Dogs have an additional eyelid to protect and disseminate tear film. Cherry eye causes the third eyelid to protrude in some dogs. Surgery can replace a dog's damaged third eyelid.
Dogs
The owl's cranium holds its eyes so it can only look straight ahead. Owls can't blink, thus their third eyelid protects them. Seeing through their eyelids helps owls hunt.
Owls
Semi-transparent third eyelids can conceal a frog's eye. This additional eyelid protects frogs' eyes underwater. Since the lid is translucent, they can hunt with their eyes shut.
Frogs
In deserts, sand can easily get in camels' eyes. Camels have a third eyelid that keeps dust and sand out of their eyes. A camel's third eyelid can work like a contact lens, improving vision.
Camels
Beavers can see with closed eyelids thanks to their third eyelid. Beavers use their third eyelid during diving. Beavers can swim underwater with their eyes, ears, & nose closed.
Beavers
Bald eagles close their eyes upward because their bottom eyelid is larger than their top. Eagles' third eyelid keeps their eyes moist. Closed, this eyelid is transparent to eagles.
Bald Eagles
Polar Bears live on ice. Snow can reflect UV radiation, putting polar bears at risk in cold weather. Polar bears' third eyelids act as sunglasses.
Polar Bears
Most lizards live near the ground, thus debris can easily get in their eyes. Lizards can use their third eyelid to shield and clean their eyes. Iguanas have a third eye and a third eyelid.
Lizards
Seals snooze underwater. Seals sleep submerged but keep their snouts above water to breathe. Their third eyelids safeguard their eyes while awake or asleep.