152 PHYSIOLOGY 



enters the body through their thin walls and carbon 

 dioxide passes off. 



The starfish eats oysters and similar animals which 

 contain the five food substances which it requires. It 

 wraps itself around the oyster, attaching itself firmly 

 to the shell by means of the tube feet. It then pulls 

 until the oyster becomes so fatigued that it can not 

 hold its shell shut any longer. As soon as the shell 

 opens the starfish pushes its stomach out through its 

 mouth, surrounds the flesh of the oyster, and with the 

 aid of the juices secreted by the livers, digests it. AVhen 

 it has finished the stomach is drawn back into the ani- 

 mal by the retractor muscles and the undigested material 

 is left behind. Through this method of feeding some 

 starfish have so degenerated that the alimentary canal 

 has lost its dorsal opening. 



Reproduction. — Reproduction is sexual. The organs 

 which form the sexual products lie at the junction of 

 the rays and open to the outside through ducts. They 

 are paired and during the breeding season extend well 

 into adjoining rays. As in coelenterata, eggs and sperm 

 are formed in separate animals. They are turned 

 loose in the water, where they meet and unite, forming 

 a new cell with the power of cell division and differ- 

 entiation. The young are free-swimming and, as they 

 differ greatly from the adult, they are called larvae. 

 They are bilaterally symmetrical but show not a trace of 

 radial symmetry. 



Regeneration. — The only form of non-sexual repro- 

 duction that seems to be present is commonly called 

 regeneration. It is present to a remarkable degree in the 



