142 THE OCEAN WORLD. 
inches in length, and from one to three broad. Its appearance is 
glassy and transparent, its colour an imperfect purple, passing to a 
violet, then to an azure above. It is surmounted by a crest, limpid 
and pure as crystal, veined with purple and violet in decreasing 
tints. Under the vesicle float the fleshy filaments, waving and 
contorted into a spiral form, which sometimes descend perpendi- 
cularly like so many threads of celestial blue. Sailors believe that 
the crest which surmounts the vesicle performs the office of a sail, 
and that it tells the navigator “how the wind blows,” as they say. 
This bladder-like form, with its aérial crest, is only a hydrostatic 
apparatus, whose office is to lighten the animal, and modify its 
specific gravity. 
“This bladder,” says Gosse, in his “ Year by the Sea-side,” “is 
filled with air, and therefore floats almost wholly on the surface. 
Along the upper side, nearly from end to end, runs a thin edge 
of membrane, which is capable of being erected at will to a 
considerable height, fully equal at times to the entire width of the 
bladder, when it represents an arched fore-and-aft sail, the bladder 
being the hull. From the bottom of the bladder, near the thickest 
extremity, where there is a denser portion of the membrane, depends 
a crowded mass of organs, most of which take the form of very 
slender, highly contractile, movable threads, which hang down 
into the deep to a depth of many feet, or occasionally of several 
yards. 
“The colours of this curious creature are very vivid; the bladder, 
though in some parts transparent and colourless, and in some 
specimens almost entirely so, is in general painted with richest blue 
and purple, mingled with green and crimson to a smaller extent, 
these all being, not as sometimes described, iridescent or changeable, 
but positive colours independent of the incidence of light, and, for 
the most part, possessing great depth and fulness. ‘The sail-like 
erectile membrane is transparent, tinted towards the edge witha 
lovely rose-pink hue, the colours arranged in a peculiar fringe-like 
manner. When examined anatomically, the bladder is found to be 
composed of two walls of membrane, which are lined with cilia, and 
have between them the nutritive fluid which supplies the place of 
the blood. Besides this, the double membrane is turned in or 
inverted like a stocking prepared for putting on; and thus there is a 
bladder within a bladder, both having double walls; the inner 
(pneumatocyst) much smaller than the outer (pneumatophore), and con- 
tracted at the point where it is turned into the almost imperceptible 
orifice. The inner sends up closed tubular folds into the crest, 
