THE STARFISH AND SOME ALLIES 237 



Definition of Echinoderma 



Each of the five animals described in this chapter repre- 

 sents one of the five classes that make up the phylum 

 Echinoder'ma (Gr. echinos, "hedgehog"; derma, "skin"). 



The most important characteristics of the phylum are the 

 radial symmetry of the body; the occurrence of repeated 

 divisions or organs of the body to the number of five, or 

 in multiples of five ; the existence of a water- vascular sys- 

 tem ; and the presence of limy plates in the skin, arranged as 

 either a continuous shell or as scattered plates. No other 

 phylum has parts occurring in fives, and none other has a 

 water-vascular system. 



