The third eyelid that some animals possess is called the nictitating membrane. This eyelid, which is clear, is closed around an animal’s eye as a way of protecting it from prey, the elements, or debris. Some of the animals that have a nictitating membrane are birds, sharks, and also some mammals.
The nictitating membrane is anatomically a part of the conjunctiva. This membrane is located in the corner of the eye when it is not used. Humans have a nictitating membrane as well but it is permanently folded in this particular corner of the eye. We see it as the pink nub in the corner. Most animals are actually able to voluntarily control the nictitating membrane (also called the haw) and be able to draw it either horizontally or diagonally over the cornea. Some scientists think the nictitating membrane evolved when animals from the ocean began to explore land. They needed a way to keep the moisture on their eyeballs and protect it from the dry air and the dust. Even predators have a great use for the nictitating membrane because they cannot blink while stalking prey or they run the risk of losing track of it. The nictitating membrane allows predators to blink while still maintaining full vision.
Flying predators like owls close its nictitating membrane when they fly at high speeds to prevent their eyes from drying out. The moisture in the eyes actually aid in improving their vision. Sharks use their nictitating membranes as way of protecting their eyes from being stabbed or poked by its prey.