扁蜷科

扁蜷科，與一般腹足綱物種的外殼形狀不同。外殼看似是右旋，但其實是螺把殼上下顛倒來揹在身上，使左旋的外殼看起來像是右旋。因其外形，本科物種又稱。由於本科物種的頭和腹足都比較細小，使得牠們頭上絲般幼的觸角顯得很細長。分類.對於部份分類元來說，是否應該歸入扁蜷科仍有爭議。這對於淡水帽貝"Ferrissia"跟"Ancylus"這兩個屬來說由其合適。這些屬有時會被歸入椎實螺科之下的Ancylini族。以下將詳述這個分類的具體內容。2005年分類.根據布歇特和洛克羅伊的腹足類分類 (2005年)，本科包括下列各個亞科：2007年本科部分物種的分類.Albrecht等(2007)分析過本科部分物種，based on sequences of mitochondrial 18S ribosomal DNA and cytochrome-c oxidase I (COI) genes, and on the basis of the results, they rearranged the taxonomy like this:"A-clade" sensu Albrecht et al. (2007)Tribus BulininiTribus Ancylini Rafinesque-Schmaltz, 1815"B-clade" sensu Albrecht et al. (2007)Tribus Camptoceratini扁蜷族 PlanorbiniTribus Segmentinini"C-Clade" sensu Albrecht et al. (2007)Cladogram.The following is a cladogram that shows the phylogenic relationships within the Planorbidae according to Albrecht 2007:屬.本科的模式屬是 "Planorbis" 。以下列出主要根據布歇特及其他人(2005)的分類，因為Albrecht (2007)的分類並未整全的包括本科所有的屬。至於每個屬的亞屬則依照 Glöer (2002) 的以下綜合三份結果：Subfamily = ? (other genera that are not yet sorted are listed here)The genus "Camptoceratops" Wenz, 1923 is no longer considered to be a planorbid. It was recognised by Curry (1965, p. 360) as a euthecosomatous pteropod (Heterobranchia) (note by Arie W. Janssen, 092507).The generic name "Taphius" H. Adams &amp; A. Adams, 1855 is a synonym for "Biomphalaria".Shell description.The shells of most species in this family are disk-like or button-like, being coiled in one plane, although several groups have shells that are more higher-spired, and some are limpet-like.All coiled shell Planorbidae are sinistral in their shell coiling, as is proved by their internal anatomy (the respiratory and the genital orifice are situated on the left side), however the animals carry their shells with what would normally be the ventral (i.e. umbilical) surface uppermost, and because of this, the shells appear to be dextral.Indeed, formerly planorbids were thought to have dextral shells, and so species of this family were figured as if they had dextral shells. Although it is now understood that these species are sinistral in shell coiling, disk-like Planorbid shells are often still shown in illustrations oriented as if they were dextral.Most species of coiled planorbids have a rather thin and moderately smooth shell, although more distinct sculpture such as a keel occurs in, and is diagnostic of, certain species. In the flat, keeled species, the whorls tend to overlap.The aperture has a sharp outer lip. A peristome can be present, but often the lip is not thickened nor reflected. Those planorbid species which have a high-spired shell may have a narrow umbilicus, but frequently this is covered by callus.In height most species vary between 6 mm and 6 cm, however, disk-like shells are usually less than about 2 cm in maximum dimension.Like all pulmonate aquatic snails, ramshorn shells do not have an operculum to close the shell aperture.Sinistral shells.Flat-coiled planorbid gastropod shells are hard to understand in terms of their coiling and orientation. Many of the shells of species in this family are almost planispiral in coiling such that one side of the shell often looks rather like the other side, but it is important to bear in mind that nonetheless there is an umbilical side and a spire side of the shell. In addition these are in fact sinistral shells, despite the fact that the snail carries its shell as if it were a normal dextral shell. To make sense of the shell coiling, the following facts are useful:However, once it is understood that the planorbid shell is sinistral, if the shell is held with the aperture on the left and facing the observer, then the sunken spire side of the shell is uppermost. This is a convenience for understanding the shell, but is the opposite of the way the shell is actually carried in life.The side of the shell which is in fact the spire (a sunken spire) faces "down" in the living animal, contrary to what is the case in almost all other shelled gastropods. Because the shell is carried "upside down" like this, the aperture of the shell is angled to face downwards also, so the aperture faces a little towards the spire, not away from it, as is usually the case in other shelled gastropods.血色素.相對於其他的軟體動物，扁卷螺科物種的血液都含有血紅素。因此，這些動物呼吸氧氣時都較其他軟體動物來得更有效；而血紅素亦令這些動物的身體帶紅色，這尤其在白化物種身上更明顯。習性.扁卷螺科的大多數物種都只能在淡水生活，只有少數物種能存活於鹹淡水交界。所以對於其他未能存活於鹹淡水的物種，亦令農夫可以用食鹽來清除牠們。Geological history.Ancestors of ramshorn snails are known with certainty since the Jurassic period. Modern taxa developed since the Cretaceous.Geographical distribution.Species in this family occur worldwide. In Northwest Europe about 20 species are known (including non-indigenous species). In this region, various extinct taxa are known to have occurred, starting in the Jurassic period.