ALTERNATION OF GENERATIONS. 29 



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 Sertularia. The ovum of Sertu- 

 laria 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," drganically 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 

 were made for its preservation. Besides these nutritive 

 zooids, however, other buds are produced which differ con- 

 siderably in appearance from the former, and which have the 

 power of generating the essential elements of reproduction. 

 These generative zooids derive their nourishment from the 

 materials collected by the nutritive zooids, but only live until 

 the ova are matured in their inJ:erior and hberated, when 

 they disappear. The ova thus produced become free-swim- 

 ming ciliated bodies, such as the one with which the cycle 

 began. 



In this case, therefore, the' "individual" Sertularia con- 

 sists of a series of nutritive zooids, collectively called the 

 " trophosome," and another series of reproductive zooids, col- 

 lectively called the "gonosome," the entire series often remain- 

 'mg in organic connection. 



In other forms nearly allied to Sertularia (such as Coryne) 

 the process advances a step further. In Coryne the genera- 

 tive buds, or zooids, do not produce the reproductive ele- 

 ments as long as they remain attached to the parent colony; 

 but they require a preliminary period of independent exist- 

 ence. For this purpose they are specially organised, and 

 when sufficiently matured they are detached from the 

 stationary colony. The generative zooid now appears as an 

 entirely independent being, described as a species of jelly-fish 



