One of the grand divisions of the animal kingdom, a phylum including the classes Cephalopoda
, and Pelecyopoda
, 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
" 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."