TREMATO'DA, or TREMATODE WoRms, constitute, according to Dr. Cobbold's system, the second order of the sub-class sterelmintha (Owen), of the class helraintka. In ordinary zoological classification, the trematoda form a division of the group of platyelmia or "flat-worms," which group is in its turn included in the class scolecida—a division of echinozoa or annuloida. This order, Is the Greek word trematodes indicates, is charac terized by the possession of certain suctorial pores or openings. All the animals included in it have soft, roundish, or flat bodies, and their visceral organs are lodged in the paren chyma of the body. Most of the. trematoda are hermaphrodites. They seldom attain to a large size (the greatest length is about 5 in.), but are usually visible to the naked eye. Like all scolecida, the trematoda possess a peculiar system of vessels ramifying through their bodies, and termed the " water-vascular" system.
The trematoda, or flukes, as they are popularly called, from their resemblance in form to small flukes or flounders, are not parasitic during the whole period of their existence; "for, while .passing through the cycle of their life-development, they frequently change their residence, at times inhabiting either open waters, or the dewy moisture of low pas ture-grounds. They perform active and passive migrations from parasitic to non parasitic abodes; and during their larval wanderings in search of a final resting-place which should prove suitable to their adult condition, they provisionally occupy the bodies of different kinds of invertebrata."—Cohboki's' Entozoa, 1864, p. 15. In his Synopsis of the Distomida3 (published in 1861), Dr. Cobbold recognizes 344 species of flukes, of which 126 belong to fishes, 47 to reptiles, 108 to birds, 58 to mammals, and 5 to the invertebrata. He now believes that, at the very lowest possible estimate, we must assume the order to contain 400 species, which may be divided into the five families of monostomidce, distomida, polystomida, and gyrodactyla—names which are based, except in the last case, on the number of their pores or oval suckers.
Van Beneden arranges these families into (a) monogenea and (b) digenea, the develop ment in the former being simple, while in the latter there is an alternation of generation, the nurses and larva living chiefly in mollusks, while the adult animals chiefly live in the bodies of vertebrate animals. The monostomidce and distomicice belong to the latter, and the others to the former group. The family of distontidce embraces the principal and best-known genera of the order trematotla, and its members are at once recognized by the presence of two pores or suckers; one, the anterior, being connected with the mouth; and the other, termed the acetabulum, being usually placed on the ventral sur face, in the middle line.
The following members of this order are of special interest, as very liable to infest man: Fasciola hepatica, described in the article FLUKE, is not only common in all varieties of grazin..-cattle, and especially the sheep, but has been found in the horse and ass, in the hare and rabbit, in the squirrel, beaver, kangaroo, etc.,. and is occasionally met with in man, not only iu the liver and gall-bladder, but beneath the skin iu various parts, as, for example, in the sole of the foot, behind the ear, and in the scalp. For an excellent account of the anatomy and development of this parasite, the reader is referred to Cobbold, op. cit. pp. See HOT. Distoma lanceolatunt. is a species which is by no means uncommon in the sheep and ox, and has been found on at least three, occasions in the human subject. Disto»ta ophthalmobium has been occasionally found in the lens of the human eye. Distoma licematobium, or Billtarzia lccematobia, as Dr. Cobbold terms it, is the only known trematode which is not hermaphroditic.
The male is a cylindrical worm, measuring only about half an inch in length; while the 2emale is tiliform, longer,, and much narrower than the male, being about t of an inch in length. The first specimens were found by Bilharz of Cairo in the portal system, and the worm has since been found in the veins of the mesentery, bladder, and other parts. This parasite is common not only along the borders of the Nile, but in south Africa and the Mauritius. It is so common in Egypt that in 363 examinations of the body after death, Griesinger found it no less than 117 times.
The principal feature of the disease caused by this worm consists in a general dis turbance of the uropoietic function. Diarrhea and hematuria occur in advanced stages of the complaint, being also frequently associated with the so-called Egyptian chlorosis, colicky pains, antemia, and great prostration of the vital powers. The true source of the disorder, however, is easily overlooked, unless a careful microscopical examination be made of the urine and other evacuations. If blood be mixed with these, and there also be a large escape of mucus, a minute inspection of the excreta will scarcely fail to reveal the presence of the characteristic ova of bilharzia.—Cobbold, op. cit. p. 202. Dr. J. Harley has published several excellent papers " On the Heinaturia of the Cape of Good Hope, produced by a Distoma," which is undoubtedly the bilharzia.
Several other trematodes occasionally occur in the human subject.