The parietal eye (very small grey oval between the regular eyes) of a juvenile bullfrog (Rana catesbeiana)

Adult carolina anole (Anolis carolinensis) clearly showing the parietal eye (small grey/clear oval) at the top of its head.

A parietal eye, also known as a parietal organ or third-eye, is a part of the epithalamus present in some animal species. The eye may be photoreceptive and is usually associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.^[1]

[edit] Function

The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina".^[2][3] Parietal eyes are also found in lizards, frogs and lampreys, as well as some species of fish, such as tuna and pelagic sharks, where it is visible as a light-sensitive spot on top of their head. A poorly developed version, often called the parapineal gland, occurs in salamanders. In birds and mammals the parietal organ (but not the pineal gland) is absent.

[edit] Anatomy

The parietal eye is a part of the epithalamus, which can be divided into two major parts; the epiphysis (the pineal organ, or pineal gland if mostly endocrine) and the parietal organ (often called the parietal eye, or third eye if it is photoreceptive). It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon. In some species, it protrudes through the skull.^[4] The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal vertebrate eye.^[5]

Many of the oldest fossil vertebrates, including ostracoderms, placoderms, crossopterygians, and even early tetrapods, had a socket in the skull that appears to have held a functional third eye. This socket remains as a foramen between the parietal bones even in many living amphibians and reptiles, although it has vanished in birds and mammals. The third eye, where present, is always much smaller than the main paired eyes, and, in living species, it is always covered by skin, and is usually not readily visible externally.^[6]

Among fish, lampreys retain two functional "third" eyes, one developed from the parietal gland, and the other from the pineal gland. These are located one behind the other in the centre of the upper surface of the braincase. Because lampreys are amongst the most primitive of all living vertebrates, it is possible that this was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above.^[6]

[edit] Analogs in other species

Crustaceans such as nauplius initially has a single eye atop of the head. This eye has lens, senses direction of light but cannot see images. Later, more capable segmented eyes develop on sides of the head but the initial eye stays for some time. Hence it is possible to say that at some stage of development crustaceans have the "third eye" as well. Some species like Artemia salina retain the primary eye, being three-eyed in the adult stage.

Many arthropods have simple eyes, frequently between the main eyes, called ocelli.

Practitioners of Hinduism frequently wear a Tilaka on the forehead between the eyes, symbolizing the "third eye." See also Bindi (decoration).

[edit] References