Nervous systems are directional signalling systems. Several components of nervous systems are present in non-metazoan organisms, and sponges are able to use electrical signals without having a nervous system. Nervous systems evolved within Eumetazoa, first as a nerve net, but there were numerous tendencies to create heterogeneity within this system by the emphasis of particular regions or pathways. This can be seen in cnidarians, and particularly occurs within bilaterians. A brain and an orthogon (the regular arrangement of longitudinal and circular nerves) are characteristic bilaterian features, and their exact evolution is discussed in this chapter. The tendency to concentrate and specialize the nervous system is very common among bilaterians. A variety of neurotransmitters are used in nervous systems, most of which are broadly distributed, while only few are of phylogenetic importance.
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