Editing Nemertea (section)

== 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
 |year       = 2009
 |volume     = 53
 |issue      = 8–9–10
 |pages      = 1427–1443 [1430]
 |doi        = 10.1387/ijdb.072409jp
 |pmid      = 19247975
 |doi-access= free
 }}</ref>

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/>