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 question; and the indications are that it began long ago to degenerate, through the loss of its primitive function.
"What was this primitive function? In attempting to answer this question we must first consider the air-bladder in relation to the fish tribe as a whole. No shark or ray possesses the air-bladder. In some few sharks, indeed, there is a diverticulum of the pharynx which may be a rudimentary approach to the air-bladder; but this is very questionable. The conditions of its occurrence in the main body of modern fishes, the Teleostean, we have already considered. But in the most ancient living orders of fishes it exists in an interesting condition. In every modern Dipnoan, Crossopterygian, and Ganoid the air-bladder has an effective pneumatic duct. This in the Ganoids opens into the dorsal side of the œsophagus, but in the Dipnoans and Crossopterygians, like the windpipe of lung-breathers, it opens into the ventral side. In the Dipnoans, also survivors from the remote past, the duct not only opens ventrally into the œsophagus, but the air-bladder does duty as a lung. Externally it differs in no particular from an air-bladder; but internally it presents a cellular structure which nearly approaches that of the lung of the batrachians. There are three existing representatives of the Dipnoans. One of these, the Australian lung-fish (Neoceratodus) has a single bladder, which, however, is provided with breathing-pouches having a symmetrical lateral arrangement. It has no pulmonary artery, but receives branches from the arteria cœliaca. In the other two forms, Lepidosiren and Protopterus, the kindred 'mudfishes' of the Amazon basin and tropical Africa, the bladder or lung is divided into two lateral chambers, as in the land animals, and is provided with a separate pulmonary artery.
"The opinion seems to have been tacitly entertained by physiologists that this employment of the air-bladder by the Dipnoans as a lung is a secondary adaptation, a side issue from its original purpose. It is more likely that this is the original purpose, and that its degeneration is due to the disappearance of the necessity of such a function. As regards the gravitative employment of the bladder, the Teleostean fishes, to which this function is confined, are of comparatively modern origin; while the Dipnoans are surviving representatives of a very ancient order of fishes, which flourished in the Devonian age of geology, and in all probability breathed air then as now; and the Crossopterygians and Ganoids, which approach them in this particular, are similarly ancient in origin, and were the ancestors of the Teleosteans. The natural presumption, therefore, is that the duty which it subserved in the most ancient fishes was its primitive function.
"The facts of embryology lend strong support to this hypothesis. For the air-bladder is found to arise in a manner very similar to the development of the lung. They each begin as an outgrowth from the fore part of the alimentary tract, the only difference being that the air-bladder usually rises dorsally and the lung ventrally. The fact already cited, that the pneumatic duct is always present in the larval form in fishes that possess a bladder, is equally significant. All the facts go to show that the introduction of external air into the body was a former function of the air-bladder, and that the atrophy of the duct in many cases, and the disappearance of the bladder in others, are results of the loss of this function.
"Such an elaborate arrangement for the introduction of air into the body could have, if we may judge from analogy, but one purpose, that of breathing, to which purpose the muscular and other apparatus for compressing and dilating the bladder, now seemingly adapted to gravitative uses, may have been originally applied. The same may be said of the great development of blood-capillaries in the inner tunic of the bladder. These may now be used only for the secretion of gas into its interior, but were perhaps originally employed in the respiratory secretion of oxygen. In fact all the circumstances mentioned—the similarity in larval development between the bladder and lung, the larval existence of the pneumatic duct, the arrangements for compressing and dilating the bladder, and the capillary vessels on its inner tunic—point to the breathing of air as its original purpose.
"It is probable that the Ganoid, as well as the Dipnoan, air-bladder is to some extent still used in breathing. The Dipnoans have both lungs and gills, and probably breathe with the latter in ordinary cases, but use their lungs when the inland waters in which they live become thick and muddy, or are charged with gases from decomposing organic matter. The Ganoid fishes to some extent breathe the air. In Polypterus the air-bladder resembles the Dipnoan lung in having lateral divisions and a ventral connection with the œsophagus, while in Lepisosteus (the American garpike) it is cellular and lung-like. This fish keeps near the surface, and may be seen to emit air-bubbles, probably taking in a fresh supply of air. The American bowfin, or mudfish (Amia), has a bladder of the same lung-like character, and has been seen to come to the surface, open its jaws widely, and apparently swallow a large quantity of air. He considers that both Lepisosteus and Amia inhale and exhale air at somewhat regular intervals, resembling in this the salamanders and tadpoles, 'which, as the gills shrink and the lungs increase, come more frequently to the surface for air.'
"As the facts stand there is no evident line of demarcation between the gas-containing bladders of many of the Teleosteans, the air-containing bladders of the others and the Ganoids, and the lung of the Dipnoans, and the indications are in favor of their having originally had the same function, and of this being the breathing of air.
"If now we ask what were the conditions of life under which this organ was developed, and what the later conditions which rendered it of no utility as a lung, some definite answer may be given. The question takes us back to the Devonian and Silurian geological periods, during which the original development of the bladder probably took place. In this era the seas were thronged with fishes of several classes, the Elasmobranchs among others, followed by the Dipnoi and Crossopterygians. The sharks were without, the Dipnoans and Crossopterygians doubtless with, an air-bladder—a difference in organization which was most likely due to some marked difference in their life-habits. The Elasmobranchs were the monarchs of the seas, against whose incursions the others put on a thick protective armor, and probably sought the shallow shore waters, while their foes held chief possession of the deeper waters without.
"We seem, then, to perceive the lung-bearing fishes, driven by their foes into bays and estuaries, and the waters of shallow coasts, ascending streams and dwelling in inland waters. Here two influences probably acted on them. The waters they dwelt in were often thick with sediment, and were doubtless in many instances poorly aerated, rendering gill-breathing difficult. And the land presented conditions likely to serve as a strong inducement to fishes to venture on shore. Its plant-life was abundant, while its only animal inhabitants seem to have been insects, worms, and snails. There can be little doubt that the active fish forms of that period, having no enemies to fear on the land, and much to gain, made active efforts to obtain a share of this vegetable and animal food. Even to-day, when they have numerous foes to fear, many fishes seek food on the shore, and some even climb trees for this purpose. Under the conditions of the period mentioned there was a powerful inducement for them to assume this habit.