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Nemertea

(Redirected fromRibbon worm)
Not to be confused with the roundworm phylumNematoda.For the Romanian village, seeGura Teghii.

Nemerteais aphylumofanimalsalso known asribbon wormsorproboscis worms,consisting of about 1300 known species.[2][3]Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below the midline of the body, theanusis at the tip of the tail, and the mouth is under the front. A little above the gut is therhynchocoel,a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have aprobosciswhich lies in the rhynchocoel when inactive butevertsto emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodiesfilter feedand have suckers at the front and back ends, with which they attach to ahost.

Nemertea
Temporal range:Middle Triassic–recentPossible Ordovician and Carboniferous records
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
(unranked): Spiralia
Superphylum: Lophotrochozoa
Phylum: Nemertea
Schultze,1851
Classes
Synonyms[1]

Nemertini
Nemertinea
Rhynchocoela

The brain is a ring of fourganglia,positioned around the rhynchocoel near the animal's front end. At least a pair ofventral nerve cordsconnect to the brain and run along the length of the body. Most nemerteans have variouschemoreceptors,and on their heads some species have a number of pigment-cupocelli,which can detect light but can not form an image. Nemerteansrespirethrough the skin. They have at least twolateralvessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid. There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products,flame cellsare embedded in the front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside.

All nemerteans move slowly, using their externalciliato glide on surfaces on a trail ofslime,while larger species use muscular waves to crawl, and some swim bydorso-ventralundulations. A few live in the open ocean while the rest find or make hiding places on the bottom. About a dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places. Most nemerteans arecarnivores,feeding onannelids,clamsandcrustaceans.Some species of nemerteans arescavengers,and a few livecommensallyinside themantlecavity ofmolluscs.

In most species the sexes are separate, but all the freshwater species arehermaphroditic.Nemerteans often have numerous temporarygonads(ovariesortestes), and build temporary gonoducts (ducts from which the ova or sperm are emitted) opening to agonopore,one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. The fertilized egg divides byspiral cleavageand grows bydeterminate development,in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of mosttaxadevelop either directly to form juveniles (like the adult but smaller) or larvae that resemble theplanulasofcnidarians.However, some form apilidiumlarva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some speciesfragmentreadily, and even parts cut off near the tail can grow full bodies.

Traditionaltaxonomydivides the phylum in twoclasses,Anopla( "unarmed" – their proboscises do not have a little dagger) with twoorders,andEnopla( "armed" with a dagger) also with two orders. However, it is now accepted that Anopla areparaphyletic,as one order of Anopla is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea itself ismonophyletic,its mainsynapomorphiesbeing the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related toflatworms,but both phyla are regarded as members of theLophotrochozoa,a very large clade, sometimes viewed as a superphylum that also includesmolluscs,annelids,brachiopods,bryozoaand many otherprotostomes.

History

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In 1555Olaus Magnuswrote of a marine worm which was apparently 17.76 metres (58.3 ft) long ( "40 cubits" ), about the width of a child's arm, and whose touch made a hand swell.William Borlasewrote in 1758 of a "sea long worm", and in 1770Gunneruswrote a formal description of this animal, which he calledAscaris longissima.Its current name,Lineus longissimus,was first used in 1806 by Sowerby.[4]In 1995, a total of 1,149 species had been described and grouped into 250 genera.[5]

Nemertea are named after the Greek sea-nymphNemertes, one of the daughters ofNereusandDoris.[6]Alternative names for the phylum have includedNemertini,Nemertinea,andRhynchocoela.[1]TheNemertodermatidaare a separate phylum, whose closest relatives appear to be theAcoela.[7][8]

Description

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Body structure and major cavities

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The typical nemertean body is very thin in proportion to its length.[9]The smallest are a few millimeters long,[10]most are less than 20 centimetres (7.9 in), and several exceed 1 metre (3.3 ft). The longest animal ever found, at 54 metres (177 ft) long, may be a specimen ofLineus longissimus,[9]Ruppert, Fox and Barnes refer to aLineus longissimus54 metres (177 ft) long, washed ashore after a storm offSt Andrewsin Scotland.[11]Other estimates are about 30 metres (98 ft).[12]Zoologists find it extremely difficult to measure this species.[13]For comparison:

