Final answer:
The primary lining cells in the peritoneal cavity are mesothelial cells, but neutrophils, lymphocytes, and erythrocytes can also be found.
Step-by-step explanation:
The cells seen lining the peritoneal cavity are known as mesothelial cells. Mesothelial cells form the epithelial layer of the serous membranes such as the peritoneum, which line the body cavities. Other cells that can be found in peritoneal fluid under certain conditions include neutrophils, particularly during inflammation or infection, and lymphocytes that are part of the immune response. Erythrocytes (red blood cells) may also be present, especially if there is bleeding within the cavity. However, the main cells that line the peritoneum are the mesothelial cells, and they are crucial for providing a smooth, lubricated surface for the organs within the abdominal cavity.