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 (c), 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 Fig. m-NBKVous SYSTEM OF PECTEN. 

 movements Of the animal, the A A, cephalic ganglia ; B, branchial 



pedal and branchial ganglia fg? ; c ' pedal ganglion; e ' 



minister 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 



Fig. 183. NERVOUS SYSTEM OB 

 APLYSIA. 



