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Archaeogastropoda An order of Gastropoda which appears in the Lower Cambrian and also includes the extant Patella vulgata (limpet). Gastropods may be subdivided according to their respiratory structures; archaeogastropods, the most primitive gastropods, possess just two gills. Some forms possess a marginal slit near the aperture to facilitate the removal of exhaled water and wastes. This may extend posteriorly for some distance but eventually it is filled with shell material as the animal grows and extends its shell. The filled slit (selenizone) is usually completely plugged. In some species (e.g. Haliotis) the selenizone is represented by a linearly arranged series of apertures (tremata, sing. trema).
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.