For organisms such as birds and lizards living in extraordinarily dry environments, keeping the eye wet and clean would mean constant blinking and thus, many moments without sight. Loosing one's sight for even short periods of time can be a highly risky experience in a world where predators are ready to pounce. We find various forms of the nictitating membrane in a large number of terrestrial vertebrates such as birds, lizards, crocodilians, and even mammals. For example, the aardvark uses its nictitating membrane for protection against its termite prey - insects that bite at the eye. Polar bears use their membranes to prevent snow-blindness.
Seals have exceptionally large eyes which aid in finding fast-swimming prey in dim underwater light.
But, as seals also spend a considerable amount of time hauled out on the rocks, their eyes must also have some aerial adaptations. The nictitating membrane keeps their eyes wet and clean while on land and it also cove rs the eyes underwater during rapid swimming. No matter where an animal lives it will come into contact with dust and debris. A spectacle is a transparent membrane covering the eye that still allows free motion and unobstructed vision. There are three types of spectacles.