Phylum

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. 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 among phyla within 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"). 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"): "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., stock] (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. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.

Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan. 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). 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.^{[citation needed]}

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 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). 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.

This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades 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 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. 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. 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

Animals

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.

Protostome	Bilateria	Nephrozoa
Deuterostome
Basal/disputed		Non-Bilateria
Vendobionta
Parazoa
Others

Phylum	Meaning	Common name	Distinguishing characteristic	Taxa described
Acanthocephala	Thorn head	Acanthocephalans, thorny-headed worms, spiny-headed worms	Worm with a proboscis armed with hooks	1420
Annelida	Little ring ^: 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; 20,000+ extinct
Brachiopoda	Arm foot^: 336	Lampshells^: 336	Lophophore and pedicle	300–500 extant; 12,000+ extinct
Bryozoa (Ectoprocta)	Moss animals	Moss animals, sea mats, ectoprocts^: 332	Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia	6,000 extant
Chaetognatha	Longhair jaw	Arrow worms^: 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+
Cnidaria	Stinging nettle	Cnidarians	Nematocysts (stinging cells)	approx. 16,000
Ctenophora	Comb bearer	Comb jellies^: 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^: 348	Fivefold radial symmetry in living forms, mesodermal calcified spines	approx. 7,500 extant; approx. 13,000 extinct
Entoprocta	Inside anus^: 292	Goblet worms	Anus inside ring of cilia	approx. 150
Gastrotricha	Hairy stomach^: 288	Gastrotrich worms	Two terminal adhesive tubes	approx. 690
Gnathostomulida	Jaw orifice	Jaw worms^: 260	Tiny worms related to rotifers with no body cavity	approx. 100
Hemichordata	Half cord^: 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	Armour bearer	Brush heads	Umbrella-like scales at each end	approx. 122
Micrognathozoa	Tiny jaw animals	None	Accordion-like extensible thorax	1
Mollusca	Soft^: 320	Mollusks / molluscs	Muscular foot and mantle round shell	85,000+ extant; 80,000+ extinct
Monoblastozoa (Nomen inquirendum)		None	distinct anterior/posterior parts and being densely ciliated, especially around the "mouth" and "anus".	1
Nematoda	Thread like	Round worms, thread worms^: 274	Round cross section, keratin cuticle	25,000
Nematomorpha	Thread form^: 276	Horsehair worms, Gordian worms^: 276	Long, thin parasitic worms closely related to nematodes	approx. 320
Nemertea	A sea nymph^: 270	Ribbon worms, rhynchocoela^: 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^: 328	Worm-like animal with legs tipped by chitinous claws	approx. 200 extant
Orthonectida		Orthonectid	Parasitic, microscopic, simple, wormlike organisms	20
Petalonamae	Shaped like leaves	None	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^: 242	Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between	4+
Platyhelminthes	Flat worm^: 262	Flatworms^: 262	Flattened worms with no body cavity. Many are parasitic.	approx. 29,500
Porifera	Pore bearer	Sponges^: 246	Perforated interior wall, simplest of all known animals	10,800 extant
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^: 264	Single anteroposterior axial celled endoparasites, surrounded by ciliated cells	100+
Rotifera	Wheel bearer	Rotifers^: 282	Anterior crown of cilia	approx. 2,000
Saccorhytida	Saccus : "pocket" and "wrinkle"	None	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 or an early ecdysozoan.	2 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	400+
Total: 40				1,525,000

Plants

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, 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, 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), 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.

	Land plants	Viridiplantae
	Green algae	Viridiplantae
	Other algae (Biliphyta)

Division	Meaning	Common name	Distinguishing characteristics	Species described
Anthocerotophyta	Anthoceros-like plants	Hornworts	Horn-shaped sporophytes, no vascular system	100-300+
Bryophyta	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^: 200	Chlorophytes		approx. 7,000
Cycadophyta	Cycas-like plants, palm-like plants	Cycads	Seeds, crown of compound leaves	approx. 100-200
Ginkgophyta	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	Gnetum-like plants	Gnetophytes	Seeds and woody vascular system with vessels	approx. 70
Lycopodiophyta, Lycophyta	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, Hepatophyta	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
Picozoa	Extremely small animals	Picozoans, picobiliphytes, biliphytes		1
Pinophyta, Coniferophyta	Pinus-like plants Cone-bearing plant	Conifers	Cones containing seeds and wood composed of tracheids	629 extant
Prasinodermophyta	Prasinoderma-like plants	Picozoans, picobiliphytes, biliphytes		8
Rhodophyta	Rose plants	Red algae	Use phycobiliproteins as accessory pigments.	approx. 7,000
Total: 14

Fungi

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

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (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 incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.

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.^: 120 Protista is a paraphyletic 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 Protozoa and Chromista in the Cavalier-Smith system.

Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by the Catalogue of Life, and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith. Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP.

