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| − | {{pp-semi-vandalism|small=yes}}
| + | An '''animal''' is an organism that belongs to either the kingdom Animalia or Metazoa. |
| − | {{redirect|Animalia|the book|Animalia (book)}}
| + | |
| − | {{otheruses}}
| + | |
| − | {{Taxobox
| + | |
| − | | color = #{{taxobox color|[[animalia]]}}
| + | |
| − | | name = Animals
| + | |
| − | | fossil_range = [[Ediacaran]] - Recent
| + | |
| − | | image = Animalia diversity.jpg
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| − | | image_width = 250px
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| − | | image_caption = Clockwise from top-left: ''[[European Squid|Loligo vulgaris]]'' (a [[Mollusca|mollusk]]), ''[[sea nettle|Chrysaora quinquecirrha]]'' (a [[cnidaria]]n), ''[[Aphthona flava]]'' (an [[arthropod]]), ''[[Nereis|Eunereis longissima]]'' (an [[annelid]]), and ''[[tiger|Panthera tigris]]'' (a [[chordate]]).
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| − | | domain = [[Eukaryote|Eukaryota]]
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| − | | unranked_regnum = [[Opisthokont]]a
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| − | | regnum = '''Animalia'''
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| − | | regnum_authority = [[Carolus Linnaeus|Linnaeus]], [[Systema Naturae|1758]]
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| − | | subdivision_ranks = [[Phylum|Phyla]]
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| − | | subdivision =
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| − | * '''''Subkingdom [[Parazoa]]
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| − | ** [[Sponge|Porifera]]
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| − | * '''''Subkingdom [[Eumetazoa]]
| + | |
| − | **[[Trichoplax|Placozoa]]
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| − | ** '''''[[Radiata]] (unranked)'''''
| + | |
| − | *** [[Ctenophora]]
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| − | *** [[Cnidaria]]
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| − | ** '''''[[Bilateria]] (unranked)'''''
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| − | *** [[Orthonectida]]
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| − | *** [[Rhombozoa]]
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| − | *** [[Acoelomorpha]]
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| − | *** [[Chaetognatha]]
| + | |
| − | *** '''Superphylum [[Deuterostome|Deuterostomia]]'''
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| − | **** [[Chordate|Chordata]]
| + | |
| − | **** [[Hemichordata]]
| + | |
| − | **** [[Echinoderm]]ata
| + | |
| − | **** [[Xenoturbellida]]
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| − | **** [[Vetulicolia]] [[extinction|†]]
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| − | *** '''''[[Protostomia]] (unranked)'''''
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| − | **** '''Superphylum [[Ecdysozoa]]'''
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| − | ***** [[Kinorhyncha]]
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| − | ***** [[Loricifera]]
| + | |
| − | ***** [[Priapulida]]
| + | |
| − | ***** [[Nematoda]]
| + | |
| − | ***** [[Nematomorpha]]
| + | |
| − | ***** [[Lobopodia]] [[extinction|†]]
| + | |
| − | ***** [[Tardigrada]]
| + | |
| − | ***** [[Onychophora]]
| + | |
| − | ***** [[Arthropoda]]
| + | |
| − | **** '''Superphylum [[Platyzoa]]'''
| + | |
| − | ***** [[Platyhelminthes]]
| + | |
| − | ***** [[Gastrotricha]]
| + | |
| − | ***** [[Rotifera]]
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| − | ***** [[Acanthocephala]]
| + | |
| − | ***** [[Gnathostomulida]]
| + | |
| − | ***** [[Micrognathozoa]]
| + | |
| − | ***** [[Cycliophora]]
| + | |
| − | **** '''Superphylum [[Lophotrochozoa]]'''
| + | |
| − | ***** [[Annelida]]
| + | |
| − | ****** [[Echiura]]
| + | |
| − | ****** [[Sipuncula]]
| + | |
| − | ***** [[Nemertea]]
| + | |
| − | ***** [[Phoronida]]
| + | |
| − | ***** [[Brachiopoda]]
| + | |
| − | ***** [[Mollusca]]
| + | |
| − | ***** [[Bryozoa]]
| + | |
| − | ***** [[Entoprocta]]
| + | |
| − | ***** [[Hyolitha]] [[extinction|†]]
| + | |
| − | }}
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| | | | |
| − | '''Animals''' are a major group of [[multicellular]], [[eukaryotic]] [[organism]]s of the [[Kingdom (biology)|kingdom]] '''Animalia''' or '''Metazoa'''. Their [[body plan]] becomes fixed as they develop, although some undergo a process of [[metamorphosis (biology)|metamorphosis]] later on in their life. Most animals are [[motile]] - they can move spontaneously and independently. Animals are [[heterotroph]]s - they are dependent on other organisms (e.g. [[plant]]s) for [[sustenance]].
