VII 



STREPTONEURA AND EUTHYNEURA 



205 



that of Physa just described, illustrates this grouping of the 

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

 visceral ganglia 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 cerebral ganglia by 



Fig. 103. — Nervous system of Physa 

 acuta Drap., showing the massing 

 of the ganglia at the hinder end of 

 the pharynx : e, e, eyes ; m, mouth ; 

 m.l, m.l, mantle lappets; o.f, fe- 

 male generative orilice ; o.ni, male 

 generative orifice ; os, osphradium. 

 (After Lacaze-Duthiers.) 



Fig. 104. — Example of a streptoneurous 

 Gasteropod (Cyclostoma elegans Drap.): 

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

 osphradium; ot, ot, otocysts; p.g, p.g, 

 pedal ganglia ; pl.g, pl.g, pleural ganglia ; 

 sp.g, supraintestinal ganglion; sh.g, sub- 

 intestinal ganglion; t.n, tentacle nerve; 

 v.g, visceral ganglion. (After Lacaze- 

 Duthiers.) 



long commissures ; the visceral loop is rather long, and the two 

 visceral 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 

 considerable distance apart. There are, as a rule, three distinct 

 pairs of ganglia, the cerebral (cerebro-pleural), pedal, and visce- 

 ral. 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, 



1 This fusion of the cerebral and pleural ganglia and the consequent union of 



