Uiplozoon para-
doxum." Zeit.f. wiss. Zool. Vol. xxn. 1872.

(216) E. Zeller. " Untersuchungen ii. d. Entwick. u. Ban d. Polystomum inte-
gerrimum." Zeit.f. wiss. Zool. Vol. xxn. 1872.

(217) E. Zeller. "Weitere Beitrage z. Kenntniss d. Polystomen." Zeit. f.
wiss. Zool. Vol. xxvn. 1876.

Cestoda.

(218) Ed. van Beneden. " Recherches sur la composition et la signification d.
Toeuf." Mem. cour. Acad. roy. Belgique. Vol. xxxiv. 1868.

(219) P. J. van Beneden. " Les vers Cestoides considered sous le rapport
physiologique embryogenique, etc." Bui. Acad. Scien. Bruxelles. Vol. xvn. 1850.

(220) T. S. Cob bo Id. Entozoa. Groombridge and Son, 1864.

(221) Parasites ; a treatise on the Entozoa, etc. Churchill, 1879.

(222) Th. H. Huxley. "On the Anatomy and Development of Echinococcus
veterinorum." Proc. Zool. Soc. Vol. xx. 1852.

(223) J. Knoch. "Die Naturgesch. d. breiten Bandwurmer." Mini. Acad.
Imp. Pctersbourg, Vol. V. Ser. 7, 1863.

(224) F. Kiichenmeister. " Ueber d. Umwandlung d. Finnen Cysticerci in
Bandwiirmer (Taenien)." Prag Vierteljahrsschr. 1852.

(226) " Experimente Ub. d. Entstehung d. Cestoden. 2° Stufe zunachst

d. Ccenurus cerebralis." Gunsburg, Zeitsch. klin. Med. IV. 1853.

(226) R. Leuckart. Die Menschlichen Parasiten, Vol. I. Leipzig, 1863. Vide
also additions at the end of the ist and 2nd volume.

(227) R. Leuckart. "Archigetes Sieboldii, eine geschlechtsreife Cestodenam-
me." Zeit.f. wiss. Zool., Vol. XXX. Supplement, 1878.

(228) El. Metschnikoff. "Observations sur le developpement de quelques
animaux ( Bothriocephalus proboscideus)." Bull. Acad. Imp. St Petersbourg, Vol.

XIII. 1869.

(229) W. Salensky. "Ueb. d. Bau u. d. Entwicklungsgeschichte d. Amphi-
lina." Zeit. f. wiss. Zool., Vol. xxiv. 1874.

(230) Von Siebold. Burdach's Physiologie.

(231) R. von Willemoes-Suhm. "Helminthologische Notizen." Zeit. f.
wiss. Zool., Vol. xix. xx. xxn. 1869, 70 and 73.

CHAPTER VIII.

ROTIFERA.

FOR many reasons a complete knowledge of the ontogeny of
the Rotifera is desirable. They constitute a group which retain
in the trochal disc an organ common to the embryos of many
other groups, but which in most other instances is lost in the
adult state. In the character of the excretory organs they
exhibit affinities with the Platyelminthes, while in other respects
they possibly approach the Arthropoda (e.g. Pedalion ?). The
interesting Trochosphcera cequatorialis of Semper closely re-
sembles a monotrochal polychaetous larva.

Up to the present time our embryological knowledge is
mainly confined to a series of observations by Salensky on
Brachionus urceolaris, and to scattered statements on other larval
forms by Huxley, etc.

In many cases Rotifers lay summer and winter eggs of a
different character. The former are always provided with a thin
membrane, and frequently undergo development within the
oviduct. They are hatched in the autumn. The winter eggs
are always provided with a thick shell.

The summer eggs are of two kinds, viz. smaller eggs which
become males, and larger, females. On the authority of Cohn
(No. 232) they are believed to develope parthenogenetically.
Males are not found in summer, and only seem to be produced
from the summer eggs. Cohn's observations, especially on
Conochilus volvox, are however not quite satisfactory. Huxley
(No. 234) came to the conclusion that the winter eggs of Lacinu-
laria developed without previous fertilization.

The following are the more important results of Salensky's
observations (No. 236) on Brachionus urceolaris.

The ovum is attached by a short stalk to the hind end of the
body of the female, in which position it undergoes its develop-
ment. It will be convenient to treat separately the development
of the female and male, and to commence with the former. The

HRACHIONUS.

female ovum divides into two unequal spheres, of which the
smaller in the subsequent stages segments more rapidly than the
larger. The segmentation ends with the formation of an epibolic
gastrula. The solid inner mass of cells derived from the larger
sphere constitutes the hypoblast, and is more granular than the
epiblast The evolution of the embryo commences with the
formation of a depression on the ventral surface, at the bottom
of which the stomodaeum is formed by an invagination. At the
hinder part of the depression there rises up a rounded protuber-
ance which eventually becomes the caudal appendage or foot.
Immediately behind the mouth is formed an underlip.

On the sides of the ventral depression are two ridges which
form the lateral boundaries of the trochal disc. They appear to
unite with the under lip.

In a later stage the anterior part of the body becomes marked
off from the posterior as a praeoral lobe, and the hypoblast is at
the same time confined to the posterior part. The supra-oeso-
phageal ganglion is early formed as an epiblastic thickening on
the dorsal side of the praeoral lobe.

The first cilia to appear arise at the apex of the praeoral lobe.
At a later period the lateral
ridges of the trochal disc meet
dorsally and so enclose the prae-
oral lobe. They then become
coated by a ring of cilia, to which
a second ring, completing the
double ring of the adult, is added
later.

In the trunk an indication of
a division into two segments
makes its appearance shortly
after the development of the
praeoral lobe. Before this period
the proctodaeum is established as
a shallow pit immediately behind
the insertion of the foot. The
latter structure soon becomes
pointed and forked (fig. 100, /).

The complete establishment

FIG. 100. EMBRYO OF BRACHIO-
NUS URCEOLARIS SHORTLY BEFORE IT
is HATCHED. (After Salensky.)

m. mouth ; ms. masticatory appa-
ratus ; me. mesenteron ; an. anus ; Id.
lateral gland ; ov. ovary ; /. tail, ;'. e.
foot ; tr. trochal disc ; sg. supra-ceso-
phageal ganglion.

ROTIFERA. 223

of the alimentary canal occurs late. The stomodaeum (fig. 100)
gives rise to the mouth (m), oesophagus and masticatory appara-
tus (ins). The mesenteron is formed from the median part of
the hypoblast ; the lateral parts of which appear to give rise to
the great lateral glandular structures (Id) which open into the
stomach, and to the ovaries (?) (ov) etc. The proctodaeum
becomes the cloaca and anus (an). The origin of the mesoblast
is not certainly known. The shell is formed before the larva is
hatched — an occurrence which does not take place till the larva
closely resembles the adult.

The early developmental stages of the male are closely
similar to those of the female ; and the chief difference between
the two appears to consist in the development of the male being
arrested at a certain point.

The larvae of Lacinularia (Huxley, No. 234) are provided
with a praeoral circlet of cilia containing two eye-spots1, and a
perianal patch of cilia. They closely resemble some telotrochal
polychaetous larvae.

Salensky has compared the larva of Brachionus to that of a
cephalophorous Mollusc, more especially to the larva of Calyp-
traea on which he has made important observations. The
praeoral lobe, with the ciliated band, no doubt admits of a
comparison with the velum of the larva of Molluscs ; but it does
so equally, as was first pointed out by Huxley, with the ciliated
praeoral lobe of the larvae of many Vermes. It further deserves
to be noted that the trochal disc of a Rotifer differs from the
velum of a Mollusc in that the eyes and ganglia are placed
dorsally to it, and not, as in the velum of a Mollusc, within it.
The larva of Lacinularia appears to be an exception to this,
since two eye-spots are stated to lie within the circlet of cilia.
More important in the comparison is the so-called foot (tail),
which arises in the embryo as a prominence between the mouth
and anus, and in this respect exactly corresponds with the
Molluscan foot.

If Salensky 's comparison is correct, and there is something
to be said for it, the foot or tail of Rotifers is not a post-anal
portion of the trunk, but a ventral appendage, and the segmen-

1 In Leydig's figure of the larva, Zeit, f. iviss. Zool. Vol. ill. 1851, the eye-spots
lie just outside the ciliated ring.

224 BRACHIONUS.

tation which it frequently exhibits is not to be compared with a
true segmentation of the trunk. If the Rotifers, as seems not
impossible, exhibit crustacean affinities, the ' foot ' may perhaps
be best compared with the peculiar ventral spine of the Nauplius
larva of Lepas fascicularis (vide Chapter on Crustacea) which in
the arrangement of its spines and other points also exhibits a
kind of segmentation.

BIBLIOGRAPHY.

(232) F. Cohn. "Ueb. d. Fortpflanzung von Raderthiere." Zeit.f. wiss. Zool.
Vol. vii. 1856.

(233) F. Cohn. " Bemerkungen ii. Raderthiere." Zeit. f. wiss. Zool. Vol. ix.
1858, and Vol. xn. 1862.

(234) T. H. Huxley. " Lacinularia socialis." Trans, of the Microscopical
Society, 1853.

(235) Fr. Leydig. "Ueb. d. Bau u. d. systematische Stellung d. Rader-
thiere." Zeit.f. wiss. Zool. Vol. vi. 1854.

(236) W. Salensky. " Beit. z. Entwick. von Brachionus urceolaris." Zeit.f.
wiss. Zool. Vol. xxii. 1872.

(237) C. Semper. " Zoologische Aphorismen. Trochosphaera sequatorialis."
Zeit.f. wiss. Zool. Vol. xxn. 1877.

CHAPTER IX.

MOLLUSCA1.

ALTHOUGH the majority of important developmental features
are common to the whole of the Mollusca, yet at the same time
many of the subdivisions have well-marked larval types of their
own. It will for this reason be convenient in considering the
larval characters to deal successively with the different sub-
divisions, but to take the whole group at once in considering the
development of the organs.

Formation of the layers and larval characters.
ODONTOPHORA.

Gasteropoda and Pteropoda. There is a very close agree-
ment amongst the Gasteropoda and Pteropoda in the general
characters of the larva ; but owing to the fact that the eggs of

1 The classification of the Mollusca adopted in the present chapter is shewn in the
subjoined table :

I. ODONTOPHORA. II. LAMELLIBRANCHIATA.

1. Gasteropoda. «. Dimya.

a. Prosobranchiata. b. Monomya.

b. Opisthobranchiata.

c. Pulmonata.

d. Heteropoda.

2. Pteropoda.

a. Gymnosomata.

b. Thecosomata.

3. Cephalopoda.

a. Tetrabranchiata.

b. Dibranchiata.

4- Polyplacophora.

5- Scaphopoda.

B, II. 15

226 GASTEROPODA AND PTEROPODA.

the various species differ immensely as to the amount of food-
yolk, considerable differences obtain in the mode of formation of
the layers and of the alimentary tract.

The spheres at a very early stage of segmentation1 become
divided into two categories, one of them destined to give rise
mainly to the hypoblast, the other mainly to the epiblast. Ac-
cording as there is much or little food-yolk the hypoblast spheres
are either very bulky or the reverse. In all cases the epiblast
cells lie at one pole, which may be called the formative pole,
and the hypoblast cells at the opposite pole. When the bulk of
the food-yolk is very great, the number of hypoblast spheres is
small. Thus in Aplysia there are only two such spheres. In other
cases, where there is but little food-yolk, they may be nearly
as numerous as the epiblast cells. In all these cases, however,
as was first shewn by Lankester and Selenka, a gastrula becomes
formed either by normal invagination as in the case of Paludina
(fig. 107), or by epibole as in Nassa mutabilis (fig. 105). In both
cases the hypoblast becomes completely enclosed by the epiblast.
T/ie blastopore is always situated opposite the original formative
pole. In the large majority of cases (i.e. Marine Gasteropoda,
Heteropoda, and Pteropoda) the blastopore becomes gradually
narrowed to a circular opening which eventually occupies the
position of the mouth. It either closes or remains permanently
open at this point. In some cases the blastopore remains per-
manently open and becomes the anus. The best authenticated
instance of this is Paludina vivipara, as was first shewn by
Lankester (No. 263).

In some instances the blastopore assumes before closing a
very narrow slit-like form, and would seem to extend along the
future ventral region of the body from the mouth to the anus.
This appears, according to Lankester (No. 262), to be the con-
dition in Lymnaeus, but while Lankester believes that the closure
proceeds from the oral towards the anal extremity, other inves-
tigators hold that it does so in the reverse direction. Fol (No.
*2 4!); has also described a similar type of blastopore. In an un-
determined marine Gasteropod, with an embolic gastrula, observed
by myself at Valparaiso, the blastopore had the same elongated

1 The reader is referred for the segmentation to pp. 98 — 101, and to the special
description of separate types.

MOLLUSCA. 227

form as in Lymnaeus, but the whole of it soon became closed
except the oral extremity ; but whether this finally closed could
not be determined. It is probable that the typical form of the
blastopore is the elongated form observed by Lankester and my-
self, in which an unclosed portion can indifferently remain at
either extremity; and that from this primitive condition the
various modifications above described have been derived1.

Before the blastopore closes or becomes converted into the
oral or anal aperture, a number of very important embryonic
organs make their appearance ; but before describing these it
will be convenient to state what is known with reference to the
third' embryonic layer or mesoblast.

This layer generally originates in a number of cells at the lips
of the blastopore, which then gradually make their way dorsal-
wards and forwards, and form a complete layer between the epi-
blast and hypoblast. The above general mode of formation of
the mesoblast may be seen in fig. 107, representing three stages
in the development of Paludina.

In some cases the mesoblast arises from certain of the seg-
mentation spheres intermediate in size between the epiblast and
hypoblast spheres. This is the case in Nassa mutabilis, where
the mesoblast appears when the epiblast only forms a very small
cap at the formative pole of the ovum ; and in this case the meso-
blast cells accompany the epiblast cells in their growth over the
hypoblast (fig. 105).

In other cases the exact derivation of the mesoblast cells is
quite uncertain. The evidence is perhaps in favour of their
originating from the hypoblast. It is also uncertain whether the
mesoblast is bilaterally symmetrical at the time of its origin. It
is stated by Rabl to be so in Lymnaeus2.

In the case of Paludina the mesoblast becomes two layers

1 Rabl (No. 268) describes a blastopore of this form in Planorbis which closes at
the mouth.

2 Rabl (No. 268) has quite recently given a more detailed account than previous
observers of the origin of the mesoblast in Planorbis. He finds that it originates
from the posterior one of the four large cells which remain distinct throughout the
segmentation. By the division of this cell two ' mesoblasts ' are formed, one on each
side of the middle line at the hinder end of the embryo. Each of these again divides
into two, an anterior and a posterior. By the division of the mesoblasts there arise
two linear rows of mesoblastic cells — the mesoblastic bands — which are directed

15—2

228

GASTEROPODA AND PTEROPODA.

thick, and tlun splits into a splanchnic and somatic layer, of
which the former attaches itself to the hypoblast, and gives rise
to the muscular and connective-tissue wall of the alimentary tract,
and the latter attaches itself to the epiblast, and forms the mus-
cular and connective-tissue wall of the body and other structures.
The two layers remain connected by protoplasmic strands, and
the space between them forms the body cavity (fig. 107). In
most instances there would appear to be at first no such definite
splitting of the mesoblast, but the layer has the form of a scattered
network of cells between the epiblast and the hypoblast. Finally
certain of the cells form a definite layer over the walls of the
alimentary canal, and constitute the splanchnic mesoblast, and
the remaining cells constitute the somatic mesoblast.

We must now return to the embryo at the time when the
blastopore is becoming narrowed. First of all it will be necessary
to define the terms to be applied to the various regions of the
body — and these will
best be understood by
taking a fully formed
larva such as that re-
presented in fig. 101.
The ventral surface I
consider to be that
comprised between the
mouth (m) and the
anus, which is very
nearly in the position
(i) in the figure. As a
great protuberance on
the ventral surface is
placed the foot/ The
long axis of the body,
at this period though not necessarily in the adult, is that passing

FIG. 101. DIAGRAM OF AN EMBRYO OF I'I.KI-
ROBRANCHIDIUM. (From Lankester.)

f. foot; ot. otocyst ; m. mouth; v. velum;
ng. nerve ganglion ; ry. residual yolk spheres ; s/is.
shell-gland ; i. intestine.

forwards and divided transversely into two parts, an anterior continued from the front
mesoblast, and a posterior from the hinder mesoblast.

If Rabl's account is correct, there is a striking similarity between the origin of the
mesoblast in Mollusca and in Chaetopoda. It appears to me very probable that the
mesoblastic bands are formed (as in Lumbricus) not only from the products of the
division of the mesoblasts, but also from cells budded off from one or both of the
primary germinal layers.

MOLLUSCA. 229

through the mouth and the shell-gland (shs.) : while the dorsal
surface is that opposite the ventral as already defined.

Before the blastopore has attained its final condition three
organs make their appearance, which are eminently characteristic
of the typical molluscan larva. These organs are (i) the velum,
(2) the shell-gland, (3) the foot.

The velum is a provisional larval organ, which has the form
of a praeoral ring of cilia, supported by a ridge of cells, often in
the form of a double row, the ventral end of which lies immedi-
ately dorsal to the mouth. Its typical position is shewn in fig.
101, v. There are considerable variations in its mode and extent
of development etc., but there is no reason to think that it is
entirely absent in any group of Gasteropoda or Pteropoda. In a
few individual instances, especially amongst viviparous forms
and land Pulmonata, it has been stated to be absent. Semper
(No. 274) failed to find it in Vitrina, Bulimus citrinus, Vaginulus
luzonicus, and Paludina costata. It is very probably absent in
Helix, etc.

In some cases, e.g. Limax (Gegenbaur), Neritina (Claparede),
Pterotrachaea (Gegenbaur), the larva is stated to be coated by an
uniform covering of cilia before the formation of the velum, but
the researches of Fol have thrown very considerable doubt on
these statements. In some cases amongst the Nudibranchiata
(Haddon) and Pteropoda there are one or two long cilia in the
middle of the velar area. In many Nudibranchiata (Haddon)
there is present a more or less complete post-oral ring of small
cilia, which belongs to the velum.

The cilia on the velum cause a rotation of the larva within
the egg capsule. Cilia are in most cases (Paludina, etc.) developed
on the foot and on a small anal area.

The shell-gland arises as an epiblastic thickening on the pos-
terior and dorsal side. In this thickening a deep invagination
(fig. 101, shs.) is soon formed, in which a chitinous plug may
become developed (Paludina, Cymbulia ? etc.), and in abnormal
larvae such a chitinous plug is generally formed.

The foot is a simple prominence of epiblast on the ventral
surface, in the cavity of which there are usually a number of meso-
blast cells (fig. ioi,y). The larval form just described has been
named by Lankester the trochosphere larva.

230 GASTEROPODA AND PTEROPODA.

Before considering the further external changes which the
larva undergoes, it will be well to complete the history of the
invaginated hypoblast.

The hypoblast has after its invagination either the form of a
sack (fig. 102) or of a solid mass (fig. 101). Whether the mouth
be the blastopore or no, the permanent
oesophagus is formed of epiblast cells, so
that the oesophagus and buccal cavity
are always lined by epiblast. When the
blastopore remains permanently open the
outer part of the oesophagus grows as a
prominent ridge round the opening.

The mesenteric sack itself becomes

IMG. 102. EMBRYO OF
differentiated into a stomach adjoining A HETEROPOD. (Fom Ge-

the oesophagus, a liver opening immedi- genbaur ; after Foh)

. 0. mouth; v. velum; g.

ately behind this, and an intestine. The archenteron ; p. foot ; c. body
cells forming the hepatic diverticula and cavity ; s' shell-Sland-
sometimes also those of the stomach may during larval life
secrete in their interior peculiar albuminous products, similar to
ordinary food-yolk.

The proctodaeum, except when it is the blastopore, arises
later than the mouth. It is frequently developed from a pair of
projecting epiblast cells symmetrically placed in the median
ventral line behind the foot. It eventually forms a very shallow
invagination meeting the intestine. Its opening is the anus.
The anus, though at first always symmetrical and ventral, subse-
quently, on the formation of the pallial cavity, opens into this
usually on the right and dorsal side.

In the cases where the hypoblast is not invaginated in the
form of a sack the formation of the mesenteron is somewhat
complicated, and is described in the sequel.

From the trochosphere stage the larva passes into what has
been called by Lankester the veliger stage (fig. 103), which is
especially characteristic of Gasteropod and Pteropod Mollusca.

The shell-gland (with a few exceptions to be spoken of subse-
quently) of the previous stage flattens out, forming a disc-like
area, on the surface of which a delicate shell becomes developed,
while the epiblast of the edges of the disc becomes thickened.
The disc-like area is the mantle. The edge of the area and with

MOLLUSCA.

231

it the shell now rapidly extend, especially in a dorsal direction.
Up to this time the embryo has been symmetrical, but in most
Gasteropods the shell and mantle extend very much more to-
wards the left than towards the right side, and a commencement
of the permanent spiral shell is thus produced.

The edge of the mantle forms a projecting lip separating the
dorsal visceral sack from the head and foot. An invagination
appears, usually on the right in Gasteropods, and eventually
extends to the dorsal side (fig. 103 B). It gives rise to the

FIG. 103. LARVAE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE.
(From Gegenbaur.)

A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia).
v, velum ; c. shell ; p. foot ; op. operculum ; /. tentacle.

pallial or branchial cavity, and receives also the openings of the
digestive, generative and urinary organs. In most Pteropods it
is also formed to the right, and usually eventually extends after-
wards towards the ventral surface (fig. 103 C). In the pallial
cavity the gills are formed, in those groups in which they are
present, as solid processes frequently ciliated. They are coated
by epiblast and contain a core of mesoblast. They soon become
hollow and contractile.

The velum in the more typical forms loses its simple circular
form, and becomes a projecting bilobed organ, which serves the
larva after it is hatched as the organ of locomotion (fig. 103 B
and C). The extent of the development of the velum varies
greatly. In the Heteropods especially it becomes very large, and
in Atlanta it becomes six-lobed, each lateral half presenting
three subdivisions. It is usually armed on its projecting edge
with several rows of long cilia, and below this with short cilia

232 GASTEROPODA AND PTEROPODA.

which bring food to the mouth. It persists in many forms for a
very long period. Within the area of the velum there appear the
tentacles and eyes (fig. 103 B). The latter are usually formed at
the base of the tentacles.

The foot grows in most forms to a very considerable size.
On its hinder and dorsal surface is formed the operculum as a
chitinos plate which originates in a depression lined by thick-
ened epiblast, much in the same way as the shell (fig. 103 B and
C, op}. In the typical larval forms it is only possible to distin-
guish the anterior flattened surface of the foot for locomotion and
the posterior opercular region, but special modifications of the
foot are found in the Pteropods and Heteropods, which are
described with those groups. The foot very often becomes richly
ciliated, and otic vesicles are early developed in it (fig. 101, of).

All the Gasteropods and Pteropods have a shell-bearing
larval form like that first described, with the exception of a few
forms, such as Limax and perhaps some other Pulmonata, in
which the "shell-gland closes up and gives rise to an internal
shell.

The subsequent metamorphosis in the different groups is very
various, but in all cases it is accompanied by the disappearance
of the velum, though in some cases remnants of the velum may
persist as the subtentacular lobes (Lymnaeus, Lankester) or the
lip tentacles (Tergipes, Nordmann). In prosobranchiate Gaste-
ropods the larval shell is gradually added to, and frequently
replaced by, a permanent shell, though the free-swimming velig-
crous larva may have a long existence. In many of the Opis-
thobranchiata the larval shell is lost in the adult and in others
reduced. Lankester, who has especially worked at the early
stages of this group, has shewn that the larvae are in almost
every respect identical with those of prosobranchiate Gastero-
pods. They are all provided with a subnautiloid shell, an oper-
culated foot, etc. The metamorphosis has unfortunately been
satisfactorily observed in but few instances. In Heteropods and
Pteropods the embryonic shell is in many cases lost in the adult.

The following sections contain a special account of the develop-
ment in the various groups of Gasteropoda and Pteropoda
which will complete the necessarily sketchy account of the pre-
ceding pages.

MOLLUSCA.

233

Gasteropoda. To illustrate the development of the Gasteropoda I
have given a detailed description of two types, viz. Nassa mutabilis and
Paludina vivipara.

Nassa mutabilis. This form, the development of which has been
very thoroughly worked out by Bobretzky (No. 242), will serve as an
example of a marine Gasteropod with a large food-yolk. The segmentation

FIG. 104. SEGMENTATION OF NASSA MUTABILIS. (From Bobretzky.)
A. Upper half divided into two segments. B. One of these has fused with the
large lower segment. C. Four small and one large segment, one of the former fusing
with the large segment. D. Each of the four segments has given rise to a fresh
small segment. E. Small segments have increased to thirty-six.

has already been described, p. 102. It will be convenient to take up the de-
velopment at a late stage of the segmentation. The embryo is then formed
of a cap of small cells which may be spoken of as the blastoderm resting
upon four large yolk-cells of which one is considerably larger than the
others (fig, 104 A). The small and the large cells are separated by a
segmentation cavity. The general features at this stage are shewn in
fig. 105 A, representing a longitudinal section through the largest yolk-
cell and a smaller yolk- cell opposite to it. The blastoderm is for the most
part one cell thick, but it will be noticed that, at the edge of the blastoderm
adjoining the largest yolk-cell, there are placed two cells underneath
the edge of the blastoderm (me). These cells are the commencement of
the mesoblast. In the later stages of development the blastoderm con-
tinues to grow over the yolk-cells, and as it grows the three smaller yolk-
cells travel round the side of the largest yolk-cell with it. As they do
so they give rise to a layer of protoplasmic cells (fig. 105, hy) which form
a thickened layer at the edge of the blastoderm and therefore round the

234

GASTEROPODA.

lips of the blastopore. These cells form the hypoblast. The whole of the
protoplasmic matter of the yolk-cells is employed in the formation of the
hypoblast. The rest of them remains as a mass of yolk. A longitudinal
section of the embryo at a slightly later stage, when the blastopore has
become quite narrowed, is represented in fig. 105 C. The greater part of
the dorsal surface is not represented.

Two definite organs have already become established. One of these is a
pit lined by thickened epiblast on the posterior and dorsal side (sg). This

FIG. 105. LONGITUDINAL SECTION THROUGH THE EMBRYO OF NASSA MUTA-
BILIS. (After Bobretzky.)

A. Stage when the mesoblast is commencing to be formed.

I'.. Stage when the yolk is half enclosed. The hypoblast is seen at the lips of the
blastopore.

C. Stage when the blastopore (bp} is nearly obliterated.

D. The blastopore is closed.

ep. epiblast ; me. mesoblast ; hy. hypoblast ; bp. blastopore ; in. intestine ;
st. stomach ; /. foot ; sg. shell-gland ; m. mouth.

is the shell-gland. The other is the foot (/) which arises as a ventral
prominence of thickened epiblast immediately behind the blastopore. The
hypoblast forms a ring of columnar cells round the blastopore. On the

MOLLUSCA.

235

posterior side its cells have bent over so as to form a narrow tube (*«), the
rudiment of the intestine.

In the next stage (fig. 105 D) the blastopore completely closes, but its
position is marked by a shallow pit (;;z) where the stomodaeum is eventually
formed. The foot (/) is more prominent, and on its hinder border is
formed the operculum. The shell-gland (not shewn in the figure) has
flattened out, and its thickened borders commence to extend especially over
the dorsal side of the embryo. A delicate shell has become formed. In
front of and dorsal to the mouth, a ciliated ring-shaped ridge of cells, which
is however incomplete dorsally, gives rise to the velum. On each side of the
foot there appears a protuberance of epiblast cells, which forms a provisional
renal organ. The hypoblast now forms a complete layer ventrally, bound-
ing a cavity which may be conveniently spoken of as the stomach (.$•/),
which is open to the yolk above. Posteriorly however a completely closed
intestine is present, which ends blindly behind (in).

The shell and with it the mantle grow rapidly, and the primitive
symmetry is early interfered with by the shell extending much more
towards the left than the right. The anus soon becomes formed and places
the intestine in communication with the exterior.

With the growth of the shell and mantle the foot and the head become
sharply separated from the visceral
sack (fig. 1 06). The oesophagus (m]
becomes elongated. The eyes and
auditory sacks become formed.

With further growth the asym-
metry of the embryo becomes more
marked. The intestine takes a trans-
verse direction to the right side of
the body, and the anus opens on the
right side and close to the foot in the
mantle cavity which is formed by an
epiblastic invagination in this region.
The cavity of the stomach (fig. 106,
st} increases enormously and passes

ce.v

FIG. 106. LONGITUDINAL SECTION

THROUGH AN ADVANCED EMBRYO OF

NASSA MUTABILIS. (After Bobretzky.)

f. foot ; m. mouth ;
vesicle ; sf. stomach.

cephalic

to the left side of the body, pushing
the food-yolk at the same time to
the right side, and the point where
it communicates with the intestine becomes carried towards the posterior
dorsal end of the visceral sack. The walls of the stomach gradually extend
so as to narrow the opening to the yolk. The part of it adjoining the
oesophagus becomes the true stomach, the remainder the liver ; its interior is
filled with coagulable fluid.

Paludina. Paludina— Lankester (No. 263) and Butschli (No 244)— is
a viviparous form characterised by the small amount of food-yolk. The
hypoblast and epiblast cells are distinguished very early, but soon become of
nearly the same size.

236

GASTEROPODA.

In the later stages of segmentation the epiblast cells differ from the
hypoblast cells in the absence of pigment. The segmentation cavity, if
developed, is small. A perfectly regular gastrula is formed (fig. 107 A and
B), which is preceded by the embryo assuming a flattened form. The
blastopore is at first wide, but gradually narrows, and finally assumes a
slightly excentric position. // becomes not the mouth, but the anus.

When the blastopore has become fairly narrow, mesoblast cells (B, me.}
appear around it, between the epiblast and hypoblast. Whether they are

FlG. 107. FOUR STAGES IN THE DEVELOPMENT OF PALUDINA VIVIPARA.

(Copied from Biitschli.)

ep. epiblast ; hy. hypoblast ; me. mesoblast ; bl. blastopore ; an. anus ; st. stomo-
dceum ; sh. shell-gland ; V. velum; x. primitive excretory organ.

bilaterally arranged or no is not clear; and though coloured like the
hypoblast, their actual development from this layer has not been followed.

The velum appears about the same time as the mesoblast, in the form of
a double ring of ciliated cells at about the middle of the body (B and C, V\
The mesoblast rapidly extends so as to occupy the whole space between
the epiblast and hypoblast, and at the same time becomes divided into two
layers (C). Shortly afterwards a space— the body cavity— appears be-
tween the two layers (D) which then attach themselves respectively to the
epiblast and hypoblast, and constitute the somatic and splanchnic layers of
mesoblast. The two layers remain connected by transverse strands.

MOLLUSCA. 237

By a change in the relations of the various parts and especially by
the growth of the posterior region of the body, the velum now occupies a
position at the end of the body opposite the blast opore. Immediately
behind it there appear two organs, one on the dorsal and one on the
ventral side. That on the dorsal side (sh) is a deep pit — the shell-gland —
which is continuous with a layer of columnar epiblast which ends near the
anus. The other organ (j/), situated on the ventral side, is a simple de-
pression, and is the rudiment of the stomodaeum. Between it and the
dorsally placed anus is a slight prominence— the rudiment of the foot. On
the two sides of the body, between the epiblast and hypoblast on a level
with the shell-gland are placed two masses of excretory cells, the pro-
visional kidneys (D, x). These are probably not homologous with the
provisional renal organ of Nassa and other marine Prosobranchiata. At
a later period a ciliated cavity appears in them, which probably communi-
cates with the exterior at the side of the throat.

In the later stages the foot grows rapidly, and forms a very prominent
mass between the mouth and the anus. An operculum is developed some-
what late in a shallow groove lined by thickened epiblast.

A provisional chitinous plug is formed in the shell-gland which soon
becomes everted. The shell is formed in the usual way on the everted
surface of the shell-gland. The thickened edge of this part becomes the
edge of the mantle, and soon projects in the neighbourhood of the anus as
a marked fold.

With the rapid growth of the larva the invaginated mesenteron becomes
relatively reduced in size. In its central part yolk spherules become
deposited, while the part adjoining the blastopore (anus) becomes elongated
to give rise to the intestine. The stomodaeum grows greatly in length and
joins the dorsal part of the archenteron which then becomes the stomach.
The part of the mesenteron with yolk spherules forms the liver. With
the development of the visceral sack the anus shifts its position. It first
passes somewhat to the left, and is then carried completely to the right.

The development of Entoconcha mirabilis (Joh. Miiller, No. 265), a
remarkable Prosobranchiate parasitic in the body cavity of Synapta, which
in the adult state is reduced to little more than an hermaphrodite generative
sack, deserves a short description. It is viviparous, and the ovum gives
rise to a larva which from the hardly sufficient characters of the foot and
shell is supposed to be related to Natica.

There is nothing very striking in the development. The food yolk is
scanty. The velum, as might be anticipated from the viviparous develop-
ment, is small. The tentacles are placed not within, but behind the velar
area. There is a natica-like shell, a large mantle-cavity, and a large two-
lobed foot.

In Buccinum, and Neritina only one out of the many ova included
in each egg-capsule develops. The rest atrophy and are used as food
by the one which develops.

Opisthobranchiata. It will be convenient to take a species of

238 GASTEROPODA.

Pleurobranchidium (Aplysia), observed by Lankester (No. 239), as a type of
Nudibranchiate development. The ovum first divides into two segments,
and from these small segments are budded off, which gradually grow
round and enclose the two large segments. The small segments now form
the epiblast.

At the aboral pole the epiblast becomes thickened and invaginated to
form the shell-gland, and shortly afterwards the velum and foot are formed
in the normal way, and a stomodaeum appears close to the ventral edge of
the velum (fig. 101). The two yolk cells (ry) still remain distinct, but a
true hypoblastic layer (probably derived from them, though this has not
been made out) soon becomes established. Prominent cells early make
their appearance at the base of the foot, which become at a later period
invaginated to form the anus. Otolithic sacks (of) become formed in the
foot, and the supraoesophageal ganglia from a differentiation of the epiblast

(ng\

At a later period the shell-gland becomes everted, and a nautiloid shell
developed. The alimentary tract becomes completed, though the two yolk
cells long retain their original distinctness. The shell-muscle is developed,
and peculiar pigmented bodies are formed below the velum. The foot
becomes prominent and acquires an operculum.

The metamorphosis of Tergipes has been more or less completely worked
out by Nordmann and by Schultze (No. 271).

In Tergipes Edwardsii worked out by the former author, the larva when
hatched is provided with a large velum, eyes, tentacles, an elongated
operculated foot, and mantle. In the next stage both shell and operculum
are thrown off, and the body becomes elongated and pointed behind. Still
later a pair of gill-processes with hepatic diverticula becomes formed.

The velum next becomes reduced, and two small processes, which give
rise to the lip tentacles and a second pair of gills, sprout out. An ecdysis
now takes place, and leads to further changes which soon result in the
attainment of the adult form.

In Tergipes lacinulatus, observed by Schultze, the velum atrophies before
the shell and operculum are thrown off.

Pulmonata. The development of the fresh-water Pulmonata appears
from Lankester's observations on the pond-snail (Lymnaeus) to be very
similar in all important particulars to that of marine Branchiogasteropoda.
The velum is however less developed than in most marine forms. The
shell-gland, etc. have the normal development. In Lymnaeus the blasto-
pore has an elongated form and it is still a matter of dispute whether it
closes at the mouth or anus.

In the Helicidae there is a gastrula by epibole. The shell-gland, as
may be gathered from Von Jhering's figures, has the usual form, and an
external shell of the usual larval type is developed. There is a ciliated
process above the mouth, which extends into the lumen of the mouth. This
process is often regarded as a rudimentary velum, but probably has not this
value. There is no other organ which can be homologous with the velum.

MOLLUSCA. 239

The development of Limax presents some peculiarities. The yolk-
spheres (hypoblast) form a large mass enclosed by the epiblast cells. A
shell-gland is formed in the usual situation, which however, instead of being
everted, as in ordinary forms, becomes closed, and in its interior are
deposited calcareous plates which give rise to the permanently internal
shell. The foot grows out posteriorly, and contains a large provisional
contractile vesicle, traversed by muscular strands which contract rhyth-
mically.

Although an external shell is present in Clausilia in the adult, the
shell-gland becomes closed in the embryo as in Limax, and an internal
plate-like shell is developed. The shell is at first covered by a complete
epithelium, which eventually gives way in the centre, leaving covered only
the edges of the shell. It thus comes about that the original internal shell
becomes an external one. It is very difficult to bring this mode of develop-
ment of the external shell into relation with that of other forms. Clausilia
like Limax develops a large pedal sinus.

In both Limax and Clausilia cilia are early developed and cause a
rotation of the embryo, but how far they give rise to a distinct velum is
not clear.

Heteropoda. The Heteropod embryos present in their early develop-
ment the closest resemblance to those of other Gasteropods. The seg-
mentation takes place according to the most usual Gasteropod type ; (vide
p. 99) and after the yolk cells have ceased to give origin to epiblast cells
they divide towards the nutritive pole, become invaginated, and line a
spacious archenteron. The epiblast cells at the formative pole gradually
envelop the yolk (hypoblast) cells, and the blastopore very early narrows
and becomes the permanent mouth.

Simultaneously with the narrowing of the blastopore, the shell-gland is
formed at the aboral pole, and the foot on the ventral side. The velum
appears as a patch of cilia on the dorsal side, which then gradually extends
ventrally so as to form a complete circle just dorsal to the mouth.

The larva, after these changes have been completed, is represented in
fig. 102.

In later stages the shell-gland becomes everted, and a shell is developed
in all the forms both with and without shells in the adult. The foot grows
very rapidly, and an operculum is in all cases formed behind. A bilobed
invagination in front gives rise to the mucous gland. The velum enlarges
and becomes bilobed.

Though the blastopore remains permanently open as the mouth, the
oesophagus is formed as an epiblastic ingrowth. The rudiment of the
proctodaeum appears as two epiblastic cells symmetrically placed behind the
foot, which subsequently pass to the right side, and give rise to a shallow
invagination which meets the meseriteric sack. In the latter structure the
cells of part of the wall develop a peculiar nutritive material, and form a
nutritive sack which eventually becomes the liver. The part of the sack
connected with the epiblastic oesophagus becomes constricted off as the

240

HETEROPODA.

stomach. The remainder, which unites with the proctodaeum, forms the
intestine.

The structural peculiarities of the adult are formed by a post-larval
metamorphosis. The caudal appendage of Pterotrachea and Firoloidea is
formed as an outgrowth of the upper border of the hind end of the foot.
The so-called fin arises as a cylindrical process in front of the base of the
foot, which is eventually flattened laterally. In the Atlantidae it is in some
cases at first vermiform, and in other cases attains directly its adult struc-
ture. The embryonic foot itself gives rise in Pterotrachea, Firoloidea and
Carinaria to the tail, on the dorsal and posterior side of which the operculum
may still be seen in young specimens. In Atlanta it forms the posterior part
of the foot on which the operculum persists through life.

The embryonic shell is completely lost in Pterotrachea and Firoloidea,
and the shell is rudimentary in Carinaria. With its atrophy the mantle
region also becomes much reduced.

The velum is enormously developed in many Heteropods. In Atlanta it
is six-lobed, each of the two primitive lateral lobes being prolonged into
three processes, two in front, and one behind. As in all other cases, it
atrophies in the course of the post-larval metamorphosis.

Pteropoda. The early larval
form of the Pteropods is closely
similar to that of marine Gastero-
pods. There are usually only three
hypoblastic spheres at the close of
the segmentation in the Thecoso-
mata, and a somewhat larger num-
ber in the Gymnosomata. The blas-
topore closes at the oral region, on
the nutritive side of the ovum, and
the shell-gland is placed at the
original formative pole. The velum,
shell-gland and foot have the usual
relations. Although many of the
adult forms are symmetrical, there is
very early an asymmetry visible in
the larva, shewing that the Pteropods
are descended from asymmetrical
ancestors. In the Gymnosomata
there is a second larval stage after
the loss of the shell when the larva
is provided with three rings of cilia
(fig. 109). In most forms of Ptero-
pods the dorsal part of the body,
covered by the mantle, is produced
into a visceral sack like that of the

of

-mr-

Cephalopoda (fig. 108).

FIG. 108. EMBRYO OF CAVOLINIA
(HYALEA) TRIDENTATA. (After Fol.)

M. mouth ; a. anus ; s. stomach ; /'.
intestine ; <r. nutritive sack ; tub. mantle ;
me. mantle cavity ; fCn. contractile sinus ;
ft. heart ; r. renal sack : f. foot ; pn. epi-
podia ; q. shell ; of. otolithic sack,

MOLLUSCA.

24I

The velum varies considerably in its development in different forms. In
the Hyaleidee it is comparatively small and atrophies early; while in
Cymbulia (fig. 103) and the Gymnosomata it is large and bilobed, and
persists till after the foot has attained its full development.

The free edge of the velum is provided with long motor cilia, and its
lower border with small cilia which bring the food to the mouth. In
Cleodora there is a median bunch of cilia in the centre of the velum like that
in the Lamellibranchiata, Nudibranchiata, etc.

The shell-gland forms a pit at the aboral end of the body, and in
Cymbulia a chitinous plug appears to be normally formed in this pit. The
pit afterwards everts itself. The edge of the everted area becomes thickened
and gradually travels towards the anterior end of the body. On this everted
area a small plate is developed, which forms the commencement of the
embryonic shell with which the larvae of all Pteropods are provided.

The remainder of the embryonic shell is secreted in successive rings by
the thickened edge of the mantle, and grows with this till it reaches the
neck (fig. 108). The permanent shell is added subsequently, usually on a
very different model to the larval shell. The fate of the embryonic shell is
very various in different forms. In the Hyaleidae the animal withdraws itself
from the larval shell, which becomes shut off from the permanent shell by a
diaphragm. The larval shell then becomes detached.

In the Styliolidae the per-
manent shell becomes twice
the size of the embryonic
shell while the animal is still
in an embryonic condition,
but the larval shell persists
for life. In the Cymbulidae
there is an embryonic and
secondary shell, which per-
sist together during larval
life. They are eventually
cast off at the same time
and replaced by a perma-
nent shell.

In the Gymnosomata an
embryonic shell is develop-
ed, and a secondary shell
added to it during embryo-
nic life. Both are cast off
before the adult condition
is attained. After the shell
has been cast off three cili-
ated rings are developed (fig. 109). The anterior of these is placed between
the velum and the foot, and the two hinder ones on the elongated posterior
part of the body.

B. II. 16

FIG. 109. FREE SWIMMING PNEUMODERMON
LARVAE. (After Gegenbaur, copied from Bronn.)

The velum has atrophied in both larvae.

In A three ciliated bands are present, and the
auditory vesicles are visible.

In B the tentacles with suckers and the epi-
podia have become developed.

an. anus.

242 CEPHALOPODA.

The ciliated rings give to these larvae a resemblance to Chcetopod larvae ;
but there can be no doubt that this resemblance is a purely superficial one-
The anterior ring atrophies early (fig. 109 B), and the second one soon
follows suit. It is probable that the hindermost one does not persist
through life, although it has been observed in forms with fully developed
sexual organs. Most of these larvae have not been traced to their adult
forms. They have been referred to Pneumodermon, Clio, etc.

The most characteristic organ of the Pteropods is the foot, which is
prolonged into two enormous lateral wings, the epipodia. These develope
at different periods in different larvae, but are always distinct lateral out-
growths of the foot.

In the Hyaleidic the foot is early conspicuous, and soon sends out two
lateral prolongations (fig. 108 pn.} which develop with enormous rapidity
as compared with the medium portion, and give rise to the epipodia. The
whole of the foot becomes ciliated.

In the Cymbulidae, though not in other forms, an operculum is developed
on the hinder surface of the foot (fig. 103 C). The epipodia are late in
appearing.

In the Gymnosomata the foot is developed very early, but remains small.
The epipodia do not appear till very late in larval life (fig. 109 B).

In Pneumodermon and some other Gymnosomata there appear on the
hinder part of the head peculiar tentacles with suckers like those of the
Cephalopoda (fig. 109 B). It is not certain that these tentacles are geneti-
cally related to the arms of the Cephalopoda.

Cephalopoda. The eggs of the Cephalopoda are usually
laid in special capsules formed in the oviduct, which differ
considerably in the different members of the group.

In the case of Argonauta each egg is enveloped in an elongated capsule
provided with a stalk. By means of the stalk the eggs are attached together
in bunches, and these again are connected together and form transparent
masses, which are placed in the back of the shell. In octopus the eggs are
small and transparent : each of them is enclosed in a stalked capsule. In
Loligo the eggs are enveloped in elongated sack-like gelatinous cords,
each containing about thirty or forty eggs. The cords are attached in
bunches to submarine objects. In Sepia each egg is independently en-
veloped in a spindle-shaped black capsule, which is attached to a stone
or other object.

In a decapod form with pelagic larvae, described by Grenacher (No. 280),
the eggs were enclosed in a somewhat cylindrical gelatinous mass. In each
mass there were an immense number of eggs arranged in spirals. Each
ovum was enclosed in a structureless membrane, within which it floated in a
colourless albumen.

The ovum itself within the capsule is a nearly homogeneous
granular mass, without a distinct envelope. Development com-

MOLLUSCA.

243

mences by the segregation, at the narrow pole of the ovum oppo-
site the egg-stalk, of the greater part of the protoplasmic forma-
tive material1. This material forms a disc equivalent to the
germinal disc of meroblastic vertebrate ova. The germinal disc
in Sepia and Loligo does not, however, undergo a quite symme-
trical segmentation (Bobretzky, No. 279). When eight segments
are present, two of them close together are much smaller and
narrower than the remainder ; and when, in the succeeding
stages small segments are formed from the inner ends of the
large ones, those derived from the two smaller segments continue
to be smaller than the remainder: so that throughout the seg-
mentation one pole of the blastoderm is formed of smaller
segments, and the blastoderm exhibits a bilateral symmetry2.
The partial segmentation results in the formation of a blastoderm
covering one pole of the egg, but, unlike the vertebrate blasto-
derm, formed of a single row of cells. This blastoderm very
soon becomes two or three cells deep at its edge, and the cells
below the surface constitute the layer from which the mesoblast
and hypoblast originate (fig. 1 10 ms). The origin of the meso-
blast at the edge of the blastoderm is a phenomenon equivalent
to its origin at the lips of the blastopore in so many other types.
The external layer forms the epiblast.

The whole blastoderm does not take its origin from the seg-
mentation spheres, but, as was discovered by Lankester (282), a
number of nuclei arise spontaneously in the yolk outside the
blastoderm, around which cell-bodies become subsequently
formed. They make their appearance near to, but not at the
surface, extending first in a ring-like series in advance of the
margin of the blastoderm, but subsequently appearing indiscrim-
inately over all parts of the egg. They take no share in form-
ing the epiblast, but would seem, according to Lankester, to
assist in giving rise to the lower layer cells, and also to a
layer of flattened cells which eventually completely encloses the
yolk, and may be called the yolk membrane. The cells of the
yolk membrane first of all appear at the thickened edge of the

1 In Octopus and Argonauta (Lankester) as soon as the blastoderm is completed
the egg reverses its position in the egg-shell ; the cleavage pole taking up a position
nearest the stalk.

2 I do not know the relation of this axis of symmetry to the future embryo.

1 6— 2

244 CEPHALOPODA.

blastoderm. From this point they spread inwards under the centre

of the blastoderm (fig.

115 tti), and, together

with the epiblast cells,

outwards over theyolk

generally ; so that

before long (on the

tenth day in Loligo) FIG. no. SECTION THROUGH THE BLASTODERM

jay "5VJ op A LoLIGO OVUM AT THE BEGINNING OF THE

the yolk becomes com- FOURTH DAY. (After Bobretzky.)

pletely invested by a ms. mesoblast ; d. cell at the edge of the blasto-

i r 11 derm; c. one of the segmentation cells.

In the non-germinal region the blastoderm is formed of two
layers, (i) a flattened epiblast, and (2) the yolk membrane. In
the region of the original germinal disc the epiblast cells become
columnar, and below them is placed a ring of lower layer cells,
which gradually extends towards the centre so as finally to form
a complete layer. Below this again comes the yolk membrane
just spoken of.

Before describing the further fate of the separate layers it is
necessary to say a few words as to the external features of the
embryo. In the adult Cephalopod it is convenient, for the sake
of comparison with other Mollusca, to speak of the narrow space
enclosed in the arms, which contains the mouth, as the ventral
surface ; the aboral apex as the dorsal surface ; and what is
usually called the upper surface as the anterior and the lower
one as the posterior.

Employing this terminology the centre of the original blasto-
derm is the dorsal apex of the embryo. In the typical forms
with a large yolk-sack the whole embryo is formed out of the
original germinal disc ; the part of the blastoderm which is
continued as a thin layer over the remainder of the egg forms a
large ventral yolk-sack appended to the head of the embryo.
The following description applies especially to two types, which
form the extremes of the series in reference to the development
of the yolk-sack. The first of these with a large yolk-sack is
Sepia, of which Kolliker in his classical memoir (No. 281) has
published a series of beautiful figures. The second, with a small
yolk-sack, is the pelagic larva of an unknown adult described by
Grenadier (No. 280).

MOLLUSCA. 245

In a young blastoderm of Sepia viewed from the dorsal
surface, a series of structures appear which are represented in
fig. in A. In the middle is a somewhat rhomboid prominence
which forms the rudiment of the mantle (mt). In its centre is a
pit which forms the shell-gland. On each side of the mantle is
a somewhat curved fold (/). These folds eventually coalesce to
form the funnel. They are divided into two parts by a small
body which forms the cartilage of the funnel. The smaller part
of the fold behind this body gives rise to the true funnel, the part
in front becomes (Kolliker) the strong muscle connecting the
funnel with the neck-cartilage. In front and to the sides are
two kidney-.shaped bodies (oc) the optic pits. Behind the mantle
are two buds (br}} the rudiments of the gills.

FlG. in. TWO SURFACE VIEWS OF THE GERMINAL DISC OF SEPIA.

(After Kolliker.)

mt. mantle ; oc. eye ; /. folds of funnel ; br. branchiae ; an. posterior portion of
alimentary tract ; m. mouth, i, 2,3, 4, 5, arms ; /. cephalic lobe.

In the somewhat later stage rudiments of the two posterior
pairs of arms make their appearance outside and behind the
rudiments of the funnel. The head is indicated by a pair of
lateral swellings on each side, the outer of which carries the eyes.
The whole embryo now becomes ciliated, though the ciliation
does not cause the usual rotation. At a slightly later stage the
second, third, and fourth pairs of arms make their appearance
slightly in front of those already present. The posterior parts of
the funnel rudiments approach each other, and the anterior meet
the rudiments of the neck-cartilage. The gills begin to be
covered by the mantle-edge, which now projects as a marked
fold. At a slightly later period two fresh rudiments may be

246

CEPHALOPODA.

noted, viz. the oral (fig. 1 1 1 B, m) and anal invaginations, the
latter of which is extremely shallow and appears at the apex of
a small papilla which may be spoken of as the anal papilla.
These invaginations appear at the two opposite poles (anterior
and posterior) of the blastoderm. Shortly after this the rudi-
ment of the first pair of arms arises considerably in front of the
other rudiments, at the sides of the outer pair of cephalic swell-
ings (fig. 1 1 1 B, i).

Fig. 1 1 1 B represents a view from the dorsal surface of an
embryo at this stage. In the centre is the mantle with the shell-
gland which is now very considerably raised beyond the general
surface. Concentric with the edge of the mantle are the two

FIG. 112. SIDE VIEWS OF THREE LATE STAGES IN THE DEVELOPMENT OF

SEPIA. (After Kolliker.)
/;/. mouth ; yk. yolk-sack ; oc. eye ; tut. mantle.

halves of the funnel, the anterior half meeting the dorsal or neck-
cartilage and the posterior halves approaching each other. The
oral invagination is shewn at ;;/ and the anal immediately in
front of an. The gills, nearly covered by the mantle, are seen at
br. At /are the cephalic swellings, and the eye is seen at oc.

MOLLUSCA. 247

The arms I — 5 form a ring outside these parts. The whole of
the embryo, with the exception of the gills, the funnel, and the
outer border of the blastoderm, is richly ciliated.

The embryo up to this time has had the form of a disc or
saucer on the surface of the yolk. After this stage it rapidly
assumes its permanent dome-like form, and becomes at the same
time folded off from the yolk. The blastoderm is very slow in
enveloping the yolk, and the whole yolk is not completely in-
vested till a considerably later stage than that represented in fig.
1 1 1 B. As soon as the blastoderm covers the yolk-sack cilia
appear upon it. The mantle grows very rapidly, and its free
border soon projects over the funnel and gills. After the two
halves of the funnel have coalesced into a tube, it comes to pro-
ject again beyond the edge of the mantle.

On the completion of the above changes the resemblance of
the embryo to a Cuttle-fish becomes quite obvious. Three of
the stages in the accomplishment of these changes are represent-
ed in fig. 112.

To the ventral side of the embryo is attached the enormous
external yolk-sack (yk}y which is continuous with an internal
section situated within the body of the embryo. The general
relations of the embryo to the yolk will best be understood by
reference to the longitudinal section of Loligo, fig. 1 27.

The arms gradually increase in length, and the second pair
passes in front of the first so as eventually to lie completely in
front of the mouth. The arms thus come to form a complete
ring surrounding the mouth, of which the original second pair,
and not, as might be anticipated, the first, completes the circle
in front. The second pair develops into the long arms of the
adult.

After the embryo has attained more or less completely its
definite form (fig. 112 C) it grows rapidly in size as compared
with the yolk-sack. The latter structure is at first four or five
times as big as the embryo, but, by the time of hatching, the em-
bryo is two to three times as big as the yolk-sack.

Loligo mainly differs from Sepia in the early enclosure of the yolk by the
blastoderm, and in the embryo exhibiting the phenomena of rotation within
the egg-capsule so characteristic of other Mollusca.

In Argonauta the yolk-sack is still smaller than in Loligo, and the yolk is

248 CEPHALOPODA.

early completely enclosed by the blastoderm. A well developed outer yolk-
sack is present during early embryonic life, but is completely absorbed
within the body before its close. Cilia appear on the blastoderm very early,
but vanish again when the yolk is about two-thirds enclosed. There is,
during embryonic life, no trace of a shell, but the mantle and other parts of
the body become covered by peculiar bunches of fine setae. The shell-gland
develops normally in both Octopus and Argonauta, but disappears again
without closing up to form a sack (Lankester).

The pelagic Decapod larva described by Grenacher, which
forms my second type, must be placed with reference to the de-
velopment of the yolk-sack at the opposite pole to Sepia. Seg-
mentation, as in other Cephalopods, is partial, but the blastoderm
almost completely envelops the yolk before any organs are de-
veloped ; and no external yolk-sack is present. At a stage
slightly before the closure of the yolk-blastopore the mantle is
formed as a slight prominence at the blastodermic pole of the
egg, and even at this early stage is marked by the presence of
chromatophores. The edge of the blastoderm is ciliated. At a
slightly later stage the embryo becomes more cylindrical, the
edge of the mantle becomes marked by a fold, which divides the
embryo transversely into two unequal parts, a smaller region
covered by the mantle, and a larger region beyond this. The
yolk is still exposed, but rudiments of the optic pit and of two
pairs of arms have appeared. The first-formed arms are appa-
rently the anterior, and not, as in Sepia, the posterior.

At a still later stage, represented in lateral and posterior
views in fig. 113 A and B, considerable changes are effected.
The yolk-blastopore is nearly though not quite closed. The
mantle fold (int) is much more prominent, and on the posterior
side on a level with its edge may be seen the rudiments of the
gills (br). The funnel is formed as two independent folds on
each side (in/1 and mf*), which apparently correspond with the
two divisions of the funnel rudiments in Sepia. The eye has
undergone considerable changes. Close to each rudiment of the
funnel may be seen a fresh sense-organ — the auditory sack (ac).
The ventral (upper in the figure) end of the body now forms a
marked protuberance, probably equivalent to the foot of other
Mollusca (vide p. 225), at the sides of which are seen the rudi-
ments of the arms (i, 2, 3). To the two previously present a
third one, on the posterior side, has been added. The blastopore

MOLLUSCA.

249

is placed on the anterior side of the ventral protuberance, and
immediately dorsal to this is an invagination (os) which gives
rise to the stomodaeum. The ciliation at the edge of the blas-
topore still persists, but does not lead to the rotation of the
embryo.

In later stages (fig. 1130 the blastopore becomes closed,
and the mantle region increases in length as compared with the
remainder of the body. The ventral halves of the funnel, each
in the form of a half tube, coalesce together to form a single

FIG. 113. THREE EMBRYOS OF A CEPHALOPOD WITH A VERY SMALL YOLK-SACK.
(After Grenacher.)

a. blastopore ; br. branchiae ; inf.1 and mf.2 posterior and anterior folds of the
funnel ; g. op. optic ganglion (?) ; oc. eye ; wk. white body ; ac. auditory pit ; os.
stomodaeum; an. anus; mt. mantle; i, 2, 3. ist, 2nd, and 3rd pairs of arms.

tube (inf) in the same manner as in Sepia. A shallow procto-
daeum (an} is formed between the two branchiae. The eyes (oc)
stand out as lateral projections, and the arms become much
longer.

Still later a fourth pair of arms is added as a bud from each
of the posterior pair, and with the growth in length of the arms
the suckers make their appearance. The mouth is gradually
carried up so as to be surrounded by the arms. The ciliation of
the surface becomes more extensive.

During the whole of the above development the interior of
the embryo is filled with yolk, although no external yolk-sack is

250 CEPHALOPODA.

present. The internal yolk-sack falls into three sections ; a
cephalic section, a section in the neck, and an abdominal section.
Of these, that in the neck is the first to be absorbed. The
cephalic portion fills out the ventral protuberance already spoken
of. The hinder section becomes occupied by the liver which
exactly fits itself into this space as it absorbs the material pre-
viously there.

It will be convenient at this point to complete the account of
the Cephalopoda by a short history of their germinal layers, and
by a fuller description of the mantle, shell, and funnel than that
given in the preceding pages.

It has already been shewn that in the region of the germinal
disc a thick layer of cells becomes interposed between the epi-
blast and the yolk membrane. This layer (fig. 115 m) is mainly
mesoblastic, but also contains the elements which form the

c7is

FIG. 114. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM

WHEN THE MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED.

(After Bobretzky. )

gls. salivary gland ; brd. sheath of radula ; oe. oesophagus; ds. yolk -sack ; c/is.
shell-gland ; int. mantle ; pdh. mesenteron ; x. epiblastic thickening between the
folds of the funnel.

lining of the alimentary tract. Its cells first become differenti-
ated into mesoblast and hypoblast after the shell-gland has
become a fairly deep pit. The mode of differentiation is shewn
in fig. 1 14. On the posterior side of the mantle, at the point
marked in fig. 1 1 1 B, an, a cavity is formed between the yolk
membrane and the mesoblast cells (fig. n^pd/i). This cavity
is the commencement of the anal extremity of the mesenteron,
and the columnar cells lining it constitute the hypoblast. The

MOLLUSCA. 251

remainder of the lower layer cells are the mesoblast. The
mesenteron gradually extends itself till it meets the stomodaeum
(fig. 127). The proctodaeum is formed as a shallow pit close to
the first formed part of the mesenteron.

The mesoblast gives rise not only to the organs usually
formed in this layer, but also to the nervous centres, etc.

The mantle and shell. The mantle first arises as a thick-
ening of the epiblast on the dorsal surface of the embryo. The
thickened integument, with the subjacent mesoblast, soon forms
a definite projection, in the centre of which appears a circular
pit (figs. 1 14 chs and 115 shs). This pit, which has already been
spoken of as the shell-gland, resembles very closely the shell-
gland of other Mollusca. The fold around the edge of the shell-

FIG. 115. DIAGRAM OF A VERTICAL SECTION THROUGH THE MANTLE REGION OK

AN EMBRYO LOLIGO. (From Lankester.)

[This figure is turned the reverse way up to fig. 114.]

ep. epiblast ; y. food-yolk ; m. mesoblast ; m'. cellular yolk membrane ; shs. shell-
gland.

gland grows inwards so as gradually to circumscribe its opening,
which before long becomes completely obliterated ; and the
gland forms a closed sack lined by epiblast which grows in an
anterior direction (figs. 114 and 127 cctt).

The edges of the mantle now begin to project, especially on
the posterior side (fig. 127), and within the cavity formed by this
projecting lip there are placed the anus (an], gills, etc. The pro-
jecting lip of the mantle is formed both of epiblast and meso-
blast. The whole of the anterior side of the mantle is filled by
the elongated shell-sack (cck), within which the shell or pen soon
becomes secreted.

252 CEPHALOPODA.

There are certain difficulties in comparing the shell-gland of the Cephalo-
poda with that of other Mollusca which will best be rendered clear by the
following quotation from Lankester1:

"The position and mode of development of the shell-gland of the Cepha-
lopoda exactly agree with that of the shell-gland as seen in the other Mol-
luscan embryos figured in this paper. We are therefore fairly entitled to
conclude from the embryological evidence that the pen-sack of Cephalopoda
is identical with the shell gland of other Mollusca.

" But here — forming an interesting example of the interaction of the
various sources of evidence in genealogical biology — palaeontology crosses
the path of embryology. I think it is certain that if we possessed no fossil
remains of Cephalopoda the conclusion that the pen-sack is a special develop-
ment of the shell-gland would have to be accepted.

" But the consideration of the nature of the shell of the Belemnites and
its relation to the pen of living Cuttle-fish brings a new light to bear on the
matter. Reserving anything like a decided opinion as to the question in
hand, I may briefly state the hypothesis suggested by the facts ascertained as
to the Belemnitidae. The complete shell of a Belemnite is essentially a
straightened nautilus-shell (therefore an external shell inherited from a
nautilus-like ancestor), which, like the nautiloid shell of Spirula, has become
enclosed by growths of the mantle, and unlike the shell of Spirula, has
received large additions of calcareous matter from those enclosing over-
growths. On the lower surface of the enclosed nautilus-shell of the Belemnite
— the phragmacone — a series of layers of calcareous matter have been
thrown down forming the guard ; above, the shell has been continued into
the extensive chamber formed by the folds of the mantle, so as to form the
flattened pen-like pro-ostracum of Huxley.

" Whether in the Belemnites the folds of the mantle which thus covered
in and added to the original chambered shell, were completely closed so as
to form a sack or remained partially open with contiguous flaps must be
doubtful.

" In Spirula we have an originally external shell enclosed but not added
to by the enclosing mantle sack.

" In Spirulirostra, a tertiary fossil, we have a shell very similar to that
of Spirula, with a small guard of laminated structure developed as in the
Belemnite (see the figures in Bronn Classen u. Ordnungen des Thierreichs}.

" In the Belemnites the original nautiloid shell is small as compared with
Spirulirostra. It appears to be largest in Huxley's genus Xiphoteuthis.
Hence in the series Spirula, Spirulirostra, Xiphoteuthis, Belemnites, we
have evidence of the enclosure of an external shell by growths from the
mantle (as in Aplysia), of the addition to that shell of calcareous matter from
the walls of its enclosing sack, and of the gradual change of the relative
proportions of the original nucleus (the nautiloid phragmacone) and its

1 "Development of Pond Snail." Quart. J. of Micro. Science, 1874, pp.

MOLLUSCA. 253

superadded pro-ostracal and rostral elements tending to the disappearance
of the nucleus (the original external shell). If this view be correct as to the
nature of these shells, it is clear that the shell-gland and its plug has nothing
to do with them. The shell-gland must have preceded the original nautiloid
shell, and must be looked for in such a relation whenever the embryology of the
pearly Nautilus can be studied. Now, everything points to the close agreement
of the Belemnitidie with the living Dibranchiata. The hooklets on the arms,
the ink-bag, the horny jaws, and general form of the body, leave no room for
doubt on that point ; it is more than probable that the living Dibranchiata
are modified descendants of the mesozoic Belemnitidae. If this be so, the
pens of Loligo and Sepia must be traced to the more complex shell of the
Belemnite. This is not difficult if we suppose the originally external shell
the phragmacone, around which as a nucleus the guard and pro-ostracum
were developed, to have finally disappeared. The enclosing folds of the
mantle remain as a sack and perform their part, producing the chitino-
calcareous pen of the living Dibranch, in which parts can be recognised as
corresponding to the pro-ostracum, and probably also to the guard of the
Belemnite. If this be the case, if the pen of Sepia and Loligo correspond to
the entire Belemnite shell minus the phragmacone-nucleus, it is clear that
the sack which develops so early in Loligo and which appears to correspond
to the shell-gland of the other Molluscs cannot be held to do so. The sack
thus formed in Loligo must be held to represent the sack formed by the
primaeval up-growth of mantle-folds over the young nautiloid shell of its
Belemnitoid ancestors, and has accordingly no general significance for the
whole Molluscan group, but is a special organ belonging only to the Dibran-
chiate stem, similar to— but not necessarily genetically connected with — the
mantle-fold in which the shell of the adult Aplysia and its congeners is con-
cealed. The pen, then, of Cephalopods would not represent the plug of the
shell-gland. In regard to this view of the case, it may be remarked that I
have found no trace in the embryonic history of the living Dibranchiata of a
structure representing the phragmacone ; and further, it is possible, though
little importance can be attached to this suggestion, that the Dibranchiate
pen-sack, as seen in its earliest stage in the embryo Loligo, etc., is fused
with the surviving remnants of an embryonic shell-gland. When the
embryology of Nautilus pompilius is worked out, we shall probably know
with some certainty the fate of the Molluscan shell-gland in the group of the
Cephalopoda."

The funnel. The general development of the funnel has
already been sufficiently indicated. The folds of which it is
formed are composed both of epiblast and mesoblast. The
mesoblast of the anterior part of each half of the funnel would
appear to give rise to a muscle passing from the cartilage of the
neck to the funnel proper. The posterior parts gradually
approximate, but meet in the first instance ventrally. The two

254

POLYPLACOPHORA.

folds at first merely form the side of a groove or imperfect tube
(fig. 1 13 C and 124 ff.), but soon the free edges unite and so give
rise to a perfect tube, the primitive origin of which by the coal-
escence of two halves would not be suspected. In Nautilus the
two halves remain permanently separate but overlap each other,
so as to form a functional tube.

Polyplacophora. The external characters of the embryo of
Chiton have long been known through the classical observations
of Loven (No. 285), while the formation of the layers and the in-
ternal phenomena of development have recently been elucidated
by Kowalevsky (No. 284). The eggs are laid in April, May,
and June, and are enclosed in a kind of chorion with calcareous

FIG. 1 1 6.

I. CHITON WOSSNESSENSKII. (After Middendorf.)

II. CHITON DISSECTED to shew o. the mouth ; g. the nervous ring ; ao. the
aorta; c. the ventricle; ^. an auricle; br. the left branchiae; od. oviducts. (After
Cuvier.)

III., IV., V. STAGES OF DEVELOPMENT OF CHITON CINEREUS. (After Loven.)
The figure is taken from Huxley.

protuberances. The segmentation remains regular till sixty-
four segments are formed. The cells composing the formative

MOLLUSCA. 255

half of the ovum then divide more rapidly than the remainder ;
there is in this way formed an elongated sphere, half of which is
composed of small cells and half of larger cells. In the interior
is a small segmentation cavity. From its eventual fate the
hemisphere of the smaller cells may be called the anterior pole,
and that of the larger cells the posterior. An involution of the
cells at the apex of the posterior pole (though not of the whole
hemisphere of larger cells) now takes place, and gives rise to the
archenteron. At the same time an equatorial double ring of
large cells appears on the surface between the two poles, which
becomes ciliated and forms the velum. At the apex of the an-
terior pole a tuft of cilia, or at first a single flagellum, is estab-
lished (fig. 116 in. and IV.).

In the succeeding developmental period the blastopore, which
has so far had the form of a circular pore at the posterior ex-
tremity of the body, undergoes a series of very remarkable
changes. In conjunction with a gradual elongation of the larva
it travels to the ventral side, and is prolonged forwards to the
velum as a groove. The middle part of the groove is next con-
verted into a tube, which opens externally in front, and post-
eriorly communicates with the archenteron. The walls of this
tube subsequently fuse together, obliterating the lumen, and
necessarily causing at the same time the closure of the blasto-
pore. The tube itself becomes thereby converted into a plate
of cells on the ventral surface between the epiblast and the
hypoblast1.

While the above changes have been taking place the meso-
blast has become established. It is derived from the lateral and
ventral cells of the hypoblast.

After the establishment of the germinal layers the further
evolution of the larva makes rapid progress. A transverse
groove is formed immediately behind the velum, which is
especially deep on the ventral surface ; and the stomodaeum is
formed as an invagination of the anterior wall of the deeper
section of the groove. Behind the stomodaeum the remainder
of the ventral surface grows out as a flattened foot.

1 There is a striking similarity between the changes of the blastopore in Chiton
and the formation of the neurenteric canal of Chordata ; especially if Kowalevsky is
correct in stating that the pedal nerves are developed from the ventral plate.

256 POLYPLACOPHORA.

The dorsal surface behind the velum constitutes the mantle,
and becomes divided by six or seven transverse grooves into
segment-like areas, which may be called mantle plates (fig. 116
IV.). These areas would seem (?) to correspond to so many
flattened-out shell-glands. Immediately behind the velum the
eyes appear as two black spots (fig. 1 16 IV.).

While the above external changes take place the archenteron
undergoes considerable modifications. Its anterior section gives
rise, according to Kowalevsky, to a dorsal (?) sack in which the
radula is formed ; while the liver arises from it as two lateral
diverticula.

From the above statements it would appear that Kowalevsky holds that
the oesophagus and radula sack are both derived from the walls of the
archenteron and not from the stomodaeum. Such an origin for these organs
is without parallel amongst Mollusca.

The larva becomes about this time hatched, and after swim-
ming about for some time attaches itself by the foot, throws off
its larval organs, cilia, etc., and develops the shell.

The shell appears first of all during larval life in the form of spicula on
the middle and sides of the head, and later on the middle and sides of the
post-oral mantle plates (fig. 116 v.). The permanent shell arises somewhat
later as a series of median and lateral calcareous plates, first of all on the
posterior part of the velar area, and subsequently on the mantle plates behind.
The three calcareous patches of each plate fuse together and give rise to the
permanent shell plates. The original spicula are displaced to the sides,
where they partly remain, and are partly replaced by new spicula.

The nervous system is formed during larval life as four longitudinal
cords : — two lateral — the branchial cords, and two ventral — the pedal.
Paired anterior thickenings of the pedal cords meet in front of the mouth to
form the cesophageal ring. The pedal cords and their derivatives are
believed by Kowalevsky to be developed from the lateral parts of the plate
formed by the metamorphosis of the blastopore. The median part of the
plate is still visible after the formation of these parts.

The chief peculiarity of the larva of Chiton (apart from the
peculiar ventral plate) consists in the elongation and dorsal
segmentation of the posterior part of the body. The velum has
the normal situation and relation to its mouth. The position of
the eyes behind it is however abnormal.

The elongation and segmentation of the posterior part of the
trunk is probably to be regarded as indicating that Chiton has

MOLLUSCA. 257

early branched off from the main group of the Odontophora
along a special line of its own, and not that the remaining Odon-
tophora are descended from Chiton-like ancestral forms. The
shell of Mollusca on this view is not to be derived from one of
the plates of Chiton, but the plates of Chiton are to be derived
from the segmentation of a primitive simple shell. The segmen-
tation exhibited is of a kind which all the trochosphere larval
forms seem to have been capable of acquiring. The bilateral
symmetry of Chiton, which is quite as well marked as that of
the Lamellibranchiata, indicates that it is a primitive phylum of
the Odontophora.

Scaphopoda. The external characters of the peculiar larva
of this interesting group have been fully worked out by Lacaze
Duthiers (No. 286).

The segmentation is unequal and conforms to the usual
molluscan type. At its close the embryo becomes somewhat
elongated, and there appears on its surface a series of transverse
ciliated rings. As soon as these become formed the larva is
hatched, and swims about by means of its cilia. Six ciliated
bands are formed in all, and in addition a tuft of cilia is formed
in a depression at the anterior extremity.

The larva thus constituted is very different in appearance to
the larvae already described, and its parts very difficult to
identify ; the next stages in the development shew however that
the whole region of the body taken up by the ciliated rings is
part of the velar area, while the small papilliform region behind
is the post-velar part of the embryo. This latter part grows
rapidly, and at the same time the ciliated rings become reduced
to four ; which gradually approach each other, while the region
on which they are placed grows in diameter. The rings finally
unite, and form a single ring on a projecting velar ridge. In
the centre of this ring is placed the terminal tuft of cilia on a
much reduced prominence.

By the time that these changes have been effected in the
velum, the post-velar part of the embryo has become by far the
largest section of the embryo, so that the velum forms a project-
ing disc at the front end of an elongated body. The mantle is
formed as two lateral outgrowths near the hinder extremity of
the body which leave between them a ventral groove lined by
B. II. 1 7

258 LAMELLIBRANCHIATA.

cilia ; on their dorsal side is formed a delicate shell. The
mantle lobes continue to grow, and by the time the above
changes in the velum are effected they meet and unite in the
ventral line and convert the groove between them into a com-
plete tube open in front and behind. A stream of water is
driven through this tube by the action of the cilia. The shell,
which is at first disc-shaped like the shell of other molluscan
larvae, moulds itself upon the mantle and is so converted into a
tube. At the front end of the mantle tube, which does not at
first cover the velum, there is formed the foot. It arises as a
protuberance of the ventral wall of the body, which rapidly
grows forwards, becomes trilobed as in the adult, and ciliated.

On the completion of these changes the larva mainly differs
in appearance from the adult by the projection of the velum
beyond the edge of the shell. The velum soon however begins
to atrophy ; and the larva sinks to the bottom. The mantle tube
and shell grow forward and completely envelop the velum,
which shortly afterwards disappears. The mouth is formed on
the ventral side of the velum at the base of the foot ; at its sides
arise the peculiar tentacles so characteristic of the adult Denta-
lium.

LAMELLIBRANCHIATA.

The larvae of Lamellibranchiata have in a general way the
same characters as those of Gasteropods and Pteropods. A
trochosphere stage with a velum but without a shell is succeeded
by a veliger stage with a still more developed velum, a dorsal
shell, and a ventral foot.

The segmentation is unequal, and in a general way like that
of Gasteropoda, but the specially characteristic Gasteropodan
type with four large yolk spheres is only known to occur in
Pisidium, and a type of segmentation similar to that of Anodon
(p. 100) appears to be the most frequent.

There is an epibolic or embolic gastrula, but the further
history of the formation of the germinal layers has been worked
out so imperfectly, and for so few types, that it is not possible to
make general statements about it. What is known on this head

MOLLUSCA.

259

is mentioned in connection with the description of the develop-
ment of special types.

The blastopore in some cases closes at the point where the
anus (Pisidium), and probably in other cases where the mouth, is
eventually formed. In Anodon it is stated to close at a point
corresponding neither with the mouth nor the anus, but on the
dorsal surface !

The embryo assumes a somewhat oval form, and in the free
marine forms there appears very early in front of the mouth a
well-developed velum. This is formed according to Love"n from
two papillae, and takes the form of a circular ridge armed with
long cilia. In the centre of the velar area there is usually

FIG. 117. THREE STAGES IN THE DEVELOPMENT OF CARDIUM. (After Loven.)
hy. hypoblast ; b. foot ; m. mouth ; an. anus ; V. velum ; cm. anterior adductor
muscle.

present a single long flagellum (fig. 117 B and C). The velum
never becomes bilobed.

In the later stages, after the development of the shell, the
velum becomes highly retractile and can be nearly completely
withdrawn within the mantle by special muscles. It forms the
chief organ of locomotion of the free larva.

In some fresh-water forms, which have no free larval exist-
ence, the velum is very much reduced (Anodon, Unio, Cyclas) or
even aborted (Pisidium). In these forms as well as in Teredo
and probably other marine forms (e.g. Ostrea) the central flagel-
lum is absent. It has been suggested by Loven, though without
any direct evidence, that the labial tentacles of adult Lamelli-
branchiata are the remains of the velum. The velar area is in
any case the only representative of the head. In some marine
forms a general covering of cilia arises before the formation of

17-2

260

LAMELLIBRANCHIATA.

the velum ; and in Montacuta and other types there is developed,
as in many Gasteropoda, a circum-anal patch of cilia.

A shell-gland appears at a very early period on the dorsal
surface in Pisidium, Cyclas and Ostrea, and probably in most
marine forms (fig. 118, s/is). It is somewhat saddle-shaped, and
formed of elongated non-ciliated cells bounding a groove. It
flattens out and on its surface is formed the shell, which appears
usually to have the form of an unpaired saddle-shaped cuticle, on
the two sides of which the valves are subsequently formed by a
deposit of calcareous salts. In Pisidium the two valves are
stated by Lankester to be at first quite independent and widely
separated, and it has been suggested by Lankester, though not
proved, that the ligament of the shell is developed in the median
part of the groove of the shell-gland.

The mantle lobes are developed as lateral outgrowths of the
body : they usually have a considerable extension before they
are covered by the shell. In Anodon and Unio the larval
mantle lobes are, however, formed in a somewhat exceptional
way, and are from the first completely covered by the valves of
the larval shell. The larval mantle lobes and shell in Anodon
and Unio are subsequently replaced by the permanent structures.

The adductor muscles are formed soon after the appearance
of the shell. The
posterior sometimes
appears first, e.g.
Mytilus,andat other
times the anterior,
e.g. Cardium.

The foot arises
in the usual way as
a prominence be-
tween the mouth
and anus. In com-
parison with Gaster-
opoda it is late in
appearing, and in
many cases does not
become prominent

till the shell has at- /'-~intestine J *hs- shell-gland

P'lG. Il8. AN EMBRYO OF PlSIDIUM PUSILLUM.

(From Lankester.)

/. foot ; m. mouth ; ph. pharynx ; gs. bilobecl stomach ;
ell-j

MOLLUSCA. 261

tained a considerable size. In its hinder part a provisional
paired byssus-gland is developed from the epidermis in Cyclas
and other forms. In other cases, e.g. Mytilus, the byssus-gland
is permanent. The byssus-gland occupies very much the position
of the Gasteropod operculum, and would appear very probably
to correspond with this organ. The anterior part of the foot is
usually ciliated.

The gills appear rather late in larval development along the
base of the foot on either side, between the mantle and the foot
(fig. 1 20, br). They arise as a linear row of separate ciliated
somewhat knobbed papillae. A second row appears later. The
two rows give rise respectively to the two gill lamellae of each
side.

The further history of the development of the gills has been studied by
Lacaze Duthiers (No. 297) in Mytilus. The first row of gill papillae formed
becomes the innermost of the two lamellae of the adult. The number of
papillae goes on increasing from before backwards. When about eleven
have been formed, their somewhat swollen free extremities unite together,
the basal portions being separated by slits.

The free limb is formed by the free end of the gill lamella bending upon
itself towards the inner side and growing towards the line of attachment of
the lamella. The free limb is at first not composed of separate bars, but of
a continuous membrane. Before this membrane has grown very wide,
perforations are formed in it corresponding to the spaces between the bars of
the attached limb.

The outer gill lamella develops in precisely the same way as, but some-
what later than, the inner. The rudiments of it appear when about twenty
papillae of the inner lamella are formed. Its first papillae are formed near
the hind border of the inner lamella, and new papillae are added both in
front and behind. Its free limb is on the outer side.

In Mytilus the two limbs (free and attached) of each bar of the gill are
joined at wide intervals by extensile processes, the ' inter-lamellar junctions,'
and the successive bars are attached together by ciliated junctions. In
many other types the concrescences between the various parts of the gills
are carried much further ; the maximum of concrescence being perhaps
attained in Anodon and Unio1.

Large paired auditory sacks seem always to be developed in
the foot; and clearly correspond with the auditory sacks in
Gasteropoda.

1 R. H. Peck, "Gills of Lamellibranch Mollusca." Quart. J. of M. Science,
Vol. xvn. 1877.

262 LAMELL1BRANCHIATA.

Eyes are frequently present in the larva, though they dis-
appear in the adult. In Montacuta and other types a pair of
these organs is formed at the base of the velum on each side of
the oesophagus, not far from the auditory sacks. They are
provided with a lens.

A row of similar organs is present in the larva of Teredo in
front of the foot.

Cardium. As an example of a marine Lamellibranchiate I may take
Cardium pygmaeum, the development of which has been studied by Lovdn
(No. 291). The ova, surrounded by a thickish capsule, are impregnated in
the cloaca. The segmentation takes place much as in Nassa (vide p. 101),
and the small segments gradually envelop the large hypoblast spheres ; so
that there would seem to be a gastrula by epibole. After the hypoblast has
become enveloped by the epiblast, one side of the embryo is somewhat flat-
tened and marked by a deepish depression (fig. 117 A). From LoveVs
description it appears to me probable that the depression on the flattened
side occupies the position of the blastopore, and that the depression itself is
the stomodaeum. At this stage the embryo becomes covered with short cilia
which cause it to rotate within the egg-capsule.

Close above the mouth there appear two small papillae. These gradually
separate and give rise to a circular ridge covered with long cilia, which
encircles the embryo anteriorly to the ventrally-placed mouth. This
structure is the velum. In its centre is a single long flagellum (fig. 117 B).
Shortly after this the shell appears as a saddle-shaped structure on the
hinder part of the dorsal surface of the embryo. It is formed at first of
two halves which meet behind without the trace of a hinge (fig. 117 C).
The two valves rapidly grow and partially cover over the velum, and below
them the mantle-folds soon sprout out as lateral flaps.

The alimentary tract has by this tirhe become differentiated (fig. 117 C).
It consists of a mouth (;;/) and ciliated oesophagus probably derived from
the stomodaeum, a stomach and intestine derived from the true hypoblast,
and an hepatic organ consisting of two separate lobes opening into the
stomach. The anus (an] appears not far behind the mouth, and between
the two is a very slightly developed rudiment of the foot (V). The anterior
adductor muscle (cm] appears at this stage, though the posterior is not yet
differentiated.

The larva is now ready to be hatched, but the further stages of its
development were not followed.

Ostrea. The larvae of Ostrea, figured by Salensky (No. 293), shew a
close resemblance to those of Cardium. The velum is however a simple
ring of cilia without a central flagellum. The proctodacum would appear to
be formed later than the stomodaeum, and the earliest stage figured is too
far advanced to throw light on the position of the blastopore.

Pisidium. The development of Pisidium has been investigated by

MOLLUSCA.

263

Lankester (No. 239). The ovum is invested by a vitelline membrane and
undergoes development in a brood-pouch at the base of the inner gill lamella.

The segmentation commences by a division into four equal spheres, each
of which, as in so many other Mollusca, then gives rise by budding to a
small sphere. The later stages of segmentation have not been followed in
detail, but the result of segmentation is a blastosphere. An invagination,
presumably at the lower pole, now takes place, and gives rise to an
archenteric sack.

The embryo now rapidly grows in size. The blastopore becomes closed
and the archenteric sack forms a small mass attached at one point to the
walls of the embryonic vesicle (fig. 119, hy). In the space between the walls
of the archenteron and those of the embryonic vesicle stellate mesoblast cells

FIG. 119. THREE VIEWS OF AN EMBRYO OF PISIDIUM IMMEDIATELY AFTER
THE CLOSURE OF THE BLASTOPORE. (After Lankester.)

A. View from the surface.

B. Optical section through the median plane.

C. Optical section through a plane a little below the surface.

ep. epiblast ; me. mesoblast ; hy. hypoblast ; p. cells apparently budding from the
hypoblast to form mesoblastic elements.

264

LAMELLIBRANCHIATA.

make their appearance, derived in the main from the epiblast, though
probably in part also from the hypoblastic vesicle (vide fig. 119 C, p}. The
cavity between the hypoblast and epiblast, which contains these cells, is the
body cavity. Fig. 1 19 represents three views of the embryo at this stage.
A is a surface view shewing the epiblast ; B is an optical section through
the median plane shewing the hypoblast and some of the mesoblast cells ;
and C is an optical section shewing the mesoblast cells. A prominence on
one side of the embryo now develops which forms the commencement of the
foot, and the archenteric sack grows out at its free extremity into two lobes,
but remains attached to the epiblast by an imperforate pedicle. The next
organ to appear is the stomodasum. It arises as a ciliated epiblastic in-
growth which meets the free end of the archenteric sack, fuses with it, and
shortly afterwards opens into it (fig. 118, ph). Between the mouth and the
attachment of the enteric pedicle is placed the foot (/), which becomes
ciliated. On the dorsal side of the enteric pedicle there appears a saddle-
shaped patch of epiblast cells bounding the sides of a groove (shs). This is
the rudiment of the shell-gland.

The enteric pedicle, or intestine as it may now be called, soon acquires a
lumen, though still imperforate at its termination where the anus is
eventually formed. Ventral to the intestine is placed a mass of cells — the
rudiment of the organ of Bojanus. It is stated to be developed as an
ingrowth of the epiblast.

In a slightly later stage the shell-gland rapidly increases in size and
flattens out, and on the two sides of it there appear the rudiments of the two
valves, which are at first quite distinct, and separated by a considerable
interval (fig. 120). Before the appearance of the valves of the shell, the
mantle folds have already grown out from the sides of the body.

At a somewhat later stage the gills
appear as a linear series of small inde-
pendent buds within the folds of the mantle
behind the foot (fig. 120, br). The ante-
rior adductor also becomes differentiated.

The alimentary tract in the meantime
has undergone considerable changes. The
primitive lateral lobes dilate enormously
and become ciliated. At a still later stage
their walls undergo peculiar changes, the
nature of which is somewhat obscure, but
they appear to me to be of the same charac-
ter as those in many Pteropods and Gas-
teropods, where the cells of the hepatic
diverticula, to which the lobes of Pisidium
apparently correspond, become filled with
an albuminous material.

The later stages in Pisidium have not
been followed.

FIG. 120. DIAGRAMMATIC VIEW
OF ADVANCED LARVA OF PiSIDIUM.
(Copied from Lankesler.)

tti. mouth ; a. anus ; B. organ
of Bojanus ; mn. mantle ; f. foot.

MOLLUSCA. 265

It is remarkable that in Pisidium a veliger stage does not occur. This is
probably due to the development taking place within the brood-pouch. The
late development of the otocysts is also remarkable. A byssus-gland was
not formed up to the stage observed. In Cyclas calyculata (Schmidt),
a byssus-gland also appears to be absent.

Cyclas. The development of Cyclas as described by Von Jhering is
very unlike that of Pisidium, and the differences would seem to be too great
to be accounted for except by errors of observation.

The segmentation of Cyclas is similar to that of Anodon (vide p. 82), and
a mass of large cells enclosed by the smaller cells gives rise to the hypoblast.
In the interior of this mass there appears a lumen, and a process from it
grows towards and meets the epiblast, and gives rise to the oesophagus and
mouth, — a mode of development of these parts without parallel amongst
Mollusca. A very rudimentary velum would appear, according to Leydig
(No. 290), to be developed at the cephalic extremity. A , shell-gland is
formed of the same character as in Gasteropods. According to Leydig the
shell appears as a single saddle-like structure on the dorsal surface ; the
lateral parts of this become calcified, and give rise to the two valves, but are
united in the middle by the membranous median portion. At the two sides
of the body the mantle lobes are formed, as in Pisidium.

Very shortly after the formation of the shell the byssus-gland appears as
a pair of small follicles in the hinder part of the foot. It rapidly grows
larger and becomes a paired pyriform gland, in which are secreted the byssus
threads which serve to attach all the embryos at a common point to the
walls of the brood-pouch.

The foot is large, and ciliated anteriorly. Otolithic sacks and peda
ganglia are developed in it very early.

Unio. The ovum of Anodonta and Unio is enveloped in a vitelline
membrane, the surface of which is raised into a projecting trumpet-like tube
perforated at its extremity (fig. 12). This structure is the micropyle. The
micropyle disappears in Anodonta piscinalis when the egg is ripe, but in
Unio persists during the whole development. The ova are transported, in a
manner not certainly made out, into the space between the two limbs of the
outer gills of the mother, and there undergo their early development. The
animal or upper pole of the egg is placed at the pole opposite to the
micropyle.

The segmentation is unequal (vide p. 100) and results in the formation of
a blastosphere with a large segmentation cavity. The greater part of the
circumference of the egg is formed of small uniform spheres, but the lower
(with reference to the segmentation) pole is taken up by a single large cell.
The small spheres become the epiblast, and the large cell gives rise to
hypoblast and mesoblast1.

1 The account of the remainder of the development till the larva becomes hatched
is taken from Rabl, No. 292.

266 LAMELLIBRANCHIATA.

The single large cell next divides into two, and then four, and finally into
about ten to fifteen cells. These cells form an especial area of more granular
cells than the other cells of the blastosphere. Most of them are nearly of
the same size, but two of them (according to Rabl), in contact with each
other, but placed on the future right and left sides of the embryo, are con-
siderably larger than the remainder. These two cells soon pass into the
cavity of the blastosphere, while at the same time the area of granular cells
becomes flattened out, and then becomes involuted as a small sack with a
transversely elongated opening, which does not nearly fill up the cavity of
the blastosphere. This involuted sack is the archenteron.

The two large cells, which lie in immediate contact with what, following
Rabl, I shall call the anterior lip of the blastopore, next bud off small cells,
which first form a layer covering the walls of the archenteron, but sub-
sequently develop into a network filling up the whole cavity of the primitive
blastosphere. The space between these cells is the primitive body cavity.
For a long time the two primitive mesoblast cells retain their preponderating
size1. At the hinder end of the body, and at the end opposite therefore
to the two mesoblast cells, are placed three especially large epiblast cells.

In Anodonta and Unio tumidus there appears at this period a patch of
long cilia at the anterior end of the body. These cilia cause a rotation of
the embryo and would appear to be the velum. In Unio pictorum they
do not appear till much later.

Immediately following this stage the changes in the embryo take place
with great rapidity. In the first place a special mass of mesoblast cells
appears at the hinder end of the archenteric sack ; and becoming elongated
transversely gives rise to the single adductor muscle. On the subsequent
formation of the shell the muscle becomes inserted in its two valves.
The blastopore next becomes closed, and the small archenteron grows for-
wards till it meets the epiblast anteriorly, and at the same time detaches
itself from the epiblast in the region where the blastopore was placed.
Where it comes in contact with the wall of the body in front a small
epiblastic invagination arises, which meets and opens into the archenteric
sack and forms the permanent mouth.

While these changes have been taking place the shell is formed as a
continuous saddle-shaped plate on the dorsal surface. From this plate the
two valves are subsequently differentiated. On the dorsal surface they
meet with a straight hinge-line. Each valve is at first rounded, but subse-
quently becomes triangular with the hinge-line as base. The valves are not
quite equi-sided, but the anterior side is less convex than the posterior. At
a later period a beak-shaped organ is formed at the apex of each valve in the
same manner as the remainder of the shell. This organ is placed at about
a right angle with the main portion of the valve. It is pointed at its ex-

1 In this description I follow Rabl's nomenclature. According to his statements
the ventral part of the body is the original animal pole— the dorsal the lower pole ;
the anterior end the mesoblastic side of the opening of invagination.

MOLLUSCA. 267

tremity and bears numerous sharp spines on its outer side, which are
especially large in the median line (vide fig. 121 A). It is employed in fixing
the larva, after it is hatched, on to the fish on which it is for some time
parasitic. The shell is perforated by numerous pores.

After the shell has become formed a new structure makes its appearance
which is known as the byssus-gland. It is developed as an invagination of
the epiblast at the hinder end of the body : Rabl was unable to determine
whether it was formed from the three large epiblastic cells present there or
no, It subsequently forms an elongated gland with three coils or so round
the adductor muscle on the left side of the body, but opening in the median
ventral line. It secretes an elongated cord by which the larva becomes
suspended after hatching.

For some time the ventral portion of the body projects behind the ends
of the valves of the shell, but before these are completely formed a median
invagination of the body wall takes place, which obliterates to a large extent
the body cavity, and gives rise to two great lateral lobes, one for each valve.
These lobes are the mantle lobes.

Before the mantle lobes are fully formed peculiar sense-organs, usually
four in number, make their appearance on each lobe. Each of them consists
of a columnar cell, bearing at its free end a cuticle from which numerous
fine bristles proceed. Covering the cell and the parts adjoining it is a
delicate membrane perforated for the passage of the bristles. The largest
and first formed of these organs is placed near the anterior and dorsal part
of the mantle. The three others are placed near the free end of the mantle
(vide fig. 121 A). These organs probably have the function of enabling the
larva to detect the passage of a fish in its vicinity, and to assist it therefore
in attaching itself. When the embryo is nearly ripe there appears im-
mediately ventral to and behind the velum a shallow pit on each side of the
middle line, and the two pits appear to be connected by a median transverse
bridge. These structures have been the cause of great perplexity to different
investigators, and their meaning is not yet clear. According to Rabl the
median structure is the somewhat bilobed archenteron, and according to
his view it is not really connected with the laterally placed pits. The cilia
of the velum overlie these latter structures and make them appear as if their
edges were ciliated. They are regarded by Rabl as the rudiments of the
nervous system.

With the development of the shell, the mantle, and the sense-organs, the
young mussel reaches its full larval development, and is now known as a
Glochidium (fig. 121 A).

If the parent, with Glochidia in its gills, is placed in a tank with fish, it
very soon (as I have found from numerous experiments) ejects the larvae
from its gills, and as soon as this occurs the larvae become free from the egg-
membrane, attach themselves by the byssus-cord, and when suspended in
this position continually close and open their shells by the contraction of the
adductor muscle. If the mussels are not placed in a tank with fish the larvae
may remain for a long time in the gills.

268 LAMELLIBRANCHIATA.

•p. ad

B.
FIG. 121.

A. GLOCHIDIUM IMMEDIATELY AFTER IT is HATCHED.
ad. adductor ; s/t. shell ; by. byssus cord ; s. sense organs.

B. GLOCHIDIUM AFTER IT HAS BEEN ON THE FISH FOR SOME WEEKS.
br. branchiae ; au. v. auditory sack ; f. foot ; a. ad. and p. ad. anterior and posterior
adductors ; al. mesenteron ; mt. mantle.

Before passing on to state what is known with reference to the larval
metamorphosis, it may be well to call attention to certain, and to my mind
not inconsiderable, difficulties in the way of accepting in all particulars
Rabl's account of the development.

In all Gasteropod Molluscs the lower or vegetative pole of the ovum is
ventral, not dorsal as Rabl would make it in Unio. The blastopore in other
Molluscs always coincides either with the mouth or anus, or extends between
the two. The surface on which the foot is formed is the ventral surface.
On the dorsal surface are placed, (i) the velum near the mouth, (2) the shell-
gland near the anus. In Anodon the velum is placed just dorsal to the
mouth, then according to Rabl follows the blastopore, and in the region of
the blastopore is formed the shell. The blastopore is therefore dorsal in
position. It occupies in fact the ordinary place of the shell-gland, and looks
very much like this organ (which is not otherwise present in Anodon and
Unio). Without necessarily considering Rabl's interpretations false, I think
that the above difficulties should have been at any rate discussed in his paper.
More especially is this the case when there is no doubt that Rabl has
made in his paper on Lymnaeus a confusion between the mouth and the
shell-gland.

Investigations on the post-embryonic metamorphosis of Glochidium have
been made by Braun (No. 287), and several years ago I made a series of
observations on this subject, the results of which agree in most points with
those of Braun. I was however unsuccessful in carrying on my observations
till the young mussel left its host.

The free Glochidia very soon attach themselves to the gills, fins, or other
parts of fish which are placed in the tank containing them; after attachment
they become covered by a growth of the epidermic cells of their host, and
undergo their metamorphosis.

MOLLUSCA. 269

The first change that takes place is the disappearance of the byssus and
the byssus organ. This occurs very soon ; shortly afterwards all traces of
the velum and sense organs also become lost.

At the time of the disappearance of these bodies, at the point of the
projection from which the byssus cord arose, and very possibly from this
very projection, the foot arises as a rounded process which rapidly grows
and soon becomes ciliated (fig. 121 B,/).

The single adductor muscle begins to atrophy very early, but before its
entire disappearance rudiments are formed at the two ends of the body,
which at a later period can be distinctly recognised as the anterior and
posterior adductor muscles (fig. 121 B, a.ad a.n& p.ad).

After the formation of these parts the gills arise as solid and at first
somewhat knobbed papillae covered with a ciliated epidermis, on each side
of, but somewhat in front of (!) the foot (fig. 121 B, br). In the foot there
soon appear the auditory sacks (au.v\ and the foot itself becomes a long
tongue-like ciliated organ projecting backwards1.

The mantle lobes undergo great changes, and indeed by Braun the
mantle lobes are stated to be formed almost entirely de novo. The perma-
nent shell is (Braun) formed on the dorsal surface of the still parasitic larva
in the form of two small independent plates. I have not followed the changes
of the alimentary canal, etc., but at an early stage there is visible, dorsal to
the foot, a simple enteric sack.

By the time the larva leaves its host all the organs of the adult, except
the generative organs, have become established.

The post-embryonic development of the organs of Glochidium is similar
in the main to that of other Lamellibranchiata. This fact is of some
importance on account of the peculiarities of the earlier developmental
stages.

The byssus organ, the toothed processes of the shell, and the sense organs
of the Glochidium can hardly be ancestral rudiments, but must be organs
which have been specially developed for the peculiar mode of life of the
Glochidium. Whether the single muscle is to be counted amongst such
provisional organs is perhaps a more doubtful point, but I am inclined to
think that it ought to be so.

If however the single muscle is an ancestral organ, it is important to
observe that it entirely disappears as development goes on and the two
adductor muscles in the adult are developed independently of it.

1 The position of the foot and gills in the larva represented in Fig. 119 B would be
more normal if the convex and not the natter side of the shell were the anterior. I
have followed Rabl and Flemming in the determinations of the anterior and posterior
end of the embryo, but failed to rear my larvae up to a stage at which the presence of
the heart or some other organ would definitely confirm their interpretation. I ori-
ginally adopted myself the other view, and in case they are mistaken, the so-called
velum would be a circum-anal patch of cilia, while the position of the primitive meso-
blast cells as well as of the byssus would better suit my view than that adopted in the
text on the authority of the above observers.

2/0 SUMMARY.

General review of the characters of the Molhiscan larvae.

The typical larva of a Mollusc, as has been more especially
pointed out by Lankester, is essentially similar to the larva of a
number of invertebrate types, and especially the Chaetopoda,
with the addition of certain special organs characteristic of the
Mollusca.

