126 GEORGE DELWIX ALLEN. 



EXPLANATION OF PLATES. 

 PLATE I. 



FIG. i. The positive spiral. Photograph of worms giving a positive reaction 

 to a current of water stirred in the clockwise direction. 



FIG. 2. The negative spiral. Photograph of worms giving a negative reaction 

 to a current of water stirred in the clockwise direction. The worms near the center 

 are reacting negatively while those about the outer part of the pan, in the stronger 

 current, are reacting positively. 



