87-512: Planarians (triclads) are free-living flatworms of the class Turbellaria , order Tricladida , which includes hundreds of species, found in freshwater, marine, and terrestrial habitats. Planarians are characterized by a three-branched intestine, including a single anterior and two posterior branches. Their body is populated by adult stem cells called neoblasts , which planarians use for regenerating missing body parts. Many species are able to regenerate any missing organ, which has made planarians
174-426: A fluid-filled body cavity , and the space between their organ systems is filled with parenchyma . Planarians lack a circulatory system, and absorb oxygen through their body wall. They uptake food to their gut using a muscular pharynx, and nutrients diffuse to internal tissues. A three-branched intestine runs across almost the entire body, and includes a single anterior and two posterior branches. The planarian intestine
261-508: 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
348-977: A phylogenetically more correct classification, where the massively polyphyletic "Turbellaria" was split into a dozen orders, and Trematoda, Monogenea and Cestoda were joined in the new order Neodermata . However, the classification presented here is the early, traditional, classification, as it still is the one used everywhere except in scientific articles. These have about 4,500 species, are mostly free-living, and range from 1 mm (0.04 in) to 600 mm (24 in) in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded, humid locations, such as leaf litter or rotting wood. However, some are symbiotes of other animals, such as crustaceans , and some are parasites . Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored. The Acoela and Nemertodermatida were traditionally regarded as turbellarians, but are now regarded as members of
435-428: A bilobed shaped cerebral ganglion that is referred to as the planarian brain . Longitudinal ventral nerve chords extend from the brain to the tail. Transverse nerves, commissure , connect the ventral nerve chords forming ladder-like nerve system. The brain has been shown to exhibit spontaneous electrophysiological oscillations, similar to the electroencephalographic ( EEG ) activity of other animals. The planarian has
522-415: A complete organism over the course of a few weeks. New tissues can grow due to pluripotent stem cells that have the ability to create all the various cell types. These adult stem cells are called neoblasts , and comprise 20% or more of the cells in the adult animal. They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. In addition, existing tissue
609-604: A discipline. Planarians are also an emerging model organism for aging research . These animals have an apparently limitless regenerative capacity, and asexual Schmidtea mediterranea has been shown to maintain its telomere length through regeneration. Planarian regeneration combines new tissue production with reorganization to the existing anatomy, morphallaxis. The rate of tissue regrowth varies between species, but in frequently used lab species, functional regenerated tissues are available already 7–10 days following tissue amputation. Regeneration starts following an injury that require
696-409: A few are internal parasites. Adult monogeneans have large attachment organs at the rear, known as haptors (Greek ἅπτειν, haptein , means "catch"), which have suckers , clamps , and hooks. They often have flattened bodies. In some species, the pharynx secretes enzymes to digest the host's skin, allowing the parasite to feed on blood and cellular debris. Others graze externally on mucus and flakes of
783-500: A fish where it penetrates the body and encysts in the flesh, then migrating to the small intestine of a land animal that eats the fish raw, finally generating eggs that are excreted and ingested by snails, thereby completing the cycle. A similar life cycle occurs with Opisthorchis viverrini , which is found in South East Asia and can infect the liver of humans, causing Cholangiocarcinoma (bile duct cancer). Schistosomes, which cause
870-417: A free-living flatworm. In addition, the intermediate stages that live in snails reproduce asexually. Adults of different species infest different parts of the definitive host - for example the intestine , lungs , large blood vessels, and liver. The adults use a relatively large, muscular pharynx to ingest cells, cell fragments, mucus , body fluids or blood. In both the adult and snail-inhabiting stages,
957-482: A large sub-population of dormant or slow-cycling cells. The life history of planarians make them a model system for investigating a number of biological processes, many of which may have implications for human health and disease. Advances in molecular genetic technologies has made the study of gene function possible in these animals and scientists are studying them worldwide. Like other invertebrate model organisms, for example C. elegans and D. melanogaster ,
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#17328919053861044-528: 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
1131-413: A penis. A female spotted hyena's apparent penis is in fact an enlarged clitoris, which contains an external birth canal. It can be difficult to determine the sex of spotted hyenas until sexual maturity , when they may become pregnant. When a female spotted hyena gives birth, she passes the cub through the cervix internally, but then passes it out through the elongated clitoris. The term hermaphrodite
