Balanidae

Balanidae, or Acorn-shells, one of the families of Cirripedia, which are sessile, i.e. not provided with a stalk. The body is protected by a ring of from four to eight plates forming a short tube which is attached by its base to rocks, shells, etc., and is closed above by two pairs of small plates between which the arms can be protruded; by the movements of these arms the food is obtained as in the barnacle. The young are free-swimming forms, and resemble in structure the mature forms of some lower groups of Crustacea; they possess eyes and other organs not found in the adults, which, it is considered, have been lost owing to the animals having adopted a fixed mode of life. The young belong to the type known as the Nauplius (q.v.). All the Balanidae are marine. Two genera, Protobalanus and Palaeocreusia, are Devonian, and several living genera occur in the Chalk and the Tertiary rocks. Balanus is the commonest English genus.