In all fishes the arteries of the air-bladder take their origin from the aorta or the system of the aorta, and its veins return either to the portal, or vertebral, or hepatic veins; like the other organs of the abdominal cavity it receives arterial blood and returns venous blood. However, in many fishes the arteries as well as veins break up below the inner membrane into _retia mirabilia_ in various ways. The terminal ramifications of the arteries may dissolve into fan-like tufts of capillaries over almost every part of the inner surface, as in Cyprinoids. Or these tufts of radiating capillaries are more localised at various places, as in _Esocidæ_; or the tufts are so aggregated as to form gland-like, _red bodies_, the capillaries reuniting into larger vessels, which again ramify freely round the border of the red body; the red bodies are formed not only by minute arteries but also by minute veins, both freely anastomosing with its kind, and being inextricably interwoven. The rest of the inner surface of the air-bladder receives its blood, not from the red bodies, but from normally ramifying vessels. This kind of rete mirabile or “vaso-ganglion” is found in the Perch and Gadoids; it is generally distributed in closed air-bladders, but also sometimes observed in air-bladders’ with pneumatic duct. In _Anguilla_ and _Conger_ two similar vaso-ganglia are situated at the sides of the opening of the pneumatic duct. [Illustration: Fig. 65.--Lung of Ceratodus opened in its lower half to show its cellular pouches. _a_, Right half; _b_, Left half; _c_, Cellular pouches; _e_, Vena pulmonalis; _f_, Arterial blood-vessel; _oe_, Œsophagus opened, to show glottis (_gl._)] Whilst the air-bladders of some _Ganoids_, anatomically as well as functionally, closely adhere to the Teleosteous type, that of _Amia_ is more cellular and lung-like in its interior than the Teleosteous air-bladder, and _Polypterus_ approaches the Dipnoi not only in having a laterally divided air-bladder but also in its pneumatic duct entering the _ventral_ side of the œsophagus. The air-bladder of the _Dipnoi_ possesses still more the anatomical characteristics of a lung and assumes its functions, though, as it co-exists with gills, only periodically or in an auxiliary manner. The ductus pneumaticus is a membranous bronchus, entering the ventral side of the œsophagus, and provided at its entrance with a _glottis_. In _Ceratodus_ (Fig. 65) the lung is still a single cavity, but with a symmetrical arrangement of its internal pouches; it has no pulmonal artery, but receives branches from the _arteria cœliaca_. Finally, in _Lepidosiren_ and _Protopterus_ the lung is completely divided into lateral halves, and by its cellular structure approaches most nearly that of a reptile; it is supplied with venous blood by a true pulmonary artery. [Illustration: Fig. 66.--Heart of Lepidosteus osseus. I. External aspect. II. Conus arteriosus opened.