Fish have it easy...at least when it comes to keeping their eyes moist and free of particles. Their eyes are constantly bathed in a wet medium where dust and debris are not frequently encountered. For them, washing the eye is as simple as swimming forward.

Although fish and other aquatic animals evolved adequate vision in their watery world, it was not until animals moved onto dry land that truly sharp vision evolved. By moving out of the water, the ancestors of land animals exposed themselves to a much greater amount of light. As a result, visual acuity increased and color vision developed. But not without consequences! Living in the air means that the eye is at a high risk of drying out. When this happens, not only does the chance of infection increase, the animal's vision becomes blurry or lost.

Thus, terrestrial animals, had to evolve a way to protect the eyes from drying air and dust particles. The result — adaptations such as the lacrimal system, eyelids, the nictitating membrane and the spectacle.

The Lacrimal System:
The lacrimal system refers to tissues that make tears. Tears evolved in vertebrates to replace the saltwater bath that fish eyes constantly experience. The human lacrimal gland produces about 1 cubic cm of tears a day under normal conditions. These tears serve to wet and clean the cornea. When the eye becomes irritated by a foreign body (such as a dust particle on the cornea), tears are produced in excess and can effectively wash away the irritant. Tear fluids are rich in sugars and protein and act as a significant nutrition source for the cornea. Tears also have anti-bacterial properties making them effective at fighting infections. Tears are salty to create an osmotic balance with the fluid inside the eye.

The lacrimal system is a complicated series of ducts surrounding the eye. Tears secreted from above the upper eyelid mix first with mucus before covering the cornea. Excess fluid drains into the lacrimal sac (at the inner corner of the eye), which empties into the nasal cavity. This is why our nose gets stuffed up when we cry!

Interestingly, psychological or emotional crying is unique to humans and a certain species of bears. It appears to serve no physiological purpose and is one of the mysteries of biology.

Eyelids and the Nictitating Membrane:
Most aquatic or mostly-aquatic animals have no eyelids at all. But, as animals moved to land, eyelids evolved with the function of sweeping the tear fluid across the eye, clearing any debris from the corneal surface and protecting the eye from sudden incoming particles. The first eyelids developed in semi-aquatic animals — ancestors to such modern animals as salamanders and frogs. As animals evolved more terrestrial lifestyles, eyelids became opaque to increase protection from excess light as well as foreign particles. This means that for an instant during the blink, there is a moment of blindness. 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. Thus, the stage was set for the evolution of a third, transparent or translucent eye-lid that could sweep the eye clean without impairing vision: the nictitating membrane. The nictitating membrane is stored in the lower corner of the eye nearest the nose and it sweeps outward to clean and wet the eye, while still allowing light to penetrate.

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.

The Spectacle:
No matter where an animal lives it will come into contact with dust and debris. In cases where eyelids are not enough (or simply not present as in fishes), vertebrate animals rely on a set of protective goggles termed the spectacle. A spectacle is a transparent membrane covering the eye that still allows free motion and unobstructed vision.

There are three types of spectacles. A primary spectacle is a modification of the corneal skin to create an unattached covering over the eye. It is found in lampreys. The secondary spectacle is similar to the primary one, but with a different evolutionary origin. The secondary spectacle is formed from skin that had at some point been fused to the cornea and has since separated. It is found in fish that feed in waters where sandy or muddy debris is present and in fish whose bulging eyes would otherwise be an impediment to streamlining.