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NATK >NAL ACADEMY OF SCIKNC'KS.

Volume VIII.

FOURTH MEMOIR.

OPHIURA BREVISPINA.

79

As my name appears upon the title-page of this memoir, it is proper for me to state that my
share in the work has been that of the instructor under whose direction the work has been done.
The discovery that this Ophiuran is of peculiar interest and that it is unusually favorable for the
study of the problems of the morphology of Echiuoderms, was made by Dr. Grave; and the
results which are here detailed are his work.

W. K. BKOOKS.
80

CONTENTS.

Page.

1.

Introduction

83

2.

Historical sketch

84

3.

Distribution and habits

84

4.

Physiological notes

85

5.

Early stages

87

6.

Sta're "A.'' Origin of the anterior enterocreles

87

7.

Stage "B." Origin of the liydrocirle

88

8.

Sta^e "C." Closing of the blastopore and formation of the mouth

89

9.

Sta"e "D." Eolation of the hydrocu'le completed

9G

10.

Stage "E." Second pair tentacles formed

92

11.

Stage "F." luvagiuatiun of the nervous system

94

12.

Stage " G." Degeneration in the larval organ begun

95

13-

Stage "H." Formation of subucural sinus

96

11.

Relation of the larva to adult

96

15.

Larva of Antedon rogacea and Ophiura brevispiua compared

98

16.

Literature cited

98

17.

1 Ascription of figures

99

81

OPHIURA BREVISPINA.

By W. K. BROOKS and OASWELL GRAVE.

INTRODUCTION.

During the summer of 1808 it was my privilege to occupy the table of the Johns Hopkins
University in the United States Fish Commission laboratory at Woods Hole, and while here I
rediscovered the peculiar Ophitiran larva, which was first found and figured by KROHN (7).

Finding the larva? he described in the open sea KROHN did not know to what species they
belonged ; but the larvre, the development of which is the subject of the greater part of this paper,
came from eggs laid m aquaria by Ophlura brevispina. It is not likely that the same species of
Ophitiran occurs both at Funchal, where KROHN did his work, and also at North Falmouth, where
my material was obtained, but it is very probable that species belonging to the genus Ophiura
have similar larval forms.

Among Echinoderms, where a direct development from the larva to adult occurs, that is,
without the usual highly specialized intermediate pelagic larva, we usually have to do with a
species which in some manner takes care of its brood; but in O. brevispina the larv;e are free
swimming, they being provided with a well developed locomotor apparatus, yet the usual Ophiurid
pluteus larva is as completely omitted as it is from the life history of the viviparous Amphii<rn
squamata.

From the fact that the usual pluteus skeleton is begun in the larva? of O. brcrixpinn one is led
to suspect, however, that at some period in its history the species possessed a larva more nearly
like a pluteus than at the present time. On the other hand, on account of the resemblances which
exist between the larvre of 0. brevispinn and Aiitedon rosacea (treated of in another place) we may
suppose a close phylogenetic relationship exists between them. If, as many zoologists believe,
the criuoids have retained more nearly than any other group the characters of the primitive
Ecliinoderm stock, then in the larva of 0. brevispina we may have one which has retained
unmodified its primitive characteristics.

In this paper, however, the facts only of development are taken up, and the question of the
bearing which this larva may have on any theoretical discussion concerning the interrelationships
of the Echiuoderms is suggested here in order that the reader may keep the subject before him
while studying the paper. The points of resemblance between the Ophiurau and Antedon larvaj
are enumerated in a chapter further on.

The method used in the preparation of the material for microscopical study, and which gave
good results, is as follows: The larva- were taken up into a pipet with as little water as possible,
and squirted into a small bottle containing a solution of sublimate-acetic (98 parts of a sat. sol.
HgCl2 being used to 2 parts of glacial acetic acid). After from two to five minutes the sublimate
solution was drawn off gently, leaving the larva? at the bottom where they had settled. Then 50
per cent alcohol was added, which in a few minutes (5) was drawn oft' and replaced by 70 per cent
alcohol, in which a little iodine had been dissolved. In a few hours (3-12) this was changed for
clear 75 per cent alcohol, in which the larva- remained until needed for laboratory study. After
staining lightly in acid carmine, so as to facilitate their orientation, the larvre were dehydrated in
the usual way and cleared in oil of cloves. From the clove oil they were oriented by a modifica-
tion of the PATTON method. After an impregnation with 55° paraffin, series of sections three

83

84 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

microns in thickness were made in three planes, transverse, longitudinal sagittal, and longi-
tudinal horizontal. The sections were stained on the slide with KLEINENBERG'S ba-matoxylon.
Other methods were tried, but none proved so satisfactory as the one just described. The
shrinkage in echinoderm tissue, which usually accompanies the unmodified paraffin method, was
not to be seen in the tissue of these larva?, due, no doubt, to its unusual thickness.

It has been thought best to make the following list of terms which are used synonymously in
the text of this paper in the description of the larva}. Those in the same line can, in most cases,
be interchangeably used.

Dorsal-aboral-above-over.
Ventral-oral-below-under.
Auterior-forward-before.
Posterior-back ward-behind.

In the drawings of the larva1, when the ventral side is up and the anterior end is nearest the
top of the page, then the reader's left is also left in the figure.

For convenience in description, the various stages taken to illustrate the life history of the
species have been designated by letters of the alphabet, this method seeming preferable to one in
which age is used as a distinguishing character, since the progress of development at any age
depends so intimately on the varying conditions of environment.

I take this opportunity to acknowledge my indebtedness to Dr. C. P. SIGERFOOS, at whose
suggestion I began the study of Ophiiirau development.

I was aided very materially while at the Fish Commission laboratory by Prof. H. C. BTJMPUS,
who placed at my disposal every facility for work at his command, and to him, also, I am greatly
indebted for many suggestions in methods of rearing larva1 at the seashore.

To Professor BROOKS, under whose direction my work has been done, are due my warmest
thanks for the interest with which he has followed me in my studies and for the many valuable
suggestions he has offered from time to time during the year.

HISTORICAL SKETCH.

The species of Ophiuran, Ophium bfrrixpinn, the life history of which is the subject of this
dissertation, was first discovered and described by Thomas Say in 1825 (12).

Since this time the species has been rediscovered and renamed as many as three times. It is
probably best known at present by one of its synonyms, Oplihini <>lir«c<'«, which was given to it
in 1865 by THEODORE LYMAN (8). In his earlier works LYMAN distinguished between <>. oUi'iin-n
and 0. brevispina, but in his ChaUeniji'r report on the 0)>hiitri<Itr and Astrophytidai (0) he places
the. two species together as one under its earlier name, which, although less descriptive of the
species than that given by Lyman, it is probably best to retain.

In 1852 ATERS described the species under the name Ophiodertun olirnwinn in Vol. IV of the
Proc. Bost. Soc. Nat. Hist.

LUTKEN also described it as Ophioderma scrpens in 1856.

DISTRIBUTION AND HABITS.

OpMura brrvispina is a very widely distributed species, it having been reported from points
along the Atlantic coast from Brazil to New England.
It has been taken from the following localities:

1. Bahia, Brazil. 6. Beaufort, North Carolina.

2. Port Antonio. Jamaica. 7. Old Point Comfort, Virginia.

3. St. Thomas, Bahamas. 8. Sag Harbor, New York.

4. Cape Florida, Florida. 9. Dartmouth, Massachusetts.

5. Tortugas. 10. New Bedford, Massachusetts.

11. North Falmouth, Massachusetts.

That part of North Falmouth Harbor which is inhabited by the species is very shallow, its
depth at low tide not exceeding 1 fathom.

The bottom is covered with a mat of living and dead grasses and alga>, and in this tangle
the ophiuraus live, together with a great variety of crustaceans, molusks, and worms.

MEMOIRS OF THE NATIONAL ACADEMY OK SCIENCES. 85

The usual color of the species is an olive green, with darker bunds on the, arms and sometimes
with a clouded disk.

Through the blending of their colors with the seaweed the ophiurans are greatly protected
from their enemies, and it is difficult, even when looking for them, to see them among the seaweed
so long as they do not move.

It is quite common to find a small Amphipod crustacean clinging to the arms of dredged
specimens, and from the structure of the crustacean it is probable that the two species live
together commensally. What benefit either animal can derive from the association it is difficult
to see.

