Morley Read / Slow motion clip of a Forest whiptail lizard (Kentropyx pelviceps) blinking its eye, showing nictitating membrane crossing the eye, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) underwater view of group feeding, Lake Kirkini, Greece. Highly commended in the GDT competition 2013. Commended in the Wildlife Photographer of the Year Awards c...

Alex Hyde / Golden eagle (Aquila chrysaetos) sequence showing protective nictitating membrane over eye. Captive bird, UK.

Anup Shah / Ostrich yawning, eye shut, beak open {Struthio camelus}, Masai Mara Game Reserve, Kenya

Tony Wu / Manchurian crane (Grus japonensis) closing nictitating membrane of eye, portrait. Hokkaido, Japan. March. Sequence 3/3.

Tony Wu / Manchurian crane (Grus japonensis) closing nictitating membrane of eye, portrait. Hokkaido, Japan. March. Sequence 2/3.

Tony Wu / Manchurian crane (Grus japonensis) closing nictitating membrane of eye, portrait. Hokkaido, Japan. March. Sequence 1/3.

Alex Hyde / Nictitating membrane covering eye of Map Treefrog (Hypsiboas geographicus). The membrane affords camouflage and protection from dryness whilst allowing the frog to see. Manu Biosphere Reserve, Amazoni...

Jack Dykinga / Raven (Corvus corax) with nictitating membrane over eye, in a blizzard on Hurricane Ridge, Olympic National Park, Washington, USA. Sequence 2/2

Jack Dykinga / Raven (Corvus corax) in a blizzard on Hurricane Ridge, Olympic National Park, Washington, USA. Nictitating membrane sequence 1/2

David Fleetham / Galapagos shark (Carcharhinus galapagensis) surfacing, with nictitating membrane across it eye. Oahu, Hawaii.

Roger Powell / Eurasian magpie (Pica pica) cleaning its beak on a fencepost. Protecting its eye by lowering the nictitating membrane. Druridge Bay, Northumberland, UK, April.

Jordi Chias / Whale shark (Rhincodon typus), closing nictitating membrane, South Ari Atoll, Maldives Islands, Indian Ocean.

Nick Upton / Rook (Corvus frugilegus) calling while perched in a Willow tree and blinking to show its blue nictitating membrane, Gloucestershire, UK, February.

Dave Bevan / European common frog (Rana temporaria) in a pond, closing its nictating membranes to lubricate its eyes, Carmarthenshire, Wales, UK, February.

Juan Carlos Munoz / Bateleur eagle (Terathopius ecaudatus) portrait with nictitating membrane closed, Captive, occurs in Africa.

Gerrit Vyn / Sandhill crane (Grus canadensis) blinking nictitating membrane, British Columbia, Canada. December.

Richard Robinson / Blue shark (Prionace glauca) with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / Blue shark (Prionace glauca) with mouth open and nictitating membrane almost completely shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

David Tipling / Rook (Corvus frugilegus) calling, Norfolk, February.

Ammonite / Asian openbill stork (Anastomus oscitans) fishing and catching food in waterhole, showing nictitating membrane, Yala National Park, Sri Lanka. Sequence 3/3.

Dave Watts / Bateleur (Terathopius ecaudatus) showing nictitating membrane. Kgalagadi National Park, South Africa

Dave Watts / Bateleur (Terathopius ecaudatus) showing nictitating membrane. Kgalagadi National Park, South Africa

Mark Carwardine / Southern boobook owl / Morepork / Ruru (Ninox novaeseelandiae) showing nictitating membranes, Kauri Park, North Island, New Zealand, endemic

Paul Williams / Jabiru stork (Jabiru mycteria) showing nictitating membrane to protect eyes whilst fishing, Pantanal, Pocone, Brazil

Thomas Marent / Red-eyed Tree Frog (Agalychnis callidryas) with half closed eyelid, Tortuguero National Park, Costa Rica

Philippe Clement / Common Blackbird (Turdus merula) female feeding juvenile whilst using the nictitating membrane for protection of the eye. Belgium, June.

Jeff Rotman / Eye of Tiger shark (Galeocerdo cuvier), nictitating membrane closing to protect eye, Great Barrier Reef, Australia, Coral Sea.

Jeff Rotman / Eye of Whitetip Reef Shark (Triaenodon obesus), nictitating membrane closing to protect eye, Cocos Island, Costa Rica, Pacific Ocean.

LYNN M. STONE / King vulture (Sarcoramphus papa) showing nictitating membrane, captive

Graham Eaton / White tailed sea eagle (Haliaeetus albicilla) portrait, captive bred bird, showing nictitating membrane, UK, September

Angelo Gandolfi / Eagle owl (Bubo bubo) portrait with translucent nictitating membrane in mid-blink, captive

Michael Hutchinson / Dipper (Cinclus cinclus), yawning / gaping with nictitating membrane over eye, Derbyshire, UK

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

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

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas), the golden webbed skin is a nictitating membrane, part of the eye lid and enables the frog to see without giving up its camouflage

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

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye with retracting nictitating membrane

Jabruson / Emu (Dromaius novaehollandiae) showing nictitating membrane response, UK, captive

Tony Heald / Giant eagle owl {Bubo lacteus} showing pink eyelids, Laikipia, Kenya

Heidi & Hans-Juergen Koch / Red-eyed Tree Frog (Agalychnis callidryas) male with nictitating membrane over eyes, clings onto female in amplexus, stimulating her to lay eggs, native to the tropical rainforests of Central America

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 / Red-eyed Tree Frog (Agalychnis callidryas) close-up portrait showing nictitating membrane which protects the eyes from danger but does not completely restrict vision, native to tropical rainforests of...

