%peritoneum%

n. [ Pleuro- + peritoneum. ] (Anat.) The pleural and peritoneal membranes, or the membrane lining the body cavity and covering the surface of the inclosed viscera; the peritoneum; -- used especially in the case of those animals in which the body cavity is not divided. [ 1913 Webster ]

☞ Peritoneum is now often used in the sense of pleuroperitoneum, the pleuræ being regarded as a part of the peritoneum, when the body cavity is undivided. [ 1913 Webster ]