1913] 6ee: Behavior of Leeches 261 



In applying the conception of chain reflexes to the responses 

 of the leech jnst considered, it is well to bear in mind the quali- 

 fication of Jennings (1906) : "If this term [chain reflex] is 

 used, it needs to be kept in mind that in most cases the succeeding 

 phase is not invariably and irrevocably called up by the pre- 

 ceding one, as is implied by this term. On the contrary, the 

 relation between the two is extremely variable. One type of 

 action may be repeated many times before the second type comes 

 into play, and the order of the different actions is by no means 

 always the same." 



III. ACCLIMATIZATION TO STIMULI 

 1. Acclimatization to Repeated Jars 



The extreme sensitiveness of the leech to the various classes 

 of stimuli is a very characteristic feature of the behavior of 

 these animals, and one that is easily overlooked if proper care 

 is not exercised. The nature of the response to a slight jar of 

 the dish in which the animal is kept varies much with the strength 

 of the shock, and the position and internal states of the organism. 

 If the animal is in a resting condition, the slight stimulus of 

 the jar may cause it to move forward by looping. If the leech 

 is already moderately active, the stimulus may excite it to greater 

 activity, such as swimming or more rapid looping movements. 

 "When it is executing undulatory respiratory movements it may 

 contract and hug the bottom of the dish, later resuming the same 

 tj-pe of reaction. Again, it may react by contraction, cease the 

 undulatory movements, and follow up the line of orientation 

 resulting from random movements evoked by the stimulus. The 

 tendency towards the temporary reversal of geotaxis has been 

 previously discussed in a consideration of the general reactions 

 of leeches. 



It is often a difficult matter to secure a leech which will con- 

 tinue its rhythmical movement after the cessation caused by the 

 stimulus of a slight jar or shadow. The most satisfactory way 

 found was as follows: Place a stone in the aquarium on the 

 side of the dish away from the window. The leeches will accum- 

 ulate underneath this shelter and, protruding the anterior two- 



