(g) and (h) The structural modifications involved in the connexion of the air-bladder with the auditory organ, and its adaptation for sound-production, as well as its use in respiration, are considered elsewhere.[[353]]
The Gases of the Air-Bladder.—The gaseous contents of the air-bladder consist of oxygen and nitrogen, but the relative proportions of the two gases differ in different Fishes, and even in the same Fish, under different conditions. Normally the proportion of oxygen is considerably less in freshwater than in marine Fishes, and amongst the latter the proportion of oxygen is often enormously greater, amounting in some cases to 87 per cent., in deep-sea species as compared with their shallow water congeners. A trace of carbon dioxide is also usually present. The gases are derived from the blood as the latter circulates through the capillaries in the walls of the bladder, and it is highly probable that the "red glands" take an important part in the process; at all events, experimental research has shown that the "secretion" or diffusion of gas into the air-bladder, as well as the absorption of gas from the bladder into the blood, take place most rapidly in those Fishes in which "red glands" or "red bodies" are present.[[354]]
Fig. 185.—Vertical section through a "red gland" (diagrammatic). c, Capillary blood-vessels; g, tubular glands. (From Vincent and Barnes.)
The Functions of the Air-Bladder.—Probably no single organ in any group of Vertebrata is associated with the performance of a greater variety of functions than the air-bladder of Fishes. Originally evolved, it may be, as a glandular caecum in certain Sharks, the air-bladder in the Dipnoi, and some of the more generalised Teleostomi (e.g. Amia and Lepidosteus), and perhaps also in a few of the more specialised members of the latter group (e.g. certain Teleosts), is to a greater or less extent an accessory respiratory organ. In not a few Teleosts it is an organ for sound-production, and in others again it is sometimes regarded as having an important relation to the sense of hearing. But omitting such subordinate functions which, as it were, have been grafted on to the air-bladder, there can be no doubt that in the great majority of Fishes its primary use is to act as a hydrostatic organ or "float." From this point of view experimental investigations[[355]] seem to justify the following conclusions:—
The function of the air-bladder is to render the Fish, bulk for bulk, of the same weight as the water in which it lives. In this condition of equilibrium, or plane of least effort, the Fish floats in the water, and therefore it is able to swim with a minimum of muscular effort. It is obvious, however, that as a Fish rises or sinks it becomes exposed to an increase or a diminution of hydrostatic pressure, which will necessarily bring about the expansion or contraction of the volume of gas in the air-bladder, and, therefore, by decreasing or increasing the specific gravity of the animal, will tend to remove the Fish from its plane of least effort. To counteract this, and to restore the Fish to a plane of equilibrium at the new level, gas is either absorbed from the air-bladder, or more gas is secreted into the bladder, as the case may be. According to Moreau, by this process of automatic adjustment a Fish will always find, sooner or later, a plane of least effort, whatever may be its depth in the water; and further, this process takes place much more readily in those Fishes which possess "red glands" or "red bodies", and with extreme slowness in those in which these organs are absent. Nevertheless, it seems doubtful if this process of adjustment can be of much use to a Fish in ordinary vertical movements, inasmuch as gaseous secretion and absorption are comparatively slow processes, the length of which in different Fishes, and under different conditions, varies from a few hours to several days. On the whole it seems more probable that adjustment to the varying pressures of different depths by such means is far more likely to be useful during such slow and gradual changes of level as are encountered in the course of diurnal, seasonal, or other periodic migrations than during the rapid changes of level which may take place in ordinary vertical locomotion.[[356]] In the generality of Fishes, and more especially in the Physoclisti, it may be concluded that the possession of an air-bladder restricts freedom of movement in the vertical direction, and confines ordinary locomotion within more or less well-defined vertical limits above or below the plane of least effort for the time being. As illustrating this point, and as a proof of the danger incurred by a too rapid rise in the water, the following remarks with reference to the "Kilch," a small Salmonoid (Coregonus) inhabiting the Lake of Constance, and a favourite article of food, may be quoted:[[357]]—The Fish "are caught in nets, and brought to the surface of the water; they come up invariably with the belly much distended, the air in the swimming-bladder, being relieved from the pressure of the column of water, has expanded greatly and occasioned this unnatural distension, which renders the Fish quite incapable of swimming. Under these conditions the Fish is naturally unable to live for any length of time. But the fishermen of the lake have a very simple remedy; they prick into the air-bladder with a fine needle; the air escapes with some force, the distension subsides, and the fishes are enabled to live under totally changed conditions as to pressure, even in quite shallow water and at the surface, swimming quite as freely as their companions, the natives of the surface water. Hence the Kilch is confined to a certain depth, because it is not capable of accommodating the tension of its swimming-bladder to the change of pressure in the column of superincumbent water."
It is not improbable that the Physostomi, or at any rate most of them, are somewhat more advantageously placed in this respect. From the general absence of "red glands" in this group, it may be inferred that whatever capacity for gaseous secretion or absorption they possess must be exercised with exceptional slowness, and, therefore, as a means of pressure-adjustment may be neglected. On the other hand, they seem to possess the compensating advantage of being able to substitute for absorption the mechanical liberation of gas through the ductus pneumaticus. It would seem, therefore, that the Physostomi have a distinct advantage over the Physoclisti in that during ascent in the water they can more readily adapt themselves to the diminished pressure of a higher level by ejecting the needful amount of gas than by relying upon the process of gaseous absorption.[[358]] This conclusion is in harmony with the results of experiment and with much that is known of the habits of these Fishes and their greater freedom of locomotion in the vertical direction.
These briefly summarised conclusions as to the hydrostatic function of the air-bladder must, however, be accepted only in a general sense. There are many structural anomalies in the air-bladder of Fishes which are very difficult to explain, or to correlate with any variations in the habits or in the locomotor activities of its possessor.
In this connexion it may be mentioned that the presence or absence of an air-bladder in different Fishes seems to some extent to be governed by two causes. First, whenever the requirements of a Fish necessitate exceptional freedom of locomotion in all directions the restrictions imposed by the presence of an air-bladder are removed by its partial or complete suppression; a result produced, secondly, by the assumption of a bottom feeding or ground habit on the part of the Fish. Fishes like the Flat Fishes or Pleuronectidae, when not in motion by the exercise of their fins, habitually rest on the sea-bottom, and, as an air-bladder is useless under such conditions, it has, in consequence, undergone complete atrophy. Not a few Siluridae, and some Cyprinidae, inhabit the comparatively shallow waters of rapidly flowing mountain torrents, and are often provided with suckers for attachment to stones or rocks. To such Fishes as these a hydrostatic organ is obviously useless, and it has hence become greatly reduced in size, and in other respects approaches the condition of a vestigial organ.