The lung of Neoceratodus forsteri consists of a single elongated sac dorsal to the gut and attached firmly along the dorsal mid-line in the region of the vertebral column. It communicates with the gut through the pneumatic duct which opens ventrolaterally via the glottis, on the right side of the pharynx. The embryological origin of the lung as a ventral outgrowth from the gut is reflected in the marked similarity between their tissues, and in the unusual configuration of the duct and blood vessels. Internally, the lung is divided into compartments formed by septa resulting from infolding of the walls. These compartments are further subdivided to form a spongy alveolar region. In this region of increased surface area run blood capillaries in proximity to the respiratory medium, close enough to allow gaseous exchange with it. Filling the lung is accomplished by a buccal force-pump, as in Amphibia, consisting of the hyoid apparatus and the muscular walls of the buccal cavity. Exhalation of air is effected by contraction of the smooth muscle components of the lung, assisted by its natural elasticity provided by elastin fibres present in both connective tissue and smooth muscle. The structure of the lung, its spongy walls, vascular supply with capillaries close to the air space, open pneumatic duct, regular exchange of air at the surface of the water, and the ability of the fish to survive out of water if kept moist, all point to the function of the lung as a respiratory organ. This is borne out by lung-gas analyses which consistently show lower oxygen level compared with air.