Flatworms, sometimes spelled “flat worms” (in defiance of the standard spelling, we might add), are unsegmented, bilaterian (their bodies have bilateral symmetry), soft-bodied invertebrate 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.
The traditional way to classify the phylum Platyhelminthes is to break it into two parts: one non-parasitic (for the most part), the other entirely parasitic. The mostly non-parasitic group is a subdivision of the phylum Platyhelminthes called Turbellaria, and it includes animals like planarians, a common flatworm that lives in many different parts of the world. Turbellarians tend to be predators that live in water or moist areas on land, like piles of wet leaves. The parasitic flatworms are divided into three groups: Cestoda, Trematoda and Monogenea. Trematodes (a.k.a. flukes) and Cestodes (a.k.a. tapeworms) are similar to one another in that their complicated life cycles share many features. In their mature stages, both live as parasites in a host; more specifically, they live in the digestive systems of fish and vertebrates that live on land. (Most people have probably heard about tapeworms in this context.) Both groups also infest secondary hosts during the intermediate stage of their life cycles, and both produce large numbers of eggs, although they accomplish this in a different fashion. Monogeneas, in contrast, are external parasites. They infest aquatic animals, with their larvae growing into adult form once they have attached themselves to a host.
While there are tons of different types of flatworms (scientists have identified over 20,000 species, and there are presumed to be many more), some common characteristics link them together. For example, flatworms are versatile and can live all over the world in wildly different habitats. Some live in extremely hot locations, while others live in the freezing cold. Temperature, whether hot or cold, isn’t much of an impediment to flatworms. What is an impediment, however, is moisture. Like most other worms, flatworms cannot withstand a dry environment. Flatworms also have what might be described as a “slimy” body, as they secrete mucus from glands on their underside. This allows flatworms that live on land to move about in a gliding motion.
A couple of interesting facts about flatworms before we conclude: first, they vary greatly in size. Some flatworms are microscopic, while others can grow to over 20 inches long. Another interesting thing about flatworms is that if their bodies are split apart in some way, the individual parts will regenerate into new worms. They must be almost completely dissolved to prevent this regeneration from taking place.
So, flatworms are simple creatures, but they are no less resilient because of it. Not many other animals can withstand bitter cold or oppressive heat, and still fewer can be cut to pieces and still survive.
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