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60-547: The lophophore ( / ˈ l ɒ f ə ˌ f ɔːr , ˈ l oʊ f ə -/ ) is a characteristic feeding organ possessed by four major groups of animals : the Brachiopoda , Bryozoa , Hyolitha , and Phoronida , which collectively constitute the protostome group Lophophorata . All lophophores are found in aquatic organisms. Lophophore is derived from the Greek lophos (crest, tuft) and -phore , -phoros (φορος) (bearing),

120-459: A coelomic space thought to be a mesocoel . The gut is U-shaped with the anterior mouth at the center of the lophophore. The anus , where present, is also anterior, but is dorsal to the mouth. In the Bryozoa , it is outside the ring of the lophophore. The inarticulate brachiopods do not have an anus. Groups with lophophores are called lophophorates . In the old view of metazoan phylogeny ,

180-493: A derivative of phérein (φέρειν) (to bear); thus crest-bearing. The lophophore can most easily be described as a ring of ciliated tentacles surrounding the mouth, but it is often horseshoe-shaped or coiled. Phoronids have their lophophores in plain view, but the valves of brachiopods must be opened wide to get a good view of their lophophore. The lophophore surrounds the mouth and is an upstream collecting system for suspension feeding. Its tentacles are hollow, with extensions of

240-504: 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

300-404: 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

360-546: 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

420-675: 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 ,

480-438: 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

540-473: 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

600-416: A variety of organ systems . These specific systems are widely studied in human anatomy . The functions of these organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus . For this reason, the two systems are combined and studied as the neuroendocrine system . The same is true for the musculoskeletal system because of

660-687: A whole, are referred to as the Organon because logic is a tool for philosophical thinking. Earlier thinkers, such as those who wrote texts in the Hippocratic corpus , generally did not believe that there were organs of the body but only different parts of the body. Some alchemists (e.g. Paracelsus ) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows: Chinese traditional medicine recognizes eleven organs, associated with

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720-492: 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 ,

780-405: 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 the most basal animals, thus, constituting

840-526: 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

900-520: 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

960-406: Is the placenta , which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa. Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues,

1020-408: 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

1080-463: 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

1140-575: 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

1200-623: 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

1260-640: The Hyolitha has long been disputed, but as of 2017, it has been assigned to the Lophophorata as finely-preserved specimens in the Burgess Shale can be seen to carry lophophores. Lophophorates did appear paraphyletic, but that is contested. Organ (biology) An organ's tissues can be broadly categorized as parenchyma , the functional tissue, and stroma , the structural tissue with supportive, connective, or ancillary functions. For example,

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1320-433: 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

1380-494: The five Chinese traditional elements and with yin and yang , as follows: The Chinese associated the five elements with the five planets (Jupiter, Mars, Venus, Saturn, and Mercury) similar to the way the classical planets were associated with different metals. The yin and yang distinction approximates the modern notion of solid and hollow organs. Placozoa Placozoa ( / ˌ p l æ k ə ˈ z oʊ ə / PLAK -ə- ZOH -ə ; lit.   ' flat animals ' )

1440-426: The gland 's tissue that makes the hormones is the parenchyma , whereas the stroma includes the nerves that innervate the parenchyma, the blood vessels that oxygenate and nourish it and carry away its metabolic wastes, and the connective tissues that provide a suitable place for it to be situated and anchored. The main tissues that make up an organ tend to have common embryologic origins, such as arising from

1500-418: The haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. This practice remains an important ritual in some remote, tribal societies. The term "visceral" is contrasted with the term " parietal ", meaning "of or relating to the wall of a body part, organ or cavity " The two terms are often used in describing a membrane or piece of connective tissue, referring to

1560-417: The stomach , intestines , gallbladder , bladder , and rectum . In the thoracic cavity , the heart is a hollow, muscular organ. Splanchnology is the study of the viscera. The term "visceral" is contrasted with the term " parietal ", meaning "of or relating to the wall of a body part, organ or cavity ". The two terms are often used in describing a membrane or piece of connective tissue, referring to

