Phylum

In biology, a phylum (/ˈfaɪləm/; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany 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.[1][2][3] Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

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 (/ˈfaɪləm/; plural: phyla also known as a subkingdom) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany 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.[1][4][3] Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

General description

The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon (φῦλον, "race, stock"), related to phyle (φυλή, "tribe, clan").[5][6] 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., race] (Phylon). In plant taxonomy, August W. Eichler (1883) classified plants into five groups named divisions, a term that remains in use today for groups of plants, algae and fungi.[1][7] The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.[8]

Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan.[9] 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 phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[10] 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.[10] 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 bearded worms 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 annelids, so the phyla were merged (the bearded worms are now an annelid family).[11] On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla (Orthonectida and Rhombozoa) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes.[12]

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 which groups are placed on a "family tree" without any formal ranking of group size.[10]

Definition based on body plan

A definition of a phylum based on body plan has been proposed by paleontologists Graham Budd 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 "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities.[10] 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.[10] 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.[13]

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.[14]

Known phyla

Animals
Main article: Animal

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[15] 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.[16]

Protostome Bilateria Nephrozoa
Deuterostome
Basal/disputed Non-Bilateria
Vendobionta
Parazoa
Others
PhylumMeaningCommon nameDistinguishing characteristicTaxa described
Annelida Little ring [17]: 306  Segmented worms Multiple circular segments 22,000 + extant
Agmata Fragmented Agmates Calcareous conical shells 5 species, extinct
Archaeocyatha Ancient cups Archaeocyathids An extinct taxon of sponge-grade, reef-building organisms living in warm tropical and subtropical waters during the Early Cambrian. 3 known classes (Extinct)
Arthropoda Jointed foot Arthropods Segmented bodies and jointed limbs, with Chitin exoskeleton 1,250,000+ extant;[15] 20,000+ extinct
Brachiopoda Arm foot[17]: 336  Lampshells[17]: 336  Lophophore and pedicle 300-500 extant; 12,000+ extinct
Bryozoa (Ectoprocta) Moss animals Moss animals, sea mats, ectoprocts[17]: 332  Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia 6,000 extant[15]
Chaetognatha Longhair jaw Arrow worms[17]: 342  Chitinous spines either side of head, fins approx. 100 extant
Chordata With a cord Chordates Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail approx. 55,000+[15]
Cnidaria Stinging nettle Cnidarians Nematocysts (stinging cells) approx. 16,000[15]
Ctenophora Comb bearer Comb jellies[17]: 256  Eight "comb rows" of fused cilia approx. 100-150 extant
Cycliophora Wheel carrying Symbion Circular mouth surrounded by small cilia, sac-like bodies 3+
Echinodermata Spiny skin Echinoderms[17]: 348  Fivefold radial symmetry in living forms, mesodermal calcified spines approx. 7,500 extant;[15] approx. 13,000 extinct
Entoprocta Inside anus[17]: 292  Goblet worms Anus inside ring of cilia approx. 150
Gastrotricha Hairy stomach[17]: 288  Gastrotrich worms Two terminal adhesive tubes approx. 690
Gnathostomulida Jaw orifice Jaw worms[17]: 260  Tiny worms related to rotifers with no body cavity approx. 100
Hemichordata Half cord[17]: 344  Acorn worms, hemichordates Stomochord in collar, pharyngeal slits approx. 130 extant
Kinorhyncha Motion snout Mud dragons Eleven segments, each with a dorsal plate approx. 150
Loricifera Corset bearer Brush heads Umbrella-like scales at each end approx. 122
Micrognathozoa Tiny jaw animals Limnognathia Accordion-like extensible thorax 1
Mollusca Soft[17]: 320  Mollusks / molluscs Muscular foot and mantle round shell 85,000+ extant;[15] 80,000+ extinct[18]
Nematoda Thread like Round worms, thread worms[17]: 274  Round cross section, keratin cuticle 25,000[15]
Nematomorpha Thread form[17]: 276  Horsehair worms, gordian worms[17]: 276  Long, thin parasitic worms closely related to nematodes approx. 320
Nemertea A sea nymph[17]: 270  Ribbon worms, rhynchocoela[17]: 270  Unsegmented worms, with a proboscis housed in a cavity derived from the coelom called the rhynchocoel approx. 1,200
Onychophora Claw bearer Velvet worms[17]: 328  Worm-like animal with legs tipped by chitinous claws approx. 200 extant
Petalonamae Shaped like leaves No An extinct phylum from the Ediacaran. They are bottom-dwelling and immobile, shaped like leaves (frondomorphs), feathers or spindles. 3 classes, extinct
Phoronida Zeus's mistress Horseshoe worms U-shaped gut 11
Placozoa Plate animals Trichoplaxes[17]: 242  Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between 3
Platyhelminthes Flat worm[17]: 262  Flatworms[17]: 262  Flattened worms with no body cavity. Many are parasitic. approx. 29,500[15]
Porifera Pore bearer Sponges[17]: 246  Perforated interior wall, simplest of all known animals 10,800 extant[15]
Priapulida Little Priapus Penis worms Penis-shaped worms approx. 20
Proarticulata Before articulates Proarticulates An extinct group of mattress-like organisms that display "glide symmetry." Found during the Ediacaran. 3 classes, extinct
Rhombozoa (Dicyemida) Lozenge animal Rhombozoans[17]: 264  Single anteroposterior axial celled endoparasites, surrounded by ciliated cells 100+
Rotifera Wheel bearer Rotifers[17]: 282  Anterior crown of cilia approx. 2,000[15]
Saccorhytida Saccus : "pocket" and "wrinkle" Saccorhytus Saccorhytus is only about 1 mm (1.3 mm) in size and is characterized by a spherical or hemispherical body with a prominent mouth. Its body is covered by a thick but flexible cuticle. It has a nodule above its mouth. Around its body are 8 openings in a truncated cone with radial folds. Considered to be a deuterostome[19] or an early ecdysozoan.[20] 1 species, extinct
Tardigrada Slow step Water bears, Moss piglets Microscopic relatives of the arthropods, with a four segmented body and head 1,000
Trilobozoa Three-lobed animal Trilobozoan A taxon of mostly discoidal organisms exhibiting tricentric symmetry. All are Ediacaran-aged 18 genera, extinct
Vetulicolia Ancient dweller Vetulicolian Might possibly be a subphylum of the chordates. Their body consists of two parts: a large front part and covered with a large "mouth" and a hundred round objects on each side that have been interpreted as gills or openings near the pharynx. Their posterior pharynx consists of 7 segments. 15 species, extinct
Xenacoelomorpha Strange hollow form Subphylum Acoelomorpha and xenoturbellida Small, simple animals. Bilaterian, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems[21] 400+
Total: 40 1,525,000[15]

