View source for Coelom

{{Short description|Main body cavity in many animals}}
{{Infobox anatomy
|Name = Coelom
|Greek = koilōma
|Image = Annelid redone w white background.svg
|Caption = Cross-section of an  [[oligochaete worm]]. The worm's '''body cavity''' surrounds the central digestive tract.
| pronunciation = ({{IPAc-en|ˈ|s|iː|l|əm}} {{respell|SEE|ləm}}, plural '''coeloms''' or '''coelomata''' {{IPAc-en|s|iː|ˈ|l|oʊ|m|ə|t|ə}} {{respell|see|LOH|mə-tə}})
}}

The '''coelom''' (or '''celom''')<ref>{{cite Merriam-Webster|celom}}</ref> is the main [[body cavity]] in many animals<ref>{{cite web |url=http://encyclopedia2.thefreedictionary.com/coelom |title=coelom |via=[[The Free Dictionary]]}}</ref> and is positioned inside the [[Body (biology)|body]] to surround and contain the [[digestive tract]] and other [[Organ (biology)|organs]]. In some [[animal]]s, it is lined with [[mesothelium]]. In other animals, such as [[molluscs]], it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify [[bilaterian]] animal phyla into informal groups.

== Etymology ==
The term ''coelom'' derives from the [[Ancient Greek]] word {{wikt-lang|grc|κοιλία}} ({{grc-transl|κοιλία}}) 'cavity'.<ref>{{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>{{Cite web | url=http://www.tabularium.be/bailly/ | title=Greek-french dictionary online | last=Bailly | first=Anatole | website=www.tabularium.be | access-date=2018-01-14}}</ref><ref>{{cite EB1911|wstitle=Coelom and Serous Membranes|volume=6|page=642}}</ref>

== Structure ==

=== Development ===
The coelom is the mesodermally lined cavity between the gut and the outer body wall.

During the [[embryogenesis|development of the embryo]], coelom formation begins in the [[gastrulation]] stage. The developing digestive tube of an [[embryo]] forms as a blind pouch called the [[archenteron]].

In [[protostomes]], the coelom forms by a process known as [[schizocoely]].<ref name="Lüter2000">{{Cite journal |last=Lüter |first=Carsten |date=2000-06-01 |title=The origin of the coelom in Brachiopoda and its phylogenetic significance |journal=Zoomorphology |language=en |volume=120 |issue=1 |pages=15–28 |doi=10.1007/s004359900019 |s2cid=24929317 |issn=1432-234X}}</ref> The [[archenteron]] initially forms, and the [[mesoderm]] splits into two layers: the first attaches to the body wall or [[ectoderm]], forming the [[parietal layer]] and the second surrounds the [[endoderm]] or [[alimentary canal]] forming the [[visceral layer]]. The space between the parietal layer and the visceral layer is known as the coelom or body cavity.

In [[deuterostomes]], the coelom forms by [[enterocoely]].<ref name="Lüter2000"/> The archenteron wall produces buds of [[mesoderm]], and these mesodermal [[diverticula]] hollow to become the coelomic cavities. Deuterostomes are therefore known as ''enterocoelomates''. Examples of deuterostome coelomates belong to three major clades: [[chordates]] ([[vertebrate]]s, [[tunicate]]s, and [[lancelet]]s), [[echinoderms]] ([[starfish]], [[sea urchin]]s, [[Holothuroidea|sea cucumber]]s), and [[hemichordates]] ([[acorn worm]]s and [[graptolite]]s).

=== Origins ===
The [[evolution]]ary origin of the coelom is uncertain. The oldest fossil presumed to have been an animal with a body cavity was the ''[[Vernanimalcula]]''. Current hypothesis include:{{citation needed|date=November 2018}}<ref>{{Cite web|url=http://faculty.college-prep.org/~bernie/sciproject/project/Kingdoms/Animal%20Kingdom%20-%205/Local%20copy/evolution.html |title=Origins and Evolution of Animals |archive-url=https://web.archive.org/web/20181112211737/http://faculty.college-prep.org/~bernie/sciproject/project/Kingdoms/Animal%20Kingdom%20-%205/Local%20copy/evolution.html#coelomates |archive-date=2018-11-12}}</ref>
* The acoelomate theory, which states that coelom evolved from an acoelomate ancestor. 
* The enterocoel theory, which states that coelom evolved from gastric pouches of [[cnidaria]]n ancestors. This is supported by research on [[flatworm]]s{{citation needed|date=March 2021}} and small worms recently discovered{{clarification needed|date=March 2021}} in marine fauna{{citation needed|date=March 2021}} ("coelom"<ref>{{Cite web|url=http://www.answers.com/body%20cavity |title=McGraw-Hill Dictionary of Scientific and Technical Terms |website=[[Answers.com]] |archive-url=https://web.archive.org/web/20141220091027/http://www.answers.com/topic/body-cavity-1 |archive-date=2014-12-20}}</ref>).