1218-605: A pharynx that is eversible (can be extended by being turned inside-out), and the mouths of different species can be anywhere along the underside. The freshwater species Microstomum caudatum can open its mouth almost as wide as its body is long, to swallow prey about as large as itself. Predatory species in suborder Kalyptorhynchia often have a muscular pharynx equipped with hooks or teeth used for seizing prey. Most turbellarians have pigment-cup ocelli ("little eyes"); one pair in most species, but two or even three pairs in others. A few large species have many eyes in clusters over
1305-576: A popular model in research of regeneration and stem cell biology. The genome sequences of several species are available, as are tools for molecular biology analysis. The order Tricladida is split into three suborders, according to their phylogenetic relationships: Maricola , Cavernicola and Continenticola . Formerly, the Tricladida was split according to their habitat: Maricola (marine planarians); Paludicola (freshwater planarian); and Terricola (land planarians). Planarians move by beating cilia on
1392-649: A result, the food can not 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
1479-420: A ring around the mouth and a larger sucker midway along what would be the underside in a free-living flatworm. Although the name "Digeneans" means "two generations", most have very complex life cycles with up to seven stages, depending on what combinations of environments the early stages encounter – the most important factor being whether the eggs are deposited on land or in water. The intermediate stages transfer
1566-463: A separate phylum, the Acoelomorpha , or as two separate phyla. Xenoturbella , a genus of very simple animals, has also been reclassified as a separate phylum. Some turbellarians have a simple pharynx lined with cilia and generally feed by using cilia to sweep food particles and small prey into their mouths, which are usually in the middle of their undersides. Most other turbellarians have
1653-472: A sequential hermaphrodite is also stipulated to only change sex once. ) This contrasts with simultaneous hermaphrodites, in which an individual possesses fully functional male and female genitalia. Sequential hermaphroditism is common in fish (particularly teleost fish) and many gastropods (such as the common slipper shell ). Sequential hermaphroditism can best be understood in terms of behavioral ecology and evolutionary life history theory , as described in
1740-522: A soft, flat, wedge-shaped body that may be black, brown, blue, gray, or white. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves
1827-710: A subgroup of seriates, are famous for their ability to regenerate if divided by cuts across their bodies. Experiments show that (in fragments that do not already have a head) a new head grows most quickly on those fragments which were originally located closest to the original head. This suggests the growth of a head is controlled by a chemical whose concentration diminishes throughout the organism, from head to tail. Many turbellarians clone themselves by transverse or longitudinal division, whilst others, reproduce by budding . The vast majority of turbellarians are hermaphrodites (they have both female and male reproductive cells) which fertilize eggs internally by copulation . Some of
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#17328919053861914-399: Is a blind sac , having no exit cavity, and therefore planarians uptake food and egest waste through the same orifice, located near the middle of the ventral body surface. The excretory system is made of many tubes with many flame cells and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste
2001-452: Is a sexually reproducing organism that produces both male and female gametes . Animal species in which individuals are either male or female are gonochoric , which is the opposite of hermaphroditic. The individuals of many taxonomic groups of animals , primarily invertebrates, are hermaphrodites, capable of producing viable gametes of both sexes . In the great majority of tunicates , mollusks , and earthworms , hermaphroditism
2088-481: Is a syncitium , which is a layer of cells that shares a single external membrane . Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea). These are often called flukes, as most have flat rhomboid shapes like that of a flounder (Old English flóc ). There are about 11,000 species, more than all other platyhelminthes combined, and second only to roundworms among parasites on metazoans . Adults usually have two holdfasts:
2175-430: Is a normal condition, enabling a form of sexual reproduction in which either partner can act as the female or male. Hermaphroditism is also found in some fish species , but is rare in other vertebrate groups. Most hermaphroditic species exhibit some degree of self-fertilization. The distribution of self-fertilization rates among animals is similar to that of plants, suggesting that similar pressures are operating to direct
2262-805: Is both male and female. And an hermaphrodite (which is also called Androgynus ) shall be heire, either as male or female, according to that kind of sexe which doth prevaile." During the Victorian era , medical authors attempted to ascertain whether or not humans could be hermaphrodites, adopting a precise biological definition to the term. From that period until the early 21st century, individuals with ovotesticular syndrome were termed true hermaphrodites if their gonadal tissue contained both testicular and ovarian tissue, and pseudohermaphrodites if their external appearance ( phenotype ) differed from sex expected from internal gonads. This language has fallen out of favor due to misconceptions and stigma associated with
