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]

WordNet (r) 3.0 (2006):

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