| + | In everyday general usage, the word "animal" excludes [[human]]s and only includes members of the Kingdom Metazoa that are not human. |
| | | | |
| − | Most known animal phyla appeared in the fossil record as marine species during the [[Cambrian explosion]], about 542 million years ago.
| + | ==See also== |
| − | | + | *[[animal rights]] |
| − | ==Etymology==
| + | |
| − | The word "animal" comes from the [[Latin]] word ''animale'', [[Grammatical gender|neuter]] of ''animalis'', and is derived from ''anima'', meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-[[human]] animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.<ref name="americanheritage_animal>{{cite encyclopedia |year=2006 |title=Animal |encyclopedia=The American Heritage Dictionary |publisher=Houghton Mifflin Company |edition=Forth }}</ref>
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| − | ==Characteristics==
| + | |
| − | Animals have several characteristics that set them apart from other living things. Animals are [[eukaryote|eukaryotic]] and usually [[multicellular]]<ref name="NationalZoo">{{citeweb|url=http://nationalzoo.si.edu/Animals/GiantPandas/PandasForKids/classification/classification.htm|author=National Zoo|title=Panda Classroom|accessmonthday=[[September 30]]|accessyear=2007|language=English}}</ref> (although see [[Myxozoa]]), which separates them from [[bacteria]] and most [[protist]]s. They are [[heterotroph]]ic,<ref name=â€Windowsâ€>{{citeweb|url=http://www.windows.ucar.edu/tour/link=/earth/Life/heterotrophs.html&edu=high|author=Jennifer Bergman|title=Heterotrophs|accessmonthday=[[September 30]]|accessyear=2007|language=English}}</ref> generally digesting food in an internal chamber, which separates them from [[plant]]s and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s.<ref name=â€AnimalCellsâ€>{{citeweb|url=http://micro.magnet.fsu.edu/cells/animalcell.html|author=Davidson, Michael W.|title=Animal Cell Structure|accessmonthday=[[September 20]]|accessyear=2007|language=English}}</ref> All animals are motile,<ref name=â€Conceptsâ€>{{citeweb|url=http://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm|author=Saupe, S.G|title=Concepts of Biology|accessmonthday=[[September 30]]|accessyear=2007|language=English}}</ref> if only at certain life stages. In most animals, [[embryo]]s pass through a [[Blastula|blastula stage]], which is a characteristic exclusive to animals.
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| − | ===Structure===
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| − | With a few exceptions, most notably the [[sponge]]s (Phylum Porifera), animals have bodies differentiated into separate [[biological tissue|tissues]]. These include [[muscle]]s, which are able to contract and control locomotion, and [[nervous system|nerve tissue]], which sends and processes signals. There is also typically an internal [[digestion|digestive]] chamber, with one or two openings. Animals with this sort of organization are called metazoans, or [[eumetazoan]]s when the former is used for animals in general.
