396 ZOOLOGY. 



the circlet of tentacles grows out in a circle below the 

 mouth. By gemmation the hydroid gemmarium is 

 produced and so the life- cycle is completed and starts 

 again. 



11. Metagenesis. It will be seen that in the life- 

 history of Obelia there is a regular alternation of hydroid 

 and medusoid. In this species and many others the 

 medusa never gives rise to medusae, but its eggs develop 

 into hydroids, from which in turn the medusae are pro- 

 duced by budding. The phenomenon was originally 

 called an alternation of generations. This term as first 

 used referred more to the organisms than to the mode in 

 which they were produced, just as we speak of the mem- 

 bers of a family as of the first, second, third, or fourth 

 generation. The fact of the alternation is a matter of obser- 

 vation : the zoophyte produces medusae and the medusae 

 produce zoophytes. Objections, however, have been raised 

 to this use of the term generation, and if we restrict this 

 term to the production of new organisms by fertilised 

 germ cells or gametes it will be seen that there is only 

 one process of generation in the life-history of Obelia, 

 namely the sexual reproduction of the medusa. The 

 medusa, however, is not produced directly from the egg, 

 but is budded off from a hydroid form which develops 

 from the egg. Generation, therefore, is, as it were, post- 

 poned, and the process has been called metagenesis 

 (meta, after ; genesis, generation). There is, therefore, an 

 alternation of gemmation and generation or sexual repro- 

 duction. In gemmation not single cells but whole tissues 

 grow to give rise to the new organism. 



Further, we see that there are not one but several 

 successive processes of gemmation before the medusa is 

 produced. The medusa is not to be compared with the 

 zoophyte, but with the zooid. The zoophyte is a com- 

 pound structure, a gemmarium, consisting of several 

 zooids. The medusa is not budded from the first zooid 

 developed from the egg, but from a blastostyle which is 

 itself a modified zooid, and the blastostyle is budded from 

 a zooid, which may be the second or third in succession 



