Arthropod
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/* Basic arthropod structure */ Restructure; link to [[arthropod exoskeleton]]
{{Taxobox
| name = Arthropods
| fossil_range = [[Cambrian]] or earlier – Recent
| image = Brachypelma_edit.jpg
|image_caption = Tarantula ''Brachypelma sp.''
| regnum = [[Animal]]ia
| phylum = '''Arthropoda'''
| phylum_authority = [[Pierre André Latreille|Latreille]], 1829
| subdivision_ranks = Subphyla and Classes
| subdivision =
*'''Subphylum [[Trilobitomorpha]]'''
**[[Trilobita]] — trilobites (extinct)
*'''Subphylum [[Chelicerata]]'''
**[[Arachnida]] — [[spider]]s,[[scorpion]]s, etc.
**[[Xiphosura]] — [[horseshoe crab]]s, etc.
**[[sea spider|Pycnogonida]] — sea spiders
**[[Eurypterid]]a — sea scorpions (extinct)
*'''Subphylum [[Myriapoda]]'''
**[[Chilopoda]] — [[centipede]]s
**[[Diplopoda]] — [[millipede]]s
**[[Pauropoda]]
**[[Symphyla]]
*'''Subphylum [[Hexapoda]]'''
**[[Insect]]a — insects
**[[Entognatha]]
*'''Subphylum [[Crustacean|Crustacea]]'''
**[[Branchiopoda]] – [[brine shrimp]] etc.
**[[Remipedia]]
**[[Cephalocarida]] – horseshoe shrimp
**[[Maxillopoda]] — [[barnacle]]s, [[fish louse|fish lice]], etc.
**[[Ostracoda]] – seed shrimp
**[[Malacostraca]] — [[lobster]]s, [[crab]]s, [[shrimp]], etc.
}}
'''Arthropods''' are [[animal]]s belonging to the [[Scientific classification|Phylum]] '''Arthropoda''' (from [[Greek language|Greek]] [[wikt:ἄρθρον|ἄρθρον]] ''arthron'', "[[joint]]", and [[wikt:ποδός|ποδός]] ''podos''"[[foot]]", which together mean "jointed feet") and include the [[insect]]s, [[arachnid]]s, [[crustacean]]s and others. Arthropods are characterized by the possession of a [[Segment (biology)|segment]]ed body with [[appendage]]s on at least one segment. They have a [[Dorsum (biology)|dorsal]] [[heart]] and a [[ventral nervous system]]. All arthropods are covered by a hard [[exoskeleton]] made of [[chitin]], a [[polysaccharide]], which provides physical protection and resistance to [[desiccation]]. Arthropods grow by shedding this covering in what are termed [[molt]]s.
They are the largest phylum in the Animal Kingdom with more than a million described species making up more than 80% of all described living species,<ref>{{cite web |author=Anna Thanukos |url=http://evolution.berkeley.edu/evolibrary/article/arthropodstory |title=The Arthropod Story |publisher=[[University of California, Berkeley]]}}</ref> and a [[fossil record]] reaching back to the late [[proterozoic]] era. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various [[symbiosis|symbiotic]] and [[parasite|parasitic]] forms. They range in size from microscopic [[plankton]] (~¼ [[millimetre|mm]]) up to forms several [[metre]]s long. The largest living arthropod is the [[Japanese spider crab]], with a leg span up to 3½ [[metre|m]] (12 [[foot (unit of length)|ft]]), and some prehistoric arthropods were even larger, such as ''[[Jaekelopterus]]'' and ''[[Arthropleura]]''.
== Basic arthropod structure ==
[[Image:Blue_crab_on_market_in_Piraeus_-_Callinectes_sapidus_Rathbun_20020819-317.jpg|left|thumb|250px|[[Blue crab]] (''Callinectes sapidus''), a crustacean]]
The success of arthropods is related to their hard '''[[exoskeleton]]''', segmentation, and jointed [[appendage]]s. The appendages are used for feeding, sensory reception, defense, and locomotion. The muscle system is more or less assisted by hydraulics originated from the blood pressure, created by the hearts of the animals.<ref>{{cite news |url=http://www.straightdope.com/mailbag/mspiderhydraulic.html |title=Do spiders have hydraulic legs? |publisher=[[The Straight Dope]] |date=[[2004-09-27]]}}</ref> The hydraulic system in spiders is especially well developed.
Arthropods are [[protostome]]s, which posess a [[body cavity|coelom]], but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a [[hemocoel|haemocoel]], filled with [[hemolymph|haemolymph]], which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a pre-segmental ''acron'', which usually supports [[compound eyes|compound]] and simple eyes and a post-segmental ''[[telson]]''. These are grouped into distinct, specialised body regions called ''tagmata''. Each segment, at least primitively, supports a pair of [[appendage]]s.
