Chelicerata
50884
220470709
2008-06-20T00:46:43Z
Wloveral
3573669
Subphylum and phylum w/o caps
{{Taxobox
| name = Chelicerata
| fossil_range = [[Cambrian]] - recent
| image = horseshoe crab female.jpg
| image_width = 250px
| image_caption = [[Horseshoe crab]]
| regnum = [[Animal]]ia
| phylum = [[Arthropod]]a
| subphylum = '''Chelicerata'''
| subphylum_authority = [[Richard Heymons|Heymons]], 1901
| subdivision_ranks = Classes
| subdivision = [[Arachnida]]<br />
[[Xiphosura]]<br />
[[Pycnogonida]]<br />
†[[Eurypterida]]
}}
The subphylum '''Chelicerata''' constitutes one of the major subdivisions of the phylum [[Arthropod]]a, including the [[arachnid]]s, [[horseshoe crab]]s, and related forms. These mainly [[predator]]y arthropods ultimately outlasted the now extinct [[trilobite]]s, the common marine arthropod of the [[Cambrian]] era. Most of the marine chelicerates, including all of the [[Eurypterus|eurypterid]]s, are now extinct. The chelicerates and their closest fossil relatives (mostly originally included in the [[Xiphosura]]) are grouped together with the trilobites to form the [[taxon]] [[Arachnomorpha]].
==Characteristics==
In the Chelicerata, the body is divided into an [[anterior]] '''prosoma''' (or [[cephalothorax]]) composed of eight segments plus a presegmental acron and a [[posterior]] '''[[opisthosoma]]''' (or ''abdomen'') composed of twelve segments plus a postsegmental [[telson]]. As in other arthropods, the mouth lies between the second and third segments, but whereas in other groups there is usually a pair of antennae on the last preoral segment, here there are none. The prosoma usually has eyes. The first two segments of the prosoma bear no appendages; the third bears the [[chelicerae]]. the fourth segment bears [[arthropod leg|legs]] or [[pedipalp]]s, and all subsequent segments bear legs.
The chelicerae, which give the group its name, are pointed appendages that grasp the food in place of the chewing mandibles most other arthropods have. Most are unable to ingest anything solid, so they drink blood or spit or inject digestive [[enzyme]]s into their prey. The legs on the prosoma are either [[uniramous]] or have a very reduced [[gill]] branch, and are adapted for walking or swimming. The appendages on the opisthosoma, in contrast, are either absent or are reduced to their gill branch.
==Classification==
[[Image:Yellow mite (Tydeidae) Lorryia formosa 2 edit.jpg|thumb|Yellow mite<br>''Lorryia formosa'']]
The Chelicerata are divided into five [[Class (biology)|classes]]:
* [[Arachnida]] ([[spider]]s, [[scorpion]]s, [[mite]]s, etc.)
* [[Xiphosura]] ([[horseshoe crab]]s)
* [[Eurypterida]] ([[sea scorpion]]s, extinct)
* Chasmataspidida (chasmataspids, extinct)
* [[Pycnogonida]] ([[sea spider]]s)
The Pycnogonida actually show some strong differences from the body plan as described above, and it has been suggested that they represent an independent line of arthropods. They may have diverged from the other chelicerates early on, or represent highly modified forms. Sometimes they are excluded from the Chelicerata but grouped with them as the Cheliceriformes. The name [[Merostomata]] should be avoided because in all recent cladistic hypotheses it refers to a [[paraphyletic]] group composed by the Xiphosura + Eurypterida.
The [[Burgess shale]] animal ''[[Sanctacaris]]'', and perhaps the [[aglaspid]]s, may also belong here. These are extinct forms that arose in the [[Cambrian]]; the aglaspids are believed to have died out during the [[Silurian]]. After them, the oldest group of chelicerates are the [[Eurypterida]], found from the [[Ordovician]] onwards. When young, these show a resemblance to the [[trilobite]]s, suggesting a possible relationship between these two groups.
{{Arthropods}}
[[Category:Arthropods]]
[[ca:Quelicerat]]
[[cs:Klepítkatci]]
[[da:Chelicerata]]
[[de:Kieferklauenträger]]
[[et:Lõugtundlased]]
[[es:Chelicerata]]
[[eo:Chelicerata]]
[[fr:Chelicerata]]
[[it:Chelicerata]]
[[he:בעלי כליצרות]]
[[lv:Helicerāti]]
[[mk:Клештари]]
[[nl:Chelicerata]]
[[ja:鋏角亜門]]
[[oc:Chelicerata]]
[[pl:Szczękoczułkowce]]
[[pt:Chelicerata]]
[[ru:Хелицеровые]]
[[sk:Klepietkavce]]
[[sr:Пауколике животиње]]
[[fi:Leukakoukulliset]]
[[sv:Palpkäkar]]