Fig. 3.

The hypothesis of monophyletic origin of the central nervous system in Bilateria. This is an alternative scenario of the neuronal evolution in animals as outlined in Fig. 2. Here, the ancestral urbilateria had a well-defined central nervous system in the form of a ventral cord. However, multiple animal lineages both within Deuterostomes, Ecdysozoa and Lophotrochozoa independently lost their central nervous system (yet many representatives of these groups are still free living organisms, sometimes with complex behavioral repertoires). Ctenophore and Cnidaria nervous systems might be related/homologous to bilaterian neural organization (although at least some parts of them may have evolved independently). Red, orange and green colors schematically illustrate different neuronal structures. Open white circles indicate the position of the mouth. See text and [54] for details