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The phyla discussed in the preceding section include about 20,000 species, a tiny fraction of all animal diversity. The final stop in this broad tour is the bilaterian branch, on which most animal species reside. Earlier, we pointed out that basic patterns of embryo formation divide bilaterians into two major groups, the Protostomia and the Deuterostomia (see Fig. 44.5). Molecular sequence data support the protostome–deuterostome groupings of bilaterian animals and show that the protostome animals can be further divided into two groups: the Lophotrochozoa (which include mollusks and annelid worms) and the Ecdysozoa (which include insects and other arthropods).

As noted above, bilaterian animals have bilateral symmetry and complex organs that develop from a triploblastic (three-germ layer) embryo. The anatomical complexity of bilaterian animals makes possible types of locomotion, feeding, gas exchange, behavior, and reproduction that are unknown in earlier branching groups. These capabilities underpin the remarkable diversity of bilaterian animals.