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The 'gastropods', also previously known as 'gasteropods', or 'univalves', and more commonly known as snails are the largest and most successful class of mollusks, with 60,000-75,000 known living species. This class of animals is second only to insects in its size and diversity.
The class gastropoda includes very large numbers of marine snails and sea slugs, as well as the freshwater snails, and the terrestrial (land) snails and slugs. Although the word snail can be applied to all the members of this class, commonly the word snail is restricted only to those species that have an external shell. Those without a shell or with only an internal shell are often known as slugs.
The marine shelled species of gastropod include abalone, cowries, conches, periwinkles, whelks, and the majority of other sea snails that have coiled seashells, as well as some families of species where the shell is coiled only in the larval stage, such as the limpets.

Contents

Description

Geological history

Taxonomy

Proposed classification, down to the level of superfamily

Footnotes

References

External links

Description

Main articles: Gastropod shell

Snails are distinguished by torsion, a process where the visceral mass of the animal permanently rotates 180º to one side during development, such that the anus is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon. Torsion is present in all gastropods, but the opistobranch gastropods are secondarily de-torted.
Gastropods typically have a well-defined head with two or four sensory tentacles, and a ventral foot, which gives them their name (Greek ''gaster'', stomach, and ''poda'', feet). The eyes that may be present at the tip of the tentacles range from simple ocelli that cannot project an image (simply distinguishing light and dark), to more complex pit and even lens eyes ^[1]. The larval shell of a gastropod is called a protoconch.

Most members have a shell, which is in one piece and is typically coiled or spiraled. This usually opens on the right hand side (as viewed with the shell apex pointing upward). Several species have an operculum which in many species operates as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.
While some of the better-known gastropods are terrestrial, more than two thirds of all species live in a marine environment. Marine gastropods include herbivores, detritus feeders, carnivores and a few ciliary feeders, in which the radula is reduced or absent. In some species which have evolved into endoparasites, such as Parenteroxenos doglieli, the gastropod traits are strongly reduced or absent. The radula is usually adapted to the food that a species eats. The simplest gastropods are the limpets and abalones, both herbivores that use their hard radulas to rasp at seaweeds on rocks.
Many marine gastropods are burrowers and have soft siphons or tubes that extend from the mantle. Sometimes the shell has a siphonal canal to accommodate this structure. These siphons act as snorkels, enabling the animal to continue to draw in a water current containing oxygen and food into their bodies. The siphons are also used to "taste" the water, in order to detect prey from a distance.
Almost all marine gastropods breathe with gills, but many freshwater species, and the majority of terrestrial species, have a pallial lung. The gastropods which have a lung all belong to one group with common descent, the Pulmonata, however, the gastropods with gills are paraphyletic.
Some Sea slugs are brightly coloured, either as a warning, if they are poisonous or contain stinging cells, or to camouflage them on the hydroids, sponges and seaweeds on which many of the species are found. In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs. Some nudibranchs have smooth or warty backs have no visible gill mechanism, such that respiration may likely take place directly through the skin. A few of the sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in close association with their food species.

Geological history

The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian (''Chippewaella'', ''Strepsodiscus''). Early Cambrian forms like ''Helcionella'' and ''Scenella'' are no longer considered gastropods, and the tiny coiled ''Aldanella'' of earliest Cambrian time is probably not even a mollusk. By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. Commonly, fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian genus ''Poleumita'' contains fifteen identified species. Fossil gastropods are less common during the Palaeozoic era than bivalves.
Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive today. By the Carboniferous period many of the shapes we see in living gastropods can be matched in the fossil record, but despite these similarities in appearance the majority of these older forms are not directly related to living forms. It was during the Mesozoic era that the ancestors of many of the living gastropods evolved.
One of the earliest known terrestrial (land-dwelling) gastropods is ''Maturipupa'' which is found in the Coal Measures of the Carboniferous period in Europe, but relatives of the modern land snails are rare before the Cretaceous period when the familiar ''Helix'' first appeared.

