Molluscs: Evolving and keeping the shell
An early group of molluscs retained the protection of a shell yet were still able to maintain a high degree of mobility. This was achieved through the development of a gas-filled floatation tanks. The prototype forms had a flat-coiled shell with an end walled-off to form a gas chamber. As the animal grew it added buoyancy with the development of new chambers. Such animals survive today and are known as nautiluses. A tube runs from the body chamber of the nautilus to the floatation tanks in the shell. The nautilus is an active carnivore eating animals such as crabs and moves in a form of jet-propulsion where water is squirted through a siphon. In this animal the original muscular foot is divided into long grasping tentacles with which it secures its prey. The mouthparts are modified to form a horny beak with which the nautilus is able to crack shells of other animals. Variations on the float chamber theme gave rise to the enormously successful group of animals called the ammonites whose circular shells were up to 2 meter in size.