393 CEPHALOPODA — TETRABRANCHIATA chap. 



funnel consisting of two separate lobes ; tentacles numerous, 

 without suckers or hooks ; no ink-sac. 



The shell consists of two layers, the outer being porcellanous, 

 and the inner, as well as the walls of the chambers or septa, 

 nacreous. The septa vary greatly in shape. In most of the 

 Nautiloidea they are regularly curved, as in Nautilus, or 

 straight, as in Orthoceras, but in the Ammonoidea they are often 

 exceedingly complex. The edge of the septum, where it unites 

 with the shell-wall, is called the suture, and the sutural line, 

 which is not seen until the porcellanous layer is removed, varies 

 in shape with the septum. 



The septa are traversed by a membranous tube known as the 

 siphuncle, which in Nautilus is said by Owen to connect ulti- 

 mately with the pericardium. The septal necks, or short tubular 



Fig. 252. — Nautilus pompiliiis L., in section, showing the septa {s, s), the septal necks 

 {s.n, s.7i), the siphuncle dotted in (si), and the large body chamber (ch). 



prolongations of the septa where they are perforated by the 

 siphuncle, are in the great majority of the Nautiloidea directed 

 backwards (Fig. 252), i.e., they project from the front wall of 

 each chamber, while in nearly all Ammonoidea they are directed 

 forwards. When the siphuncle is narrow, as in the Ammon- 

 oidea, it is simple, but when wide, as in many of the Nautiloidea, 



