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A parietal eye (third eye, pineal eye) is a part of the epithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland, which regulates circadian rhythmicity and hormone production for thermoregulation.^[1] The hole that contains the eye is known as the pineal foramen or parietal foramen, because it is often enclosed by the parietal bones.

The parietal eye was discovered by Franz Leydig, in 1872, from work with lizards.^[2]

Discovery

Franz Leydig, a professor of zoology at the University of Tübingen, dissected four species of European lizards—the slow worm (Anguis fragilis) and three species of Lacerta.^[2] in 1872;^[3] He found cup-like protrusions under the middles of their brains. He believed the protrusions to be glandular and called them frontal organs (German Stirnorgan).^[2]

In 1886, Walter Baldwin Spencer, an anatomist at the University of Oxford, reported the results of his dissection of 29 species of lizards; he noted the presence of the same structure that Leydig had described. Spencer called it the pineal eye or parietal eye and noticed that it was associated with the parietal foramen and the pineal stalk.^[4] In 1918, Nils Holmgren, a Swedish zoologist, found the pineal eye in frogs and dogfish.^[5] He noted that the structure contained sensory cells that looked like the cone cells of the retina,^[6] and hypothesised that the pineal eye could be a primitive light-sensing organ (photoreceptor). The organ has become popularly known as the "third eye".^[5]

Presence in various animals

The parietal eye is found in the tuatara, most lizards, frogs, salamanders, certain bony fish, sharks, and lampreys.^[7]^[8]^[9] It is absent in mammals but was present in their closest extinct relatives, the therapsids, suggesting that it was lost during the course of the mammalian evolution due to it being useless in endothermic animals.^[10] It is also absent in the ancestrally endothermic ("warm-blooded") archosaurs such as birds. The parietal eye is also lost in ectothermic ("cold-blooded") archosaurs like crocodilians, and in turtles, which may be grouped with archosaurs in Archelosauria.^[11] Despite being lepidosaurs, as lizards and tuatara are, snakes lack a parietal eye.^[12]^[13]

Anatomy

The third eye is much smaller than the main paired eyes; in living species, it is always covered by skin, and is usually not readily visible externally.^[14] The parietal eye is a part of the epithalamus, which can be divided into two major parts—the epiphysis (the pineal organ; or the pineal gland, if it is mostly endocrine) and the parapineal organ (often called the parietal eye or, if it is photoreceptive, the third eye). The structures arise as a single anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon; during development, it divides into two bilaterally somewhat symmetric organs, which rotate their location to become a caudal pineal organ and a parapineal organ. In some species, the parietal eye protrudes through the skull.^[15]^[16] The parietal eye's way ^{[further explanation needed]} of detecting light differs from the use of rod cells and cone cells in a normal vertebrate eye.^[17]

Many of the oldest fossil vertebrates, including ostracoderms, placoderms, crossopterygians, and early tetrapods, have in their skulls sockets that appear to have held functional third eyes. The socket remains as a foramen between the parietal bones in many living amphibians and reptiles, although it has vanished in birds and mammals.

Lampreys have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. 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 was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above.^[14] Saniwa, an extinct varanid lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ. Saniwa is the only known jawed vertebrate to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in lepidosaurs it is formed from the parapineal organ, which suggests that Saniwa re-evolved the pineal eye.^[18]

Comparative anatomy

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 further, 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.^[19] 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.^[20] 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.^[21]

Analogs in other species

Crustaceans at the nauplius stage (first-stage larva) have a single eye atop the head. The eye has a lens and senses the direction of light but can not resolve details. More sophisticated segmented eyes develop later on the sides of their heads, but the initial eye also stays for some time. Thus it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like the brine shrimp, retain the primary eye throughout all stages of their life. Most arthropods have one or more simple eyes, called ocelli, between their main, compound eyes.^[22]

