In biology, a phylum (/ˈfaɪləm/; 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.[1][2][3] 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.

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

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"). "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)." which translates as: However, 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., race] (Phylon).) 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.

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 cladistics, a method in which groups are placed on a "family tree" 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


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


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,[19] 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 Pteridophyta,[20] 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),[21] 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.[22]

Land plants Viridiplantae
Green algae
Other algae (Biliphyta)[19]
Division Meaning Common name Distinguishing characteristics Species described
Anthocerotophyta[23] Anthoceros-like plant Hornworts Horn-shaped sporophytes, no vascular system 100-300+
Bryophyta[24] Bryum-like plant, moss plant Mosses Persistent unbranched sporophytes, no vascular system approx. 12,000
Charophyta Chara-like plant Charophytes approx. 1,000
Chlorophyta Yellow-green plant[16]:200 Chlorophytes approx. 7,000
Cycadophyta[25] Cycas-like plant, palm-like plant Cycads Seeds, crown of compound leaves approx. 100-200
Ginkgophyta[26] Ginkgo-like plant Ginkgo, maidenhair tree Seeds not protected by fruit (single living species) only 1 extant; 50+ extinct
Glaucophyta Blue-green plant Glaucophytes 13
Gnetophyta[27] Gnetum-like plant Gnetophytes Seeds and woody vascular system with vessels approx. 70


Lycopodium-like plant

Wolf plant

Clubmosses & spikemosses Microphyll leaves, vascular system 1,290 extant
Magnoliophyta Magnolia-like plant Flowering plants, angiosperms Flowers and fruit, vascular system with vessels 300,000


Marchantia-like plant

Liver plant

Liverworts Ephemeral unbranched sporophytes, no vascular system approx. 9,000


Pinus-like plant

Cone-bearing plant

Conifers Cones containing seeds and wood composed of tracheids 629 extant
Total: 14


Division Meaning Common name Distinguishing characteristics
Ascomycota Bladder fungus[16]:396 Ascomycetes,[16]:396 sac fungi Tend to have fruiting bodies (ascocarp). [31] Filamentous, producing hyphae separated by septa. Asexual reproduction.[32]
Basidiomycota Small base fungus[16]:402 Basidiomycetes[16]:402 Bracket fungi, toadstools, smuts and rust. Sexual reproduction.[33]
Blastocladiomycota Offshoot branch fungus[34] Blastoclads
Chytridiomycota Little cooking pot fungus[35] Chytrids Predominantly Aquatic saprotrophic or parasitic. Have a posterior flagellum. Tend to be single celled but can also be multicellular. [36][37][38]
Glomeromycota Ball of yarn fungus[16]:394 Glomeromycetes, AM fungi[16]:394 Mainly arbuscular mycorrhizae present, terrestrial with a small presence on wetlands. Reproduction is asexual but requires plant roots. [39]
Microsporidia Small seeds[40] Microsporans[16]:390
Neocallimastigomycota New beautiful whip fungus[41] Neocallimastigomycetes Predominantly located in digestive tract of herbivorus animals. Anaerobic, terrestrial and aquatic. [42]
Zygomycota Pair fungus[16]:392 Zygomycetes[16]:392 Most are saprobes and reproduce sexually and asexually. [43]
Total: 8

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[44] and it is considered a protozoan by the International Society of Protistologists[45] (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (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 subphyla incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.[44]


Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi.[16]:120 Protista is a polyphyletic taxon[47] (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.[48]

Protist taxonomy has long been unstable,[49] 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,[50] adapted from the system used by the International Society of Protistologists.[45]

Phylum/Division Meaning Common name Distinguishing characteristics Example
Amoebozoa Amorphous animal Amoebas Amoeba
Bigyra Two ring
Choanozoa Funnel animal
Ciliophora Cilia bearer Ciliates Paramecium
Euglenozoa True eye animal Euglena
Foraminifera Hole bearers Forams Complex shells with one or more chambers Forams
Loukozoa Groove animal
Metamonada Giardia
Microsporidia Small spore
Myzozoa Suckling animal
Mycetozoa Slime molds
Ochrophyta Yellow plant Diatoms Diatoms
Oomycota Egg fungus[16]:184 Oomycetes
Radiozoa Ray animal Radiolarians
Total: 20

The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae,[50] but other systems consider these phyla part of Protista.[51]


Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)[52]

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


Currently there are 5 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN).[52]

  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


  1. ^ Paraphyletic


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External links


Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft) and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing the echinoderms and chordates (including the vertebrates). Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.

