The
chambered nautilus,
Nautilus pompilius also called the
pearly nautilus, is the best-known
species of
nautilus. The shell, when cut away, reveals a lining of lustrous
nacre and displays a nearly perfect
equiangular spiral, although it is not a
golden spiral. The shell exhibits
countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above.