Covering the abdominal cavity is the parietal peritoneum. The mesentery is a double layer of visceral peritoneum that attaches to the gastrointestinal tract. Primary peritoneal cancer is a cancer of the cells lining the peritoneum. The mesentery is part of the peritoneum. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum. The perimetrium is a serous layer of visceral peritoneum. Peritonitis is the inflammation of the peritoneum. The viscera are also covered by visceral peritoneum. The inside wall is covered by the parietal peritoneum. The abdominal cavity is lined with a protective membrane termed the peritoneum. If the infection spreads to the lining of the abdominal cavity (the peritoneum), peritonitis results. Organs are retroperitoneal if they have peritoneum on their anterior side only. It is thinner than the parietal peritoneum. It represents aggravation of the parietal layer of peritoneum by stretching or moving. Spread is usually to regional lymph nodes, peritoneum, and particularly the ovary.