扁蜷科

扁蜷科（學名：Planorbidae），亦作扁卷螺科，是腹足綱之下的一個單系群支序，本科物種皆為可呼吸空氣的淡水螺。原屬有肺目基眼亞目椎實螺總科，今屬異鰓類泛有肺類喜溼螺類支序。
本科多個物種的外殼都是平旋殼（planispiral），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Ramshorn snail。
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（Lymnaeidae Rafinesque, 1815）；另外亦有文獻把上述兩個屬放進各自的一個科裡。然而，在布歇特和洛克羅伊的腹足類分類 (2005年)的分類，這些屬被歸為扁蜷亞科（Planorbinae）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：- 扁蜷亞科 Planorbinae Rafinesque, 1815
** 扁蜷族 Planorbini Rafinesque, 1815：synonyms: Choanomphalinae P. Fisher & Crosse, 1880; Orygoceratidae Brusina, 1882
** tribe Ancylini Rafinesque, 1815 - synonym: Pseudancylinae Walker, 1923 (inv.)
** tribe Biomphalariini H. Watson, 1954 - synonyms: Acrorbini Starobogatov, 1958; Drepanothrematini Zilch, 1959; Taphiinae Harry & Hubendick, 1964
** tribe Planorbulini Pilsbry, 1934
** tribe Segmentinini F.C. Baker, 1945- subfamily Bulininae P. Fischer & Crosse, 1880
** tribe Bulinini P. Fischer & Crosse, 1880 - synonyms: Laevapicinae Hannibal, 1912; Isidorinae Annandale, 1922; Gundlachiinae Starobogatov, 1967
** tribe Coretini Gray, 1847 - synonyms: Pompholicinae Dall, 1866 (inv.); Camptoceratinae Dall, 1870; Megasystrophinae Tryon, 1871 (inv.); Pompholycodeinae Lindholm, 1927; Helisomatinae F. C. Baker, 1928; Bayardellini Starobogatov & Prozorova, 1990; Planorbariini Starobogatov, 1990
** tribe Miratestini P. & F. Sarasin, 1897 - synonyms: Ferrissiinae Walker, 1917; Ancylastrinae Walker, 1923; Protancylinae Walker, 1923; Physastrinae Starobogatov, 1958; Ameriannini Zilch, 1959; Patelloplanorbidae Franc, 1968
** tribe Plesiophysini Bequaert & Clench, 1939- subfamily Neoplanorbinae Hannibal, 1912 - synonym: Payettiinae Dall, 1924- subfamily Rhodacmeinae Walker, 1917

**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)- Burnupia Walker, 1912
Tribus Bulinini- Bulinus O. F. Müller, 1781- Indoplanorbis Annandale, 1921
Tribus Ancylini Rafinesque-Schmaltz, 1815- Ancylus O. F. Müller, 1774- Ferrissia Walker, 1903- Gundlachia Pfeiffer, 1849- Laevapex Walker, 1903- Hebetancylus Pilsbry, 1914
"B-clade" sensu Albrecht et al. (2007)- Glyptophysa Crosse, 1872- Protancylus Sarasin, 1897- Kessneria Walker & Ponder 2001- Leichhardtia Walker, 1988
Tribus Camptoceratini- Planorbarius Duméril 1806
扁蜷族 Planorbini- 多旋螺屬 Anisus Studer, 1820- Bathyomphalus Charpentier, 1837- 旋螺屬 Gyraulus Charpentier, 1837- Choanomphalus Gerstfeldt, 1859- 扁蜷屬 Planorbis O. F. Müller, 1774
Tribus Segmentinini- Segmentina Fleming, 1818- 平扁蜷屬 Hippeutis Charpentier, 1837- 類扁蜷屬 Polypylis Pilsbry, 1906，多脈扁螺屬
"C-Clade" sensu Albrecht et al. (2007)- Biomphalaria Preston, 1910- Menetus H. & A. Adams, 1855- Planorbella Haldeman, 1843- Planorbula Haldeman, 1843

