Wordnet 3.0
NOUN (1)
1. 
 any mainly aquatic arthropod usually having a segmented body and chitinous exoskeleton; 
ADJECTIVE (1)
1. 
 of or belonging to the class Crustacea; 
[syn: crustaceous, crustacean]
The Collaborative International Dictionary of English v.0.48:
Crustacean \Crus*ta"cean\ (kr?s-t?"shan; 97), a. (Zool.)
   Of or pertaining to the Crustacea; crustaceous. -- n. An
   animal belonging to the class Crustacea.
   [1913 Webster]
WordNet (r) 3.0 (2006):
crustacean
    adj 1: of or belonging to the class Crustacea [syn:
           crustaceous, crustacean]
    n 1: any mainly aquatic arthropod usually having a segmented
         body and chitinous exoskeleton