=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. [Illustration: FIG. 105.--Nervous system of Pelecypoda: _A_, _Teredo_; _B_, _Anodonta_; _C_, _Pecten_; _a_, _a_, cerebral ganglia; _b_, pedal ganglia; _c_, visceral ganglia. (After Gegenbaur.)]