Echinoderms are an exclusively marine group of animals with a rich evolutionary history extending back more than 540 million years. Whereas five major groups of echinoderms are living today (starfish, brittle stars, sea urchins, sea cucumbers, and sea lilies), more than 20 other, equally distinctive extinct echinoderms groups lived during the past.
Echinoderm body plans range from stalked flower-like sea lilies, to ambulatory stellate starfish and brittle stars, to soft-bodied sea cucumbers, to spiked, armored, and globose sea urchins, to flat sand dollars. The benthic adult forms of these diverse animals share a water vascular system in which a central coelomic ring extends to form five (and sometimes more) radial canals bearing tube feet. In contrast with the pentaradial form of benthic adults, most echinoderm larvae are bilaterally symmetric. The transformation from bilateral larvae to pentaradial adults occurs at metamorphosis, in a process that is coupled to settlement to the sea floor in the case of pelagic larvae or to release from the brood in benthic young.
The diversity of echinoderm body plans, their various larval ecologies, drastic metamorphosis, and their sister taxon relationship to chordates make echinoderms important models in a variety of comparative disciplines including studies of: the evolution of development (Jägersten, 1972; Strathman, 1978; McEdward & Janies, 1993; 1997; Lowe & Wray, 1997; Peterson & Davidson, 2000), regulatory regions of genomes (Davidson, 2001; 2006) and immune systems (Rast et al., 2006). However, the reliability of echinoderms as a model system for comparative studies can easily undermined by their unresolved phylogeny (McEdward & Miner, 2001).
More about the extant groups of echinoderms
Asteroidea (sea stars): Tree of Life Web Project
Ophiuroidea (brittle stars): overview by Rebecca L. Hunter
Holothuroidea (sea cucumbers): Tree of Life Web Project
Echinoidea (sea urchins): Natural History Museum, London
Crinoidea (sea lilies and feather stars): Tree of Life Web Project