Protostomia

Protostomes are a group of animals in the taxonomic group bilateria, and include animals such as arthropods, mollusks, and nematodes. They are most often compared with deuterostomes, the other major group of bilateria.

The major distinctions between deuterostomes and protostomes are found in embryonic development. Protostomes undergo spiral cleavage, the blastopore develops into the mouth, and their embryonic cells exhibit determinate cleavage.

Current molecular data suggest that protostome animals can be divided into two major groups: lophotrochozoa and ecdysozoa.

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