View source for Planarian

{{Short description|Flatworms of the Turbellaria class}}
{{Distinguish|Planarium|Planetarium}}
{{Lead rewrite|date=December 2023|reason=it does not summarise the body}}
{{Automatic taxobox
| image = Dugesia subtentaculata 1.jpg
| image_caption = ''[[Dugesia subtentaculata]]'', a [[dugesiid]].
| taxon = Tricladida
| authority = Lang, 1884
| subdivision_ranks = Subdivis
| subdivision_ref = <ref name="Sluys09">{{Cite journal | last1 = Sluys | first1 = R. | last2 = Kawakatsu | first2 = M. | last3 = Riutort | first3 = M. | last4 = Baguñà | first4 = J. | title = A new higher classification of planarian flatworms (Platyhelminthes, Tricladida) | doi = 10.1080/00222930902741669 | journal = Journal of Natural History | volume = 43 | issue = 29–30 | pages = 1763–1777| year = 2009 | bibcode = 2009JNatH..43.1763S | s2cid = 85174457 }}</ref>
| subdivision =
* Suborder [[Continenticola]]
* Suborder [[Maricola]]
* Suborder [[Cavernicola (suborder)|Cavernicola]]
}}
[[File:Unidentified planarian.ogv|thumb|Unidentified planarian]]

'''Planarians''' (triclads) are free-living [[flatworm]]s of the class [[Turbellaria]],<ref name="Encyclopedia">{{cite web |url=http://www.britannica.com/EBchecked/topic/462868/planarian |title=Planarian (flatworm) – Britannica Online Encyclopedia |publisher=[[Encyclopædia Britannica, Inc.]]|access-date=2010-05-01 }}</ref><ref name="Campbell">{{cite book|title=Biology|last1=Campbell|first1=Neil A.|last2=Reece|first2=Jane B.|publisher=[[Benjamin Cummings]]|year=2019|isbn=978-0-8053-7146-8|pages=1230 pp|name-list-style=vanc|author-link=Neil Campbell (scientist)|author-link2=Jane B. Reece}}</ref> order '''Tricladida''',<ref>{{ITIS | id = 54468 | taxon = Tricladida | access-date=July 23, 2007 }}</ref> which includes hundreds of species, found in freshwater, marine, and terrestrial habitats.<ref name=":3">{{Citation |last1=Sluys |first1=Ronald |title=Planarian Diversity and Phylogeny |date=2018 |work=Planarian Regeneration: Methods and Protocols |pages=1–56 |editor-last=Rink |editor-first=Jochen C. |series=Methods in Molecular Biology |place=New York, NY |publisher=Springer |language=en |doi=10.1007/978-1-4939-7802-1_1 |isbn=978-1-4939-7802-1 |last2=Riutort |first2=Marta|volume=1774 |pmid=29916154 }}pp 3., "Planarians (the popular name for the group as a whole), or triclad flatworms (the more scientific designation of the same group), are acoelomate bilaterians".</ref> Planarians are characterized by a three-branched intestine, including a single anterior and two posterior branches.<ref name=":3" /> Their body is populated by [[adult stem cell]]s called [[neoblast]]s, which planarians use for regenerating missing body parts.<ref>{{Cite journal |last1=Vila-Farré |first1=Miquel |last2=Rozanski |first2=Andrei |last3=Ivanković |first3=Mario |last4=Cleland |first4=James |last5=Brand |first5=Jeremias N. |last6=Thalen |first6=Felix |last7=Grohme |first7=Markus A. |last8=von Kannen |first8=Stephanie |last9=Grosbusch |first9=Alexandra L. |last10=Vu |first10=Hanh T.-K. |last11=Prieto |first11=Carlos E. |last12=Carbayo |first12=Fernando |last13=Egger |first13=Bernhard |last14=Bleidorn |first14=Christoph |last15=Rasko |first15=John E. J. |date=2023-10-19 |title=Evolutionary dynamics of whole-body regeneration across planarian flatworms |journal=Nature Ecology & Evolution |volume=7 |issue=12 |language=en |pages=2108–2124 |doi=10.1038/s41559-023-02221-7 |pmid=37857891 |s2cid=264347538 |issn=2397-334X|doi-access=free |pmc=10697840 |bibcode=2023NatEE...7.2108V }}</ref> Many species are able to [[Regeneration (biology)|regenerate]] any missing organ, which has made planarians a popular model in research of regeneration and stem cell biology.<ref name=":4">{{cite journal |vauthors=Newmark PA, Sánchez Alvarado A |date=March 2002 |title=Not your father's planarian: a classic model enters the era of functional genomics |journal=Nature Reviews. Genetics |volume=3 |issue=3 |pages=210–9 |doi=10.1038/nrg759 |pmid=11972158 |s2cid=28379017}}</ref>  The genome sequences of several species are available, as are tools for molecular biology analysis.<ref name=":4" /><ref>{{Cite journal |last1=Rozanski |first1=Andrei |last2=Moon |first2=HongKee |last3=Brandl |first3=Holger |last4=Martín-Durán |first4=José M |last5=Grohme |first5=Markus A |last6=Hüttner |first6=Katja |last7=Bartscherer |first7=Kerstin |last8=Henry |first8=Ian |last9=Rink |first9=Jochen C |date=2019-01-08 |title=PlanMine 3.0—improvements to a mineable resource of flatworm biology and biodiversity |url=https://academic.oup.com/nar/article/47/D1/D812/5212006 |journal=Nucleic Acids Research |language=en |volume=47 |issue=D1 |pages=D812–D820 |doi=10.1093/nar/gky1070 |issn=0305-1048 |pmc=6324014 |pmid=30496475}}</ref>

