Katushki

Katushki 

(Russian word meaning “coils”), gastropods of thefamily Planorbidae of the subclass Pulmonata. Most of them are small in size (diameter of the shell measures 2–15 mm). Only onespecies— Planorbarius corneus —is large, measuring up to 30mm. The shell is coiled in one plane. The lung serves as itsrespiratory organ, but a katushka needs less atmospheric airthan other Pulmonata because it has a supplementary organ of aquatic respiration (an adaptive gill). Katushki were formerly all classed in the single genus Planorbis, but they are now dividedinto many genera. (In the USSR alone there are up to eightgenera.) They are widely distributed.