Bilateria

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Image:SymmetryOfLifeFormsOnEarth.jpg {{Taxobox | color = pink | name = Bilateria | regnum = Animalia | subregnum = Eumetazoa | unranked_phylum = Bilateria | unranked_phylum_authority = Hatschek, 1888 | subdivision_ranks = Subgroups | subdivision =

• Deuterostomia
  • Echinodermata
  • Hemichordata
  • Chordata
• Protostomia
  • Ecdysozoa
  • Lophotrochozoa
  • Platyzoa
• Chaetognatha (affiliation uncertain)
• Cycliophora
• Mesozoa

}}

The Bilateria, having bilateral symmetry, are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges and cnidarians. 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 are radially symmetrical as adults, but are bilaterally symmetrical as larvae.

Except for a few primitive 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 it now appears that in the main acoelomate phyla (flatworms and gastrotrichs) the absence is secondary. Even the oldest known bilaterian animal, Vernanimalcula had a coelom.

There are two or more superphyla (main lineages) of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and possibly a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among the protostomes, Ecdysozoa (molting animals) and Lophotrochozoa. Some taxonomists also recognize another protostome superphyla, Platyzoa, while others would include the Platyzoans in Lophotrochozoa[1] or not include them in any superphyla. Some taxonomists place the Chaetognatha among the deuterostomes, while others place them in the protostomes.

References

• Tree of Life web project - Bilateria
• University of California Museum of Paleontology - Systematics of the Metazoaar:ثنائيات التناظر

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