38 THE AMERICAN NATURALIST. 
In this case the density of the surrounding medium might be 
expected to effect little, if any, change in the direction in which 
the organism moves. 
Results. 
(1) The specific gravity of living Spirostomum lies between 
1.016 and 1.018, or is near to 1.017. 
(2) The specific gravity of Paramzcium killed by acetic e 
or by the fumes of osmic acid, differs little, if any, from the 
specific gravity of living Spirostomum, and is about 1.017. 
(3) Small tadpoles that are negatively geotactic do not 
become positively geotactic when placed in solutions heavier 
than their own specific gravity, as one would expect were their 
upward motion in direct response to the action of gravity on 
the organism as a whole. These tadpoles show constant neg- 
ative geotaxis in water, in a solution of their own specific 
gravity, and in heavier solutions. 
RADCLIFFE COLLEGE, CAMBRIDGE, 
June 11, 1896. 
LIST- OF PAPERS REFERRED TO: 
JENSEN, P. '93. Die absolute Kraft einer Flimmerzelle. Arch. f. d. ges. 
Physiol. Vol. liv, pp. 537-551. June 24, 1893. 
Logs, J. ‘91. Ueber Geotropismus bei Thieren. Arch. f.d. ges. Physiol. 
Vol. xlix, pp. 175-189. 
ScHWARZ, F. '84. Der Einfluss der Schwerkraft auf die Bewegungsrich- — 
tung von Chlamidomonas und Euglena. Berichte d. deutschen botan. 
Ges. Vol. ii, pp. 51-72. 
