Life-Histories of the Polystomatidae.[[74]]—Polystomum integerrimum. After the mutual fertilisation of two individuals, the eggs are laid in the water by the protrusion of the body of the parent through the urinary aperture of the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day for ten days. After about six weeks, the larva (.3 mm. long) hatches out, and swims about freely by means of bands of large ciliated cells (Fig. 26, A); but if it does not meet with a tadpole within twenty-four hours, it dies. Should it, however, encounter one, the larva creeps along it in a looping fashion until it approaches the opercular spout, or opening of the branchial chamber, on the left side; into this it darts suddenly, fixes itself, and throws off its cilia. Here it remains eight or ten weeks, feeding, increasing in size, and forming the suckers from behind forwards. At the time of the tadpole's metamorphosis, the young Polystomum works its way down the pharynx into the oesophagus and along the intestine, till it reaches and enters the opening of the bladder. Three years afterwards it becomes mature.
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.
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.
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.)
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.
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.
