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HABIT-FORMATION IN THE GREEN CRAB, 

 CARCINUS GRANULATUS. 



ROBERT YERKES, 

 MARINE BIOLOGICAL LABORATORY, WOODS ROLL, MASS. 



In his excellent paper l on the structure of the nervous system 

 of Carcinns incenas Albrecht Bethe describes certain experiments 

 the results of which he believes to justify the conclusion that the 

 green crab does not profit by experience, and therefore does not 

 possess " psychischen Qualitaten." 



The following are two of the experiments by which Bethe 

 tested the ability of Carcinns to learn. A crab was placed in an 

 aquarium which contained an Elcdonc in the darkest corner. 

 Immediately the crab scurried off into the dark region and was 

 seized by the Elcdonc and drawn under its mantle. The experi- 

 menter then interfered by quickly freeing the crab from its enemy 

 and returning it to the opposite end of the basin. Again the 

 crab hurried into the shaded region, was seized, freed and re- 

 turned to the light. This experiment Bethe says he repeated 

 with one individual five times and with another six times without 

 obtaining any evidence that the crabs were learning to avoid the 

 danger. In another test crabs in a basin were baited with meat, 

 and each time they tried to get the food they were seized and 

 maltreated by the experimenter. Thus an attempt was made to 

 teach them that meat, under certain conditions, meant danger. 

 But in this test also several repetitions of the experiment failed to 

 teach the crabs to avoid the danger. 



Bethe's tests are unsatisfactory because in each he attempts to 

 inhibit an instinctive action. In the first test the animal natur- 

 ally sought to hide in the dark ; and it would be absurd to ex- 

 pect to modify this instinct by giving the animal five, six or even 

 twenty-five experiences of the disagreeable consequences of going 

 into the neighborhood of the Elcdonc. And in the second test 

 the reflex food-seeking reaction to a particular kind of chemical 



i Bethe, Albrecht : Das Centralnerven-system von Carcinns mccnas. II. Theil., 

 Arch. f. Microscop. Anat., Bd. 51, 1898, S. 447. 



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