Nemertea is aphylum ofanimals also known asribbon worms orproboscis 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, theanus is 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 aproboscis which lies in the rhynchocoel when inactive buteverts to 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 feed and have suckers at the front and back ends, with which they attach to ahost.
The brain is a ring of fourganglia, positioned around the rhynchocoel near the animal's front end. At least a pair ofventral nerve cords connect 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. Nemerteansrespire through the skin. They have at least twolateral vessels 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 cells are 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 externalcilia to glide on surfaces on a trail ofslime, while larger species use muscular waves to crawl, and some swim bydorso-ventral undulations. 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,clams andcrustaceans. Some species of nemerteans arescavengers, and a few livecommensally inside themantle cavity ofmolluscs.
In most species the sexes are separate, but all the freshwater species arehermaphroditic. Nemerteans often have numerous temporarygonads (ovaries ortestes), 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 cleavage and 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 mosttaxa develop either directly to form juveniles (like the adult but smaller) or larvae that resemble theplanulas ofcnidarians. However, some form apilidium larva, 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 speciesfragment readily, and even parts cut off near the tail can grow full bodies.
Traditionaltaxonomy divides 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 mainsynapomorphies being 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,bryozoa and many otherprotostomes.
In 1555Olaus Magnus wrote 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 Borlase wrote in 1758 of a "sea long worm", and in 1770Gunnerus wrote 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-nymph Nemertes, one of the daughters ofNereus andDoris.[6] Alternative names for the phylum have includedNemertini,Nemertinea, andRhynchocoela.[1] TheNemertodermatida are a separate phylum, whose closest relatives appear to be theAcoela.[7][8]
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 longissimus 54 metres (177 ft) long, washed ashore after a storm offSt Andrews in Scotland.[11] Other estimates are about 30 metres (98 ft).[12] Zoologists find it extremely difficult to measure this species.[13] For comparison:
The longest recordedblue whale was 33.58 metres (110.2 ft).[14]
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 grossa is 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 aciliated andglandularepithelium containingrhabdites,[10] which form themucus in 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] Inpelagic species 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 theanus is 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 classAnopla has 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]
Theproboscis is 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 theclassAnopla exits 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, acalcareous barb,[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 theeverted proboscis, 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, thePolystilifera have 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 feeding is found only among the specialized symbioticbdellonemerteans,[23] which have a proboscis but no stylet, and use suckers to attach themselves tobivalves.[26]
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 fluidcirculation rather thandiffusion to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels,[27] while theirblood is contained in the main body cavity.[28] The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluidskeleton in 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 containhemoglobin and carry oxygen, but the function of the other pigments is unknown.[27]
A schematic representation of aflame cell and other associated structures
Nemertea use organs calledprotonephridia[27] to excrete soluble waste products, especiallynitrogenous by-products of cellularmetabolism.[31] In nemertean protonephridia,flame cells which 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 morenephridiopores through which the wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be thatosmoregulation is more difficult in non-marine environments.[27]
Brain and neural cords of hoplonemerteanAmphiporus ochraceus. Several clusters of dark eyespots and the opening of one cerebral organ are also visible.
Thecentral nervous-system consists of abrain and pairedventral nerve cords that connect to the brain and run along the length of the body. Thebrain is aring of fourganglia, masses of nerve cells, positioned round the rhynchocoel near its front end[32] – while the brains of mostprotostome invertebrates 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 storesoxygen for peak activity or when the animal experiencesanoxia, for example whileburrowing in oxygen-freesediments.[32]
Some species have pairedcerebral organs, 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]
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]
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 (ovaries ortestes), forming a row down each side of the body in themesenchyme.[28][38] Temporarygonoducts (ducts from which theova orsperm are 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 bycocoons orgelatinous strings.[38] Somebathypelagic (deep sea) species haveinternal fertilization, and some of these areviviparous, growing theirembryos in the female's body.[28][38]