It has a bent alimentary tract, with a mouth on the ventral
surface and a terminal or ventral anus. The alimentary tract is
divided into three regions : oesophagus, stomach, and intestine.
There is a variously developed praeoral lobe with a ring of cilia
— the velum, and a perianal lobe, often with a patch of cilia
(Paludina, etc.). In all these characters it is essentially similar
to a Chaetopod larva. The two characteristic molluscan organs
are (i) a foot between the mouth and anus, and (2) an in-
vagination of the epiblast on the dorsal side at the hinder
end of the body, which is connected with the formation of the
shell.

The larvae of most Gasteropoda, Pteropoda, and Lamelli-
branchiata present no features which call for special remark ;
but the larvae of the Gymnosomata amongst the Pteropoda, and
of the Scaphopoda, Polyplacophora and Cephalopoda present
interesting peculiarities.

The larvae of the Gymnosomata are peculiar in the presence
of three transverse ciliated rings, situated behind the velum (Fig.
109). These rings might be regarded as indications of a rudi-
mentary segmentation ; but, as already indicated, this view
is not satisfactory. There is every reason for thinking that
these rings have been specially acquired by these larvae.

At first sight the larvae of the Gymnosomata might be
supposed to resemble those of the Scaphopoda, which are also
provided with transverse ciliated rings ; but, as shewn above, the
rings of the Scaphopoda are merely parts of the extended velar
ring.

Thus, the ciliated rings of the two larvae — so similar in
appearance — are in reality structures of entirely different values,
being in the one case parts of the velum, and in the other special
developments of cilia behind the velum.

MOLLUSCA. 271

The great peculiarity of the early larva of the Scaphopoda
is the enormous development of the praeoral lobe, which gives
room for the development of the ciliated rings. In the pre-
sence of a central tuft of cilia, at the anterior extremity, the
larva of the Scaphopoda resembles that of the Lamellibranchi-
ata, etc.

The larva of the Polyplacophora resembles that of Lamelli-
branchiata in its anterior flagellum, and that of the Scaphopoda
in the large development of the praeoral lobe ; but is quite pecu-
liar amongst Mollusca in the transverse segmentation of the
mantle area.

The embryo of the Cephalopoda agrees very closely with
that of normal Odontophora in the formation of the mantle and
(?) of the shell-gland, but is quite exceptional (i) in the almost
invariable presence of a more or less developed external yolk-
sack, (2) in the absence of a velum, (3) in the absence of a
median foot, and in the presence of the arms.

The presence of a yolk-sack may most conveniently be spoken
of in connection with the foot, and we may therefore pass on to
the question of the velum.

The velum is one of the most characteristic embryonic
appendages of the Mollusca, and its absence in the Cephalopoda
is certainly very striking. By some investigators the arms have
been regarded as representing the velum, but considering that
they are primitively placed on the posterior and ventral side of
the mouth, and that the velum is essentially an organ on the
dorsal side of the mouth, this view cannot, in my opinion, be
maintained with any plausibility.

Various views have been put forward with reference to the
Cephalopod foot. Huxley's view, which is the one most gene-
rally adopted, is given in the following quotation1.

" But that which particularly distinguishes the Cephalopoda
" is the form and disposition of the foot. The margins of this
" organ are, in fact, produced into eight or more processes termed
"arms, or brachia ; and its antero-lateral portions have grown
" over and united in front of the mouth, which thus comes,
" apparently, to be placed in the centre of the pedal disk. More-

1 The Anatomy of Invertebrated Animals, p. 519.

2/2 SUMMARY.

" over, two muscular lobes which correspond with the epipodia of
"the Pteropods and Branchiogasteropods, developed from the
" sides of the foot, unite posteriorly, and, folding over, give rise to
" a more or less completely tubular organ — the funnel or infun-

Grenacher, from his observations on the development of
Cephalopoda, argues strongly against this view, and maintains
that no median structure comparable with the foot is present in
this group : and that the arms cannot be regarded as taking the
place of the foot, but are more probably representatives of the
velum.

The difficulty of arriving at a decision on this subject is
mainly due to the presence of the yolk-sack, which, amongst the
Cephalopoda as amongst the Vertebrata, is the cause of consider-
able modifications in the course of the development. The foot is
essentially a protuberance on the ventral surface, between the
mouth and the anus. In Gasteropods it is usually not filled with
yolk, but contains a cavity, traversed by contractile mesoblastic
cells. In this group the blastopore is a slit-like opening (vide
p. 187) extending over the region of the foot, from the mouth to
the anus, the final point of the closure of which is usually at the
oral but sometimes at the anal extremity. In Cephalopods the
position of the Gasteropod foot is occupied by the external yolk-
sack. In normal forms the blastopore closes at the apex of the
yolk-sack, and at the two sides of the yolk-sack the arms grow
out. These considerations seem to point to the conclusion that
the normal Gasteropod foot is represented in the Cephalopod
embryo by the yolk-sack, which has, owing to the immense bulk
of food-yolk present in the ovum, become filled with food-yolk
and enormously dilated. The closure of the blastopore at the
apex of the yolk-sack, and not at its oral or anal side, is what
might naturally be anticipated from the great extension of this
part.

Grenacher's type of larva, where the external yolk-sack is
practically absent, appears to me to lend confirmation to this
view. If the reader will turn to fig. 1 13, he will observe a promi-
nence between the mouth and anus, which exactly resembles the
ordinary Gasteropod foot. At the sides of this prominence are
placed the rudiments of the arms. This prominence is filled

MOLLUSCA. 273

with yolk, and represents the rudiment of the external yolk-sack
of the typical Cephalopod embryo. The blastopore, owing to
the smaller bulk of the food-yolk, reverts more nearly to its
normal position on the oral side of this prominence.

If the above considerations have the weight which I attribute
to them, the unpaired part of the Cephalopod foot has been over-
looked in the embryo on account of the enormous dilatation it
has undergone from being filled with food-yolk ; and also owing
to the fact that in the adult the median part of the foot is unre-
presented. The arms are clearly, as Huxley states, processes of
the margin of the foot.

Both Grenacher and Huxley agree in regarding the funnel as
representing the coalesced epipodia; but Grenacher points out
that the anterior folds which assist in forming the funnel (vide
p. 253) represent the great lateral epipodia of the Pteropod foot,
and the posterior folds the so-called horse-shoe shaped portion of
the Pteropod foot.

Development of Organs.

The epiblast. With reference to the general structure of
the epiblast there is nothing very specially deserving of notice.
It gives rise to the whole of the general epidermis and to the
epithelium of the organs of sense. The most remarkable feature
about it is a negative one, viz. that it does not, in all cases at any
rate, give rise to the nervous system.

The epiblast of the mantle has the special capacity of secret-
ing a shell, and the integument of the foot has also a more or
less similar property in that it forms the operculum, and a
byssus in some Lamellibranchiata, other parts of the integument
form the radula, setae in Chiton, and other similar structures.

Nervous system. The origin of the nervous system in
Mollusca is still involved in some obscurity. It is the general
opinion amongst the majority of investigators that the nervous
ganglia in Gasteropods and Pteropods are formed from detached
thickenings of the epiblast. Both Lankester (No. 239) and Fol
(No. 249 — 251) have arrived at this conclusion, and Rabl has
shewn by sections that in Planorbis there are two lateral thick-
enings of the epiblast in the velar area ; from which the supra-
B. II. 1 8

2/4 NERVOUS SYSTEM.

cesophageal ganglia become subsequently separated off. The
observations on the pedal ganglia are less precise : they very
probably arise as thickenings of the epiblast of the side of the
foot.

According to Fol, the nervous system in the Hyaleacea amongst the
Pteropoda originates in a somewhat different way. A disc-like area appears
in the centre of the velum, which soon becomes nearly divided into two
halves. From each of these there is formed by invagination a small sack.
The axes of invagination of the two sacks meet at an angle on the surface.
The cavities of the sacks become obliterated ; the sacks themselves become
detached from the surface, fuse in the middle line, and come to lie astride of
the oesophagus. Fol has detected a similar process in Limax. The exact
origin of the pedal ganglia was not observed, but Fol is inclined to believe
that they develop from the mesoblast of the foot.

A very different view is held by Bobretzky (No. 242), whose observations
were made by means of sections.

The supra- cesophageal and pedal ganglia are formed according to this
author as independent and ill-defined local thickenings of cells which are
apparently mesoblastic. The two sets of ganglia appear nearly simultane-
ously, and later than the rudiments of the auditory and optic organs.

In the Cephalopoda there seems to be but little doubt, as
first pointed out by Lankester, that the various ganglia originate
in what is apparently mesoblastic tissue.

There is still very much requiring to be made out with
reference to their origin, unless details on this subject are given
in Bobretzky's Russian memoir. It would seem however that
each ganglion develops as an independent differentiation of the
mesoblast (unless the optic and cerebral ganglia are from the
first continuous)1. The corresponding ganglia of the two sides
become subsequently united and the various ganglia become
connected by their proper commissural cords. The ganglia are
shewn in figures 124, 126, and 127.

In Lamellibranchiata the development of the nervous system
has not been worked out.

The two points which are most striking in the development of the
nervous system of Mollusca are (i) the fact that in the Cephalopoda at any
rate it is developed from tissue apparently mesoblastic ; and (2) the fact that
the several ganglia frequently originate quite independently, and subse-
quently become connected.

1 Ussow states that they are independent.

MOLLUSCA.

2/5

With reference to the first of these points it should be noticed that the
supra-cesophageal and pedal ganglia are at first respectively connected with
the optic and auditory organs, and that these sense organs are in some cases
at any rate developed anteriorly in point of time to the ganglia. It seems
perhaps not impossible that primitively the ganglia may have been simply
differentiations of the walls of the sense organ, and perhaps their apparent
derivation from the mesoblast is really a derivation from cells which
primitively belonged to the walls of these sense organs. Bobretzky's
observations on Fusus fit in well with this view.

In the Hyaleacea and in other Pteropods, where the eyes are absent in
the adult, Fol finds the supra-cesophageal ganglia resulting from a pair of
epiblastic invaginations. May not these invaginations be really rudiments
of the eyes as well as of the ganglia ? Fol also, it is true, describes a similar
mode of origin for these ganglia in Limax. It would be interesting to have
further observations on this subject. The independent origin of the pedal
and supra-cesophageal ganglia finds its parallel amongst the Chaetopoda.

Pal,

N.op

FIG. 122. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA.

(After Grenacher.)

A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.
Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; Int. Int1...
Znt4. different parts of the integument ; /. lens ; I1, outer segment of lens ; R. retina;
N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous
stratum of retina.

The supra-cesophageal ganglia appear always to develop within the
region of the velar area. This area corresponds with the prae-oral lobe of
the Chaetopod larva, at the apex of which is developed the supra-cesophageal
ganglion. Embryology thus confirms the results of Comparative Anatomy
in reference to the homology of these ganglia in the two groups.

Optic organs1. An eye is present in most Gasteropods and

1 For a fuller account of this subject the reader is referred to the chapter on ' The
Development of the Eye.'

1 8— 2

276

OPTIC ORGANS.

in many larval Pteropods. Although its development has not
been fully worked out, yet it has clearly been shewn by
Bobretzky and other investigators that it originates as an involu-
tion of the epidermis, which first forms a cup and eventually a
closed vesicle. The posterior wall of the vesicle gives rise to the
retina, the anterior to the inner epithelium of the cornea. The
external epidermis becomes continued over the outer surface of
the vesicle.

The lens is formed in the interior of the vesicle, probably as
a cuticular deposit, which increases by the addition of concentric
layers. Pigment becomes deposited between the cells of the
retina. Fig. 122 B is a diagrammatic representation of the adult
eye of a Gasteropod.

The Cephalopod eye is formed, as first shewn by Lankester,
as a pit in the epiblast round which a fold arises (fig. 123 A) and
gradually grows over the mouth of the pit so as to shut it off
from communication with the exterior (fig. 123 B).

The epiblast lining the posterior region of the vesicle gives
rise to the retina, that
lining the anterior region
to the ciliary body and
processes. It is impor-
tant to notice that the
condition of the eye just
before the above pit be-
comes closed is exactly
that which is permanent
in Nautilus (vide fig. 122
A). After the pit has
become closed a meso-
blastic layer grows in
between its wall and the
external epiblast.

The lens becomes formed in two independent segments.
The inner and larger of these arises as a rod- like process (fig.
124) projecting from the front wall of the optic vesicle into the
cavity of the vesicle. It is a cuticular structure and therefore
without cells. By the deposition of a series of concentric layers
it soon assumes a spherical form (fig. 125, hi}. The condition

FlG. 123. TWO SECTIONS THROUGH THE
DEVELOPING EYE OF A CEPHALOPOD TO SHEW
THE FORMATION OF THE OPTIC CUP. (After

Lankester.)

MOLLUSCA.

277

of the eye, with a closed optic vesicle and the lens projecting
into it, is that which is permanent in the majority of Gasteropods
(vide fig. 122 B). At about the time when the lens first becomes
formed a fold composed of epiblast and mesoblast appears round
the edge of the optic cup (fig. 124,^), and gives rise to a structure

*d ffc

adk

FIG. 124. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO

OF LOLIGO. (After Bobretzky.)

vd. oesophagus ; gls. salivary gland ; g.us. visceral ganglion ; gc. cerebral ganglion;
g.op. optic ganglion ; adk. optic cartilage ; ak. and y. lateral cartilage or (?) white
body ; rt. retina ; gm. limiting membrane ; vk. ciliary region of eye ; cc. iris ; ac.
auditory sack (the epithelium lining the auditory sacks is not represented) ; vc. vena
cava ; ff. folds of funnel.

known in the adult as the iris. Shortly afterwards this becomes
more prominent (fig. 125, if), and at the same time the layers of
cells of the ciliary region in front of the inner segment of the
lens become reduced to the condition of mere membranes (fig.
125 B); and in front of them the anterior or outer segment of
the lens becomes formed as a cuticular deposit (fig. 125 B, vl).
At a still later period a fresh fold of epiblast and mesoblast
appears round the eye and gradually constitutes the anterior
optic chamber (vide fig. 122 C, Co). In most forms this chamber
communicates with the exterior by a small aperture, but in
some it is completely closed. The fold itself gives rise to the
cornea in front and to the sclerotic at the sides. At a later

278 AUDITORY ORGANS.

period another fold may appear forming the eyelids (fig. 122 C,
Pal).

Auditory organs. A pair of auditory sacks is found in the
larvae of almost all Gasteropods and Pteropods, and usually
originates very early. They are placed in the front part of the
foot, and on the formation of the pedal ganglia come into close
connection with it, though they receive their nervous supply in
the adult from the supra-cesophageal ganglia.

In a very considerable number of cases amongst Gasteropods
and Pteropods the auditory organs have been observed to develop
as invaginations of the epiblast, which give rise to closed vesicles
lying in the foot, e.g. Paludina, Nassa, Heteropods, Limax, some
Pteropods (Clio).

This is no doubt the primitive mode of origin, but in other
cases, which perhaps require confirmation, the sacks are stated
to originate from a differentiation of solid thickenings of the epi-
dermis or of the tissues subjacent to it.

The auditory sacks are provided with an otolith, which
according to Fol's observations is first formed in the wall of the
sack.

In Cephalopods the auditory organs are formed as epiblastic
pits on the posterior surface of the embryo, and are at first
widely separated (fig. 113, ac). The openings of the pits become
narrowed, and finally the original pits form small sacks lined
by an epithelium, and communicating with the exterior by
narrow ducts, equivalent to the recessus vestibuli of Vertebrates,
and named, after their discoverer, Kolliker's ducts. The ex-
ternal openings of these ducts become completely closed at
about the same time as the shell-gland, and the ducts remain as
ciliated diverticula of the auditory pits. The widely separated
auditory sacks gradually approach in the middle ventral line,
and are immediately invested by the visceral ganglia (fig. 124,
ac). They finally come to lie in contact on the inner side of
the funnel.

On the side opposite Kolliker's duct, an epithelial ridge is
formed — the crista acustica — the cells of which give rise to an
otolith connected with the crista by a granular material. At a
later period of development three regions of the epithelium of
the sack become especially differentiated. Each of these regions

MOLLUSCA.

279

is provided with two rows
of cells, bearing on their free
edges numerous very short
auditory hairs. The cells of
each row are placed nearly
at right angles to those of
the adjoining row.

Muscular system. The
muscular system in all
groups of Molluscs is de-
rived entirely from the
mesoblast.

The greater part of the
system takes its origin from
the somatic mesoblast. In
almost all Gasteropod and
Pteropod larvae there is pre-
sent a well-developed spin-
dle muscle attaching the
embryo to the shell. This
muscle appears to be absent
in the Cephalopoda.

Body cavity and vas-
cular system. The body
cavity in Gasteropods and
Pteropods originates either
by a definite splitting of
the mesoblast, or by the ap-

B

FIG. 125. SECTIONS THROUGH THE DE-
VELOPING EYE OF LOLIGO AT TWO STAGES.
(After Bobretzky.)

hi. inner segment of lens ; vl. outer segment
of lens; a and a . epithelium lining the anterior
optic chamber; gz. large epiblast cells of
ciliary body ; cc. small epiblast cells of ciliary
body; ms. layer of mesoblast between the two
epiblastic layers of the ciliary body; af. and
if. fold of iris; rt. retina; rt" . inner layer

pearance of intercellular of retina; "' rods ; aq' equatorial cartilase"
spaces. It becomes divided into numerous sinuses which freely
communicate with the vascular system.

Very different accounts have been given by different investi-
gators of the development of the heart in the Gasteropoda and
Pteropoda.

It would seem however in most cases to arise as a solid mass
of mesoblast cells at the hind end of the pallial cavity, which
subsequently becomes hollowed out and divided into an auricle
and ventricle. Bobretzky's careful observations have fully estab-
lished this mode of development for Nassa.

280 RENAL ORGANS.

In Pteropods the heart is formed (Fol) close to the anus, but slightly
dorsal to it (fig. 108, h). The pericardium is formed from the mesoblast at
a considerably later period than the heart.

A very different account of the formation of the heart is given by
Hiitschli for Paludina. He states that there appears an immense contrac-
tile sack on the left side of the body. This becomes subsequently reduced
in size, and in the middle of it appears the heart, probably from a fold
of its wall. The original sack would appear to give rise to the pericardium.

In connection with the vascular system mention may be
made of certain contractile sinuses frequently found in the larvae
of Gasteropoda and Pteropoda. One of these is placed at the
base of the foot, and the other on the dorsal surface within the
mantle cavity immediately below the velum1. The completeness
of the differentiation of these sinuses varies considerably; in
some forms they are true sacks with definite walls, in other cases
mere spaces traversed by muscular strands. They are found in
the majority of marine Gasteropods, Heteropods and Pteropods.
In Limax a large posteriorly placed pedal sinus is well developed,
and there is also a sinus in the visceral sack. The rhythmical
contraction of the yolk-sack of Cephalopods appears to be a
phenomenon of the same nature as the contraction of the foot
sinus of Limax.

In Calyptraea (Salensky) there is an enormous provisional
cephalic dilatation within the velum which does not appear to be
contractile. Similar though less marked cephalic vesicles are
found in Fusus, Buccinum and most marine Gasteropods.

In Cephalopods the vascular system is formed by a series of
independent (?) spaces originating in the mesoblast, the cells
around which give rise to the walls of the vessels. The branchial
hearts are formed at about the time at which the shell-gland
becomes closed. The aortic heart (fig. 127, c) is formed of two
independent halves which subsequently coalesce (Bobretzky).

The true body cavity arises as a space in the mesoblast sub-
sequently to the formation of the main vascular trunks.

Renal organs. Amongst the Gasteropods and Pteropods
there are present provisional renal organs, which may be of two
kinds, and a permanent renal organ.

1 Rabl holds that there is no contractile dorsal sinus, but that the appearance of
contraction there is due to the contractions of the foot.

MOLLUSCA. 28 1

The provisional organs consist of either (i) an external
paired mass of excretory cells or (2) an internal organ provided
with a duct, which is not in all cases certainly known to open
externally. The former structure is found especially in the
marine Prosobranchiates (Nassa, etc.) where it has been fully
studied by Bobretzky. It consists of a mass of cells on each
side of the body, close to the base of the foot, and not far
behind the velum. This mass grows very large, and below it
may be seen a continuous layer of epiblast. The cells forming
it fuse together, their nuclei disappear, and numerous vacuoles
containing concretions arise in them. At a later stage all the
vacuoles unite together and form a cavity filled with a brown
granular mass.

The provisional internal renal organ is found in many pulmo-
nate Gasteropods — Lymnaeus, Planorbis, etc. It consists of a
paired V-shaped ciliated tube with a pedal and cephalic limb.
The former has an external opening, but the termination of the
latter is still in doubt.

It consists, according to Biitschli's description (No. 244), in the fresh-
water Pulmonata (Lymnasus, Planorbis) of a round sack, close to the head,
opening by an elongated and richly ciliated tube in the neighbourhood of
the eye. From the sack a second shorter tube passes off towards the foot,
which seems however to end blindly. The cells lining the sack contain
concretions, and there is one especially large cell in the lumen of the sack
attached on the side turned towards the eye. It coexists in Lymnasus with
provisional renal organs of the type of those in marine Prosobranchiata.

A somewhat different description of the structure and development of
this organ in Planorbis has recently been given by Rabl (No. 268). It
consists of a V-shaped tube on each side with both extremities opening into
the body cavity. The one limb is directed towards the velar area, the other
towards the foot. It is developed from the mesoblast cells of the anterior
part of the mesoblastic band. The large mesoblast (p. 227) of each side
grows into two processes, the two limbs of the future organ. A lumen in
the cell is continued into each limb, while continuations of the two limbs of
the V are formed from the hollowing out of the central parts of the adjoining
mesoblast cells.

In Limax embryos Gegenbaur found a pair of elongated
provisional branched renal sacks, the walls of which contained
concretions. These sacks are provided with anteriorly directed
ducts opening on the dorsal side of the mouth. This organ is

282 GENERATIVE GLANDS.

probably of the same nature as the provisional renal organ in
other Pulmonata.

Permanent renal organ. According to the most recent ob-
server (Rabl, No. 268), whose statements are supported by the
sections figured, the permanent renal organ in Gasteropods is
developed from a mass of mesoblast cells close to the end of the
intestine. This is first carried somewhat to the left side, and
then becomes elongated and hollow, and attaches itself to the
epiblast on the left side of the anus (fig. 108, r). After the
formation of the heart the inner end opens into the pericardium
and becomes ciliated, the median part becomes glandular and
concrements appear in its lining cells, and the terminal part
forms the duct.

Previous observers have usually derived this organ from the epiblast;
according to Rabl this is owing to their having studied too late a stage in the
development.

In Cephalopoda the excretory sacks or organ of Bojanus are
apparently differentiations of the mesoblast1. At an early stage
part of their walls envelops the branchial veins. From this
part of the wall the true glandular section of the organ would
seem to be formed. The epithelium forming the inner wall of
each sack is at an early age very columnar.

The development of the organ of Bojanus in Lamellibranchiata
has been studied by Lankester. He finds that it develops as a
paired invagination of the epiblast immediately ventral to the
anus.

Generative glands. The generative glands in Mollusca
would appear to be usually developed in the post-larval period,
but our knowledge on this subject is extremely scanty.

In Pteropods Fol believes that he has proved that the hermaphrodite
gland originates from two independent formations, one (the testicular)
epiblastic in origin, and the other (the ovarian) hypoblastic.

These views of Fol do not appear to me nearly sufficiently substantiated
to be at present accepted.

The generative glands in Cephalopoda appear to be simple
differentiations of the mesoblast. They are at first very closely

1 I conclude this from Bobretzky's figures.

MOLLUSCA. 283

connected with the aortic heart (fig. 127, &/), but soon become
completely separated from it.

Alimentary tract. The formation of the archenteron, and
the relation of its opening to the permanent mouth and anus, has
already been described and needs no further elucidation. It will
be convenient to treat the subject of this section under three
headings for each group — viz. (i) the mesenteron, (2) the sto-
modaeum, and (3) the proctodaeum.

The mesenteron. In the Gasteropoda and Pteropoda the
mesenteron, as has already been mentioned, forms a simple sack,
which may however, owing to the presence of food-yolk, be at
first without a lumen. Of this sack an anterior portion gives
rise to the stomach and liver, and a posterior to the intestine.
This latter portion is the first to be distinctly differentiated as
such, and forms a narrowish tube connecting the anterior dila-
tation with the anus. In the meantime the cells of a great
part of the anterior portion of the mesenteron undergo peculiar
changes. They enlarge, and in each of them a deposit of food
material appears, which is often at any rate derived from the
absorption of the albumen in which the embryo floats. The cells
on the dorsal side, adjoining the cesophageal invagination, and
the whole of the cells on the ventral side do not however undergo
these changes. There thus arises an anterior and ventral region
adjoining the oesophagus, which becomes completely enclosed by
small cells and forms the true stomach. The part behind and
dorsal to the stomach is lined by the large nutritive cells and
forms the liver. It opens into the stomach at the junction of the
latter with the intestine, which in the later stages becomes bent
somewhat forwards and to the right. Still later the hepatic
region becomes branched, the albuminous contents of its cells
are replaced by a coloured secretion, and it becomes bodily
converted into the liver. The stomach is usually richly ciliated.

The various modifications of the above type of development of the
alimentary tract are to be regarded as due to the disturbing influence of
food-yolk. Where primitively the hypoblast cells are very bulky, though
invaginated in a normal way, the wall of the hepatic region becomes
immensely swollen with food-yolk, e.g. Natica. In other cases amongst
certain Pteropods (Fol, No. 249) where the hypoblast is still more bulky,
part of the archenteric walls becomes converted into a bilobed sack opening

284 ALIMENTARY TRACT.

into the pyloric region, in the walls of which a large deposit of food material
is stored, which gradually passes into the remainder of the alimentary tract
and is there digested. The bilobed nutritive sack, as it is called by Fol, is
eventually completely absorbed, though the liver in some, if not all cases,
grows out as a fresh sack from its duct.

The formation of the permanent alimentary tract, when the hypoblast is
so bulky that there is no true archenteric cavity, has been especially investi-
gated by Bobretzky (No. 242).

In the case of a species of Fusus the hypoblast, when enclosed by the
epiblast, is composed of four cells only. The blastopore remains perma-
nently open at the oral region, and around it the oesophagus grows in a
wall-like fashion. The protoplasmic portions of the four hypoblast cells are
turned towards the cesophageal opening, and from them are budded off
small cells which are continuous at the blastopore with the epiblast of the
oesophagus. These cells give rise posteriorly to the intestine and anteriorly
to the sack, which becomes the stomach and liver. This sack always
remains open towards the four primitive yolk cells. The cells of the
posterior part of it become larger and larger and form the hepatic sack,
which fills up the left and posterior part of the visceral sack, pushing the
yolk cells to the right. The cells lining the hepatic sack become pyramidal
in shape, and each of them is filled with a peculiar mass of albuminous
material. The cells adjoining the opening of the oesophagus remain small,
become ciliated, and form the stomach. They are not sharply separated off
from the cells of the hepatic sack. The yolk cells remain distinct on the
right side of the body during larval life, and their food material is gradually
absorbed for the nutrition of the embryo.

A modification of the above mode of development, where the food
material is still more bulky and the blastopore closed, is found in Nassa,
and has already been described (vide p. 233).

The stomodceum. The stomodaeum in most cases is formed
as a simple epiblastic imagination which meets and opens into
the mesenteron. When the blastopore remains permanently
open at the oral region the stomodaeum is formed as an epiblastic
wall round its opening. In all cases the stomodaeum gives rise
to the mouth and oesophagus. At a subsequent period there are
developed in the oral region of the stomodaeum the radula in a
special ventral pit, and the salivary glands — the latter as simple
outgrowths.

The oesophagus is usually ciliated.

The proctod&um. Except where the blastopore remains as
the permanent anus (Paludina) the proctodseum is always formed
subsequently to the mouth. Its formation is usually preluded
by the appearance of two projecting epiblast cells, but it is

MOLLUSCA. 285

always developed as a very shallow epiblastic invagination, which
does not give rise to any part of the true intestine.

In the Cephalopods the alimentary tract is formed, as in
other cephalophorous Mollusca, of three sections, (i) A stomo-
daeum, formed by an epiblastic invagination, which gives rise to
the mouth, oesophagus and salivary glands. (2) A proctodseum,
which is an extremely small epiblastic invagination. (3) A
mesenteron, lined by true hypoblast, which forms the main

cTts

•mf

brd

c,h

FIG. 126. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM WHEN

THE MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED. (After Bobretzky.)

gls. salivary gland ; brd. sheath of radula ; oe. oesophagus ; ds. yolk-sack ; chs.
shell-gland ; mt. mantle ; pdh. mesenteron ; x. epiblastic thickening between the folds
of the funnel.

section of the alimentary tract, viz. the stomach, intestine, the
liver, and ink sack1.

The mesenteron. The mesenteron is first visible from the
surface as a small tubercle on the posterior side of the mantle
between the rudiments of the two gills (fig. 1 1 1 B, an). Within
this, as was first shewn by Lankester, a cavity appears.

This cavity is as in Gasteropods open to the yolk-sack, and
only separated from the yolk itself by the yolk membrane
already spoken of. It is at first lined by indifferent cells of the
lower layer of the blastoderm, which however soon become
columnar and form a definite hypoblastic layer (fig. 126, pdk).
Between the hypoblast and epiblast there is a very well marked
layer of mesoblast. As the mesenteric cavity extends, its walls

1 The following description applies specially to Loligo.

286

ALIMENTARY TRACT.

meet the epiblast, and at the point of contact of the two layers
the epiblast becomes slightly pitted in. At this point the anus
is formed at a considerably later period (fig. 127, an}.

On the ventral side of the primitive mesenteron an outgrowth
appears very early, which becomes the ink sack (fig. 127, bi).

The mesenteric cavity, still open to the yolk, gradually ex-
tends itself in a dorsal di-
rection over the yolk-sack,
but remains for some time
completely open to it ven-
trally, and only separated
from the actual yolk by
theyolk membrane. There
early grow out from the
walls of the mesenteron
a pair of hepatic diver-
ticula.

As the mesenteric
cavity extends it dilates
at its distal extremity
into a chamber destined
to form the stomach (fig.
127, mg). At about this
time the anus becomes
perforated. Shortly af-
terwards the mesenteron
meets and opens into
the oesophagus at the
dorsal extremity of the
yolk sack, but at the time
when this takes place the
hypoblast has extended
round the entire cavity,
and has shut it off from

—tr

ypd

the yolk. The yolk mem-
brane throughout the
whole of this period is
quite passive, and has no
share in forming the walls of the alimentary tract.

FIG. 127. LONGITUDINAL SECTION THROUGH

AN ADVANCED EMBRYO OF LOLIGO. (After Bo-

bretzky.)

os. mouth ; gls. salivary gland ; brd. sheath of
radula ; ao. anterior aorta; ao1. posterior aorta;
ra. branch of posterior aorta to shell sack ; ma.
branch of posterior aorta to mantle ; c. aortic
heart ; oe. oesophagus ; mg. stomach ; an. anus ;
bi. ink sack ; kd. germinal tissue ; eih. shell sack ;
•vc. vena cava ; g.vs. visceral ganglion ; g-pd. pedal
ganglion ; ac. auditory sack ; tr. funnel.

MOLLUSCA. 287

The stomodaum. The stomodaeum appears as an epiblastic
imagination at the anterior side of the blastoderm, before any
trace of the mesenteron is present. It rapidly grows deeper,
and, shortly after the mesenteric cavity becomes formed, an
outgrowth arises from its wall adjoining the yolk-sack, which
gives rise to the salivary glands (figs. 126 and 127, gls). Im-
mediately behind the opening of the salivary glands there
appears on its floor a swelling which becomes the odontophore,
and behind this a pocket of the stomodaeal wall forms the
sheath of the radula (figs. 126 and 127, brd}. Behind this again
the oesophagus is continued dorsalwards as a very narrow tube,
which eventually opens into the stomach (fig. 127).

The terminal portion of the rudiment of the salivary gland
divides into two parts, each of which sends out numerous diver-
ticula which constitute the permanent glands. The greater part
of the original outgrowth remains as the unpaired duct of the
two glands1.

In the larva observed by Grenacher the anterior pair of
salivary glands originated from independent lateral outgrowths
of the floor of the mouth, close to the opening of the posterior
salivary glands.

The yolk-sack of the Cephalopoda. The yolk, as has already been stated,
becomes at an early period completely enclosed in a membrane formed of
flattened cells, which constitutes a definite yolk-sack. It is, in the more
typical forms of Cephalopoda, divided into an external and an internal
section, of which the former is probably a special differentiation of the
median part of the foot of other cephalophorous Mollusca (vide p. 272). At
no period does the yolk-sack communicate with the alimentary tract. The
two sections of the yolk-sack are at first not separated by a constriction. In
the second half of embryonic life the condition of the yolk-sack undergoes
considerable changes. The internal part grows greatly in size at the expense
of the external, and the latter diminishes very rapidly and becomes con-
stricted off from the internal part of the sack, with which it remains con-
nected by a narrow vitelline duct.

The internal yolk-sack becomes divided into three sections : a dilated
section in the head, a narrow section in the neck, and an enormously
developed portion in the mantle region. It is the latter part which mainly
grows at the expense of the external yolk-sack. It gives off at its dorsal
end two lobes, which pass round and embrace the lower part of the cesopha-

1 In Loligo only a single pair of salivary glands is present.

288 ALIMENTARY CANAL.

gus. The passage of the yolk from the external to the internal yolk-sack is
probably largely due to the contractions of the former.

The external yolk-sack is not vascular, and probably the absorption of
the yolk for the nutrition of the embryo can only take place in the internal
yolk-sack. The most remarkable feature of the Cephalopod yolk-sack is the
fact that it lies on the opposite side of the alimentary tract to the yolk cells,
which form a rudimentary yolk-sack in such Gasteropoda as Nassa and
Fusus. In these forms, the yolk-sack is at first dorsal, but subsequently is
carried by the growth of the liver to the right side. In Cephalopoda on the
contrary, the yolk-sack is placed on the ventral side of the body.

What is known of the development of the alimentary tract in
the Polyplacophora has already been mentioned.

In the Lamellibranchiata (Lankester, No. 239), the mesen-
teron early grows out into two lateral lobes which form the liver,
while the part between them forms the stomach.

In Pisidium the intestine is formed from the original pedicle
of invagination, which remains permanently attached to the
epiblast. The stomodaeum is formed by the usual epiblastic
invagination, and becomes the mouth and cesophagus. The
development of the crystalline rod and its sack do not appear to
be known. In the adult the sack of the crystalline rod opens
into a part of the alimentary tract which appears to belong to
the mesenteron. Were however the development to shew them
to be really derived from the stomodaeum they might be inter-
preted as rudiments of the organ which constitutes the odonto-
phore and its sack in cephalophorous Mollusca — an interpretation
which would be of considerable phylogenetic interest.

BIBLIOGRAPHY.

General.

(238) T. H. Huxley. "On the Morphol. of the Cephal. Mollusca." Phil.
Trans. 1853.

(239) E. R. Lankester. "On the developmental history of the Mollusca."
Phil. Trans. 1875.

(240) H. G. Bronn and W. Keferstein. Die Klassen u. Oninungen d. Thier-
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Gasteropoda and Pteropoda.

(241) J. Alder and A. Hancock. "Devel. of Nudibr." Ann. and Magaz.
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BIBLIOGRAPHY. 289

(242) N. Bobretzky. " Studien uber die embryonale Entwicklung d. Gastero-
poden." Archivf. micr. Anat., Vol. xin.

(243) W. K. Brooks. " Preliminary Observations on the Development of
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1879.

(244) O. Butschli. "Entwicklungsgeschichtliche Beitrage (Paludina vivipara)."
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(245) W. Carpenter. "On the devel. of the embr. of Purpura lapillus."
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(246) W. Carpenter. " On the devel. of the Purpura." Ann. and Mag. of
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(247) E. Claparede. "Anatomic u. Entwickl. der Neritina fluviatilis."
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(248) H. Eisig. "Beitr. z. Anat. u. Entwickl. der Geschlechtsorg. von
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(249) H. Fol. "Sur le developpement des Pteropodes." Archiv. de Zool.
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(250) H. Fol. "Sur le developpement des Gasteropodes pulmones." Compt.
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(251) H. Fol. "Sur le developpement des Heteropodes." Archiv. deZool. ex-
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(252) C. Gegenbaur. " Beit. z. Entwicklungsgesch. der Landgasteropoden. "
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(253) C. Gegenbaur. Untersttch. iib. Pteropoden u. Heteropoden, Leipzig,

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(254) H. vonjhering. " Entwicklungsgeschichte von Helix. " Jenaische Zeit-
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(255) W. Keferstein and E. Ehlers. "Beob. iib. d. Entwick. v. ^Eolis
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(256) J. Koren and D. C. Danielssen. "Bemark. til Mollusk Udvikling."
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(257) J. Koren and D. C. Danielssen. Bidrag til Pectinibr. Udvikl. Bergen,
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(258) A. Krohn. " Beobacht. aus d. Entwickl. der Pteropoden u. Heterop."
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(259) A. Krohn. Beitr.zur Entwickl. der Pteropoden u. Heteropoden. Leipzig,
1860.

(260) H. de Lacaze-Duthiers. "Mem. sur 1'anat. et 1'embryog. des Ver-
mets." 2e partie. Ann. sc. nat., 4° serie, T. xin. 1860.

(261) P. Langerhans. " Zur Entwickl. der Gasterop. Opisthobr." Zeitschr.
f. w. Zool., Vol. xxin. 1873.

(262) E. R. Lankester. " On the development of the Pond-Snail." Quart. J.
of Micr. Scie., Vol. xiv. 1874.

(263) E. R. Lankester. "On the coincidence of the blastopore and anus in
Paludina vivipara." Quart. J. of Micr. Scie., Vol. xvi. 1876.

(264) F. Leydig. " Ueber Paludina vivipara." Zeitschr. f. w. Zool., Vol. n.
1850.

(265) J. Muller. Ueber Synapta dig. u. iib. d. Erzeug. v. Schnecken in Holoth.,
1852.

B. II. 19

290 BIBLIOGRAPHY.

(266) J. Muller. "Bemerk. aus d. Entwickl. der Pteropoden." Monatsber.
Berl. Akad., 1857.

(267) C. Rabl. " Die Ontogenie d. Susswasser-Pulmonaten. " Jenaische Zeit-
schrift, Vol. IX. 1875.

(268) C. Rabl. " Ueb. d. Entwick. d. Tellerschnecke (Planorbis)." Morph.
Jahrbuch, Vol. v. 1879.

(269) W. Salensky. " Beitr. zur Entwickl. d. Prosobr." Zeitschr. f. w. Zool.,
Vol. xxn. 1872.

(270) O. Schmidt. "Ueb. Entwick. von Limax agrestis." Muller's Archiv,
1851.

(271) Max S. Schultze. "Ueber d. Entwick. des Tergipes lacinulatus." Arch,
f. Naturg., Jahrg. xv. 1849.

(272) E. Selenka. " Entwick. von Tergipes claviger." Niederl. Arch.f. Zool.,
Vol. I. 1871.

(273) E. Selenka. "Die Anlage d. Keimbl. bei Purpura lapillus." NiederL
Arch.f. Zool., Vol. I. 1872.

(274) C. Semper. "Entwickl. der Ampullaria polita, etc." Natuurk. Ver-
handl. Utrechts Genootsch., 1862.

(275) An. Sleeker. " Furchung u. Keimblatterbildung bei Calyptrsea." Mor-
phol. Jahrbuch, Vol. II. 1876.

(276) A.Stuart. " Ueb. d. Entwickl. einiger Opisthobr." Zeitschr. f. 10. Zool. ,
Vol. xv. 1865.

(277) N. A. Warneck. "Ueber d. Bild. u. Entwick. d. Embryos bei Gaste-
rop." Bullet. Soc. natural, de Moscou, T. xxm. 1850.

Cephalopoda.

(278) P. J. van Beneden. " Recherches sur 1'Embryogenie des Sepioles."
Nouv. Mini. Acad. Roy. de Bruxelles, Vol. xiv. 1841.

(279) N. Bobretzky. Observation on the development of the Cephalopoda
(Russian). Nachrichten d. kaiserlichen Gesell. d. Freunde der Naturwiss. Anthropolog.
Ethnogr. bei d. Universitdt Moskau.

(280) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
/. wiss. Zool., Bd. xxiv. 1874.

(281) A. Kolliker. Entwicklungsgeschichte d. Cephalopoden. Zurich, 1844.

(282) E. R. Lank ester. "Observations on the development of the Cepha-
lopoda." Quart. J. of Micr. Science, Vol. xv. 1875.

(283) E. Metschnikoff. *• Le developpement des Sepioles." Archiv d. Sc.
phys, et nat.y Vol. XXX. Genfcve, 1867.

Polyplacophora.

(284) A. Kowalevsky. "Ueb. d. Entwick. d. Chitonen." Zoologischer An-
zeiger, No. 37. 1879.

(285) S. L. Loven. " Om utvecklingcn hos slagtet Chiton." Stockholm bfver-
sigt, xn. 1855. [Vide also Ann. and Mag. of Nat. Hist., Vol. xvn. 1856, and
Archiv f. Naturgeschichte, 1856.]

BIBLIOGRAPHY. 29 1

Scaphopoda.

H. Lacaze-Duthiers. " Developpement du Dentale." Ann. d. Sci.
Nat., Series iv. Vol. VH. 1857.

L amellibranchiata.

(287) M. Braun. " Postembryonale Entwicklung d. Siisswasser-Muscheln."
Zoologischer Garten.

(288) C. G. C arus. " Neue Untersuch. lib. d. Entwickl. unserer Flussmuschel."
Verh. Leop.-Car. Akad., Vol. xvi. 1832.

(289) W. Flemming. " Studien in d. Entwicklungsgeschichte der Najaden."
Sitz. d. k. Akad. Wiss. Wien, Vol. LXXI. 1875.

(290) F. Leydig. " Ueber Cyclas Cornea." Muller's Archiv, 1855.

(291) S. L. Loven. " Bidrag til Kanned. om Utweckl. af Moll. Acephala
Lamellibr." Vetensk. Akad. Hand!., 1848. [Vide also Arch. f. Naturg., 1849.]

(292) C. Rabl. «' Ueber d. Entwicklungsgeschichte d. Malermuschel." Jena-
ische Zeitschrift, Vol. x. 1876.

(293) W. Salensky. " Bemerkungen iiber Haeckels Gastrsea-Theorie (Ostrea)."
Arch. f. Naturg., 1874.

(294) O. Schmidt. "Ueb. d. Entwick. von Cyclas calyculata." Muller's
Arch., 1854.

(295) O.Schmidt. " Zur Entwickl. der Najaden." Wien,Sitzungsber.math.-
nat. CL, Vol. xix. 1856.

(296) P. Stepanoff. "Ueber die Geschlechtsorgane u. die Entwicklung von
Cyclas." Archiv f. Naturgeschichte, 1865.

(297) H. Lacaze-Duthiers. "Developpement d. branchies d. Mollusques
Acephales." An. Sc. Nat., Ser. iv. Vol. v. 1856.

I9—2

CHAPTER X.
POLYZOA1.

ENTOPROCTA.

THE development of the larvae of Pedicellina is known from
the researches of Hatschek (No. 299) far more completely than
that of Loxosoma, though it does not apparently differ from it
except in certain details. In both the known Entoproctous
genera the segmentation is regular or nearly so, though Hatschek
believes that he has detected in Pedicellina a slight difference
between the two first segmentation spheres, and regards them as
constituting the animal and vegetative poles of the embryo. The
segmentation in Pedicellina, to which genus alone the remainder
of the description applies, results in the formation of a single-
layered blastosphere, with a small segmentation cavity, in which
the animal and vegetative poles can readily be distinguished
owing to the smaller size of the cells at the animal pole.

The hypoblast cells and the vegetative2 pole become invagi-

1 The classification of the Polyzoa adopted in this chapter is shewn in the sub-
joined table :

I. Entoprocta.
II. Ectoprocta.

fa. Chilostomata.

1. GYMNOUEMATA \b. Ctenostomata.

v. Cyclostomata.

2. PHYLACTOL^EMATA.

3. PODOSTOMATA (Rhabdopleuro).

2 The succeeding statements about the gastrula are derived from Hatschek.
According to Salensky a segmentation cavity is not present, and the hypoblast would
seem to be formed by delamination or epibole. Barrois finds a gastrula in both
Loxosoma and Pedicellina, but gives no details. Uljanin finds a segmentation cavity
in Pedicellina, and Schmidt would appear to have observed a gastrula stage in
Loxosoma. None of the accounts we have can be compared in fulness of detail to
that of Hatschek.

. POLYZOA. 293

nated in the normal manner (fig. 128 A), the blastopore becomes
narrowed to a slit with an antero-posterior direction, i.e. parallel
to the line connecting the mouth and anus in the adult. At
the hinder extremity of the blastopore there are present two
conspicuously large cells (fig. 128 B, me), one on each side of
the middle line. These cells give rise to the fnesoblast. On
the completion of the invagination the mesoblasts become
covered by the epiblast (fig. 128 C, me). The blastopore then
closes, but in the position it occupied the epiblast becomes
thickened to form the rudiment of the vestibule, which at this
stage constitutes a disc marked off by a shallow groove from the
remainder of the body.

FIG. 128. THREE STAGES IN THE DEVELOPMENT OF PEDICELLINA ECHINATA.
(After Hatschek.)

s.c. segmentation cavity ; a.e. archenteron ; ep. epiblast ; me. mesoblast ; hy.
hypoblast.

A is the commencing gastrula stage from the side in optical section.

B is a slightly later stage from above in optical section. It shews the two primi-
tive mesoblast cells.

C is a later stage after the closure of the blastopore, viewed from the side in
optical section.

At the anterior extremity of this disc an invagination arises
to form the oesophagus (fig. 129 A, oe) ; and not long afterwards
a posterior invagination to form the rectum (fig. 129 B, an.i).
The oral disc and the oesophagus are richly ciliated. The
cesophagus first, and afterwards the rectum unite with the
archenteron (fig. 130), the walls of which soon become differ-
entiated into a stomach and intestine, and on the upper wall
of the former the hepatic cells become especially conspicuous
(fig. 130).

294 ENTOPROCTA.

During the completion of the alimentary canal a number of
important structures is formed. The disc in which the oral and
anal apertures are situated becomes converted into a true vesti-

FlG. 129. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After

Hatschek.)

oe. oesophagus ; ae. archenteron ; an.i. anal invagination ; f. fold of epiblast ;
f.g. ciliated disc ; x. problematical body derived from hypoblast (probably a bud).

bule. On its floor, between the mouth and the anus, there arises
a marked prominence with a tuft of cilia (fig. 130 B), which
persists in the adult.

This prominence is perhaps equivalent to the epistome of the
Phylactolsemata and the disc-like organ of Rhabdopleura, which
Lankester has compared to the molluscan foot1.

Very shortly after the first formation of the vestibule there
appears at the opposite end of the larva a thickening of the
epiblast, which soon becomes invaginated, and forms an eversible
pit (fig. 129 A and B, /.£-.). Round its mouth there is formed a
ring of stiff cilia (fig. 130, f.g.). This organ is very possibly
equivalent to the cement gland described by Kowalevsky in the
adult Loxosoma. I shall speak of it as the ciliated disc.

The epiblast cells early secrete a cuticle.

The two mesoblast cells soon increase by division, and
occupy the space between the alimentary canal and the body
wall. They do not become divided into a splanchnic and
somatic layer ; but give rise to the interstitial connective tissue

1 Lankester. "Remarks on the Affinities of Rhabdopleura." Quart. J. of
Micro. Science, Vol. XIV. 1874.

POLYZOA. 295

and muscles. From the mesoblast there is also formed, ac-
cording to Hatschek, a pair of ciliated excretory canals, in the
space between the mouth and anus (fig. 130 B, npk.). The
development of the nervous system has not been observed.

At a comparatively late stage in the development there is
formed round the edge of the vestibule a ring of long cilia (fig.
1306, m.).

A remarkable organ makes its appearance on the dorsal side
of the oesophagus (the side opposite the adult ganglion) formed
of an oval mass of cells attached to the epiblast at the apex of a
small ciliated papilla (fig. 130 A and B, *.). This organ will be

FlG. 130. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After

Hatschek.)

v. vestibule ; m. mouth ; /. liver ; kg. hind-gut ; a. anus ; and. anal invagination ;
nph. duct of kidney \ fg. ciliated disc ; x. dorsal organ (probably bud).

spoken of as the dorsal organ. According to Hatschek it
develops as a solid outgrowth of the hypoblastic walls of the
mesenteron shortly before the mesenteron joins the oesophagus
(fig. 129 B, jr.). The cells composing it arrange themselves as a
sack, which acquires an external opening on the dorsal surface
(fig. 130 A, jr.). In a later stage the lumen of the sack dis-
appears, but at the junction of the organ with the epiblast a pit

296 ENTOPROCTA.

is formed, lined with ciliated cells, which is capable of being
protruded as a papilla. The organ itself becomes invested by a
lining of cells, which Hatschek regards as mesoblastic. A nearly
similar organ to this is found in the embryo of Loxosoma [Vogt
(No. 302) and Barrois (No. 298)]. Here however it is double,
and forms a kind of disc connected with two eye spots.

Hatschek has made with reference to the dorsal organ the
extremely plausible suggestion that it is a rudimentary bud, and
that the hypoblastic sack it contains gives rise to the hypoblast
of the young polype developed from the bud. Although, owing
to the deficiency of our observations on the attachment of the
larva, this suggestion has not received direct confirmation, yet
the relations of dorsal organs in Pedicellina and Loxosoma
respectively strongly confirm Hatschek's view of their nature.
Both of these forms increase in the adult state by budding : in
Pedicellina there is a single row of buds formed successively
on the dorsal side of the stem, corresponding with the single
dorsal organ of the embryo ; while in Loxosoma a double row
of buds, right and left, is formed, in correspondence with the
double nature of the dorsal organ.

As to the mode of attachment of the embryo next to nothing is known,
the few observations we have being due to Barrois. From these observa-
tions it would appear probable that the larva, as is usual amongst Polyzoa,
does not become directly converted into the permanent form, but that,
on becoming fixed, it undergoes a metamorphosis in the course of which its
organs atrophy. I would venture to suggest that the whole free-swimming
larva atrophies, while the dorsal organ alone develops into the fixed form 1.

Although the changes which take place during budding do not fall within
the province of this work, it may be well to state that Hatschek has
observed during this process the development of the nervous system and
the generative organs. The nervous system arises as an unpaired thickening
of the epiblastic floor of the vestibule, between the mouth and the anus.
On becoming constricted off from the epiblast the nerve ganglion contains a
central cavity which afterwards vanishes.

The generative organs originate as a pair of specially large mesoblast
cells in the space between the stomach and the floor of the vestibule. These
two cells, surrounded by an investment of flattened mesoblast cells, sub-

1 My view of the metamorphosis which takes place during the fixation of the
larva involves the supposition that in Loxosoma, about the attachment of which we
know absolutely nothing, two buds are directly formed in accordance with the double
nature of the dorsal organ.

POLYZOA. 297

sequently divide and form two masses. At a still later period each mass
divides into an anterior and a posterior part ; the former giving rise to the
ovary, the latter to the testis. The similarity of this mode of development
of the generative organs to that observed by Butschli in Sagitta, which
is described in the sequel, is very striking.

ECTOPROCTA.

Although the embryology of the Ectoprocta has been in-
vestigated by a very considerable number of the distinguished
naturalists of the century, many points connected with it still
stand in great need of further elucidation. The original nature
of the embryo was rightly interpreted by Grant, Dalyell and
other naturalists, but it was not till Huxley demonstrated the
presence of both the ovary and testis that the true sexual origin
of the embryo in the ovicells became an established fact in
science. The recent memoir of Barrois (No. 298), though it
contains the record of a vast amount of research, and marks a
great advance in our knowledge, still leaves a great number of
points, both with reference to the early development and to the
larval metamorphosis in a very unsatisfactory condition.

Four larval forms can be distinguished, viz.

(1) A larval form which with slight modifications is common
to all the genera of the Chilostomata (except Membranipora and
Flustrella) and of the Ctenostomata.

(2) A bivalved larva of Membranipora known as Cyphon-
autesy the true nature of which was first recognized by Schneider
(No. 322), and the closely allied larva of Flustrella.

(3) The typical Cyclostomatous larva, for the first full
description of which we are indebted to Barrois (No. 298).

(4) The larva of the Phylactolaemata.

Chilostomata and Ctenostomata. As an example of the
first type of larvae, Alcyonidium my till, one of the Ctenostomata,
may be conveniently selected for description, as having been
more completely worked out by Barrois than perhaps any other
form. The segmentation commences in the normal manner by
the appearance of two vertical furrows followed by an equatorial
furrow, which divide the ovum into eight equal spheres. The
stage with eight spheres is followed, according to Barrois, by one
with sixteen, formed in a remarkable manner by the simultaneous

298 ECTOPROCTA.

appearance of two vertical furrows, both parallel to one of the
original vertical furrows, so that the segmentation spheres at this
stage are arranged in two layers of eight each. In the next
stage segmentation takes place along two fresh vertical planes,
similar to those of the last stage, but at right angles to them, and
therefore parallel to the second of the two primitive vertical
furrows. At the close of this stage there are thirty-two cells
arranged in two layers of sixteen eachj and when viewed from
the surface each of these layers presents a regularly symmetrical
pattern. Up to the stage with sixteen cells the two poles of the
egg, separated by the primitive equatorial plane of segmentation,
remain equal, but during the stage with thirty-two cells a
peculiar change takes place in the character of the cells at the
two poles. At the one pole, which will be spoken of as the oral
pole, the four central cells become much larger than the twelve
peripheral cells.

The stages immediately following are still involved in much
obscurity, and have been described very differently by Barrois in
his original memoir (No. 298), and in a subsequent note (No.
307)1. In the latter he states that the four large cells of the
oral face become enclosed by the division and growth of the
twelve peripheral cells. They are thus carried into the interior
of the ovum ; and there divide into a central vitelline mass — the
hypoblast — and a peripheral mesoblastic layer.

The eight peripheral cells of the aboral pole divide vertically,
and, owing to the eight central cells at the aboral pole dividing
transversely so as to form a protuberance on the aboral surface,
they constitute a transverse ring of large cells round the ovum,
which become ciliated and constitute the main ciliated band of
the embryo, corresponding to the ciliated band at the edge of
the vestibule of the entoproctous larvae. They divide the embryo
into an aboral and an oral region. The central part of the
aboral projection forms a structure which I shall speak of as the
ciliated disc. It probably corresponds with the ciliated disc in
the Entoprocta. An invagination is next formed on the oral

1 The note (No. 307) refers in the first instance to the changes in the larvae of the
Chilostomata, but the similarity of the larvoe of the Ctenostomata to those of the
Chilostomata renders it practically certain that the corrections, in so far as they apply
to the one group, apply also to the other.

POLYZOA. 299

surface, which gives rise to a sack opening to the exterior
(fig. 131, st.). This was originally held by Barrois to be the
stomach ; but Barrois now prefers to call it ' the internal sack.'
To my mind it is probably the stomodaeum. The embryo has
become in the meantime laterally compressed, and, at what
I shall call the anterior end of the oral disc, a structure makes
its appearance (fig. 131, m), which is probably homologous with
the dorsal organ of the larva of Pedicellina and may go by the
same name. It was originally interpreted by Barrois as the
pharynx1.

The larva, having now acquired all the important structures
it is destined to possess, becomes free. It is shewn in fig. 131 ;
the oral face being turned upwards. There are two rings of

FIG. 131. FREE-SWIMMING LARVA OF ALCYONIDIUM MYTILI. (After Barrois.)
m (?) dorsal organ ; st. stomodseum (?) ; s. ciliated disc.

cilia, one round the edge of the ciliated disc, and a second with
larger cilia on the ring of large cells described above. This
ring projects somewhat ; its projecting edge being directed
towards the ciliated disc. The dorsal organ (m?) is placed on
the oral face at the bottom of an elongated groove, in front
of which is a bunch of long cilia or flagella. Two long flagella
are also developed at the posterior extremity of the oral face,
and two pairs (an anterior and a posterior) of eye-spots also
appear. Towards the posterior extremity of the oral face is
seen a body marked st, which forms the internal sack. If I am

1 The interpretation of the larvae given in the text must be regarded as somewhat
tentative. The opacity of the free larvae is very great, and almost every one of the
numerous authors who have worked on these larvae have arrived at different conclu-
sions, as to the physiological significance of the various parts.

300 ECTOPROCTA.

right in regarding this as the stomodaeum, it is probable that it
never unites with the invaginated hypoblast, and that the
alimentary tract of the larva remains therefore permanently in
an imperfect condition.

Careful observations have been made by Repiachoff (No. 318) on the
early development of Tendra, which accord in some respects with the
results arrived at by Barrois in his second memoir. The observations are
not, unfortunately, carried down to the complete development of the larva.

The ovum divides in the normal way into two and then four uniform
segments. These four next become divided by an equatorial furrow into
four dorsal and four ventral segments, the former constituting the aboral
pole and forming the epiblast, and the latter the oral pole. The stages with
sixteen and thirty-two cells appear to be formed in the same manner as in
Alcyonidium — but between the two layers of cells forming the oral and
aboral poles a well-marked segmentation cavity arises at the stage with
sixteen segments. At the stage with thirty-two cells the four middle cells of
the oral side, which are larger than the others, become divided into two
tiers, in such a manner as to form a prominence projecting into the
segmentation cavity. By the appearance of a lumen in this prominence
it becomes converted into an archenteron, which communicates with the
exterior by a blastopore in the middle of the oral surface. The blastopore
becomes eventually closed.

The archenteric sack of Repiachoff is clearly the same structure as
Barrois' four invaginated cells of the oral face, their further history has
unfortunately not been followed out by Repiachoff.

The free larva swims about for some time, and then fixes
itself and undergoes a metamorphosis; but the exact course of
this metamorphosis is still very imperfectly known.

According to the latest statements of Barrois the attachment
takes place by the oral face1. The ciliated disc, which in the
free larva forms a kind of cup directed towards the aboral end,
turns in upon itself towards the oral face. It subsequently
undergoes degeneration and forms a nutritive or yolk-mass.
The skin of the larva after these changes gives rise to the
ectocyst or cell of the future polype. The future polype itself
appears to originate, in part at any rate, from the so-called
dorsal organ*.

1 Barrois himself held the opposite view in his earlier memoir, and other observers
have done the same.

a The statements on this head are so unsatisfactory and contradictory that it does
not appear to me worth while quoting them here ; even the latest accounts of Barrois,
which entirely contradict his early statements, can hardly be regarded as satisfactory.

POLYZOA. 301

The first distinct rudiment of the polype appears as a white
body, which gradually develops into the alimentary canal and
lophophore. While this is developing the ectocyst grows rapidly
larger, and the yolk in its interior separates from the walls and
occupies a position in the body cavity of the future polype,
usually behind the developing alimentary canal. According
to Nitsche (No. 316) it is attached to a protoplasmic cord
(funiculus) which connects the fundus of the stomach with the
wall of the cell. It is probably (Nitsche, etc.) simply employed
as nutritive material, but, according to Barrois, becomes con-
verted into the muscles, especially the retractor muscles.

Adopting the hypothesis already suggested in the case of the
Entoprocta the metamorphosis just described would seem to be
a case of budding accompanied by the destruction of the original
larva.

This view of the nature of the post-embryonic metamorphosis is appa-
rently that of Claparede and Salensky, and is supported by Claparede's
statement that the formation of the first polype ' resembles to a hair ' that of
the subsequent buds. The mode of budding would, however, appear to
present certain peculiarities, in that the whole larval skin passes directly into
the bud, while from the rudimentary bud of the larva the lophophore and
alimentary tract only of the fixed polype are formed.

Flustrella and Cyphonautes. The next group of larval
forms is that of which Cyphonautes is the best known type.
The larvae composing it at first sight appear to have but little in
common with the larvae hitherto described. The researches
of Barrois (No. 298) and Metschnikoff (No. 314), (but especially
those of the former on the early stages of Flustrella hispida, the
larva of which is very similar in form to Cyphonautes, though
without so great a complexity of organisation), have given a
satisfactory basis for a general comparison of Cyphonautes with
other ectoproctous larvae.

The segmentation and early stages of the embryo of Flus-
trella resemble closely those of Alcyonidium. A projecting ring
of large cells is formed, dividing the larva into oral and aboral
parts. The oral part soon however becomes very small as com-
pared with the aboral, and becomes vertically flattened so as to be
nearly on a level with the ring of large cells. In the next stage
the flattening becomes completed ; and the ring of large cells

302 ECTOPROCTA.

surrounds, like the vestibule of the Entoprocta, a flat oral disc.
The aboral side is dome-shaped, and forms the greater part
of the embryo.

FIG. 132. ADVANCED LARVA OF FLUSTRELLA HISPIDA. (After Barrois.)
m (?) groove above dorsal organ ; Ph. dorsal organ ; st. stomodoeum (?) ; s. ciliated
disc at aboral end of body.

In the next stage a small disc — the ciliated disc — is formed
in the middle of the aboral dome. The larva becomes laterally
compressed. The ring of large cells which now constitute the
edge of the vestibule is covered, as in the larva of Pedicellina, by
cilia, which are specially long in front of the dorsal organ.

In the next stage the ciliated disc (fig. 132, s.) becomes
reduced in size, but surmounted by a ring of cilia round the
edge, and a tuft of cilia in the centre. The chief difference
between this larva and that of Alcyonidium depends on the
small size of the ciliated disc, and the oral position of the ciliated
ring in the former. There are intermediate types between these
forms of larvae.

This stage immediately precedes the liberation of the larva.
The free larva differs from that in the ovicell mainly in the
possession of a shell formed as a cuticular structure, composed
of two valves placed on the two sides of the embryo. The
aboral ciliated disc, still more reduced in size, loses its cilia, and
becomes enclosed between the two valves of the shell.

The post-embryonic metamorphosis follows, so far as is known,
the course already described for the larva of Alcyonidium.

POLYZOA.

303

Cyphonautes (fig. 133) forms at certain seasons of the year
one of the commonest captures in the surface net. It was origi-
nally described by Ehrenberg, but the important discovery of its
true nature as the larva of Membranipora (the common species C.
compressus is the larva of Mem. pilosa), a genus of the chilosto-
matous Polyzoa, was made by Schneider (No. 322). The younger
stages of the larva have not been worked out, but from a
comparison with the last described larva it is easy to make out
the general relationship of the parts. The larva has a triangular
form with an aboral apex, corresponding with the summit of the
dome of the Flustrella larva, and an oral base. It is enclosed in
a bivalve shell, the two valves of which meet along the two sides,
but are separate along the base. At the apex an opening is left
between the two valves, through which a ciliated disc (/. g) of
the same character and nature as that of previous larvae can be
protruded.

The oral side or base is girthed by a somewhat sinuous
ciliated edge, which is continued round the anterior and posterior
extremities of the oral disc. It is no doubt equivalent to the
ciliated ring of other larvae. Two openings are present on the
oral face, both enclosed in a special lobe of the ciliated ring.
The larger of these leads
into a depression, which
may be called the ves-
tibule; and is situated
on the posterior side of
the oral surface. The
smaller of the two, on
the anterior side, leads
into a cavity which is
apparently (Hatschek)
equivalent to the rudi-
mentary bud or dorsal
organ of other larvae.
The deeper part of the
vestibule leads into the
mouth (m) and oesopha-
gus ; the latter is con-
tinued till close to the

CYPHONAUTES (LARVA OF MEM-
( After Hatschek.)

FIG. 133
BRANIPORA).

m. mouth; a. anus; f.g. ciliated disc; x.
problematical body (probably a bud).

apex of the larva, there bends upon

304 ECTOPROCTA.

itself, dilates into a stomach, and is continued parallel to the
oesophagus as the rectum which opens by an anus (a1) at the
posterior end of the vestibule. A peculiar paired organ is
situated on each side nearly above the stomach. Its nature
is somewhat doubtful. It was regarded as muscular by Clapa-
rede (No. 309), though this, as shewn by Schneider, is no doubt
a mistake. Allman (No. 305) regards it as hepatic, and Hat-
schek as a thickening of the epidermis. Close to each of these
organs is a small body regarded by Claparede as an accessory
muscle. It is placed in the normal position for a Polyzoon
ganglion, and may perhaps be therefore regarded as nervous in
nature. Allman points out its similarity to a bilobed ganglion,
but is not inclined to take this view of it. The constitution
of the parts contained in the anterior cavity (x) is somewhat
obscure. The most elaborate descriptions of them are given by
Schneider and Allman. Lining the cavity is apparently a mass
of spherical bodies, connected with which is a tongue-like process
provided with long cilia, which can be protruded from the orifice.
Internal to this is a striated body. A good figure of the whole
structure is given by Schneider.

The general similarity of Cyphonautes to the other larvae is
quite obvious from the above description and figure. In the
presence of an anus, a vestibule, and possibly a nervous
system, it clearly exhibits a far more complicated organisa-
tion than any other Polyzoon larvae except those of the
Entoprocta.

The post-embryonic metamorphosis of Cyphonautes, ad-
mirably investigated by Schneider, takes place in the same
manner as that of other larvae, and is accompanied by the de-
generation of the larval organs, and the formation of a clear
body, which gives rise to the alimentary cavity and lophophore
of the fixed polype. The larval shell takes part in the formation
of the ectocyst of the polype.

Cyclostomata. We owe to Barrois by far the fullest account of the
development of the Cyclostomata, but how far his interpretations are to be
trusted is very doubtful. The larvae differ very considerably from the
normal larvae of the Chilostomata and Ctenostomata ; the difference being
mainly due to the enormous development of the ciliated disc. Barrois has
investigated the larvae of three genera, Phalangella, Crisia, and Diastopora,

POLYZOA. 305

and states that they very closely resemble each other. The ovum is ex-
tremely minute.

The segmentation, so far as it has been made out, is regular. During
the segmentation growth is very rapid, and eventually there is formed a
blastosphere many times larger than the original ovum. The blastosphere
becomes flattened, and is converted into a gastrula by bending up into
a cup-like form. The gastrula opening is stated to remain as the permanent
mouth, which has a terminal and central position. A transverse ring-like
thickening is formed round the larva, which probably corresponds with the
ciliated ring of previous larvae ; and the body of the larva in front of this
ring becomes ciliated. The aboral end of the larva becomes thickened, and
grows out into an elongated prominence, which probably corresponds to the
ciliated disc. The ring before mentioned becomes at the same time more
prominent, and forms a cylindrical sheath for the ciliated disc. At the
time when the larva becomes liberated from the maternal cell it has the
form of a barrel with a slight constriction in the middle separating the oral
from the aboral end. At the centre of the oral face is situated the mouth,
leading into a wide stomach, while the aboral end is formed of the ciliated
disc enclosed in its sheath. The whole surface is now ciliated. No
structure equivalent to the dorsal organ or bud is described by Barrois, but
in other respects, if the ciliated disc is really equivalent in the two forms, a
general comparison on the line indicated above between this larva and the
normal larvae of the Ctenostomata and Chilostomata seems quite possible.
The fixation and subsequent development of the larva take place in the
normal manner.

Phylactolaemata. The development of the phylactolaema-
tous Polyzoa has been studied by Metschnikoff (No. 315), who
describes the eggs as undergoing a complete segmentation within
a peculiar brood-pouch developed from the walls of the body of
the parent. After segmentation the cells of the embryo arrange
themselves in two layers round a central cavity. The embryo
then forms the well-known cyst, from which a colony is formed
by a process of budding.

General considerations on the Larva of the Polyzoa.

The different forms of embryo amongst the Polyzoa are
represented in figs. 130 B, 131, 132, and 133 in what I regard as
identical positions, and fig. 133 A is a figure of what may be
regarded as an idealized larval Polyzoon. In all the larvae there
is present a ciliated ring, which separates an oral from an aboral
face, and is apparently homologous throughout the series. In
the adult it is probably represented by the lophophore. On the
B. IT 20

306 SUMMARY.

oral face is situated in all cases the mouth, and in the entoproc-
tous larvae and Cyphonautes also the anus. It thus appears that
Cyphonautes, though the larva of an ectoproctous form, is itself
entoproctous — a fact which tends to shew that the Entoprocta
are the more primitive forms. In all the larvae, except possibly
those of the Cyclostomata, there is present on the anterior side
of the mouth, in the Ectoprocta on the oral, and in the Ento-
procta on the aboral side of the ciliated ring, an organ, to which
is attached externally a plume of long cilia. This organ has
been identified throughout the series
in accordance with Hatschek's view
as the dorsal organ or rudimentary
bud ; but it is well to bear in mind
that this identification is of a purely
hypothetical character.

On the aboral side of the ciliated
ring there is present in all the larv*
an organ, which has been called the ,„. m0uth; an. anus;^. sto-
ciliated disc, which is probably homo- mach' *- ciliated disc-
logous throughout the series. It perhaps remains in the adult of
Loxosoma as the cement gland, but not in other forms.

The Polyzoa present a simple and almost certainly degraded
organisation in the adult state ; it is therefore more than usually
necessary to turn to their larvae for the elucidation of their
affinities, and various plausible suggestions have been made as
to the interpretation of the characters of the larvae.

Lankester1 has suggested that the larvae are essentially
similar to those of Molluscs. He compares the main ciliated
ring to the velum, but has ingeniously suggested that it repre-
sents not the simple velar ring of most molluscan larvae, but
a more extended longitudinal ring, of which the gills of Lamel-
libranchiata are supposed by him to be remnants, and to which
the Echinoderm larvae with one continuous ciliated band furnish
a parallel.

The foot he finds in the epistome of the Phylactolaemata,
and the disc of Rhabdopleura — both situated between the
mouth and anus, and therefore in the situation of the molluscan

1 Lankester. "Remarks on the affinities of Rhabdopleura." Quart. J. of
Mitro. Science, Vol. XIV. 1874.

POLYZOA. 307

foot. The peculiar prominence between the mouth and the
anus in Pedicellina (vide fig. 130 B) and Loxosoma is probably
the same structure.

Finally he identifies my ciliated disc, which as mentioned
above is perhaps equivalent to the cement gland in the adult
Loxosoma, as the molluscan shell-gland. Lankester's interpre-
tations are very plausible, but at the same time they appear to
me to involve considerable difficulties.

There is absolutely no evidence amongst the Mollusca of the
existence of a primitive longitudinal ciliated ring, such as he
supposes to have existed, and Lankester is debarred from
regarding the ciliated ring of the Polyzoa as equivalent to the
simple velar ring of the Mollusca, because his shell-gland lies in
the centre and not as it should do on the posterior side of the
ciliated ring.

Another difficulty which I find is the invariable ciliation of
Lankester's shell-gland — a ciliation which never occurs amongst
Mollusca.

It appears to me that a more satisfactory comparison of the
larvae of the Polyzoa with those of the Mollusca is obtained by
dropping the view that the ciliated disc is the shell-gland, and
by regarding the ciliated ring as equivalent to the velum. This
mode of comparison has been adopted by Hatschek.

The larva ceases however on this view to have any special
molluscan characters (except possibly the organ which Lankester
has identified as the foot), and only resembles a molluscan larva
to the same extent as it does a larva of the Polychaeta. The
ciliated disc lies according to this view in the centre of the velar
area or prae-oral lobe, and therefore in the situation in which a
tuft of cilia is often present in lamellibranchiate and other
molluscan larvae, and also in the larvae of most Chaetopoda. It
is moreover at this point that the supra-cesophageal ganglion is
always formed in the Mollusca and Chaetopoda as a thickening
of the epiblast (fig. 134, .$£-.), so that the thickening of the
epiblast in the ciliated disc of the Polyzoa may perhaps be a
rudiment of the supra-cesophageal ganglion, which entirely
atrophies in the adult after the attachment has been effected in
the region of this disc.

The comparison between the Polyzoon larva and that of a

20 — 1

308

POLYZOA.

Chaetopod becomes very much strengthened by taking as types
Mitraria1 (fig. 134) and Cyphonautes (fig. 133). The similarity

FIG. 134. Two STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metsch-
nikoff.)

m. mouth; an. anus; sg. supra-oesophageal ganglion; br. and b. provisional
bristles; pr.b. prae-oral ciliated band.

between these two forms is so striking that I am certainly
inclined to view the larvae of the Polyzoa as trochospheres similar
to those of Chaetopods, Rotifera, etc., which 'become fixed in the
adult by the extremity of their prce-oral lobe.

The attachment of the larva by the prae-oral lobe is not more
extraordinary than the attachment of a Barnacle by its head,
and after such a mode of attachment the atrophy of the supra-
cesophageal ganglion would be only natural.

There is one important fact which deserves to be noted in
the development of the Polyzoa, viz. that if the suggestion in the
text as to the mode of development of the adult from the so-
called larva is accepted, the Polyzoa exhibit universally the
phenomenon of alternations of generations. The ovum gives rise
to a free form which never becomes sexual, but produces by
budding the sexual attached form.

1 The larva of Mitraria is figured with the aboral surface turned upwards, instead
of downwards, as in the figure of Cyphonautes. The ciliated band is also diagramma-
tical ly put in black for greater distinctness.

BIBLIOGRAPHY. 309

BIBLIOGRAPHY.
General.

J. Barrels. Recherches sur P embryologie des Bryozoaires. Lille, 1877.

Entoprocta.

(299) B. Hatschek. "Embryonalentwicklung u. Knospung d. Pedicellina
echinata." Zeitschrift fur wiss. Zool., Bd. xxix. 1877.

(300) M. Salensky. " Etudes sur les Bryozoaires entoproctes." Ann. Scien.
Nat., 6th Ser. Tom. v. 1877.

(301) O. Schmidt. "Die Gattung Loxosoma." Archiv f. mik. Anat., Bd.
xii. 1876.

(302) C. Vogt. "Sur le Loxosome des Phascolosomes." Archives de Zool.
exper. et gener., Tom. v. 1876.

(303) C. Vogt. "Bemerkungen zu Dr Hatschek's Aufsatz iib. Embryonalent-
wicklung u. Knospung von Pedicellina echinata." Zeit. f. wiss. Zool., Bd. XXX.
1878.

Ectoprocta.

(304) G. J. Allman. Monograph of fresh water Polyzoa. Ray Society.

(305) G. J. Allman. " On the structure of Cyphonautes." Quart. J. of Micr.
Scie., Vol. xii. 1872.

(306) G. J. Allman. "On the structure and development of the Phylactoke-
matous Polyzoa." Journal of 'the Linnean Society, Vol. xiv. No. 77. 1878.

(307) J. Barrois. " Le developpement d. Bryozoaires Chilostomes." Comptes
rendus, Sept. 23, 1878.

(308) E. Claparede. "Beitrage zur Anatomic u. Entwicklungsgeschichte d.
Seebryozoen." Zeit.fiir wiss. Zool., Bd. xxi. 1871.

(309) E. Claparede. "Cyphonautes." Anat. u. Entwick. wirbell. Thiere.
Leipzig, 1864.

(310) R. E. Grant. "Observations on the structure and nature of Flustrse."
Edinburgh New Philosoph. Journal, 1827.

(311) B. Hatschek. "Embryonalentwicklung u. Knospung d. Pedicellina
echinata" (Description of Cyphonautes). Zeit.f. wiss. Zool., Bd. xxix. 1877.

(312) T. H. Huxley. "Note on the reproductive organs of the Cheilostome
Polyzoa." Quart. Jour, of Micr. Science, Vol. IV. 1856.

(313) L. Joliet. "Contributions a 1'histoire naturelle des Bryozoaires des cotes
de France." Archives de Zoologie Experimental, Vol. vi. 1877.

(314) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere." Got-
tingische Nachrichten, 1869.

<315) E. Metschnikoff. Bull, de tAcad. de St Petersbourg, XV. 1871, p. 507.

(316) H. Nitsche. "Beitrage zur Kenntniss d. Bryozoen." Zeit. f. wiss.
Zool., Bd. xx. 1870.

(317) W. Repiachoff. "Zur Naturgeschichte d. chilostomen Seebryozoen."
Zeit.f. wiss. Zool., Bd. xxvi. 1876.

310 BIBLIOGRAPHY.

(318) W. Repiachoff. " Ueber die ersten Entwicklungsvorgange bei Tendra
zostericola." Zeit.f. wiss. Zool., Bd. xxx. 1878. Supplement.

(319) W. Repiachoff. " Zur Kenntniss der Bryozoen." Zoologischer Anzeiger,
No. 10, Vol. I. 1878.

(320) W. Repiachoff. " Bemerkungen ub. Cyphonautes." Zoologischer An-
*eiger. Vol. II. 1879.

(321) M. Salensky. " Untersuchung an Seebryozoen." Zeit. fur wiss. ZooL,
Bd. xxiv. 1874.

(322) A. Schneider. "Die Entwicklung u. syst. Stellung d. Bryozoen u.
Gephyreen." Archiv /. mikr. Anat., Vol. v. 1869.

(323) Smitt. " Om Hafsbryozoernas utveckling och fettkroppar. " Aftryck ur
ijfvtrs. af Kong. Vet. Akad. Fork. Stockholm, 1865.

(324) T. Hi neks. British Marine and Polyzoa. Van Voorst, 1880.
[Conf. also works by Farre, Hincks, Van Beneden, Dalyell, Nordmann.]

CHAPTER XL

BRACHIOPODA1.

THE observations which have been made on the develop-
mental history of the Brachiopoda have thrown very consider-
able light on the systematic position of this somewhat isolated
group.

Development of the Layers.

For our knowledge of the early stages in the development of
the Brachiopoda we are almost entirely indebted to Kowalevsky2
(No. 326). His researches extend to four forms, Argiope,
Terebratula, Terebratulina, and Thecidium. The early develop-
ment of the first three of these takes place on one plan, and
that of Thecidium on a second plan.

In Argiope, which may be taken as typical of the first group,
the ova are transported into the oviducts (segmental organs)
where they undergo their early development. The segmentation
leads to the formation of a blastosphere, which then becomes a
gastrula by invagination. The Slastopore gradually narrows,
and finally closes, while at the same time the archenteric cavity

1 The classification of the Brachiopoda adopted in the present chapter is shewn in
the subjoined table :

,. , , (a. Rhynchonellidae.

I. Articulata. L _.£.

\b. Terebratuhdse.
/a. Lingulidae.

II. InartiCUlata. \b. Craniadae.

I c. Discinidae.

2 Kowalevsky's Memoir is unfortunately written in Russian. The account in the
text is derived from an inspection of his figures, and from an abstract in Hoffmann
and Schwalbe's Jahresberichte for 1873.

12

ARTICULATA.

(fig. 135 A) becomes divided into three lobes, a median (me)
and two lateral (pv). These lobes next become completely
separated, and the middle one forms the mesenteron, while
the two lateral ones give rise to the body cavity, their outer
walls forming the somatic mesoblast, and their inner the
splanchnic (fig. 135 B). The embryo now elongates, and
becomes divided into three successive segments (fig. 135 B),
which are usually, though on insufficient grounds (vide Thecid-
ium), regarded as equivalent to the segments of the Cheetopoda.
The alimentary tract is not continued into the hindermost of
them.

In Thecidium the ova are very large, and development takes
place in a special incubatory pouch in the ventral valve. The
embryos are attached by suspenders to
the two cirri of the arms which immedi-
ately adjoin the mouth. There is a nearly
regular segmentation, and a very small
segmentation cavity is developed. There
is no invagination ; but cells are budded
off from the walls of the blastosphere,
which soon form a solid central mass,
enclosed by an external layer — the epi-
blast. In this central mass three cavities
are developed, which constitute the me-
senteron and the two halves of the body
cavity. Around these cavities distinct
walls become differentiated. The body
(Lacaze Duthiers, No. 327) soon after
becomes divided into two segments, of
which the posterior is the smaller. The
hinder part of the large anterior segment
next becomes constricted off as a fresh
segment, and subsequently the remaining
part becomes divided into two, of which the anterior is the
smallest. The embryo thus becomes divided into four segments,
of which the two foremost appear (?) together to correspond to
the cephalic segment of Argiope ; but these segments are formed
not, as in Chaetopoda and other truly segmented forms, by the
addition of fresh segments between the last-formed segment and

pv.

FIG. 135. Two STAGES

IN THE DEVELOPMENT OF

ARGIOPE. (After Kowa-
levsky.)

A. Late gastrula stage.

B. Stage after the larva
has become divided into
three segments.

bl. blastopore ; me. me-
senteron ; pv. body cavity ;
b. temporary bristles.

BRACHIOPODA.

313

the unsegmented end of the body, but by the interpolation of
fresh segments at the cephalic end of the body as in Cestodes ;
so that the hindermost segment is the oldest. Assuming the
correctness1 of Lacaze Duthiers' observations, the mode of
formation of these segments appears to me to render it probable
that they are not identical with the segments of a Chaetopod.
A suspender is attached to the front end of each embryo. Before
the four segments are established the whole embryo is covered
with cilia2, and two and then four rudimentary eyes are
developed on the anterior segment of the body.

The history of the Larva and the development of the organs of

the Adult.

Articulata. The observations of Kowalevsky and Morse
have given us a fairly complete history of the larval metamor-
phosis of some of the Articulata, while some of the later larval
stages in the history of the Inarticulata have been made known
to us from the researches of Fritz M tiller, Brooks, etc. The
embryo of Argiope, which may be taken
as the type for the Articulata, was left
(fig. 135 B) as a three lobed organism
with a closed mesenteron and a body
cavity divided into two lateral compart-
ments. On the middle segment of the
body dorsal and ventral folds, destined
to form the mantle lobes, make their
appearance, and on the latter two pairs
of bundles of setae are present (fig. 135 B).
The setae together with the mantle folds
grow greatly, and the setae resemble in ap-
pearance the provisional setae of many
Chaetopods (fig. 152). On the hinder
border of the mantle cilia make their
appearance. The anterior or cephalic
segment assumes a somewhat umbrella-

FIG. 136. LARVA OF AR-
GIOPE. (From Gegenbaur,
after Kowalevsky.)

. . , L m. mantle; b. setae; </.

like form, and round its edge is a circlet archenteron.

1 It should be stated that it is by no means clear from Kowalevsky's figures that
he agrees with Lacaze Duthiers as to the succession of the segments.

2 Kowalevsky in his figures leaves the penultimate lobe unciliated.

3H

ARTICULATA.

of long cilia, while elsewhere it is provided with a coating of short
cilia. Two pairs of eyes also arise on its anterior surface (fig. 136).

After swimming about for some time the larva becomes fixed
by its hind lobe, and becomes gradually transformed into the
adult. The hind lobe itself becomes the peduncle. After
attachment the mantle lobes bend forward (fig. 137 A, m\ and
enclose the ce-
phalic lobe.
The valves of
the shell are
formed on their
outer surface as
two delicate
chitinous plates
(fig. 1 37 B). At
a somewhat la-
ter stage the
provisional bri-
stles are thrown
off, and are
eventually re-
placed by per-
manent setae
round the edge
of the mantle.
The cephalic
lobe becomes
located in the
dorsal valve of
the shell, and
the mouth is
formed near the
apex of the ce-
phalic lobe im-
mediately ven-
tral to the eye-spots, by an epiblastic invagination. The per-
manent muscles are formed out of the muscles already present
in the embryo.

Around the mouth there arises a ring of tentacles, very

FlG. 137. TWO STAGES IN THE DEVELOPMENT OF
ARGIOPE, SHEWING THE FOLDS OF THE MANTLE GROWING
OVER THE CEPHALIC LOBE. (After Kowalevsky.)

m. mantle fold ; me. mesenteron ; pd. peduncle ; b.
visional setae.

pro-

BRACHIOPODA.

315

possibly derived from the ciliated ring visible in fig. I361. The
ring of tentacles is placed obliquely, and the mouth is situated
near its ventral side. The tentacles appear to form a post-oral
circlet, like that of Phoronis (Actinotrocha): they gradually
increase in number as the larva grows older.

Some of the later stages in the development
of the Terebratulidse have been made known to
us by the observations of Morse (No. 328 — 9)
on Terebratulina septentrionalis.

The most interesting point inMorse's observa-
tions on the later stages is the description of
the gradual conversion of the disc bearing the
circlet of tentacles into the arms of the adult.
The tentacles, six in number, first form a ring
round the edge of a disc springing from the
dorsal lobe of the mantle ; in their centre is
the mouth. In the later stages calcareous
spicula become developed on the tentacles.
When the embryo is far advanced the tentacles
begin to assume a horse-shoe arrangement,
which bears a striking, though probably acciden-
tal, resemblance to that of the tentacles on the
lophophore of the fresh-water Polyzoa. The
disc bearing the tentacles is prolonged anteriorly
into two processes, the free ends of the future
arms. By this change of shape in the disc the
tentacles form two rows, one on the anterior and
one on the posterior border of the disc, and
eventually become the cirri of the arms. The
mouth is placed between the two rows of tenta-
cles, where the two arms of the lophophore meet
behind. The position of the mouth was the
original centre of the ring of tentacles before
they became pulled out into a horse-shoe form.
In front of the mouth is a lip. The arms grow
greatly in length in the adult Terebratulina. In
Thecidium the oral disc retains the horse-shoe
form, while in Argiope the embryonic circular
arrangement of the tentacles is only interfered
with by the appearance of marginal sinuations.

FIG. 138. DIAGRAM OF
A LONGITUDINAL VERTICAL
SECTION OF AN ADVANCED

EMBRYO OFLlNGULA. (After

Brooks.)

a. end of valves ; b. thick-
ened margin of mantle ; c.
mantle ; d. dorsal median
tentacle ; e. lophophore ; /.
lip ; g. mouth ; h. mantle
cavity ; i. body cavity ; k.
wall of oesophagus ; /. oeso-
phagus; m. hepatic cham-
ber of stomach ; n. intesti-
nal chamber of stomach ; o.
intestine ; q. ventral gang-
lion ; r. posterior muscle ;
s. dorsal valve of shell; /.
ventral valve of shell.

1 In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the
tentacles spring from the border of the mantle. This can hardly be a correct account
of what he states, since it does not fit in with the adult anatomy of the parts. The
figures he gives might lead to the supposition that they sprang from the edge of the
cephalic lobe, or perhaps from the dorsal lobe of the mantle.

316 ARTICULATA.

The shell is deposited as to chitinous plates, which subse-
quently become calcified. It undergoes in the different genera
great changes of form during its growth.

With reference to the larval stages of other Articulata, a few points may
be noted.

The three-lobed larva of Terebratulina septentrionalis is provided with
a special tuft of cilia at the apex of the front lobe. The arms appear to
originate, in Terebratulina caput serpentis, as two processes at the sides of
the mouth, on which the tentacles are formed.

Provisional setae do not appear to be formed in the lobed embryos of
Thecidium and Terebratulina, but they appear at a later stage at the edge of
the mantle in the latter form. The third lobe of Thecidium gives rise to the
dorsal and ventral mantle lobes.

Inarticulate. The youngest stages in the development of
the Inarticulata are not known, and in the earliest stages
observed the shell is already developed. The young larvae with
shells differ however from those of the Articulata in the fact
that they are free swimming, and that the peduncle is not
developed.

The larva of Discina radiata has been described by Fritz Miiller (No.
331). It resembles generally a larva of the Articulata shortly after the
tentacles have become developed. Five pairs of long provisional setae are
present, of which all but the hindermost are seated on the ventral lobe of
the mantle. Shorter setae are also lodged on the edge of the dorsal lobe.
The mouth is placed on the ventral side of a protrusible oral lobe. It is
imperfectly surrounded by four pairs of tentacles, which form a swimming
apparatus.

A fuller history of the development of Lingula has been recently supplied
by Brooks (No. 325). The youngest larva is enveloped in two nearly similar
plate-like valves, covering the two mantle lobes. The mouth is placed at
the centre of a disc, attached to the dorsal valve, on the margin of which is
a ring of ciliated tentacles. The general position of the disc and its
relations may be gathered from fig. 138, which represents a diagrammatic
longitudinal vertical section of the embryo.

With the growth of the embryo the tentacles increase in number, the
new pairs being always added between the odd dorsal tentacle and the next
pair. There is an axial cavity in the tentacles which, unlike the cavity in
the tentacles of the Polyzoa, does not communicate with the perivisceral
cavity. As the tentacles increase in number, the lateral parts of the
tentacular disc grow out into the two lateral arms of the adult, while the
dorsal margin forms the median coiled arm. These changes are not effected
till the larva has become fixed.

The attachment of the larva was not observed ; but the peduncle, of

BRACHIOPODA. 317

which there is no trace in the young stages, grows out as a simple prolonga-
tion of the hinder end of the body while the larva is still free. It had already
reached a very great length in the youngest fixed larva observed.

Development of Organs.

The alimentary tract after the obliteration of the blastopore
forms a closed sack, which becomes subsequently placed in
communication with the exterior by the stomodaeal imagination.
The liver is formed as a pair of dorsal outgrowths of the mesen-
teron. From Brooks' observations on Lingula it would appear
that the primitive mesenteron forms the stomach of the adult
only, and that the intestine grows out from this as a solid
process : this eventually meets the skin, and here the anus is
formed. In the Articulata the mesenteron is aproctous.

The origin of the body cavity as paired archenteric diverticula
has already been described. Its somatic wall becomes in Lingula
ciliated, and its cavity filled with a corpusculated fluid, as in
many Chsetopods. It is eventually prolonged into the dorsal
and ventral mantle lobes as a pair of horn-like prolongations
into each lobe, which communicate with the body cavity by
large ciliated openings. Some incomplete observations of Brooks
on the development of the nervous system in Lingula shew that
it arises in the embryo as a ring round the oesophagus with
a ventral sub-cesophageal (fig. 138 q\ and two lateral ganglia,
and two dorsal otocysts. The ventral ganglion is formed as a
thickening of the epiblast, with which it remains in continuity
for life. The remainder of the ring grows out from the ventral
ganglion as two cords, which gradually meet on the dorsal side
of the oesophagus.

General observations on the Affinity of the Brachiopoda.

The larva of Argiope, as has been noticed by many observers, has
undoubtedly very close affinities with the Chaetopoda. It resembles, in fact,
a mesotrochal larval Chaetopod with provisional setae (vide Chapter on
Chaetopoda). Lacaze Duthiers' observations point to the lobes of the larva
not being true segments, and certainly the mesoblast does not in the embryo
become segmented as it ought to do were these lobes true segments. If this
view is correct the larva is to be compared to an unsegmented Chastopod
larva. In Rhynchonella, however, indications of two segments are afforded
in the adult in the two pairs of segmental organs.

Though the larval Brachiopod resembles a mesotrochal Chastopod larva,

318 ARTICULATA.

it does not appear to resemble the trochosphere larvae so far described, or
the more typical larvae of the Chaetopoda, in that the ring of tentacles, which
is probably, as already mentioned, derived from the ciliated ring shewn in
fig. 137, is post-oral, and not prce-oral. The ring of tentacles is like the
ring in Actinotrocha (the larva of Phoronis) amongst the Gephyrea.
Although there is no doubt a striking resemblance between the tentacular
disc of a larval Brachiopod and the lophophore of a Polyzoon, which has
been pointed out by Lankester, Morse, Brooks, etc., their homology is
rendered, to my mind, very doubtful (i) by the fact that the lophophore is
prae-oral in Polyzoa1 and post-oral in Brachiopoda ; and (2) by the fact that
the concave side of the lophophore is turned in nearly opposite directions
in the two forms. In Brachiopods it is turned dorsalwards, and in phylacto-
laematous Polyzoa ventralwards.

The view of Morse, that the Brachiopoda are degraded tubicolous
Chaetopods, is not so far supported by any definite embryological facts.
The development of the tentacular ring as well as its innervation from the
sub-cesophageal ganglion prohibit us, as has been pointed out by Gegenbaur,
from comparing it with the tentacles of tubicolous Chaetopoda.

BIBLIOGRAPHY.

(325) W. K. Brooks. "Development of Lingula." Chesapeake Zoological
Laboratory, Scientific Results of the Session of 1878. Baltimore, J. Murphy and Co.

(326) A. Kowalevsky. " Development of the Brachiopoda." Protocol of the
First Session of the United Sections of Anatomy, Physiology1, and Comparative Ana-
tomy at the Meeting of Russian Naturalists in Kasan, 1873. (Russian.)

(327) H. Lacaze Duthiers. " Histoire de la Thecidie." Ann. Scien. Nat.
etc. Ser. 4, Vol. xv. 1861.

(328) Morse. " On the Early Stages of Terebratulina septentrionalis." Mem.
Boston Soc. Nat. History, Vol. II. 1869, also Ann. & Mag. of Nat. Hist., Series 4,
Vol. viii. 1871.

(329) "On the Embryology of Terebratulina." Mem. Boston Soc. Nat.

History, Vol. III., 1873.

(330) - - "On the Systematic Position of the Brachiopoda." Proceedings of
the Boston Soc. of Nat. Hist , 1873.

(331) Fritz Miiller. " Beschreibung einer Brachiopoden Larve." Mutter's
Archiv, 1860.

1 For the ectoproctous Polyzoa it might be held that the ciliated ring of tentacles
is post-oral, but the facts of development recorded in the previous chapter appear to
me to shew that this view is untenable.

CHAPTER XII.

CH^TOPODA1.

Formation of the Germinal Layers.

MOST Chaetopoda deposit their eggs before development.
The Oligochaeta lay them in peculiar cocoons or sacks formed
by a secretion of the integument. Some marine Polychaeta carry
them about during their development. Autolytus cornutus has
a special sack on the ventral surface in which they are hatched.
In Spirorbis Pagenstecheri they develop inside the opercular
tentacle, and in Spirorbis spirillum inside the tube of the
parent.

A few forms (e.g. Eunice sanguinea, Syllis vivipara, Nereis
diversicolor) are viviparous.

Perhaps the most primitive type of Chaetopod development
so far observed is that of Serpula (Stossich, No. 357) 2. There is
a regular segmentation resulting in the formation of a blasto-
sphere with a central segmentation cavity. An invagination of
the normal type now ensues. The blastopore soon narrows to
become the permanent anus, while the invaginated hypoblast
forms a small prominence with an imperfectly developed lumen,
which does not nearly fill up the segmentation cavity (fig. 139 A).
The embryo, which has in the meantime become completely

1 The following classification of the Chaetopoda is adopted in the present section.

I. Achseta (Polygordius).

(Sedentaria.

ii. Polychseta. jErrantia
ill. Oligochgeta.

2 The observations of Stossich are not thoroughly satisfactory.

320 FORMATION OF THE LAYERS.

covered with cilia, now assumes more or less the form of a cone,
at the apex of which is the anus, while the base forms the
rudiment of a large prae-oral lobe. The alimentary sack grows
forwards and then bends upon itself nearly at right angles, and
meets a stomodaeal invagination from the ventral side some way
from the front end of the body.

The alimentary canal soon differentiates itself into three
regions (i) oesophagus, (2) stomach, and (3) intestine. With

FlG. 139. TWO STAGES IN THE DEVELOPMENT OF SfiRPULA. (After StOSsich.)

m. mouth ; an. anus ; al. archenteron.

these changes the larva, which in the meantime becomes hatched,
assumes the characters of a typical Annelid larva (fig. 139 B).
In front is a large prae-oral lobe, at the sides of which the eye-
spots soon appear. The primitive segmentation cavity remains
as a wide space between the curved alimentary tract and the
body walls, and becomes traversed by muscular fibres passing
between the two. The original chorion appears to serve as
cuticle, and is perforated by the cilia.

The further changes in this larval form do not present features of
general importance. A peculiar vesicle, which in anomalous cases is
double, is formed near the anus. If it were shewn to occur widely
amongst Chaetopoda, it might be perhaps regarded as homologous with
the anal vesicles of the Gephyrea.

Serpula is one of the few Chaetopoda at present known in

CH^TOPODA. 321

which the segmentation is quite regular1. In other forms it is
more or less unequal. The formation of the germinal layers
has been far more fully studied in the Oligochaeta than in the
Polychaeta, and though unfortunately the development is much
abbreviated in the former group, they nevertheless have to serve
as our type ; and unless the contrary is indicated the statements
in the remainder of the section apply to the Oligochaeta. The
segmentation is nearly regular in Lumbricus agricola (Kowa-
levsky) and results in the formation of a flattened blastosphere,
one of the sides of which is hypoblastic and the other epiblastic,
the hypoblast cells being easily distinguished from the epiblast
cells by their clearer aspect. An invagination takes place,
in the course of which the hypoblast becomes enclosed by the
epiblast, and a somewhat cylindrical two-layered gastrula is
formed. The opening of this gastrula at first extends over
the whole of what becomes the ventral surface of the future
worm, but gradually narrows to a small pore — the permanent
mouth — near the front end. The central cavity of the gas-
trula is lined by hypoblast cells, but the oral opening, which
leads by a narrow passage into the gastric cavity, is lined by
epiblast cells.

The segmentation of Lumbricus trapezoides (Kleinenberg, No. 341), and
of Criodrilus (Hatschek, No. 339), is more unequal and more irregular than
that of Lumbricus agricola, and there is an invagination which is inter-
mediate between the embolic and epibolic types.

The segmentation of Lumbricus trapezoides is especially remarkable. It
is strangely irregular and at one period the segmentation cavity communi-
cates by a pore with the exterior. Before the completion of the gastrula
stage the ovum becomes partially divided into two halves, each of which
gives rise to a complete embryo. The two embryos are at first united
by an epiblast cord which connects their necks (fig. 141 A), but this cord is
very early ruptured, and the two embryos then become quite independent.
Some of the peculiarities of the segmentation may no doubt be explained
by this remarkable embryonic fission.

The gastrula opening in both Lumbricus trapezoides and Criodrilus is
placed on the ventral surface, and eventually narrows to form the mouth
or possibly (Criodrilus) closes at the position of the mouth. In Lumbricus
trapezoides the oral opening is at first lined by hypoblast, and in Criodrilus
is bounded anteriorly by three large peculiar epiblast cells, which are

1 According to Willemoes-Suhm, Terebellides stroemii is also characterised by a
regular segmentation,

3, II, 21

322 FORMATION OF THE LAYERS.

believed by Hatschek to assist in absorbing the albuminous fluid in which
the eggs are suspended. These large cells are eventually covered by the
normal epiblast cells and subsequently disappear. In both these types
the hypoblast cells undergo, during their invagination, peculiar changes
connected with their nutritive function.

In Euaxes (Kowalevsky) the segmentation is far more unequal than in
the other types ; a typical epibolic invagination takes place (fig. 140), and
the blastopore closes completely along the ventral surface.

