 The idea of Hox genes and their work was developed in the studies of classical model animals belonging to the lineages of Ecdysozoa (Drosophila, Tribolium, Caenorhabditis) and Deuterostomia (mouse, chicken, Xenopus, and zebrafish). Subsequently the list of objects was continued by Spiralia (mollusks, polychaetes, brachiopods, rotiferans) and expanded every year by animals with complicated phylogenetic positions or interesting developmental programs. To date, a sufficient set of data has been accumulated to search for ancestral, i.e., constant, features in representatives of different taxa, based on similarities and dissimilarities in the usage of Hox genes to try to reconstruct UrBilateria—a common ancestor of bilateral animals.