A barnacle is a type of marine crustacean that belongs to the class Cirripedia. These small, hard-shelled animals typically attach themselves to surfaces like rocks, ships, and even the shells of other sea creatures. They have a unique feeding mechanism, using their feathery appendages called cirri to filter plankton and other small particles from the water. Barnacles are known for their distinctive calcareous shells, which provide protection from predators and harsh environmental conditions. They reproduce by releasing eggs into the water, where the larvae eventually settle and grow into adult barnacles. Their presence can impact marine ecosystems and human activities, such as shipping and fishing.