The major coelomate protostome phyla include the annelids, molluscs, and arthropods. Echinoderms and chordates are deuterostomes. In protostomes, the coelom arises as an enlarging space within the mesoderm. In deuterostomes, the coelom is formed from an evagination of the archenteron. With the roundworms came the advent of a body cavity, permitting some degree of internal circulation of gases.
Segmentation allows for greater evolutionary potential and flexibility.
Segments can be duplicated or modified into specialized regions of the body. Jointed appendages confer a great degree of motility upon an animal. Arthropods and chordates possess jointed appendages.