300 University of California Publications in Zoology [Vol. 11 



not seem to be due to muscular fatigue, but to either a dulling 

 of the sensibility of the receptors or to slight changes in the 

 nerve centers concerned, or perhaps both of these latter factors 

 may be involved. 



23. Repeated contact stimulation of the posterior end induces 

 at first a gradual increase in activity followed by a gradual de- 

 crease, concluding with complete depression. 



24. The effects of strychnine, nicotine, cocaine, chloretone, 

 and magnesium sulphate on the leech appear in their funda- 

 mental characteristics the same as those produced in the higher 

 animals. 



25. Carbon dioxide, mono-potassium phosphate, and lactic 

 acid, the substances probably causing fatigue in the higher ani- 

 mals, are found experimentally to produce at first a much in- 

 creased activity in the leech, followed by a complete depression. 

 This parallelism in the effects of these substances suggest that 

 they are very likely contributory to the production of fatigue 

 in the leech. 



26. The presence of diffusing food juices intensifies the ten- 

 dency to positive reaction to light contact stimulation. This is 

 particularly the case in the starved leech. Thigmotactic pro- 

 clivities very materially affect, at times, the reactions of the 

 animal to light. 



27. The internal states of the leech exercise a profound in- 

 fluence upon its behavior. The chief effect of hunger and satiety 

 seems to be that of altering the irritability of the animal ; hunger 

 producing a greater relative responsiveness; satiety, a tendency 

 towards sluggishness. It is suggested that the presence of a 

 superfluity of foodstuffs within the body leads to the accumu- 

 lation of intermediate metabolic products which may react on 

 the animal as do the fatigue substances. 



28. The attempt in this paper on the modifiability of be- 

 havior in the leech has been to analyze, so far as possible, this 

 behavior into its component physiological factors. In the opinion 

 of the writer, such a method should lead to the kind of results 

 most needed in the science of animal behavior. 



