82-470: Placozoa ( / ˌ p l æ k ə ˈ z oʊ ə / PLAK -ə- ZOH -ə ; lit. ' flat animals ' ) is a phylum of marine and free-living (non-parasitic) animals. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion , eating food by engulfment , reproducing by fission or budding , placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among
164-422: A "characteristic tight pavement-like appearance". But this is not always the case, such as when the cells are derived from a tumor. In these cases, it is often necessary to use certain biochemical markers to make a positive identification. The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, so they are often used for this purpose. Cancers originating from
246-474: A cell and the extracellular matrix , or they build up the paracellular barrier of epithelia and control the paracellular transport . Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions: tight junctions , adherens junctions , desmosomes , hemidesmosomes , and gap junctions . Tight junctions are a pair of trans-membrane protein fused on outer plasma membrane. Adherens junctions are
328-447: 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
410-440: A connexion). Epithelial tissues are derived from all of the embryological germ layers : However, pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas , whereas true epithelial cancers are called carcinomas . Additionally,
492-503: A functional-morphology model, all or most animals are descended from a gallertoid , a free-living ( pelagic ) sphere in seawater, consisting of a single ciliated layer of cells supported by a thin, noncellular separating layer, the basal lamina . The interior of the sphere is filled with contractile fibrous cells and a gelatinous extracellular matrix . Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes: While
574-403: A hypothesis that Trichoplax is not a distinct animal but that it is a form of the planula larva of the anemone -like hydrozoan Eleutheria krohni . Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s. The development of electron microscopy in
656-477: 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 the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement
738-534: A marginal level of statistical significance. A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues ( Eumetazoa ). The taxon they share, called the Epitheliozoa , is itself construed to be a sister group to the sponges ( Porifera ): Porifera Placozoa Eumetazoa The above view could be correct, although there
820-422: 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
902-689: 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
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#1732852705542984-500: A plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes resemble desmosomes on a section. They are made up of the integrin (a transmembrane protein) instead of cadherin. They attach the epithelial cell to the basement membrane. Gap junctions connect the cytoplasm of two cells and are made up of proteins called connexins (six of which come together to make
1066-438: A relationship comes from special cell to cell junctions – belt desmosomes – that occur not just in the Placozoa but in all animals except the sponges: They enable the cells to join together in an unbroken layer like the epitheloid of the Placozoa. Trichoplax adhaerens also shares the ventral gland cells with most eumetazoans. Both characteristics can be considered evolutionarily derived features ( apomorphies ), and thus form
1148-438: A sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen ." Primary cilia on epithelial cells provide chemosensation, thermoception , and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of
1230-472: A single-celled Amoeba , it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more complex animals. The epithelial cells bear cilia , which the animal uses to help it creep along the seafloor. The lower surface engulfs small particles of organic detritus, on which
1312-747: A singular layer of cells as simple epithelium, either simple squamous, simple columnar, or simple cuboidal, or in layers of two or more cells deep as stratified (layered), or compound , either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. This sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include diffusion , filtration, secretion , selective absorption , germination , and transcellular transport . Compound epithelium has protective functions. Epithelial layers contain no blood vessels ( avascular ), so they must receive nourishment via diffusion of substances from
1394-490: Is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination. Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behaviour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion. In 1893, Italian zoologist Francesco Saverio Monticelli described another animal which he named Treptoplax ,
1476-467: 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
1558-472: 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
1640-522: Is some evidence that the ctenophores , traditionally seen as Eumetazoa , may be the sister to all other animals. This is now a disputed classification. Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650 million years ago (many of the genes expressed in modern neurons are absent in ctenopheres, although some of these missing genes are present in placozoans). The principal support for such
1722-459: 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 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,
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#17328527055421804-513: Is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is only known for parasitic organisms, but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens . This version is supported by statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to
