A terrestrial habit has evolved at least three times in Gastropoda, as three (Neritimorpha, Caenogastropoda, and Heterobranchia) of the six main gastropod lineages (see below) contain species with a terrestrial or semi-terrestrial lifestyle. The following tree of major gastropod groups follows Bouchet & Rocroi, 2005 [Classification and Nomenclature of Gastropod Families.Malacologia 47 (1-2): 1-397]. This simple tree recognizes the monophyly of the six major groups, but indicates that relationships among the groups are not known with certainty. Their opinion is that no shared derived characters (synapomorphies) have been identified that reliably group any of these lineages with another.
However, some authorities contend there are enough synapomorphies to group some of the lineages. The most common hypothesis is shown in the tree below with the most common name for the two sub-clades, Orthogastropoda and Apogastropoda. This nomenclator describes other igher-level gastropod names and relationships not recognized by Bouchet & Rocroi.
Species of terrestrial snails are not equally distributed among Neritimorpha, Caenogastropoda, and Heterobranchia. The clade Neritimorpha contains two families with terrestrial species, Helicinidae and Hydrocenidae, both in the neritimorh group Cycloneritomorpha. The clade Caenogastropoda contains several families with terrestrial species, within two diferent clades, the Architaenioglossa and the Hypogastropoda. Within the Heterobranchia terrestrial species are limited to a large, diverse clade, the Pulmonata.