94 BEHAVIOR OF THE LOWER ORGANISMS 



The contact reaction may completely prevent the reaction to gravity. 

 Paramecia placed in a tube which contains many bits of solid matter, or 

 has its walls rough or dirty, usually do not rise to the top, but settle 

 against the solid matter on the wall and remain. They may thus re- 

 main scattered through all parts of the tube, or may gather in any por- 

 tion of it where the material inducing the reaction is found. Specimens 

 at rest against a solid may occupy any position with reference to gravity. 

 In similar ways the contact reaction may prevent the usual reaction 

 to water currents. 



The interference between the contact reaction and the reaction to 

 the electric current produces a number of peculiar results. If a weak 

 electric current is passed through a preparation containing many speci- 

 mens attached to a bit of debris or to the surface of the glass, the free 

 specimens swim at once toward the cathode, while the attached speci- 

 mens do not react at all. If the current is made stronger, it produces 

 for an instant the usual effect on the cilia of the attached specimens. 

 The cathodic cilia strike forward, the anodic cilia backward. But this 

 does not continue; after a moment the contact reaction resumes its 

 sway, and the cilia have their usual positions. If the current continues, 

 after a short time the cilia are again affected as before; then resume 

 their original positions. This may occur many times, — the two stimuli 

 alternating in their control of the cilia. If the current is made much 

 stronger, the animal finally leaves the solid. It then swims directly 

 to the cathode in the usual way. To induce this reaction in a resting 

 specimen, it requires as a rule two or three times as intense a current as 

 that needed for producing the same effect on free swimming animals. 



If the electric stimulus is first in action and the Paramecium then 

 comes in contact with a solid, somewhat different results are produced. 

 If the current is weak, often the animal, swimming toward the cathode, 

 ceases to react to the electricity on coming against the solid ; it may 

 then take up any position on the surface of the solid. If it comes against 

 the surface film of the water, or the surface of the glass slide, it may 

 cease its forward movement only for an instant, then, becoming free, it 

 may swim again toward the cathode. If the current is a little stronger 

 (such as to produce the maximum rapidity of movement toward the 

 cathode, in free swimming specimens), a different effect is produced. 

 The Paramecium stops against the surface of the solid, and places itself 

 transversely or obliquely to the current, with the oral surface toward the 

 cathode (Fig. 67). Here it remains, the current produced by the cilia 

 being everywhere backward save in the oral groove, where it is forward. 

 If the electric current is reversed, the oral cilia strike strongly backward, 

 and the animal at once turns on its short axis till the oral surface 



