PBBSH- WATER CEUSTACEANS. 289 



the aquariiun. In a natural state they do good service, by 

 helping to ventilate stagnant water by the means of their 

 almost constant movements ; by (many of them) acting the 

 part of scavengers; and by, owing to their very great proli- 

 ficacy, supplying fish and other animals with most nntritions 

 food. They may be found in either stagnant or slow-running 

 water during the greater portion of the year. They will 

 breed freely in captivity, and their presence should always 

 be encouraged in the aquarium. It is a good plan to keep 

 a small tank especially for them, and so they will be always 

 at hand for supplying food to interesting carnivorous 

 aquatic animals. Entomostraca can easily be obtained from 

 their native waters by the help of a very simple contHvance, 

 as explained on page 37. Though they are very small, they 

 can, as a rule, be readily seen with the naked eye : but a 

 microscope, or at any rate a magnifying-glass, is generally 

 necessary for observing their different parts. The commonest, 

 and perhaps the most interesting and useful, of these small 

 crustaceans, are the DaphnioB, the Cyprides, and the Cyclops. 

 The DaphnicB, owing to their curious antennae, belong to 

 the order Cladocera ("branching homed"). Their jerky or 

 jumping movements in the water have given them the common 

 name of Water-fleas. Of these Water-fleas, Daphnia Fulex 

 is by far the commonest. Its body, and that of all its near 

 relatives, may be said to be divided into two parts : the 

 smaller part is the head, a portion of which is produced in 

 front to a kind of beak; and the larger part, which consists 

 of the thorax and abdomen, is inclosed in a shelly envelope. 

 AH the Dwphnice possess five pairs of legs, which by the 

 help of a microscope may be seen in almost constant motion 

 through the transparent shell. The female Water-flea carries 

 her eggs for a time between her shell and the back part of 

 her body. These small crustaceans are frequently found in 

 stagnant water in great numbers, especially where there is 

 plenty of duckweed. I have sometimes caught what appeared 

 to be thousands of them with one dip of a large bottle in 

 that part of a pond where they seemed to be the most 

 numerous. Dr. Baird, in his "British Entomostraca," in 



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