NORMAL MOVEMENTS IN SEA-WATER. 9 



but as we shall see, disks without sense-organs pulsate at the maxi- 

 mum rate at which their tissue is capable of transmitting the wave 

 of pulsation, and they can therefore exhibit "excitement" only by 

 an increase in amplitude. 



It is worthy of note that if the forceps used to stimulate the Medusa 

 be made to seize upon only a small area of tissue, the Medusa will not 

 respond, but on bringing a larger area between the forceps the response 

 is sudden and violent. In this connection it will be recalled that 

 Romanes showed that the bell of Sarsia, when deprived of its margin, 

 will respond to mechanical shocks by pulsations, each stimulus usually 

 giving rise to one or two pulsations, and this is also true of the par- 

 alyzed disk of Cassiopea. We must conclude that the presence of 

 marginal sense-organs is not necessary for the display of that sudden 

 increase in activity which we have called "excitement," and that this 

 response may come from many or all parts of the undifferentiated tissue 

 of the sub-umbrella.* 



a 



C3 



Fig. 2. 



Romanes showed that in Aurelia annular cuts separating the margin 

 from the center of the disk caused the rhythm to become slower, and he 

 was led to suspect (1885, p. 163) that a stimulus of an afferent character 

 emanates from all parts of the sensory surfaces of the sub-umbrella 

 to the marginal sense-organs, although of this he had no direct proof. 

 I think we can prove that this is the case in Cassiopea^ for if we cut 

 off all but one marginal sense-organ, and then make cuts through 

 the sub-umbrella tissue (fig. i) radiating outward from the sense-organ 

 and therefore not interfering with any stimulus which may travel by the 

 shortest path from any point in the disk to the sense-organ, the final 

 rate of pulsation, after the excitement due to the operation has sub- 



* It is interesting to observe that Bancroft and Esterly (1903) find that while con- 

 tractions normally originate from the ganglionated ends of the heart of Ciona, they 

 may originate from any other region. 