The order Tricladida is split into three suborders, according to their phylogenetic relationships: [[Maricola]], [[Dimarcusidae|Cavernicola]] and [[Continenticola]]. Formerly, the Tricladida was split according to their habitat: [[Maricola]] (marine planarians); [[Dimarcusidae|Paludicola]] (freshwater planarian); and [[Geoplanidae|Terricola]] (land planarians).<ref>Hallez P. (1892). Classification des Ticlades, Bulletin de la Société Zoologique de France.</ref>

Planarians move by beating [[cilia]] on the [[ventral]] [[dermis]], allowing them to glide along on a film of [[mucus]]. Some also can move by undulations of the whole body by the contractions of muscles built into the body membrane.<ref>{{cite book | vauthors = Rompolas P, Patel-King RS, King SM | title = Schmidtea mediterranea: a model system for analysis of motile cilia | series = Methods in Cell Biology | volume = 93 | pages = 81–98 | year = 2009 | pmid = 20409812 | doi = 10.1016/S0091-679X(08)93004-1 | isbn = 978-0-12-381377-0 }}</ref>

Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators.<ref>{{cite journal | vauthors = Manenti R | year = 2010 | title = – Effect of landscape features and water quality on Triclads inhabiting head waters: the example of ''Polycelis felina'' | url = http://acta-zoologica-bulgarica.eu/downloads/acta-zoologica-bulgarica/2014/66-2-271-275.pdf | journal = Revue Écologie Terre et Vie | volume = 65 | issue = 2 | pages = 279–285 | doi = 10.3406/revec.2010.1533 | hdl = 2434/147984 | s2cid = 54499235 }}</ref>

==Phylogeny and taxonomy==

===Phylogeny===
Phylogenetic supertree after Sluys et al., 2009:<ref name="Sluys09"/>
{{clade| style=font-size:100%;line-height:80%
|label1=Tricladida
 |1={{Clade
   |1=[[Maricola]]
   |2={{Clade
     |1=[[Dimarcusidae|Cavernicola]]
     |label2=[[Continenticola]]
     |2={{Clade
         |label1=[[Planarioidea]]
         |1={{Clade
           |1=[[Planariidae]]
           |2={{Clade
             |1=[[Kenkiidae]]
             |2=[[Dendrocoelidae]]
             }} 
           }} 
         |label2=[[Geoplanoidea]]
         |2={{Clade
            |1=[[Dugesiidae]]
            |2=[[Geoplanidae]]
           }}
        }}
      }}
   }}
}}

