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. 2015 Dec 19;370(1684):20150286. doi: 10.1098/rstb.2015.0286

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© 2015 The Authors.

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

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Figure 7. — The evolution of the bilaterian nervous system. (a) Neuralian ancestor. A nerve net covers the gastrula-shaped animal. Nerve net neurons situated around the digestive opening (red) control feeding. White arrows indicate direction of water flow. (b) Cnidarian–bilaterian ancestor. Specialized parts of the nerve net are centred on the slit-like digestive opening: the nerve of the digestive sole (red) and the sensory-integrative centre (yellow), controlling contraction of a bilaterial series of contractile gastric pouches. White arrows indicate direction of water flow. (c) Urbilaterian. Motor centre of the ventral nerve cord (red) and sensory-integrative centre (yellow). White arrows indicate direction of water flow.