$\textbf{{\color{#4257b2}Mucous membranes }}$line the body cavities that open to the external environment. They secrete mucus, which has a protective function. The mouth and nose are examples of cavities that are lined by mucous membranes.

$\textbf{{\color{#4257b2}Serous membranes }}$ line body cavities that do not communicate with the external environment. They secrete serous fluid, which acts as a lubricant and reduces friction during movement. Pericardium and pleura are examples of cavities lined by serous membranes.