754 



ZOOLOGY 



SECT 



set delicate lamellae ; the median canal opens into the mantle- 

 cavity by a slit bounded by a number of plaits situated at the 

 narrower oral end. The nidamental glands secrete the viscid 

 material by means of which the eggs when deposited adhere together 

 in masses. A glandular mass of unknown function, known as the 

 accessory nidamental glands (ac. nid.), lies at the sides and around 

 the oral ends of the nidamental glands proper. 



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 



FIG. 683. Section of the shell of Nautilus pompilius, showing the septa (s, s), the ?eptal 

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

 chamber (ch). (From the Cambridge Natural History.) 



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

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

 coiled shell (Fig. 683), 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 

 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 



