Body bilaterally symmetrical; unsegmented; usually with a definite head. Ventral body wall specialised as a muscular foot, used chiefly for locomotion. Dorsal body wall forms the mantle, which encloses the mantle cavity, Surface epithelium usually ciliated, bearing mucous glands and sensory nerve endings. Coelom mainly limited to area around heart. Complex digestive system; rasping organ (radula) usually present. Open circulatory system of heart, blood vessels and sinuses. Gaseous exchange by gills, lung, mantle or body surface. Sensory organs of touch, smell, taste, equilibrium, and vision (in some); eyes highly developed in cephalopods.
