J. M. D. OLMSTED 63 



wave appeared only in the old tissue, never in the new, and that 

 it appeared infrequently and only after several waves had passed 

 down the uninjured side. I should explain the appearance of 

 the wave on the side lacking its half of the brain on the ground 

 that through the physiological principles of ^facilitation' and 

 'summation,' the movement of a wave down the uninjured side 

 was able to serve as a stimulus, which, upon being transmitted 

 through the connecting commissure immediately behind the 

 brain to the other nerve cord, was sufficient to initiate an occa- 

 sional wave. (Cf. the diagram of the ventral nervous system of 

 L. saxicola given by Heath and McGregor, '12, fig. 21.) 



If the stepping movement in ditaxic locomotion is controlled 

 by the cephalic ganglia, the question arises as to the function of 

 the longitudinal nerve cords. Various operations were tried in 

 order to answer this question. It was found necessary to make 

 observations fairly soon after cutting the nerve cords, for within 

 three or four days a wound extending almost the entire width of 

 the body would be joined together and perfect coordination re- 

 stored. For example, in a specimen of L. saxicola a horizontal 

 slit was made posterior to the brain, severing both nerve cords. 

 The part anterior to the wound at once moved rapidly by the 

 ditaxic method of locomotion and also attempted to swim freely 

 in the water. The posterior part moved slowly by the ataxic 

 method. The consequence was that the pulling of the anterior 

 end caused the wound to gape in a wide circle. When the worm 

 came to rest, the edges of the wound closed. Two days later the 

 wound was healed and on the next day perfectly coordinated al- 

 ternate waves passed down the entire length of the worm in a 

 normal manner. 



The nerve cords serve as conductors of impulses which cause 

 progression of the waves in ditaxic locomotion. This is nicely 

 show^n by cutting a portion from the side of a worm without in- 

 juring the nerve cord. A wave starting at the head end of the 

 body stops when it reaches the upper edge of the wound, for the 

 muscles which would carry the wave on have been cut away. 

 But the wave again makes its appearance at the proper time at 

 the lower margin of the wound, and then continues on to the 



