The habits, anatomy, and embryology of the giant scallop, (Pecten tenuicostatus, Mighels)

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The habits, anatomy and embryology of the
giant scallop (Pecten tenuicostatus Mighels)

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The Habits, Anatomy, and Em-
bryology of the Giant Scallop,
(Pecten tenuicostatus,

Mighels)

GILMAN ARTHUR DREW

Professor of Biology

ORONO, MAINE
September, 1906

THE GIANT SCALLOP (Pecten tenuicostatus, Mighels.)

Most of the material upon which these observations were made
was secured near Bass Harbor, Mount Desert Island, Maine,
during the summer of 1got. Scallops in that region are fairly
-abundant but are now for the most part confined to water 40
fathoms or more in depth and are accordingly only to be
obtained by dredging.

The scallop industry of Maine has undergone some changes
since the appearance of Smith’s (28) paper in 1889, but on the
whole the changes are not very important. Some of the beds
have run out so they are not profitable to work, and others have
been discovered. Most of the good beds are now in water of
from 40 to 60 fathoms and the scallops are somewhat smaller
than in former years but the methods of catching them do not
seem to have changed much.

The majority of lamellibranchs are adapted for burrowing
more or less in mud or sand, are elongated, a shape that makes
burrowing easy, and are provided with two adductor muscles for
closing the shell, a desirable number for a shell and body of this
shape. Such animals usually move very slowly, by thrusting
the foot out into the mud, swelling its end to form an anchor,
and then drawing the shell with the inclosed body to the position
of the anchored foot. ‘The shell may, or may not be capable of
closing tightly, this being dependent in part upon how deep the
animal burrows, and accordingly how well it is protected by the
overlying mud, and in part by other conditions. Such forms,
when disturbed, usually close the shell rather slowly and keep it
closed until probable danger is past. ,

It is common for lamellibranchs that live in or about the
mud or sand to make use of a current of water, which they

R)

throw from the shell by rather suddenly closing the shell, to free
the mantle chamber of the dirt that gains entrance. The ability
to form a strong current is much better developed in some forms
than in others, the difference apparently depending upon the
ease with which dirt can be dislodged. Thus in the fresh-water
mussel which lives near the surface of the mud, with at least a
portion of the shell valves protruding above the surface, the
current produced is not nearly so strong as in the case of the
soft-shelled clam, Mya arenaria, which lives deeply buried in the
mud where the dirt that gains entrance has to be elevated some
distance, and where mud in suspension, caused by waves and
tidal currents combined, is frequently considerable.

Many forms of lamellibranchs when put into aquaria may
move some distance along the bottom by thus squirting water
from the shell. This is true with Yoldia, Angulus, Mya and a
large number of other forms, and at least one form, Solenomya,
(6) swims freely in the water by producing a somewhat similar
current of water. As such a current of water is primarily of
service in cleaning the mantle chamber and is used habitually for
locomotion by only a few forms, it seems quite possible that
those forms that do use it for locomotion may have simply per-
fected an already existing mechanism primarily designed for
another purpose.

Pecten is one of the ablest swimmers among lamellibranchs.
The whole structure of the animal is modified for this purpose.
The valves have become rounded in outline, flattened, and com-
paratively light. The anterior adductor muscle has been lost,
and the posterior adductor muscle, which is very powerful, is
situated near the middle of the body. The cartilage has become
well developed so the shell may be opened quickly when the
muscle relaxes, and the hinge line is straight so there may be no
unnecessary strains in opening and in closing the shell. Each
gill is attached by one lamella only, so water in the temporary
cloacal chamber may be thrown out without injuring the gills,
and the gills and margins of the mantle are provided with
muscles to withdraw them from the margins of the shell when
the shell is closed. Furthermore the margins of the mantle are
provided with in-folded ridges and with circular muscles so it is
possible to direct the current of water which issues from the
shell in the required direction.

That Pecten makes use of its ability to swim in escaping from
its enemies no one can doubt who has seen the shallow water
species, Pecten irradians, scatter when disturbed. How it
becomes acquainted with the presence of enemies is more a
matter of surmise than of experiment. That the tentacles are
exceedingly sensitive to touch is well known, and they may be of
great service in detecting disturbances in the water. That the
eyes are physically optical instruments that may produce distinct
images there is every reason to believe, but it seems to be diffi-
cult to get responses from specimens in aquaria that are without
question due to the sense of vision. No experiments definitely
to test their sense of vision have been devised, but it is very
probable that the eyes are important organs in detecting the
approach of enemies in shallow water where light is sufficient.
The shallow water scallop is known to migrate between shallow
and deeper water at different seasons of the year. Whether the
migration is due to search for food, temperature of the water,
enemies, or location for spawning, or to a combination of factors,
is not known. The giant scallops seem to shift their position
from time to time, but as they always remain in comparatively
deep water, the movement cannot be accounted for by either a
change in temperature or spawning. It is most likely due to
either continued disturbances by dredging, or enemies, or to lack
of food. The latter is the usual reason assigned.

It is not entirely certain what relationship Pecten bears to the
usual form of lamellibranch as regards positions of parts. In
lamellibranchs that are supplied with two practically equal
‘adductor muscles, a line connecting the two adductors runs
nearly lengthwise of the animal. In such a case the hinge line
is more or less dorsal, one end is anterior, and the other pos-
terior. When one of the muscles disappears, as is the case with
Pecten, one of the landmarks disappears and it becomes more
difficult to locate the direction of parts. Inasmuch as the hinge
line is usually dorsal, it is very natural to look at the hinge line
of this form as dorsal, and for matters of description it is con-
venient to so consider it. If, however, the position that the
anterior adductor would have occupied, had it been retained, be
considered, the position of the mouth, foot and heart indicate
that it would have to be placed much nearer the hinge line than

5

the present position of the posterior adductor muscle, the muscle
that is retained. If this is the case, it becomes evident that the
loss of the anterior adductor muscle has been accompanied by a
general reduction of the anterior part of the body, so a large part
of the body of Pecten is to be considered morphologically pos-
terior. This supposition seems to be borne out by the nervous
system, and the vascular system of the mantle, as well as by the
extent and position of organs. In most forms the margin of
each lobe of the mantle is supplied with a posterior and an ante-
rior pallial nerve of approximately equal size. These nerves
supply the muscles and sense organs of the margins, and, in
many forms at least, unite with each other so they form a con-
tinuous connection between the cerebral and the visceral ganglia.
In Pecten, not only is this the case, but the nerve in the margin
of the mantle is joined at intervals for nearly its whole length
by nerves from the visceral ganglia (fig. 15). On the other
hand, it is joined only in the region of the anterior ear by
nerves from the cerebral ganglia. The visceral ganglia are the
important ganglia of the animal, and both the cerebral and pedal
ganglia are greatly reduced.

The blood is supplied to the mantle very largely by the pos-
terior pallial arteries (fig.°g). The anterior pallial arteries are
comparatively small, and while they are connected with the pos-
terior pallial arteries, the size and character of the vessels
indicates that the junction is probably very near the anterior
wing.

Considering everything, it seems likely that the longitudinal
axis of the body could be morphologically represented by a line
drawn from near the hinge extremity of the anterior ear to the
middle of the adductor muscle, and that a very small portion of
the scallop is anterior.

The only portion of the scallop that is eaten is the adductor
muscle, which is proportionately very large but certainly forms
less than one-half of the bulk of the soft parts of the animal.
There seems to be no good reason why the remainder of the
animal should not be eaten. The margins of the mantle are
rather tough but not more so than the neck (siphons) of the
soft shelled clam. The probable reason that all but the muscle
of the scallop is discarded, while in other forms the whole body

6

is eaten, is that in the scallop many portions are highly colored.
The visceral mass of the female is bright pink and the margins
of the mantle are usually very conspicuously colored with yel-
lows, browns, and black. Although such colors are not con-
sidered to be objectionable in many foods, they are not the
usual colors for “ shell-fish”” and accordingly are looked upon
with disfavor.

SHELL.

All members of the genus Pecten are provided with shells of
a very characteristic shape. The shells are rounded, inequi-
valved and eared; the hinge line is straight and runs the length
of the margin formed by the ears, is not provided with teeth or
with a conspicuous hinge ligament and is provided with a large
cartilage that is placed immediately between the beaks of the
‘two valves.

All of the members of the genus have somewhat similar habits,
are provided with numerous sense organs and are capable of
swimming by clapping the shell valves together. A very heavy
shell, massive teeth, or a small cartilage would not be adapted
for swimming in this manner, where lightness and speed of
movement are essential. A rounded outline is consistent with
such progression, and the presence of numerous sense organs is
important, for these animals live in positions unusually exposed
to the attacks of enemies.

The shell of this particular species (figs. 1 to 7) is somewhat
longer than wide and rather flat. The dimensions of a good
sized specimen may be given as length 7 inches, width 6% inches
and thickness 114 inches. The proportional thickness differs
more than the other two dimensions, varying from a little under
1! inches to a little over 134 inches in specimens corresponding
to the other dimensions given. The two valves are of about the
same weight, but the right valve, that on which the animal
habitually lies, is much flatter than the left (fig. 19), lighter in
color and has a rather prominent rounded notch where the
anterior wing joins the main body of the shell (fig. 4).-* I Rave
been unable to satisfy myself as to the function performed
by this notch. The sense tentacles on the mantle margin oppo-
site the notch are somewhat longer than those adjacent, but I

7

have been unable to determine that they have a special function
or that they are specially advantageously placed.

The outer surfaces of the valves (figs. 1 to 4), are marked
by fine ridges and grooves that radiate from the beaks to the
margin, and by rather prominent lines of growth that run out
along the hinge line. Not all of the radial markings start from
the beak but new ones are added at intervals so that the number
remains approximately from 30 to 35 per inch on any portion of
the surface. The lines generally have a somewhat wavy or zig-
zag course. Those on the upper, left valve, are more rounded
and prominent than those on the under, probably because of the
difference in wear in the two-cases. The wear is further indi-
cated by the fact that the markings may be nearly or quite
obliterated on the most convex portion of the lower valve.

The lines of growth are visible as very fine lines all over the
surface, but are much more prominent in some places than
others. The larger ones may appear as ridges, which would
seem to indicate that marginal growth occasionally stops as the
shell thickens along a line, or they may appear as a series of
rather jagged depressions that indicate where the old margins
have been broken. These latter markings are rather more
abundant on the lower than on the upper valve. Their relative
abundance compared to the shells of other lamellibranchs is
easily accounted for by the use to which the shell is put in swim-
ming and as the upper valve usually extends over the margin
of the lower (fig. 19) each blow when the valves are clapped
together would be directed by the more solid inner portion of
the upper valve directly onto the margin of the lower valve,
thus being more likely to break off small bits from it.

The outer surface of the shell, especially of the upper valve,
seems to be more than ordinarily subject to the attacks of the
boring sponge, Cliona sulphurea, which frequently riddles the
shell so that hardly a trace of the original surface is left (fig. 3).
The reasons for the attacks on this shell more than on the shells
of other mollusks found in the region is probably due to the fact
that this animal does not burrow and that the shell is not pro-
vided with anything like an adequate cuticle, a layer that could
not be formed because the enlarged margin of the mantle must
necessarily be withdrawn far into the shell whenever the valves

8

are brought together. The borings of the sponge frequently
weaken the shells decidedly and it is not uncommon to find bor-
ings that come so near the inner surface that light may readily
be seen through them, but I have never found actual perforation
of the shells. It seems probable that the secretion from the
sponge that acts in dissolving the shell soaks through the shell
sufficiently to stimulate the mantle to renewed secretion of shell
substance before the perforations are actually complete. This is
indicated by the fact that shells that are badly attacked are quite
universally thicker in these places than those that are not so badly
attacked, and that their inner surfaces are likely to be rough-
ened (fig. 7). The meaning of the roughened appearance is
especially striking if shells that are bored only in patches are
examined. In such cases the end of each of the deep burrows
is marked on the inner surface of the shell by a rounded eleva-
tion, each of which reminds one at first sight of a minute pearl
that has become fastened to the shell, and partially covered up
by later secretions of nacre.

The markings on the inner surface of the shell are much more
distinct in some individuals than in others and they differ some-
what on the two valves.

The adductor muscle scar is quite large and fairly distinct,
and is placed somewhere near the center of the shell, a little
nearer the posterior than the anterior border. The scar is much
larger on the left (fig. 5), than in the right valve (fig. 6),
where it is distinctly double, consisting of a large rounded ante-
rior portion, (pas.) and a small somewhat crescent shaped
posterior portion (pas’.). The scars usually form depressions,
but sometimes elevatons, on the inner surface of the shell, and
are more or less distinctly marked by lines that indicate the
larger bundles into which the muscle is divided.

The foot retractor muscle is attached to the left valve of the
shell along the dorsal border of the adductor muscle, just ante-
rior to the upper end of the posterior crescent shaped portion,
and the scar is not distinguishable from the scar of the adductor
muscle,

The mantle is very firmly attached to each valve along a rather
broad and indistinct pallial line, (pl.) that is very far removed
from the margin of the shell. The muscles attached along the

9

greater portion of this line radiate toward the free edge of the
mantle. The margin of the mantle is also well supplied with
circular muscles that, near the hinge line, are collected into large
bundles and attached to the shell, (aps. and pps.). The scars
on the shell caused by the attachment of these muscles are some-
what larger and more distinct posteriorly than anteriorly.

Immediately beneath the scars of the adductor muscle there
is frequently to be seen an indistinct line that marks the attach-
ment of the muscles by which the gills are elevated (fig. 7, sms.)

The inner surface of the shell is sometimes quite smooth, in
which case fine radiating lines, possibly corresponding to the
radial pallial muscles, may sometimes be seen running from the
pallial line toward the free margins of the shell. It more fre-
quently happens, however, that the inner surface of the shell
is roughened. These roughenings are no doubt always due to
the secretion of nacre caused by irritation, probably sometimes
due to the boring sponges before mentioned, but also to other
causes. Not infrequently the inner surface of the shell is
marked by minute elevations each of which has a dark spot at
its tip. In such shells it has frequently been noticed that the
mantle is spotted as if diseased but whether the spots are due to
parasites as was determined by Jameson (13) has not been
determined, neither has the relationship of the spots on the
mantle to the spots on the shell been determind.

Pearls are sometimes very numerous, several dozen having
been taken from a single shell. When these become attached to
the inner surface of the shell, they give it a very rough appear-
ance.

This inner surface of the shell is further frequently marked
by dark streaks and blotches that are usually caused by worms
and other marine animals that inhabit the holes that have been
formed by the boring sponges.

The structure of the shell does not correspond to the structure
of a large majority of mollusk shells. Most shells are com-
posed of a nacreous inside layer, a prismatic middle layer and an
outside cuticle. In this form, and this holds true for the other
scallops that I have examined, only the nacreous layer seems to
be present. I have not been able to find a trace of either of the
other layers in the sections that I have made, but the shell is so

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brittle that it is hard to grind satisfactory sections. A cuticular
gland is present along the margins of the mantle (fig. 26, cgl.),
and in this gland fragments of a weak layer that no doubt is
cuticle is present but the frequent removal of the mantle margin
far into’the interior of the shell would seriously interfere with
the formation of a continuous cuticle. As might be expected in
an animal where so large a portion of the mantle is movable, the
nacre is not formed of such uniform delicate layers as are found
in some shells. Layers are present but the carbonate of lime
seems to have become partly crystalized and the layers inter-
rupted in places.

