| Goose barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages.
Barnacles are encrusters, attaching themselves permanently to a hard substrate. The most common, "acorn barnacles" (Sessilia) are sessile, growing their shells directly onto the substrate The order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.
Most barnacles are suspension feeders; they dwell continually in their shell — which is usually constructed of six plates — and reach into the water column with modified legs. These feathery appendages beat rhythmically to draw plankton and detritus into the shell for consumption
Free-living barnacles are attached to the substratum by cement glands that form from the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long muscular stalk, but in most they are part of a flat membrane or calcified plate. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. The plates are held together by various means, depending on species, in some cases being solidly fused
Inside the carapace, the animal lies on its back, with its limbs projecting upwards. Segmentation is usually indistinct, and the body is more or less evenly divided between the head and thorax, with little, if any, abdomen. Adult barnacles have few appendages on the head, with only a single, vestigial, pair of antennae, attached to the cement gland. There are six pairs of thoracic limbs, referred to as "cirri", which are feathery and very long, being used to filter food from the water and move it towards the mouth
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