L. longissimus,however, is usually only a few millimeters wide.[17]The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species,[17][9]but reduce their length to 50% and increase their width to 300% when disturbed.[12]A few have relatively short but wide bodies, for exampleMalacobdella grossais up to 3.5 centimetres (1.4 in) long and 1 centimetre (0.39 in) wide,[9][18]and some of these are much less stretchy.[17]Smaller nemerteans are approximately cylindrical, but larger species are flatteneddorso-ventrally.Many have visible patterns in various combinations of yellow, orange, red and green.[9]

The outermost layer of the body has nocuticle,but consists of aciliatedandglandularepitheliumcontainingrhabdites,[10]which form themucusin which the cilia glide.[19]Each ciliated cell has many cilia andmicrovilli.[9]The outermost layer rests on a thickenedbasement membrane,thedermis.[10]Next to the dermis are at least three layers of muscles, some circular and some longitudinal.[9]The combinations of muscle types vary between the differentclasses,but these are not associated with differences in movement.[10]Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species.[9]Inside the concentric tubes of these layers ismesenchyme,a kind ofconnective tissue.[10]Inpelagicspecies this tissue is gelatinous and buoyant.[9]

They are unsegmented, but at least one species, Annulonemertes minusculus, is segmented. But this is assumed to be a derived trait. The segmentation does not include the coelom and body wall, and is therefore referred to as pseudosegmentation.[20][21]

The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and theanusis at the tip of the tail.[22]Above the gut and separated from the gut by mesenchyme is therhynchocoel,a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of classAnoplahas an orifice a little to the front of the mouth, but still under the front of the body. In the other class,Enopla,the mouth and the front of the rhynchocoel share an orifice.[9]The rhynchocoel is acoelom,as it is lined byepithelium.[10]

Proboscis and feeding

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Theproboscisis an infolding of the body wall, and sits in the rhynchocoel when inactive.[10]When muscles in the wall of the rhynchocoel compress the fluid inside, the pressure makes the proboscis jump inside-out along a canal called the rhynchodeum and through an orifice, the proboscis pore. The proboscis has a muscle which attaches to the back of the rhynchocoel, can stretch up to 30 times its inactive length and acts to retract the proboscis.[9]

Gorgonorhynchus repens,a species within class Anopla, discharges a sticky branched proboscis.

The proboscis of theclassAnoplaexits from an orifice which is separate from the mouth,[9]coils around the prey and immobilizes it by sticky, toxic secretions.[22]The Anopla can attack as soon as the prey moves into the range of the proboscis.[23]Some Anopla have branched proboscises which can be described as "a mass of sticky spaghetti".[9]The animal then draws its prey into its mouth.[10]

Stylet-containing part of proboscis of "armed" nemerteanAmphiporus ochraceus.

In most of the classEnopla,the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has astylet,acalcareousbarb,[9]with which the animal stabs the prey many times to inject toxins and digestive secretions. The prey is then swallowed whole or, after partial digestion, its tissues are sucked into the mouth.[22]The stylet is attached about one-third of the distance from the end of theevertedproboscis, which extends only enough to expose the stylet. On either side of the active stylet are sacs containing back-up stylets to replace the active one as the animal grows or an active one is lost.[9]Instead of one stylet, thePolystiliferahave a pad that bears many tiny stylets, and these animals have separate orifices for the proboscis and mouth, unlike other Enopla.[24][25]The Enopla can only attack after contacting the prey.[23]

Some nemerteans, such asL. longissimus,absorb organic food in solution through their skins, which may make the long, slim bodies an advantage.[17]Suspension feedingis found only among the specialized symbioticbdellonemerteans,[23]which have a proboscis but no stylet, and use suckers to attach themselves tobivalves.[26]

