Unisegmental Vermes, with a flattish leaflike or more or less cylindrical body provided with organs of adhesion in the shape of suckers, and sometimes of chitinoid hooks. The cuticle so-called appears to be a metamorphosed layer of cells. There is a well-developed nervous system, the ganglia of which are entirely supra-pharyngeal, i. e. dorsal. There is a mouth, and an alimentary canal which is usually forked, but no anus. The excretory system has the form of more or less branching tubes commencing with flame-cells, and either ending in a contractile vesicle, or opening by two independent orifices. Hermaphrodite. Self-impregnation occurs, as well as reciprocal impregnation. The embryo either developes direct into the sexual form - monogenetic Trematoda, or gives origin to a series of intermediate non-sexual dimorphic forms - digenetic Trematoda. Parasitic.

The monogenetic Trematoda are either roundish or elongated in shape. Two small oral suckers are often present, one on either side the mouth, while the posterior end of the body is either provided with a single sucker, usually large and sometimes armed with chitinoid hooks, or it is expanded and furnished with suckers, or hooks, or with both hooks and suckers or claspers. The digenetic Trematoda are usually elongate, and either flattish or rounded. They have an oral sucker, and there is, except in Monostomidae, a second, usually ventral, sometimes posterior as in Amphistomum, but anterior in Gasterostomum where the mouth is sub-median and ventral. A large number of suckers is rare. The suckers themselves are usually cup-shaped, sometimes compartmented as in the large ventral sucker of Tristomum and in Aspidogaster. They are either sessile or stalked. When numerous they may be set close together in a series of rows on the ventral aspect, e. g. Gastrodiscus from the horse; in three dorsal (? ventral) rows as in Notocotyle (= Monostomum verru-cosum) from the duck, coot, etc.; in a single ventral row as in Stichocotyle Nephropis found encysted in the Crustacean Nephrops, or in a row on the margins of the body as in some monogenetic Trematoda. The chitinoid hooks of the group just named are very generally situated on the suckers.

The monogenetic family Gyrodactylidae is remarkable for possessing either 2-4 retractile adoral processes, or an incomplete circum-oral lobe (Calceostoma) as well as one or two posterior terminal discs armed with hooks (= retinacula). The claspers found in some Polystomeae are provided with two muscular valves supported by chitinoid bars: one valve is fixed, the other free: and the free valve has a snapping action. These last named structures have been accurately investigated only in Axine and Microcotyle.

There is a cuticle, variable in thickness, sometimes perforated by pores, and sometimes spiniferous. The spines are occasionally very conspicuous, e. g. on the body of Fasciola hepaticay on the oral extremity of some species of Distomum and especially on the body and round the oral sucker of Echinostowium. The cuticle is prolonged inwards at the apertures of the different organs opening externally, and it is said in various instances to be underlain either by a layer of cells, by a granular layer, a layer of elastic fibres, or by the circular layer of body muscles1. Unicellular cutaneous glands are said to be present in some instances, e.g. on the posterior sucker of Tristotnum on the surface of the body in Polystomum integerrimum and Aspidogaster, etc. Glands open on the adoral processes of Gyrodactylus, and on the lateral lobes of Holostontutn, and probably secrete an adhesive material. The musculature of the body consists typically of an external layer of circular fibres, followed by a layer of longitudinal fibres, and this in turn by diagonal fibres, crossing from right to left and vice versa. But there are variations from this typical arrangement.

Bundles of dorso-ventral fibres pass vertically from the dorsal to the ventral surface, and their ends are attached to internal prominences of the cuticle. Special bundles of muscles are sometimes present in connection with the oral sucker, and the terminations of the genital organs. Cells disposed in groups or in a layer, are often present internally to the muscular layers. Their significance is not known. The suckers are limited internally by a distinct membrane, and are composed of muscular fibres, arranged equatorially, meridionally, and radially, the last-named constituting the bulk of the organ. The substance of the body is made up of a tissue variable in character; either simply cellular; or composed of a cell-network in which the cells are more or less distinct with other cells of a rounded aspect contained in their meshes; or consisting of a matrix imbedding distinct or feebly indicated cells and fibres. Spaces between the cells or in the matrix have been held to represent the coelome.

The nervous system consists of a pair of ganglia placed anteriorly, and connected by a transverse commissure dorsal to the pharynx. A subpharyngeal ganglion, connected by two commissures with the supra-pharyngeal ganglia, has been described in Fasciola hepatica (Sommer), and a pair of sub-oesophageal nervous commissures in Distomum isostomum . (Gaffron). Two pairs of nerves usually pass forwards from the ganglia, two or three pairs backwards. Of the latter, one pair is dorsal and submedian, and extends in length to a variable degree, sometimes anastomosing posteriorly (D. isostomum). A second pair is perhaps best described as ventral or ventro-lateral, and the third, present e. g. in Tristomum Molae, D. isostomum, D. palliatum, may be termed lateral. The two ventro-lateral nerves may unite posteriorly. In Tristomum Molae D. isostomum, D. clavatum circular commissures connect these longitudinal nerves from place to place. In some species of Distomum the ventral nerves possess a strong transverse commissure in front, and another behind the ventral sucker to which fibres pass from them. It is probable that the Trematoda in general will be found to conform to the type of nervous system described. Ganglion cells occur in the nerves as well as in the ganglia.