PuHyYsrtoLoGicaL Errects or Lack or Oxygen 417 
diac activity and oxygen in the larve of a fresh-water 
mussel (Cyclas). In this animal the frequency of the heart- 
beat steadily decreases from 50 heart-beats to 0 in the course 
of one and one-half hours in an atmosphere of hydrogen (at 
24° C.). In this case, therefore, we have neither a sudden 
standstill of the heart without an appreciable decrease in the 
frequency, as in Ctenolabrus, nor a long-continued steady 
beat of low frequency, as in Fundulus, but a decrease in 
cardiac activity which runs parallel with the removal of 
oxygen, as if processes of oxidation are the sole source of 
energy for the activity of the heart. 
XI. ON THE TRANSFORMATION OF NEGATIVELY HELIOTROPIC 
ANIMALS INTO POSITIVELY HELIOTROPIC THROUGH LACK 
OF OXYGEN 
A series of papers have proved that it is possible to 
change the sign of heliotropism in certain animals at will 
through external conditions." It is an easy matter, for 
example, to render negatively heliotropic Copepods posi- 
tively heliotropic by cooling, and to keep them permanently 
positively heliotropic at a low temperature; while it is also 
possible to render positively heliotropic Copepods negatively 
heliotropic by an increase in temperature. The same 
experiments can be made on larve of Polygordius. In 
order to determine the cause of this change in the sign of 
heliotropism, and also the conditions upon which the latter 
depends, I tried to see whether other conditions could bring 
about similar changes. Groom and I had previously found 
that the positively heliotropic Nauplii of Balanus perforatus 
rapidly became negatively heliotropic when exposed to 
strong light. 
I also found that the same effect can be produced upon 
Copepods and Polygordius larve by properly diluting the 
1GRoom UND LOEB, Biologisches Centralblatt, Vol. X; Loxs, Vol. I, pp. 265 ff. 
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