The
nictitating membrane (from Latin , to blink) is a transparent or
translucent third
eyelid present in some animals that can be drawn across the
eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and
sharks have full nictitating membranes; in many
mammals, a small,
vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as
camels,
polar bears,
seals and
aardvarks, have full nictitating membranes. Often called a
third eyelid or
haw, it may be referred to in scientific terminology as the
plica semilunaris,
membrana nictitans or
palpebra tertia.