142 MODIFICATIONS OF BEHAVIOR 
the behavior of the worm was the same as before. The 
writer has found that mosquito larvee which descend quickly 
from the surface of the water when a shadow passes over 
them, cease to react after a number of trials but become 
as responsive as ever after a short rest. 
Many forms which react by contraction to a sudden increase 
or decrease of illumination show similar modifications. The 
leech Clepsine, when a shadow is cast upon it, raises and 
elongates the anterior end of the body, but after a number of 
such reactions shadows produce no result. Mrs. Yerkes in 
experimenting on a tube dwelling annelid Hydroides found 
that shadows which at first caused the worm to retract into 
its tube, would, if repeated at brief intervals, produce no 
response. With longer intervals the response was more 
regular. Hargitt in studying the same form arranged a 
pendulum so that it would throw a shadow on the worm 
at regular intervals and found that “with the full second 
movement there was more or less constant reaction with 
each passing shadow. With the half second movement it 
was found that, after the first few beats, a considerable 
portion of the worms failed to respond at all, and with 
the quarter second beats almost all the colonies became 
indifferent to the presence of the passing shadows.” A 
condition which Hargitt considers akin to fatigue is finally 
produced, in which the organism becomes comparatively 
irresponsive to light. Walter in his studies on the reactions 
of planarians to light finds that “when worms were placed 
in a field of non-directive light, parts of which were of two 
different intensities, the number of wigwag responses 
made at the critical line separating the two intensities 
grew less after the animals had repeatedly crossed the line. 
At first the new condition of sharply contrasted light in- 
tensities in the worm’s field of locomotion called out a large 
