LIV

3005

Liverpool flftarine Bioloa^ Committee.

L.M.B.C. MEMOIRS

on Typical British Marine Plants & Animals

EDITED BY W. A. HERDMAN, B.Sc, F.R.S.

ii.

CARDIUM

BY

J. JOHNSTONE,

Fisheries Assistant, University College, Liverpool

(With 7 Plates)

Price Two Shillings

LIVERPOOL

T. Dobb & Co., Printers, 229 Brownlow Hill

December, 1899

HARVARD UNIVERSITY.

LIBRARY

MUSEUM OF COMPARATIVE ZOOLOGY.

^5UJL/wOUr^ ^ ,\^10.

L.M.B.C. MEMOIRS

II.
CARDIUM

NOTICE.

The Committee desire to intimate that no copies of these
Memoirs will be presented or exchanged, as the prices
have been fixed so low that most of the copies will have
to be sold to meet the cost of production.

The Memoirs may be obtained, post free at the nett prices
stated, from the Hon. Treasurer, Mr. I. C. Thompson, 53,
Croxteth Koad, Liverpool; Prof. Herdman, University
College, Liverpool; or the Curator, Biological Station,
Port Erin, Isle of Man.

Memoir I. Ascidia — published in October, 60 pp. and

five plates, price 1/6.
,, II. Cardium — now ready, 92 pp., six plates and a

map, price 2/-.
,, III. Echinus — in a few weeks, ... pp. and three

plates, price 1/-.
,, IV. Codium — January, 1900, price 1/-

Liverpool flftarine Biologg Committee.
L.M.B.C. MEMOIRS

on Typical British Marine Plants & Animals

EDITED BY IV. A. HERDMAN, D.Sc, F.R.S.

ii.
CARDIUM

BY

J. JOHNSTONE,

Fisheries Assistant, University College, Liverpool

(With 7 Plates)

Price Two Shillings

J LIVERPOOL
T. Dobb & Co., Printers, 229 Brownlow Hill
December, 1899

EDITOK'S PEEFACE.

The Liverpool Marine Biology Committee was constituted
in 1886, with the object of investigating the Fauna and
Flora of the Irish Sea.

The dredging, trawling, and other collecting expeditions
organised by the Committee have been carried on inter-
mittently since that time, and a considerable amount
of material, both published and unpublished, has been
accumulated. Thirteen Annual Eeports of the Committee
and four volumes dealing with the " Fauna and Flora "
have been issued. At an early stage of the investigations
it became evident that a Biological Station or Laboratory
on the sea-shore nearer the usual collecting grounds than
Liverpool would be a material assistance in the work.
Consequently the Committee, in 1887, established the
Puffin Island Biological Station on the North Coast of
Anglesey, and later on, in 1892, moved to the more
commodious and convenient Station at Port Erin in the
centre of the rich collecting grounds of the south end of
the Isle of Man.

In our twelve years experience of a Biological Station
(five years at Puffin Island and seven at Port Erin), where
College students and young amateurs formed a large
proportion of the workers, the want has been constantly
felt of a series of detailed descriptions of the structure
of certain common typical animals and plants, chosen
as representatives of their groups, and dealt with by
specialists. The same want has probably been felt in other
similar institutions and in many College laboratories.

VI.

The objects of our Committee and of the workers at our
Biological Station have hitherto been chiefly faunistic and
speciographic. The work must necessarily be so at first
when opening up a new district. Some of our workers
have published papers on morphological points, or on
embryology and observations on life-histories and habits ;
but the majority of the papers in our volumes on the
" Fauna and Flora of Liverpool Bay " have been, as was
intended from the first, occupied with the names and
characteristics and distribution of the many different kinds
of marine plants and animals in our district. And this
faunistic work will still go on. It is far from finished, and
the Committee hope in the future to add greatly to the
records of the Fauna and Flora. But the papers in the
present series are quite distinct from these previous
publications in name, in treatment, and in purpose. They
will be called the "L.M.B.C. Memoirs," each will treat
of one type, and they will be issued separately as they are
ready, and will be obtainable Memoir by Memoir as they
appear, or later bound up in convenient volumes. It is
hoped that such a series of special studies, written by
those who are thoroughly familiar with the forms of which
they treat, will be found of value by students of Biology
in our laboratories and in Marine Stations, and will be
welcomed by many others working privately at Marine
Natural History.

It is proposed that the forms selected should, as far as
possible, be common L.M.B.C. (Irish Sea) animals and
plants, of which no adequate account already exists in any
text-book. Probably most of the specialists who have
taken part in the L.M.B.C. work in the past, will prepare
accounts of one or more representatives of their groups.
The following have already promised their services, and
in some cases the Memoir is already far advanced. The

Vll.

first Memoir appeared in October, the second is now pub-
lished, the third and fourth will be issued early in 1900,
and others will follow, it is hoped, in rapid succession.
Memoir I. Ascidia, W. A. Herdman, 60 pp., 5 Pis., Is. 6d.
II. Cardium, J. Johnstone, 92 pp , 7 Pis., 2s.

III. Echinus, H. C. Chadwick, ... pp., 3 Pis., Is.

IV. Codium, E. J. H. Gibson and Helen Auld.
Dendronotus, J. A. Clubb.
Peridinian, G. Murray and F. G. Whitting.
Zostera, E. J. Harvey Gibson.
Himanthalia, C. E. Jones.

Diatoms, F. E. Weiss.

Gigartina, 0. V. Darbishire.

Alcyonium, S. J. Hickson.

Plaice, F. J. Cole and J. Johnstone.

Botrylloides, W. A. Herdman.

Cuttle-Fish, W. E. Hoyle.

Ostracod, Andrew Scott.

Patella, J. E. Ainsworth Davis.

Calanus, I. C. Thompson.

Actinia, J. A. Clubb.

Bugula, Laura E. Thornely.

Calcareous Sponge, E. Hanitsch.

Porpoise, A. M. Paterson.

Arenicola, J. H. Ash worth.

Antedon, H. C. Chadwick.

Oyster, W. A. Herdman and J. T. Jenkins.
As announced in the preface to Ascidia, a donation
from Mr. F. H. Gossage of Woolton has met the expense
of preparing the plates in illustration of the first few
Memoirs, and so has enabled the Committee to commence
the publication of the series sooner than would otherwise
have been possible. W. A. Herdman.

University College, Liverpool,
December, 1899.

L.M.B.C. MEMOIRS.

No. II. CARDIUM

(the cockle).

BY

JAMES JOHNSTONE,

Fisheries Assistant, University College, Liverpool.

The edible cockle {Cardium edule) is by far the commonest
member of the genus Cardium, a group of eulamelli-
branchiate Mollusca having a world-wide distribution and
containing a great number (about 200) of species. The
number of British species is, however, limited to ten,
most of which {C. echinatum, C. fasciatum, C. edule, C.
minimum, C. norvegicum, C. nodosum) are recorded as
being present in the Irish Sea ; of these the only abundant
species is C. edule ; C. norvegicum (Lcevicardium) is fairly
common, the others being only occasionally found. The
different species seem to have a fairly well-marked bathy-
metrical range, but C. edule is found from between tide
marks out to 1360 fathoms. Here and over the greater
part of Europe C. edule is the only species of any economic
importance ; in Jersey, however, C. norvegicum is used for
food, and in the Mediterranean various other species are
eaten.

The edible cockle is gregarious all along the coast line
where suitable bottoms exist, but the great cockling beds

2

are, as a rule, found only in sheltered waters, in shallow
bays, and at the mouths of estuaries. There is great con-
stancy in the characters of the cockles from the various
parts of the Lancashire and Cheshire coasts, no well-marked
varieties being found. The difference in size observed in
specimens from various parts of this district are most
probably due to the extent to which the beds have been
fished or disturbed in late years. Generally the influence
of some fresh water* seems to be favourable in that cockles
are more abundant in the neighbourhood of the mouths of
rivers, but the largest specimens are found only in areas
far removed from the influence of fresh water. Large
cockles, with shells two inches in length, are found on
some parts of the North Coast of Devon, in Barra, in the
Western Hebrides, and in the Scilly Isles. In Barra
these large cockles are sufficiently numerous to form the
material for an important fishery. Such giant forms are
not found on the Lancashire coast, where the average
length is about one and a half inches. Here the market-
able cockle has a minimum breadth of f inch, the size being
fixed by the Sea-Fisheries Committee's Bye-law. With
the exercise of proper care on the part of fishermen to
take only well-grown animals, there can be no doubt that
the Lancashire and Cheshire cockle fisheries might be
more largely developed, since the physical conditions are
so suitable.

The cockle inhabits the topmost layer of the sand,
burying itself to the depth of an inch at most. It lies in an
oblique position, and, when the bed is covered with water,
with the siphons projecting slightly above the surface.

* The influence of extreme salinity or freshness, as might be expected, is to
produce well-marked variations in the character of the shell. See Bateson,
Phil. Trans. Roy. Soc, vol. CLXXX. B., pp. 297-330, 1889 ; and Forbes
and Hanley, History of the British Mollusca, 1850, vol. II. , p. 21,

An exceptional habit has been noted by R. D. Darbishire*
when the cockles become anchored by the byssus threads
of Mijtllus and grow and develop freely in the water.
Only on the posterior margin of the shell, from which the-
siphons protrude, are there any other attached animals or
plants. Among these are green or brown Alga? (Sphace-
laria), Zoophytes (Obelia), small anemones (Actinia), and
rarely, barnacles (Balanus). In some places the cockles
commonly bear a tuft of algae, and the position of the animal
in the sand can be determined by the presence of this
projecting tuft. The animal, as a rule, remains in nearly
the same position, but is able to shift about by means of
the strongly developed muscular foot. Where the sand is
not much disturbed by the tidal current, as round the
stake of a net for instance, the cockles are generally more
abundant.

A current of water continually entering the mantle
cavity by the lower, and passing out again by the upper
siphon, bears the food supply in the form of suspended
microscopic animals and plants. The cockle feeds on
spores and other young stages of lower algae, fragments of
filamentous algae, vegetable debris, Foraminifera, Diatoms,
and probably also the smaller micro-crustacea. The
animal exercises no selective action on the food taken in ;
all that is contained in the entering current of water,
including a large quantity of sand and suspended inorganic
matter, is carried in by the cilia of the labial palps and
passed on into the stomach. The greater part, therefore,
of the contents of stomach and intestine is sand and fine
mud.

The chief enemies of the cockle are fishes and birds.
They also are eaten by starfish and bored by dog whelks.
They form an important food for many fishes, chiefly

* Fauna of Liverpool Bay. Report I., p. 241.

plaice and dab. There is said to be enormous destruction
by the larger sea-birds. According to the fishermen who
gave evidence to the Commissioners of 1879, the decrease
in some cockle beds may be directly attributed to the
increase in sea-gulls due to the operation of the Sea-Birds
Preservation Act. Whole cockle beds may be destroyed by
a hard frost or by an encroachment of sand.

At the beginning of the year the reproductive glands
are nearly ripe, and spawning commences at the end of
February or the beginning of March. The spawning
period is prolonged, and ends about June or July. The
reproductive glands then pass into the " spent " condition
and after a short time begin to ripen again. Eggs and
milt are shed freely into the water, where fertilisation and
development go on. The minute larva swims freely in the
water for some time, then settles down in the sand as a
small shelled cockle.

After a detailed account of the anatomy of the cockle as
a typical Lamellibranchiate Mollusc, an Appendix follows,
containing an account of the animal from the economic
aspect, with special reference to the cockle fisheries in the
Lancashire Sea-Fisheries district.

The Shell.

As in the case of most Lamellibranchs, the shell is
equivalve, that is, the two valves are precisely alike in
shape ; it is inequilateral, the straight central rib or groove
on each valve dividing it into unequal anterior and posterior
parts ; as regards the form and relative proportion of
the two parts so delimited, and the ratio of length to
lateral breadth or depth, great variability is found. The
number of ribs varies from 20 to 24. The external surface
is marked by a series of fine concentric grooves and ridges
indicating the growth of the shell margin. Some of these

grooves are very prominent, and, in a full grown shell, 3
to 6 such can readily be observed which probably mark the
limits of as many successive years of growth. Occasion-
ally these grooves are very distinct. Looked at from the
outside the margin is nearly even ; on the inside it is
deeply notched, the depressions corresponding in position
with the ribs on the outside (see fig. 1, PI. I., and fig. 10,
PI. II.).

The hinge line is gently curved, and the concavity
of the shell is continued dorsally beyond the hinge into
the umbo ; along this hinge line is a series of double teeth
on each valve which interlock when the valves are closed.
On the right valve the central cardinal tooth has the form
of a deep depression with sharp cusps arranged anteriorly
and posteriorly. On the left valve this arrangement is
reversed, there being a single median cusp with depressions
in front and behind ; similarly on the right valve each of
the two lateral teeth (which are really anterior and pos-
terior) consists of a prominent ventral and a smaller
dorsal cusp with an elongated depression between. On
the left valve this arrangement is reversed. The hinge
ligament, which represents the dorsal uncalcified portion
of the shell, lies posterior to the umbones and is external;
it is hollow and arched, underneath it the dorsal margin
of the valves do not come into contact when the shell is
closed, and a median glandular fold of the mantle projects
up into the cavity beneath the arch of the ligament, and is
in contact with the internal surface of the latter. The
ligament is very elastic and serves for the divarication of
the valves when the adductor muscles are relaxed.

There is little pigmentation on the shell except at the
posterior margin, where, particularly on its internal
surface, it is tinted a chocolate brown or green. This
pigmentation may extend on to the scar of attachment

6

of the posterior adductor muscle, which is often striped
with brownish yellow and white. This posterior margin is
much denser than any other part of the shell, and dissolves
slowly in dilute acid, always remaining after the rest of
the shell has disappeared ; the organic matrix is more
abundant here than at any other part.

The scars indicating the attachment of the muscles
show plainly on the dry shell (fig. 10). The posterior
adductor scar (Add.p'.) is large and usually pigmented.
The anterior adductor scar (Add.a'.) is rather smaller,
and is always unpigmented. Both approach very near the
margin of the shell, and lie just beneath the hinge line.
On the dorsal margin of the posterior adductor is a small
oval scar (Bet.p'.), sometimes not very obvious. This
indicates the place of attachment of the posterior retractor
of the foot. Two scars are to be seen in a similar position
over the anterior adductor scar. The more dorsal of these
{Bet. a'.) is the scar of attachment of the anterior retractor
pedis. The other (Pro'.) shows the attachment of the
protractor pedis. The pallial line indicating the place of
attachment of the radial series of muscle fibres serving
for the retraction of the mantle edge runs parallel to the
ventral shell margin, at a distance of about 8 mm., and is
slightly indented in correspondence with the notches on
the margin. There is no siphonal sinus, but at the
posterior margin of the shell the pallial line becomes
much broader as the retractor muscle of the mantle edge
passes into the retractor of the siphons. A small scar
hidden in the umbo serves for the attachment of a small
bundle of muscle fibres attached to the dorsal margin of
the wall of the viscero-pedal mass.

The shell in the region of the umbones is always thin,
and the periostracum is worn off in the fully grown
specimen. Towards the margin, and especially at the

posterior margin, it is much thicker, and both periostracum
and organic matrix are present. The internal structure is
very peculiar, differing from that of most lamellibranch
shells, and corresponding closely to what Ehrenbaum,*
who has investigated various species of Gardium, describes
as the gastropod type of shell structure. The calcareous
substance is distributed in two ill-defined layers (fig. 29,
PL V., Sh.i., Sh.e.), which must be termed inner and
outer shell layers since the terms prismatic and nacreous
layers are not applicable here. The shell is composed of
a great number of exceedingly thin laminae which lie, for
the most part, parallel to the shell surface. But since the
mantle edge is folded over the shell edge, each lamina
begins as a curved plate, the convexity of which is turned
towards the margin, and since the whole shell grows by
the addition of successive laminae to those already formed,
its most external layer is formed by the edges of the
laminae coming out on the surface at an angle of from 4.V
to 60°. The deposition of calcareous matter seems to be
effected principally by a rather wide zone of the external
surface of the mantle, extending back from the margin.
Hence the dorsal parts of the shell are thin, since there
seems to be little, if any, formation of lime over the general
mantle surface.

Each lamina has a very fine fibrous structure, the fibres
lying longitudinally in the thickness of the layer. In any
two successive laminae the directions of the fibres are at
right angles to each other. Thus, starting from any one,
in laminae 1, 3, 5, 7, the fibres are parallel to each other,
but in laminae 2, 4, 6, 8, they are at right angles to the
fibres in the first set. This is not easily observed in
sections perpendicular to the surface of the shell and to
the margin, owing to the excessive thinness of the laminae.
* Zeitsclir. f. wissensch. Zoul. Bil. XLI., \>\>. 1 — 47, 1SS5.

