THE BEHAVIOR OF INFUSORIA; PARAMECIUM 



53 



the Paramecium turns in the same manner as before, toward the 

 aboral side, and hence toward the point touched. This experiment is 

 more easily performed, and the results are more striking, with certain 

 of the Hypotricha, 1 because these animals do not continually revolve on 

 the long axis, as Paramecium does. 



The general effect of the avoiding reaction is to cause the animals 

 to avoid and escape from the region in which the stimulus is acting. 

 This may be illustrated for the different classes of stimuli in the follow- 

 ing ways. 



The effects of this reaction to chemicals may best be seen by intro- 

 ducing a little \ per cent solution of sodium chloride into the water 



containing the ani- 



mals. For this pur- 

 pose water with many 

 Paramecia is placed 

 on a slide and covered 

 with a long cover- 

 glass supported near 

 its end by glass rods. 



chemical into a slide 



FIG. 40. Method of introducing 

 of infusoria. 



A medicine dropper 



is drawn to a long, 



slender point, and with 



this a drop of the salt 



solution is introduced 



beneath the cover-glass, as illustrated in Fig. 40. The Paramecia are 



swimming about in all directions, but as soon as they come to the 



region of the salt solution, the 

 avoiding reaction is given in the 

 way already described, and 

 the animals swim elsewhere. 

 Thus the drop of salt solution 

 remains empty (Fig. 41). 



Practically all strong chemicals 



FIG. 41. -Slide of Paramecia four minutes ""luce the avoiding reaction, SO 



after, the introduction of a drop of 

 NaCl. The drop remains empty. 



per cent 



that Paramecia do not enter them. 



^^ ^ b ^ n sh()wn for many 



alkalies, neutral salts, and organic substances, and for strong acids. 

 In the case of acids the reaction differs in certain respects from the 

 behavior under the influence of other chemicals ; this will be brought 

 out later. 



The reaction to heat or cold may easily be shown by placing a drop 



i See Chapter VII. 



