THE ECHINODERMS 



193 



of five. Radial symmetry is best suited to sessile animals, 

 since the similarity of the parts enables them to obtain food or 

 to repel enemies from all sides. 



Starfishes. — The starfishes are common along many sea- 

 coasts, where they may be found usually upon the rocks with 



Fig. 113. — A, the oral surface of a starfish. 



B, a spine bearing three pedicellaria:. 



C, tube feet expanded and contracted. (From Cambridge Natural History.) 



the mouth down. On the surface are many spines of various 

 sizes, and on the under side are five grooves, one in each arm, 

 from which two or four rows of tube feet extend (Fig. 113). 

 The skeleton is made up of calcareous plates or ossicles bound 

 together. The arms, however, are not rigid, but they may be 

 bent slowly by a few muscle fibers in the body wall. The 

 tube feet are also supplied with muscle fibers. 



