SYNAPTA VIVIPARA

A CONTRIBUTION TO THE MORPHOLOGY OF ECHINODERMS

A DISSERTATION

V

ACCEPTED FOK THE DEGREE OF DOCTOR OF PHILOSOPHY BY THE BOARD OF
UNIVERSITY STUDIES OF THE JOHNS HOPKINS UNIVERSITY

1897

BY HUBERT LYMAN CLARK

(Reprinted from Volume V, No. 2, of the Memoirs of the Boston Society of Natural History,

and Volume IV of the Memoirs from the Biological Laboratory of

the Johns Hopkins ITnietrrity)

BALTIMORE
1898

BIOLOGY
LIB*

Q

3. SYNAPTA VIVIPARA : A CONTRIBUTION TO THE MORPHOLOGY OF ECHINODERMS.

BY HUBERT LYMAN CLARK.

(Read November 3, 1897.)

CONTENTS.

1. Introductory 53 0. The development of the pentactula . . 63

2. History and systematic position of Synapta vivipara 64 7. The development of the adult Synapta . . 68

3. Distribution and habits 55 8. The anatomy of the adult 75

4. Fertilization and segmentation of the egg . . 58 9. Conclusion 79

6. Gastrulation and formation of the hydrocoel and 10. Literature .83

coelomic vesicles ' . til 11. Explanation of plates 86

1. INTRODUCTORY.

In February, 1890, through the kindness of Dr. W. K. Brooks, there were placed in
my hands for study a number of specimens of a small brown Synapta from Jamaica. In
their body-cavities there were numerous young ones in all stages of development, so that
an excellent opportunity was offered for working out the embryology. So interesting
did this prove that I gladly availed myself of the privilege of spending the months of
May, June, and July, 1896, at Port Henderson, Jamaica, in the marine biological labora-
tory of the Johns Hopkins University. For such a privilege I am under the greatest
obligations to the authorities of that institution. During those months, I studied the
segmentation of the egg and the early stages of development from living material and
obtained an abundance of preserved material for further investigations. Since my return
I have been engaged in a detailed study of the development and anatomy of the animal
under the direction of Dr. Brooks, and it gives me great pleasure to acknowledge the debt
I am under to him for his suggestions and help.

The young were easily procured by cutting off the heads of the adults and thus
setting free the contents of the body-cavity. They could then be killed as desired, but
young taken from the body-cavities of preserved adults were fully as satisfactory for all
purposes. The best results were obtained by the use as a fixing agent of " corrosive-
acetic " (four parts corrosive-sublimate and one part glacial acetic acid), but excellent
preservation, especially for the adults, was secured by the use of picro-sulphuric or picro-

173213

54 HUBERT LYMAN CLARK ON

nitric acid. Corrosive-sublimate alone gave poor results, while Perenyi's fluid proved
very unsatisfactory, the material prepared with it showing only fair preservation, and
staining very poorly. Very good material for the study of the calcareous bodies and
the development of the calcareous ring was obtained by the use of absolute alcohol. In
staining, it was found useless to try any solution which contained less than seventy per
cent alcohol, collapse and distortion of the tissues always resulting. Kleinenberg's
haematoxylin and eosin gave good results, but acid borax-carmine (70% alcohol) and
Lyons blue (70% alcohol) proved the most satisfactory. The early stages of the larvae
were oriented and imbedded in celloiden before imbedding in paraffine, and thus kept
their shape very well. Although material was so abundant and easily obtained, it is
impossible to tell the age of the different embryos, for none of the eggs obtained
developed beyond the blastula, and later stages would live outside of the mother for only
a very short time. Accordingly it cannot be determined how many days or weeks are
required for the growth of the pentactula or older stages, though there is reason to
believe that growth, at least early in life, is very rapid.

2. HISTORY AND SYSTEMATIC POSITION OF SYNAPTA VIVIPARA.

The Danish naturalist Orsted ('50) mentioned the discovery in the West Indies of
a new genus of the Synaptidae, the chief characteristic of which was its being viviparous.
To this genus he gave the name Synajjtula, and the type species he called vivipam. In
his very brief and unsatisfactory account, for which I am indebted to Ludwig's German
translation ('81), he mentions the occurrence of the form in " shallow-water," describes
the color as "greenish," and speaks of the presence of " eyes," "skin-glands," and
"anchors." He never published anything further in regard to the species, and later
writers, as Bronn ('60), Selenka ('67), and Semper ('68), accepted the genus Synaptula
without comment, But Ludwig ('81), in describing a viviparous Chirodota (G. rotifera)
from the coast of BraziJ, suggested that it might be the species on which Orsted had
based his new genus. This conclusion was adopted by Lamport ('85), and in his work
we find Synaptula vivipara given as a synonym under hirodota rotifera. Theel ('86)
placed Synaptula vivipara in the list of Synaptidae about which little or nothing is known,
but he suggests in agreement with Ludwig ('81) that the stage of development which the
eggs reach before leaving the mother, is not a satisfactory character upon which to base
a new genus. Ludwig ('86) described a small Synapta, found in floating seaweed, west
of the Abrolhos Reef, Brazil, which he regarded as identical with Orsted's species, since
it contained young in the body-cavity and answered Orsted's description, except in color

SYNAPTA VIVIPARA. 55

In 1891, students of the Johns Hopkins University found at Port Royal, Jamaica, a
viviparous Synapta, and, in 1893, another party from the same institution brought back a
large amount of preserved material of this species, which they found abundantly about
Port Royal. It was this material which came into my hands, and after examining it and
studying the living animal at Port Royal, I had no difficulty in satisfying myself that it,
as well as Ludwig's Abrolhos specimen, was indeed Orsted's Synaptula vivipara. A
brief description to establish its position in the genus Synapta, was published by Clark ('96) ,
and attention was there called to its similarity to S. picta Theel ('96). This species was
described from a single specimen in the Challenger collection, from Bermuda, and the
agreement of all its characters with those of S. vivipara was very striking. Dr. The"el
writes me that he has specimens of a viviparous Synapta from Bermuda in his hands at
the present time, which agree with S. picta and S. vivipara so completely that he has no
doubt that all three are the same species; but the evidence is incomplete, owing to the
absence of the anchors and plates in his Bermuda specimens, caused by the killing agent
used. Although, as we shall see, Synapta vivipara differs in several important particulars
from all other Synaptas hitherto described, they are not sufficiently obvious to warrant its
separation, under the existing classification of the Synaptidae, as a distinct genus. Synap-
tula must therefore become a synonym of Synapta, and the synonomy of S. vivipara will
be as follows : —

Synapta vivipara (Orst.) Ludw. Zool. Jahrbiicher. Bd. 2, p. 28. 1886.
Synaptula vivipara Orsted. Vid. med. fra d. nat. For. i Kjobenhavn for 1849-50, p. vii.
Synaptula vivipara Bronn. Klas. und Ord. d. Thier., Bd. 2, p. 403. 1860.
Synaptula vivipara Selenka. Zeit. fur wiss. Zool., Bd. 17, p. 365. 1867.
Synaptula vivipara Semper. Reis. in Arch. Phil., 2. Theil, 1. Bd., p. 24. 1868.
Chirodota rotifera (in part only) Lampert. Die Seewalzen. Wiesbaden. 1885.
Synaptula vivipara Theel. Report on the Holothurioidea. "Challenger" Reports.

Zool., Vol. 14, p. 32. 1886.
Synapta picta The"el., p. 10. Report on the Holothurioidea. "Challenger" Reports.

Zool., Vol. 14, p. 32. 1886.

3. DISTRIBUTION AND HABITS.

Orsted's specimens of Synapta vivipara, he tells us, were from the West Indies,
and all of the specimens I have seen, came from Jamaica. Ludwig's single specimen came
from the Abrolhos Reef off the coast of Brazil (18° S. lat.), while all of The"el's specimens
are from Bermuda (32° N. lat.). We may therefore conclude that the species is pretty

56 HUBERT LYMAN CLARK ON

widely distributed throughout the eastern Atlantic ocean, wherever suitable conditions
are found. In Jamaica, however, the species is extremely local and was found in only
one place, the so-called " lakes " at Port Royal. These " lakes " are parts of the harbor
which have been wholly or in part cut off by the growth of mangroves, so that they are
very quiet bodies of water, though not at all stagnant or brackish. On the roots of the
mangroves, which hang down in the water on all sides, is an abundant growth of
vegetable and animal life. In some places, a particular sea-weed, one of the Florideae,
crowds out all the other Algae. In this weed Synapta vivipara makes its home, and though
carefully looked for elsewhere, it was found in numbers only in such situations. The
late Dr. J. E. Humphrey kindly identified this alga for me, as Acantliopliora thierii
Lamouroux. This weed also grows in large bunches on the bottom in the shallow water
of the harbor just outside the " lakes," and I was told that in 1893 the Synapta was
found in great quantities there, but in 1896 it seemed to have entirely disappeared
from that place. At Montego Bay, on the northwest coast of Jamaica, Acanthophora is
very abundant on the mangrove roots, but a thorough search revealed no sign of
Synaptas there. Even in the Port Royal "lakes," their distribution was very capricious,
and only certain favored masses of Acanthophora contained them in any numbers. They
seem to be quite social in their habits, and usually if one or two were found, there would
be a whole colony of them. They are very sensitive to changed conditions, and I was
unable to keep them alive in aquaria more than twenty-four hours. The anchors in the
body-wall are so abundant and prominent that they cling very tenaciously to anything
with which they come in contact, especially the hands, and it is accordingly no easy task
to disentangle them from the sea-weed without injury. They seem to be able to swim
very little, and it is doubtful if they ever leave the bunch of sea-weed, in which they
have once settled. Their food consists largely of vegetable matter, diatoms being
abundant in the stomach, but probably many small crustaceans and worms are also eaten.
The tentacles are kept constantly in motion, and it was very common to find small
amphipods caught among them, but I was unable to find evidence that these crustaceans
ever served as food. Semon ('87) has called attention to what he considers a mimicry
of coloration in Synapta inhaerans, in relation to the bottom on which it is found. In
this connection, it is interesting to note that the reddish and greenish brown shades
of S. vivipara are almost exactly those of the Acanthophora in which it lives. So close
is the resemblance that it is very easy to overlook Synaptas, even when the sea-weed is
in one's hand. Whether this coloration is actually protective or not is doubtful, for they
seem to have few, if any, enemies. No internal parasites were observed ; externally
however a small brown calcareous sponge, like Grantia, was found firmly attached

SYNAPTA VIVIPARA. 57

to the skin, just behind the circle of tentacles. When placed in aquaria, this
Synapta does not break up by muscular contractions, like S. inhaerans, nor does it
ordinarily eviscerate like many holothurians, but after crawling about restlessly for a
while, it stretches out on the bottom and dies, almost without the contraction of a muscle.
In a few cases, evisceration at the mouth took place, when they were thrown into a killing
agent.

Breeding goes on all through the spring and summer ; and there is no evidence to
show that it does not go on all the year round. My earliest specimens, collected April 30
contained many well-developed young, and up to the end of July in all the specimens
obtained, young were found, while Ludwig's specimen from Abrolhos was collected in
September and contained very young embryos. The number of young in the body-
cavity of a single adult varies greatly, depending more or less on the size of the individ-
ual. Specimens not over a centimeter long may contain a few, while some very large
ones have scores. The largest number I have found is 176. It is a curious fact that the
young are almost always in two broods ; that is, a certain proportion of them will all
have reached a given age, say that of the pentactula, while the remainder will be much
younger, say about that of the gastrula. When the young are very few, they will all be
the same age, while if they are very numerous, they will sometimes show three different
stages. This fact seems to indicate that the eggs ripen and pass into the body-cavity in
lots of from six to a hundred, and that several days elapse before another lot is ripened.
Regarding the length of time during which the young remain in the body-cavity, it is
impossible to make even an estimate. Animals kept in aquaria frequently gave birth to
young only five mm. long, and it was not usual to find much larger specimens in the adults
examined, but sometimes a young one, fifteen or twenty mm. long, with all the charac-
ters of the adult, would be found still inside its mother. Observations made on the living
animals showed that birth occurs normally at the posterior end of the body, apparently
through the anus. Investigation showed that this was accomplished by a rupture of the
body- wall, which may be through the skin some little distance from the anus (Fig. 29),
or, as seems to be more generally the case, through the wall of the rectum close to the
point where it joins the external body-wall, the young passing out through the anal
opening. It may be that the openings in the wall of the rectum (Figs. 30 and 31), to be
described later, are concerned in the birth of the young, but they seem to be too small to
be of any service in this connection, except possibly as starting points for the rupture of
the rectum wall. Under abnormal conditions, I found by experiment, rupture of the
body-wall and consequent birth of the young may occur at other points than near the anus.

