The Animal Kingdom - 663 



Fig. 32-35. Structure of the common starfish. Above, left; diagram of internal structures. Below, right: the water- 

 vascular system. (From General Zoology, by Miller and Haub. Holt, Rinehart and Winston, Inc.) 



portance of this perihaemal system has not 

 been ascertained. 



Classes of Echinodermata. The major 

 natural subdivisions of the phylum are: 



Class 1. The Asferoidea (the Starfishes). 

 Body, star-shaped; arms, five, single or mul- 

 tiple, not sharply demarcated from the cen- 

 tral disc; spines, short and blunt; two to four 

 rows of tube feet, with suckers, extending 

 along undersurface of each arm; distribution, 

 Cambrian period to present time (Fig. 32-33). 



Class 2. The Echinoidea (the Sea Urchins, 



Sand Dollars, etc.). Body, hemispherical, 

 disclike or egg-shaped; no arms; ossicles inter- 

 linking and firmly sutured together; spines 

 numerous, relatively long and movable; tube 

 feet, numerous, with suckers; Ordovician 

 period to present time (Fig. 32-33). 



Class 3. The Holothuroidea (the Sea Cu- 

 cumbers). Body, sausage-shaped; no arms or 

 spines; ossicles, numerous, of microscopic 

 size; mouth, usually surrounded by retractile 

 tentacles; Cambrian period to present time 

 (Fig. 32-33). 



