View source for Nemertea

{{Short description|Phylum of invertebrates, ribbon worms}}
{{Hatnote|Not to be confused with the roundworm phylum [[Nematoda]]. For the Romanian village, see [[Gura Teghii]]. }}
{{Good article}}
{{Automatic taxobox
| fossil_range = {{fossil range|earliest=Katian|Middle Triassic|recent}} <small>Possible Ordovician and Carboniferous records</small>
| image = Nemertea by Joubin.jpg
| image_caption = 
| display_parents = 8
| taxon = Nemertea
| authority = [[Max Johann Sigismund Schultze|Schultze]], 1851
| synonyms = Nemertini<br />Nemertinea<br />Rhynchocoela
| synonyms_ref =<ref name="Scott"/>
| subdivision_ranks = Classes
| subdivision = 
* [[Palaeonemertea]]
* [[Pilidiophora]]
* [[Hoplonemertea]] (= [[Enopla]])
}}

'''Nemertea''' is a [[phylum]] of [[animal]]s also known as '''ribbon worms''' or '''proboscis worms''', consisting of about 1300 known species.<ref name="ITIS">{{ITIS |id=57411 |taxon=Nemertea |access-date=February 18, 2011}}</ref><ref>{{cite journal | url=https://www.nature.com/articles/d41586-022-01484-7 | doi=10.1038/d41586-022-01484-7 | title=A poisonous shield, a potent venom: These worms mean business | journal=Nature | year=2022 | volume=606 | issue=7913 | page=230 | bibcode=2022Natur.606R.230. | s2cid=249434440 | url-access=subscription }}</ref>  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.
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Under the skin are usually at least three layers of muscles, some circular and some longitudinal. The mouth is [[ventral]] and a little behind the front of the body.
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The foregut, stomach and intestine run a little below the midline of the body, the [[anus]] is at the tip of the tail, and the mouth is under the front. A little above the gut is the {{visible anchor|rhynchocoel}}, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a [[proboscis]] which lies in the rhynchocoel when inactive but [[wikt:eversion|everts]] 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 bodies [[Filter feeders|filter feed]] and have suckers at the front and back ends, with which they attach to a [[Host (biology)|host]].

The brain is a ring of four [[ganglia]], positioned around the rhynchocoel near the animal's front end. At least a pair of [[ventral nerve cord]]s connect to the brain and run along the length of the body. Most nemerteans have various [[chemoreceptors]], and on their heads some species have a number of pigment-cup [[ocelli]], which can detect light but can not form an image. Nemerteans [[Respiration (physiology)|respire]] through the skin. They have at least two [[Lateral (anatomy)|lateral]] 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 cell]]s 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 external [[cilia]] to glide on surfaces on a trail of [[Mucus|slime]], while larger species use muscular waves to crawl, and some swim by [[Anatomical terms of location#Directional terms|dorso-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 are [[carnivores]], feeding on [[annelid]]s, [[clam]]s and [[crustacean]]s. Some species of nemerteans are [[scavenger]]s, and a few live [[commensal]]ly inside the [[Mantle (mollusc)|mantle]] cavity of [[mollusc]]s. 

In most species the sexes are separate, but all the freshwater species are [[hermaphroditic]]. Nemerteans often have numerous temporary [[gonad]]s ([[ovary|ovaries]] or [[testes]]), and build temporary gonoducts (ducts from which the ova or sperm are emitted) opening to a [[gonopore]], 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 by [[spiral cleavage]] and grows by [[determinate development]], in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most [[taxon|taxa]] develop either directly to form juveniles (like the adult but smaller) or larvae that resemble the [[planula]]s of [[cnidaria]]ns. However, some form a [[pilidium (zoology)|pilidium]] 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 species [[Fragmentation (reproduction)|fragment]] readily, and even parts cut off near the tail can grow full bodies.

Traditional [[Taxonomy (biology)|taxonomy]] divides the phylum in two [[Class (biology)|classes]], [[Anopla]] ("unarmed" – their proboscises do not have a little dagger) with two [[Order (biology)|orders]], and [[Enopla]] ("armed" with a dagger) also with two orders. However, it is now accepted that Anopla are [[paraphyletic]], as one order of Anopla is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea itself is [[monophyletic]], its main [[synapomorphy|synapomorphies]] being the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related to [[flatworms]], but both phyla are regarded as members of the [[Lophotrochozoa]], a very large clade, sometimes viewed as a superphylum that also includes [[mollusc]]s, [[annelid]]s, [[brachiopod]]s, [[bryozoa]] and many other [[protostome]]s.

==History==

In 1555 [[Olaus Magnus]] wrote of a marine worm which was apparently {{convert|17.76|m|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 1770 [[Johan Ernst Gunnerus|Gunnerus]] wrote a formal description of this animal, which he called ''Ascaris longissima''. Its current name, ''[[Lineus longissimus]]'', was first used in 1806 by Sowerby.<ref>{{cite journal|last=Cedhagen|first=Tomas|author2=Per Sundberg|year=1986|title=A previously unrecognized report of a nemertean in the literature|journal=[[Archives of Natural History]] |volume=13|pages=7–8|issn=0260-9541|doi=10.3366/anh.1986.13.1.7}}</ref> In 1995, a total of 1,149 species had been described and grouped into 250 genera.<ref name="Gibson">{{cite journal |author=R. Gibson |year=1995 |title=Nemertean genera and species of the world: an annotated checklist of original names and description citations, synonyms, current taxonomic status, habitats and recorded zoogeographic distribution |journal=[[Journal of Natural History]] |volume=29 |issue=2 |pages=271–561 |doi=10.1080/00222939500770161}}</ref>

Nemertea are named after the Greek sea-[[nymph]] Nemertes, one of the daughters of [[Nereus]] and [[Doris (Oceanid)|Doris]].<ref>{{cite book|last=Barnes|first=Richard Stephen Kent|title=The Invertebrates: a Synthesis|chapter-url=https://books.google.com/books?id=TBMsbe9efPgC&q=rhynchocoel&pg=PA82|access-date=27 Jan 2011|year=2001|publisher=Wiley-Blackwell|pages=81–83|chapter=The worms|isbn=978-0-632-04761-1}}</ref> Alternative names for the phylum have included ''Nemertini'', ''Nemertinea'', and ''Rhynchocoela''.<ref name="Scott">{{cite book|last=Scott|first=Thomas|title=Concise Encyclopedia of Biology|year=1996|publisher=Walter de Gruyter|isbn=978-3-11-010661-9|pages=[https://archive.org/details/conciseencyclope00scot/page/815 815–816]|chapter=Nemertini, Rhynchocoela, Nemertea, Nemertinea|chapter-url=https://archive.org/details/conciseencyclope00scot/page/815}}</ref><!-- [[Libbie Hyman]] named them Rhynchocoela, a name used primarily in North America but gradually abandoned since the 1980s. --> The [[Nemertodermatida]] are a separate phylum, whose closest relatives appear to be the [[Acoela]].<ref name="PapsBaguñàEtc2009">{{cite journal |vauthors=Paps J, Baguñà J, Riutort M |title=Lophotrochozoa internal phylogeny: new insights from an up-to-date analysis of nuclear ribosomal genes |journal=[[Proceedings of the Royal Society B]] |volume=276 |issue=1660 |pages=1245–54 |date=April 2009 |pmid=19129141 |doi=10.1098/rspb.2008.1574  |pmc=2660960}}</ref><ref name="Jimenez-GuriPapsEtc2006">{{cite journal |vauthors=Jimenez-Guri E, Paps J, Garcia-Fernandez J, Salo E |title=Hox and ParaHox genes in Nemertodermatida, a basal bilaterian clade |journal=[[International Journal of Developmental Biology]] |volume=50 |issue=8 |pages=675–9 |year=2006 |pmid=17051477 |doi=10.1387/ijdb.062167ej |doi-access=free }}</ref>