=== Taxonomy ===
[[File:Sabussowia ronaldi 2.jpg|thumb|right|200px|''Sabussowia ronaldi'', a [[Maricola]].]]
[[File:Polycelis felina.jpg|thumb|right|200px|''Polycelis felina'', a [[planariid]].]]
[[File:Peerj-297-fig-1 Platydemus manokwari.png|thumb|right|200px|''[[Platydemus manokwari]]'', a [[Geoplanidae|geoplanid]].]]
[[Linnaean taxonomy|Linnaean]] ranks after Sluys ''et al.'', 2009:<ref name="Sluys09"/>
* Order '''Tricladida'''
** Suborder [[Maricola]]
*** Superfamily [[Cercyroidea]]
**** Family [[Centrovarioplanidae]]
**** Family [[Cercyridae]]
**** Family [[Meixnerididae]]
*** Superfamily [[Bdellouroidea]]
**** Family [[Uteriporidae]]
**** Family [[Bdellouridae]]
*** Superfamily [[Procerodoidea]]
**** Family [[Procerodidae]]
** Suborder [[Dimarcusidae|Cavernicola]]<ref name="Sluys90">{{Cite journal | last1 = Sluys | first1 = R. | doi = 10.1111/j.1463-6409.1990.tb00237.x | title = A monograph of the Dimarcusidae (Platyhelminthes, Seriata, Tricladida) | journal = Zoologica Scripta | volume = 19 | issue = 1| pages = 13–29| year = 1990 | s2cid = 84915439 }}</ref>
*** Family [[Dimarcusidae]]
** Suborder [[Continenticola]]
*** Superfamily [[Planarioidea]]
**** Family [[Planariidae]]
**** Family [[Dendrocoelidae]]
**** Family [[Kenkiidae]]
*** Superfamily [[Geoplanoidea]]
**** Family [[Dugesiidae]]
**** Family [[Geoplanidae]]

==Anatomy and physiology==
Planarians are [[bilateria]]n flatworms that lack a [[Acoelomates|fluid-filled body cavity]], and the space between their organ systems is filled with [[parenchyma]].<ref name=":3" /><ref>{{Cite journal |last1=Riutort |first1=Marta |last2=Álvarez-Presas |first2=Marta |last3=Lázaro |first3=Eva |last4=Sol |first4=Eduard |last5=Paps |first5=Jordi |date=2012 |title=Evolutionary history of the Tricladida and the Platyhelminthes: an up-to-date phylogenetic and systematic account |url=http://www.intjdevbiol.com/paper.php?doi=113441mr |journal=The International Journal of Developmental Biology |language=en |volume=56 |issue=1–2–3 |pages=5–17 |doi=10.1387/ijdb.113441mr |issn=0214-6282|doi-access=free |pmid=22450992 }}</ref> Planarians lack a circulatory system, and absorb oxygen through their body wall. They uptake food to their gut using a muscular pharynx, and nutrients diffuse to internal tissues. A three-branched intestine runs across almost the entire body, and includes a single anterior and two posterior branches. The planarian intestine is a [[Blind sac body|blind sac]], having no exit cavity, and therefore planarians uptake food and egest waste through the same orifice, located near the middle of the ventral body surface.<ref name=":3" />

The excretory system is made of many tubes with many [[flame cell]]s and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian.

The triclads have an anterior end or head where sense organs, such as eyes and [[chemoreceptor]]s, are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors.<ref name="Kenk1972">Kenk, R., 1972. Freshwater planarians (Turbellarians) of North America.</ref>

The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. ''[[Dugesia]]'' or ''[[Microplana]]''), others have many more distributed along the body (e.g. most [[Geoplaninae]]). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind.<ref name="Kenk1972" />

The body of the triclads is covered by a ciliated epidermis that contains [[rhabdite]]s. Between the epidermis and the gastrodermis there is a parenchymatous tissue or [[mesenchyme]].<ref name="Kenk1972" />

===Nervous system===
[[File:Planaria nervous.png|Planaria nervous system|thumb]]
The planarian nervous systems consists of a bilobed shaped cerebral [[ganglion]] that is referred to as the planarian [[brain]].<ref>{{Cite journal |last1=Cebrià |first1=Francesc |last2=Nakazawa |first2=Masumi |last3=Mineta |first3=Katsuhiko |last4=Ikeo |first4=Kazuho |last5=Gojobori |first5=Takashi |last6=Agata |first6=Kiyokazu |date=2002-04-03 |title=Dissecting planarian central nervous system regeneration by the expression of neural-specific genes |journal=Development, Growth & Differentiation |volume=44 |issue=2 |pages=135–146 |doi=10.1046/j.1440-169x.2002.00629.x |issn=0012-1592 |pmid=11940100|doi-access=free }}</ref> Longitudinal ventral nerve chords extend from the brain to the tail. Transverse nerves, [[commissure]], connect the ventral nerve chords forming ladder-like nerve system.<ref name=":3" /> The brain has been shown to exhibit spontaneous electrophysiological oscillations,<ref>
{{cite journal
 | vauthors = Aoki R, Wake H, Sasaki H, Agata K
 | date = March 2009
 | title = Recording and spectrum analysis of the planarian electroencephalogram
 | journal = Neuroscience
 | volume = 159 | issue = 2 | pages = 908–914
 | pmid = 19063945 | s2cid = 207244874
 | doi = 10.1016/j.neuroscience.2008.11.011
}}
</ref> similar to the electroencephalographic ([[EEG]]) activity of other animals.