One pair of the thoracic legs of the crustacean is so modified as to form a structure beauti-
fully adapted for clinging to the round ophiuran arms. The last segment but one of each of this
pair of legs is Y-shaped. At the end of one arm of the Y is attached a movable segment, the end
segment of the leg, which when shut down upon the end of the other arm of the Y incloses a
triangular space in which the ophiuran arm is held.

The body of the crustacean is colored and banded in such a manner as to simulate closely the
color and banding of the ophiuran arms.

When placed in aquaria with their host, the crustaceans cling to the ophiuran arms until the
water becomes depleted of oxygen, when they leave the arms and swim about the edge of the dish
apparently much alarmed.

In examining the stomachs of the ophiurans one finds bits of other animals, such as crustacean
appendages and the skeletons of young horseshoe crabs. From this it is probable that the crea-
tures are scavengers, since an active crustacean would hardly be captured by so slow and poorly
armed an animal as an ophiuran. None were ever observed to eat anything when kept in the
laboratory, and it is quite out of the question to observe them in their natural habitat, since they
are nocturnal animals remaining hidden during the day.

The ophiurans were first examined for sexual elements early in June, and at that time the
eggs were very large but. adhered closely together in the gonads. The sperm appeared to be fully
formed but were nonmotile.

From this time on until the middle of August the species was regularly watched and exam-
ined, and on July 16 the first ripe eggs and sperm were obtained. A great number of specimens
had that day been dredged and placed in aquaria dishes of fresh, filtered sea water. One week
later a great number of adults were again brought in and placed under the same conditions as
those which had spawned in the laboratory the week before, but this time very few eggs were
obtained, and all subsequent attempts to get the ophiurans to spawn were unsuccessful.

From this it would seem that the breeding season is extremely short.1

The time of day at which spawning occurred corresponds well with the time at which I have.
noted it to take place in Ophiophilus aculeata and Ophiocoma echiiitttti, that is, between s and 10
o'clock p.. m.

PHYSIOLOGICAL NOTES.

The locomotor movements of an ophiurid, upon a casual observation, seem to consist of an
uncoordinated writhing and twisting not calculated to bring the creature to food or a place of
safety except by chance; but a more careful study shows them to be the result of an orderly and
nicely coordinated mechanism.

The rapid strides which characterize the movements of a brittle star are in strong contrast

1 During the summer of 1899, after this paper had gone to press, my experience with the species was very dif-
ferent from the above. Specimens brought into the laboratory early in June threw eggs and sperm, but the eggs,
after passing through the early segmentation stages, ceased to develop. The eggs were probably immature, and
were spawned only because of the bad condition of the water in the aquaria, but spawning always occurred early
in the evening at the time when it would have, occurred under normal conditions. Why unripe eggs should develop
at all, or why eggs mature enough to begin their development should not be mature enough to complete it, is ail
interesting question.

This phenomenon was repeated every few days until July 26, when about one-fourth the number of eggs
spawned developed into normal larvn-. This is ten days Inter than the date when eggs became mature at Woods
lloll. From the fact that the water is much warmer at Beaufort than at Woods lloll one would expect to find the
spawning season earlier at the latter place.
10396 li

86

MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES.

with the slow creeping movements of a starfish or sea-urchin, the difference being due to the
employment of different locomotor mechanisms in the two cases; the starfish and sea-urchin
depending entirely upon their tube feet and spines while in the ophiurids, the arms themselves
are the efficient locomotor organs, they being used much as we use our arms in swimming.

The arm of au ophiurid consists of u large number of segments, each of which contains a
central calcarious ossicle. The calcarious ossicles of adjacent segments articulate witli each other
like the vertebne of the spinal column, and are joined together by two pairs of muscles in such
a manner that motion is possible in all directions. This mechanism is aided in producing the
locomotion of the creature not only by the arm spines, where they are present, but by the foot
tentacles. These latter organs, which are the homologues of the tube feet of other echinoderms,
have been previously regarded as having given up their locomotor function entirely, but I shall
show further on that this is not true in the genus Opliiiini.

The experiments I carried on last summer on the movements of ophiurans resulted in little
that is new, but on account of the confirmation my notes and photographs give to I'UEYERS' work
(11) on the same subject, it has been thought advisable to publish them.

In the usual method of progression one arm precedes, it taking no other part, apparently, than
to point out the way; the two arms adjacent to and behind the anterior arm make the stroke; the
remaining arms are dragged behind, acting as a rudder.

Fio. 1.

Fiii. 2.

Flu. 3.

Flu. 4.

FIG. 5.

No preference as to which arm should precede could be found in au adult ophiuran, each arm
being equally capable of going before, making the stroke, or following behind.

If greater speed is needed, for example, to get away from a strong stimulus, the arm which
precedes may also take part in the stroke, its contractions being made simultaneously with those
of the side arms. This added force, if produced repeatedly on one side, would soon change the
course of progression, but this difficulty is overcome by au alternation of the stroke of the preced-
ing arm, first on one side, then on the other (text fig. -).

In a third method of normal locomotion the arms are arranged as is seen in text tig. f>, in
which only one arm follows, acting as the rudder. This leaves two pairs of arms for the stroke,
but the anterior pair is usually most vigorous in its contractions.

Since no physiological differentiation into anterior, posterior, or lateral parts is to be found in
ophiurids, the creatures are under no necessity of turning the body when a change in the direction
of progression is to be made. The arm which finds itself pointing in the new direction to be
traveled takes the lead, although it may have been either lateral or posterior in position in the
previous movements.

As has been mentioned before, the foot tentacles aid in making the strokes of the arms efficient
in propelling the body. After a stroke has been made, while the arms are being drawn forward
and extended for a new stroke, the tentacles can be seen moving actively about, but as the arms
come to rest for the backward movement the tentacles are thrust down against the substratum
and cease to move. The tentacles thus n't themselves into the inequalities of the surface and
afford fixed points for the arms to pull against. The tentacles of the posterior arms act in the
same way, and are efficient in preventing the force of the stroke being lost in side motion.

In ophiurans with long arm spines these latter structures may perform the function just
described for the foot tentacles, but iu the genus Ophiura the arm spines are very minute and
closely applied to the sides of the arms.

MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 87

It is interesting to note the wonderful coordination of locomotor movements immediately
following tlie amputation of three of the arms. In this case if the. nerve ring has been uninjured
one of the remaining arms takes up the part of guiding and balancing, while the other strokes
first on one side then on the other (text flgs. 3 and 4).

When the central nerve ring is cut at any point the coordination in movement is impaired, and
when cut in h' ve places, between the arms, it is lost entirely.

When placed on its aboral surface an opliiuran quickly turns over. The method used is quite
definite; two adjacent arms straighten out so that together they form a straight line. On these
arms as an axis the body revolves, being pushed over by the three remaining arms, but mostly by
the median one of the three.

EARLY STAGES.

The mature eggs are opaque and vary in color from an olive greoi to an orange yellow.
Those of the same individual, however, are constant in their coloration. Until quite well
developed the larva' retain the color which was on the eggs at the time they were laid.

For echinoderms the eggs are very large, being 0.3 millimeter in diameter.

Soon after they are fertilized the eggs throw off two membranes, the first of which is much
thicker than the second.

When first laid and during their early development the eggs float, but when their cilia are
formed the larv;e are able to swim below the surface.

As I did not know that any special interest would be, found in the life history of the species,
I did not carefully observe the early stages while living, nor preserve material for future study,
and as I have stated elsewhere, all later attempts to get other material were unsuccessful.

This makes it necessary to begin this paper with the description of a late gastrnla in which
the first pair of enterocreles have already begun to form as lateral pouches from, the anterior free
end of the archeiiteron (figs. 1-3).

Larv;e in this stage of development will be designated as "A."

STAGE " A," 36 HOURS OLD.
(Figures 1, 2, ami 3.)

At the age of 36 hours the larvae swim actively, they being uniformly covered -with cilia

(fig. 1).

The shape of the larva? is an oval, the length being to the shorter diameter as 2 is to 1.

The animal or anterior pole is slightly more pointed than the posterior vegetative one. The
ventral surface is distinguished by the presence of the blastopore, which latter has been pushed
from its posterior position to a ventral one by the rapid growth of the ectoderm of the dorsal
surface of the larva.

