REACTIONS OF CRAYFISHES. 



modification is psychological then the animals must respond from 

 associated memory. Shelford and Alice ('13, '14) have pointed 

 out that it is hard to locate the things associated. Besides asso- 

 ciation formation is usually very slow forYerkes ('08) states that 

 50 to 100 trials are necessary for the crayfish to form a perfect 

 association in a simple labyrinth. The same slowness of modifica- 

 tion would be expected of association due to a stimulus unless the 

 sensitiveness of the animals was in some way progressively 

 increased. 



The modification is rapid, the number of invasions being some- 

 times but one before complete avoidance of the high end fol- 

 lowed. This modification is probably due, as Shelford and Alice 

 ('13) have pointed out, to increased sensitiveness on the part 

 of the crayfishes and as they have further suggested, the greater 

 sensitiveness may be the result of an increase in the hydrogen 

 ion content of the blood of the animals. 



In the cases where propinquus came to rest upon the screen 

 in the low end and remained there for the rest of the experiment, 

 the reaction may be considered the climax of the behavior 

 modification, especially since the animals made this reaction more 

 quickly in the presence of high total concentrations of acid than 

 in the low. These points are shown by propinquus, Chart I., 

 Expts. 5, 14. The explanation as to why the animals came to 

 rest at all after being made more sensitive by the acid is not clear, 

 but probably a combination of factors, one of which is thigmo- 

 taxis, were acting. 



Shelford and Alice ('13) suggest carbon dioxide as a factor 

 in determining the distribution of fishes and that the same may 

 be true for crayfishes is suggested by the foregoing experiments. 

 Crayfishes react to very weak concentrations of carbon dioxide 

 and acetic acid and they were not overcome by the carbon dioxide 

 except in concentrations higher than usually appear in natural 

 waters. There seems also to be a correlation of the specific 

 reactions of the two species with their respective habits. Pro- 

 pinquus is a pond form and its reactions were directive, while 

 virilis, a rapid stream form, gave reactions which were much 

 less directive. In natural waters carbon dioxide would be en- 

 countered in rather high concentrations by propinquus and that 

 this species may react to these concentrations to its own advan- 



