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Bilateral means symmetrical across a central axis, or concerning two sides. The word is used in international diplomacy to mean "between two countries," and can be used to describe geometric figures, but this article will focus on the term bilateral as it is used in biology.
Bilateral organisms are all members of the unranked group Bilateria, a scientific classification above superphylum but below subkingdom. Bilateral organisms include humans, fish, insects, flatworms, birds, reptiles, amphibians, and many others. The bilateral nature of most animals is evolutionarily fixed — because of the way embryos develop, the descendants of bilateral animals will always be bilateral. The oldest known bilateral animal is Vernanimalcula, an oval-shaped animal between 0.1 and 0.2 mm across that lived 600 million years ago, during the early Ediacaran period, 60 million years before the appearance of most modern phyla, called the Cambrian explosion.
The primary alternative to bilateral symmetry in organisms is radial symmetry, displayed by sea anemones, jellyfish, corals, and many flowers. Starfish and sea urchins have five-fold radial symmetry (pentamerism) as adults, but at their essence are bilateral animals, as is demonstrated by the bilateral symmetry of their larvae. Some primitive animals, such as sponges, Trichoplax, and mesozoans have no symmetry at all, being referred to as asymmetrical. Sponges are believed to form the stem group from which all other animals originally evolved.
Nearly all bilaterans are tripoblastic, meaning their embryos develop in three layers, called the endoderm, mesoderm, and ectoderm. Almost all bilaterans have complete digestive tracts with separate anus and mouth, and an internal body cavity, called a coelom. Bilaterans may be either sessile (stationary) or mobile, though usually the latter. Bilaterans colonize every conceivable environment on Earth, from the bottom of the oceans to the peaks of Mt. Everest.
There are two main groups within Bilateria, called superphyla: the deuterostomes and protostomes. The main difference between these two groups is that in the latter, the first hole to form in the embryo becomes the mouth, while in the latter, it becomes the anus. In deuterostomes there is also less predetermination of cell locations during embryogenesis.