Trematoda

Trematoda is a class within the phylum Platyhelminthes. It includes two groups of parasitic flatworms, known as flukes. They are commonly referred to as trematodes. The class Trematoda is divided into two subclasses, Digenea and Aspidogastrea, which are morphologically distinct.

Morphology

Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 1 millimeter (0.039 in) are known. Their most distinctive external feature is the presence of two suckers, one close to the mouth, and the other on the underside of the animal.

Life Cycle

The life cycle of a typical trematode begins with an egg. After the egg hatches, it releases a larval form, the miracidium, which invades a suitable snail intermediate host. The miracidium transforms into a sporocyst, which produces rediae, from which cercariae are derived. These are released from the snail and encyst as metacercariae on vegetation, or directly on the skin of the definitive host, where they lie in wait to be eaten or, in the case of skin-penetrating species, to burrow into the host.

Classification

The class Trematoda is divided into two subclasses. The Digenea, the majority of trematode species, are complex internal parasites, infecting all classes of vertebrates, including humans. The Aspidogastrea, a much smaller group, are internal parasites of molluscs and vertebrates.

Human Health

Trematodes are responsible for serious human diseases including schistosomiasis, clonorchiasis, opisthorchiasis, fascioliasis, and paragonimiasis. The diseases are caused by the adult stage of the fluke in the human body.

See Also

• Schistosomiasis
• Clonorchiasis
• Opisthorchiasis
• Fascioliasis
• Paragonimiasis

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