The Acorn Barnacle is a barnacle without stalks in the clade Sessilia. They form a monophyletic group and are probably derived from stalked barnacles. Sessilia is divided into two orders: Verrucomorpha contains one living family, Verrucidae, and Balanomorpha contains 13 families.
The acorn barnacle Semibalanus balanoides, also known as the common barnacle, common rock barnacle, northern rock barnacle, or northern acorn barnacle, is a common and widespread boreo-arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western Europe and both coasts of North America.