View source for Biology:Lancelet

{{Short description|Order of chordates}}
{{Automatic taxobox
| name = Lancelet
| image = Branchiostoma_lanceolatum.jpg
| image_caption = ''[[Biology:Branchiostoma lanceolatum|Branchiostoma lanceolatum]]''
| display_parents = 3
| parent_authority = Müller, 1845
| taxon = Amphioxiformes
| authority = Anonymous, 1886
| synonyms = *Branchiostomiformes<ref>{{cite web|url=http://data.gbif.org/species/browse/resource/344/taxon/7498708|title=Species Search Results for browse/resource/344/taxon/7498708|last=GBIF|access-date=2009-09-25|archive-url=https://web.archive.org/web/20150716023922/http://data.gbif.org/species/browse/resource/344/taxon/7498708|archive-date=2015-07-16|url-status=dead}}</ref>
*Amphioxi
| subdivision_ranks = Families
| subdivision = * [[Biology:Asymmetron|Asymmetron]]idae
* [[Biology:Branchiostomatidae|Branchiostomatidae]]
}}

The '''lancelets''' ({{IPAc-en|ˈ|l|æ|n|s|l|ᵻ|t|s|,_|ˈ|l|ɑː|n|-}} {{respell|LA(H)N|slits}}), also known as '''amphioxi''' ({{singular}}: '''amphioxus''' {{IPAc-en|ˌ|æ|m|f|i|ˈ|ɒ|k|s|ə|s}} {{respell|AM|fee|OK|səs}}), consist of some 30 to 35 species of "fish-like" benthic filter feeding [[Biology:Chordate|chordates]]<ref>{{Cite journal|last1=Poss|first1=Stuart G.|last2=Boschung|first2=Herbert T.|date=1996-01-01|title=Lancelets (cephalochordata: Branchiostomattdae): How Many Species Are Valid?|url=https://www.tandfonline.com/doi/abs/10.1080/00212210.1996.10688872|journal=Israel Journal of Zoology|volume=42|issue=sup1|pages=S13–S66|doi=10.1080/00212210.1996.10688872|issn=0021-2210|doi-broken-date=1 August 2023}}</ref> in the order '''Amphioxiformes'''. They are modern representatives of the subphylum [[Biology:Cephalochordate|Cephalochordata]]. Lancelets closely resemble 530-million-year-old ''[[Biology:Pikaia|Pikaia]]'', fossils of which are known from the [[Earth:Burgess Shale|Burgess Shale]]. However, according to phylogenetic analysis, the lancelet group itself probably evolved around the [[Earth:Cretaceous|Cretaceous]], 97.7 million years ago for Pacific species and 112 million years ago for Atlantic species.<ref>{{Cite journal |last1=Nohara |first1=Masahiro |last2=Nishida |first2=Mutsumi |last3=Manthacitra |first3=Vipoosit |last4=Nishikawa |first4=Teruaki |date=2004 |title=Ancient Phylogenetic Separation between Pacific and Atlantic Cephalochordates as Revealed by Mitochondrial Genome Analysis |journal=Zoological Science |volume=21 |issue=2 |pages=203–210 |doi=10.2108/zsj.21.203 |pmid=14993833 |s2cid=23356580 |issn=0289-0003|doi-access=free }}</ref><ref>{{Cite journal |last1=Donoghue |first1=Philip C. J. |last2=Keating |first2=Joseph N. |date=2014 |editor-last=Smith |editor-first=Andrew |title=Early vertebrate evolution |url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12125 |journal=Palaeontology |language=en |volume=57 |issue=5 |pages=879–893 |doi=10.1111/pala.12125|bibcode=2014Palgy..57..879D |s2cid=23828862 }}</ref> ''[[Biology:Palaeobranchiostoma|Palaeobranchiostoma]]'' from the [[Earth:Permian|Permian]] may be part of the fossil record of lancelets; however, due to poor preservation, some doubt about its nature remains.<ref>{{Cite journal |last1=Briggs |first1=Derek E.G. |last2=Kear |first2=Amanda J. |date=1993 |title=Decay of Branchiostoma: implications for soft-tissue preservation in conodonts and other primitive chordates |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1993.tb01532.x |journal=Lethaia |language=en |volume=26 |issue=4 |pages=275–287 |doi=10.1111/j.1502-3931.1993.tb01532.x |issn=0024-1164}}</ref>

Zoologists are interested in them because they provide evolutionary insight into the origins of [[Biology:Vertebrate|vertebrate]]s. As [[Biology:Living fossil|living fossil]]s, lancelets contain many organs and organ systems that are [[Biology:Homology|homologous]] to those of modern fish, but in a more primitive form. Therefore, they provide a number of examples of possible evolutionary [[Biology:Exaptation|exaptation]]. For example, the gill-slits of lancelets are used for feeding only, and not for respiration. The circulatory system carries food throughout their body, but does not have red blood cells or hemoglobin for transporting oxygen. Lancelet [[Biology:Genome|genome]]s hold clues about the early [[Biology:Evolution|evolution]] of vertebrates: by comparing genes from lancelets with the same genes in vertebrates, changes in gene expression, function and  number as vertebrates evolved can be discovered.<ref>[http://newswise.com/articles/view/541865/ Worm-like Marine Animal Providing Fresh Clues About Human Evolution] Newswise, Retrieved on July 8, 2008.</ref><ref>{{Cite journal|last=Holland|first=PWH|date=1992|title=An amphioxus homeobox gene: sequence conservation, spatial expression during development and insights into vertebrate evolution|url=http://dx.doi.org/10.1016/0168-9525(93)90180-p|journal=Development|volume=116|issue=2|pages=653–661|doi=10.1016/0168-9525(93)90180-p|s2cid=7298022 |issn=0168-9525}}</ref> The genome of a few species in the genus ''[[Biology:Branchiostoma|Branchiostoma]]'' have been sequenced: ''B. floridae,''<ref>{{Cite journal|last1=Rokhsar|first1=Daniel S.|last2=Satoh|first2=Nori|last3=Holland|first3=Peter W. H.|last4=Holland|first4=Linda Z.|last5=Fujiyama|first5=Asao|last6=Bronner-Fraser|first6=Marianne|last7=Toyoda|first7=Atsushi|last8=Shin-I|first8=Tadasu|last9=Schmutz|first9=Jeremy|year=2008|title=The amphioxus genome and the evolution of the chordate karyotype|journal=Nature|language=en|volume=453|issue=7198|pages=1064–1071|doi=10.1038/nature06967|pmid=18563158|issn=1476-4687|bibcode=2008Natur.453.1064P|s2cid=4418548|doi-access=free}}</ref> ''B. belcheri'',<ref>{{Cite journal|last1=Xu|first1=Anlong|last2=Chen|first2=Shangwu|last3=Dong|first3=Meiling|last4=Wu|first4=Fenfang|last5=Fu|first5=Yonggui|last6=Yuan|first6=Shaochun|last7=You|first7=Leiming|last8=Zhou|first8=Sisi|last9=Qiujin Zhang|date=2014-12-19|title=Decelerated genome evolution in modern vertebrates revealed by analysis of multiple lancelet genomes|journal=Nature Communications|language=en|volume=5|pages=5896|doi=10.1038/ncomms6896|issn=2041-1723|pmc=4284660|pmid=25523484|bibcode=2014NatCo...5.5896H}}</ref> and ''B. lanceolatum''.<ref>{{Cite journal|last1=Marlétaz|first1=Ferdinand|last2=Firbas|first2=Panos N.|last3=Maeso|first3=Ignacio|last4=Tena|first4=Juan J.|last5=Bogdanovic|first5=Ozren|last6=Perry|first6=Malcolm|last7=Wyatt|first7=Christopher D. R.|last8=de la Calle-Mustienes|first8=Elisa|last9=Bertrand|first9=Stephanie|last10=Burguera|first10=Demian|last11=Acemel|first11=Rafael D.|date=December 2018|title=Amphioxus functional genomics and the origins of vertebrate gene regulation|url= |journal=Nature|language=en|volume=564|issue=7734|pages=64–70|doi=10.1038/s41586-018-0734-6|issn=1476-4687|pmc=6292497|pmid=30464347|bibcode=2018Natur.564...64M}}</ref>

