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In biology, a phylum (/ˈfaɪləm/; pl.: 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.
General description
The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon (φῦλον, "race, stock"), related to phyle (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 Stamm (Phylon)."[a] 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][6] The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.[7]
Informally, phyla can be thought of as groupings of organisms based on general specialization of body 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 (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[9] 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.[citation needed]
Definition based on genetic relation
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, 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).[10] 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.[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 in clades without any formal ranking of group size.[9]
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.[9] 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.[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]
Known phyla
Animals
This section needs additional citations for verification. (February 2013) |
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]
Protostome | Bilateria | Nephrozoa | |
Deuterostome | |||
Basal/disputed | Non-Bilateria | ||
Vendobionta | |||
Parazoa | |||
Others |
Phylum | Meaning | Common name | Distinguishing characteristic | Taxa described |
---|---|---|---|---|
Acanthocephala | Thorn head | Acanthocephalans, thorny-headed worms, spiny-headed worms | Worm with a proboscis armed with hooks | 1420 |
Annelida | Little ring [16]: 306 | Segmented worms | Multiple circular segments | 22,000+ 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 | 1,250,000+ extant;[14] 20,000+ extinct |
Brachiopoda | Arm foot[16]: 336 | Lampshells[16]: 336 | Lophophore and pedicle | 300–500 extant; 12,000+ extinct |
Bryozoa (Ectoprocta) | Moss animals | Moss animals, sea mats, ectoprocts[16]: 332 | Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia | 6,000 extant[14] |
Chaetognatha | Longhair jaw | Arrow worms[16]: 342 | 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. 55,000+[14] |
Cnidaria | Stinging nettle | Cnidarians | Nematocysts (stinging cells) | approx. 16,000[14] |
Ctenophora | Comb bearer | Comb jellies[16]: 256 | Eight "comb rows" of fused cilia | approx. 100–150 extant |
Cycliophora | Wheel carrying | Symbion | Circular mouth surrounded by small cilia, sac-like bodies | 3+ |
Echinodermata | Spiny skin | Echinoderms[16]: 348 | Fivefold radial symmetry in living forms, mesodermal calcified spines | approx. 7,500 extant;[14] approx. 13,000 extinct |
Entoprocta | Inside anus[16]: 292 | Goblet worms | Anus inside ring of cilia | approx. 150 |
Gastrotricha | Hairy stomach[16]: 288 | Gastrotrich worms | Two terminal adhesive tubes | approx. 690 |
Gnathostomulida | Jaw orifice | Jaw worms[16]: 260 | Tiny worms related to rotifers with no body cavity | approx. 100 |
Hemichordata | Half cord[16]: 344 | Acorn worms, hemichordates | Stomochord in collar, pharyngeal slits | approx. 130 extant |
Kinorhyncha | Motion snout | Mud dragons | Eleven segments, each with a dorsal plate | approx. 150 |
Loricifera | Armour bearer | Brush heads | Umbrella-like scales at each end | approx. 122 |
Micrognathozoa | Tiny jaw animals | None | Accordion-like extensible thorax | 1 |
Mollusca | Soft[16]: 320 | Mollusks / molluscs | Muscular foot and mantle round shell | 85,000+ extant;[14] 80,000+ extinct[17] |
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[16]: 274 | Round cross section, keratin cuticle | 25,000[14] |
Nematomorpha | Thread form[16]: 276 | Horsehair worms, Gordian worms[16]: 276 | Long, thin parasitic worms closely related to nematodes | approx. 320 |
Nemertea | A sea nymph[16]: 270 | Ribbon worms, rhynchocoela[16]: 270 | Unsegmented worms, with a proboscis housed in a cavity derived from the coelom called the rhynchocoel | approx. 1,200 |
Onychophora | Claw bearer | Velvet worms[16]: 328 | Worm-like animal with legs tipped by chitinous claws | approx. 200 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 | 11 |
Placozoa | Plate animals | Trichoplaxes[16]: 242 | Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between | 4+ |
Platyhelminthes | Flat worm[16]: 262 | Flatworms[16]: 262 | Flattened worms with no body cavity. Many are parasitic. | approx. 29,500[14] |
Porifera | Pore bearer | Sponges[16]: 246 | Perforated interior wall, simplest of all known animals | 10,800 extant[14] |
Priapulida | Little Priapus | Penis worms | Penis-shaped worms | approx. 20 |
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[16]: 264 | Single anteroposterior axial celled endoparasites, surrounded by ciliated cells | 100+ |
Rotifera | Wheel bearer | Rotifers[16]: 282 | Anterior crown of cilia | approx. 2,000[14] |
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[18] or an early ecdysozoan.[19] | 2 species, extinct |
Tardigrada | Slow step | Water bears, moss piglets | Microscopic relatives of the arthropods, with a four segmented body and head | 1,000 |
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[20] | 400+ |
Total: 40 | 1,525,000[14] |
Plants
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,[21] 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,[22] 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),[23] 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.[24]
Land plants | Viridiplantae | |
Green algae | ||
Other algae (Biliphyta)[21] |
Division | Meaning | Common name | Distinguishing characteristics | Species described |
---|---|---|---|---|
Anthocerotophyta[25] | Anthoceros-like plants | Hornworts | Horn-shaped sporophytes, no vascular system | 100-300+ |
Bryophyta[25] | Bryum-like plants, moss plants | Mosses | Persistent unbranched sporophytes, no vascular system | approx. 12,000 |
Charophyta | Chara-like plants | Charophytes | approx. 1,000 | |
Chlorophyta | (Yellow-)green plants[16]: 200 | Chlorophytes | approx. 7,000 | |
Cycadophyta[26] | Cycas-like plants, palm-like plants | Cycads | Seeds, crown of compound leaves | approx. 100-200 |
Ginkgophyta[27] | Ginkgo-like plants | Ginkgo, maidenhair tree | Seeds not protected by fruit (single living species) | only 1 extant; 50+ extinct |
Glaucophyta | Blue-green plants | Glaucophytes | 15 | |
Gnetophyta[28] | Gnetum-like plants | Gnetophytes | Seeds and woody vascular system with vessels | approx. 70 |
Lycopodiophyta,[23] |
Lycopodium-like plants Wolf plants |
Clubmosses & spikemosses | Microphyll leaves, vascular system | 1,290 extant |
Magnoliophyta | Magnolia-like plants | Flowering plants, angiosperms | Flowers and fruit, vascular system with vessels | 300,000 |
Marchantiophyta,[30] Hepatophyta[25] |
Marchantia-like plants Liver plants |
Liverworts | Ephemeral unbranched sporophytes, no vascular system | approx. 9,000 |
Polypodiophyta, | Polypodium-like plants |
Ferns | Megaphyll leaves, vascular system | approx. 10,560 |
Picozoa | Extremely small animals | Picozoans, picobiliphytes, biliphytes | 1 | |
Pinophyta,[23] Coniferophyta[31] |
Pinus-like plants Cone-bearing plant |
Conifers | Cones containing seeds and wood composed of tracheids | 629 extant |
Prasinodermophyta | Prasinoderma-like plants | Picozoans, picobiliphytes, biliphytes | 8
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