THE BEHAVIOR OF INFUSORIA; PARAMECIUM 



97 



shows that as soon as they reach the boundary of the area of carbon 

 dioxide, they give the avoiding reaction, in the usual way, and pass back 

 into the area. Here they become oriented again by the electric current, 

 and pass again to the boundary, where they react as before. Thus the 

 reaction to the electric current prevails until a region of a sudden change 

 in chemical character is reached; the reaction to this then supplants 

 the reaction to the current. If the current is reversed, the animals 

 gather in the same way at the opposite side of the area of carbon dioxide 

 (Fig. 68, C). If the current is made very powerful and is long continued, 



Fig. 68. — Interference of chemicals with the reaction to the electric current. At A Para- 

 mecia have gathered in an area containing COj. At B an electric current is passed through 

 the preparation with cathode at the left; the animals gather at the left edge of the area of CO2. 

 At C the current has been reversed; the animals are therefore gathered at the right edge of the 



the Paramecia are one by one caused to cross the boundary of the acid 

 area and to swim to the cathode. If a drop of some repellent chemical 

 — as sodium chloride or an alkah — is introduced into a preparation 

 (Fig. 41), the Paramecia of course leave this vacant. If the electric 

 current is passed through the preparation, the Paramecia swim toward 

 the cathode ; coming to the boundary of the drop, they swim around it, 

 leaving it empty, and thus reach the cathode. In this case the path 

 followed is a resultant of the operation of the two stimuh, — the orienta- 

 tion due to the electric current and the avoiding reaction produced by 



the chemical. 



If the entire region next the cathode is occupied by a repellent chemi- 

 cal, the Paramecia may be forced by a strong and long-continued cur- 

 rent to enter it till they are destroyed. 



A very pecuHar interaction of chemicals and the electric current is 

 seen when Paramecia are placed in physiological salt solution (0.7 per 



