No. 385.] THE PROBLEM OF GEOTAXIS. 35 
extent of the change (from 1.017 to 1.25) was surprising, for I 
had already found that Spirostomum changed its weight slightly 
when living in a medium denser than water, and had also found 
that, when killed with any one of several reagents, the specific 
gravity of Spirostomum increases. For example, when Spiro- 
stomum is killed with quinine, it sinks in solutions of gum arabic 
of 1.028 specific gravity, although the living but motionless 
Spirostomum quickly rises to the surface in a solution of this 
density. 
On placing Spirostomum in a solution of gum arabic slightly 
denser than its own protoplasm, I found that the infusorian did 
not remain still in a vertical position, as is its habit in water, 
but wandered restlessly about. Of those individuals which at 
the time of observation occupied a vertical, or nearly vertical 
position, the number with the anterior extremity directed up- 
wards was 48. The number of those with the-anterior extremity 
directed downwards was 81. The larger number of individuals 
having their anterior extremity directed downwards suggests 
a tendency to reverse the normal position maintained in water, 
where the anterior extremity is directed upwards. I doubt, 
however, if the difference in the numbers should be thus inter- 
preted; for if Spirostomum merely wandered back and forth 
between the upper and lower surfaces of the liquid, it would 
rise in a dense solution more rapidly than it would descend. 
There would thus be at any moment more individuals going 
downwards than upwards. The power of nice adjustment 
between the force of the cilia and the force of gravity, which 
maintains Spirostomum suspended in water, was lost in those 
individuals which I observed when placed in solutions of greater 
specific gravity than theirown. My conclusions regarding their 
reaction to gravity are, therefore, purely negative. 
Paramecium. 
To obtain the specific gravity of Parameecia, I tried to para- 
lyze them with ether, quinine, and nicotine, but invariably killed 
the Paramezcia, which in dying went to pieces. I therefore 
added a few drops of 0.5 per cent acetic acid to the water con- 
