The anatomy of some new archaeogastropod limpets (Order Patellogastropoda, Suborder Lepetopsina) from hydrothermal vents

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Abstract
The anatomy of one species of neolepetopsid limpet, Eulepetopsis vitrea (Order Patellogastropoda, Suborder Lepetopsina), is described, and this is followed by comments on differences shown by four other members of the Suborder, Neolepetopsis occulta, N. densata, N. gordensis and Paralepetopsis fioridensis.These limpets possess features linking them directly with limpets from other habitats, in this case with patellogastropods. Shared external features relate to the shallow pallial cavity with no gill; the single fold of the mantle edge, which is sensory, glandular, and has a respiratory function; in the gut the presence of inner as well as outer lips, with a strongly arched dorsal jaw associated with the former; the absence of an epipodium; and the cavity of the protoconch sealed from that of the teleoconch by a septum. The relationship displayed by internal anatomy is even more striking: the docoglossate radula and licker; the oesophageal glands forming a series of pouches, each with its own opening to the oesophagus; the stomach a simple ciliated sac lacking gastric shield, sorting area, and caecum; the enlarged initial part of the intestine; the orientation of the chambers of the heart, which is not traversed by the rectum; the lack of haemocyanin; the opening of the gonad directly to the base of the papilla of the right kidney; the absence of secondary sexual structures; the small left kidney displaying characters of a nephridial gland; and the degree of concentration of the main ganglia and the arrangement of the visceral loop. This aggregate of characters shows that the neolepetopsids cannot be separated from patellogastropods.The radula of the neolepetopsid differs from that of other patellogastropods in its unmineralized and articulating teeth and in the presence of a rachidian: the first feature appears to be a derived character and the other two ancestral. These radular differences contrast with the anatomical homogeneity exhibited not only here but throughout the patellogastropods. Such anatomical differences as do occur are most easily explained as mosaic evolution.