In rocks of the Mesozoic era gastropods are slightly more common as fossils, their shell often well preserved. Their fossils occur in beds which were deposited in both freshwater and marine environments. The "Purbeck Marble" of the Jurassic period and the "Sussex Marble" of the early Cretaceous period which both occur in southern England are limestones containing the tightly packed remains of the pond snail ''Viviparus''.
Rocks of the Cenozoic era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.
Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these trails are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods. An example of this is ''Bellerophon'' from the limestones of the Carboniferous period in Europe which may be mistaken for a cephalopod!
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets during the Pleistocene epoch.

Taxonomy

The taxonomy of the Gastropoda is under constant revision, and more and more of the old taxonomy is being abandoned. Nevertheless terms as "opisthobranch" and "prosobranch" are still being used in a descriptive way. In a sense, we can speak of a taxonomic jungle when we go down to the lower taxonomic levels. The taxonomy of the Gastropoda can be different from author to author. But with the arrival of DNA-sequencing, further revisions of the higher taxonomic levels are to be expected in the near future.
According to the traditional classification there are four subclasses. :

★ Prosobranchia (gills in front of the heart).

★ Opisthobranchia (gills to the right and behind the heart).

★ Gymnomorpha (no shell)

★ Pulmonata (with lungs instead of gills)
According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten in terms of strictly monophyletic groups. Integrating these findings into a working taxonomy will be a true challenge in the coming years. At present, it is impossible to give a classification of the Gastropoda that has consistent ranks and also reflects current usage. Convergent evolution, observed at especially high frequency in the Gastropods, may account for the observed differences between phylogenies obtained from morphological data and more recent studies based on gene sequences.
New changes in systematics are made by (Bouchet & Rocroi, 2005).^[2][3]

Proposed classification, down to the level of superfamily

'Class Gastropoda' (''Cuvier, 1797'')

'''Incertæ sedis'''

★ Order Bellerophontida (''fossil'')

★ Order Mimospirina (''fossil'')
'Subclass Eogastropoda' (''Ponder & Lindberg, 1996'') (earlier: Prosobranchia)

★ Order Euomphalida de Koninck 1881 (''fossil'')

★
★
★ Superfamily Macluritoidea

★
★
★ Superfamily Euomphaloidea

★
★
★ Superfamily Platyceratoidea

★ Order Patellogastropoda Lindberg, 1986 (true limpets)

★
★ Suborder Patellina Van Ihering, 1876

★
★
★ Superfamily Patelloidea Rafinesque, 1815

★
★ Suborder Nacellina Lindberg, 1988

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★
★ Superfamily Acmaeoidea Carpenter, 1857

★
★
★ Superfamily Nacelloidea Thiele, 1891

★
★ Suborder Lepetopsina McLean, 1990

★
★
★ Superfamily Lepetopsoidea McLean, 1990
'Subclass Orthogastropoda ' Ponder & Lindberg, 1996 (earlier Prosobranchia, Opisthobranchia)

'''Incertæ sedis'''

★ Order Murchisoniina Cox & Knight, 1960 (''fossil'')

★
★
★ Superfamily Murchisonioidea Koken, 1889

★
★
★ Superfamily Loxonematoidea Koken, 1889

★
★
★ Superfamily Lophospiroidea Wenz, 1938

★
★
★ Superfamily Straparollinoidea

★
★ Grade Subulitoidea Lindström, 1884
Superorder Cocculiniformia Haszprunar, 1987

★
★
★ Superfamily Cocculinoidea Dall, 1882

★
★
★ Superfamily Lepetelloidea Dall, 1882 (deep sea limpets)
Superorder ‘Hot Vent Taxa' Ponder & Lindberg, 1997

★ Order Neomphaloida Sitnikova & Starobogatov, 1983

★
★
★ Superfamily Neomphaloidea McLean, 1981 (hydrothermal vents limpets)

