Protostomes (from the Greek: first the mouth) are a major group 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 protostomes development, the mouth forms at the site of the blastopore, and the anus forms as a second opening.

Current molecular data suggest that protostome animals can be divided into three major groups:

• Ecdysozoa, e.g. arthropods and roundworms
• Lophotrochozoa, e.g. mollusks and annelids
• Platyzoa, e.g. flatworms

Of these, the latter two make up the Spiralia, including most animals where the embryo undergoes spiral cleavage.