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The Basommatophora is an “informal group” of pulmonate gastropod snails. Research indicates that this is not a monophyletic grouping. Most are freshwater snails of worldwide distribution, found in ponds, creeks and shallow lakes, which derive from terrestrial pulmonate snails but secondarily evolved to live back in aquatic environments during the Mesozoic. Almost all basommatophoran snails have retained their lung for breathing air, although some use their lung as a gill to extract oxygen from water, and some (e.g. snails of tribe Ancylini, which have a limpet-like shell) have evolved a secondary "pseudogill" for this purpose. Most have thin translucent shells, and most lack an operculum. They are characterized by having their eyes at the base of their retractable tenticular stalks, rather than at the tips, as do the land snails and slugs in the clade Stylommatophora. (Bouchet et al. 2004; Kozloff 1990; Wikipedia 2013; Wood 1982).