== Functions ==
A coelom can absorb shock or provide a [[hydrostatic skeleton]]. It can also support an [[immune system]] in the form of [[coelomocyte]]s that may either be attached to the wall of the coelom or may float about in it freely. The coelom allows muscles to grow independently of the body wall — this feature can be seen in the digestive tract of [[tardigrade]]s (water bears) which is suspended within the body in the [[mesentery]] derived from a mesoderm-lined coelom.

== Coelomic fluid ==
The fluid inside the coelom is known as coelomic fluid. This is circulated by mesothelial [[cilia]] or by contraction of muscles in the body wall.{{clarify|date=June 2018|reason=http://www.citycollegekolkata.org/online_course_materials/Evolution_of_Coelom.pdf}}<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |page=205 }}</ref> The coelomic fluid serves several functions: it acts as a hydroskeleton; it allows free movement and growth of internal organs; it serves for transport of gases, nutrients and waste products around the body; it allows storage of sperm and eggs during maturation; and it acts as a reservoir for waste.<ref name=Dorit>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last1=Dorit |first1=R. L. |last2=Walker |first2=W. F. |last3=Barnes |first3=R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |page=[https://archive.org/details/zoology0000dori/page/190 190] }}</ref>

== Classification in zoology ==
{{Further|Body cavity}}
In the past, some zoologists grouped [[bilaterian]] animal phyla based on characteristics related to the coelom for practical purposes, knowing, and explicitly stating, that these groups were ''not'' [[Phylogenetics|phylogenetically]] related. Animals were classified in three informal groups according to the type of body cavity they possess, in a non-taxonomic, utilitarian way, as the Acoelomata, Pseudocoelomata, and Coelomata. These groups were never intended to represent related animals, or a sequence of evolutionary traits.

However, although this scheme was followed by a number of college textbooks and some general classifications, it is now almost totally abandoned as a formal classification. Indeed, as late as 2010, one author of a [[molecular phylogeny]] study mistakenly called this classification scheme the "traditional, morphology-based phylogeny".<ref>Nielsen, C. (2010). "[http://www.palaeodiversity.org/pdf/03Suppl/Supplement_Nielsen.pdf The 'new phylogeny'. What is new about it?]" ''Palaeodiversity'' 3, 149–150.</ref>
[[File:Figure 27 02 05.jpg|thumb|right|upright=2|alt=An illustration describing the classification of tripoblasts.|Classification of tripoblasts based on body cavities]]

'''Coelomate''' animals or '''Coelomata''' (also known as eucoelomates – "true coelom") have a body cavity called a coelom with a complete lining called [[peritoneum]] derived from mesoderm (one of the three [[germ layer|primary tissue layers]]). The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity. Most bilateral animals, including all the [[vertebrate]]s, are coelomates.

'''Pseudocoelomate''' animals have a '''pseudocoelom''' (literally "false cavity"), which is a fluid filled body cavity. Tissue derived from mesoderm partly lines the fluid filled body cavity of these animals. Thus, although organs are held in place loosely, they are not as well organized as in a coelomate. All pseudocoelomates are [[protostome]]s; however, not all protostomes are pseudocoelomates. An example of a pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as [[Blastocoele|blastocoelomate]].

'''Acoelomate''' animals, like [[flatworm]]s, have no body cavity at all. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.

== Coelomates ==
Coeloms developed in [[triploblast]]s but were subsequently lost in several lineages. The lack of a coelom is correlated with a reduction in body size. Coelom is sometimes incorrectly used to refer to any developed digestive tract. Some organisms may not possess a coelom or may have a false coelom ([[Body cavity#Pseudocoelom|pseudocoelom]]). Animals having coeloms are called [[coelomate]]s, and those without are called [[acoelomate]]s. There are also subtypes of coelom:{{citation needed|date=February 2015}}

* schizocoelom: develops from split in [[mesoderm]] found in [[annelids]], [[arthropods]] and [[molluscs]]
* haemocoelom: true coelom reduced and cavity filled with blood found from [[Arthropoda]] to [[Mollusca]]
* enterocoelom: develops from wall of embryonic gut found from [[Echinodermata]] to [[Chordata]]