Respiration and circulatory system

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Nemerteans lack specializedgills,and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluidcirculationrather thandiffusionto move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels,[27]while theirbloodis contained in the main body cavity.[28]The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluidskeletonin everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation.[27]The circulatory vessels are a system of coeloms.[29]

In the simplest type of circulatory system, two lateral vessels are joined at the ends to form a loop. However, many species have additional long-wise and cross-wise vessels. There is no heart nor pumping vessels,[30]and the flow of fluid depends on contraction of both the vessels and the body wall's muscles. In some species, circulation is intermittent, and fluid ebbs and flows in the long-wise vessels.[27]The fluid in the vessels is usually colorless, but in some species it contains cells that are yellow, orange, green or red. The red type containhemoglobinand carry oxygen, but the function of the other pigments is unknown.[27]

Excretion

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A schematic representation of aflame celland other associated structures

Nemertea use organs calledprotonephridia[27]to excrete soluble waste products, especiallynitrogenousby-products of cellularmetabolism.[31]In nemertean protonephridia,flame cellswhich filter out the wastes are embedded in the front part of the two lateral fluid vessels. The flame cells remove the wastes into two collecting ducts, one on either side, and each duct has one or morenephridioporesthrough which the wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be thatosmoregulationis more difficult in non-marine environments.[27]

Nervous-system and senses

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Brain and neural cords of hoplonemerteanAmphiporus ochraceus.Several clusters of dark eyespots and the opening of one cerebral organ are also visible.

Thecentral nervous-systemconsists of abrainand pairedventral nerve cordsthat connect to the brain and run along the length of the body. Thebrainis aringof fourganglia,masses of nerve cells, positioned round the rhynchocoel near its front end[32]– while the brains of mostprotostomeinvertebrates encircle the foregut.[33]Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord.[32]In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers.[34]The central nervous-system is often red or pink because it containshemoglobin.This storesoxygenfor peak activity or when the animal experiencesanoxia,for example whileburrowingin oxygen-freesediments.[32]

Some species have pairedcerebralorgans, sacs whose only openings are to the outside. Others species have unpaired evertible organs on the front of their heads. Some have slits along the side of the head or grooves obliquely across the head, and these may be associated with paired cerebral organs. All of these are thought to bechemoreceptors,and the cerebral organs may also aidingosmoregulation.Small pits in the epidermis appear to be sensors.[32]On their head, some species have a number of pigment-cupocelli,[32]which can detect light but not form an image.[35]Most nemerteans have two to six ocelli, although some have hundreds.[34]A few tiny species that live between grains of sand havestatocysts,[32]which sense balance.[36]

Paranemertes peregrina,which feeds on polychaetes, can follow the prey's trails of mucus, and find its burrow by backtracking along its own trail of mucus.[22]

Movement

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The nemerteanGeonemertes pelaensis(right) being inspected by a spider, which it then captures.
Lineus longissimusinGrevelingen

Nemerteans generally move slowly,[10]though they have occasionally been documented to successfully prey on spiders or insects.[37]Most nemerteans use their external cilia to glide on surfaces on a trail ofslime,some of which is produced by glands in the head. Larger species use muscular waves to crawl, and some aquatic species swim by dorso-ventral undulations. Some species burrow by means of muscularperistalsis,and have powerful muscles.[9]Some species of thesuborderMonostilifera,whose proboscis have one active stylet, move by extending the proboscis, sticking it to an object and pulling the animal toward the object.[24]

Reproduction and life-cycle

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Larger species often break up when stimulated, and the fragments often grow into full individuals. Some species fragment routinely and even parts near the tail can grow full bodies.[38]But this kind of extreme regeneration is restricted to only a few types of nemerteans, and is assumed to be a derived feature.[39]Allreproduce sexually,and most species aregonochoric(the sexes are separate),[10][38]but all the freshwater forms arehermaphroditic.[28]

