Üçşaxəlilər və ya üçbarmaqlılar (lat. Tricladida) — Kirpikli qurdlar sinfinə aid yarımdəstə.
Bu qrupun xarakterik əlaməti üçbarmaq bağırsaqlardır.
Els triclàdides (Tricladida,[2] del grec antic tri/τρι-, "tres"; i klados/κλάδος, "branca") constitueixen un ordre de platihelmints turbel·laris de vida lliure. Es tracta d'un grup molt ben conegut, car s'han realitzat molts estudis de regeneració i expressió gènica. Els triclàdides han colonitzat el medi marí (marícoles), l'aigua dolça (planàrids, kènkids, dendrocèlids i dugèsids) i el medi terrestre (geoplànids). També se'n troben a coves (cavernícoles). Es desconeix quan van aparèixer a causa de l'abscència de registre fòssil.
Els triclàdides es caracteritzen per presentar un intestí trifurcat i uns ovaris situats anteriorment, propers al cervell. Les espècies d'aquest grup s'alimenten a través d'un tub retràctil anomenat faringe. Aquesta faringe es comunica amb les tres branques principals de l'intestí, una branca està situada a l'eix mitjà de la part anterior de l'organisme i les altres dues branques es dirigeixen cap enrere. La faringe també els serveix per a l'excreció.
S'han descrit quatre característiques morfològiques que indiquen que els triclàdides són monofilètics: presenten un desenvolupament embrionari únic, els ovaris situats prop del cervell, una disposició en sèrie de nombrosos nefridiòpors i presenten zones adhesives marginals.[3]
Els triclàdides es coneixen des de fa segles per la seva capacitat de regeneració. De fet, consten entre els primers animals en els que es va estudiar la regeneració. L'any 1774 Pallas va ser el primer a adonar-se que un petit fragment de cap de triclàdide era capaç de regenerar l'organisme sencer.[4] L'any 1814, Dalyell després de realitzar experiments amb Polycelis nigra va escriure que aquesta espècie podia ser "gairebé considerada immortal sota la fulla d'un ganivet".[5][6]
La capacitat de regeneració en els triclàdides prové d'unes cèl·lules mare pluripotents anomenades neoblasts, que poden diferenciar-se en qualsevol altre tipus cel·lular.[7] Els triclàdides no són els únics platihelmints de vida lliure o planàries que són capaços de dur a terme la regeneració.[8][6]
Els triclàdides d'aigua dolça adults són molt fràgils i sensibles als extrems de temperatures i a la desecació. Passen tot el cicle vital en entorns aquàtics.[9]
Les planàries d'aigua dolça tenen poca capacitat de dispersió, s'escampen per la seva pròpia activitat a través de masses d'aigua dolça contigus.[9]
Existeixen evidències que els ocells han ajudat a dispersar-se en distàncies curtes a Crenobia alpina i Polycelis felina al nord-oest d'Europa.[10][11] Tanmateix, sembla que aquest tipus de dispersió no és important a grans distàncies.[11]
Es té constància de la dispersió de càpsules embrionàries (altrament dites cocoons o ous) i adults de triclàdides d'aigua dolça per aigües d'inundació i per objectes flotants.[12] La resistència d'aquests animals a l'aigua salada és molt baixa, pel que s'ha descartat que puguin dispersar-se a través de l'aigua marina. Tanmateix, es coneixen casos de triclàdides d'aigua dolça que han envaït hàbitats d'aigües salobres[13][14] i d'algunes marícoles que es poden adaptar quasi totalment a l'aigua dolça.[14][15]
És probable que la hidrocòria no hagi representat un paper important en la dispersió dels triclàdides d'aigua dolça.[9]
Actualment l'ordre dels triclàdides es subdivideix en tres subordres, Maricola, Continenticola i Cavernicola.[1] Els marícola inclouen triclàdides marins, mentre els continentícola inclouen quatre famílies d'aigua dolça i una família terrestre, els geoplànids.
