EFFECTS OF CARBON DIOXIDE 529 
on account of the presence of chlorophyll in its body it is accus- 
tomed to rather a low concentration of carbon dioxide, since this 
substance is constantly being removed by it from the surrounding 
medium. Experiments to determine whether it was more, or 
less, resistant in bright light than in the dark have as yet not 
given very positive results, chiefly on account of the difficulty of 
controlling the temperature factor. 
4. Colpidium colpoda. This in every case proved to be the 
most resistant species studied, living long after the other forms in 
the same drop had succumbed. The general effects on it of the 
carbon dioxide are as follows. When the stream of gas is turned 
on a strong negative response is shown and the animals collect in 
the center of the drop in the same manner as the forms already 
mentioned; soon, however, they become uniformly distributed 
and thereafter behave normally almost to the time of death which 
hardly ever occurs in less than six hours, and may take place much 
later. In anumber of experiments the carbon dioxide was allowed 
to flow for six or seven hours and was then shut off without how- 
ever admitting the air, and the animals were found to be in a 
normal condition the next day. In one such experiment they 
remained alive for a week, but this experiment was somewhat 
vitiated by the fact that a small quantity of chlorophyll derived 
from disintegrated Euglena cells was present in the drop, and 
during the hours of daylight could have furnished a certain amount 
of oxygen, though only a small portion of the carbon dioxide in 
the chamber could have been gotten rid of in thisway. However, 
even if these last results be discarded the fact remains that Col- 
- pidium is exceedingly resistant to carbon dioxide and may remain 
alive in a drop saturated with it formany hours. Prowazek (’03) 
found in an experiment of a different sort that Colpidium survives 
a simple lack of oxygen far better than Paramecium caudatum. 
Carbon dioxide is not without its effects on Colpidium, how- 
ever. Locomotion, while not so markedly affected as in most of 
the other forms, nevertheless becomes less active than normally. 
Before death occurs the animals become quiescent, often at the 
edge of the drop, and the cilia beat more and more slowly until 
they stop. One point of interest is that the animals, while they 
