it is actually a crustacean, closely related to crabs and lobsters.
Barnacles are most often seen as roughly circular sessile invertebrates and are permanently attached to the substrate they live on. In their juvenile form they are free-floating, but eventually they attach themselves to any nearby rock, shell, or other object and stay there for the rest of their lives. Their shells are composed of calcite.
Barnacles are often seen on crabs, whales, boats, rocks and on the shells of sea turtles. Although some species of barnacle are parasitic, most barnacle species are harmless, because they are filter feeders and do not interfere with an animal's normal diet and do not harm that animal that they live on in any way. Many species of barnacle are so harmless that in fact, an animal that is covered in them, may not even notice!