MOLLUSKS 157 
duced to a mere dot and is practically invisible, but when the walls 
expand the cell enlarges to fully twenty-five times its former area, 
and the sudden enlargement of thousands of these little pigment 
cells, or chromatophores, as they are called, produces a flash of color. 
There are several sets of these chromatophores, some rosin colored, 
others yellow, blue-green or brown. Each set may expand inde- 
pendently or in combination with the others and thus a varied play 
of color is produced. 
In Cephalopods the sexes are separate, the male being often 
much smaller than the female. In some forms, such as the paper 
nautilus, one of the 
arms of the male un- 
dergoes a curious 
transformation. It 
develops within a 
large sac which 
bursts leavinga part 
of the sides of the 
sac still attached to 
thearm. The male 
then places a packet 
of spermatoza upon 

the arm, and after 
seizing the female 
the arm breaks off. Fig. 114; THE CHAMBERED, OR PEARLY NAUTILUS. 
From the Tropical Pacitic. 
and becomes at- 
tached to her body within the mantle cavity, thus conveying the 
spermatoza for the fertilization of the eggs. The eggs are usually 
laid enclosed in gelatinous capsules and the development is direct 
without any free swimming larval stage such as is characteristic of 
other mollusks. 
A good general account of Cephalopods is given by Professor 
J. S. Kingsley in the “ Riverside Natural History,” Vol. I. 
The Chambered Nautilus, (Nautilus pompilius, Fig. 114). 
This most interesting creature is found in the western parts of the 
tropical Pacific, and in the Indian Ocean. It lives upon the bottom, 
and is not usually found in water less than 100 feet deep, being 
most abundant at a depth of about 1000 feet. Contrary to popular 
