The Acorn Barnacle is a barnacle without stalks in the order Sessilia. They form a monophyletic group and are probably derived from stalked barnacles. The order is divided into 3 suborders: Brachylepadomorpha contains a single family, Neobrachylepadidae, while Verrucomorpha contains 2 families, Verrucidae and Neoverrucidae. The remaining 12 families are in the suborder Balanomorpha.
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.