A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

"Such conditions must have strongly tended to induce fishes to breathe the air, and have acted to develop an organ for this purpose. In addition to the influences of foul or muddy water and of visits to land may be named that of the drying-out of pools, by which fishes are sometimes left in the moist mud till the recurrence of rains, or are even buried in the dried mud during the rainless season. This is the case with the modern Dipnoi, which use their lungs under such circumstances. In certain other fresh-water fishes, of the family Ophiocephalidæ, air is breathed while the mud continues soft enough for the fish to come to the surface, but during the dry period the animal remains in a torpid state. These fishes have no lungs, but breathe the air into a simple cavity in the pharynx, whose opening is partly closed by a fold of the mucous membrane. Other Labyrinthici, of similar habits, possess a more developed breathing organ. This is a cavity formed by the walls of the pharynx, in which are thin laminæ, or plates, which undoubtedly perform an oxygenating function. The most interesting member of this family is Anabas scandens, the climbing perch. In this fish, which not only leaves the water, but is said to climb trees, the air-breathing organ is greatly developed. The labyrinthici, moreover, have usually large air-bladders. As regards the occasional breathing of air by fishes, even in species which do not leave the water, it is quite common, particularly among fresh-water species. Cuvier remarks that air is perhaps necessary to every kind of fish; and that, particularly when the atmosphere is warm, most of our lacustrine species sport on the surface for no other purpose.

"It is not difficult to draw a hypothetical plan of the development of the air-bladder as a breathing organ. In the two families of fishes just mentioned, whose air-bladders indicate that they once possessed the air-breathing function and have lost it, we perceive the process of formation of an air-breathing organ beginning over again under stress of similar circumstances. The larval development of the air-bladder points significantly in the same direction. In fact we have strong reason to believe that air-breathing in fishes was originally performed, as it probably often is now, by the unchanged walls of the œsophagus. Then these walls expanded inwardly, forming a simple cavity, partly closed by a fold of membrane, like that of the Ophiocephalidæ. A step further reduced this membranous fold to a narrow opening, leading to an inner pouch. As the air-breathing function developed, the opening became a tube, and the pouch a simple lung, with compressing muscles and capillary vessels. By a continuation of the process the smooth-walled pouch became sacculated, its surface being increased by folding into breathing cells. Finally, a longitudinal constriction divided it into two lateral pouches, such as we find in the lung of the Dipnoans. This brings us to the verge of the lung of the amphibians, which is but a step in advance, and from that the line of progress is unbroken to the more intricate lung of the higher land animals.

"The dorsal position of the bladder and its duct would be a difficulty in this inquiry, but for the fact that the duct is occasionally ventral. This dorsal position may have arisen from the upward pressure of air in the swimming fish, which would tend to lift the original pouch. But in the case of fishes which made frequent visits to the shore new influences must have come into play. The effect of gravity tended to draw the organ and its duct downward, as we find in the Crossopterygians and in all the Dipnoans, and its increased use in breathing required a more extended surface. Through this requirement came the pouched and cellular lung of the Dipnoans. Of every stage of the process here outlined examples exist, and there is great reason to believe that the development of the lung followed the path above pointed out.

"When the carboniferous era opened there may have been many lung- and gill-breathing fishes which spent much of their time on land, and some of which, by a gradual improvement of their organs of locomotion, changed into batrachians. But with the appearance of the latter, and of their successors, the reptiles, the relations of the fish to the land radically changed. The fin, or the simple locomotor organ, of the Dipnoans could not compete with the leg and foot as organs of land locomotion, and the fish tribe ceased to be lords of the land, where, instead of feeble prey, they now found powerful foes, and were driven back to their native habitat, the water. Nor did the change end here. In time the waters were invaded by the reptiles, numerous swimming forms appearing, which it is likely were abundant in the shallower shore-line of the ocean, while they sent many representatives far out to sea. These were actively carnivorous, making the fish their prey, the great mass of whom were doubtless driven into the deeper waters, beyond the reach of their air-breathing foes.

"In this change of conditions we seem to perceive an adequate cause for the loss of air-breathing habits in those fishes in which the lung development had not far progressed. It may indeed have been a leading influence in the development of the Teleostean or bony fishes, as it doubtless was in the loss of its primitive function by, and the subsequent changes of, the air-bladder.

"Such of the Crossopterygians and Dipnoans as survived in their old condition had to contend with adverse circumstances. Most of them in time vanished, while their descendants which still exist have lost in great measure their air-breathing powers, and the Dipnoans, in which the development of the lung had gone too far for reversal, have degenerated into eel-like, mud-haunting creatures, in which the organs of locomotion have become converted into the feeble paddle-like limbs of Neoceratodus and the filamentary appendages of the other species.

"As regards the presence of a large quantity of oxygen in the bladders of deep-swimming marine fishes, it not unlikely has a respiratory purpose, the bladder being, as suggested by Semper, used as a reservoir for oxygen, to serve the fish when sleeping, or when, from any cause, not actively breathing. The excess of oxygen is not due to any like excess in the gaseous contents of sea-water, for the percentage of oxygen decreases from the surface downward, while that of nitrogen remains nearly unchanged. In all cases, indeed, the bladder may preserve a share of its old function, and act as an aid in respiration. Speaking of this, Cuvier says: 'With regard to the presumed assistance which the swim-bladder affords in respiration, it is a fact that when a fish is deprived of that organ, the production of carbonic acid by the branchiæ is very trifling,' thus strongly indicating that the bladder still plays a part in the oxygenation of the blood.

"Under the hypothesis here presented the process of evolution involved may be thus summed up. Air-breathing in fishes was originally performed by the unchanged walls of the œsophagus perhaps at specially vascular localities. Then the wall folded inward, and a pouch was finally formed, opening to the air. The pouch next became constricted off, with a duct of connection. Then the pouch became an air-bladder with respiratory function, and finally developed into a simple lung. These air-breathing fishes haunted the shores, their fins becoming converted into limbs suitable for land locomotion, and in time developed into the lung- and gill-breathing batrachia, and these in their turn into the lung-breathing reptilia, the locomotor organs gradually increasing in efficiency. Of these pre-batrachia we have existing representatives in the mud-haunting Dipnoi, with their feeble limbs. In the great majority of the Ganoid fishes the bladder served but a minor purpose as a breathing organ, the gills doing the bulk of the work. In the Teleostean descendants of the Ganoids the respiratory function of the bladder in great measure or wholly ceased, in the majority of cases the duct closing up or disappearing, leaving the pouch as a closed internal sac, far removed from its place of origin. In this condition it served as an aid in swimming, perhaps as a survival of one of its ancient uses. It gained also in certain cases some connection with the organ of hearing. But these were makeshift and unimportant functions, as we may gather from the fact that many fishes found no need for them, the bladder, in these cases, decreasing in size until too small to be of use in swimming, and in other cases completely disappearing after having travelled far from its point of origin. In some other cases, above cited, the process seems to have begun again, in modern times, in an eversion of the wall of the œsophagus for respiratory purposes. The whole process, if I have correctly conceived it, certainly forms a remarkable organic cycle of development and degeneration, which perhaps has no counterpart of similarly striking character in the whole range of organic life."

The Heart of the Fish.—The heart of the fish is simple in structure, small in size, and usually placed far forward, just behind the branchial cavity, and separated from the abdominal cavity by a sort of "diaphragm" formed of thickened peritoneum. In certain eels the heart is remote from the head.