GASTROPODA. 



365 



or without forming a ganglion gives off a strong branchial nerve> 

 which passes to a ganglion 

 the branchial ganglion, 

 close under the osphra- 

 dium ; "while the commis- 

 sure from the left pleural 

 ganglion passes ventral to 

 the alimentary canal to the 

 right side, and there gives 

 rise to the sub -intestinal 

 ganglion (sb, 16), which 

 supplies the right side, or 

 gives off a nerve to the 

 right branchial ganglion. 

 The part of the commissure 

 connecting these two gan- 

 glia generally contains one 

 or more purely visceral 

 ganglia. The supra -intes- 

 tinal ganglion supplies the 

 left osphradium, which is 

 generally present, while 

 the sub -intestinal supplies 

 the right osphradium, 

 which is absent in all 

 except the Zygobranchiate 

 forms. 



The nervous system has 

 not been seen in the adult 

 of Entoconcha and Ento- 

 colax. 



In the Streptoneura each side 

 of the mantle is usually inner- 

 vated by a mantle-nerve from the 

 pleural of the same side, and by 

 a mantle-nerve from the supra- 

 (left side of mantle) or sub- 

 intestinal (right side of mantle) 

 ganglia, or, if they are absent, 

 from the corresponding parts of 

 the visceral commissure. The 

 nerves from these two sources 

 generally anastomose in the 



FIG. 286. Nervous system of Cassidaria (after Haller), 

 illustrating clialyneury on the left side, and zygo- 

 neury on the right. The pallial nerves from the 

 left pleural ganglion Pig anastomose with the pallial 

 nerves from the supra - intestinal ganglion Gsp 

 (dialyneury), whereas the pallial nerve from the 

 right pleural (not lettered) runs to the subin- 

 testinal ganglion Gsb (zygoneury), from which all 

 the mantle-nerves of the right side arise. Cg 

 cerebral ; Pig left pleural ; Pg right pedal ; Gsp 

 supra-intestinal ; Gsb subintestinal ; Vg abdominal 

 ganglion ; Ot otocyst. 



