148 



PHYSIOLOGY 



animal, irritability, assimilation, and reproduction, 

 manifest themselves. 



The Starfish. — Perhaps the best-known echinoderm 

 is the starfish. It has a symmetrical, flattened, star- 



Fie. 83 A.— Starfish, dorsal surface. (From McMurrich.) 



shaped body (Fig. 83 A) with, as a rule, five rays, 

 though there may be as many as twenty. It is covered 

 with a protective skeleton made of movable plates which 

 allow the rays to bend easily in any direction. A 

 mouth is in the center of the ventral side (Fig. 83 B). 

 On this side, in each arm, is a groove in which are 

 situated the locomotor organs, commonly called tube 

 feet. These tubes are connected with a system which is 

 a peculiar characteristic of the echinoderms. 



Symmetry. — The similarity of the rays of the star- 