An apical plate of taller cells is present at the anterior end, but I could not see that the cilia
at this point were any longer than those which cover the other parts of the larva (fig. 3, ap).

From the blastopore, through which its cavity opens to the exterior, a large archenteron
projects forward into the blastociele.

The remainder of the blastocaile, not taken up by the archenteron or its pouches, is filled with
a close network of mesenchyme cells. This meseuchyme tissue is shown in fig. 3, mes, which is a
longitudinal sagittal section of "A."

From the auterior free end of the archenteron a large pouch is in process of being cut oil'.
This pouch projects to the right and left as horn-like processes, which latter are to be considered
the rudiments of the right and left auterior euterocojles (fig. 2, aer and ael).

As to the method of gastrulation 1 can not at present speak from observation on larva? in
which it is just taking place, but from a study of the stage now under consideration some idea can
be gotten as to how it has proceeded. In figs. 1 and 3 we see a cellular plug (cp) protruding from
the blastopore and also extending far into the archenteric cavity. In some cases it extends even
into the enteroccele pouch. The contour of this cellular mass is ragged, which is also true of both
the outer and inner surfaces of the wall of the archenteron and the inner surface of the ectoderm.

These facts seem to indicate that gastrulation does not take place by invagination, as is usual
in eehiuoderuis, but that the larva before gastrulatiou is a solid, plaiiula-like affair, and later the

88 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

archenteron is formed by a splitting away of the central core. Ill the same way the plug of cells
is probably formed by the hollowing out of the solid archenterou.

Beside their ragged outline the walls of the larva have another peculiarity in their structure,
for, judging by the number and position of the nuclei, they are from two to three cells in thickness
(fig. 3).

Cell walls are not distinguishable in any stage of development.

STAGE " B," 42 HOURS OLD.
(Figures 4 and 5.)

According to BURY (2) the hydrocoele does not have the same origin in all the groups- of
echinoderms. lie found that it originates in the crinoids, sea-urchins, and starfishes from
the left anterior enteroctele, but in the ophiurids it grows out from the anterior end of the left
posterior enterocoile.

This observation, which BURY records with apparent hesitation, I can completely confirm, as
will be seen in the description and figures of UB."

Externally the appearance is the same as in "A," but the internal structures have undergone
a great change.

The anterior pouches, the cavities of which in "A" were connected both with each other and
with the cavity of the archenteron, are now separate and distinct. The connection between these
structures still continues, however, in their fused walls. The left pouch is a little larger than the
right and lies behind and to the left of the latter (fig. 4, ael).

Just below the anterior pouches there is to be found a third pouch, which is growing out from
the left side and anterior end of the archenteron (fig. 4, hy). It protrudes anteriorly and partially
covers the two anterior enterocujles. The cavity of this pouch, which is the rudiment of the
hydroco'le, is in wide communication with the archenteron.

From the wall forming the convex sides of the hydroco?le there are, even at this early stage
in its formation, five outgrowths which are the beginnings of the radial canals of the adult
ophiuran (fig. 4. 1, 2, 3, 4, and 5).

The whole hydrocujle is curving round to the right to encircle the o?sophagus, which latter is
making its first appearance in this stage as a shallow but definite pit in the central part of the
ventral ectodermal wall (fig. 4, oe).

To avoid confusion the hydroccele was spoken of above as arising from the archenteron, but,
as will be seen in the transverse section (tig. 5), taken in a plane posterior to the origin of the
hydroriL'le, a differentiation is taking place in the archenteron which enables us to distinguish in
it the rudiments of two structures, the posterior enterocceles and the stomach. By a longitudinal
circular furrow the archenteron is being cut horizontally into a large ventral pouch, the posterior
enterocceles (pe) and a smaller dorsal one, the stomach (s). This stomach rudiment bends around
the posterior end of the posterior euteroccele and opens to the exterior through the blastopore

(fig- 4).

It is from the left side and anterior end of the ventral pouch that the hydrociele grows out,
hence the confirmation of BURY'S statement that it arises from the left posterior enterocffile in
ophiurids.

In most I'chiuoderms the posterior euterocoales originate as paired structures, and if the
statements of BURY and McBRiDE are correct, that the left posterior eiiteroctele of the larva
forms the hypogastric body cavity of the adult, and the right posterior enterocit-le goes to form
the epigastric ccelom, then, according to this, the large ventral pouch, which I regard as the
fused right and left posterior enteroctples, really represents the left only, because it takes no part
in the formation of the epigastric body cavity of the adult ophiurid, but, with the left, does pass
directly into the hypogastric.

The origin of the epigastric enterocojle is discussed in the description of Stage " C," in which
its rudiment is first found over the stomach.

My reason for regarding the ventral pouch of " B " as the fused right and left posterior entero-
cieles, is that at the time of its origin it is symmetrically disposed on either side of the plane of
larval bilateral symmetry.

MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 89

GOTO (5), too, has shown that the hypogastric enteroco'le in the starfishes is not formed from
the left alone, but in it are to be found the left and the greater part of the right posterior entero-
cu-les.

The cellular plug of cells, which in "A'' tills the archenteric cavity, becomes divided by the
furrow which separates the archenterou into enteroecele and stomach and a part of it becomes
inclosed iu the cavities of each of these structures (fig. 5, cp).

STACK "C," 48 Horns OLD.
(Figures 6, 7, and 8.)

The external form of the larva, which in this series of embryos is six hours older than "B,"
has been changed by the appearance of two lateral thickenings of the, ectoderm a little posterior
to the median transverse plane (tig. 0).

The blastopore, which in "15" was open to the exterior, has closed, leaving no trace of its
former position.

The mouth and oesophagus, which existed iu "B" only as a shallow ectodermal pit, now have
the form of a deep, hollow tube (figs. 0 and 7, m and oe), which projects vertically inward until it
passes through the hydroco'le and beyond the posterior enterocoe.le, when it curves back under
the latter to fuse with the anterior wall of the stomach.

The stomach and posterior enteroccele are still iu open communication, as in "B,'1 but the
furrow in "C" has deepened, and the process by which the two structures are being separated is
almost complete (fig. 7).

Although the walls of the (esophagus and stomach are fused, their cavities are still separate.

This condition renders it easy to see just what part is played by the ectoderm in the formation
of the alimentary canal, the entire cesophageal cavity being surrounded by ectoderm.

In "B" the hydroccele communicates with the posterior enteroccele by a wide opening, and
at the same point in " C " the two structures are still in communication, but the connection has
been narrowed down to a small tube (fig. 7, he).

Beside this connection with the posterior euterocoele, a second tube has been formed, joining
the left anterior enterocoale with the hydroccele (fig. 7, st). This new tube, which is the rudiment
of the stone canal, enters the hydrociele at the same point with the tube connecting the latter
with the posterior enteroccele.

The left anterior enterocoele lies to the left of the oesophagus, and dorsal to the left half of
the hydrocnele (figs. 6 and 7, ael).

It is to be noted that, although we now have a larva possessing both hydrocu-le and stone
canal, there has been as yet no pore canal formed. This is a marked reversal in the sequence of
the formation of these structures from what might be expected from the order of their appearance
in other known echinoderms, the pore canal arising usually before the formation of the hydroccele,
while the-stone canal appears much later than either.

Returning to the hydroctele, we find it a horseshoe-shaped structure astride the oesophagus
(tigs. G and 7, hy). The bulging areas which are to form the radial canals of the adult are much
longer and more regular in size than in "B." The radial pouch, which lies to the right of the
oesophagus and at the end of the right horn of the horseshoe, will hereafter be spoken of as radial
canal 1, since it arises from that part of the hydroccele which was first to bud out from the
posterior euterocoele. The other radial canals, passing to the left over the oesophagus, will be
designated as 2, 3, 4, and 5. Radial canal 5 lies in this stage over the opening of the stone canal.

The rotation of the hydrocoele around the oesophagus from its original left position, which
was begun in "B," has continued to such an extent in "C" that half of it lies to the right of the
median sagittal plane of the larva and half to the left. Radial canal 3 lies in this plane and
points directly toward the anterior end of the larva (fig. 6).

With its rotation the hydroccele also moves bodily toward the posterior end of the larva,
carrying with it the (esophagus. The cesophagus, coming in contact with the anterior wall of the
united posterior enterocceles, causes the latter to be pushed iu at the point of contact. As the
process continues, those parts of the posterior enteiociples lying on either side of this in-pushing
area are forced to How forward around the oesophagus and under the hydroccele; thus we have

90 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

the posterior enterocu'les becoming horseshoe-shaped, the two horns of which lie under the horns
of the hydrocu'le (tigs, f! and 7, he and hy).