Humans make use of many other animal species for food, including meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals for power and transport. Dogs have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.


The annelids (Annelida, from Latin anellus, "little ring"), also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.

The annelids are bilaterally symmetrical, triploblastic, coelomate, invertebrate organisms. They also have parapodia for locomotion. Most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species. Cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.

The basic annelid form consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is used less frequently by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.

Since annelids are soft-bodied, their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Palaeontologists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Tertiary period, which began 66 million years ago.


The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of plastid called an apicoplast, and an apical complex structure. The organelle is an adaptation that the apicomplexan applies in penetration of a host cell.

The Apicomplexa are unicellular and spore-forming. All species are obligate endoparasites of animals, except Nephromyces, a symbiont in marine animals, originally classified as a chytrid fungus. Motile structures such as flagella or pseudopods are present only in certain gamete stages.

The Apicomplexa are a diverse group that includes organisms such as the coccidia, gregarines, piroplasms, haemogregarines, and plasmodia.

Diseases caused by Apicomplexa include:

Babesiosis (Babesia)

Malaria (Plasmodium)

Cryptosporidiosis (Cryptosporidium parvum)

Cyclosporiasis (Cyclospora cayetanensis)

Cystoisosporiasis (Cystoisospora belli (formerly known as "Isospora Belli"))

Toxoplasmosis (Toxoplasma gondii)The name of the taxon Apicomplexa derives from two Latin words—apex (top) and complexus (infolds)—and refers to a set of organelles in the sporozoite. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, by use of a gliding mechanism

that uses adhesions and small static myosin motors. The other main lines were the Ascetosporea (now in Rhizaria), the Myxozoa (now known to be highly derived cnidarian animals), and the Microsporidia (now known to be derived from fungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.


An arthropod (, from Greek ἄρθρον arthron, "joint" and πούς pous, "foot" (gen. ποδός)) is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.

Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80 percent of all described living animal species, some of which, unlike most other animals, are very successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.

Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, and through which their haemolymph – analogue of blood – circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.

Arthropods contribute to the human food supply both directly as food, and more importantly indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops.


Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches, and others are found in polar waters. One class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. Over 4,000 living species are known. One genus is solitary and the rest are colonial.

The phylum was originally called "Polyzoa", but this term was superseded by "Bryozoa" in 1831. Another group of animals discovered subsequently, whose filtering mechanism looked similar, was also included in "Bryozoa" until 1869, when the two groups were noted to be very different internally. The more recently discovered group was given the name Entoprocta, while the original "Bryozoa" were called "Ectoprocta". However, "Bryozoa" has remained the more widely used term for the latter group.

Individuals in bryozoan (ectoproct) colonies are called zooids, since they are not fully independent animals. All colonies contain autozooids, which are responsible for feeding and excretion. Colonies of some classes have various types of non-feeding specialist zooids, some of which are hatcheries for fertilized eggs, and some classes also have special zooids for defense of the colony. The class Cheilostomata have the largest number of species, possibly because they have the widest range of specialist zooids. A few species can creep very slowly by using spiny defensive zooids as legs. Autozooids supply nutrients to non-feeding zooids by channels that vary between classes. All zooids, including those of the solitary species, consist of a cystid that provides the body wall and produces the exoskeleton and a polypide that contains the internal organs and the lophophore or other specialist extensions. Zooids have no special excretory organs, and the polypides of autozooids are scrapped when the polypides become overloaded by waste products; usually the body wall then grows a replacement polypide. In autozooids the gut is U-shaped, with the mouth inside the "crown" of tentacles and the anus outside it. Colonies take a variety of forms, including fans, bushes and sheets. The Cheilostomata produce mineralized exoskeletons and form single-layered sheets which encrust over surfaces.

