The peritoneum was the serous membrane that formed the lining of the abdominal cavity or the coelom.
In 2369, Mullibok's peritoneum was punctured by a phaser beam during a scuffle with a Bajoran Security officer. Upon arriving at Kira Nerys' behest, Bashir warned him not to move or risk aggravating his injuries. He wanted to remove Mullibok to Deep Space 9 for further medical care, but Kira refused, saying she'd remain with him on Jeraddo and see to his care. (DS9: "Progress")