A barnacle is a type of marine crustacean that belongs to the subclass Cirripedia. They are known for their hard, calcareous shells, which they use to attach themselves permanently to various surfaces, such as rocks, ships, and even other animals like whales. Barnacles are filter feeders, using their feathery appendages to capture plankton and other small particles from the water. Barnacles have a unique life cycle that includes a free-swimming larval stage called a nauplius. After several molts, the larvae settle on a surface and undergo metamorphosis, transforming into the adult form. They play an important role in marine ecosystems, providing habitat for other organisms and contributing to the food web.