In Asia, lancelets are harvested commercially as food for humans. In Japan, amphioxus (''B. belcheri'') has been listed in the registry of "Endangered Animals of Japanese Marine and Fresh Water Organisms".<ref>{{Cite journal|last1=Tomiyama|first1=Minoru|last2=Azuma|first2=Nobuyuki|last3=Kubokawa|first3=Kaoru|date=1998|title=A New Population of the Amphioxus (Branchiostoma belcheri) in the Enshu-Nada Sea in Japan|journal=Zoological Science|volume=15|issue=5|pages=799–803|doi=10.2108/zsj.15.799|s2cid=85834803|issn=0289-0003|doi-access=free}}</ref>

== Ecology ==

=== Habitat ===
'''Amphioxi''' are distributed in shallow [[Earth:Neritic zone|subtidal]] sand flats in [[Earth:Temperate climate|temperate]] (as far north as {{wipe|Norway}}<ref>{{cite journal| author1= Tambs-Lyche, H.|year=1967 | title= Branchiostoma lanceolatum (Pallas) in Norway | journal=Sarsia | volume=29 | issue=1 | pages=177–182 | doi= 10.1080/00364827.1967.10411078 }}</ref>), [[Earth:Subtropics|subtropical]] and tropical seas around the world.<ref name="Carvalho2017">{{cite journal| author1=Carvalho, J.E.| author2=F. Lahaye| author3=M. Schubert| year=2017 | title=Keeping amphioxus in the laboratory: an update on available husbandry methods | journal=Int. J. Dev. Biol. | volume=61 | issue=10–11–12| pages=773–783 | doi=10.1387/ijdb.170192ms | pmid= 29319123| doi-access=free }}</ref> The only exception is ''Asymmetron inferum'', a species known from the vicinity of [[Biology:Whale fall|whale fall]]s at a depth of about {{convert|225|m|ft|abbr=on}}.<ref name="Wanninger2015" /> Although they are able to swim, adult amphioxi are mostly [[Earth:Benthos|benthic]]. They live in sandy bottoms whose granulometry depends on the species and the site,<ref name=":0">{{Cite journal|last=Escriva|first=Hector|date=2018|title=My Favorite Animal, Amphioxus: Unparalleled for Studying Early Vertebrate Evolution| journal= BioEssays| language= en| volume= 40| issue= 12|pages= 1800130| doi= 10.1002/bies.201800130|pmid=30328120|s2cid=53528269|issn=1521-1878| url=https://hal.sorbonne-universite.fr/hal-01974406/file/83-Escriva-2018-BioEssays_sans%20marque.pdf}}</ref> and they are usually found half-buried in sand.<ref name="Carvalho2017" /> When disturbed, they quickly leave their burrow, swim a short distance, and then rapidly burrow again, posterior end first, into the sand. Adults (''B. floridae'') can tolerate salinities as low as 6‰ and temperatures from {{cvt|3|to|37|C}}.<ref name=":2" />

=== Feeding ===
Their habitat preference reflects their feeding method: they only expose the front end to the water and filter-feed on [[Earth:Plankton|plankton]] by means of a branchial ciliary current that passes water through a mucous sheet. ''Branchiostoma floridae'' is capable of trapping particles from microbial to small phytoplankton size,<ref>{{Cite journal |last1=Smith|first1=Allison J.| last2= Nash|first2=Troy R. |last3= Ruppert| first3=Edward E.|date=2000|title=The size range of suspended particles trapped and ingested by the filter-feeding lancelet Branchiostoma floridae (Cephalochordata: Acrania)| url= https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/div-classtitlethe-size-range-of-suspended-particles-trapped-and-ingested-by-the-filter-feeding-lancelet-span-classitalicbranchiostoma-floridaespan-cephalochordata-acraniadiv/78AD799D990BE3517E0D77BD557580E0|journal=Journal of the Marine Biological Association of the United Kingdom| language= en| volume= 80|issue=2|pages=329–332|issn=1469-7769| doi= 10.1017/S0025315499001903|s2cid=85696980}}</ref> while ''B. lanceolatum'' preferentially traps bigger particles (>4&nbsp;µm).<ref>{{Cite journal|last1=Riisgard|first1=Hans Ulrik|last2=Svane|first2=Ib|date=1999| title= Filter Feeding in Lancelets (Amphioxus), vertebrate Biology| journal= Invertebrate Biology| volume= 118|issue=4|pages=423|doi=10.2307/3227011|issn=1077-8306| jstor= 3227011}}</ref>

=== Reproduction and spawning ===
Lancelets are [[Biology:Gonochorism|gonochoric]] animals, i.e. having two sexes, and they reproduce via external [[Biology:Fertilisation|fertilization]]. They only reproduce during their [[Biology:Spawn|spawning]] season, which varies slightly between species — usually corresponding to spring and summer months.<ref name=":0" /> All lancelets species spawn shortly after sunset, either synchronously (e.g. ''Branchiostoma floridae'', about once every two weeks during spawning season<ref name=":2">{{Cite journal |last1= Stokes|first1=M. Dale| last2= Holland|first2=Nicholas D.| date= 1996| title=Reproduction of the Florida Lancelet (Branchiostoma floridae): Spawning Patterns and Fluctuations in Gonad Indexes and Nutritional Reserves| journal=Invertebrate Biology| volume=115|issue=4|pages=349|doi=10.2307/3227024|issn=1077-8306| jstor=3227024}}</ref>) or asynchronously (''Branchiostoma lanceolatum'', gradual spawning through the season<ref>{{Cite journal| last1=Fuentes|first1=Michael|last2=Benito|first2=Elia|last3=Bertrand|first3=Stephanie|last4=Paris|first4=Mathilde|last5=Mignardot|first5=Aurelie|last6=Godoy|first6=Laura|last7=Jimenez-Delgado| first7=Senda|last8=Oliveri|first8=Diana|last9=Candiani|first9=Simona|date=2007| display-authors=3| title= Insights into spawning behavior and development of the european amphioxus (Branchiostoma lanceolatum)|journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution| language= en| volume= 308B| issue= 4|pages=484–493| doi= 10.1002/jez.b.21179|pmid=17520703|issn= 1552-5015}}</ref>).