In all the oligochaetous types, with the exception of Euaxes,
where the blastopore closes completely, the blastopore becomes,
or coincides with the mouth. In
Serpula it is stated (Stossich),
as we have seen, to coincide
with the anus : a statement
which receives confirmation
from the similar statements of
Willemoes-Suhm (No. 358). It
is necessary either to suppose ^

a mistake on the part of Stos-
sirh or that we have in Chaeto- FlG- l*°' TRANSVEKSE SECTION

S1Cn>01 THROUGH THE OVUM OF EUAXES

pods a case like that of Gas- DURING AN EARLY STAGE OF DEVELOP-
, . 1-1 f. 1-1 MENT. (After Kowalevsky.)

teropods in which a slit-like ^ epiblast; ms. mesoblastic band;
blastopore originally extending h- hypoblast.
along the ventral surface may in some forms become reduced
to a pore at the oral, or in other forms at the anal extremity.

So far only two germinal layers — the epiblast and the hypo-
blast — have been spoken of. Before the invagination of the
hypoblast is completed the mesoblast makes its appearance in
the form of two bands or streaks, extending longitudinally for
the whole length of the embryo. These are usually spoken
of as germinal streaks, but to avoid the ambiguity of this term
they will be spoken of as mesoblastic bands.

Their origin and growth has been most fully studied by
Kleinenberg (No. 341) in Lum. trapezoides. They commence
in this species shortly before the gastrula stage as two large
cells on the surface of the blastoderm, which may be called
mesoblasts. These cells lie one on each side of the median
line at the hind end of the embryo. They soon travel inwards
and become covered by the epiblast (fig. 141 A, m'\ while on
their inner and anterior side a row of small cells appears (ms).

CH^TOPODA.

323

These rows of cells form the commencement of the mesoblastic
bands, and in the succeeding stages they extend one on each
side of the body (fig. 141 B, ms) till they reach the sides of the
mouth. Their forward growth takes place mainly at the
expense of the superjacent epiblast cells, but the two mesoblasts

FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS

TRAPEZOIDES. (After Kleinenberg.)
ms. mesoblastic band ; m' . mesoblast ; al. archenteron ; pp. body cavity.

A. Horizontal and longitudinal section of an embryo which is dividing into two
embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands
proceeding from them.

B. Transverse section shewing the two widely-separated mesoblastic bands.

C. Transverse section at a later stage shewing the mesoblastic bands which have
approached the ventral line and developed a body cavity^/.

at their hinder extremities probably assist in their growth.
Each mesoblastic band is at first composed of only a single row
of cells, but soon becomes thicker, first of all in front, and
becomes composed of two, three or more rows of cells abreast.
From the above it is clear that the mesoblastic bands have, in
L. trapezoides at any rate, in a large measure an epiblastic
origin.

At first the two bands end in front at the sides of the mouth,
but subsequently their front ends grow dorsalwards at the

21 — 2

324 FORMATION OF THE LAYERS.

expense of the adjoining epiblast cells, and meet above the
mouth, forming in this way a mesoblastic dorsal commissure.

The mesoblastic bands soon travel from the lateral position,
which they at first occupy, towards the ventral surface. They
do not however meet ventrally for some time, but form two
bands, one on each side of the median ventral line (fig.
141 C).

The usual accounts of the origin and growth of the bands differ some-
what from the above. By Kowalevsky (No. 342) and Hatschek (No. 339)
they are believed to increase in Lumbricus rubellus and Criodrilus entirely
at the expense of the mesoblasts. Kowalevsky moreover holds that in L.
rubellus the original mesoblasts spring from the hypoblast. In some forms,
e.g. Lumbricus agricola, the mesoblasts are not present.

In Euaxes the origin of the mesoblast bands is somewhat interesting
as illustrating the relation of the Chaetopod mesoblastic bands to the
mesoblast of other forms. To render intelligible the origin of the mesoblast
in this form, it is necessary to say a few words about the segmentation.

By a somewhat abnormal process of segmentation the ovum divides
into four spheres, of which one is larger than the others, and occupies
a position corresponding with the future hind end of the embryo. The
three smaller spheres give rise on their dorsal side by a kind of budding
to small cells, which become the epiblast ; and the epiblast is also partly
formed from the hinder large cell in that this cell produces by budding
a small cell, which again divides into two. The anterior of the two
cells so formed divides still further and becomes incorporated in the
epiblast ; the posterior only divides into two which form the two mesoblasts.
The remainder of the mesoblast is formed by further division of the three
smaller of the primitive large spheres, and at first forms a continuous
layer between the dorsal cap of epiblast and the four largest cells which,
after giving rise to the epiblast and mesoblast, constitute the hypoblast.
As the epiblast spreads over the hypoblast the mesoblastic sheet gives way
in the middle, and the mesoblast remains as a ridge of cells at the edge of
the epiblastic cup. It forms in fact a thickening of the lips of the blasto-
pore. Behind the thickening is completed by the two mesoblasts. The
appearance of the mesoblast in section is shewn in fig. 140. As the
epiblast accompanied by the mesoblast grows round the hypoblast, the
blastopore assumes an oval form, and the mesoblast appears as two bands
forming the sides of the oval. The epiblast travels over the hypoblast
more rapidly than the mesoblast, so that when the blastopore becomes
closed ventrally the mesoblastic bands are still some little way apart on
the ventral side.

In Euaxes the mesoblast originates in a manner which is very
similar to that in some of the Gasteropoda, e.g. Nassa, vide p. 234,
and Vermes, e.g. Bonellia, etc. As mentioned in the chapter on the

CH^TOPODA. 325

Mollusca the origin of the mesoblast in Planorbis, p. 227, is very similar to
that in Lumbricus.

Hatschek has shewn that in Polygordius the mesoblast arises in funda-
mentally the same way as in the Oligochaeta.

Besides the mesoblast which arises from the mesoblastic bands, there
is evidence of the existence of further mesoblast in the larvae of many
Polychaeta in the form of muscular fibres which traverse the space between
the body wall and the wall of the enteric cavity prior to the formation
of the permanent body cavity. These fibres have already been described
in the embryo of Serpula, and are probably represented by stellate cells
in the cephalic region (pras-oral lobe) of the Oligochaeta. These cells are
probably of the same nature as the amoeboid cells in the larvae of Echino-
dermata, some Mollusca and other types.

The Larval form.

True larval forms are not found in the Oligochaeta where the
development is abbreviated. They occur however in the ma-
jority of the marine Polychseta.

They present a great variety of characters with variously
arranged ciliated bands. Most of these forms can be more or
less satisfactorily derived from a larval form, like that of Serpula
(fig. 139 B) or Polygordius (fig. 142); and the constant recur-
rence of this form amongst the Chsetopoda, combined with the
fact that it presents many points of resemblance to the larval
forms of many Rotifers, Molluscs, and Gephyreans, seems to
point to its being a primitive ancestral form for all these
groups.

The important characters of this larval form are (i) the
division of the body into a large prae-oral lobe and a relatively
small post-oral region containing the greater part of the alimen-
tary tract ; (2) the presence of a curved alimentary canal
divided into stomodaeum (oesophagus), stomach and intestine,
and opening by a ventrally placed mouth, and an anus near the
hind end of the body. To these may be added the frequent
presence of (i) a ganglion at the apex of the prae-oral lobe,
(2) a large cavity between the wall of the gut and the skin,
which is the remnant of the segmentation cavity, and is usually
traversed by muscular strands, of which one connecting the apex
of the prae-oral lobe and the stomach or oesophagus is very
commonly present (fig. 142).

The arrangement of the ciliated bands presents great varia-

326

THE LARVAL FORM.

me.p

nph

tions, though in some instances it is constant through large
groups. In Chaetopods there is a widely distributed prae-oral
ciliated band, which is similarly placed to the ring constantly
found in the larvae of Molluscs, Rotifers, etc. In many of these
forms the band is practically double, the opening of the mouth
being placed between its two component rings (vide fig. 142).
The best introduction to the study of the Chaetopod larval forms
will be the history of the changes of a typical larval form in
becoming converted into the adult.

For this purpose no better form can be selected than the interesting
larva of Polygordius (vide Agassiz, No. 332,
Schneider, No. 352, and Hatschek, No. 339),
which was first discovered by Lovdn, and
believed by him to be the larva of an ordinary
Chaetopod. Its true nature was determined
by Schneider.

At a very young stage the larva has the
form (fig. 142) of a flattened sphere, with a
small conical knob at the posterior ex-
tremity.

At the equator are situated two parallel
ciliated bands1, between which lies the ven-
trally placed mouth (ni). The more conspicu-
ous ciliated band is formed of a double row
of cilia, and is situated in front of the mouth. The thinner ciliated band
behind the mouth appears to be absent in the American species.

The mouth leads into an oesophagus, and this into a globular stomach
(<?/), which is continuous with a rectum terminating
by an anus (an) placed at the hind end of the
posterior conical knob. The whole alimentary
tract is ciliated. In the American form of larva
there is a ring of cilia round the anus, which is
developed at a somewhat later stage in the form
observed by Hatschek.

The position of the ciliated bands and the
alimentary tract enables us to divide the embryo
into three regions : a prae-oral region bounded by
the anterior ciliated band, a gastric region in
which the embryonic stomach is situated, and an
abdominal region formed of the posterior conical

FlG. 142. POLYGORDIUS

LARVA. (After Hatschek.)

m. mouth; sg. supra-ceso-
phageal ganglion ; nph. neph-
ridion ; me.p. mesoblastic band ;
an. anus ; ol. stomach.

FIG. 143. POLYGOR-
DIUS LARVA . ( From Alex .
Agassiz.)

1 These two rings are at first (Hatschek) not quite closed dorsally, calling to mind
the early condition of the Echinoderm larva with a prae-oral and post-oral ciliated

CH^ETOPODA.

327

FlG. 144. POLYGOR-
DIUS LARVA. (From
Alex. Agassiz.)

portion, which by its subsequent elongation gives rise to the whole
segmented portion of the future Polygordius.

At the front end of the prae-oral lobe is situated the early formed supra-
cesophageal ganglion (sg) (first noticed by Agassiz) in connection with
which is a pair of eyes, and a ramified system of nerves. The ganglion is
marked externally by a crown of cilia.

The larval epidermis bears a delicate cuticula, and is separated by a
considerable interval from the walls of the alimentary tract. The space
between the two represents a provisional body cavity, which is eventually
replaced by the permanent body cavity formed between the two layers
of the mesoblast. It is doubtful when the replace-
ment takes place in the head. It probably does so
very early. The mesoblast is present in the usual
form of two bands (me.p] (germinal streaks), which
are anteriorly continued into two muscular bands
which pass through the embryonic body cavity to
the front end of the prae-oral lobe. Another pair of
contractile bands passes from the same region of the
prse-oral lobe to the oesophagus.

There is no trace of the ventral nerve cord. The
most remarkable organ of the larva is a paired excre-
tory organ (npti) discovered by Hatschek. This is a
ciliated canal with at first one and subsequently
several funnel-shaped openings into the body cavity in front and an
external opening behind. It is situated immediately anterior to the lateral
band of mesoblast, and is parallel with, and dorsal
to, the contractile band which passes off from
this. It occupies therefore a position in front
of the segmented region of the adult Polygordius.

The changes by which this peculiar larval form
reaches the adult condition will be easily gathered
from an inspection of figs. 143 — 148. They con-
sist essentially in the elongation of what has been
termed the abdominal region of the body, and the
appearance of a segmentation in the mesoblast ;
the segments being formed from before backwards,
and each fresh segment being interpolated between
the anus-bearing end of the body and the last
segment.

As the hind portion of the body becomes
elongated the stomach extends into it, and gives rise to the mesenteron
of the adult (figs. 143, 144, and 145). For a long time the anterior spherical
dilated portion of the larva remains very large, consisting of a prae-oral
lobe and a post-oral section. The two together may be regarded as con-
stituting the head.

At a comparatively late stage a pair of tentacles arises from the front

FIG. 145. POLYGOR-
DIUS LARVA. (From
Alex. Agassiz.)

328

THE LARVAL FORM.

end of the prae-oral lobe (fig. 146), and finally the head becomes relatively
reduced as compared with the body, and gives rise to the simple head of

FIG. 146. POLYGORDIUS LARVA. (From Alex.
Agassiz.)

the fully formed worm (fig. 148). The two ciliated bands disappear, the
posterior vanishing first. The ciliated band at the hind end of the body
also atrophies ; and just in front of it the ring of wart-like prominences used
by the adult to attach itself becomes developed.

At the sides of the head there is formed a pair of ciliated pits, also
found by Hatschek in the embryo of
Criodrilus, and characteristic of many
Chaetopod larvae, but persistent in the
adult Polygordius, Saccocirrus, Poly-
ophthalmus, etc. They are perhaps the
same structures as the ciliated pits in
Nemertines.

During the external changes above
described, by which the adult form of
Polygordius is reached, a series of in-
ternal changes also takes place which are
for the most part the same as in other
Chaetopoda ; and do not require a detailed
description. The nervous1 and muscular
systems have precisely the normal de-
velopment. The division of the meso-
blast into somites is not externally in-
dicated. The organs most worthy of
notice are the excretory organs.

FIG. 147. POLYGORDIUS LARVA.
(From Alex. Agassiz.)

The essential points in the above development of Polygor-
dius are (i) the gradual elongation and corresponding segmenta-
tion of the post-cephalic part of the body ; and (2) the relative
reduction in size of the prae-oral lobe and its conversion to-
gether with the oral region into the head ; (3) the atrophy of
the ciliated bands. The conversion of the larva into the adult
takes place in fact by the intercalation of a segmented region

1 The structure of the ventral cord in the adult requires further elucidation.

CH^TOPODA.

329

between a large mouth-bearing portion of the primitive body
and a small anus-bearing portion1.

The general mode of development of Chsetopod larvae is
similar to the above except in details, which are however no
doubt often of great importance. The history of the larvae may

FIG. 148. POLYGORDIUS LARVA. (From Alex. Agassiz.)

be conveniently treated under three heads, (i) The form of
the primitive unsegmented larva; (2) the arrangement of the
cilia on the unsegmented larva, and on the larva at later stages ;
(3) the character of the metamorphosis and the development of
the permanent external organs.

A larva similar to the Polygordius larva with a greatly
developed prae-oral lobe is widely distributed amongst the
Annelids.

An almost identical form is that of Nepthys
scolopendroides (Claparede and Metschnikoff, No.
336) ; that of Phyllodoce (fig. 149) is also very
similar, and that of Saccocirrus (Metsch. and Clap.
No. 336, PL XIII. fig. i), a very primitive form most
nearly related to Polygordius, clearly belongs to the
same type. Many other larval forms, such as that
of Spio fuliginosus (Metsch. and Clap. No. 336), Tere-
bella, Nerine, etc., also closely approach this form.

FIG. 149. LARVA OF
PHYLLODOCE. (From
Alex. Agassiz.)

Other really similar forms at first sight
appear very different, but this is mainly
owing to the fact that their prae-oral lobe never attains a
considerable development. Its smallness, though obviously of
no deep morphological significance, at once produces a very
different appearance in a larva.

1 For Semper's view as to the intercalation of segments in the cephalic region,
vide note on p. 333.

330

THE LARVAL FORM.

A good example of a larval form with a small prae-oral lobe is afforded by
Capitella, which is figured by Clap, and Metsch. (No. 336, PI. xvn. fig. 2).
The imperfect development of the prae-oral lobe is also generally character-
istic of the Oligochasta. The persistence of a relatively large pras-oral lobe
for so long a time as in Polygordius is very unusual.

The arrangement of the cilia in Chaetopod larvae has been
employed as a means of classifying them. Although a classifi-
cation so framed has no morphological value, yet the terms
themselves which have been invented are convenient. The
terms most usually employed are Atrochae, Monotrochae,
Telotrochae, Polytrochae, Mesotrochae. The polytrochae
may again be subdivided into Polytrochae proper, Nototrochae,
Gasterotrochae, and Amphitrochae.

The atrochae contain forms (fig. 139) in which the larva is at
first coated by an uniform covering of cilia, which, though it
may subsequently disappear from certain areas, does not break
up into a series of definite bands.

The monotrochae or cephalotrochae are larvae in which only a
single prae-oral ring is developed (fig. 150 B).

In the telotrochae there is
present a prae-oral and a post-
oral, i.e. peri-anal ring (fig. 150
A) ; the latter sometimes hav-
ing the form of a peri-anal
patch.

The polytrochae are seg-
mented larvae with perfect or
imperfect rings of cilia on the
segments of the body — usually
one ring to each segment —
between the two characteristic

FIG. 150. Two CH>ETOPOD LARVAE.

(From Gegenbaur.)

o. mouth ; i. intestine ; a. anus ;
•v. pne-oral ciliated band ; w. peri-anal
ciliated band.

telotrochal rings. When these

rings are complete the larvae

are polytrochae proper, when they are only half rings they are

either nototrochae or gasterotrochae. Sometimes there are both

dorsal and ventral half rings which do not however correspond,

such forms constitute the amphitrochae.

In the mesotrochae one or two rings are present in the middle
of the body, and the characteristic telotrochal rings are absent.

CKLETOPODA. 331

Larvae do not necessarily continue to belong to the same group
at all ages. A larva may commence as a monotrochal form and
then become telotrochal and from this pass into a polytrochal
condition, etc.

The atrochal forms are to be regarded as larvae which never
pass beyond the primitive stage of uniform ciliation, which in
other instances may precede that of definite rings. They usually
lose their cilia early, as in the cases of Serpula and other larvae
described below.

The atrochal larvae are not common. The following history of an
Eunicidan larva (probably Lumbriconereis) from Claparede and Metschni-
koff (No. 336) will illustrate their general history.

In the earliest stage noticed the larva has a spherical form, the prae-oral
lobe not being very well marked. In the interior is a globular digestive
tract. The cilia form a broad central band leaving free a narrow space at
the apex of the prae-oral lobe, and also a circumanal space. At the apex of
the pras-oral lobe is placed a bunch of long cilia, and a patch of cilia also
marks out the anal area.

As the larva grows older it becomes elongated, and the anterior bunch of
cilia is absorbed. The alimentary canal divides itself into pharynx and
intestine. The former opens (?) by the mouth in the middle of the central
band of cilia, the latter in the anal patch. The setae indicating the segmen-
tation are formed successively in the posterior ring-like area free from cilia.
The cilia disappear after the formation of two segments.

In Lumbricus, the embryo of which ought perhaps to be grouped with
the atrochae, the cilia (Kleinenberg) cover a ventral tract of epiblast between
the two mesoblastic cords, and are continued anteriorly to form a circle
round the mouth.

The monotrochal larvae are provided only with the important
prae-oral ciliated ring before mentioned. In the majority of
cases they are transitional forms destined very shortly to become
telotrochal, and in such instances they usually have a more or
less spherical body which is nearly divided into two equal halves
by a ciliated ring. In some few instances, such as Polynoe,
Dasychone, etc., the monotrochal characters are not lost till the
larval cilia are exuviated.

The telotrochal forms (of which examples are shewn in figs.
144, 150, etc.) may (i) start as monotrochal; or (2) from the
first have a telotrochal character ; or (3) be derived from atrochal
forms. The last mode of origin probably represents the ances-
tral one.

332 LARVAL FORMS.

Their mode of development is well illustrated by the case of Terebella
nebulosa (vide Milne-Edwards, No. 347). The embryo is at first a nearly
spherical ciliated mass. One end slightly elongates and becomes free from
cilia, and, acquiring dorsally two eye-spots, constitutes a prse-oral lobe.
The elongation continues at the opposite end, and near this is formed a
narrow area free from cilia. The larva now has the same characters as the
atrochal Eunicidan larva described above. It consists of a non-ciliated
prae-oral lobe, followed by a wide ciliated band, behind which is a ring-like
area free from cilia ; and behind this again a peri-anal patch of cilia. The
ring-like area free from cilia is, as in the Eunicidan larva, the region which
becomes segmented. It soon becomes longer, and is then divided into two
segments ; a third and fourth etc. non-ciliated segment becomes succes-
sively interposed immediately in front of the peri-anal patch ; and, after
a certain number of segments have become formed, there appear on some of
the hinder of them short tubercles, provided with single setae (the notopodia),
which are formed from before backwards, like the segments.

The mouth, anus, and intestine become in the meantime clearly visible.
The mouth is on the posterior side of the ciliated band, and the anus in the
centre of the peri-anal patch.

The ciliated band in front now becomes contracted and provided with
long cilia. It passes below completely in front of the mouth, and constitutes,
in fact, a well-marked pras-oral ring, while the cilia behind constitute an
equally marked peri-anal ring. The larva has in fact now acquired all the
characters of a true telotrochal form.

Only a comparatively small number of Chsetopod larvae
remain permanently telotrochal. Of these Terebella nebulosa,
already cited (though not Terebella conchilega), is one ; Poly-
gordius, Saccocirrus and Capitella are other examples of the
same, though in the latter form the whole ventral surface
becomes ciliated.

The majority of the originally telotrochal forms become
polytrochal.

In most cases the ciliated rings or half rings of the polytro-
chal forms are placed at equal distances, one for each segment.
They are especially prominent in surface-swimming larvae, and
are in rare cases preserved in the adult. In some instances
(e.g. Nerine and Spio) the ventral half rings, instead of being
segmentally arranged, are somewhat irregularly distributed
amongst the segments, so that there does not seem to be a
necessary correspondence between the ciliated rings and the
segments. This is further shewn by the fact that the ciliated
rings are not precursors of the true segmentation, but are

CH^TOPODA. 333

developed after the establishment of the segments, and thus
seem rather to be secondarily adapted to the segments than
primarily indicative of them.

In most Polytrochae the rings are incomplete, so that they
fall under the category of Nototrochae or Gasterotrochae.

The larva of Odontosyllis is an example of the former, and that of
Magelona of the latter. The larvae of Nerine and Spio, already quoted as
examples of an unsegmented arrangement of the ventral ciliated half rings,
are both amphitrochal forms.

As an example of a polytrochal form with complete ciliated rings Oph-
ryotrocha puerilis may be cited. This form, discovered by Claparede and
Metschnikoff, develops a complete ciliated ring on each segment : and the
prae-oral ring, though at first single, becomes at a later period divided into
two. This form is further exceptional in that the ciliated rings are persistent
in the adult.

The unimportance of the character of the rings in the polytrochal forms
is shewn by such facts as the absence of these rings in Terebella nebulosa
and the presence of dorsal half rings in Terebella conchilega.

The mesotrochal forms are the rarest of Chaetopod larvae,
and would seem to be confined to the Chaetopteridae.

Their most striking character is the presence of one or two complete
ciliated rings which girth the body between the mouth and anus. The
whole body is further covered with short cilia. The anus has a distinct
dorsal situation, while on its ventral side there projects backwards a peculiar
papilla.

The total absence of the typical prae-oral and of the peri-anal
bands separates the mesotrochal larvae very sharply from all the
previous types.

A characteristic of many Chaetopod larvae is the presence of
a bunch of cilia or a single flagellum at the apex of the prae-oral
lobe. The presence of such a structure is characteristic of the
larval forms of many other groups, Turbellarians, Nemertines,
Molluscs, etc.

In the preceding section the mode of multiplication of the
segments has already been sufficiently described1.

1 It has been insisted by Semper (No. 355) that certain of the anterior segments,
belonging to what he regards as the head region in opposition to the trunk, become
interpolated between the trunk and the head. The general evidence, founded on ob-
servations of budding, which he brings forward, cannot be discussed here. But the
special instance which he cites (founded on Milne-Edwards's (No. 347) observations)

334

LARVAL FORMS.

FIG. 151. LARVA OF
PHYLLODOCE FROM THE

VENTRAL SIDE. (From

Alex. Agassiz.)

Apart from the formation of the segments the larval meta-
morphosis consists in the atrophy of the
provisional ciliated rings and other provi-
sional organs, and in the acquirement of
the organs of the adult.

The great variations in the nature of the
Chaetopod appendages render it impossible
to treat this part of the developmental
history of the Chaetopoda in a systematic
way.

The mode of development of the append-
ages is not constant, so that it is difficult to
draw conclusions as to the primitive form
from which the existing types of appendages
are derived.

In a large number of cases the primitive rudiments of the
feet exhibit no indication of a division into notopodium and
neuropodium ; while in other instances (e.g. Terebella and Nerine,
fig. 152) the notopodium is first
developed, and subsequently the
neuropodium quite independently.

In many cases the setae appear
before there are any other visible
rudiments of the feet (e.g. Lumbri-
conereis) ; while in other cases the
reverse holds good. The gills arc
usually the last parts to appear.

Not only does the mode of
development of the feet differ
greatly in different types, but also the period. The appearance
of setae may afford the first external indication of segmentation,
or the rudiments of the feet may not appear till a large number
of segments are definitely established.

A very considerable number of Chaetopod larvae are provided
with very long provisional setae (figs. 152 and 153). These setae

of the interpolation of the head segments, bearing the gills, in Terebella appears to
me quite unjustified from Milne-Edwards's own statements ; and is clearly shewn to
be unfounded by the careful observations of Claparede on Ter. conchilega, where the
segments in question are demonstrated to be present from the first.

FIG. 152. LARVA OF NERINE,

WITH PROVISIONAL SET>E. (From

Alex. Agassiz.)

CH^iTOPODA.

335

are usually placed at the sides of the anterior part of the body,
immediately behind the head, and also sometimes on the
posterior parts of the body. In some instances (e.g. fig. 153)

FIG. 153. EMRRYO CH^ETOPOU WITH PROVISIONAL SEIVE. (From Agassiz.)

they form the only appendages of the trunk. Alex. Agassiz .has
pointed out that setae of this kind, though not found in existing
Chaetopods, are characteristic of the fossil forms. Setae of this
kind are found in chaetopod-like larvae of some Brachiopods
(Argiope, fig. 136).

It is tempting to suppose that the long provisional bristles
springing from the oral region are the setiform appendages
handed down from the unsegmented ancestors of the existing
Chaetopod forms. Claparede has divided Chaetopod larvae into
two great groups of Metachaetae and Perennichaetae, according as
they possess or are without provisional setae.

With reference to the head and its appendages it has already
been stated that the head is primarily formed of the prae-oral
lobe and of the peristomial region.

The embryological facts are opposed to the view that the
prae-oral region either represents a segment or is composed of
segments equivalent to those of the trunk. The embryonic
peristomial region may, on the other hand, be regarded as in a
certain sense the first segment. Its exact relations to the
succeeding segments become frequently more or less modified in
the adult. The prae-oral region is in most larvae bounded
behind by the ciliated ring already described. On the dorsal
part of the prae-oral lobe in front of this ring are placed the
eyes, and from it there may spring a variable number of
processes which form antennae or cephalic tentacles. The
number and position of these latter are very variable. They
appear as simple processes, sometimes arising in pairs, and at

336 LARVAL FORMS.

other times alternating on the two sides. There is frequently a
median unpaired tentacle.

The development of the median tentacle in Terebella, where there is in
the adult a great number of similar tentacles, is sufficiently remarkable to
deserve special notice ; vide Milne-Edwards, Claparede, etc. It arises long
before any of the other tentacles as a single anterior prolongation of the
prae-oral lobe containing a parenchymatous cavity, which communicates
freely with the general perivisceral cavity. It soon becomes partially con-
stricted off at its base from the procephalic lobe, but continues to grow
till it becomes fully half as long as the remainder of the body. A very
characteristic figure of the larva at this stage is given by Claparede and
Metschnikoff, PI. XVII., Fig. I E. It now strikingly resembles the larval
proboscis of Balanoglossus, and it is not easy to avoid the conclusion that
they are homologous structures.

Another peculiar cephalic structure which deserves notice is the gill
apparatus of the Serpulidae.

In Dasychone (Sabella) the gill apparatus arises (Claparede and
Metschnikoff, No. 336) as a pair of membranous wing-like organs on the
dorsal side of the prae-oral lobe immediately in front of the ciliated ring.
Each subsequently becomes divided into
two rays, and new rays then begin to sprout
on the ventral side of the two pairs already
present. A cartilaginous axis soon becomes
formed in these rays, and after this is
formed fresh rays sprout irregularly from
the cartilaginous skeleton.

In Spirorbis spirillum as observed by
Alex. Agassiz, the right gill-tentacle (fig.
154, /) first appears, and then the left, and
subsequently the odd opercular tentacle
which covers the right original tentacle. FIG. 154. LARVA OF SPIROR-

The third and fourth tentacles are formed BIS. (From Alex. Agassiz.)
successively on the two sides, and rapidly The first odd tentacle (t) is shewn

become branched in the succeeding stages. on J\?eht,side-

Behind the prse-oral ciliated ring

With reference to the sense

organs it may be noted that the eyes, or at any rate the cephalic
pigment spots, are generally more numerous in the embryo than
in the adult, and that they are usually present in the larvae of
the Sedentaria, though absent in the adults of these forms. The
Sedentaria thus pass through a larval stage in which they
resemble the Errantia.

Paired auditory vesicles of a provisional character have been
found on the ventral side of the body, in the fourth segment

CH^LTOPODA.

337

behind the mouth, in the larva of Terebella conchilega
(Claparede).

Mitraria. A peculiar larval Ch?etopod form known as Mitraria, the
metamorphosis of which was first worked out by Metschnikoff, deserves a
special notice.

This form (fig. 155 A) in spite of its remarkable appearance can easily be
reduced to the normal type of larva.

The mouth (m) and anus (an) (fig. 155 A) are closely approximated, and
situated within a vestibule the edge of which is lined by a simple or lobed
ciliated ring. The shape of the body is somewhat conical. The cavity of

FlG. 155. TWO STAGES IN THE DEVELOPMENT OF MlTRARIA.

(After Metschnikoff.)

m. mouth; an. anus; sg. supra-oesophageal ganglion; br. provisional bristles;
pr.b. prse-oral ciliated band.

the vestibule forms the base of the cone, and at the apex is placed a ciliated
patch (sg). A pair of lobes (br) bear provisional setae. The alimentary
canal is formed of the three normal parts, oesophagus, stomach, and
intestine.

To compare this larva with an ordinary Chsetopod larva one must
suppose that the alimentary canal is abnormally bent, so that the post-oral
ventral surface is reduced to the small space between the mouth and the
anus. The ciliated band surrounding the vestibule is merely the usual
prae-oral band, borne on the very much extended edge of the pras-oral lobe.
The apex of the larva is the front end of the pras-oral lobe with the
usual ciliated patch. The two lobes with provisional bristles are really
dorsal and not posterior.

B. II.

22

338 FORMATION OF ORGANS.

The correctness of the above interpretation is clearly shewn by the
metamorphosis.

The first change consists in the pushing in of a fold of skin, between the
mouth and anus, towards the intestine, which at the same time rapidly
elongates, and forms the axis of a conical projection, which thereupon
becomes segmented and is thereby shewn to be the rudiment of the trunk
(fig. 155 B). On the elongation of the trunk in this way the prae-oral lobe
and its ciliated ring assume an appearance not very dissimilar to the same
structures in Polygordius. At the ciliated apex of the prae-oral lobe a paired
thickening of epiblast gives rise to the supra-cesophageal ganglia (sg). In
the further metamorphosis, the prae-oral lobe and its ciliated ring gradually
become reduced, and finally atrophy in the normal way, while the trunk
elongates and acquires setae. The dorsally situated processes with provisional
setae last for some time, but finally disappear. The young worm then
develops a tube and shews itself as a normal tubicolous Chaetopod.

Formation of Organs.

Except in the case of a few organs our knowledge of the
formation of the organs in the Chaetopoda is derived from
investigations on the Oligochaeta.

The embryo of the Oligochaeta. has a more or less spherical
form, but it soon elongates, and becoming segmented acquires a
distinct vermiform character. The ventral surface is however
for a considerable time markedly convex as compared to the
dorsal.

The ventrally placed mouth is surrounded by a well-marked
lip, and in front of it is placed a small prae-oral lobe.

The epiblast. The epiblast cells at the commencement of
the gastrula stage become much flattened, and on the comple-
tion of the invagination form an invest-
ment of flattened cells, only thickened in
the neighbourhood of the mesoblastic
bands (fig. 141 B and C). In the Poly-
chaeta at any rate the statements of
several investigators would seem to in-

dicate that the cuticle is derived from the FJG i 6 SFCTION
chorion. It is difficult to accept this THROUGH THE HEAD OF A

conclusion, but it deserves further in-

VCStigation. Kleinenberg. )

Nervous system. The most im- '-&• cephalic ganglion;

, .. cc. cephalic portion of the

portant organ derived from the epiblast body cavity ; x. cesophagus.

CH^TOPODA.

339

FIG. 157. SECTION THROUGH PART

OF THE VENTRAL WALL OF THE
TRUNK OF AN EMBRYO OF LUMBRI-

is the nervous system ; the origin of which from this layer was
first established by Kowalevsky (No. 342).

It arises1 (Kleinenberg, No. 341) from two at first quite
distinct structures, viz. (i) the supra-cesophageal rudiment
and (2) the rudiment of the ventral cord. The former of
these takes its origin as an unpaired dorsal thickening of
the epiblast at the front end of the head (fig. 1 56, e.g.], which
sends two prolongations downwards and backwards to meet the
ventral cord. The latter arises as two independent thickenings
of the epiblast, one on each side
of the ventral furrow (fig. 157, Vg].
These soon unite underneath the
furrow, in the median line, and
after being differentiated into seg-
mentally arranged ganglionic and
interganglionic regions become
separated from the epiblast. Both
the supra-cesophageal and ven-
tral cord become surrounded by a £us TRAPEZOIDES. (After Kleinen-

berg.)

layer of somatic mesoblast. The Mm longitudinal muscles. S0m so.

junction between the tWO parts of matic mesoblast ; sp. splanchnic me-
!« i . ' i soblast; hy. hypoblast ; Vg. ventral

the central nervous System takes nerve cord ; w. ventral vessel.

place comparatively late.

The mesoblast. It is to Kowalevsky (No. 342) and Klein-
enberg (No. 341) that we mainly owe our knowledge of the history
of the mesoblast. The fundamental processes which take place are
(i) the splitting of the mesoblast into splanchnic and somatic
layers with the body cavity between them, (2) the transverse
division of the mesoblast of the trunk into distinct somites.

The former process commences in the cephalic mesoblastic
commissure, where it results in the formation of a pair of cavities
each with a thin somatic and thick splanchnic layer (fig. 156,
cc) ; and thence extends gradually backwards into the trunk
(fig. 141 C, //). In the trunk however the division into somites
precedes the horizontal splitting of the mesoblast. The former
process commences when the mesoblastic bands form widish
columns quite separate from each other. These columns become

1 For further details, vide general chapter on Nervous System.

22 — 2

340 FORMATION OF ORGANS.

broken up successively from before backwards into somewhat
cubical bodies, in the centre of which a cavity soon appears.
The cavity in each somite is obviously bounded by four walls,
(i) an outer, the somatic, which is the thickest; (2) an inner, the
splanchnic ; and (3, 4) an anterior and posterior. The adjoining
anterior and posterior walls of successive somites unite together
to form the transverse dissepiments of the adult, which subse-
quently become very thin and are perforated in numerous places,
thus placing in communication the separate compartments
of the body cavity. The somites, though at first confined to a
small area on the ventral side, gradually extend so as to meet
their fellows above and below and form complete rings (fig. 157)
of which the splanchnic layer (sp) attaches itself to the enteric
wall and the somatic (so) to the epiblast. In Polygordius and
probably also Saccocirrus and other forms the cavities of the
somites of the two sides do not coalesce ; and the walls which
separate them constitute dorsal and ventral mesenteries. The
two cavities in the cephalic commissure unite dorsally, but
ventrally open into the first somite of the trunk.

The mesoblastic masses of the head are probably not to be regarded as
forming a pair of somites equivalent to those in the trunk, but as forming
the mesoblastic part of the pras-oral lobe, of which so much has been said in
the preceding pages. Kleinenberg's observations are however of great im-
portance as shewing that the cephalic cavities are simply an anterior part of
the true body cavity.

The splanchnic layer of the head cavity gives rise to the
musculature of the oesophagus.

The somatic layer of the trunk somites becomes converted
into the musculature of the body wall and the external peri-
toneal layer of body cavity. The first part of the muscular
system to be definitely formed is the ventral band of longitudi-
nal muscles which arises on each side of the nervous system in
contact with the epidermis (fig. 157, m). How the circular
muscles become subsequently formed outside these muscles has
not been made out.

The splanchnic layer of the trunk somites gives rise to the
muscular and connective-tissue wall of the mcscntcron, and also
to the walls of the vascular trunks. The ventral vessel is first
formed (Kowalevsky) as a solid mass of cells which subsequently

CILKTOPODA. 34!

becomes hollowed out. The dorsal vessel in Lumbricus and
Criodrilus is stated by Kowalevsky and Vejdovsky to be formed
by the coalescence of two lateral vessels ; a peculiarity which is
probably to be explained by the late extension of the mesoblast
into the dorsal region.

The layer from which the sacks for the setae and the
segmental organs spring is still doubtful. The sacks for the setae
are believed by Kowalevsky (No. 342) to be epiblastic invagina-
tions, but are stated by Hatschek (No. 339) to be mesoblastic
products. For the development of the segmental organs the
reader is referred to the chapter on the excretory system.

In marine Polychaeta the generative organs are no doubt
mesoblastic products, as they usually spring from the peritoneal
epithelium, especially the parts of it covering the vascular
trunks.

The Alimentary Canal, In Lumbricus the enteric cavity
is formed during the gastrula stage. In Criodrilus the hypoblast
has at first no lumen, but this becomes very soon established.
In Euaxes on the other hand, where there is a true epibolic
gastrula, the mesenteron is at first represented by a solid mass
of yolk (i.e. hypoblastj cells. As the central amongst these
become absorbed a cavity is formed. The protoplasm of the
yolk cells which line this cavity unites into a continuous polynu-
clear layer containing at intervals masses of yolk. These masses
become gradually absorbed, and the protoplasmic wall of the
mesenteron then breaks up into a cylindrical glandular epithelium
similar to that of the other types.

In Lumbricus and Criodrilus the blastopore remains as the
mouth, but in Euaxes a new mouth or rather stomodaeum is
formed by an epiblastic invagination between the front end of
the two mesoblastic bands. This epiblastic invagination forms
the permanent oesophagus; and in Lumbricus trapezoides and
Criodrilus, where the oral opening is at first lined by hypoblast,
the epiblast soon becomes inflected so as to line the cesophageal
region. The splanchnic mesoblast of the cephalic region subse-
quently invests the oesophagus, and some of its cells penetrating
between the adjoining epiblast cells give rise to a thick wall for
this part of the alimentary tract ; the original epiblast cells being
reduced to a thin membrane. This mesoblastic wall is sharply

342 ALTERNATIONS OF GENERATIONS.

separated from the muscular wall outside, which is also formed
of splanchnic mesoblast.

The anus is a late formation.

Alternations of generations.

Amongst Chaetopoda a considerable number of forms exhibit
the phenomenon of alternations of generations, which in the
same general way as in the case of the Ccelenterata, is second-
arily caused by budding or fission.

The process of fission essentially consists in the division of a
parent form into two zooids by the formation of a zone of fission
between two old rings, which becomes differentiated (i) into an
anal zone in front which forms the anal region of the anterior
zooid, and (2) into a cephalic zone behind which forms the head
and some of the succeeding segments of the posterior zooid.
The anal zone is capable, by growth and successive segmenta-
tion, of giving rise to an indefinite number of fresh segments.

In Protula Dysteri, as shewn by Huxley, there is a simple
fission into two in the way described. Sexual reproduction does
not take place at the same time as reproduction by fission,
but both zooids produced are quite similar and multiply
sexually.

In the freshwater forms Nais and Chaetogaster a more or
less similar phenomenon takes place. By a continual process of
growth in the anal zones, and the formation of fresh zones of
fission whenever four or five segments are added in front of an
anal zone, complicated chains of adhering zooids are produced,
each with a small number of segments. As long as the process
of fission continues sexual products are not developed, but even-
tually the chains break up, the individuals derived from them
cease to go on budding, and, after developing a considerably
greater number of segments than in the asexual state, reproduce
themselves sexually. The forms developed from the ovum then
repeat again the phenomenon of budding, etc., and so the cycle
is continued1.

The phenomena so far can hardly be described as cases of

1 Reproduction by budding and formation of the sexual products to some extent
overlap.

CH/ETOPODA. 343

alternation of generations. The process is however in certain
types further differentiated. In Syllis (Quatrefages) fission
takes place, the parent form dividing into two, of which only the
posterior after its detachment develops sexual organs. The
anterior asexual zooid continues to produce fresh sexual zooids
by fission. In Myrianida also, where a chain of zooids is formed,
the sexual elements seem to be confined to the individuals
produced by budding.

The cases of Syllis and Myrianida seem to be genuine
examples of alternations of generations, but a still better
instance is afforded by Autolytus (Krohn, No. 343, and Agassiz,
No. 333).

In Autolytus cornutus the parent stock, produced directly
from the egg, acquires about 40 — 45 segments, and then gives
rise by fission, with the production of a zone of fission between
about the I3th and I4th rings, to a fresh zooid behind. This
after becoming fully developed into either a male or a female is
detached from the parent stock, from which it very markedly
differs. The males and females are moreover very different from
each other. In the female zooid the eggs are carried into a
kind of pouch where they undergo their development and give
rise to asexual parent stocks. After the young are hatched the
female dies. The asexual stock, after budding off one asexual
zooid, elongates again and buds off a second zooid. It never
develops generative organs.

The life history of some species of the genus Nereis presents certain very
striking peculiarities which have not yet been completely elucidated.

As was first shewn by Malmgren asexual examples of various species of
Nereis may acquire the characters of Heteronereis and become sexually
mature.

The metamorphosis of Nereis Dumerilii has been investigated by
Claparede, who has arrived at certain very remarkable conclusions. He
finds that there are two distinct sexual generations of the Nereis form of
this species, and two distinct sexual generations of the Heteronereis form.

One sexual Nereis, characterized by its small size, is dioecious, the other
discovered by Metschnikoff is hermaphrodite.

Of the Heteronereis sexual forms, both are dioecious, one is small, and
swims on the surface, the other is larger and lives at the bottom.

How these various generations are mutually related has not been made
out ; but Claparede traced the passage of large asexual examples of the
Nereis form into the large Heteronereis form.

344 CH^iTOPODA.

BIBLIOGRAPHY.

(332) Alex. Agassiz. "On the young stages of a few Annelid.^." Annuls
Lyceum Nat. Hist, of New York, Vol. vin. 1866.

(333) Alex. Agassiz. " On the embr}'ology of Autolytus cornutus and alterna-
tions of generations, etc." Boston Journal of Nat. History, Vol. VII. 1859 — 63.

(334) W. Busch. Beobachtnngcn ii. Anat. u. Entwick. einiger wirbel loser See-
thiere, 1851.

(335) Ed. Claparede. Beobachlungen ii. Anat. it. Entwick. wirbelloser Thiere
an d. Kiiste von Normandie. Leipzig, 1863.

(336) Ed. Claparede u. E. Metschnikoff. " Beitrage z. Kenntniss iib. Ent-
wicklungsgeschichte d. Chrctopoden." Zeit.f. wt'ss. Zool. Vol. xix. 1869.

(337) E. Grube. Untersuchungen iib. Entwicklung d. Anneliden. Konigsberg,
1844-

(338) B. Hatschek. "Beitrage z. Entwick. u. Morphol. d. Anneliden." Sitz.
d. k. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(339) B. Hatschek. " Studien iiber Entwicklungsgeschichte der Anneliden."
Arbeiten aus d. zoologischen Institute d. Universitdt Wien. Von C. Claus. Heft III.
1878.

(340) Th. H. Huxley. "On hermaphrodite and fissiparous species of tubicolar
Annelidae (Protula)." Edinburgh New Phil. Journal, Vol. I. 1855.

(341) N. Kleinenberg. "The development of the earthworm Lumbricus tra-
pezoides." Quart. J. of Micr. Science, Vol. Xix. 1879. Sullo sviluppo del Lumbri-
cus trapezoides. Napoli, 1878.

(342) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropo-
den." Mem. Acad. Petersbourg, Series VII. Vol. XVI. 1871.

(343) A. Krohn. " Ueber die Erscheinungen bei d. Fortpflanzung von Syllis
prolifera u. Autolytus prolifer." Archiv f. Naturgesch. 1852.

(344) R. Leuckart. " Ueb. d. Jugendzustande ein. Anneliden, etc." Archiv
f. Naturgesch. 1855.

(345) S. Loven. " Beobachtungen u. die Metamorphose von Anneliden."
Weigmann's Archiv, 1842.

(346) E. Metschnikoff. " Ueber die Metamorphose einiger Seethiere (Mitra-
ria)." Zeit.f. wiss. Zool. Vol. xxi. 1871.

(347) M. Milne-Edwards. " Recherches zoologiques, etc." Ann. Scie.
Natttr. in. Serie, Vol. in. 1845.

(348) J. M tiller. " Ueb. d. Jugendzustande einiger Seethiere." Monats. d.
k. Akad. Wiss. Berlin, 1851.

(349) Max Muller. "Ueber d. weit. Entwick. von Mesotrocha sexoculata."
Muller's Archiv, 1855.

(350) Quatrefages. " Me"moire s. 1'embryogenie des Annelides." Ann. Scie.
Natur. in. Serie, Vol. x. 1848.

(351) M. Sars. "Zur Entwicklung d. Anneliden." A re hiv f. Naturgeschichte,
Vol. xi. 1845.

(352) A. Schneider. "Ueber Bau u. Entwicklung von Polygordius." Muller's
Archiv, 1868.

(353) A.Schneider. " Entwicklung u. system. Stell. d. Bryozoen u. Gephy-
reen (Mitraria)." Archiv f. mikr. Anat. Vol. v. 1869.

CHyfcTOPODA. 345

(354) M. Schultze. Ueb. die Entwicklitng von Arenicola piscatorum u. anderer
Kiemenwiirmer . Halle, 1856.

(355) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
Arbeiten a. d. zool.-zoot. Instit. Wurzburg, Vol. in. 1876-7.

(356) C. Semper. " Beitrage z. Biologic d. Oligochseten." Arbeiten a. d. zool.-
zoot. Instit. Wurzburg, Vol. IV. 1877-8.

(357) M. Stossich. "Beitrage zur Entwicklung d. Chaetopoden." Sitz. d. k.
k. Akad. Whs. Wien, B. LXXVII. 1878.

(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.

CHAPTER XIII.

DISCOPHORA1.

THE eggs of the Discophora, each enclosed in a delicate
membrane, are enveloped in a kind of mucous case formed by a
secretion of the integument, which hardens into a capsule or
cocoon. In each cocoon there are a limited number of eggs
surrounded by albumen. The cocoons are attached to water-
plants, etc. In Clepsine the embryos leave the cocoon very
soon after they get rid of the egg membrane, but in Nephelis
they remain within the cocoon for a very much longer period
(27 — 28 days after hatching). The young of Clepsine, after
their liberation, attach themselves to the ventral surface of their
parent.

Our knowledge of the development of the Discophora is in a
very unsatisfactory state ; but sufficient is known to shew that it
has very many points in common with that of the Oligochaeta,
and that the Discophora are therefore closely related to the
Chaetopoda. In Clepsine there is an epibolic gastrula, and
mesoblastic bands like those in Euaxes are also formed. In
Nephelis however the segmentation is very abnormal, and the
formation of the germinal layers cannot easily be reduced to an
invaginate gastrula type, though probably it is modified from
such a type. Mesoblastic bands similar to those in the Oligo-
chaeta occur in this form also.

The embryology of Clepsine, which will serve as type for the
Leeches without jaws (Rhyncobdellidae), has recently been
studied by Whitman (No. 365), and that of Nephelis, which will

1 The Discophora are divided into the following groups.
I. Rhyncobdellidse.
II. Gnathobdellidae.
III. Branchiobdellidffi.

DISCOPHORA.

347

serve as type for the Leeches with jaws (Gnathobdellidae), has
been studied by Butschli (No. 359). The early history of both
types is imperfectly known1.

Formation of the layers.

Clepsine. It is necessary to give a full account of the segmentation
of Clepsine, as the formation of the germinal layers would be otherwise
unintelligible.

Segmentation commences with the division of the ovum into two unequal
spheres by a vertical cleavage passing from the animal to the vegetative
pole. By a second vertical cleavage the large segment is divided into two
unequal parts, and the small one into two equal parts. Of the four segments
so produced three are relatively small, and one, placed at the posterior end,
is large. Each of the four segments next gives rise to a small cell at the
animal pole. These small cells form the commencement of the epiblast,
and, according to Whitman, the mouth is eventually placed in their centre.
Such a position for the mouth, at the animal pole, is extremely unusual, and
the statements on this head require further confirmation.

The posterior large segment now divides into two, one of which is dorsal,
and the other and larger ven-
tral. The former I shall call
with Whitman the neuroblast,
and the latter the mesoblast.
The mesoblast very shortly
divides again. During the for-
mation of the neuroblast and
mesoblast additional epiblastic
small cells are added from the
three spheres which give rise
to three of the primitive epi-
blast cells, which may now be
called the vitelline spheres.

The neuroblast next divides
into ten cells, of which the two
smaller are soon broken up
into epiblastic cells, while the
remaining eight arrange them-
selves in two groups of four
each, one group on each side
at the posterior border of the epiblastic cap. The two mesoblasts also take up
a position on the right and left sides immediately ventral to the four neuro-
blasts of each side. The neuroblasts and mesoblasts now commence to

FlG. 158. TWO VIEWS OF THE LARVA OF

CLEPSINE. (After Whitman.)

o. oral extremity ; m mouth ; pr. germinal
streak.

A. This figure shews the blastoderm (shaded)
with a thickened edge formed by the primitive
(i.e. mesoblastic) streaks with the four so-called
neuroblasts posteriorly. The vitelline spheres
are left without shading.

B. represents an embryo in which the blas-
toderm has enclosed the yolk, and in which the
division into segments has taken place. At the
hind end are shewn the so-called neuroblasts
forming the termination of the germinal streak.

1 Hoffmann's account (No. 36) is so different from that of other observers that
I have been unable to make any use of it.

348 CLEPSINE.

proliferate at their anterior border, and produce on each side a thickened
band of cells underneath the edge of the cap of epiblast cells. Each of these
bands is formed of a superficial quadruple1 row of neuroblasts budded off
from the four primary neuroblasts, and a deeper row of mesoblasts. The
compound streaks so formed may be called the germinal streaks.

The general appearance of the embryo as seen from the dorsal surface,
after the appearance of the two germinal streaks, may be gathered from
fig. 158 A. The epiblastic cap in this figure is shaded. The epiblastic cap,
accompanied by the germinal streaks, now rapidly extends and encloses the
three vitelline spheres by a process equivalent to that of an ordinary epibolic
gastrula; but the front and hind ends of the streaks remain practically
stationary. Owing to this mode of growth the edges of the epiblastic cap
and the germinal streaks meet in a linear fashion along the ventral surface
of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly
(B) and their junction is then gradually continued backwards. The process
is completed at about the time of hatching.

During the above changes the nuclei of the vitelline spheres pass to the
surface and rapidly divide. Eventually, together with part of the protoplasm
of the vitelline spheres, they appear to give rise to a layer of hypoblastic
cells. This layer encloses the remains of the vitelline spheres, which
become the yolk.

At the front end of the germinal streaks, in a position corresponding with that
of the four original epiblast cells,
two depressions appear which
coalesce to form the single oral
invagination ; in the centre of
which are formed the mouth and
pharynx by a second epiblastic
invagination.

The most important point in FIG. 159. Two EMBRYOS OF CLEPSINE IN

connection with the above history WHICH THE GERMINAL STREAKS HAVE PARTI-
is the fate of what have been ALLY^MET ALONG THE VENTRAL LINE. (After

called the germinal streaks. Ac- ^ germinal, i.e. mesoblastic streaks,

cording to Whitman they are The area covered by epiblast is shaded.

composed of two kinds of cells, The so-called neuroblasts at the end of the
viz. four rows of smaller super- germinal streaks are shewn in B.
ficial cells, which he calls neuroblasts, and, in the later stages at any rate, a
row of deeper large cells, which he calls mesoblasts. As to the eventual fate
of these cells he states that the neuroblasts uniting together in the median
line form the rudiment of the ventral ganglionic chain, while the mesoblasts
equally coalesce and give rise to the mesoblast. Such a mode of origin for a
ventral ganglionic chain is, so far as I know, without a parallel in the whole
animal kingdom ; and whatever evidence Whitman may have that the cells

1 According to Robin it is more usual for there to be only a triple row of primary
neuroblasts.

DISCOPHORA. 349

in question really do give rise to the nervous system he has not thought fit to
produce it in his paper. He figures a section with the eight neuroblastic cells
in the middle ventral line, and in the next stage described the nervous
system is divided up into ganglia ! The first stage, in which the so-called
nervous system has the form of a single row of eight cells, is quite unlike
any rudiment of the nervous system such as is usually met with in the
Chaetopoda, and not a single stage between this and a ganglionated cord is
described or figured. Whitman, whose views seem to have been influenced
by a peculiar, and in my opinion erroneous, theory of Rauber's about the
relation of the neural groove of Vertebrata to the blastopore, does not seem
to be aware that his determination of the fate of his neuroblasts requires any
special support.

He quotes the formation of these parts in Euaxes (vide preceding
Chapter, p. 324) as similar to that in Clepsine. In this comparison it
appears to me probable that he may be quite correct, but the result of the
comparison would be to shew that the neuroblasts and mesoblasts composed
together a mesoblastic band similar to that of the Oligochaeta. Till more
evidence is brought forward by Whitman or some other observer in support
of the view that the so-called neuroblasts have any share in forming the
nervous system, they must in my opinion be regarded as probably forming,
in conjunction with the mesoblasts, two simple mesoblastic bands. Kowa-
levsky has moreover briefly stated that he has satisfied himself that the
nervous system in Clepsine originates from the epiblast — a statement which
certainly could not be brought into harmony with Whitman's account.

Nephelis. Nephelis will form my type of the Gnathobdellidae. The
segmentation of this form has not yet been thoroughly investigated, but
Biitschli's (No. 359) observations are probably the most trustworthy.

The ovum first divides into two, and then into four segments of which
two are slightly smaller than the others. Four small cells which form the
commencement of the epiblast are now formed. Three of them are derived
by budding from the two larger and one of the smaller of the four cells,
and the fourth from a subsequent division of one of the larger cells1.
The three cells which assisted in the formation of the epiblast cells again
give rise each to a small cell ; and the small cells so formed constitute a
layer underneath the epiblast which is the commencement of the hypoblast,
while the cells from which they originated form the vitelline spheres.
Shortly after the formation of the hypoblast, the large sphere which has
hitherto been quiescent divides into two, one of which then gives rise
in succession to two small epiblastic elements.

The two large spheres, resulting from the division of the originally
quiescent sphere, next divide again on the opposite side of the embryo,
and form a layer of epiblast there ; so that there is now on one side of
the embryo (the ventral according to Robin) a layer of epiblast formed

1 Doubts have been cast by Whitman on the above account of the origin of the
four epiblast cells.

350 CLEPSINE.

of six cells, and on the opposite side a layer formed of four cells. The
two layers meet at the front border of the embryo and between them are
placed the three large vitelline spheres. The two patches of epiblast cells
now rapidly increase, and gradually spread over the three large vitelline
spheres. Except where they meet -each other at the front edge they leave
uncovered a large part of the margin of the vitelline spheres.

While these changes have been taking place on the exterior, the
hypoblast cells have increased in number (additional cells being probably
derived from the three large vitelline spheres) and fill up in a column-like
fashion a space which is bounded behind by the three vitelline spheres, and
in front by the epiblast of the anterior end of the embryo. At the sides of
the hypoblast the mesoblast has become established, probably as two lateral
bands. The origin of the cells forming it has not yet been determined.
The hypoblast cells in the succeeding stage arrange themselves round a
central archenteric cavity, and at the same time rapidly increase in size
and become filled with a secondary deposit of food-yolk. Shortly after-
wards a mouth and thick-walled oesophagus are formed, probably from an
epiblastic invagination. The mesoblast now forms two curved lateral
bands at the two sides of the body, equivalent to the mesoblastic bands
of the Chaetopoda. The three vitelline spheres, still largely uncovered by
the epiblast, lie at the posterior end of the body. The embryo grows
rapidly, especially anteriorly, and the three vitelline spheres become
covered by a layer of flattened epiblast cells. Around the oesophagus a
cavity traversed by muscular fibres is established. Elsewhere there is no
trace of such a cavity. The cephalic region becomes ciliated, and the
dorsal part of it, which represents a rudimentary prae-oral lobe, is especially
prominent. The cilia of the oral region are continued into the lumen of
the oesophagus, and at a later period are prolonged, as in Lumbricus, along
the median line of the ventral surface.

The mesoblastic bands would seem from Biitschli's observations, which
receive confirmation from Kleinenberg's researches on Lumbricus, to be pro-
longed dorsally to the oesophagus into the cephalic region. Posteriorly they
abut on the large vitelline spheres, which were supposed by Kowalevsky
to give origin to them, and to play the same part as the large meso-
blasts in Lumbricus. It has already been shewn that the function of the
large cells in Lumbricus has been exaggerated, and Biitschli denies to
them in Nephelis any share in the production of the mesoblast. It seems
in fact probable that they are homologous with the three vitelline spheres
of Clepsine ; and that their primitive function is to give origin to the
hypoblast. They are visible for a long time at the hind end of the embryo,
but eventually break up into smaller cells, the fate of which is unknown.

The embryo of Hirudo would appear from the researches of Robin
to develop in nearly the same way as that of Nephelis. The anterior
part is not however ciliated. The three large posterior cells disappear
relatively early.

DISCOPHORA. 351

General history of the larva.

The larva of Clepsine, at the time when the mesoblastic
bands have met along the ventral line, is represented in fig.
158 B. It is seen to be already segmented, the process having
proceeded pari passu with the ventral coalescence of the meso-
blastic bands. The segments are formed from before backwards
as in Chaetopoda. The dorsal surface is flat and short, and the
ventral very convex. The embryo about this time leaves its
capsule, and attaches itself to its parent. It rapidly elongates,
and the dorsal surface, growing more rapidly than the ventral,
becomes at last the more convex. Eventually thirty-three post-
oral segments become formed ; of which the eight last coalesce
to form the posterior sucker.

The general development of the body of Nephelis and
Hirudo is nearly the same as that of Clepsine. The embryo
passes from a spherical to an oval, and then to a vermiform
shape. For full details the reader is referred to Robin's
memoir.

The presence of a well-marked protuberance above the
oesophagus, which forms the rudiment of a prae-oral lobe, has
already been mentioned as characteristic of the embryo of
Nephelis ; no such structure is found in Clepsine.

History of the germinal layers and development of organs.

The epiblast. The epiblast is formed of a single layer of
cells and early develops a delicate cuticle which is clearly formed
quite independently of the egg membrane. It becomes raised
into a series of transverse rings which bear no relation to the
true somites of the mesoblast.

The nervous system. The nervous system is probably
derived from the epiblast, but its origin still requires further
investigation. The ventral cord breaks up into a series of
ganglia, which at first correspond exactly with the somites of
the mesoblast. Of these, four or perhaps three eventually coal-
esce to form the sub-cesophageal ganglion, and seven or eight
become united in the posterior sucker.

It would appear from Biitschli's statements that the supra-

352 NEPHELIS.

cesophageal ganglion arises, as in Oligochaeta, independently of
the ventral cord.

Mesoblast. It has already been indicated that the meso-
blast probably takes its origin both in Nephelis and Clepsine
from the two mesoblastic bands which unite in the median
ventral line. The further history of these bands is only im-
perfectly known. They become segmented from before back-
wards. The somites formed by the segmentation gradually
grow upwards and meet in the dorsal line. Septa are formed
between the somites probably in the same way as in the
Oligochaeta.

In Clepsine the mesoblastic bands are stated by Kowalevsky to be-
come split into somatic and splanchnic layers, between which are placed
the so-called lateral sinuses. These sinuses form, according to Whitman,
a single continuous tube investing the alimentary tract ; a tube which
differs therefore to a very small extent from the normal body cavity of
the Chaetopoda. The somatic layer of mesoblast no doubt gives rise to
the circular and longitudinal muscular layers of the embryo. The former
is stated to appear the earliest, while the latter, as in the Oligochaeta,
first takes its origin on the ventral side.

A delicate musculature, formed mainly of transverse but also of longi-
tudinal fibres, would appear to be developed independently of the meso-
blastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and
Biitschli). It develops apparently from certain stellate cells which are
found between the walls of the alimentary tract and the skin, and which
probably correspond to the system of contractile fibres which pass from
the body wall to the alimentary tract through the segmentation cavity in
the larva of Chaetopoda, various Vermes and Mollusca1.

The mesoblast, so far as is known, gives rise, in addition to
the parts already mentioned, to the excretory organs, generative
organs, vascular system, etc.

Excretory organs. There are found in the embryo of Nephe-
lis and Hirudo certain remarkable provisional excretory organs
the origin and history of which is not yet fully made out. In
Nephelis they appear as one (according to Robin, No. 364), or
(according to Biitschli, No. 359) as two successive pairs of

1 According to Robin this system of muscles becomes gradually strengthened and
converted into the permanent system. Rathke on the other hand states that it is
provisional, and that it is replaced by the muscles developed from the mesoblastic
somites. It is possible to suppose that it may really become incorporated in the latter
system.

DISCOPHORA. 353

convoluted tubes on the dorsal side of the embryo, which are
stated by the latter author to develop from the scattered meso-
blast cells underneath the skin. At their fullest development
they extend, according to Robin, from close to the head to near
the ventral sucker. Each of them is U-shaped, with the open
end forwards, each limb of the U being formed by two tubes
united in front. No external opening has been clearly made
out. Semper believed that the tubes were continuous with
the three posterior vitelline cells, but this has been shewn not
to be the case. Fiirbringer1 is inclined from his own re-
searches to believe that they open laterally. They contain a
clear fluid.

In Hirudo, Leuckart (No. 362) has described three similar
pairs of organs the structure of which he has fully elucidated.
They are situated in the posterior part of the body, and each of
them commences with an enlargement from which a convoluted
tube is continued for some distance backwards ; it then turns
forwards again and afterwards bends upon itself to open to the
exterior. The anterior part is broken up into a kind of laby-
rinthic network.

The true segmental organs are found in a certain number
of the segments and are stated (Whitman) to develop from
groups of mesoblast cells. Their origin requires however further
investigation.

A double row of colossal cells on each side of the body has been
described in Clepsine by Whitman as derived from the mesoblastic plates.
These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of
the septa. The inner row is stated to be connected with the segmental
organs. Their eventual history is unknown, but they are conjectured
by Whitman to be the mother cells of the testes.

The alimentary tract. This is formed primitively of two
parts — the epiblastic stomodaeum — forming mouth, pharynx,
and oesophagus, and the hypoblastic mesenteron. The anus is
formed very late as a simple perforation immediately dorsal to
the posterior sucker.

In Clepsine, where there is an epibolic gastrula, the rudiment

1 Morphologisches Jahrbuch, Vol. iv. p. 676. He further speaks of the tube as
" feinverzweigt u. netzformig verastelt," but whether from his own observations is
not clear.

B. II. 23

354 DEVELOPMENT OF ORGANS.

of the mesenteron is at first formed of the three vitelline
spheres, from the surface of which a true hypoblastic layer
enclosing a central yolk mass becomes differentiated, as already
described. The mesenteric sack so formed is constricted by the
growth of the mesoblastic septa into a series of lobes, while the
posterior part forms a narrow and at first very short tube open-
ing by the anus.

The lobed region forms the sacculated stomach of the adult.
The sacculations of the stomach by their mode of origin neces-
sarily correspond with the segments. In the adult however the
anterior lobe is really double and has two divisions for the two
segments it fills, while the posterior lobe, which, as is well
known, extends backwards parallel with the rectum, is composed
of five segmental sacculations. In connection with the stomo-
daeum a protrusible pharynx is developed.

In Hirudo and Nephelis the mesenteron has from the first a
sack-like form. The cells which compose the sack give rise to a
secondary deposit of food-yolk. The further changes are prac-
tically the same as in Clepsine. In Hirudo the posterior saccu-
lation of the stomach is primitively unpaired. The jaws are
formed at about the same time as the eyes as protuberances on
the wall of the oral cavity.

BIBLIOGRAPHY.

(359) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit.
f. wiss. Zool. Vol. xxix. 1877.

(360) E. Grube. Untersuchnngen iib. d. Entwicklung d. Aniiclidcu. Konigs-
l)crg, 1844.

(361) C.K.Hoffmann. " Zur Entwicklungsgeschichte d. Clepsineen." Nie-
derldnd. Archiv f. Zool. Vol. iv. 1877.

(362) R. Leuckart. Die mcnschlichen Parasiten (Hirudo), Vol. i. |>. 686,
et seq.

(363) II. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.

(364) Ch. Robin. Mfm. sur le Dhjeloppcment embryogenique des Hirudwccs.
1'aris, 1875.

(365) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micro.
Science, Vol. xvm. 1878.

[Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.]

CHAPTER XIV.

GEPHYREA1.

IT is convenient for the purposes of embryology to divide
the Gephyrea into two groups, viz. (i) Gephyrea nuda or true
Gephyrea; and (2) Gephyrea tubicola formed by the genus
Phoronis.

GEPHYREA NUDA.
Segmentation and formation of the layers.

An embolic or epibolic gastrula is characteristic of the
Gephyrea, and the blastopore appears, in some cases at any rate
(Phascolosoma, Thalassema), to become the mouth.

Bonellia. In Bonellia (Spengel, No. 370) the segmentation
is unequal but complete, and, as in many Molluscs etc., the
ovum exhibits before its commencement a distinction into a
protoplasmic and a yoke pole. The ovum first divides into four
equal segments, each of them formed of the same constituents as
the original ovum. At the animal pole four small cells, entirely
formed of protoplasm, are next formed by an equatorial furrow.
They soon place themselves in the intervals between the large
spheres. Four small cells are again budded off from the large
spheres and the eight small cells then divide. By a further
continuation of the division of the existing small cells, and the
formation of fresh ones from the large spheres, a layer of small

1 The following scheme shews the classification of the Gephyrea adopted in the
present chapter : —

i. Gephyrea nuda. {«
ii. Gephyrea tubicola (Phoronis).

23—2

356

SEGMENTATION.

cells is eventually formed, which completely envelops the four
large spheres except for a small blastopore at the vegetative pole
of the ovum (fig. 160 A). The large spheres continue to give
rise to smaller cells which however no longer take a superficial
position but lie within the layer of small cells, and give rise to
the hypoblast (fig. r6o B). The small cells become the epiblast,
and at the blastopore they curl inwards (fig. 160 B) and give

FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)

A. Stage when the four hypoblast cells are nearly enclosed.

B. Stage after the formation of the mesoblast has commenced by an infolding of
the lips of the blastopore.

ep. epiblast ; me. mesoblast ; bl. blastopore.

rise to a layer of cells, which appears to extend as an unbroken
sheet between the epiblast and hypoblast, and to form the
mesoblast. The blastopore now closes up, but its position in
relation to the parts of the embryo has not been made out.

In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a
porous zona radiata, divides into two unequal spheres, of which
the smaller next divides into two and then into four. An
invagination takes place which is intermediate between the
embolic and the epibolic types. The small cells, the number of
which is increased by additions from the large sphere, divide, and
grow round the large sphere. The latter in the meantime also
divides, and the cells produced from it form on the one hand a
small sack which opens by the blastopore, and on the other they
fill up the segmentation cavity, and become the mesoblast and
blood corpuscles. Tin- Mastoporc becomes the permanent
mouth.

GEPHYREA.

357

Larval forms and development of organs.

Amongst the Gephyrea armata the larva has as a rule
(Thalassema, Echiurus) the characters of a trochosphere, and
closely approaches the typical form characteristic of the larva of
Polygordius, often known as Loven's larva. In Bonellia this
larval form is less perfectly preserved.

Echiurus. In Echiurus (Salensky, No. 368) the youngest
known larva has all the typical trochosphere characters (fig. 161).
It is covered with cilia and divided into a prae-oral lobe and
post-oral region of nearly equal dimensions. There is a double
ciliated ring which separates the two sections of the body as in
the larva of Polygordius : the mouth (m) opens between its two
elements. The alimentary canal is divided into a stomodaeum
with a ventral opening, a large stomach, and a short intestine
opening by a terminal anus (an). Connecting the oesophagus
with the apex of the prae-oral lobe is the usual contractile band,
and at the insertion of this band -is a thickening of the epiblast
which probably represents the rudiment of the supra-oesophageal
ganglion. A ventral nerve cord is stated by Salensky to be
present, but his observations on this point are not quite satis-
factory.

The metamorphosis is accompanied by the loss of swimming
power, and consists in the
enlargement of the post-oral
portion of the trunk, and in
the simultaneous reduction
of the prae-oral lobe, which
remains however perma-
nently as the cylindrical
proboscis. A groove which
terminates posteriorly at the
mouth is very early formed
on its ventral side. The
ciliated rings gradually dis-
appear during the metamor-
phosis.

FIG. 161. LARVA OF ECHIURUS.
(After Salensky.)

„ _ m. mouth ; an. anus ; sg. supra-ceso

Of the further external phageal ganglion (?).

358

LARVAL FORMS.

changes the most important are (i) the early appearance
round the anal end of the body of a ring of bristles ; and (2) the
appearance of a pair of ventral setae in the anterior part of the
body. The anterior ring of bristles characteristic of the adult
Echiurus does not appear till a late period.

Of the internal changes the earliest is the formation of the
anal respiratory sacks. With the growth of the posterior part
of the trunk the intestine elongates, and becomes coiled.

Bonellia. The embryo of Bonellia, while still within the
egg, retains a spherical form and acquires an equatorial band of
cilia, behind which a second narrower band is soon established,
while in front of the first one a pair of eye-spots becomes

KM;. 162. THREE STAGES IN THE DEVELOPMENT OF BONELLIA. (After Spengel.)

A. Larva with two ciliated bands and two eye-spots.

B. Ripe larva from the dorsal surface.

C. Young female Bonellia from the side.

a/, alimentary tract ; m. mouth ; sc. provisional excretory tube ; s. ventral hook ;
an.-', anal vesicle.

formed (fig. 162 A). The embryo on becoming hatched rapidly
elongates, while at the same time it becomes dorso-vcntrally
flattened and acquires a complete coating of cilia (fig. 162 B).
According to Spengel it resembles at this time in its form and
habits a rhabdoccelous Turbcllarian. The anterior part is
however somewhat swollen and presents an indication of a
pre-oral lobe.

GEPHYREA. 359

During the above changes important advances are made in the forma-
tion of the organs from the embryonic layers.

The epiblast acquires a superficial cuticula, which is perhaps directly
derived from the vitelline membrane. The nervous system is also formed,
probably from the epiblast. The band-like supra-cesophageal ganglion is
the first part of the nervous system formed, and appears to be undoubtedly
derived from the epiblast. The ventral cord arises somewhat later, but the
first stages in its development have not been satisfactorily traced. It is
continuous with the supra-cesophageal band which completely girths the
oesophagus without exhibiting any special dorsal enlargement. After the
ventral cord has become completely separated from the epiblast a central
fibrous mass becomes differentiated in it, while the lateral parts are composed
of ganglion cells. In the arrangement of its cells it presents indications of
being composed of two lateral halves. It is, however, without ganglionic
swellings.

The mesoblast, though at first very thin, soon exhibits a differentiation
into a splanchnic and somatic layer — though the two do not become
distinctly separated by a body cavity. The somatic layer rapidly becomes
thicker, and enlarges laterally to form two bands united dorsally and
ventrally by narrow, thinner bands. The outermost parts of each of these
bands become differentiated into an external circular and an internal
longitudinal layer of muscles. In the pras-oral lobe the mesoblast assumes
a somewhat vacuolated character.

The hypoblast cells form a complete layer round the four yolk cells from
which they arise (fig. 162 B, al\ but at first no alimentary lumen is developed.
The oesophagus appears during this period as an, at first solid, but subse-
quently hollow, outgrowth of the hypoblast towards the epiblast.

The metamorphosis of the larva into the adult female
Bonellia commences with the conversion of many of the in-
different mesoblast cells into blood corpuscles, and the intro-
duction into the body cavity of a large amount of fluid, which
separates the splanchnic and somatic layers of mesoblast. The
fluid is believed by Spengel to be sea-water, introduced by two
anal pouches, the development of which is described below.

The body cavity is lined by a peritoneum, and very soon
distinct vessels, formed by folds of the peritoneum, become
established. Of these there are three trunks, two lateral and a
median in the prae-oral lobe (proboscis), and in the body a
ventral trunk above the nerve cord, and an intestinal trunk
opening anteriorly into the ventral one. The vessels appear
to communicate with the body cavity.

In the course of the above changes the two ciliated bands

360 LARVAL FOR. MX

disappear, the hinder one first. The cilia covering the general
surface become atrophied, with the exception of those on the
ventral side of the prae-oral lobe. The latter structure becomes
more prominent ; the stellate mesoblast cells, which fill up its
interior, become contractile, and it gives rise to the proboscis
(fig. 162 C).

At the point where the cesophageal protuberance joined the epiblast at
a previous stage the mouth becomes established (fig. 162 C, ;//), and though
it is formed subsequently to the atrophy of the anterior ciliated band, yet
there is evidence that it is potentially situated behind this band. The lumen
of the alimentary canal becomes established by the absorption of the
remains of the four central cells. The anus is formed on the ventral side
of the posterior end of the body, and close to it the pouches already noticed
grow out from the hindermost part of the alimentary tract (fig. 162 C, an.v\
They are at first simple blind pouches, but subsequently open into the
body cavity1. They become the anal pouches of the adult. There is present
when the mouth is first formed a peculiar process of the alimentary tract
projecting into the prae-oral lobe, which appears to atrophy shortly after-
wards.

After the formation of the mouth, there are formed on the ventral side of
and slightly behind it (i) anteriorly a pair of tubes, which appear to be
provisional excretory organs and soon disappear (fig. 162 C, sc}\ and (2)
behind them a pair of bristles (s) which remain in the adult. The formation
of the permanent excretory (?) organ (oviduct and uterus) has not been
followed out. The ovary appears very early as a differentiation of the
epithelium lining the ventral vessel.

The larvae, which become the minute parasitic males, undergo
a very different and far less complete metamorphosis than those
which become females. They attach themselves to the pro-
boscis of an adult female, and lose their ciliated bands. Germi-
nal cells make their appearance in the mesoblast, which form
spherical masses, and, like the germinal balls in the female
ovary, consist of a central cell, and an epithelium around it.
The central cell becomes very large, while the peripheral cells
give rise to the spermatozoa. A body cavity becomes developed
in the larvae, into which the spermatic balls are dehisced.
Neither mouth nor anus is formed. The further changes have
not been followed out.

1 The fact that these pouches are outgrowths of the alimentary tract appears to
preclude the possibility of their being homologous with excretory tubes of the Plaly-
elminthes and Rotifera.

GEPHYREA. 361

The larval males make their way into the oesophagus of the
female, where they no doubt live for some time, and probably
become mature, though the seminal pouch of the adult is not
found in many of the males living in the oesophagus. When
mature the males leave the oesophagus, and pass into the
uterus.

Phascolosoma. Cilia appear in Phascolosoma (Selenka,
No. 369) while the ovum is still segmenting. After segmentation
they form a definite band immediately behind the mouth, which
divides the Jarva into two hemispheres — a prae-oral and a post-
oral. A prae-oral band of cilia is soon formed close to the post-
oral band, and at the apex of the prae-oral lobe a tuft of cilia
also appears.

The larva has now the characters of a trochosphere, but
differs from the typical trochosphere in the post-oral part of the
ciliated equatorial ring being more important than the prae-oral,
and in the absence of an anus.

The metamorphosis commences very early. The trunk
rapidly elongates, and the prae-oral lobe becomes relatively less
and less conspicuous. The zona radiata becomes the larval
cuticle.

Three pairs of bristles are formed on the trunk, of which the
posterior pair appears first, then the anterior, and finally the
middle pair : an order of succession which clearly proves they
can have no connection with a true segmentation.

The tentacles become developed between the two parts of the
ciliated ring, and finally the prse-oral lobe, unlike what takes
place in the Gephyrea armata, nearly completely vanishes.

The anus appears fairly late on the dorsal surface, and the
ventral nerve cord is established as an unganglionated thickening
of the ventral epiblast.

GEPHYREA TUBICOLA.

The larva of Phoronis was known as Actinotrocha long
before its connection with Phoronis was established by Kowa-
levsky (No. 372). There is a complete segmentation leading to
the formation of a blastosphere, which is followed by an invagi-
nation, the opening of which is said by Kowalevsky to remain as

362 ACTINOTROCHA.

the mouth1. It is at first terminal, but on the development of a
large prse-oral lobe it assumes a ventral position. The anus
is formed at a later period at the posterior end of the body.

FlG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PlIORONIS FROM

ACTINOTROCHA. (After Metschnikoff.)

A. Young larva.

H. Larva after the formation of post-oral ring of tentacles.

C. Larva with commencing invagination to form the body of Phoronis.

D. Invagination partially everted.

E. Invagination completely everted.

m. mouth ; an. anus; iv. invagination to form the body of Phoronis.

The youngest free larva observed by Metschnikoff (No. 373)
was less developed than the oldest larva found by Kowalevsky.

1 Kowalevsky states that whnt I have called the mouth is the anus, but his sub-
sequent descriptions shew that he has transposed the mouth and anus in the embryo,
and that the opening, which he asserts to be the anus, is in reality the mouth.

GEPHYREA. 363

It probably belongs to a different species. The body is uni-
formly ciliated (fig. 163 A). There is a large contractile prse-oral
lobe, and the body ends behind in two processes. The mouth
(m) is ventral, and the anus (an) dorsal, and not terminal as in
Kowalevsky's larva.

The alimentary tract is divided into stomodseum, stomach
and intestine. The two processes at the hind end of the body
are the rudiments of the first-formed pair of the arms which are
so characteristic of the fully developed Actinotrocha. A second
pair of arms next become established on the dorsal side of the
previously existing pair, and the region where the anus is placed
grows out as a special process. New pairs of arms continue to
be formed in succession dorsalwards and forwards, and soon
constitute a complete oblique post-oral ring (fig. 163 B). They are
covered by long cilia. Round the anal process a very conspicuous
ciliated ring also becomes established.

At the period when five pairs of arms are present a delicate membrane
becomes visible on the ventral side of the intestine which joins the somatic
mesoblast anteriorly. This membrane is the rudiment of the future ventral
vessel. The somatic mesoblast is present even before this period as a
delicate layer of circular muscular fibres.

When six pairs of arms have become formed an involution
(fig. 163 C, iv) appears on the ventral side, immediately behind
the ring of arms. This involution consists both of the epiblast
and somatic mesoblast. It grows inwards towards the intestine,
and, increasing greatly in length, becomes at the same time
much folded.

When it has reached its full development the critical period
of the metamorphosis of Actinotrocha into Phoronis is reached,
and is completed in about a quarter of an hour. The ventral
involution becomes evoluted (fig. 163 D), just as one might turn
out the finger of a glove which had been pulled inwards. When
the involution has been to a certain extent everted, the alimen-
tary canal passes into it, and at the same time the body of the
larva becomes violently contracted. By the time the evagi-
nation is completed it forms (fig. 162 E) a long conical body,
containing the greater part of the alimentary tract, and consti-
tuting the body of the young- Phoronis. The original anal process
remains on the dorsal side as a small papilla (fig. 162 E, an).

364 \< T1NOTROCHA.

While these changes have been taking place the prae-oral
lobe has become much contracted, and partly withdrawn into
the stomodajum. At the same time the arms have become bent
forward, so as to form a ring round the mouth. Their bases
become much thickened. The metamorphosis is completed by
the entire withdrawal of the prae-oral lobe within the oesophagus,
and by the casting off of the ends of the arms, their bases
remaining as the circumoral ring of tentacles, which form
however a lophophore rather than a complete ring. The
perianal ring of cilia is also thrown off, and the anal process
withdrawn into the body of the young Phoronis. There are now
three longitudinal vascular trunks, united anteriorly by a circular
vessel which is prolonged into the tentacles.

General Considerations.

The development of Phoronis is so different from that of the
other Gephyrea that further investigations are required to shew
whether Phoronis is a true Gephyrean. Apart from its peculiar
metamorphosis Actinotrocha is a very interesting larval form, in
that it is without a prae-oral ciliated ring, and that the tentacles
of the adult are derived from a true post-oral ring, prolonged
into arm-like processes.

The other Gephyrea present in their development an obvious
similarity to the normal Chaetopoda, but their development stops
short of that of the Chaetopoda, in that they are clearly without
any indications of a true segmentation. In the face of what is
known of their development it is hardly credible that they can
represent a degenerate Chaetopod phylum in which segmentation
has become lost. Further than this the Gephyrea armata seem
in one respect to be a very primitive type in that they retain
through life a well-developed pra-oral lobe, which constitutes
their proboscis. In almost all other forms, except Balanoglossus,
the larval prae-oral lobe becomes reduced to a relatively in-
significant anterior part of the head.

BIBLIOGRAPHY.
Gephyrea nuda.

(366) A. Kowalevsky. Sitz. d. zool. Abth. d. III. Vcrsam. russ. Naturj.
(Thalasscma). Zeit.f. wiss. Zool. Vol. xxn. 1872, p. 284.

GEPHYREA. 365

(367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen,"
etc. Miiller's Archiv, 1857.

(368) M. Salensky. "Ueber die Metamorphose d. Echiurus." Morphologisches
Jahrbuch, Bd. 11.

(369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma elonga-
tum." Zeit.f. wiss. ZooL 1875, Bd. xxv. p. i.

(370) J. W. Spengel. "Beitrage z. Kenntniss d. Gephyreen (Bonellia)." Mit-
theil. a. d. zool. Station z. Neapel, Vol. I. 1879.

Gephyrea tubicola (Actinotrocha).

(371) A. Krohn. " Ueb. Pilidium u. Actinotrocha." Miiller's Archiv, 1858.

(372) A. Kowalevsky. "On anatomy and development of Phoronis," Peters-
bourg, 1867. 2 PI. Russian. Vide Leuckart's Bericht, 1866-7.

(373) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere (Actino-
trocha)." Zeit.f. wiss. Zool. Bd. xxi. 1871.

(374) J. Miiller. " Bericht lib. ein. Thierformen d. Nordsee." Miiller's Archiv,
1846.

(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata."
Miiller's Arch. 1862.

CHAPTER XV.

CH/ETOGNATHA, MYZOSTOMEA AND GASTROTRICHA.

THE present chapter deals with three small isolated groups,
which only resemble each other in that the systematic position of
all of them is equally obscure.

Chatognatha.

The discoveries of Kowalevsky (No. 378) confirmed by
Btitschli (No. 376) with reference to the development of Sagitta,
though they have not brought us nearer to a knowledge of the
systematic position of this remarkable form, are nevertheless of

FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
Hiitschli and B after Kowalevsky.) The three embryos are represented in the same
positions.

A. The gastrula stage.

li. A succeeding stage in which the primitive archenteron is commencing to be
divided into three parts, the two lateral of which are destined to form the body
cavity.

C. A later stage in which the mouth involution (/;/) has become continuous with
the alimentary tract, and the blastopore has become closed.

m. mouth; al. alimentary canal ; ae. archenteron ; bl.p. blastopore; pv. perivisceral
cavity; sf>. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative
organs.

CH^TOGNATHA. 367

great value for the more general problems of embryology. The
development commences after the eggs are laid. The segmen-
tation is uniform, and a blastosphere, formed of a single layer of
columnar cells, is the product of it. An invagination takes
place, the opening of which narrows to a blastopore situated
at the pole of the embryo opposite that at which the mouth
subsequently appears (fig. 164 A). The simple archenteron soon
becomes anteriorly divided into three lobes, which communicate
freely with the still single cavity behind (fig. 164 B). The two
lateral lobes are destined to form the body cavity, and the
median lobe the alimentary tract of the adult. An invagination
soon arises at the opposite pole of the embryo to the blastopore
and forms the mouth and oesophagus (fig. 164 B and C, m).

At the gastrula stage there is formed a paired mass destined
to give rise to the generative organs. It arises as a prominence
of six cells, projecting from the hypoblast at the anterior pole of
the archenteron, and soon separates itself as a mass, or probably
a pair of masses, lying freely in the cavity of the archenteron
(fig. 164 A.yge). When the folding of the primitive cavity takes
place the generative rudiment is situated at the hind end of the
median lobe of the archenteron in the position represented
in fig. 164 C, ge.

An elongation of the posterior end of the embryo now takes
place, and the embryo becomes coiled up in the egg, and when
eventually hatched sufficiently resembles the adult to be recog-
nisable as a young Sagitta.

Before hatching takes place various important changes
become manifest. The blastopore disappears after being carried
to the ventral surface. The middle section of the trilobed region
of the archenteron becomes separated from the unpaired
posterior part, and forms a tube, blind behind, but opening
in front by the mouth (fig. 165 A, al). It constitutes the perma-
nent alimentary tract, and is formed of a pharyngeal epiblastic
invagination, and a posterior hypoblastic section derived from
the primitive archenteron. The anus is apparently not formed
till comparatively late. After the isolation of the alimentary
tract the remainder of the archenteron is formed of two cavities
in front, which open freely into a single cavity behind (fig.
165 A). The whole of it constitutes the body cavity and its walls

368

CH/ETOGNATHA.

f/ic mesoblast. The anterior paired part becomes partitioned off
into a head section and a trunk section (fig. 165 A and B). The
former constitutes a pair of distinct cavities (c.pv) in the head,
and the latter two cavities opening freely into the unpaired
portion behind. At the junction of the paired cavities with the
unpaired cavity are situated the generative organs (ge). The
inner wall of each of the paired cavities forms the splanchno-
pleuric mesoblast, and the outer wall of the whole the somatic
mesoblast. The inner walls of the posterior cavities unite above
and below the alimentary tract, and form the dorsal and ventral
mesenteries, which divide the body cavity into two compartments
in the adult. Before the hatching of the embryo takes place
this mesentery is continued backwards so as to divide the
primitively unpaired caudal part of the body cavity in the
same way.

From the somatic mesoblast of the trunk is derived the
single layer of longitudinal muscles of Sagitta, and part of the
epithelioid lining of the body cavity. The anterior termination
of the trunk division of the body cavity is marked in the adult
by the mesentery dividing into two laminae, which bend outwards
to join the body wall.

The cephalic sec-
tion of the body cavity
seems to atrophy, and
its walls to become con-
verted into the compli-
cated system of muscles
present in the head of
the adult Sagitta.

In the presence of
a section of the body
cavity in the head the
embryo of Sagitta re
sembles Lumbricus,
Spiders, etc.

The generative ru-
diment of each side
divides into an anterior
and a posterior part

In;. [65. Two VIEWS OF A LATE EMBRYO OF
SV.ITTA. A. from the dorsal surface. I?, from the
tide. (After 15iitschli.)

m. mouth ; al. alimentary canal ; v.g. ventral
ganglion (thickening of epiblast) ; rp. epiblast ; c.pv,
cephalic section of body cavity; so. somatopleure ;
s/>. splanchnopleure ; ,;v. generative

CH^ETOGNATHA. 369

(fig. 165, ge]. The former constitutes the ovary, and is situated
in front of the septum dividing the tail from the body ; and the
latter, in the caudal region of the trunk, forms the testis.

The nervous system originates from the epiblast. There is a
ventral thickening (fig. 165 B, v.g) in the anterior region of the
trunk, and a dorsal one in the head. The two are at first
continuous, and on becoming separated from the epiblast remain
united by thin cords.

The ventral ganglion is far more prominent during embryonic
life than in the adult. Its position and early prominence in the
embryo perhaps indicate that it is the homologue of the ventral
cord of Chaetopoda1.

BIBLIOGRAPHY.

(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f.
wiss. Zoo!., Vol. xxni. 1873.

(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. na-
turforschenden Gesellschaft in Halle, 1857.

(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropo-
den." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871.

MYZOSTOMEA.

The development of these peculiar parasites on Crinoids has been
investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff
(No. 379).

The segmentation is unequal, and would appear to be followed by an
epibolic invagination. The outer layer of cells (epiblast) becomes covered
with cilia, and the inner is transformed into a non-cellular (?) central yolk
mass. At this stage the larva is hatched, and commences to lead a free
existence. In the next stage observed by Metschnikoff, the mouth, oeso-
phagus, stomach, and anus had become developed ; and two pairs of feet
were present. In both of these feet Chaetopod-like setae were present, which
in the hinder pair were simple fine bristles without a terminal hook. The
papilliform portion of the foot is at first undeveloped. The feet become
successively added, like Chaetopod segments, and the stomach does not
become dendriform till the whole complement of feet (5 pairs) are present.

In the primitive covering of cilia, combined with a subsequent indication

1 Langerhans has recently made some important investigations on the nervous
system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic gan-
glia of Molluscs, while he has found a pair of new ganglia, the development of which
is unknown, which he calls the suboesophageal or pedal ganglia. The embryolo-
gical facts do not appear to be in favour of these interpretations.

B. II. 24

3/0 MYZOSTOMEA.

of segments in the formation of the feet and setae, the larva of the Myzo-
stomea shews an approximation to the Chaetopoda, and the group is
probably to be regarded as an early Chactopod type specially modified in
connection with its parasitic habits.

BIBLIOGRAPHY.

(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.

(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
Zfit.f. wiss. Zool.y Vol. XVI. 1866.

(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Ztit.f. wiss.
Zool., Vol. ix. 1858.

GASTROTRICHA.

A few observations of Ludwig on the winter eggs of Ichthydium larus
shew that the segmentation is a total and apparently a regular one. It
leads to the formation of a solid morula. The embryo has a ventral
curvature, and the caudal forks are early formed as cuticular structures. By
the time the embryo leaves the egg, it has almost reached the adult state.
The ventral cilia arise some little time prior to the hatching.

BIBLIOGRAPHY.

(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss.
Zool., Vol. xxvi. 1876.

CHAPTER XVI.

NEMATELMINTHES AND ACANTHOCEPHALA.

NEM ATELMINTHES '.

Nematoidea. Although the ova of various Nematodes have
formed some of the earliest, as well as the most frequent objects
of embryological observation, their development is still but very
imperfectly known. Both viviparous and oviparous forms are
common, and in the case of the oviparous forms the eggs are
usually enveloped in a hard shell. The segmentation is total
and nearly regular, though the two first segments are often
unequal. The relation of the segmentation spheres to the
germinal layers is however only satisfactorily established (through
the researches of Butschli (No. 383)) in the case of Cucullanus
elegans, a form parasitic in the Perch2.

The early development of this embryo takes place within
the body of the parent, and the egg is enveloped in a delicate
membrane. After the completion of the early stages of seg-
mentation the embryo acquires the form of a thin flat plate
composed of two layers of cells (fig. 166 A and B). The two
layers of this plate give rise respectively to the epiblast and
hypoblast, and at a certain stage the hypoblastic layer ceases to

1 The following classification of the Nematoda is employed in this chapter :

r Ascaridae.
Strongylidae.

Trichinidse. II. Gordioidea.

I. Nematoidea. , Filarid8B. m. Chaetosomoidea.

Mermithidae.
[_ Anguillulidse.

2 The ova of Anguillula aceti are stated by Hallez to undergo a similar develop-
ment to those of Cucullanus.

24—2

372

CUCULLANUS.

grow, while the growth of the epiblastic layer continues. As a
consequence of this the sides of the plate begin to fold over
towards the side of the hypoblast (fig. 166 D.) This folding
results in the formation of a remarkably constituted gastrula,
which has the form of a hollow two-layered cylinder with an
incompletely closed slit on one side (fig. 166 E, bl.p}. This slit
has the value of a blastopore. It becomes closed by the coales-
cence of the two edges, a process which commences posteriorly,

FIG. 166.

A.
B.
C.

VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS.

(From Biitschli.)

Surface view of flattened embryo at an early stage in the segmentation.
Side view of an embryo at a somewhat later stage, in optical section.
Flattened embryo at the completion of segmentation.

D. Embryo at the commencement of the gastrula stage.

E. Embryo when the blastopore is reduced to a mere slit.

F. Vermiform embryo after the division of the alimentary tract into oesophageal
and glandular divisions.

m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, oesophagus; bl.p. blas-
topore.

and then gradually extends forwards. In front the blastopore
never becomes completely closed, but remains as the permanent
mouth. The embryo after these changes has a worm-like form,
which becomes the more obvious as it grows in length and
becomes curved (fig. 166 F).

The hypoblast of the embryo gives rise to the alimentary

NEMATELM1NTHES. 373

canal, and soon becomes divided into an cesophageal section
(fig. 1 66 F, ce) formed of granular cells, and a posterior division
formed of clear cells. The mesoblast (fig. 166, me) takes its
origin from certain special hypoblast cells around the mouth,
and thence grows backwards towards the posterior end of the
body.

The young Cucullanus becomes hatched while still in the
generative ducts of its parent, and is distinguished by the
presence of a remarkable thread-like tail. On the dorsal surface
is a provisional boring apparatus in the form of a conical papilla.
A firm cuticle enveloping the body is already present. In this
condition it leaves its parent and host, and leads for a time a
free existence in the water. Its metamorphosis is dealt with in
another section.

The ova of the Oxyuridae parasitic in Insects are stated by Galeb (No.
386) to take the form of a blastosphere at the close of segmentation. An
inner layer is then formed by delamination. What the inner layer gives rise
to is not clear, since the whole alimentary canal is stated to be derived from
two buds, which arise at opposite ends of the body, and grow inwards till
they meet.

The generative organs. The study of the development of
the generative organs of Nematodes has led to some interesting
results. In the case of both sexes the generative organs origi-
nate (Schneider, No. 390) from a single cell. This cell elongates
and its nuclei multiply. After assuming a somewhat columnar
form, it divides into (i) a superficial investing layer, and (2) an
axial portion.

In the female the superficial layer is only developed dis-
tinctly in the median part of the column. In the course of the
further development the two ends of the column become the
blind ends of the ovary, and the axial tissue they contain forms
the germinal tissue of nucleated protoplasm. The superficial
layer gives rise to the epithelium of the uterus and oviduct.
The germinal tissue, which is originally continuous, is interrupted
in the middle part (where the superficial layer gives rise to the
uterus and oviduct), and is confined to the two blind extremities
of the tube.

In the male the superficial layer, which gives rise to the epi-
thelium of the vas deferens, is only formed at the hinder end of

374 METAMORPHOSIS.

the original column. In other respects the development takes
place as in the female.

Gordioidea. The ovum of Gordius undergoes a regular segmentation.
According to Villot (No. 391) it forms at the close of segmentation a morula,
which becomes two-layered by delamination. The embryo is at first
spherical, but soon becomes elongated.

By an invagination at the anterior extremity the head is formed. It
consists of a basal portion, armed with three rings of stylets, and a conical
proboscis, armed with three large stylets. When the larva becomes free
the head becomes everted, though it remains retractile. By the time the
embryo is hatched a complete alimentary tract is formed with an oral opening
at the end of the proboscis, and a subterminal ventral anal opening. It is
divided into an oesophagus and stomach, and a large gland opens into it at
the base of the proboscis.

The body has a number of transverse folds, which give it a ringed
appearance.

Metamorphosis and life history.

Nematoidea. Although a large number of Nematodes have
a free existence and simple life history, yet the greater number
of known genera are parasitic, and undergo a more or less com-
plicated metamorphosis1. According to this metamorphosis
they may be divided into two groups (which by no means
closely correspond with the natural divisions), viz. those which
have a single host, and those with two hosts. Each of these
main divisions may be subdivided again into two.

In the first group with one host the simplest cases are those
in which the adult sexual form of parasite lays its eggs in the
alimentary tract of its host, and the eggs are thence transported
to the exterior. The embryo still in the egg, if favoured by
sufficient warmth and moisture, completes its development up
to a certain point, and, if then swallowed by an individual of the
species in which it is parasitic in the adult condition, it is
denuded of its shell by the action of the gastric juice, and
develops directly into the sexual form.

Leuckart has experimentally established this metamorphosis in the case
of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis.
The Oxyuridae of Blatta and Hydrophilus have a similar life history

1 The following facts are mainly derived from Leuckart's exhaustive treatise
(No. 388).

NEMATELMINTHES. 375

(Caleb, No. 386), and it is almost certain that the metamorphosis of the
human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this
nature.

A slightly more complicated metamorphosis is common in
the genera Ascaris and Strongylus. In these cases the egg-shell
is thin, and the embryo becomes free externally, and enjoys for
a shorter or longer period a free existence in water or moist
earth. During this period it grows in size, and though not
sexual usually closely resembles the adult form of the perma-
nently free genus Rhabditis. In some cases the free larva
becomes parasitic in a freshwater Mollusc, but without thereby
undergoing any change. It eventually enters the alimentary
tract of its proper host and there become sexual.

As examples of this form of development worked out by Leuckart may
be mentioned Uochmius trigonocephalus, parasitic in the dog, and Ascaris
acuminata, in the frog. The human parasite Dochmius duodenale under-
goes the same metamorphosis as Dochmius trigonocephalus.

A remarkable modification of this type of metamorphosis is found in
Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition
is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). The
embryos pass through their first developmental phases in the body of the
parent. They have the typical Rhabditis form, and make their way after
birth into the frog's rectum. From this they pass to the exterior, and then
living either in moist earth, or the faeces of the frog, develop into a sexual
form, but are very much smaller than in the adult condition. The sexes are
distinct, and the males are distinguished from the females by their smaller
size, shorter and rounded tails, and thinner bodies. The females have
paired ovaries with a very small number of eggs, but the testis of the males
is unpaired. Impregnation takes place in the usual way, and in summer
time about four embryos are developed in each female, which soon burst
their egg-capsules, and then move freely in the uterus. Their active move-
ments soon burst the uterine walls, and they then come to lie freely in the
body cavity. The remaining viscera of the mother are next reduced to a
finely granular material, which serves for the nutrition of the young forms
which continue to live in the maternal skin. The larvae eventually become
free, and though in many respects different from the parent form which gave
rise to them, have nevertheless the Rhabditis form. They live in water or
slime, and sometimes become parasitic in water-snails ; in neither case how-
ever do they undergo important changes unless eventually swallowed by a
frog. They then pass down the trachea into the lungs and there rapidly
develop into the adult form. No separate males have been found in the
lungs of the frog, but it has been shewn by Schneider (No. 390) that the
so-called females are really hermaphrodites ; the same gland giving origin

376 METAMORPHOSIS.

to both spermatozoa and ova, the former being developed before the latter1.
The remarkable feature of the above life history is the fact that in the stage
corresponding with the free larval stage of the previous forms the larvae of
this species become sexual, and give rise to a second free larval generation,
which develops into the adult form on again becoming parasitic in the
original host. It constitutes a somewhat exceptional case of heterogamy as
defined in the introduction.

