A major division of the animal kingdom comprising all forms with a bilaterally symmetrical body plan (that is, their left and right sides are mirror images, or nearly so) and organ systems that develop from three tissue layers. The animal kingdom, Metazoa, is subdivided into about 30 major animal groups, or phyla, each constructed on a different architectural plan. Most of these phyla are grouped as the Bilateria, comprising a major early branch on the family tree of animals that may have originated nearly 600 million years ago. All bilaterians possess some features that are not perfectly symmetrical (for example, the human heart and liver), but all have descended from an essentially bilateral ancestor. The nonbilaterian phyla include sponges (which lack muscles and nerves), jellyfish, and sea anemones and their allies, which have quasiradial symmetry and organ systems that develop from only two tissue layers (see illustration).