Final answer:
Cuttlefish have an internal, porous shell known as a cuttlebone, which is used for buoyancy and as an external source of calcium carbonate for other animals.
Step-by-step explanation:
The cuttlefish, a marine animal within the class Cephalopoda, possesses an internal shell known as a cuttlebone. This structure is not an external shell like that of a snail but is instead a reduced, internal shell which performs a variety of functions. Cuttlebones are lightweight, porous, and composed of aragonite. They serve multiple roles, including providing buoyancy control for the animal in water and serving as a source of calcium carbonate for other purposes, such as smoothing bird beaks or as a dietary supplement for birds like chickens and quail. While other members of the class Cephalopoda, such as the Nautilus, have an external multi-chambered shell, the cuttlefish showcases an evolutionary path where the shell is not external but internal and greatly reduced in size.