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57-411: The gastrotrichs ( phylum Gastrotricha ), commonly referred to as hairybellies or hairybacks , are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals , and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton , the layer of tiny organisms and detritus that is found on the seabed and

114-408: A "looper" caterpillar . In response to a threat, the head and trunk can be rapidly pulled backwards, or the creeping movement can be reversed. Muscular action is important when the animal turns sideways and during copulation, when two individuals twine around each other. Gastrotrich reproduction and reproductive behaviour has been little studied. That of macrodasiyds probably most represents that of

171-417: A hydrostatic skeleton . The mouth is at the anterior end and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen , lined by myoepithelial cells . The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The anus is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to

228-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in

285-407: 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

342-470: A de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body. The body wall consists of a cuticle , an epidermis and longitudinal and circular bands of muscle fibres. In some primitive species, each epidermal cell has a single cilium, a feature shared only by the gnathostomulans . The whole ventral surface of

399-402: 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 [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,

456-602: 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, 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), or for conifers alone as below. Since

513-564: A phylogenetic tree, or cladogram, where varying relatedness amongst species is evidently depicted. Through this tree, organisms can be categorized by divergence from the common ancestor, and primitive characters, to clades of organisms with shared derived character states. Furthermore, cladograms allow researchers to view the changes and evolutionary alterations occurring in a species over time as they move from primitive characters to varying derived character states. Cladograms are important for scientists as they allow them to classify and hypothesize

570-400: 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

627-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae

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684-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to

741-401: 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

798-552: A simple body plan with a head region, with a brain and sensory organs , and a trunk with a simple gut and the reproductive organs . They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx . They are hermaphrodites , the marine species producing eggs which develop directly into miniature adults . The freshwater species are parthenogenetic , producing unfertilised eggs, and at least one species

855-401: A single outlet cell which passes the excreted material into the protonephridial duct. As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two ganglia , one on either side of the pharynx, connected by a commissure . From these lead a pair of nerve cords which run along either side of the body beside

912-401: 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

969-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with

1026-457: Is viviparous . Gastrotrichs mature with great rapidity and have lifespans of only a few days. The name gastrotrich comes from Greek γαστήρ, gaster 'stomach' and θρίξ, thrix 'hair'. The name was coined by the Russian zoologist Élie Metchnikoff in 1865. The common name hairyback apparently arises from a mistranslation of gastrotrich . The relationship of gastrotrichs to other phyla

1083-474: Is a paraphyletic taxon, 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 . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by

1140-511: Is a feature that appears in the clade. However, in the clade of tetrapods, legs are primitive since they were inherited from a common ancestor. The terms "primitive" and "advanced", etc., are not properly used in referring to a species or an organism as any species or organism is a mosaic of primitive and derived traits. Using "primitive" and "advanced" may lead to "ladder thinking" (compare the Latin term scala naturae 'ladder of nature'), which

1197-484: Is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and

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1254-405: 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 ,

1311-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish

1368-440: Is internal. The fertilised eggs are released by rupture of the body wall which afterwards repairs itself. As is the case in most protostomes , development of the embryo is determinate , with each cell destined to become a specific part of the animal's body. At least one species of gastrotrich, Urodasys viviparus , is viviparous . Many species of chaetotonid gastrotrichs reproduce entirely by parthenogenesis . In these species,

1425-405: Is not clearly defined as a head but contains the sense organs, brain and pharynx. Cilia are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the posterior end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes

1482-427: Is solely due to cell enlargement. Gastrotricha is divided into two orders and a number of families: Order Macrodasyida Remane, 1925 [Rao and Clausen, 1970] Order Chaetonotida Remane, 1925 [Rao and Clausen, 1970] Suborder Multitubulatina d'Hondt, 1971 Suborder Paucitubulatina d'Hondt, 1971 Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla )

1539-503: Is the thought that all species are evolving because they are striving toward supremacy. When this form of thinking is used, humans are typically considered perfect and all other organisms are of less quality than them. This can cause the misconception of one species being an ancestor to another species, when in fact both species are extant. Homo sapiens , for example have large brains (a derived trait) and five fingers (a primitive trait) in their lineage. Species are constantly evolving, so

1596-918: Is unclear. Morphology suggests that they are close to the Gnathostomulida , the Rotifera , or the Nematoda . On the other hand, genetic studies place them as close relatives of the Platyhelminthes , the Ecdysozoa or the Lophotrochozoa . As of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the Macrodasyida and the Chaetonotida . Edward Ruppert et al. report that

1653-440: Is used in reference to characters and character state. In doing so, a derived character is depicted as a character procured through evolution from the previous ancestral state, and persisting due to fixation of derived alleles. Whereas, a primitive character is one that is originally present in the ancestral population. Primitive characters are avoided as they depict the ancestral character state. Conversely, derived characters depict

1710-601: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Primitive (phylogenetics) A primitive trait is the original condition of that trait in the common ancestor; advanced indicates a notable change from the original condition. These terms in biology contain no judgement about

1767-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to

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1824-473: The Macrodasyida are wholly marine, but two rare and poorly known species, Marinellina flagellata and Redudasys fornerise , are known from fresh water. The Chaetonotida comprises both marine and freshwater species. Gastrotrichs vary in size from about 0.06 to 3 mm (0.002 to 0.118 in) in body length. They are bilaterally symmetrical , with a transparent strap-shaped or bowling pin -shaped body, arched dorsally and flattened ventrally. The anterior end

1881-497: 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 . 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. Protista

1938-458: 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. 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