8

In sections taken in a plane perpendicular to this, that is,
perpendicular to the surface of the shell and tangential to
a line of growth, it is evident that in the centre of the
section the laminae must lie in planes approximately
parallel to the surface, since, owing to their upward
bending near the surface of the shell, they must lie at one
point in planes perpendicular to that surface. So here the
section is marked out into irregular areas representing
small portions of the bent laminae. In any two such
contiguous areas the directions of the fibres are at right
angles to each other. Further, since the laminae are very
thin, several are superposed in the thickness of the section,
and in any one such area, by focussing, a system of parallel
lines crossing each other at nearly a right angle may be
easily seen.

The structure of the shell is greatly complicated by the
sculpturing at the margin. Once the formation of the
definitive ribs and grooves has been initiated, the deposi-
tion of successive laminae proceeds upon the surface so
laid down, and so at the edge of the shell the surface of a
lamina is a very irregular one. Further back from the
margin, as one observes in a vertical section tangential to
the edge, the laminae are very regularly crumpled, the
contour of a single lamina being concentric with that of
the internal surface of the shell at its extreme margin.
It is obvious that this arrangement causes great irregu-
larity in the appearance presented by a vertical section
made with the intention of passing perpendicular to the
shell edge, for it is difficult or impossible to make the
section pass exactly through a rib or hollow without cut-
ting, in some part, the margin of the rib where the planes
of the laminae are approximately perpendicular to the
surface and margin of the shell. This causes the coarse
pseudo-prismatic appearance observed in a vertical section

9

perpendicular to the margin (fig. 29). These apparent
prisms are, of course, the obliquely cut edges of the
laminae.

The shell is everywhere penetrated by very fine tubes
and irregular channels. These are more apparent in the
vertical section taken perpendicular to the shell margin,
where they seem to be cut, for the most part, transversely.
In a section at right angles to this they are by no means
so obvious. It is evident from this that the black appear-
ance of these cavities is due not so much to pigment, as
Ehrenbaum supposes, as to the air included in the process
of preparation of the section. They are very regular,
following the planes of deposition of the shell laminae.

The periostracum (Epic, fig. 29) appears in section, not
as a regular layer on the external surface, but rather in
irregular fragments and patches.

General Organisation, Mantle and Foot.

In the ordinary cockle not preserved by any special
method, the animal is completely retracted within the
shell. The shell margins fit together very closely except
at the posterior extremity where, in the relaxed condition,
the siphons protrude. Even here on account of the con-
traction of the siphonal tubes, the mantle cavity is
completely closed. In this condition it is difficult or
impossible to open the shell without injury to the soft
parts, and, when this is done, the animal is so much
distorted owing to muscular contraction that the true
relations of the parts are not evident. The animal is best
prepared for dissection either by gradually adding a 4%
solution of cocaine to the water in which the expanded
animal lies, or by placing it at once in a 1% solution, and
allowing it to extend, which generally happens in the
course of an hour. The irritability of the siphons and

10

mantle' edge is first lost; the foot never becomes quite
insensitive. The animal is then killed in a 20% solution
of formol, a small piece of wood having been previously
placed between the edges of the valves to prevent the slow
contraction of the adductor muscles which occurs after
death. Only a slight amount of contraction takes place
in the formol, the siphons and foot being generally moder-
ately extended. If it is desired to prepare the animal for
sectioning, both valves are removed by placing it in a 10%
solution of nitric acid in 70% spirit; if for dissection, it is
propped up on a couple of glass slides in a dish with the
shell margin horizontal, and the acid solution poured in so
as to cover one valve. Either of the two valves can be
dissolved off in this way, the other being left to fix the
animal in the dissecting dish. A great inconvenience is
caused by the accumulation of carbon dioxide, resulting
from the decomposition of the shell, within the cavities of
the body. If the specimen is being prepared for section-
ing, it is best to remove this gas by allowing it to remain
for some days in 70% spirit containing a little ammonia.

Except at the margin, and for a reddish strip at the
dorsal surface which is the pallial portion of Keber's organ,
the mantle lobe is thin and transparent. Removal of this
by cutting along the line Mn'. seen in fig. 2, exposes the
gills and labial palps. The gills pass obliquely backwards
from the dorsal surface of the body beneath the umbo at an
angle of about 30° to the vertical axis of the viscero-pedal
mass.

The labial palps are triangular in shape. Their shorter
or anterior sides are attached to the body-wall, and their
most anterior extremities pass into the upper and lower
lips respectively. The dorsal margins are thin, smooth,
and slightly uneven. The internal surface of the outer,
and the external surface of the inner palps are marked

11

with deep grooves parallel to the anterior margins, and are
ciliated. The other surfaces are smooth and non-ciliated.
Cilia are present on all surfaces of the grooves and ridges.
Each ridge is roughly quadrangular in section (fig. 24).
The outer faces and the faces turned towards the apices
of the palps are covered with long columnar cells bearing
long cilia, and having abundant eosinophilous cells. The
rest of the surface of the ridges and furrows is covered
with cubical cells, carrying shorter cilia. The smooth
surfaces of the palps have an epidermis of flat, almost
squamous cells. Within the palp there is a very loose
connective tissue, rather denser in the interior of the
ridges and containing blood corpuscles in its interspaces.

The mantle lobe of each side (Mn., fig. 4, PI. II.) is
inserted into the extreme dorsal margin of the body, and
round the lower margin of each adductor ; anteriorly the
right and left lobes fuse together at the dorsal surface of
the anterior adductor. Posteriorly there are two fusions ;
dorsal to the posterior adductor the mantle roofs in a
portion of the mantle cavity which passes upwards over the
adductor (Mc.l, figs. 3 and 6), and in which the terminal
portion of the rectum and the anus lie. The first fusion
forms the septum between the dorsal and ventral siphons.
It is prolonged inwards from the mantle edge forming
a horizontal shelf (Mn.l, fig. 3), which separates the
cavity of the dorsal siphon (Mc.2, figs. 3 and 6) from the
general mantle cavity (Mc.3, figs. 3 and 6). The second
fusion forms the lower wall of the ventral siphon. Between
the posterior margin of the mantle and fusion one, and
between the first and second fusions, the matitle edges are
prolonged outwards to form the siphons. In the full grown
animal these have a maximum length, when extended, of
about 1 cm. In the young cockle their length is relatively
much greater.

12

The siphons differ slight]}7 in structure ; the dorsal, or
exhalent one (Si.d., figs. 2 and 3), is the shorter of the two.
Its free edge is quite even and a small portion of the
wall of the tip is exceedingly thin. This thin tip is
contractile, and is generally closed forming a little cone at
the end of the siphon. The tentacles, which are rather
over 1 mm. in length when fully extended, are situated in
an irregular ring at the base of this cone. Other and
larger tentacles are borne on the wall behind this ring.
At intervals the conical tip of the siphon opens and water
and faecal matter are suddenly expelled. The ventral or
inhalent siphon [Si.v., figs. 2 and 3) remains permanently
open. The free edge bears a great number of very delicate
tentacles, smaller and thinner than those borne on the
outer wall and differing slightly in structure. Behind
this, as in the upper siphon, is a ring of tentacles with
others scattered irregularly upon the wall. At the tip of
both upper and lower siphons are a number of brown pig-
ment spots which are the openings of little pigmented
crypts or glands.

The mantle edge (fig. 23) is thrown into longitudinal
folds extending along its entire length. There is an inner
strong fold projecting into the mantle cavity, a smaller
median fold, and an outer fold which dips into the
grooves on the shell margin. On the surface applied
to the shell, the epidermis consists of rather irregular
cubical cells, except towards the extreme edge, where the
cells become spindle shaped and are crowded with brownish
pigment granules. Over the rest of the mantle edge
(ventral and internal surfaces), there is an epidermis
consisting of a very regular layer of cubical cells with very
distinct nuclei and a strong cuticle. On the inner of the
three folds mentioned above, and in the ' deep groove
separating this from the small median fold, the cuticle

13

becomes very strongly developed. From it a cuticular
structureless membrane passes off over the free edge of the
shell forming the epicuticula or periostracum (Epic).
When the shell is dissolved off in acid, this cuticular
investment is seen to be really continuous with the mantle
edge, so that the latter is firmly attached to the outer
surface of the shell.

On the inner surface is a ciliated zone which begins a
little way back from the free edge of the inner fold, and
ceases or begins to die out opposite to the line of attach-
ment of the pallial muscles to the shell. Coincident with
the distribution of the cilia is that of a zone of mucous
glands opening on to this ciliated surface on the mantle
margin. These glands are very similar to those which
will be described as present on the ciliated tip of the
foot (p. 17), but are, as a rule, unicellular, the body and
conducting stalk being formed from a single cell. Occa-
sionally several cohere together forming a structure of the
same nature as those found on the foot, but they are very
generally much smaller individually. Their stalks passing
out through the epidermis give the latter an irregular
appearance. The tips of these stalks project out among
the cilia as prominent little knobs.

The pallial muscles (Rct.m., fig. 23) have a narrow zone
of attachment to the shell (Ret.vi'., fig. 10). Here the
epidermic cells disappear completely from the mantle
surface. The muscle bundles, passing ventrally, parallel
to the surface, are attached to the shell at a very acute
angle. The bundles run along the outer surface for a
short distance, then divide into two series. One of these
continues to run along the outer surface, and terminates
in the connective tissue in the outer and median folds,
the other series, consisting of stronger bundles, crosses over
to the inner surface and breaks up into smaller bundles,

14

and isolated fibres which terminate in the inner mantle
fold.

Within the mantle there is, at the margin, a filling tissue
composed of fine connective tissue fibres, in which ran the
muscle bundles and the trunks and finer branches of the
pallial nerve plexus ; passing inwards, this begins to
include large irregular communicating spaces, and is soon
reduced to a mere lining to the epidermal surfaces from
which trabecule pass inward, forming a very coarse net-
work. The trabecule consist of rather dense fibrous tissue
with scattered nuclei. Far back from the edge this
becomes reduced to little more than a layer of small
nuclei and a few fine fibres. Delicate bridges of fibrous
tissue unite the two epidermes, so that the whole cavity
in the interior of the mantle lobe is divided into a system
of inter-communicating spaces which are generally empty
in sections, but are most probably blood sinuses. The
inner epidermis is composed of flat squamous cells. Near
the point of attachment of the mantle lobe to the body-
wall the former becomes much thicker, and the spongy
tissue in its interior attains a greater development.

If now, the mantle lobe being removed, the labial palps
be cut away along their attached borders, and the gills be
carefully removed by cutting close to their bases, the
portion of the body lying between the adductors is laid
bare. The base of the ctenidium (Br1., fig. 3) extends
downwards obliquely from the region of the body extend-
ing up into the umbones, to the lower horizontal level of
the posterior adductor. Here the bases of the right and
left ctenidia become free from the body-wall, and continue
to pass ventrally and posteriorly till their posterior extrem-
ities fuse with each other in the middle line, and with the
horizontal shelf, referred to above, as formed by the
extension inwards of the first fusion of the mantle lobes

15

between the siphons. At this point of concrescence of the
two ctenidia, a peculiar little semicircular flap of mem-
branous tissue projects downwards and forwards into the
general mantle cavity.

This horizontal shelf is farther prolonged anteriorly by
the inner lamellae of the internal branchiae. Part of these
inner lamellae (Br. 1. 2, figs. 3 and 4) have the ordinary
filamentar structure and are connected to the outer lamellae
of the same branchiae by the inter -lamellar junctions.
But the remainder of the inner lamellae are simply mem-
branous, and fuse with each other across the middle line
of the body, and thus form the floor of the ventral supra-
branchial chamber {M.cA, fig. 4), which continues forward
the cavity of the dorsal siphon. The outer reflected
lamella? of the external branchiae, on the other hand,
fuse with the body-wall just beneath the origin of the
mantle lobes. It is evident from a consideration of fig. 4
that the inner and outer lamellae of each external branchia
must enclose a cavity, which is also continuous with the
cavity of the dorsal siphon ; this is the dorsal suprabran-
chial chamber (M.c.5, fig. 4). The ventral suprabranchial
chamber is single and median. There are paired, right
and left, dorsal suprabranchial chambers.

Since the inner lamellae of the internal branchiae do not
fuse with the body-wall but with each other, the ventral
suprabranchial chamber is incomplete anteriorly; the dorsal
chambers end blindly in front. Evidently water entering
the general mantle cavity by the ventral siphon or through
the ventral opening of the shell has three courses open to
it. It may pass forward between the labial palps into the
mouth and so reach the dorsal siphon per anum ; it may
enter the ventral suprabranchial chamber through the
space included between the posterior surface of the foot
and the anterior margin of the fused inner lamellae of the

16

inner branchiae ; or, finally, it may pass through the inter-
filamentar gaps in the branchial lamellae into either dorsal
or ventral suprabranchial chambers and so into the dorsal
siphon.

Two regions of the body can be distinguished (figs. 2 and
3, PI. I.), the viscero-pedal mass (Ped.l and Ped.2) and the
portion of the body lying behind this and in front of the
posterior adductor. This latter portion contains the peri-
cardium {Per.) and heart, and the renal organ (Ben.) with
the terminal portion of the rectum. The viscero-pedal
mass contains, besides the muscular foot, the greater part
of the alimentary canal, the digestive gland, and the
gonads. It is sharply marked off from the posterior region
by the differentiation of a sub-epidermal muscular sheath,
but the same epidermal layer covers both portions of the
body.

The pericardium is situated dorsally, occupying the
whole dorsal area between the viscero-pedal mass and the
posterior adductor, beneath it is the renal organ, the
ventral wall of which forms the roof of the ventral supra-
branchial chamber.

The viscero-pedal mass is defined by the continuous
muscular sheath (c/., fig. 11) extending ventrally from the
dorsal body-wall. In horizontal section it is elliptical.
It consists of a proximal or vertical, and a distal or
horizontal limb which, both in the contracted and relaxed
condition, form an angle of about 90° with each other.
The distal limb is directed forward, it is very much
flattened laterally and has a deep groove, the pedal groove,
(By'., fig. 3) running along its ventral edge from near the
tip to a point beneath the axis of the vertical limb. About
a third of the length of the distal limb, from the tip pos-
teriorly, has an epidermis composed of short columnar
ciliated cells which also form the lining to the pedal

17

groove. The rest of the epidermis of the viscero-pedal
mass consists of short columnar, non-ciliated cells, with a
continuous thick cuticle.

As on the internal surface of the mantle edge, this
ciliated portion of the foot is also an area provided with
unicellular mucus-secreting glands. These form a con-
tinuous dense layer beneath the ciliated epidermis, and are
imbedded in the loose muscular sheath of this part of the
foot. Each of these glands (figs. 21 and 22, PL IV.) consists
of either a single cell or a group of from 2 to 6 cells aggre-
gated together. Single-celled glands are uncommon, and
are only found near the posterior limit of the glandular
area. Towards the tip of the foot they are more complex ;
the largest groups measure about 0*3 mm., the stalks
being about half that length. Such a complex gland con-
sists of a group of cells forming a bulb with a long stalk.
The limits of the separate cells are not always clearly
distinguishable. The cell bodies consist of a reticulum,
some of the bars of which are rather coarse ; these are,
however, continuous with a very fine meshwork, which
apparently makes up the cell substance. This reticulum
is continuous from cell to cell. Nuclei are not evident,
but in every cell there is a nodal point at the intersection of
several of the coarser bars of the reticulum ; reticulum
and nodal points, and the finest ground substance, stain a
intense blue with haematoxylin. The stalk is non-tubular,
has the same structure as the bulb, and breaks up at its
free end into a small number of branches which penetrate
between the epidermal cells and form little knobs among
the cilia on the surface of the foot. The structure and
staining reaction of these bodies, coupled with the habit
which the very young cockle has of pulling itself along a
smooth surface by causing the tip of its foot to adhere to

18

the surface, indicate their probable nature — that of a
simple mucus-secreting apparatus.

The pedal groove itself is not a glandular structure, nor
do the glands above described open on to its surface.
Passing backwards from the tip of the foot the cilia dis-
appear, and the groove is lined with an epidermis consist-
ing of simple columnar cells. The groove becomes deeper,
and may be thrown into longitudinal folds. Finally, it
passes into a duct which runs upwards and backwards
into the proximal limb of the viscero-pedal mass, and
terminates in a swollen bulbous head, which lies on the
right side of the caecal prolongation of the straight intes-
tine {By.g., fig. 3). A single delicate hyaline fibre may
sometimes be seen projecting from the opening of this duct,
and indicates, what the histological character of the gland
in fact demonstrates, that the apparatus is a byssus secret-
ing structure. Sections of the duct show only an
epithelium consisting of short columnar, ciliated cells.
Further up, the duct expands into a wide cavity (fig. 19)
from which lateral diverticula are given off; these may
branch again. Their lumina are always restricted. Each
of these secreting alveoli (fig. 20) is lined with a epithelium
of rather club-shaped cells which do not seem to bear cilia,
but from between which a number of fine threads pass out
into the lumen where they become agglutinated together,
forming a filament. The filaments from the various
alveoli unite together in the duct to form the single byssus
thread.

