Why are most flatworms thin and flattened rather than thick-bodied?

Flatworms (phylum Platyhelminthes) are thin and flattened primarily because they lack a specialized circulatory system to transport oxygen and nutrients throughout their bodies.12

Because they lack this system, these substances must reach all tissues via diffusion, which is only efficient over very short distances. Their flattened, leaf-like or ribbon-like body shape minimizes the distance between the body surface and internal cells, ensuring that oxygen and fuel can reach tissues at a sufficient rate to maintain their metabolic needs.13

Additionally, flatworms lack a fluid-filled body cavity (coelom), meaning the space between their gut and body wall is filled with solid mesenchyme (parenchyma) tissue. This structural constraint further necessitates their thin profile, as any significant increase in body thickness would prevent essential materials from diffusing effectively to internal organs. While some larger free-living species have evolved extensively divided guts to help distribute nutrients, the fundamental requirement to minimize diffusion distances remains a primary constraint on their body shape.132