No. 385.] THE PROBLEM OF GEOTAXIS. 33 
suspended in the water with their long axis in a vertical plane, 
and with the anterior end of the body directed upwards. As 
it is possible that the maintenance of this position is in reaction 
to gravity, I tried the effect of placing Spirostoma in solutions 
denser than water. When Spirostomum moves, it usually 
advances first the smaller end of its elongated body, or the 
.end which is opposite the large contractile vacuole; and this I 
consequently call the anterior end of the body. Spirostomum 
occasionally reverses its cilia and moves backwards, but the 
regression is of momentary duration, and the small organism 
returns directly to its usual manner of motion. 
` A fortunate accident by which a faint trace of some un- 
known poison must have been present in the culture jar to 
which a number of Spirostoma were transferred, rendered these 
Infusoria motionless without killing them. When examined 
under the microscope it was found that their cilia did not 
move, yet an occasional spasmodic contraction demonstrated 
that life still continued. I was thus enabled to obtain the 
weight of ving Infusoria, and the result was subsequently 
confirmed by similar experiments with Spirostoma that were 
not poisoned, but merely exhausted by long cultivation in the 
laboratory. 
To avoid currents that would arise from introducing water 
with the Spirostoma into the gum arabic solution, they were 
first drawn from the culture jar with a pipette and ejected into 
a watch glass, which was gently shaken so that the Spirostoma 
gathered at the bottom. The water that had been taken up 
with them was then withdrawn, and gum arabic of the same 
density as the solution to which they were about to be trans- 
ferred was substituted. In this solution of gum arabic the 
Spirostoma were again drawn into a pipette and ejected gently 
into the midst of the solution with which their specific gravity 
was to be compared. 
In determining the specific gravity of these Infusoria, the 
reaction during the first fifteen or twenty minutes is alone to 
be trusted, since Spirostoma, and also Paramzecia, whether liv- 
ing or dead, increase their specific gravity on remaining long 
in a solution of greater density than water. The paralyzed, 
