DIMINUTION OF REACTION 143 



percentage of wigwag responses. Later, however, by 

 repeated experiences, the worm became familiar with this 

 feature of its environment and made fewer wigwag motions/' 

 the percentage in a specimen of Planaria gonocephala be- 

 coming gradually reduced from 84 per cent, in the first 

 twenty-five crossings to 32 per cent, hi the fourth twenty- 

 five. 



There are many bivalve molluscs which retract their 

 extended siphons or close their shells upon being stimulated 

 by shadows, or hi some cases when subjected to increased 

 illumination, but after one or more repetitions of the stimulus, 

 the number varying greatly in different species, the response 

 is no longer made. Gradual cessation of response to a given 

 stimulus may be due to (1) a fatigue of the motor apparatus, 

 (2) to a dulling of the sensibility of receptors, or (3) perhaps 

 to other states of the organism. With the first of these is 

 not improbably to be classed the gradual diminution of the 

 duration of death feint, which many animals show in succes- 

 sive feints. The death feigning attitude is one in which 

 the muscles are usually rigid and the condition naturally 

 involves more or less fatigue. As the feints decrease in 

 length the body also becomes more relaxed and the attitude 

 of the body less constant and characteristic. In lower 

 forms the death feint but slowly wears itself out. In higher 

 ones feigning is carried out only once or a very few times be- 

 fore the animal refuses to feign longer. Hence in many cases 

 the factor of intelligence may come into play, but whether 

 this will explain all cases where the response is given up sud- 

 denly appears doubtful. 



A considerable part of the cases of cessation of reaction to a 

 given stimulus are probably due, as Bohn contends, to a 

 "fatigue sensorielle" or dulling of sensibility. This sup- 

 position is supported by the fact that the central apparatus 



