Any of a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the anus. At this stage in their development, the later specialized function of any given embryonic cell has not yet been determined. Deuterostomes are one of the two groups of animals that have true body cavities (coeloms), and are believed to share a common ancestor. They include the echinoderms, chaetognaths, hemichordates, and chordates. Compare protostome.