48 EDWARD C. DAY 



for the last 5, totaling 2 minutes and 17 seconds. After a pause 

 of 8 ", a reversal again occurred. 



It will be noticed that there is close correspondence between 

 the time-relations for the two series except in the matter of the 

 intervening pauses. The fact that the landmark selected on the 

 tunic as the point for observing the passage of the impulses, did 

 not lie equidistant from the two ends of the heart was found, 

 however, to account for this difference in length between the 

 pause following a ventrad series and the pause following a dorsad 

 series. 



The beat of the heart varied from day to day even under 

 normal conditions as may be seen from a glance at table 4. For 

 Ascidia mentula no. 15 the number of beats per ventrad series 

 ranges from a minimum number of 25 up to a maximum of 42, 

 and the number per dorsad series ranges from 21 to 35. These 

 extremes in the range of beats are no doubt the expression of 

 varying degrees of stimulation of some sort to which the animal 

 is subjected. The average number of beats per series, ventrad 

 and dorsad alike, when the extremes are disregarded is 25. 

 There are, however, individual differences, as comparison of the 

 records of the three animals in the table reveals. The normal 

 number of beats runs higher for both nos. 16 and 18, the latter 

 having the highest average record of the three. 



The fifth feature listed for the normal heart-beat, acceleration 

 during the course of a series, is best seen in a long series such as 

 is exhibited by Ascidia m. no. 18. In graph 18\ table 3, the first 

 ventrad series has 39 beats which required 3 minutes and 23 

 seconds for their execution. The number of seconds per every 

 consecutive 10 beats runs 53, 51, 49, and then 50" for the last 

 nine beats. The second and third 10 beats, therefore, required 

 less time than either the first 10 or the last 10 of the series. In 

 the case of the first dorsad series where the 38 beats required 3 

 minutes and 30 seconds, the acceleration is less pronounced, 

 the third 10 beats requiring but one second less than the first 

 10 and seven less than the last 10. In both instances it takes 

 the first 30 beats in which to develop the acceleration and the last 

 8 or 10 beats to check it. In other words, there is a rather 



