FIELD BOOK OF PONDS AND STREAMS 



groups of crustaceans; very small ones, the Entomostraca, and 

 the large ones, the Malacostraca. Among the entomostracans 

 are the temperamental fairy shrimps, swarms of little "water- 

 fleas," and a host of similar minute animals; the malacostra- 

 cans include such larger ones as the crayfishes, scuds, and 

 sow-bugs. 



A great host of crustaceans are microscopic animals. Many 

 more are just visible with a hand-lens but cannot be recog- 

 nized with any certainty; and in still others, like the water- 

 fiea, Daphnia, their general identity can be made out pretty 

 clearly. Except in considering their general importance in 

 water life this book does not attempt to deal with things which 

 are too small to be seen with the naked eye or through a pocket- 

 lens. The great group of minute crustaceans can only be given 

 slight consideration, and the largest and commonest have 

 been chosen in the hope that these may at least usually be the 

 ones which are found. Habits, food, and associates of crusta- 

 ceans will be mentioned under separate groups, only the most 

 important of which are here named. 



Identification. — Key to orders of crustaceans described here, 

 modified from Pratt (see Bibliography, p. 407). 



Subclasses of Crustacea 



Small, often minute crustaceans without abdominal appen- 

 dages. I. Entomostraca (p. 161). 

 Larger crustaceans usually with abdominal appendages. 



2, Malacostraca (p. 168). 



I. Orders of Entomostraca 



Thoracic appendages flattened and leaflike. Phyllopoda 

 (p. 161), suborders Branchiopoda (p. 161) and Cladocera 



(p. 164). 

 Animals minute; body elongate and segmented with cylindri- 

 cal thoracic appendages. Copepoda (p. 166). 



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