The salt gland is an organ for excreting excess salt. It is found in elasmobranchs, marine birds, and some reptiles. In sharks, salt glands are found in the rectum, but in birds and reptiles, they are found in the skull, in the area of the eyes or nostrils. Such glands work by active transport via sodium-potassium pump that moves salt from the blood into the gland, where it can be excreted as a concentrated solution. Salt glands function to keep salt balance, and allow marine reptiles to drink seawater.
The need for salt glands in reptiles and birds stems from the fact that their kidneys are much less efficient than those of mammals. Unlike the skin of amphibians, reptile and bird skin is impermeable to salt, meaning that the transition to a tougher skin meant a loss in salt-releasing ability. The evolution of a salt gland would have allowed early reptiles and birds to eat aquatic plants and animals, who have high salt concentrations. This does not, however, explain the evolution of the gland in the elasmobranchs, suggesting convergent evolution.
Some theories suggest that mammalian tear ducts and sweat glands may be evolutionarily related to salt glands. Human tears are high in potassium, lending support to this theory, however most phylogenists disagree with this idea.