Nicholas and Linda Holland were the first researchers to describe a method of obtaining amphioxus embryos by induction of spawning in captivity and in vitro fertilization.<ref>{{Cite journal |title=Fine Structural Study of the Cortical Reaction and Formation of the Egg Coats in a Lancelet|journal = The Biological Bulletin|volume = 176|issue = 2|pages = 111–122| first1= Nicholas D. | last1= Holland | first2= Linda Z. | last2= Holland |doi= 10.2307/1541578| jstor= 1541578|year = 1989|url = https://www.biodiversitylibrary.org/part/10948}}</ref> Spawning can be artificially induced in the lab by electric or thermal shock.<ref>{{Cite journal| last=Guang Li, ZongHuang Shu, Yiquan Wang|title=Year-Round Reproduction and Induced Spawning of Chinese Amphioxus, Branchiostoma belcheri, in Laboratory|journal=PLOS ONE| volume= 8|issue =9| pages= e75461| doi= 10.1371/journal.pone.0075461| pmid=24086537|pmc=3784433|year=2013|bibcode=2013PLoSO...875461L|doi-access= free}}</ref>

== History ==

=== Taxonomy ===

The first representative organism of the group to be described was ''[[Biology:Branchiostoma lanceolatum|Branchiostoma lanceolatum]]''. It was described by Peter Simon Pallas in 1774 as [[Biology:Mollusc|mollusc]]an slugs in the genus ''[[Biology:Limax|Limax]]''. It was not until 1834 that Gabriel Costa brought the phylogenetic position of the group closer to the agnathan vertebrates ([[Biology:Hagfish|hagfish]] and lampreys), including it in the new genus ''[[Biology:Branchiostoma|Branchiostoma]]'' (from the Greek, branchio = "gills", stoma = "mouth").<ref name="Garcia-Fernàndez">{{Cite journal | first1 = Jordi | last1 = Garcia-Fernàndez  | first2 = Èlia | last2 = Benito-Gutierrez | title = It's a long way from amphioxus: descendants of the earliest chordate | journal = BioEssays | volume = 31 | issue = 6 | pages = 665–675| date = June 2008 | doi = 10.1002/bies.200800110 | pmid=19408244| s2cid = 9292134 }}</ref> In 1836, Yarrel renamed the genus as ''Amphioxus'' (from the Greek: "pointed on both sides"), now considered an obsolete [[Biology:Synonym (taxonomy)|synonym]] of the genus ''Branchiostoma''. Today, the term "amphioxus" is still used as a [[Biology:Common name|common name]] for the Amphioxiformes, along with "lancelet", especially in the English language. The order Amphioxiformes was apparently named in 1886 in the ''Jahresbericht und Abhandlungen des Naturwissenschaftlichen Vereins in Magdeburg''.<ref>{{Cite book|last1=Naturwissenschaftlicher Verein in Magdeburg.|url=https://www.biodiversitylibrary.org/item/39425|title=Jahresbericht und Abhandlungen des Naturwissenschaftlichen Vereins in Magdeburg.|last2=Magdeburg|first2=Naturwissenschaftlicher Verein in|date=1886|publisher=The Verein|volume=1886|location=Magdeburg}}</ref><ref>{{Cite web|url=http://zoobank.org/NomenclaturalActs/bcc11d8b-e39a-4726-9296-3926481003a8|title=Amphioxiformes|website=zoobank.org|access-date=2020-03-07}}</ref>

=== Anatomy ===
Observations of amphioxus anatomy began in the middle of the 19th century. First, the adult then the embryonic anatomy were described.<ref name=":1">{{Cite book|title=Entwickelungsgeschichte des Amphioxus lanceolatus|last=Kowalevsky AO.|publisher=Mém Acad Sci St Petersburg.|year=1867}}</ref>

Alexander Kowalevsky first described the key anatomical features of the adult amphioxus (hollow dorsal nerve tube, endostyle, segmented body, postanal tail).<ref name=":1" /> De Quatrefages first completely described the nervous system of amphioxus.<ref>{{Cite book|title=Annales des sciences naturelles|last=de Quatrefages|first=M. A.|publisher=Libraires-editeurs|year=1845}}</ref> Other important contributions to amphioxus adult anatomy were given by Heinrich Rathke <ref>{{Cite book|title=Bemerkungen uber den Bau des Amphioxus lanceolatus eines Fisches aus der Ordnung der Cyclostomen|url=https://archive.org/details/b22392531|last=H. Rathke|publisher=Gebrüder Bornträger|year=1841}}</ref> and John Goodsir.<ref>{{Cite book|title=Proc. R. Soc. Edinb|last=J. Goodsir|year=1844}}</ref>

Kowalevsky also released the first complete description of amphioxus embryos,<ref name=":1" /> while Schultze and Leuckart were the first to describe the larvae.<ref>{{Cite book|title=Z. Wiss. Zool.|last=M. Schultze|year=1851}}</ref> Other important contributions to amphioxus embryonic anatomy were given by Hatschek, Conklin<ref>{{Cite journal|title=The Amphioxus and Its Development|journal=Nature|volume=48|issue=1252|last=B. Hatschek|year=1893|pages=613|bibcode=1893Natur..48..613E|doi=10.1038/048613a0|hdl=2027/hvd.hn25lj|s2cid=4016509|url=https://archive.org/details/amphioxusitsdeve00hats|doi-access=free}}</ref> and later by Tung (experimental embryology).<ref>{{Cite book|title=Dev. Growth. Differ|last=S. Yan|publisher=1999}}</ref>

== Anatomy ==
[[File:Lancelet_Anatomy.png|alt=Anatomy of the Lancelet|thumb|500x500px|Anatomy of the Lancelet]]
Depending on the exact species involved, the maximum length of lancelets is typically {{convert|2.5|to(-)|8|cm|in|abbr=on|1}}.<ref name="Wanninger2015" /><ref>{{cite web |author=Barnes, M.K.S |date=7 June 2015 |editor1=Tyler-Walters, H. |editor2=K. Hiscock |url=http://www.marlin.ac.uk/species/detail/85 |title=Lancelet (Branchiostoma lanceolatum) |publisher=Marine Life Information Network: Biology and Sensitivity Key Information Reviews |access-date=7 January 2018 |df=dmy-all}}</ref> ''Branchiostoma belcheri'' and ''[[Biology:Branchiostoma lanceolatum|B. lanceolatum]]'' are among the largest.<ref name="Wanninger2015" /> Except for the size, the species are very similar in general appearance, differing mainly in the number of [[Biology:Myotome|myotome]]s and the pigmentation of their larvae.<ref name="Wanninger2015" /> They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some [[Biology:Cartilage|cartilage]] material stiffening the [[Biology:Gill slit|gill slit]]s, mouth, and tail, they have no true skeleton.<ref name="VB">{{cite book |author=Romer, Alfred Sherwood |author2=Parsons, Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA |pages= 18–21 |isbn=978-0-03-910284-5 |df=dmy-all}}</ref>

=== Nervous system and notochord ===
In common with vertebrates, lancelets have a hollow nerve cord running along the back, [[Biology:Pharyngeal slit|pharyngeal slit]]s and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called [[Biology:Myomere|myomere]]s.

Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of [[Biology:Cell|cells]] that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate [[Biology:Vertebral column|spine]], extends into the head. This gives the subphylum its name ({{math|κεφαλή}}, ''kephalē'' means 'head'). The fine structure of the notochord and the cellular basis of its adult growth are best known for the Bahamas lancelet, ''Asymmetron lucayanum''<ref name="HollandSomorjai2020">{{cite journal |last1=Holland |first1=Nicholas |last2=Somorjai |first2=Ildiko |year=2020 |title=Serial blockface SEM suggests that stem cells may participate in adult notochord growth in an invertebrate chordate, the Bahamas lancelet |journal=EvoDevo |volume=11 |issue=22 |page=22 |doi=10.1186/s13227-020-00167-6 |pmid=33088474 |pmc=7568382 |doi-access=free }}</ref>