Amongst the Nematodes with but a single host a remarkable parasite in
wheat has its place. This form, known as Anguillula scandens, inhabits in
the adult condition the ears of wheat, in which it lays its eggs. After
hatching, the larvae become encysted, but become free on the death of the
plant. They now inhabit moist earth, but eventually make their way into
the ears of the young wheat and become sexually mature.

The second group of parasitic Nematodes with two hosts
may be divided into two groups, according to whether the larva
has a free existence before passing into its first or intermediate
host, or is taken into it while still in the egg. In the majority
of cases the larval forms live in special connective tissue cap-
sules, or sometimes free in the tissues of their intermediate
hosts ; but the adults, as in the cases of other parasitic Nema-
todes, inhabit the alimentary tract.

The life history of Spiroptera obtusa may be cited as an example of a
Nematode with two hosts in which the embryo is transported into its
intermediate host while still within the egg. The adult of this form is
parasitic in the mouse, and the ova pass out of the alimentary tract with the
excreta, and may commonly be found in barns, etc. If one of the ova is
now eaten by the meal-worm (larva of Tenebrio), it passes into the body
cavity of this worm and undergoes further development. After about five
weeks it becomes encapsuled between the ' fat bodies ' of the meal-worm.
It then undergoes an ecdysis, and, if the meal-worm with its parasites is
now eaten by the mouse, the parasites leave their capsule and develop into
the sexual form.

As examples of life histories in which a free state intervenes before the
intermediate host, Cucullanus elegans and Dracunculus may be selected.
The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch
and other freshwater fishes. It is a viviparous form, and the young after
birth pass out into the water. They next become parasitic in Cyclops,
passing in through the mouth, so into the alimentary tract, and thence into
the body cavity. They soon undergo an ecdysis, in the course of which the
oesophagus becomes divided into a muscular pharynx and true glandular

1 Leuckart does not appear to be satisfied as to the hermaphroditism of these
forms ; and holds that it is quite possible that the ova may develop parthenogeneti-
cally.

NEMATELMINTHES. 377

oesophagus. They then grow rapidly in length, and at a second ecdysis
acquire a peculiar beaker-like mouth cavity approaching that of the adult.
They do not become encapsuled. No further development of the worm
takes place so long as it remains in the Cyclops, but, if the Cyclops is now
swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly
attains to sexual maturity.

The observations of Fedschenko on Dracunculus medinensis1, which is
parasitic in the subcutaneous connective tissue in Man, would seem to shew
that it undergoes a metamorphosis very similar to that of Cucullanus. There
is moreover a striking resemblance between the larvae of the two forms.
The larvae of Dracunculus become transported into water, and then make
their way into the body cavity of a Cyclops by boring through the soft skin
between the segments on the ventral surface of the body. In the body cavity
the larvae undergo an ecdysis and further development. But on reaching
a certain stage of development, though they remain a long time in the
Cyclops, they grow no further. The remaining history is unknown, but
probably the next host is man, in which the larva comes to maturity. In the
adult condition only females of Dracunculus are known, and it has been
suggested by various writers that the apparent females are in reality herma-
phrodites, like Ascaris nigrovenosa, in which the male organs come to
maturity before the female.

Another very remarkable human parasite belonging to the same group
as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria
Bancrofti2.

The sexual form is parasitic in warm climates in the human tissues, and
produces multitudes of larvae which pass into the blood, and are sometimes
voided with the urine. The larvae in the blood do not undergo a further deve-
lopment, and unless transported to an intermediate host die before very long.
Some, though as yet hardly sufficient, evidence has been brought forward to
shew that if the blood of an infected patient is sucked by a mosquito the
larvae develop further in the alimentary tract of the mosquito, pass through a
more or less quiescent stage, and eventually grow considerably in size, and
on the death of the mosquito pass into the water. From the water they are
probably transported directly or indirectly into the human intestines, and
then bore their way into the tissues in which they are parasitic, and become
sexually mature.

The well-known Trichina spiralis has a life history unlike that of other
known Nematodes, though there can be little doubt that this form should
be classified in respect to its life history with the last- described forms.
The peculiarity of the life history of Trichina is that the embryos set free
in the alimentary canal pass through the walls into the muscular tissues and
there encyst ; but do not in a general way pass out from the alimentary

1 Vide Leuckart, D. men. Par., Vol. II. p. 704.

2 Vide D. P. Manson, " On the development of Filaria sanguinis hominis."
Journal of the Linnean Society, Vol. xiv. No. 75.

378 MKTAMORPHOSIS.

canal of one host and thence into a fresh host to encyst. It occasionally
however happens that this migration does take place, and the life history
of Trichina spiralis then becomes almost identical with that of some of the
forms of the third type. Trichina is parasitic in man, and in swine, and
also in the rat, mouse, cat, fox and other forms which feed upon them.
Artificially it can be introduced into various herbivorous forms (rabbit,
guinea-pig, horse) and even birds.

The sexual form inhabits the alimentary canal. The female is vivi-
parous, and produces myriads of embryos, which pass into the alimentary
canal of their host, through the walls of which they make their way, and
travelling along lines of connective tissue pass into the muscles. Here the
embryos, which are born in a very imperfect condition, rapidly develop,
and eventually assume a quiescent condition in a space inclosed by sarco-
lemma. Within the sarcolemma a firm capsule is developed for each larva,
which after some months becomes calcified ; and after the atrophy of the
sarcolemma a connective tissue layer is formed around it. Within its
capsule the larva can live for many years, even ten or more, without
undergoing further development, but if at last the infected flesh is eaten by
a suitable form, e.g. the infected flesh of the pig by man, the quiescent
state of the larva is brought to a close, and sexual maturity is attained in
the alimentary tract of the new host.

Gordioidea. The free larva of Gordius already described usually pene-
trates into the larva of Chironomus where it becomes encysted. On the
Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus laevis or
Cobitis barbatula), it penetrates into the wall of the intestine of its second
host, becomes again encysted and remains quiescent for some time. Event-
ually in the spring it leaves its capsule, and enters the intestine, and
passes to the exterior with the faeces. It then undergoes a gradual meta-
morphosis, in the course of which it loses its ringed structure and cephalic
armature, grows in length, acquires its ventral cord, and on the develop-
ment of the generative organs loses the greater part of its alimentary tract.

Young examples of Gordius have often been found in various terrestrial
carnivorous Insecta, but the meaning of this fact is not yet clear.

BIBLIOGRAPHY.

(383) O. Biitschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zdt.j.
wiss. Zool., B. xxvi. 1876.

(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.

(385) T. S. Cobbold. Parasites; A Treatise on the Entozoa of Man mn/
Animals. Churchill, 1879.

(386) O. Galeb. "Organisation et developpement des Oxyurides," &c. Arch-
ives de Zool. expcr. et getter. , Vol. vn. 1878.

(387) R. Leu ck art. Untcrsufkutigcn itb. Trichina spiralis. 2nd ed. Leip/ig,
1866.

(388) R. Leuckart. Die tnenschlichcn Parasitcn, Bd. II. 1876.

NEMATELMINTHES. 379

(389) H. A. Pagenstecher. Die Trichinen nach Versitchen dargestellt. Leip-
zig, 1865.

(390) A.Schneider. Monographic d. Nemaioden. Berlin, 1866.

(391) A. Villot. "Monographic des Dragoneaux" (Gordioidea). Archives de
Zool. exper. et gener., Vol. ill. 1874.

ACANTHOCEPHALA.

The Acanthocephala appear to be always viviparous. At the time of
impregnation the ovum is a naked cell, and undergoes in this condition the
earlier phases of segmentation.

The segmentation is unequal (Leuckart, No. 393), but whether there is an
epibolic gastrula has not clearly been made out.

Before segmentation is completed there are formed round the ovum
thick protecting membranes, which are usually three in number, the middle
one being the strongest. After segmentation the central cells of the ovum
fuse together to give rise to a granular mass, while the peripheral cells at a
slightly later period form a more transparent syncytium. At the anterior
end of the embryo there appears a superficial cuticle bearing in front a ring
of hooks.

The embryo is now carried out with the excreta from the intestine of
the vertebrate host in which its parent lives. It is then swallowed by some
invertebrate host1.

In the intestine of the invertebrate host the larva is freed from its
membranes, and is found to have a somewhat elongated conical form, ter-
minating anteriorly in an obliquely placed disc, turned slightly towards the
ventral surface and armed with hooks. Between this disc and the granular
mass, already described as formed from the central cells of the embryo, is a
rather conspicuous solid body. Leuckart supposes that this body may re-
present a rudimentary functionless pharynx, while the granular mass in
his opinion is an equally rudimentary and functionless intestine. The body
wall is formed of a semifluid internal layer surrounding the rudimentary
intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
The adult Echinorhyncus is formed by a remarkable process of develop-
ment within the body of the larva, and the skin is the only part of the
larva which is carried over to the adult.

In Echinorhyncus proteus the larva remains mobile during the forma-
tion of the adult, but in other forms the metamorphosis takes place during
a quiescent condition of the larva.

The organs of the adult are differentiated from a mass of cells which
appears to be a product of the central embryonic granular mass, and is

1 Echin. proteus, which is parasitic in the adult state in many freshwater fish,
passes through its larval condition in the body cavity of Gammarus pulex. Ech.
angustatus, parasitic in the Perch, is found in the larval condition in the body cavity
of Asellus aquaticus. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394)
to pass through its larval stages in maggots.

380 ACANTHOCEPHALA.

called by Leuckart the embryonic nucleus. The embryonic nucleus becomes
divided into four linearly arranged groups of cells, of which the hindermost
but one is the largest, and very early differentiates itself into (i) a peripheral
layer, and (2) a central mass formed of two distinct bodies. The peripheral
layer of this segment grows forwards and backwards, and embraces the
other segments, with the exception of the front end of the first one which
is left uncovered. The envelope so formed gives rise to the splanchnic and
somatic mesoblast of the adult worm. Of the four groups of cells within it
the anterior gives rise to the proboscis, the next to the nerve ganglion, the
third, formed of two bodies, to the paired generatives, and the fourth to the
generative ducts. The whole of the above complex rapidly elongates, and
as it does so the enveloping membrane becomes split into two layers ; of
which the outer forms the muscular wall of the body (somatic mesoblast),
and the inner the muscular sheath of the proboscis and the so-called gene-
rative ligament enveloping the generative organs. The inner layer may be
called the splanchnic mesoblast in spite of the absence of an intestine.
The cavity between the two mesoblastic layers forms the body cavity.

The various parts of the adult continue to differentiate themselves as
the whole increases in size. The generative masses very early shew traces
of becoming differentiated into testes or ovaries. In the male the two
generative masses remain spherical, but in the female become elongated :
the rudiment of the generative ducts becomes divided into three sections
in both sexes. The most remarkable changes are, however, those undergone
by the rudiment of the proboscis.

In its interior there is formed a cavity, but the wall bounding the front
end of the cavity soon disappears. By the time that this has taken place
the body of the adult completely fills up the larval skin, to which it very
soon attaches itself. The hollow rudiment of the proboscis then becomes
everted, and forms a papilla at the end of the body, immediately ad-
joining the larval skin. This papilla, with the larval skin covering it,
constitutes the permanent proboscis. The original larval cuticle is either
now or at an earlier period thrown off and a fresh cuticle developed. The
hooks of the proboscis are formed from cells of the above papilla, which
grow through the larval skin as conical prominences, on the apex of which
a chitinous hook is modelled. The remainder of the larval skin forms the
skin of the adult, and at a later period develops in its deeper layer the
peculiar plexus of vessels so characteristic of the Acanthocephala. The
anterior oval appendages of the adult cutis, known as the lemnisci, are
outgrowths from the larval skin.

The Echinorhyncus has with the completion of these changes practically
acquired its adult structure ; but in the female the ovaries undergo at this
period remarkable changes, in that they break up into a number of spherical
masses, which lie in the lumen of the generative ligaments, and also make
their way into the body cavity.

The young Echinorhyncus requires to be transported to its permanent
host, which feeds on its larval host, before attaining to sexual maturity.

ACANTHOCEPHALA. 381

BIBLIOGRAPHY.

(392) R. Greeff. " Untersuchungen ii. d. Bau u. Entwicklung des Echin. milia-
rius." Archiv f. Naturgesch. 1864.

(393) R. Leuckart. Die menschlichen Parasiten. Vol. n. p. 80 1 et seq.
1876.

(394) An. Schneider. " Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat.
u, Phys. 1868.

(395) G. R. Wagener. Beitrdge z. Entwicklungsgeschichte d. Eingeweidewiir-
mer. Haarlem, 1865.

CHAPTER XVII.

TRACHEATA.

PROTOTRACH EAT A.
THE remarkable researches of Moseley (No. 396) on Peripatus

FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size.
(From Moseley.)

capensis have brought clearly to light the affinities of this form
with the tracheate Arthropoda ; and its numerous primitive

FIG. 168. Two STAGES IN THE DEVELOPMENT OF PERIPATUS CAPENSIS.
(After Moseley.)

A. Youngest stage hitherto observed before the appearance of the legs.

B. Later stage after the legs and antennae have become developed.
Both figures represent the larva as it appears within the egg.

i and i. First and second post-oral appendages.

characters, such as the generally distributed tracheal apertures,
the imperfectly segmented limbs, the diverging ventral nerve

TRACHEATA.

383

cords with imperfectly marked ganglia, and the nephridia (seg-
mental organs1), would render its embryology of peculiar in-
terest. Unfortunately Moseley was unable, from want of
material, to make so complete a study of its development as of
its anatomy. The youngest embryo observed was in part
distinctly segmented, and coiled up within the egg (fig. 168 A).
The procephalic lobes resemble those of the Arthropoda gene-
rally, and are unlike the prae-oral lobe of
Chaetopods or Discophora. They are not
marked off by a transverse constriction
from the succeeding segments. The three
embryonic layers are differentiated, and
the interior is filled with a brownish mass
— the remnant of the yolk — which is pro-
bably enclosed in a distinct intestinal wall,
and is lobed in correspondence with the
segmentation of the body. The mouth
invagination is not present, and but two
pairs of slight prominences mark the rudi-
ments of the two anterior post-oral ap-
pendages.

The single pair of antennae is formed
in the next stage, and is followed by the
remaining post-oral appendages, which
arise in succession from before backwards
somewhat later than the segments to which
they appertain.

The posterior part of the embryo be-
comes uncoiled, and the whole embryo
bent double in the egg (fig. 168 B).

The mouth appears as a slit-like open-
ing between and below the procephalic
lobes. On each side and somewhat behind it there grows out
an appendage — the first post-oral pair (fig. 169, i) — while in
front and behind it are formed the upper and lower lips. These
two appendages next turn inwards towards the mouth, and their

FIG. 169. EMBRYO
OF PERIPATUS CAPENSIS.
Slightly older than A in
fig. 168; unrolled. (After
Moseley.)

a. antennae ; o. mouth ;
i. intestine ; c. procephalic
lobe, i, 2, 3, etc., post-
oral appendages.

1 F. M. Balfour, "On certain points in the Anatomy of Peripatus capensis."
Quart. Journ. of Micros. Science, Vol. xix. 1879.

PROTOTRACHEATA.

bases become gradually closed over by two processes of the
procephalic region (fig. 170, m)
The whole of these structures
assist in forming a kind of
secondary mouth cavity, which
is at a later period further
completed by the processes of
the procephalic region meeting
above the mouth, covering over
the labrum, and growing back-
wards to near the origin of the
second pair of post-oral appen-
dages.

The antennae early become
jointed, and fresh joints con-
tinue to be added throughout
embryonic life ; in the adult
there are present fully thirty
joints. It appears to me probable (though Mr Moseley takes
the contrary view) from the late development of the paired
processes of the procephalic lobes, which give rise to the circular
lip of the adult, that they
are not true appendages.
The next pair therefore
to the antennae is the first
post-oral pair. It is the
only pair connected with
the mouth. At their ex-
tremities there is formed a
pair of claws similar to
those of the ambulatory

legs (fig. 171). The next FIG. 171. HEAD OF AN EMBRYO PERIPA-
, . . r TUS. (From Moseley.)

and largest pair of appen- The figure shews the jaws (mamlil)lcs)> and

dagCS in the embryo are close to them epiblastic involutions, which

FIG. 170. VENTRAL VIEW OF THE
HEAD OF AN EMBRYO OF PERIPATUS CA-
PENSIS AT A LATE STAGE OF DEVELOP-
MENT.

/. thickening of epiblast of procepha-
lic lobe to form supra-oesophageal gan-
glion ; ///. process from procephalic lobe
growing over the first post-oral appen-
dage ; o. mouth; e. eye; i and 2, first
and second pair of post-oral appendages.

the oral papillae. They

grow into the supra-oesophageal ganglia. The
antennae, oral cavity, and oral papilhe are also

are chiefly remarkable for shewn.

containing the ducts of the slime glands which open at their
bases. They are without claws. The succeeding appendages
become eventually imperfectly five-jointed ; two claws are

TRACHEATA.

385

formed as cuticular investments of papillae in pockets of the
skin at the ends of their terminal joints.

I have been able to make a few observations on the internal structure of
the embryos from specimens supplied to me by Moseley. These are so far
confined to a few stages, one slightly earlier, the others slightly later, than
the embryo represented in fig. 168 B. The epiblast is formed of a layer of
columnar cells, two deep on the ventral surface, except along the median line
where there is a well-marked groove and the epiblast is much thinner (fig. 172).

The ventral cords of the trunk are formed as two independent epiblastic
thickenings. In my earlier stage these are barely separated from the
epiblast, but in the later ones are quite independent (fig. 172, v.n), and
partly surrounded by mesoblast.

The supra-cesophageal ganglia are formed as thickenings of the epiblast
of the ventral side of the procephalic lobes in front of the stomodaeum.
They are shewn at / in fig. 170. The thickenings of the two sides are at
first independent. At a somewhat later period an invagination of the
epiblast grows into each of these lobes. The openings of these invaginations
extend from the oral cavity forwards; and they are shewn in fig. 171 l.
Their openings become closed, and the walls of the invaginations constitute
a large part of the embryonic supra-cesophageal ganglia.

Similar epiblastic invaginations assist in forming the supra-cesophageal
ganglia of other Tracheata.
They are described in the sequel
for Insects, Spiders and Scor-
pions. The position of the supra-
cesophageal ganglia on the ven-
tral side of the procephalic lobes
is the same as that in other
Tracheata.

The mesoblast is formed, in
the earliest of my embryos, of
scattered cells in the fairly wide
space between the mesenteron
and the epiblast. There are two
distinct bands of mesoblast on
the outer sides of the nervous
cords. In the later stage the
mesoblast is divided into dis-
tinct somatic and splanchnic lay-
ers, both very thin ; but the two
layers are connected by trans-
verse strands (fig. 172). There

sp.w

$.m

FIG. 172. SECTION THROUGH THE TRUNK
OF AN EMBRYO OF PERIPATUS. The embryo
from which the section is taken was somewhat
younger than fig. 171.

sp.m. splanchnic mesoblast.

s.m. somatic mesoblast.

me. median section of body cavity.

k. lateral section of body cavity.

v.n. ventral nerve cord.

me. mesenteron.

1 This figure is taken from Moseley. The epiblastic invaginations are represented
in it very accurately, and though not mentioned in the text of the paper, Moseley
informs me that he has long been aware of the homology of these folds with those in
various other Tracheata.

B. II.

25

386 PROTOTRACHEATA.

are two special longitudinal septa dividing the body cavity into three
compartments, a median (me), containing the mesenteron, and two lateral
(Ic) containing the nerve cords. This division of the body cavity persists,
as I have elsewhere shewn, in the adult. A similar division is found in
some Chaetopoda, e.g. Polygordius.

I failed to make out that the mesoblast was divided into somites, and
feel fairly confident that it is not so in the stages I have investigated.

There is a section of the body cavity in the limbs as in embryo Myria-
pods, Spiders, etc.

In the procephalic lobe there is a well-developed section of the body
cavity, which lies dorsal to and in front of the rudiment of the supra-
cesophageal ganglia.

The alimentary tract is formed of a mesenteron (fig. 172), a stomo-
daeum, and proctodaeum. The wall of the mesenteron is formed, in the
stages investigated by me, of a single layer of cells with yolk particles,
and encloses a lumen free from yolk. The forward extension of the
mesenteron is remarkable.

The stomodaeum in the earlier stage is a simple pit, which meets but does
not open into the mesenteron. In the later stage the external opening of
the pit is complicated by the structures already described. The procto-
daeum is a moderately deep pit near the hinder end of the body.

The existence of a tracheal system1 is in itself almost sufficient to
demonstrate the affinities of Peripatus with the Tracheata, in spite of the
presence of nephridia. The embryological characters of the procephalic
lobes, of the limbs and claws, place however this conclusion beyond
the reach of scepticism. If the reader will compare the figure of Peripatus
with that of an embryo Scorpion (fig. 196 A) or Spider (fig. 200 C) or better
still with Metschnikoffs figure of Geophilus (No. 399) PI. xxi. fig. u,he
will be satisfied on this point.

The homologies of the anterior appendages are not very easy
to determine ; but since there does not appear to me to be suffi-
cient evidence to shew that any of the anterior appendages have
become aborted, the first post-oral appendages embedded in the
lips may provisionally be regarded as equivalent to the mandibles,
and the oral papillae to the first pair of maxillae, etc. Moseley is
somewhat doubtful about the homologies of the appendages,
and hesitates between considering the oral papillae as equivalent
to the second pair of maxillae (on account of their containing the
openings of the mucous glands, which he compares with the
spinning glands of caterpillars), or to the poison claws (fourth

1 The specimens shewing tracheae which Moseley has placed in my hands are
quite sufficient to leave no doubt whatever in my mind as to the general accuracy of
his description of the tracheal system.

TRACHEATA. 387

post-oral appendages) of the Chilopoda (on account of the
poison-glands which he thinks may be homologous with the
mucous glands).

The arguments for either of these views do not appear to me con-
clusive. There are glands opening into various anterior appendages in
the Tracheata, such as the poison glands in the Chelicerae (mandibles) of
Spiders, and there is some evidence in Insects for the existence of a gland
belonging to the first pair of maxillae, which might be compared with the
mucous gland of Peripatus. For reasons already stated I do not regard
the processes of the cephalic lobes, which form the lips, as a pair of true
appendages.

BIBLIOGRAPHY.

(396) H. N. Moseley. "On the Structure and Development of Peripatus
capensis." Phil. Trans. Vol. 164, 1874.

MYRIAPODA1.

Chilognatha. The first stages in the development of the
Chilognatha have been investigated by Metschnikoffand Stecker,
but their accounts are so contradictory as hardly to admit of
reconciliation.

According to Metschnikoff, by whom the following four
species have been investigated, viz., Strongylosoma Guerinii,
Polydesmus complanatus, Polyxenus lagurus, and Julus Mone-
letei, the segmentation is at first regular and complete, but,
when the segments are still fairly large, the regular segmentation
is supplemented by the appearance of a number of small cells at
various points on the surface, which in time give rise to a
continuous blastoderm.

The blastoderm becomes thickened on the ventral surface,
and so forms a ventral plate2.

1 The classification of the Myriapoda employed in the present section is

I. Chilognatha. (Millipedes.)
II. Chilopoda. (Centipedes.)

2 Stecker's (No. 400) observations were made on the eggs of Julus fasciatus, Julus
fcetidus, Craspedosoma marmoratum, Polydesmus complanatus, and Strongylosoma
pallipes, and though carried on by means of sections, still leave some points very
obscure, and do not appear to me deserving of much confidence. The two species of
Julus and Craspedosoma undergo, according to Stecker, a nearly identical develop-
ment. The egg before segmentation is constituted of two substances, a central proto-
plasmic, and a peripheral deutoplastic. It first divides into two equal segments, and
coincidentally with their formation part of the central protoplasm travels to the

25—2

388 CHILOGNATHA.

FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII.
(After Metschnikoff.)

A. Embryo on eleventh day with commencing ventral flexure (*).

B. Embryo with three pairs of post-oral appendages.

C. Embryo with five pairs of post-oral appendages.

gs. ventral plate; at. antenme; 1—5 post-oral appendages; x. point of flexure of
the ventral plate.

surface as two clear fluid segments. The ovum is thus composed of two yolk segments
to two protoplasmic segments. The two former next divide into four, with the pro-
duction of two fresh protoplasmic segments. The four protoplasmic segments now
constitute the upper or animal pole of the egg, and occupy the position of the future
ventral plate. The yolk segments form the lower pole, which is however dorsal in
relation to the future animal. The protoplasmic segments increase in number by a
regular division, and arrange themselves in three rows, of which the two outermost
rapidly grow over the yolk segments. A large segmentation cavity is stated to be
present in the interior of the ovum.

It would appear from Stecker's description that the yolk segments (hypoblast)
next become regularly invaginated, so as to enclose a gastric cavity, opening externally
by a blastopore; but it is difficult to believe that a typical gastrula, such as that
represented by Sleeker, really comes into the cycle of development of the Chilo-
gnatha.

The mesoblast is stated to be derived mainly from the epiblast. This layer in the
region of the future ventral plate becomes reduced to two rows of cells, and the inner
of these by the division of its constituent elements gives rise to the mesoblast. The
development of Polydesmus and Strongylosoma is not very different from that of Julus.
The protoplasm at the upper pole occupies from the first a superficial position.
Segmentation commences at the lower pole, where the food yolk is mainly present !
The gastrula is stated to be similar to that of Julus, The mesoblast is formed in
Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an
outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his
paper as to the origin of the mesoblast from the epiblast, sums up at the end to the
effect that both the primary layers have a share in the formation of the mesoblast,
which originates by a process of endogenous cell-division !

It may be noted that the closure of the blastopore takes place, according to
Stecker, on the dorsal side of the embryo.

TRACHEATA. 389

The most important sources of information for the general
embryology of the Chilognatha are the papers of Newport (No.
397) and Metschnikoff (No. 398). The development of Strongy-
losoma may be taken as fairly typical for the group ; and the
subsequent statements, unless the reverse is stated, apply to the
species of Strongylosoma investigated by Metschnikoff.

After the segmentation and formation of the layers the first
observable structure is a transverse furrow in the thickening of
the epiblast on the ventral surface of the embryo. This furrow
rapidly deepens, and gives rise to a ventral flexure of the embryo
(fig. 173 A, x\ which is much later in making its appearance in
Julus than in Strongylosoma and Polyxenus. A pair of ap-
pendages, which become the antennae, makes its appearance
shortly after the formation of the transverse furrow, and there
soon follow in order the next three pairs of appendages. All
these parts are formed in the infolded portion of the ventral
thickening of the blastoderm (fig. 173 B). The ventral thicken-
ing has in the meantime become marked by a longitudinal
furrow, but whether this is connected with the formation of
the nervous system, or is equivalent to the mesoblastic furrow in
Insects, and connected with the formation of the mesoblast, has
not been made out. Shortly after the appearance of the three
pairs of appendages behind the antennae two further pairs become
added, and at the same time oral and anal invaginations become
formed '(fig- 173 Q. In front of the oral opening an unpaired
upper lip is developed. The prse-oral part of the ventral plate
develops into the bilobed procephalic lobes, the epiblast of
which is mainly employed in the formation of the supra-cesopha-
geal ganglia. The next important change which takes place is
the segmentation of the body of the embryo (fig. 174 A), the
most essential feature in which is the division of the mesoblast
into somites. Segments are formed in order from before back-
wards, and soon extend to the region behind the appendages.
On the appearance of segmentation the appendages commence
to assume their permanent form. The two anterior pairs of
post-oral appendages become jaws ; and the part of the embryo
which carries them and the antennae is marked off from the
trunk as the head. The three following pairs of appendages
grow in length and assume a form suited for locomotion. Behind

390 CHILOGNATHA.

the three existing pairs of limbs there are developed three fresh
pairs, of whicJi tJie two anterior belong to a single primitive seg-
ment. While the above changes take place in the appendages
the embryo undergoes an ecdysis, which gives rise to a cuticular
membrane within the single egg-membrane (chorion, Metschni-
koff\ On this cuticle a tooth-like process is developed, the
function of which is to assist in the hatching of the embryo
(fig. 174 A).

In Polyxenus a cuticular membrane is present as in Strongylosoma,
but it is not provided with a tooth-like process. In the same form amoeboid
cells separate themselves from the blastoderm at an early period. These
cells have been compared to the embryonic envelopes of Insects described
below.

In Julus two cuticular membranes are present at the time of hatching :
the inner one is very strongly developed and encloses the embryo after
hatching. After leaving the chorion the embryo Julus remains connected
with it by a structureless membrane which is probably the outer of the two
cuticular membranes.

At the time when the embryo of Strongylosoma is hatched
(fig. 174 B) nine post-cephalic segments appear to be present.

FlG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUEKINll.

(After Metschnikoff.)

A. A seventeen days' embryo, already segmented.

B. A just-hatched larva.

Of these segments the second is apparently (from MetschnikofT's
figure, 174 B) without a pair of appendages; the third and

TRACHEATA. 391

fourth are each provided with a single functional pair of limbs ;
the fifth segment is provided with two pairs of rudimentary
limbs, which are involuted in a single sack and not visible with-
out preparation, and therefore not shewn in the figure. The
sixth segment is provided with but a single pair of" appendages,
though a second pair is subsequently developed on it1.

Julus, at the time it leaves the chorion, is imperfectly segmented, but is
provided with antennas, mandibles, and maxillae, and seven pairs of limbs,
of which the first three are much more developed than the remainder.
Segmentation soon makes its appearance, and the head becomes distinct
from the trunk, and on each of the three anterior trunk segments a single
pair of limbs is very conspicuous (Metschnikoff)2. Each of the succeeding
segments bears eventually two pairs of appendages. At the time when
the inner embryonic cuticle is cast off, the larva appears to be hexapodous,
like the young Strongylosoma, but there are in reality four pairs of rudi-
mentary appendages behind the three functional pairs. The latter only
appear on the surface after the first post-embryonic ecdysis. Pauropus
(Lubbock) is hexapodous in a young stage. At the next moult two pairs
of appendages are added, and subsequently one pair at each moult.

There appear to be eight post-oral segments in Julus at the
time of hatching. According to Newport fresh segments are
added in post-embryonic life by successive budding from a
blastema between the penultimate segment and that in front of
it. They arise in batches of six at the successive ecdyses, till
the full number is completed. A functional, though not a real
hexapodous condition, appears to be characteristic of Chilognatha
generally at the time of hatching.

The most interesting anatomical feature of the Chilognatha
is the double character of their segments, the feet (except the
first three or four, or more), the circulatory, the respiratory, and
the nervous systems shewing this peculiarity. Newport's and

1 Though the superficially hexapodous larva of Strongylosoma and other Chilo-
gnatha has a striking resemblance to some larval Insects, no real comparison is pos-
sible between them, even on the assumption that the three functional appendages of
both are homologous, because Embryology clearly proves that the hexapodous Insect
type has originated from an ancestor with numerous appendages by the atrophy of
those appendages, and not from an hexapodous larval form prior to the development
of the full number of adult appendages.

2 Newport states however that a pair of limbs is present on the first, second, and
fourth post-oral segments, but that the third segment is apodous ; and this is un-
doubtedly the case in the adult.

392

CHILOPODA.

Metschnikoff's observations have not thrown as much light on
the nature of the double segments as might have been hoped,
but it appears probable that they have not originated from a
fusion of two primitively distinct segments, but from a later
imperfect division of each of the primitive segments into two,
and the supply to each of the divisions of a primitive segment of
a complete set of organs.

Chilopoda. Up to the present time the development of only
one type of Chilopoda, viz. that of Geophilus, has been worked
out. Most forms lay their eggs, but Scolopendra is viviparous.

a u . i

FlG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS.

(After Metschnikoff.)

A. Side-view of embryo at the stage when the segments are beginning to be formed.

B. Later stage after the appendages have become established.

at. antenna.-; an.t. proctodseum.

The segmentation appears to resemble that in the Chilognatha,
and at its close there is present a blastoderm surrounding a
central mass of yolk cells. A ventral thickening of the blasto-
derm is soon formed. It becomes divided into numerous seg-
ments, which continue to be formed successively from the
posterior unsegmented part. The antennae are the first append-
ages to appear, and are well developed when eighteen segments
have become visible (fig. 175 A). The post-oral appendages
are formed slightly later, and in order from before backwards.
As the embryo grows in length, and fresh segments continue to
be formed, the posterior part of it becomes bent over so as to
face the ventral surface of the anterior, and it acquires an

TRACHEATA. 393

appearance something like that of many embryo Crustaceans
(fig. 175 B). Between forty and fifty segments are formed while
the embryo is still in the egg. The appendages long remain
unjointed. The fourth post-oral appendage, which becomes the
poison-claw, is early marked out by its greater size : on the
third post-oral there is formed a temporary spine to open the
egg membrane.

It does not appear, from Metschnikoff's figures of Geophilus, that any
of the anterior segments are without appendages, and it is very probable
that Newport is mistaken in supposing that the embryo has a segment with-
out appendages behind that with the poison claws, which coalesces with the
segment of the latter. It also appears to me rather doubtful whether the
third pair of post-oral appendages, i.e. those in front of the poison-claws, can
fairly be considered as forming part of the basilar plate. The basilar plate
is really the segment of the poison-claws, and may fuse more or less com-
pletely with the segment in front and behind it, and the latter is sometimes
without a pair of appendages (Lithobius, Scutigera).

Geophilus, at the time of birth, has a rounded form like that
of the Chilognatha.

The young of Lithobius is born with only six pairs of limbs.

General observation on the homologies of the appendages of
Myriapoda.

The chief difficulty in this connection is the homology of the third pair of
post-oral appendages.

In adult Chilognatha there is present behind the mandibles a four-lobed
plate, which is usually regarded as representing two pairs of appendages,
viz. the first and second pairs of maxillae of Insects. Metschnikoff's ob-
servations seem however to shew that this plate represents but a single
pair of appendages, which clearly corresponds with the first pair of maxillae
in Insects. The pair of appendages behind this plate is ambulatory, but
turned towards the head ; it is in the embryo the foremost of the three
functional pairs of legs with which the larva is born. Is it equivalent to
the second pair of maxillae of Insects or to the first pair of limbs of Insects?
In favour of the former view is the fact (i) that in embryo Insects the
second pair of maxillae sometimes resembles the limbs rather than the
jaws, so that it might be supposed that in Chilognatha a primitive
ambulatory condition of the third pair of appendages has been retained ;
(2) that the disappearance of a pair of appendages would have to be
postulated if the second alternative is adopted, and that if Insects are
descended from forms related to the Myriapods it is surprising to find a
pair of appendages always present in the former, absent in the latter.

394

MYRIAPODA.

The arguments which can be urged for the opposite view do not appear
to me to have much weight, so that the homology of the appendages in
question with the second pair of maxillae may be provisionally assumed.

The third pair of post-oral appendages of the Chilopoda may probably
also be assumed to be equivalent to the second pair of maxillae, though they
are limb-like and not connected with the head. The subjoined table shews
the probable homologies of the appendages.

CHILOGNATHA(Strongylo-

so ma at time of birth).

CHILOPODA (Scolopendra
adult).

Pre-oral region.

Antennae.

Antennas.

ist Post-oral segment.

Mandibles.

Mandibles.

2nd ,, ,,

Maxillae i. (Four-lobed
plate in adult, but a sim-
ple pair of appendages
in embryo).

Maxillie i.
(Palp and bilobed median
process).

3rd „
(probably equivalent to
segment bearing 2nd pair
of maxillae in Insects).

ist pair of ambulatory
limbs.

Limb-like appendages with
basal parts in contact.

4th ,, ,,

(?) Apodous.

Poison claws.

5th

2nd pair of ambulatory
limbs.

ist pair of ambulatory
limbs.

6th

3rd „ ,, „

2nd „ ,,

7th

4th and sth „ „
(rudimentary. )

3rd- „ „ „

8th ,, ,,

6th
(the 7th pair is developed
in this segment later).

4th „ „ „

9th

Apodous.

5th

loth ,,

,, (last segment in
embryo).

6th

The germinal layers and formation of organs.

The development of the organs of the Myriapoda, and the origin of the
germinal layers, are very imperfectly known : Myriapoda appear however
to be closely similar to Insects in this part of their development, and the
general question of the layers will be treated more fully in connection with
that group.

The greater part of the blastoderm gives rise to the epiblast, which
furnishes the skin, nervous system, tracheal system, and the stomodacum
and proctodaeum.

TRACHEATA. 395

The mesoblast arises in connection with the ventral thickening of the
blastoderm, but the details of its formation are not known. Metschnikoff
describes a longitudinal furrow which appears very early in Strongylosoma,
which is perhaps equivalent to the mesoblastic furrows of Insects, and so
connected with the formation of the mesoblast.

The mesoblast is divided up into a series of protovertebra-like bodies —
the mesoblastic somites — the cavities of which become the body cavity and
the walls the muscles and probably the heart. They are (Metschnikoff)
prolonged into the legs, though the prolongations become subsequently
segmented off from the main masses. The splanchnic mesoblast is,
according to Metschnikoff, formed independently of the somites, but this
point requires further observation.

The origin of the hypoblast remains uncertain, but it appears probable
that it originates, in a large measure at least, from the yolk segments. In
the Chilognatha the mesenteron is formed in the interior of the yolk seg-
ments, so that those yolk segments which are not employed in the formation
of the alimentary canal lie freely in the body cavity. In the relation of
the yolk segments to the alimentary canal the Chilopoda present a strong
contrast to the Chilognatha, in that the greater part of the yolk lies
within their mesenteron. The mesenteron is at first a closed sack, but is
eventually placed in communication with the stomodaeum and the procto-
dasum. The Malpighian bodies arise as outgrowths from the blind ex-
tremity of the latter.

BIBLIOGRAPHY.

(397) G. N e wp or t. " On the Organs of Reproduction and Development of the
Myriapoda." Philosophical Transactions, 1841.

(398) E. Metschnikoff. ' ' Embryologie der doppeltflissigen Myriapoden (Chi-
lognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.

(399) ' ' Embryologisches iiber Geophilus." Zeit. f. wiss. ZooLy Vol. xxv.

1875-

(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
Archivf. mik. Anatomie, Bd. xiv. 1877.

INSECTA1.

The formation of the embryonic layers in Insects has not
been followed out in detail in a large number of types ; but, as

1 The following classification of the Insecta is employed in this chapter,
((i) Collembola.

I. Aptera. |(a) Thysanura.

!(i) Orthoptera genuina (Blatta, Locusta, etc.).
(2) „ pseudoneuroptera (Termes, Ephemera,

Libellula).

!(i) Hemiptera heteroptera (Cimex, Notonecta, etc.).
(2) ,, homoptera (Aphis, Cicada, etc.).

(3) ,, parasita (Pediculus, etc.).

396

INSECTA.

in so many other instances, some of the most complete histories
we have are due to Kowalevsky (No. 416). The development

FiG. 176. FOUR EMBRYOS OF llYDROPHlLUS P1CEUS VIEWED FROM THE

VENTRAL SURFACE. (After Kowalevsky.)
The upper end is the anterior, gg. germinal groove; am. amnion.

of Hydrophilus has been worked out by him more fully than
that of any other form, and will serve as a type for comparison
with other forms.

The segmentation has not been studied, but no doubt belongs
to the centrolecithal type (vide pp. no — 120). At its close
there is an uniform layer of cells enclosing a central mass of
yolk. These cells, in the earliest observed stage, were flat on
the dorsal, but columnar on part of the ventral surface of the
egg, where they form a thickening which will be called the ven-
tral plate. At the posterior part of the ventral plate two folds,
with a furrow between them, make their appearance. They form
a structure which may be spoken of as the germinal groove (fig.

!(i) Diptera genuina (Musca, Tipula, etc.).
(2) „ aphaniptera (Pulex, etc.).
(3) ,, pupipara (Braula, etc.).

v .. ( (i) Neuroptera planipennia (Myrniclcon, etc.)-

TOptera. j (a) ^ trichoptera (Phryganea, etc.).

VI. Coleoptera.
VII. Lepidoptera.

(i) Hymenoptera aculeata (Apis, Formica, etc.).
(a) ,, entomophaga (Ichneumon, Platy-

gaster, etc).
(3) ,, phytophaga ( Tenthredo, Sirex, etc.).

VIII. Hymenoptera.

TRACHEATA.

397

y*

FlG. 177. TWO TRANSVERSE SECTIONS THROUGH

EMBRYOS OF HvDROPHiLUS piCEUS. (After Kowa-
levsky.)

A. Section through an embryo of the stage repre-
sented in fig. 176 B, at the point where the two
germinal folds most approximate.

B. Section through an embryo somewhat later
than the stage fig. 176 D, through the anterior region
where the amnion has not completely closed over the
embryo.

). The cells
which form the floor
of the groove are far
more columnar than
those of other parts
of the blastoderm (fig.
177 A). The two
folds on each side of
it gradually approach
each other. They do
so at first behind, and
then in the middle;
from the latter point
the approximation
gradually extends
backwards and for-
wards (fig. 176 B and
C). In the middle
and hinder parts of
the ventral plate the
groove becomes, by
the coalescence of the folds, converted into a canal (fig. 178 A,
gg), the central cavity of which soon disappears, while at the
same time the cells of the wall undergo division, become more
rounded, and form a definite layer (me} — the mesoblast — beneath
the columnar cells of the surface. Anteriorly the process is
slightly different, though it leads to the similar formation of
mesoblast (fig. 177 B). The flat floor of the groove becomes in
front bodily converted into the mesoblast, but the groove itself
is never converted into a canal. The two folds simply meet
above, and form a continuous superficial layer.

During the later stages of the process last described remark-
able structures, eminently characteristic of the Insecta, have
made their first appearance. These structures are certain
embryonic membranes or coverings, which present in their mode
of formation and arrangement a startling similarity to the true
and false amnion of the Vertebrata. They appear as a double
fold of the blastoderm round the edge of the germinal area,
which spreads over the ventral plate, from behind forwards, in a

gg. germinal groove ;
nion ; yk. yolk.

me. mesoblast ; am. am-

INSECT A.

general way in the same
manner as the amnion in,
for instance, the chick.
The folds at their origin
are shewn in surface view
in fig. 176 D, am, and in
section in fig. 177 B, am.
The folds eventually
meet, coalesce (fig. 178,
am) and give rise to two
membranes covering the
ventral plate, viz. an
inner one, which is con-
tinuous with the edge of
the ventral plate ; and
an outer, continuous with
the remainder of the
blastoderm. The verte-
brate nomenclature may
be conveniently employ-
ed for these membranes.
The inner limb of the
fold will therefore be spoken of as the amnion, and the outer
one, including the dorsal part of the blastoderm, as the
serous envelope1. A slight consideration of the mode of
formation of the membranes, or an inspection of the figures
illustrating their formation, makes it at once clear that the yolk
can pass in freely between the amnion and serous envelope (vide
fig. 181). At the hind end of the embryo this actually takes
place, so that the ventral plate covered by the amnion appears to
become completely imbedded in the yolk: elsewhere the two
membranes are in contact. At first (fig. 176) the ventral plate
occupies but a small portion of the ventral surface of the egg, but
during the changes above described it extends over the whole
ventral surface, and even slightly on the dorsal surface both in
front and behind. It becomes at the same time (fig. 179) divided

FIG. 178. SECTIONS THROUGH TWO EMBRYOS
OF HYDROPHILUS PICEUS. (After Kowalevsky.)

A. Section through the posterior part of the
embryo fig. 1 76 D, shewing the completely closed
amnion and the germinal groove.

B. Section through an older embryo in which
the mesoblast has grown out into a continuous
plate beneath the epiblast.

gg. germinal groove ; am. amnion ; yk. yolk ;
cp. epiblast.

1 The reverse nomenclature to this is rather inconveniently employed by Metsch-
nikoff.

TRACHEATA.

399

FIG. 179. EMBRYO OF
HYDROPHILUS PICEUS

VIEWED FROM THE VEN-

TRAL SURFACE. (After
Kowalevsky.)

pc.L procephalic lobe.

by a series of transverse lines into segments, which increase in
number and finally amount in all to seven-
teen, not including the most anterior section,
which gives off as lateral outgrowths the
two procephalic lobes (pc.l). The changes
so far described are included within what
Kowalevsky calls his first embryonic period;
at its close the parts contained within the
chorion have the arrangement shewn in fig.
178 B. The whole of the body of the
embryo is formed from the ventral plate,
and no part from the amnion or serous
envelope.

The general history of the succeeding
stages may be briefly told.

The appendages appear as very small
rudiments at the close of the last stage, but
soon become much more prominent (fig.
1 80 A). They are formed as outgrowths of both layers, and
arise nearly simultaneously. There
are in all eight pairs of appendages.
The anterior or antennae (at) spring
from the procephalic lobes, and
the succeeding appendages from
the segments following. The last
pair of embryonic appendages,
which disappears very early, is
formed behind the third pair of
the future thoracic limbs. Paired
epiblastic involutions, shewn as pits
in the posterior segments in fig.
1 80 A, give rise to the tracheae;
and the nervous system is formed
as two lateral epiblastic thicken-
ings, one on each side of the mid-
ventral line. These eventually be-
come split off from the skin ; while
between them there passes in a
median invagination of the skin

FlG. 1 80. TWO STAGES IN THE
DEVELOPMENT OF HYDROPHILUS

PICEUS. (From Gegenbaur, after
Kowalevsky.)

Is. labrum ; at. antenna ; md.

400 INSECTA.

(fig. 189 C). The two nervous strands are continuous in front
with the supra-oesophageal ganglia, which are formed of the
epiblast of the procephalic lobes. These plates gradually grow
round the dorsal side of the embryo, and there is formed
immediately behind them an oral invagination, in front of which
there appears an upper lip (fig. 180, Is). A proctodaeum is formed
at the hind end of the body slightly later than the stomodaeum.
The mesoblast cells become divided into two bands, one on
each side of the middle line (fig. 189 A), and split into
splanchnic and somatic layers. The central yolk mass at about
the stage represented in fig. 179 begins to break up into
yolk spheres. The hypoblast is formed first on the ventral
side at the junction of the mesoblast and the yolk, and
gradually extends and forms a complete sack-like mesenteron,
enveloping the yolk (fig. 185 al). The amnion and serous
membrane retain their primitive constitution for some time, but
gradually become thinner on the ventral surface, where a rupture
appears eventually to take place. The greater part of them
disappears, but in the closure of the dorsal parietes the serous
envelope plays a peculiar part, which is not yet understood. It
is described on p. 404. The heart is formed from the mesoblas-
tic layers, where they meet in the middle dorsal line (fig. 185 C,
hi]. The somatic mesoblast gives rise to the muscles and
connective tissue, and the splanchnic mesoblast to the muscular
part of the wall of the alimentary tract, which accompanies the
hypoblast in its growth round the yolk. The proctodaeum
forms the rectum and Malpighian bodies1, and the stomodseum
the oesophagus and proventriculus. The two epiblastic sections
of the alimentary tract are eventually placed in communication
with the mesenteron.

The development of Hydrophilus is a fair type of that of
Insects generally, but it is necessary to follow with somewhat
greater detail the comparative history of the various parts which
have been briefly described for this type.

TJte embryonic membranes and the formation of the layers.

All Insects have at the close of segmentation a blastoderm
formed of a single row of cells enclosing a central yolk mass,

1 This has not been shewn in the case of Hydrophilus,

TRACHEATA.

401

which usually contains nuclei, and in the Poduridae is divided up
in the ordinary segmentation into distinct yolk cells. The first
definite structure formed is a thickening of the blastoderm,
which forms a ventral plate.

The ventral plate is very differently situated in relation to the yolk in
different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phry-
ganea) it forms from the first a thickening extending over nearly the
whole ventral surface of the ovum, and in many cases extends in its sub-
sequent growth not only over the whole ventral surface, but over a con-
siderable part of the apparent dorsal surface as well (Chironomus, Simulia,
Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less
extended thickening either of the central part (Donacia) or posterior part
(Hydrophilus) of the ventral surface, and gradually grows in both directions,
passing over to the dorsal surface behind.

Embryonic membranes. In the majority of Insects there
are developed enveloping membranes like those of Hydrophilus.

The typical mode of formation of these membranes is repre-
sented diagrammatically in fig. 181 A and B. A fold of the
blastoderm arises round the edge of the ventral plate. This
fold, like the am-
niotic fold of the
higher Vertebrata,
is formed of two
limbs, an outer,
the serous mem-
brane (se), and an
inner, the true am-
nion (am). Both
limbs extend so
as to cover over
the ventral plate,
and finally meet
and coalesce, so
thatadouble mem-
brane is present
over the ventral
plate. At the same
time (fig. 181 B)
the point where the fold originates is carried dorsalwards by the
B. II. 26

Sf

FIG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS
OF AN INSECT EMBRYO AT TWO STAGES TO SHEW THE

DEVELOPMENT OF THE EMBRYONIC ENVELOPES.

In A the amniotic folds have not quite met so as to
cover the ventral plate. The yolk is represented as divided
into yolk cells. In B the sides of the ventral plate have
extended so as nearly to complete the dorsal integument.
The mesenteron is represented as a closed sack filled with
yolk cells, am. amnion; se. serous envelope; v.p. ven-
tral plate ; d. i. dorsal integument ; me. mesenteron ; st.
stomodaeum ; an i. proctodaeum.

4O2 INSECTA.

dorsal extension of the edges of the ventral plate, which give
rise to the dorsal integument (d.i). This process continues
till the whole dorsal surface is covered by the integument.
The amnion then separates from the dorsal integument, and the
embryo becomes enveloped in two membranes — an inner, the
amnion, and an outer, the serous membrane. In fig. 181 B the
embryo is represented at the stage immediately preceding the
closure of the dorsal surface.

By the time that these changes are effected, the serous
membrane and amnion are both very thin and not easily
separable. The amnion appears to be usually absorbed before
hatching; but in hatching both membranes, if present, are either
absorbed, or else ruptured and thrown off.

The above mode of development of the embryonic membranes has been
especially established by the researches of Kowalevsky (No. 416) and Graber
(No. 412) for various Hymenoptera (Apis), Diptera (Chironomus\ Lepido-
ptera and Coleoptera (Melolontha, Lino).

Considerable variations in the development of the enveloping membranes
are known.

When the fold which gives rise to the membranes is first formed, there
is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk can
pass in between the amnion and serous membrane. Such a passage of the
yolk between the two membranes takes place posteriorly in Hydrophilus and
Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form
the ventral plate becomes first of all imbedded in the yolk, and finally, on the
completion of the dorsal integument, the embryo is enclosed in a complete
envelope of yolk contained between the amnion and the serous membrane.
During the formation of the dorsal integument the external yolk sack com-
municates by a dorsally situated umbilical canal with the yolk cavity within
the body. On the rupture of the amnion the embryo is nourished at the
expense of the yolk contained in the external yolk sack.

In the Hemiptera and the Libellulidae the ventral plate also becomes
imbedded in the yolk, but in a somewhat different fashion to the Lepido-
ptera, which more resembles on an exaggerated scale what takes place in
Hydrophilus.

In the Libellulidas (Calopteryx) there is first of all formed (Brandt, No.
403) a small ventral and posterior thickening of the blastoderm (fig. 182 A).
The hinder part of this becomes infolded into the yolk as a projection (fig.
182 B), which consists of two laminae, an anterior and a posterior, continuous
at the apex of the invagination. The whole structure, which is completely
imbedded within the yolk, rapidly grows in length, and turns towards
the front end of the egg (fig. 182 C). Its anterior lamina remains thick and
gives rise to the ventral plate (ps), the posterior (am) on the other hand

TRACHEATA.

403

becomes very thin, and
forms a covering corre-
sponding with the amnion
of the more ordinary types.
The remainder of the blas-
toderm covering the yolk
(se) forms the homologue
of the serous membrane
of other types. The ven-
tral surface of the ventral
plate is turned towards
the dorsal side (retaining
the same nomenclature as
in ordinary cases) of the
egg, and the cephalic
extremity is situated at
the point of origin of the
infolding.

The further history is
however somewhat pecu-
liar. The amnion is at first
(fig. 182 C) continuous with
the serous envelope on the
posterior side only, so that
the serous envelope does
not form a continuous sack,
but has an opening close
to the head of the embryo.
In the Hemiptera parasita this opening (Melnikow, No. 422) remains per-
manent, and the embryo, after it has reached a certain stage of development,
becomes everted through it, while the yolk, enclosed in the continuous mem-
brane formed by the amnion and serous envelope, forms a yolk sack on the
dorsal surface. In the Libellulidae however and most Hemiptera, a fusion of
the two limbs of the serous membrane takes place in the usual way, so as to
convert it into a completely closed sack (fig. 183 A). After the formation of
the appendages a fusion takes place between the amnion and serous enve-
lope over a small area close to the head of the embryo. In the middle of
this area a rupture is then effected, and the head of the embryo followed by
the body is gradually pushed through the opening (fig. 183 B and C). The
embryo becomes in the process completely rotated, and carried into a
position in the egg-shell identical with that of the embryos of other orders of
Insects (fig. 183 C).

Owing to the rupture of the embryonic envelopes taking place at the
point where they are fused into one, the yolk does not escape in the above
process, but is carried into a kind of yolk sack, on the dorsal surface of the
embryo, formed of the remains of the amnion and serous envelope. The

26—2

FIG. 182. THREE STAGES IN THE DEVELOPMENT

OF THE EMBRYO OF CALOPTERYX. (After Brandt.)

The embryo is represented in the egg-shell.

A. Embryo with ventral plate.

B. Commencing involution of ventral plate.

C. Involution of ventral plate completed.

ps. vefitral plate; g. edge of ventral plate; am.
amnion ; se- serous envelope.

404

INSECTA.

walls of the yolk sack either
assist in forming the dorsal
parietes of the body, or are
more probably enclosed
within the body by the
growth of the dorsal pari-
etes from the edge of the
ventral plate.

In Hydrophilus and
apparently in the Phry-
ganidae also, there are cer-
tain remarkable peculiari-
ties in the closure of the
dorsal surface. The fullest
observations on the subject
have been made by Kowa-
levsky (No. 416), but Dohrn
(No. 408) has with some
probability thrown doubts
on Kowalevsky's interpreta-
tions. According to Dohrn
the part of the serous enve-
lope which covers the dor-
sal surface becomes thick-
ened, and gives rise to a
peculiar dorsal plate which
is shewn in surface view in
ventral parts of the amnion
and serous membrane have
either been ruptured or
have disappeared. While
the dorsal plate is being
formed, the mesoblast, and
somewhat later the lateral
parts of the epiblast of the
ventral plate gradually
grow towards the dorsal
side and enclose the dorsal
plate, the wall of which in
the process appears to be
folded over so as first of
all to form a groove and
finally a canal. The stages
in this growth are shewn
from the surface in fig. 184
B and C and in section in

FlG. 183. THREE STAGES IN THE DEVELOPMENT

OF CALOPTERYX. (After Brandt.)

The embryo is represented in the egg-shell; B.
and C. shew the inversion of the embryo.

sf. serous envelope ; am. amnion ; ab. abdomen ;
v. anterior end of head ; at. antennae ; md. mandible ;
mxl. maxilla i ; mx*. maxilla 2 ; p1—^. three pairs
of legs; oe. oesophagus.

fig. 184 A, doi and in section in fig. 185 A, do. The

FIG. 184. THREE LARVAL STAGES OF HYDRO-
PHILUS FROM THE DORSAL SIDE, SHEWING THE
GRADUAL CLOSING IN OF THE DORSAL REGION WITH
THE FORMATION < >!• THK I'l.CULIAR DORSAL ORGAN

do. (After Kowalevsky.)

do. dorsal organ ; at. antennae.

TRACHEATA.

405

fig. 185 B, do. The canal is buried on the dorsal part of the yolk, but for
some time remains open by a round aperture in front (fig. 184 C). The
whole structure is known as the dorsal canal. It appears to atrophy without
leaving a trace. The heart when formed lies immediately dorsal to it1.

A.
B.
C.

vn

FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED

EMBRYOS OF HYDROPHILUS.

Section through the posterior part of the body of the same age as fig. 184 A.
Section through the embryo of the same age as fig. 184 C.
Section through a still older embryo.
do. dorsal plate ; vn. ventral nerve cord ; al. mesenteron ; ht. heart.
The large spaces at the sides are parts of the body cavity.

In the Poduridas the embryonic membranes appear to be at any rate
imperfect. Metschnikoff states in his paper on Geophilus that in some ants
no true embryonic membranes are found, but merely scattered cells which
take their place. In the Ichneumonidas the existence of two embryonic
membranes is very doubtful.

Formation of the embryonic layers. The formation of the
layers has been studied in sections by Kowalevsky (No. 416),

1 According to Kowalevsky the history of the dorsal plate is somewhat different.
He believes that on the absorption of the amnion the ventral plate unites with the
serous membrane, and that the latter directly gives rise to the dorsal integument,
while the thickened part of it becomes involuted to form the dorsal tube already
described.

406 INSECTA.

Hatschek (No. 414), and Graber (No. 412), etc. From their
researches it would appear that the formation of the mesoblast
always takes place in a manner closely resembling that in
Hydrophilus. The essential features of the process (figs. 177
and 178) appear to be that a groove is formed along the median
line of the ventral plate, and that the sides of this groove either
(i) simply close over like the walls of the medullary groove in
Vertebrates, and so convert the groove into a tube, which soon
becomes solid and forms a mass or plate of cells internal to the
epiblast ; or (2) that the cells on each side of the groove grow
over it and meet in the middle line, forming a layer external
to the cells which lined the groove. The former of these
processes is the most usual ; and in the Muscidae the dimensions
of the groove are very considerable (Graber, No. 411). In both
cases the process is fundamentally the same, and causes the
ventral plate to become divided into two layers1. The external
layer or epiblast is an uniform sheet forming the main part of
the ventral plate (fig. 178 B, ep). It is continuous at its edge
with the amnion. The inner layer or mesoblast constitutes an
independent plate of cells internal to the epiblast (fig. 178 B, me).
The mesoblast soon becomes divided into two lateral bands.

The origin of the hypoblast is still in dispute. It will be
remembered (vide pp. 1 14 and 1 16) that after the segmentation a
number of nuclei remain in the yolk ; and that eventually a
secondary segmentation of the yolk takes place around these
nuclei, and gives rise to a mass of yolk cells, which fill up the
interior of the embryo. These cells are diagrammatically shewn
in figs. 181 and 189, and it is probable that they constitute the
true hypoblast. Their further history is given below.

Formation of the organs and their relation to the germinal

layers.

The segments and appendages. One of the earliest
phenomena in the development is the appearance of transverse
lines indicating segmentation (fig. 186). The transverse lines
are apparently caused by shallow superficial grooves, and also in

1 Tichomiroff (No. 420) denies the existence of a true invagination to form the
mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can
take place at other parts besides the median ventral line.

TRACHEATA.

407

many cases by the division of the mesoblastic bands into
separate somites. The most anterior line marks off a prae-oral
segment, which soon sends out two lateral wings — the procephalic
lobes. The remaining segments are at first fairly uniform.
Their number does not, however, appear to be very constant.
So far as is known they never exceed seventeen, and this
number is probably the typical one (figs. 186 and 187).

In Diptera the number appears to be usually fifteen though it may be
only fourteen. In Lepidoptera and in Apis there appear to be sixteen
segments. These and other variations affect only the number of the segments
which form the abdomen of the adult.

The appendages arise as paired pouch-
like outgrowths of the epiblast and meso-
blast ; and their number and the order of
their appearance are subject to considerable
variation, the meaning of which is not yet
clear. As a rule they arise subsequently to
the segmentation of the parts of the body
to which they belong. There is always
formed one pair of appendages which spring
from the lateral lobes of the procephalic
region, or from the boundary line between
these and the median ventral part of this
region. These appendages are the antennae.
They have in the embryo a distinctly ven-
tral position as compared to that which
they have in the adult.

In the median ventral part of the pro-
cephalic region there arises the labrum (fig. 187, Is}. It is formed
by the coalescence of a pair of prominences very similar to true
appendages, though it is probable that they have not this
value1.

1 If these structures are equivalent to appendages, they may correspond to one of
the pairs of antennae of Crustacea. From a figure by Fritz Miiller of the larva of
Calotermes (Jenaische Zeit. Vol. XI. pi. n, fig. 12) it would appear that they lie in
front of the true antennae, and would therefore on the above hypothesis correspond to
the first pair of antennae of Crustacea. Biitschli (No. 405) describes in the Bee a pair
of prominences immediately in front of the mandibles which eventually unite to form
a kind of underlip ; they in some ways resemble true appendages.

FIG. 1 86. EMBRYO
OF HYDROPHILUS PI-

CEUS VIEWED FROM THE
VENTRAL SURFACE.

(After Kowalevsky.)
pc. I. procephalic lobe.

408

INSECTA.

The antennae themselves can hardly be considered to have
the same morphological value as the succeeding appendages.
They are rather equivalent to paired processes of the prae-oral
lobes of the Chaetopoda.

From the first three post-oral segments there grow out the
mandibles and two pairs of maxillae, and from the three following
segments the three pairs of thoracic appendages. In many
Insects (cf. Hydrophilus) a certain .number of appendages of the
same nature as the anterior ones are visible in the embryo on
the abdominal segments, a fact which shews that Insects are
descended from ancestors with more than three pairs of ambu-
latory appendages.

In Apis according to Biitschli (No. 405) all the abdominal segments are
provided with appendages, which always
remain in a very rudimentary condition.
All trace of them as well as of the thoracic
appendages is lost by the time the embryo
is hatched. In the phytophagous Hy-
menoptera the larva is provided with
9 — ii pairs of legs.

In the embryo of Lepidoptera there
would appear from Kowalevsky's figures
to be rudiments of ten pairs of post-tho-
racic appendages. In the caterpillar of
this group there are at the maximum five
pairs of such rudimentary feet, viz. a pair
on the 3rd, 4th, 5th, and 6th, and on the
last abdominal segment. The embryos
of Hydrophilus (fig. 187), Mantis, etc. are
also provided with additional appendages.
In various Thysanura small prominences
are present on more or fewer of the abdo-
minal segments (fig. 192), which may
probably be regarded as rudimentary
feet.

Whether all or any of the appendages
of various kinds connected with the
hindermost segments belong to the same
category as the legs is very doubtful. Their usual absence in the embryo or
in any case their late appearance appears to me against so regarding them ;
but Biitschli is of opinion that in the Bee the parts of the sting are related
genetically to the appendages of the penultimate and antepenultimate abdo-
minal segments, and this view is to some extent supported by more recent

FlG. 187. TWO STAGES IN THE
DEVELOPMENT OF HYDROPHILUS

PICEUS. (From Gegenbaur, after
Kowalevsky. )

Is. labrum; at. antenna; tnd.
mandible; nix. maxilla I.; li. max-
illa II.; //>"/"• feet; a. anus.

TRACHEATA.

409

observations (Kraepelin, etc.), and if it holds true for the Bee must be regarded
as correct for other cases also.

As to the order of the appearance of the appendages observations are as
yet too scanty to form any complete scheme. In many cases all the appen-
dages appear approximately at the same moment, e.g. Hydrophilus, but
whether this holds good for all Coleoptera is by no means certain. In Apis
the appendages are stated by Biitschli to arise simultaneously, but according
to Kowalevsky the two mouth appendages first appear, then the antennae,
and still later the thoracic appendages. In the Diptera the mouth appen-
dages are first formed, and either simultaneously with these, or slightly later,
the antennae. In the Hemiptera and Libellulidae the thoracic appendages
are the first to be formed, and the second pair of maxillae makes its appear-
ance before the other cephalic appendages.

The history of the changes in the embryonic appendages during the
attainment of the 'adult con- .

dition is beyond the scope
of this treatise, but it may
be noted that the second
pair of maxillae are rela-
tively very large in the
embryo, and not infre-
quently (Libellula, etc.)
have more resemblance to
the ambulatory than to the
masticatory appendages.

The exact nature of the
wings and their relation to
the other segments is still
very obscure. They ap-
pear as dorsal leaf-like ap-
pendages on the 2nd and
3rd thoracic segments, and
are in many respects simi-
lar to the tracheal gills
of the larvae of Epheme-
ridae and Phryganidae (fig.
1 88 A), of which they are
supposed by Gegenbaur
and Lubbock to be modifi-
cations. The undoubtedly
secondary character of the
closed tracheal system of
larvae with tracheal gills
tells against this view.
Fritz Miiller finds in the
larvae of Calotermes ru-

FIG. 188. FIGURES ILLUSTRATING AQUATIC RE-
SPIRATION IN INSECTS. (After Gegenbaur.)

A. Hinder portion of the body of Ephemera
vulgata. a. longitudinal tracheal trunks; b. alimen-
tary canal ; c. tracheal gills.

B. Larva of ^Eschna grandis. a. superior longi-
tudinal tracheal trunks ; b. their anterior end ; c. por-
tion branching on proctodaeum ; o. eyes.

C. Alimentary canal of the same larva from the
side, a, b, and c. as in B ; d. inferior tracheal trunk ;
e. transverse branches between upper and lower
tracheal trunks.

410 INSECTA.

gosus (one of the Termites) that peculiar and similar dorsal appendages are
present on the two anterior of the thoracic segments. They are without
tracheae. The anterior atrophies, and the posterior acquires tracheas and gives
rise to the first pair of wings. The second pair of wings is formed from
small processes on the third thoracic segment like those on the other two.
Fritz Miiller concludes from these facts that the wings of Insects are
developed from dorsal processes of the body, not equivalent to the ventral
appendages. What the primitive function of these appendages was is not
clear. Fritz Miiller suggests that they may have been employed as respira-
tory organs in the passage from an aqueous to a terrestrial existence, when
the Termite ancestors lived in moist habitations — a function for which pro-
cesses supplied with blood-channels would be well adapted. The undoubted
affinity of Insects to Myriapods, coupled with the discovery by Moseley of a
tracheal system in Peripatus, is however nearly fatal to the view that Insects
can have sprung directly from aquatic ancestors not provided with tracheae.
But although this suggestion of Fritz Miiller cannot be accepted, it is still
possible that the processes discovered by him may have been the earliest
rudiments of wings, which were employed first as organs of propulsion by a
water-inhabiting Insect ancestor which had not yet acquired the power of
flying.

The nervous system. The nervous system arises entirely
from the epiblast; but the development of the prae-oral and
post-oral sections may be best considered separately.

The post-oral section, or ventral cord of the adult, arises as
two longitudinal thickenings of the epiblast, one on each side of
the median line (fig. 189 B, vn), which are subsequently split ofif
from the superficial skin and give rise to the two lateral strands
of the ventral cord. At a later period they undergo a differenti-
ation into ganglia and connecting cords.

Between these two embryonic nerve cords there is at first a shallow
furrow, which soon becomes a deep groove (fig. 189 C). At this stage the
differentiation of the lateral elements into ganglia and commissures takes
place, and, according to Hatschek (No. 414), the median groove becomes in
the region of the ganglia converted into a canal, the walls of which soon fuse
with those of the ganglionic enlargements of the lateral cords, and connect
them across the middle line. Between the ganglia on the other hand the
median groove undergoes atrophy, becoming first a solid cord interposed
between the lateral strands of the nervous system, and finally disappearing
without giving rise to any part of the nervous system. It is probable that
Hatschek is entirely mistaken about the entrance of a median element into
the ventral cord, and that the appearances he has described are due to
shrinkage. In Spiders the absence of a median element can be shewn with
great certainty, and, as already stated, this element is not present in

TRACHEATA.

411

Peripatus. Hatschek states that in the mandibular segment the median
element is absorbed, and that the two lateral cords of that part give rise to
the oesophageal commissures, while the sub-cesophageal ganglion is formed
from the fusion of the ganglia of the two maxillary segments.

The prae-oral portion of the nervous system consists entirely
of the supra-cesophageal ganglion. It is formed, according to
Hatschek, of three parts. Firstly and mainly, of a layer sepa-

FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
HYDROPHILUS. (After Kowalevsky.)

A. Transverse section through the larva represented in fig. 187 A.

B. Transverse section through a somewhat older embryo in the region of one of
the stigmata.

C. Transverse section through the larva represented in fig. 187 B.

vn. ventral nerve cord; am. amnion and serous membrane ; me. mesoblast ; me.s.
somatic mesoblast ; hy. hypoblast (?) ; yk. yolk cells (true hypoblast) ; st. stigma of
trachea.

rated from the thickened inner part of the cephalic lobe on each
side ; secondly, of an anterior continuation of the lateral cords ;
and thirdly, of a pit of skin invaginated on each side close to the

412 IN SECT A.

dorsal border of the antennae. This pit is at first provided with
a lumen, which is subsequently obliterated; while the walls of
the pit become converted into true ganglion cells. The two
supra-cesophageal ganglia remain disconnected on the dorsal
side till quite the close of embryonic life.

The tracheae and salivary glands. The tracheae, as was
first shewn by Butschli (No. 405), arise as independent segment-
ally arranged paired invaginations of the epiblast (fig. 189 B and
C, st). Their openings are always placed on the outer sides of
the appendages of their segments, where such are present.

Although in the adult stigmata are never found in the space
between the prothorax and head1, in the embryo and the larva
tracheal invaginations may be developed in all the thoracic (and
possibly in the three jaw-bearing segments) and in all the
abdominal segments except the two posterior.

In the embryo of the Lepidoptera, according to Hatschek (No. 414),
there are 14 pairs of stigmata, belonging to the 14 segments of the body
behind the mouth ; but Tichomiroff states that Hatschek is in error in
making this statement for the foremost post-oral segments. The last two
segments are without stigmata. In the larvae of Lepidoptera as well as those
of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all
the postcephalic segments except the 2nd and 3rd thoracic and the two last
abdominal. In Apis there are eleven pairs of tracheal invaginations accord-
ing to Kowalevsky (No. 416), but according to Butschli (No. 405) only ten,
the prothorax being without one. In the Bee they appear simultaneously,
and before the appendages.

The blind ends of the tracheal invaginations frequently (e.g.
Apis) unite together into a common longitudinal canal, which
forms a longitudinal tracheal stem. In other cases (eg. Gryllo-
talpa, Dohrn, No. 408) they remain distinct, and each tracheal
stem has a system of branches of its own.

The development of the tracheae strongly supports the view,
arrived at by Moseley from his investigations on Peripatus, that
they are modifications of cutaneous glands.

The salivary and spinning glands are epiblastic structures,
which in their mode of development are very similar to the
tracheae, and perhaps have a similar origin. The salivary glands

1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only
two stigmata, which are placed on the head.

TRACHEATA. 413

arise as paired epiblastic imaginations, not, as might be
expected, of the Stomodaeum, but of the ventral plate behind
the mouth on the inner side of the mandibles. At first indepen-
dent, they eventually unite in a common duct, which falls into
the mouth. The spinning glands arise on the inner side of the
second pair of maxillae in Apis and Lepidoptera, and form
elongated glands extending through nearly the whole length
of the body. They are very similar in their structure and deve-
lopment to salivary glands, and are only employed during larval
life. They no doubt resemble the mucous glands of the oral
papillae of Peripatus, with which they have been compared by
Moseley. The mucous glands of Peripatus may perhaps be the
homologous organs of the first pair of maxillae, for the existence
of which there appears to be some evidence amongst Insects.

Mesoblast. It has been stated that the mesoblast becomes
divided in the region of the body into two lateral bands (fig. 189
A). These bands in many, if not all forms, become divided
into a series of somites corresponding with the segments of the
body. In each of them a cavity appears — the commencing
perivisceral cavity — which divides them into a somatic plate in
contact with the epiblast, and a splanchnic plate in contact with
the hypoblast (fig. 189). In the interspaces between the
segments the mesoblast is continuous across the median ventral
line. The mesoblast is prolonged into each of the appendages
as these are formed, and in the appendages there is present a
central cavity. By Metschnikoff these cavities are stated to be
continuous, as in Myriapods and Arachnida, with those of the
somites ; but by Hatschek (No. 414) they are stated to be
independent of those in the somites and to be open to the yolk.

The further details of the history of the mesoblast are very imperfectly
known, and the fullest account' we have is that by Dohrn (No. 408) for
Gryllotalpa. It would appear that the mesoblast grows round and encloses
the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms
a pulsating membrane. As the epiblast extends dorsalwards the median
dorsal part of the membrane is constricted off as a tube which forms the
heart. At the same time the free space between the pulsating membrane
and the yolk is obliterated, but transverse passages are left at the lines
between the somites, through which the blood passes from the ventral part of
the body to corresponding openings in the wall of the heart. The greater
part of the membrane gives rise to the muscles of the trunk.

414 INSECTA.

Ventrally the mesoblastic bands soon meet across the median line. The
cavities in the appendages become obliterated and their mesoblastic walls
form the muscles, etc. The cavities in the separate mesoblastic somites also
cease to be distinctly circumscribed.

The splanchnic mesoblast follows the hypoblast in its growth, and gives
rise to the connective tissue and muscular parts of the walls of the aliment-
ary tract. The mesoblastic wall of the proctodaeum is probably formed
independently of the mesoblastic somites. In the head the mesoblast is
stated to form at first a median ventral mass, which does not pass into the
procephalic lobe ; though it assists in forming both the antennae and upper
lip.

The alimentary canal. The alimentary tract of Insects is
formed of three distinct sections (fig. 181) — a mesenteron or
middle section (me), a stomodaeum (st) and a proctodaeum (an).
The stomodaeum and proctodaeum are invaginations of the
epiblast, while the mesenteron is lined by the hypoblast. The
distinction between the three is usually well marked in the adult
by the epiblastic derivatives being lined by chitin. The stomo-
daeum consists of mouth, oesophagus, crop, and proventriculus or
gizzard, when such are present. The mesenteron includes the
stomach, and is sometimes (Orthoptera, etc.) provided at its
front end with pyloric diverticula — posteriorly it terminates just
in front of the Malpighian bodies. These latter fall into the
proctodaeum, which includes the whole of the region from their
insertion to the anus.

The oral invagination appears nearly coincidently. with the
first formation of segments at the front end of the groove
between the lateral nerve cords, and the anal invagination
appears slightly later at the hindermost end of the ventral plate.

The Malpighian bodies arise as two pairs of outgrowths of the
epiblast of t/te proctodceum, whether solid at first is not certain.
The subsequent increase which usually takes place in their
number is due to sproutings (at first solid) of the two original
vessels.

The glandular walls of the mesenteron are formed from the hypoblast ;
but the exact origin of the layer has not been thoroughly worked out in all
cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as
two sheets split off from the lateral masses of mesoblast, which gradually
grow round the yolk, and a similar mode of formation would seem to hold
good for Apis. Tichomiroff (No. 420) confirms Kowalevsky on this point,

TR ACHE AT A. 415

and further states that these two masses meet first ventrally and much later
on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that
the hypoblast arises as a median mass of polygonal cells in the anterior part
of the ventral plate. These cells increase by absorbing material from the
yolk, and then gradually extend themselves and grow round the yolk.

Dohrn (No. 408) believes that the yolk cells, the origin of which has
already been spoken of, give rise to the hypoblastic walls of the mesenteron,
and this view appears to be shared by Graber (No. 412), though the latter
author holds that some of the yolk cells are derived by budding from the
blastoderm1.

From the analogy of Spiders I am inclined to accept Dohrn's and
Graber's view. It appears to me probable that Kowalevsky's observations
are to be explained by supposing that the hypoblast plates which he believes
to be split off from the mesoblast are really separated from the yolk.

.It will be convenient to add here a few details to what has already been
stated as to the origin of the yolk cells. As mentioned above, the central
yolk breaks up at a period, which is not constant in the different forms, into
polygonal or rounded masses, in each of which a nucleus has in many
instances been clearly demonstrated although in others such nuclei have not
been made out. It is probable however that nuclei are in all cases really
present, and that these masses must be therefore regarded as cells. They
constitute in fact the yolk cells. The periphery of the yolk breaks up into
cells while the centre is still quite homogeneous.

The hypoblastic walls of the mesenteron appear to be formed
in the first instance laterally (fig. 189 B and C, hy). They then
meet ventrally (fig. 185 A and B), and finally close in the
mesenteron on the dorsal side.

The mesenteron is at first a closed sack, independent of both
stomodaeum and proctodaeum ; and in the case of the Bee it so
remains even after the close of embryonic life. The only gland-
ular organs of the mesenteron are the not unfrequent pyloric
tubes, which are simple outgrowths of its anterior end. It is
possible that in some instances they may be formed in situ
around the lateral parts of the yolk.

In many instances the whole of the yolk is enclosed in the walls of the
mesenteron, but in other cases, as in Chironomus and Simulia (Weismann,
No. 430 ; Metschnikoff, No. 423), part of the yolk may be left between the
ventral wall of the mesenteron and the ventral plate. In Chironomus the

1 Graber's view on this point may probably be explained by supposing that he has
mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells
into the yolk. The former occurrence takes place, as I have found, largely in Spiders,
and probably therefore also occurs in Insects.

41 6 INSECTA.

mass of yolk external to the mesenteron takes the form of a median and two
lateral streaks. Some of the yolk cells either prior to the establishment of
the mesenteron, or derived from the unenclosed portions of the yolk, pass
into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell.
They also form blood corpuscles and connective-tissue elements. Such yolk
cells may be compared to the peculiar bodies described by Reichenbach in
Astacus, which form the secondary mesoblast. Similar cells play a very
important part in the development of Spiders.

Generative organs. The observations on the development of the
generative organs are somewhat scanty. In Diptera certain cells — known
as the pole cells — are stated by both Metschnikoff (No. 423) and Leuckart to
give rise to the generative organs. The cells in question (in Chironomus
and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the
ovum before any other cells of the blastoderm. They soon separate from
the blastoderm and increase by division. In the embryo, produced by the
viviparous larva of Cecidomyia, there is at first a single pole cell, which
eventually divides into four, and the resulting cells become enclosed within
the blastoderm. They next divide into two masses, which are stated by
Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells1.
Their protoplasm then fuses, and their nuclei divide, and they give rise to
the larval ovaries, for which the enclosing cells form the tunics.

In Aphis Metschnikoff (No. 423) detected at a very early stage a mass
of cells which give rise to the generative organs. These cells are situated
at the hind end of the ventral plate ; and, except in the case of one of the
cells which gives rise by division to a green mass adjoining the fat body,
the protoplasm of the separate cells fuses into a syncytium. Towards the
close of embryonic life the syncytium assumes a horse-shoe form. The mass
is next divided into two, and the peripheral layer of each part gives rise
to the tunic, while from the hinder extremity of each part an at first solid
duct— the egg- tube — grows out. The masses themselves form the ger-
mogens. The oviduct is formed by a coalescence of the ducts from each
germogen.

Ganin derives the generative organs in Platygaster (vide p. 347) from
the hind end of the ventral plate close to the proctodaeum ; while Suckow
states that the generative organs are outgrowths of the proctodicum.
According to these two sets of observations the generative organs would
appear to have an epiblastic origin — an origin which is not incompatible
with that from the pole cells.

In Lepidoptera the genital organs are present in the later periods of
embryonic life as distinct paired organs, one on each side of the heart, in
the eighth postcephalic segment. They are elliptical bodies with a duct
passing off from the posterior end in the female or from the middle in the
male. The egg-tubes or seminal tubes are outgrowths of the elliptical
bodies.

1 This point requires further observation.

TRACHEATA.

417

In other Insects the later stages in the development of the generative
organs closely resemble those in the Lepidoptera, and the organs are usually
distinctly visible in the later stages of embryonic life.

It may probably be laid down, in spite of some of Metschnikoff's
observations above quoted, that the original generative mass gives rise to
both the true genital glands and their ducts. It appears also to be fairly
clear that the genital glands of both sexes have an identical origin.

Special types of larva.

Certain of the Hymenopterous forms, which deposit their eggs in the
eggs or larvae of other Insects, present very peculiar modifications in their
development. Platygaster, which lays its egg in the larvae of Cecidomyia,
undergoes perhaps the most remarkable development amongst these forms.
It has been studied especially by Ganin (No. 410), from whom the following
account is taken.

The very first stages are unfortunately but imperfectly known, and the
interpretations offered by Ganin do not in all cases appear quite satis-
factory. In the earliest stage after being laid the egg is enclosed in a
capsule produced into a stalk (fig. 190 A). In the interior of the egg
there soon appears a single spherical body, regarded by Ganin as a cell
(fig. 190 B). In the next stage three similar bodies appear in the vitellus,
no doubt derived from the first one (fig. 190 C). The central one presents
somewhat different characters to the two others, and, according to Ganin,
gives rise to the whole embryo. The two peripheral bodies increase by
division, and soon ap-
pear as nuclei imbed-
ded in a layer of pro-
toplasm (fig. 190 D,
E, F). The layer so
formed serves as a
covering for the em-
bryo, regarded by
Ganin as equivalent
to the amnion (? se-
rous membrane) of
other Insect em-
bryos. In the em-
bryo cell new cells
are stated to be
formed by a process
of endogenous cell formation (fig. 190 D, E). It appears probable that
Ganin has mistaken nuclei for cells in the earlier stages, and that a blasto-
derm is formed as in other Insects, and that this becomes divided in a way
not explained into a superficial layer which gives rise to the serous
envelope, and a deeper layer which forms the embryo. However this

B. II. 27

FlG. 190. A SERIES OF STAGES IN THE DEVELOPMENT

OF PLATYGASTER. (From Lubbock ; after Ganin.)

41 8 INSECTA.

may be, a differentiation into an epiblastic layer of columnar cells and
a hypoblastic layer of more rounded cells soon becomes apparent in the body
of the embryo. Subsequently to this the embryo grows rapidly, till by a
deep transverse constriction on the ventral surface it becomes divided into an
anterior cephalothoracic portion and a posterior caudal portion (fig. 190 F).
The cephalothorax grows in breadth, and near its anterior end an in-
vagination appears, which gives rise to the mouth and cesophagus. On
the ventral side of the cephalothorax there is first formed a pair of
claw-like appendages on each side of the mouth, then a posterior pair of
appendages near the junction of the cephalothorax and abdomen, and
lastly a pair of short conical antennae in front.

At the same time the hind end of the abdomen becomes bifid, and gives
rise to a fork-like caudal appendage ; and at a slightly later period four
grooves make their appearance in the caudal region, and divide this part of
the embryo into successive segments. While these changes have been
taking place in the general form of the embryo, the epiblast has given rise
to a cuticle, and the hypoblastic cells have become differentiated into a
central hypoblastic axis — the mesenteron — and a surrounding layer of
mesoblast, some of the cells of which form longitudinal muscles.

With this stage closes what may be regarded as the embryonic develop-
ment of Platygaster. The embryo becomes free from the amnion, and pre-
sents itself as a larva, which from its very remarkable characters has been
spoken of as the Cyclops larva by Ganin.

The larvae of three species have been described by Ganin, which are repre-
sented in fig. 1 9 1 A, B, C. These larvae are strangely dissimilar to the ordinary
Hexapod type, whether larval or adult. They are formed of a cephalothoracic
shield with the three pairs of appendages (a, kf, lfg\ the development of
which has already been described, and of an abdomen formed of five seg-
ments, the last of which bears the somewhat varying caudal appendages.
The nervous system is as yet undeveloped.

The larvae move about in the tissues of their hosts by means of their
claws.

The first larval condition is succeeded by a second with very different
characters, and the passage from the first to the second is accompanied by
an ecdysis.

The ecdysis commences at the caudal extremity, and the whole of the
last segment is completely thrown off. As the ecdysis extends forwards
the tail loses its segmentation and becomes strongly compressed, the
appendages of the cephalothorax are thrown off, and the whole embryo
assumes an oval form without any sharp distinction into different regions
and without the slightest indication of segmentation (fig. 191 D). Of the
internal changes which take place during the shedding of the cuticle, the
first is the formation of a proctodaeum (gfi) by an invagination, which ends
blindly in contact with the mesenteron. Shortly after this a thickening of
the epiblast (bsm} appears along the ventral surface, which gives rise mainly
to the ventral nerve cord ; this thickening is continuous behind with the

TRACHEATA.

419

epiblast which is invaginated to form the proctodaeum, and in front is pro-
longed on each side into two procephalic lobes, in which there are also
thickenings of the epiblast (gsae), which become converted into supra-
oesophageal ganglia, and possibly other parts.

Towards the close of the second larval period the muscles (/;«) become
segmentally arranged, and give indications of the segmentation which

FlG. 191. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.

(From Lubbock ; after Ganin.)

A. B. C. Cyclops larvae of three species of Platygaster.
D. Second larval stage. E. Third larval stage.

mo. mouth ; a. antenna ; kf. hooked feet ; Ifg. lateral feet ; /. branches of tail ;
ul. lower lip ; slkf. oesophagus ; gsae. supra- oesophageal ganglion ; bsm. ventral epi-
blastic plate ; Im. lateral muscles (the letters also point in D to the salivary glands) ;
gh. proctodseum ; ga. generative organs ; md. mandibles ; ag. ducts of salivary glands ;
sp. (in E) salivary glands ; mis. stomach ; ed. intestine ; ew. rectum ; ao. anus ;
tr. tracheae ; fk. fat body.

becomes apparent in the third larval period. The third and last larval
stage (fig. 191 E) of Platygaster, during which it still remains in the tissues
of its host, presents no very peculiar features. The passage from the second
to the third form is accompanied by an ecdysis.

Remarkable as are the larvae just described, there can I think be
no reason, considering their parasitic habits, for regarding them as ancestral.

27—2

420 INSECTA.

Metamorphosis and heterogamy.

Metamorphosis. The majority of Insects are born in a
condition in which they obviously differ from their parents. The
extent of this difference is subject to very great variations, but
as a rule the larvae pass through a very marked metamorphosis
before reaching the adult state. The complete history of this
metamorphosis in the different orders of Insects involves a far
too considerable amount of zoological detail to be dealt with in
this work ; and I shall confine myself to a few observations on
the general characters and origin of the metamorphosis, and of
the histological processes which take place during its occur-
rence1.

In the Aptera the larva differs from the adult only in the
number of facets in the cornea and joints in the antennae.

In most Orthoptera and Hemiptera the larvae differ from the
adult in the absence of wings and in other points. The wings,
etc., are gradually acquired in the course of a series of successive
moultings. In the Ephemeridae and Libellulidae, however, the
metamorphosis is more complicated, in that the larvae have
provisional tracheal gills which are exuviated before the final
moult. In the Ephemeridae there are usually a great number of
moultings ; the tracheal gills appear after the second moult, and
the rudiments of the wings when the larva is about half grown.
Larval life may last for a very long period.

In all the other groups of Insects, viz. the Diptera, Neuro-
ptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva
passes — with a few exceptions — through a quiescent stage, in
which it is known as a pupa, before it attains the adult stage.
These forms are known as the Holometabola.

In the Diptera the larvae are apodous. In the true flies (Muscidae) they
are without a distinct head and have the jaws replaced by hooks. In the
Tipulidae there is on the other hand a well-developed head with the normal
appendages. The pupae of the Muscidae are quiescent, and are enclosed in
the skin of the larva which shrinks and forms a firm oval case. In the

1 For a systematic account of this subject the reader is referred to Lubbock (No.
420) and to Graber (No. 411). He will find in Weismann (Nos. 430 and 431) a detailed
account of the internal changes which take place.

TRACHEATA. 42 1

Tipulidae the larval skin is thrown off at the pupa stage, and in some cases
the pupae continue to move about.

The larvae of the Neuroptera are hexapodous voracious forms. When the
larva becomes a pupa all the external organs of the imago are already
established. The pupa is often invested in a cocoon. It is usually quiescent,
though sometimes it begins to move about shortly before the imago emerges.

In the Coleoptera there is considerable variety in the larval forms. As a
rule the larvae are hexapodous and resemble wingless Insects. But some
herbivorous larvae (e.g. the larva of Melolontha) closely resemble true
caterpillars, and there are also grub-like larvae without feet (Curculio) which
resemble the larvae of Hymenoptera. The pupa is quiescent, but has all
the parts of the future beetle plainly visible. The most interesting larvae
among the Coleoptera are those of Sitaris, one of the Meloidae (Fabre, No.
409). They leave the egg as active hexapodous larvae which attach them-
selves to the bodies of Hymenoptera, and are thence transported to a cell
filled with honey. Here they eat the ovum of the Hymenopterous form.
They then undergo an ecdysis, in which they functionally lose their append-
ages, retaining however small rudiments of them, and become grubs. They
feed on the honey and after a further ecdysis become pupae.

In the Lepidoptera the larva has the well-known form of a caterpillar.
The caterpillars have strong jaws, adapted for biting vegetable tissues,
which are quite unlike the oral appendages of the adult. They have three
pairs of jointed thoracic legs, and a variable number (usually five) of pairs
of rudimentary abdominal legs — the so-called pro-legs. The larva undergoes
numerous ecdyses, and the external parts of the adult such as the wings, etc.,
are formed underneath the chitinous exoskeleton before the pupa stage.
The pupa is known as a chrysalis and in some Lepidoptera is enveloped in
a cocoon.

The Hymenoptera present considerable variations in the character of the
larvae. In the Aculeata, many Entomophaga, the Cynipidae, etc., the larvae
are apodous grubs, incapable of going in search of their food ; but in the
Siricidse they are hexapodous forms like caterpillars, which are sometimes
even provided with pro-legs. In some of the Entomophaga the larvae have
very remarkable characters which have already been described in a special
section, 'vide pp. 418, 419.

Before proceeding to the consideration of the value of the
various larval forms thus shortly enumerated, it is necessary to
say a few words as to the internal changes which take place
during the occurrence of the above metamorphosis. In the
simplest cases, such as those of the Orthoptera and Hemiptera,
where the metamorphosis is confined to the gradual formation
of the wings, etc. in a series of moults, the wings first appear as
two folds of the epidermis beneath the cuticle on the two
posterior thoracic segments. At the next moult these processes

422 INSECTA.

become covered by the freshly formed cuticle, and appear as
small projections. At every successive moult these projections
become more prominent owing to a growth in the epidermis
which has taken place in the preceding interval. Accompanying
the formation of such organs as the wings, internal changes
necessarily take place in the arrangement of the muscles, etc. of
the thorax, which proceed pari passu with the formation of the
organs to which they belong. The characters of the metamor-
phosis in such forms as the Ephemeridae only differ from the
above in the fact that provisional organs are thrown off at the
same time that the new ones are formed.

In the case of the Holometabola the internal phenomena of
the metamorphosis are of a very much more remarkable cha-
racter. The details of our knowledge are largely due to Weis-
mann (Nos. 430 and 431). The larvae of the Holometabola have
for the most part a very different mode of life to the adults.
A simple series of transitions between the two is impossible,
because intermediate forms would be for the most part incapable
of existing. The transition from the larval to the adult state is
therefore necessarily a more or less sudden one, and takes place
during the quiescent pupa condition. Many of the external
adult organs are however formed prior to the pupa stage, but do
not become visible on the surface. The simplest mode of Holo-
metabolic metamorphosis may be illustrated by the development
of Corethra plumicornis, one of the Tipulidae. This larva, like
that of other Tipulidae, is without thoracic appendages, but
before the last larval moult, and therefore shortly before the
pupa stage, certain structures are formed, which Weismann has
called imaginal discs. These imaginal discs are in Corethra
simply invaginations of the epidermis. There are in the thorax
six pairs of such structures, three dorsal and three ventral. The
three ventral are attached to the terminations of the sensory
nerves, and the limbs of the imago are formed as simple
outgrowths of them, which as they grow in length take a spiral
form. In the interior of these outgrowths are formed the
muscles, tracheae, etc., of the limbs; which are believed by
Weismann (it appears to me without sufficient ground) to be
derived from a proliferation of the cells of the neurilemma.
The wings are formed from the two posterior dorsal imaginal

TRACHEATA. 423

discs. The hypodermis of the larva passes directly into that of
the imago.

The pupa stage of Corethra is relatively very short, and the
changes in the internal parts which take place during it are not
considerable. The larval abdominal muscles pass for the most
part unchanged into those of the imago, while the special
thoracic muscles connected with the wings, etc., develop directly
during the latest larval period from cords of cells already formed
in the embryo.

In the Lepidoptera the changes in the passage from the
larval to the adult state are not very much more considerable
than those in Corethra. Similar imaginal discs give rise during
the later larval periods to the wings, etc. The internal changes
during the longer pupa period are somewhat more considerable.
Important modifications and new formations arise in connec-
tion with the alimentary tract, the nervous and muscular
systems.

The changes which take place in the true flies (Muscidse) are
far more complicated than either those in Corethra or in the
Lepidoptera. The abdomen of the larva of Musca becomes
bodily converted into the abdomen of the imago as in the above
types, but the whole epidermis and appendages of the head and
thorax are derived from imaginal discs which are formed within
and (so far as is known) independently of the epidermis of the
larva or embryo. These imaginal discs are simple masses of
apparently indifferent cells, which for the most part appear at
the close of embryonic life, and are attached to nerves or
tracheae. They grow in size during larval life, but during the
relatively long pupa stage they unite together to give rise to a
continuous epidermis, from which the appendages grow out as
processes. The epidermis of the anterior part of the larva is
simply thrown off, and has no share in forming the epidermis of
the adult.

There are a pair of cephalic imaginal discs and six pairs of
thoracic discs. Two pairs, a dorsal and a ventral, give rise to
each thoracic ring, and the appendages attached to it.

Though, as mentioned above, no evidence has yet been
produced to shew that the imaginal discs of Musca are derived
from the embryonic epiblast, yet their mode of growth and

424 1NSECTA.

eventual fate proves beyond the shadow of a doubt that they are
homologous with the imaginal discs of Corethra. Their earliest
origin is well worth further investigation.

The metamorphosis of the internal organs is still more
striking than that of the external. There is a disruption, total
or partial, of all the internal organs except the generative
organs. In the case of the alimentary tract, the Malpighian
vessels, the heart and the central nervous system, the disruption
is of a partial kind, which has been called by Weismann
histolysis. The cells of these organs undergo a fatty degenera-
tion, the nuclei alone in some cases remaining. The kind of
plasma resulting from this degeneration retains the shape of the
organs, and finally becomes built up again into the correspond-
ing organs of the imago. The tracheae, muscles and peripheral
nerves, and an anterior part of the alimentary tract, are entirely
disrupted. They seem to be formed again from granular cells
derived from the enormous fat body.

The phenomena of the development of the Muscidse are undoubtedly of
rather a surprising character. Leaving for the moment the question of the
origin of the pupa stage to which I return below, it will be admitted on all
hands that during the pupa stage the larva undergoes a series of changes
which, had they taken place by slow degrees, would have involved, in such a
case as Musca, a complete though gradual renewal of the tissues. Such
being the case, the cells of the organs common to the larva and the imago
would, in the natural course of things, not be the same cells as those of the
larva but descendants of them. We might therefore expect to find in the
rapid conversion of the larval organs into those of the adult some condensa-
tion, so to speak, of the process of ordinary cell division. Such condensations
are probably represented in the histolysis in the case of the internal organs,
and in the formation of imaginal discs in the case of the external ones, and
I think it probable that further investigation will shew that the imaginal
discs of the Muscidae are derivatives of the embryonic epiblast. The above
considerations by no means explain the whole of Weismann's interesting
observations, but an explanation is I believe to be found by following up
these lines.

More or less parallel phenomena to those in Insects are found in the
development of the Platyelminthes and Echinoderms. The four disc-like
invaginations of the skin in many larval Nemertines (vide p. 198), which
give rise to the permanent body wall of the Nemertine, may be compared to
the imaginal discs. The subsequent throwing off of the skin of Pilidium or
larva of Desor is a phenomenon like the absorption of part of the larval
skin of Musca. The formation of an independent skin within the first larval

TR ACHE AT A.

425

form in the Distomeaeand in the Cestoda may be compared to the apparently
independent formation of the imaginal discs in Musca.

The fact that in a majority of instances it is possible to trace
an intimate connection between the surroundings of a larva and
its organization proves in the clearest way that the characters of
the majority of existing larval forms of Insects have owed their
origin to secondary adaptations. A few instances will illustrate
this point.

In the simplest types of metamorphosis, e.g. those of the
Orthoptera genuina, the larva has precisely the same habits as
the adult. We find that a caterpillar
form is assumed by phytophagous larvae
amongst the Lepidoptera, Hymenoptera
and Coleoptera. Where the larva has
not to go in search of its nutriment the
grub-like apodous form is assumed. The
existence of such an apodous larva is
especially striking in the Hymenoptera,
in that rudiments of thoracic and abdo-
minal appendages are present in the
embryo and disappear again in the larva.
The case of the larva of Sitaris, already
described (p. 421), affords another very
striking proof that the organization of
the larva is adapted to its habits.

It follows from the above that the
development of such forms as the Or-
thoptera genuina is more primitive than
that of the holometabolous forms; a
conclusion which tallies with the fact

FIG. 102.

ANTERIOR

HALF OF CAMPODEA FRAGI-
LIS. (From Gegenbaur; af-
ter Palmen.)

a. antennae ; p. feet ; j> ',
post-tho
feet; s.

stigma.

that both palaeontological and anatomical evidence shew the
Orthoptera to be a very primitive group of Insects.

The above argument probably applies with still greater force
to the case of the Thysanura ; and it seems to be probable that
this group is more nearly related than any other to the primitive
wingless ancestors of Insects1. The characters of the oral

1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these
forms, especially of Campodea.

426 INSECTA.

appendages in this group, the simplicity of their metamorphosis,
and the presence of abdominal appendages (fig. 192), all tell in
favour of this view, while the resemblance of the adult to the
larvae of the Pseudoneuroptera, etc., points in the same direction.
The Thysanura and Collembola are not however to be regarded
as belonging to the true stock of the ancestors of Insects, but as
degenerated relations of this stock ; much as Amphioxus and
the Ascidians are degenerate relations of the ancestral stock of
Vertebrates, and Peripatus of that of the Tracheata. It is
probable that all these forms have succeeded in retaining their
primitive characters from their degenerate habits, which pre-
vented them from entering into competition in the struggle for
existence with their more highly endowed relatives. While in a
general way it is clear that the larval forms of Insects cannot be
expected to throw much light on the nature of Insect ancestors,
it does nevertheless appear to me probable that such forms as
the caterpillars of the Lepidoptera are not without a meaning in
this respect. It is easy to conceive that even a secondary larval
form may have been produced by the prolongation of one of the
embryonic stages ; and the general similarity of a caterpillar to
Peripatus, and the retention by it of post-thoracic appendages, are
facts which appear to favour this view of the origin of the cater-
pillar form.

The two most obscure points which still remain to be dealt
with in the metamorphosis of Insects are (i) the origin of the
quiescent pupa stage ; (2) the frequent dissimilarity between the
masticatory apparatus of the larva and adult.

