2.6 



HYDRA. 



by the usual number of tentacles ; if these be again and again and 

 again divided, each division will become a new head, thus form- 

 ing a realization of the fable of the Lernean Hydra. Every one 



FIG. 18 HYDRA VIVIDIS. 



of the tentacles of this newly-formed monster will capture food, 

 and all the mouths will devour it. If two be cut across, and the 

 fore part of one be applied to the hind part of the other, the two 

 parts will unite and form a perfect Polype without leaving a scar. 

 They may even be turned inside out like a glove without injury, 

 and in this state will remain, that which was the external surface 

 now being the stomach. 



Their ordinary mode of increase is by the young ones budding 

 from the side of the adult ; but previously to their separation, the 

 offsets themselves often send out side-buds, so that several gene- 

 rations may sometimes be found branching from one parent : nine- 

 teen young of various ages have been seen thus connected, their 

 numerous long tentacles twining about in inextricable confusion. 



To the earlier observers of the habits of the Hydrae, nothing 

 could be more mysterious than their power of seizing and retain- 

 ing active prey ; but this is now readily explained. The whole 

 surface of their tentacula is densely provided with a set of organs 

 that appear, under high powers of the microscope, to be minute 

 oval vesicles, something like little soda-water bottles, in each of 



