430 THE NEEVOUS SYSTEM IN THE LOWEK ANIMALS 



fibers is found in the coelenterates. In the hydra are cer- 

 tain scattered cells which are partly muscular and partly 

 nervous in function. In the jellyfishes we find a network 

 of fibers and cells scattered over the disk. The general 

 purpose of such systems seems to be to give sensitiveness 

 to the animals and thus aid them in securing food. 



Echinoderms. — In the starfish and its allies we have a 

 true system. In these animals \ve find a ring of five gan- 

 glia surrounding the mouth and connected by a nerve cord. 

 Each ganglion gives rise to radial nerves which extend 



along the arm opposite the 

 ganglion. The peculiarity of 

 this system is that it actually 

 consists of three systems, each 

 of which has its own combina- 

 tion of cells and fibers and sup- 

 plies a special part of the body. 

 These three systems are abso- 

 lutely independent of one another 

 in action. We may compare the 

 nervous control of the starfish to 

 a city with three absolutely in- 

 dependent telephone companies, 

 each company with its own "central" and wires. Such 

 an arrangement is absolutely unknown in any other group 

 of animals. 



Mollusks. — The nervous system of such animals as the 

 clam and its allies consists of several ganglia located in 

 different parts of the body and interconnected by nerve 

 trunks. Each ganglion controls special parts of the body, 

 and this arrangement is spoken of as a scattered system 

 of control. The arrangement in the clam coasists of two 



Fig. 199 — Diagram of nervous 

 system of starfisli; r, nervous 

 ring around mouth; «., radial 

 nerves to eacli arm, ending in 

 the eye. 



