Barnacle—Anthony Mercieca/Root Resources

Any of a majority of the 1,000 species of the subclass Cirripedia of marine crustaceans that, as adults, are covered with a shell made of hard calcium-containing plates and are permanently cemented, head down, to rocks, pilings, ships' hulls, driftwood, or seaweed or to the bodies of larger sea creatures, from clams to whales. Barnacles trap tiny particles of food with their cirri, feathery retractable organs that emerge from openings between the shell plates. Adult barnacles commonly are hermaphrodites.

This entry comes from Encyclopædia Britannica Concise.
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