166 PRACTICAL ZOOLOGY 



in successive stages of its growth. The compartments are filled 

 with gas and are connected by a calcareous tube in which is a 

 cylindrical growth of the animal called the siphon (Fig. 95, 7). 

 The gas in the compartments counterbalances the weight of the 

 shell. 



The paper nautilus, Argonauta argo, is a sort of octopus, the 

 female of which secretes a delicate, slightly coiled shell. The 

 true octopus or devilfish (Fig. 94, B) lives in the Mediterranean 

 Sea and West Indies. It may reach a length of over ten feet 

 and a weight of seventy-five pounds. Devilfishes have been 

 accused of serious attacks on man, but are probably not so bad 

 as generally supposed. 



The Relations of Mollusks to Man. The bivalves are of 

 great economic importance because of their value as food. The 

 oyster is, of course, the most valuable (see p. 153). The other 

 bivalves that are commonly eaten by human beings are the 

 soft-shell clam, razor-shells, hen clams, mussels, and scallops. 

 Certain large snails are considered a delicate article of food, 

 especially by the French. Squids are eaten by some people, 

 particularly the Chinese and Italians. 



As SCAVENGERS. The fresh-water mussels are considered 

 inedible, but their beneficial qualities do not depend upon their 

 food value. They are, first of all, excellent scavengers. All 

 sorts of animal and vegetable particles that pollute the water are 

 drawn into the mantle cavity and thence into the mouth. For 

 this reason, a couple of mussels in a fresh-water aquarium are 

 almost indispensable in. keeping the vessel in good condition. 



PEARL BUTTONS. The shells of mussels are used extensively 

 in the manufacture of pearl buttons, and so freely have these 

 mollusks been captured in the upper Mississippi River for this 

 purpose, and for the pearls they sometimes contain, that the 

 United States Bureau of Fisheries is making strenuous efforts 

 to restock the depleted waters by artificially rearing bivalves. 

 Recently a biological station has been built at Fairport, Iowa, 

 largely with this end in view. 



