Fig. 11.

Scenario of the evolution of segmentation in the ventral nerve cord within ecdysozoans. Simplified diagrams of the ventral nerve cords of putative last common ancestors of Ecdysozoa (left), Onychophora (middle) and Arthropoda (right). Note the two alternative hypotheses regarding the architecture of the ventral nervous system in the last common ancestor of ecdysozoans with either a paired ventral nerve cord (1) or a median unpaired ventral nerve cord (2; modified from Yang et al. [7]). Note also the stepwise increase in the degree of segmentation of the ventral nervous system. Diagrams of the onychophoran and arthropod nervous systems according to Mayer and Harzsch [11] and Richter et al. [39], respectively. Neuronal cell bodies of the ventral nerve cord are indicated in blue and somata of motor neurons of leg nerves are indicated in red. LCA, last common ancestor