Which of the following is not a part of the diffusion membrane (respiratory membrane)?

Mesothelium is the epithelium that lines the coelom. It is not a part of the respiratory membrane. The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.