References

^ Eakin, R. M (1973). The Third Eye. Berkeley: University of California Press.
^ ^a ^b ^c Eakin, Richard M. (1973), "3 Structure", The Third Eye, University of California Press, pp. 32–84, doi:10.1525/9780520326323-004, ISBN 978-0-520-32632-3, retrieved 2023-03-28
^ Flemming, A. F. (1991). "A third eye". Culna (40): 26–27 – via Sabinet.
^ Spencer, Sir Baldwin (1885). "On the Presence and Structure of the Pineal Eye in Lacertilia". Quarterly Journal of Microscopy. London. pp. 1–76.{{cite book}}: CS1 maint: location missing publisher (link)
^ ^a ^b Wurtman, R. J.; Axelrod, J. (1965). "The pineal gland". Scientific American. 213 (1): 50–60. Bibcode:1965SciAm.213a..50W. doi:10.1038/scientificamerican0765-50. ISSN 0036-8733. PMID 14298722.
^ Pearce, J.M.S. (2022). "The pineal: seat of the soul". Hektoen International. ISSN 2155-3017. Retrieved 2023-03-28.
^ Dodt, Eberhard (1973). "The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates". Visual Centers in the Brain. Handbook of Sensory Physiology. Vol. 7 / 3 / 3 B. Springer, Berlin, Heidelberg. pp. 113–140. doi:10.1007/978-3-642-65495-4_4. ISBN 9783642654978.
^ Uetz, Peter (2003-10-07). "Sphenodontidae". The EMBL reptile database. European Molecular Biology Laboratory, heidelberg. Archived from the original on 2007-02-21. Retrieved 2007-02-22.
^ Gundy, GC; Wurst, GZ (1976). "The occurrence of parietal eyes in recent Lacertilia (Reptilia)". Journal of Herpetology. 10 (2): 113–121. doi:10.2307/1562791. JSTOR 1562791.
^ Benoit, Julien; Abdala, Fernando; Manger, Paul R.; Rubidge, Bruce S. (2016-03-17). "The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids". Acta Palaeontologica Polonica. 61 (4): 777–789. doi:10.4202/app.00219.2015. ISSN 0567-7920.
^ Emerling, Christopher A. (2017-03-01). "Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck". Molecular Biology and Evolution. 34 (3): 666–676. doi:10.1093/molbev/msw265. ISSN 1537-1719. PMID 27940498.
^ Infectious diseases and pathology of reptiles : color atlas and text. Elliott R. Jacobson. Boca Raton. 2007. p. 21. ISBN 978-1-4200-0403-8. OCLC 317753687.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
^ Tosini, Gianluca (1997-10-01). "The pineal complex of reptiles: Physiological and behavioral roles". Ethology Ecology & Evolution - ETHOL ECOL EVOL. 9 (4): 314. Bibcode:1997EtEcE...9..313T. doi:10.1080/08927014.1997.9522875.
^ ^a ^b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 471–473. ISBN 978-0-03-910284-5.
^ Light-sensitive organs that evaginate from the diencephalon - NCBI
^ Zug, George; Vitt, Laurie Vitt; Caldwell, Janalee (2002). Herpetology: An introductory biology of amphibians and reptiles (Second ed.). San Diego, CA: Academic Press. p. 75. ISBN 978-0-12-782622-6.
^ Xiong, Wei-Hong; Solessio, Eduardo C.; Yau, King-Wai (1998). "An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor". Nature Neuroscience. 1 (5): 359–365. doi:10.1038/1570. PMID 10196524. Retrieved 2007-02-22.
^ Smith, Krister T.; Bhullar, Bhart-Anjan S.; Köhler, Gunther; Habersetzer, Jörg (2 April 2018). "The only known jawed vertebrate with four eyes and the bauplan of the pineal complex". Current Biology. 28 (7): 1101–1107.e2. Bibcode:2018CBio...28E1101S. doi:10.1016/j.cub.2018.02.021. ISSN 0960-9822. PMID 29614279.
^ "FRONTAL AND PARIETAL BONES=". Retrieved 2011-09-08.
^ "Edible Frog Brain Clipart". Etc.usf.edu. Retrieved 2011-09-08.
^ Journal of morphology - Google Books. 1887. Retrieved 2011-09-08.
^ Mayer, Georg (2006-12-01). "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?". Arthropod Structure & Development. 35 (4): 231–245. Bibcode:2006ArtSD..35..231M. doi:10.1016/j.asd.2006.06.003. ISSN 1467-8039. PMID 18089073.

[:0-1] Eakin, R. M (1973). The Third Eye. Berkeley: University of California Press.

