EXPERIMENTS ON MEDUSA. 25 



the margin and the nerve-ring, no disturbance of 

 coordination ensues. It seems that the continuity of 

 the structures located in the marginal portions of the 

 umbrella is necessary for the coordinated activity. 

 Now how does it happen that so long as the continuity 

 is preserved all the elements act synchronically, while 

 the synchronism disappears if the continuity is inter- 

 rupted? 1 In order to answer this question, we must 

 turn our attention to an organ which shows the phe- 

 nomena of coordinated rhythmical activity in a strik- 

 ing manner namely, the heart. If the heart of a frog 

 be divided into several pieces, they will all be rhythm- 

 ically active, but the number of contractions will vary 

 in the different pieces. The sinus venosus beats most 

 rapidly, and the number of its contractions in a unit 

 of time equals that of the heart before it was divided. 

 T Juts we see that the whole heart beats in the rhytJim 

 of the part that has the maximum number of contrac- 

 tions per minute. From this we must assume that the 

 coordination of the heart's activity is due to the fact 

 that the part which contracts most frequently, forces 

 the other parts to contract in the same rhythm. They 

 will be forced to do this if the activity of the sinus 

 venosus acts as a stimulus upon the other parts. A 

 centre of coordination is therefore entirely unneces- 

 sary. 



Porter succeeded by an ingenious method in causing 



1 It should be emphasised that incisions through the margin alone do not 

 interfere with coordination in Gonionemus, but that it is necessary to continue 

 the incisions to the centre of the swimming-bell. But even under such circum- 

 stances the animal may still contract in a coordinated way. 



