6i8 



ECHINODERMATA 



The 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 



Fia. 294. — Three views of the development of Antedon rosacea. A, free-swimming larva ; 

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

 a.c, Anterior coelom ; avib, ambulacral groove ; a}), apical plate of sensory and 

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

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

 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 pari 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 