**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 Müller。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) 的文獻。以下綜合三份文獻的結果：
spire of the shell of Anisus septemgyratus- 扁蜷亞科 Planorbinae Rafinesque, 1815
** ?
***多旋螺屬 Anisus S. Studer, 1820
**** subgenus Disculifer C. Boettger, 1944
*** Bathyomphalus Charpentier, 1837
*** 旋螺屬 Gyraulus Charpentier, 1837
**** subgenus Torquis Dall, 1905
**** subgenus Lamorbis Starobogatov, 1967
**** subgenus Armiger Hartmann, 1843
*** 平扁蜷屬 Hippeutis Charpentier, 1837：又名圓扁螺屬
** tribe Ancylini Rafinesque, 1815 
*** Ancylus O. F. Müller, 1773 - type genus of tribe Ancylini
** tribe Biomphalariini H. Watson, 1954
*** Biomphalaria Preston, 1910 - type genus of tribe Biomphalariini
*** Drepanotrema Crosse & P. Fischer, 1880
** 扁卷螺族 Planorbini Rafinesque, 1815
*** 扁卷螺屬 Planorbis Müller, 1773
** tribe Planorbulini Pilsbry, 1934
*** Planorbula Haldeman, 1840 - type genus of tribe Planorbulini
** tribe Segmentinini F.C. Baker, 1945
*** Segmentina Fleming, 1818 - type genus of tribe Segmentinini
*淤泥螺亞科 Bulininae P. Fischer & Crosse, 1880
**印度扁卷螺屬 Indoplanorbis Annandale & Prashad, 1920：單型屬，只有Indoplanorbis exustus一個物種。
** Planorbarius Duméril, 1806
** Planorbella Haldeman, 1842
** Menetus H. Adams & A. Adams, 1855
*** subgenus Dilatata Clessin, 1885
** tribe Bulinini P. Fischer & Crosse, 1880
*** Bulinus O. F. Müller, 1781 - type genus of subfamily Bulininae
*** Gundlachia Pfeiffer, 1849
** tribe Coretini Gray, 1847 
*** Coretus Gray, 1847 - type genus of tribe Coretini
** tribe Miratestini P. & F. Sarasin, 1897
*** Miratesta P. & F. Sarasin, 1897 - type genus of tribe Miratestini
*** Amerianna Strand, 1928
*** Ferrissia Walker, 1903date=January 2009
**** Pettancyclus Iredale, 1943
** tribe Plesiophysini Bequaert & Clench, 1939
*** Plesiophysa P. Fischer, 1883 - type genus of tribe Plesiophysini- subfamily Neoplanorbinae Hannibal, 1912
*** Neoplanorbis Pilsbry, 1906 - type genus of subfamily Neoplanorbinae- subfamily Rhodacmeinae Walker, 1917
*** Rhodacmea Walker, 1917 - type genus of subfamily Rhodacmeinae
亞科地位未定(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).
屬名 Taphius H. Adams & A. Adams, 1855 為Biomphalaria的異名。

**Shell description**
shell of Biomphalaria tenagophila. Scale bar is 3 mm.
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**
Planorbarius corneus. View of the sunken spire (held facing downwards in life)
Planorbarius corneus. View of the umbilicus (held uppermost in life)
白化了的扁卷螺科物種，令帶有血紅素的身體組織在身體內顯得紅紅啡啡的。
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:- In life, these pond snails hold their shells upside down compared to the normal gastropod shell orientation, with the umbilicus facing upwards- The spire of the shell is quite sunken in many species, in addition it is carried facing downwards- The umbilicus of the shell is very wide and shallow- In some species the umbilicus is not as deeply "dished" as the sunken spire is, so superficially it can be hard to tell one from the other
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.

**參看**
膀胱螺科

**參考文獻**

**外部連結**
   NCBI Taxonomy Browser: Planorbidae date=20181216163553 -   Systema naturae 2000 (classification) - Taxon: Planorbidae date=20070929121933 - taxon=Planorbidae-  http://mkohl1.net/Planorbidae.html date=20160401071403 -  Frank Collins Baker, The Molluscan Family Planorbidae, The University of Illinois press, Urbana, 1945