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46-454: Triploblasts Lankester, 1873 Bilateria ( / ˌ b aɪ l ə ˈ t ɪər i ə / BY -lə- TEER -ee-ə ) is a large clade or infrakingdom of animals called bilaterians ( / ˌ b aɪ l ə ˈ t ɪər i ə n / BY -lə- TEER -ee-ən ), characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development . This means their body plans are laid around

92-405: A few phyla (i.e. flatworms and gnathostomulids ), bilaterians have complete digestive tracts with a separate mouth and anus . Some bilaterians lack body cavities ( acoelomates , i.e. Platyhelminthes , Gastrotricha and Gnathostomulida ), while others display primary body cavities (deriving from the blastocoel , as pseudocoeloms ) or secondary cavities (that appear de novo , for example

138-443: A head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation , the development of a head with sense organs and a mouth. The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict

184-408: A left side and a right side. Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation , the development of a head with sense organs and a mouth. The body stretches back from the head, and many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten

230-422: A longitudinal axis ( rostral – caudal axis) with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly ( ventral ) and back ( dorsal ) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms , which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo . Cephalization

276-431: A period that is much longer compared to other microorganisms. Helminth eggs are resistant to various environmental conditions due to the composition of the egg shell. Each helminth egg species has 3 to 4 layers with different physical and chemical characteristics: Larvae hatch from eggs, either inside or outside the host, depending on the type of helminth. For eggs in moist soil at optimal temperature and oxygen levels,

322-412: A rear (or "tail") end, as well as a left–right–symmetrical belly ( ventral ) and back ( dorsal ) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms , which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo . Cephalization is also a characteristic feature among most bilaterians, where

368-490: A round or flattened body, respectively. Ringworm (dermatophytosis) is actually caused by various fungi , and not by a parasitic worm. The lifetime of adult worms varies tremendously from one species to another but is generally in the range of 1 to 8 years (see following table). This lifetime of several years is a result of their ability to manipulate the immune response of their hosts by secreting immunomodulatory products. Helminths can be either hermaphroditic (having

414-447: A strong shell that protects them against a range of environmental conditions. The eggs can therefore survive in the environment for many months or years. Many of the worms referred to as helminths are intestinal parasites. An infection by a helminth is known as helminthiasis , helminth infection, or intestinal worm infection. There is a naming convention which applies to all helminths: the ending "-asis" (or in veterinary science: "-osis")

460-413: Is a large clade or infrakingdom of animals called bilaterians ( / ˌ b aɪ l ə ˈ t ɪər i ə n / BY -lə- TEER -ee-ən ), characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development . This means their body plans are laid around a longitudinal axis ( rostral – caudal axis) with a front (or "head") and

506-415: Is added at the end of the name of the worm to denote the infection with that particular worm. For example, Ascaris is the name of a type of helminth, and ascariasis is the name of the infection caused by that helminth. Helminths are a group of organisms which share a similar form but are not necessarily evolutionarily related . The term "helminth" is an artificial term. There is no real consensus on

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552-410: Is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end. Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera (sponges), Cnidaria ( jellyfish , hydrae , sea anemones and corals ), Ctenophora (comb jellies) and Placozoa (tiny "flat animals"). For

598-590: Is termed the " urbilaterian ". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff , Elie Metchnikoff , Libbie Hyman , or Luitfried von Salvini-Plawen  [ nl ] ), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla ( flatworms and gastrotrichs ) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as

644-531: Is termed the " urbilaterian ". The nature of the first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by Ludwig von Graff , Elie Metchnikoff , Libbie Hyman , or Luitfried von Salvini-Plawen  [ nl ] ), while the other poses that the first bilaterian was a coelomate organism and the main acoelomate phyla ( flatworms and gastrotrichs ) have lost body cavities secondarily (the Archicoelomata hypothesis and its variations such as

690-514: Is that Ambulacraria are sister to Xenacoelomorpha together forming Xenambulacraria . Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or to Chordata + Protostomia). The phylogenetic tree shown below depicts the latter proposal. Also, the validity of Deuterostomia (without Protostomia emerging from it) is under discussion. The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya). While

736-512: Is that Ambulacraria are sister to Xenacoelomorpha together forming Xenambulacraria . Xenambulacraria may be sister to Chordata or to Centroneuralia (corresponding to Nephrozoa without Ambulacraria, or to Chordata + Protostomia). The phylogenetic tree shown below depicts the latter proposal. Also, the validity of Deuterostomia (without Protostomia emerging from it) is under discussion. The cladogram indicates approximately when some clades radiated into newer clades, in millions of years ago (Mya). While

782-408: The blastocoel , as pseudocoeloms ) or secondary cavities (that appear de novo , for example the coelom ). Some of the earliest bilaterians were wormlike, and their body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have

828-443: The cestodes (tapeworms) and the trematodes (flukes and blood flukes ), depending on whether or not they have segmented bodies. There may be as many as 300,000 species of parasites affecting vertebrates, and as many as 300 affecting humans alone. Helminths of importance in the sanitation field are the human parasites, and are classified as Nemathelminthes (nematodes) and Platyhelminthes , depending on whether they possess