These two points may be conveniently dealt with together,
and some valuable remarks about them will be found in Lubbock
(No. 420).

On grounds already indicated it may be considered certain
that the groups of Insects without a pupa stage, and with a larva
very similarly organised to the adult, preceded the existing
holometabolic groups. The starting-point in the metamorphosis
of the latter groups was therefore something like that of the
Orthoptera. Suppose it became an advantage to a species that
the larva and adult should feed in a somewhat different way, a
difference in the character of their mouth parts would soon make
itself manifest ; and, since an intermediate type of mouth parts

TRACHEATA. 427

would probably be disadvantageous, there would be a tendency
to concentrate into a single moult the transition from the larval
to the adult form of mouth parts. At each ordinary moult there
is a short period of quiescence, and this period of quiescence
would naturally become longer in the important moult at which
the change in the mouth parts was effected. In this way a
rudimentary pupa stage might be started. The pupa stage,
once started, might easily become a more important factor in
the metamorphosis. If the larva and imago diverged still more
from each other, a continually increasing amount of change
would have to be effected at the pupa stage. It would probably
be advantageous to the species that the larva should not have
rudimentary functionless wings ; and the establishment of the
wings as external organs would therefore become deferred to
the pupa stage. The same would probably apply to other
organs.

Insects usually pass through the pupa stage in winter in cold
climates and during the dry season in the tropics, this stage
serving therefore apparently for the protection of the species
during the inclement season of the year. These facts are easily
explained on the supposition that the pupa stage has become
secondarily adapted to play a part in the economy of the
species quite different from that to which it owes its origin.

Heterogamy. The cases of alternations of generations
amongst Insects all fall under the heading already defined in
the introduction as Heterogamy. Heterogamy amongst Insects
has been rendered possible by the existence of parthenogenesis,
which, as stated in the introduction, has been taken hold of by
natural selection, and has led to the production of generations of
parthenogenetic forms, by which a clear economy in reproduction
is effected. Parthenogenesis without heterogamy occurs in a
large number of forms. In Bees, Wasps, and a Sawfly (Nematus
ventricosus) the unfertilized ova give rise to males. In two
Lepidopterous genera (Psyche and Solenobia) the unfertilized
ova give rise mainly, if not entirely, to females. Heterogamy
occurs in none of the above types, but in Psyche and Solenobia
males are only occasionally found, so that a series of generations
producing female young from unfertilized ova are followed by a
generation producing young of both sexes from fertilized ova. It

428 INSECTA.

would be interesting to know if the unimpregnated female would
not after a certain number of generations give rise to both males
and females ; such an occurrence might be anticipated on
grounds of analogy. In the cases of true heterogamy partheno-
genesis has become confined to special generations, which differ
in their character from the generations which reproduce them-
selves sexually. The parthenogenetic generations generally
flourish during the season when food is abundant; while the
sexual generations occur at intervals which are often secondarily
regulated by the season, supply of food, etc.

A very simple case of this kind occurs, if we may trust the
recent researches of Lichtenstein1, in certain Gall Insects
(Cynipidae). He finds that the female of a form known as
Spathegaster baccarum, of which both males and females are
plentiful, pricks a characteristic gall in certain leaves, in which
she deposits the fertilized eggs. The eggs from these galls give
rise to a winged and apparently adult form, which is not, how-
ever, Spathegaster, but is a species considered to belong to a
distinct genus known as Neuroterus ventricularis. Only females
of Neuroterus are found, and they lay unfertilized ova in peculiar
galls which develop into Spathegaster baccarum. Here we have
a true case of heterogamy, the females which produce partheno-
genetically having become differentiated from those which pro-
duce sexually. Another interesting type of heterogamy is that
which has been long known in the Aphides. In the autumn
impregnated eggs are deposited by females, which give rise in
the course of the spring to females which produce partheno-
genetically and viviparously. The viviparous females always
differ from the females which lay the fertilized eggs. The gene-
rative organs are of course differently constituted, and the ova of
the viviparous females are much smaller than those of the ovi-
parous females, as is generally the case in closely allied vivi-
parous and oviparous forms; but in addition the former are
usually without wings, while the latter are winged. The reverse
is however occasionally the case. An indefinite number of gene-
rations of viviparous females may be produced if they are arti-
ficially kept warm and supplied with food ; but in the course of

1 Petites Nouvelles Entomolog iyues, May, 1878.

TRACHEATA. 429

nature the viviparous females produce in the autumn males and
females which lay eggs with firm shells, and so preserve the
species through the winter. The heterogamy of the allied
Coccidae is practically the same as that of the Aphidae. In the
case of Chermes and Phylloxera the parthenogenetic generations
lay their eggs in the normal way.

The complete history of Phylloxera quercus has been worked
out by Balbiani (No. 401). The apterous females during the
summer lay eggs developing parthenogenetically into apterous
females, which continue the same mode of reproduction. In the
autumn, however, the eggs which are laid give rise in part to
winged forms and in part to apterous forms. Both of these
forms lay smaller and larger eggs, which develop respectively
into very minute males and females without digestive organs.
The fertilized eggs laid by these forms probably give rise to the
parthenogenetic females.

A remarkable case of heterogamy accompanied by paedo-
genesis was discovered by Wagner to take place in certain
species of Cecydomyia (Miastor), a genus of the Diptera. The
female lays a few eggs in the bark of trees, etc. These eggs
develop in the winter into larvae, in which ovaries are early
formed. The ova become detached into the body cavity,
surrounded by their follicles, and grow at the cost of the
follicles. They soon commence to undergo a true development,
and on becoming hatched they remain for some time in the
body cavity of the parent, and are nourished at the expense of
its viscera. They finally leave the empty skin of their parent,
and subsequently reproduce a fresh batch of larvae in the same
way. After several generations the larvae undergo in the
following summer a metamorphosis, and develop into the sexual
form.

Another case of paedogenesis is that of the larvae of Chiro-
nomus, which have been shewn by Grimm (No. 413) to lay eggs
which develop exactly in the same way as fertilized eggs into
larvae.

BIBLIOGRAPHY.

(401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene."
An. Sc. Nat. Ser. v. Vol. xix. 1874.

430 INSECTA.

(402) E. Bess els. " Studien u. d. Entwicklung d. Sexualdriisen bei den Lepi-
doptera." Ztit.f. wiss. Zool. Bd. xvii. 1867.

(403) Alex. Brandt. "Beitrage zur Entwicklungsgeschichte d. Libellulida u.
Hemiptera, mil besonderer Berucksichtigung d. Embryonalhiillen derselben." Mem.
Ac. Petersbourg, Ser. vn. Vol. xm. 1869.

(404) Alex. Brandt. Ueber das Ei u. seine Bildungsstdttt. Leipzig, 1878.

(405) O. Biitschli. "Zur Entwicklungsgeschichte d. Biene." Zeit. f. wiss.
Zool. Bd. xx. 1870.

(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool.
Vols. xxv. and xxvin. 1875 and 1877.

(407) H. Dewitz. "Beitrage zur Kenntniss d. Postembryonalentwicklung d.
Gliedmassen bei den Insecten." Zeit.f. wiss. Zool. xxx. Supplement. 1878.

(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeitschrift
f. wiss. Zool. Bd. xxvi. 1876.

(409) M. Fabre. " L'hypermetamorphose et lesmoeursdes Meloides." An.Sci.
Nat. Series iv. Vol. vn. 1857.

(410) Ganin. " Beitrage zur Erkenntniss d. Entwicklungsgeschichte d. Insecten."
Zeit.f. wiss. Zool. Bd. xix. 1869.

(411) V. Graber. Die Insecten. MUnchen, 1877.

(412) V. Graber. "Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten."
Archivf. mikr. Anat. Vol. XV. 1878.

(413) O. v. Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus Art-u.
deren Entwicklung aus dem unbefruchteten Ei." Mem. Acad. Petersbourg. 1870.

(414) B. Hatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaische
Zeitschrift, Bd. XI.

(415) A. K 6 1 1 i k e r. " Observationes de prima insectorum genese, etc. " Ann. Sc.
Nat. Vol. xx. 1843.

(416) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
Mem. Ac. imp. Petersbourg, Ser. vn. Vol. xvi. 1871.

(417) C. Kraepelin. 4 ' Untersuchungen Ub. d. Bau, Mechanismus u. d. Ent-
wick. des Stachels d. bienartigen Thiere." Zeit.f. wiss. Zool. Vol. xxni. 1873.

(418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus."
Arch.f. mikr. Anat. Vol. u. 1866.

(419) R. Leuckart. Zur Kenntniss d. Generationswechsels u. d. Parthenogenese
b. d. Insecten. Frankfurt, 1858.

(420) Lubbock. Origin and Metamorphosis of Insects. 1874.

(421) Lubbock. Monograph on Collembola and Thysanura. Ray Society, 1873.

(422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv
f. Naturgeschichte, Bd. xxxv. 1869.

(423) E. Metschnikoff. "Embryologische Studien an Insecten." Zeit. f.
wiss. Zool. Bd. xvi. 1866.

(424) P. Meyer. "Ontogenie und Phylogenie d. Insecten." Jenaische Zeit-
schrift, Vol. x. 1876.

(425) FritzMiiller. " Beitrage z. Kenntniss d. Termiten." Jenaische Zeit-
schrift, Vol. IX. 1875.

(426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and
the Thysanurous genus Isotoma." Mem. Peabody Acad. Science, I. i. 1871.

(427) Suckow. " Geschlechtsorgane d. Insecten." Ileusinger's Zeitschrift f.
organ. Physik, Bd. n. 1828.

TRACHEATA.

431

(428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms."
Zoologischer Anzeiger, n. Jahr. No. 20 (Preliminary Notice).

(429) Aug. Weismann. "Zur Embryologie d. Insecten." Archiv f. Anat.
und Phys. 1864.

(430) Aug. Weismann. " Entwicklung d. Dipteren." Zeit. f. wiss. Zool.
Vols. xin. and xiv. Leipzig, 1863 — 4.

(431) Aug. Weismann. " Die Metamorphose d. Corethra plumicornis. " Zeit.
f. wiss. Zool. Vol. xvi. 1866.

(432) N. Wagner. "Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven."
Zeit.f. wiss. Zool. Vol. xin. 1860.

(433) Zaddach. Untersuchungen iib. d. Bau u. d. Entwicklungd. Gliederthiere.
Berlin, 1854.

ARACHNIDA1.

The development of several divisions of this interesting
group has been worked out ; and it will be convenient to deal in
the first instance with the special history of each of these
divisions, and then to treat in a
separate section the develop-
ment of the organs for the
whole group.

Scorpionidae. The embry-
onic development always takes
place within the female Scor-
pion. In Buthus it takes place
within follicle-like protuber-
ances of the wall of the ovary.
In Scorpio also development
commences while the egg is
still in the follicle, but when the
trunk becomes segmented the
embryo passes into the ovarian
tube. The chief authority for
the development of the Scorpio-
nidae is Metschnikoff (No. 434).

At the pole of the ovum facing the ovarian tube there is

FIG. 193. OVUM OF SCORPION WITH
THE ALREADY -FORMED BLASTODERM
SHEWING THE PARTIAL SEGMENTATION.
(After Metschnikoff.)

bl. blastoderm.

1 The classification of the Arachnida adopted in the present work is shewn below.
c Scorpionidse. . . ( Tetrapneumones.

Pedipalpi. IL Aranema- JDipneumones.

I. ArthrOgastra. \ Pseudoscorpionidae.

I Soiifugse. in. Acarina,

^ Phalangidse.

432

SCORPIONID^E.

formed a germinal disc which undergoes a partial segmentation
(fig. 193 bl). A somewhat saucer-shaped one-layered blasto-
derm is then formed, which soon becomes thickened in the
centre and then divided into two layers. The outer of these
is the epiblast, the inner the mesoblast. Beneath the mesoblast
there subsequently appear granular cells, which form the
commencement of the hypoblast1.

During the formation of the blastoderm a cellular envelope is formed
round the embryo. Its origin is doubtful, though it is regarded by
Metschnikoff as probably derived from the blastoderm and homologous
with the amnion of Insects. It becomes double in the later stages (fig. 195).

During the differentiation of the three embryonic layers the
germinal disc becomes somewhat pyriform, the pointed end
being the posterior. At this extremity there is a special thick-
ening which is perhaps
equivalent to the prim-
itive cumulus of Spiders.
The germinal disc con-
tinues gradually to spread
over the yolk, but the
original pyriform area is
thicker than the remain-
der, and is marked off
anteriorly and posterior-
ly by a shallow furrow.
It constitutes a structure
corresponding with the
ventral plate of other
Tracheata. It soon be-
comes grooved by a FIG. 194. THREE SURFACE VIEWS OF THE

.A ,. , f VENTRAL PLATE OF A DEVELOPING SCORPION.

shallow longitudinal fur- (After Metschnikoff.)

A. Before segmentation.

B. After five segments have become formed.

C. After the appendages have begun to be

formed.

row (fig. 194 A) which
subsequently becomes
less distinct. It is then
divided by two transverse lines into three parts2.

1 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff
doubtfully derives them from the blastoderm cells ; from my investigations on Spiders
it appears to me more probable that they originate in the yolk.

* The exact fate of the three original segments is left somewhat obscure by

TRACHEATA.

433

In succeeding stages the anterior of the three parts is clearly
marked out as the procephalic lobe, and soon becomes somewhat
broader. Fresh segments are added from before backwards,
and the whole ventral plate increases rapidly in length (fig.
194 B).

When ten segments have become formed, appendages appear
as paired outgrowths of the nine posterior segments (fig. 194 C).
The second segment bears the pedipalpi, the four succeeding
segments the four ambulatory appendages, and the four hinder-
most segments smaller provisional appendages which subse-
quently disappear, with the possible exception of the second.
The foremost segment, immediately behind the procephalic
lobes, is very small, and still without a rudiment of the cheli-
cerae, which are subsequently formed on it. It would appear
from Metschnikoff's figures to
be developed later than the
other post-oral segments pre-
sent at this stage. The still
unsegmented tail has become
very prominent and makes an
angle of 180° with the re-
mainder of the body, over the
ventral surface of which it is
flexed.

By the time that twelve
segments are definitely form-
ed, the procephalic region is
distinctly bilobed, and in the
median groove extending
along it the stomodaeum has
become formed (fig. 196 A).
The chelicerae (ck) appear as
small rudiments on the first
post-oral segment, and the

FlG. 195. A FAIRLY-ADVANCED EM-
BRYO OF THE SCORPION ENVELOPED IN

ITS MEMBRANES. (After Metschnikoff. )

ch. chelicerae ; pd. pedipalpi ; p^—p4.
ambulatory appendages ; al>. post-abdomen
(tail).

Metschnikoff. He believes however that the anterior segment forms the procephalic
lobes, the posterior probably the telson and five adjoining caudal segments, and
the middle one the remainder of the body. This view does not appear to me quite
satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites
ought to be added by a continuous segmentation of the posterior lobe.

B. II. 28

434 1 SEUDOSCORPIONID^E.

nerve cords are distinctly differentiated and ganglionated. In
the embryonic state there is one ganglion for each segment.
The ganglion in the first segment (that bearing the chelicerse) is
very small, but is undoubtedly post-oral.

At this stage, by a growth in which all the three germinal
layers have a share, the yolk is completely closed in by the
blastoderm. It is a remarkable fact with only few parallels, and
those amongst the Arthropoda, that the blastopore, or point
where the embryonic membranes meet in closing in the yolk, is
situated on the dorsal surface of the embryo.

The general relations of the embryo at about this stage are
shewn in fig. 195, where the embryo enclosed in its double
cellular membrane is seen in a side view. This embryo is about
the same age as that seen from the ventral surface in fig. 196 A.

The general nature of the further changes may easily be
gathered from an inspection of fig. 196 B and C, but a few
points may be noted.

An upper lip or labrum is formed as an unpaired organ in
the line between the procephalic lobes. The pedipalpi become
chelate before becoming jointed, and the chelicerae also early
acquire their characteristic form. Rudimentary appendages
appear on the six segments behind the ambulatory legs, five of
which are distinctly shewn in fig. 195 ; they persist only on the
second segment, where they appear to form the comb-like
organs or pectines. The last abdominal segment, Le. that next
the tail, is without provisional appendages. The embryonic tail
is divided into six segments including the telson (fig. 196 C, ab).
The lungs (st) are formed by paired invaginations, the walls of
which subsequently become plicated, on the four last segments
which bear rudimentary limbs, and simultaneously with the
disappearance of the rudimentary limbs.

PseudoscorpionidaB. The development of Qielifer has been in-
vestigated by Metschnikoff (436), and although (except that it is provided
with tracheae instead of pulmonary sacks) it might be supposed to be closely
related to Scorpio, yet in its development is strikingly different.

The eggs after being laid are carried by the female attached to the first
segment of the abdomen. The segmentation (vide p. 93) is intermediate
between the types of complete and superficial segmentation. The ovum,
mainly formed of food-yolk, divides into two, four, and eight equal segments

TRACHEATA.

435

(fig. 197 A). There then appear one or more clear segments on the surface
of these, and finally a complete layer of cells is formed round the central
yolk spheres (fig. 197 B), which latter subsequently agglomerate into a
central mass. The superficial cells form what may be called a blastoderm,
which soon becomes divided into two layers (fig. 197 C). There now
appears a single pair of appendages (the pedipalpi) (fig. 198 A,/^/), while at
the same time the front end of the embryo grows out into a remarkable
proboscis-like prominence— a temporary upper lip (concealed in the figure

flf

ab-

FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE

EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE.

(After Metschnikoff.)

ch. chelicerae ; pd. pedipalpi ; pl — />*. ambulatory appendages ; pe. pecten ; st.
stigmata ; ab. post abdomen (tail).

behind the pedipalpus), and the abdomen (ab) becomes bent forwards to-
wards the ventral surface. In this very rudimentary condition, after under-
going an ecdysis, the larva is hatched, although it still remains attached to
its parent. After hatching it grows rapidly, and becomes filled with a
peculiar transparent material. The first pair of ambulatory appendages is
formed behind the pedipalpi and then the three suceeding pairs, while at the
same time the chelicerae appear as small rudiments in front. External signs
of segmentation have not yet appeared, but about this period the nervous
system is formed. The supra-cesophageal ganglia are especially distinct,
and provided with a central cavity, probably formed by an invagination, as
in other Arachnida. In the succeeding stages (fig. 198 B) four provisional

28—2

ARANETNA.

pairs of appendages (shewn as small knobs at ati] appear behind the ambu-
latory feet. The abdomen is bent forwards so as to reach almost to the
pedipalpi. In the later stages (fig. 198 C) the adult form is gradually
attained. The enormous upper lip persists for some time, but subsequently
atrophies and is replaced by a normal labrum. The appendages behind the

FIG. igj. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.

(After Metschnikoff.)

In A the ovum is divided into a number of separate segments. In B a number of
small cells have appeared (bl) which form a blastoderm enveloping the large yolk
spheres. In C the blastoderm has become divided into two layers.

ambulatory feet atrophy, and the tail is gradually bent back into its final
position. The segmentation and the gradual growth of the limbs do not call
for special description, and the formation of the organs, so far as is known,
agrees with other types.

The segmentation of Chthonius is apparently similar to that of Chelifer
(Stecker, No. 437).

Phalangidae. Our knowledge of the development of Phalangium is
unfortunately confined to the later stages (Balbiani, No. 438). These stages
do not appear however to differ very greatly from those of true Spiders.

Araneina. The eggs of true Spiders are either deposited in
nests made specially for them, or are carried about by the
females. Species belonging to a considerable number of genera,
viz. Pholcus, Epeira, Lycosa, Clubione, Tegenaria and Agelcna

TRACHEATA.

437

have been studied by Claparede (No. 442), Balbiani (No. 439),
Barrois (No. 441) and myself (No. 440), and the close similarity
between their embryos leaves but little doubt that there are no
great variations in development within the group.

The ovum is enclosed in a delicate vitelline membrane,
enveloped in its turn by a chorion secreted by the walls of the
oviduct. The chorion is covered by numerous rounded promi-
nences, and occasionally exhibits a pattern corresponding with
the areas of the cells which formed it. The segmentation has
already been fully described, pp. 1 18 and 1 19. At its close there
is present an enveloping blastoderm formed of a single layer of
large flattened cells. The yolk within is formed of a number of

' °r.°v-ii~-cr^ ^H1—

o°o°o£°^ o°^*afe

Cll

ab

FIG. 198. THREE STAGES IN THE DEVELOPMENT OF CHELIFKR.

(After Metschnikoff.)
pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; c/i. chelicerse.

large polygonal segments ; each of which is composed of large
yolk spherules, and contains a nucleus surrounded by a layer of
protoplasm, which is prolonged into stellate processes holding
together the yolk spherules. The nucleus, surrounded by the
major part of the protoplasm of each yolk cell, appears, as a rule,

438 ARANEINA.

to be situated not at the centre, but on one side of its yolk
segment.

The further description of the development of Spiders applies
more especially to Agelena labyrinthica, the species which
formed the subject of my own investigations.

The first differentiation of the blastoderm consists in the
cells of nearly the whole of one hemisphere becoming somewhat
more columnar than those of the other hemisphere, and in the
cells of a small area near one end of the thickened hemisphere
becoming distinctly more columnar than elsewhere, and two
layers thick. This area forms a protuberance on the surface of
the ovum, originally discovered by Claparede, and called by him
the primitive cumulus. In the next stage the cells of the
thickened hemisphere of the blastoderm become still more
columnar; and a second area, at first connected by a whitish
streak with the cumulus, makes its appearance. In the second
area the blastoderm is also more than one cell deep (fig. 199).
It will be noticed that the blastoderm, though more than one
cell thick over a large part of the ventral surface, is not divided
into distinct layers. The second area appears as a white patch
and soon becomes more distinct, while the streak continued to
it from the cumulus is no longer visible. It is shewn in surface
view in fig. 200 A. Though my observations on this stage are
not quite satisfactory, yet it appears to me probable that there
is a longitudinal thickened ridge of the blastoderm extending
from the primitive cumulus to the large white area. The section
represented in fig. 199, which I believe to be oblique, passes
through this ridge at its most projecting part.

The nuclei of the yolk cells during the above stages multiply
rapidly, and cells are formed in the yolk which join the blasto-
derm ; there can however be no doubt that the main increase in
the cells of the blastoderm has been due to the division of the
original blastoderm cells.

In the next stage I have been able to observe there is, in the
place of the previous thickened half of the blastoderm, a well
developed ventral plate with a procephalic lobe in front, a
caudal lobe behind, and an intermediate region marked by
about three transverse grooves, indicating a division into
segments. This plate is throughout two or more rows of

TRACHEATA.

439

FIG. 199. SECTION THROUGH THE EM-
BRYO OF AGELENA LABYRINTHICA.

The section is from an embryo of the
same age as fig. 200 A, and is represented
with the ventral plate upwards. In the
ventral plate is seen a keel-like thickening,
which gives rise to the main mass of the
mesoblast.

yk. yolk divided into large polygonal
cells, in several of which nuclei are shewn.

cells thick, and the cells
which form it are divided into
two distinct layers — a colum-
nar superficial layer of epiblast
cells, and a deeper layer of
mesoblast cells (fig. 203 A).
In the latter layer there are
several very large cells which
are in the act of passing from
the yolk into the blastoderm.
The identification of the struc-
tures visible in the previous
stage with those visible in
the present stage is to a
great extent a matter of
guess-work, but it appears
to me probable that the
primitive cumulus is still present as a slight prominence visible
in surface views on the caudal lobe, and that the other thickened
patch persists as the procephalic lobe. However this may be,
the significance of the primitive cumulus appears to be that it is
the part of the blastoderm where two rows of cells become first
established \

The whole region of the blastoderm other than the ventral
plate is formed of a single row of flattened epiblast cells. The
yolk retains its original constitution.

By this stage the epiblast and mesoblast are distinctly
differentiated, and the homologue of the hypoblast is to be
sought for in the yolk-cells. The yolk-cells are not however
entirely hypoblastic, since they continue for the greater part of
the development to give rise to fresh cells which join the meso-
blast.

The Spider's blastoderm now resembles that of an Insect
(except for the amnion) after the establishment of the mesoblast,
and the mode of origin of the mesoblast in both groups is very
similar, in that the longitudinal ridge-like thickening of the

1 Various views have been put forward by Claparfede and Balbiani about the
position and significance of the primitive cumulus. For a discussion of which vide
self, No. 440.

440 AKANEINA.

mesoblast shewn in fig. 199 is probably the homologue of the
mesoblastic groove of the Insects' blastoderm.

The ventral plate continues to grow rapidly, and at a some-
what later stage (fig. 200 B) there are six segments interposed
between the procephalic and caudal lobes. The two anterior of
these (ch and pd), especially the foremost, are less distinct than
the remainder ; and it is probable that both of them, and in any
case the anterior one, are formed later than the three segments
following. These two segments are the segments of the chelicenc
and pedipalpi. The four segments following belong to the four
pairs of ambulatory legs. The segments form raised transverse
bands separated by transverse grooves. There is at this stage a
faintly marked groove extending along the median line of the
ventral plate. This groove is mainly caused by the originally
single mesoblastic plate having become divided throughout the
whole region of the ventral plate, except possibly the procephalic
lobes, into two bands, one on each side of the middle line (fig.
203 B).

The segments continue to increase in number by the con-
tinuous addition of fresh segments between the one last formed
and the caudal lobe. By the stage with nine segments the first
rudiments of the limbs make their appearance. The first
rudiments to appear are those of the pedipalpi and four ambu-
latory limbs : the chelicerae, like the segment to which they
belong, lag behind in development. The limbs appear as small
protuberances at the borders of their segments. By the stage
when they are formed the procephalic region has become
bilobed, and the two lobes of which it is composed are separated
by a shallow groove.

By a continuous elongation the ventral plate comes to form
a nearly complete equatorial ring round the ovum, the pro-
cephalic and caudal lobes being only separated by a very narrow
space, the undeveloped dorsal region of the embryo. This is
shewn in longitudinal section in fig. 204. In this condition the
embryo may be spoken of as having a dorsal flexure. By the
time that this stage is reached (fig. 200 C) the full number of
segments and appendages has become established. There are
in all sixteen segments (including the caudal lobe). The first
six of these bear the permanent appendages of the adult ; the

TRACHEATA. 44!

next four are provided with provisional appendages ; while the
last six are without appendages. The further features of this
stage which deserve notice are (i) the appearance of a shallow
depression (st) — the rudiment of the stomodaeum — between the
hinder part of the two procephalic lobes ; (2) the appearance of

FIG. aoo. FOUR STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.

A. Stage when the ventral plate is very imperfectly differentiated, pr.c. primitive
cumulus.

B. Ovum viewed from the side when the ventral plate has become divided into
six segments, ch. segment of chelicerae imperfectly separated from procephalic lobe ;
pd. segment of pedipalpi.

C. Ventral plate ideally unrolled after the full number of segments and
appendages are established, st. stomodoeum between the two proe-oral lobes.
Behind the six pairs of permanent appendages are seen four pairs of provisional
appendages.

D and E. Two views of an embryo at the same stage. D ideally unrolled,
E seen from the side. st. stomodseum ; ch. chelicerse ; on their inner side is seen
the ganglion belonging to them. pd. pedipalpi ; pr.p. provisional appendages.

raised areas on the inner side of the six anterior appendage-
bearing segments. These are the rudiments of the ventral
ganglia. It deserves to be especially noted that the segment of

442 AKANEINA.

the chelicera, like the succeeding segments, is provided with
ganglia ; and that the ganglia of the chelicerae are quite distinct
from the supra-cesophageal ganglia derived from the procephalic
lobes. (3) The pointed form of the caudal lobe. In Pholcus
(Claparede, No. 442) the caudal lobe forms a projecting structure
which, like the caudal lobe of the Scorpion, bends forward so as
to face the ventral surface of the part of the body immediately
in front. In most Spiders such a projecting caudal lobe is not
found. While the embryo still retains its dorsal flexure con-
siderable changes are effected in its general constitution. The
appendages (fig. 200 D and E) become imperfectly jointed, and
grow inwards so as to approach each other in the middle line.
Even in the stage before this, the ventral integument between
the rudiments of the ganglia had become very much thinner,
and had in this way divided the ventral plate into two halves.
At the present stage the two halves of the ventral plate are still
further separated, and there is a wide space on the ventral side
only covered by a delicate layer of epiblast. This is shewn in
surface view (fig. 200 D) and in section in fig. 203 C.

The stomodaeum (j/) is much more conspicuous, and is
bounded in front by a prominent upper lip, and by a less
marked lip behind. The upper lip becomes less conspicuous in
later stages, and is perhaps to be compared with the provisional
upper lip of Chelifer. Each procephalic lobe is now marked by
a deep semicircular groove.

The next period in the development is characterised by the
gradual change in the flexure of the embryo from a dorsal to a
ventral one ; accompanied by the division of the body into an
abdomen and cephalo-thorax, and the gradual assumption of the
adult characters.

The change in the flexure of the embryo is caused by the
elongation of the dorsal region, which has hitherto been hardly
developed. Such an elongation increases the space on the
dorsal surface between the procephalic and caudal regions, and
therefore necessarily separates the caudal and procephalic lobes ;
but, since the ventral plate does not become shortened in the
process, and the embryo cannot straighten itself in the egg-shell,
it necessarily becomes ventrally flexed.

If there were but little food yolk this flexure would naturally

TRACHEATA. 443

cause the whole embryo to be bent in so as to have the ventral
surface concave. But instead of this the flexure is at first con-
fined to the two bands which form the ventral plate. These
bands, as shewn in fig. 201 A, acquire a true ventral flexure, but
the yolk forms a projection — a kind of yolk sack as Barrois
(No. 441) calls it — distending the thin integument between the
two ventral bands. This yolk sack is shewn in surface view in

FlG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.

A. Embryo from the side at the stage when there is a large ventral protuberance
of yolk. The angle between the line of insertion of the permanent and provisional
appendages shews the extent of the ventral flexure.

B. Embryo nearly ready to be hatched. The abdomen which has not quite
acquired its permanent form is seen to be pressed against the ventral side of the
thorax.

prJ. procephalic lobe; pd. pedipalpi ; ch. chelicerae ; c,L caudal lobe; pr.p. pro-
visional appendages.

fig. 20 1 A and in section in fig. 206. At a later period, when
the yolk has become largely absorbed, the true nature of the
ventral flexure becomes quite obvious, since the abdomen of the
young Spider, while still in the egg, is found to be bent over so
as to press against the ventral surface of the thorax (fig. 201 B).
The general character of the changes which take place
during this period in the development is shewn in fig. 201 A and
B representing two stages in it. In the first of these stages
there is no constriction between the future thorax and abdomen.

444 ACARINA.

The four pairs of provisional appendages exhibit no signs of
atrophy ; and the extent of the ventral flexure is shewn by the
angle formed between the line of their insertion and that of the
appendages in front. The yolk has enormously distended the
integument between the two halves of the ventral plate, as is
illustrated by the fact that, at a somewhat earlier stage than
that figured, the limbs cross each other in the median ventral
line, while at this stage they do not nearly meet The limbs
have acquired their full complement of joints, and the pedipalpi
bear a cutting blade on their basal joint.

The dorsal surface between the prominent caudal lobe and
the procephalic lobes forms more than a semicircle. The terga
are fully established, and the boundaries between them, especially
in the abdomen, are indicated by transverse markings. A large
lower lip now bounds the stomodaeum, and the upper lip has
somewhat atrophied. In the later stage (fig. 201 B) the greater
part of the yolk has passed into the abdomen, which is now to
some extent constricted off from the cephalo-thorax. The
appendages of the four anterior abdominal somites have dis-
appeared, and the caudal lobe has become very small. In front
of it are placed two pairs of spinning mammillae. A delicate
cuticle has become established, which is very soon moulted.

Acarina. The development of the Acarina, which has been mainly
investigated by Claparede (No. 446), is chiefly remarkable from the frequent
occurrence of several larval forms following each other after successive
ecdyses. The segmentation (vide p. 116) ends in the formation of a blasto-
derm of a single layer of cells enclosing a central yolk mass.

A ventral plate is soon formed as a thickening of the blastoderm, in which
an indistinct segmentation becomes early observable. In Myobia, which is
parasitic on the common mouse, the ventral plate becomes divided by five
constrictions into six segments (fig. 202 A), from the five anterior of which
paired appendages very soon grow out (fig. 202 B) The appendages are the
chelicerae (ch} and pedipalpi (pd] and the first three pairs of limbs (p^—fi1}.
On the dorsal side of the chelicerae a thickened prominence of the ventral
plate appears to correspond to the procephalic lobes of other Arachnida.
The part of the body behind the five primitive appendage-bearing segments
appears to become divided into at least two segments. In other mites the
same appendages are formed as in Myobia, but the preceding segmentation
of the ventral plate is not always very obvious.

In Myobia two moultings take place while the embryo is still within the
primitive egg-shell. The first of these is accompanied by the apparently
total disappearance of the three pediform appendages, and the complete

TRACK EAT A.

445

coalescence of the two gnathiform appendages into a proboscis (fig. 202 C).
The feet next grow out again, and a second ecdysis then takes place. The
embryo becomes thus inclosed within three successive membranes, viz. the
original egg-shell and two cuticular membranes (fig. 202 D). After the
second ecdysis the appendages assume their final form, and the embryo
leaves the egg as an hexapodous larva. The fourth pair of appendages is

FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA MUSCULI.-

(After Claparede.)

J1 — j4. post-oral segments ; ch. chelicerae ; pd. pedipalpi ; pr. proboscis formed by
the coalescence of the chelicerse and pedipalpi ; pl, /*, etc. ambulatory appendages.

acquired by a post-embryonic metamorphosis. From the proboscis are
formed the rudimentary palpi of the second pair of appendages, and two
elongated needles representing the chelicerae.

In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are
not accompanied by the peculiar changes observable in Myobia : the
cheliceras and pedipalpi fuse however to form the proboscis. The first
larval form is hexapodous, and the last pair of appendages is formed at a
subsequent ecdysis.

In Atax Bonzi, a form parasitic on Unio, the development and meta-
morphosis are even more complicated than in Myobia. The first ecdysis
occurs before the formation of the limbs, and shortly after the ventral plate
has become divided into segments. Within the cuticular membrane resulting
from the first ecdysis the anterior five pairs of limbs spring out in the usual
fashion. They undergo considerable differentiation ; the chelicerae and
pedipalpi approaching each other at the anterior extremity of the body, and
the three ambulatory legs becoming segmented and clawed. An oesophagus,
a stomach, and an oesophageal nerve-ring are also formed. When the larva

446 ACARINA.

has attained this stage the original egg-shell is split into two valves and
eventually cast off, but the embryo remains enclosed within the cuticular
membrane shed at the first ecdysis. This cuticular membrane is spoken of
by Claparede as the deutovum. In the deutovum the embryo undergoes
further changes ; the chelicerae and pedipalpi coalesce and form the
proboscis ; a spacious body cavity with blood corpuscles appears ; and the
alimentary canal enclosing the yolk is formed.

The larva now begins to move, the cuticular membrane enclosing it is
ruptured, and the larva becomes free. It does not long remain active, but
soon bores its way into the gills of its host, undergoes a fresh moult, and
becomes quiescent. The cuticular membrane of the moult just effected
swells up by the absorption of water and becomes spherical. Peculiar
changes take place in the tissues, and the limbs become, as in Myobia,
nearly absorbed, remaining however as small knobs. The larva swims
about as a spherical body within its shell. The feet next grow out afresh,
and the posterior pair is added. From the proboscis the palpi (of the
pedipalpi) grow out below. The larva again becomes free, and amongst
other changes the chelicerae grow out from the proboscis. A further ecdysis,
with a period of quiescence, intervenes between this second larval form and
the adult state.

The changes in the appendages which appear common to the Mites
generally are (i) the late development of the fourth pair of appendages, which
results in the constant occurrence of an hexapodous larva ; and (2) the early
fusion of the chelicerae and pedipalpi to form a proboscis in which no trace
of the original appendages can be discerned. In most instances palpi and
stilets of variable form are subsequently developed in connexion with the
proboscis, and, as indicated in the above descriptions, are assumed to cor-
respond with the two original embryonic appendages.

TJie history of tJie germinal layers.

It is a somewhat remarkable fact that each of the groups of
the Arachnida so far studied has a different form of segmenta-
tion. The types of Chelifer and the Spiders are simple modi-
fications of the centrolecithal type, while that of Scorpio, though
apparently meroblastic, is probably to be regarded in the same
light (vide p. 120 and p. 434). The early development begins in
the Scorpion and Spiders with the formation of a ventral plate,
and there can be but little doubt that Chelifer is provided
with an homologous structure, though very probably modified,
owing to the small amount of food-yolk and early period of
hatching.

The history of the layers and their conversion into the organs
has been studied in the case of the Scorpion (Metschnikoff, No.

TRACHEATA. 447

434), and of the Spiders ; and a close agreement has been found
to obtain between them.

It will be convenient to take the latter group as type, and
simply to call attention to any points in which the two groups
differ.

The epiblast. The epiblast, besides giving rise to the skin
(hypodermis and cuticle), also supplies the elements for the
nervous system and organs of sense, and for the respiratory
sacks, the stomodaeum and proctodaeum.

At the period when the mesoblast is definitely established,
the epiblast is formed of a single layer of columnar cells in the
region of the ventral plate, and of a layer of flat cells over other
parts of the yolk.

When about six segments are present the first changes take
place. The epiblast of the ventral plate then becomes somewhat
thinner in the median line than at the two sides (fig. 203 B). In
succeeding stages the contrast between the median and the
lateral parts becomes still more marked, so that the epiblast
becomes finally constituted of two lateral thickened bands, which
meet in front in the procephalic lobes, and behind in the caudal
lobe, and are elsewhere connected by a very thin layer (fig.
203 C). Shortly after the appendages begin to be formed, the
first rudiments of the ventral nerve-cord become established as
epiblastic thickenings on the inner side of each of the lateral
bands. The thickenings of the epiblast of the two sides are
quite independent, as may be seen in fig. 203 C, vn, taken from a
stage somewhat subsequent to their first appearance. They are
developed from before backwards, but either from the first, or in
any case very soon afterwards, cease to form uniform thickenings,
but constitute a linear series of swellings — the future ganglia —
connected by very short less prominent thickenings of the epi-
blast (fig. 200 C). The rudiments of the ventral nerve-cord are
for a long time continuous with the epiblast, but shortly after the
establishment of the dorsal surface of the embryo they become
separated from the epiblast and constitute two independent
cords, the histological structure of which is the same as in other
Tracheata (fig. 206, vn\

The ventral cords are at first composed of as many ganglia
as there are segments. The foremost pair, belonging to the

448

ARACHNIDA.

segment of the chelicerae, lie immediately behind the stomodaeum,
and are as independent of each other as the remaining ganglia.
Anteriorly they border on the supra-cesophageal ganglia. When
the yolk sack is formed in connection with the ventral flexure of
the embryo, the two nerve-cords become very widely separated
(fig. 206, vn) in their middle region. At a later period, at the
stage represented in fig. 201 B, they again become approximated
in the ventral line, and delicate commissures are formed uniting

FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELBNA
LABYRINTHICA AT THREE STAGES.

A. Stage when about three segments are formed. The mesoblastic plate is not
divided into two bands.

B. Stage when six segments are present (fig. ?oo B). The mesoblast is now
divided into two bands.

C. Stage represented in fig. 200 D. The ventral cords have begun to be formed
on thickenings of the epiblast, and the limbs are established.

ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; 7>n. ventral nerve-cord ;
yk. yolk.

the ganglia of the two sides, but there is no trace at this or any
other period of a median invagination of epiblast between the
two cords, such as Hatschek and other observers have attempted
to establish for various Arthropoda and Chaetopoda. At the
stage represented in fig. 201 A the nerve ganglia are still present
in the abdomen, though only about four ganglia can be distin-
guished. At a later stage these ganglia fuse into two continuous

TRACHEATA. 449

cords, united however by commissures corresponding with the
previous ganglia.

The ganglia of the chelicerae have, by the stage represented
in fig. 20 1 B, completely fused with the supra-oesophageal ganglia
and form part of the oesophageal commissure. The cesophageal
commissure is however completed ventrally by the ganglia of
the pedipalpi.

The supra-cesophageal ganglia are formed independently of
the ventral cords as two thickenings of the procephalic lobes (fig.
205). The thickenings of the two lobes are independent, and
each of them becomes early marked out by a semicircular groove
(fig. 200 D) running outwards from the upper lip. Each thick-
ening eventually becomes detached from the superficial epiblast,
but before this takes place the two grooves become deeper,
and on the separation of the ganglia from the epiblast, the
cells lining the grooves become involuted and detached from
the skin, and form an integral part of the supra-oesophageal
ganglia.

At the stage represented in fig. 201 B the supra-oesophageal ganglia
are completely detached from the epiblast, and are constituted of the
following parts : (i) A dorsal section formed of two hemispherical lobes,
mainly formed of the invaginated lining of the semicircular grooves. The
original lumen of the groove is still present on the outer side of these
lobes. (2) Two central masses, one for each ganglion, formed of puncti-
form tissue, and connected by a transverse commissure. (3) A ventral
anterior lobe. (4) The original ganglia of the chelicerae, which form the
ventral parts of the ganglia1.

The later stages in the development of the nervous system have not
been worked out.

The development of the nervous system in the Scorpion is almost
identical with that in Spiders, but Metschnikoff believes, though without
adducing satisfactory evidence, that the median integument between the
two nerve cords assists in forming the ventral nerve cord. Grooves are
present in the supra-cesophageal ganglia similar to those in Spiders.

The mesoblast. The history of the mesoblast, up to the
formation of a ventral plate subjacent to the thickened plate of
epiblast, has been already given. The ventral plate is shewn
in fig. 203 A. It is seen to be formed mainly of small cells,

1 For further details vide self, No. 440.
B. II. 29

45O ARACHNIDA.

but some large cells are shewn in the act of passing into it
from the yolk. During a considerable section of the subse-
quent development the mesoblast is confined to the ventral
plate.

The first important change takes place when about six
somites are established ; the mesoblast then becomes divided

f/0

FIG. 204. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELENA

LABYRINTHICA.

The section is through an embryo of the same age as that represented in fig.
200 C, and is taken slightly to one side of the middle line so as to shew the relation
of the mesoblastic somites to the limbs. In the interior are seen the yolk segments
and their nuclei.

i — 16. the segments; pr.l. procephalic lobe ; do. dorsal integument.

into two lateral bands, shewn in section in fig. 203 B, which meet
however in front in the procephalic lobes, and behind in the
caudal lobes. Very shortly afterwards these bands become
broken up into a number of parts corresponding to the segments,
each of which soon becomes divided into two layers, which
enclose a cavity between them (vide fig. 204 and fig. 207). The
outer layer (somatic) is thicker and attached to the epiblast,
and the inner layer (splanchnic) is thinner and mainly, if not
entirely, derived (in Agelena) from cells which originate in the
yolk. These structures constitute the mesoblastic somites. In
the appendage-bearing segments the somatic layer of each of
them, together with a prolongation of the cavity, is continued

TRACHEATA.

451

into the appendage (fig. 203 C). Since the cavity of the meso-
blastic somites is part of the body cavity, all the appendages
contain prolongations of the body cavity. Not only is a pair of
mesoblastic somites formed for each segment of the body, but
also for the procephalic lobes (fig. 205). The mesoblastic somites
for these lobes are established somewhat later than for the true
segments, but only differ from them in the fact that the somites
of the two sides are united by a median bridge of undivided meso-
blast. The development of a somite for the procephalic lobes
is similar to what has been described by Kleinenberg for Lum-
bricus (p. 339),
but must not be
necessarily sup-
posed to indicate
that the procepha-
lic lobes form a
segment equiva-
lent to the seg-
ments of the trunk.
They are -rather
equivalent to the

ce.s

FIG. 205. SECTION THROUGH THE PROCEPHALIC
LOBES OF AN EMBRYO OF AGELENA LABYRINTHICA.

The section is taken from an embryo of the same age
as fig. 200 D.

Drae oral lobe of groove

stomodseum ; gr. section through semi-circular
procephalic lobe ; ce.s. cephalic section of body
cavitv.

Chaetopod larvae.
When the dorsal surface of the embryo is established a thick
layer of mesoblast becomes formed below the epiblast. This
layer is not however derived from an upgrowth of the mesoblast
of the somites, but from cells which originate in the yolk. The
first traces of the layer are seen in fig. 204, do, and it is fully
established as a layer of large round cells in the stage shewn in
fig. 206. This layer of cells is seen to be quite independent of
the mesoblastic somites (ine.s). The mesoblast of the dorsal
surface becomes at the stage represented in fig. 201 B divided
into splanchnic and somatic layers, and in the abdomen at any
rate into somites continuous with those of the ventral part of the
mesoblast. At the lines of junction of successive somites the
splanchnic layer of mesoblast dips into the yolk, and forms a
number of transverse septa, which do not reach the middle of
the yolk, but leave a central part free, in which the mesenteron
is subsequently formed. At the insertion of these septa there

29 — 2

452

ARACHNIDA.

me.s

are developed widish spaces between the layers of somatic
and splanchnic mesoblast, which form transversely directed
channels passing
from the heart out-
wards. They are
probably venous.
At a later stage
the septa send out
lateral offshoots,
and divide the
peripheral part of
the abdominal cav-
ity into a number
of compartments
filled with yolk. It
is probable that
the hepatic diverti-
cula are eventually
formed in these
compartments.

The somatic
layer of mesoblast

FIG. 206. TRANSVERSE SECTION THROUGH THE THO-
RACIC REGION OF AN EMBRYO OF AGELENA LABYRINTHICA.

The section is taken from an embryo of the same age
as fig. 201 A, and passes through the maximum pro-
tuberance of the ventral yolk sack.

vn. ventral nerve cord ; yk. yolk ; me.s. mesoblastic
somite ; ao. aorta.

is converted into the muscles, both of the limbs and trunk, the
superficial connective tissue, nervous sheath, etc. It probably
also gives rise to the three muscles attached to the suctorial
apparatus of the oesophagus.

The heart and aorta are formed as a solid rod of cells of the
dorsal mesoblast, before it is distinctly divided into splanchnic
and somatic layers. Eventually the central cells of the heart
become blood corpuscles, while its walls are constituted of an
outer muscular and inner epithelioid layer. It becomes func-
tional, and acquires its valves, arterial branches, etc., by the
stage represented in fig. 201 B.

The history of the mesoblast, more especially of the mesoblastic somites,
of the Scorpion is very similar to that in Spiders : their cavity is continued
in the same way into the limbs. The general character of the somites
in the tail is shewn in fig. 207. The caudal aorta is stated by MetschnikofT
to be formed from part of the mesenteron, but this is too improbable to be
accepted without further confirmation.

TRACHEATA.

453

The hypoblast and alimentary tract. It has already
been stated that the yolk is to be regarded as corresponding to
the hypoblast of other types.

For a considerable period it is composed of the polygonal
yolk cells already described and shewn in figs. 203, 204, and 205.
The yolk cells divide and be-
come somewhat smaller as de-
velopment proceeds ; but the
main products of the division
of the yolk nuclei and the pro-
toplasm around them are un-
doubtedly cells which join the
mesoblast (fig. 203 A). The
permanent alimentary tract is
formed of three sections, viz.
stomodaeum, proctodaeum, and
mesenteron. The stomodaeum
and proctodaeum are both
formed before the mesenteron.
The stomodaeum is formed as
an epiblastic pit between the
two procephalic lobes (figs. 200
and 205, st). It becomes
deeper, and by the latest stage
figured is a deep pit lined by a
cuticle and ending blindly. To
its hinder section, which forms
the suctorial apparatus of the adult, three powerful muscles (a
dorsal and two lateral) are attached.

The proctodaeum is formed considerably later than the
stomodaeum. It is a comparatively shallow involution, which
forms the rectum of the adult. It is dilated at its extremity, and
two Malpighian vessels early grow out from it.

The mesenteron is formed in the interior of the yolk. Its
walls are derived from the cellular elements of the yolk, and the
first section to be formed is the hinder extremity, which appears
as a short tube ending blindly behind in contact with the procto-
daeum, and open to the yolk in front. The later history of the
mesenteron has not been followed, but it undoubtedly includes

FlG. 207. TAIL OF AN ADVANCED EM-
BRYO OF THE SCORPION TO ILLUSTRATE
THE STRUCTURE OF THE MESOBLASTIC

SOMITES. (After Metschnikoff.)

al. alimentary tract; an.i. anal in-
vagination ; ep. epiblast ; me.s. meso-
blastic somite.

454 ARACHNIDA.

the whole of the abdominal section of the alimentary canal of
the adult, except the rectum, and probably also the thoracic
section. The later history of the yolk which encloses the mesen-
teron has not been satisfactorily studied, though it no doubt
gives rise to the hepatic tubes, and probably also to the thoracic
diverticula of the alimentary tract.

The general history of the alimentary tract in Scorpio is much the same
as in Spiders. The hypoblast, the origin of which as mentioned above is
somewhat uncertain, first appears on the ventral side and gradually spreads
so as to envelop the yolk, and form the wall of the mesenteron, from
which the liver is formed as a pair of lateral outgrowths. The procto-
daeum and stomodseum are both short, especially the former (vide fig. 207).

Summary and general conclusions.

The embryonic forms of Scorpio and Spiders are very
similar, but in spite of the general similarity of Chelifer to
Scorpio, the embryo of the former differs far more from that of
Scorpio than the latter does from Spiders. This peculiarity is
probably to be explained by the early period at which Chelifer
is hatched ; and though a more thorough investigation of this
interesting form is much to be desired, it does not seem probable
that its larva is a primitive type.

The larvae of the Acarina with their peculiar ecdyses are to
be regarded as much modified larval forms. It is not however
easy to assign a meaning to the hexapodous stage through
which they generally pass.

With reference to the segments and appendages, some inter-
esting points are brought out by the embryological study of
these forms.

The maximum number of segments is present in the
Scorpion, in which nineteen segments (not including the prae-
oral lobes, but including the telson) are developed. Of these the
first twelve segments have traces of appendages, but the append-
ages of the six last of these (unless the pecten is an appendage)
atrophy. In Spiders there are indications in the embryo of
sixteen segments ; and in all the Arachnida, except the Acarina,
at the least four segments bear appendages in the embryo
which are without them in the adult. The morphological bear-
ings of this fact are obvious.

TRACHEATA. 455

It deserves to be noted that, in both Scorpio and the Spider,
the chelicerae are borne in the embryo by the first post-oral
segment, and provided with a distinct ganglion, so that they
cannot correspond (as they are usually supposed to do) with the
antennae of Insects, which are always developed on the prae-oral
lobes, and never supplied by an independent ganglion.

The chelicerae would seem probably to correspond with the
mandibles of Insects, and the antennae to be absent. In favour
of this view is the fact that the embryonic ganglion of the
mandibles of Insects is stated (cf. Lepidoptera, Hatschek, p. 340)
to become, like the ganglion of the chelicerae, converted into
part of the cesophageal commissure.

If the above considerations are correct, the appendages of
the Arachnida retain in many respects a very much more prim-
itive condition than those of Insects. In the first place, both the
chelicerae and pedipalpi are much less differentiated than the
mandibles and first pair of maxillae with which they correspond.
In the second place, the first pair of ambulatory limbs must be
equivalent to the second pair of maxillae of Insects, which, for
reasons stated above, were probably originally ambulatory. It
seems in fact a necessary deduction from the arguments stated
that the ancestors of the present Insecta and Arachnida must
have diverged from a common stem of the Tracheata at a time
when the second pair of maxillae were still ambulatory in
function.

With reference to the order of the development of the appendages
and segments, very considerable differences are noticeable in the different
Arachnoid types. This fact alone appears to me to be sufficient to prove
that the order of appearance of the appendages is often a matter of
embryonic convenience, without any deep morphological significance. In
Scorpio the segments develop successively, except perhaps the first post-
oral, which is developed after some of the succeeded segments have
been formed. In Spiders the segment of the chelicerae, and probably also
of the pedipalpi, appears later than the next three or four. In both these
types the segments arise before the appendages, but the reverse appears to
be the case in Chelifer. The permanent appendages, except the chelicerae,
appear simultaneously in Scorpions and Spiders. The second pair appears
long before the others in Chelifer, then the third, next the first, and finally
the three hindermost.

456 ARACHNIDA.

BIBLIOGRAPHY.

Scorpionidcz.

(434) El. Metschnikoff. " Embryologie des Scorpions." Zeit.f.wiss. Zool.
Bd. xxi. 1870.

(435) H. Rathke. Reisebemerkungen aus Taurien (Scorpio), Leipzig, 1837.

Pseudoscorpionidce.

(436) El. Metschnikoff. " Entwicklungsgeschichte d. Chelifer." Zeit.f.wiss.
Zool., Bd. xxi. 1870.

(437) A. Stecker. " Entwicklung der Chthonius-Eier im Mutterleibe und die
Bildung des Blastoderms." Sitzung. konigl. bohmisch. Gesellschaft Wissensch., 1876,
3. Heft, and Aimed, and Mag. Nat. History, 1876, xvm. 197.

Phalangida.

(438) M. Balbiani. " Memoire sur le developpement des Phalangides." Ann.
Scien. Nat. Series v. Vol. xvi. 1872.

A raneina.

(439) M. Balbiani. "Memoire sur le developpement des Araneides." Ann.
Scien. Nat. Series v. Vol. xvn. 1873.

(440) F. M. Balfour. "Notes on the development of the Araneina." Quart.
Journ. of Micr. Science, Vol. xx. 1880.

(441) J. Barrois. " Recherches s. 1. developpement des Araigndes. " Journal
de 1'Anat. et de la Physiol. 1878.

(442) E. Claparede. Recherches s. t evolution des Araignees. Utrecht, 1862.

(443) Hero Id. De generatione Araneorum in Ovo. Marburg, 1824.

(444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den Spinnen."
Zeit.f. wiss. Zool., Vol. xxvi. 1876.

Acarina.

(445) P. van Beneden. " Developpement de 1'Atax ypsilophora." Acad. Bru-
xelles, t. xxiv.

(446) Ed. Claparede. "Studien iiber Acarinen." Zeit.f. wiss. Zool., Bd.
xvm. 1868.

Formation of the layers and the embryonic envelopes in the

Tracheata.

There is a striking constancy in the mode of formation of
the layers throughout the group. In the first place the hypo-
blast is not formed by a process which can be reduced to
invagination : in other words, there is no gastrula stage.

TRACHEATA. 457

Efforts have been made to shew that the mesoblastic groove of Insects
implies a modified gastrula, but since it is the essence of a gastrula that it
should directly or indirectly give rise to the archenteron, the groove in
question cannot fall under this category. Although the mesoblastic groove
of Insects is not a gastrula, it is quite possible that it is the rudiment of a
blastopore, the gastrula corresponding to which has now vanished from
the development. It would thus be analogous to the primitive streak of
Vertebrates1.

The growth of the blastoderm over the yolk in Scorpions admits no
doubt of being regarded as an epibolic gastrula. The blastopore would
however be situated dorsally, a position which it does not occupy in any
gastrula type so far dealt with. This fact, coupled with the consideration
that the partial segmentation of Scorpio can be derived without difficulty
from the ordinary Arachnidan type (vide p. 120), seems to shew that there
is no true epibolic invagination in the development of Scorpio.

On the formation of the blastoderm traces of two embryonic
layers are established. The blastoderm itself is essentially the
epiblast, while the central yolk is the hypoblast. The formation
of the embryo commences in connection with a thickening of the
blastoderm, known as the ventral plate. The mesoblast is
formed as an unpaired plate split off from the epiblast of the
ventral plate. This process takes place in at any rate two ways.
In Insects a groove is formed, which becomes constricted off to
form the mesoblastic plate : in Spiders there is a keel-like
thickening of the blastoderm, which takes the place of the
groove.

The unpaired mesoblastic plate becomes in all forms very
soon divided into two mesoblastic bands.

The mesoblastic bands are very similar to, and probably
homologous with, those of Chaetopoda ; but the different modes
by which they arise in these two groups are very striking, and
probably indicate that profound modifications have taken place
in the early development of the Tracheata. In the Chaetopoda
the bands are from the first widely separated, and gradually
approach each other ventrally, though without meeting. In the
Tracheata they arise from the division of an unpaired ventral
plate.

The further history of the mesoblastic bands is nearly the

1 The primitive streak of Vertebrates, as will appear in the sequel, has no con-
nection with the medullary groove, and is the rudiment of the blastopore.

458 TRACHEATA.

same for all the Tracheata so far investigated, and is also very
much the same as for the Chaetopoda. There is a division into
somites; each containing a section of the body cavity. In the
cephalic section of the mesoblastic bands a section of the body
cavity is also formed. In Arachnida, Myriapoda, and probably
also Insecta, the body cavity is primitively prolonged into the
limbs.

In Spiders at any rate, and very probably in the other groups
of the Tracheata, a large part of the mesoblast is not derived
from the mesoblastic plate, but is secondarily added from the
yolk-cells.

In all Tracheata the yolk-cells give rise to the mesenteron
which, in opposition, as will hereafter appear, to the mesenteron
of the Crustacea, forms the main section of the permanent
alimentary tract.

One of the points which is still most obscure in connection
with the embryology of the Tracheata is the origin of the
embryonic membranes. Amongst Insects, with the exception
of the Thysanura, such membranes are well developed. In the
other groups definite membranes like those of Insects are never
found, but in the Scorpion a cellular envelope appears to be
formed round the embryo from the cells of the blastoderm, and
more or less similar structures have been described in some
Myriapods (vide p. 390). These structures no doubt further
require investigation, but may provisionally be regarded as
homologous with the amnion and serous membrane of Insects.
In the present state of our knowledge it does not seem easy to
give any explanation of the origin of these membranes, but they
may be in some way derived from an early ecdysis.

CHAPTER XVIII

CRUSTACEA1.

History of the larval forms1 '.

THE larval forms of the Crustacea appear to have more faith-
fully preserved their primitive characters than those of almost
any other group.

BRANCHIOPODA.

The Branchiopoda, comprising under that term the Phyllo-
poda and Cladocera, contain the Crustacea with the maximum
number of segments and the least differentiation of the separate
appendages. This and other considerations render it probable
that they are to be regarded as the most central group of the
Crustaceans, and as in many respects least modified from the
ancestral type from which all the groups have originated.

1 The following is the classification of the Crustacea employed in the present
chapter.

i Phyllopoda. ( Natantia.

I. Branchiopoda. ciadocenu III. Copepoda. Euc°PeP°da Iparasita.

( Branchiura
T Nebaliadse. jThoracica.

M f Sat- <v- wdi, paminai ia

II. Malacostraca. ] Stomatopoda. ULocephaia.

I Cumacese. v. Ostracoda.

I Edriophthalmata.

2 The importance of the larval history of the Crustacea, coupled with our compara-
tive ignorance of the formation of the layers, has rendered it necessary for me to
diverge somewhat from the general plan of the work, and to defer the account of the
formation of the layers till after that of the larval forms.

460 PHYLLOPODA.

The free larval stages when such exist commence with a
larval form known as the Nauplius.

The term Nauplius was applied by O. F. Muller to certain
larval forms of the Copepoda (fig. 229) in the belief that they
were adult.

The term has now been extended to a very large number of
larvae which have certain definite characters in common. They
are provided (fig. 208 A) with three pairs of appendages, the
future two pairs of antennae and mandibles. The first pair of
antennae (an1) is uniramous and mainly sensory in function, the
second pair of antennae (an*) and mandibles (md) are biramous

A qn

FlG. 208. TWO STAGES IN THE DEVELOPMENT OF APUS CANCRIFORM1S.

(After Claus.)

A. Nauplius stage at the time of hatching.

B. Stage after first ecdysis.

an1, and a«2. First and second antennae ; md. mandible ; MX. maxilla ; /. labrum;
fr. frontal sense organ ; /. caudal fork ; s. segments.

swimming appendages, and the mandibles are without the future
cutting blade. The Nauplius mandibles represent in fact the
palp. The two posterior appendages are both provided with
hook-like prominences on their basal joints, used in mastication.
The body in most cases is unsegmented, and bears anteriorly a
single median eye. There is a large upper lip, and an aliment-
ary canal formed of cesophagus, stomach and rectum. The anus
opens near the hind end of the body. On the dorsal surface
small folds of skin frequently represent the commencement of a
dorsal shield. One very striking peculiarity of the Nauplius
according to Claus and Dohrn is the fact that the second pair
of antennae is innervated from a sub-oesophageal ganglion. A
larval form with the above characters occurs with more or less
frequency in all the Crustacean groups. In most instances it

CRUSTACEA. 461

does not exactly conform to the above type, and the divergences
are more considerable in the Phyllopods than in most other
groups. Its characters in each case are described in the sequel.
Phyllopoda. For the Phyllopoda the development of Apus
cancriformis may conveniently be taken as type (Claus, No. 454).
The embryo at the time it leaves the egg (fig. 208 A) is some-
what oval in outline, and narrowed posteriorly. There is a
slight V-shaped indentation behind, at the apex of which is
situated the anus. The body, unlike that of the typical
Nauplius, is already divided into two regions, a cephalic and
post-cephalic. On the ventral side of the cephalic region there
are present the three normal pairs of appendages. Foremost
there are the small anterior antennae (an1), which are simple
unjointed rod-like bodies with two moveable hairs at their
extremities. They are inserted at the sides of the large upper-
lip or labrum (/). Behind these are the posterior antennae, which
are enormously developed and serve as the most important
larval organs of locomotion. They are biramous, being formed
of a basal portion with a strong hook-like bristle projecting
from its inner side, an inner unjointed branch with three bristles,
and an outer large imperfectly five-jointed branch with five long
lateral bristles. The hook-like organ attached to this pair of
appendages would seem to imply that it served in some ancestral
form as jaws (Claus). This character is apparently universal in
the embryos of true Phyllopods, and constantly occurs in the
Copepoda, etc.

The third pair of appendages or mandibles (md) is attached
close below the upper lip. They are as yet unprovided with
cutting blades, and terminate in two short branches, the inner
with two and the outer with three bristles.

At the front of the head there is the typical unpaired eye.
On the dorsal surface there is already present a rudiment of the
cephalic shield, continuous anteriorly with the labrum (/) or
upper lip, the extraordinary size of which is characteristic of the
larvae of Phyllopods. The post-cephalic region, which afterwards
becomes the thorax and abdomen, contains underneath the skin
rudiments of the five anterior thoracic segments and their
appendages, and presents in this respect an important variation
from the typical Nauplius form. After the first ecdysis the

462 PHYLLOPODA.

larva (fig. 208 B) loses its oval form, mainly owing to the elong-
ation of the hinder part of the body and the lateral extension of
the cephalic shield, which moreover now completely covers over
the head and has begun to grow backwards so as to cover over
the thoracic region. At the second ecdysis there appears at its
side a rudimentary shell gland. In the cephalic region two
small papillae (fr) are now present at the front of the head close
to the unpaired eye. They are of the nature of sense organs,
and may be called the frontal sense papillae. They have been
shewn by Claus to be of some phylogenetic importance. The
three pairs of Nauplius appendages have not altered much, but
a rudimentary cutting blade has grown out from the basal joint
of the mandible. A gland opening at the base of the antennae
is now present, which is probably equivalent to the green gland
often present in the Malacostraca. Behind the mandibles a pair
of simple processes has appeared, which forms the rudiment of
the first pair of maxillae (mx).

In the thoracic region more segments have been added
posteriorly, and the appendages of the three anterior segments
are very distinctly formed. The tail is distinctly forked. The
heart is formed at the second ecdysis, and then extends to the
sixth thoracic segment : the posterior chambers are successively
added from before backwards.

At the successive ecdyses which the larva undergoes new
segments continue to be formed at the posterior end of the body,
and limbs arise on the segments already formed. These limbs
probably represent the primitive form of an important type of
Crustacean appendage, which is of value for interpreting the
parts of the various malacostracan appendages. They consist
(fig. 209) of a basal portion (protopodite of Huxley) bearing two
rami. The basal portion has two projections on the inner side.
To the outer side of the basal portion there is attached a
dorsally directed branchial sack (br) (epipodite of Huxley). The
outer ramus (ex) (exopodite of Huxley) is formed of a single plate
with marginal setae. The inner one (en) (endopodite of Huxley)
is four-jointed, and a process similar to those of the basal joint
is given off from the inner side of the three proximal joints.

At the third ecdysis several new features appear in the
cephalic region, which becomes more prominent in the succeeding

CRUSTACEA. 463

stages. In the first place the paired eyes are formed at each side

of and behind the unpaired eye, second-

ly the posterior pair of maxillae is

formed though it always remains very

rudimentary. The shell gland becomes

fully developed opening at the base of

the first pair of maxillae. The dorsal

shield gradually grows backwards till it

covers its full complement of segments.

After the fifth ecdysis the Nauplius FIG. 209. TYPICAL PHYL-

. , . , , LOPOD APPENDAGE. (Copied

appendages undergo a rapid atrophy. from ciaus.)

The second pair of antennae especially ex. exopodite ; en. endo-

becomes reduced in size, and the man-

dibular palp — the primitive Nauplius portion bearing the two proxi-
r . ..... , mal projections is not sharply

portion of the mandible— is contracted separated from the endopo-
to a mere rudiment, which eventually dite-
completely disappears, while the blade is correspondingly en-
larged and also becomes toothed. The adult condition is only
gradually attained after a very large number of successive changes
of skin.

The chief point of interest in the above development is the
fact of the primitive Nauplius form becoming gradually convert-
ed without any special metamorphosis into the adult condition1.

Branchipus like Apus is hatched as a somewhat modified Nauplius,
which however differs from that of Apus in the hinder region of the body
having no indications of segments. It goes through a very similar meta-
morphosis, but is at no period of its metamorphosis provided with a dorsal
shield : the second pair of antennae does not abort, and in the male is pro-
vided with clasping organs, which are perhaps remnants of the embryonic
hooks so characteristic of this pair of antennas.

The larva of Estheria when hatched has a Nauplius form, a large
upper lip, caudal fork and single eye. There are two functional pairs of
swimming appendages — the second pair of antennae and mandibles. The
first pair of antennae has not been detected, and a dorsal mantle to form
the shell is not developed. At the first moult the anterior pair of
antennae arises as small stump-like structures, and a small dorsal shield
is also formed. Rudiments of six or seven pairs of appendages sprout

1 Nothing appears to be known with reference to the manner in which it comes
about that more than one appendage is borne on each of the segments from the
eleventh to the twentieth. An investigation of this point would be of some interest
with reference to the meaning of segmentation-

464

CLADOCERA.

out in the usual way, and continue to increase in number at successive
moults : the shell is rapidly developed. The chief point of interest in
the development of this form is the close resemblance of the young larva
to a typical adult Cladocera (Claus). This is shewn in the form of the
shell, which has not reached its full anterior extension, the rudimentary
anterior antennae, the large locomotor second pair of antennas, which differ
however from the corresponding organs in the Cladocera in the presence
of typical larval hooks. Even the abdomen resembles that of Daphnia.
These features perhaps indicate that the Cladocera are to be derived
from some Phyllopod form like Estheria by a process of retrogressive
metamorphosis. The posterior antennas in the adult Estheria are large
biramous appendages, and are used for swimming ; and though they
have lost the embryonic hook, they still retain to a larger extent than
in other Phyllopod families their Nauplius characteristics.

The Nauplius form of the Phyllopods is marked by several
definite peculiarities. Its body is distinctly divided into a ceph-
alic and post-cephalic region. The upper lip is extraordinarily
large, relatively very much more so than at the later stages.
The first pair of antennae is usually rudimentary and sometimes
even absent ; while the second pair is exceptionally large, and
would seem to be capable of functioning not only as a swimming
organ, but even as a masticating organ. A dorsal shield is
nearly or quite absent.

Cladocera. The probable derivation of the Cladocera from a form
similar to Estheria has already been mentioned, and it might have been
anticipated that the deve-
lopment would be similar
to that of the Phyllopods.
The development of the ma-
jority of the Cladocera takes
place however in the egg,
and the young when hatched
closely resembles their pa-
rents, though in the egg they
pass through a Nauplius
stage (Dohrn). An excep-
tion to the general rule is
however offered by the case
of the winter eggs of Lepto-
dora, one of the most primi-
tive of the Cladoceran

families. The summer eggs after Sars.)

FIG. 709 A. NAUPLIUS LARVA OF LEPTODORA
IIYAI.INA FROM wiNTKR EGG. (Copied from Bronn ;

develop without metamor-

«;/'. antenna of first pair; an*, antenna of

phosis, but Sars (No. 461) second pair; ntd. mandible;/ caudal fork.

CRUSTACEA.

465

has discovered that the larva leaves the winter eggs in the form of a
Nauplius (fig. 209). This Nauplius closely resembles that of the Phyllopods.
The body is elongated and in addition to normal Nauplius appendages
is marked by six pairs of ridges — the indications of the future feet. The
anterior antennae are as usual small ; the second large and biramous,
but the masticatory bristle characteristic of the Phyllopods is not present.
The mandibles are without a cutting blade. A large upper lip and unpaired
eye are present.

The adult form is attained in the same manner as amongst the Phyllo-
pods after the third moult.

MALACOSTRACA.

Owing to the size and importance of the various forms
included in the Malacostraca, greater attention has been paid to
their embryology than to that of any other division of the
Crustacea ; and the proper interpretation of their larval forms
involves some of the most interesting problems in the whole
range of Embryology.

The majority of Malacostraca pass through a more or less
complicated metamorphosis, though in the Nebaliadae, the
Cumaceae, some of the Schizopoda, a few Decapoda (Astacus,
Gecarcinus, etc.), and in the Edriophthalmata, the larva on
leaving the egg has nearly the form of the adult. In contradis-
tinction to the lower groups of Crustacea the Nauplius form of
larva is rare, though it occurs in the case of one of the Schizopods
(Euphausia, fig. 212), in some of the lower forms of the Decapods
(Penaeus, fig. 214), and
perhaps also, though this
has not been made out, in
some of the Stomatopoda.

In the majority of the
Decapoda the larva leaves
the egg in a form known
as the Zoaea (fig. 210).
This larval form is
characterised by the pre-
sence of a large cephalo-

thoracic t shield usually FIG. 210. ZO^EAOFTHIAPOLITA. (After'Claus.)
, ., , , , , mxp*. second maxillipede.

armed with lateral, an-
terior, and dorsal spines. The caudal segments are well de-

B. II. 30

466 SCHIZOPODA.

veloped, though wit/tout appendages, and the tail, which functions
in swimming, is usually forked. The six posterior thoracic seg-
ments are, on the other hand, rudimentary or non-existent. There
are seven anterior pairs of appendages shewn in detail in fig. 21 1,
viz. the two pairs of antennae (At. I. and At. II.), neither of them
used as swimming organs, the mandibles without a palp (ma7),
well-developed maxillae (two pairs, mx I and mx 2), and two or
sometimes (Macrura) three pairs of biramous natatory maxilli-
peds (mxp I and mxp 2). Two lateral compound stalked eyes
are present, together with a median Nauplius eye. The heart
has in the majority of cases only one or two (Brachyura) pairs of
ostia.

The Zoaea larva, though typically developed in the Decapoda,
is not always present (e.g. Astacus and Homarus), and some-

FIG. 211. THE APPENDAGES OF A CRAB Z<VEA.

.-//./. first antenna ; At. I I. second antenna ; md. mandible (without a palp); mx.
\. first maxilla; mx. i. second maxilla; mxp. \. first maxilliped ; mxp. i. second
maxilliped.

ex. exopodite ; en. endopodite.

times occurs in a very modified form. It makes its appearance
in an altered garb in the ontogeny of some of the other groups.

The two Malacostracan forms, amongst those so far studied,
in which the phylogenetic record is most fully preserved in the
ontogeny, are Euphausia amongst the Schizopods and Penaeus
amongst the Decapods.

Schizopoda. Euphausia leaves the egg (MetschnikofT, No. 468—9)
as a true Nauplius with only three pairs of appendages, the two hinder

CRUSTACEA.

467

biramous, and an unsegmented body. The second pair of antennae has not
however the colossal dimensions so common in the lower types. A mouth is
present, but the anus is undeveloped.

After the first moult three pairs of prominences — the rudiments of the
two maxillae and ist maxillipeds arise behind the Nauplius appendages
(fig. 212). At the same time an anus appears between the two limbs of
a rudimentary caudal fork ; and an unpaired eye and upper lip appear in
front. After another moult (fig. 212) a lower lip is formed (UL) as a
pair of prominences very similar to true appendages ; and a delicate
cephalo-thoracic shield also becomes developed. Still later the cutting blade
of the mandible is formed, and the palp (Nauplius appendage) is greatly

FIG. 212. NAUPLIUS OF EUPHAUSIA. (From Glaus; after Metschnikoff.)
The Nauplius is represented shortly before an ecdysis, and in addition to the

proper appendages rudiments of the three following pairs are present.

OL. upper lip ; UL. lower lip ; Md. mandible ; MX', and MX", two pairs of

maxillae ; mf . maxilliped i .

reduced. The cephalo-thoracic shield grows over the front part of the
embryo, and becomes characteristically toothed at its edge. There are also

30—2

468 SCHIZOPODA.

two frontal papillae very similar to those already described in the Phyllopod
larvae. Rudiments of the compound eyes make their appearance, and
though no new appendages are added, those already present undergo further
differentiations. They remain however very simple ; the maxillipeds
especially are very short and resemble somewhat Phyllopod appendages.

Up to this stage the tail has remained rudimentary and short, but
after a further ecdysis (Claus) it grows greatly in length. At the same
time the cephalo-thoracic shield acquires a short spine directed backwards.
The larva is now in a condition to which Claus has given the name of
Protozoasa (fig. 213 A).

Very shortly afterwards the region immediately following the segments
already formed becomes indistinctly segmented, while the tail is still with-
out a trace of segmentation. The region of the thorax proper soon be-
comes distinctly divided into seven very short segments, while at the same
time the now elongated caudal region has become divided into its normal
number of segments (fig. 213 B). By this stage the larva has become

FIG. 213. LARVAE OF EUPHAUSIA. (After Claus.) From the side.

A. Protozorea larva. B. Zonea larva.

mx'. and tux", maxillre I and 2 ; mxp^. maxilliped r.

a true Zoaea — though differing from the normal Zoaea in the fact that
the thoracic region is segmented, and in the absence of a second pair of
maxillipeds.

The adult characters are very gradually acquired in a series of suc-
cessive moults ; the later development of Euphausia resembling in this
respect that of the Phyllopods. On the other hand Euphausia differs from
that group in the fact that the abdominal (caudal) and thoracic appendages
develop as two independent series from before backwards, of which the
abdominal series is the earliest to attain maturity.

CRUSTACEA.

469

This is shewn in the following table compiled from Claus' observations.

LENGTH OF LARVA.

APPENDAGES OF THORACIC
REGION ; viz. the 2nd and
3rd maxilliped and 5 ambu
latory appendages.

APPENDAGES OF ABDOMEN.

3 — 3^ mm.

2nd maxilliped, rudimentary.

ist abdominal appendage.

3£— 4 mm.

•2nd maxilliped, biramous.
3rd rudimentary,
ist and 2nd ambulatory ap-
pendages, rudimentary.

2nd and 3rd abdominal ap-
pendages.
4th and 5th rudimentary.

4^ — 5 mm.

3rd maxilliped, biramous.

4th, 5th, and 6th fully de-
veloped.

5—5^ mm.

3rd and 4th ambulatory ap-
pendages.

6 mm.

5th ambulatory appendage.

All the appendages following the second pair of maxillas are biramous,
and the first eight of these bear branched gills as their epipodites. It is
remarkable that the epipodite is developed on all the appendages anteriorly
in point of time to the outer ramus (exopodite).

Although in Mysis there is no free larval stage, and the development
takes place in a maternal incubatory pouch, yet a stage may be detected
which clearly corresponds with the Nauplius stage of Euphausia (E. van
Beneden, No. 465). At this stage, in which only the three Nauplius
appendages are developed, the Mysis embryo is hatched. An ecdysis
takes place, but the Nauplius skin is not completely thrown off, and
remains as an envelope surrounding the larva during its later develop-
ment.

Decapoda. Amongst the Decapoda the larva usually leaves
the egg in the Zoaea form, but a remarkable exception to this
general rule is afforded by the case of one or more species of
Penseus. Fritz M tiller was the first to shew that the larva of
these forms leaves the egg as a typical Nauplius, and it is
probable that in the successive larval stages of these forms the
ancestral history of the Decapoda is most fully preserved1.

The youngest known larva of Penaeus (fig. 214) has a some-
what oval unsegmented body. There spring from it the three
typical pairs of Nauplius appendages. The first is uniramous,
the second and third are biramous, and both of them adapted

1 The doubts which have been thrown upon Miiller's observations appear to be
quite unfounded.

470 DECAPODA.

for swimming, and the third of them (mandibles) is without a
trace of the future blade. The body has no carapace, and bears
anteriorly a single median simple eye. Posteriorly it is produced
into two bristles.

After the first moult the larva has a rudiment of a forked
tail, while a dorsal fold of skin indicates the commencement of

FIG. 214. NAUPLIUS STAGE OF PEN^EUS. (After Fritz Miiller.)

the cephalo-thoracic shield. A large provisional helmet-shaped
upper lip like that in Phyllopods has also appeared. Behind
the appendages already formed there are stump-like rudiments
of the four succeeding pairs (two pairs of maxillae and two pairs
of maxillipeds) ; and in a slightly older larva the formation of
the mandibular blade has commenced, together with the atrophy
of the palp or Nauplius appendage.

Between this and the next observed stage there is possibly a
slight lacuna. The next stage (fig. 215) at any rate represents
the commencement of the Zoaea series. The cephalo-thoracic
shield has greatly grown, and eventually acquires the usual
dorsal spine. The posterior region of the body is prolonged
into a tail, which is quite as long as the whole of the remainder
of the body. The four appendages which were quite functionless
at the last stage have now sprouted into full activity. The

CRUSTACEA.

471

region immediately be-
hind them is divided (fig.
215) into six segments
(the six thoracic seg-
ments) without appen-
dages, while somewhat
later the five anterior
abdominal segments be-
come indicated, but are
equally with the thoracic
segments without feet.
The mode of appearance
of these segments shews
that the thoracic and
abdominal segments de-
velop in regular succes-
sion from before back-
wards (Claus). Of the
palp of the mandibles,
as is usual amongst Zosea
forms, not a trace remains,
though in the youngest
Zoaea caught by Fritz
Miiller a very small rudiment of the palp was present. The
first pair of antennae is unusually long, and the second pair
continues to function as a biramous swimming organ ; the
outer ramus is multiarticulate. The other appendages are fully
jointed, and the two maxillipeds biramous. On the dorsal
surface of the body the unpaired eye is still present, but on each
side of it traces of the stalked eyes have appeared. Frontal
sense organs like those of Phyllopods are also present.

From the Protozoaea form the larva passes into that of a true
Zoaea with the usual appendages and spines, characterised how-
ever by certain remarkable peculiarities. Of these the most
important are (i) the large size of the two pairs of antennae and
the retention of its Nauplius function by the second of them ;
(2) the fact that the appendages of the six thoracic segments
appear as small biramous Schizopod legs, while the abdominal
appendages, with the exception of the sixth, are still without

FIG.

215.

PROTOZO^EA STAGE OF PEN/EUS.
(After Fritz Miiller.)

472 DECAPODA.

their swimming feet. The early appearance of the appendages
of the sixth abdominal segment is probably correlated with
their natatory function in connection with the tail. As a point
of smaller importance which may be mentioned is the fact that
both pairs of maxillae are provided with small respiratory plates
(exopodites) for regulating the flow of water under the dorsal
shield. From the Zoaea form the larva passes into a Mysis or
Schizopod stage (fig. 216), characterised by the thoracic feet and
maxillipeds resembling in form and function the biramous feet
of Mysis, the outer ramus being at first in many cases much
larger than the inner. The gill pouches appear at the base of
these feet nearly at the same time as the endopodites become
functional. At the same time the antennae become profoundly
modified. The anterior antennae shed their long hairs, and from
the inner side of the fourth joint there springs a new process,

FIG. 216. PEN^EUS LARVA IN THE MYSIS STAGE. (After Claus.)

which eventually elongates and becomes the inner flagellum.
The outer ramus of the posterior antennae is reduced to a scale,
while the flagellum is developed from a stump-like rudiment of
the inner ramus (Claus). A palp sprouts on the mandible and
the median eye disappears.

The abdominal feet do not appear till the commencement of
the Mysis stage, and hardly become functional till its close.

From the Mysis stage the larva passes quite simply into the
adult form. The outer ramus of the thoracic feet is more or less
completely lost. The maxillipeds, or the two anterior pairs at
any rate, lose their ambulatory function, cutting plates develop
on the inner side of their basal joints, and the two rami persist

CRUSTACEA.

473

as small appendages on their outer side. Gill pouches also
sprout from their outer side.

The respiratory plate of the second maxilla attains its full
development and that on the first maxilla disappears1. The
Nauplius, so far as is known, does not occur in any other
Decapod form except Penaeus.

The next most primitive
larval history known is
that which appears in the
Sergestidae. The larval
history, which has been
fully elucidated by Claus,
commences with a Proto-
zoaea form (fig. 217), which
develops into a remarkable
Zoaea first described by
Dohrn as Elaphocaris.
This develops into a form
originally described by
Claus as Acanthosoma,
and this into a form known
as Mastigopus (fig. 218)
from which it is easy to
pass to the adult.

The remarkable Proto-
zoaea (fig. 217) is charac-
terised by the presence on
the dorsal shield of a fron-
tal, dorsal and two lateral
spikes, each richly armed
with long side spines. The

FIG. 217. LATEST PROTOZO^A STAGE OF SEK-

GESTES LARVA (ELAPHOCARIS). (After Claus.)

mxp'" '. third pair of maxillipeds.

normal Zoasa appendages are present, and in addition to them a small third
pair of maxillipeds. The thoracic region is divided into five short rings, but
the abdomen is unsegmented. The tail is forked and provided with long
spines. The antennae, like those of Penasus, are long — the second pair
biramous ; the mandibles unpalped. Both pairs of maxillae are provided
with respiratory plates ; the second pair is footlike, and has at its base a
glandular mass believed by Claus to be the equivalent of the Entomostracan
shell-gland. The maxillipeds have the usual biramous characters. A

1 From Claus' observations (No. 448) it would appear that the respiratory plate
is only the exopodite and not, as is usually stated, the coalesced exopodite and
epipodite. Huxley in his Comparative Anatomy reserves this point for embryological
elucidation.

474

DECAPOD A.

FIG. 218. MASTIGOPUS STAGE
OF SERGESTES. (From Claus.)
Mf". maxilliped 3.

helmet-shaped upper lip like
that of a typical Nauplius is
present, and the eyes are situa-
ted on very long stalks.

In the true Zoaea stage there
appear on the five thoracic

segments pouch-like biramous rudiments of the limbs. The
tail becomes segmented; but the segments, with the
exception of the sixth, remain without appendages. On
the sixth a very long bilobed pouch appears as the com-
mencement of the swimming feet of this segment. The
segments of the abdomen are armed with lateral spines.

From the Zoaea stage the larva passes into the form
known as Acanthosoma, which represents the Mysis stage
of Penaeus. The complex spikes on the dorsal shield of
the Zoaea stage are reduced to simple spines, but the
spines of the tail still retain their full size. In the appendages the chief
changes consist (i) in the reduction of the jointed outer ramus of the
second pair of antennae to a stump representing the scale, and the elon-
gation of the inner one to the flagellum ; (2) in the elongation of the five
ambulatory thoracic appendages into biramous feet, like the maxillipeds,
and in the sprouting forth of rudimentary abdominal feet.

CRUSTACEA.

475

The most obvious external indica-
tions of the passage from the Acantho-
soma to the Mastigopus stage (fig. 218)
are to be found in the elongation of the
abdomen, the reduction and flattening
of the cephalo-thoracic shield, and the
nearly complete obliteration of all the
spines but the anterior. The eyes on
their elongated stalks are still very
characteristic, and the elongation of
the flagellum of the second pair of
antennae is very striking.

The maxillae and maxillipeds un-
dergo considerable metamorphosis, the
abdominal feet attain their adult form,
and the three anterior thoracic ambu-
latory legs lose their outer rami. The
most remarkable change of all concerns
the two last pairs of thoracic appen-
dages, which, instead of being meta-
morphosed like the preceding ones, are
completely or nearly completely thrown
off in the moult which inaugurates the
Mastigopus stage, and are subsequently
redeveloped. With the reappearance
of these appendages, and the changes
in the other appendages already indi-
cated, the adult form is practically
attained.

FIG. 219. LARVA OF HIPPOLYTE
IN ZO/EA STAGE. (From Claus.)

MX', and MX", maxillae i and 2 ;
Mf. Mf. Mf". maxillipeds.

FIG. 220.

OLDER LARVA OF HIPPOLYTE AFTER THE THORACIC APPENDAGES HAVE
BECOME FORMED. (From Claus.)

476 DECAPODA.

With reference to the development of the majority of the
Carabidae, Penaeinae, Palaemoninae, Crangoninae, it may be stated
generally that they leave the egg in the Zoaea stage (fig. 219)
with anterior appendages up to the third pair of maxillipeds.
The thorax is unsegmented and indeed almost unrepresented,
but the abdomen is long and divided into distinct segments.
Both thoracic and abdominal appendages are absent, and the
tail is formed by a simple plate with numerous bristles, not
forked, as in the case of the Zoaea of Fritz M tiller's Penaeus and
Sergestes. A dorsal spine is frequently found on the second
abdominal segment. From the Zoaea form the embryo passes
into a Mysis stage (fig. 220), during which the thoracic ap-
pendages gradually appear as biramous swimming feet; they

FIG. 221. NEWLY-HATCHED LARVA OF THE AMERICAN LOBSTER. (After Smith.)
are all developed before any of the abdominal appendages,
except the last. In some cases the development is still further
abbreviated. Thus the larvae of Crangon and Palaemonetes
(Faxon, No. 476) possess at hatching the rudiments of the two
anterior pairs of thoracic feet, and Palaemon of three pairs'.

Amongst the other Macrura the larva generally leaves the
egg as a Zoaea similar to that of the prawns. In the case of the

1 Fritz Miiller has recently (Zoologisrher Anzeiger^ No. 52) described a still more
abbreviated development of a Pala-mon living in brooks near Blumenau.

CRUSTACEA. 477

Thalassinidae and Paguridae a Mysis stage has disappeared.
The most remarkable abbreviations of the typical development
are presented on the one hand by Homarus and Astacus, and on
the other by the Loricata.

The development of Homarus has been fully worked out by S. J. Smith
(No. 491) for the American lobster (Homarus americanus). The larva (fig.
221) leaves the egg in an advanced Mysis stage. The cephalo-thoracic
shield is fully developed, and armed with a rostrum in front. The first pair
of antennae is unjointed but the second is biramous, the outer ramus forming
a large Mysis-like scale. The mandibles, which are palped, the maxillae,
and the two anterior maxillipeds differ only in minor details from the same
appendages of the adult. The third pair of maxillipeds is Mysis-like and
biramous, and the five ambulatory legs closely resemble them, the endopo-
dite of the first being imperfectly chelate. The abdomen is well developed
but without appendages. The second, third, fourth and fifth segments are
armed with dorsal and lateral spines.

In the next stage swimming feet have appeared on the second, third,
fourth and fifth abdominal segments, and the appendages already present
have approached their adult form. Still later, when the larva is about half
an inch in length, the approach to the adult form is more marked, and the
exopodites of the ambulatory legs though present are relatively much
reduced in size. The swimmerets of the sixth abdominal segment are
formed. In the next stage observed the larva has entirely lost its Schizopod
characters, and though still retaining its free swimming habits differs from
the adult form only in generic characters.

As has been already stated, no free larval stages occur in the develop-
ment of Astacus, but the young is hatched in a form in which it differs only
in unimportant details from the adult.

The peculiar larval form of the Loricata (Scyllarus, Palinurus) has long
been known under the name Phyllosoma (fig. 222 C), but its true nature was
first shewn by Couch (No. 474) [Couch did not however recognise the
identity of his larva with Phyllosoma ; this was first done by Gerstacker]
and shortly afterwards by Gerbe and Coste. These observations were
however for a long time not generally accepted, till Dohrn (No. 477)
published his valuable memoir giving an account of how he succeeded in
actually rearing Phyllosoma from the eggs of Scyllarus and Palinurus, and
shewing that some of the most remarkable features of the metamorphosis of
the Loricata occur before the larva is hatched.

The embryo of Scyllarus in the egg first of all passes through the usual
Nauplius stage, and then after the formation of a cuticle develops an
elongated thoracico-abdominal region bent completely over the anterior
part of the body. There appear moreover a number of appendages and the
rudiments of various organs ; and the embryo passes into a form which may
be described as the embryonic Phyllosoma stage. In this stage there are
present on the anterior part of the body, in front of the ventral flexure, two

478 DECAPODA.

pairs of antennae, mandibles, two pairs of maxillae, the second commencing
to be biramous, and a small stump representing the first pair of maxillipeds.
The part of the body bent over consists of a small quadrate caudal plate,
and an appendage-bearing region to which are attached anteriorly three
pairs of biramous appendages— the second and third maxillipeds, and the
anterior pair of ambulatory legs — and two pairs of undivided appendages —
the second and third pairs of ambulatory legs. In a slightly later stage the
first pair of maxillae becomes biramous, as also does the first pair of maxilli-
peds in a very rudimentary fashion. The second and third pairs of ambu-
latory legs become biramous, while the second and third maxilliped nearly
completely lose their outer ramus. Very small rudiments of the two hinder
ambulatory legs become formed. If the embryo is taken at this stage (vide
fig. 222 A, which represents a nearly similar larva of Palinurus) out of the
egg, it is seen to consist of (i) an anterior enlargement with a vaulted dorsal
shield enclosing the yolk, two stalked eyes, and a median eye ; (2) a thoracic
region in which the indications of segmentation are visible with the two

FIG. 222. LARWE OF THE LORICATA. (After Claus.)

A. Embryo of Palinurus shortly before hatching.

B. Young Phyllosoma larva of Scyllarus, without the first maxilliped, the two
last thoracic appendages, or the abdominal appendages.

C. Fully-grown Phyllosoma with all the Decapod appendages.

at*, antenna of first pair ; at*, antenna of second pair ; md. mandible ; ntx1. first
maxilla; mx1. second maxilla; mx^—mxf. maxillipeds; /1— /3. thoracic
appendages.

posterior pairs of maxillipeds (mxfp and wr/3) and the ambulatory legs (/l);
(3) an abdominal region distinctly divided into segments and ending in a fork.
Before the embryo becomes hatched the first pair of maxillipeds becomes
reduced in size and finally vanishes. The second pair of maxillae becomes
reduced to simple stumps with a few bristles, the second pair of antennae

CRUSTACEA. 479

also appears to undergo a retrogressive change, while the two last thoracic
segments cease to be distinguishable. It thus appears that during embryonic
life the second pair of antennae, the second pair of maxillae, and the second
and third pair of maxillipeds and the two hinder ambulatory appendages
undergo retrogressive changes, while the first pair of maxillipeds is completely
obliterated !

The general form of the larva when hatched (fig. 222 B) is not very
different from that which it has during the later stages within the egg. The
body is divided into three regions: (i) an anterior cephalic; (2) a middle
thoracic, and (3) a small posterior abdominal portion ; and all of them are
characterised by their extreme dorso-ventral compression, so that the whole
animal has the form of a three-lobed disc, the strange appearance of which
is much increased by its glass-like transparency.

The cephalic portion is oval and projects slightly behind so as to overlap
the thorax. Its upper surface constitutes the dorsal shield, from which there
spring anteriorly the two compound eyes on long stalks, between which is a
median Nauplius eye. The mouth is situated about the middle of the under
surface of the anterior disc. It leads into a stomach from which an anterior
and a lateral hepatic diverticulum springs out on each side. The former
remains as a simple diverticulum through larval life, but the latter becomes
an extremely complicated glandular structure.

At the front border of the disc is placed the unjointed but elongated
first pair of antennae (rt/1). Externally to and behind these there spring the
short posterior antennae (at'*}. At the base of which the green gland is
already formed. Surrounding the mouth are the mandibles (md) and anterior
pair of maxillae (mx1), and some distance behind the second pair of maxillae
(mx*), consisting of a cylindrical basal joint and short terminal joint armed
with bristles. The first pair of maxillipeds is absent.

The thoracic region is formed of an oval segmented disc attached to the
under surface of the cephalic disc. From its front segment arises the second
pair of maxillipeds (inxpl£} as single five-jointed appendages, and from the
next segment springs the five-jointed elongated but uniramous third pair
of maxillipeds (mxfl3}, and behind this there arise three pairs of six-jointed
ambulatory appendages (p\ /2, p3, of which only the basal joint is represented
in the figure) with an exopodite springing from their second joint. The two
posterior thoracic rings and their appendages cannot be made out.

The abdomen is reduced to a short imperfectly segmented stump, ending
in a fork, between the prongs of which the anus opens. Even the youngest
larval Phyllosoma, such as has just been described, cannot be compared with a
Zoaea, but belongs rather, in the possession of biramous thoracic feet, to a
Mysis stage. In the forked tail and Nauplius eye there appear however to
be certain very primitive characters carried on to this stage.

The passage of this young larva to the fully formed Phyllosoma (fig.
222 C) is very simple. It consists essentially in the fresh development of
the first pair of maxillipeds and the two last ambulatory appendages, the
growth and segmentation of the abdomen, and the sprouting on it of biramous

480 DECAPODA.

swimming feet. In the course of these changes the larva becomes a true
Decapod in the arrangement and number of its appendages ; and indeed it
was united with this group before its larval character was made out. In
addition to the appearance of new appendages certain changes take place in
those already present. The two posterior maxillipeds, in the Palinurus
Phyllosoma at any rate, acquire again an exopodite, and together with the
biramous ambulatory feet develop epipodites in the form of gill pouches.

The mode of passage of the Phyllosoma to the adult is not known, but
it can easily be seen from the oldest Phyllosoma forms that the dorsal
cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic
shield of the adult.

There are slight structural differences, especially in the antennae, between
the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference
in development is that the first pair of maxillipeds of the Palinurus embryo,
though reduced in the embryonic state, does not completely vanish, at any
rate till after the free larval state has commenced ; and it is doubtful if
it does so even then. The freshly hatched Palinurus Phyllosoma is very
considerably more developed than that of Scyllarus.

Brachyura. All the Brachyura, with the exception of one or
more species of land crabs1, leave the egg in the Zoaia condition,
and though there are slight variations of structure, yet on the

FIG. 223. THE APPENDAGES OF A CRAB ZOJEA.

At. I. first antenna ; At. //. second antenna ; md. mandible (without a palp) ; mx.
i. first maxilla ; mx. i. second maxilla ; w.r. 3. third maxilla ; mxp. i. first maxilliped ;
mxp. i. second maxilliped.

ex. exopodite ; ett. cndopodite.

whole the Crab Zoaea is a very well marked form. Immediately
after leaving the egg (fig. 210) it has a somewhat oval shape

1 It has been clearly demonstrated that the majority of land-crabs leave the egg in
the 7.oxa. form.

CRUSTACEA. 481

with a long distinctly-segmented abdomen bent underneath the
thorax. The cephalo-thoracic shield covers over the front part
of the body, and is prolonged into a long frontal spine pointing
forwards, and springing from the region between the two eyes ;
a long dorsal spine pointing backwards ; and two lateral spines.

To the under surface of the body are attached the anterior
appendages up to the second maxilliped, while the six following
pairs of thoracic appendages are either absent or represented
only in a very rudimentary form. The abdomen is without
appendages.

The anterior antennae are single and unjointed, but provided
at their extremity with a few olfactory hairs (only two in
Carcinus Mcenas) and one or two bristles. The rudiment of the
secondary flageltum appears in very young Zoaeae on the inner
side of the antennules (fig. 223 At. /.). The posterior antennae
are without the flagellum, but are provided with a scale repre-
senting the exopodite (fig. 223 At. II. ex] and usually a spinous

FIG. 224. CRAB ZO^EA AFTER TH.. THIRD PAIR OF MAXILLIPEDS AND THE

THORACIC AND ABDOMINAL APPENDAGES HAVE BECOME DEVELOPED.

at1, antenna of first pair ; atz. antenna of second pair ; mxl. first maxilla ; mop.
second maxilla ; mxp1. first maxilliped ; mxjP. second maxilliped ; mxf. third max-
illiped ; oc. eye ; ht. heart.

process. The flagellum is very early developed and is repre-
sented in fig. 223, At. II. en. The mandibles (md) are large but
without a palp. The anterior maxillae (mx i) have a short two-
jointed endopodite (palp) with a few hairs, and a basal portion
B. II. 31

--•

482 DECAPODA.

with two blades, of which the distal is the largest, both armed
with stiff bristles. The posterior maxillae have a small respira-
tory plate (exopodite), an endopodite (palp) shaped like a
double blade, and two basal joints each continued into a double
blade. The two maxillipeds (inxp i and mxp 2) have the form
and function of biramous swimming feet. The exopodite of
both is two-jointed and bears long bristles at its extremity ; the
endopodite of the anterior is five-jointed and long, that of the
second is three-jointed and comparatively short.

In the six-jointed tail the second segment has usually two
dorsally directed spines, and the three succeeding segments each
of them two posteriorly directed. The telson or swimming plate
is not at first separated from the sixth segment ; on each side it
is prolonged into two well-marked prongs ; and to each prong
three bristles are usually attached (fig. 224). The heart (fig.
224 ht) lies under the dorsal spine and is prolonged into an
anterior, posterior, and dorsal aorta. It has only two pairs of
venous ostia.

During the Zoaea stage the larva rapidly grows in size, and
undergoes considerable changes in its appendages which reach
the full Decapod number (fig. 224). On both pairs of antennae
a flagellum becomes developed and grows considerably in length.
Before the close of the Zoaea condition a small and unjointed
palp appears on the mandible. Behind the second maxilliped
the third maxilliped (inxp*} early appears as a small biramous
appendage, and the five ambulatory feet become distinctly
formed as uniramous appendages — the exopodites not being
present. The third pair of maxillipeds and three following
ambulatory appendages develop gill pouches. The abdominal
feet are formed on the second to the sixth segments of the tail
as simple pouches.

The oldest Zoaea is transmuted at its moult into a form
known as Megalopa, which is really almost identical with an
anomurous Decapod. No Schizopod stage is intercalated, which
shews that the development is in many respects greatly abbrevi-
ated. The essential characters of the Megalopa are to be found
in (i) the reduction of the two anterior maxillipeds, which
cease to function as swimming feet, and together with the
appendages in front of them assume the adult form ; (2) the full

CRUSTACEA. 483

functional development of the five ambulatory appendages ;
(3) the reduction of the forked telson to an oval swimming
plate, and the growth in size of the abdominal feet, which
become large swimming plates and are at the same time
provided with short endopodites which serve to lock the feet of
the two sides.

