1911 Encyclopædia Britannica/Lamellibranchia

by Edwin Ray Lankester and Joseph Thomas Cunningham

LAMELLIBRANCHIA (Lat. lamella, a small or thin plate, and Gr. βράγχια, gills), the fourth of the five classes of animals constituting the phylum Mollusca (q.v.). The Lamellibranchia are mainly characterized by the rudimentary condition of the head, and the retention of the primitive bilateral symmetry, the latter feature being accentuated by the lateral compression of the body and the development of the shell as two bilaterally symmetrical plates or valves covering each one side of the animal. The foot is commonly a simple cylindrical or ploughshare-shaped organ, used for boring in sand and mud, and more rarely presents a crawling disk similar to that of Gastropoda; in some forms it is aborted. The paired ctenidia are very greatly developed right and left of the elongated body, and form the most prominent organ of the group. Their function is chiefly not respiratory but nutritive, since it is by the currents produced by their ciliated surface that food-particles are brought to the feebly-developed mouth and buccal cavity.

The Lamellibranchia present as a whole a somewhat uniform structure. The chief points in which they vary are—(1) in the structure of the ctenidia or branchial plates; (2) in the presence of one or of two chief muscles, the fibres of which run across the animal’s body from one valve of the shell to the other (adductors); (3) in the greater or less elaboration of the posterior portion of the mantle-skirt so as to form a pair of tubes, by one of which water is introduced into the sub-pallial chamber, whilst by the other it is expelled; (4) in the perfect or deficient symmetry of the two valves of the shell and the connected soft parts, as compared with one another; (5) in the development of the foot as a disk-like crawling organ (Arca, Nucula, Pectunculus, Trigonia, Lepton, Galeomma), as a simple plough-like or tongue-shaped organ (Unionidae, &c.), as a re-curved saltatory organ (Cardium, &c.), as a long burrowing cylinder (Solenidae, &c.), or its partial (Mytilacea) or even complete abortion (Ostraeacea).

The essential Molluscan organs are, with these exceptions, uniformly well developed. The mantle-skirt is always long, and hides the rest of the animal from view, its dependent margins meeting in the middle line below the ventral surface when the animal is retracted; it is, as it were, slit in the median line before and behind so as to form two flaps, a right and a left; on these the right and the left calcareous valves of the shell are borne respectively, connected by an uncalcified part of the shell called the ligament. In many embryo Lamellibranchs a centro-dorsal primitive shell-gland or follicle has been detected. The mouth lies in the median line anteriorly, the anus in the median line posteriorly.

Both ctenidia, right and left, are invariably present, the axis of each taking origin from the side of the body as in the schematic archi-Mollusc (see fig. 15). A pair of renal tubes opening right and left, rather far forward on the sides of the body, are always present. Each opens by its internal extremity into the pericardium. A pair of genital apertures, connected by genital ducts with the paired gonads, are found right and left near the nephridial pores, except in a few cases where the genital duct joins that of the renal organ (Spondylus). The sexes are often, but not always, distinct. No accessory glands or copulatory organs are ever present in Lamellibranchs. The ctenidia often act as brood-pouches.

A dorsal contractile heart, with symmetrical right and left auricles receiving aerated blood from the ctenidia and mantle-skirt, is present, being unequally developed only in those few forms which are inequivalve. The typical pericardium is well developed. It, as in other Mollusca, is not a blood-space but develops from the coelom, and it communicates with the exterior by the pair of renal tubes. As in Cephalopoda (and possibly other Mollusca) water can be introduced through the nephridia into this space. The alimentary canal keeps very nearly to the median vertical plane whilst exhibiting a number of flexures and loopings in this plane. A pair of large glandular outgrowths, the so-called “liver” or great digestive gland, exists as in other Molluscs. A pair of pedal otocysts, and a pair of osphradia at the base of the gills, appear to be always present. A typical nervous system is present (fig. 19), consisting of a cerebro-pleural ganglion-pair, united by connectives to a pedal ganglion-pair and a visceral ganglion-pair (parieto-splanchnic).

