The chambered nautilus is the only type that has a complete shell. The squid and cuttlefish have small shells that are located inside of the body, rather than outside. The octopus, on the other hand, has lost its shell completely. All of the cephalopods are marine animals and all are carnivores. While cephalopods appear to be very different from their mollusk relatives, they all have the same basic body plan.
All mollusks have a soft body with a special covering called the mantle.
The mantle encloses all of the body organs such as the heart, stom ach and gills. The tough skinlike layer is also responsible for making the shell, if it is present. Most mollusks have a muscular foot to aid in movement and a feeding structure called a radula. In the cephalopods, instead of one muscular foot, there are at least eight arms projecting from the head region. In this class, the mantle is present but the body is more elongated.