FUNDAMENTAL EXPERIMENTS. oo 
specimens of Sarsia, I have tried, by cutting out 
all the margin besides, to ascertain how minute a 
portion of intertentacular tissue is sufficient to 
perform this function, and I find that this portion 
may be so small as to be quite invisible without the 
aid of a lens. 
From numerous observations, then, upon Sarsia, 
I conclude that in this genus (and so, from analogy, 
probably in all the other genera of the true 
Medusze) locomotor centres are situated in every 
part of the extreme margin of a nectocalyx, but that 
there is a greater supply of such centres in the 
marginal bodies than elsewhere. 
Effects of excising Certain Portions of the Margin 
of Umbrellas. 
Coming now to the covered-eyed Meduse, I find 
that the concentration of the locomotor centres of 
the margin into the marginal bodies, or lithocysts, 
is still more decided than it is in the case of Sarsia. 
Taking Aurelia aurita as a type of the group, I 
cannot say that, either by excising the lthocysts 
alone or by leaving the lithocysts in sitw and 
excising all the rest of the marginal tissue, I have 
ever detected the slightest indications of locomotor 
centres being present in any part of the margin of 
the umbrella other than the eight lithocysts; so 
that all the remarks previously made upon this 
species, while we were dealing with the effects of 
excising the entire margin of umbrellas, are equally 
applicable to the experiment we are now consider- 
ing, viz. that of excising the lithocysts alone. In 