The shell material may vary in color considerably in individ-
uals, or in the same individual, but this seems to be a variation
in the nacre and not caused by the presence of other layers.
The variation is especially well marked in the smaller more
southern scallop, Pecten irradians, where the outer portions of
each of the ridges that are so prominent in this species is deeply
colored while the inner surface of the ridges and the portions
forming the valleys between the ridges are white. It all seems
to correspond in structure to nacre however.

The valves are attached to each other along the hinge line by
a very weak ligament which represents a modification of a
cuticle that serves simply to hold the two valves together along
this line. Near the middle of the hinge line, just beneath the
beaks, there is in each valve a deep and broad somewhat tri-
angular depression that in old shells opens quite broadly on the
hinge line. This depression, (fig. 5, cp.) forms the cartilage
pit, into which is set the end of an elastic pad, the so-called
cartilage, (fig. 10, c.) which is a modification of the ligament.

When the adductor muscle contracts and the valves are drawn
together, this pad of cartilage is compressed and serves, when
the muscle relaxes, to separate the valves again. The cartilage
is composed of the same material as the enlarged elastic liga-
ments of such forms as the fresh-water mussels, and no doubt
they have had a.common origin. They both serve to open the
shell by their elasticity, in the one case by expanding after being
compressed and in the other by contracting after being stretched.

Teh

MANTLE.

The inside of each valve of the shell is lined by a thin fold
of tissue, a mantle lobe (fig. Io, m.). The mantle lobes are
united to each other along the hinge line and anteriorly and
posteriorly for a space that practically corresponds to the width
of the ears of the shell.

With the exception of the free borders of the mantle, each
lobe consists of a very thin membrane that is closely applied to
the inner surface of the shell which it entirely covers in the
living extended animal. In specimens that have been disturbed
so the shell valves are closed together, the margins of the mantle
lobes are drawn far back into the shell so there may be a strip
of three-quarters of an inch or more of the inner border of each
shell vaive that is left uncovered. This retraction of the mantle
is necessary in order that the thickened and highly modified
margins of the mantle may not be injured by the closing of the
shell.

Each mantle lobe is free from the rest of the body except
where it covers and is fused with the adductor muscle and diges-
tive gland, where it is joined by the posterior border of each of
the outer labial palps, and where it is joined by the membranes
that support the gills.

The formation of the shell is due entirely to the secretion of
materials by the mantle. The nacre, which forms the inner
surface and most, if not the entire thickness of the shell is
secreted by the whole of the applied surface of the mantle and
in healthy perfect shells is quite smooth and white. As it is
constantly being added to, it is thicker in old than in young
shells and in the older part of the shell, toward the middle of
the hinge than elsewhere. Certain portions of the mantle, as
that along the margins between the pallial lines and the free
borders, and in the region of the hinge line and wings, are more
active than others in secreting nacre. In these regions the
epithelium on the shell side of the mantle is composed of espec-
ially large apparently actively secreting cells, while the remain-
der of the surface is composed of small less active cells. This
arrangement is in accord with the especially thick portion of
the shell along the hinge line including the ears, and with the

I2

fact that the margins of the valves‘are thicker than would other-
wise be expected.

If a prismatic layer is present it must be secreted by the
extreme margin of the mantle but there is nothing in the appear-
ance of the epethelium of this region that would indicate that it
has a different function than that a little further removed from
the margin.

A gland for the formation of the cuticle occupies a grove
along the margin of the mantle, (fig. 26, cgl.) and fragments of
a thin and apparently not very tough cuticle may nearly always
be seen in it attached at the inner end of the gland to the epethe-
lium that secretes it, but the withdrawal of the mantle margins
far into the shell when the shell is closed must interfere with
the formation of anything like a continuous cuticle. The liga-
ment and cartilage are both present, although the former is
quite thin, and may be looked upon as modifications of the
cuticle. They are secreted by adjacent portions of the mantle.

The free margin of each lobe is very muscular, is abundantly
supplied with organs in the form of tactile tentacles and eyes,
and is provided with a large flattened ridge that is turned away
from the shell valve on which the mantle lobe rests (figs. 10, 19,
20, and 26). The ridges on the two mantle lobes may be brought
into contact with each other at any or all points along the
margin, when the shell valves are slightly separated and thus
regulate the currents of water formed by the cilia on the gills,
or in swimming.

The muscles of the mantle, (figs. 16 and 26) may be grouped
as the radial pallial, which are attached to the shell at the pallial
lines and radiate out toward the margins, the circular pallial
which are very strong, attached to the shell valves near the hinge
line and run along the borders of the mantle, the muscles of
the pallial ridge that are largely circular but contain also radial
muscles connected with the radial muscles that have been
referred to, and the suspensory muscles of the gills which are
really continuation of a fold of the mantle and are attached to
the shell valves between the pallial scars and the adductor muscle
scars. All but the last group of these muscles, which will be
described in connection with the gills, are confined to the borders
of the mantle and, together with the infolded ridge, sense organs

13

and pigmentation of this region, form a thickened portion that is
very striking in appearance when compared with similar parts
of most other forms.

The radial muscles are much longer and more powerful in
the region opposite the hinge line than elsewhere and have for
their chief function the withdrawal of the mantle margins into
the shell in order that the margins may not be injured in closing
the shell, which is closed both rapidly and powerfully, especially
when the animal is swimming. The radial muscles of the
infolded ridge are continuous with these muscles and serve to
contract the width of the ridge or to extend the ridge out in the
same plane with the rest of the mantle, that is to extend it out of
the shell. This is no doubt done in conjunction with relaxing
the circular muscles of the ridge, and extending the margin by
blood pressure. |

The circular pallial muscles of each mantle lobe form a very
strong band that is attached to the corresponding shell valve
’ anteriorly and posteriorly just beneath the ridge along the hinge
line, at the dorsal ends of the pallial lines. They are spread out
between the attachment of the radial muscles and the margins
of the mantle lobes but are strongest some distance away from
the attachment of the radial muscles and they nearly or quite
disappear before the pallial nerve is reached. They serve as
constrictors that are important in withdrawing the margins into
the shell. In this they act in conjunction with the radial mus-
cles. Their attachment to the shell along the dorsal limits of
the pallial line suggests that they may be regarded as extended,
modified radial muscles of this region of the mantle.

The circular muscles of the infolded ridges are connected with
this band especially near the hinge line. Here the circular
muscles become continuous with the other circular muscles.
This leaves the dorsal inch and a half or two inches of the ridges
both anteriorly and posteriorly without well developed circular
muscles. The radial muscles of the ridges of these regions are
also poorly developed so there is but little independent move-
ment of the ridges near the hinge line. It is interesting to
notice further that the tentacles on the ridges are not developed
in this region and that with the exception of the extreme dorsal
margin, the parts covered by the ears, the eyes are absent or

14

very few in number. The circular muscles of the ridges are
important in adjusting the positions of the ridges to each other
and thus in regulating the currents of water in respiration and
feeding, and in swimming.

The sense organs of the mantle are of two distinct kinds,
tactile tentacles and eyes. The tentacles are distributed in two
bands along the margin of each lobe (figs. 10 and 20). The
largest forms a broad band, several tentacles deep that runs
along the inner face of the margin of each lobe slightly removed
from the extreme edge, about where the base of the infolded
ridge joins the lobe. The tentacles in this ridge vary greatly
in size, those placed farthest from the margin usually being the
largest and those next to the free margins being smallest (fig.
26). Along the borders of the ears of the shell the tentacles
are somewhat longer and more slender than elsewhere, and they
are perhaps longest near the notch at the base of the anterior
ear. A large individual may possess several thousand ten-
tacles for there are from 75 to 100 on an inch of border.

The other band runs along the face of the ridge near its free
border. In appearance these tentacles correspond to those of
the other band, but they are not nearly as numerous and are not
as large as the largest in the other band. They are most
abundant in the portion farthest removed from the hinge line
and are not found on the portions adjacent to the hinge.

All of the tentacles of both bands are capable of being greatly
lengthened so they sometimes form a fringe along the border,
an inch or more in length. When the animal is disturbed they
are immediately withdrawn and form conical projections hardly
more than a sixteenth of an inch in length for the largest.

The structure of the tentacles will be considered under the
head of sense organs.

The eyes are placed along the margin of the larger band of
tentacles, on the side that is turned away from the free border
of the lobe of the mantle. They form a single scattered row in
which they are set at irregular intervals but fairly close together.
They are most abundant along the border farthest from the
hinge and are absent or very few in number for an interval near
the ears of the shell both anteriorly and posteriorly. In the

15

space covered by the ears a few eyes are present. In all there
are frequently as many as one hundred on each of the lobes of
the mantle. Their size even in old individuals is noticeably
unequal and they are not arranged in any order of size. As
there are many more eyes in large than in small individuals, new
ones must be added during the growth of the animal and their
size may be an indication of their age. If this is true, new eyes
are not added in accordance with any plan but make their
appearance as spaces for them occur. The outer ends of the
stalks on which the eyes are set are deeply pigmented with black
or brown pigment, and the eyes themselves are blue and exceed-
ingly brilliant. The structure of the eyes will be considered
under the head of sense organs.

The mantle margins, including the infolded ridges and ten-
tacles, are usually highly pigmented. Yellow and brown, either
light or so dark as to approach black, are conspicuous in this
pigmentation. Sometimes the margins are nearly of the same
color throughout their extent but they are frequently blotched
with different colors and with different shades of the same color
arranged in irregular patterns so that with the infolded ridges,
the tentacles and the brilliant eyes, the margins make very strik-
ing objects. What purpose the brilliant pigmentation may
serve I cannot say. Perhaps they are not as conspicuous among
the yellow incrusting sponges and the other variously colored
incrusting growths among which they live. Living in deep
water as the animal does, these are matters that are not easily
studied.

The distribution of nerves and blood vessels in the mantle will
be described in the general consideration of the nervous system
and the vascular system of the animal.

VISCERAL MASS AND FOOT.

It will hardly be necessary to describe the general shape and
positions of these portions of the animal as reference to figures
will make the relation of parts much clearer than description
(figs. 11 and 12). It will be noticed that the portion containing
the digestive gland or liver, and the reproductive portion of the
viscera are not broadly connected, and that the foot is placed
anterior to the heart and ventral to the connecting portion.

16

This arrangement is such that the large adductor muscle is
pretty well inclosed, there being only a small postero-ventral
portion of the muscle that is not surrounded by the other organs.

The portion near the hinge consists almost entirely of the
digestive gland, commonly called the liver, with the cesophagus,
stomach and first part of the intestine inclosed in it. During
the season of reproduction, a thin layer of gonads extends over
its surfaces laterally as well as anteriorly and posteriorly. The
portion ventral to the foot consists almost entirely of the gonads
with the coils of the intestines running through them. The foot
is largely muscle with a rather extensive byssal gland inclosed
in it. The liver region extends from valve to valve of the shell
and is covered closely with a very thin portion of the mantle.
The reproductive portion and foot are comparatively narrow and
are suspended between the gills, being supported in large part
by the adductor muscle. These portions do not occupy a posi-
tion midway between the two valves but are nearer the right
than the left valve.

The foot is a comparatively slender, roughly cylindrical organ
somewhat larger where it is attached to the body than elsewhere,
cleft at its free end, so it may be spread to form a “ sole” as in
Nucula and Yoldia, and showing the large opening of the byssal
gland to the right of the middle line, on the ventral surface some-
what behind the sole (fig. 8).

The foot has lost its symmetry, being twisted so the ventral
surface is directed somewhat toward the right valve. It seems
to be in a large measure a degenerate organ that is practically of
no service as an organ of locomotion. The animal depends
upon swimming by clapping the shell valves together to change
position. The foot may however be greatly extended and
thrust between the valves of the shell. When protruded it leaves
the shell just ventral to the notch at the base of the anterior
ear, and may be moved from place to place. The flaps on the
sides of the foot are moveable and are frequently separated
somewhat but there is no such active movement as in Yoldia
(6) or other forms with this type of foot, with which I am
acquainted. In the species under consideration I have ever
seen the foot protrude far out of the shell and have never seen
the animal attach itself with a byssus.

2 17

Individuals of the smaller species, Pectens irradians, do attach
themselves with a byssus and I have no reason to doubt that indi-
viduals of this species attach themselves. An individual of
Pecten irradians placed in a glass dish of sea water will some-
times protrude its foot from the shell, apply it closely to the
bottom of the dish and after a short time slowly withdraw it,
leaving a rather broad band of slightly yellowish material
attached to the glass and connected with the foot at the byssal
gland. This is not composed of small tough threads as in the
mussels Mytilus and Modiola, but it may be sufficiently tough
to support the weight of the animal if, after a few minutes, the
dish is carefully turned over. The animals seem never to remain
attached for long periods, but after a few hours at most the
attachment is dropped at the byssal gland. Whether this is
passive or due to a sudden strain caused by forcing strong cur-
rents of water from the shell as in swiniming has not been
observed.

As in other lamellibranchs the foot is largely composed of
crossing muscle fibres that by individual or combined action may
press upon blood that may be confined in a rather large blood
space in the foot and so cause the elastic foot to be extended.
The foot is attached to the shell by a single retractor muscle
which runs along the dorsal portion of the foot posteriorly,
dorsal to the posterior adductor muscle, to be inserted on the
left shell valve at about the point where the adductor muscle is
Separated into two parts (fig. 10, fm.). This muscle extends
along the dorsal border of the foot and is about equally in evi-
dence on its right and left sides. Above the opening of the
byssal gland, which lies somewhat to the right of the median
line on the ventral side, the muscle loses its individuality and
becomes merged with the general foot muscles.‘ It leaves the
foot along the median line flattens a little and gradually runs
Over on the left side to be attached to the left shell valve. Why
the right muscle should have degenerated is not clear but the
position, attached to the left shell valve, which is uppermost,
gives the muscle a straighter pull when the foot is attached to
the bottom by the byssus than would be the case if the right
muscle had persisted instead.

18

The byssal gland is quite extensive and not only permeates
a considerable portion of the foot but extends some distance
dorsally and posteriorly ventral to the retractor muscle of the
foot. It is a racemose gland of the usual character.

ALIMENTARY CANAL,

The cesophagus is rather short and extends from the mouth,
which lies beneath the anterior protruding portion of the liver,
to the antero-dorsal portion of the stomach. The stomach
fig. 12, s.), is fairly large and receives two large ducts from
the liver which surrounds it. The openings of these ducts into
the stomach are so large as to form two latteral diverticula of
the stomach into which the smaller ducts from the liver empty.
The portion of the alimentary canal that extends from the
rounded stomach to near the posterior portion of the body is of
greater diameter than the remaining portion and resembles in
microscopic appearance the stomach rather than the intestine.
A portion of the lining epithelium of this and of the lower end
of the stomach resembles that usually concerned in the forma-
tion of a crystalline style. While a definite, well formed rod-
like style, such as is so well formed in the soft clam, Mya, is not
present, a large quantity of mucous is secreted. This mucous
entangles the food that is swallowed and very likely performs
the same function that is performed by the dissolving of the
crystalline style which Kellogg (14) with much reason thinks
may be to keep the cilia of the alimentary canal from forcing
the food through the canal before it has had time to digest.

The remaining portion of the alimentary canal, is of about
even diameter throughout its length and is lined by epithe-
lium of the character ordinarily found in this portion in lamelli-
branchs. It is a ciliated epithelium, the cells of which stain
deeply and probably have some secretory function. Undoubt-
edly the chief reason for having the canal so elongated is to give
time and surface for absorbing digested food.