With these essential characters the form of the Megalopa differs con-
siderably in different cases. In some instances (e.g. Carcinus mcenas) the
Zoasa spines of the youngest Megalopa are so large that the larva appears
almost more like a Zoasa than a Megalopa (Spence Bate, No. 470). In other
cases, e.g. that represented on fig. 225, the Zoasa spines are still present but
much reduced; and the cephalo-thoracic shield has very much the adult
form. In other cases again (e.g. Portunus) the Zoasa spines are completely
thrown off at the youngest Megalopa stage.

There is a gradual passage from the youngest Megalopa to
the adult form by a series of moults.

Some of the brachyu-
rous Zoasa forms exhibit
considerable divergences
from the described type,
more espcially in the ar-
mature of the shield. In
some forms the spines are
altogether absent, e.g. Maja
(Couch, No. 474) and Eu-
rynome. In other forms
the frontal spine may be
much reduced or absent
(Inachus and Achasus).
The dorsal spine may also
be absent, and in one form
described by Dohrn (No.
478) there is a long frontal
spine and two pairs of
lateral spines, but no dorsal ^ MEGALOPA STAGE OF CRAB LARVA.

spine. Both dorsal and

frontal spines may attain enormous dimensions and be swollen at their extre-
mities (Dohrn). A form has been described by Claus as Pterocaris in which
the cephalo-thoracic shield is laterally expanded into two wing-like processes.

The Zoasa of Porcellana presents on the whole the most remarkable
peculiarities and, as might be anticipated from the systematic position of the
adult, is in some respects intermediate between the macrurous Zoasa and that
of the Brachyura. It is characterized by the oval form of the body, and by

31-2

484 STOMATOPODA.

the presence of one enormously long frontal spine and two posterior spines.
The usual dorsal spine is absent. The tail plate is rounded and has the
character of the tail of a macrurous Zoaea, but in the young Zoasa the third
pair of maxillipeds is absent and the appendages generally have a brachyu-
rous character. A Megalopa stage is hardly represented, since the adult
may almost be regarded as a permanent Megalopa.

Stomatopoda. The history of the larval forms of the Stomatopoda
(Squilla etc.) has not unfortunately been thoroughly worked out, but what is
known from the researches of Fritz Miiller (No. 495) and Claus (No. 494) is
of very great importance. There are it appears two types, both of which
used to be described as adult forms under the respective names Erichthus
and Alima.

The youngest known Erichthus form is about two millimetres in length,
and has the characters of a modified Zoaea (fig. 226). The body is divided
into three regions, an anterior unsegmented region to which are attached
two pairs of antennas, mandibles, and maxillae (two pairs). This portion
has a dorsal shield covering the next or middle region, which consists of
five segments each with a pair of biramous appendages. These appendages
represent the five maxillipeds of the adult1. The portion of the body
behind this is without appendages. It consists of three short anterior
segments,— the three posterior thoracic segments of the adult, — and a long
unsegmented tail. The three footless thoracic segments are covered by the
dorsal shield. Both pairs of antennae are uniramous and comparatively
short. The mandibles, like those of Phyllopods, are without palps, and the
two following pairs of maxillae are small. The five maxillipeds have the
characters of normal biramous Zoaea feet. From the front of the head
spring a pair of compound eyes with short stalks, which grow longer
in the succeeding stages ; between them is a median eye. The dorsal
shield is attached just behind this eye, and is provided, as in the typical
Zoaea, with a frontal spike — while its hinder border is produced into two
lateral spikes and one median. In a larva of about three millimetres a pair
of biramous appendages arises behind the three footless thoracic segments.
It is the anterior pair of ab-
dominal feet (fig. 226). The
inner ramus of the second pair
of maxillipeds soon grows
greatly in length, indicating
its subsequent larger size and
prehensile form (fig. 227 g).
When the larva after one or

two moults attains a length FlG- ™6- SECOND STAGE OF ERICHTHUS
of six millimetres Cfitr 227 1 LARVA OFSQUII.LA WITH FIVE MAXILLIPEDS AND

(tig. 227) THE FIRST PAIR OF ABDOMINAL APPENDAGES.

the abdomen has six segments (From Claus.)

1 These five maxillipeds correspond with the three maxillipeds and two anterior
ambulatory appendages of the Decapoda.

CRUSTACEA.

485

(the sixth hardly differentiated), each with a pair of appendages (the two
hindermost still rudimentary) which have become gradually developed from
before backwards. The three hindermost thoracic segments are still without
appendages.

Some changes of importance have occurred in the other parts. Both
antennas have acquired a second flagellum, but the mandible is still without

FIG. 227. ADVANCED ERICHTHUS LARVA OF SQUILLA WITH FIVE PAIRS OF

ABDOMINAL APPENDAGES. (From Claus.)

f. first maxilliped ; g. second maxilliped.

a palp. The first and second pair of maxillipeds have both undergone
important modifications. Their outer ramus (exopodite) has been thrown
off, and a gill-plate (epipodite) has appeared as an outgrowth from their
basal joint. Each of them is composed of six joints. The three following
biramous appendages have retained their earlier characters but have become
much reduced in size. In the subsequent moults the most remarkable new
features concern the three posterior maxillipeds, which undergo atrophy, and
are either completely lost or reduced to mere unjointed sacks (fig. 228). In

FIG. 228. ADVANCED ERICHTHUS LARVA OF SQUILLA WHEN THE THREE
POSTERIOR MAXILLIPEDS HAVE BECOME REDUCED TO MINUTE POUCHES.

(From Claus.)

the stage where the complete Erichthus type has been reached, these
three appendages have again sprouted forth in their permanent form and
each of them is provided with a gill-sack on its coxal joint. Behind them
the three ambulatory appendages of the thorax have also appeared, first
as simple buds, which subsequently however become biramous. On their
development the full number of adult appendages is acquired.

The most noteworthy points in the developmental history detailed above
are the following :

(i) The thoracic and abdominal segments (apart from their appendages)
develop successively from before backwards.

486 STOMATOPODA.

(2) The three last maxillipeds develop before the abdominal feet, as
biramous appendages, but subsequently completely atrophy, and then sprout
out again in their permanent form.

(3) The abdominal feet develop in succession from before backwards,
and the whole series of them is fully formed before a trace of the appendages
of the three hindermost thoracic segments has appeared. It may be
mentioned as a point of some importance that the Zoaea of Squilla has
an elongated many-chambered heart, and not the short compact heart
usually found in the Zoaea.

The younger stages of the Alima larva are not known1, but the earliest
stage observed is remarkable for presenting no trace of the three posterior
pairs of maxillipeds, or of the three following pairs of thoracic appendages.
The segments belonging to these appendages are however well developed.
The tail has its full complement of segments with the normal number of
well developed swimming feet. The larva represents in fact the stage of
the Erichthus larva when the three posterior pairs of maxillipeds have
undergone atrophy ; but it is probable that these appendages never become
developed in this form of larva.

Apart from the above peculiarities the Alima form of larva closely
resembles the Erichthus form.

Nebaliadae. The development of Nebalia is abbreviated, but from
MetschnikofFs figures2 may be seen to resemble closely that of Mysis.
The abdomen has comparatively little yolk, and is bent over the ventral
surface of the thorax. There is in the egg a Nauplius stage with three
appendages, and subsequently a stage with the Zoaea appendages.

The larva when it leaves the egg has the majority of its appendages
formed, but is still enveloped in a larval skin, and like Mysis bends its
abdomen towards the dorsal side. When the larva is finally hatched it does
not differ greatly from the adult.

Cum ace ae. The development of the Cumaceae takes place for the
most part within the egg, and has been shewn by Dohrn (No. 496) to
resemble in many points that of the Isopods. A dorsal organ is present,
and a fold is formed immediately behind this which gives to the embryo a
dorsal flexure. Both of these features are eminently characteristic of the
Isopoda.

The formation of the two pairs of antennie, mandibles, and two pairs
of maxillae and the following seven pairs of appendages takes place very
early. The pair of appendages behind the second maxilku assumes an
ambulatory form, and exhibits a Schizopod character very early, differing
in both these respects from the homologous appendages in the Isopoda.
The cephalo-thoracic shield commences to be formed when the appendages
are still quite rudimentary as a pair of folds in the maxillary region. The

1 The observations of Brooks (No. 493) render it probable that the Alima larva
leaves the egg in a form not very dissimilar to the youngest known larva.
3 His paper is unfortunately in Russian.

CRUSTACEA. 487

eyes are formed slightly later on each side of the head, and only coalesce at
a subsequent period to form the peculiar median sessile eye of the adult.

The two pairs of appendages behind the second maxillae become con-
verted into maxillipeds, and the exopodite of the first of them becomes the
main ramus, while in the externally similar second maxilliped the exopodite
atrophies and the endopodite alone remains.

The larva is hatched without the last pair of thoracic limbs or the
abdominal appendages (which are never developed in the female), but in
other respects closely resembles the adult. Before hatching the dorsal
flexure is exchanged for a ventral one, and the larva acquires a character
more like that of a Decapod.

COPEPODA.

Natantia. The free Copepoda are undoubtedly amongst
the lowest forms of those Crustacea which are free or do not
lead a parasitic existence. Although some features of their
anatomy, such for instance as the frequent absence of a heart,
may be put down to a retrogressive development, yet, from their
retention of the median frontal eye of the Nauplius as the sole
organ of vision1, their simple biramous swimming legs, and other
characters, they may claim to be very primitive forms, which
have diverged to no great extent from the main line of Crus-
tacean development. They supply a long series of transitional
steps from the Nauplius stage to the adult condition.

While still within the egg-shell the embryo is divided by two
transverse constrictions into three segments, on which the three
Nauplius appendages are developed, viz. the two pairs of
antennae and the mandibles. When the embryo is hatched the
indication of a division into segments has vanished, but the
larva is in the fullest sense a typical Nauplius2. There are
slight variations in the shape of the Nauplius in different genera,
but its general form and character are very constant. It has
(fig. 229 A) an oval unsegmented body with three pairs of
appendages springing from the ventral surface. The anterior of
these (at i) is uniramous, and usually formed of three joints
which bear bristles on their under surface. The two posterior

1 The Pontellidse form an exception to this statement, in that they are provided
with paired lateral eyes in addition to the median one.

2 The term Nauplius was applied to the larva of Cyclops and allied organisms by
O. F. Muller under the impression that they were adult forms.

488

COPEPODA.

pairs of appendages are both biramous. The second pair of
antennae (at 2) is the largest. Its basal portion (protopodite)
bears on its inner side a powerful hook-like bristle. The outer
ramus is the longest and many-jointed ; the inner ramus has
only two joints. The mandibles (md), though smaller than the
second pair of antennae, have a nearly identical structure. No
blade-like projection is as yet developed on their protopodite.
Between the points of insertion of the first pair of antennae is
the median eye (oc), which originates by the coalescence of two
distinct parts. The mouth is ventral, and placed in the middle
line between the second pair of antennae and the mandibles : it

FIG. 229. SUCCESSIVE STAGES IN THE DEVELOPMENT OF CYCLOPS TENUICORMS.

(Copied from Bronn ; after Claus.)

A. B. and C. Nauplius stages. D. Youngest Copepod stage. In this figure maxillae
and the two rami of the maxilliped are seen immediately behind the mandible md.

oc. eye ; at1, first pair of antennae ; a/8, second pair of antennre ; md. mandible ;
/*. first pair of feet ; /2. second pair of feet ; f. third pair of feet ; //. excretory con-
cretions in the intestine.

is provided with an unpaired upper lip. There are two bristles
at the hind end of the embryo between which the anus is placed,
and in some cases there is at this part a slight indication of the
future caudal fork.

The larva undergoes a number of successive ecdyses, at each
of which the body becomes more elongated, and certain other

CRUSTACEA. 489

changes take place. First of all a pair of appendages arises
behind the mandibles, which form the maxillae (fig. 229 B) ; at
the same time the basal joint of the maxillae develops a cutting-
blade. Three successive pairs of appendages (fig. 229 C) next
become formed — the so-called maxillipeds (the homologues of
the second pair of maxillae), and the two first thoracic limbs.
Each of these though very rudimentary is nevertheless bifid.
The body becomes greatly elongated, and the caudal fork more
developed.

Up to this stage of development the Nauplius appendages
have retained their primitive character almost unaltered ; but
after a few more ecdyses a sudden change takes place ; a cephalo-
thoracic shield becomes fully developed, and the larva comes to
resemble in character an adult Copepod, from which it mainly
differs in the smaller number of segments and appendages. In
the earliest 'Cyclops' stage the same number of appendages are
present as in the last Nauplius stage. There (fig. 229 D) is a
well developed cephalo-thorax, and four free segments behind it.
To the cephalo-thoracic region the antennae, mandibles, maxillae,
the now double pair of maxillipeds (derived from the original
single pair of appendages), and first pair of thoracic appendages
(pl) are attached. The second pair of thoracic appendages (/2)
is fixed to the first free segment, and the rudiment of a third
pair (/3) projects from the second free segment. The first pair
of antennae has grown longer by the addition of new joints, and
continues to increase in length in the following ecdyses till it
attains its full adult development, and then forms the chief
organ of locomotion. The second pair of antennae is much
reduced and has lost one of its rami. The two rami of the
mandibles are reduced to a simple palp, while the blade has
assumed its full importance. The maxillae and following appen-
dages have greatly increased in size. They are all biramous,
though the two rami are not as yet jointed. The adult state is
gradually attained after a number of successive ecdyses, at
which new segments and appendages are added, while new
joints are formed for those already present.

Parasita. The earliest developmental stages of the parasitic types
of Copepoda closely resemble those of the free forms, but, as might be
expected from the peculiarly modified forms of the adult, they present a

490

COPEPODA.

large number of secondary characters. So far as is known a more or less
modified Nauplius larva is usually preserved.

The development of Achtheres percarum, one of the Lernaeopoda parasitic
in the mouth, etc. of the common Perch, may be selected to illustrate the
mode of development of these forms. The larva leaves the egg as a much
simplified Nauplius (fig. 230 A). It has an oval body with only the two
anterior pairs of Nauplius appendages ; both of them in the rudimentary
condition of unjointed rods. The usual median eye is present, and there is
also found a peculiar sternal papilla, on which opens a spiral canal filled
with a glutinous material, which is probably derived from a gland which
disappears on the completion of the duct. The probable function of this

FIG. 330. SUCCESSIVE STAGES IN THE DEVELOPMENT OF ACHTHERES PERCARUM.
(Copied from Bronn ; after Claus. )

A. Modified Nauplius stage. B. Cyclops stage. C. Late stage of male
embryo. D. Sexually mature female. E. Sexually mature male.

at1, first pair of antennae; at3, second pair of antennae; tnd. mandible; tnx.
maxillae ; ptn1. outer pair of maxillipeds ; ftn^. inner pair of maxillipeds ; J>1. first pair
of legs ; /*. second pair of legs ; z. frontal organ ; i. intestine ; o. larval eye ; b.
glandular body ; t. organ of touch ; ov. ovary ; /. rod projecting from coalesced maxil-
lipeds ; g. cement gland ; rs. receptaculurn seminis ; n. nervous system ; te. testis ; v.
vas deferens.

organ is to assist at a later period in the attachment of the parasite to its
host. Underneath the Nauplius skin a number of appendages are visible,
which become functional after the first ecdysis. This takes place within a
few hours after the hatching of the Nauplius, and the larva then passes from

CRUSTACEA. 491

this rudimentary Nauplius stage into a stage corresponding with the Cyclops
stage of the free forms (fig. 230 B). In the Cyclops stage the larva has an
elongated body with a large cephalo-thoracic shield, and four free posterior
segments, the last of which bears a forked tail.

There are now present eight pairs of appendages, viz. antennae (two
pairs), mandibles, maxillae, maxillipeds, and three pairs of swimming feet.
The Nauplius appendages are greatly modified. The first pair of antennae is
three-jointed, and the second biramous. The outer ramus is the longest, and
bears a claw-like bristle at its extremity. This pair of appendages is used
by the larva for fixing itself. The mandibles are small and connected with
the proboscidiform mouth ; and the single pair of maxillae is small and palped.
The maxillipeds (pm* and flm2) are believed by Claus to be primitively
a single biramous appendage, but early appear as two distinct structures1,
the outer and larger of which becomes the main organ by which the larva is
fixed. Both are at this stage simple two-jointed appendages. The two
anterior pairs of swimming feet have the typical structure, and consist of a
protopodite bearing an unjointed exopodite and endopodite. The first pair
is attached to the cephalo-thorax and the second (p*} to the first free thoracic
segment. The third pair is very small and attached to the second free
segment. The mouth is situated at the end of a kind of proboscis formed
by prolongations of the upper and lower lips. The alimentary tract is fairly
simple, and the anus opens between the caudal forks.

Between this and the next known stage it is quite possible that one
or more may intervene. However this may be the larva in the next stage
observed (fig. 230 C) has already become parasitic in the mouth of the Perch,
and has acquired an elongated vermiform aspect. The body is divided into
two sections, an anterior unsegmented, and a posterior formed of five
segments, of which the foremost is the first thoracic segment which in the
earlier stage was fused with the cephalo-thorax. The tail bears a rudimen-
tary fork between the prongs of which the anus opens. The swimming feet
have disappeared, so also has the eye and the spiral duct of the embryonic
frontal organ. The outer of the two divisions of the maxilliped have under-
gone the most important modification, in that they have become united at
their ends, where they form an organ from which an elongated rod (_/)
projects, and attaches the larva to the mouth or gills of its host. The
antennae and jaws have nearly acquired their adult form. The nervous
system consists of supra- and infra-cesophageal ganglia and two lateral
trunks given off from the latter. At this stage the males and females can
already be distinguished, not only by certain differences in the rudimen-
tary generative organs, but also by the fact that the outer branch of the
maxillipeds is much longer in the female than in the male, and projects
beyond the head.

In the next ecdysis the adult condition is reached. The outer maxilli-

1 Van Beneden (No. 506) in the genera investigated by him finds that the two
maxillipeds are really distinct pairs of appendages.

492 CIRRIPEDIA.

peds of the male (fig. 230 £,/>#*) separate again ; while in the female (fig.
230 D) they remain fused and develop a sucker. The male is only about
one-fifth the length of the female. In both sexes the abdomen is much
reduced.

In the genera Anchorella, Lernaeopoda, Brachiella and Hessia, Ed. van
BenecUn (No. 506) has shewn that the embryo, although it passes through
a crypto-Nauplius stage in the egg, is when hatched already in the Cyclops
stage.

Branchiura. The peculiar parasite Argulus, the affinities of which
with the Copepoda have been demonstrated by Claus (No. 511), is hatched
in a Cyclops stage, and has no Nauplius stage. At the time of hatching it
closely resembles the adult in general form. Its appendages are however
very nearly those of a typical larval Copepod. The body is composed of
a cephalo-thorax and free region behind this. The cephalo-thorax bears
on its under surface antennae (two pairs), mandibles, maxillipeds, and the
first pair of thoracic feet.

The first pair of antennae is three-jointed, but the basal joint bears a
hook. The second pair is biramous, the inner ramus terminating in a hook.
The mandible is palped, but the palp is completely separated from the
cutting blade1. The maxilla would, according to Claus, appear to be
absent.

The two typical divisions of the Copepod maxillipeds are present, viz. an
outer and anterior larger division, and an inner and posterior smaller one.
The first pair of thoracic feet, as is usual amongst Copepoda, is attached
to the cephalo-thorax. It has not the typical biramous Copepod character.
There are four free segments behind the cephalo-thorax, the last of which
ends in a fork. Three of them bear appendages, which are rudimentary in
this early larval stage. On the dorsal surface are present paired eyes as
well as an unpaired median eye.

Between the larval condition and that of the adult a number of ecdyses
intervene.

CIRRIPEDIA.

The larvae of all the Cirripedia, with one or two exceptions,
leave the egg in the Nauplius condition. The Nauplii differ
somewhat in the separate groups, and the post-nauplial stages
vary not inconsiderably.

It will be most convenient to treat successively the larval

1 It seems not impossible that the appendage regarded by Claus as the mandibular
palp may really represent the maxilla, which would otherwise seem to be absent.
This mode of interpretation would bring the appendages of Argulus into a much
closer agreement with those of the parasitic Copepoda. It does not seem incompatible
with the existence of the stylet-like maxillse detected by Claus in the adult.

CRUSTACEA. 493

history of the four sub-orders, viz. Thoracica, Abdominalia,
Apoda, and Rhizocephala.

Thoracica. The just hatched larvae at once leave the egg
lamellae of their parent. They pass out through an opening in
the mantle near the mouth, and during this passage the shell of
the parent is opened and the movements of the cirriform feet
cease.

The larval stages commence with a Nauplius1 which, though
regarded by Claus as closely resembling the Copepod Nauplius
(figs. 231 and 232 A), certainly has very marked pecularities of
its own, and in some respects approaches the Phyllopod
Nauplius. It is in the youngest stage somewhat triangular in
form, and covered on the dorsal side by a very delicate and
hardly perceptible dorsal shield, which is prolonged laterally
into two very peculiar conical horns (fig. 231 Ik), which are the
most characteristic structures of the Cirriped Nauplius. They
are connected with a glandular mass, the secretion from which
passes out at their apex. Anteriorly the dorsal shield has the
same extension as the body, but posteriorly it projects slightly.

An unpaired eye is situated on the ventral surface of the
head, and immediately behind it there springs a more or less
considerable upper lip (Ib), which resembles the Phyllopod
labrum rather than that of the Copepoda. Both mouth and
anus are present, and the hind end of the body is slightly forked
in some forms, but ends in others, e.g. Lepas fascicularis, in an
elongated spine. The anterior of the three pairs of Nauplius
appendages (At*) is uniramous, and the two posterior (Af and
md) are biramous. From the protopodites of both the latter
spring strong hooks like those of the Copepod and Phyllopod
Nauplii. In some Nauplii, e.g. that of Balanus, the appendages
are at first not jointed, but in other Nauplii, e.g. that of Lepas
fascicularis, the jointing is well marked. In Lepas fascicularis
the earliest free Nauplius is enveloped in a larval skin, which is
thrown off after a few hours. The Nauplii of all the Thoracica
undergo a considerable number of moults before their appendages
increase in number or segmentation of the body appears. During
these moults they grow larger, and the posterior part of the

1 Alepas squalicola is stated by Koren and Danielssen to form an exception to
this rule, and to leave the egg with six pairs of appendages.

494

CIRRIPEDIA.

body — the future thoracic and abdominal region — grows re-
latively in length. There also appear close to the sides of the
unpaired eye two conical bodies, which correspond with the
frontal sense organs of the Phyllopods. During their growth
the different larvae undergo changes varying greatly in degree.

In Balanus the changes consist for the most part in the full
segmentation of the appendages and the growth and distinctness

FIG. 231. NAUPLIUS LARVA OF LEPAS FASCICULARIS VIEWED FROM THE SIDE.
oc. eye ; At. i. antenna of first pair ; At. 2. antenna of second pair ; md. mandible ;
Ib. labrum ; an. anus; me. mesenteron; d.sp. dorsal spine; c.sp. caudal spine;
Vp. ventral spine ; Ih. lateral horns.

of the dorsal shield, which forms a somewhat blunt triangular
plate, broadest in front, with the anterior horns very long, and
two short posterior spines. The tail also becomes produced into
a long spine.

In Lepas fascicularis the changes in appearance of the
Nauplius, owing to a great spinous development on its shield,
are very considerable ; and, together with its enormous size,
render it a very remarkable form. Dohrn (No. 520), who was
the first to describe it, named it Archizoaea gigas.

CRUSTACEA. 495

The dorsal shield of the Nauplius of Lepas fascicularis (fig. 231) becomes
somewhat hexagonal, and there springs from the middle of the dorsal surface
an enormously long spine (d,sp], like the dorsal spine of a Zoa^a. The hind
end of the shield is also produced into a long caudal spine (c.sfi] between
which and the dorsal spine are some feather-like processes. From its edge
there spring in addition to the primitive frontal horns three main pairs of
horns, one pair anterior, one lateral, and one posterior, and smaller ones in
addition. All these processes (with the exception of the dorsal and posterior
spines) are hollow and open at their extremities, and like the primitive
frontal horns contain the ducts of glands situated under the shield. On the
under surface of the larva is situated the unpaired eye (pc] on each side of
which spring the two-jointed frontal sense organs. Immediately behind
these is the enormous upper lip (lb] which covers the mouth1. At the sides
of the lip lie the three pairs of Nauplius appendages, which are very
characteristic but present no special peculiarities. Posteriorly the body is
produced into a long ventral spine-like process ( Vfi) homologous with that
of other more normal Nauplii. At the base of this process large moveable
paired spines appear at successive moults, six pairs being eventually formed.
These spines give to the region in which they are situated a segmented
appearance, and perhaps similar structures have given rise to the appear-
ance of segmentation in Spence Bate's figures. The anus is situated on
the dorsal side of this ventral process, and between it and the caudal
spine of the shield above. The fact that the anus occupies this position
appears to indicate that the ventral process is homologous with the
caudal fork of the Copepoda, on the dorsal side of which the anus so
often opens2.

From the Nauplius condition the larvae pass at a single
moult into an entirely different condition known as the Cypris
stage. In preparation for this condition there appear, during
the last Nauplius moults, the rudiments of several fresh organs,
which are more or less developed in different types. In the
first place a compound eye is formed on each side of the
median eye. Secondly there appears behind the mandibles a
fourth pair of appendages — the first pair of maxillae — and
internal to these a pair of small prominences, which are perhaps

1 Willemoes Suhm (No. 530) states that the mouth is situated at the free end of the
upper lip, and that the oesophagus passes through it. From an examination of some
specimens of this Nauplius, for which I am indebted to Moseley, I am inclined to
think that this is a mistake, and that a groove on the surface of the upper lip has been
taken by Suhm for the oesophagus.

2 The enormous spinous development of the larva of Lepas fascicularis is probably
to be explained as a secondary protective adaptation, and has no genetic connection
with the somewhat similar spinous armature of the Zosea.

496" CIRRIPEDIA.

equivalent to the second pair of maxillae, and give rise to the
third pair of jaws in the adult (sometimes spoken of as the
lower lip).

Behind these appendages there are moreover formed the rudi-
ments of six pairs of feet. Under the cuticle of the first pair of
antennae there may be seen just before the final moult the four-
jointed antennae of the Cypris stage with the rudiment of a disc
on the second joint by which the larvae eventually become
attached.

By the free Cypris stage, into which the larva next passes, a
very complete metamorphosis has been effected. The median
and paired eyes are present as before, but the dorsal shield has
become a bivalve shell, the two valves of which are united along
their dorsal, anterior, and posterior margins. The two valves
are further kept in place by an adductor muscle situated close
below the mouth. Remains of the lateral horns still persist. The
anterior antennae have undergone the metamorphosis already
indicated. They are four-jointed, the two basal joints being
long, and the second provided with a suctorial disc, in the centre
of which is the opening of the duct of the so-called antennary or
cement gland, which is a granular mass lying on the ventral
side of the anterior region of the body. The gland arises
(Willemoes Suhm) during the Nauplius stage in the large upper
lip. The two distal joints of the antennae are short, and the
last of them is provided with olfactory hairs. The great upper
lip and second pair of antennae and mandibles have disappeared,
but a small papilla, forming the commencement of the adult
mandibles, is perhaps developed in the base of the Nauplius
mandibles. The first pair of maxillae have become small papillae
and the second pair probably remain. The six posterior pairs
of appendages have grown out as functional biramous swimming
feet, which can project beyond the shell and are used in the
locomotion of the larva. They are composed of two basal
joints, and two rami with swimming hairs, each two-jointed.
These feet resemble Copepod feet, and form the main ground
for the views of Claus and others that the Copepoda and
Cirripedia are closely related. They are regarded by Claus as
representing the five pairs of natatory feet of Copepoda, and the
generative appendages of the segment behind these. Between

CRUSTACEA.

497

the natatory feet are delicate chitinous lamellae, in the spaces
between which the cirriform feet of the adult become developed.
The ventral spinous process of the Nauplius stage is much reduced,
though usually three-jointed. It becomes completely aborted
after the larva is fixed.

In addition to the antennary gland there is present, near the
dorsal side of the body above the natatory feet, a peculiar paired
glandular mass, the origin of which has not been clearly made
out, but which is perhaps equivalent to the entomostracan shell
gland. It probably supplies the material for the shell in suc-
ceeding stages1.

The free Cypris stage is not of long duration ; and during it
the larva does not take food. It is succeeded by a stage known
as the pupa stage (fig. 232 B), in which the larva becomes fixed,
while underneath the larval skin the adult structures are de-
veloped. This stage fully deserves its name, since it is a quies-
cent stage during which no nutriment is taken. The attachment
takes place by the sucker of the antennae, and the cement gland
(/) supplies the cementing material for effecting it. A retro-
gressive metamorphosis of a large number of the organs sets in,
while at the same time the for-
mation of new adult structures
is proceeded with. The eyes
become gradually lost, but the
Nauplius eye is retained,though
in a rudimentary state, and the
terminal joints of the antennae
with their olfactory hairs are
thrown off. The bivalve shell
is moulted about the same time
as the eyes, the skin below it
remaining as the mantle. The
caudal process becomes abor-
ted. Underneath the natatory

FIG. 232. LARVAL FORMS OF THE
THORACICA. (From Huxley.)

A. Nauplius of Balanus balanoides.
(After Sp. Bate.) B. Pupa stage of Lepas
australis. (After Darwin.)

n. antennary apodemes ; /. cement
gland with duct to antenna.

1 There is considerable confusion about the shell gland and antennary gland. In
my account Willemoes Suhm has been followed. Claus however regards what I have
called the antennary gland as the shell gland, and states that it does not open into the
antennae till a later period. He does not clearly describe its opening, nor the organ
which I have called the shell gland.

B. II. 32

498 CIRRIPEDIA.

feet, and between the above-mentioned chitinous lamellae, the
cirriform feet are formed ; and on their completion the natatory
feet become thrown off and replaced by the permanent feet. In
the Lepadidae, in which the metamorphosis of the pupa stages
has been most fully studied, the anterior part of the body with
the antennae gradually grows out into an elongated stalk, into
which pass the ovaries, which are formed during the Cypris
stage. At the base of the stalk is the protuberant mouth, the
appendages of which soon attain a higher development than in
the Cypris stage. At the front part of it a large upper lip
becomes formed. Above the mantle and between it and the
shell there are formed in the Lepadidae the provisional valves of
the shell. These valves are chitinous, and have a fenestrated
structure, owing to the chitin being deposited round the margin
of the separate epidermis (hypodermis) cells. These valves in
the Lepadidae " prefigure in shape, size, and direction of growth,
the shelly valves to be formed under and around them" (Darwin,
No. 519, p. 129).

Whatever may be the number of valves in the adult the provisional
valves never exceed five, viz. the two scuta, the two terga and the carina.
They are relatively far smaller than the permanent valves and are therefore
separated by considerable membranous intervals. They are often preserved
for a long time on the permanent calcareous valves. In the Balanidce
the embryonic valves are membranous and do not overlap, but do not
present the peculiar fenestrated structure of the primordial valves of the
Lepadidae.

In connection with the moult of the pupa skin, and the
conversion of the pupa into the adult form, a remarkable change
in the position takes place. The pupa lies with the ventral side
parallel to and adjoining the surface of attachment, while the
long axis of the body of the young Cirriped is placed nearly at
right angles to the surface of attachment. This change is
connected with the ecdyses of the antennary apodemes («),
which leave a deep bay on the ventral surface behind the
peduncle. The chitinous skin of the Cirriped passes round
the head of this bay, but on the moult of the pupa skin
taking place becomes stretched out, owing to the posterior
part of the larva bending dorsalwards. It is this flexure which
causes the change in the position of the larva.

CRUSTACEA.

499

In addition to the remarkable external metamorphosis
undergone during the pupa stage, a series of hardly less con-
siderable internal changes take place, such as the atrophy of
the muscles of the antennae, a change in the position of the
stomach, etc.

Abdominalia. In the Alcippidae the larva leaves the egg as a
Nauplius, and this stage is eventually followed by a pupa stage closely
resembling that of the Thoracica. There are six pairs of thoracic natatory
legs (Darwin, No. 519). Of these only the first and the last three are pre-
served in the adult, the first being bent forward in connection with the
mouth. The body moreover partially preserves its segmentation, and the
mantle does not secrete calcareous valves.

The very remarkable genus Cryptophialus, the development of which is
described by Darwin (No. 519) in his classical memoir, is without a free
Nauplius stage. The embryo is at first oval but soon acquires two anterior
processes, apparently the first pair of antennae, and a posterior prominence,
the abdomen. In a later stage the abdominal prominence disappears, and
the antennary processes, within which the true antennas are now visible, are
carried more towards the ventral
surface. The larva next passes into
the free Cypris stage, during which it
creeps about the mantle cavity of its
parent. It is enveloped in a bivalve
shell, and the antennae have the nor-
mal cirriped structure. There are no
other true appendages, but posteriorly
three pairs of bristles are attached to
a rudimentary abdomen. Paired com-
pound eyes are present. During the
succeeding pupa stage the metamor-
phosis into the adult form takes place,
but this has not been followed out in
detail.

In Kochlorine, a form discovered
by Noll (No. 526) and closely related
to Cryptophialus, the larvae found
within the mantle represent ap-
parently two larval stages, similar to
two of the larval stages described by
Darwin.

Rhizocephala. The Rhizo-
cephala, as might have been antici-

FIG. 233. STAGES IN THE DEVELOP-
MENT OF THE RHIZOCEPHALA. (From
Huxley, after Fritz Miiller.)

A. Nauplius of Sacculina purpurea.
B. Cypris stage of Lernseodiscus por-
cellanae. C. Adult of Peltogaster paguri.

II, III. IV. Two pairs of antennae
and mandibles; cp. carapace; a. anterior
end of body; b. generative aperture; c.
root-like processes.

pated from their close relationship to Anelasma squalicola amongst the
Thoracica, undergo a development differing much less from the type of the
Thoracica than that of Cryptophialus and Kochlorine.

32—2

5oo

OSTRACODA.

Sacculina leaves the egg as a Nauplius (fig. 233 A), which differs from
the ordinary type mainly (i) in the large development of an oval dorsal
shield (eft] which projects far beyond the edge of the body, but is provided
with the typical sternal horns, etc. ; and (2) in the absence of a mouth.
The Cypris and pupa stages of Sacculina and other Rhizocephala (fig. 233 B)
are closely similar to those of the Thoracica, but the paired eye is absent.
The attachment takes place in the usual way, but the subsequent metamor-
phosis leads to the loss of the thoracic feet and generally to retrogressive
changes.

OSTRACODA.

Our knowledge of the development of this remarkable group is entirely
due to the investigations of Claus.

Some forms of Cythere are viviparous, and in the marine form Cypridina
the embryo develops within the valves of the shell. Cypris attaches its
eggs to water plants. The larvae of Cypris are free, and their development
is somewhat complicated. The whole development is completed in nine
ecdyses, each of them accompanied by more or less important changes in
the constitution of the larva.

In the earliest free stage the larva has the characters of a true Nauplius
with three pairs of appendages (fig. 234 A). The Nauplius presents how-
B A

-A'

MX SM

FlG. 234. TWO STAGES IN THE DEVELOPMENT OF CYPRIS. (From ChlUS.)

A. Earliest (Nauplius) stage. B. Second stage.

A'. A". First and second pairs of antennae ; Md. mandibles ; OL. labrum ;
MX,', first pair of maxilla; /". first pair of feet.

ever one or two very marked secondary characters. In the first place it is
completely enveloped in a fully formed bivalve shell, differing in unessential
points from the shell of the adult. An adductor muscle (SM] for the shell
is present. Again the second and third appendages, though locomotive in
function are neither of them biramous, and the third one already contains
a rudiment of the future mandibular blade, and terminates in an anteriorly
directed hook-like bristle. The first pair of antenna? is moreover very
similar to the second and is used in progression. Neither of the pairs of

CRUSTACEA.

501

antennae become much modified in the subsequent metamorphosis. The
Nauplius has a single median eye, as in the adult Cypris, and a fully
developed alimentary tract.

The second stage (fig. 234 B), inaugurated by the first moult, is mainly
characterized by the appearance of two fresh pairs of appendages, viz. the
first pair of maxillae and the first pair of feet ; the second pair of maxillae
not being developed till later. The first pair appear as leaf-like curved

FIG. 235. STAGES IN THE DEVELOPMENT OF CYPRIS. (From Claus.)

A. Fourth stage. B. Fifth stage.

MX', first maxilla ; MX", and/', second maxilla ; /". first pair of feet ; L. liver.

plates (Mx'} more or less like Phyllopod appendages (Claus) though at this
stage without an exopodite. The first pair of feet (/"} terminates in a
curved claw and is used for adhering. The mandibles have by this stage
fully developed blades, and have practically attained their adult form, con-
sisting of a powerful toothed blade and a four-jointed palp.

During the third and fourth stages the first pair of maxillae acquire
their pectinated gill plate (epipodite) and four blades ; and in the fourth
stage (fig. 235 A) the second pair of maxillae (Mx"} arises, as a pair of
curved plates, similar to the first pair of maxillae at their first appearance.
The forked tail is indicated during the fourth stage by two bristles. During
the fifth stage (fig. 235 B) the number of joints of the first pair of antennae
becomes increased, and the posterior maxillae develop a blade and become

502

PHYLOGENY OF THE CRUSTACEA.

four-jointed ambulatory appendages terminating in a hook. The caudal fork
becomes more distinct.

In the sixth stage (fig. 236) the second and hindermost pair of feet be-
comes formed (/"') and the maxillae of the second pair lose their ambulatory
function, and begin to be converted into definite masticatory appendages by
the reduced jointing of their palp, and the increase of their cutting blades.
By the seventh stage all the appendages have practically attained their

Fu

FIG. 236. SIXTH STAGE IN THE DEVELOPMENT OF CYPRIS. (From Claus.)
MX!, first maxilla ; Mx".f. second maxilla; /'. and/"', first and second pair oi
feet ; Fu. caudal fork ; L. liver ; S.D. shell gland.

permanent form ; the second pair of maxillae has acquired small branchial
plates, and the two following feet have become jointed. In the eighth and
ninth stages the generative organs attain their mature form.

The larva of Cythere at the time of birth has rudiments of all the limbs,
but the mandibular palp still functions as a limb, and the three feet (2nd
pair of maxillae and two following appendages) are very rudimentary.

The larvae of Cypridina are hatched in a condition which to all intents
and purposes resembles the adult.

Phylogeny of the Crustacea.

The classical work of Fritz Miiller (No. 452) on the phylogeny of the
Crustacea has given a great impetus to the study of their larval forms, and
the interpretations of these forms which he has offered have been the subject
of a very large amount of criticism and discussion. A great step forward
in this discussion has been recently made by Claus in his memoir (No. 448).

The most fundamental question concerns the meaning of the Nauplius.
Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz
Miiller and Claus, or are its peculiarities and constant occurrence due to
some other cause ? The most plausible explanation on the second hypothesis

CRUSTACEA. 503

would seem to be the following. The segments with their appendages of
Arthropoda and Annelida are normally formed from before backwards,
therefore every member of these two groups with more than three segments
must necessarily pass through a stage with only three segments, and the fact
that in a particular group this stage is often reached on the larva being
hatched is in itself no proof that the ancestor of the group had only three
segments with their appendages. This explanation appears to me, so far
as it goes, quite valid ; but though it relieves us from the necessity of
supposing that the primitive Crustacea had only three pairs of appendages,
it does not explain several other peculiarities of the Nauplius1. The more
important of these are the following.

1. That the mandibles have the form of biramous swimming feet and
are not provided with a cutting blade.

2. That the second pair of antennae are biramous swimming feet with a
hook used in mastication, and are innervated (?) from the subcesophageal
ganglion.

3. The absence of segmentation in the Nauplius body. An absence
which is the more striking in that before the Nauplius stage is fully reached
the body of the embryo is frequently divided into three segments, e.g.
Copepoda and Cirripedia

4. The absence of a heart.

5. The presence of a median single eye as the sole organ of vision.

Of these points the first, second, and fifth appear only to be capable of
being explained phylogenetically, while with reference to the absence of a
heart it appears very improbable that the ancestral Crustacea were without
a central organ of circulation. If the above positions are accepted the
conclusion would seem to follow that in a certain sense the Nauplius is
an ancestral form — but that, while it no doubt had its three anterior pairs
of appendages similar to those of existing Nauplii, it may perhaps have
been provided with a segmented body behind provided with simple biramous
appendages. A heart and cephalo-thoracic shield may also have been
present, though the existence of the latter is perhaps doubtful. There was
no doubt a median single eye, but it is difficult to decide whether or no
paired compound eyes were also present. The tail ended in a fork between
the prongs of which the anus opened ; and the mouth was protected by a
large upper lip. In fact, it may very probably turn out that the most
primitive Crustacea more resembled an Apus larva at the moult immediately
before the appendages lose their Nauplius characters (fig. 208 B), or a
Cyclops larva just before the Cyclops stage (fig. 229), than the earliest
Nauplius of either of these forms.

If the Nauplius ancestor thus reconstructed is admitted to have existed,
the next question in the phylogeny of the Crustacea concerns the relations
of the various phyla to the Nauplius. Are the different phyla descended
from the Nauplius direct, or have they branched at a later period from

1 For the characters of Nauplius vide p. 460.

504 PHYLOGENY OF THE CRUSTACEA.

some central stem? It is perhaps hardly possible as yet to give a full and
satisfactory answer to this question, which requires to be dealt with for each
separate phylum ; but it may probably be safely maintained that the existing
Phyllopods are members of a group which was previously much larger, and
the most central of all the Crustacean groups; and which more nearly
retains in the characters of the second pair of antennae etc. the Nauplius
peculiarities. This view is shared both by Claus and Dohrn, and appears
to be in accordance with all the evidence we have both palaeontological and
morphological. Claus indeed carries this view still further, and believes
that the later Nauplius stages of the different Entomostracan groups and
the Malacostraca (Penaeus larva) exhibit undoubted Phyllopod affinities.
He therefore postulates the earlier existence of a Protophyllopod form, which
would correspond very closely with the Nauplius as reconstructed above,
from which he believes all the Crustacean groups to have diverged.

It is beyond the scope of this work to attempt to grapple with all the
difficulties which arise in connection with the origin and relationships of the
various phyla, but I confine myself to a few suggestions arising out of the
developmental histories recorded above.

Malacostraca. In attempting to reconstitute from the evidence in
our possession the ancestral history of the Malacostraca we may omit from
consideration the larval history of all those types which leave the egg in
nearly the adult form, and confine our attention to those types in which the
larval history is most completely preserved.

There are three forms which are of special value in this respect, viz.
Euphausia, Penaeus and Squilla. From the history of these which has
already been given it appears that in the case of the Decapoda four stages
(Claus) may be traced in the best preserved larval histories.

1. A Nauplius stage with the usual Nauplius characters.

2. A Protozoaea stage in which the maxillae and first pair of maxillipeds
are formed behind the Nauplius appendages ; but in which the tail is still
unsegmented. This stage is comparatively rarely preserved and usually not
very well marked.

3. A Zoaea stage the chief features of which have already been fully
characterised (vide p. 465). Three more or less distinct types of Zosea are
distinguished by Claus. (a) That of Penaeus, in which the appendages up
to the third pair of maxillipeds are formed, and the thorax and abdomen are
segmented, the former being however very short. The heart is oval, with
one pair of ostia. From this type the Zoaea forms of the other Decapoda
are believed by Claus to be derived, (b} That of Euphausia, with but one
pair of maxillipeds and those short and Phyllopod-like. The heart oval
with one pair of ostia. (c) That of Squilla, with an elongated many-
chambered heart, two pairs of maxillipeds and the abdominal appendages in
full activity.

4. A Mysis stage, which is only found in the macrurous Decapod
larvie.

The embryological questions requiring to be settled concern the value

CRUSTACEA. 50$

of the above stages. Do they represent stages in the actual evolution of
the present types, or have their characters been secondarily acquired in
larval life ?

With reference to the first stage this question has already been discussed,
and the conclusion arrived at, that the Nauplius does in a much modified
form represent an ancestral type. As to the fourth stage there can be no
doubt that it is also ancestral, considering that it is almost the repetition of
an actually existing form.

The second stage can clearly only be regarded as an embryonic prepara-
tion for the third ; and the great difficulty concerns the third stage.

The natural view is that this stage like the others has an ancestral value,
and this view was originally put forward by Fritz Miiller and has been
argued for also by Dohrn. On the other hand the opposite side has been
taken by Claus, who has dealt with the question very ably and at great
length, and has clearly shewn that some of Fritz Miiller's positions are
untenable. Though Claus' opinion is entitled to very great weight, an
answer can perhaps be given to some of his objections. The view adopted
in this section can best be explained by setting forth the chief points which
Claus urges against Fritz Miiller's view.

The primary question which needs to be settled is whether the Malacos-
traca have diverged very early from the Nauplius root, or later in the history
of the Crustacea from the Phyllopod stem. On this question Claus1 brings
arguments, which appear to me very conclusive, to shew that the Malacos-
traca are derived from a late Protophyllopod type, and Claus' view on this
point is shared also by Dohrn. The Phyllopoda present so many characters
(not possessed by the Nauplius) in common with the Malacostraca or their
larval forms, that it is incredible that the whole of these should have
originated independently in the two groups. The more important of these
characters are the following.

1. The compound eyes, so often stalked in both groups.

2. The absence of a palp on the mandible, a very marked character of
the Zoasa as well as of the Phyllopoda.

3. The presence of a pair of frontal sense knobs.

4. The Phyllopod character of many of the appendages. Cf. first pair of
maxillipeds of the Euphausia Zosea.

1 Claus speaks of the various Crustacean phyla as having sprung from a Protophyl-
lopod form, and it might be supposed that he considered that they all diverged from
the same form. It is clear however from the context that he regards the Protophyl-
lopod type from which the Malacostraca originated as far more like existing Phyl-
lopods than that from which the Entomostracan groups have sprung. It is not quite
easy to get a consistent view of his position on the question, since he states (p. 77) that
the Malacostraca and the Copepods diverged from a similar form, which is represented
in their respective developments by the Protozosea and earliest Cyclops stage. Yet if
I understand him rightly, he does not consider the Protozosea stage to be the Proto-
phyllopod stage from which the Malacostraca have diverged, but states on p. 71 that
it was not an ancestral form at all.

506 PHYLOGENY OF THE CRUSTACEA.

5. The presence of gill pouches (epipodites) on many of the append-
ages1.

In addition to these points, to which others might be added, Claus
attempts to shew that Nebalia must be regarded as a type intermediate
between the Phyllopods and Malacostraca. This view seems fairly esta-
blished, and if true is conclusive in favour of the Phyllopod origin of the
Malacostraca. If the Protophyllopod origin of the Malacostraca is admitted,
it seems clear that the ancestral forms of the Malacostraca must have de-
veloped their segments regularly from before backwards, and been provided
with nearly similar appendages on all the segments. This however is far
from the case in existing Malacostraca, and Fritz Miiller commences his
summary of the characters of the Zoaea in the following words2. "The
middle body with its appendages, those five pairs of feet to which these
animals owe their name, is either entirely wanting or scarcely indicated."
This he regards as an ancestral character of the Malacostraca, and is of
opinion that their thorax is to be regarded as a later acquirement than the
head or abdomen. Claus' answer on this point is that in the most primitive
Zoasas, viz. those already spoken of as types, the thoracic and abdominal
segments actually develop, in regular succession from before backwards,
and he therefore concludes that the late development of the thorax in the
majority of Zoaea forms is secondary and not an ancestral Phyllopod
peculiarity.

This is the main argument used by Claus against the Zosea having any
ancestral meaning. His view as to the meaning of the Zoaea may be
gathered from the following passage. After assuming that none of the
existing Zoaea types could have been adult animals, he says—" Much more
"probably the process of alteration of the metamorphosis, which the Mala-
" costracan phylum underwent in the course of time and in conjunction
" with the divergence of the later Malacostracan groups, led secondarily
" to the three different Zoaea configurations to which probably later modifica-
" tions were added, as for instance in the young form of the Cumaceae. We
"might with the same justice conclude that adult Insects existed as cater-
" pillars or pupae as that the primitive form of the Malacostraca was a
" Protozoaea or Zoaea."

Granting Claus' two main positions, viz. that the Malacostraca are
derived from Protophyllopods, and that the segments were in the primary
ancestral forms developed from before backwards, it does not appear im-
possible that a secondary and later ancestral form may have existed with a
reduced thorax. This reduction may only have been partial, so that the
Zoaea ancestor would have had the following form. A large cephalo-thorax
and well-developed tail (?) with swimming appendages. The appendages up
to the second pair of maxillipeds fully developed, but the thorax very

1 Claus appears to consider it doubtful whether the Malacostracan gills can be
compared with the Phyllopod gill-pouches.
3 Facts for Darwin, p. 49.

CRUSTACEA. 507

imperfect and provided only with delicate foliaceous appendages not pro-
jecting beyond the edge of the cephalo-thoracic shield.

Another hypothesis for which there is perhaps still more to be said is
that there was a true ancestral Zoaea stage in which the thoracic appendages
were completely aborted. Claus maintains that the Zoaea form with
aborted thorax is only a larval form ; but he would probably admit that its
larval characters were acquired to enable the larva to swim better. If this
much be admitted it is not easy to see why an actual member of the
ancestral series of Crustacea should not have developed the Zoaea pecu-
liarities when the mud-dwelling habits of the Phyllopod ancestors were
abandoned, and a swimming mode of life adopted. This view, which
involves the supposition that the five (or six including the third maxillipeds)
thoracic appendages were lost in the adult (for they may be supposed to
have been retained in the larva) for a series of generations, and reappeared
again in the adult condition, at a later period, may at first sight appear very
improbable, but there are, especially in the larval history of the Stomatopoda,
some actual facts which receive their most plausible explanation on this
hypothesis.

These facts consist in cases of the actual loss of appendages during
development, and their subsequent reappearance. The two most striking
cases are the following.

1. In the Erichthus form of the Squilla larva the appendages corre-
sponding to the third pair of maxillipeds and first two pairs of ambulatory
appendages of the Decapoda are developed in the Protozosea stage, but
completely aborted in the Zoasa stage, and subsequently redeveloped.

2. In the case of the larva of Sergestes in the passage from the Acan-
thosoma (Mysis) stage to the Mastigopus stage the two hindermost thoracic
appendages become atrophied and redevelop again later.

Both of these cases clearly fit in very well with the view that there was an
actual period in the history of the Malacostraca in which the ancestors of
the present forms were without the appendages which are aborted and
redeveloped again in these larval forms. Claus' hypothesis affords no
explanation of these remarkable cases.

It is however always possible to maintain that the loss and reappearance
of the appendages in these cases may have no ancestral meaning ; and the
abortion of the first pair of maxillipeds and reduction of some of the other
appendages in the case of the Loricata is in favour of this explanation.
Similar examples of the abortion and reappearance of appendages, which
cannot be explained in the way attempted above, are afforded by the Mites
and also by the Insects, e.g. Bees.

On the other hand there is almost a conclusive indication that the loss
of the appendages in Sergestes has really the meaning assigned to it, in that
in the allied genius Leucifer the two appendages in question are actually
absent in the adult, so that the stage with these appendages absent is
permanently retained in an adult form. In the absence of the mandibular
palp in all the Zoaea forms, its actual atrophy in the Penaeus Zoasa, and its

508 PHYLOGENY OF THE CRUSTACEA.

universal reappearance in adult Malacostraca, are cases which tell in favour
of the above explanation. The mandibular palp is permanently absent in
Phyllopods, which clearly shews that its absence in the Zoaea stage is due to
the retention of an ancestral peculiarity, and that its reappearance in the
adult forms was a late occurrence in the Malacostracan history.

The chief obvious difficulty of this view is the redevelopment of the
thoracic feet after their disappearance for a certain number of generations.
The possibility of such an occurrence appears to me however clearly demon-
strated by the case of the mandibular palp, which has undoubtedly been
reacquired by the Malacostraca, and by the case of the two last thoracic
appendages of Sergestes just mentioned. The above difficulty may be
diminished if we suppose that the larvae of the Zoaea ancestors always
developed the appendages in question. Such appendages might first only
partially atrophy in a particular Zoaea form and then gradually come to
be functional again ; so that, as a form with functional thoracic limbs
came to be developed out of the Zoaea, we should find in the larval history
of this form that the limbs were developed in the pre-zoaeal larval stages,
partially atrophied in the Zoaea stage, and redeveloped in the adult. From
this condition it would not be difficult to pass to a further one in which the
development of the thoracic limbs became deferred till after the Zoaea stage.

The general arguments in favour of a Zoaea ancestor with partially or
completely aborted thoracic appendages having actually existed in the past
appear to me very powerful. In all the Malacostracan groups in which
the larva leaves the egg in an imperfect form a true Zoaea stage is found.
That the forms of these Zoaeas should differ considerably is only what might
be expected, considering that they lead a free existence and are liable to
be acted upon by natural selection, and it is probable that none of those
at present existing closely resemble the ancestral form. The spines from
their carapace, which vary so much, were probably originally developed,
as suggested by Fritz Miiller, as a means of defence. The simplicity of
the heart— so different from that of Phyllopods— in most forms of Zoaea
is a difficulty, but the reduction in the length of the heart may very
probably be a secondary modification ; the primitive condition being retained
in the Squilla Zoaea. In any case this difficulty is not greater on the
hypothesis of the Zoaea being an ancestral form, than on that of its being a
purely larval one.

The points of agreement in the number and character of the appendages,
form of the abdomen, etc. between the various types of Zoaea appear to me
too striking to be explained in the manner attempted by Claus. It seems
improbable that a peculiarity of form acquired by the larva of some ancestral
Malacostracan should have been retained so permanently in so many groups l

1 A secondary larval form is less likely to be repeated in development than an
ancestral adult stage, because there is always a strong tendency for the former, which
is a secondarily intercalated link in the chain, to drop out by the occurrence of a
reversion to the original type of development.

CRUSTACEA. 509

—more permanently indeed than undoubtedly ancestral forms like that of
Mysis — and it would be still more remarkable that a Zoaea form should
have been two or more times independently developed.

There are perhaps not sufficient materials to reconstruct the characters of
the Zoaea ancestor, but it probably was provided with the anterior appen-
dages up to the second pair of maxillipeds, and (?) with abdominal swim-
ming feet. The heart may very likely have been many-chambered.
Whether gill pouches were present on the maxillipeds and abdominal feet
does not appear to me capable of being decided. The carapace and general
shape were probably the same as in existing Zoaeas. It must be left an open
question whether the six hindermost thoracic appendages were absent or
only very much reduced in size.

On the whole then it may be regarded as probable that the Malacostraca
are descended from Protophyllopod forms, in which, on the adoption of
swimming habits, six appendages of the middle region of the body were
reduced or aborted, and a Zoaea form acquired, and that subsequently the
lost appendages were redeveloped in the descendants of these forms, and
have finally become the most typical appendages of the group.

The relationship of the various Malacostracan groups is too difficult
a subject to be discussed here, but it seems to me most likely that in
addition to the groups with a Zoaea stage the Edriophthalmata and Cumaceae
are also post-zoaeal forms which have lost the Zoasa stage. Nebalia is
however very probably to be regarded as a prae-zoaeal form which has
survived to the present day ; and one might easily fancy that its eight thin
thoracic segments with their small Phyllopod-like feet might become nearly
aborted.

Copepoda. The Copepoda certainly appear to have diverged very
early from the main stem, as is shewn by their simple biramous feet and the
retention of the median eye as the sole organ of vision. It may be argued
that they have lost the eye by retrogressive changes, and in favour of this
view cases of the Pontellidae and of Argulus may be cited. It is however
more than doubtful whether the lateral eyes of the Pontellidae are related to
the compound Phyllopod eye, and the affinities of Argulus are still uncertain.
It would moreover be a great paradox if in a large group of Crustacea the
lateral eyes had been retained in a parasitic form only (Argulus), but lost in
all the free forms.

Cirripedia. The Cirripedia are believed by Claus to belong to the
same phylum as the Copepoda. This view does not appear to be completely
borne out by their larval history. The Nauplius differs very markedly from
that of the Copepoda, and this is still more true of the Cypris stage. The
Copepod-like appendages of this stage are chiefly relied upon to support the
above view, but this form of appendages was probably very primitive
and general, and the number (without taking into consideration the doubtful
case of Cryptophialus) does not correspond to that in Copepoda. On the
other hand the paired eyes and the bivalve shell form great difficulties in the
way of Claus' view. It is clear that the Cypris stage represents more or less

510

PHYLOGENY OF THE CRUSTACEA.

closely an ancestral form of the Cirripedia, and that both the large bivalve
shell and the compound eyes were ancestral characters. These characters
would seem incompatible with Copepod affinities, but point to the indepen-
dent derivation of the Cirripedia from some early bivalve Phyllopod form.

Ostracoda. The independent origin of the Ostracoda from the main
Crustacean stem seems probable. Claus points out that the Ostracoda
present by no means a simple organisation, and concludes that they were
not descended from a form with a more complex organisation and a larger
number of appendages. Some simplifications have however undoubtedly
taken place, as the loss of the heart, and of the compound eyes in many
forms. These simplifications are probably to be explained (as is done by
Claus) as adaptations due to the small size of body and its enclosure in a
thick bivalve shell. Although Claus is strongly opposed to the view that

I)

FIG. 737. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From
Parker ; after Reichenbach.)

A. Section through part of the ovum during segmentation, n. nuclei ; w.y. white
yolk ; y.p. yolk pyramids ; c. central yolk mass.

B and C. Longitudinal sections during the gastrula stage, a. archenteron ;
b. blastopore ; ms. mesoblast ; ec. epiblast ; en. hypoblast distinguished from epiblast
by shading.

I '. Highly magnified view of the anterior lip of blastopore to shew the origin of
the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast ;
ec. epiblast ; en. hypoblast.

I Two hypoblast cells to shew the amoeba-like absorption of yolk spheres.
y. yolk ; ». nucleus ; /. pseudopodial process.

F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.nts.).
tt. nuclei.

CRUSTACEA. 5 1

the number of the appendages has been reduced, yet the very fact of the
(in some respects) complex organisation of this group might seem to indicate
that it cannot have diverged from the Phyllopod stem at so early a stage as
(on Claus' view of the Nauplius) would seem to be implied by the very small
number of appendages which is characteristic of it, and it therefore appears
most probable that the present number may be smaller than that of the
ancestral forms.

The formation of the germinal layers.

The formation of the germinal layers has been more fully
studied in various Malacostraca, more especially in the Decapoda,
than in other groups.

Decapoda. To Bobretzky (No. 472) is due the credit of
having been the pioneer in this line of investigation ; and his
researches have been followed up and enlarged by Haeckel,
Reichenbach (No. 488), and Mayer (No. 482). The segmentation
is centrolecithal and regular (fig. 237 A). At its close the
blastoderm is formed of a single uniform layer of lens-shaped
cells enclosing a central sphere of yolk, in which as a rule all
trace of the division into columns, present during the earlier
stages of segmentation, has disappeared ; though in Palaemon
the columns remain for a long period distinct. The cells of the
blastoderm are at first uniform, but in Astacus, Eupagurus,
and most Decapoda, soon become more columnar for a small
area, and form a circular patch. The whole patch either
becomes at once invaginated (Eupagurus, Palaemon, fig. 239 A)
or else the edge of it is invaginated as a roughly speaking
circular groove deeper anteriorly than posteriorly, within which
the remainder of the patch forms a kind of central plug, which
does not become invaginated till a somewhat later period
(Astacus, fig. 237 B and C). After the invagination of the
above patch the remainder of the blastoderm cells form the
epiblast.

The invaginated sack appears to be the archenteron and its
mouth the blastopore. The mouth finally becomes closed1, and
the sack itself then forms the mesenteron.

In Astacus the archenteron gradually grows forwards, its
opening is at first wide, but becomes continuously narrowed

1 Bobretzky first stated that the invagination remained open, but subsequently
corrected himself. Zeit. /. Wiss. Zool., Bd. xxiv. p. 186.

512

FORMATION OF THE LAYERS.

and is finally obliterated. Very shortly after this occurrence
there is formed, slightly in front of the point where the last trace
of the blastopore was observable, a fresh epiblastic invagination,
which gives rise to the proctodaeum, and the opening of which
remains as the definite anus. The proctodaeum (fig. 238 A, kg)
is very soon placed in communication with the mesenteron (mg).
The stomodaeum (fg) is formed during the same stage as the
proctodaeum. It gives rise to the oesophagus and stomach.
The hypoblast cells which form the wall of the archenteron
grow with remarkable rapidity at the expense of the yolk ; the
spherules of which they absorb and digest in an amceba-like
fashion by means of their pseudopodia. They become longer
and longer, and finally, after ab-
sorbing the whole yolk, acquire
a form almost exactly similar to
that of the yolk pyramids dur-
ing segmentation (fig. 238 B).
They enclose the cavity of the
mesenteron, and their nuclei
and protoplasm are situated ex-
ternally. The cells of the me-
senteron close to its junction
with the proctodaeum differ
from those elsewhere in being
nearly flat.

In Palaemon (Bobretzky)
the primitive invagination (fig.
239 A) has far smaller dimen-
sions than in Astacus, and ap-
pears before the blastoderm
cells have separated from the
yolk pyramids. The cells which
are situated at the bottom of it
pass into the yolk, increase in
number, and absorb the whole
yolk, forming a solid mass of
hypoblast in which the outlines
of the individual cells would
seem at first not to be distinct.

FlG. 238. TWO LONGITUDINAL SEC-
TIONS OF THE EMBRYO OF ASTACUS.

(From Parker ; after Bobretzky.)

A. Nauplius stage. B. Stage after
the hypoblast cells have absorbed the
food yolk. The ventral surface is turned
upwards, fg. stomodseum ; hg. procto-
dccum ; an. anus ; m. mouth ; mg. me-
senteron ; abd. abdomen ; h. heart.

The blastopore in the mean-

CRUSTACEA. 513

time becomes closed. Some of the nuclei now pass to the
periphery of the yolk mass ; the cells appertaining to them
gradually become distinct and assume a pyramidal form (fig.
239 B, hy\ the inner ends of the cells losing themselves in a
central mass of yolk, in the interior of which nuclei are at first
present but soon disappear. The mesenteron thus becomes
constituted of a layer of pyramidal cells which merge into
a central mass of yolk. Some of the hypoblast cells adjoining
the junction of the proctodaeum and mesenteron become
flattened, and in the neighbourhood of these cells a lumen

FlG. 239. TWO STAGES IN THE DEVELOPMENT OF PAL^MON SEEN IN SECTION.

(After Bobretzky.)

A. Gastrula stage.

B. Longitudinal section through a late stage, hy. hypoblast ; sg. supra-resopha-
geal ganglion ; vg. ventral nerve cord ; hd. proctodseum ; st. stomodseum.

first appears. The stomodaeum and proctodaeum are formed as
in Astacus. Fig. 239 B shews the relative positions of the
proctodaeum, stomodaeum, and mesenteron. Although the
process of formation of the hypoblast and mesenteron is
essentially the same in Astacus and Palaemon, yet the differences
between these two forms are very interesting, in that the yolk is
external to the mesenteron in Astacus, but enclosed within it in
Palaemon. This difference in the position of the yolk is rendered
possible by the fact that the invaginated hypoblast cells in
Palaemon do not, at first, form a continuous layer enclosing a
central cavity, while they do so in Astacus.

The mesoblast appears to be formed of cells budded off
from the anterior wall of the archenteron (Astacus, fig. 237 D),
B. II. 33

514 FORMATION OF THE LAYERS.

or from its lateral walls generally (Palaemon). They make
their first appearance soon after the imagination of the hypo-
blast has commenced. The mesoblast cells are at first spherical,
and gradually spread, especially in an anterior direction, from
their point of origin.

According to Reichenbach there are formed in Astacus at the Nauplius
stage a number of peculiar cells which he speaks of as * secondary mesoblast
cells.' His account is not very clear or satisfactory, but it appears that they
originate (fig. 237 F) in the hypoblast cells by a kind of endogenous growth,
and though they have at first certain peculiar characters they soon become
indistinguishable from the remaining mesoblast cells.

Towards the end of the Nauplius period the secondary mesoblast cells
aggregate themselves into a rod close to the epiblast in the median ventral
line, and even bifurcate round the mouth and extend forwards to the
extremity of the procephalic lobes. This rod of cells very soon vanishes,
and the secondary mesoblast cells become indistinguishable from the
primary. Reichenbach believes, on not very clear evidence, that these cells
have to do with the formation of the blood.

General form of the body. The ventral thickening of epitlast
or ventral plate, continuous with the invaginated patch already
mentioned, forms the first indication of the embryo. It is at
first oval, but soon becomes elongated and extended anteriorly
into two lateral lobes — the procephalic lobes. Its bilateral
symmetry is further indicated by a median longitudinal furrow.
The posterior end of the ventral plate next becomes raised into
a distinct lobe — the abdomen — which in Astacus at first lies in
front of the still open blastopore. This lobe rapidly grows in size,
and at its extremity is placed the narrow anal opening. It soon
forms a well-marked abdomen bent forwards over the region in
front (figs. 239 B, and 240 A and B). Its early development
as a distinct outgrowth causes it to be without yolk ; and so to
contrast very forcibly with the anterior thoracic and cephalic
regions of the body. In most cases this process corresponds to
the future abdomen, but in some cases (Loricata) it appears to
include part of the thorax. Before it has reached a considerable
development, three pairs of appendages spring from the region
of the head, viz. two pairs of antennae and the mandibles, and
inaugurate a so-called Nauplius stage (fig. 240 A). These three
appendages are formed nearly simultaneously, but the hinder-
most appears to become visible slightly before the two others

CRUSTACEA. 515

(Bobretzky). The mouth lies slightly behind the anterior pair
of antennae, but distinctly in front of the posterior pair. The
other appendages, the number of which at the time of hatching
varies greatly in the different Decapods (vide section on larval
development), sprout in succession from before backwards (fig.
240 B). The food yolk in the head and thoracic region
gradually becomes reduced in quantity with the growth of the
embryo, and by the time of hatching the disparity in size between
the thorax and abdomen has ceased to exist.

Isopoda. The early embryonic phases of the Isopoda have
been studied by means of sections by Bobretzky (No. 498) and
Bullar (No. 499) and have been found to present considerable

FlG. 240. TWO STAGES IN THE DEVELOPMENT OF

A. Nauplius stage.

B. Stage with eight pairs of appendages, op. eyes ; at1, and at*, first and second
antennae; md. mandibles; mxl, mx2. first and second maxillae; mxp*. third maxilli-
peds ; Ib. upper lip.

variations. When laid the egg is enclosed in a chorion, but
shortly after the commencement of segmentation (Ed. van
Beneden and Bullar) a second membrane appears, which is
probably of the nature of a larval membrane.

In all the forms the segmentation is followed by the
formation of a blastoderm, completely enclosing the yolk, and
thickened along an area which will become the ventral surface of
the embryo. In this area the blastoderm is formed of at least
two layers of cells — an external columnar epiblast, and an
internal layer of scattered cells which form the mesoblast and
probably in part also the hypoblast (Oniscus, Bobretzky ; Cymo-
thoa, Bullar).

33—2

516 FORMATION OF THE LAYERS.

In Asellus aquaticus there is a centrolecithal segmentation,
ending in the formation of a blastoderm, which appears first
on the ventral surface and subsequently extends to the dorsal.

In Oniscus murarius, and Cymothoa the segmentation is
partial [for its peculiarities and relationship vide p. 120] and a
disc, formed of a single layer of cells, appears at a pole of the
egg which corresponds to the future ventral surface (Bobretzky).
This layer gradually grows round the yolk partly by division of
its cells, though a formation of fresh cells from the yolk may
also take place. Before it has extended far round the yolk, the
central part of it becomes two or more layers deep, and the cells
of the deeper layers rapidly increase in number, and are destined
to give rise to the mesoblast and probably also to part or the
whole of the hypoblast. In Cymothoa this layer does not at
first undergo any important change, but in Oniscus it becomes
very thick, and its innermost cells (Bobretzky) become imbedded
in the yolk, which they rapidly absorb; and increasing in
number first of all form a layer in the periphery of the yolk, and
finally fill up the whole of the interior of the yolk (fig. 241 A),
absorbing it in the process.

It appears possible that these cells do not, as Bobretzky believes, origin-
ate from the blastoderm, but from nuclei in the yolk which have escaped
his observation. This mode of origin would be similar to that by which yolk
cells originate in the eggs of the Insecta, etc. If Bobretzky's account is
correct we must look to Palaemon, as he himself suggests, to find an explana-
tion of the passage of the hypoblast cells into the yolk. The thickening of
the primitive germinal disc would, according to this view, be equivalent to
the invagination of the archenteron in Astacus, Palaemon, etc.

Whatever may be the origin of the cells in the yolk they no
doubt correspond to the hypoblast of other types. In Cymothoa
nothing similar to them has been met with, but the hypoblast
has a somewhat different origin ; being apparently formed from
some of the indifferent cells below the epiblast, which collect as
a solid mass on the ventral surface, and then divide into two
masses which become hollow and give rise to the liver caeca.
Their fate, as well as that of the hypoblast in Oniscus, is dealt
with in connection with the alimentary tract. The completion
of the enclosure of the yolk by the blastoderm takes place on
the dorsal surface. In all the Isopods which have been carefully

CRUSTACEA. 517

studied, there appears before any other organ a provisional
structure formed from the epiblast and known as the dorsal
organ. An account of it is given in connection with the de-
velopment of the organs. The general external changes under-
gone by the larva in its development are as follows. The
ventral thickened area of the blastoderm (ventral plate) shapes
itself and girths nearly the whole circumference of the ovum in
Oniscus (fig. 241 A) but is relatively much shorter in Cymothoa.
Anteriorly it dilates into the two procephalic lobes. In
Cymothoa it next becomes segmented; and the anterior seg-
ments are formed nearly simultaneously, and those of the
abdomen somewhat later. At the same time a median depres-

FlG. 241. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS

MURARIUS. (After Bobretzky.)

st. stomodaeum ; pr. proctodseum ; hy. hypoblast formed of large nucleated cells
imbedded in the yolk ; m. mesoblast ; vg. ventral nerve cord ; sg. supra- oesophageal
ganglion ; li. liver ; do. dorsal organ ; zp. rudiment of masticatory apparatus ; ol. upper
lip.

sion appears dividing the blastoderm longitudinally into two
halves. The appendages are formed later than their segments,
and the whole of them are formed nearly simultaneously, with
the exception of the last thoracic, which does not appear till
comparatively late after the hatching of the embryo. The late
development of the seventh thoracic segment and appendage is
a feature common to the majority of the Isopoda (Fritz Miiller).
In Oniscus the limbs are formed in nearly the same way as in
Cymothoa, but in Asellus they do not arise quite simultaneously.
First of all, the two antennae and mandibles (the future palp)
appear, inaugurating a stage often spoken of as the Nauplius
stage, which is supposed to correspond with the free Nauplius

5l8 FORMATION OF THE LAYERS.

stage of Penaeus and Euphausia. At this stage a cuticle is shed
(Van Beneden) which remains as an envelope surrounding the
larva till the time of hatching. Similar cuticular envelopes are
formed in many Isopoda. Subsequently the appendages of the
thorax appear, and finally those of the abdomen. Later than
the appendages there arise behind the mouth two prominences
which resemble appendages, but give rise to a bilobed lower lip
(Dohrn).

In Asellus and Oniscus the ventral plate moulds itself to the
shape of the egg, and covers the greater part of the dorsal as
well as of the ventral side (fig. 241 A). As a result of this the
ventral surface of the embryo is throughout convex ; and in
Asellus a deep fold appears on the back of the embryo, so that
the embryo appears coiled up within the egg with its ventral
side outwards and its head and tail in contact. In Oniscus the
ventral surface is convex, but the dorsal surface is never bent in
as in Asellus. In Cymothoa the egg is very big and the
ventral plate does not extend nearly so far round to the dorsal
side as in Asellus, in consequence of which the ventral surface
is not nearly so convex as in other Isopoda. At the same time
the telson is early formed, and is bent forwards so as to lie
on the under side of the part of the blastoderm in front. In
having this ventral curvature of the telson Cymothoa forms
an exception amongst Isopods ; and in this respect is interme-
diate between the embryos of Asellus and those of the
Amphipoda.

Amphipoda. Amongst the Amphipoda the segmentation
is usually centrolecithal. In the case of Gammarus locusta
(Ed. van Beneden and Bessels, No. 503) it commences with
an unequal but total segmentation like that of the Frog (vide p.
97), and the separation of a central yolk mass is a late occur-
rence ; and it is noticeable that the part of the egg with the
small segments eventually becomes the ventral surface. In the
fresh-water species of Gammarus (G. pulex and fluviatilis) the
segmentation is more like that of Insects ; the blastoderm cells
being formed nearly simultaneously over a large part of the
surface of the egg.

Both forms of segmentation give rise to a blastoderm cover-
ing the whole egg, which soon becomes thickened on the ventral

CRUSTACEA. 519

surface. There is formed, as in the Isopoda, a larval membrane
at about the time when the blastoderm is completed. Very
soon after this the egg loses its spherical shape, and becomes
produced into a pointed extremity — the future abdomen — which
is immediately bent over the ventral surface of the part in front.
The ventral curvature of the hinder part of the embryo at so
early an age stands in marked contrast to the usual condition of
Isopod embryos, and is only approached in this group, so far as
is known, in the case of Cymothoa.

At the formation of the first larval membrane the blastoderm
cells separate themselves from it, except at one part on the
dorsal surface. The patch of cells adherent at this part gives rise
to a dorsal organ, comparable with that in Oniscus, connecting
the embryo and its first larval skin. A perforation appears in it
at a later period.

The segments and limbs of the Amphipoda are all formed
before the larva leaves the egg.

Cladocera. The segmentation (Grobben, No. 455) takes place on the
normal centrolecithal type, but is somewhat unequal. Before the close of
the segmentation there may be seen at the apex of the vegetative pole one
cell marked off from the remainder by its granular aspect. It gives rise
to the generative organs. One of the cells adjoining it gives rise to the
hypoblast, and the other cells which surround it form the commencement
of the mesoblast. The remaining cells of the ovum form the epiblast. By
a later stage the hypoblast cell is divided into thirty-two cells and the genital
cell into four, while the mesoblast forms a circle of twelve cells round the
genital mass.

The hypoblast soon becomes involuted ; the blastopore probably closes,
and the hypoblast forms a solid cord of cells which eventually becomes the
mesenteron. The stomodaeum is said to be formed at the point of closure
of the blastopore. The mesoblast passes inwards and forms a mass ad-
joining the hypoblast, and somewhat later the genital mass also becomes
covered by the epiblast. The proctodseum appears to be formed later than
the stomodasum.

The embryo as first shewn by Dohrn passes through a Nauplius stage
in the brood-pouch, but is hatched, except in the case of the winter eggs of
Leptodora, in a form closely resembling the adult.

Copepoda. Amongst the free Copepoda the segmentation and
formation of the layers have recently been investigated by Hoek (No. 512).
He finds that there is, in both the fresh-water and marine forms studied
by him, a centrolecithal segmentation similar to that of Palaemon and
Pagurus (vide p. 112), which might from the surface be supposed to be

520 FORMATION OF THE LAYERS.

complete and nearly regular. After the formation of the blastoderm an
invagination of some of its cells takes place and is completed in about a
quarter of an hour. The opening becomes closed. This invagination is
compared by Hoek to the invagination in Astacus, and is believed by him
to give rise to the mesenteron. Its point of closing corresponds with the
hind end of the embryo. On the ventral surface there appear two trans-
verse furrows dividing the embryo into three segments, and a median
longitudinal furrow which does not extend to the front end of the foremost
segment. The three pairs of Nauplius appendages and upper lip become
subsequently formed as outgrowths from the sides of the ventral blasto-
dermic thickening.

Amongst the parasitic Copepoda there are found two distinct types of
segmentation, analogous to those in the Isopoda. In the case of Condra-
canthus the segmentation is somewhat irregular, but on the type of Eupa-
gurus, etc. (vide p. 112). In the other group (Anchorella, Clavella, Congeri-
cola, Caligus, Lerneopoda) the segmentation nearly resembles the ordinary
meroblastic type (vide p. 120), and is to be explained in the same manner as
in the cases of Oniscus and Cymothoa. The first blastodermic cells some-
times appear in a position corresponding with the head end of the embryo
(Anchorella), at other times at the hind end (Clavella), and sometimes in the
middle of the ventral surface. The dorsal surface of the yolk is always
the latest to be inclosed by the blastoderm cells. A larval cuticle similar
to that of the Isopoda is formed at the same time as the blastoderm. At
the sides of the ventral thickening of the blastoderm there grow out the
Nauplius appendages, of which only the first two appear in Anchorella.
In Anchorella and Lerneopoda the embryos are not hatched at the
Nauplius stage, but after the Nauplius appendages have been formed
a fresh cuticle — the Nauplius cuticle — is shed, and within it the embryo
develops till it reaches the so-called Cyclops stage (vide p. 490). The
embryo within the egg has its abdomen curved dorsalwards as amongst the
Isopoda.

Cinipedia. The segmentation of Balanus and Lepas commences by
the segregation of the constituents of the egg into a more protoplasmic
portion, and a portion formed mainly of food material. The former sepa-
rates from the latter as a distinct segment, and then divides into two not
quite equal portions. The division of the protoplasmic part of the embryo
continues, and the resulting segments grow round the single yolk segment.
The point where they finally enclose it is situated on the ventral surface
(Lang) at about the position of the mouth (?).

After being enclosed by the protoplasmic cells the yolk divides, and gives
rise to a number of cells, which probably supply the material for the walls of
the mesenteron. The external layer of protoplasm forms the so-called
blastoderm, and soon (Arnold, Lang) becomes thickened on the dorsal
surface.

The embryo is next divided by two constrictions into three segments ;
and there are formed the three appendages corresponding to these, which are

CRUSTACEA. 52!

at first simple. The two posterior soon become biramous. The larva leaves
the egg before any further appendages become formed.

Comparative development of the organs.

Central nervous system. The ventral nerve cord of the
Crustacea develops as a thickening of the epiblast along the
median ventral line ; the differentiation of which commences in
front, and thence extends backwards. The ventral cord is at
first unsegmented. The supra-oesophageal ganglia originate as
thickenings of the epiblast of the procephalic lobes.

The details of the above processes are still in most cases very imper-
fectly known. The fullest account we have is that of Reichenbach (No. 488)
for Astacus. He finds that the supra- cesophageal ganglia and ventral cord
arise as a continuous formation, and not independently as would seem to be
the case in Chsetopoda. The supra-cesophageal ganglia are formed from the
procephalic lobes. The first trace of them is visible in the form of a pair of
pits, one on each side of the middle line. These pits become in the
Nauplius stage very deep, and their walls are then continued into two ridges
where the epiblast is several cells deep, which pass backwards one on each
side of the mouth. The walls of the pits are believed by Reichenbach to
give rise to the optic portions of the supra-cesophageal ganglia, and the
epiblastic ridges to the remainder of the ganglia and to the circum-cesopha-
geal commissures. At a much later stage, when the ambulatory feet have
become formed, a median involution of epiblast in front of the mouth and
between the two epiblast ridges gives rise to a central part of the supra-
cesophageal ganglia. Five elements are thus believed by Reichenbach to be
concerned in the formation of these ganglia, viz. two epiblast pits, two
epiblast ridges, and an involution of epiblast between the latter. It should
be noted however that the fate neither of the pair of pits, nor of the median
involution, appears to have been satisfactorily worked out. The two
epiblast ridges, which pass back from the supra-cesophageal ganglia on
each side of the mouth, are continued as a pair of thickenings of the epiblast
along the sides of a median ventral groove. This groove is deep in front
and shallows out posteriorly. The thickenings on the sides of this groove
no doubt give rise to the lateral halves of the ventral cord, and the cells of
the groove itself are believed by Reichenbach, but it appears to me without
sufficient evidence, to become invaginated also and to assist in forming the
ventral cord. When the ventral cord becomes separated from the epiblast
the two halves of it are united in the middle line, but it is markedly bilobed
in section.

In the Isopoda it would appear both from Bobretzky's and Bullar's
observations that the ventral nerve cord arises as an unpaired thickening of
the epiblast in which there is no trace of anything like a median involution.
After this thickening has become separated from the epiblast a slight

522 DEVELOPMENT OF ORGANS.

median furrow indicates its constitution out of two lateral cords. The
supra-oesophageal ganglia are stated to be developed quite simply as a pair
of thickenings of the procephalic lobes, but whether they are from the
first continuous with the ventral cord does not appear to have been deter-
mined.

The later stages in the differentiation of the ventral cord are,
so far as is known, very similar throughout the Crustacea. The
ventral cord is, as has been stated, at first unsegmented (fig. 241
A, vg\ but soon becomes divided by a series of constrictions into
as many ganglia as there are pairs of appendages or segments
(fig. 241 B, vg).

There appears either on the ventral side (Oniscus) or in the
centre (Astacus, Palaemon) of the two halves of each segment or
ganglion a space filled with finely punctuated material, which is
the commencement of the commissural portion of the cords.
The commissural tissue soon becomes continuous through the
length of the ventral cord, and is also prolonged into the supra-
cesophageal ganglia.

After the formation of the commissural tissue the remaining
cells of the cord form the true ganglion cells. A gradual
separation of the ganglia next takes place, and the cells become
confined to the ganglia, which are finally only connected by a
double band of commissural tissue. The commissural tissue not
only gives rise to the longitudinal cords connecting the successive
ganglia, but also to the transverse commissures which unite the
two halves of the individual ganglia.

The ganglia usually, if not always, appear at first to corre-
spond in number with the segments, and the smaller number so
often present in the adult is due to the coalescence of originally
distinct ganglia.

Organs of special sense. Comparatively little is known on
this head. The compound eyes are developed from the coales-
cence of two structures, both however epiblastic, viz. (i) part of
the superficial epiblast of the procephalic lobes ; (2) part of the
supra-cesophageal ganglia. The former gives rise to the corneal
lenses, the crystalline cones, and the pigment surrounding
them ; the latter to the rhabdoms and the cells which encircle
them. Between these two parts a mesoblastic pigment is inter-
posed.

CRUSTACEA.

523

Of the development of the auditory and olfactory organs
almost nothing is known.

Dorsal organ. In a considerable number of the Malacostraca
and Branchiopoda a peculiar organ is developed from the epiblast
in the anterior dorsal region. This organ has been called the
dorsal organ. It appears to be of a glandular nature, and is
usually very large in the embryo or larva and disappears in the
adult ; but in some Branchiopoda it persists through life. In
most cases it is unpaired, but in some instances a paired organ
appears to take its place.

Various views as to its nature have been put forward. There
is but little doubt of its being glandular, and it is possible that it
is a provisional renal organ, though so far as I know concretions
have not yet been found in it.

Its development has been most fully studied in the Isopoda.

In Cymothoa (Bullar, No. 499) there appears on the dorsal surface, in the
region which afterwards becomes the first thoracic segment, an unpaired
linear thickening of the blastoderm. This soon becomes a circular patch,
the central part of which is inva-
ginated so as to communicate
with the exterior by a narrow
opening only (fig. 242). It be-
comes at the same time attached
to the inner egg membrane. It
retains this condition till the close
of larval life.

In Oniscus (Dohrn, No. 500 ;
Bobretzky, No. 498) there appears
very early a dorsal patch of thick-
ened cells. These cells become
attached at their edge to the
inner egg membrane and gradu-
ally separated from the embryo,
with which they finally only re- , FlG- W- DIAGRAMMATIC SECTION OF
. , ... CYMOTHOA SHEWING THE DORSAL ORGAN.
main in connection by a hollow (From Bullar.)

column of cells (fig. 241 A, do).

The original patch now gradually spreads over the inner egg membrane, and
forms a transverse saddle-shaped band of flattened cells which engirths the
embryo on all but the ventral side.

In the Amphipods the epiblast cells remain attached for a small area on
the dorsal surface to the first larval skin, when this is formed. This patch
of cells, often spoken of as a micropyle apparatus, forms a dorsal organ
equivalent to that in Oniscus. A perforation is formed in it at a later

524

DEVELOPMENT OF ORGANS.

period. A perhaps homologous structure is found in the embryos of Euphau-
sia, Cuma, etc.

In many Branchiopoda a dorsal organ is found. Its development has
been studied by Grob-
ben in Moina. It
persists in the adult
in Branchipus, Lim-
nadia, Estherea, etc.

In the Copepoda
a dorsal organ is
sometimes found in
the embryo ; Grob-
ben at any rate be-
lieves that he has
detected an organ of
this nature in the
embryo of Cyclops
serrulatus.

A paired organ
which appears to be

FIG. 243. DIAGRAMMATIC SECTION OF AN EMBRYO
OF ASELLUS AQUATICUS TO SHEW THE PAIRED DORSAL
ORGAN. (From Bullar ; after E. van Beneden.)

of the same nature
has been found in
Asellus and Mysis.

In Asellus (Rathke (No. 501), Dohrn (No. 500), Van Beneden (No. 497))
this organ originates as two cellular masses at the sides of the body just
behind the region of the procephalic lobes. Each of them becomes trifoliate
and bends towards the ventral surface. In each of their lobes a cavity
arises and finally the three cavities unite, forming a trilobed cavity open to
the yolk. This organ eventually becomes so large that it breaks through the
egg membranes and projects at the sides of the embryo (fig. 243\ Though
formed before the appendages it does not attain its full development till
considerably after the latter have become well established.

In Mysis it appears during the Nauplius stage as a pair of cavities lined
by columnar cells, which atrophy very early.

Various attempts have been made to identify organs in other Arthropod
embryos with the dorsal organ of the Crustacea, but the only organ at all
similar which has so far been described is one found in the embryo of Lingu-
atula (vide Chapter XIX.), but there is no reason to think that this organ is
really homologous with the dorsal organ of the Crustacea.

The mesoblast. The mesoblast in the types so far investi-
gated arises from the same cells as the hypoblast, and appears
as a somewhat irregular layer between the epiblast and the
hypoblast. It gives rise to the same parts as in other forms, but
it is remarkable that it does not, in most Decapods and Isopods

CRUSTACEA. 525

(and so far we do not know about other forms), become divided
into somites, at any rate with the same distinctness that is usual
in Annelids and Arthropods. Not only so, but there is at first
no marked division into a somatic and splanchnic layer with an
intervening body cavity. Some of the cells become differentiated
into the muscles of the body wall and limbs ; and other cells,
usually in the form of a very thin layer, into the muscles of the
alimentary tract. In the tail of Palcsmon Bobretzky noticed
that the cells about to form the muscles of the body were
imperfectly divided into cubical masses corresponding with the
segments ; which however, in the absence of a central cavity,
differed from typical mesoblastic somites. In Mysis Metschni-
koff states that the mesoblast becomes broken up into distinct
somites. Further investigations on this subject are required.
The body cavity has the form of irregular blood sinuses amongst
the internal organs.

Heart. The origin and development of the heart and vascular system
are but very imperfectly known.

In Phyllopods (Branchipus) Claus (No. 454) has shewn that the heart is
formed by the coalescence of the lateral parts of the mesoblast of the ventral
plates. The chambers are formed successively as the segments to which
they belong are established, and the anterior chambers are in full activity
while the posterior are not yet formed.

In Astacus and Palaemon, Bobretzky finds that at the stage before the
heart definitely appears there may be seen a solid mass of mesoblast cells
in the position which it eventually occupies1; and considers it probable that
the heart originates from this mass. At the time when the heart can first
be made out and before it has begun to beat, it has the form of an oval sack
with delicate walls separated from the mesenteron by a layer of splanchnic
mesoblast. Its cavity is filled with a peculiar plasma which also fills up the
various cavities in the mesoblast. Around it a pericardial sack is soon
formed, and the walls of the heart become greatly thickened. Four bands
pass off from the heart, two dorsalwards which become fixed to the
integument, and two ventralwards. There is also a median band of cells
connecting the heart with the dorsal integument. The main arteries arise
as direct prolongations of the heart. Dohrn's observations on Asellus
greatly strengthen the view that the heart originates from a solid meso-
blastic mass, in that he was able to observe the hollowing out of the mass in

1 Reichenbach describes these cells, and states that there is a thickening of the
epiblast adjoining them. In one place he states that the heart arises from this thicken-
ing of epiblast, and in another that it arises from the mesoblast. An epiblastic origin
of the heart is extremely improbable.

526 DEVELOPMENT OF ORGANS.

the living embryo (cf. the development of the heart in Spiders). Some of the
central cells (nuclei, Dohrn) become blood corpuscles. The formation of
these is not, according to Dohrn, confined to the heart, but takes place in
situ in all the parts of the body (antennae, appendages, etc.). The corpuscles
are formed as free nuclei and are primarily derived from the yolk, which at
first freely communicates with the cavities of the appendages.

Alimentary tract. In Astacus the formation of the mesenteron by
invagination, and the absorption of the yolk by the hypoblast cells, have
already been described. On the absorption of the yolk the mesenteron has
the form of a sack, the walls of which are formed of immensely long cells —
the yolk pyramids — at the base of which the nucleus is placed (fig. 238 B).
This sack gives rise both to the portion of the alimentary canal between the
abdomen and the stomach and to the liver. The epithelial wall of both of
these parts is formed by the outermost portions of the pyramids with the
nuclei and protoplasm becoming separated off from the yolk as a layer of
flat epithelial cells. The yolk then breaks up and forms a mass of nutritive
material filling up the cavity of the mesenteron.

The differentiation both of the liver and alimentary tract proper first
takes place on the ventral side, and commences close to the point where the
proctodasum ends, and extends forward from this point. A layer of epithelial
cells is thus formed on the ventral side of the mesenteron which very soon
becomes raised into a series of longitudinal folds, one of which in the
middle line is very conspicuous. The median fold eventually, by uniting
with a corresponding fold on the dorsal side, gives rise to the true mesente-
ron ; while the lateral folds form parallel hepatic cylinders, which in front
are not constricted off from the alimentary tract. The lateral parts of the
dorsal side of the mesenteron similarly give rise to hepatic cylinders. The
yolk pyramids of the anterior part of the mesenteron, which projects
forwards as a pair of diverticula on each side to the level of the stomach, are
not converted into hepatic cylinders till after the larva is hatched.

The proctodasum very early opens into the mesenteron, but the stomo-
daeum remains closed till the differentiation of the mid-gut is nearly
completed. The proctodaeum gives rise to the abdominal part of the intes-
tine, and the stomodaeum to the oesophagus and stomach. The commence-
ment of the masticatory apparatus in the latter appears very early as a
dorsal thickening of the epithelium.

The primitive mesenteron in Palaemon differentiates itself into the
permanent mid-gut and liver in a manner generally similar to that in
Astacus, though the process is considerably less complicated. A distinct
layer of cells separates itself from the outer part of the yolk pyramids,
and gives rise to the glandular lining both of the mid-gut and of the liver.
The differentiation of this layer commences behind, and the mid-gut very
soon communicates freely with the proctodasum. The lateral parts of
the primitive mesenteron become constricted into four wings, two directed
forwards and two backwards ; these, after the yolk in them has become
absorbed, constitute the liver. The median part simply becomes the me-

CRUSTACEA. 527

senteron. The stomachic end of the stomodaeum lies in contact with the
mesenteron close to the point where it is continued into the hepatic
diverticula, and, though the partition-wall between the two becomes early
very thin, a free communication is not established till the yolk has been
completely absorbed.

The alimentary tract in the Isopoda is mainly if not entirely formed
from the proctodaeum and stomodaeum, both of which arise before any other
part of the alimentary system as epiblastic invaginations, and gradually
grow inwards (fig. 244). In Oniscus the liver is formed as two discs
at the surface of the yolk on each side of the anterior part of the body.
Their walls are composed of cubical cells derived from the yolk cells, the

pr

sr "aqcagga»w.rt-j_ .-. f.i~T':. -^a^Mi^ • . - .. >va^^^

Vff

FlG. 244. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS

MURARIUS. (After Bobretzky.)

st. stomodaeum ; pr. proctodseum ; hy. hypoblast formed of large nucleated cells
imbedded in yolk ; m. mesoblast ; vg. ventral nerve cord ; jr^. supra- oesophageal gan-
glion ; li. liver; do. dorsal organ; zp. rudiment of masticatory apparatus.

origin of which was spoken of on p. 516. These two discs gradually take
the form of sacks (fig. 244 B, li.) freely open on their inner side to the
yolk. As these sacks continue to grow the stomodaeum and proctodaeum
do not remain passive. The stomodaeum, which gives rise to the oesophagus
and stomach of the adult, soon exhibits a posterior dilatation destined to
become the stomach, on the dorsal wall of which a well-marked prominence
— the earliest trace of the future armature — is soon formed (fig. 244 B,
xp}. The proctodaeum (pr) grows with much greater rapidity than the
stomodaeum, and its end adjoining the yolk becomes extremely thin or even
broken through. In the earliest stages it was surrounded by the yolk cells,
but in its later growth the yolk cells become gradually reduced in number
and appear to recede before it — so much so that one is led to conclude
that the later growth of the proctodaeum takes place at the expense of the
yolk cells.

The liver sacks become filled with a granular material without a trace
of cells ; their posterior wall is continuous with the yolk cells, and their
anterior lies close behind the stomach. The proctodaeum continually
grows forwards till it approaches close to the stomodaeum, and the two

528 DEVELOPMENT OF ORGANS.

liver sacks, now united into one at their base, become directly continuous
with the proctodaeum. By the stage when this junction is effected the yolk
cells have completely disappeared. It seems then that in Oniscus the yolk
cells (hypoblast) are mainly employed in giving rise to the walls of the
liver ; but that they probably also supply the material for the later growth
of the apparent proctodaeum. It becomes therefore necessary to conclude
that the latter, which might seem, together with the stomodasum, to form
the whole alimentary tract, does in reality correspond to the proctodaeum
and mesenteron together, though the digestive fluids are no doubt mainly
secreted not in the mesenteron but in the hepatic diverticula. The procto-
daeum and stomodaeum at first meet each other without communicating, but
before long the partition between the two is broken through.

In Cymothoa (Bullar, No. 499) the proctodaeum and stomodaeum
develop in the same manner as in Oniscus, but the hypoblast has quite
a different form. The main mass of the yolk, which is much greater than
in Oniscus, is not contained in definite yolk cells, but the hypoblast is
represented by (i) two solid masses of cells, derived apparently from the
inner layer of blastoderm cells, which give rise to the liver ; and (2) by a
membrane enclosing the yolk in which nuclei are present.

The two hepatic masses lie on the surface of the yolk, and each of them
becomes divided into three short caecal tubes freely open to the yolk.
The stomodaeum soon reaches its full length, but the proctodaeum grows
forwards above the yolk till it meets the stomodaeum. By the time this
takes place the liver caeca have grown into three large tubes filled with
fluid, and provided with a muscular wall. They now lie above the yolk,
and no longer communicate directly with the cavity of the yolk sack,
but open together with the yolk sack into the point of junction of the
proctodaeum and stomodaeum. The yolk sack of Cymothoa no doubt
represents part of the mesenteron, but there is no evidence in favour of
any part of the apparent proctodaeum representing it also, though it is
quite possible that it may do so. The relations of the yolk sack and hepatic
diverticula in Cymothoa appear to hold good for Asellus and probably for
most Isopoda.

The differences between the Decapods and Isopods in the development
of the mesenteron are not inconsiderable, but they are probably to be
explained by the relatively larger amount of food yolk in the latter forms.
The solid yolk in the Isopods on this view represents the primitive mesen-
teron of Decapods after the yolk has been absorbed by the hypoblast cells.
Starting from this standpoint we find that in both groups the lateral parts of
the mesenteron become the liver. In Decapods the middle part becomes
directly converted into the mid-gut, the differentiation of it commencing
behind and proceeding forwards. In the Isopods, owing to the mesenteron
not having a distinct cavity, the differentiation of it, which proceeds forwards
as in Decapods, appears simply like a prolongation forwards of the procto-
da?um, the cells for the prolongation being probably supplied from the yolk.
In Cymothoa the food yolk is so bulky that a special yolk sack is developed

CRUSTACEA.

529

for its retention, which is not completely absorbed till some time after the
alimentary canal has the form of a continuous tube. The walls of this yolk
sack are morphologically a specially developed part of the mesenteron.

BIBLIOGRAPHY.

General Works.

(447) C. Spence Bate. " Report on the present state of our knowledge of the
Crustacea." Report of the British Association for 1878.

(448) C. Claus. Untersuchungen zur Erforschung der genealogischen Grundlage
des Crustaceen- Systems. Wien, 1876.

(449) A. Dohrn. "Geschichte des Krebsstammes. " Jenaische Zeitschrift,
Vol. VI. 1871.

(450) A. Gerstaecker. Bronris Thierreich, Bd. v. Arthropoda, 1866.

(451) Th. H. Huxley. The Anatomy of Invertebrated Animals. London,
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(452) Fritz Mliller. Fur Darwin, 1864. Translation, Facts for Darwin.
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Branchiopoda.

(453) Brauer. "Vorlaufige Mittheilung iiber die Entwicklung u. Lebensweise
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(455) C. Grobben. "Zur Entwicklungsgeschichte d. Moina rectirostris."
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(456) E. Grube. " Bemerkungen uber die Phyllopoden nebst einer Uebersicht
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(457) N. Joly. " Histoire d'un petit Crustace (Artemia salina, Leach} etc." An-
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(458) N. Joly. " Recherches zoologiques anatomiques et physiologiques sur
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(459) Lereboullet. " Observations sur la generation et le developpement de la
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(460) F. Leydig. " Ueber Artemia salina u. Branchipus stagnalis." Zeit. f.
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(461) G. O. Sars. " Om en dimorph Udvikling samt Generationsvexel hos
Leptodora." Vidensk. Selskab. Forhand, 1873.

(462) G. Zaddach. De apodis cancreformis Schaeff. anatome et historia evolu-
tionis. Dissertatio inanguralis zootomica. Bonnse, 1841.

Nebaliadce.

(463) C. Claus. " Ueber den Bau u. die systematische Stellung von Nebalia."
Zeit.f. wiss. Zool., Bd. xxn. 1872.

(464) E. Metschnikoff. Development of Nebalia (Russian), 1868.

B. II. 34

530 BIBLIOGRAPHY.

Schizopoda.

(465) E. van Beneden, " Recherches sur 1'Embryogenie des Crustaces. n.
DeVeloppement des Mysis." Bullet, de rAcadtmie roy. de Belgique, second series,
Tom. xxvin. 1869.

