Pectinodontidae family Lottiidae family Neolepetopsidae † family Daminilidae † family Lepetopsidae In 2007, two years following Bouchet & Rocroi, 2005, Tomoyuki Nakano and Tomowo Ozawa referred to the order Patellogastropoda. Description 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. 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. The teeth are composed of goethite, an iron-based mineral, woven in a particular way into grouped 1μ thick bundles. Many limpets create a home "scar" on the rock to which they always return between tides, the scar becoming a perfect fit even for the always growing cone-shaped shell and providing excellent protection from predators as well as helping to prevent dehydration, though dehydration remains one of the greatest risks to which these molluscs are exposed and may be why they only survive in temperate waters, not tropical ones where they simply become too hot during low tides. They adhere to the substratum via the adhesion/ suction of the stiffened foot against the rock surface to which it bonds each time with a layer of pedal mucus. The majority of limpet species have shells that are less than 3 in (8 cm) in maximum length and many are much smaller. Anatomy The true limpets have an internal structure much like that of other members of Mollusca. 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. The pleural and pedal ganglia each send a nerve cord back through the rest of the body, the pleural nerve cords and the pedal or ventral nerve cords (the latter are embedded in the foot musculature in Patellagastropoda). Just outside the pedal ganglia are each of the two statocysts (though see Bathyacmaea secunda as an exception to this rule). Like the keyhole limpets, the true limpets have retained both kidneys though in Patellagastropoda the kidneys both lie on the animal's right side and the further right of the two— the "right" kidney— is