An apertural view of a shell of the small, air-breathing, freshwater snail Physella acuta in the family Physidae. This an example of a sinistral or left-handed shell, which means that if the shell is held with the aperture facing the observer, and the spire pointing up, then the aperture is on the left-hand side. In this genus the aperture is long and large, and the spire is pointed. The shell is thin and corneous, and rather transparent.
The family Physidae and the family Planorbidae are within the superfamily Planorboidea; all the species within these families have shells that are left-handed. In general however, sinistral shell coiling is quite rare among the gastropods.