== Description ==

=== Body structure and major cavities ===

The typical nemertean body is very thin in proportion to its length.<ref name="RFB2004FormBodyProboscis">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | pages=[https://archive.org/details/isbn_9780030259821/page/271 271–274] | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | url=https://archive.org/details/isbn_9780030259821/page/271 }}</ref> The smallest are a few millimeters long,<ref name="WalkerAnderson1998Nemertea">{{cite book|last1=Walker|first1=J.C.|last2=Anderson|first2=D.T.|editor=D.T. Anderson|title=Invertebrate Zoology|edition=1|year=1998|publisher=Oxford University Press Australia|isbn=978-0-19-553941-7|pages=79–85|chapter=The Platyhelminthes, Nemertea, Entoprocta and Gnathostomulida}}</ref> most are less than {{convert|20|cm|in}}, and several exceed {{convert|1|m|ft}}. The longest animal ever found, at {{convert|54|m|ft}} long, may be a specimen of ''[[Lineus longissimus]]'',<ref name="RFB2004FormBodyProboscis" /> Ruppert, Fox and Barnes refer to a ''[[Lineus longissimus]]'' {{convert|54|m|ft}} long, washed ashore after a storm off [[St Andrews]] in Scotland.<ref>{{cite book|last=Carwardine|first=Mark|title=The Guinness Book of Animal Records|year=1995|publisher=Guinness Publishing|isbn=978-0-85112-658-6|page=232}}</ref> Other estimates are about {{convert|30|m|ft}}.<ref name="GibsonMBL">{{cite web|last=Gibson|first=Ray|title=Phylum Nemertea (Nemertinea, Nemertini, Rhynchocoela)|url=http://www.mbl.edu/publications/biobull/keys/7/index.html|url-status=dead|archive-url=https://web.archive.org/web/20110927084717/http://www.mbl.edu/publications/biobull/keys/7/index.html|archive-date=2011-09-27|access-date=30 March 2011|publisher=The Marine Biological Laboratory|location=[[Woods Hole]], [[Massachusetts]]}}</ref> Zoologists find it extremely difficult to measure this species.<ref>{{cite book|title=Encyclopedia of the Aquatic World: Starfish|year=2004|publisher=Marshall Cavendish Corporation|isbn=978-0-7614-7418-0|chapter-url=https://books.google.com/books?id=fqgZX5VIQMQC&q=%22bootlace+worm%22&pg=PA1420|access-date=30 March 2011|page=1420|chapter=King of the Worms}}</ref> For comparison:
* The longest recorded [[blue whale]] was {{convert|33.58|m|ft}}.<ref>{{cite book|last=Simmonds|first=Mark|title=Whales and Dolphins of the World|url=https://books.google.com/books?id=TQxyo8O5KD8C&q=longest+animal&pg=PA155|access-date=27 Jan 2011|year=2007|publisher=New Holland Publishers|isbn=978-1-84537-820-2|page=155}}</ref>
* The [[dinosaur]]s ''[[Argentinosaurus]]'' and ''[[Patagotitan]]'' are estimated at approximately {{convert|35|m|ft}} and {{convert|31|m|ft}} respectively.<ref name="Paul2019">{{cite journal|author=Paul, Gregory S. |title=Determining the largest known land animal: A critical comparison of differing methods for restoring the volume and mass of extinct animals |journal=Annals of the Carnegie Museum |year=2019 |volume=85 |issue=4 |pages=335–358 |doi=10.2992/007.085.0403 |s2cid=210840060 |url=http://www.gspauldino.com/Titanomass.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://www.gspauldino.com/Titanomass.pdf |archive-date=2022-10-10 |url-status=live }}</ref>
* A specimen of the Arctic giant jellyfish ''[[Cyanea capillata arctica]]'' was {{convert|36.5|m|ft}} long.<ref>{{cite book|last=Carwardine|first=Mark|title=Animal Records|url=https://books.google.com/books?id=T3FEKopUFkUC&q=longest+jellyfish&pg=PA241|access-date=27 Jan 2011|year=2008|publisher=Sterling Publishing Company |isbn=978-1-4027-5623-8|page=241}}</ref>

''L. longissimus'', however, is usually only a few millimeters wide.<ref name="MooreOverhill2006" /> The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species,<ref name="MooreOverhill2006" /><ref name="RFB2004FormBodyProboscis" /> but reduce their length to 50% and increase their width to 300% when disturbed.<ref name="GibsonMBL" /> A few have relatively short but wide bodies, for example ''Malacobdella grossa'' is up to {{convert|3.5|cm|in}} long and {{convert|1|cm|in}} wide,<ref name="RFB2004FormBodyProboscis" /><ref>{{cite book|last=Haderlie|first=Eugene Clinton|url=https://books.google.com/books?id=NAybxQZvWI0C&q=Malacobdella+grossa&pg=PA89|title=Intertidal invertebrates of California|publisher=Stanford University Press|year=1980|isbn=978-0-8047-1045-9|editor=Robert H. Morris|pages=85–90|access-date=26 Jan 2011}}</ref> and some of these are much less stretchy.<ref name="MooreOverhill2006" /> Smaller nemerteans are approximately cylindrical, but larger species are flattened [[Anatomical terms of location#Dorsal and ventral|dorso-ventrally]]. Many have visible patterns in various combinations of yellow, orange, red and green.<ref name="RFB2004FormBodyProboscis" />

The outermost layer of the body has no [[cuticle]], but consists of a [[cilia]]ted and [[gland]]ular [[epithelium]] containing [[rhabdite]]s,<ref name="WalkerAnderson1998Nemertea" /> which form the [[mucus]] in which the cilia glide.<ref>{{cite journal|last=Martin|first=Gary G.|year=1978|title=A new function of rhabdites: mucus production for ciliary gliding |journal=[[Zoomorphology]] |volume=91|issue=3|pages=235–248|doi=10.1007/BF00999813|s2cid=206787592}}</ref> Each ciliated cell has many cilia and [[microvilli]].<ref name="RFB2004FormBodyProboscis" /> The outermost layer rests on a thickened [[basement membrane]], the [[dermis]].<ref name="WalkerAnderson1998Nemertea" /> Next to the dermis are at least three layers of muscles, some circular and some longitudinal.<ref name="RFB2004FormBodyProboscis" /> The combinations of muscle types vary between the different [[Class (biology)|classes]], but these are not associated with differences in movement.<ref name="WalkerAnderson1998Nemertea" /> Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species.<ref name="RFB2004FormBodyProboscis" /> Inside the concentric tubes of these layers is [[mesenchyme]], a kind of [[connective tissue]].<ref name="WalkerAnderson1998Nemertea" /> In [[pelagic zone|pelagic]] species this tissue is gelatinous and buoyant.<ref name="RFB2004FormBodyProboscis" />

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.<ref>{{cite journal | pmc=1975765 | year=2007 | last1=Sundberg | first1=P. | last2=Strand | first2=M. | title=Annulonemertes (Phylum Nemertea): When segments do not count | journal=Biology Letters | volume=3 | issue=5 | pages=570–573 | doi=10.1098/rsbl.2007.0306 | pmid=17686756 }}</ref><ref>{{Cite book |last1=Giribet |first1=Gonzalo |url=https://books.google.com/books?id=anetDwAAQBAJ&dq=Annulonemertes+minusculus&pg=PA413 |title=The Invertebrate Tree of Life |last2=Edgecombe |first2=Gregory D. |date=2020-03-03 |publisher=Princeton University Press |isbn=978-0-691-19706-7 |language=en}}</ref>

