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Mesogastropoda (class Gastropoda, subclass Prosobranchia) An order of prosobranch gastropods in which individuals possess one pectinibranch gill and one kidney. The shell is generally helical, the aperture being entire. The morphology of the radula is variable, and the sexes are separate. This large order contains terrestrial, marine, and freshwater forms, and first appeared in the Lower Ordovician.