Camels have not one, not two, but three eyelids. Called a nictitating membrane, the transparent lid helps keep out sand and dust; it can even improve vision, like a contact lens. Many animals, including dogs, cats, sharks, and some birds and amphibians, also have third eyelids. Animals with a third eyelid use the thin, protective membrane to blink away debris, sharpen vision and enable themselves to see clearly underwater.
Some of our most popular pets sport third eyelids, though you may not have noticed. Dogs and cats generally hide their third eyelids. If you happen to see an opaque film covering your buddy’s eye, that’s his nictitating membrane. Some dogs and cats let their third eyelids down, so to speak, when they’re totally relaxed or falling asleep; however, they generally retract them when alerted or awoken. If you notice your dog or cat’s third eyelid for a prolonged period, this may indicate a medical problem. Cherry eye is one of the most common problems associated with the third eyelid. This ailment presents itself as a reddened bulge in the inside corner of your pet’s eye.
Picture Credit : Google