300 



ESSENTIALS OF ZOOLOGY 



seems not to be affected by ordinary light. It reacts either posi- 

 tively or negatively to contact, change of chemical constitution, 

 change in temperature, to gravity, and to electric current. The re- 

 sponse to contact is positive, negative to ultraviolet light, negative 

 to sodium chloride, positive to weak acetic acid, and positive to th« 

 negative pole of a weak, galvanic electric current. The optimum 

 temperature for Paramecium ranges between 24° and 28° C. (71° 

 F.). Gravity causes the anterior end to point upward, and when 

 placed in moving water, the animals will swim upstream. If Para- 

 mecium comes in contact with a solid object when it is moving, it 

 will back away, swing on its posterior end to a slightly different 



10' 



25' 



Fig. 102. — Reactions of paramecia to temperature, a, Paramecia in a trough 

 with a uniform temperature of 19° C. The animals are evenly scattered through 

 the water. In &, the temperature is held at 26° C. at the left end and 38° C. at the 

 right. The animals are congregated at the end of the lower temperature. In c the 

 temperature is 25° C. at one end and 10° C. at the other and the animals have all 

 collected in the region of the higher. An optimum temperature for these animals is 

 evident. (From Jennings, Behavio7' of the Lower Organisms, The Columbia Univer- 

 sity Press.) 



direction and try again. This may be repeated, and is known as 

 the ''avoiding reaction." Such a reaction really involves simply 

 one or more negative responses. These animals are constantly sam- 

 pling the water and avoiding the conditions which are least favor- 

 able. This may be repeated in all directions. The same type of 

 persistence is practiced in attempting to surmount a solid barrier. 

 Such successive attempts to gain the result desired constitute what 

 is kno^vn as the *' trial and error'' mode of behavior. 



