INTERDEPENDENCE OF SUB-UMBRELLA AND SENSE-ORGANS. 1 1 



upon the sense-organs even after the sense-organs have become too 

 exhausted to themselves maintain an "excited" rate. Moreover, if 

 we stimulate the sub-umbrella surface by touching it repeatedly with a 

 crystal of K 2 SO 4 the disk responds by active contractions and forces 

 the sense-organs to respond at the same rate. Then after the stimu- 

 lus is withdrawn the sense-organs are found to have been exhausted 

 by the contractions of the disk and can not again resume pulsation 

 until after a long interval of rest. 



Direct evidence showing that the sub-umbrella may exert a control- 

 ling influence on all parts of the sensory tissues of the Medusa is also 

 afforded by the following experiment : If we cut off the basal plate 

 with the 8 mouth-arms, the mouth-arms remain normally expanded 

 in sea- water. If now we place the mouth-arms in a solution which 

 resembles sea- water, but lacks potassium, the arms contract into a 

 close bunch, and will not again expand as long as they remain in the 

 solution. If, however, we place a perfect Medusa in the solution it 

 exhibits periods of active pulsation alternating with periods of rest. 

 Immediately after it comes to rest its mouth -arms contract into a 

 close bunch, but they always expand again as soon as the Medusa 

 resumes pulsation. It will be remembered that Romanes showed 

 that removal of the margin of the bell in Sarsia caused the manu- 

 brium to elongate and lose its muscular tonus. He also found that 

 in Sarsia stimulation of the sub-umbrella caused the manubrium to 

 contract, and that the manubrium of Tiaropsis indicans would apply 

 its mouth to any stimulated part of the sub-umbrella, provided the 

 stimulus could travel radially inward from the stimulated spot to the 

 manubrium. Otherwise the manubrium executed ill-directed or 

 wandering movements. 



We will soon show that any difference between the physiological 

 action of the marginal sense-organs and that of the general sensory 

 tissue of the sub-umbrella is one of degree, not of kind. 



CONTROL OVER PULSATION EXERCISED BY THE MARGINAL 

 SENSE-ORGANS. 



Romanes found that the potency of the marginal sense-organ attached 

 to a segment of the disk has more to do with its rate of pulsation than 

 has the size of the segment; nevertheless small segments usually pul- 

 sate slower than large ones. 



In Cassiopea xamachana there are 13 to 23 marginal sense-organs, 

 and I find that the average rate of the perfect Medusa is apt to be the 

 same as the rate of its most rapidly working sense-organ . As Romanes 

 saw in Aurelia, the sense-organs tend to initiate stimuli at various 



