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The roles of celomic membranes and celomic structures in preventing cancer metastasis were examined. The relationship between a tunnel in the peritoneal cavity of hens and the cancer metastasis was also investigated. A tunnel between the right and left dorsal hepatic cavities was found in almost half of healthy hens. The tunnel was also found in 17% of hens with cancer. Neoplastic cells were detected in the ascitic fluid of 79% of hens with effusions. Metastasis was most severe in the intestinal peritoneal and left dorsal hepatic cavities, whereas the right dorsal hepatic cavity showed the least involvement. Severe inflammatory responses were found within the metastatic membranes, resulting in an increase in their thickness. In tumor-bearing hens, crevasse-like splits were present within the membranes at the layers of loose connective tissues. It appears that the splits prevent further invasion by the cancer cells. There were no successful lesions on the inner surfaces of air sacs. It is suggested that the tunnel in the peritoneal cavity enhances cancer metastasis. We also conclude that the celomic membranes and their structure in hens retard the spread of cancer by functioning as a barrier against direct spread of the cancer cells.