A pyloric caecum connected with the stomach is commonly found, containing a tough flexible cylinder of transparent cartilaginous appearance, called the “crystalline style” (Mactra). In many Lamellibranchs a gland is found on the hinder surface of the foot in the mid line, which secretes a substance which sets into the form of threads—the so-called “byssus”—by means of which the animal can fix itself. Sometimes this gland is found in the young and not in the adult (Anodonta, Unio, Cyclas). In some Lamellibranchs (Pecten, Spondylus, Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there. There are no pores in the foot or elsewhere in Lamellibranchia by which water can pass into and out of the vascular system, as formerly asserted.

The Lamellibranchia live chiefly in the sea, some in fresh waters. A very few have the power of swimming by opening and shutting the valves of the shell (Pecten, Lima); most can crawl slowly or burrow rapidly; others are, when adult, permanently fixed to stones or rocks either by the shell or the byssus. In development some Lamellibranchia pass through a free-swimming trochosphere stage with pre-oral ciliated band; other fresh-water forms which carry the young in brood-pouches formed by the ctenidia have suppressed this larval phase.

Fig. 1.—Diagrams of the external form and anatomy of Anodonta cygnea, the Pond-Mussel; in figures 1, 3, 4, 5, 6 the animal is seen from the left side, the centro-dorsal region uppermost. (1) Animal removed from its shell, a probe g passed into the sub-pallial chamber through the excurrent siphonal notch. (2) View from the ventral surface of an Anodon with its foot expanded and issuing from between the gaping shells. (3) The left mantle-flap reflected upwards so as to expose the sides of the body. (4) Diagrammatic section of Anodon to show the course of the alimentary canal. (5) The two gill-plates of the left side reflected upwards so as to expose the fissure between foot and gill where the probe g passes. (6) Diagram to show the positions of the nerve-ganglia, heart and nephridia. Letters in all the figures as follows:
a,	Centro-dorsal area.
b,	Margin of the left mantle-flap.
c,	Margin of the right mantle-flap.
d,	Excurrent siphonal notch of the mantle margin.
e,	Incurrent siphonal notch of the mantle margin.
f,	Foot.
g,	Probe passed into the superior division of the sub-pallial chamber through the excurrent siphonal notch, and issuing by the side of the foot into the inferior division of the sub-pallial chamber.
h,	Anterior (pallial) adductor muscle of the shells.
i,	Anterior retractor muscle of the foot.
k,	Protractor muscle of the foot.
l,	Posterior (pedal) adductor muscle of the shells.
m,	Posterior retractor muscle of the foot.
n,	Anterior labial tentacle.
o,	Posterior labial tentacle.
p,	Base-line of origin of the reflected mantle-flap from the side of the body.
q,	Left external gill-plate.
r,	Left internal gill-plate.
rr,	Internal lamella of the right inner gill-plate.
rg,	Right outer gill-plate.
s,	Line of concrescence of the outer lamella of the left outer gill-plate with the left mantle-flap.
t,	Pallial tentacles.
u,	The thickened muscular pallial margin which adheres to the shell and forms the pallial line of the left side.
v,	That of the right side.
w,	The mouth.
x,	Aperture of the left organ of Bojanus (nephridium) exposed by cutting the attachment of the inner lamella of the inner gill-plate.
y,	Aperture of the genital duct.
z,	Fissure between the free edge of the inner lamella of the inner gill-plate and the side of the foot, through which the probe g passes into the upper division of the sub-pallial space.
aa,	Line of concrescence of the inner lamella of the right inner gill-plate with the inner lamella of the left inner gill-plate.
ab, ac, ad,	Three pit-like depressions in the median line of the foot supposed by some writers to be pores admitting water into the vascular system.
ae,	Left shell valve.
af,	Space occupied by liver.
ag,	Space occupied by gonad.
ah,	Muscular substance of the foot.
ai,	Duct of the liver on the wall of the stomach.
ak,	Stomach.
al,	Rectum traversing the ventricle of the heart.
am,	Pericardium.
an,	Glandular portion of the left nephridium.
ap,	Ventricle of the heart.
aq,	Aperture by which the left auricle joins the ventricle.
ar,	Non-glandular portion of the left nephridium.
as,	Anus.
at,	Pore leading from the pericardium into the glandular sac of the left nephridium.
au,	Pore leading from the glandular into the non-glandular portion of the left nephridium.
av,	Internal pore leading from the non-glandular portion of the left nephridium to the external pore x.
aw,	Left cerebro-pleuro-visceral ganglion.
ax,	Left pedal ganglion.
ay,	Left otocyst.
az,	Left olfactory ganglion (parieto-splanchnic).
bb,	Floor of the pericardium separating that space from the non-glandular portion of the nephridia.

