CILIARY CURRENT IN PARAMECIA 287 



is in reality a weak avoiding reaction, it appeared that the cone 

 was produced after the animals began to turn aside, rather than 

 before. To test this a small amount of hydrochloric acid was 

 added to the ink-suspension so that the animals gave a weak 

 avoiding reaction at the edge of the freshly added ink. In mixed 

 cultures the reactions of different paramecia to such a prepara- 

 tion differ greatly. Some give a violent avoiding reaction, 

 others enter the acid without a pause. Many, however, give a 

 weak avoiding reaction, i.e., they stop and swerve toward the 

 aboral side without swimming backward. These animals always 

 draw out a cone-shaped mass of ink from the edge of the cloud. 



The ciliary movements in reactions of this sort were studied in 

 detail and this gave a clue to the method by which the cone is 

 produced. The animals, without producing a cone, swim up to 

 the edge of the region containing acid until the anterior end is in 

 contact with the ink particles; then the body cilia reverse, but 

 those of the oral groove continue to beat backward. In conse- 

 quence the forward progress is stopped and the animal swerves 

 to the aboral side. Since the body is prevented from advanc- 

 ing (owing to the reversal of its cilia), the backward stroking 

 cilia of the oral groove draw out a cone of ink which follows the 

 swerving animal until forward progress is resumed as represented 

 in figure 3. 



To demonstrate this reaction methylene blue is most favorable. 

 Crystals dropped upon the surface of the water containing para- 

 mecia sink to the bottom, dissolving slowly and forming dense 

 blue vertical columns of solution. The paramecia respond to 

 this with a weak avoiding reaction, but they do not respond until 

 after the anterior end has actually come in contact with the 

 blue solution, then they turn and the blue solution is drawn 

 out in the form of a cone as previously described. The feeding 

 cone is, however, not always produced when paramecia respond 

 to acids. 



After a drop of acidified ink has been in a culture of paramecia 

 a few minutes, they no longer respond to it as they did at first. 

 They collect in a zone around it and swim far into the cloud 

 before giving a reaction; then they respond with a strong avoid- 



