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In biology, a phylum (; : phyla) is a level of classification or below kingdom and above class. Traditionally, in the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent.

(2007). 9780534466695, Cengage Learning. .
Depending on definitions, the animal kingdom or Metazoa contains approximately 35 phyla; the plant kingdom contains about 14, and the fungus kingdom contains about 8 phyla. Current research in is uncovering the relationships between phyla, which are contained in larger , like and .

General description
The term phylum was coined in 1866 by from the Greek (, "race, stock"), related to (, "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as a group ("a self-contained unity"). "Wohl aber ist eine solche reale und vollkommen abgeschlossene Einheit die Summe aller Species, welche aus einer und derselben gemeinschaftlichen Stammform allmählig sich entwickelt haben, wie z. B. alle Wirbelthiere. Diese Summe nennen wir Stamm (Phylon)." which translates as: However, perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate a Stamm i.e., ( Phylon). In , August W. Eichler (1883) classified plants into named divisions, a term that remains in use today for groups of plants, algae and fungi.
(1984). 9780074517888, Tata McGraw-Hill. .
The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of .Collins AG, Valentine JW (2001). "Defining phyla: evolutionary pathways to metazoan body plans." Evol. Dev. 3: 432-442.

Informally, phyla can be thought of as groupings of organisms based on general specialization of .

(2021). 9780226845487, University of Chicago Press.
At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the definition), or a group of organisms with a certain degree of evolutionary relatedness (the definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.

Definition based on genetic relation
The most important objective measure in the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine 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, the were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of , so the phyla were merged (the bearded worms are now an annelid family). On the other hand, the highly parasitic phylum was divided into two phyla ( and ) when it was discovered the Orthonectida are probably and the Rhombozoa .

This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of , a method in which groups are placed on a "family tree" without any formal ranking of group size.

Definition based on body plan
A definition of a phylum based on body plan has been proposed by paleontologists and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.

This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is 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 greater problem is that it relies on a subjective decision about which groups of organisms should be considered as phyla.

The approach is useful because it makes it easy to classify extinct organisms as "" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the of a phylum is difficult, as it must display a character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.

A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise.

Known phyla

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.
(2021). 9781603442695, Texas A&M University Press. .

Thorny headThorny-headed worms
(2021). 9780123736215, Academic Press. .
Reversible spiny that bears many rows of hooked spines
Little ring Segmented wormsMultiple circular segment+ extant
Jointed footArthropodsSegmented bodies and jointed limbs, with + extant; 20,000+ extinct
Arm footLampshells and pedicle
Moss animalsMoss animals, sea mats, ectoproctsLophophore, no pedicle, , anus outside ring of ciliaextant
Longhair jawArrow worms spines either side of head, finsextant
With a cordChordatesHollow dorsal nerve cord, , , , post- +
Stinging nettleCnidarians (stinging cells)
Comb bearerComb jelliesEight "comb rows" of fused cilia
Wheel carryingSymbionCircular mouth surrounded by small cilia, sac-like bodies+
Spiny skinEchinodermsFivefold radial in living forms, mesodermal calcified spinesextant; approx. 13,000 extinct
Inside Goblet wormsAnus inside ring of cilia
Hairy stomachGastrotrich wormsTwo terminal adhesive tubes
Jaw orificeJaw worms
Half cordAcorn worms, hemichordates in collar, extant
Motion snoutMud dragonsEleven segments, each with a dorsal plate
Corset bearerBrush headsUmbrella-like scales at each end
Tiny jaw animalsLimnognathia-like extensible
SoftMollusks / molluscsMuscular foot and mantle round shell+ extant; 80,000+ extinctFeldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
Thread likeRound worms, thread wormsRound cross section,
Thread formHorsehair worms, gordian worms
A sea nymphRibbon worms, rhynchocoela
Claw bearerVelvet wormsLegs tipped by chitinous clawsextant
Straight swimmingOrthonectidsSingle layer of ciliated cells surrounding a mass of sex cells
Zeus's mistressHorseshoe wormsU-shaped gut
Plate animalsTrichoplaxesDifferentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between
Flat wormFlatworms
Pore bearerSpongesPerforated interior wallextant
Little Penis worms
Lozenge animalRhombozoansSingle anteroposterior axial cell surrounded by ciliated cells+
Wheel bearerRotifersAnterior crown of cilia
Small tubePeanut wormsMouth surrounded by invertible tentacles
Slow stepWater bears, Moss pigletsFour segmented body and head
Strange hollow formAcoels, xenoturbellids, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems+
Total: 34 1,525,000

The kingdom Plantae is defined in various ways by different biologists (see Current definitions of Plantae). All definitions include the living (land plants), to which may be added the two green algae divisions, and , to form the clade . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with , a group containing Viridiplantae and the algal and divisions.