The planarian has a soft, flat, wedge-shaped body that may be black, brown, blue, gray, or white. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall.

===Reproduction===

{{Main article|Reproductive system of planarians}}
{{More citations needed|date=November 2016}}
[[File:Planarian reproductive system.png|Planarian reproductive system|thumb]]

Triclads reproduce sexually and asexually, and different species may be able to reproduce by one or both modes.<ref name=":3" /> Planarians are [[Hermaphrodism|hermaphrodites]]. In sexual reproduction, the mating generally involves mutual insemination.

Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to the other planarian, giving and receiving [[sperm]]. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. In asexual reproduction, the planarian fissions and each fragment regenerates its missing tissues, generating complete anatomy and restoring functions.<ref name=":0">{{Cite journal |last1=Reddien |first1=Peter W. |last2=Alvarado |first2=Alejandro Sánchez |date=2004-11-01 |title=Fundamentals of Planarian Regeneration |url=https://www.annualreviews.org/doi/10.1146/annurev.cellbio.20.010403.095114 |journal=Annual Review of Cell and Developmental Biology |language=en |volume=20 |issue=1 |pages=725–757 |doi=10.1146/annurev.cellbio.20.010403.095114 |pmid=15473858 |issn=1081-0706|url-access=subscription }}</ref> Asexual reproduction, similar to regeneration following injury, requires [[neoblast]]s, adult stem cells, which proliferate and produce differentiated cells.<ref name=":0" /> Some researchers claim that the products derived from bisecting a planarian are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planarians and its effect on the population are ongoing.<ref>{{cite journal | vauthors = Neuhof M, Levin M, Rechavi O | title = Vertically- and horizontally-transmitted memories - the fading boundaries between regeneration and inheritance in planaria | journal = Biology Open | volume = 5 | issue = 9 | pages = 1177–88 | date = September 2016 | pmid = 27565761 | pmc = 5051648 | doi = 10.1242/bio.020149 }}</ref> Some species of planarian are exclusively asexual, whereas some can reproduce both sexually and asexually.<ref>{{Cite journal |last1=Newmark |first1=Phillip A. |last2=Alvarado |first2=Alejandro Sánchez |date=2002-03-01 |title=Not your father's planarian: a classic model enters the era of functional genomics |url=https://www.nature.com/articles/nrg759 |journal=Nature Reviews Genetics |language=en |volume=3 |issue=3 |pages=210–219 |doi=10.1038/nrg759 |pmid=11972158 |s2cid=28379017 |issn=1471-0056|url-access=subscription }}</ref> In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in ''[[Cura foremanii]]'').<ref name="Kenk1972" />

