Cirripedia

Cirripedia is a subgroup of crustaceans that comprises the barnacles, a diverse group of exclusively marine, mostly sessile organisms. Traditionally, barnacles are divided into two broad lifestyle groups: the stalked pedunculate barnacles and the directly attached acorn barnacles (sessile). Adults are typically encased in a calcareous shell formed from several hardened plates, and many species attach firmly to hard substrates such as rocks, ships, whales, or other living organisms.

Morphology and feeding are adapted for a sessile life. The body is enclosed within a shell that can be opened and closed by the plates. The anterior part bears feathery thoracic appendages called cirri, which are extended to sweep plankton and detritus from the water for feeding. Cirripedes often have a reduced abdomen and a specialized thorax, reflecting their specialized lifestyle.

Life cycle and reproduction involve free-swimming larval stages. Most species are hermaphroditic, although some have separate sexes. Fertilization is followed by planktonic larval stages, beginning with nauplius and later cypris larvae, which settle onto suitable substrates and metamorphose into the adult form.

Distribution and ecological role: Cirripedes occur in oceans worldwide, from the intertidal zone to deep water. They are important biofoulers on ships and underwater equipment and serve as indicators of marine environments. The fossil record is rich, particularly in the Mesozoic and later, reflecting their long evolutionary history and morphological diversification.