FUNCTIONS OF THE MARGINAL SENSE-ORGANS. 15 



determined by cutting spiral strips reaching from the margin inward, 

 in the manner of Romanes. It was observed that when the spiral was 

 made 5 mm. or less in width only powerful stimuli would travel from 

 one end of the strip to the other, and if under these conditions a single 

 sense-organ was left at the outer end of the strip, waves of contraction 

 which started from this sense-organ might or might not reach the cen- 

 tral part of the disk. If, however, the end containing this sense-organ 

 were touched with a crystal of K 2 SO 4 , or any other potassium salt, a 

 powerful wave of contraction immediately ensued and always traveled 

 completely through the spiral. But if the inner end of the spiral were 

 touched with the crystal of potassium salt, not only did the wave not 

 always reach the sense-organ, but it traveled only three-quarters as 

 fast as did the waves from the sense-organ. When the sense-organ 

 was cut off, however, the waves traveled at the same rate from either 

 end of the spiral strip, and this rate was the slower of the two men- 

 tioned above. Evidently the sense-organ reinforced the stimulus 

 given by the potassium salt. 



In this connection Romanes showed that in Aurelia strong stimuli 

 may initiate waves that may travel over the disk at twice the rate of 

 weak ones. 



Peripheral parts of the disk transmit stimuli at a faster rate than do 

 parts near the center of the disk. This was shown by Romanes to be 

 the case in Aurelia. Altogether the outer parts of the sub-umbrella 

 are more sensitive than the inner. 



As Romanes showed, there must be an appreciable interval of rest 

 between two successive responses to stimuli, and rhythmical waves 

 can not follow one after another faster than a certain frequency. 

 Waves traveling in opposite directions through the same strip of tissue 

 meet and reinforce, but do not pass each other, for a stimulus can not 

 produce a contraction over the tissue that has been in contraction only 

 the instant before. 



The sensory field of the Medusa is confined to the sub-umbrella 

 and the mouth-arms. The ex-umbrella surface exhibits no reactions 

 to stimuli, and indeed the epithelium of the ex-umbrella may be killed 

 by such penetrating reagents as Gilson's fluid, and, provided the poi- 

 sonous liquid does not reach the sub-umbrella, the rhythmical move- 

 ment will not be altered in rate. Even near the margin of the disk 

 close to the sub-umbrella surface, the ex-umbrella is inert to stimuli 

 of all sorts. The action of the sucker-like concavity at the aboral 

 center of the ex-umbrella is entirely passive, and a Medusa deprived of 

 all marginal sense-organs will still "cling" to the bottom or side 

 the aquarium, although paralyzed and motionless. 



