Aquatic and algae eating snails are of course able to breathe underwater as they reside here and breathe using their gills. Terrestrial snails such as your average garden snail however cannot breathe if placed under water and will most likely drown. I.e. not all snails are meant to live underwater. Aquatic snails on the other hand are very fast, active creatures when it comes to being in the water; however they are very slow creatures when on land. They will also die in about a week’s time if they do not have water.
Those snails that are designed with gills can often be found living at the bottom of ponds. Those that do not have gills and are in water will have to come to the surface often in order to breathe and it is for this reason that they often reside on the surface of the water.
Snails also have a thin skin that is permeable for water. On one hand this means that aquatic snails can also breathe through their skin. However it also means that they have to face the loss of water through their skin. These snails also breathe by using their gills (as mentioned above). These gills are located in the pallial cavity and resemble a double comb, with a stem and feathery protrusions that are responsible for the general process of gas exchange. Gas exchange is the absorption of oxygen from the water and the diffusion of carbon dioxide into the water. These gills are called cetenidia. Some species of snails are also designed with a secondary set of gills. These gills are located in the groove between the foot, mantle and shell.