330 



MOLLUSCA 



that the pleural and parietal ganglia are separate. If the commissures 

 be short, the ganglia are collected near the pharynx and, thus freed from 

 the body torsion, are symmetrical (orthoneurous, fig. 336, //). If the 



FIG. T.T.Z,. Row of teeth from the radula of Trochus cincran'us (after Schmarda). 



1L 



FIG. 336. FIG. 337. 



FIG. 336. 7, streptoneurous nervous system of Paludina (after Hering, from 

 Gegenbaur). II, orthoneurous system of Limna-a (after Lacaze Duthiers). A, 

 visceral; 5, buccal; C, cerebral; p, pedal; PI, pleural; sb, sp, sub- and supraintestinal 

 ganglia; n, olfactory nerve; o, otocyst. 



FIG. 337. Diagram of circulation in Doris (after Leuckart). a, auricle; 

 c, gills around anus; /, tentacle; v, ventricle; x, vessels returning venous blood from 

 the body. 



cerebrovisceral commissures be longer, the result is almost always strep- 

 toneury. Pleural and visceral ganglia hold their place, but the right 

 parietal ganglion crosses above the intestine to the left side (hence called 



