EFFECTS OF CARBON DIOXIDE O2/ 
ance as the average Coleps hirtus for instance, or as high a one 
as the average Colpidium colpoda. Furthermore in a given 
culture, if P. caudatum has a higher resistance than usual, the 
other forms present also will, and their relative resistance remains 
practically constant. It is possible, therefore, to attribute to 
P caudatum as well as to the other forms a specific resistance, 
remembering only that its absolute value is somewhat subject to 
variation under different conditions and that in comparing differ- 
ent forms it is well to have them either from the same culture or 
at least to make a considerable number of independent observa- 
tions on different cultures. 
2. Paramecium aurelia. A comparison of this form with the 
preceding one will illustrate the statement just made. P. aurelia 
is in general considerably less resistant than P. caudatum. When 
the two forms are present in the same culture the former is always 
killed sooner than the latter, though rarely, when different cul- 
tures are studied, some strains of P. aurelia are encountered which 
show a higher resistance than some of the most susceptible strains 
of P. caudatum. In general, however, P. aurelia is killed in less 
than a half hour while P. caudatum nearly always survives several 
times as long. The average time of death in the two extreme 
experiments on P. aurelia was a little over ten minutes on the one 
hand and over two hours on the other, in most of the experiments, 
however, lying, as already stated, below thirty minutes. Loeb 
and Hardesty state that P. aurelia is killed by carbon dioxide in 
two-and-a-half to three-and-a-half hours. Perhaps their culture 
was an abnormally resistant one, or possibly the application of 
the gas was slower than in these experiments, or the temperature 
lower In view of the fact, however, that the figures given by 
them are quite typical of the rather more common P. caudatum 
at ordinary room temyerature, and also that the distinction be- 
tween caudatum and aurelia formerly was not very sharply drawn, 
it is possible that they were dealing with the former rather than 
the latter species. The difference in size between the two forms 
is probably not the reason for their different powers or resistance, 
since in the same species no constant relation could be found 
between the time of death and the size of the animal _-Further- 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, NO. 4 
