Editing Sea urchin (section)

=== Nervous system and senses ===
The nervous system of sea urchins has a relatively simple layout. With no true brain, the neural center is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and [[pedicellariae]].<ref name=IZ />

Sea urchins are sensitive to touch, light, and chemicals. There are numerous sensitive cells in the epithelium, especially in the spines, pedicellaria and tube feet, and around the mouth.<ref name=Ruppert/> Although they do not have eyes or eye spots (except for [[Diadematidae|diadematids]], which can follow a threat with their spines), the entire body of most regular sea urchins might function as a compound eye.<ref>{{cite journal |last1=Knight |first1=K. |title=Sea Urchins Use Whole Body As Eye |journal=Journal of Experimental Biology |volume=213 |pages=i–ii |year=2009 |doi=10.1242/jeb.041715 |issue=2|doi-access=free }}
*{{cite press release |author=Charles Q. Choi |date=December 28, 2009 |title=Body of Sea Urchin is One Big Eye |website=[[LiveScience]] |url=http://www.livescience.com/animals/091228-sea-urchin-eye.html}}</ref> In general, sea urchins are negatively attracted to light, and seek to hide themselves in crevices or under objects. Most species, apart from [[Cidaris|pencil urchins]], have [[statocyst]]s in globular organs called spheridia. These are stalked structures and are located within the ambulacral areas; their function is to help in gravitational orientation.<ref name=IZ />