=Origin of the Air-bladder.=--In the more primitive forms, and probably in the embryos of all species, the air-bladder is joined to the oesophagus by an air-duct. This duct is lost entirely in the adult of all or nearly all of the thoracic and jugular fishes, and in some of the abdominal forms. The lancelets, lampreys, sharks, rays, and chimæras have no air-bladder, but in the most primitive forms of true fishes (Dipnoans and Crossopterygians), having the air-bladder cellular or lung-like, the duct is well developed, freely admitting the external air which the fish may rise to the surface to swallow. In most fishes the duct opens into the oesophagus from the dorsal side, but in the more primitive forms it enters from the ventral side, like the windpipe of the higher vertebrates. In some of the Dipnoans the air-bladder divides into two parts, in further resemblance to the true lungs. =The Origin of the Lungs.=--The following account of the function of the air-bladder and of its development and decline is condensed from an article by Mr. Charles Morris:[5] "If now we seek to discover the original purpose of this organ, there is abundant reason to believe that it had nothing to do with swimming. Certainly the great family of the sharks, which have no bladder, are at no disadvantage in changing their depth or position in the water. Yet if the bladder is necessary to any fish as an aid in swimming, why not to all? And if this were its primary purpose, how shall we explain its remarkable variability? No animal organ with a function of essential importance presents such extraordinary modifications in related species and genera. In the heart, brain, and other organs there is one shape, position, and condition of greatest efficiency, and throughout the lower forms we find a steady advance towards this condition. Great variation, on the other hand, usually indicates that the organ is of little functional importance, or that it has lost its original function. Such we conceive to be the case with the air-bladder. The fact of its absence from some and its presence in other fishes of closely related species goes far to prove that it is a degenerating organ; and the same is shown by the fact that it is useless in some species for the purpose to which it is applied in others. That it had, at some time in the past, a function of essential importance there can be no question. That it exists at all is proof of this. But its modern variations strongly indicate that it has lost this function and is on the road towards extinction. Larval conditions show that it had originally a pneumatic duct as one of its essential parts, but this has in most cases disappeared. The bladder itself has in many cases partly or wholly disappeared. Where preserved, it seems to be through its utility for some secondary purpose, such as an aid in swimming or in hearing. That its evolution began very long ago there can be no