That Cardium possesses a true byssus was demonstrated
by Gosse,* who shewed that in the young C. acnleatum
this was actually functional. Carrieret and BarroisJ have

* Ann. and Mag. Nat. Hist. Ser. II., vol. XVIII., pp. 257—8.

t Arb. Zool.-Zoot. Institut, Wurtzbnrg. Bd. V., pp. 56—92, 1882.

X Comptes Rendus Acad. Sci, T. C, pp. 188—190, 1885.

19

also shown that in Gardium " oblong um" C. echinatu?n,
and G. rusticum a rudimentary by ssus gland, corresponding
in all respects to the structure above described, is present.
Finally, Gwyn Jeffreys* mentions a case of G. edule itself
in possession of a functional byssus.

Of the apertures of the body, the mouth (ilf., fig. 3) is a
wide slit lying between the viscero-pedal mass and the
anterior adductor; the anus {An.) lies on the dorsal surface
of the posterior adductor. The apertures of the gonad and
renal organ are rather difficult to observe. They may be
seen by folding back the internal branchia (see fig. 3). The
ureter (Ben1., fig. 4) is a mere slit in the wall of the
renal vestibule, and lies on the lateral wall of the renal
organ just dorsal to the origin of the posterior retractor
muscles of the foot. The external opening of the gonad
lies in nearly the same position, but on the lateral and
posterior wall of the viscero-pedal mass ; it is slightly
elongated and sometimes has tumid lips.

The Musculature.

The muscles may be conveniently arranged into four
groups : (1) the adductor muscles of the shell ; (2) the
extrinsic muscles of the foot ; (3) the intrinsic muscles of
the foot ; and (4) the pallial muscles. The extrinsic and
intrinsic pedal musculature form really one system. The
pallial muscles may be divided into the pallial muscles
proper and the muscles of the siphons.

(1) There are two adductor muscles of the shell (Add.a.
and Add.p., fig. 3), anterior and posterior. Each is a
strong bundle inserted on the dorsal oblique surface of the
shell near the margin, and running across in an exact trans-
verse direction from valve to valve. Owing to the curve
of the shell the scars of attachment (Add.a'. and Add.p'.,

* British Conehology, vol. II., p. 208.

20

fig. 10) are much larger than the areas of transverse
sections of the muscles. The posterior muscle forms a
thicker bundle than the anterior. In the extended condi-
tion of the animal, the structure of the muscle bundle is
rather loose, being penetrated by blood spaces usually
filled with corpuscles.

The force of contraction of the adductor muscles is
very great. Plateau* has measured this in terms of the
weight required to force open the valves. Two hooks
were inserted under the ventral edges of the valves. The
hook sustaining the upper valve was fixed to a support.
The other, which was carried by the lower valve, supported
a scale pan. Weights were placed in the scale pan till
the valves were separated to the extent of 1 mm. As the
mean of eight such experiments it was found that the
weight required was 1134 grms. And taking the transverse
area of both adductors into account, this gives the force
necessary to overcome the contraction per sq. cm. of the
sectional area of the adductor muscles of Gardium as
equal to the weight of 2856 grms. Plateau also made the
converse experiment. An animal in a completely relaxed
condition, with the shell gaping, was supported with the
lower valve resting on a firm support. A loop was passed
round the upper valve, from the lower end of which was
suspended a scale pan. The mean weight required to
overcome the elasticity of the hinge ligament was found
to be 106 grms.

(2) The extrinsic muscles of the foot take their origin
from the superficial muscular sheath of the viscero-pedal
mass. The posterior retractors of the foot {Bet. p., figs. 3
and ID originate in the posterior margin of the proximal
limb of the latter, and run backward as a short apparently
single median bundle (Bet. p., fig. 4), this bifurcates into

* Pull. Acad. Roy. Sci. de Belgique. Ser, III., t. VI., pp. 226—259, 1883.

21

the right and left muscle bundles, which are inserted, one
into each valve, at the dorsal margin of the posterior
adductor scar (Bet.p'., fig. 10). There is a well-marked
decussation of the fibres forming each muscle. That is,
the fibres forming the left retractor originate in a flattened
band, lying on the right side of the middle line, on the
inside of the muscular posterior wall of the viscero-pedal
mass, and vice versa. This crossing of the fibres is effected
by two or three smaller inter-digitating bundles from
each side, and is complete, none of the fibres remaining
uncrossed. The band from which these muscles take
the origin (Bet.p., fig. 11) can be traced round the bend of
foot into the ventral and lateral walls of the distal limb,
after which its further course becomes very complex.

The auterior retractors of the foot lie through almost all
their course in the interior of the viscero-pedal mass.
Together they form a flattened band of fibres on the inside
of the anterior wall (Bet. a., fig. 11). At the upper level of
the anterior adductor they come to the surface as two
paired bundles which are inserted into the shell on the
dorsal margin of the anterior adductor scar (Bet. a'., fig. 10).
As in the case of the posterior retractors, a certain amount
of crossing takes place, though this is not so evident as in
the case of the other. Below the bend of the foot the
further internal course of the fibres is difficult to make
out.

The protractor muscles of the foot form right and left
short bundles, which are inserted into the shell near the
scars of attachment of the anterior retractors. Internally
the fibres spread out in a fan-shaped sheet on the lateral
dorsal walls of the proximal limb of the viscero-pedal mass,
but there are no obviously distinct bundles, as in the case
of the retractor muscles. Many of the fibres seem to pass

22

round and meet on the posterior margin, and the bundles
may be regarded as being constituted by a break in the
continuity of the circular muscular sheath of the proximal
limb, the free ends being gathered up into two short
bundles and attached to the shell. Judging from the
direction of the fibres, the only effect of the contraction of
these muscles will be to slightly rotate the whole viscero-
pedal mass about its dorsal attachment on the shell, so
that the term " protractor " is rather a misnomer.

The superior retractors of the foot (elevatores pedis,
Pelseneer) form two paired muscle bundles, which are
inserted one into each valve in the most dorsal part of the
umbo. The scars cannot be easily seen without breaking
the shell. Each bundle consists of fibres gathered up
from the transverse and oblique musculature of the dorsal
body- wall.

(3) The intrinsic muscles of the foot include all the pedal
muscles, which are not inserted into the shell, but have
their attachments within the viscero-pedal mass itself.
There is a thick hypodermal muscular sheath (fig. 25) in
which typically four muscle layers can be distinguished.
Beneath the epidermis is a thin layer of fibrous connective
tissue within which is a layer of muscle fibres running trans-
versely round the foot. This is succeeded by a thick layer
of obliquely running fibres, which passes into another layer
of transverse fibres, and internal to all is an irregular
sheath of longitudinal fibres. The precise arrangement is
variable at different levels, and all the layers may not be
present. The oblique and circular layers are always
represented. Here and there in a transverse section
through the proximal limb of the viscero-pedal mass,
strong muscle bundles (M.p.i., fig. 11) may be seen passing
across between the lateral walls in a transverse direction.
In the extended condition the structure of these is rather

23

loose, but the ends of each bundle are gathered up into
a tendinous root, which penetrates the muscular layers
of the body-wall, and has its attachment in the fibrous
connective tissue which is present among the muscle
fibres. These straight transverse bundles occupy the
greater portion of the cavity of the proximal limb. Towards
the axial portion they separate, leaving a space in which
the convoluted and straight portions of the intestine
are lodged. Between them penetrate the tubules of the
gonads.

(4) The pallial muscles consist of the retractor muscles
of the mantle edge and siphons and the intrinsic muscula-
ture of the siphons. The former (Bet.m., fig. 3) form a
radial series which extend round the mantle edge from
adductor to adductor. They are inserted into the shell
along the pallial line (Bet.m'.; Bet.s'.; fig. 10), and
extend into the folds in the mantle edge where their
distribution has already been described. Their length in
the extended specimen is about 0*5 cm. Towards the
posterior margin of the shell they become very much
stronger in correlation with the development of the
siphons, for the retraction of which they serve. In the
walls of the siphons they form a dense longitudinal sheath
which extends outwards to the tips. This sheath lies
principally on the inner portion of the wall. Special
circular and radial intrinsic fibres are present in the
siphonal walls. The former are distributed in bundles
tying just beneath the outer wall, and less evident bundles
situated midway between outer and inner walls. The
radial fibres pass across from inner to outer epidermis.
The outer zone of the siphonal wall consists of connective
tissue with included blood spaces.

24

The Alimentary Canal.

By far the greater portion of the alimentary canal lies
entirely within the proximal limb of the viscero-pedal
mass, and may be easily dissected in a specimen hardened
with formol by removing the muscular body-wall of one
side, cutting through the attachment of the transverse
muscle bundles as close to the integument as possible.
The attachments of these to the opposite wall are then
cut through beneath the intestine and the bundles removed,
the digestive gland and the gonad are picked away, and
the stomach and intestine lie exposed to view.

The mouth (.1/., fig. 3, PI. I.) is at first a wide slit
extending across the body between the anterior body-wall
and the anterior adductor. It is bounded dorsally and
ventrally by the prominent upper and lower lips, the outer
extremities of which are produced laterally to form the
dorsal and ventral labial palps respectively. The opening
soon contracts, so that the perforation in the muscular
body leading into the oesophagus is oval in form. The
latter (Al.c.l) is a short tube passing backwards and
slightly upwards towards the posterior and dorsal part of
the viscero-pedal mass, where it expands into the stomach
{Al.c.2), which forms a capacious sac, increasing in
diameter from before backwards. The stomach is sur-
rounded on all sides by the dark -green mass of the
digestive gland [D.g.). This is really paired, and forms a
thick lobe on each side. Each lateral lobe, however,
expands over the dorsal anterior and posterior sides of the
stomach, so that it seems to nearly envelope the latter.
If the digestive gland is carefully picked away prior to
laying bare the stomach, two ducts may be seen opening
into the latter. One is small, and opens on the posterior
and dorsal margin ; the other is much larger, and opens
into the stomach at its junction with the oesophagus ; it

25

passes at first forwards and downwards, then comes round
into the mass of the gland. A great number of smaller
lateral ductules open into these principal ducts, and on
them are arranged in clusters the secreting alveoli of the
gland.

The epithelium of the stomach passes gradually into
that of the ductules of the gland. The tricuspid body
disappears ; the long spindle cells become shorter, and a
corresponding decrease in length of the cilia takes place.
But the epithelium of the ductules (fig. 17, PL III.) always
consists of spindle cells carrying cilia, and their walls are
thrown into slight longitudinal folds ; at the passage of
the lumen of the ductule into that of a secreting alveolus
a rapid transition from this ciliated epithelium into that
characteristic of the alveoli takes place (fig. 15, PI. III.).

The fixation and subsequent treatment of the digestive
gland, so as to exhibit the nature of the secretory epithe-
lium, is difficult, but good results are to be obtained by
McMunn's method. A very small piece of the gland is
rapidly removed from the living animal, and at once put into
a 20% solution of commercial formaldehyde in water. The
tissue is allowed to remain in this for about 24 hours, and
is then transferred to 70% spirit and dehydrated, embedded
and cut in the ordinary way. The sections are stained
with Mayer's glychaemalum and eosin, or with Heiden-
hain's iron haematoxylin. With fixing reagents of less
penetrative power the epithelium breaks up in the process.

The lumen of the alveolus is always a very restricted one,
and is usually cruciform in transverse section. The spindle
cells becomes cubical and the cilia disappear (fig. 15, PI.
III.). Then the cubical epithelium becomes replaced by
four groups of large club-shaped cells (fig. 16, PI. III.). In
tangential sections of the alveolus these cells, which are
then cut transversely, have very definite polygonal out-

26

lines. Their cell substance is coarsely granular, with
many round clear spaces ; the nuclei are placed at their
lower extremities. Only four to six cells are found in
each group. The groups are delimited by the arms of the
cross-shaped lumen, which extend nearly to the walls of
the tubule, and here at the thinnest portion of the wall
the cells composing it are small and irregular, and have
relatively large nuclei. If the section has been stained
with Heidenhain's haematoxylin a very distinct basement
membrane, staining dense black, can be seen investing
each alveolus. The whole mass of the gland is bound
together by fibrous connective tissue, in the interspaces of
which are crowded corpuscles of various kinds.

The lining epithelium at the animal's mouth consists of
elongated columnar cells bearing cilia, and supported on a
rather distinct basement membrane. Passing inwards this
epithelium is thrown into a close series of longitudinal
folds, and the height of the cells diminishes. The cells
have distinct striated free borders, the nuclei are situated
about their middle, the lower ends are rather loose and
seem separated from each other ; rounded faintly granular
eosinophilous cells are found here and there wedged in
between the columnar cells.

As the oesophagus widens out to form the stomach
these cells gradually elongate to form the epithelium
lining the cavity of the latter. This gastric epithelium
(fig. 18, PI. III.) is of variable thickness, but the cells are
always longer than in the oesophagus. A gelatinous
looking substance — the " fleche tricuspide " (F.trL,
fig. 18) — lines a large portion of the stomach-wall, and
underneath this the epithelium becomes much thicker,
consisting of long spindle-shaped cells, the long oval nuclei
of which occupy any position within a rather wide zone
about their middle. The lower ends of these cells are

27

very distinctly rounded off, forming an uneven lower
margin, which rests on a fibrous sheath consisting of
several layers, and which passes into the loose connective
tissue surrounding the digestive gland tubules.

The thickness and extent of the tricuspid body is
variable, some portions of the stomach-wall being bare;
the latter is thrown into folds and pads, into which the
substance of the tricuspid body dips. In most parts this
substance is closely adherent to the gastric epithelium, in
other parts it is distinctly separated ; where it lies in
close contact with the epithelium the striated free border
of the latter is very evident. Where it is separated the
epithelium is seen to bear cilia which, at other places,
must be embedded in its substance. It stains slightly
with eosin. No obvious structure can be discerned in it
except that in favourable preparations, delicate striae,
parallel to the surface of the epithelium, and other striae
perpendicular to the surface may be seen, which seem to
indicate that it has been deposited round the cilia and in
laminae, perpendicular to the surface of the epithelium.
Where the tricuspid body is wanting the gastric epithelium
is composed of much shorter spindle cells than are found
elsewhere. The average length of the cells beneath the
tricuspid body is 0'08 mm., on the rest of the stomach-
wall 0'04 mm.

The whole posterior part of the stomach contracts to
form the straight portion of the intestine (Al.c.3, fig. 3).
This extends downwards nearly in the axial line of the
proximal limb of the viscero-pedal mass. It is slightly
curved, the concavity being anterior. The diameter, the
average value is 1*25 mm., decreases from above down-
wards. At the lower end and on the anterior side, the
spiral portion of the intestine takes origin, below this there
is a short anteriorly directed caecum.

28

As the stomach contracts to form this division of the
intestine, two folds of its wall (fig. 11, PI. III.), which
are anterior and posterior, are formed, and are continued
down the straight intestine dividing the lumen of the
latter into two longitudinal cavities, both of which com-
municate with the stomach by wide openings and with
each other by a long wide slit. Of the two communicating
semi-tubes so formed, the left {Al.c.3') is the larger, and
is exactly circular in transverse section ; it lodges the
crystalline style. The right semi-tube {Al.c.3") is irre-
gular in section, and forms the channel along which the
ingested food travels. Morphologically, this is the portion
of the intestine immediately following the stomach. The
left semi-tube is a diverticulum of the stomach cavity —
the pyloric caecum* (sac of the crystalline style). Pyloric
caecum and intestine are separate in some Lamellibranchs
(Pholas, Donax), but in Cardium and others have fused
together, the anterior and posterior folds being the remains
of the primitively adjacent walls. At the tip of the straight
intestine, in the short caecum already referred to, is a
vestige of the originally separate condition of the two
channels.

Three very distinct kinds of epithelium are present in
the straight portion of the intestine. On the wall of the sac
of the crystalline style there is a single layer of spindle-
shaped cells (Ep.I.l, fig. 12, PI. III.), having an average
height of about 0"03 mm. These bear a very close set
series of long and stiff cilia, having an average length of
f that of the cells carrying them. The cell bodies are
finely granular, with rather highly refractive free borders,
the nuclei are situated at their lower extremities ; the cells
fit together very closely, except at their lower extremities,

* Purdie, A. Studies in Biology for New Zealand Students. No. 3.
Anatomy of the" common Mussels. Wellington, 1887.

