HYDRA, AND HYDROID ZOOPHYTES. 131 



for into whatever number of parts its body may be cut up, 

 each, under favourable circumstances, can give origin to 

 a new and entire polype, so that thirty or forty individuals 

 may thus be produced by the division of one. 



123. The proper generative process, here reduced to its 

 utmost simplicity, consists in the development of a germ-cell 

 and of sperm-cells in the substance of the wall of the stomach, 

 the former being produced near the footstalk, the latter just 

 beneath the arms. The egg which is evolved from the former, 

 being fertilized by the products set free from the latter, gives 

 origin to a young Hydra, which resembles its parent. The 

 two reproductive processes, however, are performed under 

 very different conditions; 



for whilst multiplication by 

 gemmation is favoured by 

 warmth and a copious sup- 

 ply of food, the true gene- 

 rative process seems to be 

 brought about by a lower- 

 ing of the temperature, and 

 to have for its object the 

 perpetuation of the race 

 through the winter, the egg 

 being capable of enduring a 

 degree of cold which would 

 be fatal to the polype itself. 



124. The group of Hy- 

 drozoa is for the most part 

 made up of composite fabrics 

 more or less resembling the 

 Campanularia (fig. 72), 

 which may be likened to 

 a Hydra whose buds do not 

 detach themselves, but re- 

 main in connexion with the 

 stock that produced them ; 



the whole plant-like StniC- Fig. 72. CAMPANULARIA. 



ture, moreover, being strengthened by the consolidation of its 

 external layer into a horny sheath, which retains its form 

 after the destruction of the soft parts. Thus each comes to 

 consist of a stem and branches, on the sides or ends of which 



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