182 E. H. HARPER 



and out with a pipette; or the whole may be placed in a small 

 vial and turned back and forth. After this treatment the Para- 

 moecia may be recovered without the iron by pouring the whole 

 into a Syracuse watch glass with water and the Paramoecia 

 removed with a pipette by several rinsings. They were then 

 placed for experimentation in a flat rectangular jar about 3 x 

 1 ', x ; l inches. This was supported with its narrow face against 

 the core of the magnet. The water used is preferably boiled to 

 drive away gases. The normal geotropic reaction of Paramoe- 

 cium, as Jennings points out, is a delicate one and is best elicited 

 in pure water in the absence of disturbing chemical factors and 

 mechanical agitation. But as pointed out in the previous paper 

 ingestion of iron accentuates the geotropic response so that it 

 is not so readily masked by chemical and other factors. 



EXPERIMENTS 



1 . Ingestion of iron for a relatively long period, five minutes or 

 more. 



Three controls are needed in connection with this experiment. 

 First a culture of normal Paramoecia in a similar vessel and 

 subjected to similar shaking, as mechanical agitation has a ten- 

 dency to drive them to the bottom. The control Paramoecia 

 used in this set of experiments were taken from a dense ring 

 round the edge of the water in the stock jar. They showed a 

 slight normal upward tendency after a time, but were apt to 

 remain rather evenly diffused and finally to settle all over the 

 walls of the jar, which was probably due to the large extent of 

 its inner surface. Other cultures sometimes display a tendency 

 to go downward or more commonly aggregate toward the top 

 and occasionally form a ring. 



As a second control such a jar of Paramoecia may be brought 

 into the field of the magnet, which gave no effect, and would 

 disprove the existence of anything in the field itself to influence 

 the movements. 



Specimens of the treated culture under the microscope showed 

 a heavy load of iron particles. The behavior of such a culture 

 without the magnet furnishes the third control experiment. 

 These moved downward and soon left the upper third of the jar 

 fairly free from Paramoecia. A considerable number would 

 come to rest on the bottohi where they would remain for short 



