In biology, a phylum (//; plural: phyla)[note 1] is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division is used instead of "phylum", although in 1993 the International Botanical Congress accepted the designation "phylum". The kingdom Animalia contains approximately 35 phyla; the kingdom Plantae contains 12 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.
Concepts of Phyla have changed importantly from their origins in the six Linnaean classes and the four "embranchements" of Georges Cuvier. Haeckel introduced the term phylum, based on the Greek word phylon. In plant taxonomy, Eichler (1883) classified all plants into five groups, named divisions.  Informally, phyla can be thought of as grouping 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.
The largest objective measure in the above definitions is the "certain degree"—how unrelated do organisms need to be to be members of different phyla? The minimal requirement is that all organisms in a phylum should be related closely enough for them to be clearly more closely related to one another than to any other group. Even this is problematic as the requirement depends on knowledge of organisms' relationships: As more data become available, particularly from molecular studies, we are better able to judge 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 as a group of annelids and merged the phyla, so that the bearded worms are now an annelid family. Likewise, 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 cladistics, a method in which groups are placed on a "family tree" without any formal ranking of group size.
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 typically hardest to classify; they can be off-shoots 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, characters common to most members of a phylum may be secondarily lost by some members. It is also defined 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 an subjective decision of which groups of organisms should be considered as phyla.
Its utility is that 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 is strongly affected by the chance survival of rare groups, which vastly increase the size of phyla. Representatives of many modern phyla did not appear until long after the Cambrian.
|This section needs additional citations for verification. (February 2013)|
|Phylum||Meaning||Common name||Distinguishing characteristic||Species described|
|Acanthocephala||Thorny headed worms||Thorny-headed worms||Reversible spiny proboscis that bears many rows of hooked spines||approx. 756|
|Acoelomorpha||Without gut||Acoels||No mouth or alimentary canal (alimentary canal = digestive tract in digestive system)|
|Annelida||Little ring||Segmented worms||Multiple circular segment||17,000+ extant|
|Arthropoda||Jointed foot||Arthropods||Segmented bodies and jointed limbs, with Chitin exoskeleton||1,134,000+|
|Brachiopoda||Arm foot||Lamp shells||Lophophore and pedicle||300-500 extant|
|Bryozoa||Moss animals||Moss animals, sea mats||Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia||5,000 extant|
|Chaetognatha||Longhair jaw||Arrow worms||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. 100,000+|
|Cnidaria||Stinging nettle||—||Nematocysts (stinging cells)||approx. 11,000|
|Ctenophora||Comb bearer||Comb jellies||Eight "comb rows" of fused cilia||approx. 100 extant|
|Cycliophora||Wheel carrying||Symbion||Circular mouth surrounded by small cilia, sac-like bodies||3+|
|Echinodermata||Spiny skin||Echinoderms||Fivefold radial symmetry in living forms, mesodermal calcified spines||approx. 7,000 extant; approx. 13,000 extinct|
|Entoprocta||Inside anus||Goblet worm||Anus inside ring of cilia||approx. 150|
|Gastrotricha||Hair stomach||—||Two terminal adhesive tubes||approx. 690|
|Gnathostomulida||Jaw orifice||Jaw worms||approx. 100|
|Hemichordata||Half cord||Acorn worms, pterobranchs||Stomochord in collar, pharyngeal slits||approx. 100 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||—||Accordion-like extensible thorax||1|
|Mollusca||Soft||Mollusks / molluscs||Muscular foot and mantle round shell||112,000|
|Nematoda||Thread like||Round worms||Round cross section, keratin cuticle||80,000–100,000|
|Nematomorpha||Thread form||Horsehair worms||approx. 320|
|Nemertea||A sea nymph||Ribbon worms||approx. 1,200|
|Onychophora||Claw bearer||Velvet worms||Legs tipped by chitinous claws||approx. 200 extant|
|Orthonectida||Straight swim||Single layer of ciliated cells surrounding a mass of sex cells||approx. 20|
|Phoronida||Zeus's mistress||Horseshoe worms||U-shaped gut||11|
|Placozoa||Plate animals||Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between||1|
|Platyhelminthes||Flat worm||Flatworms||approx. 25,000|
|Porifera*||Pore bearer||Sponges||Perforated interior wall||5,000+ extant|
|Rhombozoa||Lozenge animal||—||Single anteroposterior axial cell surrounded by ciliated cells||75|
|Rotifera||Wheel bearer||Rotifers||Anterior crown of cilia||approx. 2,000|
|Sipuncula||Small tube||Peanut worms||Mouth surrounded by invertible tentacles||144–320|
|Tardigrada||Slow step||Water bears||Four segmented body and head||1,000+|
|Xenoturbellida||Strange flatworm||—||Ciliated deuterostome||2|
|Others (Radiata or Parazoa)|
|This section needs additional citations for verification. (July 2011)|
The ten Divisions into which plants, here defined as living (extant) land plants, may be placed are shown in the table below. This definition excludes the algal Divisions Chlorophyta and Charophyta which are included in the clade Viridiplantae (see also current definitions of Plantae). The definition and classification of plants at this level varies from source to source. 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 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 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.
|Division||Meaning||Common name||Distinguishing characteristics|
|Anthocerotophyta||Anthoceros-like plants||Hornworts||Horn-shaped sporophytes, no vascular system|
|Bryophyta||Bryum-like plants, moss plants||Mosses||Persistent unbranched sporophytes, no vascular system|
|Liverworts||Ephemeral unbranched sporophytes, no vascular system|
|Clubmosses & spikemosses||Microphyll leaves, vascular system|
|Pteridophyta||Pteris-like plants, fern plants||Ferns & horsetails||Prothallus gametophytes, vascular system|
|Conifers||Cones containing seeds and wood composed of tracheids|
|Cycadophyta||Cycas-like plants, palm-like plants||Cycads||Seeds, crown of compound leaves|
|Ginkgophyta||Ginkgo-like plants||Ginkgo, Maidenhair||Seeds not protected by fruit (single living species)|
|Gnetophyta||Gnetum-like plants||Gnetophytes||Seeds and woody vascular system with vessels|
|Flowering plant||flowering plants||Flowering plants, angiosperms||Flowers and fruit, vascular system with vessels|
|This section does not cite any references or sources. (July 2011)|
|Phylum||Meaning||Common name||Distinguishing characteristics|
|Chytridiomycota||Little pot mushroom||Chytrids||Cellulose in cell walls, flagellated gametes|
|Deuteromycota||Second mushroom||Imperfect fungi||Unclassified fungi; only asexual reproduction observed no other major distinguishments|
|Zygomycota||Yolk mushroom||Zygomycetes||Blend gametangia to form a zygosporangium|
|Glomeromycota||Ball mushroom||None||Form arbuscular mycorrhizae with plants|
|Ascomycota||Bag/Wineskin Mushroom||Sac fungi||Produce spores in an 'ascus' which is a kind of fruiting bud|
|Basidiomycota||Basidium Mushroom||Club Fungi||Produce spores from a 'basidium' which is a kind of fruiting bud|
Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)
|This section does not cite any references or sources. (July 2011)|
Phyla in the plant kingdom are frequently called divisions.
|Look up Phylum in Wiktionary, the free dictionary.|