The position of the loops of the intestine are shown in figure
12, i, and needs no special description. It is worth noticing
that the loops are practically in the same position in Pecten irra-
dians, the only other species of scallop that I have examined.
In the figure given by Pelseneer, which is copied in Parker &

19

Haswell’s Text-Book of Zoology, page 648, the reverse loop of
the intestine in the posterior portion of the visceral mass is not
given. This may not be present in the species figured, but it
seems quite possible that it might have been overlooked in dis-
sections as the backward turn is so abrupt and takes place so
near the other portion of the intestine. After running dorsally
nearly to the hinge line, the intestine bends rather abruptly pos-
tero-ventrally, perforates the ventricle of the heart, and termi-
nates posterior to the adductor muscle. The last inch or more
of the intestine protrudes from the general body and ends in a
dorsal turn that directs the faeces toward the edge of the shell in
the same direction that is taken by the excurrent stream of water
that is coming from the gills. It is important that the feces
should be voided, as they are, where they may be promptly
removed by the current of water coming from the gills.

Throughout the length of the alimentary canal its epithelium
is ciliated and movements of its contents are dependent upon the
action of the cilia. The general movements caused by the
muscles of the body wall may have some effect, but there is no
special muscular provision to aid in the movement of ingested
material.

The muscles covering the pericardium are continued down
over the otherwise free extremity of the intestine and may have
the action of a sphincter but in the living individuals and in the
sections examined, the lumen of the intestine seems to be quite
as unobstructed in this region as in other regions.

LABIAL PALPS.

These organs are essentially lips and have for their chief, if
not their only, function the conducting of food into the mouth.
There is as in other forms a pair of palps on each side of the

body. The palps on one side of the body are connected with
those of the other above and below the mouth so they resemble
large drawn out lips with the upper lip of each side covering the
under lip of the corresponding side so their inner surfaces, that
is the surfaces continuous with the epithenium of the cesophagus,
are applied to each other. In many kinds of lamellibranchs the
palps consist of inconspicuous smooth flaps of tissue that have
their inner or opposed surfaces thrown into series of ridges and

20

grooves which are densely ciliated. In this form, (figs. 10 and
12, lp.) the portions of the palps that lie along the sides of the
body are of this character but the portions above and below the
mouth are ruffled so they form a large conspicuous mass that
entirely conceals the mouth. What purpose is served by this
ruffled portion that would not be served as well by the simpler
arrangement has not been determined. d

Both palps are free only along their ventral borders. The
outer palps, which correspond to the upper lips, are united to
the body wall above the mouth and along their dorsal borders.
The inner palps, which correspond to the lower lip, are united
to the body wall below the mouth along their dorsal borders and
have their inner surfaces continuous with the inner surfaces of
the outer palps. Their posterior borders, which are not as
extensive as the corresponding posterior borders of the outer
palps are united to the body wall.

Food from the gills passed between the palps is conducted by
the cilia covering their opposing surfaces to the corners of the
mouth, of which the grooves formed by the union of the dorsal
borders of the outer and inner palps are continuations.

GILLS,

As in most lamellibranchs the gills (figs. 11, 12 and 19), are
four in number, there being two on each side of the body. Each
of these gills appears as a thin and delicate striated membrane
that runs from near the mouth, around the ventral side of the
adductor muscle to and a little beyond the anal opening. The
gills are very similar in appearance, pointed anteriorly and pos-
teriorly and marked by distinct striations that radiate from their
lines of attachment near the adductor muscle, toward their free
borders. With proper illumination very fine striations may be
seen crossing these at right angles.

Each gill consists of two thin membranes, called lamellze
(figs. 17, 18 and 19), that lie very close together and are
attached to each other at intervals corresponding to the radial
striations of the gill. Each pair of gills is suspended by a mus-
cular membrane (figs. 19 and 20, sm.), most of the muscles of
which are inserted on the corresponding shell valve near the

21

border of the adductor muscle (fig. 7, sms.). The membrane
however has the appearance of being suspended from the
adductor muscle as connective tissue fibers extend along the
surface of the muscle and bend it toward the visceral mass.
Most of the muscles of the membrane run from the region of the
adductor muscle directly toward the borders of the gills attached
to it, but two distinct bands of muscles are present (fig. 18 lm.),
that occupy positions along the sides of the blood space which
runs along the dorsal borders of the gills and receives blood
from the gills. When the gills are elevated these muscles con-
tract and shorten the gills, at the same time throwing them into
a series of plaits. Each of the gills that are attached to this
membrane is attached by one lamella only. The outer gill is
attached by its inner lamella and the inner gill by its outer
lamella. The remaining lamelle are free along their borders
which are usually somewhat reflected.

The gills of lamellibranchs are usually attached so the dorsal
borders of the outer lamelle of the outer gills are attached to
the mantle and the dorsal borders of the inner lamellz of the
inner gills are attached to the visceral mass, or behind the vis-
ceral mass, to each other. In this way the gills divide the space
between the lobes of the mantle into a ventral space, the
branchial chamber, into which the gills hang, and a dorsal
space, the cloacal chamber, above the gills. This dorsal space
is divided throughout the greater portion of its extent by the
visceral mass and by the membranes that support the gills on
each side, which in the scallop are muscular.

Although the gills on each side do not form the attachments
described, the free edges of the gills are pushed out and make
contacts that correspond with the attachments in other forms
that have been described. It is important that such contacts
should be made as the water that passes through the gills for
purposes of respiration and feeding, in a manner to be described
later, must be constantly renewed from the outside to be effec-
tive for either purpose, and if the outsides of the gills and the
spaces between their lamellz did not communicate with separate
cavities, a current could not be formed. It is also important for
a scallop, which swims by throwing powerful currents of water
from the mantle chamber to have no permanent division between

22

the branchial and the cloacal chambers as the pressure of the
water in the cloacal chamber caused by the rapid closing of the
shell would be certain to injure the gills. At such times the gills
are kept from injury by the contraction of the muscles of the
interlamellar junctions, so the lamellae of each gill are drawn
together and by the contraction of the suspensory membranes
of the gills which draw them away from the margins of the shell
and keep them from being crushed. No doubt the arrangement
of the gills in this form. is to be explained by its exceptional
habits.

As before stated the two lamellz of a gill are attached to each
other at intervals corresponding to the striations that run the
width of the gill. These lines of attachment (fig. 17, ilj.), the
interlamellar junctions, form complete partitions so the space
between the lamellz is divided into a series of tubes, the water
tubes, that are closed, except for minute openings in the sides,
(io.) the inhalent ostia, and where they open into the cloacal
chamber. Each tube extends from the free border of the gill
(figs. 18 and 20), where it is closed by the joined lamellz, to
its opening in the cloacal chamber, and is bounded by the
lamella and by the interlamellar junctions. Of these water
tubes there are several hundred in the length of each gill.

Each lamella is composed of a series of delicate filaments, (fig.
17, gf.)the gill filaments, that run the width of the gill parallel
to the more prominent striations. These filaments are of two
kinds, large ones concerned in the formation of the inter-lamellar
junctions, and small ones. They are all connected at intervals
by cross bars, (ifj.) the inter-filamentar junctions, that run at
right angles to them.

The crossing bars (the filaments and the inter-filamentar
junctions), leave spaces, the inhalent ostia (fig. 17, io.) between
them, that are the openings that have been referred to as
leading into the water tubes. The inhalent ostia are much larger
and more regular in the scallop than in most other lamelli-
branchs, as the lines of fusion that form the inter-filamentar
junctions are not nearly as extensive as in most other forms.

Usually the filaments of one lamella are continuous with those
of the other at the free margin of the gill, so it is quite possible
to trace a filament from the suspensory membrane down one

23

lamella around the margin of the gill and up to the free border
of the other lamella. Whether filaments are always continuous
in this manner or not has not been determined, but in the sec-
tions that have been examined the same number of filaments are
constantly present on the two sides of any given water-tube.

The filaments are very similar in size and appearance, except
those that are concerned in the formation of the inter-lamellar
junctions. These are many times as large as the others and differ
decidedly in shape as well as structure. Those placed next to
these modified filaments are somewhat larger than the remainder
but they do not otherwise differ in appearance or structure.
The number of filaments concerned in the formation of water
tubes is not entirely constant. Nineteen of the small filaments,
between the large modified filaments, is a very common number
but as few as seventeen and as many as twenty-two have been
noticed. No attempt has been made to determine the relative
number of filaments for each water tube in different parts of the
gill but the variations mentioned occur within a space of ten ora
dozen tubes.

Each of the smaller filaments is composed of a layer of sur-
face epithetium that incloses some connective tissue and a large
blood space. The connective tissue is so arranged that quite
universally a strand of tissue extends across the blood space (fig.
21, fs.) from one side of the filament to the other, so in cross
sections of the filament the blood space appears divided into two
nearly equal portions. This Kellogg (14) has quite naturally
taken for a functional division that allows the blood to pass
down one side of the filament and back the other. That this
is not actually its function is indicated by injections of the vas-
cular system that I have made, and by the connections of the
blood spaces of the filaments to the afferent and efferent. vessels
of the gill. There is every indication that the blood moves in
the same direction on each side of the partition, if it is a com-
plete partition. The only reason that I can suggest to explain its
constant presence is that each acts as a brace to keep the filament
from swelling into a cylinder with the pressure of the blood, and
so partially close and interfere with the flow of water through
the inhalent ostia. That there is great need for braces of this
character in filaments shaped like these, where they are not

24

supported along their sides by extensive inter-filamentar junc-
tions as is commonly the case, is evident, but I have not
examined other forms in which the filaments are similar, to find
if similar braces. exist.

The epithelial cells on the outsides of the filaments bear
numerous rather short cilia (fig. 21, fc.) that have to do with
moving currents over the surface of the gill. Between the
filaments, but near their outer borders, the epithilial cells are
modified in shape so they collectively give rise to a strong band
of cilia (oc.) on each side of each filament. In _ transverse
sections of filaments each of these bands appears as a bunch.
These cilia are concerned in forcing water through the ostia and,
thus in creating the currents of water that furnish the food and
oxygen for the animal.

The large modified filaments are roughly triangular in cross
section and like the smaller filaments each has a surface layer
of epithelium. The epithelium on the outer surface of the fila-
ment is thickly covered with cilia that correspond to the surface
cilia of the other filaments but there are no bands of cilia along
the sides. Inside the layer of epithelium is a pair of chitinous
rods (fig. 21, cr.) that run nearly to the free margin of the gill.
These rods are elastic and quite stiff and serve to keep the gill
in shape. Similar rods are present in each filament in many
forms but there seems to be no sign of them in the scallop in
any but these enlarged filaments. Considerable connective
tissue and well developed bands of muscle (mf.) are present,
that together cut up the large blood space (bv’. into a number
of small ones which are, however, connected with each other at
frequent intervals, so they may be regarded collectively as one
blood space.

With corn starch injecting mass these spaces will frequently
inject for half the width of the gill. With gelatine injecting
mass it is quite possible to inject the vessels of the smaller fila-
ments as well and get the connection through the interfilamentar
junctions. In these injections it frequently happens that a fila-
ment will be injected for only a portion of its length and in such
cases the whole cavity of the filament is filled as far as the injec-
tion extends. This indicates that the apparent partition is not
functional as a division between vessels.

25

The muscles of the large filaments are for the most part con-
tinuous with the muscular suspensory membranes. Fibers
extend through the inter-lamellar junctions to the free lamella
and serve to draw the lamella together. Other fibers
extend through the inter-filamentar junctions and serve to draw
the filaments together and so shorten the gill. A nerve
(fig. 21, n.) is frequently present near the inner border (the
border away from the outer surface of the gill) of the filament.
How universally this is true has not been determined but I have
frequently been unable to discover such nerves. Again each of
a series of filaments may have its nerve. Branches from these
nerves have been traced into the inter-filamentar junctions and
presumably give out branches in turn to the filaments. They
probably also control the muscles of the larger filaments, inter-
lamellar junctions and inter-filamentar junctions. The inter-
filamentar muscles are especially active in gills hat have been cut
from the animal, and keep the gills in almost constant move-
ment, folding the lamellz and allowing them to straighten, as
they contract and relax in different portions. The large fila-
ments of one lamella are united to the large filaments of the
other lamella by rather thin membranes, the inter-lamellar junc-
tions (figs. 17 and 18, ilj.) that are thickened along their free
borders, where a large blood vessel is present. They are likewise
more extensive along this border so the lamella may be sepa-
rated quite a distance along the upper border of the gill. The
shape of the membranes is such as to allow greater separation
than would be the case if the attachment extended straight
across from one lamella to the other. The bend that is made,
allows great freedom of movement to the free edge of the
lamella, which may thus be separated from the attached border
of the other lamella of the gill for a space of half an inch or
more.

The inter-filamentar junctions (fig. 18, ifj.) are much heavier
along the upper margins of the lamelle than toward the free
edge of the gill. These junctions are very muscular, and are
much heavier near the large filaments than they are toward the
middle of the water tube (fig. 17, ifj.). They join the filaments
to each other and extend into the cavity of the water tubes as
rather prominent ridges. Each inter-filamentar junction con-

26

tains a rather large blood vessel that is connected with the ves-
sels of the large and the small filaments, and thus serves to
distribute blood either from or to the large vessels of the large
filaments, which are connected in their turn to the vessels that
supply blood to or take blood from the gills.

As the free edges of the gills are approached the inter-fila-
mentar junctions become less and less prominent until, near the
margin, the filaments near the middle of the water tubes are
connected only by bunches of cilia like those in the mussel,
Mytilus, and some other forms. That the junctions of this
region should be less prominent is what might be expected, for
the margins of the gills, after the gills have become sufficiently
developed to show adult structure, are the growing and conse-
quently the youngest portions. That the filaments should at
first be connected by cilia only, may be looked upon as an indi-
cation of past history. The scallops presumably have had
ancestors in which the gill filaments were united by cilia only.

Inasmuch as the gills are respiratory orgens, the arrangement
of the blood spaces in them is of more than ordinary interest.

Two blood vessels are present in each of the suspensory mem-
branes of the gills, near the borders of the gills that it supports.
These vessels follow along the borders of the gills from near
their anterior to their posterior ends. One, the dorsal, that is
the one farthest away from the gills, supplies both of the gills
with blood, the other, the ventral, which is very near the borders
of the gills, receives the blood that is returned from both of the
gills. The blood enters each gill by branches from the supply-
ing vessel (figs. 17 and 18, ba’.) that run along the upper bor-
ders of the inter-lamellar junctions to the edge of the free
lamella. Here they enter the large modified filaments of this
lamella (fig. 17, ba”.) and are continued down to the margin
of the gill, giving off vessels to each of the inter-filamentar junc-
tions except those near the margin of the gill, which consist of
cilia only and are accordingly not vascular.

Through the inter-filamentar junctions the blood is supplied
to the small filaments, so the blood vessels become a net work
that corresponds to the structure of the gill itself. The blood
makes its way around the margin of the gill, through somewhat
broken passages to the other lamella. This takes place all along

27

the margin in both large and small filaments. The vessels in
the other lamella are similar to those already described, the blood
being collected on this side into the vessels of the large filaments
(fig. 17, bv’.), and finally poured into the vessel at the bases of
the gills, which conducts the blood back to the heart (fig.
18, bv.).

It is quite possible to make out all of the connections
described, in sections of the gills but the arrangement of vessels
has been further demonstrated by injections of the gills with
starch and with gelatine injecting masses. This can readily be
accomplished through the supplying and receiving vessels
of the gill with a hypodermic syringe. The animals are large
and the vessels are fairly distinct so with a little practice it is
quite easy to make successful injections. With a starch mass
the vessels may readily be injected different colors as the mass
is too coarse to pass out into the smaller vessels and complete
the circuit. By this method it was easy to determine that the
blood passing to the gill all passed along the borders of the
inter-lamellar junctions to the free lamella and that all of the
blood entering the vessel that carries the blood away from the
gill comes from the attached lamella.