Zooids of all the freshwater species are simultaneous hermaphrodites. Although those of many marine species function first as males and then as females, their colonies always contain a combination of zooids that are in their male and female stages. All species emit sperm into the water. Some also release ova into the water, while others capture sperm via their tentacles to fertilize their ova internally. In some species the larvae have large yolks, go to feed, and quickly settle on a surface. Others produce larvae that have little yolk but swim and feed for a few days before settling. After settling, all larvae undergo a radical metamorphosis that destroys and rebuilds almost all the internal tissues. Freshwater species also produce statoblasts that lie dormant until conditions are favorable, which enables a colony's lineage to survive even if severe conditions kill the mother colony.

Predators of marine bryozoans include nudibranchs (sea slugs), fish, sea urchins, pycnogonids, crustaceans, mites and starfish. Freshwater bryozoans are preyed on by snails, insects, and fish. In Thailand, many populations of one freshwater species have been wiped out by an introduced species of snail. A fast-growing invasive bryozoan off the northeast and northwest coasts of the US has reduced kelp forests so much that it has affected local fish and invertebrate populations. Bryozoans have spread diseases to fish farms and fishermen. Chemicals extracted from a marine bryozoan species have been investigated for treatment of cancer (citation missing) and Alzheimer's disease (citation missing), but analyses have not been encouraging (citation missing).

Mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period, making it the last major phylum to appear in the fossil record. This has led researchers to suspect that bryozoans arose earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms. Early fossils are mainly of erect forms, but encrusting forms gradually became dominant. It is uncertain whether the phylum is monophyletic. Bryozoans' evolutionary relationships to other phyla are also unclear, partly because scientists' view of the family tree of animals is mainly influenced by better-known phyla. Both morphological and molecular phylogeny analyses disagree over bryozoans' relationships with entoprocts, about whether bryozoans should be grouped with brachiopods and phoronids in Lophophorata, and whether bryozoans should be considered protostomes or deuterostomes.


A chordate () is an animal constituting the phylum Chordata ([kɔrˈdata]). During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail: these five anatomical features define this phylum. Chordates are also bilaterally symmetric; and have a coelom, metameric segmentation, and a circulatory system.

The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds, and mammals); Tunicata or Urochordata (sea squirts,salps); and Cephalochordata (which includes lancelets). There are also extinct taxa such as the Vetulicolia. Hemichordata (which includes the acorn worms) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Pisces,class Osteichthyes.

Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull. The Craniata and Tunicata compose the clade Olfactores. (See diagram under Phylogeny.)

Class (biology)

In biological classification, class (Latin: classis) is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order.


Cnidaria ([knɪˈdarɪ.a]) is a phylum under Kingdom Animalia containing over 11,000 species of animals found exclusively in aquatic (freshwater and marine) environments: they are predominantly marine.

Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.

They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage.

Cnidarians were formerly grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members, such as Hydra, and colonial swimmers, such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa and Polypodiozoa were firmly recognized as cnidarians in 2007.Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, and a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish, turtles, and even other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. While reef-forming corals are almost entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, and in freshwater.

Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians. Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago, almost 200 million years before the Cambrian period as well as any fossils.


Ctenophora (; singular ctenophore, or ; from Ancient Greek: κτείς, romanized: kteis, lit. 'comb' and φέρω, pherō, 'to carry'; commonly known as comb jellies) comprise a phylum of invertebrate animals that live in marine waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are the largest animals to swim with the help of cilia.

Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (4 ft 11 in) in size. Only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("little tentacles") that are covered with colloblasts, sticky cells that capture prey. Their bodies consist of a mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with retractable tentacles that capture prey, the flat generally combless platyctenids, and the large-mouthed beroids, which prey on other ctenophores.

