Barnacle, a name applied to Crustacea of the division Cirripedia or Thyrostraca. Originally, the name was given to the stalked barnacles (Lepadidae of C. Darwin), which attach themselves in great numbers to drift-wood and other objects floating in the sea and are one of the chief agents in the fouling of ships' bottoms during long voyages. The sessile barnacles (Balanidae of Darwin) or "acorn-shells" are found in myriads, encrusting the rocks between tide-marks on all coasts. One of the most extraordinary and persistent myths of medieval natural history, dating back to the 12th century at least, was the cause of transferring to these organisms the name of the barnack or bernacle goose (Bernicla branta). This bird is a winter visitor to Britain, and its Arctic nesting-places being then unknown, it was fabled to originate within the shell-like fruit of a tree growing by the sea-shore. In some variants of the story this shell is said to grow as a kind of mushroom on rotting timber in the sea, and is obviously one of the barnacles of the genus Lepas. Even after the scientific study of zoology had replaced the fabulous tales of medieval writers, it was a long time before the true affinities of the barnacles were appreciated, and they were at first classed with the Mollusca, some of which they closely resemble in external appearance.
It was not till Vaughan Thompson demonstrated, in 1830, their development from a free-swimming and typically Crustacean larva that it came to be recognized that, in Huxley's graphic phrase, "a barnacle may be said to be a Crustacean fixed by its head and kicking the food into its mouth with its legs." For a systematic account of the barnacles and their allies, see the article Thyrostraca.
(W. T. Ca.)