102 TEXTBOOK OP ZOOLOGY 



Such successive attempts to gain the result desired constitute what 

 is known as the "trial and error'' mode of behavior. 



In an effort to defend itself when severely irritated, Paramecium 

 will discharge the contents of the trichocysts, which harden on con- 

 tact with the water and form a mass of fine threads. These threads 

 will entangle many of the aquatic enemies of these animals. 



Locomotion 



The beating action of the cilia against the water serves as the 

 principal means of locomotion. The stroke of the cilia is rather 

 oblique and this coupled with the increased length of the cilia along 

 the oral groove causes the body to turn on its long axis while swim- 

 ming. The total effect of these activities causes the course followed 

 through the water to be that of a spiral. Paramecium may reverse 

 the direction of the stroke of the cilia and thus move backward just 

 as a car can be thrown in reverse. 



The cilia are contractile outgrowths of the ectosarc. Each has an 

 elastic sheath and a fibrillar core. Contraction of the protoplasmic 

 substance on one side, bends the cilium in that direction. The re- 

 verse stroke is much more passive. The movement of one tier of 

 cilia seems to stimulate the adjacent ones to bring about coordi- 

 nated, rhythmic ciliary activity and movement. 



