Protosome is a multicellular organism whose mouth grows before the anus from a primary embryonic opening which includesphyla mollusk, arthropod or annelid. Protosomes are developed by spiral cleavage during embryo development.
Protosome animals have bilateral symmetry and three germ layers. Some of the examples of protostomes are snails and slugs (mollusk), earthworms (annelids), and spiders, lobsters, barnacles (arthropods).
