Chapter XXI: Part 21

The gill-plates have a structure very different from that of the
labial tentacles, and one which in _Anodonta_ is singularly
complicated as compared with the condition presented by these organs
in some other Lamellibranchs, and with what must have been their
original condition in the ancestors of the whole series of living
Lamellibranchia. The phenomenon of "concrescence" which we have
already had to note as showing itself so importantly in regard to the
free edges of the mantle-skirt and the formation of the siphons, is
what, above all things, has complicated the structure of the
Lamellibranch ctenidium. Our present knowledge of the interesting
series of modifications through which the Lamellibranch gill-plates
have developed to their most complicated form is due to R. H. Peck, K.
Mitsukuri and W. G. Ridewood. The Molluscan ctenidium is typically a
plume-like structure, consisting of a vascular axis, on each side of
which is set a row of numerous lamelliform or filamentous processes.
These processes are hollow, and receive the venous blood from, and
return it again aerated into, the hollow axis, in which an afferent
and an efferent blood-vessel may be differentiated. In the genus
_Nucula_ (fig. 10) we have an example of a Lamellibranch retaining
this plume-like form of gill. In the Arcacea (e.g. _Arca_ and
_Pectunculus_) the lateral processes which are set on the axis of the
ctenidium are not lamellae, but are slightly flattened, very long
tubes or hollow filaments. These filaments are so fine and are set so
closely together that they appear to form a continuous membrane until
examined with a lens. The microscope shows that the neighbouring
filaments are held together by patches of cilia, called "ciliated
junctions," which interlock with one another just as two brushes may
be made to do. In fig. 11, A a portion of four filaments of a
ctenidium of the sea-mussel (_Mytilus_) is represented, having
precisely the same structure as those of _Arca_. The filaments of the
gill (ctenidium) of _Mytilus_ and _Arca_ thus form two closely set
rows which depend from the axis of the gill like two parallel plates.
Further, their structure is profoundly modified by the curious
condition of the free ends of the depending filaments. These are
actually reflected at a sharp angle--doubled on themselves in
fact--and thus form an additional row of filaments (see fig. 11 B).
Consequently, each primitive filament has a descending and an
ascending ramus, and instead of each row forming a simple plate, the
plate is double, consisting of a descending and an ascending lamella.
As the axis of the ctenidium lies by the side of the body, and is very
frequently connate with the body, as so often happens in Gastropods
also, we find it convenient to speak of the two plate-like structures
formed on each ctenidial axis as the outer and the inner gill-plate;
each of these is composed of two lamellae, an outer (the reflected)
and an adaxial in the case of the outer gill-plate, and an adaxial and
an inner (the reflected) in the case of the inner gill-plate. This is
the condition seen in _Arca_ and _Mytilus_, the so-called plates
dividing upon the slightest touch into their constituent filaments,
which are but loosely conjoined by their "ciliated junctions."
Complications follow upon this in other forms. Even in _Mytilus_ and
_Arca_ a connexion is here and there formed between the ascending and
descending rami of a filament by hollow extensible outgrowths called
"interlamellar junctions" (_il._ j in B, fig. 11). Nevertheless the
filament is a complete tube formed of chitinous substance and clothed
externally by ciliated epithelium, internally by endothelium and
lacunar tissue--a form of connective tissue--as shown in fig. 11, C.
Now let us suppose as happens in the genus _Dreissensia_--a genus not
far removed from _Mytilus_--that the ciliated inter-filamentar
junctions (fig. 12) give place to solid permanent inter-filamentar
junctions, so that the filaments are converted, as it were, into a
trellis-work. Then let us suppose that the interlamellar junctions
already noted in _Mytilus_ become very numerous, large and irregular;
by them the two trellis-works of filaments would be united so as to
leave only a sponge-like set of spaces between them. Within the
trabeculae of the sponge-work blood circulates, and between the
trabeculae the water passes, having entered by the apertures left in
the trellis-work formed by the united gill-filaments (fig. 14). The
larger the intralamellar spongy growth becomes, the more do the
original gill-filaments lose the character of blood-holding tubes, and
tend to become dense elastic rods for the simple purpose of supporting
the spongy growth. This is seen both in the section of _Dreissensia_
gill (fig. 12) and in those of _Anodonta_ (fig. 13, A, B, C). In the
drawing of _Dreissensia_ the individual filaments f, f, f are cut
across in one lamella at the horizon of an inter-filamentar junction,
in the other (lower in the figure) at a point where they are free. The
chitinous substance ch is observed to be greatly thickened as compared
with what it is in fig. 11, C, tending in fact to obliterate
altogether the lumen of the filament. And in _Anodonta_ (fig. 13, C)
this obliteration is effected. In _Anodonta_, besides being thickened,
the skeletal substance of the filament develops a specially dense,
rod-like body on each side of each filament. Although the structure of
the ctenidium is thus highly complicated in _Anodonta_, it is yet more
so in some of the siphonate genera of Lamellibranchs. The filaments
take on a secondary grouping, the surface of the lamella being thrown
into a series of half-cylindrical ridges, each consisting of ten or
twenty filaments; a filament of much greater strength and thickness
than the others may be placed between each pair of groups. In
_Anodonta_, as in many other Lamellibranchs, the ova and hatched
embryos are carried for a time in the ctenidia or gill apparatus, and
in this particular case the space between the two lamellae of the
outer gill-plate is that which serves to receive the ova (fig. 13, A).
The young are nourished by a substance formed by the cells which cover
the spongy interlamellar outgrowths.

A. Section across the axis of a ctenidium with a pair of
plates--flattened and shortened filaments--attached.
i, j, k, g, Are placed on or near the membrane which attaches the
axis of the ctenidium to the side of the body.
a, b, Free extremities of the plates (filaments).
d, Mid-line of the inferior border.
e, Surface of the plate.
t, Its upper border.
h, Chitinous lining of the plate.
r, Dilated blood-space.
u, Fibrous tract.
o, Upper blood-vessel of the axis.
n, Lower blood-vessel of the axis.
s, Chitinous framework of the axis.
cp, Canal in the same.
A, B, Line along which the cross-section C of the plate is taken.
B. Animal of a male _Nucula proxima_, Say, as seen when the left
valve of the shell and the left half of the mantle-skirt are
removed.
a, a, Anterior adductor muscle.
p.a, Posterior adductor muscle.
v.m, Visceral mass.
f, Foot.
g, Gill.
l, Labial Tentacle.
l.a, Filamentous appendage of the labial tentacle.
lb, Hood-like appendage of the labial tentacle.
m, Membrane suspending the gill and attached to the body along the
line x, y, z, w.
p, Posterior end of the gill (ctenidium).
C. Section across one of the gill-plates (A, B, in A) comparable
with fig. 11 C.
i.a, Outer border.
d.a, Axial border.
l.f, Latero-frontal epithelium.
e, Epithelium of general surface.
r, Dilated blood-space.
h, Chitinous lining (compare A).]

A, Part of four filaments seen from the outer face in order to show
the ciliated junctions c.j.
B, Diagram of the posterior face of a single complete filament with
descending ramus and ascending ramus ending in a hook-like process;
ep., ep., the ciliated junctions; il, j., interlamellar junction.
C, Transverse section of a filament taken so as to cut neither a
ciliated junction nor an interlamellar junction. f.e., Frontal
epithelium; l.f.e´., l.f.e´´., the two rows of latero-frontal
epithelial cells with long cilia; ch, chitinous tubular lining of
the filament; lac., blood lacuna traversed by a few processes of
connective tissue cells; b.c., blood-corpuscle.]