Almost all ctenophores function as predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed.

Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in lagerstätten dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with sponges being the sister-group to all other multicellular animals. Other biologists once believed that ctenophores were emerging earlier than the sponges, which themselves appeared before the split between cnidarians and bilaterians. However reanalysis of the data showed that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species.

A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous–Paleogene extinction event 66 million years ago.

Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words they do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups descend from various cydippids.


Echinoderm is the common name given to any member of the phylum Echinodermata (from Ancient Greek, ἐχῖνος, echinos – "hedgehog" and δέρμα, derma – "skin") of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.

The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.

Aside from the hard-to-classify Arkarua (a Precambrian animal with echinoderm-like pentamerous radial symmetry), the first definitive members of the phylum appeared near the start of the Cambrian. One group of Cambrian echinoderms, the cinctans (Homalozoa), which are close to the base of the echinoderm origin, have been found to possess external gills used for filter feeding, similar to those possessed by chordates and hemichordates.The echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. Most echinoderms are able to reproduce asexually and regenerate tissue, organs, and limbs; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries. Further, it is held by some scientists that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution.


The Firmicutes (Latin: firmus, strong, and cutis, skin, referring to the cell wall) are a phylum of bacteria, most of which have gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer membrane that causes them to stain gram-negative. Scientists once classified the Firmicutes to include all gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus).

Many Firmicutes produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through anoxygenic photosynthesis. Firmicutes play an important role in beer, wine, and cider spoilage.


The flatworms, flat worms, Platyhelminthes, Plathelminthes, or platyhelminths (from the Greek πλατύ, platy, meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth-, meaning "worm") are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates (having no body cavity), and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, the food cannot be processed continuously.

In traditional medicinal texts, Platyhelminthes are divided into Turbellaria, which are mostly non-parasitic animals such as planarians, and three entirely parasitic groups: Cestoda, Trematoda and Monogenea; however, since the turbellarians have since been proven not to be monophyletic, this classification is now deprecated. Free-living flatworms are mostly predators, and live in water or in shaded, humid terrestrial environments, such as leaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems of fish or land vertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers of hermaphroditic, segment-like proglottids that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infesting aquatic animals, and their larvae metamorphose into the adult form after attaching to a suitable host.

Because they do not have internal body cavities, Platyhelminthes were regarded as a primitive stage in the evolution of bilaterians (animals with bilateral symmetry and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one subgroup, the Acoelomorpha, as basal bilaterians—closer to the original bilaterians than to any other modern groups. The remaining Platyhelminthes form a monophyletic group, one that contains all and only descendants of a common ancestor that is itself a member of the group. The redefined Platyhelminthes is part of the Lophotrochozoa, one of the three main groups of more complex bilaterians. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups, Catenulida and Rhabditophora, with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded as paraphyletic, since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians".

Two planarian species have been used successfully in the Philippines, Indonesia, Hawaii, New Guinea, and Guam to control populations of the imported giant African snail Achatina fulica, which was displacing native snails. However, there is now concern that these planarians may themselves become a serious threat to native snails. In northwest Europe, there are concerns about the spread of the New Zealand planarian Arthurdendyus triangulatus, which preys on earthworms.


The gastropods (), more commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca, called Gastropoda. This class comprises snails and slugs from saltwater, from freshwater, and from the land. There are many thousands of species of sea snails and slugs, as well as freshwater snails, freshwater limpets, and land snails and slugs.

The class Gastropoda contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian. As of 2017, 721 families of gastropods are known, of which 245 are extinct and appear only in the fossil record, while 476 are currently extant with or without a fossil record.Gastropoda (previously known as univalves and sometimes spelled "Gasteropoda") are a major part of the phylum Mollusca, and are the most highly diversified class in the phylum, with 65,000 to 80,000 living snail and slug species. The anatomy, behavior, feeding, and reproductive adaptations of gastropods vary significantly from one clade or group to another. Therefore, it is difficult to state many generalities for all gastropods.