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 the [[anus]] is at the tip of the tail.<ref name="RFB2004NutritionDigestive">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | pages=[https://archive.org/details/isbn_9780030259821/page/274 274–275] | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | url=https://archive.org/details/isbn_9780030259821/page/274 }}</ref> Above the gut and separated from the gut by mesenchyme is the [[rhynchocoel]], a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class [[Anopla]] 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.<ref name="RFB2004FormBodyProboscis" /> The rhynchocoel is a [[coelom]], as it is lined by [[epithelium]].<ref name="WalkerAnderson1998Nemertea" />

=== Proboscis and feeding ===

The [[proboscis]] is an infolding of the body wall, and sits in the rhynchocoel when inactive.<ref name="WalkerAnderson1998Nemertea" /> 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.<ref name="RFB2004FormBodyProboscis" />

[[File:Gorgonorhynchus repens.jpg|''[[Gorgonorhynchus repens]]'', a species within class Anopla, discharges a sticky branched proboscis.|245px|right|thumb]]The proboscis of the [[Class (biology)|class]] [[Anopla]] exits from an orifice which is separate from the mouth,<ref name="RFB2004FormBodyProboscis" /> coils around the prey and immobilizes it by sticky, toxic secretions.<ref name="RFB2004NutritionDigestive" /> The Anopla can attack as soon as the prey moves into the range of the proboscis.<ref name="McDermottRoe1985" /> Some Anopla have branched proboscises which can be described as "a mass of sticky spaghetti".<ref name="RFB2004FormBodyProboscis" /> The animal then draws its prey into its mouth.<ref name="WalkerAnderson1998Nemertea" />

[[File:Amphiporus ochraceus stylet region.jpg|Stylet-containing part of proboscis of "armed" nemertean ''Amphiporus ochraceus''.|245px|right|thumb]]
In most of the class [[Enopla]], the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has a [[Stylet (anatomy)|stylet]], a [[calcareous]] barb,<ref name="RFB2004FormBodyProboscis" /> 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.<ref name="RFB2004NutritionDigestive" /> The stylet is attached about one-third of the distance from the end of the [[wikt:eversion|evert]]ed 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.<ref name="RFB2004FormBodyProboscis" /> Instead of one stylet, the [[Polystilifera]] have a pad that bears many tiny stylets, and these animals have separate orifices for the proboscis and mouth, unlike other Enopla.<ref name="RFB2004Diversity" /><ref>{{cite book|last1=Roe|first1=Pamela |last2=Norenburg|first2=Jon L. |last3=Maslakova|first3=Svetlana |editor=Sol Felty Light |editor2=James T. Carlton|title=The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon |url=https://books.google.com/books?id=64jgZ1CfmB8C&q=Polystilifera&pg=PA223|access-date=4 Feb 2011|year=2007|publisher=University of California Press|isbn=978-0-520-23939-5|pages=221–233}}</ref> The Enopla can only attack after contacting the prey.<ref name="McDermottRoe1985" />

Some nemerteans, such as ''L. longissimus'', absorb organic food in solution through their skins, which may make the long, slim bodies an advantage.<ref name="MooreOverhill2006" /> [[Suspension feeding]] is found only among the specialized symbiotic [[bdellonemertea]]ns,<ref name="McDermottRoe1985" /> which have a proboscis but no stylet, and use suckers to attach themselves to [[bivalve]]s.<ref name="Light1974">{{cite book|last=Light|first=Sol Felty|title=Intertidal Invertebrates of the Central California Coast|year=1974|publisher=University of California Press|isbn=978-0-520-00750-5|pages=55–58|chapter-url=https://books.google.com/books?id=2Qnod-98-q8C&q=bdellonemertea&pg=PA56|edition=2|access-date=22 February 2011|chapter=Phylum Nemertea (Rhynchocoela)}}</ref>

=== Respiration and circulatory system ===
Nemerteans lack specialized [[gills]], and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid [[Circulatory system|circulation]] rather than [[diffusion]] to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels,<ref name="RFB2004RespirationCirculatoryExcretion" /> while their [[Blood#Invertebrates|blood]] is contained in the main body cavity.<ref name="Moore" /> The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluid [[skeleton]] in everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation.<ref name="RFB2004RespirationCirculatoryExcretion">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | pages=[https://archive.org/details/isbn_9780030259821/page/275 275–276] | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | url=https://archive.org/details/isbn_9780030259821/page/275 }}</ref> The circulatory vessels are a system of coeloms.<ref name="Pérez-PomaresEtc2009Card">{{cite journal
 |last       = Pérez-Pomares
 |first      = José M.
 |author2    = Juan M. González-Rosa
 |author3    = Ramón Muñoz-Chápuli
 |title      = Building the vertebrate heart - an evolutionary approach to cardiac development
 |journal    = The International Journal of Developmental Biology
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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,<ref name="Anderson1998InvertebratePhyla">{{cite book|last1=Anderson|first1=D.T.|editor=D.T. Anderson|title=Invertebrate Zoology|edition=1|year=1998|publisher=Oxford University Press Australia|isbn=978-0-19-553941-7|page=4|chapter=The invertebrate phyla}}</ref> 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.<ref name="RFB2004RespirationCirculatoryExcretion" /> 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 contain [[hemoglobin]] and carry oxygen, but the function of the other pigments is unknown.<ref name="RFB2004RespirationCirculatoryExcretion" />

=== Excretion ===
[[File:Flamecell.jpg|thumb|right|A schematic representation of a [[flame cell]] and other associated structures]]
Nemertea use organs called [[protonephridia]]<ref name="RFB2004RespirationCirculatoryExcretion" /> to excrete soluble waste products, especially [[nitrogen]]ous by-products of cellular [[metabolism]].<ref name="RFB2004BilateriaExcrete">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Introduction to Bilateria | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | pages=[https://archive.org/details/isbn_9780030259821/page/212 212–214] | url=https://archive.org/details/isbn_9780030259821/page/212 }}</ref> In nemertean protonephridia, [[flame cell]]s 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 more [[nephridiopore]]s through which the wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be that [[osmoregulation]] is more difficult in non-marine environments.<ref name="RFB2004RespirationCirculatoryExcretion" />

=== Nervous-system and senses ===

[[File:Amphiporus ochraceus brain.jpg|245px|Brain and neural cords of hoplonemertean ''Amphiporus ochraceus''. Several clusters of dark eyespots and the opening of one cerebral organ are also visible.|thumb]]

The [[central nervous system|central nervous-system]] consists of a [[brain]] and paired [[ventral nerve cord]]s that connect to the brain and run along the length of the body. The [[brain]] is a [[nerve ring|ring]] of four [[ganglia]], masses of nerve cells, positioned round the rhynchocoel near its front end<ref name="RFB2004NervousSenses" /> – while the brains of most [[protostome]] invertebrates encircle the foregut.<ref name="Maslakova2010Invention">{{cite journal|last=Maslakova|first=Svetlana A.|date=July 2010|title=The invention of the pilidium larva in an otherwise perfectly good spiralian phylum Nemertea|journal=[[Integrative and Comparative Biology]]|volume=50|issue=5|pages=734–743|doi=10.1093/icb/icq096|pmid=21558236|doi-access=free}}</ref> Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord.<ref name="RFB2004NervousSenses" /> In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers.<ref name=IZ/> The central nervous-system is often red or pink because it contains [[hemoglobin]]. This stores [[oxygen]] for peak activity or when the animal experiences [[Hypoxia (medical)|anoxia]], for example while [[burrowing]] in oxygen-free [[sediment]]s.<ref name="RFB2004NervousSenses">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | page=[https://archive.org/details/isbn_9780030259821/page/276 276] | url=https://archive.org/details/isbn_9780030259821/page/276 }}</ref>

