Goose barnacles (Lepas anatifera) feeding underwater. These cirripedian crustaceans attach themselves to boats and driftwood. Their long stalk is derived from the elongated head and cements the barnacle to its host surface. The rest of the body is a carapace or mantle of five calcified plates. When the valves of the mantle open, the thoracic limbs (black), hang out. The limbs, which are covered in fine bristles (cirri), are thrust in and out allowing the cirri to filter out small plankton which are then conveyed to its mouth.