254 



SYSTEMATIC ZOOLOGY 



living genus, the pearly or chambered nautilus (Fig. 263), which 

 lives in the Indian and Pacific oceans. As the name of the 

 group suggests, there are four gills, — two pairs. The lobes 

 about the mouth, which represent a part of the foot, are not long 

 and bear numerous slender tentacles. The funnel, likewise a 

 part of the foot, is not a closed tube, but is open along the lower 

 side, though the two edges may be approximated. The eyes 

 are extremely simple in structure. The ink sac, present in 

 most Cephalopoda, is wanting in the nautilus. The shell, which 



Fig. 263. Nautilus pompilius. Shell in section, ch, body chamber occupied by living 

 animal; s, septum ; si, siphuncle; s.n, septal neck. (From the Cambridge Natural His- 

 tory.) 



has a pearly lining, is a spiral, symmetrically coiled in one 

 plane, and consists of a series of chambers separated from one 

 another by calcareous partitions or septa. As the shell is made 

 larger and the animal grows, it moves forward, leaving a g 

 filled chamber behind it, and then secretes the partition over 

 the deeper surface of its bodv. In this way all the chambers 

 have been formed. In the center of each partition is an open- 

 ing through which a cylindrical stalk passes from the end of 

 the animal's body to the extreme inner end of the spiral, thus 

 traversing all the chambers. 



