Editing Flatworm (section)

==Description==

[[File:Haeckel Platodes.jpg|thumb|Various parasitic flatworms from Haeckel's ''[[Kunstformen der Natur]]'' (1904)]]

===Distinguishing features===

Platyhelminthes are [[Symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]] [[animal]]s: their left and right sides are mirror images of each other; this also implies they have distinct top and bottom surfaces and distinct head and tail ends. Like other [[bilateria]]ns, they have three main [[cell (biology)|cell]] layers (endoderm, [[mesoderm]], and [[ectoderm]]),<ref name="WalkerAnderson2001PlatyhelminthesInAnderson">{{cite book
|author1=Walker, J.C. |author2=Anderson, D.T. |year=2001 |chapter=The Platyhelminthes |pages=58–80
|editor=Anderson, D.T. |title=Invertebrate Zoology |publisher=Oxford University Press
|isbn=978-0-19-551368-4
}}</ref> while the [[Symmetry (biology)#Radial symmetry|radially symmetrical]] [[cnidarian]]s and [[ctenophore]]s (comb jellies) have only two cell layers.<ref name="Hinde2001CnidariaAndCtenophoraInAnderson">{{cite book |last=Hinde |first=R.T. |year=2001 |chapter=The Cnidaria and Ctenophora |pages=28–57 |editor=Anderson, D.T. |title=Invertebrate Zoology |publisher=Oxford University Press |isbn=978-0-19-551368-4}}</ref> Beyond that, they are "defined more by what they do not have than by any particular series of specializations."<ref name="Barnes1998DiversityOfLifePlatyhelminthes">{{cite book |title=The Diversity of Living Organisms |author=Barnes, R.S.K. |publisher=Blackwell Publishing |year=1998 |isbn=978-0-632-04917-2 |pages=194–195 |url=https://books.google.com/books?id=Xvk-yrjXH70C&q=Platyhelminthes&pg=PA194 |access-date=2008-12-21 |archive-date=2023-03-23 |archive-url=https://web.archive.org/web/20230323154437/https://books.google.com/books?id=Xvk-yrjXH70C&q=Platyhelminthes&pg=PA194 |url-status=live }}</ref> Unlike most other bilaterians, Platyhelminthes have no internal body cavity, so are described as [[acoelomate]]s. Although the absence of a [[coelom]] also occurs in other bilaterians: [[gnathostomulid]]s, [[gastrotrich]]s, [[Xenacoelomorpha|xenacoelomorphs]], [[Symbion|cycliophorans]], [[entoprocta]]ns and the parastic [[mesozoa]]ns.<ref>{{Cite journal |title=Gnathostomulid phylogeny inferred from a combined approach of four molecular loci and morphology |year=2006 |doi=10.1111/j.1096-0031.2006.00085.x |last1=Sorensen |first1=Martin V. |last2=Sterrer |first2=Wolfgang |last3=Giribet |first3=Gonzalo |journal=Cladistics |volume=22 |issue=1 |pages=32–58 |pmid=34892893 |s2cid=85570246 |doi-access=free }}</ref><ref>{{Cite journal |title=An Introduction to the Study of Gastrotricha, with a Taxonomic Key to Families and Genera of the Group |year=2019 |doi=10.3390/d11070117 |doi-access=free |last1=Todaro |first1=M. Antonio |last2=Sibaja-Cordero |first2=Jeffrey Alejandro |last3=Segura-Bermúdez |first3=Oscar A. |last4=Coto-Delgado |first4=Génesis |last5=Goebel-Otárola |first5=Nathalie |last6=Barquero |first6=Juan D. |last7=Cullell-Delgado |first7=Mariana |last8=Dal Zotto |first8=Matteo |journal=Diversity |volume=11 |issue=7 |page=117 |hdl=11380/1179356 |hdl-access=free }}</ref><ref>{{Cite journal |title=Xenacoelomorpha: a case of independent nervous system centralization? |year=2016 |pmc=4685578 |last1=Gavilán |first1=B. |last2=Perea-Atienza |first2=E. |last3=Martínez |first3=P. |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=371 |issue=1685 |doi=10.1098/rstb.2015.0039 |pmid=26598722 }}</ref><ref>{{Cite journal |title=Muscular anatomy of an entoproct creeping-type larva reveals extraordinary high complexity and potential shared characters with mollusks |year=2015 |doi=10.1186/s12862-015-0394-1 |last1=Merkel |first1=Julia |last2=Lieb |first2=Bernhard |last3=Wanninger |first3=Andreas |journal=BMC Evolutionary Biology |volume=15 |issue=1 |page=130 |pmid=26138503 |pmc=4490756 |doi-access=free |bibcode=2015BMCEE..15..130M }}</ref><ref>{{Cite journal |title=Dicyemida and Orthonectida: Two Stories of Body Plan Simplification |year=2019 |doi=10.3389/fgene.2019.00443 |doi-access=free |last1=Zverkov |first1=Oleg A. |last2=Mikhailov |first2=Kirill V. |last3=Isaev |first3=Sergey V. |last4=Rusin |first4=Leonid Y. |last5=Popova |first5=Olga V. |last6=Logacheva |first6=Maria D. |last7=Penin |first7=Alexey A. |last8=Moroz |first8=Leonid L. |last9=Panchin |first9=Yuri V. |last10=Lyubetsky |first10=Vassily A. |last11=Aleoshin |first11=Vladimir V. |journal=Frontiers in Genetics |volume=10 |page=443 |pmid=31178892 |pmc=6543705 }}</ref> They also lack specialized [[circulatory system|circulatory]] and [[respiratory system|respiratory]] organs, both of these facts are defining features when classifying a flatworm's [[organ (anatomy)|anatomy]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes">{{cite book |author1=Ruppert, E.E. |author2=Fox, R.S. |author3=Barnes, R.D. |name-list-style=amp |title=Invertebrate Zoology |publisher=Brooks / Cole |edition=7 |isbn=978-0-03-025982-1 |year=2004 |pages=[https://archive.org/details/isbn_9780030259821/page/226 226–269] |url-access=registration |url=https://archive.org/details/isbn_9780030259821/page/226 }}</ref> Their bodies are soft and unsegmented.<ref name="Rhode2001PlatyhelminthesInEncLifeSci" />

