Solvin Zankl / Common European toads (Bufo bufo) mating, with membranes closed over eyes for protection, Solling, Central Germany.

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

Chris Mattison / Common Lizard (Zootoca vivipara), giving birth to young in thin egg membranes, Derbyshire, England, UK. This species can both lay eggs and also give birth to live young in a thin egg membrane.

Chris Mattison / Common Lizard (Zootoca vivipara), giving birth to young in thin egg membranes, Derbyshire, England, UK. This species can both lay eggs and also give birth to live young in a thin egg membrane.

Chris Mattison / Common Lizard (Zootoca vivipara), giving birth to young in thin egg membranes, Derbyshire, England, UK. This species can both lay eggs and also give birth to live young in a thin egg membrane.

Nick Upton / Serotine bat (Eptosicus serotinus) held to show its tail extending beyond its wing membranes, North Devon Bat Care, Barnstaple, Devon, UK, October 2015. Model released.

D. Parer & E. Parer-Cook / Tammar Wallaby (Macropus eugenii) birth, baby is born enclosed in fetal membranes, Australia

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

Juergen Freund / Spectacled flying foxes (Pteropus conspicillatus) Hospital President Jenny Maclean stretches out the wing of a bat, Tolga Bat Hospital, Atherton, North Queensland. January 2008.

Chris Gomersall / 2020VISION / Sea angler landing Great blue shark (Prionace glauca) on boat prior to tagging and release, Irish Sea off Pembrokeshire, Wales, UK, August 2010.

Chien Lee / Wallace's Flying Frog (Rhacophorus nigropalmatus) showing webbed membranes between toes, Kubah National Park, Malaysia

Chien Lee / Wallace's Flying Frog (Rhacophorus nigropalmatus) showing webbed membranes between toes, Kubah National Park, Malaysia