58 THE TROPISMS 
THERMOTAXIS 
Almost all organisms which are free to move go away from 
regions in which the temperature is injuriously high, and 
many, although a less number, withdraw from regions which 
are colder than a certain optimum. If the water near an 
Amceba is heated by a hot needle the animal will contract 
on the side nearest the needle, send out pseudopods in some 
other direction and crawl away. Paramcecium and other 
infusoria form groups in a region of optimum temperature by 
the same method employed in reacting to chemicals. When 
a Paramcecium swims into a region above the optimum it 
gives the motor reflex and goes inanother direction. Whenit 
encounters a region below a certain temperature it behaves 

Sf a 
Fig. 6.—Reactions of Parameecia to heat and cold. One end of the 
slide is heated to 35° C. while the other end is kept on ice. The Par- 
amoecia gather in an intermediate zone dc. 
in the same way. If Paramcecia are placed in a trough one 
end of which is heated to 35° C. while the other is placed upon 
ice the infusorians will form a band near the middle where 
the temperature ranges from 24° to 28° C. Previous ex- 
posure to higher or lower temperatures than those usually 
experienced changes the optimum to a considerable degree. 
Most organisms above the protozoa turn directly away 
from hot or cold objects. Sometimes the animal may escape 
undue stimulation by making a number of random move- 
