120 THE NATURE AND VARIETIES OF 



the species presents but a single form, very slightly modi- 

 fied even at the breeding period. The single fonn of the 

 Hydra may, of course, be as fairly taken to indicate the 

 type of the allied polypes, as that of the viviparous Trema- 

 toda to determine the type of the order of worms to which 

 they belong. So long, therefore, as we confine our atten- 

 tion to the orders of Hydrozoa above mentioned, there 

 seems no difficulty in determining that here the " alterna- 

 tion" is due to gemmation in the gamomorphic stage, or in 

 the course of the evolution of the sexual organs — that the 

 oviparous zooid is not here the typical form, but a sub- 

 sidiary offset from it, for the development of those organs of 

 reproduction which never make their appearance in the 

 former — and that the gemmiparous zooid is not here the re- 

 presentative of the gemmiparous or protomorphic zooid of 

 the Trematoda, but is itself the typical form, which in 

 polypes is produced by the transformation rather than the 

 gemmation of the primary infusorial product of the ovum ; 

 or, as we recognise in both cases a principal or typical 

 and a subsidiary form, the difference might otherwise be 

 expressed by saying, that in the Trematoda the subsidiary 

 zooid is gemmiparous and preliminary to the typical, while 

 in the Polypifera it is oviparous and supplementary. 



But there is one order of Hydrozoa — the Lucernariadse — 

 some of the reproductive phenomena of which seem at first 

 sight much opposed to this view. Certain Lucernarian 

 polypes, particularly that termed Scyphistoma by Sars, 

 (Hydra Tuba of Dalzell) give off gemmae, which are even- 

 tually developed into Hood-eyed Medusse — the common 

 Jelly-fishes of our seas — animals of colossal dimensions, 

 compared with the original polype stock, and, apparently at 

 least, of much higher organization.* Here it seems natural 



* On the Lucernarian affinities of the polype phase of the large Me- 

 dusge, see Huxley in Med. Times, XII., 506, and Oceanic Hydrozoa, p. 21. 

 See also Dr. J. Reid's Physiolog. and Patholog. Researches, p. 445. 



