GENERAL SURVEY. 



135 



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. 64), which is often 

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

 plants. The structure here is extremely simple, but the 



FIG. 62. Diagram of Coelenterate structure, cndoderni 

 darker throughout. 



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



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



2. 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 ; gl., gullet. 



Xote anatomical correspondence of the polypoid and medu- 

 soid forms. 



simplicity is probably due to degeneration. In favourable 

 conditions the polype may give off daughter buds, which 

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

 as independent Hydra, 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 

 imperfect separation of the units, continued indefinitely, we 

 can understand the formation of hydroid colonies, such as 



