Phylum III 



Coelenterata, Class Hydrozoa 



HYDRAS 



Libbie H. Hyman, New York City 



Collection. Hydras most commonly occur attached to the submerged 

 vegetation, fallen leaves, or other objects in pools, ponds, lakes, and 

 the slow portions of rivers. They are collected from such habitats by 

 gathering a quantity of vegetation and placing it in jars with a relatively 

 small amount of water. As the vegetation begins to decay, the hydras 

 usually come to the top of the jar or the surface film and may be picked 

 out and transferred to suitable containers. One species, Hydra littoralis, 

 occurs in enormous numbers on the under surface of stones in streams, 

 spillways, and along shores subject to wave action. When the stones 

 are turned over, this species appears like an orange jelly on the stone. 

 They may be obtained in large numbers by squirting the animals from 

 the stone into a pan by means of a squirter made from an atomizer bulb 

 and a glass tube. Unfortunately this species is not very suitable for 

 laboratory cultivation, but is useful when large numbers are needed 

 for a short time, or when large quantities of hydras are wanted for pres- 

 ervation. Hydras are sometimes found in large numbers attached to 

 the surface film and in such situations are easily gathered. They do not 

 in general live in stagnant water but require rather clean water with 

 adequate oxygen supply. 



Laboratory cultivation. The most suitable hydra for laboratory cul- 

 tivation is the brown hydra, Pelmato hydra oligactis, but any of the 

 species which live in standing water may be grown successfully in the 

 laboratory. The species which inhabits moving water, Hydra littoralis, 

 mentioned above, may be grown in the laboratory if the water con- 

 tains a supply of algae or other vegetation to keep up a high oxygen 

 content. The author has not tried bubbling air through a culture of 

 this species but such a procedure would probably be successful. The 

 green hydra, Chlorohydra viridissima, is one of the most hardy hydras, 

 very common everywhere, and easy to maintain in the laboratory except 

 for one difficulty. It is difficult to find a food crustacean which is small 



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