412 



ZOOLOGY 



SECT 



oral, and not on the aboral surface as in the Asteroidea. In the 

 Euryalida there are five madreporites and five madreporic canals. 



In the Echinoidea the body is either globular, or heart-shaped, 

 or flattened and disc-like. The exoskeleton is in the form of a 

 rigidly articulated system of calcareous plates, fitting closely 

 together by sutures, so as to form a continuous shell or corona. 

 Only Asthenosoma and allies, deep-sea forms, differ from all the rest 

 in having a corona possessing a certain degree of flexibility and 

 performing movements which are brought about by the contrac- 



FIG. 342. Astrophyton arborescens, aboral surface. (After Liid\vi>_'.> 



tions of five longitudinal bands of muscle running along the 

 ambulacral areas on the inner surface. 



In the globular forms, or regular Sea-urchins, the mouth is 

 situated at the oral pole of the globe, the anus at the aboral, and 

 the plates of the corona are in twenty regular meridional rows, 

 arranged in ten zones, five ambulacral and five inter-ambulacra!, 

 as described in the account of Echinus, with peristome, periproct, 

 ocular and genital plates, and madreporite. Spines (Fig. 343), 

 pedicellaricc (Fig. 344), and sphceridia are present, as already 

 described (p. 387), the last-named appendages, however, being 



