ECHINODERMS 143 



has resulted in the production of a thin, cake-like form 

 (Fig. 91). 



If the spherical body of a sea-urchin were to be stretched 

 in the direction of a line joining the mouth and the center 



FIG. 89. Basket-star (Astrophyton). One-half natural size. 



of the dorsal surface, a form resembling a sea-cucumber 

 (Fig. 92) would be the result. These latter organisms live 

 among crevices of the rocks, embedded in the mud or bur- 

 rowing in the sand at the bottom of the sea. In such situa- 

 tions they are well protected, and a highly developed skele- 

 ton, such as that of the sea-urchin, would not only be of 

 little value, but a positive hindrance to locomotion. The 

 skeleton, therefore, is much reduced, consisting of a few 

 scattered calcareous plates embedded in the fleshy body- 

 wall. Another peculiar feature is almost universally pres- 

 ent, in the form of a circlet of tentacles surrounding the 

 mouth, which serve either for the purpose of respiration, 

 for locomotion, or to convey food to the mouth. 



A very good imitation of the general plan of a sea-lily 

 or crinoid (Fig. 93) could be made by attaching a serpent- 



