174 A HISTORY OF FISHES 



will be considered in greater detail in the chapter devoted to 

 breeding {cf. p. 279). 



It has been previously pointed out that the air-bladder is an 

 organ with more than one duty, and its function as a lung 

 {cf. p. 49) and as a sound-producing organ (cf. p. 155) has 

 been already considered, while in the next chapter its connection 

 with the inner ear will be described. Its original function was 

 probably a respiratory one, but in the vast majority of Bony 

 Fishes in which this organ is present it has discarded this duty 

 and taken on another, namely, that of a hydrostatic organ or 

 float, enabling the fish to adapt itself to the changing pressure 

 encountered at different depths. The walls of the bladder 

 are richly supplied with fine blood-vessels, and at certain areas 

 these are accumulated to form the so-called red bodies or red 

 glands, masses of interlacing and tightly packed arteries and 

 veins. As far as is known, these glands occur only in those 

 fishes in which there is no duct connecting the bladder with the 

 gullet, and it is believed that they secrete gases into the bladder. 

 Another remarkable structure, the oval, seems to have the power 

 of absorbing some of the excess gas when necessary. The gas 

 has been analysed and found to contain oxygen, nitrogen, and 

 a small trace of carbon dioxide. These are the same constituents 

 as are contained in atmospheric air, but the proportion of 

 oxygen is very much greater. Further, the amount of oxygen 

 varies somewhat in different fishes, generally being greater 

 in marine than in fresh-water forms; it is considerably greater 

 in fishes living at any depth, in some cases as much as eighty- 

 seven per cent, of the whole. 



The hydrostatic function of the air-bladder is to render the 

 weight of the fish the same as the surrounding water in which 

 it lives. In this stable condition it floats in the water without 

 any tendency either to rise or to sink, and is able to swim with 

 the minimum of muscular effort. It is clear that if the fish 

 rises to a higher level the pressure of the surrounding water is 

 diminished, the gas in the bladder expands, and the body tends 

 to shoot upwards. To counteract this, and to restore the fish to 

 a plane of equilibrium at the new level, gas is absorbed by the 

 oval, or, if the bladder is connected with the gullet, some of the 

 gas is allowed to escape through this channel. In the same way, 

 if the fish swims downwards the hydrostatic pressure increases, 

 the body becomes heavier and tends to sink rapidly towards 

 the bottom, but by increasing the volume of gas in the air- 

 bladder the necessary adjustment is attained. This power of 



