In traditional taxonomy, there are five classes of living echinoderms: Crinoidea (sea lilies), Asteroidea (starfish), Ophiuroidea (brittle stars or snake stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers). An unusual echinoderm, Xyloplax , was found in 1986, on sunken wood in the deep sea. It may represent a sixth class, the Concentricycloidea (Baker et al., 1986), but many zoologists now consider Xyloplax to be an aberrant asteroid.
Fossil echinoderms were much more diverse in form than living ones. Many Paleozoic forms, now extinct, had very unusual morphologies. There is still debate over how they fit into the scheme of echinoderm phylogeny; in fact, some Cambrian echinoderm-like fossils in the Homalozoa have been suspected of being chordates. The cladogram you see here is a composite of molecular and morphological data. Like all cladograms, it is subject to revision as new data comes in.
David, B. & R. Mooi. 1998. Major events in the evolution of echinoderms viewed by the light of embryology. Pp. 21-28 in Echinoderms: San Francisco.
Raff, R. A., Field, K. G., Ghiselin, M. T., Lae, D. J., Olsen, G. J., Pace, N. R., Parks, A. L., Parr, B. A., and Raff, E. C. 1988. Molecular analysis of distant phylogenetic relationships in echinoderms. In: Paul, C. R. C. and Smith, A. B. (eds.) Echinoderm Physiology and Evolutionary Biology. Clarendon Press, Oxford.
Sprinkle, J. 1992. Radiation of Echinodermata. In: Lipps, J. H. and Signor, P. W. (eds.) Origin and Early Evolution of the Metazoa. Plenum Press, New York.