874-437: The coelom ). Some of the earliest bilaterians were wormlike, and their body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have

920-450: The echinoderms , hemichordates , chordates , and the extinct Vetulicolia . The protostomes include most of the rest, such as arthropods , annelids , mollusks , flatworms , and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among

966-450: The echinoderms , hemichordates , chordates , and the extinct Vetulicolia . The protostomes include most of the rest, such as arthropods , annelids , mollusks , flatworms , and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists now recognize at least two more superphyla among

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1012-479: The special sense organs and central nerve ganglia become concentrated at the front/rostral end. Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera (sponges), Cnidaria ( jellyfish , hydrae , sea anemones and corals ), Ctenophora (comb jellies) and Placozoa (tiny "flat animals"). For the most part, bilateral embryos are triploblastic , having three germ layers : endoderm , mesoderm and ectoderm . Except for

1058-573: The Gastrea by Haeckel or Sedgwick , the Bilaterosgastrea by Gösta Jägersten  [ sv ] , or the Trochaea by Nielsen). One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella . Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry. However, there is evidence that it

1104-411: The Gastrea by Haeckel or Sedgwick , the Bilaterosgastrea by Gösta Jägersten  [ sv ] , or the Trochaea by Nielsen). One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella . Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry. However, there is evidence that it

1150-1452: The Nephrozoa. Subsequently the acoelomorphs were placed in phylum Xenacoelomorpha , together with the xenoturbellids , and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably since 2000. Cnidaria [REDACTED] Placozoa [REDACTED] Proarticulata ? † [REDACTED] Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Cephalochordata [REDACTED] Tunicata [REDACTED] Vertebrata [REDACTED] † Saccorhytus coronarius † Vetulocystids [REDACTED] † Vetulicolians [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Loricifera [REDACTED] Onychophora [REDACTED] Tardigrada [REDACTED] Arthropoda [REDACTED] Priapulida [REDACTED] Kinorhyncha [REDACTED] Rotifera and allies [REDACTED] Chaetognatha [REDACTED] Platyhelminthes and allies [REDACTED] Mollusca [REDACTED] Annelida and allies [REDACTED] A different hypothesis

1196-1392: The Nephrozoa. Subsequently the acoelomorphs were placed in phylum Xenacoelomorpha , together with the xenoturbellids , and the sister relationship between Xenacoelomorpha and Nephrozoa confirmed in phylogenomic analyses. A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably since 2000. Cnidaria [REDACTED] Placozoa [REDACTED] Proarticulata ? † [REDACTED] Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Cephalochordata [REDACTED] Tunicata [REDACTED] Vertebrata [REDACTED] † Saccorhytus coronarius † Vetulocystids [REDACTED] † Vetulicolians [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Loricifera [REDACTED] Onychophora [REDACTED] Tardigrada [REDACTED] Arthropoda [REDACTED] Priapulida [REDACTED] Kinorhyncha [REDACTED] Rotifera and allies [REDACTED] Chaetognatha [REDACTED] Platyhelminthes and allies [REDACTED] Mollusca [REDACTED] Annelida and allies [REDACTED] A different hypothesis

1242-942: The below tree depicts Chordata as a sister group to Protostomia according to analyses by Philippe et al., the authors nonetheless caution that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy". Cnidaria [REDACTED] Placozoa [REDACTED] † Proarticulata Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Helminths Some parasitic worms, including leeches and monogeneans , are ectoparasites  – thus, they are not classified as helminths, which are endoparasites . Parasitic worms live in and feed in living hosts . They receive nourishment and protection while disrupting their hosts' ability to absorb nutrients . This can cause weakness and disease in

1288-695: The below tree depicts Chordata as a sister group to Protostomia according to analyses by Philippe et al., the authors nonetheless caution that "the support values are very low, meaning there is no solid evidence to refute the traditional protostome and deuterostome dichotomy". Cnidaria [REDACTED] Placozoa [REDACTED] † Proarticulata Xenoturbellida [REDACTED] Nemertodermatida Acoela [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] † Cambroernida [REDACTED] Triploblasts Triploblasts Lankester, 1873 Bilateria ( / ˌ b aɪ l ə ˈ t ɪər i ə / BY -lə- TEER -ee-ə )

1334-735: The body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis . Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus, while xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely plesiomorphic body structures. The hypothetical most recent common ancestor of all Bilateria

1380-653: The body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis . Most bilaterians (nephrozoans) have a gut that extends through the body from mouth to anus, while xenacoelomorphs have a bag gut with one opening. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), and certain parasitic worms have extremely plesiomorphic body structures. The hypothetical most recent common ancestor of all Bilateria

1426-685: The earliest known bilaterian, but may also represent an infilled bubble. Fossil embryos are known from around the time of Vernanimalcula ( 580  million years ago ), but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old. However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran. Bilateria has traditionally been divided into two main lineages or superphyla . The deuterostomes traditionally include

