The coeloms' (/ˈsiːləm/ SEE-ləm, singular coelom"or animals with coeloms "'coelomata' /siːˈloʊmətə/ see-LOH-mə-tə) (also celom, -s) (Greek koilōma, hollow, cavity) refer to the initially paired secondary [body cavities]]in most multicellular animals and are positioned inside the body to surround and contain the digestive tract and other organs. By definition, coeloms are lined with a mesodermal epithelium. They develop bilaterally after formation of the primary body cavity, the blastocoel, which surrounds the alimentary canal, and the coeloms may partially or completely obliterate the blastocoel which may persist as mid-line blood vessels as in e.g. Annellda.According to Brusca and Brusca, the following phyla possess coeloms, though in some cases the initially paired coeloms fuse dorsally or ventrally or longitudinally and, in others, pairs of coeloms along the body are associated with metameric segmentation. However, the existence of metameric segmentation is not always associated with the presence of coeloms, as in the tail of vertebrates or the segmented bauplan of Arthropoda.