Air-bladders with a pneumatic duct are found in Ganoids and Physostomes, the duct entering the dorsal side of the intestinal tract, with the exception of _Polyp__terus_ and the _Dipnoi_, in which it enters on the ventral side of the œsophagus. In the majority the orifice is in the œsophagus, but in some, as in _Acipenser_, in the cardiac portion of the stomach, or in its blind sac, as in many Clupeoids. The air-bladder may be single, or consist of two divisions situated one behind the other (Fig. 52); its inner surface may be perfectly smooth, or form manifold pouches and cells. If two divisions are present the anterior possesses a middle elastic membrane which is absent in the posterior; each division has a muscular layer, by which it can be separately compressed, so that part of the contents of the posterior may be driven into the elastic anterior division, and _vice versa_. The posterior division being provided with the ductus pneumaticus does not require the elasticity of the anterior. [Illustration: Fig. 64.--Air-bladder of Pogonias chromis.] Some Siluroids possess a peculiar apparatus for voluntarily exercising a pressure upon the air-bladder. From the first vertebra a process takes its origin on each side, expanding at its end into a large round plate; this is applied to the side of the air-bladder, and by pressing upon it expels the air through the duct; the small muscle moving the plate rises from the skull. The connection of the air-bladder with the organ of hearing in some Physostomes has been described above, p. 117. In the modifications of the air-bladder, hitherto mentioned, the chief and most general function is a mechanical one; this organ serves to regulate the specific gravity of the fish, to aid it in maintaining a particular level in the water, in rising or sinking, in raising or depressing the front part of its body as occasion may serve. Yet a secretion of gas from the blood into its cavity must take place; and if this be so, it is not at all impossible that also an exchange of gases between the two kinds of blood is effected by means of the extraordinary development of _retia mirabilia_ in many air-bladders.