An eye is part of the animalanatomy. They are what give them their sense of sight. Most animal species have two eyes, but some have only one, and some, such as the fly, have several. In all cases, the retina of the eye is what takes in light, therefore allowing sight.Eye doctors specialize in the treatment of the eye.
When Wendy tossed an egg that she said was a FRERP egg at Marvin, causing it to splat all over his face, Marvin tells her he deduced it wasn't a FRERP egg, and that it might have been a real egg, based on the fact that he recognized the handwriting on the egg was hers. He goes on to tell her that he was certain because of the "twinkle in [her] eye and the egg on [his] face."