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1472-620: The earliest known bilaterian, but may also represent an infilled bubble. Fossil embryos are known from around the time of Vernanimalcula ( 580  million years ago ), but none of these have bilaterian affinities. Burrows believed to have been created by bilaterian life forms have been found in the Tacuarí Formation of Uruguay, and were believed to be at least 585 million years old. However, more recent evidence shows these fossils are actually late Paleozoic instead of Ediacaran. Bilateria has traditionally been divided into two main lineages or superphyla . The deuterostomes traditionally include

1518-434: The embryo develops into an infective larva after 2 to 4 weeks, named "second-stage larva". Once ingested by a host, this larva has the ability to get out of the egg, hatch in the small intestine and migrate to different organs. These infective larvae (or "infective eggs") may remain viable in soil for two years or longer. The process of larval maturation in the host can take from about two weeks up to four months, depending on

1564-483: The helminth species. The following table shows the principal morphological and reproductive distinctions for three helminth groups: Fasciola hepatica Number of species Registered > 25,000 Hymenolepis nana Taenia solium / Taenia saginata Fasciola hepatica Ascaris lumbricoides Hookworm Trichuris trichiura Some days (eggs can survive for months) 9–15 days 18 days to several weeks 1–2 days 15–30 days After hatching,

1610-468: The host, and poses a global health and economic problem. Parasitic worms cannot reproduce entirely within their host's body; they have a life cycle that includes some stages that need to take place outside of the host. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the host's immune response by secreting immunomodulatory products. All parasitic worms produce eggs during reproduction. These eggs have

1656-542: The larvae develop into cysticercoid, which can survive for years in an animal 5–7 weeks as cercariae in snails and longer periods in wet environments as encysted metacercariae 10–14 days 5–10 days (after maturing can survive for weeks outside the host) 60–70 days (from hatching to mature state) 5–6 days 2 months (from cysticercoid to adult) 3–4 months 2–3 months 2–8 weeks (can become dormant for months) 4–6 weeks Several years 8–10 years 1–2 years Several years 1 year up to 6 times

1702-441: The most part, bilateral embryos are triploblastic , having three germ layers : endoderm , mesoderm and ectoderm . Except for a few phyla (i.e. flatworms and gnathostomulids ), bilaterians have complete digestive tracts with a separate mouth and anus . Some bilaterians lack body cavities ( acoelomates , i.e. Platyhelminthes , Gastrotricha and Gnathostomulida ), while others display primary body cavities (deriving from

1748-516: The protostomes, Ecdysozoa (molting animals) and Spiralia . The arrow worms ( Chaetognatha ) have proven difficult to classify; recent studies place them in the Gnathifera . The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha ), and

1794-456: The protostomes, Ecdysozoa (molting animals) and Spiralia . The arrow worms ( Chaetognatha ) have proven difficult to classify; recent studies place them in the Gnathifera . The traditional division of Bilateria into Deuterostomia and Protostomia was challenged when new morphological and molecular evidence found support for a sister relationship between the acoelomate taxa, Acoela and Nemertodermatida (together called Acoelomorpha ), and

1840-645: The range of 3,000 to 700,000. The frequency of egg deposition from an adult helminth is generally daily, and can occur up to six times per day for some Taenia species. Adult trematodes lay smaller numbers of eggs compared to cestodes or nematodes. However, the egg develops into a miracidia from which thousands of cercariae , or swimming larvae, develop. This means that one egg may produce thousands of adult worms. Helminth eggs remain viable for 1–2 months in crops and for many months in soil, fresh water, and sewage , or even for several years in feces , fecal sludge (historically called night soil ), and sewage sludge –

1886-443: The remaining bilaterians. The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004). The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in

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1932-441: The remaining bilaterians. The latter clade was called Nephrozoa by Jondelius et al. (2002) and Eubilateria by Baguña and Riutort (2004). The acoelomorph taxa had previously been considered flatworms with secondarily lost characteristics, but the new relationship suggested that the simple acoelomate worm form was the original bilaterian body plan and that the coelom, the digestive tract, excretory organs, and nerve cords developed in

1978-476: The sex organs of both sexes), like tapeworms and flukes (not including the blood fluke), or have their sexes differentiated, like the roundworms. All helminths produce eggs (also called ova) for reproduction. Generally, thousands or even hundreds of thousands of eggs are produced each time the female worm deposits its eggs - a process called oviposition . There is a large variation in the number of eggs produced by different species of worm at one time; it varies in

2024-530: The taxonomy (or groupings) of the helminths, particularly within the nematodes . The term "helminth" contains a number of phyla , many of which are completely unrelated. However, for practical considerations the term is currently used to describe four phyla with superficial similarities: Annelida (ringed or segmented worms), Platyhelminthes ( flatworms ), Nematoda (roundworms), and Acanthocephala (thorny-headed worms). The phylum Platyhelminthes includes two classes of worms of particular medical significance:

2070-465: Was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes). The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella , dating to 555  million years ago . Earlier fossils are controversial; the fossil Vernanimalcula may be

2116-412: Was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes). The first evidence of Bilateria in the fossil record comes from trace fossils in Ediacaran sediments, and the first bona fide bilaterian fossil is Kimberella , dating to 555  million years ago . Earlier fossils are controversial; the fossil Vernanimalcula may be

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