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Coelom

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Cœlom
Cross-section of an oligochaete worm. The worm's body cavity surrounds the central typhlosole.
Details
Pronunciation(/ˈsiːləm/ SEE-ləm, plural cœloms or cœlomata /siːˈloʊmətə/ see-LOH-mə-tə)
Identifiers
Greekkoilōma
Anatomical terminology

The cœlom (or celom)[1] is the main body cavity in many animals[2] and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated. In the past, and for practical purposes, cœlom characteristics have been used to classify bilaterian animal phyla into informal groups.

Etymology[edit]

The term cœlom derives from the Ancient Greek word κοιλία (koilía) 'cavity'.[3][4][5]

Structure[edit]

Development[edit]

The cœlom is the mesodermally lined cavity between the gut and the outer body wall.

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

In protostomes, the cœlom forms by a process known as schizocoely.[6] 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 cœlom or body cavity.

In deuterostomes, the cœlom forms by enterocoely.[6] The archenteron wall produces buds of mesoderm, and these mesodermal diverticula hollow to become the cœlomic cavities. Deuterostomes are therefore known as enterocœlomates. Examples of deuterostome cœlomates belong to three major clades: chordates (vertebrates, tunicates, and lancelets), echinoderms (starfish, sea urchins, sea cucumbers), and hemichordates (acorn worms and graptolites).

Origins[edit]

The evolutionary origin of the cœlom is uncertain. The oldest known animal to have had a body cavity was the Vernanimalcula. Current hypothesis include:[citation needed][7]

Functions[edit]

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

Cœlomic fluid[edit]

The fluid inside the cœlom is known as cœlomic fluid. This is circulated by mesothelial cilia or by contraction of muscles in the body wall.[clarification needed][9] The cœlomic 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.[10]

Classification in zoology[edit]

Further information: Body cavity

In the past, some zoologists grouped bilaterian animal phyla based on characteristics related to the cœlom for practical purposes, knowing, and explicitly stating, that these groups were not 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 Acœlomata, Pseudocœlomata, and Cœlomata. 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".[11]

An illustration describing the classification of tripoblasts.
Classification of tripoblasts based on body cavities

Cœlomate animals or Cœlomata (also known as eucœlomates – "true cœlom") have a body cavity called a cœlom with a complete lining called peritoneum derived from mesoderm (one of the three 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 vertebrates, are cœlomates.

Pseudocœlomate animals have a pseudocœlom (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 cœlomate. All pseudocœlomates are protostomes; however, not all protostomes are pseudocœlomates. An example of a pseudocœlomate is the roundworm. pseudocœlomate animals are also referred to as blastocœlomate.

Acœlomate animals, like flatworms, have no body cavity at all. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.

Cœlomates[edit]

Cœloms developed in triploblasts but were subsequently lost in several lineages. The lack of a cœlom is correlated with a reduction in body size. Cœlom is sometimes incorrectly used to refer to any developed digestive tract. Some organisms may not possess a cœlom or may have a false cœlom (pseudocœlom). Animals having cœloms are called cœlomates, and those without are called acœlomates. There are also subtypes of cœlom:[citation needed]

Cœlomate phyla[edit]

According to Brusca and Brusca,[12] the following bilaterian phyla possess a coelom:

Pseudocœlomates[edit]

In some protostomes, 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 organs serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton.

A pseudocœlomate or blastocœlomate is any invertebrate animal with a three-layered body and a pseudocœl. The cœlom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocœlomates evolved from cœlomates.[15] "Pseudocœlomate" is no longer considered a valid 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

Pseudocœlomate phyla[edit]

Bilaterian pseudocœlomate phyla according to Brusca and Brusca,:[12]

Acœlomates[edit]

Acœlomates 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, acœlomate organs are not protected from crushing forces applied to the animal's outer surface. The cœlom 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.

A body cavity is also absent in placozoans, cnidarians (jellyfish and allies) and the ctenophores (comb jellies), but these animals are neither bilaterians or triploblastic.

See also[edit]

References[edit]

  1. ^ "celom". Merriam-Webster.com Dictionary.
  2. ^ "coelom" – via The Free Dictionary.
  3. ^ Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 2010035283. OCLC 461974285.
  4. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 2018-01-14.
  5. ^ Chisholm, Hugh, ed. (1911). "Cœlom and Serous Membranes" . Encyclopædia Britannica. Vol. 6 (11th ed.). Cambridge University Press. p. 642.
  6. ^ a b Lüter, Carsten (2000-06-01). "The origin of the cœlom in Brachiopoda and its phylogenetic significance". Zoomorphology. 120 (1): 15–28. doi:10.1007/s004359900019. ISSN 1432-234X. S2CID 24929317.
  7. ^ "Origins and Evolution of Animals". Archived from the original on 2018-11-12.
  8. ^ "McGraw-Hill Dictionary of Scientific and Technical Terms". Answers.com. Archived from the original on 2014-12-20.
  9. ^ Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning. p. 205. ISBN 978-81-315-0104-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
  10. ^ Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology. Saunders College Publishing. p. 190. ISBN 978-0-03-030504-7.
  11. ^ Nielsen, C. (2010). "The 'new phylogeny'. What is new about it?" Palaeodiversity 3, 149–150.
  12. ^ a b R. C. Brusca, G. J. Brusca. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, 2003 (2nd ed.), p. 47, ISBN 0-87893-097-3.
  13. ^ Nemertea
  14. ^ Cœlom development in the priapulid worm Priapulus caudatus
  15. ^ Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.
  16. ^ Sielaff, Malte; Schmidt, Hanno; Struck, Torsten H.; Rosenkranz, David; Mark Welch, David B.; Hankeln, Thomas; Herlyn, Holger (2016). "Phylogeny of Syndermata (Syn. Rotifera): Mitochondrial gene order verifies epizoic Seisonidea as sister to endoparasitic Acanthocephala within monophyletic Hemirotifera". Molecular Phylogenetics and Evolution. 96: 79–92. Bibcode:2016MolPE..96...79S. doi:10.1016/j.ympev.2015.11.017.
  17. ^ Study of Rotifers of Safari Zoo Lake Lahore in Relation to Physico-chemical Parameters
  18. ^ Biology of Non-Chordates. PHI Learning Pvt. November 2017. ISBN 978-93-87472-01-3.
  19. ^ Comparative genomic studies on Dicyema japonicum: the phylogenetic position of dicyemids and the genomic adaptations to parasitic lifestyle
  20. ^ Nakano, Hiroaki; Lundin, Kennet; Bourlat, Sarah J.; Telford, Maximilian J.; Funch, Peter; Nyengaard, Jens R.; Obst, Matthias; Thorndyke, Michael C. (2013). "Xenoturbella bocki exhibits direct development with similarities to Acœlomorpha". Nature Communications. 4: 1537. Bibcode:2013NatCo...4.1537N. doi:10.1038/ncomms2556. PMC 3586728.
  21. ^ Xenacoelomorph-Specific Hox Peptides: Insights into the Phylogeny of Acoels, Nemertodermatids, and Xenoturbellids
  22. ^ R.C.Brusca, G.J.Brusca 2003, p. 379.

Further reading[edit]

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