See text The Patellogastropoda, common name true limpets and historically called the Docoglossa, are members of a major phylogenetic group of marine gastropods, treated by experts either as a clade[2] or as a taxonomic order.

Families that are exclusively fossil are indicated with a dagger †: With the exception of calling Patellogastropoda a clade rather than an order, as was previously the case in Ponder and Lindberg, 1997 the taxon has not changed much, differing more in the arrangement of its content rather than in the overall composition.

[citation needed] Nakano & Ozawa (2007)[3] made many changes in the taxonomy of the Patellogastropoda, based on molecular phylogeny research: Acmaeidae is a synonym of Lottiidae; Pectinodontinae is elevated to Pectinodontidae; new family Eoacmaeidae with the new type genus Eoacmaea is established.

[3] A cladogram based on sequences of mitochondrial 12S ribosomal RNA, 16S ribosomal RNA and cytochrome-c oxidase I (COI) genes showing phylogenic relations of Patellogastropoda by Nakano & Ozawa (2007)[3] and superfamilies based on World Register of Marine Species:[6] Lottiidae (including Acmaea and Niveotectura) Pectinodontidae Lepetidae Nacellidae Patellidae Eoacmaeidae Note that the family Neolepetopsidae is not in the cladogram above, because its members were not genetically analyzed by Nakano & Ozawa (2007).

[3] However, two Neolepetosidae species Eulepetopsis vitrea and Paralepetopsis floridensis were previously analyzed by Harasewych & McArthur (2000),[7] who confirmed their placement within Acmaeoidea/Lottioidea based on analysis of partial 18S rDNA.

[3] Patellogastropoda have flattened, cone-shaped shells, and the majority of species are commonly found adhering strongly to rocks or other hard substrates.

Many limpet shells are covered in microscopic growths of green marine algae, which can make them even harder to see, as they can closely resemble the rock surface itself.

[citation needed] The substance making up the teeth in the radula of limpets is among the strongest biological materials known, with a tensile strength about five times stronger than that of spider silk.

Their diffuse nervous system is oriented around three principal pairs of ganglia—the cerebral, pleural (which are hypoathroid), and pedal—located in the animal's snout and surrounding its esophagus in a ring.

[11] They do not have ctenidia, instead obtaining oxygen through a ring of gill lamellae that encircle the mantle just inside the shell edge and from the surface of the roof of the nuchal cavity which is exposed to air when the animal is no longer under water and which is covered in a network of blood vessels all of which eventually carry oxygenated blood and connect to the auricle through a series of veinlets on the animal's left side.

Between these papillae and the heart lies the neural "visceral twist", a nervous condition called streptoneury or chiastoneury, which characterizes many molluscs and all gastropods whose ancient ancestor had an anus located posterior to its head but which now have it positioned much closer because of a change in the arrangement of the shell.

At the posterior ventral end is the large gonad organ which, when ripe, bursts and empties its gametes into the right kidney from which they are then expelled directly into the surrounding water.

They can also "clamp down" against the rock surface with very considerable force when necessary, and this ability enables them to remain safely attached, despite the dangerous wave action on exposed rocky shores.

In some parts of the world, certain smaller species of true limpet are specialized to live on seagrasses and graze on the microscopic algae which grow there.