\Mol*lus"ca\, n. pl. [NL. See {Mollusk}.] (Zo["o]l.)
One of the grand divisions of the animal kingdom, including
the classes Cephalopoda, Gastropoda, PteropodaScaphopoda, and
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.
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 {Molluscoudea}.