Editing Chordate (section)

==Subphyla==
{{see also|List of chordate orders}}
[[File:Branchiostoma lanceolatum.jpg|thumb|right| Cephalochordate: lancelet. Pictured species: ''[[Branchiostoma lanceolatum]]'']]

===Cephalochordata: Lancelets===
{{main|Lancelet}}

[[Cephalochordate]]s, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.<ref>{{cite book
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | page=6
|  url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate
| access-date=2008-09-22
}}</ref> These burrowing filter-feeders compose the earliest-branching chordate subphylum.<ref name="Gee2008AmphioxusUnleashed">{{cite journal
| author=Gee, H. | title=Evolutionary biology: The amphioxus unleashed
| journal=Nature | volume=453 | pages=999–1000 |date=19 June 2008 | doi=10.1038/453999a
| pmid=18563145
| issue=7198
|bibcode = 2008Natur.453..999G | s2cid=4402585
| doi-access=free }}</ref><ref>{{cite web
| url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/branchiostoma.html | access-date=2016-02-05
| title=Branchiostoma | publisher=Lander University
}}</ref>

===Tunicata (Urochordata)===
{{main|Tunicate|l1= Tunicate (Urochordata)}}

[[File:BU Bio.jpg|thumb|right|Tunicates: sea squirts]]

The [[tunicates]] have three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicate [[larva]]e have the standard chordate features, including long, [[tadpole]]-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors.<ref name="Benton2002VertebratePalaeontologyP5" /> 

The smallest of the three groups of tunicates is the [[Larvacean|Appendicularia]]. They retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of the other two groups.<ref>{{cite web| title=Appendicularia| publisher=Australian Government Department of the Environment, Water, Heritage and the Arts| url=http://www.environment.gov.au/biodiversity/abrs/publications/electronic-books/pubs/tunicates/05-appendicularia.pdf| access-date=2008-10-28| archive-url=https://web.archive.org/web/20110320223518/http://www.environment.gov.au/biodiversity/abrs/publications/electronic-books/pubs/tunicates/05-appendicularia.pdf| archive-date=20 March 2011}}</ref> 

The other two groups, the sea squirts and the salps, metamorphize into adult forms which lose the notochord, nerve cord, and post anal tail. Both are soft-bodied filter feeders with multiple gill slits. They feed on [[plankton]] which they collect in their mucus.

Sea squirts are [[Sessility (motility)|sessile]] and consist mainly of water pumps and filter-feeding apparatus.<ref name="Benton2002VertebratePalaeontologyP5" /> Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.

The [[salp]]s float in mid-water, feeding on [[plankton]], and have a two-generation cycle in which one generation is solitary and the next forms chain-like [[Colony (biology)|colonies]].<ref>{{cite web | url=https://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml | access-date=2008-09-22 | title=Animal fact files: salp | publisher=BBC | archive-date=21 June 2013 | archive-url=https://web.archive.org/web/20130621073155/http://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml }}</ref> 

The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.<ref>Oxford English Dictionary, Third Edition, January 2009: Urochordata</ref> The term '''Tunicata''' (Lamarck 1816) is recognised as having precedence and is now more commonly used.<ref name="Benton2002VertebratePalaeontologyP5">{{cite book| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | page=5| url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate}}</ref>

{{multiple image
 | align = center
 | caption_align = center
 | direction = vertical
 | width     = 320
 | header    = Comparison of two<!--only 2 species shown--> invertebrate chordates
 | image1    = Comparison of Three Invertebrate Chordates.svg 
 | alt1      = 
 | caption1  = A. Lancelet, B. Larval tunicate, C. Adult tunicate<br /><small>--------------------------------------------------------<br />1. [[Notochord]], 2. Nerve chord, 3. Buccal [[Cirrus (biology)|cirri]], 4. [[Pharynx]], 5. [[Gill slit]], 6. [[Gonad]], 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant [[syphon]], 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. [[Esophagus]], 15. Intestines, 16. Tail, 17. Atrium, 18. [[Tunica (biology)|Tunic]]</small>
}}