58 HUBERT LYMAN CLARK ON

4. FERTILIZATION AND SEGMENTATION OF THE EGG.

Like all other known Synaptas, S. vivipara is hermaphroditic. Ripe spermatozoa and
ova are found in the same genital organ and even in the same branch, and to judge both
from living material and preserved specimens, even at the same time; but this latter
point could not be proven since it is impossible to determine simply by observation, when
the egg is mature. The branches of the genital gland are nile-green in color in the living
animal, while the fully grown ova are brownish yellow, and the spermatozoa, when in
any quantity, appear to be white. In shape, the latter are like those figured by Jourdan
('83) for Holothuria tubulosa. The mature egg (Fig. 1) is about '20(V in diameter and
before fertilization is not provided with any membrane. It is full of yolk material but
comparatively transparent, so that the internal changes could be watched up to a late
stage of development. Artificial fertilization proved unsuccessful, although attempted
several times. Whether self-fertilization takes place or not could not be positively
decided, but that it is at least very improbable seems clear from the structure of the
genital organs and the probable manner of egg-laying. The genital gland of the adult
Synapta lies just above the oesophagus, with one or two branches on each side of the
dorsal mesentery and with the genital duct lying in that mesentery and opening to the
exterior close to the base of one of the mid-dorsal tentacles. A cross-section of one of
the branches (Fig. 40) shows that it consists of an external covering, the continuation of
the epithelium, a very scanty connective-tissue layer consisting only of a few scattered
mesenchyme cells, and a much folded germinal epithelium surrounding the lumen of the
gland. Of the layer of circular muscle fibers which was found in the genital gland
of S. digitata and S. inhaerans by Quatrefages ('42), Baur ('64), and Hamnnn ('84), I
have found no trace in S. vivipara. The branches of the gland contract vigorously after
being cut from the body, but so far as I could see, these contractions were always longi-
tudinal. The germinal epithelium is more or less plainly made up of two or more layers
of cells of which the external are the larger. As will be seen from figures 39 and 40,
from this external layer of cells the ova arise and so come to lie between the germinal
epithelium and the epithelium of the body-cavity, and do not pass into the lumen of the
gland at all. This arrangement is quite the reverse of what Cuenot ('91) has shown to
exist in S. inhaerans, and gives us a clue as to how the eggs get into the body-cavity of
the mother. When the eggs are mature, they press so closely against the external
epithelium of the gland that it bulges out sufficiently to be seen with the naked eye, in
the living gland. Sections show that the epithelium over such ova is so stretched as to
be thinner than elsewhere (Fig. 39), and probably the eggs enter the body-cavity simply

SYNAPTA VIVIPARA. 59

by the rupture of the epithelium at that point. While this has never been actually
observed, the theory is supported by a further examination of the genital gland. The
smaller internal cells of the germinal epithelium give rise to spermatozoa, which are
almost invariably to be found in the lumen of the gland, never on the outside with the
ova. If we trace the lumen forward we find it passes directly into the lumen of the
genital duct, the internal walls of the latter actually being formed by a continuation of
the germinal epithelium, which has become of uniform thickness and ciliated internally
(Figs. 36 and 37). The lumina of the two or three branches of the gland unite on
entering the duct (Fig. 36) and pass through the latter upwards towards the body-wall-
At no point is there any sign of communication between the genital duct and the space
external to the germinal epithelium in which the ova lie. If we follow the genital duct
upwards to where it comes in contact with the body-wall, we find it does not fuse with
the ectoderm and open at once to the exterior, but simply lies in the connective tissue
with its end against the ectoderm (Fig. 38), and although there are probably openings
through which the spermatozoa pass out, they are so extremely small I have not been
able to demonstrate them satisfactorily. In nearly all the specimens examined the
genital duct contained large quantities of spermatozoa, but in no case was there any trace
of an ovum. That the openings at the end of the duct are not directly continuous with
openings through the body-wall is indicated by the occurrence of spermatozoa, sometimes
in large quantities, in the connective tissue surrounding the terminus of the duct (Fig.
38). From all these facts I am convinced that the ova pass into the body-cavity by a rup-
ture of the peritoneal epithelium, while the spermatozoa pass outward through the genital
duct to the exterior. In Cucumaria glacialis, the only other viviparous holothurian
concerning whose breeding we have any information, Mortensen ('94) thinks the eggs
are laid on the bottom and taken up afterwards into the brood-sacks of the mother. But
it is manifestly impossible that the eggs of S. vivipara could get into the body-cavity in
any such way. The next question that arises is, how do the spermatozoa reach the ova
inside the body-cavity of the mother, and the answer brings to light another interesting
modification of structure, adapted to the viviparous habit. Careful examination of the
rectum shows that through its wall there are direct channels of communication between
the body-cavity and the exterior through the anus. The wall of the rectum is folded
and ridged longitudinally, and at certain places, parts of these ridges have pushed out and
fused with invaginations from the surrounding coelomic wall, forming distinct tubes
connecting the interior of the rectum with the interior of the coelom (Figs.
30 and 31). No trace of valves or cilia was found in any of these tubes, but
the passage of water in and out could be easily regulated by the opening and

60 HUBERT LYMAN CLARK ON

closing of the anus. With the anus open, each muscular contraction of the animal
would tend to either force water out or draw it in through these openings, and in
that way spermatozoa could easily get into the body-cavity and thus fertilization
could take place within. This will appear more probable when it is remembered
that the animals are very social, and that the water around a mangrove root on which
there are hundreds of them, must contain countless spermatozoa. There is also a
possibility that spermatozoa enter the body-cavity, through the water-pore and stone-
canal, which, as we shall see, remain open in the adult, and also open into the body-cavity.
After fertilization a membrane forms around the egg and segmentation begins. It
seems probable that the extrusion of the polar bodies occurs before the formation of this
membrane, as Selenka ('83) found no trace of them in the segmenting eggs of &'. digitata,
and I could not find them in any of the eggs of S. vivipara which I examined, although
they were looked for with special care. Segmentation and the formation of the blastula
occur in practically the same manner as has been so well figured by Selenka ('83) for
S. digitata. The first plane of division forms two blastomeres of equal size and appear-
ance (Fig. 2). After a resting period of about twenty minutes, the second plane of
division occurs at right angles to the first, giving rise to four similar blastomeres (Fig.
3). The third plane is at right angles to the first two, and we now have an embryo of
eight equal cells with a segmentation-cavity between them (Fig. 4). The sixteen-cell
stage (Fig. 5) soon follows, the division plane being at right angles to the preceding.
The appearance of the embryo at this stage is very peculiar and characteristic, the cells
being arranged in a band or ring and the segmentation-cavity being open at each pole.
Another plane of division, again at right angles to the preceding, doubles the width of
the band and decreases the openings at the poles, but it does not divide the cells exactly
equally, so that the upper- and lowermost rows are of somewhat smaller cells than the two
middle rows and have a less diameter (Fig. 6 and 7). The subsequent divisions occur
with a fair degree of regularity in alternating planes, each division decreasing the
openings at the poles until at last they are entirely closed. This occurs when the embryo
consists of approximately 256 cells, and so the blastula is formed (Fig. 8). The cells of
the four equatorial rows are somewhat larger than the rest, but the difference is not at
all noticeable and apparently has no significance. The divisions have followed on each
other with great rapidity so that the complete blastula is formed after about four hours,
while in S. digitata, according to Selenka ('83), the blastula is the result of twelve hours'
growth. It is for this reason, that I am inclined to think that the whole process of devel-
opment in S. vivipara is very rapid.

SYNAPTA VIVIPARA. 61

5. GASTRULATION AND FORMATION or THE HYDROCOEL AND COELOMIC VESICLES.

Invagination of one of the poles of the blastula soon forms the archenteron from the
blind end of which the mesenchyme cells now begin to arise, some of the endodermal
cells being simply crowded out into the segmentation-cavity. I could find no evidence at
all of Selenka's ('83) two primitive mesenchyme cells, but on the contrary, I found cells
all over the archenteron which were in various stages of passing into the segmentation
cavity. On the other hand in not a single case were there found any of the ectoderma-
cells forming mesenchyme, and I feel no hesitation in affirming that the mesenchymel
arises exclusively from the endodermal cells, contrary to Ludwig's ('91) observations on
Cucumaria. The number of mesenchyme cells is comparatively small, and they never
become so numerous or play so important a part in larval structures as they do in S.
digitata, judging from Semon's ('88) account and figures. The completed gastrula (Fig.
9) is covered with cilia which are easily seen in the living specimens, but in none of my
preserved material has it been possible to demonstrate them. In a few cases gastrula
were found free from the egg-membrane and moving about actively in the fluid of the
body-cavity, but the great majority are still enclosed in the membrane within which they
rotate by means of the cilia. The membrane may be retained, as preserved specimens
show, until long after the coelomic vesicles are formed, and I am inclined to believe that
there is no definite time when it is cast off, but that it ruptures and is lost whenever the
larva has grown too big for it. As the archenteron increases in length it bends to one side
and unites with the wall of what subsequently becomes the dorsal surface of the larva.
Its lumen breaks through the surface, and thus the water-pore is formed, as described by
Selenka ('83) for S. digitata. Meanwhile the gastrula loses its spherical shape and
becomes more or less elongated (Fig. 10). The cells at the end opposite the blastopore
are already somewhat different in form from those elsewhere and make a sort of plate of
thickened ectoderm (Fig. 21) which may correspond to the so-called "neural plate" of
other echinoderm larvae. This plate, however, does not lie exactly opposite the blastopore,
but somewhat toward the ventral side of the larva, and as the gastrula increases in length,
it comes to lie more and more on that side. At the same time, the archenteron continues
to increase in length and grows forward and at the same time ventralward, thus drawing
away from the water-pore. In so doing, that part of it which grows forward pushes by
the part opening through the water-pore, on the right-hand side (looked at from the
dorsal surface) so that the latter comes to lie on the left side of the larva (Figs. Hand
12). As the archenteron grows it completely severs its connection with this vesicle, and

62 HUBERT LYMAN CLARK ON

pushing onward and downward against the thickened part of the ectoderm, becomes
attached to it, and the mouth breaks through at that point. The mouth may be formed
before the separation between the archenteron and the other vesicle is completed. Mean-
while the latter increases in size, and its walls become thinner. It grows backward toward
the blastopore especially and soon becomes constricted and divided into two vesicle?, the
anterior of which is connected with the exterior by the water-pore, while the posterior is
entirely closed and lies beside the posterior part of the stomach, if we may so designate
the middle section of the archenteron (Fig. 13). Soon afterwards this posterior vesicle
grows out laterally to the right, across the dorsal surface of the larval hind-gut, and forms
on the right-hand side a vesicle like itself, which soon becomes entirely separate from it
(Fig. 14). The anterior of these three vesicles is the rudimentary hydrocoel, while the
pair of posterior poiiches represent the right and left coelomic vesicles. Their mode of
formation is essentially the same as that described by Selenka ('83) for S. digitata, but
the relative size of the various organs is markedly different, so that the figures of the same
stages show almost no resemblance. If figure 15 of Synapta vivipara be compared with
the auricularia of about the same age which Semon ('88, Plate 6, Fig. 2) figures, this
difference will appear in many ways. The rudiments of the two coelomic pouches and
especially of the hydrocoel are very much larger relatively in S. vivipara than in S. digi-
tata, while the difference in the digestive tract is even more noticeable. In the European
species there is a well-marked differentiation into fore-, mid-, and hind-gut, while in the
Jamaican form there is no such distinction evident. A remarkable difference is also to
be seen in the external form of the larvae. That of S. vivipara has retained its elliptical
shape and shows no trace of ciliated bands, while the other has assumed the familiar
auricularia form. Furthermore, there is no trace of calcareous structures of any kind in
S. vivipara to correspond with the plates at the posterior end of auricularia. In the
latter also, Semon ('88) has figured and described a larval nervous system, but after care-
ful investigation of this point, I am convinced there is none in the larva of S. vivipara.
In external form, the latter is very regular, but with the growth of the coelomic pouches
the ectoderm of the dorsal surface begins to become thinner and more flattened, while that
around the mouth, especially posterior to it, shows a tendency to increase in thickness.
This difference between the dorsal and ventral surfaces may sometimes be seen in very
young larvae and is clearly shown in figure 22. Before the two coelomic vesicles have
entirely separated, the hydrocoel has become considerably larger and begins to grow
anteriorly and toward the right, while about the same time five outgrowths begin to appear
on that side which is furthest from the fore-gut (Fig. 15). Soon after their appearance
five other much smaller outgrowths' arise, one at the right of each of the first five. The

