Final answer:
Most flatworms have three embryonic germ layers: ectoderm, endoderm, and mesoderm, which give rise to different tissues and organ systems in their bodies.
Step-by-step explanation:
Most flatworms have three embryonic germ layers that give rise to different tissues in their bodies.
The three layers are the ectoderm, which forms the outer layer of cells; the endoderm, which forms the inner layer of cells and the lining of the digestive system; and the mesoderm, which forms the middle layer between the ectoderm and the endoderm.
The mesoderm allows flatworms to develop organ systems like muscles and excretory systems.