The Trematodes are a class of flatworms in the phylum Platyhelminthes. They are called flukes, and are internal parasites of molluscs and vertebrates. Most trematodes have a complex life cycle with at least two hosts. The primary host, where the flukes sexually reproduce, is a vertebrate. The intermediate host, which is the agent of dispersal, is usually a snail.
The trematodes or flukes are estimated to include 18,000 to 24,000 species