72 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



we cut the mass, as long as we do not actually 

 divide it, contraction-waves will pass throughout 

 the whole mass. But this very reasonable hypo- 

 thesis of the contraction-waves in the Medusae 

 being possibly nothing more than muscle- waves is 

 negatived by other facts, which I shall now proceed 

 to state. 



In the first place, if a number of experiments be 

 tried in any of the three modes of section above 

 described, it will be found that extreme variations 

 are manifested as regards the degree of tolerance. 

 In the spiral mode of section, for instance, it will 

 sometimes happen that the contraction-wave will 

 become blocked when the contractile strip is only 

 an inch lono- while in other cases (as in the one 

 represented) the wave will continue to pass through 

 a strip more than thirty times that length ; and 

 between these two extremes there are all possible 

 grades of tolerance. Now it seems to me that if 

 the tissue through which these contraction-waves 

 pass is supposed (as far as they are concerned) to be 

 of a functionally homogeneous nature, no reason 

 can be assigned why there should be such great 

 differences in the endurance of the tissue in different 

 individual cases ; while, if we suppose that the 

 passage of the contraction-waves is more or less 

 dependent on the functional activity of the nervous 

 plexus which we know from microscopical examina- 

 tion to be present, Ave encounter no such difficulty ; 

 for it is almost to be expected that in some cases it 

 would happen that important nerves would soon be 

 encountered by the section, while in other cases it 



