Why do many flatworms have only one opening to the gut?

Many flatworms possess only one opening to the gut because they lack a complete digestive tract with both a mouth and an anus. In these organisms, the mouth serves as a common opening for both the intake of food and the elimination of undigested residue.12

This anatomical structure is characteristic of their status as acoelomates, meaning they lack a body cavity. Because flatworms do not have a circulatory system, they rely on diffusion to transport nutrients and oxygen throughout their bodies. To compensate for this limitation, many flatworms have evolved extensively branched guts that bring nutrients close to all internal tissues. While a few species have developed anal pores, the vast majority of flatworms maintain this "blind-ending" digestive system, which is sufficient for their relatively small or thin body plans.341