258 ELEMENTARY ZOOLOGY 



which secretes a thin shell (not homologous with the 

 shell of the other molluscs) to protect her eggs, is a mem- 

 ber of the Octopod group. In fine weather the argonauts 

 sail in fleets on the surface of the ocean. 



The pearly nautilus (Nautilus pompilius) is a Cephalo- 

 pod with four gills instead of two, as with the Decapoda 

 and Octopoda, and is the only existing member of what 

 was in the earlier times of the earth's history a large 

 group of animals. The nautili live in rather shallow 

 water usually creeping over the bottom feeding on small 

 marine animals. They make a many-chambered spiral 

 shell with its inner surface lined with beautiful pearly 

 nacre. 



