218 
THE OCEAN WORLD. 
Hebrides are chiefly nourished by Medusae which have been tranS' 
ported by the waves in innumerable swarms from the coast of 
Atlantic to the region of whales. “ The locomotion of the medus®> 
which is very slow,” says De Blainville, “ and denotes a very feeble 
muscular energy, appears, on the other hand, to be unceasing. Si» cti 
their specific gravity considerably exceeds the water in which they a 1 ® 
immerged, these creatures, which are so soft that they probably cou 1 
not repose on solid ground, require to agitate constantly in order 1° 
sustain themselves in the fluid which they inhabit. They require als° 
to maintain a continual state of expansion and contraction, of systol® 
and diastole. Spallanzani, who observed their movements with gi'» x 
care, says that those of translation are executed by the edges of th<j 
disk approaching so near to each other that the diameter is diminish® 
in a very sensible degree ; by this movement a certain quantity 0 
water contained in the body is ejected with more or less force, by which 
the body is projected in the inverse direction. Renovated by th 1 ' 
cessation of force in its first state of development, it contracts its® 
again, and makes another step in advance. If the body is perpendieulf 1 ' 
to the horizon, these successive movements of contraction and dikn a 
tion cause it to ascend ; if it is more or loss oblique, it advances rao* 6 
or less horizontally. In order to descend, it is only necessary for tin 
animal to cease its movements ; its specific gravity secures its descent- 
It is, then, by a series of contractions and dilatations of their bodi^ 
that the Medusa; make their long voyages on the surface of 
waters. This double movement of their light skeleton had already 
been remarked by the ancients, who compared it to the action 0 
respiration in the human chest. From this notion the ancients caU et 
them Sea Lungs. * 
The Medusas usually inhabit the deep seas. They are ibX^J 
solitary, but seem to wander about in considerable battalions in t ® 
latitudes to which they belong. During their journey they proce® 
forward, with a course slightly oblique to the convex part of the 
m, 
, folded 
body. If an obstacle arrests them, if an enemy touches them, 
umbrella contracts, and is diminished in volume, the tentacles are J 
up, and the timid animal descends into the depths of the ocean. ^ 
We have said that the Medusae constitute in the Arctic seas one ^ 
the principal supports of the whale. Their innumerable masses soi»^ 
times cover many square leagues in extent. They show themse u 
