536 MERKEL HENRY JACOBS 
particular Euglena studied, which was close to, but probably not 
identical with, E. viridis, was not one which very actively changes 
its form, and consequently was not a very favorable one in which 
to observe the effect of carbon dioxide on the contractile move- 
ments of the body. It may be said, however, that while ‘eugle- 
noid’ movements were observed in many individuals before the 
beginning of the experiment and also after recovery, they never 
occurred during its progress, consequently the conditions here 
probably are the same, even if less striking, than those found in 
Peranema. 
After the organisms have settled to the bottom of the drop and 
become motionless the only change that can be observed is a 
gradual slow swelling of the body. At the same time there is a 
slight shortening which, however, is not sufficient to account for 
the greater thickness of the organisms as careful -measurements 
show. This swelling continues until the shape of the body has 
changed from cylindrical to broadly elliptical in outline and the 
chlorophyll bodies appear forced apart from each other. In 
extreme cases the cell may appear to be at the point of rupture, 
though this rarely occurs, the pellicle being very tough and elastic. 
Although all movements cease in Euglena in ten minutes or 
less, it requires a much longer time to kill the organisms. Even 
after an exposure of three hours about a third of the individuals 
eventually recovered, though the time required was considerable. — 
The recovery of Euglena is far slower than that of Peranema. 
After an exposure of seven minutes no recoveries could be noticed 
a half hour after removal to the air, although they began to occur 
soon after that, and in an hour and a quarter practically all the 
individuals were normal. After an exposure of two or three hours, 
the time required for recovery is three or four hours or more. 
3. Chilomonas paramecium. This form shows great individual 
and also cultural variation. While in a few cases the animals 
become motionless in fifteen or twenty minutes, the average time 
required generally is three-quarters of an hour or more. Many 
resistant individuals retain their movements for several hours. | 
In general, therefore, this may be said to be a form with a high 
resistance. Unlike the two previous flagellates, Chilomonas 