===Craniata (Vertebrata)===
{{main|Craniata|Vertebrata}}
[[File:Pacific hagfish Myxine.jpg|thumb|right|Craniate: [[hagfish]]]]

[[Craniate]]s all have distinct [[skull]]s. They include the [[hagfish]], which have no [[vertebra]]e. [[Michael Benton|Michael J. Benton]] commented that "craniates are characterized by their heads, just as chordates, or possibly all [[deuterostome]]s, are by their tails".<ref>{{cite book
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | pages=12–13
| url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate
| access-date=2008-09-22}}</ref>

Most craniates are [[vertebrate]]s, in which the [[notochord]] is replaced by the [[vertebral column]].<ref name="UCMPMorphologyOfVertebrates">{{cite web | url=http://www.ucmp.berkeley.edu/vertebrates/vertmm.html | access-date=2008-09-23 | title=Morphology of the Vertebrates | publisher=University of California Museum of Paleontology}}</ref> It consists of a series of bony or cartilaginous [[cylinder|cylindrical]] vertebrae, generally with [[neural arch]]es that protect the [[spinal cord]], and with projections that link the vertebrae. [[hagfish|Hagfishes]] have incomplete [[braincase]]s and no vertebrae, and are therefore not regarded as vertebrates,<ref>{{cite web | title=Introduction to the Myxini | publisher=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html | access-date=2008-10-28 | archive-url=https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html | archive-date=15 December 2017 }}</ref> but they are members of the craniates, the group within which vertebrates are thought to have [[evolution|evolved]].<ref>{{cite book |author=[[Neil A. Campbell|Campbell, N.A.]] |author2=[[Jane Reece|Reece, J.B.]] | year=2005 | title=Biology | edition=7th | publisher=Benjamin Cummings | location=San Francisco, California | isbn=978-0-8053-7171-0}}</ref> However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.<ref name=r3>{{cite journal |last=Janvier |first=P. |author-link=Philippe Janvier |year= 2010|title=MicroRNAs revive old views about jawless vertebrate divergence and evolution |journal= Proceedings of the National Academy of Sciences |volume=107 |pages=19137–19138 |quote=Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time|doi=10.1073/pnas.1014583107|pmid= 21041649 |issue=45 |bibcode=2010PNAS..10719137J |pmc=2984170 |doi-access= free}}</ref>

Before [[molecular phylogenetics]], the position of [[lamprey]]s was ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true [[fish]].<ref>{{cite web | title=Introduction to the Petromyzontiformes | publisher=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/petro.html | access-date=2008-10-28 | archive-url=https://web.archive.org/web/20180124115940/http://www.ucmp.berkeley.edu/vertebrates/basalfish/petro.html | archive-date=24 January 2018 }}</ref> However, molecular phylogenetics, which uses [[DNA]] to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.<ref>{{cite journal |author1=Shigehiro Kuraku, S. |author2=Hoshiyama, D. |author3=Katoh, K. |author4=Suga, H. |author5=Miyata, T. | title=Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA-Coded Genes | journal=Journal of Molecular Evolution | volume=49 | issue=6 | pages=729–735 |date=December 1999 | doi=10.1007/PL00006595 | pmid=10594174 |bibcode=1999JMolE..49..729K |s2cid=5613153 }}</ref> If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a [[clade]], which has been named the [[Cyclostomata]].<ref name="Delarbre 2002">{{cite journal | last= Delabre |first=Christiane |display-authors=etal | title = Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strngly Supports the Cyclostome Monophyly | year = 2002 | journal = Molecular Phylogenetics and Evolution | volume = 22 | issue = 2 | pages = 184–192 | doi = 10.1006/mpev.2001.1045 | pmid = 11820840| bibcode = 2002MolPE..22..184D}}</ref>

{{anchor|Taxonomy}}