THE HYDRAS, FRESH WATER JELLYFISHES 



If hydras are present they will soon be hanging out into the 

 water (Fig. 87). They should be kept out of the direct sun- 

 light. Like other animals hydras will have a better chance 

 to live if they get used to water gradually so only a little new 

 water should be given them at a time. Sprays of water plants 

 such as Nitella, Myriophyllum, or Elodea will give off enough 

 oxygen for them and small crustaceans will be their best food. 

 Swarms of these can be dipped out of the water close to the 

 pond shore and hydras will live on them the year round. 



In the aquarium their feeding habits can be watched; they 

 will even take a small worm or larva from forceps if it is moved 

 toward them very gently. They will hang from the side of 

 the aquarium, stretching their bodies and tentacles into tenu- 

 ous slenderness, circling slowly around within a radius of an 

 inch or more. Occasionally, one will attach its tentacles to 

 the glass, loosen its foot and move it over to the tentacles, 

 which are loosened, stretched, and pressed to the glass again; 

 then the foot is lifted again and extended and thus, the 

 whole animal moves, looping like an inch worm. Again the 

 same animal may elect to travel by turning slow motion 

 somersaults. 



Identification. — The commoner fresh water Hydrozoa (Hy- 

 dras). Animals which have tentacles in a circlet about the 

 mouth and do not form colonies. 



Fig. 90 — Green hydra. Hydra viridissima. 



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