418 STUDIES IN GENERAL PHYSIOLOGY 
sea-water as by increasing the temperature, while a proper 
increase in the concentration of the sea-water brings about 
the same effect as cooling. 
The majority of Copepods were, immediately after being 
caught, positively heliotropic. It seemed as if the majority 
of the negatively heliotropic Copepods belonged to one and 
the same species. When the Copepods were allowed to 
remain for a long time in a vessel containing sea-water, the 
number of negatively heliotropic animals decreased, becom- 
ing positively heliotropic with time, while the reverse change 
occurred only rarely. The experiments on the effect of lack 
of oxygen were made under a small bell-jar, the contents of 
which were separated from the air on the outside by mer- 
cury. Two tubes extended into the bell-jar, one of which 
conducted the hydrogen into the bell, while the other con- 
ducted it away from the bell. Two vessels were placed 
under the bell-jar, of which the one contained freshly 
selected positively heliotropic Copepods, while the other 
contained negatively heliotropic Copepods. While the 
positively heliotropic animals remained positively heliotropic 
during the course of the experiment, the negatively helio- 
tropic Copepods within fifteen to twenty minutes after the 
hydrogen was turned on began, in part, to leave the room 
side of the vessel and to distribute themselves irregularly 
throughout the vessel, in part to collect at the window side 
of the vessel. The number of animals collected near the 
window steadily increased, while the number of Copepods at 
the room side of the vessel steadily decreased. In about 
thirty to forty-five minutes after the current of hydrogen 
had been turned on all the Copepods lay quietly on the 
bottom of the vessel. The Copepods which had from the 
beginning been positively heliotropic died at the side of 
the vessel nearest the source of light. Most (if not all) of 
the Copepods which had at first been negatively heliotropic 
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