The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection
and to moisten the eye while also keeping visibility. Various reptiles, birds, and sharks have a full nictitating membrane, whereas, in
many mammals, there is a small vestigial remnant of the membrane present in the corner of the eye. Some mammals, such
as camels, polar bears, seals, and aardvarks, also have a full nictitating membrane. The membrane also serves to
protect the puffin's eye while it dives under water.

I was totally unaware while we were photographing the puffins that they had this membrane until I saw my images
on the computer.