CEPHALOPODA 87 



grasps its prey. " Devil-fishes '' are found in all seas. They are gregarious 

 when young, but the adult is solitary. They creep about among the rocks 

 upon the extremities of their arms, generally moving sideways; or swim 

 rapidly, either forward or backward. The arms are somewhat webbed at 

 the bases. 



Some devil-fishes measure 12 to 15 feet, others but a few inches. They 

 are found on our western coast and in the Pacific islands. They are much 

 used for food along the Mediterranean Sea and by the Chinese and Italians 

 of San Francisco. 



The Nautilus (Fig. 64). — This Cephalopod has a many-chambered, 

 spiral, univalved shell, lined with pearly nacre, hence is often called the 

 " pearly nautilus." It has four gills instead of two. It crawls about 

 on the sea bottom by means of its many (about forty) small tentacles. 

 It has no suckers. The outer chamber of the shell is a large compartment 

 in which the animal lives.. As it grows, the nautilus partitions off the space 

 behind it and moves forward. A calcareous tube containing the siphuncle, 

 a slender tubular continuation of the body, extends through all the septa. 

 The abandoned compartments are filled with air. 



The nautilus has a beak and a rasping tongue, like those of the squid. 

 Each of its two disk-shaped eyes is attached by its convex side to a short 

 thick stalk. The aperture of the eye is small, and there is no cornea, no 

 iris, nor vitreous humor, but simply the retina at the base of a disk or pit. 

 The nautilus has not the power of changing its color, and has no ink sac. 



It lives in the deep water in the south Pacific Ocean, and has been but 

 little studied. Many of the species of former ages are extinct. This is the 

 " chambered nautilus," immortalized by Oliver Wendell Holmes. 



Economic Importance. — Mollusks are probably of more 

 direct use to man than any other invertebrate branch. The 

 oyster industry is of vast importance, giving employment to 

 thousands of persons and bringmg an annual income of millions 

 of dollars. Clams are also used extensively for food, and peri- 

 winkles and snails less extensively. We get also pearls, and 

 the mother-of-pearl for the making of buttons, knife-handles, 

 and novelties. Factories have been established in Illinois and 

 Iowa for making buttons on a large scale from the fresh-water 

 mussel shell. This industry threatens to exterminate these 

 bivalves unless means are taken to protect and perpetuate them. 



The squid is extensively used as bait in cod-fishing, while 

 both the squid and the cuttlefish furnish the sepia ink used by 

 artists. The cuttlebone used for canaries is another product 

 of the cuttlefishes. 



The ship-worm does much harm to dikes, wharves, and piles, 

 or any wooden structures which have been in water some time. 



Important Biologic Facts. — The mollusks are the most highly 

 organized of any of the invertebrates except the Arthropoda, 



