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animals, but vary greatly in complexity. [ ] the only multicellular animals that have no nervous system at all are sponges , placozoans , and mesozoans , which have very simple body plans. the nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include
anemones , hydras , corals and jellyfish ) consist of a diffuse nerve net . all other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel ), and nerves radiating from the brain and central cord. the size...
https://en.wikipedia.org/wiki/Nervous_system