ECHINODERMS 141 
for the protection of the internal organs, and at the same 
time admit of considerable movement. 
In the brittle-stars (Fig. 88) the central disk is much 
more sharply defined 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. 87.—Starfish (Asterias ocracea), ventral view. One-half natural size. 
in Fig. 88, 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. 90) 
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 
