SENSE-ORGANS AND PUI^ATION. 33 



waves travel faster than weak ones, and that strong stimuli repeated 

 at short intervals soon tired the tissue, so that it failed to respond. 



The rate of pulsation of disks is greater than their most excited 

 rate when the sense-organs are intact ; in other words, the disk itself 

 can maintain pulsation at a faster rate than can the marginal sense- 

 organs. The rate of pulsation in the disks deprived of sense-organs 

 depends simply upon the time required for the waves to traverse the 

 circuit and restimulate the center. The wave travels faster through 

 peripheral than through the inner annuli of the disk. When pulsating 

 disks are suddenly seized, moved, or otherwise stimulated, the ampli- 

 tude of their rhythmical movement suddenly increases, but the rate 

 remains practically the same, and thus the presence of the marginal 

 sense-organs is not necessary for the display of excitement. The disks 

 of small Medusae pulsate at a faster rate than do those of large ones, 

 other things being equal. 



These pulsating disks may continue to give regular rhythmical con- 

 tractions in sea- water for 140 hours or more, but at the end of that 

 time, if they have been deprived of their mouth-arms and central 

 stomach, they become exhausted, and the amplitude of their pulsa- 

 tion decreases, although the rate remains practically constant. Sud- 

 denly the center fails to restimulate the returning wave, all movement 

 ceases, and the disk can not be re-stimulated until after a period of 

 rest. Indeed, the tissue appears much exhausted and responds feebly 

 even to the strongest stimuli, such as K 2 SO 4 , KC1, etc. Complete 

 recovery takes place, however, in normal sea-water, so that disks may 

 be maintained in condition to pulsate for weeks. 



While in sea- water it is almost impossible to set a Medusa, with mar- 

 ginal sense-organs intact, into any form of pulsation other than that 

 controlled by the sense-organs. If, however, we cut partial rings in 

 the sub-umbrella of a Cassiopea, leaving the sense-organs and margin 

 intact, and then place the Medusa in a solution resembling sea-water 

 but lacking calcium,* all pulsations will cease in from 2 to 6 minutes. 

 Then, after the Medusa has remained motionless in the solution for 

 one hour, if we touch the disk for an instant with a crystal of K 2 SO 4 

 it immediately springs into a rapid rhythmical pulsation at a much 

 faster rate than that previously maintained by the sense-organs. This 

 pulsation, indeed, exhibits all of the features shown by disks with- 

 out sense-organs, and therefore we see that the absence of calcium has 



*965 H 2 O + 26.74 NaCl -f 3.75 MgCl 2 + 1.64 MgSO* + 0.85 K 2 SO 4 + 0.07 MgBr, 

 or Van 't Hoff's solution consisting of 100 NaCl + 2.2 KC1 + 7.8 MgClg + 3.8 

 MgSOi, all of >6n concentration. 



