ALTERNATION OF GENERATIONS. 2/ 



the death of the parent organism. When thus liberated, the 

 statoblast, after a longer or shorter period, ruptures and gives 

 exit to a young Polyzoon, which has essentially the same 

 structure as the adult. It is, however, simple, and has to 

 undergo a process of continuous gemmation before it can 

 assume the compound form proper to the adult. 



As regards the nature of these singular bodies, "the in- 

 variable absence of germinal vesicle and germinal spot, and 

 their never exhibiting the phenomena of yelk-cleavage, inde- 

 pendently of the conclusive fact that true ova and ovary occur 

 elsewhere in the same individual, are quite decisive against 

 their being eggs. We must then look upon them as gemmce 

 peculiarly encysted, and destined to remain for a period in a 

 quiescent or pupa-like state." (Allman.) 



c. Alternation of Generations. In the case of the Hydra and 

 the sea-mat, which we have considered above, fresh zooids 

 are produced by a primordial organism by gemmation ; the 

 beings thus produced (as well as the parent) being capable 

 not only of repeating the gemmiparous process, but also of 

 producing new individuals by a true generative act. We 

 have now to consider a much more complex series of pheno- 

 mena, in which the organism which is developed from the 

 primitive ovum produces by gemmation two sets of zooids, one 

 of which is destitute of sexual organs, and is capable of per- 

 forming no other function than that of nutrition, whilst the 

 other is provided with reproductive organs, and is destined 

 for the perpetuation of the species. In the former case the 

 produced zooids all resembled each other, and the parent 

 organism which gave rise to them ; in the latter case, the pro- 

 duced zooids are often utterly unlike each other and unlike the 

 parent, since their functions are entirely different. 



The simplest form of the process is seen in certain of the 

 Hydroid Polypes, such as Campanularia. The ovum of Cam- 

 panularia is a free-swimming ciliated body, which, after a 

 short locomotive existence, attaches itself to some submarine 

 object, develops a mouth and tentacles, and commences to pro- 

 duce zooids like itself by a process of continuous gemmation. 

 These remain permanently attached to one another, with the 

 result that a compound organism is produced, consisting of a 

 number of zooids, or " polypites," organically connected to- 

 gether, but enjoying an independent existence. None of the 

 zooids, however, are provided with sexual organs ; and though 

 there is theoretically no limit to the size which the colony may 

 reach by gemmation, its buds are not detached, and the 

 species would, therefore, die out, unless some special provision 