[:1-2] Eakin, Richard M. (1973), "3 Structure", The Third Eye, University of California Press, pp. 32–84, doi:10.1525/9780520326323-004, ISBN 978-0-520-32632-3, retrieved 2023-03-28

[3] Flemming, A. F. (1991). "A third eye". Culna (40): 26–27 – via Sabinet.

[4] Spencer, Sir Baldwin (1885). "On the Presence and Structure of the Pineal Eye in Lacertilia". Quarterly Journal of Microscopy. London. pp. 1–76.{{cite book}}: CS1 maint: location missing publisher (link)

[:2-5] Wurtman, R. J.; Axelrod, J. (1965). "The pineal gland". Scientific American. 213 (1): 50–60. Bibcode:1965SciAm.213a..50W. doi:10.1038/scientificamerican0765-50. ISSN 0036-8733. PMID 14298722.

[6] Pearce, J.M.S. (2022). "The pineal: seat of the soul". Hektoen International. ISSN 2155-3017. Retrieved 2023-03-28.

[7] Dodt, Eberhard (1973). "The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates". Visual Centers in the Brain. Handbook of Sensory Physiology. Vol. 7 / 3 / 3 B. Springer, Berlin, Heidelberg. pp. 113–140. doi:10.1007/978-3-642-65495-4_4. ISBN 9783642654978.

[8] Uetz, Peter (2003-10-07). "Sphenodontidae". The EMBL reptile database. European Molecular Biology Laboratory, heidelberg. Archived from the original on 2007-02-21. Retrieved 2007-02-22.

[9] Gundy, GC; Wurst, GZ (1976). "The occurrence of parietal eyes in recent Lacertilia (Reptilia)". Journal of Herpetology. 10 (2): 113–121. doi:10.2307/1562791. JSTOR 1562791.

[10] Benoit, Julien; Abdala, Fernando; Manger, Paul R.; Rubidge, Bruce S. (2016-03-17). "The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids". Acta Palaeontologica Polonica. 61 (4): 777–789. doi:10.4202/app.00219.2015. ISSN 0567-7920.

[11] Emerling, Christopher A. (2017-03-01). "Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck". Molecular Biology and Evolution. 34 (3): 666–676. doi:10.1093/molbev/msw265. ISSN 1537-1719. PMID 27940498.

[12] Infectious diseases and pathology of reptiles : color atlas and text. Elliott R. Jacobson. Boca Raton. 2007. p. 21. ISBN 978-1-4200-0403-8. OCLC 317753687.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)

[13] Tosini, Gianluca (1997-10-01). "The pineal complex of reptiles: Physiological and behavioral roles". Ethology Ecology & Evolution - ETHOL ECOL EVOL. 9 (4): 314. Bibcode:1997EtEcE...9..313T. doi:10.1080/08927014.1997.9522875.

[VB-14] Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 471–473. ISBN 978-0-03-910284-5.

[15] Light-sensitive organs that evaginate from the diencephalon - NCBI

[16] Zug, George; Vitt, Laurie Vitt; Caldwell, Janalee (2002). Herpetology: An introductory biology of amphibians and reptiles (Second ed.). San Diego, CA: Academic Press. p. 75. ISBN 978-0-12-782622-6.

[17] Xiong, Wei-Hong; Solessio, Eduardo C.; Yau, King-Wai (1998). "An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor". Nature Neuroscience. 1 (5): 359–365. doi:10.1038/1570. PMID 10196524. Retrieved 2007-02-22.

[18] Smith, Krister T.; Bhullar, Bhart-Anjan S.; Köhler, Gunther; Habersetzer, Jörg (2 April 2018). "The only known jawed vertebrate with four eyes and the bauplan of the pineal complex". Current Biology. 28 (7): 1101–1107.e2. Bibcode:2018CBio...28E1101S. doi:10.1016/j.cub.2018.02.021. ISSN 0960-9822. PMID 29614279.

[19] "FRONTAL AND PARIETAL BONES=". Retrieved 2011-09-08.

[20] "Edible Frog Brain Clipart". Etc.usf.edu. Retrieved 2011-09-08.

[21] Journal of morphology - Google Books. 1887. Retrieved 2011-09-08.

