46 GLIMPSES OF NATURE. 



inhabits. As it grows too big for its receptacle, it 

 partitions off the old apartment, and adds a new 

 chamber to its house, living in the last-formed and 

 biggest of the rooms. Now, this perfect shell of the 

 pearly nautilus is made by the "mantle," just as the 

 shell of the snail or oyster is fabricated by that layer 

 of the molluscan body. 



Turn we now to the argonaut, or paper nautilus. 

 Here, also, we find a shell ; but it is not a true shell, 

 because it is not formed by the " mantle " of the 



Fig. 14. Pearly Nautilus Shell. 



animal. The delicate papery shell of the argonaut is 

 made by two of the arms of the animal, and therefore 

 does not agree with the true shell of the other nautilus. 

 Cuttlefishes, however, have undergone a very large 

 amount of modification as living forms. To-day it is 

 the sepias and the octopi and their kith and kin which 

 swarm in the seas. They are active, free-swimming 

 animals, which do not depend on a shell for protec- 



