mollusk

from WordNet (r) 3.0 (2006)

mollusk
    n 1: invertebrate having a soft unsegmented body usually
         enclosed in a shell [syn: {mollusk}, {mollusc},
         {shellfish}]

from The Collaborative International Dictionary of English v.0.48

Mollusk \Mol"lusk\, n. [F. mollusque, L. mollusca a kind of soft
   nut with a thin shell, fr. molluscus soft, mollis soft. See
   {Mollify}.] (Zool.)
   One of the Mollusca. [Written also {mollusc}.]
   [1913 Webster]