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, 