The nerve cord is only slightly larger in the head region than in the rest of the body, so that lancelets do not appear to possess a true brain. However, developmental gene expression and [[Engineering:Transmission electron microscopy|transmission electron microscopy]] indicate the presence of a diencephalic [[Biology:Forebrain|forebrain]], a possible [[Biology:Midbrain|midbrain]], and a hindbrain.<ref name="CandianiMoronti2012">{{cite journal |last1=Candiani |first1=Simona |last2=Moronti |first2=Luca |last3=Ramoino |first3=Paola |last4=Schubert |first4=Michael |last5=Pestarino |first5=Mario |year=2012 |title=A neurochemical map of the developing amphioxus nervous system |journal=BMC Neuroscience |volume=13 |issue=1 |page=59 |issn=1471-2202 |doi=10.1186/1471-2202-13-59 |pmid=22676056 |pmc=3484041|df=dmy-all |doi-access=free }}</ref><ref name="Holland2015">{{cite journal |last=Holland |first=L.Z. |year=2015 |title=The origin and evolution of chordate nervous systems |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=370 |issue=1684 |page=20150048 |issn=0962-8436 |doi=10.1098/rstb.2015.0048 |pmid=26554041 |pmc=4650125 |df=dmy-all}}</ref> Recent studies involving a comparison with vertebrates indicate that the vertebrate [[Biology:Thalamus|thalamus]], pretectum, and [[Biology:Midbrain|midbrain]] areas jointly correspond to a single, combined region in the amphioxus, which has been termed ''di-mesencephalic primordium'' (DiMes).<ref>{{cite journal |vauthors = Albuixech-Crespo B, López-Blanch L, Burguera D, Maeso I, Sánchez-Arrones L |display-authors=0 |year=2017 |title=Molecular regionalization of the developing amphioxus neural tube challenges major partitions of the vertebrate brain |journal= PLOS Biology |volume=15 |issue=4 |pages=e2001573 |doi=10.1371/journal.pbio.2001573 |pmid=28422959 |pmc=5396861 |doi-access=free }}</ref>

=== Visual system ===
Lancelets have four known kinds of light-sensing structures: Joseph cells, Hesse organs, an unpaired anterior eye and lamellar body, all of which utilize [[Biology:Opsin|opsin]]s as light receptors. All of these organs and structures are located in the neural tube, with the frontal eye at the front, followed by the lamellar body, the Joseph cells, and the Hesse organs.<ref name="NieuwenhuysDonkelaar2014">{{cite book |last1=Nieuwenhuys |first1=Rudolf |last2=ten Donkelaar |first2=Hans J. |author3=Charles Nicholson |title=The Central Nervous System of Vertebrates |url=https://books.google.com/books?id=gsDqCAAAQBAJ&pg=PA371 |access-date=25 November 2015 |date=14 November 2014 |publisher=Springer |isbn=978-3-642-18262-4 |page=371 |df=dmy-all}}</ref><ref name="Wanninger2015">{{cite book |last=Wanninger |first=Andreas |title=Evolutionary Developmental Biology of Invertebrates 6: Deuterostomia |url=https://books.google.com/books?id=GxZcCgAAQBAJ&pg=PA108 |access-date=21 November 2015 |date=11 August 2015|publisher=Springer |isbn=978-3-7091-1856-6 |pages=93–94, 108–109 |df=dmy-all}}</ref><ref name="Lamb2013">{{cite journal |last1=Lamb |first1=Trevor D.|title=Evolution of phototransduction, vertebrate photoreceptors and retina |journal=Progress in Retinal and Eye Research |volume=36 |year=2013 |pages=52–119 |issn=1350-9462 |doi=10.1016/j.preteyeres.2013.06.001 |pmid=23792002 |s2cid=38219705|df=dmy-all|doi-access=free |hdl=1885/84715 |hdl-access=free }}</ref>

==== Joseph cells and Hesse organs ====
Joseph cells are bare photoreceptors surrounded by a band of microvilli. These cells bear the opsin [[Biology:Melanopsin|melanopsin]]. The Hesse organs (also known as dorsal ocelli) consist of a photoreceptor cell surrounded by a band of microvilli and bearing melanopsin, but half enveloped by a cup-shaped pigment cell. The peak sensitivity of both cells is ~470&nbsp;nm<ref name="del Pilar GomezAngueyra2009">{{cite journal |last1=del Pilar Gomez |first1=M. |last2=Anyfgueyra |first2=J. M. |last3=Nasi|first3=E. |title=Light-transduction in melanopsin-expressing photoreceptors of Amphioxus |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=22 |year=2009 |pages=9081–9086 |issn=0027-8424 |doi=10.1073/pnas.0900708106 |pmid=19451628 |pmc=2690026 |bibcode=2009PNAS..106.9081D |df=dmy-all|doi-access=free }}</ref> (blue).

Both the Joseph cells and Hesse organs are in the neural tube, the Joseph cells forming a dorsal column, the Hesse organs in the ventral part along the length of the tube. The Joseph cells extend from the caudal end of the anterior vesicle (or cerebral vesicle) to the boundary between myomeres&nbsp;3 and 4, where the Hesse organs begin and continue nearly to the tail.<ref name="Trainor2013">{{cite book |last1= Le Douarin |first1=Nicole Marthe|last2=Dupin |first2=Elisabeth |editor=Paul Trainor |title=Neural Crest Cells: Evolution, development and disease |url=https://books.google.com/books?id=QckxAQAAQBAJ&pg=PA10 |access-date=25 November 2015 |date=23 November 2013 |publisher=Academic Press |isbn=978-0-12-404586-6 |page=10 |df=dmy-all}}</ref><ref name="WichtLacalli2005">{{cite journal |last1=Wicht |first1=Helmut |last2=Lacalli |first2=Thurston C. |title=The nervous system of amphioxus: Structure, development, and evolutionary significance |journal=Canadian Journal of Zoology |volume=83 |issue=1 |year=2005 |pages=122–150 |issn=0008-4301 |doi=10.1139/z04-163 |df=dmy-all}}</ref>

==== Frontal eye ====
The frontal eye consists of a pigment cup, a group of putative photoreceptor cells (termed ''Row&nbsp;1''), three rows of neurons (''Rows&nbsp;2–4''), and glial cells. The frontal eye, which expresses the [[Biology:PAX6|PAX6]] gene, has been proposed as the homolog of vertebrate paired eyes, the pigment cup as the homolog of the RPE ([[Medicine:Retinal pigment epithelium|retinal pigment epithelium]]), the putative photoreceptors as homologs of vertebrate [[Biology:Rod cell|rods]] and [[Biology:Cone cell|cones]], and Row 2 neurons as homologs of the retinal ganglion cells.<ref name="VopalenskyPergner2012">{{cite journal |last1=Vopalensky |first1=P. |last2=Pergner |first2=J. |last3=Liegertova |first3=M. |last4=Benito-Gutierrez |first4=E. |last5=Arendt |first5=D. |last6=Kozmik |first6=Z. |title=Molecular analysis of the amphioxus frontal eye unravels the evolutionary origin of the retina and pigment cells of the vertebrate eye |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=38 |date=18 September 2012 |pages=15383–15388 |issn=0027-8424 |doi=10.1073/pnas.1207580109 |pmid=22949670 |pmc=3458357 |bibcode=2012PNAS..10915383V |df=dmy-all|doi-access=free }}</ref>

The pigment cup is oriented concave dorsally. Its cells contain the pigment [[Chemistry:Melanin|melanin]].<ref name="VopalenskyPergner2012"/><ref name="Jankowski2013">{{cite book |last=Jankowski |first=Roger |title=The Evo-Devo Origin of the Nose, Anterior Skull Base and Midface |url=https://books.google.com/books?id=tfpGAAAAQBAJ&pg=PA152 |access-date=7 December 2015 |date=19 March 2013 |publisher=Springer Science & Business Media |isbn=978-2-8178-0422-4 |page=152 |df=dmy-all}}</ref>

