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Animal



Roman snail, Helix pomatia
Roman snail, Helix pomatia

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida.[11][12] The former includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods,[13] because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[14]

The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore.[15] These were traditionally grouped together as the lophophorates.[16] but it now appears they are paraphyletic,[17] some closer to the Nemertea and some to the Mollusca and Annelida.[18][19] They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.[20]

Model organisms

Main articles: Model organism and Animal testing

Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first lifeforms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but the double-edged sword here is that with many genes, introns and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.[21]

An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.[22]

Other model organisms belonging to the animal kingdom include the mouse (Mus musculus) and zebrafish (Danio rerio}.

History of classification

Carolus Linnaeus known as the father of modern taxonomy
Carolus Linnaeus known as the father of modern taxonomy

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.

In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

See also

Look up Animal in
Wiktionary, the free dictionary.

Notes

  1. ^ "Animal". The American Heritage Dictionary (Forth). (2006). Houghton Mifflin Company. 
  2. ^ National Zoo. Panda Classroom (English). Retrieved on September 30, 2007.
  3. ^ Jennifer Bergman. Heterotrophs (English). Retrieved on September 30, 2007.
  4. ^ Davidson, Michael W.. Animal Cell Structure (English). Retrieved on September 20, 2007.
  5. ^ Saupe, S.G. Concepts of Biology (English). Retrieved on September 30, 2007.
  6. ^ Seilacher, A., Bose, P.K. and Pflüger, F. (1998). "Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India". Science 282: 80–83. doi:10.1126/science.282.5386.80. 
  7. ^ Dunn et al. 2008. "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature 06614.
  8. ^ Ruiz-Trillo, I.; Ruiz-Trillo, Iñaki; Riutort, Marta; Littlewood, D. Timothy J.; Herniou, Elisabeth A.; Baguñà, Jaume (March 1999). "Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes". Science 283 (5409): 1919–1923. doi:10.1126/science.283.5409.1919. 
  9. ^ Todaro, Antonio. Gastrotricha: Overview. Gastrotricha: World Portal. University of Modena & Reggio Emilia. Retrieved on 2008-01-26.
  10. ^ Kristensen, Reinhardt Møbjerg (July 2002). "An Introduction to Loricifera, Cycliophora, and Micrognathozoa". Integrative and Comparative Biology 42 (3): 641–651. Oxford Journals. doi:10.1093/icb/42.3.641. 
  11. ^ Biodiversity: Mollusca. The Scottish Association for Marine Science. Retrieved on 2007-11-19.
  12. ^ Russell, Bruce J. (Writer), Denning, David (Writer). Branches on the Tree of Life: Annelids [VHS]. BioMEDIA ASSOCIATES.
  13. ^ Eernisse, Douglas J.; Eernisse, Douglas J.; Albert, James S.; Anderson , Frank E. (1992). "Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology". Systematic Biology 41 (3): 305–330. doi:10.2307/2992569. 
  14. ^ Eernisse, Douglas J.; Kim, Chang Bae; Moon, Seung Yeo; Gelder, Stuart R.; Kim, Won (September, 1996). "Phylogenetic Relationships of Annelids, Molluscs, and Arthropods Evidenced from Molecules and Morphology". Journal of Molecular Evolution 43 (3): 207–215. New York: Springer. doi:10.1007/PL00006079. ISSN 0022-2844. 
  15. ^ [|Collins, Allen G.] (1995), The Lophophore, University of California Museum of Paleontology, <http://www.ucmp.berkeley.edu/glossary/gloss7/lophophore.html> 
  16. ^ Adoutte, A.; Adoutte, André; Balavoine, Guillaume; Lartillot, Nicolas; Lespinet, Olivier; Prud'homme, Benjamin; de Rosa, Renaud (April, 25 2000). "The new animal phylogeny: Reliability and implications". Proceedings of the National Academy of Sciences 97 (9): 4453–4456. doi:10.1073/pnas.97.9.4453. ISSN 0022-2844. PMID 10781043. 
  17. ^ Passamaneck, Yale J. (2003), “Woods Hole Oceanographic Institution”, Molecular Phylogenetics of the Metazoan Clade Lophotrochozoa, pp. 124, <http://handle.dtic.mil/100.2/ADA417356> 
  18. ^ Adoutte, A.; Sundberg, Per; Turbevilleb, J. M.; Lindha, Susanne (September, 2001). "Phylogenetic relationships among higher nemertean (Nemertea) taxa inferred from 18S rDNA sequences". Molecular Phylogenetics and Evolution 20 (3): 327–334. doi:10.1006/mpev.2001.0982. 
  19. ^ "The mitochondrial genome of the Sipunculid Phascolopsis gouldii supports its association with Annelida rather than Mollusca" (PDF) (February, 2002). Molecular Biology and Evolution 19 (2): 127–137. ISSN 0022-2844. PMID 11801741. 
  20. ^ Nielsen, Claus (April 2001). "Bryozoa (Ectoprocta: ‘Moss’ Animals)". Encyclopedia of Life Sciences. John Wiley & Sons, Ltd. doi:10.1038/npg.els.0001613. 
  21. ^ N.H. Putnam, et al. (Jul 2007). "Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization". Science 317 (5834): 86–94. doi:10.1126/science.1139158. 
  22. ^ Wang, X.; Wang, Xiujuan; Lavrov Dennis V. (2006-10-27). "Mitochondrial Genome of the Homoscleromorph Oscarella carmela (Porifera, Demospongiae) Reveals Unexpected Complexity in the Common Ancestor of Sponges and Other Animals". Molecular Biology and Evolution 24 (2): 363–373. Oxford Journals. doi:10.1093/molbev/msl167. 

References

  • Klaus Nielsen. Animal Evolution: Interrelationships of the Living Phyla (2nd edition). Oxford Univ. Press, 2001.
  • Knut Schmidt-Nielsen. Animal Physiology: Adaptation and Environment. (5th edition). Cambridge Univ. Press, 1997.

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