Barnacles are crustaceans, which was only established in the nineteenth century once their larval stages were described. A free-swimming nauplius larva develops into a non-feeding cyprid, whose sole task is to find a site. The cyprid tests surfaces with its antennules, responding to texture, water flow and chemical traces of other barnacles, then secretes a proteinaceous cement from glands near the antennules and attaches head-first and permanently.
After settlement it metamorphoses, building the familiar calcareous plates and reorienting so the thoracic limbs project upward through the opening. Those cirri sweep rhythmically through the water, filtering plankton and detritus. The animal inside is essentially a shrimp-like body lying on its back within a mineral house.
Being cemented in place creates a reproductive problem, solved by a very long penis, which in some barnacles exceeds several times body length and is the longest relative to body size of any animal. Most barnacles are hermaphroditic and fertilise neighbours within reach, and some species can also capture drifting sperm from the water.