These parasitic flatworms have evolved to thrive inside the bodies of other living creatures.

Nature · Published 3 October 2026 Swipe more like this →

In short: Trematoda, or flukes, are a class of parasitic flatworms that have mastered the art of living within hosts. Ranging from microscopic to several centimetres long, these organisms lack complex respiratory or circulatory systems, instead relying on specialized body structures to survive and reproduce across diverse, often multi-host, life cycles.

The class Trematoda belongs to the phylum Platyhelminthes, a group of soft-bodied, bilaterally symmetrical invertebrates. Unlike more complex animals, flukes lack a body cavity and specialized respiratory or circulatory systems. Their bodies are filled with a spongy connective tissue known as parenchyma, and they typically possess a head region containing a brain and concentrated sense organs. While some flatworms are free-living, trematodes are obligate internal parasites, often requiring multiple hosts—such as mollusks, arthropods, and vertebrates—to complete their complex life cycles.

Physically, trematodes are characterized by their flattened, often oval shape and the presence of two suckers, which allow them to attach to and feed upon their hosts. Their body surface is protected by a tough syncytial tegument, an adaptation that helps them withstand the digestive enzymes encountered within a host's gut. Internally, they possess a muscular pharynx and blind-ending caeca for digestion, though they lack an anus, meaning waste must be egested through the mouth. Their nervous system is relatively simple, consisting of a pair of ganglia in the head and nerve cords running the length of the body.

The impact of these parasites on human and animal health is significant. More than 36 species of flukes have been reported as parasitic in humans, causing conditions ranging from schistosomiasis to liver and lung infections. Some species, such as those in the genus Schistosoma, are even linked to cancer. Because these parasites often rely on specific intermediate hosts like snails or crustaceans, their distribution is closely tied to the presence of these organisms. Despite attempts to control fluke populations by eradicating their mollusk hosts, these parasites remain a persistent global health challenge.

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