STARFISH AND OTHER ECHINODERMS 359 



are some species which do not adhere to this pentamerous condition 

 and have up to forty rays. The rays are not usually sharply con- 

 stricted from the central disc. Asterias, Astropecten, SolasteVy Greas- 

 ier and Echmaster are representative genera. 



Class Ophiuroidea. — Brittle stars and serpent stars. There is a 

 small central disc with five long, slender rays which are clearly 

 marked off from the disc. The rays are lacking in ambulacral grooves. 

 The tube feet do not serve in locomotor functions but are tactile 

 only. Both the madreporite and mouth are located ventrally. The 

 ability of autotoimj (self -mutilation) is so well developed here that 

 arms will become detached by merely grasping them. Ophioderma, 

 OpMura, Ophiothrix and Gorgonocephalus are common Atlantic and 

 Gulf genera. 



Fig. 132. — Oral view of a basket star belonging to class Ophiuroidea. (By courtesy 



of General Biological Supply House.) 



Class Echinoidea.— Sea urchins and sand dollars are representa- 

 tives of this group the members of which have lost the rays but 

 still retain the pentamerous (five division) condition of the body. 

 The sea urchins are globular or hemispherical, while the sand dol- 

 lars are disc-shaped. The skeleton or test is composed of five rows 

 of closely fitting plates which are usually arranged into five pairs 

 of inter-ambulacral rows. The position and condition of these rows 

 of plates can be compared to a starfish with its arms turned up over 

 its body until the tips all touch each other. The surface of the 

 skeleton bears processes which support movable spines. Tube feet 

 may be thrust out through perforations in the plates of the ambu- 



