310 FISHES CHAP. 



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 hydro- 

 static organ or " float." From this point of view experimental 

 investigations * 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 adjust- 

 ment 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 slow- 

 ness 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 



1 Moreau, op. cit. 



