The lung of marine snakes varies in structure and function related to diversity of phylogeny, behavior, and environment. All species are secondarily marine and retain dependence on aerial breathing, although some also exchange gases across the skin. Generally, there is an elongated functional right lung, and the left lung is vestigial or absent. Respiratory gases are exchanged in the 'vascular lung' segment, which may include the trachea, whereas the more distal 'saccular lung' has a poorly vascularized muscular wall, terminates blindly, and facilitates storage of oxygen. In terrestrial scansorial species of snakes, the vascularized segment of the lung is relatively short to prevent gravity-induced edema when the body is vertical or upright. In contrast, the vascular lung in marine snakes is relatively long. In the file snake