(46G) C. Glaus. " Ueber einige Schizopoden u. niedere Malakostraken." Zett.
f. wiss. Zoologie, Bd. XII I., 1863.

(467) A. Dohrn. " Untersuchungen Ub. Bau u. Entwicklung d. Arthropoden."
Zeit.f. wiss. Zool.y Bd. XXL, 1871, .p. 375. Peneus zoaea (larva of Euphausia).

(468) E. Metschnikoff. " Ueber ein Larvenstadium von Euphausia." Zeit.
fiir wiss. Zool., Bd. xix., 1869.

(469) E. Metschnikoff. " Ueber den Naupliuszustand von Euphausia. " Zeit.
fiir wiss. Zool., Bd. XXI., 1871.

Decapoda.

(470) S pence Bate. "On the development of Decapod Crustacea." Phil.
Trans., 1858.

(471) Spence Bate. " On the development of Pagurus." Ann. and Mag. Nat.
History, Series 4, Vol. II., 1868.

(472) N. Bobretzky. Development of Astacus and Palamon. Kiew, 1873.
(Russian.)

(473) C. Glaus. "Zur Kenntniss d. Malakostrakenlarven. " Wiirzb. naturw.
Zeitschrift, 1861.

(474) R. Q. Couch. "On the Metamorphosis of the Decapod Crustaceans."
Report Cornwall Polyt. Society. 1848.

(475) Du Cane. "On the Metamorphosis of Crustacea." Ann. and Mag. of
Nat. History, 1839.

(476) Walter Faxon. " On the development of Palsemonetes vulgaris." Bull,
of the Mus. of Camp. Anat. Harvard, Cambridge, Mass., Vol. v., 1879.

(477) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden."
" Zur Entwicklungsgeschichte der Panzerkrebse. Scyllarus Palinurus." Zeit. f.
wiss. Zool., Bd. xx., 1870.

(478) A. Dohrn. "Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Erster Beitrag z. Kenntniss d. Malacostraken u. ihrer Larven Amphion Reynaudi,
Lophogaster, Portunus, Porcellanus, Elaphocaris. " Zeit. f. wiss. Zool., Bd. xx.,
1870.

(479) A. Dohrn. "Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Zweiter Beitrag, etc." Zeit.f. wiss. Zool., Bd. xxi., 1871.

(480) N. Joly. " Sur la Caridina Desmarestii." Ann. Scien. Nat., Tom. xix.,
1843.

(481) Lereboullet. " Recherches d . 1'embryologie comparee sur le developpement
du Brochet, de la Perche et de 1'Ecrevisse." Mem. Savans ktrang. Paris, Vol. xvn.,
1862.

(482) P. Mayer. "Zur Entwicklungsgeschichte d. Dekapoden." Jenaische
Zeitschrift, Vol. XI., 1877.

(483) F r i t z M u 1 1 e r. " Die Verwandlung der Porcellana." Archivf. Natnrge-
schichte, 1862.

CRUSTACEA. 531

(484) Fritz Muller. " Die Verwandlungen d. Garneelen," Archiv f. Natur-
gesch., Tom. xxix.

(485) Fritz Muller. " Ueber die Naupliusbrut d. Garneelen." Zeit f. wiss.
Zool., Bd. xxx., 1878.

(486) T. J. Parker. "An account of Reichenbach's researches on the early
development of the Fresh-water Crayfish." Quart. J. of M. Science, Vol. xvin.,
1878.

(487) H. Rathke. Ueber die Bildung u. Entivicklung d. Flusskrebses. Leip-
zig, 1829.

(488) H. Reichenbach. " Die Embryoanlage u. erste Entwicklung d. Fluss-
krebses." Zeit.f. wiss. Zool., Vol. xxix., 1877.

(489) F. Richters. " Ein Beitrag zur Entwicklungsgeschichte d. Loricaten."
Zeit.f. wiss. Zool., Bd. xxiil., 1873.

(490) G. O. Sars. " Om Hummers posiembryonale Udvikling. " Vidensk Selsk.
Fork. Christiania, 1874.

(491) Sidney J. Smith. " The early stages of the American Lobster. " Trans,
of the Connecticut Acad. of Arts and Sciences, Vol. n., Part 2, 1873.

(492) R. v. Willemoes Suhm. " Preliminary note on the development of some
pelagic Decapoda." Proc. of Royal Society, 1876.

Stomatopoda.

(493) W. K. Brooks. " On the larval stages of Squilla empusa." Chesapeake
Zoological Laboratory, Scientific results of the Session of 1878. Baltimore, 1879-

(494) C. Claus. "Die Metamorphose der Squilliden." Abhand. der kbnigl.
Gesell. der Wiss. zu Gbttingen, 1871.

(495) Fr. Muller. " Bruchstuck a. der Entwicklungsgeschichte d. Maulfusser I.
und II." Archiv f. Naturgeschichte, Vol. xxvin., 1862, and Vol. xxix., 1863.

Cumacea.

(496) A. Dohrn. " Ueber den Bau u. Entwicklung d. Cumaceen." Jenaische
Zeitschrift, Vol. v., 1870.

Isopoda.

(497) Ed. van Beneden. " Recherches sur 1'Embryogenie des Crustaces. I.
Asellus aquaticus." Bull, de FAcad. roy Belgique, 2me serie, Tom. xxvni., No. 7,
1869.

(498) N. Bobretzky. " Zur Embryologie des Oniscus murarius." Zeit. fur
wiss. Zool., Bd. xxiv., 1874.

(499) J. F. Bullar. "On the development of the parasitic Isopoda." Phil.
Trans., Part II., 1878.

(500) A. Dohrn. " Die embryonale Entwicklung des Asellus aquaticus." Zeit.
f. wiss. Zool., Vol. xvn., 1867.

(501) H. Rathke. Untersuchungen iiber die Bildung tmd Entwicklung der
Wasser-Assel. Leipzig, 1832.

(502) H. Rathke. Zur Morphologic. Reisebemerkungen aus Taurien. Riga u.
Leipzig, 1837. (Bopyrus, Idothea, Ligia, lanira.)

34—2

532 BIBLIOGRAPHY.

A mphipoda.

(503) Ed. van Beneden and E. Bessels. "M&noire sur la formation du
blastoderme chez les Amphipodes, les Lerneens et les Cope"podes." Classe des Sciences
deTAcad. roy. de Belgique, Vol. xxxiv., 1868.

(504) De la Valletta St George. " Studien iiber die Entwicklung der Amphi-
poden." Abhand. d. naturfor. Gesell. zu Halle, Bd. v., 1860.

Copepoda.

(505) E. van Beneden and E. Bessels. " Me*moire sur la formation du blas-
toderme chez les Amphipodes, les Lerndens et Copepodes." Classe des Sciences de
FAcad. roy. de Belgique, Vol. xxxiv., 1868.

(506) E. van Beneden. " Recherches sur 1'Embryogenie des Crustaces iv. An-
chorella, Lerneopoda, Branchiella, Hessia." Bull, de FAcad. roy. de Belgique, 2me
serie, T. xxix., 1870.

(507) C. Claus. Zur Anatomie u. Entwicklungsgeschichte d. Copepoden.

(508) C. Claus. " Untersuchungen Uber die Organisation u. Verwandschaft d.
Copepoden." Wiirzburger naturwiss. Zeitschrift, Bd. III., 1862.

(509) C. Claus. " Ueber den Bau u. d. Entwicklung von Achtheres percarum."
Zeit.f. wiss. Zool., Bd. XL, 1862.

(510) C. Claus. Die freilebenden Copepoden mit besonderer Berucksichtigung der
Fauna Deutschlands, des Nordsee u. des Mittelmeeres. Leipzig, 1863.

(511) C. Claus. " Ueber d. Entwicklung, Organisation u. systematische Stellung
d. Argulidse." Zeit.f. wiss. Zool., Bd. xxv., 1875.

(512) P. P. C. Hoek. " Zur Entwicklungsgeschichte d. Entomostracen." Nie-
derldndisches Archiv, Vol. IV., 1877.

(513) N o r d m a n n. Mikrographische Beitrdge zur Naturgeschichte der ivirbellosen
l^hiere. Zweites Heft. 1832.

(514) Salensky. " Sphseronella Leuckartii." Archivf. Naturgeschichte, 1868.

(515) F. Vejdovsky. "Untersuchungen Ub. d. Anat. u. Metamorph. v. Trache-
liastes polycolpus." Zeit.f. wiss. Zool., Vol. xxix., 1877.

Cirripedia.

(516) C. Spence Bate. "On the development of the Cirripedia." Annals
and Mag. of Natur. History. Second Series, Vin., 1851.

(517) E. van Beneden. " DeVeloppement des Sacculines." Bull, de I" Acad.
roy. de Belg., 1870.

(518) C. Claus. Die Cypris-dhnliche Larve der Cifripedien. Marburg, 1869.

(519) Ch. Darwin. A monograph of the sub-class Cirripedia, i Vols., Ray
Society, 1851—4.

(520) A. Dohrn. •' Untersuchungen iiber Bau u. Entwicklung d. Arthropoden
ix. Eine neue Naupliusform (Archizoea gigas)." Zeit. f. wiss. Zool., Bd. xx.,
1870.

(521) P. P. C. Hoek. "Zur Entwicklungsgeschichte der Entomostraken i.
Kinbryologie von Balanus." Niederldndisches Archiv fur Zoologie, Vol. III., 1876 — 7.

(522) R. Kossmann. "Suctoria u. Lepadidoc." Arbeiten a. d. zool.-zoot. Insti-
tuted. Univer. Wiirz., Vol. I., 1873.

CRUSTACEA. 533

(523) Aug. Krohn. " Beobachtungen iiber die Entwicklung der Cirripedien."
Wiegmanris Archiv fur Naturgesch., xxvi., 1860.

(524) E. Metschnikoff. Sitzungsberichte d. Versammlung deutscher Naturfors-
cher zu Hannover, 1865. (Balanus balanoides.)

(525) Fritz Muller. "Die Rhizocephalen." Archiv f. Naturgeschichte,
1862-3.

(526) F. C. Noll. " Kochlorine hamata, ein bohrendes Cirriped." Zeit.f. wiss.
Zool., Bd. xxv., 1875.

(527) A. Pagenstecher. " Beitrage zur Anatomic und Entwicklungsgeschichte
von Lepas pectinata." Zeit.f. wiss. ZooL, Vol. xni., 1863.

(528) J. V. Thompson. Zoological Researches and Illustrations, Vol. I., Part I.
Memoir IV. On the Cirripedes or Barnacles. 8vo. Cork, 1830.

(529) J. V. Thompson. " Discovery of the Metamorphosis in the second type
of the Cirripedes, viz. the Lepades completing the natural history of these singular
animals, and confirming their affinity with the Crustacea." Phil. Trans. 1835. Part
n.

(530) R. von Willemoes Suhm. "On the development of Lepas fascicularis."
Phil. Trans., Vol. 166, 1876.

Ostracoda.

(531) C. Glaus. " Zur naheren Kenntniss der Jugendformen von Cypris ovum."
Zeit.f. wiss. ZooL, Bd. xv., 1865.

(532) C. Glaus. "Beitrage zur Kenntniss d. Ostracoden. Entwicklungsges-
chichte von Cypris ovum." Schriften d. Gesell. zur Befdrderung d. gesamm. Natur-
wiss. zu Marburg, Vol. IX., 1868.

CHAPTER XIX.

PCECILOPODA, PYCNOGONIDA, TARDIGRADA, AND LIN-
GUATULIDA; AND COMPARATIVE SUMMARY OF
ARTHROPODAN DEVELOPMENT.

THE groups dealt with in the present Chapter undoubtedly
belong to the Arthropoda. They are not closely related, and in
the case of each group it is still uncertain with which of the
main phyla they should be united. It is possible that they may
all be offshoots from the Arachnidan phylum.

PCECILOPODA.

The development of Limulus has been studied by Dohrn (No. 533) and
Packard (No. 534). The ova are laid in the sand near the spring-tide
marks. They are enveloped in a thick chorion formed of several layers ;
and (during the later stages of development at any rate) there is a mem-
brane within the chorion which exhibits clear indications of cell outlines1.

There is a centrolecithal segmentation, which ends in the formation of
a blastoderm enclosing a central yolk mass. A ventral plate is then
formed, which is thicker in the region where the abdomen is eventually
developed. Six segments soon become faintly indicated in the cephalo-
thoracic region, the ends of which grow out into prominent appendages
(fig. 245 A) ; of these there are six pairs, which increase in size from before
backwards. A stomodaeum (m) is by this time established and is placed well
in front of the foremost pair of appendages'*-.

In the course of the next few days the two first appendages of the
abdominal region become formed (vide fig. 245 C shewing those abdominal
appendages at a later stage), and have a very different shape and direction
to those of the cephalothorax. The appendages of the latter become

1 The nature of the inner membrane is obscure. It is believed by Packard to be
moulted after the formation of the limbs, and to be equivalent to the amnion of Insects,
while by Dohrn it is regarded as a product of the follicle cells.

2 Dohrn finds at first only five appendages, but thinks that the sixth (the anterior
one) may have been present but invisible.

PCECILOPODA.

535

flexed in the middle in such a way that their ends become directed towards
the median line (fig. 245 B). The body of the embryo (fig. 245 B) is
now distinctly divided into two regions— the cephalothoracic in front, and
the abdominal behind, both divided into segments.

FIG. 245. THREE STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS.
(Somewhat modified from Packard.)

A. Embryo in which the thoracic limbs and mouth have become developed on
the ventral plate. The outer line represents what Packard believes to be the amnion.

B. Later embryo from the ventral surface.

C. Later embryo, just before the splitting of the chorion from the side. The full
number of segments of the abdomen, and three abdominal appendages, have become
established ; m. mouth ; I — IX. appendages.

Round the edge of the ventral plate there is a distinct ridge — the
rudiment of the cephalothoracic shield.

With the further growth of the embryo the chorion becomes split
and cast off, the embryo being left enclosed within the inner membrane.
The embryo has a decided ventral flexure, and the abdominal region
grows greatly and forms a kind of cap at the hinder end, while its
vaulted dorsal side becomes divided into segments (fig. 245 C). Of these
there are according to Dohrn seven, but according to Packard nine, of
which the last forms the rudiment of the caudal spine.

In the thoracic region the nervous system is by this stage formed as
a ganglionated cord (Dohrn), with no resemblance to the peculiar cesopha-
geal ring of the adult. The mouth is stated by Dohrn to lie between the
second pair of limbs, so that, if the descriptions we have are correct, it must
have by this stage changed its position with reference to the appendages.
Between the thorax and abdomen two papillae have arisen which form the

536

PCEC1LOPODA.

so-called lower lip of the adult, but from their position and late development
they can hardly be regarded as segmental appendages. In the course of
further changes all the parts become more distinct, while the membrane in
which the larva is placed becomes enormously distended (fig. 246 A). The
rudiments of the compound eyes are formed on the third (Packard) or fourth
(Dohrn) segment of the cephalothorax, and the simple eyes near the median
line in front. The rudiments of the inner process of the chelae of the cepha-
lothoracic appendages arise as buds. The abdominal appendages become
more plate-like, and the rudiments of a third pair appear behind the two
already present. The heart appears on the dorsal surface.

An ecdysis now takes place, and in the stage following the limbs have
approached far more closely to their adult state (fig. 246 A). The
cephalothoracic appendages become fully jointed ; the two anterior ab-
dominal appendages (vn.) have approached, and begin to resemble the oper-

ce.

VIII

FlO. 246. TWO STAGES IN THE DEVELOPMENT OF LlMULUS POLYPHEMUS.

(After Dohrn.)

A. An advanced embryo enveloped in the distended inner membrane shortly
before hatching ; from the ventral side.

B. A later embryo at the Trilobite stage, from the dorsal side.
I., vii., VIII. First, seventh, and eight appendages.

cs. caudal spine ; se. simple eye ; ce. compound eye.

culum of the adult, and on the second pair is formed a small inner ramus.
The segmentation of the now vaulted cephalothorax becomes less obvious,
though still indicated by the arrangement of the yolk masses which form
the future hepatic diverticula.

Shortly after this stage the embryo is hatched, and at about the time of
hatching acquires a form (fig. 246 B) in which it bears, as pointed out by
Dohrn and Packard, the most striking resemblance to a Trilobite.

Viewed from the dorsal surface (fig. 246 B) it is divided into two
distinct regions, the cephalothoracic in front and the abdominal behind.
The cephalothoracic has become much flatter and wider, has lost all trace
of its previous segmentation, and has become distinctly trilobed. The

PCECILOPODA. 537

central lobe forms a well-marked keel, and at the line of insertion of the
rim-like edge of the lateral lobes are placed the two pairs of eyes (se and
ce). The abdominal region is also distinctly trilobed and divided into nine
segments ; the last, which is merely formed of a median process, being the
rudiment of the caudal spine. The edges of the second to the seventh are
armed with a spine. The changes in the appendages are not very con-
siderable. The anterior pair nearly meet in the middle line in front or
the mouth ; and the latter structure is completely covered by an upper
lip. Each abdominal appendage of the second pair is provided with four
gill-lamellas, attached close to its base.

Three weeks after hatching an ecdysis takes place, and the larva passes
from a trilobite into a limuloid form. The segmentation of the abdomen
has become much less obvious, and this part of the embryo closely resem-
bles its permanent form. The caudal spine is longer, but is still relatively
short. A fourth pair of abdominal appendages is established, and the first
pair have partially coalesced, while the second and third pairs have become
jointed, their outer ramus containing four and their inner three joints.
Additional gill-lamellae attached to the two basal joints of the second and
third abdominal appendages have appeared.

The further changes are not of great importance. They are effected in
a series of successive moults. The young larvae swim actively at the
surface.

Our, in many respects, imperfect knowledge of the development of
Limulus is not sufficient to shew whether it is more closely related to the
Crustacea or to the Arachnida, or is an independent phylum.

The somewhat Crustacean character of biramous abdominal feet, etc.
is not to be denied, but at the same time the characters of the embryo
appear to me to be decidedly more arachnidan than crustacean. The
embryo, when the appendages are first formed, has a decidedly arach-
nidan facies. It will be remembered that when the limbs are first formed
they are all post-oral. They resemble in this respect the limbs of the
Arachnida, and it seems to be probable that the anterior pair is equivalent
to the cheliceras of Arachnida, which, as shewn in a previous section, are
really post-oral appendages in no way homologous with antennae1.

The six thoracic appendages may thus be compared with the six
Arachnidan appendages; which they resemble in their relation to the
mouth, their basal cutting blades, etc.

The existence of abdominal appendages behind the six cephalothoracic
does not militate against the Arachnidan affinities of Limulus, because in
the Arachnida rudimentary abdominal appendages are always present in
the embryo. The character of the abdominal appendages is probably

1 Dohrn believes that he has succeeded in shewing that the first pair of appendages
of Limulus is innervated in the embryo from the supra-cesophageal ganglia. His
observations do not appear to me conclusive, and, arguing from what we know of the
development of the Arachnida, the innervation of these appendages in the adult can be
of no morphological importance.

538 PYCNOGONIDA.

secondarily adapted to an aquatic respiration, since it is likely (for the
reasons already mentioned in connection with the Tracheata) that if Limulus
has any affinities with the stock of the Tracheata it is descended from air-
breathing forms, and has acquired its aquatic mode of respiration. The
anastomosis of the two halves of the generative glands is an Arachnidan
character, and the position of the generative openings in Limulus is more
like that in the Scorpion than in Crustacea.

A fuller study of the development would be very likely to throw
further light on the affinities of Limulus, and if Packard's view about the
nature of the inner egg membrane were to be confirmed, strong evidence
would thereby be produced in favour of the Arachnidan affinities.

(533) A. Dohrn. "Untersuch. Ub. Bau u. Entwick. d. Arthropoden (Limulus
polyphemus)." Jenaische Zeitschrift, Vol. vi., 1871.

(534) A. S. Packard. "The development of Limulus polyphemus." Mem.
Boston Soc. Nat. History, Vol. II., 1872.

PYCNOGONIDA.

The embryos, during the first phases of their development, are always
carried by the male in sacks which are attached to a pair of appendages
(the third) specially formed for this purpose. The segmentation of the
ovum is complete, and there is in most forms developed within the egg-
shell a larva with three pairs of two-jointed appendages, and a rostrum
placed between the front pair.

It will be convenient to take Achelia kevis, studied by Dohrn (No. 536),
as type.

The larva of Achelia when hatched is provided with the typical three
pairs of appendages. The foremost of them is chelate, and the two follow-
ing pairs are each provided with a claw. Of the three pairs of larval-
appendages Dohrn states that he has satisfied himself that the anterior is
innervated by the supra-cesophageal ganglion, and the two posterior by
separate nerves coming from two imperfectly united ventral ganglia. The
larva is provided with a median eye formed of two coalesced pigment
spots, and with a simple stomach.

The gradual conversion of the larva into the adult takes place by the
elongation of the posterior end of the body into a papilla, and the forma-
tion there, at a later period, of the anus ; while at the two sides of the
anal papilla rudiments of a fresh pair of appendages — the first pair of am-
bulatory limbs of the adult — make their appearance. The three remaining
pairs of limbs become formed successively as lateral outgrowths, and their
development is accomplished in a number of successive ecdyses. As they
are formed caeca from the stomach become prolonged into them. For each
of them there appears a special ganglion. While the above changes are
taking place the three pairs of larval appendages undergo considerable
reduction. The anterior pair singly becomes smaller, the second loses
its claw, and the third becomes reduced to a mere stump. In the adult the

PENTASTOMIDA. 539

second pair of appendages becomes enlarged again and forms the so-called
palpi, while the third pair develops in the male into the egg-carrying append-
ages, but is aborted in the female. The first pair form appendages lying
parallel to the rostrum, which are sometimes called pedipalpi and some-
times antennae.

The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn,
contains rudiments of two pairs of ganglia.

The larvae of Phoxichilidium are parasitic in various Hydrozoa (Hydrac-
tinia, etc.). After hatching they crawl into the Hydractinia stock. They
are at first provided with the three normal pairs of larval appendages. The
two hinder of these are soon thrown off, and the posterior part of the trunk,
with the four ambulatory appendages belonging to it, becomes gradually
developed in a series of moults. The legs, with the exception of the hinder-
most pair, are fully formed at the first ecdysis after the larva has become
free. In the genus Pallene the metamorphosis is abbreviated, and the'
young are hatched with the full complement of appendages.

The position of the Pycnogonida is not as yet satisfactorily settled.
The six-legged larva has none of the characteristic features of the Nauplius,
except the possession of the same number of appendages.

The number of appendages (7) of the Pycnogonida does not coincide
with that of the Arachnida. On the other hand, the presence of chelate
appendages innervated in the adult by the supra-cesophageal ganglia rather
points to a common phylum for the Pycnogonida and Arachnida ; though as
shewn above (p. 455) all the appendages in the embryo of true Arachnida
are innervated by post-oral ganglia. The innervation of these appendages
in . the larvae of Pycnogonida requires further investigation. Against
such a relationship the extra pair of appendages in the Pycnogonida is
no argument, since the embryos of most Arachnida are provided with four
such extra pairs. The two groups must no doubt have diverged very
early.

BIBLIOGRAPHY.

(535) G. Cavanna. " Studie e ricerche sui Picnogonidi." Pubblicazioni del R.
Institute di Studi stiperiori in Firenze, 1877.

(536) An. Dohrn. " Ueber Entwickhuig u. Baud. Pycnogoniden." Jenaische
Zeitschrift, Vol. v. 1870, and " Neue Untersuchungen lib. Pycnogoniden." Mitthdl.
a. d. zoologischen Station zu Neafel, Bd. I. 1878.

(537) G. Hodge. " Observations on a species of Pycnogon, etc." Annal. and
Mag. of Nat. Hist. Vol. ix. 1862.

(538) C. Semper. " Ueber Pycnogoniden u. ihre in Hydroiden schmarotzenden
Larvenformen." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. I. 1874.

PENTASTOMIDA.

The development and metamorphosis of Pentastomum taenoides have
been thoroughly worked out by Leuckart (No. 540) and will serve as type
for the group.

540 PENTASTOMIDA.

In the sexual state it inhabits the nasal cavities of the dog. The early
embryonic development takes place as the ovum gradually passes down the
uterus. The segmentation appears to be complete ; and gives rise to an
oval mass in which the separate cells can hardly be distinguished. This
gradually differentiates itself into a characteristic embryo, divided into a tail
and trunk. The tail is applied to the ventral surface of the trunk, and on
the latter two pairs of stump-like unsegmented appendages arise, each
provided with a pair of claws. At the anterior extremity of the body is
formed the mouth, with a ventral spine and lateral hook, which are perhaps
degenerated jaws. The spine functions as a boring apparatus, and an
apparatus with a similar function is formed at the end of the tail. A larval
cuticle now appears, which soon becomes detached from the embryo, except
on the dorsal surface, where it remains firmly united to a peculiar papilla.
This papilla becomes eventually divided into two parts, one of which remains
attached to the cuticle, while the part connected with the embryo forms a
raised cross placed in a cup- shaped groove. The whole structure has been
compared, on insufficient grounds, to the dorsal organ of the Crustacea.

The eggs, containing the embryo in the condition above described, are
eventually carried out with the nasal slime, and, if transported thence into
the alimentary cavity of a rabbit or hare, the embryos become hatched by
the action of the gastric juice. From the alimentary tract of their new host
they make their way into the lungs or liver. They here become enveloped
in a cyst, in the interior of which they undergo a very remarkable metamor-
phosis. They are, however, so minute and delicate that Leuckart was
unable to elucidate their structure till eight weeks after they had been
swallowed. At this period they are irregularly-shaped organisms, with a
most distant resemblance to the earlier embryos. They are without their
previous appendages, but the alimentary tract is now distinctly differentiated.
The remains of two cuticles in the cyst seem to shew that the above changes
are effected in two ecdyses.

In the course of a series of ecdyses the various organs of the larval form
known as Pentastomum denticulatum continue to become differentiated.
After the first (= third) ecdysis the cesophageal nerve-ring and sexually
undifferentiated generative organs are developed. At the fourth (=sixth)
ecdysis the two pairs of hooks of the adult are formed in pockets which
appeared at a somewhat earlier stage ; and the body acquires an annulated
character. At a somewhat earlier period rudiments of the external genera-
tive organs indicate the sex of the larva.

After a number of further ecdyses, which are completed in about six
months after the introduction of the embryos into the intermediate host, the
larva attains its full development, and acquires a form in which it has long
been known as Pentastomum denticulatum. It now leaves its cyst and
begins to move about. It is in a state fit to be introduced into its final host ;
but if it be not so introduced it may become encysted afresh.

If the part of a rabbit or hare infected by a Pentastomum denticulatum
be eaten by a dog or wolf, the parasite passes into the nasal cavity of the

TARDIGRADA. 541

latter, and after further changes of cuticle becomes a fully-developed sexual
Pentastomum taenioides, which does not differ to any very marked extent
from P. denticulatum.

In their general characters the larval migrations of Pentastomum are
similar to those of the Cestodes.

The internal anatomy of the adult Pentastomum, as well as the
characters of the larva with two pairs of clawed appendages, are perhaps
sufficient to warrant us in placing it with the Arthropoda, though it would
be difficult to shew that it ought not to be placed with such a form as
Myzostomum (vide p. 369). There do not appear to be any sufficient
grounds to justify its being placed with the Mites amongst the Arachnida.
If indeed the rings of the body of the Pentastomida are to be taken as
implying a true segmentation, it is clear that the Pentastomida cannot be
associated with the Mites.

BIBLIOGRAPHY.

(539) P. J. van Beneden. " Recherches s. 1'organisation et le developpement d.
Linguatules." Ann. d. Sden. Nat., 3 Ser., Vol. XI.

(540) R. Leuckart. " Bau u. Entwicklungsgeschichte d. Pentastomen." Leipzig
and Heidelberg. 1860.

TARDIGRADA.

Very little is known with reference to the development of the Tardigrada.
A complete and regular segmentation (von Siebold, Kaufmann, No. 541) is
followed by the appearance of a groove on the ventral side indicating a
ventral flexure. At about the time of the appearance of the groove the cells
become divided into an epiblastic investing layer and a central hypoblastic
mass.

The armature of the pharynx is formed very early at the anterior
extremity, and the limbs arise in succession from before backwards.

The above imperfect details throw no light on the systematic position of
this group.

Tardigrada.

(541) J. Kaufmann. " Ueber die Entwicklung u. systematische Stellung d.
Tardigraden." Zeit.f. wiss, ZooL, Bd. HI. 1851.

Summary of Arthropodan Development.
The numerous characters common to the whole of the
Arthropoda led naturalists to unite them in a common phylum,
but the later researches on the genealogy of the Tracheata and
Crustacea tend to throw doubts on this conclusion, while there
is not as yet sufficient evidence to assign with certainty a
definite position in either of these classes to the smaller groups
described in the present chapter. There seems to be but little

542 SUMMARY.

doubt that the Tracheata are descended from a terrestrial Anne-
lidan type related to Peripatus. The affinities of Peripatus to
the Tracheata are, as pointed out in a previous chapter (p. 386),
very clear, while at the same time it is not possible to regard
Peripatus simply as a degraded Tracheate, owing to the fact
that it is provided with such distinctly Annelidan organs as
nephridia, and that its geographical distribution shews it to be a
very ancient form.

The Crustacea on the other hand are clearly descended from
a Phyllopod-like ancestor, which can be in no way related to
Peripatus.

The somewhat unexpected conclusion that the Arthropoda
have a double phylum is on the whole borne out by the anatomy
of the two groups. Without attempting to prove this in detail,
it may be pointed out that the Crustacean appendages are
typically biramous, while those of the Tracheata are never at
any stage of development biramous1; and the similarity between
the appendages of some of the higher Crustacea and those of
many Tracheata is an adaptive one, and could in no case be
used as an argument for the affinity of the two groups.

The similarity of many organs is to be explained by both
groups being descendants of Annelidan ancestors. The simi-
larity of the compound eye in the two groups cannot however
be explained in this way, and is one of the greatest difficulties
of the above view. It is moreover remarkable that the eye of
Peripatus2 is formed on a different type to either the single or
compound eyes of most Arthropoda.

The conclusion that the Crustacea and Tracheata belong to
two distinct phyla is confirmed by a consideration of their
development. They have no doubt in common a centrolecithal
segmentation, but, as already insisted on, the segmentation is
no safe guide to the affinities.

In the Tracheata the archenteron is never, so far as we
know, formed by an invagination3, while in Crustacea the

1 The biflagellate antennae of Pauropus amongst the Myriapocls can hardly be
considered as constituting an exception to this rule.

3 I hope to shew this in a paper I am preparing on the anatomy of Peripatus.

8 Stecker's description of an invagination in the Chilognatha cannot be accepted
without further confirmation ; -vide p. 388.

SUMMARY. 543

evidence is in favour of such an invagination being the usual,
and, without doubt, the primitive, mode of origin.

The mesoblast in the Tracheata is formed in connection with
a median thickening of the ventral plate. The unpaired plate
of mesoblast so formed becomes divided into two bands, one on
each side of the middle line.

In both Spiders and Myriopods, and probably Insects, the
two plates of mesoblast are subsequently divided into somites,
the lumen of which is continued into the limbs.

In Crustacea the mesoblast usually originates from the walls
of the invagination, which gives rise to the mesenteron.

It does not become divided into two distinct bands, but
forms a layer of scattered cells between the epiblast and hypo-
blast, and does not usually break up into somites ; and though
somites are stated in some cases to be found they do not
resemble those in the Tracheata.

The proctodaeum is usually formed in Crustacea before and
rarely later1 than the stomodaeum. The reverse is true for the
Tracheata. In Crustacea the proctodseum and stomodaeum,
especially the former, are very long, and usually give rise to the
greater part of the alimentary tract, while the mesenteron is
usually short.

In the Tracheata the mesenteron is always considerable, and
the proctodaeum is always short. The derivation of the Mal-
pighian bodies from the proctodaeum is common to most
Tracheata. Such diverticula of the proctodaeum are not found
in Crustacea.

1 This is stated to be the case in Moina (Grobben).

CHAPTER XX.

ECHINODERMATA1.

THE development of the Echinodermata naturally falls into
two sections: —

(i) The development of the germinal layers and of the
systems of organs; (2) the development of the larval appendages
and the metamorphosis.

The Development of the Germinal Layers and of tJie Systems

of Organs.

The development of the systems of organs presents no very
important variations within the limits of the group.

Holothuroidea. The Holothurians have been most fully
studied (Selenka, No. 563), and may be conveniently taken as
type.

The segmentation is nearly regular, though towards its close,
and in some instances still earlier, a difference becomes apparent
between the upper and the lower poles.

At the close of segmentation (fig. 247 A) the egg has a
nearly spherical form, and is constituted of a single layer of
columnar cells enclosing a small segmentation cavity. The
lower pole is slightly thickened, and the egg rotates by means of
fine cilia.

An invagination now makes its appearance at the lower
pole (fig. 247 B), and simultaneously there become budded off
from tJie cells undergoing the invagination amoeboid cells, which

1 The following classification of the Echinodermata is employed in this chapter.

I. Holothuroidea. IV. Echinoidea.

II. Asteroidea. V. Crinoidea.

III. Ophiuroidea.

ECHINODERMATA. 545

eventually form the muscular system and the connective tissue.
These cells very probably have a bilaterally symmetrical origin.
This stage represents the gastrula stage which is common to all
Echinoderms. The invaginated sack is the archenteron. As it
grows larger one side of the embryo becomes flattened, and the
other more convex. On the flattened side a fresh invagination

FIG. 247. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA

VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Blastosphere stage at the close of segmentation. B. Gastrula stage.
mr. micropyle ; //. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast;
hy. hypoblast; ms. amoeboid cells derived from hypoblast ; a.e. archenteron.

arises, the opening of which forms the permanent mouth, the
opening of the first invagination remaining as the permanent
anus (fig. 248 A).

These changes give us the means of attaching definite names
to the various parts of the embryo. It deserves to be noted in
the first place that the embryo has assumed a distinctly bilateral
form. There is present a more or less concave surface ex-
tending from the mouth to near the anus, which will be spoken
of as the ventral surface. The anus is situated at the posterior
extremity. The convex surface opposite the ventral surface
forms the dorsal surface, which terminates anteriorly in a
rounded prse-oral prominence.

It will be noticed in fig. 248 A that in addition to the
primitive anal invagination there is present a vesicle (?/.).
This vesicle is directly formed by a constriction of the primitive
B. II. 35

546

HOLOTHUROIDEA.

archenteron (fig. 249 Vpv.), and is called by Selenka the vaso-
peritoneal vesicle. It gives origin to the epithelioid lining of
the body cavity and water-vascular system of the adult1. In the
parts now developed we have the rudiments of all the adult organs.
The mouth and anal involutions (after the separation of the
vaso-peritoneal vesicle) meet and unite, a constriction indicating
their point of junction (fig. 248 B). Eventually the former gives

FIG. 248. THREE STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA

VIEWED FROM THE SIDE IN OPTICAL SECTION. (After Selenka.)
tn. mouth; oe. oesophagus; st. stomach; i. intestine; a. anus; I.e. longitudinal
ciliated band; v.p. vaso-peritoneal vesicle; p.v. peritoneal vesicle; p.r. right peri-
toneal vesicle ; //. left peritoneal vesicle ; w.v. water- vascular vesicle ; p. dorsal pore
of water- vascular system ; ms. muscle cells.

rise to the mouth and cesophagus, and the latter to the re-
mainder of the alimentary canal2.

The vaso-peritoneal vesicle undergoes a series of remarkable
changes. After its separation from the archenteron it takes
up a position on the left side of this, elongates in an antero-
posterior direction, and from about its middle sends a narrow
diverticulum towards the dorsal surface of the body, where an

1 The origin of the vaso-peritoneal vesicle is not quite the same in all the species.
In Holothuria tubulosa it is separated from the csecal end of the archenteron; the
remainder of which then grows towards the oral invagination. In Cucumaria the
archenteron forks (fig. 249) ; and one fork forms the vaso-peritoneal vesicle, and the
other the major part of the mesenteron.

2 There appears to be some uncertainty as to how much of the larval cesophagus is
derived from the stomodaeal invagination.

ECHINODERMATA.

547

opening to the exterior becomes formed (fig. 248 B, /.). The
diverticulum becomes the madreporic canal, and the opening
the dorsal pore.

The vaso-peritoneal vesicle next divides into two, an an-
terior vesicle (fig. 248 B, w.v.), from which is derived the
epithelium of the water-vascular system, and a posterior (fig.
248 B, /.?;.), which gives rise to the epithelioid lining of the body
cavity. The anterior vesicle (fig. 248 C, w.v.) becomes five-
lobed, takes a horseshoe-shaped form, and grows round the
oesophagus (fig. 256, w.v.r). The five lobes form the rudiments
of the water-vascular prolongations into the tentacles. The
remaining parts of the water-vascular system are also developed
as outgrowths of the original vesicle. Five of these, alternating
with the original diverticula, form the five ambulacral canals,
from which diverticula are produced into the ambulacral feet ; a
sixth gives rise to the Polian vesicle. The remaining parts of
the original vesicle form the water-vascular ring.

We must suppose that eventually the madreporic canal loses
its connection with the exterior so as to hang loosely in the
interior, though the steps of this process do not appear to
have been made out.

The original hinder peri-
toneal vesicle grows rapidly,
and divides into two (fig. 248 C,
pi. and pr.}, which encircle the
two sides of the alimentary
canal, and meet above and
below it. The outer wall of
each of them attaches itself to
the skin, and the inner one to
the alimentary canal and water-
vascular system ; in both cases
the walls remain separated
from the adjacent parts by a
layer of the amoeboid cells
already spoken of. The cavity
of the peritoneal vesicles be-
comes the permanent body
cavity. Where the walls of

-ME

FIG. 249. LONGITUDINAL SECTION

THROUGH AN EMBRYO OF CUCUMARIA
DOLIOLUM AT THE END OF THE FOURTH
DAY.

Vpv. vaso-peritoneal vesicle; ME.
mesenteron; Blp., Ptd. blastopore, proc-
todaeum.

35—2

548 HOLOTHUROIDEA.

the two vesicles meet on the dorsal side, a mesentery, suspend-
ing the alimentary canal and dividing the body cavity longitu-
dinally, is often formed. In other parts the partition walls
between the two sacks appear to be absorbed.

The amoeboid cells, which were derived from the invaginated
cells, arrange themselves as a layer round all the organs (fig.
249). Some of them remain amoeboid, attach themselves to the
skin, and form part of the cutis; and in these cells the cal-
careous spicula of the larva and adult are formed. Others
form the musculature of the larval alimentary tract, while the
remainder give rise to the musculature and connective tissue of
the adult.

The development of the vascular system is not known, but the discovery
of Kowalevsky, confirmed by Selenka, that from the walls of the water-
vascular system corpuscles are developed, identical with those in the blood-
vessels, indicates that it probably develops in connection with the water-
vascular system. The observations of Hoffmann and Perrier on the commu-
nication of the two systems in the Echinoidea point to the same conclusion.
Though nothing very definite is known with reference to the development of
the nervous system, Metschnikoff suggests that it develops in connection
with the thickened bands of epiblast which are formed by a metamorphosis
of the ciliated bands of the embryo, and accompany the five radial tubes
(vide p. 555). In any case its condition in the adult leaves no doubt of its
being a derivative of the epiblast.

From the above description the following general conclusions
may be drawn : —

(1) The blastosphere stage is followed by a gastrula stage.

(2) The gastrula opening forms the permanent anus, and the
mouth is formed by a fresh invagination.

(3) The mesoblast arises entirely from the invaginated cells,
but in two ways : —

(a) As scattered amoeboid cells, which give origin to the
muscles and connective tissue (including the cutis) of the body
wall and alimentary tract.

(&) As a portion separated off from the archenteron,
which gives rise both to the epithelioid lining of the body cavity,
and of the water-vascular system.

(4) The oesophagus is derived from an invagination of the
epiblast, and the remainder of the alimentary canal from the
archenteron.

ECHINODERMATA. 549

(5) The embryonic systems of organs pass directly into those
of the adult.

The development of Synapta diverges, as might be expected, to a very
small extent from that of Holothuria.

Asteroidea. In Asterias the early stages of development conform to
our type. There arise, however, two bilaterally symmetrical vaso-peritoneal
diverticula from the archenteron. These diverticula give rise both to the
lining of the body cavity and water-vascular system. With reference to
the exact changes they undergo there is, however, some difference of opinion.
Agassiz (543) maintains that both vesicles are concerned in the formation of
the water-vascular system, while Metschnikoff (560) holds that the water-
vascular system is entirely derived from the anterior part of the larger left
vesicle, while the right and remainder of the left vesicle form the body
cavity. MetschnikofFs statements appear to be the most probable. The
anterior part of the left vesicle, after separating from the posterior, grows
into a five-lobed rosette (fig. 260, /), and a madreporic canal (h] with a dorsal
pore opening to the exterior. The rosette appears not to grow round the
oesophagus, as in the cases hitherto described. But the latter is stated to
disappear, and a new oesophagus to be formed, which pierces the rosette,
and places the old mouth in communication with the stomach. Except
where the anus is absent in the adult, the larval anus probably persists.

Ophiuroidea. The early development of the Ophiuroidea is not so
fully known as that of other types. Most species have a free-swimming
larva, but some (Amphiura) are viviparous.

The early stages of the free-swimming larvae have not been described,
but I have myself observed in the case of Ophiothrix fragilis that the
segmentation is uniform, and is followed by the normal invagination. The
opening of this no doubt remains as the larval anus, and there are probably
two outgrowths from this to form the vaso-peritoneal vesicles. Each of these
divides into two parts, an anterior lying close to the oesophagus, and a
posterior close to the stomach. The anterior on the right side aborts ; that
on the left side becomes the water-vascular vesicle, early opens to the
exterior, and eventually grows round the oesophagus, which, as in Holothu-
rians, becomes the oesophagus of the adult. The posterior vesicles give rise
to the lining of the body cavity, but are stated by Metschnikoff to be at first
solid, and only subsequently to acquire a cavity— the permanent body cavity.
The anus naturally disappears, since it is absent in the adult. In the
viviparous type the first stages are imperfectly known, but it appears that
the blastopore vanishes before the appearance of the mouth. The develop-
ment of the ^vaso-peritoneal bodies takes place as in the free-swimming
larvae.

Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564)
there is a regular segmentation and the normal invagination (fig. 250 A).
The amoeboid mesoblast cells arise as two laterally placed masses, and give
rise to the usual parts. The archenteron grows forward and bends towards

550

CRINOIDEA.

the ventral side (fig. 250 B). It becomes (fig. 250 C) divided into three
chambers, of which the two hindermost (d and c) form the stomach and
intestine ; while the anterior forms the oesophagus, and gives rise to the

FIG. 250. THREE SIDE VIEWS OF EARLY STAGES IN THE DEVELOPMENT OF

STRONGYLOCENTRUS. (From Agassiz.)

a, anus (blastopore) ; d. stomach ; o. oesophagus ; c . rectum ; w. vaso-peritoneal
vesicle ; v. ciliated ridge ; r. calcareous rod.

vaso-peritoneal vesicles. These latter appear as a pair of outgrowths
(fig. 251), but become constricted off as a single two-horned vesicle, which
subsequently divides into two. The left of these
is eventually divided, as in Asteroids, into a
peritoneal and water-vascular sack, while the
right forms the right peritoneal sack. An oral
invagination on the flattened ventral side meets
the mesenteron after its separation from the
vaso-peritoneal vesicle. The larval anus per-
sists, as also does the larval mouth, but owing
to the manner in which the water-vascular
rosette is established the larval oesophagus ap-
pears to be absorbed, and to be replaced by a
fresh oesophagus.

Crinoidea. Antedon, the only Crinoid
so far studied (Gotte, No. 549), presents some
not inconsiderable variations from the usual
Echinoderm type. The blastopore is placed on
the somewhat flattened side of the oval blasto-
sphere, and not, as is usual, at the hinder end.

The blastopore completely closes, and is not converted into the perma-
nent anus. The archenteron gives rise to the epithelioid lining of both body
cavity and water-vascular system. These parts do not, however, appear as
a single or paired outgrowth from the archenteron, but as three distinct
outgrowths which are not formed contemporaneously. Two of them are first

FIG. -251. DORSO-VEN-
TRAL VIEW OF AN EARLY
LARVA OF STRONGYLOCEN-
TRUS. (From Agassiz.)

a. anus ; d. stomach ; o.
oesophagus ; w. vaso-perito-
neal vesicle; r. calcareous
rod.

ECHINODERMATA.

551

formed and become the future body cavity; but their lumens remain distinct.
Jngmally appearing as lateral outgrowths, the right one assumes a dorsal
position and sends a prolongation into the stalk (fig. 252 rp'\ and
the left one assumes first a ventral, and then an oral position (fur
252 lp\

The third outgrowth of the archenteron gives rise to the water-vascular
vesicle. It first grows round the region of the future oesophagus and so
forms the water-vascular ring.
The wall of the ring then
grows towards the body wall
so as to divide the oral (left)
peritoneal vesicle into two
distinct vesicles, an anterior
and a posterior, shewn in fig.
253, lp' and lp. Before this
division is completed, the
water-vascular ring is pro-
duced in front into five pro-

FIG. 252. LONGITUDINAL SECTION THROUGH
AN ANTEDON LARVA. (From Carpenter: after
Gotte.)

al. mesenteron ; -wv. water- vascular ring ;
lp. left (oral) peritoneal vesicle; rp. right peri-
toneal vesicle ; rp'. continuation of right vesicle
into the stalk ; st. stalk.

cesses—the future tentacles
(fig. 252, wv)— which project
into the cavity of the oral
vesicle (lp\ After the oral
peritoneal space has become
completely divided into two parts, the anterior dilates (fig. 253, //) greatly,
and forms a large vestibule at the anterior end of the body. This vestibule
(lp'} next acquires a communication with the mesenteron, shewn in fig. 253
at m. The anterior wall of this vestibule is finally broken through. By this
rupture the mesenteron is placed in communication with the exterior by the
opening at m, while at the same time the tentacles of the water-vascular ring
(/) project freely to the exterior. Such is Gotte's account of the prge-oral
body space, but, as he himself points out, it involves our believing that the
lining of the diverticulum derived from the primitive alimentary vesicle
becomes part of the external skin. This occurrence is so remarkable, that
more evidence appears to me requisite before accepting it.

The formation of the anus occurs late. Its position appears to be the
same as that of the blastopore, and is indicated by a papilla of the mesente-
ron attaching itself to the skin on the ventral side (fig. 253, an). It event-
ually becomes placed in an interradial space within the oral disc of the adult.
The water-vascular ring has no direct communication with the exterior, but
the place of the madreporic canal of other types appears to be taken in
the larva by a single tube leading from the exterior into the body cavity, the
external opening of which is placed on one of the oral plates (vide p. 571) in
the next interradial space to the right of the anus, and a corresponding
diverticulum of the water-vascular ring opening into the body cavity. The
line of junction between the left and right peritoneal vesicles forms in the
larva a ring-like mesentery dividing the oral from the aboral part of the body

552

CRINOIDEA.

cavity. In the adult1 the oral section of the larval body cavity becomes the
ventral part of the circumvisceral division of the body cavity, and the
subtentacular canals of the arms and disc ; while the aboral section becomes
the dorsal part of the circumvisceral division of the body cavity, the cceliac
canals of the arms, and the cavity of the centro-dorsal piece. The primitive

,+wr

FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX OF AN ADVANCED
PENTRACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE.

(From Carpenter ; after Gotte.)

ae. epithelium of oral vestibule; ;//. mouth; al. mesenteron; an. rudiment of
permanent anus; lp. posterior part of left (oral) peritoneal sack; lp' '. anterior part of
left (oral) peritoneal sack; wr. water-vascular ring; /. tentacle; mt. mesentery;
rp. right peritoneal sack; rp '. continuation of right peritoneal sack into the stalk;
r. roof of tentacular vestibule.

distinction between the sections of the larval body cavity becomes to a large
extent obliterated, while the axial and intervisceral sections of the body-
cavity of the adult are late developments.

The more important points in the development indicated in
the preceding pages are as follows :

(i) The blastosphere is usually elongated in the direction
of the axis of invagination, but in Comatula it is elongated
transversely to this axis.

1 Vide P. H. Carpenter, "On the genus Actinometra." Linnean Trans., and
Series, Zoology, Vol. n., Part I., 1879.

ECHINODERMATA. 553

(2) The blastopore usually becomes the permanent anus,
but it closes at the end of larval life (there being no anus in the
adult) in Ophiuroids and some Asteroids, while in Comatula it
closes very early, and a fresh anus is formed at the point where
the blastopore was placed.

(3) The larval mouth always becomes the mouth of the
adult.

(4) The archenteron always gives rise to outgrowths which
form the peritoneal membrane and water-vascular systems. In
Comatula there are three such outgrowths, two paired, which
form the peritoneal vesicles, and one unpaired, which forms the
water-vascular vesicle. In Asteroids and Ophiuroids there are
two outgrowths. In Ophiuroids both of these are divided into a
peritoneal and a water-vascular vesicle, but the right water-
vascular vesicle atrophies. In Asteroids only one water-vascular
vesicle is formed, which is derived from the left peritoneal vesicle.
In Echinoids and Holothuroids there is a single vaso-peritoneal
vesicle.

(5) The water- vascular vesicle grows round the larval
oesophagus in Holothuroids, Ophiuroids, and Comatula ; in
these cases the larval oesophagus is carried on into the adult.
In other forms the water-vascular vesicle forms a ring which
does not enclose the cesophagus (Asteroids and Echinoids);
in such cases a new oesophagus is formed, which perforates this
ring.

Development of the larval appendages and metamorphosis.

Holothuroidea. The young larva of Synapta, to which J.
Muller gave the name Auricularia (fig. 255), is in many respects
the simplest form of Echinoderm larva. With a few exceptions
the Auricularia type of larva is common to the Holothuria.

It is (fig. 254 A and fig. 255) bilaterally symmetrical, pre-
senting a flattened ventral surface, and a convex dorsal one.
The anus (an) is situated nearly at the hinder pole, and the
mouth (m) about the middle of the ventral surface. In front
of the mouth is a considerable process, the prae-oral lobe.
Between the mouth and anus is a space, more or less concave
according to the age of the embryo, interrupted by a ciliated

554

AURICULARIA.

A similar ciliated ridge is
A E

ridge a little in front of the anus,
present on the ventral surface
of the prae-oral lobe immedi-
ately in front of the mouth.
The anal and oral ridges are
connected by two lateral cili-
ated bands, the whole forming
a continuous band, which,
since the mouth lies in the
centre of it (fig. 255), may be
regarded as a ring completely
surrounding the body behind
the mouth, or more naturally
as a longitudinal ring.

The bilateral Auricularia
is developed from a slightly
elongated gastrula with an uniform covering of cilia. The
gastrula becomes flattened on the oral side. At the same time
the cilia become specially developed on the oral and anal ridges,
and then on the remainder of the ciliated ring, while they are

FIG. 254. A. THE LARVA OF A HOLO-
THUROID. B. THE LARVA OF AN ASTER-
OID.

;//. mouth; st. stomach; a. anus; l.c>
primitive longitudinal ciliated band; pr.c.
prae-oral ciliated band.

FIG. 155. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN
AURICULARIA FROM THE SIMPLEST ECHINODERM LARVAL FORM. (Copied from
MUller.)

The black line represents the ciliated ridge. The shaded part is the oral side of
the ring, the clear part the aboral side.

/;;. mouth; an. anus.

simultaneously obliterated elsewhere ; and so a complete Auricu-
laria is developed. The water-vascular ring in the fully-developed
larva has already considerably advanced in the growth round the
oesophagus (fig. 256 w.v.r).

Most Holothurian larvae, in their transformation from the
bilateral Auricularia form to the radial form of the adult, pass
through a stage in which the cilia form a number of transverse

ECHINODERMATA.

555

-2>.v

rings, usually five in number, surrounding the body. The
stages in this metamorphosis are shewn in figs. 256, 257, and
258.

The primitive ciliated band,
at a certain stage of the meta-
morphosis, breaks up into a
number of separate portions
(fig. 256), the whole of which are
placed on the ventral surface.
Four of these (fig. 257 A and B)
arrange themselves in the form
of an angular ring round the
mouth, which at this period pro-
jects considerably. The remain-
ing portions of the primitive
band change their direction from
a longitudinal one to a trans-
verse (fig. 257 B), and eventually
grow into complete rings (fig.
2570). Of these there are five.
The middle one (257 B) is the
first to develop, and is formed
from the dorsal parts of the
primitive ring. The two hinder
rings develop next, and last of
all the two anterior ones, one of
which appears to be in front of the mouth (fig. 257 C).

The later development of the mouth, and of the ciliated ridge
surrounding it, is involved in some obscurity. It appears from
Metschnikoff (No. 560) that an invagination of the oesophagus
takes place, carrying with it the ciliated ridge around the mouth.
This ridge becomes eventually converted into the covering for
the five tentacular outgrowths of the water- vascular ring (fig.
258), and possibly also forms the nervous system.

The opening of the cesophageal invagination is at first behind
the foremost ciliated ring, but eventually comes to lie in front of
it, and assumes a nearly terminal though slightly ventral position
(fig. 258). No account has been given of the process by which
this takes place, but the mouth is stated by Metschnikoff (though

FIG. 256. FULL-GROWN LARVA OF
SYNAPTA. (After Metschnikoff.)

m. mouth ; st. stomach ; a. anus ;
p.v. left division of perivisceral cavity,
which is still connected with the water-
vascular system ; w.v.r. water-vascular
ring which has not yet completely en-
circled the oesophagus; I.e. longitudinal
part of ciliated band ; pr.c. prae-oral part
of ciliated band.

556

BIPINNARIA.

Miiller differs from him on this point) to remain open through-
out. The further changes in the metamorphosis are not con-
siderable. The ciliated bands disappear, and a calcareous ring
of ten pieces, five ambulacral and five interambulacral, is formed
round the oesophagus. A provisional calcareous skeleton is also
developed.

All the embryonic systems of organs pass in this case
directly into those of the adult.

The metamorphosis of most Holothuroidea is similar to that just
described. In Cucumaria (Selenka) there is however no Auricularia stage,
and the uniformly ciliated stage is succeeded by one with five transverse

FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B
are viewed from the ventral surface, and C from the side. (After Metschnikoff.)

m. mouth; oe. oesophagus; pv. walls of the perivisceral cavity; wv. longitudinal
vessel of the water- vascular system; p. dorsal pore of water-vascular system;
cr. ciliated ring formed round the mouth from parts of the primitive ciliated
band.

bands of cilia, and a prae-oral and an anal ciliated cap. The mouth is at
first situated ventrally behind the prse-oral cap of cilia, but the prae-oral
cap becomes gradually absorbed, and the mouth assumes a terminal
position.

In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the
adult condition is attained without even a metamorphosis. There appear to

ECHINODERMATA.

557

be five plates surrounding the
mouth, which are developed before
any other part of the skeleton, and
are regarded by P. H. Carpenter
(No. 548) as equivalent to the five
oral plates of the Crinoidea. The
larval condition with ciliated bands
is often spoken of as the pupa stage,
and during it the larvae of Holo-
thurians proper use their embryonic
tube feet to creep about.

Asteroidea. The com-
monest and most thoroughly
investigated form of Asteroid
larva is a free swimming form
known as Bipinnaria.

This form in passing from
the spherical to the bilateral
condition passes through at
first almost identical changes
to the Auricularian larva.
The cilia become at an early
period confined to an oral
and anal ridge.

The anal ridge gradually extends dorsalwards, and finally
forms a complete longitudinal post-oral ring (fig. 259 A) ; the
oral ridge also extends dorsalwards, and forms a closed prae-oral
ring (fig. 259 A), the space within which is left unshaded in my
figure.

The presence of two rings instead of one distinguishes the
Bipinnaria from the Auricularia. The two larvae are shewn side
by side in fig. 254, and it is obvious that the two bands of the
Bipinnaria are (as pointed out by Gegenbaur) equivalent to the
single band of the Auricularia divided into two. Ontologically,
however, the two bands of Bipinnaria do not appear to arise
from the division of a single band.

As the Bipinnaria grows older, a series of arms grows out
along lines of the two ciliated bands (fig. 259 C), and, in many
cases, three special arms are formed, not connected with the
ciliated bands, and covered with warts. These latter arms are

FlG. 258. A LATE STAGE IN THE DE-
VELOPMENT OF SYNAPTA. (After Metschni-
koff.)

The figure shews the vestibular cavity
with retracted tentacles ; the ciliated bands ;
the water-vascular system, etc.

p. dorsal pore of water-vascular system ;
pv. walls of perivisceral cavity; ms. amoe-
boid cells.

558

BIPINNARIA.

known as brachiolar arms, and the larvae provided with them
as Brachiolaria (fig. 259 D).

As a rule the following arms can be distinguished (fig. 259 C and D), on
the hinder ring (Agassiz' nomenclature) a median anal pair, a dorsal anal
pair, and a ventral anal pair, a dorsal oral pair, and an unpaired anterior
dorsal arm ; on the prae-oral ring a ventral oral pair, and sometimes (Miiller)
an unpaired anterior ventral arm.

The three brachiolar arms arise as processes from the base of the
unpaired dorsal arm, and the two ventral oral arms. The extent of the
development of the arms varies with the species.

FIG. 259. DIAGRAMMATIC REPRESENTATION OF VARIOUS FORMS OF ASTEROID
LARWE. A, B, C, BIPINNARIA; D, BRACHIOLARIA. (Copied from Muller.)
The black lines represent the ciliated bands ; and the shading the space between
the prae-oral and the post-oral bands.

m. mouth; an. anus.

The changes by which the Bipinnaria or Brachiolaria becomes
converted into the adult starfish are very much more complicated
than those which take place in Holothurians. For an accurate
knowledge of them we are largely indebted to Alex. Agassiz
(No. 543). The development of the starfish takes place entirely
at the posterior end of the larva close to the stomach.

On the right and dorsal side of the stomach, and externally
to the rig/it peritoneal space, are formed five radially situated
calcareous rods arranged in the form of a somewhat irregular
pentagon. The surface on which they are deposited has a
spiral form, and constitutes together with its calcareous rods, the

ECHINODERMATA. 559

abactinal or dorsal surface of the future starfish. Close to its
dorsal, i.e. embryonic dorsal, edge lies the dorsal pore of the
water-vascular system (madreporic canal), and close to its ventral
edge the anus. On the left and ventral side of the stomach is
placed the water-vascular rosette, the development of which was
described on p. 549. It is situated on the actinal or ventral surface
of the future starfish, and is related to the left peritoneal vesicle.

Metschnikoff (No. 560) and Agassiz (No. 543) differ slightly as to the
constitution of the water- vascular rosette. The former describes and figures
it as a completely closed rosette, the latter states that ' it does not form a
completely closed curve but is always open, forming a sort of twisted
crescent-shaped arc.'

The water-vascular rosette is provided with five lobes, corre-
sponding to which are folds in the larval skin, and each lobe
corresponds to one of the calcareous plates developed on the
abactinal disc. The plane of the actinal surface at first meets
that of the abactinal at an acute or nearly right angle. The two
surfaces are separated by the whole width of the stomach. The
general appearance of the larva from the ventral surface after
the development of the water-vascular rosette (i) and abactinal
disc (A) is shewn in fig. 260.

As development proceeds the abactinal surface becomes a
firm and definite disc, owing to the growth of the original
calcareous spicules into more or less definite plates, and to the
development of five fresh plates nearer the centre of the disc and
interradial in position. Still later a central calcareous plate
appears on the abactinal surface, which is thus formed of a
central plate, surrounded by a ring of five interradial plates, and
then again by a ring of five radial plates. The abactinal disc
now also grows out into five short processes, separated by five
shallow notches. These processes are the rudiments of the five
arms, and each of them corresponds to one of the lobes of the
water-vascular rosette. A calcareous deposit is formed round
the opening of the water-vascular canal, which becomes the
madreporic tubercle1. At about this stage the absorption of the
larval appendages takes place. The whole anterior part of the

1 The exact position of the madreporic tubercle in relation to the abactinal plates
does not seem to have been made out. It might have been anticipated that it would
be placed in one of the primary interradial plates, but this does not seem to be the
case. The position of the anus is also obscure.

56o

BIPINNARIA.

larva with the great prae-oral lobe has hitherto remained
unchanged, but now it contracts and undergoes absorption, and
becomes completely withdrawn into the disc of the future starfish.
The larval mouth is transported into
the centre of the actinal disc. In the
larvae observed by Agassiz and Met-
schnikoff nothing was cast off, but the
whole absorbed.

According to M tiller and Koren and
Danielssen this is not the case in the larva
observed by them, but part of the larva is
thrown off, and lives for some time indepen-
dently.

After the absorption of the larval
appendages the actinal and abactinal
surfaces of the young starfish approach
each other, owing to the flattening of
the stomach ; at the same time they
lose their spiral form, and become flat
discs, which fit each other. Each of
the lobes of the rosette of the water-
vascular system becomes one of the
radial water-vascular canals. It first
becomes five-lobed, each lobe forming
a rudimentary tube foot, and on each ^dctinal disc of youn£ Aste'
side of the middle lobe two fresh ones

next spring out, and so on in succession. The terminal median
lobe forms the tentacle at the end of the arm, and the eye is
developed at its base. The growth of the water-vascular canals
keeps pace with that of the arms, and the tube feet become
supported at their base by an ingrowth of calcareous matter.
The whole of the calcareous skeleton of the larva passes directly
into that of the adult, and spines are very soon formed on the
plates of the abactinal surface. The original radial plates,
together with the spines which they have, are gradually pushed
outwards with the growth of the arms by the continual addition
of fresh rows of spines between the terminal plate and the plate
next to it. It thus comes about that the original radial plates
persist at the end of the arms, in connection with the unpaired

FIG. 260. BIPINNARIA
LARVA OF AN ASTEROID. (From
Gegenbaur ; after Miiller.)

b. mouth ; a. anus ; h. ma-
dreporic canal ; t. ambulacral
rosette ; c . stomach ; d. g. e.
etc. arms of Bipinnaria ; A.

ECHINODERMATA. 561

tentacles which form the apex of the radial water-vascular
tubes.

It has already been mentioned that according to Metschnikoff (No. 560)
a new oesophagus is formed which perforates the water-vascular ring, and
connects the original stomach with the original mouth. Agassiz (No. 543)
maintains that the water-vascular ring grows round the primitive oesophagus.
He says — " During the shrinking of the larva the long oesophagus becomes
" shortened and contracted, bringing the opening of the mouth of the larva
" to the level of the opening of the oesophagus, which eventually becomes
"the true mouth of the starfish." The primitive anus is believed by
Metschnikoff to disappear, but by Agassiz to remain. This discrepancy
very possibly depends upon these investigators having worked at different
species.

There is no doubt that the whole of the larval organs, with
the possible exception of the oesophagus, and anus (where absent
in the adult), pass directly into the corresponding organs of the
starfish — and that the prae-oral part of the body and arms of the
larva are absorbed and not cast off.

In addition to the Bipinnarian type of Asteroid larva a series of other
forms has been described by Miiller (No. 561), Sars, Keren, and Danielssen
(No. 554) and other investigators, which are however very imperfectly
known. The best-known form is one first of all discovered by Sars in
Echinaster Sarsii, and the more or less similar larvae subsequently investi-
gated by Agassiz, Busch, Miiller, Wyville Thomson, etc. of another species
of Echinaster and of Asteracanthion. These larvae on leaving the egg have
an oval form, and are uniformly covered by cilia. Four processes (or in
Agassiz' type one process) grow out from the body ; by these the larvae fix
themselves. In the case of Echinaster the larvae are fixed in the ventral
concavity of the disc of the mother, between the five arms, where a tempo-
rary brood-pouch is established. The main part of the body is converted
directly into the disc of the young starfish, while the four processes come to
spring from the ventral surface, and are attached to the water- vascular ring.
Eventually they atrophy completely. Of the internal structure but little is
known ; till the permanent mouth is formed, after the development of the
young starfish is pretty well advanced, the stomach has no communication
with the exterior.

A second abnormal type of development is presented by the embryo of
Pteraster miliaris, as described by Koren and Danielssen1. The larvae to
the number of eight to twenty develop in a peculiar pouch on the dorsal
surface of the body. The early stages are not known, but in the later ones
the whole body assumes a pentagonal appearance with a mouth at one edge

1 The following statements are taken from the abstract in Bronn's Thierreichs.
B. II. 36

562

OPHIUROID PLUTEUS.

of the disc. At a later stage the anus is formed on the dorsal side of an arm
opposite the mouth. The stomach is surrounded by a water-vascular ring,
from which the madreporic canal passes to the dorsal surface, but does not
open. At a later stage the embryonic mouth and anus vanish, to be replaced
by a permanent mouth and anus in the normal positions.

A third, and in some respects very curious, form is a worm like larva of
Miiller, which is without bands of cilia. The dorsal surface of the youngest
larva is divided by transverse constrictions into five segments. On the
under side of the first of these is a five-lobed disc, each lobe being provided
with a pair of tube feet.

At a later period only three segments are visible on the dorsal surface,
but the ventral surface has assumed a pentagonal aspect. The later stages
are not known.

Ophiuroidea. The full-grown larva of the Ophiuroids is
known as a Pluteus. It commences with the usual more or less
spherical form ; from this it passes to a form closely resembling

FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OK AN OPHIU-
ROID PLUTEUS FROM A SIMPLE ECHINODERM LARVA. (Copied from Miiller.) The
calcareous skeleton is not represented.

///. mouth; an. anus; d. anterior arms; d'. lateral arms; e'. posterior arms; tf.
anterolateral arms.

that of Auricularia with a rounded dorsal surface, and a flattened
ventral one. Soon however it becomes distinguished by the
growth of a post-anal lobe and the absence of a prae-oral lobe
(fig. 261 B). The post-anal lobe forms the somewhat rounded
apex of the body. In front of the mouth, and between the
mouth and anus, arise the anal and oral ciliated ridges, which
soon become continued into a single longitudinal ciliated ring.
At the same time the body becomes prolonged into a series of

ECHINODERMATA.

563

processes along the ciliated band, which is continued to the
extremity of each. The primitive ciliated ring never becomes
broken up into two or more rings. A ciliated crown is usually
developed at the extremity of the post-anal lobe. The arms are
arranged in the form of a ring surrounding the mouth, and are
all directed forwards.

The first arms to appear are two lateral ones, which usually remain the
most conspicuous (fig. 261 B and C, cf\ Next arises a pair on the sides of
the mouth, which may be called the mouth or anterior arms (C, d}. A pair
ventral to and behind the lateral arms is then formed, constituting the
posterior arms (D, e'\ and finally a pair between the lateral arms and the
anterior, constituting the anterolateral arms (D,^).

The concave area between the arms forms the greater part of
the ventral surface of the body. Even before the appearance of
any of the arms, and before the formation of the mouth, two
calcareous rods are formed, which meet behind at the apex of
the post-anal lobe, and are continued as a central support into
each of the arms as they are successively formed. These rods
are shewn at their full development in fig. 262. The important
points which distinguish a Pluteus
larva from the Auricularia or
Bipinnaria are the following :

(i) The presence of the post-
anal lobe at the hind end of the
body. (2) The slight develop-
ment of a prae-oral lobe. (3) The
provisional calcareous skeleton in
the larval arms.

Great variations are presented
in the development of the arms
and provisional skeleton. The
presence of lateral arms is however
a distinctive characteristic of the
Ophiuroid Pluteus. The other
arms may be quite absent, but
the lateral arms never.

The formation of the perma-
nent Ophiuroid takes place in
much the same way as in the Asteroidea.

36-2

FIG. 262.
OPHIUROID.
after Miiller.)

PLUTEUS LARVA OF AN
(From Gegenbaur ;

A. rudiment of young Ophiuroid ;
(?. lateral arms; d. anterior arms;
e . posterior arms.

564

OPHIUROID PLUTEUS.

There is formed (fig. 262) on the right and dorsal side of stomach the
abactinal disc supported by calcareous plates, at first only five in number
and radial in position1. The disc is at first not symmetrical, but becomes so
at the time of the resorption of the larval arms. It grows out into five
processes — the five future rays. The original five radial plates remain as the
terminal segments of the adult rays, and new plates are always added
between the ultimate and penultimate plate (Mu'ller), though it is probable
that in the later stages fresh plates are added in the disc.

The ventral surface of the permanent Ophiuroid is formed by the concave
surface between the mouth and anus. Between this and the stomach is

FIG. 263. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF ECHINOID
PLUTEI. (Copied from Miiller.) The calcareous skeleton is not represented. E.
Pluteus of Spatangus.

m. mouth; an. anus; d. anterior arms; d' . point where lateral arms arise in the
Ophiuroid Pluteus; e. anterointernal arms; e. posterior arms; g'. anterolateral arms;
g. anteroexternal arms.

situated the water-vascular ring. It is at first not closed, but is horseshoe-
shaped, with five blind appendages (fig. 262). It eventually grows round
the cesophagus, which, together with the larval mouth, is retained in the
adult. The five blind appendages become themselves lobed in the same
way as in Asterias, and grow out along the five arms of the disc and become
the radial canals and tentacles. All these parts of the water-vascular system
are of course covered by skin, and probably also surrounded by mesoblast
cells, in which at a later period the calcareous plates which lie ventral to the
radial canal are formed. The larval anus disappears. As long as the larval
appendages are not absorbed the ventral and dorsal discs of the permanent
Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain
period the appendages are absorbed. The calcareous rods of the larval arms

1 Whether interradial plates are developed as in Asterias is not clear. They seem
to be found in Ophiopholis bellis, Agassiz, but have not been recognised in other
forms (vide Carpenter, No. 548, p. 369).

ECHINODERMATA. 565

break up, the arms and anal lobe become absorbed, and the dorsal and
ventral discs, with the intervening stomach and other organs, are alone left.
After this the discs fit together, and there is thus formed a complete young
Ophiuroid.

The whole of the internal organs of the larva (except the anus), including
the mouth, cesophagus, the body cavity, etc. are carried on directly into the
adult.

The larval skeleton is, as above stated, absorbed.

The viviparous larva of Amphiura squamata does not differ very greatly
from the larvae with very imperfect arms. It does not develop a distinct
ciliated band, and the provisional skeleton is very imperfect. The absence
of these parts, as well as of the anus, mentioned on p. 549, may probably be
correlated with the viviparous habits of the larva. With reference to the
passage of this larva into the adult there is practically nothing to add to
what has just been stated. When the development of the adult is fairly
advanced the part of the body with the provisional skeleton forms an
elongated rod-like process attached to the developing disc. It becomes
eventually absorbed.

Echinoidea. The Echinus larva (fig. 263} has a Pluteus
form like that of the Ophiuroids, and in most points, such as the

FIG. 264. Two LARV/E OF STRONGYLOCENTRUS. (From Agassiz.)
m. mouth; a. anus; o. cesophagus; d. stomach; c. intestine; »'. and v. ciliated
ridges; iv. water- vascular tube; r. calcareous rods.

presence of the anal lobe, the ciliated band, the provisional
skeleton, etc., develops in the same manner. The chief difference
between the two Pluteus forms concerns the development of the
lateral arms. These, which form the most prominent arms in
the Ophiuroid Pluteus, are entirely absent in the Echinoid

566

ECHINOID PLUTEUS.

Pluteus, which accordingly has, as a rule, a much narrower form
than the Ophiuroid Pluteus.

A pair of ciliated epaulettes on each side of and behind the
ciliated ring is very characteristic of some Echinoid larvae.
They are originally developed from the ciliated ring (fig. 266 A

FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS.

(From Agassiz.) General references as in fig. 264.

b. dorsal opening of madreporic canal; e '. posterior arms ; e'". anterior arms;
flV. anterointernal arms.

and B, z>"). The presence of three processes from the anal lobe
supported by calcareous rods is characteristic of the Spatangoid
Pluteus (fig. 263 E).

The first two pairs of arms to develop, employing the same names as in
Ophiuroids, are the anterior attached to the oral process (fig. 263 C, d] and
the posterior pair (*?')• A pair of anterolateral arms next becomes developed
(j^). A fourth pair (not represented in Ophiuroids) appears on the inner
side of the anterior pair forming an anterointernal pair (e}, and in the
Spatangoid Pluteus a fifth pair may be added on the external side of the
anterior pair forming an anteroexternal pair (g).

Each of the first-formed paired calcareous rods is composed of three
processes, two of which extend into the anterior and posterior arms ; and the
third and strongest passes into the anal lobe, and there meets its fellow
(fig. 265). A transverse bar in front of the arms joins the rods of the two
sides meeting them at the point where the three processes diverge. The
process in the anterolateral arm (fig. 266 B) is at first independent of this
system of rods, but eventually unites with it. Although our knowledge of

ECHINODERMATA. 567

the Pluteus types in the different groups is not sufficient to generalise with
great confidence, a few points seem to have been fairly determined1. The
Plutei of Strongylocentrus (figs. 266 and 267) and Echinus have eight arms
and four ciliated epaulettes. The only Cidaris-like form, the Pluteus of
which is known, is Arbacia : it presents certain peculiarities. The anal lobe
develops a pair of posterior (auricular) appendages, and the ciliated ring,
besides growing out into the normal eight appendages, has a pair of short
blunt anterior and posterior lobes. An extra pair of non-ciliated accessory
mouth arms appears also to be developed. Ciliated epaulettes are not
present. So far as is known the Clypeastroid larva is chiefly characterized
by the round form of the anal lobe. The calcareous rods are latticed. In the
Pluteus of Spatangoids there are (fig. 263) five pairs of arms around the
mouth pointing forwards, and three arms developed from the anal lobe
pointing backwards. One of these is unpaired, and starts from the apex of
the anal lobe. All the arms have calcareous rods which, in the case of the
posterior pair, the anterolateral pair, and the unpaired arm of the anal lobe,
are latticed. Ciliated epaulettes are not developed.

Viviparous larvae of Echinoids have been described by Agassiz2.

The development of the permanent Echinus has been chiefly worked out
by Agassiz and Metschnikoff.

In the Pluteus of Echinus lividus the first indication of the adult arises,
when three pairs of arms are already developed, as an invagination of the
skin on the left side, between the posterior and anterolateral arms, the
bottom of which is placed close to the water-vascular vesicle (fig. 266 B, u/\
The base of this invagination becomes very thick, and forms the ventral disc
of the future Echinus. The parts connecting this disc with the external
skin become however thin, and, on the narrowing of the external aperture of
invagination and the growth of the thickened disc, constitute a covering for
the disc, called by Metschnikoff the amnion. The water- vascular vesicle
adjoining this disc grows out into five processes, forming as many tube feet,
which cause the surface of the involuted disc to be produced into the same
number of processes. The external opening of the invagination of the disc
never closes, and after the development of the tube feet begins to widen
again, and the amnion to atrophy. Through the opening of the invagination
the tube feet now project. The dorsal and right surface of the Pluteus,
which extends so as to embrace the opening of the madreporic canal and
the anus, forms the abactinal or dorsal surface of the future Echinus
(fig. 267, a). This disc fits on to the actinal invaginated surface which arises
on the left side of the Pluteus. On the right surface of the larva (dorsal of
permanent Echinus) two pedicellariae appear, and at a later period spines
are formed, which are at first arranged in a ring-like form round the edge of
the primitively flat test. While these changes are taking place, and the two
surfaces of the future Echinus are gradually moulding themselves so as to

1 Vide especially Muller, Agassiz, and Metschnikoff.

2 For viviparous Echini vide Agassiz, Proc. Amer. Acad. 1876.

568

ECHINOID PLUTEUS.

form what is obviously a young Echinus, the arms of the Pluteus with their
contained skeleton have been gradually undergoing atrophy. They become
irregular in form, their contained skeleton breaks up into small pieces, and
they are gradually absorbed.

The water-vascular ring is from the first complete, so that, as in
Asterias, it is perforated through the centre by a new oesophagus. According

FIG. 266. SIDE AND DORSAL VIEW OF A LARVA OF STRONGYLOCENTRUS.

(From Agassiz.) General reference letters as in figs. 264 and 265.
e" . anterolateral arms; v" '. ciliated epaulettes; ?&'. invagination to form the disc
of Echinus.

to Agassiz the first five tentacles or tube feet grow into the radial canals,
and form the odd terminal tentacles exactly as in Asterias1. Spatangus
only differs in development from Echinus in the fact that the opening of the
invagination to form the ventral disc becomes completely closed, and that
the tube feet have eventually to force their way through the larval epidermis
of the amnion, which is ruptured in the process and eventually thrown
off.

Crinoidea. The larva of Antedon, while still within the
egg-shell, assumes an oval form and uniform ciliation. Before it

1 Gotte (No. 549) supported by Muller's and Krohn's older, and in some points
extremely erroneous observations, has enunciated the view that the radial canals in
Echinoids and Holothuroids have a different nature from those in Asteroids and
Ophiuroids.

ECHINODERMATA.

569

becomes hatched the uniform layer of cilia is replaced by four
transverse bands of cilia, and a tuft of cilia at the posterior
extremity. In this condition it escapes from the egg-shell

FIG. 267. FULL-GROWN LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
The figure shews the largely-developed abactinal disc of the young Echinus
enclosing the larval stomach. Reference letters as in previous figs.

(fig. 268 A), and becomes bilateral, owing to a flattening of the
ventral surface. On the flattened surface appears a ciliated

570

CRINOID LARVA.

depression corresponding in position with the now closed blas-
topore (vide p. 550). The third ciliated band bends forward
to pass in front of this (fig. 269). Behind the last ciliated band
there is present a small depression of unknown function, also

FIG. 768. THREB STAGES IN THE DEVELOPMENT OF ANTEDON (COMATULA.)

(From Lubbock; after Thomson.)

A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Penta-
crinoid larva.

ECHINODERMATA.

571

situated on the ventral surface. The posterior extremity of the
embryo elongates to form the rudiment of the future stem, and
a fresh depression, marking the position of the future mouth,
makes its appearance on the anterior and ventral part.

While the ciliated bands are still at their full development,
the calcareous skeleton of the future calyx makes its appearance
in the form of two rows, each of five plates, formed of a network
of spicula (figs. 268 B and 269). The plates of the anterior ring
are known as the orals, those of the posterior as the basals.
The former surround the left, i.e. anterior
peritoneal sack ; the latter the right, i.e.
posterior peritoneal sack. The two rows
of plates are at first not quite transverse,
but form two oblique circles, the dorsal
end being in advance of the ventral.
The rows soon become transverse, while
the originally somewhat ventral oral
surface is carried into the centre of the
area enclosed by the oral plates.

By the change in position of the
original ventral surface relatively to the
axis of the body, the bilateral symmetry
of the larva passes into a radial sym-
metry. While the first skeletal elements
of the calyx are being formed, the
skeleton of the stem is also established.
The terminal plate is first of all esta-
blished, then the joints, eight at first, of
the stem. The centro-dorsal plate is
stated by Thomson to be formed as the
uppermost joint of the stem1. The larva, after the completion
of the above changes, is shewn in fig. 268 B, and somewhat more
diagrammatically in fig. 269.

After the above elements of the skeleton have become es-
tablished the ciliated bands undergo atrophy, and shortly after-

1 Gotte (No. 549) on the other hand holds that the centro-dorsal plate is developed
by the coalescence of a series of at first independent rods, which originate simul-
taneously with, and close to, the lower edges of the basals, and that it is therefore
similar in its origin to the basals.

FIG. 269. LARVA OF
ANTEDON WITH RUDIMENTS
OF CALCAREOUS SKELETON.
(From Carpenter; after
Thomson.)

i. Terminal plate at the
end of the stem ; 3. basals ;
or. orals ; bl. position of blas-
topore.

572

CRINOID LARVA.

wards the larva becomes attached by the terminal plate of its
stem. It then passes into the Pentacrinoid stage! The larva in
this stage is shewn in fig. 268 C and fig. 270. New joints are
added at the upper end of the stem next the calyx, and a new
element — the radials — makes its appearance as a ring of five
small plates, placed in the space between the basals and orals,
and in the intervals alternating with them
(fig. 270, 4). The roof of the oral vesti-
bule (vide fig. 253 and p. 551) has in
the meantime become ruptured ; and
the external opening of the mouth thus
becomes established. Surrounding the
mouth are five petal-like lobes, each of
them supported by an oral plate (fig.
268 C). In the intervals between them
five branched and highly contractile ten-
tacles, which were previously enclosed
within the vestibule, now sprout out :
they mark the position of the future
radial canals, and are outgrowths of the
water-vascular ring. At the base of each
of them a pair of additional tentacles is
soon formed. Each primary tentacle cor-
responds to one of the radials. These
latter are therefore, as their name implies,
radial in position; while the basals and
orals are interradial. In addition to the
contractile radial tentacles ten non-con-
tractile tentacles, also diverticula of the
water- vascular ring, are soon formed, two
for each interradius.

In the course of the further develop-
ment the equatorial space between the FlG- 27<>. YOUNG PEN-

. TACRINOID LARVA OF AN

orals and the basals enlarges, and gives TEDON. (From Carpenter ;
rise to a wide oral disc, the sides of which after w>'ville Thoms°"-)

- , , . ... . i. terminal plate of stem;

are formed by the radials resting on the cd. centro-donal plate; 3.
basals; while in the centre of it are bftsalsJ 4- radials; or. orals.
placed the five orals, each with its special lobe.

The anus, which is formed on the ventral side in the position

ECHINODERMATA. 573

of the blastopore (p. 551), becomes surrounded by an anal plate,
which is interradial in position, and lies on the surface of the
oral disc between the orals and radials. On the oral plate in
the next interradius is placed the opening of a single funnel
leading into the body cavity, which Ludwig regards as equiva-
lent to the opening of the madreporic canal (vide p. 55 1)1.

From the edge of the vestibule the arms grow out, carrying
with them the tentacular prolongation of the water-vascular ring.
Two additional rows of radials are soon added.

The stalked Pentacrinoid larva becomes converted, on the
absorption of the stalk, into the adult Antedon. The stalk is
functionally replaced by a number of short cirri springing from
the centro-dorsal plate. The five basals coalesce into a single
plate, known as the rosette, and the five orals disappear, though
the lobes on which they were placed persist. In some stalked
forms, e.g. Rhizocrinus Hyocrinus, the orals are permanently
retained. The arms bifurcate at the end of the third radial, and
the first radial becomes in Antedon rosacea (though not in all
species of Antedon) concealed from the surface by the growth of
the centro-dorsal plate. An immense number of funnels, leading
into the body cavity, are formed in addition to the single one
present in the young larva. These are regarded by Ludwig as
equivalent to so many openings of the madreporic canal ; and
there are developed, in correspondence with them, diverticula of
the water-vascular ring.

Comparison of Echinoderm Larvce and General Conclusions.

In any comparison of the various types of Echinoderm larvae
it is necessary to distinguish between the free-swimming forms,
and the viviparous or fixed forms. A very superficial examina-
tion suffices to shew that the free-swimming forms agree very
much more closely amongst themselves than the viviparous

1 I have made no attempt to discuss the homologies of the plates of the larval
Echinodermata because the criteria for such a discussion are still in dispute. The
suggestive memoirs of P. H. Carpenter (No. 548) on this subject may be consulted by
the reader. Carpenter attempts to found his homologies on the relation of the plates
to the primitive peritoneal vesicles, and I am inclined to believe that this method of
dealing with these homologies is the right one. Ludwig (No. 559) by regarding the
opening of the madreporic canal as a fixed point has arrived at very different results.

574

COMPARISON OF ECHINODERM LARV.-E.

forms. We are therefore justified in concluding that in the
viviparous forms the development is abbreviated and modified.

All the free forms are nearly alike in their earliest stage after
the formation of the archenteron. The surface between the
anus and the future mouth becomes flattened, and (except in
Antedon, Cucumaria, Psolinus, etc. which practically have an
abbreviated development like that of the viviparous forms) a
ridge of cilia becomes established in front of the mouth, and a
second ridge between the mouth and the anus. This larval
form, which is shewn in fig. 264 A, is the type from which the
various forms of Echinoderm larvae start.

In all cases, except in Bipinnaria, the two ciliated ridges
soon become united, and constitute a single longitudinal post-
oral ciliated ring.

The larvae in their further growth undergo various changes,
and in the later stages they may be divided into two groups :

(1) The Pluteus larva of Echinoids and Ophiuroids.

(2) The Auricularia (Holothuroids) and Bipinnaria (Aster-
oids) type.

The first group is characterized by the growth of a number
of arms more or less surrounding the mouth, and supported
by calcareous rods. The ciliated band retains its primitive
condition as a simple longitudinal band throughout larval life.
There is a very small prae-oral lobe, while an anal lobe is very
largely developed.

The Auricularia and Bi- A. B

pinnaria resemble each other
in shape, in the development
of a large prae-oral lobe, and
in the absence of provisional
calcareous rods ; but differ in
the fact that the ciliated band
is single in Auricularia (fig
271 A), and is double in Bi-
pinnaria (fig. 271 B).

TheBipinnarialarvashews

THUROID. B. THE LARVA OF AN ASTE-
a great tendency to develop RIAS.

soft arms; while in the Auri- . »'• mouth; st. stomach; a. anus; I.e.
, . ,_, , •*_ •• 1-1- primitive longitudinal ciliated band; pr.c.

cularia the longitudinal ciliat- pr3e-oral ciliated band.

FlG-

THE LARVA OF A

ECHINODERMATA. 575

ed band breaks up into a number of transverse ciliated bands.
This condition is in .some instances reached directly, and such
larvae undoubtedly approximate to the larvae of Antedon, in
which the uniformly ciliated condition is succeeded by one with
four transverse bands, of which one is prae-oral.

All or nearly all Echinoderm larvae are bilaterally symmetrical,
and since all Echinodermata eventually attain a radial sym-
metry, a change necessarily takes place from the bilateral to the
radial type.

In the case of the Holothurians and Antedon, and generally
in the viviparous types, this change is more or less completely
effected in the embryonic condition ; but in the Bipinnaria and
Pluteus types a radial symmetry does not become apparent till
after the absorption of the larval appendages. It is a re-
markable fact, which seems to hold for the Asteroids, Ophiur-
oids, Echinoids, and Crinoids, that the dorsal side of the larva is
not directly converted into the dorsal disc of the adult; but
the dorsal and right side becomes the adult dorsal or abactinal
surface, while the ventral and left becomes the actinal or ventral
surface.

It is interesting to note with reference to the larvae of the
Echinodermata that the various existing types of larvae must
have been formed after the differentiation of the existing groups
of the Echinodermata ; otherwise it would be necessary to adopt
the impossible position that the different groups of Echinoder-
mata were severally descended from the different types of larvae.
The various special appendages, etc. of the different larvae have
therefore a purely secondary significance; and their atrophy
at the time of the passage of the larva into the adult, which
is nothing else but a complicated metamorphosis, is easily ex-
plained.

Originally, no doubt, the transition from the larva to the
adult was very simple, as it is at present in most Holothurians ;
but as the larvae developed various provisional appendages, it
became necessary that these should be absorbed in the passage
to the adult state.

It would obviously be advantageous that their absorption
should be as rapid as possible, since the larva in a state of
transition to the adult would be in a very disadvantageous

576 COMPARISON OF ECHINODERM

position. The rapid metamorphosis, which we find in Asteroids,
Ophiuroids, and Echinoids in the passage from the larval to the
adult state, has no doubt arisen for this reason.

In spite of the varying provisional appendages possessed by
Echinoderm larvae it is possible, as stated above (p. 574), to
recognise a type of larva, of which all the existing Echinoderm
larval forms are modifications. This type does not appear to
me to be closely related to that of the larvae of any group
described in the preceding pages. It has no doubt certain
resemblances to the trochosphere larva of Chaetopoda, Mollusca,
etc., but the differences between the two types are more striking
than the resemblances. It firstly differs from the trochosphere
larva in the character of the ciliation. Both larvae start from the
uniformly ciliated condition, but while the prae-oral ring is almost
invariable, and a peri-anal ring very common in the trochosphere;
in the Echinoderm larva such rings are rarely found ; and even
when present, i.e. the prae-oral ring of Bipinnaria and the terminal
though hardly peri-anal patch of Antedon, do not resemble
closely the more or less similar structures of the trochosphere.
The two ciliated ridges (fig. 264 A) common to all the Echino-
derm larvae, and subsequently continued into a longitudinal ring,
have not yet been found in any trochosphere. The transverse
ciliated rings of the Holothurian and Crinoid larvae are of no
importance in the comparison between the trochosphere larvae
and the larvae of Echinodermata, since such rings are frequently
secondarily developed. Cf. Pneumodermon and Dentalium a-
mongst Mollusca.

In the character of the prae-oral lobe the two types again
differ. Though the prae-oral lobe is often found in Echinoderm
larvae it is never the seat of an important (supra-oesophageal)
ganglion and organs of special sense, as it invariably is in the
trochosphere.

Nothing like the vaso-peritoneal vesicles of the Echinoderm
larvae has been found in the trochosphere ; nor have the charac-
teristic trochosphere excretory organs been found in the Echino-
derm larvae.

The larva which most nearly approaches those of the Echino-
dermata is the larva of Balanoglossus described in the next
chapter.

ECHINODERMATA. 577

BIBLIOGRAPHY.

(542) Alex. Agassiz. Revision of the Echini. Cambridge, U.S. 1872— 74.

(543) Alex. Agassiz. " North American Starfishes." Memoirs of the Museum
of Comparative Anatomy and Zoology at Harvard College, Vol. v., No. i. 1877
(originally published in 1864).

(544) J. Barrois. " Embryogenie de 1'Asteriscus verruculatus " Journal dc
VAnat. et Phys. 1879.

(545) A. Baur. Beitrdge zur Naturgeschichte d. Synapta digitata. Dresden,
1864.

(546) H. G. Bronn. Klassen u. Ordnungen etc. Strahlenthiere, Vol. II. 1860.

(547) W. B. Carpenter. "Researches on the structure, physiology and de-
velopment of Antedon." Phil. Trans. CLVI. 1866, and Proceedings of the Roy. Soc.,
No. 166. 1876.

(548) P. H. Carpenter. " On the oral and apical systems of the Echinoderms."
Quart. J. of Micr. Science, Vol. xvm. and xix. 1878—9.

(549) A. Gotte. " Vergleichende Entwicklungsgeschichte d. Comatula medi-
terranea." Arch, fur micr. Anat., Vol. xn. 1876.

(550) R. Greeff. "Ueber die Entwicklung des Asteracanthion rubens vom Ei
bis zur Bipinnaria u. Brachiolaria." Schriften d. Gesellschaft zur Beforderung d. ge-
sammten Natunvissenschaften zu Marburg, Bd. xn. 1876.

(551) R. Greeff. "Ueber den Bau u. die Entwicklung d. Echinodermen." Sitz.
d. Gesell. z. Beforderung d. gesam. Naturwiss. zu Marburg, No. 4. 1879.

(552) T. H. Huxley. "Report upon the researches of Miiller into the anat.
anddevel. of the Echinoderms." Ann. and Mag. of Nat. Hist., 2nd Ser., Vol. vin.
1851.

(553) Koren and Danielssen. "Observations sur la Bipinnaria asterigera.
Ann. Scien. Nat., Ser. in., Vol. vii. 1847.

(554) Koren and Danielssen. "Observations on the development of the Star-
fishes." Ann. and Mag. of Nat. Hist., Vol. XX. 1857.

(555) A. Kowalevsky. " Entwicklungsgeschichte d. Holothurien. " Mhn.Ac.
Petersbourg, Ser. VII., Tom. XL, No. 6.

(556) A. Krohn. "Beobacht. a. d. Entwick. d. Holothurien u. Seeigel."
Miillers Archiv, 1851.

(557) A. Krohn. "Ueb. d. Entwick. d. Seesterne u. Holothurien." Miillcr's
Archiv, 1853.

(558) A. Krohn. "Beobacht. lib. Echinodermenlarven." Mailer's Archiv,
1854.

(559) H. Ludwig. "Ueb. d. primar. Steinkanal d. Crinoideen, nebst vergl.
anat. Bemerk. lib. d. Echinodermen." Zeit.f. wiss. ZooL, Vol. xxxiv. 1880.

(560) E. Metschnikoff. "Studien iib. d. Entwick. d. Echinodermen u.
Nemertinen." Mem. Ac. Petersboiirg, Series vii., Tom. xiv., No. 8. 1869.

(561)1 Joh. Miiller. "Ueb. d. Larven u. d. Metamorphosed. Echinodermen."
Abhandlungen d. Berlin. Akad. (Five Memoirs), 1848, 49, 50, 52 (two Memoirs).

(562) Joh. Mtiller. "Allgemeiner Plan d. Entwicklung d. Echinodermen."
Abhandl. d. Berlin. Akad., 1853.

1 The dates in this reference are the dates of publication.
B. II. 37

578 BIBLIOGRAPHY.

(563) E. Selenka. "Zur Entwicklung d. Holothurien." Zeit. f. wiss. Zool.,
Bd. xxvii. 1876.

(564) E. Selenka. "Keimblatter u. Organanlage bei Echiniden." Zeit.f.-wiss.
Zool., Vol. xxxin. 1879.

(565) Sir Wyville Thomson. "On the Embryology of the Echinodermata."
Natural History Review, 1 864.

(566) Sir Wyville Thomson. "On the Embryogeny of Antedon rosaceus."
Phil. Trans. 1865.

CHAPTER XXI.

ENTEROPNEUSTA.

THE larva of Balanoglossus is known as Tornaria. The prse-
larval development is not known, and the youngest stage (fig.
272) so far described (Gotte, No. 569) has
many remarkable points of resemblance to
a young Bipinnaria.

A mouth (m\ situated on the ventral
surface, leads into an alimentary canal with
a terminal anus (an). A prae-oral lobe is
well developed, as in Bipinnaria, but there
is no post-anal lobe. The bands of cilia
have the same general form as in Bipin-
naria. There is a prae-oral band, and a
longitudinal post-oral band ; and the two
bands nearly meet at the apex of the prae-
oral lobe (fig. 273). A contractile band

an

FIG. 272. EARLY
STAGE IN THE DEVELOP-
MENT OF TORNARIA.
(After Gotte.)

W. so-called water-
vascular vesicle develop-
ing as an outgrowth
of the mesenteron; m.

passes from the oesophagus to the apex of mouth; an. anus,
the prae-oral lobe, and a diverticulum (fig. 272, W) from the
alimentary tract, directed towards the dorsal surface, is present.
Contractile cells are scattered in the space between the body
wall and the gut.

In the following stage (fig. 274 A) a conspicuous transverse
post-oral band of a single row of long cilia is formed, and the
original bands become more sinuous. The alimentary diverti-
culum of the last stage becomes an independent vesicle opening
by a pore on the dorsal surface (fig. 274 A, w). The contractile
cord is now inserted on this vesicle. Where this cord joins the
apex of the prae-oral lobe between the two anterior bands of
cilia a thickening of the epiblast (? a ganglion) has become

37—2

580

ENTEROPNEUSTA.

C.C.

an.

FIG. 273. YOUNG TORNARIA.

(After Miiller.)

m. mouth ; an. anus ; w. water-
vascular vesicle ; oc. eye-spots ; c.c.
contractile cord.

established, and on it are placed
two eye-spots (fig. 273 oc, and
fig. 274 A). A deep bay is
formed on the ventral surface of
the larva.

As the larva grows older the
original bands of cilia become
more sinuous, and a second
transverse band with small cilia
is formed (in the Mediterranean
larva) between the previous
transverse band and the anus.
The water-vascular vesicle is
prolonged into two spurs, one
on each side of the stomach.
A pulsating vesicle or heart is
also formed (fig. 274 B, ht), and arises, according to Spcngel
(No. 572), as a thicken-
ing of the epidermis.
It subsequently be-
comes enveloped in a
pericardium, and is
placed in a depression
in the water-vascular
vesicle. Two pairs of
diverticula, one behind
the other, grow out
(Agassiz, No. 568) from
the gastric region of
the alimentary canal.
The two parts of each
pair form flattened
compartments, which
together give rise to a
complete investment of
the adjoining parts of
the alimentary tract.
The two parts of each
coalesce, and thus form

FlG. 274. TWO STAGKS IN THK 1 >KY KI.< >I'M KN I

OF TORNARIA. (After Metschnikoff.)

The black lines represent the ciliated hands.
m. mouth; an. anus; br. branchial cleft; ///.

heart ; c. Ixxly cavity between splanchnic and

somatic mesoblast layers; 7.-'. watcr-vascvilar vesicle:

v. circular blood-vessel.

ENTEROPNEUSTA.

58l

a double-walled cylinder round the alimentary tract, but their
cavities remain separated by a dorsal and ventral septum.

Eventually (Spengel) the cavity of the anterior cylinder
forms the section of the body cavity in the collar of the adult,
and that of the posterior (fig. 274 B, c) the remainder of the
body cavity. The septa, separating the two halves of each,
remain as dorsal and ventral mesenteries.

The conversion of Tornaria (fig. 274 A) into Balanoglossus
(fig. 274 B) is effected in a few hours, and consists mainly in
certain changes in configuration, and in the disappearance of
the longitudinal ciliated band.

The body of the young Balanoglossus (fig. 274 B) is divided
into three regions (i) the proboscidian region, (2) the collar,
(3) the trunk proper. The proboscidian region is formed by the
elongation of the prae-oral lobe into an oval body with the eye-
spots at its extremity, and provided with strong longitudinal
muscles. The heart (hi) and water-vascular vesicle lie near its
base, but the contractile cord con-
nected with the latter is no longer
present. The mouth is placed on
the ventral side at the base of the
prae-oral lobe, and immediately be-
hind it is the collar. The remainder
of the body is more or less conical,
and is still girt with the larval
transverse ciliated band, which lies
in the middle of the gastric region
in the Mediterranean species, but
in the cesophageal region in the
American one.

The whole of the body, including
the proboscis, becomes richly cili-
ated.

One of the most important cha- Sus WITH FOUR BRANCHIAL
racters of the adult Balanoglossus CLEFTS* (After Alex. Agossiz.)

r . m. mouth ; an. anus ; br. bran-

consists in the presence of respira- chial cleft . hL heart ; IV. water-
tory structures comparable with the vascular vesicle,
vertebrate gill slits. The earliest traces of these structures
are distinctly formed while the larva is still in the Tornaria

FIG. 275. LATE STAGE IN THE
DEVELOPMENT OF BALANOGLOS-

582 I'N I'KUOl'NKUSTA.

condition, as one pair of pouches from the oesophagus in the
Mediterranean species, and four pairs in the American one
(fig. 275, br).

