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THE ANASOMT, HABITS, AUD EMBHZOLOGY OF YOLDLA. LIMATULA, SAT.

A DISSERTATION SUBMITTED TO THE BOARD OF UNP/SSSITY
STUDIES OF THE JOHNS HOPKINS UNP/ESSITY, FOR THE DEGREE OF DOCTOR
OF PHILOSOPHY.

by

GIU'IAN A. DREW.

BAIjTMORS
1898.

^tjili^

.bc: .f

THE ALTATOMT, HABITS, AND EMBSYOLOGT OP YOLDIA LIMATULA, SAY.

Along tiie coast of Maine, wliere Yoldia limatiila is very
abundant, and gro'ws to be especially large, specimens are widely
distributed. Tlieir principal habitat, liOTTevor, is in tlie shallow
coves and inlets, wliere the tidal currents sweep by without enter-
ing. ^^

^— Here, soft rmid has accumulated, and is constantly being
added to, by the decay of plants and animals swept in from the
surrounding sea and land. They are most abundant in water from
one to five fathoms deep, and probably never occur above low tijle
mark.

During the two seasons of my stay at Casco Bay Maine,
Prof. C. B. Wilson generously allowed me the free use of his equip-
ment, which was of great value to me. Previous to this, speci-
mens from Woods Holl, Mass., were furnished me through the kind-
ness of Dr. W. K, Brooks, Mr. Richard Rathbum and Dr. James L.
Kelloga Most of these specimens were collected by Mr. Vinal

Edwards.

(11)
1^ own work, like the work of Drs. Mitsukuri and

Kellog^'^) upon this animal, has been carried on under the direc-
tion of Dr. Brooks who has for many years manifested a great in-

;'•.; ■■' li'L.' ■'

■Jt

§.;

fii ,-■ >4

•..•>v;r ii^jL

2:

terest in tliis rather peculiar lamellibranch.. It is a pleasure
to ackno-wledge my indebtedness to liim, I wish publicly to ac-
knowledge my indebtedness to ray wife, wlio has materially aided
me in securiiig, tending and preserving specimens,

ANATOIIT AND HABITS.
The distincti-re characters for the genus Yoldia as given
by Verrill and Bush ^^■'•) are : "shell nearly smooth, compressed,
lanceolate, gaping, more or less prolonged and tapering posterior-
ly, with a well- defined wide rostrum, generally withotit carina-
tions. The external liganent is marginal, feebly developed,
continuous under the beaks and not much differentiated from the gen-
eral epidermis. The Chondrophore is large, concave, and pro-
jects within the margin. The pallial sinus is large and deep.
The siphon-tubes and posterior pallial tantacle are long. The
palpal tentacle^ are long and tapered ; in life they may extend
nearly to the end of the expanded siphon.'

SHELL.
Figures 1 and 2.
To the above characters of the shell will be added a few
others, part of which, no doubt, are comriion to all the species
of the genus, while others are specific.

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V ;. 1- '

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i'/.Q? p'y.^s ?

3.

liVlien tlie animal is taken, from the mud. in ^Afiaich it lives,
the anterior portion of the shell is jet black. Posteriorly it
gradually assiunes olive greeai. The black is no doubt a stain,
as it bleaches out in animals kept in aquaria, and the shell as-
sumes a rather laniforra olivaceous tint, somewhat streal^ed along
some of the more prominent, lines of gro"VTth with yellow or brown.

On each valve, two more or less prono\xncod radial stripes
extend from the beal-c to the ventral margin, one anterior, the
other posterior, figure 1. The margin of the mantle opposite
the extremity of each of these stripes is specially modified
and sen.sitive. Internally, near the dorsal margin of each valve
are two rows of toxodont teeth, one extending anteriorly a:ad the
other posteriorly from the cartilage pit, figure 2. Near tlie car-
tilage pit these teeth are very small and closely placed. They
attain their greatest length about midway along each row, and be-
come short and rather widely separated at the ends furth^est from
the cartilage pit.

Internal markings are rather obscure. Beside the foot
and adductor muscle scars, and the pallial line with its deep and
broad sinus, there is on each valve" a rather distinct, cur^red
line, extending from the ventral margin of the anterior adductor
muscle scar, nearly to the cartilage pit. Dissection shows that
this line marks the limit of the genital mass and digestive glaid.

4.

(20)

Tryon refers to this mark on the right valve, and

thinks it is caused by the loop of the intestine that extends for-
ward, on the right side, very near the shell. That this view is
wrong is shown by there being similar markings on both valves.

MANTLE.
Figures 1 and 3.

The mantle lobes are free along their ventral borders,
and are ciliated in patches on their inner surfaces. They are
modified to form the siphons, the marginal tentacles^ ar» unpair-
ed, very rouch elongated tentacle situated near the base of the
siphons, a flattened expansion opposite the e>rbremity of each
posterior shell stripe, and a rounded projection opposite the ex-
tremity of each anterior shell stripe. These will be treated
in turn. There are also two pairs of apparently glandular patch-
es, one situated ventral to the anterior adductor muscle and the
other ventral to the base of the siphons.

Siphons-- The oldest specimens reared from eggs have not begun
to develop siphons. The youngest specimen collected with a dredge
(about 1/2 m m. long) had already formed the exhalent siphon.

This siphon, figure 11, seems to have been formed by
the union of the margins of the mantle lobes, followed by their
gro-vrbh into a tube and the withdrawal of the tube between the

5.

mantle lobes. In withdrawing the tube its dorsal surface, cor-
responding to tlie dorsal surface of the united mantle lobes, is
drawn, in, forming a complete septum. A ridge on each mantle lobe
indicates ^j.ere the point of union of the mantle margins has been
drawn along its inner surface.

The ventral surface of the base of this siphon arches
dorsally, figure 12, the ridges on the mantle lobes near its base
thicken and finally fuse. Thus a second tube is formed lying
ventral to the extialent siphon, figure 13. The wall separating
the two siphons reinains arcZaed upward for some t^ime, but ffabse-
quent growth straiglitens it. Even in the adult the line of
fusion along tlie ventral side of the inhalent siphon remains dis-
tinct, figure 14, and offers little resistance to splitting. Al-
thoufTh the inlialent siphon is formed between the mantle lobes, at
the base of the exhalep.t siphon, it may morphologically be con-
sidered marginal in formation, as it is formed by thickenings of,
and growth from, ridges that seem to have been carried back from
the margin. The adult siphons are united along their Tfoole
length, figures 1 and 3, and may be extended beyond the shell to
a distance considerably exceeding the shell's length. Normally
the inhalent siphon is shorter, broader, and has thicker walls

6.

tiian the exhalent siplion, but they are frequently rendered of
eqiial length by inj-'ory. Both siphons, figure 14, have betiween
their outer and inner layers of epithelium, large bundles of
longitudinal muscle fibers, separated by sl\eets of radial muscles.
A few circular fibers lie near the epithelium but they are not
numerous. Ihe siphons are extended by forcing blood into the
spaces holloT/red out in the connective tissue. If the siphon
of a young specimen is examined with a moderately high power of
the microscope, small, conical papillae mil be seen projecting
from its surface, figure 15. Each papilla bears at its tip a

long, rigid filament. Older specimens show similar papillae,
but instead of bearing single elongated filaments, several short-
er filaments are borne on the tip of each. Similar papillae are
found in vsirious places on the mantle and its modifications. They
probably correspond to the "pinselzellen" of FlemmingC^) later
described by Dorst^^) and Rawitz i^'^) ,
Siphonal tentacle and marginal tentacles, -

The youngest specimen collected with a dredge, shows a
little rounded knob, figure 11, st. lying on one side, between
the mantle and the siphon. This knob, the rudiment of the si-
phonal tentacle, is an outgrowth from the line of imion of the

7.

mantle with the base of the siphon, which line, as has beeii sho?m.,
seems Tnorphologically to be a portion of the mantle's margin.

During the development of the siphons the tentacle is
carried ventrally and in the adult, comes to lie nearly opposite
the ventral border of the inhalent siphon. Ihis unpaired tenta-
cle was first described by Brookp(l) and has frequently been re-
ferred to since. Pelseneer^ ■'■'*) fo-und that it occurred on either
the right or left side of Yoldia isonota, an obser\ration that al-
so h.olds true for Y. limatula. From its position and iner^ration
Pelseneer(l^) was led to conclude that it might be cojTpared to
the osphradium of gastropods, but this conclusion does not seem
to be borne out by a further knowledge of the subject. Another
organ corresponding in enervation and position to the osphradium
of other lamellibranchs, is present and will be described later.

Ifflhen extended, figures 1, 3 and 10, st . the siphonal
tentacle is a long and slender filament, gradually tapering to
its free extremity, and generally lying loosely coiled on the bot-
tom of the aquariiim, or on the surface of the mud in -v^-ich the
animal lives. [Throughout its length it is set with small conic-
al, papillae, figures 6 and 7, sucin as have already been describ-
ed for the siphons. Each papilla, like the papillae on the

8.

adult siphon, beaxs a cluster o£ filaments at its tip.

"When retracted tlie tentacle presents a series of trans-
verse ivrinkles, figure 6, Beneath the layer of epithelium,
figure 8, ep. run strands of longitudinal muscle fibers, Im. im-
bedded in connective tissue. On the side of the tentacle near-
est the mantle lobe to which it is attached, inside the muscle
layer, is a large ner\re, tn, which can be easily traced to the
tentacle's tip. This nerve is a branch of the pallial zierve.
It is not given off directly opposite the base of the tentacle,
but some distance above it, and the two ner^res continue along,
side by side, until the tentacle is reached. The appearance of
the two ner\'-es lying side by side, is cjaite like^a ganglioxv an.d
has, no doubt, been mistaken for one. On the side of the ten-
tacle opposite the nerve, also within the muscle layer, is a more
or less definite blood space, bs.

Extension of the tentacle seems to be accomplished sole-
ly by forcing blood into this space. In favorable cases blood
corpuscles can be seen moving along it wflien the tentacle is being
extended, -x

^If considerable force is exerted in extendeing the ten-
tacle, S77el lings, figure 7, may occur in it. Such swellings are

9.

filled with blood an.d disappear when the presfjure is remoA'ed.
"When the muscles of the tentacle contract, the blood is forced
back, and the blood-space Tnay be completelj obliterated.

The tentacles that fringe the postero -ventral margins
of the mantle, have, beneath their epithelium, both longitudinal
and transverse strands of muscle fibers, an.d generally several
blood spaces. "iThile branches of the pallial nerves have not been
traced into these tentacles, such branches can be traced to their
bases, and there can be no doubt that the tentacles are s^ipplied
by fibers from these nerves.

