Editing Tuatara (section)

===Sensory organs===
[[File:Tuatara (7714490358).jpg|thumb|left|Close-up of a tuatara's head]]

==== Eyes ====
The eyes can [[Accommodation (eye)|focus]] independently, and are specialised with three types of photoreceptive cells, all with fine structural characteristics of retinal [[cone cell]]s<ref>{{cite journal |vauthors=Meyer-Rochow VB, Wohlfahrt S, Ahnelt PK |year=2005 |title=Photoreceptor cell types in the retina of the tuatara (Sphenodon punctatus) have cone characteristics |url=https://www.sciencedirect.com/science/article/abs/pii/S0968432805000521 |journal=Micron |volume=36 |issue=5 |pages=423–428 |doi=10.1016/j.micron.2005.03.009 |pmid=15896966}}</ref> used for both day and night vision, and a ''[[tapetum lucidum]]'' which reflects onto the retina to enhance vision in the dark. There is also a third eyelid on each eye, the [[nictitating membrane]]. Five visual [[opsin]] genes are present, suggesting good [[colour vision]], possibly even at low light levels.<ref name="Gemmell2020">{{cite journal |vauthors=Gemmell NJ, Rutherford K, Prost S, Tollis M, Winter D, Macey JR, Adelson DL, Suh A, Bertozzi T, Grau JH, Organ C, Gardner PP, Muffato M, Patricio M, Billis K, Martin FJ, Flicek P, Petersen B, Kang L, Michalak P, Buckley TR, Wilson M, Cheng Y, Miller H, Schott RK, Jordan MD, Newcomb RD, Arroyo JI, Valenzuela N, Hore TA, Renart J, Peona V, Peart CR, Warmuth VM, Zeng L, Kortschak RD, Raison JM, Zapata VV, Wu Z, Santesmasses D, Mariotti M, Guigó R, Rupp SM, Twort VG, Dussex N, Taylor H, Abe H, Bond DM, Paterson JM, Mulcahy DG, Gonzalez VL, Barbieri CG, DeMeo DP, Pabinger S, Van Stijn T, Clarke S, Ryder O, Edwards SV, Salzberg SL, Anderson L, Nelson N, Stone C |date=August 2020 |title=The tuatara genome reveals ancient features of amniote evolution |journal=Nature |volume=584 |issue=7821 |pages=403–409 |doi=10.1038/s41586-020-2561-9 |pmc=7116210 |pmid=32760000 |doi-access=free}}</ref>

==== Parietal eye (third eye) ====
Like some other living vertebrates, including some lizards, the tuatara has a third eye on the top of its head called the [[parietal eye]] (also called a pineal or third eye) formed by the parapineal organ, with an accompanying opening in the skull roof called the pineal or parietal foramen, enclosed by the [[parietal bone]]s.<ref name=":1">{{Cite journal |last1=Smith |first1=Krister T. |last2=Bhullar |first2=Bhart-Anjan S. |last3=Köhler |first3=Gunther |last4=Habersetzer |first4=Jörg |date=April 2018 |title=The Only Known Jawed Vertebrate with Four Eyes and the Bauplan of the Pineal Complex |journal=Current Biology |volume=28 |issue=7 |pages=1101–1107.e2 |doi=10.1016/j.cub.2018.02.021 |issn=0960-9822|doi-access=free |pmid=29614279 |bibcode=2018CBio...28E1101S }}</ref> It has its own lens, a parietal plug which resembles a [[cornea]],<ref>{{cite journal |author1=Schwab, I.R. |author2=O'Connor, G.R. |date=March 2005 |title=The lonely eye |journal=The British Journal of Ophthalmology |volume=89 |issue=3 |pages=256 |doi=10.1136/bjo.2004.059105 |pmc=1772576 |pmid=15751188}}</ref> [[retina]] with rod-like structures, and degenerated nerve connection to the brain. The parietal eye is visible only in hatchlings, which have a translucent patch at the top centre of the skull. After four to six months, it becomes covered with opaque scales and pigment.<ref name="encyclo" /> While capable of detecting light, it is probably not capable of detecting movement or forming an image.<ref>{{Cite journal |last=Jones |first=Marc E.H. |last2=Cree |first2=Alison |date=December 2012 |title=Tuatara |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982212013036 |journal=Current Biology |language=en |volume=22 |issue=23 |pages=R986–R987 |doi=10.1016/j.cub.2012.10.049}}</ref> It likely serves to regulate the [[circadian rhythm]] and possibly detect seasonal changes, and help with [[thermoregulation]].<ref name="encyclo" /><ref name=":1" />

Of all extant tetrapods, the parietal eye is most pronounced in the tuatara. It is part of the pineal complex, another part of which is the [[pineal gland]], which in tuatara secretes melatonin at night.<ref name="encyclo" /> Some [[salamander]]s have been shown to use their pineal bodies to perceive polarised light, and thus determine the position of the sun, even under cloud cover, aiding [[Animal navigation|navigation]].<ref>{{cite book |author=Halliday, T.R. |year=2002 |chapter=Salamanders and newts: Finding breeding ponds |editor1=Halliday, T. |editor2=Adler, K. |title=The New Encyclopedia of Reptiles and Amphibians |place=Oxford, UK |publisher=Oxford University Press |isbn=0-19-852507-9|page=52}}</ref>

==== Hearing ====
Together with [[turtle]]s, the tuatara has the most primitive hearing organs among the amniotes. There is no tympanum ([[eardrum]]) and no earhole,<ref name="lutzp27" /> and the [[middle ear]] cavity is filled with loose tissue, mostly [[adipose tissue|adipose (fatty) tissue]]. The [[stapes]] comes into contact with the [[Quadrate bone|quadrate]] (which is immovable), as well as the [[hyoid bone|hyoid]] and [[squamosal]]. The [[hair cell]]s are unspecialised, innervated by both [[Afferent nerve|afferent]] and [[efferent nerve]] fibres, and respond only to low frequencies. Though the hearing organs are poorly developed and primitive with no visible external ears, they can still show a frequency response from 100 to 800&nbsp;[[Hertz|Hz]], with peak sensitivity of 40&nbsp;[[Decibel|dB]] at 200&nbsp;Hz.<ref>{{cite web |author=Kaplan, Melissa |date=6 September 2003 |title=Reptile Hearing |website=Melissa Kaplan's herp care collection |url=http://www.anapsid.org/reptilehearing.html |access-date=24 July 2006}}</ref>

==== Odorant receptors ====
Animals that depend on the sense of smell to capture prey, escape from predators or simply interact with the environment they inhabit, usually have many odorant receptors. These receptors are expressed in the dendritic membranes of the neurons for the detection of odours. The tuatara has around 472 receptors, a number more similar to what birds have than to the large number of receptors that turtles and crocodiles may have.<ref name="Gemmell2020" />