1886-407: Is the precursor to the external skin layer ( epidermis ), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians. As noted above, this hypothesis was supported in a statistical analysis of
1968-461: Is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and endosymbiotic bacteria in the endoplasmic reticulum . The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a secretory granule . The body axes of Hoilungia and Trichoplax are overtly similar to
2050-443: Is the type of epithelium that forms the glands from the infolding of epithelium and subsequent growth in the underlying connective tissue. They may be specialized columnar or cuboidal tissues consisting of goblet cells , which secrete mucus . There are two major classifications of glands: endocrine glands and exocrine glands : Some epithelial cells are ciliated , especially in respiratory epithelium , and they commonly exist as
2132-541: 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: Epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix . An example is the epidermis , the outermost layer of
2214-557: The Bilateria . Currently, they are typically placed according to the cladogram below: Porifera Ctenophora Placozoa Cnidaria Bilateria / Triploblasts In this cladogram the Epitheliozoa and Eumetazoa are synonyms to each other and to the Diploblasts , and the Ctenophora are basal to them. An argument raised against the proposed scenario
2296-552: 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
2378-613: The Cnidaria and once in the Bilateria . If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups. On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals ( Metazoa ) and are thus considered a sister taxon to all other metazoans: Placozoa Sponges (Porifera) Animals with tissues (Eumetazoa) According to
2460-432: The Trichoplax adhaerens whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species. A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified eumetazoans . According to this, Trichoplax adhaerens is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as
2542-459: The bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered
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2624-488: The epigenome of these cells, which enables a rapid response to immunological challenges. The slide shows at (1) an epithelial cell infected by Chlamydia pneumoniae ; their inclusion bodies shown at (3); an uninfected cell shown at (2) and (4) showing the difference between an infected cell nucleus and an uninfected cell nucleus. Epithelium grown in culture can be identified by examining its morphological characteristics. Epithelial cells tend to cluster together, and have
2706-538: The skin . Epithelial ( mesothelial ) tissues line the outer surfaces of many internal organs , the corresponding inner surfaces of body cavities , and the inner surfaces of blood vessels . Epithelial tissue is one of the four basic types of animal tissue , along with connective tissue , muscle tissue and nervous tissue . These tissues also lack blood or lymph supply. The tissue is supplied by nerves. There are three principal shapes of epithelial cell: squamous (scaly), columnar, and cuboidal. These can be arranged in
2788-661: The American copperhead and the West African carpet viper. The Placozoa show substantial evolutionary radiation in regard to sodium channels , of which they have 5–7 different types, more than any other invertebrate species studied to date. Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from
2870-495: 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 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
2952-510: 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 a phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done
3034-491: 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
3116-589: The animal feeds . All placozoa can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme . Sexual reproduction has been reported to occur in one clade of placozoans, whose strain H8 was later found to belong to genus Cladtertia , where intergenic recombination was observed as well as other hallmarks of sexual reproduction. Some Trichoplax species contain Rickettsiales bacteria as endosymbionts . One of
3198-521: The at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described Margulisbacteria , that lives inside cells used for algal digestion . It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return. Studies suggest that aragonite crystals in crystal cells have
3280-414: The basal lamina of the epithelium , were accordingly lost more recently by radical secondary simplification. Various studies in this regard so far yield differing results for identifying the exact sister group: In one case the Placozoa would qualify as the nearest relatives of the Cnidaria , while in another they would be a sister group to the Ctenophora , and occasionally they are placed directly next to
3362-475: The basis of a common taxon for all animals that possess them. One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in Trichoplax adhaerens evolved by reduction of the collars in the collar cells ( choanocytes ) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as
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3444-512: The blood and lymphatic vessels are of a specialised form of epithelium called endothelium . Epithelium lines both the outside ( skin ) and the inside cavities and lumina of bodies. The outermost layer of human skin is composed of dead stratified squamous , keratinized epithelial cells. Tissues that line the inside of the mouth, the esophagus , the vagina , and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from
3526-537: The blood vessels in the underlying tissue. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium. The basal lamina is made up of laminin (glycoproteins) secreted by epithelial cells. The reticular lamina beneath the basal lamina is made up of collagen proteins secreted by connective tissue . Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between