Each tentacle has at least one papilla and frequently
there are several papillae upon its surface. In development the
tentacle is preceded by a single sense papilla which is carried
out by the growth of tlie mantle near its base, into a conspicuous
projection. As this projection grovrs, other sorise papillae make
their appearance on its sides, and the papilla at its tip may or
may not retain its position. In a few cases the tentacles divide
or brarich. This is most common witli the tentacles fringing the
posterior expansions of the mantle, soon to be described.

Considering the origin, structure, and inervation of
the siphonal tentacle and the marginal tentacles, there can be

10.

but little doubt that the siphonal tentacle is a greatly enlarged
atid specialized marginal tentacle.

Both stnacture and experiment agree in assigning to
these tentacles a tactile fimction. The siphonal tentacle is
not, as might be supposed, the most sensitive to toucli of any of
the organs. It seeins to be rather more sensitive than the mar-
ginal tentacles, about as sensitive as the expansions of the man-
tle opposite the extremities of the posterior shell stripes, and
rather less sensitive than the projections opposite the extremi-
ties of the anterior shell stripes, the foot, and the siphons. It
may be moved about with a pencil-point without causing much dis-
turbance, but if it be pressed slightly, moved qxxickly, or Jarred,
it is quickly withdraTOi, and the withdrawal may be accompanied by
the partial or complete retraction of the siphons, the closing
of the shell, and, in some cases, by the activity of tlie foot. It
seems especially sensitive to sudden movements or Jars, and its
special function may lie in this direction, in enabling the
creature to detect the approacJi of enemies, of -^^lich flounders
seem to be a^.ong the most dangerous.

If a vessel, containing several specimens partially
buried in the mud, is not disturbed for some hours and^^then Jar-
red, all will generally disappear with astonishing rapidity.

11.

Ihetlier the stirmiUis is transmitted through, the siphonal tentacle
or not, has not been determined, but its position, coiled on the
surface of the mud, suggests the possibility.

All of the er-cperiinents that were tried to determine the
function of the tentacle resulted in failure, inasmuch as specimens
in Tffoich the tentacle Zaad been removed, seemed to be as sensitive
to jars as uninjured specimens. It may be remarked, however,
that the Jars of a small vessel, can. at best bear only a slight
resemblance to Jars af.fecting the surface of the mud on the bot-
tom of the ocean, such as would be caused, for instance, by the
swimming of a floundei'.
Posterior expansions of the mant 1 e , -

These e:<pansions, figures 1, 3 and 10, pe., lie oppo-
site the extremities of the posterior shell stripes. They con-
sist of flat, somewhat triangular extensions of the edges of
the mantle. The anterior border of each is fringed with ten-
tacles, resembling the ordinary marginal tentacles, but more
frequently branched, and usually tiiey have a rather larger pro-
portion of sense-papillae. Tlae inner surfaces of the expansions
are very densely ciliated. Near the base of eao.l\ is a fold or
ridge, along the side of -vtxich such dirt as gains access to the

mantle chamber is swept. This dirt is swept back over the ex-

12.

paasions an.d so out of tlie cavity. The ricli nervous supply comes
as branches of the pallial nerves.
■Anterior projections of the nantl_e,-

The ant ero -ventral margins of the mantle may be pro-
truded a short distance beyond the margin of the shell, figiore 1,
and seems to aid in cleaning the foot when it is withdrawn from
the mud. These margins are destitute of tentacles b\xt are

well siappl led with sense-papillae.

Arising from each margin, opposite the extremity of the
anterior shell stripe, is an oblong projection, figure 1, a e.
This projection carries a row of sense-papillae, and is ciliated
on either side, figures 16 and 17. Branches of the anterior
pallial nerves can be traced very near, and they undoubtedly en-
ter, these projections. Enlarged pallial muscles, figure 17,
pm. act as their retractors.

These organs are very sensitive to mechanical stimula-
tion, and probably serve as tactile organs, but their function
is not known. They are placed so near the edges of the retract-
ed foot that it is easy to think that they may serve some function
in connection with it, but no observations were made that support
this sT.irmise or that give a clue to any other fonction.

13.

I? 0 0 T.
Figures 3, 5 and 9, f.

The foot, figure 3, f, is very large and powerful. Dur-
ing development its sides grow out into two muscular f Isps, that
lie side by side, and are capable of being extended laterallj^
30- that tth.-e4x— iaa^g— s44»s-^^a— aar— alfne-st flat or— archgdr'suTfaco,
figure 9. It is this part of the foot that is called the sole.

Notwithstanding its being a rigid muscular organ, known
to bo adapted for burrowing C* ) and executing movements with re-
markable rapidity ^1 ^^^ 21)^ .^ is gtill referred to as a
"creeping sole" (10-12-15 and 21).

The free margins of the flaps are thin, and are fringed
witli flattened papillae that are very sensitive to touch. There
are four pairs of foot muscles, one posterior arj.d tliree anterior.
The posterior foot muscles, figure 3, pfra, are inserted on the
shell, just in front of the posterior adductor muscle and very
near the bases of the teeth. They are very large and send all
of their fibers forward, along the sides of tlie foot, to be dis-
tributed to its anterior and ventral portions. Their function
is to retract the foot.

The anterior foot-muscles, a fm, are inserted on the

14.

shell just behind the anterior adductor rmiscle and very near the
bases of the teeth. Of these nuscles there are tliree pairs.
The anterior pair pass ventral ly and posteriorly, along the sides
of the foot, and are distributed to its posterior portion. The
middle pair pass ventral ly, between the pair just mentioned, and
are distributed to an intermediate portion. The posterior pair
pass ventral ly and anteriorly, between both of the preceding pairs
and are distributed to the anterior and ventral portion of the
foot, including the muscular flaps.

These three pairs of muscles are undoubtedly used in
producing the A^aried and energetic movements of the foot. Be-
side these special foot muscles vdiich together form a large part
of its outer walls, there are transverse strands of muscle fibers
extending from one wall to the other. Large blood spaces are
hollowed out between these strands.

Movements o^ 'th.e foot are always very rapid and are re-
markably diversified. The movements of burrowing consist of
thrusting the closed foot, -wgiich is wedge-shaped, antorio?ly deep
into the mud, reflecting its margins to form an anchor, and then
withdrawing it. These movements follow eacJi other in qiiick suc-
cession and enable an animal to bury itself with great rapidity.
"jyhen the animal is placed upon its side, the closed foot is bent

15.

back vmder the shell, its margins are reflected, and it is then
withdra-wn. If these movements are executed on mud, the lower,
reflected margin of the foot buries itself, and gives the necess-
ary purchase for the animal to right itself. If instead, they
are executed on a smooth, hard surface, where no purchase can be
obtai'16^* these movements are likely to be replaced by leaping
movements.

The leaping movements consist of bending the foot back
vmder the shell, turning the shell nearly on its dorsal margin,
planting the sTirface of the expa;T.ded "sole" on the bottoi^i* and
giving a quick downward movement.

If the foot does not slip, the shell is generally turned
end for end and thrown some incZaes. If the foot slips, as more
frequently happens in smooth -bottomed aquaria, it ahoots^out aa-
t-05H:^9Pi^ with wonderful rapidity and causes a posterior movement.
Other movements are common but will not be described. It seems
very hard to imagine that the foot could possibly be used as a
creeping organ, and its present function, for which its structiire
so admirabl;r fits it, is tiH burro^^ift^ in soft mud.

BYSSAL GLAtro.
Figures 4 and 5 bg.
In none of the specimens that came under rny obser^j-a- "

'»•

16.

tion, did this organ seem to be fvmctiorLal. The youngest speci-
men collected with a dredge shows a small pouch, lined with
epithelium, which opens into the posterior end of the groove that
separates the rauscvilar flaps of the foot. The gland receives a
rather large nerve from each pedal ganglion, and often contains a
few fine threads. These thjreads have never been seen protruding
from the -gis^tds duct.

.t^jlMMTASX CANAL.
Figures 4 and 5.

The rather broad and long oesophagus opens into the
dorsal end of a capacoous stomach, figure 4, sto . which extends
from near the dorsal margin of the animal, well Into the foot.
The stomach is divided transversely by a well marked constriction,
and internally by a prominent ridge, formed by elongated epithe-
lial cells. Above this ridge the epithelial cells are rather
slender, carry cilia, as do the other epithelial cells of the ali-
mentary canal, stain deeply, and have the appearance of secretory
cells. Bel077- the ridge the epithelial cells are broader and
stain less deeply.

Prom the ventral end of this portion of the stomacli,
the intestine bends back along its posterior side, ascends to a

17.

point just heneatli the pericardiiam, makes a forward turn, coTnes
very near the sirrface on the right side, followe the edge of
the genital mass and digestive gland nearly to the anterior ad-
ductor muscle, then turns dorsal ly and posteriorly, gradually as-
sumes a median position again, passes througli the ventricle, and
opens into the mantle chamber at the base of the er-chalent siphon,
just behind the porterior adductor muscle. Ihe intestinal
epithelium is composed of short, deeply-staining cells, which are
frequently arranged in longitudinal ridges. One of these ridges,
rather more prominent than the others, occupies the position of the
tjrplilosole.

DIGESTIVS GLAHDS.
The digestive glands consist of a pair of racemose
glands, the ducts from which open into the stomaod^ near its dorsal
end. The lobes are very numerous and form a rather compact
mass, in the dorsal region of the body, which laterally over-w-
hangs the dorsal part of tlie foot.

LABIAI. PALPS.
Figures 3 arxd 5, Ip.
Ihe palps of the adult an.inal are very large, figure 3,
Ip. Each united pair is suspended from the body wall by a thin

18.

membrane, containing a few muscle-fibers, Tflftiioh is attached to
the body -wall beneath the overhanging digestive gland. The palps
are triangular in shape and have their inner surfaces ciliated and
thrcwn into a series of ridges and grooves, that do not extend
quite to their free margins. Large blood spaces follow along
the bases of many of these ridges. The siipposition that the
large expanded palps, serve in respiration, seems probable.

Each outer palp is supplied with a long appendage,
figure 3, pap, which originates from its posterior end, near its
dorsal margin, and can be extended far beyond the posterior mar-
gin of the shell.

In young specimens, this appendage is flat and is en-
tirely confined to the external palp, figure 5, p'^a^p. As it
grows, it folds longitudinally so as to foim a groove on its in-
ner side, and, at the same time, twists so that it may appear like
a continuation of the united dorsal margins of the outer and inner
palps.

Each palp appendage is supplied with longitudinal mus-
cles, figure 18, Im, a large nerve, p'^n, that is continued into
if from the dorsal margins of the palps, and a continuous blood
space, b s, lying beside, and morphologically ventral to it. Un-
der favorable conditions, blood corpuscles ma^'' be seen moving

19.

along this space, v&ien the appendage is being extended. Other
blood spaces occ-ar in the connective tiss^ae of the appendage, but
they are much smaller and anastomose freely.

