ECHINODERMS 



151 



for the protection of the internal organs, and at the same 

 time admit of considerable movement. 



In the brittle-stars (Fig. 91) the central disk is much 

 more sharply denned than in the preceding forms, and the 

 long snake-like arms are capable of a very great freedom of 

 movement, enabling the animal to glide over the sea-bottom, 

 or through the crevices of the rocks, at a surprising rate. 



In several species, otherwise closely resembling those 



FIG. 90. Starfish (Asterias ocracea), ventral view. One-naif natural size. 



in Fig. 91, the arms divide repeatedly. These are the so- 

 called basket-stars, living in the deeper waters of the sea, 

 where they, like other brittle-stars, act as scavengers and 

 devour large quantities of decomposing plant or animal 

 remains. 



At first sight the globular spiny sea-urchins (Fig. 93) 

 would scarcely be recognized as close relatives of the star- 

 fishes. A closer examination, however, shows the mouth to 

 be located on the under side of the body ; from it five rows 

 of feet radiate and terminate close to the center of the 

 dorsal side, and the arrangement of the plates forming the 



