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Crustacean barnacles are marine arthropods that belong to the subclass Cirripedia. They are characterized by their hard calcareous shells, which are made up of multiple plates and secreted by the animal itself. These sedentary filter feeders attach themselves to hard surfaces such as rocks, pilings, and the hulls of ships using a cement gland located on their heads. Barnacles have feathery appendages called cirri that they use to capture plankton and other small organisms from the water for food. They are often found in intertidal zones and can vary in size, shape, and color depending on the species.