TREM'ATO'DA (Neo-Lat. nom pl., from Gk. rpiluaruiain, trematodes, having many holes, porous, from rpihua, tffma, hole). A class of Platyhelminthes, the flukes, characterized by the possession of certain suctorial pores or openings. They have soft. roundish or flat bodies, and their visceral organs are lodged in the paren chyma of the body. Most of them are her maphrodites. They seldom reach a large size (the greatest length is •about five inches), but are usually visible to the naked eye. The color is usually dull gray, green, or brown, sometimes mottled. The reproductive organs are com plicated and developed to a remarkable degree, occupying a very large part of the body. These worms are not all parasitic, nor are the parasitic forms confined to a single host during the whole of their existence. Many of them, indeed, have a most remarkable life history, showing an ex traordinary succession of generations, in which the young resemble not the parents or grand parents, but the great-great-grandparents. The
first three or four generations live in the body of some invertehrate, especially water-snails; but the next generation is a free-swimming one, and its offspring are like the original form and para sitic within sonic vertebrate.
TheTrematoda are divided into two orders: ( I ) Ectoparasitica. trematodes with three or more suckers, living on the outside of their hosts, and with a direct development from the egg; (2) Endaparasitica, trematodes with not more than two suckers, living in the hlood-vessels, alimen tary canal, or other spaces of the higher animals, and undergoing a complicated alternation of generations. This order includes a large num ber of species, many of which are dangerous, not only to the domestic animals, but even to man himself. See FLUKE (WORM).