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 
Fig. 95. 
Nervous system of Aplysia 
(dorsal aspect), as a type of the 
long-looped Euthyneurous con- 
dition. The untwisted visceral 
loop is lightly shaded.  ab.sp, 
visceral ganglion which repre- 
sents the abdominal + the supra- 
intestinal ganglia of Streptoneura, 
and gives off the nerve to the 
osphradium and another to an 
unlettered so-called “ genital ” 
ganglion; ce, cerebral ganglion ; 
o, osphradium ; pe, pedal ganglion 
and double pedal commissure ; 
pl, pleural ganglion (the stomato- 
gastric commissure and ganglia 
are omitted). (From Lankester, 
after Spengel.) 
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 
Fic. 96. 
Latia neritoides, central nervous system, dorsal 
view (the buccal mass is indicated by a dotted line). 
bu, buceal mass; co.vi, visceral commissure; 9.@, 
abdominal ganglion; g.bu, buccal ganglion; 4g.ce, 
cerebral ganglion; g.i.i, infra-intestinal ganglion ; 
g.pa, parietal ganglion; g.pe, pedal ganglion and 
double pedal commissure; g.p/, pleural ganglion ; 
g.s.i, supra-intestinal ganglion; 7.p.i, nerve of the 
inferior pallial lobe ; osp, osphradium ; rad, 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 Plewrobranchus and 
