114 



THE GASTROPODA 



seen in the Opisthobranchia (Figs. 95 and 159) and the Pulmonata 



(Figs. 96 and 97). 



Further, in all the Euthyneura but those which are the most 



primitive from this point of view, such as the Bullomorpha and 

 Aplysia among the Opisthobranchs, the 

 Auriculidae, Chilina, and Latia among the 

 Pulmonata, there is a tendency to the 

 approximation of the ganglionic centres 

 and at the same time a shortening of the 

 visceral commissure. This is carried so 

 far that the ganglia come into contact 

 and form a chain of several united nerve- 

 centres between the pleural ganglia (Fig. 

 97). When it has reached this stage of 

 evolution the whole nervous system is 



<tb. 



Fio. 05. 



Nervous system of Aylysiu 

 (dorsal aspect), as a type of tin- 

 long -looped Euthyneurous con- 

 dition. The untwisted visceral 

 loop is lightly shaded. ab.pi>, 

 visceral ganglion which repre- 

 sents the abdominal + the supra- 

 intestinal ganglia of Streptoneuru, 

 and gives off the nerve to the 

 osphradinm and another to an 

 unlettered so - called " genital " 

 ganglion ; ce, cerebral ganglion ; 

 o, osphradium ; pt, pedal ganglion 

 and double pedal commissure ; 

 i>t, pleural ganglion (the stomato- 

 gastric commissure and ganglia 

 are omitted). (From Lankester, 

 after Spengel.) 



Fie. 06. 



iMia neritoide*, central nervous system, dorsal 

 view (the buccal mass is indicated by a dotted line), 

 bit, buccal mass ; co.vi, visceral commissure ; y.a, 

 abdominal ganglion ; <j.bii, buccal ganglion ; y.ce, 

 cerebral ganglion; g.i.i, infra - intestinal ganglion; 

 f/.jw, parietal ganglion ; g.pe, pedal ganglion and 

 double pedal commissure ; g.rf, pleural ganglion ; 

 g.s.i, supra-intestinal ganglion ; n.p.i, nerve of the 

 inferior pallia! lobe; osp, osphradima ; nul, radula. 



concentrated in the cephalic region round the anterior part of the 

 oesophagus, and finally all the ganglia cerebral, pleural, pedal, and 

 visceral are intimately united and localised on the dorsal surface of 

 the oesophagus, a condition which may be seen in Pleurdbranchus and 



