Digenea

The Digenea are a subclass of flatworms, specifically trematodes, characterized by complex life cycles involving at least two hosts. Typically, they reproduce sexually as internal parasites within a vertebrate host and then reproduce asexually within a mollusk, such as a snail.123

The life cycle begins when fertilized eggs are released, often hatching in water. The first larval stage, the miracidium, penetrates a mollusk, where it undergoes asexual reproduction to produce numerous cercariae. These cercariae then exit the mollusk and must find a vertebrate host to continue the cycle. Many Digenea require a second intermediate host, like a fish or amphibian, before reaching their definitive vertebrate host. This parasitic lifestyle is thought to have evolved from free-living flatworms that initially lived commensally with mollusks before becoming parasitic.34