CCELENTERATA. 



153 



one extremity of it begins to elongate. It soon becomes free, 

 and rapidly assumes an elongated cylindrical form, while a 

 coating of cilia, by means of which it moves sluggishly about, 

 appears on its outer surface. A central cavity appears in the 

 interior, and the inner cells form themselves into a definite 

 hypoblast. The larva has now become a planula, and consists of 

 a closed sack with double walls. It continues for some few days 

 to move about, but eventually drops its cilia, and becomes 

 dilated at one extremity, by which it then becomes attached. 

 The base of attachment becomes gradually enlarged so as to 

 form a disc, which spreads out and is frequently divided by 

 fissures into radiating lobes. The free extremity becomes en- 

 larged to form the eventual calyx. 



Over the whole exterior a delicate pellicle the future peri- 

 sarc now becomes secreted. Round the edge of the anterior 

 enlargement a row of tentacles makes its appearance. These, in 

 the embryos of the Tubularian genera, lie some little way behind 

 the apex of the body. After a certain time the perisarc, which 

 has hitherto been continuous, becomes ruptured in the region of 



C 



FIG. 68. THREE LARVA STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.) 



A. Blastosphere stage with hypoblast spheres becoming budded off into the 

 central cavity. 



B. Planula stage with solid hypoblast. 



C. Planula stage with a gastric cavity. 



ep. epiblast ; hy. hypoblast ; a!, gastric cavity. 



the calyx, and the tentacles become quite free. At about the 

 same period a mouth is formed at the oral apex. 



