NERVOUS SYSTEM OF TUNICATE 47 



NORMAL BEAT OF THE HEART 



The heart-beat of an Ascidia mentula lying undisturbed in a 

 vessel of fresh sea-water exhibits the following characteristics: 



1. Alternating ventrad and dorsad series of pulsation-waves. 



2. Each series consists of about the same number of waves, 

 twenty to forty according to the size and condition of the animal. 



3. Each requires about the same length of time for its com- 

 pletion, two to four minutes depending on the length of the 

 series. 



4. Each pulsation-series is followed by a pause of from ten to 

 fifteen seconds before reversal occurs. 



5. An acceleration of beat occurs usually during the course of 

 a series. 



The heart is looped around the base of the pharyngeal sac in 

 the posterior end of the body in such a way that one end lies 

 under the nerve ganglion and dorsal and the other end lies on 

 the opposite side of the pharyngeal sac and ventral, A ventrad 

 series, therefore, is one in which the pulsation-waves originate at 

 the dorsal end and travel around the loop to the ventral end, 

 and a dorsad series is one in which the direction of the waves is 

 reversed, commencing at the ventral end and traveling around 

 the loop to the dorsal. 



The graphic records have been analyzed and the results set 

 forth in the accompanying tables. Graph 15^ of table 1 repre- 

 sents a typical sequence of events for the heart-beat of a normal, 

 uninjured, and unstimulated animal. The first line states the 

 number of beats in alternating ventrad and dorsad series, and 

 the next three lines give an analysis of the first ventrad and of 

 the first dorsad series of beats. 



The ventrad series required 2 minutes and 19 seconds for the 

 25 beats comprising it. This interval of time was distributed 

 over the series as follows : 55 " for the first 10 beats, 55 " for the 

 second 10 beats, and 29 " for the last 5 beats of the series. A 

 pause of 18" followed, and succeeding this came a normal 

 reversal with the beats traveling dorsad. The dorsad series 

 required 54" for the first 10 beats, 55" for the second 10, and 28" 



