[Illustration: FIG. 9.--Lung of _Neoceratodus_, opened in its lower half to show its cellular pouches. a, Right half; b, Left half; c, Cellular pouches; e, Pulmonary vein; f, Arterial blood-vessel; oe, Oesophagus, opened to show glottis (gl.)] In the Crossopterygian the ventrally placed slit-like glottis leads into a common chamber produced anteriorly into two horns and continued backwards into two "lungs." These are smooth, thin-walled, saccular structures, the right one small, the left very large and extending to the hind end of the splanchnocoele. In the Dipnoans the lung has taken a dorsal position close under the vertebral column and above the splanchnocoele. Its walls are sacculated, almost spongy in _Lepidosiren_ and _Protopterus_, so as to give increase to the respiratory surface. In _Nexeratodus_ (fig. 9) an indication of division into two halves is seen in the presence of two prominent longitudinal ridges, one dorsal and one ventral. In _Lepidosiren_ and _Protopterus_ the organ is completely divided except at its anterior end into a right and a left lung. The anterior portion of the lung or lungs is connected with the median ventral glottis by a short wide vestibule which lies on the right side of the oesophagus. In the Teleostei the representative of the lung, here termed the swimbladder, has for its predominant function a hydrostatic one; it acts as a float. It arises as a diverticulum of the gut-wall which may retain a tubular connexion with the gut (_physostomatous_ condition) or may in the adult completely lose such connexion (_physoclistic_). It shows two conspicuous differences from the lung of other forms: (1) it arises in the young fish as a dorsal instead of as a ventral diverticulum, and (2) it derives its blood-supply not from the sixth aortic arch but from branches of the dorsal aorta.