Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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