Explanation

Flatworms (Platyhelminthes) and roundworms (Aschelminthes) both possess triploblastic body, bilateral symmetry and metamorphosis in the life history. But flat worms differ from all roundworms in having solid mesoderm. The mesodermally derived tissue includes a loose tissue called parenchyma and this tissue includes fills the body space, i.e., space between the body wall and more specialized tissue or organs.