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| − | All animals have [[eukaryotic]] cells, surrounded by a characteristic extracellular matrix composed of [[collagen]] and elastic [[glycoprotein]]s. This may be calcified to form structures like [[animal shell|shell]]s, [[bone]]s, and [[spicule]]s. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: [[tight junction]]s, [[gap junction]]s, and [[desmosome]]s.
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| − | | + | |
| − | ===Reproduction and development===
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| − | Nearly all animals undergo some form of sexual reproduction. Adults are [[diploid]] or [[polyploid]]. They have a few specialized reproductive cells, which undergo [[meiosis]] to produce smaller motile [[spermatozoon|spermatozoa]] or larger non-motile [[ovum|ova]]. These fuse to form [[zygote]]s, which develop into new individuals.
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| − | [[Image:Mitosis-flourescent.jpg|thumb|left|200px|A [[newt]] [[lung]] [[cell (biology)|cell]] [[staining (biology)|stained]] with [[fluorescent]] [[dye]]s undergoing [[mitosis]], specifically early [[anaphase]].]]
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| − | Many animals are also capable of [[asexual reproduction]]. This may take place through [[parthenogenesis]], where fertile eggs are produced without mating, or in some cases through [[fragmentation (biology)|fragmentation]].
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| − | A [[zygote]] initially develops into a hollow sphere, called a [[blastula]], which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first [[invagination|invaginates]] to form a [[gastrula]] with a digestive chamber, and two separate [[germ layer]]s - an external [[ectoderm]] and an internal [[endoderm]]. In most cases, a [[mesoderm]] also develops between them. These germ layers then differentiate to form tissues and organs.
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| − | Most animals grow by indirectly using the energy of [[sunlight]]. Plants use this [[energy]] to convert sunlight into simple [[sugars]] using a process known as [[photosynthesis]]. Starting with the molecules [[carbon dioxide]] (CO<sub>2</sub>) and [[water]] (H<sub>2</sub>O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of [[glucose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>) and releases [[oxygen]] (O<sub>2</sub>). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as [[glycolysis]].
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| − | Animals who live close to [[hydrothermal vent]]s and [[cold seep]]s on the ocean floor are not dependent on the energy of sunlight. Instead, [[chemosynthesis|chemosynthetic]] [[archaea]] and [[eubacteria]] form the base of the food chain.
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| − | ==Origin and fossil record==
| + | |
| − | Animals are generally considered to have [[Evolution|evolved]] from a [[flagellate]]d eukaryote. Their closest known living relatives are the [[choanoflagellate]]s, collared flagellates that have a morphology similar to the choanocytes of certain sponges. [[Molecular]] studies place animals in a supergroup called the [[opisthokont]]s, which also include the choanoflagellates, [[fungus|fungi]] and a few small parasitic [[protist]]s. The name comes from the posterior location of the [[flagellum]] in motile cells, such as most animal spermatozoa, whereas other [[eukaryotes]] tend to have anterior flagella.
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| − | The first fossils that might represent animals appear towards the end of the [[Precambrian]], around 610 million years ago, and are known as the [[Ediacaran biota|Ediacaran or Vendian biota]]. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous appearance during the [[Cambrian]] period, about 542 million years ago. It is still disputed whether this event, called the [[Cambrian explosion]], represents a rapid divergence between different groups or a change in conditions that made fossilization possible. However some paleontologists and geologists would suggest that animals appeared much earlier than previously thought, possibly even as early as 1 billion years ago. Trace fossils such as tracks and burrows found in [[Tonian]] era strata in India indicate the presence of [[triploblastic]] worm like [[metazoans]] roughly as large (about 5mm wide) and complex as [[earthworms]].<ref name="Seilacher1998">{{cite journal
| + | |
| − | | title=Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India
| + | |
| − | | authors=Seilacher, A., Bose, P.K. and Pflüger, F.
