Fig. 17

Alternative scenarios on the evolution of mediolateral patterning of nerve cords among bilaterians. Scenarios A and B are modified from Ref. [17]. A Convergent evolution of similarities of the mediolateral patterning in vertebrates, arthropods and some annelids. The reconstruction of the ancestral morphology of the nervous system in the “urnephrozoan” is still unresolved due to the diversity of nerve cord arrangements in the nephrozoan lineages (white question mark). B The “urnephrozoan” already possessed a medially condensed nerve cord and mediolateral patterning. Thus, the medially condensed nerve cords of vertebrates, arthropods and annelids are homologous, implying multiple losses of mediolateral patterning. C The “urnephrozoan” already possessed a medially condensed nerve cord and mediolateral patterning. The medially condensed nerve cords of vertebrates and annelids are homologous. As a result, multiple independent losses of mediolateral patterning would have occurred, including the ecdysozoan ancestor, which would imply an independent evolution of medially condensed nerve cords in arthropods