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 Aydractinia 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 Aydrac- 
asi Fic. 67.—Diagram of a typical 
The same general con- Hydrozoon polyp.—After Allman. 
clusions apply tothe jelly- 5c, Ectoderm; £W., endoderm; C., the 
fish and sea-anemones. cavity of the gut (coelenteron); G., a re- 
The jellyfish present a  prosuctive tnd: 7g tenadies #4. hype 
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 (Zéedagia) always locomotor, another 
(Auzelia) whose early life is sedentary, and others (Lu- 
cernarians) which in their adult life are predominantly 
passive, and attach themselves by a stalk. 