Other points in the modification of the typical ctenidium must be
noted in order to understand the ctenidium of _Anodonta_. The axis of
each ctenidium, right and left, starts from a point well forward near
the labial tentacles, but it is at first only a ridge, and does not
project as a free cylindrical axis until the back part of the foot is
reached. This is difficult to see in _Anodonta_, but if the
mantle-skirt be entirely cleared away, and if the dependent lamellae
which spring from the ctenidial axis be carefully cropped so as to
leave the axis itself intact, we obtain the form shown in fig. 15,
where g and h are respectively the left and the right ctenidial axes
projecting freely beyond the body. In _Arca_ this can be seen with far
less trouble, for the filaments are more easily removed than are the
consolidated lamellae formed by the filaments of _Anodonta_, and in
_Arca_ the free axes of the ctenidia are large and firm in texture
(fig. 9, c, d).

f, Constituent gill-filaments.
ff, Fibrous sub-epidermic tissue.
ch, Chitonous substance of the filaments.
nch, Cells related to the chitonous substance.
lac, Lacunar tissue.
pig, Pigment-cells.
bc, Blood-corpuscles.
fe, Frontal epithelium.
lfe´, lfe´´, Two rows of latero-frontal epithelial cells with long
cilia.
lrf, Fibrous, possibly muscular, substance of the inter-filamentar
junctions.]

A, Outer gill-plate.
B, Inner gill-plate.
C, A portion of B more highly magnified.
o.l, Outer lamella.
i.l, Inner lamella.
v, Blood-vessel.
f, Constituent filaments.
lac, Lacunar tissue.
ch, Chitonous substance of the filament.
chr, Chitonous rod embedded in the softer substance ch.]

Diagram of a block cut from the outer lamella of the outer gill-plate
and seen from the interlamellar surface. f, Constituent filaments;
trf, fibrous tissue of the transverse inter-filamentar junctions; v,
blood-vessel _ilj_, Inter-lamellar junction. The series of oval holes
on the back of the lamella are the water-pores which open between the
filaments in irregular rows separated horizontally by the transverse
inter-filamentar junctions.]

a, Centro-dorsal area.
b, Anterior adductor muscle.
c, Posterior adductor muscle.
d, Mouth.
e, Anus.
f, Foot.
g, Free portion of the axis of left ctenidium.
h, Axis of right ctenidium.
k, Portion of the axis of the left ctenidium which is fused with the
base of the foot, the two dotted lines indicating the origins of the
two rows of gill-filaments.
m, Line of origin of the anterior labial tentacle.
n, Nephridial aperture.
o, Genital aperture.
r, Line of origin of the posterior labial tentacle.]

If we were to make a vertical section across the long axis of a
Lamellibranch which had the axis of its ctenidium free from its origin
onwards, we should find such relations as are shown in the diagram
fig. 16, A. The gill axis d is seen lying in the sub-pallial chamber
between the foot b and the mantle c. From it depend the gill-filaments
or lamellae--formed by united filaments--drawn as black lines f. On
the left side these lamellae are represented as having only a small
reflected growth, on the right side the reflected ramus or lamella is
complete (fr and er). The actual condition in _Anodonta_ at the region
where the gills begin anteriorly is shown in fig. 16, B. The axis of
the ctenidium is seen to be adherent to, or fused by concrescence
with, the body-wall, and moreover on each side the outer lamella of
the outer gill-plate is fused to the mantle, whilst the inner lamella
of the inner gill-plate is fused to the foot. If we take another
section nearer the hinder margin of the foot, we get the arrangement
shown diagrammatically in fig. 16, C, and more correctly in fig. 17.
In this region the inner lamellae of the inner gill-plates are no
longer affixed to the foot. Passing still farther back behind the
foot, we find in _Anodonta_ the condition shown in the section D, fig.
16. The axes i are now free; the outer lamellae of the outer
gill-plates (er) still adhere by concrescence to the mantle-skirt,
whilst the inner lamellae of the inner gill-plates meet one another
and fuse by concrescence at g. In the lateral view of the animal with
reflected mantle-skirt and gill-plates, the line of concrescence of
the inner lamellae of the inner gill-plates is readily seen; it is
marked aa in fig. 1 (5). In the same figure the free part of the inner
lamella of the inner gill-plate resting on the foot is marked z,
whilst the attached part--the most anterior--has been snipped with
scissors so as to show the genital and nephridial apertures x and y.
The concrescence, then, of the free edge of the reflected lamellae of
the gill-plates of Anodon is very extensive. It is important, because
such a concrescence is by no means universal, and does not occur, for
example, in _Mytilus_ or in _Arca_; further, because when its
occurrence is once appreciated, the reduction of the gill-plates of
_Anodonta_ to the plume-type of the simplest ctenidium presents no
difficulty; and, lastly, it has importance in reference to its
physiological significance. The mechanical result of the concrescence
of the outer lamellae to the mantle-flap, and of the inner lamellae to
one another as shown in section D, fig. 16, is that the sub-pallial
space is divided into two spaces by a horizontal septum. The upper
space (i) communicates with the outer world by the excurrent or
superior siphonal notch of the mantle (fig. 1, d); the lower space
communicates by the lower siphonal notch (e in fig. 1). The only
communication between the two spaces, excepting through the
trellis-work of the gill-plates, is by the slit (z in fig. 1 (5)) left
by the non-concrescence of a part of the inner lamella of the inner
gill-plate with the foot. A probe (g) is introduced through this
slit-like passage, and it is seen to pass out by the excurrent
siphonal notch. It is through this passage, or indirectly through the
pores of the gill-plates, that the water introduced into the lower
sub-pallial space must pass on its way to the excurrent siphonal
notch. Such a subdivision of the pallial chamber, and direction of the
currents set up within it do not exist in a number of Lamellibranchs
which have the gill-lamellae comparatively free (_Mytilus_, _Arca_,
_Trigonia_, &c.), and it is in these forms that there is least
modification by concrescence of the primary filamentous elements of
the lamellae.

A, Shows two conditions with free gill-axis.
B, Condition at foremost region in _Anodonta_.
C, Hind region of foot in _Anodonta_.
D, Region altogether posterior to the foot in _Anodonta_.
a, Visceral mass.
b, Foot.
c, Mantle flap.
d, Axis of gill or ctenidium.
e, Adaxial lamella of outer gill-plate.
er, Reflected lamella of outer gill-plate.
f, Adaxial lamella of inner gill-plate.
fr, Reflected lamella of inner gill-plate.
g, Line of concrescence of the reflected lamellae of the two inner
gill-plates.
h, Rectum.
i, Supra-branchial space of the sub-pallial chamber.]

m, Mantle-flap.
br, Outer, b´r´, inner gill-plate--each composed of two lamellae.
f, Foot.
v, Ventricle of the heart.
a, Auricle.
p, p´, Pericardial cavity.
i, Intestine.]

In the 9th edition of this Encyclopaedia Professor (Sir) E. R.
Lankester suggested that these differences of gill-structure would
furnish characters of classificatory value, and this suggestion has
been followed out by Dr Paul Pelseneer in the classification now
generally adopted.

