ECHINODERMATA 153 



ichinoderms. If a ray is injured the animal throws 

 >ff the whole ray and then makes another to take its 

 )lace. In some species a single ray with perhaps a 

 small portion of the central disk attached is able to 

 form an entire animal. 



Summary. — The echinoderms are marine forms with 

 calcareous skin. They show marked radial symmetry as 

 well as a rudimentary bilateral symmetry. They have 

 a well-developed alimentary canal with two openings to 

 the exterior. They breathe through thin-walled out- 

 growths on the dorsal surface. A well-developed cir- 

 culatory system carries food and oxygen to all parts 

 of the body. Locomotion is effected through the water- 

 vascular system with the aid of muscles and nerves. 

 Reproduction takes place sexually; a free-swimming 

 larva is formed which in order to reach the adult form 

 must undergo a metamorphosis. The power of regen- 

 eration is marked. 



