GENERAL SURVEY. 



141 



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 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 predominance of 

 activity is exhibited in 

 the permanent medus- 

 oids, e.g. Geryonia, where 

 the sedentary hydroid 

 stage is omitted, and the 

 embryo becomes at once 

 medusoid. Finally, the 

 medusoids themselves 

 may become colonial, 

 and we have active float- 

 ing colonies, like those 

 of the Portuguese man- 

 of-war, which show, on a 

 different plane, as much 

 polymorphism as Hydrac- 

 tinia. 



The same general con- 

 clusions apply to the jelly- 



J ' EC., Ectoderm ; EN., endoderm ; C. t the 



nsn and Sea-anemoneS. cavity of the gut (coelenteron) ; G., a. re- 



The iellv-fkh nresenr a productive bud; 7\, a tentacle; //..hypo- 



e J ei V n stome or oral cone ; M., mouth. 



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 alternation of active and 

 passive forms, though as real, is less obvious. But even 

 here we find one type (Pelagio) always locomotor, another 

 (Aurelid) whose early life is sedentary, and others (Lu- 

 cernarians) which in their adult life are predominantly 

 passive, and attach themselves by a stalk. 



FIG. 67. Diagram of a typical 

 Hydrozoon polyp. After Allman. 