The putative photoreceptor cells, Row&nbsp;1, are arranged in two diagonal rows, one on either side of the pigment cup, symmetrically positioned with respect to the ventral midline. The cells are flask-shaped, with long, slender ciliary processes (one cilium per cell). The main bodies of the cells lie outside of the pigment cup, while the cilia extend into the pigment cup before turning and exiting. The cells bear the opsin ''c-opsin 1'', except for a few which carry ''c-opsin 3''.<ref name="VopalenskyPergner2012"/><ref name="Lacalli1996">{{cite journal |last1=Lacalli |first1=T. C. |title=Frontal Eye Circuitry, Rostral Sensory Pathways and Brain Organization in Amphioxus Larvae: Evidence from 3D Reconstructions |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=351 |issue=1337 |date=29 March 1996 |pages=243–263 |url=http://royalsocietypublishing.org/content/royptb/351/1337/243.full.pdf |issn=0962-8436 |doi=10.1098/rstb.1996.0022 |df=dmy-all |bibcode=1996RSPTB.351..243L |access-date=14 December 2015 |archive-url=https://web.archive.org/web/20181021235426/http://royalsocietypublishing.org/content/royptb/351/1337/243.full.pdf |archive-date=21 October 2018 |url-status=dead }}</ref>

The Row&nbsp;2 cells are [[Chemistry:Serotonin|serotonergic]] neurons in direct contact with Row 1 cells. Row&nbsp;3 and 4 cells are also neurons. Cells of all four rows have axons that project into the left and right ventrolateral nerves. For Row&nbsp;2 neurons, axon projections have been traced to the tegmental [[Biology:Neuropil|neuropil]]. The tegmental neuropil has been compared with [[Biology:Animal locomotion|locomotor]] control regions of the vertebrate [[Biology:Hypothalamus|hypothalamus]], where paracrine release modulates locomotor patterns such as feeding and swimming.<ref name="VopalenskyPergner2012"/>

=== Fluorescent proteins ===
[[File:Lancelet GFP.png|alt=Green Fluorescence in Lancelets|thumb|Green fluorescence in Lancelets. (a. ''Branchiostoma floridae'' GFP near the eye spot and in the oral tentacles.) (b. ''Asymmetron lucayanum'' green fluorescence in the gonads.)]]

Lancelets naturally express [[Biology:Green fluorescent protein|green fluorescent protein]]s (GFP) inside their oral tentacles and near the eye spot.<ref>{{Cite journal|last1=Deheyn|first1=Dimitri D.|last2=Kubokawa|first2=Kaoru|last3=McCarthy|first3=James K.|last4=Murakami|first4=Akio|last5=Porrachia|first5=Magali|last6=Rouse|first6=Greg W.|last7=Holland|first7=Nicholas D.|date=2007-10-01|title=Endogenous Green Fluorescent Protein (GFP) in Amphioxus|url=https://www.journals.uchicago.edu/doi/10.2307/25066625|journal=The Biological Bulletin|volume=213|issue=2|pages=95–100|doi=10.2307/25066625|jstor=25066625|pmid=17928516|s2cid=45913388|issn=0006-3185}}</ref> Depending on the species, it can also be expressed in the tail and gonads, though this is only reported in the Asymmetron genus.<ref>{{Cite journal|last1=Yue|first1=Jia-Xing|last2=Holland|first2=Nicholas D.|last3=Holland|first3=Linda Z.|last4=Deheyn|first4=Dimitri D.|date=2016-06-17|title=The evolution of genes encoding for green fluorescent proteins: insights from cephalochordates (amphioxus)|url= |journal=Scientific Reports|language=en|volume=6|issue=1|pages=28350|doi=10.1038/srep28350|issn=2045-2322|pmc=4911609|pmid=27311567|bibcode=2016NatSR...628350Y}}</ref> Multiple fluorescent protein [[Biology:Gene|gene]]s have been recorded in lancelet species throughout the world. ''[[Biology:Branchiostoma floridae|Branchiostoma floridae]]'' alone has 16 GFP-encoding genes. However, the GFP produced by lancelets is more similar to GFP produced by [[Biology:Copepod|copepod]]s than jellyfish (''[[Biology:Aequorea victoria|Aequorea victoria]]'').

It is suspected GFP plays multiple roles with lancelets such as attracting plankton towards their mouth. Considering that lancelets are filter feeders, the natural current would draw nearby plankton into the digestive tract. GFP is also expressed in [[Biology:Larva|larva]]e, signifying it may be used for [[Physics:Photoprotection|photoprotection]] by converting higher energy blue light to less harmful green light.
[[File:Lancelet GFP GIF.gif|alt=Live lancelet (B. floridae) under a fluorescent microscope.|thumb|Live lancelet (''B. floridae'') under a fluorescent microscope.]]
The fluorescent proteins from lancelets have been adapted for use in molecular biology and microscopy. The [https://www.ncbi.nlm.nih.gov/nuccore/EU482389 yellow fluorescent protein] from ''[[Biology:Branchiostoma lanceolatum|Branchiostoma lanceolatum]]'' exhibits unusually high [[Physics:Quantum yield|quantum yield]] (~0.95).<ref>{{Cite journal|last1=Shaner|first1=Nathan C.|last2=Lambert|first2=Gerard G.|last3=Chammas|first3=Andrew|last4=Ni|first4=Yuhui|last5=Cranfill|first5=Paula J.|last6=Baird|first6=Michelle A.|last7=Sell|first7=Brittney R.|last8=Allen|first8=John R.|last9=Day|first9=Richard N.|last10=Israelsson|first10=Maria|last11=Davidson|first11=Michael W.|date=May 2013|title=A bright monomeric green fluorescent protein derived from Branchiostoma lanceolatum|url= |journal=Nature Methods|language=en|publication-date=24 March 2013|volume=10|issue=5|pages=407–409|doi=10.1038/nmeth.2413|pmid=23524392|pmc=3811051|issn=1548-7105}}</ref> It has been [[Biology:Genetic engineering|engineered]] into a [[Chemistry:Monomer|monomer]]ic green fluorescent protein known as [https://www.fpbase.org/protein/mneongreen/ mNeonGreen], which is the brightest known monomeric green or yellow fluorescent protein.

=== Feeding and digestive system ===
Lancelets are passive [[Biology:Filter feeder|filter feeder]]s,<ref name=Igawa2017>{{cite journal |author1=Igawa, T. | author2=M. Nozawa |author3=D.G. Suzuki |author4=J.D. Reimer |author5=A.R. Morov |author6=Y. Wang |author7=Y. Henmi |author8=K. Yasui |year=2017 |title=Evolutionary history of the extant amphioxus lineage with shallow-branching diversification |journal=Scientific Reports |volume=7 |issue=1 |page=1157 |doi=10.1038/s41598-017-00786-5 |pmid=28442709 |pmc=5430900 |bibcode=2017NatSR...7.1157I |df=dmy-all}}</ref> spending most of the time half-buried in sand with only their frontal part protruding.<ref>{{cite book |author=Kotpal, R.L. |title=Modern Text Book of Zoology: Vertebrates |year=2008–2009 |edition=3 |isbn=978-81-7133-891-7 |pages=76 |publisher=Rastogi Publications |df=dmy-all}}</ref> They eat a wide variety of small [[Earth:Plankton|plankton]]ic organisms, such as bacteria, [[Biology:Fungi|fungi]], [[Biology:Diatom|diatom]]s, [[Biology:Dinoflagellate|dinoflagellate]]s and [[Earth:Zooplankton|zooplankton]], and they will also take [[Earth:Detritus|detritus]].<ref name=Carvalho2017/> Little is known about the diet of the lancelet larvae in the wild, but captive larvae of several species can be maintained on a diet of [[Earth:Phytoplankton|phytoplankton]], although this apparently is not optimal for ''Asymmetron lucayanum''.<ref name=Carvalho2017/>

