Treated as plural. A former class of invertebrates lacking distinct heads, broadly equivalent to Bivalvia (bivalve molluscs) but also including tunicates; (also in form acephala) invertebrates of this class collectively. Occasionally with singular concord (also in form acephala): an invertebrate of this class.
((n. pl.) 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.)
Origin:
Early 19th century. From scientific Latin Acephala, plural noun, use as noun of post-classical Latin acephalus, after French acéphales, plural noun; compare scientific Latin -a.
