Platyhelminthes (from Ancient Greek πλατύ platy'flat' and ἕλμινς helmins'parasitic worm')[4] is aphylum of relatively simplebilaterian,unsegmented, soft-bodiedinvertebrates commonly calledflatworms orflat worms. Beingacoelomates (having nobody cavity), and having no specialisedcirculatory andrespiratoryorgans, they are restricted to having flattened shapes that allowoxygen and nutrients to pass through their bodies bydiffusion. The digestive cavity has only one opening for both ingestion (intake of nutrients) and egestion (removal of undigested wastes); as a result, thefood can not be processed continuously.
In traditional medicinal texts, Platyhelminthes are divided intoTurbellaria, which are mostly non-parasiticanimals such asplanarians, and three entirely parasitic groups:Cestoda,Trematoda andMonogenea; however, since the turbellarians have since been proven not to bemonophyletic, this classification is now deprecated. Free-living flatworms are mostly predators, and live inwater or in shaded, humid terrestrial environments, such asleaf litter. Cestodes (tapeworms) and trematodes (flukes) have complex life-cycles, with mature stages that live as parasites in the digestive systems offish or landvertebrates, and intermediate stages that infest secondary hosts. The eggs of trematodes are excreted from their main hosts, whereas adult cestodes generate vast numbers ofhermaphroditic, segment-likeproglottids that detach when mature, are excreted, and then release eggs. Unlike the other parasitic groups, the monogeneans are external parasites infestingaquatic animals, and theirlarvae metamorphose into the adult form after attaching to a suitable host.
Because they do not have internalbody cavities, Platyhelminthes were regarded as a primitive stage in theevolution of bilaterians (animals withbilateral symmetry and hence with distinct front and rear ends). However, analyses since the mid-1980s have separated out one subgroup, theAcoelomorpha, asbasal bilaterians – closer to the originalbilaterians than to any other modern groups. The remaining Platyhelminthes form amonophyletic group, one that contains all and only descendants of acommon ancestor that is itself a member of the group. The redefined Platyhelminthes is part of theSpiralia, one of the two main groups ofProtostomia. These analyses had concluded the redefined Platyhelminthes, excluding Acoelomorpha, consists of two monophyletic subgroups,Catenulida andRhabditophora, with Cestoda, Trematoda and Monogenea forming a monophyletic subgroup within one branch of the Rhabditophora. Hence, the traditional platyhelminth subgroup "Turbellaria" is now regarded asparaphyletic, since it excludes the wholly parasitic groups, although these are descended from one group of "turbellarians".
A planarian species has been used in thePhilippines and theMaldives in an attempt tocontrol populations of theimported giant African snail (Achatina fulica), which was eating agricultural crops. Success was initially reported for the Maldives but this was only temporary and the role of flatworms has been questioned.[5] These planarians have now spread very widely throughout the tropics and are themselves a serious threat to native snails, and should not be used for biological control. InNorthwestern Europe, there are concerns about the spread of theNew Zealand planarianArthurdendyus triangulatus, which preys onearthworms.
