limpet, marine gastropod mollusk with a simple, flattened, conical shell, found in cooler waters of the Atlantic and the Pacific oceans. Certain species creep over rocks, feeding on algae during high tides, but when the tide recedes they return instinctively to the same spot occupied previously, to await the return of high water. The muscular foot clings so powerfully that limpets are found in wave-swept areas where few other forms of life can survive. The keyhole limpet is named for its central opening, through which respiratory currents pass. Limpets range up to 4 in. (10 cm) in length, but most are smaller; there are several freshwater species. Limpets are classified in the phylum Mollusca, class Gastropoda, order Archeogastropoda.