Colin Seddon / Portrait of Great horned owl {Bubo virginianus} showing one nictitating membrane, UK, captive

David Fleetham / Galapagos shark (Carcharhinus galapagensis) with mouth open at surface, Hawaii.

David Fleetham / Galapagos shark (Carcharhinus galapagensis), with head out of the water biting photographer's camera housing, Hawaii.

David Fleetham / Nictitating membrane over eye of oceanic whitetip shark (Carcharhinus longimanus), Hawaii.

Grant McDowell / Common raven {Corvus corax} soiled from mobbing thrushes. Note nictitating membrane. Not available for ringtone/wallpaper use.

Klaus Nigge / Philippine eagle (Pithecophaga jefferyi) blinking with crest raised, showing nictitating membrane, Philippine Eagle Centre, Malagos, Davao, Mindanao, Philippines, August 2006. Critically endangered....

Andy Rouse / Brown fish owl (Ketoupa zeylonensis) fishing in shallow water, with nictitating membrane visible on one eye, Ranthambhore, India.

Andy Rouse / Brown fish owl (Ketoupa zeylonensis) spreading wings whilst fishing in shallow water, Ranthambhore, India.

Eric Baccega / Buzzard (Buteo buteo) with nictitating membrane visible, perched on tree stump, Lorraine, France. March.

Daniel Nunez / Red eyed tree frog (Agalychnis callidryas) pair in amplexus at night, Calakmul Biosphere Reserve, Campeche, Mexico. Cropped.

Daniel Nunez / Black-eyed tree frogs (Agalychnis moreleti) pair in amplexus, male with nictitating membrane visible, Finca El Amate, Escuintla, Guatemala.

Daniel Nunez / Black-eyed tree frogs (Agalychnis moreleti) pair in amplexus, male with nictitating membrane visible, Finca El Amate, Escuintla, Guatemala.

Manuel Gracia / Booted eagle (Hieraaetus pennatus) looking around and calling. The animal blinks showing its nictitating membrane. Seville, Spain.

Manuel Gracia / Booted eagle (Hieraaetus pennatus) head-shot. The animal blinks showing its nictitating membrane. Seville, Spain.

Manuel Gracia / Tracking shot of a Booted eagle (Hieraaetus pennatus) standing on a log and cleaning its beak. The animal blinks repeatedly showing its nictitating membrane. Seville, Spain.

Joel Sartore / Photo Ark / Starrett's frog (Isthmohyla tica) profile. The animal seated, breathing. The animal then closes its eye, showing its nictitating membrane. La Libia, Costa Rica. Critically Endangered. Controlled condi...

Joel Sartore / Photo Ark / Mountain yellow-legged frog (Rana muscosa) lying down, breathing. The animal opens its eye, showing its nictitating membrane. Then the frog jumps out of the frame. Oakland Zoo, California, USA. Captiv...

Joel Sartore / Photo Ark / Mountain yellow-legged frog (Rana muscosa) lying down and opening its eye, showing its nictitating membrane. The animal is showing respiration. The frog then jumps out of the frame. Oakland Zoo, Calif...

Joel Sartore / Photo Ark / Foothill yellow-legged frog (Rana boylii), sitting upright, breathing. The animal is opening and closing its eye, showing its nictitating membrane. Oakland Zoo, California, USA. Captive.

Franco Banfi / Alpine newt (Ichthyosaura alpestris) swimming in lake, nictitating membrane visible, portrait, upper Verzasca valley, Ticino, Switzerland. August.

Harry Collins / Belted kingfisher (Megaceryle alcyon) perched overlooking a small pond. Animal is showing nictitating membrane. Florida, USA.

Doug Perrine / Tiger shark (Galeocerdo cuvier) with remora / sharksucker attached to lower jaw, and nictitating membrane partially covering eye to protect it; shark has slight injury to eye socket, revealing more of...

Neil Aldridge / Common toad (Bufo bufo) in a meadow closing and opening eye showing nictitating membrane and eyelid, Bunloit Estate, Scotland, summer.

Harry Collins / Peregrine falcon (Falco peregrinus) perched and closing eye showing nictitating membrane and eyelid, New Jersey, USA, June.

Roland Seitre / King vulture (Sarcoramphus papa) yawning, showing its protective nictitating membrane or "third eyelid", head portrait, Parque Jaime Duque, Bogota, Colombia. Captive.

David Fleetham / Galapagos shark (Carcharhinus galapagensis), listed as potentially dangerous, at sea surface, with nictitating membrane covering its eye, Hawaii, Pacific Ocean.

David Weiller / Long-nosed horned frog (Pelobatrachus nasutus) juvenile close up portrait, closing its eyes, showing the nictitating membrane, Crocker Range, Sabah, Borneo, July.

Tony Wu / Slaty-backed gull (Larus schistisagus) eating Righteye flounder (Pleuronectidae) stolen from Common merganser (Mergus merganser). The bird's nictitating membrane closed to protect its eyes while...

Tony Wu / Slaty-backed gull (Larus schistisagus) eating Righteye flounder (Pleuronectidae) stolen from Common merganser (Mergus merganser). The bird's nictitating membrane closed to protect its eyes while...

Nick Upton / Common / Eurasian crane (Grus grus) blinking close up, with nictitating membrane covering the eye, captive, WWT Slimbridge, Gloucestershire, UK, February.