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Symmetry
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===In biology=== {{Further|symmetry in biology|facial symmetry}} [[File:Chance and a Half, Posing.jpg|thumb|upright=0.8|Many animals are approximately mirror-symmetric, though internal organs are often arranged asymmetrically.]] In biology, the notion of symmetry is mostly used explicitly to describe body shapes. [[Bilateria|Bilateral animals]], including humans, are more or less symmetric with respect to the [[sagittal plane]] which divides the body into left and right halves.<ref>{{cite web |last=Valentine |first=James W. |title=Bilateria |url=http://www.accessscience.com/abstract.aspx?id=802620&referURL=http%3a%2f%2fwww.accessscience.com%2fcontent.aspx%3fid%3d802620 |publisher=AccessScience |access-date=29 May 2013 |url-status=dead |archive-url=https://web.archive.org/web/20080118213208/http://www.accessscience.com/abstract.aspx?id=802620&referURL=http%3A%2F%2Fwww.accessscience.com%2Fcontent.aspx%3Fid%3D802620 |archive-date=18 January 2008 }}</ref> Animals that move in one direction necessarily have upper and lower sides, head and tail ends, and therefore a left and a right. The [[cephalisation|head becomes specialized]] with a mouth and sense organs, and the body becomes bilaterally symmetric for the purpose of movement, with symmetrical pairs of muscles and skeletal elements, though internal organs often remain asymmetric.<ref>{{cite web | url=http://biocongroup.eu/DA/Calendario_files/Bilateria.pdf | title=Animal Diversity (Third Edition) | publisher=McGraw-Hill | work=Chapter 8: Acoelomate Bilateral Animals | year=2002 | access-date=October 25, 2012 | author1=Hickman, Cleveland P. | author2=Roberts, Larry S. | author3=Larson, Allan | page=139 | archive-date=May 17, 2016 | archive-url=http://arquivo.pt/wayback/20160517212058/http://biocongroup.eu/DA/Calendario_files/Bilateria.pdf | url-status=dead }}</ref> Plants and sessile (attached) animals such as [[sea anemone]]s often have radial or [[rotational symmetry]], which suits them because food or threats may arrive from any direction. Fivefold symmetry is found in the [[echinoderms]], the group that includes [[starfish]], [[sea urchin]]s, and [[sea lilies]].<ref>{{cite book | title=What Shape is a Snowflake? Magical Numbers in Nature | publisher=Weidenfeld & Nicolson | author=Stewart, Ian | year=2001 | pages=64–65}}</ref> In biology, the notion of symmetry is also used as in physics, that is to say to describe the properties of the objects studied, including their interactions. A remarkable property of biological evolution is the changes of symmetry corresponding to the appearance of new parts and dynamics.<ref>{{cite book |url=https://www.springer.com/la/book/9783642359378 |title=Perspectives on Organisms: Biological time, Symmetries and Singularities |last1=Longo |first1=Giuseppe |last2=Montévil |first2=Maël |date=2016 |publisher=Springer |isbn=978-3-662-51229-6 |language=en}}</ref><ref>{{cite journal |last1=Montévil |first1=Maël |last2=Mossio |first2=Matteo |last3=Pocheville |first3=Arnaud |last4=Longo |first4=Giuseppe |date=2016 |title=Theoretical principles for biology: Variation |url=https://www.academia.edu/27942089 |journal=Progress in Biophysics and Molecular Biology |series=From the Century of the Genome to the Century of the Organism: New Theoretical Approaches |volume=122 |issue=1 |pages=36–50 |doi=10.1016/j.pbiomolbio.2016.08.005|pmid=27530930 |s2cid=3671068 }}</ref>
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