146 



CCELENTERA. 



found swimming in the gastro-vascular canals, and also 

 within the shelter of the long lips. 



Life History of Aurelia.—AccorAmg to the most recent investigation, 

 the fertilised ovum divides completely, but not quite equally, to form a 

 blastosphere with a very narrow slit-like cavity. From the larger celled 

 hemisphere, single cells migrate into the cavity, and fill ^ this up 

 entirely with a solid mass of endoderra. The archenteron arises as a 

 central cleft in this cell mass, and opens to the exterior temporarily by 

 the primitive mouth. During these processes the embryo elongates, the 



Fig. 44.- 



-Diagram of life history of Aurelia. (After 



H/1-;CKEL.) 



T, Free swimming embryo; 2-6, various stages of Hydra-tuba; 

 7-8, Strobila stage; g, liberation of Ephyrse ; lo-ii, growth of 

 Ephyrae into Medusse. 



outer cells become ciliated, the mouth closes, and the embr)'0 swims 

 freely as an oval planiila. 



After a short period of free life, this planula settles down on a 

 stone or seaweed, attaching itself by the pole where the inouth formerly 

 opened. At a very early stage the mesoglcea appears between the two 

 layers. At the free pole an ectodermie invagination next occurs, an 

 opening breaks through at its lower end, and thus a gullet lined with 

 ectoderm ' is formed, which hangs freely in the general cavity. During 



' The statement as to the ectodermie gullet is due to Gotte (1887) ; its 

 existence is denied by Claris, who is followed by Chun. 



