STARFISH AND OTHER ECHINODERMS ii6J 



areas. Strongylocentrotus, Arbacia, Trip7ieustes, Clypeaster, and 

 Echinarachinus (sand dollar) are representative genera of the gronp. 

 Class Holothurioidea. — The echinoderms of this class have only 

 an incomplete skeleton, the body is^elongated, the mouth surrounded 

 by tentacles is located at one end and the anus is at the other. 

 These animals are called sea cucumbers because of their shape and 

 color. There is some remnant of the pentamerous condition in that 

 there are five double rows of tube feet extending lengthwise on 

 five sides of the body of some forms, others have less or none. The 

 expanded body of a holothurian is soft like a bladder partly filled 

 with liquid and the body wall is very muscular. Representative 

 genera of this class include Thyone, Holothuria, Cucumaria, Lepto- 

 synapta, Aphelodactyla and Caudina. 





Fig. 134. — Thyone, the common sea cucumber. 



Class Crinoidea. — Most of the sea lilies live attached by long 

 stalks, but a few are free. At the free end of the stalk are located 

 the five, many-branched arms. The mouth is located in the upper- 

 most center of the calyx and is surrounded by the anus. The anus 

 is also to be found on the oral side within the enclosure made by the 

 arms. Neocomatella, Pentacrimis, Rhizocrinus, Metacrmus and Ante- 

 don are representative genera. 



Habitat and Behavior of Starfish 



The starfish lives along the shores and in the shore waters (to a 

 depth of over 125 feet) of our stony coasts of the Atlantic and 

 Pacific, with scattered ones occurring in the Gulf of Mexico. A few 