The general relation of the blood spaces of the gills to the rest
of the circulatory system will be discussed under the head of the
circulatory system.

The movement of water for respiration and feeding is depend-
ent entirely upon the bands of cilia on the sides of the filaments.
These, acting like so many small paddles, force the water through
the inhalent ostia into the water tubes, thus driving the water
along the water tubes into the space above the gills that corres-
ponds to a cloacal chamber, and so out of the shell along the
margin posterior to the adductor muscle and dorsal to the gills.
The current of water which enters to take the place of that
driven out is taken in anywhere along the ventral and anterior
borders of the animal. By changing the position of the margins
of the mantle so access is given at one place and denied at
another, it is possible for the animal to vary the places where
water is admitted and ejected. Whether this serves any definite
purpose or is more by way of accident caused by performing
other functions of the body, is not known, but powdered carmine

28

allowed to settle in the water past the margins of the shell of an
extended individual will show that such variations in the cur-
rents do occur.

This current of water not only supplies the means of respira-
tion, allowing the blood that is passing through the gills to
become charged with oxygen and to rid itself of carbon dioxide,
but it serves to supply the animals with food.

The food for the most part consists of microscopic plants
which are strained out of the water that passes through the
inhalent ostia. These are passed along the surfaces of the gills
by the cilia that cover the surfaces of the filaments, to their free
margins and along the margins to the anterior ends of the gills.
Here they are passed between the labial palps which inclose the
anterior ends of the gills, and so on into the mouth.

Attention has not been given to the action of the feeding cilia
in this form, but Kellogg (15) and Stenta (31) have found that
the action is apparently under control in many forms, so food
that is passed over the surface of the gills may be carried to the
palps or may be passed onto definite tracts of cilia on the mantle
that carry it away and finally eject it from the mantle chamber.
It would thus seem that while the cilia on a gill are active, food
is being strained out but that the animal may or may not eat
the food gathered.

The ability to accept or to reject solid material that is brought
to the gills in the current of water that is formed by the cilia on
the gills is indicated by examining the stomach contents, as well
as by the observations made by Kellogg and Stenta. The study
of the food that is in the stomach shows that there has actually
been selection of materials and that the ability to reject is not
simply to allow the animal to continue respiration without feed-
ing, for there are many forms of diatoms that are abundant in
the water in which the animal lives that are not present in its
stomach. The observations of Grave (8) on the food of the
oyster indicate how great this selection may be and no one who
has given any attention to the stomach contents of lamellibranchs
will doubt that food selection is common among members of the
group. No observations have been made on the speed with
which food is gathered by scallops but as the method of feeding
is so similar for most lamellibranchs the observations made by

29

Grave (8) on the oyster are of great interest. By a series of
careful experiments he determined that oysters that had been
kept out of water and in filtered water until most of the food
had been digested or passed through the alimentary canal, col-
lected on an average, upon being returned to the bottom from
which they were taken, “ 385 diatoms during the first hour, 550
during the second, 1,406 during the third and 4,301 during the
fourth. This increasing rate of feeding is probably due to the
gradual recovery on the part of the oysters from the shock of
their unusual treatment in the laboratory. The rate at which
feeding took place during the fourth hour is probably nearer the
rate at which it occurs with oysters living undisturbed on the
beds.”

From these and other observations Grave draws conclusions
regarding the length of time that an oyster must feed and the
amount of food that water must contain, in order that oysters
shall get proper food supply, that are open to criticism. He
says, ““ The work on the food resources of Newport river show
the average number of diatoms per liter (or about a quart)
available to oysters on the natural beds, during the summers of
1900, 1901 and 1902 to be 23,432, and that the oysters of salable
size examined during this time contained, on an average, 11,453
diatoms. If the usual rate of feeding under natural conditions
is near the figure obtained from the above experiment, 4,301
diatoms per hour, then three hours is ample feeding time for an
oyster; and taking 23,432 as the average amount of food con-
tained in a liter of water over the natural oyster ground, it
follows that in collecting its daily meal (11,453 diatoms) an
oyster must filter altogether about 500 cc., or 16 oz. of water,
and that about 167 cc., or 5% oz., are filtered per hour.” The
error is in taking 11,453 diatoms, the average number to be
found in an oyster’s stomach at one time, as the average daily
ineal. This does not take into account the rate of digestion and
accordingly the number that actually pass into the alimentary
canal in a day is an unknown quantity. The observations indi-
cate, however, that the number of diatoms used by an oyster is
enormous and that the part taken by lamellibranchs in convert-
ing this great wealth of food material into a form that is avail-
able for the higher animals is very considerable. (See
Brooks, 3.)

30

MUSCULAR SYSTEM.

The muscles of each of the organs are best described in con-
nection with the structure of the organ concerned. As _ the
muscle that functions in closing the shell is the only one that
would not naturally receive attention in describing the organs,
it will be necessary here to describe in detail only the adductor
muscle.

There is but one adductor muscle in the adult scallop and this
corresponds to the posterior muscle of those forms that possess
two muscles. In the very young scallop the anterigr adductor
muscle is present (fig. 35, aam.) and for a time is the only
functional one, but the posterior muscle soon makes its appear-
ance. At what stage of development the anterior adductor
muscle is lost has not been determined but a scallop a centimetre
(half an inch) in diameter shows no indication of such a muscle.

In the adult scallop the adductor muscle has a greater area
of attachment on the left, upper, valve than on the right, and
the scar on the shell is comparatively smooth and _ indistinctly
bounded. Usually there: are some markings indicating the
division of the muscle into bundles but they are not nearly as
prominent as on the other valve.

The muscle is quite definitely divided into two portions. An
anterior large rounded portion (fig. 10, pa.) and a posterior
somewhat crescent shaped portion (pa’.) that is applied on its
concave side to the anterior portion. The left end of the pos-
terior portion is a somewhat narrower and longer crescent than
the right end and is applied much more closely to the anterior
portion of the muscle than at the right end (figs. 9 and
16). Where the muscle is attached to the right shell
valve the separation of the anterior and posterior portions is
marked by a deep cleft on the ventral side of the muscle. This
cleft extends along the ventral side of the muscle for nearly half
the length of the muscle where it becomes a very distinct line
that may readily be followed to the other end of the muscle.
The two portions of the muscle differ in color, the posterior
portion being darker, and their physiology is quite different.
The large anterior portion may be entirely severed and the pos-
terior will close the shell with nearly as great rapidity as was the
case before the muscle was injured, but if the posterior portion

31

is severed and the anterior portion is left intact, the animal will
not close its shell. If the shell valves are pressed together the
muscle will not hold them, but they separate immediately when
they are released. The meaning of this is not clear. I feel sure
that it is not due to severing any nerve as the muscle has been
carefully scraped from the shell with the same results. That
there is some explanation that investigation will reveal seems
likely. My own work has for the most part been carried on at
some distance from the seashore, and the opportunity to inves-
tigate the action of the muscle has not yet presented itself.

Other prominent muscles of the body beside the usual inter-
lacing muscles of the body wall are the radial and circular
muscles of the margins of the mantle (fig. 16, rpm. and cpm.)
the muscles of the suspensory membranes of the gills (fig. 20,
sm.) and the retractor muscle of the foot (fig. 10, fm.) which is
here confined to the left side. All of these are described in con-
nection with the organs with which they are associated.

EXCRETORY ORGANS.

These organs lie just anterior to the adductor muscle, against
which they are flattened, between the visceral mass and the sus-
pensory membranes of the gills (figs. 12 and 20, e.). Each
organ forms an elongated sac like body that runs from the
extreme latteral prolongations of the pericardium ventrally,
around the adductor muscle, and opens into’ the mantle chamber,
above the gills and about one-third of the diameter of the
adductor muscle from its ventral margin. The openings of the
kidneys into the mantle chamber are large, slit like, and guarded
by somewhat thickened lips. Not uncommonly the excretory
organs of lamellibranchs consist of long coiled tubes, each organ
being a single tube which may be nearly or quite cylindrical and
of nearly even diameter, or the tube may be greatly sacculated
or have certain enlargements. Such long coiled tubes strongly
suggest nephridia, and they may be looked upon as
modifications of this structure. Not uncommonly the organ
is divided into a glandular and a non-glandular portion as in the
fresh-water muscle, but it is usually coiled to the extent of
possessing at least one loop.

32

In the scallop, however, the organ is of a calibre that suggests
a sac more than a tube, that curves only to follow the curvature
of the muscle and opens at one end by a rather broad opening
into the pericardial cavity and at the other, by the slit like open-
ing already described, into the mantle chamber. The pericardial
opening of the right organ is a little more dorsal and a little
nearer the mantle than the other. This seems to be caused by the
shape of the adductor muscle which spreads out near its attach-
ment to the left valve so the left excretory organ is forced in
toward the median line of the body.

In structure the excretory organs are practically racemose
glands. The pockets that are frequently found in the walls of
the nephridia of other forms are here greatly extended and
branched. The thick walls of the organs allow this without
causing a roughened exterior. There is no division of the organ
into glandular and non-glandular portions but it is glandular
throughout.

The excretory organs are joined by the genital ducts near
their inner, pericardial ends. The relationship of the two is
further described in connection with the genital organs.

GENITAL ORGANS.

These organs occupy the greater part of the portion that has
been called the visceral mass (fig. 12, vm.).

In this species of scallop the sexes are separate and may
easily be distinguished by the color of the sexual products which
give the color to the parts of the body containing them. In the
female the color is bright salmon pink to dull pink, apparently
differing with the number of eggs and possibly also with the
maturity of the eggs. In the male the color is white or with a
tinge of yellow. Pecten irradians is hermaphroditic, with the
male portions of the organ dorsal, that is, near the foot, and the
female portion in the remaining large, ventral and posterior
portions of the visceral mass. The male portion is here white,
the female brownish yellow or orange. Both kinds of sexual
products are matured at the same time and there is considerable
reason to believe that individual fertilization may be and possi-
bly frequently is accomplished.

33

In the giant scallop the distribution of the gential organs is
the same in both sexes. The organs occupy nearly the whole
of that portion of the body that lies beneath the foot that is not
occupied by the alimentary canal, and extend up dorsal to the
foot so as to form a thin layer over the surface of a portion of
the liver. When the organs are gorged with their products the
portions of the body that contain them are plump and compara-
tively large. When spawning. has been completed they are
shriveled and small. In the adult there is no apparent separa-
tion into a pair of organs farther than by the possession of a
pair of ducts. These are not very conspicuous and enter the
kidneys of the respective sides near their dorsal ends. From
this point out, the sexual products traverse the lumen of the
kidney, so they are finally expelled into the water through the
external opening of the kidney.

The products are expelled from the openings of the kidneys
in streams. The animal occasionally flaps its valves together dur-
ing the process so the products are thrown out of the shell and
dispersed in the water. For the most part the animal lies with
the valves separated and is rather indifferent to outside stimula-
tions. At such times it is sometimes possible to pick a specimen
up out of the water without causing it to close its shell. Soon
after removal from the water, however, the animal recovers and
responds as usual. Replaced in the water it may or may not
immediately begin to spawn again.

The relation of the genital ducts and kidneys of lamellibranchs
has long been considered as important for its possible bearing
on the relation of the kidneys to nephridia and the pericardium
to a ccelom. In the adult of this form the sexual ducts open
into the kidneys near their pericardial ends. Nothing is known
about their developmental relation. The openings are much
farther from the external openings of the kidneys than I have
found to be the case in Yoldia lamatula (5) or Nucula delphino-
donta (7) but not so far as in the case of Solenoma where, as
Stempell has found (30) for one species (Solemya togata) and
I have verified for another one (Solenomya velum) the ducts
open very near the pericardial ends of the kidneys. In view of
what we know about these and other species studied and
reported by Pelseneer, (22) Stempell (29) and others, where it

34

would seem that we have all gradations from separate openings,
near the outer end of the kidneys, openings near the peri-
cardial end of the kidneys, ‘and double openings, so the genital
ducts may be connected with the kidneys by branchs and
be continued to the outside as well, most of which arrangements
are present among members of the Protobranchia, it is still very
doubtful whether any significance can be attached to the rela-
tionship of the genital ducts and kidneys in different forms.

CIRCULATORY SYSTEM.

The animal is large enough to allow one to successfully inject
the chief vessels with starch or gelatin injecting masses, and
then by dissection and microscopic preparations to trace the dis-
tribution of the vessels of the different organs and to determine
quite definitely the course taken by the blood in its circulation.

The heart is a typical, symmetrical lamellibranch heart with
two auricles and one ventricle (fig. 11, 13 and 20) the latter
perforated by the intestine which enters it near one end and
leaves it near the other end. Dorsally the ventricle is prolonged
somewhat, posterior to the intestine, where the morphologically
anterior aorta is given off, and ventrally to a less extent it is
prolonged anterior to the intestine, where the much smaller
morphologically posterior aorta is given off. The walls of the
ventricle are of about even thickness throughout their extent,
and are quite smooth outside and inside. The auricles join the
ventricle on each side near its middle, are somewhat triangular
in shape, with the most acute angle receiving blood from the
gills and mantle, at a point dorsal to the adductor muscle, and
directly ventral to, but some distance from, the cartilage.
The opening of each auricle into the ventricle is near the middle
of the side of the auricle that lies next to the ventricle and
farthest away from the opening where the auricle receives its
blood. The muscles around the openings of the auricles into
the ventricle, and to a less extent around the openings through
which the auricles receive blood, are well developed and must
act as spinctors that tend to keep the blood from being regurg-
itated. The walls of the auricles, unlike those of the ventricles,
are roughened by pits that open into the cavities of the auricles.
These seem to be formed by the arrangement of bands of muscle

68)

fibers along the borders of the pits. The arrangement, gives the
outside of the auricle a pebbly appearance that is very striking.
Both auricles and venticle are composed of interlacing muscle
fibers, and are capable of great extension. In preserved speci-
mens, the heart is usually contracted and is not very conspic-
uous. In such contracted hearts the cavities of both auricles
and ventricle are practically obliterated.

The heart lies in a somewhat triangular, spacious, pericardial
cavity that is dorsal to the posterior half of the adductor muscle,
and ventral to the posterior portion of the liver. Posteriorly,
it is covered only by a somewhat thick, muscular membrane
which separates it from the mantle chamber.

As already mentioned, two blood vessels leave the ventricle
(figs. 11 and 13), one from each end. Although they are not
so placed in reference to the ways the terms have been used in
describing this form, the two ends correspond to the anterior
and posterior ends of the ventricle in most forms of lamelli-
branchs. The posterior aorta is much the smaller of the two,
leaves the heart ventral to the intestine (actually anterior to it)
and divides immediately after leaving the heart, into two
vessels, one of which, the smaller, follows along the intestine
supplying it and surrounding portions with blood. The other
vessel turns almost at right angles upon leaving the aorta and
enters the adductor muscle, where it divides into a system of
vessels that supply the muscle with blood.

