Polycladida

{{Short description|Order of free-living marine flatworms}} {{Automatic taxobox | taxon = Polycladida | fossil_range = {{fossil range| 508|0}} | image = Bedford's Flatworm.jpg | image_caption = ''Pseudobiceros bedfordi'' (Persian carpet flatworm) | authority = Lang, 1884 <ref name="Lang1884"/> | subdivision_ranks = Suborder | subdivision = * Cotylea * Acotylea * †Platydendron (not placed in a phylum) }} {{More citations needed|date=March 2026}} The '''Polycladida'''<ref name="Lang1884">{{cite book |author= Lang, Arnold |author-link=Arnold Lang |year=1884 |title=Die Polycladen (Seeplanarien) des Golfes von Neapel und der angrenzenden Meeresabschnitte. Eine Monographie. Fauna und Flora des Golfes von Neapel und der angrenzenden Meeresabschnitte, herausgegeben von der Zoologische Station in Neapel |publisher=W. Engelmann |location=Leipzig |page= 688}}</ref> represents a highly diverse clade of free-living marine flatworms. They are known from the littoral to the sublittoral zone (extending to the deep hot vents), and many species are common from coral reefs. Only a few species are found in freshwater habitats.

== Description == Polyclads range from {{convert|3|mm|in}} to {{convert|10|cm|in}} in length with a flattened, roughly oval, body shape and, in many cases, a pair of short tentacles on the head. They are distinguished from other related animals by the presence of a folded pharynx, an elongated intestine with numerous complex diverticula, and multiple ocelli.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|page= 229|isbn= 0-03-056747-5}}</ref>

The etymology of the order name ''Polycladida'' corresponds to the two ancient Greek words {{wikt-lang|grc|πολύς}} ({{grc-transl|πολύς}}), meaning "numerous", and {{wikt-lang|grc|κλάδος}} ({{grc-transl|κλάδος}}), meaning "branch".<ref name="BaillyBook">{{Cite book |title=Abrégé du dictionnaire grec français |last=Bailly |first=Anatole |date=1981-01-01 |publisher=Hachette |isbn=2010035283 |location=Paris |oclc=461974285 }}</ref><ref name="BaillyWeb">{{Cite web |url=http://www.tabularium.be/bailly/ |title=Greek-french dictionary online |last=Bailly |first=Anatole |website=www.tabularium.be |access-date=November 25, 2018}}</ref> It refers to the ramified shape of the intestine in these flatworms.<ref>{{Citation|last=Bulnes|first=Veronica|title=Polycladida-Marine Flatworms|date=2010-01-01|pages=356–368|publisher=Nature in Focus |url=https://www.researchgate.net/publication/313656350|isbn=9789563322446|access-date=2018-11-25}}</ref>

Most polyclads hide away from direct light. However, some of the brightly colored species often are active during the day. With their flamboyant coloring they advertise their potential toxicity to visual predators such as fish.{{Citation needed|date=March 2026}}

== Development == Some polyclads develop through a Müller's larval stage.{{Explain|reason=Explain what a "Müller's larval stage" is.|date=March 2026}}

== Ecology == Often polyclads are associated with other invertebrates, such as bivalve mollusks, sponges, corals, or ascidians. In such associations, the worms may use the invertebrates as a source of food, or they may find protection from predators inside the structural framework of these hosts.{{Citation needed|date=March 2026}}

== Systematics == The order Polycladida is divided into the two suborders, Cotylea and Acotylea, based on the character "presence/absence of a cotyl or sucker". Of the two, the Acotylea is the larger group with over 26 families worldwide. Acotyleans are major predators of sessile marine invertebrates such as all commercial bivalves species (including pearl and rock oysters), mussels, scallops and giant clams. Acotyleans are dull in coloration, and cryptic in their behavior, hiding in crevices and under coral during the day.

Cotyleans, on the other hand, with as many as 16 families, are prominent members of tropical coral reef communities and have bright, colorful bodies. Although cotylean flatworms are conspicuous predators in subtropical and tropical ecosystems, they are difficult to study. These worms are very fragile and when disturbed can break apart.

Some examples of Polycladida families are as follows: * Discocelidae * Cestoplanidae * Planoceridae * Stylochidae * Ditremageniidae * Prosthiostomidae * Opisthogeniidae * Pseudocerotidae

== Phylogeny == In 2017, molecular analyses produced well-supported phylogenetic hypotheses that confirmed the monophyly of Polycladida,<ref name="Bahia2017">{{Cite journal|last1=Bahia|first1=Juliana|last2=Padula|first2=Vinicius|last3=Schrödl|first3=Michael|date=2017-05-11|title=Polycladida phylogeny and evolution: integrating evidence from 28S rDNA and morphology|journal=Organisms Diversity & Evolution|language=en|volume=17|issue=3|pages=653–678|doi=10.1007/s13127-017-0327-5|s2cid=34263342 |issn=1439-6092}}</ref><ref name="AguadoNoreña2017">{{cite journal|last1=Aguado|first1=M. T.|last2=Noreña|first2=C.|last3=Alcaraz|first3=L.|last4=Marquina|first4=D.|last5=Brusa|first5=F.|last6=Damborenea|first6=C.|last7=Almon|first7=B.|last8=Bleidorn|first8=C.|last9=Grande|first9=C.|title=Phylogeny of Polycladida (Platyhelminthes) based on mtDNA data|journal=Organisms Diversity & Evolution|volume=17|issue=4|pages=767–778|year=2017|issn=1439-6092|doi=10.1007/s13127-017-0344-4|s2cid=26677216 |url=http://sedici.unlp.edu.ar/handle/10915/133598 |hdl=11336/50141|hdl-access=free}}</ref> as well as that of Acotylea and of Cotylea.<ref name="AguadoNoreña2017"/>

== Images == <gallery> File:Pseudobiceros hancockanus.jpg|Marine flatworm,<br />''Pseudobiceros sp.'' File:Yellow papillae flatworm (Thysanozoon nigropapillosum).jpg|Yellow papillae flatworm,<br />''Thysanozoon nigropapillosum'' </gallery>

== References == {{Reflist}}

Category:Turbellaria