264 BONY FISHES ix. n- 



and presumably containing venous blood (Fig. 158). Blood returns 

 to the heart by pulmonary veins. Essentially the same arrangement 

 is found in Amia, but in all other Actinopterygii oxygenated blood is 

 supplied to the bladder from the dorsal aorta (or sometimes from the 

 coeliac artery). Probably, then, the original function of the air-bladder 

 was respiratory, and this may still be its main function not only in 

 Amia and Lepisosteus but also in some of the physostomatous Tele- 

 ostei. However, in the majority of teleosts its dorsal position, closed 

 duct, and arterial blood-supply show that it has some other function 

 and it has long been supposed that this is concerned in some way with 

 flotation. The air-bladder is absent from bottom-living forms, such 

 as flat-fishes, Lophius and Uranoscopus, though it may be present in 

 their pelagic larvae. 



The bladder is provided with special glands by which it is filled 

 and the gas they secrete is mostly oxygen; only in some physosto- 

 matous forms is the bladder filled by gulping air. Nitrogen and carbon 

 dioxide are also present and the former even constitutes the main gas 

 in some freshwater fishes at great depths. In the more primitive forms 

 gas is secreted all over the surface of the bladder, but later there 

 develop special anterior oxygen-secreting and posterior oxygen- 

 absorbing regions. The former, known as the red gland, has a special 

 apparatus of blood-vessels, the rete mirabile, and the latter, or 'oval', 

 which may be developed from the closed end of the pneumatic duct, 

 has a special sphincter by means of which it can be closed off. 



The pressure of the gases in the swim-bladder is adjusted to make 

 the fish neutrally buoyant, which is achieved when the bladder 

 occupies 7-10 per cent of the total volume in fresh-water and 5 per 

 cent in marine fishes. This may involve partial pressures of oxygen, 

 carbon dioxide, and nitrogen many times greater than those in the 

 blood. The mechanism by which the gases are secreted against a 

 diffusion gradient of several atmospheres has been much discussed. 

 Carbonic anhydrase is present in the gas gland and the oxyhemo- 

 globin of fish blood is especially sensitive to carbon dioxide, giving up 

 its oxygen even at high oxygen concentration. 



If weights or floats are attached to a fish it maintains its position 

 in mid-water by swimming while gas is secreted or absorbed. The 

 receptors concerned are therefore not activated by the tension in the 

 bladder but perhaps by the movements that are necessary when the 

 fish is not in equilibrium. The bladder is innervated by the vagus and 

 sympathetic nerves and after severing the former gas secretion ceases. 



The various diverticula connecting the bladder with the ear (and 



