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définition - Phylum

phylum (n.)

1.(biology) the major taxonomic group of animals and plants; contains classes

2.(linguistics) a large group of languages that are historically related

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Merriam Webster

PhylumPhy"lum (?), n.; pl. Phyla (#). [NL. See Phylon.]
1. (Zoöl.) One of the larger divisions of the animal kingdom; a branch; a grand division.

2. (Biol.) A series of animals or plants genetically connected.

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définition (complément)

voir la définition de Wikipedia

locutions

dictionnaire analogique

 

MESH root[Thème]

phylum [MeSH]




Wikipedia

Phylum

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<imagemap>File:Biological_classification_L_Pengo_vflip.svg|150px|The various levels of the scientific classification system.rect 100 15 225 57 Liferect 100 78 225 120 Domainrect 100 142 225 184 Kingdomrect 100 205 225 247 Phylumrect 100 268 225 310 Classrect 100 332 225 374 Orderrect 100 395 225 437 Familyrect 100 459 225 501 Genusrect 100 522 225 564 Species

default File:Biological_classification_L_Pengo_vflip.svgdesc none</imagemap>

The hierarchy of biological classification's eight major taxonomic ranks. A kingdom contains one or more phyla. Intermediate minor rankings are not shown.

In biology, a phylum (plural: phyla)[note 1] is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division.[1]

Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum" may be a misnomer indicative of ignorance.[2] Consequently the number of phyla varies from one author to the next. The relationships among phyla are becoming increasingly well known, and larger clades can be found to contain many of the phyla.

Contents

General description and familiar examples

Informally, phyla can be thought of as grouping animals based on general body plan,[3] as well as developmental or internal organizations.[4] For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "phylum" in reference to plants, the term "Division" is almost always used by botanists.

The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum to which humans belong, along with all other vertebrate species, as well as some invertebrates such as the lamprey. Although there are approximately 35 phyla, these nine include over 96% of animal species. Many phyla are exclusively marine, and only one phylum, the Onychophora (velvet worms) is entirely absent from the world's oceans–although ancestral onycophorans were marine.[5]

Earliest known phyla

The origin of phf the time were mainly similar to, but not strictly members of, modern phyla.[6] The significance of this event depends on two factors:

  1. How long did modern phyla exist prior to the Cambrian fossil embryos? New interpretations of the pre-Cambrian Ediacara biota suggest that there could have been an origin for some phyla earlier than the Cambrian.
  2. How soon did phyla appear in the Cambrian? This factor depends on both the definition of a phylum and on interpretation of early fossils, which may not display characteristics needed to determine membership in a modern phylum – non-mineralised parts of organisms are rarely preserved.[7]

The magnitude of the event was also overestimated as early authors felt it necessary to erect a new phylum for any organism that could not be accommodated in modern phyla. This approach is misleading and unhelpful; by one definition, such organisms do not fall into any phylum, but are classified as "aunts" of a phylum.[6]

Defining a phylum

At the most basic level, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[6] Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is an unsatisfactory approach, but the phenetic definition is more useful when addressing questions of a morphological nature – such as how successful different body plans were.

Definition based on genetic relation

The largest objective measure in the above definitions is the "certain degree" – how unrelated do organisms need to be to be members of different phyla? The minimal requirement is that all organisms in a phylum should be related closely enough for them to be clearly more closely related to one another than to any other group.[6] However, even this is problematic, as the requirement depends on our current knowledge about organisms' relationships: As more data becomes available, particularly from molecular studies, we are better able to judge the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not; for example, since the onychophora and the tardigrada have now been accepted as stem groups of the arthropods, these three phyla should be combined.

This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of cladistics, a method in groups are placed on a "family tree" without any formal ranking of group size.[6] So as to provide a handle on the size and significance of groups, a "body-plan" based definition of a phylum has been proposed by paleontologists Graham Budd and Sören Jensen. The definition was posited by paleontologists because it is extinct organisms that are typically hardest to classify, because they can be extinct off-shoots that diverged from a phylum's history before the characters that define the modern phylum were all acquired.

Definition based on body plan

By Budd and Jensen's definition, phyla are defined by a set of characters shared by all their living representatives. This has a couple of small problems – for instance, characters common to most members of a phylum may be secondarily lost by some members. It is also defined based on an arbitrary point of time (the present). However, as it is character based, it is easy to apply to the fossil record. A more major problem is that it relies on an objective decision of which group of organisms should be considered a phylum.

