Ciliated funnels have been detected in several monogenetic Tre-matoda, e. g. Polystomum integerrimum. There is generally a system of canals corresponding to the two sides of the body which branch and anastomose. In Aspidogaster and Stichocotyle there is a posterior terminal pore, from which two contractile (?) vessels originate. But in Tristomum, Pseudocotyle, Epibdella, etc, each main lateral canal terminates in a small vesicle, and the two vesicles open on the ventral aspect of the body anteriorly, one on either side of the pharynx. The two apertures are said to be dorsal in Polystomum integerrimum and Axine, but in Onckoeotyle they are posterior, and situated respectively at the ends of the two contractile processes of the body. In most Polystomeae the excretory system is imperfectly known 1.

The Trematoda are hermaphrodite with the exception of Bilharzia 2. The male organs consist of the testes and efferent canals. The former usually lie near the centre of the body, and just behind the ovary. There is but one testis in Udonella Caligorum, Octobothrium lanceolatum and Calceostoma elegans. In other instances there are two, one somewhat in front of the other. They are generally more or less globular, sometimes lobed. In most of the monogenetic Trematoda the lobes are well marked, and the testes are frequently broken up into independent follicles, e. g. Tristomum, Polystomum. The digenetic genus Fasciola alone has branched tubular testes. The vas deferens is formed by the union of two or more efferent vessels according to the number of testes or testicular follicles. It is dilated more or less, for the first part of its course forming a vesicula seminalis; and in some Distomidae the part of the canal following the vesicula has cellular walls, and is surrounded by a number of unicellular glands which open into it, thus constituting a prostatic region. Similar unicellular glands are present in some monogenetic Trematoda, e. g.

Polystomum. The terminal portion of the canal is generally more or less evaginable, and is surrounded by a muscular cirrus-sac, sometimes absent, e. g. D. clavatum, which may in some Distomidae include only the prostatic region, or the vesicula seminalis also, together with the terminal portion. The sac appears to be composed of external longitudinal and internal circular muscular fibres. Among monogenetic Trematoda the genera Calicotyle and Pseudocotyle have the vas deferens terminating in a perforated chitinoid piece: while Axine, Microcotyle, Polystomum, Gyro-dactylus, etc, have a circlet of hooks (?) surrounding its aperture. The female organs consist of a germarium, vitellarium, their ducts, a shell-gland, and oviduct. The germarium is always single, usually globular or elongate, and even folded on itself, e. g. Octobothrium. sometimes lobed, e.g. in Tristomum, the lobes being extremely well-developed in some instances, e.g. in the genus Onchocotyle. This gland gives origin solely to the germ by the segmentation of which the embryo is formed. The vitellarium is a symmetrical gland right and left.

It consists very commonly of a number, generally very great indeed, of grape-like follicles connected to ducts which unite, and finally open into two longitudinal canals, one right, the other left, connected by a cross canal, usually situated just in front of the germarium. At the centre point of this canal a single duct which, as a rule, commences with a dilatation serving as a yolk reservoir, leads to the duct of the germarium. Instead of grapelike follicles, the gland may consist of two tubes provided with short caeca, e. g. Calceostoma, some Distomidae; or of two sacs as in D. ventricosum; or of several saccules arranged in a star-like fashion as in D. rufo-viride, D. globosum; or even of a net-work as in Monostomum mutabile, and M. reticulare (P. J. van Beneden). A canal, the Laurer-Stieda canal or vagina, which is double in Polystomum, Calicotyle, Pseudocotyle, single in all other instances where it has been recognised, opens externally in various positions, dorsally in Distomidae and Microcotyle, laterally in Epibdella, Axine, and Polystomum, ventrally in Pseudocotyle and Calicotyle, and near the uterine aperture in Tristomum. Internally it opens, when single, into the vitello-duct, or germ-duct, somewhere near the union of these ducts; when double into the lateral vitello-ducts in Polystomum, the united vitello- and germ-ducts in Pseudocotyle and Calicotyle. The canal itself is sometimes dilated, or there is a dilatation near its inner extremity in which spermatozoa are usually found1 its function is doubtful.

It has been supposed (I) to serve for copulatory purposes; (2) to act as a safety-tube for the escape of over-abundant or altered vitelline products and spermatozoa. Copulation has been actually observed to take place by its means in Polystomum (Zeller); and its structure in Axine and Microcotyle appears to favour that view. On the other hand its calibre is said to be too small to admit of copulation in some Distomidae (Poirier) and in Calicotyle. Both vitelline cells as well as spermatozoa and even germs have been observed in it in the former. The united vitello- and germ-ducts are surrounded-where they merge into the oviduct by a set of unicellular glands which secrete the shell. In the monogenetic Trematoda the portion of the oviduct into which the shell-glands open is dilated, variously shaped, and endowed with peristallic movement. To this portion the name 'Ootype' was given by P. J. van Beneden, or 'uterus' by Taschenberg. A germ, a certain quantity of vitelline cells and spermatozoa are moulded together in it into an egg, and provided with a shell. The ovum is then passed on.

A number of ova may accumulate in the terminal portion of the oviduct which is then dilated, or each ovum is laid singly as soon as formed, e. g. in Epibdella, Calceostoma. The ootype is not represented in the digenetic Trematoda with perhaps the exception of Gasterostomum, but the oviduct is dilated, thrown into convolutions which occupy a greater or less extent of the body, and retain a large number of ova. In some Distomidae, e. g. D. clavatum, the terminal part of the oviduct is surrounded by unicellular glands. The oviducal aperture may in some monogenetic Trematoda, e. g. Axine, Microcotyle, be surrounded by chitinoid hooks. The male and female apertures are always close together. They are usually placed anteriorly on the ventral surface a little to one side, e. g. Tristomum, or medianly, and in the Distomidae as a rule in front of the ventral sucker. They are, however, sometimes placed posteriorly to it, e.g. D.ocreatum, etc, or even on the posterior margin of the body as in D. macrostomum, Opisthotrema, Holostomum; rarely on the lateral margin as in D. clavi-gerum from the frog and the Tristomidan Epibdella. In many Distomidae the two orifices are surrounded by a sexual cloaca, and the male orifice may be situated on a projecting muscular papilla which bears in some instances the female aperture as well 1.

1 Calcareous bodies are stated by Cunningham to occur in the two main canals of Stichocotyle. 2 It is possible that D. filicolle (= Kollikeria of Cobbold, D. Okeni of Kolliker) is another example. See P. J. van Beneden, C. R. Suppl. ii. 1861, p. 186.

1A receptaculum seminis ( = inner vesicula seminalis auctt) may be present as (I) a dilatation of the Laurer-Stieda canal (Tristomum, D. clavatum); (2) a pear-shaped vesicle near the inner end of the same (D. palliatum, etc.); (3) a dilatation of the germ-duct (Onchocotyle appendiculatd). In many Distomidae the sperm is aggregated at the spot where the shell-glands open. The term 'outer vesicula seminalis' is often applied to the dilated region of the vas deferens.