Any of a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the mouth.

At this stage of development, the later specialization of any given embryonic cell has already been determined.

Protostomes are on of the two groups of animals having a true body cavity (coelom) and are believed to share a common ancestor. They include the mollusks (nails, slugs, clams, mussels, cuttlefish, and octopuses), annelids (segmented worms, earthworms, lugworms, and leeches), and arthropods (insects; including mosquitoes, ticks, centipedes, spiders and many other species).