| + | |
| − | | journal=Science
| + | |
| − | | volume=282
| + | |
| − | | number=5386
| + | |
| − | | pages=80-83
| + | |
| − | | year=1998
| + | |
| − | | url=http://www.sciencemag.org/cgi/content/abstract/282/5386/80
| + | |
| − | | accessdate=2007-08-20
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| − | }}
| + | |
| − | </ref> In addition during the beginning of the Tonian period around 1 billion years ago (roughly the same time that the trace fossils previously discussed in this article date back to) there was a decrease in [[Stromatolite]]
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| − | diversity which may indicate the appearance of grazing animals during this time as Stromatolites also increased in diversity shortly after the end-Ordovician and end-Permian rendered large amounts of grazing marine animals extinct and decreased shortly after their populations recovered. However some other scientists doubt that these fossils are authentic and have suggested these trace fossils are just the result of natural processes such as erosion.{{Fact|date=December 2007}} <!--ref name=Budd2000/--><!--I left that in case it's important-->
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| − | ==Groups of animals==
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| − | [[Image:Elephant-ear-sponge.jpg|thumb|left|Orange elephant ear sponge, ''[[Agelas clathrodes]]'', in foreground. Two corals in the background: a [[sea fan]], ''Iciligorgia schrammi'', and a sea rod, ''Plexaurella nutans''.]]
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| − | The sponges ([[Porifera]]) were long thought to have diverged from other animals early. As mentioned above, they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are [[sessile]] and typically feed by drawing in water through pores. [[Archaeocyatha]], which have fused skeletons, may represent sponges or a separate phylum. However, a phylogenomic study in 2008 of 150 genes in 21 genera<ref name=class>Dunn ''et al.'' 2008. "Broad phylogenomic sampling improves resolution of the animal tree of life". ''Nature'' 06614.</ref> revealed that it is the [[Ctenophora]] or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.
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| − | Among the other phyla, the Ctenophora and the [[Cnidaria]], which includes [[sea anemone]]s, [[coral]]s, and [[jellyfish]], are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into [[organ (anatomy)|organs]]. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called [[diploblastic]]. The tiny [[Placozoa]]ns are similar, but they do not have a permanent digestive chamber.
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| − | The remaining animals form a [[monophyletic]] group called the [[Bilateria]]. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is [[triploblastic]], i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a [[coelom]] or pseudocoelom. There are exceptions to each of these characteristics, however - for instance adult [[echinoderm]]s are radially symmetric, and certain parasitic worms have extremely simplified body structures.
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| − | Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the [[Deuterostomes]] and [[Protostomes]], which includes the [[Ecdysozoa]], [[Platyzoa]], and [[Lophotrochozoa]]. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the [[Acoelomorpha]], [[Rhombozoa]], and [[Orthonectida]]. The [[Myxozoa]], single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Bilateria as well.
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| − | ===Deuterostomes===
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| − | [[Image:Superb fairy wren2 LiquidGhoul.jpg|thumb|[[Superb Fairy-wren]], ''Malurus cyaneus'']]
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| − | [[Deuterostome]]s differ from the other Bilateria, called [[protostome]]s, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the [[archenteron]]) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through [[invagination]] of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.
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| − | All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the [[Echinodermata]] and [[Chordate|Chordata]]. The former are radially symmetric and exclusively marine, such as [[starfish]], [[sea urchin]]s, and [[sea cucumber]]s. The latter are dominated by the [[vertebrate]]s, animals with backbones. These include [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s, and [[mammal]]s.
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| − | In addition to these, the deuterostomes also include the [[Hemichordata]] or acorn worms. Although they are not especially prominent today, the important fossil [[graptolite]]s may belong to this group.
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| − | The [[Chaetognatha]] or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.
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| − | ===Ecdysozoa===
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| − | [[Image:Sympetrum flaveolum - side (aka).jpg|thumb|[[Yellow-winged Darter]], ''Sympetrum flaveolum'']]
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| − | The [[Ecdysozoa]] are protostomes, named after the common trait of growth by moulting or [[ecdysis]]. The largest animal phylum belongs here, the [[Arthropoda]], including [[insect]]s, [[spider]]s, [[crab]]s, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the [[Onychophora]] and [[Tardigrada]], are close relatives of the arthropods and share these traits.
