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Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks. Many of the species in this family have coiled shells that are planispiral, in other words, the shells are more or less coiled flat, rather than having an elevated spire as is the case in most gastropod shells. Although they carry their shell in a way that makes it appear to be dextral, the shell of coiled planorbids is in fact sinistral in coiling, but is carried upside down, which makes it appear to be dextral. The foot and head of planorbids are rather small, while their thread-like tentacles are relatively long. For information on what are known as "ramshorn snails" in the aquarium trade, some of which (but not all of which) are planorbids, please see ramshorn snail. ==General taxonomic context== For several taxa, no consensus exists as to whether the taxa should even be assigned to the family Planorbidae. This is certainly the case with the freshwater limpets ''Ferrissia'', and ''Ancylus''. Both of these genera have sometimes been assigned to the family Lymnaeidae. Alternatively sometimes each one of them is raised to the level of a family. However, according to the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), these genera are currently placed in the tribe Ancylini within the family Planorbidae, and that is the taxonomic system that is followed here. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Planorbidae」の詳細全文を読む スポンサード リンク
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