Why is a blood system unnecessary in either free-living or parasitic flatworms?

Flatworms, both free-living and parasitic, lack a blood system because they have a flat body structure that allows for efficient diffusion of nutrients and gases directly through their skin. Their bodies are thin enough that all cells are close to the external environment, facilitating the exchange of oxygen and waste without the need for a circulatory system. Additionally, many parasitic flatworms absorb nutrients directly from their host, reducing the need for an internal transport system.