Some species have paired [[cerebrum|cerebral]] 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 be [[chemoreceptor]]s, and the cerebral organs may also aiding [[osmoregulation]]. Small pits in the epidermis appear to be sensors.<ref name="RFB2004NervousSenses" /> On their head, some species have a number of pigment-cup [[ocelli]],<ref name="RFB2004NervousSenses" /> which can detect light but not form an image.<ref>{{cite book|last1=Russell|first1=Peter J. |last2=Wolfe|first2=Stephen L. |last3=Hertz|first3=Paul E. |author4=Cecie Starr|title=Biology: the dynamic science|volume=3|year=2008|publisher=Cengage Learning |isbn=978-0-495-01034-0|pages=894–895|chapter=Photoreceptors and vision | chapter-url=https://books.google.com/books?id=VqkrMPg6E7EC&q=Russell,+Peter+J.;+Wolfe,+Stephen+L.;+Hertz,+Paul+E.;+Cecie+Starr+pigment-cup+ocelli&pg=PA894 | access-date=31 Jan 2011}}</ref> Most nemerteans have two to six ocelli, although some have hundreds.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 252–262|isbn= 978-0-03-056747-6}}</ref> A few tiny species that live between grains of sand have [[statocyst]]s,<ref name="RFB2004NervousSenses" /> which sense balance.<ref>{{cite book|last1=Russell|first1=Peter J. |last2=Wolfe|first2=Stephen L. |last3=Hertz|first3=Paul E. |author4=Cecie Starr|title=Biology: the dynamic science|volume=3|year=2008|publisher=Cengage Learning |isbn=978-0-495-01034-0|page=889|chapter=Photoreceptors and vision | chapter-url=https://books.google.com/books?id=VqkrMPg6E7EC&q=Russell,+Peter+J.;+Wolfe,+Stephen+L.;+Hertz,+Paul+E.;+Cecie+Starr+pigment-cup+ocelli&pg=PA894 | access-date=31 Jan 2011}}</ref>

''[[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.<ref name="RFB2004NutritionDigestive" />

=== Movement ===
[[File:Declining-soil-Crustacea-in-a-World-Heritage-Site-caused-by-land-nemertean-41598 2017 12653 MOESM2 ESM.ogv|thumb|The nemertean ''[[Geonemertes pelaensis]]'' (right) being inspected by a spider, which it then captures.]]
[[File:Lineus longissimus Grevelingen.jpg|200px|thumb|right|''Lineus longissimus'' in [[Grevelingen]]]]
Nemerteans generally move slowly,<ref name="WalkerAnderson1998Nemertea" /> though they have occasionally been documented to successfully prey on spiders or insects.<ref name="ShinobeUchida2017">{{cite journal|last1=Shinobe|first1=Shotaro|last2=Uchida|first2=Shota|last3=Mori|first3=Hideaki|last4=Okochi|first4=Isamu|last5=Chiba|first5=Satoshi|title=Declining soil Crustacea in a World Heritage Site caused by land nemertean|journal=Scientific Reports|volume=7|issue=1|pages=12400|year=2017|doi=10.1038/s41598-017-12653-4|pmid=28963523|pmc=5622052|bibcode=2017NatSR...712400S }}</ref> Most nemerteans use their external cilia to glide on surfaces on a trail of [[Mucus|slime]], 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 muscular [[peristalsis]], and have powerful muscles.<ref name="RFB2004FormBodyProboscis" /> Some species of the [[Order (biology)|suborder]] [[Monostilifera]], whose proboscis have one active stylet, move by extending the proboscis, sticking it to an object and pulling the animal toward the object.<ref name="RFB2004Diversity" />

=== Reproduction and life-cycle ===
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.<ref name="RFB2004ReproductionLifeCycle">{{cite book | last1=Ruppert | first1 =E.E | last2 = Fox| first2 =R.S. |last3 = Barnes | first3 = R.D. | title=Invertebrate Zoology | chapter=Nemertea | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | pages=[https://archive.org/details/isbn_9780030259821/page/276 276–278] | url=https://archive.org/details/isbn_9780030259821/page/276 }}</ref> But this kind of extreme regeneration is restricted to only a few types of nemerteans, and is assumed to be a derived feature.<ref>[https://royalsocietypublishing.org/doi/10.1098/rspb.2018.2524 A phylum-wide survey reveals multiple independent gains of head regeneration in Nemertea]</ref> All [[Sexual reproduction|reproduce sexually]], and most species are [[gonochoric]] (the sexes are separate),<ref name="WalkerAnderson1998Nemertea" /><ref name="RFB2004ReproductionLifeCycle" /> but all the freshwater forms are [[hermaphroditic]].<ref name="Moore" />

Nemerteans often have numerous temporary [[gonad]]s ([[ovaries]] or [[testes]]), forming a row down each side of the body in the [[mesenchyme]].<ref name="Moore" /><ref name="RFB2004ReproductionLifeCycle" /> Temporary [[gonoduct]]s (ducts from which the [[Ovum|ova]] or [[sperm]] are emitted<ref>{{cite web|url=http://www.merriam-webster.com/medical/gonoduct?show=0&t=1296496594|title=Gonoduct – Medical Definition|publisher=Merriam-Webster, Incorporated|access-date=31 January 2011}}</ref>), one per gonad, are built when the ova and sperm are ready.<ref name="RFB2004ReproductionLifeCycle" /> 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 by [[Pupa#Cocoon|cocoon]]s or [[gelatinous]] strings.<ref name="RFB2004ReproductionLifeCycle" /> Some [[bathypelagic]] (deep sea) species have [[internal fertilization]], and some of these are [[viviparous]], growing their [[embryo]]s in the female's body.<ref name="Moore">{{cite encyclopedia|last1=Moore|first1=Janet|last2=Gibson|first2=Ray |title=Nemertea| encyclopedia=[[Encyclopedia of Life Sciences]] |doi=10.1038/npg.els.0001586 |year=2001|publisher=John Wiley & Son|isbn=978-0470016176}}</ref><ref name="RFB2004ReproductionLifeCycle" />

The [[zygote]] (fertilised egg) divides by [[spiral cleavage]] and grows by [[determinate development]],<ref name="RFB2004ReproductionLifeCycle" /> in which the fate of a cell can usually be predicted from its predecessors in the process of division.<ref name="MooreOverhill2006">{{cite book|last1=Moore|first1=Janet |last2=Overhill|first2=Raith |editor=Raith Overhill|title=An Introduction to the Invertebrates |chapter-url=https://books.google.com/books?id=bZw-ntFxp-YC&q=determinate+development&pg=PA96|access-date=31 Jan 2011|edition=2|year=2006|publisher=Cambridge University Press|isbn=978-0-521-85736-9|pages=75–84|chapter=Chapter 7 – Nemertea}}</ref> The embryos of most [[taxa]] develop either directly to form [[Juvenile (organism)|juveniles]] (like the adult but smaller) or to form [[planula|planuliform]] [[larva]]e. The planuliform larva stage may be short-lived and [[lecithotrophic]] ("yolky") before becoming a juvenile,<ref name="RFB2004ReproductionLifeCycle" /> or may be [[planktotrophic]], swimming for some time and eating prey larger than microscopic particles.<ref name="Maslakova2010Invention" /> However, many members of the order [[Heteronemertea]] and the [[palaeonemertea]]n [[Family (biology)|family]] [[Hubrechtiidae]] form a [[pilidium (zoology)|pilidium]] larva, which can capture [[unicellular]] [[algae]] and which Maslakova describes as like a [[deerstalker]] 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 of [[imaginal disc]]s form, encircling the [[archenteron]] (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 catastrophic [[metamorphosis]].<ref name="Maslakova2010Invention" /> This larval stage is unique in that there are no [[Hox gene]]s involved during development, which are only found in the juveniles developing inside the larvae.<ref>{{Cite journal|title=Hox genes pattern the anterior-posterior axis of the juvenile but not the larva in a maximally indirect developing invertebrate, Micrura alaskensis (Nemertea)|first1=Laurel S.|last1=Hiebert|first2=Svetlana A.|last2=Maslakova|date=April 11, 2015|journal=BMC Biology|volume=13|issue=1|pages=23|doi=10.1186/s12915-015-0133-5|pmid=25888821|pmc=4426647 |doi-access=free }}</ref>

