acephala

‖n. pl. [ NL., fr. Gr. &unr_;, adj. neut. pl., headless. See Acephal. ] (Zool.) 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. [ 1913 Webster ]