Alexander Dyce, A General Glossary to Shakespeare's Works

barnacles THE TEMPEST, iv. 1. 247. “Caliban's barnacle is the clakis or tree-goose” (DOUCE) . “Barnacle. A multivalve shell-fish [lepas anatifera, Linn.] growing on a flexible stem, and adhering to loose timber, bottoms of ships, etc.; anciently supposed to turn into a Solan goose; possibly because the name was the same. . . . Sometimes the barnacles were supposed to grow on trees, and thence to drop into the sea, and become geese; as in Drayton's account of Furness, Polyolb. Song 27, p. 1190 [p. 136, ed. 1622]. From this fable Linnæus has formed his trivial name anatifera, Goose or Duckling-bearing. See Donovan's British Shells, Plate 7, where is a good description of the real animal, and an excellent specimen of the fabulous account from Gerard's Herbal.” Nares's Gloss.