39 
the pressure. The alteration in the volume can be compensated 
by means of a secretion or absorption of gas. 
In the Corethra larva the regulation is of another nature. 
Krogh has shown that a larva sinks to the bottom when the 
pressure in the experimental vessel is raised. By direct measuring 
it was shown that the volume of the air-sacs was diminished, the 
specific gravity of the larva became greater and the animal will 
therefore sink whenever it keeps quiet. 
But the change in the volume was far from being in the in¬ 
verse ratio to the pressure. 
The wall of the air-sacs is in other words able to make a cer- 
tain resistance against the augmented pressure, but this resistance 
is far from being complete; when the pressure is raised the volume 
of the air-sac will diminish, but not in the same degree as if the 
wall had been only a thin membrane. 
Krogh has shown that when the pressure is augmented with 
one atmosphere the volume of the air-sac will diminish, but only to 
0,8—0,9 of its normal size, whereas the volume of an air-bubble 
will be diminished to half its size. 
When the pressure has been augmented the equilibrium of the 
larva must be disturbed, the larva — when not swimming — will 
sink. But after a certain time the animal will again have obtained 
the equilibrium. 
The function is consequently analogous to the swimming biadder 
of fishes, the mechanism however a different one. 
By means of microscopical gas analyses it was shown that the 
regulating mechanism was not of the nature of a gas secretion. 
According to Krogh the mechanism is the following: When the 
volume of the air-sac has been diminished, as a consequence of 
the augmented pressure, water — which must be supposed to be 
in the biadder — will be secreted out from the biadder until the 
air in the sac has its original volume and pressure. 
In this way the animal attains equilibrium under the new pres¬ 
sure. But the experiments show moreover that the larva only was 
able to endure an alteration in pressure inside rather narrow limits. 
When the pressure was raised more than one atmosphere the biad¬ 
der became filled with a fluid and the animal died in a few days. 
Therefore it is of great interest that Wesenberg-Lund has 
