Some time ago, a reader wrote to us to ask a blessedly straightforward question: why are planarians and flukes called flatworms? Taken one way, this is the simplest question imaginable: planarians and flukes are called flatworms because they are flatworms (just as, say, trout and salmon are fish because they are fish). In other words, both planarians and flukes are simply members of the phylum Platyhelminthes, which are more commonly known as flatworms, and that’s why they are called flatworms. We suspect, however, that our reader wasn’t driving at such a simple question. Instead, we suspect he meant to ask why any flatworm (planarians, flukes, or otherwise) is called a flatworm. And so, without further ado, why are flatworms called flatworms?
Flatworms, sometimes spelled “flat worms” (in defiance of the standard spelling, we might add), are unsegmented, bilaterian (their bodies have bilateral symmetry), soft-bodied inveterate animals that belong to the phylum Platyhelminthes. They have no specialized respiratory or circulatory organs, and they lack a body cavity. Through the process of diffusion, their flattened bodies (hence the name “flatworms”) absorb oxygen and nutrients. Because of these biological features, flatworms are considered very simple animals.