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Encyclopedia—crustaceanTypes of CrustaceansCrustaceans can be divided according to size into two main groups. The larger group, which is divided into the classes Branchiopoda, Copepoda, Ostracoda, and Cirripedia, includes the familiar shrimp, crayfish, lobsters, and crabs. The smaller group includes species that are either microscopic or range up to a few inches (about 5 cm) in size. Most of the smaller marine forms can be found in plankton (see marine biology) and thereby occupy an important position in the marine food chain. Other copepods supply food for small fish, and still others exist as parasites on the skin and gills of fish. Best known of the smaller freshwater crustaceans are members of the genus Daphnia (water fleas), the fairy shrimp (a phyllopod that swims inverted), and Cyclops (a copepod). The order Isopoda includes the only large group of truly terrestrial crustaceans. Known as wood lice, sow bugs, or pillbugs, these small animals can be found under the bark of trees, beneath stones and rocks, and in other damp places. When disturbed they curl up armadillolike, withdrawing into the exoskeleton. Sections in this article: The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press. All rights reserved.
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