Barnacles (Class Cirripedia) are unusual crustaceans that are sometimes mistaken for molluscs. Unlike most other crustaceans, which are mobile as adults, barnacles typically settle permanently on hard substrata and secrete a hard calcareous shell for protection. Acorn barnacles are the most common group, and secrete four to eight shell plates arranged in conical shape. Stalked barnacles (Goose barnacles) have long flexible stalks and often grow attached attach to floating objects such as logs, buoys and debris. The animal inside is attached by its head, and the legs develop into long, feathery cirri that extend outside the shell and beat to create a current that directs food to the mouth. Most barnacles are hermaphrodites and fertilisation is internal. Fertilised eggs are brooded in the mantle cavity and develop into larvae that are released into the sea for dispersal.