The class Gastropoda has an extraordinary diversification of habitats. Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries, mudflats, the rocky intertidal, the sandy subtidal, in the abyssal depths of the oceans including the hydrothermal vents, and numerous other ecological niches, including parasitic ones.

Although the name "snail" can be, and often is, applied to all the members of this class, commonly this word means only those species with an external shell big enough that the soft parts can withdraw completely into it. Those gastropods without a shell, and those with only a very reduced or internal shell, are usually known as slugs; those with a shell into which they can partly but not completely withdraw are termed semi-slugs.

The marine shelled species of gastropod include species such as abalone, conches, periwinkles, whelks, and numerous other sea snails that produce seashells that are coiled in the adult stage—though in some, the coiling may not be very visible, for example in cowries. In a number of families of species, such as all the various limpets, the shell is coiled only in the larval stage, and is a simple conical structure after that.

Language family

A language family is a group of languages related through descent from a common ancestral language or parental language, called the proto-language of that family. The term "family" reflects the tree model of language origination in historical linguistics, which makes use of a metaphor comparing languages to people in a biological family tree, or in a subsequent modification, to species in a phylogenetic tree of evolutionary taxonomy. Linguists therefore describe the daughter languages within a language family as being genetically related.According to Ethnologue the 7,111 living human languages are distributed in 141 different language families. A "living language" is simply one that is used as the primary form of communication of a group of people. There are also many dead and extinct languages, as well as some that are still insufficiently studied to be classified, or are even unknown outside their respective speech communities.

Membership of languages in a language family is established by comparative linguistics. Sister languages are said to have a "genetic" or "genealogical" relationship. The latter term is older. Speakers of a language family belong to a common speech community. The divergence of a proto-language into daughter languages typically occurs through geographical separation, with the original speech community gradually evolving into distinct linguistic units. Individuals belonging to other speech communities may also adopt languages from a different language family through the language shift process.Genealogically related languages present shared retentions; that is, features of the proto-language (or reflexes of such features) that cannot be explained by chance or borrowing (convergence). Membership in a branch or group within a language family is established by shared innovations; that is, common features of those languages that are not found in the common ancestor of the entire family. For example, Germanic languages are "Germanic" in that they share vocabulary and grammatical features that are not believed to have been present in the Proto-Indo-European language. These features are believed to be innovations that took place in Proto-Germanic, a descendant of Proto-Indo-European that was the source of all Germanic languages.


Mollusca is the second-largest phylum of invertebrate animals. The members are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 8 or 9 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.

The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small.

The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.

Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for anatomically modern humans. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some preindustrial societies.

Mollusc species can also represent hazards or pests for human activities. The bite of the blue-ringed octopus is often fatal, and that of Octopus apollyon causes inflammation that can last over a month. Stings from a few species of large tropical cone shells can also kill, but their sophisticated, though easily produced, venoms have become important tools in neurological research. Schistosomiasis (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some ecosystems.


The nematodes (UK: , US: ) or roundworms constitute the phylum Nematoda (also called Nemathelminthes). They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends.

Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications, without additional investigation, in an attempt to accentuate the importance and ubiquity of nematodes in the global ecosystem (rather than as a sign of agreement with the estimated taxonomic figure). Many other publications have since vigorously refuted this claim on the grounds that it is unsupported by fact, and is the result of speculation and sensationalism. More recent, fact-based estimates have placed the true figure closer to 40,000 species worldwide.Nematodes have successfully adapted to nearly every ecosystem: from marine (salt) to fresh water, soils, from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts, and oceanic trenches. They are found in every part of the earth's lithosphere, even at great depths, 0.9–3.6 km (3,000–12,000 ft) below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystem. The roughly 2,271 genera are placed in 256 families. The many parasitic forms include pathogens in most plants and animals. A third of the genera occur as parasites of vertebrates; about 35 nematode species occur in humans.

Nathan Cobb, a nematologist, described the ubiquity of nematodes on Earth as thus:In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.


