i;2 MANUAL OF ZOOLOGY SECT. 



(peripr.) at the aboral pole, in the middle of which is the anus ; 



FIG. 89. Corona of Sea-urchin with the spines removed to show the arrangement 

 of the plates, lateral view. A mb. ambulacral zone with its perforated plates ; 

 Ap. apical (aboral) pole ; Int. amb. inter-ambulacra! zones. (From Bronn's 

 Thierreich.) 



a madreporite is amalgamated with one of the genital plates. 



In the "Heart-urchins" 

 the body is heart-shaped in- 

 stead of globular, and in the 

 " Cake-urchins " it is flat- 

 tened and disc-like. In most 

 respects, however, these 

 irregular sea-urchins are 

 very closely allied to the 

 ordinary or regular forms, 

 and with the latter they 

 constitute the third class 

 of Echinodermata, the 

 Echinoidea. 



