Barnacles both attach themselves to and burrow inside objects. You may have seen them on a variety of surfaces—covering pilings and rocks or clinging to the hull of a ship or the outside of a crab; they even attach themselves to larger animals like whales. Barnacles begin their lives as swimming larvae and eventually attach to a hard surface (such as rocks, wood, animal shells, coral, etc.—and sometimes, for parasitic species, softer homes like a shark’s belly!). Once anchored, they metamorphose some of their swimming legs into feathery retractile organs called cirri, which they use to capture little food particles floating by.

Source: Seattle Aquarium