The alimentary canal of _Anodonta_ is shown in fig. 1 (4). The mouth
is placed between the anterior adductor and the foot; the anus opens
on a median papilla overlying the posterior adductor, and discharges
into the superior pallial chamber along which the excurrent stream
passes. The coil of the intestine in _Anodonta_ is similar to that of
other Lamellibranchs. The rectum traverses the pericardium, and has
the ventricle of the heart wrapped, as it were, around it. This is not
an unusual arrangement in Lamellibranchs, and a similar disposition
occurs in some Gastropoda (_Haliotis_). A pair of ducts (ai) lead from
the first enlargement of the alimentary tract called stomach into a
pair of large digestive glands, the so-called liver, the branches of
which are closely packed in this region (af). The food of the
_Anodonta_, as of other Lamellibranchs, consists of microscopic animal
and vegetable organisms, brought to the mouth by the stream which sets
into the sub-pallial chamber at the lower siphonal notch (e in fig.
1). Probably a straining of water from solid particles is effected by
the lattice-work of the ctenidia or gill-plates.

The heart of _Anodonta_ consists of a median ventricle embracing the
rectum (fig. 18, A), and giving off an anterior and a posterior
artery, and of two auricles which open into the ventricle by orifices
protected by valves.

A, Pericardium opened dorsally so as to expose the heart and the
floor of the pericardial chamber d.
B, Heart removed and floor of the pericardium cut away on the left
side so as to open the non-glandular sac of the nephridium,
exposing the glandular sac b, which is also cut into so as to show
the probe f.
C, Ideal pericardium and nephridium viewed laterally.
D, Lateral view showing the actual relation of the glandular and
non-glandular sacs of the nephridium. The arrows indicate the
course of fluid from the pericardium outwards.
a, Ventricle of the heart.
b, Auricle.
bb, Cut remnant of the auricle.
c, Dorsal wall of the pericardium cut and reflected.
e, Reno-pericardial orifice.
f, Probe introduced into the left reno-pericardial orifice.
g, Non-glandular sac of the left nephridium.
h, Glandular sac of the left nephridium.
i, Pore leading from the glandular into the non-glandular sac of
the left nephridium.
k, Pore leading from the non-glandular sac to the exterior.
ac, Anterior.
ab, Posterior, cut remnants of the intestine and ventricle.]

The blood is colourless, and has colourless amoeboid corpuscles
floating in it. In _Ceratisolen legumen_, various species of _Arca_
and a few other species the blood is crimson, owing to the presence of
corpuscles impregnated with haemoglobin. In _Anodonta_ the blood is
driven by the ventricle through the arteries into vessel-like spaces,
which soon become irregular lacunae surrounding the viscera, but in
parts--e.g. the labial tentacles and walls of the gut--very fine
vessels with endothelial cell-lining are found. The blood makes its
way by large veins to a venous sinus which lies in the middle line
below the heart, having the paired renal organs (nephridia) placed
between it and that organ. Hence it passes through the vessels of the
glandular walls of the nephridia right and left into the
gill-lamellae, whence it returns through many openings into the
widely-stretched auricles. In the filaments of the gill of
Protobranchia and many Filibranchia the tubular cavity is divided by a
more or less complete fibrous septum into two channels, for an
afferent and efferent blood-current. The ventricle and auricles of
_Anodonta_ lie in a pericardium which is clothed with a pavement
endothelium (d, fig. 18). It does not contain blood or communicate
directly with the blood-system; this isolation of the pericardium we
have noted already in Gastropods and Cephalopods. A good case for the
examination of the question as to whether blood enters the pericardium
of Lamellibranchs, or escapes from the foot, or by the renal organs
when the animal suddenly contracts, is furnished by the _Ceratisolen
legumen_, which has red blood-corpuscles. According to observations
made by Penrose on an uninjured _Ceratisolen legumen_, no red
corpuscles are to be seen in the pericardial space, although the heart
is filled with them, and no such corpuscles are ever discharged by the
animal when it is irritated.

A, Of _Teredo_.
B, Of _Anodonta_.
C, Of _Pecten_.
a, Cerebral ganglion-pair (= cerebro-pleuro-visceral).
b, Pedal ganglion-pair.
c, Olfactory (osphradial) ganglion-pair.]

c, Capsule.
e, Ciliated cells lining the same.
o, Otolith.]

The pair of renal organs of _Anodonta_, called in Lamellibranchs the
organs of Bojanus, lie below the membranous floor of the pericardium,
and open into it by two well-marked apertures (e and f in fig. 18).
Each nephridium, after being bent upon itself as shown in fig. 18, C,
D, opens to the exterior by a pore placed at the point marked x in
fig. 1 (5) (6). One half of each nephridium is of a dark-green colour
and glandular (h in fig. 18). This opens into the reflected portion
which overlies it as shown in the diagram fig. 18, D, i; the latter
has non-glandular walls, and opens by the pore k to the exterior. The
renal organs may be more ramified in other Lamellibranchs than they
are in _Anodonta_. In some they are difficult to discover. That of the
common oyster was described by Hoek. Each nephridium in the oyster is
a pyriform sac, which communicates by a narrow canal with the
urino-genital groove placed to the front of the great adductor muscle;
by a second narrow canal it communicates with the pericardium. From
all parts of the pyriform sac narrow stalk-like tubes are given off,
ending in abundant widely-spread branching glandular caeca, which form
the essential renal secreting apparatus. The genital duct opens by a
pore into the urino-genital groove of the oyster (the same arrangement
being repeated on each side of the body) close to but distinct from
the aperture of the nephridial canal. Hence, except for the formation
of a urino-genital groove, the apertures are placed as they are in
_Anodonta_. Previously to Hoek's discovery a brown-coloured investment
of the auricles of the heart of the oyster had been supposed to
represent the nephridia in a rudimentary state. This investment, which
occurs also in many Filibranchia, forms the pericardial glands,
comparable to the pericardial accessory glandular growths of
Cephalopoda. In _Unionidae_ and several other forms the pericardial
glands are extended into diverticula of the pericardium which
penetrate the mantle and constitute the organ of Heber. The glands
secrete hippuric acid which passes from the pericardium into the renal
organs.

_Nervous System and Sense-Organs._--In _Anodonta_ there are three
well-developed pairs of nerve ganglia (fig. 19, B, and fig. 1 (6)). An
anterior pair, lying one on each side of the mouth (fig. 19, B, a) and
connected in front of it by a commissure, are the representatives of
the cerebral and pleural ganglia of the typical Mollusc, which are not
here differentiated as they are in Gastropods. A pair placed close
together in the foot (fig. 19, B, b, and fig. 1 (6), ax) are the
typical pedal ganglia; they are joined to the cerebro-pleural ganglia
by connectives.

