The peritoneum is a membrane that lines the abdominal cavity and covers the abdominal organs. It acts as a protective barrier and helps to support and stabilize the organs within the abdomen. Additionally, it produces a lubricating fluid that allows the organs to move and function smoothly.
A membrane covering the abdominal cavity.
The mesentry is fold of peritonuem and inclosed tissue which conect intestine (jejunum n ileum) n their appendages with dorsal body wall of abdominal cavity.it is 15cm long and extend from duodonojejunal flexure just lateral(left) to midline and ends at sacro-iliac joint.
The peritoneal cavity is just space which the abdominal organs occupy. So basically the membrane would be called the peritoneal membrane or shortly known as the peritonuem hope this helps ^_^