Thoughts about snails
Did you know that freshwater snails evolved from land snails? Landsnails evolved from marine snails. The gills became adapted into simple lungs. When some of these land snails moved into freshwater environments they did not re-evolve the ability to breathe underwater, and so essentially just hold their breath.
linoleum jet 
How long can they hold their breath (go without taking in new air)? Do they actually have lungs, or some other sort of organs that process the oxygen from the air? The image of snails with tiny mammalian lungs seems unlike to me, but I don’t know what an aternative would look like.
They don’t have lungs like we do. I just turned to this not quite authoritative site and learned that gilled snails can also live in freshwater, interestingly.