Editing Lancelet (section)

=== 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>