However, each protostome cell would develop only into a part of that organism. The protostome lineage includes the Annelida, Mollusca, and Arthropoda, while the deuterostomes include the Echinodermata and the Chordata. These five phyla are distinct from each other on the basis of segmentation, quite a controversial evolutionary event. The Arthropoda differ from the Annelida primarily in their possession of jointed appendages. On the deuterostome branch, the chordates are segmented, while Echinodermata are not.
Evolutionary biologists continue to debate the phylogenetic origins of segmentation.
Some believe it evolved separately in the protostome and deuterostome lineages after their divergence, while others believe it evolved very early and was later lost by the Echinoderms and Mollusks. All of the organisms of Animalia are, perhaps contrary to the belief of most people, invertebrates. The only vertebrates are in the subphylum Vertebrata of the Phylum Chordata.