Protozoa (also protozoan, plural protozoans) is an informal term for single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microorganisms or organic tissues and debris. Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviors, such as motility and predation, and lack a cell wall, as found in plants and many algae. Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.

In some systems of biological classification, Protozoa is a high-level taxonomic group. When first introduced in 1818, Protozoa was erected as a taxonomic class, but in later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom. In a series of classifications proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa has been ranked as a kingdom. The seven-kingdom scheme presented by Ruggiero et al. in 2015, places eight phyla under Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. Notably, this kingdom excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, all of which are classified under Kingdom Chromista. Kingdom Protozoa, as defined in this scheme, does not form a natural group or clade, but a paraphyletic group or evolutionary grade, within which the members of Fungi, Animalia and Chromista are thought to have evolved.


Sponges, the members of the phylum Porifera (; meaning "pore bearer"), are a basal Metazoa (animal) clade as a sister of the Diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. The branch of zoology that studies sponges is known as spongiology.

Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Sponges were first to branch off the evolutionary tree from the common ancestor of all animals, making them the sister group of all other animals.

Taxonomic rank

In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, order, class, phylum, kingdom, domain, etc.

A given rank subsumes under it less general categories, that is, more specific descriptions of life forms. Above it, each rank is classified within more general categories of organisms and groups of organisms related to each other through inheritance of traits or features from common ancestors. The rank of any species and the description of its genus is basic; which means that to identify a particular organism, it is usually not necessary to specify ranks other than these first two.Consider a particular species, the red fox, Vulpes vulpes: the next rank above, the genus Vulpes, comprises all the "true" foxes. Their closest relatives are in the immediately higher rank, the family Canidae, which includes dogs, wolves, jackals, and all foxes; the next higher rank, the order Carnivora, includes caniforms (bears, seals, weasels, skunks, raccoons and all those mentioned above), and feliforms (cats, civets, hyenas, mongooses). Carnivorans are one group of the hairy, warm-blooded, nursing members of the class Mammalia, which are classified among animals with backbones in the phylum Chordata, and with them among all animals in the kingdom Animalia. Finally, at the highest rank all of these are grouped together with all other organisms possessing cell nuclei in the domain Eukarya.

The International Code of Zoological Nomenclature defines rank as: "The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy (e.g. all families are for nomenclatural purposes at the same rank, which lies between superfamily and subfamily)."

Phylum Meaning Common name Distinguishing characteristic Species described
Acanthocephala Thorny head Thorny-headed worms[16]:278 Reversible spiny proboscis that bears many rows of hooked spines 1,420
Annelida Little ring :306 Segmented worms Multiple circular segment 17,000 + extant
Arthropoda Jointed foot 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 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 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 Corset bearer Brush heads Umbrella-like scales at each end approx. 122
Micrognathozoa Tiny jaw animals Limnognathia 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]
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 approx. 320
Nemertea A sea nymph[16]:270 Ribbon worms, Rhynchocoela[16]:270 approx. 1,200
Onychophora Claw bearer Velvet worms[16]:328 Legs tipped by chitinous claws approx. 200 extant
Orthonectida Straight swimming[16]:268 Orthonectids[16]:268 Single layer of ciliated cells surrounding a mass of sex cells approx. 26
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 1
Platyhelminthes Flat worm[16]:262 Flatworms[16]:262 approx. 29,500[14]
Porifera [a] Pore bearer Sponges[16]:246 Perforated interior wall 10,800 extant[14]
Priapulida Little Priapus Penis worms approx. 20
Rhombozoa Lozenge animal Rhombozoans[16]:264 Single anteroposterior axial cell surrounded by ciliated cells 100+
Rotifera Wheel bearer Rotifers[16]:282 Anterior crown of cilia approx. 2,000[14]
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 form without gut Bilaterian, but lacking typical bilaterian structures such as gut cavities, anuses, and circulatory systems[18] 400+
Total: 34 1,525,000[14]
Extant Life phyla/divisions by domain


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