172 BEHAVIOR OF THE LOWER ORGANISMS 



not as a rule take place save in response to well-marked stimuli. When 



not disturbed in any way, the animal remains extended, with cilia in 



active operation. 



Let us try the effect of disturbing the animal very slightly. While 



the disk is widely spread and the cilia are actively at work, we cause a 

 fine current of water to act upon the disk, in the 

 following way. A long tube is drawn to a very fine 

 capillary point and filled with water. The capillary 

 tip is brought near the Stentor, while the long tube 

 is held nearly perpendicular. The pressure causes 

 a jet of water from the tip to strike the disk of the 

 animal. Like a flash it contracts into its tube. In 

 about half a minute it extends again, and the cilia 



*IG. no. — Stentor . . i 



rceseia contracted into resume their activity. Now we cause the current to 

 lts tube - act again upon the disk. This time the animal does 



not contract, but continues its normal activities without regard to 

 the current of water. This experiment may be repeated on other indi- 

 viduals; invariably they react to the current the first time, then no 

 longer react. The same results are obtained with other fixed infuso- 

 ria: Epistylis and Carchesium. By using other very faint stimuli, 

 such as that produced by touching the surface film of the water close 

 to the organism, or by slightly jarring the object to which it is at- 

 tached, the same results are obtained. To the first stimulus they 

 respond sharply ; to the second and following ones they do not respond 

 at all, even if long continued. 



Thus the organism becomes changed in some way after its first 

 reaction, for to the same stimulus, under the same external conditions, 

 it no longer reacts. What is the nature of this internal change? The 

 first suggestion that rises to the mind in explanation of such a cessation 

 of reaction is that it may be due to fatigue. The distinction between 

 fatigue and other changes of condition is an important one, for the 

 following reason. Fatigue is due to what may be called a failure. It 

 is an imperfection inherent perhaps in the nature of the material of 

 which organisms are composed, preventing them from doing what might 

 be to their advantage. Changes of reaction due to other causes might 

 on the other hand be regulatory, tending to the advantage of the or- 

 ganism. Higher animals often react strongly by a "start," to the first 

 incidence of sudden harmless stimuli, then no longer react, and this 

 cessation is evidently a regulation of behavior that is to the interest of 

 the organism. We must then determine whether the failure of the in- 

 fusorian to react to the second stimulation is due to fatigue or to some 

 other cause. 



