Cats and dogs have a third eyelid

Actually, polar bears, kangaroos, beavers and seals also have a third eyelid, which is really a membrane intended to keep the eyeball moist. Unlike lids which move up and down, this membrane tracks across the eye from side to side.

Learn more from these articles:

Why do dogs and cats have inner eyelids? The Toronto Star
Why do dogs have a third eyelid? Animal Planet (includes common issues)

Caleb Woods