272 University of California Publications in Zoology [Vol. 11 



mented upon were allowed in each case to remain in the dish 

 until the next morning, and in each ease had practically entirely 

 recovered their normal responsiveness. 



V. INDUCED DEPRESSION 



According to Cushny (1910), fatigne due to the prolonged 

 exercise of the normal organ, and as the result of depressants, 

 has in several instances been shown to be much the same. Quot- 

 ing him: "Depression, whether induced directly or following 

 on stimulation, has been shown in several instances to resemble 

 the fatigue induced by the prolonged exercise of the normal 

 organ, and it is probable that depresison and fatigue are in all 

 instances identical in appearance, although not necessarily iden- 

 tical in cause." 



In order to throw some light on the nature of fatigue in the 

 leech, since this condition seems an important factor in the ex- 

 planation of some of the features of modifiability, a number of 

 tests were made with stimulants and depressants, the general 

 effects of which are known on the system of higher and to a certain 

 extent lower organisms. The results secured shed some light 

 on the processes involved in normal fatigue in the leech, and are 

 consequently given in some detail. 



1. General Methods 



With the various substances used the necessary strength to 

 be effective was determined by adding a drop at a time of a 

 known strength of solution to the water of a definite volume in 

 a glass dish, the stimulating effects being closely observed. With 

 this known, for convenience and accuracy measurement was then 

 expressed in terms of cubic centimeters. A leech was placed in 

 a freshly made solution and upon coming to attach itself to the 

 bottom of the dish was stimulated posteriorly, the response given 

 representing the first interval recorded. A stop watch was used 

 in timing the duration of the swimming response. The speci- 

 mens were stimulated until the animal showed no further pro- 

 gressive movement. Upon reaching this condition the leech was 



