144 



ANIMAL FORMS 



star, especially one of the basket-stars, by its dorsal side 

 to a stalk. In the crinoids the numerous branches of the 



arms are compara- 

 tively short, and in 

 the arrangement of 

 the internal organs 

 there are numer- 

 ous differences, but 

 for all that the re- 

 semblance of these 

 organisms to the 

 other echinoderms 

 is undoubted. 



138. Haunts, 

 The greater num- 

 ber of starfishes 

 occur alongshore, 

 slowly crawling 

 about in search of 

 food, or concealed 

 in dark crevices of 

 the rocks, where they may often be found as the tide goes 

 out, and we know that in gradually lessening numbers other 

 species lead similar lives at different levels far down in the 

 dark and gloomy depths. In these same locations the sea- 

 urchins occur, sometimes singly, but more usually associa- 

 ted in great numbers, several species excavating hollows in 

 the rocks, within which they obtain protection. The brit- 

 tle-stars and sea-cucumbers may also be found occasionally 

 in open view, but more often they make their way about in 

 search of food buried in the sand. The crinoids are usual- 

 ly inhabitants of deeper water, where they are found asso- 

 ciated often in great numbers. A few species upon attain- 

 ing the adult condition separate from the stalk, and are 

 able to move about (Fig. 94), but the remaining species 

 never shift their position. 



FIG. 90. Sea-urchin (Strongylocentrotus purpuratus). 

 Natural size. 



