very little studied, largely because of their small size and the fact that micro- 

 scopic study is usually necessary in order to determine specific differences. 

 Authorities do not agree as to the division of the Entomostraca into orders and 

 suborders. A common division makes three orders of the fresh-water forms: 

 the Brayichiopoda or leaf-footed; the Ostracoda, with cylindrical appendages 

 and bivalve shells; and the Copepoda, which have cylindrical appendages and 

 no shcU-likc coverings. 



The Branchwpoda or leaf-footed Crustacea are further divided into two 

 groups, one of which is characterized by an elongate, distinctly segmented body 

 and the other by a short, indistinctly segmented body, usually partly covered 

 by an apparently bivalve shell. The best known and most common members 

 of the first group or Phyllopoda are the fairy shrimps, which sometimes appear 

 in huge numbers in temporary ponds in early spring and disappear within a 

 few weeks or even days. These dainty creatures, like most of the Phyllopoda, 

 swim on their backs. The males have the second pair of antennae much enlarged 

 to serve as clasping organs in mating. A study of these modified antennae is 

 usually necessary in order to identify genera and species. The females are 

 usually burdened with large sacs of eggs. These eggs soon fall to the bottom 

 of the pond and are able to endure both desiccation and freezing before they 

 hatch. Indeed it is reported that the eggs of some species will not hatch until 

 such strenuous conditions have been met. The frequent abundance of fairy 

 shrimps in small temporary ponds and their rarity in or absence from larger, 

 more permanent ponds support this idea. 



The other suborder of Branchwpoda, the Cladocera, were observed by 

 the naturalist Swammerdam in 1669, when the microscope was in its infancy. 

 He called them "water fleas", a name which has since been incorporated into 

 the scientific name of one of the most common forms, Daphnia pulex. In recent 

 years the importance of these tiny Crustacea as food for small fish has been 

 discovered, and the species of the genus Daphnja and other genera have been 

 widely collected and grown in cultures as food for aquarium fishes. They are 

 excellent subjects for examination under the low power of the microscope, for 

 they are usually so transparent that the internal organs are easily seen. One 

 striking feature is the brood case or space between the top of the body and the 

 shell. Here the eggs undergo their development, so that often several well- 

 developed young, miniatures of the parent, may be seen. During most of the 

 year eggs are formed and develop without fertilization. As a matter of fact 

 males are rare and in some species have never been observed. Sexual reproduc- 

 tion appears to occur only when living conditions become unfavorable, and then 

 only one or two eggs are produced, the shell of the parent becoming thickened 

 and modified and being shed with the egg or eggs still in it. The eggs then lie 

 dormant until the next favorable season arrives. 



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