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'{{paraphyletic group |name = Agnathans |image = Lampetra fluviatilis.jpg |image_caption = ''[[Lampetra fluviatilis]]'' | fossil_range = {{fossil range|530|0|ref=<ref name=Xian-guang2002>{{Cite journal | last = Xian-guang | first = H. | coauthors =Richard J. Aldridge2, David J. Siveter2,Derek J. Siveter3,4 and Feng Xiang-hong | year = 2002 | title = New evidence on the anatomy and phylogeny of the earliest vertebrates | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 269 | issue = 1503 | pages = 1865–1869 | doi = 10.1098/rspb.2002.2104 | url = http://journals.royalsociety.org/index/JA88X875KH2VC0E8.pdf|format=PDF | pmid = 12350247 | pmc = 1691108 | postscript = <!--None-->}}</ref>}} | regnum = [[Animal]]ia | phylum = [[Chordate|Chordata]] | subphylum = [[Vertebrata]] | superclassis = '''Agnatha''' | superclassis_authority = [[Edward Drinker Cope|Cope]], 1889 | excludes = [[Gnathostomata]]<ref>''[[Edwin Harris Colbert|Colbert, E.H.]] & Morales, M. (2001): [[Evolution of the Vertebrates|Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time]]''. 4th edition. John Wiley & Sons, Inc, New York - ISBN 978-0-471-38461-8.</ref> }} '''Agnatha''' ([[Greek Language|Greek]], "no jaws") is a [[superclass (biology)|superclass]] of jawless fish in the [[phylum (biology)|phylum]] [[Chordata]], subphylum [[Vertebrata]]. The group excludes all vertebrates with jaws, known as [[gnathostome]]s. The agnathans as a whole are [[paraphyletic]],<ref>{{cite book|last=Purnell|first=M. A.|authorlink=|editor=[[Derek Briggs|Derek E. G. Briggs]] and Peter R. Crowther|year=2001|title=Palaeobiology II|publisher=[[Blackwell Publishing]]|location=[[Oxford]]|isbn=0-632-05149-3|page=401}}</ref> because most extinct agnathans belong to the [[stem group]] of gnathostomes.<ref name="Zhao & Zhu 2007">{{cite journal | year = 2007 | title = Diversification and faunal shift of Siluro-Devonian vertebrates of China | journal = Geological Journal | volume = 42 | issue = (3-4) | pages = 351–369 | url = http://www3.interscience.wiley.com/journal/114129423/abstract | doi = 10.1002/gj.1072 | author = Zhao Wen-Jin, Zhu Min}}</ref><ref name="Sansom 2009">{{cite journal | author = Sansom, Robert S. | year = 2009 | title = Phylogeny, classification, & character polarity of the Osteostraci (Vertebrata) | journal = Journal of Systematic Palaeontology | volume = 7 | pages = 95–115 | url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=3978288 | doi = 10.1017/S1477201908002551}}</ref> Recent molecular data, both from rRNA<ref name="Mallatt, J., and J. Sullivan. 1998.">{{cite journal | author = Mallatt, J., and J. Sullivan. 1998. | year = 1998 | title = 28S and 18S ribosomal DNA sequences support the monophyly of lampreys and hagfishes. | journal = Molecular Biology and Evolution | volume = 15 | pages = 1706–1718 | pmid = 9866205 | issue = 12}}</ref> and from mtDNA<ref name="DeLarbre et al. 2002">{{cite journal | author = DeLarbre Christiane ; Gallut Cyril ; Barriel Veronique ; Janvier Philippe ; Gachelin Gabriel | year = 2002 | title = Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. | journal = Molecular phylogenetics and evolution | volume = 22 | issue = 2 | pages = 184–192 | pmid = 11820840 | doi = 10.1006/mpev.2001.1045}}</ref> strongly supports the theory that living agnathans, known as [[cyclostome]]s, are [[monophyletic]]. The oldest fossil agnathans appeared in the [[Cambrian]], and two groups still survive today: the [[lamprey]]s and the [[hagfish]], with about 100 [[species]] in total. Hagfish are not members of the subphylum Vertebrata, as hagfish do not have vertebrae; they are rather classified in the more inclusive group [[Craniata]]. In addition to the absence of jaws, modern agnathans are characterised by absence of paired [[fin]]s; the presence of a [[notochord]] both in larvae and adults; and seven or more paired [[gill]] pouches. There is a light sensitive [[Pineal_gland#In_lower_vertebrates|pineal eye]] (homologous to the [[pineal gland]] in [[mammal]]s). All living and most extinct Agnatha do not have an identifiable [[stomach]] or any [[appendages]]. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are [[ectothermic]] or cold blooded, with a [[Cartilage|cartilaginous]] [[skeleton]], and the [[heart]] contains 2 chambers. While a few scientists still regard the living agnaths as only superficially similar, and argue that many of these similarities are probably shared [[Basal (phylogenetics)|basal]] characteristics of ancient vertebrates, recent classifications clearly place hagfish (the Myxini or Hyperotreti), with the [[lampreys]] (Hyperoartii) as being more closely related to each other than either is to the jawed fishes. ==Metabolism== Agnathans are [[ectothermic]], meaning they do not regulate their own body temperature. Agnathan metabolism is slow in cold water, and therefore they do not have to eat