SYPATA VIVIPARA. 63

first series gives rise to the five primary tentacles, and the second series corresponds to
those which in S. digitata give rise to the radial water-canals. In S. digitata however,
there is a sixth outgrowth at the extreme left, the rudiment of the Polian vesicle, but in
S. vivipara this vesicle is not formed until after the closure of the hydrocoel ring. The
water-canal enters this ring at a point just between the fourth primary tentacle (counting
from before backwards) and the fourth secondary outgrowth (Fig. 23). In this particu-
lar my observations confirm Bury ('89) in opposition to the statements of Semon ('88).
Before the outgrowths of the hydrocoel are very evident, that part of it which lies
dorsally and in immediate connection with the water-canal biilges out, becomes thinner-
walled than the rest of the hydrocoel, and gradually separates from it, but before
the separation becomes marked, the outgrowth diminishes in size and with the
increasing growth of the water-canal disappears. This structure, I believe, is the
"anterior coelom " which Bury ('95) has shown to exist in the auricularia of
S. digitata. It is very marked in some specimens of S. vivipara (Fig. 24), and I
see no reason to doubt Bury's interpretation. He does not make very clear what the
ultimate fate of this coelom is in S. digitata, but leaves the impression that it is connected
with the subsequent formation of the madrepore plate, as Ludwig ('91) considers it to be
in Cucumaria. In S. vivipara however, there is seldom any trace of it left after the
hydrocoel ring closes, and there is no reason to suppose that it has any connection with
the much later madrepore openings. About the time of the appearance of the primary
tentacles, the larval anus, which was the original blastopore, closes entirely and the
digestive tract ends blindly. Accordingly we now have a regular elliptical larva, about
a third of a millimeter long, with the ventral ectoderm much thicker than the dorsal,
without ciliated bands, calcareous particles, or nervous system, a mouth on the anterior
ventral surface but no anus, a well-developed coelomic pouch on each side of the digestive
tract, and a hydrocoel with five primary tentacles and five secondary outgrowths, opening
to the exterior through the dorsal pore, by means of an adradial water-canal, upon
which may still be seen the vestige of an anterior coelom.

6. THE DEVELOPMENT OF THE PENTACTULA.

In such larvae, the ectoderm of the ventral surface continues to thicken and before
long is sharply set off from the ectoderm of the rest of the body, which consists of a
single layer of cells. The thickened ectoderm forms a circular field around the mouth
though the latter does not lie at its center, but nearer to its anterior edge. This circular
disc gradually sinks below the level of the rest of the ectoderm, and the latter grows in

64 HUBERT LYMAN CLARK ON

over it from all sides, until the disc lies at the bottom of a shallow cavity, the so-called
" atrium," which opens on the ventral surface through a small poi'e (Fig. 16). This
pore does not lie directly over the mouth, but either before or behind or somewhat to
one side. While these external changes are taking place, the hydrocoel has continued its
growth, the anterior end passing across on to the right side>of the larva where it bends
downward and backward around the oesophagus to meet the posterior end near the middle
line. The primary tentacles have increased considerably in size and are growing up
around the floor of the atrium, while the secondary outgrowths also grow upward beside
them. No Polian vesicle has yet appeared, and no marked distinctions between the
different parts of the digestive tract are to be seen, but the latter has become very much
arched toward the dorsal surface, and the lumen of the middle section is larger than at
either end. A new anus may have been formed by the end of the hind-gut growing to
the body-wall on the ventral side and an opening breaking through, but in some cases
the definitive anus does not appear until the pentactula form is nearly attained. The
most important changes have been going on meanwhile in the coelomic pouches,
the growth of which has been very rapid. The left vesicle has grown more ante-
riorly than the right, and sends forward two finger-like processes, one of which passes
across the median line into the right side of the larva, while the other grows up to
the inner side of the hydrocoel ring, above the most posterior tentacle, and follows the
course of that ring around the oesophagus (Figs. 26-28). These anterior prolongations
of the left coelom were observed by Bury ('89 and '95) in 8. digitata, but have appar-
ently been overlooked by other investigators. The one which passes on to the right,
side of the body fuses so soon with the right coelom that I have been unable to confirm
Bury's further observations regarding it, and in S. vivipara its appearance might easily
be entirely overlooked. But the prolongation which passes to the hydrocoel remains
distinct through all the later stages of the larva, and its subsequent changes are easy to
trace. It soon loses its connection with the left coelom and forms a tube, lying on the
oral surface of the hydrocoel ring. Meanwhile the right and left coelomic pouches have
met and fused on the ventral side of the digestive tract, while dorsally they are still
separate, the right pouch extending considerably further back than the left. The larva
has now reached the condition shown in Fig. 16. In its subsequent growth the atrium,
with its thick ectodermal floor and narrow opening to the exterior, moves to the anterior
end of the larva, where it finally comes to lie, along with the mouth, oesophagus, and
hydrocoel ring. The latter has now closed, without the formation of a Polian vesicle,
somewhat to the left of the middle line apparently (Fig. 25), but I am not sure that the
point of closure is always on the left side, for it is by no means easy to determine the

SYNAPTA VIVIPARA. 65

exact mid-line. With the closure of the hydrocoel, occurs the union of the two ends of
that coelomic tube which lies on its oral surface, so that we now have a circular sinus
around the oesophagus just above the water-ring. This sinus is very evident in young
Synaptas, and Bury's ('95) surmise regarding its origin from the left coelom is entirely
correct. The primary tentacles are growing upward, not pushing the floor of the atrium
out before them, as Semon ('88) says, but enclosing the atrium within their circle so that
the thickened sensory epithelium, which they subsequently possess, does not arise as
Semon describes. It is clear, from Figs. 16 and 17, that his description could not possibly
apply to S. vimpara. The secondary outgrowths remain nearly unchanged in size and
show no sign of bending backward to form radial canals. The Polian vessel is formed as
an outgrowth on the inner side of the hydrocoel ring in the left dorsal interradius, as
Ludwig ('23) found it to be in Cucumaria.

The digestive tract grows with greater rapidity relatively than any of the other
organs. Accordingly, the oesophagus pushes upward toward the atrial opening, so that
the thickened ectodermal floor of the atrium lies surrounding it, in the form of a poorly
defined circumoral ring. Continued growth pushes the oesophagus against the upper
ectodermal wall of the atrium, and with that it fuses, leaving the circumoral ring entirely
cut off from the outer ectoderm of the body (Figs. 87 and 88). Meanwhile the growth
of the primary tentacles has pushed this body-wall upward and outward, so that the
narrow slit-like opening of the atrium is gradually widened until it finally disappears,
leaving the ectodermal-covered, anterior end of the oesophagus to form the definitive
mouth, in the center of the circle of tentacles. This process is not completed however,
until the pentactula form is fully assumed. The differences between this development
of the mouth and circumoral ring and that given by Semon ('88) for S. digitata are
almost irreconcilable, but they are all dependent on the question, whether the five
primary tentacles push up through the floor of the atrium or grow up around it. The
latter is certainly the case in S. vimpara. While these changes are taking place anteri-
orly, the hind-gut has increased in length so that it has not only arched still more toward
the dorsal surface but has bent on itself and formed a loop lying to the left of the
stomach. The coelomic pouches already united and forming a single cavity ventrally,
have met in, or close to, the mid-dorsal line and by the union of their walls have formed
the dorsal mesentery. This mesentery follows pretty closely the curve of the intestine
and attaches it throughout its course, to the body-wall. The water-canal lies in the
anterior part of the mesentery, but whether that part was formed in a different manner,
as Bury ('95) thinks probable, it is impossible to say from observations on S. mmpara.

Meantime most important changes are going on in the circumoral ring of ectoderm

HE

UNIVERSITY ,

OF

-•

66 HUBERT LYMAN CLARK ON

which we have seen was formed from the floor of the atrium. This ring is the
beginning of the central nervous system, and from it the tentacle and radial nerves arise.
As the primary tentacles push upward past the atrium, they lie closely appressed to its
floor and wall. They retain this position even after the circumoral ring is pretty clearly
denned, and before they have grown much above it, the radial nerves appear between
each pair of them as outgrowths of the central ring. These outgrowths pass directly
over the secondary outgrowths of the water-vascular system and bend backwards to run
toward the aboral pole of the body. Very soon after they appear (not before them, as
Semon ('88) describes for S. digitata), that part of the circumoral ring appressed to each
primary tentacle begins to grow upward with it on its inner side, forming the tentacle
nerves. As the tentacles continue to grow and press the anterior body wall outward, the
ectoderm which covers them at the tip becomes noticeably thickened, especially on the
outer side (Fig. 54), and apparently assumes a sensory function, probably in connection
with the tentacle nerve. The formation of the otocysts of Thomson ('62), the " horor-
gane " of Baur ('b'4), takes place as described by Semon ('88). They arise by evagina-
tions from the outer side of the circumoral ring close beside the outgrowths which form
the radial nerves. With the growth of the latter, the otocysts come to lie external to
them at the point where they bend backward, and in this position the sense-organs remain
throughout life. It is a very evident and noteworthy fact that the development of the
radial nerves and otocysts does not take place at the same time in the five radii, but there
is a marked difference between them. The first to appear is that nerve which subse-
quently indicates the mid-ventral radius, and with it appear its two otocysts. The two
lateral ventral nerves appear next, and with them the otocyst which accompanies each one
on its ventral side. The lateral dorsal nerves next appear (Fig. 1), and very soon after-
wards the two other otocysts of the right and left ventral radii are formed. Last of all
to develop are the otocysts of the right and left dorsal radii. This sequence in the
appearance of these nerves and sense-organs is probably connected with the fact already
mentioned, that the thickened ectoderm which made up the floor of the atrium did not lie
symmetrically around the mouth, but the greater part of it was posterior or, when the
mouth lies at the anterior end of the larva, ventral to it. What the significance of this
condition may be, I am unable to suggest, but it is interesting to note that in Cucumaria
Ludwig ('91) found the ventral radius the most advanced in development. Any possible
similarity ends here however, for the development of the other radii was quite the reverse
in Cucumaria of what it is in Synapta.

About the time of the completion of the pentactula form, there appear in the ecto-
derm of various parts of the body peculiar invaginations (Fig. 41) which are finally

SYNAPTA VIVIPARA. 67

connected with the exterior only by a very narrow canal (Figs. 42 and 43, and Fig. 17,
Igo.) . These organs appear, from their- structure and the great variation which they show
in staining, to be of a glandular nature, and I am inclined to think they may be connected
with the absorption of nourishment from the fluid of the body-cavity of the mother, for
they never increase in size, are most abundant in the young with ten tentacles, and seem
to have entirely disappeared in the adult, and finally, nothing of the kind has been described
for any other holothurian. When fully grown they measure about fifty mikrons in diam-
eter and somewhat less in depth. They consist of very long, clear cells, with nuclei at
the extreme distal ends, surrounding a more or less spacious lumen which opens to the
exterior by a narrow canal of ordinary epithelial cells. Sometimes the clear cells stained
heavily, but often they did not stain at all.

