^^^^] Gee: Behavior of Leeches 295 



The general responsiveness of the leeches in normal, starved, 

 and well-fed states indicates the influence that metabolic condi- 

 tions may have upon the reactions of the animal which has formed 

 the subject for this investigation. The effect of feeding to satiety 

 seems a generally depressing one, perhaps due, as has been shown 

 in the case of fatigue, to substances produced in the body tissues 

 of the animal incident to its digestive processes. In this con- 

 nection an idea of Lee (1910) is highly suggestive: "It is im- 

 po.ssible to avoid a strong suspicion that the presence of a super- 

 fluity of foodstufi's within the body leads to an accumulation 

 of intermediate metabolic products which in themselves act on 

 the tissues as fatigue substances." In the starved condition, the 

 animal has remained quiet sufficiently long for the system to 

 have become largely rid of such substances, and at first is some- 

 what low in responsiveness. As the fatigue substances increase, 

 irritability is increased to the stage where the accumulation of 

 these substances and the depleted stores of energy furnishing 

 materials cause depression to set in. 



VIII. GENERAL CONSIDERATIONS ON MODIFIABILITY 

 IN LEECHES 



Much stress has been placed in recent years upon the great 

 variability of reactions in organisms. Not only has there been 

 shown in numerous instances marked differences in the behavior 

 of different individuals of a particular species, but there have 

 been demonstrated wide variations in the responses given by the 

 same individual to a particular stimulus at different times. This 

 variability of reaction has been shown to be largely due to in- 

 ternal changes in the animal. Attention has been directed ((uite 

 properly towards this plastic element in an organism's respon- 

 siveness ; for the work which has been the outcome of the pro- 

 jection of such a view has served to place the behavior of an 

 animal in its true perspective. Par from projecting the analysis 

 of behavior into the realm of vagueness and mysticism, it has 

 only made more apparent the fact that the problem of this 

 analysis is one much more complex than was formerly supposed. 



The most interesting and significant feature in animal be- 

 havior is the regulatory nature of the responses. The fact that 



