Barnacle
bar'na-kl, a degenerate marine animal. Barnacles are related closely to the crayfish, but in appearance they more nearly resemble the clam. In fact, naturalists formerly considered them mollusks. There are at least three classes of barnacles. The common barnacle is known also as the ship barnacle and goose barnacle. The second kind is the acorn barnacle. The third class includes a number of small parasitic forms.
The common barnacle has a peculiar life history. Larvae escape from the egg cases of the parent, and, after moulting several times, become very much like little water fleas. After a period of free swimming, the little "flea" settles down, head first, on some floating object to which it becomes firmly glued by a secretion from cement glands. As the body develops, it assumes a stalk-like form from two to several inches in length. The end of the body, the tail end, we should remember, secretes a shell of five valves, giving the barnacle an appearance not unlike that of a clam clinging to an object by a long foot.
The body is indistinctly jointed. The mouth is situated on one side of the body and is sheltered by the "shell." There is no perceptible abdomen; there is no well-defined circulatory system, no special breathing organs. As the animal has no further use for legs, the three pairs of organs that are used by the crayfish for scrambling have degenerated into flippers used for waving food into the mouth and the food canal. Huxley describes the barnacle as a crustacean, fastened by its head, and kicking the food into its mouth with its legs. The food consists of small marine animals.
Early naturalists in some way connected this barnacle with the barnacle goose, whence the name goose barnacle. Barnacles attach themselves to the bottoms of ships in such numbers as to impede their motion in the water. A ship returning from a voyage of a few weeks or months may be fouled with a perfect forest of these clinging, swinging creatures. When the U. S. battleship South Dakota was drydocked at Mare Island in 1909, 600 tons of barnacles were scraped from its bottom. The cost of docking a ship and cleaning its hull is an expense to be expected by the merchant.
The acorn barnacle has no stalk. Its shell is composed of more pieces. It does not attach itself to ships as a rule, but it may be found in great numbers attached to rocks along the seashore.
Close relatives of the common or goose barnacle and of the acorn barnacle live in or upon the bodies of other living animals, and are therefore said to be parasitic. Some of these parasites resemble in appearance the common or acorn barnacle; but others have lost all their limbs and even the mouth, so that they appear like a formless lump or tumor on the body of the host. They absorb nourishment by means of a number of thread-like projections which proceed from their bodies into the body of the host.
The term barnacle has been extended to almost anything that hangs on and becomes an impediment. In some harbors, where no pilots are needed, local laws require an incoming ship to take aboard a pilot and pay him for his services. A hanger-on of this sort may be called a barnacle. An officeholder who retains his office tenaciously and beyond his term of usefulness, may be termed a barnacle. A needless dependent of any sort, in fact, may be characterized in the same way.