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.

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.