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In biology, a phylum (; 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. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, 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.
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.
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.
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 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.
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 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.
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 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.
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.
The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista.
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.
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]]
[[Deuterostome]]
Basal/disputed
Others
Acanthocephala Thorny head Thorny-headed worms Reversible spiny proboscis that bears many rows of hooked spines Annelida Little ring :306 Segmented worms Multiple circular segment + extant Arthropoda Jointed foot Insects, crustaceans Segmented bodies and jointed limbs, with Chitin exoskeleton + extant; 20,000+ extinct Brachiopoda Arm foot Lampshells Lophophore and pedicle Bryozoa Moss animals Moss animals, sea mats, ectoprocts Lophophore, no pedicle, , anus outside ring of cilia extant Chaetognatha Longhair jaw Arrow worms spines either side of head, fins extant Chordata With a cord Chordates Hollow dorsal nerve cord, notochord, , endostyle, post-anus tail + Cnidaria Stinging nettle Cnidarians Nematocysts (stinging cells) Ctenophora Comb bearer Comb jellies Eight "comb rows" of fused cilia 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 Hemichordata Half cord Acorn worms, hemichordates Stomochord in collar, extant Kinorhyncha Motion snout Mud dragons Eleven segments, each with a dorsal plate Loricifera Corset bearer Brush heads Umbrella-like scales at each end Limnognathia Tiny jaw animals Limnognathia Accordion-like extensible thorax Mollusca Soft Mollusks / molluscs Muscular foot and mantle round shell + extant; 80,000+ extinctFeldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725) Nematoda Thread like Round worms, thread worms Round cross section, keratin cuticle Nematomorpha Thread form Horsehair worms, gordian worms Nemertea A sea nymph Ribbon worms, rhynchocoela Onychophora Claw bearer Velvet worms Legs tipped by chitinous claws extant Orthonectida Straight swimming Orthonectids Single layer of ciliated cells surrounding a mass of sex cells 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 Porifera Pore bearer Sponges Perforated interior wall extant Priapulida Little Priapus Penis worms Rhombozoa Lozenge animal Rhombozoans Single anteroposterior axial cell surrounded by ciliated cells + Rotifera Wheel bearer Rotifers Anterior crown of cilia Sipuncula Small tube Peanut worms Mouth surrounded by invertible tentacles Tardigrada Slow step Water bears Four segmented body and head Xenacoelomorpha Strange form without gut Acoels, xenoturbellids , but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems + '''Total: 34 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 (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.
Embryophyte]] [[Viridiplantae]]
[[Green algae]]
Other algae ([[Biliphyta]])
Anthocerotophyta p. 489 Anthoceros-like plant Hornworts Horn-shaped , no vascular system -300+ Moss Bryum-like plant, moss plant Mosses Persistent unbranched , no vascular system Charophyta Chara-like plant Charophytes Chlorophyta Yellow-green plant Chlorophytes Cycadophyta Cycas-like plant, palm-like plant Cycads Seeds, crown of compound leaves -200 Ginkgophyta Ginkgo-like plant Ginkgo, maidenhair tree Seeds not protected by fruit (single living species) extant; 50+ extinct Glaucophyta Blue-green plant Glaucophytes Gnetophyta Gnetum-like plant Gnetophytes Seeds and woody vascular system with vessels Lycopodiophyta,
LycophytaLycopodium-like plant
Wolf plantClubmosses & spikemosses Microphyll leaf, vascular system extant Flowering plant Magnolia-like plant Flowering plants, angiosperms Flowers and fruit, vascular system with vessels Marchantiophyta, (2020). 9780521660976, Cambridge University Press. ISBN 9780521660976
HepatophytaMarchantia-like plant
Liver plantLiverworts Ephemeral unbranched , no vascular system Pinophyta,
ConiferophytaPinus-like plant
Cone-bearing plantConifers Cones containing seeds and wood composed of tracheids extant Total: 12
Fungi
Ascomycota Bladder fungus Ascomycetes, sac fungi Tend to have fruiting bodies (ascocarp). Filamentous, producing hyphae separated by septa. Asexual reproduction. Basidiomycota Small base fungus Basidiomycetes Bracket fungi, toadstools, smuts and rust. Sexual reproduction. Blastocladiomycota Offshoot branch fungus Blastoclads 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. 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. Microsporidia Small seeds Microsporans Neocallimastigomycota New beautiful whip fungus Neocallimastigomycetes Predominantly located in digestive tract of herbivorus animals. Anaerobic, terrestrial and aquatic. Zygomycota Pair fungus Zygomycetes Most are saprobes and reproduce sexually and asexually. Total: 8
Protista
Kingdom Protista (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 polyphyletic taxon (it includes groups not directly related to one another), 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.
[[Chromista]]
[[Protozoa]]
Amoebozoa Amorphous animal Amoebas Amoeba Bigyra Two ring Cercozoa Choanozoa Funnel animal Ciliophora Cilia bearer Ciliates Paramecium Cryptista Euglenozoa True eye animal Euglena Foraminifera Hole bearers Forams Complex shells with one or more chambers Forams Haptophyta Loukozoa Groove animal Metamonada Giardia Microsporidia Small spore Myzozoa Suckling animal Mycetozoa Slime molds Ochrophyta Yellow plant Diatoms Diatoms Oomycota Egg fungus Oomycetes Percolozoa Radiozoa Ray animal Radiolarians Sarcomastigophora Sulcozoa Total: 20
Bacteria
Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)
Archaea
Currently there are five phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN).
See also
Notes
External links
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