MOLLUSKS 89 
upon which they rest. In the skin are embedded multi- 
tudes of small spherical sacs filled with pigments of various 
colors, chiefly shades of red, brown, and blue, each sac be- 
ing connected with a nerve.and a series of delicate muscles. 
If the animal settles upon a red surface, for example, a 
nerve impulse is sent to each of the hundreds of color sacs 
of corresponding shade, causing the muscles to contract 
and flatten the bag like a coin, and thus exposing a far 
greater surface than before, they give the animal a reddish 
hue. In the twinkling of an eye they may completely 
change to another tint, or present a mottled look, and some 
may even throw the surface of the skin into numerous 
small projections that make the animal appear part of the 
rock upon which it rests. These devices not only serve for 
protection, but they also aid in enabling these mollusks to 
steal upon their prey, chiefly fishes, which they destroy in 
great numbers with lionlike ferocity. 
The devil-fishes -and a number of other species are usu- 
ally found creeping along the sea bottom, generally near 
shore, and are solitary in their habits, while the squids re- 
main near the surface and frequently travel in great com- 
panies, sometimes numbering hundreds of thousands. In 
size they usually range from a few inches to a foot or two 
in length, but a few devil-fishes and squids attain a greater 
size, some of the latter reaching the enormous length of 
from forty to sixty feet. There are many stories of their 
great strength and of their voluntarily attacking people 
and even overturning boats, but the latter are in almost 
every case sailors’ yarns. 
In their external organization the cephalopods have 
little to remind one of any of the preceding mollusks, and 
their internal structure shows only a distant resemblance. 
In the Octopi (Fig. 52) the shell is lacking ; in the squid it 
is called the pen, and consists of a horn-like substance with- 
out any lime deposit; in the cuttlefishes it is spongy and 
plate-like, and is a familiar object in the shops ; and, finally, 