The anterior aorta is much larger than the fasted: aorta,
and supplies all of the remainder of the body. It leaves the
ventricle dorsal to (actually posterior to) the intestine and very
soon gives rise to a vessel which passes into and supplies the
wall that separates the pericardial cavity from the mantle cham-
ber. From the pericardium the anterior aorta follows along the
postero-dorsal border of the liver to the base of the ear. Here
it gives rise to a branch (fig. 13, ppa.) which passes posteriorly
to the extreme upper margin of the mantle that lines the ear,
giving off along its course a number of branches which supply
this portion of the mantle. Here it divides into two vessels, a
right and a left, each of which bends abruptly ventrally (fig.
9, ppa.) and follows along the margin of the respective mantle
lobe about opposite the line of attachment of the infolded ridge

36

of the mantle, alongside but external to the pallial nerve.
Very fine branches are given off from these vessels all along
their courses, which further divide to form systems of capillary
spaces that are finest and most numerous near the margins.
Some of these capillary spaces are large enough to be injected
with starch mass, and | have a preparation of the mantle lobe
from which only the infolded ridge has been removed that was
dehydrated, cleared, and mounted in balsam, in which the whole
system of vessels can be traced. A gelatin mass not only fills the
spaces mentioned, but passes out between the cells so that in
sections it may be seen to be diffused throughout the tissue.
This seems to hold good for all other parts of the body with the
exception of the gills, in which organs the mass is more com-
pletely, but not entirely, confined to the blood spaces. The indi-
cation therefore is, that the blood spaces are not confined ves-
sels, and that the blood functions as both blood and lymph.
The posterior pallial vessel may be traced far anteriorily, grad-
ually diminishing in size along its course. Here it finally joins
the anterior pallial vessel. The anterior pallial artery (fig. 13,
apa.) leaves the anterior aorta very near the cartilage and runs
directly to the anterior border of the hinge region of the mantle,
giving off vessels to this portion of the mantle on the way.
Here it branches into right and left vessels, each of which bend
abruptly ventrally (fig. 9, apa.) and pursues a course along the
anterior border of the mantle similar to that taken by the pos-
terior pallial artery at the other extremity of the animal.

Along the anterior border of the mantle, near the dorsal line,
the vessel is rather small and slightly broken in its course. It
may be possible that this represents the border line between the
posterior and the anterior pallial arteries. There are other
reasons for believing that a large share of the animal is mor-
phologically equivalent to the posterior portions of other forms,
and that the anterior portion is greatly reduced. This has
received attention in another place.

Several vessels leave the anterior aorta to supply the liver and
stomach. Most prominent among these is a vessel which leaves
the aorta between the points of origin of the anterior and pos-
terior pallial arteries. This bends out toward the left side of
the liver, where, in injected specimens, it is very conspicuous,

A

d/

passes ventrally and sends branches to the major part of the liver
and to the stomach.

A short distance in front of the cartilage the anterior aorta
bends ventrally, passes through the liver and gives off a few
small branches to it, sends a vessel to the palps in passing, and
passes on to supply the foot and the visceral mass. The vessel
that supplies the foot (fig. 13, fa.) leaves the aorta a short dis-
tance ventral to the mouth, passes along the body wall until the
foot is reached and extends into the foot along its dorsal border.
Just before entering the foot this, the pedal artery, gives rise to
a small vessel that passes posteriorly along the single retractor
muscle of the foot supplying it with blood. From the point of
origin of the pedal artery the aorta extends into the visceral
mass following along the enlarged portion of the intestine that
leads away from the stomach, and supplying this and other por-
tions of the intestine and the reproductive organs with small and
with large branches. The enlarged portion of the intestine that
comes from the stomach is especially well supplied (compare
figs. 12 and 13), there being numerous small branches that are
given out directly from the aorta, and large branches that follow
along on the different sides of this portion of the intestine and
likewise supply it with branches. A short distance ventral to
the foot a large branch leaves the aorta and passes postero-
ventrally to divide again and form small branches that supply
the remaining loops of the intestine and the postero-ventral por-
tions of the reproductive organs.

This completes what might be called the systemic arterial
system. Beginning with the heart the system ends in the capil-
lary spaces of the various organs. This system is most easily
injected through the vessel in the suspensory membrane
of the gills that is farthest from the adductor muscle,
(fig. I1, bv.) with a hypodermic syringe, injecting toward
the heart. If a starch mass that will not pass through the
capillary spaces is used, all of the vessels thus far described will
be injected, as will also the veins that return blood from the
gills, as this vessel is the one that returns blood from the gills
to the heart. If a gelatin mass is used all of the systems may
be injected, but as the injecting mass may pass out of the spaces,
between the cells of the various organs, such injection does not
aid in tracing the course of blood flow.

38

The systemic veins (fig. 14) that collect the blood that is sup-
plied by the systemic arteries from the various organs of the
body, may be injected from several different vessels. They may
be injected by pushing the needle beneath the mmbrane that
covers the posterior surface of the adductor muscle. A large
blood space occupies this position, into which the needle is
inserted and the mass injected fills the systemic veins. Another
point from which these veins may be injected is from one of the
superficial vessels of the visceral mass. ‘These vessels are very
conspicuous, and may be very easily picked up with the needle.
Still another vessel is the vein that returns blood from the liver,
which may be seen on the left side of the animal anterior to, but
near the large artery that supplies the liver. Injecting any one
of these vessels will to a greater or less extent inject the others,
but there does not seem to be an entirely free communication
between them. They all carry blood to the kidneys, and seem
to empty into a common sinus on either side, that lies alongside
the kidney in the walls of the visceral mass. The sinuses of
the two sides are conneected beneath the adductor muscle, but
it frequently happens that a complete injection of the system is
not obtained from an injection from any one of the veins men-
tioned. Just where the obstruction lies in such cases has not
been determined. It has been noticed that obstructions are more
likely to be encountered in injecting from the veins of the vis-
ceral mass than in injecting any of the others.

Inasmuch as blood spaces are cut in removing the muscle from
the shell it has been found desirable in injecting this system of
vessels to wedge the valves open and to inject from the posterior
surface of the adductor muscle. In injecting after the animal
is removed a considerable quantity of the injecting mass is sure
to escape at the ends of the muscle.

The position of the veins may be seen in figure 14. A large
vein comes from the liver, another from the foot, and the
veins in the muscle unite to form a more or less definite sinus
along the dorsal border of the muscle, and two smaller ones on
the anterior and ventral side of the muscle. ‘These sinuses unite
near the anterior end of the kidneys. A series of vessels from
the visceral mass unite along the borders of the kidneys and
finally connect with these sinuses. Most of the blood from all of

39

these organs is distributed to the kidneys through systems of
capillary spaces. The branching of these vessels is not conspic-
uous on the surface of the kidneys, but is better seen by
cutting the kidneys open. ‘That not all of the blood necessarily
traverses the capillary spaces of the kidneys is indicated by the
fact that injections of the systemic veins frequently fill the vein
that carries blood away from the kidneys as well as those lead-
ing to it. This is much more frequently the case when injecting
from the posterior surface of the adductor muscle than when
injecting from other places, and seems to be dependent upon a
direct connection between the vessel in question and the sinuses
on the anterior and ventral surface of the adductor muscle near
the dorsal ends of the kidneys.

Of the blood that leaves the heart, only that which goes to the
mantle remains to be accounted for. This is collected and
returned directly to the heart (fig. 9, pv.).

All of the blood that leaves the kidneys is conducted to the
gills. The blood from each kidney is collected into a sinus that
runs along the border of the kidney that is applied to the adduc-
tor muscle. This sinus, which also seems to receive blood from
the sinuses on the anterior and ventral surface of the adductor
muscle, bends abruptly ventrally over the anterior end of the
kidney and is continued on the lower border of the suspensory
membrane of the gill (fig. 11, ba.) to the posterior end of the
gill, supplying the gill with branches throughout its length.

The blood vessels of the gills have been described in connec-
tion with the structure of the gills, but for the sake of com-
pleteness the course of the blood through the gills will be traced
in this connection.

Blood vessels leave the vessel that carries blood from the
kidney, opposite each of the inter-lamellar junctions of each of
the gills supperted by the suspensory membrane. Each of these
branches is continued along the free border of the membrane
that forms the inter-lamellar junction (figs. 17 and 18, ba’.)
until it reaches the free edge of the lamella, the edge that is not
attached to the suspensory membrane. That is, if the branch
supplies an outer gill, it leaves the suspensory membrane along
the free border of an inter-lamellar junction and crosses over
to the free border of the outer lamella of this gill. Here the

40

vessel is continued down the enlarged, modified filament that is
concerned in the formation of the inter-lamellar junction (fig.
17, ba”.) giving out side branches through each. of the inter-
filamenter junctions (as long as these are composed of tissue
that can carry blood vessels) and so supplies the various fila-
ments of the lamella. The blood thus distributed finds its way
around the margin of the gill through small blood spaces and is
continued up the other lamella of the gill, the blood of the small
filaments being gradually collected through the vessels of the
inter-filamenter junctions into the vessels of the large filaments,
(fig. 17, bv’.) and by these poured into a vessel that lies just
beneath the vessel that supplies the gill and runs parallel with
it (figs. 11 and 18, bv.). This vessel receives all of the blood
from both of the gills of the side, and carries it directly to the
corresponding auricle of the heart. Just before the vessel
empties into the heart it receives a rather large vessel from the
corresponding lobe of the mantle which returns the blood that
was sent to the mantle, back of the heart.

To sum up the course of the circulation of the blood briefly,
st will be seen that of the blood that leaves the heart only that
which is sent to the mantle is returned to the heart after travers-
ing a single get of capillary spaces ; that a small portion of the
blood sent to the adductor muscle (that which is collected by
the sinuses on the antro-ventral portion of the muscle) may be
returned after traversing two sets of capillaries—those of the
adductor muscle and those of the gills; and that the greater por-
tion is returned only after traversing three sets of capillaries—
those of the general system, those of the kidneys, and those of
the gills. ’

The reasons for this arrangement of the circulatory system
are at least in part not hard to explain. The blood which
passes to the mantle loses some of its nourishing materials,
but as the mantle lobes are thin and are bathed over such a
large portion of their surfaces by a current of water, in which
there is an abundance of dissolved oxygen, respiration, no
doubt, takes place direct, and the blood has no need to pass
through the gills to get a supply. Again the work of the
mantle is not of such an active nature as to load the blood with
nitrogenous wastes. It seems likely that the amount of nitro-

AI

genous waste in the blood that has traversed the mantle is so
small that it would diminish the proportion of nitrogenous
waste in the blood, if this blood were added to the blood that
passes through the kidneys.

The blood that goes to the general system must in its progress
lose a considerable portion of its oxygen, and (in all portions
except around the alimentary canal, where there is, of course,
a decided gain) food materials, and gain from the waste of the
tissues a considerable amount of nitrogenous and carbonaceous
wastes. It is then essential that such blood should go to the
excretory and respiratory organs to get rid of these waste prod-
ucts and to gain oxygen. Inasmuch as the heart provides for
but a single circulation it is, of course, necessary that the capil-
laries of these organs be traversed before the blood is returned
to the heart. Why it is arranged so part of the blood may
dodge the kidneys and be carried directly to the gills is not
nearly so evident. Possibly the periodically great activity of
the adductor muscle causes the blood to move through it so
rapidly that the small kidneys cannot take care of it and prop-
erly perform their function, and the other channel is provided
to carry the surplus away to the comparatively extensive gills
where the increased flow can be taken care of with greater ease.
It is, of course, essential that the amount of orygen in the blood
at such times shall not be reduced. It is at any rate evident
that there is a possibility that part of the blood that is returned
from the muscle, liver, etc., may not pass through the kidneys,
for when starch injecting mass is injected through a vessel that
carries blood from one of the kidneys to the gills not only are the
kidney and the gill injected, but part-of the mass usually finds
its way into the adductor muscle, liver, and other organs of the
body.

The rate of the heart beat is slow, and as in other lamelli-
branchs is, no doubt, dependent upon the temperature of the
animal as well as on other factors. The auricles and ventricle
become very greatly distended during diastole, and contract so
that their cavities are almost entirely obliterated in systole.

42

NERVOUS SYSTEM.

The three pairs of ganglia that are usually found in lamelli-
branchs are present in this form, but they differ greatly in size
and they are not all placed in the usual positions.

The cerebral ganglia (fig. 15, cg.) are placed some distance
ventral to the mouth, just beneath the outer covering of the
body. They, like the other ganglia, are yellowish in color, and
may frequently be faintly seen through the covering of the body.
Each cerebral ganglion is somewhat elliptical in outline with
the long axis directed dorso-ventrally and has a rather distinct
swelling on the ventral (actually anterior) and outer side (the
side away from the median plane of the body) (fig. 24, cg).
The anterior end of each cerebral ganglion presents a forked
appearance, due to the origin of two large nerve cords. The
inner and ventral one of these two cords (figs. 23 and 24, cc.)
is the commissure that joins the two cerebral ganglia. As the
ganglia lie some distance ventral to the cesophagus, this com-
missure forms a long loop that passes dorsally around the
cesophagus just posterior to the mouth. The outer and pos-
terior of the two large cords that leave the anterior end of each
ganglion is the anteror pallial nerve (figs. 15, 23 and 24, apn).
This runs parallel with the commissure as far as the cesophagus
and is then continued along the side of the liver and in the
mantle, to the margin of the mantle in the region of the anterior
ear of the shell, where it joins by several branches the circum-
pallial nerve (cpn.) that follows along the margin of the mantle
near the bases of the tentacles and eyes. The circum-pallial
nerve will receive attention later.

Between the points of origin of the cerebral commissure and
the pallial nerve, a small nerve (figs. 23 and 24, pn.) leaves the
ganglion to be continued dorsally, and to supply the labial palp.

From the inner, ventral surface of each cerebral ganglion, a
little in front of the middle, the cerebro-pedal connective leaves
to join the pedal ganglion of the same side. The cerebro-pedal
connective is smaller near the cerebral than the pedal ganglion
(fig. 24, cpe.) and bears a ganglionic swelling on its outer side
very near the pedal ganglion.

In the acute angle formed by the surface of the cerebral
ganglion and the cerebro-pedal connective, a small nerve (otn.),

43

the otocystic nerve, leaves the ganglion to be continued around
the dorsal surface of the cerebro-pedal connective to the otocyst
of the same side. This nerve will receive attention later.

Posteriorly the cerebral-ganglia taper rather gradually into
the cerebral-visceral connectives which run along the sides of
the visceral mass very near the adductor muscle until the vis-
ceral ganglia are reached.

The pedal ganglia lie very near each other (fig. 24, pg.), so
the commissure that connects them is short and broad and pre-
sents ordinary ganglionic structure. They are separated from
the cerebral ganglia only by a short interval, and lie anterior
and slightly ventral to them, some distance dorsal to the base
of the foot. They lie so near the surface that their color may
frequently be distinguished through the body wall beneath the
mouth. Two large nerves (fn.) leave each pedal ganglion to
be continued into the foot where they supply the muscles of the
foot and probably the byssal gland. The swellings on the cere-
bro-pedal connectives near the pedal ganglia have already been
described. The otocystic nerves which usually leave the cerebro-
pedal connectives near the pedal ganglia, in this form originate
directly from the cerebral ganglia near the point where the con-
nectives leave the ganglia.

