Temporary covering protects from dirt and impact: mammals
The eyes of mammals are protected from dirt and impacts by eyelids.
"Being a particularly delicate instrument, the eye needs protection -- usually, an eyelid. Most mammals have two eyelids, one above and one below, but some - such as horses and deer - have a third, inner eyelid, the nictitating membrane, which may move upwards or sideways across the eyeball. Both types of eyelid can be closed to protect the eye from a blow, or from dirt; in closing - blinking - they wipe the eyeball clean and lubricate it with teardrops." (Foy and Oxford Scientific Films 1982:124)
- Foy, Sally; Oxford Scientific Films. 1982. The Grand Design: Form and Colour in Animals. Lingfield, Surrey, U.K.: BLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, London. 238 p.