Editing Nervous system (section)

====Bilateria====
[[File:Bilaterian-plan.svg|thumb|right|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 bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front]]

The vast majority of existing animals are [[bilateria]]ns, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appear as fossils beginning in the Ediacaran period, 550–600 million years ago.<ref name=Balavoine/> The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to 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".

[[File:Gray797.png|thumb|left|125px|Area of the human body surface innervated by each spinal nerve]]

Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.<ref name=Ghysen>{{Cite journal |author=Ghysen A |title=The origin and evolution of the nervous system |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=555–562 |year=2003 |pmid=14756331 |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756331 |citeseerx=10.1.1.511.5106}}</ref>

Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups ([[superphylum|superphyla]]) called [[protostomia|protostomes]] and [[deuterostome]]s.<ref name=Erwin/> Deuterostomes include vertebrates as well as [[echinoderm]]s, [[hemichordata|hemichordates]] (mainly acorn worms), and [[Xenoturbellida]]ns.<ref name=Bourlat/> Protostomes, the more diverse group, include [[arthropod]]s, [[mollusc]]s, and numerous phyla of "worms". There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by [[Étienne Geoffroy Saint-Hilaire|Geoffroy Saint-Hilaire]] for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline.<ref name=Lichtneckert/>