The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enableoxygen to reach andcarbon dioxide to leave all parts of their bodies by simplediffusion. Hence, many are microscopic, and 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.[8] Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments wheredehydration is unlikely: sea and freshwater, moist terrestrial environments such asleaf litter or between grains of soil, and asparasites within other animals.[6]
The space between the skin and gut is filled withmesenchyme, also known asparenchyma, aconnective tissue made ofcells and reinforced bycollagen fibers that act as a type ofskeleton, providing attachment points formuscles. 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-filledvacuoles; andstem cells, which can transform into any other type of cell, and are used in regenerating tissues after injury orasexual reproduction.[6]
Most platyhelminths have noanus and regurgitate undigested material through the mouth. The genusParacatenula, whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut.[17] 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.[14] The gut is lined with a single layer ofendodermal cells that absorb and digest food. Some species break up and soften food first by secretingenzymes in the gut orpharynx (throat).[6]
All animals need to keep theconcentration 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 the same system tocontrol the concentration of their body fluids.Flame cells, so called because the beating of theirflagella 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 andmicrovilli. The tube cells' flagella drive the water towards exits callednephridiopores, while their microvilli reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are calledprotonephridia.[6][16]
In all platyhelminths, thenervous system is concentrated at the head end. Other platyhelminths have rings ofganglia in the head and main nerve trunks running along their bodies.[6][14]
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[18] proposed aphylogenetically more correct classification, where the massivelypolyphyletic "Turbellaria" was split into a dozen orders, and Trematoda, Monogenea and Cestoda were joined in the new orderNeodermata. However, the classification presented here is the early, traditional, classification, as it still is the one used everywhere except in scientific articles.[6][19]
These have about 4,500 species,[14] 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 aresymbiotes of other animals, such ascrustaceans, and some areparasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.[6] TheAcoela andNemertodermatida were traditionally regarded as turbellarians,[14][20] but are now regarded as members of a separate phylum, theAcoelomorpha,[21][22] or as two separate phyla.[23]Xenoturbella, agenus of very simple animals,[24] has also been reclassified as a separate phylum.[25]
Some turbellarians have a simplepharynx lined withcilia 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 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.[6] The freshwater speciesMicrostomum caudatum can open its mouth almost as wide as its body is long, to swallow prey about as large as itself.[14] Predatory species in suborderKalyptorhynchia often have a muscular pharynx equipped with hooks or teeth used for seizing prey.[26]
Most turbellarians have pigment-cupocelli ('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 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 havestatocysts – 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 the same way incnidarianmedusae and inctenophores. 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.[14]
Planarians, 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 turbellariansclone themselves by transverse or longitudinal division, whilst others, reproduce bybudding.[14]
The vast majority of turbellarians arehermaphrodites (they have both female and male reproductive cells) which fertilize eggsinternally bycopulation.[14] Some of the larger aquatic species mate bypenis fencing – a duel in which each tries to impregnate the other, and the loser adopts the female role of developing the eggs.[27] In most species, "miniature adults" emerge when the eggs hatch, but a few large species produceplankton-likelarvae.[14]
These parasites' name refers to the cavities in theirholdfasts (Greekτρῆμαtrêma 'hole'),[6] which resemble suckers and anchor them within their hosts.[15] The skin of all species is asyncitium, which is a layer of cells that shares a single externalmembrane. Trematodes are divided into two groups, Digenea and Aspidogastrea (also known as Aspodibothrea).[14]
These are often called flukes, as most have flatrhomboid shapes like that of aflounder (Old Englishflóc). There are about 11,000 species, more than all other platyhelminthes combined, and second only toroundworms among parasites onmetazoans.[14] Adults usually have two holdfasts: a ring around the mouth and a larger sucker midway along what would be the underside in a free-living flatworm.[6]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 the parasites from one host to another. Thedefinitive 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.[14] For example, the adjoining illustration shows the life cycle of the intestinal flukemetagonimus, which hatches in the intestine of a snail, then moves to 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 withOpisthorchis viverrini, which is found in South East Asia and can infect the liver of humans, causingCholangiocarcinoma (bile duct cancer). Schistosomes, which cause the devastating tropical diseasebilharzia, also belong to this group.[28]
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 a free-living flatworm. In addition, the intermediate stages that live in snails reproduce asexually.[14]
Adults of different species infest different parts of the definitive host - for example theintestine,lungs, large blood vessels,[6] and liver.[14] The adults use a relatively large, muscularpharynx to ingest cells, cell fragments,mucus, body fluids or blood. In both the adult and snail-inhabiting stages, the external syncytium absorbs dissolved nutrients from the host. Adult digeneans can live without oxygen for long periods.[14]
Members of this small group have either a single divided sucker or a row of suckers that cover the underside.[14] They infest the guts ofbony orcartilaginous fish, turtles, or the body cavities of marine and freshwaterbivalves andgastropods.[6] Their eggs produceciliated swimming larvae, and the life cycle has one or two hosts.[14]
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.[14]
Of about 1,100 species ofmonogeneans, most are external parasites that require particular host species - mainly fish, but in some cases amphibians or aquatic reptiles. However, a few are internal parasites. Adult monogeneans have large attachment organs at the rear, known ashaptors (Greek ἅπτειν,haptein, means "catch"), which havesuckers,clamps, and hooks. They often have flattened bodies. In some species, thepharynx 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 the hosts' skins. The name "Monogenea" is based on the fact that these parasites have only one nonlarval generation.[14]
Life cycle of the eucestodeTaenia: Inset 5 shows the scolex, which has fourTaenia solium, a disk with hooks on the end. Inset 6 shows the tapeworm's whole body, in which the scolex is the tiny, round tip in the top left corner, and a mature proglottid has just detached.
