Webster's English Dictionary

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crustacean

crus.ta.cean \.kr*-'sta--sh*n\ n [deriv. of L crusta crust, shell] : any of 
   a large class (Crustacea) of mostly aquatic arthropods having a c hitinous 
   or calcareous and chitinous exoskeleton and including the lobsters, 
   shrimps, crabs, wood lice, water fleas, and barnacles - crustacean aj