236 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



have in their body a horny calcareous substance known as 

 the "bone" or "pen." This is the cuttlefish bone that is 

 used to feed canary birds. True sepia ink is also a product 

 of these creatures. The ink is a dark secretion which the 

 cuttlefish discharges when it is irritated or frightened. 

 This clouds the water and allows the animal to escape from 

 its enemies. 



The chambered pearly nautilus, genus Nautilus, belongs to 

 the only living genus of a group which was much better 

 represented in former geologic times. 



