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Definition of CRUSTACEAN
: any of a large class (Crustacea) of mostly aquatic mandibulate arthropods that have a chitinous or calcareous and chitinous exoskeleton, a pair of often much modified appendages on each segment, and two pairs of antennae and that include the lobsters, shrimps, crabs, wood lice, water fleas, and barnacles
Any member of the 45,000 arthropod species in the subphylum Crustacea. Distributed worldwide, crustaceans are distinguished by having two pairs of antenna-like appendages in front of the mouth and other paired appendages near the mouth that act like jaws. Most species are marine, including shrimps and barnacles. Some, including crayfishes, live in freshwater habitats; others (e.g., sand fleas, land crabs, and sow bugs) live in moist terrestrial environments. The typical adult body is composed of a series of segments (somites) either fused or linked to each other by flexible areas that form movable joints. The carapace (shell) varies in thickness among species and must be periodically molted to allow growth. Many species of marine crustaceans are scavengers, and many (including copepods and krill) are significant components of the diets of larger organisms. See alsodecapod.