EXTERNAL FEATURES 287 



region between two arms is an interradius. Along each arm runs 

 on the oral side a deep ambulacral groove, and the grooves meet 

 around the mouth, which has a membranous Hp or peristome. 



Fig. 214. — Parts of an oral view of a starfish [Asterias nibens). 



a An arm with the ambulacral groove widely open ; b, an arm with the ambulacral groove closed by the 

 ' contraction of its sides and the bending over of the adambulacral spines ; c, an ambulacral spme, 

 with its tuft of large pedicellarije enlarged. 



The surface of the body is soft and cihated, but below it is a tough 

 body-wall, strengthened by a meshwork of rod-shaped calcareous 

 ossicles, which can be felt and seen through it. Over the inter- 

 spaces between the ossicles the skin is raised into delicate, 

 hollow outgrowths, the dermal gills, into which the body cavity 