[22] Mayer, Georg (2006-12-01). "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?". Arthropod Structure & Development. 35 (4): 231–245. Bibcode:2006ArtSD..35..231M. doi:10.1016/j.asd.2006.06.003. ISSN 1467-8039. PMID 18089073.

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@@ Line 1: / Line 1: @@
+{{short description|Part of the epithalamus}}
-[[Image:Frog parietal eye.JPG|thumb|right|250px|The parietal [[eye]] (very small grey oval between the regular eyes) of a juvenile bullfrog (''[[Rana catesbeiana]]'')]]
-[[Image:Anolis carolinensis parietal eye.JPG|thumb|right|250px|Adult [[green anole]] (''Anolis carolinensis'') clearly showing the parietal eye (small grey/clear oval) at the top of its head.]]
+[[Image:Frog parietal eye.JPG|thumb|right|250px|The parietal [[eye]] (very small grey oval between the regular eyes) of a juvenile bullfrog (''[[Lithobates catesbeianus]]'')]]
+[[Image:Anolis carolinensis parietal eye.JPG|thumb|right|250px|Adult [[green anole]] (''Anolis carolinensis'') clearly showing the parietal eye (small grey/clear oval) at the top of its head]]
 [[Image:Madagascar spiny tailed iguana cropped.jpg|thumb|right|250px|Parietal eye of the Merrem's Madagascar swift (''[[Oplurus cyclurus]]'') is surrounded by a black-and-white spot on the skin, giving it the "three-eyed" appearance]]
-A '''parietal eye''', also known as a '''third eye''' or '''pineal eye''', is a part of the [[epithalamus]] present in some species of fish, amphibians and reptiles. The eye is located at the top of the head, is photoreceptive and is associated with the [[pineal gland]], regulating [[circadian rhythm]]icity and hormone production for [[thermoregulation]].<ref name=":0">{{cite book
+A '''parietal eye''' ('''third eye''', '''pineal eye''') is a part of the [[epithalamus]] in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the [[pineal gland]], which regulates [[circadian rhythm]]icity and hormone production for [[thermoregulation]].<ref name=":0">{{cite book
   | last =Eakin
   | first =R. M
@@ Line 11: / Line 12: @@
   | year =1973
   | location =Berkeley
+}}</ref> The hole that contains the eye is known as the '''pineal foramen''' or '''parietal foramen''', because it is often enclosed by the [[Parietal bone|parietal bones]].
-}}</ref>
+The parietal eye was discovered by [[Franz Leydig]], in 1872, from work with [[Lizard|lizards]].<ref name=":1">{{Citation |last=Eakin |first=Richard M. |title=3 Structure |date=1973 |url=https://www.degruyter.com/document/doi/10.1525/9780520326323-004/html |work=The Third Eye |pages=32–84 |access-date=2023-03-28 |publisher=University of California Press |doi=10.1525/9780520326323-004 |isbn=978-0-520-32632-3}}</ref>
+== Discovery ==
+Franz Leydig, a professor of zoology at the [[University of Tübingen]], dissected four species of European lizards—the [[slow worm]] (''[[Anguis fragilis]]'') and three species of ''[[Lacerta (genus)|Lacerta]]''.<ref name=":1" /> in 1872;<ref>{{Cite journal |last=Flemming |first=A. F. |date=1991 |title=A third eye |url=https://journals.co.za/doi/pdf/10.10520/AJA10162275_149 |journal=Culna |issue=40 |pages=26–27 |via=Sabinet}}</ref> He found cup-like protrusions under the middles of their brains. He believed the protrusions to be glandular and called them frontal organs (German ''Stirnorgan'').<ref name=":1" />
