MEDUSID. 161 
swarms from the coast of the Atlantic to the region of whales. ‘The 
locomotion of the Medusz, which is very slow,” says De Blainville, 
“and denotes a very feeble muscular energy, appears, on the other 
hand, to be unceasing. Since their specific gravity considerably 
exceeds the water in which they are immersed, these creatures, which 
are so soft that they probably could not repose on solid ground, 
require to keep themselves constantly moving in order to sustain 
themselves in the fluid which they inhabit. They require also to 
maintain a continual state of expansion and contraction, of systole 
and diastole. Spallanzani, who observed their movements with great 
care, says that those of locomotion are executed by the edges of the 
disc approaching so near to each other that the diameter is 
diminished in a very sensible degree; by this movement a certain 
quantity of water contained in the body is ejected with more or less 
force, by which the body is projected in the inverse direction. 
Renovated by the cessation of force in its first state of expansion, it 
contracts itself again, and makes another movement in advance. If 
the body is perpendicular to the horizon, these successive movements 
of contraction and dilatation cause it to ascend ; if it is more or less 
oblique, it advances more or less horizontally. In order to descend, 
it is only necessary for the animal to cease its movements ; its 
specific gravity secures its descent.” 
It is, then, by a series of contractions and dilatations of their bodies 
that the Medusz make their long voyages on the surface of the 
waters. This double movement of their light skeleton had already 
been remarked by the ancients, who compared it to the action of 
respiration in the human chest. From this notion the ancients 
called them Sea Lungs. 
The Medusz usually inhabit the deep seas. They are rarely 
solitary, but seem to wander about in considerable battalions in the 
latitudes to which they belong. During their journey they proceed 
forward, with a course slightly oblique to the convex part of their 
body. If an obstacle arrests them, if an enemy touches them, the 
umbrella contracts, and is diminished in volume, the tentacles are 
folded up, and the timid animal descends into the depths of the 
ocean. 
We have said that the Medusz constitute in the Arctic seas one 
of the principal supports of the whale. Their innumerable masses 
sometimes cover many square leagues in extent. They show them- 
selves and disappear by turns in the same region, at determinate 
epochs—alternations which depend, no doubt, on the ruling of the 
winds and: currents which carry or lead them. ‘The barks which 
L 
