Opisthobranchs (/əˈpɪsθəˌbræŋks, -θoʊ-/) 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.
Opisthobranch means "gills behind" (and to the right) of the heart. In contrast, Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart.