Editing Chelicerata (section)

===Segmentation and cuticle===
The Chelicerata are [[arthropod]]s as they have: [[Segmentation (biology)|segmented]] bodies with jointed limbs, all covered in a [[cuticle]] made of [[chitin]] and [[protein]]s; heads that are composed of several segments that fuse during the development of the [[embryo]]; a much reduced [[coelom]]; a [[hemocoel]] through which the [[blood]] circulates, driven by a tube-like heart.<ref name="RuppertFoxBarnes2004P518">{{harvnb|Ruppert|Fox|Barnes|2004|pp=518–522}}</ref> Chelicerates' bodies consist of two [[Tagma (arthropod anatomy)|tagmata]], sets of segments that serve similar functions: the foremost one, called the [[prosoma]] or [[cephalothorax]], and the rear tagma is called the [[opisthosoma]] or [[abdomen]].<ref name="RuppertFoxBarnes2004ChelicerataGen">{{harvnb|Ruppert|Fox|Barnes|2004|pp=554–555}}</ref> However, in the [[Acari]] (mites and ticks) there is no visible division between these sections.<ref name="RuppertFoxBarnes2004Acari" />

The [[prosoma]] is formed in the embryo by fusion of the ocular somite (referred as "acron" in previous literatures), which carries the eyes and [[Labrum (arthropod mouthpart)|labrum]],<ref name=":3" /> with six post-ocular segments (somite 1 to 6),<ref name=":1">{{Cite journal|last1=Dunlop|first1=Jason A.|last2=Lamsdell|first2=James C.|title=Segmentation and tagmosis in Chelicerata|url=https://www.academia.edu/28212892|journal=Arthropod Structure & Development|language=en|volume=46|issue=3|pages=395–418|issn=1467-8039|doi=10.1016/j.asd.2016.05.002|pmid=27240897|year=2017|bibcode=2017ArtSD..46..395D }}</ref> which all have paired appendages. It was previously thought that chelicerates had lost the antennae-bearing somite 1,<ref>{{cite book |author1=Willmer, P. |author2=Willmer, P.G. |title=Invertebrate Relationships: Patterns in animal evolution |publisher=Cambridge University Press |year=1990 |isbn=978-0-521-33712-0 |page=275 |via=Google Books |url=https://books.google.com/books?id=uRrfn_5UNUQC&q=chelicerate+mouth&pg=PA275 |access-date=2008-10-14 |df=dmy-all}}</ref> but later investigations reveal that it is retained and corresponds to a pair of [[chelicera]]e or chelifores,<ref name=":2">{{Cite journal|last1=Telford|first1=Maximilian J.|last2=Thomas|first2=Richard H.|date=1998-09-01|title=Expression of homeobox genes shows chelicerate arthropods retain their deutocerebral segment|journal=Proceedings of the National Academy of Sciences|language=en|volume=95|issue=18|pages=10671–10675|issn=0027-8424|pmid=9724762|pmc=27953|doi=10.1073/pnas.95.18.10671|bibcode=1998PNAS...9510671T|doi-access=free}}</ref> small appendages that often form [[pincer (biology)|pincer]]s. Somite 2 has a pair of [[pedipalp]]s that in most sub-groups perform sensory functions, while the remaining four [[cephalothorax]] segments (somite 4 to 6) have pairs of legs.<ref name=":1" /> In basal forms the ocular somite has a pair of [[compound eye]]s on the sides and four pigment-cup [[ocelli]] ("little eyes") in the middle.<ref name="RuppertFoxBarnes2004ChelicerataGen" /> The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The [[opisthosoma]] consists of thirteen or fewer segments, may or may not end with a [[telson]].<ref name=":1" /> In some taxa such as [[scorpion]] and [[eurypterid]] the opisthosoma divided into two groups, ''mesosoma'' and ''metasoma''.<ref name=":1" /> The abdominal appendages of modern chelicerates are missing or heavily modified<ref name="RuppertFoxBarnes2004ChelicerataGen" /> – for example in [[spider]]s the remaining appendages form [[spinneret (spider)|spinnerets]] that extrude [[spider silk|silk]],<ref name="RuppertFoxBarnes2004Spiders">{{harvnb|Ruppert|Fox|Barnes|2004|pp=571–584}}</ref> while those of [[horseshoe crab]]s (Xiphosura) form [[gill]]s.<ref name="RuppertFoxBarnes2004Xiphosura">{{harvnb|Ruppert|Fox|Barnes|2004|pp=555–559}}</ref><ref name=":1" />

Like all arthropods, chelicerates' bodies and appendages are covered with a tough [[cuticle]] made mainly of chitin and chemically hardened proteins. Since this cannot stretch, the animals must [[molt]] to grow. In other words, they grow new but still soft cuticles, then cast off the old one and wait for the new one to harden. Until the new cuticle hardens the animals are defenseless and almost immobilized.<ref name="RuppertFoxBarnes2004ArthropodCuticle">{{harvnb|Ruppert|Fox|Barnes|2004|pp=521–525}}</ref>
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