+In 1886, [[Walter Baldwin Spencer]], an anatomist at the [[University of Oxford]], reported the results of his dissection of 29 species of lizards; he noted the presence of the same structure that Leydig had described. Spencer called it the pineal eye or parietal eye and noticed that it was associated with the [[Parietal eye|parietal foramen]] and the pineal stalk.<ref>{{Cite book |last=Spencer |first=Sir Baldwin |url=https://books.google.com/books?id=kvwXAAAAYAAJ&dq=baldwin+spencer+third+eye+1886&pg=PA1 |title=Quarterly Journal of Microscopy |date=1885 |location=London |pages=1–76 |language=en |chapter=On the Presence and Structure of the Pineal Eye in Lacertilia}}</ref> In 1918, [[Nils Holmgren]], a [[Swedes|Swedish]] zoologist, found the pineal eye in [[Frog|frogs]] and [[Plectorhinchus lineatus|dogfish]].<ref name=":2">{{Cite journal |last1=Wurtman |first1=R. J. |last2=Axelrod |first2=J. |date=1965 |title=The pineal gland |url=https://pubmed.ncbi.nlm.nih.gov/14298722 |journal=Scientific American |volume=213 |issue=1 |pages=50–60 |doi=10.1038/scientificamerican0765-50 |issn=0036-8733 |pmid=14298722|bibcode=1965SciAm.213a..50W }}</ref> He noted that the structure contained sensory cells that looked like the [[cone cells]] of the retina,<ref>{{Cite news |last=Pearce |first=J.M.S. |date=2022 |title=The pineal: seat of the soul |language=en-US |work=Hektoen International |url=https://hekint.org/2022/04/20/the-pineal-seat-of-the-soul/ |access-date=2023-03-28 |issn=2155-3017}}</ref> and hypothesised that the pineal eye could be a primitive light-sensing organ ([[Photoreceptor cell|photoreceptor]]). The organ has become popularly known as the "third eye".<ref name=":2" />
 ==Presence in various animals==
-The parietal eye is found in the [[tuatara]], most [[lizard]]s, [[frog]]s, [[salamander]]s, certain bony fish, [[shark]]s, and [[lamprey]]s (a kind of [[Agnatha|jawless fish]]).<ref>{{Cite book|title=Visual Centers in the Brain|volume = 7 / 3 / 3 B|last=Dodt|first=Eberhard|date=1973|publisher=Springer, Berlin, Heidelberg|isbn=9783642654978|series=Handbook of Sensory Physiology|pages=113–140|language=en|doi=10.1007/978-3-642-65495-4_4|chapter = The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates}}</ref><ref>{{cite web|url=http://www.embl-heidelberg.de/~uetz/families/Sphenodontidae.html|title=Sphenodontidae|last=Uetz|first=Peter|date=2003-10-07|work=The EMBL reptile database|publisher=European Molecular Biology Laboratory, heidelberg|archive-url=https://web.archive.org/web/20070221195027/http://www.embl-heidelberg.de/~uetz/families/Sphenodontidae.html <!-- Bot retrieved archive -->|archive-date=2007-02-21|access-date=2007-02-22}}</ref><ref>{{cite journal|last1=Gundy|first1=GC|last2=Wurst|first2=GZ|date=1976|title=The occurrence of parietal eyes in recent Lacertilia (Reptilia)|journal=Journal of Herpetology|volume=10|issue=2|pages=113–121|doi=10.2307/1562791|jstor=1562791}}</ref> It is absent in [[mammal]]s, but was present in their closest [[Extinction|extinct]] relatives, the [[therapsid]]s, suggesting it was lost during the course of the mammalian evolution due to it being useless in [[endothermic]] animals.<ref>{{Cite journal|last1=Benoit|first1=Julien|last2=Abdala|first2=Fernando|last3=Manger|first3=Paul R.|last4=Rubidge|first4=Bruce S.|date=2016-03-17|title=The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids|journal=Acta Palaeontologica Polonica|volume=61|issue=4|pages=777–789|doi=10.4202/app.00219.2015|issn=0567-7920|doi-access=free}}</ref> It is also absent in [[turtle]]s and in [[archosaur]]s, which includes [[bird]]s and [[crocodilia]]ns, and [[Dinosaur|their extinct relatives]].<ref>{{Cite journal|last=Emerling|first=Christopher A.|date=2017-03-01|title=Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck|journal=Molecular Biology and Evolution|volume=34|issue=3|pages=666–676|doi=10.1093/molbev/msw265|issn=1537-1719|pmid=27940498|doi-access=free}}</ref>
