Artropodoj: Malsamoj inter versioj

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La tri-parta aspekto de multaj [[insekto]]-korpoj kaj la du-parta aspekto de [[araneo]]j estas rezulto de tiu grupigo;<ref name="WonderfulLifeP102To106">[[#refGould|Gould (1990)]], pp. 102–106</ref> fakte ne estas eksteraj signoj de segmentado en [[akaroj]].<ref name="RuppertFoxBarnes2004P518" /> Artropodoj havas ankaŭ du korpelementojn kiuj ne estas parto de tiu serie ripetita modelo de segmentoj, nome ''prostomium'' ĉe la fronto, antaŭ la buŝo, kaj ''telson'' malantaŭe, fakte malantaŭ la [[anuso]]. La okuloj estas muntitaj sur la ''prostomium''.<ref name="RuppertFoxBarnes2004P518" />
 
OriginallyOrigine itŝajnas seemske thatĉiu eachapendico appendagekiu bearinghavas segmentsegmentojn hadhavas twodu separateseparatajn pairsparojn ofde appendagesmembroj; andkaj uppersupra and akaj lowermalsupra pairparo. TheseTiuj wouldposte laterfuziiĝis fuseen intounusola aparo singlede pairdubranĉaj of [[biramous]] appendagesapendizoj, withel thekiuj upperla branchsupra actingbranĉo asagadas akiel [[gillbranko]] whiledum thela lowermalsupra branchbranĉo wasestas useduzata forpor locomotiontranslokigo.<ref>{{cite web|url=http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution|title=Giant sea creature hints at early arthropod evolution|publisher=}}</ref> InĈe somekelkaj segmentssegmentoj ofde allĉiuj knownkonataj arthropods theartropodoj appendagesla haveapendizoj beenestis modifiedmodifitaj, forpor exampleekzemplo topor formformi gillsbrankojn, mouthbuŝo-partspartojn, [[AntennaAnteno (biologybiologio)|antennaeantenojn]] forpor collectingkolekti informationinformaron,<ref name="WonderfulLifeP102To106" /> or clawsungojn forpor graspingkaptado;<ref>{{citation |author1=Shubin, N. |author2=Tabin, C. |author3=Carroll, S. |last-author-amp=yes |chapter=Fossils, Genes and the Evolution of Animal Limbs |title=Shaking the Tree: Readings from Nature in the History of Life |editor=Gee, H. |publisher=[[University of Chicago Press]] |year=2000 |isbn=0-226-28497-2 |page=110 |url=https://books.google.com/?id=M6yF0pU4eCsC&pg=PA110&dq=arthropod+diversity}}</ref> arthropodsartropodoj areestas "likekvazaŭ [[SwissSvisa Army knifetranĉilo|Swiss Armysvisarmeaj knivestranĉiloj]], eachĉiu equippedekuipita withper aunika uniquearo setde of specializedspecializitaj toolsiloj."<ref name="RuppertFoxBarnes2004P518" /> InĈe manymultaj arthropodsartropodoj, appendagesapendicoj havevanuis vanishedel fromkelkaj someregionoj regionsde ofla the bodykorpo, and itkaj isestas particularlypartikulare commonofta forĉe abdominalabdomenaj appendagesapendicoj toke haveili disappearedestas ormalaperintaj be highlytre modifiedmodifitaj.<ref name="RuppertFoxBarnes2004P518" />
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The most conspicuous specialization of segments is in the head. The four major groups of arthropods&nbsp;– [[Chelicerata]] (includes [[spider]]s and [[scorpion]]s), [[Crustacean|Crustacea]] ([[shrimp]]s, [[lobster]]s, [[crab]]s, etc.), [[Tracheata]] (arthropods that breathe via channels into their bodies; includes [[insect]]s and [[myriapod]]s), and the extinct [[trilobite]]s&nbsp;– have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.<ref name="RuppertFoxBarnes2004P518" /> In addition some extinct arthropods, such as ''[[Marrella]]'', belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.<ref name="Whittington1971Marrella">{{citation |last=Whittington |first=H. B. |title=Redescription of ''Marrella splendens'' (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia |journal=[[Geological Survey of Canada Bulletin]] |volume=209 |pages=1–24 |year=1971}} Summarised in [[#refGould|Gould (1990)]], pp. 107–121.</ref>
Originally it seems that each appendage bearing segment had two separate pairs of appendages; and upper and a lower pair. These would later fuse into a single pair of [[biramous]] appendages, with the upper branch acting as a [[gill]] while the lower branch was used for locomotion.<ref>{{cite web|url=http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution|title=Giant sea creature hints at early arthropod evolution|publisher=}}</ref> In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, [[Antenna (biology)|antennae]] for collecting information,<ref name="WonderfulLifeP102To106" /> or claws for grasping;<ref>{{citation |author1=Shubin, N. |author2=Tabin, C. |author3=Carroll, S. |last-author-amp=yes |chapter=Fossils, Genes and the Evolution of Animal Limbs |title=Shaking the Tree: Readings from Nature in the History of Life |editor=Gee, H. |publisher=[[University of Chicago Press]] |year=2000 |isbn=0-226-28497-2 |page=110 |url=https://books.google.com/?id=M6yF0pU4eCsC&pg=PA110&dq=arthropod+diversity}}</ref> arthropods are "like [[Swiss Army knife|Swiss Army knives]], each equipped with a unique set of specialized tools."<ref name="RuppertFoxBarnes2004P518" /> In many arthropods, appendages have vanished from some regions of the body, and it is particularly common for abdominal appendages to have disappeared or be highly modified.<ref name="RuppertFoxBarnes2004P518" />
 
Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the [[arthropod head problem]]".<ref>{{citation| author=Budd, G. E. |date=16 May 2002 |title=A palaeontological solution to the arthropod head problem| journal=[[Nature (journal)|Nature]] |volume=417 |issue=6886 |pages=271–275 |url=http://www.nature.com/nature/journal/v417/n6886/full/417271a.html |doi=10.1038/417271a |pmid=12015599|bibcode=2002Natur.417..271B }}</ref> In 1960 R. E. Snodgrass even hoped it would not be solved, as trying to work out solutions was so much fun.{{refn|"It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise."<ref>{{citation |author=Snodgrass, R. E. |year=1960 |title=Facts and theories concerning the insect head |journal=Smithsonian Miscellaneous Collections |volume=142 |pages=1–61}}</ref>|group=Note}}
 
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