HYDROIDS AND JELLY-FISH 41 



paralyse the microscopic creature that has brushed against the 

 tentacle. These remarkable thread-cells, or nematocysts, are very 

 characteristic organs of the Ccelenterates. 



It is quite possible by keeping the jar of water in a cool place 

 near a window to watch the habits of the Hydra at leisure, for 

 the little creature is very hardy, and so long as the weed flourishes, 

 oxygenating the water and keeping it pure, all will be well. It 

 will then be seen that the Hydra does not remain permanently 

 attached to one spot, but that it sometimes goes for a walk, its 

 mode of progress being a series of slow and cautious somersaults. 

 When about to change its quarters the Hydra elongates its body, 

 and, bending over, brings its crown of tentacles into contact with 

 the base to which it is attached. Having fixed itself by its ten- 

 tacles, the Hydra then detaches the sucker end of its body, and 

 literally, for a second or two, stands upon its head. Then the 

 body is slowly moved over until the sucker is brought into contact 

 with the base, and the whole process is again repeated. Some- 

 times, however, the Hydra will get as close to the surface of the 

 water as it possibly can, and then, releasing its hold, turn the base 

 of its body upwards and swim along just beneath the surface, head 

 downwards, and by waving movements of its tentacles. 



During the summer the Hydra multiplies by a curious budding 

 process. A little roundish swelling makes its appearance on the 

 side of the slender body of the Hydra, and soon becomes furnished 

 at its free end with a crown of tentacles, so that it closely resembles 

 its parent in appearance. A second may form, and the parent stem 

 and buds thus constitute a miniature colony. But the buds do not 

 remain permanently attached ; after a little while they drop off, 

 and, developing a detachable base, become free individuals cap- 

 able of capturing food and fending for themselves. In the autumn 

 ova and sperms are developed by the Hydra, and after fertilisation 

 the eggs, which form little wart-like swellings on the surface of the 

 body, escape into the surrounding water ; from them, in due 

 course, another generation develops. 



The Hydra has a most remarkable power of reparation of 

 injuries, and the reproduction of individuals out of portions of 

 itself. Thus, if a Hydra should lose two or three tentacles, new 

 ones are quickly grown to replace them ; while should the animal 

 itself be cut into several pieces, provided each contains a portion 



