Editing Crab (section)

==Description==
[[File:Gecarcinus quadratus (Nosara).jpg|thumb|left|''[[Gecarcinus quadratus]]'', a [[land crab]] from [[Central America]]]]
Crabs are generally covered with a thick [[exoskeleton]], composed primarily of highly mineralized [[chitin]].<ref>{{cite journal |author1=F. Boßelmann |author2=P. Romano |author3=H. Fabritius |author4=D. Raabe |author5=M. Epple |date=October 25, 2007 |title=The composition of the exoskeleton of two crustacea: The American lobster Homarus americanus and the edible crab ''Cancer pagurus'' |journal=[[Thermochimica Acta]] |volume=463 |issue=1–2 |pages=65–68 |doi=10.1016/j.tca.2007.07.018|bibcode=2007TcAc..463...65B }}</ref><ref>{{cite journal |author1=P. Chen |author2=A.Y. Lin |author3=J. McKittrick |author4=M.A. Meyers |date=May 2008 |title=Structure and mechanical properties of crab exoskeletons |journal=[[Acta Biomaterialia]] |volume=4 |issue=3 |pages=587–596 |doi=10.1016/j.actbio.2007.12.010|pmid=18299257 }}</ref> Behind their pair of [[chelae]] (claws) are six walking legs and then two swimming legs. The crab [[breathing|breathes]] through [[gills]] on its underside; gills must be at least moist to work.

Crabs vary in size from the [[pea crab]], a few millimeters wide, to the [[Japanese spider crab]], with a leg span up to {{convert|4|m|ft|abbr=on}}.<ref>{{cite web |url=http://na.oceana.org/en/explore/creatures/japanese-spider-crab |title=Japanese spider crab ''Macrocheira kaempferi'' |publisher=[[Oceana (non-profit group)|Oceana North America]] |access-date=2009-01-02 |archive-url=https://web.archive.org/web/20091114041143/http://na.oceana.org/en/explore/creatures/japanese-spider-crab |archive-date=2009-11-14 |url-status=dead }}</ref> Several other groups of crustaceans with similar appearances – such as [[king crab]]s and [[porcelain crab]]s – are not true crabs, but have evolved features similar to true crabs through a process known as [[carcinisation]].<ref>{{cite journal|author=Borradaile LA|title=Crustacea. Part II. Porcellanopagurus: an instance of carcinization|journal=British Antarctic ("Terra Nova") Expedition, 1910. Natural History Report. Zoology|year=1916|volume=3|number=3|pages=111–126}}</ref><ref>{{cite journal|author1=Martin JW|author2=Abele LG|title=Phylogenetic relationships of the genus ''Aegla'' (Decapoda: Anomura: Aeglidae), with comments on anomuran phylogeny|journal=Journal of Crustacean Biology|year=1986|volume=6|number=3|pages=576–612|doi= 10.1163/193724086X00406}}</ref><ref>{{cite journal|author1=McLaughlin PA|author2=Lemaitre R|title=Carcinization in the Anomura - fact or fiction? I. Evidence from adult morphology|journal=Contributions to Zoology|year=1997|volume=67|number=2|pages=79–123|doi=10.1163/18759866-06702001|doi-access=free}}</ref><ref>{{cite journal|author=Scholtz G|title=Evolution of crabs - history and deconstruction of a prime example of convergence|journal=Contributions to Zoology|year=2014|volume=83|number=2|pages=87–105|doi=10.1163/18759866-08302001|doi-access=free}}</ref>