Antigament, seguint l'esquema de Hallez, els triclàdides es classificaven segons la seva ecologia, així es distingien els Paludicola (triclàdides d'aigua dolça), els Terricola (triclàdides terrestres) i els Maricola (triclàdides marins).[16] El grup Paludicola, en realitat, és parafilètic, ja que segons proves moleculars[17][18] i morfològiques els Terricola haurien evolucionat a partir d'un grup de planàries d'aigua dolça i, per tant, s'haurien d'agrupar amb aquestes. Les planàries terrestres serien el grup germà dels dugèsids. L'any 1998 Carranza et al. van proposar englobar els triclàdides terrestres i d'aigua dolça sota un grup monofilètic anomenat Continenticola.[17] Aquest grup s'assentà l'any 2009.[1]
Arbre filogenètic dels triclàdides basant-se en dades moleculars, Carranza i col·laboradors (1998):[17]
TricladidaSuperarbre filogenètic que inclou tots els grans grups coneguts de triclàdides, realitzat basant-se en diversos estudis publicats, Sluys i col·laboradors (2009):[1]
TricladidaClassificació taxonòmica dels triclàdides segons un estudi de Sluys i col·laboradors de l'any 2009:
Els triclàdides (Tricladida, del grec antic tri/τρι-, "tres"; i klados/κλάδος, "branca") constitueixen un ordre de platihelmints turbel·laris de vida lliure. Es tracta d'un grup molt ben conegut, car s'han realitzat molts estudis de regeneració i expressió gènica. Els triclàdides han colonitzat el medi marí (marícoles), l'aigua dolça (planàrids, kènkids, dendrocèlids i dugèsids) i el medi terrestre (geoplànids). També se'n troben a coves (cavernícoles). Es desconeix quan van aparèixer a causa de l'abscència de registre fòssil.
A planarian is one of the many flatworms of the traditional class Turbellaria.[2][3] It usually describes free-living flatworms of the order Tricladida (triclads),[4] although this common name is also used for a wide number of free-living platyhelminthes.[2] Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.
The triclads are characterized by triply branched intestine and anteriorly situated ovaries, next to the brain. Today the order Tricladida is split into three suborders, according to their phylogenetic relationships: Maricola, Cavernicola and Continenticola. Formerly, the Tricladida was split according to habitats: Maricola, which is marine; Paludicola which inhabits freshwater; and Terricola, which is land-dwelling.[5]
Planaria exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots (also known as ocelli) that can detect the intensity of light, while others have several eye-spots. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (they have a very solid body with no body cavity). They have a single-opening digestive tract; in Tricladida planarians this consists of one anterior branch and two posterior branches.
Planarians move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some also may move by undulations of the whole body by the contractions of muscles built into the body membrane.[6]
Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators.[7]
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. Recently, however, the species Schmidtea mediterranea has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and the existence of both asexual and sexual strains.[8] Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Many of these genes have orthologs in the human genome.[9]
The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the middle of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities.[3] Planaria eat living or dead small animals that they suck up with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested by the cells lining the intestines. Then its nutrients diffuse to the rest of the planaria.
Planaria receive oxygen and release carbon dioxide by diffusion. 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 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.[10]
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 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.[10]
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.[10]
The head of the planarian evolved through cephalisation;[11] the planarian head has a ganglion under the eyespots. The cerebral ganglia, a bi-lobed mass of nerve tissue, is sometimes referred to as the planarian “brain”[12] and has been shown to exhibit spontaneous electrophysiological oscillations,[13] similar to the electroencephalographic (EEG) activity of other animals. From the ganglion there are two nerve cords which extend along the length of the tail. There are many transverse nerves connected to the nerve cords extending from the brain, which makes the nerve system look like a ladder. With a ladder-like nerve system, it is able to respond in a coordinated manner.
The planarian has 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 the planarian's body by diffusing through the body wall.
There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to 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 detaches its tail end and each half regrows the lost parts by regeneration, allowing endoblasts (adult stem cells) to divide and differentiate, thus resulting in two worms. Some researchers claim that the products derived from bisecting planaria are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planaria and its effect on the population are ongoing.[14] 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).[10]
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, 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 a discipline.
Planarians are also an emerging model organism for aging research. These animals have an apparently limitless regenerative capacity, and the asexual animals seem to maintain their telomerase levels throughout their lifetime, making them "effectively immortal".[15]
Planarian can be cut into pieces, and each piece can regenerate into a complete organism. Cells at the location of the wound site proliferate to form a blastema that will differentiate into new tissues and regenerate the missing parts of the piece of the cut planaria. It's this feature that gave them the famous designation of being "immortal under the edge of a knife."[16] 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 a complete organism over the course of a few weeks.[17] New tissues can grow due to pluripotent stem cells that have the ability to create all the various cell types.[18] These adult stem cells are called endoblasts, and comprise 20% or more of the cells in the adult animal.[19] They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. In addition, existing tissue is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism.[19][20]
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.[21] Researchers, including those from Tufts University in 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 the International Space Station (ISS) – though regeneration of amputated worms as double-headed heteromorphosis is not a rare phenomenon unique to a microgravity environment.[22] 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,[23] as well as by genetic expression blocks (interference RNA) to the canonical Wnt/β-Catenin signalling pathway.[24]
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 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 show that memory could be transferred chemically. The experiment 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.[25] 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.[26][27] 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 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.[28]
Phylogenetic supertree after Sluys et al., 2009:[1]
Tricladida Continenticola Planarioidea Geoplanoidea
Linnaean ranks after Sluys et al., 2009:[1]
{{cite web}}: CS1 maint: url-status (link) A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelminthes. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.
