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...

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

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.

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.

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

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.

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...