A. Genital ducts pass on the ventral side of the nerve and of the two longitudinal excretory vessels _T. crassicollis_ Rud. B. Genital ducts pass between the dorsal and ventral longitudinal vessels. _a._ Nerve present on dorsal side of genital ducts. α. Head armed _T. solium_ Rud. β. Head unarmed _T. saginata_ Goeze. _b._ Nerve on ventral side of genital ducts. DOG-TAENIAE[109] {90} Head armed; genital pore marginal and — Single Many proglottides; strobila several centimetres long; small hooks with guard. Bifid hooks, which are — 230µ-260µ long[110]; genital pore very distinct _T. serrata_ Goeze. — 136µ-157µ long; genital pore not very salient _T. serialis_ Ball. Entire large hooks, which are — 180µ-220µ long; length of mature segments double their width _T. marginata_ Batsch. — 150µ-170µ long; length of mature segments treble their width _T. coenurus_ Küch. 3-4 segments; a few mm. long _T. echinococcus_ v. Sieb. — Double and bilateral _Dipylidium_ caninum L. Head unarmed; two genital pores on ventral surface _Mesocestoides lineatus_ Goeze. II. Scolex without hooks; one or two transverse uteri present; one or two genital pores and yolk-glands, the latter never median; genital ducts pass on the dorsal side of the nerve; eggs with pyriform apparatus. A. One transverse uterus present. _a._ Uterus with bullate egg-sacs; pyriform apparatus without horns; genital ducts between dorsal and ventral vessels THYSANOSOMA Dies. α. Head large (1.5 mm.); square lobed testes in median field; posterior margin of segments fimbriated; genital pore double _T. fimbriata_ Dies. β. Head small; no fimbriae; pore rarely double _T. giardii_ Riv. _b._ Uterus without saccular dilatations; segments short, thick, and slightly imbricate ANOPLOCEPHALA E. Blanch. HORSE-TAENIAE. α. Head very large — No posterior lobes _A. plicata_ Zed. — Four posterior lobes _A. perfoliata_ Goeze. β. Head small, without posterior lobes _A. mamillana_ Mehl.