188 JELLY-FISH, 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 Medusze, 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, Cyanza, and the covered-eyed 
Medusze generally are the best species for obtaining 
artificial rhythm under the influence of faradaic 
stimulation, some of the naked-eyed Medusz 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 
