CHAPTER XVI. 



HYORJE. 



HEBE is nothing in connection witli the fresh-water 

 aquaritun more extraordinary and interesting than 

 the Hydras. These polypes were first noticed, I 

 believe, hy Leenwenhoek, in 1703 ; but M. Trembley, of Geneva, 

 about 1740, was the first to describe their life-history. The 

 three species known in Britain are the Green Hydra {Hydra 

 viridis), the Orange-brown {Hydra vulgaris), and the Brown 

 (if. fused). These interesting creatures, of which H. vulgaris is 

 about the commonest, may be found in ponds or slow-running 

 streams nearly everywhere. They are generally taken adhering 

 to such plants as the duckweed or water-crowfoot. The best 

 way to procure Hydrse for the aquarium is to gather a little 

 of the weed among which these tiny creatures are supposed 

 to live, and take it home. Then fill a few tumblers, or other 

 transparent glass vessels, with clear water, put them in a sunny 

 window, and in each of these receptacles place a small portion 

 of the weed. If there are any polypes at all, their presence will 

 be easily detected in less than an hour. When startled or 

 frightened in any way, these creatures contract themselves until 

 they assume the appearance of minute lumps of jeUy, which are 

 not likely to be noticed by an inexperienced eye; but under 

 the influence of quiet and sunshine they will soon begin to 

 extend themselves in such a way that even a novice will be 

 easily able to see them. Their bodies may be likened in size 



