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.