Up to this time the mesenchyme cells have played no part in the development of the
larva. In Fig. 16, they are shown as they appear scattered almost uniformly through the
segmentation cavity. Shortly after this, however, they begin to gather around the lower
and outer edge of the water-vascular ring, and by the time the pentactula stage is reached
they have begun the formation of the calcareous ring. Contrary to Semon's ('88) views
on S. digitata, and in accordance with Ludwig's ('91) observations on Cucumaria, I have
found no evidence at all of any mesenchymatous musculature on the oesophagus. The
first products of the mesenchyme cells to appear are five small straight rods between the
primary tentacles but outside and somewhat below the hydrocoel ring. Soon after these,
five more appear below the bases of the tentacles, so that there are now ten rods, five
radial and five interradial, and they continue in this position so long as there are only five
tentacles. I saw no evidence at all of any such shifting of position of the first five rods
as Semon ('88) records for S. digitata, nor could I consider the position of the second
series as agreeing at all with his description. Very soon after the appearance of the ten
rods, they fork at the ends and begin to branch very irregularly. As will be seen from
Fig. 46 a-i, the divisions occur at all sorts of angles and not only differ decidedly from
Semon's figures of the same rods in S. digitata but show no sign at all of following his
law ('87) for the formation of calcareous plates in Echinoderms. The much-branched
ends of the rods come into very close contact but evidently never mingle, for the plates
into which they develop are at all times easily separable and the line of division between
them is practically straight. Very soon after the appearance of the first five calcareous
rods, the radial nerves grow backward over them and before the completion of the pentac-
tula, run to the posterior end of the animal.

We have now reached the complete pentactula form, a slightly older stage of which
appears in Fig. 18. The pentactula is about half a millimeter long and its characteristics

68 HUBERT LYMAN CLARK ON

nfay be briefly summed up as follows: — Water- vascular system consisting of a closed
hydrocoel ring or circumoral water-tube with five primary tentacles, between which are
five very much smaller but equally erect secondary outgrowths; a water-tube in the mid-
dorsal interradius connecting the circumoral ring with the exterior; and a Polian vessel
in the left dorsal interradius. Nervous system consisting of a circumoral ring ; five ten-
tacle nerves on the inner face of the primary tentacles, the ectoderm of which is consid-
erably thickened, especially on the outer side ; five radial nerves bending backward over
the secondary outgrowths of the water-ring and over the radial pieces of the calcareous
ring, and running to the posterior end of the body; and five pairs of otocysts, lying
external to the radial nerves, where they bend backward. Digestive system, consisting
of a short oesophagus with the mouth opening anteriorly in the center of the circle of
tentacles, a large stomach, a comparatively short intestine with a single loop in it, and
usually an anus formed secondarily near or at the aboral pole. Digestive system attached
to the wall throughout its whole course by a mesentery, formed by the union of the two
walls of the right and left coelomic pouches. Calcareous ring consisting of five radial
and five interradial pieces with much-branched ends. A few scattered glandular organs
of doubtful function in various parts of the ectoderm.

7. THE DEVELOPMENT OF THE ADULT SYNAPTA.

Since there is no cessation of growth nor any resting period on the assumption of
the pentactula form, it is impossible to draw any hard and fast lines which will always
serve to distinguish that stage. For many larvae, which appear to have only five tenta-
cles, show on careful examination the rudiments of new ones, and other larvae which
show no tentacles externally show the perfect pentactula form, when sectioned. As soon
as the five primary tentacles have pushed out so far as to entirely obliterate the original
opening of the atrium, the secondary outgrowths of the water-ring, which have hitherto
scarcely shown any indication of growth, begin to develop and push upward. As we have
already seen, the radial nerves lie directly over them, so that they cannot grow straight
up but push out to one side or the other of the nerve. The outgrowth which lies in the
mid-ventral radius, however, develops very slightly and does not normally push out on
either side of the nerve which overlies it. The outgrowths which lie in the right and
left ventral radii take the opposite course, and broadening out laterally, grow up on both
sides of the nerves which overlie them, to form accessory tentacles. The outgrowth of
the right dorsal radius pushes out on the dorsal side of its overlying nerve and forms an
accessory tentacle in the mid-dorsal interradius, while the outgrowth of the left dorsal

SYNAPTA VIVIPARA. 69

radius shows as yet little tendency to develop either way (Fig. 85) . Consequently we
now have a larval form with ten tentacles, two in each interradius. As the accessory
tentacles grow very rapidly they are soon equal in size to, and cannot be distinguished
from, the five primary tentacles. It is hard to decide positively which of the five accessory
tentacles develops first, for apparently they all begin to grow at about the same time.
In Chirodota rotifera, Ludwig ('81) found the first two accessory tentacles in the lateral
dorsal interradii, and he does not speak of finding any trace of additional tentacles in the
other interradii. I have not found any stage similar to that in S. vivipara, and I think
the five accessory tentacles appear at practically the same time. It is an important and
interesting fact, however, that the five accessory tentacles are formed in precisely the
same manner and from the same radii as the second series of five tentacles in Cucumaria,
(Ludwig, '91). It seems to me that this fact proves satisfactorily that the radial canals
in Synaptidae are homologous with those of the other holothurians or, more accurately,
the secondary outgrowths of the hydrocoel ring in the Synaptidae are homologous with
the five outgrowths of the hydrocoel ring in the true holothurians. In both cases, the
ten-tentacled young has one primary and one accessory tentacle in each interradius.
While this change is taking place in the number and arrangement of the tentacles, a cor-
responding change is going on in the calcareous ring. As the accessory tentacles push
out into the interradii, the calcareous rod which lies at the base of the primary tentacle
cornes to lie between it and the accessory tentacle. I could not see that this came about
by any actual movement of the rod itself, but was due simply to the increase of width
in each interradius. In the further growth of the calcareous ring, the interradial pieces
send up projections between the two tentacles (Fig. 46 h) and at the same time branch
and divide so rapidly and irregularly that they soon become plates, with straight sides
but pointed anteriorly and notched behind, made up of a very fine irregular network of
calcareous strands (Fig. 44). The radial plates develop in the same way but send up two
projections, one on each side of the radial nerve (Fig. 46 i). which finally fuse together
above it and thus form the perforated plates of the ring (Fig. 45). About the same
time, the mesenchyme cells lying between the ectoderm and the wall of the coelom
begin to gather in groups close to the ectoderm and there give rise to anchors
and anchor-plates so characteristic of the adult Synapta. The development of the cal-
careous bodies from a straight rod takes place as described by Semon ('87) for Synapta
inhaerans. While these deposits appear in the body-wall as far anteriorly as the base of
the tentacles, in the walls of the latter, lying parallel to the long axis, there appear
numerous rather long, more or less, knobbed rods (Fig. 48) similiar to those described

70 HUBERT LYMAN CLARK ON

by Seruon ('87) from the tentacles of various synaptids. These become very abundant
in the older ten-tentacled larvae.

The digestive tract meantime has increased in length, and the stomach is more
clearly marked off from the intestine and oesophagus. The nervous system has not
undergone any marked changes, but each of the accessory tentacles is supplied with a
nerve on its inner side, as in the case of the primary tentaclesi In various parts of the
skin, especially anteriorly, clusters of ectoderm cells are to be found which later form
the so-called sense-papillae (" Tastpapillen " of Hamann, '83). I have been unable to
find any connection between these spots and the nerves until a very much later period,
and I cannot decide how or when this connection is made. The glandular organs
previously described are very abundant at this stage, especially posteriorly. Soon after
the pentactula form is complete, the walls of the coelom and of the hydrocoel begin the
formation of muscle fibers, always on the side turned from the cavity which they enclose.
The first to appear are the longitudinal muscles of the tentacles and radii. The former
appear as fibers on the outside of the tentacle canals and they soon form quite a thick
layer. The radial muscles arise within a fold of the coelomic wall, which appears along
the inner side of the radial nerves. This fold begins anteriorly near the calcareous ring
and runs backwards with the nerve, enclosing a considerable space between its walls. In
this space the muscle fibers arise from the endodermal cells of the coelom. Later on,
the circular muscles of the body appear, arising from the outer side of the coelomic wall
also. They cross the space in which the longitudinal muscles lie, forming a layer
between the latter and the nerve. At the same time, the longitudinal and circular
muscles of the digestive tract, and the muscles of the water-ring, begin to appear, so that
by the time the ten-tentacled stage is reached, the musculature is practically that of the
adult. With the appearance of the longitudinal muscles of the tentacles, comes the
development of the valves at the openings of the tentacular canals, close to the upper
edge of the calcareous ring.

Before the accessory tentacles have begun to appear, there arises on the right hand
side of the mesentery which fastens the intestine to the wall of the left interradius a
longitudinal fold or evagination of the epithelium close to the intestine (Fig. 58). This
fold follows the course of the intestine, with the mesentery on the dorsal side, and grows
forward along the stomach and backward toward the anus. Later a similar fold appears
in the coelomic epithelium on the opposite (ventral) side of the digestive tract and the
two folds soon become connected around the intestine and stomach by numerous
ill-defined lacunae between the coelomic wall and that of the digestive organ itself.
These vessels are the first stages of the blood vascular system and into them cells from

SYNAPTA VIVIPARA. 71

the coelomic epithelium pass to form the blood corpuscles. Theoretically the vessels
ought to be lined with connective tissue of mesenchyme cells but these are so few in the
early stages of the larva, that the connective tissue between the laminae of the mesen-
teries or even around the digestive tract is very hard to demonstrate. The main blood
vessels appear to be purely entodermal in origin and their walls seem to be made up
solely of the coelomic wall. At any rate, the dorsal vessel does not arise in S. vivipara
iu the way given by Semon ('88) for its origin in S. digitata, by a simple split in the
mesenchyme where the two coelomic folds unite above the intestine to form the
mesentery.

Very soon after the pentactula stage is reached the first rudiment of the genital
system appears. It arises on the inner side of the right-hand lamina of the dorsal
mesentery between the stone-canal and the oesophagus. The first appearance is simply
the increased size of the cells at this point, resulting in a thickening of the wall (Fig.
32), but the cells soon multiply rapidly and form a more or less spherical mass within
the mesentery (Fig. 33). As this mass increases in size, cavities appear within it
(Fig. 34) and these increase in size and begin to unite together until they form one
central lumen for the gland (Fig. 35). Meanwhile the right lamina of the mesentery
forms an outer epithelium which soon becomes quite distinct from the central mass of
cells. Between this epithelium and the remainder of the gland a few mesenchyme cells
form an extremely scanty connective-tissue layer. In the full-grown ten-tentaculed
larva, the genital gland is plainly seen (Fig. 19), lying entirely on the right-hand side
of the mesentery, near the stone-canal.

