BEHAVIOR OF UNICELLULAR ORGANISMS. 99 



of such a protoplasmic body would be passive in the same sense 

 as the movements of iron filings are passive when acted upon 

 by a magnet. Delage and Herouard J . actually state that the 

 Flagellata have, in addition to their usual active movements, 

 also a passive motion, due to the attraction of chemical sub- 

 stances. Perhaps the majority of biologists hold less radical 

 views than this ; yet the opinion seems widespread that in 

 chemotaxis we are dealing with a simple primary phenomenon. 

 Coming now to an examination of the phenomena as exhib- 

 ited by Paramecium, we will first take up positive chemotaxis, 

 or attraction toward chemical substances. The phenomenon 

 to be explained shows itself as follows. If into a slide of Para- 

 mecia a drop of some attractive substance (as a weak acid) is 

 introduced, the Infusoria soon collect in the drop, forming there 

 a dense assemblage. Now, what is the exact action of the 

 attractive substance on the Paramecia to cause them to turn 

 and enter the drop ? Observing carefully the conduct of the 

 animals, we find, first, that they do not turn toward the drop. 

 Owing to its slow diffusion, the margin of a drop thus intro- 

 duced beneath the cover glass is evident, and the Paramecia, 

 swimming in every direction throughout the preparation, may 

 be seen in their random course to graze almost the edge of the 

 drop, without their motion being changed in the least ; they 

 keep on straight past the drop and swim to another part of the 

 slide. But of course some of the Paramecia in their random 

 swimming come directly against the edge of the drop. These 

 do not change their motion, but keep on undisturbed across the 

 drop. But when they come to the opposite margin, where they 

 would if unchecked pass out again into the surrounding me- 

 dium, a marked reaction is caused ; the Paramecium jerks back 

 and turns again into the drop. Such an animal then swims 

 across the drop in the new direction till it again comes to the 

 margin, when it reacts negatively, as before. This continues, 

 so that the animal appears as if caught in the drop as in a trap. 

 Other Paramecia enter the drop in the same way and are im- 

 prisoned like the first, so that in time the drop swarms with the 

 animals. As a result of their swift, random movements when 



1 Traite de zoologie concrete, tome i, p. 305. 