Its utility is that it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities.[6] However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group.[6]Further, organisms in the stem group to a phylum can bear all the aspects of the "body plan" of the phylum without all the characters necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.[8]

Based upon this definition, which some say is unreasonably affected by the chance survival of rare groups, which vastly increase the size of phyla, representatives of many modern phyla did not appear until long after the Cambrian – as late as the Carboniferous in the case of the Priapulids.[7]

Lists

Animal phyla

'Phylum'MeaningCommon NameDistinguishing characteristicSpecies described
AcanthocephalaThorny headed wormsThorny-headed wormsReversible spiny proboscisabout 750
AcoelomorphaWithout gutA colonNo mouth or alimentary canal
AnnelidaLittle ringSegmented wormsMultiple circular segmentabout 16,300 modern
ArthropodaJointed footArthropodsChitin exoskeleton1,134,000+
BrachiopodaArm footLamp shellsLophophore and pediclebetween 300 and 500 extant
BryozoaMoss animalsMoss animals, sea matsLophophore, no pedicle, ciliated tentaclesabout 5,000 living species
ChaetognathaLonghair jawArrow wormsChitinous spines either side of head, finsabout 100 modern species
ChordataCordChordatesHollow dorsal nervous chord, notochord, pharyngeal slits, endostyle, post-anal tailabout 100,000+
CnidariaStinging nettleCoelenteratesNematocysts (stinging cells)about 11,000
CtenophoraComb bearerComb jelliesEight "comb rows" of fused ciliaabout 100 modern species
CycliophoraWheel carryingSymbionCircular mouth surrounded by small ciliaat least 3
EchinodermataSpiny skinEchinodermsFive-fold radial symmetry in living forms, mesodermal calcified spinesabout 7,000 extant and 13,000 extinct species
EchiuraSpine tailSpoon wormsSet of hooks at posterior endabout 140
EntoproctaInside anusGoblet wormAnus inside ring of ciliaabout 150
GastrotrichaHair stomachMeiofaunaTwo terminal adhesive tubesabout 690
GnathostomulidaJaw orificeJaw wormsabout 100
HemichordataHalf cordAcorn worms, pterobranchsStomochord in collar, pharyngeal slitsabout 100 living species
KinorhynchaMotion snoutMud dragonsEleven segments, each with a dorsal plateabout 150
LoriciferaCorset bearerBrush headsUmbrella-like scales at each endabout 122
MicrognathozoaTiny jaw animalsAccordion like extensible thorax1
MolluscaThin shellMollusks / molluscsMuscular foot and mantle round shell112,000[9]
NematodaThread likeRound wormsRound cross section, keratin cuticle80,000 – 1 million
NematomorphaThread formHorsehair wormsabout 320
NemerteaA sea nymphRibbon wormsabout 1200
OnychophoraClaw bearerVelvet wormsLegs tipped by chitinous clawsabout 200 modern
OrthonectidaStraight swimSingle layer of ciliated cells surrounding a mass of sex cellsabout 20
PhoronidaZeus's mistressHorseshoe wormsU-shaped gut20
PlacozoaPlate animals1
PlatyhelminthesFlat wormsFlat wormsabout 25,000[10]
PoriferaPore bearerSpongesPerforated interior wallover 5,000 modern
PriapulidaPenisPriapulid wormsRetractable proboscis surrounded by papillae17
RhombozoaLozenge animalSingle axial cell surrounded by ciliated cells75
RotiferaWheel bearerRotifersAnterior crown of ciliaabout 2000
SipunculaSmall tubePeanut wormsMouth surrounded by invertible tentacles144–320
TardigradaSlow stepWater bearsFour segmented body and head1,000+
XenoturbellidaStrange flatwormCiliated deuterostome2
Total: 362,000,000-

Groups formerly ranked as phyla

Name as phylumCommon nameCurrent consensus
AschelminthesPseudocoelomatesDivided into several pseudocoelomate phyla.
CraniataSubgroup of phylum Chordata; perhaps synonymous with Vertebrata.
CephalochordataLanceletsSubphylum of phylum Chordata.
CephalorhynchaSuperphylum Scalidophora.
EnterepneustaAcorn wormsClass of phylum Hemichordata.
GephyraPeanut worms and spoon wormsDivided into phyla Sipuncula and Echiura.
MesozoaMesozoansDivided into phyla Orthonectida and Rhombozoa.
MyxozoaSeverely modified Cnidarians.
PentastomidaTongue wormsSubclass of Maxillopoda of phylum Arthropoda.
PogonophoraBeard wormsPart of family Siboglinidae of phylum Annelida.
PterobranchiaClass of phylum Hemichordata.
SymplasmaGlass spongesClass Hexactinellida of phylum Porifera.
UrochordataTunicatesSubphylum of phylum Chordata.
VestimentiferaVent wormsPart of family Siboglinidae of phylum Annelida.

