NERVOUS SYSTEM OF MOLLUSKS. 
353 
guard the mouth; and they evidently correspond, both in 
position and functions, to the sensory ganglia of higher ani¬ 
mals, whilst the posterior gan¬ 
glion has for its office to regulate 
the respiratory movements. In 
the Pecten , however, as in other 
Conchifera which possess a 
foot (fig. 62), we find an addi¬ 
tional ganglion (o), the pedal , 
connected with the cephalic 
ganglia, and sending nerve- 
trunks to that organ. There 
is good reason to believe that, 
whilst the cephalic ganglia alone 
are the instruments of sensation, 
so that they exert a general 
control and direction over the 
ig. 182 .—Nervous System of Pecten. 
movements of the animal, the A a, cephalic ganglia ; B, branchial 
pedal and branchial ganglia phafu ° n J C ’ pedal gangllon * e ' ceso * 
min ister to the reflex actions 
(§ 433) of the organs which they supply. 
438. A similar arrangement is 
found in the higher Mollusks, 
among which the ganglia are more 
numerous, in accordance with the 
greater variety of functions to be 
performed. Of this we have an 
example in the Aplysia, a sort of 
sea-slug somewhat resembling those 
formerly alluded to (§ 316). In 
proportion as we ascend the scale, 
we find the cephalic ganglia rising 
higher and higher on the sides of 
the oesophagus; and in the Aplysia 
they meet on the central line above 
it, forming the single mass (a, fig. 
183), which receives the nerves of 
the eyes, tentacula, &c., and sends 
branches of communication to the 
other ganglia. The branches which 
it sends backwards are three on each 
A A 
