256 Vniversitij of Calif ornia Publications in Zoology [Vol. 11 



the more common and important of these are given. The great 

 predominance of the adaptive reaction, a turn to the left away 

 from the stimulus, is very plainly indicated. A decided ten- 

 dency for a reaction to be repeated several times in succession 

 is also to be observed. The significance of these facts in the 

 explanation of the nature of the reactions is considered in a 

 succeeding paragraph. 



3. Reactions to Stimulation of the Posterior End 

 To contact stimulation of the posterior end the responses are 

 much more stereotyped, due partly perhaps to the fact that the 

 posterior sucker is attached when the .stimulus is applied. So far 

 as I have been able to determine, there are only two types of 

 reactions when the stimulus is applied to the posterior region of 

 the body. These are as follows: 



1. By the looping response, much as a measuring worm moves 

 forward. 



2. By the release of attachment to the substratum, and the 

 execution of eel-like swimming movements. 



The first response is that usually given upon the application 

 of a slight stimulus to the animal when it is in a resting condition, 

 or in a state of low sensitivity produced from some such cause 

 as fatigue. The swimming response is the more commonly evoked, 

 and is the expression of an active condition of the organism. 

 The rapidity of its execution serves to remove the animal speedily 

 from the region of stimulation. 



4. Determining Factors of the Different Response to the 

 Same Stimulus 

 (a) Intensity of stimulus. — It is obviously impossible to stim- 

 ulate the leech each successive time to the same degree. This 

 fact alone plays its part in determining the character of the 

 response to contact stimulation of a certain degree, though its 

 influence has been eliminated so far as practicable. Under uni- 

 form conditions, when the body of the animal is extended, stim- 

 ulation with a fine hair bristle or a capillary glass rod on the 

 right side of the anterior end of the body will usually produce 



