A barnacle is a type of arthropod constituting the 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 (nonmobile) suspension feeders, and have four nektonic (active swimming) larval stages. Around 70 barnacle species are currently known. The name "Cirripedia" is Greek, meaning "curl-footed". The study of barnacles is called cirripedology.

Barnacles have no true heart, although a sinus close to the esophagus performs a similar function, with blood being pumped through it by a series of muscles. The blood vascular system is minimal. Similarly, they have no gills, absorbing oxygen from the water through their limbs and the inner membrane of their carapaces. The excretory organs of barnacles are maxillary glands.

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