6i8 



ECHINODERMATA 



Tlie eggs are comparatively large and full of food-yolk, and 

 they adhere for a considerable period to the pinnules. They 

 pass through a large portion of the development within the egg- 

 membrane. The blastula and gastrula are formed in the usual 

 way, but the formation of the coelom is most remarkable (Fig. 287, 

 E\ E"). The archenteron divides into anterior and posterior 



Flu. 294. — Three views of the development of An fedon rosacea. A, free-swimming larva. ; 

 B, longitudinal section of free-swimming larva ; C, oral view of young fixed form. 

 a.c. Anterior coelom ; amb, ambulacral groove ; ap, apical plate of sensory and 

 nervous tissue ; cH, ciliated ring ; hi/, hydrocoel ; l.p.c, left posterior coelom ; mad, 

 primary pore-canal ; pod, podia ; r.p.c, right posterior coelom ; stcmi, larval stomo- 

 daeum. (A and B after Bury ; C after Perrier. ) 



divisions. The posterior divides into right and left posterior 

 coelomic sacs, but before the division is complete a dorsal and a 

 ventral tongue grow out from the anterior division and unite 

 posteriorly, encircling the band of connexion between right and 

 left posterior coelomic sacs like a ring. This band of connexion 

 becomes solid and is absorbed, and ^>aW passu the ring becomes 

 converted, by the disappearance of its central opening, into a sac, 

 which is the definitive gut (Fig. 287, E). The rest of the 



