Siphuncle

At the same time gasses, mostly nitrogen, oxygen, and carbon dioxide, diffuse from the blood in the siphuncle into the emptying chamber.

Typically, cephalopods maintain a density close to that of sea water, allowing them to keep a stable buoyancy with minimal effort.

Around these holes, the rim of the septum is bent into a stout aragonitic tube known as a septal neck (or siphuncle notch).

In living nautiluses, the connecting ring is a simple, thin-walled cylinder, with organic or thinly calcitic layers secreted from the tissues of the siphuncle.

Connecting rings are strongly variable in morphology, from narrow homogenous tubes to bulbous, segmented cavities.

These may include horizontal partitions (diaphragms), stacked conical structures (endocones), longitudinal rods, and various other concretions.

Without these deposits, the apex of the buoyant shell would have pointed upwards and the heavier body downwards, making horizontal swimming difficult.

A cross-section through a Nautilus shell, showing a narrow siphuncle connecting the chambers of the shell
Simplified structure and mechanism of cephalopodic siphuncle.
A shell of Nautilus pompilius in cross section. Septal necks are preserved, but the thin connecting rings have been degraded and lost.
These polished orthocerid nautiloid cephalopod fossils from Morocco have fully preserved septal necks and connecting rings outlining the shape of the siphuncle.