THE CIRCULATION OF BODY FLUIDS 115 



minutes and consists of from twenty to forty beats according 

 to the size and condition of the animal. In any individual 

 the number of beats in each advisceral or abvisceral series 

 tends to be approximately constant. Each pulsation series is 

 followed by a pause of from ten to twenty seconds before 

 reversal of the direction of 

 the contraction wave occurs. 

 Acceleration is caused by sever- 

 ing the nerves. Extirpation of 

 the ganglion or incisions in the 

 tunic produce an increase in 

 the number of beats per series, 

 a slight acceleration of the pul- 

 sation rate and a diminution of 

 the pause before reversal. After 

 a time varying from an hour 

 to a day the original rhythm 

 returns. In Ascidia atra, which 

 has been investigated by Hecht 

 (19 1 8), the heart has a well- 

 marked nodal constriction about 

 halfway along its length. Most 

 individuals show a marked pre- 

 ponderance of advisceral over 

 abvisceral beats. Thus in one 



series of observations the beats succeeded one another as 

 follows : 



Fig. 29. — Anatomy of Ascidian to 

 show circulatory system (after 

 Hecht). 



Abvisceral 

 Advisceral 



17 



15 



36 36 



44 51 65 



Various hypotheses have been suggested to account for this 

 phenomenon of reversal. In particular one may mention the 

 suggestion that the significant factor is back-pressure from the 

 peripheral circulation. This is dismissed by Hecht on three 

 grounds. First, because it occurs in the isolated heart. 

 Secondly, because it can be abolished by raising the tempera- 

 ture to a certain height (about 35° C. in Hecht 's experiments), 

 when the direction of the contraction wave remains constant. 

 Thirdly, because the peripheral resistance cannot be very 