29

where large clear spaces are seen, which are either inter-
cellular spaces or cell vacuoles, probably both. The
epithelium in the intestinal division (Ep.1.3) is made up
of much shorter cells, which bear short cilia. The free
extremities of these cells fit closely together, but the
lower ends are rather loose. Scattered abundantly
throughout the epithelium are large, round, oily masses,
staining faintly with eosin, and quite homogeneous in
structure. Similar bodies can be seen in the tissue lying
outside the epithelium. Here they are seen to be true
cells, with the nucleus compressed against one portion of
the cell wall and the greater part of the cell space filled up
with an oily globule. In many the cell contents are very
coarsely granular, and stain strongly with eosin. Others
are finely granular, and all transition stages between the
coarsely granular condition and the homogeneous appear-
ance, which the bodies in the epithelium present, can be
found. Similar oily globules can be found projecting into
the lumen of the intestine, or even lying free in the cavity.
Nuclei are not evident in these structures in the latter
positions. The nuclei of the ordinary cells are found near
their bases. Above each nucleus is a large clear cell
vacuole, the occurrence and position of which seem to be
fairly constant.

In the sac of the crystalline style and near the anterior
fold is a very remarkable strip of epithelium {Ep.1.2),
which extends all the way from the opening of the sac
into the stomach to the point from which the spiral
division of the intestine takes origin. This appears in
transverse section as a crescentic area of the wall made up
entirely of very long spindle cells. On the left side these
long cells pass gradually into the epithelium lining the
sac of the crystalline style. On the right side they are
very sharply demarcated from the short cells lining the

30

intestinal division of the straight intestine. Their maxi-
mum length is about 0*2 mm. They are finely granular,
with the nucleus at any level from near the free extremity
to near the bottom of the cell. They bear a covering of
very short cilia. Sometimes, at the middle of the free
surface, there is a little groove in which the cilia are
matted together. This is not constantly present, and it
is probably due to the action of reagents.

The epithelium, of the intestinal portion especially, rests
on a layer of dense connective tissue, which fills up the
spaces between the epithelia forming the anterior and
posterior folds, and is found in patches all round the
intestine. This presents no very obvious structure. It
seems to be largely fibrous, with nuclei scattered through
it. It stains densely with haematoxylin. It includes
large irregular spaces, evidently blood channels ; in its
substance are seen corpuscles of various kinds, blood
corpuscles and corpuscles containing a greenish substance.

The crystalline style (St, fig. 12, PL III.) completely
fills the lumen of the left division of the straight portion
of the gut. In sections made from hardened specimens,
the style is usually seen to be retracted away from the
wall of the sheath. But since, in such preparations, it
may be observed that the superficial layer of the ciliated
epithelium is in some parts torn away and adherent to the
substance of the style, it is evident that this contracted
condition is due to the process of embedding ; and the
same cause most probably gives rise to cavities sometimes
observed in its marginal part. In hand sections of the
animal, simply killed with formol and examined under a
low magnification, the style appears perfectly homogen-
eous, and completely fills the sac.

No obvious structure is to be observed in the style
except a very delicate concentric lamination. It is per-

31

fectly transparent, and seen singly, colourless ; in mass
the substance is very slightly yellowish. The length,
when taken from full-grown specimens, varies from 20 to
26 mm. The diameter decreases from above downwards,
and may be taken as about 1 mm. The proximal
extremity always projects into the cavity of the stomach,
and is opaque and slightly frayed ; the distal extremity
does not fill the lumen of the ventral portion of the sheath,
but remains adherent to one portion of the wall. It is
firm, somewhat elastic, but breaks easily.

Barrois,* in an exhaustive memoir on the Morpho-
logy and Physiology of the Lamellibranch Style, gives an
account of the chemical composition and reactions of the
substance of which it is composed, which may be briefly
summarized here. Barrois made his analysis and experi-
ments on the crystalline styles of Cardiuni edule. The
style has an average weight of 0'026 grm. It is a colloidal
substance. A number placed together coalesce to form a
transparent jelly, which takes the form of the vessel in
which it is contained. Dried at a temperature of 120°, the
mass contracts considerably in volume, but still remains
perfectly transparent and somewhat moist. On ignition a
small amount of inorganic ash remains. The fresh style
is rapidly soluble in concentrated hydrochloric acid to a
bluish solution ; it is slowly soluble in water, forming a
slightly opaque and viscous solution. Millon's reagent
gives a red colouration in the warm. Treated with copper
sulphate solution and caustic potash, a fine blue colouration
is obtained. These reactions indicate the presence of an
albuminoid substance.

When the solid styles are boiled with dilute sulphuric
acid, and the acid solution neutralized and precipitated
with alcohol, a solution in the latter solvent is obtained.

* Revue Biologique du Nord de la France. T. I. and II., 1889—90,

32

This solution is filtered off from the flocculent precipitate,
formed by the addition of the alcohol. It is evaporated
nearly to dryness, and the dissolved matter again taken
up by water ; the aqueous solution so obtained reduces
Fehling's solution. This series of reactions is charac-
teristic of mucine and chondrine, since glycogen or other
carbohydrates capable of yielding sugar on treatment with
dilute acid, and consequently reducing Fehling's solution,
are absent.

Further the addition of crystals of magnesium sulphate
in excess to the aqueous solution of the styles, gives an
abundant precipitate which contains practically all the
proteid matter present in the solution. This behaviour
with magnesium sulphate, which agrees with that of a
globulin, and the reaction with dilute acid, indicate the
nature of the substance. It is allied to, but apparently
not identical with mucine.

Leaving out of account the older views concerning the
function of the crystalline style, such as that of von
Heide, that it was an accessory genital organ, or that it
was a skeletal structure (Cams and Garner), or the
representative of the radula of the Glossophora, and
consequently a masticatory organ (Meckel), only two
hypotheses as to its nature seem to survive modern
investigation. Hazay* as the result of a series of observa-
tions and experiments, concluded that it represented a
store of reserve food material, resulting from the meta-
morphosed excess of food matters taken in during the warm
season, and lodged in the pyloric caecum to be utilized by
the animal during periods of hibernation. Practically the
same conclusion was arrived at by Haselofft from a series

* Die Mollusken-Fauna von Budapest II. Biologischen Theil. Cassel, 1881.
t Ueber den Krystallstiel der Muscheln nach Untersuchungen verschiedener
Aiten der Kielcr Buclit. Osterode, 1888.

33

of experiments carried out on Mytilus, from which it
appears that not only is the style absorbed during the
starvation of the animal by keeping it in filtered sea-water,
but it is formed anew on abundant food being supplied.
Haseloff inferred that the style "is the product of the
chemical transformation of the excess of food material
taken in by the animal, the change being effected by the
agency of the digestive ferments." More lately, Woodward*
working on the same mollusc, was able to confirm Haseloff' s
experiments.

Barrois rejects the conclusion of Hazay and Haseloff,
basing his objections chiefly on the chemical composition
of the style which is very different from that of most reserve
food materials, and on the unusual form in which the
substance is stored up. In both these points it differs
notably from all undoubted physiological reserves. In
Helix poviatia, which undergoes a true hibernation, abun-
dant reserve food material is stored up in the liver in the
form of glycogen. This substance undergoes a gradual
change into sugar in the course of the hibernation period,
and ultimately disappears completely. Moreover, neither
in Mytilus nor in Cardium taken during all times in the
year was he able to observe any change in the volume of
the style. Specimens of Cardium were placed in filtered
sea-water and starved for eleven days. Only after the
eighth day was any diminution in volume observed, and in
general complete disappearance only occurred on the death
and partial decomposition of the animal. The disappear-
ance of the style during this experiment Barrois regards
as due, not only to the solution of its substance in the
stomach which goes on, as under normal conditions, but
also to the profound bodily disturbance brought about by

* On the Anatomy of Pterocera, with some notes on the crystalline
style. Proc. Malacologieal Soc, London. Vol. I., pt. 4. 1894.

34

the experiment. At the same time, the secretion of the
substance from the walls of the sac ceases, and the remains
of the style are pushed into the stomach by the action of
the cilia.

Barrois' own view, based chiefly on the chemical
composition of the style, is that it is acuticular structure
secreted by the wall of the sac, which acts as a lubricating
material and invests sharp particles of sand, &c, with a
viscous coating which prevents damage to the intestinal
wall. As it is formed it is moved forward into the cavity
of the stomach, where its free extremity is continually
being worn away by the attrition of the food particles and
the solvent action of the digestive fluids. The viscous
fluid so formed also unites the food matter into a coherent
bolus which easily traverses the intestine.

The view that the style is a cuticular structure secreted
by the wall of the pyloric caecum seems rather difficult to
reconcile with the appearance of that epithelium, for,
with the possible exception of the longitudinal strip of
elongated cells in the caecum, the wall of the latter does
not present the appearance usually associated with a
secretory surface. The compact layer of columnar cells,
the refractile free border, and the dense layer of long
stiff cilia contrast strikingly with the wall of the right
division of the lumen of the straight portion of the gut,
where secretion into the cavity of the intestine is most
probably taking place, and is far more suggestive of a
surface performing a mechanical function than of an
actively secreting epithelium. And it seems unnecessary
to locate the mucus-secreting epithelium in the wall of
the crystalline style sac. All along the course of the
intestine there is abundant evidence of some substance
being poured out into the lumen, in the rounded homo-
geneous bodies found in the wall or projecting from the

35

wall into the cavity, or even free in the cavity of the
intestine itself. Many of these are possibly migrating
cells taking up food matters and again passing back
through the intestine into the blood stream, but there can
be little doubt that many are the products of secretion
of cells in the intestinal wall itself or in the tissue lying
round that wall. Masses of a dense tissue staining with
haematoxylin in the same manner as the mucus-secreting
cells in the foot and mantle are to be found along the
whole length of the intestine.

The faecal matter is expelled from the intestine in the
form of coherent strings, frequently of great length, in
which the particles are certainly bound together by some
viscous material. On Barrois' view this fascal matter
ought to contain a substance chemically identical with the
substance of the style, otherwise, transformation of the
latter goes on in the intestine, and the substance of the
style must function otherwise than as a simple lubricant.
On the whole it would seem as if the presence of the style
were associated with the ingestion of a large quantity of
foreign matter, such as mud and sand, and the separation,
to some extent, of the nutrient material therefrom. The
substance of the style need not be regarded as physiolo-
gically a store of reserve material, but as a first separation
out of some constituents of the food which are continuously
lodged in a portion of the stomach by the action of the
ciliated wall of the latter, and as continuously dissolved
away.

A narrow slit on the anterior surface of the straight
portion of the gut leads into the next division — the spiral
portion of the intestine. This lies nearly in the axis of
the proximal limb of the viscero-pedal mass, and anterior
to the latter. It is twisted into a close spiral of five or six
turns {Al.c.4, figs. 3 and 11) the planes of which are

36

nearly horizontal. At its upper extremity the coiling
becomes rather irregular, and the tube passes into the
succeeding coiled portion of the intestine near the axis of
the latter. The crystalline style passes the opening of the
spiral intestine, and its narrowed end is lodged in the
short caecum already referred to. The anterior fold in
the straight intestine continues on into the spiral portion,
and passes into a thick pad, nearly filling up the lumen
of the latter (Tij., fig. 13, PI. III.). This pad or typhlosole
is formed by the same tissue which fills up the space in
the anterior and posterior folds which divide the straight
gut into right and left divisions. Owing to the presence
of this typhlosole the spiral gut appears externally as a
round tube, although in section the lumen is contracted
and crescentric in form ; the tissue filling the typhlosole is
continuous with a narrow layer surrounding the gut and
with the general connective tissue of the viscero-pedal
mass.

After making about six turns the typhlosole disappears,
and the intestinal tube passes into a loose coil of four or
five turns, which may be described as the coiled portion
of the intestine (Al.c.5, figs. 3 and 11), and which lies
anterior to the spiral gut. The average diameter of this
coil is from 0"6 to 1 cm. Its most anterior turn joins the
spiral intestine ; its most posterior one passes off into the
rectum, which passes to the right side of the straight
intestine, and runs up along the posterior part of the
viscero-pedal mass (Al.c.6, figs. 3 and 11) to near the
dorsal portion of the latter, where it pierces the muscular
body -wall and enters the pericardium. After passing
through the ventricle of the heart, the rectum runs along
in the dorsal body-wall over the posterior adductor, and
terminates in the anus {An., fig. 3).

The histological character of the epithelium of the intes-

37

tine behind the typhlosolar portion resembles that described
as present in the right division of the straight intestine.
The wall may be smooth or thrown into three or four
longitudinal ridges. The whole surface is ciliated. The
oily eosinophilous globules already referred to are particu-
larly abundant in the coiled intestine, and many may be
found lying freely in the cavity. The average diameter of
this portion of the intestine is about 0'5 mm.

Towards the anus the epithelium becomes arranged in
a very peculiar manner (fig. 14, PI. III.). Passing over the
posterior adductor the wall becomes thrown into longi-
tudinal folds, which, towards the anus, become more
complex, secondary foldings being developed, and the
bases being narrowed till some folds have, in transverse
section, an almost dendritic appearance. Within they are
filled up by the dense tissue already noted. A continuous
sheet of connective tissue with a few muscle fibres
surrounds the gut, but does not enter into the folds.
Near the anus this dense filling tissue becomes restricted
to the dorsal half of the intestinal tube, where it forms a
crescentic pad lying on the epithelium beneath. This
epithelium differs completely from that forming the
ventral half. While the latter is thrown into complex
foldings and bears long cilia which, on account of the
proximity of the folds to each other, become matted
together in the lumen, the upper epithelium is smooth,
is non-ciliated, and consists of long clear spindle cells,
with nuclei lying at their lower extremities, which form
a sharp contrast with the cubical epithelium of the lower
half. The transition from upper to lower epithelium is
quite a sharp one. The tissue forming the pad lying on
the upper half of the tube differs somewhat from the
dense tissue lying outside the intestinal tube in its more
anterior parts. Here it seems to consist of a dense mass

38

of cells, the bodies of which stain with haematoxylin,
so that nuclei and cell bodies are not clearly distinguish-
able.

The Vasculae System.

The pericardium {Per., figs. 3 and 4) is a spacious sac
which occupies the whole dorsal surface of the body
between the posterior wall of the viscero-pedal mass and
the posterior adductor. Its anterior wall is closely applied
to the former. Its ventral wall rests on the upper surface
of the renal organ. Dorsally and laterally the pericardial
wall is also the body wall, and is thin and delicate, except
in the median dorsal line where it is produced upwards
into a strong ridge, and also on the lateral dorsal borders
where the mantle lobes take origin. It is widest in front
and contracts as it approaches the posterior adductor. It
is nearly filled by the heart consisting of the median
ventricle and the two auricles.

In the animal which has been killed after treatment
with cocaine, the ventricle is usually fixed in the condition
of diastole and then, together with the two auricles, fills
almost the whole pericardial cavity. The ventricle (Veil.,
figs. 3 and 4) is constricted medially, forming two rather
well-marked lateral lobes. The walls are muscular -but
rather thin. Abundant muscular trabecular run across the
cavity in all directions. The rectum passes through its
cavity, suspending the ventricle in the centre of the peri-
cardium. The auricles (Aw., figs. 3 and 4) are triangular
in shape, the apices being attached to the ventricle, the
bases to the bases of the ctenidia. Their walls are very thin,
except at the apical portions, where they are thickened
and composed of dense fibrous tissue ; these portions pro-
ject into the ventricle ; the openings are horizontal slits
bounded by the thickened tissue which forms a pair of

39

valves preventing the reflux of the blood into the auri-
cles during the ventricular systole. On the floor of the
pericardium, beneath the ventricle, are the openings of the
reno-pericardial canals. On the anterior part of the floor
are several openings which are the terminal portions of
systems of tubules forming the paired pericardial glands.
The tubules are lined with cells containing brownish-red
concretions, and are distributed over a wedge-shaped area of
the mantle extending, ventrally, from the most dorsal
portion.* The ventricle is prolonged backwards into a
short narrow neck which still contains the rectum. A
transverse membranous partition, beginning at the dorsal
surface of this neck, extends backwards and downwards
across its cavity, embracing the rectum, and ends so that
its free edge lies near the ventral surface. This must form
a valve preventing the reflux of blood into the ventricle
from the posterior part of the body. Behind this valve the
tube expands forming a "bulbns arteriosus" (B.a., fig. 30)
with thin muscular walls. This terminates in two lateral
branches, the right and left posterior pallial arteries which
diverge from each other and pass backwards beneath the
posterior adductor. Here their walls become very ill-
defined and communicate freely with a system of lacunas
between the bundles of the muscle. The arteries, which
are now difficult to trace, reach the mantle margin and
terminate in the sinuses there and in the walls of the
siphons.

Anteriorly the ventricle passes into a single median vessel
with well defined walls, the anterior aorta (Ao., fig. 30);
this pierces the posterior wall of the viscero-pedal mass, and
travels along in the dorsal region of the latter, giving off,
in its course, small vessels to the digestive gland. Near

* Grobben, Dr. C. Die Pericaidialdiiise der Lamellibranchiaten. Albeit.
Zool. Inst. Wien. Bd. VII. 1888.