The triclads are characterized by triply branched intestine and anteriorly situated ovaries, next to the brain. Today the order Tricladida is split into three suborders, according to their phylogenetic relationships: Maricola, Cavernicola and Continenticola. Formerly, the Tricladida was split according to habitats: Maricola, which is marine; Paludicola which inhabits freshwater; and Terricola, which is land-dwelling.
Planaria exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. Some planarian species have two eye-spots (also known as ocelli) that can detect the intensity of light, while others have several eye-spots. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (they have a very solid body with no body cavity). They have a single-opening digestive tract; in Tricladida planarians this consists of one anterior branch and two posterior branches.
Planarians move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some also may 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.
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. Recently, however, the species Schmidtea mediterranea has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and the existence of both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Many of these genes have orthologs in the human genome.
Los tricládidos (Tricladida,[2] del griego antiguo tri / τρι-, "tres", y klados / κλάδος, "rama") constituyen un orden de platelmintos turbelarios de vida libre. Se trata de un grupo muy bien conocido, pues se han realizado muchos estudios de regeneración y expresión génica. Los tricládidos han colonizado el medio marino (marícolas), el agua dulce (planáridos, Kenkiidae, Dendrocoelidae y Dugesiidae) y el medio terrestre (geoplánidos). También se encuentran en cuevas (Dimarcusidae). Se desconoce cuándo aparecieron debido a la ausencia de registro fósil.
Los tricládidos se caracterizan por presentar un intestino trifurcado y unos ovarios situados anteriormente, cercanos al cerebro. Las especies de este grupo se alimentan a través de un tubo retráctil llamado faringe. Esta faringe se comunica con las tres ramas principales del intestino, una rama está situada en el eje medio de la parte anterior del organismo y las otras dos ramas se dirigen hacia atrás. La faringe también les sirve para la excreción.
Se han descrito cuatro características morfológicas que indican que los Tricladida son monofiléticos: presentan un desarrollo embrionario único, los ovarios situados cerca del cerebro, una disposición en serie de numerosos nefridioporos y presentan zonas adhesivas marginales.[3]
Los tricládidos se conocen desde hace siglos por su capacidad de regeneración. De hecho, constan entre los primeros animales en los que se estudió la regeneración. En 1774 Pallas fue el primero en darse cuenta de que un pequeño fragmento de cabeza de tricládido era capaz de regenerar el organismo entero.[4] En 1814, Dalyell después de realizar experimentos con Polycelis nigra escribió que esta especie podía ser "casi considerada inmortal bajo la hoja de un cuchillo".[5][6]
La capacidad de regeneración en los tricládidos proviene de unas células madre pluripotentes llamadas neoblastos, que pueden diferenciarse en cualquier otro tipo celular.[7] Los tricládidos no son los únicos platelmintos de vida libre o planarias que son capaces de llevar a cabo la regeneración.[8][6]
Los tricládidos de agua dulce adultos son muy frágiles y sensibles a los extremos de temperaturas y la desecación. Pasan todo el ciclo vital en entornos acuáticos.[9]
Las planarias de agua dulce, en general, tienen poca capacidad de dispersión, se esparcen por su propia actividad a través de cuerpos de agua dulce contiguos.[9]
Superarbol filogenético que incluye todos los grandes grupos conocidos de tricládidos, realizado basándose en diversos estudios publicados, Sluys y colaboradores (2009):[1]
Tricladida Continenticola Planarioidea GeoplanoideaClasificación taxonómica de los tricládidos según un estudio de Sluys y colaboradores del 2009:
Los tricládidos (Tricladida, del griego antiguo tri / τρι-, "tres", y klados / κλάδος, "rama") constituyen un orden de platelmintos turbelarios de vida libre. Se trata de un grupo muy bien conocido, pues se han realizado muchos estudios de regeneración y expresión génica. Los tricládidos han colonizado el medio marino (marícolas), el agua dulce (planáridos, Kenkiidae, Dendrocoelidae y Dugesiidae) y el medio terrestre (geoplánidos). También se encuentran en cuevas (Dimarcusidae). Se desconoce cuándo aparecieron debido a la ausencia de registro fósil.
