FUNDAMENTAL EXPERIMENTS. 29 
genus Sarsia (Fig. 1) be cut completely through, so 
that the swimming-bell instead of being closed at 
the top is converted into an open tube, this open 
tube continues its rhythmical contractions for an 
indefinitely long time, notwithstanding that the 
organism so mutilated is, of course, unable to pro- 
gress. Thus it is a matter of no consequence how 
small or how large a portion of contractile tissue is 
left adhering to the severed margin of the swim- 
ming-bell; for whether this portion be large or 
small, the locomotor centres contained in the 
margin are alike sufficient to supply the stimulus 
to contraction. Indeed, if only the tiniest piece of 
contractile tissue be left adhering to a single mar- 
ginal body cut out of the bell of Sarsia, this tiny 
piece of tissue, in this isolated state, will continue 
its contractions for hours, or even for days. 
Effects of excising the entire Margins of Umbrellas. 
Turning now to the covered-eyed division of the 
Medusz, I find, in all the species I have come across, 
that excision of the margins of umbrellas produces 
an effect analogous to that which is produced by 
excision of the margins of swimming-bells. There 
is an important difference, however, between the 
two cases, in that the paralyzing effect of the opera- 
tion on umbrellas is neither so certain nor so com- 
plete as it is on swimming-bells. That is to say, 
although in the majority of experiments such 
mutilation of umbrellas is followed by immediate 
paralysis, this is not invariably the case; so that 
