1913] Gee: Behavior of Leeches 251 



composed of invariable reflexes, occiirriDg- always in the same 

 way under the same external circumstances. This is far from 

 the truth and leads, as it seems to the writer, to a fundamentally 

 false conception of the nature of animal behavior. Inner states 

 and changes are full.y as important in determining behavior as 

 are external stimuli, modifying fundamentally the reactions which 

 the latter produce." 



The works of Jennings (1902, 1905, 1906), Bolm (1907, 1909), 

 Yerkes (1906), Holmes (1905, 1907, 1911), Hargitt (1906), and 

 Allee (1912) are the chief contributions to this phase of animal 

 behavior. Considering its importance, the field is a rather neg- 

 lected one, and certainly its complexity will afford work enough 

 for some time to come. The following study was undertaken to 

 learn, so far as possible, the general features of modifiability in 

 the leech, Dina microstoma Moore. Except where specifically 

 mentioned, no other species of leech was employed in the experi- 

 ments discussed in the section of this paper dealing with the 

 sub.ject of modifiability. 



II. DIFFERENT RESPONSES TO THE SAME STIMULUS 



Every animal with any degree of complexity of structure 

 shows a considerable variation, qualitative as well as quantitaive, 

 to successive applications of the same stimulus. Not only is this 

 true in the behavior of the more complex organisms, but it holds 

 with equal force in certain of the lower forms. A case in point, 

 which has become classic in the literature on animal behavior, 

 is the modified responsiveness of 8 tent or (Jennings, 1906) to 

 particles of carmine repeatedly applied to its di.sk. The work 

 of Pearl (1903) has served to emphasize this feature of behavior 

 in the planarian. Jennings (1906) has furnished a very sug- 

 gestive analysis of the factors determining the direction and 

 character of the movements in the earthworm. The leech, how- 

 ever, is an aquatic annelid, and its repertoire of responses difl'ers 

 considerably from that of the eartliw'orm. For this reason a 

 classification and analj'sis of its different reactions to the same 

 stimulus seem desirable. 



