MOLLUSKS 



431 



compound ganglia (cerebro-pleural) located on the 

 right and left sides of the esophagus and connected by 

 a nerve cord called a commissure. Each 

 of these ganglia actually consists of two 

 cell masses. The two cerebral masses give 

 off nerves which control the head parts 

 while the pleural masses control the neigh- 

 boring parts of the body. Each com- 

 pound ganglion gives off also two nerve 

 trimks which pass backward along the 

 same side of the body as the ganglion 

 from which they originate. The right and 

 left trunks of one pair unite with a gang- 

 lion (visceral) near the siphon end of the 

 animal. The right and left trunks of the 

 other pair unite with a ganglion in the foot, 

 called the pedal ganglion. The pedal gang- 

 lion sends out fibers which direct the 

 movements of the foot, while the visceral 

 ganglion, through its fibers, controls the 

 internal organs and the parts near it. In 

 such a system it is evident that we have 

 control stations located near their seat of 

 control and, unlike those of the starfish, all 

 are interconnected into one system by the nerve trunks. 

 We may compare such a system with a city telephone 

 system in which each district has its own "central" but 

 all the centrals are connected. 



In the snail we find an interesting modification of the 

 clam system. Here the main ganglia are collected in 

 a ring about the esophagus, while additional ganglia are 

 found scattered through the body. In the cuttlefish this 



Fig. 200 — Ner- 

 vous system of 

 clam; c, cere- 

 bro-pl eur al 

 ganglia; c', 

 cerebral com- 

 missure; Pf 

 pedal gangliou; 

 ps, visceral 

 ganglion;^' 

 and ps', nerve 

 trunks. 



