GENERAL SURVEY. 



127 



activity, for among them there is no sessile hydroid 

 type. 



In our survey of the series, however, we must pass over 

 these familiar types, and begin with the little fresh-water 

 Hydra (Fig. 56), which is often 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 



Fig. 55. — Diagram of Coslenterate structure, endoderm 

 darker throughout. 



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

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



-.:. 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. 



Note anatomical correspondence of the polypoid and medu- 

 soid forms. 



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 the zoophytes. 



