Despite their shelly texture and shell-like appearance, barnacles are not molluscs but belong to the Crustacea, which also include crabs, shrimps, crayfish, and lobsters. The larval history of a barnacle shows that it is related to the shrimp; in fact, at first the larval barnacle is a free-swimming shrimplike creature that later fastens itself upside down to a solid surface and then proceeds to grow a shell about itself. The feelerlike processes which often protrude from the top of a barnacle are the former legs modified to rake in food to the mouth. The SMALL BARNACLE (Elminius modestus) is the trouble-some pest that quickly covers most surfaces immersed in the sea: rocks, wharf piles, bottoms of boats; in fact, any available smooth surface. A very different looking creature (Lepas anatifera) or goose barnacle belongs to the open sea and attaches itself to drifting objects by means of a long fleshy “stalk”. Another curious barnacle (Coronula diadema) is found attached only to the edges of the jaws and flippers of one species of whale, the humpback. The basal edge of this large shelly barnacle is formed of thin plates which become embedded into the skin of the whale.
by Arthur William Baden Powell, Assistant Director, Auckland Institute and Museum.