The species ''[[Paranemertes peregrina]]'' has been reported as having a life span of around 18 months.<ref name=IZ/>

== Ecological significance ==
[[File:Nemertea Terrestrial W Java.png|right|thumb|A terrestrial nemertean from West [[Java]]. The animal is {{convert|1.5|cm|in}} long, of which the anterior {{convert|1|cm|in}} is visible.]]
[[File:Geonemertes sp., Mindanao, Philippines - 20110515-02.jpg|right|thumb|A terrestrial ''[[Geonemertes]]'' sp. on a rotting log, from [[Mindanao]] Island, the [[Philippines]]]]
Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or the [[holdfast (biology)|holdfast]]s of [[algae]] or [[Sessility (zoology)|sessile]] animals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined with [[mucus]] or produce [[cellophane]]-like tubes. Mainly in the tropics and subtropics, about 12 species appear in freshwater,<ref name="RFB2004FormBodyProboscis" /> and about a dozen species live on land in cool, damp places, for example under rotting logs.<ref name="MooreOverhill2006" />

The terrestrial ''Argonemertes dendyi'' is a native of [[Australia]] but has been found in the [[British Isles]], in [[São Miguel Island|Sao Miguel]] in the [[Azores]], in [[Gran Canaria]], and in a [[lava tube]] at [[Kaumana]] on the [[Hawaii (island)|Island 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 a [[protandrous]] [[hermaphrodite]].<ref name="HowarthMoore1983">{{cite journal|last=Howarth|first=Francis G.|author2=Janet Moore|year=1983|title=The land nemertine ''Argonemertes dendyi'' (Dakin) in Hawaii (Nemertinea: Hoplonemertinea: Prosorhochmidae)|journal=[[Pacific Science]]|volume=37|issue=2|pages=141–144|url=http://scholarspace.manoa.hawaii.edu/bitstream/10125/654/1/v37n2-141-144.pdf|access-date=2011-02-17|archive-url=https://web.archive.org/web/20181104001908/https://scholarspace.manoa.hawaii.edu/bitstream/10125/654/1/v37n2-141-144.pdf|archive-date=2018-11-04|url-status=dead}}</ref> Another terrestrial [[genus]], ''[[Geonemertes]]'', is mostly found in [[Australasian realm|Australasia]] but has species in the [[Seychelles]], widely across the [[Indo-Pacific]], in [[Tristan da Cunha]] in the South Atlantic, in [[Frankfurt]], in the [[Canary Islands]], in [[Madeira]] and in the Azores.<ref name="Gibson"/>  ''[[Geonemertes pelaensis]]'' has been implicated in the decline of native arthropod species on the [[Ogasawara Islands]], where it was [[Introduced species|introduced]] in the 1980s.<ref>{{Cite journal
| last1=Shinobe |first1=S |last2=Uchida |first2=S |last3=Mori |first3=H |last4=Okochi |first4=I |last5=Chiba |first5=S
| title = Declining soil Crustacea in a World Heritage Site caused by land nemertean
| doi = 10.1038/s41598-017-12653-4
| journal = Scientific Reports
|volume=7 |issue=1 |pages=12400 | year = 2017
| pmid = 28963523
| pmc = 5622052
|bibcode=2017NatSR...712400S }}</ref>

Most are [[carnivore]]s, feeding on [[annelid]]s, [[clam]]s and [[crustacean]]s,<ref name="RFB2004NutritionDigestive" /> and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects and [[myriapod]]s are the only known prey of the two terrestrial species of ''Argonemertes''.<ref name="McDermottRoe1985">{{cite journal|last=McDermott|first=J.|author2=Pamela Roe|year=1985|title=Food, feeding behavior and feeding ecology of nemerteans |journal=[[American Zoologist]] |volume=25|issue=1|pages=113–125|doi=10.1093/icb/25.1.113|doi-access=free}}</ref>
A few nemerteans are [[scavenger]]s,<ref name="RFB2004NutritionDigestive" /> and these generally have good distance [[chemoreception]] ("smell") and are not selective about their prey.<ref name="McDermottRoe1985" /> A few species live [[commensal]]ly inside the [[mantle (mollusc)|mantle]] cavity of molluscs and feed on micro-organisms filtered out by the host.<ref name="Waggoner">{{cite web |url=http://www.ucmp.berkeley.edu/nemertini/nemertini.html |access-date=3 Feb 2011 |title=Introduction to the Nemertini |publisher=[[University of California, Berkeley]] |date=June 13, 2001 |author1=Ben Waggoner |author2=Allen G. Collins |archive-url=https://web.archive.org/web/20101210034443/http://www.ucmp.berkeley.edu/nemertini/nemertini.html |archive-date=10 December 2010 |url-status=dead }}</ref>

Near [[San Francisco]] the nemertean ''[[Carcinonemertes errans]]'' has consumed about 55% of the total egg production of its host, the [[dungeness crab]] ''Metacarcinus magister''. ''C. errans'' is considered a significant factor in the collapse of the dungeness crab fishery.<ref name="McDermottRoe1985" /> Other coastal nemerteans have devastated [[clam]] beds.<ref name="RFB2004FormBodyProboscis" />

The few predators on nemerteans include bottom-feeding fish, some sea birds, a few invertebrates including [[horseshoe crab]]s, and other nemerteans.<ref name="RFB2004FormBodyProboscis" /> Nemerteans' skins secrete toxins that deter many predators, but some crabs may clean nemerteans with one claw before eating them.<ref name="Moore" /> The American ''[[Cerebratulus lacteus]]'' and the South African ''Polybrachiorhynchus dayi'', both called "tapeworms" in their respective localities, are sold as fish bait.<ref name="RFB2004FormBodyProboscis" />

