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a group of invertebrates, including Plathel-minthes, Nemathelminthes, Nemertini, Annelides, Mollusca, and Arthropoda. Protostomes are bilaterally symmetrical animals. Their development is characterized by the formation of a mouth from the primitive blastopore or from its anterior part. The anal opening emerges from the posterior part of the body. There is an exoskeleton of cuticle, chitin, or shell. The foregut and hindgut develop by invagination of the ectoderm. Blastulation is spiral unless it has undergone a secondary change.