In biology, a phylum (; : 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 among phyla within larger clades like Ecdysozoa and Embryophyta.
The term phylum was coined in 1866 by Ernst Haeckel from the Greek ("race, stock"), related to ("tribe, clan").[4] 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] German: Stamm [i.e., stock] (German: 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] [5] The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four French: embranchements of Georges Cuvier.[6]
Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan.[7] 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).[8] 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.
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.[8] 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).[9] 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.[10]
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.[8]
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.[8] 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.[8] 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.[11]
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.[12]
See main article: Animal.
Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[13] 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.[14]
| Nephrozoa | |||
| Deuterostome | |||
| Basal/disputed | Non-Bilateria | ||
| Vendobionta | |||
| Parazoa | |||
| Others | |||
| Phylum | Meaning | Common name | Distinguishing characteristic | Taxa described | |
|---|---|---|---|---|---|
| Annelida | Little ring [15] | Segmented worms | Multiple circular segments | + 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 | + extant; 20,000+ extinct | |
| Brachiopoda | Arm foot | Lampshells | Lophophore and pedicle | –500 extant; 12,000+ extinct | |
| Bryozoa (Ectoprocta) | Moss animals | Moss animals, sea mats, ectoprocts | Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia | extant | |
| Chaetognatha | Longhair jaw | Arrow worms | Chitinous spines either side of head, fins | extant | |
| Chordata | With a cord | Chordates | Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail | + | |
| Cnidaria | Stinging nettle | Cnidarians | Nematocysts (stinging cells) | ||
| Ctenophora | Comb bearer | Comb jellies | Eight "comb rows" of fused cilia | –150 extant | |
| Cycliophora | Wheel carrying | Symbion | Circular mouth surrounded by small cilia, sac-like bodies | + | |
| Echinodermata | Spiny skin | Echinoderms | Fivefold radial symmetry in living forms, mesodermal calcified spines | extant; approx. 13,000 extinct | |
| Entoprocta | Inside anus | Goblet worms | Anus inside ring of cilia | ||
| Gastrotricha | Hairy stomach | Gastrotrich worms | Two terminal adhesive tubes | ||
| Gnathostomulida | Jaw orifice | Jaw worms | Tiny worms related to rotifers with no body cavity | ||
| Hemichordata | Half cord | Acorn worms, hemichordates | Stomochord in collar, pharyngeal slits | extant | |
| Kinorhyncha | Motion snout | Mud dragons | Eleven segments, each with a dorsal plate | ||
| Loricifera | Armour bearer | Brush heads | Umbrella-like scales at each end | ||
| Micrognathozoa | Tiny jaw animals | None | Accordion-like extensible thorax | ||
| Mollusca | Soft | Mollusks / molluscs | Muscular foot and mantle round shell | + extant; 80,000+ extinct[16] | |
| 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 | Round cross section, keratin cuticle | ||
| Nematomorpha | Thread form | Horsehair worms, Gordian worms | Long, thin parasitic worms closely related to nematodes | ||
| Nemertea | A sea nymph | Ribbon worms, rhynchocoela | Unsegmented worms, with a proboscis housed in a cavity derived from the coelom called the rhynchocoel | ||
| Onychophora | Claw bearer | Velvet worms | Worm-like animal with legs tipped by chitinous claws | 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 | ||
| Placozoa | Plate animals | Trichoplaxes | Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between | + | |
| Platyhelminthes | Flat worm | Flatworms | Flattened worms with no body cavity. Many are parasitic. | ||
| Porifera | Pore bearer | Sponges | Perforated interior wall, simplest of all known animals | extant | |
| Priapulida | Little Priapus | Penis worms | Penis-shaped worms | ||
| 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 | Single anteroposterior axial celled endoparasites, surrounded by ciliated cells | + | |
| Rotifera | Wheel bearer | Rotifers | Anterior crown of cilia | ||
| 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[17] or an early ecdysozoan.[18] | 2 species, extinct | |
| Tardigrada | Slow step | Water bears, moss piglets | Microscopic relatives of the arthropods, with a four segmented body and head | ||
| 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[19] | + | |
| Total: 40 | 1,525,000 |
See 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,[20] 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),[21] 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.
| Viridiplantae | |||
| Green algae | |||
| Other algae (Biliphyta) | |||
| Division | Meaning | Common name | Distinguishing characteristics | Species described |
|---|---|---|---|---|
| Anthocerotophyta[22] | Anthoceros-like plants | Hornworts | Horn-shaped sporophytes, no vascular system | -300+ |
| Bryophyta | Bryum-like plants, moss plants | Mosses | Persistent unbranched sporophytes, no vascular system | |
| Charophyta | Chara-like plants | Charophytes | ||
| Chlorophyta | (Yellow-)green plants | Chlorophytes | ||
| Cycadophyta | Cycas-like plants, palm-like plants | Cycads | Seeds, crown of compound leaves | -200 |
| Ginkgophyta | Ginkgo-like plants | Ginkgo, maidenhair tree | Seeds not protected by fruit (single living species) | extant; 50+ extinct |
| Glaucophyta | Blue-green plants | Glaucophytes | ||
| Gnetophyta | Gnetum-like plants | Gnetophytes | Seeds and woody vascular system with vessels | |
| Lycopodiophyta, Lycophyta | Lycopodium-like plants Wolf plants | Clubmosses & spikemosses | Microphyll leaves, vascular system | extant |
| Magnoliophyta | Magnolia-like plants | Flowering plants, angiosperms | Flowers and fruit, vascular system with vessels | |
| Marchantiophyta,[23] Hepatophyta | Marchantia-like plants Liver plants | Liverworts | Ephemeral unbranched sporophytes, no vascular system | |
| Polypodiophyta,Monilophyta | Polypodium-like plants | Ferns | Megaphyll leaves, vascular system | |
| 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 | extant |
| Prasinodermophyta | Prasinoderma-like plants | Picozoans, picobiliphytes, biliphytes | 8 | |
| Rhodophyta | Rose plants | Red algae | Use phycobiliproteins as accessory pigments. | |
| Total: 14 |
See main article: Fungi.
