These bivalves possessing a foot present three pairs of ganglia instead of two—the anterior or oval, the posterior or branchial, and the inferior or pedal. It occasionally happens, however, that the ganglia of the posterior or of the inferior pair become approximated or even fused into one. The fusion of the posterior pair takes place, as in the oyster, when the branchiæ from which they receive nerves come close together posteriorly. On the other hand, in those mollusks in which the branchiæ are farther apart, the two ganglia remain separate, and are connected only by a short commissure, as in the mussel (Fig. 9, b).
 | ||
| Fig. 8.—Nervous System of the Oyster. | Fig. 9.—Nervous System of the Common Mussel. | |
The separate existence or fusion of the inferior or pedal ganglia depends upon the size and shape of the foot. The nerves in relation with these ganglia are distributed almost wholly to this organ and its retractor muscles. Where the foot is broad the ganglia remain separate, and are merely connected by a commissure. But where the foot is small and narrow, as in the mussel, the two ganglia become fused into one (Fig. 9, p).
Some of the special senses are unquestionably represented among these headless Mollusca, though the distribution of the different organs is very peculiar. Thus in Pecten, Pinna, Spondulus, the oyster, and many others, very distinct and often pedunculated ocelli are distributed over both margins of the pallium or mantle. These vary in number from forty to two hundred or more, and are in connection with distinct branches of the circumpallial nerves. In the razor-fish,