Lying dorsal to the stomach we find a small enteroco-le which was not present in "B," or if
present, not in this position. It is the rndimeut of the body cavity, which in the adult lies aboral
to the stomach and which has been recently appropriately termed the epigastric euterocu-le (figs.
fl and 7, ee).

As to the origin of this structure I have no direct observations to give, but certain facts have
led me to believe that it is formed from the right anterior enterowele. These facts may be summed
upas follows: In "15" no epigastric enterocu'le exists, but the two anterior en terocoeles (fig. 4,
aer and ael) he side by side anterior to the stomach and the posterior enterocu'les. In " C " (figs.
0 and 7, ee) an epigastric pouch, equal in size to the right anterior enterocn-le of "B" is to be
found, but by the side of the (esophagus only the left anterior enteroccele remains (figs. 6 and 7,
ael).

During the six hours which intervene between " B" and "C" it seems hardly possible that a
complete formation of the epigastric enterocu'le should have taken place or that there should
have been time for the complete degeneration and disappearance of the right anterior pouch;
sufficient time may have elapsed, however, for the migration of the right anterior enterocodle to a
position behind the stomach.

Against such an interpretation as the above there is the fact that in no other case has the
epigastric enterocu'le been observed to take its origin from the right anterior pouch. It has been
described as arising from the right posterior enterocu'le, however, as has been referred to before, in
all the groups by BURY, and his observations have been corroborated by both Me BRIDE and
GOTO in the starfishes.

STAGE "D," GO HOURS OLD.
(Figures 9-14.)

The changes which have taken place in "C" to produce "D" are very marked.

The cilia have disappeared, except in four transverse rings or bands, three of which extend
entirely around the body of the larva. The third ring, counting from the anterior end, is inter-
rupted by the aboral disk on the ventral surface.

This third ciliated ring first appears on the lateral bulges, which were described in "C," and
the fourth ring appears on a second pair of lateral bulges which originate behind the first pair
near the posterior end of the larva.

The shape of the larva is no longer oval, but the posterior end has widened laterally and
become somewhat dorso ventrally compressed (fig. 9). The anterior end has not changed in shape
and may be thought of as forming the handle of the now club-shaped larva.

The enlarged posterior end of the larva contains all its organs and is the part which will enter
directly into the formation of the adult ophiurid.

From its homology with the preoral lobe and larval organ ofAsterhm gibbona I have called the
anterior end of the larva the larval organ. It disappears with the metamorphosis into the adult
form.

The larval organ is also homologous with the stalk of the Antedon larva, although in the
ophinrid larva it never functions as an attachment organ. When swimming, the larval organ
precedes. It is filled with a network of meseuchyme cells (fig. 11, mes).

Internally the changes have been even greater than the external ones we have just considered,
for it is during this period of development that the rotation and readjustment of organs takes
place, which is present in all echinoderins at some stage of their development.

The hydroccele, which has begun its rotation about the (esophagus as an axis in "C," has
completed it in "D'' and reached its definite position.

That part of the hydrocu-le which in "Cr was situated on the left of the plane dividing the
larva into bilaterally symmetrical halves, now lies on the right side of the same plane and vice
versa. (Compare figs. G and 9.)

A revolution of 180° has taken place in the hydroccele since "C," to which if the 180° of
rotation be added, which took place up to the time of "C," we have a total rotation of ,'360 ^ in the

MEMOIRS OF TUB NATIONAL ACADEMY OF SCIENCES.

hydrocn'le. Kadial pouch 1, linally, after having passed around tin- (esophagus, conies to rest
at the point where it originated. Kadial pouch 5, it will be. noted, is carried only half as t'a
as radial pouch 1, or from its point of origin on the left to a point opposite on the right of th<
oesophagus. (Compare tigs, (i and 9, ( 1 ) and (5).)

This great amount of rotation seemed so peculiar that I hesitated for some time to believe it,
and was led to suppose instead that while the hydrocu-le moved to the right the other organs
lying above it rotated an equal amount to the left.

The early closure of the blastopore anil the central position of the mouth in the early stages
make such a view as the latter seem possible, and as it may suggest itself to those who study figs.
C and 7, 1 will give below the points which seem to me, directly or indirectly, to prove that the
hydrocu-le revolves under the enterocieles and stomach, rather than that the latter twist over
the hydrocu-le:

(a) The ectodermal bulges, nearer the posterior end in "C" (tig. C), are the same as those
nearer the posterior end of " 1>" (fig. 9), on which the third ciliated band is situated.

(b) If the latter view is the correct one then radial canal 3 points toward the same end of
the larva in both "C" and "D" (iigs. 0 and 9), but in "C" the end toward which it points is
anteriorly directed in swimming and in "D" it points away from the end which precedes. It is
hardly thinkable that in any stage in its development the anterior end of a larva should change
its physiological function and become the posterior end.

(c) By any other view than the one I have adopted the blastopore, or the point where it
existed before closing, would be anterior and the larval organ posterior in position. In all known
echinoderm larva?, however, the blastopore marks the posterior end, and in all cases where it
occurs the larval organ originates from the anterior end of the larva1.

(<?) It may be recalled, also, that in the readjustment of parts which takes place during the
metamorphosis of other echinoderm larva- the rotation is almost entirely confined to the hydrocu-le.

As the hydrocu-le passes around the (esophagus the tube connecting it with the left horn of
the liypogastric enterocu-le. becomes broken and the left anterior enterocu-le, together with the
tube connecting it with the left horn of the hydroc<ele, are carried anteriorly around the (esophagus
(fig. 9, ael and st). In "D," then, we find the stone canal on the right side of a line dividing the
larva into symmetrical halves, instead of to the left of the same line as it is in "C." (Compare
iigs. 0 and 7 with 9.) The anterior enterocoale comes to rest immediately in front of the stomach
and (esophagus.

From the point where the stone canal enters the anterior enterocu-le the pore canal grows
out, passes dorsally to the ectoderm, with which latter its walls fuse, and an opening the water
pore (figs. 9 and 11, pc.) breaks through. Thus in this stage the co-loin and hydrocu-le are first
connected with the exterior.

In "C'' the circular water-canal had not closed, but existed in the form of a horseshoe, the
concave side of which opened posteriorly, but as the rotation of the hydrociele takes place its
horns grow toward each other until they meet. A fusion of their walls then takes place at the
point of contact and a complete ring is thus formed. The part of the ring canal, the formation of
which has just been described, lies between radial canals 1 and ."• in tig. 9. The opening of the
stone canal into the water ring is situated in "C" at the base ol radial canal 5, but by means of
the rotation of the hydrocu-le about the (esophagus, together with the growth of the ends of the
horseshoe, this opening is carried away from its position at the base of radial canal 5 toward
radial canal 1. It always remains, however, nearer the former than the latter; in other words, it
comes to lie definitively in the right tulraiUtts between radial canals 5 and 1. (Compare figs. 6
and 9.)

The radial canals, which existed in "C" as simple pouches from the convex side of the hydro-
coele, have in " D'' each become three- lobed. Near the tip and from the sides of each canal a pair
of pouches has budded out, each of which is about equal in size to the end of the canal which lies
between and beyond them (fig. 9, et and tl). In these three structures we have the rudiments of
the end tentacle and the first pair of foot tentacles of the ophiurid arm.

When we were last considering the liypogastric enterocirle it was in the form of a crescent,
the horns of which were very short and its central part very wide. Into its concavity, which was

92 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

anteriorly directed, the oesophagus fitted. The horns of this enteroccele, beginning in "C"to
grow over the hydrocu-le, continue the process during the rotation of the latter, the horns of the
crescent growing at the expense of the thickness of its central part, and in "D" we have this
enteroc(jele lying directly over the hydrocoele in the form of a perfect horseshoe (tig. 9, he).

Between the ends of the horns of the hypogastric ctelom lies the anterior enteroccele. The
walls of these structures come together and fuse in such a way that they together form a hollow
circular ccelom surrounding the stomach and lying over the somewhat smaller water vascular ring

(fig. 9).

