Do dogs have three eyelids?

Yes, dogs have a third eyelid which gives the eye extra protection. Underneath the eyelid is a piece of cartilage which clears debris from the cornea, much like a window washer. Tissue secretions from the third eyelid help fight infection. The third eyelid's gland also produces tear film, bathing the eye. While humans and pigs don't have a third eyelid, many animals do.