As an example of the organization of a Lamellibranch, we shall review the structure of the common pond-mussel or swan mussel (Anodonta cygnea), comparing it with other Lamellibranchia.

The swan-mussel has superficially a perfectly developed bilateral symmetry. The left side of the animal is seen as when removed from its shell in fig. 1 (1). The valves of the shell have been removed by severing their adhesions to the muscular areae h, i, k, l, m, u. The free edge of the left half of the mantle-skirt b is represented as a little contracted in order to show the exactly similar free edge of the right half of the mantle-skirt c. These edges are not attached to, although they touch, one another; each flap (right or left) can be freely thrown back in the way carried out in fig. 1 (3) for that of the left side. This is not always the case with Lamellibranchs; there is in the group a tendency for the corresponding edges of the mantle-skirt to fuse together by concrescence, and so to form a more or less completely closed bag, as in the Scaphopoda (Dentalium). In this way the notches d, e of the hinder part of the mantle-skirt of Anodonta are in the siphonate forms converted into two separate holes, the edges of the mantle being elsewhere fused together along this hinder margin. Further than this, the part of the mantle-skirt bounding the two holes is frequently drawn out so as to form a pair of tubes which project from the shell (figs. 8, 29). In such Lamellibranchs as the oysters, scallops and many others which have the edges of the mantle-skirt quite free, there are numerous tentacles upon those edges. In Anodonta these pallial tentacles are confined to a small area surrounding the inferior siphonal notch (fig. 1 [3], t). When the edges of the mantle ventral to the inhalant orifice are united, an anterior aperture is left for the protrusion of the foot, and thus there are three pallial apertures altogether, and species in this condition are called “Tripora.” This is the usual condition in the Eulamellibranchia and Septibranchia. When the pedal aperture is small and far forward there may be a fourth aperture in the region of the fusion behind the pedal aperture. This occurs in Solen, and such forms are called “Quadrifora.”

Fig. 2.—View of the two Valves of the Shell of Cytherea (one of the Sinupalliate Isomya), from the dorsal aspect.

Fig. 3.—Right Valve of the same Shell from the Outer Face.

The centro-dorsal point a of the animal of Anodonta (fig. 1 [1]) is called the umbonal area; the great anterior muscular surface h is that of the anterior adductor muscle, the posterior similar surface i is that of the posterior adductor muscle; the long line of attachment u is the simple “pallial muscle,”—a thickened ridge which is seen to run parallel to the margin of the mantle-skirt in this Lamellibranch. In siphonate forms the pallial muscle is not simple, but is indented posteriorly by a sinus formed by the muscles which retract the siphons.

It is the approximate equality in the size of the anterior and posterior adductor muscles which led to the name Isomya for the group to which Anodonta belongs. The hinder adductor muscle is always large in Lamellibranchs, but the anterior adductor may be very small (Heteromya), or absent altogether (Monomya). The anterior adductor muscle is in front of the mouth and alimentary tract altogether, and must be regarded as a special and peculiar development of the median anterior part of the mantle-flap. The posterior adductor is ventral and anterior to the anus. The former classification based on these differences in the adductor muscles is now abandoned, having proved to be an unnatural one. A single family may include isomyarian, anisomyarian and monomyarian forms, and the latter in development pass through stages in which they resemble the first two. In fact all Lamellibranchs begin with a condition in which there is only one adductor, and that not the posterior but the anterior. This is called the protomonomyarian stage. Then the posterior adductor develops, and becomes equal to the anterior, and finally in some cases the anterior becomes smaller or disappears. The single adductor muscle of the Monomya is separated by a difference of fibre into two portions, but neither of these can be regarded as possibly representing the anterior adductor of the other Lamellibranchs. One of these portions is more ligamentous and serves to keep the two shells constantly attached to one another, whilst the more fleshy portion serves to close the shell rapidly when it has been gaping.

