Phylogenetic systematics and classification of the Sacoglossa (Mollusca, Gastropoda, Opisthobranchia)

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Abstract
Character analysis is performed based on anatomical information on 95 species of Sacoglossa (Mollusca, Opisthobranchia). Functional and developmental information is included, where available. Phylogenetic analysis to generic level is performed. The shelled Oxynoacea is a monophyletic group containing three families, Volvatellidae, Juliidae and Oxynoidae. The family Cylindrobullidae is excluded from the Sacoglossa, but forms a sister taxon. The non-shelled Placobranchacea is also a monophyletic group composed of two monophyletic superfamilies, Placobranchoidea (= Elysioidea) and Limapontioidea (= Stiligeroidea; = Polybranchoidea). Relationships among most genera included in the Placobranchoidea remain unresolved. The family Boselliidae is retained until further examination of the included species has shown whether it should be included in the Placobranchidae. The Platyhedylidae, comprising the generaPlatyhedyleandGascoignella, is included in the Placobranchoidea. Of the families included in the Limapontioidea, the ‘Polybranchiidae’ appears to be paraphyletic, though monophyly could be ‘forced’ by applying extra weight to four characters. The Hermaeidae forms a monophyletic group as does the Limapontiidae. The latter family may contain two or three subfamilies, but this must await further anatomical studies. The families Alderiidae and Costasiellidae are superfluous, the genera clustering within the Limapontiidae.