Compared to platyhelminthes which physiological problem have the cylindrical body of nematodes brought? How was that problem solved?

The cylindrical shape of nematodes made impossible the respiration exclusively by simple diffusion among cells since there are tissues far from the exterior. This problem was solved by the presence of an inner cavity in the body filled with fluid, the pseudocoelom. The pseudocoelom has the function of distributing gases and nutrients to the body and to collect residuals, besides serving as a hydrostatic base to keep the worm shape.