Protostomes are a major group of animals characterized by their 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 diverse organisms such as arthropods, mollusks, and annelids. Protostomes are divided into two main groups: lophotrochozoans and ecdysozoans. Lophotrochozoans include animals like snails and earthworms, which often have a larval stage called a trochophore. Ecdysozoans, on the other hand, include insects and crustaceans, which grow by molting their exoskeletons.