Posteriorly beneath the posterior adductors, and covered only by a
thin layer of elongated epidermal cells, are the visceral ganglia.
United with these ganglia on the outer sides are the osphradial
ganglia, above which the epithelium is modified to form a pair of
sense-organs, corresponding to the osphradia of other Molluscs. In
some Lamellibranchs the osphradial ganglia receive nerve-fibres, not
from the visceral ganglia, but from the cerebral ganglia along the
visceral commissure. Formerly the posterior pair of ganglia were
identified as simply the osphradial ganglia, and the anterior pair as
the cerebral, pleural and visceral ganglia united into a single pair.
But it has since been discovered that in the Protobranchia the
cerebral ganglia and the pleural are distinct, each giving origin to
its own connective which runs to the pedal ganglion. The cerebro-pedal
and pleuro-pedal connectives, however, in these cases are only
separate in the initial parts of their course, and unite together for
the lower half of their length, or for nearly the whole length.
Moreover, in many forms, in which in the adult condition there is only
a single pair of anterior ganglia and a single pedal connective, a
pleural ganglion distinct from the cerebral has been recognized in the
course of development. There is, however, no evidence of the union of
a visceral pair with the cerebro-pleural.

a, Prae-corneal epithelium.
b, Cellular lens.
c, Retinal body.
d, Tapetum.
e, Pigment.
f, Retinal nerve.
g, Complementary nerve.
h, Epithelial cells filled with pigment.
k, Tentacle.]

The sense-organs of _Anodonta_ other than the osphradia consist of a
pair of otocysts attached to the pedal ganglia (fig. 1 (6), ay). The
otocysts of _Cyclas_ are peculiarly favourable for study on account of
the transparency of the small foot in which they lie, and may be taken
as typical of those of Lamellibranchs generally. The structure of one
is exhibited in fig. 20. A single otolith is present as in the veliger
embryos of Opisthobranchia. In Filibranchia and many Protobranchia the
otocyst (or statocyst) contains numerous particles (otoconia). The
organs are developed as invaginations of the epidermis of the foot,
and in the majority of the Protobranchia the orifice of invagination
remains open throughout life; this is also the case in _Mytilus_
including the common mussel.

A, When free swimming, shows the two dentigerous valves widely open.
B, A later stage, after fixture to the fin of a fish.
sh, Shell.
ad, Adductor muscle.
s, Teeth of the shell.
by, Byssus.
a.ad, Anterior adductor.
p.ad, Posterior adductor.
mt, Mantle-flap.
f, Foot.
br, Branchial filaments.
au.v, Otocyst.
al, Alimentary canal.]

_Anodonta_ has no eyes of any sort, and the tentacles on the mantle
edge are limited to its posterior border. This deficiency is very
usual in the class; at the same time, many Lamellibranchs have
tentacles on the edge of the mantle supplied by a pair of large
well-developed nerves, which are given off from the cerebro-pleural
ganglion-pair, and very frequently some of these tentacles have
undergone a special metamorphosis converting them into
highly-organized eyes. Such eyes on the mantle-edge are found in
_Pecten_, _Spondylus_, _Lima_, _Pinna_, _Pectunculus_, _Modiola_,
_Cardium_, _Tellina_, _Mactra_, _Venus_, _Solen_, _Pholas_ and
_Galeomma_. They are totally distinct from the cephalic eyes of
typical Mollusca, and have a different structure and historical
development. They have originated not as pits but as tentacles. They
agree with the dorsal eyes of _Oncidium_ (Pulmonata) in the curious
fact that the optic nerve penetrates the capsule of the eye and passes
in front of the retinal body (fig. 21), so that its fibres join the
anterior faces of the nerve-end cells as in Vertebrates, instead of
their posterior faces as in the cephalic eyes of Mollusca and
Arthropoda; moreover, the lens is not a cuticular product but a
cellular structure, which, again, is a feature of agreement with the
Vertebrate eye. It must, however, be distinctly borne in mind that
there is a fundamental difference between the eye of Vertebrates and
of all other groups in the fact that in the Vertebrata the retinal
body is itself a part of the central nervous system, and not a
separate modification of the epidermis--myelonic as opposed to
epidermic. The structure of the reputed eyes of several of the
above-named genera has not been carefully examined. In _Pecten_ and
_Spondylus_, however, they have been fully studied (see fig. 21, and
explanation). Rudimentary cephalic eyes occur in the _Mytilidae_ and
in _Avicula_ at the base of the first filament of the inner gill, each
consisting of a pigmented epithelial fossa containing a cuticular
lens. In the _Arcidae_ the pallial eyes are compound or faceted
somewhat like those of Arthropods.

A, Blastula stage (one-cell-layered sac), with commencing
invagination of the wall of the sac at bl, the blastopore.
B, Optical section of a somewhat later stage, in which a second
invagination has begun--namely, that of the shell-gland sk.
bl, Blastopore.
en, Invaginated endoderm (wall of the future arch-enteron).
ec, Ectoderm.
C, Similar optical section at a little later stage. The
invagination connected with the blastopore is now more contracted,
d; and cells, me, forming the mesoblast from which the coelom and
muscular and skeleto-trophic tissues develop, are separated.
D, Similar section of a later stage. The blastopore, bl, has
closed; the anus will subsequently perforate the corresponding
area. A new aperture, m, the mouth, has eaten its way into the
invaginated endodermal sac, and the cells pushed in with it
constitute the stomodaeum. The shell-gland, sk, is flattened out,
and a delicate shell, s, appears on its surface. The ciliated velar
ring is cut in the section, as shown by the two projecting cilia on
the upper part of the figure. The embryo is now a Trochosphere.
E, Surface view of an embryo at a period almost identical with that
of D.
F, Later embryo seen as a transparent object.
m, Mouth.
ft, Foot.
a, Anus.
e, Intestine.
st, Stomach.
tp, Velar area of the prostomium. The extent of the shell and
commencing upgrowth of the mantle-skirt is indicated by a line
forming a curve from a to F.

_N.B._--In this development, as in that of _Pisidium_ (fig. 25), no
part of the blastopore persists either as mouth or as anus, but the
aperture closes--the pedicle of invagination, or narrow neck of the
invaginated arch-enteron, becoming the intestine. The mouth and the
anus are formed as independent in-pushings, the mouth with
stomodaeum first, and the short anal proctodaeum much later. This
interpretation of the appearances is contrary to that of Horst, from
whom our drawings of the oyster's development are taken. The account
given by the American William K. Brooks differs greatly as to matter
of fact from that of Horst, and appears to be erroneous in some
respects.]

_Generative Organs._--The gonads of _Anodonta_ are placed in distinct
male and female individuals. In some Lamellibranchs--for instance, the
European Oyster and the _Pisidium pusillum_--the sexes are united in
the same individual; but here, as in most hermaphrodite animals, the
two sexual elements are not ripe in the same individual at the same
moment. It has been conclusively shown that the _Ostrea edulis_ does
not fertilize itself. The American Oyster (_O. virginiana_) and the
Portuguese Oyster (_O. angulata_) have the sexes separate, and
fertilization is effected in the open water after the discharge of the
ova and the spermatozoa from the females and males respectively. In
the _Ostrea edulis_ fertilization of the eggs is effected at the
moment of their escape from the uro-genital groove, or even before, by
means of spermatozoa drawn into the sub-pallial chamber by the
incurrent ciliary stream, and the embryos pass through the early
stages of development whilst entangled between the gill-lamellae of
the female parent (fig. 23). In _Anodonta_ the eggs pass into the
space between the two lamellae of the outer gill-plate, and are there
fertilized, and advance whilst still in this position to the
glochidium phase of development (fig. 22). They may be found here in
thousands in the summer and autumn months. The gonads themselves are
extremely simple arborescent glands which open to the exterior by two
simple ducts, one right and one left, continuous with the tubular
branches of the gonads. In the most primitive Lamellibranchs there is
no separate generative aperture but the gonads discharge into the
renal cavity, as in _Patella_ among Gastropods. This is the case in
the Protobranchia, e.g. _Solenomya_, in which the gonad opens into the
reno-pericardial duct. But the generative products do not pass through
the whole length of the renal tube: there is a direct opening from the
pericardial end of the tube to the distal end, and the ova or sperms
pass through this. In _Arca_, in _Anomiidae_ and in _Pectinidae_ the
gonad opens into the external part of the renal tube. The next stage
of modification is seen in _Ostraea_, _Cyclas_ and some _Lucinidae_,
in which the generative and renal ducts open into a cloacal slit on
the surface of the body. In _Mytilus_ the two apertures are on a
common papilla, in other cases the two apertures are as in _Anodonta_.
The Anatinacea and _Poromya_ among the Septibranchia are, however,
peculiar in having two genital apertures on each side, one male and
one female. These forms are hermaphrodite, with an ovary and testis
completely separate from each other on each side of the body, each
having its own duct and aperture.

