Unlike most molluscs, the blood of ram's horn snails contains iron-based hemoglobin instead of copper-based hemocyanin.

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.

According to the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), this family consists of the following subfamilies: Albrecht et al. (2007)[4] analyzed a limited number of genera of Planorbidae, 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)[4] Tribus Bulinini Tribus Ancylini Rafinesque-Schmaltz, 1815 "B-clade" sensu Albrecht et al., (2007)[4] Tribus Camptoceratini Tribus Planorbini Tribus Segmentinini "C-Clade" sensu Albrecht et al., (2007)[4] The following is a cladogram that shows the phylogenic relationships within the Planorbidae according to Albrecht 2007:[4] Burnupia; Burnupia cafra and Burnupia stenochorias Bulinus tropicus and Indoplanorbis exustus Ancylus fluviatilis Ferrissia Laevapex fuscus Gundlachia radiata Hebetancylus excentricus Glyptophysa sp.

Protancylus adhaerens Kessneria papillosa Leichhardtia sisurnius Planorbarius corneus Anisus spirorbis Bathyomphalus contortus Gyraulus (Caronogyraulus) sp.

(other genera that are not yet sorted are listed here) The genus Camptoceratops Wenz, 1923 is no longer considered to be a planorbid.

Those planorbid species which have a high-spired shell may have a narrow umbilicus, but frequently this is covered by callus.

Flat-coiled planorbid gastropod shells are hard to understand in terms of their coiling and orientation.

Most species of planorbids live only in fresh water, such as ponds, lakes, and slow moving rivers.

[citation needed] Ancestors of ramshorn snails are known with certainty since the Jurassic period.