THE GEOTROPISM OF PARAMOECIUM 
EK. H. HARPER 
From the Zoological Laboratory, Northwestern University 
FIVE FIGURES 
Are there any free-swimming organisms which are oriented to 
gravity by means of the shape or contents of the body in the same 
way as the axis of orientation of certain eggs is determined by 
the difference of specific gravity of the opposite poles? The 
body of Paramoecium caudatum suggests the possibility of its 
being oriented to gravity by the difference in buoyancy of the 
two ends. The posterior end is broader and the anterior end 
indented by a deep groove. Is the body of Paramoecium ‘stern- 
heavy,’ and if so, does it account for the ordinary negative geo- 
tropism of the animal, the anterior end having a tendency nor- 
mally to point upward? Various explanations have been pro- 
pounded for the geotropism of Paramoecium, such as difference 
in water pressure at different depths, difference in pressure on 
the lower and upper surfaces and finally Lyon,! ascribes the 
geotropic response to the internal st muli of heavy particles, 
making the body of Paramoecium the analogue of the statocyst 
of higher forms. 
To test the writer’s hypothesis, it was thought it might be 
possible to increase the difference in specific gravity of the two 
ends by making the animals ingest heavy particles. These would 
at first lhe toward the posterior end near the mouth, making the 
body more ‘stern-heavy,’ and so possibly increasing the negative 
geotropic tendency. 
‘Lyon, E. P., 1905. On the theory of geotropism in Paramoecium: Amer. 
Journ. Physiol., vol. 14, pp. 421-482. 
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