BRITISH ZOOPHYTES. 27 



The ordinary method of reproduction iu the hydrae 

 is by gemmation, or budding, a small bud appearing 

 upon some part of the body and ultimately developing 

 tentacles, and becoming exactly like its parent, upon 

 which it drops off and commences a separate exist- 

 ence. Occasionally, however, another bud grows from 

 the former one before it drops off, and in this case 

 three generations appear attached together. In the 

 warm period towards the end of summer, reproduction 

 takes place by means of ova. The ovary {ov) is 

 situated on the exterior of the body, and coutains the 

 ova, which are fertilized from the sperm cells (tes), 

 which are produced in the neighbourhood of the 

 tentacles. Both elements are developed in the ecto- 

 derm. Unlike other hydroids, the embryological pro- 

 cess is abbreviated in the hydree, and they do not 

 go through the planula stage, which will be described 

 later. 



The hydra, in common with many other hydrozoa, 

 possesses remarkable recuperative powers. In fact, 

 injury seems to be rather beneficial than otherwise : 

 cutting a hydra in two has no worse effect than that 

 of causing two hydree to be produced. 



The experiments of Trerabley, Baker, and others on 

 this question were very minute. They appear to have 

 mutilated the hydra in all possible ways, without 

 exhausting its recuperative power ; and these experi- 

 ments have been recently i-epeated by Mr. Duukerley, 

 of Manchester, who has described (in the " Micro- 

 scopical News," vol. iii. p. 272, October, 1883) several 

 successful dissections which resulted in the production 

 of perfect hydras from the mutilated parts. 



The polypites or bydrauths of the compound hydroids 



