SEA-URCHINS. 



479 



comlition of the cycle-restoration on this plan. In that genus tlie 

 postero-lateral genitals close in, so as to bring the system to an 





H 





P5 O 



Ah 



approximately circular shape, and the place of the posterior plate 

 is taken by a })i-olongation of the madreporic genital througli the 



