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Nictitating membrane

The nictitating membrane of a Masked Lapwing as it closes. The membrane 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, sharks, and mammals have full nictitating membranes. Most mammals, however, have a small, vestigial portion of it remaining in the corner of the eye.

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