Editing Chelicerata (section)

===Nervous system===
{| align="right" class="wikitable" width="40%" style="margin-left:2px"
! &nbsp; !! Cephalothorax ganglia fused into brain !! Abdominal ganglia fused into brain
|- align="center"
! [[Horseshoe crabs]]
| All || First two segments only
|- align="center"
! [[Scorpion]]s
| All || None
|- align="center"
! [[Mesothelae]]
| First two pairs only || None
|- align="center"
! Other [[arachnid]]s
| All || All
|}
Chelicerate nervous systems are based on the standard arthropod model of a pair of [[Ventral nerve cord|nerve cord]]s, each with a [[ganglion]] per segment, and a [[arthropod brain|brain]] formed by fusion of the ganglia just behind the mouth with those ahead of it.<ref name="RuppertFoxBarnes2004P531To532">{{harvnb|Ruppert|Fox|Barnes|2004|pp=531–532}}</ref> If one assume that chelicerates lose the first segment, which bears [[Antenna (biology)|antenna]]e in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two.<ref name="RuppertFoxBarnes2004ChelicerataGen" /> However, there is evidence that the first segment is indeed available and bears the cheliceres.<ref>{{cite journal |author1=Mittmann, B. |author2=Scholtz, G. |year=2003 |title=Development of the nervous system in the "head" of Limulus polyphemus (Chelicerata: Xiphosura): Morphological evidence for a correspondence between the segments of the chelicerae and of the (first) antennae of Mandibulata |journal=[[Dev Genes Evol]] |volume=213 |issue=1 |pages=9–17 |doi=10.1007/s00427-002-0285-5 |pmid=12590348 |s2cid=13101102 |df=dmy-all}}</ref><ref name=":2" />

There is a notable but variable trend towards fusion of other ganglia into the brain. The brains of [[horseshoe crabs]] include all the ganglia of the [[prosoma]] plus those of the first two opisthosomal segments, while the other opisthosomal segments retain separate pairs of ganglia.<ref name="RuppertFoxBarnes2004Xiphosura" /> In most living [[arachnid]]s, except [[scorpion]]s if they are true arachnids, ''all'' the ganglia, including those that would normally be in the opisthosoma, are fused into a single mass in the prosoma and there are no ganglia in the opisthosoma.<ref name="RuppertFoxBarnes2004ArachnidaGen" /> However, in the [[Mesothelae]], which are regarded as the most basal living spiders, the ganglia of the opisthosoma and the rear part of the prosoma remain unfused,<ref name="CoddingtonLevi1991">{{cite journal |author1=Coddington, J.A. |author2=Levi, H.W. |s2cid=55647804 |year=1991 |title=Systematics and Evolution of Spiders (Araneae) |journal=[[Annu. Rev. Ecol. Syst.]] |volume=22 |pages=565–592 |doi=10.1146/annurev.es.22.110191.003025}}</ref> and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia.<ref name="RuppertFoxBarnes2004ArachnidaGen" />