Plant divisions

DivisionMeaningCommon nameDistinguishing characteristics
AnthocerotophytaFlower-horn plantsHornwortsHorn-shaped sporophytes, no vascular system
BryophytaMoss plantsMossesPersistent unbranched sporophytes, no vascular system
MarchantiophytaMarchantia plantsLiverwortsEphemeral unbranched sporophytes, no vascular system
LycopodiophytaWolf foot plantsClubmosses & SpikemossesMicrophyll leaves, vascular system
PteridophytaFern plantsFerns & HorsetailsProthallus gametophytes, vascular system
PteridospermatophytaFern with seeds plantSeed fernsOnly known from fossils, mostly Devonian, ranking in dispute[11]
PinophytaSap/pitch plantsConifersCones containing seeds and wood composed of tracheids
CycadophytaPalm plantsCycadsSeeds, crown of compound leaves
GinkgophytaGinkgo plantsGinkgo, MaidenhairSeeds not protected by fruit (single species)
GnetophytaGnetophytesSeeds and woody vascular system with vessels
Anthophyta (or MagnoliophytaFlower plantFlowering plantsFlowers and fruit, vascular system with vessels

Fungal divisions

PhylumMeaningCommon nameDistinguishing characteristics
ChytridiomycotaLittle pot mushroomChytridsCellulose in cell walls, flagellated gametes
DeuteromycotaSecond mushroomImperfect fungiUnclassified fungi; only asexual reproduction observed
ZygomycotaYolk mushroomZygomycetesBlend gametangia to form a zygosporangium
GlomeromycotaBall mushroomNoneForm arbuscular mycorrhizae with plants
AscomycotaBag/Wineskin MushroomSac fungiProduce spores in an 'ascus'
BasidiomycotaBasidium MushroomClub FungiProduce spores from a 'basidium'

See also

Notes

  1. ^ "Phylum" is adopted from the Greek φυλαί phylai, the clan-based voting groups in Greek city-states.

References

  1. ^ "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. http://dictionary.reference.com/browse/phylum. Retrieved 2008-10-04. "Phyla in the plant kingdom are frequently called divisions." 
  2. ^ Bengtson, (1986). "Introduction: The problem of the problematica,". in Hoffman, A.; Nitecki, M. H.. Problematic Fossil Taxa. Oxford,: Oxford University Press,. pp. 3–11. 
  3. ^ Valentine, James W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. pp. 7. ISBN 0226845486. "Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy."
  4. ^ Parker, Andrew (2003). In the blink of an eye: How vision kick-started the big bang of evolution. Sydney: Free Press. pp. 1–4. ISBN 0743257332. "The job of an evolutionary biologist is to create dinosaurs for the devil to consume, make sense of the conflicting diversity of form – there is not always a relationship between internal and external parts. Early in the history of the subject, it became obvious that internal organisations were generally more important to the higher classification of animals than are external shapes. The internal organisation puts general restrictions on how an animal can exchange gases, obtain nutrients and reproduce."
  5. ^ Davidson, E. H; Erwin, D. H (2006). "Gene Regulatory Networks and the Evolution of Animal Body Plans". Science (American Association for the Advancement of Science) 311 (5762): 796–800. doi:10.1126/science.1113832. PMID 16469913. http://www.sciencemag.org/cgi/content/abstract/311/5762/796. 
  6. ^ a b c d e f g Budd, G.E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews 75 (02): 253–295. doi:10.1017/S000632310000548X. http://www.journals.cambridge.org/abstract_S000632310000548X. Retrieved 2007-05-26. 
  7. ^ a b Briggs, D. E. G; Fortey, R. A (2005). [Expression error: Missing operand for > "Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation"]. Paleobiology 31 (2 (Suppl)): 94–112. doi:10.1666/0094-8373(2005)031[0094:WSSSGA]2.0.CO;2. 
  8. ^ Budd, G.E. (1998). Lethaia (Blackwell Synergy) 31 (3): 197–210. doi:10.1111/j.1502-3931.1998.tb00508.x (inactive 2008-10-21). http://www.blackwell-synergy.com/doi/abs/10.1111/j.1502-3931.1998.tb00508.x. 
  9. ^ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
  10. ^ Species Register. "Flatworms — Phylum Platyhelminthes". Marine Discovery Centres. http://www.woodbridge.tased.edu.au/mdc/Species%20Register/phylum_platyhelminthes.htm. Retrieved 2007-04-09. 
  11. ^ ""Kingdom Plantae Tree of Life"". http://www.fossilmuseum.net/Tree_of_Life/KingdomPlantae.htm. 

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