136 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



and an external, chambered shell. The partitions, or 

 septa, of the shell are united by a tube called " siphun- 



FIG. 109. Female paper nautilus (A rgonauta argo) : i, swimming toward a by ejecting 

 water from funnel, b ; 2, crawling on the bottom; 3, coiled within its shell, which is 

 one fourth natural size. Mediterranean. 



cle," and the animal lives in the last and largest 

 chamber. 33 The living nautilus has a smooth, pearly 



FIG. no. Pearly nautilus, with shell bisected; one half natural size. Indian Ocean. 



shell, a head retractile within the mantle or "hood," 

 and calcareous mandibles, well fitted for masticating 



