Figure 2.

Phylogenetic tree of the deuterostomes when sequences with similar evolutionary rates (16 taxa ≤0.12 substitutions per site) were analyzed with gambit. Key characters are mapped to the deeper nodes. The deuterostomes form two great clades, one containing the hemichordates and echinoderms and the other made up of urochordates and chordates (cephalochordates and vertebrates). Major differences in adult body plan between Cephalochordata + Vertebrata (myotomes) and Urochordata (tunic) are marked. These results, combined with morphological data, suggest that Chordata should be restricted to Cephalochordata + Vertebrata and that Urochordata is an independent phylum and the sister group to Chordata. Note that the tripartate coelom of hemichordates is considered homologous to the three pairs of echinoderm coeloms.