40 
shown that Corethra plumicornis lives in our larger lakes 30—40 
m. below the surface, under a total pressure of 4—5 atm. 
It is obvious that the lake form must be another phenotype than 
the pond form; it would namely be quite impossible for the pond 
form to live under the same pressure as the lake form. Whether 
it is another biotype or not future experiments must solve. 
Wesenberg-Lund writes that the larva seems to be quite 
uninfluenced by changes of pressure from 1 to 5 atm.; when the 
larva taken on a depth of c. 40 m comes to the surface the bladder 
is still filled with air and the animals behave well in an aquarium. 
The tracheal bladder of the lake form consequently reacts against 
the pressuré in quite another way than the pond form. 
I have made a few experiments in order to examine the diffe- 
rences between the air-sacs in the lake form and in the pond form. 
Firstly I measured the alteration of the volume under different 
pressure on a detached bladder. !) 
The result of my measurings was that the air-sac of the lake 
form was not completely uninfluenced by alteration in pressure, 
but that it was so rigid, that even considerable change of pressure 
only effected a very small alteration in the volume. In the table 
I have given the changes under different pressures in the volume 
of the air-sac of the lake form of Corethra plumicornis compared 
with those of a pond form and with a common air bubble. 
The table shows that in order to effect the same change in 
volume it is necessary to expose the air-sac of a lake form to three 
times as great a pressure as necessary for the pond form. (Com- 
pare 3 with 4.) 
The air-sacs of the lake form however are not completely rigid; 
we must therefore expect that the equilibrium of the animal must 
be disturbed when exposed to considerable alterations in pressure. 
In order to examine this I exposed the larva to sudden and 
considerable change of pressure. The method employed was the 
same as described by Krogh. 
It was shown that the equilibrium of a larva was not disturbed 
in any perceivable degree by pressure between one and two at- 
) The bladder was enclosed in a pressure chamber consisting of a glass 
tube placed in water between two plates of plane glass. 
