HYDRA AND OTHER FRESH-WATER HYDROZOA 319 



various distances from the surface, but not infrequently is sus- 

 pended from the surface film or even drifts about unattached 

 and thus often becomes a component of the plankton. The hydras 

 multiply so rapidly when conditions are favorable that they often 

 take heavy toll from the plankton organisms, especially the ento- 

 mostracans and small worms. Since they are probably little used 

 as food by animals useful to man and since they compete with 

 young fish for food, their economic relations to man are unfavor- 

 able. 



The most favorable conditions for Cordylophora are in brackish 

 water and there it attains most luxuriant development but it 

 thrives also in fresh water, although the colonies are there less 

 stalwart and the ascending branches are usually not more than 

 half as large as in colonies from brackish water. It was first known 

 as a brackish water form from Europe and its appearance in fresh 

 water is of comparatively recent date. It has been known for a 

 number of years in the United States, near the Atlantic Coast, where 

 it occurs in both brackish and fresh water. The first recorded 

 appearance in the Mississippi Valley was in the Illinois River in 

 1909, but it is now known in several states of that region. 



This form is a plankton feeder and thus competes with young 

 fish for food. Its most vigorous colonies are found where there is 

 considerable current and in company with Bryozoa it not infre- 

 quently invades the pipes of water systems, impedes the flow, and 

 at times vitiates the water itself. Microhydra is found associated 

 with bryozoans on the surface of stones in running water near 

 Philadelphia, and is apparently not an abundant form. 



In the search after hydra if pond-lily leaves and coarse sub- 

 merged vegetation be collected from bodies of water in which 

 hydra occurs, and allowed to stand a few hours or days in glass 

 jars, specimens are likely to be found attached to the vegetation 

 or to the sides of the jar or even suspended from the surface film. 

 Hydra may be kept in good condition for long periods of time in 

 well aerated aquaria, if supphed with sufficient food, preferably 

 small entomostracans and worms. At the proper season and tem- 

 perature they may reproduce sexually as well as by budding. 



For ordinary purposes a corrosive sublimate and acetic acid 



