Why is a worm long?

One of the reasons worms are long are because of their general biological makeup. They have a very simple design lacking in lungs. They acquire their needed oxygen by diffusion through their skin. In order for a worm to acquire more oxygen a greater surface area is needed. This increased surface area can be accomplished by the worm being long and thin, but not by the worm being short and fat. it's an equation of surface area to volume. In being long and thin the oxygen can easily and rapidly diffuse across the worms membranes.