These are parasites of all classes of vertebrates. Adults have great morphological diversity so there are no unifying features. This is the largest order of digeneans, with between 20 and 50 families depending on the classification scheme used!
A variety of life cycles is exhibited, involving three hosts, in aquatic or terrestrial habitats. The molluscan first intermediate host harbors sporocysts and often rediae as well. Adults can be found in the lumen of the digestive system, lungs, and other internal organs.
Adults exhibit the basic digenean body plan, and often possess a spined body. Cercariae usually possess an oral stylet which helps them penetrate their second intermediate host.