Lattanat (Tricladida) ovat laakamatojen (Platyhelminthes) värysmatoihin (Turbellaria) kuuluva lahko. Lattanoiden ruumis on jaokkeeton ja eläin on kaksikylkinen. Lattanoilla ei ole verenkiertoa eli niiltä puuttuvat sydän ja verisuonet. Suoleen johtaa vain suuaukko, kuten muillakin värysmadoilla. Lattanat elävät kosteissa, vesipitoisissa ympäristöissä, kivien alla.
Lattanat (Tricladida) ovat laakamatojen (Platyhelminthes) värysmatoihin (Turbellaria) kuuluva lahko. Lattanoiden ruumis on jaokkeeton ja eläin on kaksikylkinen. Lattanoilla ei ole verenkiertoa eli niiltä puuttuvat sydän ja verisuonet. Suoleen johtaa vain suuaukko, kuten muillakin värysmadoilla. Lattanat elävät kosteissa, vesipitoisissa ympäristöissä, kivien alla.
Les Tricladida sont un ordre de vers plats.
Les anciens sous-ordres Paludicola et Terricola sont désormais regroupés dans les Continenticola.
Selon Turbellarian Taxonomic Database, Version 2.0[2],[3].
PlathelminthesLes Tricladida sont un ordre de vers plats.
I tricladi (Tricladida) sono un ordine di Turbellari, un gruppo di platelminti a vita libera, comunemente noti come planarie
L'ordine Tricladida è suddiviso in 3 sottordini:[1]
I tricladi (Tricladida) sono un ordine di Turbellari, un gruppo di platelminti a vita libera, comunemente noti come planarie
Planarier er en gruppe flatormer. De har tredelt tarm.[1]
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Os tricládidos (Tricladida) constituem uma ordem biológica de turbelários não parasitas, de vida livre. Podem viver em ambiente aquáticos marinhos (subordem Maricola), de água doce (famílias Planariidae, Dendrocoelidae, Kenkiidae, Dugesiidae) e terrestres (família Geoplanidae).
Os tricládidos caracterizam-se por apresentar um intestino tubular ramificado em três partes: uma anterior e duas posteriores, cada uma possuindo cecos e ovários situados anteriormente, próximos à faringe. Esta faringe comunica-se com os três ramos principais do intestino e pode servir para a excreção.
Atualmente a ordem Tricladida é subdividida em três subordens, Maricola, Continenticola e Cavernicola[1], mas deste último pouco se sabe. Maricola é constituido pelas planarias marinhas, enquanto Continenticola é formado por quatro famílias de água doce e uma família terrestre, chamada Geoplanidae.
Antigamente os tricládidos eram classificados segundo a sua ecologia, Paludicola (planárias de água doce), Terricola (plánarias terrestres) e Maricola (plánarias marinhas).[2] . Hoje, sabe-se que o grupo Paludicola é parafilético, já que segundo estudos moleculares[3][4] e morfólogicos o grupo Terricola seria derivado a partir de plánarias de água doce e portanto deveria agrupar-se com estas. As plánarias terrestres seriam assim grupo irmão de Dugesidea. Em 1998 Carranza et al. propuseram que tanto os tricládidos terrestres quanto os dulcíquolas fossem agrupados em um clado monofilético denominado Continenticola.[3] Este grupo foi finalmente criado em 2009.[1]
Classificação taxonômica de Tricladida segundo Sluys et al. 2009:
Os tricládidos (Tricladida) constituem uma ordem biológica de turbelários não parasitas, de vida livre. Podem viver em ambiente aquáticos marinhos (subordem Maricola), de água doce (famílias Planariidae, Dendrocoelidae, Kenkiidae, Dugesiidae) e terrestres (família Geoplanidae).
Tricladida (triclads)[2] là một bộ trong lớp Turbellaria, một nhóm giun dẹp sống tự do. Chúng cũng được biết đến như planarian, mặc dù tên chung này cũng được sử dụng cho một số lượng lớn các loài giun dẹp (Platyhelminthes) sống tự do.[3]
Bộ giun này có đặc trưng là ruột chia ba nhánh (từ đó mà có tên gọi Tricladida) và các buồng trứng nằm ở phía trước, ngay sau não.