2349-434: Is common in bryophytes and some vascular plants. Historically, the term hermaphrodite was used in law to refer to people whose sex was in doubt. The 12th-century Decretum Gratiani states that "Whether an hermaphrodite may witness a testament, depends on which sex prevails" ("Hermafroditus an ad testamentum adhiberi possit, qualitas sexus incalescentis ostendit."). Alexander ab Alexandro (1461–1523) stated, using
2436-530: Is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called protonephridia . In all platyhelminths, the nervous system is concentrated at the head end. Other platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies. Early classification divided the flatworms in four groups: Turbellaria, Trematoda, Monogenea and Cestoda. This classification had long been recognized to be artificial, and in 1985, Ehlers proposed
2523-422: Is not to be confused with ovotesticular syndrome in mammals, which is a separate and unrelated phenomenon. While people with the condition were previously called " true hermaphrodites" in medical literature, this usage is now considered to be outdated as of 2006 and misleading, as people with ovotesticular syndrome do not have functional sets of both male and female organs. The term hermaphrodite derives from
2610-494: Is released on the dorsal surface of the planarian. The triclads have an anterior end or head where sense organs, such as eyes and chemoreceptors , are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors. The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. Dugesia or Microplana ), others have many more distributed along
2697-483: Is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism. The organism itself does not have to be completely cut into separate pieces for the regeneration phenomenon to be witnessed. In fact, if the head of a planarian is cut in half down its center, and each side retained on the organism, it is possible for the planarian to regenerate two heads and continue to live. Researchers, including those from Tufts University in
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2784-544: Is used in botany to describe, for example, a perfect flower that has both staminate (male, pollen-producing) and carpellate (female, ovule-producing) parts. The overwhelming majority of flowering plant species are hermaphroditic. Flowering plant species with separate, imperfect, male and female flowers on the same individual are called monoecious . Monoecy only occurs in about 7% of flowering plant species. Monoecious plants are often referred to as hermaphroditic because they produce both male and female gametes. However,
2871-615: The Greek πλατύ, platy , meaning "flat" and ἕλμινς (root: ἑλμινθ-), helminth- , meaning " worm ") are a phylum of relatively simple bilaterian , unsegmented , soft-bodied invertebrates . Being acoelomates (having no body cavity ), and having no specialised circulatory and respiratory organs , they are restricted 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
2958-459: The Latin : hermaphroditus , from Ancient Greek : ἑρμαφρόδιτος , romanized : hermaphroditos , which derives from Hermaphroditus (Ἑρμαφρόδιτος), the son of Hermes and Aphrodite in Greek mythology . According to Ovid , he fused with the nymph Salmacis resulting in one individual possessing physical traits of male and female sexes. According to the earlier Diodorus Siculus , he
3045-742: The imported giant African snail Achatina fulica , which was displacing native snails. However, these planarians are themselves a serious threat to native snails and should not be used for biological control. In northwest Europe , there are concerns about the spread of the New Zealand planarian Arthurdendyus triangulatus , which preys on earthworms . Platyhelminthes are bilaterally symmetrical animals : their left and right sides are mirror images of each other; this also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other bilaterians , they have three main cell layers (endoderm, mesoderm , and ectoderm ), while
3132-677: The phylogenetic tree : Acoelomorpha [REDACTED] Deuterostomia [REDACTED] Ecdysozoa [REDACTED] Gnathifera [REDACTED] Gastrotricha [REDACTED] Platyhelminthes [REDACTED] Mollusca [REDACTED] Annelida [REDACTED] The internal relationships of Platyhelminthes are shown below. The tree is not fully resolved. Catenulida Haplopharyngida Macrostomida Prorhynchida Polycladida Gnosonesimida Kalyptorhynchia Dalytyphloplanida Proseriata Prolecithophora Fecampiida Hermaphrodism A hermaphrodite ( / h ər ˈ m æ f r ə ˌ d aɪ t / )
3219-484: The radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. Beyond that, they are "defined more by what they do not have than by any particular series of specializations." Unlike most other bilaterians, Platyhelminthes have no internal body cavity, so are described as acoelomates . Although the absence of a coelom also occurs in other bilaterians: gnathostomulids , gastrotrichs , xenacoelomorphs , cycliophorans , entoproctans and
3306-581: The sexual differentiation stage; however, this is now known to be incorrect, as humans are simply undifferentiated before this stage and possess a paramesonephric duct , a mesonephric duct , and a genital tubercle . The evolution of anisogamy may have contributed to the evolution of simultaneous hermaphroditism and sequential hermaphroditism, it remains unclear if the evolution of anisogamy first led to hermaphroditism or gonochorism . A 2023 study argued that hermaphroditism can evolve directly from mating types under certain circumstances, such as if
