62 LOCOMOTION OF POLYCLADS 



Splitting the head through the brain usually resulted in loss 

 of this ditaxic type of movement. In two cases, however, parts 

 of the cerebral ganglia remained intact. In one of these the cut 

 removed a portion of the right half of the brain. For a week 

 after the operation this worm performed the walking movement 

 with its left side perfectly and moved in circles to the right. 

 Later, an occasional wave on the right alternated with those on 

 the left. This imperfect coordination continued even after a 

 month, so that the animal was never able to move in a straight 

 line. In the other case the brain was evenly divided. Each 

 piece regenerated all its parts except the missing half of the 

 brain. The smaller piece which contained the left half of the 

 brain moved in circles to the right by means of a single wave 

 which took its origin at the usual point and passed down the 

 left margin. There was never any indication of a wave in the 

 new tissue on the right side during the month it was under ob- 

 servation. In the larger piece in which the left half of the brain 

 was missing, waves passed down the right side quite normally 

 with only an occasional alternate wave down the left side, so 

 that this piece moved in circles to the left. By the end of a week, 

 however, the waves on the left side made their appearance some- 

 what more frequently and alternated some two or three times 

 with waves on the right side. These waves never appeared in 

 the new tissue, they began posterior to the cut. The worm 

 remained in this condition for more than a month, still circling 

 to the left and lacking perfect coordination between the right 

 and left halves of its body. 



These experiments indicate that each cerebral ganglion con- 

 trols the 'stepping movement' on its own side, each half being 

 independent of the other. But for perfectly coordinated move- 

 ments involving both sides of the body, the connection between 

 the halves of the brain must be intact. In view of the fact that 

 a few coordinated movements were possible in the two cases just 

 described, where a portion or all of the brain had been removed 

 from one side of the body, it is evident that a wave may appear 

 on a side lacking the brain. But it must be remembered that 

 there was at least one-haK the brain present in the body, that the 



