Editing Nervous system (section)

====Radiata====
[[Cnidaria|Jellyfish]], [[ctenophore|comb jellies]], and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete [[ganglion|ganglia]].<ref name=Ruppert/>

The development of the nervous system in [[radiata]] is relatively unstructured. Unlike [[bilaterians]], radiata only have two primordial cell layers, [[endoderm]] and [[ectoderm]]. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.<ref name=Sanes3>{{Cite book |title=Development of the nervous system |url=https://archive.org/details/developmentnervo00sane |url-access=limited |publisher=Academic Press |year=2006 |isbn=978-0-12-618621-5 |pages=[https://archive.org/details/developmentnervo00sane/page/n17 3]–4 |vauthors=Sanes DH, Reh TA, Harris WA}}</ref>