Nemerteans often have numerous temporarygonads(ovariesortestes), forming a row down each side of the body in themesenchyme.[28][38]Temporarygonoducts(ducts from which theovaorspermare emitted[40]), one per gonad, are built when the ova and sperm are ready.[38]The eggs are generally fertilised externally. Some species shed them into the water, some lay them in a burrow or tube, and some protect them bycocoonsorgelatinousstrings.[38]Somebathypelagic(deep sea) species haveinternal fertilization,and some of these areviviparous,growing theirembryosin the female's body.[28][38]

Thezygote(fertilised egg) divides byspiral cleavageand grows bydeterminate development,[38]in which the fate of a cell can usually be predicted from its predecessors in the process of division.[17]The embryos of mosttaxadevelop either directly to formjuveniles(like the adult but smaller) or to formplanuliformlarvae.The planuliform larva stage may be short-lived andlecithotrophic( "yolky" ) before becoming a juvenile,[38]or may beplanktotrophic,swimming for some time and eating prey larger than microscopic particles.[33]However, many members of the orderHeteronemerteaand thepalaeonemerteanfamilyHubrechtiidaeform apilidiumlarva, which can captureunicellularalgaeand which Maslakova describes as like adeerstalkercap with the ear flaps pulled down. It has a gut which lies across the body, a mouth between the "ear flaps", but no anus. A small number ofimaginal discsform, encircling thearchenteron(developing gut) and coalesce to form the juvenile. When it is fully formed, the juvenile bursts out of the larva body and usually eats it during this catastrophicmetamorphosis.[33]This larval stage is unique in that there are noHox genesinvolved during development, which are only found in the juveniles developing inside the larvae.[41]

The speciesParanemertes peregrinahas been reported as having a life span of around 18 months.[34]

Ecological significance

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A terrestrial nemertean from WestJava.The animal is 1.5 centimetres (0.59 in) long, of which the anterior 1 centimetre (0.39 in) is visible.
A terrestrialGeonemertessp. on a rotting log, fromMindanaoIsland, thePhilippines

Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or theholdfastsofalgaeorsessileanimals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined withmucusor producecellophane-like tubes. Mainly in the tropics and subtropics, about 12 species appear in freshwater,[9]and about a dozen species live on land in cool, damp places, for example under rotting logs.[17]

The terrestrialArgonemertes dendyiis a native ofAustraliabut has been found in theBritish Isles,inSao Miguelin theAzores,inGran Canaria,and in alava tubeatKaumanaon theIsland of Hawaii.It can build a cocoon, which allows it to avoid desiccation while being transported, and it may be able to build populations quickly in new areas as it is aprotandroushermaphrodite.[42]Another terrestrialgenus,Geonemertes,is mostly found inAustralasiabut has species in theSeychelles,widely across theIndo-Pacific,inTristan da Cunhain the South Atlantic, inFrankfurt,in theCanary Islands,inMadeiraand in the Azores.[5]Geonemertes pelaensishas been implicated in the decline of native arthropod species on theOgasawara Islands,where it wasintroducedin the 1980s.[43]

Most arecarnivores,feeding onannelids,clamsandcrustaceans,[22]and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects andmyriapodsare the only known prey of the two terrestrial species ofArgonemertes.[23] A few nemerteans arescavengers,[22]and these generally have good distancechemoreception( "smell" ) and are not selective about their prey.[23]A few species livecommensallyinside themantlecavity of molluscs and feed on micro-organisms filtered out by the host.[44]

NearSan Franciscothe nemerteanCarcinonemertes erranshas consumed about 55% of the total egg production of its host, thedungeness crabMetacarcinus magister.C. erransis considered a significant factor in the collapse of the dungeness crab fishery.[23]Other coastal nemerteans have devastatedclambeds.[9]

The few predators on nemerteans include bottom-feeding fish, some sea birds, a few invertebrates includinghorseshoe crabs,and other nemerteans.[9]Nemerteans' skins secrete toxins that deter many predators, but some crabs may clean nemerteans with one claw before eating them.[28]The AmericanCerebratulus lacteusand the South AfricanPolybrachiorhynchus dayi,both called "tapeworms" in their respective localities, are sold as fish bait.[9]

Taxonomy

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See also:List of bilaterial animal orders

Traditional taxonomic classification has divided the group into two classes and four orders:

