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.  From the lower part of the body hang numerous tentacles (cirri), amidst which the mouth is placed.  Probably these multitudinous arms assist in locomotion; and, hence the name of the family, Cirrigrades.  Amongst the creatures of this division we meet with some very interesting locomotive apparatus.  There are some of them by no means obliged to trust to their oars alone—­they have also sails.  The Velella, large fleets of which visit our seas at times, has a plate (the mast) rising from its bluish disk or deck, covered with a delicate membrane (the sail) of snowy whiteness, by means of which it traverses the ocean.  This sail, it has been noticed, ’is set at the same angle as the lateen-sail’ of the Malays.  We cannot doubt that it is admirably suited to its purpose, and the Malays may be proud of having nature as a voucher for their contrivance.

We find in another species a still more perfect rigging.  In it (Rataria) the crest is supplied with muscular bands, by means of which the sail can be lowered or raised at pleasure.  These adaptations of structure are full of interest.  Nothing can be more admirable than the sailing-gear of these little creatures.  They have to traverse the surface of the ocean amidst all diversities of weather.  Paddles alone would not suffice for them.  They must be enabled to take advantage of the winds.  Sails, therefore, are added, and the mightiest agents in nature are commissioned to speed the little voyagers on their way.