SENSE-ORGANS AND PUPATION. 35 



to pulsate without alteration in their rates. They both ceased in- 

 stantly as soon as a radial cut was completed from center to margin, 

 thus breaking the circuit of the waves of contraction. 



We have seen that a center of pulsation in the undifferentiated sub- 

 umbrella tissue sends out its stimulus only when the contraction wave 

 returns to it through the circuit, and that therefore the rate must be 

 constant, for it depends only upon the length of the circuit and the 

 rapidity of the wave ; and no pulsation can be maintained by a center 

 in the sub-umbrella tissue unless the contraction wave can pass through 

 a circuit and finally travel back to restimulate the center. 



The marginal sense-organs behave differently. They send forth the 

 stimulus, which produces contraction, at a slow, irregular rate, and 

 they are not restimulated into immediate action by a returning wave, 

 and can maintain tissue in pulsation even if its shape is not that of a 

 closed circuit. They function only when calcium is present in solu- 

 tion in the sea-water, and if lifted out of water and dried with blotting 

 paper they cease in a few minutes to initiate pulsations ; but if then 

 they be moistened with distilled water containing the amount of cal- 

 cium found in sea-water, they recommence pulsation. Indeed, the 

 sense-organs behave as if a slow chemical change takes place within 

 them, the result being a contraction-stimulus; and this state of con- 

 traction in turn reducing the built-up compounds to their original 

 condition. Calcium has the peculiar power to offset the stupefying 

 influence of the magnesium of the sea- water, but calcium is of primary 

 importance only when magnesium is present. If magnesium be absent 

 the presence of calcium is relatively unimportant in the pulsation of 

 Cassiope.a. Indeed, the Medusa pulsates longer and faster in a solution 

 containing the amounts and proportions of NaCl + KCL, found in sea- 

 water than it does in NaCl + CaCl 2 . 



Before closing the account of these experiments upon disks it should 

 be stated that the disks of Aurelia flavidula and Dactylometra quin- 

 quecirra may also be set into sustained and regular rhythm by cutting 

 partial rings, as has been described in the case of Cassiopea. These 

 Scyphomedusse, however, soon recover to some extent from the loss 

 of their marginal sense-organs, and the chief difference between their 

 usual behavior after the loss of the margin and their behavior when 

 cut by partial rings and then set into pulsation is that in the latter 

 case the pulsation is of machine-like regularity and without pauses, 

 whereas under normal conditions it is irregular. Dactylometra is more 

 favorable for these experiments than Aurelia, for Aurelia is extremely 

 sensitive to mechanical shocks and to chemical stimuli. It is of 

 interest to observe that the rate at which the tissues of the disk of 



