344 ORGANIC GROWTH sec. 



the nerve-centres on the margin. Gradually the movements 

 of the animal became regular, they succeeded one another in 

 a definite rhythm, as in the entire animal, and the animal 

 thenceforth behaved completely like an uninjured specimen. 

 Thus a new nerve-centre had been formed in place of the old 

 from the nerve-cells scattered over the body-surface, and had 

 taken upon itself the movement and direction of the animal. 

 [ have repeated this experiment many times with the same 

 result, and have observed mutilated animals living in this way 

 more than eight days. 



The movements of Medusae are, as I have shown, involun- 

 tary, but they can be retarded or hastened, diminished or in- 

 tensified voluntarily. As involuntary movements they are 

 respiratory, as voluntary they are the means of locomotion. 



The only possible explanation of the reappearance of the 

 movements is that the need of respiration in the still living 

 but motionless animal first produces convulsive contractions, 

 which gradually become rhythmical. But how it happens 

 that they become rhythmical, and that this rhythm may be 

 controlled by some portion of the nerve-cells scattered over the 

 body, and how these cells are able to influence the movements 

 by volition remains a mystery, unless we assume that all the 

 nerve-cells of the Medusa still possess, as an inherited and 

 originally acquired character, the faculty of directing motion, 

 even after the function of direction has been assigned to the 

 nerve-centres of the margin of the umbrella. In order to 

 explain the facts, it must be further assumed that some of the 

 nerve-cells of the surface have retained this faculty in a higher 

 degree than others, or that they happen to be more vigorous at 

 a given moment, and are therefore able to exert their inlierited 

 faculty more powerfully than the rest. It would thus only 

 require external stimulation to bring this faculty into action : 

 Medusae which were left after the operation in unchanged 

 water deficient in oxygen, did not recover their power of motion. 



I 