=== Neoblasts ===
{{Main article|Neoblast}}
Neoblasts are abundant [[adult stem cells]] that are found in the planarian parenchyma across the planarian body.<ref name=":5">{{Citation |last=Rink |first=Jochen C. |title=Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale |date=2018 |work=Planarian Regeneration: Methods and Protocols |pages=57–172 |editor-last=Rink |editor-first=Jochen C. |series=Methods in Molecular Biology |volume=1774 |place=New York, NY |publisher=Springer |language=en |doi=10.1007/978-1-4939-7802-1_2 |pmid=29916155 |isbn=978-1-4939-7802-1}}</ref> They are small and round cells, 5 to 10&nbsp;μm, and characterized by a large nucleus, which is surrounded by little cytoplasm.<ref name=":5" /> Neoblasts are required for regenerating missing tissues and organs, and they continuously replenish tissues by producing new cells.<ref name=":0" /> Neoblasts can self-renew and generate progenitors for different cell types. In contrast to adult vertebrate stem cells (e.g., [[hematopoietic stem cell]]), neoblasts are [[pluripotent]] (i.e., producing all somatic cell types).<ref>{{Cite journal |last1=Wagner |first1=Daniel E. |last2=Wang |first2=Irving E. |last3=Reddien |first3=Peter W. |date=2011-05-13 |title=Clonogenic Neoblasts Are Pluripotent Adult Stem Cells That Underlie Planarian Regeneration |journal=Science |language=en |volume=332 |issue=6031 |pages=811–816 |doi=10.1126/science.1203983 |issn=0036-8075 |pmc=3338249 |pmid=21566185|bibcode=2011Sci...332..811W }}</ref> Moreover, they give rise to differentiating, post-mitotic, cells directly,<ref>{{Cite journal |last1=Raz |first1=Amelie A. |last2=Wurtzel |first2=Omri |last3=Reddien |first3=Peter W. |date=2021-04-20 |title=Planarian stem cells specify fate yet retain potency during the cell cycle |journal=Cell Stem Cell |language=en |volume=28 |issue=7 |pages=1307–1322.e5 |doi=10.1016/j.stem.2021.03.021 |pmc=8254784 |pmid=33882291}}</ref> and not by producing rapidly-dividing [[transit amplifying cell]]s.<ref name=":5" /> Consequently, neoblasts divide frequently, and apparently lack a large sub-population of dormant or slow-cycling cells.<ref>{{Cite journal |last1=Newmark |first1=Phillip A. |last2=Sánchez Alvarado |first2=Alejandro |date=2000-04-15 |title=Bromodeoxyuridine Specifically Labels the Regenerative Stem Cells of Planarians |journal=Developmental Biology |volume=220 |issue=2 |pages=142–153 |doi=10.1006/dbio.2000.9645 |issn=0012-1606|doi-access=free |pmid=10753506 }}</ref>

== As a model system in biological and biomedical research ==
The [[Biological life cycle|life history]] of planarians make them a model system for investigating a number of biological processes, many of which may have implications for human health and disease. Advances in [[molecular]] [[Genetics|genetic]] technologies has made the study of [[gene]] function possible in these animals and scientists are studying them worldwide. Like other invertebrate model organisms, for example ''[[Caenorhabditis elegans|C. elegans]]'' and ''[[Drosophila melanogaster|D. melanogaster]]'', the relative simplicity of planarians facilitates experimental study.

Planarians have a number of cell types, tissues and simple organs that are [[Homology (biology)|homologous]] to human [[cell (biology)|cells]], [[Tissue (biology)|tissues]] and [[Organ (anatomy)|organs]]. However, [[Regeneration (biology)|regeneration]] has attracted the most attention. [[Thomas Hunt Morgan]] was responsible for some of the first systematic studies (that still underpin modern research) before the advent of [[molecular biology]] as a discipline.

Planarians are also an emerging model organism for [[aging research]]. These animals have an apparently limitless regenerative capacity, and asexual ''Schmidtea mediterranea'' has been shown to maintain its telomere length through regeneration.<ref>{{cite journal | vauthors = Tan TC, Rahman R, Jaber-Hijazi F, Felix DA, Chen C, Louis EJ, Aboobaker A | title = Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 11 | pages = 4209–14 | date = March 2012 | pmid = 22371573 | pmc = 3306686 | doi = 10.1073/pnas.1118885109 | bibcode = 2012PNAS..109.4209T | doi-access = free }}</ref>

Live planarians are increasingly used in toxicological research due to their regenerative capabilities, simple anatomy, and sensitivity to environmental changes. Their ability to regenerate lost body parts provides a unique model to study the effects of chemical exposures on cellular processes, while their rapid response to toxins makes them an efficient tool for screening potential environmental and pharmaceutical hazards.  An example of this application is a fluorescence-based skin irritability assay, where planaria are exposed to various chemicals, and fluorescence dye is used to evaluate their epithelial damage in response to irritation, providing an effective screening method.<ref>{{Cite journal |last1=Shah |first1=Syed Ibrahim |last2=Williams |first2=Adrian C. |last3=Lau |first3=Wing Man |last4=Khutoryanskiy |first4=Vitaliy V. |date=2020-12-01 |title=Planarian toxicity fluorescent assay: A rapid and cheap pre-screening tool for potential skin irritants |url=https://www.sciencedirect.com/science/article/abs/pii/S0887233320305543 |journal=Toxicology in Vitro |volume=69 |article-number=105004 |doi=10.1016/j.tiv.2020.105004 |pmid=33010358 |bibcode=2020ToxVi..6905004S |issn=0887-2333|url-access=subscription }}</ref>

