The echinoderm (meaning spiny-skinned) is a diverse group of invertebrates belonging to the phylum Echinodermata. The Phylum Echinodermata consists of five main classes divided into 7,500 extant species and approximately 13,000 known extinct species, including several classes discovered through fossils. The five major classes are proposed to have evolved from a common metazoan ancestor around 500 million years ago, during the early Paleozoic period (Britannica, 2014). Echinoderms are primarily benthic (seabed dwellers) and only found in marine ecosystems. It is the largest phylum not to contain any divergent terrestrial or freshwater organisms (Selden, PA 2001). This discussion examines various similarities and variations in echinoderm body structure, their feeding behaviors, the habitats in which they reside, and how they have achieved such diversity among marine habitats despite their seemingly primitive characteristics. Shared Characteristics Between Classes Although these five classes are all derived from the same phylum, they are considerably unique from each other. Most of these classes do not look similar visually, but there are multiple common characteristics that determine their place within the echinoderm phylum. Unlike most animals today, echinoderms still exhibit characteristics that might consider them primitive. They show a lack of cephalization (compartmentalization of particular areas of the body), pentamerous radial organization and symmetry in adults and a lack of specialized organs (Morris VB, 2002). There are some rather interesting features among echinoderms, however, such as some having the ability to regenerate and a unique feature exhibited only in echinoderms, called the water vascular system...... middle of paper ...... leave , RB, McEdward, LR and Strathmann, RR (1987) Larval ecology of the echinoderm seen from the egg. In M. Jangoux and JM Lawrence (eds.). Studies on echinoderms.Vol. 2. AA Balkema, Rotterdam, pp. 55-136. Thorson, G. (1950) Reproduction and larval ecology of seafloor invertebrates. Biol. Reverend Cambr. Philos. Soc. 25:1–45.Barnes, R. (1987). Invertebrate zoology. Orlando, FL: Dryden Press. Brusca, R., G. Brusca. (2003). Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc. Mladenov, PV and Chia, FS (1983) Development, sedimentation behavior, metamorphosis and feeding of pentacrinoids and growth of the feather star Florometra serratissima. Marine Biology 73: 319-323. Hendler, GL, MP Kier, JE Miller, and DL Pawson. Class Ophiuroidea, pages 89-195 in Sea Stars, Sea Urchins and Allies. Washington: Smithsonian Institution Press. 1995.