In removing the valves of the shell from an Anodonta, it is necessary not only to cut through the muscular attachments of the body-wall to the shell but to sever also a strong elastic ligament, or spring resembling india-rubber, joining the two shells about the umbonal area. The shell of Anodonta does not present these parts in the most strongly marked condition, and accordingly our figures (figs. 2, 3, 4) represent the valves of the sinupalliate genus Cytherea. The corresponding parts are recognizable in Anodonta. Referring to the figures (2, 3) for an explanation of terms applicable to the parts of the valve and the markings on its inner surface—corresponding to the muscular areas already noted on the surface of the animal’s body—we must specially note here the position of that denticulated thickening of the dorsal margin of the valve which is called the hinge (fig. 4). By this hinge one valve is closely fitted to the other. Below this hinge each shell becomes concave, above it each shell rises a little to form the umbo, and it is into this ridge-like upgrowth of each valve that the elastic ligament or spring is fixed (fig. 4). As shown in the diagram (fig. 5) representing a transverse section of the two valves of a Lamellibranch, the two shells form a double lever, of which the toothed-hinge is the fulcrum. The adductor muscles placed in the concavity of the shells act upon the long arms of the lever at a mechanical advantage; their contraction keeps the shells shut, and stretches the ligament or spring h. On the other hand, the ligament h acts upon the short arm formed by the umbonal ridge of the shells; whenever the adductors relax, the elastic substance of the ligament contracts, and the shells gape. It is on this account that the valves of a dead Lamellibranch always gape; the elastic ligament is no longer counteracted by the effort of the adductors. The state of closure of the valves of the shell is not, therefore, one of rest; when it is at rest—that is, when there is no muscular effort—the valves of a Lamellibranch are slightly gaping, and are closed by the action of the adductors when the animal is disturbed. The ligament is simple in Anodonta; in many Lamellibranchs it is separated into two layers, an outer and an inner (thicker and denser). That the condition of gaping of the shell-valves is essential to the life of the Lamellibranch appears from the fact that food to nourish it, water to aerate its blood, and spermatozoa to fertilize its eggs, are all introduced into this gaping chamber by currents of water, set going by the highly-developed ctenidia. The current of water enters into the sub-pallial space at the spot marked e in fig. 1 (1), and, after passing as far forward as the mouth w in fig. 1 (5), takes an outward course and leaves the sub-pallial space by the upper notch d. These notches are known in Anodonta as the afferent and efferent siphonal notches respectively, and correspond to the long tube-like afferent inferior and efferent superior “siphons” formed by the mantle in many other Lamellibranchs (fig. 8).

Fig. 4.—Left Valve of the same Shell from the Inner Face. (Figs. 2, 3, 4 from Owen.)

Fig. 5.—Diagram of a section of a Lamellibranch’s shells, ligament and adductor muscle. a, b, right and left valves of the shell; c, d, the umbones or short arms of the lever; e, f, the long arms of the lever; g, the hinge; h, the ligament; i, the adductor muscle.

Whilst the valves of the shell are equal in Anodonta we find in many Lamellibranchs (Ostraea, Chama, Corbula, &c.) one valve larger, and the other smaller and sometimes flat, whilst the larger shell may be fixed to rock or to stones (Ostraea, &c.). A further variation consists in the development of additional shelly plates upon the dorsal line between the two large valves (Pholadidae). In Pholas dactylus we find a pair of umbonal plates, a dors-umbonal plate and a dorsal plate. It is to be remembered that the whole of the cuticular hard product produced on the dorsal surface and on the mantle-flaps is to be regarded as the “shell,” of which a median band-like area, the ligament, usually remains uncalcified, so as to result in the production of two valves united by the elastic ligament. But the shelly substance does not always in boring forms adhere to this form after its first growth. In Aspergillum the whole of the tubular mantle area secretes a continuous shelly tube, although in the young condition two valves were present. These are seen (fig. 7) set in the firm substance of the adult tubular shell, which has even replaced the ligament, so that the tube is complete. In Teredo a similar tube is formed as the animal elongates (boring in wood), the original shell-valves not adhering to it but remaining movable and provided with a special muscular apparatus in place of a ligament. In the shell of Lamellibranchs three distinct layers can be distinguished: an external chitinous, non-calcified layer, the periostracum; a middle layer composed of calcareous prisms perpendicular to the surface, the prismatic layer; and an internal layer composed of laminae parallel to the surface, the nacreous layer. The last is secreted by the whole surface of the mantle except the border, and additions to its thickness continue to be made through life. The periostracum is produced by the extreme edge of the mantle border, the prismatic layer by the part of the border within the edge. These two layers, therefore, when once formed cannot increase in thickness; as the mantle grows in extent its border passes beyond the formed parts of the two outer layers, and the latter are covered internally by a deposit of nacreous matter. Special deposits of the nacreous matter around foreign bodies form pearls, the foreign nucleus being usually of parasitic origin (see Pearl).

