1 5 6 



PHYLUM CCELENTERA. 



processes the embryo elongates, the outer cells become ciliated, and 

 the mouth closes. Thus the embryo becomes a free-swimming oval 

 planula. 



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

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

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

 layers. At the free pole an ectodermic imagination 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 

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



FIG. 75. Diagram of life history of Aurelia. After 

 Haeckel. 



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

 7, 8, Strobila stage; 9, liberation of Ephyrae ; 10, IT, 

 growth of Ephyrae into Medusae. 



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 tseniolae. 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 

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

 this development with that of the hydroid polyp, Goette specially 