The visceral ganglia (figs. 15, 23, and 25, vg.) are by far the
largest and most complicated of the ganglia, and from them
nerves are sent to most parts of the body. They are situated
on the antro-ventral surface of the adductor muscle, nearly oppo-
site the external opening of the kidneys. They are imbedded in a
mass of connective tissue and are fused to each other so the
commissure that connects them is nearly as broad as the ganglia
themselves and shows ganglionic structure. The chief indica-
tion of the presence of a pair of ganglia is the arrangement of
the nerves that leave them, and of the celebro-visceral connec-
tives that join them. The ganglia are divided into very definite
regions, each of which is connected with definite bundles of
nerve fibers and, no doubt, has a particular function to perform.
I have not had time to make a detailed study of the structure
and nerve tracts of the ganglia, but I am satisfied that there is
much more complexity than is ordinarily attributed to the gan-
glia of lamellibranchs. The dorsal surfaces of the ganglia are

44

quite smooth, but when seen from the ventral surface (fig. 25)
‘the regions that are indicated in the figure are always visible.
On each cerebro-visceral connective, just before it joins the
ganglion proper, there is a ganglionic swelling (x.) that sup-
plies one of two roots of a nerve (figs. 15, 23, and 25, bn.) that
leaves in an antro-dorsal direction along the border of the excre-
tory organ, to bend ventrally and posteriorly in the suspensory
membrane of the gills, and supply the gills of the correspond-
ing side. Between the points where the cerebro-visceral con-
nectives join the visceral ganglia, on the ventral side, there are
four rather distinct swellings, with three less distinct swellings
posterior to them. Extending laterally from the outer side of each
ganglion is a somewhat flattened ridge (fig. 25. y.) from which
all of the pallial nerves from this ganglion originate. These
nerves (figs. 15 and 23, ppn.) pass laterally, posteriorly and
anteriorly along the surface of the adductor muscle, to meet
the mantle lobe and to be continued to the margins, where they
unite with the circum-pallial nerve. It will be noticed that they
unite with the circum-pallia! nerve at intervals throughout the
greater length of these nerves. As the pallial nerves that leave
the visceral ganglia are in most forms distributed to the pos-
terior portion of the mantle only, the distribution in this form
may be looked upon as evidence that all of this portion of the
mantle belongs morphologically to the posterior portion of the
animal.

Other nerves leave the dorsal surface of the visceral ganglia
near their posterior ends, and enter the adductor muscle directly.
The nerves that supply the posterior division of the muscle are —
continued along the ventral surface of the anterior portion of
the adductor muscle until this posterior portion is reached.
Small nerves also leave the ventral side of the ganglia and pene-
trate the visceral mass.

All of the ganglia are well supplied with nerve cells, there
being very many large polar cells present, but the number of the
cells is far greater and their arrangement more complicated inthe
visceral than in any of the other ganglia.

Nerve cells are also to be found in the circum-pallial nerves
and in the branchial nerves. So abundant are the nerve cells
in the circum-pallial nerves that they assume the structure of

45

ganglia. The nerves by which they are connected with the
visceral and cerebral ganglia contain no ganglionic cells. From
the structural standpoint we would accordingly be justified in
considering the circum-pallial nerves as separate ganglia, and
the nerves connecting them with the visceral and cerebral gan-
glia as connectives.

_ The circum-pallial nerves of the two lobes of the mantle are
connected with each other anteriorly and posteriorly near the
hinge line (fig. 23, cpn). They are not of constant diameter but
suddenly increase or diminish in size so that they have a rather
irregular appearance. They lie just inside, that is, toward the
median plane of the body, of the large pallial arteries that supply
the mantle margin (fig. 26, cpn.) about opposite the line ot
attachment of the infolded ridge. From them nerves are sent
to the eyes and tentacles, to the infolded ridges and to the pallial
muscles. Very likely the pallial muscles are partially supplied
from the pallial nerves that come from the visceral ganglia, but
of this I am not sure.

It seems probable that the ganglionic structure of these nerves
has been developed to meet the needs of the very complex mar-
gins of the mantle. The development of such structure in the
immediate region of the sense organs, is an indication of the
ease with which such centers may be established when need
arises. The branchial nerves are supplied with ganglionic cells
throughout their length. These are present not only along the
borders of the gills, but from the points where the nerves origi-
_ nate to their extremities. The almost constant activity of the
gills no doubt renders such an arrangement desirable. No other
nerves or connectives in the body seem to be supplied with
ganglion cells.

The whole nervous system is modfied to meet the special
needs of the animal. ‘The cerebral and pedal ganglia are small,
corresponding with the slight development of the anterior parts
of the body and of the foot. The visceral ganglia are highly
developed, corresponding to the excessive development of the
parts that are supplied by these ganglia. Accessory centers
have also been developed in the margins of the mantle and in
the gills.

46

It seems that students of Mollusca quite commonly hold that
the lamellibranch ganglia have been derived from a gastropod-
like type, a type that possess at least one pair of ganglia, the
pleural, that are not commonly found in lamellibranchs. This
view seems to be based largely upon the acceptance of a hypo-
thetical type for a primitive mollusk that seems to me to be a
much better ancestor for the gastropods than for the other classes
of the Mollusca. The discussion of this hypothetical form may
be left for another place, but the discussion of the nervous sys-
tem properly belongs here. About all of the actual evidence
that we have of the presence of pleural ganglia in lamellibranchs
is that in Nucula (22) and some other forms the anterior gan-
glionic mass is so shaped that it is possible to consider it as
two ganglionic masses, and further that the connective that runs
from this mass to the pedal ganglion is connected with this
mass by two roots. The interpretation (22) that has been put
on this is that the two apparent divisions of the ganglion repre-
sent respectively the cerebral and pleural ganglion, and that
the roots of the connective represent the cerebro-pedal and
pleuro-pedal connectives that have become fused before reach-
ing the pedal ganglion. My own view, discussed in another
paper (7) is that the apparent division into two ganglionic
masses is superficial, and due to the swellings accompanying the
origins of nerves, and that one of the cerebral ends of the con-
nective may be the central end of the otocystic nerve which
is fused for the greater part of its length with the connective,
but, unlike most forms, is free near the ganglion. This view
seemed to me most reasonable as Stempell (30) has found that
in Soleyma togata, a supposed near relative of Nucula, the oto-
cystic nerve arises directly from the cerebral ganglion and is
separate from the connective throughout its length. So far as
I know, the instance given by Stempell is the only one that has
heretofore been reported where the otocystic nerves originate
from the cerebral ganglia, and are free from the cerebro-pedal
connectives throughout their length. Pecten tenuicostatus has
the same arrangement. In this form the position of the ganglia,
connectives and otocysts is stich that it is a very simple matter
for the otocystic nerves to make direct connection with the cere-
bral ganglia, but they do not join the ganglia at their nearest

47

point. Instead they are continued around the connectives to
join the ganglia in contact with, and posterior to them.

To me it seems probable that the separation into the two
groups that have developed into the classes Lamellibranchiata
and Gastropoda took place at an early date in the history of the
Mollusca, probably before a complicated head apparatus was
developed, and while the nervous system was of a very simple
nature. If this was the case, we have no reason to search for
pleural ganglia in lamellibranchs, for it is very probable that they
never had them. In fact were ganglia ever present in this region
in lamellibranchs, it would be more reasonable to view them as
new formations for special purposes than as direct descendants
from, and accordingly homologous with, the pleural ganglia of
gastropods. The gastropod and lamellibranch are so different in
structure and habits that we may reasonably expect important
differences in their nervous systems. Gastropods and Cephalo-
pods possess accessory ganglia that have evidently been devel-
oped to perform special functions. That such centers may be
comparatively easily developed is indicated by the fact that the
circum-pallial nerves of the scallop are essentially such centers.
Is it not then more likely that pleural ganglia have been devel-
oped in the groups that need them than that lamellibranchs,
which, so far as we know have never been more complicated
than they are to-day, should have formerly possessed these
ganglia and have since quite uniformly lost them?

Eyes.— SENSE ORGANS.

The number and position of the eyes has been discussed in |
connection with the structure of the mantle, on the lobes of
which they are borne. They have been so frequently and well
described by other investigators that it does not seem necessary
to give a detailed description here. Each eye (fig. 26) is club-
shaped, pigmented near its cuter end, and its position and gen-
eral appearance indicate that it is probably a modified tentacle.
The extreme end is occupied by the cornea, (co.) which con-
sists of a single layer of transparent epithelial cells that are con-
tinuous with the layer of somewhat thicker cells that forms the
remainder of the covering of the tentacle. Near the free
extremity these cells are completely filled with a dark-brown or

48

nearly black pigment, which gives the color already referred to,
so that in sections of large eyes, where the pigmentation is
deepest, the nuclei of the cells are not easily found. The pigmen-
tation becomes less dense toward the base of the tentacle and
gradually disappears. Muscle fibers that extend back into the
eye stalk are attached to the edges of the cornea so in preserved
specimens it is not uncommon to find the cornea pulled back
so the pigmented portion extends around it as a ridge. These
muscles may be of use in changing the focus of the eye. So
far as I know, there is no other provision for focussing.

The lens (le.) is cellular in structure, and except for a thin
layer of muscle and connective tissue fibers that cover its outer
surface and are continuous with the muscle fibers at the edge
of the cornea, it is in contact with the inner surface of the cor-
nea. There is no space between them so the cornea, muscles
and lens form a single optical lens. The inner surface of the
lens is applied to the retina, (r.) but as part of the nerve (on.)
that supplies the retina enters the eye from one side, the nerve
is continued between the retina and the lens. The edges of the
lens are bounded by a blood space.

In sections the lens varies greatly in shape. It may be nearly
circular, indicating that the lens is nearly globular, or either its
anterior or its posterior face may be greatly flattened. The
shape shown in figure 26 is not uncommon, but sections in which
the posterior face is drawn out and is very convex are not at
all rare. It is a question whether these shapes indicate a pos-
sible focal range, or whether they are to a considerable extent
distortions due to preservation.

The flattening of the outer face of the lens may be accom-
plished by the muscles that are attached to the margin of the
cornea, and that are continued over the surface of the lens
between it and the cornea. It is possible that injecting the blood
space with blood and contracting muscles in the eye stalk which
surround this blood space, may lengthen the lens—that is, make
it more convex. The mechanism is not very complete, but it is
hardly to be expected that focal changes take place with great
rapidity.

The retina is rather thick, and is slightly concave toward the
lens, with which it is always in contact. The exceedingly con-

4 49

vex lens is no doubt sufficient to bring the light to a focus on
the retina, although the retina is in contact with the surface of
the lens. Judging from section it seems likely that the refrac-
tive indices of the cornea, lens, and retina are practically the
same. If this is the case, the only refraction that takes place
is when the light enters the cornea. In this case the relative
convexity of the outer surface of the cornea determines the
focal distance, and the shape of the inner portion of the eye is
immaterial as long as the lens and retina are kept in contact. ©
With such an arrangement the more convex the cornea the
shorter the lens must be in order to place the retina at the focal
distance, and vice versa.

The nerve that enters the eye on the side next to the shell,
just in front of the surface of the retina that is applied to the
lens, supplies the retina. The layer that resembles rods is placed
on the side turned away from the lens, and it is to these that
the nerve fibers are apparently distributed. A pigment layer,
often of considerable thickness, lies next to the rods. Another
nerve, a branch of the one already described, reaches the eye
near its optical axis, and spreads out beneath this pigment layer.
I have not traced its distribution.

Looking directly into it, the eye universally appears blue.
The color is probably due to the breaking up of the light
reflected from it by the small elements of which it is composed
as there seems to be no blue pigment. .

The development of the eye has not been carefully followed.
It is noticeable that the small, presumably young eyes, have
proportionately much thicker corneas than the large eyes have.

It is a difficult matter to determine by experiment how well
a scallop sees. If an animal is placed in a position that is illu-
minated from one side, and allowed to remain undisturbed for
some time, and then a sudden shadow is made to fall over it,
it is almost sure to suddenly close its shell. If this be tried
several times at short intervals the animal usually soon fails
to respond. It is also to be noticed that Pecten irradians when
approached in shallow water will either start to swim or close
its valves. It is, of course, not at all certain that the stimula-
tion that leads to this action is received through the eyes. The
response is much more noticeable than with most other shallow

50

water forms, but the scallop is naturally more active, and is well
supplied with tactile tentacles as well as with eyes. _

Quick motions outside of an aquarium made so the illumi-
nation is not materially affected, and so the aquarium is not
jarred, frequently seem to cause response, but the results are
so frequently negative that apparent responses may be acci-
dental. Experiments to test the power of vision have not been
devised.

Tentacles.—

The number and arrangement of the tentacles has been dis-
cussed in treating the mantle. Although the size, shape, and
position of the tentacles differ considerably, they are all essen-
tially of the same structure. As they are included in the color
pattern of the margins of the mantle, some are pigmented and
others are not. The tentacles are quite smooth when they are
extended, and short, wrinkled, and conical when retracted. Each
tentacle is covered by a layer of epithelium and bears near its
free extremity several conical projections, ‘‘pinselzellen,’’ each
of which bears a cluster of sense cilia at its tip. These projec-
tions are always more numerous near the extremities of the ten-
tacles than elsewhere, but they are scattered pretty well over
their surfaces and may occur on the mantle also. Each tentacle
(fig. 22) is supplied with a large nerve (n.) derived from the
circum-pallial nerve, that runs the whole length of the tentacle
near its middle line. Nerve cells are present in this nerve
throughout its length. A connective tissue framework divides
the interior into a number of spaces. Muscle fibers (mf.) that
run lengthwise of the tentacle lie alongside the framework
and surround the blood spaces (bs.). The nerve lies very near
the center of the framework and occupies one side of a large
blood space. The structure is very much the same as that of
the special sense tentacle of Yoldia (5). The chief difference
is, that in this tentacle there are a number of blood spaces, while
in Yoldia there is only one.

The blood spaces serve to lengthen the tentacle, by having
blood forced into them, and the muscle fibers shorten it.

Otocysts.—
The otocysts (figs. 15 and 24, ot.) are placed very near, and
almost dorsal to the pedal ganglia, and accordingly not far from

51

the cerebral ganglia, but a little ventral, anterior and nearer the
median plane of the body than these ganglia. The otocysts are
imbedded in a mass of connective tissue that surrounds the pedal
ganglia, and may with comparative ease be dissected out with
the pedal and the cerebral ganglia, and studied in total mounts.

Each otocyst consists of a nearly spherical pouch formed of
epithelial cells that is connected with the exterior by a small
canal (fig. 24, otc.) that opens almost opposite the cerebral gan-
glion of the corresponding side. Similar canals are present
either as complete canals or as rudiments in different species
of the Protobranchia, but so far as I know, have never before
been described for any species outside of this group. As oto-
cysts uniformly originate as invaginations from the surface
epithelium of the animal possessing them, it is reasonable to
suppose that these canals are simply persistent from the embryo-
logical condition, but this has not yet been proved. The oto-
cysts are ciliated (whether the cilia are vibratile or not has not
been observed) and usually contain a considerable mass of fine
granular material that may be scattered, or collected into a very
definte ball. It is not at all uncommon to find one otocyst nearly
filled with this material, while the other is nearly empty. On
the other hand, both may be nearly filled, or both may be nearly
empty. The origin of this material is doubtful. In appearance
it resembles fine fragments of debris such as is found on the
bottoms where the scallops live, and there is sometimes some
variation in the color of these particles. All of the particles
seem to be sufficiently smal! to have been introduced through
the otocystic canals, but I have not thus far found any of the
shells of diatoms although the mud on which the animals live
is full of them and many of them are as small as the particles
that are found in the otocysts. The irregular, broken appear-
ance of the particles and the fact that frequently there are many
more particles in one otocyst than in the other rather points to
their being foreign particles than to their being products of
secretion.

The otocysts are usually considered to be static organs. So
far as I know, there are no experiments that bear on the func-
tion of these organs in lamellibranchs, and the supposition that
they are organs for determining position in space is based upon

52

experiments on supposedly similar organs in other forms. The
scallop normally lies on the right shell valve, but I am not sure
that it makes any particular effort to turn over when it is placed
on the other valve. When it swims and settles to the bottom,
it settles uniformly on the right valve. Whether this is due to
the shape of the shell or to some determining factor outside of
the nervous system of the scallop or not, is not known.

