THE BEHAVIOR OF PROTOZOA 87 
creature had great difficulty in reversing its course, having 
to bend its body in the form of a U to get around. If the 
diameter of the tube is not too small the time which it takes 
the Parameecium to turn, says Smith, “may gradually be 
shortened and a most surprising aptitude of turning be 
developed. . . . I have found a reduction of turning 
time, after the animals have been in the tube for twelve 
hours or more, from four to five minutes to a second or two, 
which is the minimum time in which the turns can be 
made.” Although Paramecia kept without food for twelve 
hours would diminish sufficiently in size to enable them to 
turn within the tube with much greater ease—a fact which 
Mr. Smith apparently has not considered—Day and Bentley, 
who have repeated Smith’s experiments, have found that the 
greater facility in turning is acquired within a few minutes. 
It should be borne in mind that Paramecium is an organ- 
ism which takes in and excretes water many times more 
rapidly than even the specialized organs of excretion of 
higher animals, and that the abnormal conditions resulting 
from confinement within a very small amount of water 
may possibly cause a certain change in size within a short 
time. I have often observed a marked shrinkage in Para- 
moecia when they are placed in a medium of somewhat 
higher osmotic pressure. In abnormal conditions Paramecia 
become more plump and the body seems softer and more 
flexible, and it is also possible that, since Parameecium is 
endowed with a certain degree of contractility, the stimuli. 
encountered through its frequent efforts to turn within the 
tube might cause a shortening of the body which would cer- 
tainly occur in a more marked way in a Stentor and many 
other infusorians under these conditions. The experiment 
has its practical drawbacks as a means of testing habit for- 
mation or “learning,” since the changes of size, form and con- 