The epitheliiim lining the groove formed by the longi-
tudinal folding of the appendage, is densely covered with cilia.

Ihe an.irial while feeding, figure 10, is sliglitly tipped
ventral ly from the perpendicular, and frequently has about one
third of its posterior end above the mud. IThile in this posi-
tion, the palp-appendages are thirust out of the shell and one, at
least-, bends over and inserts its tip in the mud. The cilia lin-
ing its longitudinal groove immediately begin to elevate the
mud, "vSiich is rich in living organisms. The streaan of particles
passing along the groove is large enough to be distinguished at a
distance of some feet. In this way, foraminifers, ostracods,
and even small lamellibranchs, and gastropods, together with the
smaller forms and mud, are passed along the groove, finally be-
tween the palps, and so on into the mouth. Thus vie find that

( 11 1
Mits"akuri's surmise^ ' , based on finding sand in the grooves

of the palp-appendages of preserved specimens, was riglit. The

palp appendages are food collectors.

20.

GILLS.
Figures 3, 19 and 20.

Althovigli the gills lia-re been well treated by Mitsu-
kuri, ^ ) Kellog^C'') , and ot?aers, some additional facts botli on
tlie structrire and function of tJieso organs, seem to make it allow-
able to go over the already rather familiar ground.

Each gill, figure 3, g, consists of a double series
of rather broad, flat plates, placed laterally edge to edge, and
antero-posteriorly with their flat surfaces opposed.

This double series of plates is suspended from the body
wall by a thin muscular membrane. Anteriorly the gills pass be-
neath the oyerha:aging digestive glands, on the under sides of
which the plates become gradually smaller and smaller, and final-
ly fade away. Ihe extreme anterior ends are continued as
ridges. These ridges seen to represent continuations of the
suspensory membranes rather than gill plates, and point to the
probable origin of the membranes as folds from the body T/all.

Posteriorly the gills likewise become smaller, and are
finally attached to the wall that separates the inhal ent^cuad ex-
halent siphon.

ffith the exception of a few of the plates at the extreme
anterior ends of the gills, which are sometimes much distorted

21.

and s'^70llen, all of tlie plates are alike in shape and structixre.
Laterally tlie plates of eacii gill lie opposite each other. Gen-
erally in the length of each gill, one or more plates are inter-
polated on one side or the other, figure 20, ip, and thus the
plates are made to alternate for a short distance, but they soon
become opposite again.

The plates on the two sides of each gill are separated
from each other except belov^, -viftiere they are fused, either leav-
ing a connecting ridge, as frequently happens anteriorly, or a
smooth interiiediate portion, as is generally the case further
back.

Each plate is also Joined to the s^ispensory nembrariC,
figure 19 su, abo\'-e, and to two longitudinal muscles, 11m and ulm,
that are continuous the length of the gill. Each plate is hollow
and may be likened to a loosely folded sheet of paper, leaving
a space between ; -vfoich space is traversed by a few fibers of
connective tissue. Ihe unshaded line, y, figure 19, represents
1/fliere the wall has been cut in separating the plate from the
plate lying in front of it. liie opposite wall of the plate is
continuous, in a like manner, with the wall of the plate lying
behind. Along each side of the ventral portion of each plate.

22.

lying just inside, and closely applied, to the wall, ia a ciiittaous
rod or plate, figure, 19, cr.. These rods are much heavier near
the median line^become thinner laterally, and finally disappear.
FurtZaermore the chittnoiis rod on each side of each plate is con-
tinuous with the chittnous rod of the opposing side of the plate
adjoining. Each rod, might then be described as hairpin shaped,
with two prongs, each of which extends along the innery,of the wall
of one side of one plate. Each plate receives one prong from
each of two hairpin shaped rods, figure 20 cr.

Many of the muscle fibers of the suspensory membrane
are continued dOT/n along the inner surface of the anterior wall
of each plate, fig. 19 rap. These were supposed by Mitsukuri C^^)
to be chitenous rods but they were later sliown by Kellogg ('*)to be
muscles. They start from the suspensory membrane as bundles of
fibres, which branch and spread out as they descend. into a fan
•ehftpe-. Manj'- of the fibers apparently find insertion in the wall
of the plate itself, but not a few are extended to the chitenous
rods and are inserted over thfir s^arfaces. Kellogg, no doubt
rightly, oonnootoi movements of the plates, which are common in the
gills of mutilated specimens, and in the excised gills themselves,
to these muscles.

It can be seen in young specimens, where the gills can

23.

be watcJied tlirougli the transparent shell, that these movements
are not cornmon. The norrnal function of these m-uscles mil be
made apparent later, in connection with the special function of
the gills.

Each plate bears cilia along its ventral margin, and for
a short distance along each side, but not over its general surface*

The cilia between the plates are arranged in lines,
one or two lines on each side of each plate. Near the ends of
the chittnous rods, the cilia on the edges of the plates disappear,
and those between the plates becorne modified, as described by
Kellogg for holding the plates together. They do not seem
to be hooked, but are very numerous, rather stout, and ^fullV inter-
mingle with the cluster on the adjoining plate. The area covered
by these cilia is ratlxer limited, and above them the plates are not
ciliated. The cilia of the gills seem to drive water between the
plates and aid in lowering the gills.

Each gill has two, rather large bundles of muscles run-
ning its entire length, one above, figure 19, ulm, and one below,
11m. The lower bundle is somewhat crescent shaped in cross sec-
tion, the concave surface being directed upward and forming part
-f of the ventral wall of a large blood space, lbs, tiiat is also

continuous the length of the gills. These longitudinal muscles

• ( ^ 1 )
were supposed by Mits"akurt , "v^iose material was poorly preserv-

24

ed, to be fibrous tissue, but there seems to be no doubt that
they are m^^scles. Their contraction cause the gills to sliorten,
a coiTimon movement, already described by Kellogg ^'^), and a move-
ment necessarily of ser^/ice in their special function.

The cavities of the plates on the two sides of each gill
open ^1 ly into each other, and the cavities of all of the plates
are put into communication through the blood space, lbs, ^Afcich
is continued upward between the plates as a narrow slit reaching
to the upper longitudinal muscle. Above this muscle, at the
base of the suspensory membrane, is another blood apace, ubs, also
running the length of the gills^t separated from the blood spaces
between the muscular walls of the suspensorj?- mejubrane only by oc-
casional strands of connective tissue. This space comnuinicates

,^^ti- est^^-rCtu^ a-^ (11)

laterally with.each of the gill plates OTavitios. Mitsul-iuri

supposed that these blood channels^ were connected with some defi-

nite blood circulation (in ITucula^

In Yoldia there seems to be no evidence that the blood
follows a very definite course. The continuovis longitudinal
spaces seem to be the means of equalizing pressure throughout the
gills, by allowing a free movement of the blood to different
parts as circijmstances demand.

ShOTild the blood be confined in any part of the gills
when pressure is applied, the walls of the plates would probably

"',■.:? "a i-:-"'j- i\

■-■t :fv-;;.

V ; «

25.

be ruptured, but with these continuous oa-vities the pressure must
necessarily remain nearly equal in all the plates.

Before bringing the discussion of the structure of the
gills to a close, reference will be made to Kellogg's opinion

that the plates of each gill are alternate aiid not opposite.

His opinion is based on horizontal sections taken
below the upper longitudinal muscle, in which position the inner
edges of the plates are alternate. This seems to be due to bend-
ing caused by the pressure of blood in them. Horizontal sections
through the bases of the gills, or Just beneath the upper longitud-
inal muscle, as well as whole specimens^ show the plates to be op-
posite as a rule.

It will be remanbered that the gills are suspended by
muscular membranes, which probabljr represent folds of the body
wall. They are composed of wide plates that are sufficiently
V7ide to span the space between the foot and the mantle and, behind
the foot, unitedl^r to span the Tnan tie- chamber itself. Anteriorly
they gradually diminish in size and finally disappear. Poster-
iorly they are attached to the wall that separates the inhalent
apid exhalent siphon. The mantle chamber is thus divided by a
movable partition, into a ventral chamber, open.i'ng througli tlie
inhalent siphon and a dorsal chamber, opening through the exhal-

•.^ * ,^1-V

.:;-^ \\-.

26.

ent siphon.

In young speciemns the niovenonts of the browi gills are
visible through the shell. They are gradually pressed ventrally,
probably by the blood forced into them, possibly aided by the
powerful cilia, which, as Kellogg has pointed out, are so

powerfiil in their action, as frequently to cause excised gills to
move about in the water. As the gil is descend the water passes
between the plates. After reaching their greatest ventral de-
pression the gills may remain quiet for a longer or s}iorter tine.
Then comes a quick contraction of the suspensory mcanbranes, ac-
companied by a vigorous discharge of water through the exhalent
siphon, as the chamber dorsal to the gills diminishes, and a
corresponding influx of water through the inhalent siphon, as the
chamber ventral to the gills enlarges. During this exchange,
the siphons are made quite rigid, probably by having blood forced
into them. The movements of the siphons acco^npa-ying the move-
ments of the gills are very conspicuous and have been mentioned

(1)
by Brooks ' .

The movements of the gills are more or less rliythnic,

the time varying with the needs of the animal. Thus the rhythm

is much faster after a disturbance that has caused all movements

to be suspended for sometime, than when, no such disturbance has

occurred.

"•(*■::

/ • . • .-i -■-

.. 'I

/C:.::

27.

The currents of water are probably for respirator7 pur-
poses, 830 d to keep the 'nantle-chanber clear of the dirt that is
constantly finding its way in, r^ore especially the faeces, Aifiiich,
being so largely composed of sand and other hea^^?- particles,
would otherxvise drop into, and soon clog, this chamber.

It seems to be a ju.stifiable siiggestion that the con-
tract i02i of the gills may aid in the movements of blood as well
as/. water.

The blood spaces of the plates, and especially of the
suspensory raembran.es are considerably diminished in size during
each contraction and, as the openings where the spaces of the
suspensory membranes communicate with the auricles, are guarded
by ridges that probably act as valves, each contraction must modi-
fy the blood supply.

A little study will show how admirably the gills are
fitted for the function of pumping water.

In shape they exactly fit the ms-ntle chamber, in which
they form a moveable partition. Contact is insured by the pres-
sure of the blood inside the plates, an.d by the soft dorsal pro-
jections. These projections must act much like the leather on
the plunger of a suction pump, making good contact when there is

pressure from above but not hindering its descent.