==Taxonomy==
{{See also|List of bilaterial animal orders}}
Traditional taxonomic classification has divided the group into two classes and four orders:
* [[Class (biology)|Class]] [[Anopla]] ("unarmed"). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.<ref name="RFB2004Diversity">{{cite book | author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology | chapter=Nemertea | publisher=Brooks / Cole | edition=7 | isbn=978-0-03-025982-1 | year=2004 | page=[https://archive.org/details/isbn_9780030259821/page/279 279] | url=https://archive.org/details/isbn_9780030259821/page/279 }}</ref>
** [[Order (biology)|Order]] [[Palaeonemertea]]. 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 the [[epidermis (zoology)|epidermis]]; the extra muscle layers seem to be needed for [[burrowing]] by [[peristalsis]].<ref name="RFB2004Diversity" />
** Order [[Heteronemertea]]. 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. Two [[genus|genera]] have branched proboscises.<ref name="RFB2004Diversity" />
* Class [[Enopla]] ("armed"). All have [[stylet (anatomy)|stylet]]s except order [[Bdellonemertea]]. Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.<ref name="RFB2004Diversity" />
** Order [[Bdellonemertea]]. Includes seven species, of which six live as [[commensal]]s in the [[Mantle (mollusc)|mantle]] of large [[clam]]s and one in that of a freshwater snail. The hosts [[Filter feeders|filter feed]] and all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a sucking [[pharynx]] and a posterior stucker, with which they move like [[Geometer moth#Caterpillars|inchworm]]s.<ref name="RFB2004Diversity" />
** Order [[Hoplonemertea]]. Comprises 650 species. They live in [[benthic]] and [[pelagic]] sea water, in freshwater and on land. They feed by commensalism and [[parasitism]], and are armed with [[stylet (anatomy)|stylet]](s)<ref name="RFB2004Diversity" />
*** Suborder [[Monostilifera]]. Includes 500 species with a single central [[stylet (anatomy)|stylet]]. Some use the stylet for locomotion as well as for capturing prey.<ref name="RFB2004Diversity" />
*** Suborder [[Polystilifera]]. Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets.<ref name="RFB2004Diversity" />

Recent molecular phylogenetic studies divided the group into two superclasses, three classes, and eight orders:<ref name="Chernyshev2021">{{cite journal|last=Chernyshev |first=A.V. |year=2021 |title=An updated classification of the phylum Nemertea |journal=Invertebrate Zoology |volume=18 |issue=3 |pages=188–196 |doi=10.15298/invertzool.18.3.01|s2cid=239872311 |doi-access=free }}</ref>
*Superclass  Pronemertea
**Class [[Palaeonemertea]]
***Order [[Carinomiformes]]
***Order [[Tubulaniformes]]
***Order [[Archinemertea]]
*Superclass  [[Neonemertea]]
**Class [[Pilidiophora]]
***Order [[Hubrechtiiformes]]
***Order [[Heteronemertea]]
**Class [[Hoplonemertea]] (= Enopla)
***Order [[Polystilifera]]
***Order [[Monostilifera]] (includes Bdellonemertea)
*''incertae sedis''
** Order [[Arhynchonemertea]] (provisionally has been separated its own class [[Arhynchocoela]] in 1995)

== Evolutionary history ==

=== Fossil record ===
<!-- No fossil nermeteans were known in late 1990s.<ref name=WalkerAnderson1998Nemertea/> -->
As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare.<ref name=WalkerAnderson1998Nemertea/><ref name=Waggoner/> One might expect the stylet of a nemertean to be preserved, since it is made of [[calcium phosphate]], but no fossil stylets have yet been found.<ref name=WalkerAnderson1998Nemertea/><ref name=Waggoner/> {{harvp|Knaust|2010}} reported nemertean fossils and traces from the [[Middle Triassic]] of [[Germany]].<ref>
{{cite journal
 |first=Dirk |last=Knaust
 |year=2010
 |title=Remarkably preserved benthic organisms and their traces from a middle Triassic (Muschelkalk) mud flat
 |journal=Lethaia
 |volume=43 |issue=3 |pages=344–356
 |doi=10.1111/j.1502-3931.2009.00196.x
}}
</ref>

The [[Middle Cambrian#Subdivision|Middle Cambrian]] fossil ''[[Amiskwia]]'' from the [[Burgess 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.<ref name=Waggoner/><ref>
{{cite book
 |last=Dzik |first=Jerzy
 |year=1999
 |chapter=Evolutionary origin of asymmetry in early metazoan animals
 |editor1-first=Gyula   |editor1-last=Pályi
 |editor2-first=Luciano |editor2-last=Caglioti
 |editor3-first=Claudia |editor3-last=Zucchi
 |title=Advances in BioChirality
 |publisher=Elsevier
 |isbn=978-0-08-043404-9
 |page=165
 |chapter-url=https://books.google.com/books?id=DS68n3zIqwAC&q=amiskwia+nemertean+nemertini&pg=PA165
 |via=Google books |access-date=11 Feb 2011
}}
</ref>

{{harvp|Knaust|Desrochers|2019}} reported fossils of [[vermiform]] organisms with a wide range of [[Morphology (biology)|morphologies]] occurring on bedding planes from the Late [[Ordovician]] ([[Katian]]) [[Vaureal Formation|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 of [[turbellaria]]ns or [[annelid]]s.<ref name=GRKnaustDesrochers>
{{cite journal
 |first1=Dirk  |last1=Knaust
 |first2=André |last2=Desrochers
 |year=2019
 |title=Exceptionally preserved soft-bodied assemblage in Ordovician carbonates of Anticosti Island, eastern Canada
 |journal=Gondwana Research
 |volume=71 |pages=117–128
 |doi=10.1016/j.gr.2019.01.016
 |bibcode=2019GondR..71..117K |s2cid=134814852
}}
</ref>

It has been suggested that ''[[Archisymplectes]]'', one of the [[Pennsylvanian (geology)|Pennsylvanian]]-age animals from [[Mazon Creek]] in northern and central [[Illinois]], may be a nemertean.<ref name=Schram1973>
{{cite journal
 |last=Schram |first=Frederick R.
 |date=September 1973
 |title=Pseudocoelomates and a Nemertine from the Illinois Pennsylvanian
 |journal=[[Journal of Paleontology]]
 |volume=47 |issue=5 |pages=985–989
 |jstor=1303083
}}
</ref>
This fossil, however, only preserves the outline of the "worm",<ref name=Waggoner/> and there is no evidence of a proboscis,<ref name=Nielsen2001>
{{cite book
 |last=Nielsen |first=Claus
 |year=2001
 |chapter=Phylum Nemertini
 |title=Animal Evolution: Interrelationships of the living phyla |edition=2
 |publisher=Oxford University Press
 |isbn=978-0-19-850681-2
 |pages=281–282
 |chapter-url=https://books.google.com/books?id=UmCg6c0HkqMC&q=Archisymplectes&pg=PA281
 |access-date=11 Feb 2011
}}
</ref>
so there is no certainty that it represents a nemertean.<ref name=Waggoner/>

==== Within Nemertea ====
{{thumb
|content=
<div style="text-align:left">{{clade
 |style=font-size:80%;
 |label1=Nemertea
 |1={{clade
     |label1=
     |1={{bg|yellow|[[Palaeonemertea]]}}
     |2={{clade
         |1={{clade
             |label1=
             |1={{bg|yellow|[[Heteronemertea]]}}
            }}
         |2={{clade
             |label1=[[Enopla]]
             |1={{clade
                 |label1=
                 |1={{clade
                     |label1=
                     |1=[[Bdellonemertea]]
                    }}
                 |2={{clade
                     |label1=[[Hoplonemertea]]
                     |1=[[Monostilifera]]
                     |2=[[Polystilifera]]
                    }}
                }}
            }}
        }}
    }}
 }}</div>
|caption = Groups within Nemertea, by {{harvp|Ruppert|Fox|Barnes|2004a}}.<ref name=RFB2004Phylogeny>
{{cite book
 | last1=Ruppert | first1=E.E.
 | last2=Fox     | first2=R.S.
 | last3=Barnes  | first3=R.D.
 | year=2004b
 | title=Invertebrate Zoology | edition=7
 | chapter=Nemertea
 | pages=[https://archive.org/details/isbn_9780030259821/page/279 279–280]
 | publisher=Brooks / Cole
 | isbn=978-0-03-025982-1 | url=https://archive.org/details/isbn_9780030259821/page/279 }}</ref><br />{{legend2|yellow}} highlights the "Anopla", which are [[paraphyletic]].<ref name=RFB2004Phylogeny/>
}}
There is no doubt that the phylum Nemertea is [[Monophyly|monophyletic]] (meaning that the phylum includes all and only descendants of one ancestor that was also a member of the phylum).<ref name=RFB2004Phylogeny/>{{rp|pp=2-3}} The [[synapomorphy|synapomorphies]] (trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel.<ref name=ThollessonNorenburg2003>
{{cite journal
 | last1=Thollesson | first1=Mikael
 | last2=Norenburg  | first2=Jon L.
 | date = February 2003
 | title=Ribbon worm relationships: A phylogeny of the phylum Nemertea
 | journal=[[Proceedings of the Royal Society B]]
 | volume=270 | issue=1513 | pages=407–415
 | doi=10.1098/rspb.2002.2254
 |pmid=12639321 |pmc=1691252
}}
</ref>

