third eyelid
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A nictitating membrane or third eyelid is a transparent or translucent additional eyelid present in some animals that can be drawn across the eye to protect and moisten it while maintaining vision. Animals that possess a full nictitating membrane are tailless amphibians, some reptiles, birds, and sharks. For most mammals the nictitating membrane only has a vestigial portion remaining, but mammals such as cats, camels, polar bears, seals and aardvarks possess full ones.