+The parietal eye is found in the [[tuatara]], most lizards, frogs, [[salamander]]s, certain [[osteichthyes|bony fish]], [[shark]]s, and [[lamprey]]s.<ref>{{Cite book|title=Visual Centers in the Brain|volume = 7 / 3 / 3 B|last=Dodt|first=Eberhard|date=1973|publisher=Springer, Berlin, Heidelberg|isbn=9783642654978|series=Handbook of Sensory Physiology|pages=113–140|language=en|doi=10.1007/978-3-642-65495-4_4|chapter = The Parietal Eye (Pineal and Parietal Organs) of Lower Vertebrates}}</ref><ref>{{cite web|url=http://www.embl-heidelberg.de/~uetz/families/Sphenodontidae.html|title=Sphenodontidae|last=Uetz|first=Peter|date=2003-10-07|work=The EMBL reptile database|publisher=European Molecular Biology Laboratory, heidelberg|archive-url=https://web.archive.org/web/20070221195027/http://www.embl-heidelberg.de/~uetz/families/Sphenodontidae.html <!-- Bot retrieved archive -->|archive-date=2007-02-21|access-date=2007-02-22}}</ref><ref>{{cite journal|last1=Gundy|first1=GC|last2=Wurst|first2=GZ|date=1976|title=The occurrence of parietal eyes in recent Lacertilia (Reptilia)|journal=Journal of Herpetology|volume=10|issue=2|pages=113–121|doi=10.2307/1562791|jstor=1562791}}</ref> It is absent in [[mammal]]s but was present in their closest [[Extinction|extinct]] relatives, the [[therapsid]]s, suggesting that it was lost during the course of the mammalian evolution due to it being useless in [[endotherm|endothermic]] animals.<ref>{{Cite journal|last1=Benoit|first1=Julien|last2=Abdala|first2=Fernando|last3=Manger|first3=Paul R.|last4=Rubidge|first4=Bruce S.|date=2016-03-17|title=The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids|journal=Acta Palaeontologica Polonica|volume=61|issue=4|pages=777–789|doi=10.4202/app.00219.2015|issn=0567-7920|doi-access=free}}</ref> It is also absent in the ancestrally endothermic ("warm-blooded") [[archosaur]]s such as [[birds]]. The parietal eye is also lost in [[Ectotherm|ectothermic]] ("cold-blooded") archosaurs like [[Crocodilia|crocodilians]], and in [[Turtle|turtles]], which may be grouped with archosaurs in [[Archelosauria]].<ref>{{Cite journal|last=Emerling|first=Christopher A.|date=2017-03-01|title=Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck|journal=Molecular Biology and Evolution|volume=34|issue=3|pages=666–676|doi=10.1093/molbev/msw265|issn=1537-1719|pmid=27940498|doi-access=free}}</ref> Despite being [[Lepidosauria|lepidosaurs]], as lizards and tuatara are, [[snake]]s lack a parietal eye.<ref>{{Cite book|url=https://www.worldcat.org/oclc/317753687|title=Infectious diseases and pathology of reptiles : color atlas and text|date=2007|others=Elliott R. Jacobson|isbn=978-1-4200-0403-8|location=Boca Raton|pages=21|oclc=317753687}}</ref><ref>{{Cite journal|last=Tosini|first=Gianluca|date=1997-10-01|title=The pineal complex of reptiles: Physiological and behavioral roles|url=https://www.researchgate.net/publication/228648887|journal=Ethology Ecology & Evolution - ETHOL ECOL EVOL|volume=9|issue=4 |pages=314|doi=10.1080/08927014.1997.9522875|bibcode=1997EtEcE...9..313T }}</ref>
 ==Anatomy==
-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.<ref name=VB>
+The third eye is much smaller than the main paired eyes; in living species, it is always covered by skin, and is usually not readily visible externally.<ref name=VB>
 {{cite book
  |author1=Romer, Alfred Sherwood
@@ Line 28: / Line 36: @@
  |location=Philadelphia, PA
 }}
+</ref> The parietal eye is a part of the [[epithalamus]], which can be divided into two major parts—the [[pineal organ|epiphysis]] (the pineal organ; or the pineal gland, if it is mostly endocrine) and the parapineal organ (often called the ''parietal eye'' or, if it is photoreceptive, the ''third eye''). The structures arise as a single anterior [[evagination]] of the [[Pineal gland|pineal]] organ or as a separate outgrowth of the roof of the [[diencephalon]]; during development, it divides into two bilaterally somewhat symmetric organs, which rotate their location to become a caudal pineal organ and a parapineal organ. In some species, the parietal eye protrudes through the [[skull]].<ref>[https://www.ncbi.nlm.nih.gov/books/NBK153508/figure/Lambevolution.F28/ Light-sensitive organs that evaginate from the diencephalon - NCBI]</ref><ref>
-</ref>
