128 CCELENTERA. 



In such cases the colony is usually supported by an 

 organic sheath of varying complexity. 



The members of such a colony would, however, with an 

 exception which we will consider later, be all similar and 

 equivalent, and this is by no means true of all hydroid 

 colonies. In Hydractinia, for example, which is common 

 on shells at the shore, the colony consists of polypes of 

 varied structure and function. It may be that these 

 differences are caused by differences in nutrition, the fact 

 at any rate is that some of the polypes are nutritive "per- 

 sons," like Hydra in appearance; some are mouthless (?) 

 reproductive "persons," which produce sperms and eggs, 

 and so eventually start a new colony ; others, with a mouth, 

 are long, slender, sensitive, and abundantly furnished with 

 stinging cells; while the little protecting spines at the 

 base of the colony may perhaps be abortive "persons." 

 All these polypes are united by connecting canals at the 

 base, and all are fed at the expense of the nutritive " per- 

 sons." Hydractinia thus exhibits division of labour among 

 the members of the colony, and a tendency towards this is 

 common in the Coelentera. 



If we now return to the simpler zoophyte colony, we 

 find that this tendency Can be recognised even here. 

 Like Hydractinia, the colony at intervals exhibits repro- 

 ductive "persons," different from the ordinary polypes. 

 These, as in Hydractinia, may be sessile and mouth- 

 less, or they may 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 alternation of generations (see 



P- 54)- 



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 colonial 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 