While {{harvp|Ruppert|Fox|Barnes|2004a}} treat the Palaeonemertea as monophyletic,<ref name=RFB2004Phylogeny/> {{harvp|Thollesson|Norenburg|2003}} regard them as [[paraphyletic]] and [[Basal (phylogenetics)|basal]] (contains the ancestors of the more recent clades).<ref name=ThollessonNorenburg2003/> The [[Anopla]] ("unarmed") represent an [[evolutionary grade]] of nemerteans without stylets (comprising the [[Heteronemertea]] and the Palaeonemerteans), while [[Enopla]] ("armed") are monophyletic, but find that Palaeonemertea is doubly paraphyletic, having given rise to both the Heteronemertea and the Enopla.<ref name=RFB2004Phylogeny/><ref name=ThollessonNorenburg2003/> {{harvp|Ruppert|Fox|Barnes|2004a}} treat the Bdellonemertea as a [[clade]] separate from the [[Hoplonemertea]],<ref name=RFB2004Phylogeny/> while {{harvp|Thollesson|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.<ref name=ThollessonNorenburg2003/> The Polystilifera (with many tiny stylets) are monophyletic.<ref name=RFB2004Phylogeny/><ref name=ThollessonNorenburg2003/>

==== Relationships with other phyla ====
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.<ref name=Turbeville2002/> However, multiciliated epidermal cells and epidermal gland cells are also found in [[Ctenophora]], [[Annelida]], Mollusca and other [[taxon|taxa]]. 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 and [[entoproct]] larvae. The protonephridia of nemertea and flatworms are different in structure,<ref name=Turbeville2002/> and in position – the [[flame cell]]s 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,<ref name=RFB2004RespirationCirculatoryExcretion/> while the flame cells of flatworms are scattered throughout the body.<ref name=RFB2004FlatwExcret>
{{cite book
 | last1=Ruppert | first1=E.E.
 | last2=Fox     | first2=R.S.
 | last3=Barnes  | first3=R.D.
 | year=2004a
 | title=Invertebrate Zoology | edition=7
 | publisher=Brooks / Cole
 | url=https://archive.org/details/isbn_9780030259821
}}
</ref>{{rp|p=[https://archive.org/details/isbn_9780030259821/page/239  239 ] }}
Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia.<ref name=Turbeville2002/>

According to more recent analyses, in the development of nemertean embryos, ecto[[mesoderm]] (outer part of the mesoderm, which is the layer in which most of the internal organs are built) is derived from cells<!-- blastomeres --> labelled 3a and 3b, and endomesoderm (inner part of the mesoderm) is derived from the 4d cell. Some of the ectomesoderm in [[annelids]], [[echiuran]]s and [[mollusc]]s is derived from cells 3a and 3b, while the ectomesoderm of [[polyclad]] [[flatworm]]s is derived from the 2b cell and [[acoel]] flatworms produce no ectomesoderm. In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellular [[connective tissue]]. This structure is similar to that found in larger flatworms such as [[polyclad]]s and [[triclad]]s, but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among the [[Spiralia]] (animals in which the early cell divisions make a spiral pattern) such as [[sipuncula]]ns, [[echiura]]ns and many annelids.<ref name=Turbeville2002>
{{cite journal
 | last=Turbeville | first=J.M.
 | year=2002
 | title=Progress in nemertean biology: Development and phylogeny
 | journal=[[Integrative and Comparative Biology]]
 | volume=42 | issue=3 | pages=692–703
 | doi=10.1093/icb/42.3.692  | doi-access=free
 | issn=1540-7063  | pmid=21708766
}}
</ref>
{{thumb
|content=
<div style="text-align:left">{{clade
  |style=font-size:80%;
  |label1=[[Bilateria]]
  |1={{clade
      |label1=
      |1={{clade
           |label1=[[Acoelomorpha]]
           |1={{clade
               |label1=
               |1=[[Acoela]]
               |2=[[Nemertodermatida]] 
              }}
          }}
       |2={{clade
           |label1=[[Deuterostomia]]
           |1=([[Echinoderm]]s, [[chordate]]s, etc.)
           |label2=[[Protostomia]]
           |2={{clade
                 |label1=[[Ecdysozoa]]
                 |1=([[Arthropod]]s, [[nematode]]s, [[priapulid]]s, etc.)
                 |label2=[[Lophotrochozoa]]
                 |2={{clade
                      |label1=
                      |1=[[Bryozoa]]
                      |2={{clade
                           |label1=
                           |1=[[Annelida]] (and phyla merged into them)
                           |2=[[Mollusca]]
                           |3={{clade
                               |label1=
                               |1=[[Phoronida]]
                               |2=[[Brachiopoda]]
                              }}
                           |4=Nemertea
                           |5=[[Dicyemida]]
                           |6=[[Myzostomida]]
                           |7={{clade
                               |label1=[[Platyzoa]]
                               |1={{clade
                                   |label1=
                                   |1={{clade
                                       |label1=
                                       |1=[[Platyhelminthes]]
                                       |2=[[Gastrotricha]]
                                      }}
                                   |2=(other Platyzoans)
                                  }}
                              }}
                         }}
                    }}
               }}
          }}
  }}
}}</div>
|caption=Relationships of Nemertea to other [[Bilateria]]:<ref name=Halanych2004NewView/>{{#tag:ref|group=Note|
Sipuncula were merged into Annelida in 2007.<ref name=StruckEtAl2007AnnelidPhylogeny>
{{cite journal
 | last1=Struck   | first1=T.H.       | last2=Schult  | first2=N.
 | last3=Kusen    | first3=T.         | last4=Hickman | first4=E.
 | last5=Bleidorn | first5=C.         | last6=McHugh  | first6=D.
 | last7=Halanych | first7=K.M.
 | year=2007
 | title=Annelid phylogeny and the status of Sipuncula and Echiura
 | journal=[[BMC Evolutionary Biology]]
 | volume=7  | issue=57 | page=57
 | pmid=17411434 | pmc=1855331
 | doi=10.1186/1471-2148-7-57  | doi-access=free
}}
</ref>}}
}}

