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POPULAR SCIENCE MONTHLY 



The question is, why is it that the central disk of the medusa does 

 not pulsate in sea-water when its sense-clubs are removed? Curiously 

 enough, if we stimulate the disk in any manner, such as by a mechanical 

 or electrical shock, or by touching it with a crystal of common salt, 

 it gives a few vigorous pulsations and then lapses into quiescence. 



But if Ave cut out the center of the medusa and also remove the 

 rim, thus forming a ring tissue without sense-organs (Fig. 2), tills- 



Fig. 1. Living Medus.e of Cassiupca xamachana ox a Sandy Bottom. The- 

 large medusa in the middle is in the natural attitude with its mouth-arms uppermost. 

 The smaller medusae have been turned over in order to show their pulsating disks. 



ring remains quiescent in sea-water unless we stimulate it at any point 

 such as at S with a single momentary touch of a crystal of potassium, 

 or in some other manner, when a contraction-wave starts out from the 

 point touched. In a narrow ring, however, tbe waves can go only in 

 opposite directions from the stimulated point. Now one of these waves 

 is apt to be strong and the otber weak ; for the nervous network which 