These are often called tapeworms because of their flat, slender but very long bodies – the name "cestode" is derived from theLatin wordcestus, which means 'tape'. The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and thesyncitial skin absorbs nutrients – mainlycarbohydrates andamino acids – from the host, and also disguises it chemically to avoid attacks by the host'simmune system.[14] Shortage of carbohydrates in the host's diet stunts the growth of parasites and may even kill them. Theirmetabolisms generally use simple but inefficient chemical processes, compensating for this inefficiency by consuming large amounts of food relative to their physical size.[6]
In the majority of species, known as eucestodes ('true tapeworms'), the neck produces a chain of segments called proglottids via a process known asstrobilation. As a result, the most mature proglottids are furthest from the scolex. Adults ofTaenia 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 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 ofdigeneans, but varies depending on the species. For example:
Adults ofDiphyllobothrium infest fish, and the juveniles usecopepod crustaceans as intermediate hosts. Excreted proglottids release their eggs into the water where the eggs hatch intociliated, swimming larvae. If a larva is swallowed by a copepod, it sheds the cilia and the skin becomes a syncitium; the larva then makes its way into the copepod'shemocoel (an internal cavity which is the central part of thecirculatory system) where it attaches itself using three small hooks. If the copepod is eaten by a fish, the larvametamorphoses into a small, unsegmented tapeworm, drills through to the gut and grows into an adult.[14]
Various species ofTaenia infest the guts of humans, cats and dogs. The juveniles use herbivores – such as pigs, cattle and rabbits – as intermediate hosts. Excreted proglottids release eggs that stick to grass leaves and hatch after being swallowed by a herbivore. The larva then makes its way to the herbivore's muscle tissue, where it metamorphoses into an oval worm about 10 millimetres (0.39 in) long, with a scolex that is kept internally. When the definitive host eats infested raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, when the adult tapeworm develops.[14]
Members of the smaller group known asCestodaria have no scolex, do not produce proglottids, and have body shapes similar to those of diageneans. Cestodarians parasitize fish and turtles.[6]
The oldest confidently identified parasitic flatworm fossils arecestode eggs found in aPermian sharkcoprolite, but helminth hooks still attached toDevonianacanthodians andplacoderms might also represent parasitic flatworms with simple life cycles.[33] The oldest known free-living platyhelminth specimen is a fossil preserved inEocene ageBaltic amber and placed in the monotypic speciesMicropalaeosoma balticus,[34] whilst the oldest subfossil specimens areschistosome eggs discovered in ancient Egyptianmummies.[15] The Platyhelminthes have very fewsynapomorphies - distinguishing features that all Platyhelminthes (but no other animals) exhibit. This makes it difficult to work out their relationships with other groups of animals, as well as the relationships between different groups that are described as members of the Platyhelminthes.[35]
