Protostomia (from Greek meaning "mouth first") are a clade of animals. Together with the deuterostomes and a few smaller 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. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it used to be thought that in their embryos the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus. However, this idea has been challenged, because the platyhelminthes, a group which forms a sister group to the rest of the bilaterian animals, have a single mouth which leads into a blind gut (with no anus). The genes employed in the embryonic construction of this mouth are the same as those expressed around the protostome mouth, 
There are other significant differences between the protostome and deuterostome patterns of development:
- Most protostomes are schizocoelomates, meaning a solid mass of the embryonic mesoderm splits to form a coelom. A few, such as Priapulids, have no coelom, but they may have descended from schizocoelomate ancestors. On the other hand all known deuterostomes are enterocoelous, meaning that the coelom is formed from longitudinal pouches of the archenteron which then become separate cavities.
- Within the Protostomes a number of phyla undergo what is known as spiral cleavage which is determinate, meaning that the fate of the cells is determined as they are formed. This is in contrast to deuterostomes which have radial cleavage that is indeterminate.
Current molecular data suggest that protostome animals can be divided into three major groups:
as well as a number of minor taxa of basal or ambiguous affinity.
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