GENERAL SURVEY. 137 



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 (see p. 57). 



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 

 medusoids, 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 floating colonies, like those of the 

 Portuguese man-of-war, which show, on a different plane, 

 as much polymorphism as Hydractinia. 



The same general conclusions 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 alternation 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 (Lu- 

 cernarians) which in their adult life are predominantly 

 passive, and attach themselves by a stalk. 



The sea-anemones and their numerous allies may be 

 regarded as bearing a relation to the jelly-fish, somewhat 

 similar to that which the hydroid polypes bear to the 

 swimming-bells (Fig. 62). They are, however, much more 

 complicated in structure than trie hydroids. Solitary forms 

 are much commoner than in the hydroids, but the colonial 

 type is nevertheless very frequent. The colonies may be 

 supported by an organic framework only, but very commonly 

 there is a tendency to accumulate lime in the tissues, which 

 results in the formation of " corals." It should be noted, 

 however, that various quite distinct polypoid types may 

 form " corals." Thus, while the most important reef-building 

 corals are included in the Anthozoa, the Millepore-corals 

 are hydroids. 



