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?

As the title of this article suggests in a vaguely patronizing way (which is absolutely not intentional), flatworms are called flatworms for the very simple reason that they are in fact flat worms. They have no body cavity, nor do they have specialized respiratory or circulatory organs, and as a consequence their bodies are flat because this body design facilitates the diffusion of nutrients and oxygen.

The thousands of different species of flatworms are traditionally classified by whether they are parasitic. Over half of them are, and these parasitic flatworms fit into three different classes: Cestoda, Monogenea, and Trematoda. It is this last class of flatworms – Trematoda – that are commonly known as flukes. The Platyhelminthes that are not parasitic (or at least are largely not parasitic) belong to the Turbellaria class. Some (but not all) of the non-parasitic species in this class are commonly referred to as planarians. So, as we have just made clear, planarians and flukes are both members of classes that belong to the phylum Platyhelminthes, and this is the case precisely because they have flat bodies.

That is really all there is to be said on the matter. Flatworms are called flatworms because the are, once more, flat worms. Since planarians and flukes are both flat-bodied worms in the phylum Platyhelminthes, they are called flatworms (as are all Platyhelminthes). Hopefully we answered our reader’s question to his satisfaction, and if he (or any other reader) is interested in learning more about flatworms, he can check out our article about Platyhelminthes.

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