Editing Exoskeleton (section)

== Role ==
Exoskeletons contain rigid and resistant components that fulfil a set of functional roles in addition to [[structural support]] in many animals, including protection, respiration, excretion, sensation, feeding and [[courtship display]], and as an osmotic barrier against [[desiccation]] in terrestrial organisms. Exoskeletons have roles in defence from parasites and predators and in providing attachment points for [[muscle|musculature]].<ref name="Bengtson2004">{{Cite conference |title=Early skeletal fossils | author=S. Bengtson |editor1=J. H. Lipps |editor2=B. M. Waggoner |book-title=Neoproterozoic–Cambrian Biological Revolutions |year=2004 |journal=Paleontological Society Papers |volume=10 |pages=67–78 |archive-url=https://web.archive.org/web/20081003122817/http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |archive-date=2008-10-03|url=http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf}}</ref>

[[Arthropod exoskeleton]]s contain [[chitin]]; the addition of [[calcium carbonate]] makes them harder and stronger, at the price of increased weight.<ref name="Nedin 1999">{{cite journal |author=Nedin, C. |year=1999 | title=''Anomalocaris'' predation on nonmineralized and mineralized trilobites |journal=Geology |volume=27 | issue=11 |pages=987–990 |doi=10.1130/0091-7613(1999)027<0987:APONAM>2.3.CO;2 |bibcode=1999Geo....27..987N}}</ref> Ingrowths of the [[arthropod exoskeleton]] known as '''apodemes''' serve as attachment sites for muscles. These structures are composed of chitin and are approximately six times stronger and twice the stiffness of vertebrate [[tendon]]s. Similar to tendons, apodemes can stretch to store [[elastic energy]] for jumping, notably in [[locust]]s.<ref>{{cite journal |author=H. C. Bennet-Clark |year=1975 |title=The energetics of the jump of the locust, ''Schistocerca gregaria'' |journal=[[Journal of Experimental Biology]] |volume=63 |pages=53–83 |pmid=1159370 |url=http://jeb.biologists.org/cgi/reprint/63/1/53.pdf |issue=1|doi=10.1242/jeb.63.1.53 }}</ref> Calcium carbonates constitute the shells of molluscs, [[brachiopod]]s, and some tube-building [[polychaete]] worms. [[Silicon dioxide|Silica]] forms the exoskeleton in the microscopic [[diatom]]s and [[Radiolarian|radiolaria]]. One mollusc species, the [[scaly-foot gastropod]], even uses the iron sulfides [[greigite]] and [[pyrite]].{{cn|date=May 2023}}

Some organisms, such as some [[foraminifera]], [[agglutination (biology)|agglutinate]] exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, [[echinoderm]]s do not possess an exoskeleton and their [[Test (biology)|test]] is always contained within a layer of living tissue.{{cn|date=May 2023}}

Exoskeletons have evolved independently many times; 18 lineages evolved [[Calcification|calcified]] exoskeletons alone.<ref name=Porter2007>{{cite journal | author=Susannah M. Porter | year=2007 | title=Seawater chemistry and early carbonate biomineralization | journal=[[Science (journal)|Science]] | volume=316 | issue=5829 | page=1302 | doi=10.1126/science.1137284 | pmid=17540895 | bibcode=2007Sci...316.1302P| s2cid=27418253 }}</ref> Further, other lineages have produced tough outer coatings, such as some mammals, that are analogous to an exoskeleton. This coating is constructed from bone in the [[armadillo]], and hair in the [[pangolin]]. The armour of reptiles like turtles and dinosaurs like [[Ankylosauria|Ankylosaur]]s is constructed of bone; [[crocodile]]s have bony [[scute]]s and [[Horn (anatomy)|horn]]y scales.