206 



VALUE IN CLASSIFICATION 



CHAP. 



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 to- 

 gether, 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 rudimen- 

 tary or becomes aborted tFrough 

 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 compara- 

 tively short loop (compare Fig. 

 106, c.v.c'}, while a long commis- 

 sure, which runs round the entire edge of the mantle, and supplies 

 branching nerves to the mantle border and siphons (Fig. 106, 

 c.u.<?), may also connect the visceral and cerebral ganglia. 



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, 



the cerebro-pedal and pleuro-pedal commissures can be recognised by sections 

 of the mass (Pelseneer, Comptcs Bendus^ cxi. p. 245). 



Fig. 105. — Nervous system of Pelecy- 

 poda : A, Teredo ; B, Anodonta ; 

 C, Pecten; a, a, cerebral ganglia; 

 b, pedal ganglia; c, visceral gang- 

 lia. (After Gegenbaur.) 