Plants
Main article: Plant

The kingdom Plantae is defined in various ways by different biologists (see Current definitions of Plantae). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta, to form the clade Viridiplantae. The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida,[22] a group containing Viridiplantae and the algal Rhodophyta and Glaucophyta 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 Monilophyta,[23] while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes),[24] or for conifers alone as below.

Since the first publication of the APG system 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.[25]

Land plants Viridiplantae
Green algae
Other algae (Biliphyta)[22]
DivisionMeaningCommon nameDistinguishing characteristicsSpecies described
Anthocerotophyta[26] Anthoceros-like plants Hornworts Horn-shaped sporophytes, no vascular system 100-300+
Bryophyta[26] Bryum-like plants, moss plants Mosses Persistent unbranched sporophytes, no vascular system approx. 12,000
Charophyta Chara-like plants Charophytes approx. 1,000
Chlorophyta (Yellow-)green plants[17]: 200  Chlorophytes approx. 7,000
Cycadophyta[27] Cycas-like plants, palm-like plants Cycads Seeds, crown of compound leaves approx. 100-200
Ginkgophyta[28] Ginkgo-like plants Ginkgo, maidenhair tree Seeds not protected by fruit (single living species) only 1 extant; 50+ extinct
Glaucophyta Blue-green plants Glaucophytes 15
Gnetophyta[29] Gnetum-like plants Gnetophytes Seeds and woody vascular system with vessels approx. 70
Lycopodiophyta,[24]

Lycophyta[30]

 Lycopodium-like plants

Wolf plants

 Clubmosses & spikemosses Microphyll leaves, vascular system 1,290 extant
Magnoliophyta Magnolia-like plants Flowering plants, angiosperms Flowers and fruit, vascular system with vessels 300,000
Marchantiophyta,[31]

Hepatophyta[26]

 Marchantia-like plants

Liver plants

 Liverworts Ephemeral unbranched sporophytes, no vascular system approx. 9,000
Polypodiophyta,

Monilophyta

 Polypodium-like plants
 Ferns Megaphyll leaves, vascular system approx. 10,560
Pinophyta,[24]

Coniferophyta[32]

 Pinus-like plants

Cone-bearing plant

 Conifers Cones containing seeds and wood composed of tracheids 629 extant
Rhodophyta Rose plants Red algae Use phycobiliproteins as accessory pigments. approx. 7,000
Total: 14

