ON ACALEPHA. 565 
tain themselves in the fluid which they inhabit. Accordingly, 
they are in a continual motion of systole, and diastole. 
Spallanzani, who has observed them carefully in their mo- 
tions, says that those by which they change place, are ex- 
ecuted by the approximation of the edges of the umbrella, so 
that its diameter diminishes in a very considerable manner. 
By this a certain quantity of water contained in the stomachs, 
and in the cavity of the umbrella, is expelled with greater or 
less force, and the body is projected in an inverse direction. 
Returned by the cessation of the muscular force to its first 
state of development, it contracts itself afresh, and makes a 
new step. If the body is perpendicular to the horizon, this 
succession of contraction and dilatation causes it to ascend; 
if it is more or less oblique, it advances more or less horizon- 
tally. To descend, it is sufficient for the animal to cease its 
movements ; its weight alone draws itdown. It is never in an 
inverted position, with the convexity of the umbella under- 
neath. Neither the tentacula nor the arms appear to be em- 
ployed in these movements of transportation, at least the 
latter, according to Spallanzani, are always extended follow- 
ing the body. Some ingenious experiments related by this 
observer, prove that it is only the muscles of the marginal 
zone of the umbrella, which cause it totally to contract, since, 
on removing them, the remainder of the umbrella undergoes 
no change, while the removed zone continues its movements 
of systole and diastole. In spite of this almost continual 
action of the locomotive faculty, the medusz do not appear 
to be able to overcome the smallest current, but are constantly 
carried away by it. 
According to all observers, the medusz feed on little ani- 
mals, on mollusca, worms, crustacea, and even fish, which 
they attract towards their mouth, by means of the appendages 
with which itis armed. Spallanzani has supposed this, be- 
cause he saw a small fish which was attached to one of the 
