'^^1^] Gee: Behavior of Leeches 297 



elusions, still the aim throughout the second part of this paper 

 has been to present, so far as possible, a causal explanation of 

 the behavior described. 



The different responses to the same stimulus have been shown 

 in their essential features to be in accord with our knowledge of 

 reflex-are structure and what might be expected of its conduc- 

 tivity in the various stages of excitement of the leech. Accli- 

 matization to slight stimuli has been explained on the basis of 

 the dulled sensibility of the receptors and slight changes in the 

 nerve centers involved. It has been shown that the phenomenon 

 of fatigue in the leech possesses the same fundamental charac- 

 teristics as fatigue in skeletal muscle. An important factor in 

 explaining the behavior of the organism at a given moment has 

 been shown to be the consideration of the concurrent stimuli 

 operative at that moment. Perhaps intermediate metabolic pro- 

 ducts are the cause of much of the difference in responsiveness 

 between normal and well fed leeches. The increased irritability 

 of starved leeches is likely due to much the same cause. 



The stage has been reached in the investigation of animal 

 behavior where vague generalizations cease to be of value. The 

 most urgent need of the science is a deliberate and extended 

 study of the component factors of the physiological states so 

 profoundly modifying the character of reaction to a given stim- 

 ulus. Each animal, as well as each group of animals, possesses 

 its own characteristic types of reaction, which demand for their 

 proper understanding a careful physiological analysis. 



IX. SUjMMARY 



A. The General Reactions of the Leeches Dina microstoma 



Moore and Glossiphonia stagnalis Linnaeus 



1. The most abundant species of leeches occurring in the 

 vicinity of Berkeley appear to be Glossiphonia stagnalis Linnaeus 

 and Dina microstoma Moore. It was upon these two kinds of 

 leeches that the major portion of the work embodied in this paper 

 was performed. 



2. The chief interest manifested in the literature on the re- 

 actions of leeches has been centered in the blood-leech because 

 of its former important relation to medicine. 



