Siphuncle

The siphuncle is the segmented tube that runs through the chambers in a cephalopod shell, through which during life blood vessels, nerves, and other viscera connected to the animal run. Its primary function is to remove water from the latest chamber formed so as to maintain or re-establish neutral buoyancy.

Siphuncles are composed of two parts that are repeated, septal necks and connecting rings. Septal necks are a part of the septa that either point toward the apex, in the case of nautiliods, or point toward the aperture in the case of most ammonoids. Terms are respectively retrochoanitic and prochoanitic.

Necks are impervious, and vary in length from short to as long or longer than the chamber length, and in form from straight to flared to recumbent, but generally inveriable in a specific individual or species. Connecting rings connect necks with the next septum, and vary in form and construction, sometimes overlaying, other times providing an extension. Rings are generally permiable, it is through them that communication with the chambers is made. Some, as in the early ellesmerocerids, endocerids, actinocerids, and tarphycerids are thick and layered. Others as in the discosorids are zoned. Others as in the later orthocerids, barrandeocerids, oncocerids, and nautilids are thin; along with the Ammonoidea

Siphuncles are an essential character, if not the essential character in determining and classifying nautiloid cephalopods, especially those of the early Paleozoic.

Selected References
Formation and Function of the Siphuncle - Septal Neck Structures in Two Mesozoic Ammonites. Kazushige Tanabe, Yoshio Fukuda, and Ikuwo Obata 1982 

The siphuncle of nautiloids controls buoyancy by active transport of ions and osmosis between the siphuncle and shell chamber.