Nictitating membrane
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 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. The nictitating membrane of a blue shark Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates. In humans, the plica semilunaris and its associated muscles are homologous to the nictitating membranes seen in some other mammals and other vertebrates. In most primate species, a plica semilunaris is present, although fully developed nictitating membranes can be found in lemurs and lorisoid primates.