Ao the .QJ-llg lio at ro-st- before thre- •«©*»*^:'«;e%4.-o*i

'v?.l- :>

itr,.-

23

^/^ ^^f<^^^ ^^^-^ .^/^-'i--^

-eu95'«ise«:?->^aei^»xaa*««,.tlie plates are loosely opposed and alloTisr
vTater to pass freely between them. In tliis position the gills
eire eiucIi longer than during contraction, an.d lie in a:' curve*,
figure 3, g, "When the suspensory membranes contract, the longi-
tudinal muscles, figure 19, ulm, and 11m, also contract, the gills
are shortened, and the plates more closely opposed.

Sach plate is rendered rather rigid centrally chit-mous
rods that are so disposed as to allow flexibility ; and many
of the muscle fibers of the plate are attached to these rods in
such a way as to insure an even strain and keep the plate from
bending ventral ly when pressure is applied.

E3cperiments were tried to determine, if possible, the
part taken by the gills in the collection of food. For this
puipose both mutilated and uninjured specimens were used. No

definite resvilts were reached, but in no case was Kelloa^s obser-

(7)

vat ion , that the gills are extremely active food collectors,

confirmed. Considering the remarkable activity of the palps as
collectors of food, such activity for the gills seems rather
unnecessary, an.d it would also seem that the p-uinping action of the
gills would seriously interfere with their normally performing
such a function.

. ■ -f

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o.

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29.

EXCEETOKI ORG^TS.
Figures 4-. and 23.
In tlie adult aninal the excretory organs are very exten-
sive. They consist of many loops, the walls of -vfliich are mucli
sacc^^lated. They lie ventral to the pericardiim arid are con-

tinued into the foot along the posterior side of the stomach,
figure 4, ex. ^Ji^ .^^.^.^.^.^^ , ^;^:^-^«:^3.^^=^U^ , .^^c^-^U^^ ^ ^^^.^

Figure 23 is a drawing of a wax inodel of the 'ends of
the left excretorjr orgaa and genital duct, seen from the -^rentral
side.

The inner end, i- e, opens into the pericardium -ef
■^ife^eh a-Bortion of— the-w-al4 , p c, is shown, by a rather large open-
ing, turns abruptly towards the middle line of the body, crosses
the outer end, o'~^e, and is continued anteriorly close to the
pericardium. After passing through the many sacculated loops
already mentioned, the tube again comes near the pericardial ^irall,
enlarges, runs alongside the anterior i^um of the inner end, nar-
rows down again and opens into the mantle chamber through an
an antero-posteriorly elongated opening, mco, just posterior to
the point where the outer and inner ends of the tube cross.

.H"ir«>.*u^

•J r«f:

'J.0

30.

The genital duct, gd, following the course of the cerebro-
visceral commissare, turns ventral I7 wlxen almost in contact with,
the inner end of the excretory organ, meets the outer end and opens
With IX,, The common opening of the excretory opening and genital
duct is, as has been noticed, elongated in an antero-posterior di-
rection, figure 65, mco, and may represent a fusion of the two
rather than an opening of one Lnto the other.

As has been seen, the condition described by Pelseneer^ '
viz : the genital duct opening into the excretory organ near
its pericardial opening, is not borne out by these obser\rations.

In material that is not well preserved, it not unfre-
quently happens that the walls of the genital duct and the inner
end of the excx-otory organ., are ruptured in sectioning, at the
point where they come in contact, thus placing th.eir cavities in
c oitcnun i c a t i on .

It may be well to state in passing that the genital ducts
of Nucula proxima open into or with the outer end of the excretory
organs in a manner similar to that Just described for Yoldia
1 imatula.

GENITAL GLAiroS.
These, when distended with th.eir products are very ex-
tensive. They pusQi in between the kidneys, below the pericardium^

31.

between the stomacla and intestine nearly to tlie pedal ganglia,
and surroiind tlie digestive glands as well as dip in between their
lobes. The opening of the genital duct, figure 23, gd, has al-
ready been described in connection with the excretory organ.

The sexes are separate aiid, when the genital products
are abundant, they may easily be distinguished by the color of
the genital mass surrounding the digestive glands. This portion
is chocolate brown in fe^nales and yellow in males.

NERVOUS SYSTEM.
Figures 4 and 5.
The cerebral ganglia fig. 4, eg. united in front of the
oesophagus by a very broad commissure, which differs very little
in structure from the ganglia themselves, lie just posterior to
the ventral side of the anterior adductor raascle. Each ganglion
is almost circular in transverse section and gradually tapers
posteriorly into the cerebro-visceral commissure. So gradually
does this tapering take place, that it is impossible to deterroine
where the ganglion ends and the commissure begins. Even in struct-
ure there is onl3'- a difference in degree, there being proportional-
ly more nuclei in the ganglion than in the commissure. Posterior
ly each commissure gives place, in the same gradual way, to a visce3
al ganglion. The visceral ganglia, vg, lie anterior to the

'■■ ^^h

CO

xr.v.no IV-'

. ' ©/■ .1

ac

V .1 ...

■x . ;;( J

Tr.C .:■■

r _>».-.-) •-,;j _>

32.

ventral side of tlie posterior adductor muscle, and almost equal
tlie cerebral ganglia in size. Tlie commissure connecting these
ganglia is also verv broad an.d, like tlio cerebral commissure,
tends to share the structure of ganglia.

The pedal ganglia, pg, are roxtnded bodies and are ap-
parently larger than either the cerebral or Tisceral, Thev lie
very close together and are connected by a very broad commissure.

The cerebro-pedal, ^onlike the cerebro -vise oral commiss-ores
are not surrounded by nuclei and show ordinary commissural struct-
ure.

Although a great many specimens of widely differing ages
have been examined, the double origin of each cerebro-pedal com-
missure from the cerebral ganglia, has not been observed. Neither
is a separation into cerebral and pleural ganglia evident. It
is true that the cerebral ganglia are not quite smooth and, in
some places, the outer layer of nuc'lei dips into the fibrous layer,
but this seems to be accounted for by the origin of nerves in
the immediate vicinity. The only place that can be looked upon
as a constriction, lies between the origin of the large anterior
pallial nerve, on the outer dorsal surface o C the ganglion, and
the origins of the cerebro-pedal comraissure, the large palpal
nerve, and a small f-oot maoolo nerve^. all of which leave the 4'.c^<-^«-v^

i.''r>-

<^f.x

■ <;/.. lys-^

r.!/ o . 1 f

V ^.D . Y.^i: ■■:.:•

<-v

[<'<^''Oii '3fcc;va

83.

together, on the inner, near tlie -ventral side, of the ganglion.
The origin of the corebro-pedal conunissure is entirely anterior
to tliis depression. The bulgings at the bases of the ner^re seem
sufficient to account for the irregularity.

Each cerebral ganglion giv^es rise to several nerves.
A large herve passes forward, under the anterior adductor muscle,
branches^and supplies the anterior portion of the corresponding
inantl© lobe. Another large ner^.'-e passes dovm to the dorsal mar-
gins of the palps, follows them posteriorly and is finally continued
the whole length of the palp appendage. Of the smaller no37ves,
one supplies the anterior adductor muscle and another passes to
the body wal 1 .

Each visceral ganglion gives rise anteriorly to a small
osphradial ner^.'-e, and posteriorly to a nerve that branches almost
immediately. Sometimes these branches originate as separate
nerves. One of them sends a small neirve to the posterior ad-
ductor muscle and another belo?/ the adductor muscle to the postero-
dorsal portion of the man.tle. The main nerve bends ventral ly and
is distributed to the siphons. The other nex^re passed ventrally,
Reside the ner^;-e just described, sends a large noirve to the siphon—
al tentacle^, on the side where this tentacle is present, and is

contiraued to supply the postero-ventral margin of the mantle.

.-.In.

34.

Several branches are given to t'ne posterior extension of the
mantle.

Each pedal ganglion gives rise to a nerve that runs
direct^ to the byssal gland, and to six or nore ner^.'^s that supply
the muscles of the foot. Frequently one neirre and sometimes sev-
eral ner^^es may run up the cerebro-pedal commissure for some dis-
tance before iss'aing to be distributed to the muscles of the foot.

The nerve that supplies the otocyst issues from the
cerebro-pedal commissure but^ judging from the direction of the
fibres in the commissure, probably has its origin in the cerebral
ganglion.

OTOCYSTS.
Figixres 4-., 21 and 22.

The otocysts are rather large and, as Kellogg ' has
pointed out, each contains a single large otolith, whicia plainly
■< shows concentric structure. The epithelial cells of the otocysts
carry rather long arid slender cilia. In adult- specimens there
IS a well defined canal connected with each otocyst. Pelseneer v"' J
describes this canal as opening at the surface of the foot and
gives a figure indicating the position of its opening.

I have found that, by the careful dissection of speci-
mens preserved in formaline, the cerebro- pedal coirmissures, with
the otocysts, otocystic ner^res, and otocystic canals, all held

35.

together by connecti-"-e tissue, can be removed. Study of these
preparations, of Mtr.icli I have :made nearly 30, shoW the usual
form of tlie otocvstic canals to be cylindrical and of a uniform
size for aboiit three-fourths the length of the otocystic nerves,
figure 21; ot, at 7/hich point theire is nearly always a swelling,
beyond which there continues a thin strand of fibrous tissue,
probably connective tissue. At the distal end of the swollen por-
tion of each canal there is generally, probably always, a wall
that can easily be seen in dissected preparations, and has been
verified in sections. This wall is the rounded end of the
otocystic canal.

Sometimes one or more little closed patch eg are found,
lying in the strand of tissue that is continued on past the swol-
len portion of the canal ^

^ Two such pouches are shown in figure 22, which repre-
sents the distal swollen portion of a canal and part of tlie con-
tinuing strand of tissue. These pouclies, when foimd, shov^ the
same structure that is shoiTO. by the canals, and like them are
lined with niLmerous verj?- long cilia.

It seems to me that these walls and pouches sh-ovr con-
clusively that in each of the several cases tiie whole canal has
been removed send, in this species, that the canals do not reach
the surface of the foot.

86.

In Nucula proxima there is a strand of tissue passing

from eacli otocj^st to the surface of the foot. This corresponds

(15)
in position to the canal described by Polseneer for Nucula

mus/culxis, and is probably an open canal, but I have not had tine

to demonstrate this point.

In Yoldia limatula, the strand of tissue, with the
series of pouches which it sorae times encloses, seems to indicate
that these canals have undergone^ and perhaps are still undergoing
degeneration.

The otocystic nerves lie alongside the canals and seem
to send fibers to their walls. In fact it is difficult to demon-
strate that the ner^res reach the otocj'-sts other tlian through, the
walls of the canals.

OSPHRADIUM.

On the inner side of each suspens or;;'- membrane of the
gills, lying just beneath the visceral ganglion of the corres-
ponding side, is a rather large patch of modified epithelium
which probably represents an osphradixtm.

