Swim Bladder is the answer for sure
The pleural cavity is the thin fluid-filled space between the two pulmonary pleurae (known as visceral and parietal) of each lung. A pleura is a serous membrane which folds back onto itself to form a two-layered membranouspleural sac. The outer pleura (parietal pleura) is attached to the chest wall, but is separated from it by the endothoracic fascia. The inner pleura (visceral pleura) covers the lungs and adjoining structures, including blood vessels, bronchi and nerves. The pleural cavity can be viewed as a potential space because the two pleurae adhere to each other (through the thin film of serous liquid) under all normal conditions.
