Extant animals are classified into about 26 phyla, each of which capture variations in a basic body plan (Bauplan) that, with only one exception, dates back to the Cambrian. On the basis of molecular criteria, most of these are grouped into three “superphyla” (Fig. 3.1)—Ecdysozoa (the “moulting” animals, within which the arthropods and nematodes are the major component phyla), Lophotrochozoa (crest or wheel animals; mollusks, annelids, and platyhelminths are the major groups), and Deuterostomia (echinoderms, hemichordates, urochordates, and chordates). The overwhelming majority of extant animals have two obvious body axes—anterior/posterior and dorsal/ventral—and hence the phyla to which they belong are known as the Bilateria (or bilaterally symmetric animals), the “higher” animals. That the Bilateria have a single origin (i.e., are a monophyletic group) is most convincingly shown by the fact that the molecular mechanisms involved in patterning along these two axes are conserved even between such different kinds of animals as vertebrates and insects.
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