  • ClassAnopla( "unarmed" ). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.[24]
    • OrderPalaeonemertea.Comprises 100 marine species. Their body wall has outer circular and inner length-wise muscles. In addition,Carinoma tremaphoroshas circular and inner length-wise muscles in theepidermis;the extra muscle layers seem to be needed forburrowingbyperistalsis.[24]
    • OrderHeteronemertea.Comprises about 400 species. The majority are marine, but three are freshwater. Their body-wall muscles are disposed in four layers, alternately circular and length-wise starting from the outermost layer. The order includes the strongest swimmers. Twogenerahave branched proboscises.[24]
  • ClassEnopla( "armed" ). All havestyletsexcept orderBdellonemertea.Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.[24]
    • OrderBdellonemertea.Includes seven species, of which six live ascommensalsin themantleof largeclamsand one in that of a freshwater snail. The hostsfilter feedand all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a suckingpharynxand a posterior stucker, with which they move likeinchworms.[24]
    • OrderHoplonemertea.Comprises 650 species. They live inbenthicandpelagicsea water, in freshwater and on land. They feed by commensalism andparasitism,and are armed withstylet(s)[24]
      • SuborderMonostilifera.Includes 500 species with a single centralstylet.Some use the stylet for locomotion as well as for capturing prey.[24]
      • SuborderPolystilifera.Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets.[24]

Recent molecular phylogenetic studies divided the group into two superclasses, three classes, and eight orders:[45]

Evolutionary history

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Fossil record

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As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare.[10][44]One might expect the stylet of a nemertean to be preserved, since it is made ofcalcium phosphate,but no fossil stylets have yet been found.[10][44]Knaust (2010)reported nemertean fossils and traces from theMiddle TriassicofGermany.[46]

TheMiddle CambrianfossilAmiskwiafrom theBurgess Shalehas been classed as a nemertean, based on a resemblance to some unusual deep-sea swimming nemerteans, but few paleontologists accept this classification as the Burgess Shale fossils show no evidence of rhynchocoel nor intestinal caeca.[44][47]

Knaust & Desrochers (2019)reported fossils ofvermiformorganisms with a wide range ofmorphologiesoccurring on bedding planes from the LateOrdovician(Katian)Vauréal Formation(Canada). In the specimens preserving the anterior end of the body, this end is pointed or rounded, bearing a rhynchocoel with the proboscis, which is characteristic for nemerteans. The authors attributed these fossils to nemerteans and interpreted them as the oldest record of the group reported so far. However, Knaust & Desrochers cautioned that partly preserved putative nemertean fossils might ultimately turn out to be fossils ofturbellariansorannelids.[48]

It has been suggested thatArchisymplectes,one of thePennsylvanian-age animals fromMazon Creekin northern and centralIllinois,may be a nemertean.[49] This fossil, however, only preserves the outline of the "worm",[44]and there is no evidence of a proboscis,[50] so there is no certainty that it represents a nemertean.[44]

Within Nemertea

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Nemertea
Groups within Nemertea, byRuppert, Fox & Barnes (2004a).[51]
highlights the "Anopla", which areparaphyletic.[51]

There is no doubt that the phylum Nemertea ismonophyletic(meaning that the phylum includes all and only descendants of one ancestor that was also a member of the phylum).[51]: 2–3 Thesynapomorphies(trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel.[52]

WhileRuppert, Fox & Barnes (2004a)treat the Palaeonemertea as monophyletic,[51]Thollesson & Norenburg (2003)regard them asparaphyleticandbasal(contains the ancestors of the more recent clades).[52]TheAnopla( "unarmed" ) represent anevolutionary gradeof nemerteans without stylets (comprising theHeteronemerteaand the Palaeonemerteans), whileEnopla( "armed" ) are monophyletic, but find that Palaeonemertea is doubly paraphyletic, having given rise to both the Heteronemertea and the Enopla.[51][52]Ruppert, Fox & Barnes (2004a)treat the Bdellonemertea as acladeseparate from theHoplonemertea,[51]whileThollesson & Norenburg (2003)believe the Bdellonemertea are a part of the Monostilifera (with one active stylet), which are within the Hoplonemertea – which implies that "Enopla" and "Hoplonemertea" are synonyms for the same branch of the tree.[52]The Polystilifera (with many tiny stylets) are monophyletic.[51][52]