In the four interradii, marked by their positions between radial canals 1 and 2, 2 and 3, 3 and
4, and 4 and ;">, four pouches of the hypogastric enterocude grow downward, outside the water
vascular ring, forcing themselves between the radial canals; a fifth pouch, similar to those just
described, is formed from the left anterior enterocci-le in the remaining interradius between radial
canals 5 and 1 (figs. 9, 12, 13, and 14, hip 1-2, 2-3, 3-4, 4-5, and ipax 5-1). These five pouches
are the rudiments of the ouler periha-mal ring, which will be more fully considered in the
succeeding stages.

The stomach, after being entirely cut off from the hypogastric enteroccele, was drawn forward
during the rotation of the hydrocoele, and the (esophagus was carried in the opposite direction,
so that in " D " the stomach lies almost directly over the u-sopbagus (figs. 8 and 13, oe and s). The
partition, which in "C" separated the cavities of these two structures, has disappeared in "D,"
and the cesophageal cavity opens into that of the stomach. There is present, then, in "D" the
definitive alimentary canal of the adult ophinrid.

The "cellular mass," which in "U" and "C" was being divided into two parts by the
constriction separating the archenteron into enteroccele and stomach, is to be found, in sections
of "D," in the cavities of both the above structures (figs. 11, 12, 13, and 14 cp).

Lying immediately above, or aboral to, the stomach is to be found the epigastric euteroc<rle.
It has enlarged considerably during the interval between " C " and " D," but is not yet of suflir.ient
size for its walls to touch those of the hypogastric cu'lom, and hence in this stage no circular
aboral mesentery is to be found.

STAOK "K," (16 Houus OLD.
(Figures 15-21.)

The thickening of the ventral ectoderm which was begun in"!)" has continued during the
six hours which intervene between "D" and "E" and has spread to tin- sides of the larva (tigs.
15 and 19-21).

Near the edge of this thickened oral disk are to be found five groups of rounded elevations of
the ectoderm (fig. 15, I, II, III, IV, and V). The three elevations, of which each group consists,
form the angles of an isosceles triangle the apex of which points away from the mouth of the
larva (tig. 15). These elevations or evaginated papilla' lie immediately below and inclose the tips
of those branches of the radial water canals which form the rudiments of the end tentacle and
first pair of foot tentacles of each arm (figs. 19 and 21). In this way each tentacle grows into its
ectoderm, the latter closing around it as it pushes out.

The function of these tentacles in the adult being mainly a sensory one, it is interesting to
note that they receive their ectoderm from part of the same thickened oral area which gives rise
later to the adult nervous system.

The ciliated bands in "E" do not differ in appearance and position from those in "D," but
since they were not figured in the earlier stage it may be well to refer to them again in connection
with figs. 15 and Ki, cb 1, 2, 3, and 4. The first or most anterior baud surrounds the larval organ
quite near its tip.

Near the first band, and parallel to it, runs the second one also around the larval organ. The
third ciliated band is separated from the second by a much wider space than that which separates
the first and second bauds. Were it not interrupted on the oral disk the third ciliated baud would
lie in the line separating the bivium and trivium — that is, between arms I and II on the one side
and IV and V on the other. The fourth baud, passing just posterior to the group of ectoderma!
elevations lying under the branches of the third radial water tube, surrounds the posterior end of
the larva.

MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 93

In "E" the cavities of the oesophagus and stomach have become obliterated, and the two
structures appear in section as one solid mass of cells (fig. 20, st and oe). No degeneration in
their size, however, is to be observed, and their outer walls remain well defined, the oesophagus
retaining its connection with the ectoderm. As will be seen later, their lumen reappear and they
become the definitive alimentary organs of the adult ophiurid.

Returning to the consideration of the water system, we find in "E" instead of one pair of
tentacles on each radial canal, as in " 1),'' there are two pairs present (fig. 17, tl and t2), the
second pair having grown out of the radial canal between the first pair and the water ring. The
second pair is much smaller than those which were first to be formed, and, contrary to what
one would expect, this discrepancy in size does not disappear as time goes on. This is also true
in the sea-urchins, in which the primary tube feet in the larva are enormously larger than those
which are subsequently formed. The primary tube feet in this case gradually diminish in size
after the adult form is reached.

As a rule, among echiuoderms the tube feet or tentacles are formed centrifuyally from the
radial canals; that is, between the end tentacle and the last pair of tube feet or tentacles already
formed. This process keeps the undiffereutiated growing point of each arm at its tip, but in this
ophiuran, and the same is true of Antedou, the formation of the tentacles begins in a centripetal
manner; that is, the second pair of tentacles appears, not between the end tentacle and the first
pair, but between the first pair and the ring canal.

This second pair of tentacles is the rudiment of the buccal tentacles, and although differing in
both function and position in the adult from that of the foot tentacles, is nevertheless entirely
homologous with the latter. This homology is shown by their origin and the fact that for a time
after forming they are directed away from the mouth toward the end of the arm just as is the case
with the foot tentacles. After a time, however, as will be seen later, they turn back and point
toward the mouth, thus showing that in this second pair of outgrowths from the radial canals
we have to do with the first pair of buccal tentacles of the adult. After budding, as we see, from
the radial canals, they migrate to a position on the ring canal, with which we find them connected
in the adult.

In " E" the buccal tentacles have no ectoderm nor rudiment of such, the ectoderm under their
tips being as yet undiffereutiated from the oral disk.

The hypogastric enterocoele has assumed a more pentagonal shape than in "D," it having
grown out over the radial water canals (fig. 17, he). These projections of the hypogastric eutero-
ccele will continue to grow with the growth of the arms and become the brachial extensions of
the body ccelom.

The interradial pouches of the hypogastric enteroccele, which were beginning to form in "D,"
have pushed down further and further between the radial canals until, coming in contact with the
ventral ectoderm, they bend over, inserting themselves between the ring canal and the oral disk
(figs. 17-21, hip 1-2, 2-3, 3-4, and 4-5). In the same way the pouch from the anterior enteroctele in
the stone canal interradius has grown under the water ring. In these five interradial enterocffilic
outgrowths, as has been mentioned before, we have the rudiments of the outer perihremal sinus
of the adult. The process by which this periLuernal sinus is formed in Ophiura ln-evispina agrees
in every detail with its method of origin in Asterina gibbosa as described by McBuiDE (10).

The epigastric euterocffle is in much the same condition as that in which we left it in ;'D," it
being as yet too small to meet and form a mesentery with the dorsal edges of the hypogastric
co^lom (fig. 19, ee).

The stone and pore canals, too, have changed very little during the interval between "D"
and " E." From the ring canal at a point to the left of the origin of radial canal 1 the stone canal
passes upward and opens into the right postero-dorsal part of the anterior enterocode. The pore
canal begins at the same point where the stone canal ends, the two canals thus having a common
opening into the anterior euterocosle or ampulla}. The pore canal extends from the enterocoale to
the dorsal surface of the larva, where it empties through the dorsal pore at a point a little to the
right of the median sagittal plane. These two canals, although extending in the same direction,
do not lie in the same straight line, the pore canal being set a little anterior to and to the right of
the stone canal (figs. 17, IS, and 21, st and pc; also fig. 11).
10396 3

94 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

STAGE "F," 5 DAYS OLD.
(Figures 22-30.)

Although " F " is separated from the stage last described by a considerable space of time, the
changes in the larva which have been brought about are easy to follow.

The larva is considerably larger than in '< E," and has reached its full development. From
this time on the larval organ gradually'degenerates and is finally completely absorbed by the
developiug star (Compare figs. 22 and 31 lo.)

The external form of the larva has been changed by the appearance of a number of elevations
and depressions in its outer surface, the ciliated bauds being elevated upon circular ridges (fig.
22, cb 1, 2, 3, and 4), while at points on the sides of the disk beyond the end tentacles projections
in the ectoderm have made their appearance, these being the rudiments of the ophiurau arms (fig.
22,1, II, 111, IV, and V).

The larval organ is cylindrical, but the disk has continued its dorso-veutral flattening.
(Compare figs. 22 and 26.)

The first and second ciliated bands are situated in the same places as in " E." The third,
while retaining its old position, has grown in upon the ventral disk toward the mouth (fig. 22, cb 3).
< )n the ventral side of the larva the fourth band has shifted from its old position behind the third
radial canal to one on the interradii between arms II and III and III and IV. It has also become
interrupted on the oral disk in a manner similar to the third ciliated band (fig. 22, cb 4).