| Division | Meaning | Common name | Distinguishing characteristics | Species described |
|---|---|---|---|---|
| Ascomycota | Bladder fungus | Ascomycetes, sac fungi | Tend to have fruiting bodies (ascocarp).[24] Filamentous, producing hyphae separated by septa. Can reproduce asexually.[25] | 30,000 |
| Basidiomycota | Small base fungus | Basidiomycetes, club fungi | Bracket fungi, toadstools, smuts and rust. Sexual reproduction.[26] | 31,515 |
| Blastocladiomycota | Offshoot branch fungus[27] | Blastoclads | Less than 200 | |
| Chytridiomycota | Little cooking pot fungus[28] | Chytrids | Predominantly Aquatic saprotrophic or parasitic. Have a posterior flagellum. Tend to be single celled but can also be multicellular.[29] [30] [31] | 1000+ |
| Glomeromycota | Ball of yarn fungus | Glomeromycetes, fungi | Mainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots. | 284 |
| Microsporidia | Small seeds[32] | Microsporans | 1400 | |
| Neocallimastigomycota | New beautiful whip fungus[33] | Neocallimastigomycetes | Predominantly located in digestive tract of herbivorous animals. Anaerobic, terrestrial and aquatic.[34] | approx. 20 [35] |
| Zygomycota | Pair fungus | Zygomycetes | 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,[36] 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),[37] 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.
See main article: Protista taxonomy.
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. Protista is a paraphyletic taxon,[38] 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.[39]
Protist taxonomy has long been unstable,[40] with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by the Catalogue of Life,[41] and correspond to the Protozoa-Chromista scheme,[42] with updates from the latest (2022) publication by Cavalier-Smith.[43] 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.[44]
| Stramenopiles | "Chromista" | ||
| Alveolata | |||
| Rhizaria | |||
| "Hacrobia" | |||
| "Sarcomastigota" | "Protozoa" | ||
| "Excavata" | |||
| Orphan groups | |||
| Phylum | Meaning | Common name | Distinguishing characteristics | Species described | Image | |
|---|---|---|---|---|---|---|
| Amoebozoa | Amorphous animals | Amoebozoans | Presence of pseudopodia for amoeboid movement, tubular cristae. | [45] | ||
| 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. | [46] | ||
| Apusozoa | 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 | Funnel animals | Opisthokont protists | Filose pseudopods
| |||
| Ciliophora | Cilia bearers | Ciliates | Presence of multiple cilia and a cytostome. | [47] | ||
| Cryptista | Hidden | Defined by molecular phylogeny, flat cristae. | ||||
| 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. | extant955 fossil | ||
| Endomyxa | Within mucus | Defined by molecular phylogeny, typically plasmodial endoparasites of other eukaryotes. | ||||
| Eolouka | Early groove[48] | Heterotrophic biflagellates with ventral feeding groove. | ||||
| Euglenozoa | True eye animals | Biflagellates, one of the two cilia inserted into an apical or subapical pocket, unique ciliary configuration. | extant20 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. | extant2,262 fossil | ||
| Haptista | Fasten | Thin microtubule-based appendages for feeding (haptonema in haptophytes, axopodia in centrohelids), complex mineralized scales. | extant1,205 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. | [49] | ||
| Malawimonada | Malawimonas-like organisms | Malawimonads | Small free-living bicilates with two kinetosomes, one or two vanes in posterior cilium. | [50] | ||
| Metamonada | Middle monads | Metamonads | Anaerobic or microaerophilic, some without mitochondria; four kinetosomes per kinetid | |||
| Opisthosporidia | Opisthokont spores[51] | 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. | |||
| Provora | Devouring voracious protists | Defined by molecular phylogeny, free-living eukaryovorous heterotrophic biflagellates with ventral groove and extrusomes. | ||||
| Pseudofungi | False fungi | Defined by molecular phylogeny, phagotrophic heterokonts with a helical ciliary transition zone. | [52] | |||
| Retaria | Reticulopodia-bearing organisms | Feeding by reticulopodia (or axopodia) typically projected through various types of skeleton, closed mitosis. | extant50,000 fossil | |||
| Sulcozoa | 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. | + | |||
| Telonemia | Telonema-like organisms | Telonemids | Phagotrophic pyriform biflagellates with a unique complex cytoskeleton, tubular cristae, tripartite mastigonemes, cortical alveoli.[53] [54] | |||
| 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.[55] 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.[43] 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.[44]
See main article: Bacterial phyla.
Currently there are 40 bacterial phyla (not including "Cyanobacteria") that have been validly published according to the Bacteriological Code[56]
See main article: Archaea. Currently there are 2 phyla that have been validly published according to the Bacteriological Code
Other phyla that have been proposed, but not validly named, include: