274 SELIG HECHT 
act of adaptation, it is not possible to state with any degree of 
confidence on the basis of the mere observation of the animal. It 
seemed therefore advisable to record the amplitude of response 
during the period of repeated stimulation. In this way it was 
hoped to gain at least a clearer presentation of the problem. 
Accordingly, an apparatus was arranged by means of which 
the amplitude of any reaction would be recorded on the slowly 
moving drum of a kymograph. The animal was stimulated by 
the vibrations produced in the water by the impact against the 
glass aquarium of the pendulum bob previously used. It was . 
found more satisfactory to apply a stimulus just as the animal 
had recovered from the effects of the preceding stimulus. This, 
in fact, amounted practically to a stimulation at regular intervals 
of about one minute. 
In figure 1, is shown the record! of a series of contractions pro- 
duced by the impact of the bob swinging from a distance of 30 
em. The animal failed to contract after a half hour of regular 
application of the stimulus. If the shape of the record be com- 
pared with the fatigue curve of a voluntary muscle within the 
organism (Howell, ’12, p. 48), their similarities with regard to the 
presence of a fatigue level and of a rapid final decrease in ampli- 
tude will hardly be doubted. 
The presence of an appearance like that of the fatigue level of 
vertebrate muscle may be shown better when the stimulation is 
not so intense. For instance, figure 2 records the results of a 
regular stimulation by the bob swinging from a distance of only 
20 cms. The animal responded for an hour, the arrow at point a 
indicating when stimuli applied at intervals of 15 seconds failed 
to call forth a reaction. An inspection of the record shows that 
for about three-quarters of an hour the amplitude of response to 
each stimulus was practically thesame. At the close of the period 
the amplitude decreased suddenly and the animal ceased to re- 
spond. These are precisely the effects exhibited by a muscle 
under similar conditions. 
4 For the apparatus by means of which these records were made, see figure 7 
in the first paper of this series (Hecht, 717). 
