carpoid, member of an extinct group of unusual echinoderms (modern echinoderms include starfish, sea urchins, and sea lilies), known as fossils from rocks of Middle Cambrian to Early Devonian age (the Cambrian Period began about 542 million years ago, and the Devonian Period began 416 million years ago). Unlike other echinoderms, the carpoids display no radial symmetry, nor do they seem to have had a water-vascular system. They do possess a calcitic system of plates, however, as well as stemlike or armlike appendages; carpoids generally have flat bodies. Some seem to possess gill slits, a feature found in primitive chordates; the carpoids may be related to the most primitive chordates or vertebrates and are ancestral to the more advanced echinoderms.