524 MERKEL HENRY JACOBS 
seen therefore that while there is not very much difference between 
the medium sized drops and the large one, the small one shows 
a decided difference. This is probably due to the greater sudden- 
ness with which the animals were subjected to the carbon dioxide 
in the latter case rather than to any difference in its concentra- 
tion ultimately in the drops, since these probably all became prac- 
tically fully saturated long before the end of the experiment. In 
order to guard against this source of error the drops were all made 
as nearly the same size as possible, the standard being a drop about 
8 mm. in diameter with a moderate curvature. Results obtained 
in this way were constantly compared with those obtained when 
a number of the forms in question were present in the same drop. 
The final precaution has already been mentioned, namely, not 
to base far-reaching conclusions on results obtained from a single 
culture. In the case of some of the forms studied more than a 
dozen cultures were employed; in all of them except one form, 
obtained very late, the number of cultures was at least three. 
III. OBSERVATIONS AND EXPERIMENTS 
A. Ciliates 
1. Paramecium caudatum. The effects of carbon dioxide on 
this form are briefly as follows. Immediately after the current of 
gas has been turned on, the animals exhibit a general restlessness 
and begin to seek the center of the drop, or rather, to avoid its 
edges, doubtless because the concentration of the carbon dioxide 
is greatest there. Inside a minute, as a rule, in a drop of the size 
used in these experiments, they have collected in its central and 
thickest part. Here they swim about actively but in very short 
paths, since the ‘avoiding reaction’ occurs whenever their move- 
ments have a tendency to carry them into the thinner and con- 
sequently more saturated part of the drop. Soon, however, 
generally within two or three minutes, they cease to be able to 
discriminate between the concentrations in different parts of the 
drop and spread out again until they are uniformly distributed. 
Sometimes toward the end of the experiment, for unknown reasons, 
they collect about its edge. Previous workers have noticed the 
