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 Paramoecium 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 Paramoecia 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, 

 w r ho 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 Paramoscium is an organ- 

 ism which takes in and excretes water many tunes 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- 

 mcEcia when they are placed hi a medium of somewhat 

 higher osmotic pressure. In abnormal conditions Paramoecia 

 become more plump and the body seems softer and more 

 flexible, and it is also possible that, since Paramoecium 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- 



