1091 
for the experiment, by the side of the latter. No fish was placed 
in it. It only served to verify. Before and after the experiment the 
temperature of the water was taken with a sensitive BECKMANN- 
thermometer. Thus it could be established whether the changes o1 
the two menisci were due to temperature-fluctuations or not. 
My experiments showed that active volume-changes of fishes occur 
indeed, though in quite another manner than it was supposed by 
Borerrt and others. Slow meniscus-movements, namely, manifested 
themselves, which did not run parallel to movements of the fish in 
the horizontal plane. The meniscus-movements in the bottle could 
not be explained by expansion of the water as a result of heat- 
production by the fish, for in all experiments the temperature in 
the two bottles was the same. What may be the cause of these 
volume-changes of the fish, which, it must be admitted, are never 
very great; viz. up to +0.3°/, of its body-volume? Satisfactory 
explanations might be found in: 1. change of the tonus of the 
muscles in the surface of the body, 2. changes of pressure in the 
air-bladder. It seems unlikely that the latter should be the cause. 
The changes in the gas-pressure found by Morrav and others, took 
place very slowly, whilst the volume-changes in my experiments 
occurred in a rather short time. 
A calculation showed that the maximum volume changes, ascer- 
tained by me for a fish whose static plane lay at a depth of one 
metre, caused this plane to move 16 centimetres. 
On the ground of the facts mentioned 1 do not wish to declare 
myself an adherent of the theory of BoreLrt; I only wish to point 
out that Morwau’s classical experiments do not sufficiently refute 
Boreui’s theory. 
We saw already that the fish responds to slight changes in the 
pressure to which it is subjected namely by well-coordinated swimming- 
motions. BAGLIONr asked himself where the stimuli arise that awaken 
these movements, which he looks upon as averting-reflexes. He 
supposes this to take place in the surface of the air-bladder. The 
numerous nerve-endiugs described by DerNeka would, according to 
him, be stimulated when the tension in the bladder-surface was 
modified. In those fishes which possess the well-known organ of 
WeBer (the 4 pair of bones connecting the surface of the air-bladder 
with the perilymphatic space of the vestibular apparatus) this organ 
might be an important factor in the perception of pressure-modi- 
fications. 
Evidently it is difficult to give direct proofs for the tempting 
theory of BAGLIONT. It cannot be proved that the upward or down- 
72 
Proceedings Royal Acad. Amsterdam. Vol. XVII. 
