median eye (pineal eye) An eyelike structure, with a lens and retina, found on the top of the head of some lizards, Sphenodon, and the Cyclostomata (lampreys) as well as in many fossil vertebrates. It corresponds to the pineal gland of other vertebrates and is thought to act as a photoreceptor, detecting changes in light intensity and modifying the physiology and behaviour of the animal.

median eye See PINEAL EYE.