3393-552: The ventral dermis , allowing them to glide along on a film of mucus . Some also can move by undulations of the whole body by the contractions of muscles built into the body membrane. Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators. Phylogenetic supertree after Sluys et al., 2009: Maricola Cavernicola Planariidae Kenkiidae Dendrocoelidae Dugesiidae Geoplanidae Linnaean ranks after Sluys et al. , 2009: Planarians are bilaterian flatworms that lack
3480-684: The International Space Station (ISS) – though regeneration of amputated worms as double-headed heteromorphosis is not a rare phenomenon unique to a microgravity environment. In contrast, two-headed planaria regenerates can be induced by exposing amputated fragments to electrical fields. Such exposure with opposite polarity can induce a planarian with 2 tails. Two-headed planaria regenerates can be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells, as well as by genetic expression blocks (interference RNA) to
3567-427: The U.S., sought to determine how microgravity and micro-geomagnetic fields would affect the growth and regeneration of planarian flatworms , Dugesia japonica . They discovered that one of the amputated fragments sent to space regenerated into a double-headed worm. The majority of such amputated worms (95%) did not do so, however. An amputated worm regenerated into a double-head creature after spending five weeks aboard
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3654-490: 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
3741-574: The adults take several years to change from female to male, the broodstock are extremely valuable individuals. Simultaneous hermaphrodites (or homogamous hermaphrodites) are individuals in which both male and female sexual organs are present and functional at the same time. Self-fertilization often occurs. When spotted hyenas were first scientifically observed by explorers, they were thought to be hermaphrodites. Early observations of wild spotted hyenas led researchers to believe that all spotted hyenas, male or female, were born with what looked to be
3828-407: The body (e.g. most Geoplaninae ). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind. The body of the triclads is covered by a ciliated epidermis that contains rhabdites . Between the epidermis and the gastrodermis there is a parenchymatous tissue or mesenchyme . The planarian nervous systems consists of
3915-436: The body can alter the expression of genes that encode molecules that provide positional information. Therefore, the activities of neoblasts and muscle cells following injuries are essential for successful regeneration. Historically, planarians have been considered "immortal under the edge of a knife." Very small pieces of the planarian, estimated to be as little as 1/279th of the organism it is cut from, can regenerate back into
4002-399: The brain, mounted on tentacles, or spaced uniformly around the edge of the body. The ocelli can only distinguish the direction from which light is coming to enable the animals to avoid it. A few groups have statocysts - fluid-filled chambers containing a small, solid particle or, in a few groups, two. These statocysts are thought to function as balance and acceleration sensors, as they perform
4089-417: The canonical Wnt/β-Catenin signalling pathway. In 1955, Robert Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut
4176-477: The devastating tropical disease bilharzia , also belong to this group. Adults range between 0.2 mm (0.0079 in) and 6 mm (0.24 in) in length. Individual adult digeneans are of a single sex, and in some species slender females live in enclosed grooves that run along the bodies of the males, partially emerging to lay eggs. In all species the adults have complex reproductive systems, capable of producing between 10,000 and 100,000 times as many eggs as
4263-461: 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
4350-561: The evolution of selfing in animals and plants. A rough estimate of the number of hermaphroditic animal species is 65,000, about 5% of all animal species, or 33% excluding insects. Insects are almost exclusively gonochoric, and no definitive cases of hermaphroditism have been demonstrated in this group. There are no known hermaphroditic species among mammals or birds . About 94% of flowering plant species are either hermaphroditic (all flowers produce both male and female gametes) or monoecious , where both male and female flowers occur on
4437-429: The external syncytium absorbs dissolved nutrients from the host. Adult digeneans can live without oxygen for long periods. Members of this small group have either a single divided sucker or a row of suckers that cover the underside. They infest the guts of bony or cartilaginous fish, turtles, or the body cavities of marine and freshwater bivalves and gastropods . Their eggs produce ciliated swimming larvae, and
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#17328919053864524-439: The fertilization is well organized and the average size of groups is small. Simultaneous hermaphroditism that exclusively reproduces through self-fertilization has evolved many times in plants and animals, but it might not last long evolutionarily. Joan Roughgarden and Priya Iyer argued that the last common ancestor for animals was hermaphroditic and that transitions from hermaphroditism to gonochorism were more numerous than
