This is one of the six major gastropod lineages and contains a large number of terrestrial species in two of the three main subgroups, Architaenioglossa and Hypogastropoda. The third subgroup, Sorbeoconcha, are mostly marine snails, but a few members of the family Thiaridae have a pulmonary cavity and inhabit brackish, muddy waters.
The relationships among the three subgroups are in dispute. One hypothesis is shown below. Within the Architaenioglossa terrestrial species are found in several closely related families, the classification and taxonomy of which are somewhat unsettled. Bouchet & Rocroi, 2005 [Classification and Nomenclature of Gastropod Families. Malacologia 47 (1-2): 1-397] recognize six families under the superfamily Cyclophoroidea with terrestrial species, Cyclophoridae, Diplommatinidae, Maizaniidae, Megalomastomatidae, Neocyclotidae and Pupinidae. The Hypogastropoda contains three subgroups and only one of these, Littorinimorpha, contains terrestrial species. They are found in two superfamilies, the Littorinoidea and Rissoidea. The terrestrial members of the former are found in the family Pomatiidae and the latter in the families Assimineidae, Pomatiopsidae, and Truncatellidae.