Bilateria

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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