Jeff Rotman / Close-up of nictating membrane over eye of Tiger shark. (Galeocerdo cuvieri) Great Barrier reef.

LYNN M. STONE / Eastern Brown Pelican (Pelecanus occidentalis carolinensis) in breeding plumage, diving for fish prey; note the closed nictating membrane over eye at the moment of impact. Indian Rocks Beach, Florida,...

Ann & Steve Toon / Verreaux's / Giant eagle owl (Bubo lacteus) perched with eyes closed showing nictating membrane, Kgalagadi Transfrontier Park, South Africa, January

Ann & Steve Toon / Verreaux's / Giant eagle owl (Bubo lacteus) perched with eyes closing showing nictating membrane, Kgalagadi Transfrontier Park, South Africa, November

Mike Wilkes / Gaint eagle owl (Bubo lacteus) on nest with nictating membrane covering eye, Baringo NP, Kenya

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye covered with nictating membrane

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye covered with nictating membrane

Philippe Clement / Arizona toad (Bufo microscaphus) burrowing in mud and showing the transparent inner eyelid or nictating membrane, Arizona, USA. Captive

Philippe Clement / Arizona toad (Bufo microscaphus) burrowing in mud and showing the transparent inner eyelid or nictating membrane, Arizona, USA. Captive

Jane Burton / Common Frog (Rana temporaria) wiping eye with nictating membrane

Heidi & Hans-Juergen Koch / Red-eyed Tree Frog (Agalychnis callidryas) close-up portrait showing tympanum or eardrum, and nictitating membrane which protects the eyes from danger but does not completely restrict vision, native t...

Heidi & Hans-Juergen Koch / Northern Leopard Frog (Rana pipiens) partly submerged, face reflected in water's surface, native to North America

Heidi & Hans-Juergen Koch / Northern Leopard Frog (Rana pipiens) partly submerged, face reflected in water's surface, native to North America

Heidi & Hans-Juergen Koch / Northern Leopard Frog (Rana pipiens) partly submerged, face reflected in water's surface, native to North America

Jeff Rotman / Eye of Caribbean reef shark {Carcharhinus perezi} Bahamas, nictating membrane closing

Jeff Rotman / Eye of Tiger shark with nictating membrane half closed {Galeocerdo cuvieri} Red Sea

Jeff Rotman / Eye of Caribbean reef shark {Carcharhinus perezi} Bahamas, nictating membrane closed

Jeff Rotman / Eye of Caribbean reef shark nictating membrane closed {Carcharhinus perezi} Bahamas

Pete Oxford / Madagascar fish eagle, note nictating membrane {Haliaeetus vociferoides} Madagascar

Jeff Rotman / Whitetip reef shark eye protected by nictating membrane {Triaenodon obesus} Cocos Is, Costa Rica, Pacific Ocean

Jeff Rotman / Eye of Tiger shark with nictating membrane closed {Galeocerdo cuvieri} Red Sea