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 
Fie. 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- 
