Figure 1.

Animal phylogeny. Phylogenetic diagram showing the position of the phylum Echinodermata (shown in red; e.g. starfish) in the deuterostomian branch of the animal kingdom. The Bilateria comprise two super-phyla—the deuterostomes and the protostomes. The deuterostomes comprise the chordates (vertebrates, urochordates and cephalochordates) and the ambulacrarians (hemichordates and echinoderms). The protostomes comprise the lophotrochozoans (e.g. molluscs and annelids) and the ecdysozoans (e.g. arthropods and nematodes). The Cnidaria (e.g. sea anemones) are basal to the Bilateria. Images of representative animals from each phylum were obtained from http://phylopic.org or were created by the authors or by M. Zandawala (Stockholm University).