any member of the lower invertebrate phyla in which the mouth appears before the anus during development, cleavage is spiral and determinate, and the coelom forms as a splitting of the mesoderm.
Any of a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the mouth. At this stage of development, the later specialization of any given embryonic cell has already been determined. Protostomes are one of the two groups of animals having a true body cavity (coelom) and are believed to share a common ancestor. They include the mollusks, annelids, and arthropods. Compare deuterostome.
