X 



THE FRESH-WATER HYDRA. 79 



light, but not in the sun, in a few hours you will find 

 many of these little creatures clinging to the side of the 

 glass toward the window. They hold on to the glass by 

 one end, and all around the other end, which is wider, 

 are a number of long threads called tentacles hanging 

 down gracefully in the water (Fig. 78). At first you 

 might think them whiskers, as they grow out around 

 the mouth ; but tentacle means a feeler, or holder, 

 and with its tentacles you will see how our little friend 

 feels and holds its food and carries it to its mouth, just 

 as you use your fingers. These little animals are called 

 hydros, because, if you cut them up into pieces, each 

 piece will grow again as did the heads of the old Greek 

 monster. If you look at the hydra under the micro- 

 scope, you will find all these parts (Fig. 78). First 

 there is the part by which it holds on ; it is round and 

 hollow, something like the bottom of a fly's foot, and it 

 changes its size whenever the body of the hydra 

 changes its form. When the hydra is stretched out to 

 its full length, the foot is smaller than the body, and, 

 when the hydra shrinks back against the weed or glass, 

 the foot is larger than the body. When it is stretched 

 out, the body is round and hollow like a pipe-stem, or 

 more like a very slender funnel, and the large opening 

 surrounded by tentacles, or feelers, is the mouth (Fig. 

 79). The hydra's tentacles are not all the same length, 



