Why do cats have two eyelids?

Question

Have you ever opened your cats eyes while it was asleep? And saw this white thing covering the eyes, then it wakes up and the white thing moves to the side.

I want to know if its supposed to protect them or something o_O?

Answer 1

the eyelid acts much like a windshield wiper blade by removing debris from the surface and when the cat is alert the bulk of the eyelid is hidden and only a small portion is visible in the inner corner of the eye When relaxed, during sleep or during blinking it is revealed

Answer 2

The "third" eyelid (the inner one that moves side to side) is believed to remove debris if anything gets in the eye.  It is very important for wild cats to keep their eyes clean because it is a vital tool for tracking and hunting prey.   It is also believed to be used for protection.  If a cat is moving through tall grass or brush, and also when it captures prey.  

Many species of animals have a third eyelids. humans, and other primates are actually an oddity for not having one.  It makes sense for us to evolve without one though, we don't attack with our faces like other species so we don't need to protect our eyes as much.

Answer 3

Cats have three eyelids.

They have a top and bottom one and also one that moves across the eye from side to side.

The third lid is to protect and lubricate their eyes.

Cats' upper and lower eyelids, like those of humans, sheathe the eyeballs. For further protection, all cats have an opaque, white third eyelid, called the nictitating membrane, between the lower lid and inside corner of each eye. This layer helps moisten the eye and clear dust from the surface of the cornea. When dozing, this third eyelid closes, perhaps to act as a shade. As soon as the sleeping cat is alerted by any sound, the nictitating membrane flicks back to the inside corner of the eye.

Hidden from view behind the outer eyelids of a sleeping cat and barely noticeable in the corner of the eyes while the cat is awake, the nictitating membranes are sometimes visible in felines that doze with open eyelids. While most cats do sleep with their eyes completely closed, some rest with their eyes partially open.

Cats with very short noses, such as Persians, may not be physically able to close their eyes completely because the eyes bulge more, causing the nictitating membranes to be visible during sleep.

The membranes also are seen when cats blink, which, quite mysteriously, they do infrequently — sometimes as little as once every few minutes.

http://animal.discovery.com/guides/cats/...