The "traditional" view before the 1990s was that Platyhelminthes formed thesister group to all the other bilaterians, which include, for instance,arthropods,molluscs,annelids andchordates. Since then,molecular phylogenetics, which aims to work out evolutionary "family trees" by comparing different organisms'biochemicals such asDNA,RNA andproteins, has radically changed scientists' view of evolutionary relationships between animals.[21] Flatworms are now recognized as secondarily simplified bilaterians.[36]
Detailedmorphological analyses of anatomical features in the mid-1980s, as well as molecular phylogenetics analyses since 2000 using different sections of DNA, agree thatAcoelomorpha, consisting ofAcoela (traditionally regarded as very simple "turbellarians"[14]) andNemertodermatida (another small group previously classified as "turbellarians"[20]) are the sister group to all other bilaterians.[21][22] However, a 2007 study concluded that Acoela and Nemertodermatida were two distinct groups of bilaterians.[23]
Xenoturbella, a bilaterian whose only well-defined organ is astatocyst, was originally classified as a "primitive turbellarian".[24] Later studies suggested it may instead be adeuterostome,[25][37] but more detailed molecular phylogenetics have led to its classification as sister-group to the Acoelomorpha.[38]
The Platyhelminthes excluding Acoelomorpha contain two main groups –Catenulida andRhabditophora – both of which are generally agreed to be monophyletic (each contains all and only the descendants of an ancestor that is a member of the same group).[22][30] Early molecular phylogenetics analyses of the Catenulida and Rhabditophora left uncertainties about whether these could be combined in a single monophyletic group; a study in 2008 concluded that they could, therefore Platyhelminthes could be redefined as Catenulida plus Rhabditophora, excluding the Acoelomorpha.[22]
Other molecular phylogenetics analyses agree the redefined Platyhelminthes are most closely related toGastrotricha, and both are part of a grouping known asPlatyzoa. Platyzoa are generally agreed to be at least closely related to theLophotrochozoa, a superphylum that includes molluscs andannelid worms. The majority view is that Platyzoa are part of Lophotrochozoa, but a significant minority of researchers regard Platyzoa as a sister group of Lophotrochozoa.[21]
It has been agreed since 1985 that each of the wholly parasitic platyhelminth groups (Cestoda,Monogenea andTrematoda) is monophyletic, and that together these form a larger monophyletic grouping, theNeodermata, in which the adults of all members havesyncytial skins.[39] However, there is debate about whether theCestoda andMonogenea can be combined as an intermediate monophyletic group, theCercomeromorpha, within the Neodermata.[39][40] It is generally agreed that the Neodermata are a sub-group a few levels down in the "family tree" of the Rhabditophora.[22] Hence the traditional sub-phylum "Turbellaria" isparaphyletic, since it does not include the Neodermata although these are descendants of a sub-group of "turbellarians".[41]
An outline of the origins of the parasitic lifestyle has been proposed;[42] epithelial feedingmonopisthocotyleans on fish hosts are basal in the Neodermata and were the first shift to parasitism from free living ancestors. The next evolutionary step was a dietary change fromepithelium toblood. The last common ancestor of Digenea + Cestoda was monogenean and most likely sanguinivorous.
