Many other fresh water snails, though, do not breathe using gills. Their gills are reduced, instead leaving to the snail the thin wall of the mantle cavity to assimilate oxygen through. This organ is called the snails' lung, so this type of snails is also referred to as pulmonate (lung) snails (Pulmonata). Those lung-breathing snails make, by far, the largest part of known gastropod species. The advantage of breathing with lungs is obvious: Those snails can breathe oxygen from dry air and do not have to rely on water.
To be able to do this, fresh water snails breathing with lungs have to regularly go to the surface. They climb plants or simply float to the surface. Some freshwater snails even are able to crawl on the lower side of the water surface, eat algae while doing so and breathe oxygen from the air. This is possible because of the water's surface tension, which makes it possible to a water strider to run over the water surface.
Not only pulmonate snails float at the water surface (apart from the Physidae in the left picture, it is for example also theLymnaeidae and the Planorbidae), but also gill-breathing snails, such as the lesser mud snails (Hydrobiidae), among which the behaviour might rather serve the purpose of finding food than of breathing.
Great pond snails:
Among those fresh water snails breathing with lungs, for example, are pond snails (Lymnaeidae). With up to 60 mm shell length the great pond snail (Lymnaea stagnalis L. 1758) is actually the largest land snail in Europe. In contrary to that are the largest European terrestrial snail, the Roman Snail (Helix pomatia) with 50 mm shell height at most and the largest gastropod of all in Europe, which is a slug, the ash-black slug(Limax cinereoniger) with a body length of more than 20 cm or 8 in. (
Pond snails have got an elongate shell that is often extended characteristically in the aperture area. They mainly live in calm waters such as ponds and backwaters, where they can graze plant matter and decaying organic matter (detritus). Compared to nerites, mud snails and Bithynias, pond snails lack a proboscis, as well as an operculum. But then again, none of the pulmonate snails have one.
On the other hand, both have something in common: Pond snails as well have only two tentacles, which they cannot withdraw. At the tentacles' base there are two eyes, that can be seen in the picture as black dots in front of each tentacle
. A pond snail's triangular tentacle lobes are full of blood vessels, thanks to which the snail is able to breathe by tissue respiration under water. As a result, it does not need to surface as often. The respiratory hole is elongated to form a respiratory tube, serving the snail as a snorkel to get fresh air, without the need to surface.
https://www.sciencenews.org/blog/science-ticker/how-snails-breathe-through-snorkels-land