PHYSIOLOGY OF THE INVERTEBRATA. 303 



from this point downwards to the margin there is a beautiful 

 and uninterrupted progression of excitability, the latter being 

 greatest of all when the electrodes are placed upon the 

 string of cells described by Agassiz as nerve-cells." In regard 

 to " the marginal tract of excitable tissue, the degree of ex- 

 citability differs slightly in different parts." In other parts 

 of the nectocalyx there is " a marked difference between the 

 excitability of this organ when the electrodes are placed upon 

 any one of the four radiating canals (and so upon the ascend- 

 ing nerve-chains described by Agassiz), and when the 

 electrodes are placed upon the tissue between any of 

 the canals. The ratio is generally about 9 centims. to 

 6| centims." 



Concerning the action of electrical stimulation the following 

 conclusions have been arrived at by Romanes : — 



(i) " The excitable tissues of the MedioscB, in their behaviour 

 towards electrical stimulation, conform in all respects to the 

 rules which are followed by the excitable tissues of other 

 animals. Thus closure of the constant current acts as a 

 much stronger stimulus than does opening of the same, while 

 the reverse is true of the induction-shock. 



(2) " Different species of the Medusce manifest different 

 degrees of sensitiveness to electrical stimulation, though in 

 all cases the degree of sensitiveness is wonderfally high. 



(3) "When the constant current is passing in a portion 

 of the strip of a severed margin, the nectocalyx sometimes 

 manifests uneasy motions during the time the current is 

 passing. It is possible, however, that these motions may be 

 merely due to accidental variations in the intensity of the 

 current. 



(4) "When the intrapolar portion of the severed margin 

 of Staurophora laciniata happens to be spontaneously contract- 

 ing prior to the passage of the constant current, the moment 

 this current is thrown in, these spontaneous contractions 

 often cease, and are then seldom resumed until the current 

 again is broken, when they are almost sure to recommence. 