The earliest known fossils confidently classified as tapeworms have been dated to270 million years ago, after being found incoprolites (fossilised faeces) from anelasmobranch.[1] Putative older fossils include a ribbon-shaped, bilaterally symmetrical organism namedRugosusivitta orthogonia from the Early Cambrian ofChina,[2] brownish bodies on the bedding planes reported from the LateOrdovician (Katian)Vauréal Formation (Canada) by Knaust & Desrochers (2019), tentatively interpreted as turbellarians (though the authors cautioned that they might ultimately turn out to be fossils ofacoelomorphs ornemerteans)[3] and circlets of fossil hooks preserved withplacoderm andacanthodian fossils from theDevonian ofLatvia, at least some of which might represent parasitic monogeneans.[44]
Cestodes (tapeworms) anddigeneans (flukes) cause diseases in humans and theirlivestock, whilstmonogeneans can cause serious losses of stocks infish farms.[45]Schistosomiasis, also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behindmalaria. TheCarter Center estimated 200 million people in 74 countries are infected with the disease, and half the victims live in Africa. The condition has a lowmortality rate, but usually presents as achronic illness that can damage internal organs. It can impair the growth andcognitive development of children, increasing the risk ofbladder cancer in adults. The disease is caused by several flukes of the genusSchistosoma, which can bore through human skin; those most at risk use infected bodies of water for recreation orlaundry.[28]
In 2000, an estimated 45 million people were infected with the beef tapewormTaenia saginata and 3 million with the pork tapewormTaenia solium.[45] Infection of the digestive system by adult tapeworms causes abdominal symptoms that, whilst unpleasant, are seldom disabling or life-threatening.[46][47] However,neurocysticercosis resulting from penetration ofT. solium larvae into thecentral nervous system is the major cause of acquiredepilepsy worldwide.[48] In 2000, about 39 million people were infected withtrematodes (flukes) that naturally parasitize fish and crustaceans, but can pass to humans who eat raw or lightly cooked seafood. Infection of humans by the broad fish tapewormDiphyllobothrium latum occasionally causesvitamin B12 deficiency and, in severe cases,megaloblastic anemia.[45]
The threat to humans in developed countries is rising as a result of social trends: the increase inorganic farming, which usesmanure andsewage sludge rather than artificialfertilizers, spreads parasites both directly and via the droppings ofseagulls which feed on manure and sludge; the increasing popularity of raw or lightly cooked foods; imports of meat,seafood andsalad vegetables from high-risk areas; and, as an underlying cause, reduced awareness of parasites compared with otherpublic health issues such aspollution. In less-developed countries, inadequate sanitation and the use of humanfeces (night soil) as fertilizer or to enrich fish farm ponds continues to spread parasitic platyhelminths, whilst poorly designed water-supply andirrigation projects have provided additional channels for their spread. People in these countries usually cannot afford the cost of fuel required to cook food thoroughly enough to kill parasites. Controlling parasites that infect humans and livestock has become more difficult, as many species have becomeresistant to drugs that used to be effective, mainly for killing juveniles in meat.[45] While poorer countries still struggle with unintentional infection, cases have been reported of intentional infection in the US by dieters who are desperate for rapid weight-loss.[49]
There is concern in northwest Europe (including the British Isles) regarding the possible proliferation of the New ZealandplanarianArthurdendyus triangulatus and the Australian flatwormAustraloplana sanguinea, both of which prey on earthworms.[50]A. triangulatus is thought to have reached Europe in containers of plants imported bybotanical gardens.[51]
The planarianPlatydemus manokwari has been deliberately released in an attempt at control of the giant AfricansnailAchatina fulica which damages agricultural plants. It was first observed as an invasive species onGuam and was then released in thePhilippines andMaldives. It has now spread to many islands in the Pacific and Caribbean and is now spreading across theSouthern United States as well as southern and eastern Asia, including mainland China. It was initially claimed thatP. manokwari severely reduced, and in places exterminated,A. fulica – achieving much greater success than mostbiological pest control programs, which generally aim for a low, stable population of the pest species. However, the decline ofA. fulica is no longer thought to have been due to flatworm predation.[52] These planarians are a serious threat to native snails and should never be used for biological control.[53][54]
A study in Argentina shows the potential for planarians such asGirardia anceps,Mesostoma ehrenbergii, andBothromesostoma evelinae to reduce populations of the mosquito speciesAedes aegypti andCulex pipiens. The experiment showed thatG. anceps can prey on allinstars of both mosquito species, yet maintain a steady predation rate over time. The ability of these flatworms to live in artificial containers demonstrated the potential of placing these species in popular mosquito breeding sites, which might reduce the amount ofmosquito-borne disease.[55]
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