Planorbis

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Planorbis

Planorbis planorbis

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Orthogastropoda
Superorder:	Heterobranchia
Order:	Pulmonata
Family:	Planorbidae
Genus:	Planorbis O. F. Müller, 1774

Species

See text.

Planorbis is a genus of freshwater air-breathing snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails, or planorbids. All planorbids have sinistral or left-coiling shells.

Planorbis gastropod shells are hard to make sense of in terms of their coiling and orientation. Most of the shells in this genus are almost planispiral in coiling, and when examining the shell, it is important to bear in mind the fact that all planorbids have sinistral shells.

To complicate matters further however:

In life, these pond snails hold their shells upside down, with the umbilicus facing upward
The spire of the shell is quite sunken in many species
The umbilicus of the shell is very wide, and it in some species the umbilicus only as deeply "dished" as the sunken spire is.

Once you understand that a planorbid shell is sinistral, you can hold the shell with the aperture on the left and facing you, then the sunken spire side of the shell will be uppermost.

To repeat: 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.

World-wide distribution. Its shell coils in a flat plane.