Nine Species That Have Extra Eyelids
Cats have a third eyelid and lacrimal gland. It produces 50% of a cat's tears. You can see a cat's third eyelid when it's really relaxed.
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.
An owl's cranium holds its eyes so it can only see ahead. Their third eyelid shields their non-moving eyes. Owls can hunt through their eyelids.
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 closed.
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.
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, and nose closed.
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.
Polar bears inhabit icy environments. Snow can reflect UV radiation, putting polar bears at risk in cold weather. Polar bears' third eyelids act as sunglasses.
Seals snooze underwater. Seals sleep submerged but keep their snouts above water to breathe. Their third eyelids safeguard their eyes while awake or asleep.