Trematodes live free and active within the organs attacked, though they may attach themselves by suction for a longer or shorter period; in other cases, however, they bore more or less deeply into the intestinal wall with their anterior end, or lie in cysts of the intestinal wall which only communicate with the lumen through a small opening; in those species living in the lungs of mammals the host likewise produces a cyst, which usually encloses two specimens; such association of a pair is also observed in other situations, and, though this is the rule in species sexually distinct, it is not entirely confined to these. As regards the AGE attained by endoparasitic Trematodes, there are but few reliable records, and these differ considerably; the overwhelming majority of species certainly live about a year, or perhaps a little longer, but there are some whose term of life extends to several or many years. Trematodes are but rarely found encysted in the higher vertebrate animals; the condition, however, is more frequent in amphibians, and especially in fishes, as well as in numerous invertebrate animals. CLASSIFICATION OF THE TREMATODES OF MAN. The following classification, partly artificial, partly natural, embraces only the flukes found in man:-- Order. *Digenea*, v. Beneden, 1858. Anterior sucker single and median, present. Eggs few. The (specialized) terminal portion of the uterus serves as a vagina. Development indirect, _i.e._, an intermediate host is required. Sub-order. *Prostomata*, Odhner, 1905. Mouth surrounded by the anterior sucker. Group. *Amphistomata*, Rudolphi, 1801, ep., Nitzsch, 1819. Gut forked, two suckers, the posterior sucker (acetabulum) terminal or ventro-terminal behind the genitalia, or at most embraced by the vitellaria. Skin with no spines. Excretory bladder a simple sac opening dorsally near hind end. Testes in front of ovary. Genital pore, median in anterior third of body. Thick flukes, almost circular in cross section. Family. *Paramphistomidæ*, Fischoeder, 1901. Amphistomata: Body not divided into a conical anterior portion and disc-like caudal portion. Ventral pouch absent. Sub-family. *Paramphistominæ*, Fisch., 1901. Paramphistomidæ: Oral sucker without evaginations. Not in man. Sub-family. *Cladorchiinæ*, Fisch., 1901. Paramphistomidæ: Oral sucker with evaginations; testes, two, deeply cleft (fig. 137). Genera: _Watsonius_, _Cladorchis_, etc. Family. *Gastrodisciidæ*, Stiles and Goldberger, 1910. Amphistomata: With body divided into a conical cephalic and disc-like caudal portion (fig. 138). Posterior sucker ventro-terminal. Oral sucker with evaginations. Genera: _Gastrodiscus_ and _Homalogaster_. Group. *Distomata*, Retzius, 1782. Gut forked, two suckers, the posterior sucker (acetabulum) ventral. It is always separated from the hind end by at least a part of the genitalia. Family. *Fasciolidæ*, Railliet, 1895.