The nervous system of the Lamellibranchiata is composed of the three normal ganglia - the cephalic, the pedal, and the parieto - splanchnic or branchial. The principal organs of sense are the tactile labial palpi, otocysts, and eye-spots. The otocysts are not always present, and the ocelli, when present, are almost always placed round the edge of the mantle.
Fig. 210. - Shells of Lamellibranchiata. 1. Cyclas amnica, a dimyary shell with an entire pallial line. 2. Tapes pillastra, a dimyary shell with an indented pallial line. 3. Perna ephippiun, a monomyary shell (after Woodward). a Pallial line ; b Muscular impressions left by the adductors ; c Siphonal impression.
The so-called " organ of Bojanus " of the bivalves is doubtless mainly concerned in excretion, and in all probability represents the kidney. There is one of these organs on each side of the body, each composed of two sacs separated from those of the opposite side by a venous sinus. Or it may be looked upon as a double organ composed of two bilaterally symmetrical halves. It is situated just below the "pericardium" and communicates with it, and also with the mantle-cavity. Though undoubtedly performing the functions of a kidney, the organ of Bojanus is also connected in some cases with reproduction, and it appears to correspond to the " pseudo-hearts " of the Brachiopoda.
The majority of the bivalves are dioecious, but in some the sexes are united in the same individual. The young are hatched before they leave the parent, and are, when first liberated, free-swimming, and furnished with a single or double ciliated lobe, constituting what is called the "velum." A long lash-like filament or flagellum is also often present. The velum is wanting in some forms.
The muscular system of the Lamelli-branchs is well developed. Besides the muscular margin of the mantle, and the muscles of the siphons (when these exist), there are also present other muscles, of which the most important are the muscles which close the shell and those which form the "foot" (figs. 208 and 209,/). The "foot" is present in the majority of bivalves, though it is not such a striking feature as in the Gasteropoda. It is essentially a muscular organ, developed upon the ventral surface of the body, its retractor muscles usually leaving distinct impressions or scars (the "pedal impressions") in the interior of the shell. In many, the foot, which is usually compressed, and often sickle-shaped, subserves locomotion, but in the attached bivalves it is rudimentary, and in others (as in the Scallops) locomotion is effected by the alternate opening and closure of the valves. In some - such as the ordinary Mussel - the foot is subsidiary to a special gland, which secretes the tuft of silky threads ("byssus") whereby the shell is attached to foreign objects. This gland secretes a viscous material, which is moulded into threads by grooves on its external surface.
The valves of the shell are brought together by one or two muscles, which are called the "adductor muscles" - those bivalves with only one being called Monomyaria, whilst those which possess two are termed Dimyaria. In most there are two adductor muscles (fig. 208, a a') passing between the inner surfaces of the valves, one being placed anteriorly in front of the mouth, the other posteriorly on the neural side of the intestine. In the monomyary bivalves the posterior adductor is the one which remains, and the anterior adductor is absent. The adductors leave distinct " muscular impressions" in the interior of the shell, so that it is easy to determine whether there has been one only in any given specimen, or whether two were present.
Fig. 211. - Embryo of Cockle (Cardium), after Loven. v Ciliated velum; fl Flagellum.
The habits of the Lamellibranchiata are very various. Some, such as the Oyster (Ostrea), and the Scallop (Pecten), habitually lie on one side, the lower valve being the deepest, and the foot being wanting, or rudimentary. The former is fixed by the substance of the valve, but the latter. swims by rapidly opening and closing the shell. Others, such as the Mussel (Mytilus) and the Pinna, are attached to some foreign object by an apparatus of threads, which is called the "byssus," and is secreted by a special gland. Others are fixed to some solid body by the substance of one of the valves. Many, such as the Myas, spend their existence sunk in the sand of the seashore or in the mud of estuaries. Others, as the Pholades and Lithodomi, bore holes in rock or wood, in which they live. Finally, many are permanently free and locomotive. The Lamellibranchiata may be divided into two sections, according as respiratory siphons are absent or present, as follows: