Phylum

For other uses, see Phyla.

The hierarchy of biological classification's eight major taxonomic ranks. A kingdom contains one or more phyla. Intermediate minor rankings are not shown.

In biology, a phylum (/ˈfaɪləm/; plural: phyla) is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division was used instead of "phylum", although from 1993 the International Code of Nomenclature for algae, fungi, and plants accepted the designation "phylum". The kingdom Animalia contains approximately 35 phyla, Plantae contains 12, and Fungi contains 7. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

General description and familiar examples

The definitions of zoological 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 ('tribe' or 'stock'). In plant taxonomy, Eichler (1883) classified 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.

Definition based on genetic relation

The most important 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 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 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 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 cladistics, a method in which groups are placed on a "family tree" 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. Representatives of many modern phyla did not appear until long after the Cambrian.

Known phyla

Animal phyla

	Protostome	Bilateria
	Deuterostome
	Basal/disputed
	Others (Radiata or Parazoa)

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. 1,100
Acoelomorpha	Without gut	Acoels	No mouth or alimentary canal (alimentary canal = digestive tract in digestive system)	approx. 350
Annelida	Little ring	Annelids	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	Anemones / Jellyfish	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	Hairybacks	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	25,000–1,000,000
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
Priapulida	Little Priapus			approx. 16
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+
Xenacoelomorpha	Strange flatworm	—	Ciliated deuterostome	2
Total: 35				2,000,000+

Land plant phyla (divisions)

The ten Divisions into which the living embryophytes (land plants) are often placed are shown in the table below. To these may be added two algal Divisions, Chlorophyta and Charophyta, which are included with land plants in the clade Viridiplantae (see also current definitions of Plantae). The definition and classification of plants at this level 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 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.

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
Marchantiophyta, Hepatophyta	Marchantia-like plants liver plants	liverworts	ephemeral unbranched sporophytes, no vascular system
Lycopodiophyta, Lycophyta	Lycopodium-like plants "wolf" plants	clubmosses & spikemosses	microphyll leaves, vascular system
Pteridophyta	Pteris-like plants, fern plants	ferns & horsetails	prothallus gametophytes, vascular system
Pinophyta, Coniferophyta	Pinus-like plants cone-bearing plants	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 tree	seeds not protected by fruit (single living species)
Gnetophyta	Gnetum-like plants	gnetophytes	seeds and woody vascular system with vessels
Magnoliophyta	Magnolia-like plants	flowering plants, angiosperms	flowers and fruit, vascular system with vessels
Total: 10

Fungal divisions

Main article: Fungi

1. Chytridiomycota
2. Blastocladiomycota
3. Zygomycota
4. Glomeromycota
5. Ascomycota
6. Basidiomycota
7. Microsporidia
8. Neocallimastigomycota

Protista phyla (Divisions)

Group Description	Phylum	Meaning	Common name	Distinguishing characteristics	Example
Heterotrophs (no locomotor apparatus)	Rhizopoda	root-foot	Amoeba	Amoeboids have the ability to change their shape.	Amoeba
	Actinopoda	ray-foot	-------	long, thin axopodia	Radiolarians
	Foraminifera	hole bearers	Forams	Complex shells with one or more chambers	Forams
Photosynthetic protists	Dinoflagellata	whirling scourge	Dinoflagellates	Unicellular, have two dissimilar flagella	Red tides
	Euglenophyta	Good-eyed plant	Euglenids	Have a pellicle, which gives shape to the cell.	Euglena
	Chrysophyta	golden plant	golden algae		Diatoma
	Rhodophyta	rose plant	red algae	Cells do not have flagella or centrioles; use phycobiliproteins which gives red tint	Coralline Algae
	Phaeophyta	gray plant	brown algae	chloroplasts surrounded by four membranes - form differentiated tissues	Kelp
Heterotrophs (flagella)	Sarcomastigophora				Trypanosoma cruzi
	Ciliophora				Paramecium
Non-motile spore-formers	Apicomplexa				Plasmodium
Heterotrophs (restricted mobility)	Oomycota				Water Molds
	Acrasiomycota				Dictyostelium
	Myxomycota				Fuligo
Total: 14

Bacterial phyla/divisions

Main article: Bacterial phyla

Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)

1. Acidobacteria, phenotipically diverse and mostly uncultured
2. Actinobacteria, High-G+C Gram positive species
3. Aquificae, only 14 thermophilic genera, deep branching
4. Bacteroidetes
5. Caldiserica, formerly candidate division OP5, Caldisericum exile is the sole representative
6. Chlamydiae, only 6 genera
7. Chlorobi, only 7 genera, green sulphur bacteria
8. Chloroflexi, green non-sulphur bacteria
9. Chrysiogenetes, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
10. Cyanobacteria, also known as the blue-green algae
11. Deferribacteres
12. Deinococcus-Thermus, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
13. Dictyoglomi
14. Elusimicrobia, formerly candidate division Thermite Group 1
15. Fibrobacteres
16. Firmicutes, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
17. Fusobacteria
18. Gemmatimonadetes
19. Lentisphaerae, formerly clade VadinBE97
20. Nitrospira
21. Planctomycetes
22. Proteobacteria, the most known phyla, containing species such as Escherichia coli or Pseudomonas aeruginosa
23. Spirochaetes, species include Borrelia burgdorferi, which causes Lyme disease
24. Synergistetes
25. Tenericutes, alternatively class Mollicutes in phylum Firmicutes (notable genus: Mycoplasma)
26. Thermodesulfobacteria
27. Thermomicrobia
28. Thermotogae, deep branching
29. Verrucomicrobia

Archaeal phyla/division/kingdoms

1. Crenarchaeota, Second most common archaeal phylum
2. Euryarchaeota, most common archaeal phylum
3. Korarchaeota
4. Nanoarchaeota, ultra-small symbiotes, single known species
5. Thaumarchaeota

See also

• Cladistics
• Phylogenetics
• Systematics
• Taxonomy

External links

• Are phyla "real"? Is there really a well-defined "number of animal phyla" extant and in the fossil record?
• Major Phyla Of Animals