[Illustration: FIG. 20.—_Temnocephala novae-zealandiae_ Has. × 10. Ventral view to show the digestive and reproductive systems. (After Haswell.) FIG. 21.—The same from the dorsal surface, to show the excretory system (double line), and the nervous system (black and shaded). (After Haswell.) _d_, Intestine; _dln_, dorso-lateral nerve; _dn_, dorsal nerve; _ex.o_, excretory aperture on dorsal surface; _ex.s_, terminal excretory sac; _m_, mouth; _ov_, ovary; _ovd_, oviduct; _ph_, pharynx; _rh_, rhabdites; _rh.c_, cells in which the rhabdites are formed; _rv_, yolk receptacle; _sc_, sucker; _sh_, shell-gland; _te_, testes; _ut_, uterus; _vg_, vagina; _vn_, ventral nerve; _vs_, vesicula seminalis; _yd_, yolk-duct; _yg_, yolk-gland. ♀, ♂, common genital pore.] The reproductive system is very similar to that of certain Rhabdocoels. An armed penis and the female genital duct open into a genital atrium, and this by a single aperture (♀, ♂, Fig. 20) to the exterior. The fertilised ovum and yolk are enclosed in a stalked shell formed in the uterus. The interest and importance of the Temnocephalidae lies in the fact that they are almost as much Turbellaria as Trematodes. {55}In habits, in the character of the skin, the muscular, digestive, and reproductive systems, they find their nearest allies in Rhabdocoels (Vorticidae). But in the excretory and nervous systems, the latter composed of two dorsal, two lateral, and two ventral trunks all connected together (Fig. 21), they are Tristomid Trematodes. Thus they may fitly connect an account of the two great groups. [Illustration: FIG. 22.—A, _Nematobothrium filarina_ van Bened. Nat. size. Two individuals (_a_ and _b_) are found together, encysted on the branchial chamber of the Tunny. B, _Udonella caligorum_ Johns. A Tristomid, several of which are attached to the ovary of a Copepod (_Caligus_), itself a parasite on the gills of the Hake. × 8. C, _Epibdella hippoglossi_ O. F. M. A Tristomid found on the body of the Halibut. Nat. size. _m_, Mouth; _ms_, lateral suckers; _ov_, ovary; _ps_, posterior sucker; _te_, testes. (All after P. J. van Beneden.)]