1620-461: The 20th century, organ transplants began to take place as scientists knew more about the anatomy of organs. These came later in time as procedures were often dangerous and difficult. Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, including distributive justice , are developed in

1680-676: 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

1740-441: The acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types. The study of plant organs is covered in plant morphology . Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs include roots , stems , and leaves . The reproductive organs are variable. In flowering plants , they are represented by the flower , seed and fruit . In conifers ,

1800-604: 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

1860-522: 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

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1920-415: 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

1980-482: 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

2040-402: The ethical analysis. This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing. The English word "organ" dates back to the twelfth century and refers to any musical instrument. By the late 14th century, the musical term's meaning had narrowed to refer specifically to the keyboard-based instrument . At

2100-468: The liver and heart evolved in the chordates about 550-500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, molluscs, and worms about 700–650 million years ago. Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research

2160-856: The lophophorates were placed within the Deuterostomia . Now, they have been reassessed and placed in a new superphylum known as the Lophotrochozoa in the Protostomia , which includes the Mollusca and Annelida . Newer phylogeny place the bryozoans in the group Polyzoa, which also includes entoproctans and Cycliophora, while molluscs, brachiozoans and annelids make up their own group, with brachiozoans and annelids as possible sister taxa. The extinct hederelloids , microconchids , cornulitids , and tentaculitids were likely lophophorates based on their biomineralization. The position of

2220-556: 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

2280-467: 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

2340-647: 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

2400-412: 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

2460-683: 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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2520-467: The new generation of plants (see clonal colony ). Many societies have a system for organ donation , in which a living or deceased donor's organ are transplanted into a person with a failing organ. The transplantation of larger solid organs often requires immunosuppression to prevent organ rejection or graft-versus-host disease . There is considerable interest throughout the world in creating laboratory-grown or artificial organs . Beginning in

2580-448: 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

2640-403: The opposing sides. Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal) because, in ancient Greek, the word ' organon ' means 'tool', and Aristotle believed that the organs of the body were tools for us by means of which we can do things. For similar reasons, his logical works, taken as

2700-406: The opposing sides. The organ level of organisation in animals can be first detected in flatworms and the more derived phyla , i.e. the bilaterians . The less-advanced taxa (i.e. Placozoa , Porifera , Ctenophora and Cnidaria ) do not show consolidation of their tissues into organs. More complex animals are composed of different organs, which have evolved over time. For example,

2760-494: 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

2820-514: The organ that bears the reproductive structures is called a cone . In other divisions ( phyla ) of plants, the reproductive organs are called strobili , in Lycopodiophyta , or simply gametophores in mosses . Common organ system designations in plants include the differentiation of shoot and root. All parts of the plant above ground (in non- epiphytes ), including the functionally distinct leaf and flower organs, may be classified together as

2880-554: 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

2940-427: 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

3000-413: The relationship between the muscular and skeletal systems . In the study of anatomy , viscera ( sg. : viscus ) refers to the internal organs of the abdominal , thoracic , and pelvic cavities . The abdominal organs may be classified as solid organs or hollow organs . The solid organs are the liver , pancreas , spleen , kidneys , and adrenal glands . The hollow organs of the abdomen are

3060-534: The same germ layer . Organs exist in most multicellular organisms . In single-celled organisms such as members of the eukaryotes , the functional analogue of an organ is known as an organelle . In plants, there are three main organs. The number of organs in any organism depends on the definition used. There are approxiamately 79 Organs in the human body,but it is something that is debated as not all scientist agree on what counts as an organ. Except for placozoans , multicellular animals including humans have

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3120-466: 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

3180-489: The same time, a second meaning arose, in reference to a "body part adapted to a certain function". Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal. When three or more organs are present, it is called an organ system. The adjective visceral , also splanchnic , is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like

3240-409: The shoot organ system. The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis , while the reproductive organs are essential in reproduction . However, if there is asexual vegetative reproduction , the vegetative organs are those that create

3300-439: 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

3360-418: 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

3420-410: 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

3480-506: 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

3540-501: 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

3600-402: 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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