From this fact and the anatomical relations of the vagina, it is becoming increasingly probable that the so-called uterus of Trematodes is an organ corresponding to the vagina of Cestodes, and not to the uterus of Cestodes. The latter opens to the exterior in _Schistocephalus_, _Bothriocephalus_, and some other Cestodes of fishes by a special pore (Fig. 41, _uto_). Through this, some of the eggs (which in these genera give rise to ciliated larvae) are enabled to escape, and need not wait for the detachment of the proglottis, as must happen in the Taeniidae, where the uterus is closed. This uterus, a true physiological one, is probably the homologue of the "canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The fertilised ovum and yolk are brought together into the "ootype," where the shell-gland forms the egg-shell around them (Fig. 41, _sh.gl_) and the egg is then passed into the uterus. The ovum segments to form a minute six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be ciliated. Thus in _Taenia serrata_ the proglottides are shed with the faeces of the host (dog), and they protect the young from the desiccating influence of the surroundings. If inadvertently eaten by a rabbit along with herbs, the proglottis and larval envelope are digested, and by its six hooks the tiny larva bores through the gastric wall into the portal vein, and so into the liver. Here the hooks are thrown off, and the solid mass of cells becomes vacuolated. {88}[Illustration: FIG. 43.—Stages in the development of _Dipylidium caninum_ L. (= _Taenia elliptica_ Batsch, _T. cucumerina_ Bloch), the commonest of the Dog-Taeniae; compare Fig. 44. A, Six-hooked larva (now often spoken of as an "Onchosphaera"); B, larva elongating; formation of a central lacuna; C, larva further advanced; D, distinction between body and tail is visible; E, invagination of the rostellum is commencing; F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels. _c_, Calcareous concretions in cells of the larva; _ex.o_, excretory aperture; _ex.v_, excretory vessels; _inv_, invagination commencing; _rost_, rostellum; _sc_, suckers. (After Grassi and Rovelli; highly magnified.)]