Coelom

Skin
(ectoderm)

Muscle
(mesoderm)

Coelom

Internal
organ

Membrane
(peritoneum)

Gut
(endoderm)

A coelomate animal is basically a set of concentric tubes, with a gap between the gut and the outer tubes.

The coelom (/ˈsiːləm/ SEE-ləm, plural coeloms or coelomata /siːˈloʊmətə/ see-LOH-mə-tə) (also celom, -s) (Greek koiloma, cavity) is a fluid-filled body cavity, formed between the intestines and the body wall of some higher metazoans. It is lined with a mesodermal epithelium. The coelom is formed within the mesoderm of the embryo. In mammals, the coelom then develops into the peritoneal, pleural, and pericardial cavities. The term usually refers to the space located between an animal’s outer covering (epidermis), and the outer lining of the gut cavity, where internal organs develop. The human body cavity, normally refers to the ventral body cavity, because it is the much largest cavity in volume. In less developed animals, such as molluscs, it remains undifferentiated.

Physiology

Functionally, a coelom can absorb shock or provide a hydrostatic skeleton. In 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.

Biology and zoology

Coeloms developed in diploblasts 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.

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.^[1] 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 humans and everything in between).^[2]

See also

• Abdominal cavity

References

Further reading

• Dudek, Ronald W.; Fix, James D. (2004). "Body Cavities". Embryology. Lippincott Williams & Wilkins. ISBN 978-0-7817-5726-3. 
• Hall, B.K. et al. (2008). "Animals Based on Three Germ Layers and a Coelem". Strickberger's evolution: the integration of genes, organisms and populations. Jones & Bartlett Learning. ISBN 978-0-7637-0066-9. 
• Overhill, Raith, ed. (2006). "What are the advantages of the coelem and metamarism?". An introduction to the invertebrates (2nd ed.). Cambridge University Press. ISBN 978-0-521-85736-9.