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'Protostomes' (from the Greek: ''mouth first'') are a taxon 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 protostome development, the first opening in development, the blastopore, becomes the animal's mouth. In deuterostome development, the blastopore becomes the animal's anus. Protostomes have what is known as spiral cleavage which is determinate, this 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.
Another contrast resides in the formation of the coelom. Protostomes are schizocoelomates, meaning a solid mass of the embryonic mesoderm split to form a coelom. Deuterostomes are enterocoelous, meaning the folds of the archenteron form the coelom.
Current molecular data suggest that protostome animals can be divided into three major groups as follows:

★ Ecdysozoa, e.g. arthropods and roundworms

★ Lophotrochozoa, e.g. molluscs and annelids

★ Platyzoa, e.g. flatworms
Of these, the latter two make up the Spiralia, including most animals where the embryo undergoes spiral cleavage.