Thezygote (fertilised egg) divides byspiral cleavage and 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 mosttaxa develop 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 orderHeteronemertea and thepalaeonemerteanfamilyHubrechtiidae form apilidium larva, which can captureunicellularalgae and which Maslakova describes as like adeerstalker cap 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 discs form, 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 genes involved during development, which are only found in the juveniles developing inside the larvae.[41]
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 terrestrialGeonemertes sp. on a rotting log, fromMindanao Island, thePhilippines
Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or theholdfasts ofalgae orsessile animals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined withmucus or 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]
Most arecarnivores, feeding onannelids,clams andcrustaceans,[22] and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects andmyriapods are 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 livecommensally inside themantle cavity of molluscs and feed on micro-organisms filtered out by the host.[44]
NearSan Francisco the nemerteanCarcinonemertes errans has consumed about 55% of the total egg production of its host, thedungeness crabMetacarcinus magister.C. errans is considered a significant factor in the collapse of the dungeness crab fishery.[23] Other coastal nemerteans have devastatedclam beds.[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 lacteus and the South AfricanPolybrachiorhynchus dayi, both called "tapeworms" in their respective localities, are sold as fish bait.[9]
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 tremaphoros has circular and inner length-wise muscles in theepidermis; the extra muscle layers seem to be needed forburrowing byperistalsis.[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. Twogenera have branched proboscises.[24]
ClassEnopla ("armed"). All havestylets except 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 ascommensals in themantle of largeclams and one in that of a freshwater snail. The hostsfilter feed and all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a suckingpharynx and a posterior stucker, with which they move likeinchworms.[24]
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 Triassic ofGermany.[46]
TheMiddle Cambrian fossilAmiskwia from theBurgess Shale has 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 ofvermiform organisms with a wide range ofmorphologies occurring 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 ofturbellarians orannelids.[48]
It has been suggested thatArchisymplectes, one of thePennsylvanian-age animals fromMazon Creek in 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]
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 asparaphyletic andbasal (contains the ancestors of the more recent clades).[52] TheAnopla ("unarmed") represent anevolutionary grade of nemerteans without stylets (comprising theHeteronemertea and 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 aclade separate 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]
English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre-cladistics analyses 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,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 andentoproct larvae. The protonephridia of nemertea and flatworms are different in structure,[53] and in position – theflame cells of 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]: 239Rigorous 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,echiurans andmolluscs is derived from cells 3a and 3b, while the ectomesoderm ofpolycladflatworms is derived from the 2b cell andacoel flatworms 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 aspolyclads andtriclads, 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,echiurans and many annelids.[53]
Nemerteans' affinities with Annelida (including Echiura,Pogonophora,Vestimentifera and 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 intotrochophore larvae (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 organismsDNA andRNA. 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]
^Barnes, Richard Stephen Kent (2001)."The worms".The Invertebrates: a Synthesis. Wiley-Blackwell. pp. 81–83.ISBN978-0-632-04761-1. Retrieved27 Jan 2011.
^abcdefghijklWalker, J.C.; Anderson, D.T. (1998). "The Platyhelminthes, Nemertea, Entoprocta and Gnathostomulida". In D.T. Anderson (ed.).Invertebrate Zoology (1 ed.). Oxford University Press Australia. pp. 79–85.ISBN978-0-19-553941-7.
^Carwardine, Mark (1995).The Guinness Book of Animal Records. Guinness Publishing. p. 232.ISBN978-0-85112-658-6.
^abcdefMoore, Janet; Overhill, Raith (2006)."Chapter 7 – Nemertea". In Raith Overhill (ed.).An Introduction to the Invertebrates (2 ed.). Cambridge University Press. pp. 75–84.ISBN978-0-521-85736-9. Retrieved31 Jan 2011.
^Anderson, D.T. (1998). "The invertebrate phyla". In D.T. Anderson (ed.).Invertebrate Zoology (1 ed.). Oxford University Press Australia. p. 4.ISBN978-0-19-553941-7.
^abcBarnes, Robert D. (1982).Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 252–262.ISBN978-0-03-056747-6.
^Russell, Peter J.; Wolfe, Stephen L.; Hertz, Paul E.; Cecie Starr (2008)."Photoreceptors and vision".Biology: the dynamic science. Vol. 3. Cengage Learning. pp. 894–895.ISBN978-0-495-01034-0. Retrieved31 Jan 2011.
^Russell, Peter J.; Wolfe, Stephen L.; Hertz, Paul E.; Cecie Starr (2008)."Photoreceptors and vision".Biology: the dynamic science. Vol. 3. Cengage Learning. p. 889.ISBN978-0-495-01034-0. Retrieved31 Jan 2011.
^Schram, Frederick R. (September 1973). "Pseudocoelomates and a Nemertine from the Illinois Pennsylvanian".Journal of Paleontology.47 (5):985–989.JSTOR1303083.
^Nielsen, Claus (2001)."Phylum Nemertini".Animal Evolution: Interrelationships of the living phyla (2 ed.). Oxford University Press. pp. 281–282.ISBN978-0-19-850681-2. Retrieved11 Feb 2011.
