An order or suborder of the gastropod molluscan subclass Pulmonata containing about 20,500 species that are grouped into 56 families. Nearly all land snails without an operculum are stylommatophorans. They have eyespots on the tips of a pair of retractile tentacles, hermaphroditic reproduction with partial fusion of the male and female system, a single external genital orifice (or, rarely, two openings that are side by side), and, normally, a second pair of retractile tentacles that function as chemoreceptors. All are air-breathing and most are truly terrestrial. Three orders, based on excretory structures, are recognized. The more primitive Orthurethra and Mesurethra have less efficient water conservation devices than do the more specialized members of the order Sigmurethra, in which a closed ureter functions in water conservation. The latter specialization apparently is a prerequisite for evolution toward a sluglike structure, since all 16 families with slugs or sluglike taxa belong to the Sigmurethra.