The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Pteridophyta, while others place them both in Pteridophyta, as shown below. The division Pinophyta may be used for all (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below.

Since the first publication of the in 1998, which proposed a classification of angiosperms up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses.

[[Green algae]]
Other algae ([[Biliphyta]])

(2021). 9781449665807, Jones and Bartlett Learning.
p. 489
-like plantsHornwortsHorn-shaped , no vascular system-300+
-like plants, moss plantsMossesPersistent unbranched , no vascular system
Chara-like plantsCharophytes
(Yellow-)green plantsChlorophytes
-like plants, palm-like plantsCycadsSeeds, crown of compound leaves-200
-like plantsGinkgo, maidenhair treeSeeds not protected by fruit (single living species)extant; 50+ extinct
Blue-green plantsGlaucophytes
-like plantsGnetophytesSeeds and woody vascular system with vessels
-like plants
Wolf plants
Clubmosses & spikemosses , vascular systemextant
-like plantsFlowering plants, angiospermsFlowers and fruit, vascular system with vessels
(2021). 9780521660976, Cambridge University Press.

-like plants
Liver plants
LiverwortsEphemeral unbranched , no vascular system
-like plants
Cone-bearing plant
ConifersCones containing seeds and wood composed of tracheidsextant
Rose plantsRed algaeUse as accessory pigments.
Total: 13

Bladder fungusAscomycetes, sac fungiTend to have fruiting bodies (ascocarp). Filamentous, producing hyphae separated by septa. Can reproduce asexually.
Small base fungusBasidiomycetesBracket fungi, toadstools, smuts and rust. Sexual reproduction.
BlastocladiomycotaOffshoot branch fungusBlastoclads
Little cooking pot fungusChytridsPredominantly Aquatic saprotrophic or parasitic. Have a posterior . Tend to be single celled but can also be multicellular.
(2021). 9780128012383
Ball of yarn fungusGlomeromycetes, fungiMainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots.
Small seedsMicrosporans
NeocallimastigomycotaNew beautiful whip fungusNeocallimastigomycetesPredominantly located in digestive tract of herbivorus animals. Anaerobic, terrestrial and aquatic.
Pair fungusZygomycetesMost are saprobes and reproduce sexually and asexually.
Total: 8

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a by the International Society of Protistologists (see Protista, below). Molecular analysis of Zygomycota has found it to be (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish the Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, , and .

Kingdom (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all that are not plants, animals, or fungi. Protista is a taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as and in the Cavalier-Smith system.

Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. The phyla listed here are used for Chromista and Protozoa by the Catalogue of Life, adapted from the system used by the International Society of Protistologists.


Amorphous animalAmoebas Amoeba2400
Two ring
Funnel animal 125
Cilia bearerCiliates Paramecium4500
True eye animal Euglena800
Hole bearersForamsComplex shells with one or more chambersForams10000, 50000 extinct
Groove animal
Small spore
Suckling animal 1555+
Yellow plant Diatoms
Egg fungusOomycetes
Ray animalRadiolarians
Total: 20

The Catalogue of Life includes and in kingdom Plantae, but other systems consider these phyla part of Protista.

Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)
  1. , phenotypically diverse and mostly uncultured
  2. , High-G+C Gram positive species
  3. , only 14 thermophilic genera, deep branching
  4. , formerly candidate division OP5, Caldisericum exile is the sole representative
  5. , only 6 genera
  6. , only 7 genera, green sulphur bacteria
  7. Chloroflexi, green non-sulphur bacteria
  8. , only 3 genera ( Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
  9. , also known as the blue-green algae
  10. Deinococcus-Thermus, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
  11. , formerly candidate division Thermite Group 1
  12. , Low-G+C Gram positive species, such as the spore-formers (aerobic) and (anaerobic)
  13. , formerly clade VadinBE97
  14. , the most known phyla, containing species such as or Pseudomonas aeruginosa
  15. , species include Borrelia burgdorferi, which causes Lyme disease
  16. , alternatively class in phylum (notable genus: )
  17. Thermodesulfobacteria
  18. , deep branching

Currently there are five phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN).
  1. , second most common archaeal phylum
  2. , most common archaeal phylum
  3. , ultra-small symbiotes, single known species

See also


External links

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