928 MERKEL HENRY JACOBS 
more, the larger P. busaria is less resistant than P. aurelia and the 
somewhat smaller Colpidium colpoda far more resistant. The 
cause of the difference is evidently of a more deep seated nature. 
In the general effects produced on it by carbon dioxide, P. 
aurelia closely resembles the preceding species. It shows the same 
negative response at first, which disappears in about the same 
time. The body swells in about the same way. The nuclei 
often become very distinct, and this fact frequently renders an 
accurate identification possible without staining. The interval 
between the time when locomotion ceases and the cilia stop beat- 
ing is both relatively and absolutely shorter than in P. caudatum 
There is also a much greater tendency for the pellicle to rupture, 
this occurring in some cultures in almost every individual. + Per- 
haps this apparently greater delicacy of the body wall may be 
correlated with the lower powers of resistance of this form. 
3. Paramecium bursaria. In a number of the cultures used 
in these experiments this species was found associated with the 
two preceding ones and therefore a favorable opportunity was 
presented to compare it with them. Such a comparison shows 
that it is the least resistant of the three. The average time of 
death was generally ten to twenty minutes, though in a number of 
cultures it was less than five and only rarely ran as high as thirty. 
The most resistant ¢ndividual found lived over an hour but this 
was a most exceptional case. (It was in this culture that P. 
aurelia also showed its highest resistance—over two hours.) 
When the three forms in question are present in the same culture 
in every case observed the relative resistance was: bursaria, 
aurelia, caudatum, and might perhaps be represented numerically 
very roughly as 1: 2:4. The general effects of the carbon dioxide 
on this form are on the whole s milar to those already described 
in the case of the other two species. The pellicle apparently is 
very delicate and nearly always ruptures while the cilia are more 
markedly affected than those of the other species; as their move- 
ments cease they become matted together and very quickly be- 
come partly disintegrated, being represented only by an indis- 
tinct zone about the animal. It is rather interesting that this 
green form is less resistant than the colorless ones. Doubtless 
