(from the article `cake urchin`) any of the echinoid marine invertebrates of the order Clypeastroida (phylum Echinodermata), in which the body is flattened. The surface is covered ... any of the invertebrate marine animals of the order Clypeastroida (class Echinoidea, phylum Echinodermata) that has a flat, disk-shaped body. It is ... ...