31G 



COMPARATIVE ANATOMY. 



tlie cerebral ganglia (a), which are connected together by a deeply- 

 curved commissure. The extent to which the pedal ganglia are 

 developed is dependent on the development of the foot. As a 

 rule they are intimately connected, though always separate. The 

 peripheral nerves of the cerebral ganglia are principally dis- 

 tributed to those parts of the body which lie nearest the mouth, 

 while they also give off branches to the mantle. In some forms 

 these pallial nerves (Fig. 181, t') have the appearance of two large 

 trunks, in which case they are connected with other nerves given 

 off from the visceral ganglia at the edge of the mantle ; and the 

 connected trunks either form a single larger marginal nerve, or 



a regular plexus of nerves. The 

 visceral ganglion is often the largest 

 of the whole nervous system. It 

 lies behind the posterior adductor 

 (Fig. 181, c; Fig. 182, c), and is 

 connected with the cerebral ganglia 

 by long" commissures. We may 

 regard this ganglion as consisting 

 of two halves connected together 

 by a short transverse commissure, 

 which become more or less ap- 

 proximated, and finally form a 

 single quadrangular knot, accord- 

 ing as the bilateral gills of these 

 animals are free or fused together. 

 This fact points to the relation be- 

 tween this ganglion and the gills, 

 which indeed is made still clearer 

 by the large nerve-trunks which 

 are given off from it, and which 

 innervate the branchiae. In addi- 



Fig. 182. Nervous system of the 

 Larnellibranchiata. A Of Teredo. 

 13 Of Anodonta. C Of Pecten. 

 « Supra-oDsophageal (cerebral) ganglia. 

 b Infra-cesophageal (pedal) ganglia 



tion to the branches which go to 



c Branchial or visceral ganglia. 



the neighbouring portions of the 

 mantle, there are two other large 

 nerves which pass to the edge of 

 the mantle in many Lamellibranchiata, and take part in forming the 

 above-mentioned plexus. 



When siphons are developed, large nerves are given off from the 

 visceral ganglia, which do not only ramify all along the respiratory 

 tubes, but also go to form a special ganglion, which is placed at the 

 base of the siphons (Fig. 181, d) (Solen, Mactra, Mya, Lutraria, 

 Cytherea, etc.). Very little is known about the nerves which pass 

 from the visceral ganglia to the various organs. of the body. Such 

 nerves have been observed in Pinna and Anomia, as well as in Area 

 and Solen, where they arise either from the ganglion or the commis- 

 sures. It is a difficult matter to compare the chords which pass to the 

 visceral ganglion (cerebro- visceral commissure) with the pallial nerves 

 of the Placophora, because of their relation to a ganglion ; on the 