	Stramenopiles	"Chromista"
	Alveolata
	Rhizaria
	"Hacrobia"
	"Sarcomastigota"	"Protozoa"
	"Excavata"	"Protozoa"
	Orphan groups

Phylum	Meaning	Common name	Distinguishing characteristics	Species described
Amoebozoa	Amorphous animals	Amoebozoans	Presence of pseudopodia for amoeboid movement, tubular cristae.	approx. 2,400
Apicomplexa	Apical infolds	Apicomplexans, sporozoans	Mostly parasitic, at least one stage of the life cycle with flattened subpellicular vesicles and a complete apical complex, non-photosynthetic apicoplast.	over 6,000
Apusozoa (paraphyletic)	Apusomonas-like animals		Gliding biciliates with two or three connectors between centrioles	32
Bigyra	Two rings		Stramenopiles with a double helix in ciliary transition zone
Cercozoa	Flagellated animal	Cercozoans	Defined by molecular phylogeny, lacking distinctive morphological or behavioural characters.
Chromerida	Chromera-like organisms	Chrompodellids, chromerids, colpodellids	Biflagellates, chloroplasts with four membranes, incomplete apical complex, cortical alveoli, tubular cristae.	8
Choanozoa (paraphyletic)	Funnel animals	Opisthokont protists	Filose pseudopods; some with a colar of microvilli surrounding a flagellum	approx. 300
Ciliophora	Cilia bearers	Ciliates	Presence of multiple cilia and a cytostome.	approx. 4,500
Cryptista	Hidden		Defined by molecular phylogeny, flat cristae.	246
Dinoflagellata	Whirling flagellates	Dinoflagellates	Biflagellates with a transverse ribbon-like flagellum with multiple waves beating to the cell’s left and a longitudinal flagellum beating posteriorly with only one or few waves.	2,957 extant 955 fossil
Endomyxa	Within mucus		Defined by molecular phylogeny, typically plasmodial endoparasites of other eukaryotes.
Eolouka (paraphyletic)	Early groove		Heterotrophic biflagellates with ventral feeding groove.	23
Euglenozoa	True eye animals		Biflagellates, one of the two cilia inserted into an apical or subapical pocket, unique ciliary configuration.	2,037 extant 20 fossil
Ochrophyta, Heterokontophyta	Ochre plants, heterokont plants	Heterokont algae, stramenochromes, ochrophytes, heterokontophytes	Biflagellates with tripartite mastigonemes, chloroplasts with four membranes and chlorophylls a and c, tubular cristae.	21,052 extant 2,262 fossil
Haptista	Fasten		Thin microtubule-based appendages for feeding (haptonema in haptophytes, axopodia in centrohelids), complex mineralized scales.	517 extant 1,205 fossil
Hemimastigophora	Incomplete or atypical flagellates	Hemimastigotes	Ellipsoid or vermiform phagotrophs, two slightly spiraling rows of around 12 cilia each, thecal plates below the membrane supported by microtubules and rotationally symmetrical, tubular and saccular cristae.	10
Malawimonada	Malawimonas-like organisms	Malawimonads	Small free-living bicilates with two kinetosomes, one or two vanes in posterior cilium.	3
Metamonada	Middle monads	Metamonads	Anaerobic or microaerophilic, some without mitochondria; four kinetosomes per kinetid
Opisthosporidia (often considered fungi)	Opisthokont spores		Parasites with chitinous spores and extrusive host-invasion apparatus
Percolozoa	Percolomonas-like animals		Complex life cycle containing amoebae, flagellates and cysts.
Perkinsozoa	Perkinsus-like animals	Perkinsozoans, perkinsids	Parasitic biflagellates, incomplete apical complex, formation of zoosporangia or undifferentiated cells via a hypha-like tube.	26
Provora	Devouring voracious protists		Defined by molecular phylogeny, free-living eukaryovorous heterotrophic biflagellates with ventral groove and extrusomes.	7
Pseudofungi	False fungi		Defined by molecular phylogeny, phagotrophic heterokonts with a helical ciliary transition zone.	over 1,200
Retaria	Reticulopodia-bearing organisms		Feeding by reticulopodia (or axopodia) typically projected through various types of skeleton, closed mitosis.	10,000 extant 50,000 fossil
Sulcozoa (paraphyletic)	Groove-bearing animals		Aerobic flagellates (none, 1, 2 or 4 flagella) with dorsal semi-rigid pellicle of one or two submembrane dense layers, ventral feeding groove, branching ventral pseudopodia, typically filose.	40+
Telonemia	Telonema-like organisms	Telonemids	Phagotrophic pyriform biflagellates with a unique complex cytoskeleton, tubular cristae, tripartite mastigonemes, cortical alveoli.	7
Total: 26, but see below.

The number of protist phyla varies greatly from one classification to the next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista, and all alveolates except ciliates in one phylum Myzozoa, later lowered in rank and included in a paraphyletic phylum Miozoa. Even within a phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta. These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete.

Bacteria

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

Abditibacteriota
Acidobacteriota, phenotypically diverse and mostly uncultured
Actinomycetota, High-G+C Gram positive species
Aquificota, deep-branching
Armatimonadota
Atribacterota
Bacillota, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
Bacteroidota
Balneolota
Bdellovibrionota
Caldisericota, formerly candidate division OP5, Caldisericum exile is the sole representative
Calditrichota
Campylobacterota
Chlamydiota
Chlorobiota, green sulphur bacteria
Chloroflexota, green non-sulphur bacteria
Chrysiogenota, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
Coprothermobacterota
Deferribacterota
Deinococcota, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
Dictyoglomota
Elusimicrobiota, formerly candidate division Thermite Group 1
Fibrobacterota
Fusobacteriota
Gemmatimonadota
Ignavibacteriota
Kiritimatiellota
Lentisphaerota, formerly clade VadinBE97
Mycoplasmatota, notable genus: Mycoplasma
Myxococcota
Nitrospinota
Nitrospirota
Planctomycetota
Pseudomonadota, the most well-known phylum, containing species such as Escherichia coli or Pseudomonas aeruginosa
Rhodothermota
Spirochaetota, species include Borrelia burgdorferi, which causes Lyme disease
Synergistota
Thermodesulfobacteriota
Thermomicrobiota
Thermotogota, deep-branching
Verrucomicrobiota

Archaea

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

Nitrososphaerota
Thermoproteota, second most common archaeal phylum

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

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

Notes