XVI 



ECHINODEEMATA 



521 



organs. The rudiment of the genital stolon, as we have already 

 noted, is formed as a bud from the wall of the left posterior coelom, 

 just as it is formed in Asteroidea. As it grows downwards in the 

 wall of the axial sinus it becomes more extensive than in Asteroidea, 

 since it does not merely fill a fold in its wall, but almost completely 

 surrounds the cavity. The madreporic vesicle also grows downwards 



int 



a. a 



a.stom 



astern 



FIG. 392. A series of diagrams showing the changes which the gut undergoes in Echinus 

 esculentus after metamorphosis. The diagrams represent horizontal sections through 

 the young imago. 



A, the condition just after metamorphosis. B, the condition at the time of formation of the adult 

 anus. C, the beginning of the formation of the recurrent coil of the intestine. D, the condition in the 

 adult, a. a, adult anus ; a.stom, adult stomodaeum ; int, intestine ; La, larval anus ; l.stom, larval stomo- 

 daeum ; m.p, position of madreporite ; st, stomach. 



alongside the axial sinus. As in Asteroidea, the originally single 

 madreporic pore becomes divided up by folds into the numerous 

 pores of the adult. 



When we compare the life-history just described with the life- 

 histories of the two Echinoderm types previously described, we see 

 that the Echinopluteus and Ophiopluteus resemble one another in 



