For information on the anti-aircraft weapons system, see Sea Slug missile

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. Sea Slug was a Royal Navy surface-to-air missile system built by Hawker Siddeley which came into service in the 1950s and was still in use at the time of the Falklands War. ... Zoology (Greek zoon = animal and logos = word) is the biological discipline which involves the study of animals. ... Henri Milne-Edwards (October 23, 1800 - July 29, 1885) was an eminent French zoologist. ... In biology, a subclass is one level below a class. ... Subclass Subclass Eogastropoda     Patellogastropoda Subclass Orthogastropoda   Superorder Cocculiniformia   Superorder Hot Vent Taxa     Neomphaolida   Superorder Vetigastropoda   Superorder Neritaemorphi     Neritopsina   Superorder Caenogastropoda     Architaenioglossa     Sorbeoconcha   Superorder Heterobranchia     Heterostropha     Opisthobranchia     Pulmonata The gastropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 extant species known, comprising the... Phylum (plural: phyla) is a taxon used in the classification of animals, adopted from the Greek phylai the clan-based voting groups in Greek city-states. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. ...

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. Subclass Subclass Eogastropoda     Patellogastropoda Subclass Orthogastropoda   Superorder Cocculiniformia   Superorder Hot Vent Taxa     Neomphaolida   Superorder Vetigastropoda   Superorder Neritaemorphi     Neritopsina   Superorder Caenogastropoda     Architaenioglossa     Sorbeoconcha   Superorder Heterobranchia     Heterostropha     Opisthobranchia     Pulmonata The gastropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 species, and second largest class... gills of a Smooth Newt In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. ... Tentacles can refer to the elongated flexible organs that are present in many invertebrates, and sometimes to the hairs of the leaves of some insectivorous plants. ...

They are principally soft-bodied marine creatures with a small or absent shell and no operculum. Their bodies have undergone detorsion, an evolutionary reversal of the 180° torsion of their immediate ancestors. They have essentially evolved back to the bilateral linear symmetry of their primitive 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. In Biology operculum (Latin for little lid) has been used to describe several completely separate features. ...

Many have brilliant colours, 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 what are commonly known as sea slugs and more specifically groups such as the canoe shells, sea butterflies, sea hares, and nudibranchs. Families Superfamily 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. ... Families Limacinidae Cavoliniidae Clioidae Creseidae Cuvierinidae Praecuvierinidae Peraclididae Cymbuliidae Desmopteridae Sea butterflies, or flapping snails, are holoplanktonic mollusks (Mollusca, Gasteropoda), belonging to the suborder Thecosomata (Blainville, 1824). ... Families Superfamily 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. ... Infraorders See text Nudibranchs (Nudibranchia), form the largest suborder of the order Opisthobranchia, subclass Orthogastropoda, class Gastropoda in the phylum Mollusca. ...

The taxonomy of the gastropods, and their phylogenetic understanding has been evolving rapidly in the last few years. The old classification (J. Thiele 1929-1935), with the class Gastropoda divided into three subclasses Prosobranchia, 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. Taxonomy (from Greek ταξινομία (taxinomia) from the words taxis = order and nomos = law) may refer to either the classification of things, or the principles underlying the classification. ... In biology, Phylogenetics (Greek: phylon = race and genetic = birth) is the taxonomical classification of organisms based on how closely they are related in terms of evolutionary differences. ... Johannes Thiele (1860 - 1935) was a German zoologist specialized in malacology. ... Prosobranch 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. ... Suborders Systellommatophora Basommatophora Eupulmonata Taxonomy Order Pulmonata Cuvier in Blainville, 1814 (pulmonates) Suborder Systellommatophora Pilsbry, 1948 Superfamily Onchidioidea Rafinesque, 1815 Superfamily Otinoidea H. & A. Adams, 1855 Superfamily Rathouisioidea Sarasin, 1889 Suborder Basommatophora Keferstein in Bronn, 1864 (freshwater pulmonates, pond snails) Superfamily Acroloxoidea Thiele, 1931 Superfamily Amphiboloidea J.E. Gray, 1840... This cladogram shows the relationship among various insect groups. ... Taxonomy (from Greek ταξινομία (taxinomia) from the words taxis = order and nomos = law) may refer to either the classification of things, or the principles underlying the classification. ... This cladogram shows the relationship among various insect groups. ... Superorders Cocculiniformia Hot Vent taxa Vetigastropoda Neritaemorphi Caenogastropoda Heterobranchia In their work, which has become a standard reference in the field, Ponder and Lindberg (1997) showed that the Orthogastropoda is one of two subclasses of the Gastropoda the class of molluscs, the other subclass being the Eogastropoda. ... Subclass Subclass Eogastropoda     Patellogastropoda Subclass Orthogastropoda   Superorder Cocculiniformia   Superorder Hot Vent Taxa     Neomphaolida   Superorder Vetigastropoda   Superorder Neritaemorphi     Neritopsina   Superorder Caenogastropoda     Architaenioglossa     Sorbeoconcha   Superorder Heterobranchia     Heterostropha     Opisthobranchia     Pulmonata The gastropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 extant species known, comprising the...

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”

Taxonomy

Order Opisthobranchia Milne-Edwards, 1848 (sea slugs) Alphonse Milne-Edwards (October 13, 1835 _ April 21, 1900) was a French ornithologist. ...

• Suborder Cephalaspidea P. Fischer, 1883 (headshield slugs)
• Suborder Sacoglossa Von Ihering, 1876 (sap-sucking slugs)
• Suborder Anaspidea P. Fischer, 1883 (sea hares)
• Suborder Notaspidea P. Fischer, 1883 (sidegill slugs)
• Suborder Thecosomata Blainville, 1824 (sea butterflies)
• Suborder Gymnosomata Blainville, 1824 (sea angels)
• Suborder Nudibranchia Blainville, 1814 (nudibranchs)
  • Infraorder Anthobranchia Férussac, 1819
  • Infraorder Cladobranchia Willan & Morton, 1984

Reference

• The Sea Slug Forum, a resource from the Australian Museum.
• Haszprunar G., 1985 The Heterobranchia - a new concept of the phylogeny of the higher Gastropoda. Z. f. zool. Systematik u. Evolutionforschung Bd. 23 H. 1:15-37
• Bieler, R., 1990. Haszprunar's "clado-evolutionary" classification of the Gastropoda -- a critique. Malacologia, 31(2): 371-380, 2 tabs. [28 May; G, Haszprunar's response published in Malacologia, 1990, 32(1): 195-202].
• Bieler, R., 1992. Gastropod phylogeny and systematics. Annual Review of Ecology and Systematics, 23: 311-338.