The ten-tentacled larva (Fig. 19) is a more clearly defined stage in the development
of Synapta vivipara than is the pentacula; that is, it seems to last longer, and the rela-
tive condition of development of the various organs is more constant. The changes
which occur subsequently in the assumption of the adult form must now be considered.
The most obvious of these is the increase in the number of tentacles which arises from
the addition of another tentacle to the right and left dorsal interradii (Fig. 86). The
extra tentacle of the left side arises from the left dorsal " secondary outgrowth " of the
hydrocoel ring, which has hitherto remained in its original position beneath the left
dorsal nerve but now pushes out on its lower or ventral side and forms the eleventh ten-
tacle. At the same time, the extra tentacle of the right side arises from the lower or
ventral side of the right dorsal " secondary outgrowth " which pushes out on that side
of the right dorsal nerve and forms the twelfth tentacle. Not infrequently individuals
are found with thirteen tentacles, and in these the extra tentacle is usually in one of the
ventral interradii. In such cases it is probable that the mid-ventral " secondary out-

72 HUBERT LYMAN CLARK ON

growth" has grown up on one side or the other of the mid-ventral nerve, although in
normal specimens it does not develop at all. Occasionally the extra tentacle is in the mid-
dorsal interradius, and in such cases it is probable that the left dorsal " secondary
outgrowth," which normally develops only a single tentacle, has given rise to a second
on the dorsal side of the left dorsal nerve. Corresponding to these changes in the num-
ber of tentacles additional plates appear in the calcareous ring, but these plates do not
arise by interpolation of new rods. On the contrary, the plate of the same radius with
the new tentacle increases its length and sends upward a new projection for the support
of the tentacle (Fig. 47), and this subsequently forms the center of the new accessory
plate. In specimens of the calcareous rings cleaned with caustic soda, it seemed to me
that the calcareous plates of the right and left dorsal radii were less easily separated from
those plates on their ventral side than from those on the dorsal, so I am inclined to think
that for a time, if not throughout life, these two plates remain in closer union than any
of the others. In specimens with thirteen tentacles, there is an additional plate in the
calcareous ring corresponding to the extra tentacle. About this same time the miliary
granules (Fig. 50) begin to appear in various parts of the body-wall and in the tentacles.
Like all other calcareous concretions of Synapta, they are formed by mesenchyme cells.
They usually appear in clusters of several hundred, which continue to increase in number
afterwards until it may reach thousands. By the time twelve tentacles have appeared,
the genital gland begins to push over on the left hand side, but it is not until long after
the adult form is assumed in all other respects that the left branch of the gland equals the
right in size. As soon as the left branch is well started, the germinal epithelium begins
to push upward in the mesentery beside the stone-canal, and forms the genital duct, but
does not reach the outer body-wall for some time. This account of the development of
the genital duct accords with Mortensen's ('94) observations on Cucumaria glacialis,
although his account of the origin of the genital gland itself differs considerably from my
observations. Important changes are going on meanwhile in the nervous system. From
the inner side of the circumoral ring nerves or bands of nervous tissue arise and pass
inward to the oesophagus. These will be referred to more fully in describing the nervous
system of the adult. Even in the ten-tentacled stage, before the remaining two tentacles
have made a fair start, there arises on each side of the tentacle nerve at its base a knob-
like outgrowth which becomes covered over with a peculiar mesenchyme layer, and these
form the " eyes," which also appear at the base of the eleventh and twelfth tentacles,
after they receive their nerves from the circumoral ring. With the appearance of these
eyes, the first trace of pigment appears in the mesenchyme not only about them but in
various parts of the body, especially around the calcareous ring. This pigment on its

SYNAPTA VIVIPARA. 73

first appearance is bright green, even about the eyes, so that at this stage the eyes of
the young Synapta are very conspicuous as large green spots at the base of the tentacles.
Very soon, however, a dark reddish brown pigment appears, but this is probably an older
stage of the green, and not a different pigment; for the pigment around the eyes soon
loses its green color and turns brown, and there is no reason to assume that the pigment
in other parts of the body is any different from that around the eyes. All of the pigment
arises in the connective tissue, and is apparently a product of the mesenchyrne cells. It
is especially abundant at the anterior end of the body, and above all other places in and
around the calcareous ring.

Before the number of tentacles is complete the ciliated funnels so characteristic of
Synapta begin to appear on the mesentery, near the body-wall. These funnels arise
from a large cell or group of cells in the endodennal epithelium of the mesentery (Figs.
61 and 62a). The multiplication of these cells soon forms a hemispherical outgrowth
(Fig. G2d) which increases in size and becomes more and more spherical in shape, until
it is finally attached to the mesentery by only a narrow stalk (Fig. 62e). It then begins
to flatten on one side, and the cells of its outer layers become smaller and stain more
heavily than those nearer the stalk (Fig. 62f). The flattened surface at last becomes
concave and the funnel shape begins to be assumed. At the same time, the stalk
becomes elongated and draws up within it some of the connective-tissue layer of the
mesentery, which becomes the supporting layer of the funnel. Even during the ten-
tentacled stage the digits of the tentacles begin to be formed, but they do not become
prominent until the twelve tentacles are all developed. The digits arise as evaginations
of the water-canal of the tentacle (Fig. 56) which very soon become shut off from the
main canal and in the adult have no connection with it (Fig. 55). The earliest ones to
appear are near the middle of the tentacle, and the later ones appear both proximally
and distally to them. The digits form longitudinal muscles on the outer side of the
central cavity in the same way as the tentacles themselves. They are also supplied with
nerves from the main tentacle nerve. The peculiar glandular organs of the larva are no
longer forming but seem rather to be disappearing, and the longitudinal rods of the
tentacles reach their maximum number at this time. The circumoral sinus, which was
entirely cut off from the rest of the coelom in the pentactula stage, has increased greatly
in size (Fig. 89) but is now in open communication with the body-cavity, though strands
of connective tissue traverse it, uniting the oesophagus to the water-ring and, higher up,
to the coelomic wall. With the greatly increased size of the young Synapta, comes a
considerable change in the relative position of the organs in the body-cavity. The body
has grown much posteriorly, drawing out with it that part of the intestine which lies in

74 HUBERT LYMAN CLARK ON

the right ventral interradius. The nerve ring is drawn upward with the growth of the
tentacles, so that it comes to lie very near the ectoderm at their base. The increased
length of the tentacular canals has pushed the water-ring downward so that it lies some
distance below the calcareous ring (Figs. 90 .and 91), but it is still in open communica-
tion with the exterior by means of the water-canal (Fig. 66). Mesenchyme cells have
formed a few calcareous rods about the latter (Fig. 49), especially near the point where
it passes into the body-wall. Just within the body-cavity from this point, openings have
appeared on it which place its interior in direct communication with the body-cavity, so
that the water-vascular system combines the primitive external opening of the pentactula
with the internal madreporitic openings of the other holothurians. The mesenchyme
cells around the calcareous ring have formed on its posterior edge a connective-tissue
ring, which later becomes so prominent in the adult as the cartilaginous ring.

In concluding this account of the embryology it may be well to summarize briefly
the derivation of the different organs from the germ layers of the gastrula.

Ectoderm. From the gastrula-ectoderm arise the ectoderm of the adult, the sensory
epithelium of the tentacles, the entire nervous system including all the sense organs,
the larval glandular organs, and a small part of the oesophagus. Possibly the
extreme posterior part of the rectum is also ectodermal.

Endoderm. From the gastrula-endoderm arise first of all the scattered mesenchyme cells
which make up the mesoderm. Soon afterwards the hydroenterocoel is divided off.
The remainder of the archenteron forms simply the lining of the digestive tract,
including most of the oesophagus. From the hydroenterocoel, the coelomic pouches
are constricted off, leaving behind the hydrocoel, from which the entire water-
vascular system, and also the cavities of the digits, arise. The longitudinal muscles
of the tentacles and digits come from the epithelium of the hydrocoel. The coelomic
pouches form the peritoneal lining of the body-cavity and the epithelial covering for
the various organs contained in it. All the muscles of the body-wall, gut, genital
glands, water-ring, and Polian vessels are also derivatives of the endoderm. The
genital organ, including the genital duct, and the ciliated funnels are likewise
derived from the wall of the coelom. The haemal system is also covered by the
epithelium of the coelom and apparently arises as evaginations of the same, while
the blood-corpuscles certainly come from that layer.

Mesoderm. From the mesenchyme cells, arising from the archenteron of the gastrula,
come all the connective tissue of the body, the pigment, the covering of the eyes,
all the calcareous concretions (including the calcareous ring), and the cartilaginous

SYNAPTA VIVIPARA. 75

ring. No trace of mesenchymatous musculature was found anywhere, and the part
which the mesoderm takes in the formation of the haemal system is certainly incon-
siderable.

8. THE ANATOMY OF THE ADULT.

Although the anatomy of the European Synaptas is so well known, thanks to the
investigations of Baur ('64), Semon ('87), Hamann ('83 and '89), Cue"not ('91), and
others, there are so many points in which. Synapta vivipara differs from the forms
hitherto examined, it seems desirable to add a few words concerning these and other
points. Except in the case of sense-organs, no attempt has been made to go into the
histology, but my attention has been confined to the more general features of the minute
anatomy. In the structure of the body-wall and the muscular system, there are no
important features to mention, aside from the shape of the longitudinal radial muscle
bands. Each of these bands is forked at its anterior extremity, and the two branches
are attached to the radial calcareous plate, one on each side of the radial nerve. These
branches soon unite as they pass backward, and form a single narrow band, which
extends far out into the body-cavity. But still further back, it decreases in depth and
increases correspondingly in width, and the epithelium which covers it tends to fuse at
the outer edges with the epithelium of the body-cavity, so that at numerous points in its
course the muscle has acquired secondary attachments to the body-wall. During the
greater part of its course, it is a nearly flat band, but as it approaches the extreme
posterior region of the body, it tends to become cylindrical, and where it ends near the
anus the cross-section is circular. These changes in shape will be made clear from Figs.
94-100. The structure of the genital glands has already been given in detail, and the
openings in the wall of the rectum have also been sufficiently described. The blood-
vascular or haemal system is very simple, consisting of a dorsal and ventral vessel on the
intestine and stomach with connecting lacunae in their walls. Posteriorly, both vessels
end about half way down that section of the intestine which lies in the right ventral
interradius (Fig. 92) . Anteriorly, the ventral vessel ends a little in front of the stomach,
on the oesophagus. The dorsal vessel runs forward to the water-ring and forms on its
inner side a circumoesophageal ring, from which branches pass on to each tentacular
vessel. The dorsal blood-vessel also seems to open out in the mesentery to form broad
lacunae about the genital gland, such as Cuenot ('91) found in European Synaptas, but I
never found coagulated blood there as in the dorsal vessel, and I do not feel sure that

76 HUBERT LYMAN CLARK ON

such lacunae actually exist. The ventral vessel of the stomach does not lie appressed to
its wall, but entirely free from it and connected with it by several small branches. It is
also connected by a large transverse vessel with the ventral vessel of the intestine (Fig.
92), and the two sections of the latter are also connected by a similar vessel. These
transverse vessels do not appear until the animal is several centimeters long, when they
arise by outgrowths of the coelomic epithelium of stomach and intestine which, lying
close together as they do in the loops of the digestive tract, touch and fuse (Figs. 59 and
60) and with the increased growth of the intestine are finally drawn out to slender con-
necting vessels. Like the vessels of the young Synapta, these are supposed to be lined
with connective tissue, but I have been unable to detect it in their walls.

The ciliated funnels of Synapta vivipara differ considerably in appearance from those
of S. digitata or S. inhaerans, though they do not differ essentially in structure. Only
one sort seems to be present and these are quite small but extremely numerous on all
three of the mesenteries. They measure from 40/x to 75/t in length, and from 30/x to
4(V in breadth and depth, which is only about half the size of those of /S. diyitata. They
are broad funnel- or cornucopia-shaped in outline and usually have a short stalk.
Their general structure will be easily understood from Figs. 63-65. The water-vascular
system consists of a circumoesophageal ring from which canals arise and pass to the ten-
tacles, into which their entrance is guarded by well-developed valves. Each tentacle
rests on the calcareous ring in such a way that the outer half of the base is on the out-
side of the calcareous plate, forming a sort of rudimentary ampulla (Fig. 90). There is
not in the adult, any more than in any of the larval stages, the slightest trace of radia.
water-canals. Dependent from the ring-canal there is always present in the left dorsal
interradius a slender Polian vessel five or six millimeters long, and in nearly all adults
additional Polian vessels, sometimes as many as six, are present in the ventral interradii.
The stone-canal leaves the water-ring on the left-hand side of the mid-dorsal interradius
and does not lie in the dorsal mesentery but clearly separate from, and to the left of it.
It soon passes into it, however, on its outward course and runs to the body-wall close
beside the genital duct (Figs. 66 and 67). It enters the body-wall on the right of the
mesentery and bends upward more or less abruptly, opening finally to the exterior close
behind the circle of tentacles (Figs. 66-70). In exceptional cases there are two open-
ings (Figs. 71-73) or rarely the reverse happens and the canal closes before the exterior
is reached. Besides this external opening, the stone-canal also opens into the body-cavity
through a well-developed madrepore (Figs. 66 and 74). Throughout its course the canal
is heavily ciliated, and especially so around these madreporitic openings, the whole
arrangement being admirably adapted for keeping the body-cavity fluid well aerated.