==Internal organs==
[[Image:Harpaphe haydeniana 002.jpg|thumb||left|250px|''[[Harpaphe haydeniana]]'', a myriapod]]
[[Image:Adult citrus root weevil, Diaprepes abbreviatus.jpg|thumb|left|250px|[[Diaprepes abbreviatus|Citrus root weevil]], an insect]]
[[Aquatic animal|Aquatic]] arthropods use [[gills]] to exchange gases. These gills have an extensive [[surface area]] in contact with the surrounding water. [[Terrestrial animal|Terrestrial]] arthropods have internal surfaces that are specialised for [[gas exchange]]. Insects and most other terrestrial species have [[invertebrate trachea|trachea]]l systems: air sacs leading into the body from pores called [[spiracles]] in the epidermis [[cuticle]]. Others use [[book lungs]], or [[gills]] modified for breathing air as seen in species like the [[coconut crab]]. Some areas of the legs of [[soldier crab]]s are covered with an oxygen absorbing membrane. The [[gill chamber]]s in terrestrial crabs sometimes have two different structures: one that is gilled and used for breathing underwater, and another specially adapted to take up oxygen from the air (a pseudolung). Arthropods also have a complete digestive system with both a mouth and anus.
Arthropods have an [[Circulatory system#Open circulatory system|open circulatory system]]. [[Haemolymph]] containing [[haemocyanin]], a [[copper]]-based oxygen-carrying protein (the copper makes the blood blue, unlike humans, which use hemoglobin, which uses iron, which makes it red). The blood is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues.
==Exoskeleton==
{{main|Arthropod exoskeleton}}
The cuticle in arthropods forms a rigid [[exoskeleton]], composed mainly of [[chitin]], which is periodically shed as the animal grows. They contain an inner zone ([[procuticle]]), which is made of protein and chitin and is responsible for the strength of the exoskeleton. The outer zone ([[epicuticle]]) lies on the surface of the procuticle. It is nonchitinous and is a complex of [[proteins]] and [[lipids]]. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called ''[[sclerite]]s'' on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name — jointed feet — and separates them from their relatives, the [[Onychophora]] and [[Tardigrada]], also called [[Lobopoda]] (and which is sometimes included in a group called [[Panarthropoda]] that also includes arthropods). The exoskeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, [[ecdysis]], is expensive in terms of energy, and during the molting period, an arthropod is vulnerable.
==Classification of arthropods==
<!--[[Image:Common blue damselfly02.jpg|thumb|right|250px|[[Common Blue Damselfly]] (''Enallagama cyathigerum''), an insect]]-->
<!--<div style="align:right; float:right;">-->
{{userboxtop| toptext= }}
{{clade| style=font-size:75%;line-height:80%
|label1=Arthropoda
|1={{clade
|label1=[[Paradoxopoda]]
|1={{clade
|1=[[Myriapoda]]
|2=[[Chelicerata]]
}}
|label2=[[Pancrustacea]]
|2={{clade
|label1=
|1={{clade
|1={{fontcolor||pink| [[Cirripedia]] }}
|2={{clade
|1={{fontcolor||pink| [[Remipedia]] }}
|2=[[Collembola]]
}}
}}
|label2=
|2={{clade
|label1=
|1={{clade
|1={{fontcolor||Pink|[[Branchiopoda]] }}
|2={{fontcolor||Pink|[[Cephalocarida]] }}
|3={{fontcolor||Pink|[[Malacostraca]] }}
}}
|2=[[Insecta]]
}}
}}
}}
}}
<center><small>Phylogenetic relationships of the major extant arthropod groups, derived from [[mitochondrial DNA]] sequences.<ref name="Hassanin">{{cite journal |quotes=no |author=Alexandre Hassanin |title=Phylogeny of Arthropoda inferred from mitochondrial sequences: Strategies for limiting the misleading effects of multiple changes in pattern and rates of substitution |year=2006 |journal=Molecular Phylogenetics and Evolution |volume=38 |pages=100–116 |doi=10.1016/j.ympev.2005.09.012}}</ref> Taxa in pink are parts of the subphylum [[Crustacea]].</small></center>
{{userboxbottom}}
Arthropods are typically [[scientific classification|classified]] into five [[subphylum|subphyla]], of which one is extinct:<ref>{{ITIS|ID=82696|taxon=Arthropoda|date=August 15|year=2006}}</ref>
# '''[[Trilobite]]s''' are a group of formerly numerous marine animals that disappeared in the [[Permian-Triassic extinction event]], though they were in decline prior to this killing blow, having been reduced to one order in the [[Late Devonian extinction]].