In the Mediterranean Tornaria the two pouches meet the
skin dorsally, and in the young Balanoglossus (fig. 274 B, br)
acquire an external opening on the dorsal side. In the American
species the first four pouches are without external openings
till additional pouches have been formed. Fresh gill pouches
continue to be formed both in the American and probably
the Mediterranean species, but the conversion of the simple
pouches into the complicated gill structure of the adult
has only been studied by Agassiz (No. 568) in the American
species. It would seem in the first place that the structure of
the adult gill slits is much less complicated in the American than
in the Mediterranean species. The simple pouches of the young
become fairly numerous. They are at first circular ; they then
become elliptical, and the dorsal wall of each slit becomes folded ;
subsequently fresh folds are formed which greatly increase the
complexity of the gills. The external openings are not acquired
till comparatively late.

Our knowledge of the development of the internal organs, mainly
derived from Agassiz, is still imperfect. The vascular system appears early
in the form of a dorsal and a ventral vessel, both pointed, and apparently
ending blindly at their two extremities. The two spurs of the water-vascular
vesicle, which in the Tornaria stage rested upon the stomach, now grow
round the oesophagus, and form an anterior vascular ring, which Agassiz
describes as becoming connected with the heart, though it still communicates
with the exterior by the dorsal pore and seems to become connected with the
remainder of the vascular system. According to Spengel (No. 572) the
dorsal vessel becomes connected with the heart, which remains through life
in the proboscis : the cavity of the water-vascular vesicle forms the cavity of
the proboscis in the adult, and its pore remains as a dorsal (not, as usually
stated, ventral) pore leading to the exterior.

The eye-spots disappear.

Tornaria is a very interesting larval form, since it is inter-
mediate in structure between the larva of an Echinoderm and
trochosphere type common to the Mollusca, Chxtopoda, etc.
The shape of the body especially the form of the ventral
depression, the character of the longitudinal ciliated band, the
structure and derivation of the water-vascular vesicle, and the

ENTEROPNEUSTA. 583

formation of the walls of the body cavity as gastric diverticula,
are all characters which point to a connection with Echinodcrm
larvae.

On the other hand the eye-spots at the end of the prae-oral
lobe1, the contractile band passing from the oesophagus to the
eye-spots (fig. 273), the two posterior bands of cilia, and the
terminal anus are all trochosphere characters.

The persistence of the prae-oral lobe as the proboscis is
interesting, as tending to shew that Balanoglossus is the sur-
viving representative of a primitive group.

*

BIBLIOGRAPHY.

(567) A. Agassiz. "Tornaria." Ann. Lyceum Nat. Hist.\u\. New York,
1866.

(568) A. Agassiz. "The History of Balanoglossus and Tornaria." Mem.
Amer. Acad. of Arts and Stien., Vol. IX. 1873.

(569) A. Gotte. " Entwicklangsgeschichte d. Comatula Mediterranea." Archiv
fur mikr. Anat., Bd. xii., 1876, p. 641.

(570) E. Metschnikoff. " Untersuchungen iib d. Metamorphose, etc. (Tor-
naria)." Zeit.fiir wiss. ZooL, Bd. xx. 1870.

(571) J. M tiller. " Ueb. d. Larven u. Metamor. d. Echinodermen." Berlin
Akad., 1849 and 1850.

(572) J. W. Spengel. "Ban u. Entwicklung von Balanoglossus. Tagebl. d.
Naturf. Vers. Miinchen, 1877.

1 It would be interesting to have further information about the fate of the thicken-
ing of epiblast in the vicinity of the eye-spots. The thickening should by rights be the
supra-oesophageal ganglion, and it does not seem absolutely impossible that it may give
rise to the dorso-median cord in the region of the collar, which constitutes, according
to Spengel, the main ganglion of the adult.

INDEX.

Abdominalia, 459, 493, 499

Acanthocephala, 379

Acanthosoma, 473, 474, 475

Acarina, 444, 454

Accipenser, 102

Achaeta, 319

Achelia, 538

Achtheres percarum, 490

Acineta, 7, 8

Acraspeda, 152, 165, 167, 178, 179, 182,

185, 186

Actinia, 169, 171, 179
Actinophrys, 9

Actinotrocha, 315, 318, 363, 364
Actinozoa, 26, 102, 152, j66, 170, 171,

172, 176, 178, 179, 181, 182, 186
Actinula, 155
Aculeata, 421
^Egineta flavescens, 158
yEginidae, 156, 158
^Eginopsis Mediterranea, 158
/Equorea Mitrocoma, 182
Agalma, 163
Agelena, 436, 450
Agelena labyrinthica, 119, 438
Alciope, 74
Alcippidae, 499
Alcyonaria, 152
Alcyonidse, 167, 168
Alcyonidium mytili, 297, 300, 302
Alcyonium palmatum, 119, 148, 167, 182
Alima, 484, 486
Amoeba, 19, 20
Amphibia, 22, 54, 56, 59, 60, 63, 66, 74,

83, 102

Amphilina, 218

Amphioxus, 54, 56, 59, 61, 66, 93, 426
Amphipoda, 518
Amphiporus lactifloreus, 202
Amphistomum, 31

,, subclavatum, 205

Amphitrochae, 330
Amphiura squamata, 565

Anchorella, 108, 492, 520

Anelasma squalicola, 499

Anguillulidse, 371

Annelida, 14, 25, 98, 503, 525

Anodon, 37, 38, 39, 100, 107, 259, 260,

265, 266, 268
Anopla, 189, 202
Anura, 5

Antedon, 568, 573, 574
Aphides, 15, 16, 76, 79, 116, 428, 429
Aphrodite, 42

Apis, 402, 407, 408, 412, 413
Aplysia, 99, 226, 238, 252, 253
Aplysinidaa, 146
Apoda, 459, 493
Aptera, 395, 420
Apus, 1 6, 79, 460, 463
Arachnida, 22, 114, 119, 413, 4.51, 435,

444, 454, 455, 458, 537, 539
Arachnitis, 171
Araneina, 50, 51, 436
Arbacia, 567
Area, 38
Archigetes, 218
Archizosea gigas, 494
Arenicola, 42

Argiope, 311, 312, 315, 317
Argonauta, 247, 248
Argulus, 492
Armata, 355
Arthropoda, 12, 16, 18, 22, 75, 77,79, 83,

108, no, 221, 382, 383, 434, 448,503,

525> 534» 54', 542
Articulata, 311, 313, 316, 317
Ascaridiae, 371
Ascaris nigrovenosa, 16, 82

,, lumbricoides, 375
Ascetta, 144
Ascidia canina, 53
Ascidians, 74, 102, 208, 426
Asellus aquaticus, 112,120, 516
Astacus, 66, 465, 477, 511, 512, 513,

525

586

INDKX.

Asteracanthion, 69, 70, 561

Asterias, 20, 68, 69, 71, 78, 80, 84, 549,

564
Asteroidea, 35, 36, 544, 549, 557, 563,

576
Astnea, 169

Astroides, 169

Atax Bonzi, 445

Atlanta, 231, 240

Atrochae, 330

Aurelia, 167

Auricularia, 553, 554, 562, 574

Autolytus cornutus, 319, 343

Aves, 56, 59, 61, 64, 107. 109

Axolotl, 1 6

Balanoglossus, 576, 579, 581

Balanus balanoides, 75, 493

Belemnites, 252, 253

Bipinnaria, 557, 563, 574, 576, 579

Blatta, 374, 395

Bojanus, organ of, 264, 282

Bonellia, 20, 43, 44, 98, 324, 355, 358,

359
Bothriocephalus salmonis, 211

,, proboscideus, 212

Brachiella, 492
Brachiolaria, 558, 564
Brachiopoda, 311, 317, 318
Brachyura, 466, 480, 483
Branchiobdella, 42, 43, 346
Branchiogasteropoda, 272
Branchiopoda, 79, 459, 523, 524
Branchipus, 463, 524
Branchiura, 459, 492
Branchionus urceolaris, 221
Braula, 396
Uuccinum, 237, 280
Bulimus citrinus, 229
Bunodes, 169, 171
Buthus, 431

Calcispongiae, 138, 148

Calopteryx, 402

Calycophoridce, 152, 159

Calyptoblastic Hydroids, 184, 185

Calyptraea, 223, 280

Campanularidse, 183, 184

Capitclla, 330, 332

Carabidae, 476

Carcinus Mcenas, 481, 483

Cardium, 260, 262

" pygmaeum, 262

Carinaria, 240

Caryophyllium, 168, 171
•pea, 165, 167

Cecidomyia, 15, 79, 416, 417, 429

Cephalopoda, 20, 40, 41, 102, 108, 109,
135. "5. 240, 242, 244, 250, 252, 253,
270, 271, 272, 274, 279, 282, 287

Cephalothrix galatheae, 202

Ceratosponguc, 146

Cercariae, 207, 208, 209

Cerianthus, 168, 171

Cestodes, 14, 29, 31, 32, 33, 189, 210,
212, 218, 313, 425, 541

Chsetogaster, 342

Chaetopoda, 5, 18, 23, 41, 43, 44, 54,
67, 209, 215, 270, 275, 307, 312, 317,
318, 319, 320, 326, 334, 33<S, 342, 346,
349, 350, 351, 364. 369. 3«3, 3«6, 408,
448, 457, 458, 521, 576,582

ChiXitopteridte, 333

Cha^tosomoidea, 371

Chelifer, 434, 436, 442, 446, 454

Chermes, 15, 429

Chilognatha, 113, 387, 389, 391, 393,

395

Chilopoda, 387, 392, 394
Chilostomata, 292, 297, 298, 304, 305
Chironomus, 15, 378, 401, 402, 415, 416,

429

Chiton, 254, 256, 257, 273
Chordata, 5
Chrysaora, 165
Chthonius, 436
Cicada, 395

Cirripedia, 459, 492, 496,503, 509, 520
Cladocera, 459, 464, 519
Clausilia, 239
Clavella, 520
Clavularia crassa, 167
Cleodora, 241
Clepsine, 73, 346, 347, 349, 351, 352,

353, 354
Clio, 242, 278
Clubione, 436
Clupeidae, 64
Cobitis barbatula, 378
Coccida;, 429
Coccus, 50
Ccelebogyne, 79
Coelenterata, 3, 5, 13, 18, 26, 27, 2«S, 35,

74, 93, 94, 126, 148, 170, 178,179, 1 80,

181, 191, 342

Ccenurus cerebralis, 213, 214
Coleochaete, 1 1

Coleoptera, 396, 402, 409, 412, 420, 421,
^5

Collembola, 395, 426
Comatula, 5, 552, 553
Condracanthus, ill, 120, 520
Conochilus volvox, 22 1
Convoluta, 32
Copepoda, 109, 120, 459, 460, 487, 489,

493, 496, 503, 509, 519
Corallium rubrum, 168, 182
Corethra, 422, 423, 424
Crangoninoe, 476
Crnniiuhv, 311
Craniata, 5, 6, 19, 20, 54, 56, 59, 6l, 62,

64, 74, 102

Crinoidea, 35, 36, 544, 550, 568, 576
Criodilus, 321, 324, 328, 341

INDEX.

Crisia, 304
Crocodilia, 63

Crustacea, 5, 6, 18, 51, 66, 102, 109, 120,
458, 465> 487* 5<>2, 521, 524, 537, 541
Cryptophialus, 499, 509
Crystalloides, 163
Ctenophora, 26, 93, 102, 152, 173, 175,

177, 178, 179, 180, 181, 182
Ctenostomata, 292, 297, 298, 304, 305
Cucullarms elegans, 46, 75, 82, 371, 376
Cucumaria, 546, 556, 574
Cumaceae, 459, 465, 486, 506
Curculio, 421

Cyclas, 259, 260, 261, 265
Cyclops, 376, 377, 418, 489, 503
Cyclostomata, 102, -292, 304
Cymbulia, 241, 242

Cymothoa, 516, 517, 519, 520,524, 528
Cynipidae, 15, 421, 428
Cyphonautes, 297, 301, 304, 306, 308
Cypridina, 500, 502
Cysticercus cellulosce, 214, 217

,, fasciolaris, 216

,, limacis, 213

Daphnia, 79, 464

Dasychone, 331, 336

Decapoda, 66, 248, 459, 465, 469, 504,

511

Dendroccela, 32, 33, 189, 195, 196
Dentalium, 258, 576
Desmacidon, 147
Desor, type of, 196, 197, 201, 202, 204,

212, 424
Diastopora, 304
Dibranchiata, 225, 253
Dicyema, 9, 131, 134, 135, 136
Dimya, 225
Diphyes, 159
Diplozoon, 11, 209, 210
Diporpa, 210
Diptera, 49, 194, 204, 396, 401,402,407,

409, 412, 416, 420, 429
Discina radiata, 317
Discinidse, 311

Discophora, 18, 42, 165, 346, 383
Distomese, 189, 205, 425
Distomum, 31

,, cygnoides, 209

,, globiparum, 207

,, lanceolatum, 205
Dochmius duodenale, 375

,, trigonocephalus, 375
Donacia, 401
Dracunculus, 376, 377

Echinaster fallax, 23

,, Sarsii, 102, 561
Echinodermata, 5, 18, 24, 35, 74, 102,

325, 424, 544, 573, 574, 576> 582
Echinoidea, 35, 36, 544, 549, 565, 576
Echinorhyncus, 379, 380

Echinus lividus, 83, 84, 88

Echiurus, 44, 357, 358

Ectoprocta, 297, 306

Edriophthalmata, 459, 465

Elaphocaris, 473

Elasmobranchii, 23, 56, 59, 61, 62, 64,

67, 105, 106. 107, 108, 109
Enopla, 189, 202
Entoconcha mirabilis, 237
Entomophaga, 421

Entoprocta, 292, 298, 300, 302, 304, 306
Epeira, 436

Ephemera, 395, 409, 420, 422
Ephyra, 186

Epibulia auranliaca, 159, 165
Erichthus, 484, 507
Errantia, 319, 336
Esperia, 147
Estheria, 463, 464
Euaxes, lol, 322, 324, 341, 346,349
Eucharis, 178 •

,, multicornis, 178

Eucopepoda, 459
Eucope polystyla, 23, 154

Eunice sanguinea, 319

Eupagurus prideauxii, 112, 113, 115, 511,
520

Euphausia, 465, 468, 504, 505, 518, 523

Eurostomata, 176

Eurylepta auriculata, 192

Eurynome, 483

Euspongia, 146, 147

Filaria, 377
Filaridae, 371
Firoloidea, 240
Flagellata, 7, 8
Flustrella, 301, 303
Formica, 396
Fungia, 182, 186
Fusus, 275, 280, 284, 288

Gammarus, 122, 518

,, fluviatilis, 117
,, locusta, no, 112
Ganoids, 54, 102
Gasteropoda, 39, 41, 98, 225, 226, 229,

230, 232, 233, 240, 258, 260, 261, 270,

272, 275, 279, 283, 324
Gasterosteus, 64, 210
Gastrotricha, 370
Gasterotrochce, 330, 333
Gecarcinus, 465
Geophilus, 392, 393
Gephyrea, 5, 18, 24, 44, 54, 67, 102,

318, 320, 325, 355, 357, 361, 364
Germogen, 134
Geryonia hastata, 156
Geryonidse, 156
Glochidia, 267, 268
Gnathobdellidas, 346, 349
Gordiacea, 94

588

INDEX.

Cimlioidca, 371, 374, 378

;»nia, 168
Gorgonidce, 181
Gorgoninrc, 181
Gregarinidae, 8
Gryllotalpa, 401, 412, 413
Gunnnineiv, 147, 148
Gymnoblastic Hydroids, 184, 185
Gymnoloemata, 292

Gymnosomata, 225, 240, 241, 242, 270
Gyrodactylus, 210

Halichondria, 147

Ilalisarca, 22, 66, 145

Halistemma, 165

Helicidce, 238

Helioporidae, 182

Helix, 67, 229

Hemiptera, 395, 402, 403, 409, 420, 421

Hessia, 108, 492

Heterakis vermicularis, .374

Heteronereis, 343

Heteropoda, 71, 72, 225, 226, 231, 278

Hexacoralla, 152, 179, 182

Hippopodius gleba, 27, 159

Hirudinea, 74, 84

Hirudo, 350, 351, 352, 353, 354

Holometabola, 420, 422

Holostomum, 205

Holothuria, 19, 25, 35, 549, 558, 576

Holothuroidea, 35, 544, 553, 556

Homarus, 477

Hyaleacea, 273, 275

Hyaleidce, 241

Hydra, 21, 22, 26, 28, 29, 34, 152, 154,

155. 179, 183
Hydractinia, 539
Hydrocoralla, 152, 181, 185
Hydroidea, 152
Hydromedusae, 152, 179, 182, 183, 184,

185, 186, 187
Hydrophilus, 374, 396, 400, 401, 402,

404, 408, 409
Hydrozoa, 14, 19, 26, 27, 67, 102, 152,

155. 165. 179, 1 80, 181, 182, 539
Ilymenoptera, 396, 401, 402, 412, 420,

421, 425

Ichneumon, 396

Inarticulata, 311, 316

Incrmi

Infusoria, 7, 8

Insecta, 5, 15, 18, 19, 25, 46, 395, 396,

4^5, 455. 45«
Intoshia gigas, 136
Isidimc, 181
Ixxlyctia, 147
Isopoda, 109, 515, 519, 521, 523, 527

Julus Moneletei, 387, 388, 389
Kochlorine, 499

Lacertilia, 64
Lacinularia, 221, 223

„ socialis, 75
Lamellibranchiata, 23, 25, 37, 39, 98,

225, 241, 257, 258, 259, 269, 270, 271,

273, 274, 288
Lepadkue, 498

Lepas fascicularis, 224, 493, 494, 495
Lepidoptera, 79, 396, 402, 407, 408, 412,

413, 415, 417, 420, 421, 423, 415, 426.

455

Leptodora, 16, 51
Leptoplana, 74, 189, 192, 193
Lernseopoda, 490, 492, 520
Leucifer, 507

Libellulidae, 402, 403, 409, 420
Limax, 229, 232, 239, 278, 280
Limnadia, 79, 524
Limulus, 534
Lina, 402

Lingulidae, 311, 316
Lithobius, 393
Lobatse, 178

Loligo, 242, 243, 244, 247, 253
Loricata, 507, 514
Lota, 105

Loxosoma, 292, 294, 296, 306, 307
Lucernaria, 185
Lumbricus, 341, 368

,, agricola, 321

,, rubellus, 324

„ trapczoides, 13, 321, 323
Lumbriconereis, 334
Lymnseus, 82, 98, 226, 227, 232, 238,

281
Lycosa, 436

Macrostomum, 32, 34

Macrura, 476

Malacobdella, 203

Malacodermata, 171

Malacostraca, 66, 459, 462, 465, 504,

505, 506, 511, 523
Mammalia, 56, 58, 59, 64, 66
Marsipobranchii, 59
Mastigopus, 473, 474
Medusoe, 27, 154, 157, i.^s, 16;, 164, 176,

178, 181, 182, 183, 184, 185, 186
Megalopa, 482, 483, 484
Melolontha, 402, 421
Membranipora, 297, 303
Mermithido;, 371
Mesotrochoe, 330
Metachoetoe, 335
Metazoa, Q, 10, 12,67, 86, 125, 135, 14^,

ISO, 179

Millepora, 152, 181

Mitraria, 308, 337

Molgula, 102

Mollusca, 5, 18, 24, 66, 74, 84, 99, 225,
247, 248, 251, 256, 257, 262, 271, 285,
288, 307, 325, 333, 352, 576, 582

INDEX.

589

Monomya, -225
Monostomum capitellum, 205

,, mutabile, 205, 206

Monotrochse, 330
Montacuta, 260, 262
Musca, 396
Muscidae, 420, 423
Myobia, 444, 445
Myrianida, 343
Myriapoda, 22, iir, 113, 387, 394, 395,

4i.3» 458
Mynothela, 155
Myrmeleon, 396

Mysis, 120, 469, 472, 486, 504, 509, 525
Mytilus, 260, 261
Myxinoids, 5
Myxispongise, 145
Myzostomea, 369

Nais, 342

Nassa mutabilis, 101, 226, 227, 233, 262,

278, 279, 288, 3^4
Natantia, 487
Natica, 237, 283
Nauplius, 5, 16, 460, 461, 463, 465, 466,

469, 473, 490, 491, 493, 497
Nautilus pompilius, 253, 276
Nebaliadse, 459, 465, 486
Nematoda, 45, 46, 50, 66, 74, 75, 371,

373. 374> 376
Nematogens, 131
Nematoidea, 18, 84, 94, 371, 374
Nematus ventricosus, 13, 427
Nemertea, 94, 189, 196, 202, 204
Nemertines, 30, 31, 33, 93, 136, 195,

202, 328, 333, 424
Nephelis, 82, 346, 349, 350, 351, 352,

354

Nereis, 343

,, diversicolor, 319

,, Dumerilii, 343
Neritina, 229, 237
Neuroptera, 396, 401, 420, 421
Neuroterus ventricularis, 428
Notonecta, 395
Nototrochse, 330, 353
Nudibranchiata, 229, 241

Ocellata, 184

Octocoralla, 152, 179

Octopus, 248

Odontophora, 225, 257, 271

Odontosyllis, 333

Oedogonium, 1 1

Oligochseta, 42, 319, 321, 325, 330, 338,

346, 352
Olynthus, 144
Oniscus murarius, 107, 108, 109, 120,

516, 520, 528
Opercula, 31
Ophiothryx, 36, 549
Ophidia, 64

Ophiuroidea, 136, 544, 553, 562, 565,

576

Ophryotrochoe puerilis, 333
Opisthobranchiata, 225, 232, 237
Ornithodelphia, 109
Orthonectidae, 136

Orthoptera, 395, 414, 420, 421, 425, 426
Ostracoda, 459, 500, 510
Ostrea, 259, 260, 262
Oxyuridse, 46, 373, 374
Oxyurus ambigua, 374
,, vermicularis, 375

PcEcilopoda, 534
Paguridse, 477
Pakemon, no
Palaemonetes, 476
Pakemoninre, 476, 511, 512
Palinurus, 478, 480

Paludina, 66, 227, 229, 235, 270, 278,
280

,, costata, 229

,, vivipara, 226
Pandorina, n
Parasita, 489
Pedalion, 221

Pedicellina, 98, 292, 296, 299, 307
Pelagia, 167, 185
Penseinse, 476
Penaeus, no, 113, 465, 469, 473, 474,

504, 518

Pennatulidae, 181
Pentacrinus, 5
Pentastomida, 539, 540
Pentastomum denticulatum, 540, 54!

tsenoicles, 539, 540, 541
Percidae, 64
PerennichaetcE, 335
Peripatus, 5, 386, 411, 412, 413, 542
Petromyzon, 61, 63, 64, 74, 83
Phalangella, 304
Phalangidse, 436
Phallusia, 83

Phascolosoma, 44, 355, 356, 361
Pholcus, 436, 442
Phoronis, 315, 355, 363, 364
Phoxinus laevis, 378
Phryganea, 396, 401, 409
Phylactokemata, 292, 294, 297, 305, 306
Phyllobothrium, 218
Phyllodoce, 329
Phyllopoda, 16, 459, 461, 505
Phyllosoma, 479, 480
Phylloxera, 429
Physophoridoe, 152, 16-2, 164
Pilidium, type of, 196, 200, 201, 202, 704,

424

Pisces, 5
Piscicola, 20, 43
Pisidium, 259, 260, 262, 264
Planaria Neapolitana, 193
Planorbis, 273, 281, 325

590

INDEX.

Platyelminthes, 18, 20, 24, 221, 424
Platygaster, 396, 416, 417, 418, 419
Pleurohrachia, 176, 177, 238
Pneumodermon, 242, 576
Podostomata, 292
Poduridce, 401, 405
Polychaeta, 42, 319, 325, 338
Polydesmus complanatus, 387, 388
Polygordius, 319, 325, 326, 327, 328,

332, 357» 386
Polynoe, 42, 331
Polyophthalmus, 328
Polyplacophora, 225, 254, 270, 271, 288
Polystomeas, 189, 205, 209
Polystomum, 209

,, integerrimum, 30, 31, 210

Polytrochne, 330, 333
Polyxenia leucostyla, 158
Polyxenus lagurus, 387
Polyzoa, 98, 303, 305, 306. 308> 3!5. 3^
Porcellana, 483
Porifera, 102, 138, 148
Porthesia, 115
Prorhyncus, 32, 34
Prosobranchiata, 225, 237, 281
Prostomum, 32, 34, 38, 196
Protozoa, 8, 9, lo, n, 86, 135, 149
Protozoaea, 471
Protula Dysteri, 342
Pseudoneuroptera, 426
Pseudoscorpionid;e, 434
Psolinus, 556, 574
Psychidae, 16
Pteraster miliaris, 561
Pteropoda, 98, 225, 226, 229, 230, 232,

240, 258, 270, 272, 279, 283
Pterotrachcea, 71, 229, 240
Pulex, 396

Pulmonata, 39, 225, 232, 238, 281, 282
I'urpura lapillus, 78
Pycnogonida, 538
Pyrosoma, 13, 53, 109

Rana temporaria, 210
Kaspailia, 147
Rcdia, 206, 207, 208, 209
Reniera, 147

Kcptilia, 56, 59, 60, 61, 62, 64, 109
Rhabditis dolichura, 82
Khabdoccela, 32, 33, iSy, ic/>
Khnbdopleura, 294, 306
Rhi/occphala, 459, 493, 499, 500
Klii/.ocrinus, 5
klii/.ostoma, 167
Rhomlx>gens, 131, 134
Khynchoncllidaj, 311
Rhyncdbddlkbe, 346
Rotifera, 5, 12, 18, 75, 76, 77, 79, 83,
102, 221, 308, 325

Saccocirrus, 328, 329, 332, 340
Sacculina, 500

Sagartia, 169, 171

Sagitta, 33, 74, 94, 130, 366, 367, 368

Salmonidrc, 64

Salpa, 102

Sarcia, 164

Seaphopoda, 225, 257, 270, 271

Schistocephalus, 2 1 1

Schizopoda, 459, 465, 466

Scolopendra, 392

Scorpio, 120, 43 r, 446, 454, 455, 457

Scrobicularia, 38, 39

Scyllarus, 477

Scyphistoma, 179, 185, 186

Sedentaria, 319, 336

Sepia, 20, 40, 41, 242, 243, 244, 245,

247> 249> 253
Sergestidce, 473, 507
Serpula, 319. 325, 331
Sertularia, 152, 183, 184
Silicispongia.', 147
Simulia, 401, 415
Siphonophora, 13, 77, 152, 159, 163,

165, 179, 1 80, 182, 185
Sipunculida, 24
Sipunculus, 44
Sirex, 396
Sitaris, 42!

Spathegaster baccarum, 428
Spjo, 42> 332> 333
Spiroptera obtusa, 376
Spirorbis Pagenstecheri, 319

„ spirillum, 319, 336
Spirula, 252
Spirulirostra, 252
Spongelia, 147
Spongida, 138, 144, 148, 149
Spongilla, 147, 150
Sporocysts, 206, 207, 208, 209
Squilla, 66, 504, 507
Stephanomia pictum, 162, 165
Stomalopoda, 459, 465, 4X4
Stomodoeum, 413
Strongylidrc, 371, 375
Strongylocentrus, 567
Strongysoloma Guerinii, 3<S7, 388, 390
Stylasterictae, 152, r8r
Styliolidic, 24!
Stylochopsis ponticus, 193
Sycandra, 93, 138, 144, 145, 147, 150

,, raphanus, i^S, 174
Syllis, 343

„ vivipara, 319
Sympodium coralloidcs, 168

Taeniatoe, 178
Tardigrada, 541
Teoenaria, 436

'I'clcDsti'i, IS, 25, 5^), 59, C>4. 107, io<)
I'r].)troch;i.-, 330
Tcndra, 300
'I '(.'nth reds, 396
Tcrcbdla concliilcga, 332, 333, 337

INDEX.

591

Terebella nebulosa, 332, 333
Terebratula, 311, 315
Terebratulina, 311, 315, 316

,, septentrionalis, 315, 316

Teredo, larva of, 262
Tergipes, 232, 238

,, Edwardsii, 238
,, lacinulatus, 238
Tethya, 147
Tetrabranchiata, 225
Tetranychus telarius, 116
Tetrastemma varicolor, 203
Thalassema, 44, 355, 357
Thalassinidae, 477
Thallophytes, n
Thecidium, 311, 312, 315, 316
Thecosomata, 225
Thoracica, 459, 493, 499, 500
Thysanozoon, 192, 193
Thysanura, 395, 408, 425, 458
Tichogonia, 39
Tipula, 396
Tipulidae, 420, 421
Toenia cosnurus, 214

,, echinococcus, 215, 217

„ solium, 217
Tornaria, 579, 581
Toxopneustes, 22, 24, 35, 85, 88, 89
Tracheata, 385, 426, 432, 448, 455, 457,

458, 538, 54i

Trachymedusae, 152, 156, 179, 185
Trematodes, 14, 16, 29, 30, 31, 32, 33,

46, 94, 189, 205, 208, 210, 212, 216

Trichina, 377, 378

Trichinidse, 371

Trichocepha'lus affinis, 374

Trochosphsera aequatorialis, 221

TubiporidcE, 182

Tubularia, 34, 38, 152, 154, 158

Tubularidse, 29, 179, 183

Tunicata, 5, I4, 53

Turbellaria, 5, 30, 31, 33, 74, 98, 102,

136, 179, 189, 193, 333
Tyroglyphus, 445

Unio, 37, 38, 39, 100, 101, 259, 260,265,
266, 445

Vaginulus luzonicus, 229

Vermes, 5, 74, 102, 223, 324, 352

Verongia rosea, 146

Vertebrata, 14, 18, 19, 24, 59, 64, 83,

272, 349. 397' 4^6
Vesiculata, 184
Vitrina, 229
Vorticella, 8, 9, 10

Wilsia, 164
Xiphoteuthis, 252

Zoantharia, 152, 168, 169
Zooea, 465, 468, 471, 474, 482, 483, 484,
486, 504

BIBLIOGRAPHY.

THE OVUM.

General Works.

(1)} Ed. van Beneden. "Recherches sur la composition et la signification de
,A « T m' cour' d' l Acad" r°y- des Sci<™<* de Belgique, Vol. xxxiv. 1870.

/ '%, R- Leuckart. Artikel "Zeugung," R. WagMsfs Handworterbtek d. Physio-

logte, Vol. iv. 1853.

(3^ Fr' L/ydig- , "Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt
u. n. ihrer Bedeutung." Oken. Isis, 1848.

(4) Ludwig. "Ueber d. Eibildung im Thierreiche." Arbeiten a. d. zool.-zoot
Institiit. Wiirzburg, Vol. I. rSy^.

(5) AllenThomson. Article ' ' Ovum " in Todd's Cyclopedia of Anatomy and
Physiology, Vol. v. 1859.

(6) W. Waldeyer. Eierstock u. EL Leipzig, 1870.

THE OVUM OF CCELENTERATA.

(7) Ed. van Beneden. "De la distinction originelle d. testicule et de
1'ovaire." Bull. Acad. roy. Belgique, f serie, Vol. xxxvu. 1874.

(8) R. and O. Hertwig. Der Organismus d. Medusen. Jena, 1878.

(9) N. Kleinenberg. Hydra. Leipzig, 1872.

THE OVUM OF PLATYELMINTHES.

(10) P. Hallez. Contributions a fHistoire naturelle des Turbellarih. Lille,
1879.

(11) S. MaxSchultze. Beitrdge z. Naturgeschichte d. Turbellarien. Greifs-
wald, 1851.

(12) C. Th. von Siebold. ' ' Helminthologische Beitrage." Miiller's Archiv,
1836.

(13) C. Th. von Siebold. Lehrbuch d. vergleich. Anat.d. wirbellosen Thiere.
Berlin, 1848.

(14) E. Zeller. " Weitere Beitrage z. Kenntniss d. Polystomen." Zeit. f.
wiss. ZooL, Bd. xxvu. 1876.

[Vide also Ed. van Beneden (No. i).]

THE OVUM OF ECHINODERMATA.

(15) C. K. Hoffmann. " Zur Anatomic d. Echiniden u. Spatangen." Nieder-
llindisch. Archiv f. Zoologie, Vol. I. 1871.

(16) C. K. Hoffmann. " Zur Anatomic d. Asteriden. Niederldndisch. Ardiiv
/. Zoologie, Vol. n. 1873.

(17) H. Ludwig. "Beitrage zur Anat. d. Crinoiden." Zeil. f. wiss. Zool.,
Vol. xxvin. 1877.

(18) Job. Miiller. "Ueber d. Canal in d. Eiern d. Holothurien." Miiller's
Archiv, 1854.

(19) C. Semper. Holothurien. Leipzig, 1868.

(20) E. Selenka. Befruchtung d. Eies v. Toxopneustes variegalits, 1878.

[Vide also Ludwig (No. 4), etc.]

1 A very complete and critical account of the literature is contained in this paper.
B. II. a

BIBLIOGRAPHY.

THE OVUM OF MOLLUSC A.
Lamellibranchiata.

(21) II. Lacaze-Duthiers. " Organes genitaux des Acephales Lamelli-
branches." Ann. Set. Nat., 4mc serie, Vol. 1 1. 1854.

(22) W. F lemming. " Ueb. d. er. Entwick. am Ei d. Teichmuschel." Archiv
f. mikr. Anat., Vol. x. 1874.

(23) W. Flamming. "Studien lib. d. Entwick. d. Najaden." Sitz. d. t: Akad.
Wiss. men, Vol. LXXI. 1875.

(24) Th. von Hassling. " Einige Bemerkungen, etc." Zeit. f. wiss. ZooL,
Bd. v. 1854.

(25) H. von Jhering. "Zur Kenntniss d. Eibildung bei d. Muscheln." Zeit.
f. wiss. ZooL, Vol. xxix. 1877.

(26) Keber. De Introihi Spermatozoorum in ovula, etc. Konigsberg, 1853.

(27) Fr. Leydig. " Kleinere Mittheilung etc." Miiller's Archiv, 1854.

Gasteropoda.

(28) C. Semper. "Beitrage z. Anat. u. Physiol. d. Pulmonaten." Zeit. f.
wiss. ZooL, Vol. vni. 1857.

(29) H. Eisig. " Beitrage z. Anat. u. Entwick. d. Pulmonaten." Zeit.f. wiss.
ZooL, Vol. xix. 1869.

(30) Fr. Leydig. " Ueb. Paludina vivipara." Zeit.f. wiss. ZooL, Vol. u. 1850.

Cephalopoda.

(31) Al. Kolliker. Entwicklungsgeschichte d. Cephalopoden. Zurich, 1844.

(32) E. R. Lankester. "On the Developmental History of the Mollusca."
Phil. Trans., 1875.

THE OVUM OF THE CHJETOPODA.

(33) Ed. Claparede. " Les Annelides Chaetopodes d. Golfe de Naples."
Mem.d. 1. Soctit. phys. eld1 hist. nat. de Geneve, 1868 — 9 and 1870.

(34) E. Ehlers. Die Borstcnwiirmer nach system, und anat. Untersuchungen.
Leipzig, 1864—68.

(35) E. Selenka. " Das Gefass-System d. Aphrodite aculeata." Niedcrldndi-
sches Archiv f. ZooL, Vol. n. 1873.

THE OVUM OF DISCOPHORA.

(36) H. Dorner. " Ueber d. Gattung Branchiobdella." Zeit.f. wiss. ZooL,
Vol. xv. 1865.

(37) R. Leuckart. Die menschlichen Parasiten.

(38) Fr. Leydig. "Zur Anatomie v. Piscicola eeometrica, etc." Zeit. f. wiss.
ZooL, Vol. I. 1849.

(30) C. O. Whitman. "Embryology of Clepsine." Quart. 7. of Alter.
Sci., Vol. xvin. 1878.

THE OVUM OF GEPHYREA.

(40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861.

(41) C. Semper. Holothurien, 1868, p. 145.

(42) J. W. Spengel. " Beitrage z. Kenntniss d Gephyreen." Beitriigc a. d.
zool. Stationz. Neapcl, Vol. I. 1879.

(43) J. W. Spengel. " Anatomische Mittheilungen lib. Gephyreen." Tagcbl.
d. Naturf. Vers. Munchen, 1877.

THE OVUM OF NEMATODA.

(44) Ed. Claparede. De la formation ct de la fccondaiiou dcs- n-uf.\ chcz Ics
I'crs Ntmatodcs. (ienevc, 1859.

(J-r)) K. I. (.-nek art. Hif nirnsf/i lichen Paras! ten.

BIBLIOGRAPHY. jjj

d.Nematoden."

^' Nels0n* "On the reproduction of Ascaris mystax, etc." Phil.
(48) A.Schneider. Monographie d.' Nematoden. Berlin, 1866.
THE OVUM OF INSECT A.

£Sm £ ' T? run d V Ueb,?'*a5 Ei u' seine Bildungsstdtte. Leipzig, 1 878.
(50) T. H. Huxley. " On the agamic reproduction and morphology of Aphis.
Ltnnean Trans., Vol. xxn. 1858. Vide also Manual of Invertebrate* Animals, 1877.

1 * ^ ^ ^ *

(51)
bei den *,++,*„»„

/-a\ ?r',key<MS' Der Eierstock u. die Samentasche d. Insecten. Dresden, 1866.
tSl ~ub. bock- " The ova and pseudova of Insects." Phil. Trans. 1850.
(o4) Stem. Die weiblichen Geschlcchtsorgane d. Ktifer. Berlin, 1847.
[Conf. also Glaus, Landois, Weismann, Ludwig (No. 4).]

THE OVUM OF ARANEINA.

(55) Victor Cams. " Ueb. d. Entwick. d. Spinneneies." Zeit. f. wiss. Zool.t
Vol. ii. 1850.

(56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc."
Miiller s Archiv, 1849.

[Conf. Leydig, Balbiani, Ludwig (No. 4), etc.]

THE OVUM OF CRUSTACEA.

(57) Aug. Weismann. "Ueb. d. Bildung von Wintereiern bei Leptodora
hyalina." Zeit.f. wiss.ZooL, Vol. xxvn. 1876.

[For general literature vide Ludwig, No. 4, and Ed. van Beneden, No. i.]

THE OVUM OF CHORD ATA.

Urochorda (Tunicata).

(58) A. Kowalevsky. " Weitere Studien ii. d. Entwicklung d. Ascidien."
Archiv f. micr. Anat., Vol. VII. 1871.

(59) A. Kowalevsky. "Ueber Entwicklungsgeschichte d. Pyrosoma."
Arch.f. micr. Anat., Vol. xi. 1875.

(60) Kupffer. " Stammverwandtschaft zwischen Ascidien u. Wirbelthieren."
Arch. f. micr. Anat., Vol. VI. 1870.

(61) Giard. " Etudes critiques des travaux, etc. " Archives Zool. experiment.,
Vol. I. 1872.

(62) C. Semper. " Ueber die Entstehung, etc." Arbeiten a. d. zool.-zoot.
Institut Wiirzburg, Bd. II. 1875.

Cephalochorda.

(63) P. Langerhans. "Z. Anatomic d. Amphioxus lanceolatus," pp. 330 — 3.
Archiv f. mikr. Anat., Vol. xil. 1876.

Craniata.

(64) F. M. Balfour. "On the structure and development of the Vertebrate
Ovary." Quart. J. of Micr. Science, Vol. xvm. 1878.

(65) Th. Eimer. " Untersuchungen ii. d. Eier d. Reptilien." Arckiv f.
mikr. Anat., Vol. vni. 1872.

(66) Pfliiger. Die Eierstbcke d. Sdugethiere u. d. Menschen. Leipzig, 1863.

(67) J. Foulis. " On the development of the ova and structure of the ovary in
Man and other Mammalia." Quart. J. of Micr. Science, Vol. XVI. 1876.

(68) J. Foulis. " The development of the ova, etc." Journal of Anat. and
Phys., Vol. xni. 1878—9.

a 2

IV BIBLIOGRAPHY.

(69) C. Gegenbaur. " Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit
partieller Dottertheilung." Muller's Archiv, 1861.

(70) Alex. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(71) W. His. Untersuchungen iib. d. Ei u. d. Eientwicklung bei Knochenfischcn.
Leipzig, 1873.

(72) A. Kolliker. Entwicklungsgeschichte d. Menschen u. hoherer Thicre,
Leipzig, 1878.

(73) J. Miiller. " Ueber d. zahlreichen Porenkanale in d. Eikapsel d. Fische."
Muller's Archiv, 1854.

(74) W. H. Ransom. " On the impregnation of the ovum in the Stickleback."
Pro. K. Society, Vol. vn. 1854.

(75) C. Semper. " Das Urogenitalsystem d. Plagiostomen etc." Arbeiten a.
d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

[Cf. Ludwig, No. 4, Ed. van Beneden, No. i, Waldeyer, No. 6, etc.]

MATURATION AND IMPREGNATION OF THE OVUM.

(76) Auerbach. Organologische Studien, Heft 2. Breslau, 1874.

(77) Bambeke. " Recherches s. Embryologie des Batraciens." Bull, de
royale de Belgique, 2me ser., T. LXI. 1876.

(78) E. van Beneden. " La Maturation de 1'CEufdes Mammiferes." Bull,
de fAcad. royale de Belgique, 2me ser., T. XL. No. 12, 1875.

(79) Id em. " Contributions a 1'Histoire de la Vesicule Germinative, &c." Bull,
de fAcad. royale de Belgique, sme ser., T. XLI. No. i, 1876.

(80) O. Biitschli. Eizelle, Zelltheilung, und Conjugation der Infusorien.
Frankfurt, 1876.

(81) F. M. Balfour. " On the Phenomena accompanying the Maturation and
Impregnation of the Ovum." Quart. J. of Micros. Science, Vol. xvm. 1878.

(82) Calberla. " Befruchtungsvorgang beim Ei von Petromyzon Planeri.*'
Zeit. f. iviss. Zool., Vol. xxx.

(83) W. Flemming. "Studien in d. Entwickelungsgeschichte der Najaden."
Sitz. d. k. Akad. Wiett, B. LXXI. 1875.

(84) H. Fol. "Die erste Entwickelung des Geryonideneies. " Jenaische
Zeitschrift, Vol. vn. 1873.

(85) Idem. " Sur le Developpement des Pte"ropodes." Archives de Zoologic
Experimental et Gtnerale, Vol. iv. and v. 1875 — 6.

(86) Idem. " Sur le Commencement de 1'Henog^nie." Archives des Sciences
Physiques et Naturelles. Geneve, 1877.

(87) Idem. Recherches s. I. Ftcondation etl. comrnen. d. rHcnogcnic. Geneve, 1879.

(88) R. Greeff. " Ueb. d. Bau u. d. Entwickelung d. Echinodermen." Sitzun.
der Gesellschaft z. Befonlerung d. gesammten Naturwiss. z. Marburg, No. 5, 1876.

(89) Oscar Hertwig. " Beit. z. Kenntniss d. Bildung, &c., d. thier. Eies."
Morphologisches Jahrbuch, Vol. I. 1876.

(90) Idem. Ibid. Morphologisches Jahrlntch, Vol. ill. Heft i, 1877.

(91) Idem. " Weitere Beitrage, &c." Morphologisches Jahrbuch, Vol. in. 1877.
Heft 3.

(92) Idem. "Beit. z. Kenntniss, &c." Morphologisches Jahrbuch, Vol. iv.
Heft i and 2. 1878.

(93) N. Kleinenberg. Hydra. Leipzig, 1872.

(94) C. Kupffer u. B. Benecke. Der Vorgang d. llcfnichtinig am Eie d.
Neunaugen. Konigsberg, 1878.

(95) J. Oellacher. "Beitrage zur Geschichte des Keimblaschens im Wirbel-
thicreie." Archiv f. micr. Anat., Bd. VIII. 1872.

(%) W. Salensky. " Befruchtung u. P^urchung d. Sterlets-Eies." Zoolo-
gischer Anzeigcr, No. 11, 1878.

(97) E. Selenka. Befruchtung des Eies von Toxopncustcs variegatus. Leipzig,
1878.

fl Strasburger. Ucber Zclllnldu n- n. /.clltln •////;/;••. Ji-na, 1876.

Idem. Utber Befrvehtung u. Zdlthdhing. Jena, 1X78.

(HiO) C. (). \V hi tin.in. "Tlic- Kniliryology of Clepsine." Quart. J. of A/i<r.
Science, Vol. xvm. 1878.

BIBLIOGRAPHY.

DIVISION OF NUCLEUS.

(101) W. Flamming. "Beitrage z. Kenntniss d. Xclle u. ihrcr Lcbun.scrschci-
nungen." Archiv f. mikr. Anat., Vol. xvi. 1878.

(102) E. Klein. "Observations on the glandular epithelium and division of
nuclei in the skin of the Newt." Quart, y. of Micr. Science, VoL XIX. 1879.

(103) Peremeschko. "Ueber d. Theilung d. thierischen Zellen." Archiv f.
mikr. Anat., Vol. xvi. 1878.

(104) E. Strasburger. "Ueber ein z. Demonstration geeignetes Zclltheilungs-
Object." Sitz. d. Jenaischen Gesell.f. Med. u. Naturwiss., July 18, 1879.

SEGMENTATION.

(105) E. Haeckel. "Die Gastrula u. Eifurchung." Jenaische Ztitschrift, Vol.
ix. 1877.

(106) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d. Thier-
welt u. n. ihrer Bedeutung." Oken. /sis, 1848.

GENERAL WORKS ON EMBRYOLOGY.

(107) K. E. von Baer. " Ueb. Entwicklungsgeschichte d. Thiere." Konigs-
berg, 1828—37.

(108) C. Glaus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.

(109) C. Gegenbaur. Grundriss d. vergleichenden Anatomie. Leipzig, 1878.
Vide also Translation. Elements of Comparative Anatomy. Macmillan and Co.,
1878.

(110) E. Haeckel. Studien z. Gastrcea-Theorie. Jena, 1877, and also Jena-
ische Zeitschrift, Vols. vni. and ix.

(111) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation.
The History of Creation. King and Co., London, 1876.

(112) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthro-
pogeny (Translation). Kegan Paul and Co., London, 1878.

(113) Th. H.Huxley. The Anatomy of Invertebrated Animals. Churchill,
1877.

(114) E. R. Lankester. "Notes on Embryology and Classification." Quart.
J. of Micr. Science, Vol. xvi I. 1877.

(115) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
of Comparative Embryology. Holt and Co., New York, 1876.

(116) H. Rathke. Abhandlungen z. Bildung- und Entwicklungsgesch. d. Men-
schen u. d. Thiere. Leipzig, 1833.

DICYEMID.E.

(117) E van Beneden. "Recherches sur les Dicyemides." Bull. d. FAca-
dtmie roy. de Belgique, f ser. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this
paper for a full account of the literature.

(118) A. K 611 ike r. Ueber Dicyema paradoxum den Schmarotzer der Venenan-
hiinge der Cephalopoden. ,

(119) Aug. Krohn. "Ueb. d. Vorkommen von Entozoen, etc. Fronep

Notizen, vn. 1839.

ORTHONECTID^E.

(120) A If. Giard. "Les Orthonectida classe nouv. d. Phylum des Vers."
journal de tAnat. et de la Physiol., Vol. XV. 1879.

(121) El. Metschnikoff. "Zur Naturgeschichte d. OrthonecUdae." Zoologi-

scher Anzeiger, No. 40— 43» l879-

PORIFERA.
'(122) C Barrois. " Embryologie de quelques eponges de la Manche. " An-

""$) &£&™ZS^£^«'<£t*>* SP°"6es." A~* ^ M«g. cf
Nat. Hist., 4th series, Vol. xiv. 1874.

vi BIBLIOGRAPHY.

(124) Ganin1. " Zur Entwicklung d. Spongilla fluviatilis." Zoologischer Anzei-
gtr, Vol. i. No. 9, 1878.

(125) Robert Grant. "Observations and Experiments on the Structure and
Functions of the Sponge." Edinburgh Phil. jf., Vol. xm. and XIV., 1825, 1816.

(126) E. Haeckel. Die Kalkschwamme, 1872.

(127) E. Haeckel. Studien zur Gastraa- Theorie. Jena, 1877.

(128) C. Keller. Unterstichungen iiber Anatomic und Entwicklungsgeschichte
einiger Spongien. Basel, 1876.

(129) C. Keller. "Studien lib. Organisation u. Entwick. d. Chalineen." Zeit.
f. wiss. Zoo/., Bd. xxvin. 1879.

(130) LieberkUhn. "Beitr. z. Entwick. d. Spongillen." Muller's Archiv,
1856.

(131) LieberkUhn. "Neue Beitrage zur Anatomie der Spongien." Miiller's
Archiv, 1859.

(132) El. Metschnikoff. " Zur Entwicklungsgeschichte der Kalkschwamme. "
Zeit.f. wiss. Zool., Bd. xxiv. 1874.

(133) El. Metschnikoff. "Beitrage zur Morphologic der Spongien." Zeit.
f. wiss. Zool., Bd. xxvii. 1876.

(134) El. Metschnikoff. " Spongeologische Studien." Zeit. f. wiss. Zool.,
Bd. xxxn. 1879.

(135) Miklucho Maklay. "Beitrage zur Kenntniss der Spongien." Jenaische
Zeitschrift, Bd. iv. 1868.

(136) O. Schmidt. "Zur Orientirung iiber die Entwicklung der Schwamme."
Zeit.f. wiss. Zool., Bd. xxv. 1875.

(137) O. Schmidt. "Nochmals die Gastrula der Kalkschwamme." Archiv
fur mikrosk. Anat., Bd. XII. 1876.

(138) O. Schmidt. "Das Larvenstadium von Ascetta primordialis und Asc.
clathrus." Archiv fur mikrosk. Anatomie, Bd. xiv. 1877.

(139) F. E. Schulze. "Ueber den Bau und die Entwicklung von Sycandra
raphanus." Zeit.f. wiss. Zool., Bd. xxv. 1875.

(140) F. E. Schulze. "Zur Entwicklungsgeschichte von Sycandra." Zeit. f.
wiss. Zool., Bd. XXVII. 1876.

(141) F. E. S chulze. " Untersuchung Ub. d. Bau, etc. Die Gattung Halisarca."
Zeit.f. wiss. Zoo/., Bd. xxvin. 1877.

(142) F. E. Schulze. "Untersuchungen iib. d. Bau, etc. Die Metamorphose
von Sycandra raphanus." Zeit.f. wiss. Zool., Bd. xxxi. 1878.

(143) F. E. Schulze. "Untersuchungen u. d. Bau, etc. Die Familie Aply-
sinidae." Zeit.f. wiss. Zool., Bd. xxx. 1878.

(144) F. E. Schulze. "Untersuchungen u. d. Bau, etc. Die Gattung Sponge-
lia." Zeit.f. wiss. Zool., Bd. xxxn. 1878.

CCELENTERATA.
General.

(145) Alex. Agassi z. Illustrated Catalogue of the Museum of Comparative
Anatomy at Harvard College, No. II. American Acalephac. Cambridge, U. S., 1865.

(140) O. and R. Hertwig. Der Organismus d. Medusa: u, seine Stellung z.
Keimblattertheorie. Jena, 1878.

(147) A. Kowalevsky. "Untersuchungen lib. d. Entwicklung d. Coelente-
raten." Nachrichten d. kaiser. Gcsell. d. Freunde d. Nattirerkenntniss d. Antliropo-
logie u. Ethnographie. Moskau, 1873. (Russian). For abstract vide Jahresberichtc
d. Anat. u. Phys. (Hoffman u. Schwalbe), 1873.

Hydrozoa.

(148) L. A gas si z. Contributions to the Natural History of the United States of
America. Boston, 1862. Vol. IV.

(149) G. J. Allman. A Monograph of the Gymnoblastic or Tubularian Hy-
drotds. Ray Society, 1871-2.

1 There is a Russian paper by the same author, containing a full account, with
clear illustrations, of his observations.

BIBLIOGRAPHY, vii

(150) G. J. All man. "On the structure and development of Myriothela." Phil.
Trans., Vol. CLXV. p. 2.

(Iff), P- J- van Beneden. "Mem. sur les Campanulaires de la Cote d'Ostende
consideres sous le rapport physiologique, embryogenique, et zoologique." Nouv. Mini.
de PAcad. de Brux., Tom. xvn. 1844.

(^2) p- J- van Beneden. "Recherches sur 1'Embryogenie des Tubulaires et
1 histoire naturelle des differents genres de cette famille qui habitent la Cote d'Ostende."
Nouv. Mem. de P Acad. de Brux., Tom. xvii. 1844.

(153) C. Claus. "Polypen u. Quallen d. Adria." Denk. d. math.-naturwiss.
Classe d. k. k. Akad. d. Wiss. Wien, Vol. xxxvin. 1877.

(154) J. G. Dal yell. Rare and Remarkable Animals of Scotland. London,
1847.

(*55) ,H- Fo1- "Die erste Entwicklung d. Geryonideneies." J 'enaische Zeit-

schrift, Vol. vn. 1873.

(156) Carl Gegenbaur. Zur Lehre vom Generationswechsel und der Fortpfian-
zung bei Medusen und Polypen. Wiirzburg, 1854.

(157) Thomas Hincks. "On the development of the Hydroid Polypes, Clava-
tella and Stauridia ; with remarks on the relation between the Polype and the Medu-
soid, and between the Polype and the Medusa." Brit. Assoc. Rep., 1861.

(158) E. Haeckel. Zur Entwicklungsgeschichte d. Siphonophoren. Utrecht,
1869.

(159) Th. H. Huxley. Oceanic Hydrozoa. Ray Society, 1858.

(160) Geo. Johnston. A History of British Zoophytes. Edin. 1838. 2nd
Edition, 1847.

(161) N. Kleinenberg. Hydra, eine anatomisch-entwicklungsgeschichtliche Un-
tersuchung. Leipzig, 1872.

(162) El. Metschnikoff. "Ueber die Entwicklung einiger Ccelenteraten."
Bull, de FAcad. de St Petersbourg, XV. 1870.

(163) El. Metschnikoff. "Studien liber Entwicklungsgeschichte d. Medusen
u. Siphonophoren." Zeit.f. wiss. ZooL, Bd. xxiv. 1874.

(164) H. N. Moseley. "On the structure of the Sty lasteridse." Phil. Trans.,

(165) F. E. Schulze. Ueber den Bau und die Entwicklung von Cordylophora
lacustris. Leipzig, 1871.

Actinozoa.

(166) Al. Agassiz. "Arachnitis (Edwarsia) brachiolata." Proc. Boston Nat.
Hist. Society, 1860.

(167) Koch. "Das Skelet d. Alcyonarien." Morpholog. Jahrbuch, Bd. iv.
1878.

(168) A. Kowalevsky. "Z. Entwicklung d. Alcyoniden, Sympodium coralloi-
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(169) H. Lacaze Duthiers. Histoire nat.du Cor ail. Paris, 1864.

(170) H. Lacaze Duthiers. " Developpement des Coralliaires." Archives
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(171) C. Semper. " Ueber Generationswechsel bei Steinkorallen etc." Zeit. f.
wiss. ZooL, Bd. xxii. 1872.

Ctenophora.

(172) Alex. Agassiz. "Embryology of the Ctenophorae." Mem. of the Anur.
Acad. of Arts and Sciences, Vol. X. No. 1 1 1. 1874.

(173) G. J. All man. "Contributions to our knowledge of the structure and de-
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(174) C. Chun. "Das Nervensystem u. die Musculatur d. Rippenquallen."
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(175) C. Claus. "Bemerkungen u. Ctenophoren u. Medusen." Zeit. f. wiss.
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(176) H. Fol. Ein Beitrag z. Anat. u. Entwickl. einiger Rippenquallen. 1869.

(177) C. Gegenbaur. "Studien u. Organis. u. System d. Ctenophoren."
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(178) A. Kowalevsky. " Entwicklungsgeschichte d. Rippenquallen. " Mtm.
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Vlii BIBLIOGRAPHY.

(179) J. Price. "Embryology of Ciliogrades." Proceed, of British Assoc.,
1846.

(180) C. Semper. "Entwicklung d. Eucharis multicornis." Zeit. f. wtss.
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PLATYELMINTHES.
Turbellaria.

(181) Alex. Agassiz. "On the young stages of a few Annelids" (Planaria an-
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(182) Dalyell. "Powers of the Creator."

(183) C. Girard. "Embryonic development of Planocera elliptica." J our. of
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(184) Alex. Gotte. "Zur Entwicklungsgeschichte d. Seeplanarien." Zoologi-
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(185) P. Halle z. Contributions a Thistoire naturelle des Turbellarits. Thesis a
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(186) Knappert. "Bijdragen tot de Ontwikkelings-Geschiedenis der Zoet-
water-Planarien." Provinciaal Ulrechtsch Genootschap van Kunsten en Wetenschap-
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(187) W. Keferstein. " Beitrage z. Anat. u. Entwick. ein. Seeplanarien von
St. Malo." Abh. d. konig. Gesell. d. Wiss. zu Gottingcn. Bd. XI v. 1868.

(188) El. Metschnikoff. " Untersuchungen lib. d. Entwicklung d. Planarien."
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Hoffman and Schwalbe's Bericht for 1878.

(189) H. N. Moseley. "On Stylochus pelagicus and a new species of pelagic
Planarian, with notes on other pelagic species, on the larval forms of Thysanozoon,
etc." Quart. Journ. of Micr. Science, Vol. xvn. 1877.

(190) J. Miiller. "Ueber eine eigenthiimliche Wurmlarva a. d. Classe d. Tur-
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(191) J. Miiller. "Ueber verschiedene Formen von Seethieren." Miiller's
Archiv f. Anat. und Phys. 1854.

Nemertea.

(192) J. Barrois. " L'Embryologie des Nemertes." An. Sci. Nat., Vol. VI.
1877.

(193) O. BUtschli. Archiv f. Naturgeschichte, 1873.

(194) A. Krohn. "Ueb. Pilidium u. Actinotrocha." Miiller's Archiv, 1 858.

(195) E. Desor. "Embryology of Nemertes." Proceedings of the Boston Nat.
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(196) G. Dieck. "Entwicklungsgeschichte d. Nemertinen." Jenaische Zeit-
schrift, Vol. vin. 1874.

(197) C. Gegenbaur. "Bemerkungen lib. Pilidium gyrans, etc." Zeitschrift
furwiss. Zool., Bd. v. 1854.

(198) C. K. Hoffman. "Entwicklungsgeschichte von Tetrastemma tricolor."
Niederldndisches Archiv, Vol. ill. 1876, 1877.

(199) C. K. Hoffman. "Zur Anatomie und Ontogenie von Malacobdella."
Niederldndisches Archiv, Vol. IV. 1877.

(200) W. C. Mc Intosh. British Annelids. The Nemerteans. Ray Society,
J873-4.

(201) Leuckart u. Pagenstecher. "Untersuchungen lib. niedere Seethiere."
Miiller's Archiv, 1858.

(202) E. Metschnikoff. "Studien lib. die Entwicklung d. Echinodermen u.
Nemertinen." Mhn. Acad. imp. Pttersbourg, vn. Ser., Tom. xiv. No. 8, 1869.

Trematoda.

(203) T. S. Cobbold. Kntozoa. Groombridge and Son, 1864.

(204) T. S. Cobbold. Parasites; a Treatise on the Entozoa, etc. Churchill,
1879.

(205) F i 1 i p p i. " Mem. p. servir a 1'histoire geneHique des Tre"matodes." Ann.
Scien. Nat., 4th Series, Vol. II. 1854, and Mem. Accad. Torino, 1855-1859.

BIBLIOGRAPHY. ix

206) R. Leuckart. Die menschlichen Parasilen, Vol. I. 1863, p. 485 ct seq.

207) H. A. Pagenstecher. Trematodtn u. Trematodenlarven. Heidelberg,

1857.

(208) C. Th. von Siebold. Lehrbuch d. vergleich. Anat. wirbelloser Thicre.
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(209) J- J- S. Steenstrup. Generationswechsel. 1842.

(210) R. v. Willemoes-Suhm. "Zur Naturgeschichte d. Polystomuiu intc-
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(211) R. v. Willemoes-Suhm. ''Helminthologische Notizcn III." Zeit. f.
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and literature.

(212) G. R. Wagener. Beitrdge zur Entwicklungsgeschichte d. Eingeweidewiir-
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(213) G. R. W age n e r. " Helminthologische Bemerkungen, etc." Zeit. f. wiss.
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(214) G. R. Wagener. "Ueb. Gyrodactylus elegans." Archiv f. Anat. u.
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(215) E. Zeller. " Untersuchungen ub. d. Entwicklung d. Diplozoon para-
doxum." Zeit.f. wiss. Zool., Vol. xxn. 1872.

(216) E. Zeller. "Untersuchungen u. d. Entwick. u. Bau d. Polystomum inte-
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(217) E. Zeller. "Weitere Beitrage z. Kenntniss d. Polystomen." Zeit.f.
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Cestoda.

(218) Ed. van Beneden. "Recherches sur la composition et la signification
d. 1'oeuf." Mem. cour. Acad. roy. Belgique. Vol. xxxiv. 1868.

(219) P. J. van Beneden. "Les vers Cestoi'des consideres sous le rapport
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1850.

(220) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.

(221) T. S. Cobbold. Parasites; a treatise on the Entozoa, etc. Churchill,
1879.

(222) Th. H. Huxley. "On the Anatomy and Development of Echinococcus
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(223) J. Knoch. "Die Naturgesch. d. breiten Bandwiirmer." Mem. Acad.
Imp. Petersbourg, Vol. v. Ser. 7, 1863.

(224) F. Kiichenmeister. "Ueber d. Umwandlung d. Finnen Cysticerci in
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(225) F. Kiichenmeister. "Experimente iib. d. Entstehung d. Cestoden.
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(226) R. Leuckart. Die menschlichen Parasiten, Vol. I. Leipzig, 1863. Vide
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(227) R. Leuckart. "Archigetes Sieboldii, eine geschlechtsreife Cestoden-
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(228) El. Metschnikoff. "Observations sur le developpement de quelques
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(229) 'w. Salensky. "Ueb. d. Bau u. d. Entwicklungsgeschichte d. Amphi-
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(230) Von Siebold. Burdach's Physiologie.

(231) R. von Willemoes-Suhm. "Helminthologische Notizen." Zfit. /.
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ROTIFERA.

(232) F. Cohn. "Ueb. d. Fortpflanzung von Raderthiere." Zeit.f. wiss.

^(233)° F. Cohn. "Bemerkungen u. Raderthiere." Zeit.f. wiss. Zool., Vol. IX.
1858, and Vol. xn. 1862.

(234) T. H. Huxley. "Lacinularia socialis." Trans, of the Microscopical

Society, 1853.

BIBLIOGRAPHY.

(235) Fr. Leyclig. " Ueb. d. Bau u. d. systematische Stelluny; d. Radcrthiere."
Ztit.f. unss. Zool., Vol. vi. 1854.

(236) W. Salensky. "Beit. z. Entwick. von Brachionus urceolaris." Zeit. /.
itnss. Zool., Vol. xxn. 1872.

(237) C. Semper. " Zoologische Aphorismen. Trochosphuera axjuatorialis."
Zeit.f. wiss. Zool., Vol. xxn. 1872.

MOLLUSCA.

General.

(238) T. H. Huxley. "On the Morphol. of the Cephal. Mollusca." Phil.
Trans., 1853.

(239) E. R. Lankester. "On the developmental history of the Mollusca."
Phil. Trans., 1875.

(240) H. G. Bronn and W. Keferstein. Die Klasscn u. Ordnungcn d.
Thierreichs, Vol. III. 1862-1866.

Gasteropoda and Pteropoda.

(241) J. Alder and A. Hancock. "Devel. of Nudibr." Ann. and Magaz.
Nat. Hist., Vol. XH. 1843.

(242) N. Bobretzky. "Studien iiber die embryonale Entwicklung d. Gastero-
poden." Archivf. micr. Anat., Vol. xin.

(243) W. K. Brooks. "Preliminary Observations on the Development of
Marine Gasteropods." Chesapeake Zoological Laboratory, Session of 1878. Balti-
more, 1879.

(244) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Paludina vivipara)."
Zeit.f. wiss. Zool., Vol. xxix. 1877.

(245) W. Carpenter. "On the devel. of the embr. of Purpura lapillus."
Trans. Micros. Soc., 2d series, Vol. ill. 1855.

(246) W. Carpenter. "On the devel. of the Purpura." Ann. and Mag. of
Nat. Hist., 2d series, Vol. xx. 1857.

(247) E. Claparede. "Anatomic u. Entwickl. der Neritina fluviatilis."
MUller's Archiv, 1857.

(248) H. Eisig. "Beitr. z. Anat. u. Entwickl. der Geschlechtsorg. von Lym-
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(249) H. Fol. " Sur le developpement des Pteropodes." Archives de Zool. ex-
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(250) H. Fol. " Sur le developpement des Gasteropodes pulmones." Compt.
rend., 1875, pp. 523—526.

(251) H. Fol. "Sur le developpement des Heteropodes." Archives de Zool.
expe"rim.etgtn<b-ale,\o\.v. 1876.

(252) C. Gegenbaur. "Beit. z. Entwicklungsgesch. der Landgasteropoden."
Zeitschr. f. w. Zool., Vol. ill. 1851.

(253) C. Gegenbaur. Untersuch. iib. Pteropoden u. Hetcropoden. Leipzig,
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(254) H. von Jhering. "Entwicklungsgeschichte von Helix." Jcnaische Zcit-
schrift, Vol. IX. 1875.

(255) W. Keferstein and E. Ehlers. "Beob. lib. d. Entwick. v. Molis
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(256) J. Koren and D. C. Danielssen. "Benuerk. til Mollusk. Udvikling."
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(257) J. Koren and D. C. Danielssen. liidrag til Pectinibr. Udvikl. licrgcn,
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(258) A. Krohn. "Beobacht. aus d. Entwickl. der Pteropoden u. Heterop."
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(259) A. Krohn. Beitr. zur Entwickl. der Pteropoden u. Heteropoden. Leipzig,
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(260) H. de Lacaze-Duthiers. "Mem. sur 1'anat.et 1'embryog. des Vermets."
2» partie. Ann. sc. not., 4" s£rie, T. xm. 1860.

(261) P. Langerhans. "Zur Entwickl. der Gasterop. Opisthobr." Zeitschr.
f. w. Zool., Vol. xxni. 1873.

BIBLIOGRAPHY. xi

. E. R. Lankester. "On the development of the Pond-Snail." Quart.

J. of Micr. Scie., Vol. xiv. 1874.

(263) E. R. Lankester. "On the coincidence of the blastopore and anus in
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(264) F. Leydig. "Ueber Paludina vivipara." Zeitschr. f. w. Zool., Vol. 11.
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(265) J. MUller. Ueber Synapta dig. u. iib. d. Erzeug. v. Schnecken in Holoth.,
1852.

(266) J. Miiller. "Bemerk. aus d. Entwickl. der Pteropoden." Monatsber.
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(267) C. Rabl. "Die Ontogenie d. Siisswasser-Pulmonaten." Jenaische Zeit-
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(268) C. Rabl. "Ueb. d. Entwick. d. Tellerschnecke (Planorbis)." Morph.
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(269) W. Salensky. " Beitr. zur Entwickl. d. Prosobr." Zeitschr. f. iv. Zool. ,
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(270) O. Schmidt. "Ueb. Entwick. von Limax agrestis." Miillcr's Archiv,
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(271) Max S. Schultze. "Ueber d. Entwick. des Tergipes lacinulatus."
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(272) E. Selenka. "Entwick. von Tergipes claviger." Niederl. Arch.f. Zool.,
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(273) E. Selenka. "Die Anlage d. Keimbl. bei Purpura lapillus." Niederl.
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(274) C. Semper. "Entwickl. der Ampullaria polita, etc." Natuurk. Ver-
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(275) An. Stecker. "Furchung u. Keimblatterbildung bei Calyptraa."
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(276) A.Stuart. " Ueb. d. Entwickl. einiger Opisthobr." Zeitschr. f. w. Zool.,
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(277) N. A. Warneck. "Ueber d. Bild. u. Entwick. d. Embryos bei Gas-
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Cephalopoda.

(278) P. J. van Beneden. " Recherches sur 1'Embryogenie des Sepioles."
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(279) N. Bobretzky. Observation on the Development of the Cephalopoda
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(280) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
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(281) A. K6 Hiker. Entwicklungsgeschichte d. Cephalopoden. Zurich, 1844.

(282) E. R. Lankester. "Observations on the development of the Cephalo-
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(283) E. Metschnikoff. " Le developpement des Sepioles." Archives d. Sc.
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Polyplacophora.

(284) A. Kowalevsky. "Ueb. d. Entwick. d. Chitonen." Zoologischer
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(285) S L. Loven. " Om utvecklingen hos sliigtet Chiton." Stockholm
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Scaphopoda.

(286) H. Lacaze-Duthiers. "Developpement du Dentale." Ann. d. Sci.

Nat., Series iv. Vol. VII. 1857.

Lamellibranchiata.

(287) M. Braun. " Postembryonale Entwicklung d. Susswasser-Muscheln."

Zoologischer Garten.

xii BIBLIOGRAPHY.

(288) C. G. Carus. " Neue Untersuch. lib. d. Entvvickl. unscrer Flussmu-
Vcrh. Leop.-Car. Akad., Vol. xvi. 1832.

(289) W. Flemming. " Studien in d. Entwicklungsgeschichte der Najadcn."
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(290) F. Ley dig. " Ueber Cyclas Cornea." Miiller's Archiv, 1855.

(2111) S. L. Loven. " Bidrag til Kanned. om Utveckl. af Moll. Acephala
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(292) C. Rabl. "Ueber d. Entwicklungsgeschichte d. Malermuschel." Je-
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(293) W. Salensky. " Bemerkungen uber Haeckels Gastraea-Theorie (Ostrea)."
Arch. f. Naturg., 1874.

(294) O. Schmidt. " Ueb. d. Entwick. von Cyclas calyculata." Muller's
Arch., 1854.

(295) O. Schmidt. "Zur Entwickl. der Najaden." Wien. Sitzungsber.
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(296) P. Stepanoff. " Ueber die Geschlechtsorgane u. die Entwicklung von
Cyclas." Archivf. Naturgeschichte, 1865.

(297) H. Lacaze-Duthiers. " Ueveloppement d. branchies d. Mollusques
Acephales." An. Sc. Nat., Ser. iv. Vol. v. 1856.

POLYZOA.
General.

(298) J. Barrois. Recherches sur Cembi yologie des Bryozoaires. Lille, 1877.

Entoprocta.

(299) B. Hatschek. " Embryonalentwicklung u. Knospung d. Pedicellina
echinata." Zeitschrift fiir wiss. Zool., Bd. xxix. 1877.

(300) M. Salensky. " Etudes sur les Bryozoaires entoproctes." Ann. Scien.
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(301) O. Schmidt. "Die Gattung Loxosoma." Archivf. mik. Anat.,Rd.
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(302) C. Vogt. "Sur le Loxosome des Phascolosomes." Archives de Zool.
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(303) C. Vogt. "Bemerkungen zu Dr Hatschek's Aufsatz lib. Embryonalent-
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1878.

Ectoprocta.

(304) G. J. A 11 man. Monograph of fresh water Polyzoa. Ray Society.

(305) G. J. Allman. " On the structure of Cyphonautes." Quart. J. of Micr.
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(306) G. J. Allman. "On the structure and development of the Phylactola>
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(307) J. Barrois. " Le developpement d. Bryozoaires Chilostomes." Comptes
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(308) E. Claparede. " Beitrage zur Anatomic u. Entwicklungsgeschichte d.
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(309) E. Claparede. "Cyphonautes." Anat. u. Entwick. wirbell. Thiere.
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(310) R. E. Grant. "Observations on the structure and nature of Flustrae."
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(311) B. Hatschek. "Embryonalentwicklung u. Knospung d. Pedicellina
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(312) T. II. Huxley. "Note on the reproductive organs of the Cheilostome
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(313) L. Joliet. "Contributions a 1'histoire naturelle des Bryozoaires des
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(314) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere." Got-
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BIBLIOGRAPHY. xiii

(315) E. Metschnikoff. Bull. deTAcad. de St Pttersbourg, XV. 1871, p. 507.

(316) H. Nitsche. " Beitrage zur Kenntniss d. Bryozoen." Zrit. f. wiss.

Zool., Bd. xx. 1870.

(317) W. Repiachoff. "Zur Naturgeschichte d. chilostomen Seebryozoen."
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(318) W. Repiachoff. " Ueber die ersten Entwicklungsvorgange bei Tendra
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(319) W. Repiachoff. "Zur Kenntniss der Bryozoen." Zoologischer An-
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(320) W. Repiachoff. " Bemerkungen lib. Cyphonautes. " Zoologischer An-
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(321) M. Salensky. " Untersuchung an Seebryozoen." Zeit. fur wiss. Zool..
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(322) A. Schneider. "Die Entwicklung u. syst. Stellung d. Bryozoen u.
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(323) Smitt. " Om Hafsbryozoernas utveckling och fettkroppar. " Aftryck ur
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(324) T. Hincks. British Marine Polyzoa. Van Voorst, 1880.
[Conf. also works by Farre, Hincks, Van Beneden, Dalyell, Nordmann.]

BRACHIOPODA.

(325) W. K. Brooks. " Development of Lingula." Chesapeake Zoological
Laboratory, Scientific Results of the Session of 1878. Baltimore, J. Murphy and Co.

(326) A. Kowalevsky. "Development of the Brachiopoda." Protocol of the
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(327) H. Lacaze-Duthiers. " Histoire de la Thecidie." Ann. Scien. Nat.
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(328) Morse. " On the Early Stages of Terebratulina septentrionalis." Mem.
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(329) Morse. "On the Embryology of Terebratulina." Mem. Boston Soc.
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(330) Morse. " On the Systematic Position of the Brachiopoda." Proceedings
of the Boston Soc. of Nat. Hist., 1873.

(331) Fritz Miiller. " Beschreibung einer Brachiopoden-Larve." Miiller's
Archiv, 1860.

CKLETOPODA.

(332) Alex. Agassiz. "On the young stages of a few Annelids." Annals
Lyceum Nat. Hist, of New York, Vol. vm. 1866.

(333) Alex. Agassiz. " On the embryology of Autolytus cornutus and alter-
nations of generations, etc." Boston Journal of Nat. History, Vol. VH. 1859-63.

(334) W. Busch. Beobachtungen it. Anaf. u. Entwick. einiger wirbelloser See-
thiere, 1851.

(335) Ed. Claparede. Beobachtungen u. Anat. u. Entwick. 'wirbelloser Thiert
an d. Kiiste von Normandie. Leipzig, 1 863.

(336) Ed. Claparede u. E. Metschnikoff. "Beitrage z. Kenntniss lib.
Entwicklungsgeschichte d. Chsetopoden." Zeit.f. wiss. Zool., Vol. xix. 1869.

(337) E. Grube. Untersuchungen ub. Entivicklung d. Anneliden. Komgsberg,

4(338) B. Hatschek. " Beitrage z. Entwick. u. Morphol. d Anneliden." Si/*.
d. k. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(339) B. Hatschek. "Studien liber Entwicklungsgeschichte der Anneliden.
Arbeiten aus d. zoologischcn Institute d. Universitiit Wien. Von C. Claus. Heft in.

OwQ

(340) Th. H. Huxley. "On hermaphrodite and fissiparous species of tubi-
colar Annelidse (Protula)." Edinburgh New Phil. Journal, Vol. I. 1855.

(341) N. Kleinenberg. "The development of the earthworm Lumbncus
trapezoides." Quart. J, of Micr. Science, Vol. xix. 1879 Sullo ariAtfff del tn»<

bricus trapezoides. Napoli, 1878.

XIV BIBLIOGRAPHY.

(342) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthro-
poden." Mem. Acad. Pttersbourg, Series VH. Vol. xvi. 1871.

(343) A. Krohn. " Ueber die Erscheinungen bei d. Fortpfianzung von Syllis
prolifera u. Autolytus prolifer." Archiv f. Naturgesch. 1852.

(344) R. Leuckart. " Ueb. d. Jugendzustande ein. Anneliden, etc." Archiv
f. Naturgesch. 1855.

(345) S. Love"n. " Beobachtungen ii. die Metamorphose von Anneliden."
Wiegmann's Archiv, 1842.

(346) E. Metschnikoff. "Ueber die Metamorphose einiger Seethiere (Mi-
traria)." Zeit.f. tuiss. Zool., Vol. XXI. 1871.

(347) M. Milne- Ed\vards. " Recherches zoologiques, etc." Ann. Scie.
Natur. HI. Se"rie, Vol. ill. 1845.

(348) J. Miiller. "Ueb. d. Jugendzustande einiger Seethiere." Monats. d.
k.Akad. Wiss. Berlin, 1851.

(349) Max Miiller. "Ueber d. weit. Entwick. von Mesotrocha sexoculata."
Miiller's Archiv, 1855.

(350) Quatrefages. " Memoire s. 1'embryogenie des Annelides." Ann. Scie.
Natur. HI. Serie, Vol. x. 1848.

(351) M. Sars. " Zur Entwicklung d. Anneliden." Archiv f. Naturgeschichte,
Vol. xi. 1845.

(352) A. Schneider. " Ueber Bau u. Entwicklung von Polygordius." Miiller's
Archiv, 1868.

(353) A. Schneider. "Entwicklung u. system. Stell. d. Bryozoen u. Gephy-
reen (Mitraria)." Archiv f. mikr. Anat. Vol. v. 1869.

(354) M. Schultze. Ueb. die Entwicklung von Arcnicola piscatorum u. anderer
Kiemenwurmer. Halle, 1856.

(355) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876-7.

(356) C. Semper. "Beitrage z. Biologie d. Oligochjeten." Arbeiten a. d.
zool.-zoot. Instit. Wiirzburg, Vol. IV. 1877-8.

(357) M. Stossich. " Beitrage zur Entwicklung d. Chaetopoden." Sitz. d. k.
k. Akad. Wiss. Wien, B. LXXVII. 1878.

(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen ii. niedrige
Meeresthiere." Zeit.f. wiss. Zool. Bd. xxi. 1871.

DlSCOPHORA.

(359) O. BUtschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit.
f. wiss. Zool. Vol. xxix. 1877.

(360) E. Grube. Untersuchungen ub. d. Entwicklung d. Anneliden. Konigs-
berg, 1844.

(361) C. K. Hoffmann. "Zur Entwicklungsgeschichte d. Clepsineen." NIC-
derland. Archiv f. Zool. Vol. IV. 1877.

(362) R. Leuckart. Die menschlichen Parasiten (Hirudo}, Vol. I. p. 686,
et seq.

(363) H. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.

(364) Ch. Robin. Mem. snr le Developpement embryogenique des Ilirndiih'c*.
Paris, 1875.

(365) C. O. Whitman. "Embryology of Clepsine." Quart. J. of Micro.
Science, Vol. xvm. 1878.

[ Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observa-
tions.]

GEPHYREA.
GepJiyrea nuda.

(366) A. Kowalevsky. Sitz. d. zool. Abth. d. Iff. Versam. rtiss. Naturf
(Thalassema). Zeit.f. wiss. Zool. Vol. xxn. 1872, p. 284.

(367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus ncbst I'.c UK iknii^cn, '
etc. Miiller's Archiv, 1857.

BIBLIOGRAPHY. xv

(368) M. Salensky. " Ueber die Metamorphose d. Echiurus." Morfhohgisches
Jahrbuch, Bd. 11.

(369) E. Selenka. "Eifurchung u. Larvenbilflung von Phascolosoma elonga-
tum." Zeit.f. wiss. Zool. 1875, Bd- xxv. p. i.

(370) J. W. Spengel. " Beitrage z. Kenntniss d. Gephyreen (lionellia)."
MitlheiL a. d. zool. Station z. Neapel, Vol. i. 1879.

Gephyrea tubicola (Actinotroc/ia).

(371) A. Krohn. " Ueb. Pilidium u. Actinotrocha." Muller's Archiv, 1858.

(372) A. Kowalevsky. " On anatomy and development of Phoronis," Peters-
burg, 1867. i PI. Russian. Vide Leuckart's Bcricht, 1866-7.

(373) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere (Actino-
trocha)." Zeit.f. wiss. Zool. Bd. XXI. 1871.

(374) J. Miiller. " Bericht ub. ein. Thierformen d. Nordsee." Muller's Archiv,
1846.

(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata."
Muller's Arch. 1862.

CH^TOGNATHA.

(376) O. Butschli. " Zur Entwicklungsgeschichte der Sagitta." Zeilschrifl f.
wiss. Zool., Vol. xxin. 1873.

(377) C. Gegenbaur. "Ueber die Entwicklung der Sagitta." Abhand. d.
natiirforschenden Gesellschaft in Halle, 1857.

(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropo-
den." Mem. Acad. Petersbourg, vn. ser., Tom. xvi., No. 12. 1871.

MYZOSTOMEA.

(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.

(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
Zeit.f. wiss. Zool., Vol. xvi. 1866.

(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Zeit.f.
wiss. Zool., Vol. IX. 1858.

GASTROTRICHA.

(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Metschn." Zeit.f. wiss.
Zool., Vol. xxvi. 1876.

NEMATELMINTHES.

(383) O. Butschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zeit.
f. wiss. Zool., B. xxvi. 1876.

(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.

(385) T. S. Cobbold. Parasites; a Treatise on the Entozoa of Man and
Animals. Churchill, 1879.

(386) O. Caleb. "Organisation et developpement des Oxyurides, etc.
Archives de Zool. exper. et gener., Vol. VII. 1878.

(387) R. Leuckart. Untersuchungeniib. Trichina spiralis. 2nd ed. Leipzig,
1866.

(388) R. Leuckart. Die menschlichen Parasiten, Bd. u. 1876.

(389) H. A. Pagenstecher. Die Trichinen nach Versuchen dargestellt.

(390) A.Schneider. Monographie d. Nematoden. Berlin, 1866.

(391) A. Villot. " Monographie des Dragoneaux " (Gordioidea). Archives de
Zool. exptr. et gtner., Vol. in. 1874.

ACANTHOCEPHALA.

(392) R. Greeff. " Untersuchungen u. d. Ban u. Entwicklung des Echin.
miliarius." Archiv f. Naturgesch. 1864.

(393) R. Leuckart. Die menschlichen Parasiten. Vol. 11. p Soi ol

xvi BIBLIOGRAPHY.

(394) An. Schneider. " Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat.
w. Phys. 1868.

(395) G. R. Wagener. Beitrdge z. Entrvicklungsgeschichte d. Eingeweidewiir-
mer. Haarlem, 1865.

TRACHEATA.

PRO TO TRA CHE A TA .

(396) H. N. Moseley. "On the Structure and Development of Peripatus
capensis." Phil. Trans. Vol. 164, 1874.

MYRIAPODA.

(397) G. Newport. "On the Organs of Reproduction and Development of the
Myriapoda." Philosophical Transactions, 1841.

(398) E. Metschnikoff. " Embryologie der doppeltfiissigen Myriapoclen
(Chilognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.

(399) E. Metschnikoff. " Embryologisches iiber Geophilus." Zeit. f. wiss.
'Zool., Vol. xxv. 1875.

(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
Archiv f. mik. Anatomic, Bd. xiv. 1877.

INSECTA.

(401) M Balbiani. " Observations s. la reproduction d. Phylloxera du Chene."
An. Sc. Nat. Ser. v. Vol. xix. 1874.

(402) E. Bessels. "Studien u. d. Entwicklung d. Sexualdrtisen bei den Lepi-
doptera." Zeit.f. miss. Zool. Bd. xvn. 1867.

(403) Alex. Brandt. " Beitrage zur Entwicklungsgeschichte d. Libellulida u.
Hemiptera, mit besonderer Beriicksichtigung d. Embryonalhiillen derselben." Mem.
Ac. Pttersbourg, Ser. vn. Vol. xin. 1869.

(404) Alex. Brandt. Ueber das Ei u. seine Bildungsstdtte. Leipzig, 1878.

(405) O. Butschli. " Zur Entwicklungsgeschichte d. Biene." Zeit. f. wiss.
Zool. Bd. xx. 1870.

(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool.
Vols. xxv. and xxvin. 1875 and 1877.

(407) H. Dewitz. "Beitrage zur Kenntniss d. Postembryonalentwicklung d.
Gliedmassen bei den Insecten." Zeit. /. wiss. Zool. xxx. Supplement. 1878.

(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeit-
schriftf. wiss. Zool. Bd. xxvi. 1876.

(409) M. Fabre. " L'hypermetamorphose et les mceurs des Meloides." An.
Sci. Nat. Series iv. Vol. vn. 1857.

(410) Ganin. "Beitrage zur Erkenntniss d. Entwicklungsgeschichte d. In-
secten." Zeit. f. wiss. Zool. Bd. xix. 1869.

(411) V. Graber. Die Insecten. Miinchen, 1877.

(412) V. Graber. " Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten."
Archiv f. mikr. Anat. Vol. XV. 1878.

(413) O.v.Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus-Art
u. deren Entwicklung aus dem unbefruchteten Ei." Mhn. Acad. Pttcrsbourg. 1X70.

(414) B. Ilatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaischc
Zeitschrift, Bd. xi.

(415) A. Kollikcr. " Observationes de prima insectorum gcnese, etc. " Ann.
Sc. Nat. Vol. xx. 1843.

(11(5) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropo-
den." Mtm. Ac. imp. J\'(,-rstn>nr«, Ser. VII. Vol. XVI. iSji.

(417) C. Kraepelin. " Untersuchungen lib. d. Bau, Mechanismus u. d.
Entwick. des Stachels d. l.icnartigai Tliicrc." Zeit.f. wiss. Zool. Vol. xxm. 1X7.5.

(418) C. Kupffcr. " Faltcnblatt nn d. Embryoncn d. Gattung Chirononnis."
Arch.f. mikr. Anat. Vol. n. iS66.

(419) R. Leuckart. Zur Kemituiss d. Gi'ncratiomxi'ffhscls it. d. /'/; -Ihetii^ •, <
b. d. Insecten. Frankfurt, iH.nS.

BIBLIOGRAPHY. xvii

(420) Lubbock. Origin and Metamorphosis of Insi-cts. 1874.

(421) Lubbock. Monograph on Collembola ami Thysanura. Ray Society,
1873-

(422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv
f. Naturgeschichte, Bd. XXXV. 1869.

(423) E. Metschnikoff. ' ' Embryologische Studien an Insecten." '/.fit. /.
wiss. Zool Bd. xvi. 1866.

(424) P. Meyer. " Ontogenie und Phylogenie d. Insecten." Jcnaischt Zfit-
schrift, Vol. X. 1876.

(425) Fritz Miiller. " Beitrage z. Kenntniss d. Termiten." Jcnaische Zeit-
schrift, Vol. IX. 1875.

(426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and the
Thysanurous genus Isotoma." Mem. Pea body Acad. Science, \. 2. 1871.

(427) Suckow. " Geschlechtsorgane d. Insecten." Heusinger's Zeitschrift f.
organ. Physik, Bd. II. 1828.

(428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms."
Zoologischer Anzeigcr, n. Jahr. No. 20 (Preliminary Notice).

(429) Aug. Weismann. "Zur Embryologie d. Insecten." Arehiv f. Anat.
und Phys. 1864.

(430) Aug. Weismann. " Entwicklung d. Dipteren." Zeit. f. wiss. Zool.
Vols. xni. and xiv. Leipzig, 1863 — 4.

(431) Aug. Weismann. " Die Metamorphose d. Corethra plumicornis." Ztit.
f. unss. Zool. Vol. xvi. 1866.

(432) N. Wagner. " Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven."
Zeit.f. wiss. Zool. Vol. Xlll. 1860.

(433) Zaddach. Untersuchnngen tib. d. Bau u. d. Enhuicklung d. Gliederlhifre.
Berlin, 1854.

ARACHNID A.
Scorpionidce.

(434) El. Metschnikoff. " Embryologie des Scorpions." Zeit.f. unss. Zool.
Bd. xxi. 1870.

(435) H. Rathke. Reisebemerknngen aus Taurien (Scorpio), Leipzig, 1837.

Pseudoscorpionidce.

(436) El. Metschnikoff. " Entwicklungsgeschichte d. Chelifer." Zeit.f. unss.
Zool. Bd. xxi. 1870.

(437) A. Stecker. "Entwicklung der Chthonius-Eier im Mutterleibe und c
Bildung des Blastoderms." Sitzung. konigl. bohmisch. Gesellschaft Wissensch.t 1876,
3. Heft, and Annal. and Mag. Nat. History, 1876, xvill. 197.

Phalangida.

(438) M. Balbiani. " Memoire sur le developpement des Phalangides." Ann.
Scien. Nat. Series v. Vol. xvi. 1872.

Araneina.

(439) M. Balbiani. "Memoire sur le developpement des Araneides." Ann.
Scien. Nat. Series v. Vol. xvn. 1873. •»/->./

(440) F. M. Balfour. "Notes on the development of the Arane

Journ. of Micr. Science, Vol. XX. 1880. „ v ,

(441) J. Barrois. " Recherches s. 1. developpement des Araign^es.

de I' Anat. et de la Physiol. 1878. , , fiA

(442) E. Claparede. Recherches s. revolution des Aratgnees. Jtrecht, 1862.

(443) Her old. De generation Araneorum in Ovo. Marburg, 1824.

(444) H. Ludwig. "Ueber die Bildung des Blastoderms bei den
Zeit.f. wiss. Zool. Vol. xxvi. 1876.

B. II. b

xviii BIBLIOGRAPHY.

Acariua.

(445) P. van Beneden. " Developpement de 1'Atax ypsilophora." Acad. Bru-
xelles, t. xxiv.

(446) Ed. Claparede. "Studien uber Acarinen." Zeit. /. wits. Zoo/., Bd.
xvin. 1868.

CRUSTACEA.
General Works.

(447) C. Spence Bate. " Report on the present state of our knowledge of the
Crustacea." Report of the British Association for 1878.

(448) C. Claus. Untersuchungen zur Erforschung der genealogischen Grundlage
des Crustaceen -Systems. Wien, 1876.

(449) A. Dohrn. "Geschichte des Krebsstammes. " Jenaische Zeitschrift,
Vol. vi. 1871.

(450) A. Gerstaecker. Bronris Thierreich, Bd. v. Arthropoda, 1866.

(451) Th. II. Huxley. The Anatomy of Invertebrated Animals. London,
1877.

(452) Fritz Miiller. Fiir Darwin, 1864. Translation, Facts for Darwin.
London, 1869.

Branchiopoda.

(453) Brauer. "Vorlaufige Mittheilung iiber die Entwicklung u. Lebensweise
des Lepidurus (Apus) productus." Sitz. der Ak. d. Wiss. Wien, Vol. LXIX., 1874.

(454) C. Claus. '• Zur Kenntniss d. Baues u. d. Entwicklung von Branchipus
stagnalisu. Apuscancriformis." Abh. d. konig. Gesell. der Wiss. Gbttingen, Vol. xviii.

l8'3-_

(455) C. Grobben. "Zur Entwicklungsgeschichte d. Moina rectirostris."
Arbeit, a. d. zoologisch. Institute Wier., Vol. II., 1879.

(456) E. Grube. " Bemerkungen iiber die Phyllopoden nebst einer Uebersicht
etc." Archivf. Naturgcschichte, Vol. xix., 1853.

(457) N. Joly. " Histoire d'un petit Crustace (Artemia salina, Leach) etc." An-
nales d. Sciences Natur., 2nd ser., Vol. xiir., 1840.

(458) N. Joly. " Recherches zoologiques anatomiques et physiologiques sur
1' I sauracy clad oides ( = Esther ia) nouveau genre, etc." Annales d. Sciences Nat., 2nd
ser., Vol. xvii., 1842.

(459) Lereboullet. " Observations sur la generation et le de veloppement de la
Limnadia de Hermann." Annales d. Sciences Nattir., ^th ser., Vol. v., 1866.

(460) F. Ley dig. " Ueber Artemia salina u. Branchipus stagnalis." Zeit. f.
wiss. Zool., Vol. in., 1851.

(461) G. O. Sars. "Om en dimorph Udvikling samt Generationsvexel hos
I^eptodora." Vidensk. Selskab. For hand, 1873.

(462) G. Zaddach. De apodis cancrefortnis Schaeff. anatome ct historia evolu-
tionis. Dissertatio inanguralis zootomica. Bonnae, 1841.

Nebaliadce.

(463) C. Claus. " Ueber den Bau u. die systematische Stellung von Nebalia."
Zeit.f. wiss. Zool., 15d. xxn. 1872.

(464) E.Metschnikoff. Development of Nebalia ( Russian), 1 868.

Schizopoda.

(465) E. van Beneden. "Recherches sur 1'Embryogenie des Crustace's. u.
Developpement des Mysis." liullet. de rAcadc!mie roy. de Belgique, second series,
Tom. xxvin. 1869.

(46H) C. Claus. " Ueber einige Schizopoden u. niedere Malakostraken." Zeit.
/. -t'tss. Zoologie, Bd. XIII., 1863.

BIBLIOGRAPHY. xix

(467) A. Dohrn. " Untersuchungen iib. Bau u. Entwicklung d. Arthropoden."
Zeit. f. wiss. Zool., Bd. xxi., 1871, p. 375. Peneus zoaea (larva of Euphausia).

(468) E. Metschnikoff. " Ueber ein Larvenstadium von Euphausia." Zeit.
fiir wiss. Zool., Bd. xix., 1869.

(469) E. Metschnikoff. " Ueber den Naupliuszustand von Euphausia." Zeit.
fiir wiss. Zool., Bd. xxi., 1871.

Decapoda.

(470) Spence Bate. "On the development of Decapod Crustacea." Phil.
Trans., 1858.

(471) Spence Bate. " On the development of Pagurus." Ann. and Mag. Nat.
History, Series 4, Vol. 1 1., 1868.

(472) N. Bobretzky. Development of Astacus and Paluemon. Kiew, 1873.
(Russian.)

(473) C. Claus. "Zur Kenntniss d. Malakostrakenlarven." Wiirzb. naturw.
Zeitschrift, 1861.

(474) R. Q. Couch. "On the Metamorphosis of the Decapod Crustaceans."
Report Cornwall Polyt. Society, 1848.

(475) Du Cane. "On the Metamorphosis of Crustacea." Ann. and Mag. of
Nat. History, 1839.

(476) Walter Faxon. " On the development of Paloemonetes vulgaris." Bull.
of the Mus. of Comp. Anat. Harvard, Cambridge, Mass., Vol. V., 1879.

(477) A. Dohrn. " Untersuchungen iib. Bau u. Entwicklung d. Arthropoden."
" Zur Entwicklungsgeschichte der Panzerkrebse. Scyllarns Palinurus." Zeit. f.
wiss. Zool., Bd. xix., 1870.

(478) A. Dohrn. "Untersuchungen iib. Bau u. Entwicklung d. Arthropoden.
Erster Beitrag z. Kenntniss d. Malacostrakcn u. ilirer Larven Amphion Reynaudi,
Lophogaster, Portunus, Porcellanus, Elaphocaris. " Zeit. f. ^viss. Zool., Bd. XX.,
1870.

(479) A. Dohrn. "Untersuchungen iib. Bau u. Entwicklung d. Arthropoden.
Zweiter Beitrag, etc." Zeit.f. wiss. Zool., Bd. XXI., 1871.

(480) N. J oly. " Sur la Caridina Desmarestii." Ann. Scien. Nat., Tom. xix.,
1843.

(481) Lereboullet. " Recherches d. 1'embryologie comparee sur le developpe-
ment du Brochet, de la Perche et de 1'Ecrevisse." Mem. Savuns Etrang. Paris, Vol.
xvii., 1862.

(482) P. Mayer. "Zur Entwicklungsgeschichte d. Dekapoden." Jenaische
Zeitschrift, Vol. XI., 1877.

(483) Fritz Miiller. '* Die Verwandlung der Porcellana." Archivf. Naturge-
schichte, 1862.

(484) Fritz Miiller. " Die Verwandlungen d. Garneelen." Archivf. Natur-
gesch., Tom. xxix.

(485) Fritz Miiller. " Ueber die Naupliusbrut d. Garneelen." Zeit. f. wiss.
Zool., Bd. xxx., 1878.

(486) T. J. Parker. "An account of Reichenbach's researches on the early
development of the Fresh-water Crayfish." Quart. J. of M. Science, Vol. xvui.,
1878.

(487) H. Rathke. Ueber die Bildung u. Entwicklung d. Flusskrebses. Leip-
zig, 1829.

(488) H. Reichenbach. " Die Embryoanlage u. erste Entwicklung d. Fluss-
krebses." Zeit. f. wiss. Zool., Vol. xxix., 1877.

(489) F. Richters. "Ein Beitrag zur Entwicklungsgeschichte d. Loricaten."
Zeit.f. wiss. Zool., Bd. xxiii., 1873.

(490) G. O. Sars. " Om Hummers postembryonale Udvikling." Vidcnsk Selsk.
Forh. Christiania, 1874.

(491) Sidney J. Smith. " The early stages of the American Lobster. " Tratts.
of the Connecticut Acad. of Arts and Sciences, Vol. II., Part i, 1873.

(492) R. v. Willemoes Suhm. " Preliminary note on the development of some
pelagic Decapoda." Proc. of Royal Society, 1876.

XX BIBLIOGRAPHY.

Stomatopoda.

(41KI) \V. K. Brooks. " On the larval stages of Squilla empusa.'' Chesapeake
Zoological Laboratory^ Scientific results of the Session ^1878. Baltimore, 1879.

(494) C. Claus. "Die Metamorphose der Squilliden." Abhand. dcr konigl.
Gesell. der IViss. ztt Gottingen^ 1*7-.

( 1 '.!">) Fr. M tiller. 4i Bruchstuck a. der Entwicklungsgeschichte d. Maulfiisser I.
und II." Archivf. Naturgeschichte, Vol. xxvni., 1862, and Vol. XXIX., 1863.

Cumacea*

( 1%) A. Dohrn. " Ueber den Bau u. Entwicklung d. Cumaceen." Jenaische
Zeitschrift, Vol. v., 1870.

hopoda.

(497) Ed. van Beneden. " Recherches sur 1'Embryogenie des Crustaces. i.
Asellus aquaticus." BiuL de FAcad. roy. Belgique, 2me serie, Tom. XXVIII., No. 7,
1869.

(498) N. Bobretzky. "Zur Embryologie des Oniscus murarius." Ztit. fur
wiss. Zool., Bd. xxiv., 1874.

(4119) J. F. Bullar. "On the development of the parasitic Isopoda." Phil.
Trans., Part n., 1878.

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BIBLIOGRAPHY. xxi

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Ostracoda.

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PCECILOPODA.

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1 The dates in this reference are the dates of publication.

BIBLIOGRAPHY. xxiil

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