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 