{|class="wikitable" style="margin-left:4px"
! Attribute !! [[Cnidarian]]s and [[Ctenophore]]s<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /> !! Platyhelminthes (flatworms)<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes"/> ||More "advanced" [[bilateria]]ns<ref name="RuppertBarnes2004Bilateria">{{cite book |author1=Ruppert, E.E. |author2=Fox, R.S. |author3=Barnes, R.D. |name-list-style=amp |title=Invertebrate Zoology |publisher=Brooks / Cole |edition=7 |isbn=978-0-03-025982-1 |year=2004 |pages=[https://archive.org/details/isbn_9780030259821/page/196 196–224] |url-access=registration |url=https://archive.org/details/isbn_9780030259821/page/196 }}</ref>
|- align="center"
! Bilateral symmetry
|No ||colspan="2" |Yes
|- align="center"
! Number of main cell layers
|[[Diploblasty|Two]], with jelly-like layer between them ([[mesoglea]]) ||colspan="2" |[[Triploblasty|Three]]
|- align="center"
! Distinct [[brain]]
|No ||colspan="2" |Yes
|- align="center"
! Specialized [[digestive system]]
|No ||colspan="2" |Yes
|- align="center"
! Specialized [[excretory system]]
|No ||colspan="2" |Yes
|- align="center"
! Body cavity containing internal organs
|colspan="2" |No ||Yes
|- align="center"
! Specialized [[circulatory system|circulatory]] and [[respiratory system|respiratory]] [[organ (anatomy)|organ]]s
|colspan="2" |No ||Yes
|}

