A protostome is a type of animal characterized by its embryonic development. In protostomes, the first opening that forms during the early stages of development becomes the mouth, while the anus develops later. This group includes a wide variety of organisms, such as arthropods, mollusks, and annelids. Protostomes are part of a larger classification called bilateria, which includes animals with bilateral symmetry. They are distinguished from deuterostomes, where the first opening becomes the anus. The protostome group is diverse, encompassing both invertebrates and some of the most complex animal forms.