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| − | The ecdysozoans also include the [[Nematoda]] or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the [[Nematomorpha]] or horsehair worms, and the [[Kinorhyncha]], [[Priapulida]], and [[Loricifera]]. These groups have a reduced coelom, called a pseudocoelom.
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| − | The remaining two groups of protostomes are sometimes grouped together as the [[Spiralia]], since in both embryos develop with spiral cleavage.
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| − | ===Platyzoa===
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| − | [[Image:Bedford's Flatworm.jpg|thumb|[[Bedford's flatworm]], ''Pseudobiceros bedfordi'']]
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| − | The [[Platyzoa]] include the phylum [[Platyhelminthes]], the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.<ref>{{cite journal |coauthors=Ruiz-Trillo, Iñaki; Riutort, Marta; Littlewood, D. Timothy J.; Herniou, Elisabeth A.; Baguñà , Jaume |year= 1999 |month= March |title=Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes |journal=Science |volume=283 |issue=5409 |pages=1919 - 1923 |doi=10.1126/science.283.5409.1919 |accessdate= 2007-12-19 }}</ref>
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| − | A number of [[parasites]] are included in this group, such as the [[fluke]]s and [[tapeworm]]s. Flatworms are [[Body cavity#No coelom (Acoelomate)|acoelomates]], lacking a body cavity, as are their closest relatives, the microscopic [[Gastrotricha]].<ref name="umodena">{{cite web |url=http://www.gastrotricha.unimore.it/overview.htm |title=Gastrotricha: Overview |accessdate=2008-01-26 |last=Todaro |first=Antonio |work=Gastrotricha: World Portal |publisher=University of Modena & Reggio Emilia}}</ref>
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| − | The other platyzoan phyla are mostly microscopic and [[Body cavity#Pseudocoelomates|pseudocoelomate]]. The most prominent are the [[Rotifera]] or rotifers, which are common in aqueous environments. They also include the [[Acanthocephala]] or spiny-headed worms, the [[Gnathostomulida]], [[Micrognathozoa]], and possibly the [[Cycliophora]].<ref name="IntroCyclio">{{cite journal |last=Kristensen |first= Reinhardt Møbjerg |year=2002 |month=July |title=An Introduction to Loricifera, Cycliophora, and Micrognathozoa |journal=Integrative and Comparative Biology |volume=42 |issue=3 |pages=641-651 |doi =10.1093/icb/42.3.641 |url=http://icb.oxfordjournals.org/cgi/content/full/42/3/641 |accessdate= 2008-01-26 |publisher = Oxford Journals }}</ref> These groups share the presence of complex jaws, from which they are called the [[Gnathifera]].
