J. M. D. OLMSTED 59 



piece. The brain must therefore be intact for this movement to 

 take place. The wave of muscular contraction in this method of 

 locomotion involves the coordinated movements of both sides of 

 the body, for the wave extends across the entire width of the 

 animal. One might imagine that each cerebral ganglion could 

 control the movements on its own side, and in those individuals 

 with the brain split in halves each side might act independently. 

 But this proved not to be the case. It was necessary that the 

 entire brain be undisturbed. Again, it would seem reasonable 

 that the impulse to start ofT this method of locomotion might 

 originate from some stimulus at the anterior end of the worm, 

 since preliminary movements are made by this portion before 

 releasing the rest of the body from the substrate in preparation 

 for swimming. But if the anterior ends were removed by a 

 transverse cut immediately in front of the cephalic ganglia, it 

 was found that the anterior brainless pieces were unable to swim, 

 while the posterior pieces retained this power perfectly. The 

 impulse to initiate the wave seems therefore not to occur as a 

 stimulus from the anterior part of the body, but from within the 

 brain itself. 



When the cephalic ganglia were removed by cutting out a 

 small disc of tissue in the vicinity of the eyes, and the hole had 

 filled in by regeneration, it was found that the brain could not 

 be regenerated. Such brainless worms were unable to swim 

 although no part was lacking except the brain. 



Ciliarj^ movement does not play a prominent part in the 

 locomotion of these four species of polyclads. When they are 

 apparently at rest and making absolutely no progress at all, 

 the cilia can be seen still beating. When the worms are 

 placed in chloretone, the cilia (Continue to beat, and if a worm 

 becomes detached from the substrate to which it often adheres 

 even when anesthetized, it will be carried along at a very slow 

 rate by the cilia. The speed is the same whether the worm is on 

 its dorsal or ventral surface, and is slower than any method of 

 locomotion by muscular action. I am convinced that the cilia 

 never function as the sole organs of locomotion under normal 

 conditions. They may aid muscular locomotion, but do not act 



