Figure 1.

Evolutionary relationships of select metazoans. Taxa are arranged in descending order of phylogenetic emergence relative to vertebrates. The protostomes/deuterostomes split is indicated by a red circle. The blue shading is used to distinguish deuterostomes from all other animals. One branch of the deuterostomes includes the chordates (shown against a light blue background) and the other includes the echinoderms (shown against a deep blue background). Times of phylogenetic divergence are not to scale, and the tree branches are intended only to depict general relationships. The phylogenetic relationships between chordates described here are based on the current view that the cephalochordate is the most basal group in the extant chordate lineage [19-21].