| Coelom | |
|---|---|
 Cross-section of anoligochaete worm. The worm'sbody cavity surrounds the centraltyphlosole. | |
| Details | |
| Pronunciation | (/ˈsiːləm/SEE-ləm, pluralcoeloms orcoelomata/siːˈloʊmətə/see-LOH-mə-tə) |
| Identifiers | |
| Greek | koilōma |
| Anatomical terminology | |
Thecoelom (orcelom)[1] is the mainbody cavity in many animals[2] and is positioned inside thebody to surround and contain thedigestive tract and otherorgans. In someanimals, it is lined withmesothelium. In other animals, such asmolluscs, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classifybilaterian animal phyla into informal groups.
The termcoelom derives from theAncient Greek wordκοιλία (koilía) 'cavity'.[3][4][5]
The coelom is the mesodermally lined cavity between the gut and the outer body wall.
During thedevelopment of the embryo, coelom formation begins in thegastrulation stage. The developing digestive tube of anembryo forms as a blind pouch called thearchenteron.
Inprotostomes, the coelom forms by a process known asschizocoely.[6] Thearchenteron initially forms, and themesoderm splits into two layers: the first attaches to the body wall orectoderm, forming theparietal layer and the second surrounds theendoderm oralimentary canal forming thevisceral layer. The space between the parietal layer and the visceral layer is known as the coelom or body cavity.
Indeuterostomes, the coelom forms byenterocoely.[6] The archenteron wall produces buds ofmesoderm, and these mesodermaldiverticula hollow to become the coelomic cavities. Deuterostomes are therefore known asenterocoelomates. Examples of deuterostome coelomates belong to three major clades:chordates (vertebrates,tunicates, andlancelets),echinoderms (starfish,sea urchins,sea cucumbers), andhemichordates (acorn worms andgraptolites).
Theevolutionary origin of the coelom is uncertain. The oldest known animal to have had a body cavity was theVernanimalcula. Current hypothesis include:[citation needed][7]
A coelom can absorb shock or provide ahydrostatic skeleton. It can also support animmune system in the form ofcoelomocytes 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 oftardigrades (water bears) which is suspended within the body in themesentery derived from a mesoderm-lined coelom.
The fluid inside the coelom is known as coelomic fluid. This is circulated by mesothelialcilia or by contraction of muscles in the body wall.[clarification needed][9] 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.[10]
In the past, some zoologists groupedbilaterian animal phyla based on characteristics related to the coelom for practical purposes, knowing, and explicitly stating, that these groups werenotphylogenetically 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 amolecular phylogeny study mistakenly called this classification scheme the "traditional, morphology-based phylogeny".[11]
Coelomate animals orCoelomata (also known as eucoelomates – "true coelom") have a body cavity called a coelom with a complete lining calledperitoneum derived from mesoderm (one of the threeprimary 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 thevertebrates, are coelomates.
Pseudocoelomate animals have apseudocoelom (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 areprotostomes; however, not all protostomes are pseudocoelomates. An example of a pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to asblastocoelomate.
Acoelomate animals, likeflatworms, have no body cavity at all. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.
Coeloms developed intriploblasts 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 (pseudocoelom). Animals having coeloms are calledcoelomates, and those without are calledacoelomates. There are also subtypes of coelom:[citation needed]
According toBrusca and Brusca,[12] the followingbilaterianphyla possess a coelom:
In someprotostomes, the embryonicblastocoele 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 internalorgans serves several functions like distribution of nutrients and removal of waste or supporting the body as ahydrostatic skeleton.
Apseudocoelomate orblastocoelomate is anyinvertebrateanimal with a three-layered body and apseudocoel. The coelom was apparently lost or reduced as a result ofmutations in certain types ofgenes that affected early development. Thus, pseudocoelomates evolved from coelomates.[15] "Pseudocoelomate" is no longer considered a validtaxonomic group, since it is notmonophyletic. However, it is still used as a descriptive term.
Important characteristics:
Bilaterian pseudocoelomate phyla according to Brusca and Brusca,:[12]
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.
A body cavity is also absent inplacozoans,cnidarians (jellyfish and allies) and thectenophores (comb jellies), but these animals are neitherbilaterians ortriploblastic.
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