GENERAL SURVEY OF CCELENTERA. 131 



these familiar types, and begin with the little fresh water 

 Hydra, which is often to be found attached to the stems 

 and leaves of water plants. The structure here is extremely 

 simple, but the simplicity is probably due to degeneration. 

 This Hydra was first described in 1703 by Leeuwenhoek, 

 and was studied by some of the older naturalists, such as 

 Rosel von Rosenhof and the Abbe Trembley, with much 

 eagerness. In favourable conditions it may be frequently 

 observed giving off daughter buds, which remain for a time 

 attached to the parent, and then separate as independent 

 Hydra. The bud itself, before leaving the parent, may 

 also bud, so that three generations are present. If we 

 suppose this process of gemmation, combined with imper- 

 fect separation of the units, to continue indefinitely, we can 

 understand the formation of hydroid colonies, such as the 

 zoophytes, where the colony is supported by an organic 

 axis 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 Hydr actinia, 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 differ- 

 ences are caused by differences in nutrition, the fact at any 

 rate is that some of the polypes are nutritive " persons," 

 like Hydra in appearance ; some are mouthless reproductive 

 u persons," which produce sperms and eggs, and so eventu- 

 ally start a new colony ; others also mouthless, 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 "persons." 

 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 reproductive 

 " persons," different from the ordinary polypes. These, as 

 in Hydractinia, may be sessile and mouthless, or they may 



