Phylum EchinodermataEchinodermata is a phylum of marine bottom-dwelling species having external skeletons just beneath the skin. The calcareous plates the external skeletons are made of may be solidly fused together, like in sea urchins, loosely used to facilitate movement, as in sea stars (starfish), and etc. The skins of echinodermata species usually have warty projections or spines, or both. Echinoderms display "pentamerous radial symmetry" which is when the body can be divided into five, more or less, similar portions. Echinoderms do not have a head, the surface containing the mouth (the underside, in sea stars and most others) is called the oral surface, and the opposite side, which usually contains the anus, the aboral surface. Finally, there are five classes of echinoderms, Asteroidea, Ophiuroidea, Echinoidea, Crinoidea, and Holothuroidea.
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