very much. They have no distinct stomach, but rather a long gut, more or less homogenous throughout its length. Lampreys are parasitic, feeding off other fish and mammals. They rely on a row of sharp teeth to shred their host. Fluids preventing clotting are injected into the host,causing the host to yield more blood. Hagfish are decomposers, eating mostly dead animals. They also use a sharp set of teeth to break down the animal. Agnathan feeding habits have limited their ability to advance evolutionarily. The fact that all Agnathan teeth are not able to move up and down limit their possible food types. ==Body covering== The only modern Agnathan body covering is skin, with neither dermal or epidermal [[Scale (zoology)|scales]]. The skin of [[hagfish]] has copious slime glands, the slime constituting their defence mechanism. Many extinct agnathans sported heavy dermal [[armour (zoology)|armour]] or small mineralized scales (see [[Agnatha#Fossil_agnathans|below]]). ==Appendages== Most agnathans, including all those living today have no paired appendages, although they do have a [[tail]] and a [[caudal fin]]. Some fossil agnathans, such as [[osteostracan]]s, did have paired fins, a trait inherited in their [[Gnathostomata|jawed penisdescendants]].<ref name="Romer & Parson">[[Alfred Romer|Romer, A.S]]. & Parsons, T.S. (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia.</ref> ==Reproduction== Fertilization is external, as is development. There is no parental care. Not much is known about the hagfish reproductive process. It is believed that hagfish only have 30 eggs over a lifetime{{Citation needed|date=May 2010}}. Most species are [[hermaphrodites]]. There is very little of the larval stage that characterizes the lamprey. Lampreys can only reproduce once. After external fertilization, the lamprey's cloacas remain open, allowing a fungus to enter their intestines, killing them. Lampreys reproduce in freshwater river beds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment. The resulting hatchlings go through four years of larval development before becoming adults. They also have a certain unusual form of reproduction. ==Fossil agnathans== Although a minor element of modern marine [[Fauna (animals)|fauna]], Agnatha were prominent among the early fish in the early [[Paleozoic]]. Two types of Early [[Cambrian]] animal apparently having fins, [[vertebrate]] musculature, and gills are known from the early Cambrian [[Maotianshan shales]] of [[China]]: ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]''. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is ''[[Haikouella]]''. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian [[Burgess Shale]] of [[British Columbia]]. Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the [[Ostracoderm]]s, precursors to the [[bony fish]] and hence to the [[tetrapods]] (including [[human]]s)—are known from the middle [[Ordovician]], and by the Late [[Silurian]] the agnathans had reached the high point of their evolution. Most of the ostraoderms, such as [[thelodont]]s, [[osteostracan]]s, and [[galeaspid]]s, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes. Cyclostomes apparently split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including [[conodonts]].<ref name="Bapenisker 2008">{{cite journal | year = 2008 | title = The evolution and elaboration of vertebrate neural crest cells | journal = Current Opinion in Genetics & Development | volume = 18 | pages = 536–543 cock| doi = 10.1016/j.gde.2008.11.006 | author = Baker, Clare V.H. | pmid = 19121930 | issue = 6}}</ref> Agnathans declined in the [[Devonian]] and never recovered. == Groups == *[[Cyclostomata|Cyclostomes]] **[[Myxini]] (hagfish) **[[Hyperoartia]] (Petromyzontida) ***Petromyzontidae ([[lamprey]]s) *[[Ostracoderm]]s **†[[Pteraspidomorphi]] **†[[Thelodonti]] **†[[Anaspida]] **[[Cephalaspidomorphi]] ***†[[Galeaspida]] ***†[[Pituriaspida]] ***†[[Osteostraci]] ==References== <references/> ==See also== {{wikispecies|Agnatha}} *[[Gnathostomata]] *[[Amphirhina]], an alternate name for the above parallel, or sister, classification [[Category:Vertebrates without jaws| ]] [[az:Çənəsizlər]] [[be-x-old:Бяссківічныя]] [[ca:Àgnat]] [[de:Kieferlose]] [[et:Lõuatud]] [[es:Agnatha]] [[eo:Senmakzeluloj]] [[fr:Agnathe]] [[ko:무악류]] [[hr:Besčeljusnjače]] [[id:Agnatha]] [[is:Vankjálkar]] [[it:Agnatha]] [[he:חסרי לסתות]] [[ka:უყბოები]] [[kk:Агнаттар]] [[la:Agnatha]] [[lv:Bezžokļaiņi]] [[lt:Bežandžiai]] [[lij:Agnatha]] [[hu:Állkapocs nélküliek]] [[mk:Безвилични ‘рбетници]] [[ml:അഗ്നാത്ത]] [[nl:Kaakloze vissen]] [[ja:無顎類]] [[nn:Kjevelause fisk]] [[oc:Agnatha]] [[pl:Bezżuchwowce]] [[pt:Agnatha]] [[ro:Agnatha]] [[ru:Бесчелюстные]] [[simple:Agnatha]] [[sk:Kruhoústnice]] [[sl:Brezčeljustnice]] [[sr:Кичмењаци без вилица]] [[fi:Leuattomat selkärankaiset]] [[sv:Käklösa fiskar]] [[tr:Çenesiz balıklar]] [[uk:Безщелепні]] [[vi:Siêu lớp Cá không hàm]] [[zh:無頷總綱]]'