244 



HIGH SCHOOL ZOOLOGY. 



bilateral. The Echinoidea, 

 on the other hand, may be 

 globular (Fig. 170), or discoid 

 (Fig. 172), but the skeleton is 

 -, made up of regular rows of 

 plates (generally twenty) some 

 of which are perforated for 

 the tube-feet. The Star-fishes 

 (Asteroidea) and Brittle-Stars 

 Fig. 174.-Brittie.star. OpMotJ^nx Sragiiis. ^^^-^^^^^^^^ resemble eacK 



other in having a cen- 

 tral disc and projecting 

 arms, which bear tube- 

 feet only on the ventral 

 (lower) surface, but 

 they differ in that the 

 Starfish arms (Fig. 173) 

 contain processes from 

 the intestine, while this 

 is not the case in the 

 Brittle-stars (Fig. 174). 

 Finally, the Crinoidea 

 resemble the preceding 

 in having a disc with 

 arms, but the ventral 

 surface with the mouth 

 is uppermost, and the 

 dorsal surface is tem- 

 porarily or jiermanently 

 fixed by a stalk (Fig. 

 175). Along with cer- 

 tain allied forms, which 

 are now quite extinct, the Crinoids were much more abundant 

 in the earlier geological periods, than they are at the present day. 



Fig. 175. — Living Crinoid (Pentacrinus} with part 



of its stalk. 



b. Upper surface of the calyx with the amis cut off, 

 showing the mouth in the centre, and the furrows 

 converging to it from the arms. 



