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Limpet is a common name most often applied to true limpets, a taxonomic group of sea snails, marine gastropod molluscs which have a simple broadly conical shell that, unlike the shell of most snails, is not coiled.
True limpets are often found living on hard surfaces in the intertidal zone. Although true limpets are not permanently attached to a hard surface like a barnacle, when they are disturbed or need to resist strong wave action, limpets cling extremely tightly and strongly to the hard surface on which they live, using their muscular foot. A true limpet will often allow itself to be destroyed rather than give up its hold on a rock.
All of the true limpets are marine and have gills. However, because the feature of a simple conical shell has arisen and re-arisen independently many times in gastropod evolution, limpets form part of many different evolutionary lineages and are found in many different environments. There are some saltwater limpets that breathe air, and there are freshwater limpets which originally had a lung, being descended from air-breathing land snails. In the small freshwater limpets the lung has been secondarily modified to be able to absorb oxygen from water.
The common name "limpet" is also used for a number of different (not very closely related) groups of sea snails and freshwater snails (aquatic gastropod mollusks). Thus the common name "limpet" has very little taxonomic significance in and of itself; the name is applied not only to true limpets (the Patellogastropoda), but also to all snails that have a simple shell that is broadly conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snail. In other words the shell of all limpets is "patelliform", which means the shell is shaped more or less like the shell of most true limpets. The term "false limpets" is used for some (but not all) of the other groups that have a conical shell but are not true limpets.
Thus, the name limpet is used to describe various extremely diverse groups of gastropods that have independently evolved a shell of the same basic shape (see convergent evolution). And although the name "limpet" is given on the basis of a limpet-like or "patelliform" shell, the several groups of snails that have a shell of this type are not at all closely related to one another.
The limpets are grouped into the following clades:
Most marine limpets have gills, whereas all freshwater limpets and a few marine limpets have a mantle cavity adapted to breathe air and function as a lung (and in some cases again adapted to absorb oxygen from water). All these kinds of snail are only very distantly related.
The humorous author Edward Lear wrote "Cheer up, as the limpet said to the weeping willow" in one of his letters. Simon Grindle wrote an illustrated book said to be "in the great tradition of Edward Lear and Lewis Carroll" called The Loving Limpet and Other Peculiarities.