776 ZOOLOGY SECT. 



ii. THE PEARLY NAUTILUS (Nautilus pompilius). 



The three living species of Nautilus, of which N. pompilius is 

 the best known, are inhabitants of moderately shallow water about 

 the shores and coral-reefs of the South Pacific, usually swimming 

 near the bottom, and probably rarely, if ever, coming voluntarily 

 to the surface. The body is enclosed in a calcareous, spirally- 

 coiled shell (Fig. 674), into which the entire animal can be with- 

 drawn for protection. The cavity of the shell is divided by a 

 system of septa into a series of chambers, the last and largest of 

 which, opening widely on the exterior, alone lodges the body of the 



ch 



Fio. 674. Section of the shell of Nautilus pompilius, showing the septa (*, *\ the septal 

 necks (s. n.,s. .), the siphnncle, si. (represented by dotted lines), and the large body-chambei 

 (ch). (From the Cambridge Natural Hlstort/.) 



animal. Between the animal and its shell there is a direct organic 

 connection through the intermediation of a narrow, tubular, vascular 

 prolongation of the visceral region, which perforates the entire 

 series of the septa to the apex of the spiral. This tube, which is 

 termed the siphuncle (si.\ has its wall supported by scattered 

 spicules of carbonate of lime ; but, in addition, as it passes through 

 each septum, there is produced over it for some distance a shelly 

 tube the septal neck (s. n.) continuous with the substance of the 

 septum. The apical or initial chamber presents a small scar, the 

 cicatrix, which may indicate the original presence of the larval 

 shell, or protoconch, which has fallen off in the course of develop- 

 ment. 