The depressions before referred to are caused by the iuvagination of the nervous system, which
structure has been forming since "D" in the thickened oral disk of ectoderm. Immediately below
the water ring and radial water canals the thickening has increased more rapidly than at other
points, thus producing a ring-shaped internal ridge, from which extend five radial thickened
ridges. These rudiments of the nerve ring and radial nerves bulge inwardly, no evidence of their
presence being apparent on the outside. When the thickening process has been completed the
whole nervous system gradually sinks in, leaving a circular groove from which five radial grooves
pass out. This is the stage in the formation of the nervous system which has been reached in
"F" (fig. 22, eg and rg). The invagiuatiou process begins at the ends of the radial nerves, just
inside of the curved tips of the end tentacles, the nerve ring being invagiuated last of all.
(Compare figs. 23-27.) As development goes on the edges of the grooves gradually close over the
nerves, the closure taking place in the same order as the invagination proceeded — that is, first
over the ends of the radial nerves, then finally, after gradually traveling up the radial nerves,
closing over the nerve ring.

By the meeting and subsequent fusion of the edges of the grooves, part of their cavity
becomes cut oft' from the exterior and is left below the nervous system as the subneural space.
But this will be referred to again iu an older larva, in which the process of its formation is more
nearly completed,, it having begun in a few only of the most advanced larva' of Stage " F."

The nervous system shows a differentiation into two distinct layers, a fibrous one nearest the
water system and a cellular layer lying below the fibers (figs. 23-29). The nuclei of the cellular
layer are oval, with their long diameter perpendicular to the fibrous layer.

Above the nervous system, separating it from the water system, is to be found the outer
periha'inal space (figs. 23-30, opr). Recalling the condition of the perihrcrnal system in Stage "E,''
we see that the ends of the interradial projections from the hypogastric and anterior euteroco3les
have grown out over the nervous system, spreading iu both directions until the outgrowths of each
interradial pouch meet those of its adjacent fellows in the radii over the origins of the radial
nerves; here the ends of the pouches fuse, and together they grow out over the radial nerves as
the radial perihreirjal sinuses.

In the starfishes, where the formation of this perih;rmal system has been observed, it is said
that no fusion takes place between the diverticula of the iuterradial pouches of the hypogastric
and anterior euterocosles when they meet in the radii, but that a mesentery is formed at the points
where the diverticula come iu contact. This mesentery is described as continuing to the end of
the arms, separating the radial spaces into two parallel cavities.

Nowhere could I find such a mesentery iu sections of the larvse of 0. brevispina, nor could I
feel sure that it exists in the adult ophiurau.

MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 95

MCBEIDE (10) and GOTO (5) both agree that in starfishes the inner perihaemal ring sinus
arises from the anterior enteroccele, although they differ as to the method of its formation. In
noue of the larvre I have is the structure in question fully formed, but in Stage "F" a cavity is
arising, as an outgrowth from the anterior enterocajle in the stone canal interradius, which I
take to be the rudiment of the inner periLuernal ring space. It lies to the left of the stone canal
near the median sagittal plane of the larva. From the posterior side of the ventral end of the
anterior enterocajle the outgrowth takes its origin, then extending posteriorly until past the water
ring it bends over and grows down until its end reaches the nerve ring at a point inside the outer
periha'inai sinus. Here the end of the pouch in question begins to spread under the uerve ring
in both directions, parallel to the outer perihajinal riug (figs. 24 and 26, ips). This coincides
exactly with its method of origin in Asterina gibbosa as described by McBRiDE.

Although the outer periluemal ring is entirely cut off from the body cavities at this stage,
there still remain traces of the interradial pouches which gave rise to it (fig. 29, hip 1-2 and 4-5).

The hypogastric enterocoele itself has changed very little since Stage "E," but the epigastric
has enlarged to such an extent that its edges now meet the edges of the hypogastric and a circular
aboral mesentery is formed (figs. 23-29. cm).

In the water system considerable, growth is to be noted in the tentacles, the end and first
pair of foot tentacles being capable of protrusion considerably beyond the disk. By means of
these tentacles the larva- are able to cling tenaciously to the surfaces of foreign bodies, it requiring
a strong jet of water from a pipette to detach them. Special notice was taken to be sure that it
was the tentacles and not the larval organ which was used as a means of attachment.

The second pair of tentacles (buccal tentacles) have acquired their ectoderm in this stage and
they protrude, like the other tentacles, over the radial nerves into the radial grooves (figs. 22
and 20, t2).

The axial sinus or ampulla is present in "F," it being that part of the anterior enterocoele
which remains after the pouches have been cut off, which will form the inner periha-iual and part
of the outer periuwmal systems (figs. 24, 25, and 26, ax sin).

It will be noted that beside the ampulla, which is situated anteriorly to the stone canal, there
are two other cavities near the stone canal to be accounted for (fig. 20, sin a and sin b). I can
not be sure of their origin, but I believe that they also come from the anterior enteroco?le. I have
distinguished them by the letters a and b, as they are probably the same cavities as those so let-
tered by McBEiDE (10) in his figures of Amphiura.

The cavity McBRiDE has lettered b', and which he thinks represents the degenerated right
hydroca-le, I have been unable to find in any of my sections.

The stomach and oesophagus are in a condition the same as we found them, in "E;" that is,
without lumen.

STAGE "G," 5} DAYS OLD.
(Figures 31 and 32.)

Larv?e a few hours older than "F " show a decided degeneration in the larval organ (fig. 31, lo),
but otherwise the external appearance of the two stages is about the same.

The grooves caused by the iuvagiuation of the nervous system have begun to disappear by
the growing together of their edges, and instead of the deep furrows we find a slightly depressed
line where the edges of the grooves have met (fig. 28, rg).

In the nervous system a pair of tentacle nerves has been formed from each radial nerve (fig.
31, nl). They grow out laterally from the radial nerves at points proximal to the first pair of foot
tentacles, around which latter they grow and to which they belong. No nerves as yet have
appeared to supply the buccal tentacles.

In "F" the buccal tentacles had only begun to curve away from the end of the arm; but in
"G" this proximal bending has continued until they curve over the uerve ring and point toward
the mouth.

Beside this change in the water vascular system we find in "G" the first appearance of the
rudiments of the poliau vesicles. They are four in number and are in the form of small interra-
dial pouches growing distally from the convex wall of the water ring (figs. 31 aiid. 32, pv). There
is no polian vesicle present in this species in the stone canal interradius.

96 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

As no perceptible change has taken place since " F" in the organs not referred to above, the
description of them given in the previous chapter will serve equally as well for UG" as for " F,"
and the ligures of these organs in "G" may be examined in connection with their description
in " F."

• STAGE " H," 8 DAYS OLD.

(Figures 33 and 34.)

In the oldest larva- I have, the metamorphosis has been almost completed. The larval organ
has nearly disappeared, that part of it which yet remains being found sticking to the edge of the
aboral disk of the young pentagonal star.

When living the little ophiurids clung to the bottom and sides of the aquaria dishes. Although
the ciliated bauds were still evident on the disk their free swimming habits had been wholly
given up.

The pore canal still opens on the aboral surface, but with the growth of the latter it is
traveling toward the edge of the disk, and by a continuation of this process the oral surface will
ultimately be reached.

As the closure of the grooves over the nervous system took place, circular areas below the
tips of the tentacles were left open, the tentacle pores, and through these the tentacles, were able
to protrude and withdraw themselves.

The subneural sinuses which had begun to be formed in "F" have been completed in the
eight-day larva (figs. 31 and 34 ss). In "H," then, the nervous system is cushioned below by the
subneural sinus and above by the outer periha-inal ring.

The stomach, which for so long a period has been at a standstill in its development, has begun
to grow, its sides pushing out between the epigastric and hypogastric body cavities. The lumen
of both stomach and oesophagus have reappeared (fig. 34 s). The glandular structure which makes
the walls of the stomach so complicated in the adult has not begun to form in "II," the walls being-
simple and one cell in thickness.

No figure of " H " as a whole object has been made for the reason that the skeletal plates should
be included, and material adequate to a complete study of them is at present not in my possession.

RELATION OF LARVA TO ADULT.

The hydrocode is the first organ to show radial symmetry in the developing larva of Ophiura
breoispina, and from the, time when this organ has completed its rotation about the u-sophagus it
shows a definite relation to the plane of bilateral symmetry of the larva.

