Fig. 104.—Example of a streptoneurous Gasteropod (Cyclostoma elegans Drap.): c.g, c.g, cerebral ganglia; e, e, eyes; os, osphradium; ot, ot, otocysts; p.g, p.g, pedal ganglia; pl.g, pl.g, pleural ganglia; sp.g, supra-intestinal ganglion; sb.g, sub-intestinal ganglion; t.n, tentacle nerve; v.g, visceral ganglion. (After Lacaze-Duthiers.)
Scaphopoda.—In the Scaphopoda the nervous system resembles that of the Pelecypoda. The cerebral and pleural ganglia lie close together, while the pedal ganglia are placed in the anterior part of the foot, connected with the cerebral ganglia by long commissures; the visceral loop is rather long, and the two visceral ganglia are adjacent to the anus.
Pelecypoda.—The nervous system in the Pelecypoda is the simplest type in which well-marked ganglionic centres occur. The ganglia are few, symmetrically placed, and are usually at a considerable distance apart. There are, as a rule, three distinct pairs of ganglia, the cerebral (cerebro-pleural), pedal, and visceral. The cerebral are formed by the fusion of the cerebral and pleural ganglia, which, however, in some cases (Protobranchiata) continue distinct.[317] They lie above or on each side of the mouth, united by a commissure of varying length. Another pair of commissures unites them with the pedal ganglia, which are placed at the base of the foot, and are usually very close together, sometimes (as in Anodonta) becoming partially fused. The length of these commissures depends upon the distance between mouth and foot; thus they are very long in Mya and Modiola, and very short in Pecten. In cases where the foot is rudimentary or becomes aborted through disuse (e.g. Ostrea), the pedal ganglia may dwindle or disappear altogether. The visceral ganglia are on the ventral side of the posterior adductor muscle, beneath the rectum, and innervate the branchiae, osphradia, and the whole of the visceral sac. A pair of cerebro-visceral commissures traverses the base of the foot, surrounding it with a comparatively short loop (compare Fig. [106], c.v.c´), while a long commissure, which runs round the entire edge of the mantle, and supplies branching nerves to the mantle border and siphons (Fig. [106], c.v.c), may also connect the visceral and cerebral ganglia.
Fig. 105.—Nervous system of Pelecypoda: A, Teredo; B, Anodonta; C, Pecten; a, a, cerebral ganglia; b, pedal ganglia; c, visceral ganglia. (After Gegenbaur.)
Cephalopoda.—In the Cephalopoda the concentration of ganglia attains its maximum, and may perhaps be regarded as approaching the point at which a definite brain may be said to exist. Another point of distinction is the formation of special small ganglia upon the nerve-cords in different parts of the body. In the Tetrabranchiata (Nautilus) the cerebral and pedal ganglia form a broad ring which surrounds the oesophagus, the former giving out the optic nerves, with their special optic ganglion, and a pair each of buccal and pharyngeal ganglia, the latter the nerves of the arms and funnel. The visceral loop is still present in the form of a separate band, which innervates the branchiae, osphradia, and viscera generally, forming a special genital ganglion in connexion with the reproductive organs. The principal ganglia of the Dibranchiata are still more concentrated, even the visceral loop being possibly united with the rest in forming an unbroken mass in which scarcely any trace of commissures can be detected. The pedal ganglion becomes separated into two portions, one of which innervates the arms, the other the funnel. Two peculiar ganglia (the stellate ganglia) supply a number of branching nerves to the mantle.
Fig. 106.—Nervous system of Cardium edule L.: a.m, anterior adductor muscle; br, branchiae; br.n, branchial nerve; c.g, c.g, cerebral ganglia; c.p.c, cerebro-pedal commissure; c.v.c’, cerebro-visceral commissure; c.v.c, cerebro-visceral commissure of mantle; l.p, labial palps: m, mouth; p.g, pedal ganglion; p.m, posterior adductor muscle; v.g, visceral ganglion. (After Drost, × 3.)