SYNAPTA VIVIPARA. 77

The nervous system consists as in all Synaptidae of the central circumoral ring with the
five radial branches and the smaller branches to each of the tentacles, but there are some
additional nerves and certain of the sense-organs which have not been figured hitherto.
Each of the radial nerves is divided longitudinally into an outer and an inner band as in
other Synaptas, but, unlike them, there are no canals or vessels of any kind accompanying
the nerves. There are, therefore, in the radii of S. vivipara, no spaces or lacunae in
connection with either the blood, water, or nervous systems, but they are marked simply
by the longitudinal muscles and nerves (Fig. 99). Each tentacle-nerve sends off branches
to the digits (Fig. 55), so that almost the whole surface of the tentacle becomes sensory.
On the base of the tentacles and in various parts of the ectoderm all over the body, there
are numerous sense-buds or " taste-papillae," (Fig. 84), such as were first described by
Hamann ('83). The structure of these organs has been well described by him and still
more recently by Cuenot ('91). My observations support the opinion of the latter, that
under each one of these sense papillae there lies a small ganglion. From the lower side
of the circumoral ring, there arises between every two tentacles a broad band-like nerve
(Figs. 75 and 76) which runs inwards towards the mouth, innervating the ectoderm of the
oral disc as well as the muscles of the oesophagus. Hamann ('83) describes a single
nerve to the oesophagus, and Semon ('88) speaks of it in S. digitata, but so far as I can
learn no other nerves from the inner side of the ring have been described in holothurians.
At the base of each tentacle, there are easily seen a pair of reddish brown spots, the so-
called eyes (Figs. 77 and 78). Similar spots are mentioned in various Synaptas by
Miiller ('50), Baur ('64), and Semper ('68), but Semon ('87) and Hamann ('84) seem
to doubt their visual function. There can be little doubt, however, that in Synapta
vivipara these eyes are actually of service as light-detecting organs. In position and
general structure they resemble those described by Ludwig and Barthels ('91) for Synapta
oittata. They consist of a distinct, rather horny mesodermal layer, of a light brown
color, containing scattered nuclei, overlying the swollen end of a large nerve which arises
on each side from the base of the tentacle-nerve (Fig. 77). The ends of these nerves
are made up of" large nerve-cells with large nuclei, which are somewhat swollen and
apparently vacuolated at their outer extremities. They are polygonal in outline, when
seen in cross-section (Fig. 79), and the inner ends taper off into fibers which run out into
the nerve (Fig. 80) . The mesodermal covering, which also has the appearance of being
vacuolated, is clearly a continuation of the thin mesoderm layer which surrounds all the
nerves. The eyes are about 60^ in diameter, the mesodermal covering being six or eight
mikrons thick. That this covering may be affected by light is probable, for its color is
due to the pigment it contains. The other noteworthy sense-organs are the otocysts

78 HUBERT LYMAN CLARK ON

(Figs. 81-83), already mentioned as lying external to the radial nerves at the point where
they bend backwards over the calcareous ring. They are much smaller than those figured
by Cuenot ('91) for S. inhaerans and differ from them in having only a single large
vesiculated cell enclosed within them, instead of a number of small ones. The otocysts
of S. vivipara measure only about 60-70/1 in diameter, while the contained cell is almost a
quarter as large. In no case have I found more than one cell enclosed in an otocyst of
this species. Hamann ('84) suspected that they were larval organs having no function
in the adult, but Semon ('87) has already proved that idea erroneous, as Hamann ('89,
p. 308) has since admitted. If any further evidence were needed, it could be found in
the increase of size of the organs during the development of the animal (Figs. 81-83) as
well as in the very obvious connection with the radial nerves. But I am inclined to the
view that these so-called otocysts do not function as hearing organs at all, but are of use
to indicate the animal's position. Semon ('87) was unable to find any cilia in them, and
his experiments on living Synaptas brought him to the conclusion that they were deaf to
sound waves. If the enclosed cell is vesiculated, as it appears to be, it must float in the
fluid with which the otocyst is filled and so presses on that part of the wall which is
uppermost. Any change in the position of the animal would cause a corresponding
change in the position of the enclosed cell and thus give rise to a changed sensation.

The fully grown Synapta mvipara (Fig. 20) measures from ten to fifteen centi-
meters in length and from four to nine millimeters in thickness ; the size depending
largely on the state of contraction of the muscles. In color they vary from a pale
reddish brown to a very dark greenish brown more or less spotted and blotched with
white. The ground color is due to the pigment in the connective tissue of the body-wall
and varies greatly with the amount of that pigment, but the white spots and blotches are
due to the aggregation of great numbers of the miliary granules, just beneath the
ectoderm. The pigment is not affected to any extent by pure alcohol, but corrosive
sublimate and all acids destroy or greatly modify it. Just posterior to the calcareous
ring and in connection with it there is a ring of cartilage-like connective tissue (Fig.
90). This structure was described and figured by Theel ('86), who also figured the
anchors and plates from the body-wall, in his account of S. picta. The anchors (Fig. 51)
lie close under the ectoderm and parallel with it, at right angles to the main axis of the
body. Each anchor is much curved or bowed inwards, while its arms or flukes are curved
outward so that the points of the arms are always projecting. The vertex is not toothed
but has five or six almost spherical knobs on its edge. The posterior end is broadened
out into several short, very finely-toothed branches. Beneath the anchors lie the rounded,
smooth-edged, somewhat arched plates, which normally possess seven large toothed holes

SYNAPTA VIVIPARA. 79

(Fig. 52) and two large and three small smooth holes. On the side of the plate next
to the anchor and near the posterior end is an arched bow, which bears a few teeth on
its anterior edge. Increased growth of the plate often increases the number of holes
(Fig. 53), but as a rule they are very constant. The calcareous rods which were so
abundant in the tentacles of the young larvae are so few that for a long time I was led
to consider them entirely wanting. The tentacles (Fig. 57) of the adults are long and
slender with from 12 to 18 pairs of digits, but the number varies greatly with the age
and size of the animal. The glandular organs which characterize the young ten-
tentacled stage seem to be entirely wanting now ; at any rate, I have never found any
trace of them in an adult.

A number of interesting monstrosities were found, chiefly among the older embryos.
One of these is shown in Fig. 93, but some of the others were much more complicated,
consisting of three, four, and, in one case, five young, which had grown together, or
budded from each other in various ways. Among adults, besides the rather common
addition of an extra tentacle, the only peculiar specimen found was one which -had only
three radial muscles and nerves and only eleven tentacles. There were three tentacles
in the mid-dorsal interradius (indicated by the mesentery), and four tentacles in each of
the lateral interradii.

9. CONCLUSIONS.

Probably no theory of echinoderm phylogeny has attracted more attention or seems
more plausible than that upon which Semon ('88) determined, as the result of his
studies on the development of the auricularia larva, of Synapta digitata. Although it
still finds supporters at the present time, the investigations of Ludwig ('91) on Cucu-
maria and of Ludwig and Barthels ('91) on the anatomy of the Synaptidae have shown
the incorrectness of Semon's views, while the observations of Bury ('89 and '95) have cast
doubt on his interpretation of some of the conditions in auricularia. It is not my inten-
tion to enter here into any discussion of the phylogeny of the echinoderms but only to
suggest some of the points in the phylogeny of the holothurians, upon which the history
of Synapta vivipara seems to throw some light, and to indicate some of the particulars in
which my studies have apparently offered support to Bury's ('95) theory of the ances-
tral form of the echinoderms.

There are three possible opinions concerning the relationship of the Synaptidae to
the other holothurians : first, Semon's ('88) view that Synapta represents a primitive
form, from which the other holothurians have been derived; second, Cuenot's ('91) view

gO HUBERT LYMAN CLARK ON

that the Synaptas represent a more primitive branch of the echinoderms than and dif-
ferent from the true holothurians ; third, Ludwig's (*89-'92) view that the Synaptidae
are degenerate, pedate holothurians. Semon bases his opinion on the high development
of the nervous system in Synapta, the absence of anything in their manner of life to
cause degeneration, and the fact that no organs appear in the development of the young
Synapta which are not present in the adult. His own observations on the nervous sys-
tem of Synaptas as well as Hamann's ('83) and Cuenot's ('91) show that there is some
tendency to diversity in the nervous system, especially as regards sense-organs, among
the Synaptidae, and it also shows a considerable degree of adaptability to changing
conditions. Moreover, I have found in S. vivipara that the sense-organs are highly
developed, and there are additional nerves to the oesophagus, indicating modifications
to suit the mode of life. It seems from these facts, that too much stress must not
be placed on the opinion that the nervous system of Synapta digitata is primitive.
As regards degeneracy and the absence of anything in the mode of life to cause it in
SynaptaSj it seems that Semon has expressed an erroneous opinion of the causes of
degeneration. He says that we know of only three reasons for its occurrence, parasitic,
fixed, or subterranean life, and, since none of these are characteristic of Synaptas, they
cannot be degenerate. Had he given the matter more careful consideration he would
have seen how untenable his position is. Certainly no one will deny that the loss of the
power of flight in certain birds, as the famous New Zealand ground parrot, is degenera-
tion, yet they are neither parasitic, fixed, nor subterranean. Any change in the mode of
life, due to a change in environment, may result in degeneracy. The word has corne to
have a bad significance so that we think of it as indicating that the animal is on the
down-hill road, whereas it strictly means that the animal has lost some organ or group of
organs which its ancestors possessed and so has become less complex than they. Such a
loss must necessarily, however, always be a gain to the species involved, otherwise it
could never have come about. Now, it is entirely conceivable that in certain conditions
of life on the ocean bottom, in shallow water near shore, the loss of numerous ambulacral
appendages and the concentration of the water-vascular system in the circumoral ten-
tacles would be a distinct advantage to the animal. Such has certainly been the case
in Caudina (Gerould, '96), for instance, and it is probably true of all the Molpadiidae,
although in these cases it may have been brought about by subterranean life. But
Semon has by no means proved his point that the Synaptas are not, as a rule, subterranean.
Whatever may be the condition at Naples, both on the New England coast and in Jamaica
Synapta inkaerans and its allied forms are found normally buried deep in the sand, while
the large Synaptas, like S. lappa, are found under stones, which is practically a sub-

SYNAPTA VIVIPARA. 81

terranean mode of life. The absence of anything, therefore, in their manner of life to
cause degeneration is by no means proven and will hardly stand as a good test for con-
sidering the Synaptas primitive. The statement that there is no structure developed in
the young Synapta which does not appear in the adult is completely refuted by the
careful observations of Ludwig and Barthels ('91) on the absence of radial water-canals
in the Synaptidae. Since all observers are agreed that radial canals are developed in the
embryo of S. diyitata, it is clear that we have here a most important structure lost in the
adult. For these reasons, it seems to me that Semon's view is no longer tenable.
Cuenot's ('91) view is based chiefly on the important differences in the embryology of the
Synaptidae and that of other holothurians, but it seems to me that he does not take suffi-
cient account of the important evidences of degeneration in the Synaptas.