== Regeneration ==
Planarian regeneration combines new tissue production with reorganization to the existing anatomy, morphallaxis.<ref name=":0" /> The rate of tissue regrowth varies between species, but in frequently used lab species, functional regenerated tissues are available already 7–10 days following tissue amputation.<ref name=":0" /> Regeneration starts following an injury that require the growth of a new tissue.<ref>{{Cite journal |last1=Wenemoser |first1=Danielle |last2=Reddien |first2=Peter{{nbsp}}W. |date=2010-08-15 |title=Planarian regeneration involves distinct stem cell responses to wounds and tissue absence |journal=Developmental Biology |language=en |volume=344 |issue=2 |pages=979–991 |doi=10.1016/j.ydbio.2010.06.017 |pmc=2950745 |pmid=20599901}}</ref> Neoblasts localized near the injury site proliferate to generate a structure of differentiating cells called [[blastema]]. Neoblasts are required for new cell production, and they therefore provide the cellular basis for planarian regeneration.<ref>{{Cite journal |last1=Hayashi |first1=Tetsutaro |last2=Asami |first2=Maki |last3=Higuchi |first3=Sayaka |last4=Shibata |first4=Norito |last5=Agata |first5=Kiyokazu |date=2006-07-13 |title=Isolation of planarian X-ray-sensitive stem cells by fluorescence-activated cell sorting |journal=Development, Growth & Differentiation |language=en |volume=48 |issue=6 |pages=371–380 |doi=10.1111/j.1440-169X.2006.00876.x |pmid=16872450 |s2cid=10048289 |issn=0012-1592|doi-access=free }}</ref> Cell signaling mechanisms provide positional information that regulates the cell types and tissues that are produced from the neoblasts in regeneration.<ref name=":1" /> Many signaling molecules that provide positional information to neoblasts, in regeneration and homeostasis, are expressed in muscle cells.<ref name=":2">{{Cite journal |last1=Witchley |first1=Jessica N. |last2=Mayer |first2=Mirjam |last3=Wagner |first3=Daniel E. |last4=Owen |first4=Jared H. |last5=Reddien |first5=Peter W. |date=2013-08-29 |title=Muscle cells provide instructions for planarian regeneration |journal=Cell Reports |volume=4 |issue=4 |pages=633–641 |doi=10.1016/j.celrep.2013.07.022 |issn=2211-1247 |pmc=4101538 |pmid=23954785}}</ref> Following injury, muscle cells throughout the body can alter the expression of genes that encode molecules that provide positional information.<ref name=":2" /> Therefore, the activities of neoblasts and muscle cells following injuries are essential for successful regeneration.<ref>{{Cite journal |last=Reddien |first=Peter W. |date=2018-10-04 |title=The cellular and molecular basis for planarian regeneration |journal=Cell |volume=175 |issue=2 |pages=327–345 |doi=10.1016/j.cell.2018.09.021 |issn=0092-8674 |pmc=7706840 |pmid=30290140}}</ref>

Historically, planarians have been considered "immortal under the edge of a knife."<ref>{{cite book | vauthors = Dalyell JG|title=Observations on some interesting phenomena in animal physiology, exhibited by several species of planariae.|year=1814|location=Edinburgh}}</ref>  Very small pieces of the planarian, estimated to be as little as 1/279th of the organism it is cut from, can regenerate back into a complete organism over the course of a few weeks.<ref name="pmid18508666">{{cite journal | vauthors = Handberg-Thorsager M, Fernandez E, Salo E | title = Stem cells and regeneration in planarians | journal = Frontiers in Bioscience | volume = 13 | issue = 13| pages = 6374–94 | year = 2008 | pmid = 18508666 | doi = 10.2741/3160| doi-access = free }}</ref> New tissues can grow due to [[cell potency|pluripotent]] [[stem cell]]s that have the ability to create all the various cell types.<ref>{{cite journal | vauthors = Saló E, Abril JF, Adell T, Cebrià F, Eckelt K, Fernandez-Taboada E, Handberg-Thorsager M, Iglesias M, Molina MD, Rodríguez-Esteban G | title = Planarian regeneration: achievements and future directions after 20 years of research | journal = The International Journal of Developmental Biology | volume = 53 | issue = 8–10 | pages = 1317–27 | year = 2009 | pmid = 19247944 | doi = 10.1387/ijdb.072414es | doi-access = free | hdl = 2445/192658 | hdl-access = free }}</ref> These adult stem cells are called [[neoblast]]s, and comprise 20% or more of the cells in the adult animal.<ref name="TiCB">{{cite journal | vauthors = Aboobaker AA | title = Planarian stem cells: a simple paradigm for regeneration | journal = Trends in Cell Biology | volume = 21 | issue = 5 | pages = 304–11 | date = May 2011 | pmid = 21353778 | doi = 10.1016/j.tcb.2011.01.005 }}</ref> They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. In addition, existing [[tissue (biology)|tissue]] is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism.<ref name="TiCB" /><ref name=":0" />

The organism itself does not have to be completely cut into separate pieces for the regeneration phenomenon to be witnessed. In fact, if the head of a planarian is cut in half down its center, and each side retained on the organism, it is possible for the planarian to regenerate two heads and continue to live.<ref>{{cite web | work = New Scientist |title=Do it again. Round up of regenerating animals |url=https://www.newscientist.com/gallery/regeneration/3 |access-date=2012-10-21 |publisher=[[New Scientist]]}}</ref> Researchers, including those from [[Tufts University]] in the U.S., sought to determine how microgravity and micro-geomagnetic fields would affect the growth and regeneration of planarian [[flatworm]]s, ''[[Dugesia japonica]]''. They discovered that one of the amputated fragments sent to space regenerated into a double-headed worm. The majority of such amputated worms (95%) did not do so, however. An amputated worm regenerated into a double-head worm after spending five weeks aboard the International Space Station (ISS) – though regeneration of amputated worms as double-headed heteromorphosis is not a rare phenomenon unique to a microgravity environment.<ref>{{cite journal | vauthors = Morokuma J, Durant F, Williams KB, Finkelstein JM, Blackiston DJ, Clements T, Reed DW, Roberts M, Jain M, Kimel K, Trauger SA, Wolfe BE, Levin M | title = Planarian regeneration in space: Persistent anatomical, behavioral, and bacteriological changes induced by space travel | journal = Regeneration | volume = 4 | issue = 2 | pages = 85–102 | date = April 2017 | pmid = 28616247 | pmc = 5469732 | doi = 10.1002/reg2.79 }}</ref> Double-headed planaria regenerates can be induced by exposing amputated fragments to electrical fields. Electrical field exposure with opposite polarity can induce a planarian with two tails. Double-headed planaria regenerates can also be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells,<ref>{{cite journal | vauthors = Chan JD, Agbedanu PN, Zamanian M, Gruba SM, Haynes CL, Day TA, Marchant JS | title = 'Death and axes': unexpected Ca²⁺ entry phenologs predict new anti-schistosomal agents | journal = PLOS Pathogens | volume = 10 | issue = 2 | article-number = e1003942 | date = February 2014 | pmid = 24586156 | pmc = 3930560 | doi = 10.1371/journal.ppat.1003942 | doi-access = free }}</ref> as well as by genetic expression blocks (interference RNA) to the canonical Wnt/β-Catenin signalling pathway.<ref name=":1">{{cite journal | vauthors = Gurley KA, Rink JC, Sánchez Alvarado A | title = Beta-catenin defines head versus tail identity during planarian regeneration and homeostasis | journal = Science | volume = 319 | issue = 5861 | pages = 323–7 | date = January 2008 | pmid = 18063757 | pmc = 2755502 | doi = 10.1126/science.1150029 | bibcode = 2008Sci...319..323G }}</ref>

==Biochemical memory experiments==
{{Main article|Memory transfer}}
In 1955, Robert Thompson and [[James V. McConnell]] [[Classical conditioning|conditioned]] planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate, each half would develop the light-shock reaction. In 1963, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms.  He reported that the flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.

