70 ECHTNODERMATA. 



These animals abound on every coast, frequenting quiet 

 bays, wbere they are of important use. Their appetite 

 and instinct lead them to devour whatever dead or tainted 

 substance they may happen to encounter, and thus by 

 their multitudes they prevent our shores from being en- 

 cumbered by the ojBfal that would otherwise accumulate on 

 every beach. They are, moreover, highly prolific, pour- 

 ing forth at certain seasons their innumerable eggs into 

 the suiTounding water, and thus materially contributing 

 to the supply of food provided for the multitudinous inha- 

 bitants of the sea. 



In the Cake-urchins (Scutella)* (Fig. 44, 2), the rays 

 are obliterated, and the circumference acquires a circular 

 form ; moreover, their body is completely encased in a 

 hard calcareous shell, com2)osed of numerous angular 

 pieces, accurately fitted together and incapable of move- 

 ment. The Cake-Urchins bury themselves in the sand, a 

 situation in which suckers would be of little use, but for 

 which they are admirably adapted by a contrivance not 

 less calculated to excite the admiration of the observer. 



The exterior of the Scutella is entirely covered with 

 minute appendages, resembling, when seen with the naked 

 eye, delicate hairs ; but, when examined with the micro- 

 scope, they are found to be sj)ines of most elaborate 

 structure, all of which are moveably attached to the shell 

 by ball and socket-joints, and thus rendered susceptible of 

 being moved in every direction, so that by their com- 

 bined efforts the animal can sj^eedily bury itself, either 

 for the purpose of procuring food or of eluding observa- 

 tion. 



The Sea-eggs or Sea-urchins {Ecliinus)\ (Fig. 44, 1), in 

 their form resemble an orange. The mouth is a simple ori- 

 fice in the shell, placed at one extremity of its axis, and 

 through it, as represented in the figure, the points of five 

 singular teeth project externally. The instruments of 

 locomotion occupy the entire superfices of the shell, and 

 j)resent two distinct sets of organs adapted to dffierent 

 uses. The first consists of a multitude of sharp, purple 

 spines (Fig. 47), every one of which is connected by a 

 ball and socket-joint to a distinct prominence on which 

 it moves. These numerous spines, therefore, are so many 



* Scutellum, a little shield. 

 t ix^vos, echinos, a hedgehog. 



