e are composed.
He found in them the poisoned arrows or thread-cells of the Medusae,
and the same external position of the reproductive organs. And,
lastly, he separated from all other creatures, and associated with his
new group, some of the strangest and most beautiful animals of the
tropic seas, known to science as the Physophoridae and the Diphyidae.
The best-known of these is the "Portuguese man-of-war," the body of
which consists of a large pear-shaped vesicle which floats on the
water like a bladder. From the lower part of this depend into the
water large and small nutritive branches, each ending in a mouth
surrounded by a circle of waving tentacles armed with batteries of
thread-cells, while another set of hanging protrusions bear the
grape-like reproductive organs. On the upper surface of the bladder is
fixed a purple sail of the most brilliant colour, by which the
floating creature is blown through the water. When the weather is
rough, the bladder empties, and the creature sinks down into the quiet
water below the waves, to rise again when the storm is over. This,
and its equally wonderful allies, Huxley showed to be a complicated
colony of hydra-like creatures, each part being composed of two
membranes, and therefore essentially similar to Medusae. Thus, by a
great piece of constructive work, an assemblage of animals was
gathered into a new group and shewn to be organised upon one simple
and uniform plan, and, even in the most complex and aberrant forms,
reducible to the same type. The group, and Huxley's conception of its
structure, are now absolutely accepted by anatomists, and have made
one of the corner-stones of our modern idea of the arrangement of the
animal kingdom. With the exception of sponges, concerning the exact
relations of which there is still dispute, and of a few sets of
parasitic and possibly degenerate creatures, all animals, the bodies
of which are multicellular, from the simple fresh-water hydra up to
man, are divided into two great groups. The structure of the simpler
of these groups is exactly what Huxley found to be of importance in
the Medusae. The body wall, from which all the organs protrude,
consists merely of a web of cells arranged in two sheets or membranes,
and the single cavity consists of a central stomach, surrounded by
these membranes, the cavity remaining simple or giving rise to a
number of branching canals. The members of this great division of the
animal kingdom are the