Ludwig's ('89-'92) view appears to be the one best supported by the facts, and the
anatomy and embryology of S. vivipara offer no little confirmatory evidence. If we
compare its ten-tentacled stage with the hypothetical ancestor which Ludwig describes
for the Synaptidae, the resemblance is extraordinary, almost the only important differ-
ence being that the genital gland in S. vimpara is not equally developed on each side.
In fact, the ten-tentacled stage of Synapta mvipara represents an actual step in the
development of the Synaptidae from Ludwig's hypothetical ancestor. The Jamaican
species is beyond doubt a highly modified form, and, though in some respects more highly
organized than other Synaptas, in certain particulars, degeneration has gone further.
Differing from other holothurians in its manner of life and its mode of reproduction, it
has undergone various modifications to fit it for the changed conditions. Living in sea-
weed near the surface of the water, it has developed pigment in its skin to a marked
degree, and at the same time has acquired additional sense-organs in the eyes at the base
of the tentacles, and an increased innervation of the oral disc. In conformity to its
changed mode of reproduction, important changes have taken place in the structure of
the genital gland, openings have appeared in the walls of the rectum to connect the body-
cavity with the exterior, while the stone-canal has retained or has acquired secondarily
its original connection with the exterior. During the progress of these specializations,
the same causes have led to degeneration in other particulars. The changed mode of
reproduction has modified the genital duct, so that its lumen is no longer open to the
ova, and it no longer has an obvious opening to the exterior. The changed mode of
life has caused a greater concentration of the water- vascular system around the mouth
and a consequent further degeneration of the radial canals, so that they no longer appear
as such even in the embryology, but tentacles develop directly from the secondary out-
growths of the hydrocoel. And, furthermore, the mid-ventral outgrowth has degenerated

82 HUBERT LYMAN CLARK ON

a step further and normally never develops at all, but disappears altogether, which is
especially interesting as the mid-ventral radius is the first to develop its nerve and oto-
cysts, and so seems to be the leader in modifications. The manner of life has also caused
a modification of the tentacles in a way which we may consider as a degeneration from
other Synaptas. Semon ('87) describes calcareous rods in the tentacles of all the Medi-
terranean Synaptas, and these appear abundantly, as we have seen, in the young stages of
S. mvipara, but in the adult they seem to have almost entirely disappeared, the tentacles
and digits being very delicate and flexible and containing no calcareous deposits, except
some miliary granules. The changes in the larva due to the retention and development
of the ova in the body-cavity of the mother, such as closing of the blastopore and absence
of any true metamorphosis, must also be taken into consideration. For these reasons, we
must consider 8. mvipara as a highly specialized Synapta, but in its water-vascular system
it has degenerated a step further than S. digitata, although it is neither "parasitic, fixed,
nor subterranean " in its manner of life.

It is very clear from the examination of the literature on the subject that the study
of any one form or class of echinoderms is entirely insufficient to fit one to determine on
a theory of the phylogeny of the group. Notable examples of this may be seen in the
speculations of Semon ('88), Butschli ('92), and MacBride ('96). The facts added to
our knowledge of echinoderm embryology by all these writers are of real value, but their
hypotheses are for the most part of little importance. The same may be said of any
attempt to determine the entire course of echinoderm evolution by the study of palaeon-
tology alone, a notable example of which has recently appeared by no less an authority
than Haeckel ('96). The only author who has carried on original investigations on all
the classes of echinoderms and has formulated his views on the phylogeny of the group is
Bury ('80 and '95), and I cannot conclude this paper without calling attention to the
support which my observations give to him, on the questions involved in the development
of the Synaptidae. Regarding all the points on which he lays particular stress, I have
confirmed his work completely or in part. The adradial position of the water-tube, the
rudimentary left anterior enterocoel, and the growth of the left body-cavity around the
oesophagus are all very clearly marked in the development of Synapta mmpara. The
only point on which I could not entirely confirm his views was on the formation of the
mesentery of the stone-canal from the left coelom entirely, and on this point what evi-
dence I did obtain indicates the correctness of his position.

SYNAPTA VIVIPARA. 33

10. LITERATURE.

[Articles marked * contain references to Synapta vivipara.]
Agassiz, A.

'64. On the embryology of echinoderms. Mem. Amer. acad. arts and sci., Vol. 9, p. 1-30.
Baur, A.

'04. Beitrage zur naturgeschichte der Synapta digitata. Nova acta acad. Leop.-Carol., Bd. 31, 119 pp., 8 taf.
*Bronn, II. G.

'60. Die klassen und ordnungen der strahlenthiere (Actinozoa). Bronn's Klassen und ordnungen des thierreichs-

Leipzig und Heidelberg, 434 pp., 48 taf.
Btttschli, O.

'92. Versuch der ableitung des echinodenns aus einer bilateralen urform. Zeitschr. f. wiss. zool., Bd. 53, suppl., p. 136-

160, taf. 9.
Bury, H.

'89. Studies in the embryology of echinoderms. Quart, journ. micros, sci., Vol. 29, p. 409-449, pis. 37-39.
Bury, H.

'95. The metamorphosis of echinoderms. Quart, journ. micros, sci., Vol. 38, p. 45-135, pis. 3-9.
* Clark, H. L.

'96. The viviparous Synapta of the West Indies. Zool. anz., Bd. 19, p. 398-400.
Cut-not, L.

'91. Etudes morphologiques sur les echinodermes. Arch, de biol., Tom. 11, p. 313-680, pis. 24-31. See also Arch. zool.

exper., (2), Tom. 9, Notes et Rev., p. 8-16.
Danielssen, 1). C., and Koren, J.

'82. Holothurioidea. Norwegian North Atlantic "Expedition, 1876-78. 6. Zoology. Christiania, 94 pp., 13 pis., 1 map.
Gerould, J. H.

'96. The anatomy and histology of Caudina arenata Gould. Proc. Bost. soc. nat. hist., Vol. 27, p. 7-74, 8 pis. Also

Bull. mus. comp. zool., Vol. 29, p. 121-190, 8 pis.
Goette, A.

'80. Bemerkungen zur entwickelungsgeschichte der echinodermen. Zool. anz., Jahrg. 3, p. 324-326.
Haeckel, E.

'00. Die amphorideen und cystoideen. Beitrage zur morphologic und phylogenie der echinodermen. Festschrift zum

siebenzigsten geburtstage von Carl Gegenbaur, Bd. 1, p. 1-179, 4 taf.
Hamann, O.

'83. Beitrage zur histologie der echinodermen. 1. Die holothurien (Pedata) und das nervensystem der asteriden.

Zeitschr. f. wiss. zool., Bd. 39, p. 145-190, taf. 10-12.
Hamann, 0.

'84. Beitrage zur histologie der echinodermen. Heft 1, Die holothurien. Jena, 100 pp., 6 taf.
Hamann, O.

'89. Anatomie der ophiuren und crinoiden. Jena, zeitschr., Bd. 23, p. 233-388, taf. 12-23.
Hiirouard, E.

'89. Recherches sur les holothuries des cotes de France. Arch. zool. expfer., (2), Tom. 7, p. 535-704, pis. 25-32.
Jourdan, E.

'83. Recherches sur I'histologie des holothuries. Ann. mus. hist. nat. Marseille. Zool., Tom. 1, M6m. 6, 64 pp., pis. 1-6.
Kowalevsky, A.

'67. Beitrage zur entwickelungsgesehichte der holothurien. M6m. acad. sci. St. Pfetersbourg (7), Tom. 11, 8 pp., 1 pi.
* Lampert, K.

'85. Die seewalzen. Reisen in Archipel der Philippinen. Von Dr. C. Semper, Bd. 4, Heft 3, 312 pp., pi. 1. Wiesbaden.
*Ludwig, H.

'81. Ueber eine lebendiggeb&rende synaptide und zwei andere neue holothurienarten der brazilianischen ktiste. Arch.

de biol., Tom. 2, p. 41-58, pi. 3.
*Ludwig, H.

'86. Die von G. Chierchia auf der fahrt der Kgl. Ital. Corvette " Vettor Pisani" gesammelten holothurien. Zool. jahr-
biicher, Bd. 2, p. 1-36, taf. 1-2.

84 HUBERT LYMAN CLARK ON

* Ludwig, H.

'89-92. Echinodermen. Bronn's Klassen und ordnungen des thierreichs, Bd. 2, Abth. 3. Lieferungen 1-16 (Holo-

thurien).
Ludwig, H.

'91. Zur eutwickehmgsgeschichte der holothurien. Sitzungsber. k. Preuss. akad. wiss., No. 10, p. 179-192, No. 32, p. 603-
612. Transl. in Ann. and mag. nat. hist., (6), Vol. 8, p. 413-427.

* Ludwig, H., und Barthels, P.

'91. Zur anatomie der synaptiden. Zool. anz., Jahrg. 14, p. 117-119.
MacBride, E. W.

'96. The development of Asterina gibbosa. Quart, journ. micros, sci., Vol. 38, p. 339-411, pis. 18-29.
Metschnikoff, E.

'69. Studien liber die entwickelung der echinodermen und nemertinen. M£m. acad. sci. St. Petersbourg, (7), Tom. 14,

73 pp., 12 pis.
Mortensen, T.

"94. Zur anatomie und entwicklnng der Cucumaria glacialis (Ljungman). Zeitschr. f. wiss. zool., Bd. 57, p. 704-732,

taf. 31-32.
MUller, J.

'50. Anatomische studien iiber echinodermen. Miiller's archiv, Jahrg. 1850, p. 117-165.
Miiller, J.

'52. Ueber Synapta digitata und iiber die erzeugung von schnecken in holothurien. Berlin, 1852, 36 pp., 10 taf.
Orated, A. S.

'49. [Slaegt of Synapta gruppen.] Videns. medd. nat. for. Kjobenhavn, Aarene 1849 og 1860, p. vii. Translated in

Ludwig ('81), p. 48.
Quatrefages, A. de.

'42. Memoire sur la synapte de Duvernoy (S. duvernaea A de Q). Ann. sci. nat , Zool., (2), Tom. 17, p. 19-93, pi. 2-6.
Selenka, E.

'67. Beitrage zur anatomie und systematik der holothurien. Zeitschr. f. wiss. zool., Bd. 17, p. 291-372, taf. 17-20
Selenka, E.

'76. Zur entwickelung der holothurien (Holothuria tubulosa u. Cucumaria doliolum). Zeitschr. f. wiss. zool., Bd. 27,

p. 156-178.
gelenka, E.

'83. Die keirnblatter der echinodermen. Studien ueber entwickelungsgeschichte der thiere. Heft 2, p. 28-61, 6 taf.

Wiesbaden.
Senion, R.

'83. Das nervensystem der holothurien. Jena, zeitschr., Bd. 16, p. 1-23, taf. 1-2.
Semon, E.

'87. Beitrage zur naturgeschichte der synaptiden des Mittelmeeres. Mittheil. zool. station Neapel, Bd. 7, p 272-300;

p. 401-422, taf. 9, 10, u. 15.
Semon, R.

'88. Die entwickelung der Synapta digitata und die stammesgeschichte der echinodermen. Jena, zeitschr., Bd. 22, p. 175-

309, taf. 6-12.
Semon, R.

'89. Die homologien innerhalb des echinodermenstammes. Morph. jahrb., Bd. 15, p. 253-307.
Semper, C.

'68. Reisen im Archipel der Philippine!!. 2. Wissenschaftliche resultate. 1. Holothurien, 288 pp., 40 taf. Leipzig.
Theel, H.

'82. Report on the Holothuroidea. Pt. 1. " Challenger " Reports. Zoology, Vol. 14, Part 13, London.
Th6el, H.

'86. Report on the Holothuroidea. Pt. 2. " Challenger " Reports. Zoology, Vol. 14, Part 39. London.
Thomson, W.

'62. On the development of Synapta inhaerens, O. F. MUller (sp.). Quart, journ. micros, sci., Vol. 2, p. 131-146.

SYNAPTA VIVIPARA.

85

11. EXPLANATION OF PLATES.

[AH figures except 20 and 23 were drawn with the aid of a camera lucida.]
ABBREVIATIONS USED.

A. — atrium.

AE. — anterior euterocoel.
AN. — anus.
AO. — atrial opening.
AT. — accessory tentacle.
BC — body-cavity.
BL. — blastopore.
BV. — blood-vessel.
CF. — ciliated funnels.

CH. — cireumoesophageal ring of blood-system.
CM. — circular muscles of body-wall.
CR. — calcareous ring.
CT. — connective tissue.
Car.R. — cartilaginous ring.

Cir.S. — circular sinus formed from the anterior-prolon-
gation of the left coelom.
DV. — dorsal vein of blood-system.
E. — enterocoel.
Ect. — ectoderm.
Epi. — epithelium.
Ey. — eyes.
GD. — genital duct.
GG. — genital gland.
H. — hydrocoel.
I. — intestine.
LC. — left coeloin.

LGO. — larval glandular organ.

LM. — longitudinal muscles.

M. — mouth.

Mes. — mesenchyme.

MD. — madrepore.

MY. — mesentery.

NR. — circumoral nerve-ring.

