Editing Nemertea (section)

=== 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/>