A Brief Introduction to the Bilateria and Its Major Clades

This chapter introduces the Bilateria and its major clades. It describes how the phyla within Bilateria changed following the emergence of molecular phylogenetics in the 1990s. Several long-standing groups, such as Protostomia and Deuterostomia, have changed their phylum memberships, and a number of new higher-level clades have emerged. Essentially, the Bilateria is a clade of animals whose adult tissues are derived from three embryonic germ layers and whose body axis is anteroposterior. Deuterostomia includes only three phyla, (Chordata, Hemichordata, and Echinodermata), while Protosomia is a monophyletic lineage containing the two large clades, Spiralia and Ecdysozoa. The chapter also considers the recently discovered phyla, such as Xenacoelomorpha and Nephrozoa.