Final answer:
The body wall in arrow worms is composed of two layers of muscle - circular and longitudinal. Flatworms have a similar structure, but with a single layer of cells or fused cells.
Step-by-step explanation:
The body wall in arrow worms is a stratified layer composed of two layers of muscle - one circular and the other longitudinal. The mesodermal tissues include support cells and secretory cells that secrete mucus and other materials to the surface. Flatworms, on the other hand, have a single layer of cells or a layer of fused cells covering a layer of circular muscle above a layer of longitudinal muscle.