This experiment intended to test whether memory could be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed the behavior.<ref>{{cite web | first = Bob | last = Kentridge | name-list-style = vanc | title = Investigations of the cellular bases of memory | url = http://www.dur.ac.uk/robert.kentridge/bpp2mem1.html | access-date = 2007-02-08 | publisher = [[University of Durham]] | archive-url = https://web.archive.org/web/20121015043719/http://www.dur.ac.uk/robert.kentridge/bpp2mem1.html | archive-date = 2012-10-15 }}</ref> McConnell believed that this was evidence of a chemical basis for memory, which he identified as [[memory RNA]]. McConnell's results are now attributed to [[observer bias]].<ref>{{cite journal | vauthors = Rilling M | title = The mystery of the vanished citations: James McConnell's forgotten 1960s quest for planarian learning, a biochemical engram, and celebrity | journal = American Psychologist | volume = 51 | pages = 589–598 | year = 1996 | doi = 10.1037/0003-066X.51.6.589 | issue = 6 }}</ref><ref>For a general review, see also {{cite book | first = Georges | last = Chapouthier | author-link = Georges Chapouthier | chapter = Chapter 1: Behavioral studies of the molecular basis of memory | title = The Physiological Basis of Memory | veditors =  Deutsch JA | date = 1973 | publisher = Academic Press | location = New York and London  | pages = l–25 }}</ref> No [[Blind experiment|blinded experiment]] has ever reproduced his results of planarians scrunching when exposed to light.  Subsequent explanations of this scrunching behaviour associated with cannibalism of trained planarian worms were that the untrained flatworms were only following tracks left on the dirty glassware rather than absorbing the memory of their fodder.

In 2012, Tal Shomrat and Michael Levin have shown that planarians exhibit evidence of long-term memory retrieval after regenerating a new head.<ref name="Shomrat2013">{{cite journal | vauthors = Shomrat T, Levin M | title = An automated training paradigm reveals long-term memory in planarians and its persistence through head regeneration | journal = The Journal of Experimental Biology | volume = 216 | issue = Pt 20 | pages = 3799–810 | date = October 2013 | pmid = 23821717 | doi = 10.1242/jeb.087809 | doi-access = free }}</ref>

== Planarian species used for research and education ==
Several planarian species are commonly used for biological research. Popular experimental species are ''[[Schmidtea mediterranea]]'', ''[[Schmidtea polychroa]],'' and ''[[Dugesia japonica]],''<ref name=":3" /> which in addition to excellent regenerative abilities, are easy to culture in the lab.   In recent decades, ''S. mediterranea'' has emerged as the species of choice for modern molecular biology research, due to its diploid chromosomes and the availability of both asexual and sexual strains.<ref name=":4" />

The most frequently used planarian in high school and first-year college laboratories is the brownish ''[[Girardia tigrina]]''. Other common species used are the blackish ''[[Planaria maculata]]'' and ''[[Girardia dorotocephala]]''.

== See also ==
* {{annotated link|Worm Runner's Digest}}

== References ==
{{Reflist|33em}}

== External links ==
{{EB1911 poster|Planarians}}
* [https://web.archive.org/web/20181109163038/http://planaria.neuro.utah.edu/ More information on freshwater planarians and their biology]
* [https://www.wired.com/news/medtech/0,1286,67404,00.html More information on the genetic screen to identify regeneration genes]
* YouTube videos: [https://www.youtube.com/watch?v=eUmOJ6fWN9w Planaria eating worm segment], [https://www.youtube.com/watch?v=w0QzSYQGsnA Planarian]
* [http://www.geochembio.com/biology/organisms/planarian/ ''Schmidtea mediterranea'', facts, anatomy, image] {{Webarchive|url=https://web.archive.org/web/20101230230657/http://www.geochembio.com/biology/organisms/planarian/ |date=2010-12-30 }} at GeoChemBio.com
*[https://www.ibiology.org/development-and-stem-cells/planarian/ Alejandro Sanchez-Alvarado's Seminar: Regeneration in Planarians]
* [https://www.theguardian.com/science/blog/2012/feb/29/live-forever-immortal-worms Link to an article discussing some work on planarian immortality]
* [http://planform.daniel-lobo.com A user-friendly visualization tool and database of planarian regeneration experiments]
* {{cite web|last=Aboobaker|first=Aziz|title=Immortal Worms|url=http://www.test-tube.org.uk/videos/pages_aziz_immortal_worms.htm|work=Test Tube|publisher=[[Brady Haran]] for the [[University of Nottingham]]|date=27 February 2008}}
* [https://web.archive.org/web/20110813043013/http://www.eol.org/pages/3131 Tricladida on the Encyclopedia of Life (EOL)]
* [http://entomology.ifas.ufl.edu/creatures/misc/land_planarians.htm Land planarians] on the [[University of Florida|UF]] / [[Institute of Food and Agricultural Sciences|IFAS]] Featured Creatures Web site

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[[Category:Rhabditophora]]
[[Category:Animal models]]
[[Category:Negligibly senescent organisms]]
[[Category:Articles containing video clips]]
[[Category:Invertebrate common names]]