40

the anterior margin of the viscero-pedal mass the aorta
bifurcates ; one vessel runs straight forward over the
anterior adductor to the mantle lobes, forming the anterior
pallial artery (Art.p.a.), the other passes straight down-
wards as the viscero-pedal artery. In its course this gives
off a vessel from its anterior side which soon bifurcates,
forming the right and left labial arteries (A. lab.). Still
further down a large vessel is given off from the posterior
side, the visceral artery (Art. v.), this runs back horizon-
tally until it meets the straight portion of the intestine ;
branches are given off which supply blood to the rest of
the gut. The main vessel is continued beyond this branch
to the ventral surface of the foot as the pedal artery
(Art. p.).

Only the above described vessels, constituting the
arterial portion of the vascular system, have definite walls.
The further course of the circulation lies in irregular
lacunae between the various organs, between muscle
bundles, and in cavities in the connective tissues. The
first focus of this system of venous channels is the renal
sinus (Sin.ren., figs. 4 and 30), an irregular blood space
surrounding the tubules of the renal organ. Anteriorly
this begins as a pair of blood spaces lying underneath the
pericardium on each side of the middle line of the body
("venae cavae"). These unite into a large median cavity
in the middle of the renal organ, from which the blood
filters outwards round the system of tubules constituting
that organ. Blood enters the renal sinus dorsally from
the mantle lobes, and anteriorly from a vertical sinus in
the posterior part of the viscero-pedal mass (Sin.p., figs.
11 and 30). This communicates with the renal sinus
through an opening in the muscular wall of the former,
where the posterior retractor muscles of the foot take
origin from the muscular body- wall. Here the arrange-

41

ment of the muscles is such as to constitute a valve
regulating the flow of venous blood outwards from the
viscero-pedal mass.

From the renal sinus the blood reaches the heart by
passing through the gills. The precise path taken will be
considered in connection with the structure of those
organs and of the kidney.

The Eenal Oegan.

The renal organ is a single median structure. As seen
from the ventral side it forms a crescentic mass with the
convexity facing posteriorly, and the two horns, which are
anterior, embracing the posterior retractor pedis (fig. 31,
PL VI.). It forms part of the lateral and the whole ventral
wall of that portion of the body lying between the viscero-
pedal mass and the posterior adductor. Its posterior wall
lies against the adductor. Its dorsal wall is applied to,
the ventral wall of the pericardium.

In front the renal organ consists of a single wide sac
with a few secretory tubules opening into it along each
side, but the diverging retractor muscles of the foot
passing upwards through it on their way to their insertions
in the shell, break up the posterior portion of this sac into
three separate caecal divisions (Ben., fig. 7, PL II.). The
median posterior division passes backwards between the
diverging muscles, the right and left posterior divisions
pass to the outside of the right and left muscles respec-
tively. Each of these three divisions branches out behind
the muscles into a great number of irregular secreting
tubules, owing to which the mass of the organ is greatest
at its most posterior part, that is, at the convex margin of
the crescent.

It is, of course, not the actual renal sac, but the outer
body-wall that is seen from the outside : between the renal

42

sac and the outer wall is the blood sinus surrounding the
secretory tubules. The renal sinus communicates with
the pedal sinus by perforations in the muscular wall of
the viscero-pedal mass, as the pedal retractors originate
from the latter. The muscles lie actually in the blood
sinus. In front of them the sinus is a wide central
cavity lying beneath the flattened renal sac, with renal
tubules dipping into it on all sides except dorsalty.
The body-wall is gathered up into a pair of lateral folds,
which take part in the formation of the bases of the
ctenidia, and the blood from the central cavity niters
through the spaces between the tubules into these
lateral folds, and so into the gills.

The reno-pericardial canals (Ben. per., fig. 4, PI. II.) are
a pair of large tubules which take origin on the lateral
internal walls of the lateral posterior divisions of the renal
sac. In sections they may be found on the part of the
wall lying on the outside of each retractor muscle. They
open into the renal sac by wide fimbriated mouths. Their
walls near these openings are often peculiarly modified,
and are produced out into several small diverticula,
resembling the renal tubules. The canals pass down-
wards and forwards along the ventral wall of the renal sac,
and come to open into the pericardium on the floor of the
latter underneath the ventricle, by a pair of prominent
slits. All along their course the tubules carry a lining of
long cilia, and on the openings of the canals into the peri-
cardium these are very prominent, especially in young
specimens (0"5 to 1 cm.), where they sometimes form a
ciliated fringe projecting into the latter.

The form and course of the renal tubules are very
irregular, and in sections through the posterior margin of
the renal organ it is at first sight difficult to distinguish
between the tubules themselves and the blood sinus. This

43

can be done, however, by the character of the wall ; the
internal surface of the renal tubule (fig. 9) is formed by an
irregular epithelium composed of large non-ciliated cubical
cells with clear or very faintly granular contents. The
cell walls are very definite ; the nuclei lie at the bases on
the lateral walls. The epithelium is supported on a
sharply defined basement membrane, from which bridges
of delicate fibrous tissue stretch across the blood spaces.
The wall of the renal sac, other than that lining the
tubule, has the same characters, except that the cells are
smaller and more irregular.

The Branchle.

The general arrangement of the branchiae (ctenidia) with
respect to the other parts of the body, has already been
described above. There is a single ctenidium on each
side. Each of these organs consists of two branchial
plates lying side by side in the mantle cavity, attached to
a base containing blood vessels. Each branchial plate or
branchia is again folded on itself so that it consists of two
lamellae. The lamellae of the same branchia are bound
together, but there is no connection between the two
branchiae except at the base of the ctenidium from which
both take origin.

The base of the ctenidium (Br'., fig. 3, PI. I.) is a ridge of
the body-wall containing the blood vessels — the common
afferent and efferent branchial vessels. The afferent vessel
carries blood to both branchial plates, while the efferent
vessel carries away the blood oxidised by contact with the
water flowing through the mantle cavity. A flat band of
muscle fibres runs along the ctenidial base from the
posterior to the anterior extremities. The branchial
nerve terminates in the posterior portion of the base.

The ctenidium of the left side is shown in fig. 2. The

44

inner branchia (Br. I.), which is the larger of the two, is
narrowest behind where it fuses with the corresponding
structure of the right side, and increases gradually in
width towards its anterior margin which is attached to
the body-wall, and is slightly overlapped by the labial
palps. The outer branchia (Br.E.) is narrower than the
inner, and is broadest at about midway between its two
extremities. The two branchiae of each side fuse together
behind the viscero-pedal mass at their posterior extrem-
ities.

Each branchia consists of a large number of hollow
filaments connected together at intervals. The direction
of these filaments is at right angles to the ctenidial base.
At the free ventral margin, where each branchia is folded
on itself forming the two lamellae, the filaments of the
inner pass over with some modification in structure into
those of the outer lamella. The whole ctenidium is cut
obliquely to the base in fig. 4, and each of the two
branchiae is seen to be made up of a direct and a reflected
lamella. For the greater part, these two lamellae are
intimately bound together : the precise nature of the
connections is not, however, shown in the figure. In the
inner branchia the external lamella (Br.I.l) which is
inserted into the base of the ctenidium, is the direct one,
and the internal lamella (Br. 1. 2) is the reflected one.
Conversely in the outer branchia, the internal lamella
(Br.E.l) is the direct one, and the external lamella (Br.
E.2) is the reflected one. This difference in the disposition
of the two branchia will be noticed, the inner lamellae of
right and left internal branchiae fuse together in the middle
line, and here the lamellae (Br. 1. 3) are simply mem-
branous plates, showing no sign of filamentar structure.
These membranous portions of the inner lamellae are
continuous at the posterior extremities of the ctenidia

45

with the horizontal septum, which continues forward the
fusion of the mantle edge between the two siphons, and
consequently they form the floor of the ventral supra-
branchial chamber which is a continuation forward of the
cavity of the dorsal siphon. This ventral suprabranchial
chamber is continuous with a series of cavities between
the two lamellae of the right and left inner branchiae.
The reflected lamellae of the outer branchiae, on the other
hand, are filamentar in structure through all their width,
and extend dorsally beyond the ctenidial base to be
inserted into the body-wall in the angle formed by the
latter and the mantle lobes. This dorsal extension of the
reflected lamellae of the outer branchiae is common to a
number of Eulamellibranchs, of which Gardium forms
the type, and is the "appendice" of Pelseneer, which is
regarded by him as the beginning of a third lamella of the
outer branchia.*

The branchiae of Gardium are of the " fluted or com-
pound type," that is, each is thrown into a number of deep
farrows and ridges, the direction of which is perpendicular
to the base of the ctenidium. Two such ridges, with a
furrow between, are represented in fig. 26 — where the
branchia is cut in a plane at right angles to the filaments.
Each ridge consists of a group of from 20 to 30 filaments.
One or two filaments may occupy the summit of the ridge,
the bottom of the furrow is constituted by two filaments
which have opened out and fused by their edges forming a
membranous plate. In each branchia these ridges and
furrows are symmetrically opposed to each other, ridge
against ridge, and furrow against furrow. The supra-
branchial cavity extends into the interior of each ridge.

Morphologically the branchia consists only of this

* Bull. Sci. de la France et de la Belgique. Ser. III., t. XX., pp. 27—52,
1889.

46

double series of hollow filaments which were primitively
vascular channels, the wall of the filament itself serving
as the membrane through which the gaseous interchange
between the blood and the surrounding water is effected.
This simple arrangement is complicated here by the
process of folding, which is described above, and further
by a partial coherence of the filaments, which with the
development of other vascular tissues, form two series of
junctions within the branchia : — 1st, a series of inter-
filamentar junctions (Br.j.l, figs. 26 and 28, PI. V.) joining
the separate filaments in each lamella, and 2nd, a series of
inter-lamellar junctions (Br.j.2, fig. 26) joining the two
lamellse of the same branchia. It will appear from a
consideration of figs. 26 and 28 that this conjunctive
tissue is not formed simply, by the branchial filaments
themselves, but also by vascular tissue developed from
the base of the ctenidium. The whole of an inter-lamellar
junction is constituted by this vascular tissue, the inter-
filamentar junction, on the other hand, is formed both
by the union of the adjacent walls of the filaments and
by a separate vascular tissue. Wherever such an inter-
filamentar junction occurs, the filaments taking part in
it have split (fig. 28) and the adjacent edges of separate
filaments have united. But underneath this place of
union the vascular channel is completed by a sheet of
connective tissue continuous with the tissue of the efferent
or afferent vessels, as the case may be. If the whole
lamella could be flattened out, it would appear as a
trellis work of which the vertical bars would be formed by
the filaments, the horizontal bars by the vascular inter-fila-
mentar junctions. At intervals of every 40 or 50 filaments,
vertical afferent and efferent vessels occur alternately, and
between these vessels the blood circulates in the horizontal
inter-filamentar junctions. But there must also be a

47

limited circulation of the blood through the filaments
themselves.

There is a very regular segmental or repetitional
structure in each branchia, which is due to the fluting
and to the regular occurrence of the afferent and efferent
vessels. At the base of each furrow is a vessel, the
external wall of which is formed by the two flattened-
out filaments already referred to. The rest of the vessel
is formed from extra-filamentar tissue. Afferent and
efferent vessels so formed alternate with complete regu-
larity along the whole length of the branchia. There is
this difference between the two series : — the afferent series
(Br.aff' .) consists of a number of hollow plates extending
uninterruptedly from the base to the ventral edge of each
branchia, and also stretching across from external to
internal lamellae ; each afferent vessel is thus common to
the two lamellae of the branchia; the efferent series consists
of a number of nearly cylindrical vessels (Br.eff' '.), one of
which is present at the base of every alternate furrow.
They must necessarily be double the number of the
afferent vessels. The afferent vessels form the inter-
lamellar junctions. It also follows from this arrangement
that the suprabranchial cavity in the inter-lamellar space
is divided up into a series of separate cavities having no
connection with each other except at the base of the
branchia.

Fig. 26 represents such a double segment in the internal
branchia of one ctenidium. There are from 50 to 60 such
segments in the length of the organ.

The separate filament (fig. 27) is in transverse section
ellipsoidal in shape, the broader end is external, the more
pointed end is internal. The outer surface is composed of
large cubical cells, the inner surface of smaller cells.
Within is an elongated cavity across which bridges of

48

delicate fibrous tissue pass from wall to wall, and
which contains blood corpuscles. There are no skeletal
structures.

In any section of the wall four cells are very obvious.
Two of these are very large, one being situated on each
lateral wall. Their nuclei are prominent but stain lightly.
There is a very evident striated free border, and the cell
bears a number of long and coarse cilia. The two other
cells lie nearer the broad end of the filament, and resemble
those described. The nuclei are, however, very large and
stain intensely, so that they are very evident even under a
low magnifying power. Between these two cells the
outer wall of the filament is composed of cubical cells
bearing short cilia. The inner surface consists of small
cubical or even flattened cells.

Both the vertical afferent and efferent vessels and the
inter-filamentar vessels have very thin walls composed of
a flattened epithelium. Only a few fine trabecule cross
the cavities of these vessels. It is obvious from a comparison
of the area presented by this vascular tissue with the
area of the filaments themselves, and from a comparison
of the nature of the epithelia in each case that by far the
greater part of the gaseous exchange in respiration must be
effected through the wall of the vascular tissue proper and
not through that of the filaments. The latter, in fact, form
a mechanical tissue supporting the series of vascular
channels, and by the action of their ciliated epithelium,
causing the current of water from without to flow through
the bars of the trellis work of each lamella into the supra-
branchial cavities.

The Course of the Circulation.

The heart is a systemic one. Blood, with the waste
products eliminated in the renal organ, and having under-

49

gone oxidation in the gills, is distributed through the
body by two series of vessels ; posteriorly it leaves the
heart by the right and left posterior pallial arteries (Art.pp.,
Art.pp'., fig. 30, PI. V.), and reaches the siphons and the
posterior mantle margin ; anteriorly it traverses the aorta
(Ao.), which soon bifurcates; one branch, the anterior
pallial artery (Art. pa.), passes over the anterior adductor
and reaches the anterior mantle margin; the other branch
descends the anterior part of the viscero-pedal mass as the
viscero-pedal artery (Art.vp.). In its course this supplies
blood to the labial palps through the right and left labial
arteries (A.lab.), and again bifurcates, one branch, the
visceral artery (Art. v.), penetrates the visceral mass and,
passing first to the straight portion of the intestine, sup-
plies the whole length of the latter, the other continues
on to the bend of the foot.

Further than this, it is impossible to trace the afferent
blood channels. Both in the mantle margin and in the
viscero-pedal mass the arteries become lost in an irregular
system of lacunae, lying principally in the interspaces
between the muscle bundles. This lacunar system re-
presents the capillary and venous portions of the vascular
system of a more highly organised animal, and it is here
that the interchange between blood stream and tissues, in
the metabolism of the latter, is effected. Two foci exist
towards which the blood circulating in this lacunar system
converges. From the anterior and posterior margins of the
mantle lobes it flows in the ventral mantle edge towards
the centre, then dorsally through the spaces in the interior
of the thin mantle lobes towards the umbonal parts of the
latter. Here there is a direct communication between the
intrapallial lacunas and the renal sinus, but the greater
portion of the blood, after bathing the tubules of the peri-
cardial gland, reaches the anterior corners of the auricles.

50

The second focus of the venous blood is the renal sinus,
towards which all the blood circulating in the viscero-
pedal mass converges. There are two large sinuses in
the latter, an irregular sinus lying on the anterior margin
and a posterior pedal sinus (Sin. p.) lying beneath the
internal part of the posterior retractor muscles of the foot.
As the latter are gathered up into the two compact muscle
bundles which run upwards through the renal organ, they
leave an opening in the muscular wall of the foot which
leads into the two short longitudinal trunks, these, finally,
open into the large central sinus in the middle of the renal
organ (Sin.ren., figs. 7 and 30).

From this central space the venous blood flows outwards,
bathing the renal secretory tubules in its course, and
enters two longitudinal vessels which run along the bases
of the ctenidia. These are the common afferent branchial
vessels (Br.aff., fig. 30). From them a series of vessels,
running perpendicularly to the bases of the ctenidia, enters
each branchia — the afferent branchial vessels {Br.aff'., figs.
26 and 30). These communicate through the interfila-
mentar branchial junctions with a series of similarly
disposed vessels — the efferent branchial vessels (Br.eff'.,
figs. 26 and 30), which fall into a pair of common efferent
branchial vessels {Br.eff.), and these finally open into the
auricles. From the auricles the blood enters the ventricle
through the openings at the apices of the former, reflux
being prevented by the action of the valves guarding these
openings.

Menegaux* bases a theory accounting for the protrusion
of the foot, siphons, and mantle edges, on the anatomical
relationships indicated above, i.e., the presence of a valve

* Recherches Sur le Circulation des Lamellibranches Marins. 296 pp.,
Besancon, 1890. Also Comptes Kendus de l'Acad. Sci., Paris. T. CVIII.,
pp. 361—364, 1889.

