NERVOUS SYSTEM — CEREBRAL GANGLIA. 
217 
In the most primitive Streptoueures the cerebral ganglia are joined 
in front of the buccal mass, but behind it in the more specialized 
forms; in Vivipam and J^eritina there is an additional but 
sub-cesophageal cerebral commissure, distinguished as the labial 
commissure, which passes beneath the cesophagns, and forms a fourth 
nerve collar around the alimentary canal. 
In the Stylommatophora the cerebral ganglia are closely approxi- 
mated dorsally, and distinct regions, from which special groups of 
nerves originate, are distinguishable therein, which have therefore 
been differentiated as the 
Protocerebrou, Mesocerebron, 
and Metacerebron. In the 
anterior protocerebral region, 
as well as in the terminal 
ganglia of the upper and 
lower tentacles and labial 
lobes, small .spherical sensory 
ganglion cells of uniform size 
are congregated, and in 
Helix pomatia especially the 
ganglion cells of the cerebral 
ganglia are more differentiated in size and polarity than those of the 
Basommatophora. 
Electrical or mechanical stimulation of the cerebral ganglia of 
Helix produces little or no perceptible effect, and the animal can 
survive their removal for four or five weeks, although remaining 
perfectly motionless. 
In the Pelecypoda the snpra-oesophageal or cerebro-pleural ganglia 
are generally small in correlation with the diminished importance of 
the cephalic region. They are placed near the pedal protractor and 
innervate the anterior adductor, the labial lobes, and the oral region 
generally, besides giving off the anterior pallial nerves, which ramify 
within the pallial margin and unite with the posterior pallial nerves 
arising from the visceral ganglia. They also furnish the nerves to 
the otocysts and, according to Pelseneer, to the osphradia also, and 
are composed of the cerebral ganglia united with the pleural 
ganglia of the visceral centre. This combination of the ganglia is 
clearly shown in the archaic genus Niicuki, in which the pleural 
ganglia, though in close proximity, have not yet fused with the cere- 
Fic. 430. — Cerebral ganglion cells, with their 
nerve prolongations, from Anodonta anatina (L.), 
X 500 (after Rawitz). 
