from
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]