The development of _Anodonta_ is remarkable for the curious larval
form known as _glochidium_ (fig. 22). The glochidium quits the
gill-pouch of its parent and swims by alternate opening and shutting
of the valves of its shell, as do adult _Pecten_ and _Lima_, trailing
at the same time a long byssus thread. This byssus is not homologous
with that of other Lamellibranchs, but originates from a single
glandular epithelial cell embedded in the tissues on the dorsal
anterior side of the adductor muscle. By this it is brought into
contact with the fin of a fish, such as perch, stickleback or others,
and effects a hold thereon by means of the toothed edge of its shells.
Here it becomes encysted, and is nourished by the exudations of the
fish. It remains in this condition for a period of two to six weeks,
and during this time the permanent organs are developed from the cells
of two symmetrical cavities behind the adductor muscle. The early
larva of _Anodonta_ is not unlike the trochosphere of other
Lamellibranchs, but the mouth is wanting. The glochidium is formed by
the precocious development of the anterior adductor and the
retardation of all the other organs except the shell. Other
Lamellibranchs exhibit either a trochosphere larva which becomes a
veliger differing only from the Gastropod's and Pteropod's veliger in
having bilateral shell-calcifications instead of a single central one;
or, like _Anodonta_, they may develop within the gill-plates of the
mother, though without presenting such a specialized larva as the
glochidium. An example of the former is seen in the development of the
European oyster, to the figure of which and its explanation the reader
is specially referred (fig. 23). An example of the latter is seen in a
common little fresh-water bivalve, the _Pisidium pusillum_, which has
been studied by Lankester. The gastrula is formed in this case by
invagination. The embryonic cells continue to divide, and form an oval
vesicle containing liquid (fig. 24); within this, at one pole, is seen
the mass of invaginated cells (fig. 25, hy). These invaginated cells
are the arch-enteron; they proliferate and give off branching cells,
which apply themselves (fig. 25, C) to the inner face of the vesicle,
thus forming the mesoblast. The outer single layer of cells which
constitutes the surface of the vesicle is the ectoderm or epiblast.
The little mass of hypoblast or enteric cell-mass now enlarges, but
remains connected with the cicatrix of the blastopore or orifice of
invagination by a stalk, the rectal peduncle. The enteron itself
becomes bilobed and is joined by a new invagination, that of the mouth
and stomodaeum. The mesoblast multiplies its cells, which become
partly muscular and partly skeleto-trophic. Centro-dorsally now
appears the embyronic shell-gland. The pharynx or stomodaeum is still
small, the foot not yet prominent. A later stage is seen in fig. 26,
where the pharynx is widely open and the foot prominent. No ciliated
velum or pre-oral (cephalic) lobe ever develops. The shell-gland
disappears, the mantle-skirt is raised as a ridge, the paired
shell-valves are secreted, the anus opens by a proctodaeal ingrowth
into the rectal peduncle, and the rudiments of the gills (br) and of
the renal organs (B) appear (fig. 26, lateral view), and thus the
chief organs and general form of the adult are acquired. Later changes
consist in the growth of the shell-valves over the whole area of the
mantle-flaps, and in the multiplication of the gill-filaments and
their consolidation to form gill-plates. It is important to note that
the gill-filaments are formed one by one _posteriorly_. The labial
tentacles are formed late. In the allied genus _Cyclas_, a byssus
gland is formed in the foot and subsequently disappears, but no such
gland occurs in _Pisidium_.

m, Mouth.
x, Anus.
f, Foot.
br, Branchial filaments.
mn, Margin of the mantle-skirt.
B, Organ of Bojanus.]

FIG. 27.--Surface view of a forty-five hour embryo of _Yoldia
limatula_. a.c, Apical cilia. bl, Blastopore. x, Depression where the
cells that form the cerebral ganglia come to the surface.]

An extraordinary modification of the veliger occurs in the development
of _Nucula_ and _Yoldia_ and probably other members of the same
families. After the formation of the gastrula by epibole the larva
becomes enclosed by an ectodermic test covering the whole of the
original surface of the body, including the shell-gland, and leaving
only a small opening at the posterior end in which the stomodaeum and
proctodaeum are formed. In _Yoldia_ and _Nucula proxima_ the test
consists of five rows of flattened cells, the three median rows
bearing circlets of long cilia. At the anterior end of the test is the
apical plate from the centre of which projects a long flagellum as in
many other Lamellibranch larvae. In _Nucula delphinodonta_ the test is
uniformly covered with short cilia, and there is no flagellum. When
the larval development is completed the test is cast off, its cells
breaking apart and falling to pieces leaving the young animal with a
well-developed shell exposed and the internal organs in an advanced
state. The test is really a ciliated velum developed in the normal
position at the apical pole but reflected backwards in such a way as
to cover the original ectoderm except at the posterior end. In
_Yoldia_ and _Nucula proxima_ the ova are set free in the water and
the test-larvae are free-swimming, but in _Nucula delphinodonta_ the
female forms a thin-walled egg-case of mucus attached to the posterior
end of the shell and in communication with the pallial chamber; in
this case the eggs develop and the test-larva is enclosed. A similar
modification of the velum occurs in _Dentalium_ and in _Myzomenia_
among the Amphineura.

CLASSIFICATION OF LAMELLIBRANCHIA

The classification originally based on the structure of the gills by P. Pelseneer included five orders, viz.: the Protobranchia in which the gill-filaments are flattened and not reflected; the Filibranchia in which the filaments are long and reflected, with non-vascular junctions; the Pseudolamellibranchia in which the gill-lamellae are vertically folded, the inter-filamentar and interlamellar junctions being vascular or non-vascular; the Eulamellibranchia in which the inter-filamentar and interlamellar junctions are vascular; and lastly the Septibranchia in which the gills are reduced to a horizontal partition. The Pseudolamellibranchia included the oyster, scallop and their allies which formerly constituted the order Monomyaria, having only a single large adductor muscle or in addition a very small anterior adductor. The researches of W. G. Ridewood have shown that in gill-structure the Pectinacea agree with the Filibranchia and the Ostraeacea with the Eulamellibranchia, and accordingly the order Pseudolamellibranchia is now suppressed and its members divided between the two other orders mentioned. The four orders now retained exhibit successive stages in the modification of the ctenidia by reflection and concrescence of the filament, but other organs, such as the heart, adductors, renal organs, may not show corresponding stages. On the contrary considerable differences in these organs may occur within any single order. The Protobranchia, however, possess several primitive characters besides that of the branchiae. In them the foot has a flat ventral surface used for creeping, as in Gastropods, the byssus gland is but slightly developed, the pleural ganglia are distinct, there is a relic of the pharyngeal cavity, in some forms with a pair of glandular sacs, the gonads retain their primitive connexion with the renal cavities, and the otocysts are open.