★
★
★ Superfamily Peltospiroidea McLean, 1989
Superorder Vetigastropoda Salvini-Plawen, 1989 (limpets)

★
★
★ Superfamily Fissurelloidea Fleming, 1822 (keyhole limpets)

★
★
★ Superfamily Haliotoidea Rafinesque, 1815 (abalones)

★
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★ Superfamily Lepetodriloidea McLean, 1988 (hydrothermal vent limpets)

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★ Superfamily Pleurotomarioidea Swainson, 1840 (slit shells)

★
★
★ Superfamily Seguenzioidea Verrill, 1884

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★
★ Superfamily Trochoidea Rafinesque, 1815 (top shells)
Superorder Neritaemorphi Koken, 1896

★ Order Cyrtoneritomorpha (''fossil'')

★ Order Neritopsina Cox & Knight, 1960

★
★
★ Superfamily Neritoidea Lamarck, 1809
Superorder Caenogastropoda Cox, 1960

★ Order Architaenioglossa Haller, 1890

★
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★ Superfamily Ampullarioidea J.E. Gray, 1824

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★ Superfamily Cyclophoroidea J.E. Gray, 1847 (terrestrials)

★ Order Sorbeoconcha Ponder & Lindberg, 1997

★
★ Suborder Discopoda P. Fischer, 1884

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★ Superfamily Campaniloidea Douvillé, 1904

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★ Superfamily Cerithioidea Férussac, 1822

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★ Suborder Hypsogastropoda Ponder & Lindberg, 1997

★
★ Infraorder Littorinimorpha Golikov & Starobogatov, 1975

★
★
★ Superfamily Calyptraeoidea Lamarck, 1809

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★ Superfamily Capuloidea J. Fleming, 1822

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★ Superfamily Carinarioidea Blainville, 1818 (formerly called Heteropoda)

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★ Superfamily Cingulopsoidea Fretter & Patil, 1958

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★ Superfamily Cypraeoidea Rafinesque, 1815 (cowries)

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★ Superfamily Ficoidea Meek, 1864

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★ Superfamily Laubierinoidea Warén & Bouchet, 1990

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★
★ Superfamily Littorinoidea (Children), 1834 (periwinkles)

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★ Superfamily Naticoidea Forbes, 1838 (moon shells)

★
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★ Superfamily Rissooidea J.E. Gray, 1847 (Risso shells) (includes genus oncomelania, schistosomiasis transmission vector)

★
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★ Superfamily Stromboidea Rafinesque, 1815 (true conchs)

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★ Superfamily Tonnoidea Suter, 1913

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★ Superfamily Trivioidea Troschel, 1863

★
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★ Superfamily Vanikoroidea J.E. Gray, 1840

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★ Superfamily Velutinoidea J.E. Gray, 1840

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★ Superfamily Vermetoidea Rafinesque, 1815 (worm shells)

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★ Superfamily Xenophoroidea Troschel, 1852 (carrier shells)

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★ Infraorder Ptenoglossa J.E. Gray, 1853

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★
★ Superfamily Eulimoidea Philippi, 1853

★
★
★ Superfamily Janthinoidea Lamarck, 1812

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★ Superfamily Triphoroidea J.E. Gray, 1847

★
★ Infraorder Neogastropoda Thiele, 1929

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★
★ Superfamily Buccinoidea (whelks, false tritions)

★
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★ Superfamily Cancellarioidea Forbes & Hanley, 1851

★
★
★ Superfamily Conoidea Rafinesque, 1815

★
★
★ Superfamily Muricoidea Rafinesque, 1815
Superorder Heterobranchia J.E. Gray, 1840

★ Order Heterostropha P. Fischer, 1885

★
★
★ Superfamily Architectonicoidea J.E. Gray, 1840

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★
★ Superfamily Nerineoidea Zittel, 1873 (''fossil'')

★
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★ Superfamily Omalogyroidea G.O. Sars, 1878

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★ Superfamily Pyramidelloidea J.E. Gray, 1840

