The tube on the outside of an annelid is its epidermis, with flesh below that. Inside of this lies the digestive tract, the tube through which food passes while being digested and then turned into waste.
Work Step by Step
The key here is to see that almost all animals after the cnidarians are basically flesh organized around a hollow, tube-like digestive tract. Annelids, by their outer form, make this more simple to see.