190 



GASTROPODA. 



come to the other constituent parts of the nervous system. A large nerve 

 runs back from the cerebral ganglia on each side, swelling to form two lateral 

 ganglia, the pleural ganglia (A-C, pig). These are connected with the pedal 

 ganglia by the pleuro-pedal connectives. From the pleural ganglia, again, 

 two lateral strands run back and end in the one or two connected abdominal 

 ganglia (Fig. 85 B, abg). Another lateral ganglion is formed in each of these 

 lateral strands which are known as the pleuro-visceral commissures. These 

 two last ganglia may be called the visceral ganglia (B and C, vg). In the 

 Prosobranehia, the pleuro-visceral commissures undergo displacement in 

 consequence of the twisting of the body already described (cf. p. 145 and 



d. 



3h. 



Fig. 86. -A-C, Diagrams of the nervous system oi'a Prosobranch (A), an Opisthobranch 

 (/}),andaPulmonate (''). abg, abdominal ganglia ; bg, buccal gan. . cerebral 



ganglia : d, alimentary canal diagrammatically represented as a straight tube ; peg, 

 pedal ganglia : pig, pleural ganglia : -%, sub-, and spg, supra-intestinal ganglion ; 

 vg, visceral ganglia. 



Fig. 60), the right commissure coming to lie above and the left commissure 

 below the intestine (Fig. 86-4). The original right visceral ganglion is thus 

 displaced to the left side and becomes the supra-intestinal ganglion {spg), 

 while the original left visceral ganglion now lies on the right side and is 

 known as the sub-intestinal ganglion {sbg). The abdominal ganglia {abg), 

 in consequence of the twisting, come to lie dorsally to the intestine. In 

 this way arises the crossing of the pleuro-visceral commissures (chiastoneurv) 

 characteristic of the Prosobranehia. 



In the Pulmonata, the commissures are, as a rule, decidedly shorter than 

 in the other divisions, and the whole of the nervous system appears con- 

 centrated round the oesophagus (Fig. S6 C). 



