PHYLUM ECHINODERMATA 219 



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 ven- 

 trally. On the oral side of the central disc, five interradial sets of 

 skeletal plates project toward the mouth in the formation of jaws 

 which are operated by muscles in chewing food. The anus is not 

 present. The viscera are confined to the central disc. The brittle 

 stars and serpent stars are quite active and live in the shore waters. 

 At low tide they may be found under rocks and debris, but they 

 move about and feed during high tide. The ability of autotomy 



Fig. 116. — Oral view of a basket star belonging to class Ophiuroidea. (By courtesy 



of General Biological Supply House.) 



(self -mutilation) is so well developed here that arms will become de- 

 tached by merely grasping them. This makes it difficult to collect 

 entire animals alive. Ophioderma, Ophiura, Ophiothrix and Gorgono- 

 cephalus are common Atlantic and Gulf Coast genera. 



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 



