Editing Nemertea (section)

=== 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" />