All trematodes are parasitic, and most adult trematodes parasitize vertebrates. Around 9000 species have been described. Their body is covered with a tegument, a peculiar kind of epidermal arrangement in which the main cell bodies are deep, separated from the cytoplasm that lies next to the exterior by a layer of muscle (but connected to the exterior layer by cellular processes. The exterior layer is syncytial; that is, it is continuous, not broken by cell membranes. The tegument lacks cilia in adults. Unlike monogeneans, trematodes have no opisthoaptor; instead, they are characterized by one or two suckers. They are like turbellarians in having a relatively well developed alimentary canal, and their muscular, excretory, and reproductive systems are also relatively complete.
Most trematodes have complex life cycles, with larval stages parasitizing one or more species that are different from host of adults. Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria. Most trematodes are endoparasites. They include several parasites that have an enormous impact on human populations, such as human liver flukes and the blood flukes that cause schistosomiasis.
Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.