The jelly-fishes of another section are furnished with one or more
air-bags, which assist them in swimming, and hence bear the name of
_hydrostatic acalephae_. In the Portuguese man-of-war (_Physalia_), the
bag is large, and floats conspicuously on the surface of the water.
From the top of it rises a purple crest, which acts as a sail, and by
its aid the little voyager scuds gaily before the wind. But should
danger threaten--should some hungry, piratical monster in quest of a
dinner heave in sight, or the blast grow furious--the float is at once
compressed, through two minute orifices at the extremities a portion
of the air escapes, and down goes the little craft to the tranquil
depths, leaving the storm or the pirate behind. In one species
(_Cuvieria_), the floats are numerous and prettily ranged round the
margin of the body. Resting on these, the creature casts about its
long fishing-lines, and arrests the passing prey.
One more section remains to be noticed. The jelly-fishes which belong
to it have a rudimentary skeleton--a plate which supports the soft,
circular body.
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