Siphuncle

Siphuncle, the long membranous tube which passes back through the shell of a Cephalopod (q.v.), such as the Nautilus, and establishes a connection between the different chambers. Where it passes through the septa, prolongations of these, known as "collars," pass back and protect it. The position of the siphuncle is of considerable importance, as in Nautilus and its allies it passes through the centre of the septa, while in the Ammonites it cuts them at the margin.