452 



coup le renversement de la circulation s'opère; le sang accumulé 

 à la périphérie se précipite en sens inverse dans les parties centrales 

 de la colonie et les cœurs changent le sens de leurs contractions, qui 

 passent subitement par un nouveau maximum.« 



Althoug the pulsations of the isolated ampullae show that they have 

 an independent contractility, and that the flow of blood to and from 

 the periphery of the uninjured colony is not entirely due to the coor- 

 dinated contractions of the hearts, as Pizon thinks, still a series of 

 observations was made to test whether the heart beats have any effect 

 upon the contractions of the ampullae. I will discuss first the coordi- 

 nation of the heart beats and of their reversals, then the influence of 

 the heart beats on the length of the ampullar pulsations and the cha- 

 racters of the latter, and finally the coordination in the contractions of 

 the neighboring ampullae and the means by which it is accomplished. 



Coordination of the Heart Beats and their Reversals. 



Pizon writes, in the passage quoted above, that in a single colony 

 the hearts beat simultaneously, but I have been unable to confirm this 

 statement. In some cases it is even impossible for the beats to be 

 simultaneous for there is a constant and characteristic difference in 

 the lengths of the heart beats. Thus in one colony an old zoöid, the 

 greater part of which had degenerated, had an average of 76.2 heart 

 beats per minute (extremes 69 and 80) while the adult zoöid developed 

 from it had an average of 1 00,5 (extremes 99 and 102) and in the young 

 buds the heart beat faster still'-. 



In zoöids of the same generation the rapidity of the heart beats is 

 about the same; but simple inspection in transparent colonies shows 

 that here too the contractions are often not simultaneous, though they 

 may be so in isolated cases. 



Secondly Pizon says that the hearts beat all in the same direction 

 and that the reversals in the direction of the heart beats take place at 

 the same time in different zoöids. To test this matter I made 5 series 

 of observations. One on an old zoöid that had partly degenerated and 

 the adult zoöid developed from its bud, one on an adult zoöid and its 

 bud, and 3 on adjacent adult zoöids of the same generation. Of these 

 last one pair was in a colony containing 2 zoöids, one in a colony of 

 8 zoöids, and the last in a larger colony with 3 systems of zoöids. In 

 each of these series the two zoöids were carefully watched for about 

 20 consecutive minutes and the escond in which the reversal took place 



2 The colonies observed were transparent ones growing on glass slides. This 

 allowed an examination of the attached ventral surface and enabled all the details 

 of the heart beats to be seen with the greatest distinctness. 



