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The coelom (// SEE-ləm, plural coeloms or coelomata // see-LOH-mə-tə) (also celom, -s) (Greek koiloma, cavity) is a fluid-filled body cavity 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. In other animals, e.g., molluscs, it remains undifferentiated.
Functionally, a coelom can absorb shock or provide a hydrostatic skeleton. It also allows muscles to grow independently off the body wall. This can be seen in the digestive tract of water bears and other tardigrades, which is suspended within the body in a mesentery derived from a mesoderm-lined coelom.
Coeloms developed in diploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with 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 coelomata are called coelomates.
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 humans and everything in between).