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Bilateria

Metazoans (multicellular organisms) are divided, at the most basic level, into two groups: radiata (organisms with radial symmetry and two embryonic germ layers) and the bilatera (organisms with two-fold symmetry and three germ layers).

The Bilatera have two-fold symmetry (hence the name: Bi-latera, having two sides) and three germ cell layers in the embryo (the endoderm, mesoderm, and ectoderm). Bilaterians are sub-divided into the protostomes (organims in which the first opening formed during gastrulation becomes the mouth) and deuterostomes (organisms whose mouths form from a secondary opening, while the first opening becomes the anus).

Editor(s): John Bracht

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