In Sphenodontidae (tuatara), some lizards, Petromyzonidae (lamprey), and many fossil vertebrates, a structure on the top of the head that resembles an eye, with a lens and a retina, linked to the brain by a nerve. It is the larger and more developed of a pair of sacs that form by evagination of part of the brain. Above it there is a gap in the skull covered by almost transparent skin. The function of the eye is uncertain but probably it is involved in regulating diurnal rhythms.
Subjects: Zoology and Animal Sciences.