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Bilateria

The Bilateria (pronounced /ˌbaɪləˈtɪəriə/) 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 bilateralians 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 achieve near-radial symmetrical 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. The indirect evidence for the primitivity of the coelom is that the oldest known bilaterian animal, Vernanimalcula, had a structure that could be interpreted as a body cavity.

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 superphylum, Platyzoa, while others would include the Platyzoans in Lophotrochozoa^[1] or not include them in any superphylum. The arrow worms (Chaetognatha) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins (Helfenbein et al. 2004 and Papillon et al. 2004) support protostome affinities.

The hypothetical last common ancestor of all bilaterians has sometimes been called Urbilateria.^[2][3] Since the name refers to the concept of the ancestral bilaterian, it does not refer to any animal fossil, nor is it a taxonomically valid genus designation. Other terms that can be used are Last common bilaterian ancestor and Protostome-Deuterostome ancestor. The fossil Vernanimalcula may be the earliest known bilaterian.

Phylogeny

A proposed phylogeny of the Bilateria after Nielsen is as follows.^[4] An alternate phylogeny suggests a basal group called the ecdysozoa.

Notes

1. ^ The Invertebrate Animals
2. ^ Knoll, Andrew H. and Sean B. Carroll. (1999) Early Animal Evolution: Emerging Views from Comparative Biology and Geology. Science. 25 June 1999: Vol. 284. no. 5423, pp. 2129 - 2137. Found at [1] - URL retrieved November 15, 2006
3. ^ Balavoine, Guillaume, & Adoutte, Andre. 2003. The segmented Urbilateria: A testable scenario. Integrative & Comparative Biology 43: 137-147. Found at [2] - URL retrieved November 15, 2006
4. ^ Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Second Edition. Oxford University Press, Oxford.

Other references

• Helfenbein, Kevin G., H. Matthew Fourcade, Rohit G. Vanjani, and Jeffrey L. Boore (2004). The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes. Proceedings of the National Academy of Sciences of the United States of America 101(29), July 20 2004: 10639-10643.
• Papillon, Daniel, Yvan Perez, Xavier Caubit, and Yannick Le Parco (2004). Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome. Molecular Biology and Evolution 21(11), November 2004: 2122-2129.

External links

• Tree of Life web project - Bilateria
• University of California Museum of Paleontology - Systematics of the Metazoa