Relationships with other phyla

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English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre-cladisticsanalyses emphasised as shared features: multiciliated (with multiple cilia per cell), glandular epidermis; rod-shaped secretory bodies or rhabdites; frontal glands or organs;protonephridia;and acoelomate body organization.[53]However, multiciliated epidermal cells and epidermal gland cells are also found inCtenophora,Echiura,Sipuncula,Annelida,Mollusca and othertaxa.The rhabdites of nemertea have a different structure from those of flatworms at the microscopic scale. The frontal glands or organs of flatworms vary a lot in structure, and similar structures appear in small marine annelids andentoproctlarvae. The protonephridia of nemertea and flatworms are different in structure,[53]and in position – theflame cellsof nemertea are usually in the walls of the fluid vessels and are served by "drains" from which the wastes exit by a small number of tubes through the skin,[27]while the flame cells of flatworms are scattered throughout the body.[54]: 239  Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia.[53]

According to more recent analyses, in the development of nemertean embryos, ectomesoderm(outer part of the mesoderm, which is the layer in which most of the internal organs are built) is derived from cells labelled 3a and 3b, and endomesoderm (inner part of the mesoderm) is derived from the 4d cell. Some of the ectomesoderm inannelids,echiuransandmolluscsis derived from cells 3a and 3b, while the ectomesoderm ofpolycladflatwormsis derived from the 2b cell andacoelflatworms produce no ectomesoderm. In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellularconnective tissue.This structure is similar to that found in larger flatworms such aspolycladsandtriclads,but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among theSpiralia(animals in which the early cell divisions make a spiral pattern) such assipunculans,echiuransand many annelids.[53]

Relationships of Nemertea to otherBilateria:[55][56]

Nemerteans' affinities with Annelida (including Echiura,Pogonophora,Vestimentiferaand perhaps Sipuncula) and Mollusca make the ribbon-worms members ofLophotrochozoa,which include about half of the extant animal phyla.[57]Lophotrochozoa groups: those animals that feed using alophophore(Brachiopoda,Bryozoa,Phoronida,Entoprocta); phyla in which most members' embryos develop intotrochophorelarvae (for example Annelida and Mollusca); and some other phyla (such as Platyhelminthes, Sipuncula, Gastrotricha, Gnathostomulida, Micrognathozoa, Nemertea,Phoronida,Platyhelminthes, andRotifera).[55][57] These groupings are based onmolecular phylogeny,which compares sections of organismsDNAandRNA.While analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear.[55][57]

Mostprotostomephyla outside the Lophotrochozoa are members ofEcdysozoa( "animals thatmolt"), which includeArthropoda,NematodaandPriapulida.Most otherbilaterianphyla are in theDeuterostomia,which includeEchinodermataandChordata.TheAcoelomorpha,which are neither protostomes nor deuterostomes, are regarded asbasalbilaterians.[55][57][58]

See also

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Notes

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References

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  1. ^abScott, Thomas (1996)."Nemertini, Rhynchocoela, Nemertea, Nemertinea".Concise Encyclopedia of Biology.Walter de Gruyter. pp.815–816.ISBN978-3-11-010661-9.
  2. ^"Nemertea".Integrated Taxonomic Information System.RetrievedFebruary 18,2011.
  3. ^"A poisonous shield, a potent venom: These worms mean business".Nature.606(7913): 230. 2022.Bibcode:2022Natur.606R.230..doi:10.1038/d41586-022-01484-7.S2CID249434440.
  4. ^Cedhagen, Tomas; Per Sundberg (1986). "A previously unrecognized report of a nemertean in the literature".Archives of Natural History.13:7–8.doi:10.3366/anh.1986.13.1.7.ISSN0260-9541.
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