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 {56}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). [Illustration: 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 {57}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.