===Coelomate phyla===
According to [[Taxonomy of invertebrates (Brusca & Brusca, 2003)|Brusca and Brusca]],<ref name=Brusca>R. C. Brusca, G. J. Brusca. ''Invertebrates''. Sunderland, Massachusetts: Sinauer Associates, 2003 (2nd ed.), p.&nbsp;47, {{ISBN|0-87893-097-3}}.</ref> the following [[bilaterian]] [[phylum|phyla]] possess a coelom:

*[[Nemertea]], traditionally viewed as acoelomates. Its coelom, called a rhynchocoel, lies above the digestive tract instead of around it like in other coelomate animals.<ref>{{Cite web |title=Nemertea |url=https://encyclopediaofarkansas.net/entries/nemertea-12880/ |access-date=2025-05-26 |website=Encyclopedia of Arkansas |language=en-US}}</ref>
*[[Priapulida]] appears to belong to the pseudocoelomate animals, but the possibility of it having a true coelom has still not been completely dismissed.<ref>[https://www.diva-portal.org/smash/get/diva2:1679890/FULLTEXT01.pdf Coelom development in the priapulid worm Priapulus caudatus]</ref>
*[[Onychophora]]
*[[Tardigrada]]
*[[Arthropoda]]
*[[Chaetognatha]]
*[[Bryozoa]]
*[[Mollusca]]
*[[Annelida]]
*[[Brachiopoda]]
*[[Phoronida]]
*[[Echinoderm]]ata
*[[Hemichordata]]
*[[Chordata]]

== Pseudocoelomates ==
In some [[protostome]]s, the embryonic [[blastocoele]] persists as a body cavity. These protostomes have a fluid filled main body cavity unlined or partially lined with tissue derived from mesoderm.

This fluid-filled space surrounding the internal [[organ (anatomy)|organ]]s serves several functions like distribution of nutrients and removal of waste or supporting the body as a [[hydrostatic skeleton]].

A '''pseudocoelomate''' or '''blastocoelomate''' is any [[invertebrate]] [[animal]] with a three-layered body and a [[Body cavity|pseudocoel]]. The coelom was apparently lost or reduced as a result of [[mutation]]s in certain types of [[gene]]s that affected early development. Thus, pseudocoelomates evolved from coelomates.<ref>Evers, Christine A., Lisa Starr. ''Biology:Concepts and Applications.'' 6th ed. United States:Thomson, 2006. {{ISBN|0-534-46224-3}}.</ref> "Pseudocoelomate" is no longer considered a valid [[Taxonomy (biology)|taxonomic group]], since it is not [[monophyletic]]. However, it is still used as a descriptive term.

Important characteristics:
* lack a vascular blood system
** [[diffusion]] and [[osmosis]] circulate nutrients and waste products throughout the body.
* lack a skeleton
** hydrostatic pressure gives the body a supportive framework that acts as a skeleton.
* no segmentation
* body wall
** epidermis and muscle
** often [[syncytial]]
** usually covered by a secreted cuticle
* most are microscopic
* parasites of almost every form of life (although some are free living)
* [[eutely]] in some
* loss of [[larval]] stage in some
* possibly [[pedomorphism]]

===Pseudocoelomate phyla===
Bilaterian pseudocoelomate phyla according to Brusca and Brusca,:<ref name=Brusca />

*[[Nematoda]] (roundworms)
*[[Nematomorpha]] (horsehair worms)
*[[Loricifera]]
*[[Priapulida]]
*[[Kinorhyncha]]
*[[Rotifera]], including [[Acanthocephala]] (spiny-headed worms)<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/abs/pii/S105579031500370X | doi=10.1016/j.ympev.2015.11.017 | title=Phylogeny of Syndermata (Syn. Rotifera): Mitochondrial gene order verifies epizoic Seisonidea as sister to endoparasitic Acanthocephala within monophyletic Hemirotifera | date=2016 | last1=Sielaff | first1=Malte | last2=Schmidt | first2=Hanno | last3=Struck | first3=Torsten H. | last4=Rosenkranz | first4=David | last5=Mark Welch | first5=David B. | last6=Hankeln | first6=Thomas | last7=Herlyn | first7=Holger | journal=Molecular Phylogenetics and Evolution | volume=96 | pages=79–92 | pmid=26702959 | bibcode=2016MolPE..96...79S | url-access=subscription }}</ref><ref>[https://web.archive.org/web/20180410121327id_/http://biosoc.pk/wp-content/uploads/2015/12/5-50-15-Rotifers-Safari.pdf Study of Rotifers of Safari Zoo Lake Lahore in Relation to Physico-chemical Parameters]</ref>

==Acoelomates==
Acoelomates lack a fluid-filled body cavity between the body wall and digestive tract. This can cause some serious disadvantages. Fluid compression is negligible, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal's outer surface. The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorso-ventral flattening.

