Anterior–posterior gene expression in central nervous systems of three extant bilaterians and the urbilaterian. Anterior–posterior regionalization of gene expression in the central nervous systems of three extant bilaterians (an arthropod, an annelid and a vertebrate) and inferred expression in the last common bilaterian ancestor, the urbilaterian. Expression of Fez and Irx in the annelid Platynereis is unknown. For the urbilaterian, both anterior–posterior and medio-lateral gene expression domains are shown. Hypothetical posterior limits of Irx and Gbx domains in the urbilaterian brain are highlighted by a “?” and dashed lines. PC, protocerebrum; DC, deutocerebrum; TC, tritocerebrum; VC, ventral nerve cord; CG, cerebral ganglion; SG, segmental ganglia; FB, forebrain; MB, midbrain; HB, hindbrain; SC, spinal cord. Gene expression domains based on [1, 2, 9, 24, 29, 34, 42, 64],[68–80].