a class of fossil invertebrates of the phylum Echinodermata that lived from the beginning of the Ordovician to the end of the Devonian. The body was pentagonal in shape. Rays, or arms, were present only on the oral (inferior) surface, where there were double rows of ambulacral plates embracing an ambulacral groove. Diagonal rows of rod-shaped plates extended from the ambulacral plates. Spicules with three or four rays were situated on the aboral (superior) body surface. Somasteroidea lived partly burrowed in the bottom. Microscopic organisms entered the mouth through the action of cilia. Starfish and brittle stars apparently descended from Somasteroidea. Remains of Somasteroidea have been found in Europe (Scotland, France, Czechoslovakia), Australia, and North America.