51

behind the ventricle and at the opening of the posterior
pedal into the renal sinus. Since it has been proved
that there is no entrance of water from without, either
into the blood stream or into a closed water vascular
system, the mechanism of the erection of the foot and
siphon must be sought for in the intrinsic muscula-
ture of those parts or in arrangements whereby the
blood pressure in localised regions of the animal's body
can be varied at will. Since there appears to be no
arrangement of muscles which can possibly bring about
the protrusion of the siphons, the only other evident cause
of this must be their distension with blood which is forced
in from the heart through the posterior pallial arteries.
The condition of " turgescence " in the siphons is probably
initiated by the simultaneous relaxation of the constrictor
muscle fibres at their bases, and of the retractor muscles.
The lacunar spaces are thus enlarged and become distended
with blood. The radial muscles and, to a certain extent,
the constrictor muscles must function in preventing
lateral expansion so that the blood pressure is distri-
buted towards the tips of the siphons and is directed
largely towards increasing their length. The valve behind
the ventricle prevents the reflux of blood back into the
heart. Retraction is abundantly provided for by the con-
strictor fibres of the siphonal walls and by the retractor
muscle bundles.

Similarly the turgescence of the foot is initiated by the
relaxation of the posterior and anterior retractores pedis.
The entrance of the posterior pedal sinus into the renal
organ is guarded by an arrangement of muscle fibres
which is in effect a valve, and the closure of this is most
probably the first effect of the relaxation of the posterior
retractor of the foot, since in sections through this region,
in an extended specimen, the opening is difficult to find.

52

Blood which is forced into the viscero-pedal mass by the
increased action of the heart, is now retained there since
there is no other exit than that into the renal sinus, and,
as in the case of the siphons, an area of increased pressure
is established. This, of itself, must tend to straighten out
the two limbs of the foot, and this is in fact observed in
cases of extreme distension. But the action of the in-
trinsic circular and transverse fibres also aids in the
protrusion of the foot, since by their correlated contraction
both diameters can be reduced and the increased pressure
distributed towards the tip. "Within the distal limb the
courses of the intrinsic fibres are so various that no reliable
deduction as to the result of their contraction can be
made.

Retraction of the foot is provided for by the contraction
of the posterior retractor pedis. This, first of all, opens
the passage leading into the renal sinus and allows the
blood in the foot to enter the latter space. Then both
anterior and posterior retractors operate by their contrac-
tion in reducing the length of the proximal limb, and
waves of contraction passing upwards from the tip chase
the blood in the whole foot into the renal sinus. It does
not appear from the disposition of the muscle, regarded
here as the homologue of the " protractor pedis " in other
Lamellibranchs, that its contraction can have any appre-
ciable effect in the protrusion of any part of the viscero-
pedal mass.

It follows from the above that in the condition of
turgescence, the large blood spaces in parts of the body,
other than the viscero-pedal mass, are relatively emptied
of blood ; and that conversely, in the contracted condition,
those spaces are gorged. In sections made from the tur-
gescent animal the empty condition of the renal sinus in
particular is evident. In the contracted condition, blood

53

accumulates in the more dorsal intrapallial lacunae and
in the spaces round the adductors. The lacunae in the
contracted foot itself are only potential, and the circulation
there must be largely confined to that taking place in the
visceral artery and in the venous lacunae round the intes-
tine. This explanation of the condition of turgescence
assumes that the quantity of blood contained in the body is
sufficient to produce the distension of the parts in question.
Fleischman has shown that this is the case for Anadonta,
and the assumption may not unreasonably be made that
it is the case also in Cardium.

The Nervous System.

The central nervous system in Cardium is constituted
by the two separate, paired, cerebral ganglia, each of which
represents the fusion of originally distinct cerebral and
pleural ganglia ; the single median pedal ganglion, formed
by the fusion of originally lateral and paired pedal ganglia,
and the single median parieto- splanchnic ganglion also
formed from originally separate, right and left, ganglionic
masses. There are two pairs of connectives, the cerebro-
visceral connectives joining the cerebral and parieto-
splanchnic ganglia, and the cerebro-pedal which connect
the cerebral and pedal centres. The whole nervous system
is bilaterally symmetrical.

The cerebral ganglion of each side (fig. 3, PI. I., Ga.c.)
lies quite superficially in the loose connective tissue
between the bases of the labial palps, the anterior dorsal
wall of the viscero-pedal mass, and the anterior adductor ;
and is easily exposed by removing the palps close to their
attachments, and parting slightly the adductor from the
adjoining body-wall. It is oval in shape and unpigmented.
The right and left ganglia are joined together by the
long cerebral commissure {Com.) which runs in the base of

54

the dorsal palp; four conspicuous nerves leave the ganglion
on each side, and are easily observed.

The cerebro - visceral connectives (Con.cv.) run back-
wards and upwards in the loose tissue surrounding the
ganglion ; they pierce the muscular body-wall, and pass
through the upper portion of the viscero-pedal mass
embedded in the substance of the digestive gland. Leaving
the latter they again pierce the posterior muscular body-
wall near the external openings of the gonads, and slightly
above the origin of the retractor muscles of the foot. Then
they pass along the lower wall of the renal sinus to join
the parieto-splanchnic ganglion.

The pedal connective of each side (con.cp.) runs down-
wards in the loose tissue round the ganglion for a short
distance, then pierces the anterior muscular wall of the
viscero-pedal mass, and passes vertically downwards on the
internal surface of the latter to the pedal ganglion. The
connectives are quite distinct, but lie close together in the
middle line of the body ; about half-way down in the course
of each, a nerve is given off which passes backwards into
the visceral mass.

A large nerve, the anterior common pallial nerve
{N.pa.), leaves the anterior surface of each ganglion, and
passes along the lower surface of the anterior adductor,
and outwards from this on to the most anterior and dorsal
corner of the mantle lobe. Just before leaving the adductor
each pallial trunk bifurcates.

A very small nerve, the anterior adductor nerve {N.add.),
leaves the ganglion near the origin of the anterior pallial
nerve, and plunges into the posterior surface of the anterior
adductor muscle.

Lastly, there is the strong cerebral commissure running
over the dorsal wall of the mouth.

These are all the nerves that can be observed in the

55

course of dissection. In addition to these, several smaller
twigs can be seen in sections, passing off from the ganglion
into the surrounding tissue, some of these most probably
innervate the labial palps.

The parieto-splanchnic ganglion (Ga.sp., fig. 31, PI. VI.)
is best dissected by placing the animal (removed from the
shell), ventral surface uppermost, and cutting through the
wall of the lower siphon, the fused inner lamellae of the
internal branchiae, and the horizontal shelf formed by the
concrescence of the two ctenidia and the septum between
dorsal and ventral siphons. These parts are then folded
back and the ganglion and its nerves are fully exposed.

This is the largest of the ganglionic centres, it is nearly
square in shape. Indications of its origin from paired and
lateral ganglia are seen in the two little anterior lobes,
from which the connectives take origin. It is covered only
by a single layered epithelium, and lies in a cavity; three
nerves leave it on each side.

The cerebro-visceral connectives are the most delicate
of the nerves leaving the ganglion. Each connective
originates in one of the anterior lobes already referred to,
and at once plunges into the renal sinus and passes
through the latter, lying apparently free in the blood
space. The further course has been described above.

The branchial nerves (N.br.) leave the ganglion from
the anterior corners, and pass along in the wall of the
afferent branchial vessels to the bases of the ctenidia.
Where the latter become free from the body-wall the
nerves bend round at a right angle, and pass backwards
to the tip. Following Duvernoy and Drost* I have
termed these the " branchial nerves," though I have been

* Drost, K. Uber das Nerveusystemu.d. Siiinesepithelieu der Herznmschel
(Cardium edule), &c. Morphologisches Jahrbuch. Bd. XII., pp. 164 — 201 ;
Tafx., 1886—7.

56

unable to demonstrate their actual endings in the tissues
of the ctenidia.

The posterior common pallial nerves (N.pp., fig. 31), are
the stoutest of the nerves proceeding from the visceral
ganglion. They leave the latter from its most posterior
corners, and pass over the ventral surface of the adductor,
reaching the mantle at the most lateral and posterior
corners of the former. As they leave the muscle each
nerve bifurcates; and the external branch, which is the
largest, runs along the mantle edge as the external pallial
nerve (N.p.l) ; the internal branch again divides, the outer
of the two nerves so formed also runs in the muscular
tissue of the mantle margin as the median pallial nerve
(N.p.2) ; the inner one has its whole course in the thin
tissue of the mantle within the line of insertion of the
retractor muscles. This internal pallial nerve (N.p.3) is
by much the most delicate of the three.

As each common pallial nerve passes over the adductor,
two branches are given off from its external surface.
These enter the wall of the dorsal siphon. Three other
nerves leave the trunk after the branch forming the
median and internal pallial nerves is given off. These
enter the wall of the ventral siphon. Thus the two
siphons are innervated by five nerves on each side, of
which two enter the dorsal, three the ventral siphon.
Anastomoses between the first three of these siphonal
nerves are common, and gangliform enlargements may be
observed at their points of origin from the mantle nerve
or further out on their course.

The pedal ganglion (fig. 3, PL I., and fig. 33, PI. VI.) is
best exposed by removing the viscero-pedal mass, with the
bases of the labial palps, and, therefore, the cerebral
ganglia attached, and pinning it down in a dish with the
anterior margin uppermost ; the muscular body-wall is

57

then cut through in the middle line, and the two sides
reflected outwards from below upwards, the attachments
of the transverse muscle bundles being cut through as close
to the body-wall as possible. The whole course of each
cerebro-pedal connective is then exposed from the point
where it perforates the muscular body-wall to its ending
in the ganglion. The latter, with its nerves, is further
exposed by picking away the transverse muscle fibres and
the tubules of the gonad.

The ganglion (Ga.p.) is elongated in a transverse direc-
tion, and is rather oblong in shape. The cerebro-pedal
connectives {Con. cp.) leave its upper and external borders.
Half-way between the pedal and cerebral ganglia each
connective gives off a small branch from its internal
surface, which enters into the viscero-pedal mass.

Exclusive of the connectives four pairs of nerves radiate
out from the ganglion. These enter into the surrounding
tissue. One nerve, however, which is very thick, and
which leaves the lower border of the ganglion, can be
traced as far as the tip of the foot.

The pallial plexus is formed by the three pallial nerves
described above. The anterior common pallial nerve, it
has been stated, bifurcates on leaving the anterior adduc-
tor muscle ; the inner of the two branches so formed gives
off a very fine nerve on its internal side, and thus three
pallial nerves, as in the case of the posterior common
pallial trunk, are formed. These three nerves are, of
course, identical with the three formed from the posterior
trunk, and so each pallial nerve has a double origin, one
extremity proceeding from the cerebral ganglion, the other
from the visceral. At about the centre of the mantle
margin, the middle and external nerves, which have
hitherto kept widely apart from each other, come together
and separate again, and at this point of contact a gangli-

58

form enlargement is formed. The external nerve gives
off an abundant series of branches which pass outwards
to the extreme mantle edge; between external and median,
and to a less extent between median and internal nerves,
there are numerous anastomosing branches. These anas-
tomoses are particularly abundant in the neighbourhood
of the branching of the common pallial trunks.

Sense Organs.

The sensory structures in Cardium are : — (1) Sensory
epithelial cells (Flemming's cells) in the integument ;
(2) visual organs borne by the siphonal tentacles ; (3) a
pair of otocysts in the proximal limb of the viscero-pedal
mass.

(1) The sensory epithelial cells are found over the general
body surface, but are more abundant on the mantle edge
and on the siphons. On the latter they are present in
groups in the depressions at the free extremities of the
tentacles borne by the outer wall.* They are either spindle-
shaped cells with the nucleus at the middle and a bundle
of long hairs on the free extremity, or elongated cells with
the nucleus at the base and the free end swollen out into
a disk, which bears short hairs. Probably they act both
as tactile and as olfactory organs, the latter function being
subserved more especially by the cells on the mantle edge
and siphons.

(2) The structures generally regarded as visual organs
(fig. 36, PI. VI.) are found on the summits of the tentacles
borne by the outer walls of both siphons. At the free end
of each tentacle there is a small depression. The lip of
this depression which faces the siphon is gently rounded,

*Flemming. Untersuch. u.d. Sinnesepithelien der Mollusken. Arcliiv
f. Mikr. Anat. Bd. VI., pp. 439—471, 1870.

59

and bears a somewhat crescentic patch of pigmented epi-
thelium (Ep.op.) ; the other lip, which is turned away from
the wall of the siphon, is sharp, and is raised up to form a
screen. The nerve branch entering the tentacle from the
posterior pallial plexus runs in the axial part, and at the
summit swells out into a nearly globular ganglion (Ga.op.)
consisting of large, clear, oval cells and a network of fibres.
On the siphonal side of the tentacle this ganglion lies close
to the wall. It is invested on all sides except towards the
tip and the outer wall by a capsule of fibrous tissue, which
stains deeply and homogeneously with staining reagents,
has no nuclei, and shows little indication of cellular nature.
On the tip of the tentacle the hair cells, referred to above,
can be traced into the tissue of the ganglion. Nerve fibres
from the latter probably spread round the edge of the
incomplete fibrous capsule, not through its tissue.

The cells covering the tentacle generally are cubical in
form with a continuous cuticle. Towards the tip, how-
ever, they become higher and columnar in shape, and the
cuticle becomes less evident. On the pigmented spot the
outer two-thirds of each of these columnar cells is filled up
with brownish pigment, which takes the form of a dense
mass of spherical granules lying perfectly free from each
other. The nucleus is in contact with the lower layers of
this mass of granules, but is usually quite free. The cell
body seem to be composed of clear cell substance, in which
are embedded the round pigment granules. Beneath the
nucleus it is faintly fibrillar, the direction of the fibrillge
being that of the long axis of the cell. A narrow space
separates this epithelium from the capsule surrounding
the ganglion, and this space is filled by fine fibrous tissue.
It is very probable that nerve fibres from the ganglion,
passing round the edge of the capsule, form part of this
layer, and some at least terminate in or among the cells

60

on the pigment patch, though the existence of such is
difficult to demonstrate.

There can be little doubt that the structure so described
functions as a very simple eye, capable only of distinguish-
ing differences in the intensity of the incident light. Thus,
if a shadow be rapidly thrown on a cockle lying in a
shallow dish, with the siphons fully extended, retraction of
the latter generally follows, and it seems reasonable to
locate the sensitive parts in the structures on the tips of
the tentacles. Very similar organs are found in other
species of Cardium, and in C. muticum, Kishinouye* has
described organs on the tip of the siphonal tentacles which
have all the characters of an eye — a multicellular lens
composed of flattened cells, beneath which is a retinal
layer of elongated cells, and beneath this again a pigment
layer. The only other conjecture as to the nature of the
structures described in the common cockle has been made
by Brock, t who supposes them to be luminous organs;
but it does not appear that there are any observations in
support of this suggestion.

(3) A pair of otocysts (fig. 34, PI. VI.) are present in the
proximal limb of the viscero-pedal mass. They are very
small — 0"07 mm. in longest diameter, and can only be ob-
served in sections taken in the neighbourhood of the pedal
ganglion. They are situated a little way above the latter,
right and left of the middle line and internal to the
cerebro-pedal connectives. They are probably innervated
from the latter. They are oval in form, the longest dia-
meters lying parallel to the transverse axis of the foot. A

*Note on the Eyes of Cardium muticum, Meeve. Journal Roy. Coll. of
Science, Imp. University, Tokyo. Vol. VI., pt. 4, pp. 279—285, PI. IX., 1894.

t Uber die sogenannten Augen von Tridacna, &c. Zeitsch. f. Avisseusch.
Zool. Bd. LXVI., pp. 270—88, PI. XXI. (English translation in Ann. Mag.
Nat. Hist., 1888, pp. 435—52).

61

prominent otolith, marked with concentric and radiating
lines, exactly spherical in form, and about 0'02 mm. in
diameter, is present. The wall of the otocyst is composed
of a single, rather irregular, layer of cells ; at the extremities
of the long diameter are two single, nucleated cells, the
cell bodies of which stain deeply, and on either side of
each of these are several clear, apparently non-nucleated,
spindle cells. The remainder of the wall is composed of
irregular cubical cells. Hairs or cilia are not clearly
shown. The nerves seem to enter at the internal poles
of the organs. Each otocyst is surrounded by a loose
investment of fibrous connective tissue, and but for this,
lies freely among the transverse muscles of the foot.

The Eeproductive Organs.

