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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. In developed animals, it is lined with a mesodermal epithelium. In less developed animals, such as molluscs, it remains undifferentiated.[clarification needed]
Functionally, 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.
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. 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).