<p>The Asteroidea is one of the largest and most familiar classes within the Phylum Echinodermata. These animals, commonly known as sea stars or starfishes, form a diverse and speciose group. There are approximately 1600 extant species (Hyman 1955; Clark 1977; Clark and Downey 1992) which are found throughout the world's oceans. Following the classification of Blake (1987), these species are grouped into seven orders: Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida. </p> <p>Like other echinoderms, asteroids are important members of many marine benthic communities. They can be voracious predators, having significant impacts on community structure. For example, Paine (1966) used <em>Pisaster ochraceus</em> to illustrate his concept of the role keystone species play in community ecology. The crown-of-thorns starfish, <em>Acanthaster planci</em>, is particularly well-known because it can cause extreme detrimental effects to coral reefs, particularly during population outbreaks (Moran 1988). </p> <div ><!-- ToL Image #START# --> <a href="javascript: w = window.open('http://tolweb.org/media/6436', '6436', 'resizable,height=600,width=800,scrollbars=yes'); w.focus();"><img src="http://tolweb.org/tree/ToLimages/Pisaster_ochraceus1.100a.jpg"/></a><!-- ToL Image #END# --> <!-- ToL Image #START# --> <a href="javascript: w = window.open('http://tolweb.org/media/6437', '6437', 'resizable,height=600,width=800,scrollbars=yes'); w.focus();"><img src="http://tolweb.org/tree/ToLimages/Acanthaster_planci1.100a.jpg"/></a><!-- ToL Image #END# --> <p>Figure 1: <em>Pisaster ochraceus</em> and <em>Acanthaster planci</em>, two asteroids of great ecological significance.<br /> <em>Pisaster</em> image by Sherry Ballard, courtesy <a href="http://elib.cs.berkeley.edu/photos/">CalPhotos</a>, copyright &copy; 1999 California Academy of Sciences. <em>Acanthaster</em> image copyright &copy; <a href="http://www.borutfurlan.com/">Borut Furlan</a>.</p> </div> <p>The controversial Concentricycloidea (a proposed sixth class of the Echinodermata; Baker et al. 1986, Rowe et al. 1988, Pearse and Pearse 1994) have been diagnosed as unusual asteroids (Smith 1988, Belyaev 1990, Janies and Mooi 1999). Their relationship to other asteroid taxa is not well resolved, but alliances with species from the Velatida and the Forcipulatida have been proposed. The unique morphology of the concentricycloids makes it difficult to assign this group to the recognized asteroid orders and is cited as sufficient distinction for class recognition.</p>