Cirripedia

(invertebrate zoology) A subclass of the Crustacea, including the barnacles and goose barnacles; individuals are free-swimming in the larval stages but permanently fixed in the adult stage.

A subclass of the Crustacea, permanently attached when adult, and called barnacles. The carapace forms a complete covering or mantle over the rest of the body and is usually strengthened by calcareous plates. The body within the mantle consists of a mouth region and thorax. The abdomen is usually vestigial. Typically the mouth appendages are paired mandibles with palps, maxillulae, and maxillae. The thorax bears six pairs of biramous appendages (cirri) composed of numerous segments, each with a considerable armament of setae. Compound eyes occur only in the larvae, there is no heart, and typically adults are hermaphroditic. The eggs are incubated in the mantle cavity and hatch into free-swimming nauplius larvae (illus. a). By successive molts a further five nauplius larvae follow. The sixth nauplius (or metanauplius) molts into an entirely different larval form, the cypris (illus. b), which is characteristic of all Cirripedia. The Cirripedia are divided into four orders: Thoracica, Acrothoracica, Ascothoracica, and Rhizocephala. Only the Thoracica, the goose barnacles, and acorn barnacles are at all conspicuous, and only the latter of any economic significance. See also Acrothoracica; Ascothoracica; Barnacle; Rhizocephala.

Larval stages of Balanus. (a) Nauplius. (b) Cypris. (Photographs by D. P. Wilson)