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THE POPULAR SCIENCE MONTHLY. 



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. 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 his mouth, almost 

 as you use you fingers. These little animals are called hydra?, because 

 if you cut them up each piece will grow again, as did the heads of the 

 old Greek monster. If you look at your hydra under the microscope, 

 you will find all these parts : 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 chauges its size whenever the body of the hydra changes its 

 form. When the hydra is stretched out full length, the foot is smaller 

 than the body ; but, when the hydra shrinks up against the glass, it 



Young hydra 



Body 



Moutli 



Tentacle 



Fig. 35. Green Hydra. 



seems to be all foot. When the body is stretched out, it is round and 

 hollow like a pipe-stem, or more like a very slender funnel, and the 

 opening at the large end surrounded by tentacles or feelers is the 

 mouth. 



The hydra's feelers are not all the same length ; some of them are 

 prettily colored, and all are filled with wavy knobs or knuckles along 

 the sides (Fig. 35). The bag or body of the hydra is made of two 



