Illustration of a section through an adult acorn barnacle (family Balanidae). At top right is a barnacle larva at the cyprid stage. Barnacles are hermaphrodite filter-feeding crustaceans that attach themselves to rocks and other surfaces in intertidal zones. They attach using cement that is produced by cement glands (dark pink) in the barnacle's mantle. The mantle also houses the majority of the barnacle, including the testicles, ovaries, mouth and stomach, and is covered in calcerous plates. Inner to the mantle is the mantle cavity, which contains eggs and ovisacs (oval) containing young naupliar larvae.