8 



PULSATION OF JELIvYFlSHES. 



well-fed Medusae of the same size. Thus one Cassiopea was starved 

 for three months, and the diameter of its disk shrank from 78 to 21 

 millimeters, while at the same time its rate of pulsation declined from 

 about 40 to 1 6 per minute. It is also interesting to observe that if 

 we cut off the margins of the disks of Medusae of various sizes, the 

 severed rims of the small Medusae pulsate at a more rapid rate than 

 do those of the large Medusae, although in both cases this rate is 

 slower than that of the uninjured Medusa. 



TABLE I. Relation between the rates of pulsation and the diameters of the 

 disks in Medusce of Cassiopea xamachana. 



EXCITKMBNT. 



As we have said, the pulsating Medusae occasionally exhibit a sud- 

 den increase in their rate and amplitude of pulsation without apparent 

 cause. This can, however, be invariably brought about as a response 

 to any stimulus, such as a water current, a mechanical shock, or the 

 introduction of some irritating chemical into the water. When lifted 

 wholly or partially out of water, and replaced, the Medusae pulsate at 

 about twice their normal rate for two or three minutes, and the ampli- 

 tude of their pulsations is also increased. Even small fragments of 

 the disk containing a marginal sense-organ will usually display this 

 excitement, although the duration of the period of excitement is shorter 

 for small than for large pieces, and their rate of pulsation slower. 



However, the presence of marginal sense-organs is not necessary for 

 this "excitement," for, as we shall soon show, we have succeeded in 

 causing disks deprived of marginal sense-organs to pulsate constantly 

 and regularly in sea- water ; and if such disks be pinched or lifted out 

 of water or otherwise disturbed the amplitude of their pulsations 

 becomes suddenly increased, while the rate remains practically con- 

 stant. In normal uninjured Medusae both rate and amplitude increase, 



