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 the richest blues and
purples, mingled with green and crimson to some less extent; these
all being, not as sometimes described, iridescent or changeable
reflections, 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-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 blood. Besides this,
the double membrane is turned in, somewhat as the foot of a stocking
is inverted, when ready for putting on; and thus there is a bladder
within a bladder, both having double walls. The inner (_pneumatocyst_)
is much smaller than the outer (_pneumatophore_); and the point where
it is turned in is contracted to the almost imperceptible orifice that
has been mentioned. The inner bladder sends up closed tubular folds
into the crest, which, being invested by the membranous walls of the
outer sac, give to the sail that appearance of vertical wrinkles which
is conspicuous.