332 



MOLLUSCA. 



Since there is never a distinct head and sense-organs are not present upon 

 the anterior part of the body, the cerebral ganglia are but slightly developed 

 They supply mainly the regions round the mouth and the anterior part of 

 the mantle, to the margins of which two large nerves pass. They also send 

 fibres along the cerebro-pedal connective to the otocysts. The two ganglia 

 are frequently (Unio) far removed from one another laterally, and are some- 

 times approximated to the anteriorly 

 placed pedal ganglia (Pecten\ the nerves 

 of which supply the ventral region of 

 the body. The pedal ganglia are reduced 

 when the foot is atrophied. 



The visceral ganglia innervate the 

 viscera, the gills, the heart, and the 



FIG. 257. Nervous system of the pond mussel 

 Anodonta (after Keber). mouth ; A anus ; 

 K gills ; P foot ; Se labial palps ; Gg cerebral 

 ganglion ; Pg pedal ganglion ; Vg splanchnic 

 (visceral) ganglion; G generative gland ; Oe' 

 external opening of generative gland ; Oe" 

 opening of kidney. 



Fio. 258. Dorsal view of nervous 

 system of Numla, the middle part 

 of the foot is represented by dotted 

 shading (after Pelseneer). / pleural 

 ganglion ; II pleuro-pedal connective ; 

 ///combined pleuro-pedal and cerebro- 

 pedal connective; IV nerve to the 

 otocyst ; V pedal ganglion ; VI visceral 

 ganglion ; VII posterior pallial nerve ; 

 VIII osphradiurn ; IX visceral commis- 

 sure ; X otocyst ; XI canal from otocyst 

 to exterior, and XII its external open- 

 ing; XIII cerebro-pedal connective; 

 XIV anterior pallial nerve ; XV nerve 

 to palps; XVI cerebral ganglion. 



posterior part of the mantle. The nerves supplying the latter are two large 

 trunks which run in the edge of the mantle, and anastomose with the mantle 

 nerves from the cerebral ganglia. Large nerves also pass from the visceral 

 ganglia to the siphons, at the base of which they form a pair of accessory 

 ganglia. 



