LIMPET, a gastropod mollusk, with a low, conical, non-spiral shell; properly a representa tive of the families Patellidce and Actnceidce. The former has no ctenidia or true gills, but breathe by means of a ring of special •ranchial filaments between the mantle and the foot; in the latter the left cteniciium functions as a gill and there is no accessory ring. By means of a muscular, sucker-like foot, the limpets adhere so firmly to rocks near low-water mark that they defy the beating of the heaviest surf and are difficult to detach without injury. At high tide they move about in search of the alga: on which they feed, but are said to return to exactly the same place and position, the muscle in time wearing a smooth spot or ((form') on the rock, and the shell becoming adapted to its irregularities. A widely distributed circum polar species (Acme° testudinalis) is common on the New England coast, and may he recog nized by its low, conical, smooth shell with the eccentric apex slightly turned forward. In Europe limpets are utilized as food and in this country for bait. The key-hole limpets belong to the family Fissure/lids, in which the shell is usually perforated like a key-hole at the apex to permit the protrusion of a process of the mantle. Numerous species occur in the littoral
zone of warm seas. The Haliotidee, ear-lim pets or abalones (q.v.), are closely related. The cup-and-saucer limpets and slipper-limpets (Calyptrceidce) have flat shells usually provided with an internal lip or shelf. Several species of Crepidula, having interesting commensalistic habits, are abundant on our coasts, and are known to fishermen as °half-decks.° Finally the fresh-water limpets (Ancylus and Gund lachia) belong to the pulmonate family Lim Kreider Numerous species of these true air breathing limpets are found abundantly on stones and plants in the fresh-water streams and ponds of the United States. They feed on confervae. Limpets, especially Acme° and Patella, are of very ancient race, having existed almost unchanged since the Silurian Age. Consult Cooke, ((Molluscs') ((Cambridge Natu ral History,' Vol. III, London 1895) ; 'Stand ard Natural History' (Vol. I); Arnold, Beach at Ebb-tide' (New York 1901) ; Mayer, 'Sea-shore Life' (New York 1906).