NERVOUS SYSTEM OF TUNICATE 55 



The dorsad series of 18- shows a condition almost the reverse 

 of that for the ventrad series of IS''. In the latter graph the rate 

 of beat has increased and the number of beats has been reduced, 

 whereas in the former graph the number of beats has been in- 

 creased while the rate of beat has been reduced. Now, as a 

 rule, the number and the rate as both increased b}^ stimulation, 

 but here we have the two features changing not only independently 

 of each other, but in opposite directions. Apparently it is due to 

 a greater volatility or evanescence of the one phenomenon in 

 comparison with the other : the change of rate is the more evanes- 

 cent and volatile while the change in the number of beats pos- 

 sesses greater inertia; consequently, the rate waxes and wanes 

 while the number of beats continues to increase, and this change 

 crops out in a graph either as an acceleration or a retardation 

 during the time that the number of beats is developing to a 

 maximum. 



The rate of beat would seem, therefore, to be the more sus- 

 ceptible of the two phenomena to stimulation and might be 

 regarded as the best indicator of the state of excitation. One 

 would consequently expect to find that if there were any regula- 

 tory control exerted by the ganglion, it would be first expressed 

 by this delicate indicator. The change in rate of beat may be 

 regarded as the 'vernier' for determining the finer measure of 

 any existing control, while the number of beats may be regarded 

 as the coarser scale for measuring its grosser aspects. 



The number of beats, as has been pointed out, increases as a 

 direct effect of stimulation, but, irrespective of whether the 

 stimulus has been the result of incisions in the tunic or of com- 

 plete removal of the ganglion, the number subsides again to 

 normal after sufficient time has elapsed for recovery. If, after 

 recovery, the animal is again stimulated, the same rise in number 

 of beats occurs with a subsequent reversion to normal after the 

 stimulation effects have died away. Animals nos. 15 and 16 

 show this recovery the best. In graphs 15^ and IS'', both of 

 which are cases of recovery, the first after slicing the tunic and the 

 second after removing the ganglion, the number of ventrad beats 

 has dropped from 57 and 83, respectively (graphs 15^ and 15''') ^ 



