1913] Gee: Behavior of Leeches 285 



of responsiveness. From the experiments discussed in the several 

 preceding pages, it becomes clear that the modus operandi of 

 this fatigue is not essentially different from that known in the 

 higher animals. The various depressants used act upon the leech 

 in a manner fundamentally very similar to their action upon 

 vertebrate types. Carbon dioxide, mono-potassium phosphate, 

 and lactic acid have been found in the fatigued muscle of the 

 higher forms, and when applied to these artificially produce a 

 condition of depression. While the analysis of fatigued leech 

 muscle has not been made, I have found that these same sub- 

 stances produce a condition of fatigue in this form. It appears 

 very likely to the writer that in the action of these "fatigue 

 substances" is to be found the explanation of many of the "states 

 of excitement" discussed in a preceding portion of this paper. 

 Such results as those secured from the various substances used 

 on leeches lead one to extend to the lower organisms the fol- 

 lowing conclusions of Lee (1907): "The facts here reported 

 seem to emphasize anew and strikingly the great desirableness 

 of a very careful and full investigation of the physiological 

 actions on cells, tissues, and organs of the products of meta- 

 bolism, both intermediate and final products. We seem not yet 

 to realize how potent may be the influence of such substances. 

 We incur the charge of being unscientific if without experimental 

 data we deny to even the humblest katabolic product a possible 

 role as a physiological reagent. It seems to me that physiology 

 is destined to make great progress along the lines here indi- 

 cated." Such a viewpoint as this is certainly suggestive in its 

 bearing upon the future work in the field of animal behavior. 



VI. EFFECT OF COMBINED STIMULI 

 1. Influence of Food Juices on Reaction to Contact 

 (a) Normal Individuals.— Otten when a delicate glass rod 

 is brought gently alongside the extreme anterior end of the 

 leech the animal responds by inclining its anterior end in the 

 direction of the stimulus and momentarily attaching the anterior 

 sucker. An immediate recoil is produced, yet nevertheless a 

 positive reaction is involved, the adaptivene.ss of which has been 