EMBRYOLOGY.

From observations made early in the summer, it seemed prob-
able that the giant scallop spawned rather late in the season;
accordingly as soon as other duties permitted, August 20, IgoI,
quarters were procured at Bass Harbor, Mt. Desert Island,
Maine, and work begun. Examination of specimens showed
that for the most part they had not spawned, and that sperma-
toza removed from the testis and placed in sea water were
active. Many trials were made while at Bass Harbor to arti-
ficially fertilize the eggs by cutting them out and mixing them
with sea water containing sperm which had likewise been pro-
cured by cutting. Very few of the eggs showed any signs of
development, and most of them that started did not develop nor-
mally. Eggs thus removed from a lamellibranch are irregular
- in shape, due no doubt to their crowded condition in the ovaries.
For some reason that is not understood, the eggs of many species
of lamellibranchs seldom round up, are incapable of fertiliza-
tion, and soon go to pieces when they are cut out of the ovary.
Such is the case with this form. Eggs removed even during
the height of the breeding season did not develop well.

It was found that scallops that were full of eggs and sperm
when placed in a floating car on August 23rd, had, when exam-
ined the next day, thrown most of their sexual products. Speci-
mens had also been placed in large vessels of water at the same
time, but these had not spawned and did not spawn although
kept another day. The animals die rapidly in such vessels of
water, seldom living more than three days, and frequently not
more than one. A careful watch indicated that specimens put in
the car did not spawn during the middle part of the day, so night
observations were made. Fresh specimens were placed in the

53

car and in a dory that had been carefully cleaned and partly
filled with water, and left floating so that the temperature would
remain something like that of the sea water outside. At inter-
vals of a half hour up to midnight, the scallops in the car were
examined by lantern light and a little of the water in the dory
was examined microscopically, to see if eggs or sperm could
be discovered. As they were apparently not spawning, they
were left until just before sunrise, when observations were
again begun. About 8 A. M. sperm were discovered in the
water of the dory. Soon-after several specimens began to throw
sperm in such quantities that the water in their vicinity was
turbid. Upon going to the car it was found that the water in
the car and for some distance outside was so full of eggs and
sperm that they could be dipped up in such numbers that the
bottoms of white agate ware dish pans filled with the water
became pink with the eggs that settled. The water was decanted
and the eggs supplied with fresh sea water. Some of the eggs
were transferred to glass dishes that were covered by loose
glass plates that prevented undue evaporation, and excluded dirt
that was rather in evidence in the shed on the steamboat wharf
that served as a laboratory. Water was changed in all of the
vessels at intervals during the time that the embryos remained
alive.

The scallops that were put in the dory were removed as soon
as eggs were abundant, and after allowing a few minutes for
the eggs to settle, most of the water was dipped out and replaced
by pure sea water. The results were not satisfactory, however,
and as at 6 P. M. the embryos in the dory did not seem to be
doing well, not much further attention was given them. All
of them apparently died before those in the dishes were in bad
condition. This is not strange, as-a large quantity of sperm
had to be left in the dory, and it was not possible to give the
embryos as good care in the bottom of a comparatively foul dory
as in the cleaner dishes.

The development is what may be considered normal for lamel-
libranchs. There is no part of the early larval history that is
different from what might be expected for such a form, as it
differs very little from Teredo (9 and 27), Dressinia (19),
Ostrea (4), Mya (18), Cardium, and a host of others that have

54

been described, or are familiar to every worker on lamellibranch
embryology.

It will accordingly be necessary to describe the formation of
the embryos only very briefly. As the age of the eggs could not
be accurately determined it is not possible to give the exact time
that elapsed before the polar bodies made their appearance, but
probably the first polar body was given off in from half to three-
quarters of an hour after the egg was laid. The first external
sign of activity after fertilization is the formation of a promi-
nent yolk-lobe, which nearly disappears after the first polar body
is formed, to become prominent again when the second polar
body is formed (fig. 27) and to disappear again after this is
separated from the egg. It again becomes prominent when the
egg cleaves into two cells, (fig. 29) and is slightly visible during
the second cleavage. The polar bodies are given off from the side
of the egg that is opposite the yolk-lobe, and although the egg
is not inclosed in a membrane as ig the case in many forms, the
polar bodies adhere until the cells are provided with cilia and
the embryo begins to swim. The adherence is apparently due
to protoplasmic strands such as have been described by Andrews
(1 and 2).

The first plane of cleavage passes through the point where
the polar bodies were formed, and just to one side of the yolk-
lobe (fig. 29). This divides the egg into two unequal portions,
the larger of which contains the whole of the yolk-lobe. The
next cleavage plane also passes through the point where the
polar bodies were formed and nearly at right angles to the first
cleavage plane (fig. 30). This also passes a little to one side
of the yolk-lobe so at least a large portion of the yolk remains
in one cell which is larger than the others.

The division into eight cells is accomplished by cleavage
planes at right angles to the planes already described (fig. 31).
In this way each of the four cells are divided unequally, those
nearest the polar bodies being smaller than those on the opposite
side. Continued division of the cells results in the formation
of a mass of cells (fig. 32) some of which are confined to the
surface, while others are large and extend into the interior, thus
forming an almost typical epibolic gastrula. This stage of
development is reached in from 12 to 14 hours, at which time

a5

many of the surface cells have acquired cilia and the embryos
begin to roll around on the bottom of the dish.

An hour or two later the apical cilia make their appearance.
They at first are not much longer than the others, and do not
seem to be very numerous. Because of their motion and tend-
ency to bunch together they are hard to count, but only four or
five seem to be present at this early stage. They grow quite
rapidly until they are nearly as long as the diameter of the
embryo and increase in number until a considerable bunch is
formed.

About this time the embryo begins to elongate slightly in the
axis roughly corresponding to the direction of the apical tuft
of cilia and the embryo begins to swim freely in the water. The
motion is not very rapid, and is at first rolling, but as the apical
cilia elongate the embryo begins to swim in definite lines, always
with the apical cilia pointing forward. In swimming the em-
bryo varies its direction almost constantly, and continually
rotates on its longitudinal axis. The direction of the rotation
may be changed from time to time.

Sections at this stage (fig. 33) show two pouches formed by
the invagination of the surface layer of cells. On
what is to become the dorsal portion of the animal,
nearly opposite the apical cilia, is the larger of these two
pouches (sg.). It is composed of large cells that are con-
tinuous with the surface cells. This is the shell gland. It
soon spreads out and grows down on the sides to form
the lobes of the mantle and to secrete the shell. The other
invagination (ar.) is somewhat smaller than the one just
described, is composed of smaller cells, and is situated on the
ventral side. Like the other, this is continuous with the sur-
face layer of cells. It has been formed apparently partially by
the pushing in of surface cells, and partially by the division and
separation of cells on the inside of the embryo. This is the first
appearance of the alimentary canal, and probably represents a
combined archenteron and stomodeum. The inner ends of the
shell gland and archenteron lie very close to each other and may
for a time be in contact. With the spreading out of the shell
gland, which is accomplished in about 18 or 20 hours after the
egg is fertilized, the embryo elongates decidedly (fig. 34) and

56

becomes somewhat pointed behind; that is, the end directed
away from the apical cilia becomes the pointed end. The embryo
enlarges, due to the formation of a space beneath the shell gland,
which has now become the mantle, and the surface cilia become
restricted to the anterior end. The archenteron begins to grow
rapidly, enlarges to form the stomach, (s.) and grows pos-
teriorly to form the intestine (i.). The anterior adductor muscle
(aam.) makes its appearance dorsal to the apical plate (ap.)
and posterior to the dorsal margin of the portion that bears
surface cilia, which later develops into the velum. The space
between the developing alimentary canal and the body wall is
quite extensive, practically surrounding the alimentary canal
except where it joins the body wall at each end and where the
anterior end of the stomach is in contact with the apical plate.
The adductor muscle is in contact with the body wall on its
anterior surface, but is otherwise surrounded by this space. A
few greatly elongated spindle-shaped fibers resembling muscle
fibers usually extend across the space. Almost universally one
or two such fibers extend from the dorsal surface of the stomach
dorsally and posteriorly to the body wall. Similar fibers have
been noticed in the embryos of other lamellibranchs and are
quite conspicuous in Nucula, (7) but their function is not
known. The space is no doubt a schizoccele that is formed as
the result of the arching up of the shell gland to form the
mantle. This takes place much more rapidly than the internal
organs grow, and the space is accordingly formed. Its ultimate
fate has not been traced as the oldest of the embryos reared still
have a remnant of it dorsal to the alimentary canal.

The stage that has been described is practically a trochophore.
The cilia are in front of the mouth, but cover the whole area
around the apical tuft instead of being arranged to form a band.
Later, as the velum is formed, they are better developed along
the margins of its lobes and thus form a band.

A stage similar to this is probably present in all forms of
lamellibranchs that do not give protection to their embryos as
is done by the Unionide, Spherium, etc. Even here (32)
something that corresponds to the stage may be recognized. At
first sight the embryos of Yoldia (5) and Nucula (7) seem to
differ considerably from the trochophore that has been described,

57

but this is apparent rather than real. If the ciliated cells that
cover these larve and form the tests, were pushed forward: and
the stomodzum shortened so the mouth would retain its position
at the margin of the ciliated area, the two larve would be essen-
tially alike. At a slightly earlier stage in Pecten a large part
of the surface is covered with cilia, and this is changed only by
the posterior development of the embryo, beginning with the
flattening of the shell gland. Such a posterior development is
normal in many trochophores as in the case of Dondersia (23),
Dentalium (17), Chiton (10 and 16), and most lamellibranchs
and gastropods, as well as in annelids, where the posterior devel-
opment is so marked.

The shell gland spreads out laterally and forms the lobes of
the mantle which secrete the shell valves (figs. 35 and 36). The
ciliated area grows rapidly and forms the two lobes of the velum
(vl.). The cerebral ganglon (cg.) are formed near the apical
plate. The alimentary canal grows dorsally and is bent into the
shape of a U. The stomach (s.) enlarges, the intestine (i.)
acquires an anal opening, the greater part of the schizoccele
becomes filled with mesoderm and the embryo assumes the form
of a veliger. This change is accomplished inside of three or
four hours, so active veligers are formed in about thirty hours
after the eggs are laid. The shape of the embryonic shell is
quite characteristic for the embryonic shells of lamellibranchs
(12) and differs very greatly from the adult form. Risser reports
that this is not the case with Pecten irradians (24) but the shells
on very young embryos that I have reared are very similar.
Until this stage is reached the embryos take little
or no food. They now swim about actively through the
water, frequently going to the surface. The cilia on
the edges of the lobes of the velum are the means of
locomotion. The apical cilia remain bunched and are moved
rather gently in different directions but apparently func-
tion as sensory rather than locomotary cilia. Each individual
occasionally retracts its velum between the valves of its shell,
closes its shell, and slowly settles to the bottom. This is almost
always the case whenever the animal is disturbed, as by jarring
the dish in which the veliger is swimming, or when the animal
runs into anything or is run into by another animal. In such

58

cases the veliger may recover after falling a short distance, or
it may fall to the bottom and remain quiet for some time. The
response to disturbance, which is the usual response of lamelli-
branch veligers, has been taken into account by Mead and
Barnes (18) who have devised a trap whereby quantities of
the veligers of the soft-shelled clam, Mya arenaria, can be col-
lected and reared without trouble to such a size that they may
be used as seed in stocking clam ground.

The embryos of most lamellibranchs usually remain as veli-
gers and swim about freely for a number of days, or even for
some weeks. ‘The embryos of Pecten were kept alive for only
five days. Weather and an unsatisfactory place to work inter-
fered with proper care and they were apparently weakened by
starvation. This difficulty could probably easily be overcome,
as in other cases, by putting them in vessels of sea water in
which cultures of the diatoms that supply the greater part of
their natural food have been started, but cultures could not be
started at the time, and there have since been no opportunities
to return to the scallop grounds during the breeding season.
The young of Nucula proxima reared from eggs, have been kept
alive for eleven months in a small jar of sea water in which a
small quantity of mud from the bottom had been placed after
straining it through silk bolting cloth to remove forms that
might be enemies.

Many fishermen report having seen the young scallops
attached to shells by means of threads during the early winter.
In a few cases small Anomia were brought to me as young scal-
lops, but most of the fishermen to whom these were shown did
not accept them as young scallops. Their descriptions of young
scallops were in some cases quite minute, and apparently accu-
rate, and in all such cases the scallops were said to be attached
by threads. Pecten irradians is known to attach itself by a
byssus in the young (24) and even in the adult stage, and it is
very probable that the young of the giant scallop attach them-
selves in the same way. If so, when the veligers settle perma-
nently to the bottom, they must find something on which to attach
themselves in order to keep from being destroyed. If this is
true, the absence of suitable material for this purpose may be
the reason that many of the old grounds, especially the shallow

59

water grounds that have been much dredged, no longer support
scallops. The present custom is to shuck the scallop before leav-
ing the grounds, but the shells are usually badly attacked by
boring sponges, and go to pieces quickly, so it is possible that
they may not be of service when the breedng season arrives. I
have had no opportunity to examine the run-out grounds, nor to
make a careful study of existing beds, so the above are simply
surmises that may not agree with facts.

SUMMARY. a

Shell.—

The shell is adapted for swimming, in shape, in weight, in the position
and strength of the muscle, and in the possession of a large cartilage, and
a straight hinge line which will allow rapid movement without great
strains or friction. (See pp. 7-11, and Figs. 1-7.)

Mantle.—

The mantle lobes are supplied with numerous sense tentacles and
eyes which are probably of use in detecting enemies; with nerves that
possess ganglion cells; with infolded ridges that regulate the opening
of the mantle chamber when the shell is open and probably serve to
direct the current of water thrown from the shell in swimming; and
with strong pallial muscles which serve to withdraw the margins of the
mantle when the shell is closed. (See pp. 12-16 and Figs. 9, I0, 16, 20
and 26.)

Foot.—

The foot is comparatively small, split at the end, and possesses a large
byssal gland. It is probably not of much service in locomotion. The
retractor muscle of the left side only is retained. (See pp. 16-19 and
Figs. 8, 10, and 12.)

Alimentary Canal.—

The stomach lies near the hinge line surrounded by the liver. The
portion of the intestine that leaves the stomach corresponds with it in
structure. It seems probable that one loop of the intestine has been
overlooked in previous dissections of scallops. (See pp. 19 and 20, and
Big, 12.)

Labial Palps.—

Unlike most forms, the palps are ruffled above and below the mouth.
The reason for the arrangement is not known. (See pp. 20 and 21, and
figs. 10 and 12.)

61

Gills.—

Each gill is attached by one lamella to a muscular membrane that
serves to elevate the gills when the shell is closed. The other lamella
is not attached. This arrangement makes it possible for the water to
be thrown from the shell in swimming without injuring the gills. The
inter-filamenter junctions are composed of cilia near the margins of the
gills, and of tissue near the suspensory membranes. Their blood vascu-
lar supply is intricate. (See pp. 21-30, and figs. 11, 12, 17, 18, 19, 20,
and 21.)

Muscular System.—

The anterior adductor muscle is lost at an early period of develop-
ment. The posterior adductor muscle is distinctly separated into an
anterior and a posterior portion. The anterior portion, which is much
the larger of the two, may be cut without causing the shell valves to
gap. If the posterior portion is cut without injuring the anterior por-
tion, the valves immediately open.

Muscles for withdrawing the margins of the mantle and the gills are
well developed. Only the left retractor muscle of the foot is present
in the adult animal. (See pp. 31 and 32, and figs. 10, 16, 19, 20, and 26.)

