The Collaborative International Dictionary of English v.0.48:

Mollusca \Mol*lus"ca\, prop. n. pl. [NL. See Mollusk.] (Zool.)
   One of the grand divisions of the animal kingdom, a phylum
   including the classes Cephalopoda, Gastropoda, Pteropoda,
   Scaphopoda, and Pelecyopoda (syn. Bivalvia, formerly called
   Lamellibranchiata, or Conchifera). These animals have an
   unsegmented bilateral body, with most of the organs and parts
   paired, but not repeated longitudinally. Most of them develop
   a mantle, which incloses either a branchial or a pulmonary
   cavity. They are generally more or less covered and protected
   by a calcareous shell, which may be univalve, bivalve, or
   multivalve.
   [1913 Webster +PJC]

   Note: Formerly the Brachiopoda, Bryzoa, and Tunicata were
         united with the Lamellibranchiata in an artificial
         group called Acephala, which was also included under
         Mollusca. See Molluscoidea.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Mollusca
    n 1: gastropods; bivalves; cephalopods; chitons [syn:
         Mollusca, phylum Mollusca]