On the Nervous System of Cassiopea Xamachana. 



163 



chemical reaction by which sodium is Uberated, unless they are depend- 

 ent upon the other tissues of the subumbrella for some substance which 

 in the smaller areas becomes reduced to such an extent that it can no 

 longer be produced 

 rapidly enough to allow 

 the sense-organ to dis- 

 charge at the usual rate. 

 The fact that in a nor- 

 mal medusa the initia- 

 tion of pulsation is taken 



up bv one after another ^^^- ^^- — Showing arrangement of vessels by which any area 



« , , of tissue could l)e rendered inactive in order to determine 



OI tne sense-organs at whether or not the method by which the area of muscle 



irregular intervals may under the influence of a single sense-organ, was reduced 



T . ,•■ , , ., was an important factor in determining rate of pulsation. 



mdicate that, even while 



in normal relationship to the surrounding tissues, each sense-organ 

 during its period of control of the pulsation of the medusa exhausts 

 from its immediate surroundings some substance necessary for the 

 maintenance of the discharge of impulses at a regular high rate. Since 

 the pulsation-rate of a medusa is under normal conditions constant, each 

 sense-organ must give up its control as soon as its power of givmg out 

 impulses falls below that necessary to maintain the usual rate of pulsa- 

 tion. When a given sense-organ has surrendered to a more active 

 neighbor the initiation of pulsation, it will have a sufficient latent period 

 to recover the chemical equilibrium necessary for bringing its rate of 

 discharge of impulses up to that necessary to supplant any other center 

 as the controlling factor in pulsation. 



