Opisthobranchs () are a large and diverse group of specialized complex
gastropods previously united in the subclass
Opisthobranchia, but no longer considered to represent a
monophyletic grouping because it contains land snails and slugs, the
Pulmonata.
Euopisthobranchia is a revised collection of opisthobranchs that
is monophyletic, but this group leaves out some "traditional" opisthobranchs including the
Sacoglossa and the
Acochlidiacea. The subclass
Heterobranchia now contains the species which previously were assigned to Opisthobranchia plus the Pulmonata.