Excretory Organs.—

These are essentially rather large sacs with glandular walls. They
receive the genital ducts near their pericardial ends. (See pp. 32 and
33, and figs. 13 and 20.)

Genital Organs.

The genital organs are large, pink in the female, and white in the
male. The genital ducts join the excretory organs near their pericardial
ends. (See pp. 33-35.)

Circulatory System.—

The large size of the animal makes it possible to inject the vascular
system successfully. Blood from the mantle is returned immediately to
the heart. Most of the blood from other portions is returned to the kid-
neys, from which it is carried to the gills and then back to the heart.
A portion may dodge the kidneys and go to the gills. Blood seems to
act both as blood and lymph. (See pp. 35-42, and figs. 9, 11, 13, 14, 17,
and 21.)

Nervous System.—

The cerebral and pedal ganglia are small and somewhat removed from
their usual positions. The visceral ganglia are very large and compli-

62

cated in structure. The circum-pallial nerves and the branchial nerves
have ganglion cells throughout their length. The otocystic nerves origi-

nate directly from the cerebral ganglia. (See pp. 43-48, and figs. 15, 23,
24, and 25.)

Sense Organs.—

The eyes are numerous and optically arranged for the formation of
images. The sense tentacles are exceedingly numerous and of ordinary
structure. The otocysts have canals that open at the surface of the
body and the otocystic nerves join the cerebral ganglia direct. The

otoliths are composed of granular material that may have been intro-
duced from the outside. (See pp. 48-53, and figs. 10, 20, 22, 24, and 26.)

Embryology.—
The development is normal and rather rapid. (See pp. 53-60, and
figs. 27-36.)

63

“

rs.

16.

LITERATURE:

ANDREWS. Some Activities of Polar Bodies. Johns Hopkins Univ.
Circ Vol. AX MIL., INO. 132) 1807:

ANnpbrEws. :Activities of the Polar Bodies of Cerebratulus. Arch.
f. Entwicklungsmechanik. Bd. IV., 1808.

Brooks. The Origin of ihe Oldest Fossils and the Discovery of
the Bottom of the Ocean. Smithsonian Report for 1894 (also
Salpa.

Brooks. The Development of the American Oyster (Ostrea vir-
giniana). Stud. Biol. Lab. Johns Hopkins Univ., Vol. 1, 1880.
Drew. Yoldia limatula. Memoirs from the Biol. Lab. of the Johns

Hopkins Univ. Vol. 4, No. 3, 1889.

Drew. Locomotion in Solenomya and its Relatives. Anat. Anz.
Bd. XVII., No. 15, 1900.

Drew. The Life-History of Nucula delphinodonta. Quart. Jour.
of Micro. Sci. Vol. 44, Part 3, New Series, 1got.

Grave. Investigations for the Promotion of the Oyster Industry
of North Carolina. U. S. Fish Com. Report for 1903.

HatscHEeK. Ueber Entwickelungsgeschichte von Teredo. Arb.
Zool. Inst. Wien. Bd. 3, 1880.

Heatu. The Development of Ischnochiton. Zool. Jahrb., Abth. f.
Anat. u. Ontog. Bd. 12, 1899.

Hype. The Histology of the Eye of Pecten. Mark Anniversary
Volume, Harvard, 1903.

Jackson. Phylogony of the Pelecypoda. Memoirs Boston’ Soc.
Nat. Hist. Vol. IV., No. 8, 1890.

Jameson. On the Origin of Pearls. Proc. Zool. Soc. London, 1902.

Ketiocc. A Contribution to our Knowledge of the Morphology of
Lamellibranchiate Mollusks. Bul. U. S. Fish Com. Vol. X.,
1800. -

Ketiocc. The Ciliary Mechanism in the Branchial Chamber of the
Pelecypoda. Science (2), Vol. 11.

Kowa.evsky. Embryogénie du Chiton Polii (Philippi) avec
quelques Remarques sur le Développement des Autres Chitons.
Ann. Mus. Hist. Nat. Marseille. T. 1, No. 5, 1883.

64

17.

18.

ot

Lacaze-Duruters. Historie de l’Organisation et du Développement
du Dentale. Ann. des Sci. Nat. Ser. 4, VII., 1857.

MEap AND BARNES. Observations on the Soft-clam. (Fifth paper).
Rhode Island, 34th Ann. Report of the Com. of Inland Fisheries,
1904. "

MEISENHEIMER. Entwicklungsgeschichte von Dreissensia polymor-
pha. Zeit. f Wiss. Zool. Bd. LXIX., 1900.

Patren. Eyes of Molluscs and Arthropods. Mitth. Zool. Stat.
Neapel. Bd. 6, 1886.

PaTteEN. The Embryology of Patella. Arb. Zool. Inst. Univ. Wien.
Bd. VI., 1886.

PELSENEER. Contribution 4 l’Etude des Lamellibranchs. Arch. de
Biol. XI., 1801.

Pruvor. Sur le Développement d’un Solénogastre. Compt. rend.
Acad. Sci. Paris. CXI., 1890.

Risser. Habits and Life-History of the Scallop (Pecten irradians).
Rhode Island, 31st Ann. Report of the Com. of Inland Fisheries,
I9OI.

Rick. Die Systematische Verwertbarkeit der Kiemen bei- den
Lamellibranchiaten. Jen. Zeit. f. Naturwiss. Bd. XXXI., 1897.
SCHREINER. Die Augen bei Pecten und Lima. Bergens Mus. Aar-

bog, 18096.

SicerFoos. The Pholide. Notes on the Early Stages of Develop-
ment. Johns Hopkins Univ. Circ. Vol. 14, 1895.

SmitH. The Giant Scallop Fishery of Maine. Bul. U. S. Fish.
Com., Vol. IX., 1880.

STEMPELL. Beitrage zur Kenntniss der Nuculiden. Zool. Jahrb.
Sup. 4. Fauna Chilensis, Heft 2, 1808.

STEMPELL. Zur Anatomie von Solemya togata. Zool. Jahrb. Bd.
XIII., 1899.

STeENTA. Zur Kenntniss der Str6mungen im Mantelraume der
Lamellibranchiaten. Arb. Zool. Inst. Univ. Wien. Bd. XIV.,
1902.

ZEIGLER. Die Entwickelung von Cyclas cornea. Zeit. f. Wiss. Zool.
Bd. 41, 188s.

REFERENCE LETTERS.

a. Auricle.

aa. Anterior aorta.

aam. Anteroir adductor muscle.

ac. Apical cilia.

ap. Apical plate.

apa. Anterior pallial artery.

apn. Anterior pallial nerve.

aps. Anterior pallial scar.

ar. Archenteron.

ba. Branchial artery.

ba’. Branches of the branchial artery.
ba”. Branches of the branchial artery in the modified filaments.
bg. Byssal gland.

bn. Branchial nerve.

bs. Blood space.

bv. Branchial vein.

bv’. Branches of the branchial vein.
c Cartilage.

cc. Cerebral commissure.

cg. - Cerebral gangloin.

cgl. Cuticular gland.

co. Cornea.

cp. Cartilage pit.

cpc. Cerebro-pedal connective.

cpm. Circular pallial muscles.

cpm’. Circular pallial muscles of the infolded ridge.
cpn. Circum-pallial nerve.

cr. Chitinous rods.

cvc. Cerebro-visceral connective.
e. Excretory organ.

f. Foot.

fa. Foot artery.

fc. Feeding cilia.

fe. Free edge of the unattached lamella.
fm. Foot muscle.

fn. Foot nerve.

66

=

Filament support, probably to keep the filament from swelling into

a cylindrical shape with the pressure of the blood.
Foot vein.
Gill.
Gill filament.
Intestine.
Inter-filamentar junction.
Inner gill.
Inter-lamellar junction.
Inhalent ostium.
Infolded ridge of the mantle.
Liver.
Lens.
Longitudinal muscles.
Labial palps.
Left valve of the shell.
Mantle.
Muscle fibers.
Nerve.
Ostium cilia.
Csophagus.
Outer gill.
Optic nerve.
Otocyst.
Otocystic nerve.
Pericardial cavity.
Posterior adductor muscle, anterior portion.
Posterior adductor muscle, posterior portion.
Posterior adductor artery.
Posterior adductor muscle scar, anterior portion.
Posterior adductor muscle scar, posterior portion
Posterior adductor muscle vein.
Pedal ganglion.
Pallial line.
Palp nerve.
Posterior pallial artery.
Posterior pallial nerve.
Posterior pallial scar.
Pallial vein.
Retina.
Radial pallial muscles.
Right valve of the shell.
Stomach.
Shell gland.
Suspensory membranes of the gills.
Suspensory membrane scars.

67

st. Sense tentacle.

v. Ventricle.

va. Visceral arteries.

vg. Visceral ganglion.

vl. Velum.

vm. Visceral mass.

vv. Visceral veins.

x Swelling on the visceral ganglion from which the anterior root
of the branchial nerve originates.

y. Swelling on the visceral ganglion from which the posterior pallial

nerves originate.

PLATE 1.

Fic. 1.—A well preserved left shell valve showing the markings on the
oiter surface. Two-thirds natural size.

Fic. 2.—Outer surface of a left shell valve that shows distinct radial
color markings. Two-thirds natural size.

PLATE, -2.

Fic. 3.—Left shell valve badly mutilated by the attacks of boring sponges.
The large barnacle near the margin of the valve shows that
the rate of growth has not been very rapid for some time.
Two-thirds natural size.

Fic. 4.—Outer surface of a right shell valve. The valve is flatter than
its mate and has a conspicuous notch at the base of the
anterior wing. The radiating ridges are worn so they are
not as conspicuous as they are on the other valve. This valve
is usually lighter in color than the left. The relatively dark
color here is due to different printing. The round openings
on the surface are due to recent attacks of the boring sponge.
Two-thirds natural size.

PLATE, 3.

Fic. 5.—Inside of a left shell valve. The markings on the inside of the
left shell valve are never as conspicuous as they are on the
inside of the right shell valve. T'wo-thirds natural size.

Fic. 6.—Inside of a right shell valve. The division of the adductor
muscle into a large anterior and a small posterior portion is
conspicuously shown by the scar on this valve. ‘Two-thirds
natural size.

PLATE <4:

Fic. 7.—Inside of the right shell valve of a specimen that has been rather
badly attacked by boring sponges. The roughenings on the
surface seem to cover the deep borings of the sponge. T'wo-
thirds natural size.

68

Fic. 8—Ventral view of the foot showing the split end and the open-
ing of the byssal gland. Magnified three diameters.

PLATE 5.

Fic. 9.--Outer surface of the left lobe of the mantle showing the arrange-
ment of blood vessels. Two-thirds natural size.

Fic. 10.—Animal as seen from the left side with the left shell valve and
mantle lobe removed. The rounded bodies at the bases of
the marginal row of tentacles are the eyes. Two-thirds nat-
ural size.

PLATE, 6.

Fic. 11.—Animal as seen from the left side with the left shell valve and
mantle lobe removed and with a portion of the pericardial
wall cut away. A few of the blood vessels are shown. ‘T'wo-
thirds natural size.

Fic. 12.—Animal as seen from the left side with the left shell valve and
mantle lobe removed, with the alimentary canal shown. Two-
thirds natural size.

PLATE 7.

~ Fic. 13.—Animal as seen from the left side with the left shell valve and
mantle lobe removed. Drawn to show the arterial system of
blood vessels. Two-thirds natural size.

Fic. 14.—Animal as seen from the left side with the left shell valve and
mantle lobe removed. Drawn to show the systemic veins.
Two-thirds natural size.

PLATE 8

F|c. 15.—Animal as seen from the left side with the left shell valve and
mantle lobe removed. Drawn to show the nervous system.
Two-thirds natural size.

Fic. 16.—Inner surface of the right lobe of the mantle showing the
arrangement of the pallial muscles. Two-thirds natural size.

PLATE 9.

Fic. 17.—A portion of a gill showing the arrangement of parts. The
. figure indicates the inter-lamellar junctions cut at different
levels. The further lamella is the one that was attached to

the suspensory membrane and the vessel (ba’) was directly

connected with the vessel that supplied the gill with blood

(ba, Fig. 11). This vessel follows along the edge of the
inter-lamellar junction to the free edge of the unattached

lamella, (the one on the side nearest the observer in the

figure) where it bends back and passes down the modified

69

Fic.

Fic.

Fic.

Fic.

Fic.

Fic.

filament as the vessel ba”. Branches are given off from this
vessel through the inter-filamentar junctions to supply the
filaments. The vessel bv’ is the vessel into which the blood
that has traversed the gill is collected. It in turn communi-
cates with the vein of the gill (bv., Fig. 11). Magnified about
seventy diameters.

PLATE to.

18.—Vertical section (from the suspensory membrane to the free

edge) of a gill. Taken next to an inter-lamellar junction.
Magnified about fifteen diameters.

. I9.—Section of an animal taken through the plane that connects the

cartilage and the visceral ganglia. Two-thirds natural size.

. 20.—Section of an animal taken through the plane that connects the

heart and the outer ends of the excretory organs. Two-thirds
natural size.

PLATE 11.

21.—Transverse section of a modified filament (with the attached

portion of an inter-lamellar junction) and of two adjacent
filaments. The section is taken near the suspensory mem-
brane. Magnified about six hundred diameters. ;

22.—Transverse section of a sense tentacle of a young specimen.

Magnified about one hundred sixty-five diameters.

PLATE 12.

23.—Nervous system as seen from in front and a little to one side.

Natural size. (Diagramatic.)

PLATE 13.

24.—Cerebral and pedal ganglia with their nervous connections, as

seen from the antero-ventral position. These ganglia and the
otocysts lie in a mass of connective tissue and may be dis-
sected out and mounted for study without injury. Magnified
about fifteen diameters.

25.—Visceral ganglia seen from the ventral side. These may easily

be exposed for study by stripping the thin muscular covering
from their ventral surfaces. They are hard to separate from
the adductor muscle but they may be mounted with a thin
piece of the muscle and studied in position. Magnified about
fifteen diameters.

7O

FIc.

Fic.

Fic.

Fic.

Fic.
Fic.

Fic.

Fic.

Fic.

Fic.

Fic.

PLATE, 14:

26.—A section of the margin of the mantle taken through an eye.
The section is taken rather near the hinge line on the pos-
terior border, in a plane nearly corresponding with the line
pa., Fig. 11. Most of the circular muscles leave the infolded
ridge ventral to this point, and there are no tentacles on the
ridge at this level. Magnified about fifty diameters.

PEAS Bong:

27.—An egg at the time of the formation of the second polar body
showing the yolk lobe. Magnified about seven hundred diam-
eters.

28.—Two-celled stage after the yolk lobe has disappeared. Magni-
fied about seven hundred diameters.

29.—Two-celled stage soon after cleavage showing the yolk lobe.
Magnified about seven hundred diameters.

30.—Four-celled stage. Magnified about seven hundred diameters.

31.—Eight-celled stage. Magnified about seven hundred diameters.

PLATE 16.

32.—Later cleavage stage. Magnified about seven hundred diam-
eters.

33.—A sagittal section of a slightly later stage than that shown in
the preceding figure. Magnified about seven hundred diam-
eters.

34.—-A somewhat later stage seen as a transparent object from the
left side. Magnified about seven hundred diameters.

PLATE, 17.

35.—-Veliger larva seen as a transparent object from the left side,
with the velum extended. This stage is reached in about
thirty hours after the egg is laid. The one figured is slightly
older than this. Magnified about seven hundred diameters.

36.—Veliger larva seen as a transparent object from the anterior
end. Magnified about seven hundred diameters.

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