COLONIES OF CCELENTERA 



171 



cases sessile and mouthless, or they may after a time 

 become detached and float away as deHcate, 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 alternation of 

 generations. 



Again, just as the predominance of passivity is exhibited 

 in Hydractinia and some 

 zoophytes, where the 

 active swimming - bell 

 stage is left out of the 

 life-history, so the pre- 

 dominance of activity is 

 exhibited in the per- 

 manent medusoids, e.g. 

 Geryonia^ where the 

 sedentary hydroid stage 

 is omitted, and the em- 

 bryo becomes at once 

 medusoid. Finally, the 

 medusoids themselves 

 may become- colonial, 

 and we have active 

 floating colonies, like 

 those of the Portuguese 

 man-of-war, which show, 

 on a diflferent plane, as 

 much polymorphism as 

 Hydractinia. 



The same general con- 

 clusions apply to the 



jelly-fish and sea-anemones. The jelly-fish present a strong 

 resemblance to the medusoids, but are distinguished from 

 them by their usually greater size, as well as by greater 

 complexity and several anatomical differences. It is in 

 accordance with this increased complexity that the alter- 

 nation of active and passive forms, though as real, is less 

 obvious. But even here we find one type {Pelagia) always 

 locomotor, another (Aurelia) whose early life is sedentary, 

 and others (Lucernarians) which in their adult life are 



Fig. 85. — Diagram of a typical 

 Hydrozoon polyp. — After Allman. 



EC, Ectoderm ; EN., endoderm ; C, the 

 cavity of the gut (coelenteron) ; G., a re- 

 productive bud ; T., a tentacle ; H., hypo- 

 stome or oral cone ; M., mouth. 