This epithelium is s^applied hj a rather large nerve,
figure 4, on, that leaves the corresponding visceral ganglion
protty well forward* (h*^^-^ "-^ ^:x^..^^^^<^^ ^^.-^ .

:'i .1

37.

CIECULATORY SYSTEIl.

The heart, figure 4, h.^ is situated Just posterior to the
beaks of the shell, and verv near the dorsal margin. The cavity
of each auricle is connected with the blood spaces of the corres-
ponding gill, through a slit in a muscular partition that pro-
jects dorsal I7 into the cavity of the auricle. The arrangement
of this partition is such as to tend to stop the backward flow of
blood into the gills. The ventricle is perforated bv the in-
testine. "Where the ventricle joins each auricle, there is a
marked constriction and, internally, a muscular septum perforated
by a single opening. When the ventricle contracts, this septum
probably contracts and more or less completely closes the opening
in it.

Two blood vessels leave the ventricle, one anteriorly
on the left side of the intestine, the other posteriorly, beneath
the intestine.

EMBRYOLOGY.
The eggs of Y. limatule, are abo\it .15 m m. in diameter,
of a chocolate brown color and very opaque. They are laid f r&e
in the water and are not encumbered by any kind of envelope. The
polar bodies are lost soon after they are formed. The first
cleavage, which tastes place about two hours after the eggs are

38.

fertilizecl, results in tiae formation of sub equal blastomeres,
figure 24.

In the next cleavage, figure 25, tlie smaller blastomere
divides equally and the larger unequally. The eight celled
stage is formed by a cleavage at right angles to the two preced-
ing planes of cleavage, and the two sets of foiir cells each come
to lie in the position indicated by figure 26, the last division
being in the plane of the paper.

One of these cells is considerably larger than the
other seven and, with the beginning of the next cleavage, begins
to be s«6i©:wfeat— ^u-s^so^adreii by Other cells, figure 27. The small
cell formed by the unequal division of this large cell, figure 28,
becomes a surface coll. What becomes of similar cells in later
divisions has not been determined, but, from sections, it appears
probable that the next small cell cut from the large cell, figure
29, crowds in over' the tip of the large cell and divides into
two, figure 37.

Aboiit this time the outer cells, or at least part of
them, acquire cilia, figure 30, and the embryo rolls around on
the bottom of the dish. It finally rises to the surface an.d
swims freely.

Just what internal changes take place during this in-
terval is rather hard to determine. Surface cells in the region

39.
of tlie blastopo3:'e become especially active, dividing and crowding

in arouad tlie large internal cell, wlLidi novr divides into nearly

equal cells, figure 39.

Most of tiie outer cells now take on a decidedly differ-
ent appearance from tiie internal cells.

There is a rather densely staining layer of protoplasm
against the outer -wall of each cell, but the remainder of the cell
is occupied by vacuolated protoplasm. These vacuoles become
larger -with age.

Beside the vaucolated cells, there is a group of cells,
figures 38 and 39 eg, and figure 40, ap, which are largely sur-
face colls and later give rise to the cerebral ganglia and the
apical plate. The apical plate, figure 40, Ifcp, is already dis-
t ingui shab 1 e .

"Whether there is a general migration of surface cells
into the interior is doubtful, but it is certain that their num-
ber is reduced. The reduction iS probably not more than is
accounted for by the pushing in of those around the blastopore,
and the setting aside of the group from which the cerebral
ganglia and apical plate arise.

The embryo now elongates, figure 31, apical plate ac-
quires a tuft of apical cilia, ac, and the oiiter, vacuolated
cells, which will hereafter be referred to as test cells, arrange

4o;

tliemsel-ves in^ five rovrs that surround the embrvo. Frequently
the fifth, last, row of test cells, is not complete on the dorsal
side at this stage but this does not long remain so. Occasional-
Iv a si^cth more or less complete row is present. The blastopore
and the apical plate occupy nearly opposite ends of the embryo,
but for some time the blastopore may be twisted around somewhat
to one side. The position of the group of cells from which the
cerebral ganglia are formed, is indicated in tZae external view of
the embryo by a depression, x, where the colls of the first, or
anterior row of test cells, are pushed away from the second row.
As later stages show this to be the ventral side of the embryo,
it is easy to determine that the blastopore is always twisted
tovmrd the same, that is, the dorsal side.

The test cells are distinctly rounded and the cilia on
each of the three intermediate rows are collected into a band.
The embryo now swims in more or less definite lines, rotating
the while upon its longitudinal axis. The rotation is q'aite
rapid but is not always in the same direction, there being fre-
quent changes. During activity the apical cilia are b-onched
together into a sort of whip, which lashes vigorously from side
to side, and always precedes the embryo in its movements.

Inside the test the cells in the region of the blasto-
pore are still activel:'- undergoing division and are quite small.

0 ■ !.

4-''

figure 41,

The cells of tiie apical plate, ap, and those from which,
the cerebral ganglia arise, &g, are separated externally by test
cells, but internally are, and continue to be, connectel by cells
that do not take part in the formation of other organs.

The other internal cells have increased in number, and
their origin can no longer be determned. At a little later
stage, figure 42, the dorsal internal cells are arranged in a lay-
er, sg, tZaat represents the beginning of the formation of the
shell gland. Other cells soon arrange themselves over the re-
mainder of the surface inside the test and, together with the
shell gland, form a new ectoderm. This ectoderm probably is
formed from cells originally surface cells. If this view is
true, the original surface cells give rise to at least three
groupfi : those tjiat remain at the surface, acquire cilia, and
become differentiated as test cells ; those that form the cere-
bral ganglia and apical plate ; and those that form the new ec-
toderm inside the test.

In the stage of which figure 42 represents a section,
a space, seems always to be present in about the same position.

This space probably represents the beginning of the liimen

of the ifiidgut •

-.0 ."sr.

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f» .:>- ■.

>•< ■:*; ,:\; vrV ;i»<r;)ri'^ ^.yy ^ fii p

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42..

The embryo continues to elongate, figure 32, becomes
cylindrical, and the test cells flatten and becone less distinct
from one another. Between about the 32nd hour and about the
70th hour, tliere is no appreciable change in the external appear-
ance of the onbryo and, as the test cells are quite opaqiie, few
internal changes can be followed in living specimens.

The test, iiibich is now fully formed, is coiirposod of
large vacuolated cells, figure 44, t, the nuclei of which are al-
most, if not quite, in contact with the inner walls. Just be-
neath each band of cilia the protoplasm stains very deeply. Just
inside the test, between it and tiie new ectodei^, are, frequently,
scattered nuclei, lying in a very thin film of protoplasm. Their
significance is not known.

The shell gland becomes more definite, the lumen of the
midgu':, figure 43, mg, becomes surrounded by a definite wall of
rather large cells, and-imrraginat ion, std, extends into the mass
of cells from the ventral side of the blastopore. Thtis invagina-
tion is the beginning of what I have called ^-) the ventral tube,
but, as it has since been found that it is foimed as an ectodermal
invagination that does not at first communicate with the midgut
there seems to be no reason to give it a special name, and it
will hereafter be referred to as the stomodaeum,^

•:;ri,^

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43,

The stomodaeura elorxgates and soon establislies communica-
tion witli the inidg"at, fig-ores 40, and 48. It lies along the
ventral side of the enhvyo, just inside the test, and is joined,
but is not enclosed by^the layer of ectoderm, figure 49, std.
The shell glaiid beco-ines slightly invaginated, fig-ares 44, and 45,
sg, but it very soon arches Tipward preparatorjr to protruding
laterally to form the mantle, figure 49. The midget becones

prolonged posteriorly, and the mass of cells from ■t'tiich the cere-
bral ganglia form arch in-ward and form two poxiches that open to
the surface. The poviches lie side by side, f ignore 46, and gen-
erally open to the exterior together. Sometimes, however, they
open between different test cells, figtxre 47, and seem to be only
slightly connected with each other. In other cases only one
of the poiiches opens to the exterior and this opening may be
small. In some cases it may not be present at all.

The walls of the pouches are rather thick but each is
composed of a single layer of cells.

The shell gland folds ox^t at the edges and most of the
dorsal cells assume the character of mantle cells. Only those
near the edges remain especially large and glandular. This is
accompanied by the ectodermal thickenings that form the pedal
ganglia, figure 59 pg, and the invaginations that form the

U. Vi

t iV..

i '. .}. . vi;

ni''.

=■: -^ ;.•■§••

.-•J'lp ■••a ■.:!■•••■)■. •^::;v- 'XH-'^i

f -'■'■''. rC'^^-XS

I .\ V^>, ;

■.n ■*'■'■: f;-'.;;;

44.

otocj'-sts, figure 56, ot. The ne:ct stage shows the otocj^sts ap-
parently completely closed off, lying yer:;^ close to the clefts
that separate the mantle from t2ie body wall.

The formation of the mantle and shell is accompanied
by a slight lateral com^jression and a corresponding dorso-ventral
widening of the embryo. The test cells become yery much thinner
and the vacuoles, nearly or quite disappear, but tlie cilia re-
main as described and tiie moA'-ements of swinmiiig are not altered.
Several new organs ma^e their appearance.

Cleared "iiS^ole motmts show some of the internal organs,
but throughout it is necessary to resort to reconstructions from
the study of serial sections, to get the relationship of the dif-
ferent parts.

The body of the embrj'-o now lies between the shell valves
and these are enclosed hx the test. The foot, figures 33, 55 ,
and 61, f, is formed by a rapid increase of cells between the mid-
gut and the stomodaeum. The anterior adductor muscle, figures
33, 55, and 56, aa, is formed at a corresponding time (about 85
hours) and consists of a few muscle fibers which stretch from^
one shell valve to the other. It is left, by the gro7rth of the
embryo, separated from the rest of the bods'-, except at its ends,
which penetrate the majitle, and are attached to the shell. The

' ■; '"y( •'.'"■.'■)

\i,---.'^-..

l\..\>-:''\-y,'!i-:.^.-i

X ,...7

I'V; :-ivr

r,-

45.

posterior adductor muscle, figures 33 , aiid 55, pa, does not ap-
pear until some liours later. It is always siirrounded by other
tissue. The digestive glands, figures 33, 62, and 63, are formed
as tvaginations from tiie anterior end of tiie midg^at, vrliicli is nov^
extended posteriorly nearly to the blastopore. The visceral
ganglia appear as groups of nuclei, the position of which suggests
ectodermal origin. The cerebral poxiches have moved some distance
dorsally and posteriorly into the interior of the embryo, and are
situated at the end of a single pouch, figures 33 and 55, r, which
opens to the exterior between the test cells at the point where
the pouches were originally formed. The paired pouches are thus
brought to lie in the interior, as diverticula of a single elon-
gated pouch. As they are carried into the interior, the cells
f03nning their walls divide rapidly, push in, and fill up their
cavities, and a commissure is foimed connecting the tv^o masses
of colls. The relation of the unpaired pouch to the cerebral
ganglia has been 3hO¥/n by tjiree succeeding transverse sections of
the same embryo. Figure 53 shows the external opening of the
un-paired poiich, figure 59 shows the anterior portion of the cere-
bral ganglia, eg, with the laterally compressed unpaired pouch,

r
§, lying beneath them, and figure 60 shows the commissure con-

necting the ganglia.

