Barnacles are sessile crustaceans, which means they do not move around. Larvae are free-swimming for a short time, and then they attach their heads to a hard surface (shell, log, or rock). They then build a protective shell around themselves out of calcite, a mineral composed of calcium carbonate. Barnacles feed when covered by water by extending feathery legs to trap food particles, and can close off their shell entirely when water levels drop during low tide. They retain sufficent moisture inside the shell to last until water covers them again at high tide. This adaptation allows barnacles to live successfully in intertidal areas.
M. J. O'Donnell