Lancelets have oral cirri, thin [[Biology:Tentacle|tentacle]]-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of [[Biology:Mucus|mucus]]. [[Biology:Cilium|Cilia]]ry action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal groove, known as the [[Biology:Epipharyngeal groove|epipharyngeal groove]], and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.<ref name="VB" />

Both adults and larvae exhibit a "cough" reflex to clear the mouth or throat of debris or items too large to swallow. In larvae the action is mediated by the pharyngeal muscles while in the adult animal it is accomplished by atrial contraction.<ref name="RogersAndrew2002">{{cite book |author1=Rogers, Lesley J. |author2=Andrew, Richard |title=Comparative Vertebrate Lateralization |url=https://books.google.com/books?id=lbS86Sx9tLMC&pg=PA72 |date=25 March 2002 |publisher=Cambridge University Press|isbn=978-1-139-43747-9 |pages=72 ff |df=dmy-all}}</ref><ref name="RigonStach2013">{{cite journal|last1=Rigon|first1=Francesca |last2=Stach |first2=Thomas |last3=Caicci |first3=Federico |last4=Gasparini |first4=Fabio |last5=Burighel |first5=Paolo |last6=Manni |first6=Lucia |title=Evolutionary diversification of secondary mechanoreceptor cells in tunicata |journal=BMC Evolutionary Biology |volume=13 |issue=1 |year=2013 |pages=112 |issn=1471-2148 |doi=10.1186/1471-2148-13-112 |pmid=23734698 |pmc=3682859 |df=dmy-all |doi-access=free }}</ref>

The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. The [[Biology:Hepatic caecum|hepatic caecum]], a single blind-ending [[Biology:Cecum|caecum]], branches off from the underside of the gut, with a lining able to [[Biology:Phagocytosis|phagocytize]] the food particles, a feature not found in vertebrates. Although it performs many functions of a liver, it is not considered a true liver but a [[Biology:Homology|homolog]] of the vertebrate liver.<ref name="YuanRuan2015">{{cite journal |last1=Yuan |first1=Shaochun |last2=Ruan |first2=Jie |last3=Huang |first3=Shengfeng |last4=Chen |first4=Shangwu |last5=Xu |first5=Anlong |title=Amphioxus as a model for investigating evolution of the vertebrate immune system |journal=Developmental & Comparative Immunology |volume=48 |issue=2 |date=February 2015 |pages=297–305 |url=http://mosas.sysu.edu.cn/lab/refsys/uppdf/201462021511673499.pdf |issn=0145-305X |doi=10.1016/j.dci.2014.05.004 |pmid=24877655 |df=dmy-all |access-date=2015-12-16 |archive-url=https://web.archive.org/web/20151222131815/http://mosas.sysu.edu.cn/lab/refsys/uppdf/201462021511673499.pdf |archive-date=2015-12-22 |url-status=dead }}</ref><ref name="YuLecroisey2015">{{cite journal |last1=Yu |first1=Jr-Kai Sky |last2=Lecroisey |first2=Claire |last3=Le Pétillon |first3=Yann |last4=Escriva |first4=Hector |last5=Lammert |first5=Eckhard |last6=Laudet |first6=Vincent |title=Identification, Evolution and Expression of an Insulin-Like Peptide in the Cephalochordate Branchiostoma lanceolatum |journal=PLOS ONE |volume=10 |issue=3 |year=2015 |pages=e0119461 |issn=1932-6203 |doi=10.1371/journal.pone.0119461 |pmid=25774519 |pmc=4361685 |bibcode=2015PLoSO..1019461L |df=dmy-all|doi-access=free }}</ref><ref name="EscrivaChao2012">{{cite journal |last1=Escriva |first1=Hector |last2=Chao |first2=Yeqing |last3=Fan |first3=Chunxin |last4=Liang |first4=Yujun |last5=Gao |first5=Bei |last6=Zhang |first6=Shicui |title=A Novel Serpin with Antithrombin-Like Activity in Branchiostoma japonicum: Implications for the Presence of a Primitive Coagulation System |journal=PLOS ONE |volume=7 |issue=3 |year=2012 |pages=e32392 |issn=1932-6203 |doi=10.1371/journal.pone.0032392 |pmid=22427833 |pmc=3299649 |bibcode=2012PLoSO...732392C |df=dmy-all|doi-access=free }}</ref>

=== Other systems ===
Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple [[Biology:Epithelium|epithelium]]. Despite the name, little if any respiration occurs in the "gill" slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include a [[Biology:Heart|heart]]. There are no blood cells, and no [[Biology:Hemoglobin|hemoglobin]].<ref name="VB" />

The excretory system consists of segmented "kidneys" containing [[Biology:Nephridium|protonephridia]] instead of nephrons, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented gonads.<ref name="VB" />

== Model organism ==
Lancelets became famous in the 1860s when [[Biography:Ernst Haeckel|Ernst Haeckel]] began promoting them as a model for the ancestor of all vertebrates. By 1900 lancelets had become a model organism. By the mid-20th century they had fallen out of favor for a variety of reasons, including a decline of comparative anatomy and embryology, and due to the belief that lancelets were more derived than they appeared, e.g., the profound asymmetry in the larval stage.<ref name="Hopwood2015">{{cite journal|last1=Hopwood|first1=Nick|title=The cult of amphioxus in German Darwinism; or, Our gelatinous ancestors in Naples' blue and balmy bay|journal=History and Philosophy of the Life Sciences|volume=36|issue=3|date=January 2015|pages=371–393|issn=0391-9714|doi=10.1007/s40656-014-0034-x|pmid=26013195|pmc=4286652}}</ref><ref name="RefTudgeVariety">{{RefTudgeVariety}}</ref> More recently, the fundamental symmetric and twisted development of vertebrates is the topic of the axial twist theory. According to this theory, there is a deep agreement between the vertebrates and cephalochordates, and even all chordates.<ref name="Lussanet2012">{{cite journal | first1=M.H.E. | last1=de Lussanet | first2=J.W.M. | last2=Osse | year=2012 | title=An ancestral axial twist explains the contralateral forebain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2 | pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872 | s2cid=7399128}}</ref><ref name="Kinsbourne2013">{{cite journal | first1=M. | last1=Kinsbourne | s2cid=11646580  | title=Somatic twist: a model for the evolution of decussation | journal=Neuropsychology | volume=27 | issue=5 | year=2013 | pages=511–515 | doi=10.1037/a0033662 | pmid=24040928}}</ref>

With the advent of molecular genetics lancelets are once again regarded as a model of vertebrate ancestors, and are used again as a model organism.<ref name="HollandLaudet2004">{{cite journal|last1=Holland|first1=L.Z.|last2=Laudet|first2=V.|last3=Schubert|first3=M.|title=The chordate amphioxus: an emerging model organism for developmental biology|journal=Cellular and Molecular Life Sciences|volume=61|issue=18|date=September 2004|pages=2290–2308 |url=https://www.researchgate.net/publication/8337108|issn=1420-682X|doi=10.1007/s00018-004-4075-2|pmid=15378201|s2cid=28284725}}</ref><ref name="Garcia-Fernàndez" />

As a result of their use in science, methods of keeping and breeding lancelets in captivity have been developed for several of the species, initially the European ''Branchiostoma lanceolatum'', but later also the West Pacific ''Branchiostoma belcheri'' and ''Branchiostoma japonicum'', the Gulf of Mexico and West Atlantic ''Branchiostoma floridae'' and the circumtropical (however, genetic evidence suggest the Atlantic and [[Earth:Indo-Pacific|Indo-Pacific]] populations should be recognized as separate<ref name=Igawa2017/>) ''Asymmetron lucayanum''.<ref name=Carvalho2017/><ref name=EMBRCFrance>{{cite web | url=http://www.embrc-france.fr/en/our-services/supply-biological-resources/model-organisms/amphioxus-branchiostoma-lanceolatum |title=Amphioxus Branchiostoms lanceolatum | publisher=EMBRC France |access-date=7 January 2018 }}</ref> They can reach an age of up to 7–8 years.<ref name=EMBRCFrance/>

