Metazoans are divided into two basic groups: Radiata (organisms with radial symmetry, including jellyfish and their relatives) and Bilateria (organisms with twofold symmetry that gives them definite front and rear, and left and right, body surfaces). Bilaterians, the first of which likely evolved in the Precambrian, are a step up in multicellular complexity from Radiata. Whereas Radiata develop from two embryonic tissue layers (an inner endoderm and outer ectoderm), the Bilateria have a third tissue layer, the mesoderm, between the endo- and ectoderm. This layer forms the muscles and most organs located between the digestive tract and the outer covering of the animal. The vertebrate circulatory and skeletal systems also stem from the mesoderm. Most bilaterians also show cephalization: the evolutionary trend toward the concentration of sensory structures (such as the mouth, nerves, etc.) at the anterior end of the body -- the end of a moving animal that is usually first to encounter food, danger, or other stimuli.

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