Barnacles are sessile organisms that are encased in a pair of calcareous plates that form a carapace. They may be cemented directly to the substrate (such as the acorn barnacles shown above) or attached via stalks (the goose barnacles). Barnacles are filter feeders that feed using feathery, modified legs called cirri. From an economic standpoint, barnacles can cause problems by fouling the hulls or ships and boats.

Note: Some authorities consider the Class Maxillopoda to be a non-monophyletic and therefore technically invalid group that also includes such forms as ostracods and copepods, each of which is placed into its own separate class. In this view, barnacles would be placed into the Class Cirripedia. However, until the issue is resolved, this website will continue to consider barnacles as members of the Class Maxillopoda.