Archaeogastropoda

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Archaeogastropoda (class Gastropoda, subclass Prosobranchia) An order of gastropods that appeared first in the Lower Cambrian and that includes the extant Patella vulgata (limpet). Gastropods may be subdivided according to their respiratory structures; archaeogastropods, the most primitive members of the class, have just two gills, one of which may be reduced or absent, and some forms have a marginal slit near the aperture through which water and wastes are removed.

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