II. Tristomatidae and III. Polystomatidae.[[72]]—The members of these families are found on the body, or attached to the gills, of fresh-water and marine fishes. The edible and inedible fish of our coasts have each their particular ectoparasitic Trematodes; while the Minnows, Sticklebacks, and Miller's Thumbs of streams and ponds are attacked by Diplozoon, Gyrodactylus, and other forms. The aquatic Amphibia also harbour a number. Polystomum integerrimum is common in the bladder of Frogs, where it leads a practically aquatic life. Other species of Polystomum inhabit the buccal and nasal cavities of certain Chelonia, but naturally no terrestrial Vertebrates are infested externally by these Trematodes. The blood and epithelia of the host are sucked, and to this end the pharynx has frequently a chitinous armature to aid in the abrasion or inflammation of the tissues upon which the parasite feeds. In the case of a Sturgeon attacked by Nitzschia elongata, a Tristomid, the mouth of the host appeared to be highly inflamed by these attacks (v. Baer).
Fig. 23.—Octobothrium merlangi Kuhn, from the gills of the whiting, × 8. int, Intestine; ms, mouth; sc suckers with chitinoid armature; yk yolk-glands. (After v. Nordmann.)
The suckers, in the two families under consideration, vary in number and complexity. There is always a powerful apparatus at the hinder end of the body securing the Trematode firmly to the slimy body or gills of its host, and, usually in the Polystomatidae, a pair of suckers at the sides of the mouth accessory to the pumping action of the pharynx. In Axine, and to a less extent in Octobothrium (Fig. 23), the suckers are strengthened by a complex hingework of chitinoid bars or hooks, which serve as insertions for the muscles of the suckers, and thus increase their efficiency.
The mouth is invariably present just beneath the anterior end of the body. It leads into a muscular, pumping pharynx (Fig. 24, ph), and this into a bifurcated intestine which ends blindly. The two openings of the excretory system lie on the dorsal surface (as in Temnocephala), and the excretory canals branch through the substance of the body, ending usually in "flame-cells." The nervous system is highly developed, and resembles that of Temnocephala (Fig. 21) in detail. Upon the brain one or even two pairs of eye-spots are present in the larvae, and may persist throughout life. Tactile setae occur in Sphyranura, a parasite of the North American Amphibian Necturus, but a cellular epidermis is apparently rendered impossible, perhaps from the nature of the mucus in which the body is bathed, or to the attempts of the host to free itself from these parasites; and hence an investing membrane is present, which morphologically is either a modified epithelium, or a cuticle formed by the glandular secretion of the parenchyma.
Fig. 24.—Polystomum integerrimum Fröh., from the bladder of the Frog, and seen from the ventral surface. The alimentary canal is black, the white dots upon it being the yolk-glands, dvi, Ductus vitello-intestinalis (probably homologous with the Laurer's canal or "vagina" of Digenea); eh, hooks of sucking disc; int, intestine; m, mouth; ov, ovary; pe, penis; ph, pharynx; sc, suckers with an embryonic hook persisting in each; te, testes; ut, uterus with eggs; vag, left vagina; vd, vas deferens; yd, yolk-duct; yg, yolk-glands; ♂ ♀, common genital aperture. (Modified from Zeller.) × 8.
The reproductive organs of the Polystomatidae may be understood from Figs. 24, 27, and 28. At the point of union of the oviduct (Fig. 28, ovd), the vitelline ducts (yd), and the commencement of the uterus (ut), a slender duct is given off which opens into the intestine, and is known as the "vitello-intestinal canal" (Fig. 24, dvi; Fig. 28, gic). This duct has apparently the same relations as the "canal of Laurer" of Digenea,[[73]] except only that the latter opens to the exterior directly. In connexion with this vitello-intestinal canal a "vagina" is present, which in Polystomum and most Monogenea is paired (Fig. 24, vag), in Diplozoon and in one or two other forms, however, unpaired. The vagina receives the penis of another individual during copulation (Fig. 26), and does not appear to have an homologue in the liver-fluke or other Digenea.
Fig. 25.—Eggs of Monogenea. A, Eggs of Encotylabe pagelli v. Ben.-Hesse; B, eggs of Udonella pollachii v. Ben.-Hesse (with young forms just hatching out); C, egg of Microcotyle labracis v. Ben.-Hesse. (After van Beneden and Hesse.) × 50.