Jennings, UcxkiiU on Physiology of Behavior. 323 



stt'uJy tension (as (k'scrilicd alinvc), contract readily in response 

 to even a weak stiumlus, tending thns to bend tlie spine toward that 

 side on wliicli there has been tension. 



From this a nnndnu- of jieenliar facts resnlt. As we have seen, 

 a moderate stimidns at a certain point tends to cause the spines to 

 bend toward that point. If, as a result of pressure, the muscles that 

 face the point stimulated have lost their tonus, they respond readily; 

 the spine at once bends toward the side stimulated. P>ut if the 

 spines have been pn^ssed over in the opposite direction, so that their 

 muscles facing the point of stinmlation are in strong tonus, no 

 effect is produced; the spines retain their ])osition. Hence, when 

 a spot on the body is stimulated, certain s])ines will respond while 

 others will not, depending on the previous tonus of their muscles. 

 This phenomenon v. Uexkiill calls "Ivlinkung" ; those spines which 

 are in such a condition or position that they can respond to the 

 stimulus are said to be "eingekliukt" ; those which are not are ''aus- 

 geklinkt.'' These expressions may perhaps be translated by "in 

 circuit" and "out of circuit," — comparing the spines with instru 

 ments in an electric circuit. This condition of affairs has great 

 importance for the functioning of the s])ines in locomotion and else- 

 where, and parallel conditions are found in other organisms. 



A similar analysis is given by the author for the pedicellariis, 

 tube-feet, teeth, etc. 



Thus by a close and thorough study v. Uexkiill has been able to 

 analyze and formulate a number of what have been called vaguely 

 the varying "physiological states" of organs or organisms; such 

 analysis is needed for all cases. By making use of the concepts of 

 Reflexumkehr, Reflexverkettung, Wanderung des Erregungsmittel- 

 punkts, Schaltung, Klinkung, and by observing the changes in tonus 

 and the rules for its increase and decrease, one can explain some of 

 the most important features in the behavior of the sea urchin under 

 natural conditions; locomotion, negative reactions to various stimuli, 

 defence from enemies, capture of food, etc. It is, of course, no 

 disparagement of the value of this analysis that it does not exhaust 

 the matter for the sea urchin. Thus, when the animal is turned 

 on its back, its spines move in ways that would not be expected 



