[1] Euopisthobranchia is a monophyletic portion of the Opisthobranchia as that taxon was traditionally defined but is not a replacement name for that group as several marine opisthobranch orders including Nudibranchia, Sacoglossa and Acochlidiacea are not included.

[1] Euopisthobranchia consist of the following taxa:[1] Previous studies discussed the gizzard (i.e. a muscular oesophageal crop lined with cuticula) with gizzard plates as homologous apomorphic structures supporting a clade composed of Cephalaspidea s.s., Pteropoda and Anaspidea.

[1] A gizzard with gizzard plates probably originated in herbivorous taxa in which it worked like a grinding mill, thus might be secondarily reduced in carnivorous groups within Cephalaspidea s.s. and Gymnosomata.

[1] This contradicted Salvini-Plawen and Steiner, who had proposed the gizzard to be a synapomorphy of the larger clade of Paratectibranchia (Pteropoda, Cephalaspidea and Anaspidea) and Eleutherobranchia, secondarily lost in Nudipleura but still present in Umbraculoidea.

[1] A cladogram showing phylogenic relations of the Heterobranchia as proposed by Jörger et al. (2010):[1] Lower Heterobranchia (including Acteonoidea) - Lower Heterobranchia does not form a clade in the study by Jörger et al. (2010):[1] Nudibranchia Pleurobranchomorpha Umbraculoidea Cephalaspidea s.s. Runcinacea Anaspidea Pteropoda (pteropods include two clades: Gymnosomata and Thecosomata) Sacoglossa Siphonarioidea Glacidorboidea Amphiboloidea Pyramidelloidea Hygrophila Acochlidiacea Stylommatophora Systellommatophora Ellobioidea Otinoidea Trimusculoidea This article incorporates CC-BY-2.0 text from the reference.