Flukes, a group of parasitic worms constituting the order Trematoda. They are all composed of a single segment, and their form is typically flattened like a leaf, though some are cylindrical. They secure attachment to the body of their host by one or more suckers, whence they are often known as the "suctorial worms." The mouth usually leads to a pharynx, and this to a forked alimentary canal; there is, however, no anus. Except the abnormal genus Bilharzia, the Flukes are all hermaphrodite, and in some cases self - impregnation certainly occurs, as, e.y. in Polystomum inteyerrimum. The Flukes are divided into two groups according to whether they exhibit "alternation of generations" (q.v.) or not; if they do not they are known as the "monogenetic." In these development is direct, with some few exceptions, as Calicotyle, which lives in the Rays. All the members of this group are external parasites. The "digenetic" have one or more (usually many) nonsexual generations intervening between the egg and the mature sexual form: the adults are always parasitic in some vertebrate animals, while the larva? are free-swimming at another stage and live in mollusca in another. Both the monogenetic and digenetic forms are classified into families based on the number of suckers; thus the former is divided into the Tristomeae and Polystomea, and the latter into the Monostomidcp, Distomidce, etc. The best-known of the Trematoda is the Liver-Fluke (q.v.), Fasciola, (Distomum) hepatica, which is the cause of the disease known as sheep-rot or liver-rot. An account of the structure and life history of this form, serving as a general type for the class, is given under Liver-fluke (q.v.). This species occasionally occurs in man, but is mainly found in ruminants. Among other human parasites belonging to the class are Distomum ringeri, which attacks the lungs, and the African Bilharzia lucmatobia, which lives in the blood-vessels of the bladder.