Editing Lancelet (section)

== Anatomy ==

[[File:Lancelet Anatomy.png|alt=Anatomy of the Lancelet|thumb|500x500px|Anatomy of the lancelet]]

The larvae are extremely asymmetrical, with the mouth and anus on the left side, and the gill slits on the right side.<ref>{{Cite journal |last1=D'Aniello |first1=Salvatore |last2=Bertrand |first2=Stephanie |last3=Escriva |first3=Hector |date=2023-09-18 |editor-last=Pérez Valle |editor-first=Helena |editor2-last=Rodgers |editor2-first=Peter |title=Amphioxus as a model to study the evolution of development in chordates |journal=eLife |volume=12 |pages=e87028 |doi=10.7554/eLife.87028|doi-access=free |pmid=37721204 |pmc=10506793 }}</ref><ref>{{Cite journal |last1=Meulemans |first1=Daniel |last2=Bronner-Fraser |first2=Marianne |date=2007-08-29 |title=Insights from amphioxus into the evolution of vertebrate cartilage |journal=PLOS ONE |volume=2 |issue=8 |pages=e787 |doi=10.1371/journal.pone.0000787 |doi-access=free |pmc=1950077 |pmid=17726517|bibcode=2007PLoSO...2..787M }}</ref> Organs associated with the pharynx are positioned either exclusively on the left or on the right side of the body. In addition, segmented muscle blocks and parts of the nervous system are asymmetrical.<ref>{{Cite journal |last=Soukup |first=Vladimir |date=2017 |title=Left-right asymmetry specification in amphioxus: review and prospects |url=https://pubmed.ncbi.nlm.nih.gov/29319110/ |journal=The International Journal of Developmental Biology |volume=61 |issue=10–11–12 |pages=611–620 |doi=10.1387/ijdb.170251vs |pmid=29319110}}</ref> After metamorphosis the anatomy becomes more symmetrical, but some asymmetrical traits are still present also as adults, such as the nervous system and the location of the gonads which are found on the right side in Asymmetron and Epigonichthys (in Branchiostoma gonads develop on both sides of body).<ref>{{Cite journal |last1=Kaji |first1=Takao |last2=Aizawa |first2=Shinichi |last3=Uemura |first3=Masanori |last4=Yasui |first4=Kinya |date=2001-07-09 |title=Establishment of left-right asymmetric innervation in the lancelet oral region |url=https://onlinelibrary.wiley.com/doi/10.1002/cne.1039 |journal=Journal of Comparative Neurology |language=en |volume=435 |issue=4 |pages=394–405 |doi=10.1002/cne.1039 |pmid=11406821 }}</ref><ref>{{Cite journal |last1=Igawa |first1=Takeshi |last2=Nozawa |first2=Masafumi |last3=Suzuki |first3=Daichi G. |last4=Reimer |first4=James D. |last5=Morov |first5=Arseniy R. |last6=Wang |first6=Yiquan |last7=Henmi |first7=Yasuhisa |last8=Yasui |first8=Kinya |date=2017-04-25 |title=Evolutionary history of the extant amphioxus lineage with shallow-branching diversification |journal=Scientific Reports |language=en |volume=7 |issue=1 |pages=1157 |doi=10.1038/s41598-017-00786-5|pmid=28442709 |pmc=5430900 |bibcode=2017NatSR...7.1157I }}</ref>

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. Hisc ock |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 ''[[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 [[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 [[cartilage]] material stiffening the [[gill slit]]s, mouth, and tail, they have no true complex 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, [[pharyngeal slit]]s and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called [[myomere]]s.<ref>{{Cite journal |last1=Walker |first1=Warren F. |last2=Noback |first2=Charles R. |date=2021 |title=Muscular system |url=https://www.accessscience.com/content/muscular-system/440200 |journal=Access Science |language=en |doi=10.1036/1097-8542.440200|url-access=subscription }}</ref>

Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler [[notochord]] made up of a [[Cylinder (geometry)|cylinder]] of [[Cell (biology)|cells]] that are closely packed in collagen fibers to form a toughened rod. The lancelet notochord, unlike the vertebrate [[Vertebral column|spine]], extends into the head. This gives the subphylum, Cephalochordata, 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 [[transmission electron microscopy]] indicate the presence of a [[diencephalic]] [[forebrain]], a possible [[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 [[thalamus]], [[pretectum]], and [[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=etal |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 [[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 |author-link1=Rudolf Nieuwenhuys |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 [[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;three and four, where the Hesse organs begin and continue nearly to the tail.<ref name="Trainor2013">{{cite book |last1= Le Douarin |first1=Nicole Marthe|author-link1=Nicole Marthe Le Douarin |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 |bibcode=2005CaJZ...83..122W |df=dmy-all}}</ref>

==== Frontal eye ====
The frontal eye consists of a pigment cup, a group of  photoreceptor cells (termed ''Row&nbsp;1''), three rows of neurons (''Rows&nbsp;2–4''), and [[glial cells]]. The frontal eye, which expresses the [[PAX6]] gene, has been proposed as the homolog of vertebrate paired eyes,or the [[parietal eye|pineal eye]] on vertebrates, the pigment cup as the homolog of the RPE ([[retinal pigment epithelium]]), the putative photoreceptors as homologs of vertebrate [[rod cell|rods]] and [[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 [[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 [[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 [[Tegmentum|tegmental]] [[neuropil]]. The tegmental neuropil has been compared with [[Animal locomotion|locomotor]] control regions of the vertebrate [[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 [[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|url-access=subscription}}</ref> Depending on the species, it can also be expressed in the tail and [[gonad]]s, 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 [[gene]]s have been recorded in lancelet species throughout the world. ''[[Branchiostoma floridae]]'' alone has 16 GFP-encoding genes. However, the GFP produced by lancelets is more similar to GFP produced by [[copepod]]s than jellyfish (''[[Aequorea victoria]]'').{{cn|date=March 2024}}

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 [[larva]]e, signifying it may be used for [[photoprotection]] by converting higher energy blue light to less harmful green light.{{cn|date=March 2024}}
[[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 ''[[Branchiostoma lanceolatum]]'' exhibits unusually high [[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 [[Genetic engineering|engineered]] into a [[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 [[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 [[plankton]]ic organisms, such as bacteria, [[fungi]], [[diatom]]s, and [[zooplankton]], and they will also take [[detritus]].<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> 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 [[phytoplankton]], although this apparently is not optimal for ''Asymmetron lucayanum''.<ref name=Carvalho2017/>

Lancelets have oral cirri, thin [[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 [[human pharynx|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 [[mucus]]. [[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 [[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 |bibcode=2013BMCEE..13..112R }}</ref>

The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. The [[hepatic caecum]], a single blind-ending [[cecum|caecum]], branches off from the underside of the gut, with a lining able to [[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 [[homology (biology)|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 [[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 [[heart]]. There are no blood cells, and no [[hemoglobin]].<ref name="VB" />

The excretory system consists of segmented "kidneys" containing [[nephridium|protonephridia]] instead of [[nephron]]s, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented [[gonad]]s.<ref name="VB" />