Ramshorn Snails (Wheel Snails; Planorbids)
This group of freshwater snails is easy to identify at a glance, because the shell is a flat, disklike coil, similar to a ram’s horn. (In most other freshwater snails, the shell forms with a strong twist toward one side, with a spire at the apex pointing to left or right—as with a common land snail.)
Ramshorn snails are grouped with the “pulmonate” snails, for they breathe air by means of a lunglike organ, and do not breathe water via gills. Like other pulmonate snails, they lack an operculum, a hard horny “trapdoor” that other types of aquatic snails possess that closes when the animal retracts into the shell.