364 



MOLLUSCA. 



length, and its ganglia are 

 widely separate (Strepto- 

 neura and the less special- 

 ized Euthyneura). 



The principal variations 

 in this arrangement are as 

 folloAvs : the cerebral gan- 

 glia are close together, and 

 the pleural are approxi- 

 mated to them or may 

 even fuse with them (most 

 Pectinibranchs, Pteropoda 

 Thecosomata, Adaeon); the 

 pedal ganglia are concen- 

 trated and not drawn out 

 into cords ; the visceral 

 commissure is short, and 

 its ganglia approximated 

 both to each other and to 

 the pleural (Fig. 285, most 

 Euthyneura) ; finally, all 

 the ganglia (cerebral, pedal, 

 pleural, and ganglia of vis- 

 ceral commissure) may be 

 closely approximated on the dorsal side of the oesophagus (many 

 Nudibranchs). 



The visceral commis- 

 sure is in the Strepto- 

 neura twisted into a figure 

 of 8 in the following 

 manner (Figs. 282, 283); 

 the commissure from the 

 right pleural ganglion 

 passes dorsal to the ali- 

 mentary canal to the left 

 side, and there forms a 

 ganglion the supra-intes- 

 tinal ganglion (sp, 14) 

 which supplies the left 

 osphradium and ctenidium 

 and left side of the mantle, 



FIG. 284. Nervous system of Haliotis (diagrammatic 

 after Spengel). Cg cerebral ganglion; Pg fused 

 pleural and pedal ganglia; Ag abdominal gan- 

 glion ; and 0' osphradia ; Pe pedal cord ; S and S' 

 pallial nerves ; Br gills. 



FIG. 285. Nervous system of Limnaea (after L. Du- 

 thiers). Cg cerebral, Pg pedal, Pig pleural, Ag ab- 

 dominal ganglion ; osphradium. 



