Some mammals, such as camels, polar bears, and seals, have full nictitating membranes. That is the transparent or translucent third eyelid that can be drawn across the eye for protection and to moisten it while maintaining visibility.

In humans, the plica semilunaris (also known as the semilunar fold) and its associated muscles are thought to be homologous to the nictitating membranes seen in other animals. In most primate species, a plica semilunaris is present, but a fully developed nictitating membranes still exists in lemurs and lorisiform primates.

Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball (shown below). In some diving animals, such as beavers and manatees, it moves across the e ...