VI. 



THE FRESH-WATER POLYPES (Hydra viridis 

 and H. fused). 



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, 

 rarely more, and are cylindrical or slightly conical in form. 

 From the free end delicate filaments, which are often much 

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

 or less downward curve, in the water. These threads, which 

 are the tentacles, may vary in number 1 ; if touched they 

 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 usually 

 termed Hydra fusca, the green to that called H. viridis. 

 The polypes generally remain attached 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. 



1 In H. Hexactinella, an Australian species, their number is re- 

 ported to be invariably six. 



