Cirripedes are the only major group of sedentary arthropods. They have very diverse life styles: some are free-living, others live on other animals. Barnacles are hermaphroditic and use an extendable penis to copulate with their neighbours. Barnacles begin life as free-swimming nauplius larvae. These change into cypris larvae which search for and bind to suitable attachments sites with cement produced by glands on their heads.
Free living sessile barnacles - like the acorn barnacle - are completely covered by calcareous plates. Free living stalked barnacles - like the goose barnacle - may be incompletely covered and their stalks are always naked. The parasitic cirripede, Conchoderma virgatum, attaches to the copepod Penella exocoeti which grows in the skin of whales.