The coelom (// SEE-ləm, plural coeloms or coelomata // see-LOH-mə-tə) (also celom, -s) (Greek koilōma, hollow, cavity) refers to the main body cavity in most multicellular animals and is positioned inside the body to surround and contain the digestive tract and other organs. By definition, a coelom is lined with a mesodermal epithelium.
In Protostomes the coleom forms by a process known as schizocoely. 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 forming the visceral layer or alimentary canal. The space between the parietal layer and the visceral layer is known as the coelom or body cavity.
- The acoelomate theory, which states thatcoelom evolved from an acoelomate ancestor.
- The enterocoel theory, which states that coelom evolved from gastric pouches of cnidarian ancestors. This is supported by research on flatworms and small worms recently discovered in marine fauna ("coelom")
A coelom can absorb shock or provide a hydrostatic skeleton. It can also support an immune system in the form of coelomocytes 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 tardigrades (also known as water bears) which is suspended within the body in the mesentery derived from a mesoderm-lined coelom.
The fluid inside the coelum is known as coelomic fluid. This is circulated by mesothelial cila or by contraction of muscles in the body wall which are themselves of mesothelial origin. 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 between different parts of the body, it allows storage of sperm and eggs during maturation and it acts as a reservoir for waste.
Classification in biology and zoology
In the past, zoologists grouped animals based on characteristics related to the coelom. The presence or absence of a coelom and the way in which it was formed were believed to be important in understanding the phylogenetic relationships of animal phyla. However, recent molecular phylogenies have suggested this characteristic is not as informative as previously believed: the coelom may have arisen twice, once in protostomes and once among the deuterostomes. The coelomate phyla comprise Entoprocta, Ectoprocta, Phoronida, Brachiopoda, Mollusca, Priapulida, Sipuncula, Echiura, Annelida, Tardigrada, Pentastoma, Onychophora, Arthropoda, Pogonophora, Echinodermata, Chaetognatha, Hemichordata and Chordata (i.e., from tiny sessile aquatic animals to great whales and everything in between).
When used, organisms can be classified according to the type of body cavity they possess:
- Coelomate animals or Coelomata (also known as eucoelomates — "true coelom") have a fluid filled body cavity called a coelom // 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 coelomates.
- Pseudocoelomate animals have a pseudocoelom (literally “false cavity”), which is a fully functional body cavity. Tissue derived from mesoderm only 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 protostomes; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate.
- Acoelomate animals, like flatworms, have no body cavity at all. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.
Coeloms developed in triploblasts 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 called coelomates, and those without are called acoelomates.
- pseudocoelomate nemathelminthes(roundworms)
- haemocoelomate true coelom reduced & cavity filled with blood found from arthropoda and mollusca
A pseudocoelomate or blastocoelomate is any invertebrate animal with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates. "Pseudocoelomate" is no longer considered a valid taxonomic group, since it is not monophyletic. However, it is still used as a descriptive term.
- lack a vascular blood system
- 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
- Nematoda (roundworms)
- Rotifera (rotifers)
- Nematomorpha (nematomorphs or horsehair worms)
- Acanthocephala (spiny-headed worms)
According to Brusca and Brusca, pseudocoelomate phyla include:
No coelom (Acoelomates)
Acoelomates lack a fluid-filled body cavity between the body wall and digestive tract. This can present 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.
Acoelomates include the cnidarians (jellyfish and allies), and the ctenophores (comb jellies), platyhelminthes (flatworms including tapeworms, etc.), Nemertea, and Gastrotricha. According to Brusca and Brusca, phyla without a coelum include:
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