one of the grand divisions of the animal kingdom, including the classes cephalopoda, gastropoda, pteropodascaphopoda, and 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.