[Illustration: FIG. 126.--Diagram of female genitalia. _Ov._, ovary; _ovd._, oviduct; _L.c._, Laurer’s canal; _Rec. sem._, receptaculum seminis; _Vit. R._, vitellarian reservoir; _t.v.d._, transverse vitelline duct; _Oo._, oötype; _Sh. gl._, shell gland; _Rec. ut._, receptaculum uterinum; _ut._, uterus. (The various parts are not to the same scale.) (Stephens.)] [Illustration: FIG. 127.--Diagram of male and part of female genitalia. _ut._, uterus; _vag._, vagina; ♀, opening of vagina; _g.s._, genital sinus; _g.p._, genital pore; ♂, opening of ejaculatory duct or vas deferens; _c.s._, cirrus sac; _c._, cirrus; _p.p._, pars prostatica; _s.v._, seminal vesicle; _e.j._, ejaculatory duct or vas deferens; _v.e._, vas efferens; _t._, testis. (Stephens.)] _Sexual Organs._--Nearly all the Trematodes are hermaphrodites, and only a few (_Schistosomidæ_, _Koellikeria_) are sexually differentiated. The sexual organs usually lie in the “central field” limited by the gut cæca; the vitellaria, on the other hand, are, as a rule, external to the gut cæca in the “lateral fields.” The male apparatus[259] is composed of two variously formed testes (fig. 127) (globular, oval, indented, lobed, or ramified), which may lie side by side or one behind the other; from each testicle a tube (vas efferens) originates; sooner or later, both tubes as a rule unite to form the ejaculatory duct or vas deferens, which is frequently enclosed in a muscular CIRRUS SAC, or more rarely passes directly into the genital pore. The cirrus, which is the thick muscular terminal portion of the vas deferens, can be everted and protruded from the cirrus sac and serves as an organ of copulation. The walls of the muscular portion of the tube (the cirrus) are attached to the walls of the cirrus sac, and hence when the sac contracts the cirrus cannot be protruded except by evagination of its lumen. Opening into the middle portion of the vas deferens, and as a rule enclosed in the cirrus sac, is found a mass of unicellular glands (prostate), the vesicula seminalis (which is likewise within, or may also be outside the sac) being the dilated first portion of the vas. [259] The following description relates mainly to the _Distomata_.