===Features common to all subgroups===

The lack of circulatory and respiratory organs limits platyhelminths to sizes and shapes that enable [[oxygen]] to reach and [[carbon dioxide]] to leave all parts of their bodies by simple [[diffusion]]. Hence, many are microscopic, and the large species have flat ribbon-like or leaf-like shapes. Because there is no circulatory system which can transport nutrients around, the guts of large species have many branches, allowing the nutrients to diffuse to all parts of the body.<ref name="Barnes1998DiversityOfLifePlatyhelminthes" /> Respiration through the whole surface of the body makes them vulnerable to fluid loss, and restricts them to environments where [[dehydration]] is unlikely: sea and freshwater, moist terrestrial environments such as [[leaf litter]] or between grains of soil, and as [[parasite]]s within other animals.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

The space between the skin and gut is filled with [[mesenchyme]], also known as [[Parenchyma#Flatworms|parenchyma]], a [[connective tissue]] made of [[cell (biology)|cells]] and reinforced by [[collagen]] fibers that act as a type of [[skeleton]], providing attachment points for [[muscle]]s. The mesenchyme contains all the internal organs and allows the passage of oxygen, nutrients and waste products. It consists of two main types of cell: fixed cells, some of which have fluid-filled [[vacuole]]s; and [[stem cell]]s, which can transform into any other type of cell, and are used in regenerating tissues after injury or [[asexual reproduction]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

Most platyhelminths have no [[anus]] and regurgitate undigested material through the mouth. The genus ''[[Paracatenula]]'', whose members include tiny flatworms living in symbiosis with bacteria, is even missing a mouth and a gut.<ref>{{Cite web |url=https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |title=This Seabed Flatworm Got Rid Of Its Mouth And Anus, Replacing Its Entire Digestive System With Bacteria |date=8 April 2019 |access-date=2022-12-11 |archive-date=2022-12-11 |archive-url=https://web.archive.org/web/20221211200016/https://www.iflscience.com/this-seabed-flatworm-got-rid-of-its-mouth-and-anus-replacing-its-entire-digestive-system-with-bacteria-52075 |url-status=live }}</ref> However, some long species have an anus and some with complex, branched guts have more than one anus, since excretion only through the mouth would be difficult for them.<ref name="RuppertBarnes2004Platyhelminthes" /> The gut is lined with a single layer of [[endoderm]]al cells that absorb and digest food. Some species break up and soften food first by secreting [[enzyme]]s in the gut or [[pharynx]] (throat).<ref name="WalkerAnderson2001PlatyhelminthesInAnderson"/>

All animals need to keep the [[Concentration (chemistry)|concentration]] of dissolved substances in their body fluids at a fairly constant level. Internal parasites and free-living marine animals live in environments with high concentrations of dissolved material, and generally let their tissues have the same level of concentration as the environment, while freshwater animals need to prevent their body fluids from becoming too dilute. Despite this difference in environments, most platyhelminths use the same system to [[osmoregulation|control the concentration]] of their body fluids. [[Flame cell]]s, so called because the beating of their [[flagella]] looks like a flickering candle flame, extract from the mesenchyme water that contains wastes and some reusable material, and drive it into networks of tube cells which are lined with flagella and [[microvilli]]. The tube cells' flagella drive the water towards exits called [[nephridiopore]]s, while their microvilli reabsorb reusable materials and as much water as is needed to keep the body fluids at the right concentration. These combinations of flame cells and tube cells are called [[protonephridia]].<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Bilateria" />

In all platyhelminths, the [[nervous system]] is concentrated at the head end. Other platyhelminths have rings of [[Ganglion|ganglia]] in the head and main nerve trunks running along their bodies.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" /><ref name="RuppertBarnes2004Platyhelminthes" />