How do snails breathe?

  Terrestrial (or land) snails are descendants from water snails and were roaming the earth along side the dinosaurs. Their lungs have evolved from gills (Ctenidium).  This type of lung found in water animals is only able to acquire oxygen from water. To breathe on land, the tissues around the shell's opening had to evolve to acquire oxygen from dry air. 

 The mantle: the snails had to adapt to the great amount of water they loose when breathing out. Water evaporation is a big danger to the snail so its mantle grew  especially thick in terrestrial snails and formed a breathing hole (pneumostome) to let the air through. The snail has a muscle around it to be able to close its hole when it's not breathing.

 A snail's slime is another protection against evaporation. It attracts water rather than releasing it (it's hygroscopic). So even millions of years after their ancestors left the sea, terrestrial snails still wear a coat of water around it.

To further save on its slime, the snail developed the cute habit of "jumping" which is when it will allow only parts of its foot to touch the ground.  You can see it jumped where the trail that it leaves is interrupted.

 

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