The gonad is paired and consists of a branching tubular
gland. The external opening (see fig. 3, PL I.) is situated
on the lateral and posterior body-wall, near the origin of
the posterior retractor muscle of the foot. This leads into
a very short terminal duct which immediately branches
into three main divisions (fig. 3). One of these runs
dorsally along the posterior margin of the viscero-pedal
mass, the second downwards along the body-wall, the
third duct runs obliquely forwards and downwards towards
the bend of the foot. Branches are given off laterally
from all these ducts on which are borne botryoidal clusters
of secreting alveoli (fig. 37, PL VI.). These lateral
branches penetrate among the transverse muscle bundles,
between the convolutions of the intestine and between
the lobes of the digestive gland. They lie principally
in the peripheral zone of the proximal limb of the viscero-
pedal mass. There is no extension at any time into the
distal limb of the latter, nor into the mantle lobes.

In Cardium edule the sexes are separate. In at least

62

one other species of Cardium — C. serratum ( = C. nor-
vegicum), investigated by Lacaze-Duthiers,* the animal
is hermaphrodite; on the same branch duct alveoli are
present, some of which are filled with ova, some with
spermatozoa ; and both genital products may be found
even in the same alveolus. In the edible cockle there is
little difference between the gonads of different sexes apart
from their contents. The male gonad is less voluminous
and more opaque.

In the animal taken during the spring or early summer
the gonad is nearly filled with ova or spermatozoa, as the
case may be. In a transverse section through an alveolus
of the male gland (fig. 39, PL VI.) there is a peripheral
zone of small, dense cells lying close to, and obscuring the
wall. Where the latter can be observed it is seen to
consist of a single layer of small, rounded cells supported
on a delicate basement membrane. Within this is the
zone of cells referred to, which results from the pro-
liferation of the germinal epithelium forming the wall.
Towards the centre of the alveolus these become smaller
and denser as they become transformed into the mature
spermatozoa. The elongated head pieces of the latter are
arranged in radial streaks converging towards a portion of
the wall, which does not consist of germinal epithelium,
and where there is generally a slight space. The long tail
pieces are directed towards this space and alternate with
the rows of heads. In the alveoli this part of the wall,
towards which the streaks of spermatozoa converge, is thin
and presents no particular structure. In the larger ducts,
however, it consists of a strip of ciliated epithelium where
the cells are irregular and have clear cell contents. In

* Recherches sur les organes genitaux des acephales Laniellibranclies.
Annates des Sciences Nat. Ser. VI., t. II., Zool., pp. 153—248, PL V.—
IX., 1854.

63

the duct, as in the alveolus, this only forms a small strip
of the wall, the rest being composed of germinal epithelium.

In the female gonad (fig. 38), at a corresponding stage,
the cavities of the ducts and alveoli are filled with eggs in
various stages of development. As in the male gland there
is a strip of the wall which in the alveolus is thin and
membranous, and in the ducts is composed of a ciliated
epithelium ; the remainder of the duct consists of germinal
epithelium which, in some parts, is composed of small,
deeply staining cells with conspicuous nuclei. Many of
these are enlarged and project out from the wall into the
lumen ; the largest eggs which are attached have a narrow,
short stalk which forms part of the wall. The eggs lying
freely in the cavity are flattened against each other by
their mutual pressure, and are usually polygonal in section.
Their true shape is oval (fig. 35, PI. VI.). A very thick
vitelline membrane (M.vit.), secreted apparently from the
surface during the later stages of development, surrounds
each. The cell contents are coarsely granular. The nucleus
is a large, oval body, with a very sharp outline, faintly
granular in texture, and with no apparent signs of chromatic
material. A single, large, rounded, very distinct nucleolus
is always present, the contents of which sometimes show
a very regular vacuolation ; within the ovary the eggs are
frequently adherent together by their membranes.

The eggs and spermatozoa are shed in the early part of
the year (March), and spawning apparently lasts till July
or August. Fertilisation and development take place at
large in the surrounding water, resulting in the formation
of a typical veliger larva. After a short free-swimming
stage, the velum is absorbed, the shell is formed, and the
long vermiform foot is developed. The young cockle,
then still less than 1 mm. long, settles down in the sand
for the remainder of its existence.

64

APPENDIX.

The Economy of the Cockle with special reference
to the Lancashire Sea-Fisheries District.

The cockle derives its economic importance from the fact
that it is a suitable article of food, and that it is sufficiently
abundant, at the same time, on large portions of the coast
to give employment to a large number of people in gather-
ing it for the markets. To a less extent, it is economically
valuable in that it provides an abundant supply of food to
some of the more important flat fishes.

The Lancashire Cockle Fisheries.

A glance at the map of the Lancashire Sea-Fisheries
District, reproduced in this Keport, and which has been
reduced from Mr. R. A. Dawson's sketch chart of the dis-
trict, will show the great area covered by the cockle beds.
Owing to the nature of the coast, where almost every-
where there are great stretches of clean sandy shore swept
twice a day by the tides, there are very few places where
the cockle may not be found. It is not, however, the
mere occurrence of the animal that is considered in the
chart, but its occurrence in sufficient quantities to render
an area a sufficiently profitable fishing ground. The red
coloured areas on the chart indicate approximately the
position and extent of such profitable fishing grounds, and
it is evident that these are numerous enough and of suf-
ficient extent to render Cardium an animal of some
importance to Lancashire fishermen, and to justify the
regulation, to some extent, of the fishery.

65

The cockle beds in the whole district may be conven-
iently grouped into three principal divisions. At the most
northern limit is the estuary of the Duddon, practically
the whole of which is occupied by cockle beds. Then
comes Morecambe Bay, the whole northern part of which
is scattered over with cockle beds. On the western side
of the Bay are the important Baicliff and Bardsea beds,
and towards the eastward side are the Bolton-le-Sands
beds. Between these, and reaching southward as far as
Yeoman Wharf is a large stretch of sands over the whole
of which cockle beds are found and regularly fished. The
chart shows some coloured areas on the southern side of
the Bay, but there the cockle fishery is very irregular.
The Morecambe Bay and Duddon beds together form the
Northern Division.

Between Rossall Point and Blackpool the coast is too
exposed to admit of the formation of profitable cockle beds,
but from Southshore to Southport is the estuary of the
Kibble, containing some very important beds. The most
northern of these, the Crusader Bank, is of little value,
but the Salthouse and Horse Banks, lying further south,
are very valuable, as a glance at Table I. will show. Those
banks form the Central Division.

From Southport to below Formby Point is the second
barren portion of the coast. Here cockles are to be found,
but not in such quantities as to render the beds of any
commercial value. South of Formby Point is a narrow
strip of from two to three miles in length — Formby Bank —
where cockles are very abundant. There are several im-
portant beds on the Cheshire Coast. The Formby, Lea-
sowe, Hoylake, and West Hoyle beds are referred to
hereafter as forming the Southern Division. There are
also a number of areas containing cockle beds in the

66

estuary of the Dee, but these are not included in the
Lancashire Sea-Fisheries district.

I estimate the area covered by cockle beds at 69 square
miles in the Northern Division, 19 square miles in the
Central Division, and 17 square miles in the Southern
Division. Altogether there are not less than 105 square
miles of cockle bearing sands in the whole district.

It must not be supposed that each of the coloured areas
on the map represents a bank, or portion of a bank, over
the whole of which cockles are abundant, and are con-
tinually being fished for. At any one time the fishing
is practically restricted to one or more comparatively small
portions of the bank, and as this becomes exhausted, or as
the cockles become so small as to be under the legal size,
the fishing shifts to some other part. The whole of a
bank may become exhausted temporarily ; this was the
case in 1899 for the Formby Bank, though in 1897 it was a
very profitable cockle fishing ground, as much as 180 cwts.
being removed daily during the winter and spring. At
present (1899) the cockles on this bank are, as a rule, under
legal size ; they are, however, exceedingly numerous, and
a season similar to that of 1897 may be expected in 1900.
Each coloured area on the map represents, in fact, a tract
over which cockle beds are distributed. The precise posi-
tion of the beds is continually shifting to some extent, old
beds being exhausted by fishing, or being sanded up with
the shifting of the sand banks. New beds are being
formed, the position and extent of these being dependent
on the deposition of the spat. The newly-hatched cockle
leads, for a time, a free-swimming life, and with the
acquirement of its shell, settles down for the remainder of
its life in the sand. Obviously the conditions which
determine the place on which the spat ultimately settles,
and the consequent formation of a bed, are complex.

67

The Yield of the Cockle Beds.

In seeking for figures on which to base an estimate
of the productiveness of the Lancashire cockle fisheries,
one turns naturally to the published statistics of the
Board of Trade which relate to fisheries. Here, how-
ever, there is little available information, for statistics
are only collected at the most important ports, and
although the collectors discriminate between the various
animals landed, Crustacea (crabs, lobsters and shrimps),
and mollusca (oysters, mussels and cockles) are included
in the figures relating to " shell-fish." Owing to this
latter cause, comparisons between the amounts of cockles
landed at the same port during different years is im-
possible, and from the fact also that no account is taken
of the produce landed at many important cockling centres,
even an approximate estimate of the value of the Lan-
cashire cockle beds cannot be made.

In the absence of any published data, an estimate of
the value of the fishery has been attempted by finding
approximately the number of cocklers employed on the
various beds, and the amount gathered daily by each
during the time of the year when the fishing is most
active. Obviously, only an approximation to the true
output of the beds can be made by this method, and it
has been regarded as preferable to under-estimate rather
than over-estimate the produce of the fishery. The totals
given in Table IV. then represent, at the least, the value
of the Lancashire cockle fishery. The figures, as to the
number of cocklers, &c, have been given for the most part
by the officers of the Lancashire Sea-Fisheries Committee.
Those for the Northern Division are given by Mr. J.
Wright, chief fishery officer at Piel, and the corresponding
statement for the Southern Division by Mr. G-. Eccles,
chief officer at New Brighton. Mr. E. L. Ascroft has

68

supplied the information relating to the Central Division.
This consists of an accurate statement of the amount of
cockles landed, month by month, during the year June,
1898 — May, 1899, at Lytham. This does not represent
all the fishing in the Kibble estuary, and the amount of
cockles taken in the Central Division and landed elsewhere
than at Lytham is taken as approximately one and a half
times the total shown in Mr. Ascroft's table.

Table I. — Amount of Cockles landed at Lytham
during the year June, 1898— May, 1899 :—

Amount in cwts.

Relative values.

(=R)

June, 1898

July

August

September

October

November

December

January, 1899

February

March

April

May

65

114

151
1250

3983 ) g
3254 §>«
2586 ( S ||
3412 ) >
2278
1547
1316
1165

2

3-5
4-5
37'5
100
100
100
100
69
46-5
40
35

Total amount landed at Lytham
Amount taken in Central Division
and landed elsewhere ...

21121
30000

Total amount taken from Central
Division...

51121

The figures in the third column show the relative values
of the monthly takes expressed as percentages of the
average take (3309) for the four months October — January.

In the construction of tables referring to the Northern
and Southern Divisions it may be assumed, without serious
error, that during the four months, October — January, the
yield of the beds is practically constant. For the other

69

months it is further assumed that the amounts taken
vary in a similar manner to those represented in the above
table, since causes similar to those operating in the Central
Division affect the monthly yield in the Northern and
Southern Divisions. If, then, the amount taken in those
districts in October be known, it becomes possible to con-
struct a table showing the yearly take. The following
table, compiled from information supplied by the fishery
officers, shows approximately the amount taken in that
month from the beds in the districts referred to.

Table II. — Showing the number of Cocklers and the
Amount of Cockles taken in October, 1899, in the
Northern and Southern Divisions : —

No. of
Cocklers.

Amount
gathered by
each per day.

Total amount
taken during
the month
of 25 days.

Northern
Division.

Duddon

Baicliffand Bardsea
Floukburgh Sands..
Bolton-le-Sands ..

20

50

100

15

1£ cwt.

24 „

625 cwt.
2344 ,,
6250 ,,

937 „

Total take in October for Northern Division

10156 cwt.
(=M)

Southern
Division.

West Hoyle
Hoy lake ..
Leasowe

20

10

Average daily

3f cwts.
2£ ,,
take = 7J cwt.

1875 cwts.
625 ,,
187 „

Total take for October in Southern Division (except Crosby
and Formby)

2687 cwt.
(=M')

No account is taken in the above table of the small
amount of fishing which goes on on the southern side of
Morecambe Bay. It has been stated above that the
cockling here is very irregular. Mr. Ascroft informs me
that in 1861 and 1862 as many as twenty carts, with four
cocklers to each, from Bolton-le-Sands, fished regularly on

70

Pilling Sands. At present, however, the fishing there is
of little importance.

From this table, showing the extent of the fishing during
the best time of the year, and the relative monthly values
of the cockles taken, which is exhibited in Table I., an
estimate of the amount taken throughout the whole district
can now be made. This is given in Table III. ; it is there
assumed that the fishing is constant during the four
months October — January. The values for the other
months are calculated ; they vary with respect to the
value for October — January as the values E in Table I.

Table III. — Total Yield in Cwts. of the beds in the
whole Disteict during the year June, 1898 — May, 1899.

Northern
Division.

Central
Division.

Southern
Division.

June, 1898

July

August

September

October

November

December

January, 1899

February
March

April

May

203

355

457

3808

10156

10156

10156

10156

7007

4722

4062

3554

65
114
151
1250
3983
3254
2586
3412
2278
1547
1316
1165

53

94

120

1007

2687

2687

2G87

2687

1854

1249

1074

940

Total amount landed at Lytham

Amount landed in Central Division else-
where than at Lytham ..

Total yield of Formby and Crosby beds
during the year ...

21121
30000

17139
650

Totals

64792

51121

17789

The money value of the cockles taken from the beds
will depend on whether the price paid to the fishermen or

71

the price paid by the consumer is taken as the basis of
calculation. Probably 2/- per cwt. represents the average
value to the fisherman; this may possibly be too low, since
a certain proportion of the cockles landed are hawked in the
neighbourhood of the beds by the fishermen themselves.
On the other hand, supposing the greater portion to be
sent directly by rail to the markets, and adding the cost of
freight and intermediate profits, 6/- per cwt. will represent
fairly the price paid by the consumer.

Table IV. — Money value of the Cockle beds in the
whole District during the year June, 1898 — May, 1899.

Weight of Cockles
in tons.

Money value at
£2 per ton.

Money value at
£6 per ton.

Northern Division...
Central Division ...
Southern Division...

3240

2556

889

£6480
5112
1778

£19440

15336

5334

The whole District ...

6685

£13370

£40110

The striking variation in the monthly yield of the beds
will be noticed. In the table relating to the Central
Division the maximum amount is taken in October, and
the monthly yield then decreases until, in the June follow-
ing, only 2 per cent, of the amount taken in the previous
October is landed. This variation in the fishing seems to
be general in all parts of the district, and is apparently due
to the difficulties encountered in sending the cockles to
the markets, and in keeping them in the retail shops in a
fresh condition during the relatively hot weather of June,
July, and August. Other causes operate; in some parts
of the district the cocklers leave the fishing for employ-
ment in the harvests, or in working pleasure boats at tourist
resorts.

72

Eegulations and Methods of Fishing.

The Bye-laws of the Lancashire Sea-Fisheries Com-
mittee are directed towards the regulation of the cockle
fishery in accordance with local conditions in various parts
of the district, and the methods of fishing vary, to some
extent, in the three principal divisions. Over all the
district the " craam " may be used, but it is employed
chiefly in the Duddon and Morecambe Bay areas. The
"craam" is a metal three-pronged fork, fixed on a handle
of about 18 inches in length. The prongs are about six
inches in length, and about two inches of the extremity of
each is bent downwards at a right angle. The cockier
pushes a fish basket along the sand in his left hand, and
scoops the cockles, one by one, out of the sand into the
basket, with the " craam." Often the tuft of Algge on the
shell of the cockle indicates its presence in the sand, but, as
a rule, they are so abundant that the "craam" plunged
into the sand brings up a cockle. Over the whole district
the "Jumbo" may be used, but only from November to
February, both months included. Fishing by means of
the " Jumbo " is an extension of the method of treading
on the sand with heavy boots, in which process the cockles
are forced up to the surface and are then gathered. The
" Jumbo is a wooden frame, with a base consisting of a
board, the maximum dimensions of which are 4|- feet in
length, 14 inches in width, and 1 inch thick. The frame,
with this base-board resting on the sand, is rocked back-
wards and forwards, with the result that the cockles are
forced out of the sand on to the surface, and are then
gathered up.

Special regulations apply to the central and southern
parts of the district : in the part of the Central Division
lying between Formby Point and the Gut Channel in the
Kibble, the use of a spade is permitted. The spade is

73

used to remove the upper layer of sand to the depth of
about an inch and half. The cockles occupy this layer
and are thus exposed. The use of a spade is not
permitted in the other parts of the district. Southward
of " Biossall Landmark," near Fleetwood, that is, over the
whole of the Central and Southern Divisions, the use of
the cockle rake is permissible ; the cockle rake does not
differ greatly from the ordinary garden rake, but may not
exceed 12 inches in length. Thus, in the Northern Divi-
sion the craam and the limited use of the Jumbo are
permitted ; in the Central Division the craam, Jumbo,
rake, and spade ; in the Southern Division the craam,
Jumbo, and rake. Practically all the fishing in the
Southern Division is done by the rake.

