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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.

The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges, belonging to Parazoa, and cnidarians belonging to Radiata. For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few highly reduced forms, the Bilateria have complete digestive tracts with separate mouth and anus. Most Bilateria also have a type of internal body cavity, called a coelom. It was previously thought that acoelomates gave rise to the other group, but there is some evidence now that in the main acoelomate phyla (flatworms and gastrotrichs) the absence could be secondary.

See also

Embryological origins of the mouth and anus

Unless indicated otherwise, the text in this article is either based on Wikipedia article "Bilateria" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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