VII 



STREPTONEURA AND EUTHYNEURA 



205 



the twist of the visceral loop, and the position of the visceral 

 ganglion at its posterior end. 



Scaphopoda. — In the Scaphopoda the nervous system resem- 

 bles that of the Pelecypoda. The cerebral and pleural ganglia lie 

 close together, while the pedal ganglia are placed in the anterior 

 part of the foot, connected with the cerel.)ral ganglia l»y long 



Fig. 103. — Nervous system of Physa Fig. 104. — Example of a streptoiieurous 



acuta Drap., showing the massing 

 of the ganglia at the hinder end of 

 the pliarynx : e, e, eyes ; vi, mouth ; 

 m.l, 711.1, mantle lappets ; o.f, female 

 generative orifice ; o.m, male gen- 

 erative orifice ; o.f, osphradium. 

 (After Lacaze-Duthiers.) 



Gasteropod [Cyclostoma elegans Drap.) 

 eg, eg, cerebral ganglia ; e, e, eyes ; os, 

 osphradium ; at, ot, otocysts ; 2^-(h P-U) 

 pedal ganglia ; j)l.g, pl.g, pleural ganglia ; 

 sp.g, supraiutestiual ganglion ; sb.g, sub- 

 intestinal ganglion ; t.n, tentacle nerve ; 

 T.g, visceral ganglion. (After Lacaze- 

 Duthiers.) 



commissm-es ; the visceral loop is rather long, and the two vis- 

 ceral ganglia are adjacent to the anus. 



Pelecypoda. — The nervous system in the Pelecypoda is the 

 simplest type in which well-marked ganglionic centres occur. The 

 ganglia are few, symmetrically placed, and are usually at a con- 

 siderable distance apart. There are, as a rule, three distinct pairs 

 of ganglia, the cerebral (cerebro-pleural), pedal, and visceral. The 

 cerebral are formed by the fusion of the cerebral and pleural 

 ganglia, which however, in some cases (Protobranchiata) continue 

 distinct.^ They lie above or on each side of the mouth, united 

 ^ This fusion of the cerobral ami pleural ganglia and the consequent union of the 



