142 



C(ELENTERA. 



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

 this process there are formed first two and then four diverticula of the 

 general cavity, which are arranged round the gullet above, and open 

 freely into the digestive cavity below. In the gullet region these are 

 separated by broad septa, which are continued into the lower region of 

 the body as four interradial ridges or taeniolae. The tentacles bud out 

 from the region of the mouth, the first four corresponding in position to 

 the four pouches. Interradially above the four septa, four narrow 

 funnel-shaped invaginations arise ; these are produced by the ingrowth 

 of ectoderm, which then forms the muscle fibres which run down the 



Fig. 6i. — Diagram of life history of Aureiia.— After 

 Haeckel. 



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

 7, 8, Strobila stage ; 9, liberation of Ephyrae ; 10, n. growth of 

 Epbyrae into Medusse. 



tEeniolte (contrast the endodermic muscles of Anthozoa). In contrasting 

 this development with that of the hydroid polype, Gcette specially 

 emphasises the fact that the radial symmetry is first indicated by the 

 gut pockets, and the tentacles are late in development. Gcette 

 describes a quite similar process of development in certain sea- 

 anemones, and claims to have found there rudiments of septal pockets 



1 The statement as to the ectodermic gullet is due to Gcette (1887) ; 

 its existence is denied by Claus, who is followed by Chun. 



