12G 



HYDROIDEA. 



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 enlarged to form the 

 eventual calyx. 



Over the whole exterior a delicate pellicle the future perisarc 

 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 the calyx, and the tentacles 

 become quite free. At about the same period a mouth is formed 

 at the oral apex. 



The development of Eucope polystyla (fig. 68), one of the 



FIG. 68. THREE LAKVA STAGES OF EOCOPE POLYSTYLA. (After Kowalevsky.) 



A. Blastospliere stage with hypoblast spheres becoming budded off into the central 

 cavity. 



B. Planula stage with solid hypoblast. 



C. Planula stage with a gastric cavity. 



cp. epiblast; hy. hypoblast; al. gastric cavity. 



Campanularidse, deviates according to Kowalevsky (no. 147) in 

 somewhat important points from the usual type. The whole 

 development takes place after the deposition of the ovum. The 

 segmentation results in the formation of a single-walled blastosphere 

 with a large central cavity (fig. 68 A). This cavity, somewhat as 



