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The Gastropoda are the largest of the commonly-recognised molluscan classes. Ancestrally, gastropods have a dorsal, unchambered, often helically coiled, shell (lost in numerous derived groups) and a head with eyes and cephalic tentacles. Gastropods are also distinguished by the process of torsion where, during development, the gut becomes U-shaped and the initially posterior anus is moved to an anterior position (Prothero 1998). In some derived subgroups, the effects of torsion have become reduced.
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Prothero, D. R. 1998. Bringing Fossils to Life: An introduction to paleobiology. WCB McGraw-Hill: Boston.
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