188 JELLY-FISir, STAR-FISH, AND SEA-URCHINS. 



be observed as in the case of electrical stimulation. 

 The principal differences consist in the rhythm 

 being much less sustained in the former than in 

 the latter case. But, by experimenting on other 

 species of Medusae, I have been able to obtain, 

 in response to mechanical and chemical stimula- 

 tion, artificial rhythm of a much more sustained 

 character than that which, under such modes of 

 stimulation, occurs in Aurelia. I have no explana- 

 tion to offer why it is that some species, or some 

 tissues, present so much more readiness to manifest 

 sustained rhythm under certain modes of stimula- 

 tion, and less readiness to manifest it under other 

 modes, than do other species or tissues. Probably 

 these differences depend on some peculiarities in 

 the irritability of the tissues which it is hopeless 

 to ascertain; but, in any case, the facts remain, 

 that while Aurelia, Cyanaea, and the covered-eyed 

 Medusae generally are the best species for obtaining 

 artificial rhythm under the influence of faradaic 

 stimulation, some of the naked-eyed Medusae are 

 the best species for obtaining it under the influence 

 of the constant current, and also under that of 

 mechanical and chemical stimulation. I have 

 already spoken of this effect of the constant cur- 

 rent in the case of Sarsia; I shall now proceed 

 to describe the effects of mechanical and chemical 

 stimulation on the same species. 



It is but rarely that artificial rhythm can be pro- 

 duced in the paralyzed nectocalyx of Sarsia by 

 means of mechanical stimulation, but in the case 

 of the manubrium, a very decided, peculiar, and 



