294 ZOOPHYTES. 



to maturity, — many months, — not fewer than 200 days intervening from 

 production of the ovum of the Cristatella, until its sundering shells allow 

 protrusion of the nascent hydra. 



On the whole, it seems more consistent to conclude that the elements 

 of the future specimen are lodged in the germ, than that each successive 

 hydra is derived from the first or from any one preceding its own evolution. 

 The race of the Polypus, or Hydra proper, is perpetuated by gemma- 

 tion, or budding of progeny from the parent, as buds burst from the 

 tree. Various learned authors, of older date, and also those more modern, 

 among whom a skilful physiologist, Professor Allen Thomson, recently dis- 

 cusses the question of propagation being carried on by ova likewise. The 

 general confirmation of such an important fact would be interesting to 

 naturalists. 



Independently of this, however, propagation is incessantly advancing 

 by the developement of young hydra from the body of the parent, which, 

 without undergoing any sensible external metamorphosis, are perfected 

 merely by evolution of the parts. 



The fresh-water hydrae, yet believed to be in a final and perfect state 

 as such, are minute subjects ; but in the Hydra tuba, though in a state of 

 transition, this process is beautifully displayed to great advantage from its 

 infinitely superior dimensions, conjoined with its long and easy preserva- 

 tion. 



A kind of twofold generation is carried on by some zoophytes, one 

 augmenting the parent, the other perpetuating the race. A gem mule is 

 discharged, founding a new colony, as by the foliaceous Flustrae, while the 

 developement of new hydrse multiplies the original colony of the specimen. 

 A process not dissimilar may be ascribed, though less explicitly, to 

 the Sertularian Zoophytes. A planula discharged from a vesicle founds a 

 new specimen, while the old is augmented by developement of new shoots, 

 bearing cells with their hydra'. 



Generation by enlargement, or evolution of new parts augmenting 

 the specimen, becomes very clear from the nature and formation of the 

 Alcyonidium mytili. Here the cellular margin of the old subject is en- 

 larged by a parallel row of growing cells, originally empty to appearance, 



