GENERAL SURVEY. 



139 



of the antithesis between sedentary polypoid and active 

 medusoid types which is characteristic of the phylum. The 

 delicate iridescent globes, which represent the class 

 Ctenophora, illustrate the climax of activity, and have no 

 hint of a sedentary phase. 



In our preliminary survey of the series, we may begin 

 with the little fresh-water Hydra (Fig. 68), which is often 



FIG. 65. Diagram of Coelenterate structure, endoderm 

 darker throughout. 



i. To left, shows longitudinal section of Hydra; to right, of 

 sea-anemone, g. , gut ; /., incipient gullet. 



a. To left, shows cross-section of Hydra ; to right, of sea- 

 anemone, in the region of the gullet. 



3 To left, shows vertical section of Craspedote Medusoid 

 (with velum) ; to right, of Acraspedote Medusa, with- 

 out velum, g., gut ; /., gullet. 



Note anatomical correspondence of the polypoid and medu- 

 soid forms. 



to be found attached to the stems and leaves of water- 

 plants. The structure here is extremely simple, but the 

 simplicity is probably due to degeneration. In favourable 

 conditions the polyp may give off daughter buds, which 

 remain for a time attached to the parent, and then separate 

 as independent polyps. The bud itself, before leaving 

 the parent, may also bud, so that three generations are 

 present. If we picture this process of gemmation, but with 