Fungi
Main article: Fungi
DivisionMeaningCommon nameDistinguishing characteristicsSpecies described
Ascomycota Bladder fungus[17]: 396  Ascomycetes,[17]: 396  sac fungi Tend to have fruiting bodies (ascocarp).[33] Filamentous, producing hyphae separated by septa. Can reproduce asexually.[34] 30,000
Basidiomycota Small base fungus[17]: 402  Basidiomycetes,[17]: 402  club fungi Bracket fungi, toadstools, smuts and rust. Sexual reproduction.[35] 31,515
Blastocladiomycota Offshoot branch fungus[36] Blastoclads Less than 200
Chytridiomycota Little cooking pot fungus[37] Chytrids Predominantly Aquatic saprotrophic or parasitic. Have a posterior flagellum. Tend to be single celled but can also be multicellular.[38][39][40] 1000+
Glomeromycota Ball of yarn fungus[17]: 394  Glomeromycetes, AM fungi[17]: 394  Mainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots.[35] 284
Microsporidia Small seeds[41] Microsporans[17]: 390  1400
Neocallimastigomycota New beautiful whip fungus[42] Neocallimastigomycetes Predominantly located in digestive tract of herbivorous animals. Anaerobic, terrestrial and aquatic.[43] approx. 20 [44]
Zygomycota Pair fungus[17]: 392  Zygomycetes[17]: 392  Most are saprobes and reproduce sexually and asexually.[43] aprox. 1060
Total: 8

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[45] and it is considered a protozoan by the International Society of Protistologists[46] (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor),[47] 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 incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.[45]

Protista

Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi.[17]: 120  Protista is a polyphyletic taxon,[48] 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 Protozoa and Chromista in the Cavalier-Smith system.[49]

Protist taxonomy has long been unstable,[50] 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,[51] adapted from the system used by the International Society of Protistologists.[46]

Harosa
Protozoa
Phylum/DivisionMeaningCommon nameDistinguishing characteristicsExampleSpecies described
Amoebozoa Amorphous animal Amoebas Presence of pseudopodia Amoeba 2400
Bigyra Two rings
Cercozoa
Choanozoa Funnel animal Presence of a colar of microvilli surrounding a flagellum 125
Ciliophora Cilia bearer Ciliates Presence of multiple cilia and a cytostome Paramecium 4500
Cryptista Hidden
Euglenozoa True eye animal Euglena 800
Foraminifera Hole bearers Forams Complex shells with one or more chambers Forams 10000, 50000 extinct
Haptophyta
Loukozoa Groove animal
Metamonada Middle single-celled organisms Giardia
Microsporidia Small spore
Myzozoa Suckling animal 1555+
Ochrophyta Yellow plant Diatoms
Oomycota Egg fungus[17]: 184  Oomycetes
Percolozoa
Radiozoa Ray animal Radiolarians
Sarcomastigophora Flesh and whip bearer
Sulcozoa
Total: 19

The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae,[51] but other systems consider these phyla part of Protista.[52]

Bacteria

Currently there are 40 bacterial phyla (not including "Cyanobacteria") that have been validly published according to the Bacteriological Code[53]

  1. Acidobacteriota, phenotypically diverse and mostly uncultured
  2. Actinomycetota, High-G+C Gram positive species
  3. Aquificota, deep-branching
  4. Armatimonadota
  5. Atribacterota
  6. Bacillota, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
  7. Bacteroidota
  8. Balneolota
  9. Bdellovibrionota
  10. Caldisericota, formerly candidate division OP5, Caldisericum exile is the sole representative
  11. Calditrichota
  12. Campylobacterota
  13. Chlamydiota
  14. Chlorobiota, green sulphur bacteria
  15. Chloroflexota, green non-sulphur bacteria
  16. Chrysiogenota, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
  17. Coprothermobacterota
  18. Deferribacterota
  19. Deinococcota, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
  20. Dictyoglomota
  21. Elusimicrobiota, formerly candidate division Thermite Group 1
  22. Fibrobacterota
  23. Fusobacteriota
  24. Gemmatimonadota
  25. Ignavibacteriota
  26. Kiritimatiellota
  27. Lentisphaerota, formerly clade VadinBE97
  28. Mycoplasmatota, notable genus: Mycoplasma
  29. Myxococcota
  30. Nitrospinota
  31. Nitrospirota
  32. Planctomycetota
  33. Pseudomonadota, the most well-known phylum, containing species such as Escherichia coli or Pseudomonas aeruginosa
  34. Rhodothermota
  35. Spirochaetota, species include Borrelia burgdorferi, which causes Lyme disease
  36. Synergistota
  37. Thermodesulfobacteriota
  38. Thermomicrobiota
  39. Thermotogota, deep-branching
  40. Verrucomicrobiota

Archaea
Main article: Archaea

Currently there are 2 phyla that have been validly published according to the Bacteriological Code[53]

  1. Nitrososphaerota
  2. Thermoproteota, second most common archaeal phylum

Other phyla that have been proposed, but not validly named, include:

  1. "Euryarchaeota", most common archaeal phylum
  2. "Korarchaeota"
  3. "Nanoarchaeota", ultra-small symbiotes, single known species

See also

References
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  2. "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. Retrieved 4 October 2008. Phyla in the plant kingdom are frequently called divisions.
  3. Berg, Linda R. (2 March 2007). Introductory Botany: Plants, People, and the Environment (2 ed.). Cengage Learning. p. 15. ISBN 9780534466695. Retrieved 23 July 2012.
  4. "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. Retrieved 4 October 2008. Phyla in the plant kingdom are frequently called divisions.
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