FIELD BOOK OF PONDS AND STREAMS 



and occurred in cold weather when there was a lusty popula- 

 tion of small crustaceans which are their favorite food. 



Food. — Hydras are carnivorous. They thrive on a diet of 

 minute worms and such small crustaceans as Daphnia, Cypris, 

 and Cyclops (Figs, 124, 128, 126), for which they compete with 

 the young fishes. In an aquarium hydras can be seen dangling 

 their tentacles lazily into the water reaching after the 

 Daphnias and finally ensnaring them with unbelievable 

 surety. They also swallow young insects and small clams, 

 casting the shells of the latter out of their mouths after the 

 soft parts have been digested. Hickson of the University of 

 Manchester, England, saw a hydra capture and hold a very 

 small tadpole in its tentacles. Another observer saw a hydra 

 swallow a small fish which extended from its mouth after it 

 had been taken in as far as was possible. In 1740 Abbe 

 Abraham Trembley, the pioneer student of hydras, described 

 their capture of baby fishes. In modern times hydras some- 

 times appear by thousands in hatchery troughs, creating 

 havoc among the very small fry, and they no doubt act the 

 same way in the natural homes of fish fry. 



Associates. — Hydras may feed upon young fishes, but it is 

 well known that slightly larger fishes feed upon them. They 

 live on terms of a biological intimacy, known as symbiosis, 

 with at least two organisms. Green algae or zoochlorellas 

 live within the body cells of the green h^^dra as they do in 

 those of fresh water sponges and cause their brilliant green 

 color; and microscopic protozoans (Kerona pediculus) live 

 on brown hydras. There are often ten or fifteen of these 

 climbing over an animal at one time, but they are not known 

 to feed upon it nor to do it any harm. 



Collecting, aquarium study.— When one is familiar with them 

 hydras can often be seen in still pond water hanging from plant 

 stems like Nitella and Elodea or from the under sides of lily 

 leaves. But it is usually necessary to gather a few water 

 plants and let them stand in a glass dish of the pond water. 



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