==As human food==
The animals are edible and harvested in some parts of the world. They are eaten both fresh, tasting like herring, and as a food additive in dry form after being roasted in oil.{{Cn|date=February 2023}} When their gonads start to ripen in the spring it affects their flavor, making them taste bad during their breeding season.<ref>[https://www.researchgate.net/profile/Nicholas-Holland/publication/241302129_The_Lancelet/links/56b644f308ae3c1b79ad4716/The-Lancelet The Lancelet - Red or White Wine?]</ref>

== Phylogeny and taxonomy ==
[[File:Amphioxus.png|thumb|right|The lancelet is a small, translucent, fish-like animal that is one of the closest living invertebrate relatives of the vertebrates.<ref>{{cite journal | last1 = Gewin | first1 = V | year = 2005 | title = Functional genomics thickens the biological plot | journal = PLOS Biology | volume = 3 | issue = 6| page = e219 | doi = 10.1371/journal.pbio.0030219 | pmid = 15941356 | pmc=1149496 | doi-access = free }}</ref><ref>[https://arstechnica.com/science/2008/06/lancelet-amphioxus-genome-and-the-origin-of-vertebrates/ Lancelet (amphioxus) genome and the origin of vertebrates] ''Ars Technica'', 19 June 2008.</ref>]]

The [[Biology:Cephalochordate|Cephalochordata]] were traditionally seen as the sister lineage to the [[Biology:Vertebrate|vertebrate]]s; in turn, these two groups together (sometimes called Notochordata) were considered the sister group to the [[Biology:Tunicate|Tunicata]] (also called Urochordata and including sea squirts).  Consistent with this view, at least 10 morphological features are shared by lancelets and vertebrates, but not tunicates.<ref name="benton2005">Michael J. Benton (2005). ''Vertebrate Palaeontology, Third Edition'' 8. Oxford: Blackwell Publishing. {{ISBN|0-632-05637-1}}.</ref> Newer research suggests this pattern of evolutionary relationship is incorrect. Extensive [[Biology:Molecular phylogenetics|molecular phylogenetic]] analysis has shown convincingly that the Cephalochordata is the most [[Biology:Basal (phylogenetics)|basal]] subphylum of the chordates, with tunicates being the sister group of the vertebrates.<ref>{{Cite journal |first1=Frédéric |last1=Delsuc |first2=Henner |last2=Brinkmann |first3=Daniel |last3=Chourrout |first4=Hervé |last4=Philippe |url=http://worldcat.org/oclc/784007344|title=Tunicates and not cephalochordates are the closest living relatives of vertebrates.|journal=Nature|year=2006|volume=439|issue=7079|pages=965–8|doi=10.1038/nature04336|pmid=16495997|bibcode=2006Natur.439..965D|s2cid=4382758|oclc=784007344}}</ref><ref name="putnam">{{Cite journal|last1=Putnam|first1=N. H.|last2=Butts|first2=T.|last3=Ferrier|first3=D. E. K.|last4=Furlong|first4=R. F.|last5=Hellsten|first5=U.|last6=Kawashima|first6=T.|last7=Robinson-Rechavi|first7=M.|last8=Shoguchi|first8=E.|last9=Terry|first9=A.|date=Jun 2008|title=The amphioxus genome and the evolution of the chordate karyotype|journal=Nature|volume=453|issue=7198|pages=1064–1071|bibcode=2008Natur.453.1064P|doi=10.1038/nature06967|issn=0028-0836|pmid=18563158|last10=Yu|first10=J. K.|last11=Benito-Gutiérrez|first11=E. L.|last12=Dubchak|first12=I.|last13=Garcia-Fernàndez|first13=J.|last14=Gibson-Brown|first14=J. J.|last15=Grigoriev|first15=I. V.|last16=Horton|first16=A. C.|last17=De Jong|first17=P. J.|last18=Jurka|first18=J.|last19=Kapitonov|first19=V. V.|last20=Kohara|first20=Y.|last21=Kuroki|first21=Y.|last22=Lindquist|first22=E.|last23=Lucas|first23=S.|last24=Osoegawa|first24=K.|last25=Pennacchio|first25=L. A.|last26=Salamov|first26=A. A.|last27=Satou|first27=Y.|last28=Sauka-Spengler|first28=T.|last29=Schmutz|first29=J.|last30=Shin-i|first30=T.|s2cid=4418548|doi-access=free}}</ref>  This revised phylogeny of chordates suggests that tunicates have secondarily lost some of the morphological characters that were formerly considered to be synapomorphies (shared, derived characters) of vertebrates and lancelets.

Among the three extant (living) genera, ''Asymmetron'' is basal and it is placed in its own family. Genetic studies have come to separate conclusions on their divergence, with some suggesting that ''Asymmetron'' diverged from other lancelets more than 100 million years ago and others (with higher statistic support) that it occurred about 42 million years ago.<ref name=Igawa2017/> The two remaining genera, ''Branchiostoma'' and ''Epigonichthys'' diverged from each other about 36 million years ago.<ref name=Igawa2017/> Despite this deep separation, [[Biology:Hybrid|hybrids]] between ''Asymmetron lucayanum'' and ''Branchiostoma floridae'' are viable (among the deepest split species known to be able to produce such hybrids).<ref name=Carvalho2017/>

The following are the species recognised by [[Organization:World Register of Marine Species|WoRMS]]. Other sources recognize about thirty species.<ref name="RefTudgeVariety"/><ref name=Igawa2017/><ref>{{cite journal | author=WoRMS Editorial Board | title= World Register of Marine Species- Cephalochordates species list | year=2013 | url=http://www.marinespecies.org/aphia.php?p=taxlist&pid=1824&rComp=%3E%3D&tRank=220 | access-date=2013-10-22 }}</ref> It is likely that currently unrecognized cryptic species remain.<ref name=Carvalho2017/>

