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 J 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'; if touched they 

 rapidly shorten and together with the body shrink into a 

 rounded mass. After a while, the contracted body and th^ 

 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 si 

 motion similar to that of the looping caterpillar; and, 

 sometimes, they detach themselves and float passively in 

 the water. 



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

 ported to be invariably six. 



