Original: Pulmonata (with a lung instead of gills) According to newer insights based on DNA sequencing,  the taxonomy of the Gastropoda must be rewritten in terms of strictly monophyletic groups.
Simplified: Gymnomorpha (no shell) Pulmonata (with lungs instead of gills) According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten in terms of strictly monophyletic groups.