Order I. PROTOBRANCHIA

In addition to the characters given above, it may be noted that the mantle is provided with a hypobranchial gland on the outer side of each gill, the auricles are muscular, the kidneys are glandular through their whole length, the sexes are separate.

Fam. 1. _Solenomyidae._--One row of branchial filaments is directed
dorsally, the other ventrally; the mantle has a long postero-ventral
suture and a single posterior aperture; the labial palps of each side
are fused together; shell elongate; hinge without teeth; periostracum
thick. _Solenomya._

Fam. 2. _Nuculidae._--Labial palps free, very broad, and provided with
a posterior appendage; branchial filaments transverse; shell has an
angular dorsal border; mantle open along its whole border. _Nucula.
Acila. Pronucula._

Fam. 3. _Ledidae._--Like the _Nuculidae_, but mantle has two posterior
sutures and two united siphons. _Leda. Yoldia. Malletia._

Fam. 4. _Ctenodontidae._--Extinct; Silurian.

The fossil group Palaeoconcha is connected with the Protobranchia
through the Solenomyidae. It contains the following extinct families.

Fam. 1. _Praecardiidae._--Shell equivalve with hinge dentition as in
_Arca. Praecardium_; Silurian and Devonian.

Fam. 2. _Antipleuridae._--Shell inequivalve. _Antipleura_; Silurian.

Fam. 3. _Cardiolidae._--Shell equivalve and ventricose; hinge without
teeth. _Cardiola_; Silurian and Devonian.

Fam. 4. _Grammysiidae._--Shell thin, equivalve, oval or elongate;
hinge without teeth. _Grammysia_; Silurian and Devonian. _Protomya_;
Devonian. _Cardiomorpha_; Silurian to Carboniferous.

Fam. 5. _Vlastidae._--Shell very inequivalve; hinge without teeth.
_Vlasta_; Silurian.

Fam. 6. _Solenopsidae._--Shell equivalve, greatly elongated, umbones
very far forward. _Solenopsis_; Devonian to Trias.

Order II. FILIBRANCHIA

Gill-filament ventrally directed and reflected, connected by ciliated junctions. Foot generally provided with a highly developed byssogenous apparatus.

Sub-order I.--_Anomiacea._

Very asymmetrical, with a single large posterior adductor. The heart
is not contained in the pericardium, lies dorsad of the rectum and
gives off a single aorta anteriorly. The reflected borders of the
inner gill-plates of either side are fused together in the middle
line. The gonads open into the kidneys and the right gonad extends
into the mantle. Shell thin; animal fixed.

Fam. 1. _Anomiidae._--Foot small; inferior (right) valve of adult
perforated to allow passage of the byssus. _Anomia_; byssus large
and calcified; British. _Placuna_; byssus atrophied in adult.
_Hypotrema_. _Carolia_. _Ephippium_. _Placunanomia_.

Sub-order II.--_Arcacea._

Symmetrical; mantle open throughout its extent; generally with well
developed anterior and posterior adductors. The heart lies in the
pericardium and gives off two aortae. Gills without interlamellar
junctions. Renal and genital apertures separate.

Fam. 1. _Arcidae._--Borders of the mantle bear compound pallial
eyes. The labial palps are direct continuations of the lips. Hinge
pliodont, that is to say, it has numerous teeth on either side of
the umbones and the teeth are perpendicular to the edge. _Arca_;
foot byssiferous; British. _Pectunculus_; foot without byssus;
British. _Scaphula_; freshwater; India. _Argina. Bathyarca.
Barbatia. Senilia. Anadara. Adacnarca._

Fam. 2. _Parallelodontidae._--Shell as in _Arca_, but the posterior
hinge teeth elongated and parallel to the cardinal border.
_Cucullaea_; recent and fossil from the Jurassic. All the other
genera are fossil: _Parallelodon_; Devonian to Tertiary.
_Carbonaria_; Carboniferous, &c.

Fam. 3. _Limopsidae._--Shell orbicular, hinge curved, ligament
longer transversely than antero-posteriorly; foot elongate, pointed
anteriorly and posteriorly. _Limopsis. Trinacria_; Tertiary.

Fam. 4. _Philobryidae._--Shell thin, very inequilateral, anterior
part atrophied, umbones projecting. _Philobrya._

Fam. 5. _Cyrtodontidae._--Extinct; shell equivalve and
inequilateral, short, convex. _Cyrtodonta_; Silurian and Devonian.
_Cypricardites_, Silurian. _Vanuxemia_; Silurian.

Fam. 6. _Trigoniidae._--Shell thick; foot elongated, pointed in
front and behind, ventral border sharp; byssus absent. _Trigonia_;
shell sub-triangular, umbones directed backwards. This genus was
very abundant in the Secondary epoch, especially in Jurassic seas.
There are six living species, all in Australian seas. Living
specimens were first discovered in 1827. _Schizodus_; Permian.
_Myophoria_; Trias.

Fam. 7. _Lyrodesmidae._--Extinct; shell inequilateral, posterior
side shorter; hinge short, teeth in form of a fan. _Lyrodesma_;
Silurian.

Sub-order III.--_Mytilacea._

Symmetrical, the anterior adductor small or absent. Heart gives off
only an anterior aorta. Surface of gills smooth, gill-filaments all
similar, with interlamellar junctions. Gonads generally extend into
mantle and open at sides of kidneys. Foot linguiform and byssiferous.

Fam. 1. _Mytilidae._--Shell inequilateral, anterior end short; hinge
without teeth; ligament external. Mantle has a posterior suture.
Cephalic eyes present. _Mytilus_; British. _Modiola_; British.
_Lithodomus. Modiolaria_; British. _Crenella. Stavelia. Dacrydium.
Myrina. Idas. Septifer._

Fam. 2. _Modiolopsidae._--Extinct; Silurian to Cretaceous; adductor
muscles sub-equal. _Modiolopsis.--Modiomorpha. Myoconcha._

Fam. 3. _Pernidae._--Shell very inequilateral; ligament subdivided;
mantle open throughout; anterior adductor absent. _Perna.
Crenatula_; inhabits sponges. _Bakewellia. Gervilleia_; Trias to
Eocene. _Odontoperna_; Trias. _Inoceramus_; Jurassic to Cretaceous.

Sub-order IV.--_Pectinacea._

Monomyarian, with open mantle. Gills folded and the filaments at
summits and bases of the folds are different from the others. Gonads
contained in the visceral mass and generally open into renal cavities.
Foot usually rudimentary.

Fam. 1. _Vulsellidae._--Shell high; hinge toothless; foot without
byssus. _Vulsella._

Fam. 2. _Aviculidae._--Shell very inequilateral; cardinal border
straight with two auriculae, the posterior the longer. Foot with a
very stout byssus. Gills fused to the mantle. _Avicula_; British.
_Meleagrina._ Pearls are obtained from a species of this genus in
the Persian Gulf, Indian Ocean, &c. _Malleus._ Several extinct
genera.

Fam. 3. _Prasinidae._--Shell inequilateral, with anterior umbones
and prominent anterior auricula; cardinal border arched. _Prasina._

Fam. 4. _Pterineidae._--Extinct; Palaeozoic.

