The Collaborative International Dictionary of English v.0.48:

Crustaceous \Crus*ta"ceous\ (kr?s-t?"sh?s; 97), a. [NL.
   crustaceous. See crustacea.]
   1. Pertaining to, or of the nature of, crust or shell; having
      a crustlike shell.
      [1913 Webster]

   2. (Zool.) Belonging to the Crustacea; crustacean.
      [1913 Webster]

WordNet (r) 3.0 (2006):

crustaceous
    adj 1: of or belonging to the class Crustacea [syn:
           crustaceous, crustacean]
    2: being or having or resembling a hard crust or shell