1089 
resting upon the fish, increased its volume. When the fish sank to 
the bottom again the meniscus returned to 4. Such meniscus- 
fluctuations did not take place if fishes without air-bladders were 
put into the vessel. These volume-changes being always synehronal 
with the vertical movements, Mornav was led to conclude that they 
were passive. 
According to Mornav’s view, therefore, the fish could not avail 
itself of its air-bladder to change its S.G. in accordance with the 
needs of its locomotion. Morpav, discovered however, in the course 
of his further experiments that fishes can indeed modify the gas- 
pressure in the air-bladder. And this happens in such a manner that 
the fish adapts its S.G. to the water-stratum in which it is. It tries 
to remain floating in this stratum. If it is placed in deeper water 
where it is subjected to a greater pressure, the tension of the gases 
in the bladder increases. The volume which had decreased, owing 
to the greater pressure from outside, increases again till the S.G. 
is 1 again in the stratum in which the fish now finds itself. 
If the fish is suddenly placed at the surface of the water two things 
may happen; if the fish is a physostomus, then it lets escape through 
the ductus pneumaticus, which connects the air-bladder with the 
esophagus, as much gas as is necessary to bring it in a state of 
equilibrium in the upper stratum; if the fish is a-physoclistus, 
missing the above-mentioned safetyvalve, then it can get into equili- 
brium with its new.stratum by a slow resorption of the surplus gas. 
In a natural state, therefore, every fish will, according to Morgauv, 
have one certain depth where it lives by preference. This plane is 
called by him “plan des moindres efforts.” 
Subsequent investigations by CHARBONNEL-SALLE, BAGLIONL and 
Guréror strengthened Morntav’s theory. Only JarGer thinks that 
BoreLii’s view has not been conclusively disproved. 
Baauiont, who sides with Moreau, draws the attention to the 
swimming-movements of a fish when it is exposed to a higher or 
lower pressure than that to which it is adapted. Besides being a 
hydrostatic organ the air-bladder is, according to him, an organ of 
sense which enables the fish to perceive modifications in the pressure. 
Jancrr declares himself unconvinced by the experiments of 
Mornav and CHARBONNEL-SALLE, and demands that the meniscus- 
changes of the water in Morrau’s experiment shall correspond 
exactly with the physical volume-moditications of the air-bladder 
during the removal to another plane. It is not such a simple thing 
of course to demonstrate this. JAEGER tries to prove, by the following 
experiment, that the fish modifies its volume by active muscular 
