THE FRESH-WATER HYDRA. 167 



Two species of hydras are found ; one green, the other 

 brown or flesh-colored. Put the leaves and stems to 

 which the hydras are attached into shallow dishes, such as 

 fruit-dishes, and keep them in a light but shaded place ; 

 watch their behavior when thus kept undisturbed. Cut 

 off a bit of leaf bearing a hydra, and transfer it to a deep 

 watch crystal half full of water. Without the aid of any 

 lens watch the hydra for several minutes. When it is 

 expanded, gently touch it with the tip of a pencil or other 

 blunt object. 



Examine a hydra with a hand lens ; are all parts colored 

 alike ? Place the watch crystal on the stage of a micro- 

 scope and examine with a one-inch objective. The follow- 

 ing points of structure should now be made out : 



1. That the body is a hollow tube closed at one end and 

 open at the other. This opening, within the circle of 

 the tentacles, is the mouth. 



2. That the tentacles are also hollow tubes, closed at 

 their outer ends* but at the inner communicating 

 freely with the body cavity. 



3. That the body wall consists of two layers, which are 

 continuous with the corresponding layers of the ten- 

 tacles. How do these layers differ from each other ? 



The body is, then, a double-walled sac, and the ten- 

 tacles are simply extensions of this sac. Watch the 

 movements of the different parts of the body. Can 

 hydras move from place to place ? If so, how is this 

 accomplished? Look in the body cavity for foreign 

 matter which has been taken in through the rnouth 

 as food. Look also for minute particles obtained by 

 the digestion of such food matter. These particles 

 may often be seen in motion, caused by contractions 