Let us now examine the organs which lie beneath the mantle-skirt of Anodonta, and are bathed by the current of water which circulates through it. This can be done by lifting up and throwing back the left half of the mantle-skirt as is represented in fig. 1 (3). We thus expose the plough-like foot (f), the two left labial tentacles, and the two left gill-plates or left ctenidium. In fig. 1 (5), one of the labial tentacles n is also thrown back to show the mouth w, and the two left gill-plates are reflected to show the gill-plates of the right side (rr, rq) projecting behind the foot, the inner or median plate of each side being united by concrescence to its fellow of the opposite side along a continuous line (aa). The left inner gill-plate is also snipped to show the subjacent orifices of the left renal organ x, and of the genital gland (testis or ovary) y. The foot thus exposed in Anodonta is a simple muscular tongue-like organ. It can be protruded between the flaps of the mantle (fig. 1 [1] [2]) so as to issue from the shell, and by its action the Anodonta can slowly crawl or burrow in soft mud or sand. Other Lamellibranchs may have a larger foot relatively than has Anodonta. In Arca it has a sole-like surface. In Arca too and many others it carries a byssus-forming gland and a byssus-cementing gland. In the cockles, in Cardium and in Trigonia, it is capable of a sudden stroke, which causes the animal to jump when out of the water, in the latter genus to a height of four feet. In Mytilus the foot is reduced to little more than a tubercle carrying the apertures of these glands. In the oyster it is absent altogether.

Fig. 6.—Shell of Aspergillum vaginiferum. (From Owen.)

Fig. 7.—Shell of Aspergillum vaginiferum to show the original valves a, now embedded in a continuous calcification of tubular form. (From Owen.)

The labial tentacles or palps of Anodonta (n, o in fig. 1 [3], [5]) are highly vascular flat processes richly supplied with nerves. The left anterior tentacle (seen in the figure) is joined at its base in front of the mouth (w) to the right anterior tentacle, and similarly the left (o) and right posterior tentacles are joined behind the mouth. Those of Arca (i, k in fig. 9) show this relation to the mouth (a). These organs are characteristic of all Lamellibranchs; they do not vary except in size, being sometimes drawn out to streamer-like dimensions. Their appearance and position suggest that they are in some way related morphologically to the gill-plates, the anterior labial tentacle being a continuation of the outer gill-plate, and the posterior a continuation of the inner gill-plate. There is no embryological evidence to support this suggested connexion, and, as will appear immediately, the history of the gill-plates in various forms of Lamellibranchs does not directly favour it. The palps are really derived from part of the velar area of the larva.

Fig. 8.—Psammobia florida, right side, showing expanded foot e, and g incurrent and g′ excurrent siphons. (From Owen.)

Fig. 9.—View from the ventral (pedal) aspect of the animal of Arca noae, the mantle-flap and gill-filaments having been cut away. (Lankester.)
a,	Mouth.
b,	Anus.
c,	Free spirally turned extremity of the gill-axis or ctenidial axis of the right side.
d,	Do. of the left side.
e, f,	Anterior portions of these axes fused by concrescence to the wall of the body.
g,	Anterior adductor muscle.
h,	Posterior adductor.
i,	Anterior labial tentacle.
k,	Posterior labial tentacle.
l,	Base line of the foot.
m,	Sole of the foot.
n,	Callosity.

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.

Fig. 10.—Structure of the Ctenidia of Nucula. (After Mitsukuri.) See also fig. 2.
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).

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).

Fig. 11.—Filaments of the Ctenidium of Mytilus edulis. (After R. H. Peck.)
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.

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

Fig. 12.—Transverse Section of the Outer Gill-plate of Dreissensia polymorpha. (After R. H. Peck.)
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.

Fig. 13.—Transverse Sections of Gill-plates of Anodonta. (After R. H. Peck.)
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.

Fig. 14.—Gill-lamellae of Anodonta. (After R. H. Peck.)

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.

