258 University of California Publications in Zoology [Vol. 11 



side will usually produce a sharp turn to the right. If the animal 

 is crawling forward, the anterior sucker attached, stimulation 

 of the anterior end will produce usually onl.y a local contraction 

 of the part stimulated, the leech continuing its forward move- 

 ment at an accelerated rate. Such modification of behavior as 

 this holds much in common with the case of striking a man 

 when he is stretched on the ground. He must first get up be- 

 fore it is possible for him to strike back eiJeetively. The leech 

 in crawling forward is executing, as Von Uexkiill (1905) has 

 shown in the blood-leech, a sequence of reflexes. A certain 

 equilibriiun of the body is well established through this action, 

 which first necessitates the completion of the reaction started. 

 This is what is usually done except when an intensely strong 

 stimulus is applied. In such a case, the leech will release the 

 anterior sucker immediatel,y and swing violently away from the 

 stimulating agent. The average strength of stimulus applied 

 to the anterior end when the leech is crawling forward is often 

 transformed into an accelerating factor, causing the looping 

 response to be more quickly executed. 



{d) Tonus of the Organism. — From his work on Planaria, 

 Pearl (1903) concludes that the "physiological states" in this 

 form play a large part in determining its behavior. These states 

 he classifies as follows : a resting or sleeping condition ; a normal 

 undisturbed activity ; a condition of heightened activity ; and 

 an excited condition. He also records states of hunger and 

 satiety. Jennings (1906) gives as examples of similar states in 

 the earthworm : the state of rest ; a state of excitement ; a state 

 of greater excitement ; a state of still greater excitement ; and 

 a state of still more intense excitement. This classification does 

 not include "all physiological states resulting from varied states 

 of metabolism, and possible lasting modifications of physiological 

 states (habits, etc.)." The same writer says: "The movement 

 at a given time demonstrably depends not alone on present 

 external conditions, but also on former external conditions, for- 

 mer actions of the organisms, and present internal physiological 

 conditions that are determined in many ways." 



These same states are to be observed in the leech, each having 

 its own more or less characteristic mode of response. When 



