Marine crustacean of the subclass Cirripedia. The larval form is free-swimming, but when mature, it fixes itself by the head to rock or floating wood. The animal then remains attached, enclosed in a shell through which the cirri (modified legs) protrude to sweep food into the mouth. Barnacles include the stalked goose barnacle Lepas anatifera found on ships' bottoms, and the acorn barnacles, such as Balanus balanoides, common on rocks.