A minimum size, below which a cockle is not legally
saleable, is fixed by the Committee's Bye-law, and extends
to all parts of the district. This minimum size is that
of a cockle which will exactly fit into a rectangular
opening in the "gauge" carried by the fishery officer.
This opening is two inches in length, and three-quarters
of an inch in breadth ; the ratio of length to lateral
breadth in the cockle is, of course, variable within certain
limits, and the smallest legal-sized cockle may be defined
as the cockle whose lateral breadth, from valve to valve,
when the shell is closed, is just over three-quarters of an
inch. This standard of size is, of course, a purely arbi-
trary one, and has no definite relation to any particular
phase in the life-history of the animal : the cockle becomes
sexually mature before it has attained this size, and pro-
bably has spawned once. It most probably represents the
size of the animal which is over two and less than three
years of age. But the rate of growth of the cockle
certainly differs, probably to a considerable extent, on
various parts of the Lancashire and Cheshire coasts. On

74

the Baicliff and Bardsea beds the average cockle picked
out of the fisherman's basket shows, at most, only three
of the lines of growth on the shell, which are referred to
at the beginning of this Memoir; in the Southern Division,
on the Cheshire coast, four or five are generally present in
the average specimen. Growth is most rapid in the sum-
mer months ; on the Baicliff beds, according to the fisher-
men, the cockles in part of a bed, which are under gauge
size in April, may be of legal size in June or July ; in that
period the shell has increased in girth by as much as half
an inch.

The legal size is convenient and most probably eminently
useful. On the principle that a marine food animal ought
to be allowed the chance of spawning at least once before
it is captured for the market, the Bye-law must be regarded
as operating for the preservation of the cockle fishery.
The present condition of the Crosby and Formby cockle
beds is a case in point ; here the difference in the output
of the beds (180 cwts. daily in the winter of 1897 — 8, and
12£ cwts. weekly during the year 1898 — 9) points to the
over-fishing of the beds, which was, of necessity, followed
by their temporary exhaustion. But since the cockles
then became, on the average, so small as to fall under the
gauge size, the fishing of the beds practically stopped for
a time. During this period spawning of the remain-
ing cockles went on, the beds being, to a large extent,
undisturbed, and it is to be expected, from their present
condition, that the fishing will again become abundant.

No close time, as in the case of the common mussel, is
enforced. It will be seen from a consideration of the
monthly output of the Kibble beds, exhibited in Table I.,
that natural causes lead to the suspension (to a great
extent) of the fishing during the summer months. A
certain amount of spawning goes on during May, June,

75

and July, so that, as in the case of the mussel, the animal
is protected for at least a portion of its yearly spawning
period.

No recent data exist on which to base an opinion as to
whether the supply of cockles from the beds in the whole
district is increasing or decreasing ; but in 1879 Buckland
and Walpole, in the course of an examination into the
state of the Sea-Fisheries of England and Wales,* made a
special investigation into the cockle fishery in Morecambe
Bay, and obtained data which enable a comparison to be
made between the output of the beds at the beginning and
end of a period of 21 years.

The Commissioners estimated the value of the pro-
duce of the Morecambe Bay cockle beds for the year 1877
at over 3,943 tons in weight, and at £20,000 in money
value. Cockles were taken by them as worth £5^ per ton.
These figures agree fairly well with those given in this
Memoir. For the year 1898 — 99 the amount landed is
estimated at 3,240 tons, and their money value at £19,440.
Here, however, £6 per ton is taken as representing the
retail price. It is not certain from the Commissioners'
Report whether they regarded £5^ per ton as the price
received by the fishermen, or as the price paid by the
consumer. It was also stated in evidence to the Com-
missioners that 100 carts, with six or seven people to the
cart, were employed cockling in Morecambe Bay. This is
far in excess of the number estimated as at present em-
ployed in the same area.

A more exact means of forming a comparison is
furnished by the return from the Furness Railway Com-
pany, which is published in the Report for 1879. I am
indebted to the courtesy of Mr. Aslett, the present

Report of the Commissioners for Sea Fisheries on the Sea Fisheries of
England ami Wales. 1879, pp. 21—23.

76

manager of the Furness Railway Company, for a similar
statement for the year 1898, which is here reproduced.

Table V. — Weight of Cockles, in Tons, forwarded
from ten stations on the Furness Railway for the two
years 1877 and 11

Stations.

1877.

1898.

Barrow

Tiel

Dal ton

Askam

Millom

Ulverston ...

Cark

Kent's Bank
Arnside
Silverdale . . .

10

70

80

220

343

290

1160

25

5

50

0
0
0
72
0

341
1696

154
0
0

Total

2253

2263

The above Table shews that the amount of cockles for-
warded by the Furness Railway Company in 1877 from
ten of their stations does not differ by so much as a half
per cent, from the amount sent by them in 1898 from the
same stations. These figures represent the greater portion
of cockles sent from Morecambe Bay and Duddon. To
complete the comparison the amount taken from the
southern side of the Bay has to be estimated. The Com-
missioners valued this as worth £5,000, that is, about 940
tons were taken from the Bolton-le-Sands and Pilling
Sands beds in 1877. Since these areas are served by the
London and North -Western Railway Co., the amount
stated does not appear in the Table. From the data
obtainable in the course of the present inquiry, it appears
that the amount of cockles taken from the same areas does
not exceed 300 tons, and the falling off in the produce of
these beds accounts for the apparent decrease in the

77

interval between 1877 and 1899 ; that is, in 1877 the
weight of cockles taken from the whole Bay was
approximately 3,940 tons, in the year 1898 — 99, considered
in this Eeport, it was 3,240, the difference being approxi-
mately equal to the decrease in the produce of the beds
on the southern side of the Bay. But the irregular
nature of the fishing here has been already commented
on, and it is fair to conclude that the decrease is only a
temporary one.

In this comparison the change in the conditions of the
fishery must be borne in mind. In 1877 the fishing was
not regulated in any way, and cockles of any size might
be sent into the markets, but at the present time, only
cockles having the minimum breadth of three-quarter
inch are allowed to be landed and consequently sent
over rail, so that on the areas compared, cockles must
now be more abundant than in 1877.

On the whole, it does not appear that there has been
any permanent decrease in the cockle fishery of More-
cambe Bay proper. Fluctuations from year to year have
been experienced, and it may be gleaned from the Board
of Trade Statistical Tables that though there have been
some lean years, such as 1895 — 96, others — such as 1897
— have been correspondingly fat, and that, on the whole,
the fishing has been, if anything, improving. It has not
been possible to investigate the rest of the District to the
same extent, but it may be safely assumed that there also,
the yield of the beds has been, on the whole, maintained.
The Commissioners of 1879, as the result of their in-
quiries, stated that they had " been unable to trace any
decrease in the fishery." So far as the data obtained in
the course of the present investigation go, that statement
may be safely repeated.

78

Explanation of Plates.
Reference Letters.

Al.c.l. oesophagus.

Al.c.2. stomach.

Al.c.3. straight division of the
intestine.

Al.c.3' . sac of crystalline style.

Al.c.3". digestive division of the
straight intestine.

Al.c.4. spiral division of the
intestine.

Al.c.5. coiled division of the in-
testine.

Al.c.6. rectum.

An. anus.

Add. a. ant. adductor muscle.

Add. a', ant. adductor impres-
sion.

Add.p. post, adductor muscle.

Add. j)'. post, adductor impres-
sion.

Art.pa. anterior pallial artery.

Art.pp. right posterior pallial
artery.

Art.pp' . left post, pallial artery.

Art.vp. viscero-pedal artery.

Art.v. visceral artery.

Art.p. pedal artery.

A. lab. right and left labial
arteries.

Ao. aorta.

Aur. auricle.

Ba. bulbus arteriosus.

Br' . base of right ctenidium.

Br.E. external branchia.

Br.I. internal branchia.

Br.E.l. inner lamella, external
branchia.

Br.E. 2. outer ,, ,,

Br.I.l. ,, ,, internal

Br. 1. 2. inner ,, ,,

Br.1.3. „

membranous portion.

Br.aff. com. afferent branchial
vessel.

Br.eff. com. efferent branchial
vessel.

Br.eff'. efferent branch, vessel.

Br.aff'. afferent branch, vessel.

Br.j.l. inter-filamentar junct.

Br.j.2. inter-lamellar junction.

By. g. byssus gland.

By', pedal groove.

By. byssus thread.

Com. cerebral commissure.

Con.cv. cerebro -visceral con-
nective.

Con.cp. cerebro-pedal connec-
tive.

Dg. digestive gland.

Ep.I.l. epithelium of wall of
crystalline style sac.

Ep.1.2. elongated cells in wall
of crystalline style sac.

Ep.1.3. epithelium of intestinal
div. of straight intest.

Ep).op. pigment cells of an eye.

EjJ.p. epidermis of the foot.

79

Epic, epicuticula (periostracura)
F.tri. tricuspid body.
Go. right gonad.
Ga.c. right cerebral ganglion.
Ga.sp. parieto-splanchnic gang-
lion.
Ga.p. pedal ganglion.
Ga.op. optic ganglion.
Lig. hinge ligament.
Lith. otolith.
M. mouth.
Mn. mantle lobe.
Mn' . cut edge of mantle lobe.
Mn.l. anterior prolongation of

intersiphonal partition.
M.c.l. anal division of mantle

cavity.
M.c.2. cavity of dorsal siphon.
M.c.3. general mantle cavity.
M.c.4. ventral suprabranchial

cavity.
M.c.5. dorsal suprabranchial

cavity.
M.vit. vitelline membrane.
M.p.i. straight transverse pedal

muscles.
M.p.c. circular pedal muscles.
M.p.o. oblique pedal muscles.
M.p.l. long, pedal muscles.
N.add. nerve supplying ant.

adductor.
N.br. branchial nerve.
N.2>a. ant. com. pallial nerve.
N.pp. post. com. pallial nerve.
N.p.l. external pallial nerve.
N.p.2. median pallial nerve.

N.p.3. internal pallial nerve.

N.t. tentacular nerve.

Pro. protractor pedis.

Pro', protractor muscle im-
pression.

Pa.cl. dorsal labial palp.

Pa.v. ventral labial palp.

Ped.l. proximal limb of the
viscero-pedal mass.

Ped.2. distal limb ,, ,,

Per. pericardium.

Bet. a. anterior retractor pedis.

Bet.a'. scar of attachment of
anterior retractor.

Bet.p. posterior retractor pedis.

Bet.})', posterior retractor pedis
impression.

Bet. m. retractor muscles of the
mantle edge.

Bet.m'. impression of retrac-
tors of mantle edge.

Bet.s. retractor muscles of the
siphons.

Bet.s'. impression of retractors
of the siphons.

Ben. renal organ.

Ben' . external opening of renal
organ (ureter).

Ben.per. reno-pericardial canal.

Si.d. dorsal or exhalent siphon.

Si.v. ventral or inhalent siphon.

Sh.e. outer shell layer.

Sh.i. inner shell layer.

Sin.p. posterior pedal sinus.

Sin.ren. renal sinus.

St. crystalline style.

80

Ty. typhlosole.
Ven. ventricle.
A anterior.
P posterior.
D dorsal.

V ventral.
E external.
I internal.
E right.
L left.

Plate I.

Fig. 1. Cockle with the foot and siphons moderately
extended, seen from the right side. Nat. size.

Fig. 2. External anatomy ; the left valve and part of
the left mantle lobe have been removed. Nat.
size.

Fig. 3. General anatomy ; the right valve and mantle
lobe have been removed, and the right wall of
the viscero-pedal mass cut away to expose the
intestine ; the right wall of the pericardium is
cut away ; the right branchiae are removed.
Magnified 2|- diameters. The figure is slightly
diagrammatic ; the convolutions of the intes-
tine are represented as pulled apart, and for
clearness, as being less in diameter than the
magnification of the figure warrants. Their
exact relations are seen in the section repre-
sented in fig. 11.

Plate II.

Fig. 4. Section passing through the ureters transver-
sely to the long axis of the shell, and cutting
the branchiae obliquely. X 3 diam.

Fig. 5. Transverse section through the bases of the
siphons. X 4 diam.

Fig. 6. Transverse section through the posterior adduc-
tor muscle. X 4 diam.

81

Fig. 7. Section through part of the body passing through
the anterior part of the renal organ parallel to
the branchial filaments. X 12 diam.

Fig. 8. Blood corpuscles from one of the branchial
vessels. Zeiss apo. 1"5, compens. oc. 4.

Fig. 9. Transverse section through a renal tubule. X
500 diam.

Fig. 10. The left valve of the shell, seen from the inside.
Nat. size.

Plate III.

Fig. 11. Section through the middle part of the proximal
limb of the viscero-pedal mass, transverse to
the axis of the latter, and in the horizontal
plane of the body. X 7 diam.

Fig. 12. Transverse section through the straight portion
of the intestine. X about 50 diam.

Fig. 13. Transverse section through the spiral portion of
the intestine. X 50 diam.

Fig. 14. Transverse section of the rectum near the anus.
X 50.

Fig. 15. Section through part of the digestive gland,
showing the transition from ciliated to glandu-
lar epithelium. X 50 diam.

Fig. 16. Transverse section of a secreting alveolus from
the digestive gland. Zeiss apo. 1"5, compens.
oc. 4.

Fig. 17. Transverse section of part of a bile duct. X
500 diam.

Fig. 18. Vertical section through part of the stomach
wall. X 280 diam.

82

Plate IV.

Fig. 19. Transverse section of the secreting portion of
the byssus gland. The section is cut trans-
versely to the long axis of the foot. X 60
diam.

Fig. 20. Longitudinal section through a single alveolus of
the byssus gland. X 570 diam.

Fig. 21. Vertical section through part of the body- wall
of the distal limb of the viscero-pedal mass,
showing a mucous gland cell opening through
the epidermis on to the surface of the foot.
Zeiss apo. 1*5 mm., compens. oc. 12.

Fig. 22. An isolated mucous gland from the distal limb
of the viscero-pedal mass. Zeiss apo. 1*5 mm.,
compens. oc. 12.

Fig. 23. A radial vertical section of the mantle edge. X
30 diam.

Fig. 24. A section through the internal labial palp, pas-
sing parallel to the longest border. X 80 diam.

Fig. 25. Vertical section through the muscular body- wall
of the proximal limb of the viscero-pedal mass.
X 120 diam. Z

Plate V.

Fig. 26. Part of a section .through a branchia, transverse
to the branchial filaments, and including two
groups of filaments. X 80 diam.

Fig. 27. Transverse section of a single branchial filament.
Zeiss apo. 1*5 mm., compens. oc. 4.

Fig. 28. Transverse section through two adjacent bran-
chial filaments, showing the interfilamentar
junction. X about 600 diam.

83

Fig. 29. Part of a section through the shell, passing
vertically to the shell surface and perpendicu-
lar to a line of growth. X about 25.

Fig. 30. A schematic representation of the course of the
principal blood vessels and channels as seen
from the right side.

Plate VI.

Fig. 31. Dissection of a cockle from the ventral side, to
show the visceral ganglion and nerves. Only
the portion of the body behind the viscero-pedal
mass is shown. The fused portion of the inner
lamellae of the two internal branchiae is cut
through in the middle line, and the septum
between the siphons and the horizontal shelf
continuing this forward are cut likewise. X
about 2 diam.

Fig. 32. Dissection to show the right cerebral ganglion
and surrounding parts. The extremity of the
anterior adductor has been cut away and the
muscle separated slightly from the anterior
wall of the viscero-pedal mass. The right
labial palps are mostly removed. X 4 diam.

Fig. 33. Dissection of the proximal limb of the viscero-
pedal mass from its anterior margin to show
the pedal ganglion and nerves. The anterior
body- wall and the underlying muscles and geni-
tal gland tubules have been removed. X 3.

Fig. 34. The otocyst of the right side in longitudinal
section. Zeiss apo. 1"5 mm., compens. oc. 4.

Fig. 35. Section through an ovarian egg. X about 200
diam.

84

Fig. 36. Transverse section through the tip of a siphonal
tentacle passing through an "eye." Zeiss
apo. 1*5 mm., compens. oc. 4.

Fig. 37. A group of secreting alveoli from the nearly ripe
ovary. X 25 diam.

Fig. 38. Transverse section through two contiguous
alveoli from a nearly ripe ovary, showing the
germinal epithelium and the eggs lying freely
in the lumina of the alveoli. X 90 diam.

Fig. 39. Transverse section through three alveoli from a
nearly ripe testis. X 90 diam.

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