Although barnacles live in shells made of calcium carbonate plates stuck to docks, boat bottoms, and other hard surfaces, they are in fact crustaceans, cousins of crabs and shrimp. Larval barnacles ready to settle down after their planktonic babyhood seek a suitable location, usually in an intertidal habitat. Then, they land headfirst and secrete their shells. Here they will live the rest of their lives. When submerged, a barnacle's feathery appendages, called cirri, extend out from its shell and comb the water for food. At low tide, the barnacle closes its plates tightly with a trapdoor to keep from drying out.
Image above is of a species of acorn barnacle common in Europe closed up tight in their shells at low tide. "Chthamalus stellatus" by MichaelMaggs - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons -
The most common coastal barnacles are acorn barnacles, which can be found attached almost anywhere in the intertidal zone. Some acorn barnacle relatives are more specialized. The small gray barnacle Chthamalus fragilis lives in high-tide zones within the estuary. The gray barnacle can be distinguished by the tough brown membrane that it uses to stick to surfaces, rather than the white lime plate of acorn barnacles.
The shells of horseshoe crabs and blue crabs are often home to the crab barnacle, Chelonibia patula, which occurs nowhere else. A related barnacle species is found only on sea turtle shells.
Two crab barnacles that have found a suitable location on the carapace of a blue crab. "Chelonibia patula on Blue crab" by Cirriphilia - Shot pic on a Canon camera. Licensed under CC BY-SA 3.0 via Wikipedia
Goose barnacles are very different from acorn barnacles. The body of the typical goose barnacle is encased in hard white plates on a thick bluish-purple stalk. These crustaceans drift about in the open ocean, attached to planks and other floating objects. Occasionaly their floating homes wash up on the beach after a storm. In the Middle Ages, these barnacles were thought to be the young of a wild bird, the barnacle goose, because the coloration of the goose's head and neck bears a striking resemblance to the color pattern of goose barnacles.
Goose barnacles feeding underwater with cirri extended, "Entenmuscheln" by M.Buschmann - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons
Speaking of sticking to it, Charles Darwin certainly did that with his eight year-long barnacle project. After returning from the Beagle's 5-year circumnavigation of the globe, during which Darwin began developing his theory of evolution, the young naturalist decided he needed multiple lines of evidence to support his ideas. He turned to classification of barnacles, of which there were many varieties that had been collected all over the world ocean. The goal was to establish that barnacles, like other life forms, could be grouped into clades of species that were related by descent from a common ancestor. Darwin thought the effort might take a year. He toiled much longer than that, painstakingly studying an ever larger inventory of barnacles sent to him by fellow naturalists. In the end, Darwin produced four thick volumes on barnacles, which is still the definitive work on this large group of marine crustaceans. His detailed knowledge of the relationships among barnacle species gave him increased confidence as he prepared his opus, 'On the Origin of Species.'