4611-431: The growth of ovarian tissues ). Thus, testicular and ovarian tissues will both be present in the same individual. Of all total recorded cases of ovotesticular DSD, in only 8% percent of all cases was SRY present, leaving the rest of cases that could be explained to other or less common causes, with the vast majority simply being currently unexplainable. Fetuses were previously thought to be phenotypically female before
4698-604: The growth of a new tissue. Neoblasts localized near the injury site proliferate to generate a structure of differentiating cells called blastema . Neoblasts are required for new cell production, and they therefore provide the cellular basis for planarian regeneration. Cell signaling mechanisms provide positional information that regulates the cell types and tissues that are produced from the neoblasts in regeneration. Many signaling molecules that provide positional information to neoblasts, in regeneration and homeostasis, are expressed in muscle cells. Following injury, muscle cells throughout
4785-509: The gut or pharynx (throat). All animals need to keep the concentration of dissolved substances in their body fluids at a fairly constant level. Internal parasites and free-living marine animals live in environments with high concentrations of dissolved material, and generally let their tissues have the same level of concentration as the environment, while freshwater animals need to prevent their body fluids from becoming too dilute. Despite this difference in environments, most platyhelminths use
4872-442: The host, and also disguises it chemically to avoid attacks by the host's immune system . Shortage of carbohydrates in the host's diet stunts the growth of parasites and may even kill them. Their metabolisms generally use simple but inefficient chemical processes, compensating for this inefficiency by consuming large amounts of food relative to their physical size. In the majority of species, known as eucestodes ("true tapeworms"),
4959-480: The hosts' skins. The name "Monogenea" is based on the fact that these parasites have only one nonlarval generation. These are often called tapeworms because of their flat, slender but very long bodies – the name " cestode " is derived from the Latin word cestus , which means "tape". The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and the syncitial skin absorbs nutrients – mainly carbohydrates and amino acids – from
5046-553: The individual flowers are not hermaphroditic if they only produce gametes of one sex. 65% of gymnosperm species are dioecious, but conifers are almost all monoecious. Some plants can change their sex throughout their lifetime, a phenomenon called sequential hermaphroditism . In andromonoecious species, the plants produce perfect (hermaphrodite) flowers and separate fertile male flowers that are sterile as female. Andromonoecy occurs in about 4000 species of flowering plants (2% of flowering plants). In gynomonoecious species,
5133-566: The lab. In recent decades, S. mediterranea has emerged as the species of choice for modern molecular biology research, due to its diploid chromosomes and the availability of both asexual and sexual strains. The most frequently used planarian in high school and first-year college laboratories is the brownish Girardia tigrina . Other common species used are the blackish Planaria maculata and Girardia dorotocephala . Flatworm Traditional: Phylogenetic: The flatworms , flat worms , Platyhelminthes , or platyhelminths (from
5220-561: The large species have flat ribbon-like or leaf-like shapes. Because there is no circulatory system which can transport nutrients around, the guts of large species have many branches, allowing the nutrients to diffuse to all parts of the body. Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments where dehydration is unlikely: sea and freshwater, moist terrestrial environments such as leaf litter or between grains of soil, and as parasites within other animals. The space between
5307-463: The larger aquatic species mate by penis fencing – a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs. In most species, "miniature adults" emerge when the eggs hatch, but a few large species produce plankton -like larvae . These parasites' name refers to the cavities in their holdfasts (Greek τρῆμα, hole), which resemble suckers and anchor them within their hosts. The skin of all species
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#17328919053865394-454: The life cycle has one or two hosts. Cercomeromorpha contains parasites attach themselves to their hosts by means of disks that bear crescent-shaped hooks. They are divided into the Monogenea and Cestoda groupings. Of about 1,100 species of monogeneans , most are external parasites that require particular host species - mainly fish, but in some cases amphibians or aquatic reptiles. However,
5481-475: The mouth. The genus Paracatenula , whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut. However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them. The gut is lined with a single layer of endodermal cells that absorb and digest food. Some species break up and soften food first by secreting enzymes in
5568-439: The neck produces a chain of segments called proglottids via a process known as strobilation . As a result, the most mature proglottids are furthest from the scolex. Adults of Taenia saginata , which infests humans, can form proglottid chains over 20 metres (66 ft) long, although 4 metres (13 ft) is more typical. Each proglottid has both male and female reproductive organs. If the host's gut contains two or more adults of