-The parietal eye is a part of the [[epithalamus]], which can be divided into two major parts; the [[pineal organ|epiphysis]] (the pineal organ, or pineal gland if mostly endocrine) and the parapineal organ (often called the ''parietal eye'', or if it is photoreceptive, the ''third eye''). The parietal eye 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]].<ref>
 {{cite book
   | author1 = Zug, George
@@ Line 43: / Line 49: @@
   | isbn = 978-0-12-782622-6
 }}
-</ref> The parietal eye uses a different biochemical method of detecting light from that of [[rod cell]]s or [[cone cell]]s in a normal vertebrate eye.<ref>
+</ref> The parietal eye's way {{Explain|date=June 2022}} of detecting light differs from the use of [[rod cell]]s and [[cone cell]]s in a normal vertebrate eye.<ref>
 {{cite journal
   | author1=Xiong, Wei-Hong
@@ Line 58: / Line 64: @@
 </ref>
-Many of the oldest fossil vertebrates, including [[ostracoderm]]s, [[placoderm]]s, [[crossopterygian]]s, and even early [[tetrapod]]s, had a socket in the skull that appears to have held a functional third eye. This socket remains as a [[foramen]] between the [[parietal bone]]s even in many living amphibians and reptiles, although it has vanished in birds and mammals.
+Many of the oldest fossil vertebrates, including [[ostracoderm]]s, [[placoderm]]s, [[crossopterygian]]s, and early [[tetrapod]]s, have in their skulls sockets that appear to have held functional third eyes. The socket remains as a [[foramen]] between the [[parietal bone]]s in many living amphibians and reptiles, although it has vanished in birds and mammals.
-Lampreys have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. 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.<ref name=VB/>
+[[Lamprey]]s have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. 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 was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above.<ref name="VB" /> ''[[Saniwa]]'', an extinct [[Varanidae|varanid]] lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ. ''Saniwa'' is the only known [[Gnathostomata|jawed vertebrate]] to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in lepidosaurs it is formed from the parapineal organ, which suggests that ''Saniwa'' re-evolved the pineal eye.<ref>
-''[[Saniwa]]'', an extinct [[Varanidae|varanid]] lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ. ''Saniwa'' is the only known [[Gnathostomata|jawed vertebrate]] to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in [[Lepidosauria|lepidosaurs]], it is formed from the parapineal organ, which suggests that ''Saniwa'' re-evolved the pineal eye.<ref>
 {{cite journal
  |last1=Smith       |first1=Krister T.
@@ Line 74: / Line 78: @@
  |issn=0960-9822  |pmid=29614279
  |doi=10.1016/j.cub.2018.02.021 |doi-access=free
+ |bibcode=2018CBio...28E1101S
- |url=http://www.cell.com/current-biology/fulltext/S0960-9822(18)30206-9
-}}
+ }}
 </ref>
 ==Comparative anatomy==
-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 bone]]s, 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 [[sarcopterygian]]s and basal amphibians, but have moved further back in [[higher vertebrates]].<ref>{{cite web|url=http://bio.sunyorange.edu/updated2/comparative_anatomy/anat_3/s_frontal.htm |title=FRONTAL AND PARIETAL BONES= |access-date=2011-09-08}}</ref>  Likewise, in the brain of the frog, the [[diencephalon]], from which the [[pineal stalk]] arises, appears relatively further forward, as the [[cerebral hemisphere]]s are smaller but the [[Midbrain#Corpora quadrigemina|optic lobes]] are far more prominent than the human [[mesencephalon]], which is part of the [[brain stem]].<ref>{{cite web|url=http://etc.usf.edu/clipart/48500/48585/48585_frog_brain.htm |title=Edible Frog Brain Clipart |publisher=Etc.usf.edu |access-date=2011-09-08}}</ref>  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.<ref>{{cite book|url=https://books.google.com/books?id=AmqpB1W_Km0C&pg=PA260 |title=Journal of morphology - Google Books |access-date=2011-09-08|year=1887 }}</ref>
