Phragmocone

The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae.

In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a siphuncle which determines buoyancy by means of gas exchange.

Despite this benefit, such a large shell adds to the weight of the animal, and hence is not advantageous in catching fast-moving prey. Some nautiloids, such as the Silurian Ascocerida, drop the phragmocone upon maturity, presumably to increase speed and maneuverability. They thus became the early Paleozoic equivalent of coleoids. The early coleoids and belemnoids adopted a different approach. The phragmocone is retained but became internal and reduced, and so like the shell in general it tends to be vestigial or absent in most cephalopods.