Evolutionary scenario of Hox gene expression evolution in the Bilateria. The two gene model of Hox gene evolution is hypothesized and its stepwise extension in the evolution of the Bilateria. The cnidarian-bilaterian ancestor possessed two clear Hox genes which are expressed ambiguously in the taxon cnidaria. One major branch, the Anthozoa, express the anterior ortholog in the pharynx and the posterior orthologs along the oral-aboral axis. The other major branch (Medusozoa) the posterior ortholog is expressed in the oral pole, while the anterior ortholog is expressed at both ends of the body. Based on the data of Hox gene expression of the acoelomorph C. longifissura, the three Hox genes are expressed in a spatial staggered pattern along the anterior-posterior axis. The expression is switched on simultaneously in the acoel, which excludes temporal colinearity. The spatial staggered expression however, is ancestral for the Bilateria lineage. The protostome Hox expression is represented by the annelid Capitella teleta , the deuterostome expression by the hemichordate Saccoglossus . The acoel expression shares the blastoporal expression of the central and posterior class orthologs in Saccoglossus, the anterior ortholog is not expressed.