*[[Flatworm]]s
*[[Limnognathia|Micrognathozoa]]
*[[Mesozoa]]<ref>{{cite book | url=https://books.google.com/books?id=ss5ADwAAQBAJ&dq=seven+acoelomate+phyla&pg=PA156 | title=Biology of Non-Chordates | date=November 2017 | publisher=PHI Learning Pvt. | isbn=978-93-87472-01-3 }}</ref><ref>[https://core.ac.uk/download/pdf/234765327.pdf Comparative genomic studies on Dicyema japonicum: the phylogenetic position of dicyemids and the genomic adaptations to parasitic lifestyle]</ref>
*[[Xenacoelomorpha]]<ref>{{cite journal | doi=10.1038/ncomms2556 | title=Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha | date=2013 | last1=Nakano | first1=Hiroaki | last2=Lundin | first2=Kennet | last3=Bourlat | first3=Sarah J. | last4=Telford | first4=Maximilian J. | last5=Funch | first5=Peter | last6=Nyengaard | first6=Jens R. | last7=Obst | first7=Matthias | last8=Thorndyke | first8=Michael C. | journal=Nature Communications | volume=4 | article-number=1537 | pmid=23443565 | bibcode=2013NatCo...4.1537N | pmc=3586728 }}</ref><ref>{{Cite journal |last1=Ueki |first1=Tatsuya |last2=Arimoto |first2=Asuka |last3=Tagawa |first3=Kuni |last4=Satoh |first4=Noriyuki |date=October 2019 |title=Xenacoelomorph-Specific Hox Peptides: Insights into the Phylogeny of Acoels, Nemertodermatids, and Xenoturbellids |url=https://bioone.org/journals/zoological-science/volume-36/issue-5/zs190045/Xenacoelomorph-Specific-Hox-Peptides--Insights-into-the-Phylogeny-of/10.2108/zs190045.full |journal=Zoological Science |language=en |volume=36 |issue=5 |page=395 |doi=10.2108/zs190045 |pmid=33319963 |issn=0289-0003 |archive-url=http://web.archive.org/web/20250323134428/https://bioone.org/journals/zoological-science/volume-36/issue-5/zs190045/Xenacoelomorph-Specific-Hox-Peptides--Insights-into-the-Phylogeny-of/10.2108/zs190045.full |archive-date=2025-03-23}}</ref>
*[[Gastrotricha]], traditionally viewed as blastocoelomates
*[[Entoprocta]], traditionally viewed as blastocoelomates
*[[Gnathostomulida]], traditionally viewed as blastocoelomates
*[[Cycliophora]]<ref>R.C.Brusca, G.J.Brusca 2003, p.&nbsp;379.</ref>

A body cavity is also absent in [[placozoa]]ns, [[cnidaria]]ns ([[jellyfish]] and allies) and the [[Ctenophora|ctenophores]] (comb jellies), but these animals are neither [[bilateria]]ns or [[Triploblasty|triploblastic]].

== See also ==
* [[Abdominal cavity]]

== References ==
{{Reflist|2}}

== Further reading ==
* {{cite book|author1=Dudek, Ronald W. |author2=Fix, James D. |chapter=Body Cavities|title=Embryology|publisher=Lippincott Williams & Wilkins|year=2004|isbn=978-0-7817-5726-3|chapter-url=https://books.google.com/books?id=sYCuFFNe4wgC&pg=PA203}}
* {{cite book|title=Strickberger's evolution: the integration of genes, organisms and populations|chapter=Animals Based on Three Germ Layers and a Coelem|publisher=Jones & Bartlett Learning|year=2008|isbn=978-0-7637-0066-9|chapter-url=https://books.google.com/books?id=LnPeZdz5X4sC&pg=PA379|author=Hall, B.K.|display-authors=etal}}
* {{cite book|editor=Overhill, Raith|chapter=What are the advantages of the coelem and metamarism?|title=An introduction to the invertebrates|edition=2nd|publisher=Cambridge University Press|year=2006|isbn=978-0-521-85736-9|chapter-url=https://books.google.com/books?id=bZw-ntFxp-YC&pg=PA108}}

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