A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some animal species. The eye is photoreceptive and is associated with the pineal gland, regulating circadian rhythmicity and hormone production for thermoregulation.
Presence in various animals
The lizard-like reptile tuatara has a "well-developed parietal eye, with small lens and retina". 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 and in fish such as zebrafish. In extant birds and mammals the parietal organ (but not the pineal gland) is absent.
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. The parietal eye uses a different biochemical method of detecting light than rod cells or cone cells in a normal vertebrate eye.
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.
Among fish, lampreys retain two functional "third" eyes, one developed from the parietal gland, and the other from the pineal gland. These are one behind the other in the centre of the upper surface of the braincase. Because lampreys are among 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.
As shown in the accompanying figures, the parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the human pineal gland appears far away from this position, tucked away between the corpus callosum and cerebellum. Also the parietal bones, in humans, make up a portion of the rear of the skull, far from the eyes. To understand this, note that the parietal bones formed a part of the skull lying between the eyes in sarcopterygians and basal amphibians, but have moved further back in higher vertebrates. Likewise, in the brain of the frog, the diencephalon, from which the pineal stalk arises, appears relatively further forward, as the cerebral hemispheres are smaller but the optic lobes are far more prominent than the human mesencephalon, which is part of the brain stem. In humans the optic tract, commissure, and optic nerve bridge the substantial distance between eyes and diencephalon. Likewise the pineal stalk of Petromyzon elongates very considerably during metamorphosis.
Analogs in other species
Crustaceans have a single eye atop of the head as a nauplius (first stage larva). This eye has a lens and senses direction of light but cannot see images. Later, more sophisticated 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, the brine shrimp, retain the primary eye, being three-eyed in the adult stage.
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