Protostomes and deuterostomes
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage.[141] Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[142][143] Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.[144]
The main deuterostome phyla are the Ambulacraria and the Chordata.[145] Ambulacraria are exclusively marine and include acorn worms, starfish, sea urchins, and sea cucumbers.[146] The chordates are dominated by the vertebrates (animals with backbones),[147] which consist of fishes, amphibians, reptiles, birds, and mammals.[148][149][150]
The protostomes include the Ecdysozoa, named after their shared trait of ecdysis, growth by moulting,[151] Among the largest ecdysozoan phyla are the arthropods and the nematodes.[152] The rest of the protostomes are in the Spiralia, named for their pattern of developing by spiral cleavage in the early embryo. Major spiralian phyla include the annelids and molluscs.[153]