EFFECTS OF CARBON DIOXIDE 537 
shows most decided reactions to the presence of .carbon dioxide. 
The first effect is often to cause a temporary cessation of motion 
which lasts however only for a few:seconds, after which the ani- 
mals are remarkably active. They show a striking tendency to 
seek the center of the drop at first, later becoming uniformly dis- 
tributed again. Their motions at first are normal but gradually 
the animals come to rest and give evidences of life only by a slow 
rotation or by quick darting movements which they occasionally 
make. In practically every case the cell becomes circular in out- 
line and if the experiment be long continued may actually burst. 
Chilomonas like the preceding form also has considerable powers 
of recovery after all motion has ceased. In one experiment even 
after an exposure to carbon dioxide of two-and-a-half hours about 
75 per cent of the individuals eventually recovered after exposure 
to the air. In other cases, however, even after a shorter exposure 
the mortality is greater. 
4. Entostphon sulcatum. This form unfortunately was studied 
in only one experiment in which, however, a considerable number 
of individuals was present. Judging from these rather incom- 
plete data, it is by far the most resistant of the flagellates exam- 
ined. After an exposure of five hours it not only was alive,but 
the movements were not very markedly affected. Both flagella 
continued to beat, though in rather a stiff and jerky fashion, and 
slow forward progression continued. How long it would have 
survived cannot be said, but probably the time would have been 
considerably above that mentioned, since when the experiment 
had to be ended none of the animals had as yet been killed. 
IV. DISCUSSION OF RESULTS 
_ From the results given it is apparent that all of the forms 
studied are injured and eventually killed by pure carbon dioxide, 
but that the resistance of the different forms is very different. 
Colpidium colpoda can withstand without injury an exposure 
of many hours, while Coleps hirtus is killed in three or four minutes. 
Sometimes the time of cessation of visible movements and the 
point at which the cell is so severely injured that recovery cannot 
occur, may coincide, as in most of the ciliates (in Euplotes patella 
