24 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



Romanes found that if the whole margin of the 

 umbrella of a Hydromedusa be cut off, and only a tiny 

 piece left, this is sufficient to keep up the spontane- 

 ous activity of the jelly-fish in sea-water. From this 

 it would appear that any element of the margin may 

 be considered a centre for the rhythmical contractions 

 of the whole Medusa. But if this be the case, how 

 does it happen that the whole umbrella contracts sim- 

 ultaneously, and why do we not find one part of the 

 margin in systole and the other in diastole ? This 

 coordination is by no means to be taken for granted. 

 It is present only in healthy specimens, and is wanting 

 in injured or dying specimens, a fact to which Roma- 

 nes called attention. The problem of the mechanism 

 of this coordination has been dismissed by many au- 

 thors by the assumption of a " coordinating centre " 

 that is supposed to control this coordination. We 

 shall shortly be in a position to decide whether coor- 

 dination in lower animals is controlled by a special 

 " centre of coordination," or whether it is not rather 

 the result of simple laws of stimulation and conduction. 



Romanes found in Acalephae that coordination 

 ceases when all direct connection between the nervous 

 centres has been interrupted by radial incisions in the 

 umbrella, the various sectors no longer contracting 

 simultaneously. The same thing results in Hydrome- 

 dusae, if conduction through the nerve-ring is inter- 

 rupted. In such cases, the radial incision must reach 

 well toward the centre of the bell. If, however, such 

 incisions are made in the umbrella without injuring 



