Compare the external structure of gastropods and cephalopods and name the difference.

Representatives of the classes of gastropods and cephalopods have a number of differences in both internal and external structure. The body of gastropods (for example, a slug) consists of such sections as the head, torso with a mantle, and a leg. Cephalopods (such as squid) have a head, torso and leg, represented by tentacles and a funnel. The visible difference lies in the structure of the shell: in gastropods it is whole, spiral-twisted, while in cephalopods the shell is poorly developed or completely reduced.