[Illustration: PHOTOGRAPH OF THE ENTIRE SHELL OF THE PEARLY NAUTILUS
The headquarters of the Nautilus are in the Indian and Pacific Oceans.
They sometimes swim at the surface of the sea, but they usually creep
slowly about on the floor of comparatively shallow water.]
[Illustration: NAUTILUS
A section through the Pearly Nautilus, _Nautilus pompilius_, common from
Malay to Fiji. The shell is often about 9 inches long. The animal lives
in the last chamber only, but a tube (S) runs through the empty
chambers, perforating the partitions (SE). The bulk of the animal is
marked VM; the eye is shown at E; a hood is marked H; round the mouth
there are numerous lobes (L) bearing protrusible tentacles, some of
which are shown. When the animal is swimming near the surface the
tentacles radiate out in all directions, and it has been described as "a
shell with something like a cauliflower sticking out of it." The Pearly
Nautilus is a good example of a conservative type, for it began in the
Triassic Era. But the family of Nautiloids to which it belongs
illustrates very vividly what is meant by a dwindling race. The
Nautiloids began in the Cambrian, reached their golden age in the
Silurian, and began to decline markedly in the Carboniferous. There are
2,500 extinct or fossil species of Nautiloids, and only 4 living
to-day.]
[Illustration: _Photo: W. S. Berridge._