232 University of California Publications in Zoology [Vol. 11 



the responsiveness of the animal. To a localized application, 

 by means of a capillary glass rod, of water heated to 70° C, a 

 negative response witli an accentuation of random movements is 

 produced. To a higher temperature, 90°-95° C, a sharp negative 

 response accompanied by the swimming reaction is the usual 

 habit. As already stated, the effect of a uniform temperature 

 of 36° C is to produce a markedly increased activity of the 

 animals subjected to it. When currents of heated water were 

 induced in a long, shallow dish by immersing a flask of water 

 heated to 90° C, a tendency was observed for the animals to 

 collect in the regions of lower temperature, thus seeking as nearly 

 as was possible their optimum temperature conditions. 



10. Thigmotaxis 



The strongly positive thigmotactic propensities of the leech 

 have already been mentioned as a factor in the formation of 

 collections. Positive thigmotaxis is still further evidenced in 

 the large number of individuals that will find shelter underneath 

 a small stone. When placed in an aquarium with mud and 

 decaying vegetation in the bottom, Dina is often observed with 

 two-thirds of the posterior part of the body buried among this 

 debris, the anterior end projecting and exercising undulatory 

 movements. 



To a localized contact stimulus applied to the anterior end 

 the usual response is a turn in the direction opposite to that of 

 the stimulus. This varies rather widely under successive stimu- 

 lation and other factors discussed in a later section of this paper. 

 There is considerably less variability in response to stimulation 

 of the posterior end. In the posterior end stimulation, the re- 

 sponse is either by "looping" or by "swimming," just which 

 response is given depending upon several factors discussed in the 

 second portion of this paper. 



11. Ability to withstand Desiccation 



In most localities it would be comparatively rare that the 



species Dina microstoma or Glossiphonia stagnaUs would have to 



undergo desiccation in its native habitat, and it is very fortunate 



for the species that this is true. Within a relatively few minutes 