5655-491: The other planarian, giving and receiving sperm . Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. In asexual reproduction, the planarian fissions and each fragment regenerates its missing tissues, generating complete anatomy and restoring functions. Asexual reproduction, similar to regeneration following injury, requires neoblasts , adult stem cells, which proliferate and produce differentiated cells. Some researchers claim that
5742-420: The parasites from one host to another. The definitive host in which adults develop is a land vertebrate; the earliest host of juvenile stages is usually a snail that may live on land or in water, whilst in many cases, a fish or arthropod is the second host. For example, the adjoining illustration shows the life cycle of the intestinal fluke metagonimus , which hatches in the intestine of a snail, then moves to
5829-455: The parastic mesozoans . They also lack specialized circulatory and respiratory organs, both of these facts are defining features when classifying a flatworm's anatomy . Their bodies are soft and unsegmented. The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable oxygen to reach and carbon dioxide to leave all parts of their bodies by simple diffusion . Hence, many are microscopic, and
5916-739: The planarian parenchyma across the planarian body. They are small and round cells, 5 to 10 μm, and characterized by a large nucleus, which is surrounded by little cytoplasm. Neoblasts are required for regenerating missing tissues and organs, and they continuously replenish tissues by producing new cells. Neoblasts can self-renew and generate progenitors for different cell types. In contrast to adult vertebrate stem cells (e.g., hematopoietic stem cell ), neoblasts are pluripotent (i.e., producing all somatic cell types). Moreover, they give rise to differentiating, post-mitotic, cells directly, and not by producing rapidly-dividing transit amplifying cells . Consequently, neoblasts divide frequently, and apparently lack
6003-401: The planarian's body by diffusing through the body wall. Triclads reproduce sexually and asexually, and different species may be able to reproduce by one or both modes. Planarians are hermaphrodites . In sexual reproduction, the mating generally involves mutual insemination. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to
6090-513: The plants produce hermaphrodite flowers and separate male-sterile pistillate flowers. One example is the meadow saxifrage, Saxifraga granulata . Charles Darwin gave several other examples in his 1877 book "The Different Forms of Flowers on Plants of the Same Species". About 57% of moss species and 68% of liverworts are unisexual , meaning that their gametophytes produce either male or female gametes, but not both. Sequential hermaphroditism
6177-584: The products derived from bisecting a planarian are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planarians and its effect on the population are ongoing. Some species of planarian are exclusively asexual, whereas some can reproduce both sexually and asexually. In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in Cura foremanii ). Neoblasts are abundant adult stem cells that are found in
6264-421: The relative simplicity of planarians facilitates experimental study. Planarians have a number of cell types, tissues and simple organs that are homologous to our own cells , tissues and organs . However, regeneration has attracted the most attention. Thomas Hunt Morgan was responsible for some of the first systematic studies (that still underpin modern research) before the advent of molecular biology as
6351-474: The result of an atypical set of sex chromosomes. One common cause of variations in sex characteristics traits is the crossing over of the testis-determining factor (SRY) from the Y chromosome to the X chromosome during meiosis . The SRY is then activated in only certain areas, causing development of testes in some areas by beginning a series of events starting with the upregulation of the transcription factor (SOX9), and in other areas not being active (causing
6438-487: The reverse. Other scientists have criticized this argument; saying it’s based on paraphyletic Spiralia , assignments of sexual modes for the phylum level than the species level, and methods exclusively based on maximum parsimony . Hermaphroditism is polyphyletic in invertebrates where it evolved from gonochorism and gonochorism is also ancestral to hermaphroditic fishes. According to Nelson Çabej simultaneous hermaphroditism in animals most likely evolved due to
6525-618: The same cestode species they generally fertilize each other, however, proglottids of the same worm can fertilize each other and even themselves. When the eggs are fully developed, the proglottids separate and are excreted by the host. The eucestode life cycle is less complex than that of digeneans , but varies depending on the species. For example: Members of the smaller group known as Cestodaria have no scolex, do not produce proglottids, and have body shapes similar to those of diageneans. Cestodarians parasitize fish and turtles. The relationships of Platyhelminthes to other Bilateria are shown in
