FUNDAMENTAL EXPERIMENTS. 83 



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, 

 \ robably in all the other genera of the true 

 Medus[t3) 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 Umhrellas. 

 Coming now to the covered-eyed Medusae, 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 lithocysts 

 alone or by leaving the lithocysts in situ and 

 excising all the rest of the marginal tissue, T 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 



