Physa
Physa is a genus of small, left-handed or sinistral, air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the subfamily Physinae of the family Physidae. These snails eat algae, diatoms and detritus. Anatomy. Members of the freshwater pulmonate family Physidae possess a complex of muscles that is unique amongst gastropods. This complex was given the name "physid musculature". The physid musculature has two main components, the physid muscle "sensu stricto" and the fan muscle. The physid musculature is responsible for a unique ability of physids to rapidly flick their shells from side to side — a reaction that frequently  enables them to escape predation. Shell description. These small snails, like all the species in the family Physidae, have shells that are sinistral, which means that when the shell is held with the spire pointing up and the aperture facing the viewer, then the aperture is on the left-hand side. The shells of "Physa" species have a long and large aperture, a pointed spire, and no operculum. The shells are thin and corneous, and rather transparent. Species. Species in the genus include: