Barnacles are the sharp conical bumps that cover the rocks and the bottoms of boats. They are also small enough to attach to other shells such as limpets. They are actually small crustaceans, related to crabs and shrimps. You can see the kite-shaped openings, which will open up when the tide comes so their legs can catch plankton to feed on. There looks to be two different sorts of barnacle in this photo...