[Illustration: FIG. 28.—Hinder part of the body of _Diplozoon paradoxum_. The fusion of the two _Diporpa_, where they come into contact, is now complete. They now cross each other like an X, and are twisted, so that _Diporpa_ I, in front of the point of fusion, is seen from the dorsal surface; behind, from the ventral surface; and the reverse is the case with _Diporpa_ II. The compound animal is seen from the opposite surface to that shown in Fig. 27, D. The digestive and excretory organs are omitted. (After Zeller.) I _Ant. dorsal_, dorsal surface of _Diporpa_ I, facing the anterior end; I _Post. ventral_, ventral surface of _Diporpa_ I, posterior end; and similarly for II _Ant. ventral_ and II _Post. dorsal_. _d_, Piece of the intestine showing opening of, _gic_, vitello-intestinal canal; _ov_, ovary; _ovd_, point of union of female genital ducts; _sc_, suckers; _te_, testis; _ut_ (in _Diporpa_ I), "ootype" or chamber into which shell-glands open. This is continuous with the uterus (_ut_) of _Diporpa_ I; _uto_, ventral opening of uterus; _vag_, vagina, with _vd_, vas deferens, permanently inserted into it through the genital pore; _yd_, yolk-ducts; _yg_, yolk-glands.] {61}_Diplozoon paradoxum._—The life-history of _Diplozoon_ is unique. For whereas the larvae of most animals grow up, each into a single adult, in _Diplozoon_, of the few larvae that survive the dangers of their free-swimming existence, only those become mature which conjugate permanently with another individual. But although there are thus only half as many adult _Diplozoon_ as there were conjugating larvae (or _Diporpa_, as they were called when they were considered distinct forms), yet the total number of eggs produced is probably as great as if each larva became individually mature. [Illustration: FIG. 29.—_Gyrodactylus elegans_ v. Nord., from the fins of the Stickleback. (After v. Nordmann.) × 125. _emb_, Embryo.]