PHYSIOLOGICAL EFFECTS OF LACK OF OXYGEN 417 



diac activity and oxygen in the larvae of a fresh-water 

 mussel (Cyclas). In this animal the frequency of the heart- 

 beat steadily decreases from 50 heart-beats to 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. 1 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 larvae 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 larvae by properly diluting the 



i GROOM TJND LOEB, Biologisches Centralblatt, Vol. X; LOEB, Vol. I, pp. 265 ff. 



