The Air-bladder.—The air-bladder, or swim-bladder, must be classed among the organs of respiration, although in the higher fishes its functions in this regard are rudimentary, and in some cases it has taken collateral functions (as a hydrostatic organ of equilibrium, or perhaps as an organ of hearing) which have no relation to its original purpose.
Fig. 77.—Weberian apparatus and air-bladder of Carp. (From Günther, after Weber.)
The air-bladder is an internal sac possessed by many fishes, but not by all. It lies in the dorsal part of the abdominal cavity above the intestines and below the kidneys. In some cases it is closely adherent to the surrounding tissues. In others it is almost entirely free, lying almost loose in the cavity of the body. In some cases it is enclosed in a bony capsule. In the allies of the carp and catfish, which form the majority of fresh-water fishes, its anterior end is connected through a chain of modified vertebræ to the ear. Sometimes its posterior end fits into an enlarged and hollow interhæmal bone. Sometimes, again, a mass of muscle lies in front of it or is otherwise attached to it. Sometimes it is divided into two or three parts by crosswise constrictions. Sometimes it is constricted longitudinally, and at other times it has attached to it a complication of supplemental tubes of the same character as the air-bladder itself. In still other cases it is divided by many internal partitions into a cellular body, similar to the lung of the higher vertebrates, though the cells are coarser and less intricate. This condition is evidently more primitive than that of the empty sac.
The homology of the air-bladder with the lung is evident. This is often expressed in the phrase that the lung is a developed air-bladder. This is by no means true. To say that the air-bladder is a modified and degenerate lung is much nearer the truth, although we should express the fact more exactly to say that both air-bladder and lung are developed from a primitive cellular breathing-sac, originally a diverticulum from the ventral walls of the œsophagus.
The air-bladder varies in size as much as in form. In some fishes it extends from the head to the tail, while in others it is so minute as to be scarcely traceable. It often varies greatly in closely related species. The common mackerel (Scomber scombrus) has no air-bladder, while in the closely related colias or chub mackerel (Scomber japonicus) the organ is very evident. In other families, as the rockfishes (Scorpænidæ), genera with and those without the air-bladder are scarcely distinguishable externally. In general, fishes which lie on the bottom, those which inhabit great depths, and those which swim freely in the open sea, as sharks and mackerel, lack the air-bladder. In the sharks, rays, and chimæras there is no trace of an air-bladder. In the mackerel and other bony fishes without it, it is lost in the process of development.
The air-bladder is composed of two layers of membrane, the outer one shining, silvery in color, with muscular fibres, the inner well supplied by blood-vessels. The gas within the air-bladder must be in most cases secreted from the blood-vessels. In river fishes it is said to be nearly pure nitrogen. In marine fishes it is mostly oxygen, with from 6 to 10 per cent of carbonic-acid gas, while in the deep-sea fishes oxygen is greatly in excess. In Lopholatilus, a deep-sea fish, Professor R. W. Tower finds 66 to 69 per cent of oxygen. In Trigla lyra Biot records 87 per cent. In Dentex dentex, a shore fish of Europe, 40 per cent of oxygen was found in the air-bladder. Fifty per cent is recorded from the European porgy, Pagrus pagrus. In a fish dying from suffocation the amount of carbonic-acid gas (CO2) is greatly increased, amounting, according to recent researches of Professor Tower on the weak-fish, Cynoscion regalis, to 24 to 29 per cent. This shows conclusively that the air-bladder is to some degree a reservoir of oxygen secreted from the blood, to which channel it may return through a kind of respiration.
The other functions of the air-bladder have been subject to much question and are still far from understood. The following summary of the various views in this regard we copy from Professor Tower's paper on "The Gas in the Swim-bladder of Fishes":
"The function of the swim-bladder of fishes has attracted the attention of scientists for many centuries. The rôle that this structure plays in the life of the animal has been interpreted in almost as many ways as there have been investigators, and even now there is apparently much doubt as to the true functions of the swim-bladder. Consequently any additional data concerning this organ are of immediate scientific value.
"Aristotle, writing about the noises made by fishes, states that 'some produce it by rubbing the gill-arches ...; others by means of the air-bladder. Each of these fishes contains air, by rubbing and moving of which the noise is produced.' The bladder is thus considered a sound-producing organ, and it is probable that he arrived at this result by his own investigations.