Protostomes (from the Greek: first the mouth) 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.
In both protostomes and deuterostomes, the embryo consists of a little ball of cells known as a blastula. Protostomes have their early cell divisions diagonal to the polar axis forming a spiral arrangement of cells; this is called spiral cleavage. A groups of cells move inward to form an opening called the blastophore, which in protostomes develops into the mouth.
Protostomes have a determinate cleavage: the fate of how each embryonic cell will turn out to be or function is typically fixed very early; the first four cells are separate and each will develop into a fixed quarter of the larva. If a cell is removed from the blastula, a limb might not form, for the other cells don't compensate. Protostomes are schizocoely, where the mesoderm splits and the split widens into a cavity that becomes the coelom.
Current molecular data suggest that protostome animals can be divided into two major groups: lophotrochozoa and ecdysozoa.
