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The Bilateria are those animals which have a body composed of three different germ layers, called the endoderm[?], mesoderm, and ectoderm[?]. These animals are also called triploblastic.

Basically, the Bilateria include all animals that are (more or less) bilaterally symmetrical and, except for a few flatworms, have an internal body cavity (coelom or pseudocoelom) with a mouth at one end and an anus at the other. The Bilateria include the majority of animals, with the cnidarians and sponges being the principal exceptions.

The two divisions of the Bilateria are Deuterostomes (echinoderms, chordates, hemichordates, Chaetognatha and Vetulicolia) and Protostomes (arthropods, annelid worms, molluscs and many more groups). They are distinguished by how the coelom develops, in particular whether the first body opening to form in larval growth becomes the anus (deuterostomes) or mouth (protostomes). Another distinguishing characteristic is how the third set of cells to form after egg fertilization orient themselves during cleavage. Protostomes have spiral cleavage, while deuterstomes have radial cleavage.

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