ADAPTATION OF ORGANISMS 



341 



4 



It tries various new positions until its anterior end is directed 

 upward, and continues in that direc- 

 tion. It thus reaches the surface film. 

 To this it responds by the avoiding 

 reaction (Fig. 183), finding a new 

 position and swimming along near 

 the surface of the water. . . . Swim- 

 ming forward here, it approaches a 

 region where the sun has been shining 

 strongly into the pool, heating the 

 water. The Paramecium receives 

 some of this heated water in the 

 current passing from the anterior 

 end down the oral groove. (Fig. 

 184.) Thereupon it pauses, swings 

 its anterior end about in a circle, 

 and finding that the water coming 

 from one of the directions thus 

 tried is not heated, it proceeds for- 

 ward in that direction. This course 

 leads it perhaps into the region of a 

 fresh plant stem which has lately 

 been crushed and has fallen into the 

 water. The plant juice, oozing out, 

 alters markedly the chemical consti- 

 tution of the water. The Parame- 

 cium soon receives some of this 

 altered water in its ciliary current. 

 Again it pauses, or if the chemical 



was strong, swims backward a dis- 



t 



FIG. 184. Diagram to show the rotation on the long axis, and the spiral path of 

 Paramecium. 1-4, successive positions assumed. The dotted areas with small arrows 

 represent the currents of water drawn from in front. (After Jennings.) 