# '''[[Chelicerata|Chelicerates]]''' include [[spider]]s, [[mite]]s, [[scorpion]]s and related organisms. They are characterised by the presence of [[chelicerae]].
# '''[[Myriapoda|Myriapods]]''' comprise [[millipede]]s and [[centipede]]s and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
# '''[[Hexapoda|Hexapods]]''' comprise [[insect]]s and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called [[Uniramia]], though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
# '''[[Crustacean]]s''' are primarily aquatic (a notable exception being [[woodlouse|woodlice]]) and are characterised by having [[Arthropod leg#Biramous and uniramous|biramous]] appendages. They include [[lobster]]s, [[crab]]s, [[barnacle]]s, [[crayfish]], [[shrimp]] and many others.
Aside from these major groups, there are also a number of fossil forms — mostly from the lower [[Cambrian]] — including [[anomalocarid]]s, [[Euthycarcinoidea|euthycarcinoids]]<ref>{{cite web |url=http://www.abdn.ac.uk/rhynie/euthy.htm |title=The Rhynie Chert Euthycarcinoids |publisher=[[University of Aberdeen]] |accessdate=2006-08-15}}</ref> and ''[[Arthrogyrinus]]'', which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.
The phylogeny of the arthropods has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include [[Mandibulata]], [[Uniramia]] and [[Atelocerata]]. The most recent studies tend to suggest a [[paraphyly|paraphyletic]] Crustacea with different hexapod groups nested within it.<ref name="Hassanin"/><ref>{{cite journal |quotes=no |title=The position of crustaceans within Arthropoda — Evidence from nine molecular loci and morphology |author=Giribet, G., S. Richter, G. D. Edgecombe & W. C. Wheeler |year=2005 |journal=Crustacean Issues |volume=16 |pages=307–352 |url=http://www.amonline.net.au/palaeontology/pdf/giribet-etal2005.pdf}}</ref> The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or [[Myriochelata]].
Since the [[International Code of Zoological Nomenclature]] recognises no priority above the rank of family, many of the higher groups can be referred to by a variety of different names.<ref name="Campbell">{{cite web |url=http://www.peripatus.gen.nz/Taxa/Arthropoda/Index.html |author=Campbell, Reece & Mitchell |title=Arthropoda |date=[[2006-07-30]]}}</ref>
==Evolution ==
[[Image:31-Velvet Worm.JPG|thumb|left|The [[velvet worm|velvet worm (Onychophora)]] is closely related to Arthropods]]
{{userboxtop| toptext= }}
{{clade| style=font-size:75%;line-height:80%
|1={{clade
|1=[[Sipuncula]]
|label2=[[Articulata]]
|2={{clade
|1=[[Mollusca]]
|label2=[[Euarticulata]]
|2={{clade
|1=[[Annelida]]
|label2=[[Panarthropoda]]
|2={{clade
|1=[[Onychophora]]
|2={{clade
|1=[[Tardigrada]]
|2=[[Arthropoda]]
}}
}}
}}
}}
}}
}}
<center><small>A phylogeny of the arthropods after Nielsen.<ref>{{cite book |author=Nielsen, C. |year=2001 |title=Animal Evolution: Interrelationships of the Living Phyla. Second Edition |publisher=[[Oxford University Press]], Oxford |id=ISBN 978-0-19-850681-2}}</ref></small></center>
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Arthropods are today almost universally considered to be [[monophyletic]], i.e. they only arose once, a view supported by both morphological and molecular studies. Such a view contradicts the widespread view in the 1970s that the arthropods had evolved on several occasions from soft-bodied, annelid-like ancestors.
The closest relatives of the arthropods are usually considered to be the [[Tardigrada]] and [[Onychophora]], together forming the monophyletic group [[Panarthropoda]] (the crustaceans, myriapods, chelicerates and insects are often referred to as "[[Euarthropoda]]" to distinguish them from their soft-bodied relatives). Comparison between these groups suggests that the euarthropods evolved from a soft-bodied ancestor not too dissimilar to the living onychophorans, a view that has found some support from the fossil record.
Traditionally the [[Annelida]] have been considered the closest relatives of these three phyla, on account of their common segmentation. Molecular data however, is strongly against this grouping (known as the [[Articulata]]), suggesting instead that the panarthropods belong in a [[clade]] including both the arthropods and various [[body cavity|pseudocoelomates]] such as [[roundworm]]s and [[priapulid]]s that share with them growth by moulting, or [[ecdysis]], from which its name, the [[Ecdysozoa]]. is derived. If this new grouping is correct, then segmentation of arthropods and annelids has either evolved through [[Convergent evolution|convergence]], or has been inherited from a very deep ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.
==References==<!-- Palaeontology44:143 -->
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{{Arthropods}}
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