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| − | ===Lophotrochozoa===
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| − | [[Image:Grapevinesnail 01.jpg|thumb|[[Roman snail]], ''Helix pomatia'']]
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| − | The [[Lophotrochozoa]] include two of the most successful animal phyla, the [[Mollusca]] and [[Annelida]].<ref>{{cite web |url=http://www.lophelia.org/lophelia/biodiv_6.htm |title=Biodiversity: Mollusca|accessdate=2007-11-19 |publisher=The Scottish Association for Marine Science}}</ref><ref>{{cite video | people = Russell, Bruce J. (Writer), Denning, David (Writer) | title = Branches on the Tree of Life: Annelids| medium = [[VHS]] | publisher = BioMEDIA ASSOCIATES | date = 2000 }}</ref> The former includes animals such as [[snail]]s, [[clam]]s, and [[squid]]s, and the latter comprises the segmented worms, such as [[earthworm]]s and [[leech]]es. 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,<ref>{{cite journal| coauthors=Eernisse, Douglas J.; Albert, James S.; Anderson , Frank E. | title=Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology | journal=Systematic Biology | volume=41 | issue=3 | pages = 305-330 | date = [[1992]] | accessdate = 2007-11-19 }}</ref> because they are both segmented. Now this is generally considered [[convergent evolution]], owing to many morphological and genetic differences between the two phyla.<ref>{{cite journal| coauthors=Kim, Chang Bae; Moon, Seung Yeo; Gelder, Stuart R.; Kim, Won | title=Phylogenetic Relationships of Annelids, Molluscs, and Arthropods Evidenced from Molecules and Morphology | journal=Journal of Molecular Evolution | volume=43 | issue=3 | pages = 207-215 | publisher = Springer | location = [[New York]] | date = [[September]], [[1996]] | issn = 0022-2844 | url = http://www.springerlink.com/content/xptr6ga3ettxnmb9/ | doi = 10.1007/PL00006079 | accessdate = 2007-11-19 }}</ref>
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| − | 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]].<ref>{{citation | url = http://www.ucmp.berkeley.edu/glossary/gloss7/lophophore.html | title= The Lophophore| author =Collins, Allen G. | author-link = http://www.paleobio.org/agc/ | year = 1995 | publisher = University of California Museum of Paleontology}}</ref> These were traditionally grouped together as the lophophorates.<ref>{{cite journal| coauthors=Adoutte, André; Balavoine, Guillaume; Lartillot, Nicolas; Lespinet, Olivier; Prud'homme, Benjamin; de Rosa, Renaud | title=The new animal phylogeny: Reliability and implications | journal=Proceedings of the National Academy of Sciences | volume=97 | issue=9 | pages = 4453-4456 | date = [[April]], [[25]] [[2000]] | issn = 0022-2844 | url = http://www.pnas.org/cgi/content/full/97/9/4453 | pmid=10781043 | accessdate = 2007-11-19 }}</ref> but it now appears they are [[paraphyletic]],<ref>{{Citation | first = Yale J. | last = Passamaneck | contribution = Woods Hole Oceanographic Institution | title = Molecular Phylogenetics of the Metazoan Clade Lophotrochozoa | year = 2003 | pages = 124 | url = http://handle.dtic.mil/100.2/ADA417356 | format = PDF }}</ref> some closer to the Nemertea and some to the Mollusca and Annelida.<ref>{{cite journal| coauthors=Sundberg, Per; Turbevilleb, J. M.; Lindha, Susanne | title=Phylogenetic relationships among higher nemertean (Nemertea) taxa inferred from 18S rDNA sequences | journal=Molecular Phylogenetics and Evolution | volume=20 | issue=3 | pages = 327-334 | date = [[September]], [[2001]] | doi = 10.1006/mpev.2001.0982 | accessdate = 2007-11-19 }}</ref><ref>{{cite journal| coauthors=Boore, Jeffrey L.; Staton, Joseph L | title=The mitochondrial genome of the Sipunculid Phascolopsis gouldii supports its association with Annelida rather than Mollusca | journal=Molecular Biology and Evolution | volume=19 | issue=2 | pages = 127–137 | date = [[February]], [[2002]] | issn = 0022-2844 | url = http://mbe.oxfordjournals.org/cgi/reprint/19/2/127.pdf | format=PDF | pmid=11801741 | accessdate = 2007-11-19 }}</ref> 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.<ref>{{cite journal | last = Nielsen | first = Claus | year = 2001 | month = April | title = Bryozoa (Ectoprocta: ‘Moss’ Animals) | journal = Encyclopedia of Life Sciences | publisher = John Wiley & Sons, Ltd | doi = 10.1038/npg.els.0001613 | url = http://mrw.interscience.wiley.com/emrw/9780470015902/els/article/a0001613/current/abstract | accessdate = 2008-01-19 }}</ref>
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| − | == Model organisms ==
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| − | {{Mainarticle|Model organism|Animal testing}}
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| − | 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 organism]]s, and were among the first lifeforms to be genetically sequenced. This was facilitated by the severely reduced state of their [[genome]]s, but the double-edged sword here is that with many [[gene]]s, [[intron]]s and [[genetic linkage|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 project]]s currently in progress. Analysis of the [[starlet sea anemone]] genome has emphasised the importance of sponges, placozoans, and [[choanoflagellate]]s, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.<ref>
| + | |
| − | {{cite journal
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| − | |author=N.H. Putnam, ''et al.''
