Solem, G. Alan Division of Invertebrates, Field Museum of Natural History, Chicago, Illinois.
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An order (or superorder) of the gastropod molluscan subclass Pulmonata containing about 2500 species that are grouped into 11–15 families. Only a few members of the family Ellobiidae are terrestrial, with the other species today being tidal to supratidal or estuarine in habitat. The families Siphonariidae and Trimusculidae are marine limpets with caplike shells. The families Otinidae and Amphibolidae also are marine taxa. The remaining families inhabit a great variety of freshwater situations and are quite varied in shell structure and shape. Because of this great variation in habitat and form, it is difficult to find structures that are common to all taxa and thus diagnostic of the group, which indeed may not have a common origin. The most easily observable characters are having the eyespots normally located at the base of two slender tentacles that are neither contractile nor retractile, and usually having two external genital orifices. Most features of the anatomy are shared with other pulmonate superorders, or are specializations related to major changes in habitat or body form. See also: Gastropoda; Pulmonata
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