Editing Nervous system (section)

====Worms====
[[File:Earthworm nervous system.png|thumb|250px|right|Earthworm nervous system. ''Top:'' side view of the front of the worm. ''Bottom:'' nervous system in isolation, viewed from above]]

[[Worm]]s are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, [[earthworm]]s have dual [[ventral nerve cord|nerve cords]] running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by [[transverse plane|transverse]] nerves like the rungs of a ladder. These transverse nerves help [[coordinate]] the two sides of the animal. Two [[ganglion|ganglia]] at the head (the "[[nerve ring]]") end function similar to a simple [[brain]]. [[Simple eyes in arthropods|Photoreceptors]] on the animal's eyespots provide sensory information on light and dark.<ref name=Adey>{{Cite journal |author=ADEY WR |title=The nervous system of the earthworm Megascolex |journal=J. Comp. Neurol. |volume=94 |issue=1 |pages=57–103 |date=February 1951 |pmid=14814220 |doi=10.1002/cne.900940104 |s2cid=30827888}}</ref>

The nervous system of one very small roundworm, the [[nematode]] ''[[Caenorhabditis elegans]]'', has been completely mapped out in a [[connectome]] including its synapses. Every neuron and its [[fate mapping|cellular lineage]] has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is [[sexually dimorphic]]; the nervous systems of the two sexes, males and female [[hermaphrodites]], have different numbers of neurons and groups of neurons that perform sex-specific functions. In ''C. elegans'', males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.<ref name=Wormbook/>