43-687: The Lophophorata or Tentaculata are a Lophotrochozoan clade consisting of the Brachiozoa and the Bryozoa . They have a lophophore . Molecular phylogenetic analyses suggest that lophophorates are protostomes , but on morphological grounds they have been assessed as deuterostomes . Fossil finds of the " tommotiid " Wufengella suggest that they evolved from worm-like animals that resembled annelids . Cycliophora Annelida Mollusca Brachiopoda Phoronida Entoprocta Ectoprocta Nemertea This protostome -related article
86-515: 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
129-977: 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
172-436: A basal spiralian clade outside Lophotrochozoa; Cycliophora could be a gnathiferan or a lophotrochozoan phylum. One of the candidate hypotheses is presented below. The Lophotrochozoa has basal Cycliophora and Mollusca groups, and more derived Lophophorate , Nemertea and Annelida groups. With the introduction of Platytrochozoa and Rouphozoa , one candidate phylogeny is pictured below – though other studies recover
215-409: 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
258-507: 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
301-417: 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,
344-541: 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
387-605: 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
430-989: A range of alternative possibilities: In the most recent research, the three phyla Cycliophora, Entoprocta and Bryozoa makes up a single clade and are the first to branch off from the other lophotrochozoans. The second split is the molluscs, and the third consists of two sister phyla, annelids and nemerteans. Lastly remains the clade that consist of the phoronids and the brachiopods. Ecdysozoa [REDACTED] Gnathifera [REDACTED] Gastrotricha [REDACTED] Platyhelminthes [REDACTED] Mesozoa [REDACTED] Cycliophora [REDACTED] Echiura [REDACTED] Clitellata [REDACTED] Siboglinidae [REDACTED] Sipuncula [REDACTED] Mollusca [REDACTED] Brachiopoda [REDACTED] Phoronida [REDACTED] Entoprocta [REDACTED] Ectoprocta [REDACTED] Nemertea [REDACTED] Another study recovers Lophotrochozoa as equivalent to Platytrochozoa, forming
473-649: 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
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#1732783213508516-420: 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
559-469: 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
602-616: A sister group with Gnathifera at the base of Spiralia . Ecdysozoa [REDACTED] Gnathifera [REDACTED] Mollusca [REDACTED] Entoprocta [REDACTED] Gastrotricha [REDACTED] Ectoprocta [REDACTED] Phoronida [REDACTED] Brachiopoda [REDACTED] Annelida [REDACTED] Platyhelminthes [REDACTED] Nemertea [REDACTED] A number of fossil taxa can be identified as early Lophotrochozoans, even if their precise affinity remains contested. However, relevant Cambrian fossils are debated. The clade Lophotrochozoa
645-710: 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
688-478: Is a stub . You can help Misplaced Pages by expanding it . Lophotrochozoa Lophotrochozoa ( / l ə ˌ f ɒ t r oʊ k oʊ ˈ z oʊ ə / , "crest/wheel animals") is a clade of protostome animals within the Spiralia . The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids , molluscs , bryozoans , and brachiopods . Lophotrochozoa
731-481: 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:
774-547: Is named after the two distinct characteristics of its members; the lophophore , a feeding structure consisting of a ciliated crown of tentacles surrounding a mouth, and the developmental stage of the trochophore larva. Lophophorata such as Brachiozoa and Bryozoa have lophophores, while members of Trochozoa such as molluscs and annelids have trochophore larvae, although some may have none. Platyhelminthes Traditional: Phylogenetic: The flatworms , flat worms , Platyhelminthes , or platyhelminths (from
817-530: 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
860-619: 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
903-748: 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
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#1732783213508946-484: 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
989-539: The spiralian tree of life, and in some phylogenetic hypotheses, Lophotrochozoa may even be synonymous to Spiralia. Nemertea and Orthonectida (if not directly considered as part of Annelida ) are probably lophotrochozoan phyla; Dicyemida , Gastrotricha , and Platyhelminthes may be lophotrochozoans or placed in the Rouphozoa clade outside Lophotrochozoa; Chaetognatha , Gnathostomulida , Micrognathozoa , and Syndermata are probably gnathiferans and so placed as
1032-490: 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
1075-399: 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
1118-477: 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
1161-461: 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
1204-429: 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
1247-509: 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
1290-442: 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"),
1333-531: 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
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1376-561: 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
1419-463: 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
1462-459: 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,
1505-475: 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
1548-439: 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
1591-420: 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
1634-455: 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
1677-618: 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
1720-495: 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
1763-425: 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 ,
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1806-658: 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
1849-411: Was defined in 1995 as the "last common ancestor of the three traditional lophophorate taxa ( brachiopods , bryozoans , and phoronid worms), the mollusks and the annelids , and all of the descendants of that common ancestor". It is a cladistic definition (a node-based name), so the affiliation to Lophotrochozoa of spiralian groups not mentioned directly in the definition depends on the topology of
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