Editing Brain (section)

=== Generic bilaterian nervous system ===
[[Image:Bilaterian-plan.svg|thumb|right|300px|alt=A rod-shaped body contains a digestive system running from the mouth at one end to the anus at the other. Alongside the digestive system is a nerve cord with a brain at the end, near to the mouth. |Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front]]
Except for a few primitive organisms such as [[sponge]]s (which have no nervous system)<ref name=Jacobs>{{Cite journal |title=Evolution of sensory structures in basal metazoa |journal=Integrative and Comparative Biology |volume=47 |issue=5 |year=2007 |pages=712–723 |doi=10.1093/icb/icm094 |last1=Jacobs |first1=DK |pmid=21669752 |last2=Nakanishi |first2=N |last3=Yuan |first3=D |last4=Camara |first4=A. |last5=Nichols |first5=S. A. |last6=Hartenstein |first6=V.|display-authors=3|citeseerx=10.1.1.326.2233 }}</ref> and [[cnidaria]]ns (which have a diffuse nervous system consisting of a [[nerve net]]),<ref name=Jacobs/> all living multicellular animals are [[bilateria]]ns, meaning animals with a [[bilateral symmetry|bilaterally symmetric]] [[body plan]] (that is, left and right sides that are approximate mirror images of each other).<ref name=Urbilateria /> All bilaterians are thought to have descended from a common ancestor that appeared late in the [[Cryogenian]] period, 700–650 million years ago, and it has been hypothesized that this common ancestor had the shape of a simple tubeworm with a segmented body.<ref name=Urbilateria>{{cite journal |last=Balavoine |first=G |title=The segmented Urbilateria: A testable scenario |journal=Integrative and Comparative Biology |year=2003 |volume=43 |pages=137–147 |doi=10.1093/icb/43.1.137 |pmid=21680418 |issue=1|doi-access=free }}</ref> At a schematic level, that basic worm-shape continues to be reflected in the body and nervous system architecture of all modern bilaterians, including vertebrates.<ref>{{cite book |title=The Evolution of Organ Systems |url=https://archive.org/details/evolutionorgansy00schm |url-access=limited |last=Schmidt-Rhaesa |first=A |publisher=Oxford University Press |year=2007 |isbn=978-0-19-856669-4 |page=[https://archive.org/details/evolutionorgansy00schm/page/n120 110]}}</ref> The fundamental bilateral body form is a tube with a hollow gut cavity running from the mouth to the anus, and a nerve cord with an enlargement (a [[ganglion]]) for each body segment, with an especially large ganglion at the front, called the brain. The brain is small and simple in some species, such as [[nematode]] worms; in other species, such as vertebrates, it is a large and very complex organ.<ref name=Shepherd/> Some types of worms, such as [[leech]]es, also have an enlarged ganglion at the back end of the nerve cord, known as a "tail brain".<ref>{{cite journal |title=Neuronal control of leech behavior |journal=Prog Neurobiol |year=2005 |volume=76 |pages=279–327 |pmid=16260077 |last1=Kristan |first1=WB Jr. |last2=Calabrese |first2=RL |last3=Friesen |first3=WO |doi=10.1016/j.pneurobio.2005.09.004 |issue=5|s2cid=15773361 }}</ref>

There are a few types of existing bilaterians that lack a recognizable brain, including [[echinoderm]]s and [[tunicate]]s. It has not been definitively established whether the existence of these brainless species indicates that the earliest [[bilaterians]] lacked a brain, or whether their ancestors evolved in a way that led to the disappearance of a previously existing brain structure.