ACALEPHjE. 
247 
surmounts the vesicle performs the office of a sail, and that they tell 
the navigator “ how the wind blows,” as they say. With all respect to 
the sailors, the bladder-like form, with its aerial crest, is only a hydro- 
static apparatus, whose office is to lighten the animal, and modify its 
specific gravity. Mr. Gosse thinks otherwise however. 
“ This bladder,” says Grosse,in his “Year by the Sea-side,” “is filled 
with air, and therefore floats ahnost 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 repre- 
sents 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 colom-s of this curious creature are very vivid : the bladder, 
though in some parts transparent and colourless, and in some speci- 
mens almo st, entirely so, is in general painted with richest blues 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 with a lovely rose- 
Phik hue the colours arranged in a peculiar fringe-like manner. 
When examined anatomically, the bladder is found to he composed of 
two walls of membrane, which are lined with cilia, and have between 
them the nutritive food 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 ( pneumatocysi ) much smaller 
than the outer ( pneumatophone ), and contracted at the point where it 
is turned in to the almost imperceptible orifice. The inner sends up 
elosed tubular folds into the crest, which, being arrested by the 
Membranous walls of the outer sac, give to the sail that appearance 
°f verticle wrinkles which is so conspicuous. 
When it is filled with air the body is almost projected out of the 
Water. I n order to descend it is necessary to compress itself or dispel 