The hydroeitle is not only radially symmetrical, but bilaterally symmetrical, since it is divided
into symmetrical halves by the plane which passes through radial canal 3 and through the iuter-
radius of the stone canal. This plane coincides with the plane of bilateral symmetry of the larva.
The other parts of the star are built about the water vascular system; hence it, as a whole, bears
a similar relation to the larva as was initiated by the hydroccele.

No secondary twisting of the various parts of the star occurs, and its relation to the larva
remains constant as it began, and throughout the life history of the species the following state-
ments hold true: Ventral and dorsal in the larva are equivalent to oral and aboral in the adult.
Although no physiological differentiation exists, if we regard that part of the adult as anterior
which was anteriorly directed in the free swimming larva, the trivium is anterior, the bivium is
posterior.

In the foregoing I have confirmed, in an ophiurid, the conclusions drawn by GOTO from his
studies on a starfish. In his work on the development of Aslerins pallida GOTO (5) thought he
was able to prove the coincidence of bilateral symmetry, which obtains in the adult starfish, with
the plane of bilateral symmetry of the bipinnarian and brachiolarian larva-.

The study of the relation of larva to adult in the starfishes is made most difficult, however,
by the independent origin and subsequent twisting of the parts of the star. At the time of their
origin no two parts of the star bear the same relation to the larva. The relation of each part to
the larva also changes as metamorphosis proceeds.

The facts just enumerated admit of other conclusions than those deduced by GOTO, and no

MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 97

two- investigators have reached the same conclusion. The point of view from which the subject
has been approached is not the same in all cases, but the results obtained by those who have
studied the question admit of being reduced to the same basis; that is, the relation of the planes
of bilateral symmetry in larva and adult.

CUENOT (4) in his latest work denies the existence of any known relation between them.

SEMON (13), working on a holothurian, found the two planes in question to coincide, but his
conclusion is based on the supposition that the dorsal mesentery of the adult is the same as that
of the aiiricularia larva, which supposition BURY has since shown to be incorrect.

BURY (3), after working on members of all the groups of echinoderms. concluded that the
plane of bilateral symmetry of the larval form coincides not with the plane dividing the adult
form into two symmetrical halves, but with the plane of radial symmetry.

McBuiDE's (10) observation on a starfish, Asterina gibbosa, led him to adopt about the same
view as that of BURY. He found that the plane of radial symmetry of the star makes an angle
of 70° plus with the frontal plane of the larva, but may, without error, be considered as 90°. This
is equivalent to saying that the plane of radial symmetry of the star is parallel with the sagittal
plane or the plane of bilateral symmetry of the larva, and is also reducible to the statement that
the planes of bilateral symmetry of the larva and adult are at right angles to one another. Thus
right and left in the larva become aboral and oral in the adult.

The difference in results arrived at by GOTO and McBRiDE are due almost wholly to the
stages in the metamorphosis selected in each case for the study of the question, GOTO selecting a
very late stage, when the larval body had all but disappeared, while the stage chosen byMcBRiDE
is an early one, in which the rudiments of the star are just appearing.

If the five groups of echiuoderms have sprung from a common stem after radial symmetry
had been established, then in the metamorphosis which is found in all the groups there should be
discoverable a unity of relation between larva and adult. It is hard to conceive of the radial
symmetry of echiuoderms as having been independently acquired by each group, although it is
easy to see how secondary changes may have arisen in the metamorphosis since the groups separated.

The five groups of echiuoderms stand isolated from one another almost as completely as does
the echiuoderni phylum from the other phyla of the animal kingdom, and it is not my intention at
this time to enter into a discussion of the interrelationships of echiuoderms. I wish, however, to
point out an interesting series of facts presented by members of the Asterid, Criuoid, and Ophiurid
groups which may have a bearing upon the subject, and in the same connection I wish to call
attention to how well MCBRIDE'S hypothetical ancestor of the Asterids and Crinoids (10, fig. — ),
when details are not too closely compared, fits into the facts of the larva of OpMura brevispina.

In one of the Asterids GOTO has shown that toward the end of metamorphosis the almost
complete star sits as a cap at the posterior end of the larva, with its aboral end posterior, its oral
surface anterior, the bivium dorsal, and the trivium ventral.

In Autedou, like the starfish, the rotation brings the developing crinoid head to the posterior
end of the larva, but differing diametrically from the starfish in that the oral instead of the
aboral surface of the crinoid is posterior; but this difference does not in any way affect the
honiologies between the two groups as has been supposed.

In OpMura brevispina the relation of larva and adult at the time of metamorphosis is approx-
imately the same as is shown in Stage "F" (fig. 22), in which ventral in the larva is ventral
(oral) in the adult.

Now, if we take an ophiuran larva at Stage " F," and imagine the disk to rotate in such a
•way as to bring its oral surface away from the larval organ or preoral lobe, it fairly represents
that which takes place in the metamorphosis of Antedon, but if we think of it as rotating in
the opposite way, bringing the aboral surface away from the preoral lobe, then it more nearly
illustrates the starfish metamorphosis.

In Antedou, as metamorphosis proceeds, the stem is carried on to the aboral surface, while in
the starfish the preoral lobe finally disappears on the oral surface. In 0. brevispina the place of
disappearance of the larval organ more nearly recalls the crinoid than any other echinoderm, the
larval organ being found in some of my oldest specimens as a small knob near the edge of the
aboral surface between arms I and V.

98 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

COMPARISON OF THE LARVA OF ANTEDON ROSACEA WITH THAT OF OPHIURA •

BREVISPINA.

While I was studying the larva of Ophiura brevispina, characters were constantly being
found which reminded me of the larva of Antedon as described by IJruv (1). Some of these
points of resemblance are no doubt only superficial, but others are such as to make it worth the
while to devote a short chapter to the similarities of the two larvae.

The entire ciliation of the very young larvre gives place in both to a series of transverse
ciliated bauds, five in Antedou, four in Ophiura. The band nearest the anterior end of the Ante-
don larva, however, is small and incomplete. Two bauds only in each case surround that part of
the larva from which the disk is formed.

The blastopore in both larva?., after shifting from a posterior position to one on the ventral
surface, closes and the archenterou loses its connection with the ectoderm and lies free iu the body
cavity.

In the seven-day embryo of Antedon and Stage " C " of Ophiura the hydrocoele is a horseshoe-
shaped structure lying iu the posterior ventral part of the larva; with the open end directed
anteriorly, and in each case the plane of radial symmetry of the hydrocu?le is at right angles to
the plane of bilateral symmetry of the larva;.

In the formation of the paired tentacles from the radial water canals the process is begun
centripetally iu both larva1., the second pair of tentacles appearing between the first pair and the
water ring instead of between the first pair and the eiid tentacle, as is the case in the other groups
of echinoderms.

In the five-day Antedon larva and those stages represented by "D" to UF'' in Ophiura the
stalk and larval orgau are strikingly similar, both in shape and position, the two structures being
anteriorly directed in swimming.

The stem of the Antedon larva, as a result of metamorphosis, comes to be an aboral structure,
and just bctore the disappearance of the larval orgau .from the ophiuraii larva it is to be found as
a small knob, not in the center of the abor;;l disk, it is true, but on its edge. In the starfishes it
may be recalled that the preoral lobe disappears on the oral surface of the metamorphosing star.

To the above larval characteristics may be added the similarity which exists in the disposition
of the alimentary and cudomic systems iu the adult forms.

In both Crinoids and Ophiurans the digestive apparatus is confined to the disk.

The body cavity is continued into and to the ends of the arms. When a transverse section of
a pinnule of Autedou is compared with a transverse section of an ophiuild arm, the following
striking correspondence is found in the parts: Aborally, segmeutally arranged calcarious ossicles
and muscles are present; a continuation of the body citlom runs between and oral to the muscles;
connected with and on each side of this central brachial body cavity are two other cavities, the
subtentacular canals of Antedon. These latter in the Ophiuraus are connected with periha<uial
space in each vertebral segment.

The radial water tube lies between the subtentacular canals, and in each segment sends out a
pair of tentacles. The tentacles iu both the Criuoids and Ophiurids are devoid of the terminal
suckers, which are so characteristic of the other echinoderms.

Separating the radial water tube from the nerve cord is to be found the radial periluemal
sinus.

