Los opistobranquios (Opisthobranchia) son un orden de moluscos gasterópodos. El nombre deriva del griego opisthen, detrás, y significa branquias atrás, al contrario de los Prosobranchia (del griego proso, hacia adelante) que significa branquias adelante. Su masa visceral presenta solo una torsión de 90º (y no de 180º) respecto al conjunto cabeza-pie, lo que los distingue de los prosobranquios. Son exclusivamente marinos y su concha está reducida o ausente.

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Opisthobranchs (/əˈpɪsθəˌbræŋks, -θoʊ-/) are a large and diverse group of specialized complex gastropods that used to be united in the subclass Opisthobranchia, but are 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 used to be assigned to Opisthobranchia plus the Pulmonata. The Opisthobranchia included species in the order Cephalaspidea (bubble shells and headshield slugs), the sacoglossans, anaspidean sea hares, pelagic sea angels, sea butterflies, and many families of the Nudibranchia. 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. Opisthobranchia are known from as early as the Carboniferous.

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