*'''Order Branchiostomiformes'''
**'''Family''' '''[[Biology:Branchiostomatidae|Branchiostomatidae]]''' <small>Bonaparte 1841</small>
*** Genus ''[[Biology:Asymmetron|Asymmetron]]'' <small>Andrews 1893</small> [''Amphioxides'' <small>Gill 1895</small>]
****''Asymmetron inferum'' <small>Nishikawa 2004</small>
****''Asymmetron lucayanum'' <small>Andrews 1893</small> (Sharptail lancelet)
*** Genus ''[[Biology:Branchiostoma|Branchiostoma]]'' <small>Costa 1834 non Newport 1845 non Banks 1905</small> [''Amphioxus'' <small>Yarrell 1836</small>; ''Limax'' <small>Pallas 1774 non Linnaeus 1758 non Férussac 1819 non Martyn 1784</small>; ''Dolichorhynchus'' <small>Willey 1901 non Mulk & Jairajpuri 1974</small>] 
****''Branchiostoma africae'' <small>Hubbs 1927</small>
****''Branchiostoma arabiae'' <small>Webb 1957</small>
****''Branchiostoma bazarutense'' <small>Gilchrist 1923</small>
****''[[Biology:Branchiostoma belcheri|Branchiostoma belcheri]]'' <small>(Gray 1847)</small><!-- ZoolSci24:787. --> (Belcher's lancelet)
****''[[Biology:Branchiostoma bennetti|Branchiostoma bennetti]]'' <small>Boschung & Gunter 1966</small> (Mud lancelet)
****''Branchiostoma bermudae'' <small>Hubbs 1922</small>
****''Branchiostoma californiense'' <small>Andrews 1893</small> (Californian lancelet)
****''Branchiostoma capense'' <small>Gilchrist 1902</small>
****''Branchiostoma caribaeum'' <small>Sundevall 1853</small> (Caribbean lancelet)
****''Branchiostoma elongatum'' <small>Sundevall 1852</small>
****''[[Biology:Branchiostoma floridae|Branchiostoma floridae]]'' <small>Hubbs 1922</small> (Florida lancelet)
****''Branchiostoma gambiense'' <small>Webb 1958</small>
****''Branchiostoma indicum'' <small>(Willey 1901)</small>
****''Branchiostoma japonicum'' <small>(Willey 1897)</small>  (Pacific lancelet)
****''[[Biology:Branchiostoma lanceolatum|Branchiostoma lanceolatum]]'' <small>(Pallas 1774)</small> (European lancelet)
****''Branchiostoma leonense'' <small>Webb 1956</small>
****''Branchiostoma longirostrum'' <small>Boschung 1983</small> (Shellhash lancelet)
****''Branchiostoma malayanum'' <small>Webb 1956</small>
****''Branchiostoma moretonense'' <small>Kelly 1966</small>; [[Biology:Nomen dubium|nomen dubium]]<ref>{{cite web|url=http://www.marinespecies.org/urmo/aphia.php?p=taxdetails&id=266214|title=UNESCO-IOC Register of Marine Organisms (URMO) - Branchiostoma mortonense Kelly, 1966}}</ref><ref>{{cite web|url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=266214|title=WoRMS - World Register of Marine Species - Branchiostoma mortonense Kelly, 1966}}</ref>
****''Branchiostoma nigeriense'' <small>Webb 1955</small>
****''Branchiostoma platae'' <small>Hubbs 1922</small>
****''Branchiostoma senegalense'' <small>Webb 1955</small>
****''Branchiostoma tattersalli'' <small>Hubbs 1922</small>
****''Branchiostoma virginiae'' <small>Hubbs 1922</small> (Virginian lancelet)
*** Genus ''[[Biology:Epigonichthys|Epigonichthys]]'' <small>Peters 1876</small> [''Amphipleurichthys'' <small>Whitley 1932</small>; ''Bathyamphioxus'' <small>Whitley 1932</small>; ''Heteropleuron'' <small>Kirkaldy 1895</small>; ''Merscalpellus'' <small>Whitley 1932</small>; ''Notasymmetron'' <small>Whitley 1932</small>; ''Paramphioxus'' <small>Haekel 1893</small>; ''Zeamphioxus'' <small>Whitley 1932</small>] 
****''Epigonichthys australis'' <small>(Raff 1912)</small>
****''Epigonichthys bassanus'' <small>(Günther 1884)</small>
****''Epigonichthys cingalensis'' <small>(Kirkaldy 1894)</small>; [[Biology:Nomen dubium|nomen dubium]]<ref>{{cite web|url=http://www.marinespecies.org/aphia.php?p=taxdetails&id=104903|title=WoRMS - World Register of Marine Species - Epigonichthys Peters, 1876}}</ref> 
****''Epigonichthys cultellus'' <small>Peters 1877</small>
****''Epigonichthys hectori'' <small>(Benham 1901)</small> (Hector's lancelet)
****''Epigonichthys maldivensis'' <small>(Foster Cooper 1903)</small>

== References ==
{{reflist|20em}}

==Further reading==
* {{cite book |author1=Stach, T.G. |year=2004 |section=Cephalochordata (Lancelets) |editor1=M. Hutchins  |editor2=Garrison, R.W. |editor3=Geist, V. |editor4=Loiselle, P.V. |editor5=Schlager, N. |editor6=McDade, M.C. |editor7=Duellman, W.E. |title=Grzimek's Animal Life Encyclopedia |title-link=Grzimek's Animal Life Encyclopedia |edition=2nd |volume=1 |pages=485–493 |place=Detroit, MI |publisher=Gale}}
* {{cite journal |author1=Stokes, M.D. |author2=Holland, N.D. |year=1998 |title={{grey|[no title cited]}} |journal=American Scientist |volume=86 |pages=552–560|doi=10.1511/1998.43.799 }}

== External links ==
* {{cite web
 |title=Cephalochordata
 |place=Berkeley, CA
 |department=Museum of Paleontology
 |publisher=[[Organization:University of California, Berkeley|U.C. Berkeley]]
 |url=http://www.ucmp.berkeley.edu/chordata/cephalo.html
}}
* {{cite web
 |title=Branchiostoma japonicum and B. belcheri are Distinct Lancelets (Cephalochordata) in Xiamen Waters in China
 |website=
 |url=https://www.researchgate.net/publication/6935781 
 |via=[[Company:ResearchGate|ResearchGate]]
}}
* {{cite press release
 |title=Error in the genealogy of humans
 |website=sars.no
 |place=Bergen, Norway
 |department=[[Organization:Sars International Centre for Marine Molecular Biology|Sars International Centre for Marine Molecular Biology]]
 |publisher=University of Bergen
 |url=http://www.sars.no/research/chourroutPress.php 
}}
* {{cite journal
 |title=A special issue of Amphioxus research
 |volume=I
 |website=biolsci.org
 |url=http://www.biolsci.org/v2.php#i2
 |access-date=2006-06-08
 |archive-date=2012-03-05
 |archive-url=https://web.archive.org/web/20120305143024/http://www.biolsci.org/v2.php#i2
 |url-status=dead
 }}
* {{cite journal
 |title=A special issue of Amphioxus research
 |volume=II
 |website=biolsci.org
 |url=http://www.biolsci.org/v2.php#i3
 |access-date=2006-06-08
 |archive-date=2012-03-05
 |archive-url=https://web.archive.org/web/20120305143024/http://www.biolsci.org/v2.php#i3
 |url-status=dead
 }}
* {{cite press release
 |title=Amphioxus and the T-box gene
 |website=news-info.wustl.edu
 |place=St.&nbsp;Louis, MO
 |publisher=[[Organization:Washington University in St. Louis|Washington University in St. Louis]]
 |url=http://news-info.wustl.edu/tips/2002/science-tech/vertebrate-invertebrate.html 
}}
* {{YouTube
 |id=ycHJMXUT2o0
 |title= A movie of the amphioxus embryonic development
}}
* {{cite press release
 |title=Scripps scientists discover fluorescence in key marine creature
 |website=scrippsnews.ucsd.edu
 |department=[[Organization:Scripps Institution of Oceanography|Scripps Institute]]
 |place=San&nbsp;Diego, CA
 |publisher=[[Organization:University of California, San Diego|U.C. San Diego]]
 |url=http://scrippsnews.ucsd.edu/Releases/?releaseID=848
 |access-date=2007-11-01
 |archive-date=2013-05-15
 |archive-url=https://web.archive.org/web/20130515062324/http://scrippsnews.ucsd.edu/Releases/?releaseID=848
 |url-status=dead
 }}
* {{cite EB9 |wstitle = Amphioxus |volume= I  | page=774 |short=1}}
* {{cite web
 |title=Amphioxus: Taxonomy, brief facts, life cycle and embryology
 |website=GeoChemBio
 |url=http://www.geochembio.com/biology/organisms/lancelet/ 
}}
* {{UCSC genomes|braFlo1}}

{{Animalia}}
{{Chordata}}
{{Taxonbar|from1=Q1156226|from2=Q20722244|from3=Q21286775}}

[[Category:Cephalochordata]]

{{Sourceattribution|Lancelet}}