36 THE AMERICAN NATURALIST. < VOL. XXXIII. 
taining the Infusoria. As acetic acid is said to swell proto- 
plasm, I also killed Paramzecia by the fumes of osmic acid. 
Both these reagents caused the Paramzecia to change their 
shape somewhat in dying, by becoming wider and shorter. 
Their form, however, remained symmetrical in both cases, and 
seemed neither enlarged nor distorted. 
The results obtained by the two methods were similar. 
When the killed Paramzcia were placed in a gum arabic solu- 
tion of 1.018 specific gravity, they remained long suspended in 
the solution. In a solution of 1.024 specific gravity they rose 
to the surface as rapidly as they fell when placed in water. 
The reaction in solutions of nearly the same specific gravity as 
- Paramzecium is less certain than with Spirostomum, since the 
smaller size of Paramzecium makes it more liable to drift up or 
down with chance currents, and less rapid in its response to 
gravity. There is, however, no doubt that the specifie gravity 
of Paramecium thus killed differs but little from that of living 
Spirostomum, and differs very greatly from the specific gravity 
assigned this infusorian by Jensen (93). 
The Paramzcia found in the cultures of our laboratory and 
in the water of the pools about Cambridge did not show geo- 
tactic tendencies. If one were placed in clear water, and were 
drawn into a capillary tube held vertical, it would rise to the 
upper surface of the water drawn into the tube, and if the tube 
were now directly reversed, the Paramzecium would rise again ; 
but if the tube were retained in its original vertical position, 
the Paramzecium wandered back and forth between the up- 
per and lower surfaces of the water. The water of the cul- 
ture vessels, which contained dead leaves, bacteria, etc., soon 
swarmed throughout with Paramezecia, which were found in great- 
est quantities at the surface where a scum of bacteria gathered. 
If these superficial bacteria were placed in a glass of clear water, 
they wandered up and down indifferently. 
Tadpoles. 
: Some toad tadpoles that had just left their gelatinous enve- 
2 lope were observed by Dr. Davenport to swim at once to the 
