\A*ceph"a*la\, n. pl. [NL., fr. Gr. ?, adj. neut. pl.,
headless. See {Acephal}.] (Zo["o]l.)
That division of the Mollusca which includes the bivalve
shells, like the clams and oysters; -- so called because they
have no evident head. Formerly the group included the
Tunicata, Brachiopoda, and sometimes the Bryozoa. See
{Mollusca}.