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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.
group of animals-including the arthropods (e.g., insects, crabs), mollusks (clams, snails), and annelid worms-classified together on the basis of embryological development. The mouth of the Protostomia (proto, "first"; stoma, "mouth") develops from the first opening into the embryonic gut (blastopore). The coelom (body cavity) forms from a split in the embryonic mesoderm (middle tissue). Larval (immature) forms, if present, are called trochophores. The Protostomia constitute one of two divisions of the coelomates (animals with a body cavity, or coelom). Compare Deuterostomia.