How does the digestive system of a turbellarian flatworm work?

The digestive system of a turbellarian flatworm typically consists of a mouth, a pharynx, and an intestine. While most turbellarians possess this structure, members of the order Acoela lack an intestine, instead passing food directly from the mouth into the parenchyma for absorption by mesenchymal cells.1

For other turbellarians, the pharynx serves as a critical feeding organ. In highly developed species, this muscular organ can be protruded through the mouth to capture prey, either by piercing the prey's body to extract fluids and organs or by enveloping the prey entirely.2

Digestion occurs through a combination of extracellular and intracellular processes. Once food enters the gut, it is mixed with digestive enzymes that break down particles. This partially digested material is then either absorbed or engulfed by gut cells, where digestion is completed. Because these organisms generally lack an anus, undigested residue is typically expelled back through the mouth.21