Limpets are aquatic snails with a shell that is broadly conical in shape and a strong, muscular foot. Although all limpets are members of the class Gastropoda, limpets are polyphyletic, meaning the various groups which are referred to as "limpets" have descended independently from different ancestral gastropods. This general category of conical shell is known as "patelliform" (dish-shaped). All members of the large and ancient marine clade, patellogastropoda are limpets. Within that clade, the patellidae family in particular are often referred to as "true limpets". Other groups, not in the same family, are also called limpets of one type or another, due to the similarity of their shells' shape. Examples include the Fissurellidae ("keyhole limpet") family, which is part of the Vetigastropoda clade (many other members of the Vetigastropoda don't have the morphology of limpets) and the Siphonariidae ("false limpets"), which use a siphon to pump water over their gills.