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 (¢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. 