| + | |
| − | |month=Jul
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| − | |year=2007
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| − | |title=Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization
| + | |
| − | |journal=[[Science (journal)|Science]]
| + | |
| − | |volume=317
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| − | |issue=5834
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| − | |pages=86-94
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| − | |doi=10.1126/science.1139158}}</ref>
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| − | 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.<ref>{{cite journal | coauthors = Wang, Xiujuan; Lavrov Dennis V. | date = 2006-10-27 | year = 2006 | month = 27
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| − | | title = Mitochondrial Genome of the Homoscleromorph Oscarella carmela (Porifera, Demospongiae) Reveals Unexpected Complexity in the Common Ancestor of Sponges and Other Animals | journal = Molecular Biology and Evolution | volume = 24 | issue = 2 | pages = 363-373 | publisher = Oxford Journals | doi = 10.1093/molbev/msl167 | url = http://mbe.oxfordjournals.org/cgi/content/abstract/24/2/363 | accessdate = 2008-01-19 }}</ref>
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| − | Other model organisms belonging to the animal kingdom include the mouse (''[[Mus musculus]]'') and zebrafish (''[[Danio rerio]]''}.
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| − | ==History of classification==
| + | |
| − | [[Image:Carolus Linnaeus (cleaned up version).jpg|thumb|[[Carolus Linnaeus]] known as the father of modern [[taxonomy]]]]
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| − | [[Aristotle]] divided the living world between animals and [[plant]]s, 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.
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| − | In [[Carolus Linnaeus|Linnaeus]]'s original scheme, the animals were one of three kingdoms, divided into the classes of [[Vermes]], [[Insect]]a, [[Fish|Pisces]], [[Amphibia]], [[bird|Aves]], and [[Mammal]]ia. Since then the last four have all been subsumed into a single phylum, the [[chordate|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.
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| − | == See also == | + | |
| − | {{wiktionary}}
| + | |
| − | * [[Fauna]] | + | |
| − | * [[List of animal names]]
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| − | * [[Animal behavior]]
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| − | * [[Animal rights]]
| + | |
| − | * [[List of animals by number of neurons]]
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| − | * [[Holocene extinction event]]
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| − | | + | |
| − | ==Notes==<!-- Cladistics14:249,23:130 -->
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| − | {{reflist}}
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| − | | + | |
| − | ==References==
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| − | * Klaus Nielsen. ''Animal Evolution: Interrelationships of the Living Phyla'' (2nd edition). Oxford Univ. Press, 2001.
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| − | * Knut Schmidt-Nielsen. ''Animal Physiology: Adaptation and Environment''. (5th edition). Cambridge Univ. Press, 1997.
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| − | ==External links==
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| − | {{Sisterlinks|Animalia}}
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| − | {{wikispecies| Animalia}}
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| − | * [http://tolweb.org/ Tree of Life Project]
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| − | * [http://animaldiversity.ummz.umich.edu/site/index.html Animal Diversity Web] - [[University of Michigan]]'s database of animals, showing taxonomic classification, images, and other information.
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| − | * [http://www.arkive.org ARKive] - multimedia database of worldwide endangered/protected species and common species of UK.
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| − | * [http://www.sciam.com/article.cfm?chanID=sa006&articleID=000DC8B8-EA15-137C-AA1583414B7F0000 Scientific American Magazine (December 2005 Issue) - Getting a Leg Up on Land] About the evolution of four-limbed animals from fish.
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