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The gastropods, gasteropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 known living species comprising the snails and slugs as well as a vast number of marine and freshwater species.
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Description
Snails are distinguished by torsion, a process where the body coils to one side during development.
They 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 •. The larval shell of a gastropod is called a protoconch.
Most members have a shell, which is in one piece and typically coiled or spiraled that usually opens on the right hand side (as viewed with the shell apex pointing upward). Several species have an operculum that operates as a trapdoor to close the shell. This is usually made of a horny material, but in some molluscs it is calcareous. In some members, the slugs, the shell is reduced or absent, and the body is streamlined so its torsion is relatively inconspicuous.
While the best-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. 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 siphons or tubes that extend from the mantle and sometimes the shell. These 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 detect prey from a distance. These gastropods breathe with gills, but some freshwater species and almost all terrestric species have developed lungs. While the gastropods with lungs all belong to one group, Pulmonata, the gastropods with gills belong to another, paraphyletic.
Sea slugs are often flamboyantly coloured, either as a warning if they are poisonous or to camouflage them on the corals and seaweeds on which many of the species are found. Their gills are often in a form of feathery plumes on their backs which gives rise to their other name, nudibranchs. Nudibranchs with smooth or warty backs have no visible gill mechanisms and respiration may 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 association with certain species.
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Geological history
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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 mollusc. 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.
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Taxonomy
The taxonomy of the Gastropoda is under constant revision, but 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.
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.
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Proposed classification, down to the level of superfamily
Class Gastropoda (Cuvier, 1797)
Incertæ sedis
Order Mimospirina (fossil)
Subclass Eogastropoda (Ponder & Lindberg, 1996) (earlier: Prosobranchia)
Order Euomphalida de Koninck 1881 (fossil)
Superfamily Macluritoidea
Superfamily Euomphaloidea
Superfamily Platyceratoidea
Suborder Patellina Van Ihering, 1876
Suborder Nacellina Lindberg, 1988
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 Flemming, 1822 (keyhole limpets)
Superfamily Haliotoidea Rafinesque, 1815 (abalones)
Superfamily Lepetodriloidea McLean, 1988 (hydrothermal vent limpets)
Superfamily Pleurotomarioidea Swainson, 1840 (slit shells)
Superfamily Seguenzioidea Verrill, 1884
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
Superfamily Cyclophoroidea J.E. Gray, 1847 (terrestrials)
Superfamily Campaniloidea Douvillé, 1904
Superfamily Cerithioidea Férussac, 1822
Superfamily Calyptraeoidea Lamarck, 1809
Superfamily Capuloidea J. Fleming, 1822
Superfamily Carinarioidea Blainville, 1818 (formerly called Heteropoda)
Superfamily Cingulopsoidea Fretter & Patil, 1958
Superfamily Cypraeoidea Rafinesque, 1815 (cowries)
Superfamily Ficoidea Meek, 1864
Superfamily Laubierinoidea Warén & Bouchet, 1990
Superfamily Littorinoidea (Children), 1834 (periwinkles)
Superfamily Naticoidea Forbes, 1838 (moon shells)
Superfamily Rissooidea J.E. Gray, 1847 (Risso shells) (includes genus oncomelania, schistosomiasis transmission vector)
Superfamily Stromboidea Rafinesque, 1815 (true conchs)
Superfamily Tonnoidea Suter, 1913
Superfamily Trivioidea Troschel, 1863
Superfamily Vermetoidea Rafinesque, 1815 (worm shells)
Superfamily Xenophoroidea Troschel, 1852 (carrier shells)
Infraorder Ptenoglossa J.E. Gray, 1853
Superfamily Eulimoidea Philippi, 1853
Superfamily Janthinoidea Lamarck, 1812
Superorder Heterobranchia J.E. Gray, 1840
Order Heterostropha P. Fischer, 1885
Superfamily Architectonicoidea J.E. Gray, 1840
Superfamily Nerineoidea Zittel, 1873 (fossil)
Superfamily Oxynooidea H. & A. Adams, 1854
Suborder Anaspidea P. Fischer, 1883 (sea hares)
Suborder Notaspidea P. Fischer, 1883
Superfamily Pleurobranchoidea Férussac, 1822
Suborder Thecosomata Blainville, 1824 (sea butterflies)
Superfamily Peraclidoidea
Suborder Nudibranchia Blainville, 1814 (nudibranchs)
Infraorder Anthobranchia Férussac, 1819
Superfamily Doridoxoidea Bergh, 1900
Superfamily Onchidoridoidea Alder & Hancock, 1845
Superfamily Polyceroidea Alder & Hancock, 1845
Infraorder Cladobranchia Willan & Morton, 1984
Superfamily Dendronotoidea Allman, 1845
Superfamily Metarminoidea Odhner in Franc, 1968
Superfamily Otinoidea H. & A. Adams, 1855
Superfamily Rathouisioidea Sarasin, 1889
Suborder Basommatophora Keferstein in Bronn, 1864 (freshwater pulmonates, pond snails)
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
Superfamily Cochlicopoidea Pilsbry, 1900
Superfamily Partuloidea Pilsbry, 1900
Superfamily Pupilloidea Turton, 1831
Subinfraorder Sigmurethra
Superfamily Acavoidea Pilsbry, 1895
Superfamily Achatinoidea Swainson, 1840
Superfamily Aillyoidea Baker, 1960
Superfamily Arionoidea J.E. Gray in Turnton, 1840
Superfamily Camaenoidea Pilsbry, 1895
Superfamily Clausilioidea Mörch, 1864
Superfamily Dyakioidea Gude & Woodward, 1921
Superfamily Gastrodontoidea Tryon, 1866
Superfamily Helixarionoidea Bourguignat, 1877
Superfamily Oleacinoidea H. & A. Adams, 1855
Superfamily Orthalicoidea Albers-Martens, 1860
Superfamily Plectopylidoidea Moellendorf, 1900
Superfamily Polygyroidea Pilsbry, 1894
Superfamily Punctoidea Morse, 1864
Superfamily Rhytidoidea Pilsbry, 1893
Superfamily Sagdidoidera Pilsbry, 1895
Superfamily Staffordioidea Thiele, 1931
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.
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