218 



TEXTBOOK OF ZOOLOGY 



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

 stricted from the central disc. An ambulacral groove is present in 

 each ray. This groove is formed by a particular arrangement of 

 skeletal plates. Skeletal plates also support the aboral side of the 

 central disc as well as the region around the mouth. Surrounding 

 the spines on the skin are distributed some pincherlike calcareous 



^^«SlK*n, 



Fig. 115. — A brittle star, oral view. 



organs called pedicellariae. These vary considerably in different 

 species. The anus and madreporic plate are located aborally. The 

 mouth is located on the oral or ventral side of the central disc, and 

 the radiating parts of the vascular, ambulacral, and nervous, sys- 

 tems lie orally along the ambulacral groove of each arm. Asterias, 

 Astropecten, Pisaster, Solaster, Oreaster and Echinaster are repre- 

 sentative genera. 



