Protostomia (from Greek meaning mouth first) are a clade of animals. Together with the deuterostomes and other phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development.In animals at least as complex as earthworms, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. Most protostomes are schizocoelomates, meaning a solid mass of the embryonic mesoderm splits to form a coelom. 2  A few, such as Priapulids, have no coelom, but they may have descended from schizocoelomate ancestors.