The evolution of panarthropod brains. a Models depicting gene expression domains in euarthropods (left), onychophorans (center), and tardigrades (right). In the anatomical models, red ovals represent brain segments, black ovals represent ganglia, and black lines that connect to ovals represent connectives. In the models of gene expression domains, thin bars represent reduced expression or more restricted expression. Light bars represent expression domains that have been identified in some species, but not others. Gray bars labeled “tripartite” and “unipartite” underscore the segments that are homologous to the tardigrade head under these competing hypotheses. b Model for the evolution of panarthropod brains based on developmental, morphological, and paleontological data. The phylogeny is based on Campbell et al.  and Rota-Stabelli et al.