Fam. 5. _Lunulicardiidae._--Extinct; Silurian and Devonian.

Fam. 6. _Conocardiidae._--Extinct; Silurian to Carboniferous.

Fam. 7. _Ambonychiidae._--Extinct; Silurian and Devonian. The last
two families are dimyarian, with small anterior adductor.

Fam. 8. _Myalinidae._--Extinct; Silurian to Cretaceous; adductors
sub-equal.

Fam. 9. _Amussiidae._--Shell orbicular, smooth externally with
radiating costae internally. Gills without interlamellar junctions.
_Amussium._

Fam. 10. _Spondylidae._--Shell very inequivalve, fixed by the right
valve which is the larger. No byssus. _Spondylus_; shell with spiny
ribs, adherent by the spines. _Plicatula._

Fam. 11. _Pectinidae._--Shell with radiating ribs; dorsal border
with two auriculae. Foot byssiferous. Mantle borders with well
developed eyes. _Pecten_; shell orbicular, with equal auriculae;
without a byssal sinus; British. _Chlamys_; anterior auricula the
larger and with a byssal sinus; British. _Pedum. Hinnites.
Pseudamussium. Camptonectes. Hyalopecten_; abyssal.

Sub-order V.--_Dimyacea._

Dimyarian, with orbicular and almost equilateral shell; adherent;
hinge without teeth and ligament internal. Gills with free
non-reflected filaments.

Fam. _Dimyidae._--Characters of the sub-order. _Dimya_; recent in
abyssal depths and fossil since the Jurassic.

Order III. EULAMELLIBRANCHIA

Edges of the mantle generally united by one or two sutures. Two adductors usually present. Branchial filaments united by vascular inter-filamentar junctions and vascular interlamellar junctions; the latter contain the afferent vessels. The gonads always have their own proper external apertures.

Sub-order I.--_Ostraeacea._

Monomyarian or with a very small anterior adductor. Mantle open; foot
rather small; branchiae folded; shell inequivalve.

Fam. 1. _Limidae._--Shell with auriculae. Foot digitiform, with
byssus. Borders of mantle with long and numerous tentacles. Gills
not united with mantle. _Lima_; members of this genus form a nest by
means of the byssus, or swim by clapping the valves of the shell
together. _Limaea._

Fam. 2. _Ostraeidae._--Foot much reduced and without byssus. Heart
usually on the ventral side of the rectum. Gills fused to the
mantle. Shell irregular, fixed in the young by the left and larger
valve. _Ostraea_; foot absent in the adult; edible and cultivated;
some species, as the British _O. edulis_, are hermaphrodite.

Fam. 3. _Eligmidae._--Extinct; Jurassic.

Fam. 4. _Pinnidae._--Shell elongated, truncated and gaping
posteriorly. Dimyarian, with a very small anterior adductor. Foot
with byssus. _Pinna_; British. _Cyrtopinna. Aviculopinna_; fossil,
Carboniferous and Permian. _Pinnigena_; Jurassic and Cretaceous.
_Atrina_; fossil and recent, from Carboniferous to present day.

Sub-order II.--_Submytilacea._

Mantle only slightly closed; usually there is only a single suture.
Siphons absent or very short. Gills smooth. Nearly always dimyarian.
Shell equivalve, with an external ligament.

Fam. 1. _Dreissensiidae._--Shell elongated; hinge without teeth;
summits of valves with an internal septum. Siphons short.
_Dreissensia_; lives in fresh water, but originated from the Caspian
Sea; introduced into England about 1824.

Fam. 2. _Modiolarcidae._--Foot with a plantar surface; the two
branchial plates serve as incubatory pouches. _Modiolarca._

Fam. 3. _Astartidae._--Shell concentrically striated; foot elongate,
without byssus. _Astarte_; British. _Woodia. Opis_; Secondary.
_Prosocoelus_; Devonian.

Fam. 4. _Crassatellidae._--Shell thick, with concentric striae,
ligament external; foot short. _Crassatella. Cuna._

Fam. 5. _Carditidae._--Shell thick, with radiating costae; foot
carinated, often byssiferous. _Cardita. Thecalia. Milneria._
_Venericardia._

Fam. 6. _Condylocardiidae._--Like _Carditidae_, but with an external
ligament. _Condylocardia. Carditella. Carditopsis._

Fam. 7. _Cyprinidae._--Mantle open in front, with two pallial
sutures; external gill-plates smaller than the internal. _Cyprina_;
British. _Cypricardia. Pleurophorus_; Devonian to Trias.
_Anisocardia_; Jurassic to Tertiary. _Veniella_; Cretaceous to
Tertiary.

Fam. 8. _Isocardiidae._--Mantle largely closed, pedal orifice small;
gill-plates of equal size; shell globular, with prominent and coiled
umbones. _Isocardia_; British.

Fam. 9. _Callocardiidae._--Siphons present; external gill-plate
smaller than the internal; umbones not prominent. _Callocardia_;
abyssal.

Fam. 10. _Lucinidae._--Labial palps very small; gills without an
external plate. _Lucina_; British. _Montacuta_; British.
_Cryptodon._

Fam. 11. _Corbidae._--Shell thick, with denticulated borders; anal
aperture with valve but no siphon; foot elongated and pointed.
_Corbis. Gonodon_; Trias and Jurassic. _Mutiella_; Upper Cretaceous.

Fam. 12. _Ungulinidae._--Foot greatly elongated, vermiform, ending
in a glandular enlargement. _Ungulina. Diplodonta_; British.
_Axinus_; British.

Fam. 13. _Cyrenellidae._--Two elongated, united, non-retractile
siphons; freshwater. _Cyrenella. Joanisiella._

Fam. 14. _Tancrediidae._--Shell elongate, sub-triangular. Extinct.
_Tancredia_; Trias to Cretaceous. _Meekia_; Cretaceous.

Fam. 15. _Unicardiidae._--Shell sub-orbicular, nearly equilateral,
with concentric striae. Extinct, Carboniferous to Cretaceous.
_Unicardium. Scaldia. Pseudedmondia._

Fam. 16. _Leptonidae._--Shell thin; no siphons; foot long and
byssiferous; marine; hermaphrodite and incubatory. _Kellya_;
British. _Lepton_; commensal with the Crustacean _Gebia_; British.
_Erycina_; Tertiary. _Pythina. Scacchia. Sportella. Cyamium._

Fam. 17. _Galeommidae._--Mantle reflected over shell; shell thin,
gaping; adductors much reduced. _Galeomma_; British. _Scintilla.
Hindsiella. Ephippodonta_; commensal with shrimp _Axius_. The three
following genera with an internal shell probably belong to this
family:--_Chlamydoconcha_. _Scioberetia_; commensal with a
Spatangid. _Entovalva_; parasitic in _Synapta_.

Fam. 18. _Kellyellidae._--Shell ovoid; anal aperture with very short
siphon; foot elongated. _Kellyella. Turtonia_; British. _Allopagus_;
Eocene. _Lutetia_; Eocene.

br, br´, Outer and inner gill-plates.
t, Labial tentacle.
ta, tr, Upper and lower siphons.
ms, Siphonal muscle of the mantle-flap.
ma, Anterior adductor muscle.
mp, Posterior adductor muscle.
p, Foot.
c, Umbo.]