3608-412: The cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as pseudostratified columnar epithelium . Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on
3690-575: The cells, the stretch that they experience rapidly activates cell division. Alternatively, when too many cells accumulate, crowding triggers their death by activation epithelial cell extrusion . Here, cells fated for elimination are seamlessly squeezed out by contracting a band of actin and myosin around and below the cell, preventing any gaps from forming that could disrupt their barriers. Failure to do so can result in aggressive tumors and their invasion by aberrant basal cell extrusion. Epithelial tissues have as their primary functions: Glandular tissue
3772-422: The epithelium are classified as carcinomas . In contrast, sarcomas develop in connective tissue . When epithelial cells or tissues are damaged from cystic fibrosis , sweat glands are also damaged, causing a frosty coating of the skin. The word epithelium uses the Greek roots ἐπί ( epi ), "on" or "upon", and θηλή ( thēlē ), "nipple". Epithelium is so called because the name was originally used to describe
3854-488: The epithelium. Stratified or compound epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers, the cells can be squamous, cuboidal, or columnar. Stratified epithelia (of columnar, cuboidal, or squamous type) can have
3936-452: The filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is generally accepted that the epithelium arises from all three germ layers. Epithelia turn over at some of the fastest rates in the body. For epithelial layers to maintain constant cell numbers essential to their functions, the number of cells that divide must match those that die. They do this mechanically. If there are too few of
4018-504: 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
4100-447: The fluid flow, and mediation of fluid flow if the cilia are motile . Epithelial cells express many genes that encode immune mediators and proteins involved in cell-cell communication with hematopoietic immune cells. The resulting immune functions of these non-hematopoietic, structural cells contribute to the mammalian immune system ("structural immunity"). Relevant aspects of the epithelial cell response to infections are encoded in
4182-446: The following specializations: Epithelial tissue cells can adopt shapes of varying complexity from polyhedral to scutoidal to punakoidal. They are tightly packed and form a continuous sheet with almost no intercellular spaces. All epithelia is usually separated from underlying tissues by an extracellular fibrous basement membrane. The lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of
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#17328527055424264-468: 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 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
4346-536: The idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise. 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
4428-703: The influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida , 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
4510-415: The inner lining of blood vessels and the heart , and is known as vascular endothelium, and lining lymphatic vessels as lymphatic endothelium. Another type, mesothelium , forms the walls of the pericardium , pleurae , and peritoneum . In arthropods, the integument , or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle , an outer covering of chitin ,
4592-537: The lower epithelium with greater cell-type diversity. There is no convincing fossil record of the placozoa, although the Ediacaran biota (Precambrian, 550 million years ago ) organism Dickinsonia appears somewhat similar to placozoans. Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the Middle Triassic Muschelkalk ( Germany ). Traditionally, classification
4674-456: The mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest in Trichoplax starting in 1966. The most important descriptions were made by Karl Gottlieb Grell at the University of Tübingen since 1971. That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa. Grell derived
4756-708: The most basal animals, thus, constituting a primitive metazoan phylum. The first known placozoan, Trichoplax adhaerens , was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921). Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century, new species began to be added since 2018. So far, three other extant species have been described, in two distinct classes: Uniplacotomia ( Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022) and Polyplacotomia ( Polyplacotoma mediterranea ,
4838-618: The most basal, in 2019). A single putative fossil species is known, the Middle Triassic Maculicorpus microbialis . Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze , in a seawater aquarium at the Zoological Institute in Graz, Austria . The generic name is derived from the classical Greek θρίξ ( thrix ), meaning "hair", and πλάξ ( plax ), "plate". The specific epithet adhaerens
4920-412: The most reliable approximation of the placozoans' phylogeny. Their exact position on the phylogenetic tree would give important information about the origin of neurons and muscles. If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a sister group ; once in the Ctenophora , once in
5002-678: The name from the placula hypothesis, Otto Bütschli 's notion on the origin of metazoans . Placozoans do not have well-defined body plan, much like amoebas , unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob." An individual body measures about 0.55 mm in diameter. There are no body parts; as one of the researchers Michael Eitel described: "There's no mouth, there's no back, no nerve cells, nothing." Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells. Placozoans have only three anatomical parts as tissue layers inside its body:
