Reproduction is sexual, often, however, combined with asexual methods of propagation (segmentation, budding). The Platyhelminthes live partly free in fresh or salt water, exceptionally also on land. The greater part, however, live as parasites on or in animals. CLASSIFICATION OF THE PLATYHELMINTHES. _Class I._--*Turbellaria* (or Eddy Worms). Flat worms for the most part, free living, and always covered with a ciliated epithelium. _Order 1._--_Rhabdocœlida_, gut unbranched. _Order 2._--_Tricladida_, gut with three main branches. _Order 3._--_Polycladida_, a central gut with lateral cæca. Development direct or through metamorphosis. They live in fresh and salt water or on land; very seldom as parasites. _Class II._--*Trematoda* (Sucking Worms[254]). [Usually known as Flukes.--F. V. T.] Flat worms, living as ecto- or endoparasites, that are only ciliated in the larval condition, and in their adult state are covered with a cuticle, the matrix cells of which lie in the parenchyma. They have either one, a few, or several suckers,[255] and frequently also possess chitinous fixation and adhesive organs. The intestine is single, but generally bifurcated, and not uncommonly there are transverse anastomoses between the forks or diverticula on them. Excretory organs double, with two orifices at the anterior extremity or a single one at the posterior end. Development takes place by a metamorphosis or alternation of generations (p. 283). These worms are almost always hermaphroditic, with two or more female and one male sexual orifice. They live, almost without exception, as parasites on vertebrate animals, but the intermediate generations are passed in molluscs. [254] This grouping goes back to the year 1800, and was made by J. G. H. Zeder, a physician and helminthologist of Forchheim, who divided the helminths, which until 1851 were generally regarded as a special class of animals, into the groups of round, hook, sucker, tape and bladder worms, as which they are recognized up to the present time. In 1809, K. A. Rudolphi gave them the names _Nematodes_, _Acanthocephali_, _Trematodes_, _Cestodes_ and _Cystici_. [255] A sucker or acetabulum (little cup) is a round, cup-shaped muscular organ, the muscles of which are _sharply defined_ from those of the body.