Practice question

Molluscan organization shows varied cephalization levels. Active gastropods and cephalopods possess prominent head bearing eyes, tentacles, statocysts and ganglia supporting predation and locomotion. Polyplacophora have subradular sensory organ. In contrast class Bivalvia, adapted to sedentary suspension feeding enclosed within two hinged valves, demonstrates extreme regression; distinct head completely absent, mouth surrounded only by paired labial palps, radula reduced or lost, sensory roles taken by mantle siphons and osphradia. This simplification reflects adaptation to burrowing and sessile filter-feeding lifestyle where head mobility unnecessary and protected within shell valves for safety.