Fig. 15.—Diagram of a view from the left side of the animal of Anodonta cygnaea, from which the mantle-skirt, the labial tentacles and the gill-filaments have been entirely removed so as to show the relations of the axis of the gill-plumes or ctenidia g, h. (Original.)
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.

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.

Fig. 16.—Diagrams of Transverse Sections of a Lamellibranch to show the Adhesion, by Concrescence, of the Gill-Lamellae to the Mantle-flaps, to the foot and to one another. (Lankester.)
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.

Fig. 17.—Vertical Section through an Anodonta, about the mid-region of the Foot.
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.

Fig. 18.—Diagrams showing the Relations of Pericardium and Nephridia in a Lamellibranch such as Anodonta.
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.

Fig. 19.—Nerve-ganglia and Cords of three Lamellibranchs. (From Gegenbaur.)
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.

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.

Fig. 20.—Otocyst of Cyclas. (From Gegenbaur.)
c,	Capsule.
e,	Ciliated cells lining the same.
o,	Otolith.

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.

Fig. 21.—Pallial Eye of Spondylus. (From Hickson.)
a,	Prae-corneal epithelium.		f,	Retinal nerve.
b,	Cellular lens.		g,	Complementary nerve.
c,	Retinal body.		h,	Epithelial cells filled with pigment.
d,	Tapetum.		k,	Tentacle.
e,	Pigment.

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.

Fig. 22.—Two Stages in the Development of Anodonta. (From Balfour.) Both figures represent the glochidium stage.
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.

Fig. 23.—Development of the Oyster, Ostrea edulis. (Modified from Horst.)
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 cœlom 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.

Fig. 24.—Embryo of Pisidium pusillum in the diblastula stage, surface view (after Lankester). The embryo has increased in size by accumulation of liquid between the outer and the invaginated cells. The blastopore has closed.

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.

Fig. 25.—B, Same embryo as fig. 24, in optical median section, showing the invaginated cells hy which form the arch-enteron, and the mesoblastic cells me which are budded off from the surface of the mass hy, and apply themselves to the inner surface of the epiblastic cell-layer ep. C. The same embryo focused so as to show the mesoblastic cells which immediately underlie the outer cell-layer.

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.

Fig. 26.—Diagram of Embryo of Pisidium. The unshaded area gives the position of the shell-valve. (After Lankester.)
m,	Mouth.
x,	Anus.
f,	Foot.
br,	Branchial filaments.
mn,	Margin of the mantle-skirt.
B,	Organ of Bojanus.

After Drew, in Lankester’s Treatise on Zoology. (A. & C. Black.)

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.

Fig. 28.—Lateral view of a Mactra, the right valve of the shell and right mantle-flap removed, and the siphons retracted. (From Gegenbaur.)
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.

Fig. 29.—The same animal as fig. 28, with its foot and siphons expanded. Letters as in fig. 28. (From Gegenbaur.)

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.

Fam. 4. Lutrariidae.—Mantle extensively closed; a fourth pallial aperture behind the foot; siphons long and united; shell elongated, a spoon-shaped projection for the ligament on each valve. Lutraria; British. Tresus. Standella.

Fam. 5. Solenidae.—Elongated burrowing forms; foot cylindrical, powerful, without byssus; shell long, truncated and gaping at each end. Solenocurtus; British. Tagelus; estuarine. Ceratisolen; British. Cultellus; British. Siliqua. Solen; British. Ensis; British.

Fam. 6. Saxicavidae.—Mantle extensively closed, with a small pedal orifice; siphons long, united, covered by a chitinous sheath; gills prolonged into the branchial siphon; foot small; shell gaping. Saxicava; British. Glycimeris. Cyrtodaria.

Fam. 7. Gastrochaenidae.—Shell thin, gaping widely at the posterior end; anterior adductor much reduced; mantle extensively closed; siphons long, united. Gastrochaena; British. Fistulana.

Sub-order VIII.—Adesmacea.

Ligament wanting; shell gaping, with a styloid apophysis in the umbonal cavities. Gills prolonged into the branchial siphon. Mantle largely closed, siphons long, united. Foot short, truncated, discoid, without byssus.

Fam. 1. Pholadidae.—Shell containing all the organs; heart traversed by the rectum; two aortae. Shell with a pallial sinus; dorsal region protected by accessory plates. Pholas; British. Pholadidea; British. Jouannetia. Xylophaga; British. Martesia.

