A barnacle is a short salt water shellfish that attaches itself to ship hulls, rocks, docks and other underwater objects. Anyone who goes to the seashore is likely to see barnacles.
A barnacle hatches from an egg as a tiny, free-swimming creature. But soon it fastens itself to any convenient object, such as the hull of a ship, pilling, rock, or even a passing whale.
Once attached, a hard, limy shell grows around the barnacle. The barnacles stays for the rest of its life in the place where it settles. It eats by waving its feathery legs through an opening in the shell to pull tiny sea creatures and plants into its mouth.
The shell has a lid that can be closed in case of danger.
To sailors the barnacle is a trouble. Masses of them clinging to a ship’s hull reduce the ship’s speed. The only way to remove barnacles’ shell is to put the ship in dry dock and scrape its bottom. – Dick Rogers