ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
187 
all probability only partially homologous with those of Elasmobranchs 
or higher animals. 
Air-bladder of Fishes.* — Dr. J. S. Haldane gives an account of what 
has been ascertained with regard to the conditions which determine the 
separation and absorption of gas in the swimming- or air-bladder. The 
belief which has long been current is that the chief function of the air- 
bladder is to enable its possessor, by compressing or relaxing its muscles, 
to increase or diminish its specific gravity, and thus to alter the depth at 
which it swims. Delaroclie (1809) supposed that the muscles were in a 
state of tonic contraction which becomes more or less rigorous as the 
animal rises or goes down, so that the specific gravity is kept constant. 
But the careful investigations and experiments of Moreau •f showed that 
the fish makes no use of its muscles in regulating the volume of its air- 
bladder. When the fish descends, the air in the bladder is compressed, 
and the specific gravity of the body increases, so that it tends to sink 
farther and farther ; and, conversely, if the fish ascends the air expands, 
and if it goes too far it may be carried helplessly to the surface, unless 
it has a permeable air-duct. It thus behaves like the toy known as the 
“ Cartesian Diver,” and its position is at best one of unstable equilibrium 
which the smallest movement upwards or downwards could disturb. 
Compression of the air-bladder, even when it is very muscular, seems 
accidental and momentary, as by a sudden and violent movement. 
Moreau’s further experiments show that the swimming-bladder may 
assist the fish in balancing itself at any depth, but that the action is a 
very slow one. Thus, he determined the specific gravity of a fish which 
had lived at the surface for some time, sank it in a cage to a greater 
depth, and tested it again at the same pressure as before. The fish was 
much lighter, because air had been gradually secreted into the bladder 
until its specific gravity corresponded with that of the water. Again, 
when an air-bladder was artificially emptied, the fish sank at once to 
the bottom, but in a few hours was able to swim about again. A float 
was attached to one fish and a sinker to another, with the result that the 
first could not leave the surface, the second could not leave the bottom ; 
but a few hours later both were swimming about together, one having 
absorbed, the other having secreted sufficient gas to readjust matters. The 
absorption and secretion would therefore appear to be in some sense 
under voluntary control, and a fish will be able to live comfortably at 
any depth if the change be made gradually enough. 
Early in the century Biot and others discovered that the gas in the 
swimming-bladder is not air, but a variable mixture of oxygen and 
nitrogen with about 1 or 2 per cent, of carbonic acid, and that the per- 
centage of oxygen is higher the greater the depth at which the fish is 
caught. Moreau confirmed this, and showed that, if the bladder be 
artificially emptied, the gas secreted gradually into it is richer in oxygen 
than before, and much richer than air. Therefore, apparently, pure or 
nearly pure oxygen is secreted. In some fishes, however, such as 
Coregonus Acronius , the air-bladder is sometimes filled with pure nitro- 
gen. The theory of diffusion of gases does not meet the case ; for the 
* Science Progress, *vii. (1897) No. 6, pp. 120-30. ] 
f ‘ Memoires de Physiologic,’ Paris, lS77. 
