<P> The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus for protection and to moisten it while maintaining vision . 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 . </P> <P> 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 vision . The term comes from the Latin word nictare, meaning "to blink". It is often called a third eyelid or haw, and may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia . Unlike the upper and lower eyelids, the nictitating membrane moves horizontally across the eyeball . </P> <P> In many species, any stimulus to the eyeball (such as a puff of air) will result in reflex nictitating membrane response . This reflex is widely used as the basis for experiments on classical conditioning in rabbits . </P>

Where is the nictitating membrane found in a frog