Final answer:
Flatworms have tissues and internal organs, including the lining of the digestive system, derived from the three embryonic germ layers - ectoderm, endoderm, and mesoderm.
Step-by-step explanation:
Flatworms are soft, flattened worms that have tissues and internal organs. The internal organs of flatworms include the lining of the digestive system, as well as other internal tissues derived from the three embryonic germ layers - ectoderm, endoderm, and mesoderm. The mesodermal tissues contain support cells and secretory cells that produce mucus and other materials.