What does acephala mean?
The definition of acephala
A*ceph”a*la, n. pl.
Etym: [NL., fr. Gr. Acephal.] (Zoöl.)

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.