MOLLUSCA. CEPHALOPODA 261 



A shell is present in all Nautiloids and is always 

 external ; it consists of a tube, which tapers to a point at 

 one end, and may be straight, arched, or spiral. In the 

 spiral forms the whorls may be separate, or partly free, or 

 in contact throughout ; commonly they are all in one 

 plane, but in some cases they form a helicoid spiral. The 

 interior of the shell, unlike that in most gasteropods, is 

 divided into a number of chambers by means of trans- 

 verse partitions termed septa (fig. 103, b) ; generally the 

 chambers increase in size towards the aperture of the 

 shell. The body of the animal is placed in the last or 

 body-chamber (a), to the walls of which it is attached by 

 muscles ; in Nautilus there are two oval muscular im- 

 pressions, one on each side, near the last septum and 

 the inner side of the whorl; these impressions are marked 

 by faint concentric lines. The muscles are connected 

 both above and below by a band of fibres called the 

 annulus, which likewise leaves a mark on the shell. In 

 Nautilus the funnel is placed at the external margin of 

 the aperture, so that this part of the last chamber is 

 regarded as ventral. 



All the chambers, except the body-chamber, are filled 

 with air, giving buoyancy to the shell. The shell grows 

 by the addition of material at the margin of the aperture; 

 after a certain period the body of the animal moves 

 forward and a new septum is secreted behind it, thus 

 cutting off a new air chamber. This movement occurs 

 after a period of growth, and is not related, as some have 

 supposed, to periods of reproduction, since it is only after 

 the shell is completed that reproduction begins. In 

 Nautilus the last air chamber of the completed shell is 

 usually somewhat smaller than the preceding one (fig. 

 103). All the air chambers are transversed by a slender 



