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Bilateria
Fossil range: Ediacaran–Recent
A Bowfin , Amia calva
Scientific classification
Domain: Eukaryota
(unranked): Opisthokonta
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Hatschek, 1888
Phyla
Illustration of the different types of symmetry of Life Forms On Earth. Display at the Field Museum, Chicago. The forms with bilateral symmetry can have heads. Life Forms with other types of symmetry have corresponding organs, if not a head.

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 bilateralia are a subregnum (a major group) of animals, including the majority of phyla; the most notable exceptions are the sponges and cnidarians beloning 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.

Contents

Evolution

The hypothetical last common ancestor of all bilateria is termed the "Urbilaterian".[1][2] There is some debate about its appearance. The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella, dating to 555 million years ago.[3] Earlier fossils are controversial; The fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.[4] Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities.[5]

Phylogeny

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[6] 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 support protostome affinities.[7][8]

A phylogeny of the Bilateria after Nielsen (2001) is as follows.[9] (Note that this does not agree with the taxobox in this article. For example it classifies the Phoronida and Brachiopoda as Deuterostomes rather than Protostomes.) An alternate phylogeny suggests a basal group called the ecdysozoa.


Protostomia
Spiralia
Schizocoelia

Sipuncula


Articulata

Mollusca


Euarticulata

Annelida


Panarthropoda

Onychophora




Tardigrada



Arthropoda







Bryozoa

Ectoprocta



Entoprocta



Parenchymia

Platyhelminthes



Nemertini



Gnathifera

Chaetognatha




Gnathostomulida



Rotifera





Cycloneuralia

Gastrotricha


Introverta

Nematoda



Nematomorpha



Cephalorhyncha

Priapulida




Kinorhyncha



Loricifera






Deuterostomia


Phoronida



Brachiopoda





Echinodermata



Pterobranchia




Enteropneusta



Chordata






See also

Embryological origins of the mouth and anus

References

  1. ^ 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
  2. ^ 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
  3. ^ For refs see Ediacara biota
  4. ^ For refs see Vernanimalcula
  5. ^ For refs see Fossil embryos
  6. ^ The Invertebrate Animals
  7. ^ *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.
  8. ^ 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.
  9. ^ Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Second Edition. Oxford University Press, Oxford.

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