;l.^f^''

■J L'

.-ii:-;-?.

,\'.- ...

, -r)- .' .

.,•/:•■■ j.i ....•;) "Ov

I.ry '

',• i-

i^--"^'

46.

Having described the for!:aatioii of the various organs
atteiition will be given to tlie ei3ibr;;-o*s appcara;ice.

I^irtlier than tlie lateral co:tipressiori and th.e correspond-
ing dors o -ventral widening; tliere lias been little change in the
embryo's external appearance, but the internal appearance is
greatly altered.

Inside the test a shell has heeri formed, the two valves
of which gape ventral I7. The anterior adductor inuscle, figure 33,
aa, is quite large and, being isolated from tiae general niass of
the body is very conspicuous. The posterior adductor rmiscle,
pa, as yet consists of very few fibers and is enclosed in other
tissue. It is not easily seen in Tidiole mounts. The apical
plate ap, is connected with the ventral end of the group of
cells that form the wall of the unpaired pouch, r. This groiip of
cells f orraS a distinct stalk w^iich runs dorsally and posteriorly
to connect with the anterior part of the body. The midgut, ng,
and the stomodaeun, std, are widely separated by the foot, f.
The stomodaeum opens to the e:coerior through the blastopore, bl,
and the posterior end of the midgut almost reaches it but does
not open. #

# Observations since my publication in the Johns Hopkins Uni-
versity Circular (-) have not revealed the actiial opening,
up to the tine the test is cast away.

r - '-,

. «'■) • jrOf." ■'■xV.-'ii :.J

\(i :}-:f^-

^^::G^ -:-.:'

:C.v'1

s'lji- vA;:,i>.t,

'"^Bi--

47,

The stomodaeiim is ofte?a attached to the anterior, a;o.d
to part of the ventral margins, of the foot, but its posterior
end, figure 61, std, lies free between the mantle lobes, some-
times almost, if not quite, in contact with the test, sometimes
raised well abore it. The anterior end of the midgut is enlarg-
ed to form the stomach, which corimunicates with the liver-pouches.
Most of the tissue dorsal to the intestine has beer, \ised in form-
ing the mantle, which is now separated from it by a large space.
This space extends to, and around, the anterior adductor tmiscle
anteriorly, and nearly to the margin of the shell posteriorly.
The cerebral, pedal, and visceral ganglia, with their c ommis spires ,
have been formed. The otocysts, figure 61, ot, have apparently
been entirely closed off, and each contains an otolith, which
stains deeply witli haematoxj'-lin and, a little later, plainly
shows concentric structure.

As the otocysts have nearer been opai to the exter'ior,
development having taken place inside a closed test, the otolith's
cannot be foreiga particles.

The only places where the outer test is attached to the
body of the embryo lying inside it: are, around the blastopore,
the apical plate, and the opening of the unpaired pouch, figure
33, r. The embryo still swims rapidly but has periods of rest.

cr.-f.

.Ir) .= ■

."■. O'?'

:h\ :mv -F'l

•VT^C:

.-.(^ .V

S'i: (, *;

48^

Tills condition of the enbr^ro is reached in from 90 to
120 hotirs. It now stops swinmlng, settles to tlie bottom, the
apical cilia shrivel, and the test cells break apart and freqiient-
ly go to pieces at once. The process of casting the test gener-
ally begins with the posterior row of cells and woiks forward.
It generally takes but a very few minutes. Casting includes,
beside the test, the stalk which extends from it to the cerebral
ganglia, the apical plate and its connection, and the stomodae^im
from the blastopore to the position of the definitive mouth.
These parts, together with a few adhering test cells, that are no
longer stretched flat but have rounded \ip, raaj not sever tlieir
connection for a half hour or more, figure 34, but they finally
break off Just belovr the cerebral ganglia, and casting is com-
plete.

TJntil about the tine that the test is cast, the shell
valves remain gaping ventral ly and the tissues of the body ex-
tend completely across' from valve to valve. ItVlien the sliel 1. is
finally closed by the contraction of the adductor muscles, there
is no longer sufficient space for t2"ie organs in tlieir original
position. The principal change^ of importance due to this crowd-
ing, takes place in the pouches of the digestive glands, which
are forced into the space dorsal to the alimentary canal, and, in

I(; v.i. '

49.

all of tiie specinens examined, seem to imdergo ohan/^es that result
in tlie formation of large cells, figures 35, 64, and 65, Z, that
are not arranged in any definite manner.

It was thouglit possible that tJais appearance was due to
poor preservation, an explanation that -veirj naturally suggests
itself, but the large cells appear in living as well as preser^red
specinens. Reformation of the liver pouches is accompanied by
the disappearance of these cells.

The anterior adductor muscle soon becomes enclosed lij
the forward growth of the bods'". For a few hotii's a space may re-
main between it and tl-ie cerebral ganglia but this soon disappears.
The foot, which at first is capable of onlj'- feeble movements,
grows quite rapidly, and is soon ver;'- active. Locomotion is
chiefly effected by the means of long and powerful cilia, ^.ich
are arranged along the sides of the foot, figures 35, and 36, and
are very active whenever the foot is tiirust oixt of the shell.

These enable the animal to glide over the surface on
which it lies, but do not enable it to rise. For some hours
after the test is cast, the foot shoiws no indication of the modi-
fication which is later generally knovjn as the sole, figure 52.
The first indication of this modification is a groove vfiiich ap-
pears along the middle line of the ventral surface of the foot.

,i \-> "^S K^'<?'^

i... VI J

,: (-.

>-Ci ■:■-,,'■

50,

fig-aro 53. This groove deepens, apparently by the groTTtl-i of tlio
parts on each, side, figure 54, and gives rise to a "sole" resembl-
ing that of the adult, figures 2, and 9, The novenents of the
foot no^'T closely reseiible those of the adtilt, but locomotion is
still materially aided by the poTT'erf'j.l cilia along its sides.

The formation of the "sole" is accompanied by the growth
of the poster© -ventral portion of the foot into a more and more
prominent portion resembling a heel. The use of this outgrovrbh
is not evident. It remains in the adalt as a small projection,
just behind the opening of the jbyssal gland ; an organ that makes
its appearance about a day after the test is cast, but does not
seem to secrete threads.

About the tim.e that the test is cast away, a thickening
makes its appearance on the inside of each mantle lobe, near its
posterior border, figures 34 and 35 g. •

This thickening, the rudiment of the gill, accjuires
cilia and grows anteriorly so as to project over the mantloi-sur-
face. A constriction appears near the middle of each gill,
figure 36, g, deepens, and divides it into almost equal lobes.
Beneath its epitlieliiim, following the curve of its antei-ior ra8.r-

«

gin, is a chit*nous bar, the two ends of //hich lie near the ends
of the lobes, figure 36, cr.

ccJ .)'Lr

f\v> > ;l

):i

.■>.'■: :^rj:ji'-^:

51,

Figures 50 and 51 represent longitudinal sections of a
gill, taken in tlie pla^ies indicated in figure 36. Tlie cliitfnous
bar is cut in one, but tlie plane of the other lies below it.
These sections shov/ the gill to be rather evenly ciliated. Later
stages, after the lobes have elongated, show the cilia to be con-
fined to t}ie side next to the mantle. The third lobe is formed
from the ventral lobe, which broadens and then constricts in the
manner detailed above. Other lobes are foimied in a similar man-
ner.

The alimentary canal elongates, bends ventral ly, and
the stomach enlarges. In from two to four days after the test
is cast, tiie liver poviches reappear. At first they do not jiave
very definite walls, but they gradually become well defined, and
it is noticeable that the large scattered cells disappear at a
corresponding time. The two po'aches are no longer symmetrical,
the left being prolonged posteriorly, fills up the space between
the dorsal end of the stomach and the intestine, figures 36 and
66. The greater development of the left liver lobe at this early
date, ver^'- likely affects the futiore position of the intestine,
a loop of A-^^ich comes to lie on the right side of the animal, very
near the shell.

Abovit the 25th day, a ridge, corresponding in position

■ J

:r;.?M',j. .:.;r

.ii •-•>;..

52.

to the otiter labial palps, appears as an outgrovrth from the body
wall. The palps have not attained any considerable development
in any of the embryos reai'ed from eggs.

The gajiglia are all distinct in T/ifliole mounts as well
as in sections. At first the cerebral ganglia are much larger
than either of the others, figure 33, but the pedal ganglia grow
rapidly, and soon come to equal if not to exceed them in size,
figure 36. No division into cerebral and pleural ganglia has
been observed.

GENSRAL aSMASKS.
ANATOMY..
Since Mitsukuri's ^^'^j paper appeared in October 1381,

much has been ivritten on the anatomy, and itc oignifleaaoO', of

( 1 5)
members of this group. Pelseneer ^ has called attention to

certain characters, in support of the supposition that the group

is primitive. Possibly influenced by tZ-ie desire to make the case

as strong as possible, he has allowed some errors to creep in.

Thus he says on page 272 :

" The least specialized 1 ame 1 1 ibran chs : In demonstrate

ing the ph3'-i ©genetic development of the gills, part 2, IV, 6,

p. 246, I have shown that the most primitive of t}ies<i organs are

53.

found in tlie Nuculidae an.d Solenomyidae. On tne otlier Iiand sev-
eral recent autliors have considered eitlier Avicula, or Melea.irrina
and Area, as representing the most ancient forms of this group.
But, it is bo be noticed that Avicula and Melea.frrina
have already lost their anterior addiictor (this in itself is not
a sign of great antiquity) and further that these free forms have
their gills more specializ.ed than the Nuculidae and Solenomyidae.
It is not only by tiie appearance of their respiratory apparatus,
but by the existence of all the following common characters,
that are not found in otlier Lamellibranchs (include here Avicula.
Melea^^rina and Area) that these t^ro farailies are undebateably
shovm to be the most archaic of ail :

1. ^ypobranehial gland situated dorsal ly, between the
mantle and the gills.

2. Foot has a ventral "creeping" disc (not existing
in others except Pe^tunculus) .

3. Gills tCkx posterior, have free, flat, filaments,
without reflected lamellae, analagous to the bipectonate gills of
Amphineura, Cephalopoda, Bliipidoglossus, and Euthyneura.

