Nictitating membrane
（重定向自Semilunar fold）
The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as camels, polar bears, seals and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans or palpebra tertia.