Nemerteans' affinities with Annelida (including Echiura, [[Siboglinidae|Pogonophora]], [[Vestimentifera]] and perhaps Sipuncula) and Mollusca make the ribbon-worms members of [[Lophotrochozoa]], which <!-- include more animal phyla than any other [[metazoan]] clade than [[Animalia]] and --> include about half of the extant animal phyla.<ref name=Giribet2008/> Lophotrochozoa groups: those animals that feed using a [[lophophore]] ([[Brachiopoda]], [[Bryozoa]], [[Phoronida]], [[Entoprocta]]); phyla in which most members' embryos develop into [[trochophore]] larvae (for example Annelida and Mollusca); and some other phyla (such as Platyhelminthes, Sipuncula, Gastrotricha, Gnathostomulida, Micrognathozoa, Nemertea, [[Phoronida]], Platyhelminthes, and [[Rotifera]]).<ref name=Halanych2004NewView>
{{cite journal
 | last=Halanych | first=K.M.
 | date=December 2004
 | title=The new view of animal phylogeny
 | journal=[[Annual Review of Ecology, Evolution, and Systematics]]
 | volume=35  | pages=229–256
 | doi=10.1146/annurev.ecolsys.35.112202.130124
 | url= http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf
 | url-status=live
 | archive-url=https://ghostarchive.org/archive/20221010/http://mugue.narod.ru/supporting_materials/Halanych_2004.pdf
 | archive-date=2022-10-10
}}
</ref><ref name=Giribet2008/>
These groupings are based on [[molecular phylogeny]], which compares sections of organisms [[DNA]] and [[RNA]]. 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.<ref name=Halanych2004NewView/><ref name=Giribet2008>
{{cite journal
 | last=Giribet | first=Gonzalo
 | date=April 2008
 | title=Assembling the lophotrochozoan (=spiralian) tree of life
 | journal=[[Proceedings of the Royal Society B]]
 | volume=363  | issue=1496  | pages=1513–1522
 | doi=10.1098/rstb.2007.2241
 |pmid=18192183  |pmc=2614230
}}
</ref>

Most [[protostome]] phyla outside the Lophotrochozoa are members of [[Ecdysozoa]] ("animals that [[ecdysis|molt]]"), which include [[Arthropod]]a, [[Nematoda]] and [[Priapulida]]. Most other [[bilaterian]] phyla are in the [[Deuterostomia]], which include [[Echinodermata]] and [[chordate|Chordata]]. The [[Acoelomorpha]], which are neither protostomes nor deuterostomes, are regarded as [[Anatomical terms of location|basal]] bilaterians.<ref name=Halanych2004NewView/><ref name=Giribet2008/><ref name=Cannon2016>
{{cite journal
 |last1=Cannon    |first1=J.T.         |last2=Vellutini |first2=B.C.
 |last3=Smith     |first3=J.           |last4=Ronquist  |first4=F.
 |last5=Jondelius |first5=U.           |last6=Hejnol    |first6=A.
 |date=4 February 2016
 |title=Xenacoelomorpha is the sister group to Nephrozoa
 |journal=Nature
 |volume=530  |issue=7588  |pages=89–93
 |pmid=26842059  |doi=10.1038/nature16520
 |bibcode=2016Natur.530...89C  |s2cid=205247296
 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844
}}
</ref>
{{Clear}}
<!--    !!!  
With other phyla:
* Phylogenetic Position of Nemertea Derived from Phylogenomic Data - Torsten H. Struck and Frauke Fisse<ref name=StruckFisse2008>
{{cite journal
 |last1=Struck |first1=Torsten H.
 |last2=Fisse  |first2=Frauke
 |year=2008
 |title=Phylogenetic position of Nemertea derived from phylogenomic data
 |journal=[[Molecular Biology and Evolution]]
 |volume=25  |issue=4  |pages=728–736
 |doi=10.1093/molbev/msn019
<!-- |publisher=Oxford University Press --  !!!  >
}}
</ref>
* Assembling the lophotrochozoan (=spiralian) tree of life - Giribet<ref name=Giribet2008/>
* Testing the new animal phylogeny: A phylum level molecular analysis of the animal kingdom - Sarah J. Bourlat, Claus Nielsen, Andrew D. Economou, Maximilian J. Telford<ref name=BourlatNielsenEtc2008>
{{cite journal
 |first1=Sarah J.  |last1=Bourlat          |first2=Claus         |last2=Nielsen
 |first3=Andrew D. |last3=Economou         |first4=Maximilian J. |last4=Telford
 |date=October 2008
 |title=Testing the new animal phylogeny: A phylum level molecular analysis of the animal kingdom
 |journal=[[Molecular Phylogenetics and Evolution]]
 |publisher=Elsevier
 |volume=49  |issue=1  |pages=23–31
 |doi=10.1016/j.ympev.2008.07.008 |pmid=18692145
 |url=http://ent.njau.edu.cn:8008/upload/taxonomy/%E7%8E%8B%E6%99%93%E8%8A%B3_Sarah_J._Bourlat_2008.pdf
 |url-status=live
 |archive-url=https://web.archive.org/web/20101225134947/http://ent.njau.edu.cn:8008/upload/taxonomy/%E7%8E%8B%E6%99%93%E8%8A%B3_Sarah_J._Bourlat_2008.pdf
 |archive-date=2010-12-25
}}
</ref>
* There is no support for Jensen's hypothesis of nemerteans as ancestors to the vertebrates - Sundberg, Turbeville, & Härlin<ref name=SundbergTurbeville2008>
{{cite journal
 |last=Sundberg |first=Per
 |author2=J.M. Turbeville
 |author3=Mikael S. Härlin
 |year=1998
 |title=There is no support for Jensen's hypothesis of nemerteans as ancestors to the vertebrates
 | journal=[[Hydrobiologia]]
 | volume=365 | pages=47–54
 | doi=10.1023/a:1003182527183
<!-- | publisher=Kluwer Academic Publishers
 | location=Belgium --  !!!  >
 | url=http://deepblue.lib.umich.edu/bitstream/2027.42/42893/1/10750_2004_Article_159065.pdf
 | url-status=live
 | archive-url=https://web.archive.org/web/20110807132350/http://deepblue.lib.umich.edu/bitstream/2027.42/42893/1/10750_2004_Article_159065.pdf
 | archive-date=2011-08-07
}}
</ref>
* Phylogeny and mitochondrial gene order variation in Lophotrochozoa in the light of new mitogenomic data from Nemertea - Podsiadlowski, Braband, Struck, von Döhren and Bartolomaeus<ref name=PodsiadlowskiBraband2009>
{{cite journal
 |last1=Podsiadlowski |first1=Lars              |last2=Braband    |first2=Anke 
 |last3=Struck        |first3=Torsten H.        |last4=von Döhren |first4=Jörn 
 |last5=Bartolomaeus  |first5=Thomas 
 |date=August 2009
 |title=Phylogeny and mitochondrial gene order variation in Lophotrochozoa in the light of new mitogenomic data from Nemertea
 |journal=[[BMC Genomics]]
 |volume=10 |page=364
<!-- |publisher=[[Springer Science+Business Media|Springer]] --  !!!  >
 |doi=10.1186/1471-2164-10-364
 |pmid=19660126 |pmc=2728741}}</ref>
-->

==See also==
*''[[Emplectonema neesii]]''
*''[[Lineus longissimus]]''
*''[[Parborlasia corrugatus]]''

==Notes==
{{Reflist|group=Note}}

==References==
{{Reflist|30em}}

== External links ==
{{EB1911 poster|Nemertina}}
* [https://web.archive.org/web/20110927084717/http://www.mbl.edu/publications/biobull/keys/7/index.html The Marine Biological Laboratory: Phylum Nemertea (Nemertinea, Nemertini, Rhynchocoela)]
*[https://web.archive.org/web/20150227005410/http://nemertea.lifedesks.org/ Nemertea LifeDesk]
*[https://www.youtube.com/watch?v=SnkHY1CvfEA Video of a Nemertea in Puget Sound]

{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from=Q185631}}
{{Authority control}}

[[Category:Nemerteans| ]]
[[Category:Protostome phyla]]
[[Category:Extant Ordovician first appearances]]