of the Genus Equoree. 379 
contract and dilate its umbrella. If we study the relation 
between these oscillations and the progression of the zoo- 
phyte, we shall soon find that in some cases, and even 
when they are most lively, these oscillations are not fol- 
Jowed by any removal of the animal who makes them: if 
we remove the medusa from the sea, and place it in a 
‘glass vessel sufficiently provided with fresh sea-water, and 
the diameter of which is such in proportion to the size of 
the umbrella that progressions physically impossible, the 
‘oscillations will nevertheless take place with the same ap- 
“pearances as when the medusa swam at large on the 
waves. If the animal descends to the bottom of the ves- 
sel, and attaches itself to it, still in this last case the same 
motions will be continued; and although less energetic in 
appearance, they will always present the characters of this 
regular succession which we have indicated. If we remove 
‘the zoophyte from its natural element, and place it on the 
hand, on a table, on astem, or any other solid body, it 
will still continue to move: the oscillations will doubtless 
appear more feeble, because the organs which cause them 
will be as it were pressed under the weight of the body, 
but they will not be-less constant or less regular: if, with 
certain precautions to be afterwards mentioned, we cut 
into several pieces the umbrella of a medusa, cach of the 
slices will continue to move for some time. 
If, after having ascertained the existence of these oscilla- 
tions under all the relations which we have indicated, the 
observer endvavours to ascertain how far their motions are 
regular, he will scon be convinced that the contractions 
and dilatations are isochronous, 7. e. that equal intervals of 
time correspond with equal numbers of each of them ; that, 
in similar circumstances, the quantity of these oscilla- 
tions is the same for individuals of analogous proportions ; 
that it is greater, every thing being similar in other re- 
spects, as the animals of each species are smaller and pro- 
bably younger; that these pulsations become less frequent 
and fecbler in proportion as the vital energy of the animal 
diminishes; but that, in this last case also, they preserve 
their isochronous motion, that they continue even some 
time after the general déath of the individual, and that 
they may be kept up or excited by various physical and 
chemical agents. 
Spallanzani had already made the greater part of these 
experiments ; and although at the time we were strangers 
to the labours of this great man, we were Jed to the te 
results, 
