An eyelike structure, with a rudimentary lens and retina, found on the top of the head of some lizards, frogs, salamanders, sharks, fishes, the Cyclostomata (lampreys) as well as in many fossil vertebrates. The most notable example is found in the tuatara (Sphenodon) of New Zealand, a ‘living fossil’ representative of an order of early reptiles. In adults the median eye is generally covered with skin or scales, and acts as a photoreceptor, detecting changes in light intensity and modifying the physiology and behaviour of the animal, particularly its circadian rhythms. In lizards the median eye develops from part of the forebrain in close association with the pineal sac (which becomes the pineal gland in animals that have lost the median eye, such as birds and mammals).