Phragmocone

From Freepedia

Phragmocones are the chambered portions of the shell of a cephalopod. They are 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 determined 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 Ascoderida, drop the phragmocone upon maturity, presumably to increase speed and maneuvuerability. They thus became the early Paleozoic equivelant 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 vestigal or absent in most cephalopods.