Barnacles (Cirripedia)

This group gathers Crustaceans subcategory members belonging to the infraclass Cirripedia, the barnacles, crustaceans so unlike their swimming relatives in adult form that the great naturalist Charles Darwin spent nearly a decade studying them in detail before their true crustacean identity was firmly established. Adult barnacles live permanently cemented head-down to rocks, ship hulls, pilings, floating debris or even the skin of whales and other marine animals, protected by hard calcareous plates, and feed by rhythmically extending feathery, modified legs called cirri out through an opening at the top of the shell to filter plankton from passing water. A barnacle begins life as a free-swimming larva indistinguishable in body plan from other crustacean larvae, and only after settling permanently onto a hard surface does it metamorphose into the immobile, armor-plated adult form that gives the group its deceptively un-crustacean-like appearance.