The term phylum was coined in 1866 byErnst Haeckel from the Greekphylon (φῦλον, "race, stock"), related tophyle (φυλή, "tribe, clan").[4][5] 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 / tribe] (Phylon)."[a] Inplant taxonomy,August W. Eichler (1883) classified plants intofive groups named divisions, a term that remains in use today for groups of plants, algae and fungi.[1][6]The definitions of zoological phyla have changed from their origins in the sixLinnaean classes and the fourembranchements ofGeorges Cuvier.[7]
Informally, phyla can be thought of as groupings of organisms based on general specialization ofbody plan.[8] 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 (thephenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (thephylogenetic definition).[9] Attempting to define a level of theLinnean 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]
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.[9] 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, thebearded 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 ofannelids, so the phyla were merged (the bearded worms are now an annelidfamily).[10] On the other hand, the highly parasitic phylumMesozoa was divided into two phyla (Orthonectida andRhombozoa) when it was discovered the Orthonectida are probablydeuterostomes and the Rhombozoaprotostomes.[11]
This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa inclades without any formal ranking of group size.[9]
A definition of a phylum based on body plan has been proposed bypaleontologistsGraham Budd andSören 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.[9] However, proving that a fossil belongs to thecrown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group.[9] 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.[12]
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.[13]
Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[14] 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.[15]
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[18] or an earlyecdysozoan.[19]
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.
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,[22] while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for allgymnosperms (i.e. including cycads, ginkgos and gnetophytes),[23] or for conifers alone as below.
Since the first publication of theAPG system in 1998, which proposed a classification ofangiosperms up to the level oforders, 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.[24]
Most are saprobes and reproduce sexually and asexually.[42]
approx. 1060
Total: 8
Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[44] and it is considered aprotozoan by the International Society of Protistologists[45] (seeProtista, below). Molecular analysis of Zygomycota has found it to bepolyphyletic (its members do not share an immediate ancestor),[46] 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 subphylaincertae sedis (of uncertain placement):Entomophthoromycotina,Kickxellomycotina,Mucoromycotina, andZoopagomycotina.[44]
KingdomProtista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing alleukaryotes that are not plants, animals, or fungi.[16]: 120 Protista is aparaphyletic taxon,[47] which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such asProtozoa andChromista in theCavalier-Smith system.[48]
Protist taxonomy has long been unstable,[49] with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by theCatalogue of Life,[50] and correspond to the Protozoa-Chromista scheme,[45] with updates from the latest (2022) publication byCavalier-Smith.[51] 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.[52]
Mostly parasitic, at least one stage of the life cycle with flattened subpellicular vesicles and a complete apical complex, non-photosyntheticapicoplast.[52]
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.[52]
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.[52][60]
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.[59]
Phagotrophic pyriform biflagellates with a unique complex cytoskeleton, tubular cristae, tripartite mastigonemes, cortical alveoli.[69][70]
7
Total: 26, but see below.
The number of protist phyla varies greatly from one classification to the next. The Catalogue of Life includesRhodophyta andGlaucophyta in kingdom Plantae,[50] but other systems consider these phyla part of Protista.[71] In addition, less popular classification schemes uniteOchrophyta andPseudofungi under one phylum,Gyrista, and all alveolates exceptciliates in one phylumMyzozoa, later lowered in rank and included in a paraphyletic phylumMiozoa.[51] Even within a phylum, other phylum-level ranks appear, such as the case ofBacillariophyta (diatoms) withinOchrophyta. These differences became irrelevant after the adoption of acladistic 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.[52]
^"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)."
^"Life sciences".The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. Retrieved4 October 2008.Phyla in the plant kingdom are frequently called divisions.
^Haeckel, Ernst (1866).Generelle Morphologie der Organismen [The General Morphology of Organisms] (in German). Vol. 1. Berlin, (Germany): G. Reimer. pp. 28–29.
^Valentine, James W. (2004).On the Origin of Phyla. Chicago: University of Chicago Press. p. 7.ISBN978-0-226-84548-7.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.
^Budd, G. E. (September 1998). "Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle".Lethaia.31 (3):197–210.doi:10.1111/j.1502-3931.1998.tb00508.x.
^abCronquist, A.; A. Takhtajan; W. Zimmermann (April 1966). "On the higher taxa of Embryobionta".Taxon.15 (4):129–134.doi:10.2307/1217531.JSTOR1217531.
