This one-eyed snail is known variously as Fossaria parva or Galba parva or even Lymnaea humilis (family Lymnaeidae). (It all depends on which taxonomist you talk to.)
Fossaria parva is a small (~5-6 mm long) freshwater snail, although in the spring and the summer, the snails seem to spend most of their time outside the water on mud or wet rocks. Obtaining oxygen from the air is not a problem for them, because they are pulmonate snails; the vascularized roof of their mantle cavity functions as a lung. The lymnaeids are distant relatives of the fully terrestrial pulmonate snails and slugs and closer relatives of the also pulmonate semi-terrestrial ellobiids (family Ellobiidae) of seashore habitats.
The lymnaeids, like the ellobiids, appear to have descended from semi-terrestrial snails with lungs and then evolved a lifestyle probably even more aquatic than those of their ancestors.
Of course, the normal individuals have 2 eyes.