4. Heart posterior, has auricles 5-n contact with the
gills only by their anterior extranities and not along their ^^tlole

axes .

54:.

5. Absence of a posterior aorta.

6. Kidjieys glandular tlirouf^^oiit their extent, 'vvithout
internal partitions, or coTnmuni cat ions between them.

?. Genital glands opening into the pericardial ex-
tremities of the kidneys.

8. Pleural ganglia still distinct frpm the cerebral,
and pleuro-pedal connectives arising by distinct origins from tJie
cerebro-pedal .

9. Visceral ganglia little developed, much smaller
than the cerebral."

It is not my intention to enter into theoretic consid-
erations until my studies on other numbers of tlie group are more
co:nplete, but so far as Yoldia limatiila is concerned the fore-
going statement is not accurate.

Of Pelseneer's numbered statenents, numbers 2, 5, and
7 are wholly wrong and I can do no better t2ian refer the reader
to t^at has already been said under these heads (see pages /-J,^/
and J^). Number 8 I have not been able to confirm.

The theoretic value of tlie gills is, for the present,
rendered doubtful inasmuch as tiey ha^'-e a special f^anction that
accounts for their shape and structure. Here there remains the
choice of two views, viz, that they have been modified from a

55.

primitive t^/pe or tliat they have been modified from the ordinary
type.

Continuing from the quotation given above Pelseneer
says : "These common characters teach, at the same tim.e the
mutual relations of the Nuculidae and Solenomyidae which sho-ald
be united into a common group, contrary/ to the ideas up to the
present time, T^ich have placed the latter family v/ith the
Solenidae, near the Lucinidae, under the Anaticinacea, etc.

If, on the other hand, comparisons be made betvT-een the
two families Nuculidae and Solenomyidae, it is proved that :

1. ¥ith ITucula, the mantle is entirely open ventral ly,
between the two adductors, and that the margins are simple, with-
out tentacles.

2. ¥ith the Nuculidae (Leda, Yoldia, etc) one sees
again that the retractors of the foot (columellaires) , form an
almost continuous series (without specialization), between the
two adduc t o r s .

3. In the Nuculidal, the digestive tube presents a
pharyngeal cavity where two glandular pockets open.

4. With ITuciila the heart is dorsal to the rectum.

5. In ITucula the pleural ganglia reach their maximum
separation from the cerebral, in the sense that the cerebro- and

56.

pleuro-pedal connect i-ves are still separated for a part of tlrieir
ext ent .

6. ¥it3^ the Nixculidae, th.e otooysts open freely to
tlie outside.

One can. conclude frora this that the Solenomyidae are
less primitive tlian the Nuculidae, and that the Lar.elli'brai'iclis
actually the most archaic are represented^ by the latter family and
spec i all 7/ by Nucul a . "

In this, reference is made directly to Yoldia only in
number 2, aiid the statement here made does not hold good. The
foot muscles of Yoldia are surely among the most highly developed
of any to be found in the Lamellibranchia, ai^d the only reason
they occupy so much of the dorsal margin is because of their
enormous size. Numbers 3 and 6, -which refer to t}ie Nuculidae

in general, do not seem to hold good with this species.

EMBRYOLOGY.

The most striking peculiarities in tlie development of
Yoldia are connected with the formation and disappearance of the
test.

I find similar tests in the embryos of two species of
ITucula (IT. proxiiia and N. delphonodonta) , in both* of "wfiiich cases

57.

the tests are cast. In the former, the eggs are tiirovci Tree m
the water arid the embryos closely resemble those of Yoldia, but
are considerably smaller. In the latter species, the embryos are
much larger and are somewhat modified by being carried in a mucous
case, which is attached to the posterior end of the shell.

Outside the group, so far as I have been able to leam.
Bonders ia is the only other Mollusk Tdiose embryo is known to be
provided witii a similar test.

Comparing the embryo of Yoldia 7/1 th Pruvot's '^^ ac-
count of "i^e embryo of Dondei^ia, we find that the fully formed
tests of both are co:nposed of five rows of cells, all of ^Aftiich
bear cilia. Counting from the anterior, the cilia on the third
row of cells in the test of Bonders ia, and the second, third,
and fourth rows of cells in the test of Yoldia, are long aod col-
lected into bands which surround the embrjros . The bods^ of the
enbrjro of Bonders ia protrudes posteriorly during development.
No such protrusion takes place with Yoldia. Each is provided
with an apical plate, and, in either case, the test is finally
cast off. Internal changes in the embryos of Bonders ia not being
known, farther comparisons are denied \is.

The youn.g embryos of Bentalium as described and figured

by Lacaze-Biathiers (^^ and KowalevskyCSy ^^ear a certain resem-

53.

blance to those of Bonders ia and Yoldia. Tliis is largely due to
three or more rows of colls, each bearing a "band of oilia. At this
stage, these ciliated cells form tlie greater portion of the sur-
face of the embryo. As the body elongates posteriorly, these
cells are arowded forward to form the velum, but this does not
seem to be cast off.

A some^rhat similar resemblajice is noticeable in the case
of the enbr3ros of Patella as figured an.d described by Patten ^ K
In either Dentalium or Patella, if we imagine the velum to be
stretched posteriorly over the shell— gland dorsally, and tlie foot
ventrally, so as to enclose the bods'", "tlie oesophagus will be pull-
ed out into a long narrow tube ventral to the foot, and the posi-
tion of the blastopore will corresppnd to its position in Yoldia.
Furthermore the position of the foot and sl^ell gland will corres-
pond, and the alimentary canal will necessarily be bent in the
same way.

In a similar manner, if we imagine the cells of the
velum of a typical lamellibranch larva to be drawn posteriorly
over the shell, tlaere ?^ill be the same relation of pax-ts. The
same seems to be true of the larvae of gastropods if we take into
account the twisting that accompanies development in these Jforms.

Thus we may perhaps trace an homology between the test

59.

of Yoldia and the veliun of other forms. In this connection it
is of interest to notice that in a few forms the velim is known
to bo cast away.

niis has been obser^;-ed by Heatla# for Cliiton, Siger-
foos (19) for Teredo, and Mrs. Drew for Cardium.

Wilson (^^) has described a sinilar casting of the
cells of the velum for Polygordius.

It seeras ffaite possible that the embryo of Yoldia repre-
sents a rather primitive forrn, and tliat the topical molluscan
larvae may have been developed from an ancestor approaching it in
form and structure. If this is true, the probability is that
Yoldia and its relatives, when compared with other lamellibranchs,
would represent a comparatively recent departure from the primi-
tive molluscan form and, in s\ich a case, it is more likely that
organs iifiiioh now show high specialization, have been developed
from the more primitive than from the highly complex.

# Dr. Heath has kindly allowed me to notice this geint al-
though his obser\rations are not yet published.

1 This work has not been published. The species upon

■mSiich the observations were made seems to be C. pinnu-
latuun (Con.)

60.

S U M M A E. T.
A brief statement of tiie cliief points is as follov/s :

ANATOm AST) PIABITS.

1. Tlie mantle lias two pairs of sense organs, figiii'es 1,
ae, and pe, one long unpairel tentacle, st, and a fringe of
meirginal tentacles. All of tJiese organs are sensitive to me-
clianical st imiulation. The iinpaired tentacle seems to be liomol-

ogeous to one of the ordinary marginal tentacles.

2. The foot, figures 3 and 9, f, is a highly modified bur-
rowing organ.

3. The palp-appendages, figures 3, 10 and 18, pap, are food-
collectors. In tlie adult they are not assisted, to any consid-
erable extent, by any other organ.

4. The gills, figures 3, 19 and 20, are used in pumping
water, for which function they are well fitted by their structure.
They probably servo in respiration but do not secjn to aid in col-
lecting food.

5. Each genital duct, figure 29, gd, joins the outer, not
the inner, end of tlie corresponding excretory organ, very near
its op^iing into the branchial chamber.

6. No division into cerebral and pleural gan.gliaha3 been

61.

observed. Neither do tlie cerebro-pedal commissures seesn to }iavc
a double origin.

7, The otocysts are not connected with the surface by
canals. Canals, apparently in process of degeneration, figii^res
21 and 22, ot, are present in adults.

8« Both an anterior and a posterior aorta -irs- present.

EMBRYOLOGY.

1. The original surface cells seem to gi^re rise to the
test, the cerebral ganglia, the apical plate, and a new ectoderm,
inside the test.

2. The cerebral ganglia, figures 46 and 47, are formed as
invaginations from the surface.

3. The otocysts contain otoliths before the test is cast.

4. The test, the apical plate, the stalk connecting the
test with the anterior portion of the body, and the stomodaeuia^to
the position of the definitive mouth, are all cast away.

5. It seems that tii» t3rpical molluscan larvae maj'' have
been developed from embryos resembling those of Yoidia.

LITERATURE CITED.
1. Brooks : On an Organ of Special Sense in the

62.

Lanellibrancliiate Genus Yoldia. Pros. Amer. Asso. for tlie Ads.
of Sci. 1874.

2. Brooks and Drew : Notes on the Anatomy of Yoldia,
I. --Johns Hopkins Univ. Circ^alars, No. 126, 1896.

3. Dorst. Uber das Ner^^ensystem und die Sinnesepithelien
der Herzmuscliel (Cardiiim edule, L.) nebst emigen Mitteilungen
ttber den histoligischen Bau ilrrer Mantels and ilirer Siphonen.--
Morph. Jalirbucli. Bd. 12, 188?.

4. Drew, Notes on the Embrology, Aaatomy and Habits of
Yoldia limatula. Say. — Jolins Hopkins Univ. Circulars, No. 132^139?.

5. Flemming. Die Haaretrageiiden Sinneszellen in der
Oberhaut der Mollusken — Archiv ftlr Mikrs. Anat. Bd. 5, 1869.

6. Plemming. Untersuchun.gen Uber Sinnesepi the lien der
Mollusks.-- Archiv ftlr Mikro. Anat. Bd. 6, 1870.

7. Kellogg. A contribution to our knowledge of
Lamellibranchiate Mollusks.-- Bui. U. S. Pish Com. Vol. 10, 1890.

8. Kowal^sky. Stude sur I'embryogene der Dentale. An.
die :iusee d'historie naturelle de Marseille, Zool. Tome 1, 1383.

9. Lacaze-Duthiers. Historie de 1 'organisation et du
developpment du Dentale.-- An. dos Sci. Nat. Ser. 4. Tome 7. 1857

10. Lang, Lehrbuch der vergleichenden Anatomie.

63.