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#17328527055425084-411: 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
5166-504: The number of their layers and by the shape and function of the cells. The basic cell types are squamous, cuboidal, and columnar, classed by their shape. By layer, epithelium is classed as either simple epithelium, only one cell thick (unilayered), or stratified epithelium having two or more cells in thickness, or multi-layered – as stratified squamous epithelium , stratified cuboidal epithelium , and stratified columnar epithelium , and both types of layering can be made up of any of
5248-447: The oldest branch of the multicellular animals, and a relic of the Ediacaran fauna , or even the pre-Ediacara fauna. Although very successful in their ecological niche , due to the absence of extracellular matrix and basal lamina , the development potential of these animals was of course limited, which would explain the low rate of evolution of their phenotype (their outward form as adults) – referred to as bradytely . This hypothesis
5330-490: The oral–aboral axis of cnidarians , animals from another phylum with which they are most closely related. Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences. Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes. Trichoplax is a small, flattened, animal around 1 mm (0.039 in) across. An amorphous multi-celled body, analogous to
5412-431: The outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs , the gastrointestinal tract , the reproductive and urinary tracts, and make up the exocrine and endocrine glands . The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. A specialised form of epithelium, endothelium , forms
5494-436: The phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens
5576-547: The precursor to the true epithelial tissue of the eumetazoans. In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm — the two basic embryonic cell layers of the eumetazoans. The digestive gastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals ( Bilateria ) may have developed from endoderm, whereas ectoderm
5658-427: The probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the substrate , leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to
5740-408: The rigidity of which varies as per its chemical composition. The basal surface of epithelial tissue rests on a basement membrane and the free/apical surface faces body fluid or outside. The basement membrane acts as a scaffolding on which epithelium can grow and regenerate after injuries. Epithelial tissue has a nerve supply , but no blood supply and must be nourished by substances diffusing from
5822-491: The same neuropeptide antibodies despite extant placozoans not developing any neurons. Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; 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
5904-411: 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, 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
5986-459: The same function as statoliths, allowing it to use gravity for spatial orientation . Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators. Genes has been found in Trichoplax with a strong resemblance to the venom genes of some poisonous snakes, like
6068-435: The specimens of which he collected from Naples. He gave the species name T. reptans in 1896. Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is ruled as doubtful, and the species rejected. Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria). In 1907, Thilo Krumbach published
6150-418: The substrate. In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed placuloid . Three different life strategies have accordingly led to three different possible lines of development: Should any of the analyses presented above turn out to be correct, Trichoplax adhaerens would be
6232-416: The terms as equivalent. 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 . The term phylum was coined in 1866 by Ernst Haeckel from
6314-420: The total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. 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
6396-486: The underlying connective tissue, through the basement membrane . Cell junctions are especially abundant in epithelial tissues. Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes. In general, epithelial tissues are classified by
6478-407: The upper, intermediate (middle) and lower epithelia . There are at least six different cell types. The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a cilium . Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number of mitochondria . The middle layer
6560-495: The whole genome sequences of six other animals and two related non-animal species. However, Ctenophora was not included in the analyses, placing the placozoans outside of the sampled Eumetazoans. DNA comparisons suggest that placozoans are related to Cnidaria , derived from planula larva (as seen in some Cnidaria). The Bilateria also are thought to be derived from planuloids. The Cnidaria and Placozoa body axis are overtly similar, and placozoan and cnidarian cells are responsive to
6642-497: Was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss and thus is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the sponges and the eumetazoa . In such a feature-poor phylum, molecular data are considered to provide
6724-401: Was supported by a recent analysis of the Trichoplax adhaerens mitochondrial genome in comparison to those of other animals. The hypothesis was, however, rejected in a statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only at the p = 0.07 level, which indicates
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