+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 bone]]s, in humans, make up a portion of the rear of the skull, far from the eyes. To understand further, note that the parietal bones formed a part of the skull lying between the eyes in [[sarcopterygian]]s and basal amphibians, but have moved further back in [[higher vertebrates]].<ref>{{cite web|url=http://bio.sunyorange.edu/updated2/comparative_anatomy/anat_3/s_frontal.htm |title=FRONTAL AND PARIETAL BONES= |access-date=2011-09-08}}</ref>  Likewise in the brain of the frog, the [[diencephalon]], from which the [[pineal stalk]] arises, appears relatively further forward, as the [[cerebral hemisphere]]s are smaller but the [[Midbrain#Corpora quadrigemina|optic lobes]] are far more prominent than the human [[mesencephalon]], which is part of the [[brain stem]].<ref>{{cite web|url=http://etc.usf.edu/clipart/48500/48585/48585_frog_brain.htm |title=Edible Frog Brain Clipart |publisher=Etc.usf.edu |access-date=2011-09-08}}</ref> 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.<ref>{{cite book|url=https://books.google.com/books?id=AmqpB1W_Km0C&pg=PA260 |title=Journal of morphology - Google Books |access-date=2011-09-08|year=1887 }}</ref>
 ==Analogs in other species==
-[[Crustacean]]s have a single eye atop the head as a [[Nauplius (larva)|nauplius]] (first-stage larva). The eye has a lens and senses the direction of light but cannot resolve more details in images. Later, more sophisticated segmented eyes develop on sides of the head while the initial eye stays for some time. So, it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like the [[brine shrimp]], retain the primary eye, being three-eyed in the adult stage. Most [[arthropod]]s have simple eyes, called [[simple eyes in arthropods#Dorsal ocelli|''ocelli'']], between their main, [[compound eye]]s.<ref>{{cite journal |last1=Mayer |first1=Georg |date=2006-12-01 |title=Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? |journal=Arthropod Structure & Development |lang=en |volume=35 |issue=4 |pages=231–245 |doi=10.1016/j.asd.2006.06.003 |pmid=18089073 |issn=1467-8039}}</ref>
+[[Crustacean]]s at the [[Nauplius (larva)|nauplius]] stage (first-stage larva) have a single eye atop the head. The eye has a lens and senses the direction of light but can not resolve details. More sophisticated segmented eyes develop later on the sides of their heads, but the initial eye also stays for some time. Thus it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like the [[brine shrimp]], retain the primary eye throughout all stages of their life. Most [[arthropod]]s have one or more simple eyes, called [[simple eyes in arthropods#Dorsal ocelli|''ocelli'']], between their main, [[compound eye]]s.<ref>{{cite journal |last1=Mayer |first1=Georg |date=2006-12-01 |title=Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? |journal=Arthropod Structure & Development |lang=en |volume=35 |issue=4 |pages=231–245 |doi=10.1016/j.asd.2006.06.003 |pmid=18089073 |bibcode=2006ArtSD..35..231M |issn=1467-8039}}</ref>
 ==See also==
-*[[Third eye]]
 *[[Arthropod eye]]
 *[[Mollusc eye]]

v t e Vision in animals
Vision	Birds Chameleons Dinosaurs Fish Toads Mammals horses dogs cats
Eyes	Arthropod eye Compound eye Eagle eye Eye shine Simple eye in invertebrates Mammalian eye human Mollusc eye cephalopod gastropod Holochroal eye Parietal eye Schizochroal eye
Evolution	Evolution of the eye Evolution of color vision Evolution of color vision in primates
Coloration	Albinism Animal coloration Aposematism Camouflage Chromatophore Counter-illumination Countershading Crypsis Deimatic behaviour Disruptive coloration coincident Eyespot (mimicry) Mimicry Structural coloration Underwater camouflage
Related topics	Animal senses Blindness in animals Eyespot apparatus Feature detection Infrared sensing in snakes Monocular deprivation Ommatidium Palpebral (bone) Pseudopupil Rhopalium Underwater vision Visual perception