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. in the fourth twenty- 
five. 
There are many bivalve molluses which retract their 
extended siphons or close their shells upon being stimulated 
by shadows, or in some cases when subjected to increased 
illumination, but after one or more repetitions of thestimulus, 
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 
