The coelom (/ˈsiːləm/ SEE-ləm, plural coeloms or coelomata /siːˈloʊmətə/ 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 other animals, such as molluscs, it remains undifferentiated.[clarification needed]According to Brusca and Brusca, the following phyla possess a coelom: