扁蜷科

扁蜷科（学名：Planorbidae），亦作扁卷螺科，是腹足纲之下的一个单系群支序，本科物种皆为可呼吸空气的淡水螺。原属有肺目基眼亚目椎实螺总科，今属异鳃类泛有肺类喜湿螺类支序。
本科多个物种的外壳都是平旋壳（planispiral），与一般腹足纲物种的外壳形状不同。外壳看似是右旋，但其实是螺把壳上下颠倒来揹在身上，使左旋的外壳看起来像是右旋。
因其外形，本科物种又称。
由于本科物种的头和腹足都比较细小，使得牠们头上丝般幼的触角显得很细长。
分类.
对于部份分类元来说，是否应该归入扁蜷科仍有争议。这对于淡水帽贝"Ferrissia"跟"Ancylus"这两个属来说由其合适。这些属有时会被归入椎实螺科（Lymnaeidae Rafinesque, 1815）；另外亦有文献把上述两个属放进各自的一个科里。然而，在布歇特和洛克罗伊的腹足类分类 (2005年)的分类，这些属被归为扁蜷亚科（Planorbinae）之下的Ancylini族。以下将详述这个分类的具体内容。
2005年分类.
根据布歇特和洛克罗伊的腹足类分类 (2005年)，本科包括下列各个亚科：
2007年本科部分物种的分类.
Albrecht等(2007)分析过本科部分物种，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 Bulinini
Tribus Ancylini Rafinesque-Schmaltz, 1815
"B-clade" sensu Albrecht et al. (2007)
Tribus Camptoceratini
扁蜷族 Planorbini
Tribus 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:
属.
本科的模式属是 "Planorbis" 。以下列出主要根据布歇特及其他人(2005)的分类，因为Albrecht (2007)的分类并未整全的包括本科所有的属。至于每个属的亚属则依照 Glöer (2002) 的文献。以下综合三份文献的结果：
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).
属名 "Taphius" H. Adams & A. Adams, 1855 为"Biomphalaria"的异名。
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.
相对于其他的软体动物，扁卷螺科物种的血液都含有血红素。因此，这些动物呼吸氧气时都较其他软体动物来得更有效；而血红素亦令这些动物的身体带红色，这尤其在白化物种身上更明显。
习性.
扁卷螺科的大多数物种都只能在淡水生活，只有少数物种能存活于咸淡水交界。所以对于其他未能存活于咸淡水的物种，亦令农夫可以用食盐来清除牠们。
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.