Fam. 19. _Cyrenidae._--Two siphons, more or less united, with
papillose orifices; pallial line with a sinus; freshwater. _Cyrena.
Corbicula. Batissa. Velorita. Galatea. Fischeria._

Fam. 20. _Cycladidae._--One siphon or two free siphons with simple
orifices; pallial line simple; hermaphrodite, embryos incubated in
external gill-plate; freshwater, _Cyclas_; British. _Pisidium_;
British.

Fam. 21. _Rangiidae._--Two short siphons, shell with prominent
umbones and internal ligament. _Rangia_; brackish water, Florida.

Fam. 22. _Cardiniidae._--Shell elongated, inequilateral. Extinct.
_Cardinia_; Trias and Jurassic. _Anthracosia_; Carboniferous and
Permian. _Anoplophora_; Trias. _Pachycardia_; Trias.

Fam. 23. _Megalodontidae._--Shell inequilateral, thick; posterior
adductor impression on a myophorous apophysis. Extinct. _Megalodon_;
Devonian to Jurassic. _Pachyrisma_; Trias and Jurassic. _Durga_;
Jurassic. _Dicerocardium_; Jurassic.

Fam. 24. _Unionidae._--Shell equilateral; mantle with a single
pallial suture and no siphons; freshwater; larva a glochidium.
_Unio_; British. _Anodonta_; British. _Pseudodon. Quadrula.
Arconaia. Monocondylea. Solenaia. Mycetopus._

Fam. 25. _Mutelidae._--Differs from _Unionidae_ in having two
pallial sutures; freshwater. _Muleta. Pliodon. Spatha. Iridina.
Hyria. Castalia. Aplodon. Plagiodon._

Fam. 26. _Aetheriidae._--Shell irregular, generally fixed in the
adult; foot absent; freshwater. _Aetheria. Mulleria. Bartlettia._

Sub-order III.--_Tellinacea._

Mantle not extensively closed; two pallial sutures and two
well-developed siphons. Gills smooth. Foot compressed and elongated.
Labial palps very large. Dimyarian; pallial line with a deep sinus.

Fam. 1. _Tellinidae._--External gill-plate directed upwards; siphons
separate and elongated; foot with byssus; palps very large; ligament
external. _Tellina_; British. _Gastrana_; British. _Capsa. Macoma._

Fam. 2. _Scrobiculariidae._--External gill-plates directed upwards;
siphons separate and excessively long; foot without byssus.
_Scrobicularia_; estuarine; British. _Syndosmya_; British.
_Cumingia_.

Fam. 3. _Donacidae._--External gill-plate directed ventrally;
siphons separate, of moderate length, anal siphon the longer.
_Donax_; British. _Iphigeneia._

Fam. 4. _Mesodesmatidae._--External gill-plate directed ventrally;
siphons separate and equal. _Mesodesma. Ervilia_; British.

Fam. 5. _Cardiliidae._--Shell very high and short; dimyarian;
posterior adductor impression on a prominent apophysis. _Cardilia._

Fam. 6. _Mactridae._--External gill-plate directed ventrally;
siphons united, invested by a chitinous sheath; foot long, bent at
an angle, without byssus. _Mactra_; British (figs. 28, 29).
_Mulinia. Harvella. Raeta. Eastonia. Heterocardia. Vanganella._

Sub-order IV.--_Veneracea._

Two pallial sutures, siphons somewhat elongated and partially or
wholly united. Gills slightly folded. A bulb on the posterior aorta.
Ligament external.

Fam. 1. _Veneridae._--Foot well developed; pallial sinus shallow or
absent. _Venus_; British. _Dosinia_; British. _Tapes_; British.
_Cyclina. Lucinopsis_; British. _Meretrix. Circe_; British.
_Venerupis._

Fam. 2. _Petricolidae._--Boring forms with a reduced foot; shell
elongated, with deep pallial sinus. _Petricola. P. pholadiformis_,
originally an inhabitant of the coast of the United States, has been
acclimatized for some years in the North Sea.

Fam. 3. _Glaucomyidae._--Siphons very long and united; foot small;
shell thin, with deep pallial sinus; fresh or brackish water.
_Glaucomya. Tanysiphon._

Sub-order V.--_Cardiacea._

Two pallial sutures. Siphons generally short. Foot cylindrical, more
or less elongated, byssogenous. Gills much folded. Shell equivalve,
with radiating costae and external ligament.

Fam. 1. _Cardiidae._--Mantle slightly closed; siphons very short,
surrounded by papillae which often bear eyes; foot very long,
geniculated; pallial line without sinus; two adductors, _Cardium_;
British. _Pseudo-kellya. Byssocardium_; Eocene. _Lithocardium_;
Eocene.

Fam. 2. _Limnocardiidae._--Siphons very long, united throughout;
shell gaping; two adductors; brackish waters. _Limnocardium_;
Caspian Sea and fossil from the Tertiary. _Archicardium_; Tertiary.

Fam. 3. _Tridacnidae._--Mantle closed to a considerable extent;
apertures distant from each other; no siphons; a single adductor;
shell thick. _Tridacna. Hippopus._

Sub-order VI.--_Chamacea._

Asymmetrical, inequivalve, fixed, with extensive pallial sutures; no
siphons. Two adductors. Foot reduced and without byssus. Shell thick,
without pallial sinus.

Fam. 1. _Chamidae._--Shell with sub-equal valves and prominent
umbones more or less spirally coiled; ligament external. _Chama.
Diceras_; Jurassic. _Requienia_; Cretaceous. _Matheronia_;
Cretaceous.

Fam. 2. _Caprinidae._--Shell inequivalve; fixed valve spiral or
conical; free valve coiled or spiral; Cretaceous. _Caprina._
_Caprotina. Caprinula_, &c.

Fam. 3. _Monopleuridae._--Shell very inequivalve; fixed valve
conical or spiral; free valve operculiform; Cretaceous.
_Monopleuron. Baylea._ The two following families, together known
as Rudistae, are closely allied to the preceding; they are extinct
marine forms from Secondary deposits. They were fixed by the conical
elongated right valve; the free left valve is not spiral, and is
furnished with prominent apophyses to which the adductors were
attached.

Fam. 4. _Radiolitidae._--Shell conical or biconvex, without canals
in the external layer. _Radiolites. Biradiolites._

Fam. 5. _Hippuritidae._--Fixed valve long, cylindro-conical, with
three longitudinal furrows which correspond internally to two
pillars for support of the siphons. _Hippurites. Arnaudia._

Sub-order VII.--_Myacea._

Mantle closed to a considerable extent; siphons well developed; gills
much folded and frequently prolonged into the branchial siphon. Foot
compressed and generally byssiferous. Shell gaping, with a pallial
sinus.

Fam. 1. _Psammobiidae._--Siphons very long and quite separate; foot
large; shell oval, elongated, ligament external. _Psammobia_;
British. _Sanguinolaria. Asaphis. Elizia. Solenotellina._

Fam. 2. _Myidae._--Siphons united for the greater part of their
length, and with a circlet of tentacles near their extremities; foot
reduced; shell gaping; ligament internal. _Mya_; British. _Sphenia_;
British. _Tugonia. Platyodon. Cryptomya._

Fam. 3. _Corbulidae._--Shell sub-trigonal, inequivalve; pallial
sinus shallow; siphons short, united, completely retractile; foot
large, pointed, often byssiferous. _Corbulomya. Paramya. Erodona_
and _Himella_ are fluviatile forms from South America.