132 



CCELENTERA. 



after a time become detached and float away as delicate, 

 pulsating, swimming bells. These swimming bells are male 

 and female, they give rise to male and female elements, and 

 so to embryos, which, after a time, settle down and form new 

 zoophyte colonies. This is an instance of the phenomenon 

 of alternation of generations {see p. 55). 



Again, just as the predominance of passivity is exhibited 

 in Hydractinia and some zoophytes, where the active 



Fig. 38. — Diagram of Ccelenterate structure, encloderm 

 darker throughout. 



1. To left, shows longitudinal section of Hydr.a ; to right, of Sea 

 .anemone, ^''i gut ; gl., incipient gullet. 



2. To left, shows cross section of Hydra ; to right, of Sea anemone. 



3. To left, shows vertical section of Craspedote Medusoid (with 

 velum) ; to right, of Acraspedote Medusa, without velum. ^'■, gut ; 

 gt.^ gullet. 



Note anatomical correspondence of the polype and medusoid 

 forms. 



swimming bell is left out of the life history, so the pre- 

 dominance of activity is exhibited in the permanent 

 medusoids, c.i;'., Gcryonia, where the colonial hydroid stage is 

 oinitted, and the embryo becomes at once a swimming bell. 

 Finally, the medusoids themselves may become colonial, 

 and we have active floating colonies, like those of the 



