CHEMOTAXIS OF OXYTRICHA -AERUGINOSA. 2CjI 



cept that Paramecium swam further out, possibly due to the 

 greater momentum caused by their larger body and faster rate 

 of motion. But soon Oxytricha began to shun the center of the 

 drop, repelled by the carbon dioxide exuded by the Paramecia. 

 But they were held by their own attractive excretion in a ring 

 around the outside of the area. Within thirty minutes however 

 the OxytncJia had moved so far outward that the Paramecia were 

 entirely separated from them ; shortly afterward the aggregation 

 of Oxytricha, due undoubtedly to its attenuated character, broke 

 up, while the Paramecia were still densely crowded together. 

 In a second similar group, where however the relative number 

 of Paramecia was much smaller, the two organisms remained 

 evenly distributed throughout the drop, and the area was extended 

 at the same rate for both of them. Why this difference? Very 

 likely because the smaller number of Paramecia acted less rapidly 

 in making the center of the drop uninhabitable for Oxytricha, 

 while the Oxytricha in moving outward, by their own excretion 

 of carbon dioxide kept the whole area acceptable to Paramecium. 



Paramecia in culture water, after aeration, react positively to 

 distilled water or weak acid. When in distilled water they react 

 positively to weak acid but negatively to stronger acid or to 

 alkali. Paramecium therefore usually seems to seek an optimum 

 region, weakly acid in reaction. (Of certain exceptions to this I 

 hope to speak in another paper.) The relation of this to the 

 general life activities of Paramecium has often been pointed out 

 by Jennings. 



A culture of Oxytricha, aerated in similar fashion, reacts quite 

 differently. If such an aerated culture is placed under a cover 

 glass on a slide, and a drop of distilled water is introduced, the 

 center of this drop remains free from Oxytricha. Such as swim 

 into the edge of the drop give the typical motor reaction, long 

 before they reach the center, and then swim outward, but only 

 for a short distance ; for there exists now also an outer boundary 

 bringing forth the same reaction. The result is that there is 

 soon formed a ring-like aggregation, quite like that of Parame- 

 cium in presence of a drop of stronger acid. If such an aggrega- 

 tion is left undisturbed for some time, the Oxytricha usually soon 

 begin to collect in some one portion of the ring, and within five 

 or six minutes form there a very dense aggregation. 



