Final answer:
The serosa cavities are formed from the serous membranes, which protect and cushion the internal organs.
Step-by-step explanation:
The serosa cavities are formed from the serous membranes, which are thin membranes that cover the walls and organs in the thoracic and abdominopelvic cavities. These membranes consist of a parietal layer that lines the walls of the body cavity and a visceral layer that covers the organs. Between these layers is a fluid-filled serous space or cavity. The serous membranes, including the pleura, pericardium, and peritoneum, form the serosa cavities that protect and cushion the internal organs, reducing friction when they move.