O. — otocysts.

OE. — oesophagus.

OEN. — nerve-band to mouth and oesophagus.

PT. — primary tentacle.

PV. — Polian vesicle.

R. — rectum.

RC. — right coelom.

RN. — radial nerve.

SC. — stone-canal.

SO. — secondary outgrowth of the hydrocoel.

SP. — sense-papilla.

T. — tentacle.

TC. — canal of tentacle.

TN. — tentacle-nerve.

TV. — blood-vessel on inner side of tentacular canal.

V. — valves.

WP. — . water-pore.

WR. — water-ring.

86 HUBERT LYMAN CLARK ON

PLATE 11.

Fig. 1. Mature ovmn. 225x.

Fig. 2. Two-cell stage of segmenting egg. 225x.

Fig. 3. Four-cell stage. 225x.

Fig. 4. Eight-cell stage. 225x.

Fig. 5. Sixteen-cell stage. 225x.

Fig. 6. Thirty-two-cell stage, seen from the side. 225x.

Fig. 7. Thirty-two-cell stage, seen from one of the poles. 225x.

Fig. 8. Blastula, seen from the side. 225x.

Fig. 9. Gastrula, seen from the side. 225x.

Fig. 10. Older gastrula, seen from left-hand side, to show formation of the water-pore.

Fig. 11. Still older stage, seen from left-hand side, to show the drawing away of the archenteron from the water-
pore. 225x.

Fig. 12. Slightly older stage, seen from left side, to show the formation of the mouth. 225x.

Fig. 13. Older stage, seen from in front (ventrally), to show the formation of the enterocoel. 225x.

Fig. 14. Older stage, seen from in front (veutrally), to show the formation of the coelomic vesicles. 225x.

Fig. 15. Older stage, seen from in front (ventrally), to show the five primary outgrowths of the hydrocoel. 225x.

Fig. 16. Older stage, seen from left side, to show the hydrocoel, body-cavities, and atrium. 225x.

Fig. 17. Very young pentactula, from right side, to show the nerves and sense-organs. 225x.

Fig. 18. Older pentactula, seen from dorsal surface, to show the radial nerves, calcareous ring, and rudiment of accessory
tentacle. 130x.

Fig. 19. Ten-tentacled young, seen from right side, to show genital gland and arrangement of organs. Twelfth tentacle
just developing. 22x.

Fig. 20. Adult Synapta vivipara, seen from ventral surface. Nat. size.

PLATE 12.

Fig. 21. Vertical section of gastrula, to show the thickened ectoderm at apical pole. 225x.

Fig. 22. Transverse section of Fig. 13, at the line A. B., to show the thickened ventral ectoderm. 225x.

Fig. 23. Schematic outline of hydrocoel, to show the position of water-canal.

Fig. 24. Transverse sections of hydrocoels of three embryos, to show formation of anterior enterocoel. a, youngest
stage; b, somewhat older; c, oldest stage. 225x. In c, only a very small part of the hydrocoel is shown.

Fig. 25. Transverse section of larva, somewhat older than Fig. 16, to show closure of hydrocoel ring, without the forma-
tion of a Polian vesicle. 225x. The section is very oblique, and takes in a part of the floor of the atrium (A), and only a
portion of the hydrocoel.

Fig. 26. Transverse section of larva like Fig. 16, to show anterior prolongation of the left coeloin. 225x.

Fig. 27. Transverse section of same larva, somewhat higher up, to show the left prolongations of the left coelom. 225x.

Fig. 28. Longitudinal section of larva like Fig. 16, very badly preserved, to show the anterior prolongation of the left
coelom, ant. 1. c. 225x.

Fig. 29. Posterior end of an adult, to show the rupture of the body-wall, caused by birth of the young. 35x.

Fig. 30. Transverse section of the posterior end of an adult, to show one of the openings from the rectum into the body-
cavity. 65x.

Fig. 81. One of these openings more highly magnified. 500x.

Fig. 32. Part of a transverse section of a young pentactula, to show the origin of the genital gland. 500x.

Fig. 33. Similar section of an older larva, to show the increased development of the genital gland. 500x.

Fig. 34. Similar section of a still older larva, to show first appearance of lumen and covering epithelium of the genital
gland. I, beginnings of lumen; epi, outer epithelium of gland, formed secondarily from right side of mesentery. 600x.

Fig. 35. Similar section of a young twelve-tentacled larva, to show the genital gland well developed on the right-hand
side of mesentery and confined to that side. 500x.

Fig. 36. Part of a similar section of an adult, to show the formation of the lumen of the genital duct from the lumina of
the glands. 200x.

SYNAPTA VIVIPARA. 87

Fig. 37. Longitudinal section of a part of genital duct, to show its structure and position in the mesentery. I, lumen of
duct; g. e., germinal epithelium; m. e., epithelium of mesentery. 950x.

Fig. 38. Transverse section of dorsal body-wall of an adult, to show the termination of the genital duct, sp, sper-
matozoa. 225x.

Fig. 39. Longitudinal section of a small part of genital gland, to show the position of ovum, pressing against the epi-
thelium of the gland, epi, epithelium of gland ; ov, ovum. 500x.

Fig. 40. Transverse section of genital gland, to show its structure and the position of the ova (ov). 500x.

PLATE 13.

Fig. 41. Transverse section of the ectoderm of a pentactula, to show the invagination whiclrforms the larval glandular
organ. 500x.

Fig. 42. Transverse section of the same organ, fully grown, from the body-wall of a ten-tentacled larva. 500x.

Fig. 43. Longitudinal section of one of these organs, to show the peripheral position of the nuclei, and the lumen at the
center. 600x.

Fig. 44. Interradial plate from the calcareous ring of an adult. 35x.

Fig. 45. Radial plate from the same ring. 35x.

Fig. 46. The development of these calcareous plates, a, youngest stage ; b-g, successively older stages ; n, older stage
of an interradial plate ; i, older stage of a radial plate. 225x.

Fig. 47. Part of the calcareous ring of an old ten-tentacled larva, to show the growth of the radial plate (r.p.) of the
calcareous ring, in support of the developing eleventh tentacle, ip, interradial plates. 65x.

Fig. 48. Calcareous rods from the tentacles of a ten-tentacled larva. 225x.

Fig. 49. Calcareous rods from around madrepore. 225x.

Fig. 50. Miliary granules from the skin of an adult. 225x.

Fig. 61. Anchor from the skin of an adult. 120x.

Fig. 52. Normal anchor-plate from an adult. 120x.

Fig. 53. Abnormal anchor-plate from an adult. 120x.

Fig. 54. Longitudinal section of the tentacle of an old pentactula, to show the thickened ectoderm at the tip. 225x.

Fig. 55. Cross-section of a young tentacle, to show the entire separation of the canals of the digits from the central
canal of the tentacle. 225x.

Fig. 56. Longitudinal section of part of a young tentacle, to show the origin of the digits, as outgrowths of the tentacle-
canal. 226x.

Fig. 57. Tentacle of an adult. 22x.

Fig. 58. Transverse section of a mesentery and intestine of a pentactula, to show the origin of the haemal system from
the right lamina of the mesentery. 500x.

Fig. 69. Loop of intestine in a small adult, to show formation of the transverse vessels of the haemal system. 35x.

Fig. 60. The same from an older specimen. 65x.

Fig. 61. Cross-section of the dorsal mesentery of a larva one mm. long, to show the beginning of the ciliated funnels.
575x.

Fig. 62. The development of the ciliated funnels, a, the first large cells seen from above looking down on the surface
of mesentery ; 6, increased number of cells ; c, slightly older stage seen from the side ; d, older stage, surface view ; e, older
stage, seen partly from the side ; /, older stage seen from the side. 575x..

Fig. 63. Longitudinal section of a funnel, to show its structure. 950x.

Fig. 64. Ciliated funnel of an adult, seen from in front. 575x.

Fig. 65. Ciliated funnel of an adult, seen from above. 225x.

Fig. 66. Surface view of stone-canal of an adult, to show its course from water-ring to the exterior. 35x.

PLATE 14.

Fig. 67. Cross-section of stone-canal and genital duct of an adult before they enter the body-wall. 225x.

Fig. 68. Cross-section of stone-canal and genital duct of same adult in the body-wall. 225x.

Fig. 69. Cross-section of same stone-canal as it approaches the body-wall. 225x.

Fig. 70. Cross-section of the same, where it opens to the exterior. 225x.

88 HUBERT LYMAN CLARK ON SYNAPTA VIVIPARA.

Fig. 71. Cross-section of the terminus of a stone-canal with two openings, to show the uppermost opening. 500x.

Fig. 72. Cross-section from the same series, two sections lower down. 500x.

Fig. 73. Cross-section of the same series, two sections further down, to show the second opening. 500x.

Fig. 74. Cross-section of a stone-canal of an adult, taken through the madrepore to show the openings. 225x.

Fig. 75. Longitudinal section of nerve-ring of an adult (transverse section of the animal), to show the nerve-band to the
mouth and oesophagus in cross-section. 200x.

Fig. 76. Transverse section of nerve-ring (longitudinal section of animal), to show the same nerve in sagittal section.
200x.

Fig. 77. Longitudinal section of nerve-ring and transverse section of tentacle-nerve, to show the position of the eyes.
120 x.

Fig. 78. Cross-section of nerve-ring and one of the eyes, to show their relative position. 225x.

Fig. 79. Cross-section of the tip of one of the eyes, to show the vacuolated structure of the mesenchymatous covering as
well as of the polygonal neive-cells. 950x.

Fig. 80. Sagittal section of one of the eyes, to show the shape and structure of the nerve-cells. 950x.

Fig. 81. Cross-section of an otocyst and its single vesiculated cell from a pentactula. 500x.

Fig. 82. Cross-section of the otocysts and a part of the radial nerve from a twelve-tentacled young five mm. long.
500x.

Fig. 83. Cross-section of an otocyst of an adult, showing the single vesiculated cell-'within. 500x.

Fig. 84. Cross-section of a sense-papilla in the ectoderm of an adult, to show its structure and nerve connections, ga,

ganglion ; n, nerve running to radial nerve. 500x.

PLATE 15.

Fig. 85. Transverse section of an old pentactula, to show the formation of the five accessory tentacles and the oto-
cysts ; the secondary outgrowths of the left dorsal and mid-ventral radii have not developed at all as yet. The section is
slightly oblique, more posterior on the left than on the right. 225x.

Fig. 86. Transverse section of an old ten-tentacled stage, to show position of the eleventh and twelfth tentacles. The
section is somewhat oblique, more posterior on the right than on the left. 120x.

Fig. 87. Longitudinal section of the anterior end of a pentactula ; on the left through a radius ; on the right a little to
one side of a radius. /, indicates the place of fusion between the oesophagus' and roof of the atrium. 225x.

Fig. 88. Another section of the same series; on the right, through an interradius, just touching on the side of the
Polian vesicle ; on the left a little to one side of an interradius. /, indicates the fusion of the oesophagus with the roof of
the atrium. 225x.

Fig. 89. Longitudinal section of a ten-tentacled young, to show position of the various organs. 65x.

Fig. 90. Longitudinal section of an adult through one of the tentacles, to show the position of the various organs. 35x.

Fig. 91. Longitudinal section of an adult through a radius, to show position of the organs. 35x.

Fig. 92. Adult laid open in right dorsal radius, to show position of the organs and especially the blood-vessels on
digestive tract. Nat. size.

Fig. 93. Larval monstrosity, a double embryo. 225x. ,

Fig. 94. Cross-section of longitudinal muscle of adult at the point of juncture with the calcareous- ring. 65x.

Fig. 95. Cross-section of same muscle, lower down. 65x.

Fig. 96. Cross-section of same muscle, lower down. 65x.

Fig. 97. Cross-section of same muscle, lower down. 66x.

Fig. 98. Cross-section of longitudinal muscle of a smaller adult, somewhat anterior to the middle of the body. 200x.

Fig. 99. Cross-section of a longitudinal muscle in the middle of the body. 80x.

Fig. 100. Cross-section of a longitudinal muscle at extreme posterior end'of the body. 200x.

Printed, January, 1898.

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