Sometimes, however, _Polystomum_ experiences another fate. The larvae settling down on the external gills of a young, recently-hatched tadpole, and obtaining a richer supply of blood than in the previous case, grow far more rapidly, so that in five weeks they are mature, although still in the branchial chamber of the tadpole. They do not then wander into the alimentary canal, but usually, having discharged their eggs, die at the time of the tadpole's metamorphosis. Still more interesting, however, is the difference between the genitalia in these and in the normal _Polystomum_. In contrast with the latter, these possess (1) one testis and a rudimentary penis; and their spermatozoa differ in structure and shape from those of the normal _Polystomum_. (2) The vaginae are absent, a fact connected with the absence of a functional copulatory organ. (3) In compensation for the loss of these, a duct connects the single testis and the point of union of oviduct and yolk-ducts, and by this self-fertilisation occurs. (4) The uterus is absent; the "ootype" or duct into which the shell-gland opens, communicating directly with the exterior. In (1) and (4) these aberrant _Polystomum_ resemble _P. ocellatum_, from the Tortoise _Emys europaea_. [Illustration: FIG. 26.—_Polystomum integerrimum._ A, Free-swimming larva, seen from the ventral surface. × 80. B, Two mature individuals in mutual coition attached to the bladder of a Frog. × 5. (After Zeller.) _d_, Intestine; _ex.o_, excretory pore, dorsal in position, seen here by transparency; _ey_, eye-spots; _gl_, frontal glands; _m_, mouth; _ph_, pharynx; _sd_, adhering disc; _vag_, vagina.] {60}[Illustration: FIG. 27.—A, Egg of _Diplozoon paradoxum_ v. Nord., consisting of a shell enclosing _ov_, the actual ovum, surrounded by _yc_, the yolk-cells; B, larva just hatched (× 125); C, two _Diporpa_ (I and II) about to unite; D, conjugation in progress but not yet complete. _dt_, Dorsal papilla; _e_, eye; _g_, intestine; _m_, mouth; _sc_, ad-oral sucker; _th_, spirally-wound thread attaching the egg to the gill of the Minnow; _vs_, ventral sucker; (in D) I, I, one _Diporpa_, ventral view; II, II, the other, dorsal view. (After Zeller.)]