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 

 follows : 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, Act aeon); 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 



FIG. 284. Nervous system of HaKotis (diagrammatic EutliyneWO) ; filially, all 



the ganglia (cerebral, pedal, 



after Spengel). Cg cerebral ganglion ; Pg fused 

 pleural and pedal ganglia; Ay abdominal gan- 

 glion; Oand O'osphradia; Pe pedal cord ; S and 6" pleural, and ganglia of vis- 

 pallial nerves ; Br gills. . , , 



ceral commissure) may be 



closely approximated on the dorsal side of the oesophagus (many 

 Nudibranchs). 



The visceral commis- 

 sure is in the Stre}jto- 

 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, 







Flo. 285. Nervous .system of Limnaca (after L. Du- 

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

 dominal ganglion ; osphradium. 