Nhiều loài có thể sử dụng cả sinh sản vô tính và sinh sản hữu tính. Hầu hết các triclads là loài lưỡng tính, cùng một cá thể có cả hai cơ quan sinh dục đực và cái cùng một lúc. Trong hầu hết các trường hợp, sinh sản hữu tính liên quan đến hai cá thể, tự thụ tinh hiếm gặp (ví dụ Cura foremanii).[4]
Siêu cây phát sinh chủng loài lấy theo Sluys et al, 2009.[1]
Tricladida
Planarioidea
Tricladida (triclads) là một bộ trong lớp Turbellaria, một nhóm giun dẹp sống tự do. Chúng cũng được biết đến như planarian, mặc dù tên chung này cũng được sử dụng cho một số lượng lớn các loài giun dẹp (Platyhelminthes) sống tự do.
Bộ giun này có đặc trưng là ruột chia ba nhánh (từ đó mà có tên gọi Tricladida) và các buồng trứng nằm ở phía trước, ngay sau não.
Триклады, или трёхветвистые планарии (лат. Tricladida) — подотряд свободноживущих ресничных червей (Turbellaria). Характерная черта этой группы — трёхветвистый кишечник.
До недавнего времени в систематике трикладид практически безраздельно господствовала сформировавшаяся ещё в конце XIX века концепция, по которой их разделяли на три группы: Maricola (преимущественно морские формы), Paludicola (пресноводные формы) и Terricola (наземные планарии)[1]. Однако в 1990 году Рональд Слайс, проанализировав морфологию, жизненные циклы и особенности экологии одного из семейств пресноводных планарий Dimarcusidae, пришёл к выводу, что оно представляет собой монофилетическую группу, которая не может быть включена ни в одну из трёх групп подотряда, и на этом основании выделил димаркусид в самостоятельную группу Cavernicola[2].
В начале XXI века в результате внедрения в изучение систематики трикладид методов молекулярной филогенетики была подвергнута сомнению как монофилия перечисленных выше групп, так и принадлежность трикладид к отряду Seriata[en][3]. По результатам этого исследования сестринской группой для трикладид оказался другой отряд — Prolecithophora[en]. Кроме того, была выявлена парафилия пресноводных планарий группы Paludicola: семейство Dugesiidae оказалось сестринским по отношению к наземным планариям Terricola. Два других семейства Paludicola (Planariidae и Dendrocoelidae) сохранили статус монофилетической группы.
Молочно-белая планария (Dendrocoelum lacteum) — достаточно распространённый пресноводный вид. Крупные особи достигают 2,5 см в длину. Обычно скрываются под камнями, в листовых пазухах или на нижней стороне листьев водных растений. На переднем конце имеется 2 глаза. Сквозь прозрачные покровы просвечивается кишечник.
Бурая планария (Planaria torva) — обитатель стоячих и медленно текущих пресных водоёмов. Также встречается в солоноватых районах Балтийского моря. Окраска тела червей этого вида — темно-бурая, более тёмная на спинной стороне. Длина — до 1,3 см.
Характерная черта чёрной многоглазки (Polycelis nigra) — многочисленные бокаловидные глаза на переднем конце тела. Обычно не превышает в длину 1 см. Эти планарии встречаются в небольших водоемах с чистой водой, обильно заросших водными растениями.
Триклады, или трёхветвистые планарии (лат. Tricladida) — подотряд свободноживущих ресничных червей (Turbellaria). Характерная черта этой группы — трёхветвистый кишечник.
三腸目（Tricladida）[2] ，又名三歧腸目，是扁形動物門之下的一個目，在傳統生物分類被歸入渦蟲綱。
三腸目物種的特色，是其腸歧出成三個分支，卵巢位於體前（「頭部」）、腦的旁邊。過往跟現在的三腸目都可分成三個亞目，但這些亞目由於分子支序親緣學的進展，令原來按其棲息地作的分類大幅改變。
三腸目的感覺器官，即眼睛及化學接收器等都位於其前端，也可當作牠們的「頭」。
三腸目物種的身體被有纖毛的上皮細胞覆蓋。
本分類多個物種均可進行無性繁殖及有性繁殖。大多數三腸目物種皆為雌雄間性( hermaphrodites），其身體同時擁有雄性和雌性的生殖器官。大多數情況的有性繁殖都涉及兩個不同的個體，自體受精（autofecundation）的個案很罕見（例如：發生於 Cura foremanii）[3]。
Phylogenetic supertree after Sluys et al., 2009:[1]
Tricladida Continenticola Planarioidea GeoplanoideaLinnaean ranks after Sluys et al., 2009:[1]
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