Trematodes
All trematodes are parasitic.They are characterized by one or two suckers or hooks on their anterior ends by which they fasten onto their victims. Most trematodes alternately reproduce sexually and asexually. In the definitive host (the host where the parasite reaches maturity), in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimming larval forms that are infective to the intermediate host (a host that harbors the parasite for only a short period of time), in which asexual reproduction occurs.