142 MODIFICATIONS OF BEHAVIOR 



the behavior of the worm was the same as before. The 

 writer has found that mosquito larvae 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 



