CHAPTER XVII. 

 CCELENTERATA. 



THE FRESH-WATER HYDRA. 



THE fresh-water hydra has a cylindrical body, varying in 

 diameter from the size of a fine needle to that of a common pin, 

 and from one fourth to one half an inch in length. It is found 

 in fresh-water ponds and streams, usually attached by one end 

 to submerged stems, leaves, etc., frequently on the under surface 

 of a leaf. Surrounding the free end of the hydra is a circle of 

 threadlike appendages, the tentacles, which often are longer 

 than the body itself. 



Two species of hydras are found: one green, the other brown 

 or flesh colored, often whitish. 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 microscope 

 and examine with a one-inch objective. The following 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. 



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