Figure 1.

Phylogenetic tree for Metazoa, including representative protostome phyla and the three phyla that constitute the deuterostomes. Molecular phylogenies divide the protostomes into two great clades, namely the ecdysozoans (here arthropods to priapulids) and the lophotrochozoans (here flatworms to molluscs); deuterostomes consist of hemichordates and echinoderms, which are sister groups, plus chordates (vertebrate and invertebrate) (40–44). The only phyla in which Hox gene clusters have been structurally characterized at the genome level are boxed (see text for references). The echinoderm box refers to the present work.