What is the X in X-mas?
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.
(Greek: "second mouth"), group of animals-including those of the phyla Echinodermata (e.g., starfish, sea urchins), Chordata (e.g., vertebrates), Chaetognatha (e.g., arrowworms), and Brachiopoda (e.g., lamp shells)-classified together on the basis of embryological development. During that process the mouth of the Deuterostomia develops from an opening into the embryonic gut other than the blastopore, which develops into the anus. The coelom (body cavity) develops from buds off the embryonic gut. A number of Deuterostomia have larval forms. The Deuterostomia constitute one of two divisions of the coelomates (animals having a coelom). Compare Protostomia.