^Chase, Mark W. & Reveal, James L. (October 2009), "A phylogenetic classification of the land plants to accompany APG III",Botanical Journal of the Linnean Society,161 (2):122–127,doi:10.1111/j.1095-8339.2009.01002.x
^abcMauseth, James D. (2012).Botany : An Introduction to Plant Biology (5th ed.). Sudbury, MA: Jones and Bartlett Learning.ISBN978-1-4496-6580-7. p. 489
^Crandall-Stotler, Barbara; Stotler, Raymond E. (2000). "Morphology and classification of the Marchantiophyta". In A. Jonathan Shaw; Bernard Goffinet (eds.).Bryophyte Biology. Cambridge: Cambridge University Press. p. 21.ISBN978-0-521-66097-6.
^Wyatt, T.; Wösten, H.; Dijksterhuis, J. (2013). "Advances in Applied Microbiology Chapter 2 - Fungal Spores for Dispersion in Space and Time".Advances in Applied Microbiology.85:43–91.doi:10.1016/B978-0-12-407672-3.00002-2.PMID23942148.
^White, Merlin M.; James, Timothy Y.; O'Donnell, Kerry; et al. (November–December 2006). "Phylogeny of the Zygomycota Based on Nuclear Ribosomal Sequence Data".Mycologia.98 (6):872–884.doi:10.1080/15572536.2006.11832617.PMID17486964.S2CID218589354.
^abcdefghijklmnopAdl SM, Bass D, Lane CE, Lukeš J, Schoch CL, Smirnov A, Agatha S, Berney C, Brown MW, Burki F, Cárdenas P, Čepička I, Chistyakova L, del Campo J, Dunthorn M, Edvardsen B, Eglit Y, Guillou L, Hampl V, Heiss AA, Hoppenrath M, James TY, Karnkowska A, Karpov S, Kim E, Kolisko M, Kudryavtsev A, Lahr DJG, Lara E, Le Gall L, Lynn DH, Mann DG, Massana R, Mitchell EAD, Morrow C, Park JS, Pawlowski JW, Powell MJ, Richter DJ, Rueckert S, Shadwick L, Shimano S, Spiegel FW, Torruella G, Youssef N, Zlatogursky V, Zhang Q (2019)."Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes".Journal of Eukaryotic Microbiology.66 (1):4–119.doi:10.1111/jeu.12691.PMC6492006.PMID30257078.
^abcdCavalier-Smith T (2013). "Early evolution of eukaryote feeding modes, cell structural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa".European Journal of Protistology.49 (2):115–178.doi:10.1016/j.ejop.2012.06.001.PMID23085100.
^Shɨshkin, Yegor (2022). "Spironematella terricola comb. n. andSpironematella goodeyi comb. n. (Hemimastigida = Hemimastigea = Hemimastigophora) forSpironema terricola andSpironema goodeyi with diagnoses of the genus and family Spironematellidae amended".Zootaxa.5128 (2):295–297.doi:10.11646/zootaxa.5128.2.8.PMID36101172.S2CID252220401.
^Heiss AA, Warring SD, Lukacs K, Favate J, Yang A, Gyaltshen Y, Filardi C, Simpson AGB, Kim E (December 2020). "Description of Imasa heleensis, gen. nov., sp. nov. (Imasidae, fam. nov.), a Deep-Branching Marine Malawimonad and Possible Key Taxon in Understanding Early Eukaryotic Evolution".Journal of Eukaryotic Microbiology.68: e12837.doi:10.1111/jeu.12837.
^abcDenis V. Tikhonenkov, Kirill V. Mikhailov, Ryan M. R. Gawryluk, Artem O. Belyaev, Varsha Mathur, Sergey A. Karpov, Dmitry G. Zagumyonnyi, Anastasia S. Borodina, Kristina I. Prokina, Alexander P. Mylnikov, Vladimir V. Aleoshin & Patrick J. Keeling (7 December 2022). "Microbial predators form a new supergroup of eukaryotes".Nature.612:714–719.doi:10.1038/S41586-022-05511-5.ISSN1476-4687.WikidataQ115933632.{{cite journal}}: CS1 maint: multiple names: authors list (link)
.jpg)
