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In zoology, the Opisthobranchia ( Milne Edwards, 1848) (also known as opisthobranchs) used to be a subclass of gastropods, within the phylum Mollusca, but they are now treated as an order.

They are highly evolved gastropods, characterised by a single gill behind the heart, from which the subclass derives its name ( Greek opistho-, behind; brankhia, gills) and two pairs of tentacles.

They are principally soft-bodied marine creatures with a small or absent shell and no operculumIn Biology operculum (Latin for "little lid") has been used to describe several completely separate features. Gastropods The operculum (plural : opercula or operculums) of gastropods is a corneous plate at the opening of the shell, attached dorsally to th. Their shell has undergone detorsion and they have essentially evolved back to the bilateral symmetry of their ancestors. There is no marked distinction between head and mantle. The tentacles, situated close to the mouth, are used for orientation. Behind them you can find the rhinophores, olfactory organs often with complex forms. The middle part of the foot is the sole, used for locomotion. The sides of the foot have evolved into parapodia, fleshy winglike outgrowths. In several suborders, such as the Thecosomata and Gymnosomata , these parapodia are used to move in a swimming motion.

Many have brilliant colors, warning their predators to stay away. These wonderful creatures are hard to study, because their presence is so transitory, turning up, sometimes in very large numbers, at unexpected moments.

Members of this order include groups such as the canoe shells, sea butterfliesLimacinidae Cavoliniidae Clioidae Creseidae Cuvierinidae Praecuvierinidae Peraclididae Cymbuliidae Desmopteridae Sea butterflies or flapping snails, are holoplanktonic mollusks ( Mollusca, Gasteropoda), belonging to the suborder Thecosomata ( Blainville,, sea hareSuperfamily Akeroidea Akeridae Superfamily Aplysioidea Aplysiidae Sea hares (also called sea slugs are small marine gastropod molluscs of the suborder Anaspidea (P. Fisher, 1883) in the subclass Orthogastropoda, class Gastropoda, phylum Mollusca. Their cos, and nudibranchSee text Nudibranchs (Nudibranchia), form the largest suborder of the order Opisthobranchia, subclass Orthogastropoda, class Gastropoda in the phylum Mollusca. The Nudibranchs are represented by more than 3,000 species. Description These sea slugs are jels

The taxonomy of the gastropods, and their phylogenetic understandingA phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually of a species. A major task of systematics is to determine the ancestral relationships among known species (both living and extinct), and the most commonly used method has been evolving rapidly in the last few years. The old classification ( J. ThieleJohannes Thiele ( 1860 1935) was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde ( English edition published by the Smithsonian under the title Handbook of Systematic Malacology is a standard work. The classifi 1929-1935), with the class Gastropoda divided into three subclasses ProsobranchiaProsobranch signifies “ gills in front of the heart”, this in contrast with opisthobranch : “gills to the right and behind the heart” The majority of the gastropods (marine snails, land snails) are prosobranch. They have their gill, mantle cavity and anus, Opisthobranchia and Pulmonata, is no longer accepted. It is speculated that the Opisthobranchia may be paraphyletic (Haszprunar, 1985), having given rise to the Pulmonata, although evidence is still somewhat disputed. Pulmonata may be the sister group to a particular opisthobranch taxon. The Opisthobranchia are therefore a non-clade and can no longer be accepted as a taxon. They are now included in the subclass Orthogastropoda, where they have become an order. More information is given under the entry Gastropoda.

However, one can still encounter this old classification in many manuals (older and newer) and on most websites.

The term ‘opisthobranch’, when not describing the order, can still be used in a descriptive way, meaning ‘a gastropod with the “gills to the right and behind the heart”