★
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★ Superfamily Rissoelloidea J.E. Gray, 1850

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★ Superfamily Valvatoidea J.E. Gray, 1840

★ Order Opisthobranchia Milne-Edwards, 1848

★
★ Suborder Cephalaspidea P. Fischer, 1883

★
★
★ Superfamily Acteonoidea D'Orbigny, 1835

★
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★ Superfamily Bulloidea Lamarck, 1801

★
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★ Superfamily Cylindrobulloidea Thiele, 1931 (has to be included in the Sacoglossa)

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★ Superfamily Diaphanoidea Odhner, 1914

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★ Superfamily Haminoeoidea Pilsbry, 1895

★
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★ Superfamily Philinoidea J.E. Gray, 1850

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★ Superfamily Ringiculoidea Philippi, 1853

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★ Suborder Sacoglossa Von Ihering, 1876

★
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★ Superfamily Oxynooidea H. & A. Adams, 1854

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★ Suborder Anaspidea P. Fischer, 1883 (sea hares)

★
★
★ Superfamily Akeroidea Pilsbry, 1893

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★
★ Superfamily Aplysioidea Lamarck, 1809

★
★ Suborder Notaspidea P. Fischer, 1883

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★
★ Superfamily Tylodinoidea J.E. Gray, 1847

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★ Superfamily Pleurobranchoidea Férussac, 1822

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★ Suborder Thecosomata Blainville, 1824 (sea butterflies)

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★ Infraorder Euthecosomata

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★ Superfamily Limacinoidea

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★
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★ Superfamily Cavolinioidea

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★ Infraorder Pseudothecosomata

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★ Superfamily Peraclidoidea

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★
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★ Superfamily Cymbulioidea

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★ Suborder Gymnosomata Blainville, 1824 (sea angels)

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★ Family Clionidae Rafinesque, 1815

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★ Family Cliopsidae Costa, 1873

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★ Family Hydromylidae Pruvot-Fol, 1942

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★ Family Laginiopsidae Pruvot-Fol, 1922

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★ Family Notobranchaeidae Pelseneer, 1886

★
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★ Family Pneumodermatidae Latreille, 1825

★
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★ Family Thliptodontidae Kwietniewski, 1910


★
★ Suborder Nudibranchia Blainville, 1814 (nudibranchs)

★
★ Infraorder Anthobranchia Férussac, 1819

★
★
★ Superfamily Doridoidea Rafinesque, 1815

★
★
★ Superfamily Doridoxoidea Bergh, 1900

★
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★ Superfamily Onchidoridoidea Alder & Hancock, 1845

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★ Superfamily Polyceroidea Alder & Hancock, 1845

★
★ Infraorder Cladobranchia Willan & Morton, 1984

★
★
★ Superfamily Dendronotoidea Allman, 1845

★
★
★ Superfamily Arminoidea Rafinesque, 1814

★
★
★ Superfamily Metarminoidea Odhner in Franc, 1968

★
★
★ Superfamily Aeolidioidea J.E. Gray, 1827

★ 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

★
★
★ Superfamily Chilinoidea H. & A. Adams, 1855

★
★
★ Superfamily Glacidorboidea Ponder, 1986

★
★
★ Superfamily Lymnaeoidea Rafinesque, 1815

★
★
★ Superfamily Planorboidea Rafinesque, 1815

★
★
★ Superfamily Siphonarioidea J.E. Gray, 1840

★
★ Suborder Eupulmonata Haszprunar & Huber, 1990

★
★ Infraorder Acteophila Dall, 1885 (= formerly Archaeopulmonata)

★
★
★ Superfamily Melampoidea Stimpson, 1851

★
★ Infraorder Trimusculiformes Minichev & Starobogatov, 1975

★
★
★ Superfamily Trimusculoidea Zilch, 1959

★
★ Infraorder Stylommatophora A. Schmidt, 1856 (land snails)

