ECHTNOIDS. 



445 



FIG. 673. 



SUB-SECTION III. 



THE ORDER OF ECHINOLDS OR SEA-URCHINS. 



THE Ecbinoids are echinoderms which have a more or 

 less spherical or discoidal shell composed of definitely 



formed and sym- 

 metrically arrang- 

 ed plates, which are 

 covered and firmly 

 bound together by 

 a skin, and which 

 bear tubercles 

 which are crowned 

 with spines. 



The plates are 

 so arranged as to 

 divide the shell 

 into more or less distinctly marked zones, radiating from 

 the oral opening. 



In every alternate zone the plates are perforated for 

 the passage of the locomotive suckers or ambulacra, and 

 are called amhulacral plates ; and the plates of the other 

 zones are not perforated, and are called interambulacral 

 plates. 



By means of the suckers, which can be extended much 

 beyond the spines, these animals can cling firmly to other 

 bodies, and thus move about over the rocks, and even up 

 and down their smooth sides, as well as on level sur- 

 faces. 



Sea-urchin, Echinus mammillatus. 



