Gills are evaginations of the body surface. Some open directly to the environment; others, as in fishes, are enclosed in a cavity. In contrast, lungs represent invaginations of the body surface. Many invertebrates use gills as a major means of gas exchange; a few, such as the pulmonate land snail, use lungs.
Basically, gills filter the water around the animal and take in the small amount of oxegen mixed in the water. Lungs take in all of the air and almost half of it is oxgen and they can't go underwater for long. But since lungs are more efficient, animals with lungs can grow bigger than animals with gills
