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===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 that 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 such as [[gnathostomulid]]s, [[gastrotrich]]s, [[Xenacoelomorpha|xenacoelomorphs]], [[Symbion|cycliophorans]], [[entoprocta|entoprocts]] and the parasitic [[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 which 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
|}