★
★ Subinfraorder Orthurethra

★
★
★ Superfamily Achatinelloidea Gulick, 1873

★
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★ Superfamily Cochlicopoidea Pilsbry, 1900

★
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★ Superfamily Partuloidea Pilsbry, 1900

★
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★ Superfamily Pupilloidea Turton, 1831

★
★ Subinfraorder Sigmurethra

★
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★ Superfamily Acavoidea Pilsbry, 1895

★
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★ Superfamily Achatinoidea Swainson, 1840

★
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★ Superfamily Aillyoidea Baker, 1960

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★ Superfamily Arionoidea J.E. Gray in Turnton, 1840

★
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★ Superfamily Buliminoidea Clessin, 1879

★
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★ Superfamily Camaenoidea Pilsbry, 1895

★
★
★ Superfamily Clausilioidea Mörch, 1864

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★ Superfamily Dyakioidea Gude & Woodward, 1921

★
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★ Superfamily Gastrodontoidea Tryon, 1866

★
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★ Superfamily Helicoidea Rafinesque, 1815

★
★
★ Superfamily Helixarionoidea Bourguignat, 1877

★
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★ Superfamily Limacoidea Rafinesque, 1815

★
★
★ Superfamily Oleacinoidea H. & A. Adams, 1855

★
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★ Superfamily Orthalicoidea Albers-Martens, 1860

★
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★ Superfamily Plectopylidoidea Moellendorf, 1900

★
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★ Superfamily Polygyroidea Pilsbry, 1894

★
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★ Superfamily Punctoidea Morse, 1864

★
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★ Superfamily Rhytidoidea Pilsbry, 1893

★
★
★ Superfamily Sagdidoidera Pilsbry, 1895

★
★
★ Superfamily Staffordioidea Thiele, 1931

★
★
★ Superfamily Streptaxoidea J.E. Gray, 1806

★
★
★ Superfamily Strophocheiloidea Thiele, 1926

★
★
★ Superfamily Trigonochlamydoidea Hese, 1882

★
★
★ Superfamily Zonitoidea Mörch, 1864

★
★
★ ? Superfamily Athoracophoroidea P. Fischer, 1883 (= Tracheopulmonata)

★
★
★ ? Superfamily Succineoidea Beck, 1837 (= Heterurethra)
Other extant classes of the Mollusca are Bivalvia, Scaphopoda, Aplacophora, Polyplacophora, Monoplacophora and Cephalopoda.

Footnotes

1. Lexikon der Biologie, , Klaus-Jürgen, Götting, Spektrum Akademischer Verlag, 1994, ISBN 3-86025-156-2
2. Bouchet, P.; Rocroi, J.-P. (Ed.); Frýda,J.; Hausdorf,B.; Ponder, W.; Valdes, A.; Warén, A. (2005). Classification and nomenclator of gastropod families. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. ISBN 3-925919-72-4. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
3. Poppe G.T. & Tagaro S.P. 2006. The new classification of Gastropods according to Bouchet & Rocroi, 2005. Visaya, février 2006: 10 pp. http://www.journal-malaco.fr/bouchet&rocroi_2005_Visaya.pdf

References

★ Paul Jeffery. ''Suprageneric classification of class GASTROPODA''. The Natural History Museum, London, 2001

★ Ponder & Lindberg, ''Towards a phylogeny of gastropod molluscs; an analysis using morphological characters''. ''Zoological Journal of the Linnean Society'', '119' 83-2651; 1997

★ Testing the utility of partial COI sequences for phylogenetic (full text on line), Elpidio A. Remigio and Paul D.N. Hebert, , , Molecular Phylogenetics and Evolution, 2003

External links

★ Conchology.be consult over 2,100,000 figured gastropods and other shells

★ Taxonomy

★ Gastropod Classification compiled by Paul Jeffery

★ International Code of Zoological Nomenclature 4th edition, 2000

★ Gastropods in captivity

★ Reconstructions of fossil gastropods at www.emilydamstra.com