In ophiurans a subneural space is present which is not represented in the crinoid arm. This
is due to the fact that in Antedon the nervous system is superficial, while iu Ophiura it has been
iuvagiuated, and with its iuvaginatiou a space has also been carried iu below it.

LITEBATUBK CITED.

1. BURY, H. "Early Stages in the Development of Antedon rosacea." Philosophical Transactions, vol. 170. 188S.

2. BURY, H. " Studies iu the Embryology of the Eehiuoderms." Q. J. Mio. Sc., vol. 29.

3. BUHY, H. "The Metamorphosis of Ecliiuoderms." Q. J. Mic. Sc., vol. 38.

4. CUENOT, L. " Etudes Morphologic[ues sur les Echinodertnes.'' Arehiv. Biol. t. XI.

5. GOTO, S. "The Metamorphosis of Asterias pallidn, with special reference to the fate of the Body Cavities.'
Journal of the College of Science Imp. Univ. Tokyo, Japan. Vol. X, Pt. III. 1898.

6. GRAVE, C. "Notes ou the Development of Ophiura olivacea." Zool. Anzeig. Bd. XXII. No. 580. 1899.

MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 99

7. KROIIN, A. " Ueber eineu neuen Entwickelungsinodus der Ophiuren." Mueller's Archi v. f. Anat. u. Phys., 1857,
p. 369.

8. LYMAN, T. " Ophiura brevispina Say." Pt. I. 111. Cat. Mus. Coiup. Zool., Harvard Coll. 1865, p. 1C.

9. LYMAN, T. " Report on the .Ophinridse." Vol. V. Challenger Ueport.

10. McBuiDK, E.W. "The Development of Asterina gibbosa." Q. J. Mic. So., vol. 38.

11. PREYER, W. " Ueber die Bewegungen der Seesterne." Mitth. a. d. Zool. Stat. z. Neapel. dec. 1886.

12. SAY, T. " Ophiura brevispina." Journal Phil. Acad., Vol. V. 1825. Page 149.

13. SEMON, R. "Die Eutwickoliing dor SynaptadigitataunddieStauimcsgoschicliteder Echinodermen." Jonaisch.
Zeitschr. f. Naturwiss. XXII. 1888.

EXPLANATION OF PLATES.

The figures illustrating this paper were drawn to the same scale of magnification, 330 diameters, and were
reduced one-half in reproduction.

Figs. 1, 2, 4, 6, 7, 9, 17, 31, and 32 were reconstructed from series of transverse and sagittal sections.

In all cases the part of the figure which is nearest the top of the plate is either anterior or else ventral in the
larva.

ABBREVIATIONS USED.

ael Loft anterior enteroecale.

aer Eight anterior enterocoele.

ap Apical plate.

ax sin Axial Sinus.

b Blastopore.

cb 1,2, 3, and 4 Ciliated bands.

eg Circular groove.

cm Circular mesentery.

cp Cellular plug.

d Dorsal pore.

ee Epigastric euterocojle.

et End tentacle.

he Canal connecting posterior en-

terocuole and hydrocuile.

he Hypogastric enterocoele.

hip 1-2,2-3,3-4,4-5 Interradial pouches of the hypo-
gastric enteroccele.

hy Hydrocffile.

ips Inner perih;emal sinus.

ipax 51 Interradial pouch of axial sinus.

lo Larval organ.

m Mouth.

nies Mesenchyme.

ill First, pair tentacle nerves.

iir Nerve ring.

oe (Esophagus.

od Oral disk.

opr Outer perihsemal ring.

pc Pore canal.

pe Posterior enterocooles.

pv •. Polian vesicle.

rad n Eadial nerves.

rg Eadial grooves.

rps Eadial perihremal space.

sin a Sinus "a".

sinb Sinus "b".

at Stone canal.

ss Subneural sinus.

s Stomach.

tl First pair foot tentacles.

t2 •- Buccal tentacles.

wvr Water vascular ring.

I, II, III, IV, V Arm rudiments.

1,2,3,4,5 Eadial water canals.

100 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES.

PLATE I.

FIG. 1. Larva in Stage "A," seen from the right side, the right half of the ectoderm removed and the mesenchyme

omitted.

FIG. 2. The same larva, seen from the ventral side as a transparent object.
FIG. 3. Median longitudinal section of a larva in Stage "A."

FIG. 4. A larva in Stage " B," viewed from the ventral side as a transparent object.
FIG. 5. Transverse section of a larva in Stage "B" in a plane halfway between the blastoporo and the point where

the hydroeele is connected with the archentermi.
FIG. 6. Larva in Stage " C," seen from the ventral side, the ventral half of the ectoderm, the mesenchymo, and part

of the oesophagus removed.
FIG. 7. The left half of the same larva.

FIG. 8. Transverse section through Stage "C" in a plane indicated on fig. 6 by the letters a-6.
FIG. 9. The reconstructed internal anatomy of a larva in Stage " D," the veutral ectoderm removed and with it part

of the oesophagus.
FIG. 10. An outline drawing of fig. 9, on which are indicated by lines the planes of the sections which follow in

figures 11, 12, 13, and 14.

FIG. 11. Longitudinal section taken through a larva in Stage ':D'' in the plane indicated on tig. 10 by the line m-n.
FIGS. 12, 13, and fig. 14 of Plate II. Transverse sections taken through Stage "D" in planes indicated on lig. 10 by

the lines a-b, c-d, and e-f.

PLATE II.

FIG. 15. Ventral view of a larva in Stage "E," to show ciliated bands and iirst appearance of the arm rudiments.

FIG. 16. Dorsal view of Stage " E,'' showing the ciliated bands.

FIG. 17. A reconstruction of the anatomy of a larva in stage "E," the ventral ectoderm removed.

FIG. 18. An outline drawing of fig. 17, on which are indicated by lines the planes of the sections shown in figs. 19,

20, and 21.
FIGS. 19, 20, and 21. Transverse sections taken through larva in Stage " E " in planes indicated on fig. 18 by the

lines r-s, t-ii, and r-ic.
FIG. 31. Reconstruction of the anatomy of a larva in Stage " G." In this case as in all the other reconstructions

the ventral surface is up and the ventral ectoderm removed.

FIG. 32. An outline drawing of the water vascular system of a larva in Stage "G," seen from the ventral surface.
FIG. 33. Transverse section of Stage "H," taken through the region of the stone canal.
FIG. 34. Transverse section of a larva in Stage "H," taken through the stomach.

PLATE III.

FIG. 22. Ventral view of the fully developed larva before metamorphosis has begun. Stage "F."

FIG. 23. Outline drawing of fig. 22. The lines indicate the planes of the sections, which have been drawn to show

the anatomy of a larva in Stage "F."
FIGS. 24, 25, 26, and 27. Longitudinal sections of a larva in Stage " F," the planes of which are indicated on fig. 23

by the lines a-b, c-d, m-n, and jr-y.
FiGS. 28, 29, and 30. Transverse sections of a larva in Stage "F,'' the planes of which are indicated on fig. 23 by

the lines c-o, d-o and e-o.

MEMOIRS NATIONAL ACADEMY OF SCIENCES. VOLUME VIII.

PLATE I.

-VH

•" • « v

\

Fig. 1.

Fig. 6.

Fig. 2.

Fig. 5.

Fig. 8.

i'W.)M

Fig. 3.

Fig. 4.

Kto '^f

< v«> <&v

.• %v»i si

=•- '•' ://,«*
***>€>'

Fig. 11.

Fie.

'.•

f.<?^y\^.

t' K:W .?'
^.•>--,:^

•••«>*. :..',-^v

Fig. 12.

f?ic?%

Fig. 9.

Fig. 10.

Fig. 13.

MEMOIRS NATIONAL ACADEMY OF SCIENCES. VOLUME VIM.

PLATE II.

•V. •••-••:•. ••'.•
,,-"••

Pig. 14.

Pig. 20.

Pig. 21.

Pig. 15.

Pig. 18.

Pig. St.

Pig. 16.

Pig. 17.

K. 33.

Pig. 31.

Pig. 34.

MEMOIRS NATIONAL ACADEMY OF SCIENCES. VOLUME VIII.

PLATE III.

pig.

Fig. 25.

Fig. 22.

Fie-

Fig. 23.

Fig. 28.

Fig. 50.

A,