6612-442: The same plant. There are also mixed breeding systems , in both plants and animals, where hermaphrodite individuals coexist with males (called androdioecy ) or with females (called gynodioecy ), or all three exist in the same species (called trioecy ). Sometimes, both male and hermaphrodite flowers occur on the same plant ( andromonoecy ) or both female and hermaphrodite flowers occur on the same plant ( gynomonoecy ). Hermaphrodism
6699-495: The same system to control the concentration of their body fluids. Flame cells , so called because the beating of their flagella looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some reusable material, and drive it into networks of tube cells which are lined with flagella and microvilli . The tube cells' flagella drive the water towards exits called nephridiopores , while their microvilli reabsorb reusable materials and as much water as
6786-425: The same way in cnidarian medusae and in ctenophores . However, turbellarian statocysts have no sensory cilia, so the way they sense the movements and positions of solid particles is unknown. On the other hand, most have ciliated touch-sensor cells scattered over their bodies, especially on tentacles and around the edges. Specialized cells in pits or grooves on the head are most likely smell sensors. Planarians ,
6873-568: The size-advantage mode first proposed by Michael T. Ghiselin which states that if an individual of a certain sex could significantly increase its reproductive success after reaching a certain size, it would be to their advantage to switch to that sex. Sequential hermaphrodites can be divided into three broad categories: Dichogamy can have both conservation-related implications for humans, as mentioned above, as well as economic implications. For instance, groupers are favoured fish for eating in many Asian countries and are often aquacultured . Since
6960-658: The skin and gut is filled with mesenchyme , also known as parenchyma , a connective tissue made of cells and reinforced by collagen fibers that act as a type of skeleton , providing attachment points for muscles . The mesenchyme contains all the internal organs and allows the passage of oxygen, nutrients and waste products. It consists of two main types of cell: fixed cells, some of which have fluid-filled vacuoles ; and stem cells , which can transform into any other type of cell, and are used in regenerating tissues after injury or asexual reproduction . Most platyhelminths have no anus and regurgitate undigested material through
7047-631: The term hermaphrodite, that the people who bore the sexes of both man and woman were regarded by the Athenians and the Romans as monsters, and thrown into the sea at Athens and into the Tiber at Rome. Similarly, the 17th-century English jurist and judge Edward Coke (Lord Coke), wrote in his Institutes of the Lawes of England on laws of succession stating, "Every heire is either a male, a female, or an hermaphrodite, that
7134-505: The terminology " disorders of sex development " (also known as variations in sex characteristics .) This is particularly significant because of the relationship between medical terminology and medical intervention. Intersex civil society organizations , and many human rights institutions, have criticized medical interventions designed to make bodies more typically male or female. In some cases, variations in sex characteristics are caused by unusual levels of sex hormones, which may be
7221-399: The terms, and also a shift to nomenclature based on genetics. The term "intersex" described a wide variety of combinations of what are ambiguous biological characteristics. Intersex biology may include, for example, ambiguous-looking external genitalia, karyotypes that include mixed XX and XY chromosome pairs (46XX/46XY, 46XX/47XXY or 45X/XY mosaic ). Clinically, medicine currently uses
7308-535: The untrained flatworms were only following tracks left on the dirty glassware rather than absorbing the memory of their fodder. In 2012, Tal Shomrat and Michael Levin have shown that planarians exhibit evidence of long-term memory retrieval after regenerating a new head. Several planarian species are commonly used for biological research. Popular experimental species are Schmidtea mediterranea , Schmidtea polychroa , and Dugesia japonica , which in addition to excellent regenerative abilities, are easy to culture in
7395-509: The worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1963, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. He reported that the flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms. This experiment intended to test whether memory could be transferred chemically. The experiment
7482-508: Was born with a physical body combining male and female sexes. The word hermaphrodite entered the English lexicon as early as the late fourteenth century. Sequential hermaphrodites ( dichogamy ) occur in species in which the individual first develops as one sex, but can later change into the opposite sex. (Definitions differ on whether sequential hermaphroditism encompasses serial hermaphroditism; for authors who exclude serial hermaphroditism,
7569-654: Was repeated with mice, fish, and rats, but it always failed to produce the same results. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed the behavior. McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA . McConnell's results are now attributed to observer bias . No blinded experiment has ever reproduced his results of planarians scrunching when exposed to light. Subsequent explanations of this scrunching behaviour associated with cannibalism of trained planarian worms were that
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