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) (also celom) is a fluid-filled cavity formed within the mesoderm of some animals. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size. Coeloms are only ever present in triploblastic animals, though coelom is sometimes (incorrectly) used to refer to any developed digestive tract.

Functionally, a coelom can absorb shock or provide a hydrostatic skeleton. It also allows organs to grow independently off the body wall. This can be seen in the digestive tract of earthworms and other annelids, which is suspended within the body in a mesentery derived from a mesoderm-lined coelom. In mammals, the coelom forms the peritoneal, pleural, and pericardial cavities.

In the past, zoologists grouped animals based on characters related to the coelom. The presence or absence of a coelom and the way in which it was formed was 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.

See also

• Abdominal cavity
• Body cavity
• Space
• Biology

Further reading

• Dudek, Ronald W.; Fix, James D. (2004). "Body Cavities". Embryology. Lippincott Williams & Wilkins. ISBN 9780781757263. http://books.google.com/books?id=sYCuFFNe4wgC&pg=PA203. 
• 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 9780763700669. http://books.google.com/books?id=LnPeZdz5X4sC&pg=PA379. 
• Overhill, Raith, ed (2006). "What are the advantages of the coelem and metamarism?". An introduction to the invertebrates (2nd ed.). Cambridge University Press. ISBN 9780521857369. http://books.google.com/books?id=bZw-ntFxp-YC&pg=PA108.