Fam. 2. Teredinidae.—Shell globular, covering only a small portion of the vermiform body; heart on ventral side of rectum; a single aorta; siphons long, united and furnished with two posterior calcareous “pallets.” Teredo; British. Xylotrya.

Sub-order IX.—Anatinacea.

Hermaphrodite, the ovaries and testes distinct, with separate apertures. Foot rather small. Mantle frequently presents a fourth orifice. External gill-plate directed dorsally and without reflected lamella. Hinge without teeth.

Fam. 1. Thracidae.—Mantle with a fourth aperture; siphons long, quite separate, completely retractile and invertible. Thracia; British. Asthenothaerus.

Fam. 2. Periplomidae.—Siphons separate, naked, completely retractile but not invertible. Periploma. Cochlodesma. Tyleria.

Fam. 3. Anatinidae.—Siphons long, united, covered by a chitinous sheath, not completely retractile. Anatina. Plectomya; Jurassic and Cretaceous.

Fam. 4. Pholadomyidae.—Mantle with fourth aperture; siphons very long, completely united, naked, incompletely retractile; foot small, with posterior appendage. Pholadomya.

Fam. 5. Arcomyidae.—Extinct; Secondary and Tertiary. Arcomya. Goniomya.

Fam. 6. Pholadellidae.—Extinct; Palaeozoic. Pholadella. Phytimya. Allorisma.

Fam. 7. Pleuromyidae.—Extinct; Secondary. Pleuromya. Gresslya. Ceromya.

Fam. 8. Pandoridae.—Shell thin, inequivalve, free; ligament internal; siphons very short. Pandora; British. Coelodon. Clidiophora.

Fam. 9. Myochamidae.—Shell very inequivalve, solid, with a pallial sinus; siphons short; foot small. Myochama. Myodora.

Fam. 10. Chamostraeidae.—A fourth pallial aperture present; pedal aperture small; siphons very short and separate; shell fixed by the right valve, irregular. Chamostraea.

Fam. 11. Clavagellidae.—Pedal aperture very small, foot rudimentary; valves continued backwards into a calcareous tube secreted by the siphons. Clavagella. Brechites (Aspergillum).

Fam. 12. Lyonsiidae.—Foot byssiferous; siphons short, invertible. Lyonsia; British. Entodesma. Mytilimeria.

Fam. 13. Verticordiidae.—Siphons short, gills papillose; foot small; shell globular. Many species abyssal. Verlicordia. Euciroa. Lyonsiella. Halicardia.

Order IV. Septibranchia

Gills have lost their respiratory function, and are transformed into a muscular septum on each side between mantle and foot. All marine, live at considerable depths, and are carnivorous.

Fam. 1. Poromyidae.—Siphons short and separate; branchial siphon with a large valve; branchial septum bears two groups of orifices on either side; hermaphrodite. Poromya; British. Dermatomya. Liopistha; Cretaceous.

Fam. 2. Cetoconchidae.—Branchial septum with three groups of orifices on each side; siphons short, separate, branchial siphon with a valve. Cetoconcha (Silenia).

Fam. 3. Cuspidariidae.—Branchial septum with four or five pairs of very narrow symmetrical orifices; siphons long, united, their extremities surrounded by tentacles; sexes separate. Cuspidaria; British.

Authorities.—T. Barrois, “Le Stylet crystallin des Lamellibranches,” Revue biol. Nord France, i. (1890); Jameson, “On the Origin of Pearls,” Proc. Zool. Soc. (London, 1902); R. H. Peck, “The Minute Structure of the Gills of Lamellibranch Mollusca,” Quart. Journ. Micr. Sci. xvii. (1877); W. G. Ridewood, “On the Structure of the Gills of the Lamellibranchia,” Phil. Trans. B. cxcv. (1903); K. Mitsukuri, “On the Structure and Significance of some aberrant forms of Lamellibranchiate Gills,” Quart. Journ. Micr. Sci. xxi. (1881); A. H. Cooke, “Molluscs,” Cambridge Natural History, vol. iii.; Paul Pelseneer, “Mollusca,” Treatise on Zoology, edited by E. Ray Lankester, pt. v.

(E. R. L.; J. T. C.)