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.

Stephen Dalton / Close up of ear of Common green iguana (Iguana iguana) showing tympanic membrane

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

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

Tony Phelps / Newborn Adder still in membrane {Vipera berus} Dorset, UK

Tim MacMillan / John Downer Pr / Kuhl's gecko flying - every edge of body expanded into membrane that acts as a wing as it glides (Resolution restriction - image digitised from film, 'Weird Nature' tv series)

Morley Read / Close-up of Leaf frog (Cruziohyla craspedopus) tadpoles developing within an egg membrane, blood is circulating in the gills. Pastaza Province, Ecuador. (non-ex)

Morley Read / Close-up of Leaf frog (Cruziohyla craspedopus) tadpoles developing within an egg membrane, blood is circulating in the gills. Pastaza Province, Ecuador. (non-ex)

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