1995-407: The adult. The young typically reach sexual maturity in about three days. In the laboratory, Lepidodermella squamatum has lived for up to forty days, producing four or five eggs during the first ten days of life. Gastrotrichs demonstrate eutely , each species having an invariant genetically fixed number of cells as adults. Cell division ceases at the end of embryonic development and further growth

2052-438: The alteration of characters from the ancestral state because selection favored organisms with that derived trait. "Primitive" and "advanced" are relative terms. When a trait is called primitive, the determination is based on the perspective from which the trait is viewed. Any trait can be both primitive (ancestral) and advanced (derived) depending on the context. In the clade of vertebrates, legs are an advanced trait since it

2109-416: The ancestral lineage and these more primitive gastrotrichs are simultaneous hermaphrodites , possessing both male and female sex organs. There is generally a single pair of gonads , the anterior portion of which contains sperm -producing cells and the posterior portion producing ova . The sperm is sometimes packaged in spermatophores and is released through male gonopores that open, often temporarily, on

2166-412: The animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no coelom (body cavity) and the interior of the animal is filled with poorly differentiated connective tissue . In the macrodasyidans, Y-shaped cells, each containing a vacuole , surround the gut and may function as

2223-475: The beds of other water bodies . The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil . Gastrotrichs are divided into two orders , the Macrodasyida which are marine (except for two species), and the Chaetonotida , some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described. Gastrotrichs have

2280-427: The body. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation . Unusually, the protonephridia do not take the form of flame cells , but, instead, the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum . These cells, termed cyrtocytes , connect to

2337-507: 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 . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia

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2394-583: 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 . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). 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

2451-423: The longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface which function as mechanoreceptors . There are also ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors . Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments,

2508-588: The male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. Though the eggs have a diameter of less than 50 μm , they are still very large in comparison with the animals' size. Some species are capable of laying eggs that remain dormant during times of desiccation or low temperatures; these species, however, are also able to produce regular eggs, which hatch in one to four days, when environmental conditions are more favourable. The eggs of all gastrotrichs undergo direct development and hatch into miniature versions of

2565-410: The muscular action of the pharynx. They are themselves eaten by turbellarians and other small macrofauna . Like many microscopic animals, gastrotrich locomotion is primarily powered by hydrostatics , but movement occurs through different methods in different members of the group. Chaetonotids only have adhesive glands at the back and, in them, locomotion typically proceeds in a smooth gliding manner;

2622-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within

2679-498: The origin and future of organisms. Cladograms allow scientists to propose their evolutionary scenarios about the lineage from a primitive trait to a derived one. By understanding how the trait came to be, scientists can hypothesize the environment that specific organism was in and how that affected the evolutionary adaptations of the trait that came to be. Other, more technical, terms for these two conditions—for example, "plesiomorphic" and "synapomorphic"—are frequently encountered; see

2736-539: 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 placing taxa in clades without any formal ranking of group size. A definition of

2793-475: 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

2850-638: The sophistication, superiority, value or adaptiveness of the named trait. "Primitive" in biology means only that the character appeared first in the common ancestor of a clade group and has been passed on largely intact to more recent members of the clade. "Advanced" means the character has evolved within a later subgroup of the clade. Phylogenetics is utilized to determine evolutionary relationships and relatedness, to ultimately depict accurate evolutionary lineages. Evolutionary relatedness between living species can be connected by descent from common ancestry. These evolutionary lineages can thereby be portrayed through

2907-495: The surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land. They are also found in stagnant pools and anaerobic mud, where they thrive even in the presence of hydrogen sulfide . When pools dry up they can survive periods of desiccation as eggs, and some species are capable of forming cysts in harsh conditions. In marine sediments they have been known to reach 364 individuals per 10 cm (1.6 sq in) making them

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2964-407: The table below. At least three other sets of terms are synonymous with the terms "primitive" and "advanced". The technical terms are considered preferable because they are less likely to convey the sense that the trait mentioned is inferior, simpler, or less adaptive (e.g., as in non-vascular ("lower") and vascular ("higher") plants ). The terms "plesiomorphy" and "apomorphy" are typically used in

3021-546: The technical literature: for example, when a plesiomorphic trait is shared by more than one member of a clade, the trait is called a symplesiomorphy , that is, a shared primitive trait; a shared derived trait is a synapomorphy . The amount of variation of characters can assist in depicting greater relatedness amongst species, and conversely show the lack of relatedness between species. Analysis of character variation also aids in distinguishing primitive characters from derived characters. The term derived and primitive, or ancestral,

3078-525: The third most common invertebrate in the sediment after nematodes and harpacticoid copepods . In freshwater they may reach a density of 158 individuals per 10 cm (1.6 sq in) and are the fifth most abundant group of invertebrates in the sediment. In marine and freshwater environments, gastrotrichs form part of the benthic community . They are detritivores and are microphagous: they feed by sucking small dead or living organic materials, diatoms , bacteria and small protozoa into their mouths by

3135-477: The underside of the animal, roughly two-thirds of the way along the body. A copulatory organ on the tail collects the sperm and transfers it to the partner's seminal receptacle through the female gonopore. Details of the process and the behaviour involved vary with the species, and there is a range of different accessory reproductive organs. During copulation, the "male" individual uses his copulatory organ to transfer sperm to his partner's gonopore and fertilisation

3192-399: The ventral surface; these contain valves and may allow egestion of any excess water swallowed while feeding. In the chaetonotidans, the excretory system consists of a single pair of protonephridia , which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of

3249-420: The whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. In the pelagic chaetonotid genus Stylochaeta , however, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body. By contrast, with chaetonotids, macrodasyidans typically have multiple adhesive glands and move forward with a creeping action similar to that of

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