11. Mits-ulcuri. On the Structure and Signif ican.ce of
sone Aberrant Forms of LaiTiellibrancliiate Gills. Quart. Jour,
micro. Sci. Vol. 21. 1831.

12. Parker and Has-well. Text -Book of Zoolog7.

13. Pattens. The Embryology of Patella. Arb. Zool.
Institut Univ. Wein. Bd. 6, 1806.

14. Pelseneer. Report on the Anatomy of the Deep-Sea
Mollusca collected by H. M. S. Cliallenger during the years 1872-
1876. Challenger Report Vol. 27 Pt. 74, 1838.

15. Pelseneer. Contribution a' 1 'etude des Lamelli-
branchs. Archives de Biologie Tome 11, 1890.

16. Pruvot . Sur le developpment d'un Solenogastre-
Comptes rend. Acad. Paris. Tone III. 1890.

17. Rawitz. Der Mautelj/rand der Acephalen- Jen. Zeit .
fur ITissen. Bd. 28. 1888.

18. Rice. Die systematische Vearwertbarkeit der Kiemen bei
den Lamellibranchiaten — Jen. Zeit. fur ^7issen. Bd. 31, 1897.

19. Sigerfoos. Notes on the Organization, of the Larva
and the Post-larval Development of Ship-^7orrns . Johns Hopkiiis
Univ. Circulars, No. 126, 1896.

20. Tiyon. Structxiral and Systematic .^-^^vx.C'^cr^s^.

64.

OQITOIIQLiQaY.

21. Verrill and Bush. Revie-vr of the Genera of the
Ledidae an.d Nuculidae of the Atlantic Coast of tlie United States.
Am, Jour, of Sci. Vol. 3, 1897.

22. "ITilson E. B. Origin of the Me sob last -Bands in
Annelids, Jour, of Morph. Vol. 4, 1890.

KEY TO LETTERS.
aa. Anterior adductor muscle.

ac. Apical cilia.

ae. Anterior projection of the mantle margin,
afm. Anterior foot muscle,

ap. Apical plate.

bg« Bji-ssal gland.

bl. Blastopore,

bs. Blood space,

eg. Cerebral ganglia,

cr. Chit^nous rods,
cvc . Cerebro -visceral commissure,

ec. EctodeTTn.

ep . Ep i t ho 1 ium .

es, Exhalent siphon,

f . Foot .

65.

g. Gill,

gd. Genital duct,

li. Heart .

ie. Inner end of exoretorv organ.

ip. Interpolated plate,

lbs. IjOVT-er blood space.

11. Left lobe of liver.

11m Lower longitudinal imiscle.

Im. Longitudinal muscle.

Ip. Labial palp.

mco. Cornrion opening of tlio excreto^'y organ and genital duct

into tJie :nantle chamber,

mg. Mid-gat.

np. Jifascles of tlie gill plates,

oe. Outer end of the excretory organ,

on. Ospliradial nerve,

ot. Otocyst.

pa. Posterior adductor muscle.

pap. Palp appendage,

pc. Pericardial wall.

pe. Posterior expansion of the mantle margin,

pfm. Posterior foot muscles,

pg. Pedal ganglion.

J. JS'

• J \' * 1

66.

pm. Pallial muscles,
pn. Palp norve.

r. Pouch leading to the cerebral ganglia,
rl , Uight liver lobe.
s , She 1 1 .

st , Siphonal tentacle,
std. Stomodaeum.
sto. Stomach.
t. Test,
tn. Tentacle nerve,
ubs. Upper blood space.
ulm. Upper longitudinal nuscle,
vg. Visceral ganglia.
X, Depression at the point where the cerebral ganglia

are formed,
y. Cut wall of gill plate.
Z. Large cells that appear when the liver is forced into

the space dorsal to the intestine.

EXPLAMTION OP PIGUEES.

Pig. 1. Expanded adult specimen, seen from the right side.

67.

Pig, 2. Left siiell valve, seen obliquely from tlie dorsal

margin.

Pig. 3. Adult specinen represented as seen from the riglit

side, witii the right shell valve and mantle lobci
removed. The siphons are sho^wn cut longitudi-
nally.

Pig. 4, Same as the above v;ith the right palp and gill

removed, figured to show the relations of internal
organs. The neives leaving the ganglia are
shown only on the left side. (I>argoly recon-
structed) .

Pig. 5. Specimen about 1 mm. long. Represented as a

partially transparent object, seen from the right
side, after the right shell valve and mantle lobe
have been removed. (.largely reconstructed).

Pig. 6, Tip of a retracted siphonal tentacle.

Pig. ?. Portion of an extended siphonal tentacle, with a

swelling caused by the pressure of the blood, seen
as a tran.sparent object.

Fig. 8. Transverse section of a siphonal tentacle.

68,

Pig. 9, Anterior portion of tiie sliell, and tne reriected

margins of tl-ie foot just before tliey are brouglit
together and witadraism into the shell.

Fig. 10. An animal as it appears -while feeding.

Pig. 11. Posterior portion of a specimen about 1/2 mm long.

Fig, 12. Transverse section of a specimen about 1 mm. long^
showing the thickening of the ridges leading to
the base of the siphon, and the pushing in of the
ventral wall of the exhalent siphon. Taken near
the base of the siphon.

Fig. 13. Posterior portion of a specimen about 2rani. long.

Fig. 14. Transverse section of a specimen about 1 cm. long,
taken through the bases of the siphons. One of
the ridges is shovm joining the inhalent siphon.
The section also includes a portion of the -wall
attaching the exhalent siphon witli the dorsal por-
tion of the mantle.

Fig. 15. End of the exhalent siphon of a specimen ,7 ram.
in length.

Fig. 16, Projection of the mantle margin opposite the ex-
tremity of the anterior shell stripe, seen some-
•what obi i quel v.

69.

Fig. 1?. Section of tlie anterior projection of the mantle
margin ,

Fig. 18. Transverse section of a palp appendage.

Fig. 19. A pair of gill plates seen from their anterior
surfaces. The suspensory membrane, the "walls
of the plates joining the plates next in succes-
sion, the longitxidinal muscles, and the basis of

I

the chit«aous rods are shovm in sec'oion. These
are all cut across in separating the pair of
plates from the pair lying in front. (Drami from
a study of sections).

Pig. 20. Portion of a gill, seen from the ventral side.

Fig. 21. Otocvst with its canal and nerve, and a portion
of the cerebro-pedal commissure. A nerve fre-
quently leaves the commissure near to oto cystic «
nerve and is distributed to the muscles of the
foot.

Fig. 22. Portion of the extremity of an otoo^'^stic canal,
showing *J»©. two closed pouches.

Fig, 23. Inner and oxiter ends of t2ie left excretory organ
and genital duct, seen from the ventral side.

Fig. 24. Two celled stage.

>■< *■

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•O-

t::?^r

70.

Fig. 25. Four col led. stage.

Fig. 26. Eigiit celled stage seen from the animal pole.

Fig. 27. Eiglit celled stage. The cells prepax'ing for
tlae next o 1 ea^'-age .

Figs. 28 and 29. Later cleavage stages.

Fig. 30. Ciliated embrvo about 20 hoars old.

Fig. 31. Extrenal appearance of an ernbryo abo^at
hours old.

Fig. 32. External appearance of an embrvo about
hours old.

Fig. 33. Reconstruction of an .embryo about 105 hours old.
The outli2ie and test cells were obtained from a
whole mount. The cilia are indicated only at
the margins.

Fig. 34. Reconstruction of an. embryo that has just cast
the test, represented with the right cell valve
and mantle lobe removed. The stomodaeum and
the stalk extending from the cerebral ganglia,
together with adhering test cells, are still at-
tached.

Fig. 35. Reconstruction of an embryo about 10 hours after
casting. Represented mth the left shell valve
and mantle lobe removed.

^TO i;

.!"if.'T.

, . S"'-?.

71.

Pig. 36. B.e const ruction of an embrvo about 12 days old.

Represented witl-i th.e riglit sJiell valve and mantle
lobe removed.
Fig. 37, Section of an embrj'-o about / 0 liours old. Tlie

plane of tlie section lies througli tlie blastopore.
Pigs. 33, 39 and 40. Successive sections of an e:TTbr70

• about I J hours old. Some of tne surface

cells are beginning to assume the character of

test colls. The group of cells eg, figures 38

and 39, are coimected with the cells ap, figure

40, bv cells of the same character.
Pig. 41. Median vertical section of an embryo about 15

hours old.
Pig. 42. Median vertical section of an embryo abo\it 22

hours old.
Pig. 43, Median vertical section of an embrjro about 30

hours old.
Pig. 44. Median vertical section of an embryo aboxit 36

hours old.
Pig. 45, Transverse section of an embryo about 35 hours

old, taken near the third band of cilia.
Fig. 46, Transverse section of an embryo about 45 hours

old, taken through, the cerebral pouches.

■cr

i.sT;;

?2,

Pig. 47. A similar section showing the po^xchos separate.
Pig. 48. Transverse section of an embryo about 45 hours

old, ta]-cen where the storaodaeum Joins the midgut,
Pig. 49, Transverse section of the same embryo a little

further posterior.
Pig. 50. Section of the gill rudiment in the plane indi-
cated by the line 50, figure 36,
Pig, 51. Section of the same gill rudiment in the plane

indicated by the line 51, figure 36.
Pigs, 52, 53 and 54. Transverse sections showing tho

formation of the "sole" of the foot.
Pig, 55, Median, vertical section of an embryo about 95

hoxirs old. (A portion of the stomodaeum was

drawn from the next section).
Fig, 56. Transverse section of an embrj'"© about 75 hours

old, showing the invaginations that form the

otocysts.
Pig, 57, Transverse section of an embryo about 70 hotirs

old showing the forming pedal ganglia.
Pigs. 58, 59 and 60. Successive tran.sverse sections

through the anterior portion of an ombrsT'o about

95 hours old.

)r^c>-

'.. r; H

?3.

Pig. 61. Transverse section o f an enbrj'-o about 95 hears

old, taken through, the otocysts.
Pig. 62. Transverse section through the liver pouohes of

an embryo about 95 hours old.
Pig. 63. Horizontal section through the liver pouches of an

embrvo of the sa^ne age.
Pig, 64. Median vertical section of an embryo about 10

hours after casting.
Pig. 65. Transverse section of an embryo of the same age.
Pig. 66. Horizontal section through the dorsal portion of

an enbrjj-o about 12 days old.

. - i'-

74.

LIFE.

The writer of this thesis was bom in Jasper Coun.tv,
Iowa, November 15th, 1868. He received his early educational
training in the public schools of that state, and, in the year
1890 was granted the degree of Bachelor of Science at the State
University of Iowa,

Three years have since been spent in teaching, and
five years have been devoted to studj'' in this University,

•w