A cephalopod (Greek plural ΚεÆαλÃŒÀοδα (kephalópoda); "head-feet") is any member of the mollusc class Cephalopoda, characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles. Teuthology, a branch of malacology, is the study of cephalopods.