224 



AN INTRODUCTION TO ZOOLOGY 



them. They learn by experience, and modify their behavior 

 slowly or quickly, depending upon their familiarity with the 

 situation. The following experiments prove the above state- 

 ments. Crayfishes, when placed in compartment T of a labyrinth 

 like that shown in Figure 118, will, on attempting to escape, 

 pass on one side or the other of the partition P. A transparent 

 glass screen G prevents their exit from the right side of the parti- 

 tion; the other side, however, being left open, allows a free pas- 

 sage. It was found by Yerkes and Huggins (188) that after 

 sixty trials, crayfishes that at the beginning chose the closed 

 passage 50 per cent of the time, learned to avoid that side, and in 



__^ go per cent of the 



trials chose the 

 open exit. When 

 the glass plate 

 was then removed 

 from the right of 

 the partition and 

 placed on the 

 other side, the 

 crayfishes were 

 confused at first, 

 but learned the 

 new habit of es- 

 caping at the 

 right, open side 



T 



A 



FIG. 118. Labyrinth used in experiments on the cray- 

 fish. T, compartment from which animal was 

 started; P, partition at exit; G, glass plate 

 closing one exit. (From Washburn after Yerkes 

 and Huggins.) 



more quickly than before. The " chief factors in the formation 

 of such habits are the chemical sense (probably both smell and 

 taste), touch, sight, and the muscular sensations resulting from 

 the direction of turning. The animals are able to learn a path 

 when the possibility of following a scent is excluded " (188, p. 

 577). Professor S. J. Holmes writes that he has trained cray- 

 fishes to come to him for food. 



