453 



noted. In no case did the two hearts examined regularly beat in the 

 same direction and reverse at the same time. Just as the heart beats 

 were slower in the older zoöids the frequency of the reversal was also 

 less. This will be seen from Table 1 which gives the times between 

 reversals (reversal period) for the 5 pairs of zoöids observed. In the 



Table 1, 



first two lines the reversal periods of the zoöids of the older generation 

 are placed in the first column, and it will be seen that they are consi- 

 derably longer than in the corresponding younger generation. Smaller 

 differences Avere observed between the zoöids of the same generation 

 though even here they were considerable. The variation expressed by 

 the extremes in the table appears greater than it really is; for in each 

 zoöid the period during which the blood is propelled in one direction 

 is usually decidedly longer than when the blood passes in the opposite 

 direction. Thus there are really two groups of variants from which the 

 extremes are selected. 



But inspite of the variation in the lengths of the reversal periods of 

 the different zoöids a considerable amount of coordination as regards 

 the moment of the reversal and the direction of the flow of blood is 

 possible. As a matter of fact, however, it does not occur. Table 2 

 represents the coordination between the two zoöids whose reversal 

 periods are recorded in the last line of Table 1 ; these two zoöids being 

 selected because they were better coordinated than any of the others. 

 The central column represents the time, divided into minutes and 

 half-minutes. The number of the minute is indicated by the figures 

 above each line. The times of reversal of the hearts are indicated by 

 the horizontal lines on both sides of the time column; those on one 

 side always referring to the same zoöid. These lines are inserted on 

 the time column as nearly as possible at the time when the reversal 

 occurred, and for additional accuracy the second during which the 



