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X. 



THE FRESHWATER POLYPES (Hydra viridis 



and H. fusca). 



IF a waterweed, such as duckweed, from a pond, is placed 

 in a glass and allowed to remain undisturbed for a short time, 

 minute gelatinous-looking bodies of a brownish or green 

 colour may frequently be found attached to it, or to the sides 

 of the glass. They have a length of from ~ to \ of an inch, 

 and are cylindrical or slightly conical in form. From the 

 free end numerous delicate filaments, which are often much 

 longer than the body, proceed and spread out with a more 

 or less downward curve, in the water. If touched, these 

 threads, which are the tentacles, rapidly shorten and together 

 with the body shrink into a rounded mass. After a while, 

 the contracted body and the tentacles elongate and resume 

 their previous form. These are Polypes, the brown ones 

 belonging to the species termed Hydra fusca, the green 

 to that called H. viridis. The polypes usually remain at- 

 tached to one spot for a long time, but they are capable 

 of crawling about by a motion similar to that of the looping 

 caterpillar; and, sometimes, they detach themselves and float 

 passively in the water. 



When any small animal, such as a water-flea, swimming 

 through the water conies in contact with the tentacles, it is 

 grasped, and conveyed by their contraction to the aperture 



